Study On The New Extraction Process Of Aluminum And Uranium Co-associated With In Ion-Adsorption Rare Earths Low-Level Radioactive Residues

Rare earth elements（REEs）,as the strategic key resource,have attracted wide attention for the clean production and cyclic utilization under the joint promotion of the structural contradiction between global supply and demand with the national"double carbon"policy.A terrific of low radioactive REEs residues（named as ionic rare earth elements low-level radioactive residues（IREEs-LRR））have been produced throughout the development process of ionic rare earths ore（IREO）,containing radioactive nuclides such as uranium（U）,thorium（Th）and radium（Ra）and so on,which could only be stacked and disposed of for a long time.Therefore,it is necessary to explore new methods and technologies for the emission reduction of low-level radioactive pollutants and the utilization of valuable resources in IREEs-LRR,so as to break through the barriers of environmental constrained in the production process of REEs resources.In this thesis,a novel process for gradient extraction of Al and U from IREEs-LRR with unconventional media was meticulously investigated.Specifically,Al was extracted in dilute alkali media at atmospheric pressure,while U was extracted using carbonate-citrate and other media.These techniques are designed to achieve significant reductions in the emission of low-level radioactive contaminants,optimization of resource utilization for Al and U,and mitigation of radionuclide contamination.The main innovative results were summarized as follows.（1）Taking IREEs-LRR containing high content calcium（IREEs-LRR-HCa）as the object,this study delved into the extraction of Al in a dilute NaOH medium at atmospheric pressure,taking into the characteristics that the occurrence state of Al in IREEs-LRR-HCa was aqueous aluminum polymers（e.g.,Al（OH）₃）.Under the conditions of 100 g/L NaOH,the Al leaching yield reached～82wt%,engendering an78wt%loss weight of IREEs-LRR-HCa.Correspondingly,the enrichment rates of REEs and U in the residues were approximately 3.5 and 2.4,respectively.Notably,the uranium-specific activity in the Al extraction residue increased to 13.9 Bq/g.In medium systems such as NaOH-Na₃PO₄ at atmospheric pressure,the Al leaching yield improved to over 90%.The mechanism was that Al polymers could react with NaOH solution at atmospheric pressure to form soluble NaAl（OH）₄,with calcium aluminate(Ca₃Al₂（OH）₁₂)as a by-product,and phase reconstruction occurred in the NaOH-Na₃PO₄ medium,suppressing the formation of Al-containing by-product Ca₃Al₂（OH）₁₂.（2）Taking low calcium IREEs-LRR（IREEs-LRR-LCa）as the object,this study investigated the extraction of Al in a dilute NaOH medium at atmospheric pressure,considering the occurrence state of Al in the IREEs-LRR-LCa as aluminum hydroxy sulfate.Under optimal conditions,the Al leaching yield was～91%,the weight-loss yield of the IREEs-LRR-LCa was～92%,and the by-products were sodalite(Na₈Al₆Si₆O₂₄（OH）₂and Na₈Al₆Si₆O₂₄（SO₄）)and calcium silicate（Ca Si O₃）.The single nuclide specific activity of U in the residue was～1.7 Bq/g.Industrial-scale trials were conducted,achieving an Al leaching yield of～80%and a weight loss yield of～70%.Additionally,the enrichment rate of REEs in the residues was～2.9 and that of U was～2.2.（3）Addressing the higher specific activity of U（～13.9 Bq/g）in the Al extraction residue after extracting Al from IREEs-LRR-HCa using dilute NaOH at atmospheric pressure,the leaching of U in Na₂CO₃-NaCit medium at high-temperature and high-pressure was studied,based on the characteristics that the occurrence state of U was Na₂U₂O₇ precipitation and adsorption state U（VI）.Under optimal conditions,the U leaching yield was approximately 97%,and the radioactive specific activity of U in the residue reduced to 1.9 Bq/g.The leaching mechanism of U involved the dissolution and desorption of Na₂U₂O₇ and adsorbed U（VI）in Na₂CO₃-NaCit medium,which conformed to the internal diffusion control kinetic model with an activation energy of44.91 k J/mol.A serial analysis was conducted to investigate the sequential extraction of Al and U using a gradient of media,including dilute NaOH at atmospheric pressure for Al extraction and Na₂CO₃-NaCit media at high-temperature and high-pressure for U extraction.The overall leaching yields for Al and U were 97%and 94%,respectively,with a total weight reduction of～84%for the IREEs-LRR-HCa.（4）Addressing the issues of high-U-specific activityabove 1.0 Bq/g in the U-extracted residue formed from IREEs-LRR-HCa by using Na₂CO₃-NaCit media at high-temperature and high-pressure,as well as the stringent equipment and higher energy consumption caused by the elevated temperature operation,this study focused on the extraction of U from the Al extraction residue formed with dilute NaOH at atmospheric pressure（with a U the radioactive specific activity of～13.9 Bq/g）.Leveraging the complex nature of uranyl carbonate at different p H levels,the U extraction process was investigated under milder conditions by using a NaHCO₃-Na₂CO₃ mixture of carbonate media at atmospheric pressure.Under optimal conditions of 90℃and a medium of 1.0 M NaHCO₃+0.5 M Na₂CO₃,the U leaching yield reached～96%,with a U-specific activity in the residue of～0.72 Bq/g.The U leaching process was aligned with an internal diffusion controlled kinetic model and exhibited an activation energy of～44.87 k J/mol.A serial analysis was conducted to investigate the sequential extraction of Al and U using a gradient of media,including dilute NaOH for Al extraction and NaHCO₃-Na₂CO₃ for U extraction at atmospheric pressure.The overall leaching yields for Al and U were 88%and 97%,respectively,resulting in a total weight reduction of～77%for the IREEs-LRR-HCa.（5）To address the recovery of trace REEs from tailwater after REEs extraction,a study of the extraction process of ultra-low concentration REEs was performed.Using Tb（Ⅲ）,a representative of medium-heavy REEs,as a model element,the extraction process of Tb（Ⅲ）at the ppb level was thoroughly investigated using a bioadsorption approach.The synthesized modified garlic peel bio-based adsorbent was systematically characterized,and its extraction process and mechanism for ultra-low-concentration Tb（Ⅲ）were thoroughly explored.The maximum adsorption capacity was found to be84.68 mg/g at p H 5.The adsorption process adhered to the Langmuir thermodynamic model and the pseudo-second-order kinetic model.The mechanism was the exchange of Tb（Ⅲ）ion with–COOH and Ca（Ⅱ）ions.In industrial real solutions containing trace amounts of REEs at the ppb level,extraction yields of REEs exceed 99%,enabling deep extraction of trace amounts of rare earths.