Rare earth recovery from fluoride molten-salt electrolytic slag by sodium carbonate roasting-hydrochloric acid leaching

Fluorinated rare earth molten-salt electrolytic slag contains a considerable amount of rare earth elements, as well as a variety of heavy metals and fluorides that cause environmental pollution. Therefore, it is of great importance to fully utilise this resource. In this study, the transformation mechanism of fluorinated rare earth molten-salt electrolytic slag roasted with sodium carbonate, and the regulation mechanism of rare earth leaching under different roasting conditions were investigated with the help of thermodynamic calculation of the reactions and kinetic analysis. The thermodynamic and differential thermal thermogravimetric (DTA-TG) analysis shows that the transformation of rare earth fluoride to rare earth oxide is promoted at elevated temperature. Furthermore, the leaching experimental results show that increasing the temperature, time, hydrochloric acid concentration, and liquid–solid ratio can effectively promote the recovery of rare earths. The optimum experimental conditions are a roasting temperature of 700 °C, roasting time of 2 h, and sodium carbonate to molten salt electrolytic slag mass ratio of 0.6, followed by leaching at 80 °C with a liquid–solid ratio of 10:1 by adding 3 mol/L hydrochloric acid with stirring for 2 h. Under these conditions, the rare earths in the molten salt electrolytic slag are biologically transformed at a lower temperature and the leaching efficiency of rare earths exceeds 98%.