Process to prepare chlorine-containing compounds

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).