The damage of the refractory lining of the rotary kiln often affects the continuity of production. It is one of the common equipment accidents. The causes of the defects include design structure, refractory quality, masonry quality and operation and maintenance. A comprehensive analysis of various lining damage accidents is conducive to identifying some common causes, and taking pre-control measures to minimize the occurrence of accidents.
First, the role of rotary kiln refractory lining
Prevent high-temperature flame or airflow from directly damaging the kiln body, protect the kiln cylinder; prevent harmful substances (CO, SO2) from eroding the kiln; prevent material and airflow from eroding the kiln; reduce the temperature of the kiln and prevent the kiln from being Oxidative erosion; has the function of heat storage and heat preservation; can improve the performance of hanging kiln skin.
Second, the form of refractory lining damage
1. Common damage forms
The combined effects of mechanical stress, material friction, thermal stress, airflow and chemical erosion of the rotary kiln refractory lining under long-term rotation often lead to the following problems: First, the lifting block is subjected to the mechanical rotation gravity eccentricity effect for a long time. The high temperature effect and the impact friction of the stone cause the prefabricated block to be twisted, the refractory material is detached, the thickness is reduced, and the refractory brick filled between the lifting blocks is deformed and fallen off; the second is the melting loss of the high temperature calcined sintered layer; the third is the large temperature difference in the kiln body. The air flow causes the dust to be sintered at a high temperature to bond to the surface of the refractory material. When the kiln body rotates, the separation of the gravity causes the refractory material to partially peel off, the brick lining is thinned, the temperature of the kiln body is increased, and the steel structure changes to varying degrees, and the kiln is lowered. The life of the body.
2. Probability of various damages
After a large number of experimental studies on the refractory materials after use, and the statistics of the main causes of the main causes of damage are: mechanical stress accounted for 37%, due to deformation of the cylinder and thermal expansion of the brick; chemical erosion accounted for 36%, due to Caused by the erosion of clinker silicate and alkali salt; thermal stress accounted for 27% due to overheating and thermal shock.
With the different types of kiln, operation and 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. The lining is subjected to a variety of different stresses.
Third, the cause analysis of fire damage and countermeasures
1. Mechanical stress damage
(1) Thermal expansion squeezing refractory bricks
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 brick, the refractory brick surface will be peeled off. The following measures should be taken: dry refractory bricks, reasonable sideboards, 2mm fire mud joints for wet refractory bricks; suitable brick retaining rings.
(2) Iron plate stress damage
At the hot end of the refractory brick, the magnesium oxide in the veneer iron plate and the magnesia brick reacts at a high temperature to form a magnesium iron compound, which increases the volume and extrudes the refractory brick, causing horizontal fracture. In response to this situation, the refractory brick veneer should be replaced or replaced with fire mud.
(3) Large-area skewed dislocation of refractory brick
Because the masonry is too loose, the kiln is frequently opened and closed, resulting in deformation of the kiln cylinder, so that the kiln cylinder body and the cold surface of the lining brick move relatively, causing the lining brick to be twisted and misaligned and the brick surface to burst and fall off. The following measures should be taken: when building, the large surface of the refractory brick is hammered with a wooden hammer, the locking brick should be locked, and the wedge iron is added twice; the stable thermal system is maintained; the deformed part of the kiln cylinder is flattened with high temperature cement.
(4) Elliptical stress extrusion
Due to the increase of the horn clearance of the rotary kiln wheel, the cylinder body has a large ellipticity, which causes the refractory brick to be squeezed. The ellipticity of the cylinder should be checked regularly. If the ellipse value exceeds 1/10 of the diameter of the kiln, the pad should be replaced or the horn should be added to adjust the wheel gap.
(5) Locking iron stress extrusion
When the brick is locked, the iron in the lock is too tight, which will result in the formation of a brick groove at the lock. The following measures shall be taken: at the same lock, the number of irons in the lock shall not exceed 3; the spacing of the iron at the lock shall be as dispersed as possible; the outer and outer joints shall be loose when the brick is locked; the iron at the lock shall be kept away from the thin lock brick as far as possible.
(6) brick ring extrusion refractory brick
The retaining bricks (shaped bricks) at the brick blocking ring are subject to crushing and cracking, and in this case, the single-track brick ring should be changed into a double-stop brick ring, and the bricks should be built on the brick blocking ring to avoid processing the shaped bricks. .
2. Thermal damage
Local overheating of the kiln temperature causes the refractory brick to melt and form a pit. In order to avoid this, the burner should be properly adjusted and a reasonable refractory material should be selected at different locations.
(2) Thermal shock phenomenon
The thermal stress caused by sudden changes in temperature causes spalling and cracking of the brick surface, mainly due to the frequent opening and closing of extremely cold and extremely hot. Stable production operations should be established and a reasonable heating and cooling kiln system should be established.
3. Chemical erosion damage
(1) Alkali erosion
The gas phase alkali salt compound penetrates into the voids of the brick body to condense and solidify, and forms a horizontal permeation layer of the alkali salt in the brick body, and the alkali salt content in the kiln should be reduced in the production.
MgO reacts with water to form Mg(OH)2, which increases in volume and destroys the overall structure of the refractory brick. Since refractory bricks containing MgO and CaO have hydration reactions, it is necessary to ensure that moisture, water and rain are avoided during storage, transportation and masonry of refractory bricks.
It can be seen from the damage mechanism of the above refractory bricks that the standardization of refractory construction can effectively extend the service life of refractory materials, and professional and dedicated masonry personnel are important factors to ensure the quality of refractory construction.
Fourth, the quality requirements of refractory masonry
1. Pre-laying control
Refractory materials should be carefully handled during handling, and the breakage rate of refractory bricks should be controlled within 3%. The line shall be ready for laying. The vertical reference line of the kiln shall be placed in a “ten” shape and symmetry along the circumference. Each line shall be parallel to the axis of the kiln; each line of the hoop reference line shall be placed at 2 m, and each line shall be parallel and Vertical to the kiln axis. Ensure that the kiln plate is clean and remove the corroded iron. It is strictly forbidden to use the refractory brick with the edge loss and corner loss exceeding the control range.
2. Masonry process control
During the construction process, the refractory material is protected from moisture, and the processed bricks are processed by a brick cutter. After cutting, the length of the brick must exceed 50% of the length of the original brick, and the thickness must reach 70% of the original thickness. The masonry is carried out by the ring-laying method, and the bricks are closely attached to the kiln body, and it is necessary to ensure that the four corners of the brick are in contact with the kiln body. The following common problems should be avoided in masonry: upside down, lottery, mixing, misalignment, tilting, uneven crevices, climbing, detachment, heavy seams, through seams, open mouth, void, crevices, serpentine bends, Masonry bulge, missing corners.
Wood hammers or rubber hammers should be used for refractory brick masonry. It is strictly forbidden to use hammers. The refractory mud is prepared by using clean water, accurately weighed, evenly adjusted, and used with the adjustment. The prepared mud should not be used any more. The mud that has been initially set up must not be used any more, and the different quality mud should be cleaned in time. clean.
3. Brick ring lock control
Only use the original bricks to lock the bricks, and do not use the processing bricks. If several bricks are used to lock the seams, the jointed bricks shall not be used in conjunction with each other. The standard type shall be used in conjunction with each other; no more than two types of lock seam bricks per type of refractory brick. In the lock seam belt, ensure that the horizontal seam of the brick is parallel to the kiln shaft. The thickness of the metal plate of the lock seam is not more than 2 mm. Only one piece of lock seam steel plate can be used in each joint. If several steel plates are required, they should be evenly distributed in the whole lock brick zone. The number of blocks of each ring lock seam steel plate should not exceed 4.
Fifth, the principle of selection of refractory materials
When selecting refractory materials, the following requirements should be met:
(1) High temperature resistance. It can operate in high temperature environment 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 stress caused by the expansion stress at high temperature and the deformation of the rotary kiln shell. At the same time, due to the wear of the refractory material by the charge and the 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 smoke.
(4) Good thermal stability. It can withstand the alternating stress in the incineration state. When the conditions of shutdown, start-up, and rotating operation are unstable, the temperature change in the kiln is relatively large, and there is no possibility of cracking or peeling.
(5) Thermal expansion stability. The refractory material may expand larger than the rotary kiln shell and may fall off easily.
(6) The porosity is low. If the porosity is high, the fumes will penetrate the refractory material and erode the refractory material.
In summary, the configuration of the refractory bricks in the rotary kiln, the quality of the refractory bricks, the storage of the refractory bricks, the refractory brick masonry, the rotary kiln drying kiln, and the improper handling of all aspects of the production can affect the service life of the rotary kiln. A series of measures to maintain the refractory bricks helps to achieve the best results with the most economical kiln lining.