Causes and prevention measures for cracking and collapse of refractory brick blanks during drying process

Causes and prevention measures for cracking and collapse of refractory brick blanks during drying process

After refractory bricks are formed, they need to be dried in a tunnel drying kiln. During the drying process, improper operation or other reasons can easily cause quality problems such as brick cracks. The following are common causes of drying quality issues in tunnel drying kilns and corresponding prevention measures:

Excessive Drying Air Volume and Velocity Cause Cracks in Refractory Brick Blanks

The principle of the drying regime in tunnel drying kilns is low temperature, high air volume, and slight positive pressure operation. High air volume means sufficient airflow to remove a large amount of moisture from the kiln, not necessarily the larger the better.

Excessive drying air volume and velocity can easily cause rapid drying and shrinkage of the brick blank surface, resulting in cracks. Cracks caused by this generally appear in the upper-middle part of the windward side of the front row of blanks on each kiln car and on the top 1-2 layers of bricks in the entire stack.

The drying process should be carried out according to the characteristics and operational methods of the four stages of the brick blank drying process: the brick blank heating stage, the constant-rate drying stage, the decreasing-rate drying stage, and the equilibrium drying stage.

The constant-rate drying stage, also known as the free water removal stage, is the main stage of the brick blank drying process. During this stage, shrinkage water between the clay particles in the blank is expelled, and the blank shrinks. Therefore, if the operation is not done properly, the blank is prone to cracking.

Excessive Drying Temperature and Short Drying Cycle Cause Cracks in Brick Blanks

Excessive drying temperature combined with a short drying cycle can cause cracks in the brick blanks due to rapid shrinkage during drying. These cracks typically appear in the upper and middle parts of the stack of blanks.

The root cause of brick blank cracks, whether due to excessive drying air volume, air velocity, excessive drying temperature, or a short drying cycle, is improper drying process, i.e., an unreasonable drying regime. In short, the four stages of the entire drying process must be strictly followed: the brick blank heating stage, where the drying rate accelerates as the temperature rises, causing shrinkage of the blank; and the isochronous drying stage, where drying is most intense when the external diffusion rate of moisture equals the internal diffusion rate. If the external diffusion rate is much greater than the internal diffusion rate, a large moisture gradient forms in the blank, leading to significant surface shrinkage. When the stress generated by this shrinkage exceeds the strength of the blank, cracks form on the surface.

During the drying process, when the moisture content on the surface of the brick equals the moisture adsorbed by the atmosphere, the evaporation surface gradually shrinks into the capillary channels inside the brick as the moisture decreases, and the drying rate gradually decreases, entering the falling-rate drying stage. In the falling-rate drying stage, the brick only increases in pore size and does not undergo volume shrinkage; therefore, drying cracks will not occur in the brick during this stage. There is a dividing point between the constant-rate drying stage and the falling-rate drying stage, namely the drying critical point. At the drying critical point, the evaporation and release of moisture in the brick has reached a limiting moisture content, namely the critical moisture content, which is the moisture content of the brick at the critical point. At this point, the solid particles move closer together as they lose surrounding moisture until they come into contact and clump together. Therefore, after the brick drying process reaches the drying critical point, or in other words, after the brick’s moisture content reaches the critical moisture content, the brick stops shrinking.

The critical drying point is the watershed in the brick drying process. Before this point, every drop of water removed from the brick causes shrinkage, potentially leading to cracks. If the airflow is too high and the drying speed too fast, cracks will inevitably form. After the critical point, since the drying process no longer produces destructive cracks, maximum ventilation and the highest heat medium temperature should be used to rapidly remove moisture from the brick and increase the drying speed.

Therefore, accurately determining the location of the critical point in a tunnel drying kiln is crucial for both its design and operation. During production, the exact location of the critical point within the tunnel drying kiln cannot be visually determined by the operator. Instead, adjustments to the airflow and temperature at each air outlet, as well as the drying quality and speed of the bricks, are made to establish a relatively small range that meets the needs of production.

Causes and Prevention of Brick Collapse in Tunnel Drying Kilns

In northern regions, especially during winter production when outdoor temperatures drop below zero, tunnel drying kilns are most prone to brick collapse accidents. Collapses often occur in parts of the stack or even entire cartloads of bricks. There are two types of collapse: one is where parts of the stack appear to be soaked from top to bottom; the other is where parts of the stack or even entire cartloads of bricks collapse from bottom to top due to moisture. The former is caused by condensation of moisture exiting the tunnel drying kiln on the inner wall of the dew outlet, with the condensate dripping down the wall and onto the brick stack, causing it to collapse. The latter is caused by the gradual decrease in temperature of the large amount of moisture flowing towards the dew outlet within the tunnel drying kiln. When the temperature reaches the dew point, a large amount of dew forms on the surface of the bricks. When the bricks, soaked in dew, cannot withstand the pressure from the upper stack, they collapse.

The main measures to prevent brick collapse are: first, strengthen the insulation of the kiln body and kiln roof of the tunnel drying kiln; second, strengthen the sealing of the kiln door at the car inlet end of the tunnel drying kiln to prevent cold air from entering the vent and lowering the vent temperature; third, increase the air temperature and volume entering the kiln according to the weather temperature drop; fourth, identify the location of brick collapse in the tunnel drying kiln (car position number) and increase the input of high-temperature hot air. If there is no such air inlet in the design and construction, air inlets can be added on both sides of the kiln and above the kiln car platform, and hot air can be introduced by connecting a φ300mm pipe to the main heating air pipeline to ensure that the air temperature at this location exceeds the dew point temperature. In short, the effective indicator of these measures is that the temperature at the vent must be greater than 50℃.

How to Identify and Prevent Cracks in Refractory Bricks Caused by Non-Drying Processes

Non-drying process causes refer to cracks that have already formed in the refractory brick blanks during the production process before drying, and only appear after drying. Identifying cracks caused by non-drying process issues is crucial for kiln operators to address product quality problems promptly and effectively.

(1) Cracks Caused by Molding Processes

During the molding of porous bricks, if the extrusion pressure encountered by the clay in the machine head is different, it will lead to different extrusion speeds of the clay strip cross-section. This results in different densities between the edges and the center of the brick blank. During the drying process, because the density around the edges is lower than that in the center, and the temperature at the edges is higher than that in the center, the moisture evaporates faster at the edges and slower in the center, resulting in a higher dehydration rate at the edges than in the center. When the edges shrink too quickly, cracks appear at the edges of the brick blank. The root cause of this type of crack is an unreasonable structure of the brick machine head. In addition, improper core support is also a major cause of molding cracks: First, the extrusion speed of the clay strip cross-section is uneven during molding; second, during molding, if the cutter holder is too close to the die nozzle, the extrusion zone is too short, and the clay material is poorly healed after being divided by the core support, resulting in molding cracks; third, during molding, the thickness of the green body’s hole walls is uneven, and during drying, uneven shrinkage generates significant stress, causing molding cracks.

In short, micro-cracks formed in the green body due to molding issues are not easily detected during molding, but only become apparent after drying or firing due to crack expansion. The most significant characteristic of molding cracks is their regularity, which provides us with a simple method for identifying the causes of brick cracks and taking targeted and effective measures.

(2) Cracks in brick blanks caused by clay material: First, uneven mixing of clay with water can lead to excessive moisture differences between the inner and outer layers or different parts of the brick blank, resulting in cracks due to inconsistent shrinkage during the drying process. Second, uneven mixing of different raw materials (coal gangue, shale, etc.) can form clay lumps of different sizes, which can also cause cracks due to inconsistent shrinkage during the drying process. Third, high-plasticity raw materials that have not undergone barrenening can produce brick blanks with a drying sensitivity coefficient greater than 2, resulting in a large shrinkage rate and making them prone to cracking.

Environmental Issues in Tunnel Drying Kiln Production

From the perspective of tunnel drying process operation technology, firstly, clean hot air from the cooling zone of the roasting kiln should be used as the drying heat source for the tunnel drying kiln. As long as the process is operated properly, the hot air volume and temperature can fully meet the needs of drying production. Secondly, the flue gas from the tunnel roasting kiln should not be used for drying. Thirdly, the high-temperature flue gas from the rear of the preheating zone of the roasting kiln should be used sparingly or sparingly for drying. Fourthly, if sulfur-containing flue gas is used as the drying heat source, desulfurization equipment must be installed to desulfurize the waste gas discharged from the drying kiln.