Chemical Mechanism of AZS Refractory Material and Bubble Formation
Cast AZS refractories contain impurity elements that can be oxidized to form gases, such as carbon, sulfur, zirconium carbide, nitrides, and nitrogen oxides. When AZS refractory bricks are heated to above 1400℃, these impurity elements undergo oxidation reactions, forming corresponding gases such as nitrogen, carbon monoxide, carbon dioxide, and sulfur dioxide.
When the cast AZS bricks are cooled and then reheated, oxygen bubbles, or secondary bubbles, are generally generated at the contact point between the molten glass and the cast AZS bricks. This is because, under normal circumstances, cast AZS bricks contain a small amount (<0.3% (mass fraction)) of variable-valence elements, such as iron (Fe, Fe) or titanium (Ti, Ti). These variable-valence elements can change their valence state with temperature changes, thereby absorbing or releasing oxygen, as shown in the following reaction equation.
When the temperature decreases, reaction equation (1) proceeds in the forward direction, favoring the formation of Fe3+. When the temperature increases, reaction equation (1) proceeds in the reverse direction (reverse reaction), favoring the formation of O2.
Another reason for oxygen production is the electrochemical reaction (cell reaction) between the molten glass and the AZS refractory bricks, or a “remote” electrochemical reaction occurring without direct contact between the molten glass and the AZS refractory bricks. Alkali metal or alkaline earth metal ions are responsible for electron transfer. In this cell reaction, oxygen ions (O2-) in the molten glass are oxidized to oxygen (O2), i.e., the anodic reaction. High-valence iron (Fe3+) or titanium (Ti4+) in the refractory is reduced, i.e., the cathodic reaction. The reaction rate of this electrochemical reaction is directly proportional to the difference in the content of alkali metal or alkaline earth metal ions in the molten glass and the cast AZS refractory.
When the molten glass comes into contact with the cast AZS brick, the concentration of alkali metal or alkaline earth metal ions in the molten glass is much higher than that in the refractory material. Combined with the exudation of the glass phase, these metal ions, specifically Mn+, diffuse into the interior of the refractory material. As a result, they neutralize the electrons that diffuse into the AZS refractory material, maintaining its electroneutrality.
