Many cement companies have encountered the problem of newly laid refractory bricks falling during kiln drying. Despite employing professional maintenance teams, specialized tools, and high-quality refractory materials, newly laid refractory bricks often fall off near the end of the heat-up period and just before the kiln is filled. In severe cases, many bricks are found to be severely twisted and tilted in the rings after entering the kiln, forcing the kiln to be re-cooled and re-bricked. This problem never occurred before when using bulky and outdated screws.

Not to mention the negative impact of these extreme temperatures on the lifespan of refractory materials, simply addressing the problem is extremely complex and dangerous.

Site Description

Loose refractory bricks are particularly prone to falling bricks. On-site observations show: large gaps between bricks, large circumferential joints, the bottom of the bricks not firmly against the kiln shell, poorly even refractory mortar application, and uneven, "stepped" surfaces. These two common causes are:

(1) The refractory bricks in the kiln are severely twisted and deformed, and the axial and circumferential gaps between the bricks are large. The surface of the refractory bricks is uneven. The protruding refractory bricks can be smashed in with a sledgehammer, and some can even be stepped in with feet. Many locking iron plates need to be added to the gaps each time they are handled.

(2) The refractory bricks at the rear of the retaining ring often ride on the upper part of the retaining ring. The bottom of the refractory bricks and the inner wall of the cylinder are not firmly attached, and there is even a gap of several centimeters. The refractory bricks on the upper part of the retaining ring are crushed or broken.

Cause Analysis

2.1 Untightened Brick Locking

Untightened brick locking is the main reason for the falling of refractory bricks. Judging from the on-site situation, most of the brick falling is caused by this. The main reasons for the loose brick locking are:

(1) Improper use of cement. During the masonry process, glue is needed to bond the kiln bricks and also to level the kiln bricks. If the glue is not applied evenly, it is easy to cause the proportion of the refractory bricks to be out of balance, resulting in the brick surface tilting and the bricks having different tightness. The impact of using glue is particularly prominent during winter construction. Due to the lower ambient temperature, the fluidity of the refractory glue deteriorates and there is a phenomenon of freezing. When the locking iron plate is used to lock the entire ring, the glue has already frozen. During the heating process, the glue begins to melt. The glue in the brick joints is squeezed by the gravity of the refractory bricks and loses, causing the annular brick joints to expand. When the expansion reaches a certain extent, the refractory bricks loosen and fall off, causing brick falling accidents.

(2) The locking degree of each ring of refractory bricks is not enough during masonry. When using a bricklaying machine to lay bricks, a common method used to meet deadlines is not to lock each ring of bricks into place all at once. Instead, they lock them properly before starting to lay the next ring. After all bricks are laid, the bricklaying machine is used to hammer an iron plate on the top of the refractory bricks in the kiln from the kiln tail to the kiln head. This laying method has a disadvantage: since the bricks are not locked at the beginning, they will inevitably slide downward under the action of their own gravity, causing the bottom surface of the refractory bricks to not be tightly attached to the inner wall of the kiln, resulting in voids. When the voids are formed, adding a locking iron plate will only increase the tightness between the ring refractory bricks and will not change the void problem. When the kiln temperature rises to a certain level and the rotary kiln needs to be turned over intermittently, the voids are aggravated. (3) Insufficient pressure of the bricklaying machine. The working pressure of the bricklaying machine is not enough to support the refractory bricks and make them close to the kiln shell, resulting in a slight voiding phenomenon: During the construction process, it is necessary to ensure that the air supply pressure of the air compressor is between 0.55MPa and 0.65MPa, and this is our weak point: during winter construction, due to the long compressed air pipeline connecting the bricklaying machine, the different thickness of the connecting pipes, the oil-water separator problem of the air compressor, the air storage tank is not drained frequently, the connecting pipeline leaks, etc., the air supply pressure of the bricklaying machine will be insufficient. Insufficient air supply pressure will lead to insufficient pressure on the top bricks of the bricklaying machine, resulting in voiding at the bottom of the refractory bricks.

2.2 Masonry technique problems

(1) There are two standards for refractory bricks: VDZ and ISO standards. The specifications and proportions of refractory bricks in each standard vary according to the type of kiln. Construction workers cannot blindly follow the designed masonry proportions for construction. They must comprehensively consider the elliptical shape of the kiln shell and the deviation of the external dimensions of the refractory bricks.

(2) The masonry of the cut brick part is unreasonable. Due to the lack of foresight in masonry, bricks need to be processed at the end of the masonry, especially when the length of the processed bricks is less than 60% of the original brick length. The probability of twisting and falling bricks when laying a ring of processed bricks alone is very high. If the length of the processed bricks is less than 60% of the original brick length, the adjacent ring of standard bricks should be removed and standard bricks and processed bricks should be used to lay them alternately to eliminate the ring joints.

(3) The ring joints are twisted and the length of the processed bricks is greatly deviated at the end. This is mainly because the cylinder line is not laid out before masonry and the control of the ring joints is improper. There is no awareness of adjusting the ring joints when laying the bottom and sealing the brick machine, or no adjustment is made according to the ring line, resulting in the ring joints after the brick ring is laid not being on the same plane. The distances up and down and left and right are inconsistent when the end is closed. In serious cases, the deviation can reach more than 100mm, resulting in a large deviation in the length of the final processed bricks and the difficulty in ensuring the quality of masonry.

2.3 Bricklaying Machine Disadvantages

When using a screw-type screw for masonry, the locking point is always below the centerline of the rotary kiln. Due to their own weight, the bottom surfaces of the refractory bricks below the centerline are firmly against the inner wall of the kiln, effectively locking the bricks. However, after masonry is completed using a bricklaying machine, the refractory bricks above the center of the rotary kiln are unable to maintain a firm grip against the inner wall due to their own weight, resulting in slight gaps. Our circumferential tightening work only involves adding iron plates to the top locking points. However, even with these additional plates, the refractory bricks remain loose, which explains why, when using a screw-type screw, the refractory bricks only broke, not fell off.

At every step of bricklaying with a screw, the end faces of the bricks are firmly in contact with the inner wall of the kiln shell. Furthermore, at every stage of bricklaying, the bricks are positioned below the centerline of the shell. The weight of the bricks also holds the refractory bricks tightly against the inner wall. When tightening the bricks with the additional iron plates, they are always placed below the radius of the shell. This ensures that the bottom surfaces of the bricks are firmly in contact with the inner wall. Overtightening can lead to refractory brick breakage, a common problem when using a screw.

Insufficient pressure in the bricklaying machine, the weight of the refractory bricks, and the method of only attaching the additional iron plates at the top can all result in loose contact between the top refractory bricks and the inner wall. This is not noticeable during bricklaying, but during operation, relative slippage occurs, causing the refractory bricks to loosen and fall off.

Solutions

3.1 Strengthen Equipment Maintenance and Care

Insufficient pressure in the bricklaying machine requires enhanced equipment maintenance and care. To ensure the effectiveness of the oil-water separator, the air tank should be drained frequently. During construction, the air compressor should operate normally, ensuring that the compressed air pressure remains within the range of 0.55MPa to 0.65MPa.

3.2 Brick Locking Instructions

When locking bricks, ensure that the bottom surface of the kiln bricks is as close to the inner wall of the kiln as possible. After tightening one ring, begin laying the next ring. After all bricks are laid, rotate the kiln and add locking plates. When adding locking plates, tighten them below the centerline of the kiln as much as possible. Ensure that locking plates are located at all four points on the kiln circumference: 90°, 180°, 270°, and 360°. Two locking plates are not allowed in the same brick joint.

3.3 Resolving Circumferential Joint Distortion

Before laying bricks, lay circumferential lines every 2 meters inside the kiln shell. These lines must be parallel to the circumferential welds of each section of the shell. When laying the refractory bricks, the axial and circumferential lines must be used as the reference. Check every five rings to ensure the alignment of the circumferential joints with the circumferential lines. Adjust the subsequent rings based on the distance deviation. Adjustments should not be made all at once; they should be made gradually. The circumferential joints must be kept within 2mm. During adjustments, the alignment of the axes must be ensured.

3.4 Avoid Processing Bricks

Avoid processing bricks whenever possible. If the processed brick length is less than 60% of the original brick length, remove the adjacent ring of standard bricks and alternately lay standard and processed bricks to eliminate the circumferential joints. This staggered laying must be done wet, using high-temperature mortar for optimal results. If the processed brick length is less than 50% of the original brick length, extended bricks (298mm long) can be used for processing, which also achieves better results. High-temperature mortar must also be used for wet laying.

3.5 Comprehensively Consider Kiln Shell Deformation and Other Factors

During the masonry process, comprehensive consideration should be given to kiln shell deformation and irregularities in the size of the bricks. Neither a strict adherence to brick ratio nor blindly following it should be considered. In short, two principles must be adhered to: The surface of the refractory bricks must not have steps; the bottom surface of the refractory bricks must be flush with the kiln shell.