Study On The Mechanism Of Impurity Removal In The Preparation Of Rare Earth Chloride

The mixed rare earth concentrate of Baotou was leached by aluminum chloride chloride complex,and the Leach solution was precipitated by sodium sulfate double salt and then realkalized to obtain rare earth hydroxide.The rare earth hydroxide contained high contents of iron,aluminum,thorium,sodium,sulfur and phosphorus.In this thesis,rare earth chloride was prepared from the above rare earth hydroxide by impurity removal.The chemical composition and phase structure of rare earth hydroxide were analyzed by modern analytical methods By means of thermodynamic analysis and kinetic analysis,the reaction mechanism of impurity removal process was studied as follows:At 323K,La（OH）₃precipitates after PH is more than 6.16,and the initial precipitates of iron,aluminum and thorium are all less than 6.16,therefore,impurities can be separated by controlling the p H of the solution.Secondly,the optimum condition and mechanism of impurity removal by washing were studied.The results showed that the washing efficiency of Al³⁺,PO₄^3-,SO₄^2-and Na⁺reached 78.67%,38.71%,60.65%and 91.04%respectively.The removal of Al³⁺,PO₄^3-,SO₄^2-and Na⁺mainly resulted from the dissolution of sodium hydroxide,Aluminum hydroxide,Sodium aluminate,sodium phosphate and sodium sulfate in water,and mainly concentrated in the first three washing solutions.The results show that the solubility of rare earth hydroxide is 95.98%under the optimum condition.The dissolution process of hydrochloric acid accords with the reduced-size model,which can be described by 1/3ln（1-x）+[1-x^-1/3-1]=kt.The Correlation Coefficient is 0.97842,and the apparent activation energy is 29.32 k J/mol.The reaction is affected by the interface diffusion and diffusion.Combining the results of thermodynamic analysis again,the p H values of rare earth chloride solution were compared with those of ammonia,sodium hydroxide and hexamethylenetetramine.The results showed that Al³⁺,Fe⁴⁺and Th⁴⁺were separated from the rare earth solution by hydroxide precipitation.The removal rates of Fe⁴⁺and Th⁴were almost the same.When hexamethylenetetramine was used to adjust Ph,the removal rate of Al³⁺was relatively high,reaching 96.16%,and the loss rate of rare earth was 5.47%.The removal rate of Al³⁺,Fe⁴⁺and Th⁴⁺was 92.72%,96.97%and 98.5%,respectively,and the loss rate of rare earth was 13.56%.Finally,the effect of direct crystallization and non-stage crystallization on the morphology of rare earth chloride powder was studied.The results show that the re-cl crystals prepared by direct crystallization are obviously agglomerated,and the re-cl crystals prepared by stage crystallization are weakened,especially when the crystallization time is more than 1.5h,the particles have been dispersed,irregular in shape.The composition of rare earth chloride crystallized at 1.5 h was in accordance with the national standard of“Mixed rare earth chloride”.