In north India, only 60% of the bricks fired in a Bull’s Trench Kiln (BTK) are classified as Class 1 bricks. The remaining 40% are under-fired, over-fired or broken bricks.
A brick kiln owner like you could be losing revenue of Rs 10 lakh to Rs 30 lakh in a year due to such high percentage of lower quality bricks. Besides, it could also lock your capital and increase your inventory cost as you would take longer time (at least a year) to sell off the lower quality bricks.
The primary reasons for production of under-fired bricks in BTKs are:
For proper vitrification of bricks, the bricks have to be heated to a temperature of 980 °C to 1030 °C (in north India) and they should remain at this temperature for at least 12 hours. In a BTK, most of the fuel is burned at the middle level and at the bottom of the kiln. Hence, the maximum temperature reached in the top part of the kiln is usually 70 °C to 100 °C less than the maximum temperature reached in the middle and bottom parts of the kiln. Moreover, due to heat loss from the top and sides of the kiln to the surroundings, maintaining this temperature at the top part and sides of the kiln for long hours (about 12 hours) is not possible. Therefore, the bricks at the top and the sides are usually under-fired. Higher heat losses through the wicket gates and leakage of cold air into the kiln through them result in even higher production of under-fired bricks near the wicket gates.
The reasons for the production of over-fired bricks are:
In BTKs, the fuel is fed intermittently in large quantities. The size of the fuel fed is also large usually. Normally, just before the completion of fuel feeding in a line, excessive amount of fuel is fed and the fuel feed holes are then closed. These fuel feeding practices result in the accumulation of fuel on ledges and at the bottom of the kiln. Usually an iron rod is used to disperse the fuel accumulated on ledges during fuel feeding in many kilns; but, this does not happen after closing the feed holes. The accumulation of fuel results in over-firing of bricks adjacent to the ledges or at the bottom of the kiln.
The reasons for the breakage of bricks are:
When green bricks are preheated in the kiln, first the moisture contained in the bricks evaporates. During this stage, the heating rate should be slow enough so that the moisture from the surfaces of bricks and from their core gets released completely.
If the green bricks have higher moisture content, they require longer time for moisture removal; but due to the short preheating zone in a BTK, fire reaches near the bricks before complete removal of moisture can take place. This increases the heating rate, resulting in the quick removal of moisture from the brick surfaces and their hardening, while some moisture remains in the core of the bricks. As this moisture from the core evaporates and escapes from the bricks, it creates cracks, which causes breakages in the bricks.
Uneven kiln floor or the uneven setting of bricks results in uneven load on the stacked bricks leading to breakages in the bricks.