Solubility of nitrogen in molten CaO-SiO2-Al2O3 slags

The solubilities of nitrogen in CaO-SiO2-Al2O3 slags were measured using N-2-H-2 gas or silicon nitride ceramics at 1823 K. Moreover Vickers hardness of the quenched slags was measured. Based on these results, the dissolution mechanism of nitrogen into the slags was discussed. The nitride capacity measured by an equilibrium between N-2-H-2 gas and slag under a given partial pressure of oxygen decreased with increasing SiO2 content. This result suggested that nitrogen dissolved in the slags by exchanging reaction between oxygen ion and nitrogen. On the other hand, the nitride capacity by an equilibrium between silicon nitride ceramics and slag was 10(2) to 10(4) times larger than that from gas-slag equilibrium. Although the nitride capacity was calculated assuming that silicon nitride decomposed, it was not in agreement with that from gas-slag equilibrium. Therefore it was estimated that the dissolution mechanism of nitrogen using silicon nitride ceramics was different from that obtained by nitrogen gas. Vickers hardness of the solidified slag after silicon nitride dissolution decreased with increasing SiO2 content and this tendency was the same as the influence of silica on nitrogen solubility. This suggested that network formation in the slag was strengthened by the dissolution of silicon nitride, which was estimated to proceed with the formation of complex network between Si-N (or Al-N) and oxide.