Selective Stripping of Light Rare Earth Elements using Acidic Organophosphorus Reagents

Abstract

Light rare earth elements (LREEs) are essential resources widely used in modern industries. Due to their similar chemical and physical properties, a design of the streamlined separation process is important for operational efficiency and cost-effectiveness. Although solvent extraction is the most common separation method for LREEs, many stages and a high concentration of acid are needed. Therefore, in this study, a novel separation process for LREEs is developed using selective stripping. Prior to applying selective stripping to the four-LREE- component system, the optimization for stripping was carried out in the two-adjacent-LREE system. The effects of extractant type and concentration, REE concentration ratio in the feed solution, organic-phase-to-aqueous-phase (OA) volumetric ratio and the concentration of acid on the stripping behavior of two LREEs were investigated, and the optimum conditions of these factors were determined in the two-LREE- component system. 0.5M PC88A was selected as a suitable extractant, and three stages of stripping at an OA ratio of 7:4 with 0.10M HCl was needed for the selective stripping of La and Ce. In the Ce and Pr system, it was found that selective stripping could be achieved in six stages of stripping at an OA ratio of 5:3 with 0.14M HCl, while seven stages of stripping at an OA ratio of 3:2 with 0.15M HCl was required for the separation of Pr and Nd. Furthermore, the slope analysis method was adopted to determine the LREE-extractant complex structure and stoichiometry of stripping reaction. The results show that LREEs make a complex with PC88A extractant including chloride ion, and more chloride ions tend to be included in the lighter LREE-extractant complex in the organic phase. Optimization experiments with the same factors were carried out in the four-LREE-component system and compared with the results in the two-component system. In addition, a McCabe-Thiele diagram was drawn for the determination of the number of stages and OA ratio. Based on the optimization results and McCabe-Thiele diagram, a semi-continuous counter-current process using selective stripping was introduced in the four-LREE system. The process consists of three steps: four extraction stages to separate La in the aqueous phase, three stages of the first stripping to strip the extracted La to the aqueous phase, and seven stages of the second stripping to strip Ce and Pr to the aqueous phase while Nd remains in the organic phase. After this process, a 89.8% purity of La was obtained in the aqueous phase and a 90.3% purity of Nd could be achieved in the organic phase. In this research, the optimization of stripping conditions in the LREE system was studied and a novel semi-continuous counter-current process using selective stripping was designed. The experimental results suggest that LREEs can be selectively separated in the stripping step, as well as in the extraction step. Because this is the first study about the selective stripping of four LREEs, it has a significance in verifying the possibility of selective stripping of LREEs and designing the new process using selective stripping.