Activated carbons from bituminous coals by reaction with H₃PO₄: The influence of coal cleaning

A study has been made to examine the effects of coal cleaning by column flotation on the properties of carbons prepared by the phosphoric acid activation of bituminous coals. Earlier work has shown that while moderate surface areas are generated by the reaction of coals with H₃PO₄ at temperatures around 550°C, phosphoric acid also reacts with coal sulfur and mineral matter. These side reactions consume reagent that is otherwise available for reaction with the organic structure, and result in the formation of insoluble phosphates, increasing the carbon ash content and limiting the amount of recoverable reagent. Lowering the mineral matter content of the coals prior to carbon synthesis is found to have a direct influence on reducing the ash content of the derived carbons and the extent of phosphorus retention. Coal cleaning also increases the extent of sulfur removal (most of which is liberated as H₂S), the BET and mesopore surface areas, and the carbon pore volume. Possible causes of these effects are discussed. The assumption that the ash in the carbon has negligible porosity, and hence that the reduction in mineral matter content of the coal will automatically increase the specific surface area of the carbon by lowering the ash content, does not appear to fully explain the results. The consumption of phosphoric acid by side reactions lowers the effective reagent to coal ratio and limits the amount of available reagent, which can cause a reduction in surface area. Further, it is possible that ash constituents can block the pore structure, and that lowering the coal mineral matter content will improve access. It seems probable that all three explanations have some validity and can contribute to the observed changes.