Blast Furnace

Ore dug out of the ground is a reddish or blackish lump of rock. The iron inside is tightly bonded to oxygen. To get iron, you first have to pull that oxygen away.

That's the blast furnace's job. From the top, workers load iron ore and coke (hard chunks made by baking coal) in alternating layers. From the bottom, hot air is blasted in hard. As the coke burns, the gas rises through the stack and strips the oxygen off the ore.

The molten iron that pools at the bottom is tapped out every few hours. But it isn't steel yet. It's loaded with carbon, so it hardens into pig iron, which is hard but brittle. Turning it into steel takes one more step to remove carbon.

That job goes to another furnace next door. The blast furnace pulls molten iron out of ore. The converter (called a basic oxygen furnace) blows oxygen into that iron to burn off carbon and make steel. The two are front and back of one road, not rival machines.

The inside of a blast furnace is packed with ore and coke stacked from the top, and the molten iron sits at the very bottom. The iron that flows out is well over 1,400°C (about 2,550°F). That's why steel mills glow all night long.

What if you just shut the fire off without following a procedure? The iron left inside and the raw materials stacked above it cool together and fuse solid. The furnace turns into one giant block of metal. POSCO, the Korean steelmaker, explains that in that case the furnace has to be torn out and rebuilt, which takes months.

So even stopping one has a set procedure. For a short pause, crews seal the air inlets to trap the heat and restart days or weeks later. For a longer rest, they let the stack of material drop and empty the furnace. A major shutdown to replace the inner bricks is built into the furnace's life cycle.

The inside is lined with thick heat-resistant brick, and a steel shell wraps around it. That lining wasn't made to be heated and cooled again and again. So once it's lit, it runs for years. It's like a kitchen stove in that both use fire to work with metal. But you can switch a stove on and off whenever you like, while stopping and restarting a blast furnace costs a fortune.

This trait even shapes the whole industry. When demand drops, blast furnace operators can't easily cut output. Stopping and restarting is so costly that they can't shut down just because demand dipped a little. This isn't the sunk cost idea of hating to waste money already spent. It's a constraint that comes from the nature of the equipment itself.

There are two main roads to steel. The blast furnace-converter road starts from iron ore. The electric arc furnace road starts from scrap that someone else used and threw away. Where scrap was scarce, people chose the ore road. Where scrap piled up, the road that melts it grew. What separates the two roads isn't new versus old technology. It's the raw material.

There's also a road in between. Ore is reduced with gas or hydrogen into iron first, and that iron is then melted in an electric arc furnace. The raw material is ore, but the melting furnace is electric.

Both roads still run side by side. According to the World Steel Association, 69.4% of the world's steel in 2025 came from the blast furnace-converter road. Where scrap hasn't piled up, there's little to choose besides starting from ore.

The pressure on blast furnaces today isn't cost but emissions. The World Steel Association notes that emissions are concentrated in the step that makes iron from ore. The gas from burning coke does the work of removing oxygen, and that gas leaves carrying the oxygen as carbon dioxide. So people are testing ways to remove the oxygen with hydrogen instead of carbon.

In 2021 in Sweden, the first steel made from hydrogen-reduced iron was delivered. That iron is also melted in an electric arc furnace, not a blast furnace, to become steel. But it's still at the pilot stage, and the OECD notes that many low-emission projects are being delayed. All we can confirm right now is that real material has been produced.