Coke Quality and Agglomeration

Overview

Lecture 4 continues discussion of coke quality, covering five essential qualities for blast furnace use, then transitions to iron ore preparation methods including pelletization and sintering processes.

Five Essential Qualities of Coke

• Chemical composition: Fixed carbon content determines calorific value; higher carbon means less impurity (gang material)
• Reactivity: Cellular structure required for good combustibility; coke must have reasonably high reactivity despite lump form
• Size and size range: Narrow size distribution needed (typically 10–30 mm); prevents small particles from filling voids
• Physical strength: Must withstand thermal shocks and changing environments (500°C to 2000°C) without excessive degradation
• Thermal stability: Critical for resisting solution loss reactions that create temperature gradients and internal stresses

Limestone Requirements

• Primary constituents: Calcium oxide (CaO), magnesium oxide (MgO), and silica as impurity
• Undergoes calcination at approximately 900°C: CaCO₃ → CaO + CO₂
• Size range must match other materials (10–30 mm) to maintain bed permeability
• Good reactivity essential for reactions with silica and alumina in lower furnace regions
• Permissible MgO and minimal silica content required for blast furnace operation

Iron Ore Characteristics

• Primary ore: Hematite (Fe₂O₃) most common; magnetite (Fe₃O₄) also used in some regions
• Theoretical grade: Pure hematite contains 70% iron, 30% oxygen by weight
• Practical grade: Good natural hematite contains 60–65% iron; balance is gang material
• Gang materials: Silica (SiO₂), alumina (Al₂O₃), titanium dioxide (TiO₂), trace elements
• Origin: Mostly meteoritic iron oxidized over geological time scales

Oxide Reducibility and Ellingham Diagram

• Alumina (Al₂O₃): Non-reducible under blast furnace conditions; stable below ~2200°C
• Silica (SiO₂): Partially reducible; reduction possible above ~1600°C
• Titanium dioxide (TiO₂): Partially reducible, similar behavior to silica
• Iron oxides: Readily reducible; reduction proceeds in steps: Fe₂O₃ → Fe₃O₄ → FeO → Fe
• Carbon monoxide line crosses iron oxide lines at low temperatures enabling easy reduction

Particle Size and Blast Furnace Performance

• Target size range: 10–30 mm for all solid materials (coke, ore, limestone)
• Voidage relationship: ε = (Vc - Vs)/Vc, where ε is porosity, Vc is chamber volume, Vs is solid volume
• Small particles: Increase solid volume, reduce voidage, impair gas flow (permeability approaches zero)
• Large particles: Waste furnace volume due to excessive voids between particles
• Optimum exists: Critical particle diameter gives best productivity, bed porosity, and gas passage
• Coke degradation: Solution loss reaction (C + CO₂ → 2CO) reduces coke size in upper stack
• Coke charged slightly oversized to compensate for gasification-induced size reduction in furnace

Iron Ore Size Classification and Processing

• Lump ore: Exact size (10–30 mm) charged directly to blast furnace
• Oversize: Recycled to crushers (jaw or gyratory) until proper size achieved
• Undersize: Directed to agglomeration processes (pelletization or sintering)
• Closed-circuit crushing produces all three size fractions; exact size separated by screening

Pelletization Process

• Raw material: Very fine particles (40–250 micrometers) from mines
• Mix composition: Iron ore fines + water + binders (bentonite common) + limestone fines (for flux pellets)
• Green pellet formation: Disk or drum pelletizer rotates mix; moisture and binder create balls (5–20 mm diameter)
• Ball growth mechanisms: Small ball + small ball coalescence; layering onto existing balls
• Moisture critical: Optimum water content essential; too much or too little prevents proper ball formation

Pellet Firing and Bonding

• Firing temperature: 1100–1300°C in traveling grate furnace
• Zones: Pre-heating (400°C, moisture removal) → firing (peak temperature) → cooling
• Bonding mechanisms: Iron oxide reacts with silica forming fayalite; aluminum silicate and other glassy phases form
• Liquid phase: Low-melting compounds bond particles together; solidify upon cooling creating hard pellet
• Strength-reducibility tradeoff: More slag bonding increases strength but decreases reducibility (makes pellet dense)
• Travel speed and temperature control optimize balance between adequate strength and good reducibility

Basicity Definition

• Basic formula: B = (wt% CaO) / (wt% SiO₂)
• Extended formula: B = (wt% CaO + wt% MgO) / (wt% SiO₂ + wt% Al₂O₃)
• Acidic pellets: No lime added; basicity ≈ 0
• Flux pellets: Limestone fines added; basicity > 0; calcination occurs during firing

Sintering Process

• Raw material: Coarser undersize particles (3–6 mm) from iron ore crushing
• Mix components: Iron ore + coke (or petroleum coke) + return sinter + moisture + limestone
• Operating temperature: 1100–1200°C under reducing atmosphere
• Key difference from pellets: High-temperature operation with coke provides reducing conditions; partial metallization possible
• Return sinter: Unsintered or weak material recycled to next batch

Sintering Operation Details

• Bed structure: Thoroughly mixed materials (no layering) placed in bed of specific height
• Ignition: External heat source applied to top surface; suction draws hot gases downward
• Heat front travel: Carbon ignites when threshold temperature reached; heat zone progresses downward as carbon burns
• Temperature profile: Top heats first; bottom follows with time lag (ΔT lag between top and bottom thermocouples)
• Bonding: Slag bonds and recrystallization bonds join 3–6 mm particles into large "sinter cake"
• Suction rate critical: Too fast prevents complete carbon combustion; too slow reduces productivity

Process Control Parameters

• Suction rate: Must match carbon combustion rate to ensure uniform heat front movement
• Moisture role: Controls bed temperature via endothermic reaction (C + H₂O → CO + H₂); increases gas volume
• Carbon amount: Must generate sufficient heat for entire bed to reach sintering temperature
• Bed thickness: Too thick impairs permeability; too thin reduces productivity
• Residence time: Duration over heat source determines completeness of sintering
• Material and heat balance calculations required to determine optimal carbon quantity

Comparison of Agglomeration Methods

Engineered Iron-Bearing Materials

• Sinter and pellet advantages: Higher reducibility than lump ore; better engineered properties
• Limestone integration: Calcination occurs during agglomeration, reducing blast furnace thermal load
• Efficiency: Furnaces operated with sinter and pellets exhibit much higher efficiency than lump ore alone
• Global trend: Modern plants operate with 100% sinter or 100% pellets; lump ore completely phased out
• Engineered materials optimize bed permeability, gas flow, and reduction kinetics

Key Terms & Definitions

• Gang material: Impurity elements in ore (silica, alumina, etc.) or coke residue after combustion
• Metallization: Reduction of oxygen content; molar O/Fe ratio decreases (e.g., from 1.5 in Fe₂O₃ to 1.0 in FeO)
• Solution loss reaction: C + CO₂ → 2CO; endothermic gasification in upper blast furnace stack
• Voidage (ε): Fraction of bed volume not occupied by solids; critical for gas permeability
• Green pellets: Unfired pellets with insufficient strength for blast furnace use
• Flux pellets: Pellets containing limestone; provide basicity for slag formation
• Return sinter: Inadequately sintered material recycled to next sintering batch
• Sinter cake: Large bonded mass formed when particles fuse during sintering; broken to size before charging
• Reaction degradation index: Measure of material's ability to maintain strength during reduction reactions