Vanadium in phyllosilicate ores: Occurrence, crystal chemistry, and leaching behavior

This article examines the nanoscale occurrence patterns of vanadium in phyllosilicates and correlates them with amenability to leaching. A large fraction of the global vanadium resource is hosted in phyllosilicate ores such as roscoelite and V-bearing chlorites, clays, and micas. Recovery of V from these minerals in acid leaching is typically low but varies widely for unclear reasons. Transmission electron microscopy (TEM) of V-illites, V-chlorites, and roscoelites in leach heads and tails from the La Sal mine (Colorado Plateau) shows that the observed variability in recoveries correlates with intercalations of nanoscale V oxide phases within the phyllosilicate lattice. These are present in some of the V-illites in this study, which displayed higher leach recovery than V-illites without the V oxides. In the latter, as well as in V-chlorites, the V occurs as crystallographic substitutions without oxide intercalations. Electron energy loss spectroscopy (EELS) measurements in the scanning transmission electron microscope (STEM) show that this crystallographic V is mixed-valence, averaging 3.3 in V-chlorite, 3.6 in roscoelite, and 3.8 in V-illite. Unlike the oxide intercalations, this crystallographic type of V corresponds to negligible leaching recoveries irrespective of V siting, oxidation state, or type of phyllosilicate. These results shed light on the causes of a central problem in geometallurgy, which is attempting quantitative predictions of process outcomes from geological inputs. These results show that what appears to be the same mineral (in this case V-illite) can yield wildly disparate leach recoveries depending on nanoscale metal occurrence factors. Similar under-recognized variation in other ore minerals, within or between sites, may explain much of the notorious variability in their leaching behavior.