Coexistent. of the pellet do not interact; at this tem- perature there is negligibly small mutual solid solution I. Introduction

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1 Sulfur Solubility in Iron-Carbon Melts Coexistent with Solid CaO and CaS* By E. T. T URKDOGAN** and L. J. MART ONIK** Synopsis Experimental data are presented for the solubility of sulfur in liquid Fe-C alloys coexistent with solid calcium oxide, calcium sulfide and carbon monoxide of known pressures at C. The equilibrium constant is determined for the sulfur reaction in terms of the activities of sulfur and carbon in iron for composition ranges to 0.4 % S and up to 3 % C. The extrapolation of the data to hot metal compositions crucible. The oxide/sulfide pellets, 16 mm in diameter, 3 mm thick weighing about 1.2 g, were made by pressing a powder mixture of reagent grade 40 % CaS, 40 % Ca0 and 20 % MgO. It was thought that the addition of an inert oxide such as MgO may prevent the formation of a continuous layer of Ca0 or CaS at predicts for unit activities of calcium oxide, calcium sulfide and carbon monoxide, the equilibrium sulfur content of 4 ppm for C and the metal-pellet interface during the reaction. At 8 ppm for C. In relation to ladle desulfurization of hot metal, the experimental temperature of C, the ingredients selected data are presented on some slag-metal reaction equilibria. of the pellet do not interact; at this tem- perature there is negligibly small mutual solid solution I. Introduction between Ca0, CaS and MgO. The crucible assembly was suspended with a The burnt lime is often used, sometimes together molybdenum wire in the uniform hot zone of the with other additives, in the ladle desulfurization of recrystallized-alumina reaction tube. Using a molybdenum-wire-wound resistance furnace equipped hot metal. When burnt lime is intimately mixed with hot metal in the ladle, direct desulfurization with a temperature controller, the melt temperature occurs by the reaction was maintained at 1 600±2 C during 5 h reaction CaO(s)+S+C-*CaS(s)+CO(g)...(1) time. Based on past experience, a reaction time of 5 h was considered long enough for the establishment Over the years, many equilibrium studies were made of equilibrium in the system. The equilibrium of the sulfur reaction in gas-slag and metal-slag systems. measurements were made with 10%CO+90%Ar Surprisingly, no direct measurement has been and 75%CO+25%Ar purified gas mixtures flowing made of the state of equilibrium for reaction (1) in through the reaction tube at the rate of 11/mm. the simple system Fe-C-S melts coexistent with In all the experiments, the oxide/sulfide pellet solid calcium oxide and calcium sulfide at known remained intact resting on the surface of the melt. temperatures and partial pressures of carbon monoxide. As would be expected from interfacial tension con- Although the state of equilibrium for reaction siderations, at low carbon hence high sulfur contents, (1) can be estimated from compiled thermochemical the crucible and the pellet were wetted by the liquid and various gas-slag-metal equilibrium data, the metal. At high carbon and low sulfur contents, the equilibrium constant thus estimated may be in error by as much as a factor of two because of the accumulation of uncertainties in the summation of various free metal surface acquired a convex shape because of high surface tension, indicating poor wetting of pellet by melt, and the pellet could be readily detached from energy data. This reaction is of sufficient practical the surface of the solidified melt. importance to warrant experimental determination After the experiment, the metal slug was sliced of the equilibrium constant. across at three locations : near the top surface, middle II. Experiments and near the bottom. These samples were analyzed separately for carbon and sulfur to check for concentration A series of Fe-C-S alloys, each 450 g, were profiles from top to bottom of the melt. There made by adding the required amounts of high were no systematic compositional variations along purity graphite and iron sulfide to the electrolytic the length of the metal sample, variations being within iron melted in a vacuum-induction heated furnace. ± 10 % of the amount present. The melts were cast into 16 mm diameter copperfinger To check for possible side reactions that might molds. About 60 g of the alloy used in the have occurred between the melt, the crucible and the equilibrium measurement was contained in a calciumstabilized zirconia crucible, 20 mm in diameter and 40 mm deep, with a compressed oxide/sulfide pellet oxide/sulfide pellet, selected samples were examined by X-ray diffraction analysis and by X-ray scan spectra using a scanning electron microscope. resting on the top of the metal sample. In experiments at graphite saturation, the melt with the III. Results oxide/sulfide pellet was contained in a graphite The experimental results are presented in Table * Presented to the Japan-U.S. Seminar on Advances in the Science of Iron- and Steelmaking, May 1983, at Hieizan Kokusai Kanko Hotel in Kyoto. Manuscript received February 8, ISIJ ** Research Laboratory, United States Steel Corporation, Monroeville, Pa , U.S.A. (1038 ) Research Article

2 Transactions ISIJ, Vol. 23, 1983 (1039) 1, showing the initial alloy composition and the composition of metal samples after 5 h reaction with the oxide/sulfide pellet at 0.10 and 0.75 bar CO partial pressures. The effect of carbon on the equilibrium sulfur content of the melt at 0.10 and 0.75 bar CO is shown in Fig. 1. The arrows indicate the direction of change of melt composition during reaction with the oxide/sulfide pellet. For sulfur contents from about 0.4 to %, the concentration of sulfur in the melt decreases with an increase in the concentration of carbon and a decrease in the partial pressure of carbon monoxide. This observed relation between the melt composition and the partial pressure of carbon monoxide in the system is that anticipated for reaction (1) which is close to or at equilibrium. In experiments with melts containing more than about 1.5 %C at 0.10 bar CO, the sulfur content of the melt is scattered within the range to %, independent of carbon content up to saturation with graphite. At a CO partial pressure of 0.75 bar, the % S vs. % C relation is as would be expected up to about 3 % C. At graphite saturation, however, an anomalous behavior is again observed; the sulfur content is about an order of magnitude greater than the equilibrium value estimated from extrapolation of the experimental data to higher concentrations of carbon. In an attempt to resolve the anomalous behavior of the sulfur reaction at high carbon activities, i.e., at low oxygen potentials, the materials scraped from the melt-crucible and melt-pellet interfaces were examined by X-ray diffraction analysis. There were no indications of the presence of zirconium carbide or calcium carbide. In any case, the calcium carbide is not expected to form under the experimental conditions employed. On the other hand, the available thermochemical data1 suggest that at C and 0.10 bar CO, the zirconium carbide may form by the reduction of zirconia with carbon in liquid iron containing 3 to 4 % C, for which the corresponding equilibrium concentration of zirconium in the metal would be about 0.1 %. Such a reaction was evidently sluggish in our experimental system because there was no X-ray evidence of the presence of ZrC on the inside surface of the zirconia crucible that was in contact with the melt; furthermore, the chemical analysis showed that the metal contained less than 0.01 % Zr. The oxide/sulfide pellets from experiments with high-carbon melts, where there was an anomalous behavior of the sulfur reaction, were examined by scanning electron microscopy. Typical examples of the X-ray scan spectra obtained from these pellets are shown in Fig. 2: (a) top surface of the pellet exposed to gas, (b) cross section of the pellet and (c) bottom surface of the pellet in contact with the metal. The relative peak heights Ca : S : Mg for the cross section correspond to the pellet composition. However, the top surface of the pellet (a) is depleted of sulfur and magnesium and the bottom surface (c) is enriched in sulfur. In fact, the X-ray diffraction Table 1. Composition of Fe-C-S melts before and after reaction for 5 h with coexistent CaO and CaS in Ar-CO gas mixtures at C analysis showed that the surface of the pellet next to the gas phase was primarily CaO and the bottom surface next to the melt was CaS. Loss of Mg and S from the pellet surface during 5 h reaction time may be attributed to vaporization via reaction of MgO and CaS with CO in the gas. The migration of CaS to the pellet-melt interface is harder to explain. This unexpected composition change in the pellet during reaction with the liquid metal occurred only at high carbon contents and low oxygen potentials. Under these conditions, the surface tension of the alloy is much higher than that of the iron containing lower concentrations of carbon hence higher concentrations of sulfur. One possible explanation of the CaS migration to the pellet-melt interface may be associated with the interfacial tension effect. The observed behavior would suggest that at low oxygen potentials the interfacial energy between liquid iron and CaS may be lower than that between liquid iron and CaO. This is a speculative argument; it should be tested by measuring for example, contact angles between Fe-C melts and CaS, and CaO. If the interfacial energy difference a (CaS/melt)-a (CaO/

3 (1040) Transactions Is", Vol. 23, 1983 Fig. 1. Sulfur and carbon contents of liquid iron after 5 h reaction with coexistent solid CaO and CaS at C in Ar-CO gas mixtures at atmospheric pressure. melt) is negative, there would be then a driving force at the pellet/melt interface for the substitution of oxygen atoms in the CaO by the sulfur atoms from the neighboring CaS crystals. Such an 0-S exchange reaction may occur via a surface diffusion mechanism resulting in the observed depletion of CaS on the pellet/gas surface and accumulation of CaS at the pellet/melt interface. Whatever the reaction mechanism might be, the separation of contact between the melt and the oxide/ sulfide pellet by a layer of essentially pure CaS will certainly hinder reaction (1) to approach equilibrium. It is for this reason that the non-equilibrium sulfur contents of high carbon melts in Fig. 1 are much higher than the expected equilibrium values. Iv. Discussion For liquid Fe-C-S alloys coexistent with carbon monoxide and essentially pure solid CaO and CaS, the equilibrium constant for reaction (1) is represented by K_ [ Pco Pco as] [ac] - [%S][%C]fsfc where Pco is the partial pressure of CO in bar and a's are the activities defined such that the activity coefficient fs -' 1 and fc -' 1 when % C In melts containing less than 0.5 % S, the effect of sulfur on its own activity coefficient and that of the carbon Fig. 2. X-ray scan spectra of the 5 h reaction. oxide/sulfide pellet after is small enough to be neglected. The activity coefficients of carbon and sulfur as affected by the concentration of carbon in the melt are shown in Fig. 3 using the data of Rist and Chipman2,3~ for fo, and of Morris and Buehl4> for fs. These values of fs have been confirmed in a subsequent study by Ban-ya and Chipman.5~ With the use of these activity coefficients, the equilibrium data in Fig. 1 (with the exclusion of anomalous nonequilibrium sulfur results caused by CaS accumulation on the melt surface) are presented in Fig. 4 in terms of activities as a log-log plot. The lines are drawn with the theoretical slope of --1; the lines for 10 % and 75 % CO are displaced by a factor of 7.5 in accord with the equilibrium relation as given in Eq. (2). The dotted lines are calculated from other experimental data for gas-metal, gas-oxide/sulfide

4 2 Transactions Is", Vol. 23, 1983 (1041) reactions and known thermochemical data as discussed below. Rosengvist6~ determined the equilibrium ratio ph2o/ px2s for the system CaO-CaS at temperatures of 750 to 1400 C. Turkdogan et al.'s measured the equilibrium partial pressures of SO2 for the CaO- CaS system at various oxygen activities, PCo2f pco, and temperatures of 950 to C. Similar equilibrium measurements were made also by Kor and Richardson8~ at and C. Combining the results of these equilibrium measurements with the appropriate thermochemical data gives the following free energy equation for the reaction CaS (s)+ 102( ) = CaO (s) +1S2(g) 2 ~' 2 4G = T J,...(3) all the data cited being in close agreement. The free energies of solution of gaseous oxygen9,lo~ and sulfur11 in liquid iron and the equilibrium relation for C-0 reaction in liquid iron12~ are known with adequate accuracy, thus 102(g)=0 [1 wt%] Fig. 3. Effect of carbon cients of carbon at C. in the melt on the activity coeffiand sulfur dissolved in liquid iron Fig. 4. Experimentally determined equilibrium relation for reaction Ca0 + S + C = CaS + CO is compared with that calculated from other equilibrium and thermodynamic data.

5 (1042) Transactions ISIJ, Vol. 23, 1983 CaO (s)+s +C = GaS (s)+ CO (g) dg = T J...(7) The equilibrium constant thus derived from other equilibrium data for reaction (7) at C is The value of K obtained from the results in Fig. 4 is 8.33 which is lower than the value calculated from other data by a factor of In a recent study13~ of the silicon-sulfur reaction between graphite-saturated liquid iron and blast furnace-type molten slaps, a similar difference was found between the experimentally determined and calculated (from thermochemical data) values of the equilibrium constant, the experimentally determined sulfur contents of the metal being higher than the calculated values. As indicated by the entropy term in Eq. (7), the desulfurization of liquid iron with calcium oxide and carbon becomes less favorable at lower temperatures. For example, the other conditions being the same, a decrease in temperature from to C will double the equilibrium concentration of sulfur in the metal. It is obvious from reaction (7) that desulfurization would be greatly enhanced when the partial pressure of CO in the melt is lowered by injection of lime into the liquid metal with an inert gas. In relation to the ladle desulfurization of hot metal, let us consider the effect of silicon in the metal and the presence of aluminosilicate slag on the sulfur reaction which is represented by (CaO)+S+ 1 Si = (CaS) + (SiO2) 2 2 a CaS (a Sio2 )1/2 KSiS =... (8) acao [as][f Si X %Si]1/2 where fsi= 15 is the activity coefficient of silicon in graphite-saturated iron. The equilibrium constant K515 is evaluated for 1600 C by combining the equilibrium constants for reaction (6), and the experimental value of K=8.33 for reaction (7) with the known equilibrium constant for silicon deoxidation of iron K (as1o2)1/2 si= ~ao~~,f =213 si X %Si]l/2 giving Ksis=3.55. With fsi=15 and fs=6.6 for graphite-saturated iron we have the following equilibrium relation for C. acas (aslo2)1/2 acao [%S][%Sl]"2 With this equilibrium relation and the activity data compiled by Rein and Chipman,14~ we can compute the interrelated concentrations of sulfur and silicon in graphite-saturated iron in equilibrium with CaS-saturated calcium aluminosilicate slaps. These are shown in Fig. 5 for CaO-Si02 and CaO- A %Si02 slaps, both saturated with Ca2SiO4 and CaS. Also included in this diagram is the equilibrium relation, determined experimentally by Turkdogan et a1.,13~ for a blast furnace-type slag containing 43 % CaO, 35 % SiO2, 2 % S with the basicity (%GaO+%MgO)/%Si02=1.5. and Turkdogan15~ determined Recently, Ozturk the equilibrium distribution of sulfur between molten calcium aluminate and liquid iron containing aluminum and sulfur. Using their data for 1600 C, the equilibrium line is drawn in Fig. 5 for CaO- and CaS-saturated calcium aluminate, with due correction for f s = 6.6 and f A1= 3.0 in graphite-saturated melts. In this case the reaction considered is (CaO)+S+ 2 A1= (CaS)+ 1(A12Os)....(10) 3 3 As borne out by the study of reaction equilibrium and practical experience in the plants, the liquid steel containing 0.02 to 0.05 % Al is effectively desulfurized by intimate mixing of the steel in the ladle with molten calcium aluminate. This method of ladle desulfurization is even more attractive for the treatment of hot metal for two reasons ; (1) a marked increase in the activity coefficients of sulfur and aluminum by carbon in iron and (2) the equilibrium sulfur content controlled by reaction (10) is lower at lower hot metal temperatures compared to the liquid steel temperature. The equilibrium relations for reactions (8) and (10) in melts saturated with graphite and calcium sulfide are compared in Fig. 5 with the equilibrium relation for reaction (7) at 1 bar pressure of CO as a function of the concentration of carbon in the metal. At graphite saturation and 1 bar CO, the equilibrium sulfur content of the metal for reaction (7) is about 4 ppm at C and about 8 ppm for 1400 C. Although such low levels of sulfur may not be achieved in the ladle desulfurization of hot metal by lime injection, in practice about 90 % of sulfur in hot metal is removed by treatment with a mixture of, for example, 90 % lime, 5 % fluorspar and 5 % coke. An extensive desulfurization is expected also by a combined treatment of hot metal with lime and aluminum, or better still, with lime-saturated calcium aluminate and aluminum. In fact, in many melt-shop practices the aluminum is added together with burnt lime in the ladle treatment of hot metal. As indicated by the equilibrium lines 2, 5, and 3, the silicon in the metal will aid desulfurization only when the silica content of the aluminate slag is low, say about 10 % Si02 for an aluminate slag saturated with lime. V. Conclusions In the present experiments using a CaO/CaS pellet floating on the surface of molten iron, an interesting and yet a puzzling phenomenon was observed. At high carbon contents and low oxygen potentials, hence at low sulfur contents, there was a coating of CaS at the metal-oxide/sulfide pellet interface. This side reaction in the crucible assembly caused cessation of sulfur transfer from metal to the oxide/sulfide pellet. Therefore, the equilibrium measurements could not be made at high concentrations of carbon Research Article

6 Transactions ISIr, Vol. 23, 1983 (1043) Fig. 5. Equilibrium sulfur content of liquid iron at GO), Si and Al (for graphite saturation) aluminosilicate melts saturated with CaS furnace slag with (GaO+MgO)JSi02= C as a function of the concentration for melts coexistent with (1) CaO-GaS, and 2CaO. SiO2, (4) saturated aluminate, containing 2 % S. of C (at (2) silicate and (5) 1 bar, (3) blast and low partial pressures of carbon monoxide. For the composition range to 0.4 % S, up to 1.5 % C for 0.10 bar CO and up to 3 % C for 0.75 bar CO, the following equilibrium constant is determined for the sulfur reaction at C and unit activities of CaO and CaS ~co =833 [as] [act where pco is in bar and the activities are defined such that as - % S and ac --~ % C when % C -> 0. The carbon in the metal has a dual role in desulfurization of iron by lime: (1) Increasing carbon content decreases the oxygen content of the metal, hence aids desulfurization, and (2) Increasing carbon content raises the activity coefficient of sulfur in the metal, hence lowers the concentration of sulfur. However, at atmospheric pressure of carbon monoxide, a low residual sulfur in iron by lime treatment can be achieved only at high concentrations of carbon, as in hot metal. Although of limited practical interest, the lowering of the partial pressure of carbon monoxide in the melt by inert gas purging during lime injection will aid desulfurization of high-carbon steel with lime. In the ladle desulfurization of hot metal with lime injection better results are expected (1) with the addition of aluminum to the metal and (2) by minimizing slag carryover into the ladle. REFERENCES 1) JANAF Thermochemical Tables, NSRDS-NBS 37, Bur. Standards, Washington D.C., (1971). 2) A. Rist and J. Chipman : Rev. Met., 53 (1956), ) J. Chipman: Met. Trans., 1 (1970), Nat. Research Article

7 (1044) Transactions ISIJ, Vol. 23, ) 5) 6) 7) 8) 9) J. P. Morris and R. C. Buehl: Trans. AIMS, 188 (1950), 317. S. Ban-ya and J. Chipman : Trans. Met. Soc. AIME, 245 (1969), 133. T. Rosenqvist : Trans. AIME, 191 (1951), 535. E. T. Turkdogan, B. B. Rice and J. V. Vinters : Met. Trans., 5 (1974), G. J.W. Kor and F. D. Richardson : Trans. Inst. Min. Met. Ser. C, 79 (1970), 147. T. P. Floridis and J. Chipman : Trans. Met. Soc. AIME, 212 (1958), ) E. S. Tankins, N. A. Gokcen and G. R. Belton: Trans. Met. Soc. AIME, 230 (1964), ) S. Ban-ya and J. Chipman : Trans. Met. Soc. AIME, 242 (1968), ) T. Fuwa and J. Chipman: Trans. Met. Soc. AIME, 218 (1960), ) E. T. Turkdogan, G. J.W. Kor and R. J. Fruehan: Ironmaking Steelmaking, 7 (1980), ) R. H. Rein and J. Chipman: Trans. Met. Soc. AIME, 233 (1965), ) B. Ozturk and E. T. Turkdogan: Submitted to Metal Sci.