Investigation of Secondary Reactions in Bioleaching of Nickel Laterite Ores

Abstract

This thesis investigated the impact of secondary reactions; adsorption and precipitation, on nickel and cobalt recovery from bioleaching of nickel laterite ores. Although bioleaching is considered an ecologically and energy friendly technique to process laterite ores, low recoveries of target metals have inhibited its commercialisation. The secondary reactions occurrence in bioleaching of limonite, fresh and weathered saprolite ores was verified by performing chemical leaching tests using sulphuric and citric acid that would be generated by chemolithotrophs and heterotrophs respectively. The following leaching parameters were kept constant; pulp density at 10 (g/ml)%, particle size of the ore at 64 – 180 µm and stirring speed at 500 rpm. Metal loss was observed typically after 5 – 10 days and it increased as the acid to ore (A/O) ratio was reduced. At the lowest A/O ratio (0.1:1 (g/g)), up to 74% of nickel and 89% of cobalt were lost from target metals leached from saprolites while only 9% of nickel and negligible cobalt were lost from limonite. The mineralogical transformation of the residues resulting from the leaching process contributed to the alteration of the mineral isoelectric point (IEP) and consequently adsorption behaviour. The lower IEP in the saprolite residue (2.5 – 3.5) were attributed to the residual serpentine mineral (3.3) and quartz (2.2). While goethite (6.1 – 6.7) contributed to the higher IEP of the limonite residue (6.9). Systematic tests verified that adsorption can be overcome by maintaining the solution pH below the IEP of residues and adding complexing agent, 15 g/L of citric acid. A simulation of iron precipitation revealed little effect on metals recovery suggesting adsorption was as the primary secondary reaction affecting nickel laterite bioleaching. It is concluded that secondary reactions can have a huge impact on the metal recoveries and controlling its effect is critical in driving the commercialisation of the laterite bioleaching