Investigating the Phase Diagram of SiO<sub>2</sub>–CaO–CrO<i><sub>x</sub></i> System to Evaluate Distribution of Platinum between Slag and Molten Copper

Abstract

In the hydrometallurgical process used for recycling platinum group metals (PGMs), a residue containing mainly Cr2O3 and a small amount of PGMs is generated. In the present work, a pyrometallurgical process was applied in which PGMs from the residue generated in the hydrometallurgical process are concentrated in a molten copper phase as a collector-metal, and Cr2O3 is separated into a slag phase with SiO2 and CaO as a flux. To reduce the loss of PGMs to the slag, it is necessary to have a homogenous liquid and a lower dissolution of PGMs into the slag. Therefore, the phase diagram of the SiO2–CaO–CrOx system was investigated at 1773 and 1873 K in an oxygen partial pressure pO2 = 10-10 to obtain knowledge of the homogeneous melts. Based on the determined phase diagram, the distribution ratios of Pt as representative PGMs between the liquid SiO2–CaO–Al2O3–CrOx or the liquid SiO2–CaO–CrOx slag and molten copper were measured at 1773 K in pO2 = 10-10. The experimental results showed that CrOx solubility in the slag increased with decreasing slag basicity, B defined as B = (mass%CaO)/(mass%SiO2). Furthermore, the results showed that the distribution of Pt in the slag increased with increasing CrOx concentration in the slag.