Liquidus Surface of FeO-Fe 2 O 3 -SiO 2 -CaO Slags at Constant CO 2 /CO Ratios

Transcription

1 Materials Transactions, Vol. 44, No. 10 (2003) pp to 2135 #2003 The Japan Institute of Metals Liquidus Surface of FeO-Fe 2 O 3 -SiO 2 -CaO Slags at Constant CO 2 /CO Ratios Florian Kongoli 1 * and Akira Yazawa 2 1 FLOGEN Technologies Inc., Decelles Ave., Suite 511, Montreal, Quebec, H3S 2C3, Canada 2 Tohoku University, Sendai Japan Liquidus surface of FeO-Fe 2 O 3 -SiO 2 -CaO slags is an important parameter in various smelting and converting processes. It helps not only to optimize the slag chemistry of current processes and their fluxing strategies but also to determine the availability of new slags for more advanced technologies. In our previous publications, the liquidus surface of some multicomponent iron oxide slags has been quantified at several constant oxygen potentials and the effect of the latter, ignored until that moment, was quantified along with the effect of some minor components. In this work, the liquidus surface of some iron oxide slags is quantified at constant CO 2 /CO ratios. This is a new convenient way for the quantitative description of the slag liquidus surface and the effect of several fluxes, especially in those processes, such as slag solidification, where oxygen potential changes continuously. This type of diagram also describes more dynamically the effect of oxygen potential, clarifies the relation between CO 2 /CO ratio and oxygen potential in terms of the liquidus surface (not widely understood by metallurgists today) and reduces the gap between laboratory work and industrial experience. (Received June 30, 2003; Accepted August 14, 2003) Keywords: liquidus, FeOx-SiO 2 based slag, iron oxides, oxygen potential, CO 2 /CO, slag solidification, smelting, converting 1. Introduction Iron oxide slags are the most commonly used slags in sulfide smelting and steel making. They usually contain silica and lime as well as other minor oxides, which are introduced through raw materials, fluxes, dissolved refractories etc. Liquidus surface of these slags constitutes an important parameter for the sulfide smelting and converting processes. It helps not only to optimize the slag chemistry of the current processes and the fluxing strategies, but also to determine the availability of new slags for more advanced technologies. In our previous work the liquidus surface of some iron oxide slags has been quantified at low oxygen potentials, characteristic of reductive processes 1 3) and at intermediate oxygen potentials, characteristic of oxidative processes 4 7) such as direct smelting and continuous converting. This was carried out by the means of a new type of multicomponent phase diagrams 1) at constant oxygen potentials and deducted from the use of a new thermophysicochemical model. Through a series of these diagrams, the important effect of oxygen potential on the liquidus surface of multicomponent slags, ignored until that moment, was quantified along with the effect of some minor components. Considerable confusion found in literature about the effect of some minor components was also clarified. Among others, it was found that this effect could be fundamentally different in reductive and oxidative conditions. However, confusion still exists, especially for those particular processes in which oxygen potential changes dynamically mainly as a result of the continuous cooling of the slag and the use of coke in the process. An example of these processes is the settling phase of matte smelting which is a subsequent sub-process of matte oxidative smelting and/or slag solidification in which temperature drops continuously from around 1573 to 1423 K and sometimes coke breeze is used in the last stage. *Corresponding author, FKongoli@FLOGEN.COM In these processes, as well as in some others, contradictions are often found between the microscopic results of the laboratory quenching measurements and slowly cooled solidified smelting slags from the industrial practice. Contradictory assertions are also given about the effect of minor components in these processes. It seems that the sensitivity of the slag liquidus temperature toward changes of the oxygen potentials has been ignored and this is believed to be the reason of the above confusion. Following a previous proposal, 8) the purpose of this work is to quantitatively describe the effect of the dynamic changes of oxygen potential at these particular processes through a new type of phase diagrams at constant CO 2 /CO ratios, based on the previous model. This will not only help clarify the above confusion but will also shed light in the understanding of the slag solidification process and the solidified slag mineralogy, which are recently becoming important in the environmental point of view. The partial pressures of oxygen throughout the article are given as dimensionless ones defined by p O2 = (P O2 )/ ( Pa). 2. Variation of PO 2 and CO 2 /CO during Slow Cooling As stated in previous work 4 7) oxygen potentials during matte smelting and blister making converting at 1573 K can be respectively approximated as 10 8 and This is illustrated in Fig. 1, which gives the calculated equilibrium oxygen and sulfur pressures during oxidative copper smelting. However, the oxygen potentials of the slag might drop below 10 9 during matte smelting near the solidification temperature or in the reductive slag-cleaning furnace. The gradual cooling at a lower temperature and the use of coke breeze during settling change continuously the oxygen potential. Based on the previous article 8) the variations of p O2 and CO 2 /CO during cooling are given below in Figs. 2 to 4.

2 Liquidus Surface of FeO-Fe 2 O 3 -SiO 2 -CaO Slags at Constant CO 2 /CO Ratios 2131 Fig. 1 Variations of equilibrium oxygen and sulfur pressures during oxidative matte smelting and converting moles CuFeS 2 concentrate is assumed to be oxidized with air or oxygen enriched air at K and K. ( : 1573 K Air blow; K, 40%O 2 blow; 1473 K, Air blow). Fig. 3 Effect of temperature on the activity of FeO(l) coexisting with Fe 3 O 4 (s) at fixed P O2 (dashed curves) according to 3FeO(l)+O 2 (g) = Fe 3 O 4 (s) and at fixed CO 2 /CO (solid curves) according to 3FeO(l)+CO 2 (g) = Fe 3 O 4 (s)+co(g). Fig. 2 Relation between P O2 and CO 2 /CO ratio according to 2CO(g)+O 2 (g) = 2CO 2 (g) at constant CO 2 /CO (dashed lines) and constant P O2 (solid lines). Fig. 4 Variation of p O2 and CO 2 /CO ratio with temperature according to 3FeO(l)+O 2 (g) = Fe 3 O 4 (s) and 3FeO(l)+CO 2 (g) = Fe 3 O 4 (s)+ CO(g) at a constant afeo(l) and during the equilibrium cooling of a real slag X of composition 46.3 mass% FeO, 6.7 mass% Fe 2 O 3, 37 mass% SiO 2 and 10 mass% CaO from 1623 to 1373 K. Figure 2 describes the variation of p O2 with temperature when CO 2 /CO is kept constant and the variation of CO 2 /CO with temperature when p O2 is kept constant. It can be seen that a constant CO 2 /CO ratio describes more concisely the continuous change of oxygen potential during equilibrium cooling since it includes in itself many variations of p O2 depending on temperature. Figure 3 describes the variation of the activity of FeO(l) with temperature at constant p O2 and CO 2 /CO. It can be seen that the effect of temperature on the activity of FeO(l) is much less pronounced at constant CO 2 /CO ratio compared to constant p O2 ratio. Figure 4 describes the variation of p O2 and CO 2 /CO ratio according to two separate equilibrium reactions and during the equilibrium cooling of a slag X of composition 46.3 mass% FeO, 6.7 mass% Fe 2 O 3, 37 mass% SiO 2 and 10 mass% CaO from 1623 K to 1373 K. It is shown that while oxygen potential changes considerably during cooling from around 10 7 to 10 10, the resulting CO 2 /CO ratio is almost constant. Taking into account this fact, the phase diagrams at constant CO 2 /CO ratio seem to be an interesting new alternative in the quantification of the liquidus surface of a multicomponent slag at those processes such the continuous cooling and solidification where the oxygen potentials change dynamically. Some examples of this new type of diagrams at constant CO 2 /CO are given below. They have been constructed by FLOGEN TM software 9) through a new thermophysicochemical model, 4) which was already verified against all available experimental data on liquidus temperatures as well as other thermodynamic properties at several oxygen potentials.

3 2132 F. Kongoli and A. Yazawa Fig. 5 Liquidus surface of FeO-Fe 2 O 3 -SiO 2 -CaO slag at log(co 2 / CO)= 2. Fig. 6 Liquidus surface of FeO-Fe 2 O 3 -SiO 2 -CaO slag at log(co 2 / CO)= Polythermal Projection Diagrams at Constant CO 2 / CO Ratio Figure 5 presents the liquidus surface of FeO-Fe 2 O 3 -SiO 2 - CaO slag at a constant ratio of log(co 2 /CO) = 2 by the means of a new format of multicomponent phase diagrams whose basis has been previously described. 1) This is in fact the slag liquidus surface during slow equilibrium cooling from 1623 to 1373 K where the oxygen potentials change continuously from about 10 5 to around It can be seen that at these conditions several primary phases are present, i.e., magnetite (spinel), alpha-ca 2 SiO 4,Ca 3 Si 2 O 7, wollastonite/pseudo-wollastonite and silica and each of them has its own characteristics. Some important points can be easily made from this diagram. First, during continuous cooling of these slags, the magnetite spinel phase is the dominant primary precipitate phase. There is no stable olivine or fayalite at these conditions, which suggest that the name fayalite slag is not adequate at this particular case. A meaningful name would be magnetite or spinel slag if the primary precipitate phase is to be used to name the slag. At these conditions the mineralogy of a slow cooled solidified slag with an overall liquid composition of Fe/ SiO 2 =1.1 and CaO=10 mass% (point X in the diagram) would mostly contain primary magnetite. Second, lime does not decrease, but instead, increases the liquidus temperature of the slag at spinel saturation area and consequently increases the risk of the magnetite (spinel) precipitation which makes lime not a good flux in terms of the liquidus temperature. This effect is much more pronounced in this kind of diagrams compared to the diagrams at constant p O2 presented previously 4) and this reflects the sensitivity of the liquidus temperature toward changes on the oxygen potentials. It should be mentioned however that lime decreases the liquidus temperature only in the region of newly proposed FCS slag 5,6) where it may be used as a good flux. Third, at constant CaO an increase of Fe/SiO 2 ratio in the magnetite saturation area would increase the risk of magnetite precipitation. This effect is more pronounced in this diagram compared to the one given at constant p O2, which again reflects the sensitivity of the liquidus temperature toward changes on the oxygen potentials. Figure 6 presents the liquidus surface of the same slag at log(co 2 /CO)=1. Again this represents the liquidus surface of this slag during continuous cooling from 1623 to 1373 K where the oxygen potentials change continuously from about 10 7 to around It can be seen that besides the 5 primary phases mentioned above, two new stable phases are present at these particular conditions i.e. olivine and wustite. Contrary to the previous case, lime in small and limited amounts decreases the slag liquidus temperature in the olivine saturation area but increases it in the spinel(magnetite), wustite, and wollastonite saturation areas. At these conditions the mineralogy of a slow cooled solidified slag with an overall liquid composition of Fe/SiO 2 =1.1 and CaO=10 mass% (point X in the diagram) would mostly contain primary olivine. Figure 7 gives the liquidus temperature of the slag at log(co 2 /CO)=0.3 which corresponds to the continuous equilibrium cooling of the slag from 1623 to 1373 K where the oxygen potentials drop continuously from about 10 8 to around In this case spinel is not anymore a primary Fig. 7 Liquidus surface of FeO-Fe 2 O 3 -SiO 2 -CaO slag at log(co 2 / CO)= 0.3.

4 Liquidus Surface of FeO-Fe 2 O 3 -SiO 2 -CaO Slags at Constant CO 2 /CO Ratios 2133 Table 1 The relationship between logðp O2 Þ and log(co 2 /CO) ratios at various temperatures for the reaction 2CO 2 (g) = 2CO(g)+O 2 (g) T/K DeltaH DeltaS DeltaG K Log (K) Log(P O2 ) (kj) (J/K) (kj) Log(CO 2 /CO) Fig. 8 Liquidus regions of FeO-Fe 2 O 3 -SiO 2 -CaO slag at 1448 K and at various constant values of CO 2 /CO and oxygen potentials E-13 12:44 8:44 10:44 11: E-13 12:06 8:06 10:06 11: E-12 11:69 7:69 9:69 11: E-12 11:34 7:34 9:34 10: E-11 10:99 6:99 8:99 10: E-11 10:66 6:66 8:66 10: E-11 10:34 6:34 8:34 9: E-11 10:03 6:03 8:03 9: E-10 9:73 5:73 7:73 9: E-10 9:44 5:44 7:44 8: E-10 9:16 5:16 7:16 8:56 stable phase, but all other primary phases are present i.e. olivine, wustite, alpha-ca 2 SiO 4, Ca 3 Si 2 O 7, wollastonite/ pseudo-wollastonite and silica. It can be seen that lime decreases the liquidus temperature of these slags in the olivine saturation area but increases it in almost all other areas. At these conditions the mineralogy of a slow cooled solidified slag with an overall liquid composition of Fe/ SiO 2 =1.1 and CaO=10 mass% (point X in the diagram) would mostly contain primary olivine. 4. Isothermal Diagrams at Constant CO 2 /CO Ratio Figure 8 describes the effect of CO 2 /CO ratio on the liquid regions of the current slag at 1448 K. The liquid regions for constant p O2 have also been given for comparison. It can be seen that, as expected, decreasing the CO 2 /CO ratio or the p O2 increases the liquid region at this particular temperature especially in the olivine, wustite and spinel surface. The slag X with an overall liquid composition of Fe/SiO 2 =1.1 and CaO=10 mass% would be completely liquid only at log(co 2 /CO) of 1 and 0.3 as well as at p O2 of Fig. 9 Liquidus regions of FeO-Fe 2 O 3 -SiO 2 -CaO slag at 1498 K and at various constant values of CO 2 /CO and oxygen potentials. Figure 9 also describes the effect of CO 2 /CO ratio on the liquid regions of the current slag at 1498 K. The liquid regions for constant p O2 have also been given for comparison. In this case also, a decrease of CO 2 /CO ratio or p O2 increases the liquid region. The slag X with an overall liquid composition of Fe/SiO 2 =1.1 and CaO=10 mass% would be completely liquid only at log(co 2 /CO) of 1 and 0.3 as well as at p O2 of 10 8 and It should also noted that the liquidus curve at log(co 2 /CO)=0.3 coincides with the one at P O2 ¼ since at this temperature these values correspond to each other. This can be easily understood from Table 1 that gives the relationship between the log(p O2 ) values at several constant log(co 2 /CO) constant ratios at various temperatures for the reaction 2CO 2 (g) = 2CO(g)+O 2 (g). In our particular case at log(co 2 /CO) of 0.3 the corresponding value of oxygen potential is log(p O2 )of 10:06. This explains the overlapping of liquidus curves at both above-mentioned conditions. 5. Quenching and Slow Cooling As mentioned above, contradictions are often found among the microscopic results of laboratory quenching measurements and slowly cooled solidified smelting slags from industrial practice. For instance, in matte smelting, quenching experimental measurements show that magnetite is normally the primary precipitate solid phase of the quenched samples within the glass phase (the finely crystalline structure, representing frozen liquid). However, microscopic examinations of relatively big amounts of solidified slag from this process, especially from settling stage, show olivine as the dominant crystallized phase. The fluxing effect of lime in both cases is also a subject of confusion. These disagreements can now be explained and clarified in the light of the present work. Figure 10 gives the liquidus temperature of the FeO- Fe 2 O 3 -SiO 2 -CaO slag at Fe/SiO 2 =1.1 and at different constant values of CO 2 /CO and p O2 as well as at iron saturation. If the above-mentioned slag X (Fe/SiO 2 =1.1, CaO=10 mass%) is kept at constant oxygen potentials, as it is almost the case of matte smelting and continuous converting

5 2134 F. Kongoli and A. Yazawa metallic iron and in air where the oxygen potential does not change (in air) or change only slightly (in equilibrium with metallic iron). This is clearly reflected on the difference that exists between the diagrams at constant oxygen potentials 4) and those at constant CO 2 /CO, as given in this work. 6. Conclusions Fig. 10 Liquidus temperature of the FeO-Fe 2 O 3 -SiO 2 -CaO slag at Fe/ SiO 2 =1.1 and at different constant values of CO 2 /CO and p O2 and at iron saturation. (10 8 or 10 6 ) the primary precipitate phase is magnetite (spinel) within the glassy phase representing liquid. If the slag X is equilibrated at 1433 K and p O2 ¼ 10 8 and then quenched, the microscopic examination will reveal only magnetite (spinel) as a precipitate phase besides the glass. In this case lime is not a good flux in terms of liquidus temperature since it increases it and consequently increases the risk of magnetite precipitation. If the same slag X is slow cooled, as it is the case of certain relatively big amount of slags in the industrial practice or in the settling phase of matte smelting, the primary precipitate phase would now be olivine. As it can be seen in the upper part of Fig. 4, during the equilibrium cooling of the slag X, log(co 2 /CO) stays almost constant around the value of 1 and at these conditions the primary precipitate phase is olivine, as shown in Fig. 10. Although the slow cooling of the industrial slag and it solidification is not a truly equilibrium process the diagrams at constant CO 2 /CO ratio are the best approximation of these processes. The microscopic examination of many solidified slags confirms this conclusion since it reveals that many of these slags consist of mainly olivine. In this case lime in limited amounts would be a good flux in terms of the liquidus temperature, especially in the settling phase of matte smelting where the temperature may reach 1473 K and sometimes coke breeze is used in the process. In Fig. 10 it is also worth noting that at iron saturation the primary precipitate phase is still olivine although at this particular conditions oxygen potential stays almost constant around the value of or Quenching of the slag X in iron crucible from a holding temperature of 1360 K would produce only olivine as primary precipitate crystals within the field of frozen liquid. In this light, it can be said that there is a fundamental difference between quenching and slow equilibrium cooling of an iron oxide slag at intermediate oxygen potentials. The difference could be in the primary precipitate phases, on the effect of minor components, in the mineralogical composition of the solidified slag, etc. This difference has not been always understood and one of the reasons for that is that the experiments have always been carried out in equilibrium with The liquidus surface of FeO-Fe 2 O 3 -SiO 2 -CaO slag was quantified through a new type of phase diagrams at constant CO 2 /CO ratios. It was shown that this is a convenient quantitative way for the description of the slag liquidus surface and the effect of minor components in those processes, such as slag solidification, where the oxygen potential changes continuously. It describes more dynamically the effect of oxygen potential and clarifies the relation between CO 2 /CO ratio and oxygen potential in terms of the liquidus surface. The analysis of the variation of p O2 and CO 2 /CO showed that during slow equilibrium cooling although the oxygen potential changes continuously, CO 2 / CO ratio stays almost constant. Consequently, this new type of diagrams at constant CO 2 /CO ratio also simulates the slow cooling process of the industrial slag. The effect of CO 2 /CO ratios and oxygen potentials on the liquidus temperature were also quantified and the contradictions often found between the microscopic results of laboratory quenching measurements and slowly cooled solidified smelting slags from industrial practice were clarified. It was shown that there is a fundamental difference between the quenching and slow equilibrium cooling of an iron oxide slag at intermediate oxygen potentials in terms of the primary precipitate phases, the effect of minor components, the mineralogical composition of the solidified slag, etc. The effect of lime on the liquidus temperature was also quantified at both processes for FeO-Fe 2 O 3 -SiO 2 -CaO system. However, the presence of other minor components may alter this effect. The constructed diagrams shed light in the understanding of the slag solidification process and the solidified slag mineralogy, which are recently becoming important in the environmental point of view. The diagrams at constant CO 2 /CO and p O2 are complements of each other and both help in the quantification of the liquidus surface of slags in several metallurgical processes. Acknowledgment The authors wish to thank Mitsubishi Materials Corporation and Sumitomo Metal Mining Co. Ltd. for the financial support. REFERENCES 1) F. Kongoli and I. McBow: EPD Congress 2000, ed. by P. Taylor, (The Minerals, Metals and Materials Society, Warrendale, PA, 2000) pp ) F. Kongoli and I. McBow: Copper 99, Vol. VI: Smelting, Technology Development, Process Modeling and Fundamentals. ed. by C. Diaz, C. Landolt and T. Utigard, (The Minerals, Metals and Materials Society, Warrendale, PA, USA, 1999) pp ) F. Kongoli, M. Kozlowski, R. A. Berryman and N. M. Stubina: James M. Toguri Symposium on the Fundamentals of Metallurgical Processing,

6 Liquidus Surface of FeO-Fe 2 O 3 -SiO 2 -CaO Slags at Constant CO 2 /CO Ratios 2135 ed. by G. Kaiura, C. Pickles, T. Utigard and A. Vahed, (The Canadian Institute of Mining, Metallurgy and Petroleum, Ottawa, 2000) pp ) F. Kongoli and A. Yazawa: Metall. Mater. Trans. B 32 (2001) ) A. Yazawa and F. Kongoli: High Temperature Materials and Processes 20 (2001) ) F. Kongoli and A. Yazawa: James M. Toguri Symposium on the Fundamentals of Metallurgical Processing, ed. by G. Kaiura, C. Pickles, T. Utigard and A. Vahed, (The Canadian Institute of Mining, Metallurgy and Petroleum, Ottawa, 2000) pp ) A. Yazawa and F. Kongoli: Proceedings of the Sixth International Conference on Molten Slags, Fluxes and Salts, Stockholm, Sweden - Helsinki, Finland, 2000, ed. by S. Seetharaman and D. Sichen, (Trita Met 85, 2000), CD-Rom, 016.pdf. 8) A. Yazawa: Tetsu-to-Hagané 86 (2000), ) F. Kongoli, I. McBow and S. Llubani: FLOGEN Technologies Inc. (