The interaction of glass melts and refractories in glass furnaces has a large impact on the resulting glass quality. The mostly applied type of refractory in the glass industry, in direct contact with the glass melt, is fused cast AZS. Due to the interaction of the glass melt with fused cast AZS, glass defects like stones, knots and bubbles can be generated.
Fused cast AZS block exudation is thought as the leading cause of knot and cord defects. Fused cast AZS exudation is only a short-term source of glass defects. Fused cast AZS corrosion is a more potent and long-term source of defects. Glass melt contact refractory corrosion can lessen with time due to boundary layer formation and the effect of external cooling at the metal line.
Fused cast AZS block has no open porosity but there are some closed voids and pores which arise from the manufacturing process. These pores are filled with gases. The gases in these pores have an air like composition, with nitrogen and oxygen as main components and some carbon dioxide. When fused cast AZS is in contact with glass, it reacts with the glass melt and slowly dissolves in this melt. The closed pores will be opened and then release their gas content, forming bubbles.
Impurities, which can oxidize, are for example elemental carbon originating from graphite electrodes of the fused casting operation, sulfur or zirconium carbide originating from raw materials of the AZS. The dissociation/oxidation of nitrides/oxynitrides, produced by reactions with the graphite electrodes during melting of the AZS, could give nitrogen gas.
When fused cast AZS block is brought into contact with a glass melt, an electrical potential is generated between the refractory and the glass melt because generally no thermodynamic equilibrium exists between these materials. The electromotive force (emf) generated between glass melt and refractory interior can cause the formation of oxygen bubbles.
The generation of a knot is due to one or more forces, which drive the fused cast AZS glass phase towards the refractory surface and into the bulk of the glass melt. Those forces can be caused by gravity, thermal expansion of zirconia and gas formation.
Glass defects are typically unavoidable, but their occurrence must be minimized during the production of high-quality glasses. Some alternatives, such as fused cast AZS with modified chemistry and glassy phase and alpha-beta fused cast alumina block are used to replace fused cast AZS to reduce the glass defects and improve the glass quality.