Basic Oxygen furnace 2_25605171_2024_09_21_12_34.pdf

Non ferrous metal extraction

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BASIC OXYGEN
STEEL MAKING - 2

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2. There are three typical reaction periods during BOP to consider; silicon
oxidation, full decarburization and carbon diffusion.
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I
II
III

3. I.SILICON OXIDATION PERIOD
During the first third (approximately) of the blow most of the Si
is oxidized along with some Fe. A good practice is to increase the
FeO content and get a good foaming slag at the very beginning of
the blow and then stabilize it by lowering the lance until the foam
stops rising. It is also during this period that most of the P and Mn
are oxidized. The remainder of the supplied oxygen reacts with
carbon

II. FULL DECARBURIZATION PERIOD
Under normal conditions, all of the supplied oxygen reacts with
carbon. However, if the lance position is too high, a portion of the
oxygen will instead oxidize Fe. On the other hand, when the lance
is too low, the previously formed FeO (and foaming slag) is reduced
and the oxygen released reacts with C.
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4. III. CARBON DIFFUSION PERIOD
At the end of the blow when the carbon content is less than
0.8%C, the rate of decarburization decreases substantially and is
now increasingly limited by the diffusion of carbon in the steel.
An approximate equation for decarburization during period is
shown in (remaining oxygen will oxidize Fe):
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5. MELTING PATH
The melting path is a construction of information about carbon content,
temperature and time superimposed on a C-Fe phase diagram. Each dot
represents one minute while the values of temperature and carbon content can
be read out from the X- and Y-axis.
melting path is
always above
the liquidus
temperature
otherwise the
heat might
become
partially or fully
solidified.
Solidification of
the heat results,
in total waste of
the steel made.

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6. CALCULATION OF LIQUIDUS TEMPERATURE
It is imperative to prevent the steel bath temperature falling
below the liquidus temperature (i.e. the temperature at which
the steel starts to solidify). The liquidus temperature, Tliq, is
very dependent on composition and can be approximated from
the following equations:
For %C < 0.5:
Tliq (°C) = 1537 - 73.1%C - 4%Mn - 14%Si - 45%S - 30%P

For 0.5 < %C < 4.4:
Tliq (°C) = 1531 – 61.5%C - 4%Mn - 14%Si - 45%S - 30%P

For %C > 4.4:
Tliq (°C) = 389 %C - 10.5 %Mn + 105 %Si + 140 %S + 128 %P - 506
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7. Effects of different factors on the steel temperature
1. Under holding conditions, no oxygen injection, the steel cools at
1 to 2°C min-1.
2. For most additions, each tonne (1000 kg) added results in an
additional temperature drop of about 5°C.
3. Phosphorus and silicon oxidation is highly exothermic and
produces about 26 MJ/tonne per 0.1% per tonne of hot metal
oxidized, the equivalent of nearly 3 °C/tonne per 0.1%.
4. Carbon oxidation is also exothermic and produces about 13
MJ/tonne per 0.1% oxidized, the equivalent of about 1.4 °C/tonne
per 0.1%.
5. Increasing the hot metal or steel temperature requires 9.0 or 9.4
MJ/tonne, respectively.
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Slag in the LD-process, has various functions and roles. Primarily, it
is spontaneously formed by the non-volatile oxides resulting in the
oxidation of hot metal minor constituents and iron (SiO₂, MnO,
P₂O₅, and FeO). In order to flux the impurity oxides to form a low
melting, fluid slag, lime and sometimes doloma (a mixture of CaO
and MgO) and, if necessary, fluorspar (CaF₂) are charged into the
converter.
Role of Slag in Converter Steel Making

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Secondly, molten slag is a reaction medium for impurity
elimination like desulphurization and dephosphorization.
Slag, forms an emulsion with carbon monoxide and metal
droplets —slag foaming plays some role in post-combustion of
primary carbon monoxide to carbon dioxide, and affects the
radiation heat transfer from the ‘hot spot’ formed in the oxygen
jet-iron melt impingement cavity, leveling out the temperature
distribution in the furnace. Foaming slag obviously also decreases
dust generation rate by absorbing some fraction of dust

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Formation of slag in the BOF process
Slag formation starts with the dissolution of oxygen in iron melt
and simultaneous oxidation of iron and minor bath constituents in
the oxygen jet impact zone. As the bath temperature in the impact
zone is very high, over 2000°C, iron can dissolve a great amount of
oxygen (up to 1 wt%). Iron oxide forms and the primary oxidation
zone and high oxygen iron penetrate the bath and meet ‘fresh’ iron
melt with higher contents of carbon and other minor bath
constituents oxidizing them. Part of the primary reaction products
are splashed into the slag and furnace atmosphere. Iron oxide and
other nonvolatile oxidation products (SiO₂, MnO, P₂O₅, etc.) mix
with existing slag and more lime (doloma) is dissolved into the
molten slag. Slag is, accordingly, formed by a complex chain of
reactions. The overall slag forming can be presented by the
following set of reactions

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These reactions are followed by secondary oxidation reduction
reactions, especially by decarburization taking place on the
surface of metal droplets circulating in the slag.
SLAG

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From the slag formation point of view, there are two limiting
blowing practices:
 soft blowing with high lance position characterized by low iron
bath mixing intensity, and
 hard blowing with ‘low lance’ characterized by more intensive
iron bath mixing and deeper interaction of oxygen jet with the
bath.
In the first case the interaction of the oxygen jet with the iron bath
is ‘superficial’, mass transfer from the bath interior is slow due to
weak mixing, and iron is in the first place oxidized and slagged.
In the second case interaction between the oxygen jet and the
bath, as well as mass transfer from the bath interior to the
superficial layers, is more intensive and the minor elements of the
bath are in the first place oxidized.
Blowing Practice

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• soft blowing increases the slag formation rate
• results in higher FeO content in slag
(as well as raises oxygen supersaturation in the metal)
• favours slag foaming
• promotes dephosphorization
• increases the oxidation rate of Mn, and other impurities
• increases refractory wear
• raises the risk of slag slopping out of the furnace.

Effects of blowing practice i.e. soft blowing versus hard blowing

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During the first minutes of the blow, silicon and manganese
having the highest reaction affinities are oxidized. With the
progress of the blow, the affinity relations change due to changes
in the bath and slag composition and temperature effect on the
standard thermodynamic properties, as well as partial molar
properties (activity coefficients) in solution phases. Manganese
oxide in slag starts to be reduced due to these effects.
Evolution of iron bath and slag composition in the LD-blow

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The Distribution Law
If a system , composed of two immiscible liquid phases (solvent)
has a substance added to it (solute), which is readily soluble in
both the phases, the substance distribute between the two phases
according to the distribution law which states:
‘ at equilibrium, the ratio of the concentration ( or activity) of the
solute dissolved in two contacting immiscible phases is constant at
a given temperature.

L = (M)/[M] = a(M)/a[M]
In steel making, the distribution law is the basis for purification of
steel from S,P,O etc.

16. The composition of the slag is of utmost importance since it
controls many different properties, such as:
• Sulfur partition ratio, LS
• Phosphorus partition ratio, LP
• Manganese partition ratio, LMn
• Liquidus temperature of the slag
Each of these ratios indicate how the element will be
distributed between the slag and the steel, i.e.
LP = 1 indicates that the level of phosphorus in the steel, [%P],
is equal to the level in the slag, (%P).
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17. Slag Formation and Constitution
The slag contains oxides arising from some oxidation reactions
(SiO₂, P₂O₅, FeO and MnO), added fluxes (CaO, MgO) and refractory
wear (MgO).
1-2: Acid slag formed from
oxidized Si and Fe (Mn). The lime
is only very partially dissolved.
Temperature is low so that only
part of phosphorus is oxidized.
2-3: Lime is progressively dissolved with enrichment of liquid slag in
CaO and decrease in FeO content due to dilution and FeO reduction
during full decarburization. The slag is heterogeneous, and non
reactive with respect to phosphorus.
3-4: Reactive slag, suitable for final dephosphorization.
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Slag composition path during blow:

18. PHOSPHORUS PARTITION RATIO
Dephosphorization during the latter part of the basic oxygen
steelmaking process is very important because the conditions are
favorable compared to other processes within primary and
secondary steelmaking. It is therefore critical to maintain a slag
composition that improves the phosphorus removal.
Maintaining a high ratio is quite difficult since it exists only in a
very narrow composition range. Additionally, a temperature
increase of 50°C leads to a decrease of LP with a factor of 1.6 at a
basicity ratio (CaO/SiO₂) of 3.
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19. LP in the system CaO-SiO2-FeO-2%P2O5
-1.5%Al2O3-3%MnO-5%MgO at 1650 °C
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20. SULFUR PARTITION RATIO
Although desulfurization is not a priority in BOS, there will still be
a limited removal of S due to slag/metal interface reactions.
Figure shows how the sulfur partition ratio varies with slag
composition. In the domain of liquid slag, LS is practically
temperature independent.
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21. LS in the system CaO-SiO2-FeO-2%P2O5-1.5%Al2O3-
3%MnO-5%MgO at 1650 °C
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22. MANGANESE PARTITION RATIO
Most of the manganese present in the hot metal will be oxidized
during the first part of oxygen blowing. Any remaining manganese
may also be picked up by the slag or reverted back to the steel
due to oxidization/reduction reactions at the slag/metal
interface.
The manganese partition ratio is slightly temperature dependant.
An increased temperature with 50 °C leads to a decrease of LMn
with a factor of ~ 1.25.
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23. LMn in the system CaO-SiO2-FeO-2%P2O5-1.5%Al2O3-
3%MnO-5%MgO at 1650 °C
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