The process of firing refractory bricks includes three processes: loading the kiln, firing the kiln and unloading the kiln.
1: Loading the kiln
The so-called loading the kiln refers to the operation process of reasonably arranging and stacking the brick blanks that meet the technical conditions of semi-finished products in the kiln according to the structural characteristics of the kiln and the requirements of the thermal system when the products are fired. For tunnel kilns, it is also called loading (kiln) car.
1) Determine the kiln loading height and loading method according to different brick types. Usually, the kiln loading height of magnesia bricks and first-class high-aluminum bricks is 1~1.1m; silica bricks are 1~1.7m; clay bricks are between the two. Most of the brick loading methods adopt flat loading, silica bricks adopt vertical loading and clay bricks adopt side loading.
2) According to different refractory bricks types, determine the kiln loading ratio of general bricks and special-shaped bricks. Generally, the ratio of special-shaped bricks to general bricks on the same kiln car is about 4:6. At the same time, it is also necessary to determine the kiln loading position of different varieties according to different brick types. Generally, standard and general bricks are installed at the bottom, and special-shaped bricks are installed at the top. Some special-shaped bricks or bricks that are easy to crack during firing are packaged (bricks wrapped in bricks).
3) Under the premise of ensuring the firing quality, increase the brick loading density (i.e. the number of bricks loaded per kiln car) to increase production and reduce fuel consumption. Formulating a reasonable kiln loading diagram is the premise to ensure the quality of kiln loading, but if the kiln loading operation is improper, the appearance qualification rate of the fired products will also be reduced. Therefore, in the production of refractory materials, the basic requirements for kiln loading quality are to ensure that the brick stack is flat, stable, and straight, to avoid the bricks from sticking together due to high-temperature firing, and to reduce the distortion of the fired products. In order to meet the above requirements, a layer of filling sand with a particle size of 0.5 to 3 mm is usually evenly sprinkled between each layer of bricks when loading the kiln. Products of different properties have different requirements for kiln filling sand. Usually, clay bricks and high-alumina bricks use silica sand, bauxite chips, rice husks or rice husk ash; silica bricks use waste silica brick sand or silica sand; and magnesia bricks use magnesia sand or chromium ore sand.
2: Firing
During the firing process, the brick blank undergoes a series of physical and chemical reactions, making the brick blank dense, increasing its strength, stabilizing its volume, and ensuring accurate external dimensions. The entire firing process can be divided into three stages:
(1) The heating stage, which is from the time the product enters the kiln or is ignited to the time when the product reaches the highest firing temperature. As the temperature increases, the liquid phase formation temperature and the physical phase synthesis temperature are reached. Due to the diffusion, flow, dissolution, precipitation and mass transfer of the liquid phase, the particles are further brought together under the action of the liquid surface tension, which promotes the densification of the blank, increases its strength, reduces its volume, reduces its porosity, and the blank is sintered.
(2) The insulation stage at the highest firing temperature. Various reactions in the green body tend to be complete and sufficient, the amount of liquid phase increases, the crystal phase grows further, and the green body reaches densification. During the firing process, not only the surface of the product must reach the firing temperature, but also the inside of the product must reach the firing temperature. This temperature uniformity process is achieved by heat transfer, which requires a certain amount of time. It can be seen that the larger the product and the higher the kiln density, the longer this time. In addition, due to the uneven temperature of various parts in the kiln, a certain amount of insulation time is also required.
(3) The cooling stage refers to the period from the highest firing temperature to the kiln discharge temperature. In this stage, the structural and chemical changes of the product at high temperature are basically fixed. In the early stage of this stage, some physical and chemical changes are also taking place in the product, such as crystallization of the phase, crystal transformation of certain crystals, solidification of the glass phase, and generation of microcracks. The cooling system will affect the physical properties of the product, such as strength and thermal shock resistance.
In order to reasonably carry out the firing of various refractory products, the firing system of each product should be determined in advance, including: the highest firing temperature; the heating rate in each stage; the insulation time at the highest temperature; the cooling rate when the product is cooled; the atmosphere properties in the kiln in the above stages. The firing process of refractory products can be completed by two heating methods, that is, the products are fired at a lower temperature for a longer time, or at a higher temperature for 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, there is a limit to the use of fast firing refractory products. The firing system of refractory products is not only related to the variety, shape and size of the refractory products, but also closely related to the type of firing equipment. For example, a large downdraft kiln requires a slower heating rate and a longer insulation time to ensure the uniformity of the temperature of the refractory bricks. An inappropriate firing system will increase the scrap rate and reduce the quality of the products. The appropriate firing system is based on theoretical guidance and practical experience.
3. Kiln discharge
Kiln discharge is the process of taking the fired products out of the kiln after cooling, or unloading them from the kiln car. The quality of kiln discharge operation has a direct impact on the appearance quality of the finished products. In the production of refractory materials, the following points should be paid attention to in kiln discharge operation: (1) When taking and placing bricks, they should be handled with care to avoid defects in the appearance of the products caused by careless kiln discharge operation. (2) Products with different brick numbers should be stacked strictly separately, that is, products with different brick numbers should not be placed on the same brick board when they are discharged from the kiln, otherwise, it is easy to cause confusion during the inspection and selection of finished products. Because the appearance of some products with different brick numbers can be exactly the same, but there is a small difference in a certain size (a few mm or a dozen mm). If they are mixed on a brick board, it is easy to mistake the product of one brick number for waste during inspection and selection, which will not only cause waste, but also affect the use. (3) Tool bricks (brick racks, bricks, etc.) should be sorted and stacked for continued use when loading the kiln. (4) Pay attention to safe operation and labor protection when taking out the kiln and transporting refractory bricks.
