Recovery of rare earth elements adsorbed on clay minerals: II. Leaching with ammonium sulfate

The present study investigates the use of ammonium sulfate as a lixiviant in the recovery of rare earth elements (REE) from clays. Rare earth ions are physically adsorbed on clay minerals, with concentrations ranging from 0.05 to 0.5 wt.%. It was previously shown that they could be easily recovered via an ion exchange mechanism during leaching with inorganic monovalent salt solutions (such as ammonium sulfate). A standardized desorption procedure was established to systematically investigate the influence of leaching conditions such as lixiviant concentration, temperature, pH and agitation rate on desorption kinetics and REE extraction levels. It was determined that the optimum leaching conditions, leading to 80–90% total REE extraction, required pH values in the range 3–4 and moderate temperatures (< 50 °C) in order to avoid lanthanide precipitation/loss via hydrolysis. Various lixiviant concentrations above a certain “cut-off” level (about 6 times the stoichiometric requirement) did not affect extraction levels, while the agitation speed was irrelevant with regards to leaching efficiency, requiring only sufficient stirring to ensure complete slurry suspension for effective mass-transfer. Extraction kinetics were found to be very fast, with less than 5 min to reach terminal extraction, and independent of lixiviant concentration, pH, temperature and agitation speed.