Andalusite & Mullite: impact of minerals purity in refractory behavior
In its August edition, Refractory Worldforum published the article “Production and Properties of Refractory Raw Materials Based on Andalusite and Mullite – Influence of Impurities in the Refractory Behavior” written by Danilo Frulli, Marketing & Innovation Director of Imerys Refractory Minerals.
Mullite based refractories represent one of the main groups of acid refractories. Their outstanding thermo-mechanical properties and stability make them key components of many refractory solutions. This paper describes the main properties of andalusite and mullite based products, and investigates the impact of mineral purity in the enhancement of their performance in the refractory applications.
Mullite and andalusite – mineral structure and mullitization Most of the acid refractory materials can be described on the system Al2O3 – SiO2. This system is characterized by three phases: silica, mullite and corundum. They are thermodynamically stable at high temperature with a congruent melting point at respectively 1726, 1853 and 2054 °C respectively . Due to the high liquidus temperature and the high temperature of the eutectic (1595 °C), alumino-silicates are generally considered as having good refractory properties, although their performance can be significantly affected by impurities. In particular alkali, alkali-earth oxides or even iron oxides can be responsible of a significant drop of the refractory behavior, to the point that the impurity content is often considered as more critical than alumina in the determination of the refractory behavior.