A modified hot-filament microscope is used for direct observation of the solution of calcium oxide (CaO) in synthetic melts simulating slags of metallurgical interest.
In the CaO-SiO2 system the process is uncomplicated in acid slags. However, in basic slags, when the composition approaches the liquidus in equilibrium with 2CaO•SiO2, a protective film develops which has many of the characteristics of a glassy phase. The crystallized film has the composition of 2CaO•SiO2, and crystallization results in more rapid solution of the CaO, apparently on account of the associated decrease in specific volume.
This mechanism operates with or without imposed temperature gradients and convection. It is not changed significantly by small additions of other common slag components. It appears to be general and has been observed in all other binary systems studied, including MgO-SiO2 and CaO with Al2O3, P2O5, B2O3, and V2O5. Related experiments have demonstrated the release of oxygen gas as a result of the combined influence of temperature and composition gradients during CaO solution. This is due to the change in the ferrous/ferric ion ratio and associated changes in slag structure.
Presentation based on high-temperature micrographs.