The effect of silica contents on Pd, Pt and Rh solubilities in silicate melts: an experimental study

Abstract

The effect of silica contents on Pt, Pd and Rh solubilities in CaO–MgO–Al2O3–SiO2 melts was investigated at air condition. In a pseudobinary system diopside-anorthite eutectic (DA)–silica at 1450 C, the maximum solubility of Pd (391 ppm) was observed in melts with 55 wt% SiO2. In more basic and more silicic melts Pd solubility is lower, being 338 ppm at 50 wt% SiO2 and 316 ppm at 70 wt% SiO2. In contrast, Pt and Rh solubilities in these melts systematically decrease with increasing silica, from 8.9 and 35.2 ppm, respectively, in the DA composition down to 4.0 and 21.7 ppm in melts with 70 wt% SiO2. The results on Pt solubility provide a new support to the role that PtFe alloys precipitation may play during melt evolution from basaltic to silicic compositions. In silica-free CaO-Al2O3 melts the solubility of Pt and Rh at 1550 C was found to be extremely high (230 and 319 ppm,respectively). Adding silica up to 50 wt% results in dramatic decrease in Pt and Rh solubilities (to 5.5 and 29 ppm, respectively).Our results also demonstrate that the contents of trace level impurities in experimental charges (such as W, Mo and alkalis)originating from furnace contamination from earlier experimental runs decrease by two orders of magnitude with melt SiO2 contents increasing from 0 to 70 wt%. The ratio of a contaminant concentration in two different melts from a single run is approximately inversely proportional to the ratio of the contaminant activity coefficients in these melts.