Kyanite Mullite

Figure 7.1 Typical thermal expansion trace kyanite (AljCVSiOj) + quartz (Si02) at 5°C/min. The a-0 quartz inversion is apparent at 573°C. Kyanite converts to mullite (3Alj03-2Si02) and residual glass starting at 1100°C, reaching a maximum rate at 1400°C [1]. The sharp contraction starting at 1100°C is interpreted to correspond to sintering. At 1320°C, the rapid formation of the less dense decomposition products of kyanite cause a temporary expansion [2].

When intended for use in ceramics, kyanite has to be calcined, not necessarily with the other two forms owing to the smaller volume change involved in their conversion to mullite. In place of natural raw materials from the sillimanite group, use is sometimes made of synthetic mullite produced by high-temperature reaction from a mixture of clay with bauxite, or in some cases from pure oxides. 

The three sillimanite minerals are structurally similar and have structures that are related to that of mullite. It is not surprising that they all form mullite upon decomposition. Kyanite crystallizes in the triclinic system, while sillimanite, andalusite, as well as mullite have orthorhombic crystal structures. In these structures, all the Si4+ cations are in fourfold coordination with 02 anions, but the Al3+ cations exist in four-, five-, and sixfold coordination with 02 anions, and therein lie the structural differences. The fivefold coordination of some Al3+ cations within A105 polyhedra is rather unusual, perhaps the result of formation at high pressures. The other structural differences among the three minerals are quite small. They are associated with the double chain structures of these three minerals and the linkages of the chains to one another by different alumina and silica polyhedra. Those concepts are readily extended to mullite. 

H. Schneider and A. Majdic, Preliminary investigation on the kinetics of the high-temperature transformation of sillimanite to 3/2 mullite, 3Al203-2Si02, and silica, Si02, and comparison with the behavior of andalusite and kyanite, Sci. Ceramics 11, 191-196 (1981). 

High alumina Kyanite a-Al203, mullite, glass Up to 1800 Metal handling Lab ware... 

Sillimanite, kyanite, and andalusite are non-plastic and so are processed by fine grinding, with the addition of a binder or plasticizer. When heated at about ISSO C, all three minerals decompose to form mullite and cristobaUte ... 

Sintered mullite may be obtained from a mixture of kyanite (Al20Si04), a naturally occurring mineral found in metamorphic rocks, bauxite, and kaolin. This mixture (in the correct ratio) is sintered at temperatures up to about 1600 C. The sintered quality contains 85-90% mullite... 

Sillimanite, kyanite, and andalusite are edl mullite-farming minerals, that is, on firing they decompose into muUite and vitreous silica (see mullite) according to the chemical reaction ... 

However, each polymorph exhibits a different decomposition behavior. Actually, the decomposition of kyanite is unpredictable it first starts to decompose slowly at 1310 C, and the reaction disrupts at about 1350 to 1380 C with an important volume expansion of 17 vol.%. For that reason, kyanite must always be calcined prior to being incorporated into a refractory in order to avoid blistering and spalling. By contrast, andalusite decomposes gradually from 1380 tol400"C with a low volume increase of 5 to 6 vol.%, while sillimanite does not change into mullite until the temperature reaches 1545"C with a volume expansion of 5 to 6 vol.%. 

Kyanite and andalusite convert to mullite and cryistor pellets at 1360 and 1550 °C, respectively and sillimanite at 1600 °C. 

Calcine Calcination. Heat treatment intended to produce physical and/or chemical changes in a raw material. The calcination of bauxite, for example, eliminates water and yields a product that is relatively free from further firing shrinkage when kyanite is calcined, on the other hand, the change is not in chemical but in mineralogical composition, mullite and cristobalite being formed. 

Firing Expansion. The increase in size that sometimes occurs when a refractory raw material or product is fired it is usually expressed as a linear percentage expansion from the dry to the fired state. Firing expansion can be caused by a crystalline conversion (e.g. of quartz into cristobalite, or of kyanite into mullite plus cristobalite), or by bloating (q v.). 

Also known as bauxitland cement. Kurlbaum Method. A method for the determination of flame temperature by means of an optical pyrometer (F. Kurlbaum, Phys. Z., 3,187,1902). Kyanite. A mineral having the same composition (Al2SiOs) as sillimanite and andalusite, but with different physical properties. The chief sources are Virginia and S Carolina (USA), and India. When fired, kyanite breaks down at 1300°C into mullite and cristobalite with a volume expansion of about 10% it is therefore calcined before use. Calcined kyanite is used in making aluminous refractories. 

Sillimanite. A mineral having the same composition (Al2Si05) as kyanite and andalusite but with different physical properties. The chief sources are S. Africa and India. Sillimanite changes into a mixture of mullite and cristobalite when fired at a high temperature (1550°C) this change occurs without any significant alteration in volume (cf. kyanite). The mineral is used as a refractory. 

Top Pouring. See direct teeming. Topaz. Al2Si04(0H,F)2. Occurs in economic quantities in Australia, Brazil, Nigeria and USA. After it has been calcined, the material has a composition similar to that of mullite and it has been used to a small extent, either alone or mixed with calcined kyanite, for making high-alumina refractories. 

Consists of up to 95% mullite crystals. Made from pure kaolins. Chemical and physical properties are those of mullite. (See MULLITE, KYANITE, CLAY.)... 

In the manufacture of refractory and porcelain bodies, the desirable mullite is formed from the chemically similar minerals andalusite, kyanite and dumortierite, which decompose into mullite and silica at cone 13, cone 12 and cone 6, respectively. To break up siUimanite in the same way, cone 20 is required. Sintered and electro-fused synthetic mullites (including zirconium mullite) are used in kiln furniture and refractories for the glass and steel industries. The theoretical chemical analysis (wt%) of mullite and five typical mullite materials are as follows ... 

Sillimanite and mullite are sUicates with Si-Al tetrahedral chains. They occur in high-temperature metamorphic rocks and muUite is an important ceramic material. TEM analyses identify [001] as Burgers vector in sillimanite [350,351]. Dislocations are generally of screw type in the [0 01] direction [Fig. 14(b)] and sometimes are dissociated [352]. Dislocation-assisted high-temperature deformation has been documented in mullite [353]. The influence of dislocations and strain on the aluminosilicate phase transformations were investigated for kyanite [354]. 

High alumina bricks contain more than 46% alumina. The raw materials used for making such bricks are sillimanite, kyanite, and andalusite. These minerals exist as rocks and are used in as mined condition to get more alumina content. For a lower percentage of alumina, the as-mined minerals are mixed with clays. On firing, all these bricks contain mullite, glass, and cristobalite or tridymite as the phases. The same phases are also found in firebricks. 

AI2O3 or kyanite Phases on phase diagram Major phase—mullite and minor phase—glass (can contain free Si02 ) clay since clays do not contain sufficient AI2O3 Made from 60% alumina minerals and contain some fireclay. Can be made with bauxite and clay Can be used in excess of 1700°C... 

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