Hot Metal Desulfurization by CaO SiO 2 CaF 2 Na 2 O Slag Saturated with MgO

Transcription

1 ISIJ International, Vol. 50 (00), No., pp. 5 Hot Metal Desulfurization by CaO SiO CaF Na O Slag Saturated with MgO Moon Kyung CHO, ) Jin CHENG, ) Joo Hyun PARK ) and Dong Joon MIN ) ) Department of Metallurgical Engineering, Yonsei University, Seoul 0-749, Korea. chemical@yonsei.ac.kr ) School of Materials Science and Engineering, Univeristy of Ulsan, Ulsan , Korea. basicity@mail.ulsan.ac.kr (Received on August, 009; accepted on October, 009) The effect of CaF and Na O on the sulfide capacity of the CaO SiO CaF MgO satd. ( Na O) slags for hot metal desulfurization was investigated at 63 and 73 K by taking the role of MgO as an important refractory component into account. The liquidus line of the MgO-saturated system was similar to the CaO SiO CaF ternary slag system. The silica content for the CaO SiO -saturation was slightly decreased with MgO saturation. Although the sulfide capacity at MgO-saturation is lower than that of the CaO satd. SiO CaF slag system, it is greater than the ternary slag system saturated with CaO SiO phase. This suggests that MgO can increase the activity of free O ions under conditions of relatively low CaO activity. Near the composition of 3CaO SiO -saturation, MgO decreased the sulfide capacity of the slag because MgO is less basic than CaO in highly basic compositions. In addition, the addition of 5 mass% Na O significantly increases the sulfide capacity of the MgO-saturated slag especially in the composition of low CaO CaF. This suggests that the input amounts of CaO CaF can be reduced from 95 to 75 mass% for maintaining the sulfide capacity of 0.0 at 63 K, which is important considering the reduction of CaO and CaF consumption in hot metal desulfurization process with MgO saturation. KEY WORDS: CaF ; Na O; MgO; sulfide capacity; hot metal desulfurization; activity; free oxide.. Introduction The sulfide capacity for CaO-based slags is an effective evaluation concept of efficient and inexpensive processes to determine for the removal of sulfur from hot metal and has been extensively used. The sulfide capacity is well known to be a function of temperature and composition. In particular, extensive studies have been done on the effect of Na O and CaF on increasing sulfur removal for CaO-based slags. 8) However, the role of CaF in the sulfur removal reaction by CaO-based slags saturated with MgO has not been clearly understood. Only limited studies for CaObased slag systems without MgO have been done. Bronson and Pierre found CaF ( 0 mass%) additions to increase the sulfide capacity of the CaO SiO slag (basicity, B mass%cao/mass%sio.0.3) at 776 K and suggested the increase in the sulfide capacity was due to a depolymerization of silicate network by F ions, ) which increases the concentration of free O ions. Uo et al. ) directly measured the solubility of CaS in the CaO SiO CaF slag at 573 K, and concluded that slags with dual saturation of CaO and 3CaO SiO (C 3 S) phases had the highest CaS solubility and thus the highest sulfide capacity. However, in their work, the solubility of CaS decreased with increasing content of CaF at B., while it increases at B. with increasing CaF concentration. The substitution of CaO with CaF at constant SiO decreased the sulfide capacity and the substitution of SiO with CaF at constant CaO increased the sulfide capacity. 3,4) However, in the relatively lower basicity region, i.e. B.3, the sulfide capacity exhibited a maximum value at about 0 to 30 mass% CaF, and even at B.4, the CaF did not significantly affect the sulfide capacity. 3) However, the effect of CaF could not be clearly obtained from the results, Uo et al. ) and Ferguson and Pomgret. 3) Detailed work by Susaki et al. 5) showed the sulfide capacity of the CaO SiO CaF slag had increased along the liquidus saturated with CaO SiO (C S), 3CaO SiO (C 3 S), and CaO at temperatures from 473 to 63 K. The addition of CaF enhanced sulfur removal at condition of CaO saturation, and thus the sulfide capacity had the highest value at the composition of C 3 S and CaO dual saturation. Susaki et al. 5) proposed that the CaF could affect the stability, i.e. the activity coefficient, of S ions for highly basic slags. However, by combining this result with the CaS solubility measurement, Uo et al. ) proposed the iso-sulfide capacity contours of the CaO SiO CaF slag at 573 K, where the effect of CaF to 50 mass% at least on the sulfide capacity was negligible in the liquid slag area. The addition of 3 mass% MgO decreased the sulfide capacity, whereas the same amount of Na O linearly increased the sulfide capacity by a factor of 9 in the CaO satd. SiO CaF slag. 6) The addition of 3 mass% Na O could be a result from an increase in the solubility of CaO from 54 to 6 mass% for maintaining saturation at 473 K. A reciprocal action between Na O and CaF in view of sulfide capac ISIJ

2 ISIJ International, Vol. 50 (00), No. ity of CaO-based slags was reported by Niekerk and Dippenaar. 7) The addition of CaF to about 35 mass% decreased the sulfide capacity of the CaO SiO CaF Na O (mass%cao mass%na O/mass%SiO.5.9) slag at 573 K, indicating that CaF probably diluted the slag and consequently decreased the content of free O ions. These studies have focused on slags saturated with CaO, but work on CaF -containing CaO-based slags saturated with MgO has been comparatively limited. Recent studies by Choi et al. 8) on the sulfide capacity of the CaO SiO MgO CaF and CaO SiO MgO satd. CaF slags at 873 K showed additions of 0 mass% CaF increased the sulfide capacity, while higher contents of CaF had negligible effects. These experiments were carried out along the liquidus saturated with solid calcium silicates and CaO similar to Susaki et al. s and an increase in CaF content increased in the activity of CaO in the slag towards unity at about 0 mass% CaF. Therefore, in the present study, the effect of CaF and Na O on the sulfide capacity of CaO-based slags saturated with MgO for hot metal desulfurization was investigated by taking the role of MgO as an important refractory component into account.. Experimental A chemical equilibration technique was employed for the measurement of sulfur partition between the CaO SiO CaF MgO satd. ( Na O) slags and the molten Cu S alloy at 63 and 73 K. The sulfur potential (partial pressure) was calculated from the concentration of sulfur in Cu S alloy based on the reaction given in Eq. (). 9) S (g) [S] Cu,mass%, ΔG T (J/mol)...() Thus, using the slag gas reaction given in Eq. (), the sulfide capacity can be obtained from Eq. (3). C S S (g) (O ) (S ) O (g)...() K a ( ) O p (mass%s ) f p S O S...(3) where K () is the equilibrium constant of Eq. (), a O and f S are, respectively, the activity of free O and the activity coefficient of S ions in slag, and p i is the partial pressure of gaseous component i. In this study, the oxygen partial pressure was fixed by using a gas mixture of CO/CO ( 0.553/0.447) based on the following equation. 0) CO(g) O (g) CO (g), ΔG T (J/mol)...(4) Figure is a schematic diagram of the experimental apparatus used in the present study. A fused MgO crucible / Fig.. Schematic diagram of the experimental apparatus. contained in an alumina holder was placed in an electric resistance furnace with SiC heating elements and a mullite reaction tube. The temperature was controlled within K using an R-type (Pt 3mass%Rh/Pt) thermocouple and a proportional integral differential controller. 3 g of slag were equilibrated with 5 g of Cu S alloy in MgO crucible for 5 h and Na O content was changed from 0 to 5 mass%. The equilibration time was 5 h which was preliminarily determined. The concentration of Na O was analyzed after the experiment, and was confirmed to coincide with the initial content without significant reduction or volatilization as shown in Table and Table. Before placing the crucible into the hot zone of the furnace, ultra high purity Ar gas purified by passing through the magnesium turning at 73 K was flowed sufficiently to prevent the contamination of the sample from the atmosphere. And CO and CO gases were purified by passing calcium sulfate, soda lime and silica gel for removing impurities such as H O. After equilibrium was reached, the sample was quickly moved from the furnace and was quenched by Ar gas and water, and the metal and slag phase was separated. The surface of metal and slag was carefully cleaned using a grinding machine. The metal was sectioned into several pieces and slag was ground to 00 mm for chemical analysis. The sulfur content in metal and slag was analyzed by a LECO C/S combustion analyzer. The composition of slag was analyzed by X-ray fluorescence spectroscopy (XRF). 3. Results and Discussion 3.. Phase Relation of CaO SiO CaF MgO satd. ( Na O) Slags The liquid phase boundary of the present Pseudo-quaternary and quinary slag systems at 63 K is shown in Fig. and is compared to that of the CaO SiO CaF ternary system measured by Susaki et al. 5) Generally, the SiO content of slag for CaO SiO (C S)- and CaO-saturation slightly decreases with both MgO saturation and with 5 mass% Na O MgO saturation. However, the liquidus of the slag equilibrating the 3CaO SiO (C 3 S) phase is not significantly affected by MgO and Na O. Hence, in the present study, the effect of slag composition on the sulfide capacity of the 00 ISIJ 6

3 ISIJ International, Vol. 50 (00), No. Table. Experimental data for sulfide capacity of the CaO SiO CaF Na O MgO satd. slags at 63 K. Table. Experimental data for sulfide capacity of the CaO SiO CaF Na O MgO satd. slags at 73 K ISIJ

4 ISIJ International, Vol. 50 (00), No. Fig.. Liquid phase boundary of the CaO SiO CaF, 5) CaO SiO CaF MgO satd., and CaO SiO CaF 5mass% Na O MgO satd. systems at 63 K (P.S.; present study). CaO SiO CaF MgO satd. ( Na O) slag system will be discussed based on the phase relation which is similar to that of the CaO SiO CaF ternary system. Fig. 3. Sulfide capacity of the CaO SiO CaF MgO satd. slags as a function of CaO content at 63 and 73 K (C; CaO, S; SiO, F; CaF, and M; MgO. P.S.; present study). Fig. 4. Effect of basicity and MgO content on the sulfide capacity of the CaO SiO CaF Na O MgO satd. slags at 63 K. 3.. Effect of CaO, MgO and Basicity on Sulfide Capacity of CaO SiO CaF MgO satd. ( Na O) Slags The sulfide capacity of the CaO SiO CaF MgO satd. slag is shown in Fig. 3 as a function of CaO content at 63 and 73 K. The present result is also compared to that of the CaO SiO CaF slag at 63 K, which was measured by Susaki et al. 5) The relationship between the sulfide capacity and the concentration of CaO in the present study is very similar to Susaki et al., which can be expected from the phase relation shown in Fig.. As well known, there are three distinct regions in Susaki et al. s results, viz. CaO- (region I), C 3 S- (region II), and C S-saturation (region III) conditions. The sulfide capacity in region I is slightly lower than that of CaO-saturated ternary system, while region II is significantly lower (one-tenth) than that of C 3 S-saturated ternary system. Furthermore, the sulfide capacity in region III is greater than that of the C S-saturated ternary system. The lower sulfide capacities with additions of MgO in highly basic slags such as C 3 S- and CaO-saturated systems (a CaO 0.95 in the CaO SiO CaF system at 63 K) have already been discussed through discussions on the ionic bond character of MgO compared to CaO, i.e. less basic oxide. 6) However, in the relatively acidic region (III), that is low a CaO region where the a CaO in the CaO SiO CaF system saturated with C S at 63 K is between 0.03 to 0.3, 5) the role of MgO in providing free O ions into the slag (Eq. (4)) is noticeable. (MgO) (Mg ) (O )...(4) This finding is significant considering the compensation of the sulfide capacity in the C S-saturated system by maintaining the MgO saturated condition at moderately less amount of CaO. The effect of temperature on the sulfide capacity in region I is relatively larger than that in regions II and III. This indicates that the effect of CaF on the sulfur removal reaction, more rigorously, on the basicity or sulfide stability, increases by increasing the temperature at highly basic system possibly saturated with CaO and MgO. The effect of CaF and CaO on sulfide capacity will be discussed later in detail in Sec Figure 4 shows the relationship between sulfide capacity and MgO content in the CaO SiO CaF Na O MgO satd. slag at 63 K at different basicity regions. The sulfide capacity generally decreases with increasing amount of MgO irrespective of slag basicity and it seems to increase slightly with increasing basicity of slag at a constant MgO content with some scatters. However, it should be carefully considered that the present basicity index, i.e. B (Vee-ratio) is not practical at B.9 and B 5., because the concentration of CaO in the slag equilibrating solid phases such as MgO and/or CaO and calcium silicates decreases with increasing value of B in both of the regions. Hence, the effect of MgO and basicity on the sulfide capacity is meaningful at B.9 5. in the present study, which is given as follows. A reciprocal action between MgO and basicity can be analyzed by considering a relationship between MgO and ba- 00 ISIJ 8

5 ISIJ International, Vol. 50 (00), No. Fig. 5. Iso-sulfide capacity map at 63 K with basicity and MgO content as dependent variables in CaO SiO CaF Na O MgO satd. slags. sicity to maintain a constant sulfide capacity as shown in Fig. 5. 7) Taking the log C S.6(.5) contour at 63 K, as an example, an increase in the concentration of MgO from 4 to 8 mass% necessitates a simultaneous increase in basicity from about to 5 to maintain a fixed sulfide capacity. More simply for example, at a fixed basicity of 3, the extrapolated solid line requires 5 mass% of MgO to achieve log C S.6(.5) and the extrapolated dashed line requires 7 mass% of MgO to achieve log C S 3.( 3.0). This particular example suggests that addition of MgO has a negative effect on the sulfide capacity above 4 mass% of MgO. It also indicated that MgO can have a positive effect below 4 mass% of MgO. Thus, the positive slope in this relationship means that MgO has a negative effect on the sulfide capacity, which is also applied to the log C S 3.( 3.0) contour. This indicates that MgO is a less basic oxide than CaO in the composition of B Effect of CaO and CaF on Sulfide Capacity of CaO SiO CaF MgO satd. ( Na O) Slags The sulfide capacity of the CaO SiO CaF MgO satd. slag is shown in Fig. 6 as a function of the summation of CaO and CaF ( C F ) to consider the effect of the total Ca cation at 63 and 73 K. The sulfide capacity of the CaO SiO CaF ternary system at 63 K is also shown for comparison. 5) The sulfide capacity for the present study in the CaO SiO CaF MgO satd. system is relatively unchanged at C F composition less than 75 mass%, and increases at C F greater than 75( 80) mass%. However, it is interesting that the sulfide capacity of the CaO SiO CaF MgO satd. system is significantly larger than that of the CaO SiO CaF slag saturated with C S phase at C F 75 mass%. This is likely a result of the increases in the activity of free O ions with the MgO saturation condition at relatively low CaO activity in the C S saturated region. 5) 3.4. Effect of Na O on Sulfide Capacity of CaO SiO CaF Na O MgO satd. Slags The sulfide capacity of the CaO SiO CaF Na O Fig. 6. Effect of the summation of CaO and CaF contents on the sulfide capacity of the CaO SiO CaF MgO satd. slags at 63 K (C; CaO, S; SiO, F; CaF, and M; MgO. P.S.; present study). MgO satd. slags is shown in Fig. 7 as functions of CaO and Na O at 63 and 73 K. The sulfide capacity of the CaO SiO CaF ternary system at 63 K is also shown for comparison. 5) The effect of Na O on the sulfide capacity can be explained separately within region I to III. In region III, addition of mass% Na O has little effect, but addition of 5 mass% Na O significantly increases the sulfide capacity. In region II, the addition of mass% Na O increases the sulfide capacity by factor of 3. On the other hand, the Na O addition in region I is relatively ineffective. Additional study by Susaki et al. 6) at 473 K for the CaO satd. SiO CaF slag system observed increases in the sulfide capacity by a factor of 9 with 3 mass% Na O. It was assumed that the addition of Na O increased the solubility of CaO resulting in higher sulfide capacity and it may be implied from the increase in the solubility of CaO that the sulfide activity coefficient was greatly affected with additions of Na O in the results of Susaki et al. 6) The negligible effects in Region I with Na O addition for the present study is likely the result of increased MgO solubility and subsequent dilution of CaO. The effect of Na O and the summation of CaO and CaF ( C F ) on the sulfide capacity of the CaO SiO CaF Na O MgO satd. slags is shown in Fig. 8. It is suggested that the Na O increases the sulfide capacity of the MgO-saturated slag especially at the low C F region. Also, Na O seems to affect not only the activity of free O ions, but also the gradient of a O according to C F at 63 K at C F 70( 75) mass%. However, the latter phenomenon is not observed at 73 K, where the sulfide capacity is relatively unaffected by C F 75( 80) mass%. Because the activity coefficient of S ions would not significantly be affected by MgO and Na O in the present slag system due to the fact that the heat of formation of CaS (DH f,ca S 478 kj/mol) is larger than that of MgS (DH f,mgs 353 kj/mol) and Na S (DH f,na S 378 kj/mol), ) this complex effects are mainly related to the effect of Na O, MgO and temperature on the activity of CaO (a CaO ) in this region assuming that the a O is strongly dependent on a CaO. This should be further studied in future works. Consequently, the 9 00 ISIJ

6 ISIJ International, Vol. 50 (00), No. Fig. 7. Sulfide capacity of the CaO SiO CaF Na O MgO satd. slags as a function of CaO content at (a) 63 K and (b) 73 K (C; CaO, S; SiO, F; CaF, and M; MgO. P.S.; present study). Fig. 8. about 75 mass%. Effect of Na O and the summation of CaO and CaF contents on the sulfide capacity of the CaO SiO CaF Na O MgO satd. slags at (a) 63 K and (b) 73 K (C; CaO, S; SiO, F; CaF, M; MgO. P.S.; present study). addition of 5 mass% Na O to the MgO-saturated slag at log C S.0 can reduce the amount of CaO and CaF from 95 to 75 at 63 K and from 90 to 85 mass% at 73 K in maintaining the high sulfide capacity. This is important in view of the slag volume reduction in hot metal treatment. The effect of CaO CaF contents ( C F ) and temperature on the solubility of MgO in the CaO SiO CaF Na O slag is shown in Fig. 9. The MgO solubility in the CaO SiO CaF ZrO and CaO SiO CaF Al O 3 TiO slags at 873 K is also shown for comparison.,3) At high temperatures such as 873 K, the MgO solubility continuously decreases as C F increases from 35 to 65 mass%. However, a plateau region in the composition of C F mass% at 63 and 73 K is also abserved. The solubility of MgO increases about ( 3) mass% by increasing temperature from 63 to 73 K. Therefore, MgO concentration should be maintained at about 5 and 8 mass% at 63 and 73 K, respectively, for stable operation to inhibit refractory corrosion by the slag containing C F 3.5. Correlation between Theoretical Optical Basicity and Sulfide Capacity of CaO SiO CaF Na O MgO satd. Slags In Fig. 0, the relationship between the sulfide capacity and the theoretical optical basicity of CaO-based slags containing CaF, MgO, and Na O is shown. 5 7) The theoretical optical basicity of the slags (L melt ) was calculated from Eq. (5).,4,5) Λmelt xnλ i i i xn...(5) where x i, n i, and L i are, respectively, the mole fraction, the number of oxygen or fluorine in each component, and the theoretical optical basicity of each oxide or fluoride i. The theoretical optical basicity of CaF (L CaF ) was taken to be about 0.67 from an average electron density. 6) In Fig. 0, the linear equation ((Eq. (6)) for estimating the sulfide capacity in various slags proposed by Sosinsky and Sommerville is also shown for comparison. 7) i i 00 ISIJ 0

7 ISIJ International, Vol. 50 (00), No. Na O and CaF in MgO saturation slags, it maybe possible to provide an environmental friendly hot metal desulfurization process with reduces CaO and CaF input. Fig. 9. Relationship between MgO content in the slags for the saturation and the summation of CaO and CaF contents (C; CaO, S; SiO, F; CaF. P.S.; present study). 4. Conclusions The effect of CaF and Na O on the sulfide capacity of CaO-based slags saturated with MgO for the hot metal desulfurization was investigated. The sulfide capacity was calculated from the equilibrium data between the CaO SiO CaF MgO satd. ( Na O) slags and the Cu S alloy at 63 and 73 K. The following tentative conclusions could be made. () The liquid phase boundary of the MgO-saturated system is very similar to that of the CaO SiO CaF ternary system. The silica content for the CaO SiO -saturation slightly decreased with MgO saturation. () Although the sulfide capacity of the MgO-saturated slag is lower than that of the CaO satd. SiO CaF system, it is greater than that of the ternary system saturated with CaO SiO phase. This suggests that MgO can increase the activity of free O ions under condition of relatively low CaO activity. (3) In the basicity (Vee-ratio) range from about to 5, i.e. mainly 3CaO SiO -saturation conditions, MgO decreases the sulfide capacity of the slag due to the dilution effect of MgO on CaO. (4) The addition of 5 mass% Na O significantly increases the sulfide capacity of the MgO-saturated slag especially in the composition of low CaO CaF region. This suggests that the input amounts of CaO CaF can be reduced from 95 to 75 mass% for maintaining C S 0.0 at 63 K resulting in the reduction of CaO and CaF consumption for hot metal desulfurization processes at MgO saturation. Fig. 0. Relationship between sulfide capacity and theoretical optical basicity of the various CaO-based slags containing CaF, Na O, and MgO (C; CaO, S; SiO, F; CaF, M; MgO, N; Na O. P.S.; present study) Λ log CS 43. 6Λ 5....(6) T The sulfide capacity of the various CaO-based slags containing CaF and Na O for hot metal desulfurization at temperatures from 573 to 73 K shows a positive deviation from that expected from Eq. (6). Furthermore, the sulfide capacity exhibits a large scattering at intential values of L melt. This inconsistency between measured sulfide capacity and that estimated from theoretical optical basicity of the highly basic slags containing CaF and Na O has already been reported by various authors.,8,9) The work by Carter and Macfarlane 0) was investigated in CaO SiO system and Sosinsky and Sommerville did not include Na O and CaF in the slag systems. 7) The highest value of sulfide capacity in the present CaO SiO CaF Na O MgO satd. slag is approximately 50 times larger than that expected from Eq. (6) at L melt 0.8. Considering the effect of REFERENCES ) A. Bronson and G. R. st. Pierre: Metall. Trans. B, 0B (979), 375. ) M. Uo, E. Sakurai, F. Tsukihashi and N. Sano: Steel Res., 60 (989), ) G. Ferguson and R. J. Pomfret: Proc. 3rd Int. Conf. on Molten Slags and Fluxes, Inst. Met., London, (988), 33. 4) S. R. Simeonov, I. N. Ivanchev and A. V. Hainnadjiev: ISIJ Int., 3 (99), ) K. Susaki, M. Maeda and N. Sano: Metall. Trans. B, B (990),. 6) K. Susaki, M. Maeda and N. Sano: Metall. Trans. B, B (990), 08. 7) W. H. van Niekerk and R. J. Dippenaar: ISIJ Int., 33 (993), 59. 8) C. H. Choi, S. K. Jo, S. H. Kim, K. R. Lee and J. T. Kim: Metall. Mater. Trans. B, 35B (004), 5. 9) G. K. Sigworth and J. F. Elliot: Can. Metall. Q., 3 (974), ) E. T. Turkdogan: Physical Chemistry of High Temperature Technology, Academic Press, New York, NY, (980),. ) O. Kubaschewski and C. B. Alcock: Metallurgical Thermochemistry, 5th ed., Pergamon Press, Oxford, (979), 58. ) J. H. Park and D. J. Min: Mater. Trans., 47 (006), ) J. H. Park, S. B. Lee, D. S. Kim and J. J. Pak: ISIJ Int., 49 (009), ) J. A. Duffy: J. Non-Cryst. Solids, 09 (989), 35. 5) K. C. Mills: ISIJ Int., 33 (993), 48. 6) T. Nakamura, T. Yokoyama and J. M. Toguri: ISIJ Int., 33 (993), 04. 7) D. J. Sosinsky and I. D. Sommerville: Metall. Trans. B, 7B (986), 33. 8) D. R. Gaskell: Metall. Trans. B, 0B (989), 3. 9) K. Kunisada and H. Iwai: ISIJ Int., 33 (993), 43. 0) P. T. Carter and T. G. Macfarlane: J. Iron Steel Inst., 85 (957), ISIJ