Enhanced selective recovery of rare earth elements from treated acidic mine water as oxalates: experimental optimization on the crystallization stage

Treatment of acid mine water (AMW) from mining activities is an environmental challenge due to the large volumes generated. At the same time, rare earth elements (REEs), classified as Critical Raw Materials by the European Union, can be present in AMW at low concentrations, making their recovery an attractive and sustainable option to both mitigate environmental impacts and diversify the REE supply. REE recovery from AMW typically involves an initial extraction step, which can be non-selective (coprecipitation with Fe- and Al-hydroxysulphates) or selective (adsorption onto Ion Exchange (IX) resins), followed by REE-precipitation from the sulphuric acid eluates used for resin regeneration. This study investigates the optimal conditions for REE recovery through oxalate precipitation from sulphuric acid solutions representative of IX concentrates in the absence of interfering elements. Precipitation experiments were performed by varying three key parameters: oxalic acid dose, mode of acid addition and solution pH. Results show that oxalic acid dosage strongly influences REE recovery, exceeding 80 % from 2 times the stoichiometric requirement (SQ) onwards. The rate of oxalic acid addition had no measurable effect on recovery. pH also played a significant role, with optimal recoveries (>86 %) achieved in the pH range 1.2-3.0. SEM-EDX and XRD analyses confirmed the formation of REE-oxalates with similar morphology and composition across conditions, with individual REE contents ranging from 5 to 19 wt%. These results provide practical guidelines for an efficient REE recovery from AMW-derived concentrates.