Solubility of Platinum and Rhodium in Lime-Alumina-Silica Melts at 1700 K

Three oxide melts of composition 10‐20‐70, 30‐13‐57, and 40‐20‐40 (wt%) in the ternary system CaO–Al2O3–SiO2 were equilibrated at 1700 K in platinum or platinum–rhodium boats, under oxygen partial pressures (PO2) ranging from 10−13 to 1 atm, and fixed by various gaseous mixtures (H2/H2O, CO/CO2, and Ar/O2). The melts were analyzed for platinum and rhodium content after quenching. The complex dependence of solubility of these elements with PO2 was explained by their speciation. Chronopotentiometric measurements confirmed the passivation of the metallic boats. For increased PO2, this speciation followed the series Pt, PtO, PtO2 or RhO for the solid phases covering the metallic boats and Pt0, Pt2+, Pt4+, Pt6+ or Rh0, Rh2+, Rh3+, Rh4+ in solution in the oxide melt. The predominance diagrams were determined for the three compositions in the ternary system.