Process Development for the Extraction of Rare Earth Elements from an Acid Mine Drainage Treatment Sludge

The study focused on developing a separation approach to recover REEs from AMD treatment sludge. Parametric leaching tests were first conducted, and optimum leaching conditions were determined as a pH value of 0.5 and a leaching time of 8 min. Stage-wise precipitation tests were then applied to illustrate the precipitation behaviors of various groups of elements at different pH regions. Test results indicate that selectively recovering individual REEs by targeting one particular pH region was not feasible. However, impurities, specifically iron and silicon, could be selectively removed from REEs at pH 3.5. Afterward, three different solvent extraction methodologies, including regular solvent extraction with DEHPA, solvent extraction with a phase modifier of TBP, and postoxidation solvent extraction, were evaluated for enhanced rare earth oxide purity. As a result of the developed process, a mixed rare earth product with a purity of 80% by weight was produced from the AMD treatment sludge.