Thermodynamic analysis of the Fe–Ni–Co–Mg–Si–O–H–S–C–Cl system for selective sulphidation of a nickeliferous limonitic laterite ore

• Thermodynamics shows that sulphidation a nickeliferous laterite ore is feasible.
• It is proposed that a (Fe,Ni,Co)S monosulphide solid solution phase forms.
• High grades were achieved but recoveries are restricted because of low activities.
• Experimental grades and recoveries in literature are lower than calculated values.

An increasing percentage of nickel is being extracted from the laterite ores for which there exists considerable potential for new process development. In the current research, a thermodynamic analysis of the Fe–Ni–Co–Mg–Si–O–H–S–C–Cl system has been performed in order to establish the possible sulphidation conditions for the upgrading of a limonitic laterite ore. The study was performed using the equilibrium module of HSC Chemistry® 6.1, and the data available in the literature were utilised to determine the activity coefficients of the various species employed in the calculations. The effects of variables such as temperature, amount of sulphur and carbon and chlorine additions on both the nickel grade and recovery in the monosulphide solid solution (mss) were determined. Alternative sulphidizing agents were also considered. Nickel recoveries of about 85% could be achieved at a grade of 10% nickel in the mss. Limiting factors in the sulphidation process were the formation of iron sulphide (FeS) and the relatively high stability of nickel oxide in the nickel ferrite (NiFe2O4), in comparison to the other phases, which resulted in excessive sulphur requirements of over 100 kg of S/tonne of ore. Methods to overcome these restrictions were discussed and the thermodynamic results were compared to the published experimental values.