Formation of CaSO ₄ in the CaO-FeO _x-Cu ₂O slags

Continuous converting of copper matte based on calcium ferrite slag of the CaO-FeO _x-Cu ₂O system is an attractive technology for copper smelting This slag containing approximately 20 mass% CaO can prevent the excess magnetite precipitation, but sometimes tends to separate CaSO ₄ under a high oxygen potential required to produce copper metal. To clarify the mechanism of gypsum formation, the precipitation of CaSO ₄ in the CaO-FeO _x-Cu ₂O slag system equilibrating with blister copper has been determined at 1473, 1523 and 1573 K under the SO ₂ partial pressures of 0.2, 0.4, 0.6 and 1 atm. It was found that the formation of CaSO ₄ in the CaO-FeO _x-Cu ₂O slags is enhanced with decreasing temperature, increasing SO ₂ partial pressure or CaO content and also increasing oxidation degree of the blister copper. The behavior of CaSO ₄ formation in the slag is also discussed thermodynamically based on the free energy calculations combining with the activity data derived from Prof. Takeda's data on the CaO-FeO _x-Cu ₂O ferrite slag diagram. Both of experimental and thermodynamical results show good agreements in general. It is found that the decreasing temperature, increasing CaO content or SO ₂ pressure, and extensive oxidation of the blister copper tend to separate CaSO ₄.