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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