Lecture 05

Lecture 5: Thermodynamics & Kinetics of Iron Making

(Physical, Chemical, and Thermal Characteristics of the Blast Furnace)

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1. Overview of the Process

The Blast Furnace (BF) is primarily an Indirect Gaseous Reduction process.

• Direct Reduction: Reduction by solid Carbon ($C$) – happens in the lower, hotter zones.

• Indirect Reduction: Reduction by Carbon Monoxide ($CO$) gas – happens in the upper stack.

• Overall Reaction:

  $F{e}_2{O}_3(s)+3CO(g)\to 2Fe(s)+3C{O}_2(g)$

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2. Blast Furnace Environment (Thermal & Chemical)

A. Thermal Profile

• Tuyere/Raceway Zone (Bottom): $\approx 1900^{\circ }C-2000^{\circ }C$ (Combustion Zone).

• Stock Line (Top): $\approx 500^{\circ }C$.

• Heat Transfer: Ascending hot gases transfer heat to the descending cold solids (Counter-current exchange).

B. Gas Composition Profile (Critical Exam Correction)

• At Tuyeres:

  • The gas is not 100% CO by volume because of Nitrogen from the air blast.

  • Actual Composition: $\approx 35\text{-}45\%\text{ CO}$ and $\approx 55\text{-}60\%N_2$ (Inert).

  • Reducing Potential: Although diluted by $N_2$, the chemical potential is extremely high because the $C{O}_2$ content is effectively zero. Any $C{O}_2$ formed immediately reverts to $CO$ via the Boudouard Reaction ($C+C{O}_2\to 2CO$).

• Ascending Gas: As it rises and reduces the ore, $CO$ is consumed and $C{O}_2$ is generated.

• Top Gas: Contains $CO$, $C{O}_2$, and $N_2$.

C. The “Layering” Effect

The furnace is charged in alternate layers of Coke and Ore/Flux.

1. Ore Layer: Gas passes through $\to$ Reduces Iron Oxide $\to$ $C{O}_2$ increases.

2. Coke Layer: Gas enters hot coke $\to$ Boudouard Reaction ($C+C{O}_2\to 2CO$) $\to$ $CO$ regenerated.

3. Result: The gas exiting a coke layer is “refreshed” to high reducing potential before hitting the next ore layer.

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3. Thermodynamics of Reduction (Fe-O-C System)

Iron ore reduces in steps. The pathway depends strictly on temperature.

A. The Reduction Hierarchy

• Step 1: $F{e}_2{O}_3\to F{e}_3{O}_4$ (Hematite to Magnetite). Easy, occurs at top.

• Step 2 (The Temperature Split):

  • If Temp < 570°C: Reduction goes directly $F{e}_3{O}_4\to Fe$. (Wustite is unstable).

  • If Temp > 570°C: Reduction goes $F{e}_3{O}_4\to FeO$ (Wustite).

• Step 3: $FeO\to Fe$ (Wustite to Iron). Most difficult step, requires high CO and Temp > 1000°C.

B. Equilibrium Logic

• For the reaction: $FeO+CO\rightleftharpoons Fe+C{O}_2$

• Equilibrium Constant: $K_{eq}=\frac{pC{O}_2}{pCO}$.

• Condition for Reduction: The actual gas ratio in the furnace must be “richer” in CO than the equilibrium value.

  ${\left(\frac{pCO}{pC{O}_2}\right)}_{actual}>{\left(\frac{pCO}{pC{O}_2}\right)}_{equilibrium}$

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4. Kinetics of Reduction (Rate of Production)

Thermodynamics tells us if it reacts; Kinetics tells us how fast.

A. Topochemical (Shrinking Core) Model

• Assumption: Iron ore particle is spherical.

• Reaction proceeds from the surface inward.

• Structure:

  • Outer Shell: Porous reduced Iron ($Fe$).

  • Inner Core: Unreacted Oxide ($FeO$).

  • Interface: Moves toward the center over time.

B. The 5 Kinetic Resistance Steps

The reaction must overcome 5 resistances in series. The slowest step controls the rate.

1. Gas Transport: CO diffuses through the Gas Boundary Layer surrounding the particle.

2. Pore Diffusion (In): CO diffuses through pores of the reacted Iron shell.

3. Chemical Reaction: Interfacial reaction ($FeO+CO\to Fe+C{O}_2$).

4. Pore Diffusion (Out): $C{O}_2$ diffuses back out through the shell.

5. Gas Transport (Out): $C{O}_2$ diffuses away through the boundary layer.

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5. Factors Influencing Reduction Rate

1. Gas Flow Velocity:

   • Higher velocity $\to$ Thinner boundary layer $\to$ Faster external diffusion (Step 1 & 5).

   • Significance: Critical if the process is “Mass Transfer Controlled.”

2. Temperature:

   • Increases reaction rate constant ($k$) exponentially (Arrhenius Law).

   • Significance: Critical if the process is “Chemically Controlled.”

3. Particle Porosity:

   • More pores = Faster internal diffusion (Step 2 & 4).

   • Note: Fluxed sinters are highly porous $\to$ Faster reduction than dense lumps.

4. Gas Pressure:

   • Higher pressure $\to$ Increases gas density and residence time (contact time).

   • Improves reduction rate by increasing effective concentration of reducing agents.

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6. Mathematical Representation

• Fraction Reacted ($f$):

  $f=1-{\left(\frac{r_i}{r_0}\right)}^3$

  • $r_0$: Initial radius of the pellet.

  • $r_i$: Radius of the unreacted core at time $t$.

• Rate: $df/dt$ (measured in lab by weight loss / oxygen loss over time).