List of Engineering Materials and their Properties!
Engineering Material # 1. Abrasives:
An abrasive is a substance which is used to remove excess material by cutting. It polishes soft material and gives it an attractive appearance.
The abrasives are the hardest materials. They are used for the engineering purpose in the following forms:
(i) As abrasive papers or cloths,
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(ii) As abrasive stones,
(iii) As grinding pastes with oil,
(iv) In grinding wheels, and
(v) With some suitable bonding materials.
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The abrasives may broadly be divided into the following two categories:
(i) Natural abrasives, and
(ii) Artificial abrasives.
The natural abrasives occur in nature. The most commonly used natural abrasives are diamond, garnet, flint, emery, corundum, carbonado and sandstone.
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The diamond is used for polishing precious stones. The garnet is used for polishing glass. The emery is used in papers and cloths and in grinding stones. The carbonado is used in rock drills, grinding wheels and cutting stones.
With the development of industries, it was found necessary to prepare artificial or synthetic abrasives.
These abrasives possess the following advantages:
(i) The amount of abrasive material is more in synthetic wheels.
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(ii) The manufacture of synthetic abrasives permits exact duplication of the product.
(iii) The synthetic abrasive can be prepared of desired quality.
(iv) The synthetic wheels of desired size and shape can be prepared.
The most commonly used synthetic abrasives are carbides of aluminium, boric acid and silicon.
Engineering Material #2. Adhesives:
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An adhesive is a substance which is used to join two or more parts so as to form a single unit.
The application of adhesive has the following advantages over the conventional methods of bolting, riveting and welding:
(i) A wide variety of combinations in joining is possible.
(ii) It can be used for bonding the surfaces of glass, metal, plastics and wood.
(iii) It creates a massive effect.
(iv) It is possible to prevent corrosion between different metals joined by adhesive.
(v) It produces adequate strength.
(vi) The permeable joint can be made impermeable for water and gas by the application of adhesives.
(vii) The process of applying adhesive is easy, economical and speedy.
The disadvantages of adhesives are as follows:
(i) It is not possible to adopt any adhesive for all substances. Depending upon the properties of substances to be joined, suitable adhesive has to be selected.
(ii) The adhesive substance does not become strong immediately after its application. It requires some time to attain the desired strength.
(iii) The adhesive substance generally does not remain stable at high temperature.
Following factors determine the quality of an adhesive:
(i) Its degree or intensity of sticking,
(ii) Its durability,
(iii) Its resistance to heat,
(iv) The strength of bond developed after drying or setting,
(v) The time required to develop the required bond, etc.
The glue is a general term which is used to indicate an adhesive substance.
Following are the various types of adhesives:
(1) Albumin Glues:
It is a glue of better quality. It is not attacked by water. It is used for making furniture.
(2) Animal Protein Glues:
It is obtained by boiling waste pieces of skins, bones, etc. of animals with hot water. The animal glue develops strong and tough joints and it is easy to apply. But it is affected by damp and moist conditions. It is available in the form of cakes, flakes, granules, jelly, pearls and sheets. It is used in the manufacture of plywood, laminated timbers, etc.
(3) Glues from Natural Resins:
It is prepared from natural resins. It is used for labelling building paper, etc.
(4) Glues from Synthetic Resins:
These glues are based on synthetic resins. They may either be thermo-setting glues or thermo-plastic glues. The thermo-setting glues become permanent, once they are set. The thermo-plastic glues can be made plastic again by reheating.
All synthetic glues are fire-proof, strong and water-proof. The setting time of synthetic glues can be regulated by varying the type or kind and quality of the hardener. They resist the attack by fungi and they possess non-staining qualities.
The synthetic resins are mainly of four types:
(i) Melamine Resins:
They require heat and pressure for setting. They are used in the manufacture of plywood.
(ii) Phenolic Resins:
They are available either in liquid, film or powder form. They require heat and pressure to form a permanent strong bond. They are used in the manufacture of resin-bonded plywood.
(iii) Resorcinol Resins:
They are in the form of dark viscous liquids. They are resistant to bacteria, fungi, heat and moisture. They become hard in short time at low temperatures.
(iv) Urea Resins:
They are available in the form of syrups and powders. They are extensively used in joinery work to form water-resistant glue joints.
(5) Nitrocellulose Glues:
It is prepared from pyroxilin which is a nitrated cellulose. It is derived by treating cellulose with nitric acid. It produces films which strongly adhere to the glass.
(6) Rubber Glues:
It is prepared by dissolving rubber in benzene. It is used for joining rubber, plastics, glass, etc.
(7) Special Glues:
These are specially prepared to join metals. The cycle weld is a modified form of rubber and it is used to join aluminium sheets. The araldite is another variety of special glue. It is used to join the light metals.
(8) Starch Glues:
It is prepared from vegetable starch. It has good strength in dry condition. But it is not moisture resistant. It is cheap and is used for inferior quality of plywood.
(9) Vegetable Glues:
It is prepared from natural gums and starches. It is used for preparing paper board articles, labelling, etc.
Engineering Material # 3. Asbestos:
The asbestos is a naturally occurring fibrous mineral substance. It is composed of hydrous silicates of calcium and magnesium (CaSiO3, 3MgSiO3). It also contains small amounts of iron oxide and alumina.
The natural asbestos can be divided into two groups, namely, acid-resistant asbestos and non-acid-resistant asbestos. The first group comprises actinolite asbestos, amosite asbestos, anthophyllite asbestos, crocidolite asbestos and tremolite asbestos. The second group comprises only chrysolite asbestos which is of great industrial importance.
Following are the properties of asbestos:
(i) The holes can be drilled and screws can be fitted on its surface.
(ii) It can be cut into pieces.
(iii) It is an excellent insulator for heat and electricity.
(iv) It is fire-proof and acid-proof.
(v) It is flexible, soft and non-porous.
(vi) It is smooth like glass and silk.
(vii) It possesses a good adsorption capacity. When it is mixed with cement and cured with water, it adsorbs i.e. retains firmly on its surface. Thus the asbestos cement items can be considered as reinforced cement stones with reinforcement in the form of asbestos fibres.
(viii) Its colour is brown, grey or white.
(ix) Its melting point is 1200°C to 1550°C.
(x) Its molecules are strongly bound together only in one direction and that is why it possesses very high tensile strength along the fibres.
(xi) Its quality is critically affected by the length of fibres and hence this characteristic of asbestos serves as a basis for classifying asbestos into different grades.
(xii) Its specific gravity is 3.10.
Following are the uses of asbestos:
(i) The asbestos cement products have become very popular at present. They are prepared by mixing asbestos fibres with cement. They include sheets and pipes. The sheets are used as roofing material and the pipes are used to convey rainwater, seepage water, etc.
(ii) The asbestos felt can be prepared by coating asbestos fibres with bitumen and it is used as damp-proof layer.
(iii) It is used as the covering material for magnetic coils.
(iv) It is used as the lining material for fuse box and switch box.
(v) It is used for insulating boilers, furnaces, etc.
(vi) It is used for preparing fire-proof clothes, ropes, etc.
(vii) It is used to form the asbestos paint.
In general, it can be stated that the uses of asbestos are daily growing – roofing, home appliances, pipes, textiles and packing, clutch facings, brake linings, gaskets, etc.
The term asbestos cement is used to mean an artificial stone material which is obtained by hardening a mix of cement, asbestos and water. The asbestos serves as reinforcement and it assists in providing high strength in tension and bending to asbestos cement items. The percentage of asbestos in asbestos cement items will depend on the type and quality of asbestos. But it generally varies from 10% to 20%.
The quality of asbestos cement products depends greatly on the quality of asbestos and fineness of cement. The moulding of these products requires longer time than that of concrete items. Hence it is necessary that the initial setting of cement for asbestos cement products should begin somewhat later than for common Portland cement and it should preferably be not less than 90 minutes from the time it is mixed with water.
The manufacture of asbestos cement products will require a great amount of water for preparing the asbestos cement mix and for washing the machines. The water to be used for this purpose should be free from clay impurities, organic substances and mineral salts.
The properties of asbestos cement include high mechanical strength in bending, frost resistance, low heat conductivity, high resistance to attack by mineralised aqueous solutions, high water tightness and small bulk density.
However the asbestos cement is brittle and it warps due to changes in humidity and its strength is lowered when saturated by water. The coloured asbestos cement products can also be prepared by the use of dyes and coloured enamels, resins and varnishes.
As per recent research, it has been found that asbestos dust produces a disease very similar to silicosis which results in fibrosis of the lungs called the asbestosis. It is also observed that about half the persons who get asbestosis also get lung cancer. It has also been claimed by some experts that a five minute inhalation of blue asbestos dust can produce cancer upto 20 years afterwards.
It is due to the fact that the asbestos fibres are capable of being separated to extreme fineness and into dust. The minute free-floating particles may be directly inhaled or ingested through contaminated food. The diseases caused by the asbestos are self-spreading, incurable and fatal.
Thus the asbestos which finds a wide variety of uses in industry and in homes has emerged as a versatile killer in the eyes of some people. In a similar way, the exposure to the asbestos dust can have harmful effects on the health of workers engaged in industries handling asbestos. Thus the greatest danger from asbestos is to the workers in the asbestos factories and mines.
It is equally fatal for the garage mechanics who change brake linings, carpenters and masons who saw asbestos sheets, shipyard workers who handle asbestos dust, construction workers who demolish old buildings or build new ones with asbestos sheets, etc. It could be just anybody exposed to the asbestos.
It is therefore absolutely necessary to follow strictly the rules and standards laid down to prevent, control or minimize the risk of exposure in asbestos mines and factories producing asbestos based products. In almost all the developed countries, the use of asbestos is being discontinued and it is being substituted with other safe materials like galvanized steel sheets, nylon and other petro-products, etc.
The Consumer Education and Research Centre of Ahmedabad filed a writ petition against the asbestos industry and with its tire-less efforts and persistence for more than 10 years, it was able to achieve the landmark judgement from the Hon. Supreme Court of India.
The salient aspects of the judgement are as follows:
(i) All asbestos industries should maintain the health record of every worker upto a minimum period of 40 years from the beginning of the date of the employment or 15 years after retirement or cessation of employment, whichever is later.
(ii) Any worker in the industry suffering from the occupational health hazards would be entitled to a compensation of Rs. 100000/-.
(iii) It becomes compulsory to cover all its workers under the health insurance.
The above judgement will give benefit to thousands of workers in the asbestos industry and as a matter fact, the environmentalists, social action groups, etc. will remain indebted to the Hon. Supreme Court of India for this important judgement.
At a time when the global community is concerned about ecological imbalance, the above judgement will prove to be a turning point with far reaching consequences and will additionally set a precedent for other industries too.
Engineering Material # 4. Asphalt:
The asphalt is a mechanical mixture of inert mineral matter like alumina, lime, silica, etc. and the asphaltic bitumen. It is black or brownish-black in colour. It remains in solid state at low temperature and becomes liquid at a temperature of about 50°C to 100°C.
The asphalt is classified into the following two categories:
(1) Natural asphalt
(2) Residual asphalt.
(1) Natural Asphalt:
This variety of asphalt is obtained from nature. It is further subdivided into two groups, namely, lake asphalt and rock asphalt.
The lake asphalt is obtained from lakes at Trinidad and Bermudez (South America) at depths varying from 3 to 60 metres. It contains about 40% to 70% of pure bitumen. The water content is about 30%. The rest is impurities. It is refined by boiling in a tank. The water evaporates and impurities collect at the top. These impurities are removed. This refined lake asphalt is used widely for road and pavement construction.
The rock asphalt is obtained from rocks at Switzerland, France, etc. It contains about 10 to 15 per cent of pure bitumen. The rest consists of calcareous materials. The rocks are put on the road surface after being crushed and heated.
The rock softens under the influence of heat to the consistency of paste and it consolidates on cooling. The road surface is then formed by rolling. This asphalt can also be used for roofing sheets, paving tiles, etc.
(2) Residual Asphalt:
This variety is also known as the artificial asphalt. It is obtained by the fractional distillation of crude petroleum oils with an asphaltic base. Such process leaves a solid substance in the retort. This solid substance is the residual asphalt.
Following are the usual forms of asphalt:
(1) Asphaltic Cement:
It is prepared by blowing air through melted asphalt at high temperature. The asphalt is oxidized and the resulting product is plastic in nature and it is highly resistant to varying climatic conditions. It is used for flooring, roofing, water-proofing and filler in expansion joints in concrete.
(2) Asphaltic Emulsion:
It is produced by mixing asphalt with 50 to 60 per cent water in presence of 1 per cent of emulsifying agent. When water evaporates, the emulsion breaks and it forms a water-proofing layer. It can be applied in cold condition.
(3) Cut-Back Asphalt:
This is a liquid asphalt. It is prepared by dissolving asphalt in a volatile solvent. It can be applied at normal temperature in cold condition. It is used for preparing bituminous paints, for repairing roofs, etc.
(4) Mastic Asphalt:
It is produced by heating natural asphalt with sand and mineral fillers. The resulting product is a voidless impermeable mass. The hardness and melting point of mastic asphalt can be controlled during its manufacture. It may either be in solid or semi-solid state. But when heated to a suitable temperature, it can be made sufficiently fluid so as to be able to spread by a hand float. It is used as a material for damp-proofing and water-proofing.
Following are the properties of asphalt:
(i) It is a tough and durable material.
(ii) It is a water-proof material and can be easily cleaned.
(iii) It is good insulator of electricity, heat and sound.
(iv) It is non-inflammable and non-absorbent.
(v) It is not attacked by acids and is safe against vermin.
(vi) It is resilient and reasonably elastic.
Due to the properties mentioned above, the asphalt is widely used for various engineering purposes as follows:
(i) As damp-proof courses,
(ii) As water-proof layer for tanks, basements, swimming pools, etc.,
(iii) For preparing paints and roofing felts,
(iv) For constructing roads and pavements, etc.
Engineering Material # 5. Belts:
The belts are used to drive machinery or to transmit power or to run mechanical conveyors.
The brief descriptions of the materials which are usually used to make belts are given as follows:
(1) Balata:
These bells are formed by impregnating canvas with balata which is a milky fluid present in rubber trees and similar other plants. These belts are water-proof. The grease or oil does not affect them.
(2) Canvas:
These belts are prepared by impregnating canvas with different resins. They are prepared in 2 to 10 ply or layer thickness.
(3) Cotton:
These belts may or may not be impregnated with rubber compounds. They are used for driving fans in automobiles, conveyor belts, etc.
(4) Leather:
These belts should not be allowed to get dry and brittle. The good leather should not stretch by about 15% of its length and its tensile strength should be about 17.50 N/mm2. The leather belts are available in suitable widths and special water-proof leather belts which can be used in moist places are also available.
(5) Rubber:
These belts are reinforced with cotton, jute, canvas, etc. These belts are quite cheap and they are available in any lengths and widths. These belts have better grip on pulley. They also resist deterioration by moisture. Their tensile strength is quite high about 40 N/mm2. These belts should however not come in contact with grease and oils.
Engineering Material # 6. Bitumen:
The bitumen is the binding material which is present in asphalt. It is also sometimes called the mineral tar. It is obtained by partial distillation of crude petroleum. It is chemically a hydro-carbon.
It is insoluble in water, but it completely dissolves in carbon bisulphide, chloroform, benzol, coal tar, naphtha, alkalies, alkaline carbonates, petroleum spirit and oil of turpentine. It is found on analysis to compose of 87 per cent carbon, 11 per cent hydrogen and 2 per cent oxygen by weight.
The bitumen is black or brown in colour and it is obtained in solid or semi-solid state. Its applications are same as the residual asphalt.
Following are the usual forms of bitumen:
(1) Bitumen Emulsion:
It is a liquid product containing bitumen to a great extent in an aqueous medium. The bitumen is in a very finely divided state and it is suspended in the aqueous medium with the help of some suitable stabilising agents.
(2) Blown Bitumen:
It is a special type of bitumen which is obtained by passing air under pressure at a higher temperature. Such bitumen can be used as roofing and damp-proofing felts, in the manufacture of pipe asphalts and joint fillers, as heat insulating material, etc.
(3) Cut-Back Bitumen:
It is obtained by fluxing asphaltic bitumen in presence of some suitable liquid distillates of coal tar or petroleum. It can be applied cold as a bitumen paint.
(4) Plastic Bitumen:
It consists of bitumen, thinner and a suitable inert filler. The amount of inert filler is about 40% to 45%. It is used for filling cracks in masonry structures, for stopping leakages, etc.
(5) Straight Run Bitumen:
When the bitumen is being distilled to a definite viscosity or penetration without further treatment, it is known as the straight run bitumen.
Functions of Bituminous Materials:
The bituminous materials are also called binders and when they are used in combination with the mineral aggregate, they have to perform a number of functions.
The various objectives which can be achieved by using a bitumen binder in road construction can be summarized as follows:
(1) Binding Effect:
The bitumen binds the surface particles together and loss of materials from the surface by suction under the body of the moving vehicle is thus checked.
(2) Cushion:
It acts as a cushioning material on the surface and absorbs impact, friction, etc. due to movements of wheels of vehicles on road. Thus the life of pavement is considerably enhanced.
(3) Resistance to Weathering Agencies:
If a properly selected bituminous material is used, the surface can resist effectively the actions of weathering agencies like the wind and the sun, and thus a long life for the pavement is ensured.
(4) Sealing of Surface:
When used with the dense-graded granular material, it seals the surface of the road against ingress of water and thus the damage to the road due to absorption of rain water is prevented.
Engineering Material # 7. Composite Materials:
The composite materials are shortened as composites. They are formed by combining two or more different materials to make better use of their virtues and by minimizing their deficiencies. The composites can be tailored as per the requirements by using tougher and light materials. Each material retains its physical or chemical properties separate and distinct within the finished product.
The composites are made from two main constituent materials:
(1) Strong load carrying materials known as reinforcement or reinforcing fibres,
(2) Weaker material known as matrix.
(1) Reinforcing Fibres:
Following are the functions of reinforcing fibres:
(i) It provides strength and rigidity.
(ii) It helps to support structural load.
There are three most common types of reinforcing fibres:
(i) Glass fibres
(ii) Carbon
(iii) Aramid (Kevlar)
Glass fibres are the heaviest having greatest flexibility and the lowest cost. Aramid has moderate stiffness and cost. Carbon is moderate to high in cost, slightly heavier than Aramid but lighter than glass fibre. Carbon is the strongest. Hybrid composites are the composites with more than one reinforcing material.
(2) Matrix:
Following are the functions of matrix:
(i) It works as a binder.
(ii) It maintains the position and orientation of the reinforcement.
(iii) It balances loads between the reinforcement.
(iv) It protects the reinforcement degradation.
(v) It provides shape and form to the structure.
The most common type of matrix is thermosetting resins. It reacts chemically under certain time and temperature conditions. As soon as this reaction takes place, the material becomes fused in a solid state and will not melt with the application of heat. Epoxy resins are the most widely used thermosetting resins in advanced composites.
Other resins used as matrix are polyester, vinyl ester, phenolic, bismaleimide, epoxy novolac, polymide, etc. Each resin has its own practical application and is selected as per required performance and manufacturing techniques.
The composites can be natural or man-made. Wood is a natural composite of cellulose fibres with good strength and stiffness in a resinous matrix of the polysaccharide lignin. Bones, teeth and mollusc shells are also natural composites combining hard ceramic reinforcing phases in natural organic polymer matrices.
Following are the common examples of man-made composites:
(i) Wattle and daub (mud and straw) is one of the most ancient man-made composite.
(ii) Bricks made from straw and mud.
(iii) Plywood is the building block of a composite laminate. It is a thin layer of composite material in which fibre is usually oriented in one direction. Composite laminates are constructed by stacking piles in various fibre orientations as defined by the designer. The direction and number of piles are determined by analysis based upon the required strength and stiffness of the laminate in all directions.
(iv) Concrete is a classic example of a ceramic composite, which consists of the particles of sand and aggregates of graded sizes in a matrix of hydrated Portland cement.
(v) Reinforcing plastic – Fibre Reinforced Plastic (FRP), Glass Reinforced Plastic (GRP)
(vi) Disc brake pads, which consist of hard ceramic particles embedded is soft metal matrix.
(vii) Shower stalls and bathtubs, which are made of fibre glass.
(viii)Imitation granite and cultured marble sinks and countertops.
(ix) Carbon-Fibre Reinforced Plastic (CFRP) is a carbon fibre composite with an epoxy resin matrix are commonly used in the industry.
(x) Metal-Matrix Composites (MMC) like silicon-carbide-fibre-reinforced aluminium and Ceramic-Matrix Composites (CMC) like carbon-fibre-reinforced glass.
(xi) Wire-reinforced tiles, engineered wood like wood fibre board, pykrete (sawdust in ice matrix), etc.
Fig. 17-5 gives clear idea of the scope of evolution of composites from different classes of engineering materials.
Engineering Material # 8. Cork:
The cork is obtained from the bark of cork oak trees. The bark is ground, cleaned and baked. During baking, the natural resin present in cork comes out and binds the material into a homogeneous mass.
Following are the properties of cork:
(i) It can be cut and sawn without any difficulty.
(ii) It is good insulator of electricity and sound.
(iii) It is light in weight.
(iv) It is not attacked by heat and moisture.
(v) It is porous.
(vi) It is resilient and reasonably elastic.
Uses of Cork:
The cork is widely used for preparing cork sheets and boards, bottle stoppers, packing gaskets, etc. The cork carpet is available in rolls of 6 mm thickness and 1.80 m width with a variety of colours. It is suitable as a floor covering for places like churches, theatres, etc. where a noiseless covering is required.
The cork tiles are obtained by compressing cork shavings in moulds with a thickness of 12 mm. The moulded tiles are then baked. These tiles are fairly elastic, noiseless and durable. They can be used as covering for walls as well as floors.
Engineering Material # 9. Electrical Insulators:
A body is said to possess electrification when it attracts other bodies, the attraction being different from the gravitational attraction. The electrification is not a fundamental property of matter because it remains un-electrified under ordinary conditions and it becomes electrified only after the electrification has been produced by certain causes.
All substances may broadly be divided into two groups:
(1) Conductors
(2) Non-conductors.
(1) Conductors:
In case of a conductor, if one point of the body of the substance is by any means electrified, the electrification immediately spreads all over the body or in other words, the conductors are the substances which are used to produce easy and smooth flow of electricity. The conductors are metals and alloys.
The gold, silver and copper are among the best conductors of electricity and they are followed by aluminium, iron and nickel. Some semi-metals like graphite also fall in this group. The wires made of copper, aluminium, etc. are widely used as the conductors.
(2) Non-Conductors:
In case of a non-conductor, the electrification produced does not spread over the entire body, but it remains in the neighbourhood of the point where the electrification took place. The non-conductors are also known as the insulators. There is no substance which can be considered as a perfect insulator.
But glass (especially when it has been boiled in water and is then kept in a dry atmosphere), paraffin, sulphur, plastics, rubber, asbestos and fused quartz are sufficiently good insulators for all practical purposes. The petroleum, oil, etc. are the important liquid insulators.
The electric insulators may be in the form of solids, liquids or gases. But solid and liquid insulators are important from the commercial point of view. When a gas is in the normal condition, it conducts electricity to a very small extent.
But it can be made to conduct electricity freely, when it has its temperature raised sufficiently and when it has been traversed by rays given out by certain substances such as uranium, radium, thorium, etc.
The term dielectric is used to mean a substance capable of supporting an electric stress and its strength or power to resist an electric stress is known as the dielectric strength.
The properties of a good electrical insulator can be summarised as follows:
(i) Desired chemical stability,
(ii) High dielectric strength,
(iii) High electrical resistance,
(iv) High moisture resistance,
(v) High thermal resistance,
(vi) Low dielectric constant,
(vii) Low power factor,
(viii) Suitable mechanical and physical properties for efficient working, etc.
It may however be noted that there is no hard and fast line of demarcation between the two groups mentioned above. There are certain substances such as dry wood and paper which have intermediate properties and hence they are sometimes referred to as the semi-conductors. Thus the semi-conductors conduct electricity better than insulators but not so well as conductors.
Engineering Material # 10. Fly-Ash:
The fly-ash is a fairly divided residue which results from the combustion of ground or powdered bituminous coal or sub-bituminous coal like lignite and transported by the flue gases of boilers fired by pulverised coal or lignite. It is a by-product of many thermal power stations and other plants using pulverised coal or lignite as a source of heat for boilers.
On burning, nearly 30% of coal is converted into ash, 75% of which is fine fly-ash and the rest is coarse bottom ash. The fly-ash flies through the chimneys and its discharge through the chimneys can be minimised by installing and proper working of fabric filters, mechanical dust collectors and electrostatic precipitators.
The fly-ash resembles a pozzolana i.e. a substance which although not cementitious itself contains constituents which combine with the lime to form a material having cementing properties. Normally, the fly-ash contains some un-burnt carbon. It is acidic in nature and its main constituents are silica, aluminium oxide and ferrous oxide.
There are at present more than 70 thermal power plants in our country and to cope with the increasing demand of electricity, more thermal power plants may be set up in near future. It is expected that by the turn of century, the quantity of fly-ash available will touch the figure of 1000 million kN per annum. The disposal of such large quantities of fly-ash is certainly a gigantic problem and a matter of national concern.
At present, the fly-ash is disposed off in the following two ways:
(1) Dry System:
In this system, the fly-ash in dry form is carried away pneumatically into an overhead silo or a bunker at the plant. The removal of dry fly-ash by mechanical means such as screw feeders could be recommended only when the quantity of fly-ash to be handled is small.
(2) Wet System:
In this system, the fly-ash is mixed with water and sluiced to the settling ponds or dumping areas near the plant. This method is widely adopted because it is cheaper than any other method of fly-ash disposal.
It is however necessary to have the following three conditions fulfilled for satisfactory operation by this system:
(i) Availability of large areas of waste land for ponding;
(ii) Regular emptying of filled up ponds; and
(iii) Unrestricted water supply.
The existing system of disposal of fly-ash is causing the following problems:
(i) The costly land in the vicinity of the thermal power station is wasted because of the dumping of fly-ash. It is estimated that nearly 40000 hectares of land area is under ash ponds for the disposal of about 500 million kN of fly-ash.
(ii) The ecology of the region is disturbed.
(iii) The disposal of fly-ash involves heavy expenditure to the tune of 7000 million or so to transport the fly-ash to the selected ash ponds.
(iv) There are chances of health hazard for the people living near the power stations because the inhalation of fly-ash over a long period causes silicosis, fibrosis of lungs affecting the heart, bronchitis, pneumonitis, etc.
(v) There is non-utilization of a by-product which can be recycled or used for other profitable purpose.
(vi) There is risk of pollution to the surrounding environment including soil, vegetation and underground water resources.