UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

802 IMMC 2018 | 19th International Metallurgy & Materials Congress

Mineralogical Investigations on the Phase Formation of Iron Ore Sinter

Ömer Saltuk Bölükbaşı

İskenderun Technical University, Faculty of Engineering and Natural Sciences, Department of Metallurgical and Materials Engineering, Hatay, Turkey

Abstract

Iron ore sinters are considered as multi-phase materials, with a heterogeneous microstructure. The amounts of the different phases mainly hematite, magnetite and a dicalcium silicate are contributing factors to a number of important on the sinter quality. The behavior of phase formation during the sintering process has a significant of effect on the chemical composition of sinter and controlling microstructure and concentration of silicoferrites of calcium and aluminum (SFCA). The mineralogical distribution of different phases determines the microstructure of the sinter which imparts the quality of the sinter. In this study, the microstructure of the sinter has been examined by considering the chemical composition, the mineralogy, the morphology and the spatial arrangement of the various mineral phases formed during sintering. The chemical composition in the process of sintering (especially CaO, Al2O3, Fe2O3, SiO2 etc.) were made optimum control and all parameters that can effect on the quality of the process and operating of sinter were investigated. In this work, sinter phases were determined using of Panalytical Co-Kradiation X-Ray Diffraction (XRD), raw data converted into Autoquan format, interpret the results through Rietveld Method (the powder diffraction technique). Phase’s identification was done using PANalytical X’Pert Highscore plus software.

1. Introduction

Sintering is a process by which a mixture of iron ores, fluxes and coke is agglomerated in a sinter plant to manufacture a sinter product of a suitable composition, quality and granulometry to be used as burden material in the blast furnace. This process is studied and researched in the steelmaking industry in general, and in sinter plants in particular, as well as in universities and metallurgical research centres throughout the world [1]. The sintering process is used to agglomerate a mix of iron ores, return fines, fluxes and coke, with a particle size of <8 mm, so that the resulting sinter, with a screened size of 10-40 mm, can withstand pressure and temperature conditions in the blast furnace [2-5].

Sintering is a thermal agglomeration process (1300-1480°C,. of a mixture of iron ore mineral fines by-products of the iron and steelmaking industry, fluxes, slag-forming elements and coke breeze as a particle size of <3 mm [6].

The objective of the process, whereby the mixture of materials charged is partially fused at a high temperature to produce clustered lumps, is obtaining a load (5-50 mm) for the blast furnace with the suitable physical-chemical and mechanical properties with the lowest price (Fernández-González et al., 2016) ([7]. Iron ore sinters have a mineralogy which is basically composed by iron oxides and hydroxides, ferrites and silicates. Their mineralogical composition constitutes an important parameter that can be specified and controlled [8, 9].

2. Experimental Procedure

The first stage of sintering is granulation of the raw mix, which consists of its homogenisation in a mixing drum for several minutes with the addition of 6-8% water. The granulated mix is then loaded onto the permeable sinter strand grate. The sinter plant flow diagram is illustrated in figure 1.

Figure 1. Simplified diagram of a sinter plant [10].

The bed top is heated to high temperature by oil or gas burners and air is drawn through the grate. After a short ignition time, heating of the bed top is discontinued and a narrow combustion zone (flame front) moves downwards through the bed, heating each layer successively. In the bed the granules are heated to 1250-1350 °C to achieve their softening and then partial melting [3]. In a series of reactions a semi-molten material is produced which, in subsequent cooling, crystallises into several mineral phases of different chemical and morphological compositions; mainly hematite, magnetite, ferrites and gangue composed mostly of calcium silicates. Their mineralogical composition constitutes an important parameter that can be

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specified and controlled [11].

3. Results and Discussion

3.1 SEM Analysis of Sinter Samples

Sinter samples were mounted and prepared for observation by Scanning Electron Microscopy (SEM). A conventional grinding and polishing procedure was employed. SEM investigation carried out with 15 keV energy. The chemical analyses could therefore be used to determine the different types of SFCA phases by comparing these compositions to those reported in the literature

Electron microprobe analyses were performed on four of sinter samples. Elements were analyzed included O, Mg, Al, Si, K, Ca, Mn and Fe. The phases that were analyzed included larnite (Ca2SiO4-C2S), hematite, magnetite and SFCA (all phases identified with an optical microscope). Approximately 450 SEM-EDS analyses were made on sub 4 sinter samples (S-1, S-2, S-3, S-4). SEM analyses were done on the same polished sections of the sinters for identification and chemical analysis of the different phases.

Experiment results were recalculated to determine the compositions of the different phases present in the sinter samples. The results were clustered according to the elemental analyses and the results recalculated to fit the phases present (i.e., hematite, magnetite, C2S and SFCA formations). The main phase composition of sinter samples is given in table 1.

Table 1. Phase analysis of sinter samples Phases S-1 S-2 S-3 S-4

Larnite 6.80 7.46 6.72 6.40 Hematite 27.88 23.99 23.10 27.80 Magnetite 26.48 27.60 33.58 29.98 SFCA-I 14.45 16.03 14.28 5.48 Wuestite 1.01 0.00 1.02 0.00

SFCA 22.20 25.02 22.17 30.24 Total SFCA 36.65 41.05 36.45 35.85

SFCA-I/SFCA 0.65 0.64 0.64 0.18 In this investigation, the Autoquan program was used to determine the phases contained in the sample. Phases were identified using X’Pert Highscore plus software and quantification was accomplished with Autoquan/BGMN software [12]. The Autoquan software determined the parameter Rwp which is displayed as a function of the repetition steps to illustrate the calculation progress. This parameter (the weighted residual square sum) allows the user to assess the quality of fitting of the measured and calculated diagram. A smaller value stands for better fitting [13].

Figure 2. SEM micrographs of sinter bodies. (Mag: Magnetite, Silico Ferrite Calcium and Aluminum (SFCA), SFCA-I (x1000)

Figure 2 demonstrated that magnetite crystals embedded in a silicate-rich glassy matrix. The magnetite is in close association with SFCA crystals. The magnetite is present as well-defined euhedral to subhedralcrystals as well as skeletal and dendritic crystals. Magnetite crystals are present in close association with SFCA (seen in figure 2). The calculated stoichiometry of the magnetite in all the layers indicated that this phase is actually an iron rich spinel solid solution phase with an (Fe,Mg,Mn)(Fe,Al)2O4 stoichiometry. At the lower air flow (series one) the sintering time at temperatures above 1100°C was longer. It can therefore be expected that more hematite is reduced to magnetite in series one than in series two, due to the longer reaction times. Magnetite is also formed from the dissociation of SFCA at temperatures higher than 1300°C [14]. The proportion of SFCA that is decomposed may increase when the time of exposure above the decomposition temperature increases or when the maximum temperature that is reached during sintering increases.

The SFCA phase is present as dendritic SFCA, acicular SFCA and columnar SFCA, depending on the size of the crystals. Dendritic SFCA is the first type of SFCA that forms during the sintering process [15, 16]. These crystals are very small, usually smaller than 4 m (shown in figure 3). Between 8 and 16% of the total SFCA consist of dendritic SFCA. Crystal growth between 1200°C and 1300°C results in bigger crystals, larger than 4 m but smaller than 10 m, called acicular SFCA [15, 16] . In this study, more than approximately 36% of the total SFCA consists of acicular SFCA (SFCA-I). At 1300°C acicular SFCA melts, and subsequently re-precipitates as columnar SFCA during cooling. These are rather large crystals with a typical size of a10 m and bigger [16]. Between 10 and 24% of the total SFCA is columnar.

SFCA

SFCA-1

Mag.

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

804 IMMC 2018 | 19th International Metallurgy & Materials Congress

Figure 3. SEM micrographs of sinter bodies. (Hem: Hematite, Mag: Magnetite, Silico Ferrite Calcium and Aluminum (SFCA), SFCA-I (x500).

3.2 Rietveld Analysis

The rietveld method has become an efficient method of quantitative phase analysis in recent years. Its decisive advantage over the calibration sample adaptation method is the better use it makes of the information content of the diffractogram [12, 13]. Rietveld method has several advantages. You can distinguish between and determine all existing phases in just a few minutes. Figure 4 illustrates the peak positions of the major minerals. Extensive peak overlap is apparent.

Figure 4. A typical image of rietveld refinement on the sinter sample by using HighScore Plus Program.

In this investigation, the Autoquan program was used to determine the phases contained in the sample. Phases were identified using X’Pert Highscore plus software and quantification was accomplished with Autoquan/BGMN

software. The Autoquan software determined the parameter Rwp which is displayed as a function of the repetition steps to illustrate the calculation progress. This parameter (the weighted residual square sum) allows the user to assess the quality of fitting of the measured and calculated diagram. A smaller value stands for better fitting [12, 13]. Phase identification was performed using PANalytical’s HighScore Plus software package.

Figure 5. Triangular plot showing that the compositional relationship between hematite, magnetite, SFCA, SFCA-I and SFCA-II in the sinter samples.

3.3 XRD for the Characterization of Iron Ore Sinter

The EDS analyses of the SFCA phases were plotted on the Al2O3 - CaO - Fe2O3 ternary phase were seen in figure 5. This phase diagram used to locate the mineral compositions of hematite, magnetite, SFCA and SFCA-I in the sinter material. Chemical compositions of the SFCA phases were close to the regions of SFCA and SFCA-I.

Figure 6. X-ray diffraction patterns of the total sinter samples.

The Autoquan program can be used for accurate quantitative XRD phase analysis by the Rietveld method. XRD quantification of the different sinter phases was also plotted as a function of varying fine ore ratio in the sinter

Mag.

Columnar SFCA

Dentritic SFCA

Hem.

AcicularSFCA

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Journal of the Southern African Institute of Mining and Metallurgy, 103(10), 645-650. [16] P. R. Dawson, J. Ostwald, and K. M. Hayes, (1984, June). The influence of the sintering temperature profile on the mineralogy and properties of iron ore sinters. In AIMM Bull. Proc. (Vol. 289, No. 5, pp. 163-169).

samples and is given in figure 6. It can be seen that the variation of the fine iron ore fraction of the sinters influences the amount of the different sinter phases in the sintering process.

Figure 7. X-ray diffraction patterns of SFCA phase in iron ore sinter.

Figure 7 shows the SFCA diffraction patterns of collected from sinter sample. The results of studies both the X-ray diffraction and mineralogical investigated on the sinter samples with containing different ratio of fine iron ore into the sinter blend. Sinter samples have different a mineralogical structures. Especially in this study investigated magnetite (Fe3O4), hematite (Fe2O3), Larnite-C2S (Ca2SiO4), SFCA (M14O20), SFCA-I (M20O28),wuestite (FeO) phases for all sinter samples by X-ray diffraction studies and given results in figure 5.

4. Conclusion

Modern methods like the Rietveld refinement in combination with high-speed data collection allow for fast and reliable quantification of sinter. All crystalline phases can be quantified in just a few seconds. The use of XRD and the Rietveld method have resulted in the development of fast and reliable methods of phase quantification in the iron ore industry. During the Rietveld quantification crystallographic are calculated and it does not need standards or calibration. Compared with other methods, it is still the most reliable method for phase quantification, and it has the potential to make important contributions to our knowledge regarding the effect of phase relations on the properties of sinters. Several applications of the method have been discussed in some detail. Rapid XRD analysis and associated Rietveld quantification offer a fast and reliable method to determine the mineralogical phase composition iron sinters within minutes.

New developments will necessarily attempt to relate phase composition to sinter strength and reducibility, and increased use of cluster analysis can identify important relations between these parameters. In combination with the calculation of process critical parameters such as FeO, SiO2, SFCA etc. content, it can be used as a process control

method for iron ore sinter plants.

ACKNOWLEDGEMENTS

The author wishes to thank Prof. Dr. Johan P.R. De Villiers for assistance of phase analysis and valuable suggestions. This research was supported by 2219 Research Scholarship and Supports Directorate (B DEB) of The Scientific and Technological Research Council of Turkey (TUBITAK) for financial support.

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