Production of sodium hydroxide and sulfuric acid solutions from sodium sulfate by electrodialysis

Abstract

The method of electrodialysis of aqueous solutions of natural sodium sulfate (mirabilite) was used to obtain solutions of sodium hydroxide and sulfuric acid with the ultimate goal of isolating solid NaOH. Optimization of the process parameters (concentration of components, temperature, circulation rate of solutions) made it possible to achieve a unit energy consumption of 3.4 kWh/kg for NaOH. Carrying out the electrodialysis at a current of 1.1–2.7 A and a voltage of 30 V made it possible to achieve an alkali concentration of about 70 g/l and an acid concentration of 36 g/l. It was found that the optimal final concentration for NaOH solution is 14–16 wt%. The unit energy consumption for production of 1 kg NaOH strongly depends on the duration of electrodialysis; optimal time is about 5–6 hours at voltage of 30 V. It was shown that sodium chloride, which is often used as a wash solution, can be replaced with Na2SO4 in the wash solution in order to avoid unwanted chlorine formation. The purity of the target product – sodium hydroxide solution – was controlled during electrodialysis processes; for the electro-electrodialysis process, a base produced in the cathode compartment contains less than 1 wt% of sodium sulfate. A comparison of the efficiency of electrodialyzers of various designs using ion-exchange and bipolar membranes was carried out. The use of the lower-cost monopolar membranes in contrast to bipolar membranes made it possible to achieve higher concentrations of NaOH and H2SO4, but a high level of unit energy consumption is a disadvantage – 6.7 kWh/kg for NaOH.