Blast Furnace: Definition, Construction or Parts, Working Principles, Applications Advantages [Notes with PDF]

Blast furnaces were first introduced in China around the 1st century AD. They were further introduced in European countries like Durstel, Germany, Lappyhttan in the 13th century.

Due to an increase in demand for iron, it gained popularity in France in the mid 15th century and was built in Wallonia, Normandy, and then introduced to England in 1491.

Initially, charcoal was used as a fuel, but in 1709 Abraham Darby discovered that charcoal could be replaced by coke to improve efficiency.

In 1828, James Beaumont Nelson introduced an idea to improve the efficiency even more by replacing cold air blast with hot air blast for extraction of iron.

In this article, we will study the Definition, Construction or Parts, Working principles, Applications Advantages, and Disadvantages of Blast Furnace in detail.

Let’s start with the definition first,

What is a Blast furnace? 

A furnace is a device or a structure that is used to heat different elements, generally metals. A blast furnace is a device or a structure that uses a blast of hot air enriched with oxygen to provide high temperatures.

These high temperatures are used for the reduction of iron ore into pig iron. Iron is not available in its pure form in nature, it is available in the form of ore which contains various impurities. These impurities are collectively known as gangue.

Iron is available in the form of iron oxides in the ore. A blast furnace is used to extract pure iron from the iron ore. A blast furnace is a large metallic structure that is about 30 meters tall and 10 meters in diameter.

It is tapered from the top and bottom with its maximum diameter below its geometric center. The pig iron produced in the process can be used for the production of different forms of steel by mixing impurities 

Now we will look at construction,

Construction or Parts of a Blast Furnace: 

The Blast Furnace consists of various parts as follows:

  • Stainless steel cylinder
  • Refractory lining
  • Blast pipe
  • Tuyere
  • Bed or base
  • Feeder
  • Exhaust system and more

All these parts we will understand one by one in detail. So let’s start,

Blast Furnace
1. Hot blast from Cowper stoves, 2. Melting zone (bosh), 3. Reduction zone of ferrous oxide (barrel) 4. Reduction zone of ferric oxide (stack), 5. Preheating zone (throat), 6. A feed of ore, limestone, and coke, 7. Exhaust gases, 8. Column of ore, coke, and limestone, 9. Removal of slag, 10. Tapping of molten pig iron, 11. Collection of waste gases.

Stainless steel cylinder:

The main part of a blast furnace is the stainless steel cylinder which is tapered from the top and has an increasing cross-section as the height descends. All the reactions are carried out in the stainless steel cylinder.

It acts as the main supporting element to the whole structure. It also acts as a connector between various other components. 

Refractory lining:

Refractory linings are a layer of insulators generally 5-6 inches thick. The main function of these is that there must be minimum heat transfer from the surroundings to the blast furnace and vice versa.

Refractory linings are made of materials such as magnesia chrome, alumina, silica, and carbon. This also acts as a safety system that supports the hot stainless steel cylinder. In modern blast furnaces, water-cooled steel is used instead of refractory lining. 

Blast pipe:

Blast pipe is the region or inlet for the blast of hot air to enter the furnace. Oxygen enriched hot stream of air enters into the furnace through the blast pipe. The temperature inside the furnace, around the blast pipe is maximum. 

Tuyere:

A tuyere is a nozzle-like structure that is connected to the end of the blast pipe. It is used for the transfer of blast air from the pipe to the blast furnace.

The tuyere is also used for mixing certain elements with the blast air for different reactions to take place. Tuyeres are generally water-cooled as the temperature around them is extremely high which may melt the nozzle and prevent the inflow of hot air. 

Bed or base:

The bed or base of the blast furnace is the lowermost part of the furnace which is used for the removal of molten metal through an outlet.

Besides this bed also provides structural support to the whole equipment and provides rigidity. It is made of refractory materials as well to provide an isolated environment for the molten metal. 

Feeder:

A feeder is a region from where the raw materials or ores are poured into the blast furnace. It may consist of 2 or 3 bells-like structures with increasing cross-sections, which are used to distribute the powdered ore evenly.

It is designed in such a way that the powered ore must fall down from the circumference of the bell. 

Exhaust system:

Exhaust systems are chimney-like structures used for the emission of exhaust gases produced during the reactions in the blast furnace. These systems carry the hot gases for the production of hot blast air. Hence a cycle is repeated. 

Now also look at the construction side of Blast Furnace in brief,

The construction of a blast furnace is divided into 3 parts. Starting from the top the 3 parts are Top, Stack, and Hearth.

The top consists of the feeder mechanism for the entry of ores. It also consists of exhaust systems for the emissions of gases produced in the blast furnace after the reactions take place. 

The stack is the major part of the Blast furnace. Stack contributes more than half of the blast furnace. It comprises the stainless steel cylinder and the refractory linings. All the reactions take place in the stack itself. There are various zones according to the variation of temperature in the stack. 

The hearth is the lowest part of the Blast furnace; it is situated over the bed and carries all the products after combustion including molten iron and slag. There are outlets and inlets present in the Hearth for removal of molten metal and inflow of the hot blast air. 

Blast Furnace Working Principle:

The basic working principle of a blast furnace is the change in the chemical affinity between 3 different elements. For example, there are three different elements namely A, B, and C. If ‘AB’ is a compound and the affinity of C with B is greater than the affinity of A With B. And if we react AB with C then, C having greater affinity will replace B to form CB. In the case of blast furnaces, iron oxides are reacted with carbon. Oxygen having a greater affinity with carbon forms carbon monoxide or carbon dioxide and releases free iron. Hence the reaction becomes a reduction reaction. 

NOTE: Affinity of any element refers to the tendency of that element to make bonds with any other dissimilar element.

Smelting of iron is not possible when the ore is in its natural form. Therefore it must be treated before feeding into the blast furnace. Iron ore is treated in two stages before entering the blast furnace. The two-stage treatment of iron ore are. 

Note: Smelting refers to the operation of extracting iron from its ore. 

Concentration or dressing:

Iron ore extracted from the earth’s crust through mining contains various impurities. These impurities must be removed before the ore enters the Blast furnace. The process of removing the impurities from the ore is known as the concentration of ore.

In this process, the ore is first crushed into small pieces and then is again crushed to form a powder. The powdered ore is then washed in a water current in which the lighter impurities are removed and the heavier ore is settled down. 

Calcination or roasting:

Concentrated ore then undergoes a process called Calcination in which it is heated in the presence of air to remove moisture and other organic impurities. Calcination also helps in converting non-oxide components into oxides.

Compounds such as carbon and sulfur are converted to their volatile oxides which escape from the ore. It also helps in Oxidizing ferrous oxide into ferric oxide. 

Iron ore is added with a mixture of coke(around 5% of the mixture) and lime(around 20% of the mixture). Coke is the main source of carbon (reducing agent) for producing carbon monoxide and also acts as a fuel.

On the other hand, lime acts as flux ( a compound which mixes with the impurities to form slag). If iron ore is added alone it would not get reduced and there will be no slag formation as well. 

Now here are the following steps for working of Blast Furnace:

The working of a blast furnace is divided into four stages on the basis of different temperature zones. The temperature at the bottom region is maximum and the temperature drops as the height increases. For ease of understanding, we randomly discuss various zones. 

The mixture of lime and iron ore is fed through the large bell and powdered coke is fed through the small bell. The zone and reactions taking place in a blast furnace are as follows. 

#1. Zone of combustion:

The zone of combustion is the lowermost zone of the blast furnace. It takes place near the blast pipe. It is nearly 5-10m above the base of the blast furnace. The temperature here is about 2000K.

In the zone, the carbon reacts with hot oxygen from the blast pipe to produce carbon monoxide by releasing a considerable amount of heat that is about 220kJ. The carbon monoxide so produced in the combustion zone again decomposes into carbon and oxygen thus repeating the cycle and maintaining the temperature. 

Reaction:

C + 1/2O2  ~~~~> CO (at 2000K) 
2CO ~~~~> O2 + 2C

#2. Zone of reduction:

Zone of reduction levels to 22-25m from the bottom and this zone is mainly responsible for the formation of iron from its ore. The temperature here varies a lot (900K – 1500K).

Here ferric oxide Fe2O3 reacts with carbon monoxide and gets converted to Iron. The carbon produced in the zone of combustion also helps in reducing the ferric oxide to iron and produces carbon monoxide which is again used for the reduction of more ferric oxide from the top.

As discussed in the working principle, carbon makes bonds with oxygen, and iron is released. 

Reactions:

Fe2O3  + 3CO ~~~~> 2Fe + 3CO2  (900K)
Fe2O3 + 3C ~~~~> 2Fe + 3CO (1500K)

#3. Zone of slag formation:

Zone of slag formation takes place at a height of 20m from the base of the blast furnace. The temperature here is about 12000K – 1500K.

In this region, the flux that is limestone reacts with the impurities such as silicon dioxide and aluminum oxide to form Their respective calcium compounds known as slag. The slag having less density is allowed to float over the molten iron having a higher density. 

Reactions:

CaCO3 ~~~~> CaO + CO2 (1200K) 
CaO + SiO2 ~~~~> CaSiO3 (1500K)
12CaO + 2Al2O3 ~~~~> 4Ca3AlO3 + 3O2 (1500K)

#4. Zone of fusion:

A zone of fusion exists just below the zone of slag formation. The temperature here is more than 1800K. Which is more than the Melting point of iron. Hence in this zone of fusion t, the solid iron ore is converted to liquid iron. 

Reaction:

Fe(s)  ~~~~> Fe(l) 
  • The exhaust gases are released from the chimneys situated on the top of the blast furnace 
  • Slag is also removed as it floats on the molten iron. 
  • Molten iron or pig iron is removed from the outlet in the form of ingots. 

Note: When the molten iron is solidified in the form of bricks it is called iron ingots. 

Pig iron is the name given to the iron coming out of the blast furnace because, when the molten iron flows through the channel into the ingots it looks like piglets sucking their sow.

Here is a simple layout of Blast Furnace try to understand the image,

Blast Furnace Layout
1. Iron ore + limestone sinter, 2. Coke, 3. Elevator, 4. Feedstock inlet, 5. A layer of coke, 6. A layer of sinter pellets of ore and limestone, 7. Hot blast (around 1200 °C), 8. Removal of slag, 9. Tapping of molten pig iron, 10. Slag pot, 11. Torpedo car for pig iron, 12. Dust cyclone for separation of solid particle, 13.Cowper stoves for the hot blast, 14.Smokestack, 15. Feed air for Cowper stoves (air pre-heaters), 16. Powdered coal, 17. Coke oven, 18.Coke, and 19. Blast furnace gas downcomer

Blast Furnace Advantages:

The following advantages of Blast Furnace are following:

  • Cheap method for producing iron and steel thereby requiring a low cost to set it up.
  • Capable of converting huge amounts of iron into molten iron in a short duration of time. 
  • The process is continuous, so the production rate is constant. 
  • Heat is also produced by the burning of coke. Hence the temperature drops are overcome. 
  • Large-scale production. 
  • Efficiency is high. 

Blast Furnace Disadvantages:

The following disadvantages of Blast Furnace are following:

  • Coke is used as a fuel in blast furnaces and since it’s a non renewable source of energy, it’s not easily available which leads to increase in price of coal and iron extraction.
  • It is not environmental friendly because the exhaust produced, pollutes the air
  • The process takes a lot of time
  • Blast furnace process requires a lot of energy consumption.
  • The process cannot be shut down. 

Blast Furnace Application:

The main application of the blast furnace is to smelt the iron from its ore and The pig iron formed during the process is used for the production of various grades of steel. 

The slag formed in the blast furnace can be used as raw material for the production of cement.

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Conclusion:

So today we have learned about the Working Principle, Construction, Diagram, Advantages, Disadvantages, and Applications of Blast Furnace. If you like this article, don’t forget to share it on social networks.

If you have any queries or doubts about the Blast Furnace, you can ask me in the comment section. I will love to hear from you and am glad to help you. Till then enjoy rest your day. Cheers

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Amrit Kumar

Amrit Kumar is the founder of Learn Mechanical, an Advisor at The Mechanical Engineering- a content-based website in Mechanical Engineering based in Delhi. He has 5+ years of teaching experience in the Core Mechanical Field.

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