Detailed Explanation of Sodium Sulfate Evaporation and Crystallization Equipment Process

In industrial applications of sodium sulfate wastewater treatment and recovery of anhydrous Na₂SO₄ (sodium sulfate), the common approach is "evaporation and concentration → recrystallization and separation." Core equipment options include MVR (Mechanical Vapor Reduction), multi-effect evaporation, or cryogenic routes. Based on steam utilization and crystallization methods, five typical processes can be identified. The following section, based on the latest engineering cases and operational data, systematically outlines the flow, energy consumption, applicable scenarios, and design considerations for each process.

I. MVR Forced Circulation Evaporation + Cooling (or Flash Evaporation) Crystallization

1. Flowchart

① Pretreatment: Flocculation/Activated Carbon + Softening, Turbidity ≤ 5 NTU, Ca²⁺/Mg²⁺ ≤ 50 mg/L, to prevent calcium sulfate scale formation.

② Falling film concentration: Using a 316L or TA2 falling film evaporator, 3–8% Na₂SO₄ is concentrated to 25–30%, with a heat transfer coefficient of 2000–3500 W·m⁻²·K⁻¹.

③ MVR forced circulation crystallization: The concentrated liquid enters a forced circulation evaporator. The secondary steam is heated by 8–12 °C using a Roots/centrifugal compressor and then reused. The evaporation temperature is 75–85 °C, and the boiling point is increased by 7–9 °C. The solid content of the crystal slurry is 8–15%. After passing through a cyclone separator and thickener, the solid content is increased to 35–50%. Centrifugation and dehydration yield anhydrous Na₂SO₄ with a particle size of 10–50 µm. All mother liquor is refluxed.

2. Energy Consumption: 28–35 kWh of electricity is consumed per ton of water evaporated, and steam consumption is ≤1.5 kg (operational only). The operating cost is approximately 1/4 of that of triple-effect evaporation.

3. Applicability: Projects with a capacity >5 m³/h, steam price >200 RMB/t, or where a boiler is unavailable on-site; product purity ≥99%, moisture content <0.2%.

II. Multi-Effect Evaporation (Double/Triple Effect) + Flash Crystallization

1. Process:

① First Effect: Live steam 0.2–0.35 MPa, temperature 130–140 ℃;

② Second Effect: Vacuum 0.065–0.09 MPa, temperature 70–80 ℃;

③ Final effect output concentration 28–32%, enters the vacuum flash crystallizer (60–70 ℃) for further precipitation of anhydrous Na₂SO₄. 

2. Energy Consumption: 0.3–0.4 tons of steam per ton of water, 10–15 kWh of electricity; lower investment than MVR, but operating costs are 50–70% higher.

3. Applicability: Chemical and coal chemical projects with inexpensive low-pressure steam (<160 RMB/t) and a processing capacity >10 m³/h; when particle size requirements are not critical, a separate crystallizer can be eliminated, and crystals can be grown directly in the final-effect salt tank, simplifying the process.

III. MVR + Freeze-Crystallization Combined Salt Separation (Zero Discharge Version for High-Salinity Wastewater)

1. Process

① The MVR concentrates the mixed salt wastewater (Na₂SO₄ + NaCl) to TDS≈250 g/L; ② The concentrate enters the HSCC spiral freeze crystallizer, where the solubility of Na₂SO₄ drops sharply to 1.3 g/100 g water at -5 ℃, while NaCl remains at 35.7 g/100 g water, thus achieving "freeze-precipitate nitrate—centrifugation to obtain Glauber's salt—melting to obtain anhydrous Na₂SO₄"; ③ The frozen mother liquor is returned to the MVR for evaporation to remove NaCl.

2. Energy Consumption

Evaporation section 35 kWh + Freeze section 20 kWh ≈ 55 kWh/t water, steam 0.18 t/t water; however, the product purity is 97–99%, and the impurity salt rate is <3%. 3. Applications: Suitable for RO concentrate, mine water, and dyeing wastewater requiring high-purity dual recovery of Na₂SO₄/NaCl; investment payback period is approximately 4 years.

IV. Low-Temperature Evaporation (<60℃) Crystallization – Specifically for Heat-Sensitive/Corrosive Wastewater

1. Process: Vacuum degree 85–95 kPa, evaporation temperature 45–55℃, using steam or hot water as a low-grade heat source; secondary steam is heated by a heat pump or mechanical compression and reused; latent heat of condensation is recovered through a titanium alloy plate heat exchanger; the concentrate approaches saturation at low temperature and enters a stirred cooling crystallizer, where Na₂SO₄·10H₂O precipitates at 20–25℃, followed by melt drying to obtain anhydrous product. 

2. Energy Consumption: 40–50 kWh per ton of water, 0.1–0.15 t of steam; skid-mounted equipment, occupying 20–30 m², can be operated unattended.

3. Applicable to: Mother liquor containing high chlorine, fluorine, or low-boiling-point organic matter, such as lithium carbonate precipitation wastewater, pharmaceutical wastewater; highly corrosive but temperature-limited environments.

V. Nanofiltration Salt Separation + Separate Evaporation and Crystallization (Mainstream of Zero-Emission Coal Chemical Industry)

1. Process:

① High-salinity wastewater is first separated into divalent (SO₄²⁻) and monovalent (Cl⁻) salts by nanofiltration membrane;

② Nanofiltration concentrate (Na₂SO₄) is evaporated in a triple-effect evaporator or MVR to obtain anhydrous Na₂SO₄;

③ Nanofiltration permeate (NaCl) is evaporated in a triple-effect evaporator or MVR to obtain NaCl;

④ Mother liquor is cross-refluxed to ensure salt purity. 

2. Features

Na₂SO₄ purity 99%, NaCl purity 98%, water recovery rate >95%, but the membrane system requires fouling prevention and regular cleaning.

3. Applicable to

Coal chemical industry, mine water, and high-salt wastewater from refining, requiring "dual salt" resource utilization and a water volume >500 m³/d.

VI. Common Design Considerations

1.Material: TA2/2205 duplex steel for temperatures >90℃, 316L for low-temperature sections; zirconium or fluorine-lined materials for fluorine-containing applications.

2.Scale Prevention: Forced circulation flow rate ≥1.5 m/s, addition of 0.1–0.3% polyaspartic acid (PASP) dispersant, regular washing with 80℃ hot water or 5% alkaline solution. 3. Particle Size: For particles larger than 150 µm, the cooling rate should be controlled at 1–2 ℃/min in the cooling crystallization section, with a residence time ≥30 min, and 50–100 µm seed crystals should be added.

4.Automation: Evaporator level, density, and supersaturation are measured online; centrifuge torque is interlocked with mother liquor reflux, enabling continuous discharge and one-button start/stop.

Conclusions:

1.For steam shortages, low electricity prices, and production scale >5 m³/h—preferably use the MVR forced circulation route;

2.For availability of inexpensive steam, production scale >10 m³/h, and general product particle size—choose double-effect/triple-effect evaporation;

3.For high purity Na₂SO₄/NaCl—use “MVR + freeze crystallization” or “nanofiltration salt separation + double evaporation”;

4.For heat-sensitive, highly corrosive, and space-constrained applications—use a low-temperature evaporation crystallization skid-mounted device. Following the above approach, the comprehensive operating cost per ton of water can be controlled at 25–45 yuan, and the product purity is ≥99%, achieving the goal of resource utilization and "zero discharge" of sodium sulfate wastewater.