Impurity oxides of mullite insulation firebrick

Mullite insulation firebrick usually contains impurity oxides, such as TiO₂, Fe₂O₃, CaO, MgO, K₂O and Na₂O. And the impurity content of mullite produced from natural raw materials is higher than that of synthetic raw materials. These impurity oxides act as fluxes in mullite. Then can reducing the formation temperature and viscosity of the melt, increasing the amount of liquid phase generated. And they also increase the dissolution rate and amount of dissolution of the melt in the solid phase.

But the action intensity of each impurity oxide is different. Of which the K₂O and Na₂O, on the liquid phase formation temperature has the greatest impact. K₂O and Na₂O reduce the temperature of 513~724 °C respectively and also decompose mullite.

Among these impurity oxides, the effect of TiO₂ is minimal. Only the temperature at the no-parameter point is lowered by 101 to 107°C. When the content of TiO2 is low, in addition to the limited solid solution formed by partial solid solution in mullite to promote the formation of mullite and crystal growth, there are also some parts that enter the liquid phase to form a vitreous body at high temperature.

The effect of  Fe₂O₃

Fe₂O₃ has a certain degree of solid solubility in mullite and corundum at high temperature, forming a limited solid solution. Its solid solubility in corundum is higher than that in mullite. Due to the formation of solid solution, the crystal lattice of mullite and corundum grows. The effect of Fe₂O₃ on the initial melting temperature of Al₂O₃-SiO₂ material depends on the content of Al₂O₃ in the system or the ratio of Al₂O₃/ SiO₂.

When Al₂O₃/ SiO₂<2.55, the initial melting temperature is 1380°C. When Al₂O₃/SiO₂>2.55, the initial melting temperature Increased to 1460 °C. And it will gradually increase with its Al₂O₃ content. In the reducing atmosphere, Fe₂O₃ was reduced to FeO and desolved into the glass phase, and the initial melting temperature of the system decreased, dropping to 1240°C and 1380°C, respectively.

The Performance of mullite insulation firebrick

In short, with the increase of Al₂O₃ content in mullite insulation bricks, its high temperature performance is improved, while the increase of the amount of melt and the decrease of high temperature performance. Accordingly, strict control of the content of impurities, especially K₂O, Na₂O, and Fe₂O₃, is an important measure for obtaining high-performance mullite bricks.

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