The methods of granular activated carbon can be mainly divided into two categories:
1) direct carbon activation without adding binders;
1) Add a binder to process the carbonaceous precursor into shape, and then proceed with the carbon activation process.
The preparation of granular activated carbon without the need for adding binders can be divided into two types: one is to directly activate natural wood materials of a certain shape and size to obtain amorphous granular activated carbon; The second is to prepare granular activated carbon by chemical activation method without the need for adding binders. Rich natural woody plant materials, such as coconut shells, walnut shells, olive seeds and other fruit shells and cores, can be directly used to prepare amorphous granular activated carbon through physical or chemical activation methods. Poinern et al. used Australian nut shells as raw materials, first subjected them to high-temperature carbonization, and then combined them with carbon dioxide activation method to prepare granular activated carbon, and explored the effect of activation temperature on the adsorption of gold by activated carbon. The results indicate that high-temperature activation is beneficial for improving the gold absorption capacity of granular activated carbon. By crushing, soaking, floating and low-temperature drying carbonization treatment of raw materials (mainly fruit shells and cores), products with high strength and adsorption capacity can be obtained after activation. The results showed that a raw material particle size of 20-40 mesh is beneficial for the penetration of activators. Compared with the zinc chloride method, it improves the strength of amorphous carbon, and has an iodine value of 950 mg/g and 100% B caramel color. Compared with the physical method, its production cost is reduced by 40%. The self forming preparation of granular activated carbon by chemical activation method is a one-step preparation method that involves adding a certain amount of chemical drugs to carbonaceous raw materials, thoroughly kneading and plasticizing them to produce tar and other adhesive and plasticizing substances, without the need for additional binders. The process is completed by heating and activating the carbon in one step. The chemical agents used for preparing granular activated carbon mainly include ZnCl2, H3PO4, KOH, NaOH, H2SO4, CaCl2, K2CO3, etc. Different chemical activators have different activation mechanisms for raw materials.
The main purpose of adding binders is to solve the problems of non bonding of raw materials, difficulty in forming, and unsatisfactory product strength and surface smoothness in the preparation of molded activated carbon. The type and amount of binder greatly affect the strength and adsorption performance of activated carbon. Adding binders often causes partial pore blockage in granular activated carbon, resulting in an increase in mechanical strength while reducing its adsorption performance. Common binders include coal tar, starch, wood tar, sulfite pulp waste liquid, clay, waste molasses, carboxymethyl cellulose, resins, etc. Coal has the advantages of high carbon content, abundant resources, and low price, and is widely used as a raw material for the preparation of granular activated carbon. Currently, the raw coal used to manufacture coal based granular activated carbon is non cohesive and weakly cohesive coal, such as anthracite, weakly cohesive coal, lignite, etc. Therefore, a certain amount of binder needs to be added to the raw coal to press it into shape. Due to the high asphalt content of coal tar and its strong bonding ability to coal powder, most coal based activated carbon factories in China use coal tar as a binder. Du Yaping et al. used coal tar as a binder for petroleum based granular activated carbon, mixed with anthracite, extruded and activated to produce granular activated carbon with high mechanical strength and well-developed micropores, which can be used as an adsorbent for purifying air or water.
In addition to coal based raw materials, researchers have also conducted extensive research on the preparation of granular activated carbon from wood and other raw materials. Technicians used tobacco rod waste as raw material and wood tar as composite adhesive to prepare granular activated carbon using steam and carbon dioxide activation methods, respectively. The results showed that when activated at 900 ℃, the water vapor method activated carbon particles exhibited microporous morphology, with a BET specific surface area of 1037m2/g and a total pore volume of 0.8152mL/g; Carbon dioxide activated carbon also exhibits microporous morphology, with a specific surface area of 947.81m2/g and a total pore volume of 0.48mL/g, but the yield is higher than that of steam method.
