Ion exchange

Diagram An ion exchange resin is a matter capable of attracting and binding dissolved ions in water and fluids. Resins exist in nature (kitosan, zeolite), but the kind used in industry today are loosely attached little polystyrene beads with a very large inner surface (50-100 m2/g).

In principle it would be enough to let the water or the fluid, that is being treated, run down across the ion exchange resin, but for practical reasons the resin is poured into one or more tanks, and from here the water will be led through the ion exchange resin.

In small ion exchang plants (the bottle type) the water will be led in at the top of the tank and after a trip through the resin, the water will be pressed up through a pipe placed in the middle of the tank.

In larger plants the resin is stored in "columns". See picture below.

The ion exchange resin can be roughly divided into two groups:

Cation resins and
Anion resins.

Cation resins have a negative voltage and are characterised by containing a large number of acid groups binding the positive ions (cations), - typically it is ions as calcium and magnesium plus metal ions such as copper, zinc, chrome etc.

Anvendelse

HOH ion exchange plants are used for treatment of water in steam and hot water plants, district heating plants, cooling plants, humidifiers and for processes within the chemical industry etc. which require - or can use with advantage - a chemically pure water.

The ion exchange technology is also used in plants for treatment of dextrose, maltose, fructose, pectin and brine.

Anion resins have a positive voltage and are characterised by containing ammonium ions which absorb the negative ions (the anions), - typically it is Cl-, No3- and So4--

When producing demineralised water you often mix cation and anion resins in the same column (a so-called mixed bed). For waste water treatment a 2 or 3 column plant is normally used. First column is a cation exchanger, whereas the other column is an anion exchanger. If the water contains cyanide you normally use two anion exchangers - both a strong and a weak anion exchanger.

When the one ion exchanger column in a 2-column plant is almost saturated, the ions will no longer be removed as effectively as at the beginning. An increase of the conductivity will be registered and the columns will have to be regenerated. The cation exchanger is normally regenerated with hydrochloric acid and the anion exchanger with sodium hydroxide. You have to use an excess amount of regeneration chemicals to obtain a complete regeneration of the ion exchange resins. These chemicals will be flushed out afterwards with water.

ion exchange