Solubilities of individual light rare earth sulfates (lanthanum to europium) in water and H2SO4 solutions (neodymium sulfate)

Rare earth elements (REE) are critical commodities with high accessibility risk due to monopolistic practices of suppliers and increased demand. The REE ores generally require aggressive conditions to dissolve the mineral matrix, and sulfuric acid attack is the most economical preferred treatment route. The main challenge to processing REE resources efficiently is the lack of reliable and recent data regarding the solubility of individual REE sulphates in water as a function of temperature. Therefore, solubilities of light REE sulfates (LaEu) were measured for binary REE2(SO4)3–H2O systems from 25 to 95 °C following both heating and cooling approaches. Additionally, solubility isotherms were determined for the ternary system Nd2(SO4)3–H2SO4–H2O system at 25, 55 and 95 °C from 0.05 to 0.5 M H2SO4. It was found that the solubility of light rare earth sulfates decreases with increasing temperature. Within the light REE series, solubilities increase from La to a local maximum for Pr (by one order of magnitude), followed by a sharp decrease to Nd and Sm. The solubility of Nd2(SO4)3 was moderately improved by the presence of H2SO4, as the solubility values increase by up to 30% in 0.5 M H2SO4 compared to the no acid case. This was explained by the formation of soluble sulfate complexes due to increased concentration of SO42− ions in solution. The increase in solubility resulting from sulfuric acid addition was more significant at higher temperatures, although temperature continued to have a negative overall effect. The nature of the saturating solid phases was determined by XRD analysis on samples collected before and after phase change temperature for Ce, Nd, and Pr sulfates, while only solids collected at 25 and 95 °C were analyzed for the compounds known to form a single type of hydrate over the entire temperature range: usually REE2(SO4)3•8H2O except for La2(SO4)3•9H2O.