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ABSTRACT
A refractory brick or fire brick is a block of refractory ceramic material used in lining furnaces, kilns, fire boxes and fire places.
A refractory brick is built primarily to withstand high heat but should also usually have a low thermal conductivity to save energy. Usually dense fire bricks are used in the application with extreme mechanical, chemical or thermal stresses, such as the inside of a wood fired kiln or a furnace which is subjected to abrasion from wood, fluxing from ash or slag and high temperatures, in other less harsh situation such as a natural gas fire kiln, more porous bricks are a better choice. They are weaker, but they are much lighter, easier to form and insulate far better than dense bricks. In any case, fire brick should not spall under rapid temperature change and their strength should hold up well during rapid temperature changes.
Refractories are sold in the form of fire bricks, silica, magnesite, chromite bricks, silicon carbide and zirconia refractories.
This paper deals on the classification, properties, process of manufacturing, their applications and also about other special refractory bricks.
INTRODUCTION:
A fire brick or refractory brick is a block of refractory ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces.
A refractory brick is built primarily to withstand high heat, but should also usually have a low thermal conductivity to save energy. Usually dense firebricks are used in applications with extreme mechanical, chemical, or thermal stresses, such as the inside of a wood-fired kiln or a furnace, which is subject to abrasion from wood, fluxing from ash or slag, and high temperatures. In other, less harsh situations, such as a natural gas fired kiln, more porous bricks are a better choice. They are weaker, but they are much lighter, easier to form, and insulate far better than dense bricks. In any case, firebricks should not spall under rapid temperature change, and their strength should hold up well during rapid temperature changes.
To make firebrick, fireclay is baked in the kiln until it is partly vitrified, and for special purposes may also be glazed. Fire-bricks usually contain 30-40% aluminium oxide or alumina and 50% silicon dioxide or silica. They can also be made of chamotte and other materials. For bricks of extreme refractory character, the aluminium oxide content can be as high as 50-80% (with correspondingly less silica), and silicon carbide may also be present. The standard size of fire-brick is 9 x 4.5 x 2.5 in. (228 mm x 115 mm x 64 mm).
The silica firebricks that line steel-making furnaces are used at temperatures up to 1650°C (3000°F), which would melt many other types of ceramic, and in fact part of the silica firebrick liquefies. HRSI, a material with the same composition, is used to make the insulating tiles of the space shuttle.
A range of other materials find use as firebricks for lower temperature applications. Magnesium oxide is often used as a lining for furnaces.
FIRE BRICK
HISTORY:
The first application of silica "tiles" within ceramic brick kilns or furnaces is credited to William Harry of the Swansea Valley, Glamorganshire, Wales in 1817. Harry's invention served to vitrify the interior surface of ceramic brick built blast furnace. In 1820 however Quaker entrepreneur William Weston Young began experimenting with silica clay recipes, at his pottery in Nantgarw, also in Glamorganshire, for the creation of a robust, heat-proof brick from which a whole blast furnace could durably be made.
In 1822, Young, with three further investors, including David Morgan, John Player and (Young's brother) Joseph Young established The Dinas Firebrick Co. in the Vale of Neath, Glamorganshire, Wales and the first batches of firebricks began to be exported for the construction of blast furnaces across the industrialized world.
The Welsh word "Dinas," a reference to the hill where the silica was quarried in the upper Neath Valley, (Craig-y-Dinas, at Pontneddfechan) is synonymous with the word firebrick in many foreign languages as a result of the extensive influence of this industry in South Wales.
Silica bricks were also manufactured in the upper Swansea Valley by the Penwyllt Dinas Silica Brick Co.
CLASSIFICATION OF REFRACTORIES:
1. CHEMICAL CLASSIFICATION:
Chemically, refractories can be classified into the following three types.
i. Acidic Refractories-
Those which are made of clay, clay silica mixtures and of pure silica. Clay is generally used in the form of fire clay and silica in the form of flint, quartz, sandstone, gannister etc. Fire clay refractories, chief constituent of which is silicate mineral, koalinite, Al2O3 2SiO2. 2H2O have widely been used. Silica refractories rank next to fire clay refractories and are generally produced from quartzite and quartz pebbles which are almost pure form of silica SiO2.Blue bricks, sand line bricks, cement sand bricks, glass bricks are acid bricks.
ii. Basic Refractories-
Those which contain a large proportion of lime or magnesia or a mixture of these bases or other metal oxides. Examples are magnesite bricks, bauxite bricks, dolomite bricks etc.
iii. Neutral Refractories-
Neutral refractories are those formed of certain alumina silicates which contain more alumina than pure clay. Examples are chromite bricks, silicon carbide, graphite bricks etc.
Some single oxide bricks have also been developed in recent years. They are self bonded and have high fusion points. For example, alumina (2050°C), magnesia (2250°C), zirconia (2200°C) etc.
2. CLASSIFICATION BASED ON REFRACTORINESS:
Specially fire clay bricks are also classified according to their alumina contents and refractoriness. It should be noted that refractories strictly include all materials of pyrometric cone equivalent (PCE) greater than cone 26. PCE is a measure of refractoriness of raw ceramic materials, mixtures or products. It is usually calculated by comparison with mixture of known properties, i.e, pyrometric cones, which are heat work recorders.
Refractories can be classified into four types on the basis of refractoriness.
PCE No. Classes of Refractoriness.
1. Cones of PCE 26-28 Moderate heat duty
2. Cones of PCE 28-31 Intermediate heat duty
3. Cones of PCE 31-33 High duty
4. Cones of PCE 33-34 Super duty
Refractoriness is the capacity of a material to withstand the heat without appreciable deformation or softening under particular service conditions and is measured generally as the softening or the melting temperature of the material. It should be noted that refractory materials generally do not have sharp fusion temperatures, because of the fact that they are mixtures of various metallic oxides and other substances. It is, therefore, common practice to determine softening temperature, rather than fusion.
3. CLASSIFICATION BASED ON RESISTANCE TO TEMPERATURE:
The refractory materials are divided into the following two categories as per their capacity to resist temperature.
i. Low quality refractory materials.
ii. High quality refractory materials.
The low quality refractory materials are used in the manufacture of fire bricks, as lining material for furnaces, etc. The melting point of such materials is more than 1580°C.
The high quality refractory materials are stable even at high temperature and they are used in the construction of modern aeroplanes such as rockets, jets, etc. These materials are composed of either pure clay or metals or combinations of clay and metals.
The high quality refractory materials containing pure clay are pure oxides of alumina, magnesia etc or nitrides or carbides. Those metals which melt at a temperature of about 1600°C can be used as the metal refractories. Such metals are molybednum, tungsten, zirconium, etc. These materials and their alloys are used as the refractory materials.
PROPERTIES OF REFRACTORIES:
1. Fusion Point-
Since a large number of metallurgical and other operations are performed in
industries at very high temperatures using refractories, the refractories must be able to withstand high temperatures and must remain unaffected at the temperature of the chemical process being carried out. Fusion point, which is the temperature at which the refractory starts to soften, is an index of its suitability for a particular process at a given temperature, and determines the temperature upto which a particular refractory can be used.
2. Chemical Properties-
Since refractories are acidic, basic and neutral, it is extremely important to select appropriate refractories to withstand the chemical action of slags, fuel ashes, furnace gases as well as products such as glass or steel. An acidic brick should not be used in contact with an alkaline product or vice versa. Chemical interaction of a refractory with its environment is a very important factor contributing to its slow withering away on continuous use.
The rate of decay of the refractory depends mainly upon the following important factors-
(a) Composition of the environment such as nature of slags, fuel ashes, furnace gases etc, which react with the refractories.
(b) Temperature of the material which is in contact with the refractory.
© The intensity of the environmental turbulance.
3. Porosity-
Porosity is directly related to many other physical properties of brick, including resistance to chemical attack. Greater the porosity of the brick, more easily it is penetrated by molten fluxes and gases. For a given class of brick, the one with the lowest porosity has the greatest strength, thermal conductivity and heat capacity. Hence porosity of a refractory is the deciding factor of the degree of penetration by molten fluxes and gases and brings about disintegration. So, greater the porosity, greater is the susceptibility of the refractory to chemical attack by molten flux based gases.
4. Spalling-
Refractories should be able to withstand spalling, e.g. cracking and flaking of the bricks due to uneven expansion or contraction.
Spalling is thus the fracture on flaking off a refractory because of uneven expansion of a refractory due to heat. Refractories which expand unevenly by heating usually undergo flaking or disintegration, when they are subjected to rapid heating or rapid cooling.
Spalling may be due to- (1) Thermal agitation
(2) Mechanical causes
(3) Structural factors etc.
Thermal agitation are caused by rapid volume changes due to wide temperature fluctuations taking place rapidly. Mechanical causes, such as careless removal of slag and clinkers from the surface of refractories are also responsible for spalling. This damage by spalling can be reduced or minimised by decreasing the temperature gradient through suitable insulation. Spalling is also caused by structural factors creating zones of different strengths and expansion coefficients. These factors are generally created through creation of different crystallographic forms at different temperatures in reversible manner, and creating zones of different composition and properties as a result of reaction of refractories with fluxes and slags.
5. Strength-
Refractories must also be able to withstand abrasion or erosion of the furnace charge and also pressure of the load. Strength is the resistance of the refractory to compressive loads, tension and shear stresses in cold as well as hot working conditions.
6. Thermal Conductivity-
The thermal conductivity of densest and least porous refractory brick is highest. This is probably due to the absence of air in voids.
Hence least porous bricks having high density have the highest thermal conductivity which increases when the porosity decreases. The entrapped air serves as a non heating conducting material and high thermal conductivity of least porous bricks is due to the absence of air in voids. Even with same degree of porosity the conductivity of different refractories is not same. Moreover, depending upon the purpose of refractory for which it is used, sometimes, high thermal conductivity is required and sometimes low.
Conductivity curves are very helpful in the selection of refractory for a specific purpose to be served at a particular temperature.
7. Resistance to Rapid Temperature Changes-
They should also be able to withstand sudden changes in temperature during the introduction of cold charge or sudden rush of cold air in empty furnace. The refractories having low thermal expansion and coarse texture have been found to have greater resistance to rapid fluctuations in temperature. Heating of refractory for a sufficiently long time at very high temperature causes complete mineral inversion and hence brings about resistance to rapid changes in the temperature.
8. Heat Capacity-
Furnace heat capacity depends upon-
(1).Thermal conductivity
(2).Specific heat and
(3).Specific gravity of the refractory.
The light weight bricks, which absorb low quantity of heat are used in furnaces operated intermittently. This is due to the fact that working temperature of the furnace can be achieved in less time with fuel. The dense and heavy clay fire bricks, are on the other hand, suitable for regenerator checker work as in coke ovens, glass furnaces and stoves for blast furnaces.
MANUFACTURE OF REFRACTORIES:
The manufacture of refractories consists of the following important steps:-
(1) Crushing
(2) Grinding
(3) Screening
(4) Mineral Dressing
(5) Storage
(6) Mixing
(7) Moulding
(8) Drying
(9) Firing
1. Crushing-
The clays in the form of big lumps are crushed to suitable size in single or double roll crushers, jaw crushers or roll crushers. For crushing harder clays e.g. grog, jaw crushers are most suitable.
2. Grinding-
After crushing the lumps down to 25mm in size, the materials are ground in suitable grinding machines. Mechanical crushing machines such as stone crushers or jaw crushers and Alsing cylinders or ball mills are used for this purpose. Hardinge conical mills have also been used.
3. Screening-
Screening is carried out in order to separate fine particles from coarse material.
After screening, the desired size material is passed on to the brick making machine and over size material is recycled to grinding machine, in order to get again the particles of desired size. Shaking screen, closed type screen and air separators are used for this purpose.
4. Mineral Dressing-
In order to produce good refractories, it is most essential that the raw materials should be as pure as possible. So in order to purify the raw materials, material dressing is used. Some methods of concentration are given below:-
i. Tabling- The undesired foriegn materials present as impurities have different specific gravity from main raw materials. The impurities are, therefore, separated by air or water table. The table is kept inclined as well as vibrating , as a result of which fine ground materials are passed over the table and more or less a good separation is achieved.
ii. Settling- Settling is carried out in centrifuges and thickeners.
iii Floatation- In this method, the finely crushed mineral is stirred in water with a suitable froathing agent, as a result of which a multitude of fine bubbles is obtained due to froath floatation technique. By using a suitable froathing agent, it is also possible to get the bubbles so that they can carry certain types of particles with them and float them to the top of the water, where they are carried off.
iv. Magnetic and Electrical separation- The under material like iron oxides and sulphide, garnet and micas etc., may be separated by using a powerful magnet. Electrical separation is used to separate one type of mineral from another due to different attractive forces in the presence of a strong electric field.
(5). Storage-
After mineral dressing, the pure raw materials are kept in storage basins.
(6). Mixing-
The function of mixing is the distribution of plastic material so as to coat throughoutly the non-plastic constituents. Mixing provides a lubricant during the moulding operation and permits the bonding of the mass with minimum voids.
(7). Moulding-
Since refractory bricks of greater density, strength, volume and uniformity are used in large number of factories, the dry pressure method of moulding is used.
When moulding is done by hand the density and strength of the refractory is not as high as in the case of mechanical moulding. The dry pressure method is particularly applicable to batches that consist primarily of non-plastic materials.
In order to use high pressure forming, the deairing of the refractory material is essential, in order to avoid laminations and cracking when the pressure is released. The de-airing is done either by applying vaccum in the moulds, or by double pressing of the material.
(8). Drying-
Drying is carried out to remove the moisture from refractories. Drying is performed very slowly under specified conditions of humidity and temperature depending upon the refractory. Rate of drying should be so maintained that neither voids are left in the refractory nor shrinkage causing in the production of internal stresses takes place.
(9). Firing or Burning-
The refractories are fired in order to stablize and strengthen the structure. Firing or Burning is usually carried out in down draught kilns. The function of burning or firing is vitrification and development of stable mineral forms.
FIRE CLAY BRICKS-
Fire clays are the most widely used refractory materials since they are suitable for a variety of applications.
Fire clays are hydrated aluminium silicates having the general composition, Al2O3, 2SiO2, 2H2O, with impurities of sand and gravel, alkalies, oxide, sulphate and sulphide of iron, silicates and carbonates of Ca and Mg and small quantity of TiO2. Fire clays form acidic refractories and may be of three different types. These are flint or hard clay, plastic soft clays and clays of intermediate character between hard clay and soft clay. In the manufacture of fire clay bricks both flint and plastic clay have been used.
Fire clay goods are composed of fire clays or china clays, with the addition, in case of plastic clays, of grog or free silica. Grog is nothing but broken granulated fired refractory clay and is made from rejected fire clay works, broken saggers and crucibles etc. For best fire clay goods, it is desirable to use only the grog made of fire clay.
Fire clay is dug and then allowed to weather for a long time in order to increase its plasticity. Weathering may also be replaced by de-airing for the same purpose. Clay and grog are now mixed in a plug mill and requisite amount of water is added. In modern factories, clay powder and grog of different grain sizes are collected in hoppers placed in an upper story so that they come down under gravity into the plug mill or other mixing machine. A more uniform and intimate mixture is obtained, if dry clay is mixed with the grog and desired amount of water is added after mixing. If the grain size of the grog used is large, then the resulting goods are more resistant to sudden temperature changes, but grog has a very small grain size, the fire clay material is less porous and more resistant to chemical attacks.
Fire clay bricks are then shaped by any of the following methods:-
(1) Hand moulding from plastic mixture.
(2) Machine pressing from a coarse damp dust.
(3) Extrusion through dies of a plastic material.
(4) Tamping of damp dust or plastic clay by rammers into strongly made moulds.
(5) Casting.
When moulding is done by hand the mixture is rammed and beaten into moulds so that no air pockets remain in the refractory. Drying of moulded refractories is carried out by keeping the moulded refractories in a room where there is no direct sun for a number of days. In order to avoid uneven drying, the heavy refractories are sometimes covered with clothes, gunny bags etc., especially in summer days, when air is very dry. After drying, the refractories are fired by arranging in kilns, which may be single kilns or continuous kilns. Temperature is increased slowly according to a schedule and then slowly cooled.
Various types of kilns such as down draught intermittent ovens, continuous chamber kilns, tunnel kilns have been employed for firing of fire clay refractories. Usually time required for firing the fire clay refractories varies from six to ten days, according to the kiln used and the type of materials to be fired. The temperature of firing also varies from 1200 C - 1400 C and in the case of fire clay goods rich in alumina, the temperature of firing is 1400 C or even more. Cooling process also takes about seven to ten days, according to the type of kiln employed. Extreme care should be taken during cooling in order to avoid cracking of the ware as a result of sudden decrease in temperature.
VARIETIES OF FIRE CLAY BRICKS:
There are three types of fire bricks-
(1) Acidic bricks
(2) Basic bricks
(3) Neutral bricks
(1). Acidic Bricks:
These are used for acidic lining. Following are their different types:
i. Ordinary fire bricks- These bricks are prepared from natural fire clay and they provide a good material for acidic refractory lining.
ii. Silica bricks- These bricks contain a very high percentage of silica to the extent of about 95 to 97 per cent. A small quantity of lime , about 1 to 2 per cent , is added to work as binding material. These bricks can stand a high temperature upto about 2000 C. The compressive strength of such bricks is about 15 N/mm2.
(2). Basic Bricks:
These bricks are used for basic lining and basic refractory materials are used in the manufacture of such bricks. The magnesia bricks are prepared from lime and magnesia rocks. The dolomite may also be adopted for the manufacture of these bricks.
(3). Neutral Bricks:
These bricks are used for neutral lining. They offer resistance to the corrosive action of slags and acid fumes. As compared to the basic bricks, the neutral bricks are more inert to the slags. Following are the types of neutral bricks-
i. Chromite bricks- These bricks are prepared from the mixture of chrome, iron ore, ferrous oxide, bauxite and silica. Such bricks are unaffected by acidic or basic action.
ii. High alumina bricks- These bricks contain a high percentage of alumina and they are found to be more inert to the slags.
HIGH ALUMINA-
High alumina bricks are made from clay rich in bauxite and diaspore and usually embrace those which contain more than 45% alumina. The alumina content increases the refractories and the temperature of incipient vitrification. These refractories are practically inert to carbon monoxide and are not disintegrated by natural gas atmosphere upto 1000 C.
USES OF HIGH ALUMINA BRICKS:
Bauxite bricks are more costly to produce than fire clay bricks and they are, therefore, used in special circumstances. They are usually used where very high temperatures are expected or where resistance to basic slag is required. High alumina bricks are employed in the cement industry and paper mill factories. They are also used in the linings of glass furnaces, oil fired furnaces, high pressure oil stills and in generator checkers of blast furnaces.
SILICA BRICKS-
Manufacturing silica bricks contain about 95-96% SiO2 and about 2% of lime added during grinding to furnish the bond. Silica and a bonding material are thus the two important raw materials used for the manufacture of silica bricks. Gannister, a hard, dense, fine grained variety of 95-96% pure silica is a good refractory material. Silica bricks fall into pieces, if they are heated or cooled rapidly. They do not contain clay, but the physical strength of silica bricks, when heated is much higher than those made of clay. As a result, they are very suitable for arches in large furnaces.
The raw material most widely used for the manufacture of silica bricks is quartzite, which is a rock composed crystals of almost pure silica, bound together by a cement or similar material in such a manner as to produce an almost smooth surface. The binding materials are lime, clay, sodium silicate, alum, aluminium sulphate, magnesia, magnesium silicate, plaster of paris, colloidal silica, calcium phosphate, barium sulphate and even waste products such as tar, molasses, pitch, heavy mineral oils etc.
Silica bricks have a permanent expansion, which takes place during firing. When reheated, silica bricks again expand about 1.5% but the effect is reversible, the bricks returning to original size when cooled. They have a very homogeneous texture, free from air pockets and moulding defect and posses a low porosity. Furnaces using these bricks must be heated or cooled gradually, in order to reduce spalling and cracking.
USES OF SILICA BRICKS:
Open hearth furnaces have silica bricks in their main arch side walls. Because of their high thermal conductivity, silica bricks have been utilized in co-product coke oven and gas retorts. In general, silica bricks can be used where no shrinkage in refractory and high resistant to heat is required, because silica bricks have a tendency to expand on heating.
MAGNESITE BRICKS-
Magnesite is naturally occurring magnesium carbonate which constitutes the raw material for magnesite refractories and in nature it occurs as crystalline magnesium spar or crypto crystalline magnesite. The magnesite bricks are basic in nature and used in situations, where a highly basic refractory is required or where slags of basic nature are formed.
USES OF MAGNESITE BRICKS:
These are used in open hearth and electric furnace walls, in the burning zones of cement kilns, and in roofs of nonferrous reverberatory furnaces. Hard burned chrome magnesite bricks are used in the basic open hearth furnaces. On a commercial scale, magnesite refractories have been used in the construction of soaking pits, basic open hearth furnace, arc furnaces, hot metal mixers, converters for Cu, Pb, Ni etc, and furnaces for refining noble metals.
FORSTERITE BRICKS-
Forsterite is used as a base for high temperature refractories. In the manufacture of forsterite refractories, dead burned magnesite is usually added to convert some accessory minerals also to forsterite, which is the most stable silicate at higher temperatures. These refractories have high melting point and unsurpassed volume stability at higher temperatures and no calcination is necessary in their preparation.
USES OF FORSTERITE BRICKS:
Forsterite refractories are used in glass tank superstructures, open hearth end walls and copper refining furnaces.
DOLOMITE BRICKS-
Dolomite bricks are made by mixing calcined dolomite mixture in equimolecular proportions with silica as binding material. Other binding materials used for the purpose are tar, magnesium silicate, basic slags, quick lime, iron ore or iron oxide, clays etc.
In the manufacture of dolomite, having the composition CaMg(CO3)2 is calcined and mixed with bonding agent and water in an edge runner or plug mill. The mixture is allowed to age by storing in wet conditions. Finally it is moulded to bricks by hand moulds or pressing. The moulded bricks are air dried and fired at 1500°C for about 24 hours.
USES OF DOLOMITE BRICKS:
Dolomite is rarely used as a direct refractory, but is useful as a repair material. Stabilized dolomite bricks have been used for basic electric furnace linings, Bessemer converters, open hearth furnaces, laddle linings etc.
CHROMITE BRICKS-
Chromite is a neutral refractory and hence very valuable as a contact material for basic materials. The method of manufacture of chromite bricks is similar to that for magnesite bricks, but without preliminary calcination.
USES OF CHROMITE BRICKS:
Chromite bricks are used in steel furnaces, copper works, antimony and lead furnaces. It is also used in making hearths of industrial reheating furnaces, inner lining of acid converters used in the extraction of Cu from low grade ores and also as a buffer between acid and basic linings in the basic open hearth furnaces and in the bottom of soaking pits as well.
CONCLUSION:
Refractory bricks are built primarily to withstand high heat but should also usually have a low thermal conductivity to save energy.
Firebrick should not spall under rapid temperature change and their strength should hold up well during rapid temperature changes. Either dense or porous firebricks can be used depending upon the situation. Usually dense firebricks are used in applications with extreme mechanical, chemical, or thermal stresses, such as the inside of a wood-fired kiln or a furnace, which is subject to abrasion from wood, fluxing from ash or slag, and high temperatures. In other, less harsh situations, such as a natural gas fired kiln, more porous bricks are a better choice.
Thus firebricks are used for basic electric furnace linings, Bessemer converters, open hearth furnaces, laddle linings etc, and find most of its applications in industries where high temperatures have to be encountered.