Study on the efficient preparation of low radioactivity ultra-high purity lanthanum oxide (≥6 N) by a single chromatographic column using a phosphonic acid functional resin

Abstract

Ultra-high purity La₂O₃ (≥6 N, 99.9999%) with extremely low impurity elements is widely used in advanced optical and electrical fields and usually requires a complex purification process. In this study, a phosphonic acid functional resin (HEHEHP + TBP)/SiO₂–P is prepared for the efficient removal of key impurity elements such as rare earth impurities, Fe, Th and U from La in a nitric acid medium. Batch experiments showed that in HNO₃ solutions of pH ≥ 2, the removal efficiency of impurity ions by this resin was greater than 97%. The optimal separation factors (SF_Ce/La, SF_Pr/La, and SF_Nd/La) between light rare earths reached 7.55, 19.5, and 19.2, respectively. Advanced characterization confirmed the ion exchange and coordination mechanisms, involving the participation of the P–OH and PO groups. DFT calculations corroborated the higher affinity of the resin for U and heavy rare earth over light rare earth impurities. A process for the preparation of ultra-high purity La₂O₃ was proposed and verified by a continuous single column, achieving a purity increase from 99.995% to over 99.9999% with a purification ability of 180 mg La per g of resin and Th and U less than 0.05 ppm. It is worth noting that when the feed solution is only processed through a column filled with (HEHEHP + TBP)/SiO₂–P (during the non-desorption step), a purified La(III) solution can be directly obtained, effectively adsorbing trace impurities. This process does not require the introduction of desorption agents, which reduces the economic cost. Compared with the multi-stage sequential purification method, it improves the production efficiency.