Spectroscopic Study of the Behavior of Mo(VI) and W(VI) Polyanions in Sulfuric–Phosphoric Acid Mixtures

The solution chemistry of Mo­(VI) and W­(VI) in mixtures of sulfuric and phosphoric acids is relevant to the development of practicable hydrometallurgical processes for the recovery and separation of these two elements from low-grade scheelite ores. The behavior of Mo­(VI) and W­(VI) in such mixtures has been studied using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), nuclear magnetic resonance (NMR), and small-angle X-ray scattering (SAXS) spectroscopies, along with electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS). Where applicable, these techniques have produced a self-consistent picture of the similarities and differences between the chemical speciation of Mo­(VI) and W­(VI) as functions of solution composition, mostly at a constant phosphorous/metal (P/M; M = Mo­(VI) or W­(VI)) ratio of ∼1. In dilute acidic media (0.02 mol·kg^–1 H⁺, without H₂SO₄), Mo­(VI) exists mostly (∼60%) as P₂Mo₅O₂₃^6– with the remaining ∼40% as β-Mo₈O₂₆^4–. Under the same conditions, W­(VI) is largely present as NaPW₁₁O₃₉^6– (∼80%) and P₂W₅O₂₃^6– (∼10%), with the remainder probably occurring as isopolytungstates such as W₁₂O₄₂^12– and some tungstophosphate dimers such as P₂W₁₈O₆₂^6–. At higher acid concentrations (≲5 mol·kg^–1 H₂SO₄), polymeric Mo­(VI) anions are broken down to form the oxocations MoO₂²⁺ and Mo₂O₅²⁺ and their protonated forms, with the dimers becoming increasingly dominant at higher acidities (∼80% in 5 mol·kg^–1 H₂SO₄). In stark contrast, W­(VI) polyanions do not decompose at higher acidities but instead form (∼70% in 0.6 mol·kg^–1 H₂SO₄) a Keggin ion, PW₁₂O₄₀^3–. Further acidification with H₂SO₄ results in the agglomeration of this Keggin ion, forming clusters of about 50 and 100 Å in diameter that ultimately produce crystalline precipitates, which could be identified in part by their X-ray diffraction patterns. Possible application of these findings to the hydrometallurgical separation of Mo and W using acidic solutions is briefly discussed, based on a limited number of batch solvent extractions.