1. Blast Furnace Ironmaking Introduction
Materials 3F03
MARCH 23, 2015

2. Introduction
On 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
Figure Source: 1

3. Layout of a Modern BF
Figure Source: 2

4. Typical Blast Furnace Profile
Figure Source: 2

5. Iron Ores
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
Figure Source: 2

6. Iron Oxide Reduction Purpose of Blast Furnace
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
Purpose of Blast Furnace
FeO0.5 is a chemical representation used by Ironmakers to represent average oxidation state, but not a stable FeOx species in its own right.
Figure Source: 2

7. Reductant
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
Figure Source: 3

8. Iron Oxide Reduction 3Fe2O3 + CO -> 2Fe3O4 + CO2
Sequential Reduction of Iron Oxides:
3Fe2O3 + CO -> 2Fe3O4 + CO2
1.2Fe3O4 + CO -> 3.8Fe0.947O + CO2
Fe0.947O + CO = 0.947Fe + CO2
FeO0.5 is a chemical representation used by Ironmakers to represent average oxidation state, but not a stable FeOx species in its own right.
Figure Source: 2

9. 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)
Figure Source: 1

10. Oxygen Removal 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
Figure Source: 4

11. Indirect Reduction At 1200 K, equilibrium CO/CO2 with FeO/Fe = 2.3/1
(or, %CO in CO + CO2 = 70%)
Equilibrium with FeO
FeO 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.
Figure Source: 4

12. Direct Reduction 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.
Figure Source: 4

13. Wustite Reduction Rate Limiting
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.
Figure Source: 4

14. Optimum Reduction
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
Figure Source: 4

15. Reduction by Hydrogen H2/H2O system analogous to CO/CO2
No Boudouard type reaction
CO has greater reducing potential at lower temperatures (less than 821⁰C)
Figure Source: 4

16. Role of Coke in the BF
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
Figure Source: 2

17. BF Heat Requirement
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
Figure Source: 4

18. Hot Metal Chemistry
Hot Metal is saturated in C, due to hearth conditions
Hot metal in coke bed
Typical hot metal chemistry:
% C
% Si
0.1 – 0.7 % Mn
% S
% P
External desulphurization after BF is typical in industry
Figure Source: 2

19. References
1 John Peacey and Bill Davenport, The Iron Blast Furnace, Pergamon,
2 Geerdes et Al, Modern Blast furnace Ironmaking, an Introduction,
3 Gaskell: introduction to the Thermodynamics of Materials
4: A. Biswas, Principles of Blast Furnace Ironmaking, Theory and Practice, 1981, Capter
Some of the information presented taken from Ironmaking slides in Materials 4C03, prepared by Dr. Gord Irons.