Cement Rotary Kiln “longevity Secret”: Magnesium Aluminum Spinel Brick Strong Empowerment

 Cement rotary kiln "longevity secret": Magnesium Aluminum Spinel Brick strong empowerment

The cement rotary kiln is the core equipment in the cement production process, and its working environment is extremely harsh. The temperature inside the kiln usually fluctuates between 1000℃ - 1600℃, and even reaches a high temperature of about 1800℃ in the firing zone. At the same time, the material in the kiln constantly rolling, sliding, the kiln lining produces strong mechanical wear. In addition, there are complex chemical erosion in the kiln, such as alkaline substances (mainly from the alkali metal oxides in the cement raw material) and acidic substances (such as sulfur oxides, etc.), which react with the kiln lining material at high temperatures and accelerate the damage of the material.

High refractoriness and high strength

Magnesia-aluminum spinel bricks, with magnesia-aluminum spinel as the main mineral component, have a high melting point, generally up to 2135°C. This enables them to maintain a stable physical form under the high temperature environment of the cement rotary kiln. This makes it in the high temperature environment of the cement rotary kiln, can maintain a stable physical form, not easy to soften and melt. At the same time, it has high strength characteristics, compressive strength is usually between 80 - 120MPa, can withstand the frequent impact and friction of the material in the kiln, greatly reducing the loss of material due to mechanical abrasion.

Excellent thermal shock stability

During the operation of cement rotary kiln, the kiln lining will experience drastic temperature changes due to kiln start-up, kiln shutdown and production process adjustment. Magnesia-aluminum spinel bricks show excellent thermal shock stability by virtue of their special crystal structure and thermal expansion properties. With its low and uniform coefficient of thermal expansion, the brick body produces less thermal stress when the temperature fluctuates rapidly, effectively avoiding problems such as cracking and spalling caused by thermal shock. For example, after several simulated thermal shock tests, magnesia-aluminum spinel bricks can still maintain their structural integrity and their performance is basically unaffected when they are rapidly cooled down from 1000℃ to room temperature during the cyclic process.

Strong resistance to erosion

In the complex chemical environment in the cement rotary kiln, magnesia-aluminum spinel bricks show strong anti-erosion ability. On the one hand, for the erosion of alkaline substances, MgAl spinel itself has a certain degree of alkalinity, and the chemical reaction activity with the alkaline erosion medium is low, which can prevent the alkaline substances from penetrating and eroding into the brick body. On the other hand, for sulfur oxides and other acidic substances, magnesium-aluminum spinel bricks in certain components can react with acidic oxides, forming a layer of dense reaction product layer on the surface of the brick body, which can effectively block the further erosion of acidic substances, thus prolonging the service life of the brick body.

Transition zone

The transition zone is located between the high-temperature zone and the medium-temperature zone of the cement rotary kiln, where the temperature fluctuates greatly and the physical and chemical properties of the materials change significantly. Magnesia-aluminum spinel bricks are the ideal choice of lining material for the transition zone due to their excellent thermal shock stability and erosion resistance. Here, it can withstand frequent changes in temperature and the alternating erosion of alkaline and acidic components in the material, effectively reducing the number of repairs and replacement of kiln liners in the transition zone. Actual production data show that after using magnesium-aluminum spinel bricks as kiln linings in the transition zone, the service life of kiln linings in this area has been extended from 3-5 months to 8-12 months, which greatly improves the operation rate of the cement rotary kiln and reduces production costs.

Firing zone

The firing zone is the highest temperature zone in a cement rotary kiln and requires high refractoriness and erosion resistance of the kiln lining material. Magnesia-aluminum spinel bricks, with their high refractoriness, high strength and excellent erosion resistance, are also widely used in the firing zone. In the firing zone, it can not only withstand the direct scouring of high-temperature materials and the radiation of high-temperature flames, but also resist the strong erosion of high concentrations of alkalis and sulfur oxides in the materials. Compared with conventional kiln lining materials for the firing zone, the average service life of the kiln lining in the firing zone has been increased from 1 - 2 months to 3 - 6 months with the use of Mg-Al Spinel bricks, and at the same time the clinker quality of the cement has been improved to a certain extent, as the stable kiln lining environment is conducive to the homogeneous firing of the clinker.

Cooling Belt

Although the temperature in the cooling zone is reduced compared to the firing zone, it is still in the high temperature range, and the material will release some water vapor and acidic gases during the cooling process. Magnesia-aluminum spinel bricks also perform well in the cooling belt. Its good thermal shock resistance enables it to adapt to the rapid change of temperature in the cooling belt, preventing damage to the bricks due to the sudden drop in temperature. In addition, it also has a certain resistance to the erosion of water vapor and acidic gases, which ensures the long-term stable operation of the kiln lining in the cooling belt. In some cement enterprises that use magnesium-aluminum spinel bricks as cooling belt kiln linings, the maintenance cycle of the cooling belt kiln linings has been extended from once every 2-3 months to once every 6-8 months, reducing equipment downtime and improving production efficiency.

The common forms of damage to magnesium-aluminum spinel bricks in cement rotary kilns are mainly the following:

1. Thermal shock damage: cement rotary kiln in operation, the temperature changes frequently and violently. When the temperature rises or falls sharply, magnesium-aluminum spinel bricks will produce thermal stress inside. Due to the inconsistency of expansion or contraction of various parts of the brick body, this thermal stress repeatedly, it will make the brick body cracks, with the passage of time, cracks continue to expand, eventually leading to brick spalling, falling blocks, affecting its service life.

2.Chemical erosion: The cement production process will produce a variety of chemical components, such as alkali metal oxides (K₂O, Na₂O), sulfur oxides (SO₂, SO₃) and so on. These substances will have chemical reactions with magnesium aluminum spinel bricks, for example, alkali metal oxides will react with alumina in the bricks to produce low melting point compounds, reducing the refractory properties of the bricks; sulfur oxides will react with magnesium oxide in the bricks to form sulfates such as magnesium sulfate, and these sulfates will crystallize and expand at a certain temperature to make the bricks loose in structure, and then damaged.

3.Mechanical abrasion: The materials in the cement rotary kiln are in constant motion, which will produce scouring and friction on the magnesium-aluminum spinel bricks on the kiln wall. Especially in the material falling parts and kiln rotation, the material and brick body contact more frequently, mechanical wear is more serious. Prolonged wear and tear will make the brick surface gradually thin, reduce the thickness of the brick, and ultimately lead to the loss of the protective effect of the brick.

4.High-temperature creep: In the high-temperature environment of the cement rotary kiln, magnesium-aluminum spinel bricks will undergo a certain degree of creep phenomenon. With the extension of time, the brick body will gradually produce plastic deformation under the action of its own weight and external pressure. This deformation will lead to an increase in the gap between the bricks, affecting the integrity and sealing of the bricks, making it easier for high-temperature gases and materials to penetrate into the seams of the bricks, accelerating the damage of the bricks.

5.Spalling: on the one hand, due to thermal shock, chemical erosion and other factors, the surface of the brick body will form a layer of metamorphic layer, this layer of metamorphic layer and the internal non-metamorphic brick body between the bond will gradually weaken. Under the action of high temperature and material impact in the kiln, the metamorphic layer is easy to peel off from the brick body. On the other hand, when the brick body is subjected to large external force or thermal stress concentration, it may also lead to localized brick spalling, resulting in damage to the brick body.

Prevention of thermal shock damage

Control the rate of heating and cooling: in the cement rotary kiln heating process, strictly control the rate of heating, generally should not exceed 30 - 50 ℃ / hour. Avoid excessive thermal stress inside the magnesium-aluminum spinel bricks due to too fast heating, thus triggering cracking of the brick body. In the kiln cooling, the same should control the rate of cooling, slow cooling, to prevent the brick body due to the sudden drop in temperature and damage.

Reduce temperature fluctuations: Keep the temperature in the cement rotary kiln relatively stable, to avoid large fluctuations in the kiln temperature due to improper adjustment of the production process or unstable burner work. By optimizing the combustion control strategy, ensure the full and stable combustion of fuel and maintain the uniformity of the temperature field inside the kiln. At the same time, strengthen the heat preservation measures for the kiln body to reduce heat loss and lower the degree of temperature inside the kiln affected by the external environment.

Coping with chemical attack

Control of raw materials and fuels: The raw materials and fuels used in cement production are strictly tested to control the content of alkali metal oxides (such as sodium oxide and potassium oxide), sulfur oxides and other harmful components. Try to use low alkali and low sulfur raw materials and fuels to reduce the entry of chemical erosion substances from the source. For example, when the alkali content of raw materials is too high, the proportion of silica-aluminum-based raw materials can be increased by appropriately adjusting the batching program to reduce the alkalinity of the materials.

Regular cleaning of the kiln crust: During the operation of the cement rotary kiln, certain components in the kiln materials may form crusts on the surface of magnesium-aluminum spinel bricks. These crusts will have chemical reactions with the brick body at high temperatures, accelerating the erosion of the brick body. Therefore, the kiln should be cleaned regularly (e.g., once a month) by mechanical or manual methods to keep the brick surface clean and reduce the occurrence of chemical erosion.

Preventing mechanical wear

Optimize the trajectory of material movement: optimize the trajectory of material movement in the kiln by adjusting the rotary kiln's rotational speed, inclination and the arrangement of internal lifting plate, etc., so as to reduce the direct scouring of material on magnesium-aluminum spinel bricks and abrasion. Ensure that the material is evenly distributed in the kiln, to avoid excessive accumulation of local materials leading to increased wear.

Check and maintain the kiln equipment: Regularly check other equipment in the kiln, such as burners, chains, etc., to ensure that they are firmly installed and operating normally. If these equipments are loose or displaced during the working process, they may collide or friction with magnesium-aluminum spinel bricks, causing damage to the bricks. Repair or replace the damaged equipment parts in time to ensure the good operating condition of the kiln equipment.

Localized repair and replacement

Brick repair technology: when the magnesium-aluminum spinel bricks are found to have local cracks or small area spalling, hot patching technology can be used to repair. Hot patching material should be selected and magnesium aluminum spinel brick composition similar, performance matching refractory materials. Through the hot patch material heated to the appropriate temperature, quickly coated or sprayed to the damaged parts, so that it is closely combined with the original brick body to restore the integrity of the brick body. For larger areas of damage, the method of setting new bricks can be used for repair. First, carefully remove the old bricks in the damaged area, clean up the foundation surface, and then set new magnesium-aluminum spinel bricks in accordance with the correct masonry process to ensure the quality of masonry between the new bricks and the surrounding brickwork.

Replacement cycle determination: According to the daily monitoring data and actual use, the overall replacement cycle of magnesium-aluminum spinel bricks is reasonably determined. Generally speaking, in the transition zone, due to the temperature fluctuation and chemical erosion is more serious, magnesium aluminum spinel brick replacement cycle may be 6 - 12 months; firing zone due to the extremely high temperature, the wear and erosion of the brick body faster, the replacement cycle may be 3 - 6 months; cooling zone is relatively low temperature, the working environment is better, the replacement cycle may be 12 - 18 months. However, the actual replacement cycle should be adjusted according to the specific operating conditions of each enterprise's cement rotary kiln.

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