physical chemisrty of iron making slag

1. Blast Furnace Slag
 Ores – Metal + Impurity Oxides (gangue)
 Gangue is removed by the addition of flux as slag
 Impurities in iron ores and coke ash = acidic silica + alumina
 Fluxes: lime and magnesia
 Other components = less than 5% (FeO, MnO, TiO2, S and others)
Slag Structure:
Liquid slags – homogeneous melts of oxides of silicon and other elements and
possess electrical properties.
Physico-chemical properties of slag 1 Lec-4

2. Crystal analysis of solid silica  Si occupies the centre of a
tetrahedron surrounded by
4 O atoms
 Each O is bonded with 2 Si
atoms and the network is
continuous in three
dimension
 Si atoms has 4 charges
 Each tetrahedron, SiO4
carries 4 negative charges
i.e. (SiO4)-4
 Solid structure remain
unchanged
 Molten state silica structure
become distorted but most
of the corner remain shared
(a) Solid SiO2 (b) molten silica
O white atoms and Si black atoms

3. Addition of CaO, MgO oxides to molten silicon, breakdown the three dimensional
Si-O network into silicate ions (driving force- attraction between Si and O)
Two silicon-oxygen bond are opened up as the following:
 As the metallic oxides are added more, the Si-O bonds break
 The viscosity of the acidic melt decreases as the network gradually breaks
 Finally no corners are shared
 Stoichiometric composition is 2CaO/SiO2 or 2MgO/SiO2

4. Electro-chemical Nature of Slag-Metal Reaction:
 Metal is non-polar
 Molten slag is ionic in character
 Transfer of an element from metal to slag is accompanied with anodic reaction
To preserve electro-neutrality of the slag, a cathodic reaction must occur with the
deposition of an element in the metal

5. Similarly following elements from iron can transfer
Reactions occur at above 12000C in liquid state
The important elements of in ironmaking is Si, Mn and S
Silicon- the reaction:

6. Manganese- the reaction:
Sulphur- the reaction:

7. Slag Basicity
Acidic Oxides = accept oxygen and form anion complexes in melts, e.g. SiO2, Al2O3
Basic oxides = donate oxygen and breakdown the anion complexes, e.g. CaO,
MgO, FeO, MnO etc.
Basicity of the slag = CaO/SiO2 or (CaO+MgO)/(SiO2+Al2O3)
Slag Viscosity
 The entry of O ions from metal oxide the Si-O bond breakdown and the size of
the network decreases accompanied with a lowering of the viscosity of liquid
slags
 A low viscosity when the ratio O/Si exceeds 4.
 Alumina and silica increase the viscosity, while lime and magnesia decrease
the viscosity

8. Summary of Slag property:
1. Slag viscosity increases with the
decrease of temperature
2. The higher the basicity ratio
lower the viscosity, above the
liquidus temperature
3. At higher temperature the
difference in viscosity between
slags becomes less than at lower
temperature
4. The variation in the viscosity with
temperature above liquidus is
greatest for slags of low basicities

9. Kinetics of Slag-Metal Reactions
Overall reaction rate will be controlled by the slowest of the following steps
a. Transport of reactants from the bulk phase, slag or metal to the reaction
interface through a thin boundary layer
b. Reaction at the interface
c. Transport of products from interface to the bulk phase, slag or metal through
a thin boundary layer
At high temperature, chemical reaction is very high and therefore the reactions are
diffusion controlled
 Diffusivities of
elements in iron is
higher than in the
slag
 The activation
energy of diffusion
in slag is higher
than in iron

10.  Major slag-metal reactions in the blast furnace are of reduction-oxidation of Si,
Mn, S and minor of Cr and Ti
 These reactions occur at the bosh and tuyere regions and reach the hearth
when the iron droplets pass through the slag layer