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–CrO_x system was investigated at 1773 and 1873 K in an oxygen partial pressure p_O2 = 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–CrO_x or the liquid SiO₂–CaO–CrO_x slag and molten copper were measured at 1773 K in p_O2 = 10^-10. The experimental results showed that CrO_x 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 CrO_x concentration in the slag.