Manufacture of bricks

MANUFACTURE OF BRICKS

 

1.  PREPARATION OF BRICK EARTH or CLAY

1. Removal of loose soil: The top layer of loose soil should be removed for a depth of 20 cm. It should not be used for preparation of bricks as it contains lot of impurities.

2. Digging and Spreading: The earth (clay) is dug from below 20 cm. It is spread on a level ground. All undesirable materials like grass, roots, gravel, stones, etc., are removed.

3. Weathering: Spread earth is exposed to atmosphere for a few weeks. This is called Weathering. Weathering imparts plasticity and strength to the clay.

4. Blending: Blending consists of mixing different types of earths such as sandy or calcareous clays in suitable proportions with little quantities of coke, ash, etc. Blending develops the essential qualities needed for moulding, drying and burning of bricks. The whole mass is mixed uniformly and water is added.

5. Tempering: Tempering consists of kneading the earth under the feet of men or cattle after adding water. This is to make the mass stiff and homogenous, but plastic enough for moulding. For making superior bricks in large scale, the earth is tempered in a Pug Mill.

Pug Mill

See Fig. 1. Pug Mill consists of a Conical Iron Tub. A Vertical Shaft is provided with a number of Horizontal Arms with Cutting Knives for breaking clay lumps. A Long Horizontal Arm is fitted at the top of the vertical shaft.

Clay and water in the ratio 4: 1 are fed from the top. The vertical shaft is rotated by electrical power. Due to the action of the knives in the horizontal arms, the clay is mixed and tempered. The tempered clay leaves the pug mill through a Ramp at the bottom.

 

2. MOULDING OF BRICKS

Brick Mould: See Fig. 2. It is a rectangular box made of steel or wood. The box is opened at top and bottom. The mould is made 6 mm deeper than the brick size, because the raw bricks shrink during drying and burning. Moulding of brick is done by Hand or in Moulding Machine.

Hand Moulding: In this, bricks are moulded by hand, i.e., manually. Hand moulding is preferred when bricks are manufactured on small scale. Hand moulding may be done either on ground (called Ground Moulding) or on table (called Table Moulding).

• Ground Moulding: A level ground is prepared. A thin layer of sand is spread over the prepared ground. The mould is wetted in water and placed on the ground firmly. The tempered clay is filled into the mould. Then, the mould is carefully lifted up, leaving the raw brick on the ground.

• Table Moulding: Table moulding is done over a table instead of on the ground.

Machine Moulding: Machine Moulding is used for large scale manufacture. In this, the pugged clay is fed into the moulding machine. As the clay moves through it, it is compressed and cut into strips by wires. Thus, brick blocks are formed.

Advantages: Machine-moulded bricks are heavier and stronger, having regular shape and size, sharp edges and corners and smooth surface.

 

3. DRYING OF BRICKS

The bricks are dried for two weeks under the sun or under the shed. If the bricks are not properly dried before they are burnt, they may get cracked and distorted during the burning process.

 

4. BURNING OF DRY BRICKS

Purposes of Burning are (i) To impart hardness and strength to the bricks; and (ii) To increase the density of bricks so as to absorb less quantity of water. Burning is done in a Clamp or Kiln.

a. Clamp Burning

Clamp is a temporary structure. In clamp burning, both bricks and fuels (such as dried cow dung, rice husk, wood chippings and cheap wood) are placed alternately in layers. After the clamp burning is started, it takes 3 to 6 months for burning and cooling the bricks.

Initial cost is low, as no permanent structures are built. Also, fuel cost is low. The process is economical. But, it is not possible to control the fire during burning. Quality of bricks produced is poor. Only about 60% is of good quality with uniformity of shape of bricks.

Clamp burning is adopted when bricks are manufactured on small scale. For large scale manufacture of bricks, Kiln Burning is adopted.

b. Kiln Burning

Kilns are permanent structures. There are two types of kilns: Intermittent and Continuous Kilns.

 

5. INTERMITTENT KILN (ALLAHABAD KILN)

See Fig. 3. In this, the process of burning is intermittent (i.e., discontinuous). Allahabad Kiln is an overground intermittent kiln.

The Intermittent Kiln is in the form of a rectangular building with thick outside Walls. In this, the operations, namely, Loading of Bricks, Firing, Cooling and Unloading are done one after the other."

Trenches are dug in the floor with intervening Small Openings. Above the trench, the dried bricks are laid with gaps between them for the hot flue gases to flow to each brick.

Flue gases are circulated through the small openings in the Longitudinal Wall for 3 to 4 days. The bricks are then allowed to cool for 7

 

6.  HOFFMAN'S CONTINUOUS KILN (FLAME KILN) 

In this kiln, the process of burning is continuous. It is used to produce continuous supply of bricks on a large scale.

Description: (Fig. 4)

Hoffman's Kiln is circular in plan with a Chimney at its center.

Around the chimney, there are 12 Chambers in an annular shape. Each chamber consists of the following:

• A Main Door (e.g., Di) for loading and unloading of bricks.

• Communicating Doors (e.g., A₁ and B₁) for flow of flue gases between the chambers. A Radial Flue (e.g., Fı) from each chamber to the chimney.

• Fuel Holes for dropping the fuel powdered coal.

Working

First, main doors are closed. Dampers are provided for the communicating doors and the radial flues for opening and closing.

In a continuous kiln, the progress of work is so arranged that one chamber is loaded, another is unloaded and still another is freshly fired.

• Chamber 1 is loaded with raw bricks. Chamber 2 is emptied of cooled down burnt bricks. The bricks in the chambers 3,4,5 are cooling

Bricks stacked in the chambers 7 and 8 are supplied with fuel through fire holes. Raw bricks in chambers 9,10,11 and 12 are heated by the hot flue gases. Then the flue gases leave through the chimney.

• Circulation of Flue Gases: Cold air enters the chambers 1 and 2 through the doors. After passing through the cooling chambers 3,4,5 and 6, it enters in a heated condition into the burning chambers 7 and 8. The hot flue gases then flow through the chambers 9,10,11 and 12 preheat the raw bricks.

 

7. COMPARISON OF CLAMP BURNING AND KILN BURNING