Chiba Institute of Technology; Tsudanuma, Narashino-shi 275-0016 Japan
Active carbons were prepared from such low-rank coals as brown coal (Yallourn, Australia; C 64%, Ash 1.4%) and weathered coal (Inner-Mongolia, China; C 54%, Ash 19%) with a relatively large quantities of NaOH or KOH (alkali ratio to coal 2--2.5) under a lower temperature (773--973 K) than the conventional steam activation (1173--1273 K). For instance, active carbons with specific surface area of more than 1000 m2/g were obtained with high yields (33--36%), when weathered coal was activated by NaOH (ratio 2.5--3.0) at 973 K.
The effect of alkali could be improved significantly by pre-heating (773 K) of coals, affording the active carbons with higher values specific surface area (1300 m2/g) at 823 K. This would be due to that the pre-heating of the coals decreased with the degree of consumption of alkali with carbon dioxide generated by the decarboxylation during the activation process.
The removal of ash, especially silica in the weathered coal, was performed easily by this method by washing the final product. The ash content of the active carbon from weathered coal was at least 4 %, while it was almost nil for the product from brown coal.
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