Quick answer: Brine treatment for chlor-alkali purifies sodium chloride brine to the ultra-low impurity levels that membrane electrolysis cells require. Primary treatment precipitates and filters calcium and magnesium; secondary treatment uses chelating ion exchange to reach parts-per-billion hardness. Only then is the brine fed to the membrane cells, protecting the expensive ion-exchange membranes.
Frequently asked questions
Why does chlor-alkali need ultra-pure brine?
Because modern chlor-alkali plants use ion-exchange membrane cells that are highly sensitive to impurities. Calcium, magnesium and trace metals precipitate inside the membrane, raising cell voltage, cutting current efficiency and causing largely irreversible damage. Ultra-pure feed protects membrane life, which is one of the plant’s largest lifecycle costs.
What is the difference between primary and secondary brine treatment?
Primary treatment precipitates the bulk of calcium and magnesium as carbonate and hydroxide using soda ash and caustic, then filters the solids, reaching low milligram-per-litre hardness. Secondary treatment polishes the brine with chelating ion-exchange resin to parts-per-billion hardness, the final barrier before the membrane cells.
What hardness level does membrane cell feed require?
Membrane feed typically requires combined calcium and magnesium below about 20 parts per billion, with tight limits on strontium, barium, sulphate, silica and metals too. Exact targets are plant-specific, but they are consistently in the parts-per-billion range, which only chelating ion exchange can reliably achieve.
What is resaturation in a chlor-alkali plant?
Resaturation restores the sodium chloride concentration of brine that has been depleted in the electrolysis cells. Depleted brine is dechlorinated, re-saturated by dissolving solid salt, and returned through purification in a closed loop. Make-up salt quality affects the impurity load that treatment must remove.
Which impurities damage chlor-alkali membranes?
Calcium and magnesium hardness are the primary concern, along with strontium and barium, which behave similarly. Sulphate, silica, aluminium, iron, other heavy metals and iodide also need control depending on the salt source. These species foul or deposit in the membrane, increasing energy use and shortening membrane life.
Sources and further reading
- Euro Chlor, technical guidance on chlor-alkali brine and membrane operation
- European Commission, BAT reference document (BREF) for the Chlor-Alkali Manufacturing Industry
- European Commission, Industrial Emissions Directive
- European Commission, EU Water Framework Directive (Directive 2000/60/EC)
- Rijkswaterstaat and Unie van Waterschappen, discharge permitting information
Talk to Brine Consulting for an independent audit of your chlor-alkali brine purification and membrane feed quality.
Written by the Brine Consulting advisory team. The Brine Consulting advisory practice includes Wendy Garcia, PhD, an electromembrane and resource recovery expert with over 15 years in electrodialysis and ion-exchange membrane systems, and Panos Spatharos, a chemist and field engineer specialising in sludge and leachate treatment, RO, ion exchange and filtration. Meet our experts.