Manufacturing Process Of Bricks | Preparation Of Clay | Moulding Of Units | Drying | Burning

MANUFACTURE OF BRICKS

The manufacture of bricks is carried out in a number of stages. It includes the following:

1. Selection and preparation of clay

2. Shaping and moulding of units

3. Drying

4. Burning

SELECTION AND PREPARATION OF CLAY

As a practise, suitable deposits of clay are first located and thoroughly tested for the quality of brick making. Clay for bricks is prepared in the following order.

a. Unsoiling: The top layer of the soil is taken out. This is because the clay in the top layer is full of impurities and, hence, it is to be rejected for the purpose of preparing bricks.

b. Digging: The clay which is dug out is spread on level ground, just a little deeper than the general ground level. The height of the heap of clay is about 60–120 cm.

c. Cleaning: The clay should be made free from stones, pebbles and vegetable matter. If these particles are in excess, the clay is to be washed and screened, which is considered to be uneconomical.

d. Weathering: The softening of clay is done by exposing it to the atmosphere. The period of exposure varies from weeks to full seasons.

e. Blending: The clay is made loose and any ingredient to be added is spread out at its top. Blending indicates intimate mixing. A small portion of clay is taken every time for mixing.

f. Tempering: In this stage, the clay is brought to a proper degree of hardness and it is made fit for moulding. Water in the required quantity is added and the whole mass is mixed so as to form a mass of uniform character. A large-scale tempering is usually done in a pug mill. The process of grinding clay with water and making it plastic is known as pugging.

A pug mill consists of a conical iron tub with a cover at the top. It is fixed on a timber base which is made by fixing two wooden planks at right angles. The diameter of the pug mill at the bottom is about 80 cm and at the top is about 1 m. A vertical shaft with horizontal arms is provided at the centre of the iron tub. The small wedge-shaped knives of steel are fixed on the horizontal arms. Openings are provided at the top and bottom for charging clay and water and removing the mix respectively. The height of the pug mill is about 2 m.

 

Moulding

Moulding is the process of making rectangular-shaped brick units from properly tempered clay. The two types of moulding are

a. Hand moulding

b. Machine moulding.

 

Hand moulding

This is presently the most common method for brick manufacture. This is adopted where manpower is cheap and readily available. The moulds used for hand moulding are rectangular boxes made from well-seasoned wood or steel open at the top and bottom. Hand moulding is of two types:

i. Ground moulding

ii. Table moulding

 

 

Ground moulding

In this method, the ground is first levelled and fine sand is sprinkled over it. The mould is dipped in water and placed over the ground. The clay is pressed in the mould in such a way that it fills all the corners of the mould. Any surplus earth from the top of the mould is removed using a cutting wire or a metal with a sharp edge, which has to be dipped in water every time it is used. The mould is then lifted up and the brick is left on the ground. The mould is dipped in water and placed close by and another brick is moulded in the same way. If the mould is dipped in water every time, such preparation of bricks is known as slope moulded bricks. If fine sand or ash is sprinkled on the inside surface of the mould instead of dipping the mould in water, such bricks are called sand moulded bricks.

In pallet moulding, bricks of higher quality and with frogs are produced. The frogs are made using a pair of pallet boards and a wooden block. A frog is a mark of depth about 10–20 mm made on raw bricks during moulding. The frog is provided for mainly two purposes:

i. It serves as a key of mortar when other bricks are placed over it.

ii. It indicates the trade name of the manufacturer.

 

Table moulding

The process of moulding operations are carried out on a specially designed moulding table. The clay, the mould, water pots, stock board, etc. are placed on this table. The bricks are moulded similar to the ground moulding on the table. The cost of brick moulding increases when table moulding is adopted.

 

Machine moulding

The moulding can also be achieved by using machines. It is quite economical when bricks are produced in huge amounts. The machine moulding is broadly classified into two categories:

i. Plastic clay machines

ii. Dry clay machines

 

Plastic clay machines

The machines contain rectangular openings of size equal to the length and width of a brick. The pugged clay is placed in the machine and as it comes out through the openings it is cut into strips by wires fixed in frames. Hence, it is known as wire cut bricks.

Dry clay machines

In this machine, the strong clay is first converted to powder form. A small quantity of water is added to the stiff plastic paste. Such paste is placed in the mould and pressed by machines to form well-shaped hard bricks. These bricks are known as pressed bricks.

The machine moulded bricks have regular shape, sharp edges and corners; they are heavier and stronger than hand-moulded bricks.

 

DRYING

The drying of bricks is necessary, firstly to make them strong enough for rough handling during subsequent stages and secondly to save fuel during burning. For drying the bricks are laid longitudinally in stocks of bricks with the width equal to two bricks. The drying of bricks is achieved by either natural or artificial methods.

 

The important facts to be remembered while drying of bricks is as follows:

a. The bricks are generally dried by natural process. But when bricks are to be rapidly dried, artificial drying may be adopted. In artificial drying, bricks are made to pass through driers in the form of tunnels or hot channels or floors. The tunnel driers are more economical than hot floor driers.

b. The brick in stocks should be arranged in such a way that sufficient air space is left between them for circulation of air.

c. Special drying yards should be prepared and accumulation of rainwater should be prevented.

d. The period of drying depends upon the prevailing weather conditions.

 

BURNING

Burning of dried bricks is essential to develop the desired engineering properties, like hardness, durability and resistance to decay. Three chemical changes are known to take place in the brick earth during burning, namely dehydration, oxidation and vitrification.

Dehydration is completed within 425–750°C temperature range and it results in the expulsion of most of the water from the bricks.

During oxidation, carbon and sulphur are eliminated as oxides, whereas the fluxes are also oxidized.

Oxidation starts at the range of dehydration temperatures and is completed at about 900°C.

Vitrification is the extreme reaction and occurs when heating is carried out beyond 900°C. This is commonly not required in building bricks although in other clay products like sewer pipes it is necessary.

Burning of bricks is either done in clamps or kilns. Clamps are temporary structures while kilns are permanent structures. Clamps are adopted to manufacture bricks on a small scale while kilns are adopted to manufacture bricks on a large scale.

 

CLAMPS

The shape of the clamp is generally trapezoidal. The brick wall is constructed on the short end and a layer of fuel is placed on the prepared floor. The fuels generally used are cow dung, litter, husk of rice, wood, coal, etc. The thickness of the fuel layer varies from 70 to 80 cm. The layer consisting of 4 or 5 courses of raw brick are then put up. Sufficient space for the circulation of air is provided. Alternate layers of fuel and bricks are placed over this. The total height of the clamp is around 3-4 m. When nearly one-third of the height is reached, the lower portion of the clamp is ignited so as to burn the bricks in the lower part when the construction of bricks in the upper part is in progress. After construction is complete, it is completely plastered with mud in order to prevent the escape of heat. The clamp is allowed to burn for 1 or 2 months and cooling is also done for 2 months and later the burnt bricks are taken out.

 

KILNS

The kilns used for the manufacture of bricks are of two types:

i. Intermittent kilns

ii. Continuous kilns

 

Intermittent kilns

These kilns may be underground or overground in the model. They are classified in two ways:

 

Intermittent up-draught kilns These kilns are in the form of rectangular structures with thick outside walls. 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 the kiln. A roof is provided to protect the raw bricks from rain.

The quality of the bricks is not uniform; the bricks at the bottom are overburnt and at the top are under- burnt. The supply of bricks is not continuous and there is considerable wastage of fuel in the kiln.

 

Intermittent down-draught kilns These kilns are rectangular or circular in shape. They are provided with permanent walls and a closed tight roof. The floor of the kiln has openings that are connected to a common chimney stacked through flues. They are so arranged that in this kiln the hot gases are carried through the vertical flues up to the level of the roof and then released. As a result, the bricks are evenly burnt and the performance is much better than intermittent up-draught kilns. Here, there is close control of heat and the bricks obtained are evenly burnt.

 

Continuous kilns

These kilns are continuous in operation were loading, firing, cooling and unloading are simultaneously carried out.

 

Bull’s trench kiln This is one of the continuous type kilns. These kilns are rectangular, circular or oval-shaped in the plan. These kilns are constructed in a trench excavated on the ground. It may be fully underground or partially projecting above the ground. The outer and inner walls are to be constructed in bricks. Openings are provided on the outer walls to act as flue holes. Iron plates are used to divide the kiln into suitable sections. The fuel is placed in flues and is ignited after covering the top surface with earth to prevent the escape of heat. Usually, two movable iron chimneys are employed to form draught. The chimneys are placed in advance of the firing sections so that the warm gases leaving the chimney warm up the bricks in the next section. As the section has burnt, the flue holes are closed and allowed to cool down. Later the fire is advanced to the next section.

 

Hoffmann kiln This kiln is constructed underground is circular in plan and consists of a number of chambers. A permanent roof is provided so that the kiln can function even in the rainy season. The chamber in Hoffmann’s kiln is provided with the main door for loading and unloading bricks. Communicating doors should act as flues in the open condition. A radial flue connected with a central chimney and fuel holes are also provided. The advantages are that the bricks are uniformly, equally and evenly burnt and that there is no air pollution in the locality. Also, there is a saving in fuel and a high percentage of good bricks are produced.

 

Tunnel kiln  This type of kiln is in the form of a tunnel, which is oval, circular or straight in the plan. It contains a stationary source of fire. The raw bricks are placed in trolleys which are then moved from one end to another end of the tunnel. The raw bricks get dried and preheated as they approach the zone of fire and in the zone of fire, the bricks are burnt and pushed forward for cooling. Later, after cooling, they are unloaded.

shows the comparison between clamp and kiln burning.

ITEM	CLAMP BURNING	KILN BURNING
Structure	Temporary	Permanent
Initial cost	Very low as no structures are to be built	Very high as permanent structures are to be built
Cost of fuel	Low as grass, cow dung etc. is being used	High as coal dust is being used
Quality of bricks	Percentage of good bricks is less, around 60%	Percentage of good bricks is more, around 90%
Supervision	Not necessary throughout the process	Skilled supervision required
Wastage of heat	more	less
capacity	About 20,000–1,00,000 bricks at a time	25,000 bricks per day
suitability	For small scale	For large scale
Time for burning and cooling	2-6 months	24 hours for burning and 12 days for cooling

Manufacturing Process Of Bricks

Must read: Method of Laying Brickwork in Masonry Construction

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