Synthetic mullite

Bauxitic Kaolins andMullites. Deposits of bauxitic kaolins, kaolins having aluminous minerals, have been discovered that have alumina contents between 50 and 70%. These materials are made into refractory aggregates called calcines, grog, clinker, or grain. In addition to sdectivdy mined deposits, synthetic compositions can be prepared from kaolin and alumina and other minerals to produce compositions of desired alumina and mineralogical content. These synthetic mullites are readily available in the form of sintered and fused aggregates. 

A number of ceramic materials meet these specifications fairly well. Thus, synthetic mullite having a melting point of about 1835°C. and an expansion coefficient of 45 X 10-7 cm./cm./°C. between 20° and 1320°C., and synthetic zircon having a melting point of 1775°C. and an expansion coefficient of 42 X 10 7 between 20° and 1550°C. are suitable materials. However, both ceramics start to decompose under vacuum conditions above 1300°C. and show a not negligible vapor pressure of SiO and Si02 at considerably lower temperatures. In spite of this, mullite and zircon are better than fused quartz in the temperature range up to 1200°C. Moreover, they are cheaper and easier to be sealed to the vacuum system. 

McDANEL HIGH TEMPER -ATURE PORCELAIN (SYNTHETIC MULLITE)... 

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. 

Synthetic mullite is commonly used, and can be made by heating a mixture of pure AI2O3 or bauxite with clay or sillimanite. Mullite is a common constituent of fired pottery bodies and refractories and, under the microscope, appears as long prism-shaped crystals of nearly square cross-section. 

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 ... 

High Alumina Refractories. The desired alumina content, from 100% to just above 45%, is obtained by adding bauxites, synthetic aluminosihcates, and synthetic aluminas to clay and other bonding agents. These refractories are used in kilns, ladles, and furnaces that operate at temperatures or under conditions for which fireclay refractories are not suited. Phosphate-bonded alumina bricks have exceptionally high strength at low to intermediate temperatures and are employed in aluminum furnaces. High alumina and mullite are used in furnace roofs and petrochemical apphcations. 

Monochromator 663, 791 Monodentate ligand 51 Morphine (and codeine) D. of (fu) 740 Mortar agate, 155 mullite, 155 percussion, 155 synthetic sapphire, 155 Muffle furnace 97 Multielement analysis 174, 184, 775 Multiple range indicators 268 Murexide 316... 

Figure 5.11. Effect of grinding and subsequent thermal recrystallisation on the MAS NMR spectrum of synthetic crystalline mullite. Note the progressive change of the 47/32 ppm resonance. The small resonance at 13 ppm is from a residual corundum (a-alumina) impurity. After Schmiicker et al. (1998), by permission of the copyright owner.

D Al MQ MAS NMR is used to extract the isotropic chemical shifts and quadmpolar parameters of five amorphous aluminosilicates, all of approximately mullite composition (3Al(2)0(3).2Si0(2)) but of widely differing synthetic origin. Three principal types of A1 site are apparent in each sample two of these are conventionally assigned to 4- and 6-coordinate Al, while the nature of the third site, observed at a shift of delta approximate to 30 ppm, remains a subject of debate. ... 

In this chapter, developments in the understanding of mullite over the last few decades are reviewed. A discussion of crystal structures and phase stability is presented to provide the reader with an overview of certain characteristics of this material. The next part of this chapter examines the effect of process chemistry on the synthesis and microstructure of mullite. The role of various synthetic methods that are used to modify mullite formation will be discussed, followed by a compilation of selected materials properties. 

Synthetic silica-alumina catalysts containing 25% alumina are converted to gamma-alumina and mullite when thermally treated at 700-1260°. The phase transformation is accompanied by loss of catalytic activity and by collapse of the porous structure of the catalyst. The gamma-alumina phase is formed apparently by crystallization of the amorphous alumina in the catalyst, while the mullite formation apparently results from a combined silica-alumina amorphous phase. At sufficiently high temperature all alumina is converted to mullite. Silica-alumina catalysts made from more stable silicas have a greater tendency to form gamma-alumina. Such catalysts have lower initial catalytic activity and maintain relatively high catalytic activity after steam deactivation. 

The recent commercial interest in synthetic silica-alumina catalysts containing 25 % alumina prompted a reexamination of the x-ray diffraction properties of such catalysts. By use of more refined x-ray diffraction techniques, the formation of gamma-alumina and mullite (3Al20s-2Si02) in 25 % alumina catalysts has been measured after sintering at temperatures from 700 to 1250°. 

This causes problems when determining Burgers vectors of dislocations—the details of the crystal structure can be different in different mullites. 2Al203 -2Si02 has been produced synthetically. Fe and Ti can replace Al it is a very accommodating structure. 

Synthetic materials made by sintering of bauxite and kaolinite clay. After processing, the final material mineralogical composition consists of a mixture of mullite and corundum. Sometimes less common ceramics are also used, e.g., carborundum, stabilized cubic zirconia, other oxides, and silicates. 

Mullite (synthetic) Germany, Italy, Japan, USA, and the UK 60,000 Fused mullite 1000-1500 Fused zirconia mullite 1200-1500 Sintered mullite 750-1350... 

FIGURE 41.7 Cellular solids structures, after Gibson and Ashby [1988]. Left synthetic cellular solids (a) open-cell polyurethane, (b) closed-cell polyethylene, (c) foamed nickel, (d) foamed copper, (e) foamed ziiconia, (f) foamed mullite, (g) foamed glass, (h) polyester foam with both open and closed cells. Right natural cellular solids (a) cork, (b) balsa wood, (c) sponge, (d) cancellous bone, (e) coral, (f) cuttlefish bone, (g) iris leaf, (h) plant stalk... 

Mullite occurs in nearly all ceramic products containing alumina and sdica but, with the exception of refiactories, is seldom introduced as such except as calcined kyan-ite. Mullite is rare in nature, one locality being the Isle of Mull, Scotland, where argillaceous sediments have been fused by igneous intrusion. The compound was formerly thought to be synthetic siUimanite (a name that still persists in some quarters). Mullite is very refractory, breaking up into corundum and liquid silica at 1810°C. 

Zeolites consist of aluminium oxide, calcium oxide, iron oxide, magnesium oxide, potassium oxide, silicon oxide and sodium oxide within their strucmre with water molecules and/or cations in the pores and the cages [10, 20, 27, 46-48]. A certain fraction of the mass of the zeolites is lost on ignition because of loss of water. Researchers have suggested that, for a material to get zeolited, the ratio of (Si + Al)/0 in it should be equal to 0.5 [16, 46-48]. The cation exchange capacity (CEQ, adsorption properties, pH, and loss on acid immersion of zeolites are some of the chemical properties which are reported to depend on the chemical composition of the synthesized products. Table 2.4 presents typical chemical composition of a fly ash, its crystalline constituents (viz.. Quartz and Mullite), one commercial grade synthetic zeolite, a fly ash zeolite and their comparison with a natural zeolite [47, 48]. 

---