Detailed Explanation of Magnesium Sulfate Wastewater Evaporation Crystallization Process

Magnesium sulfate wastewater mainly originates from chromium chemical reduction waste liquid, sulfur black dye by-products, lithium battery recycling, and magnesium desulfurization processes. It is characterized by high solubility, a significant increase in boiling point, and a tendency to scale and clog pipes. To address these characteristics, several mature evaporation crystallization processes have been developed industrially.

I. Core Process Principle 

The solubility of magnesium sulfate (MgSO₄) varies significantly with temperature—above 48.1℃, it crystallizes as a hexahydrate (MgSO₄·6H₂O), and below that, as a heptahydrate (MgSO₄·7H₂O). The industrial product is primarily magnesium sulfate heptahydrate, and its crystallization process follows a two-step principle: "evaporation and concentration → cooling and crystallization." First, the wastewater is concentrated to a saturated or supersaturated state through evaporation, and then crystals are induced to precipitate through cooling.

II. Mainstream Process Types

1. Evaporation Concentration + Cooling Crystallization (Most Commonly Used)

Flow: Preheating → MVR/Multi-Effect Evaporation → Vacuum Flash Cooling → Thickening → Centrifugation → Drying

Advantages:

Avoids direct crystallization within the evaporation tube, completely solving the tube blockage problem.

Vacuum flash cooling is extremely fast, achieving crystal particle size of 2.0-3.5mm.

Product quality is stable, meeting the Class I Grade 1 standard for industrial magnesium sulfate (impurity content <0.5%, whiteness >85).

Equipment Combination:

Evaporation Section: MVR forced circulation evaporator or multi-effect evaporator, concentrating to a magnesium sulfate mass concentration of 28-30% (saturated state).

Crystallization Section: Vacuum flash crystallizer (DTB).

1.(Or OSLO type) Temperature precisely controlled at <48.1℃

2. Continuous Cooling Crystallization (Direct Method)

Process: High-temperature saturated solution → Vacuum flash → Secondary cooling with chilled water → Crystallization

Features: No evaporation concentration required, lower energy consumption

However, it requires high raw material concentration (close to saturation), narrow applicability

It easily produces fine crystals, requiring a fine crystal removal system

3. MVR Evaporation and Crystallization Integration

Process: Preheating → MVR evaporation → Direct crystallization at the bottom of the evaporation chamber

Advantages: Compact process, small footprint, high degree of automation

Disadvantages: Prone to scaling on heat exchange tubes, requiring frequent cleaning; crystal particle size is relatively small (0.5-1.5mm)

4. Freeze-crystallization (High-purity route)

Process: Evaporation and concentration → Freeze-crystallization (5℃ to 5℃) → Centrifugal dehydration

Applicable scenarios: Production of pharmaceutical-grade or food-grade magnesium sulfate heptahydrate

Impurities are less soluble at low temperatures, and product purity can reach over 99.5%.

However, energy consumption is high (refrigeration power consumption increases by 30-50%), and equipment investment is large.

III. Detailed Process Flow (Taking evaporation + flash crystallization as an example)

Step 1: Evaporation and Concentration

Feed Concentration: Magnesium sulfate mass concentration 1719%

Evaporation Equipment: MVR forced circulation evaporator or triple-effect countercurrent evaporator

Operating Parameters:

Evaporation Temperature: 75-85℃ (vacuum degree 0.06~0.07MPa) Boiling point elevation: The boiling point of a saturated magnesium sulfate solution increases by approximately 812℃.

Concentration endpoint: 28-30% (reaching saturation or supersaturation)

Step 2: Vacuum flash cooling crystallization

Core equipment: DTB vacuum crystallizer

Key control points:

Flash temperature: Rapidly decrease from 75℃ to 4045℃

Vacuum degree: 0.08~0.09MPa

Residence time: 24 hours, ensuring sufficient crystal growth

Supersaturation: Controlled within the metastable region (ΔC=510g/L) to avoid explosive nucleation.

Step 3: Crystal slurry thickening

Equipment: Thickener (gravity settling or hydrocyclone separation)

Target: Increase the solid content of the crystal slurry from 15%-20% to 40%-50% Mother liquor reflux: The supernatant mother liquor is returned to the evaporation system to improve yield.

Step 4: Centrifugal dehydration

Equipment: Horizontal screw centrifuge or piston pusher centrifuge

Operation: Centrifugation speed 2000-3000 rpm, time 10-15 minutes

Product moisture content: <5% (wet basis)

Step 5: Drying and packaging

Equipment: Fluidized bed dryer or rotary dryer

Temperature: <50℃, to prevent dehydration of heptahydrate

Final product: Magnesium sulfate heptahydrate crystals, particle size 2.0-3.5mm

IV. Key Equipment and Materials

1. Evaporator

Forced Circulation Type: Shell and Tube Heat Exchanger, Flow Velocity 23 m/s within Tubes, Prevents Scale Formation

Material: Titanium-Palladium Alloy or Duplex Stainless Steel 2507 (Magnesium Sulfate Solution Corrosion Rate on 316L at 85℃ >0.5 mm/a, Titanium <0.01 mm/a)

2. Crystallizer

DTB Type: With Guide Flow Tube and Baffle Plate, Internal Crystal Slurry Circulation, Narrow Particle Size Distribution

OSLO Type: Clear Liquid Circulation, Crystal Suspension Growth, Larger Product Particles (35 mm)

3. Compressor (MVR Process)

Type: Centrifugal or Roots type

Parameters: Compression ratio 1.5-2.0, temperature rise 10-15℃

Material: Titanium alloy impeller, tantalum-lined shell

V. Control Parameters and Operating Points

1. Temperature Control

Evaporation section: 75-85℃ (excessive temperature will cause magnesium sulfate to decompose or form monohydrate)

Crystallization section: <48.1℃ (critical temperature for stable existence of heptahydrate)

Freezing crystallization section: 5℃ to 5℃ (if using a freezing process)

2. Concentration Control

Feed: 17-19% (mass concentration)

Evaporation endpoint: 28-30% (saturation concentration) Supersaturation ΔC = 510 g/L

3. Supersaturation Control

Mesostable Zone Width: Magnesium sulfate has a narrow metastable zone (ΔC ≈ 812 g/L), requiring precise control of the cooling rate.

Method: Vacuum flash evaporation for rapid cooling + mother liquor circulation dilution to avoid excessively high local supersaturation.

4. pH Control

Maintain pH 6.5-7.0 to prevent accelerated equipment corrosion under acidic conditions.

If the wastewater contains free acid, it needs to be neutralized beforehand (add MgO or MgCO₃).

VI. Technical Challenges and Solutions

Challenge 1: Scale Formation and Pipe Blockage

Cause: Magnesium sulfate Crystallization occurs on the heating tube wall, causing the heat transfer coefficient K-value to drop from 2000 to 500 W/(m²·K).

Solutions:

Forced circulation: Flow rate 23 m/s, shear force flushes the tube wall to remove crystals.

Online cleaning: Automatic CIP cleaning every 8-12 hours, alternating acid and alkaline washing.

Ultrasonic descaling: 20-40 kHz ultrasonic oscillation, scale thickness <1 mm/month.

Challenge 2: Unstable crystal quality.

Cause: Excessive cooling rate leads to numerous fine crystals, product particle size <0.5 mm.

Solution: Vacuum flash evaporation: Replaces traditional coolers. Avoid scaling on heat exchange surfaces

Graded crystallization: The DTB crystallizer has built-in grader legs, allowing large particles to be discharged preferentially.

Residence time: Extended to 34 hours for sufficient crystal growth.

Challenge 3: Large boiling point rise

Data: The boiling point rise of a 30% magnesium sulfate solution is approximately 8°C, and that of a 50% solution can reach 15°C.

Solution: The MVR compressor temperature rise needs to reach 12-18°C, with a compression ratio of 1.8-2.5.

VII. Typical Case Analysis

Case: Magnesium sulfate wastewater treatment in a chemical plant

Water quality: Magnesium sulfate concentration 17-19%, containing small amounts of NaCl and CaSO₄.

Process Flow:

1. Preheating: Utilizing waste heat from condensate to raise the temperature to 60℃

2. MVR Evaporation: Forced circulation evaporator, concentrating to 28-30%

3. Vacuum Flash Crystallization: DTB crystallizer, temperature reduced from 75℃ to 42℃

4. Centrifugal Dehydration: Horizontal screw centrifuge, product moisture content 4.5%

5. Drying: Fluidized bed drying, final moisture content <0.5%

Operating Parameters:

Processing Capacity: 20 m³/h

Evaporator Material: Titanium-Palladium Alloy

Compressor Power: 560 kW (Centrifugal)

Electricity Consumption per Ton of Water: 45kWh

Product Specifications: Particle size 2.5mm, purity 99.2%, whiteness 87

Economics:

Investment: Approximately 25 million RMB (including MVR, crystallization, centrifugation, and drying)

Operating Cost: 18 RMB/ton water (mainly electricity)

Annual Revenue: 18,000 tons/year of magnesium sulfate heptahydrate, revenue approximately 4.5 million RMB

Payback Period: 5.5 years

VIII. Selection Recommendations

For industrial-grade magnesium sulfate heptahydrate production, MVR evaporation + vacuum flash crystallization is the optimal solution, combining advantages in energy consumption, product quality, and automation.