Blast Furnace IronmakingIntroduction

MATERIALS 3F03MARCH 23, 2015

IntroductionOn the highest level, the blast furnace exists to smelt iron ore

Major considerations for this introductory lecture: 1) Iron oxide reduction 2) Satisfying energy requirements

Overall materials balance illustrates the process as a starting point for discussion

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Layout of a Modern BF

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Typical Blast Furnace Profile

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Iron Ores

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Almost all industrial Ironmaking worldwide is based around iron oxide ores

Iron is the fourth most abundant element on the Earth’s crust

Generally require Fe content of >58 wt % for economical BF process Increased slag volume at higher gangue contents Leads to gas permeability, productivity reduction

Most iron ore requires processing to increase Fe content Crushing and screening (usually minimum step) Possible upgrading (ex, magnetic separation) Pelletization

Iron Oxide Reduction

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Three thermodynamically stable species of Iron oxides: Hematite: Fe2O3

Magnetite: Fe3O4

Wustite: Fe0.947O (usually just FeO for analysis)

Name Ch. Formula Wt. % Fe O / Fe

Hematite Fe2O3 70.0 1.5

Magnetite Fe3O4 72.4 1.33

Wustite Fe0.947O 76.6 1.05

Iron Fe 100 0

FeO0.5 is a chemical representation used by Ironmakers to represent average oxidation state, but not a stable FeOx species in its own right.

Purpose of Blast Furnace

Reductant

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The Ellingham diagram can be used to analyze metal oxide reduction thermodynamic capabilities

The BF process is mainly the carbothermic reduction of iron oxide C enters BF primarily as Coke

CO can theoretically be used as a reductant for all oxides above the line Why is the slope negative?

Reduction of FeO to Fe requires the most chemical work Final reduction step in BF Lowest on diagram relative to Fe2O3,

Fe3O4

Iron Oxide Reduction

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Sequential Reduction of Iron Oxides:

3Fe2O3 + CO -> 2Fe3O4 + CO2

1.2Fe3O4 + CO -> 3.8Fe0.947O + CO2

Fe0.947O + CO = 0.947Fe + CO2FeO0.5 is a chemical representation used by Ironmakers to represent average oxidation state, but not a stable FeOx species in its own right.

The Solution Loss Reaction

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The solution loss reaction: CO2 (g) + C(s) -> 2CO (g)

Key characteristics of metallurgical relevance: Very endothermic High activation energy (360 kJ/mol)

Essentially stops below 1200 K / 900⁰C

However, the reaction regenerates reducing gas by consuming coke

Other names for reaction used in Industry: Boudouard reaction Coke gasification Gas regeneration (more ambiguous naming, but used)

Oxygen Removal

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430 kg O /tonne Fe to remove from pure Fe2O3 48 kg O /tonne Fe from Fe2O3 to Fe3O4 80 kg O / tonne Fe from Fe3O4 to Fe2O3 302 kg O / tonne Fe from FeO to Fe

Indirect Reduction

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At 1200 K, equilibrium CO/CO2 with FeO/Fe = 2.3/1 (or, %CO in CO + CO2 = 70%)

Equilibrium with FeO FeO + 3.3 CO = Fe + 2.3CO + CO2

Indirect reduction is FeO reduction with no solution loss Occurs at T < 1200 K

From stoichiometry: 1 t of Fe produced indirectly requires 760 kg C burned

at tuyeres to make CO.

Inefficient use of CO, high gas volume required

Equilibrium gas composition diagram shows the same information as the Ellingham diagram, just expressed in terms of volume % CO. Also known as a “fish tail” diagram.

Direct Reduction

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FeO + C= Fe + CO

Net reaction appears as though C directly reduces FeO

FeO + CO = CO2 + Fe

CO2 (g) + C(s) -> 2CO

FeO + C = Fe + CO

Solution loss plays a role

Only 322 kg C required

From the mass balance, appears efficient use of C

Huge fuel cost to make CO by solution loss reaction

Equilibrium gas composition diagram shows the same information as the Ellingham diagram, just expressed in terms of volume % CO. Also known as a “fish tail” diagram.

Wustite Reduction Rate Limiting

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Wustite reduction is rate limiting FeO + CO = Fe + CO2

CO2 (g) + C(s) -> 2CO (g)

Consider wustite as FeO for simplicity of analysis If 1 mole of Fe is made, then 1 mole of CO2 is made By solution loss, then 2 moles of CO are regenerated 2 moles of CO reduces 7.6 moles of FeO from Fe3O4

Corresponds to 100% direct reduction 100% indirect reduction:

1 mole of FeO makes 1 mol of CO2, 2.3 mol of CO remain

Only need ¼ ratio of CO/CO2 gas in Fe3O4 reduction to FeO

Satisfying FeO reduction satisfies higher oxide reduction

FeO reduction is rate limiting in 2 ways: 1. Strength of Gas required 2. Volume of reducing gas required

Equilibrium gas composition diagram shows the same information as the Ellingham diagram, just expressed in terms of volume % CO. Also known as a “fish tail” diagram.

Optimum Reduction

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Optimum reduction (from a mass balance perspective only!!) y kg of wustite O removed directly by C CO removes the rest (302-y) y / 16 = 3.3(302-y) /16 y =232 kg O 175 kg C (lowest C use ) 54% direct reduction

Stoichiometrically, this is possible

Heat balance implications for high solution loss means this is not achievable in practice True optimum comes from combined heat and mass

balance In fact, higher direct reduction in practice usually leads

to higher coke (ie, C) rates! (heat requirement) Out of scope for Materials 3F03 Assignment in Materials 4C03

Reduction by Hydrogen

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H2/H2O system analogous to CO/CO2

No Boudouard type reaction

CO has greater reducing potential at lower temperatures (less than 821⁰C)

Role of Coke in the BF

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Coke Plays an important role in every part of the BF

Mechanical Functions: Support for smooth burden descent Maintain permeability for high productivity Coke windows provide only means gas flow

through cohesive zone

Chemical Function: Minimize Direct reduction

More reactive coke -> more direct reduction, less coke available for burning at tuyere level

Sensible Heat Input Reductant

BF Heat Requirement

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Direct reduction uses less C than indirect, but requires much more heat

Major heat requirements: Iron Reduction Metalloid reduction Evaporation of moisture Calcination of raw fluxes Sensible heat for gases Sensible heat of HM, slag Heat losses to cooling system

Major Heat inputs: Combustion of Coke Combustion of Injected fuels: coal, NG, oil Sensible heat of hot blast (up to 1300⁰C) Slag formation

Major drive is to minimize Coke input

Hot Metal Chemistry

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Hot Metal is saturated in C, due to hearth conditions Hot metal in coke bed

Typical hot metal chemistry: 4.5 - 5.0 % C 0.3-1.0 % Si 0.1 – 0.7 % Mn 0.05-0.10 % S 0.01-0.08 % P

External desulphurization after BF is typical in industry

References 1 John Peacey and Bill Davenport, The Iron Blast Furnace, Pergamon, 1979

2 Geerdes et Al, Modern Blast furnace Ironmaking, an Introduction, 2009

3 Gaskell: introduction to the Thermodynamics of Materials

4: A. Biswas, Principles of Blast Furnace Ironmaking, Theory and Practice, 1981, Capter 3.6-3.12

Some of the information presented taken from Ironmaking slides in Materials 4C03, prepared by Dr. Gord Irons.