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 P2Mo5O236– with the remaining
∼40% as β-Mo8O264–. Under the same conditions, W(VI) is largely present as NaPW11O396– (∼80%) and P2W5O236– (∼10%),
with the remainder probably occurring as isopolytungstates such as
W12O4212– and some tungstophosphate
dimers such as P2W18O626–. 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, PW12O403–. 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.