Demineralisation is where ion exchange stops being a single-vessel operation and becomes a train. A strong acid cation resin in the hydrogen form converts dissolved salts into their acids; a strong base anion resin in the hydroxide form takes those acids out and leaves water. Everything between those two points – degassing, weak resin stages, mixed bed polishing, the temperature ceiling on the anion resin – is what separates a working demineraliser from a plant that cannot hold its conductivity specification.
The two-stage train, and what each stage does
| Stage | Resin and form | What it removes | What leaves the stage |
|---|---|---|---|
| 1. Cation | Strong acid cation, hydrogen form | All cations – calcium, magnesium, sodium, iron | Mineral acids: HCl, H2SO4, HNO3, plus CO2 |
| 2. Degasser (optional) | Not a resin – a packed tower or membrane contactor | Carbon dioxide | Reduces the anion load substantially, which cuts caustic consumption |
| 3. Anion | Strong base anion, hydroxide form | All anions, including silica | Water with very low conductivity |
| 4. Mixed bed (polishing) | Cation and anion resins mixed in one vessel | The last traces of both | Ultrapure water, where the specification demands it |
Adding weak resins to cut acid and caustic
| Configuration | What it saves | Constraint |
|---|---|---|
| Weak acid cation ahead of strong acid | Removes bicarbonate hardness at close to stoichiometric acid dose; the strong acid bed then handles only the remainder | Only works on the alkaline fraction, and the bed swells a great deal on conversion |
| Weak base anion ahead of strong base | Removes the mineral acids at near-stoichiometric caustic dose; the strong base bed polishes and takes the silica | Cannot remove CO2 or silica itself |
| Both weak stages | The cheapest chemical consumption of any configuration | More vessels, more valves, and the weak beds must be sized for their swelling |
The economic logic is straightforward: strong resins deliver the water quality, weak resins deliver the chemical efficiency. A train that uses only strong resins will meet specification and cost more to run than it needs to.
Key design parameters
| Parameter | Typical basis | Why it matters |
|---|---|---|
| Service flow rate | Moderate for the cation stage, higher for polishing beds | The exchange zone must stay sharp; too fast and the ionic front smears |
| Anion resin operating temperature | Limited, and the limit is lower in the hydroxide form | Exceeding it destroys strong base capacity permanently – the single most common cause of an anion bed that will not regenerate |
| Regenerant dose – acid | Several times stoichiometric on a strong acid bed | Regeneration is fought against a favourable equilibrium |
| Regenerant dose – caustic | Well above stoichiometric on a strong base bed, less on a weak base bed | Drives both cost and effluent load |
| Silica removal target | Set by the downstream use, not by the resin datasheet | Silica is the parameter that forces strong base resin into the train |
| Conductivity specification | From about 0.1 microsiemens per centimetre for demineralised water down to far lower for ultrapure | Determines whether a mixed bed polishing stage is needed |
Mixed beds and polishing
| Point | Detail |
|---|---|
| Why mix them | A cation and an anion bead in contact remove the products of each other’s reaction immediately, so the equilibrium is driven much further than in separate beds |
| Density difference | Anion resin is lighter than cation resin, which is what allows hydraulic separation before regeneration – the basis of the whole design |
| Separation quality | If the two resins do not separate cleanly, caustic contacts cation resin and acid contacts anion resin during regeneration, and the bed never recovers its performance |
| Uniform particle size grades | Improve separation and reduce pressure drop, which is why they dominate in condensate polishing and ultrapure water |
| Resin life | Mixed beds are harder on resin – repeated regeneration cycles and thermal cycling shorten life compared with separate beds |
What goes wrong in demineralisation plants
| Symptom | Likely cause | Action |
|---|---|---|
| Conductivity rises after regeneration and does not settle | Anion resin degraded by temperature, or poor separation in a mixed bed | Check the operating temperature against the resin limit; inspect separation |
| Silica breaks through early | Insufficient caustic, low regeneration temperature, or organically fouled anion resin | Review regeneration conditions; consider macroporous anion resin or upstream organics removal |
| High acid consumption on the cation bed | Regenerating far beyond need, or resin fouled so capacity has fallen | Optimise dose against leakage; test the resin capacity |
| Pressure drop climbing | Fouling, broken beads, or resin fines accumulating | Backwash, inspect, consider replacing with uniform beads |
| Chlorine taste or oxidant damage | Free chlorine passing the upstream dechlorination | Fix the bisulphite or carbon stage – resin damage from oxidation is irreversible |
| Organic fouling of anion resin | Surface water organics | Macroporous anion resin, caustic-brine cleaning, and organics removal ahead of the train |
Putting the train together
| Requirement | Reasonable configuration |
|---|---|
| Moderate demineralisation, low organics, cost-sensitive | Strong acid cation + strong base anion, with a degasser |
| Low chemical consumption is the driver | Weak acid + strong acid + weak base + strong base |
| Ultrapure water | Two-stage train followed by a polishing mixed bed, uniform particle size resins throughout |
| High-pressure boiler feed | Demineralisation plus condensate polishing with uniform bead mixed bed |
| Feed with high silica | Strong base resin with careful caustic and temperature control at regeneration, and a silica target set by the boiler |
For the constituent resins see strong acid cation, strong base anion, weak acid cation and weak base anion; for the overall framework, the resin selection guide.