Manufacturing Of Clay Bricks(Part 2)
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|Manufacturing Of Clay Bricks(Part 2)|
Drying(Image From Book By S.K. Duggal)
- Its objective is to remove the moisture and to control the shrinkage of bricks.
- It also facilitates the saving of fuel and time during burning.
- In developing countries, bricks are normally dried in naturally open-air driers.
- Special drying yards should be prepared and accumulation of rainwater should be prevented.
- The bricks are stacked on raised ground and are prevented from bad weather and sunlight.
- The bricks in stacks should be arranged in such a way that sufficient air space is left between them for circulation of air.
- There should be a gap of about 1 m between the adjacent layers of the stacks so as to allow free movement for the workers.
- The period of drying depends upon the prevailing weather conditions.
- Artificial drying is adopted when bricks are to be dried rapidly.
- Artificial dryers are of two types which are Tunnel driers and Hot Floor driers.
- The tunnel driers are more economical than hot floor driers.
It is further divided into three main stages:-
- It is also known as water smoking stage.
- The clay loses its plasticity as the water still absorbed in its pores(after drying) is not left.
- During dehydration, some of the carbonaceous matter is burnt, a portion of sulphur is distilled from pyrites, hydrous minerals like ferric hydroxide are dehydrated and the carbonate minerals are more or less decarbonated.
- Too rapid heating causes cracking and bursting of the bricks.
- Too slow heating of clay produces scum on the surface of the bricks.
- The remainder of carbon is eliminated and the ferrous iron is oxidized to ferric form during the oxidation period.
- The removal of sulphur is completed only after the carbon has been eliminated.
- In order to avoid the black or spongy pores, oxidation must proceed at such a rate which will allow these changes to occur before the heat becomes sufficient to soften the clay and close its pores.
- Sand is often added to the raw clay to produce a more open structure and thus provide escape of gases generated in burning.
- The temperature ranges from 900–1100°C(for low melting clay) and 1000–1250°C (for high melting clay) to convert the mass into a glass-like substance.
- In order to avoid checking and cracking, great care is required in cooling the bricks below the cherry red heat.
- Vitrification period may further be divided into three types:-
- Incipient vitrification:- In incipient vitrification, the clay has softened sufficiently to cause adherence but not enough to close the pores or cause loss of space and on cooling, the material cannot be scratched by the knife.
- Complete vitrification:- It is marked by maximum shrinkage of the material.
- Viscous vitrification:- In viscous vitrification, there is a further increase in temperature which results in a soft molten mass, a gradual loss in shape, and a glassy structure after cooling. Generally, clay products are vitrified to the point of viscosity.
- Paving bricks are burnt to the stage of complete vitrification to achieve maximum hardness as well as toughness.
Burning of bricks is either done in a clamp or kiln. They are as follows:-
Burning in clamp or Pazawah:-
|Burning In Clamp Or Pazawah(Image From Book By S.K. Duggal)|
- Clamps are temporary structures.
- To manufacture bricks on a small scale, clamps are adopted.
- The shape of the clamp is generally trapezoidal.
- The total height of the clamp is around 3-4 m.
- The bricks and fuel are placed in alternate layers.
- The amount of fuel is reduced successively in the top layers.
- The fuels generally used are cow dung, litter, the husk of rice, wood, coal, etc.
- Each brick tier consists of 4–5 layers of bricks.
- Some space is left between bricks for the free circulation of hot gases.
- After 30 per cent loading of the clamp, the fuel in the lowest layer is fired and the remaining loading of bricks and fuel is carried out hurriedly.
- The top and sides of the clamp are plastered with mud.
- Then a coat of cow dung is given, which prevents the escape of heat.
- The production of bricks is 2–3 lacs and the process is completed in six months.
- This process yields about 60 per cent first class bricks.
- The kiln used for burning bricks may be underground such as the Bull’s trench kiln or overground such as the Hoffman’s kiln.
- These may be rectangular, circular or oval in shape.
- If the process of burning bricks is continuous, the kiln is known as a continuous kiln.
- If the process of burning bricks is discontinuous, the kiln is known as an intermittent kiln.
- Intermittent kiln:-After loading the kiln, it is fired, cooled and unloaded and then the next loading is done. There is wastage of fuel since the walls and sides get cooled during reloading and are to be heated again during the next firing, For example- Intermittent up-draught and Intermittent down-draught kilns.
|Intermittent Kiln(Image From Book By S.K. Duggal)|
- Continuous kiln:- In a continuous kiln, bricks are stacked in various chambers wherein the bricks undergo different treatments at the same time. When the bricks in one of the chambers are fired, the bricks in the next set of chambers are dried and preheated while bricks in the other set of chambers are loaded and in the last are cooled. For example- Tunnel, Hoffmann and Bull's Trench kilns.
*Source- Internet, Books, Self-Analysis