In this explainer, we will learn how to describe the extraction of iron from its ore in the blast furnace.
Extraction of iron is arguably one of the most important industrial processes on Earth. Iron is used to make steel, which is incredibly important for constructing transport vehicles and large buildings. Modern society would not be able to function properly if we could not extract iron metal from iron-containing ores.
Steel plants tend to contain at least one blast furnace that can produce molten iron from unrefined iron-containing ores, such as hematite. These plants generate molten iron and then convert this iron metal into steel alloy products through the basic oxygen steelmaking process. The following image shows how a steelmaking plant can produce a steel alloy product from raw materials like iron-containing ores and coal rocks.
Steel plants usually have excellent transport links because they continuously consume large amounts of fuel and raw materials. These plants are usually located near coasts, roads, and railway lines. They also tend to be located far away from built-up cities because they can produce a lot of noise and pollution.
Ironmaking and steelmaking plants are very complex systems that are made up of multiple different types of large machines and buildings. The following figure depicts some of the most important parts of most metal-manufacturing plants.
The image on the far left shows the building that houses the coking ovens. Coke is produced in the coking ovens and is then used with limestone to extract iron metal from iron-containing ores in the blast furnace.
Coke is a form of almost-pure carbon that is produced from coal rocks. It is produced by heating coal rocks to a high temperature in a coking oven. These coal rocks are usually heated to a temperature of –, and sometimes they can be heated to temperatures as high as . This process vaporizes or decomposes undesirable organic substances in the coal and drives off volatile compounds, such as water. The residue (nonvolatile) of this decomposition process is mostly made up of carbon atoms.
Example 1: Identifying the Composition of Blast Furnace Coke
Which of the following substances is the coke that takes part in essential reactions inside a blast furnace?
- Waste gas
- Iron ore
This question lists many different substances involved in the extraction of iron. The extraction of iron occurs inside a blast furnace, a large 40 m tall steel tower. The waste gases produced from this process are carbon dioxide and nitrogen, neither of which is known as coke.
Limestone is part of the charge used when extracting iron from its ore. Limestone is primarily composed of a calcium carbonate compound and is not referred to as coke. When calcium carbonate decomposes as a result of the high heat inside the furnace, it reacts with sandy impurities to form what is known as slag.
Carbon in the form of coal is delivered to steelmaking plants, and before it is mixed with limestone and the iron ore, known as hematite, to form the charge, it must be heated to a high temperature in an oven. This removes some of the impurities from the coal and forms a substance known as coke. As such, the correct answer is D.
Iron metal is usually extracted from a hematite ore, which contains an iron(III) oxide () compound. The ore must be treated before it can be placed in a blast furnace, because blast furnaces are usually optimized to burn small pieces of ore that are compacted together. Hematite is treated through mineral processing or ore-dressing processes.
The first step is commonly crushing, in which large pieces of ore are reduced to a smaller size. The following illustration shows how a jaw crusher can be used to break large rocks into smaller ones.
Different processes are then used to produce a more highly concentrated form of iron metal from the small pieces of iron-containing ore. The concentration of iron metal usually increases as impurities are removed through a variety of different magnetic and electric separation techniques.
Chemical impurities are removed during ore dressing through a process known as roasting. Roasting describes the process in which metal-containing ores are heated to a high temperature in an aerated room or oven. This process can drive water molecules off a hydrated oxide complex and can transform undesirable chemicals into more desirable chemical products. The following equation shows how roasting process can transform an undesirable iron carbonate () compound into a more useful iron(II) oxide () product:
The next chemical equation shows how roasting process can transform useful iron oxide molecules into even more desirable iron(III) oxide compounds:
The next chemical equation shows how the roasting process can also be used to drive water molecules off a hydrated iron(III) oxide complex:
Other impurities, such as sulfur and phosphorus elements, are oxidized during the roasting process. The following equation shows how sulfur can be oxidized to sulfur dioxide during roasting:
The next equation shows how phosphorus can be oxidized to a phosphorus pentoxide compound when it is roasted:
Once impurities are removed from the ore, the treated iron ore is mixed with limestone and coke in sintering beds. These sintering beds are used to compact the iron ore, limestone, and coke materials into a solid mass. Loose materials are usually compacted into a solid mass as they are heated with burners.
The sintering process ensures that the iron ore, limestone, and coke materials are mixed evenly. It also ensures that the sinter materials are of the appropriate size and structure for the blast furnace. The sintering beds have a large surface area, and the sintering process tends to happen quite quickly. The following images show how the sintering beds can be used to change the properties of sinter mixture rocks and powders.
The mixture of iron ore, coke, and limestone is known as charge. This charge travels on conveyor belts to the top of the blast furnace. The following illustration shows how conveyor belts are used to transport the charge mixture to the top of the blast furnace. These conveyor belts are run all day and night, and they constantly transport the charge to the blast furnace.
Example 2: Identifying the Raw Materials Necessary for Making Iron
In the manufacturing of iron in a blast furnace, which two raw materials are necessary for the main process?
- Hematite and limestone
- Bauxite and limestone
- Hematite and lime
- Bauxite and coke
- Hematite and water
This question is asking us to select a pair of raw materials that are necessary for the main process of extracting iron in a blast furnace.
From the different choices available to us, we have six different chemicals. The blast furnace is continually fueled during operation by a charge. This charge is a combination of different raw materials and processed chemicals necessary for the reactions in the blast furnace to take place.
Of the three materials necessary for the charge, the first is the primary metal ore. We have three metal ores to choose from in this question: hematite, bauxite, and limestone. Hematite is an ore of iron, bauxite is an ore of aluminum, and limestone is an ore of calcium. As the blast furnace is used for the extraction of iron, we are interested in answers A, C, and E, which all contain hematite.
Hematite is an iron ore that contains a lot of sandy impurities. In the blast furnace, these sandy impurities are removed by calcium oxide, which is generated inside the furnace through the decomposition of calcium carbonate. This calcium carbonate is provided in the form of limestone, and so the correct answer is A.
Blast furnaces are steel towers that are lined with heat-resistant bricks. These furnaces are incredibly tall structures that may reach up to 60 m high. The charge enters the blast furnace at the top of the tower, and hot air is blasted into the furnace from the bottom.
Multiple chemical reactions happen concurrently inside the blast furnace. We will examine these reactions in a sensible chronological order so that we can more easily understand the iron extraction processes. The reactions will be discussed one by one, and we will learn how the charge reactants are converted into molten iron and a byproduct known as slag.
Coke is initially burned with hot air (oxygen) to produce carbon dioxide and heat energy. This combustion reaction is highly exothermic and releases a lot of heat energy. This reaction increases the temperature of the furnace at the bottom, where air (oxygen) is being blasted in from outside. The following equation represents this reaction:
However, in this case, the carbon dioxide molecules react with additional coke and produce carbon monoxide gas:
The extraction techniques for metals are determined by the reactivity of the metal itself. For example, highly reactive metals, such as aluminum, must be extracted from their ores using electrolysis. For metals whose reactivity is lower than that of carbon, it is possible to extract them using reduction. This is the preferred method as carbon in the form of coal is a relatively abundant resource.
|Zinc||Using carbon or carbon monoxide|
|Copper||Using carbon or other methods|
Carbon monoxide gas acts as a reducing agent as it travels through the blast furnace. The carbon monoxide molecules reduce the iron oxide molecules that are primarily found in the middle and high sections of the blast furnace. The following is the chemical reaction resembling this:
The molten iron then trickles down the inside of the blast furnace and accumulates at the bottom of the tower.
Example 3: Selecting the Agent That Reduces Iron(III) Oxide in a Blast Furnace
Which of the following substances found inside a working blast furnace reduces iron ore?
Oxidation and reduction can be defined in many ways. However, one way would be to define reduction as the loss of oxygen. In the blast furnace, iron(III) oxide is reduced to iron metal. During this process, carbon monoxide, which is formed during the reaction between coke and oxygen from the air, reacts with the iron(III) oxide to form molten iron metal and carbon dioxide gas. As such, the correct answer is D.
At this stage of the process, there is still one important impurity in the iron ore that needs to be removed. This impurity is sand, which can be thought of as silicon dioxide ().
Silicon dioxide can be removed from the blast furnace because the charge mixture contains limestone. Limestone is mostly made up of calcium carbonate (), which decomposes into calcium oxide () and carbon dioxide molecules at high temperatures. The temperatures reached in the blast furnace are high enough to thermally decompose limestone, according to the chemical equation
The calcium oxide molecules react with the sandy impurities, and this reaction produces a molten slag byproduct. This slag byproduct is predominantly made up of calcium silicate () molecules. The following equation represents this process:
Example 4: Describing the Use of Limestone in a Blast Furnace
What is the purpose of adding limestone to a blast furnace?
- The limestone decomposes and the products of that reaction convert impurities into slag.
- The limestone decomposes (an exothermic reaction), heating up the furnace.
- The limestone reduces the iron ore to iron.
- The limestone contributes to the overall yield as it contains iron metal impurities.
- The limestone provides a source of oxygen needed to react with coke.
This question is asking us to choose a description for the purpose of adding limestone to a blast furnace. Limestone is primarily calcium carbonate. When we add limestone to a blast furnace, the calcium carbonate decomposes to form calcium oxide and carbon dioxide.
The calcium oxide then reacts with silicon dioxide in the charge mixture. This silicon dioxide is present as sandy impurities, which exist in the hematite ore. With this information, we can see that the answer is likely to be A, but we will eliminate the other options just to be sure.
Answer B states that decomposition is an exothermic reaction; however, decomposition reactions tend to be endothermic reactions. In a blast furnace, the iron ore is reduced to iron by carbon monoxide, which also eliminates answer C.
As previously stated, limestone is a form of calcium carbonate and does not contain iron. Upon decomposition, the oxygen that limestone contains is bound to calcium and carbon atoms and is not free to react with coke, therefore eliminating choices D and E. As we initially suspected, the correct answer is A.
The slag trickles down the inside of the blast furnace tower and forms a layer on the surface of the molten iron. Some of the carbon dioxide gas from the decomposition of the limestone and the reduction of the iron ore is reduced by coke to form carbon monoxide molecules. However, most of the carbon dioxide gas and nitrogen gas leave the blast furnace through the top of the tower as waste gases.
These chemical reactions, as well as an example structure of a blast furnace, can be seen in the following illustration. This illustration is a highly simplified illustration of just one type of blast furnace. It is important to realize that other blast furnaces can operate at slightly different temperatures.
Each blast furnace is tapped 10–20 times a day to remove molten iron and molten slag from its base. Although this molten slag is not a useful product in this reaction, it is not disposed of as it is used as an aggregate for cement and in road building.
The iron that comes straight out of the blast furnace is called pig iron, and it tends to be very brittle because it contains carbon and other types of impurities. Some of the pig iron is placed into molds to make what is known as cast iron. Cast iron is primarily used to make pans, drain covers, engine blocks, and fences. The rest of the pig iron is usually turned into steel because steel is harder and generally more useful than iron.
Example 5: Naming the Iron-Based Material Produced by a Blast Furnace
What is the name of the iron first produced by a blast furnace?
- Pig iron
- Wrought iron
- Cast iron
Once the iron oxide is successfully reduced by carbon monoxide in a blast furnace, the molten iron trickles down the bottom of the tower. The taps are opened many times a day on the blast furnace, and the molten iron is removed.
This molten iron is known as pig iron. If this pig iron is quickly cast into molds, it is referred to as cast iron. However, a majority of the molten iron is further processed to form steel. Slag refers to the calcium silicate formed from calcium oxide and the sandy impurities in hematite. The other option available to us is wrought iron, which is a form of iron alloy with a very low carbon content. As such, the correct answer is B.
- Extraction of iron takes place on an industrial scale in blast furnaces.
- Blast furnaces are a common part of large industrial steelmaking plants, which also include coke ovens, sintering beds, and other areas for ore dressing.
- Iron is extracted from its ore using carbon in the form of coke.
- An iron-containing ore is combined with coke and limestone to form what is known as the charge.
- The charge is added to the top of the blast furnace, and hot air is blasted in through the bottom.
- The oxygen in the air reacts with the coke in the charge to form carbon monoxide.
- The carbon monoxide in the blast furnace reduces the iron(III) oxide in the hematite ore to produce molten iron.
- Iron ores contain a large amount of sandy impurities. These impurities are removed from the blast furnace when they react with calcium oxide compounds.
- Molten iron and molten slag accumulate at the bottom of the blast furnace, while waste gases, such as carbon dioxide and nitrogen, escape through pipes at the top of the blast furnace.
- Molten iron is known as pig iron and can be cast into molds to form cast iron.
- Most of the molten iron obtained from a blast furnace is converted into steel through the basic oxygen steelmaking process.