Glossary

A blast furnace is a large, vertical, cylindrical industrial furnace used to extract iron from iron ore through a chemical reduction process. It is the central piece of equipment in traditional ironmaking, producing molten pig iron, which is later refined into steel or cast iron. The term “blast” refers to the continuous flow of hot, pressurized air (or oxygen-enriched air) that’s blown into the furnace to sustain combustion and drive the chemical reactions.

Structure and Design

A blast furnace is typically a tower-shaped structure, about 30–40 meters (100–130 feet) tall and lined with refractory firebrick to withstand the extreme internal temperatures (up to 2000°C / 3600°F). It is divided into several zones:

1. Throat (Top): Where the raw materials—iron ore, coke, and limestone (flux)—are charged into the furnace in alternating layers.

2. Stack: The middle section where the rising hot gases preheat and partially reduce the descending ore.

3. Bosh: The hottest region, where combustion occurs, and reduction reactions intensify.

4. Hearth (Bottom): The lowest part, where molten pig iron and slag collect and are periodically drained (tapped) through separate openings.

Raw Materials

A blast furnace requires three main ingredients:

- Iron ore (hematite Fe₂O₃ or magnetite Fe₃O₄): The source of metallic iron.

- Coke: A carbon-rich fuel made from coal that burns to produce heat and carbon monoxide.

- Limestone (CaCO₃): Acts as a flux, combining with impurities like silica to form slag, which floats on top of the molten iron.

Process

The blast furnace operates continuously, with materials fed from the top while air is blown in from the bottom through tuyeres (nozzles). Several key reactions occur:

1. Combustion of coke:

C + O2 ​→ CO2​

and then

CO2​ + C → 2CO

This produces carbon monoxide (CO), the main reducing agent.

2. Reduction of iron ore:

Fe2​O3​ + 3CO → 2Fe + 3CO2​

The iron oxide is reduced to metallic iron, which melts and collects at the bottom as pig iron.

3. Formation of slag:

CaCO3​ → CaO + CO2​

CaO + SiO2 ​→ CaSiO3​

The lime (CaO) reacts with silica and other impurities to form liquid slag, which floats above the molten iron.

Products

- Molten pig iron: The main product, containing about 3.5–4.5% carbon, along with silicon, manganese, sulfur, and phosphorus.

- Slag: A byproduct used in cement production, road construction, and insulation materials.

- Top gas: A mixture of CO, CO₂, and N₂ that can be recovered and reused as a fuel or reducing gas.

Significance

The blast furnace is the heart of the iron and steel industry, providing the raw pig iron that’s refined into steel in a basic oxygen furnace (BOF) or electric arc furnace (EAF). Despite being centuries old in principle—the earliest versions date back to the 14th century—it remains one of the most efficient large-scale metallurgical systems ever developed.