Chronoamperometric and chronopotentiometric investigation of Kraft black liquor

Abstract

Black liquor is an effluent of pulp and paper mills. The electrolysis of Kraft black liquor is drawing high attention, as it leads to lignin recovery by anodic electrodeposition and cathodic production of hydrogen. However, the lignin oxidation process on black liquor is still poorly understood, requiring further fundamental studies to allow its comprehension. Therefore, detailed electrochemical studies are being carried out on the topic, which will allow a deeper insight in the involved electrode processes. The Kraft black liquor is physicochemically characterised, with the determined high values of pH, conductivity and lignin content (12.4, 460 mS cm−1 and 48.8 g dm−3, respectively) demonstrating the suitability of black liquor as an electrolyte medium. Voltammetry, chronoamperometry (CA) and chronopotentiometry (CP) techniques are done in black liquor solutions using Pt, Ni and AISI 304 SS electrodes to investigate the lignin oxidation process. The number of exchanged electrons is determined for each electrode material from CA and CP measurements using classic Cottrell and Sand analyses, respectively, with the faradic efficiency of Ni being close to that of Pt. The low-cost and high efficiency suggest Ni as a promising electrode material for application in black liquor electrolysers.