Figure 1: The Alumina-Silica Phase Diagram can be used to deter- mine the hot properties of materials containing different ratios of alumina and silica.


mullite. The ratio of three parts alumina to two parts silica makes stoichiometric mullite with an alumina content of 72 weight percent. While the melting temperature of mullite is lower than alumina, it still exhibits excellent high temperature properties. Mullite has a melting point of 3435°F (1890°C) and is one of the best refractory ceramics for thermal shock resistance.1 Pure mullite is rarely found in nature due to its high temperature/low pressure formation conditions. The first samples of the 3:2 aluminosilicate were discovered by Bowen and Grieg in the 1920’s.3


They noticed the material in


lava flows on the Isle of Mull where clay sediments had come in contact with the hot magma. The new mineral was given the name after the island.3


Its rarity as a


naturally occurring mineral means that it is not mined for industrial purposes. While not stoichiometric mullite, there are many different aluminosilicate minerals. Examples include the clay minerals (kaolin, mica, pyrophyllite, etc)


®


Figure 2: This Pressure-Temperature Phase diagram shows the conditions required to form the different minerals in the sillimanite group.


and the sillimanite group of minerals. Mullite used for industrial purposes be


must created using precursor


minerals. There are two main methods: in-situ mullite formation and the creation of mullite aggregates. Examples of mullite creation in situ in ceramics go as far back as 1500-1000 BC in Chinese pottery.3


In situ formation of mullite


is a common practice in the ceramics industry to this day. The investment casting industry uses mullite in both aggregate and powder form. Most of the aggregate mullites in the industry today are made by mixing, extruding/spheredizing, and calcining clay minerals to form the mullite. These aggregates can then be ground to make the flours used in slurries. These mullites are frequently categorized by their overall alumina content. Mullites with alumina contents ranging from 40- 70% are commonly found throughout the industry. The major mineral phase of these aluminosilicates is mullite with the remainder comprised of either


quartz, cristobalite, or amorphous silica. Selecting the appropriate alumina percentage is critical in order to achieve the desired properties of the mold at temperature. It is commonly believed that the higher the alumina content, the higher the maximum usage temperature of the mullite. While this is generally true, the impurities in mullite can have a greater effect on the usage temperature than the overall alumina content. While making mullite from clays is the predominate method of obtaining mullite, it is not the only way: industrial grade mullite is also made by calcining the


sillimanite family of minerals.


The sillimanite family of minerals are aluminosilicates with a 1:1 ratio of alumina and silica. The three industrially important members of the mineral family are kyanite, sillimanite, and andalusite. These three minerals are all polymorphs of the 1:1 chemical composition. The pressure and temperature of formation


Continued February 2020 ❘ 21


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