Preventing Silica Fouling of Weak Base Anion Resins

 Occasionally, some plants experience weak base anion resin which clumps together

due to silica polymerization. Common operating symptoms include short run

length, increased pressure drop and high WBA effluent conductivity during the run

(due to channeling), and the inability to backwash the bed. When the vessel is

opened and samples are taken, the resin beads will be clumped together and a

gummy substance may even be present. This document is an explanation of the

most probable cause, and ideas on how to correct the problem.

Most demineralizers that use weak base anion resins for water treatment also use

strong base anion resins. In order to minimize the amount of caustic used to

regenerate the resins, a technique known as thoroughfare regeneration is generally

used. Thoroughfare regeneration is the use of waste caustic from the SBA vessel to

regenerate the WBA resin. Since WBA resins regenerate very efficiently, even waste

caustic will return the resin to the free base form.

The primary concern with a thoroughfare regeneration is ensuring that most of the

impurities from the SBA resin are dumped to the drain before the thoroughfare

begins. If significant amounts of silica are present when the thoroughfare begins, it

may polymerize upon contacting the acidic WBA resin, leading to the problems

described above.

First, the problem needs to be corrected. Generally, the first third of the regenerant

from the SBA vessel is discarded to the drain. A variety of changes in SBA operating

conditions may have caused the silica peak to shift, including changes in caustic

concentration, caustic temperature, or installation of new SBA resin. Correcting the

situation is usually accomplished by discarding additional caustic to the drain before

the thoroughfare regeneration begins. To be confident that the right amount is

discarded, an elution study can be performed. Using a two step caustic injection, for

example 2% and 4% NaOH, gives another alternative to dilute the silica peak.

Layered bed configurations with SBA and WBA resins in the same vessel generally

operate with lower caustic concentrations and more uniform flow rates and so they

tend to be less prone to silica precipitation.

Finally, the weak base anion resin needs to be cleaned. A series of hot caustic soaks

should remove most of the polymerized silica and allow continued operation. In

some extreme cases, the weak base anion resin may need to be replaced.

Note that WBA resin clumping can also be caused by organics build-up on the resin

or a combination of organics and silica.