Separation of rare earth elements from a South American ionic clay lixivium by sequential precipitation

Abstract

Ionic clays, formed by natural weathering of rare earth element (REE)-containing minerals and surface adsorption of liberated ions, are a critical REE resource. Here, the sequential precipitation of impurities and REEs from the desorption of a South American ionic clay is investigated. Selective sequential precipitation profiles are constructed by stepwise addition of 1.3 M ammonium bicarbonate to the lixivium. During sequential precipitation, ammonium bicarbonate first behaves as a pH adjustor to remove co-extracted impurities as hydroxides, and then behaves as both a carbonate ion source and a pH adjustor to precipitate the REEs as carbonates. Impurity precipitation is completed at pH 6.0 and REE precipitation is completed at pH 6–7.5; however, total carbonate concentration during the impurity precipitation step must be kept low to prevent REE loss. To gain a better understanding of the REE precipitation step, thermodynamic calculations using the Davis model are performed. Dysprosium is studied as a model REE and its speciation diagram as a function of pH and total carbonate and sulfate ion concentrations is constructed. It is found that the primary factors controlling REE precipitation are pH, carbonate, and sulfate concentrations. The knowledge gained in this work provides a deeper understanding of the sequential impurity and REE precipitation processes from the desorption lixivium of a unique ionic clay from South America. The results would help guide future work on REE separation from this distinct ionic clay.