Preparation of Active Charcoal

“On the basis of the above discussion, the preparation of active charcoal will evidently involve two steps:

“First.—The formation of a porous, amorphous base carbon at a relatively low temperature.

“Second.—The removal of the adsorbed hydrocarbons from the primary carbon, and the increase of its porosity.

“The first step presents no very serious difficulties. It involves, in the case of woods and similar materials, a process of destructive distillation at relatively low temperatures. The deposition of inactive carbon, resulting from the cracking of hydrocarbons at high temperatures, must be avoided. The material is therefore charged into the retorts in thin layers, so that the contact of the hydrocarbon vapors with hot charcoal is avoided as much as possible. Furthermore, most of the hydrocarbon is removed before dangerous temperatures are reached. A slight suction is maintained to prevent outward leaks, but no activation by oxidation is attempted, as this can be carried on under better control and with less loss of material in a separate treatment.

Fig. 67. Dorsey Reactor
for Activating Cocoanut Charcoal with Steam.

“The second step, that is, the removal of the absorbed hydrocarbons from the primary carbon, is a much more difficult matter. Prolonged heating, at sufficiently high temperatures, is required to remove or break up the hydrocarbon residues. On the other hand, volatilization and cracking of the hydrocarbons at high temperatures is certain to produce an inactive form of carbon more or less like graphite in its visible characteristics, which is not only inert and non-adsorbent, but is also highly resistant to oxidation. The general method of procedure which has yielded the best results, is to remove the adsorbed hydrocarbons by various processes of combined oxidation and distillation, whereby the hydrocarbons of high boiling points are broken down into more volatile substances and removed at lower temperatures, or under conditions less likely to result in subsequent deposition of inactive carbon. Thin layers of charcoal and rapid gas currents are used so that contact between the volatilized hydrocarbons and the hot active charcoal may be as brief as possible. In this way cracking of the hydrocarbons at high temperature, with consequent deposition of inactive carbon, is largely avoided.

“While the removal of the hydrocarbons by oxidation and distillation is the main object of the activation process, another important action goes on at the same time, namely, the oxidation of the primary carbon itself. This oxidation is doubtless advantageous, up to a certain point, for it probably at first enlarges, at the expense of the walls of solid carbon, cavities already present in the charcoal, thus increasing the total surface exposed. Moreover, the outer ends of the capillary pores and fissures must be somewhat enlarged by this action and a readier access thus provided to the inner portions of the charcoal. However, as soon as the eating away of the carbon wall begins to unite cavities, it decreases, rather than increases, the surface of the charcoal, and a consequent drop in volume activity, that is in the service time, of the charcoal, is found to result.

“It is obvious, therefore, that conditions of activation must be so chosen and regulated as to oxidize the hydrocarbons rapidly and the primary carbon slowly. Such a differential oxidation is not easy to secure since the hydrocarbons involved have a very low hydrogen content, and are not much more easily oxidized than the primary carbon itself. Furthermore, most of the hydrocarbons to be removed are shut up in the interior of the granule. On the one hand, a high enough temperature must be maintained to oxidize the hydrocarbons with reasonable speed; on the other hand, too high a temperature must not be employed, else the primary carbon will be unduly consumed. The permissible range is a relatively narrow one, only about 50 to 75°. The location of the optimum activating temperature depends upon the oxidizing agent employed and upon other variables as well; for air, it has been found to lie somewhere between 350 and 450°, and for steam between 800 and 1000°.

“The air activation process has the advantage of operating at a conveniently low temperature. It has the disadvantage, that local heating and an excessive consumption of primary carbon occur, so that a drop in volume activity results from that cause before the hydrocarbons have been completely eliminated. As a consequence, charcoal of the highest activity cannot be obtained by the air activation process.”

The steam activation process has the disadvantage that it operates at so high a temperature that the regulation of temperature becomes difficult and other technical difficulties are introduced. It has the advantage that local heating is eliminated. The hydrocarbons can, therefore, be largely removed without a disproportionate consumption of primary carbon. This permits the production of a very active charcoal.

It has the further advantage that it worked well with all kinds of charcoal. Inferior material, when treated with steam, gave charcoal nearly as good as the best steam-treated cocoanut charcoal. Because of the shortage of cocoanut, this was a very important consideration.

Fig. 68.—Section of Raw Cocoanut Shell.
Magnified 146½ diameters.

The air, steam and also carbon dioxide-steam activation processes have all been employed on a large scale by the Chemical Warfare Service for the manufacture of gas mask carbon.

Fig. 69.—Section of Carbonized Cocoanut Charcoal.
Magnified 146½ Diameters.

Fig. 70.—Two-Minute Charcoal not Activated.
Magnified 732 Diameters.

“The above considerations are illustrated fairly well by the photo-micrographs shown in [Figs. 68 to 71]. Fig. 68 shows a section of the original untreated cocoanut shell crosswise to the long axis of the shell. In it can be seen the closely packed, thick-walled so-called ‘stone-cells’ characteristic of all hard and dense nut shells. [Fig. 69] is a photograph of a similar section through the same cocoanut shell after it has been carbonized. As these photographs are all taken with vertical illumination against a dark background, the cavities, or voids, and depressions all appear black, while the charcoal itself appears white. It is clear from this photograph that much of the original grosser structure of the shell persists in the carbonized products. Figs. [70] and [71] are more highly magnified photographs of a carbonized charcoal before and after activation, respectively. As before, all the dark areas represent voids of little or no importance in the adsorptive activity of the charcoal, while the white areas represent the charcoal itself. In [Fig. 70] (unactivated) the charcoal itself between the voids it seen to be relatively compact, while in [Fig. 71] (activated) it is decidedly granular. This granular structure, just visible at this high magnification (1000 diameters), probably represents the grosser porous structure on which the adsorption really depends. These photographs, therefore, show how the porosity is increased by activation.”

Fig. 71.—31-Minute Steam Activated Charcoal.
Magnified 732 Diameters.

The great demand for charcoal, and the need for activating other than cocoanut charcoal led to the development of the Dressler tunnel kiln, which seemed to offer many advantages over the Dorsey type of treater.

Fig. 72.—Sectional View of Dressler Tunnel Kiln,
Adapted to Activation of Charcoal.

“The Dressler tunnel kiln is a type used in general ceramic work. The furnace consists essentially of a brick kiln about 190 ft. long, 12 ft. broad, and 9 ft. high, lined with fire brick. Charcoal is loaded in shallow, refractory trays in small tram cars, about 120 trays to the car. The cars enter the kiln through a double door and the charcoal remains in the hot zone at a temperature of about 850° C. for about 4 hrs., depending upon the nature of the material charged. Water is atomized into this kiln, and a positive pressure maintained in order to exclude entrance of air. The kiln is gas-fired and the charcoal is activated by the steam in the presence of the combustion gases.

“Under such treatment the charcoal is given a high degree of activation without the usual accompanying high losses. Seemingly the oxidizing medium used, together with the operating conditions, produce a deep penetration of the charcoal particles without increasing the extensive surface combustion experienced in the steam activators. The capacity of such a type furnace is limited only by the size of the installation.

“The advantages of this type furnace may be tabulated as follows: