Burning of bricks | Clamps | Kilns - learncivil

Burning of bricks | Clamps | Kilns - learncivil

Burning:
This is a very important operation in the manufacture of bricks. It imparts hardness and
strength to the bricks and makes them dense and durable. Bricks are heated up to 700 to 1000° C.
Burning of bricks



At this temperature alumina and silica in the clay fuse together resulting in a compound which is
strong and stable.
The purpose of burning is:

1. To impart hardness and strength to the bricks.
2. To increase the density of the bricks so that they will absorb less quantity of water.

The bricks may be burnt in any one of the following:

a) Clamps
b) Intermittent kilns
c) Continuous kilns.

Clamps are temporary structures, and they are used to burn bricks on small scale, but kilns are permanent structures and are used to manufacture bricks on large scale basis. The burning of bricks undergoes the following three main stages:

Dehydration (400 to 650°C): The dehydration or smoking stage is the process of removal of water content remained in the bricks after drying.

Oxidation period (650 to 900°C):
In this period, removal of carbon, oxidization of ferrous iron to ferric and the removal of sulphur is done.

Vitrification (900-1100°): Clay products are vitrified to the point of viscosity to achieve maximum hardness and toughness.

Kilns may be underground, eg. Bull's trench kilns or over ground eg. Hoffman's kiln. the shape of the kilns may be rectangular, circular or oval shaped. If burning is continuous, it is continuous kiln. e.g. Bull trench and Hoffman's kilns. On the other hand, if the process of burning is discontinuous, it is intermittent kiln.

Also Visit: Manufacturing process of bricks

CLAMPS

Burning of bricks

This consists of dried bricks and fuel placed in alternate layers. The operations involved in
clamp burning are as follows:


1. The floor is prepared with an upward slope of about 15'. Its shape in plan is trapezoidal.
2. The brick wall in mud is constructed on the short end and a layer of fuel (700mm to 800mm
thick) is laid on the prepared floor. The fuel consists of grass, cow dung, litter, husks of rice or
ground nuts, etc.
3. A layer of 4 to 5 courses of raw bricks, is then laid over the layer of fuel.
4. The second layer of fuel is then placed over it, another layer of raw bricks is laid over the layer
of fuel. In this way, fuel and bricks is laid in alternate layers. The thickness of fuel decreases
as the height of clamp increases.
5. The total height of clamp is 3m to 4m. When nearly one-third height is reached, the lower
portion of the clamp is ignited.
6. When clamp is completely constructed, it is plastered with mud on sides and top and filled with
earth to prevent the escape of heat.
6. The clamp is allowed to burn for a period of one to two months and then allowed to cool for
same period.
7. The burnt bricks are then taken out from the clamp.


Advantages of clamp burning:

1. It is economical.
2. This system is well understood by the uneducated brick burners.
3. The clamp bricks are tougher than kiln bricks due to gradual burning and cooling.

Disadvantages of clamp burning:

1. It is very slow method.
2. Regulation of fire is not possible.
3. Out turn of bricks is not quite uniform.
4. Large portion of bricks distorted and cracked due to their settlement when the fuel at the
bottom is burnt to ashes.

Also read: Imp Topic - Size and weight of bricks

KILNS

A kiln is a large oven which is used to burn bricks.
i) Intermittent kilns: These kilns are intermittent in operation which means that they are loaded, fired, cooled and unloaded

They are classified in two ways:

a) Intermittent Up-draught kilos
b) Intermittent down-draught kilns

a) Intermittent Up-draught kilns:

Burning of bricks

These kilns are in the form of rectangular structures with thick outside walls, The working of the kiln is as follows:

1. The raw bricks are laid in rows of thickness equal to 2 to 3 bricks and height equal to
6 to 8 bricks. A space of about 2 bricks left between rows for placing fuel.
2. The fuels are filled with brushwood.
3. An arch like opening is formed by projecting 4 to 5 rows of bricks.
4. The loading of kiln with raw bricks then carried out.
5. The end doors are built up with dry bricks and are covered with mud.
6. The kiln is then fired. The fire can be regulated by opening or closing the iron sheet doors of the fire holes and by controlling the supply of fuel.
7. The firing is kept slow for first three days. The strong fire is maintained for a period of 48 to 60 hours. The drought rises in the upward direction from bottom of kiln and brings about the burning of bricks
8. The kiln is allowed to cool down gradually for at least 7 days and the bricks are then taken out.
9. The procedure is then repeated for the next burning of bricks.


Such intermittent up-draught kiln gives three types of bricks:

(i) Lower courses over burned to hardness
(ii) Middle courses well burned.
(iii) Top courses unburned.

The bricks manufactured by the intermitted up-draught kilns are better than those obtained
from clamps.
However, such kilns have the following disadvantage :
1. Burning of bricks is not uniform, as stated above.
2. The supply is not continuous
3. There is loss of heat from the top most brick layer.
4. There is waste of fuel heat as the kiln is to be cooled down.
5. The cost of manufacture is more.


b) Intermittent down-draught kilns

These kilns are rectangular or circular in shape. They are provided with permanent walls and
closed light roof. The floor of the kiln has openings which are connected to a common chimney
stack through flues. The working of this kiln is more or less similar to the up-draught kiln. But
it is so arranged in this kiln that, hot gases are carried through vertical flues up to the level of
roof and are then released. These hot gases move downward by chimney draught and in
doing so, they burn the bricks.

The following are the advantages of the down-draught kiln over the up-draught kilns:
1. The burning of bricks is more or less uniform.
2. The thermal performance is much better and heat loss is much less.
3. Heat can be controlled in a better way, and hence such kilns are more useful for burning of
structural clay tiles, terracotta, etc.

Also read: Imp Topic - Various test on bricks


ii) Continuous kilns

These kilns are continuous in operation. This means the loading, firing. cooling and unloading
are carried out simultaneously in these kilns.
In continuous kilns while the bricks in one set of chambers are being fired, the bricks in the
next set of chambers are getting dried and preheated, the bricks in the other set of chambers being
loaded and in the last being cooled, as shown in Fig. 2.7.
The arrangement is such that air enters through the loading chambers and passes through the
stacks of bricks being cooled and gets preheated. This preheated air enters the burning zone and
the hot gases of combustion from the burning zone have to pass through the dried bricks for
preheating before entering the chimney. Thus cool air is preheated by cooling the burnt bricks and
hot gases lose their heat to the bricks which are to be burnt. This saves fuel and makes it an
economical process. Moreover, the supply of bricks is ensured throughout the year. Bull's trench
kiln and Hoffman's kiln belong to this class of kilns.


Recommended: Classification of bricks

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