ANALYSIS OF DAMAGE FORM OF REFRACTORY LINING OF CEMENT ROTARY KILN AND REQUIREMENTS FOR MASONRY SELECTION

1 Introduction
The damage of the refractory lining of the rotary kiln often affects the continuity of production and is one of the common equipment accidents. The reasons for the accident include design structure, refractory quality, masonry quality and operation and maintenance. Through a comprehensive analysis of various lining damage accidents, it is helpful to find out some common causes, and to take pre-control measures to avoid the occurrence of accidents to the greatest extent.
2. The role of rotary kiln refractory lining
(1) Prevent direct damage to the kiln body by high temperature flame or airflow, and protect the kiln cylinder body.
(2) Prevent the erosion of the kiln body by harmful substances (C0, S02).
(3) Prevent material and airflow from eroding the kiln body.
(4) Reduce the temperature of the kiln body to prevent the kiln body from being oxidatively eroded.
(5) It has the function of heat storage and heat preservation.
(6) It can improve the performance of hanging kiln skin.
3. The form of damage to the refractory lining
3.1 Common forms of damage
The refractory lining of rotary kiln is often subjected to the combined effects of mechanical stress, material friction, thermal stress, airflow and chemical erosion under the rotating state for a long time, which often leads to the occurrence of the following problems:
(1) The lifting block has been subjected to the eccentricity effect of mechanical rotation, the effect of high temperature and the impact friction of the stone for a long time, which leads to the distortion of the prefabricated block, the falling off of the refractory material, and the thinning of the thickness, which makes the refractory bricks filled between the lifting blocks deform and fall off.
(2) Melting loss of high temperature sintered layer.
(3) The airflow with a large temperature difference in the kiln body makes the dust sintered into blocks and adheres to the surface of the refractory material at high temperature. When the kiln body rotates, the separation of gravity makes the refractory material partially peel off, the brick lining is thinned, the temperature of the kiln body increases, and the steel structure Changes to varying degrees reduce the service life of the kiln body.
3.2 Probability of various damages
The German Refractory Technology Company conducted a large-scale experimental study on the used refractory materials, and calculated the probability of the main damage causes:
(1) Mechanical stress accounts for 37%: caused by the deformation of the cylinder and the thermal expansion of the brick.
(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 difference of kiln type, operation and the position 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 shell during operation, thus Subject the lining to a variety of different stresses.
4. Cause analysis and countermeasures of refractory damage
4.1 Mechanical stress damage
4.1.1 Thermal expansion squeezes out the refractory brick
When the temperature of the kiln rises to a certain extent, the thermal expansion will generate pressure in the axial direction of the kiln, causing the 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-laid refractory bricks shall be provided with reasonable side cardboards, and wet-laid refractory bricks shall be left with 2mm fire-clay joints.
(2) Leave a suitable block ring.
4.1.2 Iron plate stress damage
At the hot end of the refractory brick, the veneer iron plate and the magnesia in the magnesia brick react chemically at high temperature to form a magnesia-iron compound, which increases the volume and squeezes the refractory brick, causing horizontal fracture. In view of this situation, the practice of refractory brick veneer iron should be changed or replaced with fire clay.
4.1.3 Large area skew dislocation of refractory bricks
Because the masonry is too loose and the kiln is frequently started and stopped, the kiln shell is deformed, and the kiln shell and the cold surface of the lining brick move relative to each other, causing the lining brick to be skewed and dislocated 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 with a wooden hammer, the lock bricks should be locked, and the wedge iron should be added for the second time.
(2) Maintain a stable thermal system.
(3) The deformed part of the kiln cylinder is leveled with high temperature cement.
4.1.4 Ovality stress extrusion
Due to the increase in the gap between the rotary kiln wheel and the shim, the cylinder body has a large ovality, causing the refractory brick to be squeezed. The ovality of the cylinder should be checked regularly. If the oval value exceeds 1/10 of the diameter of the kiln, the backing plate should be replaced or the backing iron should be increased to adjust the gap between the tires.
4.1.5 Stress extrusion of locking iron
When locking the brick, too much iron at the lock mouth will lead to the formation of a brick ditch at the lock mouth. The following measures should be taken:
(1) At the same lock. The number of lock irons does not exceed 3 pieces.(2) The distance between the lock irons is as dispersed as possible.(3) The tightness of the inner and outer openings should be the same when locking the bricks.(4) The lock iron should be kept away from the thin lock brick as far as possible.
4.1.6 Extruded refractory bricks with retaining brick ring
The blocking bricks (special-shaped bricks) at the blocking brick ring are crushed and cracked due to extrusion. In this case, the single blocking block ring should be changed to a double blocking block ring, and the whole bricks should be laid on the blocking block ring to avoid processing special-shaped bricks. .
4.2 Thermal damage
4.2.1 Overheating
The local overheating of the temperature in the kiln leads to the melting of the refractory bricks and the formation of pits. In order to avoid this situation, the burner should be adjusted correctly and reasonable refractory materials should be selected in different parts.
4.2.2 Thermal shock phenomenon
Due to the thermal stress caused by the sudden temperature change, the brick surface is peeled and cracked, which is mainly caused by frequent switching on and off, extremely cold and extremely hot. The production operation should be stabilized, and a reasonable heating and cooling kiln system should be formulated.
4.3 Chemical attack damage
4.3.1 City erosion
The gas-phase alkali salt compound penetrates into the void of the brick body to condense and solidify, forming a horizontal permeable layer of alkali salt in the brick body, and the content of alkali salt entering the kiln should be reduced in production.
4.3.2 Hydration phenomenon
MgO reacts with water to form Mg(OH)2, which increases the volume and destroys the overall structure of the refractory brick. Since refractory bricks containing MgO and CaO will have a hydration reaction, it should be ensured to avoid moisture, waterproof and rain during storage, transportation and masonry of refractory bricks.
From the damage mechanism of the above refractory bricks, it can be seen that the standardization of refractory construction can effectively prolong the service life of refractory materials, and professional and dedicated masonry personnel are an important factor to ensure the quality of refractory construction.
5. Quality requirements for refractory masonry
5.1 Control before masonry
(1) During the handling of refractory materials, care should be taken to control the damage rate of refractory bricks within 3%.
(2) The work of laying out lines should be done well. The longitudinal reference line of the kiln should be placed four symmetrically along the circumference in a “cross” shape. Each line is parallel to the axis of the kiln; the circumferential reference line should be placed every 2m. parallel and perpendicular to the kiln axis.
(3) Ensure that the steel plate of the kiln body is clean, remove the corroded iron sheet, and strictly prohibit the use of refractory bricks whose edge damage and corner damage exceeds the control range.
5.2 Masonry process control
(1) During the construction process, ensure that the refractory materials are not damp, and the processed bricks are processed by a brick cutting machine. After cutting, the length of the brick must exceed 50% of the original brick length, and the thickness must reach more than 70% of the original thickness.
(2) The ring masonry method is used for masonry, the bricks are close to the kiln body, and it is necessary to ensure that all four corners of the bricks are in contact with the kiln body.
(3) The following common problems should be avoided in masonry construction: inversion of large and small heads, lottery, mixing, dislocation, inclination, uneven cement joints, climbing, off-center, heavy joints, through joints, open mouths, voids, hair joints, snakes Curved shape, masonry bulge, missing edges and corners.
(4) Use a wooden hammer or a rubber hammer when building refractory bricks, and it is strictly forbidden to use an iron hammer.
(5) The preparation of refractory mud is made of clean water, accurately weighed, evenly mixed, and used at any time. The prepared mud should not be used with any additional water, and the mud that has been initially set should not be used any more. Appliances are cleaned in a timely manner.
5.3 Brick circle lock seam control
(1) Only the original bricks can be used to lock the bricks, and the processed bricks shall not be used.
(2) If several bricks are used for lock seam, the lock seam bricks shall not be used in conjunction with each other, and the standard type shall be used alternately with each other; each type of lock seam bricks of each ring of refractory bricks shall not exceed two.
(3) Make sure that the horizontal seam of the brick is parallel to the kiln axis in the lock seam belt.
(4) The thickness of the lock seam metal plate is not more than 2mm.
(5) Only one lock-seam steel plate can be used in each seam. If several steel plates are needed, they should be evenly distributed in the entire lock-brick area, and the number of lock-seam steel plates per ring should not exceed four.
6. Principles of selection of refractory materials
When selecting refractory materials, the following requirements should be ensured:
(1) High temperature resistance. It can run in an environment above 800T for a long time.
(2) High strength and good wear resistance. The refractory material 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 material by the charge and flue gas, the refractory material needs to have good wear resistance.
(3) It has good chemical stability. To resist the erosion of chemicals in the flue gas.
(4) Good thermal stability. Able to withstand alternating stress in incineration state. When the furnace is stopped, started and the rotating operation is unstable, the temperature change in the kiln is relatively large, 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 the expansion coefficient of the rotary kiln refractory material, the shell temperature is generally around, and the temperature of the refractory material is generally above 8001, which may cause the refractory material to expand than the rotary kiln shell. To be large, easy to fall off.
(6) The porosity should be low. If the porosity is high, the flue gas will penetrate into the refractory material and erode the refractory material.
7. Conclusion
The configuration plan of the refractory bricks in the rotary kiln, the quality of the refractory bricks, the storage of the refractory bricks, the masonry of the refractory bricks, the drying of the rotary kiln, and improper handling of all aspects of production can affect the service life of the rotary kiln. Brick maintenance helps to use the most economical kiln lining to achieve the best results.