Lecture 06

Lecture 6: Thermal Aspects, Internal Zones & Slag-Metal Reactions

1. Raceway Adiabatic Flame Temperature (RAFT)

The Raceway is the combustion zone in front of the tuyeres where the hot blast reacts with coke.

Reaction: $C + \frac{1}{2} O_{2} \to C O$ (Exothermic).
- This generates intense heat ( $\approx 190 0^{\circ} C - 200 0^{\circ} C$ ).
- The gas environment here is effectively 100% CO; any $C O_{2}$ formed is instantaneously converted to CO due to the high temperature and presence of coke.
RAFT Concept: It is the theoretical maximum temperature achieved under adiabatic conditions (no heat loss).
Board Formula (Simplified Approximation):

The professor presented a simplified linear expression to estimate RAFT based on input parameters (Blast Temp, Coke Temp, etc.):

$R A F T \approx \frac{Heat form Combustion + Sensible Heat of Blast + Sensible Heat of Coke}{Volume of Gas \times Specific Heat}$

Key Takeaway: Increasing Blast Temperature ( $T_{B}$ ) increases RAFT.
Factors Affecting RAFT:
- Oxygen Enrichment: Increases RAFT.
- Moisture ( $H_{2} O$ ): Decreases RAFT (due to endothermic reaction $C + H_{2} O \to C O + H_{2}$ ).

2. Blast Furnace Thermal & Gas Profile

Gas: Rises from Tuyeres ( $\approx 200 0^{\circ} C$ ) to Top ( $\approx 30 0^{\circ} C$ ).
- Moves very fast (seconds).
Solids: Descend from Top ( $\approx 2 5^{\circ} C$ ) to Hearth ( $\approx 150 0^{\circ} C$ ).
- Moves very slowly (hours).
Efficiency Indicator:
- Lower Top Gas Temperature = Better Thermal Efficiency (more heat transferred to solids).
- Lower CO in Top Gas = Better Chemical Efficiency (more CO utilized for reduction).

3. Internal Structure of the Blast Furnace (The 4 Zones)

Board Work Description: The professor drew a cross-section of the furnace dividing it into four distinct physical zones based on the state of the materials.

Zone 1: The Granular Zone (Upper Stack)

State: Solid particles only.
Materials: Lump Ore, Coke, Limestone remains solid.
Reactions: Indirect reduction ( $F e_{2} O_{3} \to F e_{3} O_{4} \to F e O$ ).

Zone 2: The Cohesive Zone (Middle/Lower Stack)

State: Softening and Melting (Mushy Zone).
Process:
- Iron ore and gangue start to soften and fuse.
- Formation of primary slag (Fayalite: $2 F e O \cdot S i O_{2}$ ) and liquid iron.
- Coke remains solid: Acts as the only permeable window for gas to pass through (Critical for aerodynamics).
Shape: Often inverted V or W shape (though not explicitly drawn in this specific clip, it is implied by the zone description).

Zone 3: The Active Coke & Deadman Zone (Lower Bosh)

Active Coke: Coke descending into the raceway to be burned.
Deadman Zone (Central Column): A stagnant cone of coke at the very center-bottom that does not move much and does not burn (because gas doesn’t penetrate deep enough).
- Liquid Iron and Slag must trickle down through this permeable coke bed (like water through a filter).
- Carbon pickup (Carburization) happens here as iron touches the coke.

Zone 4: The Hearth

State: Liquid Pool.
Stratification:
- Slag (Top Layer): Lighter oxides ( $C a O, S i O_{2}, A l_{2} O_{3}, M g O$ ).
- Hot Metal (Bottom Layer): Liquid Fe-C-Si-Mn-S-P alloy.

(ASCII Representation of Board Diagram: Internal Zones)

Plaintext

      |   Granular Zone    |  <-- Solids (Ore+Coke)
      |     (Solids)       |
      \                    /
       \   Cohesive Zone  /   <-- Softening/Melting
        \    (Mushy)     /        (Coke is the only solid)
         \              /
          |  Active    |
          |   Coke     |
      ----|            |----  <-- Tuyeres (Blast Injection)
     |    |  Deadman   |    |
     |    | (Stagnant) |    |
     |____|____________|____|
     |      Slag Layer      |
     |____Hot_Metal_Layer___|

4. Chemical Reactions by Zone (Board Work Table)

The professor tabulated reactions occurring as the burden descends.

Region	Primary Reactions	Notes
Upper Stack	$F e_{2} O_{3} \to F e_{3} O_{4}$ $3 F e_{2} O_{3} + C O \to 2 F e_{3} O_{4} + C O_{2}$	Low Temp, Indirect Reduction.
Lower Stack	$C a C O_{3} \to C a O + C O_{2}$ (Calcination) $F e O + C O \to F e + C O_{2}$ $C + C O_{2} \to 2 C O$ (Solution Loss) Gasification rxn vol & pressure change overall $F e + C - > F e + C O$ (Feels like Direct)	Solution Loss is endothermic. Combined with FeO reduction, it mimics “Direct Reduction.” but actual 2step rxn pressure & temp ⇒ influence on rxn pressure - gasification control
Bosh	$S i O_{2} + 2 C \to \underline{S i} + 2 C O$ $S i O_{2} (g) + S (g) + C \to S i S (g) + C O (g)$ $S i S \to [S] + [S i]$ Decomposes unstable and [] indicate dissolve into metal $M n O + C \to \underline{M n} + C O$ $C a O + S + C \to C a S + C O$	High Temp & High Reduction Potential. Slag formation begins.
Hearth	Slag-Metal Reactions (Desulfurization)	Final adjustment of Si, S, Mn.

$F e + S i O 2 - > F e O . S i O 2 (l)$ Mainly Stack $F e . S i O 2 + C + C a O - > C a O . S i O 2 + C O + F e$ (As $C a O$ More Basic Oxi. e and $S i O 2$ acidic huge amount of attraction ) In Bosh

$C a O . S i O_{2} - - A l_{2} O_{3} + T i O_{2}$ Gets mix in it

Bosh + Lower Stack / Upper Hearth Region

$()$ Means in oxide phase $[]$ in dissolved phase

Note: Underlined elements ( $\underline{S i}, \underline{M n}$ ) indicate they are dissolved in the liquid metal.

5. Slag-Metal Partitioning & Chemistry

The Behavior of Elements (Exam Critical):

Phosphorus (P):
- Thermodynamics: P-oxide line is very close to Fe-oxide line.
- Result: 100% of Phosphorus in the ore reduces and goes into the Hot Metal. It cannot be removed in the Blast Furnace.
Manganese (Mn):
- Behaves similarly to Iron. Most MnO reduces to Mn and enters the metal.
- Blast furnace slag contains very little MnO or FeO (<1%).
Silicon (Si):
- Reaction: $(S i O_{2}) + 2 C ⇌ [Si] + 2 C O$ (Endothermic). Take place at lower part of furnace
- Partitioning: Partial. Some stays in slag as $S i O_{2}$ , some enters metal as Si.
- Control: Higher Temperature (High RAFT) $\to$ Drives reaction forward $\to$ High Silicon in Metal.
Sulfur (S):
- Reaction: $[S] + (C a O) + C \to (C a S) + C O$ .
- Requirement: Reducing atmosphere + High Basicity (CaO) + High Temperature.
- Result: The Blast Furnace is the ideal place to remove Sulfur (unlike the Basic Oxygen Furnace).

Slag Basicity:

Defined as the “V-Ratio”: $V = \frac{% C a O}{% S i O _{2}}$ .
Changes dynamically as the slag trickles down and assimilates ash from the coke.

$C a O, A l_{2} O_{3}, S i O_{2}, C a S$ + $(F e O)$ if any left then will react with C(rxn below) $- - - - - - - - - - - - - - - -$ Slag-Metal Interface $F e, C, S i, M n, S, P, T i$

$[C] + (F e) \to C O + [F e]$ Same for MnO if present any So HM never contains <0.01% FeO and MnO Oxygen Partial Pressure → $1 0^{- 19}$ atm no free oxygen can exist

Final Data Check (Verification)

RAFT Formula: The specific coefficients (4.6, 1.4 etc.) mentioned by the professor are empirical approximations for the adiabatic heat balance. The concept (Energy In / Heat Capacity) is physically correct.
Phosphorus: The statement that “All P goes to metal” is a standard metallurgical fact for Blast Furnaces.
Internal Zones: The division into Granular, Cohesive, Active Coke, and Hearth is the standard “Dissection Analysis” model (pioneered by Japanese researchers).

Conclusion: The notes are accurate to the lecture content.

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