Reasons and preventive measures for cracks and collapse of refractory brick adobe during drying

After the refractory bricks are formed, they need to be dried in a tunnel drying kiln. During the drying process, due to improper operation or other reasons, it is easy to cause brick cracks and other quality reasons. The following are the drying quality reasons and prevention measures that are prone to occur in tunnel drying kilns for reference:

1. Excessive drying air volume and wind speed will cause cracks in refractory bricks

The principle of tunnel drying kiln drying system is low temperature, large air volume and slight positive pressure operation. The so-called large air volume means that there must be enough air volume to take a large amount of water vapor in the kiln out of the drying kiln, not that the larger the air volume, the better.

If the drying air volume and wind speed are too large, the surface of the adobe will dry and shrink rapidly, resulting in cracks in the adobe. The cracks produced by this reason generally appear in the middle and upper part of the windward side of the front row of blank stacks of each kiln car and on the top 12 layers of bricks of the whole vehicle blank stack.

The operation should be carried out in accordance with the characteristics of the four stages of the brick drying process, namely the brick heating stage, the constant speed drying stage of the brick, the slowing down drying stage of the brick and the balanced drying stage of the brick and the process operation method.

The constant-speed drying stage of brick adobe, that is, the free water removal stage, is the main stage of the brick adobe drying process. At this stage, the shrinkage water between the mud particles of the green body is discharged, and the green body shrinks. Therefore, if it is not handled properly, the green body is prone to cracks.

2. The drying air temperature is too high and the cycle is short, resulting in cracks in the adobe

If the drying air temperature is too high and the drying cycle is short at the same time, the surface of the adobe will also cause cracks due to rapid drying and shrinkage. Such cracks generally appear in the middle and upper parts of the pile of adobes.

Regardless of whether the drying air volume, wind speed is too large, or the drying air temperature is too high and the drying cycle is short, the root cause of the cracks in the bricks is the improper drying process, that is, the unreasonable drying system. In short, the four stages of the entire drying process must be strictly followed, that is, the brick heating stage. As the temperature rises, the drying speed of the brick is accelerated, and the brick body produces drying shrinkage. During the drying process, when the moisture diffusion rate of the tiles is equal to the internal diffusion rate, that is, the drying enters the constant-speed drying stage, and at this time, the drying proceeds most intensely. If the external diffusion rate is much greater than the internal diffusion rate, the green body will form a large moisture gradient, causing the surface of the green body to shrink greatly. When the stress generated by the shrinkage is greater than the strength of the green body, cracks will form on the surface of the green body. When the moisture on the surface of the green body is equal to the moisture absorbed by the atmosphere during the drying process, the evaporation surface gradually shrinks into the capillary channels inside the green body as the water decreases, the drying speed gradually decreases, and the drying enters the stage of slowing down drying. In the stage of drying at reduced speed, the pores in the green body are only increased correspondingly, and the volume shrinkage does not occur. Therefore, the green body will not produce drying cracks in the stage of drying at reduced speed.

There is a dividing point between the constant drying stage and the falling drying stage, which is the drying critical point. The water evaporation and release of the adobe at the critical point of drying has reached a limit moisture content, that is, the critical moisture content, and the critical moisture content is the moisture content of the adobe at the critical point. At this time, as the solid particles lose the surrounding moisture, they approach each other until they touch each other and close together.

Therefore, after the brick drying process reaches the drying critical point, or in other words, after the brick moisture reaches the critical moisture content, the brick stops shrinking. The critical point of drying is the watershed in the drying process of the adobe. Before the critical point of drying, every drop of water removed from the adobe will cause shrinkage, which may cause cracks in the adobe. It will cause cracks in the green body. After the critical point, since the drying process will no longer produce destructive cracks, the maximum ventilation rate and the highest heat medium temperature should be used to quickly remove the moisture in the green body and increase the drying speed. It can be seen that it is very important to accurately determine the position of the critical point of the tunnel drying kiln for the design and production operation of the tunnel drying kiln. During the production process, the operator cannot intuitively determine the exact position of the critical point in the tunnel drying kiln, and can only test and adjust the air volume and air temperature of each air supply outlet, as well as the drying quality and drying speed of the bricks, and find out a relatively small range , to meet the needs of production operations.

3. Reasons and preventive measures for the collapse of the tunnel drying kiln

During winter production in the northern region, especially when the outdoor temperature drops below zero, the tunnel drying kiln is most prone to collapse accidents. It often happens that part of the stack or the whole vehicle blank stack collapses. There are two cases of collapse, one is that part of the stack is wet from top to bottom; the other is that part of the stack or the entire vehicle stack is soft and collapses from bottom to top. The former reason for the collapse of the bricks is: the moisture steam discharged from the tunnel drying kiln condenses on the inner wall of the moisture exhaust tube, and the condensed water drops along the wall of the tube and drops on the pile of bricks to wet the bricks; the latter reason is: the tunnel When a large amount of moisture in the drying kiln flows to the tide outlet, the temperature gradually decreases. When the temperature reaches the dew point, a large amount of dew will be generated on the surface of the adobe, and the adobe soaked by dew will not be able to bear the pressure of the upper adobe stack , the collapse occurs.

The main measures to prevent the collapse: one is to strengthen the insulation measures of the kiln body and kiln roof of the tunnel drying kiln; the other is to strengthen the sealing of the kiln door at the entry end of the tunnel drying kiln to prevent cold wind from entering the moisture outlet and reduce the temperature of the moisture discharge; the third is to According to the weather cooling situation, increase the air temperature and air volume entering the kiln; the fourth is to find out the collapsed part (parking space number) in the tunnel drying kiln, and increase the input of high-temperature hot air. If there is no such during design and construction For the air inlet, the air inlet can be added on both sides of the kiln and above the kiln car table, and a 300mm pipe is connected to the main pipe for hot air to introduce hot air, so that the air temperature at this part exceeds the dew point temperature. In short, the effective signs of these measures are: the temperature at the tide outlet must be greater than 50.

4. How to identify cracks in refractory bricks caused by non-drying process and preventive measures

The reason for the non-drying process refers to the form of cracks that have occurred in the refractory brick adobe in the production process before drying, but only after drying. How to identify the brick cracks caused by non-drying process is also very important for kiln roasting operators, so as to solve product quality problems in a timely and targeted manner.

(1) Brick cracks caused by forming

When the porous brick is formed, if the extrusion force encountered by the mud material in the machine head is different, the extrusion speed of the section of the mud strip will be different, so that the density of the edge and the middle part of the brick body will be different. , because the compactness around the green body is lower than that in the middle, and the edge temperature is higher than the middle temperature, the moisture in the edge evaporates quickly, and the moisture in the middle evaporates slowly, forming a situation where the dehydration rate of the edge is greater than that in the middle. When the edge shrinks too fast, Cracks appear on the edge of the green body. The fundamental reason for the formation of such cracks is the unreasonable structure of the brick machine head.

In addition, the improper core frame is also the main cause of forming cracks: first, when forming, the extrusion speed of the mud strip section is not uniform; After the core frame is divided, the healing is poor and forming cracks are formed; the third is that when forming, the thickness of the hole wall of the green body is uneven, and in the drying process, due to the uneven shrinkage, large stress is caused to form forming cracks.

In short, the tiny cracks formed in the green body due to molding are not easy to be found during molding, but are revealed due to the expansion of cracks after drying or firing. The biggest feature of forming cracks is regularity, which provides us with a simple identification method for the cause of brick cracks and the elimination of brick cracks, and can take effective measures in a targeted manner. (2) Cracks in brick adobe caused by mud: firstly, the mixing of mud with water is not uniform, and the difference in moisture between the inner and outer layers or parts of the brick is too large, and cracks are generated due to inconsistent shrinkage during the drying process; second, different raw materials (coal gangue, page Rock, etc.) are mixed very unevenly, forming mud clusters of different sizes. During the drying process, cracks are caused due to the inconsistent shrinkage of each part; the third is the mud with a drying sensitivity coefficient greater than 2 for high-plastic raw materials that have not been barrenized. The adobe formed by the material has a large shrinkage rate and is prone to cracks.

5. Environmental protection issues in tunnel drying kiln production

The environmental protection problem of the tunnel drying kiln refers to the pollution problem of the waste gas discharged from the drying kiln to the environment. In recent years, the environmental protection of brick factories has been listed as an important assessment index by local governments. In particular, more attention has been paid to the smoke emission and pollution of the roasting system, and the inspection of environmental protection work has gradually increased. One day, if the smoke from brick factories If the discharge does not meet the standard, it will be ordered to transform or stop production. Therefore, attaching importance to environmental protection issues should be the same as attaching importance to production. Early grasp, early planning, early action, and early results.

From the aspect of tunnel drying process operation technology, one is to use the clean hot air in the cooling zone of the roasting kiln as the drying heat source of the tunnel drying kiln. As long as the process is properly operated, the hot air volume and temperature can fully meet the needs of drying production; the other is not to use tunnel roasting The third is to use the high-temperature flue gas at the back of the preheating zone of the roasting kiln with caution or less for drying; the fourth is to use sulfur-containing flue gas as the drying heat source, and desulfurization equipment must be installed to prevent the drying from drying. The waste gas discharged from the kiln is desulfurized.