About ionic exchange Demineralization
It has been observed that organic compounds could react selectivally towards anions and cations, fixing to itself in substitution of one radical, and then go back to the originary form through a substitution process. Those compounds, of different nature, are called ionic exchange resins and are employed in the removal of positive and negative ions from different nature dissolvents. They look like little spheres having variable diameters from half to two millimetres. It is, so, possible to conceive a system to remove salts by-means those resins. A ionic exchange demineralizer is constituted by one or more chambers, filled with one or more ionic exchange resins kind, through which the fluid to be treated will flow. Considering the (most common) case of water, the demineralizer is constituted of a first chamber, called cationic exchanger, in which the metallic ions react with resins, being fixed to them, and then being substituted by hydrogen atoms, as follows:
CaSO4 + 2 H+ ------> H2SO4 + Ca++
It will be obtained a strongly acid water, practically free from metallic ions. The exchanger is called cationic as in the water will remain cations; incorrect term but of common use.
If inlet water owns much bicarbonates, we'll obtain the following reaction:
2 NaHCO3 + H+ ------> Na2CO3 + CO2 + H2O
As Carbon Dioxide could be separated through physical media, on convenience, the water flooding from the cationic exchanger could pass through a decarbonatation tower, where the Carbon Dioxide will be separated in counter-current air flow. Water coming from the cationic exchanger, eventually de-gazed, will flow into the anionic exchanger, where the acid radicals are exchanged with hydroxyls OH+, and the reaction is:
H2SO4 + 2 OH+ ------> 2 H2O + SO4--
In this way also acid radicals have been removed, obtaining pure water.
In some cases, anionic and cationic resins are mixed, and the treatment comes up into one only chamber. This kind of equipment is called mixed-bed, and has an inferior efficiency in terms of mass exchange, but much superior in terms of final purity.
Resins, cationic and anionic, has a certain number of radicals available for the exchange; once ended, the exchange ends. It is possible to reconstitute the radicals through a chemical process called regeneration. After a counter-current wash to remove any eventual solid in suspension and resins powder produced during working process, resins are put in contact with an active solution:
- in case of cationic resin, a solution of strong acid (more frequently Hydrochlorate, to times Sulfuric or, exceptionally of other nature) and will obtain the following reaction (supposing Calcium removal):
Ca++ + 2 HCl ------> CaC2 + 2 H+
- in case of anionic resin, a strong base (frequently Caustic Soda, at times ammonia) to obtain the following reaction:
SO4-- + 2 NaOH ------> Na2SO4 + 2 OH+
Using HCl and NaOH, it will be obtained an eluate composed by chlorides of different metals and Sodium salts.