Demineralisation (DM) plants in power stations and process industries use ion exchange resins to remove dissolved salts from water. The anion resins in these plants are regenerated with caustic soda. Regeneration quality decides how much treated water you get per cycle and how pure it is.

How anion regeneration works

Anion exchange resins remove anions such as chloride, sulphate, carbonate and silica, swapping them for hydroxide ions. When the resin is exhausted, it is regenerated by passing caustic soda solution through it. The hydroxide pushes the captured anions off the resin, which then goes back into service.

Typical regeneration steps

1. Backwash to loosen the resin bed and remove fines (for co-flow units).
2. Caustic injection at the strength and flow set by the resin maker, commonly a dilute solution of around 4% NaOH.
3. Slow rinse to push the caustic through the bed.
4. Fast rinse until conductivity and silica are back to service levels.

Key points for good regeneration

• Dose: use the caustic quantity per litre of resin given by the resin maker. Too little leaves the bed partly exhausted; too much wastes chemical.
• Warm caustic for silica: strong base anion resins remove silica better when regenerated with warm caustic, typically around 35 to 50 °C, within the resin's limits.
• Caustic quality: chloride in the caustic is loaded onto the resin and leaks into the treated water. Use low-chloride caustic for high-purity water.
• Dilution water: use demineralised water to dilute the caustic, not raw water.

Signs of poor regeneration

• Shorter service runs between regenerations.
• Early silica or conductivity break-through.
• Higher caustic consumption per cubic metre of water produced.

Check caustic strength at the injector, resin condition and distributor performance before changing the dose.

Safety

Bulk caustic storage and dilution areas need eyewash stations, bunding and clear labelling. Operators should wear goggles and gloves, and caustic should always be added to water when making up solutions.

Supply for plants in Oman

For utilities and industrial plants operating in Oman, caustic soda flakes 99% with low chloride (maximum 0.05%) are available from Muscat Chemical, with ready stock in Barka and delivery within Oman.