Damage of refractory lining of rotary kiln often affects the continuity of production and is one of the common equipment accidents. The causes of the accident include design structure, refractory material quality, masonry quality and operation and maintenance. Through comprehensive analysis of various lining damage accidents, it is helpful to find out some common causes and take preventive measures in advance to avoid accidents to the greatest extent.
01. Forms of refractory lining damage
1. Common damage forms
The refractory lining of rotary kiln often causes the following problems due to the combined effects of mechanical stress, material friction, thermal stress, airflow and chemical erosion under long-term rotation:
(1) The lifting block is subjected to the mechanical rotation gravity eccentricity effect, high temperature effect and stone impact friction for a long time, which leads to the distortion of prefabricated blocks, the fall of refractory materials and the thinning of thickness, and the deformation and fall of refractory bricks filled between the lifting blocks.
(2) Melting loss of high-temperature calcined layer.
(3) The airflow with large temperature difference in the kiln body causes the dust to be sintered into blocks and adhered to the surface of the refractory material through high temperature sintering. When the kiln body rotates, the separation of gravity causes the refractory material to be partially peeled off, the brick lining becomes thinner, the kiln body temperature increases, and the steel structure changes to varying degrees, reducing the service life of the kiln body.
2. Probability of various damages
German Refractory Technology Company conducted extensive experimental research on refractory materials after use and calculated the probability of the main causes of damage:
(1) Mechanical stress accounts for 37%: caused by the deformation of the cylinder and the thermal expansion of bricks.
(2) Chemical erosion accounts for 36%: caused by the erosion of clinker silicates and alkali salts.
(3) Thermal stress accounts for 27%: caused by overheating and thermal shock.
With the different kiln types, operations and different positions of the kiln lining in the kiln, the above three factors play different roles, mainly depending on the deformation state of the flame, kiln material and kiln cylinder during operation, so that the lining is subjected to various stresses.
02. Causes of refractory damage and countermeasures
1. Thermal expansion squeezes refractory bricks
When the kiln temperature rises to a certain level, thermal expansion will generate pressure in the kiln axial direction, causing adjacent refractory bricks to squeeze each other. When the pressure is greater than the strength of the refractory bricks, the refractory brick surface will peel off. The following measures should be taken:
(1) Dry-lay refractory bricks, set reasonable side paperboards, and wet-lay refractory bricks with 2mm fire mud gaps.
(2) Leave a suitable brick retaining ring.
2. Iron plate stress damage
At the hot end of the refractory brick, the veneer iron plate and the magnesium oxide in the magnesium brick react chemically at high temperature to form magnesium-iron compounds, which increase the volume and squeeze the refractory bricks, causing horizontal fractures. In response to this situation, the practice of veneer iron for refractory bricks should be changed or replaced with fire mud.
3. Large-area twisting and dislocation of refractory bricks
Due to loose masonry and frequent kiln opening and stopping, the kiln shell is deformed, causing the kiln shell and the cold surface of the lining brick to move relative to each other, causing the lining brick to twist and dislocate, and the brick surface to burst and fall off. The following measures should be taken:
(1) During masonry, the large surface of the refractory bricks should be hammered solid, the lock bricks should be locked, and wedge irons should be added for the second time.
(2) Maintain a stable thermal system.
(3) The deformed part of the kiln shell should be leveled with high-temperature cement.
4. Ovality stress extrusion
Due to the increase in the gap between the rotary kiln wheel shims, the cylinder has a larger ovality, causing the refractory bricks to be squeezed. The cylinder ovality should be checked regularly. If the ovality value exceeds 1/10 of the kiln diameter, the pad should be replaced or the shims should be added to adjust the wheel shim gap.
5. Locking iron stress extrusion
When locking bricks, too much locking iron will cause brick grooves to form at the locking point. The following measures should be taken:
(1) At the same locking point, the number of locking irons should not exceed 3.
(2) The spacing between locking irons should be as dispersed as possible.
(3) When locking bricks, the tightness of the inner and outer openings should be consistent.
(4) The locking iron should be kept as far away from thin locking bricks as possible.
6. Brick retaining ring squeezes refractory bricks
The brick retaining ring (special-shaped bricks) breaks and cracks due to extrusion. In this case, the single-track brick retaining ring should be changed to a double-track brick retaining ring, and whole bricks should be laid on the brick retaining ring to avoid processing special-shaped bricks.
7. Overheating phenomenon
The local overheating of the temperature in the kiln causes the refractory bricks to melt and form pits. In order to avoid this situation, the burner should be adjusted correctly and reasonable refractory materials should be selected in different parts.
8. Thermal shock phenomenon
The thermal stress caused by sudden temperature changes causes the brick surface to peel and crack, which is mainly caused by frequent opening and closing of extreme cold and extreme heat. The production operation should be stabilized and a reasonable heating and cooling kiln system should be formulated.
9. Chemical erosion damage
The gas-phase alkali salt compound penetrates into the gap of the brick body, condenses and solidifies, and forms a horizontal permeable layer of alkali salt in the brick body. The alkali salt content entering the kiln should be reduced during production.
From the above damage mechanism of refractory bricks, it can be seen that the standardization of refractory material construction can effectively extend the service life of refractory materials, and professional and dedicated masonry personnel are important factors in ensuring the quality of refractory material construction.
03. Principles for the selection of refractory materials
When selecting refractory materials, the following requirements should be met:
(1) High temperature resistance. Able to operate in an environment above 800T for a long time.
(2) High strength and good wear resistance. The refractory materials in the rotary kiln must have a certain mechanical strength to withstand the expansion stress at high temperature and the stress caused by the deformation of the rotary kiln shell. At the same time, due to the wear of the refractory materials by the charge and flue gas, the refractory materials must have good wear resistance.
(3) Good chemical stability. To resist the erosion of chemical substances in the flue gas.
(4) Good thermal stability. Able to withstand the alternating stress under the burning state. When the furnace is stopped, started, and the rotary operation is unstable, the temperature in the kiln changes greatly, and there should be no cracking or peeling.
(5) Thermal expansion stability. Although the thermal expansion coefficient of the rotary kiln shell is greater than that of the rotary kiln refractory material, the shell temperature is generally around 10000℃, while the temperature of the refractory material is generally above 8001℃. This may cause the refractory material to expand more than the rotary kiln shell and fall off easily.
(6) The porosity should be low. If the porosity is high, the flue gas will penetrate into the refractory material and corrode the refractory material.
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