The investigation focuses on an alternative route to metallic phase extraction from complex Cu-Co-Fe mineral sulphide concentrates, from the Copperbelt region in Zambia. In the context of developing a novel process route for metal extraction without SO2 gas emission and slag waste generation, the reduction of Cu-Co-Fe mineral sulphide concentrates via carbothermic reduction in the presence of lime (CaO) by following the equilibrium: MS + CaO + C = M + CaS + CO(g), where M represents the metallic Cu, Co and Fe, was studied. The reduction experiments were carried out in a temperature range of 1073 K - 1573 K, under argon atmosphere. The extent of metallization was analyzed by plotting the percentage reduction (%R) for each reaction against time (t). The reacted and partially reacted samples were characterised by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques. SEM-EDX analysis of the reduced samples showed that the purity of the metallic/alloy phases were over 97 wt.%. The reduced samples were subjected to magnetic separation for separating out the metallic/alloy phases from CaS, excess CaO and gangue minerals. The regeneration of CaO from CaS and utilisation of CaS were investigated.
The effects of reduction temperature, type of carbon and mole ratios of CaO and C were investigated. Complete reduction/metallisation occurred within 2 hours above 1173 K and 1273 K with carbon black and graphite, respectively. Carbon black was found to be a better reductant as there was excessive sintering between the metallic and CaS phases, in the reduction with graphite. However, the mole ratio of carbon black should be slightly higher than that of CaO, because of; (i) consumption of part of CaO by the gangue minerals (SiO2 and Al2O3) at lower mole ratio of C than CaO (e.g. MS:CaO:C = 1:2:1) and (ii) incomplete reduction at very high mole ratio of C than CaO CaO (e.g. MS:CaO:C = 1:2:4), at T ≤ 1323 K. The reaction mechanisms were studied by stopping the reduction experiments at different times and characterising the partially reacted samples. The metallisation of Fe occured via reduction of the intermediate phase of Fe-O (FeO and Fe3O4) at C(carbon black) ≥ CaO (mole ratio). There was broad agreement between the experimental results and the thermodynamic predictions.
The low temperature (≤ 1123 K) metallisation of Cu, Co and Fe was achieved via lime roast – reduction process. The mineral sulphide concentrates were roasted in air (21 % O2), in the presence of CaO by following the equilibrium: MS + CaO + 2O2 = MO + CaSO4, in the temperature range of 773 K – 923 K. The lime roast calcine was reduced at 1073 K and 1123 K with; (i) carbon black for the selective metallisation of Cu and for retaining the CaSO4 phase and (ii) activated charcoal for the complete metallisation of Cu, Co and Fe by following the equilibrium: MO + CaSO4 + 4C = M + CaS + 4CO (g).
Because copper smelting slag is the major source of Cu and Co, the low temperature recovery of Cu and Co from a 40wt.%SiO2-(30wt.%Fe,6wt.%Al)2O3-10wt%CaO-7wt%CuO-7wt%CoO slag over a temperature range of 1173 K to 1323 K, was investigated via; (i) carbothermic reduction of metal oxides according to MO + C = M + CO(g) reaction and (ii) sulphidisation in the presence of CaSO4 and C according to MO + CaSO4 +SiO2 + 4C = MS + CaSiO3 + 4CO reaction. In the direct reduction of oxides, the recovery of metallic phase was below 90 % at 1323 K. Nearly all Cu, Co and Fe converted to metal sulphides or matte phase during sulphidisation of the slag. The reaction kinetics for the sulphidisation of slag in the presence of CaSO4 and C were determined. The sulphidised slag was reduced in the presence of CaO and C to obtain metallic/alloy, CaS and CaO rich slag phases. The metallic particles produced via sulphidisation – carbothermic reduction route were larger than those produced by direct reduction of the slag. The sulphidisation of slag in the presence of CaS via MO + CaS + SiO2 = MS + CaSiO3 reaction, was also investigated.
