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 quality reasons such as brick cracks. The following are the drying quality reasons and preventive measures that are prone to occur in tunnel drying kilns for reference: 1. The dry air volume and wind speed are too large to cause cracks in the bricks The principle of the 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. Excessive drying air volume and wind speed may cause the brick surface to dry and shrink rapidly and cause brick cracks. The cracks caused by this reason generally occur in the upper middle part of the windward side of the front row of billets of each kiln car and the top 1 to 2 layers of bricks of the entire vehicle billet. It is necessary to operate according to the characteristics of the four stages of the brick drying process, namely the heating stage of the brick, the constant drying stage of the brick, the reduced-rate drying stage of the brick and the balanced drying stage of the brick and its process operation methods. The constant velocity drying stage of the bricks, namely the free water removal stage, is the main stage of the brick drying process. At this stage, the shrinkage water between the green body mud particles is discharged, and the green body shrinks. Therefore, if the operation is improper, the green body is prone to cracks. 2. The dry air temperature is too high and the cycle is short, causing brick cracks Excessively high drying air temperature and short drying cycle will also cause cracks on the brick surface due to rapid drying shrinkage. Such cracks generally appear in the middle and upper parts of the whole vehicle billet stack. Whether it is drying air volume, excessive wind speed, or excessive drying air temperature and short drying cycle, the root cause of brick cracks is caused by improper drying process, that is, unreasonable drying system. In short, the four stages of the entire drying process must be strictly followed, namely the brick heating stage. The bricks will dry up with the increase in temperature, and the body will produce drying shrinkage. In the drying process, when the external diffusion speed of the brick’s moisture is equal to the internal diffusion speed, that is, the drying enters the constant-rate drying stage. At this time, the drying is carried out most intensely. If the external diffusion speed is much greater than the internal diffusion speed, the green body will form a large moisture gradient, resulting in a large shrinkage of the green body surface. When the stress generated by the shrinkage is greater than the strength of the green body, cracks will be formed on the surface of the body. During the drying process, when the moisture on the surface of the body is equal to the moisture absorbed by the atmosphere, the evaporation surface gradually shrinks into the capillary pores inside the body as the moisture decreases, the drying speed gradually decreases, and the drying enters the reduced-rate drying stage. In the reduced-rate drying stage, only the corresponding increase in pores occurs in the green body without volume shrinkage. Therefore, the green body will not produce drying cracks during the reduced-speed drying stage. There is a dividing point between the constant-rate drying stage and the reduced-rate drying stage, that is, the critical point of drying. The moisture release of the brick at the critical point of drying has reached a limit moisture content, that is, the critical moisture content, which is the moisture content of the brick at the critical point. At this time, the solid particles get close to each other because they lose the surrounding moisture until they touch each other and stay together. Therefore, after the brick drying process reaches the critical point of drying, or in other words, after the brick moisture reaches the critical moisture content, the brick stops shrinking. The critical point of drying is a watershed in the drying process of the brick. Before the critical point of drying, every drop of water on the brick will cause shrinkage, which may cause cracks in the body. Will cause cracks in the green body. After the critical point, since the drying process will no longer produce destructive cracks, the maximum ventilation and the highest temperature of the heating medium should be used to quickly remove the moisture in the green body and increase the drying speed. It can be seen that accurately determining the location of the critical point of the tunnel drying kiln is very important for the design and production operation of the tunnel drying kiln. During the production process, the exact position of the critical point in the tunnel drying kiln cannot be determined intuitively by the operator. The operator can only try to adjust the air volume and temperature of each air outlet, as well as the drying quality and drying speed of the bricks, and find 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 billet stack collapses. There are two cases of billet collapse, one is that part of the stack is wet from top to bottom as if wet by water; the other is that part of the billet or the whole vehicle billet collapses from bottom to top. The reason for the former collapse is: the moisture exhausted from the tunnel drying kiln condenses on the inner wall of the drain cylinder, and the condensed water drips along the wall of the cylinder on the billet stack to wet the bricks; the latter reason is: tunnel A large amount of moisture in the drying kiln will gradually decrease in temperature as it flows to the moisture outlet. When the temperature reaches the dew point, a large amount of dew will be produced on the surface of the bricks. The bricks soaked by dew cannot withstand the pressure of the upper stack. When, it collapsed. The main measures to prevent billet collapse are: one is to strengthen the insulation measures of the tunnel drying kiln body and kiln roof; the other is to strengthen the sealing of the kiln door at the entrance of the tunnel drying kiln to prevent cold air from entering the moisture outlet and reduce the moisture exhaust temperature; According to the weather cooling, increase the air temperature and increase the air volume into the kiln; fourth, find out the location of the collapsed billet in the tunnel drying kiln (parking space number), increase the input of high-temperature hot air, if there is no such thing in the design and construction For the air inlet, you can add air inlets on both sides of the kiln and above the kiln car platform. Connect a φ300mm pipe from 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 sign of these measures is: the temperature at the drain outlet must be greater than 50°C. 4. How to identify the cracks of 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 been produced in the refractory bricks in the production process before drying, and the cracks only appear after drying. How to identify 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 reasons When the porous brick is formed, if the squeezing force encountered by the mud in the die is different, the extrusion speed of the mud bar will be different, and the compactness of the side and the middle part of the brick will be different. The body is in the drying process. Because the compactness around the body is lower than the middle, the edge temperature is higher than the middle temperature, which causes the edge moisture to evaporate quickly, and the middle moisture evaporates slowly, resulting in a situation where the edge dehydration rate is greater than the middle. When the edge shrinks too fast, Cracks occurred at the edges of the green body. The root cause of this kind of crack is the unreasonable structure of the brick machine head. In addition, improper core frame is also the main reason for forming cracks: first, the extrusion speed of the mud stick section is uneven during forming; The core frame is not well healed after being divided, and forming cracks are formed; thirdly, when forming, the thickness of the blank hole wall is uneven, and during the drying process, large stresses are generated due to uneven shrinkage, resulting in forming cracks. In short, the microcracks formed in the green body due to molding are not easy to find during molding, and they are only revealed due to the expansion of cracks after drying or firing. The most important feature of forming cracks is regularity, which provides us with a simple method of identifying the causes of brick cracks and taking to eliminate brick cracks, and can take effective measures in a targeted manner. (2) Brick cracks caused by mud material. First, the mud is mixed with water unevenly, and the moisture difference between the inner and outer layers of the blank or each part is too large, and the cracks are caused by inconsistent shrinkage during the drying process; the second is different raw materials (coal gangue, page Rocks, etc.) are mixed unevenly, forming mud masses of different sizes. During the drying process, cracks are caused by the inconsistent shrinkage of each part; third is the high plastic raw material that has not been barrened, and the drying sensitivity coefficient is greater than 2. The shrinkage rate of the bricks formed by the material is very large, and it is easy to produce 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 exhaust gas discharged from the drying kiln to the environment. In recent years, the environmental protection problems of brick factories have been listed as important assessment indicators by local governments, especially the flue gas emissions and pollution of the roasting system have been paid more attention to, and environmental protection work inspections have gradually increased. One day, if the smoke If the emission is not up to standard, it will be ordered to renovate or stop production. Therefore, attaching importance to environmental issues should be the same as attaching importance to production: early attention, early planning, early action, and early results. From the aspect of tunnel drying process operation technology, one is to use the clean hot air from the cooling zone of the roasting kiln as the drying heat source of the tunnel drying kiln. As long as the process is operated properly, its hot air volume and temperature can fully meet the needs of drying production; the second is not to use tunnel roasting The flue gas of the kiln is dried; the third is to be cautious or less use of the high-temperature flue gas at the rear of the roasting kiln to preheat the heat; The exhaust gas discharged from the kiln undergoes desulfurization treatment.
