Research on preparation of super activated carbon from Baosteel pitch coke

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Research on Preparation of Super Activated Carbon from Baosteel Pitch Coke Zhang Xiuyun Shan Changchun Liu Chunfa Xue Zhang (Shanghai Baosteel Chemical Co., Ltd., Shanghai 200942) Super activated carbon has become one of the most actively researched energy storage materials due to its ultra-high specific surface area, rich nanoscale pore size, stable surface physical and chemical properties and excellent adsorption performance. The United States and Japan have successively developed such products using petroleum coke as raw material and using the KOH activation method, and have achieved industrialization, selling for 850,000 to 1 million yuan/t. Mainly used in the fields of advanced environmental protection and energy storage materials. my country's market demand for this material basically relies on imports, and research on its preparation is still in the experimental research stage. The raw materials used include petroleum coke, coconut shells, bamboo, etc. There have been no reports on the preparation of super activated carbon using pitch coke as raw material. Using calcined pitch coke (hereinafter referred to as pitch coke) produced by Shanghai Baosteel Chemical Co., Ltd. as raw material, super activated carbon was prepared using the KOH activation method. The effects of different activation process conditions on the yield, pore structure and adsorption performance of activated carbon were investigated. 1 Test 1.1 Baosteel pitch coke is used as the raw material. The ash content of Baosteel pitch coke is 0. 12%. The elemental analysis results are as follows: C 94. 79%; H 3. 17%; N 0. 76%; O+S 1. 29% ； The carbon to hydrogen atomic ratio is 2.49. 1.2 Preparation method of activated carbon: After crushing and sieving the pitch coke, take 80g of 80-150 mesh coke powder, mix it evenly with KOH aqueous solution (according to a certain alkali-to-carbon ratio), and soak it. Add the soaked slurry material into the activation kettle, and under nitrogen protection, raise the temperature to 180°C for physical dehydration. ； Continue to raise the temperature to 400°C for chemical dehydration ； Then raise the temperature to 750~850℃, and react at this temperature for 1.5h. After the activation is completed, stop heating, take out the activation kettle, let it naturally cool to room temperature, wash the activated carbon with water until it is neutral, and dry it for later use. 1.3 Characterization of raw materials and activated carbon. The ASAP2020M physical automatic adsorption instrument of the American Micromeritics company was used to measure the adsorption and desorption isotherms of nitrogen in a low-temperature liquid nitrogen bath. The specific surface area was calculated by the BET method, and the DFT model was used to fit the adsorption isotherm to obtain the pore structure distribution. ； Determination of elemental composition of pitch coke using German Vario EL III elemental analyzer ； Characterization of active functional groups on the surface of activated carbon using American Perkin Elmer PHI5000C ； Determination of the microcrystalline structure of raw materials and activated carbon using a Japanese Rigaku D/max-rA X-ray diffractometer ； Analysis of thermal weight loss of raw materials using French TG-DTA/DSC thermal comprehensive analyzer ； Determine the iodine adsorption value and methylene blue adsorption value of activated carbon with reference to the GB/T7702.7-1997 and GB/T7702-6-1997 standards. ； The benzene adsorption value was measured using static adsorption method. 2 Results and Discussion 2.1 Properties of pitch coke (1) X-ray photoelectron spectroscopy (XPS). CC and CH account for the largest proportions in pitch coke, which is 63.0%, C-OR is 22.5%, -C=OR and -COOR reach 7. 3% and 7. 2% respectively (see Table 1). This shows that the surface of asphalt coke contains a large number of active functional groups. Table 1 XPS test results of samples (%) Sample CC, C-HC-OR-C=OR-COOR Pitch coke 63.022.57.37.2 Activated carbon (alkali carbon ratio 4:1) 49.918.815.316.0 (2) X-ray diffraction (XRD). The X-ray diffraction pattern of pitch coke is shown in Figure 1. Pitch coke has a diffraction peak at about 26°, but the peak appears broad and gentle, indicating that although the pitch coke already has a certain graphite lamellar structure, the crystallite size is small. Judging from the diffraction traces at the root of the peak, there is an obvious asymmetric distribution, indicating the presence of disordered carbon and amorphous carbon. 1-Asphalt coke ； 2-Activated carbon (alkali carbon ratio 2:1) ； 3-Activated carbon (alkali carbon ratio 3:1) ； 4-Activated carbon (alkali carbon ratio 4:1). Figure 1 XRD patterns of pitch coke and activated carbon with different alkali to carbon ratios Figure 2 Thermal weight loss curve of pitch coke (3) Thermogravimetric (TG) analysis. Figure 2 shows the thermogravimetric analysis results of asphalt coke. From the weight loss curve of asphalt coke, there is about 5% weight loss at 900°C, and it mainly occurs at 400-580°C and 700-800°C, which correspond to the thermal decomposition side chain removal, functional group stage and thermal polycondensation stage respectively. Through the elemental analysis, ash content analysis and X-ray diffraction analysis of Baosteel's pitch coke, it was found that Baosteel's pitch coke has the characteristics of low ash content and moderate carbon-to-hydrogen ratio. Although it has the prototype of graphite crystallites, it is not well ordered. There is a certain amount of oxygen-containing functional groups in the molecules, which will become active points for the activation reaction, which is beneficial to the progress of the activation reaction. In addition, the lower thermal weight loss before 900°C indicates that the pitch coke will have a higher yield during the activation process, which will help reduce the preparation cost. 2.2 Preparation of activated carbon During the KOH activation process, on the one hand, KOH reacts with carbon to form K2CO3 to form pores. At the same time, K2CO3 and K2O and CO2 produced by decomposition also react with carbon to form micropores. ； On the other hand, the metallic potassium generated by the reaction between K2CO3, K2O and carbon will diffuse into the carbon layer, increasing the reactivity of the carbon. During the activation process of KOH, the following reactions mainly occur: 4KOH+C → K2CO3+K2O+ (1) K2CO3 → K2O+CO2 (2) K2O +C → 2K+CO (3) C+CO2 → 2CO (4) K2CO3+2C → 2K+3CO (5) Experiments have found that the particle size of pitch coke, alkali-to-carbon ratio, activation temperature, time, etc. all have an impact on the degree of activation reaction, which in turn affects the pore structure and adsorption performance of activated carbon, but the degree of influence is different. Among them, the alkali-to-carbon ratio has the greatest impact, followed by the activation temperature. The impact of these two factors on the yield and performance of activated carbon will be discussed below. (1) Influence of activation temperature. Usually the adsorption capacity of activated carbon can be reflected according to the iodine value, methylene blue and benzene adsorption values. Since the molecular sizes of these three adsorbents are different, the methylene blue molecule is the largest and can only enter the larger transition pores. Therefore, this indicator reflects the specific surface area and pore volume of the larger pores of activated carbon. ； The benzene molecules are smaller than methylene blue and iodine molecules and are more easily adsorbed by micropores. When pitch coke is reacted under the conditions of an activation time of 1.5h, an alkali-to-carbon ratio of 4:1, and a particle size of 80 to 150 mesh, the relationship between activated carbon yield, iodine value, methylene blue and benzene adsorption values ​​and activation temperature is shown in Figures 3 and 4. Figure 3 Effect of activation temperature on activated carbon yield and iodine value Figure 4 Effect of activation temperature on activated carbon methylene blue and benzene adsorption value From Figures 3 and 4, it can be seen that when the activation temperature increases from 750°C to 850°C, the yield of activated carbon decreases from 70. 8% to 65. 8%. This shows that the higher the activation temperature, the more violent the activation reaction. As the activation temperature increases, the iodine value, methylene blue and benzene adsorption values ​​of activated carbon first increase and then decrease, and are optimal at 800°C. This shows that too high an activation temperature will cause the pore structure to collapse, the graphite microcrystalline structure to change, and the specific surface area and pore volume to decrease. (2) Influence of alkali-to-carbon ratio. When pitch coke is reacted under the conditions of an activation time of 1.5 hours, an activation temperature of 800°C, and a particle size of 80 to 150 mesh, the relationship between the yield, iodine value, methylene blue and benzene adsorption values ​​of activated carbon and the alkali-carbon ratio is shown in Figures 5 and 6. Figure 5 The effect of alkali-to-carbon ratio on the yield and iodine value of activated carbon Figure 6 The effect of alkali-to-carbon ratio on the methylene blue and benzene adsorption value of activated carbon It can be seen from Figures 5 and 6 that as the alkali-to-carbon ratio increases, the yield of activated carbon decreases significantly, from 82. 8% to 55. 3%, indicating that in the activation process conditions, the alkali-to-carbon ratio has a great impact on the activation reaction. As the mass ratio of alkali carbon increases, the iodine value, methylene blue and benzene adsorption values ​​of activated carbon show different changing patterns. When the alkali-to-carbon ratio increases from 2:1 to 5:1, the iodine and methylene blue adsorption values ​​of activated carbon first increase sharply and then decrease slightly. The benzene adsorption value increases sharply with the increase of alkali-to-carbon ratio. Especially when the alkali-to-carbon ratio reaches 6:1, the adsorption value of benzene is inconsistent with iodine adsorption and methylene blue adsorption, and the adsorption value further increases. The reason may be that as the alkali-to-carbon ratio increases, the activation points per unit volume increase, making the pore structure more conducive to the adsorption of benzene. 2.3 The influence of alkali-to-carbon ratio on the performance of activated carbon. Since the alkali-to-carbon ratio has a great influence on the yield and adsorption performance of activated carbon, the influence of alkali-to-carbon ratio on the microscopic properties of activated carbon was further investigated. The process conditions are as follows: The activation time is 1.5h, the activation temperature is 800℃, and the particle size is 80~150 mesh. (1) XPS and XRD. XPS tests were conducted on activated carbon samples with an alkali-to-carbon ratio of 4:1 (see Table 1). The XRD test results of activated carbon samples with alkali-to-carbon ratios of 2:1, 3:1 and 4:1 are shown in Figure 1. It can be seen from Table 1 that after the activation reaction of pitch coke under certain conditions, the CC and CH contents range from 63. 0% decreased to 49.9%, while C-OR also decreased from 22.5% to 18.8%, while -C=OR and -COOR increased from 7.3% and 7.2% to 15.3% and 16.0% respectively, indicating that during the activation process, C-OR, CC, CH and KOH reacted to generate a certain amount of -C=OR and -COOR. As can be seen from Figure 1, the pitch coke raw material has a sharp diffraction peak at around 26°. After KOH activation, when the alkali-to-carbon ratio is 2:1, the diffraction peak of the prepared super activated carbon decreases. ； When the alkali-to-carbon ratio is 3:1 and 4:1, the super activated carbon produces a large number of micropores, and the spacing between graphite crystallites increases, causing the diffraction peak to disappear, indicating that the original graphite crystallite structure of pitch coke has been destroyed and has become an amorphous carbon with a very disordered orientation. (2) Specific surface, pore volume and pore size distribution. Figure 7 shows the N2 isothermal adsorption curves of activated carbon with different alkali-to-carbon ratios. As the alkali-to-carbon ratio increases, the N2 adsorption value increases, and the inflection point on the shoulder of the curve obviously moves in the direction of increasing relative pressure, indicating that as the amount of KOH increases, the content of large micropores and mesopores in activated carbon increases significantly. -■-Alkali to carbon ratio 6:1 ； -●-Alkali to carbon ratio 5:1 ； -▲-Alkali to carbon ratio 4:1 ； -▼-Alkali to carbon ratio 3:1 ； -◆-Alkali to carbon ratio 2:1. Figure 7 N2 adsorption diagram of activated carbon samples with different alkali-to-carbon ratios. As the alkali-to-carbon ratio increases from 2:1 to 6:1, the maximum pore size of activated carbon gradually increases from 4nm to 5nm, and the proportion of mesopores above 2nm increases significantly. Table 2 shows the microscopic properties of activated carbon under different alkali-to-carbon ratios. Table 2 Microstructure of activated carbon with different alkali carbon ratios This post was last edited by ldy6230016 on 2009-4-17 09:25 ]