Zero Liquid Discharge Process Design for High-Salinity Wastewater in the Metallurgical Industry

I. Design Background and Water Quality Characteristics

1.1 Wastewater Sources and Characteristics

High-salinity wastewater in the metallurgical industry mainly originates from processes such as hydrometallurgy (e.g., acid/alkali leaching), flue gas desulfurization, electrolytic refining, steel pickling, cooling system blowdown, and floor washing. This type of wastewater is a key and challenging focus for achieving "Zero Liquid Discharge" (ZLD), characterized by the following complex features:

• Extremely High Salinity: Total Dissolved Solids typically range from 20,000 to 100,000 mg/L or even higher, containing high concentrations of ions like Na⁺, Cl⁻, SO₄²⁻, Ca²⁺, and Mg²⁺.

• Complex and Variable Composition: In addition to high salt content, it often contains heavy metal ions (e.g., Cu, Ni, Zn, Pb, Cd, As), fluorides, ammonia nitrogen, oils and greases, and refractory organic compounds (e.g., extractants, flotation agents).

• Strong Acidity/Alkalinity and High Corrosivity: Pickling and leaching wastewater can have very low pH (less than 1), while some desulfurization wastewater is alkaline, imposing stringent requirements on equipment materials.

• High Hardness and Silica Content: Prone to scaling in membrane systems and evaporators, representing a major obstacle for ZLD processes.

• Significant Fluctuations in Water Quality and Quantity: Closely related to production batches, ore grades, and process adjustments.

1.2 Design Objectives and Principles

• Core Objective: Achieve "Zero Liquid Discharge" for all plant wastewater. The produced crystalline salts or mixed salts are to be disposed of in a compliant and resource-oriented manner, while maximizing the recovery of high-quality freshwater for reuse.

• Design Effluent Standards:

  • Product Water: Reuse water quality meets process requirements (e.g., cooling water, wash water) or relevant standards in Standards for Urban Wastewater Reuse.

  • Solid Product: Crystalline salts (e.g., NaCl, Na₂SO₄) meet industrial-grade standards or are disposed of safely as general/hazardous waste.

• Design Principles:

  1. Quality-Based Collection, Graded Treatment: Strictly segregate high-salinity wastewater from other streams to reduce the load on the end-of-pipe ZLD system.

  2. Pretreatment Priority, Scaling Prevention: Efficiently remove scaling and poisoning factors such as calcium, magnesium, silica, fluoride, and heavy metals to protect core membrane and evaporation equipment.

  3. Membrane Concentration, Evaporation Crystallization: Employ an "integrated membrane" process for efficient pre-concentration, significantly reducing the volume of water entering the evaporation/crystallization unit, thereby lowering capital and operating energy costs.

  4. Fractional Crystallization, Resource Recovery: Strive to separate and recover major salts like sodium chloride and sodium sulfate through process control to enhance project economics.

  5. Intelligent Control, Economic and Reliable: Select mature, efficient process combinations and achieve stable, low-consumption operation through intelligent control.

II. Core ZLD Process Route Design

Addressing the complexity of metallurgical high-salinity wastewater, this scheme recommends a main technical route of "Advanced Pretreatment + Integrated Membrane Concentration/Reduction + Evaporation Crystallization/Fractional Solidification". This route aims to separate pollutants stepwise through multiple barriers, ultimately achieving salt solidification and water reuse.

2.1 Full-Process ZLD Flow Diagram

III. Detailed Design of Key Treatment Units

3.1 Advanced Pretreatment System

• Function: Provides qualified feed water for subsequent membrane systems and evaporators, forming the foundation for the long-term stable operation of the entire ZLD system.

• Core Technologies:

  • Chemical Precipitation for Heavy Metals/Fluoride: Adjust pH and dose sulfides, iron salts, calcium salts, etc., to form heavy metal sulfide/hydroxide precipitates and calcium fluoride precipitate, ensuring compliance for heavy metals and fluoride.

  • High-Efficiency Chemical Softening: Employ a multi-stage "Lime-Soda Ash-Magnesium Reagent" softening process. Precisely control pH and chemical dosing to deeply remove calcium, magnesium, and silica, reducing hardness to very low levels (e.g., Ca²⁺ < 50 mg/L, SiO₂ < 20 mg/L). This is key to preventing scaling in downstream systems.

  • Precision Filtration: Use multi-media filters and cartridge filters (5μm) to ensure effluent turbidity <1 NTU and SDI<5.

3.2 Integrated Membrane Concentration/Reduction System

• Function: Maximize brine concentration before evaporation to reduce the evaporative load, thereby significantly lowering capital and energy costs (energy for evaporating 1 ton of water is much higher than for membrane concentration).

• Core Technology Combination:

  • Ultrafiltration (UF): Serves as a precision barrier for RO.

  • Primary Reverse Osmosis (RO): Performs preliminary desalination and concentration of pretreated wastewater, with ~50% recovery; permeate is suitable for reuse.

  • High-Efficiency RO / Disc Tube RO (DTRO): Further concentrates the primary RO concentrate under high pressure. HERO prevents silica scaling by raising pH to increase silica solubility; DTRO is highly fouling-resistant and can handle high-turbidity, high-COD concentrate. This unit can concentrate TDS to 80,000 - 150,000 mg/L.

  • Electrodialysis (ED) / Nanofiltration (NF) Salt Fractionation:

    • Nanofiltration (NF): Utilizes the high rejection of NF membranes for divalent ions (SO₄²⁻) and partial permeation of monovalent ions (Cl⁻) for preliminary separation of sulfate and chloride, creating conditions for subsequent fractional crystallization.

    • Electrodialysis (ED): Under a direct current electric field, ions migrate selectively, enabling further brine concentration and partial salt fractionation. Concentrate TDS can reach 150,000 - 200,000 mg/L or higher.

3.3 Evaporation Crystallization Fractionation System

• Function: Convert dissolved solids in the membrane concentrate into solid crystalline salts, achieving zero liquid discharge.

• Core Technologies:

  • Fractional Crystallization Process:

    1. Freeze Crystallization: For concentrates high in sodium sulfate, a freeze crystallization process can be used. Sodium sulfate decahydrate (Glauber's salt) crystallizes at low temperature and, after dehydration and melting, can produce anhydrous sodium sulfate (salt cake) for resource recovery.

    2. Thermal Evaporation Crystallization:

       • Multi-Effect Distillation (MED) / Mechanical Vapor Recompression (MVR): Mainstream technologies. MVR is highly energy-efficient, recovering heat from secondary vapor via a vapor compressor, requiring only supplemental electrical energy, resulting in lower operating costs than MED.

       • Fractional Crystallization: By controlling the crystallization points at different temperatures and concentrations, sodium chloride and sodium sulfate crystallize sequentially, yielding industrial salt and sodium sulfate product respectively. This process is complex but offers the highest resource recovery value for mixed salts.

IV. Main Design Parameters and Economic Analysis

4.1 Key Unit Design Parameters

4.2 Economic Analysis (Example: 50 m³/h Treatment Capacity)

• Capital Cost Estimate:

  • Advanced Pretreatment System: Approx. 5 - 8 million RMB

  • Integrated Membrane Concentration System: Approx. 15 - 25 million RMB

  • Evaporation Crystallization Fractionation System: Approx. 20 - 35 million RMB

  • Utilities & Automation System: Approx. 5 - 8 million RMB

  • Total Estimated Investment: Approx. 45 - 76 million RMB

• Operating Cost: 25 - 45 RMB per ton of raw wastewater.

  • Electricity: 10-18 RMB/ton (mainly high-pressure pumps, MVR compressor).

  • Steam/Heat Source: 3-8 RMB/ton (if supplemental heat is needed for MVR).

  • Chemical Costs: 5-10 RMB/ton (softening agents, acid, antiscalant, etc.).

  • Membrane & Equipment Depreciation: 4-6 RMB/ton.

  • Maintenance & Labor: 3-5 RMB/ton.

• Benefit Analysis:

  • Environmental Benefit: Completely eliminates wastewater discharge, mitigating environmental compliance risk.

  • Resource Benefit: Recovers >90% high-quality freshwater, saving water resource and discharge fees; by-product crystalline salts, if marketable, can partially offset operating costs.

  • Investment Payback Period: Relatively long, typically >8 years. The primary driver is often environmental policy pressure and social responsibility rather than short-term economic return.

V. Conclusion and Implementation Recommendations

This ZLD process scheme of "Advanced Pretreatment - Integrated Membrane Concentration - Evaporation Crystallization Fractionation" is a systematic solution designed for the characteristics of high-salinity wastewater in the metallurgical industry. Keys to successful implementation include:

1. Accurate Front-End Water Quality Analysis: Comprehensive, long-term characterization of the wastewater is essential, especially accurate analysis of scaling factors (Ca, Mg, Si, Ba, Sr) and interfering factors (organics, F). This is the foundation of process design.

2. Core Importance of Enhanced Pretreatment: Underestimating the importance of pretreatment will inevitably lead to frequent scaling, fouling, or even failure of the subsequent expensive membrane and evaporation systems, drastically increasing operating costs.

3. Emphasize the Economics of Salt Fractionation: Where investment allows, strongly consider salt fractionation processes like NF or freeze crystallization. Separated, pure salts have higher resource recovery value, improving project economics.

4. Pilot Testing is Indispensable: Given the complex and variable nature of metallurgical wastewater, long-term pilot testing is mandatory to verify pretreatment efficacy, membrane selection, concentration factor, fractionation feasibility, and crystalline salt quality.

5. System Integration and Intelligent Control: The ZLD system is complex with tightly coupled units. Advanced DCS/PLC control systems are essential for achieving automated, intelligent full-process operation and early warning.

Through technology integration and optimization, this scheme provides a viable technical pathway for metallurgical enterprises to achieve wastewater zero liquid discharge. It is not only a necessary choice to meet the most stringent environmental regulations but also an inevitable direction for the industry's green and sustainable development.