Electrochemical production of polysulfides and sodium hydroxide from while liquor .2. Electrolysis in a laboratory scale flow cell

Electrochemical production of polysulfide-containing white liquor and pure sodium hydroxide solution was investigated at 90 degrees C in a laboratory scale flow cell. A mixed iridium-tantalum oxide coated titanium electrode was used as the anode and the two electrolyte compartments were separated by a cation-exchange membrane. The process was demonstrated at current densities up to 5 kA m(-2), resulting in high current efficiencies for both products. The previously reported autocatalytic effect of polysulfide ions was confirmed, and its technical implications on the use of three-dimensional electrodes were demonstrated and discussed. The current efficiency was found to depend strongly on the degree of conversion of sulfur(-II) to sulfur(0). The anode material showed favourable properties, with respect to activity and selectivity, but suffered from limited durability.