Process Overview
The blast furnace is a refractory-lined counter-current shaft reactor, 20–35 m tall and 8–15 m in hearth diameter for large modern furnaces. Burden materials — iron ore lump, sinter, and pellets mixed with metallurgical coke — are charged at the top in alternating layers. Hot blast air (900–1,300 °C) enriched with oxygen (22–28% O₂) and carrying pulverised coal injection (PCI) is blown in at tuyere level through 20–40 copper cooled tuyere nozzles evenly distributed around the hearth circumference. The blast combusts the coke and PCI at the tuyere nose to form a recirculating combustion zone called the raceway, generating CO-rich reducing gas at ~2,200 °C. This gas rises through the burden, reducing iron oxides and preheating the descending charge.
Burden Materials
The iron-bearing burden comprises three main material types: sinter (60–75% of burden at most European and Asian plants), pellets (20–35%), and lump ore (0–15%). Sinter — produced in an on-site sintering plant from iron ore fines, coke breeze, limestone, and recycled plant material — is preferred for its high softening temperature range and reducibility. The coke burden (300–350 kg/t hot metal without PCI) provides both the reducing agent and the structural support that maintains burden permeability as the charge softens at 1,000–1,200 °C in the cohesive zone. Coke quality (CSR, CRI, M40, M10) is therefore critical to blast furnace performance.
Tuyere Injection and Raceway
Pulverised coal injection (PCI) at 150–250 kg/t hot metal partially substitutes expensive metallurgical coke. The coal is ground to 70–80% below 75 µm and pneumatically conveyed to individual tuyere injectors. In the raceway — a dynamic, pear-shaped void 1.0–1.8 m deep behind the tuyere nose — the injected coal and coke combustion produces a theoretical flame temperature (RAFT) of 1,900–2,200 °C. Oxygen enrichment and moisture control are adjusted to maintain RAFT within this window; too high a RAFT causes refractory erosion, too low causes poor gas distribution and hanging burdens.
Taphole and Casting Practice
Hot metal and slag accumulate in the hearth and are tapped at 4–6 per day through the taphole, a small opening in the hearth wall drilled open with a hydraulic tap drill. Hot metal flows at 1,450–1,520 °C into torpedo ladles for transfer to the BOF steelshop. Slag, which floats above the hot metal due to its lower density (2.6–2.8 g/cm³ vs 7.0 g/cm³ for hot metal), exits through a slag notch above the taphole or is co-tapped and separated on the cast house floor. After each cast, the taphole is plugged with a mud gun that injects taphole clay. Large blast furnaces produce 8,000–15,000 t/day of hot metal.
Hot Metal Composition
Hot metal from a modern blast furnace typically contains: 4.0–4.5% C (saturated), 0.3–0.8% Si (controlled by blast temperature and slag basicity), 0.2–0.5% Mn, 0.06–0.12% P (function of ore quality and slag chemistry), and 0.02–0.04% S (reduced by slag sulphur capacity). Silicon is the key quality parameter: high Si requires more lime in the BOF and increases exothermic heat, while low Si demands precise coke and blast temperature control. Desulphurisation in a torpedo ladle or hot metal ladle to <0.002% S is standard practice before BOF charging for quality steel grades.
Blast Furnace Inner Volume and Productivity
Blast furnace size is characterised by inner volume (m³) — the volume of the working space from tuyere level to the stockline. Large modern furnaces (e.g. POSCO Pohang No. 4, 5,600 m³; China Steel No. 2, 4,932 m³) produce over 12,000 t/day. Productivity is expressed as t HM/m³/day; world-class performance is 2.3–2.7 t/m³/day. Campaign life before a major reline is 15–20 years for a well-maintained modern furnace.
CO₂ Emissions Context
Blast furnace ironmaking is inherently carbon-intensive: the process generates approximately 1.8–2.2 t CO₂ per tonne of hot metal. The coke and coal that act as reducing agents are irreplaceable with current technology unless hydrogen injection (H₂-DRI) is substituted. This is the primary driver for the global shift toward EAF-DRI steelmaking using green hydrogen as the reduction agent.