Chlor-alkali hardness removal

Chlorine gas and Caustic Soda are today produced by electrolysis of brine on membranes, consuming less energy and being more environmental friendly than previous technologies using mercury. However, these membranes and the electrodes are very sensitive to various impurities brought by the raw materials such as Alkaline earth metals (Ca2+, Mg2+, Sr2+, Ba2+), Aluminium, Iode, Boron, and the brine requires a pretreatment, which is operated in different steps of purification including the polishing on ion exchange resins, selective to these impurities.

Different type of resin will be selected, depending the impurities to be removed, and the system design. This section is describing the hardness removal, see the next section describing others possible contaminant purification.

For the removal of Ca²⁺ and Mg²⁺ , we propose Weak Acid Cation resins fixing the metal by chelation on an aminophosphonic (AMP) or an iminodiacetic (IDA) functional group. The choice between the two resins will depend on the different concentration of others impurities in the feed brine (Sr²⁺ , Ba²⁺ , Hg²⁺ , Fe³⁺ ,…), and on the end-point criteria in order to maximize the cycle.

The regeneration is operated in two steps (HCl followed by a conversion in Na⁺ form by NaOH). Most of the systems layout is based on a merry-go-round configuration (two column in service and a third in regeneration mode).

All chelating resins are not equivalent, some have uniform bead size, some others smaller particle size, not the same selectivity to the various contaminant but all of them have to show good mechanical resistance as the resin will be exposed to strong and repetitive osmotic shocks during the regeneration cycles.

Lenntech technical team can advise on the best selection of the right resin and its operating conditions.

Thanks to the high purity of the brine achieved after the right resin choice, their life time of the membranes are extended, and their efficiency higher.