Patent ID: 8038884

Claim:
A process to prepare chlorine or sodium chlorate using an aqueous salt solution containing at least 100 g/l of sodium chloride and a contaminating amount of polyvalent cations comprising the steps of (i) preparing an aqueous salt solution containing at least 100 g/l of sodium chloride and at least 0.01 ppm of polyvalent cations by dissolving a sodium chloride source containing more than 25% by weight sodium chloride in water, (ii) adding an effective amount of at least one positive retention enhancing component to the aqueous solution, (iii) subsequently subjecting the solution to a nanofiltration step—using a nanofiltration membrane having a molecular weight cut-off of between 100 Da and 10,000 Dalton, thereby separating the solution into a retentate which is enriched for polyvalent cations and a permeate which is the purified aqueous salt solution, (iv) reacting the chloride anions in the permeate to chlorine or sodium chlorate by an electrolysis step, and (v) recycling at least part of the retentate to dissolution step (i).