Sulphuric acid pressure leaching of a limonitic laterite: chemistry and kinetics

Sulphuric acid pressure leaching of limonitic laterites is the process of choice to recover nickel and cobalt from equatorial lateritic ores, replacing the energy intensive pyrometallurgical methods. This process achieves a high nickel and cobalt extraction (more than 95%) with a high selectivity due to simultaneous iron and aluminium dissolution and precipitation. Experiments were carried out using batch pressure leaching techniques. A titanium autoclave equipped with acid injection and sample withdrawal units was employed. Conditions close to the industrial practice were tested: pulp density 30%, acid to ore ratio 0.2 and temperature ranging from 230 to 270 degrees C. Raw limonite and the evolution of the nature of solid products during leaching were characterised using transmission electron microscopy. It was observed that limonite consists of aggregates of needle-like particles of goethite compacted together. Nickel was found to be predominately associated with this phase. During leaching, goethite dissolves continuously liberating nickel whilst iron re-precipitates as dense hematite particles in solution by ex situ precipitation. Several kinetic models for porous solids were also tested. The grain model was finally proposed to best describe nickel dissolution kinetics. The rate-controlling step was suggested to be pore diffusion of sulphuric acid. (C) 1998 Elsevier Science B.V. All rights reserved.