10 Sulphur Problem AkMB Rashid Professor, Department of MME BUET, Dhaka Today s Topics Introduction Behaviour of sulphur in metal and slag Oxidation of sulphur in the furnace Oxidation of sulphur in the ladle Conditions for desulphurization
Introduction It is generally considered that sulphur is of no value in steel. Sulphur contributes red-shortness and susceptibility to overheating in wrought steels ingot cracking and low ductility in cast steels. Source of sulphur in steel pig iron steel scrap, especially in oily steel chips fuel (high-sulphur fuel oil, gas produced from high-sulphur coals) In many steel specifications, for that reason, the maximum permissible S content is limited between 0.03-0.06 wt% S. 3/16 Behaviour of Sulphur in Metal and Slag S has unlimited solubility in liquid iron, but limited solubility in solid iron. sulphur contents only about 0.001-0.1 wt% are typical in steelmaking. Sulphur dissolves in iron from its gaseous state with evolution of heat: 1/2{S 2 } = [S] 1wt% ; DG = -72,000-10.25 T J/mol a high DH indicates that certain bonds existing between S and Fe in the solution S does not pass into the gaseous phase in elemental [S] form 4/16
The solutions of sulphur in pure iron exhibit appreciable negative deviations from Henry's law. activity of sulphur in molten steel depends on the composition of steel The activity coefficient of S increases with increase in C, Si and P Influence of sulphur content, and of third elements added to ironsulphur solutions, on the activity coefficient of sulphur at 1600 C 5/16 Sulphides are completely soluble with silicate slags at high temperatures, and show extensive solubility at iron- and steelmaking temperatures Solubilities of CaS in CaO.SiO 2, CaO.Al 2 O 3.2SiO 2, and 2CaO.Al 2 O 3.SiO 2 are around 20-30% at 1500 C and approximately doubled at 1650 C Similar solubilities expected in other complex commercial slags Solubility of CaS in blast furnace slag is about 9% In practice, however, S concentrations up to about 10% are found and henrian behaviour is always assumed. 6/16
Oxidation of Sulphur in the Furnace Reaction involving transfer of S from metal to slag [FeS] + (CaO) = (CaS) + [FeO] (molecular equation, considering only CaO and CaS as the slag species) or, [S] + (O 2- ) = (S 2- ) + [O] (more general ionic form) K = a CaS a FeO a FeS a CaO a S 2 a O h S a O 2 S O f S S a O 2 Partition coefficient of sulphur or index of desulphurization (S) S = K f S a O 2 O 7/16 The partitioning of sulphur between slag and metal in laboratory experiments and in basic steelmaking practice can be described as log K 1873 = 1.4 N Ca 1.9 N Fe 2.0 N Mn 3.5 N Mg + 1.6 N Na where N Ca etc. are the electrically equivalent ionic fractions The approximate dephosphorization power for some basic cations in slag : Na Ca Fe Mn Mg 1070 : 1.0 : 0.325 : 0.25 : 0.0075 Na 2 O is the most powerful, but soda ash cannot be used in steelmaking because of its tendency to attack and corrode the refractory lining of the furnace. FeO as a basic oxide may be used, but it will increase oxygen potential and hinder S removal Most common desulphurizers are CaO and MgO 8/16
Oxidation of Sulphur in the Ladle Carried out using synthetic slag with a high activity of CaO Al 2 O 3 and CaF 2 are added to reduce the melting point An essential point is that it contains no FeO Desulphurization by synthetic slags gives satisfactory results provided that all slags at the end of melting (containing high FeO) must be removed A lid cover prevents metal oxidation and lower FeO content in the bath 9/16 To increase slag-metal contact surface, liquid metal is poured from a large height into the ladle with molten synthetic slag. Blowing inert gas also induces bath agitation. If a synthetic cannot be used, a mixture of solid lime CaO and fluorspar CaF 2 can be used in the ladle before tapping. Too large addition can lower liquid temperature. Addition of deoxidiser to metal decreases [O] and a FeO in liquid and aids S removal. 10/16
Conditions for Desulphurization Conditions for S removal from the liquid metal High K of the equation Favoured by high temperature High activity coefficient (i.e., f s ) of S in metal C, Si and P increases f s hot liquid iron is better for S removal than molten steel High activity of CaO (i.e., a O 2 ) in slag which means highly basic slag. (S) S = K f S a O 2 O Low activity of FeO (i.e., [O]) in metal Reaction shows removal of S from hot metal is accompanied by oxygen transfer to metal. This oxygen must be removed for efficient desulphurization. 11/16 Effect of slag composition In acid slag, a O 2 = 0, thus (S)/[S] is zero. Increasing slag basicity (CaO/SiO 2 ), increases (S)/[S] Lowering oxygen content (FeO) increases (S)/[S] In open hearth and oxygen converter since steelmaking is carried out in oxidizing conditions (except EAF reducing slag period), low a FeO is not possible, and thus, the efficiency of desulphurization is very low. The value of index ranges from 50-100 under blast furnace conditions and drops to 5-10 under basic steelmaking conditions Efficient stirring the liquid bath helps S removal by improving metal-slag contact surface since S removal takes place at the metal-slag interface 12/16
B = (%CaO+%MgO)/(%SiO 2 +Al 2 O 3 ) The influence of basicity and of iron oxide content of the slag on the distribution of sulphur between slag and metal at 1600 C. Sulphur distribution between slag and metal depending on slag basicity at various activities of ferrous oxide in slag 13/16 Principal conditions for efficient desulphurization 1. High activity of CaO in slag 2. Low activity of FeO in slag 3. Low activity of oxygen in metal (metal must be deoxidised thoroughly) 4. Low activity of sulphur in slag 5. High temperature 6. Large contact surface between metal and desulphurizing slag 14/16
In practice, these conditions are provided by the following techniques : 1. Addition of lime (CaO) or limestone (CaCO 3 ) 2. Addition of substance to lower slag viscosity (e.g., Al 2 O 3, CaF 2, MnO, etc.) 3. Renewal of slag (removal of last slag and formation of new slag free from S and FeO) 4. Carried out desulphurization during heat when the metal has the highest temperature. At high temperature [C] + (FeO) = {CO} + [Fe] takes place vigorously; (FeO) is reduced, a CaO increased (by more CaO dissolution), diffusion process increased, and metal-slag contact surface increased 15/16 5. Treatment in ladle of metal with liquid or solid synthetic slag containing high a CaO and low a FeO 6. Use of rare-earth elements (Ce, La, etc.) for desulphurization 7. When making steel very low in sulphur (a) low S charge is selected, (b) highly basic slag (CaO and CaF 2 ) is blown into the metal in a jet of inert gas, (c) CaC 2 and other alloys containing Ca or even pure Ca (and also Mg) are introduced 16/16