In principle, the methods for preparing an activated carbon can be divided into two categories:
physical activation and chemical activation (Wei-Li et al., 2008). As would be expected, chemical methods use chemicals for activation, whereas physical processes use gases (usually carbon dioxide and air), vapours (steam), or mixtures of gases and vapours.
Physical or thermal activation is a two step process: carbonization followed by activation.
Carbonization in an essentially inert atmosphere eliminates the bulk of the volatile matter, enriches carbon content, and leads to some increase in porosity. The carbonization steps serves to modify the pore structure of the precursor. The activation stage is a controlled gasification process in the presence of activating agents such as steam, CO2, or their mixture. The activation process serves to develop further pore structure and increase surface area.
2.2.1 Chemical activation
The wet-chemical process is generally employed to convert uncarbonized cellulosic material, primarily wood, into activated carbon. This process is about impregnation with chemical (such as phosphoric acid or potassium hydroxide, sodium hydroxide or zinc chloride, sodium carbonate, sodium and calcium hydroxide, and the chloride salts of magnesium, calcium, ferric iron, and aluminium), followed by carbonization at temperature in the range 450-900 °C. (McDougall, 1991) In the chemical activation process the two steps i.e. carbonization and activation, are carried out simultaneously, with the precursor being mixed with chemical activating agents, as dehydrating agents and oxidants (McDougall, 1991). The most popular activating agent is phosphoric acid. The process using sawdust as the starting material involves mixing of the raw material and the dehydrating agent into a paste. The paste is then dried and carbonized in some type of kiln, usually a rotary kiln, at between 200 and 650 °C (McDougall, 1991). Upon carbonization, the impregnated chemicals dehydrate the raw material, resulting in charring and aromatization of the carbon skeleton, with the concomitant creation of a porous structure and extended surface area. This procedure yields a powder product. If a granular product is required, granular materials are simply impregnated with the activating agent, and the same general procedure as that described above is followed. However, these granular products are generally soft (unless they are manufactured from the powder after suitable pelletization), which limits their application (McDougall, 1991). After carbonization of the paste, the activating agents are
UNIVERSITY OF IBADAN LIBRARY
20usually recovered and recycled for economic reasons. All the commonly used chemical activators impose certain constraints on the overall activation process and the equipment employed, or on the quality of the product. Mixtures of sulphuric acid and wood cannot be heated above 200 °C, and the product, after the sulphuric acid has been recovered by leaching, has adsorptive properties only when wet. Activation with phosphoric acid requires temperatures in the range 375 to 500 °C, and the reagent is readily recovered by leaching with water.
However, corrosion of the equipment is a major problem. Activation with zinc chloride occurs in the temperature range 550 to 650 °C and, although most of the zinc chloride is recovered by leaching with dilute hydrochloric acid, problems may arise in the application of the product because it retains traces of zinc salts. The activated carbon produced with phosphoric acid and zinc chloride is dried with little, if any, loss of adsorptive capacity. The activity of the product can be controlled by alteration of the proportion of raw material to chemical reagent. For phosphoric acid, the proportion is usually between 1:0, 5 and 1:4. The activity, as measured by the number of pores created, increases as the proportion of the chemical activator increases, and is also affected by the temperature and residence time in the kiln (McDougall, 1991).
Chemical activation offers several advantages since it is carried out in a single step, combining carbonization and activation, performed at lower temperatures and therefore resulting in the development of a better porous structure. (Ioannidou and Zabaniotu, 2007)
2.2.2 Physical activation
Physical thermal activation is a two step process: carbonization followed by activation.
Carbonization in an essentially inert atmosphere eliminates the bulk of the volatile matter, enriches carbon content, and leads to some increase in porosity. The carbonization steps serves to modify the pore structure of the precursor. It involves carbonization of a carbonaceous material followed by the activation of the resulting char at elevated temperature in the presence of suitable oxidizing gases such as carbon dioxide, steam or air. The main purpose of carbonization is to reduce the volatile content of the source material (a fixed carbon content of 80% or higher is desirable) to convert it to a suitable form for activation (Balci et al., 1992). Rearrangement of the carbon atoms into graphitic-like structures also occurs during carbonization. Carbonization temperature range between 400 and 850 °C, and sometimes reaches 1000 °C, and activation
UNIVERSITY OF IBADAN LIBRARY
21temperature range between 600 and 900 °C. (Wei et al., 2008) The first step in the production of activated carbon by the thermal route is carbonization, which is the formation of a char from the source material. Carbonization is generally accomplished by heating of the source material-as lumps or pre-sized material, or in molded form, e.g. as briquettes in an inert atmosphere such as flue gas to a temperature that must not exceed 700 °C so that dehydration and devolatilization of many of the carbon atoms can occur in a controlled manner. The activation stage is a controlled gasification process in the presence of activating agents such as steam, CO2, or their mixture.
The activation process serves to develop further pore structure and increase surface area.
Physical activation is carried out most frequently by burning off some of the raw carbon in an oxidizing environment to create mesopores.
Activation is the process by which the carbonized product develops an extended surface area and a porous structure of molecular dimensions. This step is generally conducted at temperatures between 800 and 1100 °C in the presence of suitable oxidizing agent such as steam, air, carbon dioxide, or any mixture of these gases. The activation gas is usually CO2, since it is clean, easy to handle and it facilitates control of the activation process due to the slow reaction rate at temperatures around 800 °C.
The active oxygen in the activating agent burns away the more reactive components of the carbon skeleton as carbon-monoxide and carbon dioxide, depending on the oxidizing agent employed (McDougall, 1991).
C + O2 → CO2 (1) CO2 + C → 2CO (2)
2.2.3 Method of carbonization a. PIT METHOD
The shells are burned in a limited supply of air so that they do not burn away to ash but are only carbonized. The shells are often burned in pits in the ground although large steel and brickwork kilns do exist (coconut board.nich.in/charcoal.htm 2013).
UNIVERSITY OF IBADAN LIBRARY
22b. DESTRUCTIVE DISTILLATION
When coconut shells are heated in a retort to a sufficiently high temperature, they are broken down in the absence of air into a number of products. Child (1939, as cited in Brian and Ashman, 1975) used laboratory and semi commercial methods and found that 454 kg of shells yielded 153 kg of charcoal, 171 kg of pyroligenuous acid, 27 kg of settle tar and 86 kg of incondensable gases. The yield and indeed the composition of the products are likely to vary considerably with the maturity of the coconut from which the shells are derived ( Brian and Ashman, 1975).
c. DRUM METHOD
Drum kiln is used for carbonization of shells. The drum consists of three sets of six 1"
diameter holes provided at its bottom, middle and upper layers and a lid. A detachable chimney is provided which is installed on the lid after closing the drum (coconut board.nich.in/charcoal. html 2013).
2.2.4 Activation methods.
According to Smisek and Cernyl, (1970), gasification of the carbonaceous material by steam and carbon dioxide occurs in accordance with the endothermic reactions shown in equations (3) and (4):
C + H2O ⇌ CO + H2 (Mf = 29 kcaVmol) (3) C + CO2 ⇌ 2CO (Mf = 39 kcaVmol) (4)
Because the reactions of carbon with steam and carbon dioxide are endothermic, the activation process lends itself to accurate control of the conditions in the kiln. External heating is required to drive reactions (3) and (4), and to maintain the reaction temperatures.
The reaction of water vapour with carbon is accompanied by the secondary reaction of water-gas formation, which is catalysed by the carbon surface as shown in equation (5):
CO + H2O. ⇌ CO2 + H2 (Mf = -10 kcaVmol) (5)
UNIVERSITY OF IBADAN LIBRARY
23However, as shown in equation (6), the reaction of carbon with air (oxygen) is extremely exothermic:
C + O2 ⇌ CO2 (Mf = -97 kcaVmol (6)
This reaction is difficult to control, and excessive burnoff of the external carbon surface can easily occur, resulting in a decrease in average particle size, which obviously reduces the yield of final product. The reaction of steam with carbon is catalysed by certain chemicals (e.g. the oxides and carbonates of alkali metals, iron, copper, and other metals), and some commercial operations use these chemicals as catalysts. Therefore, as combustion proceeds, preferential etching occurs, which results in the development of a large internal surface area and the creation of a pore structure (McDougall, 1991).
The term burn-off is used to denote the degree of activation, which is the loss of char (in percentage by mass) that is allowed to occur. The burn-off (B) and activation yield (Y) are related as indicated in equation (7):
B = 100 – Y (7)
2.2.5 One-step and two-step activations
Rodriguez-Reinoso et al. (1984) prepared activated carbons from almond shells and olive stones using (i) the conventional carbonization (850 °C) followed by activation at 825 °C in carbon dioxide and (ii) a single (direct) activation step in carbon dioxide from room temperature to 825 or 850 °C. Both types of activation produce a very similar yield, this means that the direct reaction of the raw material with CO2 during the heating from room temperature to reaction temperature is not important and does not imply a noticeable activation; in other words, its effect is similar to carbonization under nitrogen. The volume of micropores and the surface area of the resulting activated carbons (for a common yield) are relatively similar, but slightly larger for carbons prepared by direct activation. Similar results were found for other lignocellulosic materials. This result shows that it is possible to produce activated carbon without previous carbonization of the precursor, although the two-stage process is more common in industrial manufacture of activated carbon.