Production of Bamboo Activated Carbon by Phosphoric Acid Activation
Activation methods for activated carbon include ZnCl2 method, H3PO4 method, steam method, and KOH or NaOH method. Among these, considering the technical characteristics of each method and the quality and scope of use of the activated carbon prepared, the H3PO4 method is still the main method for industrial production of high-quality activated carbon. In addition, botanical activated carbon is used instead of coal-based activated carbon in order to prevent environmental damage. And the use of various agricultural wastes, such as wheat straw, reeds, to prepare activated carbon has become the focus of current research. Although the composition of agricultural wastes is similar, the quality of the products produced varies greatly due to the different cellulose, lignin and other volatile components contained in wastes.
The following is about the production of activated carbon using bamboo as precursor materials, by H3PO4 activation, and the influence of various factors during the production on the quality of the final bamboo activated carbon products.
The preparation of activated carbon
Pass the bamboo material through 8 mesh sieves to remove large pieces. After drying, sift out the fine powder by passing the bamboo material through a 30 mesh vibrating sieve and set the powder aside. Then spray aqueous solutions of phosphoric acid on the bamboo material that are stirred for 15 minutes. Then stop stirring and move the material into a convertor. After the reaction is finished, rinse, filter, dry and grind the material to obtain the finished activated carbon products. Afterward, measure the properties of activated carbon such as iodine value, decolorizing performance, methylene blue adsorption.
Among the factors that affect the methylene blue adsorption value of activated carbon, the concentration of phosphoric acid has the greatest impact. This is because the micropores of activated carbon produced by chemical methods are mainly due to the pore structure left after the combination of activator and plant organic matter is washed away. When the concentration of phosphoric acid is low, it cannot reach the conditions for all the bamboo materials to be activated. As the concentration of phosphoric acid increases, the distribution of phosphoric acid in the bamboo material tends to be even, and more micropores are generated. When the concentration of phosphoric acid is too high, because of the poor fluidity of the phosphoric acid, it cannot be well immersed into the internal structure of the bamboo material and a good pore structure cannot be formed, resulting in a weak adsorption effect of activated carbon. In addition, according to range analysis, the values of phosphoric acid concentration, carbonization temperature, carbonization time are much larger than those of activation temperature and activation time. This shows that the influence of activation temperature and time is less than that of the other factors.
Based on the changes in the methylene blue adsorption value, the optimal tested combination is as follows: The specific gravity of the phosphoric acid solution mixed with bamboo material is 45 °Bé; The carbonization temperature is 200 °C; The carbonization time is 50 min; The activation temperature is 500 °C; the activation time is 70 min.
Effects of bamboo material pretreatment conditions on the adsorption performance of activated carbon
Moisture content of bamboo material
Activated carbon is prepared under the optimal conditions of the orthogonal experiment, and the influence of the moisture content of the pretreated bamboo material on the methylene blue adsorption of activated carbon is examined. Within a certain range, as the moisture content in the bamboo material decreases, the methylene blue adsorption value of the prepared activated carbon continues to increase. When the moisture content in the bamboo material is less than 15%, the adsorption value no longer increases significantly. This is mainly because when the moisture content in the pretreated bamboo material is too high, the phosphoric acid is not easy to be completely impregnated in the bamboo material within the limited mixing time, resulting in the uneven distribution of the activator (phosphoric acid) in the bamboo material. If the moisture content of the bamboo material is too low, it will increase the consumption of phosphoric acid and the drying cost. Therefore, it is advisable to control the moisture content of the pretreated bamboo material at 15%.
Particle size of bamboo material
The bamboo material with 15% moisture content is separated by a vibrating sieve. Under the optimal preparation conditions, the influence of the particle size of bamboo material on the ash content of activated carbon is examined.
As the particle size of bamboo material continues to decrease, the ash content of activated carbon continues to increase, which indicates that the particle size of bamboo material should not be too small. This is because if the particle size of bamboo material is too small, the material can easily become ash that has no adsorption capacity during the activation process, and the quality of activated carbon will reduce.
The particle size should not be too large as well. When the particle size of bamboo material increases from 60 mesh to 50 mesh, 40 mesh and 30 mesh, the ash content of activated carbon is reduced by 1.11%, 1.83%, 0.14% respectively. From the perspective of the change figure, combined with the impregnation efficiency and cost control, a 40 mesh vibrating sieve is the best cost-effective option for the pretreatment of dried bamboo material.
The activated carbon with high adsorption capacity is successfully prepared using bamboo as the raw material, phosphoric acid as the activator. The produced activated carbon is with 99% of the pores between 1.9 nm – 42. 5 nm, can be used in applications such as decolorization treatment, removal of metal ions. The quality exceeds the highest national standards for water purification activated carbon in China.