Laboratory experiments were carried out to identify the conditions favouring hematite precipitation over sodium jarosite formation when nickel sulphide concentrates are leached under autoclave conditions. Increasing retention times, to 5-6 h at 150°C, result in the precipitation of more hematite from a typical processing solution containing 1.3 M NiSO_4-0.2 M Fe(SO_4)_1.5-0.15 M H_2SO_4-0.3 M LiCl but no Na_2SO_4. Increasing temperatures promote the precipitation process and improve the crystallinity of the hematite. Increasing chloride concentrations slightly decrease the amount of hematite precipitated, but have no effect on its composition. Increasing H_2SO_4concentrations dramatically reduce the amount of precipitate, but have no effect on the composition of the hematite formed. Increasing concentrations of Fe(SO_4)_1.5 augment the amount of hematite precipitated but do not affect its composition. Although the mass of precipitate increases in a complex manner with increasing NiSO_4 concentrations, the composition of the precipitated hematite remains essentially constant. Notable is the fact that even modest concentrations of Na_2SO_4 result in the extensive precipitation of sodium jarosite. However, the presence of hematite seed significantly suppresses jarosite precipitation while promoting hematite precipitation. In this regard, autoclave leach residue, which contains major amounts of hematite, is an effective seed material for hematite precipitation. In contrast, neither goethite nor elemental sulphur is an effective seed material for hematite precipitation. Regardless of the experimental conditions, the hematite precipitates are poorly crystallized and typically contain ~60% Fe and 5-7% SO_4. Supporting mineralogical studies indicate that the 5-7% SO_4 is associated with the hematite and is likely adsorbed on the individual hematite crystallites.
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