Aqueous stability of thorium and rare earth metals in monazite hydrometallurgy: Eh-pH diagrams for the systems Th-, Ce-, La-, Nd- (PO ₄)-(SO ₄)-H ₂O at 25 °c

Potential vs. pH or Eh-pH diagrams for the systems Th-, La-, Nd-, Ce-, (PO ₄)-(SO ₄)-H ₂O at 25 °C were generated using the HSC5.0 software. The thermodynamic data available in the HSC software were supplemented with additional data for the rare earth metal phosphates. The diagrams for the M-H ₂O systems show that rare earth hydroxides, such as Nd(OH) ₃, La(OH) ₃, and Ce(OH) ₃ form in alkaline solutions (i.e. pH > 7.5); in contrast ThO ₂ is stable down to relatively low pH (pH ≤ 1). For the Th-, Nd-, Ce-, and La-PO ₄-H ₂O systems, the size of the stability region of a given REPO ₄ solid is highly dependent on the degree of crystallinity of the material, as reflected in the value of the corresponding solubility product. The more crystalline the RE phosphate, the larger the stability field. Addition of sulfate ions to the M-PO ₄-H ₂O systems introduces soluble metal-sulfate complexes and insoluble sulfates in the relatively acidic regions of the Eh-pH diagrams. The trends revealed by the diagrams are related to monazite hydrometallurgy.