The selective recovery of rare earth from radioactive waste residue using improved oxamic acid for environmental and resource concerns
Graphical Abstract
Introduction
RE elements (REEs) are widely used in many high-tech fields, efficient separation and purification is an important prerequisite for the application of RE [1], [2], [3]. The properties of high-tech materials may be significantly improved by the addition of high-purity RE [4], [5], [6], [7], [8], [9]. Th is a common associated element in RE minerals. The RE industry produces millions of tons of radioactive waste. With the passage of time, waste residue accumulates rapidly. If large amount of waste residue produced by RE industry is not handled properly, it may cause roughly serious environmental pollution, especially the water pollution caused by the migration of radioactive element [10]. Dissolved slag in RE production is an important radioactive issue. In addition, the RE in the waste residue is an important resource, the content of RE may be one to two orders of magnitude higher than that of RE minerals. If RE and Th in waste residue can be recycled, it do not only improve economic benefits, but also reduce the pressure of environmental protection. The common strategy is to remove Th first and then deal with other impurities. Then, the RE was enriched and prepared for further separation. Removing Th is an important issue to ensure product equality. As for the process in Th removal, it is difficult for traditional extractant to strip Th [11]. If the separation is not effective, Th may be remained in RE products. The total cost of recovering and purifying RE can be greatly reduced by effectively removing Th before the separation of individual RE [12], [13], [14]. Resin separation can be used to produce high-purity RE products and selectively remove impurities. However, compared with solvent extraction, its application in industrial production is limited by the lower yield and the longer period [13], [15], [16], [17].
In the field of solvent extraction, many extractants have been studied for Th removal. Under mild conditions, the extracting and stripping Th with less cost and pollution is a problem. It is difficult for an extractant to achieve efficient extraction and stripping at the same time. Tributyl phosphate (TBP) is a common extractant, which can be used for the separation of Li, Th and RE [18], [19], [20], [21]. In the process of Th separation, TBP needs to extract Th under higher acidity and use special stripping agent[9]. Di-(2-ethylhexyl)phosphoric acid (P204) and 2-ethylhexylphosphoric acid mono-2-ethylhexyl ester (P507) are two of the most widely used extractants in hydrometallurgy [22], [23], [24], [25]. Because of the good selectivity, they are widely used in many metal separation industries [26], [27], [28]. In order to reduce the waste water generated in the saponification process [29], [30], the non-saponified P204/P507 based extraction system was developed. This system is effective to separate RE and Th, but it is a big challenge to strip Th from the organic phase. The high concentration acid above 6 mol/L needs to be added in the stripping process [31], [32]. The primary amine N1923((CnH2n+1)2CHNH2 n = 9–11) was developed by Shanghai Institute of organic chemistry, Chinese Academy of Sciences. Some studies aimed to selectively separate Th using N1923 in sulfuric acid or nitric acid media[33]. However, its poor performance in hydrochloric acid medium limits its industrial application. Introducing N-atoms into organophosphonic extractant and redistributing electrons in molecules, several extractants were designed to separate Ce4+ and Th4+ [34], [35].
If the stripping acidity can be lower, both acid and alkali consumption will be reduced. It can also reduce the generation of waste water and inorganic salt in the separation process. In order to reduce the acidity of stripping, oxamic acid has become a potential extractant. Oxamic acid was reported in the separation of light REEs with toluene as the diluent [36]. The introduction of nitrogen atom and the addition of carbonyl group help to improve the extraction performance of oxamic acid, so as to realize the effective separation of RE and Th. However, nitrogen-containing oxamic acid with few carbon atoms is difficult to be dissolved in conventional solvents. There is a problem of poor oil solubility in the extraction and purification of RE. The loss of polar solvent used as oil phase after extraction is serious. In order to improve oil solubility, increase extraction capacity of oxamic acid and reduce the loss of organic phase, a novel extractant 2-(didecylamino)-2-oxoacetic acid (DDOA) was developed and used to separate Th and REEs from the RE waste residue leaching solution in this article.
Section snippets
Reagents
Ethyl chloroglyoxalate and dichloromethane were provided by Adamas Reagent, Ltd. Triethylamine was purchased from Sinopharm Chemical Co., Reagents. Ltd. Didecylamine was obtained from TCI (Shanghai) Development Co., Ltd. 260# kerosene was purchased from Shanghai Laiyashi Chemical Co., Ltd., China. The purities of all reactants are higher than 97%. NaOH was provided by Admas Ragent. Co., Ltd (98%). ThCl4 solution was prepared from Th(NO3)4 solution, which was precipitated with amine hydroxide,
Concentration effect of DDOA
Compared with the reported oxamic acid extractant, the novel extractant DDOA has been improved in the alkyl chain length, so that it can be completely dissolved in the common diluent such as kerosene. The loss of extractant in aqueous phase is also reduced and the extraction cycle performance is improved. With kerosene as a diluent, the extraction of Th by DDOA is better than DBOA [37] and CA-12 [42] in the prescience of RE (Fig. S5). The separation of RE and Th by DDOA with different
Conclusion
A novel extractant DDOA with better oil solubility and separation performance is synthesized to deal with the radioactive slag. IR-spectra indicates that the loaded DDOA shows clear splitting and the interaction of DDOA-Th is tighter, which may be used to clarify why Th is more difficult to be stripped than REEs. Kinetic studies show that Th4+ can be extracted more quickly and effectively, so that it can be effectively separated from REEs. Without saponification, the loading capacity of DDOA to
CRediT authorship contribution statement
Chao Bie and Xiaoqi Sun designed the research, analyzed the data, and wrote the paper; Chao Bie performed the research; Shan Wu, Hepeng Zhang, Shuainan Ni, Yun Gao and Xiaoqi Sun participated in data collection and discussion.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by National Key Research and Development Program of China (2017YFE0106900), Key Research and Development Program of Jiangxi Province (S2020ZPYFG0029), Key Program of the Chinese Academy of Science (ZDRW-CN-2021-3-1-13), Fujian Program for High-Level Entrepreneurial and Innovative Talents Introduction and Science.
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