4.7t/h desulfurization wastewater (ammonium thiocyanate, ammonium sulfate, ammonium thiosulfate, etc.) evaporation and concentration system

I. Project Background 

The owner's 2.6 million tons/year coke production line is equipped with a dry desulfurization unit, generating 4.7 t/h of desulfurization wastewater with complex components: ammonium thiocyanate (NH₄SCN) 140-160 g/L, ammonium sulfate ((NH₄)₂SO₄) 90-110 g/L, ammonium thiosulfate ((NH₄)₂S₂O₃) 40-60 g/L, suspended sulfur 2-4 g/L, COD 8000-12000 mg/L, color 5000 times, and temperature 55℃. The original vacuum drum evaporator had been operating for ten years, consuming 0.42 t/t of water per steam. The equipment suffered from severe coking and odor emission, and the low purity of the by-product salts meant they could only be sold cheaply. In 2023, the group decided to build a new "zero-emission" evaporation and concentration system, requiring steam consumption ≤0.10t/t water, output concentration ≥65% (converted to solids), and mixed salt purity ≥95%. Kang Jinghui undertook the project in an EPC model, and it successfully started operation in January 2024. It has been running continuously since then, with a 75% reduction in overall energy consumption costs and annual operating cost savings of 11 million yuan.

II. Process Route

1. Raw Liquid Characteristics

Boiling point elevation Δb=13℃ (mixed salt 55%, 70kPa·A)

Highly thermosensitive: NH₄SCN >105℃ decomposes to generate HCN and NH₃; S₂O₃²⁻ >90℃ disproportionates to generate S precipitate

High COD, high color, easily foams, contains suspended sulfur particles

2. Process Summary

Desulfurization wastewater → Cooling and desulfurization (plate cooling to 30℃, gravity sedimentation + drum filtration, suspended sulfur ≤0. 2g/L) → High-density precipitation (adding NaOH + PAC to remove heavy metals, Pb≤0.5mg/L) → Two-stage preheating (condensate + secondary steam) → Low-temperature MVR forced circulation evaporator → DTB crystallizer → Two-stage piston centrifuge → Fluidized bed drying → Automatic packaging. The secondary steam experiences a temperature rise of 18℃ via a speed-increasing box-type centrifugal compressor, with a compression ratio of 1.7, an evaporation temperature of 58℃, and a vacuum of -90kPa, effectively avoiding the high-temperature decomposition zone of NH₄SCN.

3. Key Innovations: Low-temperature forced circulation + DTB classification: Circulation rate 8000m³/h, crystal residence ≥45min, d50=0.75mm, C.V.≤6%.

Online defoaming + side-stream degassing: A wire mesh demister and defoamer injection at the top of the tower reduce foam height to <150mm; side-stream exhaust is treated with acid washing + alkali washing + activated carbon, resulting in tail gas HCN<0.3mg/m³ and NH₃<1mg/m³.

Segmented temperature control: 58℃ in the evaporator chamber, 52℃ in the crystallizer, and 48℃ at the discharge, ensuring no decomposition of NH₄SCN and a disproportionation rate of S₂O₃²⁻ <2%.

The mother liquor reflux ratio is adjustable from 0–20%. After enriching SCN⁻ to 200g/L, it is pumped into the desulfurization tower for regeneration, resulting in no waste liquid discharge from the system.

Intelligent CIP: Alternating acid-base and oxidant cleaning is triggered when the pipe wall temperature difference exceeds 2℃ or the vacuum drops by 2kPa, extending the cycle from 2 days to 12 days.

III. Equipment Configuration

Evaporator: 2205 duplex steel + TA2 titanium composite tube sheet, heat exchange area 520m², design margin 30%, ensuring five years of leak-free operation.

Compressor: One-piece milled titanium impeller, diameter 0.9m, linear velocity ≤260m/s, magnetic levitation bearing, design life 100,000h, vibration value <1.0mm/s.

Crystallizer: Φ2000×4000mm DTB, built-in guide tube + baffle ring, maintaining slurry density at 30-35%.

Centrifuge: LW530×2200-N, separation factor 1500, screen gap 0.10mm, washing section uses 5℃ chilled mother liquor to reduce (NH₄)₂SO₄ entrainment.

Drying: Closed-loop circulating fluidized bed, nitrogen protection, oxygen content <3%, product temperature ≤50℃, preventing NH₄SCN thermal decomposition.

IV. Automation and Safety

DCS+APC: Multi-variable predictive control of evaporation rate, compression ratio, mother liquor density, and online Raman analyzer for NH₄SCN, with fluctuations ±2%.

Dual Redundant SIS: 16 interlocks including compressor surge, oxygen content >8%, temperature >65℃, HCN >1mg/m³, with a response time <300ms.

V. Operating Data (Performance Assessment, April 2024)

• Evaporation Rate: 4700 kg/h (Design 4700)

• Live Steam: 0.09 t/t water (Design 0.10)

• Power Consumption: 29 kWh/t water (Compressor + Circulating Pump + Vacuum Pump + Defoaming Pump)

• Cooling Water: 1.8 t/t water (32℃→38℃)

• Output: Mixed salt 2.8 t/d, purity 95.8%, NH₄SCN≥55%, (NH₄)₂SO₄≤35%, (NH₄)₂S₂O₃≤8%, moisture ≤2%

• System Recovery Rate: Mixed salt 98.5% 100% water reuse rate, zero wastewater discharge throughout the plant

VI. Economic and Environmental Benefits

Annual steam savings of 16,000 tons, equivalent to 1,800 tons of standard coal, reducing carbon emissions by 4,700 tons.

Annual operating cost savings of 11 million yuan (steam + outsourced disposal).

Sales price of mixed salt increased from 300 yuan/ton to 900 yuan/ton, resulting in an additional 9.2 million yuan in annual revenue.

VII. Customer Feedback

"The 4.7t/h MVR system has been running for six months with zero compressor failures. The vacuum level remains at -90kPa even in summer, and the purity of the mixed salt has remained stable above 95%, completely solving the problems of odor and energy consumption in the desulfurization liquid."