Recovery of rare earth elements from acidic mine waters: a circular treatment scheme utilizing selective precipitation and ion exchange

Abstract

Acidic Mine Waters (AMW) pose significant environmental hazards due to their high acidity and metal content. Traditional treatment methods involve neutralizing the acidity and removing metals as hydroxides, but they generate hazardous sludge. However, a more sustainable approach has emerged, considering AMW as a secondary source for Critical Raw Materials (CRMs), such as Rare Earth Elements (REEs), through a circular treatment scheme integrating selective precipitation and ion exchange. This study focused on recovering REEs, with a total content of 18.6 mg/L, from AMW obtained from the Aznalcóllar open-pit in Sevilla, Spain, which also contained significant amounts of Al (216 mg/L), Fe (47 mg/L), and Zn (548 mg/L). The AMW was initially pre-treated to precipitate Fe and Al with high removal efficiencies (>99.9% and >90%, respectively). Furthermore, optimization studies were conducted to remove Zn as sulphide, achieving removals of Zn, Cd, and Cu >99% under the optimum conditions of NaHS dose and pH. The selective recovery of REEs was carried out using the commercial ion exchange resin impregnated resin containing an organophosphinic functional group (Lewatit TP272) in column mode, with a special focus on understanding the effect of Zn presence. It was observed that the presence of Zn influenced the concentration factor achieved during elution, resulting in at least two times greater concentration when Zn was removed from the solution, impacting the content and purity of the REEs oxalates obtained. This research demonstrates a promising approach for sustainable and efficient recovery of valuable REEs from AMW while mitigating environmental hazards associated with hazardous sludge generation.