SULPHUR BEHAVIOUR UNDER CHANGING PARTIAL PRESSURE OF CO IN REFINING OF FeCr ALLOY

SULPHUR BEHAVIOUR UNDER CHANGING PARTIAL PRESSURE OF CO IN REFINING OF FeCr ALLOY Xoiwei Pan and Rauf Horman Eri Shool of Proess and Materials Engineering University of the Witwatersrand, Johannesburg Private Bag 3, WITS 25, South Afria Tel + 2711716258 ABSTRACT The equilibrium distribution of sulphur between slag and metal phases enountered in refining of FeCr alloys has been investigated at.34,.5 and.83 atm of partial pressure of CO at 1773 and. It is shown that sulphur transfer from metal to slag phase inreases with dereasing partial pressure of CO, inreasing MgO ontent of the slag phase, and dereasing silia, alumina and hromium oxide ontent of the slag phase. It is also demonstrated that sulphur transfer from metal to slag phase inreases with inreasing arbon and dereasing hromium, and silion (in the range;.5 to.2 mole fration of Si) ontent of the metal phase, partiularly when the partial pressure of CO dereases. The relations between MgO and sulphur apaity, the effet of silion on the ativity oeffiients and ativities of silion and sulphur and the effets of silion, arbon and hromium on the ativity oeffiients of sulphur and arbon have been disussed and used to interpret the experimental results. 1. INTRODUCTION With the inrease in demand for low-sulphur steels, the requirement on ferrohromium of low-sulphur ontent is on the inrease as well. High arbon ferrohromium (HCFeCr) is the dominant alloy used in the steel, espeially stainless steel industry ad great efforts have been made to redue its sulphur ontent. Coke or oal used in smelting is the greatest soure of sulphur. From an eonomi point of view, desulphurization during smelting may be more important than hoosing low-sulphur oke or oal whih are in fat obtained from quite limited reserves. It is known that there are ertain relations among sulphur ontent and omposition of both slag and metal phases. The studies 1 2 have demonstrated that when silion and/ or arbon in the metal phase or CaO into the slag phase inreases, sulphur ontent of the alloy dereases. However, sulphur flutuates in most ases and it is not easy to ontrol the arbon ontent unless a suitable hromite ore is available for smelting. For ertain appliations, l?w silion (whih is unfavourable for lowering sulphur ontent) and low-sulphur ontining FeCr alloy is required. In the smeltmg of this grade alloy and/or in the refining of HCFeCr to redue arbon and silion, utilization of other parameters is vital. When refining FeCr alloys by means of oxygen-blowing proesses, the oxygen blown into the metal bath reats with arbon to produe arbon monoxide extremely fast due to favourable kineti and mass transfer onditions at high temperature. Therefore it an be regarded that arbon monoxide is the major omponent that influenes the slagmetal-gas equilibrium. In this study, the effets of the partial pressure of arbon monoxide, metal nd slg ompositions to the sulphur behaviour m refining of FeCr alloy have been investigated in terms of slag-metal-gas equilibrium at 15 and 158 C. 2. EXPERIMENTAL PROCEDURE The slags were prepared as homogeneous mixtures from premelted master slags made from analytial grde pure oxides. Chromium oxide and iron oxide MOLTEN SLAGS, FLUXES AND SALTS '97 CONFERENCE - 191

were added when neessary to these homogenized samples prior to experiments. The initial slag ompositions were as follows : MgO; 6-2%, CaO; 1-5%, Si 2 ; 4-65%, Al 2 3; 1-3%, Cr 2 3; -25%. The metal samples were also prepared as homogeneous mixtures made from analytial-grade pure omponents. The starting metal ompositions ranged as the following : Cr; 5-7%, Fe; 18-35%, Si;.1-5%, C; 3-9%, S:.1-1 %. The partial pressure of CO was ontrolled by mixing CO and Argon gases utilizing alibrated apillary flowmeters. Samples of 2g of metal and 4g of slag were plaed in a hromium oxide or silia ruible and heated within the 3mm hotzone of a vertial molybdenum-wound resistane furnae, whih maintained onstant temperature within ± 1 C at 15 C through a digital temperature ontroller ativated by a Pt- 6%Rh/Pt- 3%Rh thermoouple. Sample temperatures were measured by another thermoouple of the same type. At the end of equilibration, the ruibles were quenhed by being dropped into water. The equilibration time was determined after a series of gas-slagmetal equilibrium runs, whih were onduted at 15 C at times varying up to 15 hours with the same initial ompositions under CO atmosphere. The approah to equilibrium was established by analysis of all the seven omponents of the slag phase, namely; Al2 3, CaO, MgO, Si2, Cr2 3, Fez3 and S. After about 12 hours no signifiant hange in ompositions were observed, and all the experiments both at 15 C and 158 C were run for 13 hours. After equilibration and quenhing, the distint slag and metal phases were separated, leaned when neessary and sent for analysis to Counil for Mineral Tehnology (Mintek) of South Afria. The arbon and sulphur ontents of the alloy were analyzed by means of a Leo apparatus, the other onstituents of the metal by the ICP method. The iron an hromium in the slag were analyzed by the o- phenanthroline olorimetri and f diphenylarbazide olorimetri methods respetively, and the other ontents of the slag by x-ray fluoresene. In addition, some x-ray diffration analyses and metallographi examinations were onduted on slag/ruible interfae to detet the ruible behaviour. Three series of experiments were undertaken : (i) in hromium oxide ruibles at 15 C and Po -.5 atm; (ii) in hromium oxide ruibles at 15 C and Po -.83 atm; (iii) in silia ruibles at 158 C and Po -.34 atm. 3. RESULTS AND DISCUSSION About 12 suessful experiments were onduted. The failure of some of the experiments were due to the raking of the hromium oxide ruibles. Efforts to redue raking was one of the major hallenges during the test work and inluded preheating of the ruibles to 14 C in a muffle furnae (prior to experiments), winding Mo wire around the ruibles to form a basket and utilizing external ruibles suh as molybdenum, zironia or alumia to hold the hromium ruibles. 3.1 Influene of slag omposition on sulphur distribution The relationship between the sulphur distribution-expressed as the ratio of molar fration of sulphur in the slag to that in the metal; (Xs)/[X'.g] - and the molar frations of all the onstituents in the slag phase were investigated under different P o in equilibrium with Cr-Fe-Si-C-S alloys. The slag omposition range was as follows : -.1 %S, -25%Cr2 3, 4-6%Si2, 1-3%Al23, 1-5% CaO, 6-2% MgO, and about.5% Fe (in mass %). In the metal phase the ompositional range was 45-65 % Cr, -9%C, -5% Si, -.5% S and balane Fe by mass. 192 - MOLTEN SLAGS, FLUXES AND SALTS '97 CONFERENCE

As shown in figure 1, with the inrease of MgO ontent of the slag, its desulphurization apability inreases both in Cr 2 3 and Si 2 ruibles at 15 C and 158 C. The sulphur distribution in Cr 2 3 ruible is higher than under silia saturation even though the silia saturation data is at the higher temperature of 158 C. This learly implies the stronger power of hromium oxide in reduing the sulphur ontent of the metal. The results at 15 C in Cr 2 3 ruibles indiate that the sulphur distribution ratio inreases when P o dereases when the MgO ontent of the slag is the range from.17 to.35 by molar fration. The slag-metal equilibrium in terms of sulphur and oxygen speies an be written as : [S] + ( 2 -) = (8 2 -) + [OJ (1) where square and round brakets indiate the metal and slag phases respetively. MgO in the slag will dissoiate aording to : (MgO) = (Mg 2 +) + ( 2 -) (2) Thus an inrease in MgO ontent of the slag will inrease free 2 - anions in the slag resulting in an inrease of the ativity of oz- anions whih in tum will help to shift reation (1) to the right, in tum assisting sulphur removal from the metal. The sulphide apaity; Cs of the slag has been defined by Sharma and Rihardson 3 from the following equilibrium : s2 (g) + <2-) = <s2-) + 2 (g) (3) 1 Cs= (%8) (Po 2 /Ps 2 ) 2 (4) Cs is a funtion of several variable suh as temperature and slag omposition, but mainly temperature and basiity of the slag. From the work of Martin et al4, the following equation was obtained : log C 8 =B/A+2.28-133/T(S} where A and B are defined as : A = %Ca+1.391 % Si 2 + 5% Al 2 3 MgO+l.867% (6) B = 5.623% Ca+4.15% Mg-1.152% Si 2 + 1.457 % Al 2 3 (7) An inrease in MgO ontent of the slag will inrease the Bl A ratio and thus Cs will inrease. The relationship between Cs, Bl A ratio and % MgO is depited in Figure 2, for a slag ontaining 5% Cao, 2% Al2 3, 45% Si 2,.5% Fez 3, 5-25% MgO and balane Cr2 3. The expliit effet of Cr 2 3 in the slag on sulphur distribution is shown in Figure 3. In general when hromium ontent dereases sulphur transfer from metal to slag inreases. This indiates that Cr 2 3 in the slags behaves like an aidi oxide onsuming free 2 - ions. This result is in agreement with Xiao and Hollappa's 5 work in siliate slags ontaining hromium oxides. The higher the hromium oxide in the slag, the lower the divalent hromium fration and the higher the trivalent hromium. The results show that the slags ontain more sulphur in Cr 2 3 ruibles at 15 C then under Si 2 saturation at 158 C. This again learly demonstrates the highly aidi nature of Si 2 Thus a slag high in Cr 2 3 will inrease its desulphurization ability. Moreover, onditions beome more favourable for sulphur removal from metal at lower P o values. The effet of Al 2 3 on the partition ratio of sulphur is illustrated in Figure 4. The partition ratio dereases slightly with MOLTEN SLAGS, FLUXES AND SALTS '97 CONFERENCE - 193

2-----------------------.,..., i 1.4 111 1.2.:::: 1...8.6.4.2 Cr-oxide ruible Po.83atrr III silia ruible Po.3atm 4 Cr-oxide ruible Po.5atrr -------------+---+-----+---------1.5.1.15.2.25.3.35 MgOontent in slags, mole fration 4 Figure 1. Effet of MgO on sulphur distribution with metal of Cr-Fe-C-.25%Si-S and slag of 1.4%Ca-.7%Fe23-Mg-Cn3-Si2-Al 23-S by mass% with Al2Q3/MgO =. -4.6 -r------------------_..,. -4.65-4.7-4.75 -+-Log Cs -4.8 en u, -4.85.9-4.9 ---value BIA.7.6.5.4.3 B/A -4.95-5 -5.5 5 1 15 2 25-5.1 ------------------+------+ MgO ontent in slags, mass%.2.1 Figure 2. Relationship between BIA and MgO, sulphide apaity Cs and MgO with slag ontaining 5%Ca, 2%Al23, 45%Si2,.5% Fe23, MgO and Cr2Q3 (as balane). 194 - MOLTEN SLAGS, FLUXES AND SALTS '97 CONFERENCE

2 1.4 1.2 GI 1.5..8.6 :.4.2.1.2.3 Cr in slags, mole fration + silia ruible Po.3atm II Cr-oxide ruible Po.83atm.._ Cr-oxide ruible PoO.OSatm...4.5.6 Figure 3. Effet of Cr of slag on sulphur distribution with metal of Cr-F e-c-.25%si-s and slag of l.4%ca-.7%fe23-mgo-cr23-si2-ah3-s by mass% with AhQ3/MgO = 1. 7. 1.2 Jg.8 GI....6 in ::::.4!a. t +--i--+-...--+-.2 sili.a ruible Po.3atm... Cr-oxide ruible Po.5atm.._ Cr-oxide ruible Po.83atm.6.8.1.12.14.16 Al23 ontent in slags, mole fration Figure 4. Effet of Ah3 on sulphur distribution with metal of 55%Cr-Fe-C-.25%Si-S and slag of l.4%ca-fe23-mg-cr23-si2-ah3-s by mass% with Ah3/Mg = 1.7, 3-5%/.1-2% Si in Cr/Si oxide ruible. MOLTEN SLAGS, FLUXES AND SALTS '97 CONFERENCE - 195

inreasing Al 2 3 ontent in the slag showing that Al 2 3 when dissolved in the slag tends to onsume 2 - ions whih redues the basiity of slags via : (8) Figure 5 illustrates the effet of silia in the slag on the partition ratio of sulphur. The slag-metal equilibrium expressed in reation (1) gives the partition ratio of sulphur as : It is apparent from equation 9 that the partition ratio of sulphur inreases with inreasing ativity of oxygen ion in the slag and with dereasing ativity of oxygen dissolved in the metal. As the Si 2 in the slag inreases more free oxygen ions will be onsumed by Si 2 and in tum the sulphur partition ratio will derease as demonstrated in Figure 5. The silion ontent of the metal, meanwhile, inreases with inreasing Si 2 in the slag. This will tend to lower the ativity of dissolved oxygen in the metal, sine Si will reat with it ( deoxidation). [Si] + 2 [OJ = (Si 2 ) (1} When ativity of oxygen dissolved in the metal dereases, it may help to inrease the partition ratio somewhat ountering the effet of inreased Si 2 in the slag, but this effet is probably minimal. 3.2 Influene of metal omposition on sulphur distribution The effets o the onstituents of the metal phase, namely Cr, Si and C on the equilibrium partition ratio of sulphur are illustrated in figures 6 to 8 respetively. The partition ratio dereases with inreasing hromium and silion levels (at low Si onentrations). Aording to sulphur distribution relationship from equation (9), the fators whih derease the ativity of oxygen in the metal and inrease the ativity oeffiient of sulphur in the metal will inrease the partition ratio of sulphur. In ferrohromium alloys the inrease in arbon and silion an lower the ativity of oxygen in the metal and an inrease the ativity oeffiient of sulphur in the metal. In the present ase when Xsi <.2 the sulphur partition dereases as Si ontent inreases as shown in figure 7. It may mean that in the metal of lower silion, inrease of silion does not make remarkable ontribution to redue the ativity oeffiient of sulphur, but does derease the basiity of the slag by adding silia into slags by ombining with dissolved oxygen and thus lower the partition ratio of sulphur. Where else the behaviour of arbon is as expeted as shown in Figure 8. 3. 3 Influene of the partial pressure of CO. The effet of lowering P o on sulphur partition ratio is in fat similar to the effet of vauum treatment. The partition ratio is affeted through basiity of slag and ativity of dissolved oxygen and sulphur in the metal. When Pea is redued, the onentration of some of the omponents of the metal and slag phases hange, partiularly silion, arbon and hromium whih in tum affet the sulphur partition ratio. As a general rule, with dereasing P o the following equilibria will be more favourable : [C] + [] = CO(g) 2[C] + (Si 2 ) = [Si] + 2CO(g) [C] + (Cr 2 3 ) = 2(Cr) + CO(g) [C] + (CrO) = [Cr] + CO(g) [Si] + (CrO) = [Cr] + (Si 2 ) (11) (12) (13) (14) (15) 196 - MOLTEN SLAGS, FLUXES AND SALTS '97 CONFERENCE

2-r--- -, 1.4!!! 1.2.! 1...8 e ;:::;..6!!?..4 Cr-oxide ruible Po.83atm II Cr-oxide ruible Po.5atm 6 silia ruible Po.3atm.2 o..._-+-... -+-...;;:... --1.25.35.45.55.65.75 Si2 ontent in slags, mole fration Figure 5. Effet of Si2 on sulphur distribution with metal of 55%Cr-Fe-C-Si S and slag of 1.4%Ca-. 7%Fe2Q3-MgO-Cr23-Si2-Ah3-S by mass% with AhQ3/MgO = 1. 7. o 1.4 ts jg 1.2.!! 1...8 -.6 -.4.2 2--- --- Cr-oxide ruible Po.5atm 1773 II Cr-oxide ruible Po.83atm 1773atm silia ruible Po.34atm +--.--+-... - --------.25.35.45.55.65.75 Cr ontent in alloys, mole fration Figure 6. Effet of [Cr] on sulphur distribution with metal of Cr-Fe-C-Si-S and slag ofl.4%ca-.7fe2j-mgo-cr2j-si2-ahoj-s by mass% with AhQ3/MgO = 1.7, 3-5%/.1-2% Si in Cr/Si oxide ruible. MOLTEN SLAGS, FLUXES AND SALTS '97 CONFERENCE - 197

2-r--,1--. 1.4 t; -.. I'll 1.2.!! 1.5..8 ui'!:::'.6 -.4 Cr-oxide ruible Po.83atm II Cr-oxide ruible Po.5atm silia ruible Po.34atm.2 188 +--+-...;o,.-+--;+--+-... --+--!.1.2.3.4.5.6.7.8 Si ontent in alloys, mole fration Figure 7. Effet of [Si] on sulphur distribution with metal of 55%Cr-Fe-C-Si S and slag of l.4%ca-. 7Fe23-MgO-Cr23-Si2-Ah3-S by mass% with AlzQ3/MgO = 1. 7. 2 1.5 -QI. -.5 Cr-oxide ruible Po.83atm Cr-oxide ruible Po.5atm silia ruible Po.34atm +--...-;,---1------'I.5.1.15.2.25 C ontent in alloys, mole fration Figure 8. Effet of [C] on sulphur distribution with metal of 55%Cr-Fe-C-Si-S and slag of l.4%ca-fe23-mg-cr23-si2-ah3-s by mass% with AlzQ3/MgO = 1. 7, 3-5%/.1-2% Si in Cr/Si oxide ruible. 198 - MOLTEN SLAGS, FLUXES AND SALTS '97 CONFERENCE

Thus the effet of P o will atually follow similar lines to the effets of hanges in the omposition of the slag and metal as outlined earlier. The arbon-oxygen equilibrium in reation (11) may help explain one of the reasons that with the same arbon ontent in the metal, sulphur partition ratio inrease with a derease in P o through a possible derease in the ativity and/ or onentration of oxygen as illustrated in Figure 8. 4. CONCLUSION The distribution of sulphur between slag and metal phases enountered in refining of FeCr alloys has been investigated at.34,.5 and.83 atm partial pressure of CO at 15 using hromium oxide ruibles and at 158 C using silia ruibles. It is shown that sulphur distribution is related to P o and the ompositions of both the slag and the metal phases. Sulphur transfer from metal to slag inreases with dereasing P o, inreasing MgO ontent of the slag, and dereasing silia, alumina and hromium oxide ontent of the slag. It is also demonstrated that sulphur transfer to slag from metal inrease with inreasing arbon and dereasing hromium and silion in the metal, partiularly when the Po dereases. 1. 2. 3. 4. 5. REFERENCES L. Hu, "A Study in sulphur ontrol in HC FeCr Proess," Ferroalloys, Vol. 4, 198, pp 14. C. Pikles, "A Review of Behaviour of Impurities in HC FeCr Prodution, " Iron and Steelmaker, 1986, No. 12. pp 37-48. RA. Sharma and F.D. Rihardson, "The Solubility of Calium sulphide and Ativities in Lime-Silia Melts." JFSI Vol. 2, 1962, pp 373. E. Martin, I. Abdelkarim, I.D. Sommerville and H.B. Bell, "Metal Slag-Gas Reations and Proesses," Eletrohem So., Toronto, 1975, pp 1-7. Y. Xiao and L. Holappa, "Thermodynamis of Slags Containing Chromium Oxides," INF ACON 7, Trondheim, June 1995, pp 319-328. ACKNOWLEDGEMENTS The authors are grateful to Counil for Mineral Tehnology (Mintek) and the Ferro Alloy Produers Assoiation of South Afria for providing finanial support to undertake this investigation. MOLTEN SLAGS, FLUXES AND SALTS '97 CONFERENCE - 199