Selective thorium and uranium extraction from monazite: I. Single-stage oxalate leaching

•Selective leaching of thorium and uranium from monazite with oxalate was studied.•Thermodynamic speciation in oxalate was theoretically determined for Th(IV), UO2(II), Ce(III) and Nd(III).•Experimental data showed that thorium and uranium extractions were affected by particle size.•Complete recovery of thorium and uranium was not achieved, due to passivation.

The selective leaching of thorium and uranium from monazite with ammonium oxalate was studied at near ambient temperatures, as a function of pH, particle size, phosphate and oxalate concentration, and temperature. The thermodynamic speciation showed that the presence of phosphate seriously restricted the solubility of both metals, while the solubility of thorium was also lowered by the formation of solid thorium oxalate in the moderately to strongly acidic pH region (pH < 3). Experimental evidence indicated that once the optimum pH was determined, the extent and velocity of both thorium and uranium extractions were modestly affected by the temperature, but greatly by the particle size. Regardless of conditions, complete recovery of thorium and uranium could not be achieved, apparently because of passivation of unreacted monazite cores by recrystallized rare earth phosphates or precipitated oxalates.