Crystallisation is the final thermal stage of a zero liquid discharge train, converting concentrated brine to recoverable solid salt and high-purity condensate. Crystal purity and dewatering determine whether the product has disposal requirement or commercial value.
Slurry is circulated by an axial-flow pump through an external heat exchanger and back to the vapour body. Supersaturation is generated by controlled evaporation. Crystal size distribution (CSD) is controlled by fines dissolution and residence time. Robust and low-maintenance; the standard choice for mixed-salt brines where crystal purity is secondary. Specific energy: 12–20 kWh/m³ with MVR compression.
Internal draft tube creates a controlled upflow in the crystallisation zone; a baffle separates the settling zone. Fines are preferentially dissolved and returned, producing a narrow CSD and larger crystals (500–2,000 µm). Preferred where crystal purity and centrifuge dewaterability are critical — e.g., food-grade NaCl or pharmaceutical grade salts. Higher capital; more precise operation required.
A fluidised bed of crystals is maintained in a growth zone; feed enters below and rises through the bed. Only particles larger than a design cut-size are withdrawn, giving excellent crystal size control. Used for high-value products (potassium nitrate, ammonium sulphate). Capital and operating requirement highest of the three types.
Automated seed crystal dosing, temperature control, and cleaning-in-place (CIP) cycles minimise operator intervention and maintain consistent crystal size distribution for high-purity salt products.
| Parameter | Forced-Circulation | DTB | Oslo |
|---|---|---|---|
| Mean crystal size | 100–500 µm | 500–2,000 µm | 1,000–5,000 µm |
| Crystal purity | Moderate (mother liquor inclusion) | High | Very high |
| Dewatering (centrifuge moisture) | 3–8% | 1–3% | <1% |
| Capital cost | Lowest | Medium | Highest |
| Operational complexity | Low | Medium | High |
| Suitable for mixed salts | Yes | Partial | No — single-salt only |
| Typical application | Industrial ZLD, FGD blowdown | Food-grade salt, industrial chemicals | Pharmaceutical, specialty chemicals |
| Salt Type | Typical Purity from ZLD | Market Value (indicative) | Key Application |
|---|---|---|---|
| Sodium Chloride (NaCl) | 98–99.5% (industrial grade) | –80/tonne | Water softener salt, chemical feedstock, road de-icing |
| Sodium Sulphate (Na&sub2;SO&sub4;) | 97–99% (anhydrous) | –160/tonne | Detergent manufacture, glass, textile processing |
| Ammonium Sulphate | 95–98% | –200/tonne | Fertiliser — high-value if purity met |
| Potassium Chloride (KCl) | 95–99% | –400/tonne | Fertiliser, pharmaceutical, food grade |
| Mixed salt waste | <90% any single species | Disposal rate –200/tonne | Hazardous waste landfill; incineration |
Selective crystallisation — operating separate crystallisers at different temperatures and conditions to produce separate pure salt streams — requires detailed brine composition analysis and pilot-scale validation. Where the salt market value is sufficient to justify the additional capital, it can transform ZLD from a compliance cost to a output-generating operation. Contact us to assess feasibility for your specific brine.
Complete ZLD train design from RO through evaporation and crystallisation.
ZLD OverviewCrystallisation pilot trials to determine CSD, purity, and energy consumption before full-scale commitment.
Pilot TestingOur engineers are available to review your site conditions and recommend the most appropriate treatment solution.
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