Video Transcript
In this video, we will look at the
extraction of iron from its ore in the blast furnace. We’ll look at the chemicals used in
a blast furnace and the temperatures involved.
Iron is an abundant metal, which is
relatively cheap to produce. Ores containing a high proportion
of iron can be found throughout the world. There are many compounds of iron
that can be found in nature. But the one that’s most common for
extracting iron and the one that we’ll feature in this video is hematite. Hematite is a naturally occurring
form of iron(III) oxide, Fe2O3. And it’s about 70 percent iron by
mass. What this means is that, with
perfect extraction, putting one kilogram of ore in, we should get 0.7 kilograms or
700 grams of pure iron out. However, it’s not quite that
simple, so let’s look at the details.
Iron can be extracted from its ore
using carbon, which is great. Carbon is very cheap and very
available. The carbon used to extract iron is
commonly obtained from coal. Coal often contains hydrocarbons
and other volatile impurities. Heating coal to over 1000 degrees
Celsius will drive off some of these impurities, forming something we call coke. Coke is a much purer form of carbon
than coal, meaning it introduces fewer impurities into our iron. When hematite is reacted directly
with carbon, a redox reaction can occur.
Hematite, Fe2O3, is reduced to
iron. And at the same time, carbon is
oxidized to a mixture of carbon monoxide and carbon dioxide. The exact stoichiometry will depend
on the conditions. If we look at the reactivity
series, we’re reminded that carbon is more reactive than iron. So when we heat iron oxides with
carbon, the oxygen passes from the iron to the carbon.
So far, this seems pretty
simple. All we need to do is heat hematite
with coke and we’ll get our iron, if only it were that simple. Pure hematite melts at 1565 degrees
Celsius, while iron melts at a very similar 1538 degrees Celsius. Carbon, however, is relatively
stubborn. It won’t melt; instead, it sublimes
at a massive 4827 degrees Celsius. This means at least until we reach
about 1500 degrees Celsius, we’ll be dealing with reactants that are all solids. Solids are difficult to mix, and
the reaction will be extremely slow.
Now, even if we did heat enough to
melt the hematite, that’s not necessarily a massive improvement. Mixing in the much less dense
carbon would still be quite an intensive process. Fortunately, we don’t need to get
nearly 5000 degrees Celsius for this reaction to work quickly. Instead, we can take advantage of a
nifty feature of carbon. When carbon reacts with a limited
supply of oxygen, it forms carbon monoxide in an exothermic reaction. With further oxygen, carbon
monoxide can react to form carbon dioxide, again in an exothermic process. If under the same conditions we use
carbon monoxide instead of carbon as a reducing agent, we’ll have a much more
effective reaction because it will be much easier for gas to be mixed in.
In the blast furnace where this
reaction happens, the hematite may actually still be solid. But the iron we get out at the end
will definitely be a liquid. The most important reaction to
remember here is the reaction between hematite and carbon monoxide. The reaction between hematite and
carbon does occur, but much less so.
Next, we’re going to look at one of
the most important features of real-world chemistry, the impurities. Real coke and real iron ore contain
impurities. The problematic impurity we’ll be
looking at is sand, which is mostly silicon dioxide. You may hear silicon dioxide called
silica. If we don’t remove most of this
impurity, it will end up in the iron, giving it properties that we don’t want.
To remove silicon dioxide, we’re
going to introduce a chemical to react with it. The chemical we’ll be adding is
calcium carbonate in the form of limestone. Calcium carbonate doesn’t actually
do the job itself. But at high temperatures, it can
thermally decompose, producing calcium oxide and carbon dioxide. Calcium oxide can react with
silicon dioxide impurities, producing calcium silicate, CaSiO3. This form of calcium silicate has a
melting point of around 1540 degrees Celsius, very close to that of iron. Molten calcium silicate will
naturally separate from molten iron because it’s about half as dense. Introducing calcium carbonate
allows us to produce a layer of impurity that we can separate purely by opening a
tap. We call this layer slag.
Next, we need to go to the very
core of how iron is extracted, the blast furnace. In the whole of 2019, 2.5 billion
metric tons of iron ore was mined worldwide. That’s the equivalent of 7500 times
the mass of the Empire State Building. We need massive pieces of equipment
to process all that ore. This is where the blast furnace
comes in.
A modern blast furnace is about 15
meters wide and about 35 meters tall. It has the volume of about two
Olympic swimming pools. From one of these furnaces, you’d
expect to get about 10000 tons of molten iron a day. The full design of a blast furnace
is very complex. So we’re only going to look at a
simplified version.
In the first step, coke, ore, and
limestone are added to the furnace from the top. The coke provides the carbon, the
ore provides the iron(III) oxide, and the limestone provides the calcium
carbonate. In the next step, hot air between
900 and 1300 degrees Celsius is injected into the bottom of the furnace. It’s these blasts of hot air that
give the blast furnace its name. Oxygen from the air quickly reacts
with the solid carbon in the coke, producing carbon dioxide gas, which rises up the
furnace. This and other exothermic reactions
raise the temperature at the bottom of the furnace to between 1500 and 2000 degrees
Celsius.
As carbon dioxide rises and mixes
with more carbon, it reacts to produce carbon monoxide. This is also an exothermic
reaction. The carbon monoxide rises up the
blast furnace to do the most important job: reducing our iron oxide. The carbon monoxide reacts with the
iron ore from the top and as it falls down the blast furnace. Iron(III) oxide will react with
carbon monoxide to produce iron and carbon dioxide. The iron produced at the top is
solid because it’s not hot enough to melt. But when it gets to the bottom, it
definitely will melt. So we can consider the overall
equation to be the one that produces liquid iron.
The ore and the iron will travel
down the column and start to get extracted from the bottom. As we get closer to the bottom,
we’ll have more iron and less iron ore. We’ll also have that calcium
carbonate from the limestone traveling downwards, but I’ll free up some space so we
can talk about it properly. At about 800 degrees, the calcium
carbonate introduced by the limestone decomposes fast enough to produce the calcium
oxide we need. The decomposition of calcium
carbonate into calcium oxide and carbon dioxide is endothermic. So this process absorbs some of the
heat generated by burning the coke. The extra carbon dioxide produced
can also react with carbon to produce more carbon monoxide.
In the next step, calcium oxide
reacts with silicon dioxide impurities in the ore. This helps keep the silicon
impurity levels of the iron at the end low. The calcium silicate produced melts
at about the same temperature as iron. So at the bottom, it turns
molten. At the bottom, we have a layer of
molten iron covered by a layer of molten impurities like calcium silicate. We call this top layer the
slag. The slag is drained away separately
to the molten iron. And the molten iron produced in
this process is called pig iron.
The pig iron produced by a blast
furnace is still relatively impure, with about four percent weight by carbon, as
well as other impurities. This high level of carbon makes the
iron when it solidifies quite strong but quite brittle. In order to produce steel for
structural applications, some of this carbon needs to be removed. This can be done using the basic
oxygen process. However, this isn’t the focus of
this video. Instead, let’s do some
practice.
Which of the following substances
found inside a working blast furnace reduces iron ore? (A) CO, (B) CO2, (C) CaCO3, (D)
H2O, or (E) O2.
A blast furnace uses a mixture of
hot air and carbon in the form of coke to reduce metal ores, like iron ores and lead
ores. In the case of iron, the most
commonly used iron ore is hematite, a form of iron(III) oxide. Reduction of this iron ore involves
removing the oxygen, leaving the iron behind.
One way of approaching this
question is to consider which of these five chemicals is capable of extracting
oxygen from iron ore. For this question, I’m going to
consider reduction to be the process of removing oxygen from something. We could consider electrons, but
thinking about removing oxygen is more straightforward in this case.
As a quick test, we can see how
these substances respond to molecular oxygen. Carbon monoxide reacts with oxygen
to form carbon dioxide. So carbon monoxide is capable of
acquiring more oxygen. Therefore, we can consider carbon
monoxide a possibly effective reducing agent. On the other hand, carbon dioxide
does not react further with oxygen. So it’s unlikely to be an effective
reducing agent. And this is true of the other three
chemicals. Calcium carbonate, water, and
oxygen do not readily react with more oxygen. This process leaves us with the
answer of carbon monoxide. But let’s look at this another
way.
Let’s have a look at the role of
each of these chemicals in the blast furnace. In the blast furnace, the very role
of carbon monoxide is to reduce the iron ore, while the role of carbon dioxide is to
react with carbon to make the carbon monoxide. So carbon dioxide is involved in
the chain that produces the agent that reduces the ore. But carbon dioxide doesn’t reduce
the ore itself. Calcium carbonate, usually in the
form of limestone, is added to blast furnaces so that it decomposes, producing
calcium oxide. Calcium oxide is there to remove
silicon dioxide impurities present in the ore. In comparison to the other
chemicals, you’re unlikely to find H2O in the blast furnace. However, there may be hydrocarbons
in the coke, which when burned will produce H2O. However, water will not be involved
in the reduction of the ore.
Let’s have a look at the last
option. Oxygen is an infamous oxidant and
therefore unlikely to be a reducing agent. In the blast furnace, its role is
to react with carbon to make carbon dioxide. Therefore, our final answer for
which substance found inside a working blast furnace reduces iron ore is carbon
monoxide, CO. And here we have the balanced
chemical equation for the reduction of hematite, iron(III) oxide.
Finally, let’s have a look at the
key points. Iron is extracted from iron ore
using a blast furnace. Coke, iron ore, and limestone are
mixed and added to the furnace. The coke reacts with hot air,
producing carbon dioxide, which goes on to react with more coke to produce carbon
monoxide. The iron ore is reduced by the
carbon monoxide, forming iron, which melts at the bottom of the hot furnace. And the limestone decomposes to
form calcium oxide and carbon dioxide. The calcium oxide reacts with
silicon dioxide impurities, and the molten slag is then drained off. This leaves a molten iron layer
that is our product.
In operation, the temperature at
the bottom of a blast furnace is over 1500 degrees Celsius. We have a layer of molten iron with
a layer of molten impurities that we call the slag. At the top of the furnace, the
temperature is closer to 400 degrees Celsius. And in between, we have a region at
around 800 degrees Celsius where the calcium carbonate begins to decompose.
