Sulfurization of Fe-Ni-Cu-Co Alloy to Matte Phase by Carbothermic Reduction of Calcium Sulfate
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TH the increasing use of Cu, Ni, and Co in many applications such as electrical and thermal conductor, corrosion- and heat-resistant superalloys for aircraft gas turbines and steam turbine power plants, etc., and rapid depletion of natural land-based ores, worldwide efforts are being made to find alternative resources for these rare metals (or critical metals). One alternative resource could be manganese nodules found at a depth of 4 to 5 km in the Pacific Ocean.[1,2] The composition of a manganese nodule is listed in Table I.[3] Several methods have been proposed to recover valuable metals from manganese nodules. Among them, the reductive smelting—sulfurization (alloy-to-matte conversion)— hydrometallurgy (RSH) combinatorial process is popular.[3,4] For this process, the manganese nodules are primarily reduced by adding cokes as a reducing agent in an EUI HYUK JEONG, formerly Graduate Student with the Department of Materials Engineering, Hanyang University, Ansan 426-791, Korea, is now with the Technical Research Center, SeAH Besteel Corp. Gunsan 573-711, Korea. CHUL WOO NAM and KYUNG HO PARK, Senior Researchers, are with the Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 305-350, Korea. JOO HYUN PARK, Professor, is with the Department of Materials Engineering, Hanyang University, Ansan 426-791, Korea. E-mail: [email protected]. Manuscript submitted June 4, 2015. Article published online February 1, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
electric arc furnace (EAF) to separate Fe-Ni-Cu-Co alloy and MnO-rich slag. The composition of the alloy tapped from EAF is shown in Table II.[3] The MnO-rich slag is reused as a raw material for the production of silicomanganese alloy. The Fe-Ni-Cu-Co alloy produced is very difficult to leach because of the high energy required for grinding and the low leaching rate caused by the high iron content.[5] To address these problems, the Fe-Ni-Cu-Co alloy must be converted to the Fe-Ni-Cu-Co-S matte phase, which has a sulfur content greater than 20 mass pct.[5] Elemental sulfur is added to the molten alloy in the conventional matte-making method.[5] However, the conversion efficiency is relatively low, and the cost of pure sulfur is high. A flow chart of the RSH process is shown in Figure 1.[6] Therefore, in the present study, we propose dehydrated waste gypsum (CaSO4) as an alternative sulfur source for the conversion of the alloy to the matte phase. Gypsum is produced in large quantities as a waste in chemical, fertilizer, and desulfurization processes.[7–13] Also, waste gypsum is difficult to treat or store safely, which is an environmental concern.[8–14] Previous studies have extensively investigated the thermal decomposition of CaSO4.[7–23] Temperature, gas atmosphere, and reducing agents are important factors in the decomposition of CaSO4. The decomposition temperature varies depending on impurities in the CaSO4 as well as on the type of reducing agents, such as carbon, sulfur, and H2 gas.[7,11,14,15,18–23] The more
VOLUME 47B, APRIL 2016—1103
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