Lecture 3: Raw Materials and Burden Preparation

1. Introduction and Scope

This lecture focuses on the inputs (feed materials) to the Blast Furnace (BF), collectively known as the Burden Material.

Context: Lecture 3 of 5 on Blast Furnace Iron Making.
Objective: To understand the physical and chemical characteristics required for materials (Ore, Coke, Flux) to be charged into a modern BF.

2. Blast Furnace Inputs: The Burden

A. Material Feed (Through the Top/Throat)

These materials are charged from the top and descend by gravity.

Iron Bearing Materials:
- Lump Iron Ore: Naturally mined ore.
- Sinters & Pellets: Man-made/agglomerated iron-bearing materials (crucial for modern furnaces).
Fluxes:
- Limestone ( $C a C O_{3}$ )
- Dolomite ( $C a C O_{3} \cdot M g C O_{3}$ )
Fuel & Reductant:
- Coke: The primary fuel and structural support.

B. Fluid/Energy Feed (Through Tuyeres/Stack)

Hot Blast: Preheated air (~1200°C) injected through tuyeres.
Auxiliary Injections (Modern BF features):
- Pulverized Coal Injection (PCI): Coal dust injected to replace expensive coke.
- Natural Gas / Oil: Sometimes injected as auxiliary fuel.
- Stack Gas Injection: Cleaned BF gas recycled into the stack (advanced concept).
- Lime Dust: Sometimes injected.

C. Outputs (Products)

Hot Metal: Liquid Iron saturated with Carbon.
Slag: Liquid waste containing impurities (Gangue + Flux).
Top Gas (Off-gas): Contains $C O, C O_{2}, N_{2}$ , and dust.

3. Burden Characteristics & Preparation

Why prepare raw materials?

Mined materials (Run-of-Mine) cannot be charged directly due to:

Impurities: High gangue content.
Physical Limitations: Incorrect size, shape, or low strength.
Chemical Inconsistency: Variation in grade.

Key Characteristics Required:

Physical:
- Size & Shape: Uniform size ensures void space for gas flow (permeability).
- Strength: Must withstand the massive load of the burden column and thermal shock without crumbling into dust (fines block gas flow).
Chemical:
- Grade: High Iron content (Fe %).
  - Instructor Note: Charging low-grade ore (e.g., 35% Fe) is economically unviable as it produces more slag than metal. Modern furnaces require high grades (>60-64% Fe).
- Reducibility: Ease with which oxygen can be removed by reducing gases.

4. Coke: The Critical Input

A. Why Coke and not Coal?

Direct use of raw coal in the BF is limited because:

Low Strength: Coal crumbles under the burden load.
Volatiles: Raw coal contains tars and volatiles that would clog the gas cleaning system.
Impurities: Coal contains alkalis (bad for refractories) and sulfur.
Carbon Content: Coke has a much higher “Fixed Carbon” content than coal.

B. Functions of Coke in the Blast Furnace

Coke performs three distinct and critical roles:

Fuel (Thermal): Combustion ( $C + O_{2} \to C O_{2}$ ) provides the heat for endothermic reactions (e.g., limestone decomposition) and melting of iron/slag.
Reductant (Chemical): Generates the reducing gas ( $C O$ ) via the Boudouard reaction ( $C + C O_{2} \to 2 C O$ ) to strip oxygen from iron ore.
Permeability (Structural - Most Important):
- In the lower part of the furnace (The Hearth/Bosh), everything is liquid (Iron, Slag) except Coke.
- Coke remains solid at 2000°C.
- It forms a permeable grid/bed through which liquid iron trickles down and hot gases rise up. Without coke, the furnace would “choke” or flood.

C. Coke Rate Evolution

Definition: Kg of Coke consumed per Tonne of Hot Metal (thm).
Past (30-40 years ago): ~800 kg/thm.
Modern Best Practice: ~500 kg/thm.
Theoretical Minimum: ~300-350 kg/thm (Thermodynamic limit).
- Goal: Reduce Coke Rate to lower Carbon footprint ( $C O_{2}$ emissions).
- Method: Use auxiliary fuels (PCI, Hydrogen, Natural Gas).

5. Coke Making Process (Carbonization)

A. The Process

Feed: Specific “Coking Coal” (Bituminous grade with specific rheological properties).
Method: Destructive Distillation (Heating in the absence of air).
Conditions: 1100°C – 1200°C for 18–36 hours in Coke Ovens.
Transformation:
- Volatiles driven off (Tar, Ammonia, Benzene, Gases).
- Coal mass softens, swells, and re-solidifies into a strong, cellular, porous carbon mass called Coke.

B. Coke Oven Battery Structure (Board Work)

Large batteries containing dozens (up to 100) of narrow, tall heating chambers.
Sandwiched between heating flues where BF gas or Coke Oven gas is burnt to supply heat.

C. Quenching (Cooling the hot coke)

When coke is pushed out of the oven, it is red hot (~1100°C) and will burn if exposed to air. It must be cooled immediately.

Wet Quenching (Traditional):
- Spray water on hot coke.
- Issue: Adds moisture to the coke. Charging wet coke into the BF causes the Water Gas Reaction ( $C + H_{2} O \to C O + H_{2}$ ), which is highly endothermic (steals heat from the furnace).
Dry Quenching (CDQ - Coke Dry Quenching):
- Cool using inert gas (Nitrogen).
- Benefit: Recovers sensible heat to generate steam/electricity; produces dry coke with better strength.

6. Important Reactions Mentioned

A. The “Gasification” or “Boudouard” Reaction

$C (s) + C O_{2} (g) ⇌ 2 C O (g)$

Nature: Highly Endothermic (Consumes Heat).
Location: Occurs in the hotter parts of the stack.
Significance: This is known as “Solution Loss.” It consumes carbon (coke) without generating heat, thereby increasing the coke rate. However, it is necessary to regenerate the reducing agent ( $C O$ ).

B. Water Gas Reaction (If wet coke is used)

$C (s) + H_{2} O (g) \to C O (g) + H_{2} (g)$

Nature: Endothermic.
Instructor Note: A small amount of Hydrogen is beneficial (better diffusivity than CO), but excess moisture acts as a thermal load.

7. Quality Requirements for Coke

To survive the hostile BF environment, coke is tested for:

Strength (Cold): Resistance to breakage during handling.
CSR (Coke Strength after Reaction):
- Simulates BF conditions.
- Measures strength after the coke has reacted with $C O_{2}$ (Solution Loss) at high temperatures.
- High CSR is critical for large Blast Furnaces to maintain permeability in the lower zones.
Reactivity (CRI - Coke Reactivity Index):
- How fast it reacts with $C O_{2}$ .
- Generally, Lower Reactivity is preferred to prevent premature consumption of coke in the stack.

Quartz 4

Explorer

Lecture 03