Lecture 07

Lecture Notes: Blast Furnace Reactions, Products, and Productivity

1. Internal Zones & Chemical Reactions

1.1 The Cohesive Zone (Softening & Melting)

As the burden (ore, coke, flux) descends from the Granular Zone into the Cohesive Zone, the temperature increases significantly.

• Primary Slag Formation: The first liquid to form is a result of the reaction between Iron Oxide ($FeO$) and Silica ($Si{O}_2$).

  • Reaction Product: Fayalite ($2FeO\cdot Si{O}_2$).

  • Condition: This occurs during the initial softening/fusion phase. [00:59]

• Temperature Profile: The temperature increases descending the furnace, reaching a maximum at the Tuyere Level (Raceway) of 1900°C – 2000°C, before dropping slightly in the hearth to 1400°C – 1450°C. [01:26]

1.2 The Bosch Region: Metalloid Reduction

The Bosch region is characterized by intense heat and a reducing atmosphere, driving the Metalloid Reduction Reactions. These are endothermic reactions where stable oxides in the slag are reduced and dissolved into the hot metal. [14:12]

Key Reactions:

1. Silicon Reduction:

   $(Si{O}_2{)}_{slag}+2[C{]}_{metal}\to [Si{]}_{metal}+2CO(g)$

   • Note: Reduction is never 100% complete. Silicon partitions between the metal and slag based on furnace thermodynamics.

   • Mechanism: Often proceeds via a gaseous intermediate, Silicon Sub-oxide ($SiO$). [02:08]

2. Manganese Reduction:

   $(MnO{)}_{slag}+[C{]}_{metal}\to [Mn{]}_{metal}+CO(g)$

   • Manganese behaves similarly to Iron (atomic weight 55 vs 56) and forms an ideal solution with Iron. [15:00]

3. Phosphorus Reduction:

   • Assumption: 100% of Phosphorus in the burden enters the Hot Metal.

   • Reasoning: Phosphorus oxides are easily reduced under blast furnace conditions and have zero stability in the slag phase.

   • Material Balance Consequence: $P_{input}\approx P_{hot\_metal}$ (ignoring minor dust losses). [04:18]

4. Iron Reduction:

   • By the time the charge reaches the hearth, iron oxide reduction is nearly complete.

   • $(FeO)$ in Slag: Typically < 0.1 wt%. [05:28]

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2. Sulfur Removal (Desulfurization)

Desulfurization is a critical “Slag-Metal Interface” reaction that must occur in the Blast Furnace because it cannot be efficiently done during Steelmaking (which is an oxidizing process). [16:40]

2.1 The Reaction Equation (Board Work)

$[FeS{]}_{metal}+(CaO{)}_{slag}+[C{]}_{metal}\to [Fe{]}_{liquid}+(CaS{)}_{slag}+CO(g)$

• $[FeS]$: Sulfur dissolved in metal (Iron has high affinity for S).

• $(CaO)$: Lime in the slag (Basicity agent).

• $(CaS)$: Calcium Sulfide (Stable form of sulfur in slag).

2.2 Mechanism & Thermodynamics

• Role of Carbon: The reaction is favored in the Blast Furnace because the metal is saturated with Carbon. Carbon raises the activity coefficient of Sulfur in the metal (making S “uncomfortable” and “loose”), pushing it into the slag. [18:00]

• Favorable Conditions:

  1. High Basicity: High concentration of free Lime $(CaO)$.

  2. Reducing Atmosphere: Low Oxygen potential ($P_{O_2}\approx 10^{-19}$ atm).

  3. High Temperature: Although the reaction is exothermic (thermodynamically favored at low T), High T is required kinetically to ensure the slag is fluid enough for the reaction to proceed. [20:12]

2.3 Instructor’s Important Warning (Steelmaking vs. Ironmaking)

“Desulfurization must be accomplished in the Blast Furnace or via external pre-treatment. It cannot be done in the Basic Oxygen Furnace (Steelmaking).”

• Reason: Steelmaking is an Oxidizing process with no Carbon in the melt. Without C and a reducing environment, the desulfurization reaction cannot proceed effectively. [21:00]

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3. Operational Challenges: The “Indian Context”

The instructor highlights a specific trade-off prevalent in processing Indian Iron Ores.

3.1 The Alumina ($A{l}_2{O}_3$) Problem

• Issue: Indian ores have a high Alumina-to-Silica ratio. High Alumina increases slag viscosity (makes it thick/sticky). [10:00]

• Counter-measure: To keep the slag fluid (absorb Alumina), operators must increase the Hearth Temperature.

• The Consequence (The Trade-off):

  • High Temperature $\to$ Good Slag Fluidity $\to$ Better Desulfurization.

  • BUT High Temperature $\to$ Promotes Endothermic Silicon Reduction $\to$ High Silicon in Hot Metal.

3.2 Impact on Downstream Steelmaking

• High Si in Hot Metal (>0.7%):

  • Generates excessive heat during steelmaking (oxidation of Si is exothermic).

  • Damages refractory linings of the converter.

  • Increases slag volume in steelmaking (requires more lime to neutralize produced silica).

• Target: Steelmakers demand Hot Metal Si $\approx 0.6-0.7\%$. [16:17]

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4. The “Deadman” Zone

• Physical Description: A stagnant, packed bed of solid Coke at the bottom of the furnace.

• Function:

  1. Permeability: Since Coke is the only solid phase at $>1500^{\circ }C$, it maintains the structural integrity of the bed, allowing gas to flow up and liquid to trickle down. [07:00]

  2. Saturation: As iron trickles through this coke bed, it picks up Carbon until it reaches saturation (~4.5% C).

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5. Products of the Blast Furnace

5.1 Hot Metal (Pig Iron)

Element	Approx. Composition (Wt %)	Notes
Iron (Fe)	~93 - 94%	Balance
Carbon (C)	~4.5%	Saturated (depends on T)
Silicon (Si)	~0.6 - 1.0%	Controlling parameter
Manganese (Mn)	~1.0%	Varies with ore
Sulfur (S)	~0.05%	Must be minimized
Phosphorus (P)	~0.1%	Depends on ore

• Temperature: 1350°C – 1450°C

5.2 Blast Furnace Slag

• Composition:

  • $CaO$: ~35 - 40%

  • $Si{O}_2$: ~30 - 35%

  • $A{l}_2{O}_3$: ~20 - 25% (High in Indian context)

  • $MgO$: ~5 - 10%

• Basicity Ratio ($V$): $\frac{\%CaO}{\%Si{O}_2}\approx 1.0-1.2$ [41:27]

• Volume: ~300 - 350 kg slag per ton of hot metal.

• Utilization: Road construction, cement making (valuable byproduct).

5.3 Top Gas (Off Gas)

• Composition:

  • $N_2$: ~55%

  • $CO$: ~30% (Fuel source)

  • $C{O}_2$: ~10 - 15%

• Energy Value: Contains significant chemical energy (~3900 kJ/mol CO). Cleaned and used as fuel elsewhere in the plant. [33:45]

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6. Blast Furnace Productivity

6.1 Definition 1: Volumetric Productivity

The standard industrial definition of productivity is:

$P=\frac{\text{Daily Production}}{\text{Working Volume}}$

• Unit: Tons of Hot Metal per Day per Cubic Meter ($T/d/m^3$). [45:18]

• Formula: $P=\frac{Q_{HM}}{V_{BF}}$

6.2 Definition 2: Wind-Rate Based (Conceptual Derivation)

To understand how to increase productivity, the instructor re-defines the term $Q_{HM}$ (Daily Production) using the Blast Volume.

$Q_{HM}=\frac{\text{Total Blast Volume per Day}}{\text{Blast Rate required per Ton of Hot Metal}}$

Substituting this into the productivity equation:

$P\propto \frac{\text{Total Wind Volume (Blowing Rate)}}{\text{Specific Wind Rate}}$

6.3 Physical Interpretation

• To increase Productivity ($P$):

  1. Increase Blowing Rate: Pump more air/oxygen into the furnace per day.

  2. Decrease Specific Wind Rate: Improve efficiency so less air is needed to produce one ton of iron (e.g., via oxygen enrichment).

• Instructor’s Warning: You cannot simply increase the blowing rate arbitrarily (“Greedy Approach”). Excess blowing velocity can disrupt the packed bed, cause channeling, or “choking” of the furnace. [51:50]