Get the answer of: What is the process of making and manufacturing bricks?
Following considerations govern the selection of a brick field for the manufacture of bricks:
(i) It should be linked up with the communicating roads so that the materials can be conveyed easily.
(ii) It should be situated on a plain ground.
(iii) It should be so selected that the earth for manufacturing good quality bricks is readily and easily available.
(iv) It should offer ail the facilities to the workers employed in the manufacturing process.
Brick Manufacturing Process and Top 4 Steps
In the process of manufacturing bricks, the following four distinct operations are involved:
(1) Preparation of clay
Each of these operations of manufacturing bricks will now be studied at length.
(1) Preparation of Clay:
The clay for bricks is prepared in the following order:
The top layer of soil, about 200 mm in depth, is taken out and thrown away. The clay in top soil is full of impurities and hence it is to be rejected for the purpose of preparing bricks.
The clay is then dug out from the ground. It is spread on the levelled ground, just a little deeper than the general level of ground. The height of heaps of clay is about 600 mm to 1200 mm.
The clay, as obtained in the process of digging, should be cleaned of stones, pebbles, vegetable matter, etc. If these particles are in excess, the clay is to be washed and screened. Such a process naturally will prove to be troublesome and expensive. The lumps of clay should be converted into powder form in the earth crushing roller.
The clay is then exposed to atmosphere for softening or mellowing. The period of exposure varies from few weeks to full season. For a large project, the clay is dug out just before the monsoon and it is allowed to weather throughout the monsoon.
The clay is made loose and any ingredient to be added to it is spread out at its top. The blending indicates intimate or harmonious mixing. It is carried out by taking small portion of clay every time and by turning it up and down in vertical direction. The blending makes clay fit for the next stage of tempering.
In the process of tempering, the clay is brought to a proper degree of hardness and it is made fit for the next operation of moulding. The water in required quantity is added to clay and the whole mass is kneaded or pressed under the feet of men or cattle. The tempering should be done exhaustively to obtain homogeneous mass of clay of uniform character.
For manufacturing good bricks on a large scale, the tempering is usually done in a pug mill. A typical pug mill capable of tempering sufficient earth for a daily output of about 15000 to 20000 bricks is shown in fig. 4-1. The process of grinding clay with water and making it plastic is known as the pugging.
A pug mill consists of a conical iron tub with cover at its top. It is fixed on a timber base which is made by fixing two wooden planks at right angles to each other. The bottom of tub is covered except for the hole to take out pugged earth. The diameter of pug mill at bottom is about 800 mm and that at top is about one metre.
The provision is made in top cover to place clay inside the pug mill. A vertical shaft with horizontal arms is provided at the centre of iron tub. The small wedge-shaped knives of steel are fixed on horizontal arms.
The long arms are fixed at the top of vertical shaft to attach a pair of bullocks. The ramp is provided to collect the pugged clay. The height of pug mill is about 2 m. Its depth below ground is about 600 mm to 800 mm to lessen the rise of the barrow run and to throw out the tempered clay conveniently.
In the beginning, the hole for pugged clay is closed and clay with water is placed in pug mill from the top. When the vertical shaft is rotated or turned by a pair of bullocks, the clay is thoroughly mixed up by the actions of horizontal arms and knives and a homogeneous mass is formed.
The rotation of vertical shaft can also be achieved by using steam, diesel or electric power. When clay has been sufficiently pugged, the hole at the bottom of tub is opened out and the pugged earth is taken out from ramp by barrow i.e., a small cart with two wheels for the next operation of moulding. The pug mill is then kept moving and feeding of clay from top and taking out of pugged clay from bottom are done simultaneously.
If tempering is properly carried out, the good brick earth can then be rolled without breaking in small threads of 3 mm diameter.
The clay which is prepared as above is then sent for the next operation of moulding.
Following are the two ways of moulding:
(i) Hand moulding
(ii) Machine moulding.
(i) Hand Moulding:
In hand moulding, the bricks are moulded by hand i.e., manually. It is adopted where manpower is cheap and is readily available for the manufacturing process of bricks on a small scale. The moulds are rectangular boxes which are open at top and bottom. They may be of wood or steel.
A typical wooden mould is shown in fig. 4-2. It should be prepared from well-seasoned wood. The longer sides are kept slightly projecting to serve as handles. The strips of brass or steel are sometimes fixed on the edges of wooden moulds to make them more durable.
A typical steel mould is shown in fig. 4-3. It is prepared from the combination of steel plates and channels. It may even be prepared from steel angles and plates. The thickness of steel mould is generally 6 mm. They are used for manufacturing bricks on a large scale. The steel moulds are more durable than wooden moulds and they turn out bricks of uniform size. The bricks shrink during drying and burning.
Hence the moulds are to be made larger than the size of Steel mould fully burnt bricks. The moulds are therefore made longer by about 8 to 12 per cent in all directions. The exact percentage of increase in dimensions of mould is determined by actual experiment on clay to be used for preparing bricks.
The bricks prepared by hand moulding are of two types:
(a) Ground-moulded bricks
(b) Table-moulded bricks.
(a) Ground-Moulded Bricks:
The ground is first made level and fine sand is sprinkled over it. The mould is dipped in water and placed over the ground. The lump of tempered clay is taken and it is dashed in the mould. The clay is pressed or forced in the mould in such a way that it fills all the corners of mould.
The extra or surplus clay is removed either by wooden strike or metal strike or frame with wire. A strike is a piece of wood or metal with a sharp edge. It is to be dipped in water every time.
The mould is then lifted up and raw brick is left on the ground. The mould is dipped in water and it is placed just near the previous brick to prepare another brick. The process is repeated till the ground is covered with raw bricks.
A brick moulder can mould about 750 bricks per day with working period of 8 hours. When such bricks become sufficiently dry, they are carried and placed in the drying sheds.
The bricks prepared by dipping mould in water every time are known as the slop-moulded bricks. The fine sand or ash may be sprinkled on the inside surface of mould instead of dipping mould in water. Such bricks are known as the sand-moulded bricks and they have sharp and straight edges.
The lower faces of ground moulded bricks are rough and it is not possible to place frog on such bricks. A frog is a mark of depth about 10 mm to 20 mm which is placed on raw brick during moulding.
It serves two purposes:
(1) It indicates the trade name of the manufacturer.
(2) In brickwork, the bricks are laid with frog uppermost. It thus affords a key for mortar when the next brick is placed over it.
The ground-moulded bricks of better quality and with frogs on their surface are made by using a pair of pallet boards and a wooden block. A pallet is a piece of thin wood. The block is bigger than mould and it has a projection of about 6 mm height on its surface.
The dimensions of projection correspond to the internal dimensions of mould. The design of impression or frog is made on this block. This wooden block is also known as the moulding block or stock board.
The mould is placed to fit in the projection of wooden block and clay is then dashed inside the mould. A pallet is placed on the top and the whole thing is then turned upside down. The mould is taken out and another pallet is placed over the raw brick and it is conveyed to the drying sheds.
The bricks are placed to stand on their longer sides in drying sheds and pallet boards are brought back for using them again. As the bricks are laid on edge, they occupy less space and they dry quicker and better.
(b) Table-Moulded Bricks:
The process of moulding these bricks is just similar as above. But here the moulder stands near a table of size about 2 m x 1 m. The clay, mould, water pots, stock board, strikes and pallet boards are placed on this table. The bricks are moulded on the table and sent for the further process of drying.
However the efficiency of moulder decreases gradually because of standing at the same place for long duration. The cost of brick moulding also increases when table moulding is adopted.
(ii) Machine Moulding:
The moulding may also be achieved by machines It proves to be economical when bricks in huge quantity are to be manufactured at the same spot in a short time. It is also helpful for moulding hard and strong clay.
These machines are broadly classified in two categories:
(a) Plastic clay machines
(b) Dry clay machines.
(a) Plastic Clay Machines:
Such machines contain a rectangular opening of size equal to length and width of a brick. The pugged clay is placed in the machine and as it comes out through the opening, it is cut into strips by wires fixed in frames. The arrangement is made in such a way that strips of thickness equal to that of the brick are obtained. As the bricks are cut by wire, they are also known as the wire cut bricks.
(b) Dry Clay Machines:
In these machines, the strong clay is first converted into powder form. A small quantity of water is then added to form a stiff plastic paste. Such paste is placed in mould and pressed by machine to form hard and well-shaped bricks. These bricks are known as the pressed bricks and they do not practically require drying. They can be sent directly for the process of burning.
The wire cut and pressed bricks have regular shape, sharp edges and corners. They have smooth external surfaces. They are heavier and stronger than ordinary hand-moulded bricks. They carry distinct frogs and exhibit uniform dense texture.
The damp bricks, if burnt, are likely to be cracked and distorted. Hence the moulded bricks are dried before they are taken for the next operation of burning. For drying, the bricks are laid longitudinally in stacks of width equal to two bricks. A stack consists of eight or ten tiers.
The bricks are laid along and across the stock in alternate layers. All bricks are placed on edge. The bricks should be allowed to dry till they become leather hard or bone-dry with moisture content of about 2 per cent or so.
The important facts to be remembered in connection with the drying of bricks are as follows:
(i) Artificial Drying:
The bricks are generally dried by natural process. But when bricks are to be rapidly dried on a large scale, the artificial drying may be adopted. In such a case, the moulded bricks are allowed to pass through special dryers which are in the form of tunnels or hot channels or floors. Such dryers are heated with the help of special furnaces or by hot flue gases. The tunnel dryers are more economical than hot floor dryers and they may be either periodic or continuous.
In the former case, the bricks are filled, dried and emptied in rotation. In the latter case, the loading of bricks is done at one end and they are taken out at the other end. The temperature is usually less than 120°C and the process of drying of bricks takes about 1 to 3 days depending upon the temperature maintained in the dryer, quality of clay product, etc.
(ii) Circulation of Air:
The bricks in stacks should be arranged in such a way that sufficient air space is left between them for free circulation of air.
(iii) Drying Yard:
For the drying purpose, special drying yards should be prepared. It should be slightly on a higher level and it is desirable to cover it with sand. Such an arrangement would prevent the accumulation of rain water.
(iv) Period for Drying:
The time required by moulded bricks to dry depends on prevailing weather conditions. Usually it takes about 3 to 10 days for bricks to become dry.
It is to be seen that bricks are not directly exposed to the wind or sun for drying. Suitable screens, if necessary, may be provided to avoid such situations.
This is a very important operation in the manufacture of bricks. It imparts hardness and strength to the bricks and makes them dense and durable. The bricks should be burnt properly. If bricks are over-burnt, they will be brittle and hence break easily. If they are under-burnt, they will be soft and hence cannot carry loads.
When the temperature of dull red heat, about 650°C, is attaired, the organic matter contained in the brick is oxidized and also the water of crystallization is driven away.
But heating of bricks is done beyond this limit for the following purposes:
(i) If bricks are cooled after attaining the temperature of about 650°C, the bricks formed will absorb moisture from the air and get rehydrated.
(ii) The reactions between the mineral constituents of clay are achieved at higher temperature and these reactions are necessary to give new properties such as strength, hardness, less moisture absorption, etc. to the bricks.
When the temperature of about 1100°C is reached, the particles of two important constituents of brick clay, namely, alumina and sand, bind themselves together resulting in the increase of strength and density of bricks. Further heating is not desirable and if the temperature is raised beyond 1100°C, a great amount of fusible glassy mass is formed and the bricks are said to be vitrified. The bricks begin to loose their shape beyond a certain limit of vitrification.
The burning of bricks is done either in clamps or in kilns. The clamps are temporary structures and they are adopted to manufacture bricks on a small scale to serve a local demand or a specific purpose. The kilns are permanent structures and they are adopted to manufacture bricks on a large scale.
A typical clamp is shown in fig. 4-4.
Following procedure is adopted in construction of clamp:
(i) A piece of ground is selected. Its shape in plan is generally trapezoidal. The floor of clamp is prepared in such a way that short end is slightly in the excavation and wider end is raised at an angle of about 15° from ground level.
(ii) The brick wall in mud is constructed on the short end and a layer of fuel is laid on the prepared floor. The fuel may consist of grass, cow dung, litter, husks of rice or ground nuts, etc. The thickness of this layer is about 700 mm to 800 mm. The wood or coal dust may also be used as fuel.
(iii) A layer, consisting of 4 or 5 courses of raw bricks, is then put up. The bricks are laid on edges with small spaces between them for the circulation of air.
(iv) A second layer of fuel is then placed and over it, another layer of raw bricks is put up. Thus alternate layers of fuel and raw bricks are formed. The thickness of fuel layer gradually decreases as the height of clamp increases.
(v) The total height of a clamp is about 3 m to 4 m. When nearly one-third height is reached, the lower portion of the clamp is ignited. The object for such an action is to burn the bricks in lower part when the construction of upper part of clamp is in progress.
(vi) When clamp is completely constructed, it is plastered with mud on sides and top and filled with earth to prevent the escape of heat. If there is any sudden and violent outburst of fire, it is put down by throwing earth or ashes.
(vii) The clamp is allowed to burn for a period of about one to two months.
(viii) It is then allowed to cool for more or less the same period as burning.
(ix) The burnt bricks are then taken out from the clamp.
Advantages of Clamp Burning:
Following are the advantages of clamp burning:
(i) The burning and cooling of bricks are gradual in clamps. Hence the bricks produced are tough and strong.
(ii) The burning of bricks by clamps proves to be cheap and economical.
(iii) No skilled labour and supervision are required for the construction and working of clamps.
(iv) The clamp is not liable to injury from high wind or rain.
(v) There is considerable saving of fuel.
Disadvantages of Clamp Burning:
Following are the disadvantages of clamp burning:
(i) The bricks are not of regular shape. This may be due to the settlement of bricks when fuel near bottom is burnt and turned to ashes.
(ii) It is a very slow process.
(iii) It is not possible to regulate fire in a clamp once it starts burning and the bricks are liable to uneven burning.
(iv) The quality of bricks is not uniform. The bricks near the bottom are over-burnt and those near sides and top are under-burnt.
A kiln is a large oven which is used to burn bricks.
The kilns which are used in the manufacture of bricks are of the following two types:
(1) Intermittent kilns
(2) Continuous kilns.
(1) Intermittent Kilns:
These kilns are intermittent in operation which means that they are loaded, fired, cooled and unloaded. Such kilns may be either rectangular or circular in plan. They may be over-ground or underground.
They are classified in two ways:
(i) Intermittent up-draught kilns
(ii) Intermittent down-draught kilns.
(i) Intermittent Up-Draught Kilns:
These kilns are in the form of rectangular structures with thick outside walls. The wide doors are provided at each end for loading and unloading of kilns. The flues are channels or passages which are provided to carry flames or hot gases through the body of kiln. A temporary roof may be installed of any light material. Such roof gives protection to the raw bricks from rain while they are being placed in position. This roof is to be removed when the kiln is fired.
Fig. 4-5 shows the plan of a typical intermittent up-draught kiln.
The working of the kiln is as follows:
(a) The raw bricks are laid in rows of thickness equal to 2 to 3 bricks and of height equal to 6 to 8 bricks. A space of about 2 bricks is left between adjacent rows. This space is utilized for placing fuel.
(b) The fuels are filled with brushwood which takes up a fire easily. The interior portion is then filled with fuel of bigger size.
(c) An arch like opening is formed by projecting 4 to 5 rows of bricks. The projection of each row is about 30 mm to 40 mm.
(d) The loading of kiln with raw bricks is then carried out. The top course is finished with flat bricks. Other courses are formed by placing bricks on edge.
(e) The end doors are built up with dry bricks and are covered with mud or clay.
(f) The kiln is then fired. The fire can be regulated by opening or closing the iron sheet doors of the fire holes and by controlling the supply of fuel. The progress of burning at any instant can be seen through these holes. For the first three days, the firing is kept slow by proper manipulation of flues. The strong fire is maintained for a period of 48 to 60 hours. The draught rises in the upward direction from bottom of kiln and brings about the burning of bricks.
(g) The kiln is allowed to cool down gradually for at least seven days and the bricks are then taken out.
(h) The procedure is then repeated for the next burning of bricks. The bricks manufactured by the intermittent up-draught kilns are better than those prepared by clamps.
But such kilns have the following disadvantages:
(a) The quality of burnt bricks is not uniform. The bricks near bottom are over-burnt and those near top are under-burnt.
(b) The supply of bricks is not continuous.
(c) There is wastage of fuel heat as kiln is to be cooled down every time after burning.
(ii) Intermittent Down-Draught Kilns:
These kilns are rectangular or circular in shape. They are provided with permanent walls and closed tight roof. The floor of the kiln has openings which are connected to a common chimney stack through flues. The working of this kiln is more or less similar to the up-draught kiln.
But it is so arranged in this kiln that hot gases are carried through vertical flues upto the level of roof and they are then released. These hot gases move downward by the chimney draught and in doing so, they burn the bricks.
Following advantages are claimed for the intermittent down-draught kilns:
(a) The bricks are evenly burnt.
(b) The performance of this kiln is better than that of up-draught kiln.
(c) There is close control of heat and hence such kilns are useful for burning structural clay tiles, terra-cotta, etc.
(2) Continuous Kilns:
These kilns are continuous in operation. This means that loading, firing, cooling and unloading are carried out simultaneously in these kilns. There are various types of the continuous kilns.
Following three varieties of continuous kilns will be discussed:
(i) Bull’s trench kiln
(ii) Hoffman’s kiln
(iii) Tunnel kiln.
(i) Bull’s Trench Kiln:
This kiln may be of rectangular, circular or oval shape in plan. Fig. 4-6 shows a typical Bull’s kiln of oval shape in plan.
As the name suggests, the kiln is constructed in a trench excavated in ground. It may be fully underground or partly projecting above ground.
In latter case, the ramps of earth should be provided on outside walls. The outer and inner walls are to be constructed of bricks. The openings are generally provided in the outer walls to act as flue holes. The dampers are in the form of iron plates and they are used to divide the kilns in suitable sections as shown in fig. 4-6. This is the most widely used kiln in India and it gives continuous supply of bricks.
The bricks are arranged in sections. They are arranged in such a way that flues are formed. The fuel is placed in flues and it is ignited through flue holes after covering top surface with earth and ashes to prevent the escape of heat. The flue holes are provided in sufficient number on top to insert fuel when burning is in progress.
Usually the two movable iron chimneys are employed to form draught. These chimneys are placed in advance of section being fired. Hence the hot gases leaving the chimneys warm up the bricks in next section. Each section requires about one day to burn.
When a section has been burnt, the flue holes are closed and it is allowed to cool down gradually. The fire is advanced to the next section and the chimneys are moved forward as shown by arrows in fig. 4-6. The Bull’s trench kiln is working continuously as all the operations — loading, burning, cooling and unloading are carried out simultaneously.
Fig. 4-6 shows Bull’s kiln with two sets of sections. The two pairs of chimneys and two gangs of workers will be required to operate this kiln. A tentative arrangement for different sections may be as shown in table 4-1.
(ii) Hoffman’s Kiln:
This kiln is constructed over-ground and hence it is sometimes known as the flame kiln. Its shape is circular in plan and it is divided into a number of compartments or chambers. As a permanent roof is provided, the kiln can even function during rainy season.
Fig. 4-7 shows plan and section of the Hoffman’s kiln with 12 chambers.
Each chamber is provided with the following:
(a) A main door for loading and unloading of bricks,
(b) Communicating doors which would act as flues in open condition,
(c) A radial flue connected with a central chimney, and
(d) Fuel holes with covers to drop fuel, which may be in the form of powdered coal, into burning chambers.
The main doors are closed by dry bricks and covered with mud, when required. For communicating doors and radial flues, the dampers are provided to shut or open them. In the normal condition, only one radial flue is connected to the chimney to establish a draught.
In this type of kiln, each chamber performs various functions in succession, namely, loading, drying, burning, cooling and unloading.
As an illustration, 12 chambers shown in fig. 4-7, may be functioning as follows:
Chamber 1 — Loading
Chambers 2 to 5 — Drying and pre-heating
Chambers 6 and 7 — Burning
Chambers 8 to 11 — Cooling
Chamber 12 — Unloading
With the above arrangement, the circulation of the flue gas will be as shown by arrows in fig. 4-7. The cool air enters through chambers 1 and 12 as their main doors are open. After crossing the cooling chambers 8 to 11, it enters the burning section in a heated condition. It then moves to chambers 2 to 5 to dry and pre-heat the raw bricks. The damper of chamber 2 is in open condition and hence it escapes into atmosphere through chimney.
The initial cost of installing this kiln is high, but it possesses the following advantages:
(a) The bricks are burnt uniformly, equally and evenly. Hence the high percentage of good quality bricks can be produced.
(b) It is possible to regulate heat inside the chambers through fuel holes.
(c) The supply of bricks is continuous and regular because of the fact that the top of kiln is closed and it can be made to work during the entire year.
(d) There is considerable saving in fuel due to pre-heating of raw bricks by flue gas. Thus the hot gases are fully utilized in drying and pre-heating the raw bricks.
(e) There is no air pollution in the locality because the exhaust gases do not contain black smoke or coal dust particles.
The capacity of the kiln will depend upon the dimensions of chambers. If each chamber is of about 11 m length, 4.50 m average width and 2.50 m height, it will contain about 25000 bricks. Hence, if it is so arranged that one chamber is unloaded daily, such a kiln will manufacture about 25000 bricks daily or about 8 to 9 million bricks annually. The quantity of coal dust required for burning one lakh of bricks is about 120 to 150 kN.
It may be noted that in case of Bull’s trench kiln and Hoffman’s kiln, the chambers are zoned in accordance with the brick-processing stages, namely, loading, drying, preheating, burning, cooling and unloading. The source of fire and other zones are moving continuously along the channel of kiln while the bricks in process remain stationary.
Table 4-2 shows the comparison of Bull’s trench kiln and Hoffman’s kiln with respect to some important items.
(iii) Tunnel Kiln:
This type of kiln is in the form of tunnel which may be straight, circular or oval in plan. It contains a stationary zone of fire. The raw bricks are placed on trolleys which are then moved from one end to the other end of tunnel.
The raw bricks get dried and pre-heated as they approach zone of fire. In zone of fire, the bricks are burnt to the required degree and they are then pushed forward for cooling. When bricks are sufficiently cooled, they are unloaded.
This kiln proves to be economical when bricks are to be manufactured on a large scale. As temperature is under control, uniform bricks of better quality are produced.