Pressure Acid Leaching of Nickel Laterites: A Review

Abstract A review of the literature over the past 30 years on the processing of nickel laterites by high temperature acid leaching has been carried out to provide a better understanding of the mineralogy, leaching process chemistry and effect of operating conditions on nickel recovery, residue properties and scaling. Particular attention is paid to the leaching experience of the commercial Moa Bay plant and to the recently reported testwork and flowsheets associated with the three Western Australia lalerite plants that will be operating in 1999. It is shown that laterites can vary significantly in their mineralogy according to location, climate and depth, and that the main host minerals for nickel and cobalt can be either goethite (iron oxide) or nontronite (clay) or manganese oxides. The mechanism of leaching involves acid dissolution of the host mineral lattice followed by hydrolysis and precipitation (transformation) of a variety of insoluble oxides and sulphates of iron, aluminium and silica under the high temperature conditions. Optimum leaching conditions and final liquor composition varies according to the ore mineralogy. More fundamental studies have demonstrated that the rate of leaching and character of the residue is dependent upon the level of Mg, Mn and Cr in the ore, the Eh of the slurry and salinity of the process water. A number of studies are reviewed on the chemistry and precipitation of iron, aluminium, magnesium and silica to understand how the process conditions affect the solubility of the species and the nature of the scale which they form. Early work at Moa Bay indicates that the incorporation of chromium into alunite scale also affects the incorporation of silica and nickel and the settling of the residues. Various types of scale have been identified during different stages of leaching and possible means of minimising scale are discussed. The clay-rich nickel laterites in Western Australia differ from Moa Bay laterite in mineralogy and have comparatively high silica and low chromium content. Since no commercial plant has previously processed such ores or used saline process water, there is little published in this area. It is therefore recommended that further research be carried out on understanding the process chemistry and species equilibrium from various ore types under autoclave conditions.