The so-called kiln installation refers to the operation process of rationally arranging and stacking bricks 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 product is fired. For tunnel kilns, it is also called loading (kiln) car.
In order to achieve the above-mentioned purpose, it is necessary to formulate the kiln installation diagram and technical operation procedures for the kiln installation in order to unify the kiln installation operation. Although the layout of the tunnel kiln and the inverted flame kiln have their own characteristics, for example, the tunnel kiln installs the bricks on the kiln car, while the inverted flame kiln loads the bricks directly into the kiln. The basic principles that should be mastered when drawing are still the same. Such as heat transfer, kiln loading positions of different types of bricks, etc. The following takes the installation of a tunnel kiln as an example to illustrate some basic principles of drawing up a kiln installation diagram.
When formulating a tunnel kiln installation plan, the following issues are usually considered:
1. Determine the height and method of kiln installation according to different brick types. Generally, the kiln height of magnesia bricks and first-class aluminum bricks is 1-1.1m; silica bricks are 1-1.7m; clay bricks are somewhere in between. Most of the brick installation methods are flat installation, while silica bricks are vertical installation and clay bricks are side installation.
2. According to different brick types, determine the kiln loading ratio of general-shaped bricks and special-shaped bricks. Generally, the ratio of special-shaped bricks and general-shaped bricks on the same kiln car is about 4:6. At the same time, according to different brick types, determine different types of kiln installation positions. Generally, standard and ordinary type bricks are installed in the lower part, and special-shaped bricks are installed in the upper part, and some special-shaped bricks or bricks that are easy to crack during firing are packaged (bricks are wrapped).
3. On the premise of ensuring the quality of firing, increase the density of bricks (that is, the amount of bricks per unit of kiln car) to increase output and reduce fuel consumption.
4. Ensure normal gas flow and good heat transfer conditions during the firing of the bricks.
Therefore, in the production of refractory materials, the basic requirements for the quality of the kiln are to ensure that the brick stacks are flat, stable, and straight, and to prevent 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 sand with a grain size of 0.5-3mm is usually evenly sprinkled between each layer of bricks when installing the kiln. Products with different properties have different requirements for sand filling in the kiln. Usually, clay bricks and high alumina bricks use silica sand, bauxite shavings, rice husk or rice husk ash; silica bricks use waste silica brick sand or silica sand; magnesia bricks are used Magnesia or chrome ore.
1. Firing
The bricks undergo a series of physical-chemical reactions during the firing process to make the bricks compact, increase in strength, stable in volume, and ensure accurate external dimensions.
1. Three stages of firing process
During the firing of refractory materials, the entire firing process can be divided into three stages according to the changing characteristics of the product:
(1) The heating stage, that is, from the time the product enters the kiln or ignites to the time when the product reaches a higher temperature for firing. In this stage, the bricks are heated, the residual moisture and chemical crystallization moisture are discharged, the decomposition of certain substances and the formation of new compounds, polycrystalline transformation and liquid phase formation, etc., including the decomposition of organic and inorganic binders, additives, Oxidation and combustion, etc., release CO2, water and other small molecules. At this stage, due to the above reasons, the weight of the blank is reduced, the porosity is increased, and the strength is reduced.
As the temperature increases, the liquid phase formation temperature and the phase synthesis temperature are reached. Due to the diffusion, flow, dissolution, precipitation and mass transfer process of the liquid phase, the particles are further moved closer together under the action of the surface tension of the liquid phase to promote the densification of the green body. The strength is increased, the volume is reduced, the porosity is reduced, and the green body is sintered.
(2) The heat preservation stage at higher firing temperature. Various reactions in the green body tend to be complete and sufficient, the number of liquid phases increases, the crystalline phase further grows, and the green bricks reach densification.
During the firing process of the product, not only the surface must reach the firing temperature, but the inside of the product must also reach the firing temperature. This temperature homogenization process is achieved by heat transfer, and it takes a certain amount of time for this. It can be seen that the larger the product and the higher the kiln density, the longer this time will be. In addition, due to the uneven temperature of various parts in the kiln, a certain holding time is also required.
(3) The cooling stage refers to the temperature from the higher sintering temperature to the kiln exit 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-chemical changes are still taking place in the product, such as the crystallization of phases, the transformation of certain crystals, the solidification of the glass phase, and the generation of microcracks. The cooling system will affect the strength, thermal shock resistance and other physical properties of the product.
