Downloaded 21 times
Uploaded byNED University of Engineering and Technology
Refining Slags.pptx
1. Slag is a molten oxide byproduct formed during smelting and refining of metals like steel. It contains both acidic oxides like SiO2 and basic oxides like CaO that neutralize each other. 2. An ideal slag for steelmaking has a basicity between 1.2-2.5, is sufficiently fluid, and can act as a thermal barrier while controlling the oxidation state of the steel through its FeO content. 3. The basicity, viscosity, oxidation potential, and ability to hold inclusions determine a slag's efficiency in refining steel of non-metallic impurities like phosphorus and sulfur.
More Related Content
More from NED University of Engineering and Technology
Refining Slags.pptx
- 1. Refining Slags Steel MakingSlag “Make a slag & slag makes steel” By Dr. Muhammad Ali Siddiqui Assistant Professor Metallurgical Engineering Department NED University of Engineering and Technology, Pakistan
- 2. What is Slag?(Structure/ composition of Slag) • “Molten oxide by product of smelting and refining is known as slag” lighter than hot metal (molten steel) • SiO2, P2O5, B2O5, etc are called acid oxide. Tetrahedral • CaO, MgO, MnO, FeO, Na2O, K2O etc are called basic oxide. octahedral • The opposite chemical nature results in the formation of complex phase (silicate or phosphate) • Therefore, “molten oxide containing two opposite chemical nature of oxide (acid oxide + basic oxide) is called slag. • Where (R ) represents monovalent or divalent catains Fe2+, Mn2+, K+, Na+ etc maintain aqueous acid-base nuetralization. Solution of oxides and sulphides
- 3. • Basic oxidesare ionic solids • Acid oxides are covalent solids. • If the ionic character is predominates (be in the majority) over the covalent character of the bond, Upon melting , these dissociates; • Short range order exist in the melt and no complex ions are formed and ions behaves as fairly free mobile ions, which can further take part in refining. Ca+2, O-2 network breakers Free oxygen simple anions
- 4. • If thecovalent character is predominates over the ionic character of the bond, upon melting, the tendency of complex ion formation increases. • Because, acid oxides have dominant covalent bonding and do not dissociate into a simple ions , but it dissociate in a complex manner as: • This indicates, there is no activity of oxygen ions in the melt. Complex Oxides = Network Formers
- 5. • The majorityof the steel production comes from the basic process because the structure of the slag is reasonably well known.(…may contain the following ions depending upon the chemical composition) • Simple cations = Fe2+, Ca2+, Mg2+, Mn2+, na+, K+, etc • Simple anions = O2-, S2-, etc • Complex anions = SiO4 4-, PO4 3-, AlO3 3, FeO3 2-, etc
- 6. • Acid Slag:In acid steel making (in acid Bessemer or acid open hearth process) the slag is silica saturated has practically no Basicity, this slag acts only as a sink for oxide products it does not take part during refining, it may be very viscous or dry so that it can be readily separated from the steel melt. • Basic Slag: in basic steel making processes, the slag is thin and wet it takes part during refining, it increases mass transport during refining and thereby increase reaction rate. Thin slag has good thermal conductivity. This slag is specially required for efficient refining of non-metallic impurities, the slag acts as a thermal barrier and the rate of heat transfer to the metal bath is inversely proportional to the thickness of the slag layer.
- 7. Role of Slag •Act as sink for impurities. • Control oxidizing and reducing power. (with the help of FeO content) • Higher FeO Slag is Oxidizing (De-C, De-P) • Lower FeO Slag is Reducing (De-S) • Act as a thermal barrier – to prevent heat transfer • Prevents the passage of unwanted gases to the steel from atmosphere (N2 and H2) • Protect the steel from re-oxidation • EAF –slag prevents the radiation of heat may be to walls or roof from the electrode.
- 8. Refining Slag Properties •The main slag properties of interest are; Its ability to retain oxide product. (Basicity) Foaming Slag (basic oxygen steel making) Oxidizing potential (oxidizing/reducing power) • The following other properties are also important from the rate of reaction point of view.(Slag-metal, slag-gas and slag-refractory reaction). • Viscosity, Thermal conductivity and Surface tension etc
- 9. • It isgenerally expressed as the ratio of basic to acid oxide, expressed as – Basicity = ∑%(all basic oxides) / ∑%(all acid oxides) – V Ratio = (wt % CaO) / ( wt % SiO2) – Modified V Ratio = %CaO / ∑%(SiO2 + AL2O3 + P2O5) • In practice slag with much higher V ratio are required to effectively eliminate phosphorous and sulphur during refining. • Slag basicity between 1.2 to 2.5 is also facilitating the foaming slag also provides good properties for desulfurization.. 1. Basicity: Slag ability to retain oxide product. Basic slag V >1 Acid slag V < 1
- 11. 2. Foaming slag •Slag foaming is obtained by injecting oxygen into the liquid steel, where mainly iron is oxidized according to the reaction: O2 + 2 Fe = 2 (FeO) • Carbon powder is then injected simultaneously into the slag phase where iron oxide is reduced. (FeO) + C = Fe + CO (g) • The resulting CO gas is a critical component in order to obtain a foaming slag.
- 12. Does foaming slagsuitable for removal of Phosphorus?
- 13. 3. Viscosity () of the slag • Property of liquid (molten metal) which controls the movement of from one place to another. • Viscosity control the fluidity. (movement of the slag layer) should be fluid enough that can be removed easily from the hot metal surface. • Viscosity depends on composition, Temperature and solid percentage.
- 14. • If moreviscous = less fluid = difficult to remove. Like in Acidic slag highly viscous, dry and thick. • Viscosity Temperature Composition Basic oxide = Network Breaker = CaO, MgO, MnO, FeO, Na2O, K2O Acid oxide = Network Former = SiO2, P2O5, B2O5 •Increase Lime (CaO ), increase the viscosity. •Fluorspar (CaF2) = fluidized the slag Addition of basic oxide decreases the
- 15. • Solid fraction: Viscosity Solidfraction • If in between 5 – 10 %, then increase by 114 – 130 %. (Drastic Effect in the increase in the viscosity)
- 16. 3. The oxidation/ reduction potential • The Capability of slag either to transfer the oxygen into the steel OR to transfer the oxygen from the steel to the slag. (Why we are telling the FeO Content is important here ?) • The oxidizing / reducing power is thermodynamically equal to the partial pressure of oxygen in equilibrium with the slag. Remember = FeO Content will be controlling this potential, How?
- 17. • There aremany oxides present in slag phase (such as MnO, SiO2, etc) but the iron oxide (FeO) has the highest equilibrium partial pressure of oxygen (po2). • This means that the po2 value in equilibrium with the FeO in the slag shall automatically be in equilibrium with the rest of the oxides.
- 18. • If poo2= equilibrium partial pressure of oxygen in contact with pure iron oxide (i.e., iron oxide held in an Fe container) the equilibrium partial pressure of oxygen of a slag will be given by: • Since the value of poo2 is very high, the oxidizing power of a slag is, therefore, conventionally expressed as the activity of iron oxide in slag.
- 19. slag Hot metal Equilibriumconstant Oxidizing Potential (oxidizing slag): Reducing Potential (Reducing Slag): Backward reaction
- 20. • The Basicityand oxidizing power of slag are independent properties. • In practice Basicity is generally related to the lime (CaO) concentration and oxidizing power is related to the FeO content.
- 21. The approx. FeOand V-Ratios of the slags used in steel making practices are shown below.
- 22. Selected Ternary andQuaternary Oxide Systems • Most steelmaking slags consist primarily of CaO, MgO, SiO2 and FeO. • In low-phosphorus steelmakingpractices, the total concentration of these oxides in liquid slags is in the range 88 to 92%. • Therefore, the simplest type of steelmaking slag to be considered is the quaternary systemCaO– MgO–SiO2–FeO. • First let us consider the ternary system CaO-SiO2– FeO; the liquidus isotherms of this system is shown in Fig.01
- 23. • Fig: 01Liquidusisotherms of CaO–SiO2–FeO system.
- 24. • The isothermalsection of the composition diagram in Fig. 02 shows the phase equilibrium at 1600°C. Four two-phase regions, shown by dotted lines, the melt is saturated withSiO2, 2CaO·SiO2, 3CaO·SiO2 or CaO; two three-phase regions (2CaO·SiO2 + 3CaO·SiO2 + liquid) and (3CaO·SiO2 + CaO + liquid); and one liquid phase region.
- 25. • Magnesia isanother important ingredient of steelmaking slags, which are invariably saturated with MgO to minimize slag attack on the magnesia refractory lining of the furnace. • The effect of MgO on the solubility of calcium silicates and calcium oxide is shown in Fig 03 for the system (CaO + MgO)–SiO2–FeO, in equilibrium with liquid iron at 1600°C.
- 26. Fig. 03 Effectof MgO (wt.%) on the solubility isotherms at 1600°C in the system (CaO + MgO)–SiO2–FeO in equilibrium with liquid iron. Broken-line curve delineates the region of saturation of molten slag with solid calcium (magnesium) silicates and solid magnesio- wustite (MgO–FeO solid solution).
- 27. References 1. Modern SteelMaking Handbook R. H. Tupkary, V. R. Tupkary, 2. The Making Shaping and Treating of Steel, Steel Making Refining Volume, AISE steel Foundation. 3. Electric Arc Furnace Simulation Guide, www.steeluniversity.org