Recovery of rare earth elements, uranium, and thorium from monazite concentrate by supercritical fluid extraction

Abstract

Quantitative recovery of rare earth elements (REEs), Th, and U by supercritical fluid extraction (SCFE) with carbon dioxide containing adducts of TBP and HDEHP with HNO3 directly from monazite concentrate (MC) powder is impossible and requires the conversion of the constituent elements into more soluble compounds. Microwave (MW) radiation can be efficiently used for MC pretreatment by sintering with Na2CO3 in the presence of coal. The resulting product consists of two phases. One of them contains REEs (∼50%) recoverable by supercritical carbon dioxide (SC-CO2) containing adducts of TBP or HDEHP with HNO3. The second phase is a solid solution of CeO2 with Th and U oxides and remaining amount of REEs. It is resistant to SCFE. Conditions were determined for quantitative dissolution of this phase in a mixture of 4 M HCl with 0.05 M HF. The use of HDEHP under the SCFE conditions allows quantitative recovery of Th and U from the hydrochloric acid solution. In the process, REEs remain in the aqueous phase and are thus separated from Th and U. A possible flowsheet was suggested for the recovery REEs from MC using SCFE with their simultaneous separation from Th and U.