2. Direct and indirect reduction
Systems of importance of ironmaking
The Fe-O System
Hematite- Fe2O3 (O2-30.04%)
(- Fe2O3 hexagonal; - Fe2O3 cubic)
Magnetite- Fe3O4 (O2-27.64%)
Wustite- FeO (O2-23.2%)
3. C-O System
C + 1/2O2 = CO Ho (298oK) = -26.42 kcal ; Ho (1673oK) = -28.00 kcal
= -2200 kcal/kg.C = -2300 kcal/kg.C
C + O2 = CO2 Ho (298oK) = -94.05 kcal ; Ho (1673oK) = -97.8 kcal
= -7840 kcal/kg.C = -8150 kcal/kg.C
CO + 1/2O2 = CO2 Ho (298oK) = -67.84 kcal ; Ho (1673oK) = -69.12 kcal
= -5760 kcal/kg.C
Carbon reacts with oxygen as follows
Combustion of C to CO yield 28 % of total heat
Oxidation to the CO2 gives the largest amount of heat
In Blast furnace (BF) 30-40 % C as coke is converted to CO2 and rest converted to CO
Carbon dioxide in the absence of carbon is stable at high temperature
Carbon dioxide in the presence of carbon is unstable at high temperature
C + CO2 = 2CO Ho (298oK) = +41.21 kcal ; Ho (1273oK) = +40.3 kcal
= +3435 kcal/kg.C = +3360 kcal/kg.C
Boudouard or carbon gasification reaction
4. •Any point on the red 1 bar, concentration of CO and CO2 is 1:1 will be found at
about 680 oC
•Temperatures above 680oC favor CO, below CO2 prevails. Above 900oC there is
practically only CO around, below 500oC you have practically only CO2
•Points in the diagram corresponding to actual or measured concentration that are
not on the red line denote a system not in equilibrium. The curve then tells you if
your system is too hot or too cold for this kind of concentration.
•The pressures different from 1 bar tells you that the equilibrium concentration of CO
at a given temperature increases with decreasing pressure and decreases with
increasing pressure
Boudouard equilibrium diagram
5. The Fe-O-C System
➢ Coke is used for reduction reaction and produced heat to obtain the iron
and slag in a liquid state
➢ The reactions are:
-The reactions are
pressure independent
-The reduction by CO
is called gaseous or
indirect reduction
The reaction occurs indirectly via gas phase above 10000C, the overall reaction
being direct consumption of carbon
6. -The curve for FeO to Fe is
upward – exothermic
-The curve for Fe2O3 to FeO is
downward - Endothermic
At 9000C, Equilibrium concentration CO – 20% for Fe2O3 to FeO;
The gaseous phase must have a ratio of greater than CO/CO2 = 20/80 = 0.25 is To
convert completely magnetite to wustite
Equilibrium concentration CO – 70% for FeO to Fe ;
The gaseous phase must have a ratio of greater than CO/CO2 = 70/30 = 2.3 to
convert completely wustite to iron
7. -The extent of utilization of CO is (denoted co) at a maximum of 80% in respect
of magnetite/wustite equilibrium and 30% for wustite/iron equilibrium at 9000C.
For hematite/magnetite it is almost 100% .
-For complete reduction of 1 mole of wustite to iron we need 100/30=3.3 moles
of CO
-For complete reduction of 1 mole of magnetite to wustite we need 100/80=1.25
moles of CO
1000 kg Fe from hematite
-302 kg O from FeO
-48 Kg O from Fe2O3 to Fe3O4
-80 KgO from Fe3O4 to FeO
-in hemetitie 402kg.O/1000kg.Fe
Total gas requirement for 100% ind
reduction is
8. The Fe-O-H System
Iron oxide can be reduced by H2/H2O mixture and the reduction occurs as follows:
Different phases of iron oxide in equilibrium with
H2/H2O mixtures at various temperature (dotted
line)
At 821OC H and CO have same reducing power
over FeO
Below 821OC, the reducing power of CO is much
greater
9. Rist diagram
Graphical presentation of heat and mass balance
Rist Diagram Based on Oxygen Balance:
Following oxygen balance can be written as
Oxygen input through air blast and iron oxide of burden = oxygen going out as CO
and CO2 with the top gas
The oxygen balance per gram of iron may be written as:
(O/Fe)x + no
B = nc
g.(O/C)g
Where (O/Fe)x = (O/Fe) atomic ratio in ore = 1.5 for hematite ore
no
B = number of gram atoms of O in air blast, per gram atom Fe
(O/C)g = (O/C) atomic ratio in top gas (lies 1 and 2)
nc
g = gram atoms of carbon input through coke, per gram atom of Fe
(also known as active carbon rate)
10. The features of schematic Rist diagram are:
1. Abscissa (i.e. horizontal axis) represents
O/C atomic ratio in the gas. The top gas
composition is at the top of the diagram.
2. O/Fe atomic ratio is shown along the
ordinate. At the top, the O/Fe ratio is that
of the unreduced ore.
3. The negative side of the ordinate below
zero represents sources of oxygen other
than iron oxide, such as the air blast
4. The slop of the line is nc
g . The rotation of
the line along the arrow means lower
value of nc
g i.e. more efficient blast
furnace operation with lower coke rate
5. PA indicates the extent of indirect
reduction of iron oxide and PE that of
direct reduction
