Study On Leaching Mechanism Of Uranium,Thorium And Rare Earth Elements From Rare Earth Smelting Slag

With the rapid development of information technology,the demand for rare earth elements risen sharply which brings economic development and technological progress,but also environmental problems.The increasing number of smelting slag and the relocation and retirement of rare earth plants make the disposal of slag imminent.In this paper,the acid insoluble residue,radioactive filter residue and final neutralization residue of an ionic rare earth ore smelter in Guangdong Province were studied,in order to provide basic data for the storage and treatment of waste residue.The effect of rainwater on the leaching of harmful metals in waste residue during shallow burial was simulated,and the leaching mechanism was explored.The waste residue was characterized by scanning electron microscopy-energy disperse spectroscopy(SEM-EDS),Fourier transform infrared spectroscopy(FTIR),X-ray fluorescence(XRF).The elemental quantitative analysis and elemental speciation study were carried out by wet digestion and modified BCR-sequential extraction procedure(modified BCR)from the EU,the results are as follows:1.According to the results of SEM-EDS analysis,the main heavy metals in the three waste residues are Cu,Zn,Y,La and Sm.The contents of uranium(U)and thorium(Th)are too low,and the energy spectrum cannot be detected.2.The radioactive level of smelting slag produced by the same rare earth factory is quite different,so it is necessary to pay attention to classification storage and separate treatment and disposal when dealing with rare earth ore slag.The acid insoluble residue enriched ²³²Th obviously,while ²³⁸U slightly.The enrichment of ²³⁸U,²²⁶Ra and ²³²Th in the radioactive filter residue was obvious.The specific radioactivity of ²³⁸U and ²³²Th in acid insoluble residue were129 Bq·kg^-1 and 5356 Bq·kg^-1,which were 1.8 and 94.0 times of the average values in Guangdong Province,respectively.The specific radioactivity of ²³⁸U,²²⁶Ra and ²³²Th in radioactive filter residue were 1718 Bq·kg^-1,548 Bq·kg^-1 and 5571 Bq·kg^-1,respectively,which were 24.2,10.7 and 97.7 times higher than the average of Guangdong Province.3.BCR results showed that acid insoluble residue had the best extraction effect,followed by neutralization residue,and the extraction effect of radioactive filter residue was the worst.This method was not suitable for the speciation analysis of heavy metals in radioactive filter residue.Uranium in acid insoluble residue and final neutralization residue is mainly in weak acid-soluble fraction and reducible fraction,which is easy to be absorbed and utilized by animals and plants and endangers the environment.Th in acid insoluble residue and final neutralization residue was mainly in residual state(81%and 73%,respectively),with weak migration activity and low risk coefficient;The contents of Pr,Nd,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb and Lu in acid insoluble residue were mainly in weak acid-soluble fraction(47%—55%)and residual state(25%—42%),with high migration activity;Y and La are mainly weak acid-soluble fraction and reducible fraction,with high migration activity;Ce are in residual fraction(51%)and reducible fraction(28%),with weak migration activity and high stability;With the proportion of reducible state(68%)more than that of residual state(17%),Sm was weak migration activity.The La,Pr,Tb and Dy in the final neutralization residue were mainly in oxidizable fraction,and the migration activity is general in the environment.4.The leaching amount of uranium is large and the change is complex,but the leaching mechanism is surface dissolution.The leaching amount of thorium is very low,indicating that the risk of thorium leaching in waste residue is very low and the leaching mechanism of thorium in the three kinds of waste residue is different.The leaching mechanism of acid insoluble residue,radioactive filter residue and final neutralization residue are dissolution diffusion,surface elution and surface dissolution.Except Mn,Sm,Y and La,the leaching amount of heavy metals in the waste residue was low,which was basically at the ppb level.The leaching mechanisms of different elements in waste residues are different,and the leaching mechanisms of the same element in different waste residues are also different.5.The FTIR results of three kinds of waste residues show that there is no significant change in the spectrum before and after leaching,indicating that the basic skeleton has not changed.XRF results showed that the main oxide types in slag did not change,which was basically consistent with FTIR results.