In this article we will discuss about:- 1. Meaning of Refractories 2. Properties of Refractories 3. Refractory Fibres 4. Uses.
Meaning of Refractories:
The term refractories embraces all materials used in the arts for the construction of heat resisting containers, using the word in its broadest sense, whether it be to afford space for the evolution of gases in combustion processes or the holding of molten charges or of solids undergoing heat-treatment.
The two principal functions involved in the use of refractory materials are to those of thermal insulation and conduction. In the outside walls of a furnace, refractories serve the purpose of confining the heat and preventing excessive loss to the atmosphere; in a muffle or retort, they serve to conduct the heat through the walls to the charge.
Although, in general, the heat resisting quality of refractories is of paramount importance, this is by no means the only requirement and may at times be only a secondary consideration. Refractories may be expected to be relatively unaffected by high temperatures under stress whether negligible or heavy; to resist mechanical abrasion at various temperatures; to resist the intrusion of molten metals, slags or metallic vapours, as well the action of superheated steam and hydrocarbons, sulphurous oxide, chlorine or other gases; and to withstand sudden temperature changes.
ADVERTISEMENTS:
Under one set of conditions, high thermal conductivity may be required and under another high insulation value, while in still other cases high electrical resistance at moderately high temperatures may be demanded. It is obvious that no single refractory will completely fulfill all these functions, and hence the proper selection of a suitable material is often a complicated task.
Properties of Refractories:
A good refractory material should have the following properties:
1. It should be able to withstand high temperatures generated in the furnace.
2. It should be able to withstand sudden alternating heating and cooling, i.e., thermal shocks.
ADVERTISEMENTS:
3. It should be able to withstand abrasion and rough usage.
4. Its contraction and expansion due to the inevitable temperature variation should be minimum possible.
5. It should be able to withstand fluxing action of the slags and the corrosive action of gases.
6. It should have good heat insulating properties.
ADVERTISEMENTS:
7. It should be chemically inactive at elevated temperatures.
8. It should be impermeable to gases and liquids as far as possible.
9. If used in electric furnaces, it must have low electrical conductivity.
Types of Refractories:
According to their chemical behaviour, the refractory materials which are mainly oxides or carbides of metals or non-metals or their mixtures, are classified as follows:
ADVERTISEMENTS:
1. Acid refractories.
2. Basic refractories.
3. Neutral refractories.
ADVERTISEMENTS:
Acid refractories are those which really combine with bases and are therefore, termed “Acid”. They are materials consisting of silica or containing silica as their chief constituent greatly in excess of the bases present. The important acid refractories are quartz, sand, ganister, dinas rock etc. Most tire-clays contain SiO2 in excess of the amount indicated by the formula Al2O3.2SiO2.2H2O and are therefore, classed as acid.
Silica:
The pure silica fuses at a very high temperature, viz., 1713°C, but when heated in contact with basic substances it forms silicates, some of which fuse easily. Therefore, the presence of bases in silica to be used as refractory materials is to be guarded against. Silica occurs as quartzite, Dinas rock and ganister. Ganister is a naturally occurring high siliceous rock (98% SiO2). In the prepared state it is used in the form of bricks. Good deposits of silica in India occur in the Gaya and Monghyr districts of Bihar.
Silica bricks are made from hard, dense fine grained quartzite (known as ganister). After crushing, the rock is ground with water and mixed with about 2 percent lime. From this mixture, bricks may be moulded by hand or by power press; after drying in hot air, they are baked for proper time and at the proper temperature. The lime being base reacts chemically with acidic SiO2 and fuses at a number of places in the brick and gives its strength. The bricks expand 3.5 percent after the baking is finished.
Silica bricks possess the following properties:
1. Low porosity and free from air pockets.
2. Good thermal expansion and conductivity.
3. Remarkable load bearing capacity especially at high temperatures.
4. Not susceptible to thermal spalling at temperatures above 400°C.
5. Ability to withstand thermal shock poor.
Silica bricks are extremely suitable for those parts of a furnace which are subjected to uniformly high temperature, e.g., fire bridges, roofs of Acid and Basic Open and Hearth, Electric furnaces, copper smelting and refining furnaces etc.
Fire Clay:
The main constituent of fire-clay is kaolinite, Al2O3 . 2SiO2 . 2H2O. The fire clays, however, contain varying proportions of alumina and silica. Impurities in fire clays such as alkalies, sand, oxides of iron and silicates of calcium and magnesium adversely affect their properties. At high temperature the fire clays lose their water of hydration and mainly consist of alumina and silica.
The fireclay bricks are manufactured in the same manner as silica bricks. The clay in a finely crushed state is mixed with a definite amount of water in a pug mill. The mixture is then pressed into moulds, dried and finally burnt. To control the shrinkage, and accelerate the rate of production about 20 to 80% of burnt or calcined clay called Grog is mixed in the clay, while using flint clay, plastic clay is employed as binder. By altering the properties of flint clay and plastic clay in the brick mixture the properties of the bricks made can be controlled.
Important properties of fire-clay bricks are:
1. Coefficient of thermal expansion is low.
2. Sufficient strength at high temperature.
3. Resist spalling.
4. Not so refractory as silica bricks but much cheaper.
Fire clay bricks are used for the following:
1. Linings of blast furnaces for the melting of iron, copper, lead ores etc.
2. The linings of flues and shocks.
3. Heat treatment furnaces.
4. Glass furnaces.
5. Checker-work of regenerative furnaces.
6. Reverberatory furnaces.
7. Pottery kilns.
Basic refractories consists mainly of basic oxides without free silica and resist the action of bases. The most common basic refractories are Magnesite, Dolomite, lime etc. But lime, because of its dehydrating tendency is never used as a refractory.
Magnesite:
This is most important basic refractory material. It is made by “dead burning’ the mineral magnesite which is essentially magnesium carbonate. At any temperature above about 700°C, magnesite is dissociated to magnesia and carbon dioxide, but the properties of the resultant product differ greatly according to the temperature of burning.
Below 900°C the product is “caustic magnesia”, which is readily hydrated to magnesium hydroxide, and is used in cement. Microscopic examination shows no crystals, so that it is often called amorphous magnesia. Prolonged firing at 1800°C gives an insoluble, unreactive “dead burnt” magnesite, easily recognized as the crystalline mineral.
In the manufacture, magnesite is first “dead burnt” to remove shrinkage as much as possible, then crushed, graded for minimum porosity, bonded, and moulded under hydraulic pressure. Bonding is done by adding an organic binder, or by use of a little caustic magnesia, which sets to a hydrated cement with water, or the crushed, deal burnt magnesite itself is allowed to stand in water for some days until the surface of the particles has undergone hydration.
Magnesite possesses the following properties:
1. Thermal conductivity greater than that for fire-clays and silica.
2. Highly resistant to the action of basic slags and iron oxide.
3. Starts losing strength at temperatures above 1500°C.
Magnesite bricks are not capable of resisting sudden changes of temperature and show a tendency to spall under such conditions. They are costlier than silica and fireclay bricks and used in the construction of those parts of a furnace which are required to withstand the corrosive action of basic slags.
They are used for:
1. Hearths of basic open-hearth and copper reverberatory furnaces.
2. Electric arc and induction furnaces.
3. Lining of L.D. crucible (in the manufacture of steel).
Dolomite:
It is a double carbonate of calcium and magnesium and occurs abundantly in India. It is burnt generally in shaft furnaces. Dolomitic bricks have low conductivity and refractoriness as compared to magnesite bricks. Calcium oxide present in the bricks has a tendency to combine with water and carbon dioxide present in the atmosphere, and causes disintegration of the bricks. They are cheaper than magnesite bricks but are poor in performance. It is extensively used for repairing the banks and the bottoms of the Basic Open- hearth furnaces.
Burned bricks made from dolomite have not been successful in service. It is possible to produce brick that will not hydrate under reasonable treatment, but the stabilizing agents added cause high shrinkage when the brick is heated to high temperature. Such bricks are also sensitive to spalling influences.
Neutral refractories are substances which do not combine with either acidic or basic oxides and for this reason constitute the most satisfactory furnace lining, e.g., chromite and carbon in graphite form.
Chromite:
Chromite refractories are made from core ores which mainly contain chromite, FeO . Al2O3. The ore is finely powdered and mixed with a small amount of binding material, such as fire-clay, bauxite and magnesia. The mixture is moulded into bricks, dried and burnt in a kiln at a temperature of about 1500°C to 1700°C.
Chrome bricks possess the following properties:
1. Low porosity.
2. High resistant to both acid and basic slags
3. High thermal conductivity.
4. Low resistance to spalling.
Chrome bricks are extensively used in steel industry for lining open hearths and also in the bottom of soaking pits.
Graphite:
It is a form of pure carbon and is natural product. It occurs mixed with calcareous and siliceous rocks. It is employed in making graphite crucibles using clay as the bond.
Graphite bricks have the following properties:
1. Reduced oxidising action (on the metal) at high temperatures.
2. Resist acid and basic slags.
3. Free from scaffolds formation due to the adherence of metals to the lining.
Kaynite:
Its composition is Al2O3 . SiO2 (Disthene) and crystals are triclinic. It decomposes at a temperature of 1350°-1330°C. Its hardness is 5 to 7 mhos.
Sillimanite:
It occurs naturally in Assam and Rewa in India. Its composition is Al2O3 . Si2 (flbrolite) and occurs as long needle-shaped crystals. It has high softening point (above 1800°C). It has very low electrical conductivity. It is employed for making blocks, bricks, crucibles, refractory fittings for electrical goods and tubes for surface combustion.
In the Table 6.1 are shown the common refractory materials with their chemical composition, melting point etc.
Refractory Fibres:
These are made from high purity alumina and silica grains melted in electric furnace and blasted by high velocity gases into light fibres.
Ceramic fibres are available in the form of felts, blocks and blankets.
The heat losses through ceramic fibres are low compared to other refractory materials.
They have excellent temperature resistance, outstanding thermal stability and good resistance to vibration.
Refractory fibres, in general, are considered in four broad categories namely:
(i) Alumina silica fibres.
(ii) Pure silica fibres.
(iii) Pure metal oxide fibres.
(iv) High silica leached fired glass fibres.
Uses of Refractories:
1. Because of low thermal conductivity the refractory ceramic fibres are fast replacing and are used for lining of furnaces, kilns and other high temperature equipments.
2. Silica fibres are very expressive, hence, limited industrial use. They find applications in aerospace industry.
3. Alumina silica fibres are generally used from 1000 to 1500°C temperature because of their low cost and good thermal properties (they have average diameter ranging from 1.5 to 3.0 microns).
4. Metallic oxide, alumina fibres are used for 1500 to 1600°C.