Activated extrudates by oxidation and KOH activation of bituminous coal

Abstract

An investigation has been made of the synthesis of formed (extruded) activated carbons from a bituminous coal by KOH activation. The coal was first pretreated by oxidation with nitric acid, at different levels of oxidation severity, in an attempt to introduce properties similar to those possessed by lowrank coals that can be directly processed in this way. It is found that hard, high-surface-area activated extrudates can be successfully produced from pre-oxidized bituminous coal. The effects of nitric acid oxidation are to regenerate humic acids and introduce oxygen and NO 2 functional groups, that are more concentrated in the humic acids. The magnitude of these changes increases with the severity of oxidation, the extent of which can be followed by optical microscopy using a dye technique. At nitric acid normalities above 0.25, the treated coals formed gel-like mixtures with KOH solution, and were easily extruded. Preoxidation was found to enhance the development of surface area in the heat-treated products, which also increased with the ratio of KOH to fixed carbon in the precursor. These parameters also strongly influenced the hardness of the extrudates. For a given oxidation severity, the hardness at first increased with the ratio of KOH to fixed carbon, then passed through a maximum. At high ratios, the extrudates were weak. The maximum hardness increased with the level of oxidation: at higher severities, extrudates were produced that were harder, and had higher surface areas, than samples of commercial carbons. The reduced hardness at high ratios is attributed to the dilution of the mixture of dispersed coal and coal particles by excess KOH, reducing the ability to form an extensive network of bridging linkages. Correspondingly, the optical texture of the harder extrudates was found to be homogeneous, whereas at high KOH ratios there was reduced fusion between coal particles and the appearance of cracks.