The drying of ceramics is one of the very important processes in the production process of ceramics. Most of the quality defects of ceramic products are caused by improper drying. The drying of the ceramic industry has experienced natural drying, chamber drying, and now continuous dryers with various heat sources, far-infrared dryers, solar dryers and microwave drying technologies. Although drying is an industrial process with relatively simple technology and wide application, it not only affects the quality and yield of ceramic products, but also affects the overall energy consumption of ceramic enterprises. According to statistics, the energy consumption in the drying process accounts for 15% of the total industrial fuel consumption, while in the ceramic industry, the energy consumption for drying accounts for far more than the total fuel consumption, so the energy saving in the drying process is related to the enterprise Energy saving event. The drying speed of ceramics is fast, energy saving, high quality and no pollution are the basic requirements for drying technology in the new century.
The mechanism of ceramic drying process
Moisture in the green body
The moisture content of the ceramic body is generally between 5% and 25%. The combination of the body and the moisture, the change of the material during the drying process and the factors affecting the drying rate are the theoretical basis for analyzing and improving the dryer. When the green body is in contact with the still air of a certain temperature and humidity, it is bound to release or absorb water, so that the water content of the green body reaches a certain equilibrium value. As long as the state of air remains unchanged, the moisture content achieved in the green body will no longer change due to the increase in contact time, and this value is the equilibrium moisture of the green body in the air state. The moisture lost by the wet green body reaching equilibrium moisture is free moisture. That is to say, the moisture of the green body is composed of equilibrium moisture and free moisture. Under a certain air state, the limit of drying is to make the green body reach the equilibrium moisture.
The water contained in the green body can be divided into physical water and chemical water. The drying process only involves physical water, and physical water is divided into bound water and unbound water. Unbound water exists in the large capillary tube of the green body, and it is combined with the green body to relax. The evaporation of unbound water in the green body is like the evaporation of water on the free liquid surface, and the partial pressure of water vapor on the surface of the green body is equal to the partial pressure of saturated water vapor at its surface temperature. When the unbound water in the green body is discharged. The particles of the material are close to each other, so the volume shrinks, so the unbound water is also called shrinking water. The bound water is the water that exists in the microcapillaries (diameter less than 0.1μm) of the green body and on the surface of the colloidal particles. The partial pressure will be less than the saturated water vapor partial pressure at the body surface temperature. During the drying process, when the partial pressure of water vapor on the surface of the green body is equal to the partial pressure of water vapor on the surrounding drying medium, the drying process stops and the water cannot continue to be discharged. At this time, the water contained in the green body is the equilibrium water, and the equilibrium water is The amount of bound water depends on the temperature and relative humidity of the drying medium. When the bound water is discharged, the volume of the green body does not shrink, which is relatively safe.
Green body drying process
Taking the convective drying process as an example, the drying process of the green body can be divided into three simultaneous and interconnected processes: heat transfer process, external diffusion process, and internal diffusion process.
In the process of heat transfer, the heat of the drying medium is transferred to the surface of the body by convection, and then transferred from the surface to the interior of the body by conduction. The moisture on the surface of the green body is vaporized by heat, changing from liquid to gas.
External diffusion process: The water vapor generated on the surface of the green body moves from the surface of the green body to the drying medium by means of diffusion through the laminar bottom layer under the action of the concentration difference.
Internal diffusion process: due to the evaporation of moisture on the surface of the wet body. It generates a humidity gradient inside it, which promotes the diffusion of moisture from the inner layer with high concentration to the outer layer with lower concentration, which is called wet conduction or wet diffusion.
In the case of stable drying conditions, the surface temperature, moisture content, drying rate and time of the green body have a certain relationship. According to the changing characteristics of their relationship, the drying process can be divided into: heating stage, constant speed drying stage, cooling Three processes of quick drying stage.
In the heating stage, since the heat transferred by the drying medium to the surface of the green body per unit time is greater than the heat consumed by the evaporation of the surface water, the temperature of the heated surface gradually increases until it is equal to the wet bulb temperature of the drying medium. At this time, the surface obtains heat and evaporation. The heat consumption reaches a dynamic equilibrium, and the temperature remains unchanged. At this stage, the moisture of the green body decreases and the drying rate increases.
In the isokinetic drying stage, the discharge of unbound water continues in this stage. Due to the high water content of the green body, the amount of water evaporated on the surface can be replenished inside the green body, that is, the moving speed of water inside the green body (internal diffusion speed) is equal to the surface water evaporation speed, which is also equal to the external diffusion speed, so the surface remains wet. . In addition, the heat transferred from the medium to the surface of the green body is the heat required for the vaporization of dry water, so the surface temperature of the green body remains unchanged, which is equal to the wet bulb temperature of the medium. The partial pressure of water vapor on the surface of the green body is equal to the partial pressure of saturated water vapor at the subsurface temperature, and the drying rate is stable, so it is called the constant speed drying stage. This stage is to discharge unbound water, so the green body will shrink in volume, and the shrinkage amount is in a linear relationship with the amount of moisture reduction. At the end of the isokinetic drying phase, the material moisture is reduced to a critical value. At this time, although the interior of the material is still unbound water, bound water begins to appear in the surface layer.
Deceleration drying stage, in this stage, the water content of the green body is reduced, the internal diffusion rate cannot keep up with the surface moisture evaporation rate and the external diffusion rate, the surface is no longer kept wet, and the drying rate gradually decreases. As the heat required to evaporate the surface water decreases, the temperature of the material begins to gradually increase. The partial pressure of water vapor on the surface of the material is less than the partial pressure of saturated water vapor at the surface temperature. This stage is to discharge the bound water, the green body will not produce volume shrinkage, and will not produce dry waste. When the material drainage drops equal to the equilibrium moisture, the drying rate becomes zero and the drying process is terminated. Even if the drying time is prolonged, the material moisture will not change. At this time, the surface temperature of the material is equal to the dry bulb temperature of the medium, and the surface water vapor partial pressure is equal to the water vapor partial pressure of the medium. The drying speed of the deceleration drying stage depends on the internal diffusion rate, so it is also called the internal diffusion control stage. At this time, factors such as the structure, shape and size of the material affect the drying rate.
Factors Affecting Drying Rates
The factors that affect the drying rate are heat transfer rate, external diffusion rate, and internal diffusion rate.
(1) Speed up the heat transfer rate
In order to speed up the heat transfer rate, it should be done: ① Increase the temperature of the drying medium, such as increasing the temperature of the hot gas in the drying kiln, increase the hot blast furnace, etc., but the surface temperature of the green body cannot be increased too fast to avoid cracking, and ② Increase the heat transfer Area: If changing from single-sided drying to double-sided drying, layering the blanks or reducing the number of layers of the blanks, increasing the contact surface with the hot gas, ③ increasing the convective heat transfer coefficient.
(2) Increase the external diffusion rate When drying is in the constant speed drying stage, the external diffusion resistance becomes the main contradiction affecting the entire drying rate. Therefore, reducing the external diffusion resistance and increasing the external diffusion rate has the greatest impact on shortening the entire drying cycle. The external diffusion resistance mainly occurs in the boundary layer, so it should be done: ① Increase the flow velocity of the medium, reduce the thickness of the boundary layer, etc., and improve the convective heat transfer coefficient. It can also improve the convective mass transfer coefficient, which is beneficial to improve the drying speed. ② Reduce the water vapor concentration of the medium, increase the mass transfer area, and also improve the drying speed.
(3) Increase the internal diffusion rate of water
The internal diffusion rate of water is a combination of wet and thermal diffusion. Wet diffusion is the movement of water in materials due to humidity gradients, and thermal diffusion is the movement of water caused by temperature gradients in physics. To improve the internal diffusion rate, it should be done: ① Make the thermal diffusion and wet diffusion direction consistent, that is, try to make the material center temperature higher than the surface temperature, such as far-infrared heating, microwave heating methods; ② When the thermal diffusion and wet diffusion directions are consistent, Strengthen heat transfer and increase the temperature gradient in the material. When the two are opposite, strengthening the temperature gradient increases the resistance of thermal diffusion, but it can enhance heat transfer, increase the temperature of the material, and increase the moisture diffusion, so it can speed up drying. The thickness of the thin body is changed from single-sided drying to double-sided drying. ④Reduce the total pressure of the medium, which can improve the wet diffusion coefficient and increase the wet diffusion rate. ⑤Other factors such as the properties and shape of the green body.
Post time: Sep-22-2022