Solubilities of heavy rare earth sulfates in water (gadolinium to lutetium) and H2SO4 solutions (dysprosium)

Solubilities of rare earth element (REE) sulfates are critical in hydrometallurgical process development for REE extraction, but measurements are notably lacking for heavy REE sulfates. Therefore, solubilities of heavy REE sulfates are measured for binary REE2(SO4)3–H2O systems from 25 to 95 °C for gadolinium through lutetium. Solubility isotherms are also determined for the ternary Dy2(SO4)3–H2SO4–H2O system at 25–95 °C and 0.05–0.5 M H2SO4. Saturating solid phases are identified by X-ray diffraction and thermogravimetric analyses. For all REEs, the octahydrates, REE2(SO4)3•8H2O, are the equilibrium saturating solid phases at 25–95 °C. Solubilities decrease strongly with increasing temperature, indicative of exothermic heats of solution for REE sulfate octahydrates. Within the REE series, solubilities increase markedly (an order of magnitude) from gadolinium through lutetium sulfate. Changes in solubilities are juxtaposed with stability constants for REE sulfate complexes, REESO4+ and REE(SO4)2−. As these species are inner-sphere complexes, water of hydration is released from REE3+ during complex formation which increases water activity. Hence, heavy REEs with larger complex stability constants also display lower solubilities, and vice versa. In sulfuric acid solutions, dysprosium sulfate solubility increases and passes through a maximum with increasing sulfuric acid concentration, as with other REE sulfates. The maximum solubility is postulated to relate to the dissociation constant of bisulfate (HSO4−), which passes through a minimum with increasing sulfuric acid concentration. The formation of bisulfate consumes some free sulfate thereby increasing REE sulfate solubility. •Solubilities of heavy REE sulfates decrease strongly with increasing temperature.•Saturating solid phases at 25–95 °C are the octahydrates, REE2(SO4)3•8H2O.•Solubilities increase markedly and monotonically with atomic number from Gd to Lu.•Solubility of Dy2(SO4)3 passes through a maximum with sulfuric acid addition.•Solubilities were interpreted with respect to REE complexation in solution.