Dissolution rate of silicon nitride ceramics into molten CaO-SiO₂-Al₂O₃ slags

Dissolution rates of silicon nitride ceramics into CaO-SiO₂-10 mass%Al₂O₃ slags were measured by rotating specimen method. Experiments were conducted at various revolution speeds, temperatures, and CaO/SiO₂ ratios. Moreover, the solubility of Si₃N₄ into the slags were measured to calculate the mass transfer coefficients of Si₃N₄. Based on these results, the dissolution mechanism of silicon nitride into the slags was discussed. The dissolution rate of silicon nitride ceramics increased with increasing revolution speed and rising temperature. These results suggested that the dissolution rate was controlled by mass transport in the boundary layer of molten slags. The dissolution rate of silicon nitride increased slightly with increasing CaO/SiO₂ ratios. This increase of the dissolution rate is attributed to the increase of the diffusivity of the solute accompanied with the decrease of slag viscosity with CaO/SiO₂ ratios. The chemical analysis of the slags after immersing silicon nitride ceramics showed that almost nitrogen in slags existed as N^3- ions. On the other hand, the quantity of CN^- ions was negligible, though the existence of CN^- ions was confirmed. From the estimation of the interdiffusion coefficient D_{CaO-Si(3)N(4)} in the molten slags using J_D-factor, it was found that D_{CaO-Si(3)N(4)} was a little smaller than D_CaO-SiO(2). From this result, it was reasonable that N^3- ions existed in the molten slags incorporated into silicate network such as silicate complex anion. These results showed that the dissolution of silicon nitride proceeded by the diffusion of the N^3- ion incorporated into silicate anions.