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At present, clay products in my country generally use semi-dry pressing. Therefore, the moisture content of the billet is low. If the kiln has a drying section, it can be fired directly into the kiln without pre-drying.
Drying medium inlet temperature: 150~200℃ (standard bricks and ordinary bricks), 120~160℃ (special-shaped bricks)
Exhaust temperature: 70~80℃
The residual moisture of the brick is less than 2%
Drying time: 16~24 hours
It should be pointed out that although the moisture content of semi-dry pressed bricks is low, for special types of large hand-made plastic-shaped bricks, due to difficulty in drainage, to prevent cracking due to excessive drying, the bricks should generally be dried in the air first. After drying for about a week, it is sent to the dryer or directly into the kiln to fire single bricks according to the complexity of the shape.
During the firing process of clay products, under high temperatures and a suitable atmosphere, the brick body undergoes a series of physical and chemical changes. The heating characteristics of clay directly affect the determination of the firing system of clay products. Physical and chemical reactions occur when the bonded clay is heated, which promotes the destruction and sintering of bricks. The greater the amount of combined clay, the lower the Al2O3 content in the clay, the higher the impurity content, the more serious the reaction, the volume shrinkage, and uneven heating, the greater the internal stress, which can easily lead to brick damage and cracking during the firing stage.
The firing of clay products can be divided into four stages
1. Normal temperature to 200℃: At this time, the temperature should not be too fast to avoid body cracking. When firing in the tunnel kiln, the temperature of the first four parking spaces should not exceed 200°C.
2. Temperature in the range of 200~900℃: The temperature rise rate should be accelerated at this stage to facilitate the chemical reaction of organic matter and impurities in the green. When the temperature is in the range of 600 to 900°C, a strong oxidizing atmosphere should be maintained in the kiln to prevent the occurrence of “black core” waste products.
3. 900°C to the highest firing temperature: In the high-temperature stage, the temperature should rise steadily and continue to maintain an oxidizing atmosphere so that the undesirable body is heated evenly and it can also prevent cracking of the brick body. Since sintering shrinkage is very strong above 1100°C and the shrinkage rate is as high as 5%, it is very important to maintain the relaxation of the temperature gradient and eliminate internal stress.
The fire resistance temperature of clay products is generally 100 to 150°C higher than the sintering temperature. If the sintering temperature range of the sintered clay used is narrow, the refractory temperature should be lower, preferably around 50 to 100°C. The sintering temperature of clay products should ensure that the bonded clay is fully softened and fully reacts with the surface layer of clinker fine powder and coarse particles to bond the clinker particles so that the product obtains appropriate strength and volume stability. The sintering temperature is generally 1250~1350℃. When the Al2O3 content is high, the sintering temperature of the product should be appropriately increased, about 1350~1380℃, and the heating time is generally 2~10h to ensure sufficient reaction in the product and consistent product quality.
4. Cooling stage: According to the lattice changes of the product in the cooling section, the cooling rate should be quickly reduced when the temperature is above 800~1000℃, and the cooling rate should be slowed down below 800℃. In fact, in actual production, the cooling rates adopted do not pose a risk of cold cracking to the product.
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