Mineralogical characterisation of Indonesian laterites prior to and post atmospheric leaching

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Nickel laterite is of increasing importance as an economically viable source of Ni. However, practical strategies for low cost atmospheric leaching of laterites have been hampered in part by poor recovery and the formation of potentially environmentally harmful residues, for instance jarosite. It is important therefore to define the mineralogical forms of nickel present, using readily accessible laboratory-based approaches, in the ores and residues, and the form of secondary precipitates in the residues, as a means for future optimisation of leach parameters. To this end a detailed mineralogical study of an Indonesian laterite and residues resulting from atmospheric acid leaching has been undertaken. The leach process involved the sequential reactions of first, limonite (3h) and then saprolite (11h) in sulphuric acid leach liquor. The acid, dry limonite and saprolite ores were added in the weight ratio of 1.4:1:1. At 100°C, over 80% of the total Ni was extracted into solution during the whole leach process.The limonite ore was found to contain Ni (1.2wt.%) predominantly within Fe oxy/hydroxides, serpentine and phyllomanganate whilst in the residue Ni (0.4wt.%) is present in highly crystalline leach resistant minerals, such as spinels. The majority of the Ni (1.6wt.%) in the saprolite ore was present in serpentine with a small fraction also present in phyllomanganate. This association is further confirmed by the similar saprolite leach extraction curves of Ni and Mg. Goethite and jarosite are both found to be concentrated in the saprolite residue, as is consistent with the decrease in Fe extraction during the saprolite leach. For the final 11h residue of the saprolite leach, the Ni (0.3wt.%) is mainly present in undissolved lizardite and leach resistant pyroxene and spinel minerals.