1. Iron does not fall from the sky
A mine for iron ore in Australia (© Bloomberg, 2020)
Neither can iron be dug up directly from the ground. In nature, iron mainly exists as iron(III) oxide, which is also known as haematite. Haematite is in turn hidden in rocks underground. Therefore, to get hold of iron, we need to first mine iron ores, chemically convert haematite into iron, and remove any impurity. The entire process is energy intensive and spews out loads of carbon dioxide.
Haematite is a mineral made of iron(III) oxide, Fe2O3. It is mined for the extraction of iron.
2. Reducing haematite to iron in a blast furnace
The world has a huge appetite for iron. In 2019, we consumed more than 1.7 billion tons of iron globally. To feed this ravenous appetite, engineers process haematite into iron in a huge container called a blast furnace. The biggest blast furance is in Korea. It is like a humongous rice cooker with enough space to cook rice for 40 million people.
Three important redox reactions occur within a blast furnace:
| Reaction | Function | Equation |
|---|---|---|
| 1. Coke burns in hot air to form carbon dioxide | Releases heat to maintain the temperature above 1600 °C | C(s) + O2(g) ⟶ CO2(g) |
| 2. Excess coke reduces the carbon dioxide formed to carbon monoxide | Produces carbon monoxide, which is the reducing agent to reduce haematite | CO2(g) + C(s) ⟶ 2CO(g) |
| 3. Carbon monoxide reduces haematite to iron | Produces molten iron | Fe2O3(s) + 3CO(g) ⟶ 2Fe(l) + 3CO2(g) |
The first reaction requires oxygen and we supply it by blasting hot air into the furnace. That’s why we call it the blast furnace.
3. Blasting our climate: carbon dioxide is the main by-product of iron extraction
The star of the show in the blast furnace is coke. First, it acts a fuel to provide energy for the reduction of haematite. Second, coke forms carbon monoxide, which then serves as the reducing agent to reduce haematite to iron.
Unfortunately, as haematite is reduced to iron, coke is concomitantly oxidised to carbon dioxide.
4. Dealing with the silicon dioxide impurities
The reduction of haematite is not everything.
Haematite is found in rocks, which is a messy, solid mixture. The rocks also contain a lot of sand, which is mainly silicon dioxide.
SiO2(s) + CaO(s) ⟶ CaSiO3(l)
Therefore, we must remove the silicon dioxide impurity by reacting it with calcium oxide. This is a neutralisation reaction, as silicon dioxide is an acidic oxide and calcium oxide is a basic oxide.
The salt formed is molten calcium silicate. As molten calcium silicate is immiscible with molten iron, it can be easily separated and drained away.
5. One step forward, two step back: on-site production of calcium oxide removes impurities but adds carbon dioxide
Instead of buying a ready-to-use calcium oxide from Amazon or Taobao, we make it on site: right within the blast furnace.
CaCO3(s) ⟶ CaO(s) + CO2(g)
We dump calcium carbonate into the furnace. The intense heat will cause it to undergo thermal decomposition. This forms calcium oxide but also carbon dioxide, adding to the greenhouse gas emission from a blast furnace.
Calcium carbonate is added to the blast furnace, which decomposes into calcium oxide to react with silicon dioxide impurities. This forms molten calcium silicate, which can be easily separated and removed.
6. Turning iron into steel of different kinds
Car bodies are made of mild steel with a low proportion of carbon mixed with iron. (© Craig Adderley on Pexels.com, 2020)
Steel is an alloy that we can make by mixing iron with carbon, and sometimes also with other elements.
We can further finetune the property of steel by:
- Varying the proportion of carbon
- Adding different types of element on top of carbon
For example, decreasing the proportion of carbon makes the steel weaker but more malleable. Because it is slightly weaker, it is also called mild steel. But this weakness is a good thing, as it allows factories to mould low-carbon steel into different shapes to make car bodies and machinery.
7. Steeling ourselves for climate change
We have scaled up mining, iron extraction, and steel manufacturing to mass produce steel with minimal cost.
Cheap steel is a wondrous steal. Without it, we would not be able build skyscrapers to house enough people in a rapidly urbanising world.
Yet, cheap steel is dangerous. Its easy accessibility makes us think less about the harm it took for us to get hold of the iron for steelmaking. We forget that for every tonne of steel that goes into our new cars, we release a greater mass of carbon dioxide even before we start driving.
We could put the blame on mining companies and factories. But we could also do something about it. Yes, even young Singaporean students! Perhaps we could ask our parents to think twice before scrapping our old car, especially when tempted by low COE prices? Perhaps we could continue taking public transport instead of buying a new family car?
8. Beyond the 3Rs: reworking the extraction of iron
Be that as it may, many countries depend on cheap steel to industrialise and uplift the poor. We cannot simply force them to reduce their steel consumption or use recycled steel that is more expensive.
One way to have the cake and eat it is to change the way we reduce haematite to iron. Currently, coke is added to form carbon monoxide as the reducing agent. Scientists in Sweden are looking at using hydrogen gas as an alternative reducing agent.
Fe2O3+ 3H2 ⟶ 2Fe + 3H2O
If hydrogen can really be used to reduce haematite, the only side product will be water. However, this reaction does not happen at room temperature and pressure. The scientists are still trying to get it to work in the laboratory and at the industrial scale by 2035.
Perhaps you could study more Chemistry and join them next time?
