MVR Separate Salt Extraction Evaporation Crystallization System

The MVR separate salt extraction evaporation crystallization system is a core process for treating high-salinity wastewater and realizing salt resource recovery. It couples mechanical steam recompression technology with a separate crystallization strategy to achieve the dual goals of zero emissions and salt product recovery.

I. Working Principle of the MVR Separate Salt Extraction Evaporation Crystallization System

Core Mechanism: A steam compressor compresses and heats the secondary steam generated during evaporation (compression ratio 1:6~1:8), increasing its enthalpy. This increased enthalpy is then used as a heat source to reheat the material, achieving self-circulation of thermal energy with only a small amount of additional steam required.

Separate Salt Extraction Logic: Based on the differences in solubility of different salts (such as NaCl, Na₂SO₄) with temperature, selective crystallization is achieved within a specific temperature range through temperature control and phase diagram analysis, realizing salt separation and purification.

II. Core Process Flow (Two Main Routes)

Route A: Thermal Salt Separation + Freeze Crystallization (Suitable for NaCl/Na₂SO₄ systems)

Process Steps:

1. Pretreatment: Oil separation + air flotation + multi-media filtration, removing SS to <10mg/L, oil to <5mg/L

2. MVR Concentration: Three-stage falling film evaporation increases TDS from 50,000mg/L to 250,000mg/L, with an electricity consumption of approximately 2835kWh per ton of water

3. High-Temperature Crystallization: Na₂SO₄ crystals are preferentially precipitated at 95-110℃ (purity ≥98.5%)

4. Freeze Crystallization: The mother liquor is cooled to 5℃, utilizing the solubility difference to further precipitate Na₂SO₄·10H₂O, Na… Cl remains in the liquid phase.

5. Low-temperature evaporation: The remaining mother liquor is evaporated and crystallized using MVR to produce NaCl crystals.

6. Mother liquor treatment: A very small amount of mixed salt mother liquor (<5%) is treated by a dryer or rotary kiln (1100℃).

Typical case: Xinjiang Tianye Group 200 tons/day RO concentrate treatment project

Operating data: Electricity consumption per ton of water 55kWh (evaporation 35 + refrigeration 20), steam consumption 0.18t/t (MVR recycling)

Product purity: Na₂SO₄ 99.2%, NaCl 97.5%

Economics: Annual operating cost 3.1 million yuan, product revenue 2.05 million yuan, investment payback period 4.3 years

Route B: Nanofiltration membrane salt separation + evaporation crystallization (more thorough separation)

Process Steps:

1. Nanofiltration Pre-salt Separation: The nanofiltration membrane (PROXS2) achieves a SO₄²⁻ rejection rate >95% and a Cl⁻ permeation rate >90%, reducing the Cl⁻/SO₄²⁻ ratio from 1.5 to 0.02.

2. Membrane Concentration: DTRO concentrates the TDS to 120,000 mg/L, with a system recovery rate >92%.

3. Freeze Crystallization: High-purity Na₂SO₄·10H₂O crystals (99.3% purity) precipitate at 8℃.

4. MVR Evaporation Crystallization: The permeate is evaporated via MVR to produce NaCl crystals (96.8% purity).

5. Mother Liquor Recycling: The crystallization mother liquor is returned to the system, with a salt recovery rate >95%.

Typical Case: An 80 m³/h salt separation project in a coal chemical plant in Anhui Province. Setup: 48 nanofiltration membrane modules, 120 m³/h MVR evaporator

Results: Na₂SO₄ recovery rate 92.3% (annual production 18,000 tons), NaCl recovery rate 88.7% (annual production 23,000 tons)

Economic Benefits: Annual savings of 4.2 million yuan in hazardous waste disposal fees, and 15.8 million yuan in by-product revenue

III. Key Technologies and Equipment

1. Steam Compressor

Type: Centrifugal or Roots type

Compression ratio: 1:6~1:8, temperature rise 820℃

Material: Duplex stainless steel or titanium alloy, resistant to Cl⁻ corrosion

2. Evaporator Type

Falling film type: The solution descends in a film form on the heat exchange tube wall, heat transfer coefficient 3500-5800 W/(m²·K), suitable for low viscosity materials

Forced circulation type Circulating type: The circulating pump forces the feed liquid through the heating tube at a flow rate of 23m/s, with strong anti-scaling ability, suitable for high-salt and easily crystallizing materials.

3. Salt separation equipment

Nanofiltration system: Separation efficiency of monovalent/divalent salts >95%

Freezing crystallizer: Spiral propeller or DTB type, temperature control accuracy ±0.5℃

Centrifugal dehydration: Processing capacity 520t/h, product moisture content <0.2%

4. Material selection

316L stainless steel: Suitable for general working conditions

Titanium-palladium alloy: Withstands Cl⁻ concentration >200,000mg/L, annual corrosion rate <0.01mm, lifespan extended to 20 years

Ceramic membrane: Used for high-salt wastewater concentration concentration, flux decline rate <5%/year

IV. Technical advantages and economics

Core Advantages

1. More Energy Efficient: Energy consumption is only 20-30% of traditional multi-effect evaporation, and steam utilization rate is increased by over 90%.

2. High Salt Recovery Rate: Through mother liquor circulation, salt recovery rate can reach over 95%, and impurity salt rate is <5%.

3. High Product Purity: Na₂SO₄ purity >99%, NaCl purity >96.8%, meeting industrial salt standards.

4. Automated Operation: PLC/DCS integration, supports remote monitoring, and has high operational stability.

Operating Cost Breakdown (per ton of water):

Electricity Cost: 3555kWh (58% of operating cost)

Chemical Costs: pH adjustment, scale inhibitor (22%)

Maintenance Costs: Membrane replacement, equipment cleaning (20%)

Total Cost: 1230 RMB/m³ Compared to traditional triple-effect evaporation, this reduces efficiency by 40-60%.

V. Technical Challenges and Optimization Directions

Existing Challenges

1. Membrane Fouling and Corrosion: Nanofiltration membranes cost as much as 1500 RMB/m², with a lifespan of approximately 35 years; the corrosion problem of electrodialysis plates has not been completely resolved.

2. Organic Matter Interference: High-COD mother liquor requires incineration (rotary kiln 1100℃), increasing natural gas consumption and operating costs.

3. Scaling Control: Hardness and silicate levels need pretreatment to <20mg/L, otherwise the heat transfer coefficient drops sharply.

Optimization Trends for 2026

1. Wastewater Diversion Treatment: Separately collect high-salt, high-organic, and oily wastewater to reduce cross-contamination.

2. Enhanced Pretreatment: Two-stage softening + weak acid resin exchange, total hardness <10mg/L. 3. Ultrasonic Descaling: Integrated ultrasonic oscillation, scale thickness controlled to <1mm/month

4. Ammonia Nitrogen Resource Recovery: Ammonium sulfate recovered via stripping absorption method, resource recovery rate >90%

VI. Typical Application Areas

1. Coal Chemical Industry: Coal gasification and coal oil wastewater treatment, scale 50-200 m³/h

2. Power Plant Desulfurization Wastewater: Zero-discharge treatment, by-product industrial salt

3. Pharmaceutical Industry: High-salt mother liquor resource recovery, annual processing capacity 5.2 million tons

4. Salt Lake Chemical Industry: Lithium extraction from brine, associated salt recovery

The MVR fractional salt extraction system, through thermal energy self-circulation and precise salt separation, transforms high-salt wastewater treatment from a "cost center" to a "profit center," and by 2026, it has become the preferred zero-discharge technology route for industries such as coal chemical and power.