In the process of manufacturing bricks, the following distinct operations involved.
- Preparation of clay
Each of the above operation of the manufacturing bricks will now be studied at length.
Preparation Of Clay
The clay for brick prepared in the following order.
The top layer of the soil, about 200mm in depth, 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 then dug out from the ground. It spread on the levelled ground, just a little deeper than the general level. The height of heaps of clay is about 600mm to 1200mm.
The clay as obtained in the process of digging should be cleaned of stones, pebbles, vegetable matters. If these particles in excess, the clay to washed and screened. Such a process naturally will prove to be troublesome and expensive.
The clay then exposed to atmosphere for softening and mellowing. The period varies from few weeks to full season.
The clay made loose and any ingredient to added to it , spread out at its top. The blending indicates intimate or harmonious mixing. It carried out by taking a small amount of clay every time and 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 Kneaded or pressed under the feet of man 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, tempering is done in pug mill.A typical pug mill capable of tempering sufficient earth for a daily output of about 15000 to20000 bricks.
The clay which is prepared as above is then sent for the text operation of moulding. Following are two types of moulding
i. Hand Moulding
ii. Machine 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.It should be 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.It is prepared from the combination of steel plate and channel. It may even be prepared from steel angles and plates.
Thickness of steel mould is 6mm.They is used for manufacturing bricks on a large scale. The steel moulds are more durable than wooden one and turn out bricks of uniform size.The bricks shrink during drying and burning .Hence the moulds are therefore made larger than burnt bricks (8-
The bricks prepared by hand moulding are of two types: Ground mouldedand Tablemoulded
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 is dashed is the mould. The clay is pressed in the mould in such a way that it fills all the corners of mould. The surplus clay is removed by wooden strike or framed 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 ids 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.The lower faces of ground moulded bricks are rough and it is not possible to place frog on such bricks.A frog is mark of depth about 10mm to 20mm which is placed on raw brick during moulding.
It serves two purposes.
1.It indicates the trade name of the manufacturer
2.In brick work, 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 the mould and it has projection of about 6mm height on its surface.
The dimensions of projection correspond to internal dimensions of mould. The design of impression or frog is made on this block.The 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 turn upside down.The mould is taken out and 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.
Table Moulded Bricks
i) The process of moulding of bricks is just similar as above.But in this case, the mould stands near a table size 2m x 1m. The bricks are moulded on the table and send for further process of drying.
ii) However the efficiency of the moulder gradually decreases because of standing at some place for a longer duration.The cost of brick is also increases when table moulding is adopted.
This type of moulding is carried out by two processes
i) Plastic clay machine
ii) Dry clay machine
Plastic Clay Moulding
i) Such machine consists of a rectangular opening having length and width is equal to an ordinary bricks. The pugged clay is placed in the machine and it comes out through the rectangular opening.
ii) These are cut into strips by the wire fixed at the frame. The arrangement is made in such a way that the strips thickness is equal to that of the bricks are obtained. So it is also called as WIRE CUT BRICKS.
Dry Clay Machinemoulding
In these machines, the strong clay is finally converted in to powered 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 dry and well-shaped bricks. They do not require the process of drying.
The damp bricks, if brunt,are likely to be cracked and distorted.Hence the moulded bricks are dried before they are taken for the next operation of burning. For the drying the bricks are laid longitudinally in the stacks of width equal to two bricks,A stack consists of ten or eight tiers.The bricks are laid along and across the stock in alternate layers. All the bricks are placed on edges. The bricks are allowed to dry until the bricks are become leather hard of moisture content about 2%.
Bricks burned at high temperature to gain the strength, durability, density and red color appearance.All the water removed at the temperature of 650 degrees but they burnt at an temperature of about 1100 degrees because the fusing of sand and lime takes place at this temperature and chemical bonding takes between these materials after the temperature cooled down resulting in the hard and dense mass.
Bricks not burnt above this temperature because it will result in the melting of the bricks and will result in a distorted shape and a very hard mass when cooled which will not workable while brickwork.
It can burnt using the following methods:
(a) Clamp Burning
(b) Kiln Burning
Clamp Means a temporary structure generally constructed over the ground with a height of about 4 to 6 m. It employed when the demand of the bricks is lower scale and when it is not a monsoon season. This is generally trapezoidal in plan whose shorter edge among the parallel sides is below the ground and then the surface raising constantly at about 15 degrees to reach the other parallel edge over the ground.
A vertical brick and mud wall constructed at the lower edge to support the stack of the brick. First layer of fuel laid as the bottom most layer with the coal, wood and other locally available material like cow dung and husk.Another layer of about 4 to 5 rows of bricks laid and then again a fuel layer laid over it. The thickness of the fuel layer goes on with the height of the clamp.
After these alternate layers of the bricks and fuel the top surface covered with mud so as to preserve the heat.
Fire ignited at the bottom, once fire started it kept under fire by itself for one or two months and same time period needed for the cooling of the bricks.
Disadvantages of Clamp burning:
- Bricks at the bottom are over-burnt while at the top are under-burnt.
- Bricks loose their shape, and reason may be their descending downward once the fuel
- This method cannot-employ for the manufacturing of large number of bricks and it is
costly in terms of fuel because large amount of heat wasted.
- It cannot employed in monsoon season.
Kiln a large oven used for the burning of bricks. Generally coal and other locally available materials like wood, cow dung etc can used as fuel.
They are of two types:
• Intermittent Kilns.
• Continuous Kilns.
These are also the periodic kind of kilns, because in such kilns only one process can take place at one time. Various major processes which takes place in the kilns are:- Loading, unloading, Cooling, and Burning of bricks.
There are two kind of intermittent kilns:
(i) Up-draught Intermittent Kilns
(ii) Down draught Intermittent Kilns
Down draught kilns more efficient because the heat is utilized more by moving the hot gases in the larger area of the kiln. In up draught kilns the hot gases released after they rise up to chimney entrance.
These kilns called continuous because all the processes of loading, unloading, cooling, heating, pre-heating take place simultaneously. They used when the bricks demanded in larger scale and in short time. Bricks burning completed in one day, so it is a fast method of burning. There are two well-known continuous kilns
Bull’s Trench Kiln:
Bull’s trench kiln consists of a rectangular, circular or oval plan shape. They constructed below the ground level by excavating a trench of the required width for the given capacity of brick manufacturing.This Trench divided generally in 12 chambers so that 2 numbers of cycles of brick burning can take place at the same time for the larger production of the bricks.
Or it may happen that one cycle carried out at one time in all the 12 chambers by using single process in the 2-3 chambers at the same time.The structure under-ground so the heat conserved to a large extent so it more efficient. Once fire started it constantly travels from one chamber to the other chamber, while other operations like loading, unloading, cooling, burning and preheating taking place simultaneously. Such kilns generally constructed to have a manufacturing capacity of about 20,000 bricks per day. The drawback of this kiln is that there is not a permanent roof, so it is not easy to manufacture the bricks in the monsoon seasons.
The main difference between the Bull’s trench kiln and the Hoffman kilns are:
- Hoffman’s kiln is an over the ground structure while Bull’s Trench Kiln is an underground structure.
- Hoffman’s kiln have a permanent roof while Bull’s trench Kiln do not have so it former can used in 12 months a year to manufacture bricks but later stopped in the monsoon season.
And Hoffman’s kiln generally circular in plan, and constructed over the ground. The whole
structure divided into the 12 chambers and the entire processes takes place simultaneously
like in Bull’s trench Kiln.
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