Production of activated carbons from a washington lignite using phosphoric acid activation

Chemical activation of a Washington state lignite was studied using heat-treatment temperatures (HTT) between 250 and 650°C. Thermal blanks were generated under similar conditions for comparison. Changes in chemical composition of both the thermal and acid-treated carbons were followed by elemental analysis and FTIR spectroscopy. With increasing HTT, acid-treatment promotes a more extensive loss of hydrogen. Fourier-transform infrared spectra show a decrease in aliphatic groups, increased aromaticity and increased stability of carbonyl functions for samples that were reacted with H₃PO₄. Ulminite reflectance (%R_(ran)) of the acid-treated samples is consistently higher than the thermal counterparts up to an HTT of 550°C. Above this temperature the relationship is reversed. A rim of higher reflectance material occurs in some of the larger pieces of acid-treated carbon indicating a reaction rim. The sharp boundary may be related to the rate of diffusion of the phosphoric acid into the lignite structure. Maximum surface area developed for the acid-treated carbons (1266 m²/g) occurs at an HTT of 450°C. The surface areas developed in these carbons is intermediate between those produced from modern white oak and subbituminous coal precursors. These preliminary data for activated carbons produced from a lignite precursor stress the importance of rank in the production of high internal surface areas by acid-activation.