Oxidation of Manganese - Steel Making - Lecture Notes, Study notes for Material Science and Technology. Jiwaji University

Material Science and Technology

Description: The main Points, which I found very informative in steel making are:Oxidation of Manganese, Behaviour of Manganese, Reduction of Manganese, Oxidation of Carbon, Rimming Reaction, Solidification of Steel, Decarburization, Iron-Carbon Melt, Mole Fraction, Activity Coefficient
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Lecture 7: Oxidation of manganese and carbon
Contents:
Behaviour of manganese
Oxidation of manganese
Reduction of manganese
Oxidation of carbon
Rimming reaction
Illustration
Key words: Solidification of steel, decarburization, BOF steelmaking
Behavoiur of manganese in iron-carbon melt:
Mn is soluble in iron in any proportion
Mn forms ideal solutions in iron
Carbon lowers the activity of Mn in Fe-Mn-C system by forming Mn3C.
Oxidation of Manganese:
Mn is oxidized readily at relatively low temperatures and can form oxides like MnO, MnO2, Mn2O3 etc.
But MnO is stable at high temperature.
[Mn] + [O] = (MnO) (1)
[Mn] + (FeO) = (MnO) + [Fe] (2)
The reaction 1 occurs with dissolved oxygen in metal, whereas reaction 2 is a slag/metal reaction. Both
reactions are exothermic. Lower temperature favours oxidation of Mn from metal to slag; whereas
higher temperature favours reduction of MnO of slag and there occurs reversal of Mn. Reduction of
MnO in slag is important; we consider reaction 2
K = a(MnO )a[Fe ]
h[Mn ]a(FeO )
(3)
Replacing activity by mole fraction and using a[Fe ]= 1, we get,
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K = N(MnO )γ(MnO )
fMn [wt % Mn]× γ(FeO )N(FeO )
(4)
Grouping all activity coefficient terms and putting N(MnO ) (wt% Mn)
We get,
K= K (γFeO )fMn
γMnO
= (wt % Mn)
[wt % Mn]× NFeO
(5)
Where K is an equilibrium quotient and it depends on composition of slag. Distribution of Mn between
slag and metal can be written as
φ=(wt % Mn)
[wt % Mn]= K NFeO (6)
log K= 7940
T 3.17 (7)
According to equation 7 Kincreases with decrease in temperature (K= 9.1, 11.72 and 20.33 at
temperatures 1923K, 1873K and 1773K respectively)
Condition for oxidation of Mn according to equation 6
High activity of FeO in slag which means an oxidizing slag
Decrease in temperature increases K* according to equation 7.
Reduction of Mn in slag
Conditions for reduction of MnO, that is reversal of reaction 2 is important. The reduction of MnO in slag
transfers Mn f rom slag to metal and increases the concentration of manganese. The following are the
conditions for the reduction of MnO in slag
Low activity of FeO in slag which means a reducing slag
High temperature which decreases K
Illustration:
Consider a slag of basicity 1.8. At this basicity the activity coefficient of MnO in slag is 1.6. The mole
fraction of FeO and MnO in slag is 0.25 and 0.05 respectively. Determine the equilibrium content of Mn
and O in steel at 1873K. Given
[Mn] + (FeO) = (MnO) + [Fe] ; G°= 27800 +11.8T
Using equilibrium constant definition, we can write
ln[% Mn] = G°
RT + ln aFe γMnO NMnO
aFeO
(8)
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Substituting the values, we get at 1873K
[% Mn] = 0.048%
Using equations
G°=6880
aFeO = 0.514 (NFeO )0.2665
We get aFeO = 0.36
Hence [wt% O] = [wt% O]sat × aFeO
= 0.233 × 0.36
= 0.084%
Calculations performed at 1773K shows that [wt% Mn] is 0.032. This means that decrease in
temperature favours removal of manganese from metal to slag. The reader may perform calculations at
1973K and interpret the calculations.
Oxidation of Carbon
It is important to note that amongst all steelmaking reactions, oxidation of carbon is the reaction whose
product is gas i.e. CO. Therefore this reaction is of very much significance during steelmaking because
CO gas during escape from the molten bath can induce stirring in metal and slag phases during
steelmaking.
CO gas can cause slag to foam which leads to increase in surface area.
CO gas has a high calorific value and combustion of CO in steelmaking can contribute to energy
efficiency.
Carbon oxidation is also known as decarburizing reaction
[C] + [O] = {CO} (9)
KCO = pCO
hChO
= pCO
[wt % C]×fC×[wt % O]×fO
(10)
[wt% C]×[wt% O]= pCO
KCO
×1
fC×fO
(11)
If we assume fC= fO=unity that is at low concentration of carbon and oxygen in molten metal then
[wt% C]×[wt% O]= pCO
KCO
(12)
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According to eq. 12, the product [wt% C]×[wt% O] at a given temperature depends only on partial
pressure of CO in equilibrium with melt. It is important to note that pCO depends on the location of
nucleation of CO in steel melt. If CO nucleates deep into the bath then pCO will be greater than
atmospheric pressure.
Let us calculate equilibrium content of carbon and oxygen at 1873K for pCO = 1 atm 1.2 atm and 1.5
atm
The value of KCO is calculated from
log KCO = 1056
T+ 2.13
[wt% C]
[wt% O ]
pCO = 1
pCO = 1.2
pCO = 1.5
0.0405
0.0486
0.1
0.0202
0.0242
0.0303
0.0040
0.0048
1.0
0.0020
0.0024
0.0030
From the table we note that
Decrease in carbon content increases the oxygen dissolved in steel. This is important in
connection with production or ultra low carbon steel for certain applications. Production of
ultra low carbon steels will be accompanied with dissolved oxygen if precautions are not taken
during steelmaking.
Increase in pCO increases [wt% O] in steel
Let us consider the evolution of CO gas. According to equation 9,
12 Kg C produces 22.4 m3CO (1 atm and 273K)
1 Kg C produces 1.87 m3 CO (1 atm and 273K) which is equivalent to 12.83 m3CO (1 atm and 1873 K)
Now for 1000 Kg hot metal and 0.2% carbon in steel
CO production would be 488 m3 (1atm, 1873 K) / ton of hot metal. This volume of CO will evolve no
doubt over a period of time but at any time large amount of CO will be escaping the system. Escaping of
this gas will agitate the bath and contribute to enhanced rates of mass transfer reactions. Also care
must be taken for the easy and unhindered escape of CO gas from the vessel failing which foaming and
eventually expulsion of slag may occur.
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