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A blast furnace does not turn iron ore directly into finished steel. It reduces iron oxides in ore to liquid hot metal; in the integrated blast furnace–basic oxygen furnace (BF-BOF) route, that hot metal is then refined in a basic oxygen furnace (BOF) to make raw liquid steel. The steel is subsequently refined as needed, cast and rolled.
How the blast-furnace-to-steel process works
The process has two distinct stages: ironmaking in the blast furnace and steelmaking in the BOF. In the furnace, solid materials descend while hot gases rise, transferring heat and removing oxygen from iron ore. The result is liquid iron, not steel.
- Charge iron-bearing materials. Ore, sinter or pellets are combined with coke and fluxes such as limestone or dolomite, then added through the top of the furnace. Exact preparation and proportions vary by plant.
- Heat and reduce the burden. Preheated air enters near the furnace bottom. Coke reacts to generate heat and carbon monoxide. The hot gas rises through the descending burden, heats it and removes oxygen from iron oxides.
- Form and separate slag. Flux reacts with gangue and other oxides to form molten slag, which is less dense than iron. Both collect in the hearth, where the liquid iron and slag are tapped.
- Recover gas and move the iron. Gas leaving the furnace top is collected and cleaned for use as fuel. Liquid iron moves through refractory-lined channels to the steelmaking shop. Some plants treat it to remove unwanted elements before it reaches the BOF.
- Refine hot metal into steel. In the BOF, oxygen oxidizes carbon and phosphorus in the hot metal. These reactions release heat, which can also melt scrap or iron-ore additions. The result is raw liquid steel.
- Refine, cast and roll. Steel may undergo secondary refining before it is cast. Rolling turns cast steel into products. These are downstream operations, not blast-furnace functions.
This is a countercurrent process: solids travel down and gases travel up. That arrangement lets rising gases heat and reduce material as it descends.
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What coke, flux and the blast each do
- Coke supplies heat and helps generate carbon monoxide, the reducing gas that removes oxygen from iron oxides. It also helps keep the burden permeable so gases can pass through the furnace; it is more than just a fuel.
- Preheated air enters near the bottom and supports coke reactions that produce heat and reducing gas.
- Flux, commonly limestone or dolomite, combines with gangue and other oxides to form slag. Slag carries these materials away from the iron, but the blast furnace does not remove every unwanted element from hot metal.
Is pig iron the same as steel?
No. Hot metal—also called pig iron—is the carbon-rich liquid iron tapped from a blast furnace. It still needs refining. In the BF-BOF route, the BOF is the principal step that reduces carbon and removes some impurities to produce raw liquid steel. As steeluniversity describes the purpose of the process, “The purpose of the Basic Oxygen Steelmaking (BOS) process is to refine the hot metal produced in the blast furnace into raw liquid steel, which may be subsequently refined in the secondary steelmaking shop.”
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How much material does the route use?
World Steel Association’s 2022 Steel Facts publication gives these approximate inputs for producing 1,000 kg of crude steel by the BF-BOF route. They describe route-level inputs, not a fixed furnace recipe or measurements from a particular plant.
| Input | Approximate amount per 1,000 kg crude steel |
|---|---|
| Iron ore | 1,370 kg |
| Coal | 780 kg |
| Limestone | 270 kg |
| Steel scrap | 125 kg |
The route figures are not plant operating instructions: actual burden composition and operating conditions differ. The coal figure is not a direct statement of coke charged; coal is a route input in the association’s approximate accounting.
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Does all steel come from blast furnaces?
No. The two principal modern routes are BF-BOF and electric arc furnace (EAF). BF-BOF makes iron from ore in a carbon-based blast furnace and then refines it with oxygen. EAF chiefly uses recycled steel and electricity, though it can also use direct-reduced iron or hot metal.
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| Route | Typical metallic feedstock | Main energy input | Blast furnace required to make iron from ore? |
|---|---|---|---|
| BF-BOF | Iron ore, coal and recycled steel | Coal-based ironmaking, followed by oxygen steelmaking | Yes, in the conventional route described here |
| EAF | Chiefly recycled steel; may also use direct-reduced iron or hot metal | Electricity | No, not when using recycled steel; it can use iron produced elsewhere |
In its 2022 Steel Facts publication, the World Steel Association reported an approximate global production split of 71% BF-BOF and 29% EAF. Its current “What is steel?” and “Raw materials” pages round this to about 70% and 30%, respectively. These are global route shares, not a claim that every steel product follows one route.
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What the emissions figure does—and does not—mean
The World Steel Association’s climate page reports an average of 2.18 tonnes of CO2e per tonne of steel produced, using 2024 data and covering scopes 1, 2 and 3. This is an overall steel-production figure, not a standalone blast-furnace emissions factor. The association says most emissions in ore-based production are associated with making iron from ore.
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Sources and further reading
- World Steel Association: What is steel? Explains ironmaking, steelmaking and route context.
- Environment and Climate Change Canada: Environmental Code of Practice for integrated steel mills, section 2 Describes furnace charging, hot metal, slag and recovered gas.
- World Steel Association: Steel Facts (2022) Provides approximate route inputs and dated production shares.
- steeluniversity: Basic Oxygen Furnace module Covers the BOF stage that refines hot metal.
- World Steel Association: Climate change and the production of iron and steel Gives the reported emissions figure and its scope.
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