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 6

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 13

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 24

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 26

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