Lecture 04

Lecture 4: Iron Ore & Agglomeration

1. Coke Quality Recap (Continuation from Lecture 3)

Before moving to Iron Ore, the instructor finalizes the critical parameters for selecting Blast Furnace (BF) Coke.

Five Essential Qualities of Good Coke

1. Chemical Composition (Fixed Carbon):

   • Priority: High Fixed Carbon content is essential.

   • Reason: Fixed Carbon determines the Calorific Value. Lower fixed carbon means higher impurity (ash/gangue like silica, calcium sulfide), which acts as “garbage” filling the furnace volume without contributing energy or reduction potential.

2. Reactivity:

   • Requirement: Moderate reactivity with a cellular/porous structure.

   • Reason: Coke must burn (combust) readily at the tuyeres but should not be too reactive in the upper stack (to avoid excessive Solution Loss Reaction).

3. Size and Size Range:

   • Requirement: Narrow size distribution (Mean size ± small variation).

   • Reason:

     • Too Large: Poor volume utilization (large voids).

     • Too Small: High packing density but very low permeability (chokes gas flow).

     • Wide Distribution: Small particles fill the voids between large particles, destroying permeability.

4. Strength:

   • Requirement: High Cold Strength and High Hot Strength (CSR).

   • Reason: Must withstand the physical load of the burden column and thermal shocks (from 500°C at top to 2000°C at tuyeres) without degrading into fines.

5. Thermal Stability:

   • Requirement: Must not suffer from Reaction Degradation.

   • Mechanism: The gasification reaction ($C+C{O}_2\to 2CO$) is endothermic. It occurs at the surface, cooling the exterior of the coke particle while the core remains hot. This $\Delta T$ creates thermal stress $\to$ cracks/fissures $\to$ fines generation.

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2. Iron Ore: Types and Characteristics

A. Ore Varieties

• Hematite ($F{e}_2{O}_3$):

  • Purity: Theoretically 70% Fe, 30% O.

  • Reality: Good natural ore is 60–65% Fe. The rest is gangue.

  • Availability: Abundant in India (Chhattisgarh, MP, Karnataka, Odisha).

• Magnetite ($F{e}_3{O}_4$):

  • Used in other parts of the world; less common in India.

B. Gangue Materials (Impurities)

Natural ore is never pure. It contains oxides that behave differently in the BF:

Gangue Oxide	Chemical Formula	BF Behavior (Ellingham Diagram Logic)	Outcome
Alumina	$A{l}_2{O}_3$	Very Stable. $C$ cannot reduce it even at >2000°C.	Remains in Slag ($A{l}_2{O}_3$).
Silica	$Si{O}_2$	Partially Reducible at high T (~1600°C).	Mostly Slag, some enters Metal as $Si$.
Titania	$Ti{O}_2$	Partially Reducible.	mostly Slag, some enters Metal.

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3. The Need for Agglomeration

The Problem: Fines Generation

Mining and crushing iron ore to the required size (10–30 mm) generates huge amounts of undersize particles (Fines).

• Fines (< 10 mm): cannot be charged directly into the BF because:

  1. They block gas flow (permeability crisis).

  2. They are blown out by the high-pressure gas (dust losses).

• Environmental Hazard: Fines cause pollution if left in open piles.

The Solution: Agglomeration

Agglomeration is the process of clustering fine particles into larger, engineered lumps.

• Two Main Technologies:

  1. Pelletization: For extremely fine particles (microns, “flour-like”).

  2. Sintering: For coarser fines (3–6 mm, “chips/semolina-like”).

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4. Pelletization

Feed Material: Ultra-fine iron ore (< 100 microns) + Water + Binder (Bentonite) + Limestone (optional).

Process Overview (Step-by-Step)

1. Mixing: Ore fines + Water + Binder are mixed to form a “dough”.

2. Balling (Green Pelletization):

   • Done in Disc or Drum Pelletizers.

   • Mechanism: Rotation causes particles to bridge via capillary forces (water) and binder, growing into spheres (like rolling dough balls).

   • Product: Green Pellets (soft, weak, will crumble if dropped).

3. Induration (Firing):

   • Green pellets are fired at 1100°C – 1300°C on a moving grate furnace.

   • Stages: Pre-heating $\to$ Firing $\to$ Cooling.

Bonding Mechanism (Firing)

• At 1100–1300°C, low melting point phases (Iron silicates, Calcium ferrites) form a liquid slag phase.

• Upon cooling, this liquid solidifies, forming a Slag Bond (Glassy bridge) between ore particles.

• Instructor Note:

  • Too much slag bond: Very strong pellet, but dense and low porosity (Bad Reducibility).

  • Optimized process: Balances strength vs. porosity.

Types of Pellets

1. Acid Pellets: No Flux added. (Basicity = $\frac{CaO}{Si{O}_2}\approx 0$).

2. Fluxed Pellets: Limestone added. (Basicity > 0).

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5. Sintering

Feed Material: Iron Ore Fines (3–10 mm) + Coke Breeze (Small coke particles) + Flux (Limestone) + Moisture + Return Sinter.

Process Principle

Sintering is a thermal agglomeration process involving localized combustion and fusion.

The Charge Mix (Sinter Mix):

1. Iron Ore Fines: The main material.

2. Coke Breeze: Fuel source (internal heat generation).

3. Limestone ($CaC{O}_3$): Flux. (Gets calcined during sintering).

4. Return Sinter: Under-sintered material recycled to improve permeability.

5. Moisture: Critical for bed permeability and heat transfer control.

Mechanism (The Sintering Machine)

(Conceptual Reconstruction of Board/Verbal Description)

1. The Bed: A layer of mix is laid on a moving grate.

2. Ignition: The top surface is ignited by external burners.

3. Suction: Air is sucked downwards through the bed.

4. Heat Front: The combustion zone ($C+O_2\to C{O}_2$) travels downwards.

   • Top: Hot/Sintered.

   • Middle: Combustion Zone (Max T ~1200–1300°C).

   • Bottom: Wet/Cold mix.

5. Fusion: High T causes partial melting and recrystallization. Particles “weld” together into a porous, irregular clinker (Sinter Cake).

Chemical Changes during Sintering

• Limestone Calcination: $CaC{O}_3\to CaO+C{O}_2$. (This saves the BF from doing this endothermic work).

• Slag Formation: $CaO+Si{O}_2+FeO\to$ Complex Silicates/Ferrites.

• No Metallization: Sintering is an oxidation atmosphere process (mostly). $F{e}_2{O}_3$ remains largely oxides; it does not become metallic Fe.

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6. Comparison: Pellets vs. Sinter

Feature	Pelletization	Sintering
Feed Size	Ultra-fines (Microns)	Coarse Fines (3–10 mm)
Shape	Spherical (Balls)	Irregular chunks (Clinker)
Binder	Bentonite / Organic binders	None (Coke/Heat fusion)
Fuel	External (Gas/Oil for furnace)	Internal (Coke breeze mixed in)
Site	usually at Mines (saves transport)	usually at Steel Plant (uses plant waste)
Atmosphere	Oxidizing	Oxidizing (but local reducing zones exist)

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7. Instructor’s Final Remarks

• The Efficiency Boost: A Blast Furnace operated with 100% Sinter/Pellets is significantly more efficient (higher productivity, lower coke rate) than one using Lump Ore.

• Global Trend: Modern ironmaking has largely dispensed with Lump Ore. The burden is now engineered (Engineered Porosity, Engineered Strength, Engineered Chemistry).

• Limestone Benefit: By adding limestone to Sinter/Pellets, we perform Calcination outside the BF. This reduces the thermal load on the BF, saving expensive metallurgical coke.

• Return Sinter: We aim for 90% yield. The un-sintered 10% (Returns) is crucial to recycle as it maintains the permeability of the sinter bed.