A miscibility gap in the CaO–SiO₂–Cu₂O–Fe₃O₄ system is clarified by equilibrium experiments. the phase diagram of the miscibility gap, the activity coefficient of CuO_0.5 and the distribution ratios of minor element are presented. The experiments are performed in magnesia crucibles at 1573 K and the oxygen potentials are measured by oxygen sensors. Three layers of molten phases that are calcium silicate, cuprous oxide and metallic copper are observed clearly. The formation of a stable calcium silicate melt that is a neutralized product by acid-base reaction causes immiscibility with cuprous oxide.
The activity coefficient of CuO_0.5 that is related with copper solubility in slag is represented by (γ_{CuO0.5⁽¹⁾})=77.66×(mass% Cu)^−0.980. The activity coefficient is a terminal value of oxidized condition for the CaO–SiO₂–Cu₂O–FeO_x system saturated with metallic copper.
For each experiment, small amounts of impurities such as As, Sb, Bi, Ni, and Ag are added to the sample to determine the distribution ratios of these minor elements. The calcium silicate melt is effective in eliminating arsenic and antimony from copper because we can achieve an oxidizing condition with keeping high slag basicity. Iron, nickel and antimony are distributed almost equally to both melts of calcium silicate and cuprous oxide. On the other hand, arsenic is concentrated in calcium silicate and silver and bismuth are concentrated in cuprous oxide rather than in calcium silicate.