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| Refractory |
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| One of a number of ceramic materials for use in high-temperature structures or equipment. The term high temperatures is somewhat indefinite but usually means above about 1800°F (1000°C), or temperatures at which, because of melting or oxidation, the common metals cannot be used. In some special high-temperature applications, the so-called refractory metals such as tungsten, molybdenum, and tantalum are used. |
| The greatest use of refractories is in the steel industry, where they are used for construction of linings of equipment such as blast furnaces, hot stoves, and open-hearth furnaces. Other important uses of refractories are for cement kilns, glass tanks, nonferrous metallurgical furnaces, ceramic kilns, steam boilers, and paper plants. Special types of refractories are used in rockets, jets, and nuclear power plants. Many refractory materials, such as aluminum oxide and silicon carbide, are also very hard and are used as abrasives; some applications, for example, aircraft brake linings, use both characteristics.?See also:?Abrasive |
| Refractory materials are commonly grouped into (1) those containing mainly aluminosilicates; (2) those made predominately of silica; (3) those made of magnesite, dolomite, or chrome ore, termed basic refractories (because of their chemical behavior); and (4) a miscellaneous category usually referred to as special refractories.?See also:?Metal coatings |
Aluminosilicate refractories
| Fireclay is the raw material from which the bulk (about 70%) of refractories is manufactured. Different grades are distinguished according to the softening temperature or the pyrometric cone equivalent (PCE), the number of the standard pyrometric cone which deforms under heat treatment in the same manner as the fireclay. Thus, the minimum PCEs for low, intermediate, high, and superduty fireclays are 19, 29, 31/32, and 33, respectively. Fireclays are also classified by their working properties into two other classes: plastic (those which form a moldable mass when mixed with water), and flint (a hard, rocklike clay that does not become plastic when mixed with water). In general, flint clays have higher PCEs than plastic clays and are mixed with them to form higher-grade fireclay brick.?See also:?Clay, commercial |
| High alumina refractories are made from clays which contain, in addition to the alumina (Al2O3) in the clay minerals, hydrates of aluminum oxide. These raise the total Al2O3 content and make the material more refractory. Different grades are distinguished on the basis of the total Al2O3 content (50, 60, 70% alumina refractories). |
| Sillimanite and kyanite are anhydrous aluminosilicate minerals used to make special refractory objects, such as crucibles, tubes, and muffles, or as an addition to fireclay to increase refractoriness and to control its shrinkage during firing. |
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Silica refractories
| These account for about 15% of total production. They are made from crushed and ground quartzite (ganister) to which about 2% lime has been added to assist in bonding, both before and after firing. The quality of silica refractories is to a great extent determined by the amount of Al2O3 impurity, even small amounts having a deleterious effect on refractoriness. This is just opposite to the case of alumina in fireclays, where a higher alumina content means greater refractoriness. High-grade silica brick contains less than 0.6% Al2O3, and even the standard grade contains less than 1%. During firing, the mineral quartz transforms to cristobalite and tridymite, the high-temperature forms of silica. Since these are less dense than quartz, the brick expands on firing and the true density of the solid is often taken as a test of adequate firing. An example would be the case in which the density of acceptably fired material must be below 1.36 oz/in3 (2.35 g/cm3). The outstanding characteristic of silica is its ability to withstand high loads at elevated temperatures, for example, as a sprung-arch roof 30 to 40 ft (9–12 m) wide over an open hearth. The hearth may be operated within 120°F (50°C) of the melting point of silica.?See also:?Silicon |
| Semisilica refractories are made from clay with a high silica (sand) content (over 70% total silica); their main advantage is their dimensional stability when heated, or fired. Apparently the expansion of the silica, as sand, offsets the contraction of the clay. |
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Basic refractories
| Magnesite refractories are so named because magnesium carbonate mineral was for many years the sole raw material. Since World War II seawater has become a significant source of magnesium oxide refractory, and such material is often called seawater magnesite. In any case, the raw material is calcined to form a material largely magnesium oxide, MgO; about 5% iron oxide is usually added before calcining.?See also:?Magnesite |
| Chrome refractories are made from chrome ore, a complex mineral containing oxides of chromium, iron, magnesium, aluminum, and other oxides crystallized in the spinel structure. These crystals are usually embedded in a less refractory matrix called gangue. |
| In an attempt to combine the best properties of each, magnesite and chrome are often mixed to form chrome-magnesite or magnesite-chrome refractories (the first-named is the dominant constituent). |
| Dolomite is a mixed calcium-magnesium carbonate, CaMg(CO3)2, which, when calcined to a mixture of MgO and CaO, is used in granular form to patch the bottoms of open hearths and also to make bricks. |
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Miscellaneous materials
| Special refractories are made of a great many materials, and it is possible to mention here only a few of the more important. |
| Silicon carbide, SiC, is used for many refractory shapes, its outstanding properties being good thermal and electrical conductivity (it is used to make electric heating elements for furnaces), good heat-shock resistance, strength at high temperatures, and abrasion resistance. The first silicon carbide refractories were bonded with clay, so that the refractory properties of the bond placed the ultimate limit on the material. A method of making self-bonded silicon carbide has been developed to remove this limitation. Although silicon carbide tends to oxidize to form SiO2 and either CO or CO2, the silica-oxidation product forms a glassy coating on the remaining material and to a certain extent protects it from further oxidation. |
| Insulating firebrick is made from refractory clays to which a combustible material (sawdust, cork, coal) has been added; when this burns during the firing operation, it leaves a brick of high porosity. The low thermal conductivity of insulating brick reduces heat losses from furnaces, and the low bulk density and consequent low heat capacity reduce the amount of heat needed to bring the furnace itself up to temperature. The main disadvantage of such bricks is their low strength, but even this is useful in that they can be cut or ground to shape quite readily. |
| Pure oxides, of which alumina, Al2O3, is the prime example, are used for many special refractories. Zircon, ZrSiO2, and zirconia, ZrO2, are finding increased and significant uses as refractory materials. Some, such as beryllia, BeO, thoria, ThO2, and uranium oxide, UO2, are of particular interest for nuclear applications. |
| Carbides, nitrides, borides, silicides, and sulfides of various sorts have been considered as refractory materials, and some study made of them; aside from a few carbides and nitrides, however, none have found much use. |
| Cermets are an intimate mixture of a metal and a nonmetal, for example, Al2O3 and chromium. Although the nonmetal may be an oxide, it is more commonly a carbide or nitride (as in cemented tungsten carbide).?See also:?Cermet |
| Carbon, generally in the form of graphite, is used for such equipment as crucibles and as stopper nozzles in ladles for steel casting. A potentially very large use of carbon is in blocks for construction of blast-furnace hearths. Graphite has very good thermal-shock resistance and moderate electrical conductivity, does not melt but rather sublimes at a significant rate only at temperatures well above 5400°F (3000°C), is quite inert chemically, and is wet by very few molten materials. The main disadvantage of graphite, common to all nonoxide materials at high temperatures, is that it oxidizes; since the products are all gaseous, they offer no protection against further oxidation.?See also:?Graphite |
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Manufacture
| Standard ceramic techniques are used. Hand molding, once widely used, is used only for special shapes and small orders. The extrusion or stiff mud process is used for plastic fireclays; very often the extruded blanks are repressed or hydraulically rammed to form special shapes, for example, T-sections of refractory pipe. Power pressing of simple shapes is the most widely used forming method. Hot pressing and hydrostatic pressing are used for some special refractories. Slip casting is used for special refractory shapes. Fusion casting is commonly used for glass tank blocks; these are mainly either Al2O3 or Al2O3, with significant amounts of SiO2, ZrO2, or both.?See also:?Ceramics |
| Refractories are generally fired in tunnel kilns, but some periodic kilns are still used, particularly for special shapes.?See also:?Kiln |
| Some types of basic refractories, known as chemically bonded, are pressed with a chemical binder, such as magnesium oxychloride, and installed without firing. Some of these, the steel-clad refractories, are encased in a metal sheath at the time of pressing. When the refractory is heated after installation, the iron oxidizes and reacts with the refractory, forming a tight bond between the individual bricks. |
| In all refractory products and in unfired brick in particular, the maximum possible formed density is desired. To this end, careful crushing and sizing of raw materials are carried out so that, as far as possible, the gaps between large pieces are filled with smaller particles, and the space between these with still smaller, and so on. In the case of clay refractories, it is customary to use prefired (calcined) clay or crushed, fired rejects (both are known as grog) to increase the density and to reduce the firing shrinkage. |
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Properties
| A high melting point is of course necessary in a refractory, but many other properties must be considered in choosing a refractory for a specific application. |
| A definite melting point is characteristic of pure materials; actual minerals from which refractories are made, for example, clay, are far from pure and hence do not melt at a specific temperature. Rather, they form increasing amounts of liquid as the temperature is increased above a certain minimum temperature at which liquid first appears. This characteristic of gradual softening is indicated by the PCE of the material and the underload test. |
| High-temperature strength is important for refractories, but most materials become plastic and flow at elevated temperatures. Therefore, the rate of flow (creep rate) at a given temperature under a given load is a more important design criterion. |
| A knowledge of the thermal expansion of high-temperature materials is important, first, so that allowance can be made in furnace construction [long tunnel kilns must be built with expansion joints of several inches every 10 ft (3 m) or so], and second, because of its relation to thermal-shock resistance. |
| Thermal conductivity determines the amount of heat that will flow through a furnace wall under given conditions, and a knowledge of this property is essential to furnace design. |
| Thermal-shock resistance is the ability of a specimen to withstand, without cracking, a difference in temperature between one part and another. For example, if a red-hot brick is dropped into cold water, it is likely to shatter since the outside cools and contracts while the center is still hot. This cracking is often referred to as thermal spalling, the term spalling meaning any cracking off of large pieces of brick. Other causes of spalling are mechanical (hitting the brick and knocking off a piece) and structural (a reaction in the brick which changes the mineral structure and causes cracking). Thermal-shock resistance is enhanced by high strength, low Young's modulus, low thermal expansion, and sometimes, depending on conditions, high thermal conductivity. Whether or not a given specimen cracks under heat shock depends not only on the material of which it is made, but also on its size and shape and on the test conditions, for example, whether it is dropped into water or into still air at the same temperature. |
| Various chemical properties are important in refractories. For example, the tendency of the magnesium oxide in basic brick to hydrate, that is, to react with water to form Mg(OH)2, should be as low as possible. Turning to high-temperature chemistry, the rate of corrosion of refractories by molten slags and iron oxide fumes is vital to the length of service. Reference to the appropriate phase equilibrium diagrams may give some indication of which combinations of slag and refractory will react; but usually, actual tests are needed to make any precise predictions. The rate of corrosion depends to a great extent on such physical factors as the porosity of the refractory and whether or not the refractory is wet by the slag. |
| Carbon deposition is another chemical reaction which affects the life of refractories. The reaction is not with the refractory, but is catalyzed by substances in it. When carbon monoxide, perhaps in the top of a blast furnace, comes in contact with certain iron compounds which can occur in fireclays, its reduction to carbon is catalyzed. This carbon deposits at the site of the catalyst in the brick, and causes the brick to shatter. The effect is most pronounced around 950°F (500°C); much below this temperature, the rate of reaction is too slow, and much above it, the equilibrium oxygen pressure necessary for the reduction is lower than is found in practice. Although the reaction is not completely understood, it has been found that high-temperature firing of the fireclay refractories converts the iron to a form which does not catalyze the carbon deposition. |
| The bursting of spinel (chrome) refractories in contact with iron oxide is another high-temperature chemical reaction; it is not thoroughly understood, but appears to be related to oxidation and reduction reactions in the refractory. |
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