Blast Furnace Process: Reaction Equation & Explanation |

Using the blast furnace process, it is possible to produce steel. Pig iron is produced here from various mined iron ores. This happens via many individual redox reactions in which carbon acts as a reducing agent in the form of carbon monoxide or coke. Although blast furnaces emit gigantic amounts of carbon dioxide as reaction products, they are an integral part of our lives.

The construction of the blast furnace

The blast furnace has a typical shape that is usually easy to recognize. Together with all superstructures, it is about 100 m high. Here you can see all the important components including the temperatures that prevail inside.

Figure 1: The structure of a blast furnace Source: seilnacht.com

The blast furnace is filled by a dog (also known as a hunt). This feeds a mixture of iron ore and lime into the blast furnace. The iron ore is then turned into the desired pig iron thanks to the heat. This is done via a reduction. Carbon monoxide, which is produced by burning coke, acts as a reducing agent. Overall, the reactions in the blast furnace are redox reactions.

Coke is a fuel that is very rich in carbon. It is made from low-sulphur, low-ash carbon. Pyrolysis, i.e. the supply of heat in the absence of oxygen, is important for this process.

The ore-lime mixture and the coke enter the blast furnace in layers. Two so-called gout bells serve as a closure. This prevents gas escaping from the oven or cold air getting inside.

In the lower area, on the other hand, hot air is fed into the oven so that combustion can take place. This produces pig iron, which is removed via a line, as well as slag. This contains the remaining components of the lime, ore and coke, which are also removed. Thanks to the difference in density, they are deposited in two layers that are easily separated from each other.

The blast furnace is cooled by cooling water in the jacket. Refractory brick also prevents the blast furnace from melting even at the prevailing temperatures.

The blast furnace process simply explained

As mentioned earlier, the Blast Furnace aims to produce pig iron from iron ore, which can then be used to make steel.

Figure 2: Processes in the blast furnace Source: ruby.chemie.uni-freiburg.de

In the figure you can see the simplified structure of a blast furnace. In this case, the respective reactions are in the appropriate places.

However, to ensure that this continues to take place, various countercurrents are important for the blast furnace. A distinction is made between material and thermal counterflow.

Material countercurrent

The material countercurrent refers to the fact that ores, coke and other additives such as lime are fed into the blast furnace from above, while gaseous oxygen is fed in from below. As the gas rises, the solids fall, causing mixing.

Thermal countercurrent

At the same time, the thermal countercurrent takes place, in which all the substances involved at the corresponding level are brought to the respective temperatures. The increase in temperature is based on the strongly exothermic reaction of atmospheric oxygen and coke to form carbon dioxide. This results in the thermal energy:

This is shown in reaction 1 in Figure 2. It should be noted that different energies are specified here. The reaction itself is also slightly different. In fact, both reactions are possible. That depends on how many carbon atoms are involved in the respective reaction. It is important for you that both correct are. For the reaction that is written here, a second step is necessary. However, the goal remains the same: carbon monoxide.

The resulting temperatures melt slag and iron. The latter can then be reduced. You can find this in the figure as reactions 2 and 3. With our example, however, carbon monoxide must still be created from the carbon dioxide with the help of coke:

It is important here that a lot of carbon monoxide is produced so that it can act as a reducing agent for iron.

Figure 3: Blast furnace in Dortmund Source: wikipedia.org

Reactions 5 and 6 describe the reduction of various iron ores:

Here, too, the energies released differ due to the carbon monoxide molecules involved and the original composition of the iron ore. Furthermore, there is the possibility that even carbon itself can act as a reducing agent at the layer boundary between coke and ores:

As you can see, both reactions produce the important pig iron, which is deposited and removed as a liquid due to the temperatures. The slag floats on top due to the difference in density. This also prevents the pig iron from being oxidized again by the air supplied. The individual gases that are produced in addition to carbon dioxide and monoxide are collected via the throat at the top end and used to generate thermal energy. This prevents, among other things, too many harmful gases from escaping to the outside.

The importance of the blast furnace

About 13,000 tons of pig iron can be produced using this method today. Up to 97% of the pig iron produced is used to make steel. This is mainly what happens oxygen inflation process for use. This extracts the remaining carbon from the pig iron and supplies oxygen. This creates an alloy that can be forged. This is then the steel that is used for the construction of buildings or the like.

Gray pig iron can be made from the pig iron that is not processed into steel. This is cast or foundry iron. Due to its higher carbon content, it has different properties than steel and cannot be forged or rolled. Often it is also significantly more porous. There are three forms:

  • cast iron: This form is mainly used for housings, tubes, cylinders and stands for machine tools and power tools.
  • chilled cast iron: Rollers and excavator shovels are made from this pig iron.
  • malleable iron: Used to craft Raw Connectors, Wrenches, Lock Parts, and Sewing Machine Parts. This shape is also used in vehicle construction.

The blast furnace process and the environment

The blast furnace process is essential for industry because steel is required for numerous construction works. Nevertheless, it is also important to have an eye on the environment.

In 2019, the steel industry alone caused around 7% of global carbon dioxide emissions. Much of it comes directly from the blast furnace. In many places, people have already switched from coke to heavy oil, which, however, has to be purchased at great expense.

A climate-neutral alternative would be to replace coke and heavy oil with hydrogen. However, the processes would change in such a way that the iron could then no longer be removed in liquid form. The temperatures are not sufficient for this. The resulting iron would first have to be processed in further furnaces, in which it is melted and refined. There are currently numerous pilot projects working on implementing this concept.

However, it will be some time before such direct reduction systems are used across the board.

Blast Furnace Process – The Most Important

  • Blast furnaces are used to produce pig iron. Iron ores are burned together with coke and other substances such as lime.
  • Iron production describes a redox reaction in which iron is reduced.

  • The blast furnace works on the basis of a material and thermal countercurrent.

  • Carbon monoxide acts as a reducing agent.

  • Pig iron and slag are produced, each of which is removed from the blast furnace in liquid form.

  • 97% of pig iron is used in the steel industry. The rest becomes cast or foundry iron, which is used for pipe making and engineering itself. A distinction is made between the three forms of grey, hard and malleable cast iron.

  • 7% of global carbon dioxide emissions come from the steel industry and associated blast furnaces.

  • A possible alternative is hydrogen for use in blast furnaces. The general goal remains the complete replacement of the blast furnaces by direct reduction plants.