THE SUPRIZING SOLUBILITY OF PLATINUM IN SURFACE GEOCHEMICAL ENVIRONMENTS
Conventional wisdom presumes that platinum occurrences within the geochemical environment are essentially inert; recent peer reviewed laboratory studies clearly demonstrate that this viewpoint is both misleading and incorrect! Possibly the best described examples of anomalous Pt occurrences are heterogeneous metallic nuggets, largely of Pt composition, located in geologic areas where outcrop weathering and possibly transportation cannot account for their presence and size. Typically, in these situations only very fine-grained or microscopic to sub-microscopic Pt sources can be found in the areas within or outlying from the nugget deposit. Concentric Pt-bearing growth/accretion patterns conforming to individual nuggets found within the accumulation are frequently observed. If placer or alluvial Pt nuggets are formed by continual accretion then solubilized Pt must have been carried by groundwater at a pH of between 4 and 10; more likely between pH 5.5 to 9. Long term, static laboratory studies M. Azaroual et al have clearly demonstrated that the most likely transporting complexes are hydroxalated species of platinum (PtOH+) rather than the conventionally suggested organic, chloride, or sulfate ligand compounds. These same static studies demonstrated hydroxalated Pt solubilities ranging from 55 to 650 ppm Pt over a period ranging from 210 to 567 days. With a sufficient geochemical background source of Pt and the presence of moving groundwater, there is potentially an abundance of solubilized Pt available for mobilization, deposition, and Pt nugget growth. Another interesting example of the dissolution of Pt in weakly acidic or basic conditions is the production-scale nucleation of non-bulk metal size clusters of Pt bonded to monomers or oligomers of amorphous colloidal silica (a.c.s.). In this instance the electrolyte consists essentially of water at a pH of approximately 7.5. In order to nucleate the system, Pt is solubilized as hydroxalated compleses. The picture below shows an Fe grinding media that has been coated with Pt metal originating from the precursor Pt-bearing a.c.s. It really doesn’t require aqua regia to dissolve Pt!
Despite the misconception that Pt is a rather inert metal, Pt in both the geochemical and hydrometallurgical environments is capable of being dissolved and deposited in rather mild chemical conditions that are typical of the earth’s geochemical environment. A.C.J.
