Slag Modification in Reduction of Kiviniemi Ferrous Scandium Concentrates

Rita Kallio; Pekka Tanskanen; Eetu-Pekka Heikkinen; Tommi Kokkonen; Saija Luukkanen; Timo Fabritius

doi:10.3390/met12050709

Slag Modification in Reduction of Kiviniemi Ferrous Scandium Concentrates

Kallio, Rita;Tanskanen, Pekka;Heikkinen, Eetu-Pekka;Kokkonen, Tommi;Luukkanen, Saija;Fabritius, Timo 2022-04-21 00:00:00 metals Article Slag Modiﬁcation in Reduction of Kiviniemi Ferrous Scandium Concentrates 1 , 2 2 2 1 Rita Kallio *, Pekka Tanskanen , Eetu-Pekka Heikkinen , Tommi Kokkonen , Saija Luukkanen and Timo Fabritius Oulu Mining School, Faculty of Technology, University of Oulu, FI-90014 Oulu, Finland; saija.luukkanen@oulu.ﬁ Process Metallurgy Research Group, Faculty of Technology, University of Oulu, FI-90014 Oulu, Finland; pekka.a.tanskanen@oulu.ﬁ (P.T.); eetu.heikkinen@oulu.ﬁ (E.-P.H.); tommi.kokkonen@oulu.ﬁ (T.K.); timo.fabritius@oulu.ﬁ (T.F.) * Correspondence: rita.kallio@oulu.ﬁ; Tel.: +358-50-526-6806 Abstract: Several research projects are currently focused on the search for new sources of scandium due to its expected increasing demand in advanced technology applications. The Kiviniemi Fe-Sc- enriched maﬁc intrusion is a potential primary source for Sc. According to the recent investigations on the FeO component reduction in the Kiviniemi magnetic Sc concentrate at various end temperatures, complete FeO reduction is achieved at the highest experimental temperature (1500 C). However, efﬁcient separation of metal from the Sc O -enriched slag is hindered by the high viscosity of the 2 3 slag. In this study, investigations of the Kiviniemi-type concentrate reduction characteristics are complemented from three perspectives: (1) slag modiﬁcation with CaF and/or CaO to promote the reduction of the FeO component and metal separation, (2) reduction characteristics of the concentrates with a slightly different modal mineralogy and chemical composition, and (3) description of the main features of the progression of reduction at selected temperatures (950, 1050, 1150, 1250, and 1350 C) Citation: Kallio, R.; Tanskanen, P.; with CaO addition. Both CaF and CaO increase conversion rates at a lower temperature region Heikkinen, E.-P.; Kokkonen, T.; and promote the separation of metal from the slag. High-temperature behavior of the concentrates Luukkanen, S.; Fabritius, T. Slag Modiﬁcation in Reduction of used in this study is essentially similar, although the main reduction stage is initiated at a slightly Kiviniemi Ferrous Scandium higher temperature for concentrates with less amphibole and a higher amount of nonferrous gangue Concentrates. Metals 2022, 12, 709. minerals. Only after the complete decomposition and melting of clinopyroxene and nonferrous https://doi.org/10.3390/ minerals of the concentrate, the ﬁnal reduction of the FeO component from the slag can take place. met12050709 Keywords: ferrous scandium concentrate; reduction; slag; modiﬁcation Academic Editor: Alexander McLean Received: 15 March 2022 Accepted: 18 April 2022 1. Introduction Published: 21 April 2022 To meet the expected increasing demand of scandium, various primary and secondary Publisher’s Note: MDPI stays neutral scandium-rich resources are currently being investigated as potential new sources of this with regard to jurisdictional claims in rare-earth element. The Kiviniemi maﬁc intrusion in Finland represents one of these poten- published maps and institutional afﬁl- tial new sources with a preliminary mineral resource estimation of 13.4 Mt and an average iations. Sc grade of 163 g/t [1,2]. The main scandium-bearing minerals are ferrous amphibole and clinopyroxene [2–4]. Due to the paramagnetic nature of these minerals, a concentration stage involving low-intensity and high-gradient magnetic separation has been suggested for removing alkali-containing diamagnetic minerals with only negligible losses of Sc O to 2 3 Copyright: © 2022 by the authors. the tailings [4]. With magnetic concentration, the total ferrous oxide load in the concentrate Licensee MDPI, Basel, Switzerland. is inevitably high. Furthermore, due to the incorporation of scandium into the lattice of fer- This article is an open access article rous silicates, conventional beneﬁciation should be complemented with pyrometallurgical distributed under the terms and and/or hydrometallurgical methods. However, non-selective mobilization of Fe and Ti in conditions of the Creative Commons hydrometallurgical processing imposes challenges to the puriﬁcation and precipitation of Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ suitable Sc products [5–8]. Moreover, as reported in the preliminary beneﬁciation study 4.0/). of Kiviniemi ferrodiorite [3], large quantities of acids are required, and large volumes of Metals 2022, 12, 709. https://doi.org/10.3390/met12050709 https://www.mdpi.com/journal/metals Metals 2022, 12, 709 2 of 20 byproducts are produced. Therefore, pyrometallurgical processing is suggested as a poten- tial technique to decrease the amount of the ferrous oxide component in the concentrate prior to hydrometallurgical processing. In the ﬁrst part of our high-temperature studies, the results of reduction experiments of a ferrodiorite concentrate and characteristics of the products up to temperatures of 1500 C were described [9]. The observed increase in the derivative conversion rates during the experiments initiated gradually from ~950 C onwards with a sharp increase in the conversion rates between 1050 and 1170 C and high rates until ~1250 C. This was interpreted to signify the main melting and reduction stage, which continued as the formed FeO-containing slag reacted with graphite. Plagioclase and potassium feldspar are the main gangue minerals in the Kiviniemi concentrates [4]. In the plagioclase series, the liquidus temperature rises from the pure sodium endmember (albite 1118 C) to the pure calcium endmember (anorthite 1550 C) [10]; the plagioclase in the Kiviniemi samples is intermediate between the two endmembers, with its anorthite content being approximately 40 mol-% [2]. It is assumed that prior to complete melting of the partly reduced ferrous silicates and nonferrous gangue minerals, such as plagioclase, reduction reactions are hindered due to the formed product layers and remaining solid particles. Only after complete melting of the silicates and dissolution of unreduced FeO into the slag, the ﬁnal FeO reduction can be achieved by carbon, accompanied by segregation and accumulation of metallic iron. Liquid-phase mass transfer of iron oxide has been regarded as the major factor in the rate of reduction of iron oxide by carbonaceous material in slags [11]. Viscosity as a measure of the internal friction of a ﬂuid phase is one of the most important properties of metallurgical melts, directly affecting the kinetic conditions of the processes [12–14]. It is related to the internal structure of oxide melt and is very sensitive to changes in temperature, slag composition, and oxygen partial pressure [14,15]. Low viscosi- ties improve the transfer of mass and heat, the solubility of slag formers and modiﬁers, and the separation of metal and slag. Network former SiO has strong, highly covalent metal- oxygen bonds, leading to high liquid viscosities, whereas network transforming alkali and alkali earth oxides, such as Na O, K O, MgO, CaO, and other divalent oxides, such as MnO 2 2 and FeO, break this network, thus decreasing the viscosity [12,13,16,17]. Amphoteric oxide Al O may act either as a network former or transformer, depending on the composition of 2 3 the slag system [17,18]. In the ﬁrst part of our pyrometallurgical study, the viscosity of the slag was increased as the reduction reactions proceeded, decreasing the FeO content of the slag. This hindered the diffusion of the remaining FeO, which was particularly observed at lower ﬁnal temperatures, also causing signiﬁcant entrapment of small metal inclusions in the slag. Common techniques to lower the viscosity of the slag in metallurgical operations include the addition of CaO or CaF [13,19,20]. It is generally recognized that F in the 4+ silicate melts is not coordinated with Si , which means that CaF is merely dissociated in the molten silicate [21]. As the slags are ionic in nature, containing both covalent and ionic bonds, and as the extent of polymerization varies with the metal oxide and ﬂux contents in the slag, the viscosity of slags is extremely sensitive to the quality and quantity of ions and electrostatic interactions, as well as temperature [14,17]. In addition to viscosity, the slag composition with a suitable liquidus temperature—as based on the information from appropriate phase diagrams—is essential in industrial practices [17]. The experimental program in this second pyrometallurgical part of our study was designed to complement the characterization of the reduction behavior of Kiviniemi ferrous scandium concentrates from three perspectives: (1) modifying the slag composition with CaF and/or CaO to lower the viscosity of the slag and promote the reduction of the FeO component with enhanced separation of metallic iron, (2) comparing the reduction of a variety of concentrates from various parts of the Kiviniemi maﬁc intrusion with selected CaF and/or CaO doping, and (3) describing the main features of the progression of reduction with CaO addition at selected temperature intervals (950, 1050, 1150, 1250, and 1350 C). The main aim is to produce a modiﬁed, improved slag composition to be used Metals 2022, 12, 709 3 of 20 in hydrometallurgical processing, document the progression of reduction, and provide fundamental information for the beneﬁciation scenarios of ferrous scandium concentrates. 2. Materials and Methods Samples from three drill cores (R1, R2, and R3) from the Kiviniemi maﬁc intrusion were included in this study. Three adjacent or proximate drill core samples from four drill core intervals (altogether 12 individual samples) were combined into four composite samples. The composite samples are named R1, R2/u, R2/l, and R3 based on the drill cores they originate from; R2/u represents samples from the upper part and R2/l from the lower part of drill core R2 of the Kiviniemi intrusion [1,4]. The comminution procedure as well as magnetic separation with LIMS and SLon 100 pulsating high-gradient magnetic separation (HGMS) at Metso Outotec laboratory in Pori, Finland are described in more detail in [4]. P values after comminution are 68, 85, 92, and 78 m for R1, R2/u, R2/l, and R3 concentrates, respectively. The concentrates produced with SLon parameters (150 rpm and 1.0 T) with the highest Sc recovery and grade for each composite sample were selected for this study. Concentrate bulk compositions were measured by Euroﬁns Labtium in Kuopio, Fin- land, using an accredited inductively coupled plasma optical emission spectrometry (ICP- OES) method (721P). Dry samples were pulverized to 100%—90 m with a tungsten carbide mill at Oulu Mining School prior to sending for analysis. For ICP-OES analysis, a prepared pulp sample (0.2 g) is fused with anhydrous sodium peroxide in zirconium crucible by heating in electric furnace at 700 C for one hour. The melt is dissolved in hydrochloric acid. The ﬁnal solution is diluted with water prior to instrumental analysis. The routine method entails the analysis of 27 elements by ICP-OES Thermo Electron ICAP 6500 Duo. Detection limits with quality control details are provided in Kallio et al., 2021 [4]. Polished vertical blocks (Ø 25 and 40 mm) of the concentrate grain mounts and samples after each high- temperature experiment were prepared for ﬁeld emission scanning electron microscope (FESEM) and electron probe microanalyzer (EPMA) analysis, which were conducted at the Center of Material Analysis (CMA), University of Oulu. Polished blocks were coated with carbon prior to analyses. Data on modal mineralogy were acquired with INCAMineral software (version 5.05; Oxford Instruments, Oxford, Halifax, UK) and a Zeiss ULTRA Plus FESEM instrument (Oberkochen, Germany). The applied instrumental parameters were an acceleration voltage of 15 kV, beam current of 2.3 nA, and working distance of 8.3 mm. Postprocessing was conducted with GrainAlyzer software (Oxford Instruments, Oxford, Halifax, UK). A JEOL JXA-8530FPlus electron probe microanalyzer (JEOL Ltd., Tokyo, Japan) was employed to characterize mineral chemical compositions in concentrates as well as characteristics of the produced slag and metal, with the analytical conditions including an accelerating voltage of 15 kV, a beam current of 15 nA, and a beam diameter of 1–10 m. The peak and background counting times were set at 10 s and 5 s, respectively, for all components. For Sc O , values of 30 s and 15 s were also tested. The matrix correction with 2 3 the ZAF method (atomic number—absorption—ﬂuorescence) was applied. The standards used with EPMA are reported in [9]. X-ray diffraction (XRD) was utilized to monitor the presence of crystalline phases at selected temperatures. A Rigaku SmartLab 9 kW XRD apparatus (Rikagu Ltd., Tokyo, Japan) with Co anode was used with 40 kV and 135 mA settings. Speed of acquisition was 4 /min with 0.02 /step and 2 range of 10–130 . Data processing was performed with PDXL2 software and PDF-4 2022 database (Rikagu Ltd., Tokyo, Japan). High-temperature experiments were conducted with a thermogravimetric (TG) fur- nace at the Laboratory of Process Metallurgy, University of Oulu. The experimental set-up is described in more detail in the ﬁrst part of our pyrometallurgical study [9]. Ten grams of loose powders of the concentrate mix were prepared with calculated proportions of LIMS and SLon concentrates for each of these experiments. Based on the total ferrous oxide con- tent of the concentrates, graphite powder (Thermo-Fisher Scientiﬁc, Karlsruhe, Germany, Alfa Aesar 40797 lot: 61100109) was mixed with the concentrate in correct proportions to Metals 2022, 12, 709 4 of 20 ensure the complete ferrous component reduction. Other chemicals used for modifying the slag composition were Alfa Aesar 33299 CaO (lot: P12F022), burned at 850 C and stored in a desiccator, and Alfa Aesar 11055 CaF (lot: Z27D012). Powders were pressed to the bottom of the graphite crucibles. In addition to the similar experimental procedure used in the ﬁrst part of our pyrometallurgical study [9], quickly cooled samples at selected temperatures were included to investigate the progression of mineral reduction reactions at various stages. For these experiments, the gas composition was changed to 95% Ar and 5% H . After reaching the desired temperature in the TG furnace, the sample was raised to the upper, cooler part of the furnace, in which it was cooled with N gas ﬂow. The sample was taken out of the furnace after a few minutes of cooling. In total, the designed experimental program consists of 20 experiments, as presented in Table 1. Table 1. High-temperature experimental TG program for Kiviniemi magnetic concentrates. Focus of Experiment No. Sample Target T ( C) Isotherm, min Additives 1 R2/l 1450 120 5%C + 7%CaF Effect of CaF addition 2 R2/l 1450 120 5%C + 14%CaF 3 R2/l 1450 120 5%C + 5%CaO Effect of CaO addition 4 R2/l 1450 120 5%C + 10%CaO Effect of CaO + CaF addition 5 R2/l 1450 120 5%C + 7%CaF + 5%CaO 2 2 6 R1 1500 120 5%C + 7%CaF Effect of concentrate quality, 7 R2/u 1500 120 5%C + 7%CaF temperature and CaF addition 8 R3 1500 120 5%C + 7%CaF 9 R1 1500 120 5%C + 5%CaO Effect of concentrate quality, 10 R2/u 1500 120 5%C + 5%CaO temperature and CaO addition 11 R3 1500 120 5%C + 5%CaO 12 R1 1500 120 5%C + 7%CaF + 5%CaO Effect of concentrate quality, 13 R2/u 1500 120 5%C + 7%CaF + 5%CaO temperature and CaF + CaO addition 14 R3 1500 120 5%C + 7%CaF + 5%CaO Baseline with Ar + H (5%) 15 R3 1500 10 ~5%C + 5%CaO 16 R3 950 - ~5%C + 5%CaO 17 R3 1050 - ~5%C + 5%CaO Main features on the progression of 18 R3 1150 - ~5%C + 5%CaO reduction with CaO 19 R3 1250 - ~5%C + 5%CaO 20 R3 1350 - ~5%C + 5%CaO 3. Results and Discussion 3.1. Characteristics of the Concentrates Information on the concentrates produced with a combination of LIMS and SLon (150 rpm pulsation and 1.0 T applied magnetic induction) are presented in Table 2. Separate magnetic concentrates from LIMS and SLon were combined with correct mass proportions to produce a combined feed for high-temperature experiments to ensure the maximum possible recovery of both Sc O and FeO. 2 3 Modal compositions of the combined concentrates used in our pyrometallurgical experiments are shown in Figure 1A. Their chemical compositions, as calculated based on ICP-OES results, are reported in Table 3. Feed chemical compositions based on the modal mineralogy and EPMA analyses are also provided as a reference. This comparison allows evaluation of the reliability of the process’ mineralogical data, which are generally in agree- ment with the ICP-OES data. Chemical characteristics of the Kiviniemi intrusion include high Sc, Fe, Ti, and P content [2–4]. Despite deﬂecting apatite into the tailings in mag- netic separation, the P O grade in the concentrate remained at ~0.2–0.3 wt% (feed grade 2 5 0.6–0.74 wt%). FeO grades increased from the feed 19–23 wt% to concentrate 31–36 wt% while silica reduced from 42–47 wt% to 38–41 wt%, depending on the composite sample in question. Metals 2022, 12, x FOR PEER REVIEW 5 of 20 SLon MAGS mass % 55 62 65 52 SLon MAGS Sc ppm 230 310 250 310 SLon MAGS Sc recovery % 88 91 92 87 SLon Tailings mass % 45 39 35 49 SLon Tailings Sc ppm 40 50 40 50 TOTAL MAGS mass % 60 65 68 55 TOTAL MAGS Sc ppm 211 290 236 289 TOTAL MAGS Sc recovery % 89 91 93 88 TOTAL NMAGS mass % 40 35 32 45 TOTAL NMAGS Sc % 11 9 7 12 MAGS = magnetic concentrates, NMAGS = non-magnetic tailings. Modal compositions of the combined concentrates used in our pyrometallurgical ex- periments are shown in Figure 1A. Their chemical compositions, as calculated based on Metals 2022, 12, 709 5 of 20 ICP-OES results, are reported in Table 3. Feed chemical compositions based on the modal mineralogy and EPMA analyses are also provided as a reference. This comparison allows evaluation of the reliability of the process’ mineralogical data, which are generally in a Tg able reem 2.ent with the ICP- Details of the selected OES da concentrates ta. Chemi processed cal chawith racteri LIMS stics of and the SLon Ki using viniemi composite intrusi samples on in- clude h R1, R2/u, igh R2/l Sc, Fe, Ti (R2 upper , and andP cont lower ent sample), [2–4] and . Despit R3. e deflecting apatite into the tailings in magnetic separation, the P2O5 grade in the concentrate remained at ~0.2–0.3 wt% (feed Drill Core Sample R1 R2/u R2/l R3 grade 0.6–0.74 wt%). FeO grades increased from the feed 19–23 wt% to concentrate 31–36 Feed ppm Sc 150 210 170 180 wt% while silica reduced from 42–47 wt% to 38–41 wt%, depending on the composite sample in question. LIMS mass % 9 9 10 8 LIMS Sc ppm 110 163 153 167 Table 3. Chemical LIMS composi Sc recovery tions of % concentrates used in 7 high-temperature 7 experiments. Composi- 9 7 tions calculated based on modal mineralogy are also provided for comparison. Sc2O3 presented in SLon MAGS mass % 55 62 65 52 ppm, other oxides in wt%. SLon MAGS Sc ppm 230 310 250 310 SLon MAGS Sc recovery % 88 91 92 87 Sample SiO2 TiO2 Al2O3 FeO MnO MgO CaO K2O P2O5 S Sc2O3 (ppm) SLon Tailings mass % 45 39 35 49 R1 ICP-OES 39.8 3.08 8.33 32.7 0.61 1.81 7.99 1.52 0.32 0.28 324 SLon Tailings Sc ppm 40 50 40 50 R1 MODAL 39.3 2.68 7.84 33.1 0.58 1.62 8.57 1.43 0.48 0.32 289 R2/u ICP-OES 39.0 2.76 7.96 35.7 0.70 1.16 9.03 1.08 0.27 0.36 444 TOTAL MAGS mass % 60 65 68 55 TOTAL MAGS Sc ppm 211 290 236 289 R2/u MODAL 39.2 2.72 7.80 33.1 0.66 1.16 9.92 0.96 0.25 0.34 441 TOTAL MAGS Sc recovery % 89 91 93 88 R2/l ICP-OES 37.6 2.48 9.73 31.4 0.66 1.13 8.99 1.11 0.31 0.30 363 R2/l MODAL 40.3 2.52 9.27 31.5 0.76 1.11 9.70 0.59 0.22 0.19 380 TOTAL NMAGS mass % 40 35 32 45 R3 ICP-OES 40.7 2.45 10.1 T OTAL31.2 NMAGS 0.59 Sc % 0.97 8.71 11 1.63 9 0.22 0.28 7 444 12 R3 MODAL 39.7 2.07 MAGS 9.88 = magnetic concentrates, 29.9 0.59 NMAGS 0.95 = non-magnetic 8.88 tailings. 1.54 0.21 0.20 380 Figure 1. (A) Modal compositions of the concentrates used in pyrometallurgical experiments. Mineral abbreviations: AM = amphibole, CPX = clinopyroxene, FeOX = iron oxides (magnetite), FA = fayalite, ILM = ilmenite, SULF = sulﬁdes, CFS = clinoferrosilite, GRT = garnet, BT + CHL = biotite + chlorite, PL = plagioclase, FSP = potassium feldspar, QTZ + CAL = quartz + calcite, AP = apatite, ZRN = zircon. (B,C) Deportment of SiO , Al O , FeO, CaO, and Sc O in amphibole, clinopyroxene, fayalite, and 2 2 3 2 3 garnet for concentrates R2/l and R3. Although the feed samples represent the main lithology, with Sc-bearing amphibole and pyroxene having close to an average grade, there were differences in the amount of the main minerals between the feed composite samples and therefore also between produced concentrates. As indicated by Figure 1A, this was true particularly for the amounts of amphibole and clinopyroxene but also garnet and fayalite. Paramagnetic minerals, i.e., those with a ferrous component, accounted for 82–88 mass % of all the concentrates. Examples of the deportment of the main components—SiO , Al O , FeO, and CaO—as well 2 2 3 as Sc O within the four minerals (amphibole, clinopyroxene, fayalite, and garnet) in the 2 3 R2/l and R3 concentrates are presented in Figure 1B,C. Further details on the deportment of the main components are provided in the electronic supplementary data (Table S1). Metals 2022, 12, 709 6 of 20 Mineralogically, Sc O was mainly incorporated into the lattices of clinopyroxene and 2 3 amphibole, though with varying amounts (Figure 1B,C). Garnet and amphibole accounted for most of the Al O in the concentrates, whereas clinopyroxene introduced most of the 2 3 CaO into the system, particularly in concentrate R2/u. Fayalite was a signiﬁcant FeO- containing phase in concentrates R1 and R2/u. Compared with other concentrates, the R2/l concentrate had a signiﬁcantly higher amount of amphibole. Table 3. Chemical compositions of concentrates used in high-temperature experiments. Compositions calculated based on modal mineralogy are also provided for comparison. Sc O presented in ppm, 2 3 other oxides in wt%. Sample SiO TiO Al O FeO MnO MgO CaO K O P O S Sc O (ppm) 2 2 2 3 2 2 5 2 3 R1 ICP-OES 39.8 3.08 8.33 32.7 0.61 1.81 7.99 1.52 0.32 0.28 324 R1 MODAL 39.3 2.68 7.84 33.1 0.58 1.62 8.57 1.43 0.48 0.32 289 R2/u ICP-OES 39.0 2.76 7.96 35.7 0.70 1.16 9.03 1.08 0.27 0.36 444 R2/u MODAL 39.2 2.72 7.80 33.1 0.66 1.16 9.92 0.96 0.25 0.34 441 R2/l ICP-OES 37.6 2.48 9.73 31.4 0.66 1.13 8.99 1.11 0.31 0.30 363 R2/l MODAL 40.3 2.52 9.27 31.5 0.76 1.11 9.70 0.59 0.22 0.19 380 R3 ICP-OES 40.7 2.45 10.1 31.2 0.59 0.97 8.71 1.63 0.22 0.28 444 R3 MODAL 39.7 2.07 9.88 29.9 0.59 0.95 8.88 1.54 0.21 0.20 380 3.2. Effects of Additions on Reduction of Concentrate R2/l R2/l concentrate was used in the ﬁrst part of the pyrometallurgical study [9]; R2/l concentrate with a slightly higher Sc O grade and recovery was chosen as the ﬁrst type 2 3 of concentrate to test with selected additions. The details and results of experiments 1–5 conducted up to 1450 C are presented in Table 4. The sum of the calculated mass of oxygen for FeO and P O , the calculated sum of hydroxyl in amphibole, biotite, and chlorite, and 2 5 the mass of graphite are presented as a reference (S O + C + H O g). Hydroxyl removal from amphibole, biotite, and chlorite lattices was included assuming 1.86, 2.63, and 8.14 wt% of H O, respectively. Table 4. Results of experiments 1–5 using the R2/l concentrate with various additives. Experiment Number Sample 1 2 3 4 5 S LIMS g 1.17 1.17 1.17 1.17 1.17 Slon g 8.83 8.83 8.83 8.83 8.83 C g 0.50 0.50 0.50 0.50 0.50 CaF g 0.70 1.40 - - 0.70 CaO g - - 0.50 1.00 0.50 S g 11.20 11.90 11.00 11.50 11.70 m g 11.19 11.36 11.01 11.50 11.68 m g 9.05 8.91 9.35 9.82 9.61 Dm g 2.14 2.45 1.66 1.68 2.07 Dm % 19.12 21.57 15.08 14.61 17.72 S O + C + H O g 1.33 1.33 1.33 1.33 1.33 The calculation of the derivative conversion rates is explained in more detail in [9]. These rates exhibited acceleration at ~730–860 C for samples with CaO doping, which did not appear as distinctly in the experiments with CaF additions only (Figure 2). As presented in Figure 2, the main reaction stage was initiated with CaF - and/or CaO-doped R2/l samples at the same temperatures (~950 C) as with non-doped samples, but with doping and less nonferrous gangue minerals, the reaction rates were faster with higher rates of conversion at lower temperatures. The CaO-doped samples appeared to have Metals 2022, 12, x FOR PEER REVIEW 7 of 20 Metals 2022, 12, 709 7 of 20 doping and less nonferrous gangue minerals, the reaction rates were faster with higher rates of conversion at lower temperatures. The CaO-doped samples appeared to have their main peak in the derivative conversion rates at a slightly higher temperature (a rapid their main peak in the derivative conversion rates at a slightly higher temperature (a rapid increase between 970–1020 °C) than with CaF2 (950–990 °C). Furthermore, the derivative increase between 970–1020 C) than with CaF (950–990 C). Furthermore, the derivative conversion rates exhibited another acceleration at ~1050–1120 °C for both types of doping conversion rates exhibited another acceleration at ~1050–1120 C for both types of doping at the main reduction stage, after which the rates sharply declined. at the main reduction stage, after which the rates sharply declined. Figure 2. Derivative conversion curves for experiments 1, 3, and 5 using the R2/l concentrate. The Figure 2. Derivative conversion curves for experiments 1, 3, and 5 using the R2/l concentrate. The dotted dotted lline ine ind indicates icates aa conver conversion sion cu curve rve obta obtained ined without a without any ny slag modif slag modiﬁcation ication and wit and with h a higher a higher amount of gangue minerals [9]. amount of gangue minerals [9]. With CaF additions, another signiﬁcant increase in the conversion rates appeared With CaF2 additions, another significant increase in the conversion rates appeared approaching the ﬁnal temperature (1450 C) and particularly during the observed tempera- approaching the final temperature (1450 °C) and particularly during the observed tem- ture overshoot (Figure 2). The differences between 5 and 10% additions of CaO seemed perature overshoot (Figure 2). The differences between 5 and 10% additions of CaO negligible according to the mass loss (Table 4), whereas the sample with the highest addi- seemed negligible according to the mass loss (Table 4), whereas the sample with the high- tion of CaF (14%) exhibited the highest mass loss. This was interpreted to be caused by est addition of CaF2 (14%) exhibited the highest mass loss. This was interpreted to be ﬂuoride gas-forming reactions (Equation (1)) and increased silica reduction [22] and is one caused by fluoride gas-forming reactions (Equation (1)) and increased silica reduction [22] of the main reasons for the discrepancy between the calculated and measured mass losses and is one of the main reasons for the discrepancy between the calculated and measured (Table 4). mass losses (Table 4). 2CaF (slag) + SiO (slag) ! SiF (g) + 2CaO (slag) (1) 2 2 4 2CaF2 (slag) + SiO2 (slag) → SiF4 (g) + 2CaO (slag) (1) Although CaF has been widely used in the steel industry, it can be lost from industrial Although CaF2 has been widely used in the steel industry, it can be lost from indus- slags due to gas formation reactions, which increase with increasing temperature and set trial slags due to gas formation reactions, which increase with increasing temperature and limits to the use of CaF in industrial practice [22–24]. Depending on the temperature set limits to the use of CaF2 in industrial practice [22–24]. Depending on the temperature and composition of the slag, gaseous compounds such as KF, AlF and NaF can also be and composition of the slag, gaseous compounds such as KF, AlF3 and NaF can also be emitted. Other reasons for the ﬂuorspar substitution in the current industrial practices emitted. Other reasons for the fluorspar substitution in the current industrial practices include refractory wear, environmental issues, and problems in the supply or availability include refractory wear, environmental issues, and problems in the supply or availability of ﬂuorspar [24]. of fluorspar [24]. Figure 3A,B show back-scattered electron images of slag produced without any addi- Figure 3A,B show back-scattered electron images of slag produced without any ad- tions and with 5% addition of CaO, respectively, in experiments with R2/l concentrates at ditions and with 5% addition of CaO, respectively, in experiments with R2/l concentrates 1450 C. The positive effect of the addition of network-transforming CaO on the reduction at 1450 °C. Th of the slag FeO e positive effe component, ct of the addit as well as the ion of network-tran separation of very sform small ing CaO on the reduc- metal droplets in the tion of the slag FeO component, as well as the separation of very small metal droplets in slag, appeared to be obvious, with less metal retained in the slag. Figure 3A,B also present the sla EPMAg, analysis appeared to be obvi points with corr ous, wi esponding th less met results. al ret The aineslag d in t inhe both slag. samples Figure is 3A visually ,B also present homogeneous EPMA in ana back-scatter lysis pointsed wielectr th coon rrespondin images g but resu the lts. analytical The slag rin esults bothr s eveal amples some is variation, particularly in the SiO , FeO, and Sc O contents without doping (Figure 3A), visually homogeneous in back-scattered electron images but the analytical results reveal 2 2 3 which was diminished with CaO doping (Figure 3B). A summary of the slag analytical data some variation, particularly in the SiO2, FeO, and Sc2O3 contents without doping (Figure is provided in Table 5. 3A), which was diminished with CaO doping (Figure 3B). A summary of the slag analyt- ical data is provided in Table 5. Metals 2022, 12, x FOR PEER REVIEW 8 of 20 Table 5. Average chemical compositions determined with EPMA for slags produced in reduction experiments 1–5; Sc2O3 ppm, other oxides wt%. n = number of analysis points. Exp. No. n SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 ZrO2 Sc2O3 F 1 14 52.48 2.73 13.88 0.16 0.61 1.64 20.85 1.70 1.43 0.01 0.50 434 3.71 SD - 0.20 0.18 0.11 0.44 0.03 0.05 0.22 0.04 0.02 0.01 0.06 91 0.07 2 14 47.63 2.27 13.13 0.04 0.52 1.55 26.80 1.47 1.12 0.01 0.50 366 6.23 SD 0.27 0.11 0.07 0.02 0.03 0.04 0.12 0.05 0.03 0.01 0.05 92 0.09 3 14 55.48 3.24 13.54 0.22 0.74 1.61 19.75 1.69 1.50 0.00 0.50 399 0.00 SD - 0.23 0.11 0.11 0.03 0.04 0.04 0.16 0.05 0.02 0.01 0.09 89 0.00 4 13 51.82 3.11 12.59 0.13 0.68 1.52 24.92 1.52 1.37 0.01 0.45 420 0.00 SD - 0.31 0.14 0.08 0.04 0.04 0.10 0.14 0.04 0.02 0.01 0.04 159 0.00 Metals 2022, 12, 709 8 of 20 5 13 49.42 2.63 12.88 0.05 0.57 1.53 25.79 1.49 1.26 0.00 0.47 436 3.83 SD - 0.27 0.08 0.12 0.03 0.05 0.02 0.17 0.06 0.02 0.01 0.03 107 0.05 Figure 3. Figure 3. Back Back-scatter -scattered ed ele electr ctron on im images ages of of slag slag with with E EPMA PManalysis A analys point is poi locations nt locations an and corr d corre- espond- sponding results in wt% after reduction experiments with the R2/l concentrate conducted at 1450 ing results in wt% after reduction experiments with the R2/l concentrate conducted at 1450 C. °C. (A) Slag produced in the first part of our pyrometallurgical study without additions [9]. (B) Slag (A) Slag produced in the ﬁrst part of our pyrometallurgical study without additions [9]. (B) Slag produced with 5% addition of CaO in this study. produced with 5% addition of CaO in this study. According to the EPMA data (provided in electronic supplementary Table S2), CaF2 Table 5. Average chemical compositions determined with EPMA for slags produced in reduction addition promoted the reduction of SiO2 and solution of silicon to ferrite. None of the experiments 1–5; Sc O ppm, other oxides wt%. n = number of analysis points. 2 3 metal analyses provided Sc contents above the detection limit (100 ppm Sc). Titanium car- bide (TiC) formation was occasionally observed on the borders of graphite and ferrite. Exp. No. n SiO TiO Al O FeO MnO MgO CaO Na O K O P O ZrO Sc O F 2 2 2 3 2 2 2 5 2 2 3 Metal analyses did not indicate the presence of Fe3C. Figure 4 presents an overview of the 1 14 52.48 2.73 13.88 0.16 0.61 1.64 20.85 1.70 1.43 0.01 0.50 434 3.71 characteristics of large metal accumulation after the experiment with 10% addition of SD - 0.20 0.18 0.11 0.44 0.03 0.05 0.22 0.04 0.02 0.01 0.06 91 0.07 CaO, with a close-up of the texture and EPMA point analytical data. The black areas in 2 14 47.63 2.27 13.13 0.04 0.52 1.55 26.80 1.47 1.12 0.01 0.50 366 6.23 SD 0.27 0.11the back-scat 0.07 0.02 tered electro 0.03 n imag 0.04 es rep 0.12 resent 0.05 graphi0.03 te and t 0.01 he bright 0.05 est pha 92 se is ferr 0.09 ite while the intermediate grey color represents steadite, the eutectic of ferrite and iron phos- 3 14 55.48 3.24 13.54 0.22 0.74 1.61 19.75 1.69 1.50 0.00 0.50 399 0.00 phide (Fe3P). It solidifies during cooling from the liquid as the last constituent at the grain SD - 0.23 0.11 0.11 0.03 0.04 0.04 0.16 0.05 0.02 0.01 0.09 89 0.00 4 13 51.82 3.11 12.59 0.13 0.68 1.52 24.92 1.52 1.37 0.01 0.45 420 0.00 boundaries [25]. SD - 0.31 0.14 0.08 0.04 0.04 0.10 0.14 0.04 0.02 0.01 0.04 159 0.00 5 13 49.42 2.63 12.88 0.05 0.57 1.53 25.79 1.49 1.26 0.00 0.47 436 3.83 SD - 0.27 0.08 0.12 0.03 0.05 0.02 0.17 0.06 0.02 0.01 0.03 107 0.05 According to the EPMA data (provided in electronic supplementary Table S2), CaF addition promoted the reduction of SiO and solution of silicon to ferrite. None of the metal analyses provided Sc contents above the detection limit (100 ppm Sc). Titanium carbide (TiC) formation was occasionally observed on the borders of graphite and ferrite. Metal analyses did not indicate the presence of Fe C. Figure 4 presents an overview of the characteristics of large metal accumulation after the experiment with 10% addition of CaO, with a close-up of the texture and EPMA point analytical data. The black areas in the back-scattered electron images represent graphite and the brightest phase is ferrite while the intermediate grey color represents steadite, the eutectic of ferrite and iron phosphide (Fe P). It solidiﬁes during cooling from the liquid as the last constituent at the grain boundaries [25]. Metals 2022, 12, 709 9 of 20 Metals 2022, 12, x FOR PEER REVIEW 9 of 20 Figure 4. Example of large metal accumulation produced after reducing concentrate R2/l with 10% Figure 4. Example of large metal accumulation produced after reducing concentrate R2/l with 10% CaO addition. EPMA data on metal concentrations from points 1–5 expressed in wt%. CaO addition. EPMA data on metal concentrations from points 1–5 expressed in wt%. 3.3. Comparison of Concentrate Quality with Selected Additions 3.3. Comparison of Concentrate Quality with Selected Additions Experiments 6–14 (Table 1) were conducted to compare the high-temperature prop- Experiments 6–14 (Table 1) were conducted to compare the high-temperature prop- erties and behavior of concentrates R1, R2/u, and R3, having differences in their modal erties and behavior of concentrates R1, R2/u, and R3, having differences in their modal mineralogy (Figure 1). Additions of 7% CaF2, 5% CaO, and a combination of both of these mineralogy (Figure 1). Additions of 7% CaF , 5% CaO, and a combination of both of these two were used. The final isotherm was changed to the highest possible one in the TG two were used. The ﬁnal isotherm was changed to the highest possible one in the TG furnace (1500 °C) to further improve the separation of small metal inclusions from the slag furnace (1500 C) to further improve the separation of small metal inclusions from the slag (Figure 3). Details of the feed materials and experimental results are presented in Table 6. (Figure 3). Details of the feed materials and experimental results are presented in Table 6. Table 6. Results of experiments 6–14 conducted up to 1500 °C with R1, R2/u, and R3 concentrates. Table 6. Results of experiments 6–14 conducted up to 1500 C with R1, R2/u, and R3 concentrates. Exp. No. 6 7 8 9 10 11 12 13 14 Exp. No. 6 7 8 9 10 11 12 13 14 Sample R1 R2/u R3 R1 R2/u R3 R1 R2/u R3 Sample R1 R2/u R3 R1 R2/u R3 R1 R2/u R3 Ʃ LIMS g 1.59 1.39 1.44 1.59 1.39 1.44 1.59 1.39 1.44 S LIMS g 1.59 Slon g1.39 8.41 1.448.62 1.59 8.56 8.41 1.39 8.62 1.44 8.56 8.41 1.59 8.62 1.39 8.56 1.44 Slon g 8.41 8.62 8.56 8.41 8.62 8.56 8.41 8.62 8.56 C g 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 C g 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 CaF2 g 0.70 0.70 0.70 - - - 0.70 0.70 0.70 CaF g 0.70 0.70 0.70 - - - 0.70 0.70 0.70 CaO g - - - 0.50 0.50 0.50 0.50 0.50 0.50 CaO g - - - 0.50 0.50 0.50 0.50 0.50 0.50 Ʃ g 11.20 11.21 11.20 11.00 11.01 11.00 11.70 11.71 11.70 S g 11.20 11.21 11.20 11.00 11.01 11.00 11.70 11.71 11.70 m0 g 11.17 11.17 11.16 10.97 10.99 10.99 11.68 11.71 11.67 mf g 8.68 8.62 8.59 9.04 9.06 8.95 9.23 9.28 9.16 m0 g 11.17 11.17 11.16 10.97 10.99 10.99 11.68 11.71 11.67 mf g 8.68 Δm g 8.62 2.49 8.592.55 9.042.57 1.93 9.06 1.93 8.95 2.04 2.45 9.23 2.43 9.28 2.51 9.16 Dm g 2.49 2.55 2.57 1.93 1.93 2.04 2.45 2.43 2.51 Δm (%) 22.29 22.83 23.03 17.59 17.56 18.56 20.98 20.75 21.51 Dm (%) 22.29 22.83 23.03 17.59 17.56 18.56 20.98 20.75 21.51 Ʃ O + C + H2O g 1.33 1.38 1.28 1.33 1.38 1.28 1.33 1.38 1.28 S O + C + H O g 1.33 1.38 1.28 1.33 1.38 1.28 1.33 1.38 1.28 The obtained mass loss curves (Figure 5A) and derivative conversion rates (Figure 5B–D) exhibited essentially similar characteristics for the three concentrates under the The obtained mass loss curves (Figure 5A) and derivative conversion rates (Figure 5B–D) same experimental conditions. Compared with the other concentrates, the R2/l concen- exhibited essentially similar characteristics for the three concentrates under the same ex- trate used in the first part of our pyrometallurgical study [9] and in the first five experi- perimental conditions. Compared with the other concentrates, the R2/l concentrate used ments of this study contains a significantly higher amount of amphibole, which likely is in the ﬁrst part of our pyrometallurgical study [9] and in the ﬁrst ﬁve experiments of this the reason for the slightly lower temperature region for the higher conversion rates (Fig- study contains a signiﬁcantly higher amount of amphibole, which likely is the reason for ures 2 and 5) initiating at ~950 °C. Although the main reduction stage appeared to initiate the slightly lower temperature region for the higher conversion rates (Figures 2 and 5) at a slightly higher temperature (~1000 °C), R1, R2/u, and R3 concentrates all exhibited initiating at ~950 C. Although the main reduction stage appeared to initiate at a slightly higher rates of conversion with both additions at a lower temperature region as compared higher temperature (~1000 C), R1, R2/u, and R3 concentrates all exhibited higher rates to the R2/l concentrate without slag modification (Figure 5). Metals 2022, 12, 709 10 of 20 Metals 2022, 12, x FOR PEER REVIEW 10 of 20 of conversion with both additions at a lower temperature region as compared to the R2/l concentrate without slag modiﬁcation (Figure 5). Figure 5. (A) Mass-loss curves for experiments 6–14, with all conducted using the same temperature Figure 5. (A) Mass-loss curves for experiments 6–14, with all conducted using the same temperature program up to 1500 °C for R1, R2/u, and R3 concentrates. (B–D) Derivative conversion curves for program up to 1500 C for R1, R2/u, and R3 concentrates. (B–D) Derivative conversion curves experiments 6–8 (with 7% CaF2), 9–11 (with 5% CaO), and 12–14 (with 7% CaF2 + 5% CaO). Conver- for experiments 6–8 (with 7% CaF ), 9–11 (with 5% CaO), and 12–14 (with 7% CaF + 5% CaO). 2 2 sion rates for R2/l concentrate without additions shown as a reference [9]. Conversion rates for R2/l concentrate without additions shown as a reference [9]. The R2/u concentrate has the highest FeO content and exhibited the highest deriva- The R2/u concentrate has the highest FeO content and exhibited the highest deriva- tive conversion rates at the main stage of reduction reactions. Similar to the first set of tive conversion rates at the main stage of reduction reactions. Similar to the ﬁrst set of experiments, the conversion rates with CaF2 occurred at slightly lower temperatures than experiments, the conversion rates with CaF occurred at slightly lower temperatures than with CaO additions. Both additions alone exhibited a wider temperature region for higher with CaO additions. Both additions alone exhibited a wider temperature region for higher rates of conversion, whereas with the combined and thus the highest amount of doping, rates of conversion, whereas with the combined and thus the highest amount of doping, the rate increased and decreased very sharply (Figure 5D), occurring within a narrower the rate increased and decreased very sharply (Figure 5D), occurring within a narrower temperature range. This is interpreted to be caused by improved slag-forming reactions temperature range. This is interpreted to be caused by improved slag-forming reactions and slag characteristics due to a lower viscosity, which in turn enhances mass-transfer and slag characteristics due to a lower viscosity, which in turn enhances mass-transfer phenomena and allows faster reduction reactions within a narrower temperature interval. phenomena and allows faster reduction reactions within a narrower temperature interval. As observed in the previous and current set of experiments, with CaO additions at the As observed in the previous and current set of experiments, with CaO additions at the low-temperature regime (<900 °C) in CO atmosphere, the reversed Boudouard reaction is low-temperature regime (<900 C) in CO atmosphere, the reversed Boudouard reaction is likely to occur with the possibility of reaction with CaO, based on the mass loss curves likely to occur with the possibility of reaction with CaO, based on the mass loss curves and and derivative conversion rates (Figures 2 and 5) [26]. derivative conversion rates (Figures 2 and 5) [26]. Average slag compositions determined with EPMA are presented in Table 7. As men- Average slag compositions determined with EPMA are presented in Table 7. As tioned earlier, the counting times were 10 s for peaks and 5 s for background for each mentioned earlier, the counting times were 10 s for peaks and 5 s for background for element in th each elemente beginnin in the beginning g of our of analytic our analytical al work. This wa work. This s iniwas tially initially considered an a considerppro- ed an pri appr ate compromi opriate compr se between a omise between reasona a r beasonable le total analysis time, also co total analysis time, nsider alsoing the considering available the WDS cr available ystals WDS and detector crystals and s. Re detectors. garding the electr Regardingon m the electr icroprobe on micr analysis o oprobe analysis f trace elements of trace (< elements 1000 pp(<1000 m) for p ppm) hasefor s th phases at are st that able are under stable a den under se electron be a dense electr am, the on beam, detec thetion limit detection and precision could be decreased by using a higher acceleration voltage and beam current combined with a longer counting time [27]. Due to the observed high standard deviation for Sc2O3 in the first set of slag analyses for these samples (data provided in electronic Metals 2022, 12, 709 11 of 20 limit and precision could be decreased by using a higher acceleration voltage and beam current combined with a longer counting time [27]. Due to the observed high standard deviation for Sc O in the ﬁrst set of slag analyses for these samples (data provided in 2 3 electronic supplementary Table S3), longer peak and background counting times were tested to improve the quality of the data; the counting times of 30 s for peaks and 15 s for background were set for Sc O only, while the initial parameters were employed for 2 3 other components. This lowered the standard deviation and provided results closer to the calculated values. More speciﬁc analysis of Sc O with a higher acceleration voltage and 2 3 beam current and/or a longer counting time might provide potential to lower the detection limit and standard deviation even further [28,29]. The calculated slag compositions listed in Table 7 are normalized, excluding FeO and P O from the ICP-OES results and considering 2 5 the applied doping. As an exception to other components, the Na O values were calculated based on modal mineralogy because the samples were subjected to the sodium peroxide fusion prior to ICP-OES analysis. The differences between the analyzed and calculated slag compositions are visualized in Figure 6, which is based on the percentage of the difference of EPMA analyzed values from calculated value to provide an indication of the extent of reduction for other slag components in addition to FeO and P O . 2 5 Table 7. Average chemical compositions determined with EPMA for slags produced in reduction experiments 6–14; Sc O ppm, other oxides wt%. n = number of analysis points. Calc = calculated 2 3 values based on ICP-OES data, excluding FeO and P O components. 2 5 Exp. No. n SiO TiO Al O FeO MnO MgO CaO Na O K O P O ZrO Sc O F 2 2 2 3 2 2 2 5 2 2 3 6 20 53.17 2.71 12.80 0.03 0.52 2.83 20.74 1.42 2.02 0.01 0.12 389 3.78 SD - 0.42 0.10 0.10 0.02 0.03 0.06 0.28 0.04 0.05 0.01 0.03 64 0.05 Calc. - 54.76 4.24 11.46 0.00 0.83 2.47 17.76 1.40 * 2.09 0.00 - 446 4.56 7 20 54.04 2.30 12.16 0.03 0.64 1.85 22.53 1.17 1.46 0.01 0.38 604 3.73 SD - 0.33 0.14 0.09 0.02 0.03 0.04 0.14 0.05 0.03 0.01 0.05 58 0.08 Calc. - 54.69 3.87 11.17 0.00 0.98 1.63 19.66 1.24 * 1.52 0.00 - 623 4.72 8 20 52.71 2.31 14.99 0.03 0.49 1.44 20.90 1.54 2.09 0.01 0.16 559 3.76 SD - 0.19 0.12 0.08 0.02 0.03 0.05 0.10 0.04 0.03 0.01 0.03 71 0.08 Calc. - 54.50 3.28 13.57 0.00 0.79 1.30 18.28 1.25 * 2.19 0.00 - 595 4.46 9 ** 20 56.27 4.39 12.32 0.09 0.70 2.74 19.68 1.52 2.16 0.01 0.12 381 0.00 SD - 0.29 0.18 0.07 0.03 0.02 0.06 0.15 0.04 0.04 0.01 0.04 61 0.00 Calc. - 57.45 4.45 12.03 0.00 0.88 2.61 18.57 1.41 * 2.19 0.00 - 468 10 ** 20 56.85 3.87 11.89 0.07 0.82 1.84 21.63 1.23 1.56 0.00 0.38 594 0.00 SD - 0.24 0.13 0.08 0.03 0.04 0.04 0.09 0.05 0.03 0.01 0.03 49 0.00 Calc. - 57.51 4.07 11.61 0.00 1.03 1.72 20.63 1.29 * 1.60 0.00 - 655 11 ** 20 55.74 3.43 14.58 0.10 0.65 1.45 20.04 1.61 2.24 0.00 0.16 542 0.00 SD - 0.18 0.17 0.16 0.03 0.03 0.03 0.14 0.05 0.04 0.01 0.03 60 0.00 Calc. - 57.07 3.44 14.21 0.00 0.83 1.36 19.09 1.31 * 2.29 0.00 - 623 12 20 50.22 2.45 11.76 0.04 0.51 2.62 25.89 1.16 1.72 0.01 0.11 326 3.97 SD - 0.17 0.12 0.08 0.02 0.04 0.03 0.13 0.03 0.02 0.01 0.02 59 0.07 Calc. - 51.34 3.98 10.75 0.00 0.78 2.33 22.93 1.28 * 1.96 0.00 - 418 4.26 13 20 50.41 2.32 11.15 0.03 0.65 1.71 27.60 1.00 1.23 0.01 0.38 533 3.98 SD - 0.15 0.09 0.08 0.02 0.04 0.04 0.13 0.03 0.03 0.01 0.03 52 0.05 Calc. - 51.16 3.62 10.45 0.00 0.91 1.53 24.84 1.16 * 1.42 0.00 - 583 4.41 14 20 49.44 2.06 13.95 0.02 0.45 1.37 26.27 1.21 1.75 0.01 0.14 477 3.93 SD - 0.22 0.10 0.04 0.02 0.03 0.04 0.11 0.04 0.03 0.01 0.03 47 0.08 Calc. - 51.15 3.08 12.73 0.00 0.74 1.22 23.31 1.17 * 2.06 0.00 - 558 4.18 * Value calculated based on concentrate modal mineralogy; ** EPMA results normalized due to low totals. Metals 2022, 12, x FOR PEER REVIEW 12 of 20 Metals 2022, 12, 709 12 of 20 typical structures in large metal accumulations produced from the R3 concentrate with various doping are presented in Figure 7. Metals 2022, 12, x FOR PEER REVIEW 12 of 20 typical structures in large metal accumulations produced from the R3 concentrate with various doping are presented in Figure 7. Figure 6. Differences between the analyzed (EPMA) and calculated (ICP-OES) slag compositions. Figure 6. Differences between the analyzed (EPMA) and calculated (ICP-OES) slag compositions. Experiments using concentrates R1, R2/u, and R3 with (A) 7% CaF2, (B) 5% CaO, and (C) 7% CaF2 + Experiments using concentrates R1, R2/u, and R3 with (A) 7% CaF , (B) 5% CaO, and (C) 7% CaF + 2 2 5% CaO. 5% CaO. According to the slag and metal EPMA data, reduction of the MnO, TiO , K O, and 2 2 SiO components from slag occurred to some extent, particularly with CaF additions. The 2 2 negative values for Sc O are considered to be arising from the challenges in trace element 2 3 analysis with EPMA and because Sc O is thermodynamically very stable in comparison 2 3 to other components in the slag [30,31]. Back-scattered electron images of the slags and Figure 6. Differences between the analyzed (EPMA) and calculated (ICP-OES) slag compositions. typical structures in large metal accumulations produced from the R3 concentrate with Experiments using concentrates R1, R2/u, and R3 with (A) 7% CaF2, (B) 5% CaO, and (C) 7% CaF2 + various doping are presented in Figure 7. 5% CaO. Figure 7. Back-scattered electron images of slag and metal after experiments with concentrate R3 conducted up to 1500 °C with various doping. (A) 7% CaF2, (B) 5% CaO, and (C) 7% CaF2 + 5% CaO additions. There were no significant differences in the amount or size of small metal inclusions in the slag between the experimental runs with CaF2 and CaO additions. Large graphite- bearing metal accumulations occurred at the bottom of the crucibles with smaller metal droplets on the sides, along the contact of the slag and graphite crucible. The wetting of Figure 7. Back-scattered electron images of slag and metal after experiments with concentrate R3 Figure 7. Back-scattered electron images of slag and metal after experiments with concentrate R3 conducted up to 1500 °C with various doping. (A) 7% CaF2, (B) 5% CaO, and (C) 7% CaF2 + 5% CaO conducted up to 1500 C with various doping. (A) 7% CaF , (B) 5% CaO, and (C) 7% CaF + 5% CaO 2 2 additions. additions. There were no significant differences in the amount or size of small metal inclusions in the slag between the experimental runs with CaF2 and CaO additions. Large graphite- bearing metal accumulations occurred at the bottom of the crucibles with smaller metal droplets on the sides, along the contact of the slag and graphite crucible. The wetting of Metals 2022, 12, 709 13 of 20 There were no signiﬁcant differences in the amount or size of small metal inclusions Metals 2022, 12, x FOR PEER REVIEW 13 of 20 in the slag between the experimental runs with CaF and CaO additions. Large graphite- bearing metal accumulations occurred at the bottom of the crucibles with smaller metal droplets on the sides, along the contact of the slag and graphite crucible. The wetting of graphite–slag interfaces by slag without an FeO component has been regarded as rather graphite–slag interfaces by slag without an FeO component has been regarded as rather poor depending on the carbon-slag interfacial tension, slag and carbon surface tension, poor depending on the carbon-slag interfacial tension, slag and carbon surface tension, and the dynamic reactions occurring at the interface [32]. It is apparent from the observed and the dynamic reactions occurring at the interface [32]. It is apparent from the observed concave slag surface with CaF2 doping (Figure 7A,C), in comparison to the convex surface concave slag surface with CaF doping (Figure 7A,C), in comparison to the convex surface with only CaO doping (Figure 7B), that the surface characteristics for these slags are quite with only CaO doping (Figure 7B), that the surface characteristics for these slags are quite different. CaF2 is known to be a surface-active constituent in slags [21]. In their evaluation different. CaF is known to be a surface-active constituent in slags [21]. In their evaluation of the surface tension of molten ionic mixtures containing CaF2, Nakamoto et al. [21] stated of the surface tension of molten ionic mixtures containing CaF , Nakamoto et al. [21] stated that in the SiO2–CaO–CaF2 system at 1500 °C, the surface tension decreases with increas- that in the SiO –CaO–CaF system at 1500 C, the surface tension decreases with increasing 2 2 ing CaF2 content, apparently also applying to the slag system of this study. CaF content, apparently also applying to the slag system of this study. Metal analyses revealed similar phases as described in our previous experiments, in- Metal analyses revealed similar phases as described in our previous experiments, cluding eutectic steadite between Si-containing ferrite and flake graphite and/or interden- including eutectic steadite between Si-containing ferrite and ﬂake graphite and/or inter- dritic graphite segregations. These experiments also exhibited increased silica reduction dendritic graphite segregations. These experiments also exhibited increased silica reduction with with CaF CaF 2 do doping, ping, wh which ich isis in indicated dicated b by y an an incr inc ease rease in in the the conversion conversion rates rate at s temperatu at tempera- res of tures of >14 >1450 C50 °C (Figur (F eig 5ure 5B) B). In accor . In a dance ccordance withwi the th the previous previexperiments, ous experiments, ti titanium tanium carbide car- (TiC) was detected occasionally at the borders between ferrite and graphite. Averages of the bide (TiC) was detected occasionally at the borders between ferrite and graphite. Aver- ferrite ages of analyses the ferrite analyses are are provided inprov Figur ided e 8 in with Fig details ure 8 w pr ith details pro ovided in thev electr ided oni in the c supplemen- electronic tary data (Table S2). Graphite crystallization in cast gray iron is a complex phenomenon, supplementary data (Table S2). Graphite crystallization in cast gray iron is a complex phe- which is controlled by melt composition, temperature, and cooling rate [25,33]. According nomenon, which is controlled by melt composition, temperature, and cooling rate [25,33]. to the textures developed in large metal accumulations, the tendency to form ﬁne inter- According to the textures developed in large metal accumulations, the tendency to form dendritic graphite was promoted by a higher silicon content in the metal (Figures 7 and 8). fine interdendritic graphite was promoted by a higher silicon content in the metal (Figures Metallic iron somewhat penetrated the graphite crucible, dissolving carbon into the metal 7 and 8). Metallic iron somewhat penetrated the graphite crucible, dissolving carbon into and producing the above-mentioned textures upon cooling. Furthermore, the gas bubble the metal and producing the above-mentioned textures upon cooling. Furthermore, the formed on top of the large metal accumulation (Figure 7C) indicated gasiﬁcation reactions gas bubble formed on top of the large metal accumulation (Figure 7C) indicated gasifica- at the Fe–C surface; as proposed by Teasdale and Hayes [34,35], gasiﬁcation of the carbon tion reactions at the Fe–C surface; as proposed by Teasdale and Hayes [34,35], gasification in the alloy produces CO as one of the reaction steps involved in the reduction of slag of the carbon in the alloy produces CO as one of the reaction steps involved in the reduc- by solid carbon in the presence of liquid Fe-C. As stated by White et al. [36], liquid slags tion of slag by solid carbon in the presence of liquid Fe-C. As stated by White et al. [36], react with carbon in surprisingly complex ways, with liquid Fe-C metal and gas-forming liquid slags react with carbon in surprisingly complex ways, with liquid Fe-C metal and reactions from various slag components contributing to the whole scenario. gas-forming reactions from various slag components contributing to the whole scenario. Figure 8. Average ferrite compositions in wt% determined by EPMA in experiments on variously Figure 8. Average ferrite compositions in wt% determined by EPMA in experiments on variously doped concentrates R1, R2/u, and R3. (A) 7% CaF2, (B) 5% CaO, and (C) 7% CaF2 + 5% CaO additions. doped concentrates R1, R2/u, and R3. (A) 7% CaF , (B) 5% CaO, and (C) 7% CaF + 5% CaO 2 2 additions. The design of the type and quantity of doping used in our experiments is based on the ternary Al2O3-CaO-SiO2 phase diagram presented in Figure 9, which exhibits the target The design of the type and quantity of doping used in our experiments is based on the area with lower liquidus temperatures as indicated in the close-up. To calculate and plot ternary Al O -CaO-SiO phase diagram presented in Figure 9, which exhibits the target 2 3 2 the compositions on the phase diagram, only the main oxide components (Tables 3, 6 and area with lower liquidus temperatures as indicated in the close-up. To calculate and plot the S2) were considered. Slag compositions based on calculations from both ICP-OES and compositions on the phase diagram, only the main oxide components (Tables 3, 6 and S2) EPMA data are shown in comparison in Figure 9, which was computed with the FactSage were considered. Slag compositions based on calculations from both ICP-OES and EPMA version 7 and its FToxid database. data are shown in comparison in Figure 9, which was computed with the FactSage version 7 The amounts of the additions of CaF2 and CaO were based on the desired liquidus and its FToxid database. temperature area (<1300 °C) of the system, with the target being close to the ternary eu- tectic composition. As indicated by the close-up view of the phase diagram area, 7% CaF2 and 5% CaO were sufficient, resulting in compositions located in the target area in terms Metals 2022, 12, x FOR PEER REVIEW 14 of 20 of the main components, as confirmed with EPMA data. The commonly expressed empir- ical slag basicity based on CaO/SiO2 [15,17] varied in the experiments without doping be- tween 0.20 and 0.24, whereas in the doped experiments, it fell in the range of 0.32–0.54. Even though these values still indicate a very high acidity and therefore a high viscosity, Metals 2022, 12, 709 14 of 20 the applied relatively moderate additions of CaF2 and CaO did improve the properties of the slag by adjusting the composition to the target liquidus temperature area. Figure 9. Ternary Al2O3-CaO-SiO2 phase diagram with the target liquidus temperature area (<1300 Figure 9. Ternary Al O -CaO-SiO phase diagram with the target liquidus temperature area 2 3 2 °C) disp layed in the close-up view. Original and modified slag compositions from R2/u and R2/l (<1300 C) displayed in the close-up view. Original and modiﬁed slag compositions from R2/u concentrates computed and plotted with FactSage version 7 and its FToxid database. C = calculated and R2/l concentrates computed and plotted with FactSage version 7 and its FToxid database. from ICP-OES data and A = calculated from EPMA analysis data. C = calculated from ICP-OES data and A = calculated from EPMA analysis data. 3.4. Main Features of the Progression of Reduction at Selected Temperatures with CaO Addition The amounts of the additions of CaF and CaO were based on the desired liquidus temperature area (<1300 C) of the system, with the target being close to the ternary eutectic As the final aspect of our pyrometallurgical studies, the main features of the progres- composition. As indicated by the close-up view of the phase diagram area, 7% CaF and sion of reduction at selected temperatures (950, 1050, 1150, 1250, and 1350 °C) were i 2nves- 5% CaO were sufﬁcient, resulting in compositions located in the target area in terms of the tigated with concentrate R3 and 5% CaO addition (Table 1). Based on previous experi- main components, as conﬁrmed with EPMA data. The commonly expressed empirical slag ments, the differences between CaF2 and CaO doping are negligible, both improving the basicity based on CaO/SiO [15,17] varied in the experiments without doping between 0.20 slag properties, promoting the reduction of 2 the slag FeO component, and improving the and 0.24, whereas in the doped experiments, it fell in the range of 0.32–0.54. Even though metal segregation. However, CaO would be the likely choice to be used considering envi- these values still indicate a very high acidity and therefore a high viscosity, the applied ronmental aspects and other issues related to the CaF2 usage. A preliminary experiment relatively moderate additions of CaF and CaO did improve the properties of the slag by was conducted with a gas compositi2 on of 95% Ar + 5% H2 to 1500 °C to provide the base- adjusting the composition to the target liquidus temperature area. line against which intercepts at various temperatures could be evaluated. By replacing CO with a mixture of Ar and H2, the possibility of the reversed Boudouard reaction was elim- 3.4. Main Features of the Progression of Reduction at Selected Temperatures with CaO Addition inated. A comparison of the mass change and conversion rates with 100% CO and 95% Ar As the ﬁnal aspect of our pyrometallurgical studies, the main features of the pro- + 5% H2 are provided in the electronic supplementary data (Figure S1). Despite the differ- gression of reduction at selected temperatures (950, 1050, 1150, 1250, and 1350 C) were ences at temperatures of <900 °C, the main stage of ferrous silicate reduction reactions was investigated with concentrate R3 and 5% CaO addition (Table 1). Based on previous experi- initiated at the same temperature with both gas flows, peaking at ~1050 °C, with similar ments, the differences between CaF and CaO doping are negligible, both improving the final mass changes of 15.19 and 14.88% at 1500 °C for 100% CO and 95% Ar + 5% H2, slag properties, promoting the reduction of the slag FeO component, and improving the respectively. Therefore, the last set of experiments was conducted with a gas flow of 95% metal segregation. However, CaO would be the likely choice to be used considering envi- Ar and 5% H2. Details of mass change rates are provided in the electronic supplementary ronmental aspects and other issues related to the CaF usage. A preliminary experiment data (Figure S2). was conducted with a gas composition of 95% Ar + 5% H to 1500 C to provide the baseline As mentioned in Kallio et al. [9], mass loss curves for such a heterogeneous system against which intercepts at various temperatures could be evaluated. By replacing CO with as the Kiviniemi concentrate represent a sum of interlapping and interactive phenomena a mixture of Ar and H , the possibility of the reversed Boudouard reaction was eliminated. originating from various reactions within and from various types of crystal structures. A comparison of the mass change and conversion rates with 100% CO and 95% Ar + 5% H These include dehydration, dehydroxylation, thermal dissociation, and gas–solid reduc- are provided in the electronic supplementary data (Figure S1). Despite the differences at tion reactions at lower experimental temperatures, shifting with a rising temperature into temperatures of <900 C, the main stage of ferrous silicate reduction reactions was initiated melting, slag formation, and gas–liquid and solid–liquid reactions [37,38]. In addition to at the same temperature with both gas ﬂows, peaking at ~1050 C, with similar ﬁnal mass chemical reactions at reactant–product interfaces, diffusion and heat transfer account for changes of 15.19 and 14.88% at 1500 C for 100% CO and 95% Ar + 5% H , respectively. Therefore, the last set of experiments was conducted with a gas ﬂow of 95% Ar and 5% H . Details of mass change rates are provided in the electronic supplementary data (Figure S2). As mentioned in Kallio et al. [9], mass loss curves for such a heterogeneous system as the Kiviniemi concentrate represent a sum of interlapping and interactive phenomena originating from various reactions within and from various types of crystal structures. These include dehydration, dehydroxylation, thermal dissociation, and gas–solid reduction Metals 2022, 12, 709 15 of 20 Metals 2022, 12, x FOR PEER REVIEW 15 of 20 reactions at lower experimental temperatures, shifting with a rising temperature into melting, slag formation, and gas–liquid and solid–liquid reactions [37,38]. In addition to chemical reactions at reactant–product interfaces, diffusion and heat transfer account for the whole scenario: heat transfer from the furnace to the outer regions of the sample and the whole scenario: heat transfer from the furnace to the outer regions of the sample and into the sample, self-cooling, or self-heating of the sample during reactions, removal of into the sample, self-cooling, or self-heating of the sample during reactions, removal of evolved gaseous products, and the inﬂuence of these products on the rates of reactions evolved gaseous products, and the influence of these products on the rates of reactions all all add to the scenario [39]. Despite the complexity of the phenomena, some of the main add to the scenario [39]. Despite the complexity of the phenomena, some of the main ob- observed features at selected temperatures are presented and discussed. Figure 10 provides served features at selected temperatures are presented and discussed. Figure 10 provides an overview at different temperatures, with photographs of samples and back-scattered an overview at different temperatures, with photographs of samples and back-scattered electron images plotted on the derivative conversion curve of the baseline experiment. electron images plotted on the derivative conversion curve of the baseline experiment. Figure 10. Photographs of samples at different temperatures (950, 1050, 1150, 1250, and 1350 C), Figure 10. Photographs of samples at different temperatures (950, 1050, 1150, 1250, and 1350 °C), plotted on the derivative conversion curve of the baseline experiment with 95% Ar + 5% H . The plotted on the derivative conversion curve of the baseline experiment with 95% Ar + 5% H2. The back-scattered electron images of the samples visualize the progression of mineral decompositions, back-scattered electron images of the samples visualize the progression of mineral decompositions, slag, and metallic iron formation. slag, and metallic iron formation. FFor or the the R3 R3 cconcentrate, oncentrate, the ma the main in re rdu eduction ction ststage age wi with th a ra a rapid pid in incr crea ease se in in the the co conver nver- - sion rates was initiated at ~1030 C, with highest rates at 1050 C. Therefore, as expected, sion rates was initiated at ~1030 °C, with highest rates at 1050 °C. Therefore, as expected, the concentrate showed very limited changes at 950 C. Differences between clinopyrox- the concentrate showed very limited changes at 950 °C. Differences between clinopyrox- ene and amphibole are visualized in more detail in Figure 11. At 950 C, clinopyroxene ene and amphibole are visualized in more detail in Figure 11. At 950 °C, clinopyroxene appeared as an intact mineral with the original composition and structure, whereas de- appeared as an intact mineral with the original composition and structure, whereas dehy- hydroxylation and reaction with the reducing gas phase resulted in the destruction of droxylation and reaction with the reducing gas phase resulted in the destruction of the the original amphibole structure and formation of new solid phases, including minuscule original amphibole structure and formation of new solid phases, including minuscule me- metallic iron particles within the relict of an amphibole crystal. XRD patterns in Figure 12 tallic iron particles within the relict of an amphibole crystal. XRD patterns in Figure 12 demonstrate the complexity of concentrate crystal structures at 950 C, with identiﬁable demonstrate the complexity of concentrate crystal structures at 950 °C, with identifiable patterns for potassium feldspar, plagioclase, clinopyroxene, garnet, fayalite, ilmenite, and patterns for potassium feldspar, plagioclase, clinopyroxene, garnet, fayalite, ilmenite, and even amphibole, with relicts occasionally preserved within larger grains. even amphibole, with relicts occasionally preserved within larger grains. Metals 2022, 12, x FOR PEER REVIEW 16 of 20 Metals 2022, 12, 709 16 of 20 Figure 11. Details of mineral reactions and progression of decomposition with formation of slag and Figure 11. Details of mineral reactions and progression of decomposition with formation of slag metallic iron at 950, 1050, 1150, and 1250 °C. Mineral abbreviations: CPX = clinopyroxene, AM = and metallic iron at 950, 1050, 1150, and 1250 C. Mineral abbreviations: CPX = clinopyroxene, amphibole, ILM = ilmenite, FA = fayalite, PLG = plagioclase, Fe = metallic iron. AM = amphibole, ILM = ilmenite, FA = fayalite, PLG = plagioclase, Fe = metallic iron. Increasing the temperature to 1050 °C caused drastic changes in the sample. Particu- Increasing the temperature to 1050 C caused drastic changes in the sample. Partic- larly along the contact between the concentrate and graphite crucible, the porosity was ularly along the contact between the concentrate and graphite crucible, the porosity was increased whereas the middle and top parts of the sample exhibited much less porosity, a increased whereas the middle and top parts of the sample exhibited much less porosity, a lower amount of initial slag, and more preserved mineral grains. The height of the sample lower amount of initial slag, and more preserved mineral grains. The height of the sample was increased due to gas-forming reduction reactions, creating porosity, which was was increased due to gas-forming reduction reactions, creating porosity, which was formed formed around graphite particles, producing metal rims around the pores (Figure 10). The around graphite particles, producing metal rims around the pores (Figure 10). The forma- formation of initial slag was dominated by the decomposition of amphibole and garnet, tion of initial slag was dominated by the decomposition of amphibole and garnet, sintering sintering the sample into a solid block. The decomposition products of various types of the sample into a solid block. The decomposition products of various types of amphiboles amphiboles have been found to include different phases, such as pyroxene, spinel, olivine, have been found to include different phases, such as pyroxene, spinel, olivine, feldspars, feldspars, and silica in addition to melt [40,41]. Furthermore, the decomposition of garnet and silica in addition to melt [40,41]. Furthermore, the decomposition of garnet under under reducing conditions (>1000 °C) has been found to produce metallic iron, cristobal- reducing conditions (>1000 C) has been found to produce metallic iron, cristobalite, and ite, and hercynite, with fayalite as a secondary product [42]. In our samples, the common hercynite, with fayalite as a secondary product [42]. In our samples, the common decom- decomposition products observed were very fine-grained mixtures of dark, lath-shaped position products observed were very ﬁne-grained mixtures of dark, lath-shaped crystals crystals with a composition resembling plagioclase and FeO-rich phase, with a tendency with a composition resembling plagioclase and FeO-rich phase, with a tendency to form to form dendrites and/or formation of fayalite as an intermediate decomposition product dendrites and/or formation of fayalite as an intermediate decomposition product within within the slag phase (Figure 11). The interplay of decomposition products and mineral the slag phase (Figure 11). The interplay of decomposition products and mineral reactions reactions with evolving slag phase do provide challenges to the interpretation of individ- with evolving slag phase do provide challenges to the interpretation of individual mineral reactions, the details of which could be a subject for further studies. With respect to more Metals 2022, 12, x FOR PEER REVIEW 17 of 20 Metals 2022, 12, 709 17 of 20 ual mineral reactions, the details of which could be a subject for further studies. With re- spect to more persistent pri persistent primary minerals m ata this ry mi temperatur nerals at e, this temperature such as clinopyr , suc oxene, h as clinopyr ilmenite, potassium oxene, il- menit feldspar e, pot , and assplagioclase, ium feldsparit , and p can be lag stated ioclase that , it can reaction be statrims, ed that dissolution reaction rim str suctur , dissol es, utand ion structures, and zoning are common features, as illustrated in Figure 11. In addition to zoning are common features, as illustrated in Figure 11. In addition to metallic iron and met graphite, allic iron and clinopyr g oxene raphitand e, clinopyrox plagioclase ene ar and e identiﬁable plagioclase in are XRD ident patterns ifiable (Figur in XR eD 12 p)aa tttethis rns (Figure temperatur 12) at e. this temperature. Details Details of XRD interpretations of XRD inter are provided pretations ar in the electr e provided in onic supplementary the elec- data (Figure S3). tronic supplementary data (Figure S3). Figure 12. XRD patterns of concentrate R3 reduced at temperatures of 950, 1050, and 1150 °C. Min- Figure 12. XRD patterns of concentrate R3 reduced at temperatures of 950, 1050, and 1150 C. Mineral eral abbreviations: AM = amphibole, CPX = clinopyroxene, FA = fayalite, GRT = garnet, PL = plagi- abbreviations: AM = amphibole, CPX = clinopyroxene, FA = fayalite, GRT = garnet, PL = plagioclase, oclase, FSP = potassium feldspar, C = graphite, Fe = metallic iron. FSP = potassium feldspar, C = graphite, Fe = metallic iron. At 1150 °C, along the progression of reduction reactions, the amount of metallic iron At 1150 C, along the progression of reduction reactions, the amount of metallic iron and slag increased, with the porosity extending throughout the whole sample (Figure 10). and slag increased, with the porosity extending throughout the whole sample (Figure 10). Amorphous slag and metallic iron were the dominant phases in XRD patterns, with minor Amorphous slag and metallic iron were the dominant phases in XRD patterns, with minor peaks still identifiable for clinopyroxene and plagioclase (Figure 12). By 1250 °C, only oc- peaks still identiﬁable for clinopyroxene and plagioclase (Figure 12). By 1250 C, only casional plagioclase relicts remained in the slag, as presented in Figure 11, still with an occasional plagioclase relicts remained in the slag, as presented in Figure 11, still with an extensive porosity, which is diminished by 1350 °C (Figure 10). extensive porosity, which is diminished by 1350 C (Figure 10). The evolution of the chemical composition of the slag phase is summarized in Figure The evolution of the chemical composition of the slag phase is summarized in Figure 13, 1 displaying 3, displayibinary ng bina plots ry plof ots the of the m main components ain components and and Sc O Scvs. 2O3 vs SiO . S.iO The 2. The analytical analytic results al re- 2 3 2 sults at 950 °C are for amphibole relicts, whereas at other temperatures, the compositions at 950 C are for amphibole relicts, whereas at other temperatures, the compositions represent those of the slag represent those of the slagphase. The phase. The slag ph slag phase ase exh exhibits ibits a ste a steady ady decrease in decrease in FeO due to FeO due the progression of reduction with to the progression of reduction with incrinc easi reasing ng tempe temperatur rature. Bas e. eBased d on the on se r these esults results, , the sl the ag slag compositions at 1050 C with higher CaO content indicate the dissolution of added compositions at 1050 °C with higher CaO content indicate the dissolution of added CaO CaO into the initial slag. As the mineral reactions proceeded with increasing temperature, into the initial slag. As the mineral reactions proceeded with increasing temperature, lead- ing leading event eventually ually to the d toethe composit decomposition ion and meand ltingmelting of all clinopyroxe of all clinopyr ne, pot oxene, assium potassium feldspar, feldspar, and plagioclase into the slag by 1250 C, the composition of the slag became and plagioclase into the slag by 1250 °C, the composition of the slag became homogenized homogenized with respect to the main components, with a steady decrease in FeO. with respect to the main components, with a steady decrease in FeO. Metals 2022, 12, 709 18 of 20 Metals 2022, 12, x FOR PEER REVIEW 18 of 20 Figure 13. Binary plots of FeO, CaO, Al2O3, and Sc2O3 (wt%) for amphibole relicts (blue) at 950 °C Figure 13. Binary plots of FeO, CaO, Al O , and Sc O (wt%) for amphibole relicts (blue) at 950 C 2 3 2 3 and slag at higher temperatures. and slag at higher temperatures. 4. Conclusions 4. Conclusions If pyrometallurgical treatment is to be considered for Kiviniemi-type ferrous scandium If pyrometallurgical treatment is to be considered for Kiviniemi-type ferrous scan- concentrates, one of the challenges is to optimize and modify the composition of highly dium concentrates, one of the challenges is to optimize and modify the composition of viscous slag to promote the reduction of FeO and segregation of metal without excessively highly viscous slag to promote the reduction of FeO and segregation of metal without diluting the slag Sc O content. According to the results of this study, the reduction of the 2 3 excessively diluting the slag Sc2O3 content. According to the results of this study, the re- ferrous oxide component of the slag and segregation of metallic iron was improved with duction of the ferrous oxide component of the slag and segregation of metallic iron was moderate additions of CaF and CaO, which lowered the liquidus temperature and viscosity improved with moderate additions of CaF2 and CaO, which lowered the liquidus temper- of the slag. The Sc O component was maintained and enriched in the slag. Although CaF 2 3 2 ature and viscosity of the slag. The Sc2O3 component was maintained and enriched in the increased the derivative conversion rates at a slightly lower temperature region, the use of slag. Although CaF2 increased the derivative conversion rates at a slightly lower temper- CaO instead of CaF would be preferable in industrial applications. Despite the variations ature region, the use of CaO instead of CaF2 would be preferable in industrial applications. in the modal mineralogy of the concentrate feed used in this study, the high-temperature Despite the variations in the modal mineralogy of the concentrate feed used in this study, behavior of the concentrates is essentially similar, though the main reduction stage is the high-temperature behavior of the concentrates is essentially similar, though the main initiated at a slightly higher temperature (~1000–1030 C) for the concentrates with less reduction stage is initiated at a slightly higher temperature (~1000–1030 °C) for the con- amphibole and a higher amount of nonferrous gangue minerals. The beginning of the main centrates with less amphibole and a higher amount of nonferrous gangue minerals. The reduction stage with the formation of initial slag is dominated by the decomposition and beginning of the main reduction stage with the formation of initial slag is dominated by reduction of amphibole and garnet. The ﬁnal decomposition of clinopyroxene, the other the decomposition and reduction of amphibole and garnet. The final decomposition of main host for Sc O , occurs at a signiﬁcantly higher temperature than that of amphibole, 2 3 clinopyroxene, the other main host for Sc2O3, occurs at a significantly higher temperature with structures persisting until 1150 C. This study complements the pyrometallurgical part than that of amphibole, with structures persisting until 1150 °C. This study complements of our ongoing project, conﬁrming the smelting reduction characteristics of the Kiviniemi- the pyrometallurgical part of our ongoing project, confirming the smelting reduction char- type ferrous scandium concentrates. Only after the complete decomposition and melting of acteristics of the Kiviniemi-type ferrous scandium concentrates. Only after the complete silicates and dissolution of unreduced FeO into the slag can the ﬁnal FeO reduction from decomposition and melting of silicates and dissolution of unreduced FeO into the slag can slag be achieved by carbon, accompanied by segregation and accumulation of metallic iron. the final FeO reduction from slag be achieved by carbon, accompanied by segregation and accumulation of metallic iron. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/met12050709/s1, Table S1: Modal compositions and deportment Supplementary Materials: The following supporting information can be downloaded at: of the main components in Kiviniemi concentrates. Table S2: Summary of the metal EPMA analytical www.mdpi.com/xxx/s1, Table S1: Modal compositions and deportment of the main components in data. Table S3: Summary of the ﬁrst set of slag EPMA analytical data for experiments 6–14. Figure S1: Kiviniemi concentrates. Table S2: Summary of the metal EPMA analytical data. Table S3: Summary Comparison of the mass change and derivative conversion rates with 100% CO and 95% Ar + 5% H2. of the first set of slag EPMA analytical data for experiments 6–14. Figure S1: Comparison of the mass Figure S2: Details of mass change rates with 95% Ar + 5% H at various end temperatures. Figure S3: change and derivative conversion rates with 100% CO and 95% Ar + 5% H2. Figure S2: Details of XRD interpretations at selected temperatures. mass change rates with 95% Ar + 5% H2 at various end temperatures. Figure S3: XRD interpretations at selected temperatures. Metals 2022, 12, 709 19 of 20 Author Contributions: Conceptualization, R.K. and P.T.; methodology, R.K., P.T. and E.-P.H.; inves- tigation, R.K. and T.K.; resources, P.T. and T.F.; writing—original draft preparation, R.K.; writing— review and editing, E.-P.H., P.T., S.L. and T.F.; visualization, R.K. and E.-P.H.; supervision, S.L. and T.F.; project administration, T.F.; funding acquisition, R.K. and P.T. All authors have read and agreed to the published version of the manuscript. Funding: This research has been funded by The Foundation for Research of Natural Resources in Finland, grant number 20210019. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: This research has been funded by The Foundation for Research of Natural Resources in Finland. Support and advice from the doctoral training follow-up group members, Jussi Liipo and Tapio Halkoaho, are highly appreciated. The samples from Kiviniemi were provided by the GTK and Metso Outotec enabled the experiments with LIMS and SLon, which are all gratefully acknowledged. The authors are also thankful to Eero Hanski for comments on the manuscript and to CMA personnel for their help with analytical work. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Hokka, J.; Halkoaho, T. 3D Modelling and Mineral Resource Estimation of the Kiviniemi Scandium Deposit, Eastern Finland; Mineral Resource Estimation Report; Geological Survey of Finland: Espoo, Finland, 2017; 21p. [CrossRef] 2. Halkoaho, T.; Ahven, M.; Rämö, O.T.; Hokka, J.; Huhma, H. Petrography, geochemistry and geochronology of the Sc-enriched Kiviniemi ferrodiorite intrusion, eastern Finland. Miner. Depos. 2020, 55, 1561–1580. [CrossRef] 3. Korhonen, T.; Neitola, R.; Mörsky, P.; Laukkanen, J. 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Abstract

metals Article Slag Modiﬁcation in Reduction of Kiviniemi Ferrous Scandium Concentrates 1 , 2 2 2 1 Rita Kallio *, Pekka Tanskanen , Eetu-Pekka Heikkinen , Tommi Kokkonen , Saija Luukkanen and Timo Fabritius Oulu Mining School, Faculty of Technology, University of Oulu, FI-90014 Oulu, Finland; saija.luukkanen@oulu.ﬁ Process Metallurgy Research Group, Faculty of Technology, University of Oulu, FI-90014 Oulu, Finland; pekka.a.tanskanen@oulu.ﬁ (P.T.); eetu.heikkinen@oulu.ﬁ (E.-P.H.); tommi.kokkonen@oulu.ﬁ (T.K.); timo.fabritius@oulu.ﬁ (T.F.) * Correspondence: rita.kallio@oulu.ﬁ; Tel.: +358-50-526-6806 Abstract: Several research projects are currently focused on the search for new sources of scandium due to its expected increasing demand in advanced technology applications. The Kiviniemi Fe-Sc- enriched maﬁc intrusion is a potential primary source for Sc. According to the recent investigations on the FeO component reduction in the Kiviniemi magnetic Sc concentrate at various end temperatures, complete FeO reduction is achieved at the highest experimental temperature (1500 C). However, efﬁcient separation of metal from the Sc O -enriched slag is hindered by the high viscosity of the 2 3 slag. In this study, investigations of the Kiviniemi-type concentrate reduction characteristics are complemented from three perspectives: (1) slag modiﬁcation with CaF and/or CaO to promote the reduction of the FeO component and metal separation, (2) reduction characteristics of the concentrates with a slightly different modal mineralogy and chemical composition, and (3) description of the main features of the progression of reduction at selected temperatures (950, 1050, 1150, 1250, and 1350 C) Citation: Kallio, R.; Tanskanen, P.; with CaO addition. Both CaF and CaO increase conversion rates at a lower temperature region Heikkinen, E.-P.; Kokkonen, T.; and promote the separation of metal from the slag. High-temperature behavior of the concentrates Luukkanen, S.; Fabritius, T. Slag Modiﬁcation in Reduction of used in this study is essentially similar, although the main reduction stage is initiated at a slightly Kiviniemi Ferrous Scandium higher temperature for concentrates with less amphibole and a higher amount of nonferrous gangue Concentrates. Metals 2022, 12, 709. minerals. Only after the complete decomposition and melting of clinopyroxene and nonferrous https://doi.org/10.3390/ minerals of the concentrate, the ﬁnal reduction of the FeO component from the slag can take place. met12050709 Keywords: ferrous scandium concentrate; reduction; slag; modiﬁcation Academic Editor: Alexander McLean Received: 15 March 2022 Accepted: 18 April 2022 1. Introduction Published: 21 April 2022 To meet the expected increasing demand of scandium, various primary and secondary Publisher’s Note: MDPI stays neutral scandium-rich resources are currently being investigated as potential new sources of this with regard to jurisdictional claims in rare-earth element. The Kiviniemi maﬁc intrusion in Finland represents one of these poten- published maps and institutional afﬁl- tial new sources with a preliminary mineral resource estimation of 13.4 Mt and an average iations. Sc grade of 163 g/t [1,2]. The main scandium-bearing minerals are ferrous amphibole and clinopyroxene [2–4]. Due to the paramagnetic nature of these minerals, a concentration stage involving low-intensity and high-gradient magnetic separation has been suggested for removing alkali-containing diamagnetic minerals with only negligible losses of Sc O to 2 3 Copyright: © 2022 by the authors. the tailings [4]. With magnetic concentration, the total ferrous oxide load in the concentrate Licensee MDPI, Basel, Switzerland. is inevitably high. Furthermore, due to the incorporation of scandium into the lattice of fer- This article is an open access article rous silicates, conventional beneﬁciation should be complemented with pyrometallurgical distributed under the terms and and/or hydrometallurgical methods. However, non-selective mobilization of Fe and Ti in conditions of the Creative Commons hydrometallurgical processing imposes challenges to the puriﬁcation and precipitation of Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ suitable Sc products [5–8]. Moreover, as reported in the preliminary beneﬁciation study 4.0/). of Kiviniemi ferrodiorite [3], large quantities of acids are required, and large volumes of Metals 2022, 12, 709. https://doi.org/10.3390/met12050709 https://www.mdpi.com/journal/metals Metals 2022, 12, 709 2 of 20 byproducts are produced. Therefore, pyrometallurgical processing is suggested as a poten- tial technique to decrease the amount of the ferrous oxide component in the concentrate prior to hydrometallurgical processing. In the ﬁrst part of our high-temperature studies, the results of reduction experiments of a ferrodiorite concentrate and characteristics of the products up to temperatures of 1500 C were described [9]. The observed increase in the derivative conversion rates during the experiments initiated gradually from ~950 C onwards with a sharp increase in the conversion rates between 1050 and 1170 C and high rates until ~1250 C. This was interpreted to signify the main melting and reduction stage, which continued as the formed FeO-containing slag reacted with graphite. Plagioclase and potassium feldspar are the main gangue minerals in the Kiviniemi concentrates [4]. In the plagioclase series, the liquidus temperature rises from the pure sodium endmember (albite 1118 C) to the pure calcium endmember (anorthite 1550 C) [10]; the plagioclase in the Kiviniemi samples is intermediate between the two endmembers, with its anorthite content being approximately 40 mol-% [2]. It is assumed that prior to complete melting of the partly reduced ferrous silicates and nonferrous gangue minerals, such as plagioclase, reduction reactions are hindered due to the formed product layers and remaining solid particles. Only after complete melting of the silicates and dissolution of unreduced FeO into the slag, the ﬁnal FeO reduction can be achieved by carbon, accompanied by segregation and accumulation of metallic iron. Liquid-phase mass transfer of iron oxide has been regarded as the major factor in the rate of reduction of iron oxide by carbonaceous material in slags [11]. Viscosity as a measure of the internal friction of a ﬂuid phase is one of the most important properties of metallurgical melts, directly affecting the kinetic conditions of the processes [12–14]. It is related to the internal structure of oxide melt and is very sensitive to changes in temperature, slag composition, and oxygen partial pressure [14,15]. Low viscosi- ties improve the transfer of mass and heat, the solubility of slag formers and modiﬁers, and the separation of metal and slag. Network former SiO has strong, highly covalent metal- oxygen bonds, leading to high liquid viscosities, whereas network transforming alkali and alkali earth oxides, such as Na O, K O, MgO, CaO, and other divalent oxides, such as MnO 2 2 and FeO, break this network, thus decreasing the viscosity [12,13,16,17]. Amphoteric oxide Al O may act either as a network former or transformer, depending on the composition of 2 3 the slag system [17,18]. In the ﬁrst part of our pyrometallurgical study, the viscosity of the slag was increased as the reduction reactions proceeded, decreasing the FeO content of the slag. This hindered the diffusion of the remaining FeO, which was particularly observed at lower ﬁnal temperatures, also causing signiﬁcant entrapment of small metal inclusions in the slag. Common techniques to lower the viscosity of the slag in metallurgical operations include the addition of CaO or CaF [13,19,20]. It is generally recognized that F in the 4+ silicate melts is not coordinated with Si , which means that CaF is merely dissociated in the molten silicate [21]. As the slags are ionic in nature, containing both covalent and ionic bonds, and as the extent of polymerization varies with the metal oxide and ﬂux contents in the slag, the viscosity of slags is extremely sensitive to the quality and quantity of ions and electrostatic interactions, as well as temperature [14,17]. In addition to viscosity, the slag composition with a suitable liquidus temperature—as based on the information from appropriate phase diagrams—is essential in industrial practices [17]. The experimental program in this second pyrometallurgical part of our study was designed to complement the characterization of the reduction behavior of Kiviniemi ferrous scandium concentrates from three perspectives: (1) modifying the slag composition with CaF and/or CaO to lower the viscosity of the slag and promote the reduction of the FeO component with enhanced separation of metallic iron, (2) comparing the reduction of a variety of concentrates from various parts of the Kiviniemi maﬁc intrusion with selected CaF and/or CaO doping, and (3) describing the main features of the progression of reduction with CaO addition at selected temperature intervals (950, 1050, 1150, 1250, and 1350 C). The main aim is to produce a modiﬁed, improved slag composition to be used Metals 2022, 12, 709 3 of 20 in hydrometallurgical processing, document the progression of reduction, and provide fundamental information for the beneﬁciation scenarios of ferrous scandium concentrates. 2. Materials and Methods Samples from three drill cores (R1, R2, and R3) from the Kiviniemi maﬁc intrusion were included in this study. Three adjacent or proximate drill core samples from four drill core intervals (altogether 12 individual samples) were combined into four composite samples. The composite samples are named R1, R2/u, R2/l, and R3 based on the drill cores they originate from; R2/u represents samples from the upper part and R2/l from the lower part of drill core R2 of the Kiviniemi intrusion [1,4]. The comminution procedure as well as magnetic separation with LIMS and SLon 100 pulsating high-gradient magnetic separation (HGMS) at Metso Outotec laboratory in Pori, Finland are described in more detail in [4]. P values after comminution are 68, 85, 92, and 78 m for R1, R2/u, R2/l, and R3 concentrates, respectively. The concentrates produced with SLon parameters (150 rpm and 1.0 T) with the highest Sc recovery and grade for each composite sample were selected for this study. Concentrate bulk compositions were measured by Euroﬁns Labtium in Kuopio, Fin- land, using an accredited inductively coupled plasma optical emission spectrometry (ICP- OES) method (721P). Dry samples were pulverized to 100%—90 m with a tungsten carbide mill at Oulu Mining School prior to sending for analysis. For ICP-OES analysis, a prepared pulp sample (0.2 g) is fused with anhydrous sodium peroxide in zirconium crucible by heating in electric furnace at 700 C for one hour. The melt is dissolved in hydrochloric acid. The ﬁnal solution is diluted with water prior to instrumental analysis. The routine method entails the analysis of 27 elements by ICP-OES Thermo Electron ICAP 6500 Duo. Detection limits with quality control details are provided in Kallio et al., 2021 [4]. Polished vertical blocks (Ø 25 and 40 mm) of the concentrate grain mounts and samples after each high- temperature experiment were prepared for ﬁeld emission scanning electron microscope (FESEM) and electron probe microanalyzer (EPMA) analysis, which were conducted at the Center of Material Analysis (CMA), University of Oulu. Polished blocks were coated with carbon prior to analyses. Data on modal mineralogy were acquired with INCAMineral software (version 5.05; Oxford Instruments, Oxford, Halifax, UK) and a Zeiss ULTRA Plus FESEM instrument (Oberkochen, Germany). The applied instrumental parameters were an acceleration voltage of 15 kV, beam current of 2.3 nA, and working distance of 8.3 mm. Postprocessing was conducted with GrainAlyzer software (Oxford Instruments, Oxford, Halifax, UK). A JEOL JXA-8530FPlus electron probe microanalyzer (JEOL Ltd., Tokyo, Japan) was employed to characterize mineral chemical compositions in concentrates as well as characteristics of the produced slag and metal, with the analytical conditions including an accelerating voltage of 15 kV, a beam current of 15 nA, and a beam diameter of 1–10 m. The peak and background counting times were set at 10 s and 5 s, respectively, for all components. For Sc O , values of 30 s and 15 s were also tested. The matrix correction with 2 3 the ZAF method (atomic number—absorption—ﬂuorescence) was applied. The standards used with EPMA are reported in [9]. X-ray diffraction (XRD) was utilized to monitor the presence of crystalline phases at selected temperatures. A Rigaku SmartLab 9 kW XRD apparatus (Rikagu Ltd., Tokyo, Japan) with Co anode was used with 40 kV and 135 mA settings. Speed of acquisition was 4 /min with 0.02 /step and 2 range of 10–130 . Data processing was performed with PDXL2 software and PDF-4 2022 database (Rikagu Ltd., Tokyo, Japan). High-temperature experiments were conducted with a thermogravimetric (TG) fur- nace at the Laboratory of Process Metallurgy, University of Oulu. The experimental set-up is described in more detail in the ﬁrst part of our pyrometallurgical study [9]. Ten grams of loose powders of the concentrate mix were prepared with calculated proportions of LIMS and SLon concentrates for each of these experiments. Based on the total ferrous oxide con- tent of the concentrates, graphite powder (Thermo-Fisher Scientiﬁc, Karlsruhe, Germany, Alfa Aesar 40797 lot: 61100109) was mixed with the concentrate in correct proportions to Metals 2022, 12, 709 4 of 20 ensure the complete ferrous component reduction. Other chemicals used for modifying the slag composition were Alfa Aesar 33299 CaO (lot: P12F022), burned at 850 C and stored in a desiccator, and Alfa Aesar 11055 CaF (lot: Z27D012). Powders were pressed to the bottom of the graphite crucibles. In addition to the similar experimental procedure used in the ﬁrst part of our pyrometallurgical study [9], quickly cooled samples at selected temperatures were included to investigate the progression of mineral reduction reactions at various stages. For these experiments, the gas composition was changed to 95% Ar and 5% H . After reaching the desired temperature in the TG furnace, the sample was raised to the upper, cooler part of the furnace, in which it was cooled with N gas ﬂow. The sample was taken out of the furnace after a few minutes of cooling. In total, the designed experimental program consists of 20 experiments, as presented in Table 1. Table 1. High-temperature experimental TG program for Kiviniemi magnetic concentrates. Focus of Experiment No. Sample Target T ( C) Isotherm, min Additives 1 R2/l 1450 120 5%C + 7%CaF Effect of CaF addition 2 R2/l 1450 120 5%C + 14%CaF 3 R2/l 1450 120 5%C + 5%CaO Effect of CaO addition 4 R2/l 1450 120 5%C + 10%CaO Effect of CaO + CaF addition 5 R2/l 1450 120 5%C + 7%CaF + 5%CaO 2 2 6 R1 1500 120 5%C + 7%CaF Effect of concentrate quality, 7 R2/u 1500 120 5%C + 7%CaF temperature and CaF addition 8 R3 1500 120 5%C + 7%CaF 9 R1 1500 120 5%C + 5%CaO Effect of concentrate quality, 10 R2/u 1500 120 5%C + 5%CaO temperature and CaO addition 11 R3 1500 120 5%C + 5%CaO 12 R1 1500 120 5%C + 7%CaF + 5%CaO Effect of concentrate quality, 13 R2/u 1500 120 5%C + 7%CaF + 5%CaO temperature and CaF + CaO addition 14 R3 1500 120 5%C + 7%CaF + 5%CaO Baseline with Ar + H (5%) 15 R3 1500 10 ~5%C + 5%CaO 16 R3 950 - ~5%C + 5%CaO 17 R3 1050 - ~5%C + 5%CaO Main features on the progression of 18 R3 1150 - ~5%C + 5%CaO reduction with CaO 19 R3 1250 - ~5%C + 5%CaO 20 R3 1350 - ~5%C + 5%CaO 3. Results and Discussion 3.1. Characteristics of the Concentrates Information on the concentrates produced with a combination of LIMS and SLon (150 rpm pulsation and 1.0 T applied magnetic induction) are presented in Table 2. Separate magnetic concentrates from LIMS and SLon were combined with correct mass proportions to produce a combined feed for high-temperature experiments to ensure the maximum possible recovery of both Sc O and FeO. 2 3 Modal compositions of the combined concentrates used in our pyrometallurgical experiments are shown in Figure 1A. Their chemical compositions, as calculated based on ICP-OES results, are reported in Table 3. Feed chemical compositions based on the modal mineralogy and EPMA analyses are also provided as a reference. This comparison allows evaluation of the reliability of the process’ mineralogical data, which are generally in agree- ment with the ICP-OES data. Chemical characteristics of the Kiviniemi intrusion include high Sc, Fe, Ti, and P content [2–4]. Despite deﬂecting apatite into the tailings in mag- netic separation, the P O grade in the concentrate remained at ~0.2–0.3 wt% (feed grade 2 5 0.6–0.74 wt%). FeO grades increased from the feed 19–23 wt% to concentrate 31–36 wt% while silica reduced from 42–47 wt% to 38–41 wt%, depending on the composite sample in question. Metals 2022, 12, x FOR PEER REVIEW 5 of 20 SLon MAGS mass % 55 62 65 52 SLon MAGS Sc ppm 230 310 250 310 SLon MAGS Sc recovery % 88 91 92 87 SLon Tailings mass % 45 39 35 49 SLon Tailings Sc ppm 40 50 40 50 TOTAL MAGS mass % 60 65 68 55 TOTAL MAGS Sc ppm 211 290 236 289 TOTAL MAGS Sc recovery % 89 91 93 88 TOTAL NMAGS mass % 40 35 32 45 TOTAL NMAGS Sc % 11 9 7 12 MAGS = magnetic concentrates, NMAGS = non-magnetic tailings. Modal compositions of the combined concentrates used in our pyrometallurgical ex- periments are shown in Figure 1A. Their chemical compositions, as calculated based on Metals 2022, 12, 709 5 of 20 ICP-OES results, are reported in Table 3. Feed chemical compositions based on the modal mineralogy and EPMA analyses are also provided as a reference. This comparison allows evaluation of the reliability of the process’ mineralogical data, which are generally in a Tg able reem 2.ent with the ICP- Details of the selected OES da concentrates ta. Chemi processed cal chawith racteri LIMS stics of and the SLon Ki using viniemi composite intrusi samples on in- clude h R1, R2/u, igh R2/l Sc, Fe, Ti (R2 upper , and andP cont lower ent sample), [2–4] and . Despit R3. e deflecting apatite into the tailings in magnetic separation, the P2O5 grade in the concentrate remained at ~0.2–0.3 wt% (feed Drill Core Sample R1 R2/u R2/l R3 grade 0.6–0.74 wt%). FeO grades increased from the feed 19–23 wt% to concentrate 31–36 Feed ppm Sc 150 210 170 180 wt% while silica reduced from 42–47 wt% to 38–41 wt%, depending on the composite sample in question. LIMS mass % 9 9 10 8 LIMS Sc ppm 110 163 153 167 Table 3. Chemical LIMS composi Sc recovery tions of % concentrates used in 7 high-temperature 7 experiments. Composi- 9 7 tions calculated based on modal mineralogy are also provided for comparison. Sc2O3 presented in SLon MAGS mass % 55 62 65 52 ppm, other oxides in wt%. SLon MAGS Sc ppm 230 310 250 310 SLon MAGS Sc recovery % 88 91 92 87 Sample SiO2 TiO2 Al2O3 FeO MnO MgO CaO K2O P2O5 S Sc2O3 (ppm) SLon Tailings mass % 45 39 35 49 R1 ICP-OES 39.8 3.08 8.33 32.7 0.61 1.81 7.99 1.52 0.32 0.28 324 SLon Tailings Sc ppm 40 50 40 50 R1 MODAL 39.3 2.68 7.84 33.1 0.58 1.62 8.57 1.43 0.48 0.32 289 R2/u ICP-OES 39.0 2.76 7.96 35.7 0.70 1.16 9.03 1.08 0.27 0.36 444 TOTAL MAGS mass % 60 65 68 55 TOTAL MAGS Sc ppm 211 290 236 289 R2/u MODAL 39.2 2.72 7.80 33.1 0.66 1.16 9.92 0.96 0.25 0.34 441 TOTAL MAGS Sc recovery % 89 91 93 88 R2/l ICP-OES 37.6 2.48 9.73 31.4 0.66 1.13 8.99 1.11 0.31 0.30 363 R2/l MODAL 40.3 2.52 9.27 31.5 0.76 1.11 9.70 0.59 0.22 0.19 380 TOTAL NMAGS mass % 40 35 32 45 R3 ICP-OES 40.7 2.45 10.1 T OTAL31.2 NMAGS 0.59 Sc % 0.97 8.71 11 1.63 9 0.22 0.28 7 444 12 R3 MODAL 39.7 2.07 MAGS 9.88 = magnetic concentrates, 29.9 0.59 NMAGS 0.95 = non-magnetic 8.88 tailings. 1.54 0.21 0.20 380 Figure 1. (A) Modal compositions of the concentrates used in pyrometallurgical experiments. Mineral abbreviations: AM = amphibole, CPX = clinopyroxene, FeOX = iron oxides (magnetite), FA = fayalite, ILM = ilmenite, SULF = sulﬁdes, CFS = clinoferrosilite, GRT = garnet, BT + CHL = biotite + chlorite, PL = plagioclase, FSP = potassium feldspar, QTZ + CAL = quartz + calcite, AP = apatite, ZRN = zircon. (B,C) Deportment of SiO , Al O , FeO, CaO, and Sc O in amphibole, clinopyroxene, fayalite, and 2 2 3 2 3 garnet for concentrates R2/l and R3. Although the feed samples represent the main lithology, with Sc-bearing amphibole and pyroxene having close to an average grade, there were differences in the amount of the main minerals between the feed composite samples and therefore also between produced concentrates. As indicated by Figure 1A, this was true particularly for the amounts of amphibole and clinopyroxene but also garnet and fayalite. Paramagnetic minerals, i.e., those with a ferrous component, accounted for 82–88 mass % of all the concentrates. Examples of the deportment of the main components—SiO , Al O , FeO, and CaO—as well 2 2 3 as Sc O within the four minerals (amphibole, clinopyroxene, fayalite, and garnet) in the 2 3 R2/l and R3 concentrates are presented in Figure 1B,C. Further details on the deportment of the main components are provided in the electronic supplementary data (Table S1). Metals 2022, 12, 709 6 of 20 Mineralogically, Sc O was mainly incorporated into the lattices of clinopyroxene and 2 3 amphibole, though with varying amounts (Figure 1B,C). Garnet and amphibole accounted for most of the Al O in the concentrates, whereas clinopyroxene introduced most of the 2 3 CaO into the system, particularly in concentrate R2/u. Fayalite was a signiﬁcant FeO- containing phase in concentrates R1 and R2/u. Compared with other concentrates, the R2/l concentrate had a signiﬁcantly higher amount of amphibole. Table 3. Chemical compositions of concentrates used in high-temperature experiments. Compositions calculated based on modal mineralogy are also provided for comparison. Sc O presented in ppm, 2 3 other oxides in wt%. Sample SiO TiO Al O FeO MnO MgO CaO K O P O S Sc O (ppm) 2 2 2 3 2 2 5 2 3 R1 ICP-OES 39.8 3.08 8.33 32.7 0.61 1.81 7.99 1.52 0.32 0.28 324 R1 MODAL 39.3 2.68 7.84 33.1 0.58 1.62 8.57 1.43 0.48 0.32 289 R2/u ICP-OES 39.0 2.76 7.96 35.7 0.70 1.16 9.03 1.08 0.27 0.36 444 R2/u MODAL 39.2 2.72 7.80 33.1 0.66 1.16 9.92 0.96 0.25 0.34 441 R2/l ICP-OES 37.6 2.48 9.73 31.4 0.66 1.13 8.99 1.11 0.31 0.30 363 R2/l MODAL 40.3 2.52 9.27 31.5 0.76 1.11 9.70 0.59 0.22 0.19 380 R3 ICP-OES 40.7 2.45 10.1 31.2 0.59 0.97 8.71 1.63 0.22 0.28 444 R3 MODAL 39.7 2.07 9.88 29.9 0.59 0.95 8.88 1.54 0.21 0.20 380 3.2. Effects of Additions on Reduction of Concentrate R2/l R2/l concentrate was used in the ﬁrst part of the pyrometallurgical study [9]; R2/l concentrate with a slightly higher Sc O grade and recovery was chosen as the ﬁrst type 2 3 of concentrate to test with selected additions. The details and results of experiments 1–5 conducted up to 1450 C are presented in Table 4. The sum of the calculated mass of oxygen for FeO and P O , the calculated sum of hydroxyl in amphibole, biotite, and chlorite, and 2 5 the mass of graphite are presented as a reference (S O + C + H O g). Hydroxyl removal from amphibole, biotite, and chlorite lattices was included assuming 1.86, 2.63, and 8.14 wt% of H O, respectively. Table 4. Results of experiments 1–5 using the R2/l concentrate with various additives. Experiment Number Sample 1 2 3 4 5 S LIMS g 1.17 1.17 1.17 1.17 1.17 Slon g 8.83 8.83 8.83 8.83 8.83 C g 0.50 0.50 0.50 0.50 0.50 CaF g 0.70 1.40 - - 0.70 CaO g - - 0.50 1.00 0.50 S g 11.20 11.90 11.00 11.50 11.70 m g 11.19 11.36 11.01 11.50 11.68 m g 9.05 8.91 9.35 9.82 9.61 Dm g 2.14 2.45 1.66 1.68 2.07 Dm % 19.12 21.57 15.08 14.61 17.72 S O + C + H O g 1.33 1.33 1.33 1.33 1.33 The calculation of the derivative conversion rates is explained in more detail in [9]. These rates exhibited acceleration at ~730–860 C for samples with CaO doping, which did not appear as distinctly in the experiments with CaF additions only (Figure 2). As presented in Figure 2, the main reaction stage was initiated with CaF - and/or CaO-doped R2/l samples at the same temperatures (~950 C) as with non-doped samples, but with doping and less nonferrous gangue minerals, the reaction rates were faster with higher rates of conversion at lower temperatures. The CaO-doped samples appeared to have Metals 2022, 12, x FOR PEER REVIEW 7 of 20 Metals 2022, 12, 709 7 of 20 doping and less nonferrous gangue minerals, the reaction rates were faster with higher rates of conversion at lower temperatures. The CaO-doped samples appeared to have their main peak in the derivative conversion rates at a slightly higher temperature (a rapid their main peak in the derivative conversion rates at a slightly higher temperature (a rapid increase between 970–1020 °C) than with CaF2 (950–990 °C). Furthermore, the derivative increase between 970–1020 C) than with CaF (950–990 C). Furthermore, the derivative conversion rates exhibited another acceleration at ~1050–1120 °C for both types of doping conversion rates exhibited another acceleration at ~1050–1120 C for both types of doping at the main reduction stage, after which the rates sharply declined. at the main reduction stage, after which the rates sharply declined. Figure 2. Derivative conversion curves for experiments 1, 3, and 5 using the R2/l concentrate. The Figure 2. Derivative conversion curves for experiments 1, 3, and 5 using the R2/l concentrate. The dotted dotted lline ine ind indicates icates aa conver conversion sion cu curve rve obta obtained ined without a without any ny slag modif slag modiﬁcation ication and wit and with h a higher a higher amount of gangue minerals [9]. amount of gangue minerals [9]. With CaF additions, another signiﬁcant increase in the conversion rates appeared With CaF2 additions, another significant increase in the conversion rates appeared approaching the ﬁnal temperature (1450 C) and particularly during the observed tempera- approaching the final temperature (1450 °C) and particularly during the observed tem- ture overshoot (Figure 2). The differences between 5 and 10% additions of CaO seemed perature overshoot (Figure 2). The differences between 5 and 10% additions of CaO negligible according to the mass loss (Table 4), whereas the sample with the highest addi- seemed negligible according to the mass loss (Table 4), whereas the sample with the high- tion of CaF (14%) exhibited the highest mass loss. This was interpreted to be caused by est addition of CaF2 (14%) exhibited the highest mass loss. This was interpreted to be ﬂuoride gas-forming reactions (Equation (1)) and increased silica reduction [22] and is one caused by fluoride gas-forming reactions (Equation (1)) and increased silica reduction [22] of the main reasons for the discrepancy between the calculated and measured mass losses and is one of the main reasons for the discrepancy between the calculated and measured (Table 4). mass losses (Table 4). 2CaF (slag) + SiO (slag) ! SiF (g) + 2CaO (slag) (1) 2 2 4 2CaF2 (slag) + SiO2 (slag) → SiF4 (g) + 2CaO (slag) (1) Although CaF has been widely used in the steel industry, it can be lost from industrial Although CaF2 has been widely used in the steel industry, it can be lost from indus- slags due to gas formation reactions, which increase with increasing temperature and set trial slags due to gas formation reactions, which increase with increasing temperature and limits to the use of CaF in industrial practice [22–24]. Depending on the temperature set limits to the use of CaF2 in industrial practice [22–24]. Depending on the temperature and composition of the slag, gaseous compounds such as KF, AlF and NaF can also be and composition of the slag, gaseous compounds such as KF, AlF3 and NaF can also be emitted. Other reasons for the ﬂuorspar substitution in the current industrial practices emitted. Other reasons for the fluorspar substitution in the current industrial practices include refractory wear, environmental issues, and problems in the supply or availability include refractory wear, environmental issues, and problems in the supply or availability of ﬂuorspar [24]. of fluorspar [24]. Figure 3A,B show back-scattered electron images of slag produced without any addi- Figure 3A,B show back-scattered electron images of slag produced without any ad- tions and with 5% addition of CaO, respectively, in experiments with R2/l concentrates at ditions and with 5% addition of CaO, respectively, in experiments with R2/l concentrates 1450 C. The positive effect of the addition of network-transforming CaO on the reduction at 1450 °C. Th of the slag FeO e positive effe component, ct of the addit as well as the ion of network-tran separation of very sform small ing CaO on the reduc- metal droplets in the tion of the slag FeO component, as well as the separation of very small metal droplets in slag, appeared to be obvious, with less metal retained in the slag. Figure 3A,B also present the sla EPMAg, analysis appeared to be obvi points with corr ous, wi esponding th less met results. al ret The aineslag d in t inhe both slag. samples Figure is 3A visually ,B also present homogeneous EPMA in ana back-scatter lysis pointsed wielectr th coon rrespondin images g but resu the lts. analytical The slag rin esults bothr s eveal amples some is variation, particularly in the SiO , FeO, and Sc O contents without doping (Figure 3A), visually homogeneous in back-scattered electron images but the analytical results reveal 2 2 3 which was diminished with CaO doping (Figure 3B). A summary of the slag analytical data some variation, particularly in the SiO2, FeO, and Sc2O3 contents without doping (Figure is provided in Table 5. 3A), which was diminished with CaO doping (Figure 3B). A summary of the slag analyt- ical data is provided in Table 5. Metals 2022, 12, x FOR PEER REVIEW 8 of 20 Table 5. Average chemical compositions determined with EPMA for slags produced in reduction experiments 1–5; Sc2O3 ppm, other oxides wt%. n = number of analysis points. Exp. No. n SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 ZrO2 Sc2O3 F 1 14 52.48 2.73 13.88 0.16 0.61 1.64 20.85 1.70 1.43 0.01 0.50 434 3.71 SD - 0.20 0.18 0.11 0.44 0.03 0.05 0.22 0.04 0.02 0.01 0.06 91 0.07 2 14 47.63 2.27 13.13 0.04 0.52 1.55 26.80 1.47 1.12 0.01 0.50 366 6.23 SD 0.27 0.11 0.07 0.02 0.03 0.04 0.12 0.05 0.03 0.01 0.05 92 0.09 3 14 55.48 3.24 13.54 0.22 0.74 1.61 19.75 1.69 1.50 0.00 0.50 399 0.00 SD - 0.23 0.11 0.11 0.03 0.04 0.04 0.16 0.05 0.02 0.01 0.09 89 0.00 4 13 51.82 3.11 12.59 0.13 0.68 1.52 24.92 1.52 1.37 0.01 0.45 420 0.00 SD - 0.31 0.14 0.08 0.04 0.04 0.10 0.14 0.04 0.02 0.01 0.04 159 0.00 Metals 2022, 12, 709 8 of 20 5 13 49.42 2.63 12.88 0.05 0.57 1.53 25.79 1.49 1.26 0.00 0.47 436 3.83 SD - 0.27 0.08 0.12 0.03 0.05 0.02 0.17 0.06 0.02 0.01 0.03 107 0.05 Figure 3. Figure 3. Back Back-scatter -scattered ed ele electr ctron on im images ages of of slag slag with with E EPMA PManalysis A analys point is poi locations nt locations an and corr d corre- espond- sponding results in wt% after reduction experiments with the R2/l concentrate conducted at 1450 ing results in wt% after reduction experiments with the R2/l concentrate conducted at 1450 C. °C. (A) Slag produced in the first part of our pyrometallurgical study without additions [9]. (B) Slag (A) Slag produced in the ﬁrst part of our pyrometallurgical study without additions [9]. (B) Slag produced with 5% addition of CaO in this study. produced with 5% addition of CaO in this study. According to the EPMA data (provided in electronic supplementary Table S2), CaF2 Table 5. Average chemical compositions determined with EPMA for slags produced in reduction addition promoted the reduction of SiO2 and solution of silicon to ferrite. None of the experiments 1–5; Sc O ppm, other oxides wt%. n = number of analysis points. 2 3 metal analyses provided Sc contents above the detection limit (100 ppm Sc). Titanium car- bide (TiC) formation was occasionally observed on the borders of graphite and ferrite. Exp. No. n SiO TiO Al O FeO MnO MgO CaO Na O K O P O ZrO Sc O F 2 2 2 3 2 2 2 5 2 2 3 Metal analyses did not indicate the presence of Fe3C. Figure 4 presents an overview of the 1 14 52.48 2.73 13.88 0.16 0.61 1.64 20.85 1.70 1.43 0.01 0.50 434 3.71 characteristics of large metal accumulation after the experiment with 10% addition of SD - 0.20 0.18 0.11 0.44 0.03 0.05 0.22 0.04 0.02 0.01 0.06 91 0.07 CaO, with a close-up of the texture and EPMA point analytical data. The black areas in 2 14 47.63 2.27 13.13 0.04 0.52 1.55 26.80 1.47 1.12 0.01 0.50 366 6.23 SD 0.27 0.11the back-scat 0.07 0.02 tered electro 0.03 n imag 0.04 es rep 0.12 resent 0.05 graphi0.03 te and t 0.01 he bright 0.05 est pha 92 se is ferr 0.09 ite while the intermediate grey color represents steadite, the eutectic of ferrite and iron phos- 3 14 55.48 3.24 13.54 0.22 0.74 1.61 19.75 1.69 1.50 0.00 0.50 399 0.00 phide (Fe3P). It solidifies during cooling from the liquid as the last constituent at the grain SD - 0.23 0.11 0.11 0.03 0.04 0.04 0.16 0.05 0.02 0.01 0.09 89 0.00 4 13 51.82 3.11 12.59 0.13 0.68 1.52 24.92 1.52 1.37 0.01 0.45 420 0.00 boundaries [25]. SD - 0.31 0.14 0.08 0.04 0.04 0.10 0.14 0.04 0.02 0.01 0.04 159 0.00 5 13 49.42 2.63 12.88 0.05 0.57 1.53 25.79 1.49 1.26 0.00 0.47 436 3.83 SD - 0.27 0.08 0.12 0.03 0.05 0.02 0.17 0.06 0.02 0.01 0.03 107 0.05 According to the EPMA data (provided in electronic supplementary Table S2), CaF addition promoted the reduction of SiO and solution of silicon to ferrite. None of the metal analyses provided Sc contents above the detection limit (100 ppm Sc). Titanium carbide (TiC) formation was occasionally observed on the borders of graphite and ferrite. Metal analyses did not indicate the presence of Fe C. Figure 4 presents an overview of the characteristics of large metal accumulation after the experiment with 10% addition of CaO, with a close-up of the texture and EPMA point analytical data. The black areas in the back-scattered electron images represent graphite and the brightest phase is ferrite while the intermediate grey color represents steadite, the eutectic of ferrite and iron phosphide (Fe P). It solidiﬁes during cooling from the liquid as the last constituent at the grain boundaries [25]. Metals 2022, 12, 709 9 of 20 Metals 2022, 12, x FOR PEER REVIEW 9 of 20 Figure 4. Example of large metal accumulation produced after reducing concentrate R2/l with 10% Figure 4. Example of large metal accumulation produced after reducing concentrate R2/l with 10% CaO addition. EPMA data on metal concentrations from points 1–5 expressed in wt%. CaO addition. EPMA data on metal concentrations from points 1–5 expressed in wt%. 3.3. Comparison of Concentrate Quality with Selected Additions 3.3. Comparison of Concentrate Quality with Selected Additions Experiments 6–14 (Table 1) were conducted to compare the high-temperature prop- Experiments 6–14 (Table 1) were conducted to compare the high-temperature prop- erties and behavior of concentrates R1, R2/u, and R3, having differences in their modal erties and behavior of concentrates R1, R2/u, and R3, having differences in their modal mineralogy (Figure 1). Additions of 7% CaF2, 5% CaO, and a combination of both of these mineralogy (Figure 1). Additions of 7% CaF , 5% CaO, and a combination of both of these two were used. The final isotherm was changed to the highest possible one in the TG two were used. The ﬁnal isotherm was changed to the highest possible one in the TG furnace (1500 °C) to further improve the separation of small metal inclusions from the slag furnace (1500 C) to further improve the separation of small metal inclusions from the slag (Figure 3). Details of the feed materials and experimental results are presented in Table 6. (Figure 3). Details of the feed materials and experimental results are presented in Table 6. Table 6. Results of experiments 6–14 conducted up to 1500 °C with R1, R2/u, and R3 concentrates. Table 6. Results of experiments 6–14 conducted up to 1500 C with R1, R2/u, and R3 concentrates. Exp. No. 6 7 8 9 10 11 12 13 14 Exp. No. 6 7 8 9 10 11 12 13 14 Sample R1 R2/u R3 R1 R2/u R3 R1 R2/u R3 Sample R1 R2/u R3 R1 R2/u R3 R1 R2/u R3 Ʃ LIMS g 1.59 1.39 1.44 1.59 1.39 1.44 1.59 1.39 1.44 S LIMS g 1.59 Slon g1.39 8.41 1.448.62 1.59 8.56 8.41 1.39 8.62 1.44 8.56 8.41 1.59 8.62 1.39 8.56 1.44 Slon g 8.41 8.62 8.56 8.41 8.62 8.56 8.41 8.62 8.56 C g 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 C g 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 CaF2 g 0.70 0.70 0.70 - - - 0.70 0.70 0.70 CaF g 0.70 0.70 0.70 - - - 0.70 0.70 0.70 CaO g - - - 0.50 0.50 0.50 0.50 0.50 0.50 CaO g - - - 0.50 0.50 0.50 0.50 0.50 0.50 Ʃ g 11.20 11.21 11.20 11.00 11.01 11.00 11.70 11.71 11.70 S g 11.20 11.21 11.20 11.00 11.01 11.00 11.70 11.71 11.70 m0 g 11.17 11.17 11.16 10.97 10.99 10.99 11.68 11.71 11.67 mf g 8.68 8.62 8.59 9.04 9.06 8.95 9.23 9.28 9.16 m0 g 11.17 11.17 11.16 10.97 10.99 10.99 11.68 11.71 11.67 mf g 8.68 Δm g 8.62 2.49 8.592.55 9.042.57 1.93 9.06 1.93 8.95 2.04 2.45 9.23 2.43 9.28 2.51 9.16 Dm g 2.49 2.55 2.57 1.93 1.93 2.04 2.45 2.43 2.51 Δm (%) 22.29 22.83 23.03 17.59 17.56 18.56 20.98 20.75 21.51 Dm (%) 22.29 22.83 23.03 17.59 17.56 18.56 20.98 20.75 21.51 Ʃ O + C + H2O g 1.33 1.38 1.28 1.33 1.38 1.28 1.33 1.38 1.28 S O + C + H O g 1.33 1.38 1.28 1.33 1.38 1.28 1.33 1.38 1.28 The obtained mass loss curves (Figure 5A) and derivative conversion rates (Figure 5B–D) exhibited essentially similar characteristics for the three concentrates under the The obtained mass loss curves (Figure 5A) and derivative conversion rates (Figure 5B–D) same experimental conditions. Compared with the other concentrates, the R2/l concen- exhibited essentially similar characteristics for the three concentrates under the same ex- trate used in the first part of our pyrometallurgical study [9] and in the first five experi- perimental conditions. Compared with the other concentrates, the R2/l concentrate used ments of this study contains a significantly higher amount of amphibole, which likely is in the ﬁrst part of our pyrometallurgical study [9] and in the ﬁrst ﬁve experiments of this the reason for the slightly lower temperature region for the higher conversion rates (Fig- study contains a signiﬁcantly higher amount of amphibole, which likely is the reason for ures 2 and 5) initiating at ~950 °C. Although the main reduction stage appeared to initiate the slightly lower temperature region for the higher conversion rates (Figures 2 and 5) at a slightly higher temperature (~1000 °C), R1, R2/u, and R3 concentrates all exhibited initiating at ~950 C. Although the main reduction stage appeared to initiate at a slightly higher rates of conversion with both additions at a lower temperature region as compared higher temperature (~1000 C), R1, R2/u, and R3 concentrates all exhibited higher rates to the R2/l concentrate without slag modification (Figure 5). Metals 2022, 12, 709 10 of 20 Metals 2022, 12, x FOR PEER REVIEW 10 of 20 of conversion with both additions at a lower temperature region as compared to the R2/l concentrate without slag modiﬁcation (Figure 5). Figure 5. (A) Mass-loss curves for experiments 6–14, with all conducted using the same temperature Figure 5. (A) Mass-loss curves for experiments 6–14, with all conducted using the same temperature program up to 1500 °C for R1, R2/u, and R3 concentrates. (B–D) Derivative conversion curves for program up to 1500 C for R1, R2/u, and R3 concentrates. (B–D) Derivative conversion curves experiments 6–8 (with 7% CaF2), 9–11 (with 5% CaO), and 12–14 (with 7% CaF2 + 5% CaO). Conver- for experiments 6–8 (with 7% CaF ), 9–11 (with 5% CaO), and 12–14 (with 7% CaF + 5% CaO). 2 2 sion rates for R2/l concentrate without additions shown as a reference [9]. Conversion rates for R2/l concentrate without additions shown as a reference [9]. The R2/u concentrate has the highest FeO content and exhibited the highest deriva- The R2/u concentrate has the highest FeO content and exhibited the highest deriva- tive conversion rates at the main stage of reduction reactions. Similar to the first set of tive conversion rates at the main stage of reduction reactions. Similar to the ﬁrst set of experiments, the conversion rates with CaF2 occurred at slightly lower temperatures than experiments, the conversion rates with CaF occurred at slightly lower temperatures than with CaO additions. Both additions alone exhibited a wider temperature region for higher with CaO additions. Both additions alone exhibited a wider temperature region for higher rates of conversion, whereas with the combined and thus the highest amount of doping, rates of conversion, whereas with the combined and thus the highest amount of doping, the rate increased and decreased very sharply (Figure 5D), occurring within a narrower the rate increased and decreased very sharply (Figure 5D), occurring within a narrower temperature range. This is interpreted to be caused by improved slag-forming reactions temperature range. This is interpreted to be caused by improved slag-forming reactions and slag characteristics due to a lower viscosity, which in turn enhances mass-transfer and slag characteristics due to a lower viscosity, which in turn enhances mass-transfer phenomena and allows faster reduction reactions within a narrower temperature interval. phenomena and allows faster reduction reactions within a narrower temperature interval. As observed in the previous and current set of experiments, with CaO additions at the As observed in the previous and current set of experiments, with CaO additions at the low-temperature regime (<900 °C) in CO atmosphere, the reversed Boudouard reaction is low-temperature regime (<900 C) in CO atmosphere, the reversed Boudouard reaction is likely to occur with the possibility of reaction with CaO, based on the mass loss curves likely to occur with the possibility of reaction with CaO, based on the mass loss curves and and derivative conversion rates (Figures 2 and 5) [26]. derivative conversion rates (Figures 2 and 5) [26]. Average slag compositions determined with EPMA are presented in Table 7. As men- Average slag compositions determined with EPMA are presented in Table 7. As tioned earlier, the counting times were 10 s for peaks and 5 s for background for each mentioned earlier, the counting times were 10 s for peaks and 5 s for background for element in th each elemente beginnin in the beginning g of our of analytic our analytical al work. This wa work. This s iniwas tially initially considered an a considerppro- ed an pri appr ate compromi opriate compr se between a omise between reasona a r beasonable le total analysis time, also co total analysis time, nsider alsoing the considering available the WDS cr available ystals WDS and detector crystals and s. Re detectors. garding the electr Regardingon m the electr icroprobe on micr analysis o oprobe analysis f trace elements of trace (< elements 1000 pp(<1000 m) for p ppm) hasefor s th phases at are st that able are under stable a den under se electron be a dense electr am, the on beam, detec thetion limit detection and precision could be decreased by using a higher acceleration voltage and beam current combined with a longer counting time [27]. Due to the observed high standard deviation for Sc2O3 in the first set of slag analyses for these samples (data provided in electronic Metals 2022, 12, 709 11 of 20 limit and precision could be decreased by using a higher acceleration voltage and beam current combined with a longer counting time [27]. Due to the observed high standard deviation for Sc O in the ﬁrst set of slag analyses for these samples (data provided in 2 3 electronic supplementary Table S3), longer peak and background counting times were tested to improve the quality of the data; the counting times of 30 s for peaks and 15 s for background were set for Sc O only, while the initial parameters were employed for 2 3 other components. This lowered the standard deviation and provided results closer to the calculated values. More speciﬁc analysis of Sc O with a higher acceleration voltage and 2 3 beam current and/or a longer counting time might provide potential to lower the detection limit and standard deviation even further [28,29]. The calculated slag compositions listed in Table 7 are normalized, excluding FeO and P O from the ICP-OES results and considering 2 5 the applied doping. As an exception to other components, the Na O values were calculated based on modal mineralogy because the samples were subjected to the sodium peroxide fusion prior to ICP-OES analysis. The differences between the analyzed and calculated slag compositions are visualized in Figure 6, which is based on the percentage of the difference of EPMA analyzed values from calculated value to provide an indication of the extent of reduction for other slag components in addition to FeO and P O . 2 5 Table 7. Average chemical compositions determined with EPMA for slags produced in reduction experiments 6–14; Sc O ppm, other oxides wt%. n = number of analysis points. Calc = calculated 2 3 values based on ICP-OES data, excluding FeO and P O components. 2 5 Exp. No. n SiO TiO Al O FeO MnO MgO CaO Na O K O P O ZrO Sc O F 2 2 2 3 2 2 2 5 2 2 3 6 20 53.17 2.71 12.80 0.03 0.52 2.83 20.74 1.42 2.02 0.01 0.12 389 3.78 SD - 0.42 0.10 0.10 0.02 0.03 0.06 0.28 0.04 0.05 0.01 0.03 64 0.05 Calc. - 54.76 4.24 11.46 0.00 0.83 2.47 17.76 1.40 * 2.09 0.00 - 446 4.56 7 20 54.04 2.30 12.16 0.03 0.64 1.85 22.53 1.17 1.46 0.01 0.38 604 3.73 SD - 0.33 0.14 0.09 0.02 0.03 0.04 0.14 0.05 0.03 0.01 0.05 58 0.08 Calc. - 54.69 3.87 11.17 0.00 0.98 1.63 19.66 1.24 * 1.52 0.00 - 623 4.72 8 20 52.71 2.31 14.99 0.03 0.49 1.44 20.90 1.54 2.09 0.01 0.16 559 3.76 SD - 0.19 0.12 0.08 0.02 0.03 0.05 0.10 0.04 0.03 0.01 0.03 71 0.08 Calc. - 54.50 3.28 13.57 0.00 0.79 1.30 18.28 1.25 * 2.19 0.00 - 595 4.46 9 ** 20 56.27 4.39 12.32 0.09 0.70 2.74 19.68 1.52 2.16 0.01 0.12 381 0.00 SD - 0.29 0.18 0.07 0.03 0.02 0.06 0.15 0.04 0.04 0.01 0.04 61 0.00 Calc. - 57.45 4.45 12.03 0.00 0.88 2.61 18.57 1.41 * 2.19 0.00 - 468 10 ** 20 56.85 3.87 11.89 0.07 0.82 1.84 21.63 1.23 1.56 0.00 0.38 594 0.00 SD - 0.24 0.13 0.08 0.03 0.04 0.04 0.09 0.05 0.03 0.01 0.03 49 0.00 Calc. - 57.51 4.07 11.61 0.00 1.03 1.72 20.63 1.29 * 1.60 0.00 - 655 11 ** 20 55.74 3.43 14.58 0.10 0.65 1.45 20.04 1.61 2.24 0.00 0.16 542 0.00 SD - 0.18 0.17 0.16 0.03 0.03 0.03 0.14 0.05 0.04 0.01 0.03 60 0.00 Calc. - 57.07 3.44 14.21 0.00 0.83 1.36 19.09 1.31 * 2.29 0.00 - 623 12 20 50.22 2.45 11.76 0.04 0.51 2.62 25.89 1.16 1.72 0.01 0.11 326 3.97 SD - 0.17 0.12 0.08 0.02 0.04 0.03 0.13 0.03 0.02 0.01 0.02 59 0.07 Calc. - 51.34 3.98 10.75 0.00 0.78 2.33 22.93 1.28 * 1.96 0.00 - 418 4.26 13 20 50.41 2.32 11.15 0.03 0.65 1.71 27.60 1.00 1.23 0.01 0.38 533 3.98 SD - 0.15 0.09 0.08 0.02 0.04 0.04 0.13 0.03 0.03 0.01 0.03 52 0.05 Calc. - 51.16 3.62 10.45 0.00 0.91 1.53 24.84 1.16 * 1.42 0.00 - 583 4.41 14 20 49.44 2.06 13.95 0.02 0.45 1.37 26.27 1.21 1.75 0.01 0.14 477 3.93 SD - 0.22 0.10 0.04 0.02 0.03 0.04 0.11 0.04 0.03 0.01 0.03 47 0.08 Calc. - 51.15 3.08 12.73 0.00 0.74 1.22 23.31 1.17 * 2.06 0.00 - 558 4.18 * Value calculated based on concentrate modal mineralogy; ** EPMA results normalized due to low totals. Metals 2022, 12, x FOR PEER REVIEW 12 of 20 Metals 2022, 12, 709 12 of 20 typical structures in large metal accumulations produced from the R3 concentrate with various doping are presented in Figure 7. Metals 2022, 12, x FOR PEER REVIEW 12 of 20 typical structures in large metal accumulations produced from the R3 concentrate with various doping are presented in Figure 7. Figure 6. Differences between the analyzed (EPMA) and calculated (ICP-OES) slag compositions. Figure 6. Differences between the analyzed (EPMA) and calculated (ICP-OES) slag compositions. Experiments using concentrates R1, R2/u, and R3 with (A) 7% CaF2, (B) 5% CaO, and (C) 7% CaF2 + Experiments using concentrates R1, R2/u, and R3 with (A) 7% CaF , (B) 5% CaO, and (C) 7% CaF + 2 2 5% CaO. 5% CaO. According to the slag and metal EPMA data, reduction of the MnO, TiO , K O, and 2 2 SiO components from slag occurred to some extent, particularly with CaF additions. The 2 2 negative values for Sc O are considered to be arising from the challenges in trace element 2 3 analysis with EPMA and because Sc O is thermodynamically very stable in comparison 2 3 to other components in the slag [30,31]. Back-scattered electron images of the slags and Figure 6. Differences between the analyzed (EPMA) and calculated (ICP-OES) slag compositions. typical structures in large metal accumulations produced from the R3 concentrate with Experiments using concentrates R1, R2/u, and R3 with (A) 7% CaF2, (B) 5% CaO, and (C) 7% CaF2 + various doping are presented in Figure 7. 5% CaO. Figure 7. Back-scattered electron images of slag and metal after experiments with concentrate R3 conducted up to 1500 °C with various doping. (A) 7% CaF2, (B) 5% CaO, and (C) 7% CaF2 + 5% CaO additions. There were no significant differences in the amount or size of small metal inclusions in the slag between the experimental runs with CaF2 and CaO additions. Large graphite- bearing metal accumulations occurred at the bottom of the crucibles with smaller metal droplets on the sides, along the contact of the slag and graphite crucible. The wetting of Figure 7. Back-scattered electron images of slag and metal after experiments with concentrate R3 Figure 7. Back-scattered electron images of slag and metal after experiments with concentrate R3 conducted up to 1500 °C with various doping. (A) 7% CaF2, (B) 5% CaO, and (C) 7% CaF2 + 5% CaO conducted up to 1500 C with various doping. (A) 7% CaF , (B) 5% CaO, and (C) 7% CaF + 5% CaO 2 2 additions. additions. There were no significant differences in the amount or size of small metal inclusions in the slag between the experimental runs with CaF2 and CaO additions. Large graphite- bearing metal accumulations occurred at the bottom of the crucibles with smaller metal droplets on the sides, along the contact of the slag and graphite crucible. The wetting of Metals 2022, 12, 709 13 of 20 There were no signiﬁcant differences in the amount or size of small metal inclusions Metals 2022, 12, x FOR PEER REVIEW 13 of 20 in the slag between the experimental runs with CaF and CaO additions. Large graphite- bearing metal accumulations occurred at the bottom of the crucibles with smaller metal droplets on the sides, along the contact of the slag and graphite crucible. The wetting of graphite–slag interfaces by slag without an FeO component has been regarded as rather graphite–slag interfaces by slag without an FeO component has been regarded as rather poor depending on the carbon-slag interfacial tension, slag and carbon surface tension, poor depending on the carbon-slag interfacial tension, slag and carbon surface tension, and the dynamic reactions occurring at the interface [32]. It is apparent from the observed and the dynamic reactions occurring at the interface [32]. It is apparent from the observed concave slag surface with CaF2 doping (Figure 7A,C), in comparison to the convex surface concave slag surface with CaF doping (Figure 7A,C), in comparison to the convex surface with only CaO doping (Figure 7B), that the surface characteristics for these slags are quite with only CaO doping (Figure 7B), that the surface characteristics for these slags are quite different. CaF2 is known to be a surface-active constituent in slags [21]. In their evaluation different. CaF is known to be a surface-active constituent in slags [21]. In their evaluation of the surface tension of molten ionic mixtures containing CaF2, Nakamoto et al. [21] stated of the surface tension of molten ionic mixtures containing CaF , Nakamoto et al. [21] stated that in the SiO2–CaO–CaF2 system at 1500 °C, the surface tension decreases with increas- that in the SiO –CaO–CaF system at 1500 C, the surface tension decreases with increasing 2 2 ing CaF2 content, apparently also applying to the slag system of this study. CaF content, apparently also applying to the slag system of this study. Metal analyses revealed similar phases as described in our previous experiments, in- Metal analyses revealed similar phases as described in our previous experiments, cluding eutectic steadite between Si-containing ferrite and flake graphite and/or interden- including eutectic steadite between Si-containing ferrite and ﬂake graphite and/or inter- dritic graphite segregations. These experiments also exhibited increased silica reduction dendritic graphite segregations. These experiments also exhibited increased silica reduction with with CaF CaF 2 do doping, ping, wh which ich isis in indicated dicated b by y an an incr inc ease rease in in the the conversion conversion rates rate at s temperatu at tempera- res of tures of >14 >1450 C50 °C (Figur (F eig 5ure 5B) B). In accor . In a dance ccordance withwi the th the previous previexperiments, ous experiments, ti titanium tanium carbide car- (TiC) was detected occasionally at the borders between ferrite and graphite. Averages of the bide (TiC) was detected occasionally at the borders between ferrite and graphite. Aver- ferrite ages of analyses the ferrite analyses are are provided inprov Figur ided e 8 in with Fig details ure 8 w pr ith details pro ovided in thev electr ided oni in the c supplemen- electronic tary data (Table S2). Graphite crystallization in cast gray iron is a complex phenomenon, supplementary data (Table S2). Graphite crystallization in cast gray iron is a complex phe- which is controlled by melt composition, temperature, and cooling rate [25,33]. According nomenon, which is controlled by melt composition, temperature, and cooling rate [25,33]. to the textures developed in large metal accumulations, the tendency to form ﬁne inter- According to the textures developed in large metal accumulations, the tendency to form dendritic graphite was promoted by a higher silicon content in the metal (Figures 7 and 8). fine interdendritic graphite was promoted by a higher silicon content in the metal (Figures Metallic iron somewhat penetrated the graphite crucible, dissolving carbon into the metal 7 and 8). Metallic iron somewhat penetrated the graphite crucible, dissolving carbon into and producing the above-mentioned textures upon cooling. Furthermore, the gas bubble the metal and producing the above-mentioned textures upon cooling. Furthermore, the formed on top of the large metal accumulation (Figure 7C) indicated gasiﬁcation reactions gas bubble formed on top of the large metal accumulation (Figure 7C) indicated gasifica- at the Fe–C surface; as proposed by Teasdale and Hayes [34,35], gasiﬁcation of the carbon tion reactions at the Fe–C surface; as proposed by Teasdale and Hayes [34,35], gasification in the alloy produces CO as one of the reaction steps involved in the reduction of slag of the carbon in the alloy produces CO as one of the reaction steps involved in the reduc- by solid carbon in the presence of liquid Fe-C. As stated by White et al. [36], liquid slags tion of slag by solid carbon in the presence of liquid Fe-C. As stated by White et al. [36], react with carbon in surprisingly complex ways, with liquid Fe-C metal and gas-forming liquid slags react with carbon in surprisingly complex ways, with liquid Fe-C metal and reactions from various slag components contributing to the whole scenario. gas-forming reactions from various slag components contributing to the whole scenario. Figure 8. Average ferrite compositions in wt% determined by EPMA in experiments on variously Figure 8. Average ferrite compositions in wt% determined by EPMA in experiments on variously doped concentrates R1, R2/u, and R3. (A) 7% CaF2, (B) 5% CaO, and (C) 7% CaF2 + 5% CaO additions. doped concentrates R1, R2/u, and R3. (A) 7% CaF , (B) 5% CaO, and (C) 7% CaF + 5% CaO 2 2 additions. The design of the type and quantity of doping used in our experiments is based on the ternary Al2O3-CaO-SiO2 phase diagram presented in Figure 9, which exhibits the target The design of the type and quantity of doping used in our experiments is based on the area with lower liquidus temperatures as indicated in the close-up. To calculate and plot ternary Al O -CaO-SiO phase diagram presented in Figure 9, which exhibits the target 2 3 2 the compositions on the phase diagram, only the main oxide components (Tables 3, 6 and area with lower liquidus temperatures as indicated in the close-up. To calculate and plot the S2) were considered. Slag compositions based on calculations from both ICP-OES and compositions on the phase diagram, only the main oxide components (Tables 3, 6 and S2) EPMA data are shown in comparison in Figure 9, which was computed with the FactSage were considered. Slag compositions based on calculations from both ICP-OES and EPMA version 7 and its FToxid database. data are shown in comparison in Figure 9, which was computed with the FactSage version 7 The amounts of the additions of CaF2 and CaO were based on the desired liquidus and its FToxid database. temperature area (<1300 °C) of the system, with the target being close to the ternary eu- tectic composition. As indicated by the close-up view of the phase diagram area, 7% CaF2 and 5% CaO were sufficient, resulting in compositions located in the target area in terms Metals 2022, 12, x FOR PEER REVIEW 14 of 20 of the main components, as confirmed with EPMA data. The commonly expressed empir- ical slag basicity based on CaO/SiO2 [15,17] varied in the experiments without doping be- tween 0.20 and 0.24, whereas in the doped experiments, it fell in the range of 0.32–0.54. Even though these values still indicate a very high acidity and therefore a high viscosity, Metals 2022, 12, 709 14 of 20 the applied relatively moderate additions of CaF2 and CaO did improve the properties of the slag by adjusting the composition to the target liquidus temperature area. Figure 9. Ternary Al2O3-CaO-SiO2 phase diagram with the target liquidus temperature area (<1300 Figure 9. Ternary Al O -CaO-SiO phase diagram with the target liquidus temperature area 2 3 2 °C) disp layed in the close-up view. Original and modified slag compositions from R2/u and R2/l (<1300 C) displayed in the close-up view. Original and modiﬁed slag compositions from R2/u concentrates computed and plotted with FactSage version 7 and its FToxid database. C = calculated and R2/l concentrates computed and plotted with FactSage version 7 and its FToxid database. from ICP-OES data and A = calculated from EPMA analysis data. C = calculated from ICP-OES data and A = calculated from EPMA analysis data. 3.4. Main Features of the Progression of Reduction at Selected Temperatures with CaO Addition The amounts of the additions of CaF and CaO were based on the desired liquidus temperature area (<1300 C) of the system, with the target being close to the ternary eutectic As the final aspect of our pyrometallurgical studies, the main features of the progres- composition. As indicated by the close-up view of the phase diagram area, 7% CaF and sion of reduction at selected temperatures (950, 1050, 1150, 1250, and 1350 °C) were i 2nves- 5% CaO were sufﬁcient, resulting in compositions located in the target area in terms of the tigated with concentrate R3 and 5% CaO addition (Table 1). Based on previous experi- main components, as conﬁrmed with EPMA data. The commonly expressed empirical slag ments, the differences between CaF2 and CaO doping are negligible, both improving the basicity based on CaO/SiO [15,17] varied in the experiments without doping between 0.20 slag properties, promoting the reduction of 2 the slag FeO component, and improving the and 0.24, whereas in the doped experiments, it fell in the range of 0.32–0.54. Even though metal segregation. However, CaO would be the likely choice to be used considering envi- these values still indicate a very high acidity and therefore a high viscosity, the applied ronmental aspects and other issues related to the CaF2 usage. A preliminary experiment relatively moderate additions of CaF and CaO did improve the properties of the slag by was conducted with a gas compositi2 on of 95% Ar + 5% H2 to 1500 °C to provide the base- adjusting the composition to the target liquidus temperature area. line against which intercepts at various temperatures could be evaluated. By replacing CO with a mixture of Ar and H2, the possibility of the reversed Boudouard reaction was elim- 3.4. Main Features of the Progression of Reduction at Selected Temperatures with CaO Addition inated. A comparison of the mass change and conversion rates with 100% CO and 95% Ar As the ﬁnal aspect of our pyrometallurgical studies, the main features of the pro- + 5% H2 are provided in the electronic supplementary data (Figure S1). Despite the differ- gression of reduction at selected temperatures (950, 1050, 1150, 1250, and 1350 C) were ences at temperatures of <900 °C, the main stage of ferrous silicate reduction reactions was investigated with concentrate R3 and 5% CaO addition (Table 1). Based on previous experi- initiated at the same temperature with both gas flows, peaking at ~1050 °C, with similar ments, the differences between CaF and CaO doping are negligible, both improving the final mass changes of 15.19 and 14.88% at 1500 °C for 100% CO and 95% Ar + 5% H2, slag properties, promoting the reduction of the slag FeO component, and improving the respectively. Therefore, the last set of experiments was conducted with a gas flow of 95% metal segregation. However, CaO would be the likely choice to be used considering envi- Ar and 5% H2. Details of mass change rates are provided in the electronic supplementary ronmental aspects and other issues related to the CaF usage. A preliminary experiment data (Figure S2). was conducted with a gas composition of 95% Ar + 5% H to 1500 C to provide the baseline As mentioned in Kallio et al. [9], mass loss curves for such a heterogeneous system against which intercepts at various temperatures could be evaluated. By replacing CO with as the Kiviniemi concentrate represent a sum of interlapping and interactive phenomena a mixture of Ar and H , the possibility of the reversed Boudouard reaction was eliminated. originating from various reactions within and from various types of crystal structures. A comparison of the mass change and conversion rates with 100% CO and 95% Ar + 5% H These include dehydration, dehydroxylation, thermal dissociation, and gas–solid reduc- are provided in the electronic supplementary data (Figure S1). Despite the differences at tion reactions at lower experimental temperatures, shifting with a rising temperature into temperatures of <900 C, the main stage of ferrous silicate reduction reactions was initiated melting, slag formation, and gas–liquid and solid–liquid reactions [37,38]. In addition to at the same temperature with both gas ﬂows, peaking at ~1050 C, with similar ﬁnal mass chemical reactions at reactant–product interfaces, diffusion and heat transfer account for changes of 15.19 and 14.88% at 1500 C for 100% CO and 95% Ar + 5% H , respectively. Therefore, the last set of experiments was conducted with a gas ﬂow of 95% Ar and 5% H . Details of mass change rates are provided in the electronic supplementary data (Figure S2). As mentioned in Kallio et al. [9], mass loss curves for such a heterogeneous system as the Kiviniemi concentrate represent a sum of interlapping and interactive phenomena originating from various reactions within and from various types of crystal structures. These include dehydration, dehydroxylation, thermal dissociation, and gas–solid reduction Metals 2022, 12, 709 15 of 20 Metals 2022, 12, x FOR PEER REVIEW 15 of 20 reactions at lower experimental temperatures, shifting with a rising temperature into melting, slag formation, and gas–liquid and solid–liquid reactions [37,38]. In addition to chemical reactions at reactant–product interfaces, diffusion and heat transfer account for the whole scenario: heat transfer from the furnace to the outer regions of the sample and the whole scenario: heat transfer from the furnace to the outer regions of the sample and into the sample, self-cooling, or self-heating of the sample during reactions, removal of into the sample, self-cooling, or self-heating of the sample during reactions, removal of evolved gaseous products, and the inﬂuence of these products on the rates of reactions evolved gaseous products, and the influence of these products on the rates of reactions all all add to the scenario [39]. Despite the complexity of the phenomena, some of the main add to the scenario [39]. Despite the complexity of the phenomena, some of the main ob- observed features at selected temperatures are presented and discussed. Figure 10 provides served features at selected temperatures are presented and discussed. Figure 10 provides an overview at different temperatures, with photographs of samples and back-scattered an overview at different temperatures, with photographs of samples and back-scattered electron images plotted on the derivative conversion curve of the baseline experiment. electron images plotted on the derivative conversion curve of the baseline experiment. Figure 10. Photographs of samples at different temperatures (950, 1050, 1150, 1250, and 1350 C), Figure 10. Photographs of samples at different temperatures (950, 1050, 1150, 1250, and 1350 °C), plotted on the derivative conversion curve of the baseline experiment with 95% Ar + 5% H . The plotted on the derivative conversion curve of the baseline experiment with 95% Ar + 5% H2. The back-scattered electron images of the samples visualize the progression of mineral decompositions, back-scattered electron images of the samples visualize the progression of mineral decompositions, slag, and metallic iron formation. slag, and metallic iron formation. FFor or the the R3 R3 cconcentrate, oncentrate, the ma the main in re rdu eduction ction ststage age wi with th a ra a rapid pid in incr crea ease se in in the the co conver nver- - sion rates was initiated at ~1030 C, with highest rates at 1050 C. Therefore, as expected, sion rates was initiated at ~1030 °C, with highest rates at 1050 °C. Therefore, as expected, the concentrate showed very limited changes at 950 C. Differences between clinopyrox- the concentrate showed very limited changes at 950 °C. Differences between clinopyrox- ene and amphibole are visualized in more detail in Figure 11. At 950 C, clinopyroxene ene and amphibole are visualized in more detail in Figure 11. At 950 °C, clinopyroxene appeared as an intact mineral with the original composition and structure, whereas de- appeared as an intact mineral with the original composition and structure, whereas dehy- hydroxylation and reaction with the reducing gas phase resulted in the destruction of droxylation and reaction with the reducing gas phase resulted in the destruction of the the original amphibole structure and formation of new solid phases, including minuscule original amphibole structure and formation of new solid phases, including minuscule me- metallic iron particles within the relict of an amphibole crystal. XRD patterns in Figure 12 tallic iron particles within the relict of an amphibole crystal. XRD patterns in Figure 12 demonstrate the complexity of concentrate crystal structures at 950 C, with identiﬁable demonstrate the complexity of concentrate crystal structures at 950 °C, with identifiable patterns for potassium feldspar, plagioclase, clinopyroxene, garnet, fayalite, ilmenite, and patterns for potassium feldspar, plagioclase, clinopyroxene, garnet, fayalite, ilmenite, and even amphibole, with relicts occasionally preserved within larger grains. even amphibole, with relicts occasionally preserved within larger grains. Metals 2022, 12, x FOR PEER REVIEW 16 of 20 Metals 2022, 12, 709 16 of 20 Figure 11. Details of mineral reactions and progression of decomposition with formation of slag and Figure 11. Details of mineral reactions and progression of decomposition with formation of slag metallic iron at 950, 1050, 1150, and 1250 °C. Mineral abbreviations: CPX = clinopyroxene, AM = and metallic iron at 950, 1050, 1150, and 1250 C. Mineral abbreviations: CPX = clinopyroxene, amphibole, ILM = ilmenite, FA = fayalite, PLG = plagioclase, Fe = metallic iron. AM = amphibole, ILM = ilmenite, FA = fayalite, PLG = plagioclase, Fe = metallic iron. Increasing the temperature to 1050 °C caused drastic changes in the sample. Particu- Increasing the temperature to 1050 C caused drastic changes in the sample. Partic- larly along the contact between the concentrate and graphite crucible, the porosity was ularly along the contact between the concentrate and graphite crucible, the porosity was increased whereas the middle and top parts of the sample exhibited much less porosity, a increased whereas the middle and top parts of the sample exhibited much less porosity, a lower amount of initial slag, and more preserved mineral grains. The height of the sample lower amount of initial slag, and more preserved mineral grains. The height of the sample was increased due to gas-forming reduction reactions, creating porosity, which was was increased due to gas-forming reduction reactions, creating porosity, which was formed formed around graphite particles, producing metal rims around the pores (Figure 10). The around graphite particles, producing metal rims around the pores (Figure 10). The forma- formation of initial slag was dominated by the decomposition of amphibole and garnet, tion of initial slag was dominated by the decomposition of amphibole and garnet, sintering sintering the sample into a solid block. The decomposition products of various types of the sample into a solid block. The decomposition products of various types of amphiboles amphiboles have been found to include different phases, such as pyroxene, spinel, olivine, have been found to include different phases, such as pyroxene, spinel, olivine, feldspars, feldspars, and silica in addition to melt [40,41]. Furthermore, the decomposition of garnet and silica in addition to melt [40,41]. Furthermore, the decomposition of garnet under under reducing conditions (>1000 °C) has been found to produce metallic iron, cristobal- reducing conditions (>1000 C) has been found to produce metallic iron, cristobalite, and ite, and hercynite, with fayalite as a secondary product [42]. In our samples, the common hercynite, with fayalite as a secondary product [42]. In our samples, the common decom- decomposition products observed were very fine-grained mixtures of dark, lath-shaped position products observed were very ﬁne-grained mixtures of dark, lath-shaped crystals crystals with a composition resembling plagioclase and FeO-rich phase, with a tendency with a composition resembling plagioclase and FeO-rich phase, with a tendency to form to form dendrites and/or formation of fayalite as an intermediate decomposition product dendrites and/or formation of fayalite as an intermediate decomposition product within within the slag phase (Figure 11). The interplay of decomposition products and mineral the slag phase (Figure 11). The interplay of decomposition products and mineral reactions reactions with evolving slag phase do provide challenges to the interpretation of individ- with evolving slag phase do provide challenges to the interpretation of individual mineral reactions, the details of which could be a subject for further studies. With respect to more Metals 2022, 12, x FOR PEER REVIEW 17 of 20 Metals 2022, 12, 709 17 of 20 ual mineral reactions, the details of which could be a subject for further studies. With re- spect to more persistent pri persistent primary minerals m ata this ry mi temperatur nerals at e, this temperature such as clinopyr , suc oxene, h as clinopyr ilmenite, potassium oxene, il- menit feldspar e, pot , and assplagioclase, ium feldsparit , and p can be lag stated ioclase that , it can reaction be statrims, ed that dissolution reaction rim str suctur , dissol es, utand ion structures, and zoning are common features, as illustrated in Figure 11. In addition to zoning are common features, as illustrated in Figure 11. In addition to metallic iron and met graphite, allic iron and clinopyr g oxene raphitand e, clinopyrox plagioclase ene ar and e identiﬁable plagioclase in are XRD ident patterns ifiable (Figur in XR eD 12 p)aa tttethis rns (Figure temperatur 12) at e. this temperature. Details Details of XRD interpretations of XRD inter are provided pretations ar in the electr e provided in onic supplementary the elec- data (Figure S3). tronic supplementary data (Figure S3). Figure 12. XRD patterns of concentrate R3 reduced at temperatures of 950, 1050, and 1150 °C. Min- Figure 12. XRD patterns of concentrate R3 reduced at temperatures of 950, 1050, and 1150 C. Mineral eral abbreviations: AM = amphibole, CPX = clinopyroxene, FA = fayalite, GRT = garnet, PL = plagi- abbreviations: AM = amphibole, CPX = clinopyroxene, FA = fayalite, GRT = garnet, PL = plagioclase, oclase, FSP = potassium feldspar, C = graphite, Fe = metallic iron. FSP = potassium feldspar, C = graphite, Fe = metallic iron. At 1150 °C, along the progression of reduction reactions, the amount of metallic iron At 1150 C, along the progression of reduction reactions, the amount of metallic iron and slag increased, with the porosity extending throughout the whole sample (Figure 10). and slag increased, with the porosity extending throughout the whole sample (Figure 10). Amorphous slag and metallic iron were the dominant phases in XRD patterns, with minor Amorphous slag and metallic iron were the dominant phases in XRD patterns, with minor peaks still identifiable for clinopyroxene and plagioclase (Figure 12). By 1250 °C, only oc- peaks still identiﬁable for clinopyroxene and plagioclase (Figure 12). By 1250 C, only casional plagioclase relicts remained in the slag, as presented in Figure 11, still with an occasional plagioclase relicts remained in the slag, as presented in Figure 11, still with an extensive porosity, which is diminished by 1350 °C (Figure 10). extensive porosity, which is diminished by 1350 C (Figure 10). The evolution of the chemical composition of the slag phase is summarized in Figure The evolution of the chemical composition of the slag phase is summarized in Figure 13, 1 displaying 3, displayibinary ng bina plots ry plof ots the of the m main components ain components and and Sc O Scvs. 2O3 vs SiO . S.iO The 2. The analytical analytic results al re- 2 3 2 sults at 950 °C are for amphibole relicts, whereas at other temperatures, the compositions at 950 C are for amphibole relicts, whereas at other temperatures, the compositions represent those of the slag represent those of the slagphase. The phase. The slag ph slag phase ase exh exhibits ibits a ste a steady ady decrease in decrease in FeO due to FeO due the progression of reduction with to the progression of reduction with incrinc easi reasing ng tempe temperatur rature. Bas e. eBased d on the on se r these esults results, , the sl the ag slag compositions at 1050 C with higher CaO content indicate the dissolution of added compositions at 1050 °C with higher CaO content indicate the dissolution of added CaO CaO into the initial slag. As the mineral reactions proceeded with increasing temperature, into the initial slag. As the mineral reactions proceeded with increasing temperature, lead- ing leading event eventually ually to the d toethe composit decomposition ion and meand ltingmelting of all clinopyroxe of all clinopyr ne, pot oxene, assium potassium feldspar, feldspar, and plagioclase into the slag by 1250 C, the composition of the slag became and plagioclase into the slag by 1250 °C, the composition of the slag became homogenized homogenized with respect to the main components, with a steady decrease in FeO. with respect to the main components, with a steady decrease in FeO. Metals 2022, 12, 709 18 of 20 Metals 2022, 12, x FOR PEER REVIEW 18 of 20 Figure 13. Binary plots of FeO, CaO, Al2O3, and Sc2O3 (wt%) for amphibole relicts (blue) at 950 °C Figure 13. Binary plots of FeO, CaO, Al O , and Sc O (wt%) for amphibole relicts (blue) at 950 C 2 3 2 3 and slag at higher temperatures. and slag at higher temperatures. 4. Conclusions 4. Conclusions If pyrometallurgical treatment is to be considered for Kiviniemi-type ferrous scandium If pyrometallurgical treatment is to be considered for Kiviniemi-type ferrous scan- concentrates, one of the challenges is to optimize and modify the composition of highly dium concentrates, one of the challenges is to optimize and modify the composition of viscous slag to promote the reduction of FeO and segregation of metal without excessively highly viscous slag to promote the reduction of FeO and segregation of metal without diluting the slag Sc O content. According to the results of this study, the reduction of the 2 3 excessively diluting the slag Sc2O3 content. According to the results of this study, the re- ferrous oxide component of the slag and segregation of metallic iron was improved with duction of the ferrous oxide component of the slag and segregation of metallic iron was moderate additions of CaF and CaO, which lowered the liquidus temperature and viscosity improved with moderate additions of CaF2 and CaO, which lowered the liquidus temper- of the slag. The Sc O component was maintained and enriched in the slag. Although CaF 2 3 2 ature and viscosity of the slag. The Sc2O3 component was maintained and enriched in the increased the derivative conversion rates at a slightly lower temperature region, the use of slag. Although CaF2 increased the derivative conversion rates at a slightly lower temper- CaO instead of CaF would be preferable in industrial applications. Despite the variations ature region, the use of CaO instead of CaF2 would be preferable in industrial applications. in the modal mineralogy of the concentrate feed used in this study, the high-temperature Despite the variations in the modal mineralogy of the concentrate feed used in this study, behavior of the concentrates is essentially similar, though the main reduction stage is the high-temperature behavior of the concentrates is essentially similar, though the main initiated at a slightly higher temperature (~1000–1030 C) for the concentrates with less reduction stage is initiated at a slightly higher temperature (~1000–1030 °C) for the con- amphibole and a higher amount of nonferrous gangue minerals. The beginning of the main centrates with less amphibole and a higher amount of nonferrous gangue minerals. The reduction stage with the formation of initial slag is dominated by the decomposition and beginning of the main reduction stage with the formation of initial slag is dominated by reduction of amphibole and garnet. The ﬁnal decomposition of clinopyroxene, the other the decomposition and reduction of amphibole and garnet. The final decomposition of main host for Sc O , occurs at a signiﬁcantly higher temperature than that of amphibole, 2 3 clinopyroxene, the other main host for Sc2O3, occurs at a significantly higher temperature with structures persisting until 1150 C. This study complements the pyrometallurgical part than that of amphibole, with structures persisting until 1150 °C. This study complements of our ongoing project, conﬁrming the smelting reduction characteristics of the Kiviniemi- the pyrometallurgical part of our ongoing project, confirming the smelting reduction char- type ferrous scandium concentrates. Only after the complete decomposition and melting of acteristics of the Kiviniemi-type ferrous scandium concentrates. Only after the complete silicates and dissolution of unreduced FeO into the slag can the ﬁnal FeO reduction from decomposition and melting of silicates and dissolution of unreduced FeO into the slag can slag be achieved by carbon, accompanied by segregation and accumulation of metallic iron. the final FeO reduction from slag be achieved by carbon, accompanied by segregation and accumulation of metallic iron. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/met12050709/s1, Table S1: Modal compositions and deportment Supplementary Materials: The following supporting information can be downloaded at: of the main components in Kiviniemi concentrates. Table S2: Summary of the metal EPMA analytical www.mdpi.com/xxx/s1, Table S1: Modal compositions and deportment of the main components in data. Table S3: Summary of the ﬁrst set of slag EPMA analytical data for experiments 6–14. Figure S1: Kiviniemi concentrates. Table S2: Summary of the metal EPMA analytical data. Table S3: Summary Comparison of the mass change and derivative conversion rates with 100% CO and 95% Ar + 5% H2. of the first set of slag EPMA analytical data for experiments 6–14. Figure S1: Comparison of the mass Figure S2: Details of mass change rates with 95% Ar + 5% H at various end temperatures. Figure S3: change and derivative conversion rates with 100% CO and 95% Ar + 5% H2. Figure S2: Details of XRD interpretations at selected temperatures. mass change rates with 95% Ar + 5% H2 at various end temperatures. Figure S3: XRD interpretations at selected temperatures. Metals 2022, 12, 709 19 of 20 Author Contributions: Conceptualization, R.K. and P.T.; methodology, R.K., P.T. and E.-P.H.; inves- tigation, R.K. and T.K.; resources, P.T. and T.F.; writing—original draft preparation, R.K.; writing— review and editing, E.-P.H., P.T., S.L. and T.F.; visualization, R.K. and E.-P.H.; supervision, S.L. and T.F.; project administration, T.F.; funding acquisition, R.K. and P.T. All authors have read and agreed to the published version of the manuscript. Funding: This research has been funded by The Foundation for Research of Natural Resources in Finland, grant number 20210019. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: This research has been funded by The Foundation for Research of Natural Resources in Finland. Support and advice from the doctoral training follow-up group members, Jussi Liipo and Tapio Halkoaho, are highly appreciated. 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Published: Apr 21, 2022

Keywords: ferrous scandium concentrate; reduction; slag; modification

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Slag Modification in Reduction of Kiviniemi Ferrous Scandium Concentrates

Slag Modification in Reduction of Kiviniemi Ferrous Scandium Concentrates

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Slag Modification in Reduction of Kiviniemi Ferrous Scandium Concentrates

Slag Modification in Reduction of Kiviniemi Ferrous Scandium Concentrates

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