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

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 an overall rare earth and uranium recovery of 80 wt% and 3.1 wt%, respectively. The selective precipitation and solvent extraction test results were assessed from a fundamental point of view with respect to metal species distribution, chemical complexation reactions, and thermodynamic calculations, which all aligned well with the experimental data. The precipitation reactions were all spontaneous and exothermic for the elements studied based on calculated Gibbs energy and enthalpy values. The extraction behavior of uranium and rare earths was further studied with the distribution ratio and separation factors. Statistically, significant models were developed for uranium recovery prediction in solvent extraction, which indicates the extractant concentration, solution pH, and organic to aqueous phase ratio all played a critical role. On the contrary, the concentration of the stripping agent was identified as an insignificant parameter over the tested concentration range. As an alternative to selective precipitation and solvent extraction, an experimental design was generated to examine the potential application of zeolite adsorption on the removal of thorium and uranium. However, good selectivity was not observed, especially between uranium and rare earth elements. The kinetic adsorption studies suggest a similar pattern for rare earth elements and uranium, which creates a separation challenge. In the case of thorium adsorption, a fast adsorption behavior was observed. Freundlich adsorption isotherm was determined to be the best-fit model, and the adsorption mechanism of rare earths and thorium was identified as multilayer physisorption. Also, the kinetic adsorption data fit to the pseudo-second-order rate reaction for thorium and rare earth elements. The best separation performance was achieved using 2.5 grams of 12-µm zeolite sample at a pH value of 3 with a contact time of 2 hours. Under these conditions, the adsorption recovery of rare earths, thorium, and uranium into the solid phase was found to be 20.4 wt%, 99.2 wt%, and 89.6 wt%, respectively. As a result of comprehensive experimental and fundamental studies, a conceptual process flowsheet was developed using the metallurgical simulation software, METSIM. The developed flowsheet consists of five main unit operations. After dissolution with acid (1), it is intended to apply one-stage selective precipitation (2) for thorium removal followed by solvent extraction (3) for uranium separation, where two-stage extraction and two-stage stripping are performed. Later the remaining rare earth-containing aqueous stream is treated with oxalic acid to achieve rare earth oxalate precipitation (4). Finally, roasting (5) is applied to convert rare earth oxalate to rare earth oxide products. Lastly, the developed experimental protocol was validated using a real system rare earth oxalate sample produced at the pilot-scale processing facility