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

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 ${\mathrm{REE}}_2\left(S{O}_4\right){\left(\mathrm{OH}\right)}_4{\left(H_{2}O\right)}_2$ 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 ${\mathrm{Nd}}_2\left(S{O}_4\right){\left(\mathrm{OH}\right)}_4{\left(H_{2}O\right)}_2$. 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 ${\mathrm{Dy}}_2\left(S{O}_4\right){\left(\mathrm{OH}\right)}_4{\left(H_{2}O\right)}_2$ and $\mathrm{Dy}\left(S{O}_4\right)\left(\mathrm{OH}\right)$ 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.