Electrochemical oxidation of lignite in basic media

Abstract

Electrolysis of lignite slurries in 1N NaOH at Pt electrodes produced humic acids, CO 2, H 2, relatively small amounts of O 2 and traces of CH 4 and C 2H 2. The amount of humic acids that could be precipitated from the electrolyte increased with the applied potential, reached a maximum at E = 2.7 V, SCE and then declined with further increase in electrode potential. The rates of reaction increased with increasing temperature and activation energies computed from limiting currents and overall rates of reaction indicate that the reaction pathway is complex and involves more than one reaction. The current efficiencies for H 2 production were in the range 80–90% and > 50% of the lignite could be reacted at E = 2.0 V (SCE) in 48 h with 4 cm 2 Pt gauze electrodes. Although a graphite anode is itself electrochemically oxidized when used in this process, it increases reaction rates, ≈ 2–3 times higher with graphite than with Pt. Many aromatic organic compounds can be extracted from the residual reacted coal into suitable solvents. A significant amount of oxygenated organic compounds can also be recovered from the electrolyte as a precipitate after neutralization and by solvent extraction of the filtrate. Various products were characterized by elemental analysis, and gas chromatography-mass spectrometry. Data are also presented to show the influence of electrode potential on the rate of gas production and on the composition of the organic products. Various reaction pathways are hypothesized for the processes which lead to the products observed.