Are scandium sulfate complexes effective in mobilizing scandium?

Abstract

Sulfate has been increasingly acknowledged as a key ligand for the mobilization and enrichment of rare earth elements (REEs). Here, we report the results of an investigation of the solubility of Sc₂O₃(s), scandium speciation, and the coordination geometry of scandium species in sulfate-bearing solutions using solubility experiments and ab initio molecular dynamics (AIMD) simulations. The investigation was conducted for temperatures of 175 to 250 °C at vapor-saturated water pressure. From the results of our experiments, we conclude that Sc(SO₄)₂⁻ is the dominant scandium species, and that its formation constant (log β₁) varies from 11.20 ± 0.08 at 175 °C to 14.21 ± 0.12 at 250 °C. The AIMD simulations show that the Sc(SO₄)₂⁻ complex is either doubly monodentate or has a mixed monodentate-bidentate configuration, and is coordinated with four water molecules. The species ScSO₄ + was also identified in our experiments, but has a relatively low formation constant (log β₂) varying from 7.24 ± 0.46 at 175 °C to 9.51 ± 3.50 at 250 °C. Modeling of the transport and deposition of scandium provides convincing evidence that sulfate scandium complexes can transport scandium efficiently in acidic fluids (pH below 4). Our simulations emphasize the critical roles played by fluid-rock interaction and fluid–fluid mixing in the genesis of scandium ores. This study presents the key thermodynamic data needed to evaluate scandium mobilization in sulfate-rich hydrothermal systems.