Selective Extraction Of Rees From Rare Earth Fluoride Electrolytic Slag By Fluorine Fixation Via MgCl₂

In the existing methods of recovering rare earth（REEs）from rare earth molten salt electrolytic slag（REFES）,there are some problems such as high reagent cost,waste gas or waste water containing fluorine released are exist.Based on ideas of fluorine fixation in solid phase,a new method of REFES roasting with MgCl₂followed hydrochloric acid leaching was proposed creatively.In this method,REF₃ is transformed into acid-leachable rare earth compound,and fluorine is stably fixed as MgF₂slag,which can effectively avoid the release of dispersed fluorine pollutants.The results of this paper are concluded as follows:（1）Figures of NdCl₃-CaF₂-HCl and NdCl₃-MgF₂-HCl solution systems were drawn respectively.The concentrations of ions and precipitation regions under different pH conditions were studied.Under the condition of pH<7,when the concentration of ions is controlled as c（Nd）=0.5mol·L^-1,c（F）=1.5mol·L^-1and calcium or magnesium ions of 0.75～4.5mol/L,MgF₂can stably exist in solution,while CaF₂ are easily decomposed by acid.（2）The roasting reaction of MgCl₂ and REFES mixture was studied by TG-DSC detection.Roasting reaction can be divided into five stages.At the temperature of 50℃～78℃,MgCl₂could transform REF₃ into MgF₂ and RECl₃,and transform RE₂O₃ into MgO and REOCl,respectively.At the temperature of 90℃～130℃,MgCl₂·6H₂O lost two molecules of crystalline water to form MgCl₂·4H₂O,and the newly generated RECl₃ was converted to RE（OH）₃.At the temperature of 130℃～182℃,MgCl₂·4H₂O loses two molecules of crystalline water to form MgCl₂·2H₂O.When temperature reached 192℃and 252℃,MgCl₂·2H₂O reacted with REF₃to form REOCl and MgF₂.When temperature reached 401℃～618℃,MgCl₂·2H₂O reacted with RE（OH）₃ to form Nd OCl and MgO,and the remaining MgCl₂·2H₂O was decomposed into MgO.The reaction of roasting process can be summarized as follows:12MgCl₂+4REF₃+4RE₂O₃+4RE+3O₂+4H₂O=16REOCl+6MgF₂+8HCl+6MgO（3）The single factor experiment was proposed to investigate optimal rare-earth extraction effect.Under the conditions of the mass ratio of REFES to MgCl₂ of 10:3,roasting temperature of 700℃,roasting time of 2h,hydrochloric acid leaching concentration of 4mol/L,acid leaching temperature of 90℃,acid leaching time of 2h and acid leaching liquid-solid ratio of20:1,the total leaching efficiency of REEs can reach 99.13%,and the extraction efficiency of La,Ce,Pr and Nd are 99.20%,98.42%,99.38%and 99.08%,respectively.The concentration of fluorine in acid leaching solution was only 2.191×10^-6mol/L.