Equilibrium studies between CaO-SiO_2-10 percent MaO-Al_2O_3-TiO_(1.5)-TiO_2 slags, carbon saturated iron and a carbon monoxide atmosphere were performed at 1773 K to determine the activities of TiO_(1.5) and TiO_2 in slags. These thermodynamic parameters are required to predict the formation of titanium carbonitride in the blast furnace. In order to calculate the activity of titanium oxide the activity coefficient of titanium in carbon saturated iron-carbon-titanium alloys was determined by measuring the solubility of titanium in carbon saturated iron in equilibrium with titanium carbide. The solubility and the activity coefficient of titanium obtained was 1.3 wt.- percent and 0.023 relative to 1 wt.- percent titanium in liquid iron or 0.0013 relative to pure solid titanium at 1773 K, respectively. Over the concentration range studied the effect of the TiO_x content on its activity coefficient is small. In the slag system studied containing 35-50 percent CaO, 25-45 percent SiO_2, 7-22 percent Al_2O_3 and 10 percent MgO, the activity coefficients of TiO_(1.5) and TiO_2 relative to pure solid standard states range from 2.3 to 8.8 and from 0.1 to 0.3, respectively. Using thermodynamic data obtained, the prediction of the formation of titanium carbonitride was made. Assuming hypothetical 'TiO_2', i.e., total titanium in the slag expressed as TiO_2, and using the values of the activity coefficients of TiO_(1.5) and TiO_2 determined, the equilibrium distribution of titanium between blast furnace type slags and carbon saturated iron was computed. The value of [percent Ti]/(percent 'TiO_2') ranges from 0.1 to 0.2.
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