2. Firing system
In order to reasonably carry out the firing of various refractory products, the firing system of each product should be determined in advance, which includes: the higher temperature of firing; the heating rate at each stage; the holding time at the higher temperature; The cooling rate of the product under cooling; the nature of the atmosphere in the kiln in the above-mentioned stages.
The firing system can be made into a curve with the abscissa as time (hours) and the ordinate as temperature (°C), or it can be expressed in the form of a list with temperature range, heating rate and time as the content.
The firing temperature of refractory products mainly depends on: 1. The nature of the raw materials used. The melting point of the main mineral phase of the raw material, the lower eutectic temperature and the quantity between the mineral phases are directly related to the firing temperature. The reference firing temperature for refractory products is about 0.8 times the melting point of the main mineral phase. Therefore, the firing temperature of high-purity magnesia bricks is higher than that of magnesia spinel bricks; the firing temperature of corundum bricks is higher than that of mullite bricks; the firing temperature of alkaline refractory products is higher than that of high alumina refractory products; 2. For the same material, the purity of raw materials The higher, the higher the firing temperature. High-purity direct-bonded alkaline products have a higher firing temperature than silicate-bound alkaline products; 3. The particle size of the fine powder of raw materials. The higher the degree of dispersion, the greater the specific surface area, the greater the surface free energy, and the greater the sintering power. Therefore, the fine powder can promote sintering and reduce the sintering temperature.
When the refractory product is fired, the allowable temperature rise (fall) rate during the heating and cooling process and the necessary holding time depend on: 1. The internal stress generated during the physical and chemical changes in the product during heating and cooling 2. The temperature and time required to complete this physical and chemical change; 3. The stress caused by the temperature gradient, thermal expansion and cold shrinkage of the refractory during the firing process.
The firing process of refractory products can be completed in two heating methods, that is, the products are fired at a lower temperature in a longer time, or at a higher temperature in a shorter time. But in fact, due to the slow heat transfer in the kiln and the uneven heating of the products in the kiln, the use of fast-fired refractory products is limited.
The atmosphere in the kiln during firing includes oxidizing atmosphere, neutral atmosphere, reducing atmosphere, inert atmosphere, etc., which should be controlled according to the sintering requirements. The atmosphere in the kiln during firing will affect the chemical changes of the product during the firing process. For example, firing clay bricks in an oxidizing atmosphere can convert FeO in the product into Fe2O3, which is beneficial to improve the refractoriness of the clay products and the organic matter in the bricks to burn and volatilize. When firing silica bricks, a reducing atmosphere is usually used. The added iron mineralizer enhances the mineralization; carbon-containing products are required to be fired in a reducing atmosphere isolated from air or in controlled nitrogen to avoid carbon oxidation. Therefore, different products require different atmospheres when firing.
The firing system of refractory products is not only related to the variety, shape and size of refractory products, but also closely related to the type of firing equipment. For example, a large inverted flame kiln requires a slower heating rate and a longer holding time to ensure the uniformity of the brick temperature. Inappropriate firing system will increase the rejection rate and reduce the quality of products. The proper firing system is based on theoretical guidance and practical experience.
Three, out of the kiln
Exiting the kiln is the process of taking the fired product out of the kiln after cooling or unloading it from the kiln car. The quality of the output from the kiln has a direct impact on the quality of the finished product. In the process of leaving the kiln, if the operation is not careful or skilled, it will often cause missing edges and corners of the product, thereby reducing the qualified rate of the finished product.
