Why Use Alkaline Refractory Bricks for Precalcined Cement Kilns?

Why Use Alkaline Refractory Bricks for Precalcined Cement Kilns? During the calcination process of the pre calcining kiln clinker, a large amount of melt (liquid phase) appears in the kiln material from the back end of the upper transition zone. Some ingredients in the kiln material are calcined in the melt through the raw maturation material until it is out of the kiln. Within this range, alkaline refractory bricks that withstand high temperatures can meet their process performance requirements. The masonry length of alkaline refractory bricks is about (6~10)D of the kiln diameter, which is also the most severely damaged part of the lining brick. The use of the lining brick in this part directly affects the operation rate of the kiln. Therefore, the technological progress and product performance, and output status of basic refractory bricks are paid attention to by people. At present, the alkaline refractory bricks used in the pre-calcining kiln are mainly spinel magnesia bricks that directly join magnesia-chrome bricks and magnesia-aluminum bricks. This article mainly introduces the direct bonding of magnesia-chrome bricks.

Directly Bonded Magnesia-Chrome Brick

Brief Introduction of Directly Bonded Magnesia-Chrome Brick for Cement Kilns

Magnesia-chrome bricks appeared in the 1930s. At that time, the performance of silicate-combined magnesia-chrome bricks (common magnesia-chrome) was still relatively poor and was used in traditional rotary kilns. Beginning in the 1960s, there have been direct-joined magnesia chrome bricks (straight magnesia bricks) with better performance. Due to the high-temperature resistance, melt erosion resistance, redox resistance, mechanical stress resistance, and excellent kiln hanging performance of straight magnesia bricks, after being put into use, it is widely used in the transition zone and firing zone of the preheater kiln. Become the mainstream.

In the 1980s, the output and diameter of the pre-calcining kiln were further expanded. In addition, due to the rapid increase in crude oil prices, coal with high ash content and petroleum coke with high sulfur content gradually replaced fuel oil. The high content of alkali sulfur makes the straight magnesia bricks in the upper and lower transition zone of the kiln vulnerable to damage and shortens the service life. As a result, magnesia-aluminum spinel bricks with strong alkali-sulfur corrosion resistance have been used in the upper and lower transition zones of some large-scale pre-calcining kilns. At this point, the dominance of magnesia bricks began to break. In the late 1980s, some industrialized countries further emphasized the problem of chromium pollution, that is, when magnesia-chrome bricks are used in cement kilns, Cr3+ is transformed into Cr6+ which is harmful to the environment and human body. Industrialized countries have formulated a series of regulations on environmental protection, sanitation, and cement-related ingredients. Fully monitor the residual bricks of cement kiln magnesia-chrome bricks, the drainage of cement plants, and the chromium content of cement products.

Take Germany as an example. Its decree stipulates that stored magnesia-chromium bricks shall not pollute water sources under any climatic conditions. This only requires a strict watertight packaging method, and the packaging fee is equivalent to 1/4 to 1/3 of the brick price. In addition, Germany and the European Community both stipulate that the content of Cr6+ in drinking water should be ≤0.05mg/l, and for the drainage of cement industry into rivers, Germany stipulates that Cr6+≤0.3mg/L. Some foreign experts conservatively estimate that 1m3 of magnesia-chrome bricks will pollute about 10,000m3 of river water and about 60,000m3 of drinking water.

In the 1990s, some cement companies in industrialized countries used relatively cheap industrial waste as raw material fuel due to environmental protection needs. Increased the alkali sulfur's erosion of kiln lining materials, and put forward more demanding requirements for alkali refractories. To promote the further optimization of magnesia-chrome bricks, super-direct bonded magnesia-chrome bricks with better performance have appeared in the pre-calcining kilns with more severe working conditions. On the other hand, the process of chromium-free alkaline bricks is accelerated to further improve and improve technology. In the 21st century, chromium-free alkaline bricks have completely replaced magnesia-chrome bricks in some industrialized countries and have better performance than magnesia-chrome bricks. It is specifically reflected in the development trend of the further increase in the operation rate of the pre-calciner kiln and the further decrease in brick consumption.

In my country, in 2014, the Ministry of Environmental Protection formulated the solid waste production cement pollution control standard, which specified the limit of the pollutant content in cement products, Cr6+, to be 0.05mg/L. Its value is close to that of industrialized countries. It can be said with certainty that the limit value will definitely become stricter over time. Although restricted by environmental protection regulations, magnesia-chrome bricks are still used by some cement plants due to their excellent kiln-hanging properties and relatively low prices. They are the most consumed variety in cement production. But the general trend is to decline year by year. At present, some people of insight in my country's cement industry have raised the issue of chromium pollution in some conferences and articles, which has attracted the attention of relevant leaders.

At the same time, some design units have reduced or stopped using magnesia-chrome bricks in their drawings. Some large domestic cement groups and foreign-funded enterprises have partially or completely stopped using magnesia-chrome bricks. The above situation greatly reduces the usage of magnesia-chrome bricks. In addition, the fuel used in some areas contains high sulfur content, which intensifies the chemical erosion of magnesia-chrome bricks, forcing companies to reduce or stop using them. As the country further strengthens the environmental protection of water resources and controls the chromium content in cement. The regulation on the limit value of hexavalent chromium in magnesia-chrome brick residual bricks will be strict with time. In the end, forcing magnesia-chrome bricks to withdraw from the cement kiln, manufacturers that produce magnesia-chrome bricks must arouse great attention.

Rongsheng refractory manufacturer provides high-quality refractory materials for cement kilns. Including direct bonding of magnesia chrome bricks, magnesia spinel bricks, and various wear-resistant castables. For more information about alkaline refractory bricks, please visit our website: https://rongshengrefractory.com/several-kinds-of-alkaline-refractory-bricks/.