ACTIVITY OF CaO IN LIQUID CaO-SiO_2 AND CaO-Al_2O_3-SiO_2 SLAGS

A new method, based on the carbothermic reduction of lime, has been devised for the direct determination of the activity of lime in liquid blast-furnace type slags. The success of the method depends upon the proper choice of the solvent metal to lower the chemical potential of calcium in the melt. Owing to the expected very small activity coefficient of calcium in dilute solutions in tin, the reaction CaO (in slag)+C=Ca (in tin)+CO has been studied with a view to evaluating activities of lime in liquid CaO-SiO_2 and CaO-Al_2O_3-SiO_2 slags. The activity values in the CaO-SiO_2 system have been used to derive silica activities through the Gibbs-Duhem relationship. The shape of the activity curves is briefly discussed on the basis of a statistical mechanical model. It has been found from the results on the ternary system that the activity of lime is increased by the presence of 10% alumina in the slags for the entire basicity range under investigation. An explanation based on the ionic constitution of slags is offered for this behaviour of alumina.