Nickel, copper, and cobalt slag losses during converting
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P. TOSCANO, Engineer, is with Minnovex Technologies Inc., Toronto, ON, Canada M3K 2A2. T.A. UTIGARD, Professor, is with the Department of Materials Science and Engineering, University of Toronto, Toronto, ON, Canada M5S3E4. Contact e-mail: [email protected] Manuscript submitted December 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
[MS] ⫹
1 1 O2 ⫽ (MO) ⫹ S2 2 2
[1]
where [MS] represents metal sulfide in the matte (i.e., Ni3S2, Cu2S, or CoS) and (MO) represents metal oxides (i.e., NiO, Cu2O, or CoO) in the slag. In practice, the sulfur and oxygen pressures are determined mainly by the exchange of iron because iron is a substantial part of both the slag and matte. The exchange of metal therefore is best considered by the following reaction:[5–8] [MS] ⫹ (FeO) ⫽ (MO) ⫹ [FeS]
[2]
where [FeS] represents ferrous sulfide in matte and (FeO) represents ferrous oxide in the slag. During converting, the oxygen pressure increases as the matte FeS content decreases and the slag magnetite content increases. Several investigators[9,10,11] have found that the nickel solubility in fayalitic slag increases with increasing partial pressure of oxygen. Therefore, it is not surprising that slag losses increase as the converting process proceeds. Lee[18] determined that for a given matte grade, metal losses to slags decrease at lower sulfur pressures. Font et al.[12,13] investigated the copper, nickel, and cobalt solubilities between FeOx-SiOs-MgO slag and Cu2S-Ni3S2-FeS matte under SO2 pressures of 0.1, 0.5, and 1 atm. At a given matte grade, the solubility of copper in the slag was found to be independent of pSO2 while that of nickel and cobalt increased with increasing pSO2. These results imply that dissolved metal losses can be decreased by decreasing oxygen, sulfur, or SO2 pressures. However, in industrial matte converting operations, one does not have the freedom to vary these parameters at will. Operational variables include the oxygen enrichment in the blast, the amount and type of flux used, and the temperature. By varying the oxygen content in the blast, the SO2 pressure in bubbles formed within the matte and slag also varies. The experiments were designed to simulate the converting process by converting flash furnace matte to low levels of iron. In these tests, 500 g of the flash furnace matte (26.2 pct Fe, 22.4 pct Ni, 22.7 pct Cu, 0.74 pct Co, 27.1 pct S, and 2.86 pct O) was added to a MgO crucible with an inner diameter of 7.5 cm and a height of 15 cm. The converting was accomplished by injecting air or oxygen-enriched air into the liquid matte through a lance that was immersed into the matte with the tip at a distance of 0.5 cm from the bottom of the crucible. Silica additions were made so that the oxidized iron would form a fayalitic slag. All the experiments were carried out at 1250 ⬚C. An electrical resistance furnace was used to conduct all experimental test work. One R-type thermocouple was used to control the furnace temperature and another R-type thermocouple was used to measure the actual temperature
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