Solubility of neodymium and dysprosium sulfates at different pH and temperature and the effect of yttrium sulfate, sodium sulfate, and ammonium sulfate mixtures: Strengthening the predictive capacities of the OLI software

Abstract

This study focuses on investigating the solubilities of two rare earth element (REE) sulfate salts, Nd2(SO4)3 (representing light REEs) and Dy2(SO4)3 (representing heavy REEs), under various conditions. Four different systems are studied: 1) binary REE2(SO4)3–H2O system at natural pH and temperatures of 25, 46, 65, and 80 °C, 2) ternary REE2(SO4)3–NaOH–H2O system, covering a pH range from 7 down to neutral, at temperatures of 25, 46, 65, and 80 °C, 3) ternary REE2(SO4)3–Y2(SO4)3–H2O system, involving Y2(SO4)3 concentrations ranging from 0 to 20 g/L, pH values of 3 and 7, all at 25 °C, and 4) quaternary REE2(SO4)3–Na2SO4–(NH4)2SO4–H2O system, encompassing Na2SO4 and (NH4)2SO4 concentrations varying from 0 to 20 g/L, pH values of 3 and 7, and at 25 °C. Solubilities of Nd2(SO4)3 and Dy2(SO4)3 decrease as temperature rises, attributed to the exothermic dissolution reactions. Within pH 2 to 5, solubilities remain relatively constant, but at higher pH, they decrease due to the formation of REE2(SO4)(OH)4(H2O)2 (rare earth sulfate hydroxide hydrate). Addition of Y2(SO4)3 does not significantly affect solubilities because of the relatively low Y2(SO4)3 concentration range (below 0.03 mol/kg of water), but solubilities are lower at pH 7 due to REE2SO4OH4H2O2 formation. For Nd2(SO4)3, solubility decreases with increasing Na2SO4 and (NH4)2SO4 concentrations, especially at pH 3 due to the formation of NaNd(SO4)2(H2O) (sodium neodymium bis(sulfate) hydrate) which is also confirmed by the decrease in Na2SO4 concentration compared with the target value. However, at pH 7, the concentration of Na2SO4 is much closer to the target range. This suggests that the formation of NaNd(SO4)2(H2O) is less significant at pH 7 and most of Nd precipitates as Nd2SO4OH4H2O2. For Dy2(SO4)3, solubility increases with increasing sulfate concentrations again due to the to the formation of REESO4+ and REE(SO4)2− complexes but the solubility is lower at pH 7 due to Dy2SO4OH4H2O2 and DySO4OH formation. The outcomes of this investigation represent a notable augmentation to the existing repository of solubility data for Nd2(SO4)3 and Dy2(SO4)3. These particular systems were deliberately chosen to emulate the process of extracting rare earth elements (REEs) from ion-adsorbed clays. Nonetheless, the implications of this research extend beyond this context and hold relevance for various procedures that involve the handling of REEs in sulfate-based environments. One prominent application of these findings lies in the augmentation of thermodynamic models, notably the MSE model integrated into the OLI software. Through the incorporation of this new dataset, the OLI software is poised to become a more potent tool for forecasting and simulating the chemistry of rare earth sulfate systems. This advancement bears significant promise for forthcoming research endeavors and industrial applications where precise modeling of REE behavior is of paramount importance.