2020 Volume 60 Issue 7 Pages 1434-1437
Generally, reactions and forming phases during ironmaking can be thermodynamically predicted using equilibrium phase diagram. However, at low temperature it will likely to be different from predicted phases and deviate from equilibrium. Hence, knowledge of solid state reaction at low temperature is required to control the melting behavior of slag phase in blast furnace. Formation of 2CaO·SiO2 by the reaction between gangue SiO2 and liquid CaO–FeO phase will give negative effect to molten slag formation during ironmaking process, and enhancing the dissolution of SiO2 into CaO–FeO liquid phase is crucial.
It was found that 2CaO·SiO2 phase layer formed at the interface between SiO2 and CaO–FeO melt by rapidly heating the sample to 1423 K. Dissolution of SiO2 into CaO–FeO melt was enhanced by Al2O3 addition to the CaO–FeO melt. When the sample was rapidly heated to 1473 K, formation of 2CaO·SiO2 was not observed and the dissolution of SiO2 into CaO–FeO liquid phase was significant. Rapid heating to 1473 K will avoid formation of 2CaO·SiO2 phase and enhance melting of gangue minerals to form liquid slag.
Suppression of CO2 discharged from iron- and steel-making companies is an example of the biggest issues for the protection of global environment and sustainable growth of steelmaking industry. One of the ideas to decrease CO2 emission and energy consumption from ironmaking process is to decrease the average process temperature during ironmaking. If it is possible to produce iron at lower temperature, reaction during the process may differ from the present process. Generally, reactions and forming phases during ironmaking can be thermodynamically predicted using equilibrium phase diagram. However, at low temperature the phases will likely to be different from predicted phase and deviate from equilibrium. Raw materials charged into blast furnace includes various minerals or crystal phases. Behavior of slag formation can’t be estimated without considering the initial form, type of crystal phase and reaction between solid minerals and solid iron oxides at low temperature.
Phenomena in sinters during heating have been reported by several researchers. Matsuno1) heated Fe2O3–CaO–SiO2 mixture and concluded that phases formed in the sinter is due to the reaction between calcium ferrite liquid and SiO2. Sasaki et al.2) reported that calcium ferrite liquid is formed at temperature over 1473 K and gangue minerals dissolve into liquid phase. However, Hotta et al.3) have reported that by heating sinter and pellet having high basicity, liquid phase having composition near Olivine was found. Also, effect of gangue during heating sinters and pellets are reported.4,5,6)
In our previous study,7) reactivity between oxides were investigated by heating mixed chemical reagents under Ar atmosphere at 1373 K to understand the solid state reaction blast furnace. It was reported that reactivity of CaO and FeO was high and formation of CaO–FeO liquid phase is fast and this liquid will react with other oxides. Also, it was confirmed that 2CaO·SiO2 formed at the interface of CaO–FeO liquid phase and SiO2 solid phase. Formation of 2CaO·SiO2 that has high melting point will give negative effect to molten slag formation during ironmaking process and enhancing the dissolution of SiO2 into CaO–FeO liquid phase is crucial. However, dissolution behavior of SiO2 dissolution into CaO–FeO liquid phase have not been investigated. Therefore, dissolution behavior of SiO2 into molten CaO–FeO phase was investigated in the present work. Also, the effect of Al2O3 addition and temperature were examined.
Samples were prepared by the following procedures. FeO was made by heating reagent grade Fe2O3 at 1200 K for 2 hours under CO2-50COvol% gas. CaO was made by heating reagent grade CaCO3 at 1100 K for 4 hours under air. Also, reagent grade Al2O3 was used for experiments. Oxides were mixed and weighed 0.1 g was pressed into φ5 mm tablet shape. It was found from the previous work7) that CaO–FeO liquid phase rapidly forms when mixed CaO and FeO is heated to 1373 K. Hence, ratio of (mass%FeO)/(mass% CaO) was chosen as 3 for all samples, which is the ratio that liquid phase forms at low temperature according to the CaO–FeO binary phase diagram.8) To see the effect of Al2O3, (mass%Al2O3) in the samples were, 0, 2, 3.5, 5, 7.5, 10 and 12 mass%Al2O3, respectively. Oxide tablet was placed in a SiO2 crucible (O.D. 7 mm, I.D. 5 mm, Height 5.5 mm) on Al2O3 boat and the sample was placed in a horizontal alumina tube(O.D. 25 mm, I.D. 20 mm, L. 600 mm)) inside an electric resistance furnace. The initial thickness of the SiO2 crucible was carefully measured before experiments. The sample temperature was pushed into the hot zone heated at 1423 or 1473 K under Ar flow of 0.05 L/min. The purity of Ar was 99.9999% and was dehydrated by molecular sieve during experiment. The sample temperature was monitored by thermocouple near the sample. Initial heating rate of the sample was 550 K/min and 390 s was required to heat the sample to the desired temperature. The sample was held for 0, 5, 8, 10 min and quenched by pulling out the sample to the cold zone of the alumina tube. The cooling rate of the sample was 400 K/min.
The sample after experiment was analyzed by using SEM-EDS (Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy) coupled with an image analysis technique.
Figure 1 shows the SEM image of experiments heated to 1423 K and held for 0 min and quenched. It can be observed that CaO–FeO phase was melted in SiO2 crucible. Thickness of the SiO2 crucible was observed by SEM and the decrease of SiO2 crucible during experiment was obtained. The crucible thickness was determined by measuring 10 places of the crucible randomly.
SEM image of experiments heated to 1423 K and held for 0 min and quenched. (Online version in color.)
Figure 2 shows the time dependence of SiO2 crucible thickness change for the samples without Al2O3 addition heated to 1423 K. Thickness of SiO2 decreases with time due to SiO2 dissolution into CaO–FeO liquid phase. Error bar shows the range of thickness decrease. Figure 3 shows the effect of Al2O3 content on SiO2 dissolution into CaO–FeO(–Al2O3) liquid phase. Samples were heated to 1423 K and held for 5 mins. It can be seen that addition of Al2O3 in CaO–FeO liquid phase will enhance the dissolution of SiO2 into CaO–FeO(–Al2O3) liquid phase. Dissolution behavior seem to change around 6 mass% Al2O3 and this will be explained afterwards.
Time dependence of SiO2 crucible thickness change for the samples without Al2O3 addition heated to 1423 K. (Online version in color.)
Effect of Al2O3 content on SiO2 dissolution into CaO–FeO(–Al2O3) liquid phase. (Online version in color.)
Figure 4 shows the SEM image of the samples heated to 1423 K and kept for 5 mins. Layer was observed at the interface for the samples with low Al2O3 content. This layer phase was confirmed to be 2CaO·SiO2 phase by EDS analysis. Secondary phase precipitated during cooling period is observed the melt phase. Estimated schematic phase diagram of CaO–FeO–SiO2–Al2O3 system based on reported phase diagram8,9) is shown in Fig. 5(a). There are 2 liquid regions at 1473 K and the primary phase field of 2CaO·SiO2 lies between them. Figure 5(b) shows the diagram when the ratio of (mass%FeO)/(mass%CaO) is 3. When CaO–FeO liquid is contacted with SiO2, 2CaO·SiO2 will form. Also, by addition of Al2O3 into CaO–FeO will hinder 2CaO·SiO2 formation. This agrees with the present results. Presence of Al2O3 will enhance SiO2 dissolution into CaO–FeO melt as shown in Fig. 3.
SEM image of the samples heated to 1423 K and kept for 5 mins.
Estimated schematic phase diagram of CaO–FeO–SiO2–Al2O3 system. (Online version in color.)
Estimated schematic phase diagram of CaO–FeO–SiO2–Al2O3 system when the ratio of (mass%FeO)/(mass%CaO) is 3. (Online version in color.)
Figure 6 shows the SEM image of the sample without Al2O3 addition heated to 1423 and 1473 K and held for 5 mins. It can be clearly observed that dissolution of SiO2 was significant for the sample heated to 1473 K. The compositions of the liquid phase determined by EDS are shown in CaO–FeO–SiO2 ternary phase diagram8) shown in Fig. 7. When the sample was heated to 1423 K, SiO2 concentration in the liquid was low and the composition was on the nose of the 2CaO·SiO2 primary region. However, when the sample was rapidly heated to 1473 K, SiO2 concentration in the liquid was high and the composition was close to SiO2 saturation. No trace of 2CaO·SiO2 phase was found at the interface and liquid phase was in direct contact with SiO2 crucible. This result can be explained as follows. When sample is heated, CaO–FeO liquid phase forms and have contact with SiO2 crucible. Then SiO2 dissolution into CaO–FeO liquid phase occurs. At 1423 K, dissolution rate of SiO2 is relatively slow and 2CaO·SiO2 forms at the interface preventing further dissolution of SiO2. At 1473 K, dissolution rate of SiO2 is fast and SiO2 concentration increased before 2CaO·SiO2 formations at the interface. Competition between 2CaO·SiO2 formation and dissolution rate of SiO2 at the interface occurred and the temperature that the mechanism changes was between 1423 and 1473 K in the present work. It was found that rapid heating to 1473 K will avoid formation of 2CaO·SiO2 phase and enhance melting of gangue minerals to form liquid slag.
SEM image of the sample without Al2O3 addition heated to 1423 and 1473 K and held for 5 mins.
Compositions of the liquid phase for samples without Al2O3 addition heated to 1423 and 1473 K and held for 5 mins. (Online version in color.)
Dissolution behavior of SiO2 into molten CaO–FeO phase was investigated in the present work. It was found that 2CaO·SiO2 phase formed at the interface between SiO2 and CaO–FeO melt by rapidly heating the sample to 1423 K. Dissolution of SiO2 into CaO–FeO melt was enhanced by Al2O3 addition to the CaO–FeO melt. When the sample was rapidly heated to 1473 K, formation of 2CaO·SiO2 was not observed and the dissolution of SiO2 into CaO–FeO liquid phase was significant. It was found that rapid heating to 1473 K will avoid formation of 2CaO·SiO2 phase and enhance melting of SiO2 based gangue minerals to form liquid slag.
