The process of removing moisture from solid materials is called drying. The drying process of refractory materials is roughly divided into: drying of raw materials and drying of semi-finished products after molding. If the water content of soft clay exceeds 12%, it will be difficult to pulverize if it is not dried, which will reduce the production capacity of the mill. If the bricks enter the kiln with too much moisture, cracks are likely to occur during the firing process; while the dried semi-finished products are easier to control in the kiln and are not easy to produce waste; the mechanical strength of the bricks after drying is improved, which is conducive to transportation and kiln loading. . The dried semi-finished product is initially set, and it may also expose defects such as cracks caused during the molding process. The dried bricks will be fired in the kiln after inspection, which is beneficial to increase the yield.
The drying process is divided into three stages.
(1) Constant speed drying stage: the water is discharged at a constant speed, the residual water content of the green body is greatly reduced, and the evaporation of water is limited to the surface of the green body. The evaporation rate depends on the temperature, humidity and movement speed of the drying medium
(2) Deceleration drying stage: As the water content of the body decreases, the evaporation rate of water from the surface is higher than the rate of water diffusion from the buried body to the surface. The evaporation rate depends on the internal structure of the body, the viscosity of the water and the properties of the material. .
(3) Final drying stage: the moisture content is close to equilibrium, the drying rate gradually tends to zero, and the residual moisture in the green body is difficult to escape by evaporation.
The purpose of brick drying is to remove moisture by drying to increase the mechanical strength of the bricks, so as to reduce the mechanical loss during transportation and handling, and to make the bricks have the necessary strength when the bricks are fired after the kiln is installed; withstand certain stress. , Improve the firing yield; and provide beneficial conditions for firing.
The characteristic is that during the drying process of the bricks, some physical changes are produced, some have surface hardening (such as silica bricks); some produce volume shrinkage (such as clay bricks); when the drying is too fast, if the speed of removing water from each part is inconsistent , Cracks may occur. For example, bricks made of plastic clay will shrink to a certain extent during the drying process and produce shrinkage stress, which will cause the product to deform or even crack. The shrinkage and shrinkage stress are determined by the difference in moisture gradient between the inside and outside of the product. The greater the difference in moisture gradient, the faster the drying speed, which is likely to cause cracking and deformation. Therefore, when the bricks are dried, it is required: the drying speed during the drying process does not exceed a certain value, otherwise the product will crack; when formulating and implementing a reasonable drying system, it is necessary to make the drying speed as fast as possible without causing any increase in the bricks. Destructive stress; choosing suitable drying equipment is also very important. Common drying equipment includes tunnel dryers, drum dryers, vertical dryers, chamber dryers, drying kangs, and tunnel kiln dryers. In addition, there are; far infrared drying, microwave drying, electric drying and so on.
Far-infrared drying is a new technology developed in the 1970s. It has the advantages of fast drying speed, low power consumption, simple equipment and convenient use and maintenance. Especially for thin walls and coatings of various organic polymer materials and materials containing a large amount of water, the effect is more prominent, and the drying time can be greatly shortened. This is because these substances have a strong ability to absorb far infrared rays.
There are three types of far-infrared emitting elements: ceramic, metal and metal-clad porcelain. A far-infrared radiation layer is sintered on the surface of the emitting element to improve the radiation efficiency and obtain the required frequency. Refractory materials are commonly used as emitters with cadmium oxide coating sintered on the surface of silicon carbide components.
In order to dry non-burning bricks, a certain factory reactivated a tunnel dryer device that had been out of service for many years. The device consists of a front 24m2 exhaust gas circulating preheater connected to it. Due to the low inlet temperature of the dryer (an average of 151°C), it could not meet the drying requirements. On the basis of the original, 116 far-infrared heating plates (by calculation) were installed on the inner wall of the dryer as a supplementary heat source. The short-term load operation showed that the inlet temperature of the dryer rose from 130°C to 175°C, which met the technical requirements for product drying.
A factory uses a far-infrared electric heating tunnel dryer to dry insulation board products. The tunnel dryer is 27m long, 1.4m wide, and 1.6m high. All steel plates and angle steels are welded with added value. The kiln wall is filled with vermiculite, an insulation material. The maximum temperature of the dry insulation board is 160°C.
The whole kiln is divided into three sections: heating, drying and curing, and the temperature can be automatically controlled. The outer surface and working section of the electric heating tube are coated with far-infrared coating; coating ratio: Sic 60%, Fe2o3 25%, MnO 2%, Ni2o3 3%, Sio2 10%. The binder is water glass, and its ratio is water glass: water=1:2. The trial shows that the wet blank of the insulation board with a thickness of 30mm, a length of 1000mm, a width of 400mm, and a moisture content of about 40% is dried, and the drying time is only 5h. After drying, the moisture content of the insulation board can be reduced to 0.25%, per kilogram of the insulation board. The power consumption of the product is 1.4KW·h.
The microwave oven uses a magnetron as a heat exchanger to convert DC energy (or 50Hz AC energy.) into microwave electromagnetic oscillation energy, and then the microwave energy is drawn from the magnetron output device and enters the microwave heater (dryer). The shape of the heater depends on the physical shape of the heated object and the requirements of production efficiency. The frequency of the microwave oven is 915MC and the power is 10kV·A; the power supply is three-phase, 380V, 50Hz, and the wavelength is 320mm. The drying oven is made of 2mm thick aluminum plate. Dimensions (mm): 1750X 1750X1750 The waveguide is aluminum, long 2000mm, the section size is (mm): 240X120. It can dry 5-6 large glass bricks at a time. After demoulding, large bricks contain about 12% moisture. For safe and uniform drying, different voltage and current systems are adopted according to the size of the brick. At the same time, the position of the brick body should avoid the waveguide opening, so that the microwave will be reflected randomly after entering the drying box. Through trials, microwave dryers for large-scale refractory products have a better drying effect than alternating current drying, which not only shortens the drying time, but also saves electricity.
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