According to statistics, the refractory material consumed by the cement industry accounts for about 10% of its total production. With the further development of the cement industry, the demand for refractories has not only increased in quantity, but also in quality. In the cement plant, the development of refractories for rotary kilns has gone through a long process, which makes its service performance keep improving and the service life of kiln lining keep extending, but at the same time, there are still some problems. Moreover, with our increasing awareness of environmental protection, the high-quality spinel refractories, magnesia refractories, calcium refractories and zirconium refractories have become the development direction.
Requirements for Refractory Materials of Cement Rotary Kilns
The sintering zone has the worst working environment in the whole cement rotary kiln system. It bears more abrasion than the preheating zone and cooling zone. Its service life mainly depends on the durability of the lining refractory materials at the sintering zone. Due to the influence of the thermal-mechanical and thermo-chemical comprehensive effects, the refractories in this area should have good physical and chemical indexes.
Thermal Shock Resistance
When the kiln turns one circle, the refractory material in the rotary kiln will be exposed once in the gas. In the rest of the time, it is covered by the cement raw materials. Because the gas temperature in the kiln is higher than that of the raw material, the refractory temperature will be subject to periodic thermal shock, and the temperature difference can reach over 400℃, which requires the refractory material to have excellent thermal shock resistance.
The cement clinker has strong alkalinity, and its silicate liquid formed at a high temperature can easily infiltrate into refractory bricks, resulting in changes or even damage in chemical composition and mineral composition of refractory materials. At the same time, the gases produced by fuel combustion and the volatilized sulfur, chlorine and alkali also have a strong chemical erosion effect on refractory materials. Therefore, the refractory material used in the sintering zone of cement rotary kiln should have excellent corrosion resistance.
The Moderate Thickness of Kiln Skin
When the cement rotary kiln is in normal operation, the hot surface of lining brick in sintering zone needs stable kiln skin to play a protective role, reducing the melting loss caused by high-temperature cement raw materials. At the same time, there are some certain requirements for the thickness of kiln skin, because when the kiln skin is thin, the melting loss of the refractory material is large, but if the kiln skin is too thick, it will not only affect the normal calcination and reduce the output, but also fall off some refractory materials due to the collapse of the thick kiln skin so that reduce the service life of the whole kiln lining.
The refractory materials used in the sintering zone of cement rotary kiln should not only have the properties we mentioned above but also have the properties of low thermal expansion coefficient, good flexibility, etc., so as to achieve good service effect. Therefore, understanding the characteristics of various refractories and choosing refractories reasonably are the main ways to improve the service condition of kiln lining and reduce the consumption of refractory materials.
Some Refractory Materials for Rotary Kiln Sintering Zone
The magnesite-chrome brick has excellent corrosion resistance and spalling resistance. At the same time, it has high strength, so it is widely used in the sintering zone of cement rotary kiln. However, the residual bricks after the magnesite-chrome brick are used contain a large amount of Cr6+, which will cause long-term environmental pollution. And there are fewer and fewer high-grade chromium ores in the world. This requires us to develop other refractories that can replace magnesite-chrome. In addition, magnesite-chrome bricks have strong volatility at high temperature, which will reduce the high temperature stability, slag resistance and spalling resistance of refractories, and shorten the service life of magnesite-chrome bricks. Although the chromium-free cement rotary kiln has been required for many years, it is still impossible to completely replace magnesite-chrome bricks with chromium-free bricks.
Japan is the first country to use magnesite-spinel bricks in cement rotary kilns. The purpose is to solve the Cr6+ pollution caused by magnesite-chrome bricks in use. Compared with magnesite-chrome bricks, magnesite-spinel bricks can improve the creep resistance and thermal shock stability of rotary kiln lining, and maintain good stability in oxidation or reduction atmosphere. Although the use of magnesite-spinel brick solves the chromium-free problem of cement rotary kiln to some extent, there are still many problems. For example, it is difficult to form a stable kiln skin by using magnesite-spinel brick in the rotary kiln sintering zone, which is easily affected by the atmosphere and heat load, so its service life has always been the bottleneck of the cement kiln. Secondly, it has high thermal conductivity, wastes resources and is easy to peel off. Therefore, improving the stability and spalling resistance of magnesite-spinel brick kiln skin is one of the important measures to improve the service life of kiln lining.
Dolomite bricks are regarded as the substitute of magnesite-chrome bricks which are widely used in cement rotary kilns. The biggest characteristic of it is the price advantage and the stable kiln skin formed by it. However, this kind of brick has poor hydration resistance. It is easy to react with water vapor in the air during production, transportation and kiln shutdown, which is the main factor preventing the large-scale use of dolomite refractories. In addition, the thermal shock stability of dolomite refractories is poor. When the cement rotary kiln is operated intermittently, it is easy to produce a large number of cracks. The effective ways to improve the thermal shock stability of dolomite refractories are to adopt appropriate particle composition, improve the rotary kiln system and add appropriate ZrO2.