Causes and Solutions of Crack and Collapse in Green Brick Drying Process

The drying process of green brick refers to the heat absorption from a higher temperature environment, which converts the physical water within the brick into water vapor. The heated gas expands as the temperature increases, leading to an increase in gas pressure. The gas under higher pressure will be released into the natural environment where the pressure is lower. The water vapor evaporates from gaps and vanishes from the brick. The drying quality is a key factor in ensuring the firing quality and brick output. Without properly drying, the brick output and firing quality cannot be guaranteed. A drying chamber designed scientifically, a reasonable air supply method, and an air supply temperature suitable for the raw material characteristics are the prerequisites for ensuring the drying effectiveness.

Increasing the capacity of drying chamber, reducing the brick heating rate extending the brick drying cycle and improving the drying qualification rate, all are essential to ensuring the firing rate of kiln and to achieving rapid firing.

I. Three physical parameters affecting drying performance

(1) Drying sensitivity - The tendency of brick crack during drying process showing in three categories: low (raw material sensitivity less than 1), medium (raw material sensitivity between 1 and 2) and high (raw material sensitivity higher than 2)
(2) Critical moisture content - Brick will shrink as the free water is removed during drying process. Once the free water has evaporated and the shrinkage stops, the moisture content at this point is recognized as critical moisture content and it varies with humidity.
(3) Plasticity index - Clay can change its shape without cracking under the action of external force, and to retain the altered shape once the external force is removed.

For high sensitivity material, the drying chamber should be longer than 70m and with a drying cycle of over 45h. The air supply temperature should not exceed 120°C, and preheating length should be over 20m. The heating rate of brick in the preheating zone should be controlled between 3°C/h - 4°C/h, and relative humidity in preheating section should be maintained between 75% - 85%. The most critical step is to heat the raw material before molding, increasing the temperature of molded brick and ensuring that the temperature within the brick tends to be consistent. Additionally, the temperature of the molded green brick should be slightly higher than that at drying chamber inlet. If the above conditions are fulfilled, the drying quality can be ensured.

The air humidity of preheating section depends on critical moisture and drying sensitivity of raw material. When the critical moisture is high and the drying sensitivity is low, the process is relatively short from brick entering the drying chamber until it stops shrinking and cracking . In this case, low humidity and high temperature in preheating section can shorten the drying chamber length and achieve rapid drying. On the contrary, the drying chamber should be extended accordingly, and the preheating section should be high humidity with low temperature to reduce the drying rate, otherwise crack will appears in brick.

Ⅱ.Kiln cart
Kiln cart can ensure the uniform distribution of fuel and airflow in kiln cross section. The principle of setting brick on kiln cart should be to have a dense bottom arrangement and a looser arrangement at top, which allows for larger gap in the middle of the kiln, facilitating moisture flowing out. Solid brick is more prone to collapsing than hollow brick during drying process, which is because solid brick uses more raw material, resulting in greater weight and total moisture content. When heated, the amount of moisture generated in solid brick is large, and insufficient moisture removal will lead to collapse.

Ⅲ.Mechanical setting

Mechanical setting not only ensures the stability of brick but also ensures the consistency of the gap between brick. It can be considered that a configuration with dense edge and sparse center ensures that all kiln carts have same arrangement, facilitating heating, ventilation and moisture removal. At present, almost all new sintered brick production lines adopt mechanical setting method. Therefore, when determining the width of kiln, it is essential to design setting drawing based on the shape of the brick first. The specification of kiln cart and effective width of kiln should be based on setting type, and should not be decided arbitrarily.

Ⅳ.Green Brick preheating

Brick preheating process refers to the brick heating from surface to its inside. It should ensures that brick heat up slowly while also reducing the dehydration rate of brick surface. To achieve it, the preheating section should maintain low temperature and high humidity. If the surface heats up quickly, the moisture on it will gradually turn into water vapor and carried away by convection. If the relative humidity of surrounding air is low during this time, the moisture on the surface will evaporate rapidly, leading to a reduction in volume and causing surface to shrinkage. Meanwhile, the internal temperature of brick remains relatively low and the moisture inside does not evaporate, leading to inconsistent shrinkage between the inside and outside, which can result in surface cracking.

Therefore, it is necessary to wait until the internal temperature rises to the point where it begins to evaporate before moving on to the dewatering stage. To ensure the water on the surface is not evaporated quickly, it is necessary to keep the humidity of the air around the brick. From the stage of preheating to drying, air humidity should be maintained at 70% - 80%, the temperature rise rate should be controlled at 3℃/h - 5℃/h. If thetemperature rises too quickly, the increase of surface temperature will accelerate, while the heat conduction from the surface to the interior is relatively slow compared to the convective heating of surface. In this process, the brick should be kept in a high humidity environment, which characterizes preheating stage, the stage generally lasts 8h - 12h, with a preheating length of approximately 20m - 25m. If the air humidity around the brick in preheating stage is low, it will lead to an accelerated dehydration rate at the surface, causing inconsistent dehydration rate between inside and outside, which can result in surface cracking.




Ⅴ.Air supply volume and pressur

The application of fan frequency converter has made it easier to start the fan and do adjustment flexibly. However, the fan is not only for firing. For a tunnel kiln over 3 m, a fan with an exhaust capacity of 30,000 m³/h, a pressure of 280 Pa, and a power of 7.5 kW is sufficient to satisfy the firing requirement. However, for supplying air to the drying chamber, a fan is typically selected with a pressure of 1200 Pa, an airflow rate of 100,000 m³, and a power of more than 45 kW.

Arbitrarily reducing the fan frequency to satisfy oxygen requirement for firing can result in severely insufficient air volume and pressure into the drying chamber, which is a major cause of brick collapse. It is because the air volume is directly proportional to the speed (first-order relationship), while the air pressure has a quadratic relationship with speed (square relationship). When the frequency is reduced from rated 50 Hz to 30 Hz, the speed is only 60% of rated, resulting in 60% of rated air volume, but only 36% of rated pressure.

The reduced air volume is fail to send the air from the side into the middle of the kiln cart, nor can it effectively send the air from the top to the surface of the cart. As a result, air convection between the lower brick can not be formed, and moisture can not be properly removed.

The air supply temperature directly affects the heating rate of brick in preheating section and dehydration rate in drying section, so it should vary with different raw material and moisture content. Generally, for soft raw material, the air supply temperature should not exceed 110°C, which ensures a reasonable temperature increase during the preheating process. If the air supply temperature is too high, the moisture in the brick will evaporate quickly, leading to an excessive amount of water vapor inside the drying chamber, and if it exceeds the capacity of moisture exhaust fan, the relative humidity in the preheating section will reach saturation, causing the brick to soften and collapse.

  Air leakage is one of the main causes for brick collapse. The air leakage of lift door in inlet allows outside cold air to enter the drying chamber, resulting in the reducing of moisture exhaust fan capacity. It causes high temperature humidity to linger on the surface of the brick, softening it and leading to collapse. Now most of the drying chamber only equips with one door with not very good sealing performance. There are often large gap between the door and bottom rail, and in some cases, damaged door is there with no repair measurement. These problems inevitably reduce the suction capacity of the fan.

The main causes of crack: (1) The temperature and humidity of preheating section are inconsistent with critical moisture and drying sensitivity of raw material. (2) Low environmental temperature result in a significant temperature difference between the inside and outside of molded brick, causing surface moisture to evaporate much faster. (3) Low extruder pressure, high molding moisture and low critical moisture lead to a significant difference between molding moisture and critical moisture. (4) A high plasticity index in raw material makes it difficult for brick to dehydrate while rapid heating during drying causes surface cracking.

The occurrence of brick collapse during drying process is a common phenomenon, especially with soft raw material where collapse is even more prevalent. There are many factors that lead to collapse, such as high molding moisture, low brick strength, and the inability of lower brick to withstand the pressure from upper. However, high air supply temperature and fast heating rate are main factors.

There are many factors that lead to brick collapse in drying chamber, such as structural design and operating method. Specific problems should be identified based on actual situation, allowing for targeted solution to thoroughly resolve them. Smooth process design, reasonable kiln structure, good thermal insulation performance, and high construction quality are keys to reducing energy consumption and improving product quality.