The solubilization of nickel, cobalt and iron from laterites by means of organic chelating acids at low pH

Abstract

An economical process for recovering nickel from large quantities of low grade laterite is needed. With this in mind heterotrophic microbial leaching was investigated as an alternative to the energy expensive existing extraction process. Organic acids were screened for the ability to solubilize nickel, iron and cobalt from laterite ores and the results compared with sulphuric acid treatment of limonite type laterite at pH 2.3 and 28°C which solubilized nickel, cobalt and iron. Citric, tartaric and pyruvic acids significantly increased the yield of soluble nickel extracted at pH 2.3 with sulphuric acid. Despite their reported efficacy for other mineral systems, lactic, glycollic and oxalic acids were not effective solubilizers of nickel from Indonesian laterite and 2-ketogluconic, lactic and glycollic acids did not effectively solubilize nickel from West Australian laterite. Metabolites generated from glucose by the natural flora of limonite type laterite ore were associated with significant solubilization of metals from the ore at acid pH due to their chelating activities. The leaching solutions generated from glucose retained activity when recycled, an important consideration if the solution is to have industrial application. The leach liquors from these studies were analyzed for the extent of metal extraction and the presence of microbially produced metal chelators. Cellulose acetate electrophoresis provided proof that chelated nickel complexes were present amongst nickel ions solubilized at acid pH by metabolites of glucose generated by the natural flora of the ore. This technique also provided proof that citric acid functions as a chelator of divalent metal cations at strongly acid pH, a point hitherto the subject of conflicting reports in the literature. Atomic absorption spectrometry and X-ray diffraction studies confirmed the findings from cellulose acetate electrophoresis.