Recovery of rare earth elements from rare earth molten salt electrolytic slag via fluorine fixation by MgCl_(2)roasting

Rare earth fluoride molten-salt electrolytic slag(REFES)is a precious rare earth element(REE)secondary resource,and conside rable amounts of REEs exist in REFES as REF_(3);they are difficult to dissolve in acid or water and impede efficient REE extraction.In REFES recovery,the REF_(3)species in REFES are usually transformed into acid-soluble rare earth compounds by NaOH roasting or sulfating roasting and then extracted by acid leaching,Moreover,the fluorides in REFES are released as HF gas in the roasting process or enter the liquid phase during the water washing process;both of these processes cause fluorine pollution.Fixing the fluorine into the solid slag provides a way to avoid fluorine pollution.In this study,a novel method was proposed to extract REEs from REFES via MgCl_(2)roasting followed by HCl leaching.Thermodynamics calculations and the rmogravimetry-differential thermal analyses(TG-DTA)were conducted to investigate the reactions occurring in the roasting process,First,MgCl_(2)reacts with the REF_3and RE_(2)O_(3)to form RECl_(3)and REOCl,respectively.Second,the RECI_(3)absorbs water and forms RE(OH)_(3).Third,MgCl_(2)·6H_(2)O is gradually dehydrated to MgCl_(2)·2H_(2)O and reacts with REF_(3)and RE(OH)_(3),and REOCl,MgF_(2)and MgO are formed.Through HCl leaching,the REOCl in the roasting products is leached by HCl acid,while fluoride re mains in the solid slag as MgF_2.The optimum experimental conditions are as follows:mass ratio of MgCl_(2)to REFES of 30%,roasting temperature of 700℃,roasting time of 2 h,hydrochloride acid concentration of 4 mol/L,leaching time of 2 h,leaching temperature of 90℃and leaching L/S ratio of 20:1.The efficiencies for total leaching of the REEs,La,Ce,Pr,and Nd are 99.13%,99.20%,98.42%,99.38%,and 99.08%,respectively.Moreover,the concentration of fluoride in the leaching solution is 2.191×10^(-6)mol/L.This method has a short process flow with low reagent costs,and the problem of fluoride pollution from REFES recovery is solved;thus,our study has great industrial application potential.