Weak base anion exchange resin (WBA) carries primary, secondary and tertiary amine groups and works by protonating in acid solution – which is exactly why it belongs directly after a strong acid cation resin. It removes the mineral acids that the cation stage has just created, at close to stoichiometric regeneration efficiency, and it cannot touch silica or carbon dioxide. Values below are typical for the type; exact figures are confirmed on the grade data sheet.
Typical specification
| Parameter | Typical value | Note |
|---|---|---|
| Polymer matrix | Styrene-divinylbenzene, usually macroporous; gel grades also exist | Macroporous dominates because organic fouling tolerance matters in this duty |
| Functional group | Primary, secondary and tertiary amines | Weakly basic – requires an acidic environment to become active |
| Ionic form as shipped | Free base (or chloride depending on grade) | The free base form is the working form after caustic regeneration |
| Total exchange capacity | Of the order of 5 mmol/g dry, or 1.3-1.7 eq/L wet | Higher than a strong base resin because the amine groups are lighter |
| Moisture content | About 50-65% | Relatively high; macroporous grades hold water in the pores as well |
| Particle size | About 0.315-1.25 mm | – |
| Wet true density | About 1.02-1.10 g/mL | Lighter than cation resins – relevant in mixed bed separation |
| Operating pH | Effective up to about pH 9, optimum in acidic solution | Above the working pH the amine is unprotonated and cannot exchange anions |
| Maximum operating temperature | Generally higher than a strong base resin – confirm per grade | One of its practical advantages |
Why it is cheap to run, and where that stops
| Property | Weak base (WBA) | Strong base (SBA) |
|---|---|---|
| Capacity per litre | Higher – roughly 1.3-1.7 eq/L | Roughly 1.0-1.4 eq/L |
| Caustic consumption | Close to stoichiometric – the main commercial reason to use it | Well above stoichiometric, especially Type I |
| Removes chloride, sulphate, nitrate | Yes, efficiently | Yes |
| Removes carbon dioxide | Largely no | Yes |
| Removes silica | No | Yes |
| Organic fouling | More tolerant, especially macroporous | More prone |
| Where it fits | First anion stage, ahead of a strong base polishing stage | Final stage where silica and weak acids must go |
This is the classic two-stage anion configuration: a weak base resin takes the bulk of the strong acids cheaply, and a smaller strong base resin polishes what is left, including silica. It costs less to run than a single strong base bed of the same capacity, and it puts far less caustic into the effluent.
Applications
| Application | Configuration | Note |
|---|---|---|
| Demineralisation, first anion stage | Free base form after a strong acid cation bed | Removes the mineral acids; needs a strong base stage downstream for silica |
| Removal of strong acids from process streams | Free base form | Acid absorption from pickling, plating and chemical process liquors |
| Organic removal | Macroporous grade | Adsorbs organics while also exchanging anions; caustic regeneration removes both |
| Condensate and industrial water | Macroporous, high flow | Where silica is controlled elsewhere |
Regeneration
Regenerated with sodium hydroxide at close to the stoichiometric requirement – the adsorption of a strong acid onto a weak base is a favourable reaction, so it does not need a large excess to drive it. This is where the operating cost advantage comes from, and it is worth checking because it is the main reason plants choose WBA over SBA.
| Point | Detail |
|---|---|
| Caustic dose | Close to stoichiometric; a large excess wastes chemical and raises effluent load |
| Rinse requirement | Can be significant – allow for it in the water balance |
| Silica | Not removed, so it passes downstream; plan a strong base stage if silica limits apply |
| Organic load | Caustic regeneration also strips adsorbed organics, which is why the effluent is dirty and needs handling |
Handling and supply
Supplied moist, usually in the free base form, in bags, drums or bulk. The resin has a characteristic amine odour, which is normal but worth knowing before opening drums indoors. Store sealed, frost-free and away from acids – a weak base resin in contact with acid will take it up and heat slightly. A certificate of analysis should cover capacity, moisture, particle size and the form supplied.
For the overall framework see the ion exchange resin selection guide; for the strong base partner stage see strong base anion exchange resin.
