Engineered ZLD and mixed-salt separation for coal chemical, coking and mining operations — designed for TDS 1–25%, COD 500–50,000 mg/L, refractory organics (phenols, SCN⁻, heterocyclics) and the NaCl–Na₂SO₄ eutectic that defines salt-splitting feasibility.
Coal chemical and mining wastewater combine TDS 1–25%, refractory organics (phenols 200–2,000 mg/L, SCN⁻ 100–500 mg/L, heterocyclics), NH₃-N 50–4,000 mg/L and trace heavy metals in ways few other industries must handle simultaneously.
COD 2,000–10,000 mg/L; phenols 200–2,000 mg/L; NH₃-N 100–1,500 mg/L; SCN⁻ 100–500 mg/L; cyanide 5–50 mg/L. Solvent extraction + biological treatment removes organics, but the residual brine purge still requires evaporative ZLD for compliance.
NH₃-N 200–4,000 mg/L; organics (formate, acetate, aromatics); TDS 1–5% in the raw effluent. After ammonia stripping and biological treatment, the residual brine sits in the awkward middle band — too high for membrane, too low for direct crystallization without pre-concentration.
Coal-to-liquids and coal-to-olefins plants produce high-salinity effluent with trace organics (alcohols, ketones) and chloride-to-sulfate ratios that shift with coal feedstock. ZLD with mixed-salt separation is increasingly the default mandate in China under GB 31571 and downstream permits.
At the NaCl–Na₂SO₄ eutectic, achievable single-salt purity caps out well below reusable grade. COD >1,000 mg/L post-treatment usually forces hazardous-waste classification. The salt matrix determines whether ZLD produces two sellable fractions or a hazardous mixed-salt waste — a swing of $50–150 per tonne in net operating cost.
For coal chemical brines where sodium chloride and calcium chloride coexist, scaling control in the falling-film evaporator is not a cleaning-schedule question – it is a phase-equilibrium question. The workable
Indicative flowsheet for coal chemical wastewater ZLD with selective NaCl / Na₂SO₄ separation.
The canonical sequence: phenol + NH₃ stripping → advanced oxidation of refractory organics (O₃ / Fenton / WAO) → softening (Ca / Mg / Si removal) → membrane pre-concentration (1% → 5–8% TDS) → MVR (15–25 kWh per tonne water evaporated, depending on feed TDS and boiling-point rise) or multi-effect (0.25–0.40 t steam per tonne water, depending on effect count and boiling-point rise) or a hybrid of both → forced-circulation crystallizer (tube velocity 2–3 m/s to keep suspended solids moving) → selective salt separation → condensate reuse.
Simplified PFD for indicative routing only. Mixed-salt separation feasibility depends on the NaCl–Na₂SO₄–H₂O phase behavior, residual COD content and target salt purity. Selective separation requires laboratory phase-equilibrium testing.
Coal chemical ZLD success is decided upstream of the crystallizer. Without robust pre-treatment, even the best crystallizer fouls within weeks.
Diisopropyl ether or methyl isobutyl ketone extraction for phenols (target <50 mg/L residual); free-ammonia and fixed-ammonia stripping at elevated pH and temperature (target <15–50 mg/L NH₃-N). These protect downstream biological and evaporator stages from organic load and pH swings.
Ozone (2–5 g O₃ per g COD removed), Fenton (Fe²⁺ / H₂O₂ at pH 3–4) or wet-air oxidation (200–320°C, 5–20 MPa) breaks SCN⁻, heterocyclics and refractory aromatics that survive biological treatment before they reach the brine concentrator. Selection depends on residual COD target and flow.
Lime-soda cold or hot softening takes Ca²⁺ to <20 mg/L and Mg²⁺ to <5 mg/L; ion-exchange polishing drops hardness to <1 mg/L. Silica is limited to <150 mg/L (or lower at high recovery) by Mg(OH)₂ co-precipitation or antiscalant dosing. These prevent CaSO₄ / CaCO₃ / Mg(OH)₂ scaling in membrane and evaporator stages.
ED, HERO (High-Efficiency Reverse Osmosis) or DTRO / SPRO lifts brine from ~1% to 5–8% TDS, reducing the evaporator duty by 5–10×. Subject to scaling envelope (CaSO₄, silica) and COD / antiscalant limits — pilot testing is mandatory before commitment.
Mixed NaCl + Na₂SO₄ systems are the defining engineering problem of coal chemical ZLD. Selective separation changes project economics entirely.
| Stream Scenario | Composition Driver | Separation Feasibility | Outcome |
|---|---|---|---|
| NaCl-dominant | Cl⁻ / SO₄²⁻ molar ratio > 5; COD <500 mg/L | Favorable | Reusable-grade NaCl (>97%) + mixed salt purge |
| Na₂SO₄-dominant | SO₄²⁻ / Cl⁻ molar ratio > 3; COD <500 mg/L | Favorable | Sellable Na₂SO₄ (>95%) + NaCl purge |
| Near-eutectic | Balanced NaCl / Na₂SO₄; ~17.5% Na₂SO₄ at 100°C | Marginal | Two salt fractions, 85–92% purity max |
| High residual COD | COD >1,000 mg/L post-treatment | Difficult | Hazardous mixed salt disposal at $80–200/t |
Separation feasibility requires laboratory phase-equilibrium testing at the project’s actual brine composition. Purity targets and reuse classification depend on local regulatory acceptance.
Evaporative concentration, brine management and crystallization for mine water closure — beyond the coal chemical plant fence line.
Acid or neutral mine drainage carries metals (Fe, Mn, Zn, Cu) plus sulfate and TDS. Neutralization brine often needs evaporative ZLD.
Flotation, leaching and dewatering circuits accumulate salts and reagent residues. Closed-loop concentration prevents discharge.
Lime neutralization of acid mine drainage produces a high-TDS, high-sulfate brine that often requires forced-circulation crystallization for stable disposal.
Copper, uranium and gold heap-leach circuits generate bleed streams needing evaporative concentration and crystallization for salt management.
Where this industry is served by the solution lines — and how to reach reference projects.
Straight answers on mixed-salt economics, MVR vs multi-effect selection, stream differences and membrane limits.
Mixed NaCl + Na₂SO₄ does not crystallize cleanly — at the NaCl–Na₂SO₄–H₂O eutectic (~17.5% Na₂SO₄ at 100°C) co-crystallization is unavoidable and single-salt purity caps near 92–95%. Without separation, the mixed salt is usually classified as hazardous industrial waste at $80–200 per tonne disposal cost. Selective crystallization outside the eutectic band can produce two sellable or reusable fractions; near-eutectic brines cannot be economically split.
MVR (15–25 kWh per tonne water) typically wins when electricity is cheap relative to steam, when low-pressure steam is scarce, or when the plant has a decarbonization mandate. Multi-effect (0.25–0.40 t steam per tonne water) remains attractive where low-pressure steam is abundant (e.g., on a coal chemical site with by-product steam at <$20/t). Many projects now specify a hybrid (multi-effect + MVR) configuration for optimal lifecycle cost.
Coking wastewater is dominated by phenols, NH₃-N and SCN⁻ from coal carbonization. Gasification wastewater carries NH₃-N, organics and dissolved salts from partial oxidation. Both need pre-treatment before evaporative ZLD, but the pre-treatment trains differ — solvent extraction for coking, stripping for gasification.
No. Membrane pre-concentration is limited by scaling (CaSO₄ at Ksp >0.2 g/100 mL, silica above ~150 mg/L at neutral pH), fouling (organics, antiscalant residuals) and osmotic pressure (above ~80 bar for HERO). High-COD (>1,000 mg/L) or high-hardness streams may require softening and AOP before membranes are viable. Site-specific pilot testing (6–12 weeks minimum) is mandatory before commitment.
Send us your wastewater composition, throughput and discharge mandate. Within two business days you will receive a feasibility assessment and an indicative ZLD route.
Send your feed characterization (TDS, COD, main salts, viscosity), throughput and product targets — we respond with a preliminary process route within two business days.
