Application of temperature regulation of complex stability constants in removal of key impurities from lanthanum oxide

Abstract

In the ion-exchange process for separating and purifying rare earth elements, the separation is primarily achieved through the differences in the complexation reactions between the rare earth elements and the eluent. Therefore, enhancing these reaction differences is crucial to improving separation efficiency. In this study, the dissociation constants of ethylene diamine tetraacetic acid (EDTA) at different temperatures were determined by pH potentiometric titration. The speciation evolution of metal-EDTA complexes was elucidated through simulations conducted with Visual MINTEQ, enabling a systematic investigation into the temperature dependence of stability constants for EDTA complexes with La³⁺ and representative impurity metal ions (Ce³⁺, Mg²⁺, Al³⁺, Ca²⁺, Cu²⁺, Zn²⁺). Building upon this, static desorption studies and ion-exchange experiments were conducted to validate the removal of key impurities in lanthanum oxide. Experimental results show that, Mg²⁺ and Ca²⁺ exhibit better selective separation from La³⁺ due to the significant differences in complexation constants. The difference in complexation stabilization constants between La³⁺ and impurity ions reaches the maximum at 30 °C. The temperature primarily enhances the reaction difference in the separation system by modulating the distribution of complex species. Desorption studies indicate that increasing the temperature improves the separation coefficient between La and impurity elements. After optimization, the ion-exchange method achieves a La recovery rate of 84.91 wt% and a total impurity removal rate exceeding 92.50 wt%. Under the optimized elution conditions, an elution process was applied to the real feed solution, yielding a lanthanum oxide product with a relative purity of 99.99991 wt%. This work deepens the understanding of temperature-induced regulation of complexation stability constants and provides a novel approach for ion-exchange purification of rare earth elements.