Experimental Study On Selective Coordination For Impurity Removal During Lanthanum Chloride Liquid Precipitation

Due to the extraction limit of traditional rare earth extraction,the separation and purification of rare earth elements from the extracted solid and liquid face challenges.In order to solve this problem,this study proposes to use organic ligand functional groups to selectively form stable complexes with impurity metal ions during rare earth precipitation.The behavior of impurities,especially Fe³⁺,Cr³⁺and Al³⁺,in lanthanum chloride raw material solution was studied in oxalic acid and ammonium bicarbonate system.The removal of coordination impurities in ammonium bicarbonate system was studied.The feasibility and technology of removing Fe,Cr and Al from lanthanum chloride raw material solution by coordination method were studied systematically,and the mechanism of impurity removal was discussed by using solution chemistry and material simulation software.The results demonstrate the effectiveness of the proposed method in achieving selective impurity removal and its potential in the separation and purification of rare earth elements.The main conclusions of the paper are as follows:（1）The lanthanum chloride liquid was precipitated by oxalic acid.The results showed that the p H of rare earth precipitated by oxalic acid was lower than that of impurity ions.The oxalic acid precipitated rare earth and the precipitation rates of impurity ions were all at a very low level.（2）Iron was separated by complex coordination method in ammonium bicarbonate system.The results show that in lanthanum chloride ferric liquid,salicylic acid has the best complexation separation effect.The optimum conditions are temperature 35℃,salicylic acid dosage is 2.5 times of the theoretical amount,ammonium bicarbonate dosage is 1.75 times of the theoretical amount,reaction time 2 h,the precipitation rate of iron is the lowest,only0.21%,and the precipitation rate of lanthanum is 99.99%.Salicylic acid was effective in the separation of Fe³⁺at different concentrations.（3）The complexation and oxidation methods were used to study the lanthanum chloride liquid containing chromium.The results show that under the condition of choosing formic acid as the complexing agent,formic acid is preferentially complexed with rare earth to form rare earth formic acid precipitation,but the complexation effect of formic acid and Cr³⁺ion is not obvious.Under the optimal condition of oxidizing Cr³⁺to Cr O₄^2-with hydrogen peroxide as oxidant,the precipitation rate of chromium is only 9.36%.（4）The results show that salicylic acid can also be used to remove aluminum by complexing coordination in ammonium bicarbonate system.The optimum conditions are temperature 35℃,dosage of salicylic acid is 2.5 times of the theoretical amount,dosage of ammonium bicarbonate is 1.5 times of the theoretical amount,reaction time 3 h,the lowest precipitation rate of aluminum is 8.85%,and the precipitation rate of lanthanum is 99.78%.Salicylic acid has better effect on complex separation of high concentration Al³⁺.Salicylic acid was used to remove iron and aluminum from lanthanum chloride liquid containing iron,chromium and aluminum,and the verification test of chromium removal by oxidation method showed that under the above optimal conditions,the precipitation rates of Fe³⁺and Al³⁺in the mixture were 5.72%and 8.98%,and the precipitation rates of Cr³⁺were 18.8%.（5）The simulation analysis software Visual Minteq 3.0 and Material Studio were used to study the mechanism of complex impurity removal.The results showed that in oxalic acid system,the p H of rare earth precipitated is all below 1,and the p H of this time is lower than that of Fe³⁺,Cr³⁺and Al³⁺.A large number of free Fe³⁺,Cr³⁺,Al³⁺particles are more likely to bind to water hydroxyl group than to coordinate with chloride ion.The optimal configuration of salicylic acid combined with Fe³⁺,Cr³⁺and Al³⁺shows that salicylic acid and iron can form stable valence bonds.When Fe³⁺,Cr³⁺and Al³⁺ions are contained in the system,salicylic acid preferentially adsorbed Fe³⁺.