Reduction of a garnieritic laterite ore by CO-CO2 gas mixtures
In extraction of nickel from laterite ores, nickel has to be reduced to metallic form in pyrometallurgical processes. This paper studied the reduction of a garnieritic laterite ore in order to establish the extents and rates of reduction of nickel, cobalt and iron oxides under different conditions, reduction mechanisms, and the optimum conditions for selective reduction of nickel and cobalt oxides over iron oxides. The reduction of the garnieritic ore was examined at 700- 900°C in the CO-CO2 gas mixtures containing 20-70 vol% CO; the gas flow rate was varied in the range of 350-1050 mL/min. A ferronickel alloy was formed by reduction of the metal oxides in the ore. The extent of reduction of nickel oxide increased with increasing temperature to 740 °C and then decreased with further increasing temperature. The extents of reduction of cobalt and iron decreased slightly with increase in temperature. Increase in CO concentration also promoted reduction of the metal oxides. When CO content was lower than 60 vol%, the extent of reduction of iron oxides was below 20%. Increase in CO concentration to 70 vol% sharply increased iron metallisation (>50%). Gas flowrate above 350 mL/min had a negligible effect on the ore reduction. The effect of ore size on the reduction of the metal oxides was insignificant. The optimal conditions for selective reduction of the laterite ore included temperature 740 °Cand 60 vol% CO in the CO-CO2 gas mixture.