The treatment of zinc-containing dust and sludge from steel plants in rotary kilns is a mainstream pyrometallurgical resource recovery technology. The core of this technology is the high-temperature reduction and volatilization of zinc, the recovery of secondary zinc oxide, and the return of iron slag to production. In recent years, the main progress has been made in balling pretreatment, multi-field precise control, circle formation inhibition, deep utilization of waste heat and flue gas, and high-value product development, achieving efficient zinc removal, energy conservation, carbon reduction, and the co-processing of multiple solid wastes.

I. Technical Principles and Traditional Processes

1. Core Principles

Taking advantage of the low boiling point of zinc (907°C), in a reducing atmosphere at 1050–1200°C:

●Zinc oxides are reduced to metallic zinc and volatilized into the flue gas.

●Iron oxides are reduced to metallic iron or ferrous iron and remain in the kiln slag.

●Zinc vapor is oxidized to secondary zinc oxide (ZnO) in the flue gas and collected.

●The kiln slag is cooled, magnetically separated, and returned to the blast furnace or sintering process, achieving the separation and resource utilization of iron and zinc.

2. Traditional Rotary Kiln Process (mainly the Waelz process)

Process:

Proportioning (dust sludge + coal powder/coke powder) → Mixing and feeding into kiln → High-temperature reduction → Flue gas cooling and dust collection (zinc oxide) → Kiln slag cooling and magnetic separation → Iron slag recycling

Advantages: Zinc removal rate over 90%, mature process, low investment, utilization of self-generated carbon sources, and simple operation.

Disadvantages: Dust generation during powder feeding, severe circle formation, low metalization rate, high energy consumption, and average product purity, not suitable for low-zinc materials.

II. Recent Technological Research Progress (2020–2026)

1. Innovation in Raw Material Pretreatment: Pelletizing + Intensive Mixing Technology (Core Breakthrough)

Intensive mixing followed by pelletizing using a disc or press to form 8–15mm pellets for feeding into the kiln, replacing powder.

Effects:

● Inhibits dust generation, improves the purity of secondary zinc oxide.

●Enhances the permeability of the material layer, making the temperature and atmosphere inside the kiln more uniform.

●Reduces the formation of low-melting-point phases, significantly alleviating circle formation.

● Pellet drop strength increased to 15 times per pellet, with a thermal burst temperature reaching 850°C.

Typical: Yongfeng Steel and Bayi Steel have adopted this technology, increasing the treatment capacity by over 30%.

2. Precise Control of Multiple Fields in the Kiln (Temperature/Atmosphere/Residence Time)

Distributed and staged air supply: 12+ independent air ducts are set along the kiln body, allowing for zonal oxygen control.

●Drying section (200–400°C): 5% oxygen, promoting the combustion of volatile matter for heat generation.

●Reduction section (1000–1150°C): ≤1% oxygen, preventing re-oxidation of iron and achieving a zinc removal rate of 92%+.

●Kiln head: Micro-positive pressure + low oxygen, inhibiting re-oxidation and circle formation.

Optimization of the temperature field: High-temperature section at 1100–1150°C, residence time ≥35 minutes, balancing zinc removal and anti-slagging.

Intelligent control: Online monitoring of temperature/oxygen/pressure, with AI dynamically adjusting fuel and air volume, reducing the energy consumption per ton of slag from 1.2GJ to 0.85GJ.

3. Circle Formation Inhibition and Long-Term Operation Technology

●Pelletization + low-oxygen temperature control, reducing the formation of low-melting-point substances.

●Micro-positive pressure + sealing at the kiln head, reducing re-oxidation.

●Special anti-circle formation refractory materials + mechanical circle removal devices.

●Effects: Circle formation cycle extended from 1–2 months to over 6 months, increasing the operation rate.

4. Deep Utilization of Waste Heat and Flue Gas

High-temperature flue gas (1000°C+): Waste heat boilers generate steam or electricity, with thermal efficiency over 70%.

Purification of secondary zinc oxide: Acid washing/alkali washing + roasting, purity increased from 60–70% to 85–95%.

Flue gas synergy: Desulfurization, denitrification, defluorination, and dechlorination, producing by-products such as zinc sulfate and potassium chloride, achieving zero waste discharge.

5. High-Value Product Development and Co-processing of Multiple Solid Wastes

Secondary zinc oxide → Electrolytic zinc/zinc ingots (purity 99.99%), doubling the added value.

Metalization rate of kiln slag 80–85%, directly supplied to blast furnaces or converters, replacing part of iron ore.

Co-processing: Converter mud, blast furnace dust, steel rolling scales, and sulfuric acid slag, achieving zero solid waste discharge from the entire plant.

III. Comparison of Mainstream Processes (Rotary Kiln vs. Shaft Furnace)

IV. Engineering Application Cases

Yongfeng Steel: Pelletizing + Graded Air Inlet Rotary Kiln, processing 300,000 t/a of zinc-containing dust and sludge, zinc removal rate 93%, iron slag metalization rate 82%, annual revenue over 80 million yuan

Shougang Jingtang: Distributed Air Control Rotary Kiln, energy consumption per ton of slag 0.85 GJ, zinc recovery rate 92%, dioxin < 0.1 ng-TEQ/Nm³

Bayi Steel: New rotary kiln for the co-processing of multiple types of dust and sludge, achieving full solid waste recycling

V. Future Development Trends

1. Intelligence and Digitalization: AI full-process optimization, digital twin, unmanned operation

2. Low-carbonization: Oxygen-enriched/pure oxygen combustion, green electricity substitution, CCUS coupling

3. High-value: Sub-oxide zinc → high-purity zinc/zinc alloy, recovery of rare and precious metals (Pb/Cd/In)

4. Integration: Rotary kiln + hydrometallurgical/ pyrometallurgical purification + building materials synergy, building a closed-loop industrial chain

5. Standard Upgrade: Stricter emission and product quality standards, promoting technological iteration

VI. Conclusion

The rotary kiln treatment of steel zinc-containing dust and sludge has shifted from traditional extensive methods to new efficient processes featuring pelletizing pretreatment, multi-field precise control, caking inhibition, deep utilization of waste heat and flue gas, and high-value products. Significant improvements have been made in zinc removal rate, energy consumption, caking, and resource utilization rate, making it one of the core technologies for steel enterprises to achieve zero solid waste discharge and green low-carbon development.