While foundries are typically used for the production of cast iron and aluminum as well as molds of complex shapes in bronze, brass and other metals, steelmaking remains the backbone for a variety of industries and is commonly associated with automobile and electric appliance manufacturing. To maintain acceptable working conditions, heavy metal particulate and fumes need to be exhausted using LEVs coupled with wet / dry collection systems.
Iron ore to steel
In steelmaking, the iron ore used as raw material is found in mines across the globe, usually as magnetite (Fe3O4, 72% Fe) or hematite (Fe2O3, 70% Fe). Indeed, 98% of the mined iron ore is used to make steel. These iron oxides are reduced with carbon, such as coal or metallurgical coke, and lime (calcium oxide). Once melted, the silicates in the ore react with the lime to form a mixture or slag which floats on the molten iron. The iron produced contains impurities and is brittle, thus the need to remove the impurities (as slag or gas) by oxidation; which is usually accomplished by the direct addition of oxygen with fixed / manual lances.
Steel alloys - Oxidation - Corrosion
Alloying elements may be added with different heat treatment temperatures to produce steels used as structural components in high-rise buildings and as sheet steel for motor vehicles and domestic appliances. Its tendency to slowly return to its most stable condition or natural state, that is oxidize under moist atmospheric conditions to form rust, is the main drawback with its usage unless protected against the weather with suitable corrosion resistant paint / coating.
Steelmaking dust and sludge - Heavy metal contaminants
It comes to no surprise that steelmaking dust and sludge from blast furnaces (BF), oxygen converters (BOF) and electric arc furnaces (EAF) exhibit a high percentage of iron oxides. Due to the presence of heavy metals, such as zinc, lead as well as alkaline oxides, halides and heavy hydrocarbons, this complicates recycling and waste treatment of the finer fractions.
Heavy metal particulate and fumes - Hazards
The finer fractions with high concentrations of iron oxide and carbon (BF dust) contain hazardous volatiles – such as zinc and lead-based compounds, chlorides, fluorides, high hydrocarbons and cyanides – which preclude their usage as potential landfill. Unless you chemically attack and/or heat treat the waste effluent to isolate and possibly accelerate the decomposition of these hazardous compounds, they cannot be recycled back to the processes (sintering plant, BF or BOF). The minor constituents comprise both relatively neutral compounds (slag formers, calcium, magnesium and alkaline oxides as well as manganese oxide) and hazardous products: their composition is dependent on primary (iron concentrate) and secondary (scrap) raw materials.
Dust collector design
Steelmaking plants and foundries utilize baghouses for several applications. The coarse fractions are collected in cyclones and the finer fractions in wet scrubbers or electrostatic precipitators. Pulse-jet, shaker, and horizontal cartridge baghouses are commonplace for dry applications. We recommend working with a filtration supplier to design dust collection equipment that fits your needs.
Dust collectors adapted for individual processes
Here are some areas where Airex has supplied fume and dust collection equipment:
- Electric arc furnaces (EAF)
- Induction furnaces
- Mold cooling lines
- Shot blast/Grinding
- Sand shakeout/ Sand reclaim
A Case Study: Foundry baghouse hopper cleanliness
A recurring problem
Hopper cleanliness is a recurring problem for foundry baghouse applications. Foundry waste effluent will agglomerate and adhere to hopper sidewalls, then bridge across the exit chute, and interrupt material flow. Hoppers will fill with dust and gradually force dust back upstream onto filter bags. This will create high differential pressures with dust penetration into filter media which may generate atypical emissions during cleaning cycles.
A quick solution to this problem is the installation of a sonic blaster, mounted in the hopper: the sonic vibrations will break down the agglomerations and allow material to flow freely through the exit chute.