Ion exchange technologies are as varied as the terms used to describe them. Among these technologies are water softeners, dealkalizers, demineralizers and electrodeionization. Although this list is not exhaustive and the functioning of these technologies are not exactly the same, there are two points in common to each of them: the resin and the regenerant.
The exchange of ions takes place thanks to acid or basic radicals which are found in the molecular structure of the resin and which allow an exchange between those in the liquid to be treated and those on the resins.
What is a resin?
Resins are, for the most part, small plastic beads of a size varying between 0.3 and 1.2 mm. Even if to the naked eye, these beads appear to be smooth, they have a high porosity to increase the surface area for ion adhesion. During their manufacture, the resins are polarized to attract the desired ions.
There are several ways to categorize resin types. The most common is according to its electrical charge. There are anionic resins and cationic resins. Cationic resins allow the extraction of positively charged ions such as magnesium, while anionic resins allow the extraction of negatively charged ions such as chloride. They can also be categorized according to the materials used in their manufacture. Since the list of materials is very long, here are some of the most common ones:
- Styrene divinylbenzene copolymer
- Methacrylic acid divinylbenzene
- Phenal-formaldehyde polymer
The regenerants are the products used to restore to the resins the ions necessary to perform the permutation. In other words, when the resin is exhausted, a regeneration cycle is started, and a concentrated solution is sent into the resin bed to restore its initial processing capacity. To give a simple example, let's look at the case of water softeners which, when the resin is exhausted, perform a salt regeneration cycle.
Hence, for the regeneration of anionic resin, salt that gives sodium ions can be used. For cationic resins, we can think of hydrochloric acid (HCl) as a regenerant.