The main raw material in the manufacture of ceramic bricks and blocks is clay. Let‘s talk about the semi-dry technology for the manufacturing of ceramic products. Semi-dry technology presupposes molding with a press at high pressures. However, giving the press powder the brick shape and just drying it is not enough. Such bricks have to be burned then. The burning procedure is necessary in order for the brick to become high-quality and durable.
The purpose of green bricks’ burning is to give them the physical integrity required by the construction conditions as well as to make them water-insoluble. The raw material is burned at high temperatures of 900-1050 °, sometimes even higher. The required burning temperature depends on the clay features, i.e. on its physical, chemical, and mineralogical composition. Burning is performed in kilns (forges) that have different designs.
Kilns can be divided into two main groups – periodic and continuous. The operation of periodic kilns is based on individual completed independent periods which are called cycles. Each cycle consists of loading the kiln, heating, drying, and burning the green bricks, then their cooling, and unloading.
Periodic kilns are divided into 3 groups:
- Straight flame kilns (immediate thrust).
- Back flame kilns.
- Horizontal flame kilns (horizontal thrust)
Every kiln has 3 main elements:
- The burner, where fuel is burned.
- Work chamber where items are burned.
- Exhaust unit for releasing combustion gases (pipe or fan).
There are several stages of brick burning:
- Stage of evaporation of the remaining water. In this case, the product is heated up to 120 degrees.
- Stage of organic residue burnout. The temperature rises to 600 degrees.
- Stage of burning a ceramic product. The temperature rises from 920 to 980 degrees with the slow contraction of the clay. At this stage, the material becomes durable, as the brick block is hardened and malleablized under conditions of maximum temperature.
- Stage of cooling. The building material slowly cools down.
In case if there were no violations of the burning technology, the brick will have reddish-orange color, its structure will be homogeneous and dense.
The burning can last 6-48 hours. During this time, the brick receives the required unique features: high durability, great thermal insulation properties, moisture resistance, resistance to temperature fluctuations, and drops.
Classification of ceramic brick kilns.
According to the operating mode, the kilns are divided into:
- Periodic kilns (dome, batch, open-cut);
- Continuous kilns (tunnel, circular).
According to the design, the kilns are divided into:
- Periodic kilns;
- Gas chamber kilns;
- Tunnel kilns – with a straight line and with a circular line.
According to the method of heating the ceramic products, the kilns are divided into:
- Direct kilns;
- Indirect kilns.
According to the heating method, the kilns are divided into:
- Fuel-fired (flame) kilns;
- Electrical kilns.
Currently, the most widely used kilns are industrial tunnel and circular kilns.
The operation principle of tunnel kilns is that the green bricks move while separate heated zones of the kiln remain static. The temperature zones and the burning temperature curve remain constant. The construction of the tunnel kiln is a pass-through tunnel that plays the role of a burning line, the hearth of which is represented by a trolley string which is periodically pushed along the rails at a distance equal to the length of one trolley. In order to prevent air from sucking into the kiln and the release of hot air and gases from the kiln, the trolleys have sealing interlocking devices at the ends as well as sand seals on the sides. The end-type lines are equipped with so-called forehearth with sluice gates.
The internal dimensions of the burning line range from 48 to 120 m in length, 1.4 to 3.15 m in width, and 1.35 to 1.835 m in height.
At the moment, it became possible to make up tunnel kilns from prefabricated concrete structures made using refractory concrete. Here’s an example of a tunnel kiln.
Length – 105m with 45 trolleys with separation for loading: heating zone – 14 m, burning zone – 17 m, cooling zone – 14 m.
Kilns can operate on:
- Natural gas;
- Liquid fuel;
The advantage of circular kilns is that they are simple and cheap to make. However, they have several significant drawbacks: is hard to mechanize them, burning in them leads to a major heat loss due to the periodic shift of the burning zone and its heating to the temperature of the products’ burning. As a result, the operation of such kilns is very expensive, while the hygiene and sanitary conditions of work in such kilns are poor since the loaders of green bricks and workers who unload the ready-made products have to work in conditions of high temperatures.