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Ion exchange is often the process of choice for removing hardness ions. In domestic applications a low level of hardness can be permitted and this is why in soft water areas treatment of domestic supplies is not very common. However, in some industrial applications, particularly where the water is to be used in boilers, the total hardness has to be almost totally removed.
Ion exchange was first observed in the 19th century in naturally occurring media. It was 1940’s when synthetic ion exchangers started to be developed along the lines of the high efficiency synthetic strong acid cation resins we see today in all corners of the world.
These synthetic resins regenerated normally with brine and rely on the fact that the resin has a higher affinity for calcium and magnesium and so exchanges sodium ions for the calcium and magnesium. Some specialist softeners are also regenerated instead with potassium chloride and here the hardness is exchanged for potassium. Ions such as sodium or potassium do not cause scale and do not react with soap.
Water Softening Basics
In service operation the reaction is as follows for temporary hardness (calcium bicarbonate shown):
Ca(HCO3)2 + Resin - 2Na = 2Na(HCO3) + Resin - Ca
Temporary Hardness + Resin in Na form = 2 x Sodium Bicarbonate + Resin in Ca form
For permanent hardness (calcium sulfate shown):
CaSO4 + Resin - 2Na = Na2S04 + Resin - Ca
Permanent Hardness + Resin in Na form = 2 x Sodium Sulfate + Resin in Ca form
Softening is an easy and popular process because regeneration of the resin with a simple brine solution is possible.
In regeneration the reaction is as follows:
Resin – Ca + 2NaCl = CaCl2 + Resin - 2Na
Exhausted Resin (Ca form) + Sodium Chloride (Brine Regenerant) = Calcium Chloride + Regenerated Resin (Na form)
From the above examples place you can see that the process is reversible. If a high driving force, such as a high concentration of sodium or potassium chloride is applied to the exhausted resin it can be regenerated and reused. Usually it is a brine solution (normally 10% NaCl), the hardness on the exhausted resin is then back exchanged. Two ions of (mono-valent) sodium take the place of each calcium or magnesium ion.
This process is called “regeneration” and the NaCl solution is called the “regenerant”. So in essence a softener system, consists of a ion exchange resin bed that has the ability to remove hardness ions such as calcium and magnesium and exchange these for less problematic ions such as sodium and then, when exhausted (loaded with hardness) it can be regenerated by a high concentration salt solution (10% brine) and have its capacity restored. KCl can also be used but capacity is lower on the resin and the regenerant is more expensive.
If the system is well maintained and the resins are treated with respect, then these systems and the resins can last for many years. Softening resins operating over 10 years are not uncommon.