10t/h Sodium Sulfate and Lithium Sulfate MVR Evaporation Crystallization System

The 10t/h sodium sulfate and lithium sulfate MVR evaporation crystallization system represents another milestone for our company in the field of "zero discharge" of high-salt wastewater from rare earth functional materials. The project validated the reliability and economy of the combined "temperature-separated pressure-separated + MVR + DTB" process for complex sulfate systems. In the future, we will continue to optimize compressor efficiency and the separation depth of lithium and sodium, providing more rare earth, lithium battery, and new energy material companies with low-consumption, high-value-added integrated solutions for zero discharge of high-salt wastewater.

I. Project Background 

This owner produces 5,000 tons of high-purity yttrium oxide, terbium oxide, and lithium-ion battery-grade lithium carbonate annually. The production process generates a large amount of mixed sulfate waste liquid: Na₂SO₄ 120-140 g/L, Li₂SO₄ 45-55 g/L, COD 2000 mg/L, F⁻ 800 mg/L, at a temperature of 65℃. The original triple-effect evaporator consumed 0.33 t of steam per t of water, and the co-precipitation of Li₂SO₄ and Na₂SO₄ resulted in low product purity, making it unsellable. In 2024, the company launched a "zero-emission" upgrade. Our company delivered a 10t/h (evaporation water volume) sodium sulfate and lithium sulfate MVR evaporation crystallization system under the EPC model. It was successfully started up in April 2025 and has been running continuously for more than 8,000 hours. The steam consumption is ≤0.08t/t water, the purity of Li₂SO₄ is ≥99.2%, and Na₂SO₄ reaches the Class I standard of GB/T6009-2014. The annual revenue is 32 million yuan, which has become a demonstration model for the treatment of high-salt waste liquid in the rare earth industry.

II. Process Route

1. Characteristics of the Raw Solution

· Boiling point elevation Δb = 10℃ (mixed salt 22%, 70kPa·A)

· The solubility curves of the two salts differ significantly: Li₂SO₄ 1.34-1.54g/100gH₂O in the 80-95℃ range, Na₂SO₄ 42.7-43.8g/100gH₂O, meeting the conditions for stepwise crystallization. Contains F⁻, COD, and a small · amount of rare earth ions, requiring pretreatment to prevent scaling and product contamination.

2. Process Summary

· Raw Solution → High-density Precipitation (Ca(OH)₂ + Na₂CO₃) Removes F⁻ and calcium, Ca ≤ 20mg/L → Weak acid cation exchange bed (Li-type resin, rare earth ≤0.5mg/L) → Two-stage preheating (condensate + secondary steam) → MVR first-stage evaporation (falling film + forced circulation, 90℃) → DTB crystallizer A (Li₂SO₄ preferential precipitation) → Centrifugation → Fluidized bed drying → Battery-grade Li₂SO₄·H₂O packaging

· Mother liquor → MVR second-stage evaporation (-80kPa, 55℃) → DTB crystallizer B → Na₂SO₄ decahydrate centrifugation → Drying → Sodium sulfate packaging

· Both stages of secondary steam use a single speed-increasing box-type centrifugal compressor, with a temperature rise of 20℃, a compression ratio of 1.8, and an adiabatic efficiency ≥85%.

3. Key Innovations

· Two-stage temperature- and pressure-controlled crystallization: The high-temperature stage controls the density to 1.28g/cm³ to introduce seed crystals; the low-temperature stage uses vacuum flash precipitation of decahydrate nitrate, with a Li/Na separation coefficient ≥98%. Online washing + salt leg: Li salt section residence time 60 min, d50 = 0.9 mm; Na salt section residence time 45 min, d50 = 1.2 mm, C.V. both ≤5%.

· Lithium extraction via mother liquor reflux: Lithium-rich mother liquor is returned to the first stage to maintain a Li concentration of 55 g/L, with a system Li recovery rate >98%.

· Intelligent CIP: Automatic citric acid/EDTA cleaning is initiated when the tube wall temperature difference >2℃ or the vacuum drops by 3 kPa, extending the cleaning cycle from 4 days to 18 days.

III. Equipment Configuration

· Evaporator: 2205 duplex stainless steel + TA10 titanium composite tube sheet, total heat exchange area 960m², design margin 25%, ensuring five years of leak-free operation.

· Compressor: Speed-increasing box-type centrifuge, motor + gearbox drive, impeller diameter 1.2m, linear velocity ≤285m/s, vibration value <1.0mm/s, design life 100,000h.

· Crystallizer: Two Φ2200×4500mm DTB units, built-in guide tubes + baffle rings, maintaining slurry density at 25-30%.

· Centrifuge: LW520×2500-N, separation factor 1500, washing section uses 3℃ chilled mother liquor to reduce Na entrainment.

· Drying: Closed-loop circulating fluidized bed, nitrogen protection, oxygen content <3%, product temperature ≤55℃, preventing Li₂SO₄ dehydration phase transition.

IV. Automation and Safety

· Kang Jinghui Automatic Control System: Multivariate predictive control for evaporation rate, compression ratio, mother liquor density, and Li⁺ online analyzer (ICP-OES), with fluctuations within ±2%.

· Dual Redundant SIS: 18 interlocks including compressor surge, oxygen content >8%, temperature >100℃, and F⁻ leakage >1mg/L, with a response time <300ms.

V. Operational Data (Performance Assessment, July 2025)

· Evaporation Rate: 10000 kg/h (Design 10000)

· Live Steam: 0.08 t/t water (Design 0.10)

· Power Consumption: 26 kWh/t water (Compressor + Circulating Pump + Vacuum Pump + Refrigeration Unit)

· Cooling Water: 1.2 t/t water (32℃→38℃)

· Output: Li₂SO₄·H₂O 2.1 t/d, purity 99.3%, Na≤0.15%; Anhydrous Na₂SO₄ 14 t/d, whiteness ≥88%, Li≤0.05%.

VI. Economic and Environmental Benefits

· Annual steam savings of 26,000 tons, equivalent to 2,900 tons of standard coal, and carbon reduction of 7,600 tons.

· Annual operating cost savings of 6.5 million yuan (steam + outsourced disposal).

· Sales of battery-grade Li₂SO₄ at 18,000 yuan/ton, and sodium sulfate at 550 yuan/ton, resulting in an additional annual revenue of 32 million yuan.

· Investment payback period of 1.4 years.

VII. Customer Testimonials

"The two-stage MVR completely separates lithium and sodium. The compressor has run for 8,000 hours without any failures. The purity of lithium salts has been stable to the battery grade, completely solving the 'last mile' problem of rare earth wastewater."