Strong Base Anion

A strong base anion resin removes the anions associated with the weak acids in the effluent from a strong acid cation unit. Specifically, strong base anions remove silica (from H₂SiO₃) and carbon dioxide from H₂CO₃ (carbonic acid). In addition, the strong base anion resin removes anions associated with the strong acids in the strong acid cation effluent. Sulfate (from H₂SO₄), chloride (from HCI), and other anions are removed. The exchangeable anion is OH^-, which is picked up by the strong base anion resin during regeneration with caustic (NaOH). The cation in the water passing through unmodified (hydrogen ion is the cation) combines with the OH- liberated during the exchange and leaves the anion unit as molecules of water (H₂0). Since pure water is nonconductive, the conductivity of the effluent from a strong base anion unit is very low, about 1.9 micromhos. Sodium slippage through the strong acid cation unit combines with the liberated OH^- in the anion effluent as NaOH. Because NaOH is more conductive than H₂0, the sodium slippage through the cation increases as does the conductivity of the anion effluent.

One helpful tip to remember is that if strong acid cation resin ever gets into the anion unit (due to a bad strainer in the cation vessel bottom), it will wind up on the bottom of the anion unit because of its higher density, and will pick up sodium during the anion regeneration cycle. When the anion is put back in the service cycle, sodium will leak from the cation resin into the anion effluent and give a higher than normal anion effluent conductivity. This problem can be verified by having samples of cation effluent and anion effluent analyzed for sodium by atomic absorption. Any significant difference between the sodium levels in and out of the cation resin means that sodium has migrated into the anion vessel. This situation can be very troublesome, because most high-pressure boiler water treatment programs are adversely affected by sodium intrusion.

Strong base anion resins are quoted as having two capacities: salt-splitting capacity and total capacity. The so-called salt-splitting capacity of a strong base anion resin is a measure of its ability to remove silica and carbon dioxide. The total capacity is a combination of the salt-splitting capacity and the resin's capacity to remove SO4^-, Cl^-, NO₃^- etc.

Strong base anion resins also absorb organics from the water going through them. Most source waters contain some organic compounds. Surface waters are notorious for their propensity to carry a combination of iron and organic compounds. These compounds pass through the cation unit because the iron is not in the form of free ions. When it reaches the anion, on the other hand, the anion resin absorbs the compound and, by analysis, appears to be fouled with iron. What is really there is the iron/organic complex. This material is not removed during regeneration it occupies exchange sites normally reserved for other anions. The final result is a reduction in the capacity of the anion train.

Anion resins also remove other forms of organics during regeneration. The organics that are removed not only occupy valuable exchange sites but also react with the resin to alter its character. This alteration manifests itself in the transformation of the salt-splitting capacity of the resin to strong acid capacity. As such, the anion resin begins to lose its ability to remove silica and carbon dioxide but shows an increase in its capacity to remove the strong acid anions. The total capacity of the resin does not decrease, but its salt-splitting capacity does. The end result in a system providing water for a high-pressure boiler will be shorter and shorter run lengths, because the silica level in the effluent of the anion unit is the main control parameter.

Microbiological fouling can also occur in the anion unit. Fungi grow nicely in an anion unit. Treatment with a solution of formaldehyde is one method of sterilizing an anion train.