physical chemistry of iron making slag 2

1. The Fe-O-Si-C-Slag System
Si enters into the metallic iron in two ways
-from above the slag hearth level to the belly by SiO gas
-through oxidation/reduction when iron droplets travel through the slag
layer above the metal pool in the hearth
Two reactions are:
i. SiO can form when the coke burns before toyere and its ash-silica is reduced
The absorption of silicon by iron in the bosh occur
Physico-chemical properties of slag 2 Lec-5
Reduction of Silica by carbon can occur at high temperatures in the bosh and
hearth region
Si content in iron
reaches maximum
at above the
tuyeres

2. ii) Silica of the slag can be reduced by carbon containing liquid Fe passing through
or in contact with the hearth slag
Equilibrium Si content is several
fold higher than that usually found
in the BF
BF metal Si fails to attain
equilibrium due to the slow
reaction rate
Following assumptions are:
a. Silica reduction by C is sluggish
b. CO being a gas phase raise the
evolution pressure
c. Iron may not be saturated with
C before enetring the slag layer

3. The dependence of Si distribution between the metal and slag on the slag
basicity can be found from Ksi at any given temperature and SiO2 values for
various basicity ratios

4. Fe-O-Mn-C Slag System
 Higher oxides of Mn are reduced in the shaft and only MnO descends in lower
part of the furnace and is reduced at high temperature
 Mn absorption in iron at upper bosh is lower as compared to Si and S.
 Some Mn enters iron when the iron and MnO-containing liquid slag flow over
the incandescent coke.
 Most of the Mn is reduced in the hearth
Mn transfer occur during the C saturated iron droplets through the slag

5.  The temperature dependence of KMn is not as high as that of Ksi
 The influence of pressure is not as high since Ksi is proportional to pCO squared
 With increasing CaO/SiO2 ratio, SiO2 decreases and MnO increases

6.  S from coke is gasified in the shaft, in the bosh and in front of toyeres and
absorbed by iron
 The absorbed S is removed by the hearth slag when sulfer-laden iron droplets
pass through it
Fe-O-S-C Slag System

7. The actual S content of BF iron is much higher than the equilibrium values
High top pressure deteriorate the S removal while increasing slag basicity and
temperature facilitate S removal
The thermodynamic data show that the actual BF metal Si and Mn contents are
much below and S content much above the equilibrium values with respect to C-
saturated iron

8. The FeO-SiO2-Al2O3 System
 Gangue and flux = Silica, alumina, lime and magnesia
 Alumina, lime and magnesia are in solid state – interaction and solution is slow
 At high temperature, alumina and silica are sintered and alumina-silicates are
more stable than the individual oxide
 At low temperature, the rate of lime dissolution is extremely slow because the
materials are not finely divided
 The formation of first slag occurs through mixing of alumina-silicates by
unreduced iron oxide.
 Primary slag: molten slag flows over lumps of lime and loses its iron oxides
rapidly by the reaction of with coke at high temperature (these processes occur
at belly and bosh where most of the silica and alumina from coke ash have not
been released)
 Secondary or bosh slag: slag forms at the above of the toyere
 The final or hearth slag: When the coke ash released at the toyeres and the still
undissolved lime are incorporated in the descending bosh slag which flows down
to the hearth

9. Liquidus temperature of the
alumina-silicates containing above
40% FeO is not more than about
12000C.
As %FeO decreases, the melting
point increases.
The most favourable melting
condition- gangue of an ore
contains a SiO2/Al2O3 ratio 2.5-4/1

10. The CaO-SiO2-Al2O3 System
 The primary slag of relatively low melting- at lower part or in the belly contain
FeO-containing silicates and aluminates with varying amount of lime
 As the slag descends, ferrous oxide is reduced
 Lime is continuously absorbed, the FeO-SiO2-Al2O3 system change to CaO-
SiO2-Al2O3
 The dissolution of lime lower the viscosity and liquidus temperature
 Primary slag runs down the bosh and loses its fluxing constituent FeO and
increases the melting point
 Above the toyeres the resulting bosh slag mainly of CaO--SiO2-Al2O3 should
be liquid and sufficiently fluid

11. For a smooth operation, the slag must be free-running and above the critical hearth
temperature
Slag composition selection: BF slag
form from gangue and flux material
and controls S, Si and Mn which
need basicity ratio more than unity
Normal basic BF slags between 1.0
to 1.5 and acid slag 0.8 to 1.0
Under BF conditions slags are as
follows:

12. The CaO-MgO-SiO2-Al2O3 System
Magnesia reduces the liquidus temperature and viscosity
Desulphurization needs optimum slag composition with minimum viscosity which
decreases with the increase of ratio basic to acid
The lowest silica with optimum desulphurization will be 44% CaO, 14%MgO,
32%SiO2 and 10% Al2O3
MgO addition to CaO-SiO2-Al2O3 decrease the viscosity