What are the characteristics of ceramics? What are the advantages and disadvantages?

Ceramic drying is one of the most important processes in ceramic production, and most of the quality defects of ceramic products are caused by improper drying. The drying of ceramic industry has experienced natural drying and chamber drying, and now there are various heat source continuous dryers, far infrared dryers, solar dryers and microwave drying technologies. Although drying is a relatively simple industrial process, it is widely used, which not only affects the quality and yield of ceramics, 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, and in the ceramic industry, the proportion of energy consumption in drying is far more than that, so the energy saving in the drying process is a major event related to energy saving in enterprises. Fast drying speed, energy saving, high quality and no pollution of ceramics are the basic requirements for drying technology in the new century.

2 mechanism of ceramic drying process

2. 1 moisture in green body

The moisture content of ceramic green body is generally between 5% and 25%. The combination form of green body and moisture, the change of materials during drying and the factors affecting drying rate are the theoretical basis for analyzing and improving the dryer. When the green body comes into contact with still air with a certain temperature and humidity, it will inevitably release or absorb moisture, so that the moisture content of the green body reaches a certain equilibrium value. As long as the air state is unchanged, the water content in the green body will not change with the increase of contact time, which is the equilibrium water content of the green body in this air state. However, the moisture lost by the wet green body that reaches equilibrium moisture is free moisture. That is to say, the moisture of the green body consists of equilibrium moisture and free moisture. In a certain air state, the limit of drying is to make the blank reach equilibrium moisture.

The water contained in the green body can be divided into physical water and chemical water, and the drying process only involves physical water, which is divided into bound water and unbound water. Unbound water exists in the large capillary of the green body, which is loosely combined with the green body. The evaporation of unbound water in the green body is just like the evaporation of free surface water. The partial pressure of water vapor on the surface of green body is equal to the saturated partial pressure of water vapor at its surface temperature. When unbound wat in that green body is discharged. The particles of matter are close to each other, so the volume shrinks, so unbound water is also called shrinking water. The bound water is the water existing on the surface of the capillary tube (diameter less than 0. 1μ m) and colloidal particles of the green body, which is firmly bound with the green body (physical and chemical action), so when the bound water is discharged, the partial pressure of water vapor on the green body surface will be less than the partial pressure of saturated water vapor at the surface temperature of the green body. 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 in the surrounding drying medium, the drying process stops and the water contained in the green body is no longer discharged. At this time, the water contained in the green body is balanced water, which is a part of combined water, and its amount depends on the temperature and relative humidity of the drying medium. When the combined water is discharged, the volume of the green body does not shrink, which is safer.

2.2 Drying process of green body

Taking the convection drying process as an example, the green body drying process can be divided into three simultaneous and interrelated processes: heat transfer process, external diffusion process and internal diffusion process.

Heat transfer process, the process that the heat of the drying medium is transferred to the surface of the green body by convection, and then transferred from the surface to the inside of the green body by conduction. The moisture on the surface of the green body is heated and evaporated, changing from liquid to gas.

External diffusion process: under the action of concentration difference, the water vapor generated on the surface of the green body diffuses and moves from the surface of the green body to the drying medium through the laminar bottom layer.

Internal diffusion process: due to the evaporation of water on the surface of wet green body. It produces a humidity gradient inside and promotes the diffusion of water from the inner layer with high concentration to the outer layer with low concentration, which is called wet conduction or wet diffusion.

Under the condition of stable drying conditions, the surface temperature, moisture content and drying rate of the green body have certain relations with time. According to the changing characteristics of their relationship, the drying process can be divided into three stages: heating stage, constant speed drying stage and slow speed drying stage.

In the heating stage, because the heat transferred by the drying medium to the surface of the green body per unit time is greater than the heat consumed by surface water evaporation, the temperature of the heated surface gradually rises until it is equal to the wet bulb temperature of the drying medium. At this time, the heat obtained by the surface and the heat consumed by evaporation reach a dynamic balance, and the temperature remains unchanged. At this stage, the moisture content of the green body decreases and the drying rate increases.

In the isokinetic drying stage, the discharge of unbound water continues at this stage. Because of the high water content of the green body, the amount of water evaporated on the surface can be replenished internally, that is, the moving speed of water inside the green body (internal diffusion speed) is equal to the evaporation speed and external diffusion speed of surface moisture, so the surface remains moist. 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 isokinetic drying stage. At this stage, unbound water is discharged, so the body volume will shrink, and the shrinkage is linearly related to the water reduction. If the operation is improper and the drying is too fast, the green body is easy to deform and crack, resulting in dry waste. At the end of the isokinetic drying stage, the moisture content of the material decreases to the critical value. At this time, although the interior of the material is still unbound water, bound water begins to appear on the surface.

In the slow drying stage, the water content of the green body decreases, the internal diffusion rate can't keep up with the evaporation rate and external diffusion rate of the surface water, the surface is no longer wet, and the drying rate gradually decreases. Due to the decrease of heat required for surface water evaporation, the temperature of the material began to increase gradually. The partial pressure of water vapor on the surface of the material is less than the saturated partial pressure of water vapor at the surface temperature. At this stage, the combined water is discharged, the green body volume does not shrink, and no dry waste products are produced. When the reduction of material drainage is equal to the equilibrium moisture, the drying rate becomes zero and the drying process is terminated. Even if the drying time is prolonged, the moisture content of the material 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, the structure, shape and size of the material affect the drying speed.

2.3 Factors affecting drying speed

The factors affecting the drying rate are heat transfer rate, external diffusion rate and internal diffusion rate.

(A) to speed up heat transfer.

In order to speed up the heat transfer, we should do the following: ① increase the temperature of the drying medium, such as increasing the hot gas temperature of the drying kiln and increasing the hot blast stove, but we should not increase the surface temperature of the blank too quickly to avoid cracking; (2) Increasing the heat transfer area: if one-sided drying is changed to double-sided drying, layering or reducing the number of green bodies to increase the contact surface with hot air; ③ Increase the convective heat transfer coefficient.

(2) Improving the external diffusion rate When drying is in the constant-speed drying stage, the external diffusion resistance becomes the main contradiction about the whole drying rate, so reducing the external diffusion resistance and improving the external diffusion rate have the greatest influence on shortening the whole drying period. The external diffusion resistance mainly occurs in the boundary layer, and the following points should be achieved: ① Increase the velocity of the medium and reduce the thickness of the boundary layer. And improve the convective heat transfer coefficient. But also can improve the convection mass transfer coefficient, which is beneficial to improving the drying speed. ② The drying speed can also be improved by reducing the water vapor concentration of the medium and increasing the mass transfer area.

(3) Improve the internal diffusion speed of water.

The internal diffusion rate of water is affected by wet diffusion and thermal diffusion. Wet diffusion is the movement of water in materials caused by humidity gradient, while thermal diffusion is the movement of water caused by temperature gradient in physics. To improve the internal diffusion rate, we should do the following: ① make the thermal diffusion and wet diffusion in the same direction, that is, make the center temperature of the material higher than the surface temperature as much as possible, such as far infrared heating and microwave heating; (2) When the directions of thermal diffusion and wet diffusion are the same, strengthen heat transfer and increase the temperature gradient inside the material; When the two are opposite, strengthening the temperature gradient can increase the resistance of thermal diffusion, but it can strengthen heat transfer, increase the temperature of materials and increase the wet diffusion, so it can speed up the drying speed. (3) reducing the thickness of the green body and changing one-sided drying into double-sided drying; (4) reducing the total pressure of the medium is conducive to improving the wet diffusion coefficient, thus improving the wet diffusion rate; and (5) other factors such as the nature and shape of the green body.

3 classification of drying technology

According to whether the drying system is controlled, it can be divided into natural drying and artificial drying. Because artificial drying is to artificially control the drying process, it is also called forced drying.

According to different drying methods, it can be divided into:

① Convection drying, which is characterized by using gas as drying medium and blowing it to the surface of the blank at a certain speed to dry the blank.

(2) radiation drying, which is characterized in that the dried blank is irradiated with radiation energy of electromagnetic waves such as infrared rays and microwaves to dry it.

(3) Vacuum drying is a method of drying blanks under vacuum (negative pressure). The green body does not need to be heated, but a certain negative pressure needs to be generated by pumping equipment, so the system needs to be closed and it is difficult to produce continuously.

(4) Combined drying, characterized by comprehensive utilization of two or more drying methods, giving full play to their respective strengths and complementing each other, can often get ideal drying effect.

There are also some drying methods, which are divided into intermittent dryer and continuous dryer according to whether the drying system is continuous or not. Continuous dryers can be divided into downstream, countercurrent and mixed flow according to the moving direction of drying medium and green body, and can be divided into chamber dryers and tunnel dryers according to the different shapes of dryers.

Characteristics of dryers used in various porcelain types

4. 1 Building Sanitary Ceramic Dryer

1 The moisture content of the sanitary ware body dried in a large space at constant temperature and humidity after slight pressure is about 18%, and the strength is low at this time, so it is not suitable for moving. Generally, in-situ drying method is adopted. General manufacturers adopt boiler steam heating system, which is characterized by low fuel cost and can form a certain dry atmosphere. At the same time, there are many disadvantages, such as no transverse airflow; Poor dehumidification function and long drying time; Without ventilation system, the working conditions of workers are very poor. Therefore, a more advanced "constant temperature and humidity system" is adopted. The system does not need to change the original production process and technology, and can also accelerate the drying speed. Another main feature of the system is its forced ventilation function. This system also has a series of problems, such as high energy consumption; Parameter lag; Drying is not synchronous, etc. Especially in recent years, gypsum molds have become a major trend, so the drying time and requirements of blanks are different, in order to ensure the production arrangement of each shift. The drying of gypsum mould has become the main contradiction in production arrangement. In order to solve this problem, a sealed drying system is adopted, that is, the whole molding line is sealed after gypsum is pulled out, and a small constant temperature and humidity system is used in this narrow space.

2 Hot air rapid drying

Rapid drying means that the drying atmosphere changes according to the difference of green body and the drying degree of green body, so as to always maintain the best drying atmosphere and improve the drying speed. The rapid drying room with automatic temperature and humidity adjustment has the following characteristics: ① small space, fast response and high precision when adjusting parameters; (2) different drying curves can be set according to the condition of the blank; (3) industrial computer control, high degree of automation, reducing human error factors, and high pass rate of blank drying. The system consists of six parts: room structure, hot blast stove, air distribution system, mixing system, control system and humidity system.

3 steam rapid drying

This paper discusses the direct drying of steam, that is, after the blank is taken out of the mold, it enters the drying chamber with one end closed along the track. After the drying chamber is closed, the steam directly enters the sealed drying chamber along the pipeline at the top, and the steam expands and depressurizes in the closed chamber, and the wet steam is discharged and recovered from the pipeline at the bottom of the closed chamber. Its biggest advantage is fast drying and high rate of genuine products.

4 power frequency electric drying

That is, power frequency electricity (50Hz) is applied to the blank, and the whole blank is heated and dried uniformly due to the resistance of the blank, thus achieving the purpose of heating without temperature gradient. The disadvantage of power frequency electric drying is that the preparation before drying is very troublesome, and it is only suitable for drying single products.

4.2 wall and floor tile drying

After coming out of the press, the blank of wall and floor tiles is usually dried by the waste heat of the kiln. However, with the increasing size of the product, the maximum size reaches 1.2×2mm, or even larger, and the thickness is getting thicker and thicker. From 8mm to 60mm, the waste heat of the kiln can no longer meet the drying requirements. Moreover, with the high-grade products and the diversification of colors, the requirements for controlling the atmosphere in the kiln are becoming more and more precise and strict. When using waste heat to dry the green body, the adjustment of the drying section will cause the change of the atmosphere in the kiln, and even increase the consumption of fuel in the kiln, and some increase the fuel 1-2 tons. So vertical dryers, drying kilns and multi-layer drying kilns appeared.

1 vertical drying kiln

It is a widely used drying equipment, which occupies a small area and has a good effect on drying small wall and floor tiles.

2 drying kiln

The drying kiln is directly added before the firing kiln, which is a part of the kiln in appearance (called preheating zone) or a drying kiln with the same length and width is independently built next to the kiln. After coming out of the press or glazing, the green body directly enters the drying kiln for drying, and the dried green body directly enters the preheating zone or enters the sintering green body for sintering through transmission. It consists of a hot blast stove, an air distribution system and a kiln body structure. Generally, the heat utilization rate of drying kiln is good, and only the hot air from the kiln can meet the drying requirements, and some of them have almost low or lower drying moisture. In addition to intensive hot air, it is necessary to burn a hot blast stove, which consumes 2 ~ 3 tons of fuel every day.

3 multi-layer drying kiln

With the development of technology, the water content of green body is getting lower and lower, and it is necessary to reduce the water content from 8% to 1% during drying, which is impossible for ordinary drying kiln. Multilayer drying kiln can solve this problem. It is composed of kiln head queue, kiln tail collector and several drying units, each unit is independent, their temperature and humidity are adjustable, and the ventilation rate is also adjustable, which is independently controlled by the hot blast stove. Its advantages are: sufficient drying time; Small surface area and small heat loss; The air outlet is close to the brick surface. High drying strength; When the temperature is adjusted, the ventilation is not affected, so the speed and range of hot air blowing across the brick surface will not change due to the temperature adjustment. However, the adjustment of multi-layer drying kiln is relatively difficult, especially the increase of kiln width, which can not ensure the uniformity of temperature in the kiln, resulting in different drying effects.

4.3 Daily ceramic drying

The drying of daily-use ceramics is different from that of sanitary ceramics or wall and floor tiles, and has the following characteristics: ① There are many kinds, large quantities, small sizes and complex shapes of green bodies. Deformation and cracking are the two most common defects: (2) In the production process, the processes of demoulding, turning blank, trimming blank, connecting handle, glazing, etc. are often mixed together and become a flow process. Therefore, the drying of household porcelain mainly adopts chain dryer. According to the arrangement of chain, it can be divided into horizontal multi-layer arrangement dryer, horizontal single-layer arrangement dryer and vertical arrangement dryer.

5 far infrared drying technology

Infrared radiation drying technology has been paid more and more attention by all walks of life, which plays a huge role in food drying, tobacco, wood, Chinese herbal medicine, cardboard, cars, bicycles, metal body painting and so on. In addition, far infrared drying is also used for ceramic drying. The infrared absorption wavelength range of most objects is in the far infrared region, and water and ceramic green bodies also have strong absorption peaks in the far infrared region, which can strongly absorb far infrared rays and produce strong vibration phenomena, so that the green bodies can be quickly heated and dried. The penetration depth of far infrared to the illuminated object is greater than that of near infrared and middle infrared. Therefore, it is more reasonable to use far infrared to dry ceramics. Far-infrared drying has the advantages of high efficiency, rapid drying, energy saving, time saving, convenient use, uniform drying and small occupied area, so as to achieve excellent effects of high yield, high quality and low consumption.

According to the production practice of ceramic factory, far infrared drying can shorten the drying time by half compared with near infrared drying, which is11,the green rate is over 90%, and it saves electricity by 20 ~ 60% compared with near infrared drying. Zhengzhou Porcelain Factory has carried out far infrared drying technology for 10 inch flat plate. The results show that the production cycle is doubled, the drying time is shortened from the usual 2.5 ~ 3 hours to 1 hour, with low cost, small investment, quick effect, good sanitary conditions and small floor space. The research of far infrared materials is very active in recent years and has made great progress. There are also many successful applications in all walks of life. Why are few people interested in the drying line of building sanitary ceramics?

6 microwave drying technology

Microwave refers to electromagnetic waves between high frequency and far infrared, with a wavelength of 0.00 1- 1m and a frequency of 300-300000MHz. Microwave drying is to irradiate the wet green body with microwave, and the direction and magnitude of electromagnetic field change periodically at any time, so that the polar water molecules in the green body change with the alternating high-frequency electric field, so that the molecules rotate violently, generate friction and convert it into heat energy, thus achieving the purpose of uniform heating and drying of the green body as a whole [2, 3, 4]. The penetration ability of microwave is much greater than that of far infrared, and the lower the frequency, the greater the half-power depth of microwave. Characteristics of microwave drying:

(1) is uniform and fast, which is the main feature of microwave drying. Because of the strong penetration ability of microwave, heat can be directly generated in the medium when heating. No matter how complex the shape of the green body is, the heating is uniform and rapid, which makes the green body dehydrate quickly, demould evenly, deform little, and crack is not easy to appear.

(2) It is selective, and microwave heating is related to the properties of the material itself. In the microwave field of a certain frequency, because the dielectric loss of water is greater than other substances, the heat absorption of water is much greater than that of other dry substances; At the same time, due to microwave heating at the same time, the internal moisture can be quickly heated and directly evaporated, so that the ceramic blank can be heated and demoulded in a short time.

(3) High thermal efficiency and sensitive response. Because the heat comes directly from the inside of the dry matter, the loss of heat in the surrounding medium is very small. In addition, the microwave heating cavity itself does not absorb heat and microwaves, and all of them are emitted to the blank, so the thermal efficiency is high.

Microwave heating equipment is mainly composed of DC power supply, microwave tube, connecting waveguide, heater and cooling system. Microwave heaters can be divided into standing wave field resonance heaters, traveling wave field waveguide heaters, radiation heaters, slow wave heaters and so on according to the heating object and the form of microwave field.

6. 1 Application of Microwave Drying in Domestic Ceramics

Hunan Guoguang Porcelain Group Co., Ltd. designed a rapid dehydration and drying production line according to the technological characteristics of daily-use ceramics. Practice has proved that compared with the traditional chain drying line, the green rate has increased by more than 10%, the time for removing gypsum molds has been shortened from 35-45 minutes to 5-8 minutes, and the number of molds used has been reduced from 400-500 to 100. The microwave drying line occupies a small area and is pollution-free. It is 6.5 times more efficient than chain drying, which can not only save a lot of gypsum molds, but also reduce the total drying cost of 10.5 inch flat plate by 350 yuan per ten thousand tablets [5].

6.2 Application of Microwave Drying in Electric Porcelain

In Fushun Petrochemical Company, Liaoning Province, Li Chunyuan used microwave heating drying technology, weight reading control technology and infrared temperature reading control technology to dry electric porcelain with complex shapes. Compared with the conventional steam drying method, the productivity can be increased by 24-30 times, and the output can be increased by 65,438+05%-35%. The same output only occupies about one twentieth of the existing technology, which can greatly improve the economic benefits. This can provide reference for the drying of some special-shaped products such as building sanitary ceramics, walls and tiles.

6.3 Porous Ceramics Dry porous ceramics have the advantages of high mechanical strength, easy regeneration, good chemical stability, good heat resistance and uniform pore distribution, and have broad application prospects and are widely used in the chemical industry. Environmental protection, energy, metallurgy, electronics, petroleum, smelting, textile, pharmacy, food machinery, cement and other fields. As sound-absorbing materials, sensitive elements, artificial bones, roots and other materials have attracted more and more attention. Because porous materials contain a lot of water, there are many pores, the inner wall of the green body is particularly thin, and the heating is uneven, it is extremely difficult to dry by traditional methods. Moreover, these porous materials have poor thermal conductivity, and the drying process requirements are particularly strict, especially for environmentally friendly honeycomb ceramics for automobiles. The drying process is not well controlled, and it is easy to deform, which affects porosity and specific surface area. Microwave drying technology has been successfully applied to the drying of porous ceramics, which can easily reduce the moisture content of the green body from 18% ~ 25% to below 3%, and the precipitation rate reaches 0.7 ~ 1.5 kg, which greatly shortens the drying time and improves the yield. We also apply microwave drying to the green body drying in split brick, and the effect is also obvious.

7 outlook

Although microwave heating has many advantages, its fixed investment and pure production cost are higher than other heating methods, especially it consumes more electricity and increases production cost. Under the irradiation of large energy for a long time, microwave has adverse effects on human health, and microwave heating is selective. Therefore, using microwave drying or convection drying alone has its own advantages and disadvantages. If we combine the two methods, we will give full play to their advantages. That is, a microwave generator is added in the rapid drying room. In the blank heating stage, the microwave generator runs at the maximum power, which raises the blank temperature in a short time. Then the microwave power is gradually reduced, and the hot air drying is operated at the maximum intensity, so that the total heating time will be reduced by 50%, the total energy consumption will not increase, and the green body qualification rate is high. Moreover, it is necessary to make the microwave oven structure as reasonable as possible and take appropriate radiation protection measures, which can minimize microwave radiation and have no impact on the human body at all [6]. Therefore, in order to give full play to the advantages of microwave technology, we should not only adopt mixed heating or mixed drying technology, but also strengthen the research on the interaction mechanism between ceramic materials and microwave, strengthen the basic data test of the relationship between dielectric properties and dielectric loss of ceramic materials and microwave frequency and temperature, and improve microwave drying technology and equipment to make this technology serve the ceramic industry.