Efficient processing rare earth polishing powder solid waste by thiourea-H2SO4 system: Process optimization and thiourea decomposition effect

Abstract

Utilization of waste rare earth polishing powder (REPPW) is significant for protecting the environment and enhancing strategical rare earth resource recycling. However, efficient extraction of rare earth oxides (REOs) from REPPW in an environmental-friendly and low-cost manner remains a great challenge in this area. In this study, a reductive H2SO4 leaching process using the low toxicity thiourea as the reducing agent is proposed to extract rare earth elements (REEs) from the REPPW. Under the optimal conditions of thiourea concentration of 0.078 mol·L−1, H2SO4 concentration of 1.50 mol·L−1, temperature of 353 K, and liquid-to-solid ratio of 15 mL·g−1, the leaching ratios of Ce and La reached 93.8% and 93.2%, respectively. Moreover, the thiourea behavior during the REPPW leaching process was studied. The results indicates that the thiourea not only acted as reductant but also tended to decompose in a CS(NH2)2→(SCN2H3)2→S route. The decomposed thiourea amounted to 18.07% of its original dosage, which formed precipitation during aging of the leachate. However, it influenced neither Ce/La concentration in the leachate nor purity of the product. The recycled REOs with purities higher than 93% were prepared from the leachate by oxalate precipitation-oxidation roasting, showing no relation with the aging time.