Agua

Maintaining Ion Exchange Resin Based Systems
by
Michael C. Keller
Sybron Chemicals Inc.
Ion exchange resins use a process based upon ion selectivity. In this method, ions
of greater preference will displace ions of lower preference. For example, in water
softening, objectionable ions such as calcium and magnesium are exchanged on to
the resin and sodium or potassium ions are released intheir place. To regenerate
the bed, a strong regenerant solution must pass through the resin bed. In water
softening a 5% to saturated solution (~25%) of sodium chloride (NaCl) or
potassium chloride (KCl) is used to regenerate the bed of resin. Because of the
high concentration of sodium or potassium, hardness is displaced from the resin.
In this way, ion exchange resin can be used over andover again. Ion exchange
resin will also act as a filter to the incoming water (this is not recommended).
When substances like oil, grease, metal precipitates (iron, manganese, copper,
etc.), high molecular weight metals (aluminum, lead, etc.), chlorine, organics
(tannins, polymers etc.), silt and sand come in contact with resin, the exchange
mechanism can become inhibited. These factors canproduce poor water quality,
short service cycles, lower flow rates and a decrease the useful life of the resin.
Properly manufactured 8% crosslinked softening resin can have a life expectancy
in excess of 20 years when placed in unstressed situations. Anion resins do not last
as long with a life expectancy of 4 - 7 years. When fouling and/or degradation has
occurred, the resin can either bethrown out or it can be cleaned and put back into
service. There are many different forms of fouling/degradation that will manifest
in or on the ion exchange resin. This article will be dealing with the cleaning and
preventative maintenance of ion exchange systems.
Iron Fouling
Probably the greatest threat to softening resin is iron fouling. Cation resin will
exchange ferrous iron Fe+2 forsodium on to its structure. As long as the iron stays
in the ferrous form, salt will be able to regenerate the iron off the resin. If there are
any oxidizing agents like chlorine and/or oxygen in the water, the ferrous iron Fe+2
can be oxidized to ferric iron Fe+3 (rust). This is known as precipitation and can
occur both on the surface as well as in the internal matrix of the bead. Ferric iron
cannot be regenerated from the resin with simply salt. A quick testing procedure
for iron fouling is to obtain approximately 2 oz. (50 gm) of resin and place it into a
beaker. Add the same amount of hydrochloric acid (muriatic acid) to the beaker

and let it sit for 2 hours. If the solution is heated, a shorter contact time can be
utilized. The acid will turn from clear to a yellow / teacolor. The darker the color
the heavier the iron fouling.
Reducing agents, such as sodium hydrosulfite and sodium bisulfite, can be
effectively used to clean iron fouled beds. One half to one pound of reducing agent
can be added to one gallon of water. This mixture is then immediately poured into
the brine well, or directly into the salt, and will treat one cubic foot of resin. These
compoundsdecompose rapidly and should be used immediately after mixing.
Always remember to add the chemical to water (not water to the chemical). A
manual regeneration is performed to clean the bed.
Acids, such as hydrochloric (HCl) also known as muriatic acid, phosphoric acid,
and citric acid are commonly used to clean iron fouled resin beds. These acids will
generally produce better results thAn thehydrosulfite and bisulfite cleaning
compounds. However, precautions must be taken when utilizing acids.
Hydrochloric acid is very corrosive. In most instances when HCl comes in contact
with a metal surface, corrosion will occur. When an acid is used, some of the
exchange sites on the resin will be converted to the hydrogen form. If the resin is
not fully regenerated back to the sodium form...