The sulphide capacity and the solubility limit of CaS in CaO-Al 2 O 3 and CaO-Al 2 O 3 -CaF 2 slags have been studied at the temperatures from 1 823 to 1 923 K by the equilibrium measurement of gas-slag reaction. The sulphide capacity and the solubility of CaS in these two slag systems increase with increasing CaO content and decreasing ratio of N Al 2 O 3 /N CaF 2 . The sulphur in CaO-Al 2 O 3 -CaF 2 slag negatively deviated from Henry's law in the high concentration range of sulphur in slag, and the activity coefficient of sulphur in slag varies with sulphur content, slag composition and temperature. It was confirmed that the sulphide capacity was unable to be applied as a real measure of the slag basicity theoretically.

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