ELECTROCHEMICAL PRODUCTION OF POLYSULFIDES AND SODIUM-HYDROXIDE FROM WHITE LIQUOR .1. EXPERIMENTS WITH ROTATING-DISC AND RING-DISC ELECTRODES

Electrochemical oxidation of white liquor in a membrane cell is a proc ess of great potential for the pulp and paper industry. The process pr oduces polysulfide-containing white liquor in the anode chamber, and p ure sodium hydroxide solution in the cathode chamber. The anode reacti on has been investigated using cyclic voltammetry at temperatures betw een 25 and 90 degrees C on rotating disc and ring-disc electrodes. It was further investigated using chronoamperometry on rotating disc elec trodes at 90 degrees C. The experiments, which were mainly run in dilu te alkaline sulfide solutions, using platinum electrodes, show that th e electrochemical production of polysulfide ions, at lower anode poten tials (-0.1 to +0.1 V vs SCE), proceeds via formation of elemental sul fur on the electrode surface. The sulfur is dissolved by hydrosulfide and polysulfide ions producing (longer-chain) polysulfide ions. The ra te of dissolution, and thus the overall reaction rate, increases stron gly with temperature. Polysulfide ions have an autocatalytic effect on the anode reaction due to their ability to dissolve adsorbed sulfur. At higher anode potentials (greater than or equal to 0.2 V vs SCE), a change of reaction mechanism is observed. In this region the reaction rate depends on electrode potential and is not catalysed by polysulfid e ions.