296 



CHAPTER XVI 



ever, be activated. This activation may be effected by heating in the presence 

 (but not with active circulation) of air at temperatures variously stated, but 

 probably about 400° C. The heating may also be done in the presence of 

 superheated steam at temperatures up to 8oo--i,ooo° C. If the carbonaceous 

 material be impregnated with various materials, lime, the chlorides of zinc, 

 calcium and magnesium, soda, sulphuric acid, and be carbonized at a low 

 temperature, a very active carbon results after the removal of the impreg- 

 nating material by leaching or distillation. 



Certain materials, such as rice hulls, rich in silica, afford an active 

 carbon after removal of the silica by boiling with caustic soda. The 

 theory of these preparations is thus given by Lamb, Wilson and Chaney.' 

 Amorphous carbon exists in two forms, called primary and secondary ; 

 primary carbon is formed at lower temperatures, and may be activated. 

 Secondary carbon formed at higher temperatures is graphitic in nature and 

 cannot be activated. When charcoal is obtained as usually burnt, the 

 hydrocarbons formed in the operation are adsorbed, and an inactive carbon 

 results. Activation consists in removmg these hydrocarbons. This removal 

 is effected by heating, and is partly a process of distillation, and partly a 

 process of oxidation. At the same time the charcoal itself is oxidized on 

 the surface of the already existing capillaries, whereby the effective area 

 becomes increased. The art of the process lies in careful control of the tem- 

 perature, which if too low fails to remove the hydrocarbons, and if too high 

 causes the formation of the secondary or graphitic type of carbon. Possibly 

 also the hydrocarbons may break down at higher temperatures and deposit 

 a layer of inactive material on the surface of the charcoal. All of the im- 

 pregnating materials used are dehydrating agents, and they have the property 

 of rendering as charcoal nearly all the carbon present in the wood or other 

 material. Hydrocarbons are therefore not formed, and on removal of the 

 impregnating material an active charcoal results. A secondary action may 

 be that they penetrate into the material and on removal add to the surface 

 area. Their influence may also be catalytic. Our present knowledge of the 

 action of these bodies on sugar materials is mainly due to Schneller,^ 

 Zerban^ and Bradley. ^'^ The very detailed experiments of the last named 

 are given in abstract below. 



All experiments were made with " Norit " on solutions of Barbados or 

 Mozambique raws. Five per cent, on dry weight was used on 50 per cent, 

 sugar solutions, unless otherwise indicated. 



Effect of Size of Particles. — Norit was fractionated according to size of 

 particles by bolting through, silk sieves. 



