A 3t/h MVR evaporator for electroplating wastewater from a metal products factory

I. Project Background 

The client is a hardware electroplating industrial park, discharging 220 m³ of rinsing wastewater daily. The wastewater quality is as follows: Ni²⁺ 1800 mg/L, Cr³⁺ 600 mg/L, SO₄²⁻ 12000 mg/L, Cl⁻ 3500 mg/L, COD 400 mg/L, and conductivity 65 mS/cm. The original process of "chemical precipitation + sand filtration + RO" produced 30 m³/d of concentrated wastewater containing heavy metals, with outsourcing costs as high as 480 RMB/ton, and the nickel and chromium resources were wasted. In 2024, the company initiated a "zero-discharge" technological upgrade project. Our company delivered a 3 t/h (evaporation capacity) MVR evaporator for electroplating wastewater under an EPC model. It successfully started up in February 2025 and has been running continuously for over 6500 hours to date, achieving a heavy metal recovery rate of ≥98%, realizing the dual goals of zero wastewater discharge and high-value recovery of metal salts.

II. Process Route

1. Influent Characteristics

Boiling point elevation Δb = 12℃ (25℃ containing NiSO₄ + Cr₂(SO₄)₃, 70kPa·A)

Critical supersaturation ΔCmax = 4g/L, prone to NiSO₄·6H₂O fine crystal formation

Contains F⁻, NO₃⁻ and a small amount of brightener, posing a risk of crevice corrosion to titanium materials

2. Process Summary

Rinse water → Equalization tank (pH 3.0) → Tubular microfiltration (0.1μm, to remove...) (Removal of SS and colloids) → Chelating ion exchange (Ni²⁺≤5mg/L, Cr³⁺≤2mg/L, resin regeneration liquid returned to evaporation) → Preheating (condensate + secondary steam) → MVR forced circulation evaporator → Oslo crystallizer → Centrifugation → Fluidized bed drying → Automatic packaging. The secondary steam undergoes a 20℃ temperature rise via a high-speed centrifugal compressor, with a compression ratio of 1.8, an evaporation temperature of 55℃, and a vacuum of -91kPa, effectively avoiding high-temperature hydrolysis of NiSO₄.

3. Core Technology

Dual-phase steel + titanium composite structure: 2205 heating tubes, TA10 tube sheet and separator, resistant to Cl⁻ pitting corrosion while controlling cost.

Separate crystallization: First stage at 55℃ precipitates NiSO₄·6H₂O; second stage at 35℃ freeze crystallization precipitates Cr₂(SO₄)₃·18H₂O, achieving nickel-chromium separation.

Online washing of salt legs: Crystal residence time ≥45min, d50=1.1mm, C.V.≤4%, centrifugal moisture ≤4%.

Mother liquor reflux is adjustable from 0–25%. When the Cr/Ni ratio is enriched to 3:1, it is sent back to the electroplating tank as an "aging solution," achieving a closed loop.

Intelligent cleaning: CIP is triggered when conductivity increases by 5% or temperature difference >2℃. Citric acid + EDTA are alternated, extending the cleaning cycle from 3 days to 12 days.

III. Equipment Configuration

Evaporator: Heat exchange area 480m², design margin 25%, ensuring five years of leak-free operation.

Compressor: One-piece milled impeller of titanium material, diameter 0.8m, linear velocity ≤260m/s, magnetic levitation bearing, design life 100,000h, vibration value <1.0mm/s.

Centrifuge: Two-stage piston pusher, screen gap 0.10mm, washing section uses 5℃ chilled mother liquor to reduce Cl⁻ entrainment to 100ppm.

Drying: Closed-loop circulating fluidized bed, nitrogen protection, oxygen content <2%, product temperature ≤50℃, preventing NiSO₄ dehydration phase transformation.

IV. Automation and Safety

DCS+APC: Closed-loop control of evaporation rate, compression ratio, mother liquor density, and Ni²⁺ online analyzer, fluctuation ±1.5%.

Dual Redundant SIS: 14 interlocks including compressor surge, oxygen content >5%, temperature >60℃, Ni²⁺ leakage >0.1mg/m³, response time <200ms.

Explosion-proof Zoning: Compressor area ExdⅡCT4, all instruments, lighting, and maintenance sockets are explosion-proof certified.

Online Heavy Metal Recovery: Resin regeneration solution is processed in an electrowinning tank; nickel plate purity ≥99.5%, annual recovery of 28 tons of nickel and 8 tons of chromium, valued at 4.2 million yuan.

V. Operating Data (Performance Assessment in May 2025)

• Evaporation Rate: 3000 kg/h (Design 3000)

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

• Power Consumption: 28 kWh/t water (Compressor + Circulating Pump + Vacuum Pump + Refrigeration Unit)

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

• Output: NiSO₄·6H₂O 1.8 t/d, purity 99.1%, Cl⁻≤150ppm, Cr≤50ppm

• System Recovery Rate: Ni²⁺ 98.5%, Cr³⁺ 98.0%, water reuse rate 100%, all plant waste Zero Water Discharge

VI. Economic and Environmental Benefits

Annual steam savings of 10,000 tons, equivalent to 1,100 tons of standard coal, and carbon reduction of 2,900 tons. Savings in outsourcing costs of 4.5 million yuan/year (30 m³/d × 480 yuan/t × 330d). Annual sales revenue from metal salts and nickel plates of 6.2 million yuan, with a total operating cost reduction of 9.2 million yuan. Investment payback period of 1.6 years.

VII. Customer Feedback

"The MVR system has been running for six months with zero compressor failures, and the purity of nickel salts has consistently reached electroplating grade, completely eliminating the need for heavy metal outsourcing. The project has turned 'wastewater' into 'resources,' making a significant contribution to creating a green factory in the industrial park."