Phase Equilibria in the System CaO-SiO 2 -Al 2 O 3 -MgO-15 and 20 wt% FeO with CaO/SiO 2 Ratio of 1.3

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1 ISIJ International, Vol. 56 (2016), ISIJ International, No. 10 Vol. 56 (2016), No. 10, pp Phase Equilibria in the System CaO-SiO 2 -Al 2 O 3 -MgO-15 and 20 wt% FeO with CaO/SiO 2 Ratio of 1.3 Kyoung-oh JANG, 1) Xiaodong MA, 1) Jinming ZHU, 2) Haifa XU, 2) Geoff WANG 1) and Baojun ZHAO 1) * 1) The University of Queensland, St Lucia, QLD, 4072 Australia. 2) Baoshan Iron and Steel (Baosteel) Co. Ltd, Shanghai, China. (Received on June 2, 2016; accepted on June 21, 2016; J-STAGE Advance published date: August 30, 2016) The FeO -containing slags in the low part of blast furnace (BF) are of great importance to the operation of BF, particularly the primary and bosh slags. To optimise the slag components for the smooth operation of BF, the phase equilibria studies have been carried out in the system FeO -CaO-SiO 2 -Al 2 O 3 -MgO in equilibrium with metallic iron. High temperature equilibrations followed by quenching were conducted in experiments and electron probe X-ray microanalysis were employed to analyse the samples. For better interpretation and easy implementation of experimental results, the data obtained from measurements were symmetrically analysed and then plotted in the pseudo-ternary phase diagrams of (CaO+SiO 2 )-Al 2 O 3 - MgO with fixed CaO/SiO 2 weight ratio of 1.3 and FeO of 15 and 20 wt%, respectively. The primary phases such as melilite, Ca 2 SiO 4, merwinite, spinel and (Mg, Fe 2+ )O were observed with liquid phases and metallic iron in the composition range. The liquidus temperatures increase in melilite and spinel primary phase fields, but decrease in dicalcium silicate and merwinite primary phase fields with increasing Al 2 O 3 / (CaO+SiO 2 ) ratio. In addition, the liquidus temperatures firstly increase then decrease with increasing MgO/(CaO+SiO 2 ) ratio in dicalcium silicate and melilite primary phase fields, while they have an increasing trend in merwinite and monoxide primary phase fields. The data resulted from this study provide accurate experimental information that can be used for optimisation of the computed thermodynamic models. KEY WORDS: phase equilibrium; liquidus temperature; FeO -CaO-SiO 2 -Al 2 O 3 -MgO system; blast furnace slag. 1. Introduction The ironmaking blast furnace (BF) process is the primary technique used to produce the pig iron whilst still being affordable. The fundamental knowledge about the ironmaking BF process is well introduced by Biswas and Geerdes et al. 1,2) The increasing demand for the utilization of low-grade iron ores and poor quality of fuels brings on the new challenge of low gas permeability and formations of hearth accretions in modern BF operation. These issues can be overcome by better understanding the properties of the ironmaking BF slags (primary, bosh and final slags) and their variations which may be determined or predicted through systematic and accurate phase equilibria measurements. These slags can be formed by complicated reactions between iron ores with coke/coal and fluxes 1,2) in cohesive zone, belly, bosh and hearth of BF. It is relatively easy to determine the chemical compositions of the final slags with plenty of measured data available as most investigations which were related to the BF slags are focused on the final slags in the system CaO-SiO 2 -Al 2 O 3 -MgO. The typical CaO/SiO 2 weight ratio of the final slags is between 1.10 and ) The effect of the basicity in the system of CaO- SiO 2 -Al 2 O 3 -MgO in the steel-making process 8,9) and the effect of the basicity and FeO in the softening and melt- * Corresponding author: baojun@uq.edu.au DOI: ing temperature 10) were reported previously. However, the slag system containing FeO has been not fully understood due to a lack of a systematic investigations about the phase equilibria study of FeO -CaO-SiO 2 -Al 2 O 3 -MgO related to BF slags. In fact, for the operation of the ironmaking BF, the primary and bosh slags are generally considered to have more responsibilities than the final slags. It is therefore very important to conduct the systematic and accurate phase equilibria study in order to better understand the chemical compositions and liquidus temperatures of the primary and bosh slags. Slag Atlas presents the early results of the phase equilibrium. 11) The FeO containing phase diagrams in equilibrium with metallic iron from binary system FeO -SiO 2, 12) ternary system FeO -CaO-SiO 2, 13 15) FeO -SiO 2 -MgO 16) and FeO -SiO 2 -Al 2 O 3 17) to quaternary system CaO-SiO 2 - Al 2 O 3 - FeO 18 20) have been studied. However, only few experimental determinations have done in the system FeO -CaO-SiO 2 -Al 2 O 3 -MgO in equilibrium with metallic iron. In the authors previous research, phase equilibria study in the system FeO -CaO-SiO 2 -Al 2 O 3 -MgO with CaO/ SiO 2 ratio of 1.3 and 0, 5 and 10 wt% of FeO related to ironmaking BF slags was investigated. 6,7) The current work extends the study to the expanded systems with higher FeO concentration of 15 and 20 wt%, which are highly related to primary and bosh slags in BF. The properties of such slags with the phase equilibria will be discussed 2016 ISIJ 1728

2 systematically in terms of the chemical compositions and liquidus temperatures associated with pseudo binary and ternary phase diagrams, respectively. Both Fe 2+ and Fe 3+ are present in the quenched samples, however, only metal cation concentrations can be measured by EPMA. Since the slag was saturated with metallic iron, Fe 2+ was dominant in the slag at low oxygen partial pressure. All iron, consequently, was recalculated to FeO for presentation purpose only. 2. Experimental The experimental procedure in this investigation has been described in details in a previous paper. 7) The samples of the desired compositions were prepared by mixing highpurity chemicals of Fe 2 O 3, Fe, MgO, Al 2 O 3 and the master slag (CaO+SiO 2 ) in agate mortar. Excess Fe powder (20 wt% of the mixture) was added to certify that the slags were always in equilibrium with metallic iron. About 0.3 g of the mixture was collected, pelletized, and then put into an iron envelope made of 0.1 mm pure Fe foil. The iron envelope was then placed in an iron dish to avoid mixture spillage by the wetting flow during the equilibration. The high temperature equilibrium experiments were conducted in a vertical electric resistance furnace at the ultrahigh pure Ar gas environment. This was done for 30 mins pre-melting at 30 K higher than the desired temperature and for 3 to 10 hours melting at the predetermined liquidus temperature. The temperature was monitored using a Pt-30 pct Rh/Pt-6 pct Rh thermocouple. The overall absolute temperature accuracy of the experiments was estimated to be ±3 K. Lastly, the samples were dropped into iced water straight from the hot zone of the furnace for the quenching. The collected specimens were mounted in the epoxy resin and the resin blocks were polished for the sample examinations. The microstructures were examined by scanning electron microscopy coupled with energy-dispersive spectroscopy analysis (SEM-EDS). Compositions of the liquid and solid phases were measured by a JEOL JXA-8200 Electron Probe X-Ray Microanalyser (EPMA) with Wavelength Dispersive Spectrometers (WDS). The EPMA measurement conditions were the same as previous work. 7) The EPMA measurements have ±1 wt% accuracy in average. Fig. 1. Pseudo-ternary sections at fixed FeO concentrations in the system FeO -(CaO + SiO 2)-Al 2O 3-MgO with CaO/ SiO 2=1.3. Fig. 2. Typical microstructures of the quenched slags from primary phase fields of (a) melilite, (b) Ca 2SiO 4, (c) (Mg,Fe 2 + ) O and spinel, and (d) merwinite. 3. Results and Discussion 3.1. Description of the Pseudo-ternary Sections Over 200 phase equilibria experiments have been carried out in the system FeO -(CaO+SiO 2 )-Al 2 O 3 -MgO with CaO/SiO 2 = 1.3 and fixed FeO of 15 and 20 wt%, which are directly related to ironmaking BF slags. Figure 2 shows the typical microstructures of the analysed samples. As describe in the previous section, pre-melting was done at 30 K higher than liquidus temperature for 30 mins. During the pre-melting, the samples could be fully melted, then the crystallisation of solid phases was taken place from the fully melted liquid in equilibrium at the desired temperature. The rapid quenching technique can preserve the glass of liquid phase and the primary solid phase. Some dendritic crystals may precipitate in the liquid during the quenching depending on the slag chemistry, so the liquid composition was carefully measured in the outer well-quenched glass region. The liquid phase was in equilibrium with the five different primary solids and metallic iron. The metallic iron is present in all samples, implying that the oxygen partial pressure was controlled by the equilibration with iron. Figure 2(a) shows the equilibration of liquid with melilite and iron. Figure 2(b) presents a typical quenched microstructure of dicalcium silicate primary phase field. Figure 2(c) shows the equilibration of liquid with (Mg,Fe 2+ )O, spinel and iron. Figure 2(d) reveals that liquid was in equilibrium with merwinite and iron. Tables 1 and 2 show the chemical compositions of the analysed samples with FeO concentration in liquid at approximately 15 and 20 wt%, respectively. Melilite is a complex solid solution formed by akermanite (2CaO MgO 2SiO 2 ), (2CaO FeO 2SiO 2 ) and gehlenite (2CaO Al 2 O 3 SiO 2 ) as shown in the tables. The spinel [Al 2 O 3 (Mg,Fe 2+ )O], merwinite [3CaO (Mg,Fe 2+ ) O 2SiO 2 ] and monoxide [(Mg,Fe 2+ )O] were also observed in the investigated composition range. These accurately measured data of the solid solutions can be used invaluably for the optimisation of thermodynamic model. The pseudo-ternary sections of (CaO+SiO 2 )-Al 2 O 3 -MgO with CaO/SiO 2 ratio 1.3 and fixed FeO concentrations is an effective method to yield experimental results. However, there are usually differences between predetermined values and the experimental results in the CaO/SiO 2 ratio and FeO concentration in liquid phases, due to the precipitation of primary solid phases and the oxidation of metallic iron. A large number of experiments therefore had to be carried out and the experimental data are required carefully analysed for the construction of the phase diagram. In the present study, FeO -(CaO +SiO 2 )-Al 2 O 3 -MgO with the CaO/SiO 2 ratio 1.3 and fixed FeO as 15 and 20 wt% were investigated firstly in terms of the pseudo ternary sections as shown in Figs. 3 and 4. Thick lines in the figures indicate boundaries between primary phase fields and the thin ones are the isotherms derived from the experimental data. In addition, each experimentally determined liquid compositions are represented as symbols in the figures. At the low MgO region, the melilite and dicalcium silicate primary phases are stable, comparing with the higher ISIJ

3 Table 1. Compositions of the phases measured by EPMA in the system FeO -CaO-SiO 2-Al 2O 3-MgO with CaO/ SiO 2=1.3 and 15 wt% FeO. Experiment No. Temperature (K) Phase Composition (wt%) Assemblages CaO SiO 2 MgO Al 2O 3 FeO Liquid only CaO/ SiO Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid with one solid phase Melilite primary phase field Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Spinel primary phase field Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid ISIJ 1730

4 Spinel Liquid Spinel Liquid Spinel Liquid Spinel Ca 2SiO 4 primary phase field Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Merwinite primary phase field Liquid Merwinite Liquid Merwinite (Mg, Fe 2 + )O primary phase field Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid with more solid phases Liquid Melilite Spinel Liquid Melilite Spinel Liquid Melilite Spinel Liquid Melilite Spinel Liquid Melilite Spinel Liquid Melilite Merwinite Liquid Ca 2SiO Merwinite Liquid Ca 2SiO Merwinite Liquid Ca 2SiO Merwinite Liquid Ca 2SiO Merwinite Liquid Spinel (Mg, Fe 2 + )O Liquid Spinel (Mg, Fe 2 + )O Liquid Spinel (Mg, Fe 2 + )O Liquid Melilite Ca 2SiO Merwinite Table 2. Compositions of the phases measured by EPMA in the system FeO -CaO-SiO 2-Al 2O 3-MgO with CaO/ SiO 2=1.3 and 20 wt% FeO. Experiment No. Temperature (K) Phase Composition (wt%) Assemblages CaO SiO 2 MgO Al 2O 3 FeO Liquid only CaO/ SiO Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid ISIJ

5 Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid Liquid with one solid phase Melilite primary phase field Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Liquid Melilite Spinel primary phase field Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Liquid Spinel Ca 2SiO 4 primary phase field Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO ISIJ 1732

6 Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO Liquid Ca 2SiO (Mg, Fe 2 + )O primary phase field Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid (Mg, Fe 2 + )O Liquid with more solid phases Liquid Melilite Spinel Liquid Melilite Spinel Liquid Melilite Ca 2SiO Liquid Melilite Ca 2SiO Liquid Melilite Ca 2SiO Liquid Spinel (Mg, Fe 2 + )O Liquid Ca 2SiO Merwinite Liquid Ca 2SiO Merwinite MgO region where the spinel, merwinite and monoxide [(Mg,Fe 2+ )O] are major primary phases. Liquidus temperature increases in the melilite and spinel primary phase fields, but it decreases in the dicalcium silicate and merwinite primary phase fields with increasing Al 2 O 3 concentration and is not sensitive in monoxide primary phase field. The effects of the different components on liquidus temperatures will be discussed in details later. The present technique can determine the liquidus accurately. The equilibration temperature was accurately controlled within ±3 K using a Pt-30 pct Rh/ Fig. 3. Experimentally determined pseudo-ternary section (CaO + SiO 2)-Al 2O 3-MgO with CaO/SiO 2=1.3 and 15 wt% FeO. Fig. 4. Experimentally determined pseudo-ternary section (CaO + SiO 2)-Al 2O 3-MgO with CaO/SiO 2=1.3 and 20 wt% FeO. Pt-6 pct Rh thermocouple and the chemical compositions of the liquid and solid phases present in the quenched sample were accurately measured by EPMA. However, the experimental data was not exactly on the drawn isotherm lines. For example, the data points on the liquidus lines should be exactly 1.3 of CaO/SiO 2 ratio and 15 wt% of FeO concentration in Fig. 3. The experimentally determined points do not always have the exact CaO/SiO 2 ratio of 1.3 and FeO concentrations of 15 wt%. The location of the isotherms should be defined carefully considering the effects of CaO/ SiO 2 ratio and FeO concentration on the liquidus. FactSage, 21) a useful thermodynamic modelling program to predict slag chemistry, was employed for comparison in the present study. The databases selected in FactSage 6.2 are Fact53 and FToxide, and the solutions species selected in calculation are FToxide-SLAGA, FToxide- SPINA, FToxide-MeO_A, FToxide-bC2S, FToxideaC2S, FToxide-Mel_, and FToxide-Merwinite. In the compound species, the activity of pure solid iron was set to unity in the iron-saturated condition. The experimental data resulted from the present study and the predictions calculated by FactSage 6.2 are shown in Fig. 5. The thick solid lines are the boundaries between the primary phase fields and thin continuous lines are the K isotherms ISIJ

7 Fig. 5. Comparisons of boundaries and liquidus temperatures between FactSage 6.2 calculations and present results. determined by experimental results. The thick and thin fragmented lines are the corresponding boundaries and isotherms predicted by FactSage 6.2 calculations. As can be seen from this figure, the same primary phase fields are predicted in the composition range of the present investigation by FactSage, but the range and position of each primary phase field predicted are different with the experimental results. The experimentally determined spinel, melilite and monoxide primary phase fields are generally smaller although there are wider ranges in the dicalcium silicate and merwinite primary phase fields than those predicted by FactSage 6.2. The boundaries in addition of the primary phase fields shift to the direction of increasing Al 2 O 3 wt%. The experimentally determined liquidus temperatures are significantly lower than those predicted by FactSage 6.2 in the spinel, merwinite and monoxide primary phase fields. Only the liquidus temperatures are closed at lower MgO concentration in the melilite and dicalcium silicate primary phase fields. These considerable differences between the predictions and the experimental results indicate that the thermodynamic database for this important slag system need further improvement. In other words, the thermodynamic models dealing with the extensive solid solutions in particular will be enhanced if using the present data Effects of FeO on Primary Phase Fields and Liquidus Temperatures In order to compare the boundaries and isotherms, the K liquidus and boundary lines are projected on the pseudo-ternary section of MgO-(CaO +SiO 2 )-Al 2 O 3 with CaO/SiO 2 = 1.3 as shown in Fig. 6. These are including the previous data of FeO -free system, 6) 10 wt% FeO sections 7) as well as the present data at 20 wt% FeO. It can be seen from this figure that there are differences in the boundaries of the primary phase fields with FeO concentration variation. The melilite and dicalcium silicate primary phase fields are smaller with increasing FeO concentration, but the primary phase fields of monoxide and spinel are wider. FeO concentration does not have an effect on the range of the merwinite primary phase field. The liquidus temperatures in monoxide primary phase field are not sensitive to the FeO concentration although the addition of FeO has a significant effect on the liquidus temperatures. The isotherms move to the high-liquidus direction in the primary phase fields of the melilite, dicalcium silicate, spinel and merwinite. The effect of FeO on the liquidus temperatures is shown in Fig. 7 as a function of FeO concentration. The liquidus temperatures have a general deceasing trend with increasing FeO concentration in Fig. 6. Effects of FeO on primary phase fields and liquidus temperatures in the system (CaO + SiO 2)-Al 2O 3-MgO- FeO with CaO/SiO 2 ratio of 1.30 and 0, 10 and 20 wt%, FeO - free and 10 wt% data were from previous paper. 6,7) the primary phase fields of the dicalcium silicate, melilite, merwinite and monoxide. However, it has a less significant effect in the spinel primary phase field. The effect of FeO on the liquidus temperatures is almost similar in the melilite and spinel primary phase fields, whilst there are substantial difference in the isotherms between FeO -free and 5 wt% of FeO. This means that the variations of FeO from 0 wt% have considerable effects on the liquidus temperature in the primary phase fields of the merwinite and dicalcium silicate while only slightly change the isotherms when the FeO increases from 5 to 20 wt% in these primary phase fields. This may be due to the solubility of FeO in the solid solutions, depending on Al 2 O 3 and MgO concentrations in slags. It can be seen from Figs. 3 and 4 that in the dicalcium silicate primary phase field, the liquidus temperatures decrease with increasing Al 2 O 3 concentration at a given MgO. The Al 2 O 3 concentrations as a function of FeO concentration in liquid is shown in Fig. 8 for the K liquidus in the dicalcium silicate primary phase field. The solubilities of FeO in the corresponding dicalcium silicate as a function of FeO concentration in liquid are also present in Fig. 8 for comparison. It can be seen that FeO concentrations in the dicalcium silicate increase rapidly at low FeO level and then slowly at higher FeO level. The decrease of Al 2 O 3 concentration in the liquid with increasing FeO shows the same trend Application of Pseudo-binary Phase Diagrams While the pseudo ternary sections of phase diagrams provide a large amount of fundamental information for complex slag systems, pseudo-binary phase diagrams can be used more conveniently and easily in practice both for research and engineering applications, in particular to evaluate the effects of slag compositions on the liquidus temperature. They can be derived directly from the experimentally determined pseudo-ternary phase diagrams. Figure 9 shows the liquidus temperatures as a function of Al 2 O 3 /(CaO+SiO 2 ) ratio at fixed MgO of 5 (Fig. 9(a)) and 10 wt% (Fig. 9(b)), respectively. The continuous lines present the experimentally determined results of 15 and 20 wt% FeO, and the dotted lines the previous ones of 0, 5 and 10 wt% FeO. 6,7) It can be seen from Fig. 9(a) that more primary phases are present at 15 and 20 wt% FeO than 0, 5 and 10 wt% FeO at fixed 5 wt% MgO. At 15 and 20 wt% FeO, dicalcium silicate, merwinite, melilite 2016 ISIJ 1734

8 Fig. 8. Al 2O 3 in liquid and FeO in Ca 2SiO 4 as a function of FeO (wt%) in liquid in Ca 2SiO 4 primary phase field at K, FeO -free, 5 and 10 wt% data were from previous paper. 6,7) Fig. 7. Liquidus temperatures as a function of FeO wt% with CaO/SiO 2=1.3 at fixed MgO and Al 2O 3, FeO -free, 5 and 10 wt% data were from previous paper. 6,7) and spinel are the primary phase fields, while merwinite and spinel are not stable at 0, 5 and 10 wt% FeO. The liquidus temperatures decrease in dicalcium silicate and merwinite primary phase fields, but increase in melilite and spinel primary phase field with increasing Al 2 O 3 /(CaO+SiO 2 ) ratio. As can be seen from the figures, the higher FeO concentration in the slags leads to the lower liquidus temperatures in the primary phase fields of dicalcium silicate, melilite and spinel, slightly increased in merwinite primary phase field with incresing FeO from 15 to 20 wt%. It can be seen from Fig. 9(b) that more primary phase fields are present at fixed MgO of 10 wt% than 5 wt%. At 15 and 20 wt% FeO, dicalcium silicate and melilite primary phases are not stable, but the primary phase fields of merwinite, monoxide [(Mg,Fe 2+ )O] and spinel are existing. The liquidus temperatures in general decrease in dicalcium silicate, merwinite and monoxide, but increase in melilite and spinel Fig. 9. Pseudo-binary sections of (CaO + SiO 2)-Al 2O 3 with CaO/ SiO 2=1.3 at fixed MgO and FeO, FeO -free, 5 and 10 wt% data were from previous paper. 6,7) with increasing Al 2 O 3 /(CaO+SiO 2 ) ratio. The increase of FeO decreases the liquidus temperatures significantly, as shown by the lines of 0 and 5 wt% FeO in Fig. 9. However, there are only slight effects of FeO concentration in the slags on the liquidus temperatures from 5 to 20 wt% FeO at fixed MgO of 10 wt%. Figure 10 shows the liquidus temperatures as a function of MgO/(CaO+SiO 2 ) ratio at fixed Al 2 O 3 of 10, 15 and 20 wt%, respectively. At 10 wt% Al 2 O 3, the liquidus temperatures of FeO -free slags are very high, which were not measured. For 15 and 20 wt% Al 2 O 3, the five lines corresponding to FeO concentration of 0, 5, 10, 15 and 20 wt% are respectively shown for comparison. The previous results of 0, 5 and 10 wt% FeO are presented in these figures, ISIJ

9 shown with dash lines. 6,7) The continuous lines illustrate the present experimentally determined data corresponding to 15 and 20 wt% FeO, respectively. It can be seen from Fig. 10(a), at fixed Al 2 O 3 concentration of 10 wt%, the dicalcium silicate is the primary phase at lower MgO/(CaO +SiO 2 ) ratio, but merwinite and monoxide are stable at higher MgO/(CaO+SiO 2 ) ratio. The liquidus temperatures initially increase then decrease with increasing MgO/(CaO+SiO 2 ) ratio in dicalcium silicate primary phase field, while they exhibit increasing trend in merwinite and monoxide primary phase fields. At fixed Al 2 O 3 concentration of 15 wt%, melilite is the primary phase field at lower MgO/(CaO +SiO 2 ) ratio. In this primary phase field, liquidus temperatures increase firstly then decrease at 0, 5 and 10 wt% FeO, but there are only decrease trend in the liquidus temperatures at 15 and 20 wt% FeO with increasing MgO/(CaO+SiO 2 ) ratio. At higher MgO/(CaO+SiO 2 ) ratio, merwinite and/or spinel and monoxide are stable depending on the FeO concentrations. The liquidus temperatures increase with MgO/(CaO+SiO 2 ) ratio in those primary phase fields. Lastly, at fixed Al 2 O 3 concentration of 20 wt%, melilite and spinel primary phase fields can be the primary phase fields. The boundary between melilite and spinel moves to low MgO/(CaO+SiO 2 ) ratio direction with increasing FeO concentration. This means that, before FeO fully reduced from the slags, melilite primary phase is more stable than spinel at low Al 2 O Distribution of FeO between Liquid and Solid As discussed above, the even well-developed thermodynamic model such as FactSage still needs to be enhanced for the more accurate description of the phase equilibria and liquidus temperatures. The present study reveals there are the significant differences between the experimental data and FactSage predictions. The accurate data of the solid solution resulted from the present work provide experimental information to optimise the thermodynamics models including FactSage. It is only possible to improve the thermodynamic models with a large number of accurate experimental data including solid solutions. Since FeO concentrations in the melilite are affected by CaO, Al 2 O 3 and MgO, the distribution of FeO between liquid and melilite is more complicated than the other primary solids. Figures 11(a) and 11(b) show the distribution of FeO between liquid and dicalcium silicate at and K, respectively. The distribution curve should always pass the point of origin (0, 0). Since the data points do not have exactly 1.3 of CaO/ SiO 2 and desired MgO concentration, the effect of CaO/SiO 2 Fig. 10. Pseudo-binary sections of (CaO + SiO 2)-MgO with CaO/ SiO 2=1.3 at fixed Al 2O 3 and FeO, FeO -free, 5 and 10 wt% data were from previous paper. 6,7) Fig. 11. Distribution of FeO between liquid and Ca 2SiO 4 at (a) K and (b) K, CaO/SiO 2 ratio in liquid = ISIJ 1736

10 silicate, spinel, merwinite and monoxide. Those phase diagrams presented in present study were obtained through the carefully designed experiments with well-developed experimental techniques and systematic analyses of a large amount of measured data. The results illustrate that the liquidus temperatures of the blast furnace slags generally decrease with increasing FeO concentrations. The series of pseudo-binary phase diagrams were directly derived from the pseudo-ternary phase diagrams, providing useful tools to easily evaluate the effects of the slag compositions on the liquidus temperatures. There are significant differences in the liquidus temperatures and slag compositions between the experimental results and the FactSage 6.2 predictions. The experimentally determined data can be directly used for the optimisation of the thermodynamic models. Combined with our previously reported data, this study represents a systematic investigation on the system FeO -CaO-SiO 2 -Al 2 O 3 - MgO in equilibrium with metallic iron. It provides the extensive experimental data fulfilling the current knowledge gap that can be used for better understanding the complex reactions consisting of multi-component slag systems, such as slags in the iron-making blast furnace. Acknowledgements The authors wish to thank Baosteel through The Baosteel- Australia Joint Research and Development Centre (BAJC) for providing financial support for this project. Mr. Ron Rasch and Ms Ying Yu of the Centre for Microscopy and Microanalysis at the University of Queensland, who provided technical support for the EPMA facilities, are grateful. REFERENCES Fig. 12. Distribution of FeO between liquid and solid in (a) spinel and (b) (Mg, Fe 2 + )O primary phase fields. and MgO concentration in liquid on the solubility of FeO in the Ca 2 SiO 4 should be carefully considered to locate the curve of distribution of FeO. It can be seen from the figures that FeO concentration in solid slightly increases with increasing FeO concentration in the corresponding liquid. Figures 12(a) and 12(b) show the distribution of FeO between liquid and solid of spinel and monoxide, respectively at K and K. As seen Fig. 12, FeO concentration in the spinel increases slightly with increasing FeO concentration in the corresponding liquid while it increases significantly in monoxide primary phase. The values were also affected by the MgO concentration in the corresponding liquid, due to MgO can substitute FeO in spinel and monoxide. It can also be seen from Fig. 12 that FeO concentration decreases with increasing the liquidus temperature in spinel and monoxide primary phase fields. 4. Conclusions Phase equilibria and liquidus temperatures have been investigated in the system FeO -CaO-SiO 2 -Al 2 O 3 -MgO with CaO/SiO 2 ratio of 1.3 and at fixed FeO of 15 and 20 wt%. The FeO -containing blast furnace slags can be characterised accurately by experimentally determined phase diagrams of the liquidus temperatures and slags compositions in the primary phase fields of melilite, dicalcium 1) A. K. Biswas: Principles of Blast Furnace Ironmaking: Theory and Practice, Cootha Publishing House, Brisbane, (1981), ) M. Geerdes, R. Chaigneau, I. Kurunov, O. Lingiardi and J. Ricketts: Modern Blast Furnace Ironmaking: An Introduction, 3rd ed., IOS Press, Amsterdam, (2015), ) D. Zhang, E. Jak, P. Hayes and B. Zhao: 4th Annual High Temperature Processing Symp., Swinburne University of Technology, Melbourne, Australia, (2012), 17. 4) Q. Y. Yu, L. L. Zhang and C. C. Lin: Baosteel Technol., 3 (2002), 37. 5) X. Ma, D. Zhang, Z. Zhao, T. Evans and B. Zhao: ISIJ Int., 56 (2016), ) X. Ma, G. Wang, S. Wu, J. Zhu and B. Zhao: ISIJ Int., 55 (2015), ) K. Jang, X. Ma, G. Wang, S. Wu, J. Zhu and B. Zhao: ISIJ Int., 56 (2016), 967, DOI: JINT ) J. Gran, B. Yan and D. Sichen: Metall. Trans. B, 42B (2011), ) J. Gran, Y. Wang and D. Sichen: Calphad, 35 (2011), ) H. Chuang, W. Hwang and S. Liu: Mater. Trans., 50 (2009), ) Slag Atlas, ed. by V. D. Eisenhuttenleute, Verlag Sthaleisen GmbH, Düsseldorf, (1995), ) N. L. Bowen and J. F. Schairer: Am. J. Sci., 24 (1932), ) N. L. Bowen, J. F. Schairer and E. Posnjak: Am. J. Sci., 26 (1933A), ) N. L. Bowen, J. F. Schairer and E. Posnjak: Am. J. Sci., 25 (1933B), ) W. C. Allen and R. B. Snow: J. Am. Ceram. Soc., 38 (1955), ) N. L. Bowen and J. F. Schairer: Am. J. Sci., 29 (1935), ) J. F. Schairer and K. Yagi: Am. J. Sci., 245 (1947), ) A. Muan and E. F. Osborn: Phase Equilibria Among Oxides in Steelmaking, Addison-Wesley, New York, (1951), ) D. P. Kalmanovitch and J. Williamson: Proc. ACS Symp. Ser., 301, American Chemical Society, Washington DC, (1986), ) D. P. Kalmanovitch, A. Sanyal and J. Williamson: J. Inst. Energy, 59 (1986), ) C. Bale, E. Bélisle, P. Chartrand and S. Degterov: Calphad, 33 (2009), No. 2, ISIJ