Studies on the glassy and reaction phases given off by fused-cast AZS blocks and their effects on glass quality

Abstract

Fused-cast AZS blocks exude distinctly more glassy phase at temperature cycles in comparison with constant temperature, also for longer term. The reason for this behavior is the liberation of oxygen bubbles during temperature increase by redox equilibrium processes of polyvalent ions. In glass melting tanks for high-alkali silicate glasses this effect leads to formation of bubbles, knots and stones at temperature increase, not only with new, but also with used fused-cast AZS blocks. At the contact between high-alkali silicate glass melts and fused-cast AZS blocks a highly viscous Al2O3-enriched reaction layer with embedded ZrO2 crystals is formed on the block side of the phase boundary due to diffusion processes and chemical reaction. The composition of this glassy reaction layer corresponds to that of the knots in the glass product. During a temperature increase, the liberated bubbles of the fused-cast AZS blocks press parts of the glassy reaction layer with ZrO2 crystals (baddeleyite seam) into the base glass thus leading to the formation of knots and stones.