Industrial sabotage, government intervention, and a race against time to avoid devastating consequences — not a summary of a fictional political thriller, but the real-life drama that engulfed the UK steel-making industry in recent weeks. At the story’s centre were the formerly Chinese-owned blast furnaces in Scunthorpe, which were at risk of shutting down until they were brought under emergency government ownership.
A chemistry blog may not be the place to get into the politics of the situation, but it’s a topical opportunity to look at the multitude of reactions that transform ore to steel. After all, it’s not often that a topic commonly covered in secondary chemistry lessons makes international news!
First, the basics: the blast furnace is the main way iron is extracted from iron ore. It’s a vertical shaft, up to 60 metres tall, heated by an input of hot air near the base. Workers add raw materials at the top of the furnace, where the temperature is coolest, and molten iron is the ultimate product, tapped off at the furnace’s base.
The furnace uses three key raw materials: haematite (iron ore), coked coal (coal heated without oxygen to remove impurities) and limestone. These undergo a series of reactions in the furnace. First, the carbon in the coal reacts with oxygen in the air, producing carbon dioxide and carbon monoxide gases. The carbon monoxide then reacts with the iron oxide in iron ore, a three-stage reaction that produces molten iron.
The limestone also has an important role. It decomposes to form calcium oxide, which reacts with and removes silica impurities in the iron. This forms a lower-density molten calcium silicate layer on top of the molten iron, which can be tapped off separately. The technical term for these molten impurities is “slag”, a source of much mirth for teenagers in chemistry classes everywhere, thought to derive from a proto-Germanic term for the splinters of metal struck off during hammering.
The iron straight from the blast furnace is called “pig iron”, and it’s not particularly useful. Its relatively high carbon content (3-5%) coupled with the presence of several remaining impurities, such as iron sulfide, make it brittle. Further treatment is needed to form usable steel. First, calcium oxide or carbide is added to remove sulfur impurities, forming calcium sulfide. Then, the molten iron has oxygen blasted into it in a basic oxygen furnace, oxidising the remaining impurities and removing more of the carbon content. The result is crude steel, which can then be further alloyed with other elements for differing purposes.
Though the demise of Scunthorpe’s blast furnaces has been averted for now, their days remain numbered. According to the International Energy Agency, the blast furnace and basic oxygen furnace process accounted for 70% of global crude steel production in 2023, but it has an up-and-coming competitor: the electric arc furnace. These furnaces use arcs of electricity between graphite electrodes to produce high temperatures. Two of Scunthorpe’s four blast furnaces are already out of action, and when the remaining pair are replaced by electric arc furnaces is a question of when rather than if.
The reason for this is that blast furnace-produced steel has a sustainability problem. According to an Institute for Energy Economics and Financial Analysis report, for every metric ton of crude steel produced by the blast furnace-basic oxygen furnace process, 21.4 gigajoules of energy is required. An electric arc furnace can produce steel by using iron produced from iron ore by reaction with hydrogen and carbon monoxide from natural gas, a process which requires 17.1 gigajoules of energy per metric ton of crude steel. Alternatively, electric arc furnaces can also use scrap steel as a raw material, a process which requires just 2.1 gigajoules of energy per ton of crude steel created.
Another key factor is the carbon dioxide emissions associated with blast furnaces. According to Carbon Brief, the iron and steel industry is responsible for 11% of global carbon dioxide emissions. Per metric ton of crude steel, the blast furnace-basic oxygen furnace process emits 2.2 metric tons of carbon dioxide. This compares to 1.4 metric tons or 0.3 metric tons, respectively, for the two electric arc furnace processes detailed in the previous paragraph. Both of these figures could be further reduced with the use of green hydrogen and renewable electricity.
The International Energy Agency estimates that the percentage of steel produced by electric arc furnaces globally will need to increase from 24% in 2020 to 52% by 2050. There is also an interim target of 36% for 2030; progress towards this looks promising, with a figure of 32% reached in 2023. However, there are still challenges ahead, with the majority of iron-making capacity in development in India and China still using blast furnace and basic oxygen furnace technology.
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