Treatment Techniques

The hot BOF gases are typically treated by one of three air pollution control methods:

1. Semiwet. Water is added for conditioning furnace off-gas temperature and humidity prior to processing the gas in electrostatic precipitators or baghouses.

2. Wet—open combustion. Excess air is admitted to the off-gas collection system, allowing combustion of carbon monoxide prior to high-energy wet scrubbing for air pollution control.

3. Wet—suppressed combustion. Excess air is not admitted to the off-gas collection system prior to high-energy wet scrubbing for air pollution control, thus suppressing combustion of carbon monoxide.15

Charging and tapping emissions are controlled by a variety of evacuation systems and operating practices. Charging hoods, tapside enclosures, and full furnace enclosures are used in the industry to capture these emissions and send them to either the primary hood gas cleaner or a second gas cleaner.15,16 Pollution prevention opportunities for the reduction of heavy metals at the BOF are limited as heavy metals are an inherent part of the iron ore material stream, so the higher the iron production, the greater will be the use of the ore.

The cleaning of BOF gas is done by quenching the mixture of gas and particulate with water in the collection main to reduce the temperature. This quenching process removes the larger particles from the gas stream and entrains them in the water system. After settling in the classifier, these coarse solids can be easily dewatered via a long sloping screw conveyor or reciprocating rake and deposited in bins or hoppers. These solids are referred to as classifier sludge. The fine particulate matter remaining in the gas stream is forced through venturi scrubbers, where it is entrained in a wastewater stream and sent to thickener/flocculation tanks for settling and solids removal. This underflow slurry can be dewatered using mechanical filtration. In dry cleaning systems, the particu-late matter collected in the electrostatic precipitator or baghouse is managed as a dust.1

The rising cost of scrap and waste disposal, the scarcity of onsite landfill space, and potential environmental liabilities make it an economic necessity to recover iron units from dust and sludge. However, recycling to the blast furnace raises the hot metal phosphorus content to undesirable levels.

Also, the increasing use of galvanized scrap could increase dust and sludge zinc content. Zinc is known to form a circuit in the furnace, resulting in extra coke consumption and also increasing the risk of scaffolding.7 The quantity of zinc that can be charged to the blast furnace lies between 0.2 and 0.9 lb/t of hot metal.1 BOF dust and sludge that is not recycled is usually landfilled.

2.5.2 Electric Arc Furnace

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