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Research Regarding Iron Sludge Recovery Technology

Research Regarding Iron Sludge Recovery Technology Revista Minelor – Mining Revue ISSN-L 1220-2053 / ISSN 2247-8590 vol. 28, issue 2 / 2022, pp. 21-28 1* 2 Eugen TRAISTĂ , Camelia TRAISTĂ University of Petrosani, Petrosani, Romania, eugen_traista@yahoo.com University of Petrosani, Petrosani, Romania DOI: 10.2478/minrv-2022-0010 Abstract: One of the most important wastes in iron metallurgy is the blast furnace sludge. This sludge consists of fine particles of iron ore, coke and fine particles of flux. The furnace sludge is characterized by the chemical composition similar to that of the furnace load, the major difference being the concentration of zinc and lead. Due to the similarity with the blast furnace load, this material, after pelletization, can be recycled in the technological process. However, this recirculation is limited by the zinc content, which significantly disrupts the operation of the furnace. This paper presents tests to reduce the zinc content of the furnace sludge by hydrometallurgical and pyrometalurgical processes. Keywords: iron sludge, metallurgy, iron ore, recovery, furnace, leaching 1. Introduction As a result of industrial development, more and more industrial waste results during production processes. From the iron production processes results the furnace sludge which is one of the hazardous metallurgical wastes [1]. The production of iron and its alloys is the most important metallurgical process [2]. In the production process, in addition to Fe and C, many other elements are introduced into the furnace. Zinc is especially a problem because during the metallurgical process it distills due to the very high temperatures in the furnace and subsequently condenses on the furnace walls in areas with lower temperature, which leads to a large amount of dust and possible damage to the furnace. For a good process, the Zn concentration in the ore must not exceed 0.12 kg per tonne of cast iron produced. Partly, the evaporated zinc condenses on the dust particles in the effluent gas, the higher concentration of zinc founding in the finer dust particles. Normally each ton of cast iron results in 8 - 12 kg of dust. This dust is removed by the flue gas purification system. Large particles are removed from the gases by cyclones and filter bag filters and can be reintroduced directly into the furnace after sintering because the Zn content is generally low (<0.1% Zn). Fine particles are removed through a wet scrubber from which results a sludge [2]. Generally, blast furnace sludge contains 21-32% Fe, 15-35% C, 1.0-3.2% Zn and 0.3-1.2% Pb, reported to dry mass [3, 4]. In Europe alone, the steel industry produces about 500,000 tons of blast furnace sludge annually. Because the iron and coke content of the sludge is high, it is possible to recycle the sludge in the furnace. However, due to the zinc content of a few percent, the use of this sludge is restricted. The resulting sludge after removal of fine dust in wet filters is deposited in a tailings pond. Groundwater can contaminate groundwater with zinc and lead in this sludge. Also, the new regulations no longer allow long-term storage of waste. For this reason, research is being done for the recovery of these sludges, including by pyrometalurgical processes [3, 5]. 2. Test on Romanian iron sludge For these tests were taken into account the furnace sludge from the former steel plant Sidex Galați, stored in the ponds from Mălina. In order to compare Mălina sludge with the other, 29 samples were taken from all Corresponding author Eugen Traistă, Assoc prof. Ph.D / University of Petrosani, Petrosani, Romania (University of Petrosani, 20 University Street, eugen_traista@yahoo.com) 21 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 surface of settling ponds. The average elemental contents and the range of elemental concentration variation are presented in table 1. Table 1. The average elemental contents of Mălina sludge Content Range Element [%] Minimal Maximal MgO 0,2840 0,0720 1,1009 Al O 0,1868 0,0983 0,3976 2 3 SiO 0,9391 0,5150 1,6967 P O 0,0934 0,0592 0,1452 2 5 SO 0,4656 0,2309 1,8719 Cl 0,1428 0,0461 0,2561 K O 0,0636 0,0311 0,1198 CaO 16,3724 7,3885 25,6999 TiO 0,0420 0,0000 0,2001 V O 0,0132 0,0000 0,0632 2 5 Cr O 0,0673 0,0000 0,2547 2 3 MnO 1,2370 0,8547 1,6091 Fe O 65,9528 45,8834 86,1069 2 3 Co O 0,0047 0,0000 0,0768 2 3 NiO 0,0237 0,0000 0,1667 CuO 0,0269 0,0000 0,0740 ZnO 0,6798 0,2899 1,5912 PbO 0,1153 0,0000 0,3038 LOI 13,0700 0,1652 26,3282 This results indicate that entire amount of sludge can be valuated, if the material is correctly managed. This involve to mix pour iron content sludge with rich iron content iron. The known iron content commercial limitation is over 62% Fe O . A number of 12 samples (41%) not join with this target. Zinc content mast be 2 3 less than 0,5% (ZnO < 0,373). Just 3 samples (10%) are under this limit. If 40% of zinc is removed, 50% of sludge may be used in furnace. In order to reduce the zinc content, preliminary leaching tests are made. Table 2. Preliminary leaching tests Elements Original 1M H2SO4 leaching CO2 leaching 40% NaOH leaching MgO 0,0720 0,0000 0,1514 0,2760 Al O 0,1206 0,1449 0,1431 0,3916 2 3 SiO 0,6253 0,6321 0,8274 1,3402 P O 0,0988 0,0531 0,1059 0,0728 2 5 SO 0,2653 0,0977 0,1381 0,5524 Cl 0,1791 0,0071 0,0160 0,0164 K O 0,0570 0,0554 0,0397 0,1159 CaO 14,9346 1,0343 15,5055 17,4388 TiO 0,0000 0,0698 0,0000 0,0000 V O 0,0000 0,0000 0,0000 0,0000 2 5 Cr O 0,0540 0,0908 0,0536 0,0664 2 3 MnO 1,4683 1,6988 1,1889 1,3188 Fe O 67,4150 95,2998 66,6970 63,8999 2 3 Co O 0,0000 0,0000 0,0000 0,0000 2 3 NiO 0,0000 0,0000 0,0000 0,0370 CuO 0,0246 0,0000 0,0235 0,4124 ZnO 1,5912 0,6936 1,0825 1,5555 PbO 0,0593 0,1353 0,2476 0,5499 PC 12,7416 0,0000 13,5117 11,8073 Zn removal [%] 56,41 31,97 2,24 22 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 This results are similar with that indicated in different authors references. The relatively poor results are due to the way zinc is chemically bound in the dust particles. Zinc is mainly present in the form of ZnFe O (franklinite) and less in the form of zinc oxide ZnO (zincite). Iron, with the 2 4 exception of franklinite, is in the form of magnetite, Fe O and hematite, Fe O . Zinc oxide can be easily 3 4 2 3 recovered by acidic or alkaline leaching, unlike franklinite which is refractory to these processes. Although alkaline leaching has the advantage of a small iron extraction, the leaching medium is relatively concentrated [6]. In contrast, the use of acids does not require such a high concentration as in the case of alkaline leaching, but iron is dissolved in the solution in this case. After purification of the solution resulting from leaching, the dissolved metals can be recovered by various methods, such as precipitation, crystallization, solvent extraction, ion exchange, electrolysis, etc. In the leaching process, the selective solubility of zinc relative to that of iron is critical. Several authors (Marcos Vinícius Cantarino, Celso de Carvalho Filho and Marcelo Borges Mansur) have found an increase in the efficiency of zinc removal by heating sludge. 3. Zinc leaching methods 3.1. Acid leaching Through the leaching process, the research aims to identify such conditions of hydrometallurgical treatment so that the zinc is dissolved in the solution while the iron remains in the residue. Zinc is recovered from the solution, while solid residues can be recycled in the iron manufacturing process. The table below compares the leaching efficiency of zinc and iron in blast furnace sludge in the case of the use of different acids. Table 3. Leaching efficiency of zinc and iron by using different acids Leaching agent L/S ratio Zn extracted Fe extracted [%] [%] 1 M H2SO4 20 78.71 2.97 1 M HNO3 20 67.66 2.05 1 M HCl 20 69.84 2.96 The comparison between the use of different acids showed that sulphuric acid is an ideal leaching agent for separating zinc from blast furnace sludge. The leaching process with sulphuric acid has the advantage that zinc is extracted selectively in relation to iron at a lower cost. Our test results on Mălina sludge using 1 M H SO are shown in table 4. 2 4 Table 4. Leaching tests using 1 M H SO 2 4 Component P12 Acid leaching Acid leaching of 400C preheated material MgO 0,0720 0,0000 0,5243 Al O 0,1206 0,1449 0,5319 2 3 SiO 0,6253 0,6321 2,5284 P O 0,0988 0,0531 0,0494 2 5 SO 0,2653 0,0977 0,1710 Cl 0,1791 0,0071 0,0195 K O 0,0570 0,0554 0,0580 CaO 14,9346 1,0343 13,0213 TiO 0,0000 0,0698 0,0938 V O 0,0000 0,0000 0,0000 2 5 Cr O 0,0540 0,0908 0,2512 2 3 MnO 1,4683 1,6988 2,0132 Fe O 67,4150 95,2998 79,2751 2 3 Co O 0,0000 0,0000 0,0000 2 3 NiO 0,0000 0,0000 0,0451 CuO 0,0246 0,0000 0,0371 ZnO 1,5912 0,6936 1,1412 PbO 12,7416 0,0000 0,0000 Zn removal 56,41 39,43 23 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 Our test shown that is better to use, in acid leaching, original material. 3.2. Alcaline leaching Alkaline leaching consists of treating the furnace dust with a caustic soda solution, followed by recovery of the zinc by electrolysis simultaneously with the regeneration of the alkaline solution for reuse. The major advantage of alkaline leaching is the selectivity of zinc extraction compared to iron. Selective extraction of zinc compared to iron is demonstrated with the equilibrium diagrams, which indicate how the dissolution of iron hydroxide and zinc oxides are pH dependent. These diagrams indicate that zinc is soluble in both acidic and alkaline media, while iron is soluble in acidic media. Figure 1. ZnO Solubility as a function of pH, Figure 2. Ferrous hydroxides solubility of at 25 °C as a function of pH, at 25 °C Figure 3. Ferric hydroxides solubility of as a function of pH, at 25 °C Alkaline leaching is considered effective in dissolving heavy metals, compared to iron. Caustic soda efficiently dissolves the oxides of Zn, Pb and Al and, in limited cases, Cr and Cu. The solubility of certain amphoteric elements in alkaline solution decreases in the following sequence Zn> Pb> Al> Cr (III)> Cu. The solubility of Cr (III), Cu and Cd is negligible in the presence of zinc and lead. Also, the solubility of lead decreases with increasing zinc content. The main dissolution reactions in caustic soda are: ZnO + 2NaOH = Na2ZnO2 + H2O (1) PbO + 2NaOH = Na PbO + H O (2) 2 2 2 Instead, zinc ferrite is a very stable compound and only partially dissolves in alkaline solutions. Alkaline solutions also dissolve aluminum oxide and silica, but their solubility is limited in the case of iron dust. SiO + 2NaOH = Na SiO + H O (3) 2 2 3 2 - + Al(OH) + NaOH = Al(OH) + Na (4) 3 4 After leaching with NaOH, is obtained a residue enriched in iron and depleted in zinc and lead which corresponds to the requirements for recycling. Test made in laboratory on Mălina iron sludge using 40% NaOh solution show the results in table 5. 24 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 Table 5. Leaching tests using 40% NaOH Compound Original Orininal NaOH leaching 400C calcinated NaOH leaching MgO 0,0720 0,2760 0,2282 Al O 0,1206 0,3916 1,9908 2 3 SiO 0,6253 1,3402 9,8479 P O 0,0988 0,0728 0,1568 2 5 SO 0,2653 0,5524 1,1586 Cl 0,1791 0,0164 0,0574 K O 0,0570 0,1159 0,4562 CaO 14,9346 17,4388 14,9460 TiO 0,0000 0,0000 0,2150 V O 0,0000 0,0000 0,0000 2 5 Cr O 0,0540 0,0664 0,1541 2 3 MnO 1,4683 1,3188 2,0659 Fe O 67,4150 63,8999 66,8682 2 3 Co O 0,0000 0,0000 0,0000 2 3 NiO 0,0000 0,0370 0,0000 CuO 0,0246 0,4124 0,0510 ZnO 1,5912 1,5555 0,9420 PbO 0,0593 0,5499 0,0000 PC 12,7416 11,8073 0,3331 Zn removal 2,24 40,80 3.3. Carbon dioxide solution leaching The solubility of zinc in carbon dioxide solutions depends on the temperature, ionic strength, pH and partial pressure of carbon dioxide. The following reactions occur in an acidic medium: 2+ 2- ZnCO (s) = Zn (aq) + CO (aq) (5) 3 3 2- - H+ (aq) + CO (aq) = HCO (aq) (6) 3 3 H+ (aq) + HCO (aq) = H O + CO (aq) (7) 3 2 2 CO (aq) = CO (g) (8) 2 2 + 2+ ZnCO (s) + 2H (aq) = Zn (aq) + H O + CO (g) (9) 3 2 2 And in the aqueous carbon dioxide acidic solution: 2+ ZnCO (s) = Zn (aq) + CO (10) 3 2 2- - H+ (aq) + CO3 (aq) = HCO3 (aq) (11) CO (aq) = CO (g) (12) 2 2 - + CO (aq) + H O = HCO (aq) + H (aq) (13) 2 2 3 2+ - ZnCO (s) + CO (g) + H O = Zn (aq) + 2HCO (aq) (14) 3 2 2 3 The solubility of zinc increases as the partial pressure of carbon dioxide increases and the main species in the solution at a high partial pressure of carbon dioxide is HCO3 . The solubility values of zinc carbonate in water taken from the literature are in table 6. Table 6. The solubility of zinc carbonate in water Zinc Carbonate PCO2/bar (CCO2/mol L-1) Temperature [K] Solubility ZnCO3/mol Reference or pH L-1 0.987 1.98 x 10-3 298.15 0.00032 1.64 x 10-4 288.15 0.00032" 8 x 10-5 von Essen 1897 5.6 x 10-5 Haehnel 1924 291.15 1.56 6.7 x 10-3 Haehnel 1924 298.1 1 1.98 x 10-3 Kelley, Anderson 1935 In order to check this theory, CO leaching test are made, with results presented in table 7. 25 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 Table 7. CO leaching test Original material 400 calcinated material Compound Original material CO leacheate 2 CO leacheate MgO 0,0720 0,1514 0,339 Al O 0,1206 0,1431 1,0781 2 3 SiO 0,6253 0,8274 0,0000 P O 0,0988 0,1059 0,0447 2 5 SO 0,2653 0,1381 0,0794 Cl 0,1791 0,0160 0,0322 K O 0,0570 0,0397 0,092 CaO 14,9346 15,5055 18,7498 TiO 0,0000 0,0000 0,1837 V O 0,0000 0,0000 0,0000 2 5 Cr O 0,0540 0,0536 0,1018 2 3 MnO 1,4683 1,1889 1,2609 Fe O 67,4150 66,6970 76,0652 2 3 Co O 0,0000 0,0000 0,0000 2 3 NiO 0,0000 0,0000 0,0000 CuO 0,0246 0,0235 0,0278 ZnO 1,5912 1,0825 0,6619 PbO 0,1353 0,2476 0,0000 PC 12,7416 13,5117 0,3331 Zn removal 31,97 58,40 This results are satisfactory, but the technology requires large amounts of water because of zinc dicarbonate low solubility. 4. Pyrometalurgical method Zinc is found in sinter in the form of oxides (ZnO), ferrite (ZnO·Fe O ), silicates (2ZnO·SiO ) and sulfide 2 3 2 (ZnS). Zinc reduction, vaporization, condensation, oxidation and circulation take place in the furnace. At temperatures higher than melting points (690 K) and boiling points (1190 K) zinc oxide is easily reduced in the furnace. In the vapor phase between 1190 and 1273 K, zinc sublimates and moves to the upper areas of the furnace. ZnO(s) + [C] = Zn(g) + CO(g) (15) Volatilized zinc gas, in contact with water vapor and carbon dioxide, oxidizes and deposits in the upper part of the furnace forming dense crusts. Zn(g) + H O(CO ) = ZnO + H (CO) (16) 2 2 2 These formations adversely affect the furnace operations. Zinc is volatilized in the temperature zone of 1173-1373 K which results in a decrease in the temperature of the zone. For this reason, 11 kg of coke is consumed for every kilogram of volatilized zinc. In order to do this, the original dried material was pelletized and introduced in a melting oven. Table 8. Pre-reduced sludge composition Compound Sample I Pre-reduced sample I Sample II Pre-reduced sample I MgO 0,1277 0,2831 0,1382 0,1952 Al O 0,1316 0,7977 0,1042 1,5964 2 3 SiO 0,5977 4,5856 0,6936 7,9446 P O 0,1043 0,1643 0,0783 0,1545 2 5 SO 0,2940 0,7103 0,3474 1,0191 Cl 0,2130 0,1073 0,2041 0,0662 K O 0,0511 0,2231 0,0619 0,3818 CaO 18,1580 17,6125 17,7578 15,6204 TiO 0,0000 0,0781 0,0000 0,1881 V O 0,0000 0,0178 0,0494 0,0000 2 5 Cr O 0,0665 0,1047 0,0477 0,1426 2 3 MnO 1,6091 1,6487 1,1312 1,9561 26 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 Fe O 60,2396 71,6684 59,9655 68,6779 2 3 Co O 0,0000 0,0000 0,0000 0,0000 2 3 NiO 0,0000 0,0482 0,0385 0,0252 CuO 0,0314 0,0238 0,0280 0,0534 ZnO 0,8650 1,4813 0,8693 1,0806 PbO 0,2437 0,1617 0,2754 0,3661 PC 17,1309 17,9969 An increasing of zinc content may be observed because of coke and carbonates removing during melting process. Vitrifyed and pre-reduced sludge may be easily crushed, but not milling. We try to mill this product with the hope that zinc oxide cumulated in nonmagnetic fraction. Results obtaining for magnetic separation of crushed melted material are: Table 9. Pre-reduced sludge magnetic separation tests Fe sample I Fe sample I Fe sample II Fe sample II Compound magnetic nonmagnetic magnetic nonmagnetic MgO 0,3175 0,2602 0,1458 0,2282 Al O 0,9142 0,7201 1,0048 1,9908 2 3 SiO 4,6804 4,5224 5,0896 9,8479 P O 0,1591 0,1678 0,1510 0,1568 2 5 SO 0,6645 0,7409 0,8098 1,1586 Cl 0,1075 0,1072 0,0795 0,0574 K O 0,1929 0,2433 0,2702 0,4562 CaO 17,4584 17,7152 16,6320 14,9460 TiO 0,1064 0,0593 0,1478 0,2150 V O 0,0000 0,0296 0,0000 0,0000 2 5 Cr O 0,0949 0,1112 0,1254 0,1541 2 3 MnO 1,5834 1,6923 1,7913 2,0659 Fe O 71,3850 71,8573 71,3925 66,8682 2 3 Co O 0,0000 0,0000 0,0000 0,0000 2 3 NiO 0,0603 0,0401 0,0629 0,0000 CuO 0,0000 0,0397 0,0571 0,0510 ZnO 1,7737 1,2863 1,2886 0,9420 PbO 0,3154 0,0593 0,4155 0,3331 This material preserved initial zinc content, but it contains 7% of reduced iron and may be used, in mixture with scrap in electric furnace for steel production. We test this material for this purpose and melt it at 1600C when pig iron were obtained: Table 10. Pig iron production test result Compound Pig iron Al O 0,4188 2 3 SiO 1,2885 SO 0,1609 Cl 0,0655 K O 0,1385 CaO 0,7717 TiO 0,0627 Cr O 0,0729 2 3 MnO 0,8156 Fe 95,5262 CuO 0,0992 PbO 0,0912 We also obtain a sludge with high content of reduced iron because of high viscosity of melted sludge at 1600C. After crushing, magnetic fraction was separated: 27 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 Table 11. Pig iron sludge magnetic separation test Compound Magnetic sludge Nonmagnetic sludge MgO 0,4895 0,4279 Al O 1,5973 2,5501 2 3 SiO 9,0730 13,6901 P O 0,4057 0,4928 2 5 SO 0,8642 0,9770 Cl 0,0716 0,1250 K O 0,1108 0,5724 CaO 37,8125 54,7018 TiO 0,5704 0,8354 V O 0,1293 0,1723 2 5 Cr O 0,0679 0,0000 2 3 MnO 3,6353 5,5383 Fe O 11,5785 17,8132 2 3 Fe 34,7135 Co O 0,0000 0,0000 2 3 NiO 0,0000 0,0000 CuO 0,0000 0,0494 ZnO 0,2813 0,9260 PbO 0,0578 0,3225 Magnetic fraction may be improved by advanced crushing. This option, that allow to obtain pig iron seems to be the most suitable for the sludge with high zinc content. 5. Conclusions Leaching tests on original material extract weakly zinc because of it linking by iron oxide as franklinite. By heating at 400 degrees the zinc solubility increase. Acid leaching using sulphuric acid is the best leaching method, but this technology is certainly not environmental friendly. Sodium hydroxide leaching is the best hydrometalurgical method that may be used, but required large amounts of fresh water to wash sludge after zinc removal in order to reduce sodium content. Pig iron obtaining is, in our opinion, the best method for iron sludge valuation. References [1] Mansfeldt T., Dohrmann R., 2020 Chemical and mineralogical characterization of blast-furnace sludge from an abandoned landfill Environmental science & technology, November 15, 2004, Volume 38, Issue 22, Pages 430A-6176 [2] van Herck P., Vandecasteele C., Swennen R., Mortier R., 2000 Zinc and Lead Removal from Blast Furnace Sludge with a Hydrometallurgical Process, September 1, 2000, Volume 34, Issue 17, Pages 361A-3830 [3] Das B., Prakash S., Reddy P.S.R., Misra V.N., 2007 An overview of utilization of slag and sludge from steel industries, Resources, Conservation and Recycling, Volume 50, Issue 1, March 2007, Pages 40-57 [4] Dvořák P., Jandová J., 2005 Hydrometallurgical recovery of zinc from hot dip galvanizing ash, Hydrometallurgy, Volume 77, Issues 1–2, April 2005, Pages 29-33 [5] Asadi Zeydabadia B., Mowlaa D., Shariatb M.H., Fathi Kalajahia J., 1997 Zinc recovery from blast furnace flue dust, Hydrometallurgy, Volume 47, Issue 1, November 1997, Pages 113-125 [6] Vereš J., Jakabský S., Lovás M., 2011 Zinc recovery from iron and steel making wastes by conventional and microwave assisted leaching, Acta Montanistica Slovaca Ročník 16 (2011), číslo 3, 185-191 This article is an open access article distributed under the Creative Commons BY SA 4.0 license. Authors retain all copyrights and agree to the terms of the above-mentioned CC BY SA 4.0 license. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Mining Revue de Gruyter

Research Regarding Iron Sludge Recovery Technology

Traistă, Eugen; Traistă, Camelia
Mining Revue , Volume 28 (2): 8 – Jun 1, 2022
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Abstract

Revista Minelor – Mining Revue ISSN-L 1220-2053 / ISSN 2247-8590 vol. 28, issue 2 / 2022, pp. 21-28 1* 2 Eugen TRAISTĂ , Camelia TRAISTĂ University of Petrosani, Petrosani, Romania, eugen_traista@yahoo.com University of Petrosani, Petrosani, Romania DOI: 10.2478/minrv-2022-0010 Abstract: One of the most important wastes in iron metallurgy is the blast furnace sludge. This sludge consists of fine particles of iron ore, coke and fine particles of flux. The furnace sludge is characterized by the chemical composition similar to that of the furnace load, the major difference being the concentration of zinc and lead. Due to the similarity with the blast furnace load, this material, after pelletization, can be recycled in the technological process. However, this recirculation is limited by the zinc content, which significantly disrupts the operation of the furnace. This paper presents tests to reduce the zinc content of the furnace sludge by hydrometallurgical and pyrometalurgical processes. Keywords: iron sludge, metallurgy, iron ore, recovery, furnace, leaching 1. Introduction As a result of industrial development, more and more industrial waste results during production processes. From the iron production processes results the furnace sludge which is one of the hazardous metallurgical wastes [1]. The production of iron and its alloys is the most important metallurgical process [2]. In the production process, in addition to Fe and C, many other elements are introduced into the furnace. Zinc is especially a problem because during the metallurgical process it distills due to the very high temperatures in the furnace and subsequently condenses on the furnace walls in areas with lower temperature, which leads to a large amount of dust and possible damage to the furnace. For a good process, the Zn concentration in the ore must not exceed 0.12 kg per tonne of cast iron produced. Partly, the evaporated zinc condenses on the dust particles in the effluent gas, the higher concentration of zinc founding in the finer dust particles. Normally each ton of cast iron results in 8 - 12 kg of dust. This dust is removed by the flue gas purification system. Large particles are removed from the gases by cyclones and filter bag filters and can be reintroduced directly into the furnace after sintering because the Zn content is generally low (<0.1% Zn). Fine particles are removed through a wet scrubber from which results a sludge [2]. Generally, blast furnace sludge contains 21-32% Fe, 15-35% C, 1.0-3.2% Zn and 0.3-1.2% Pb, reported to dry mass [3, 4]. In Europe alone, the steel industry produces about 500,000 tons of blast furnace sludge annually. Because the iron and coke content of the sludge is high, it is possible to recycle the sludge in the furnace. However, due to the zinc content of a few percent, the use of this sludge is restricted. The resulting sludge after removal of fine dust in wet filters is deposited in a tailings pond. Groundwater can contaminate groundwater with zinc and lead in this sludge. Also, the new regulations no longer allow long-term storage of waste. For this reason, research is being done for the recovery of these sludges, including by pyrometalurgical processes [3, 5]. 2. Test on Romanian iron sludge For these tests were taken into account the furnace sludge from the former steel plant Sidex Galați, stored in the ponds from Mălina. In order to compare Mălina sludge with the other, 29 samples were taken from all Corresponding author Eugen Traistă, Assoc prof. Ph.D / University of Petrosani, Petrosani, Romania (University of Petrosani, 20 University Street, eugen_traista@yahoo.com) 21 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 surface of settling ponds. The average elemental contents and the range of elemental concentration variation are presented in table 1. Table 1. The average elemental contents of Mălina sludge Content Range Element [%] Minimal Maximal MgO 0,2840 0,0720 1,1009 Al O 0,1868 0,0983 0,3976 2 3 SiO 0,9391 0,5150 1,6967 P O 0,0934 0,0592 0,1452 2 5 SO 0,4656 0,2309 1,8719 Cl 0,1428 0,0461 0,2561 K O 0,0636 0,0311 0,1198 CaO 16,3724 7,3885 25,6999 TiO 0,0420 0,0000 0,2001 V O 0,0132 0,0000 0,0632 2 5 Cr O 0,0673 0,0000 0,2547 2 3 MnO 1,2370 0,8547 1,6091 Fe O 65,9528 45,8834 86,1069 2 3 Co O 0,0047 0,0000 0,0768 2 3 NiO 0,0237 0,0000 0,1667 CuO 0,0269 0,0000 0,0740 ZnO 0,6798 0,2899 1,5912 PbO 0,1153 0,0000 0,3038 LOI 13,0700 0,1652 26,3282 This results indicate that entire amount of sludge can be valuated, if the material is correctly managed. This involve to mix pour iron content sludge with rich iron content iron. The known iron content commercial limitation is over 62% Fe O . A number of 12 samples (41%) not join with this target. Zinc content mast be 2 3 less than 0,5% (ZnO < 0,373). Just 3 samples (10%) are under this limit. If 40% of zinc is removed, 50% of sludge may be used in furnace. In order to reduce the zinc content, preliminary leaching tests are made. Table 2. Preliminary leaching tests Elements Original 1M H2SO4 leaching CO2 leaching 40% NaOH leaching MgO 0,0720 0,0000 0,1514 0,2760 Al O 0,1206 0,1449 0,1431 0,3916 2 3 SiO 0,6253 0,6321 0,8274 1,3402 P O 0,0988 0,0531 0,1059 0,0728 2 5 SO 0,2653 0,0977 0,1381 0,5524 Cl 0,1791 0,0071 0,0160 0,0164 K O 0,0570 0,0554 0,0397 0,1159 CaO 14,9346 1,0343 15,5055 17,4388 TiO 0,0000 0,0698 0,0000 0,0000 V O 0,0000 0,0000 0,0000 0,0000 2 5 Cr O 0,0540 0,0908 0,0536 0,0664 2 3 MnO 1,4683 1,6988 1,1889 1,3188 Fe O 67,4150 95,2998 66,6970 63,8999 2 3 Co O 0,0000 0,0000 0,0000 0,0000 2 3 NiO 0,0000 0,0000 0,0000 0,0370 CuO 0,0246 0,0000 0,0235 0,4124 ZnO 1,5912 0,6936 1,0825 1,5555 PbO 0,0593 0,1353 0,2476 0,5499 PC 12,7416 0,0000 13,5117 11,8073 Zn removal [%] 56,41 31,97 2,24 22 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 This results are similar with that indicated in different authors references. The relatively poor results are due to the way zinc is chemically bound in the dust particles. Zinc is mainly present in the form of ZnFe O (franklinite) and less in the form of zinc oxide ZnO (zincite). Iron, with the 2 4 exception of franklinite, is in the form of magnetite, Fe O and hematite, Fe O . Zinc oxide can be easily 3 4 2 3 recovered by acidic or alkaline leaching, unlike franklinite which is refractory to these processes. Although alkaline leaching has the advantage of a small iron extraction, the leaching medium is relatively concentrated [6]. In contrast, the use of acids does not require such a high concentration as in the case of alkaline leaching, but iron is dissolved in the solution in this case. After purification of the solution resulting from leaching, the dissolved metals can be recovered by various methods, such as precipitation, crystallization, solvent extraction, ion exchange, electrolysis, etc. In the leaching process, the selective solubility of zinc relative to that of iron is critical. Several authors (Marcos Vinícius Cantarino, Celso de Carvalho Filho and Marcelo Borges Mansur) have found an increase in the efficiency of zinc removal by heating sludge. 3. Zinc leaching methods 3.1. Acid leaching Through the leaching process, the research aims to identify such conditions of hydrometallurgical treatment so that the zinc is dissolved in the solution while the iron remains in the residue. Zinc is recovered from the solution, while solid residues can be recycled in the iron manufacturing process. The table below compares the leaching efficiency of zinc and iron in blast furnace sludge in the case of the use of different acids. Table 3. Leaching efficiency of zinc and iron by using different acids Leaching agent L/S ratio Zn extracted Fe extracted [%] [%] 1 M H2SO4 20 78.71 2.97 1 M HNO3 20 67.66 2.05 1 M HCl 20 69.84 2.96 The comparison between the use of different acids showed that sulphuric acid is an ideal leaching agent for separating zinc from blast furnace sludge. The leaching process with sulphuric acid has the advantage that zinc is extracted selectively in relation to iron at a lower cost. Our test results on Mălina sludge using 1 M H SO are shown in table 4. 2 4 Table 4. Leaching tests using 1 M H SO 2 4 Component P12 Acid leaching Acid leaching of 400C preheated material MgO 0,0720 0,0000 0,5243 Al O 0,1206 0,1449 0,5319 2 3 SiO 0,6253 0,6321 2,5284 P O 0,0988 0,0531 0,0494 2 5 SO 0,2653 0,0977 0,1710 Cl 0,1791 0,0071 0,0195 K O 0,0570 0,0554 0,0580 CaO 14,9346 1,0343 13,0213 TiO 0,0000 0,0698 0,0938 V O 0,0000 0,0000 0,0000 2 5 Cr O 0,0540 0,0908 0,2512 2 3 MnO 1,4683 1,6988 2,0132 Fe O 67,4150 95,2998 79,2751 2 3 Co O 0,0000 0,0000 0,0000 2 3 NiO 0,0000 0,0000 0,0451 CuO 0,0246 0,0000 0,0371 ZnO 1,5912 0,6936 1,1412 PbO 12,7416 0,0000 0,0000 Zn removal 56,41 39,43 23 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 Our test shown that is better to use, in acid leaching, original material. 3.2. Alcaline leaching Alkaline leaching consists of treating the furnace dust with a caustic soda solution, followed by recovery of the zinc by electrolysis simultaneously with the regeneration of the alkaline solution for reuse. The major advantage of alkaline leaching is the selectivity of zinc extraction compared to iron. Selective extraction of zinc compared to iron is demonstrated with the equilibrium diagrams, which indicate how the dissolution of iron hydroxide and zinc oxides are pH dependent. These diagrams indicate that zinc is soluble in both acidic and alkaline media, while iron is soluble in acidic media. Figure 1. ZnO Solubility as a function of pH, Figure 2. Ferrous hydroxides solubility of at 25 °C as a function of pH, at 25 °C Figure 3. Ferric hydroxides solubility of as a function of pH, at 25 °C Alkaline leaching is considered effective in dissolving heavy metals, compared to iron. Caustic soda efficiently dissolves the oxides of Zn, Pb and Al and, in limited cases, Cr and Cu. The solubility of certain amphoteric elements in alkaline solution decreases in the following sequence Zn> Pb> Al> Cr (III)> Cu. The solubility of Cr (III), Cu and Cd is negligible in the presence of zinc and lead. Also, the solubility of lead decreases with increasing zinc content. The main dissolution reactions in caustic soda are: ZnO + 2NaOH = Na2ZnO2 + H2O (1) PbO + 2NaOH = Na PbO + H O (2) 2 2 2 Instead, zinc ferrite is a very stable compound and only partially dissolves in alkaline solutions. Alkaline solutions also dissolve aluminum oxide and silica, but their solubility is limited in the case of iron dust. SiO + 2NaOH = Na SiO + H O (3) 2 2 3 2 - + Al(OH) + NaOH = Al(OH) + Na (4) 3 4 After leaching with NaOH, is obtained a residue enriched in iron and depleted in zinc and lead which corresponds to the requirements for recycling. Test made in laboratory on Mălina iron sludge using 40% NaOh solution show the results in table 5. 24 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 Table 5. Leaching tests using 40% NaOH Compound Original Orininal NaOH leaching 400C calcinated NaOH leaching MgO 0,0720 0,2760 0,2282 Al O 0,1206 0,3916 1,9908 2 3 SiO 0,6253 1,3402 9,8479 P O 0,0988 0,0728 0,1568 2 5 SO 0,2653 0,5524 1,1586 Cl 0,1791 0,0164 0,0574 K O 0,0570 0,1159 0,4562 CaO 14,9346 17,4388 14,9460 TiO 0,0000 0,0000 0,2150 V O 0,0000 0,0000 0,0000 2 5 Cr O 0,0540 0,0664 0,1541 2 3 MnO 1,4683 1,3188 2,0659 Fe O 67,4150 63,8999 66,8682 2 3 Co O 0,0000 0,0000 0,0000 2 3 NiO 0,0000 0,0370 0,0000 CuO 0,0246 0,4124 0,0510 ZnO 1,5912 1,5555 0,9420 PbO 0,0593 0,5499 0,0000 PC 12,7416 11,8073 0,3331 Zn removal 2,24 40,80 3.3. Carbon dioxide solution leaching The solubility of zinc in carbon dioxide solutions depends on the temperature, ionic strength, pH and partial pressure of carbon dioxide. The following reactions occur in an acidic medium: 2+ 2- ZnCO (s) = Zn (aq) + CO (aq) (5) 3 3 2- - H+ (aq) + CO (aq) = HCO (aq) (6) 3 3 H+ (aq) + HCO (aq) = H O + CO (aq) (7) 3 2 2 CO (aq) = CO (g) (8) 2 2 + 2+ ZnCO (s) + 2H (aq) = Zn (aq) + H O + CO (g) (9) 3 2 2 And in the aqueous carbon dioxide acidic solution: 2+ ZnCO (s) = Zn (aq) + CO (10) 3 2 2- - H+ (aq) + CO3 (aq) = HCO3 (aq) (11) CO (aq) = CO (g) (12) 2 2 - + CO (aq) + H O = HCO (aq) + H (aq) (13) 2 2 3 2+ - ZnCO (s) + CO (g) + H O = Zn (aq) + 2HCO (aq) (14) 3 2 2 3 The solubility of zinc increases as the partial pressure of carbon dioxide increases and the main species in the solution at a high partial pressure of carbon dioxide is HCO3 . The solubility values of zinc carbonate in water taken from the literature are in table 6. Table 6. The solubility of zinc carbonate in water Zinc Carbonate PCO2/bar (CCO2/mol L-1) Temperature [K] Solubility ZnCO3/mol Reference or pH L-1 0.987 1.98 x 10-3 298.15 0.00032 1.64 x 10-4 288.15 0.00032" 8 x 10-5 von Essen 1897 5.6 x 10-5 Haehnel 1924 291.15 1.56 6.7 x 10-3 Haehnel 1924 298.1 1 1.98 x 10-3 Kelley, Anderson 1935 In order to check this theory, CO leaching test are made, with results presented in table 7. 25 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 Table 7. CO leaching test Original material 400 calcinated material Compound Original material CO leacheate 2 CO leacheate MgO 0,0720 0,1514 0,339 Al O 0,1206 0,1431 1,0781 2 3 SiO 0,6253 0,8274 0,0000 P O 0,0988 0,1059 0,0447 2 5 SO 0,2653 0,1381 0,0794 Cl 0,1791 0,0160 0,0322 K O 0,0570 0,0397 0,092 CaO 14,9346 15,5055 18,7498 TiO 0,0000 0,0000 0,1837 V O 0,0000 0,0000 0,0000 2 5 Cr O 0,0540 0,0536 0,1018 2 3 MnO 1,4683 1,1889 1,2609 Fe O 67,4150 66,6970 76,0652 2 3 Co O 0,0000 0,0000 0,0000 2 3 NiO 0,0000 0,0000 0,0000 CuO 0,0246 0,0235 0,0278 ZnO 1,5912 1,0825 0,6619 PbO 0,1353 0,2476 0,0000 PC 12,7416 13,5117 0,3331 Zn removal 31,97 58,40 This results are satisfactory, but the technology requires large amounts of water because of zinc dicarbonate low solubility. 4. Pyrometalurgical method Zinc is found in sinter in the form of oxides (ZnO), ferrite (ZnO·Fe O ), silicates (2ZnO·SiO ) and sulfide 2 3 2 (ZnS). Zinc reduction, vaporization, condensation, oxidation and circulation take place in the furnace. At temperatures higher than melting points (690 K) and boiling points (1190 K) zinc oxide is easily reduced in the furnace. In the vapor phase between 1190 and 1273 K, zinc sublimates and moves to the upper areas of the furnace. ZnO(s) + [C] = Zn(g) + CO(g) (15) Volatilized zinc gas, in contact with water vapor and carbon dioxide, oxidizes and deposits in the upper part of the furnace forming dense crusts. Zn(g) + H O(CO ) = ZnO + H (CO) (16) 2 2 2 These formations adversely affect the furnace operations. Zinc is volatilized in the temperature zone of 1173-1373 K which results in a decrease in the temperature of the zone. For this reason, 11 kg of coke is consumed for every kilogram of volatilized zinc. In order to do this, the original dried material was pelletized and introduced in a melting oven. Table 8. Pre-reduced sludge composition Compound Sample I Pre-reduced sample I Sample II Pre-reduced sample I MgO 0,1277 0,2831 0,1382 0,1952 Al O 0,1316 0,7977 0,1042 1,5964 2 3 SiO 0,5977 4,5856 0,6936 7,9446 P O 0,1043 0,1643 0,0783 0,1545 2 5 SO 0,2940 0,7103 0,3474 1,0191 Cl 0,2130 0,1073 0,2041 0,0662 K O 0,0511 0,2231 0,0619 0,3818 CaO 18,1580 17,6125 17,7578 15,6204 TiO 0,0000 0,0781 0,0000 0,1881 V O 0,0000 0,0178 0,0494 0,0000 2 5 Cr O 0,0665 0,1047 0,0477 0,1426 2 3 MnO 1,6091 1,6487 1,1312 1,9561 26 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 Fe O 60,2396 71,6684 59,9655 68,6779 2 3 Co O 0,0000 0,0000 0,0000 0,0000 2 3 NiO 0,0000 0,0482 0,0385 0,0252 CuO 0,0314 0,0238 0,0280 0,0534 ZnO 0,8650 1,4813 0,8693 1,0806 PbO 0,2437 0,1617 0,2754 0,3661 PC 17,1309 17,9969 An increasing of zinc content may be observed because of coke and carbonates removing during melting process. Vitrifyed and pre-reduced sludge may be easily crushed, but not milling. We try to mill this product with the hope that zinc oxide cumulated in nonmagnetic fraction. Results obtaining for magnetic separation of crushed melted material are: Table 9. Pre-reduced sludge magnetic separation tests Fe sample I Fe sample I Fe sample II Fe sample II Compound magnetic nonmagnetic magnetic nonmagnetic MgO 0,3175 0,2602 0,1458 0,2282 Al O 0,9142 0,7201 1,0048 1,9908 2 3 SiO 4,6804 4,5224 5,0896 9,8479 P O 0,1591 0,1678 0,1510 0,1568 2 5 SO 0,6645 0,7409 0,8098 1,1586 Cl 0,1075 0,1072 0,0795 0,0574 K O 0,1929 0,2433 0,2702 0,4562 CaO 17,4584 17,7152 16,6320 14,9460 TiO 0,1064 0,0593 0,1478 0,2150 V O 0,0000 0,0296 0,0000 0,0000 2 5 Cr O 0,0949 0,1112 0,1254 0,1541 2 3 MnO 1,5834 1,6923 1,7913 2,0659 Fe O 71,3850 71,8573 71,3925 66,8682 2 3 Co O 0,0000 0,0000 0,0000 0,0000 2 3 NiO 0,0603 0,0401 0,0629 0,0000 CuO 0,0000 0,0397 0,0571 0,0510 ZnO 1,7737 1,2863 1,2886 0,9420 PbO 0,3154 0,0593 0,4155 0,3331 This material preserved initial zinc content, but it contains 7% of reduced iron and may be used, in mixture with scrap in electric furnace for steel production. We test this material for this purpose and melt it at 1600C when pig iron were obtained: Table 10. Pig iron production test result Compound Pig iron Al O 0,4188 2 3 SiO 1,2885 SO 0,1609 Cl 0,0655 K O 0,1385 CaO 0,7717 TiO 0,0627 Cr O 0,0729 2 3 MnO 0,8156 Fe 95,5262 CuO 0,0992 PbO 0,0912 We also obtain a sludge with high content of reduced iron because of high viscosity of melted sludge at 1600C. After crushing, magnetic fraction was separated: 27 Revista Minelor – Mining Revue vol. 28, issue 2 / 2022 ISSN-L 1220-2053 / ISSN 2247-8590 pp. 21-28 Table 11. Pig iron sludge magnetic separation test Compound Magnetic sludge Nonmagnetic sludge MgO 0,4895 0,4279 Al O 1,5973 2,5501 2 3 SiO 9,0730 13,6901 P O 0,4057 0,4928 2 5 SO 0,8642 0,9770 Cl 0,0716 0,1250 K O 0,1108 0,5724 CaO 37,8125 54,7018 TiO 0,5704 0,8354 V O 0,1293 0,1723 2 5 Cr O 0,0679 0,0000 2 3 MnO 3,6353 5,5383 Fe O 11,5785 17,8132 2 3 Fe 34,7135 Co O 0,0000 0,0000 2 3 NiO 0,0000 0,0000 CuO 0,0000 0,0494 ZnO 0,2813 0,9260 PbO 0,0578 0,3225 Magnetic fraction may be improved by advanced crushing. This option, that allow to obtain pig iron seems to be the most suitable for the sludge with high zinc content. 5. Conclusions Leaching tests on original material extract weakly zinc because of it linking by iron oxide as franklinite. By heating at 400 degrees the zinc solubility increase. Acid leaching using sulphuric acid is the best leaching method, but this technology is certainly not environmental friendly. Sodium hydroxide leaching is the best hydrometalurgical method that may be used, but required large amounts of fresh water to wash sludge after zinc removal in order to reduce sodium content. Pig iron obtaining is, in our opinion, the best method for iron sludge valuation. References [1] Mansfeldt T., Dohrmann R., 2020 Chemical and mineralogical characterization of blast-furnace sludge from an abandoned landfill Environmental science & technology, November 15, 2004, Volume 38, Issue 22, Pages 430A-6176 [2] van Herck P., Vandecasteele C., Swennen R., Mortier R., 2000 Zinc and Lead Removal from Blast Furnace Sludge with a Hydrometallurgical Process, September 1, 2000, Volume 34, Issue 17, Pages 361A-3830 [3] Das B., Prakash S., Reddy P.S.R., Misra V.N., 2007 An overview of utilization of slag and sludge from steel industries, Resources, Conservation and Recycling, Volume 50, Issue 1, March 2007, Pages 40-57 [4] Dvořák P., Jandová J., 2005 Hydrometallurgical recovery of zinc from hot dip galvanizing ash, Hydrometallurgy, Volume 77, Issues 1–2, April 2005, Pages 29-33 [5] Asadi Zeydabadia B., Mowlaa D., Shariatb M.H., Fathi Kalajahia J., 1997 Zinc recovery from blast furnace flue dust, Hydrometallurgy, Volume 47, Issue 1, November 1997, Pages 113-125 [6] Vereš J., Jakabský S., Lovás M., 2011 Zinc recovery from iron and steel making wastes by conventional and microwave assisted leaching, Acta Montanistica Slovaca Ročník 16 (2011), číslo 3, 185-191 This article is an open access article distributed under the Creative Commons BY SA 4.0 license. Authors retain all copyrights and agree to the terms of the above-mentioned CC BY SA 4.0 license.

Journal

Mining Revue – de Gruyter

Published: Jun 1, 2022

Keywords: iron sludge; metallurgy; iron ore; recovery; furnace; leaching

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