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.