Kompendium Inhalt

 

The blast furnace is a continuously operating shaft furnace based on the counter flow principle. At the top coke and burden (sinter, pellets, lump ore and flux) are charged in different layers. Charge materials descend under the influence of gravity. In the lower part of the furnace hot blast that is produced in hot stoves is injected through tuyeres. In front of each tuyere the hot blast reacts with the coke. Carbon monoxide is formed, ascends in the furnace and reduces the iron ores. At the bottom of the hearth the molten metal is collected. Besides hot metal a slag is formed that swims due to its lower density on the hot metal bath. Liquid hot metal (1500°C) and slag (1550°C) are tapped regularly. The products of the blast furnace process are pig iron, slag, top gas and flue dust.

The blast furnace process can be divided into different temperature zones. Corresponding to each temperature interval typical reactions will take place. In the tuyere region the highest temperature can be observed. Due to the exothermic coke combustion the temperature rises up to 2200-2400°C and carbon dioxide is formed (1). Following solution loss reaction carbon dioxide reacts to carbon monoxide (2):

C + O2
C + CO2
-> CO2
-> 2 CO
- DH
+DH
(1)
(2)
At temperatures higher than approximately 1000°C the so called direct reduction takes place:
FeO + CO
C + CO2
FeO + C
-> Fe + CO2
-> 2 CO
-> Fe + C
(3)
(2)
(4)
Due to the solution loss reaction the direct reduction is endothermic. At temperatures below approximately 1000°C the solution loss reaction cannot take place and the iron oxides are reduced following equation (3). The so called indirect reduction is exothermic. Therefore the region of exothermic indirect reduction should be large compare with the region of endothermic direct reduction to reach low coke consumption. The temperature zone 900-1350°C is the region of the cohesive zone. In this region first liquids appear. Primary slag that contains high amount of wustite and molten iron drop drop downwards through the coke bed. During this process wustite is reduced to metallic iron and a carburization takes place. The carburization leads to a decreasing melting point and makes a tapping at lower temperatures possible. At temperature below 900°C the iron oxides hematite and magnetite are reduced to wustite.

The top gas leaves the furnace with a temperature 150-250°C. Because of the high pressure and calorific value the top gas can be used for power stations. After the gas is purified the blast furnace flue dust is recycled via the sinter plant. The recycling of sludge is difficult so that in general it is deposited. The cleaned top gas is than used for a regenerating turbine. In addition a combined cycle process can be installed. Another possibility is to use the cleaned top gas as fuel for the cowpers or for the coking plant or to recycle after heating and removal of CO2 in the blast furnace.

Due to ecological and economical reasons the conventional blast process has changed within the last decades. A better burden distribution, an increase in sinter and pellets quality and their rate in iron bearing burden up to 85-100%, back pressure at the furnace top, hot blast temperature of 1200-1300°C resulted in a rise of furnace productivity and drop in coke rate. Injection of auxiliary fuels as oil, natural gas and fine coal was started in order to further decrease the coke rate. The blast furnace process with pulverised coal injection rate of 180-200 kg/tHM and reduction of the coke rate from 500 to 300-320 kg/tHM is realised. The reduced coke rate leads to a decrease in the carbon dioxide emission. Injection of plastic, iron containing filter dust and other waste has technological, economical and environmental advantages.