Every tonne of steel produced in an EAF generates 15-25 kg of dust — fine particulate matter captured by the baghouse. For a 500,000-tonne-per-year meltshop, that is 7,500 to 12,500 tonnes of dust annually. This dust is classified as hazardous waste in most countries because it contains heavy metals — primarily zinc, lead, and cadmium — that can leach into groundwater if landfilled.
MONTE INTELLIGENCE has been involved in EAF dust handling systems for over a decade. The dust problem is both an environmental obligation and an economic opportunity, because EAF dust typically contains 15-30% zinc — concentrations higher than many zinc ore deposits currently being mined.
The composition of EAF dust reflects the scrap mix. Dust from plants melting galvanized scrap can contain 25-35% zinc. Dust from plants melting primarily heavy scrap and home scrap may contain only 8-15% zinc. The zinc content determines the economic viability of recycling: above about 15% zinc, dust recycling generally produces a positive return. Below 10% zinc, the recycling cost may exceed the value of the recovered zinc, and stabilization followed by landfill disposal becomes the economically rational choice — though regulatory trends are pushing against that option.
Lead content in EAF dust deserves attention because it affects both the recycling process and the marketability of the recovered zinc product. Typical lead content ranges from 1-5%. Lead and zinc are chemically similar enough that separating them economically is difficult. The Waelz kiln process, the dominant recycling technology, produces a zinc oxide product that typically contains 0.5-2% lead. This is acceptable for the zinc smelting industry, which can further refine the material. However, tighter lead specifications in some markets may require additional purification steps that add cost.
The Waelz kiln process is the workhorse of EAF dust recycling, treating approximately 80% of all recycled dust globally. The kiln is a rotating cylindrical furnace, typically 40-60 meters long and 3-4 meters in diameter, inclined at 2-3 degrees from horizontal. EAF dust is mixed with a carbonaceous reductant — usually coke breeze or coal — and fed into the kiln. As the charge moves through the kiln by rotation and gravity, temperatures reach 1100-1300°C.
At these temperatures, zinc oxide in the dust is reduced by carbon to metallic zinc vapor: ZnO + C → Zn(g) + CO. The zinc vapor exits the kiln with the off-gas and is re-oxidized to zinc oxide by air introduced above the charge: 2 Zn(g) + O2 → 2 ZnO. The zinc oxide is collected in a baghouse downstream of the kiln. The product, called Waelz oxide, typically contains 55-65% zinc and is sold to zinc smelters as a secondary feed material.
The non-volatile residue from the Waelz kiln — the slag — contains iron oxide, lime, silica, and residual heavy metals. This slag was historically landfilled, but increasingly it is being used as aggregate in road construction or as a raw material in cement manufacturing. The slag must pass leaching tests (such as the TCLP test in the United States or EN 12457 in Europe) to qualify for these beneficial uses.
Alternative technologies to the Waelz kiln include the rotary hearth furnace (RHF), the multiple hearth furnace (MHF), and plasma-based processes. The RHF uses a flat rotating hearth instead of an inclined cylinder, which allows for shorter residence times and more precise temperature control. The MHF uses stacked hearths with rabbling arms to move material between levels. Plasma processes use an electric arc or plasma torch to achieve the high temperatures needed for zinc volatilization, with the advantage of using electricity instead of fossil fuel as the energy source.
Each technology has trade-offs. The Waelz kiln is the lowest capital cost option for high throughput (above 50,000 tonnes per year), but it has higher operating costs due to coke consumption. The RHF has better energy efficiency and lower emissions, but requires more uniform feed material. Plasma processes have the lowest environmental footprint but the highest electricity cost, which can be a benefit if low-carbon electricity is available and carbon pricing is in effect.
Zinc recovery rates in commercial Waelz kiln operations typically range from 90-95%. Recovery rates below 90% usually indicate a process problem — insufficient reducing conditions in the kiln, too short a residence time, or poor mixing of dust and reductant. Recovery rates above 95% are achievable with optimized kiln operation, but the marginal cost of the last few percentage points may not be justified by the value of the additional zinc recovered.
The iron-rich slag from the kiln is a material stream that deserves more attention than it typically receives. A Waelz kiln processing 100,000 tonnes of EAF dust per year produces about 60,000 tonnes of slag. This slag contains 30-40% iron, which could be recovered and returned to the steelmaking process. Several technologies for iron recovery from Waelz slag are under development, including magnetic separation and smelting reduction, but none has yet achieved widespread commercial adoption.
MONTE INTELLIGENCE supplies dust handling and conditioning systems for EAF operations, including pneumatic conveying, pelletizing, and storage. We also offer consulting on dust recycling strategy — whether to invest in on-site recycling, ship to a central Waelz kiln, or contract with a third-party processor.
Contact helenxu@cnlymonte.com for an assessment of your EAF dust management options.
