Rare earth elements (REEs) are crucial constituents in a number of modern technologies including mobile phones, superconductors, hybrid vehicles and manufacturing industries. Increased demand for these minerals provides Australia with a unique opportunity to become a major player in the REE industry, as Western Australia’s Mt Weld mine has the world’s largest resource of rare earth elements. As conventional REE processing methods are expensive and generate corrosive waste products, we are proposing the application of phosphate solubilising microorganisms (PSMs) via bioleaching to recover REEs from monazite.
This research aimed to apply PSMs used in agriculture to phosphate solubilisation from monazite, allowing REE release into the leachate. Ten PSMs identified in literature were subjected to testing on two monazite ores. Penicillum sp. released a total concentration of 12.23 mg L-1 REE (Ce, La, Nd and Pr) from 0.5g of monazite after incubation for 192 hours on the weathered monazite, whereas cultivation on the mineral sands monazite resulted in high levels of iron and thorium and very little REEs. Three other isolates, Enterobacter aerogenes, Pantoea. agglomerans and Pseudomonas putida also successfully released REEs into leachate, at lower concentrations (1.66, 1.63 and 1.45 mg L-1 respectively) from the weathered monazite. The 6 other isolates tested were less effective with REE levels in the leachate less than 0.6 mg L-1.
HPLC examination of the leachate detected numerous low molecular weight organic acids including gluconic, acetic, formic, malic and citric acid. We hypothesize that the microbial extruded organic acids are responsible for P solubilisation and REE mobilization, however the extent of the reaction is highly dependent on the microbial attachment, monazite structure and elemental composition.
This study establishes the potential use of PSMs for the recovery of REE from monazite with successful operation of this system providing a more environmentally sound mining option.