Unit 10: Extractive Metallurgy: Iron, Steel & Specialty Alloys

The production of virgin metallic iron requires three primary solid feedstocks: iron-bearing burden, metallurgical coke, and limestone/dolomite fluxing agents.

1. Iron Ore Burden Agglomeration

Natural fine iron ores ($< 6\text{ mm}$, hematite $\text{Fe}_2\text{O}_3$, magnetite $\text{Fe}_3\text{O}_4$) cannot be charged directly into a blast furnace because fine particles blind the gas voids, causing fluidization and catastrophic gas channel blowouts:

• Sintering: A blend of fine ore ($50 - 60\%$), coke breeze ($3 - 5\%$), flux (limestone, dolomite), and moisture is ignited under suction on a traveling grate (Dwight-Lloyd sintering machine). Combustion temperatures ($1250 - 1350^\circ\text{C}$) initiate partial surface melting, fusing the fines into porous, permeable clinker-like sinter cakes.
• Pelletizing: Ultra-fine concentrates ($< 45\,\mu\text{m}$) are rolled with bentonite clay binders and water in rotating disc pelletizers into green balls ($9 - 16\text{ mm}$), then fired in an induration furnace at $1250 - 1350^\circ\text{C}$ to crystallize interlocking hematite bridges.

2. Metallurgical Coke Manufacturing Chemistries

Metallurgical coke provides three indispensable functions in the blast furnace: (1) chemical reducing agent ($\text{CO}$ gas generation), (2) thermal fuel source, and (3) permeable mechanical support grid supporting the entire burden column in the high-temperature dripping and hearth zones where all other iron materials have melted into liquid.

• Coking Process: High-volatile, medium-volatile, and low-volatile bituminous coking coals are blended and heated to $1000 - 1100^\circ\text{C}$ in the absence of air in vertical slot coke ovens for $18 - 24\text{ hours}$.
• Volatile matter ($20 - 35\%$) is driven off as coal chemicals (coke oven gas, coal tar, ammonium sulfate, crude benzol).
• Coal softens into a plastic mass ($350 - 450^\circ\text{C}$), resolidifies into semi-coke, and contracts into a rigid, highly porous ($45 - 55\%$ voidage), mechanically strong carbon matrix ($> 88 - 90\text{ wt}\%\text{ fixed carbon}$).

The iron blast furnace is a massive counter-current chemical reactor ($30 - 40\text{ m}$ height, hearth diameter up to $14 - 15\text{ m}$) operating continuously for campaigns of $15 - 20\text{ years}$ without shutdown, producing up to $10,000 - 12,000\text{ metric tons/day}$ of liquid pig iron (hot metal).

``` THE IRON BLAST FURNACE REACTOR Solid Burden Charging (Alternating Layers: Sinter/Pellets + Coke) │ ▼ ┌──────────────┐ <── Top Gas (~200°C: CO, CO2, N2 to Scrubbers) │ THROAT │ └──────┬───────┘ ▼ ┌──────────────┐ │ SHAFT / STACK│ (Upper Shaft: Indirect Reduction 400-800°C) │ │ (Lower Shaft: Wustite Reduction 800-1000°C) └──────┬───────┘ ▼ ┌──────────────┐ │ BOSH BELLY │ (Cohesive / Softening-Melting Zone: 1000-1350°C) └──────┬───────┘ ▼ ┌──────────────┐ <── Tuyeres: Hot Blast (1150-1250°C) + Pulverized Coal (PCI) │ RACEWAY ZONE │ C + 0.5 O2 ──> CO (Flame Temp ~2100-2250°C) └──────┬───────┘ ▼ ┌──────────────┐ ──> Slag Notch: Liquid Blast Furnace Slag (~1450°C) │ HEARTH │ ──> Taphole: Liquid Hot Metal / Pig Iron (~1450-1500°C) └──────────────┘ ```

1. Tuyere Combustion & Raceway Aerodynamics

Preheated air blast ($1150 - 1250^\circ\text{C}$), enriched with oxygen ($2 - 5\%\text{ O}_2$) and auxiliary pulverized coal injection (PCI), is blown through water-cooled copper tuyeres ($28 - 40$ nozzles) at sonic velocities ($200 - 250\text{ m/s}$):

• Fast combustion of coke creates a violent swirling cavity ("raceway"):

• The adiabatic flame temperature ($\text{RAFT}$) in the raceway reaches $2100 - 2250^\circ\text{C}$.
• The ascending reducing gas ("bosh gas", $35 - 40\text{ vol}\%\text{ CO}, 1 - 4\text{ vol}\%\text{ H}_2, 56 - 60\text{ vol}\%\text{ N}_2$) ascends through the burden column at $1 - 3\text{ m/s}$, transferring heat and reducing descending iron oxides.

2. Five Internal Process Zones

1. Lumpy Zone ($200 - 900^\circ\text{C}$): Solid state. Gas permeability is high through alternating coke and ore layers.

2. Cohesive Zone (Softening & Melting, $1000 - 1350^\circ\text{C}$): Iron ores soften and melt into impermeable liquid layers; gas can only pass upward through the interspersed "coke windows."

3. Active Coke Dripping Zone ($1350 - 1500^\circ\text{C}$): Molten droplets of iron and slag trickle down through the solid coke matrix.

4. Raceway Zone ($1500 - 2200^\circ\text{C}$): Intense combustion and primary gas formation.

5. Hearth ($1450 - 1500^\circ\text{C}$): Liquid hot metal ($\rho \approx 7.0\text{ g/cm}^3$) settles to the bottom, covered by an immiscible protective layer of liquid slag ($\rho \approx 2.6\text{ g/cm}^3$).

The stepwise reduction of iron oxides by carbon monoxide proceeds through three distinct oxidation states above $570^\circ\text{C}$:

Below $570^\circ\text{C}$, wüstite is thermodynamically unstable, and magnetite reduces directly to metallic iron ($\text{Fe}_3\text{O}_4 \to \text{Fe}$).

1. Indirect Reduction (Reduction by Gas)

Occurs in the upper and middle shaft ($400 - 900^\circ\text{C}$) where gaseous $\text{CO}$ acts as the reducing agent without directly consuming solid carbon:

1. $3\text{Fe}_2\text{O}_3 + \text{CO} \longrightarrow 2\text{Fe}_3\text{O}_4 + \text{CO}_2 \quad (\Delta H = -52.8\text{ kJ/mol Fe}_2\text{O}_3 \text{ [Exothermic]})$
2. $\text{Fe}_3\text{O}_4 + \text{CO} \rightleftharpoons 3\text{FeO} + \text{CO}_2 \quad (\Delta H = +36.4\text{ kJ/mol Fe}_3\text{O}_4 \text{ [Endothermic]})$
3. $\text{FeO} + \text{CO} \rightleftharpoons \text{Fe} + \text{CO}_2 \quad (\Delta H = -17.2\text{ kJ/mol FeO} \text{ [Exothermic]})$

2. Direct Reduction (Reduction by Carbon)

Occurs in the high-temperature lower shaft and bosh ($T > 950 - 1000^\circ\text{C}$). The reduction of wüstite is coupled to the endothermic Boudouard reaction (carbon gasification):

Sum of the two reactions gives the net Direct Reduction:

Direct reduction is intensely endothermic, requiring massive thermal heat supplied by burning additional coke at the tuyeres. Optimal blast furnace thermal efficiency balances the degree of direct reduction ($r_d$) at approximately $25 - 35\%$, with the remaining $65 - 75\%$ executed by indirect reduction.

The Baur-Glaessner Phase Equilibrium Diagram

Plots $\% \text{CO} / (\% \text{CO} + \% \text{CO}_2)$ vs temperature $T$:

• At $800^\circ\text{C}$, the equilibrium gas composition for wüstite reduction ($\text{FeO} + \text{CO} \rightleftharpoons \text{Fe} + \text{CO}_2$) requires at least $68\text{ vol}\%\text{ CO}$ and no more than $32\text{ vol}\%\text{ CO}_2$.
• Consequently, blast furnace top gas can never convert all $\text{CO}$ to $\text{CO}_2$; the gas discharged at the furnace throat always contains significant residual chemical energy ($20 - 24\text{ vol}\%\text{ CO}$), which is scrubbed and combusted to fire the hot blast stoves and power plant boilers.

Standard Enthalpies & Free Energies of Iron Blast Furnace Reactions

| Reaction | $\Delta H_{298}^\circ$ ($\text{kJ/mol}$) | $\Delta G^\circ(1000\text{ K})$ ($\text{kJ/mol}$) | Reaction Type | Location in Furnace | |---|---|---|---|---| | $\text{C} + \frac{1}{2}\text{O}_2 \to \text{CO}$ | $-110.5\text{ kJ}$ | $-200.3\text{ kJ}$ | Exothermic | Tuyere Raceway | | $\text{C} + \text{CO}_2 \rightleftharpoons 2\text{CO}$ | $+172.5\text{ kJ}$ | $-5.2\text{ kJ}$ | Intensely Endothermic | Lower Shaft / Bosh | | $3\text{Fe}_2\text{O}_3 + \text{CO} \to 2\text{Fe}_3\text{O}_4 + \text{CO}_2$ | $-52.8\text{ kJ}$ | $-72.1\text{ kJ}$ | Exothermic | Upper Shaft ($400 - 600^\circ\text{C}$) | | $\text{Fe}_3\text{O}_4 + \text{CO} \to 3\text{FeO} + \text{CO}_2$ | $+36.4\text{ kJ}$ | $+12.4\text{ kJ}$ | Endothermic | Middle Shaft ($600 - 800^\circ\text{C}$) | | $\text{FeO} + \text{CO} \rightleftharpoons \text{Fe} + \text{CO}_2$ | $-17.2\text{ kJ}$ | $-3.8\text{ kJ}$ | Exothermic | Shaft ($800 - 1000^\circ\text{C}$) | | $\text{FeO} + \text{C} \to \text{Fe} + \text{CO}$ | $+155.3\text{ kJ}$ | $-9.0\text{ kJ}$ | Strongly Endothermic | Bosh / Hearth ($> 1000^\circ\text{C}$) | | $\text{CaCO}_3 \rightleftharpoons \text{CaO} + \text{CO}_2$ | $+178.2\text{ kJ}$ | $+18.0\text{ kJ}$ | Endothermic | Middle Shaft ($800 - 900^\circ\text{C}$) |

The primary non-metallic product of the blast furnace is liquid slag ($250 - 350\text{ kg slag / metric ton hot metal}$), formed by the fusion of ore gangue ($\text{SiO}_2, \text{Al}_2\text{O}_3$), coke ash, and added limestone/dolomite flux ($\text{CaO}, \text{MgO}$).

1. Slag Basicity Indices

Slag chemistry is governed by the ratio of basic network-modifying oxides to acidic network-forming silica:

• Binary Basicity ($B_2$):

• Ternary Basicity ($B_3$):

• Quaternary Optical Basicity ($\Lambda$):

Calculated from individual oxide polarizabilities. For stable furnace operation, $B_2$ is targeted between $1.15$ and $1.25$. If $B_2 > 1.35$, the slag becomes overly basic; its liquidus temperature rises abruptly, precipitating dicalcium silicate ($\text{C}_2\text{S}$) crystals that make the slag viscous, crusty, and untappable. If $B_2 < 1.05$, the acidic slag exhibits excellent fluidity but loses its chemical ability to desulfurize the iron.

2. Hot Metal Desulfurization Thermodynamics

Sulfur is an intensely detrimental impurity in steel, causing hot shortness (brittleness during rolling due to low-melting intergranular $\text{FeS-Fe}$ eutectics at $988^\circ\text{C}$). Over $85 - 90\%$ of sulfur entering the blast furnace originates from coke. In the hearth, liquid slag extracts dissolved sulfur from the molten iron across the interface:

Ionic representation:

The equilibrium Sulfur Partition Ratio ($L_S$):

High desulfurization requires:

1. High basicity (high free oxygen ion activity $a_{\text{O}^{2-}}$).
2. Reducing conditions (high carbon activity $a_{[\text{C}]} \approx 1.0$, saturated in hot metal).
3. Elevated hearth temperatures ($1480 - 1520^\circ\text{C}$), which accelerate interfacial diffusion and increase $K_{\text{desulf}}$.

Under optimal operation, $L_S \approx 30 - 60$, reducing sulfur in hot metal to $0.025 - 0.040\text{ wt}\%$. Granulated blast furnace slag is vitrified with water jets and ground into GGBS for eco-friendly cement.

Hot metal tapped from the blast furnace is unsuitable for engineering structures because it contains excessive dissolved carbon ($4.0 - 4.5\text{ wt}\%$) and impurities ($0.4 - 0.8\%\text{ Si}$, $0.3 - 0.7\%\text{ Mn}$, $0.08 - 0.15\%\text{ P}$, $0.03\%\text{ S}$), rendering it extremely brittle. The Basic Oxygen Furnace (BOF / LD Converter) converts liquid pig iron into high-purity molten steel in under $16 - 20\text{ minutes}$ of supersonic oxygen blowing:

``` BASIC OXYGEN FURNACE (BOF / LD) Water-cooled Oxygen Lance (Mach 2.0-2.2 Supersonic Jets) │ ▼ ┌─────────────┐ │ CONVERTER │ <── Molten Hot Metal (75-80%) + Scrap Steel (20-25%) │ VESSEL │ <── Burnt Lime (CaO) + Dolomite Flux │ (MgO-C Ref.)│ │ │ ──> Foamy Emulsion: Gas (CO/CO2) + Liquid Slag + Droplets │ [Hot Metal]│ └──────┬──────┘ │ Bottom Tuyeres: Inert Gas Stirring (Ar / N2) ▼ Tapped Molten Steel (~1650°C, Carbon 0.04-0.08%) to Ladle Metallurgy! ```

1. Supersonic Oxygen Jet Dynamics

Pure oxygen gas ($> 99.5\%\text{ O}_2$) is injected through a multi-orifice water-cooled copper lance positioned $1.5 - 2.5\text{ m}$ above the bath at supply pressures of $10 - 14\text{ bar}$. Converging-diverging de Laval nozzles accelerate the gas to Mach $2.0 - 2.2$ ($600 - 700\text{ m/s}$):

• The supersonic jet penetrates deep into the liquid metal bath, creating an intensely turbulent hot spot ($2400 - 2600^\circ\text{C}$).
• Millions of liquid iron droplets are atomized into the slag phase, generating a high-surface-area ($> 2,000\text{ m}^2/\text{ton}$) foamy metal-slag-gas emulsion.

2. Sequential Oxidation Cascade

Oxidation reactions are fiercely exothermic, supplying all thermal energy required to melt $20 - 25\text{ wt}\%$ cold scrap steel without external fuel:

1. Silicon Oxidation ($0 - 3\text{ minutes}$):

Rapidly forms primary acidic silicate slag, requiring immediate addition of calcined lime ($\text{CaO}$) to prevent refractory dissolution.

2. Manganese Oxidation:

3. Decarburization ($3 - 14\text{ minutes}$):

At high carbon concentrations ($[\% \text{C}] > 0.3\%$), decarburization is limited solely by oxygen mass delivery rate (Constant Rate Regime, $d[\text{C}]/dt \approx 0.25 - 0.35\text{ wt}\%/\text{min}$). Below $[\% \text{C}] \approx 0.25\%$, the reaction rate transitions to liquid-phase carbon diffusion control:

4. Dephosphorization:

Favored by high slag basicity ($B_2 > 3.0$), high oxidizing potential ($(\% \text{FeO}) \approx 15 - 20\%$), and low steel temperature during the initial blow.

Liquid steel tapped from the BOF contains excess dissolved oxygen ($400 - 800\text{ ppm O}$) and dissolved gases ($[\text{H}] \approx 3 - 6\text{ ppm}, [\text{N}] \approx 40 - 80\text{ ppm}$). Secondary steelmaking refines the steel inside a refractory-lined transfer ladle prior to solidification:

1. Deoxidation (Killing of Steel)

As steel cools and solidifies, the solubility of oxygen drops precipitously, reacting with carbon to produce carbon monoxide gas bubbles that cause destructive blowholes and porosity:

• Aluminum Deoxidation: Aluminum wire or ingots are added to the tap stream:

The equilibrium constant is given by:

Dissolved oxygen plummets from $> 600\text{ ppm}$ to $< 3\text{ ppm}$.

• Inclusion Flotation: Inert argon gas is bubbled through a porous refractory plug in the ladle bottom. Rising bubble plumes capture solid alumina ($\text{Al}_2\text{O}_3$) micro-inclusions, floating them into a synthetic calcium aluminate top slag. Calcium wire injection ($\text{Ca-Si}$) converts sharp solid alumina into spherical, harmless liquid calcium aluminate inclusions ($12\text{CaO}\cdot 7\text{Al}_2\text{O}_3$).

2. Vacuum Degassing (VD & Ruhrstahl-Heraeus / RH Process)

Under vacuum ($p < 1 - 2\text{ mbar}$):

• Hydrogen Removal (Flake Elimination): Dissolved hydrogen ($[\text{H}] > 2\text{ ppm}$) causes delayed hydrogen-induced cracking and brittle shatter cracks in heavy forgings. Sieverts' law:

Exposing liquid steel to $1\text{ mbar}$ vacuum lowers dissolved hydrogen to $< 1.2\text{ ppm}$.

• Vacuum Decarburization: Enables production of Ultra-Low Carbon (ULC) steels ($[\% \text{C}] < 0.003\%$ / $30\text{ ppm}$) for automotive outer panels by shifting $[\text{C}] + [\text{O}] \rightleftharpoons \text{CO} \uparrow$ forward.

The circular scrap-based steelmaking route and modern strand casting technology represent the pinnacle of modern ferrous metallurgical efficiency:

1. Electric Arc Furnace (EAF) Technology

Modern ultra-high-power (UHP) EAFs melt $100\%$ recycled scrap steel or blended Direct Reduced Iron ($\text{DRI}$) using electrical energy:

• Three massive graphite electrodes ($600 - 750\text{ mm}$ diameter) powered by a $100 - 150\text{ MVA}$ furnace transformer strike high-current electric arcs ($40 - 60\text{ kA}$, arc temperature $> 3500 - 4000^\circ\text{C}$) directly into the scrap charge.
• Specific electrical consumption averages $350 - 420\text{ kWh/metric ton}$, assisted by oxy-fuel supersonic burners and chemical carbon/oxygen lances.

2. Direct Reduced Iron (DRI / Sponge Iron)

DRI is solid metallic iron ($92 - 96\text{ wt}\%\text{ Fe}$, Metallization $\ge 94\%$) produced by solid-state gaseous reduction of iron ore pellets without melting (e.g., Midrex or Energiron process):

Utilizing green hydrogen ($\text{H}_2$) instead of natural gas enables zero-carbon steelmaking, reducing $\text{CO}_2$ emissions by $> 95\%$.

3. Continuous Casting of Steel (Concast / Billet-Slab Casters)

Eliminating discrete ingot casting, continuous casting solidifies molten steel directly into endless semi-finished blooms, billets, or slabs:

1. Ladle to Tundish: Steel drains from the transfer ladle through a ceramic shroud into a refractory tundish that dampens fluid surges and splits flow into multiple casting strands.

2. Oscillating Copper Mold: Steel flows through a submerged entry nozzle (SEN) into a curved, water-cooled copper mold oscillating vertically ($100 - 200\text{ cycles/min}$) with synthetic mold powder lubrication. A solid steel shell ($10 - 25\text{ mm}$ thickness) freezes against the copper walls.

3. Secondary Spray Cooling Zone: The strand is withdrawn along a curved roller apron by motorized pinch rolls while high-pressure water/air-mist spray nozzles solidify the liquid steel core.

4. Torch Cutting: Flying oxy-gas torches cut the continuous strand into discrete slabs ($1.5 - 2.5\text{ m}$ width) or billets ($150 \times 150\text{ mm}$) ready for hot rolling.

Ferrous extractive metallurgy is responsible for approximately $7 - 9\%$ of total global $\text{CO}_2$ emissions ($1.85\text{ metric tons CO}_2\text{ / metric ton crude steel}$ via the traditional BF-BOF route). The transition to near-zero carbon steelmaking centers on green hydrogen reduction:

1. 100% Pure Hydrogen Direct Reduction (H2-DRI)

Replacing fossil carbon monoxide with renewable electrolytic hydrogen:

• Thermodynamic Contrasts with Carbon Monoxide Reduction:
• Reduction by $\text{H}_2$ is endothermic ($\Delta H > 0$), whereas reduction by $\text{CO}$ is exothermic ($\Delta H < 0$). Heat must be continuously supplied by electric preheating of the circulating hydrogen gas loop.
• Molecular hydrogen ($\text{H}_2$) possesses a tiny molecular radius and a gas diffusivity in porous ore pellets approximately $4\times$ faster than $\text{CO}$, accelerating reduction kinetics and permitting smaller reactor shaft volumes.
• Discharges purely non-polluting water vapor ($\text{H}_2\text{O}$) rather than greenhouse carbon dioxide ($\text{CO}_2$).

2. Hybrid Electric Arc Furnace (EAF) Melting & Slag Foaming

The resulting carbon-free Direct Reduced Iron ($\text{H}_2\text{-DRI}$) is transferred hot ($650^\circ\text{C}$) to an Electric Arc Furnace:

• Carbon Injection for Slag Foaming: In an EAF, a foamy slag is essential to shield water-cooled furnace walls from radiant arc heat and maximize arc thermal efficiency. Since $\text{H}_2\text{-DRI}$ contains $0\%\text{ C}$, a controlled charge of biogenic carbon (biochar) is injected through lances to generate foamy $\text{CO}$ bubbles ($[\text{C}] + (\text{FeO}) \to \text{Fe} + \text{CO} \uparrow$).
• Lifecycle Decarbonization: Pairing renewable solar/wind power with PEM water electrolyzers and $\text{H}_2\text{-DRI-EAF}$ slashes steel lifecycle carbon emissions to $< 0.05\text{ t CO}_2\text{ / t steel}$ ($> 97\%$ reduction).

University Honors Industrial Case Study: Hydrogen-Induced Delay Cracking (HIC) & Ladle RH Degassing

In ultra-high-strength structural steels (yield strength $> 800 - 1,000\text{ MPa}$), microscopic atomic hydrogen dissolved during steelmaking induces delayed, catastrophic brittle failure:

• Hydrogen Trapping & Recombination: In the liquid state ($1600^\circ\text{C}$), molten steel dissolves up to $6 - 8\text{ ppm}$ of atomic hydrogen ($[\text{H}]$). During cooling and solidification, hydrogen solubility plummets by an order of magnitude.
• Supersaturated interstitial hydrogen diffuses to micro-voids, non-metallic inclusions ($\text{Al}_2\text{O}_3, \text{MnS}$), and grain boundary dislocations, recombining into molecular hydrogen gas ($\text{H}_2$):

• The trapped molecular gas develops enormous internal hydrostatic pressures ($> 1,000 - 5,000\text{ MPa}$), nucleating microcracks and internal "flakes" that fracture under low external service stresses.
• Ruhrstahl-Heraeus (RH) Vacuum Circulation Degasser:

Molten steel is drawn through two refractory snorkels into a vacuum chamber ($p < 1.0\text{ mbar}$). Argon gas lift bubbles circulate the steel at $120 - 150\text{ tons/min}$. Under deep vacuum, Sieverts' law drives hydrogen desorption, stripping dissolved hydrogen down to $< 1.0 - 1.2\text{ ppm}$ in under $15 - 20\text{ minutes}$.