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The liquidus surface of the FeO-Fe 2 O 3 -SiO 2 -CaO system is of special importance in general pyrometallurgy. It determines several industrial slags, such as "fayalite" slag, calcium ferrite slag, acidic and basic steelmaking slags, as well as the newly proposed "ferrous calcium silicate slags" (FCS), which are all nominally based on this system. Nevertheless, the liquidus surface of this system has been studied only in equilibrium with metallic iron or in air, and the data at intermediate oxygen partial pressures are almost missing. In reductive smelting processes, the aforementioned slags were based on the liquidus surface of the FeO-Fe 2 O 3 -SiO 2 -CaO system in equilibrium with metallic iron. In oxidative matte smelting and in today's modern trends of nonferrous smelting, such as continuous converting or white metal production, the characterization of the liquidus surface of this system at intermediate oxygen partial pressures becomes imperative. Furthermore, the effect of minor oxides on the liquidus surface of this system has not been studied at higher oxygen potentials and confusing conclusions are found in literature even for low oxygen potentials. In this work, a quantitative description of the liquidus surface of FeO-Fe 2 O 3 -SiO 2 -CaO slag containing Al 2 O 3 , MgO, and Cu 2 O is carried out by means of a thermodynamic model at various intermediate oxygen partial pressures. Through a new type of easy-to-understand multicomponent phase diagram, it is shown that important differences exist between the liquidus surfaces of this system in reductive and oxidative conditions. It is also shown that minor components can have fundamentally different effects in reductive and oxidative conditions.

Experiments were carried out on equilibrium between calcium ferrite slag and metallic copper under a controlled partial pressure of oxygen by using a mixture of CO2 and CO. Lime, magnesia or iron crucibles were used in the temperature range from 1423 to 1623K. The results were summarized as follows:(1) The solubility of copper in the stags is affected by the oxygen potential and temperature, and the solubility amounts to 70% of that in the case of iron silicate slags.(2) The value of (Fe+++/Fe++) in slags fits the equation previously obtained in the plain FeO-FeO1.5-CaO ternary system, and this ratio in the basic calcium ferrite slags is ten times as large as that in acidic iron silicate slags.(3) The iron contents in metallic copper are related to the oxygen potential, temperature and iron content in the slag.

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