iron making

1. Reduction Process
Thermal, Physical and Chemical Profiles in Blast Furnace
-In front of tuyeres 2000 °C
-The exhaust gas temperature 200 °C
-The temperature of the hearth 1300-1500°C

2. Thermodynamic Fundamentals of Oxide Reduction
MO2 = M + O2
𝑝 𝑂2
=
𝑎 𝑀𝑂2
𝑎 𝑀
1
𝐾 𝑝
Each oxide has its own oxygen partial pressure (also known as the oxygen
potential), whose value is dependent on temperature and pressure of the system:
Shift of reaction from left to right (i.e., reduction of metal oxide MO2) can be
theoretically achieved by
(a) shift of the equilibrium by removing some of the product at constant value of 𝐾𝑝, or
(b) reduction of the equilibrium constant 𝐾𝑝 by change of pressure or temperature of the
system.
A reducing agent thus works as a “chemical pump”
MO2 = M + O2 1.1
R + O2 = RO2 1.3
MO2 + R = M + RO2 1.4
∆𝐺0
= −𝑅𝑇 ln 𝐾𝑝 = −𝑅𝑇 ln
𝑝 𝑂2
𝑎 𝑀
𝑎 𝑀𝑂2
≈ −𝑅𝑇 ln 𝑝 𝑂2
For the equation 1.1

3. In application to reduction processes
1. if the oxygen potential of oxide is higher than one for the reductant, the
reduction of oxide occurs as MO2 + R = M + RO2
2. if the oxygen potential of oxide and the reductant are the same, the above
reaction is the sate of equilibrium
3. if the oxygen potential of reductant is higher than that for the oxide, the
reduction of oxide is impossible.
Prerequisite for the reduction reaction to proceed
𝑝 𝑂2 𝑅𝑂2
< 𝑝 𝑂2 𝑀𝑂2
or, ∆𝐺0
𝑅𝑂2
< ∆𝐺0
𝑀𝑂2

4. -At 1200 °C for example, the oxygen
potential decreases and therefore stability
of oxides increases in the sequence:
Fe2O3, Cu2O, NiO, Fe3O4, FeO, Cr2O3,
MnO, V2O3, SiO2, TiO2, Al2O3, MgO,
CaO. The reduction of elements which
form several oxides takes place n stages
to oxides with lower oxygen content and
then to the element.
-Oxygen potential increases as the
temperature increases with the exception
of carbon.
-Carbon can reduce the majority of
elements from their oxides under the blast
furnace conditions except for Ca, Mg and
Al.
Ellingham Diagram

5. Reduction of Iron Oxide
At T > 570 °C Fe2O3  Fe3O4  FeO  Fe (i)
At T < 570 °C Fe2O3  Fe3O4  Fe (ii)
The iron reduction inside the blast furnace takes place mainly by the high
temperature scheme (i).
Magnetite (Fe3O4) is not reduced to FeO but to wüstite FeO1-y , where y has a
value of about 0.05 – 0.12.
Reduction of iron ore takes place following two ways:
(1) by contact with charcoal- carbon with formation of CO2 as final product is
called direct reduction
(2) by interaction with reducing gases- carbon monoxide and hydrogen with
formation of CO2 and H2O as final products is called indirect reduction.
The sum of direct and indirect reduction rates equal to unity or 100%.

6. Indirect reduction of iron oxide
3Fe2O3 + CO = 2Fe3O4 + CO2 + 52.85 kJ (1.7)
3Fe2O3 + H2 = 2Fe3O4 + H2O + 4.86 kJ (1.8)
Reduction of magnetite to wüstite to metallic iron is a reversible process
To ensure forward reactions, the ratio of CO/CO2 or H2/H2O in gaseous phase has
to exceed its stoichiometric value.
Fe3O4 + m CO = 3FeO + CO2 + (m–1) CO – 36.46 kJ (1.9)
Fe3O4 + n H2 = 3FeO + H2O + (n–1) H2 – 86.45 kJ (1.10)
FeO + p CO = Fe + CO2 + (p–1) CO + 17.13 kJ (1.11)
FeO + q H2 = Fe + H2O + (q–1) H2 – 30.86 kJ (1.12)
The oxygen potential of wüstite is lower than that for magnetite. Thus, the ratio of
CO/CO2 or H2/H2O has to be higher for wüstite reduction, i.e., p > m and q > n.
For endothermic reactions (1.9, 1.10, 1.12), m, n, and q decrease with rising
temperature. For exothermic reaction (1.11), p increases with rising temperature.

7. Summary of indirect reduction reactions can be expressed as follows:
Fe2O3 + 3CO = 2Fe + 3CO2 + 27.52 kJ (1.13)
Fe2O3 + 3H2 = 2Fe + 3H2O – 88.44 kJ (1.14)
Equilibrium of Fe-O-C system. Equilibrium of Fe-O-H-C system.
Combined equilibrium diagram of
CO-CO and H -H O mixtures
Reduction reactions in the blast furnace do
not achieve equilibrium because of a
short residence time of gases in the furnace
and CO splitting at the low temperature
(line 4)

8. Direct reduction of iron oxides
3Fe2O3 + C = 2Fe3O4 + CO – 119.62 kJ (1.16)
Fe3O4 + C = 3FeO + CO – 208.93 kJ (1.17)
FeO + C = Fe + CO – 155.34 kJ (1.18)
In the blast furnace, direct interaction between solid iron oxide and coke is negligible.
At T  950 – 1000 °C formed CO2 reacts with coke according to Boudouard reaction.
Thus direct reduction is a total reaction written according to Hess’s law.
FeO + CO = Fe + CO2 + 17.13 kJ
CO2 + C = 2CO – 172.47 kJ
FeO + C = Fe + CO – 155.34 kJ
Reduction by hydrogen at T > 950 – 1000 °C is also direct reduction, as this process
consumes coke because formed water steam reacts with coke at T > 1000 °C:
FeO + H2 = Fe + H2O – 30.86 kJ
H2O steam + C = H2 + CO – 124.48 kJ
FeO + C = Fe + CO – 155.34 kJ
In a smoothly operating blast furnace only direct reduction of wüstite takes place
because hematite and magnetite are indirectly reduced below T = 950 – 1000 °C.

9. Reduction of Accompanying Elements
Oxides present in blast furnace charge materials can be divided into three groups:
1. oxides which have a high oxygen potential, e.g., Mn2O3, Cu2O, NiO, are
reduced at relatively low temperatures by carbon monoxide and hydrogen
(indirect reduction);
2. oxides which have a lower oxygen potential that for corresponding iron oxides,
e.g., Cr2O3, MnO, SiO2, TiO2, are reduced by solid carbon (direct reduction).
Distribution of these elements between metal and slag depends on the oxygen
potential value: the lower the oxygen potential is, the less their rate in metal is.
For example, chromium mainly enters the hot metal, titanium is mainly lost in
the slag;
3. oxides which have a lower oxygen potential than that for carbon at
temperatures governing in the blast furnace (Fig. 1.2-Ellingham diagram).
These oxides (CaO, MgO, Al2O3) are not reduced under the blast furnace
conditions and transfer fully in the slag.

10. Reduction of Silicon
Silicon in the form of silica and silicates accompanies almost all charging materials.
Major sources of SiO2 are ore gangue and coke ash.
Roughly one-quarter of the SiO2 comes from coke and three- quarter from ore.
Two major routes of silicon transfer from the burden to the hot metal have been
recognised,
-from molten slag directly to molten metal
-from slag or coke ash to the gas phase in form of SiO and then into the metal.
Reduction of silica by carbon starts at high temperatures (about 1500°C):
SiO2 + 2C = Si + 2CO – 622.5 kJ (1.21)
Fresh reduced iron is a catalyst for the reaction of silica reduction and it decreases
the initial temperature of reduction down to 1050-1100 °C. Silicates of iron are
formed above the tuyere zones causing heat release, e.g.,:
Si + Fe = FeSi + 80.26 kJ (1.22)
The total reaction can be written as:
SiO2 + 2C + Fe = FeSi + 2CO – 542.24 kJ (1.23)

11. The main part of silicon is however reduced in the hearth from the liquid slag.
Silicon reaction above the tuyere zone (raceway) is obviously a reaction in the gas
phase. {SiO} volatilises at high temperatures and then condenses. Hypothetical
reaction sequence is:
in the hearth SiO2 + C = {SiO} + CO in absence of CO2 and FeO
in the belly 2{SiO} = SiO2 + Si
Low-silicon metal production is facilitated by:
 low temperature and heat potential in the lower furnace zone where Si reduction
takes place;
 high top gas pressure;
 use of high basicity and low viscosity slag as well as magnesia for lowering the
slag viscosity and liquidus temperature;
 operation with low slag volume;
 smooth operation (avoidance of fluctuation of physical and chemical properties of
charge materials).

12. Reduction of manganese
Manganese is present in the burden in the form of oxides MnO2, Mn2O3, Mn3O4 and
carbonate MnCO3.
In the furnace shaft reactions of indirect reduction take place, e.g.:
2MnO2 + CO = Mn2O3 + CO2 + 201.48 kJ (1.24)
3Mn2O3 + CO = 2Mn3O4 + CO2 + 187.26 kJ (1.25)
Mn3O4 + CO = 3MnO + CO2 + 51.83 kJ (1.26)
These reactions, especially (1.24) and (1.25), are strongly exothermic. As a result,
production of high-manganese hot metal, in particular ferromanganese, causes
increase in top temperature.
In the hearth (at T > 1100 °C) direct reduction takes place:
MnO + C = Mn + CO – 286.92 kJ (1.27)
Reduction of MnO by iron in the furnace lower zone is also possible:
MnO + Fe = Mn + FeO (1.28)

13. Manganese reduction is facilitated by:
 high temperatures in the hearth and bosh: hotter slag and metal, high coke rate,
blast temperature, enriching blast with process oxygen;
 increasing of the activity coefficient of MnO by using more basic slags; MnO
activity coefficient in the slag raises as CaO is increased and SiO2 is become
smaller;
 decreasing slag volume and increasing silicon content in hot metal.

14. Reduction of phosphorous
Phosphorus comes from coke, iron bearing materials and flux in the form of P2O5,
3CaO.P2O5 and 3FeO.P2O5.
The main phosphorus reactions in the blast furnace are as follows:
P2O5 + 5C = 2P + 5CO – 990.66 kJ (1.29)
2 (3CaO.P2O5) + 3SiO2 + 10C = 3(2CaO.P2O5) + 4P + 10CO – 2836 kJ (1.30)
P2O5 + 5Fe = 2P + 5FeO – 196.46 kJ (1.31)
3Fe + P = Fe3P + 181.24 kJ (1.32)
Phosphorus recovery makes up 90 - 100 %, i.e. nearly all phosphorous enters the
metal. Therefore production of low-phosphorus metal is only possible using low-
phosphorus coke and ores. Dephosphorisation requires an oxidising atmosphere
but in the blast furnace a reducing atmosphere governs.