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Comparison of Grinding Characteristics of Converter Steel Slag with and without Pretreatment and Grinding Aids

Comparison of Grinding Characteristics of Converter Steel Slag with and without Pretreatment and... applied sciences Article Comparison of Grinding Characteristics of Converter Steel Slag with and without Pretreatment and Grinding Aids 1 , 2 1 2 Jihui Zhao *, Dongmin Wang , Peiyu Yan and Wenping Li Institute of Building Materials, Department of Civil Engineering, Tsinghua University, Beijing 100084, China; yanpy@tsinghua.edu.cn School of Chemical & Environmental Engineering, China University of Mining & Technology, Beijing 100083, China; wangdongmin-2008@163.com (D.W.); lwpchina@163.com (W.L.) * Correspondence: zhaojihui324@163.com; Tel.: +86-159-0124-5174 Academic Editor: Stefano Invernizzi Received: 10 June 2016; Accepted: 8 August 2016; Published: 28 October 2016 Abstract: The converter steel slag cannot be widely used in building materials for its poor grindability. In this paper, the grinding characteristics of untreated and pretreated (i.e., magnetic separation) steel slag were compared. Additionally, the grinding property of pretreated steel slag was also studied after adding grinding aids. The results show that the residues (i.e., oversize substance) that passed a 0.9 mm square-hole screen can be considered as the hardly grinding phases (HGP) and its proportion is about 1.5%. After the initial 20 min grinding, the RO phase (RO phase is a continuous solid solution which is composed of some divalent metal oxides, such as FeO, MgO, MnO, CaO, etc.), calcium ferrite, and metallic iron phase made up most of the proportion of the HGP, while the metallic iron made up the most component after 70 min grinding. The D50 of untreated steel slag could only reach 32.89 m after 50 min grinding, but that of pretreated steel slag could reach 18.16 m after the same grinding time. The grinding efficiency of steel slag was obviously increased and the particle characteristics were improved after using grinding aids (GA), especially the particle proportions of 3–32 m were obviously increased by 7.24%, 7.22%, and 10.63% after 40 min, 50 min, and 60 min grinding, respectively. This is mainly because of the reduction of agglomeration and this effect of GA was evidenced by SEM (scanning electron microscope) images. Keywords: steel slag; grinding property; particle characteristics; mineral phase; hardly grinding phases; magnetic separation; grinding aids; agglomeration 1. Introduction Steel slag is an industrial solid waste generated in the steel-making process, but it is rich in dicalcium silicate (C S) and tricalcium silicate (C S) in mineral compositions, which is similar to 2 3 cement clinker [1–4]. Based on its mineral compositions, steel slag has great potential for application as supplementary cementitious materials (also known as mineral admixture) of cement and concrete [5–8]. Generally, only after being ground into powder, could steel slag have hydration activity and be used as supplementary cementitious materials [9,10]. The smaller the particle size of steel slag powder, the higher its hydration activity [11,12]. However, the grindability of steel slag is much worse than that of other mineral admixtures such as fly ash and granulated blast-furnace slag, seriously impeding the preparation efficiency of steel slag powder and its application performance in building materials. Therefore, its grinding property is an important consideration in the application of steel slag. In the grinding process of a solid, particles characteristics such as size, distribution, bulk density, and dispersion are in constant change on a macroscopic scale, and the internal chemical bonds of Appl. Sci. 2016, 6, 237; doi:10.3390/app6110237 www.mdpi.com/journal/applsci Appl. Sci. 2016, 6, 237 2 of 15 particles are constantly broken on a microscopic scale [13–15]. Steel slag has poor grindability because of a large amount of iron oxides and continuous solid solution composed of some divalent metal oxides, which form a dense structure in the steel slag [16,17]. Currently, the researches on improvement of grinding efficiency for steel slag mainly focus on the optimization of grinding equipment—such as vertical mill and roller mill, while there are few researches on other measures, such as removal of hardly grinding phases (HGP) or the use of chemical admixtures. Although vertical mills have been applied in grinding of steel slag, many application researches indicate that steel slag powder prepared by vertical mills have poor particle morphology—such as flat, needle, and irregular shapes, resulting in the ball mill still being chosen by many manufacturers to prepare steel slag powder in China. Meanwhile, some studies indicate that metallic iron in steel slag is one main reason causing its poor grinding property. However, in different grinding periods, the factors influencing grinding efficiency usually change due to different grinding characteristics [18–20], but to date these factors have not been yet been fully studied. Some researches also show that the split-phase phenomenon occurs in the grinding process of steel slag, resulting in ground steel slag powder having some differences in chemical and mineral compositions during different grinding periods [21,22]. So, it is necessary to reveal the factors influencing the grinding efficiency of steel slag from studying grinding characteristics, hardly grinding phases, and split-phase phenomenon during different grinding periods. Moreover, in the grinding process of a solid, other phenomena such as agglomeration of fine particles and re-healing of particle fracture surfaces result in a lower grinding efficiency and higher energy consumption [23–26]. In recent years, grinding aids, most of which are polar organic compounds, are extensively applied to weaken agglomeration of fine particles and improve grinding efficiency of cement [27]. Therefore, the use of grinding aids is also an important way to improve preparation efficiency of steel slag powder. Based on the abovementioned discussion, the hardly grinding phases of converter steel slag were revealed from the proportion, morphology, mineral, and chemical compositions. Then the grinding characteristics of untreated and pretreated (i.e., magnetic separation) steel slag were compared from the proportion of iron phases, grinding efficiency, particle size, and particle distribution and morphology. Further, the grinding property of pretreated steel slag was also studied after adding organic grinding aids and its positive role and mechanism were discussed here as well. 2. Materials and Methods 2.1. Materials The converter steel slag used was provided from Laiwu Steel Corporation (Laiwu, China). The chemical compositions of steel slag, which were determined by X-ray fluorescence (XRF) analysis, (Rh, 40 kV, 70 mA), are given in Table 1. The mineral phases of steel slag, which were analyzed by X-ray diffraction (XRD), using a D6000 diffractometer with nickel-filtered Cu K l radiation (= 1.5405 Å, 40 kV, and 40 mA) from Shimadzu company (Kyoto, Japan), are given in Figure 1. The organic grinding aid used, produced by Sinopharm Chemical Reagent Beijing Co., Ltd. (Beijing, China), was chemical grade glycerol. Table 1. Chemical compositions of converter steel slag (%). CaO SiO Al O Fe O MgO K O SO P O LOI 2 2 3 2 3 2 3 2 5 39.67 21.71 1.58 25.52 4.92 0.02 0.18 1.70 0.32 Appl. Sci. 2016, 6, 237 3 of 15 Appl. Sci.2016, 9, 237  3 of 16  2 1- C S 2- C S 3- C A 2 3 3 4- C AF 5-C F 6- RO 4 2 7- 3CaO Fe O  3SiO 2 3 2 8- 2CaO MgO 2SiO 9- 3CaO 2TiO 10-Fe 11- f-CaO 12- f-MgO 13- CaCO 1 7 1 10 2 4 3 1 7 1 10 13 2 1 1 2 2 10 20 30 40 50 60 70 80 2( ) Figure 1. X‐ray diffraction (XRD) pattern of converter steel slag.  Figure 1. X-ray diffraction (XRD) pattern of converter steel slag. 2.2. Experimental Methods  2.2. Experimental Methods 2.2.1. Observation and Identification of Mineral Phases for Converter Steel Slag  2.2.1. Observation and Identification of Mineral Phases for Converter Steel Slag The steel slag sample needed to be prepared for the morphology observation of mineral phase.  The steel slag sample needed to be prepared for the morphology observation of mineral phase. Firstly, the surface of the 3–8 mm blocky steel slag was burnished using P200#, P400#, P600#, P800#,  Firstly, the surface of the 3–8 mm blocky steel slag was burnished using P200#, P400#, P600#, P800#, P1000#,  and  P1200#  sandpapers  in  turn,  and  was  then  polished  by  a  polisher  (UNIPOL‐830,  P1000#, and P1200# sandpapers in turn, and was then polished by a polisher (UNIPOL-830, Shenyang Shenyang Kejing Instrument Co., Ltd., Shenyang, China). The mineral phases were observed and  Kejing Instrument Co., Ltd., Shenyang, China). The mineral phases were observed and identified by identified  by  back‐scattered  electron  (BSE)  imaging  and  energy  dispersive  X‐ray  spectroscopy  back-scattered electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDX), respectively, (EDX),  respectively,  using  a  scanning  electron  microscope  (Quanta  200  FEG,  FEI  Company,  using a scanning electron microscope (Quanta 200 FEG, FEI Company, Hillsboro, OR, USA). In addition, Hillsboro, OR, USA). In addition, the hardly grinding phases were observed using a metallographic  the hardly grinding phases were observed using a metallographic microscope (BA210Met, Motic China microscope (BA210Met, Motic China Group Co., Ltd., Xiamen, China).  Group Co., Ltd., Xiamen, China). 2.2.2. Measurement of Vickers Hardness of Mineral Phases for Converter Steel Slag  2.2.2. Measurement of Vickers Hardness of Mineral Phases for Converter Steel Slag The Vickers hardness of different mineral phases for steel slag was measured using a Vickers  The Vickers hardness of different mineral phases for steel slag was measured using a Vickers hardness tester (HV‐1000, Shanghai Materials Tester Factory, Shanghai, China), and the hardness  hardness tester (HV-1000, Shanghai Materials Tester Factory, Shanghai, China), and the hardness value value of each mineral phase was determined by taking the average of multiple points.  of each mineral phase was determined by taking the average of multiple points. 2.2.3. Procedure of Grinding Experiment for Steel Slag 2.2.3. Procedure of Grinding Experiment for Steel Slag  Grinding Grinding experiments experimentsof  ofsteel   steel slag   slag wer   were e carried   carried out  out by laboratory by  laboratory ball mill ball (SM-500, mill  (SMW‐50 uxi 0, Jianyi Wuxi  Experiment Instrument Co., Ltd., Wuxi, China). The type of the ball mill was F500 mm  500 mm, Jianyi Experiment Instrument Co., Ltd., Wuxi, China). The type of the ball mill was Ф500 mm × 500  48 mm, r/min  48 r/min , and,the  and grinding  the grin media ding media was composed  was compo of 60 sekg d of steel  60 kg balls ste (F el40 bamm, lls (ΦF40 50 mm, mm, ΦF50 60 mm mm,, Φ and 60  F70 mm) and 40 kg small steel forgings (F25 mm  35 mm). mm, and Φ70 mm) and 40 kg small steel forgings (Φ25 mm × 35 mm).   The The steel steel slag slag was was firstly firstly cr crushed ushed to to less less than than 55 mm mm by by aajaw  jawcr cr usher usher(PE  (PE60 60 × 100, 100, Shanghai Shanghai  Longshi Machinery Co., Ltd., Shanghai, China) before the grinding experiment. The weight of steel Longshi Machinery Co., Ltd., Shanghai, China) before the grinding experiment. The weight of steel  slag slag for foreach  eachgrinding  grinding experiment  experiment was  wa3s kg 3 kg and and the the grinding  grinding times  tim wer es were e 10 min,  10 min 20 min, , 20 mi 30n min, , 30 min, 40 min,  40  50 min, 60 min, and 70 min. In addition, for the grinding aids experiment, 0.05% grinding aid was min, 50 min, 60 min, and 70 min. In addition, for the grinding aids experiment, 0.05% grinding aid  added was add into ed the intotest  thegr test oup gro toucompar p to compar e with e with the blank  the blank group gro (i.e., up without (i.e., without any grinding  any grinding aids). aids).  2.2.4. Test Methods of Particle Size and Distribution of Ground Steel Slag Powder 2.2.4. Test Methods of Particle Size and Distribution of Ground Steel Slag Powder  The specific surface area of ground steel slag powder was measured by Blaine method, conforming The  specific  surface  area  of  ground  steel  slag  powder  was  measured  by  Blaine  method,  to Chinese National Standard GB/T8074-2008. The sieving residue of steel slag powder was measured conforming to Chinese National Standard GB/T8074‐2008. The sieving residue of steel slag powder  by sieving analysis method with a 45 m square-hole screen, conforming to the Chinese National was measured  by  sieving  analysis  method  with  a  45 μm  square‐hole  screen,  conforming to  the  Standard GB/T1345-2005. The particle size distribution was measured by laser-scattering method, Chinese  National  Standard  GB/T1345‐2005.  The  particle  size  distribution  was  measured  by  conforming to the Chinese Industry Standard JC/T721-2006. laser‐scattering method, conforming to the Chinese Industry Standard JC/T721‐2006.  2.2.5. Observation of Particle Morphology of Ground Steel Slag Powder  Appl. Sci. 2016, 6, 237 4 of 15 Appl. Sci.2016, 9, 237  4 of 16  2.2.5. Observation of Particle Morphology of Ground Steel Slag Powder The particle morphology of ground steel slag powder was observed with a scanning electron  The particle morphology of ground steel slag powder was observed with a scanning electron microscope (Quanta 200 FEG, FEI Company, Hillsboro, OR, USA) under high vacuum condition.  microscope (Quanta 200 FEG, FEI Company, Hillsboro, OR, USA) under high vacuum condition. 2.2.6. Test Methods of the Angle of Repose of Ground Steel Slag Powder  2.2.6. Test Methods of the Angle of Repose of Ground Steel Slag Powder The  angle  of  repose  of  ground  steel  slag  powder  was  tested  conforming  to  the  Chinese  The angle of repose of ground steel slag powder was tested conforming to the Chinese National National  Standard  GB/T11986‐1989,  as follows:  steel slag  powder  was  poured  into  a  funnel,  and  Standard GB/T11986-1989, as follows: steel slag powder was poured into a funnel, and then powder then powder from the funnel fell onto and coated the disc below the funnel. Then the height, h, of  from the funnel fell onto and coated the disc below the funnel. Then the height, h, of the powder layer the powder layer and the radius, R, of the disc were measured, thus the angle of repose, θ, of steel  and the radius, R, of the disc were measured, thus the angle of repose, , of steel slag powder was slag powder was obtained according to the formula (tan θ = h/R).  obtained according to the formula (tan  = h/R). 3. Results and Discussion  3. Results and Discussion 3.1. Mineral Phases’Characteristics of Converter Steel Slag  3.1. Mineral Phases’Characteristics of Converter Steel Slag As shown in Figure 2(a), the morphologies of mineral phases in steel slag show different grey  As shown in Figure 2a, the morphologies of mineral phases in steel slag show different grey levels levels under BSE images, such as black, grey‐black, grey, light‐grey, and white‐bright in grey level,  under BSE images, such as black, grey-black, grey, light-grey, and white-bright in grey level, which which exhibit different shapes as well, such as round shape, leaf‐like shape, hexagonal‐plate shape,  exhibit different shapes as well, such as round shape, leaf-like shape, hexagonal-plate shape, irregular irregular  shape,  and  so  on.  The  compositions  of  these  minerals  with  different  grey  levels  were  shape, and so on. The compositions of these minerals with different grey levels were determined using determined using EDX analysis, and the results are shown in Figure 2(b). It can be seen from EDX  EDX analysis, and the results are shown in Figure 2b. It can be seen from EDX analysis results that the analysis results that the minerals  with  grey  levels  of  black and grey‐black are mainly the silicate  minerals with grey levels of black and grey-black are mainly the silicate minerals phases which are minerals phases which are composed of oxygen, silicon, and calcium elements; the irregular mineral  composed of oxygen, silicon, and calcium elements; the irregular mineral phases with grey level of phases with grey level of light‐grey are mainly RO phase which is composed of oxygen, magnesium,  light-grey are mainly RO phase which is composed of oxygen, magnesium, calcium, manganese, and calcium,  manganese,  and  iron  elements;  the  irregular‐shaped  minerals  with  grey  level  of  grey,  iron elements; the irregular-shaped minerals with grey level of grey, which are filled in light-grey and which are filled in light‐grey and black minerals, are the calcium ferrite phases mainly composed of  black minerals, are the calcium ferrite phases mainly composed of oxygen, calcium, iron, aluminum, oxygen, calcium, iron, aluminum, and silicon elements; the round granular‐shaped minerals with  and silicon elements; the round granular-shaped minerals with grey level of white-bright are the grey  level  of  white‐bright  are  the  metallic  iron  phase.  The  above  results  show  that  the  mineral  metallic iron phase. The above results show that the mineral phases of steel slag mainly contain silicate phases of steel slag mainly contain silicate mineral phase, RO phase, calcium ferrite phase, and a  mineral phase, RO phase, calcium ferrite phase, and a small amount of metallic iron phase, etc. It is small amount of metallic iron phase, etc. It is also evident from XRD analysis presented in Figure 1  also evident from XRD analysis presented in Figure 1 that the mineral phases converter steel slag that the mineral phases converter steel slag mainly contains C2S, C3S, RO, 3CaO∙Fe2O3∙3SiO2 (i.e.,  mainly contains C S, C S, RO, 3CaOFe O 3SiO (i.e., calcium ferrite phases) and so on. (Comment: 2 3 2 3 2 calcium  ferrite  phases)  and  so  on.  (Comment:  RO  phase  is  a  continuous  solid  solution  which  is  RO phase is a continuous solid solution which is composed of some divalent metal oxides, such as composed of some divalent metal oxides, such as FeO, MgO, MnO, CaO, etc.)  FeO, MgO, MnO, CaO, etc.) 2 6 100μ m 100μ m 50μ m (a)  Figure 2. Cont. Appl. Sci. 2016, 6, 237 5 of 15 Appl. Sci.2016, 9, 237  5 of 16  Average result Average result Average result 1 2 3 Ele ment Wt % At % Ele ment Wt % At % Ele ment Wt % At % O K 29.34 48.97 O K 31.12 51.55 O K 17.89 35.77 MgK 00.22 00.24 MgK 01.10 01.20 MgK 22.02 28.97 Al K 00.82 00.81 AlK 01.95 01.91 CaK 03.49 02.79 Si K 14.59 13.87 SiK 11.33 10.69 MnK 05.29 03.08 P K 03.78 03.26 P K 02.12 01.81 FeK 51.31 29.39 CaK 45.35 30.21 CaK 43.91 29.03 FeK 03.06 01.47 FeK 05.32 02.52 ZnK 02.84 01.16 ZnK 03.16 01.28 Average result Ele ment Wt % At % Average result O K 22.40 43.45 Ele ment Wt % At % Average result MgK 01.24 01.58 O K 18.40 37.52 Ele ment Wt % At % AlK 06.09 07.00 MgK 18.20 24.43 O K 01.42 04.77 SiK 00.99 01.09 CaK 03.88 03.16 CaK 01.36 01.82 CaK 35.34 27.36 Cr K 01.15 00.72 FeK 97.22 93.41 Ti K 06.42 04.16 MnK 06.81 04.05 Cr K 01.33 00.79 FeK 51.55 30.11 FeK 26.19 14.56 (b)  Figure  2.  Morphologies  and  identification  of  mineral  phases  in  converter  steel  slag.  (a):  Figure 2. Morphologies and identification of mineral phases in converter steel slag. (a): Back-scattered Back‐scattered  electron  (BSE)  images  of  various  mineral  phases;  (b)  Energy  dispersive  X‐ray  electron (BSE) images of various mineral phases; (b) Energy dispersive X-ray spectroscopy (EDX) spectroscopy (EDX) analysis results of various mineral phases.  analysis results of various mineral phases. 3.2. Determination of Hardly GrindingPhases (HGP) in Converter Steel Slag   3.2. Determination of Hardly Grinding Phases (HGP) in Converter Steel Slag As the grindability of various mineral phases in  steel  slag  have great differences, the  easily  As the grindability of various mineral phases in steel slag have great differences, the easily grinding  phases  (denoted  as  EGP)  are  firstly  being  ground  to  fine  powder,  while  the  poor  grinding phases (denoted as EGP) are firstly being ground to fine powder, while the poor grindability grindability  mineral  phases  are  difficult  to  grind  down,  which  seriously  affects  the  grinding  mineral phases are difficult to grind down, which seriously affects the grinding efficiency. However, efficiency. However, different mineral phases in steel slag are very difficult to be separated out, so it  different mineral phases in steel slag are very difficult to be separated out, so it is impossible to is impossible to determine what is an easily  grinding  phase  and hardly  grinding phase  from the  determine what is an easily grinding phase and hardly grinding phase from the grindability index grindability index of various mineral phases. In this paper, a simple method for determination of  of various mineral phases. In this paper, a simple method for determination of the hardly grinding the  hardly  grinding  phase  in  steel  slag  by  oversize  substance  (i.e.,  residue)  is  provided.  After  phase in steel slag by oversize substance (i.e., residue) is provided. After grinding and then screening grinding and then screening of steel slag powder, the oversize substance obtained can be considered  of steel slag powder, the oversize substance obtained can be considered as the hardly grinding phases as the hardly grinding phases (denoted as HGP) in steel slag. In order to determine the HGP in the  (denoted as HGP) in steel slag. In order to determine the HGP in the grinding process, the steel slag grinding process, the steel slag powder after 10, 20, 30, 40, 50, 60, and 70 min grinding were screened  powder after 10, 20, 30, 40, 50, 60, and 70 min grinding were screened with a 0.9 mm square-hole with  a  0.9  mm  square‐hole  screen,  and  then  the  proportion,  morphology,  and  compositions  of  screen, and then the proportion, morphology, and compositions of oversized substances (i.e., HGP) oversized substances (i.e., HGP) were analyzed.  were analyzed. 3.2.1. Proportion of HGP in Converter Steel Slag  3.2.1. Proportion of HGP in Converter Steel Slag The proportions of oversized substances after different grinding times are shown in Figure 3.  The proportions of oversized substances after different grinding times are shown in Figure 3. Form  the  figure,  with  the  increasing  of  grinding  time  of  steel  slag,  the  proportion  of  oversized  Form the figure, with the increasing of grinding time of steel slag, the proportion of oversized substances  that passed the 0.9 mm square‐hole screen rapidly declined in the  range  of 10–20 min  substances that passed the 0.9 mm square-hole screen rapidly declined in the range of 10–20 min grinding  time,  and  then  starts  to  slowly  decline  (starting  from  30  min  grinding  time),  until  the  grinding time, and then starts to slowly decline (starting from 30 min grinding time), until the proportion finally  reaches about 1.5%, which indicates that this part of oversize  substances  is  the  proportion finally reaches about 1.5%, which indicates that this part of oversize substances is the HGP in converter steel slag. In other words, the proportion of HGP determined by residue method  HGP in converter steel slag. In other words, the proportion of HGP determined by residue method of of 0.9 mm square‐hole screen is about 1.5%.  0.9 mm square-hole screen is about 1.5%. Appl. Sci. 2016, 6, 237 6 of 15 Appl. Appl.  Sci. Sci.2016 2016,,  99,,  237 237   66  of of  16 16   4.0 4.0 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 10 20 30 40 50 60 70 10 20 30 40 50 60 70 G G rriindi nding ng t tiim m ee (m (m iin n)) Figure 3. Proportion of oversize substances of steel slag powder vs. grinding time. Figure 3. Proportion of oversize substances of steel slag powder vs. grinding time.  Figure 3. Proportion of oversize substances of steel slag powder vs. grinding time.  3.2.2. Morphology and Vickers Hardness of HGP in Converter Steel Slag 3.2.2. Morphology and Vickers Hardness of HGP in Converter Steel Slag   3.2.2. Morphology and Vickers Hardness of HGP in Converter Steel Slag   As shown in Figure 4, the morphology of HGP in converter steel slag mainly shows three kinds As shown in Figure 4, the morphology of HGP in converter steel slag mainly shows three kinds  As shown in Figure 4, the morphology of HGP in converter steel slag mainly shows three kinds  of grey levels during the initial grinding period (20 min grinding time), while basically exhibiting of grey levels during the initial grinding period (20 min grinding time), while basically exhibiting  of grey levels during the initial grinding period (20 min grinding time), while basically exhibiting  one grey level during the middle–later grinding periods (i.e., 50 min and 70 min grinding times). one  grey  level  during the middle–later grinding periods (i.e.,  50 min and 70 min grinding times).  one  grey  level  during the middle–later grinding periods (i.e.,  50 min and 70 min grinding times).  Based on the identification and analysis results of mineral phases in Section 3.1, it can be known Based on the identification and analysis results of mineral phases in section 3.1, it can be known that  Based on the identification and analysis results of mineral phases in section 3.1, it can be known that  that the mineral phases with three kinds of grey levels are mainly the RO phase, calcium ferrite, and the mineral  phases  with  three  kinds  of  grey  levels are mainly the  RO phase, calcium  ferrite, and  the mineral  phases  with  three  kinds  of  grey  levels are mainly the  RO phase, calcium  ferrite, and  metallic iron phase. So, the HGP of converter steel slag during the initial grinding period is composed metallic  iron  phase.  So,  the  HGP  of  converter  steel  slag  during  the  initial  grinding  period  is  metallic  iron  phase.  So,  the  HGP  of  converter  steel  slag  during  the  initial  grinding  period  is  of RO phase, calcium ferrite, and metallic iron phase, while the HGP is mainly the metallic iron phase composed  of  RO  phase,  calcium  ferrite,  and  metallic  iron  phase,  while  the  HGP  is  mainly  the  composed  of  RO  phase,  calcium  ferrite,  and  metallic  iron  phase,  while  the  HGP  is  mainly  the  during the later grinding period, and the longer grinding time, the higher the proportion of metallic metallic iron phase during the later grinding period, and the longer grinding time, the higher the  metallic iron phase during the later grinding period, and the longer grinding time, the higher the  iron phase. proportion of metallic iron phase.  proportion of metallic iron phase.  3 3 3 3 3 3 3 3 3 3 3 33 3 3 3 3 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 22 2 3 3 3 3 3 3 2 3 3 3 3 3 3 3 33 3 3 3 33 2 2 2 2 2 2 2 2 2 2 2 2 2 22 2 2 2 2 22 1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 22 2 2 2 22 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 1 1 1 1 1 3 3 33 3 1 1 1 1 2 2 1 1 1 1 1 1 1 11 1 1 1 2 2 22 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 22 2 2 22 2 2 2 2 22 2 2 22 2 3 3 3 3 3 3 3 3 3 33 (a)  (b) (a)  (b) 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 44 4 4 4 44 4 4 4 4 4 4 4 4 4 4 4 44 4 4 4 4 4 4 4 44 4 4 4 4 4 4 4 4 (c)  (d) (c)  (d) Figure 4. Morphologies of oversize substances of steel slag powder vs. grinding time: (a) 0 min; (b)  Figure 4. Morphologies of oversize substances of steel slag powder vs. grinding time: (a) 0 min; (b)  Figure 4. Morphologies of oversize substances of steel slag powder vs. grinding time: (a) 0 min; 20 min; (c) 50 min; (d) 70 min.  20 min; (c) 50 min; (d) 70 min.  (b) 20 min; (c) 50 min; (d) 70 min. Content of oversize products for 0.9mm screen (%) Content of oversize products for 0.9mm screen (%) Appl. Sci. 2016, 6, 237 7 of 15 To reflect HGP characteristics from another point of view, the Vickers hardness of selected regions in Figure 4 were tested and results are shown in Table 2. To some extent, the hardness of the mineral phase can reflect its grindability. Results of Vicker hardness show that the RO phase and calcium ferrite phase have high hardness compared with the silicate phase, which may be one reason why these two mineral phases are HGP. Meanwhile, metallic iron phase with very low hardness is also HGP because of its high flexibility. Thus, the HGP is explained from Vickers hardness of mineral phases. Table 2. Vickers hardness of selected region in Figure 4 (HV). Region number 1 2 3 4 Mineral phases Silicate phase Calcium ferrite phase RO phase Metallic iron phase Vickers hardness 187.3 335.0 298.1 29.4 3.2.3. Chemical Compositions of HGP in Converter Steel Slag The chemical compositions of oversized substances passed a 0.9 mm square-hole screen under different grinding times, which were determined by X-ray fluorescence analysis (XRF) and chemical titration, are shown in Table 3. The results show that the contents of CaO and SiO in HGP gradually decrease with the increase in grinding time, while the contents of metallic iron show a reverse trend, and the contents of Fe O and MgO are firstly increased and then decreased with the increasing of 2 3 grinding time. In the initial grinding period (10–20 min grinding), the chemical compositions of HGP mainly consist of CaO, SiO , Fe O , and MgO; in the middle grinding period (40–50 min grinding), 2 2 3 it mainly consists of Fe O and metallic iron; in the later grinding period (60–70 min grinding), metallic 2 3 iron takes up the most component. So the HGP have different chemical compositions during different grinding period. Table 3. Chemical compositions of hardly grinding phases (HGP) in steel slag vs. grinding time (%). Grinding time (min) CaO SiO Al O Fe O Fe MgO 2 2 3 2 3 10 32.45 12.05 1.15 34.80 3.53 8.43 20 26.12 8.26 1.06 37.07 7.60 12.29 30 20.30 5.81 0.92 38.13 13.36 13.07 40 14.70 4.16 0.75 39.08 23.10 10.13 50 6.14 3.05 0.62 32.12 45.53 7.20 60 5.19 2.03 0.51 18.09 65.33 4.12 70 4.04 1.14 0.44 11.25 76.20 3.21 3.3. Comparison of Grinding Characteristic between Untreated and Pre-treated Converter Steel Slag From the research result presented in the previous section, the iron-rich phases, especially metallic iron, are the main hardly grinding phases, so it is necessary to remove or recycle the iron-rich phases before the grinding process of steel slag. In this study, the grinding characteristics of untreated and pretreated converter steel slag were compared with respect to iron mineral phases, grinding efficiency, particle size distribution, and particle morphology (Comment: the pretreatment of steel slag is the preliminary magnetic separation and multistage screening and magnetic separation, as shown in Figure 5). Appl. Sci. 2016, 6, 237 8 of 15 Appl. Sci.2016, 9, 237  8 of 16  Untreated Steel slag after primary Steel slag after multi-stage First step multi-step converter magnetic separation screening and magnetic steel slag (PMS-steel slag) separation (MSMS-steel slag) Pre-milling Magnetic separation Crushing Magnetic separation Screening Recycli ng Recycli ng Scrap steel in slag Scrap steel and iron concentrate in slag Figure 5. The technical flow process of recycling iron‐rich phases in steel slag.  Figure 5. The technical flow process of recycling iron-rich phases in steel slag. 3.3.1. Total Analysis of Iron Mineral Phases in Converter Steel Slag After Pre‐treatment  3.3.1. Total Analysis of Iron Mineral Phases in Converter Steel Slag after Pre-treatment Total analysis results of iron mineral phases in converter steel slag after preliminary magnetic  Total analysis results of iron mineral phases in converter steel slag after preliminary magnetic separation (denoted  as PMS)  and  multistage magnetic separation (denoted  as SMS)  are shown in  Table 4.  separation (denoted as PMS) and multistage magnetic separation (denoted as SMS) are shown in Table 4. Appl. Sci.2016 From , 9, 237 the  results, it can be seen that the proportions of iron phases in steel slag are obviously 8 of  16 reduced after pretreatment. The proportion of metallic iron in steel slag is decreased from 2.38% to  Table 4. Total analysis results of iron mineral phases in converter steel slag. 1.18% by preliminary magnetic separation, and both the metallic iron proportion and total content  Untreated Steel slag after primary Steel slag after multi-stage First step multi-step converter magnetic separation screening and magnetic of iron  phases in steel slag are decreased by multistage magnetic separation, from 2.38% to 0.45%  Proportions of iron mineral phases (%) steel slag (PMS-steel slag) separation (MSMS-steel slag) Iron mineral phases and 18.26% to 9.20%, respectively, indicating that the effect of pretreatment on iron‐rich phases is  Untreated steel slag PMS-steel slag SMS-steel slag obvious. The removal of iron‐rich phases in steel slag is a very favorable factor for preparation and  Metallic iron & magnetite 2.38 1.18 0.45 application of steel slag powder.  Hematite/limonite 13.11 13.05 6.67 Sulfide 0.04 0.04 0.04 Table 4. Total analysis results of iron mineral phases in converter steel slag.  Siderite 2.52 2.26 1.98 Crushing Magnetic separation Screening Pre-milling Magnetic separation Proportions of iron mineral phases (%)  Iron silicate 0.21 0.10 0.06 Iron mineral phases  Total Untreated 18.26  steel slag PMS16.63 ‐steel slag SMS‐st 9.20 eel slag Recycli ng Recycli ng Metallic iron & magnetite  2.38  1.18  0.45  Scrap steel in slag Scrap steel and iron concentrate in slag Hematite/limonite  13.11  13.05  6.67  From the results, it can be seen that the proportions of iron phases in steel slag are obviously Sulfide  0.04  0.04  0.04  reduced after pretreatment. The proportion of metallic iron in steel slag is decreased from 2.38% to Figure 5. Siderite The t   echnical flow process 2.52 of re   cycling iron-rich p 2.h26 ases   in steel slag. 1.98  Iron silicate  0.21  0.10  0.06  1.18% by preliminary magnetic separation, and both the metallic iron proportion and total content of Total  18.26  16.63  9.20  3.3.1. Total Analysis of Iron Mineral Phases in Converter Steel Slag After Pre-treatment iron phases in steel slag are decreased by multistage magnetic separation, from 2.38% to 0.45% and 18.26% to 9.20%, respectively, indicating that the effect of pretreatment on iron-rich phases is obvious. Total analysis results of iron mineral phases in converter steel slag after preliminary magnetic 3.3.2. Grinding Efficiency of Untreated and Pretreated Steel Slag  The removal of iron-rich phases in steel slag is a very favorable factor for preparation and application separation (denoted as PMS) and multistage magnetic separation (denoted as SMS) are shown in The specific surface areas of untreated and pretreated steel slag under different grinding times  Table 4. of steel slag powder. are shown in Figure 6.  3.3.2. Grinding Efficiency of Untreated and Pretreated Steel Slag 3.3.2. Grinding Efficiency of Untreated and Pretreated Steel Slag 500 500 Untreated steel slag Untreated steel slag The specific surface areas of untreated and pretreated 450 steel slag under different grinding times 450 Pre-treated steel slag The specific surface areas of untreated and pretreated steel slag under different grinding times are Pre-treated steel slag are shown in Figure 6. 400 shown in Figure 6. SSA=157.44x-207.28 R =0.9906 500 500 Untreated steel slag Untreated steel slag 450 Pre-treated steel slag 250 SSA=124.7x-155.85 Pre-treated steel slag R =0.9605 20 400 0 SSA=157.44x-207.28 R =0.9906 2.0 2.5 3.0 3.5 4.0 4.5 10 20 30 40 50 60 70 x=lnt Grinding time (min) 250 SSA=124.7x-155.85 R =0.9605 2.0 2.5 3.0 3.5 4.0 4.5 10 20 30 40 50 60 70 x=lnt Grinding time (min) (b) (a) Figure 6. Specific surface areas (SSA) of steel slag under different grinding time: (a) SSA vs. grinding Figure 6. Specific surface areas (SSA) of steel slag under different grinding time: (a) SSA vs. grinding time; (b) linear fitting of SSA. time; (b) linear fitting of SSA. As shown in Figure 6, the specific surface areas of untreated and pretreated steel slag are gradually increased with increasing grinding time, and starting from 50 min grinding time, the tendency to increase is slowed down due to the agglomeration of fine particles. The specific surface area of pretreated steel slag is obviously higher than that of untreated steel slag after the same grinding time. For example, the specific surface area of untreated steel slag is only 360 m /kg after 60 min grinding, while that of pretreated steel slag reaches 361 m /kg after 40 min grinding, thus 20 min grinding time is saved. The relationship of specific surface area vs. grinding time was fitted, and the result is as follows: SSA=− 124.7 ln t 155.85 (R =0.9605) (Untreated steel slag) (1) SSA=− 157.44 ln t 207.28 (R =0.9906) (Pretreated steel slag) (2) Specific surface area (m /kg) Specific surface area (m /kg) 2 2 Specific surface area (m S /kg pecifi ) c surface area (m /kg) Appl. Sci. 2016, 6, 237 9 of 15 As shown in Figure 6, the specific surface areas of untreated and pretreated steel slag are gradually increased with increasing grinding time, and starting from 50 min grinding time, the tendency to increase is slowed down due to the agglomeration of fine particles. The specific surface area of pretreated steel slag is obviously higher than that of untreated steel slag after the same grinding time. For example, the specific surface area of untreated steel slag is only 360 m /kg after 60 min grinding, while that of pretreated steel slag reaches 361 m /kg after 40 min grinding, thus 20 min grinding time is saved. The relationship of specific surface area vs. grinding time was fitted, and the result is as follows: SSA = 124.7ln t 155.85 (R = 0.9605) (Untreated steel slag) (1) SSA = 157.44ln t 207.28 (R = 0.9906) (Pretreated steel slag) (2) From the above fitted result, it can be seen that the relationship of specific surface area with the logarithm of grinding time shows a good linear relationship. After applying derivation calculus to the above equations, we can get the increasing speeds of specific surface area: dSSA 124.7 = (Untreated steel slag) (3) dt t dSSA 157.44 = (Pretreated steel slag) (4) dt t It is obvious that the grinding speed of pretreated steel slag is higher than that of untreated steel slag. 3.3.3. Particle Size Distributions of Ground Untreated and Pretreated Steel Slag The particle size distribution and median diameter of ground untreated and pretreated steel slag powder are shown in Table 5 and Figure 7, respectively. Table 5. Particle size distribution of ground steel slag powder under different grinding time (%). Untreated steel slag Pretreated steel slag Grinding time (min) <3 m 3–32 m 32–65 m >65 m <3 m 3–32 m 32–65 m >65 m 10 2.77 27.88 25.24 44.11 2.44 25.42 21.95 50.19 20 4.97 37.16 27.54 30.33 5.66 40.06 30.84 23.45 30 6.64 36.62 28.04 28.70 9.00 44.22 32.10 14.68 40 8.16 38.24 27.46 26.15 10.54 48.80 30.51 10.15 50 11.18 37.90 22.19 28.73 14.87 56.99 22.65 5.49 60 8.99 32.52 26.57 31.91 16.36 53.83 20.96 8.84 70 10.91 30.99 19.12 38.97 14.50 55.74 21.47 8.29 Appl. Sci.2016, 9, 237  10 of 16  Untreated steel slag Pre-treated steel slag 10 20 30 40 50 60 70 Grinding time (min) Figure 7. Median diameter of ground steel slag powder under different grinding time.  Figure 7. Median diameter of ground steel slag powder under different grinding time. As shown in Table 5, with increasing grinding time (≤ 50 min), the proportion of particles for  more than 32 μm (especially particles for more than 65 μm) is gradually and obviously decreased,  while the proportion of particles for less than 32 μm shows a reverse tendency, which indicates that  the  particle  size  distribution  of  steel  slag  is  optimized  by  mechanical  grinding.  However,  when  grinding time is more than 50 min, it shows a complex or chaotic tendency due to the agglomeration  of fine particles. Compared with untreated steel slag, the proportion of particles for less than 32 μm  in  pretreated  steel  slag  powder  is  much  higher  after the same grinding time, indicating that the  increasing tendency of fine particles for pretreated steel slag powder is faster.   For median diameter (D50), with the increasing of grinding time, the median diameter of steel  slag is firstly decreased before 50min grinding time, and then is increased after that, which is similar  to the variation tendency of particle proportion of more than 32 μm. At 50 min grinding time, the  median diameter of untreated steel slag reaches a minimum, 32.89 μm, which is much larger than  that of pretreated  steel  slag, 18.16 μm. This shows that it is difficult for  untreated steel slag  to  be  finely ground.  3.3.4. Particle Morphologies of Ground Untreated and Pretreated Steel Slag  The particle morphologies of two kinds of ground steel slag powder after 50 min grinding are  shown in Figure 8. It can be seen from the figure that steel slag particles are mainly spherical  in  shape, and fine particles adhere to the larger particles. By comparison, the overall particle size of  pretreated steel slag powder is smaller than that of untreated steel slag, and particle uniformity in  size is also superior to that of untreated steel slag. To some extent, both the ground untreated and  pretreated steel slag powders show some agglomeration phenomenon.  5μ m 5μ m 50μ m 50μ m Median diameter (m) Appl. Sci. 2016, 6, 237 10 of 15 As shown in Table 5, with increasing grinding time ( 50 min), the proportion of particles for more than 32 m (especially particles for more than 65 m) is gradually and obviously decreased, while the proportion of particles for less than 32 m shows a reverse tendency, which indicates that the particle size distribution of steel slag is optimized by mechanical grinding. However, when grinding time is more than 50 min, it shows a complex or chaotic tendency due to the agglomeration of fine particles. Compared with untreated steel slag, the proportion of particles for less than 32 m in pretreated steel slag powder is much higher after the same grinding time, indicating that the increasing tendency of Appl. Sci.2016, 9, 237 10 of 16 fine particles for pretreated steel slag powder is faster. For median diameter (D50), with the increasing of grinding time, the median diameter of steel to the variation tendency of particle proportion of more than 32 μm. At 50 min grinding time, the slag is firstly decreased before 50min grinding time, and then is increased after that, which is similar to median diameter of untreated steel slag reaches a minimum, 32.89 μm, which is much larger than the variation tendency of particle proportion of more than 32 m. At 50 min grinding time, the median that of pretreated steel slag, 18.16 μm. This shows that it is difficult for untreated steel slag to be diameter of untreated steel slag reaches a minimum, 32.89 m, which is much larger than that of finely ground. pretreated steel slag, 18.16 m. This shows that it is difficult for untreated steel slag to be finely ground. 3.3.4. Particle Morphologies of Ground Untreated and Pretreated Steel Slag 3.3.4. Particle Morphologies of Ground Untreated and Pretreated Steel Slag The particle morphologies of two kinds of ground steel slag powder after 50 min grinding are The particle morphologies of two kinds of ground steel slag powder after 50 min grinding are shown in Figure 8. It can be seen from the figure that steel slag particles are mainly spherical in shown in Figure 8. It can be seen from the figure that steel slag particles are mainly spherical in shape, shape, and fine particles adhere to the larger particles. By comparison, the overall particle size of and fine particles adhere to the larger particles. By comparison, the overall particle size of pretreated pretreated steel slag powder is smaller than that of untreated steel slag, and particle uniformity in steel slag powder is smaller than that of untreated steel slag, and particle uniformity in size is also size is also superior to that of untreated steel slag. To some extent, both the ground untreated and superior to that of untreated steel slag. To some extent, both the ground untreated and pretreated steel pretreated steel slag powders show some agglomeration phenomenon. slag powders show some agglomeration phenomenon. 5μ m 5μ m 50μ m 50μ m (a) (b) Figure 8. Particle morphology of ground steel slag powder after 50min grinding time: (a) untreated Figure 8. Particle morphology of ground steel slag powder after 50min grinding time: (a) untreated steel slag; (b) pretreated steel slag. steel slag; (b) pretreated steel slag. 3.4. Effect of Organic Grinding Aids on the Grinding Property of Converter Steel Slag 3.4. Effect of Organic Grinding Aids on the Grinding Property of Converter Steel Slag From the above results of studies, it can be seen that the agglomeration phenomenon of fine From the above results of studies, it can be seen that the agglomeration phenomenon of fine particles will occur during deep grinding periods, which seriously affect or reduce the grinding particles will occur during deep grinding periods, which seriously affect or reduce the grinding efficiency of steel slag. In recent years, organic grinding aids (GA) have been used to improve the efficiency of steel slag. In recent years, organic grinding aids (GA) have been used to improve the grinding efficiency of cement. These researches show that organic GA molecules can be adsorbed on grinding efficiency of cement. These researches show that organic GA molecules can be adsorbed on the particles and effectively weaken the agglomeration of fine particle in the grinding process, thus the particles and effectively weaken the agglomeration of fine particle in the grinding process, thus improving the dispersion of powder and grinding efficiency [28–31]. Therefore, it is a good choice to improving the dispersion of powder and grinding efficiency [28–31]. Therefore, it is a good choice to use GA in the grinding process of steel slag for many manufacturers to improve preparation use GA in the grinding process of steel slag for many manufacturers to improve preparation efficiency efficiency of steel slag powder. In additional, on the basis of pretreatment, the grinding property of of steel slag powder. In additional, on the basis of pretreatment, the grinding property of steel slag can steel slag can be further improved by GA. In this study, glycerol, which is a common cement GA, be further improved by GA. In this study, glycerol, which is a common cement GA, was selected as the was selected as the GA of steel slag. The effects of GA on the grinding property of steel slag were GA of steel slag. The effects of GA on the grinding property of steel slag were studied from sieving studied from sieving residue, particle size distribution, angle of repose, and particle morphology. residue, particle size distribution, angle of repose, and particle morphology. 3.4.1. Effect of GA on the Sieving Residue of Steel Slag Powder As shown in Figure 9, it can be seen that the sieving residue of steel slag powder with GA is lower than that of the blank group (i.e., without any GA) after the same grinding time, indicating that ground steel slag with GA is smaller in particle size. The reducing effect of GA on the sieving residue is gradually enhanced with the increasing of grinding time before reaching 50 min grinding time. At the initial grinding period (before 20 min), the role of GA is small, but it becomes obvious after 20 min and reaches an optimum grinding role at 50 min grinding time. Appl. Sci.2016, 9, 237  11 of 16  (a)  (b) Figure 8. Particle morphology of ground steel slag powder after 50min grinding time: (a) untreated  steel slag; (b) pretreated steel slag.  3.4. Effect of Organic Grinding Aids on the Grinding Property of Converter Steel Slag  From the above results of studies, it can be seen  that the agglomeration phenomenon  of fine  particles  will  occur  during deep grinding periods, which seriously  affect  or  reduce  the grinding  efficiency of steel slag. In recent years, organic grinding aids (GA) have been used to improve the  grinding efficiency of cement. These researches show that organic GA molecules can be adsorbed on  the particles and effectively weaken the agglomeration of fine particle in the grinding process, thus  improving the dispersion of powder and grinding efficiency [28–31]. Therefore, it is a good choice to  use  GA  in  the  grinding  process  of  steel  slag  for  many  manufacturers  to  improve  preparation  efficiency of steel slag powder. In additional, on the basis of pretreatment, the grinding property of  steel slag can be further improved by GA. In this study, glycerol, which is a common cement GA,  was selected as the GA of steel slag. The effects of GA on the grinding property of steel slag were  Appl. Sci. 2016, 6, 237 11 of 15 studied from sieving residue, particle size distribution, angle of repose, and particle morphology.  3.4.1. Effect of GA on the Sieving Residue of Steel Slag Powder 3.4.1. Effect of GA on the Sieving Residue of Steel Slag Powder  As shown in Figure 9, it can be seen that the sieving residue of steel slag powder with GA is As shown in Figure 9, it can be seen that the sieving residue of steel slag powder with GA is  lower than that of the blank group (i.e., without any GA) after the same grinding time, indicating lower than that of the blank group (i.e., without any GA) after the same grinding time, indicating  that ground steel slag with GA is smaller in particle size. The reducing effect of GA on the sieving that ground steel slag with GA is smaller in particle size. The reducing effect of GA on the sieving  residue is gradually enhanced with the increasing of grinding time before reaching 50 min grinding residue is gradually enhanced with the increasing of grinding time before reaching 50 min grinding  time. At the initial grinding period (before 20 min), the role of GA is small, but it becomes obvious time. At the initial grinding period (before 20 min), the role of GA is small, but it becomes obvious  after 20 min and reaches an optimum grinding role at 50 min grinding time. after 20 min and reaches an optimum grinding role at 50 min grinding time.  Blank With 0.05% GA 40  Appl. Sci.2016, 9, 237  12 of 16  coarse and have a good dispersion at this stage, basically having no agglomeration of fine particles.  Reducing role of GA on sieve residue The  grinding  efficiency  of  steel  slag  is  basically  not  influenced  by  dispersion  of  particles,  so  the  optimization role of GA on the part 10 icle 2si 0 ze distribut 30 40ion is  5not 0  obvious. 60 70 During 40 min, 50 min, and  Grinding time (min) 60 min grinding time, the proportions of particles in the range of 3–32 μm are obviously increased  by 7.24% (absolute value, same below), 7.22%, and 10.63%, respectively, after adding GA, while the  Figure 9. Sieving residue of steel slag powder with grinding aid (GA) vs. grinding time. Figure 9. Sieving residue of steel slag powder with grinding aid (GA) vs. grinding time.  proportions of particles in the range of more than 32 μm are also obviously decreased, indicating  that GA can efficiently weaken agglomeration of fine particles, then improve the dispersion of steel  3.4.2. Effect of GA on the Particle Size Distribution of Steel Slag Powder 3.4.2. Effect of GA on the Particle Size Distribution of Steel Slag Powder  slag  powder.  Meanwhile,  many  researches  indicate  that  3–32 μm  particles  have  the  greatest  The The uni uniformi formity ty coefficient coefficient of of stee steel l slslag ag po powder wder can can  reflect reflect  thethe  extent extent  of the of the widwidth th of part of icle particle  size  contribution on the property of cement‐based materials, so the improvement of 3–32 μm particles  size distribution, and the smaller the uniformity coefficient, the wider the particle size distribution. distribution, and the smaller the uniformity coefficient, the wider the particle size distribution. From  proportion  after  using  GA  shows  that  GA  can  significantly  optimize  particle  size  distribution  of  Fig From ureFigur  10, it e can 10, itbe can  seen be th seen at the that un the ifouniformity rmity coeffic coef ient ficient  of stee ofl steel slag powder slag powder  is firs istly firstly  increincr ased eased  and  steel slag powder. At 70 min grinding time, as the agglomeration of particles is very serious, GA can  and then gradually decreased with the increasing of grinding time. By comparison, the uniformity then  gradually  decreased  with  the  increasing  of  grinding  time.  By  comparison,  the  uniformity  not  completely  resist  it  due  to  the  fixed  dosage  (0.05%),  resulting  in  the  weakening  of  GA’s  coefficient coefficient of of steel steel sl slag ag powder powder with with GA GA is is la larrger ger tha than n tha thatt witho without ut GA GA for for the the same same grind grinding ing ti time, me,  optimizing  role.  It  shows  that  the  optimizing  role  of  the  fixed  dosage  GA  on  the  particle  size  indicating that the particle size distribution of steel slag powder can be narrowed by GA. The effect is indicating that the particle size distribution of steel slag powder can be narrowed by GA. The effect  distribution of steel slag powder is different under different grinding times, and the role of 0.05%  is the the str stronge ongest s att at 50 50 min  min and and 60 60 min  migrinding n grinding times.  times.  dosage of GA is best during 40–60 min grinding time.  As shown in Table 6, it can be seen that the effect of GA on the particle size distribution of steel  slag powder is relatively small before reaching 40 min grinding time. The reason is that particles are  1.25 Blank With 0.5% GA 1.20 1.15 1.10 1.05 1.00 0.95 0.90 0.85 10 20 30 40 50 60 70 Grinding time (min) Figure 10. Uniformity coefficient of steel slag powder with GA vs. grinding time. Figure 10. Uniformity coefficient of steel slag powder with GA vs. grinding time.  Table 6. Particle size distribution of steel slag powder with GA under different grinding time (%).  Without GA  With GA  Grinding  time (min)  <3μm  >65μm <3μm >65μm 3–32μm  32–65μm 3–32μm 32–65μm  10  2.44  25.42  21.95  50.19  2.83  23.83  22.80  50.54  20  5.66  40.06  30.84  23.45  4.81  37.44  34.14  23.62  30  9.00  44.22  32.10  14.68  7.63  47.08  32.74  12.55  40  10.54  48.80  30.51  10.15  10.44  56.04  28.47  5.04  50  14.87  56.99  22.65  5.49  14.32  64.21  21.10  0.38  60  16.36  53.83  20.96  8.84  15.73  64.46  18.61  1.20  70  14.50  55.74  21.47  8.29  15.05  57.33  19.49  8.13  3.4.3. Effect of GA on the Angle of Repose of Steel Slag Powder  Powder fluidity is usually characterized by the angle of repose. The smaller the angle of repose,  the better the powder fluidity. As shown in Figure 11, it can be seen that the angle of repose of steel  slag powder is decreased after adding GA, indicating that GA can improve the fluidity of steel slag  powder. The role of GA on the angle of repose is firstly enhanced and then weakened on the whole,  which is similar to the role of GA on the sieving residue, and it is reaches maximum effectiveness at  50  min  grinding  time,  i.e.,  the  action  effect  of  GA  on  powder  fluidity  is  the  greatest  at  50  min  grinding time.  Uniformity coefficient 45m sieve residue (%) Appl. Sci. 2016, 6, 237 12 of 15 As shown in Table 6, it can be seen that the effect of GA on the particle size distribution of steel slag powder is relatively small before reaching 40 min grinding time. The reason is that particles are coarse and have a good dispersion at this stage, basically having no agglomeration of fine particles. The grinding efficiency of steel slag is basically not influenced by dispersion of particles, so the optimization role of GA on the particle size distribution is not obvious. During 40 min, 50 min, and 60 min grinding time, the proportions of particles in the range of 3–32 m are obviously increased by 7.24% (absolute value, same below), 7.22%, and 10.63%, respectively, after adding GA, while the proportions of particles in the range of more than 32 m are also obviously decreased, indicating that GA can efficiently weaken agglomeration of fine particles, then improve the dispersion of steel slag powder. Meanwhile, many researches indicate that 3–32 m particles have the greatest contribution on the property of cement-based materials, so the improvement of 3–32 m particles proportion after using GA shows that GA can significantly optimize particle size distribution of steel slag powder. At 70 min grinding time, as the agglomeration of particles is very serious, GA can not completely resist it due to the fixed dosage (0.05%), resulting in the weakening of GA’s optimizing role. It shows that the optimizing role of the fixed dosage GA on the particle size distribution of steel slag powder is different under different grinding times, and the role of 0.05% dosage of GA is best during 40–60 min grinding time. Table 6. Particle size distribution of steel slag powder with GA under different grinding time (%). Without GA With GA Grinding time (min) <3 m 3–32 m 32–65 m >65 m <3 m 3–32 m 32–65 m >65 m 10 2.44 25.42 21.95 50.19 2.83 23.83 22.80 50.54 20 5.66 40.06 30.84 23.45 4.81 37.44 34.14 23.62 30 9.00 44.22 32.10 14.68 7.63 47.08 32.74 12.55 40 10.54 48.80 30.51 10.15 10.44 56.04 28.47 5.04 50 14.87 56.99 22.65 5.49 14.32 64.21 21.10 0.38 60 16.36 53.83 20.96 8.84 15.73 64.46 18.61 1.20 70 14.50 55.74 21.47 8.29 15.05 57.33 19.49 8.13 3.4.3. Effect of GA on the Angle of Repose of Steel Slag Powder Powder fluidity is usually characterized by the angle of repose. The smaller the angle of repose, the better the powder fluidity. As shown in Figure 11, it can be seen that the angle of repose of steel slag powder is decreased after adding GA, indicating that GA can improve the fluidity of steel slag powder. The role of GA on the angle of repose is firstly enhanced and then weakened on the whole, which is similar to the role of GA on the sieving residue, and it is reaches maximum effectiveness at 50 min grinding time, i.e., the action effect of GA on powder fluidity is the greatest at 50 min grinding time. Appl. Sci.2016, 9, 237  13 of 16  Blank With 0.05wt% GA  Role of GA on angle of repose 10 20 30 40 50 60 70 Grinding time (min) Figure 11. Repose angle of steel slag powder with GA vs. grinding time. Figure 11. Repose angle of steel slag powder with GA vs. grinding time.  3.4.4. Effect of GA on the Particle Morphology of Steel Slag Powder  The particle morphologies of steel slag powder with and without GA are shown in Figure 12. It  can  be  seen  from  the  figure  that  the  particles  of  steel  slag  powder  without  GA  exhibit  the  agglomeration phenomenon where fine particles adhere to each other. Especially at 70 min grinding  time,  agglomeration  of  particles  is  very  serious  and  many  large  particles  are  regenerated  by  adhering  of  fine  particles.  By  comparison,  after  using  GA,  the  agglomeration  phenomenon  of  particles is not obvious at 50min grinding time, still keeping a relatively good dispersion. Although  the particles with GA also exhibit a little agglomeration at 70 min grinding time, their dispersion is  still  obviously  better  than  those  without  GA.  The  above  results  confirm  that  GA  can  efficiently  weaken the agglomeration of fine particles when particle size becomes small and agglomeration of  fine particles occurs.   5μ m 5μ m Agglomeration of particles Agglomeration of particles 50μ m 20μ m 50min 70min (a)  Repose angle of powder ( ) Appl. Sci.2016, 9, 237  13 of 16  Blank With 0.05wt% GA Role of GA on angle of repose 10 20 30 40 50 60 70 Grinding time (min) Figure 11. Repose angle of steel slag powder with GA vs. grinding time.  Appl. Sci. 2016, 6, 237 13 of 15 3.4.4. Effect of GA on the Particle Morphology of Steel Slag Powder  3.4.4. Effect of GA on the Particle Morphology of Steel Slag Powder The particle morphologies of steel slag powder with and without GA are shown in Figure 12. It  can  be  seen  from  the  figure  that  the  particles  of  steel  slag  powder  without  GA  exhibit  the  The particle morphologies of steel slag powder with and without GA are shown in Figure 12. agglomeration phenomenon where fine particles adhere to each other. Especially at 70 min grinding  It can be seen from the figure that the particles of steel slag powder without GA exhibit the time,  agglomeration  of  particles  is  very  serious  and  many  large  particles  are  regenerated  by  agglomeration phenomenon where fine particles adhere to each other. Especially at 70 min grinding adhering  of  fine  particles.  By  comparison,  after  using  GA,  the  agglomeration  phenomenon  of  time, agglomeration of particles is very serious and many large particles are regenerated by adhering particles is not obvious at 50min grinding time, still keeping a relatively good dispersion. Although  of fine particles. By comparison, after using GA, the agglomeration phenomenon of particles is not the particles with GA also exhibit a little agglomeration at 70 min grinding time, their dispersion is  obvious at 50min grinding time, still keeping a relatively good dispersion. Although the particles with still  obviously  better  than  those  without  GA.  The  above  results  confirm  that  GA  can  efficiently  GA also exhibit a little agglomeration at 70 min grinding time, their dispersion is still obviously better weaken the agglomeration of fine particles when particle size becomes small and agglomeration of  than those without GA. The above results confirm that GA can efficiently weaken the agglomeration fine particles occurs.   of fine particles when particle size becomes small and agglomeration of fine particles occurs. 5μ m 5μ m Agglomeration of particles Agglomeration of particles 50μ m 20μ m 50min 70min Appl. Sci.2016, 9, 237  14 of 16  (a)  3μ m 3μ m Dispersion of particles Dispersion of particles a little agglomeration 20μ m 10μ m 70min 50min (b)  Figure  12.  Particle  morphologies  of  steel slag  powder  with  and  without  GA:  (a)  without  GA;  (b)  Figure 12. Particle morphologies of steel slag powder with and without GA: (a) without GA; with GA.  (b) with GA. 4. Conclusions  4. Conclusions From this study, we can conclude that:  From this study, we can conclude that: (1) It was evident from analysis of BSE–EDX that the mineral phases of converter steel slag mainly (1)  It  was  evident  from  analysis  of  BSE–EDX  that the mineral phases of converter  steel slag  contain silicate mineral phase, RO phase, calcium ferrite phase, and a small amount of metallic iron mainly  contain  silicate  mineral  phase,  RO  phase,  calcium  ferrite  phase,  and  a  small  amount  of  phase, among others. metallic iron phase, among others.  (2) The oversize substance after screening can be considered as the hardly grinding phases (HGP), (2)The  oversize  substance  after  screening  can  be  considered  as  the  hardly  grinding  phases  which provides a simple method of determining the HGP in steel slag. The HGP proportion which (HGP), which provides a simple method of determining the HGP in steel slag. The HGP proportion  which  was  determined  by  a  0.9  mm  square‐hole  screen  is  about  1.5%.  After  the  initial  20  min  grinding, the RO phase, calcium ferrite, and metallic iron phase made up most of the proportion in  the HGP, while the metallic iron made up the most component after 70 min grinding.  (3) For steel slag powder with about 360 m /kg specific surface area (SSA), 20 min of grinding  time can be saved with pretreatment. The relationships of SSA with the logarithm of grinding time  show  a  good  linear  relationship,  but  pretreated  steel  slag  has  a  higher  grinding  efficiency.  In  addition, the D50 of untreated steel slag can only reach 32.89 μm after 50 min grinding, but that of  pretreated steel slag can reach 18.16 μm after the same grinding time.  (4) Organic grinding aids (GA) can obviously improve the grinding property of steel slag, and  the action effects of 0.05% dosage of GA on the grinding efficiency and particle characteristics were  the best during 40–60 min grinding time. especially the proportions of particles in the range of 3–32  μm, which were obviously increased by 7.24%, 7.22%, and 10.63% after 40 min, 50 min and 60 min  grinding, respectively. This is mainly because of the reduction of agglomeration after the use of GA,  as evidenced by SEM images.  Acknowledgments:  This  work  was  financially  supported  by  the  Chinaʹs  Post‐doctoral  Science  Fund  (No.  2016M591170), the Open Fund of Key Laboratory of Advanced Civil Engineering Materials (Tongji University),  Ministry of Education (No.201602).  Author Contributions: J.Z. conceived of, designed and performed the experiments. D.W. and P.Y. analyzed  the data and discussed the results. W.L. participated in writing this paper.  Conflicts of Interest: The authors declare no conflict of interest.  References  1. Yi, H.; Xu, G.P.; Cheng, H.G.; Wang, J.S.; Wan, Y.F.; Chen, H. An overview of utilization of steel slag. Proc.  Environ. Sci. 2012, 16, 791–801.  Repose angle of powder ( ) Appl. Sci. 2016, 6, 237 14 of 15 was determined by a 0.9 mm square-hole screen is about 1.5%. After the initial 20 min grinding, the RO phase, calcium ferrite, and metallic iron phase made up most of the proportion in the HGP, while the metallic iron made up the most component after 70 min grinding. (3) For steel slag powder with about 360 m /kg specific surface area (SSA), 20 min of grinding time can be saved with pretreatment. The relationships of SSA with the logarithm of grinding time show a good linear relationship, but pretreated steel slag has a higher grinding efficiency. In addition, the D50 of untreated steel slag can only reach 32.89 m after 50 min grinding, but that of pretreated steel slag can reach 18.16 m after the same grinding time. (4) Organic grinding aids (GA) can obviously improve the grinding property of steel slag, and the action effects of 0.05% dosage of GA on the grinding efficiency and particle characteristics were the best during 40–60 min grinding time. especially the proportions of particles in the range of 3–32 m, which were obviously increased by 7.24%, 7.22%, and 10.63% after 40 min, 50 min and 60 min grinding, respectively. This is mainly because of the reduction of agglomeration after the use of GA, as evidenced by SEM images. Acknowledgments: This work was financially supported by the China’s Post-doctoral Science Fund (No. 2016M591170), the Open Fund of Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education (No. 201602). Author Contributions: J.Z. conceived of, designed and performed the experiments. D.W. and P.Y. analyzed the data and discussed the results. W.L. participated in writing this paper. 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[CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Comparison of Grinding Characteristics of Converter Steel Slag with and without Pretreatment and Grinding Aids

Applied Sciences , Volume 6 (11) – Oct 28, 2016

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Multidisciplinary Digital Publishing Institute
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2076-3417
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10.3390/app6110237
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Abstract

applied sciences Article Comparison of Grinding Characteristics of Converter Steel Slag with and without Pretreatment and Grinding Aids 1 , 2 1 2 Jihui Zhao *, Dongmin Wang , Peiyu Yan and Wenping Li Institute of Building Materials, Department of Civil Engineering, Tsinghua University, Beijing 100084, China; yanpy@tsinghua.edu.cn School of Chemical & Environmental Engineering, China University of Mining & Technology, Beijing 100083, China; wangdongmin-2008@163.com (D.W.); lwpchina@163.com (W.L.) * Correspondence: zhaojihui324@163.com; Tel.: +86-159-0124-5174 Academic Editor: Stefano Invernizzi Received: 10 June 2016; Accepted: 8 August 2016; Published: 28 October 2016 Abstract: The converter steel slag cannot be widely used in building materials for its poor grindability. In this paper, the grinding characteristics of untreated and pretreated (i.e., magnetic separation) steel slag were compared. Additionally, the grinding property of pretreated steel slag was also studied after adding grinding aids. The results show that the residues (i.e., oversize substance) that passed a 0.9 mm square-hole screen can be considered as the hardly grinding phases (HGP) and its proportion is about 1.5%. After the initial 20 min grinding, the RO phase (RO phase is a continuous solid solution which is composed of some divalent metal oxides, such as FeO, MgO, MnO, CaO, etc.), calcium ferrite, and metallic iron phase made up most of the proportion of the HGP, while the metallic iron made up the most component after 70 min grinding. The D50 of untreated steel slag could only reach 32.89 m after 50 min grinding, but that of pretreated steel slag could reach 18.16 m after the same grinding time. The grinding efficiency of steel slag was obviously increased and the particle characteristics were improved after using grinding aids (GA), especially the particle proportions of 3–32 m were obviously increased by 7.24%, 7.22%, and 10.63% after 40 min, 50 min, and 60 min grinding, respectively. This is mainly because of the reduction of agglomeration and this effect of GA was evidenced by SEM (scanning electron microscope) images. Keywords: steel slag; grinding property; particle characteristics; mineral phase; hardly grinding phases; magnetic separation; grinding aids; agglomeration 1. Introduction Steel slag is an industrial solid waste generated in the steel-making process, but it is rich in dicalcium silicate (C S) and tricalcium silicate (C S) in mineral compositions, which is similar to 2 3 cement clinker [1–4]. Based on its mineral compositions, steel slag has great potential for application as supplementary cementitious materials (also known as mineral admixture) of cement and concrete [5–8]. Generally, only after being ground into powder, could steel slag have hydration activity and be used as supplementary cementitious materials [9,10]. The smaller the particle size of steel slag powder, the higher its hydration activity [11,12]. However, the grindability of steel slag is much worse than that of other mineral admixtures such as fly ash and granulated blast-furnace slag, seriously impeding the preparation efficiency of steel slag powder and its application performance in building materials. Therefore, its grinding property is an important consideration in the application of steel slag. In the grinding process of a solid, particles characteristics such as size, distribution, bulk density, and dispersion are in constant change on a macroscopic scale, and the internal chemical bonds of Appl. Sci. 2016, 6, 237; doi:10.3390/app6110237 www.mdpi.com/journal/applsci Appl. Sci. 2016, 6, 237 2 of 15 particles are constantly broken on a microscopic scale [13–15]. Steel slag has poor grindability because of a large amount of iron oxides and continuous solid solution composed of some divalent metal oxides, which form a dense structure in the steel slag [16,17]. Currently, the researches on improvement of grinding efficiency for steel slag mainly focus on the optimization of grinding equipment—such as vertical mill and roller mill, while there are few researches on other measures, such as removal of hardly grinding phases (HGP) or the use of chemical admixtures. Although vertical mills have been applied in grinding of steel slag, many application researches indicate that steel slag powder prepared by vertical mills have poor particle morphology—such as flat, needle, and irregular shapes, resulting in the ball mill still being chosen by many manufacturers to prepare steel slag powder in China. Meanwhile, some studies indicate that metallic iron in steel slag is one main reason causing its poor grinding property. However, in different grinding periods, the factors influencing grinding efficiency usually change due to different grinding characteristics [18–20], but to date these factors have not been yet been fully studied. Some researches also show that the split-phase phenomenon occurs in the grinding process of steel slag, resulting in ground steel slag powder having some differences in chemical and mineral compositions during different grinding periods [21,22]. So, it is necessary to reveal the factors influencing the grinding efficiency of steel slag from studying grinding characteristics, hardly grinding phases, and split-phase phenomenon during different grinding periods. Moreover, in the grinding process of a solid, other phenomena such as agglomeration of fine particles and re-healing of particle fracture surfaces result in a lower grinding efficiency and higher energy consumption [23–26]. In recent years, grinding aids, most of which are polar organic compounds, are extensively applied to weaken agglomeration of fine particles and improve grinding efficiency of cement [27]. Therefore, the use of grinding aids is also an important way to improve preparation efficiency of steel slag powder. Based on the abovementioned discussion, the hardly grinding phases of converter steel slag were revealed from the proportion, morphology, mineral, and chemical compositions. Then the grinding characteristics of untreated and pretreated (i.e., magnetic separation) steel slag were compared from the proportion of iron phases, grinding efficiency, particle size, and particle distribution and morphology. Further, the grinding property of pretreated steel slag was also studied after adding organic grinding aids and its positive role and mechanism were discussed here as well. 2. Materials and Methods 2.1. Materials The converter steel slag used was provided from Laiwu Steel Corporation (Laiwu, China). The chemical compositions of steel slag, which were determined by X-ray fluorescence (XRF) analysis, (Rh, 40 kV, 70 mA), are given in Table 1. The mineral phases of steel slag, which were analyzed by X-ray diffraction (XRD), using a D6000 diffractometer with nickel-filtered Cu K l radiation (= 1.5405 Å, 40 kV, and 40 mA) from Shimadzu company (Kyoto, Japan), are given in Figure 1. The organic grinding aid used, produced by Sinopharm Chemical Reagent Beijing Co., Ltd. (Beijing, China), was chemical grade glycerol. Table 1. Chemical compositions of converter steel slag (%). CaO SiO Al O Fe O MgO K O SO P O LOI 2 2 3 2 3 2 3 2 5 39.67 21.71 1.58 25.52 4.92 0.02 0.18 1.70 0.32 Appl. Sci. 2016, 6, 237 3 of 15 Appl. Sci.2016, 9, 237  3 of 16  2 1- C S 2- C S 3- C A 2 3 3 4- C AF 5-C F 6- RO 4 2 7- 3CaO Fe O  3SiO 2 3 2 8- 2CaO MgO 2SiO 9- 3CaO 2TiO 10-Fe 11- f-CaO 12- f-MgO 13- CaCO 1 7 1 10 2 4 3 1 7 1 10 13 2 1 1 2 2 10 20 30 40 50 60 70 80 2( ) Figure 1. X‐ray diffraction (XRD) pattern of converter steel slag.  Figure 1. X-ray diffraction (XRD) pattern of converter steel slag. 2.2. Experimental Methods  2.2. Experimental Methods 2.2.1. Observation and Identification of Mineral Phases for Converter Steel Slag  2.2.1. Observation and Identification of Mineral Phases for Converter Steel Slag The steel slag sample needed to be prepared for the morphology observation of mineral phase.  The steel slag sample needed to be prepared for the morphology observation of mineral phase. Firstly, the surface of the 3–8 mm blocky steel slag was burnished using P200#, P400#, P600#, P800#,  Firstly, the surface of the 3–8 mm blocky steel slag was burnished using P200#, P400#, P600#, P800#, P1000#,  and  P1200#  sandpapers  in  turn,  and  was  then  polished  by  a  polisher  (UNIPOL‐830,  P1000#, and P1200# sandpapers in turn, and was then polished by a polisher (UNIPOL-830, Shenyang Shenyang Kejing Instrument Co., Ltd., Shenyang, China). The mineral phases were observed and  Kejing Instrument Co., Ltd., Shenyang, China). The mineral phases were observed and identified by identified  by  back‐scattered  electron  (BSE)  imaging  and  energy  dispersive  X‐ray  spectroscopy  back-scattered electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDX), respectively, (EDX),  respectively,  using  a  scanning  electron  microscope  (Quanta  200  FEG,  FEI  Company,  using a scanning electron microscope (Quanta 200 FEG, FEI Company, Hillsboro, OR, USA). In addition, Hillsboro, OR, USA). In addition, the hardly grinding phases were observed using a metallographic  the hardly grinding phases were observed using a metallographic microscope (BA210Met, Motic China microscope (BA210Met, Motic China Group Co., Ltd., Xiamen, China).  Group Co., Ltd., Xiamen, China). 2.2.2. Measurement of Vickers Hardness of Mineral Phases for Converter Steel Slag  2.2.2. Measurement of Vickers Hardness of Mineral Phases for Converter Steel Slag The Vickers hardness of different mineral phases for steel slag was measured using a Vickers  The Vickers hardness of different mineral phases for steel slag was measured using a Vickers hardness tester (HV‐1000, Shanghai Materials Tester Factory, Shanghai, China), and the hardness  hardness tester (HV-1000, Shanghai Materials Tester Factory, Shanghai, China), and the hardness value value of each mineral phase was determined by taking the average of multiple points.  of each mineral phase was determined by taking the average of multiple points. 2.2.3. Procedure of Grinding Experiment for Steel Slag 2.2.3. Procedure of Grinding Experiment for Steel Slag  Grinding Grinding experiments experimentsof  ofsteel   steel slag   slag wer   were e carried   carried out  out by laboratory by  laboratory ball mill ball (SM-500, mill  (SMW‐50 uxi 0, Jianyi Wuxi  Experiment Instrument Co., Ltd., Wuxi, China). The type of the ball mill was F500 mm  500 mm, Jianyi Experiment Instrument Co., Ltd., Wuxi, China). The type of the ball mill was Ф500 mm × 500  48 mm, r/min  48 r/min , and,the  and grinding  the grin media ding media was composed  was compo of 60 sekg d of steel  60 kg balls ste (F el40 bamm, lls (ΦF40 50 mm, mm, ΦF50 60 mm mm,, Φ and 60  F70 mm) and 40 kg small steel forgings (F25 mm  35 mm). mm, and Φ70 mm) and 40 kg small steel forgings (Φ25 mm × 35 mm).   The The steel steel slag slag was was firstly firstly cr crushed ushed to to less less than than 55 mm mm by by aajaw  jawcr cr usher usher(PE  (PE60 60 × 100, 100, Shanghai Shanghai  Longshi Machinery Co., Ltd., Shanghai, China) before the grinding experiment. The weight of steel Longshi Machinery Co., Ltd., Shanghai, China) before the grinding experiment. The weight of steel  slag slag for foreach  eachgrinding  grinding experiment  experiment was  wa3s kg 3 kg and and the the grinding  grinding times  tim wer es were e 10 min,  10 min 20 min, , 20 mi 30n min, , 30 min, 40 min,  40  50 min, 60 min, and 70 min. In addition, for the grinding aids experiment, 0.05% grinding aid was min, 50 min, 60 min, and 70 min. In addition, for the grinding aids experiment, 0.05% grinding aid  added was add into ed the intotest  thegr test oup gro toucompar p to compar e with e with the blank  the blank group gro (i.e., up without (i.e., without any grinding  any grinding aids). aids).  2.2.4. Test Methods of Particle Size and Distribution of Ground Steel Slag Powder 2.2.4. Test Methods of Particle Size and Distribution of Ground Steel Slag Powder  The specific surface area of ground steel slag powder was measured by Blaine method, conforming The  specific  surface  area  of  ground  steel  slag  powder  was  measured  by  Blaine  method,  to Chinese National Standard GB/T8074-2008. The sieving residue of steel slag powder was measured conforming to Chinese National Standard GB/T8074‐2008. The sieving residue of steel slag powder  by sieving analysis method with a 45 m square-hole screen, conforming to the Chinese National was measured  by  sieving  analysis  method  with  a  45 μm  square‐hole  screen,  conforming to  the  Standard GB/T1345-2005. The particle size distribution was measured by laser-scattering method, Chinese  National  Standard  GB/T1345‐2005.  The  particle  size  distribution  was  measured  by  conforming to the Chinese Industry Standard JC/T721-2006. laser‐scattering method, conforming to the Chinese Industry Standard JC/T721‐2006.  2.2.5. Observation of Particle Morphology of Ground Steel Slag Powder  Appl. Sci. 2016, 6, 237 4 of 15 Appl. Sci.2016, 9, 237  4 of 16  2.2.5. Observation of Particle Morphology of Ground Steel Slag Powder The particle morphology of ground steel slag powder was observed with a scanning electron  The particle morphology of ground steel slag powder was observed with a scanning electron microscope (Quanta 200 FEG, FEI Company, Hillsboro, OR, USA) under high vacuum condition.  microscope (Quanta 200 FEG, FEI Company, Hillsboro, OR, USA) under high vacuum condition. 2.2.6. Test Methods of the Angle of Repose of Ground Steel Slag Powder  2.2.6. Test Methods of the Angle of Repose of Ground Steel Slag Powder The  angle  of  repose  of  ground  steel  slag  powder  was  tested  conforming  to  the  Chinese  The angle of repose of ground steel slag powder was tested conforming to the Chinese National National  Standard  GB/T11986‐1989,  as follows:  steel slag  powder  was  poured  into  a  funnel,  and  Standard GB/T11986-1989, as follows: steel slag powder was poured into a funnel, and then powder then powder from the funnel fell onto and coated the disc below the funnel. Then the height, h, of  from the funnel fell onto and coated the disc below the funnel. Then the height, h, of the powder layer the powder layer and the radius, R, of the disc were measured, thus the angle of repose, θ, of steel  and the radius, R, of the disc were measured, thus the angle of repose, , of steel slag powder was slag powder was obtained according to the formula (tan θ = h/R).  obtained according to the formula (tan  = h/R). 3. Results and Discussion  3. Results and Discussion 3.1. Mineral Phases’Characteristics of Converter Steel Slag  3.1. Mineral Phases’Characteristics of Converter Steel Slag As shown in Figure 2(a), the morphologies of mineral phases in steel slag show different grey  As shown in Figure 2a, the morphologies of mineral phases in steel slag show different grey levels levels under BSE images, such as black, grey‐black, grey, light‐grey, and white‐bright in grey level,  under BSE images, such as black, grey-black, grey, light-grey, and white-bright in grey level, which which exhibit different shapes as well, such as round shape, leaf‐like shape, hexagonal‐plate shape,  exhibit different shapes as well, such as round shape, leaf-like shape, hexagonal-plate shape, irregular irregular  shape,  and  so  on.  The  compositions  of  these  minerals  with  different  grey  levels  were  shape, and so on. The compositions of these minerals with different grey levels were determined using determined using EDX analysis, and the results are shown in Figure 2(b). It can be seen from EDX  EDX analysis, and the results are shown in Figure 2b. It can be seen from EDX analysis results that the analysis results that the minerals  with  grey  levels  of  black and grey‐black are mainly the silicate  minerals with grey levels of black and grey-black are mainly the silicate minerals phases which are minerals phases which are composed of oxygen, silicon, and calcium elements; the irregular mineral  composed of oxygen, silicon, and calcium elements; the irregular mineral phases with grey level of phases with grey level of light‐grey are mainly RO phase which is composed of oxygen, magnesium,  light-grey are mainly RO phase which is composed of oxygen, magnesium, calcium, manganese, and calcium,  manganese,  and  iron  elements;  the  irregular‐shaped  minerals  with  grey  level  of  grey,  iron elements; the irregular-shaped minerals with grey level of grey, which are filled in light-grey and which are filled in light‐grey and black minerals, are the calcium ferrite phases mainly composed of  black minerals, are the calcium ferrite phases mainly composed of oxygen, calcium, iron, aluminum, oxygen, calcium, iron, aluminum, and silicon elements; the round granular‐shaped minerals with  and silicon elements; the round granular-shaped minerals with grey level of white-bright are the grey  level  of  white‐bright  are  the  metallic  iron  phase.  The  above  results  show  that  the  mineral  metallic iron phase. The above results show that the mineral phases of steel slag mainly contain silicate phases of steel slag mainly contain silicate mineral phase, RO phase, calcium ferrite phase, and a  mineral phase, RO phase, calcium ferrite phase, and a small amount of metallic iron phase, etc. It is small amount of metallic iron phase, etc. It is also evident from XRD analysis presented in Figure 1  also evident from XRD analysis presented in Figure 1 that the mineral phases converter steel slag that the mineral phases converter steel slag mainly contains C2S, C3S, RO, 3CaO∙Fe2O3∙3SiO2 (i.e.,  mainly contains C S, C S, RO, 3CaOFe O 3SiO (i.e., calcium ferrite phases) and so on. (Comment: 2 3 2 3 2 calcium  ferrite  phases)  and  so  on.  (Comment:  RO  phase  is  a  continuous  solid  solution  which  is  RO phase is a continuous solid solution which is composed of some divalent metal oxides, such as composed of some divalent metal oxides, such as FeO, MgO, MnO, CaO, etc.)  FeO, MgO, MnO, CaO, etc.) 2 6 100μ m 100μ m 50μ m (a)  Figure 2. Cont. Appl. Sci. 2016, 6, 237 5 of 15 Appl. Sci.2016, 9, 237  5 of 16  Average result Average result Average result 1 2 3 Ele ment Wt % At % Ele ment Wt % At % Ele ment Wt % At % O K 29.34 48.97 O K 31.12 51.55 O K 17.89 35.77 MgK 00.22 00.24 MgK 01.10 01.20 MgK 22.02 28.97 Al K 00.82 00.81 AlK 01.95 01.91 CaK 03.49 02.79 Si K 14.59 13.87 SiK 11.33 10.69 MnK 05.29 03.08 P K 03.78 03.26 P K 02.12 01.81 FeK 51.31 29.39 CaK 45.35 30.21 CaK 43.91 29.03 FeK 03.06 01.47 FeK 05.32 02.52 ZnK 02.84 01.16 ZnK 03.16 01.28 Average result Ele ment Wt % At % Average result O K 22.40 43.45 Ele ment Wt % At % Average result MgK 01.24 01.58 O K 18.40 37.52 Ele ment Wt % At % AlK 06.09 07.00 MgK 18.20 24.43 O K 01.42 04.77 SiK 00.99 01.09 CaK 03.88 03.16 CaK 01.36 01.82 CaK 35.34 27.36 Cr K 01.15 00.72 FeK 97.22 93.41 Ti K 06.42 04.16 MnK 06.81 04.05 Cr K 01.33 00.79 FeK 51.55 30.11 FeK 26.19 14.56 (b)  Figure  2.  Morphologies  and  identification  of  mineral  phases  in  converter  steel  slag.  (a):  Figure 2. Morphologies and identification of mineral phases in converter steel slag. (a): Back-scattered Back‐scattered  electron  (BSE)  images  of  various  mineral  phases;  (b)  Energy  dispersive  X‐ray  electron (BSE) images of various mineral phases; (b) Energy dispersive X-ray spectroscopy (EDX) spectroscopy (EDX) analysis results of various mineral phases.  analysis results of various mineral phases. 3.2. Determination of Hardly GrindingPhases (HGP) in Converter Steel Slag   3.2. Determination of Hardly Grinding Phases (HGP) in Converter Steel Slag As the grindability of various mineral phases in  steel  slag  have great differences, the  easily  As the grindability of various mineral phases in steel slag have great differences, the easily grinding  phases  (denoted  as  EGP)  are  firstly  being  ground  to  fine  powder,  while  the  poor  grinding phases (denoted as EGP) are firstly being ground to fine powder, while the poor grindability grindability  mineral  phases  are  difficult  to  grind  down,  which  seriously  affects  the  grinding  mineral phases are difficult to grind down, which seriously affects the grinding efficiency. However, efficiency. However, different mineral phases in steel slag are very difficult to be separated out, so it  different mineral phases in steel slag are very difficult to be separated out, so it is impossible to is impossible to determine what is an easily  grinding  phase  and hardly  grinding phase  from the  determine what is an easily grinding phase and hardly grinding phase from the grindability index grindability index of various mineral phases. In this paper, a simple method for determination of  of various mineral phases. In this paper, a simple method for determination of the hardly grinding the  hardly  grinding  phase  in  steel  slag  by  oversize  substance  (i.e.,  residue)  is  provided.  After  phase in steel slag by oversize substance (i.e., residue) is provided. After grinding and then screening grinding and then screening of steel slag powder, the oversize substance obtained can be considered  of steel slag powder, the oversize substance obtained can be considered as the hardly grinding phases as the hardly grinding phases (denoted as HGP) in steel slag. In order to determine the HGP in the  (denoted as HGP) in steel slag. In order to determine the HGP in the grinding process, the steel slag grinding process, the steel slag powder after 10, 20, 30, 40, 50, 60, and 70 min grinding were screened  powder after 10, 20, 30, 40, 50, 60, and 70 min grinding were screened with a 0.9 mm square-hole with  a  0.9  mm  square‐hole  screen,  and  then  the  proportion,  morphology,  and  compositions  of  screen, and then the proportion, morphology, and compositions of oversized substances (i.e., HGP) oversized substances (i.e., HGP) were analyzed.  were analyzed. 3.2.1. Proportion of HGP in Converter Steel Slag  3.2.1. Proportion of HGP in Converter Steel Slag The proportions of oversized substances after different grinding times are shown in Figure 3.  The proportions of oversized substances after different grinding times are shown in Figure 3. Form  the  figure,  with  the  increasing  of  grinding  time  of  steel  slag,  the  proportion  of  oversized  Form the figure, with the increasing of grinding time of steel slag, the proportion of oversized substances  that passed the 0.9 mm square‐hole screen rapidly declined in the  range  of 10–20 min  substances that passed the 0.9 mm square-hole screen rapidly declined in the range of 10–20 min grinding  time,  and  then  starts  to  slowly  decline  (starting  from  30  min  grinding  time),  until  the  grinding time, and then starts to slowly decline (starting from 30 min grinding time), until the proportion finally  reaches about 1.5%, which indicates that this part of oversize  substances  is  the  proportion finally reaches about 1.5%, which indicates that this part of oversize substances is the HGP in converter steel slag. In other words, the proportion of HGP determined by residue method  HGP in converter steel slag. In other words, the proportion of HGP determined by residue method of of 0.9 mm square‐hole screen is about 1.5%.  0.9 mm square-hole screen is about 1.5%. Appl. Sci. 2016, 6, 237 6 of 15 Appl. Appl.  Sci. Sci.2016 2016,,  99,,  237 237   66  of of  16 16   4.0 4.0 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 10 20 30 40 50 60 70 10 20 30 40 50 60 70 G G rriindi nding ng t tiim m ee (m (m iin n)) Figure 3. Proportion of oversize substances of steel slag powder vs. grinding time. Figure 3. Proportion of oversize substances of steel slag powder vs. grinding time.  Figure 3. Proportion of oversize substances of steel slag powder vs. grinding time.  3.2.2. Morphology and Vickers Hardness of HGP in Converter Steel Slag 3.2.2. Morphology and Vickers Hardness of HGP in Converter Steel Slag   3.2.2. Morphology and Vickers Hardness of HGP in Converter Steel Slag   As shown in Figure 4, the morphology of HGP in converter steel slag mainly shows three kinds As shown in Figure 4, the morphology of HGP in converter steel slag mainly shows three kinds  As shown in Figure 4, the morphology of HGP in converter steel slag mainly shows three kinds  of grey levels during the initial grinding period (20 min grinding time), while basically exhibiting of grey levels during the initial grinding period (20 min grinding time), while basically exhibiting  of grey levels during the initial grinding period (20 min grinding time), while basically exhibiting  one grey level during the middle–later grinding periods (i.e., 50 min and 70 min grinding times). one  grey  level  during the middle–later grinding periods (i.e.,  50 min and 70 min grinding times).  one  grey  level  during the middle–later grinding periods (i.e.,  50 min and 70 min grinding times).  Based on the identification and analysis results of mineral phases in Section 3.1, it can be known Based on the identification and analysis results of mineral phases in section 3.1, it can be known that  Based on the identification and analysis results of mineral phases in section 3.1, it can be known that  that the mineral phases with three kinds of grey levels are mainly the RO phase, calcium ferrite, and the mineral  phases  with  three  kinds  of  grey  levels are mainly the  RO phase, calcium  ferrite, and  the mineral  phases  with  three  kinds  of  grey  levels are mainly the  RO phase, calcium  ferrite, and  metallic iron phase. So, the HGP of converter steel slag during the initial grinding period is composed metallic  iron  phase.  So,  the  HGP  of  converter  steel  slag  during  the  initial  grinding  period  is  metallic  iron  phase.  So,  the  HGP  of  converter  steel  slag  during  the  initial  grinding  period  is  of RO phase, calcium ferrite, and metallic iron phase, while the HGP is mainly the metallic iron phase composed  of  RO  phase,  calcium  ferrite,  and  metallic  iron  phase,  while  the  HGP  is  mainly  the  composed  of  RO  phase,  calcium  ferrite,  and  metallic  iron  phase,  while  the  HGP  is  mainly  the  during the later grinding period, and the longer grinding time, the higher the proportion of metallic metallic iron phase during the later grinding period, and the longer grinding time, the higher the  metallic iron phase during the later grinding period, and the longer grinding time, the higher the  iron phase. proportion of metallic iron phase.  proportion of metallic iron phase.  3 3 3 3 3 3 3 3 3 3 3 33 3 3 3 3 3 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 22 2 3 3 3 3 3 3 2 3 3 3 3 3 3 3 33 3 3 3 33 2 2 2 2 2 2 2 2 2 2 2 2 2 22 2 2 2 2 22 1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 22 2 2 2 22 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 1 1 1 1 1 1 3 3 33 3 1 1 1 1 2 2 1 1 1 1 1 1 1 11 1 1 1 2 2 22 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 22 2 2 22 2 2 2 2 22 2 2 22 2 3 3 3 3 3 3 3 3 3 33 (a)  (b) (a)  (b) 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 44 4 4 4 44 4 4 4 4 4 4 4 4 4 4 4 44 4 4 4 4 4 4 4 44 4 4 4 4 4 4 4 4 (c)  (d) (c)  (d) Figure 4. Morphologies of oversize substances of steel slag powder vs. grinding time: (a) 0 min; (b)  Figure 4. Morphologies of oversize substances of steel slag powder vs. grinding time: (a) 0 min; (b)  Figure 4. Morphologies of oversize substances of steel slag powder vs. grinding time: (a) 0 min; 20 min; (c) 50 min; (d) 70 min.  20 min; (c) 50 min; (d) 70 min.  (b) 20 min; (c) 50 min; (d) 70 min. Content of oversize products for 0.9mm screen (%) Content of oversize products for 0.9mm screen (%) Appl. Sci. 2016, 6, 237 7 of 15 To reflect HGP characteristics from another point of view, the Vickers hardness of selected regions in Figure 4 were tested and results are shown in Table 2. To some extent, the hardness of the mineral phase can reflect its grindability. Results of Vicker hardness show that the RO phase and calcium ferrite phase have high hardness compared with the silicate phase, which may be one reason why these two mineral phases are HGP. Meanwhile, metallic iron phase with very low hardness is also HGP because of its high flexibility. Thus, the HGP is explained from Vickers hardness of mineral phases. Table 2. Vickers hardness of selected region in Figure 4 (HV). Region number 1 2 3 4 Mineral phases Silicate phase Calcium ferrite phase RO phase Metallic iron phase Vickers hardness 187.3 335.0 298.1 29.4 3.2.3. Chemical Compositions of HGP in Converter Steel Slag The chemical compositions of oversized substances passed a 0.9 mm square-hole screen under different grinding times, which were determined by X-ray fluorescence analysis (XRF) and chemical titration, are shown in Table 3. The results show that the contents of CaO and SiO in HGP gradually decrease with the increase in grinding time, while the contents of metallic iron show a reverse trend, and the contents of Fe O and MgO are firstly increased and then decreased with the increasing of 2 3 grinding time. In the initial grinding period (10–20 min grinding), the chemical compositions of HGP mainly consist of CaO, SiO , Fe O , and MgO; in the middle grinding period (40–50 min grinding), 2 2 3 it mainly consists of Fe O and metallic iron; in the later grinding period (60–70 min grinding), metallic 2 3 iron takes up the most component. So the HGP have different chemical compositions during different grinding period. Table 3. Chemical compositions of hardly grinding phases (HGP) in steel slag vs. grinding time (%). Grinding time (min) CaO SiO Al O Fe O Fe MgO 2 2 3 2 3 10 32.45 12.05 1.15 34.80 3.53 8.43 20 26.12 8.26 1.06 37.07 7.60 12.29 30 20.30 5.81 0.92 38.13 13.36 13.07 40 14.70 4.16 0.75 39.08 23.10 10.13 50 6.14 3.05 0.62 32.12 45.53 7.20 60 5.19 2.03 0.51 18.09 65.33 4.12 70 4.04 1.14 0.44 11.25 76.20 3.21 3.3. Comparison of Grinding Characteristic between Untreated and Pre-treated Converter Steel Slag From the research result presented in the previous section, the iron-rich phases, especially metallic iron, are the main hardly grinding phases, so it is necessary to remove or recycle the iron-rich phases before the grinding process of steel slag. In this study, the grinding characteristics of untreated and pretreated converter steel slag were compared with respect to iron mineral phases, grinding efficiency, particle size distribution, and particle morphology (Comment: the pretreatment of steel slag is the preliminary magnetic separation and multistage screening and magnetic separation, as shown in Figure 5). Appl. Sci. 2016, 6, 237 8 of 15 Appl. Sci.2016, 9, 237  8 of 16  Untreated Steel slag after primary Steel slag after multi-stage First step multi-step converter magnetic separation screening and magnetic steel slag (PMS-steel slag) separation (MSMS-steel slag) Pre-milling Magnetic separation Crushing Magnetic separation Screening Recycli ng Recycli ng Scrap steel in slag Scrap steel and iron concentrate in slag Figure 5. The technical flow process of recycling iron‐rich phases in steel slag.  Figure 5. The technical flow process of recycling iron-rich phases in steel slag. 3.3.1. Total Analysis of Iron Mineral Phases in Converter Steel Slag After Pre‐treatment  3.3.1. Total Analysis of Iron Mineral Phases in Converter Steel Slag after Pre-treatment Total analysis results of iron mineral phases in converter steel slag after preliminary magnetic  Total analysis results of iron mineral phases in converter steel slag after preliminary magnetic separation (denoted  as PMS)  and  multistage magnetic separation (denoted  as SMS)  are shown in  Table 4.  separation (denoted as PMS) and multistage magnetic separation (denoted as SMS) are shown in Table 4. Appl. Sci.2016 From , 9, 237 the  results, it can be seen that the proportions of iron phases in steel slag are obviously 8 of  16 reduced after pretreatment. The proportion of metallic iron in steel slag is decreased from 2.38% to  Table 4. Total analysis results of iron mineral phases in converter steel slag. 1.18% by preliminary magnetic separation, and both the metallic iron proportion and total content  Untreated Steel slag after primary Steel slag after multi-stage First step multi-step converter magnetic separation screening and magnetic of iron  phases in steel slag are decreased by multistage magnetic separation, from 2.38% to 0.45%  Proportions of iron mineral phases (%) steel slag (PMS-steel slag) separation (MSMS-steel slag) Iron mineral phases and 18.26% to 9.20%, respectively, indicating that the effect of pretreatment on iron‐rich phases is  Untreated steel slag PMS-steel slag SMS-steel slag obvious. The removal of iron‐rich phases in steel slag is a very favorable factor for preparation and  Metallic iron & magnetite 2.38 1.18 0.45 application of steel slag powder.  Hematite/limonite 13.11 13.05 6.67 Sulfide 0.04 0.04 0.04 Table 4. Total analysis results of iron mineral phases in converter steel slag.  Siderite 2.52 2.26 1.98 Crushing Magnetic separation Screening Pre-milling Magnetic separation Proportions of iron mineral phases (%)  Iron silicate 0.21 0.10 0.06 Iron mineral phases  Total Untreated 18.26  steel slag PMS16.63 ‐steel slag SMS‐st 9.20 eel slag Recycli ng Recycli ng Metallic iron & magnetite  2.38  1.18  0.45  Scrap steel in slag Scrap steel and iron concentrate in slag Hematite/limonite  13.11  13.05  6.67  From the results, it can be seen that the proportions of iron phases in steel slag are obviously Sulfide  0.04  0.04  0.04  reduced after pretreatment. The proportion of metallic iron in steel slag is decreased from 2.38% to Figure 5. Siderite The t   echnical flow process 2.52 of re   cycling iron-rich p 2.h26 ases   in steel slag. 1.98  Iron silicate  0.21  0.10  0.06  1.18% by preliminary magnetic separation, and both the metallic iron proportion and total content of Total  18.26  16.63  9.20  3.3.1. Total Analysis of Iron Mineral Phases in Converter Steel Slag After Pre-treatment iron phases in steel slag are decreased by multistage magnetic separation, from 2.38% to 0.45% and 18.26% to 9.20%, respectively, indicating that the effect of pretreatment on iron-rich phases is obvious. Total analysis results of iron mineral phases in converter steel slag after preliminary magnetic 3.3.2. Grinding Efficiency of Untreated and Pretreated Steel Slag  The removal of iron-rich phases in steel slag is a very favorable factor for preparation and application separation (denoted as PMS) and multistage magnetic separation (denoted as SMS) are shown in The specific surface areas of untreated and pretreated steel slag under different grinding times  Table 4. of steel slag powder. are shown in Figure 6.  3.3.2. Grinding Efficiency of Untreated and Pretreated Steel Slag 3.3.2. Grinding Efficiency of Untreated and Pretreated Steel Slag 500 500 Untreated steel slag Untreated steel slag The specific surface areas of untreated and pretreated 450 steel slag under different grinding times 450 Pre-treated steel slag The specific surface areas of untreated and pretreated steel slag under different grinding times are Pre-treated steel slag are shown in Figure 6. 400 shown in Figure 6. SSA=157.44x-207.28 R =0.9906 500 500 Untreated steel slag Untreated steel slag 450 Pre-treated steel slag 250 SSA=124.7x-155.85 Pre-treated steel slag R =0.9605 20 400 0 SSA=157.44x-207.28 R =0.9906 2.0 2.5 3.0 3.5 4.0 4.5 10 20 30 40 50 60 70 x=lnt Grinding time (min) 250 SSA=124.7x-155.85 R =0.9605 2.0 2.5 3.0 3.5 4.0 4.5 10 20 30 40 50 60 70 x=lnt Grinding time (min) (b) (a) Figure 6. Specific surface areas (SSA) of steel slag under different grinding time: (a) SSA vs. grinding Figure 6. Specific surface areas (SSA) of steel slag under different grinding time: (a) SSA vs. grinding time; (b) linear fitting of SSA. time; (b) linear fitting of SSA. As shown in Figure 6, the specific surface areas of untreated and pretreated steel slag are gradually increased with increasing grinding time, and starting from 50 min grinding time, the tendency to increase is slowed down due to the agglomeration of fine particles. The specific surface area of pretreated steel slag is obviously higher than that of untreated steel slag after the same grinding time. For example, the specific surface area of untreated steel slag is only 360 m /kg after 60 min grinding, while that of pretreated steel slag reaches 361 m /kg after 40 min grinding, thus 20 min grinding time is saved. The relationship of specific surface area vs. grinding time was fitted, and the result is as follows: SSA=− 124.7 ln t 155.85 (R =0.9605) (Untreated steel slag) (1) SSA=− 157.44 ln t 207.28 (R =0.9906) (Pretreated steel slag) (2) Specific surface area (m /kg) Specific surface area (m /kg) 2 2 Specific surface area (m S /kg pecifi ) c surface area (m /kg) Appl. Sci. 2016, 6, 237 9 of 15 As shown in Figure 6, the specific surface areas of untreated and pretreated steel slag are gradually increased with increasing grinding time, and starting from 50 min grinding time, the tendency to increase is slowed down due to the agglomeration of fine particles. The specific surface area of pretreated steel slag is obviously higher than that of untreated steel slag after the same grinding time. For example, the specific surface area of untreated steel slag is only 360 m /kg after 60 min grinding, while that of pretreated steel slag reaches 361 m /kg after 40 min grinding, thus 20 min grinding time is saved. The relationship of specific surface area vs. grinding time was fitted, and the result is as follows: SSA = 124.7ln t 155.85 (R = 0.9605) (Untreated steel slag) (1) SSA = 157.44ln t 207.28 (R = 0.9906) (Pretreated steel slag) (2) From the above fitted result, it can be seen that the relationship of specific surface area with the logarithm of grinding time shows a good linear relationship. After applying derivation calculus to the above equations, we can get the increasing speeds of specific surface area: dSSA 124.7 = (Untreated steel slag) (3) dt t dSSA 157.44 = (Pretreated steel slag) (4) dt t It is obvious that the grinding speed of pretreated steel slag is higher than that of untreated steel slag. 3.3.3. Particle Size Distributions of Ground Untreated and Pretreated Steel Slag The particle size distribution and median diameter of ground untreated and pretreated steel slag powder are shown in Table 5 and Figure 7, respectively. Table 5. Particle size distribution of ground steel slag powder under different grinding time (%). Untreated steel slag Pretreated steel slag Grinding time (min) <3 m 3–32 m 32–65 m >65 m <3 m 3–32 m 32–65 m >65 m 10 2.77 27.88 25.24 44.11 2.44 25.42 21.95 50.19 20 4.97 37.16 27.54 30.33 5.66 40.06 30.84 23.45 30 6.64 36.62 28.04 28.70 9.00 44.22 32.10 14.68 40 8.16 38.24 27.46 26.15 10.54 48.80 30.51 10.15 50 11.18 37.90 22.19 28.73 14.87 56.99 22.65 5.49 60 8.99 32.52 26.57 31.91 16.36 53.83 20.96 8.84 70 10.91 30.99 19.12 38.97 14.50 55.74 21.47 8.29 Appl. Sci.2016, 9, 237  10 of 16  Untreated steel slag Pre-treated steel slag 10 20 30 40 50 60 70 Grinding time (min) Figure 7. Median diameter of ground steel slag powder under different grinding time.  Figure 7. Median diameter of ground steel slag powder under different grinding time. As shown in Table 5, with increasing grinding time (≤ 50 min), the proportion of particles for  more than 32 μm (especially particles for more than 65 μm) is gradually and obviously decreased,  while the proportion of particles for less than 32 μm shows a reverse tendency, which indicates that  the  particle  size  distribution  of  steel  slag  is  optimized  by  mechanical  grinding.  However,  when  grinding time is more than 50 min, it shows a complex or chaotic tendency due to the agglomeration  of fine particles. Compared with untreated steel slag, the proportion of particles for less than 32 μm  in  pretreated  steel  slag  powder  is  much  higher  after the same grinding time, indicating that the  increasing tendency of fine particles for pretreated steel slag powder is faster.   For median diameter (D50), with the increasing of grinding time, the median diameter of steel  slag is firstly decreased before 50min grinding time, and then is increased after that, which is similar  to the variation tendency of particle proportion of more than 32 μm. At 50 min grinding time, the  median diameter of untreated steel slag reaches a minimum, 32.89 μm, which is much larger than  that of pretreated  steel  slag, 18.16 μm. This shows that it is difficult for  untreated steel slag  to  be  finely ground.  3.3.4. Particle Morphologies of Ground Untreated and Pretreated Steel Slag  The particle morphologies of two kinds of ground steel slag powder after 50 min grinding are  shown in Figure 8. It can be seen from the figure that steel slag particles are mainly spherical  in  shape, and fine particles adhere to the larger particles. By comparison, the overall particle size of  pretreated steel slag powder is smaller than that of untreated steel slag, and particle uniformity in  size is also superior to that of untreated steel slag. To some extent, both the ground untreated and  pretreated steel slag powders show some agglomeration phenomenon.  5μ m 5μ m 50μ m 50μ m Median diameter (m) Appl. Sci. 2016, 6, 237 10 of 15 As shown in Table 5, with increasing grinding time ( 50 min), the proportion of particles for more than 32 m (especially particles for more than 65 m) is gradually and obviously decreased, while the proportion of particles for less than 32 m shows a reverse tendency, which indicates that the particle size distribution of steel slag is optimized by mechanical grinding. However, when grinding time is more than 50 min, it shows a complex or chaotic tendency due to the agglomeration of fine particles. Compared with untreated steel slag, the proportion of particles for less than 32 m in pretreated steel slag powder is much higher after the same grinding time, indicating that the increasing tendency of Appl. Sci.2016, 9, 237 10 of 16 fine particles for pretreated steel slag powder is faster. For median diameter (D50), with the increasing of grinding time, the median diameter of steel to the variation tendency of particle proportion of more than 32 μm. At 50 min grinding time, the slag is firstly decreased before 50min grinding time, and then is increased after that, which is similar to median diameter of untreated steel slag reaches a minimum, 32.89 μm, which is much larger than the variation tendency of particle proportion of more than 32 m. At 50 min grinding time, the median that of pretreated steel slag, 18.16 μm. This shows that it is difficult for untreated steel slag to be diameter of untreated steel slag reaches a minimum, 32.89 m, which is much larger than that of finely ground. pretreated steel slag, 18.16 m. This shows that it is difficult for untreated steel slag to be finely ground. 3.3.4. Particle Morphologies of Ground Untreated and Pretreated Steel Slag 3.3.4. Particle Morphologies of Ground Untreated and Pretreated Steel Slag The particle morphologies of two kinds of ground steel slag powder after 50 min grinding are The particle morphologies of two kinds of ground steel slag powder after 50 min grinding are shown in Figure 8. It can be seen from the figure that steel slag particles are mainly spherical in shown in Figure 8. It can be seen from the figure that steel slag particles are mainly spherical in shape, shape, and fine particles adhere to the larger particles. By comparison, the overall particle size of and fine particles adhere to the larger particles. By comparison, the overall particle size of pretreated pretreated steel slag powder is smaller than that of untreated steel slag, and particle uniformity in steel slag powder is smaller than that of untreated steel slag, and particle uniformity in size is also size is also superior to that of untreated steel slag. To some extent, both the ground untreated and superior to that of untreated steel slag. To some extent, both the ground untreated and pretreated steel pretreated steel slag powders show some agglomeration phenomenon. slag powders show some agglomeration phenomenon. 5μ m 5μ m 50μ m 50μ m (a) (b) Figure 8. Particle morphology of ground steel slag powder after 50min grinding time: (a) untreated Figure 8. Particle morphology of ground steel slag powder after 50min grinding time: (a) untreated steel slag; (b) pretreated steel slag. steel slag; (b) pretreated steel slag. 3.4. Effect of Organic Grinding Aids on the Grinding Property of Converter Steel Slag 3.4. Effect of Organic Grinding Aids on the Grinding Property of Converter Steel Slag From the above results of studies, it can be seen that the agglomeration phenomenon of fine From the above results of studies, it can be seen that the agglomeration phenomenon of fine particles will occur during deep grinding periods, which seriously affect or reduce the grinding particles will occur during deep grinding periods, which seriously affect or reduce the grinding efficiency of steel slag. In recent years, organic grinding aids (GA) have been used to improve the efficiency of steel slag. In recent years, organic grinding aids (GA) have been used to improve the grinding efficiency of cement. These researches show that organic GA molecules can be adsorbed on grinding efficiency of cement. These researches show that organic GA molecules can be adsorbed on the particles and effectively weaken the agglomeration of fine particle in the grinding process, thus the particles and effectively weaken the agglomeration of fine particle in the grinding process, thus improving the dispersion of powder and grinding efficiency [28–31]. Therefore, it is a good choice to improving the dispersion of powder and grinding efficiency [28–31]. Therefore, it is a good choice to use GA in the grinding process of steel slag for many manufacturers to improve preparation use GA in the grinding process of steel slag for many manufacturers to improve preparation efficiency efficiency of steel slag powder. In additional, on the basis of pretreatment, the grinding property of of steel slag powder. In additional, on the basis of pretreatment, the grinding property of steel slag can steel slag can be further improved by GA. In this study, glycerol, which is a common cement GA, be further improved by GA. In this study, glycerol, which is a common cement GA, was selected as the was selected as the GA of steel slag. The effects of GA on the grinding property of steel slag were GA of steel slag. The effects of GA on the grinding property of steel slag were studied from sieving studied from sieving residue, particle size distribution, angle of repose, and particle morphology. residue, particle size distribution, angle of repose, and particle morphology. 3.4.1. Effect of GA on the Sieving Residue of Steel Slag Powder As shown in Figure 9, it can be seen that the sieving residue of steel slag powder with GA is lower than that of the blank group (i.e., without any GA) after the same grinding time, indicating that ground steel slag with GA is smaller in particle size. The reducing effect of GA on the sieving residue is gradually enhanced with the increasing of grinding time before reaching 50 min grinding time. At the initial grinding period (before 20 min), the role of GA is small, but it becomes obvious after 20 min and reaches an optimum grinding role at 50 min grinding time. Appl. Sci.2016, 9, 237  11 of 16  (a)  (b) Figure 8. Particle morphology of ground steel slag powder after 50min grinding time: (a) untreated  steel slag; (b) pretreated steel slag.  3.4. Effect of Organic Grinding Aids on the Grinding Property of Converter Steel Slag  From the above results of studies, it can be seen  that the agglomeration phenomenon  of fine  particles  will  occur  during deep grinding periods, which seriously  affect  or  reduce  the grinding  efficiency of steel slag. In recent years, organic grinding aids (GA) have been used to improve the  grinding efficiency of cement. These researches show that organic GA molecules can be adsorbed on  the particles and effectively weaken the agglomeration of fine particle in the grinding process, thus  improving the dispersion of powder and grinding efficiency [28–31]. Therefore, it is a good choice to  use  GA  in  the  grinding  process  of  steel  slag  for  many  manufacturers  to  improve  preparation  efficiency of steel slag powder. In additional, on the basis of pretreatment, the grinding property of  steel slag can be further improved by GA. In this study, glycerol, which is a common cement GA,  was selected as the GA of steel slag. The effects of GA on the grinding property of steel slag were  Appl. Sci. 2016, 6, 237 11 of 15 studied from sieving residue, particle size distribution, angle of repose, and particle morphology.  3.4.1. Effect of GA on the Sieving Residue of Steel Slag Powder 3.4.1. Effect of GA on the Sieving Residue of Steel Slag Powder  As shown in Figure 9, it can be seen that the sieving residue of steel slag powder with GA is As shown in Figure 9, it can be seen that the sieving residue of steel slag powder with GA is  lower than that of the blank group (i.e., without any GA) after the same grinding time, indicating lower than that of the blank group (i.e., without any GA) after the same grinding time, indicating  that ground steel slag with GA is smaller in particle size. The reducing effect of GA on the sieving that ground steel slag with GA is smaller in particle size. The reducing effect of GA on the sieving  residue is gradually enhanced with the increasing of grinding time before reaching 50 min grinding residue is gradually enhanced with the increasing of grinding time before reaching 50 min grinding  time. At the initial grinding period (before 20 min), the role of GA is small, but it becomes obvious time. At the initial grinding period (before 20 min), the role of GA is small, but it becomes obvious  after 20 min and reaches an optimum grinding role at 50 min grinding time. after 20 min and reaches an optimum grinding role at 50 min grinding time.  Blank With 0.05% GA 40  Appl. Sci.2016, 9, 237  12 of 16  coarse and have a good dispersion at this stage, basically having no agglomeration of fine particles.  Reducing role of GA on sieve residue The  grinding  efficiency  of  steel  slag  is  basically  not  influenced  by  dispersion  of  particles,  so  the  optimization role of GA on the part 10 icle 2si 0 ze distribut 30 40ion is  5not 0  obvious. 60 70 During 40 min, 50 min, and  Grinding time (min) 60 min grinding time, the proportions of particles in the range of 3–32 μm are obviously increased  by 7.24% (absolute value, same below), 7.22%, and 10.63%, respectively, after adding GA, while the  Figure 9. Sieving residue of steel slag powder with grinding aid (GA) vs. grinding time. Figure 9. Sieving residue of steel slag powder with grinding aid (GA) vs. grinding time.  proportions of particles in the range of more than 32 μm are also obviously decreased, indicating  that GA can efficiently weaken agglomeration of fine particles, then improve the dispersion of steel  3.4.2. Effect of GA on the Particle Size Distribution of Steel Slag Powder 3.4.2. Effect of GA on the Particle Size Distribution of Steel Slag Powder  slag  powder.  Meanwhile,  many  researches  indicate  that  3–32 μm  particles  have  the  greatest  The The uni uniformi formity ty coefficient coefficient of of stee steel l slslag ag po powder wder can can  reflect reflect  thethe  extent extent  of the of the widwidth th of part of icle particle  size  contribution on the property of cement‐based materials, so the improvement of 3–32 μm particles  size distribution, and the smaller the uniformity coefficient, the wider the particle size distribution. distribution, and the smaller the uniformity coefficient, the wider the particle size distribution. From  proportion  after  using  GA  shows  that  GA  can  significantly  optimize  particle  size  distribution  of  Fig From ureFigur  10, it e can 10, itbe can  seen be th seen at the that un the ifouniformity rmity coeffic coef ient ficient  of stee ofl steel slag powder slag powder  is firs istly firstly  increincr ased eased  and  steel slag powder. At 70 min grinding time, as the agglomeration of particles is very serious, GA can  and then gradually decreased with the increasing of grinding time. By comparison, the uniformity then  gradually  decreased  with  the  increasing  of  grinding  time.  By  comparison,  the  uniformity  not  completely  resist  it  due  to  the  fixed  dosage  (0.05%),  resulting  in  the  weakening  of  GA’s  coefficient coefficient of of steel steel sl slag ag powder powder with with GA GA is is la larrger ger tha than n tha thatt witho without ut GA GA for for the the same same grind grinding ing ti time, me,  optimizing  role.  It  shows  that  the  optimizing  role  of  the  fixed  dosage  GA  on  the  particle  size  indicating that the particle size distribution of steel slag powder can be narrowed by GA. The effect is indicating that the particle size distribution of steel slag powder can be narrowed by GA. The effect  distribution of steel slag powder is different under different grinding times, and the role of 0.05%  is the the str stronge ongest s att at 50 50 min  min and and 60 60 min  migrinding n grinding times.  times.  dosage of GA is best during 40–60 min grinding time.  As shown in Table 6, it can be seen that the effect of GA on the particle size distribution of steel  slag powder is relatively small before reaching 40 min grinding time. The reason is that particles are  1.25 Blank With 0.5% GA 1.20 1.15 1.10 1.05 1.00 0.95 0.90 0.85 10 20 30 40 50 60 70 Grinding time (min) Figure 10. Uniformity coefficient of steel slag powder with GA vs. grinding time. Figure 10. Uniformity coefficient of steel slag powder with GA vs. grinding time.  Table 6. Particle size distribution of steel slag powder with GA under different grinding time (%).  Without GA  With GA  Grinding  time (min)  <3μm  >65μm <3μm >65μm 3–32μm  32–65μm 3–32μm 32–65μm  10  2.44  25.42  21.95  50.19  2.83  23.83  22.80  50.54  20  5.66  40.06  30.84  23.45  4.81  37.44  34.14  23.62  30  9.00  44.22  32.10  14.68  7.63  47.08  32.74  12.55  40  10.54  48.80  30.51  10.15  10.44  56.04  28.47  5.04  50  14.87  56.99  22.65  5.49  14.32  64.21  21.10  0.38  60  16.36  53.83  20.96  8.84  15.73  64.46  18.61  1.20  70  14.50  55.74  21.47  8.29  15.05  57.33  19.49  8.13  3.4.3. Effect of GA on the Angle of Repose of Steel Slag Powder  Powder fluidity is usually characterized by the angle of repose. The smaller the angle of repose,  the better the powder fluidity. As shown in Figure 11, it can be seen that the angle of repose of steel  slag powder is decreased after adding GA, indicating that GA can improve the fluidity of steel slag  powder. The role of GA on the angle of repose is firstly enhanced and then weakened on the whole,  which is similar to the role of GA on the sieving residue, and it is reaches maximum effectiveness at  50  min  grinding  time,  i.e.,  the  action  effect  of  GA  on  powder  fluidity  is  the  greatest  at  50  min  grinding time.  Uniformity coefficient 45m sieve residue (%) Appl. Sci. 2016, 6, 237 12 of 15 As shown in Table 6, it can be seen that the effect of GA on the particle size distribution of steel slag powder is relatively small before reaching 40 min grinding time. The reason is that particles are coarse and have a good dispersion at this stage, basically having no agglomeration of fine particles. The grinding efficiency of steel slag is basically not influenced by dispersion of particles, so the optimization role of GA on the particle size distribution is not obvious. During 40 min, 50 min, and 60 min grinding time, the proportions of particles in the range of 3–32 m are obviously increased by 7.24% (absolute value, same below), 7.22%, and 10.63%, respectively, after adding GA, while the proportions of particles in the range of more than 32 m are also obviously decreased, indicating that GA can efficiently weaken agglomeration of fine particles, then improve the dispersion of steel slag powder. Meanwhile, many researches indicate that 3–32 m particles have the greatest contribution on the property of cement-based materials, so the improvement of 3–32 m particles proportion after using GA shows that GA can significantly optimize particle size distribution of steel slag powder. At 70 min grinding time, as the agglomeration of particles is very serious, GA can not completely resist it due to the fixed dosage (0.05%), resulting in the weakening of GA’s optimizing role. It shows that the optimizing role of the fixed dosage GA on the particle size distribution of steel slag powder is different under different grinding times, and the role of 0.05% dosage of GA is best during 40–60 min grinding time. Table 6. Particle size distribution of steel slag powder with GA under different grinding time (%). Without GA With GA Grinding time (min) <3 m 3–32 m 32–65 m >65 m <3 m 3–32 m 32–65 m >65 m 10 2.44 25.42 21.95 50.19 2.83 23.83 22.80 50.54 20 5.66 40.06 30.84 23.45 4.81 37.44 34.14 23.62 30 9.00 44.22 32.10 14.68 7.63 47.08 32.74 12.55 40 10.54 48.80 30.51 10.15 10.44 56.04 28.47 5.04 50 14.87 56.99 22.65 5.49 14.32 64.21 21.10 0.38 60 16.36 53.83 20.96 8.84 15.73 64.46 18.61 1.20 70 14.50 55.74 21.47 8.29 15.05 57.33 19.49 8.13 3.4.3. Effect of GA on the Angle of Repose of Steel Slag Powder Powder fluidity is usually characterized by the angle of repose. The smaller the angle of repose, the better the powder fluidity. As shown in Figure 11, it can be seen that the angle of repose of steel slag powder is decreased after adding GA, indicating that GA can improve the fluidity of steel slag powder. The role of GA on the angle of repose is firstly enhanced and then weakened on the whole, which is similar to the role of GA on the sieving residue, and it is reaches maximum effectiveness at 50 min grinding time, i.e., the action effect of GA on powder fluidity is the greatest at 50 min grinding time. Appl. Sci.2016, 9, 237  13 of 16  Blank With 0.05wt% GA  Role of GA on angle of repose 10 20 30 40 50 60 70 Grinding time (min) Figure 11. Repose angle of steel slag powder with GA vs. grinding time. Figure 11. Repose angle of steel slag powder with GA vs. grinding time.  3.4.4. Effect of GA on the Particle Morphology of Steel Slag Powder  The particle morphologies of steel slag powder with and without GA are shown in Figure 12. It  can  be  seen  from  the  figure  that  the  particles  of  steel  slag  powder  without  GA  exhibit  the  agglomeration phenomenon where fine particles adhere to each other. Especially at 70 min grinding  time,  agglomeration  of  particles  is  very  serious  and  many  large  particles  are  regenerated  by  adhering  of  fine  particles.  By  comparison,  after  using  GA,  the  agglomeration  phenomenon  of  particles is not obvious at 50min grinding time, still keeping a relatively good dispersion. Although  the particles with GA also exhibit a little agglomeration at 70 min grinding time, their dispersion is  still  obviously  better  than  those  without  GA.  The  above  results  confirm  that  GA  can  efficiently  weaken the agglomeration of fine particles when particle size becomes small and agglomeration of  fine particles occurs.   5μ m 5μ m Agglomeration of particles Agglomeration of particles 50μ m 20μ m 50min 70min (a)  Repose angle of powder ( ) Appl. Sci.2016, 9, 237  13 of 16  Blank With 0.05wt% GA Role of GA on angle of repose 10 20 30 40 50 60 70 Grinding time (min) Figure 11. Repose angle of steel slag powder with GA vs. grinding time.  Appl. Sci. 2016, 6, 237 13 of 15 3.4.4. Effect of GA on the Particle Morphology of Steel Slag Powder  3.4.4. Effect of GA on the Particle Morphology of Steel Slag Powder The particle morphologies of steel slag powder with and without GA are shown in Figure 12. It  can  be  seen  from  the  figure  that  the  particles  of  steel  slag  powder  without  GA  exhibit  the  The particle morphologies of steel slag powder with and without GA are shown in Figure 12. agglomeration phenomenon where fine particles adhere to each other. Especially at 70 min grinding  It can be seen from the figure that the particles of steel slag powder without GA exhibit the time,  agglomeration  of  particles  is  very  serious  and  many  large  particles  are  regenerated  by  agglomeration phenomenon where fine particles adhere to each other. Especially at 70 min grinding adhering  of  fine  particles.  By  comparison,  after  using  GA,  the  agglomeration  phenomenon  of  time, agglomeration of particles is very serious and many large particles are regenerated by adhering particles is not obvious at 50min grinding time, still keeping a relatively good dispersion. Although  of fine particles. By comparison, after using GA, the agglomeration phenomenon of particles is not the particles with GA also exhibit a little agglomeration at 70 min grinding time, their dispersion is  obvious at 50min grinding time, still keeping a relatively good dispersion. Although the particles with still  obviously  better  than  those  without  GA.  The  above  results  confirm  that  GA  can  efficiently  GA also exhibit a little agglomeration at 70 min grinding time, their dispersion is still obviously better weaken the agglomeration of fine particles when particle size becomes small and agglomeration of  than those without GA. The above results confirm that GA can efficiently weaken the agglomeration fine particles occurs.   of fine particles when particle size becomes small and agglomeration of fine particles occurs. 5μ m 5μ m Agglomeration of particles Agglomeration of particles 50μ m 20μ m 50min 70min Appl. Sci.2016, 9, 237  14 of 16  (a)  3μ m 3μ m Dispersion of particles Dispersion of particles a little agglomeration 20μ m 10μ m 70min 50min (b)  Figure  12.  Particle  morphologies  of  steel slag  powder  with  and  without  GA:  (a)  without  GA;  (b)  Figure 12. Particle morphologies of steel slag powder with and without GA: (a) without GA; with GA.  (b) with GA. 4. Conclusions  4. Conclusions From this study, we can conclude that:  From this study, we can conclude that: (1) It was evident from analysis of BSE–EDX that the mineral phases of converter steel slag mainly (1)  It  was  evident  from  analysis  of  BSE–EDX  that the mineral phases of converter  steel slag  contain silicate mineral phase, RO phase, calcium ferrite phase, and a small amount of metallic iron mainly  contain  silicate  mineral  phase,  RO  phase,  calcium  ferrite  phase,  and  a  small  amount  of  phase, among others. metallic iron phase, among others.  (2) The oversize substance after screening can be considered as the hardly grinding phases (HGP), (2)The  oversize  substance  after  screening  can  be  considered  as  the  hardly  grinding  phases  which provides a simple method of determining the HGP in steel slag. The HGP proportion which (HGP), which provides a simple method of determining the HGP in steel slag. The HGP proportion  which  was  determined  by  a  0.9  mm  square‐hole  screen  is  about  1.5%.  After  the  initial  20  min  grinding, the RO phase, calcium ferrite, and metallic iron phase made up most of the proportion in  the HGP, while the metallic iron made up the most component after 70 min grinding.  (3) For steel slag powder with about 360 m /kg specific surface area (SSA), 20 min of grinding  time can be saved with pretreatment. The relationships of SSA with the logarithm of grinding time  show  a  good  linear  relationship,  but  pretreated  steel  slag  has  a  higher  grinding  efficiency.  In  addition, the D50 of untreated steel slag can only reach 32.89 μm after 50 min grinding, but that of  pretreated steel slag can reach 18.16 μm after the same grinding time.  (4) Organic grinding aids (GA) can obviously improve the grinding property of steel slag, and  the action effects of 0.05% dosage of GA on the grinding efficiency and particle characteristics were  the best during 40–60 min grinding time. especially the proportions of particles in the range of 3–32  μm, which were obviously increased by 7.24%, 7.22%, and 10.63% after 40 min, 50 min and 60 min  grinding, respectively. This is mainly because of the reduction of agglomeration after the use of GA,  as evidenced by SEM images.  Acknowledgments:  This  work  was  financially  supported  by  the  Chinaʹs  Post‐doctoral  Science  Fund  (No.  2016M591170), the Open Fund of Key Laboratory of Advanced Civil Engineering Materials (Tongji University),  Ministry of Education (No.201602).  Author Contributions: J.Z. conceived of, designed and performed the experiments. D.W. and P.Y. analyzed  the data and discussed the results. W.L. participated in writing this paper.  Conflicts of Interest: The authors declare no conflict of interest.  References  1. Yi, H.; Xu, G.P.; Cheng, H.G.; Wang, J.S.; Wan, Y.F.; Chen, H. An overview of utilization of steel slag. Proc.  Environ. Sci. 2012, 16, 791–801.  Repose angle of powder ( ) Appl. Sci. 2016, 6, 237 14 of 15 was determined by a 0.9 mm square-hole screen is about 1.5%. After the initial 20 min grinding, the RO phase, calcium ferrite, and metallic iron phase made up most of the proportion in the HGP, while the metallic iron made up the most component after 70 min grinding. (3) For steel slag powder with about 360 m /kg specific surface area (SSA), 20 min of grinding time can be saved with pretreatment. The relationships of SSA with the logarithm of grinding time show a good linear relationship, but pretreated steel slag has a higher grinding efficiency. In addition, the D50 of untreated steel slag can only reach 32.89 m after 50 min grinding, but that of pretreated steel slag can reach 18.16 m after the same grinding time. (4) Organic grinding aids (GA) can obviously improve the grinding property of steel slag, and the action effects of 0.05% dosage of GA on the grinding efficiency and particle characteristics were the best during 40–60 min grinding time. especially the proportions of particles in the range of 3–32 m, which were obviously increased by 7.24%, 7.22%, and 10.63% after 40 min, 50 min and 60 min grinding, respectively. This is mainly because of the reduction of agglomeration after the use of GA, as evidenced by SEM images. Acknowledgments: This work was financially supported by the China’s Post-doctoral Science Fund (No. 2016M591170), the Open Fund of Key Laboratory of Advanced Civil Engineering Materials (Tongji University), Ministry of Education (No. 201602). Author Contributions: J.Z. conceived of, designed and performed the experiments. D.W. and P.Y. analyzed the data and discussed the results. W.L. participated in writing this paper. 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Published: Oct 28, 2016

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