With the development of food, medicine, light industry, environmental protection and other industries, activated carbon as a adsorption decolorizing agent, its dosage has increased rapidly. In the early days of the founding of China, the annual output of activated carbon was only more than 30 tons, to 1996, the national manufacturers have more than 200, the annual output of 130,000 tons.

According to the different uses of activated carbon can be made into powder and granular, granular activated carbon is divided into shaped carbon (such as: columnar, spherical, etc.) and non-shaped; According to the different activators used in the production, it can be divided into physical activated carbon and chemical activated carbon: According to the different production materials can also be divided into wood carbon and coal carbon, because of the large porosity of wood raw materials, The internal cell tissue has many natural pores (the specific surface area of charcoal is 206-429m2 / g), so that the activator is easy to enter, and the contact reaction surface is large and easy to live, while the coal is different (the specific surface area of coal coke is only 20-80m2 / g), so the reaction surface with the activator is much smaller than the charcoal. In short, the chemical method of wood powder activated carbon decolorization performance is the best, it in the hydrolysis, fermentation, organic synthesis, sugar, pharmaceutical, water purification and environmental protection and other fields, used to remove the color body, gum, adsorption odor, prevent liquid turbidity, remove foam, improve the evaporation crystallization rate. Example: Mancheng Citric Acid Factory, Baoding Sulfonic acid Factory, Baoding Chemical Factory No. 2 xylosa workshop, Xingtai phytic acid Factory, Shijiazhuang Non-staple Food Factory No. 2 monosodium glutamate workshop, Zhaoxian Grape Sugar Factory, North China Pharmaceutical Factory starch branch factory, Xinfi malt sugar factory and lactic acid factory all use powdered wood activated carbon as refined decolorizing agent. Due to its large amount and wide scope, domestic production and scientific research units continue to explore, study and improve its production process, technology and equipment.

I. Production method of powdered wood activated carbon

1, chemical method powdered wood activated carbon

(1) Principle

The mature process of activated carbon by chemical method is zinc chloride method, most of the domestic activated carbon production plants use this method, the activation principle is: zinc chloride has a dewatering effect in carbonization activation, and the hydrogen and oxygen elements in wood raw materials are used to escape in the form of water, so that the carbonization activation temperature can be reduced and the pyrolysis route can be changed In addition, zinc chloride solution has a dissolution and erosion effect on wood raw materials, so it can smoothly penetrate into the inside of the raw materials, to the fine pores formed by the fibers in the wood raw materials, in order to achieve complete activation, zinc chloride in the activation process is mostly left in the carbon as a skeleton, and the formed carbon is deposited on the skeleton, when the zinc chloride is finally washed off with water, the carbon forms a porous Thus has adsorption, decolorization effect.

(2) Process

Kneading: First of all, the industrial zinc chloride is mixed into 45-50 baumer of aqueous solution, the p H value is 4.5-6.5 (refers to the measured value of a drop of zinc chloride solution dissolved in 25 ml of distilled water), and then put into the kneading machine with the dried wood chips at a solid-liquid ratio of 1:3-4, so that the kneading is fully uniform Kneading time is not less than 15 minutes to ensure that the zinc chloride solution fully enters the interior of the fiber. If other mixing methods are used, such as simple manual stirring or static soaking, the required mixing time should be extended.

Carbonization activation: In continuous production, internal heat rotary furnace is used as the main equipment for carbonization activation, and its heat carrier is high temperature flue gas. The heat transfer mode is: direct contact heat transfer between high temperature flue gas and material is countercurrent. The direction of material flow is: from the high end of the rotary furnace, the kneading uniform material is continuously added, and the material is slowly moved from high to low by the rotation and inclination of the rotary furnace. Because the material is constantly turning and moving in the furnace, it ensures the uniform carbonization and activation of the material and stable product quality. The reaction temperature should be strictly controlled during carbonization activation. Although increasing the temperature is beneficial to the reaction, the loss of zinc chloride increases (for example, the vapor pressure of zinc chloride at 500℃ is 100 mm Hg, and that at 610 ℃ is 100 mm Hg, respectively). 73 2℃ is 760 mm Hg), so the carbonization activation temperature should be reduced as much as possible under the premise of ensuring the quality of activated carbon. The process conditions of the section are controlled at: the temperature is 450-550 ℃, the time is 40 minutes, and the rotary furnace speed is 2-3 RPM.

Recovery: In addition to carbon and zinc chloride, the materials discharged from the rotary furnace also contain zinc oxide and zinc hydroxide generated by the reaction under high temperature and water vapor, so it needs to be treated, there are two purposes, one is to make zinc oxide and zinc hydroxide reaction into zinc chloride as an activator, and the second is to maximize the recovery of zinc chloride To reduce costs by recycling. The reactions included are:

Zn(OH)2 +2 HCl=Zn Cl2+2 H2 O

Zn O+2 HCl=Zn Cl2 +H2 O

When the concentration of zinc chloride in the lotion reaches 40 baum, it is returned to the kneading section for recycling.

Purification and impurity removal: Because most of the equipment used in the production is iron equipment, resulting in excessive iron content in the product, it is necessary to remove iron.

The process is: add the materials sent by the recycling section to the iron removal tank (non-iron equipment), then add tap water so that it is just immersed in the material, and then add industrial hydrochloric acid accounting for 4.5% of the material, stir intermittently at 95℃ for more than 2 hours, and finally rinse and filter repeatedly until the liquid reaches neutral.

Dehydration: The material washed to neutral is first removed by tripod centrifuge or plate and frame filter press. Then it is dried in a rotary dryer until the water is less than 10% and then sent to crush and pack.

Crushing packaging: Most domestic manufacturers use ball mills for crushing, and the fineness of their products should be adjusted according to user requirements. The smaller the particle size, the better the decolorization effect, but the smaller the particle size affects the filtration speed, and it is generally better to control the particle size ≥ 120 mesh.

(3) Advantages and disadvantages of this method

Advantages: The activated carbon prepared by this method has large porosity, and activated carbon with different pore sizes can be produced by adjusting the concentration of activator. In addition, zinc chloride is used as activator, which has low carbonization activity temperature, easy operation and low energy consumption, and zinc chloride can be recycled and used.

Disadvantages: Some zinc chloride enters the atmosphere during carbonization activation, and volatile hydrochloric acid is used in the recovery section, both of which pollute the atmosphere to a certain extent.

2, physical method powdered wood activated carbon

(1) Principle

The method of activating carbon with high temperature water vapor, high temperature flue gas (CO2 gas) and high temperature air is physical activation method. Its activation principle is:

H2 O(g)+Cx 80 0 -90 0℃ H2 +CO+Cx- 1 - Q

CO2 (g)+C x 80 0 -90 0℃ 2 CO+C x- 1

O2 (g)+C x 80 0 -90 0℃ 2 CO+Cx- 2 +Q

O2 (g)+C x < 6 0 0℃ CO2 +C x- 1 +Q

In actual production, most of the high temperature water vapor is used as the activator, such as Xiantai activated carbon plant in Baoding province.

(2) The production process using water vapor as the activator

Carbonization of raw materials: First of all, the wood raw materials are carbonized completely in the range of 450-60 ° C, and then broken into 2-30 mm blocks for use.

Activation of carbonized material: because the reaction of water vapor and carbon at high temperature is endothermic reaction, when the system temperature reaches 80 0-90 0 ° C, the reaction begins, but because of the reaction endothermic, the material temperature will be lower than 80 0 ° C, affecting the reaction speed, in order to ensure the temperature at 80 0-90 ° C, it is necessary to add heat. Because hydrogen and carbon monoxide are generated in the product, and the two gases can be burned to release heat, so the actual production is to use this part of the combustion heat compensation reaction caused by heat consumption. The carbonization activation reaction is carried out in a multi-tube activation furnace. The carbonized material is added from the furnace mouth of the activation furnace, the activator water vapor is passed into the upper part of the activation furnace, and the activated carbon is discharged from the lower part after the activation reaction is completed, and the gas discharged from the lower part is sent to the furnace for combustion.

(3) Advantages and disadvantages of this method

Advantages: The use of water vapor as activator, so that the surface of carbon pure, due to the reaction heat absorption, will not make the material in the activation of local overheating and affect the yield, while the method is pollution-free, fuel saving.

Disadvantages: Due to the small heat transfer coefficient of refractory materials and charcoal, it is easy to cause uneven temperature of materials in the tube, inconsistent activation and affect the stability of product quality.

Second, the process and technical improvement direction of powdered wood activated carbon

1.production of raw materials

Although the quality of activated carbon prepared with wood as raw materials is better, due to the indiscriminate cutting of trees in recent years, coupled with packaging, construction, decoration, disposable sanitary appliances and so on, a large amount of wood is consumed every year, and the regeneration rate of wood resources is very slow, less than a few years, more than ten years or decades, resulting in the current situation of wood resources shortage in China. In order to promote the healthy development of activated carbon industry, the relevant departments of our country have done a lot of work on the development and production of activated carbon with different raw materials instead of wood. For example, rice husks are used to prepare activated carbon [4], waste plants are used to produce gas, carbon and tar [5], and industrial hydrolysis residues are used to prepare activated carbon [6] [7]. The Department of Chemistry of our university has also conducted a lot of research work on local resources - wheat stalk and corn stalk instead of wood to prepare activated carbon, and its subject has been certified by the provincial level in 1999. In addition, it is also reported that the use of bagasse, organic industrial waste as raw materials to prepare activated carbon.

2. Production process

At present, most of the chemical activated carbon uses zinc chloride as activator, because of the problem of air pollution limits its development, so people are constantly exploring new production processes to replace zinc chloride with other pollution-free or less polluting activator. It has been reported that using sulfuric acid as activator can prepare a decolorizing agent with high decolorizing power, and the carbonization activation temperature is reduced from 450-550 ℃ of zinc chloride method to 2 50-300 ℃ [8], and using phosphoric acid as activator can basically eliminate pollution, and the product quality is stable, and the operating conditions are easy to control [9], according to the latest data reports A pilot study on the preparation of activated carbon by radiation process and fluidization process is under way.

With the enhancement of people's environmental awareness, the physical activated carbon process will gradually be paid attention to, in order to improve the adsorption performance of physical activated carbon, the production units have made many attempts, such as changing the high-temperature water vapor activation process to two times, which can significantly improve the decolorization power of the product.

3. Production equipment

Activation furnace is the most important equipment for activated carbon production. Although the rotary furnace is simple to operate, the labor intensity is small, the material activation is uniform, the product quality is stable, but because the flue gas temperature is high, some zinc chloride in the gas phase is brought into and the consumption is increased, if the plate furnace is used, although the efficiency is low, there are occasionally uneven carbonization activation shortcomings, but the operation flexibility is large, the pollution is easy to solve. How to weigh the two, relevant experts are exploring this, I believe that in the near future there will be a small pollution, high degree of mechanization, product quality stability of efficient activation furnace come out.

Grinding and drying equipment is the necessary equipment in the production process of activated carbon. Traditional grinding equipment mostly uses different specifications of mining ball mill, low efficiency, power consumption, serious noise pollution, some large enterprises have used Raymond machine, the machine has high efficiency, uniform crushing, fast filtering speed. Regarding drying equipment, most of them use the waste heat of carbonized activation furnace as drying heat source, using fixed box type or rotary furnace type drying equipment, but there are disadvantages of large powder and low efficiency, some enterprises have used other equipment with better effect, such as vacuum drying, fluidized fluidized bed dry noise. The crushing equipment Research Institute and the drying Equipment Research Institute under the Mechanical and Chemical Research Institute of the Ministry of Chemical Industry in China have successfully developed new equipment every year and put on the market for the reference of various enterprises.

Iii. Conclusion

Activated carbon as a product involving all walks of life adsorption decolorization, in recent years from raw materials to process equipment have been greatly improved and developed, but there are still many problems, the need for professional personnel to work together in order to continuously improve, to meet the needs of national economic development.