The cement kiln is a large rotary cylinder equipment which is the core of the cement production process. It is usually made of steel and can be up to tens of meters in length and several meters in diameter. The internal structure of a cement kiln consists of the kiln tail, kiln center, kiln head and kiln exit system etc. Fireramo presents in this article the structural characteristics of the kiln’s various working zones as well as the suitable refractory bricks.
According to the temperature of the material inside the kiln, the cement rotary kiln system can be divided into: drying zone, preheating zone, decomposition zone, exothermic reaction zone, combustion zone and cooling zone.
The drying zone is a short section at the raw meal inlet. In traditional cement rotary kiln, the material temperature in the drying belt is 20-150℃, and the gas temperature is 250-400℃.
In the new dry-process cement kiln, the kiln materials are heated to over 150℃ the moment they enter the I-stage cyclone at the end of the kiln. Therefore, the drying belt is only a short section at the raw material inlet. There is almost no drying zone in the kiln tail system of a new dry process cement kiln.
In traditional cement rotary kiln, the temperature of material in the preheat zone is 150-800℃, and the temperature of gas is 450-850℃.
In the new dry process kiln, the cyclone dust collector is a preheat zone from the first to the last stage of the countdown. In the pre-tropical zone, the organic components in the raw material begin to dry distillation and decomposition, and the clay begins to dehydrate, creating conditions for the decomposition of calcium carbonate.
In traditional cement rotary kiln, the temperature of materials in the decomposition belt is 800-1000℃, and the gas temperature is 1000-1400℃.
New dry-process cement kiln, from the decomposition furnace to the kiln from the kiln head 7D (D is the diameter of the kiln’s steel plate) are decomposition belt. For example, the size of the rotary kiln of the new dry-process kiln is Φ4m×60m, 28-60m from the kiln head are decomposition zone; the size of the rotary kiln of the new dry-process kiln is Φ4.7m×74m, 33-74m from the kiln head are decomposition zone.
In the decomposition zone, the decomposition reaction of calcium carbonate is mainly carried out.
For the drying zone, preheating zone and decomposition zone, silica mullite bricks and anti-scaling high alumina bricks are the main refractory materials for kiln construction, and these bricks have good chemical and physical properties.
The exothermic reaction zone is also known as the upper transition zone. In the traditional cement rotary kiln, the material temperature in the exothermic reaction zone is from 1000°C to 1300°C, and the gas temperature is from 1400°C to 1600°C. For the new dry-process cement kiln, the transition zone is about 7D~4D from the kiln head, and magnesia spinel bricks are preferably used in the zone, which have good slag resistance and abrasion resistance, and the thermal shock resistance is up to about 100 times.
Magnesia iron spinel brick and magnesia chromium brick are the main building materials for the exothermic reaction belt and firing belt, which is a very important part of the cement kiln, and the materials need to have good abrasion resistance and alkali resistance, and magnesia chromium brick and magnesia spinel brick are the most suitable products.
After the clinker comes out of the firing belt, it is initially solidified in the cooling belt and then enters the cooler for further cooling. If the clinker temperature is too high or too low when it comes out of the kiln, the clinker will corrode and wear out the refractory material at the kiln opening. Therefore, refractory products are very important, and in this segment, silica mullite bricks are the best choice, which have good thermal shock resistance.
However, the length of each “temperature zone” of the cement rotary kiln is not fixed, and will be affected by the preheating of raw materials, flame length, whether the raw materials are easy to burn, etc.. The distribution of each working zone should be adjusted according to the changes in production conditions.
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