Abstract:

The purity of rare earth element (REE) products is strongly influenced by the ability to selectively remove impurity elements from sulphate leach liquors prior to downstream separation and recovery. Iron, thorium, and refractory metals such as titanium, zirconium, niobium, and tantalum are commonly present in REE sulphate liquors and can negatively impact solvent extraction, selective precipitation, and product quality if not effectively managed. These impurities can hydrolyse or form secondary phases that reduce extractant capacity, promote crud formation, or result in co-precipitation losses of valuable REEs.

This study investigates the use of controlled phosphate addition as a front-end purification strategy for the selective removal of iron, thorium, and refractory metal impurities from REE sulphate liquors while minimising REE losses. The approach focuses on adjusting the Fe₂O₃/P₂O₅ ratio through targeted phosphate dosing and pH control to exploit differences in aqueous speciation and solid-phase stability between impurities and REEs.

A combination of laboratory test work and thermodynamic modelling was applied to evaluate precipitation behaviour across a range of pH conditions. Experimental results demonstrate that iron can be selectively removed at relatively low pH, with thorium precipitation occurring at higher pH values, while REEs remain predominantly in solution over the same operating window. Under optimised conditions, high levels of impurity removal were achieved with limited total rare earth oxide (TREE) losses, highlighting the potential of phosphate-assisted purification as a selective impurity management step.

Thermodynamic modelling was used to support and interpret experimental observations by predicting aqueous speciation trends and relative solubility behaviour of impurity and REE phases in sulphate-phosphate systems. The modelling results qualitatively reproduced observed precipitation sequences and provided insight into the mechanisms governing selective impurity removal. In particular, the analysis illustrates how phosphate addition alters phase stability relationships, delaying REE precipitation while promoting the formation of sparingly soluble iron- and thorium-bearing phases.

The integration of experimental test work with thermodynamic modelling provides a robust framework for understanding and optimising phosphate-controlled impurity precipitation in REE hydrometallurgical flowsheets. This combined approach supports the identification of operating windows that maximise impurity rejection while preserving REE value, and offers a transferable methodology for impurity management across variable REE sulphate liquor compositions.

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Ms. Esme Lau