High temperature acid leaching of Western Australian laterites

High temperature acid leaching is the current process of choice for greenfields nickel laterite projects. Previous knowledge of the process is limited, with only one plant operating in Cuba. Extension of the process from tropical limonites to Western Australian laterites is new technology, and as such the behaviour of the differing minerals in tropical and arid laterites in the leach is examined. Firstly, the mineralogy of a laterite from Honeymoon Well, in Western Australia was investigated, with nontronite, goethite, magnetite and lithiophorite found as key ore minerals. These four minerals hosted the nickel, whilst lithiophorite contained most of the cobalt. The fundamentals of high temperature acid leaching of iron oxides was then examined using synthetic goethite as a model ore. It was found that goethite transforms to hematite by a dissolution-reprecipitation mechanism. The leaching rates were dependent on acid concentration, the slurry oxidation potential and cations in solution. Increasing the slurry Eh decreased the reaction rate. Additionally, it was found that acid increases leaching rates where the Eh is high, but decreases leaching rates where the Eh is low due to a change in the rate limiting step. Because the Eh of the slurry was controlled by the variation of minerals in the feed, the selective removal of the high Eh (lithiophorite) component of the ore was demonstrated using atmospheric leaching with S02. At high Eh goethite dissolution is rate limiting, at low Eh hematite precipitation is rate limiting. It was also found that in general background sulphate salts increase the transformation rates and trivalent cations decrease the transformation rate by adsorption processes. Nontronite reacted more readily than iron oxides and was examined by a systematic systems analysis. Significantly for large scale operations the settling performance of the leached slurries was poor. When the settling rates of nontronite-goethite post-leach slurries were examined in detail, it was noted that this depended upon the silica content of the feed and surface charge of the particle. Settling performance was enhanced by altering the relative rates of nontronite and goethite transformation to allow silica to precipitate before transformation occurs. The residue particles were then covered by an iron oxide coating. Finally the interaction of various solution species at high temperature to form precipitates and scale are reviewed