Activated Carbons from Spanish Coals. 3. Preoxidation Effect on Anthracite Activation

Abstract

A Spanish anthracite, with 5.6 wt % ash content, has been submitted to a two-stage activation process to explore its use as an activated carbon precursor. Several oxidation treatments, with two oxidizing agents (air and nitric acid), have been carried out to study the degree of coal oxidation and its influence on both the char porosity and the char activation. Two activating agents, CO2 and steam, have been used to prepare different burn-off samples to analyze the preoxidation effect on the porosity of the resulting activated carbons. The extent of the oxidation degree, followed by TPD experiment in He, increases with the severity of the oxidation treatment in the following order: air 4 h < air 8 h < 4 M HNO3 ≪ 15 M HNO3. All the oxidation treatments carried out in this study introduce important changes in the reactivity of the resulting chars and on the porous development. The more intense the preoxidation treatment is, and hence the amount of oxygen added to the anthracite, the higher is the porosity of the resulting chars. Activated carbons prepared from preoxidized chars, using CO2 or steam, present much larger porous development than the activated carbons coming from the original coal. The results show that the anthracite needs, prior to the pyrolysis process, an oxidation treatment to be used as an activated carbon precursor, as happens with caking coals. High surface area activated carbons (about 1300 m2/g at a 50% burn-off) can be obtained using nitric acid which has proved to be the most effective preoxidation agent. Comparison of both series of activated carbons, prepared in CO2 and steam, shows that steam develops the porosity of the resulting activated carbons more than CO2.