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Iron & Organic Fouling of Resin Beds
Iron can be present in several different forms in water. For example in the case of un-aerated borehole water iron can be present in the ferrous state (Fe ++ ) but on oxidation it is converted into the ferric form (Fe +++ ). Iron can also be complexed with organic matter; in which case it is present as an anionic complex. Normally iron present in the ferric state is removed by cation resin operated either in the sodium or hydrogen forms. In the case of hydrogen form cation resin representing the first stage of a demineralization system the iron is removed from the water but eluted on regeneration with mineral acid.
With softening resin the situation is different, as the ion exchange resin removes the iron from the water but the regeneration procedure using brine does not elute the accumulated iron from the resin during the regeneration cycle. Consequently the iron accumulates on the resin from cycle to cycle and steadily causes progressive iron fouling. In the case of iron being present as organo/iron complexes the complex is present as an anion and is therefore removed from solution by the anion resin. Because the anion resin is being regenerated with caustic soda, whilst the organic matter may be substantially removed each regeneration cycle, the iron is retained on the resin. The accumulation of iron on the resin causes the anion resin to become iron fouled.
It is recommended that where the iron content of a water is high-er than 0.5ppm some form of pre-treatment is used in order to reduce the iron level down to less than 0.1ppm. Remedial Action Cation Resin When using sulphuric acid constantly while iron is present in the feedwater some accumulation of iron on the resin might take place causing a reduction in performance. In these cases, treatment with hydrochloric acid should be considered providing the internal construction of the units and attendant pipe work make this possible. In the case of accumulation of iron on base exchange softening resin, again either hydrochloric acid or sodium dithionite or a commercial resin treating solution as Rez-Up or Rez-Care treatment may be considered.
We (Atlantic Water Treatment Technologies) can install or provide you with a feeder so that the resin treatment can be injected automatically.
It is well known that anion resins are susceptible to fouling by the humic and fulvic acids sometimes found in surface waters. These organic species, because of the relatively large molecular weights, become trapped within the resin matrix (to a greater or lesser degree depending upon the resin) and specific procedures have to be employed to cause recovery of the original ion exchange properties of the resin. The symptoms of organic fouling include long rinse require-ments, poor capacity and, in the case of strong base resins, higher silica leakage. Treatment The most common forms of treatment involve the use of brine solution; the procedure is as follows :- The resin should be treated at the end of the normal exhaustion cycle. Three bed volumes of 10% w/v brine solution containing 2% w/v caustic soda should be prepared. One bed volume should be introduced into the ion exchange unit at a flow rate not exceeding 2 BV’s per hour followed by a second bed volume – this second bed volume should be retained in the unit for as long as possible, but at least 4 hours. Some agitation, if possible, should be employed periodically throughout the retention period. At the end of the retention period the last bed volume of brine should be passed through the resin at a rate of 1 BV per hour and the resin thoroughly rinsed with clean water until free from brine. The resin should be subject to at least two complete regeneration cycles before being put back on line. N.B. Brine at minimum 35°C should be employed or preferably as high as 60°C so as to produce a better organic elution effect. Iron/Organic Complexes This subject is covered to some extent in the section on iron fouling. Occasionally the presence of iron is detected on the anion resin. This can arise from an iron/organic complex being present in the raw feed water. In these cases, it is advisable to consider treatment of the anion resin with 6% hydrochloric acid immediately after the brine treat-ment. The procedure that should be followed is similar to that given for brining. It is extremely important that all traces of hydrochloric acid are removed from the unit before introduction of the caustic soda regenerant. It is important to ensure that the materials of construction are suitably resistant to hydrochloric acid.