2022 年 62 巻 8 号 p. 1624-1628
The phase evolution during heating of two Al2O3-free mixtures designed to form the SFC iron ore sinter bonding phase was investigated using in-situ X-ray diffraction over the temperature range 293–1623 K. Results showed that during heating at an oxygen partial pressure of 5 × 10−3 atm, SFC formation was preceded by the formation of γ-Ca4Fe14O25 and γ-CFF (nominal composition Ca3.0Fe14.82O25) intermediate phases. On further heating the SFC melted to form Fe3O4 in a Fe2O3–FeO–CaO–SiO2 melt. During heating of one of the mixtures in air, it was not possible to distinguish between SFCA-I and α-CFF (nominal composition Ca3.43Fe14.39O25) as intermediate phases in the formation of SFC. The improved signal-to-noise ratio and angular peak resolution afforded by synchrotron-based XRD experimentation would be required to possibly distinguish between these two phases. Regardless of whether SFCA-I or α-CFF formed as intermediate phases during heating in air, this work shows that the oxygen partial pressure has a significant effect on phase formation mechanisms in the Al2O3-free SFC system.
SFCA and SFCA-I are key iron ore sinter bonding matrix phases, and a review of their composition, structure and formation conditions has recently been undertaken.1) The review was focused on a high-Fe, low-Si form called SFCA-I (general formula M20O28, e.g., Ca3.2Fe2+0.8Fe3+14.7Al1.3O28, triclinic crystal structure), and a low-Fe form called SFCA (M14O20, e.g., Ca2.3Mg0.8Si1.1Al1.5Fe8.3O20, also triclinic). The review also briefly referenced the Al2O3-free form of SFCA, SFC, the compositional and thermal domains of which have been probed by Pownceby and Patrick and more recently by Cheng et al.2,3)
The formation mechanisms of SFCA and SFCA-I, as well as the effect of various parameters (e.g., Al2O3 concentration and source, basicity, MgO and TiO2 impurities, and oxygen partial pressure) on their formation during heating, have been extensively studied using in-situ X-ray diffraction (XRD).4,5,6,7,8,9,10,11) In the first of these studies, Scarlett et al. characterised the formation of SFCA and intermediate phases during heating under partial vacuum of sinter mixtures with compositions designed to form SFCA.4,5) The samples contained 1 and 5 mass% Al2O3 and were designated AL1 and AL5, respectively, and were compositions that were within the SFCA compositional domain established by Patrick and Pownceby.12) Results showed that SFCA and SFCA-I both formed around 1323 K through reactions involving alumina and silica with pre-cursor C2F (2CaO.Fe2O3) and CF (CaO.Fe2O3) phases. Subsequent investigation found that the phase evolution under the partial vacuum conditions was similar to the evolution observed in experiments conducted in air (i.e., at an oxygen partial pressure, pO2, of 0.21 atm).11)
Scarlett et al. also investigated phase formation during heating under partial vacuum (i.e., in air) of an Al2O3-free “SFCA” composition. This composition – 82.36 mass% Fe2O3, 14.08 mass% CaO and 3.56 mass% SiO2 – was designated AL0 and was within the SFC domain.2,3) There, SFCA-I and SFC were observed to form above ~1400 K through the solid-state reaction of Fe2O3 and CF. Whilst this work provided important information regarding the phase changes occurring in the initial stages of sintering of an Al2O3-free mixture, the application to industrial sinter systems was limited by experiments being conducted at temperatures below 1503 K and before melting of SFC. In addition, the experiments were conducted in an atmosphere not representative of industrial pO2 values. Hsieh and Whiteman found that a pO2 of 5 × 10−3 atm pO2 maximised the formation of Ca-rich ferrites while still producing mineral assemblages like those found in industrial sinters.13) Most of the recent in-situ XRD work aimed at determining SFCA and SFCA-I formation mechanisms has been conducted at this pO2 and in the range 298–1623 K.
To examine the effect of pO2 on SFC stability, Pownceby and Clout showed that in contrast to the 1513 K results in air, it was impossible to produce single-phase SFC when a reduced pO2 of 5 × 10−3 atm was employed.14) More recent phase equilibria studies conducted by Chen et al. to investigate the thermodynamic stability of SFC phase at equilibrium in 1 atm CO2 also showed that no SFC phase was formed under pO2 conditions more reducing than air.15) Both previous reduced pO2 investigations, however, were equilibrium studies and provided no information regarding the pre-cursor phases that develop during SFC formation. The elucidation of phase formation mechanisms, including observation of intermediate phases, in the SFC system would therefore be a novel contribution to the fundamental knowledge of iron ore sinter mineralogy. Here we report the phase evolution during heating, in the range 298–1623 K and at pO2 = 5 × 10−3 atm, of two Al2O3-free sinter mixtures with bulk compositions that would normally be located within the SFC domain in air.
Two sinter mixture samples were prepared – designated SFC-B and CF-17 – and their nominal bulk compositions were: 82.43 mass% Fe2O3, 14.13 mass% CaO and 3.44 mass% SiO2 (SFC-B); and 79.67 mass% Fe2O3, 15.43 mass% CaO and 4.90 mass% SiO2 (CF-17). The compositions were designed to model the reactive ultrafine (< 1 mm) component of an industrial sinter mix rather than being representative of the bulk composition of a sinter blend which contains coarse nuclei (up to 6.3 mm in size), gangue and flux particles. They were chosen because they i) were shown by Pownceby and Patrick2) to be two of only a small number of compositions to produce single-phase SFC when equilibrated in air in the range 1473–1513 K, and ii) were at opposite ends of that SFC compositional range. They were prepared by homogenising mixtures of synthetic, dehydrated Fe2O3 (99.99%), SiO2 (99.999%) and CaCO3 (99.9%).
2.2. In-situ XRD ExperimentationIn-situ XRD experiments were performed for the SFC-B and CF-17 samples using an Inel diffractometer incorporating a Co Kα X-ray tube and a CPS120 position sensitive detector. The diffractometer was fitted with an Anton Paar 16N high-temperature chamber, which incorporates a Pt heating strip. Throughout heating the Anton Paar chamber was fed by a continuous flow of a 0.5% O2 in N2 gas mixture (pO2 = 5 × 10−3 atm). A heating rate of 20 K min−1 was used for 298–873 K, then 10 K min−1 for 873–1623 K which corresponds to the region of Ca-rich ferrite phase formation and decomposition. XRD data were collected continuously, with individual datasets collected for 1 min. These conditions match those used in our previous in-situ XRD investigations of iron ore sintering reactions.6,9,10,11) Three experiments were performed, for the SFC-B and CF-17 mixtures at pO2 = 5 × 10−3 atm, and for the SFC-B mixture in air.
Figure 1(a) shows the plot of accumulated in-situ XRD data collected during the experiment performed for the SFC-B mixture at pO2 = 5 × 10−3 atm. During heating, the sequence of phase decomposition/formation events between 298 and 1323 K was: transformation of α-SiO2 to β-SiO2, which was complete by 860 K; decomposition of CaCO3 (complete by 934 K); formation of CaO; formation of dicalcium ferrite (C2F); and formation of monocalcium ferrite (CF). A similar sequence of lower-temperature phase decomposition/formation events was observed for the CF-17 mixture at pO2 = 5 × 10−3 atm (Fig. 1(b)), although the higher CaO concentration in this mixture meant that the decomposition of CaCO3 was complete by 1015 K.
In-situ XRD data collected for (a) SFC-B and (b) CF-17 during heating in the range 298–1623 K at pO2 = 5 × 10−3 atm. The plot is viewed down the intensity axis of the accumulated XRD datasets which are plotted as temperature (i.e., data set number) vs 2θ.
The phase behaviour above ~1400 K for both mixtures is shown in more detail in Fig. 2, which are stack plots of individual in-situ XRD datasets over the range 31–36° 2θ. For SFC-B in Fig. 2(a), a peak which was matched with the International Centre for Diffraction Data (ICDD) database entry no. 13-0395 for γ-Ca4Fe14O25 was observed at 34.1° 2θ and in the range 1361–1457 K.16) As temperature increased further peaks matched with the γ-CFF phase (ICDD database entry no. 04-011-0203) were observed in the range 1419–1477 K and at 32.8, 34.5 and 35.1° 2θ.17) Peaks which are thought to be indicative of SFC are evident in the range 1457–1487 K. Above 1487 K, significant melting had occurred and the phase assemblage was spinel (Fe3O4) in a Fe2O3–FeO–CaO–SiO2 melt. Based on previous work the spinel phase is considered likely to contain at least 3 mass% CaO in solid solution.6) The formation of melt is indicated by an increase in the level of the background, and this is labelled in Figs. 1(a) and 1(b).
Stack plots of in-situ XRD datasets collected for (a) SFC-B and (b) CF-17 at pO2 = 5 × 10−3 atm over the range 31 to 36° 2θ and in the range ~1400–1530 K. The datasets are offset in the intensity axis for clarity and are plotted as X-ray intensity vs 2θ.
Calcium ferrite formation and decomposition in this mixture, therefore, proceeds with increasing temperature via the mechanism in the following sequence of reactions, with intermediate calcium ferrites reacting with hematite and becoming progressively richer in Fe2O3, until reacting with SiO2 at higher temperature:
| (1) |
For CF-17 in Fig. 2(b), the peak for the γ-Ca4Fe14O25 phase was observed in the range 1421–1488 K, the peak for the γ-CFF phase in the range 1459–1507 K, and above 1527 K the phase assemblage was spinel in a Fe2O3–FeO–CaO–SiO2 melt. There was no evidence for the formation of SFC in this experiment. Pownceby and Clout determined that this composition under equilibrium conditions at pO2 = 5 × 10−3 atm did form SFC, along with Fe3O4 and melt in the phase assemblage, and so the effect of the heating profile used in these dynamic experiments was to restrict SFC formation. We assume that if a lower heating rate was used, results would be more consistent with the equilibrium study of Pownceby and Clout and SFC would form.14) The observation of peaks thought to be indicative of SFC in the experiment performed for SFC-B, but apparent absence of SFC in the experiment performed for CF-17, can be rationalised on the basis that the basicity (B, = CaO/SiO2) of SFC-B (B = 4.1) is higher than the basicity of CF-17 (B = 3.1). Previous in-situ XRD results reported for the Al2O3-containing system indicated a significant increase in the overall amount of SFCA phases (i.e., SFCA-I + SFCA) as the basicity increased from B = 2.5 to B = 4.8)
The γ-CFF phase was reported by Arakcheeva and Karpinskii and has nominal composition Ca3.0Fe14.82O25.17) It has been observed previously during in-situ XRD characterisation as an intermediate phase in the formation of SFCA-I at pO2 = 5 × 10−3 atm.18) Its observation here, along with the observation of the lower-temperature γ-Ca4Fe14O25 phase, is the main novel aspect of this current work. It demonstrates a clear effect of pO2 on phase formation under dynamic conditions during heating of Al2O3-free mixtures, since Scarlett et al. reported no evidence for the formation of either phase.4,5) It should be noted, however, that the γ-CFF phase is similar in structure and composition to the β-CFF phase which was also reported by Arakcheeva and Karpinskii.19) We consider the ICDD database entry for γ-CFF (no. 04-011-0203) to be a closer match to the observed XRD data than the entry for β-CFF (04-011-1398).17,19)
3.2. Phase Evolution in AirAs discussed in Section 1, in the in-situ XRD results of Scarlet et al. for the AL0 mixture, the SFCA-I phase was identified as forming, together with SFC, above ~1400 K through the solid-state reaction of Fe2O3 and calcium ferrite (CF). This observation appears inconsistent with the current understanding of the structure and composition of triclinic SFCA-I compounds.
Until recently, only a small number of triclinic SFCA-I compounds had been identified in the literature; Ca3.2Fe2+0.8Fe3+14.7Al1.3O28 and Ca7.12Fe2+0.88Fe3+23.82 Al8.18O56.20,21) More recently, the composition and structure of several new SFCA-I compounds - Ca2.90Mg0.95Fe10.11Al5.99O28, Ca6.67Al3.76Fe3+28.24Fe2+1.33O56, Ca6.65Al4.91Fe3+27.09Fe2+1.35O56, Ca6.57Al5.93Fe3+26.09Fe2+1.40O56, Ca6.71Al7.88Fe3+24.12Fe2+1.29O56, Ca6.66Al8.62Fe3+23.38Fe2+1.34O56, and Ca6.72Al10.47Fe3+21.53Fe2+1.28 O56 - have been reported and the extent of the triclinic SFCA-I compositional domain has become clearer.22,23) Based on the results of Kahlenberg et al., there appears to be a low Al2O3 concentration limit for SFCA-I since a mixture with nominal composition Ca8Al0.8Fe31.2O56 (i.e., 1.4 mass% Al2O3) contained the α-CFF phase (nominal composition Ca3.43Fe14.39O25)24) in addition to SFCA-I.23) They state that a pure Fe-SFCA-I (i.e., alumina-free SFCA-I) could not be obtained. In addition, based on the compositions listed above, the SFCA-I structure does not appear to accommodate a significant amount of SiO2. We also note that Ding and Guo, in their investigation of the formation process of SFC from calcium ferrite, observed the formation of α-CFF in addition to SFC after reaction of Fe2O3, SiO2 and CF in air at 1473 K.25)
We consider it to be unlikely, but it may be that SFCA-I is a metastable phase in the Al2O3-free system, and it is worthwhile noting that the SFCA-I compounds listed above were synthesised under conditions approaching equilibrium. However, we consider it more likely that the SFCA-I phase was misidentified in the experiments performed for the AL0 mixture in the Scarlett et al. study, since SFCA-I and α-CFF have several overlapped peaks within the d-spacing/2θ range which has proved most useful for characterising iron ore sintering phase decomposition, formation, and transformation events in previous in-situ XRD investigations.
To attempt to resolve this conflict, an in-situ XRD experiment was performed for the SFC-B mixture in air and the accumulated in-situ XRD data are shown in Fig. 3. The lower-temperature (i.e., in the range 298–1323 K) phase decomposition/formation events were like those observed for the SFC-B and CF-17 mixtures at pO2 = 5 × 10−3 atm (Fig. 1). Figure 4 shows a stack plot of the datasets collected over the ranges 31–36° 2θ and 1439–1527 K. A peak at 31.8° 2θ indicative of SFC is evident in the range 1468–1517 K. Above 1517 K the phase assemblage was Fe2O3 in a Fe2O3–CaO–SiO2 melt. Peaks at 31.6 and 32.4° 2θ evident in the range 1449–1507 K may be due to either SFCA-I or α-CFF (i.e., these phases have peaks which are overlapped at these positions), and the data quality is not sufficient to utilise any other non-overlapped peaks to distinguish between these two phases. Therefore, this conflict cannot be resolved with the current results, and for the Al2O3-free system, calcium ferrite formation/decomposition mechanism can only be described at this stage via the following sequence of reactions:
| (2) |
In-situ XRD data collected for SFC-B during heating in the range 298–1623 K in air.
Stack plots of in-situ XRD datasets collected for SFC-B in air over the range 31 to 36° 2θ and in the range ~1400–1530 K.
We have noted the similarity of the γ-CFF and β-CFF phases in terms of structure and composition, and the inability to distinguish between SFCA-I and α-CFF using the laboratory-based in-situ XRD experimentation utilised in this study. In both cases the improved signal-to-noise ratio and angular resolution afforded by using synchrotron-based in-situ XRD experimentation, compared to the laboratory-based experimentation employed here, may allow for conclusive determination of which of these phases forms and should be performed as future work. Regardless, the current work shows that the oxygen partial pressure has a significant effect on phase formation mechanisms under dynamic conditions in the Al2O3-free SFC system and is an important contribution to the fundamental knowledge of iron ore sinter mineralogy.
In terms of comparing the SFC system with the Al2O3-containing SFCA system, Webster et al. have previously demonstrated the effect of pO2 on phase formation during heating of a mixture with composition 77.36, 14.08, 3.56 and 5.00 mass% Fe2O3, CaO, SiO2 and Al2O3, respectively.11) There, SFCA-I and SFCA were both observed to form at pO2 = 0.21 and 5 × 10–3 atm, albeit by different mechanisms at the different pO2. Given the absence of the key iron ore sinter bonding phases at pO2 = 5 × 10–3 atm in the Al2O3-free SFC system, as demonstrated in the current study, compared to the presence of these key phases at pO2 = 5 × 10–3 atm in the Al2O3-containing system, the in-situ XRD results confirm that the presence of alumina is critical to the formation of high-quality sinter.
Rachel Pattel (formerly CSIRO Mineral Resources, Clayton, VIC, Australia) is thanked for sample preparation.
