Three Systems of Roller Kiln Firing Technology

1. Temperature regime
(1) Pre tropical and oxidation zone
The temperature of the preheating oxidation zone is generally controlled within the range of 200-1050 ℃. The green body will continue to drain residual adsorbed water in the preheating zone, while starting to drain crystalline water. Organic matter oxidation, carbonate decomposition, and quartz crystal transformation occur.
In order to avoid the generation of waste products (cracks, black cores, etc.), it is necessary to take into account that the dehydration process is carried out within a determined temperature range and at a specified clean crystal water removal rate when formulating the firing curve for the pre heating zone.
When fired in a roller kiln, dehydration reactions begin to occur when the surface temperature of the green body is between 450-700 ℃. At this time, the crystalline water is violently discharged, and the weight loss rapidly increases. The structure of the clay crystal is damaged, and the strength of the green body is reduced. Poor temperature control at this stage is extremely prone to cracking.
The allowable removal rate of crystal water from the green body largely depends on the thickness of the defective body, and the dehydration range is from 450-700 ℃ to 800-1000 ℃. During this period, ensure the necessary heating time. The related reactions of pre heating zone include: ① b-a crystal transformation of quartz at 500-600 ℃; ② Oxidative combustion of organic matter occurs at 500-800 ℃; ③ Carbonate decomposes at 700-1050 ℃, and during this period, the green body is also prone to black core formation due to rapid burning. Therefore, in the temperature range of 800-950 ℃, it is necessary to maintain the temperature for a certain period of time. In short, it is required to complete dehydration and oxidative combustion decomposition before the embryo is strongly contracted (850 ℃ - 1000 ℃).
(2) Firing zone and high temperature zone
The performance of ceramic tiles depends largely on the degree of sintering of the body, and the firing temperature and time depend on the physical and chemical reactions of the body; If the green body has the characteristics of narrow sintering range and easy deformation, then heating up and extending the holding time in the final sintering stage can improve its sintering situation.
The firing temperature of ceramic tiles is generally between 1180 and 1220 ℃, and the specific standard for their heat preservation time is to ensure that the water absorption of the product is less than 0.5%. If the upper limit of the firing temperature of the product is used, the heat preservation time can be appropriately shortened, and otherwise, the heat preservation time should be appropriately extended.
The temperature of the sintering belt is mainly controlled by appropriately adjusting the amount of oil and air in the nozzle. When the nozzle opening is normal and the indoor temperature field is stable, the purpose of automatically adjusting the temperature can be achieved only by using an automatic control valve. In most cases, the temperature can be controlled by using a fixed amount of air and adjusting the amount of fuel.
(3) Cooling belt
Generally speaking, from the highest firing temperature to 700 ℃, rapid cooling can be achieved, and the stress generated by rapid cooling is buffered by the liquid phase, without defects such as deformation and cracking.
Cooling should be slow between 700-500 ℃. This stage is accompanied by a relatively change in the crystal form of quartz. At 573 ℃, the a-b crystal form transformation occurs. The green body has a large destructive force due to a sudden change in volume of 10.82%. In addition, the product is brittle at this time, and if you do not pay attention to it, defects such as cracking may occur if the cooling is too fast or uneven.
Generally, rapid cooling can be achieved below 500 ℃, but it is still necessary to pay attention to the volume change of quartz crystal transformation between 270-200 ℃ (the volume shrinkage of a-b square carbon crystal transformation is 2.8%).
2. Pressure system
The pressure regime is an important guarantee for achieving a reasonable temperature regime and atmosphere regime, so controlling indoor pressure is an important task for stabilizing the process regime.
The pressure system of the roller table is mainly controlled by the suction force of the smoke exhaust fan and the upper and lower positions of the muffle plates on each control unit in the kiln. Generally, the positions of each section of the muffle plate are fixed after adjustment, and the pressure of the bin is mainly adjusted by the smoke exhaust fan:
① The oxidation zone must be negative pressure. If the density and strength of the brick are higher, the oxidation becomes more difficult. Therefore, it is necessary to reduce the positive pressure and increase the negative pressure;
② The pressure is based on the bottom, which represents the process of heat flow. Excessive flow velocity causes false temperature, while excessive flow velocity causes large heat loss;
③ At zero pressure, the flame goes up to the kiln roof, the negative pressure flows in the direction of the kiln wall, and the positive pressure flame flows outward. Excessive zero pressure is prone to deformation and rod breakage.
3. Atmosphere system
The atmosphere control is mainly based on the formulation of the firing curve through the air and oil ratio of each burner as required, not only considering the temperature distribution in the kiln length direction, but also paying attention to the temperature difference between the upper and lower roller tables.