High alumina

A higher density sol—gel abrasive, produced by the introduction of seed crystaUites formed by wet-milling with high alumina media or by introduction of submicrometer a-alumina particles, was patented (28) and designated Norton SG. The microstmcture of this abrasive consists of submicrometer a-alumina crystals (Fig. 1) and its bulk density approaches that of fused alumina. Norton SG has proven to be an exceptional performer in coated and bonded abrasive products it was awarded the 1989 ASM Engineering Materials Achievement Award (29). 

The attack of water is related to the leaching mechanism described for acids. Table 4 rates glasses based on their resistance to water attack. Low alkah, high alumina, or borosiUcate glasses generally have high water durabiUty. 

Flint clays and other related rocks are another potential lithium source. These are high alumina clays that are composed largely of we11-crysta11i2ed kaolinite [1318-74-1] and are used for the manufacture of refractories (qv). The lithium content ranges from <100 to 5000 ppm. Deposits occur in many states, including Missouri, Peimsylvania, and Ohio. Lithium (at ca 1.3%) is present in a chlorite mineral that is similar to cookeite [1302-92-7]. High lithium contents may be the reason why some deposits are unsatisfactory for refractory use. 

Alumina, or aluminum oxide [1344-28-17, has a thermal conductivity 20 times higher than that of most oxides (5). The flexural strength of commercial high alumina ceramics is two to four times greater than those of most oxide ceramics. The drawbacks of alumina ceramics are their relatively high thermal expansion compared to the chip material (siUcon) and their moderately high dielectric constant. 

High Alumina. The naturally occurring raw materials are bauxites, sillimanite [12141 5-6] group minerals, and diaspore clays (see Aluminum compounds). Other high alumina raw materials are made by beneficiation, blending, and other processing techniques. 

The resistance against thermal spalling of fireclay and high alumina brick is indicated in Table 5. No standard test has been adopted for basic brick. Refractories composed of 100% magnesia exhibit poor thermal shock resistance, which is improved by addition of chrome ore. So-called direct bonded basic brick, composed of magnesia and chrome additions, exhibits good thermal shock resistance. 

Year Fireday High alumina Sihca Basic ... 

Includes regular fireclay, semisilica superduty fireclay, ladle brick, and insulating firebrick (IFB) below 23. Includes high alumina, mulhte, and extra high alumina brick. 

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. 

High alumina clays refer ia the ceramic iadustries to nodular clays, budey-flint clay, budey and diaspore, gibbsitic or bauxitic kaolins (clays), abrasive clays, and others. Siace the depletion of diaspore varieties ia Missouri and Pennsylvania, most bauxitic kaolin and clay is produced ia Alabama and Arkansas. 

Aluminum chlorite, (Al,Fe)4(Si,Al)402Q(0H)g, in which a gibbsitelike interlayer proxies in part for the bmcitelike interlayer, is being discovered in increasing occurrences and abundance (11,141). Chloritelike stmctures have been synthesi2ed by precipitation of Mg and Al between montmorillonite sheets (143). Cookite [1302-92-7], an aluminous chlorite containing lithium, has been found in high alumina refractory clays and bauxite [1318-16-7] (139). 

High alumina minerals are found where intense weathering and leaching has dissolved the siHca. It is generally befleved that a very humid, subtropical climate is required for this (lateritic) stage of weathering. 

Refra.ctories, Refractory products are prepared from a wide variety of naturally occurring materials such as chromite [1308-31 -2] and magnesite [546-93-0] or from clays predominandy composed of kaolinite. Increasingly, higher purity synthetic materials are being used to obtain special properties. On the other hand, for many refractory uses, a somewhat lower fusion point than that provided by kaolinite may be adequate, so that clay materials having a moderate amount of other components as, for example, iUite, may be satisfactory. High alumina clays are also used extensively for the manufacture of special types of refractories. 

Tyjlical feeds are high alumina, kaolin carriers, molecular sieves, and gels. 

Concrete is a particulate composite of stone and sand, held together by an adhesive. The adhesive is usually a cement paste (used also as an adhesive to join bricks or stones), but asphalt or even polymers can be used to give special concretes. In this chapter we examine three cement pastes the primitive pozzolana the widespread Portland cement and the newer, and somewhat discredited, high-alumina cement. And we consider the properties of the principal cement-based composite, concrete. The chemistry will be unfamiliar, but it is not difficult. The properties are exactly those expected of a ceramic containing a high density of flaws. 

High-alumina cement is fundamentally different from Portland cement. As its name suggests, it consists mainly of CA, with very little C2S or C3S. Its attraction is its high hardening rate it achieves in a day what Portland cement achieves in a month. The... 

High-alumina cement is very quick setting, but its acid resistance is only slightly better than Portland cement, and it is rapidly attacked by alkalis. Super-sulfated cement is used for liquors high in sulfates. It is resistant to acidic conditions down to a pH of 3.5 and has alkali resistance similar to Portland cement [50]. 

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