Ironmaking 241 Process Description

In the blast furnace iron-making process, iron ore is reduced by removing oxygen, followed by melting of the resulting iron. Agglomeration processes such as pelletization and sintering help in producing coarse particles of suitable sized iron ore for easy charging into the blast furnace. In pelletization, an unbaked "green" pellet is formed from iron ore concentrate combined with a binder. These green pellets are hardened by heat treatment in an oxidizing furnace. Pelletizing is usually done at the mine site. Sintering is a crucial process in the steel mill, using natural fine iron-bearing materials as well as those recovered from ore handling and other iron and steel operations, for example, ore fines from screening operations, water treatment plant sludges, and air pollution control dusts, and fusing them into porous sinters suitable for charging to blast furnaces. In sintering, both iron ore fines and other iron-bearing materials (iron-bearing scale, dusts, and slag) are thoroughly mixed with fluxes (lime or dolomite) and approximately 5% of a finely divided fuel, such as coke breeze or anthracite.20

The mix is loaded onto a traveling grate called sinter strand, which is in the form of a shallow trough with small holes in the bottom. The bed of materials on the grate is ignited by passing under an ignition burner fired with natural gas and air. As the grate moves slowly towards the discharge end, windboxes on the underside of the strand pull down the combustion gases through the material bed into a duct to gas cleaning equipment. As the coke fines burn in the bed, the generated heat sinters the fine particles. The temperature of the bed reaches around 1300 to 1480°C. Average production rates of 22 to 43 metric tons/m2/d of grate area are expected, depending upon the characteristics of the ore materials and the sintering conditions.13

The fused sinter mass is cooled, crushed, screened, and sent to be charged, along with the ore, to the blast furnace. Approximately 2.5 t of raw materials, including water and fuel, are required to

TABLE 2.3

Constituents of Concern and Average Measured Concentrations for Wastes K141 through K145a

TABLE 2.3

Constituents of Concern and Average Measured Concentrations for Wastes K141 through K145a

Constituent

K141 (Process Residues from Coal Tar Recovery)

K142 (Tar Storage Tank Residues)

K143 (Residues from Light Oil Processing)

K144 (Wastewater Treatment Sludges from Light Oil Refining)

K145 (Residues from Naphthalene Collection and Recovery)

Benzene

3850

260

1600

3000

1000

Benz (a) anthracene

7850

6600

69

68

22

Benzo (a) pyrene

8450

6500

34

65

7

Benzo (b) fluoranthene

5450

7500

59

75

26

Chrysene

7950

6000

59

6

22

Dibenz (a,h) anthracene

1750

1000

38

15

1

Indeno (1,2,3-cd) pyrene

6140

2900

40

37

4

Naphthalene

95,000

55,000

52,000

27,000

140,000

Source: From U.S. EPA, Federal Register, Part III, U.S. EPA, Washington, DC, August 18, 1992. a Concentrations measured in mg/kg.

Source: From U.S. EPA, Federal Register, Part III, U.S. EPA, Washington, DC, August 18, 1992. a Concentrations measured in mg/kg.

APC dust Wastewater APC dust i

Particulate

Coke, fines, flux, slag, scale, sludge

4

Sinter

strand

Air, fuel

Fines

Crushing Hot screening

Fines

Crushing Hot screening

Fines

Particulate

Cooling and cold screening +

Fines

FIGURE 2.3 Sintering flow diagram. (From Energetics, Inc., Energy and Environmental Profile of the U.S. Iron and Steel Industry, DOE/EE-0229, U.S. Department of Energy, Washington, DC, 2000.)

produce 1 t of sinter product.16 Figures 2.3 and 2.4 illustrate the sintering and blast furnace iron-making processes, respectively. Blast furnaces are used to produce pig iron, which represents about three-quarters of the charge to basic oxygen steel-making furnaces. The chemical composition of pig iron typically comprises the following1:

Coal

Cokemaking

Coke oooooo

Byproduct ovens

Coke byproducts recovery

-► Coke byproducts

-► Coke byproducts

Dust

Slag w

Byproduct Waste residuals sludges

Dust

Slag

FIGURE 2.4 Flow diagram of the iron-making operation. (From U.S. EPA, Profile of the Iron and Steel Industry, EPA 310-R-95-005, U.S. EPA, Washington, DC, 1995.)

5. Sulfur before desulfurization (0.03 to 0.8%)

Iron ore, coke, flux (limestone and dolomite), and sinter are fed into the top of the blast furnace; heated air augmented with gaseous, liquid, or powdered fuel is injected into its base. As the charge materials descend, the reducing gas (containing carbon monoxide) generated by the burning coke flows upward, converting the iron oxide (FeO) in the ore to iron (Fe). The coke also provides the structural support for the unmelted burden materials. The combustion of the coke generates sufficient heat to melt the iron, which accumulates in the bottom of the furnace (hearth). The major function of the flux is to combine with unwanted impurities, that is, ash in the coke and gangue in the ores, to make a drainable fluid slag. Unreacted reducing gas (blast furnace gas) is collected at the top, cleaned, and used as a fuel. The molten iron called the "hot metal" is tapped into refractory-lined cars for transport to the basic oxygen furnace. The iron may be processed at desulfurization stations to minimize sulfur compounds before charging in the basic oxygen furnace. Molten slag, which floats on top of the molten iron, is also tapped and processed for sale as a byproduct. The production of one net ton of iron requires approximately 1.5 to 1.7 t of ore or other iron-bearing material, 0.35 to 0.55 t of coke, 0.25 t of limestone or dolomite, and 1.6 to 2.0 t of air.116 The inputs and outputs of the iron-making operation are shown in Table 2.4, with a brief overview of key environmental and energy facts of sintering and iron making in Table 2.5 and Table 2.6, respectively.

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