The solubility of nitrogen as the nitride ion in CaO-SiO2-Al2O3 slags in equilibrium with N2-CO gas mixtures and carbon was measured at 1823 K. The nitride capacity (C N3-) was calculated to compare the nitrogen contents measured under different nitrogen and oxygen potentials. C N3- decreased with increasing basicity and by replacing SiO2 with A12O3. The nitrogen partition ratio between carbon saturated iron and the slag was measured in CO gas at one atmosphere at 1823 K. By comparing the partition ratios with the corresponding nitride capacities measured by the gas-slag experiments, it was concluded that the oxygen partial pressure at the slag-metal interface was controlled by the Fe-FeO reaction. A new definition of nitride capacity was proposed based on the reaction between nitrogen and the network former, i.e., SiO2 or A12O3. This capacity could consistently explain the experimental results. Empirical equations were derived to estimate the activity coefficients of silicon and aluminum nitrides in the slags.
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