1.1 Characteristics of activated carbon
Activated carbon is an amorphous substance and is a hydrophobic non-polar adsorbent capable of selectively adsorbing non-polar substances and having a large adsorption force. Pores with different pore sizes can adsorb substances of different molecular sizes. In addition, activated carbon has good catalytic activity due to its surface and surface compounds, as well as ash and the like. The widespread use of activated carbon is also due to its stable chemical properties. It is resistant to acids, alkalis, high temperatures, insoluble in water and other solvents, and can be used in aqueous solutions and many solvents. Another characteristic of activated carbon is that it can be regenerated by various methods after being used, and can be used multiple times.
1.2 Quality indicators and applications of activated carbon
The quality of activated carbon has a number of physical and chemical indicators, such as: moisture, ash, acid solubles, various metals and acid radicals, and its adsorption properties. For different purposes of activated carbon, different substances and methods are often used to test its adsorption performance, usually with methylene blue adsorption value, iodine adsorption value, caramel adsorption value, quinine sulfate adsorption value, phenol value and so on. Among them, the adsorption value of methylene blue is the most commonly used, and its adsorption amount reflects the ability of activated carbon to adsorb small molecular substances, and has a large amount of microporous activated carbon, which is high. According to the adsorption characteristics of activated carbon, activated carbon is mainly used to remove pollutants from water, decolorize, filter and purify liquids and gases, and also be used for air purification and waste gas recovery (such as gas benzene recovery in the chemical industry), precious metals Recycling and refining (such as the absorption of gold). In recent years, it has also been used in the pharmaceutical industry. With the development of science, with the emphasis on the ecological environment, the use of activated carbon is becoming more and more widespread.
2 Status of domestic and foreign demand for activated carbon
The production and application of activated carbon at home and abroad are relatively late. Europe and the United States began to develop activated carbon production in the early 20th century [2]. China's activated carbon industry was really established in the 1950s, and it had a great development in the 1970s. At present, the annual output of activated carbon in the world is about 6.5×105t. In 1997, the Americas (Brazil, Mexico, and the United States) were 2.359×105t, and Europe (Belgium, France, Britain, and the Netherlands) reached 1.124×l05t. Asia [China (including Taiwan), India, Indonesia, Japan, South Korea, Malaysia, Philippines , Sri Lanka, Thailand] counts 3.01 × 105t. In 1997, China's total activated carbon production reached 1.2×105t, accounting for 15% of the world's annual output. According to relevant information and materials, in 1998 China's export volume was 1.004×105t, and its export volume reached US$604 million. In 2000, China's activated carbon export volume reached 1.192×105t, and its export volume reached US$685 million. In terms of the quantity of imports, the import volume of activated carbon in 1999 was 19.045 million tons, and the import volume reached 5.582 million US dollars. In 2001, the import volume of activated carbon increased to 25.673 million tons. The import value was US$ 7.271 million. It can be seen from these data that China's activated carbon industry still has broad development prospects in the 21st century. However, the domestic activated carbon industry must focus on the development trend of application of activated carbon, strengthen the development of new technologies, and promote the good development of the entire activated carbon industry.
3 Activated carbon preparation raw materials and traditional preparation methods
3.1 Preparation of activated carbon
The main raw materials used in the production of activated carbon can be divided into the following five categories: a, plant materials, such as wood, sawdust, fruit core, fruit shell, cotton straw, furfural residue, sucrose residue, etc.; b, mineral raw materials, such as various Coal and petroleum residues, etc.; c, various wastes, such as animal bones and blood, industrial waste plastics, various rubber waste products, etc.; d, synthetic fiber materials, such as polypropylene; e, organic fiber materials, such as poly Acrylonitrile fiber, viscose fiber, asphalt fiber, and the like. According to the materials used in the production, the main domestic activated carbon varieties are wood activated carbon, coal activated carbon, shell activated carbon and activated carbon fiber.
3.2 Traditional preparation method of activated carbon
Traditional methods for preparing activated carbon at home and abroad include gas activation method, chemical activation method, and chemical physical method: gas activation method is to activate carbonization, water vapor, carbon dioxide, air, flue gas, etc. at 800-900 ° C. method. The activated carbon has low yield, unstable quality, low specific surface area, high activation temperature and large equipment investment, but no pollution to the environment. The chemical activation method is to add chemicals to the raw materials, then heat them in an inert gas and simultaneously carbonize them. And a method of activation. This method is more corrosive to equipment, pollutes the environment, and has more residues. The chemical physical method adds a certain amount of chemicals (ie, additives) to the activated carbon raw material for chemical activation and immersion treatment, and forms a transmission channel inside the carbon material. It is advantageous for the gas activator to enter the pores for etching. The advantage of this method is that the activated carbon with reasonable pore size distribution can be obtained by controlling the impregnation ratio and the immersion time, and the specific surface area is large, and the yield is high.
4 Microwave preparation of activated carbon
Microwave has the advantages of fast, high efficiency, high resource recovery and utilization, no secondary pollution, and low cost. Therefore, the preparation of activated carbon by microwave method has become one of the research hotspots of scholars at home and abroad. Many researchers at home and abroad have tried to prepare activated carbon by microwave and have achieved gratifying results. For example, Kasai and Takakazu [5] used microwave heating, carbonization and activation of organic molding waste to obtain high-quality activated carbon. Peng Jinhui and other use of broad bean stalk, rice straw, black bark copy glue scrap, melon seed shell, wheat straw, corn cob, broad bean shell, bagasse, bayberry bark waste, tobacco stalk, pine sawdust, coconut shell as raw material, microwave preparation Activated carbon. Fan Xi'an uses microwave radiation bamboo joints, coconut shells and cotton stalks to produce high-quality activated carbon using phosphoric acid, water vapor and zinc chloride as activators. Zhang Libo et al. used microwave radiation tobacco stalk method to produce activated carbon with different properties.
4.1 Principle of microwave heating
The microwave frequency is approximately 300 MHz to 300 GHz, that is, electromagnetic waves having a wavelength in the range of 100 cm to 1 mm. The principle of microwave heating is based on the fact that when microwaves encounter different materials, reflection, absorption and penetration will occur depending on the properties of the material, depending on the dielectric constant, dielectric loss coefficient, specific heat, shape and water content of the material. . Generally speaking, there are two mechanisms for heating the medium in the microwave field, namely the ion conduction mechanism and the dipole rotation mechanism. In the actual heating, the microwave energy dissipation of the two mechanisms exists at the same time. Conventional heating is performed by external heat source through heat radiation from the surface and inside. Compared with microwave heating, it has the following characteristics: selective heating, fast heating, high thermal efficiency, easy control, and easy automation.
4.2 Application of microwave technology in environmental protection
Several applications of microwave technology in the field of environmental protection: microwave deoiling technology of oily sludge; microwave deoiling technology of crude oil sludge; microwave carbon reduction-flue gas desulfurization and denitration technology; microwave incineration treatment of waste circuit board to recover precious metal technology; microwave-chlorination The zinc method is used to prepare the chromium-containing wastewater by the sawdust activated carbon; the microwave method is used to prepare the high oil absorption composite technology for the leakage treatment of the crude oil; and the preparation technology of the microwave absorbent for preventing the electromagnetic radiation pollution.
4.3 Factors influencing the preparation of activated carbon by microwave method
In the preparation of activated carbon, the raw materials are different, and the efficacy of the finished product is also different. Among them, coconut shell activated carbon is recognized as the best activated carbon in the activated carbon industry. It can select various materials such as adsorption speed, adsorption capacity, strength and other activated carbon products according to different water treatment processes. The influencing factors of microwave preparation by microwave method are: microwave power, irradiation time, activator concentration, impregnation ratio, pH value and so on.
4.3.1 Effect of microwave power
The sludge raw material was fully immersed and dried in a quartz tube, and the microwave irradiation time was fixed for 4 min, the zinc chloride concentration was 35%, and the dry sludge and zinc chloride specific gravity was 1:3.0, and the microwave power was changed. When the power is low, the heating temperature does not reach the temperature of carbonization activation, so the adsorption performance is poor at this time; when the power continues to increase, the temperature is too high, because the vapor pressure of zinc chloride is high, the chemical loss is serious, so that the actual play The reduction in the action of zinc chloride results in a decrease in the performance of the adsorbent.
4.3.2 Effect of irradiation time
The sludge raw material after sufficient immersion and filtration is placed in a quartz stripping tube and fixed in a microwave oven cavity at a microwave power of 595 W, and the concentration of the activator zinc chloride is 35%, and the dry sludge and the zinc chloride solution are Under the condition of 1:3.0 weight ratio, the experimental results of changing the irradiation time are shown in Figure 2. The iodine value of the adsorbent and the adsorption value of methylene blue showed similar changes with the change of microwave time. The reason was analyzed mainly because of activation. The effect of the length of time and the rapid activation rate of the microwave, the irradiation time is too short, the carbonization is insufficient, and the excessive irradiation time will cause the partial pore diameter to be sintered.
4.3.3 Effect of activator concentration
The dried sludge material is fully impregnated with different concentrations of zinc chloride solution, placed in a quartz tube, and fixed in a microwave cavity. The microwave power is 595 W, the irradiation time is 4 min, and the medium weight of dry sludge and zinc chloride is The experiment was carried out under the conditions of 1:3.0 to study the effect of different concentrations of activator on the adsorption performance of the prepared carbon-containing adsorbent. In general, the higher the concentration of the activator, the greater the dehydration condensation effect, and the more porous the adsorbent finally obtained, the better the adsorption performance. On the other hand, zinc chloride is a strongly adsorbed substance. If the concentration is too high, the adsorption capacity of the material is enhanced, so that the activation temperature is increased, the carbon component content is decreased, the ash content is increased, and the zinc chloride crystal is blocked. The macropores are not sufficiently removed during the washing process, so that the specific surface area and adsorption capacity of the adsorbent are lowered.
4.3.4 Influence of impregnation ratio
The 40% zinc chloride solution of different weights was fully impregnated and filtered, and the sludge raw materials were placed in a quartz tube, fixed in a microwave cavity, and irradiated at 595 W microwave power for 4 min. The greater the weight ratio of dry sludge to zinc chloride solution, the activator solution can fully infiltrate the granular sludge, and the role of zinc chloride in the pyrolysis process is fully exerted, and the obtained adsorbent performance is better. However, excessive zinc chloride solution impregnation will cause the moisture content of the pyrolysis raw material to be too high, requiring a long heating time and causing waste of zinc chloride.
4.3.5 Other influencing factors
Influencing factors in the preparation of activated carbon in microwaves are also the soaking time. Generally, the longer the soaking time, the more fully the degree of impregnation, but the time exceeds a certain level, and the adsorption effect decreases. This is because the immersion time and the concentration of zinc chloride directly affect the sorption of zinc chloride on the sludge, thereby affecting the performance and yield of the product. When the zinc chloride solution is initially impregnated, the moisture content of the raw material is very low, so that the zinc chloride solution can quickly swell the raw material, and in the initial period of time, as the immersion time is prolonged, the zinc chloride in the raw material is rapidly absorbed. Increase, but when the amount of sorption reaches a certain level, the rate of adsorption of zinc chloride by the raw material becomes slower as the immersion time increases. This factor has little effect on the preparation of activated carbon from dry sludge, but the effect of immersion time on the preparation of activated carbon from plant raw materials is more significant. In addition, the activators are different, and their efficacy is also different. Zhang Libo et al. used microwave radiation tobacco stalks to obtain activated carbon with different properties. The activators used were water vapor, phosphoric acid, zinc chloride, potassium hydroxide and sulfuric acid.
5 Summary
Microwave method for the preparation of activated carbon has opened up a new way. Microwave method has the advantages of fast heating, high thermal efficiency, easy control, small equipment volume and low pollution. In addition, the prepared activated carbon has excellent adsorption performance. However, there are also deficiencies, microwave leakage will cause harm to the human body and the surrounding environment, the microwave reaction chamber should be rationally designed and appropriate use of microwave absorbing materials should be considered. Microwave-related basic theoretical research is still relatively lacking, such as the influence mechanism of microwave on chemical reaction, the temperature rise characteristics of matter in microwave field, and the temperature distribution in microwave field. Only by solving these problems, microwave technology can be industrialized on a large scale. application.
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