Study On The Enrichment Process Of Calcium Oxide Non-ammonia Precipitation In Rare Earth Leaching Solution Of Magnesium Salt System

At present,ammonium sulfate leaching-ammonium bicarbonate enrichment process is mainly used in industry to recover rare earth from ion-adsorption type rare earths ore,but the process has serious ammonia-nitrogen pollution problems.Based on the solution of ammonia-nitrogen pollution in the leaching process of ion-adsorption type rare earths ore by magnesium salt leaching agent,this paper pays more attention to ammonia-nitrogen pollution in the process of enrichment.Taking the simulated solution of rare earth leaching solution of magnesium salt system as the research object,calcium oxide is used for ammonia-free enrichment.The dissolution behavior of calcium sulfate in precipitation process,the precipitation enrichment process and the removal of sulfate radical in precipitation products were studied in order to achieve an effective cycle of calcium and magnesium,and to obtain high purity ion-type rare earth concentrate,which provides theoretical support for the final realization of green leaching and enrichment of ion-adsorption type rare earths ore.The main research contents and results are as follows:By studying the dissolution behavior of calcium sulfate during precipitation,it was found that the low concentration of rare earth had little effect on the precipitation of calcium sulfate during precipitation.At the same time,the variation of solubility of calcium sulfate with the concentration of magnesium sulfate and temperature was obtained.Finally,based on the results of solubility and precipitation experiments of calcium sulfate,it is concluded that calcium sulfate precipitation does not occur during precipitation of rare earth by calcium oxide,which indicates that the content of CaO and SO₃ in concentrate has no direct relationship with the dissolution behavior of calcium sulfate.In the process of precipitation of rare earth by calcium oxide,the Gibbs free energy of formation reaction of rare earth hydroxysulfate and rare earth hydroxide is negative,which indicates that both precipitation reactions may occur.The optimal precipitation process for calcium oxide precipitation enrichment of rare earth leaching solution was obtained by single factor experiment.The final pH of precipitation was 9.18,the concentration of rare earth?REO?was 0.8 g·L^-1,the concentration of magnesium ion was 1.0 g·L^-1,the concentration of calcium oxide slurry C^-_?OH?was 0.45 mol·L^-1,the precipitation temperature was 25?,the feeding rate of calcium oxide slurry was 1.5 mL·min^-1,and the washing volume was 200 mL.Under the technological conditions,the precipitation rate of rare earth was 99.62%,the purity of rare earth concentrates was 83.65%,and the content of SO₃ in rare earth concentrates was12.63%.Through the analysis of TG-DSC and FI-IR,it is further determined that rare earth hydroxide and rare earth hydroxysulfate are contained in rare earth precipitates,the precipitation mechanism of calcium oxide is clarified,and the introduction mechanism of sulfate radical is determined.Through the study of sulfate removal in rare earth precipitates,it is found that the selected sodium hydroxide can effectively remove sulfate in precipitates by agitation washing.The Gibbs free energy of the agitation washing reaction of rare earth hydroxysulfate with sodium hydroxide is negative,which indicates that the agitation washing reaction of sodium hydroxide is thermodynamically feasible.When the feed ratio of sodium hydroxide was 2.85,the liquid-solid ratio was 6.5 mL·g^-1,the washing temperature was 35?,and the washing time was 30 min,the purity of rare earth in the precipitate was 94.38%and the content of SO₃was only 2.90%.More importantly,after six cycles of sodium hydroxide stirring solution,the purity of rare earth concentrate can still reach 93.29%.It was further confirmed by TG-DTA and XRD that sodium hydroxide stirring can convert hydroxysulfate into rare earth hydroxide in rare earth precipitates.On this basis,a new process of calcium oxide precipitation-sodium hydroxide impurity removal was proposed.The new process was expected to eliminate the aluminium removal process before the precipitation of rare earth leaching solution,reduce rare earth loss,solve the problem of sulfate radical introduction in the process of calcium oxide precipitation,and hopefully obtain qualified rare earth concentrate.