Clay-bonded refractories

Natural graphites are unctuous and soft (hardness 0.5-1.5 Mohs), bum slowly, are chemically inert and have a sublimation temperature >3500°C. Graphite has excellent weathering properties, is hydrophobic, and tends to form water in oil-type emulsions. It is used in clay-bonded refractories and may be glazed to prevent oxidation. 

Plascast. A clay-bonded castable refractory which is claimed to combine... 

Steatite. Massive form of the mineral talc, Mg3Si40io(OH)2. The chief sources are in USA, France, Italy, India, Austria and Norway. The natural rock can be machined and the shaped parts fired for use as electroceramics. A far greater proportion is ground and shaped in the usual way with a clay bond to produce low-loss electroceramics thermal expansion (20-500°C) 7-9 x 1(H volume resistivity (300°C) 10 - 10 ohm-cm. dielectric constant (1 MHz) S.5-6.5 power factor (IMHz) 5 - 25 x ICH. Steatite is also used as a raw material for cordierite electroceramics and for refractories containing cordierite. Steelite. A high strength hotelware body, presumed to contain alumina, first introduced by Ridgeway Potteries, 1967. Now the name of a hotelware company, Steelite International Ltd, Stoke-on-Trent. 

Vitrified bonds are very widely used in the metalworking industry, especially for machinery manufacture. The basis for this glass type bond is a fusible clay, feldspar. Refractory materials and fluxing agents are added to produce the desired composition. The dried clay and other bond ingredients are pulverized, screened, and blended to form the dry bond. This is added to the water wetted abrasive in a mixer to coat each abrasive particle with the bonding medium. The resulting mix is pressed cold in compression molds to form the basic wheel shape. 

Clays can get leached out of soluble matter by water flow. The resulting deposit is low in iron, alkalies, and alkaline earth and is called china clay. It is very pure, highly refractory, and fires white. The least pure clays are derived from basic rocks. They fire red and are useful as building bricks and for making drainage pipes. Between very pure and least pure clays are the intermediate pure ones. These include plastic ball clay, bond clay, flint fireclay, fireclay, and aluminous fireclay. 

Cast and Hand-Molded Refractories. Large shapes such as burner blocks and flux blocks, and intricate shapes such as glass feeder parts saggers are produced by casting sHps, hydraulic cement bonded mixtures, or hand-molding clay or chemically bonded materials. Because these techniques are labor intensive, they are reserved for articles that caimot be satisfactorily formed in any other way, owing to complexity or small production quantities. 

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. 

A large class of fireclays associated with the coal measures of Pennsylvania, Maryland, Ohio, Kentucky, West Virginia and Indiana belong to the types softening between cones 28 to 32. These are useful for bonding purposes in the manufacture of firebricks and shapes. Such clays are available in enormous quantities in conjunction of the coals of these states. All of the plastic bond clays, whether of the No. 1 grade or not, fire to a buff color which may become more and more discolored through the presence of iron pyrites or iron oxide as we descend in the scale of refractoriness. 

The purest as well as the strongest clays are probably the so-called ball clays, the plasticity and bonding power of which is reinforced by the presence of organic matter. This type of clay seems to be preferred in the manufacture of graphite crucibles, and other special refractories. 

The results of physical tests made upon a number of refractory bond clays are given in Table 2. 

With the increase in sihca and the presence of varying amounts of fluxes the coal-measure fireclays tend to become dense or vitrify at higher temperatures than the bond clays even though the latter possess a higher ultimate refractoriness. It is evident, however, that there are also many clays of this type maturing at a low temperature, as for instance the carboniferous clays from Brazil, Ind. 

Refractories for the glass industry are made, as a rule, from a mixture of calcined clay, consisting partly of crushed old pot material, carefully cleaned, and plastic bond clay. The proportion of calcined (grog) to plastic clay usually is between 1 1 to 3 2. In some glass plants no clay but the Grossalmerode material and pot shell was used in others the plastic clay from Missouri was introduced, and, again, in some cases a smaller amount of raw flint clay was added. Some of these mixtures are as follows ... 

Certain refractories like the Marquardt porcelain used for pyrometer tubes consist essentially of sillimanite and sufficient feldspar to bring about a dense structure at about cone 18. Such a mixture would consist of a calcine fired to cone 20 having the composition 71.66 per cent kaolin and 28.34 of anhydrous alumina bonded together by clay. The mixture then would be as follows ... 

Brick made from a mixture of crushed coke and clay or from coke bonded by means of tar have been made many years ago. These materials are usually hand molded, dried and fired in muflBles with as complete exclusion of air as possible. The carbonization of the tar cements together the coke particles to a compact mass. The coke may also be replaced by graphite. Refractories of this type combine good heat-resisting power with excellent thermal conductivity. 

The function of the clay is that of a bonding material, which makes possible the shaping of the crucible and the cementing together of the graphite flakes. At the same time, it covers the particles, and thus protects them from oxidation. In order to be most effective in regard to this point, it is necessary that the clay contract and become dense at as low a temperature as is consistent with the required refractoriness. 

Chromite refractories are employed in furnaces as partings between the silica and magnesite bricks and heiice are considered neutral. They consist principally of crushed and ground chromite, and bricks are molded from this material which may be bonded together with a small amount of clay or burnt magnesia by pressing under a heavy pressure. The bricks are then dried and fired to a temperature of about cone 16. The composition of these refractories fluctuates between the following values ... 

They are normally cast in the form of brick and are sometimes bonded to assure stability. The outstanding property of these materials is their ability to act as insulators. The most important are fireclay (aluminum silicates), silica, high alumina (70-80% ALjOj), mullite (clay-sand), magnesite (chiefly MgO), dolomite (CaO-MgO), forsterite (MgO-sand), carbon, chrome ore-magnesite, zirconia, and silicon carbide. (2) Characterizing the ability to withstand extremely high temperature, e.g., tungsten and tantalum are refractory metals, clay is a refractory earth, ceramics are refractory mixtures. 

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