High-chloride (10,000–30,000 mg/L), high-suspension (TSS 1–10%), trace-heavy-metal (Hg, Se, As) wastewater from coal-fired power, steel sintering and FGD bleed streams — engineered for ZLD compliance with pre-treatment that actually handles Hg <1 µg/L and Se <50 µg/L discharge limits.
These streams sit at the intersection of chloride attack, suspended-solids loading, trace heavy-metal compliance and the zero-liquid-discharge mandate. The challenge is rarely the evaporator — it is the pre-treatment that decides whether the evaporator survives.
FGD purge water chlorides are set by the coal, and the scrubber loop adds unoxidized sulfite carryover — a reducing-acid combination that eats standard alloys.
Gypsum carryover, fly ash and unreacted limestone turn the evaporator feed into an abrasive slurry unless solids are removed first.
Coal-trace Hg and Se report to the purge stream together with associated species (As, B) — and are regulated at the µg/L level, orders of magnitude below their inlet concentrations.
Thermal power and steel sintering plants in many regions operate under ZLD or near-ZLD wastewater mandates — the system must produce reuse-grade condensate and a managed salt / solids stream, with no liquid discharge to surface water.
Evaporation–crystallization ZLD for SCR denitration catalyst manufacturing and regeneration wastewater: removing 95%+ of salts, vanadium and tungsten in one phase-change step, returning reusable condensate and concentrating V/W into a recoverable
Common feed streams in power, steel and FGD wastewater, with the technology route typically considered for first-pass screening.
| Chemical / Stream | Source & Context | Key Behavior | Typical Route | Material Focus |
|---|---|---|---|---|
| FGD purge water | Wet limestone FGD bleed, coal-fired power | Cl⁻ 10,000–30,000 mg/L · TSS 1–5% · Hg 0.05–5 µg/L · Se 0.1–2 mg/L | Pretreat + MVR + FC | Titanium / Hastelloy |
| Power plant circulating blowdown | Cooling tower bleed, water-softening residue | TDS 1–3% · Ca²⁺ 500–1,500 mg/L · Mg²⁺ 100–500 mg/L — scale-prone | Softening + MVR | Duplex 2205 |
| Steel sintering FGD wastewater | Sinter machine de-SOx bleed (limestone / seawater) | SO₄²⁻ 2,000–15,000 mg/L · Fe / Zn / Pb 10–500 mg/L · pH 2–5 | Pretreat + Crystallization | Duplex / Titanium |
| Coke-oven wastewater concentrate | Coking plant ammonia stripper bottom — see Coal Chemical & Mining | COD 2,000–10,000 mg/L · CN⁻ 5–50 mg/L · phenol 100–500 mg/L | Pre-treat + MVR | Titanium / Duplex |
| Desulfurization brine | Seawater FGD, soda-ash FGD bleed | NaCl 15–30 g/L · Na₂SO₄ 3–10 g/L · Mg²⁺ trace | MVR + FC | Titanium |
| Boiler blowdown concentrate | High-pressure boiler continuous blowdown | TDS <1% · SiO₂ 50–200 mg/L — scale-prone | Multi-Effect | 316L |
Related steel-plant streams — cold-rolling pickling rinsewater and direct-reduced-iron (DRI) scrubber water — are covered under Coal Chemical & Mining.
Routes shown are for preliminary screening only. Mercury and selenium removal requires dedicated precipitation pilot testing on a representative sample. Final material selection is confirmed by coupon testing at operating temperature.
The evaporator only sees what pre-treatment lets through. This is the canonical conditioning sequence for high-chloride, high-TSS, Hg/Se-bearing FGD feed.
Clarifier + hydrocyclone + filter press take out gypsum, fly ash and limestone solids. Target <1,000 mg/L TSS to the evaporator.
Calcium and magnesium hardness down to <20 mg/L by lime-soda dosing — protecting heat-transfer surfaces from sulfate and carbonate scale.
TMT-15 (or NaHS) at 1.5–3× stoichiometric dose at pH 5–7; ferrihydrite adsorption polishes Se(IV) — always upstream of the evaporator.
Final hardness and trace-metal trim on the softened feed — the last line of protection before MVR / multi-effect duty.
Softening and heavy-metal precipitation chemistry require site-specific jar testing on a representative sample. The crystallizer does not remove Hg or Se — both must be removed before the evaporator.
Indicative process flow for FGD purge water with high chloride, gypsum TSS and trace Hg / Se — from bleed stream to managed salt and reuse condensate.
The workhorse closing stage for scaling, high-TDS FGD brines — high liquid circulation velocity suppresses wall scaling while mixed salt is produced as a free-flowing solid.
Electric-driven concentration ahead of crystallization — strongest where power is available and steam is dear. Watch the BPE penalty at the top of the chloride range.
Falling-film (tube) effects on clarified, softened feed, finishing in FC — competitive above ~15,000 mg/L Cl⁻ when low-pressure steam ($15–25/t) is available from the power plant itself.
NaCl vs Na₂SO₄ / CaSO₄ split when a sellable clean-salt route is pursued instead of landfill — additional process complexity, justified only with verified pilot data.
Simplified PFD for indicative routing only. Actual configuration depends on chloride load, TSS profile, regulatory Hg / Se limits and site utility availability. Softening and heavy-metal precipitation chemistry require site-specific jar testing.
Chloride exposure — not TDS — picks the alloy. Map your stream to the material envelope before any evaporator geometry is discussed.
| Chloride Exposure | Typical Service | Material | Selection Logic & Watch-Outs |
|---|---|---|---|
| <200 mg/L Cl⁻ at evaporation temperature | Mild streams — boiler blowdown concentrate (TDS <1%, SiO₂ 50–200 mg/L) | 316L | Safe envelope only — pitting begins beyond it; three orders of magnitude below FGD chloride levels |
| ~1,000 mg/L Cl⁻ | Moderate streams — power-plant circulating blowdown (TDS 1–3%, Ca²⁺ 500–1,500 mg/L) | Duplex 2205 | Handles moderate Cl⁻ but fails under high Cl⁻ combined with low pH |
| 10,000–30,000 mg/L Cl⁻ — coal Cl 0.01–0.3 wt% | FGD brine — FGD purge water, seawater / soda-ash FGD bleed | Titanium Grade 2 | The workhorse material for FGD evaporator duty |
| SO₄²⁻ 2,000–15,000 mg/L · pH 2–5 — sulfate-rich | Steel sintering FGD wastewater (Fe / Zn / Pb 10–500 mg/L) | Duplex / Titanium | Sulfate-rich service shifts the choice; heavy metals removed upstream |
| >30,000 mg/L Cl⁻ + low pH + oxidizing species (Fe³⁺, Cu²⁺) | Most aggressive FGD brines | Hastelloy C-276 | Reserved for the harshest combination — a cost driver, not a default |
Final material selection is confirmed by coupon testing at operating temperature — never from a table alone.
In this industry the driver is almost always regulatory or energetic. Find yours — it decides the pre-treatment depth and the evaporation route.
EvapCryst organizes its delivery capability around four solution lines. Power, steel and FGD projects most often lead with S1 and S4.
FGD purge water ZLD, mixed-salt management and reuse-grade condensate recovery.
Where FGD by-product sodium or ammonium sulfate can be recovered as a sellable salt.
Purge optimization for sintering and coke-oven wastewater crystallizer loops.
MVR retrofits for existing steam-heated FGD concentrators — energy and carbon reduction.
FGD purge water chlorides typically sit at 10,000–30,000 mg/L — three orders of magnitude above 316L’s safe operating envelope (<200 mg/L at evaporation T). Duplex 2205 handles moderate Cl⁻ (~1,000 mg/L) but fails under high Cl⁻ plus low pH; titanium Grade 2 is the workhorse material for FGD evaporator duty. Hastelloy C-276 is reserved for the most aggressive combinations of Cl⁻ >30,000 mg/L, low pH and oxidizing species (Fe³⁺, Cu²⁺).
Mercury in FGD purge is typically present as particulate HgS (30–60%), ionic Hg²⁺ (10–40%) and a small organomercury fraction (5–20%). Standard treatment is dedicated sulfide precipitation (TMT-15 at 1.5–3× stoichiometric dose, pH 5–7) combined with ferric co-precipitation and polythiocarbonate polishing. Selenium is harder — Se(IV) is removed by ferrihydrite adsorption at pH 4–6 (Fe / Se mass ratio 10:1–20:1); Se(VI) requires prior reduction. Both must be removed before the evaporator; the crystallizer does not remove them.
In the majority of cases the mixed NaCl / CaSO₄ / heavy-metal-bearing salt from FGD ZLD must be managed as a solid waste — typically stabilized and landfilled in a compliant facility. Saleable-grade salt is achievable only when upstream separation produces a clean NaCl stream free of sulfate and heavy metals, which requires additional process complexity. Do not assume salt sales offset operating cost without verified pilot data.
High chloride raises boiling-point elevation (BPE 3–8°C at 15,000–30,000 mg/L Cl⁻) and narrows MVR’s effective ΔT. For FGD purge above ~15,000 mg/L Cl⁻ with significant concentration factor, multi-effect with motive steam (0.25–0.40 t steam per tonne water) often becomes competitive — particularly when low-pressure steam is available from the power plant itself at $15–25/t. The choice is project-specific and driven by a full energy balance including compressor electricity ($0.05–0.10/kWh) vs. steam cost.
Send us your feed analysis (Cl⁻, TSS, Hg, Se, TDS) and regulatory limits. Within 2 business days you will receive a feasibility assessment, simplified PFD and indicative scope.
Send your feed characterization (TDS, COD, main salts, viscosity), throughput and product targets — we respond with a preliminary process route within two business days.
