A review of thermodynamic data for lanthanum, iron, and thorium applied to rare earth extraction

A consolidated and revised database for thermodynamic simulation of lanthanum (representing the rare earth elements), iron, and thorium in water, sulfate, and phosphate systems was built. Thermodynamic parameters (∆Gf°, ∆Hf°, and Sf°) collected from the original sources were compared with those reported in thermochemical software databases. The inconsistencies found in the latter were mainly ascribed to typographical errors, incorrect unit conversions, and errors generated by combining data from different sources. The Eh-pH diagrams illustrate the significant effects of data discrepancies on the stability regions of aqueous and solid species. The results obtained with the revised database helped the identification of operating windows for the selective precipitation of common impurities (e.g., Fe and Th) in rare earth extraction. In the presence of sulfate, the complex LaSO4+(aq) is the only stable species up to pH = 8 and then La(OH)3(s) is formed. In phosphate solutions, lanthanum salts precipitate at lower pH. The precipitation pH varies with the nature and crystal features of the different lanthanum phosphate phases. Under oxidizing conditions, wide operational windows favor the selective precipitation of Fe2O3(s) or FePO4.2H2O(s) at pH 1–5. Thorium hydroxide is formed at pH 4. The inclusion of phosphate complexes in the simulation expands the thorium solubility region from pH 4 to 6.5 (approximately), depending on phosphate concentration.