A 6t/h sodium chloride MVR evaporator for non-ferrous metals

I. Project Background 

The client is a large-scale lithium mica smelting enterprise with an annual production capacity of 20,000 tons of battery-grade lithium carbonate. The rinsing section of the production line generates 150 m³ of high-chlorine wastewater daily, with the following composition: NaCl 180-210 g/L, Li 400-600 mg/L, F⁻2 g/L, Ca 1.2 g/L, SS 300 mg/L. The original triple-effect evaporator had been operating for ten years, with a steam consumption of 0.35 t/t water. Vacuum decay led to a 30% decrease in summer production capacity, and the mother liquor, enriched with Li, could only be sold cheaply. In 2024, the company initiated a "zero-emission" upgrade. Kang Jinghui delivered a 6 t/h (evaporation water volume) sodium chloride MVR evaporator under an EPC model. It successfully started up in March 2025, achieving a steam consumption of ≤0.09 t/t water, NaCl crystal purity ≥99.5%, Li recovery rate >92%, and annual operating cost savings of 9.2 million yuan.

II. Process Route

1. Influent Water Characteristics

Boiling point elevation Δb = 10℃ (23% NaCl, 70kPa·A)

Critical supersaturation ΔCmax = 6g/L, prone to NaCl fine crystal formation

High F⁻ and Ca, scaling factor RF = 0.9, strong scaling tendency

2. Process Summary

Raw water → High-density sedimentation (Ca(OH)₂ + Na₂CO₃, Ca ≤ 30mg/L) → Weak acid cation exchange bed (Ca ≤ 5mg/L) → Tubular microfiltration (SDI ≤ 2) → Two-stage preheating (condensate + secondary steam) → MVR forced circulation evaporator → Oslo crystallizer → Two-stage piston centrifuge → Fluidized bed dryer → Automatic packaging

Secondary steam experiences a temperature rise of 18℃ via a high-speed centrifugal compressor, compression ratio 1.7, evaporation temperature 55℃, vacuum degree -90kPa, avoiding high-temperature hydrolysis of LiCl.

3. Core Technologies

Dual-Effect Energy Recovery: The falling film section handles 75% of the evaporation, while the forced circulation section promotes crystal growth, reducing the unit heat transfer area by 20%.

Online Salt Leg Washing: Crystal residence time ≥50 min, d50=0.9 mm, C.V.≤5%, centrifugal moisture ≤3%.

Mother Lithium Extraction: Lithium-rich mother liquor (Li 8-10 g/L) is periodically pumped into a freeze crystallizer (-5℃) to precipitate LiCl·2H₂O. After centrifugation, it is returned to the lithium workshop, achieving a Li recovery rate >92%.

Intelligent Cleaning: CIP is triggered when the pipe wall temperature difference >2℃ or the vacuum drops by 2 kPa. Alternating acid/alkali/chelating agent is used, extending the cleaning cycle from 4 days to 18 days.

III. Equipment Configuration

Evaporator: 2205 duplex steel, heat exchange area 600 m², design margin 25%, ensuring five years of leak-free operation. Compressor: One-piece milled impeller made of titanium, 1.0m diameter, linear velocity ≤265m/s, magnetic levitation bearing, design life 100,000h, vibration value <1.2mm/s.

Centrifuge: Two-stage piston pusher, screen gap 0.12mm, washing section uses 3℃ chilled mother liquor to reduce Li⁺ entrainment to 50ppm.

Drying: Closed-loop circulating fluidized bed, nitrogen protection, oxygen content <3%, product temperature ≤55℃, to prevent NaCl agglomeration.

IV. Automation and Safety

DCS+APC: Closed-loop control of evaporation rate, compression ratio, mother liquor density, and online Li⁺ analyzer, fluctuation ±2%.

Dual Redundant SIS: 16 interlocks including compressor surge, oxygen content >6%, temperature >65℃, Li⁺ leakage >0.1mg/m³, response time <250ms.

V. Operating Data (Performance Assessment in June 2025)

Evaporation Rate: 6000 kg/h (Design 6000)

Live Steam: 0.09 t/t water (Design 0.12)

Power Consumption: 24 kWh/t water (Compressor + Circulating Pump + Vacuum Pump + Refrigeration Unit)

Cooling Water: 1.5 t/t water (32℃→38℃)

Output: NaCl 4.2 t/d, purity 99.6%, Ca≤80ppm, Li≤50ppm, water-insoluble matter≤0.05%, meeting the requirements for battery-grade ion-exchange membrane caustic soda salt

System Recovery Rate: NaCl 98.8%, Li 92%, water reuse rate 100%, full Zero wastewater discharge from the plant

VI. Economic and Environmental Benefits

Annual steam savings of 21,000 tons, equivalent to 2,300 tons of standard coal, and carbon reduction of 6,000 tons. Operating cost savings of 5.2 million yuan/year (steam + outsourced disposal). By-product LiCl·2H₂O of 600 tons/year, at 15,000 yuan/ton, generating an additional revenue of 9 million yuan. Investment payback period of 1.5 years.

VII. Customer Feedback

"The MVR system has been running for eight months with zero compressor failures. The vacuum level remains at -91 kPa even in summer, and the NaCl whiteness is >85, completely replacing purchased refined salt. The project transforms rinsing water into a resource, laying a solid foundation for creating a green lithium smelting plant."