PREPARATION OF ACTIVE CARBONS FROM COAL .1. OXIDATION OF COAL

The oxidation of coals of different rank, origin and particle size has been studied at temperatures between 423 and 543 K and for time inter vals between 6 h and 42 days. The chemical composition of the oxidised coal depends upon the coal rank, its particle size and the degree of oxidation as determined by the temperature and the time of oxidation, At higher degrees of oxidation, whether at higher temperatures for sho rter time intervals or at lower temperatures for longer time intervals , the oxidised coal tends to approach similar chemical compositions. T he weight of coal also changes on oxidation, the increase or decrease in weight depending upon the rank and the oxidation conditions. The ra te and extent of oxidation decrease with increase in particle size bec ause the larger particles slow down the diffusion of oxygen into the c oal particles. The oxidation can eliminate completely the plastic prop erties of bituminous coal which inhibit the formation of anisotropic s tructures and enhance the development of a primary pore structure. The helium density increases with the degree of oxidation but the mercury density initially increases and then decreases. A minimum in the merc ury density is obtained when the carbon content of the oxidised coal i s around 85-90%. The oxidation significantly enhances the porosity and the surface area, the extent of increase depending upon the nature of the coal and the degree of oxidation. The changes in chemical composi tion, porosity and surface area with the degree of oxidation indicate that the oxidation of coal involves two different mechanisms, one oper ating at lower temperatures and the other at higher temperatures.