Evaluation of Various Separation Techniques for the Removal of Actinides from A Rare Earth-Containing Solution Generated from Coarse Coal Refuse

Abstract

Rare earth elements have emerged as a vital commodity due to their significance in the production of high-tech devices as well as their utilization in defense and military systems. They are deemed critical due to potential scarcity, supply constraints, and the lack of minable concentrations. Therefore, alternative sources are needed to meet the demand and continue manufacturing rare earth-required products. Coal and coal by-products, in that regard, have presented a significant opportunity and proven to contain a considerable amount of rare earths. However, toxic elements such as thorium and uranium are frequently seen in the same mineralization as rare earths. The concentration of these hazardous trace elements is elevated due to the extraction and beneficiation processes. Unless proper separation and disposal are performed, these radionuclides accumulate on the soil’s surface or integrate with aquatic systems that raise environmental and health concerns. Consequently, there exists an urgent need to remove these radionuclides to produce high purity rare earths and diversify its supply chain as well as maintain an environmentally favorable extraction process for the surroundings. In this study, a process flowsheet was targeted to be developed for the removal of thorium and uranium from rare earth elements when coarse coal refuse was utilized as a non-traditional feedstock for rare earth production. The separation performance between these hazardous elements and rare earths was systematically evaluated. Various separation techniques, namely selective precipitation, solvent extraction, and adsorption, were applied to extract rare earth elements while minimizing the non-selective recovery of thorium and uranium into the product stream. While investigating the potential application of different separation methods, several operating parameters were tested, and the experimental test results were analyzed from a statistical and fundamental perspective to provide an in-depth understanding of each separation mechanism. Selective precipitation test results indicate that it was effective for removing thorium, while exploratory solvent extraction tests preferentially removed uranium from rare earths. Based on the findings of the initial testing, an experimental protocol consisting of both selective precipitation and solvent extraction was developed and implemented. Around pH 4.85, nearly all thorium was precipitated out with approximately 19 wt% of rare earths and 47 wt% uranium co-precipitation. The optimum separation performance between uranium and rare earth elements was achieved with double-stage solvent extraction under the following conditions: 50 v% tri-butyl phosphate, feed pH at 3.5, organic to aqueous phase ratio at 3, and DI water as the strippant, which corresponded to