Transcript Blast Furnace Ironmaking

Oxygen Steelmaking
Introduction
MATERIALS 3F03
MARCH 23, 2015
Hot Metal Chemistry
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
Carbon content of hot metal needs to be
substantially lowered to create steel
Figure Source: 2
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Oxygen Steelmaking
Refers to augeneous process for
converting hot metal into steel:
 Top blown
 LD (Linz-Donowitz)
 BOF (Basic Oxygen Furnace) or BOS
 Bottom Blown
 OBM, Q-BOP
 Combined Blowing
 KOBM, LBE
 4% C to less than 0.1 % C in
~16 minutes (~30 minutes
total)
Figure Source: 1
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Process Sequence
Figure Source: 1
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BOF Blow
Usually 16-25 minutes
Pure oxygen blown in a
supersonic rates generates
slag/metal emulsion for high
reaction rate
~100% oxygen utilization
Figure Source: 1
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Process Reactions
There are three major stages in the BOF process:
 1) Slag Formation
 2) Constant Decarburization Rate
 3) Carbon mass transfer control
Figure Source: 1
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Slag Formation
Soft blowing to start to make a SiO2-FeO rich slag (Fayalitic-type)
Once the slag is formed, harder blowing creates slag-metal emulsion
Oxidation at the end
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Mass and Energy Balance
More heat generated from
 C Oxidation
 Si Oxidation
Than required for:
◦ Heating metal
◦ Heating and melting slag
Coolants added:
 Scrap (70/30 hot metal ratio common in NA)
 Iron ore
Figure Source: 1
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Mass and Energy Balance
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Bottom Blowing
Most BOF vessels have some form of bottom
stirring to improve mixing:
 C & O closer to equilibrium
 Better dephosphorization
 Quicker slag formation
 Less iron oxide in slag for better iron and
alloy yield
Looking at mixing times, a small amount
of bottom gas is almost like total bottom
flow
 LH is lance height
 QB and QT are bottom and top flow rates
Figure Source: 1
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Bottom Blowing
Lower iron yield loss (as FeO in the
steelmaking) associated with bottom blowing
 C & O closer to equilibrium
 More decarburization before entering
carbon mass transport control regime
Figure Source: 1
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OS Reactions
Oxygen is the driver for most reactions
 Controlled by oxygen potential
 Involve oxygen directly
Figure Source: 1
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OS Reactions
Oxygen is the driver for most reactions
 Controlled by oxygen potential
 Involve oxygen directly
Figure Source: 1
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Oxidation of Silicon
Rate Controlled by mass transfer of silicon in
metal:
 [Si] + 2(FeO) = (SiO2) + 2[Fe]
Shows first order behavior until Si
content <0.05% Si
Silicon oxidation largely completely in
early stages of the blow
Figure Source: 1
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Oxidation of Manganese
By direct oxidation at hot spot, and:
[Mn] + [O] = (MnO)
[Mn] + (FeO) = (MnO) + Fe
Second reaction predominant later in blow
Figure Source: 1
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Oxidation of Phosphorous
P2O5 is acidic, so basic slags are required
Requires oxidizing conditions
Bottom blown processes closer to slagmetal equilibrium
 Bottom lime injection with O2
Initial slag has high FeO content
Mid-blow: FeO content decreases, more
reducing conditions in slag
 Possibility for P reversion back to steel
End blow: More oxidizing conditions,
opportunity for further phosphorous
oxidation
Figure Source: 1
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Sulphur Removal
Generally poor because of oxidizing
conditions
S partition is worse with acidic slags
Better to maximize desulphurization in
the BF, use external desulphurization
facility
Figure Source: 1
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Critical Carbon Content
Once carbon mass transfer control
regime commences:
 Supply of C to reaction sites is not
sufficient to consume O
 Oxygen dissolution in steel substantially
increases
 Oxidation of Fe increases, higher FeO
content in slag
Carbon content where constant
decarburization regime ends is called
Critical Carbon Content
Figure Source: 1
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Critical Carbon Content
Carbon content where constant
decarburization regime ends is
called Critical Carbon Content
1 – Slag Formation regime
2- Constant Decarburization rate
regime
3- Carbon Mass transport control
Figure Source: 1
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Critical Carbon Content
Carbon content where constant
decarburization regime ends is called
Critical Carbon Content
Options to reduce critical carbon
content:
 Slower oxygen blowing (productivity
impact)
Figure Source: 1
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Critical Carbon Content
To reduce carbon content lower than
the critical carbon content means
that higher yield loss of Fe to slag
must be accepted
 Increased oxygen dissolution into
steel
Other options include vacuum
processes for ultra-low carbon
grades
Reminder: Bottom blowing practice
means lower oxidation of metal for a
given carbon content
Figure Source: 1
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References
1 Bramha Deo and Rob Boom, Fundamentals of Steelmaking Metallurgy,
Prentice Hall, 1993, Chapters 5.1-5.2 and 6.1-6.6
2 Geerdes et Al, Blast Furnace Ironmaking: An introduction, 2009
Much of the content is taken directly from or adapted from Materials
4C03 Oxygen Steelmaking slides prepared Dr. Gord Irons.