Refractories aluminous

Aluminous Refractories.—It can bo readily seen that alumina itself, owing to its high fusion point, 2,050°C., is inherently an excellent refractory. The only difficulty in tlici way of utilizing it is its large contraction upon heating. 

Th( . fusion t( mp( raturc of the eutectic between AI2O3 and the next compound, Al iOa.SiOa, is 1,810 Xh The use of less refractory clay will lower this point still more and if t h( amount added is at all large, it will be the determining factor in the deformation tind(T th(i cfTect of Imat. Aluminous refractories should therefore contain but... 

R. E. Farris and J. E. Allen, Aluminous refractories-alkali reactions. Iron Steel Eng., February, pp. 67-74 (1973). 

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. 

Pana RP, Padhi BP, Bal AS, et al. Processing and characterization of MgAl204-calcium aluminate refractories by reaction sintering of alumina-dolomite. 13th Biennial Congress on Refractories, UNITECR. 2013 13 1153-8. 

The bond is ensured by the hydration of an aluminous refractory cement added to the product. This cement is made up of a mixture of calcium aluminates which are hydrated in the presence of water at low temperatures, by provoking hardening of the material. It therefore refers to a cold binding method. Irrespective of its content high (around 30%), low (around 2% to 10%) or ultra low (less than 2%), this cement is considered to be the typical binder for refractory castables. 

ALUMDIUMCOMPOUNDS - ALUMINIUMOXIDE(ALUMINA) - CALCINED, TABULAR, AND ALUMINATE CETffiNTS] (Vol 2) -refractories from [REFRACTORIES] (Vol 21)... 

Uses of bauxite other than for aluminum production are in refractories, abrasives, chemicals, and aluminous cements. Bauxites for these markets ... 

Calcined alumina markets consume slightly less than 50% of the specialty alumina chemicals production (1—8,20,22—115). Worldwide usage is estimated to be about 50% for refractories (qv), 20% for abrasives, and 25% for ceramics (qv). Calcined aluminas are also used in the manufacture of tabular alumina and calcium aluminate cements (CAC). Quantities are estimated to be over 200,000 and 100,000 t, respectively (7). 

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). 

There are a number or types of special brick obtainable from individual producers. High-burned kaolin refractories are particularly valuable under conditions of severe temperature and heavy load or severe spalling conditions, as in the case of high-temperature oil-fired boiler settings or piers under enamehng furnaces. Another brick for the same uses is a high-fired brick of Missouri aluminous clay. 

Castable Monolithic Refractories Standard portland cement is made of calcium hydroxide. In exposures above 427°C (800°F) the hydroxyl ion is removed from portland (water removed) below 427°C (800°F), water is added. This cyclic exposure results in spalling. Castables are made of calcium aluminate (rather than portland) without the hydroxide they are not subject to that cychc spaJhng failure. 

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. 

An alternative to silicate-based Portland cement is the calcium aluminate cement, ciment fondu, which originated with the Lafarge company in France in 1908. Ciment fondu is typically made by heating limestone with bauxite, which is mainly AIO(OH) but contains much iron oxide (see Section 17.2). As noted above, calcium aluminate hydrates and hardens much more rapidly than alite, and so ciment fondu, either as such or mixed with Portland cement, can be used whenever a rapidly setting cement is required, for example, for construction at low temperatures. Concretes made from aluminate cements remain serviceable at higher temperatures than Portland cements and so are used to make cast refractories for pyrometal-lurgical applications. 

Chromizing and Related Diffusion Processes. Chromizing is similar to aluminizing. A thin corrosion and wear resistant coaling is applied to low cost steels such as mild steel, or to a nickel-based alloy. In the related boroni/ing process, a thin boron alloy is produced for extreme hardness, wear, and corrosion resistance. Siliconizing is yet another process used especially lor coaling of the refractory metals Ti. Nb. Ta. Cr. Mo. and W. 

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