Recovery of Rare Earth and Fluorine from Fluorinated Rare Earth Molten Salt Electrolytic Slag by Ball-Milling and Sodium Hydroxide Submolten Salt Processes

Abstract

Fluorinated rare earth molten salt electrolytic slag (FRMES) is an important source of rare earths, containing large amounts of rare earth fluoride (REF₃). Traditional technologies for recovering rare earths from FRMES mainly include acid and alkali roasting, with high energy consumption and heavy pollution. In this work, a low-consumption and efficient approach for the recovery of rare earths and fluorine from FRMES by ball milling and sodium hydroxide submolten salt (SHSS) was presented. The results showed that the dissociation efficiency of REF₃ in the SHSS system was significantly improved by pre-ball milling, and the leaching efficiency of REF₃ was 99% under the optimal leaching conditions (sodium hydroxide concentration of 300 g/L, leaching temperature of 180 °C, leaching time of 3 h, and liquid–solid ratio of 4:1). The leaching process of REF₃ from ball-milled and unball-milled FRMES in the SHSS system was mainly controlled by mixed chemical reaction and internal diffusion, with apparent activation energies of 35.57 and 41.31 kJ/mol, respectively. The smaller grain size and more lattice defects of REF₃ in ball-milled FRMES than in unball-milled FRMES resulted in the exposure of more active sites and lower reaction activation energies. The product RE(OH)₃ wrapped around REF₃ was the main reason for hindering the rapid dissociation of REF₃ in unball-milled FRMES. The recovery efficiency of rare earths exceeded 96% by HCl preferential dissolution and solvent extraction processes, and F was recovered in the form of NaF. These results provide a low-energy and clean approach for the integrated treatment of FRMES with prospects for industrial application.