Blast furnaces

Various oxidic materials such as ashes, drosses, dusts and sludges, different copper-iron containing materials with an average copper content of 20 - 30 % as well as internal recycling slag from converter and anode furnace are processed in three blast furnaces using coke as fuel and reductant. Silica and limestone and iron are added for slag formation. Black copper and a slag which is poor in copper are produced. 150 - 180 tons of feed material are charged per furnace and day of operation. The raw materials are commuted or compacted if necessary. In 1996, the blast furnaces had a throughput of 148,000 t, of which 67,000 t was from secondary material. A schematic view of the blast furnace operation is given in Figure 3-8.

Water jackets

Return slags Dusts from boiler Blowing air pipe

with tuyers

Blast Furnace Settling Tower Boiler

O2

Inputs

Black copper Slag

Oxide to filter plant

Figure 3-8: Schematic view of a blast furnace at HK

Source: Nolte [54]

The black copper fraction with a copper content of 70 - 80 % is tapped into a holding furnace and then led to the converter for further processing. The blast furnace slag, which has a rather low copper content and a high density, is marketed in granulated form (15,000 t/a) or as lump slag (50,000 t/a) for construction purposes or as blasting media. In order to maintain the required mechanical properties of the slag, the chemical composition must be kept in a fixed range. Table 3-21 shows the typical chemical composition of blast furnace slag produced at HK.

Table 3-21: Chemical composition of blast furnace slag from HK

Substance FeO SiO₂ CaO Al₂O₃ MgO Na₂O Zn

Share [wt.-%]

32 - 38 20 - 25 14 - 17 10 3 2 4 - 8

Substance Cu Sn Pb Ni As Cd

Share [wt.-%]

0.7 - 1.2 0.5 - 1.0 0.2 - 0.4 0.1 < 0.1 < 0.1 Source: Meyer-Wulf/Nolte [52]

The heavy metals, which are still present in significant quantities, are hardly leachable under normal conditions, so that this slag can be used in similar ways to primary copper slag in many fields of application.

A further product from the blast furnace is zinc oxide with a zinc content of 60 - 70 %. Zinc evaporates at the existing temperatures in the blast furnace (about 1,250 °C in the lower part

of the shaft) and a part of it is carried out with the furnace process gas where it is converted to zinc oxide. Lead and tin are found in the flue dust as well. Depending on the tin content, the flue dust collected in the baghouse is further processed in the tin-lead alloy plant or, in the case of a low tin content, sold as raw material for external processing. Some further process parameters of the blast furnace are shown in Table 3-22.

Table 3-22: Process parameters of the blast furnace

Process data Typical value or range

Burden throughput [t/h] 5 - 20

Requirement of coke [kg/t burden] 100 - 120

Requirement of process air [Nm³/t burden] 5,000 - 12,000 Requirement of natural gas for support of post-combustion^*) [Nm³/t burden] 3 - 10 Intermediates and by-products

Black copper (further processed in converter [kg/t burden] 200 - 400 Slag (sold as construction material or for abrasive manufacturing) [kg/t burden] 400 - 600 Dust (processed in TLA plant) or directly sold as zinc oxide) [kg/t burden] 20 - 50

*) Natural gas necessary to support the throat gas afterburning in the case of a "cold feed".

To support the volatilisation and oxidation of the accompanying metals such as zinc, tin and lead, the furnaces are operated with top gas temperatures of about 1,000 °C and more. Figure 3-9 shows the off-gas treatment operated at HK for the three blast furnaces.

The air, which is needed for the oxidation of fugitive metals and furnace process gases (e.g.

carbon monoxide, low temperature carbonisation gases and the volatilised metals), is suctioned through the charging door. The off-gases pass a settling chamber which also serves as a post-combustion chamber. The heat content of the furnace process gas is used for producing steam in a boiler. The temperature of the off-gases is then about 300 - 400 °C.

Before the gases with a volume of 50,000 - 100,000 Nm³/h are dedusted by filtration, further cooling is necessary. This is achieved by leading the gases through a tubular cooler and by subsequent mixing with cold gas from the secondary hoods in a mixing chamber. The off-gases, which now have a temperature of about 120 °C, are dedusted in a bag house.

3

2 1 Ventilators

Filters

Mixing chamber Cooler

Blast furnaces

Secondary hoods

Hoods Settling chamber Waste heat boiler

Convective part Radiation part

Smoke flue Stack

Secondary hoods

Hoods

Figure 3-9: Off-gas treatment from blast furnaces at HK

Source: Meyer -Wulf [51]

In the past, the top gas temperature varied between 800 and 1,100 °C, and PCDD/PCDF emissions were between 2 and 30 ng/Nm³ [14]. Since 1990 additional oxygen has been injected into the top area of the blast furnace which led to a higher level of temperatures and a considerable reduction of PCDD/PCDF emissions because of the improved post-combustion¹⁴. Values lower than 0.5 ng/Nm³ have been achieved. The particulate matter content as well as its main constituents in the raw off-gases can be found in Table 3-23.

Measured and authorised emission values of the cleaned off-gases are given in Table 3-24.

Table 3-23: Particulate matter content and its main constituents in the raw off-gases from the blast furnace

Particulate matter

Total [mg/m³] 20,000 - 30,000

Pb [wt.-%] 10 - 40

Zn [wt.-%] 30 - 60

Sn [wt.-%] 1 - 5

14 A detailed description regarding the decrease of PCDD/PCDF emissions by oxygen injection can be found in Meyer-Wulf [51] and Bußmann [14].

Table 3-24: Concentrations of main constituents in the cleaned off-gases from the blast furnace

Substance Unit Measured Value Authorised value

SO_x (as SO₂) mg/m³ 50 - 150 800

NO_x (as NO₂) mg/m³ 30 - 100 500

PCDD/PCDF ng/ m³ < 0.5 < 0.5

Particulate matter^*) mg/m³ < 1 - 8 20

*) The main constituents of the particulate matter are given in Table 3-31.

Source: Emission values which are accessible to the authority

The measured values represent the typical range. However, the results of single measurements can be higher than the given range and reach the authorised values depending on different operation conditions (charging, blowing, pouring, etc.), different input materials and changes in the operating mode.