A short term program on process metallurgy of iron and steel making D E P A R T M E N T O F Metallurgical and Materials Engineering National Institute of Technology, Rourkela 1 Structure and Properties of Metallurgical Slag By Dr. S.Sarkar Associate Professor Dept. of Metallurgical and Materials Engg. National institute of Technology, Rourkela
Transcript
1. D E P A R T M E NT O F Metallurgical and Materials
Engineering National Institute of Technology, Rourkela Structure
and Properties of Metallurgical Slag By Dr. S.Sarkar Associate
Professor Dept. of Metallurgical and Materials Engg. National
institute of Technology, RourkelaA short term program on process
metallurgy of iron and steel making 1
2. D E P A R T M E NT O FPlan of Presentation Metallurgical and
Materials Engineering National Institute of Technology, Rourkela
Introduction to metallurgical slag Structure of pure oxide Role of
ionic radii Metal oxygen bond Structure of slag Properties of slag
Basicity Oxidising power Sulphide capacity Electrical and thermal
conductivity Viscosity Surface tension Constitution of slagA short
term program on process metallurgy of iron and steel making 2
3. Introduction D E P A R T M E NT O F Metallurgical and
MaterialsMetallurgical Slag Engineering National Institute of
Technology, Rourkela The slag comprising of simple and/or complex
compounds consists of solutions of oxides from gangue minerals,
sulphides from the charge or fuel and in some cases halides added
as flux. Slag cover protects the metal and from oxidation and
prevents heat losses due to its poor thermal conductivity. It
protects the melt from contamination from the furnace atmosphere
and from the combustion products of the fuel In primary extraction,
slags accept gangue and unreduced oxides, whereas in refining they
act as reservoir of chemical reactant(s) and absorber of extracted
impurities.A short term program on process metallurgy of iron and
steel making 3
4. Introduction D E P A R T M E NT O F Metallurgical and
MaterialsMetallurgical Slag (cont.) Engineering National Institute
of Technology, Rourkela In order to achieve these objectives, slag
must possess certain optimum level of physical properties: Low
melting point, Low viscosity, Low surface tension, High diffusivity
and chemical Properties: Basicity, Oxidation potential and
Thermodynamic properties The required properties of slags are
controlled by the composition and structure.A short term program on
process metallurgy of iron and steel making 4
5. Structure of Pure Oxides D E P A R T M E NT O F
Metallurgical and MaterialsRole of ionic radii Engineering National
Institute of Technology, Rourkela Relative dimensions of cations
and anions and type of bonds between them are important factors in
controlling the structure of pure oxides. Table I: Radii of common
cations, Rc and anions, RaCations K+ Ca2+ Mn2+ Fe2+ Fe3+ Mg2+ Cr3+
Al3+ Si4+ P5+Rc (nm) 0.133 0.099 0.08 0.074 0.061 0.066 0.063 0.051
0.042 0.035Anions I- S2- Cl- O2- F-Ra (nm) 0.220 0.184 0.181 0.140
0.133A short term program on process metallurgy of iron and steel
making 5
6. D E P A R T M E NT O FRole of ionic radii (cont.)
Metallurgical and Materials Engineering National Institute of
Technology, Rourkela Coordination number, Rc/Ra ratio and structure
of solid oxides Structure Coordination number Ra/Rc Examples Cubic
8 1 0.732 -- Octohedral 6 0.732 0.414 CaO, MgO, MnO, FeO
Tetrahedral 4 0.414 0.225 SiO2, P2 O5 Triangular 3 0.225 0.155 --A
short term program on process metallurgy of iron and steel making
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7. D E P A R T M E NT O FIonic radii (cont.) Metallurgical and
Materials Engineering National Institute of Technology, Rourkela In
case of SiO2 four O2- ions provide the frame of the tetrahedron and
the smaller Si4+ ion is situated within the frame as shown in Fig.
Since the neighbouring cations Structure of SiO2 (Si4+) are
mutually repellent, according to the Paulings second law the
interval between two Si4+ ions should be maximum.A short term
program on process metallurgy of iron and steel making 7
8. D E P A R T M E NT O FIonic radii (cont.) Metallurgical and
Materials Engineering National Institute of Technology, Rourkela
Structure of silica (a) solid (b) liquidA short term program on
process metallurgy of iron and steel making 8
9. Structure of Pure Oxides D E P A R T M E NT O F
Metallurgical and MaterialsMetal Oxygen bonds Engineering National
Institute of Technology, Rourkela There are two principal types of
bonds found in crystals: electrovalent and covalent. Electrovalent
bond strength is lower than the covalent bond. High temperature is
required to destroy the covalent bond. However, oxides exhibit
varying proportion of both ionic and covalent bonding in slag.
Ionic bond fraction indicates the tendency to dissociate in liquid
state.A short term program on process metallurgy of iron and steel
making 9
10. Structure of Pure Oxides D E P A R T M E NT O F
Metallurgical and MaterialsMetal Oxygen bonds Engineering National
Institute of Technology, Rourkela TiO2, SiO2 and P2O5, bonding is
mainly covalent and the electrovalent proportion is strong due to
small cations carrying higher charge with a coordination number of
4. These simple ions combine to form complex anions such as SiO4-4
and PO3-4 leading to the formation of stable hexagonal network in
slag systems. Hence they are classified as network formers or
acidic oxides. For example SiO2 + 2O2- = SiO4-4 P2O5 + 3O2- =
2(PO3-4)A short term program on process metallurgy of iron and
steel making 10
11. Structure of Pure Oxides D E P A R T M E NT O F
Metallurgical and MaterialsMetal Oxygen bonds Engineering National
Institute of Technology, Rourkela The oxides with high ionic
fraction form simple ions on heating beyond the melting point or
when incorporated into a liquid silicate slag. For example :
CaOCa2+ + O2- Na2O 2Na+ + O2- As they destroy the hexagonal network
of silica by breaking the bond they are called network
breakersorbasic oxides.A short term program on process metallurgy
of iron and steel making 11
12. Structure of Pure Oxides D E P A R T M E NT O F
Metallurgical and MaterialsMetal Oxygen bonds Engineering National
Institute of Technology, RourkelaA short term program on process
metallurgy of iron and steel making 12
13. Oxide z/(Rc+Ra) Ionic fraction Coordination Nature of the
Oxide D E P A R T M E NT O F of bond number Metallurgical and
Materials Engineering Solid- -Liquid National Institute of
Technology, RourkelaNa2O 0.18 0.65 6 6 to 8BaO 0.27 0.65 8 8 to
12SrO 0.32 0.61 8 Network breakersCaO 0.35 0.61 6 orMnO 0.42 0.47 6
6 to 8 Basic oxidesFeO 0.44 0.38 6 6ZnO 0.44 0.44 6Mgo 0.48 0.54 6
Oxides like Fe2O3, Cr2O3 andBeO 0.69 0.44 4 Al2O3 are known to be.
... ... ... ... amphoteric due to their dualCr2O3 0.72 0.41 4
characteristics because theyFe2O3 0.75 0.36 4 Amphoteric oxides
behave like acids in basic slagAl2O3 0.83 0.44 6 4 to 6 and as
bases in acidic slag.. ... .. .. . ...TiO2 0.93 0.41 4 Network
formersSiO2 1.22 0.36 4 4 orP2O5 1.66 0.28 4 4 Acidic oxides A
short term program on process metallurgy of iron and steel making
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14. D E P A R T M E NT O FStructure of Slag Metallurgical and
Materials Engineering National Institute of Technology, Rourkela It
is well known that most of the slags are silicates. When a basic
oxide is incorporated in to the hexagonal network of silica it
forms two simple ions. The fraction of basic oxide, expressed as
O/Si ratio plays an important role in destroying the number of Si-O
joints. O/Si Formula Structure 2/1 Si O2 Silica tetrahedra form a
perfect three dimensional hexagonal network 5/2 MO.2 SiO2 One
vertex joint in each tetrahedron breaks to produce two-dimensional
lamellar structure. 3/1 MO. Si O2 Two vertex joints in each
tetrahedron break to produce a fibrous structure 7/2 3MO. 2SiO2
Three vertex joints in each tetrahedron break 4/1 2MO.SiO2 All the
four joints break A short term program on process metallurgy of
iron and steel making 14
15. D E P A R T M E NT O FStructure of Slag (cont.)
Metallurgical and Materials Engineering National Institute of
Technology, Rourkela O O O O 2+ O Si O Si O + (CaO ) O Si O + Ca +
O Si O O O O O O O O O + + O Si O Si O + ( Na 2 O) O Si O + Na + Na
+ O Si O O O O O Fibrous structure of a pyroxeneA short term
program on process metallurgy of iron and steel making 15
16. Structure of fayalite2 (a) solid (b) liquid D E P A R T M E
NT O FStructure of Slag (cont.) Metallurgical and Materials
Engineering National Institute of Technology, Rourkela Structure of
fayalite (a) solid (b) liquidA short term program on process
metallurgy of iron and steel making 16
17. D E P A R T M E NT O FProperties of Slag Metallurgical and
Materials Engineering National Institute of Technology, RourkelaA
knowledge of various chemical and physicalproperties of slag is
essential in order to adjust themaccording to the need of
extraction and refiningprocesses. 1. Basicity of Slags In slag
systems, a basic oxide generates O2- anion while an acidic oxide
forms a complex by accepting one or more O2 anions: Base acid +
O2-A short term program on process metallurgy of iron and steel
making 17
18. Properties of Slag D E P A R T M E NT O F Metallurgical and
MaterialsBasicity Engineering National Institute of Technology,
Rourkela For example, SiO2, P2O5, CO2, SO3 etc are acidic oxides
because they accept O2- anions as per the reaction: (SiO2) + 2
(O2-) = SiO44- On the other hand basic oxides like CaO, Na2O, MnO
etc. generate O2- anions: (CaO) Ca2+ +O2- The amphoteric oxides
like Al2O3, Cr2O3 Fe2O3 behave as bases in the presence of acid (s)
or as acids in presence of base (s): (Al2O3) + (O2-) = 2 (Al O2-)
or (Al2 O4 2- ) (Al2O3) = 2(Al3+) + 3(O2-)A short term program on
process metallurgy of iron and steel making 18
19. Properties of Slag D E P A R T M E NT O F Metallurgical and
MaterialsBasicity Engineering National Institute of Technology,
Rourkela In a binary slag viz. CaO-SiO2 the basicity index (I) is
given as: I = wt % CaO / wt % SiO2 For example a complex slag
consisting of CaO, MgO, SiO2 and P2O5 employed in dephosphorisation
of steel, basicity index2 is estimated as follows: wt%CaO + 2 3
wt%MgO I= wt%SiO 2 + wt%P2 O 5A short term program on process
metallurgy of iron and steel making 19
20. Properties of Slag D E P A R T M E NT O F Metallurgical and
Materialsoxidising power Engineering National Institute of
Technology, Rourkela Oxidizing power means the ability of the slag
to take part in smooth transfer of oxygen from and to the metallic
bath. The oxidizing power of the slag depends on the activity of
the iron oxide present in the slag. The equilibrium between iron
oxide in slag and oxygen dissolved in metal is represented as:
(FeO) = [ Fe ] + [ O ] [ a ][ a ] Thus [ a O ] ( a FeO ) Fe O K= (a
) FeOA short term program on process metallurgy of iron and steel
making 20
21. Properties of Slag D E P A R T M E NT O F Metallurgical and
MaterialsSulphide Capacity of Slag Engineering National Institute
of Technology, Rourkela Since slags are employed to remove sulphur
from metal, chemistry of sulphur in silicate slags becomes
interesting. Sulphide is soluble in silicate melts but elemental
sulphur does not dissolve to any appreciable extent. 1 1 S 2 ( g )
+ (O 2 ) = O2 ( g ) + ( S 2 ) (18) 2 2 (a ) p S 2 1 2 x S 2 . S 2 p
O2 1 2 (a ) p O2 K= = (19) x pS O 2 H2 O 2 2 A short term program
on process metallurgy of iron and steel making 21
22. Properties of slag D E P A R T M E NT O F Metallurgical and
MaterialsSulphide capacity of slag Engineering National Institute
of Technology, Rourkela The sulphur affinity of a slag, presented
as molar sulphide capacity is defined by the equation: 1 pO 2 x 2 =
x 2 2 CS = K O (20) S pS 2 2 S or a more useful term wt % sulphide
capacity5 for technologist is defined as 1 p O2 2 C S = (wt% S)
(21) pS 2 Thus under similar conditions a slag with a high Cs will
definitely hold sulphur more strongly than the other with a low Cs
and hence will prove to be a better desulphuriser in a
metallurgical process.A short term program on process metallurgy of
iron and steel making 22
23. Electrical and thermal D E P A R T M E NT O F Metallurgical
and Materialsconductivity Engineering National Institute of
Technology, Rourkela Molten silica is a poor electrical conductor3.
However its conductivity increases to a great extent by addition of
basic oxides e.g. CaO, FeO or MnO as flux. This increase is due to
the formation of ions. The conductivity values serve as a measure
of degree of ionization of the slag. The electrical conductivity of
slags depends on the number of ions present and the viscosity of
liquid slag in which they are present. Thus conductivity will be
greater in liquid state and further increases with the temperature.
In general thermal conductivity of slag is very low but heat losses
are much higher due to convection.A short term program on process
metallurgy of iron and steel making 23
24. D E P A R T M E NT O FViscosity Metallurgical and Materials
Engineering National Institute of Technology, Rourkela Viscosity of
slags are controlled by composition and temperature. The viscosity
, of a slag of a given composition decreases exponentially with
increase of temperature according to the Arrhenius equation: = A
exp (E / RT) Basic oxides or halides with large ionic bond fraction
are more effective in reducing viscosity than those with smaller
bond fraction by breaking bonds between the silica tetrahedra.A
short term program on process metallurgy of iron and steel making
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25. D E P A R T M E NT O FViscosity Metallurgical and Materials
Engineering National Institute of Technology, Rourkela Effect of
addition of flux on activation energyA short term program on
process metallurgy of iron and steel making 25
26. D E P A R T M E NT O FViscosity Metallurgical and Materials
Engineering National Institute of Technology, Rourkela Viscosity
decreases rapidly with temperature for both basic as well as acid
slags. But basic slags with higher melting points are more
sensitive to temperature. This indicates that activation energy for
viscous flow of basic slags is much lower than for acid slags.A
short term program on process metallurgy of iron and steel making
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27. D E P A R T M E NT O FViscosity Metallurgical and Materials
Engineering National Institute of Technology, Rourkela Use of CaF2
as flux is more effective in reducing viscosity of basic slags than
that of acidic slags. This may be due to ability of F- ions to
break the hexagonal network of silica and the low melting point of
undissociated CaF2. 27A short term program on process metallurgy of
iron and steel making
28. D E P A R T M E NT O F Viscosity Metallurgical and
Materials Engineering National Institute of Technology, Rourkela
Figure shows that addition of Al2O3 to a basic slag increases
viscosity by acting as network former. Addition of Al2O3 to an
acidic slag reduces viscosity because it now acts as network
breaker. A short term program on process metallurgy of iron and
steel making 28
29. D E P A R T M E NT O FSurface tension Metallurgical and
Materials Engineering National Institute of Technology, Rourkela
The high rates of reaction in basic oxygen converters is due to the
physical conditions of the metal, slag and gaseous phases in the
converter. The theories regarding rapid reaction rates rely heavily
on the formation of slag metal emulsion and slag foams leading to
creation of the large required reaction surface. The most important
feature of emulsion and foam is the considerable increase of the
interfacial area between the two phases leading to the high rate of
reaction.A short term program on process metallurgy of iron and
steel making 29
30. D E P A R T M E NT O FSurface tension Metallurgical and
Materials Engineering National Institute of Technology, Rourkela As
surface tension is the work required to create unit area of the new
surface, the necessary energy for emulsifying a liquid or a gas in
another liquid increases with increasing surface tension value. In
a similar manner energy is liberated when interfacial area
decreases. Hence a low interfacial tension favors both formation
and retention of emulsion.A short term program on process
metallurgy of iron and steel making 30
31. D E P A R T M E NT O FSurface tension Metallurgical and
Materials Engineering National Institute of Technology, Rourkela On
this basis slag / metal and slag /gas systems are not suitable for
emulsification because of the high equilibrium slag/metal
interfacial tension. However the slag/metal interfacial tension is
considerably lowered to 1/100 of the equilibrium value due to mass
transfer. Addition of SiO2 or P2O5 to a basic oxide lowers3 the
surface tension due to the absorption of a thin layer of anions,
viz. SiO44- , PO43- on the surface. It has been reported that
lowering of surface tension of FeO by excess oxygen.A short term
program on process metallurgy of iron and steel making 31
32. Constitution of MetallurgicalMetallurgical and Materials D
E P A R T M E NT O FSlag Engineering National Institute of
Technology, Rourkela The major constituents of iron blast furnace
slags can be represented by a ternary system: SiO2 CaO Al2O3. On
the other hand all the steelmaking and many nonferrous slags are
represented by the ternary system: SiO2- CaO FeO.A short term
program on process metallurgy of iron and steel making 32
33. Constitution of MetallurgicalMetallurgical and Materials D
E P A R T M E NT O FSlag Engineering National Institute of
Technology, Rourkela1.Basic open hearth steel furnace2.Acid open
hearth steel furnace3.Basic oxygen converter4.Copper
reverberatory5.Copper oxide blast furnace6.Lead blast furnace7.Tin
smeltingA short term program on process metallurgy of iron and
steel making 33
34. Constitution of MetallurgicalMetallurgical and Materials D
E P A R T M E NT O FSlag Engineering National Institute of
Technology, RourkelaA short term program on process metallurgy of
iron and steel making 34