Electrosynthesis of sodium dithionite in a trickle-bed reactor

Sodium dithionite was generated by electroreduction of sulphur dioxide on a graphite fibre mat cathode in a continuous ‘flow-by’ trickle-bed electrochemical reactor. Reactor performance was measured with respect to variation of the feed catholyte NaOH concentration 0.5-1.0 mol/L], feed gas SO₂ concentration 4-40 vol%], product catholyte pH 2.4-12.0] and applied current 10-50 A (0.4-2.0 kA/m²)]. Product catholyte temperature was in the range 18 to 30°C. With product catholyte pH between 2.4 and 6.9, dithionite was generated at current efficiency from 42 to 100%, concentration from 0.05 to 0.78 mol/L, yield from SO₂ up to 69% and specific energy from about 2 to 5 kWh/kg Na₂S₂O₄. Current efficiency fell with increasing current and rose with increasing gas load. High levels of thiosulphate 0.07-0.3 mol/L] and sulphide 0.01-0.13 mol/L] measured in the product catholyte would compromise the use of this process in commercial applications such as brightening wood pulp.