Enhancement of the current efficiency for sodium hydroxide production from sodium sulphate in a two-compartment bipolar membrane electrodialysis system

Sodium sulphate, a by-product of chlorine dioxide generators and probably of most totally chlorine free (TCF) pulp bleaching sequences, was split into sodium hydroxide and sulphuric acid by using a two-compartment bipolar membrane electrodialysis (BME) system employing WSI4010/1030 bipolar membranes and Nafion417 cation-selective membranes. The current efficiency was enhanced by using the buffering effect of divalent anions such as sulphate to reduce the free hydrogen ion concentration in the feed compartment. In the optimized experiment, sodium hydroxide (1.08 M at 46 ml min(-1)) and sodium sesquisulphate (0.74 M at 68 ml min(-1)) were produced at a current efficiency of 78% for the production of both acid and base. The power requirement was 1986 k Wh t(-1) of sodium hydroxide at a current density of 142.9 mA cm(-2) and with an effective membrane area of 728 cm(2). Under the conditions of this experiment, it was possible to extract 25% of the sodium content of sodium sulphate in the form of sodium hydroxide. (C) 1997 Elsevier Science B.V.