Blast furnace

A blast furnace operates as a counter-current heat exchanger and chemical reactor. Iron ore (typically hematite or magnetite), coke, and limestone are charged at the top, while preheated air (often oxygen-enriched) is blown through tuyeres near the bottom. The coke combusts to carbon monoxide, which reduces iron oxides to metallic iron in a series of reactions, while limestone fluxes silica and alumina into a slag that floats on the molten iron.¹ The furnace is tapped periodically to release pig iron and slag. Modern furnaces achieve thermal efficiencies above 70% by recovering top gas for preheating the blast.²

The earliest blast furnaces appeared in China during the Han dynasty (1st century BCE), using water-powered bellows to achieve higher temperatures than bloomeries.³ In Europe, the technology spread from the 12th century, with the first English furnace at Buxted in 1491. The 19th century brought the hot blast (1828) by James Beaumont Neilson, which drastically reduced coke consumption, and the introduction of the Bessemer process that made steel affordable. By the 20th century, furnaces grew to 100-meter heights with output exceeding 10,000 tonnes per day.¹

Modern blast furnaces are integrated with sinter plants and coke ovens, and they are increasingly coupled with top-gas recycling and carbon capture to reduce CO2 emissions, which account for about 70% of the steel industry's carbon footprint.⁴ Alternative processes like direct reduced iron (DRI) and smelting reduction are being developed, but blast furnaces remain dominant due to their high productivity and economies of scale. The largest furnaces, such as those in Japan and China, produce over 10,000 tonnes of hot metal per day.⁵

Beyond iron, blast furnaces have been adapted for smelting lead, copper, and even zinc, though with different chemistries. A notable edge case is the 'low-shaft' furnace, developed in the 20th century for small-scale operations, which never achieved commercial success. The term 'blast' originally referred to the forced air, but early furnaces used natural draft; the first mechanical blowers were water-powered bellows in China.³ Another obscure fact: the tallest blast furnace ever built, at 120 meters, was in the Soviet Union in the 1970s, but it was dismantled due to operational difficulties. The 'hearth' of a furnace can last over 20 years with proper cooling, and some furnaces have been operated continuously for over 30 years between relines.²

Pig iron

High-carbon iron produced by a blast furnace, cast into ingots called pigs.

Tuyere

A nozzle through which the hot blast is injected into the furnace.

Slag

A molten byproduct of impurities and flux that floats on the iron.

Coke

A carbon-rich fuel made from coal, used as both fuel and reducing agent.