Air Drying and Burning of Bricks
(i) Natural drying
- Natural drying is done in several stages as follows.
- First bricks are allowed to dry in position as released from the mould.
- Then they are turned to stand on edge.
- Arranged in rows on the hack (rough floor of old bricks or concrete raised above the ground) with spaces of less than 1 cm between the bricks
- Stage 1 and 2 require 2 to 3 days and stage 3 about one week.
- It is to be seen that bricks are not directly exposed to the wind or sun for drying. Suitable screens may be used to avoid such situations.
(ii) Artificial drying
- Bricks are generally dried by natural drying but when bricks are to be rapidly dried on a large scale. artificial drying can be adopted.
- There are two types of driers:
(a) Hot floor driers. (b) Tunnel driers.
- Tunnel driers are more economical than hot floor driers.
- Temperature maintained by driers is usually between 20°C to 200°C and the process of drying of bricks (takes usually 1 to 3 days) depends upon the species of clay.
- The bricks in stacks should be arranged in such a way that sufficient air space is left between them for free circulation of air.
4. Burning
- It is a very important process as it provides hardness and strength to the bricks and makes them dense and durable.
- Bricks should be burnt properly. If overburnt, they will be brittle and hence break easily. If they are underburnt, they will be soft and hence cannot carry loads.
- The burning of bricks can be divided into three stages.
(i) The water retained in the pores of the clay after drying is driven off and the clay loses its plasticity.
(ii) Some of the carbonaceous matter is burnt.
(iii) A portion of sulphur is distilled from pyrites.
(iv) Hydrous minerals like ferric hydroxide are dehydrated, and
(v)The carbonate minerals are more or less decarbonated.
(i) Remained carbon is eliminated.
(ii) The ferrous iron is oxidized to the ferric form and
(iii) Removal of sulphur is completed only after the carbon has been eliminated.
3. Vitrification (1000 - 1250°C)
- To Convert the mass into a glass-like substance the temperature ranges from 900–1100°C for low melting clay and 1000-1250°C for high melting clay.
- Special care is required in cooling the bricks below the cherry red heat in order to avoid checking and cracking.
- Burning is either done in clamp ((Temporary structure for small scale manufacturing)) or kilns (Permanent structure large scale manufacturing).
Note: Too rapid heating causes cracking or bursting of the bricks. On the other hand, if alkali is contained in the clay or sulphur is present in large amounts in the clay, too slow heating of clay produces scum on the surface of the bricks.
(i) Clamp or Pazzawah
- A clamp is a temporary structure generally constructed over the ground with a height of about 4 to 6m It is employed in the non-monsoon season.
- It is trapezoidal in plan whose shorter edge (among the parallel sides) is below the ground and then the surface raising constantly at about 15° to reach the other parallel edge over the ground.
- A vertical brick and mud wall is constructed at the lower edge to support the stack of the brick. A layer of fuel is 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 is laid and then again a fuel layer is laid over it. The thickness of the fuel layer goes on with the height of the clamp.
- After these alternate layers of bricks and fuel, the top surface is covered with mud so as the pressure the heat.
- Fire is ignited at the bottom, once the fire is started it is kept under fire for 1 or 2 months and about some time period is needed for cooling the bricks.
Advantages of clamp burning:
1. Burning and cooling of bricks are gradual in clamps. Hence the bricks produced are tough and strong.
2. Burning of bricks by clamps proves to be cheap and economical. No skilled labour and supervision are required for the construction and working of clamps.
3. Clamp is not liable to injury from high wind or rain.
4. There is a considerable saving of fuel.
Disadvantages of clamp burning:
1. Bricks are not of regular shape. This may be due to the settlement of bricks when fuel near the bottom is burnt and turned to ashes.
2. Very slow process.
3. It is not possible to regulate fire in a clamp once it starts burning and the bricks are liable to uneven burning.
4. Quality of bricks is not uniform.
5. Bricks near the bottom are overburnt and those near sides and top are underburnt.
(ii) Kilns
- A kiln is a large oven used to burn bricks.
- These are of 2 types (a) Intermittent kilns (b) Continuous kilns
(a) Intermittent Kilns
- These are periodic kilns i.e., only one process takes place at a time. Various major processes are loading, unloading, cooling and burning of bricks.
- These can be either rectangular or circular in plan. They can be overground or underground.
- Intermittent kilns are further divided into two groups.
(a) Intermittent up-draught kilns
(b) Intermittent down-draught kilns
• Down draught kilns are more efficient, gives evenly burnt bricks as heat is utilized more by moving hot gases in the larger area of kiln. Whereas in up-drought kilns quality of bricks is not uniform (i.e., bottom bricks are overburnt and top bricks are underburnt) as hot gases are released after they rise up to the chimney entrance.
(b) Continuous kiln
• These kilns are continuous in operation. i.e. loading, unloading, firing and cooling are carried out simultaneously in these kilns.
- Types of the continuous kiln are
(a) Bull's trench kiln (commonly used in India).
(c) Tunnel kiln.
- Bulls Trench kiln is commonly used in India. It has an over 30 m high permanent brick chimney. It is costly to built and used outside the monsoon season.
- Hoffman's Kiln is an overground continuous kiln, which has a permanent roof due to which it can function even during the rainy season