Fire bricks have many different names in many cases, such as refectory brick, fireproof brick, or firebrick, but these names all point to one thing. It is a block of ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. A refractory brick is built primarily to withstand high temperatures but will also usually have a low thermal conductivity for greater energy efficiency.
After understanding the general situation, this article mainly divides the functions of fireproof bricks into the following three points:
Reducing the chemical erosion and mechanical wear of the cylinder by high-temperature gases and materials, and protecting the kiln shell.
Acting as a heat transfer medium, kiln bricks can absorb part of the heat from the gas and transfer it to the material through different conduction and radiation methods.
Kiln bricks can also be used for thermal insulation, reducing heat loss in the kiln body, etc. We can also categorize fire bricks according to their different materials.
- Clay bricks (insulation bricks): used in the area of the preheating calcining zone, with a refractoriness of SK35;
- High-alumina bricks: Their main component is Al2O3. The higher the content, the higher the refractory resistance of the refractory bricks, and the better the spalling resistance, thermal conductivity, mechanical strength, and chemical resistance. However, high-alumina bricks have great reversible expansion, and their slag resistance is worse than that of salt-based fire bricks. Generally used in the calcining zone (transition zone). The commonly used fire resistance index is SK37. SK37 or 36 is often used in the cooling zone.
- Salt-based fire bricks: Salt-based fire bricks have high chemical resistance to ash and slag at high temperatures, have good kiln skin hanging performance, and the kiln skin adheres relatively firmly. Because the bricks and the kiln skin are fused, if the kiln skin falls off, some of the kiln bricks will also fall with the kiln skin.
The peeling resistance of aluminum bricks is higher than that of aluminum bricks.
The type and selection of refractory bricks, whether the quality and refractory characteristics of refractory bricks are appropriately selected, and the quality of bricklaying will affect the life of the kiln bricks. The life of the refractory bricks will directly affect the cost of maintenance of the rotary kiln and the level of output. This demonstrates the importance of refractory bricks.
Depending on where the refractory materials are used in the kiln, the main requirements for kiln bricks in the rotary kiln are:
High temperature resistance, the temperature in the kiln is above 1000°C, which requires that the refractory bricks cannot melt at high temperatures and must maintain a certain strength below the melting point. At the same time, it must also have the characteristic of having a fixed volume even after being exposed to high temperatures for a long time.
Good thermal vibration stability, that is, the ability to withstand drastic changes in kiln temperature without being damaged. When the kiln is stopped or started, or the rotary kiln is operating in an unstable condition, the temperature changes in the kiln are relatively large, which requires that the kiln bricks must not crack or peel when the temperature changes drastically. This also requires that the kiln temperature be kept as stable as possible during operation.
It has a strong resistance to chemical erosion. When fired in a rotary kiln, the ash, slag, and steam formed will cause great corrosion to the kiln bricks.
Good wear resistance and mechanical strength. The sliding of feed in the kiln and the friction of dust in the airflow will cause great wear and tear on the kiln bricks. This is especially true in the early stages of opening the kiln when there is no kiln skin protection in the kiln. Kiln bricks also have to withstand the expansion stress at high temperatures and the stress caused by the elliptical deformation of the kiln shell. Kiln bricks are required to have a certain mechanical strength.
Kiln bricks have good kiln skin-hanging properties. The kiln skin is hung on the kiln bricks, which has a great protective effect on the kiln bricks. If the kiln bricks have good hanging properties and the kiln skin can be maintained for a long time, the kiln bricks can be protected from erosion and wear.
The porosity should be low. If the porosity is high, corrosive kiln gas will penetrate into the kiln bricks and condense, destroying the kiln bricks, especially alkaline gas.
The thermal expansion stability is better. Although the thermal expansion coefficient of the kiln shell is greater than that of the kiln bricks,. However, the temperature of the kiln shell is generally around 280–450 °C, while the temperature of the kiln bricks is generally above 800°C, and the temperature in the firing zone is more than 1,300°C. In this way, the thermal expansion of the kiln bricks is still greater than that of the kiln shell, and the kiln bricks are prone to peeling due to compressive stress.
When selecting fire bricks, the choice is generally based on the temperature and load of each zone. It should be noted that the expansion coefficients of the kiln bricks and the kiln barrel should be relatively close. The greater the thickness of the kiln bricks, the better the heat insulation effect of the kiln shell, but the effective cross-sectional area in the kiln will also be reduced, which will also reduce the output of the kiln. The thinner the kiln bricks are, the greater their effective volume will be, and the heat dissipation of the kiln shell will be more severe. High temperatures will cause greater damage to equipment and waste energy. Currently, two series of bricks are most commonly used in the world: ISO (international standard) and VDZ (German standard). In fact, the materials of both are the same, but the specifications and sizes are different. ISO bricks are larger, thicker, and have fewer joints, making them inconvenient to transport and operate. The bricks of the VDZ are relatively small and thin, which is convenient for operation and easy to squeeze, but they have low strength and many brick joints. Under general circumstances, the ISO of refractory bricks can be used to unify the specifications, which can reduce the number of spare parts and save investment.
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