Blast Furnace Ironmaking

Iron-bearing materials charged to the blast furnace must meet strict physical and chemical specifications to ensure adequate gas permeability through the column of material. Raw iron ore fines cannot be charged directly — they would compact under the burden weight and block the ascending gas. Three burden materials are used:

Sinter is the dominant burden type at most integrated plants. Iron ore fines (0–6 mm) are mixed with return fines, coke breeze, limestone, and dolomite, then ignited on a continuous sintering strand where partial fusion bonds the particles into a porous, self-fluxing material. High-quality sinter has a reducibility index above 70% and a tumble index (physical strength) above 65, reflecting its ability to survive the mechanical stresses of descent through the blast furnace without excessive fines generation.

Pellets, produced by balling wet ore fines into spherical green balls (9–16 mm) and indurating at 1,300 °C, offer superior reducibility and strength uniformity compared to sinter. They are the preferred burden material where high DR (direct reduction) grade requirements apply, and are mandatory in direct reduction shaft furnaces. In blast furnaces, a mixed burden of 40–70% sinter and 30–60% pellets is typical in Europe and Asia; North American practice uses a higher proportion of pellets.

Metallurgical coke — the third burden component — provides structural support to the column. The coke bed in the lower furnace (below the cohesive zone) is the only permeable pathway for liquid iron and slag drainage and for tuyere gas distribution. Coke strength after reaction (CSR, measured after CO₂ reaction at 1,100 °C for 2 hours) must exceed 60–65% in large modern furnaces to prevent coke degradation and bed collapse.

Metallurgical coke is assessed primarily by two indices: CRI (Coke Reactivity Index) and CSR (Coke Strength after Reaction). These are measured by reacting a coke sample with CO₂ at 1,100 °C for 2 hours — simulating conditions in the thermal reserve zone and upper cohesive zone — then measuring mass loss (CRI) and residual strength (CSR). The reason coke weakens in CO₂ at 1,100 °C is the Boudouard reaction: CO₂ + C → 2CO gasifies the coke carbon, preferentially attacking the cell wall pores and the disordered carbon microstructure. High-alkali coke is especially vulnerable because potassium and sodium, which circulate in a volatile cycle in the lower furnace (vaporise at the tuyeres, condense in the stack, catalyse Boudouard reaction on coke surfaces), accelerate this gasification and weaken the coke structure.

The concept of the coke slot is central to understanding BF gas flow. In the cohesive zone, softened ore layers are impermeable to gas — ascending gas is forced to flow laterally through the alternating coke layers, which remain solid through the cohesive zone temperature range. These coke layers are the coke slots. If coke degrades and collapses in the cohesive zone, the slots close, gas channels form, and the furnace loses gas distribution control. This is why minimum coke rate cannot fall below approximately 280–320 kg/tHM even with high PCI rates: the physical coke volume in the burden must maintain adequate slot thickness for gas permeation. Alkali attack on coke — where potassium and sodium carried up by the ascending gas condense on coke surfaces at 700–900 °C and catalyse carbon gasification — further limits how hard an operator can push PCI without compensating with higher-CSR coke.

Hot blast — air preheated to 1,000–1,250 °C in Cowper stoves and enriched to 23–28% oxygen — is injected through 20–42 tuyeres arranged symmetrically around the furnace circumference. The blast velocity is 150–250 m/s, creating a cavity in the coke bed around each tuyere called the raceway — an ellipsoidal void of approximately 1.0–1.5 m depth where combustion occurs.

The theoretical (adiabatic) flame temperature at the tuyere nose — the Raceway Adiabatic Flame Temperature, or RAFT — is the single most important blast furnace operating parameter. RAFT determines the energy available for ore melting and slag formation in the cohesive zone. Target RAFT is 2,000–2,300 °C, depending on ore type and slag volume. Increasing oxygen enrichment raises RAFT; adding moisture or pulverised coal injection (PCI) lowers it.

Pulverised coal injection (PCI) replaces a portion of the coke charged at the top. Coal is pulverised to <75 µm, pneumatically conveyed, and injected through a lance inside the tuyere. Modern plants inject 150–220 kg coal per tonne of hot metal (kg/tHM), reducing coke rate by approximately 0.8–0.9 kg coke per kg coal injected. PCI is economically driven — coal costs less than metallurgical coke — but also reduces CO₂ emissions on a per-heat basis since more of the carbon enters as coal rather than coke. Natural gas and oil injection are also practiced at some plants; hydrogen injection is under active development as a low-carbon ironmaking lever.