A safer and cleaner process for recovering thorium and rare earth elements from radioactive waste residue

Highlights: • The chemical composition of the radioactive residue was analyzed. • The leaching efficiency of REEs and Th were about 98.14% and 99.07%, respectively. • A multi-step leaching process was designed to reduce the use of acid and alkali. • The efficient separation of Th with CA-12 was studied in detail. • Th, lanthanides and Y in the leachate of residue were recovered respectively. Rare earth elements (REEs) have attracted widely attentions because of their excellent properties, however, radioactive waste residue generated during the REEs production has created serious environmental problems. This study aimed to develop a safer and cleaner technology, including residue leaching, thorium (Th) separating and REEs recovering, for the proper disposal of radioactive waste residue from ion-absorbed rare earth separation industry to reduce the environmental hazards. First, the chemical composition of residue was analyzed. Then, the leaching factors such as acid type, acid concentration and liquid-solid ratio were investigated and a multi-step leaching process was proposed to improve acid utilization and the leaching of REEs. After the multi-step leaching with HCl, the total leaching efficiency of REEs and Th were higher than 98.14% and 99.07%, respectively. Next, a commercial extractant of sec-octylphenoxy acetic acid (CA-12) was used to separate Th and enrich REEs from the residue leachate. The extraction factors of CA-12 toward Th were investigated in detail and a fractional extraction for separating Th and enriching lanthanides from the leachate of residue was carried out, showing that the separation efficiency of Th was higher than 99.53% and the concentration of lanthanides in the concentrated solution was 223.19 g L⁻¹.