Silico-ferrite Of Calcium Research Articles

Study of SiO2 involved in the formation process of silico-ferrite of calcium (SFC) by solid-state reactions

Silico-ferrite of calcium (SFC) is a key transitional phase in the formation process of complex silico-ferrites of calcium and aluminum (SFCA-Ι and SFCA), and SiO2 plays an important role in the formation of SFC. To study the formation mechanism of SFC by solid-state reactions is conducive to understanding the process of SiO2 involved in the formation of SFCA-I and SFCA. Experiments were carried out under air at different temperatures from 600°C to 1200°C by a certain amount of SiO2 mixing with Fe2O3 and Ca(OH)2. X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy were used to characterize the phase change of the sintered samples. The results show that the initial product of SiO2 that participated in the formation process of SFC is 2CaO·SiO2 (C2S) that mainly forms by the reaction of SiO2 and CaO·Fe2O3 (CF) at approximately 1000°C. Subsequently, C2S that disappears with it reacts with Fe2O3 and CF respectively to form SFC at approximately 1100°C. The reaction of C2S and Fe2O3 is more easily to occur compared with C2S and CF through the thorough solid-state reaction experiments between C2S, Fe2O3, and CF, and the formation of SFC will be promoted by the co-existence of Fe2O3 and CF that the optimal mole ratio of Fe2O3 to CF is approximately 1.00:0.55. Finally, melt appears at approximately 1200°C due to the effect of the eutectic structure composed of SFC and CF.

Silico-ferrite of Calcium and Aluminum Characterization by Crystal Morphology in Iron Ore Sinter Microstructure

Silico-ferrite of Calcium and Aluminum Characterization by Crystal Morphology in Iron Ore Sinter Microstructure

The Sintering Characteristics of Mixing SiO2 with Calcium Ferrite at 1473 K (1200 °C)

The liquid-generating capability in high basicity sinter was investigated by adding SiO2 to CaO·Fe2O3 (CF) at 1473 K (1200 °C) in air for 4 hours. X-ray diffraction, electron-probe microanalysis, and optical microscopy combining with etching test were used to characterize the phase change of sintered samples. The results show that the minor SiO2 additions from 0.14 wt pct to 6.49 wt pct to CF depressed the melting temperature of CF. New phases of formation included from SFC (silico-ferrite of calcium), SFC + Hem (Fe2O3) + CS (CaO·SiO2) to Hem + CS in samples with SiO2 additions from 0.05 wt pct to 21.74 wt pct. With the maximum SFC produced in sample, the amount of SiO2 addition was 10.64 wt pct. To understand the depressing mechanism of CF melting temperature, the experiments of differential scanning calorimetry for samples of CF, SFC, CF-SiO2 (3.0 wt pct), and CF-SFC (50.0 wt pct) had been carried out. The result shows that mixing of SiO2 or SFC with CF decreased the melting temperature of CF by 35 K and 34 K (35 °C and 34 °C), respectively, resulted in melting temperature approaching the eutectic temperature between CF and SFC. The phase relations between CF and SFC, as well as the intermediate products in depressing the melting temperature, will be an important guide to improve the sintering characteristics of iron ores.

Quantitative X-ray Diffraction (QXRD) analysis for revealing thermal transformations of red mud

Red mud is a worldwide environmental problem, and many authorities are trying to find an economic solution for its beneficial application or/and safe disposal. Ceramic production is one of the potential waste-to-resource strategies for using red mud as a raw material. Before implementing such a strategy, an unambiguous understanding of the reaction behavior of red mud under thermal conditions is essential. In this study, the phase compositions and transformation processes were revealed for the Pingguo red mud (PRM) heat-treated at different sintering temperatures. Hematite, perovskite, andradite, cancrinite, kaolinite, diaspore, gibbsite and calcite phases were observed in the samples. However, unlike those red mud samples from the other regions, no TiO2 (rutile or anatase) or quartz were observed. Titanium was found to exist mainly in perovskite and andradite while the iron mainly existed in hematite and andradite. A new silico-ferrite of calcium and aluminum (SFCA) phase was found in samples treated at temperatures above 1100°C, and two possible formation pathways for SFCA were suggested. This is the first SFCA phase to be reported in thermally treated red mud, and this finding may turn PRM waste into a material resource for the iron-making industry. Titanium was found to be enriched in the perovskite phase after 1200°C thermal treatment, and this observation indicated a potential strategy for the recovery of titanium from PRM. In addition to noting these various resource recovery opportunities, this is also the first study to quantitatively summarize the reaction details of PRM phase transformations at various temperatures.

Effect of SiO2 on the Crystal Structure Stability of SFC at 1473 K (1200 °C)

Silico-ferrite of calcium (SFC) is a key intermediate phase in the sintering process of fine iron ores, and SiO2 plays an important role in the formation of SFC. In this work, the crystal structure stability of SFC synthesized at 1473 K (1200 °C) has been determined by X-ray diffraction, field-emission scanning electron microscopy, and X-ray absorption spectra. Synthesis of SFC was carried out under air at 1473 K (1200 °C) by mixing different amounts of SiO2 with Fe2O3 and CaCO3. The results show that the maximum solid solubility of SiO2 in the crystal structure of SFC does not exceed 6.11 wt pct at 1473 K (1200 °C); under these conditions, Fe2O3 begins to appear. The process of Si solution is closely related to the presence of a Ca channel composed of Ca octahedron in the crystal structure of SFC based on the results from the measurements of Ca K-edge X-ray absorption spectra. Si mainly occupies the center positions of the upper and lower tetrahedron adjacent to Ca channel. The length of Ca-Ca bond in Ca channel increases with the increasing of Si content. The crystal structure stability of SFC may be related to the structure of the Ca channel.

PLSR as a new XRD method for downstream processing of ores: – case study: Fe2+ determination in iron ore sinter

The use of high-speed detectors made X-ray diffraction (XRD) become an important tool for process control in mining and metal industries. Decreasing ore qualities and increasing prices for raw materials require a better control of processed ore and a more efficient use of energy. Traditionally quality control of iron ore sinter has relied on time-consuming wet chemistry. The mineralogical composition that defines the physical properties such as hardness or reducibility is not monitored. XRD analysis in combination with Rietveld quantification and statistical data evaluation using partial least-squares regression (PLSR) has been successfully established to determine the mineralogical composition and the Fe2+ content of iron ore sinter within an analysis time of less than 10 min per sample. A total of 35 iron ore sinter samples were measured and evaluated using PLSR and the Rietveld method. The results were compared with wet chemistry data. PLSR results show accuracy for the Fe2+ content of ±0.14%. No pure phases, crystal structures, or complex modeling of peak shapes are required. The Rietveld method was used to quantify the total phase composition of the samples. The Fe2+ content could be calculated from all phases present. Both methods take the full XRD pattern into account and can be simultaneously applied on the same measurement. PLSR was found to be the more robust method if only Fe2+ results are required. The Rietveld method helps predict other parameters such as the compressional strength of the sinter by monitoring all existing phases (e.g., larnite, C2S, or silico-ferrite of calcium and aluminum phases).

Effect of flame-front speed on the pisolite-ore sintering process

Flame-front speed has a large influence on sinter quality and production. To study the influence of flame-front speed on pisolite-ore sintering, five tests were conducted in a laboratory sinter pot. Through varying mix moisture, flame-front speeds ranging from 23.9 to 27.8 mm/min were obtained. The results revealed that when the flame-front speed increased from 23.9 to 27.7 mm/min, sinter productivity increased from 34.1 to 38.6 t/m2 d at relatively unchanged sinter strength. At higher moisture and flame front speeds, while productivity and TI both deteriorated, suggesting the optimum moisture and flame-front speed for the blend has been past. Similarly, with increasing flame-front speed sinter size distribution, as indicated by the Sauter mean size, initially improved but deteriorated as flame-front speed increased further to 27.8 mm/min. Mineralogy studies carried out on the sinters showed that faster flame-front speeds favored the formation of acicular SFCA (silico-ferrite of calcium and aluminum), while columnar SFCA was more abundant at slower flame-front speeds. With decreasing flame-front speed, sinter pore area results indicated that bubble coalescence and escape from the melt increased. But when flame-front speed reached 25.1 mm/min, changes in pore area were not as large.

X – RAY DIFFRACTION ANALYSIS OF IRON SINTER

Producers of iron sinter need precise phase analysis for effective operation supervision. It is not enough to consider only chemical composition, but it is beneficial to know the iron sinter mineralogical composition. Producers can utilize this information for better sinter mechanical properties adjustment and sinter yield improvement. When iron ore fines under 6mm are mixed with limestone flux, recycled materials and coke breeze, and are heated to about 1250°C the iron sinter is obtained. Produced iron sinter is in partial melting of mixture porous, but physically strong enough heterogeneous material, in which the iron bearing minerals are bonded together by range of different phases, where complex ferrite phases are very well known as SFCA (Silico-Ferrite of Calcium and Aluminum). Sinter is used as feed material for blast furnace. Using X-ray diffraction method and Rietveld method in the TOPAS program, the existing phases were identified both qualitatively and quantitatively.

The Formation Process of Silico-Ferrite of Calcium (SFC) from Binary Calcium Ferrite

Silico-ferrite of calcium (SFC) is a significant equilibrium crystalline phase in the Fe2O3-CaO-SiO2 (FCS) ternary system and a key bonding phase in the sintering process of fine iron ore. In this work, the formation process of SFC from binary calcium ferrite has been determined by X-ray diffraction and field-emission scanning electron microscopy. Experiments were carried out under air at 1473 K (1200 °C) by adding SiO2 and Fe2O3 into CaO·Fe2O3 (CF). It was found that the formation of SFC is dominated by solid-state reactions in the FCS ternary system, in which Fe2O3 reacts with CaO·Fe2O3 to form the binary calcium ferrite phase. The chemical composition of binary calcium ferrite is Ca2.5Fe15.5O25 and approximately Ca2Fe12O20 (CaO·3Fe2O3). Then Si4+ and Ca2+ ions take the place of Fe3+ ion in preference located on the octahedral layers which belongs to (0 0 18) plane of binary calcium ferrite. The crystal structure of binary calcium ferrite gradually transforms from orthorhombic to triclinic, and the grain is refined with the addition of silica due to the smaller radius of Si4+ ion. A solid solution SFC forms completely when the content of SiO2 reaches approximately 3.37 wt pct at 1473 K (1200 °C).

Induration Process of Pellets Prepared from Mixed Magnetite–35% Hematite Concentrates

To explore the possibility of successfully processing iron ore pellets with a high content of hematite, thermogravimetric tests were performed to study the induration process of pellets composed of a mixture of iron ore concentrates (magnetite and 35% wt of hematite). Thermogravimetric tests were performed nonisothermally from 25°C to 1400°C using two heating rates, 5 and 50°C/min. To identify the reactions involved and to follow the microstructural evolution throughout the induration process, selected tests were arrested at predetermined temperatures, and samples were rapidly cooled to room temperature for later characterization using X-ray diffraction (XRD) and scanning electron microscopy (SEM) along with energy dispersive spectroscopy (EDS). It was found that the formation of phases such as calcium ferrite (CF), magnesium ferrite (MF), silico-ferrites of calcium (SCF) and silico-ferrites of calcium and aluminum (SCFA) are influenced by the heating rate. The microstructure of the fired pellet processed at 50°C/min showed compact small grains of the sintered phase of the secondary hematite (SH), partially surrounded by a slag phase. In contrast, the fired pellet processed at 5°C/min exhibited a microstructure consisting of the SH phase with a faceted morphology surrounded by a relatively large amount of the slag phase. The results suggest that the pellet processed at 50°C/min had a more satisfactory response to the induration process.

Effect of B2O3 on phase compositions of high Ti bearing titanomagnetite sinter

The phase compositions of high Ti bearing titanomagnetite sinter with different contents of B2O3 were studied at 1623 K under an oxygen partial pressure of 5×10−3 atm. The results showed that B2O3 mainly gathered in Ca2SiO4 phase and formed 2CaO.xSiO2.2/3(1−x)(B2O3) phase. The appearance of B2O3 can restrain the precipitation of CaTiO3 and 2CaO.xSiO2.2/3(1−x)(B2O3) and promote the amount of Fe bearing mineral phase in the current experimental condition. In addition, the shape of CaTiO3 changed from bulk to strip type with the increase in B2O3 content. The formula of silicoferrite of calcium and aluminium changed from 1·6CaO.4·9Fe2O3.Al2O3.1·8SiO2 to 2·5CaO.5·9Fe2O3.Al2O3.2·6SiO2, and the formula of dicalcium (boron) silicate changed from 2CaO.SiO2 to 2CaO.0·33SiO4.0·45B2O3 when B2O3 content was from 0 to 2·0 mass-% in the sinter. The change of phase compositions with the addition of B2O3 in the sinter will be beneficial for improving its metallurgical properties.

An alternative to traditional iron-ore sinter phase classification

An alternative to the traditional quantification of iron ore sinter mineralogy is presented through the use of QEMSCAN instrumentation. The classification of minerals by QEMSCAN is based on chemical composition whilst the traditional classification of iron ore sinter mineralogy, through point counting, is based on morphology. The classification of iron ore sinter minerals through XRD is based on crystal structure. Advantages of the QEMSCAN technique include the ability to distinguish magnesio ferrites from calcio magnetites and the fact that calcium ferrite and silico-ferrite of calcium and aluminium (SFCA) distinction is not dependant on the sectioning of the sample mount.

Influence of Alumina on Iron Ore Sinter Properties and Productivity in the Conventional and Selective Granulation Sintering Process

Iron ore sinter constitutes a major proportion of blast furnace burden. Hence, its quality and consistency have a significant impact on blast furnace performance. Iron ore fines are the main source for sinter, and the chemical composition of the iron ore fines, together with the thermal conditions that blends are subjected to, plays an important role in forming the primary melt during the sintering process and accordingly determines the sinter structure and quality. Therefore, considerable importance has been placed on the chemical composition and consistency of iron ore fines, particularly in terms of alumina content. Due to depletion of high grade iron ore resources, alumina content in the iron ore fines is expected to increase gradually. Ore with higher alumina content is usually expected to be detrimental in forming the sinter matrix, if sintered alone, due to the low reactivity of alumina bearing minerals and the high viscosity of primary melts. The selective granulation process is a new sintering process for high alumina iron ore fines, and can eliminate the adverse effects of ‘hard to sinter’ or ‘unsuitable – for ironmaking’ ores. In the present work laboratory sintering experiments have been carried out with iron ore fines of different alumina level (2.00 to 5.46 mass-%) to know the influence of alumina on mineralogy, productivity, physical and metallurgical properties of sinter prepared by the conventional and the selective granulation process. With increasing alumina content in sinter of both the conventional and selective granulation process, the fractions of hematite and of silico-ferrites of calcium and alumina (SFCA) as well as the pore phase increased whereas the magnetite and silicate phases decreased. With increase in alumina content sinter productivity and tumbler index (T.I.) decreased, and metallurgical properties like sinter RDI and reducibility improved. However, sinter of the selective granulation process showed better results compared to the conventional process.

Sintering and heating reduction processes of alumina containing iron ore samples

Alumina containing iron ore samples (0·7–5·5%Al2O3) were sintered in a down draft sinter pot. The structures of the produced sinters were microscopically examined and the different phases developed were identified. Electron probe micro analyser (EPMA) was used for the quantitative analysis of Fe, Ca, Si, Mn and Al in different phases. Silico ferrite of calcium and alumina (SFCA) phase sinter was identified and its stability was Al2O3 content dependent. In sinter containing ≤1·5%Al2O3, SFCA was dissociated at higher temperatures to porous magnetite and silicate melt. In samples containing ≥2·5%Al2O3, SFCA was stable and its quantity increased with increasing alumina content. The non-isothermal reduction of sinter samples was carried out using a heating reduction technique with the facility of following up the high temperature phenomena during reduction process. X-ray observations, reduction rate profile and the gasification rate were used for the determination of high temperature properties of samples. In the gas–solid reduction region, Al2O3 increased the reduction rate (RDR) at the initial stage up to 1073 K and the RDR increases with increasing Al2O3 content up to 2·5%. At >1073 K, the RDR was relatively slower due to the presence of primary low melt slag containing alumina. For sinter containing >2·5%Al2O3, the lowering in RDR and total reduction degree (TRD) starts to decrease with increasing Al2O3 content as a result of the formation of SFCA phase which has a good reducibility and as a consequence of the decrease in the quantity of the primary low melt slag containing alumina. No more than 2·5%Al2O3 in the slag results in a harmful effect on the high temperature properties while higher alumina content in sinter leads to the formation of SFCA which improves the high temperature reduction properties.

In situX-ray diffraction analysis of iron ore sinter phases

Owing to the depletion of world lump iron ore stocks, pre-treated agglomerates of fine ores are making up a growing proportion of blast-furnace feedstock (∼80%). These agglomerations, or `sinters', are generally composed of iron oxides, ferrites (most of which are silicoferrites of calcium and aluminium, SFCAs), glasses and dicalcium silicates (C2S). SFCA is the most important bonding phase in iron ore sinter, and its composition, structural type and texture greatly affect its physical properties. Despite its prevalence and importance, the mechanism of SFCA formation is not fully understood.In situpowder X-ray diffraction investigations have been conducted into the formation of SFCA, allowing the study of the mechanism of its formation and the observation of intermediate phases with respect to time and temperature. Studies have been carried out to investigate the effects of changing the substitution levels of aluminium for iron. The use of the Rietveld method for phase quantification gives an indication of the order and comparative rates of phase formation throughout the experiments.

Microstructures of industrial sinters from Quadrilatero Ferrifero’s iron ores, Minas Gerais State, Brazil

Sinters correspond to the main burden of blast furnaces for pig iron production. The industrial sinters studied were produced with iron ores from Quadrilatero Ferrifero, which are hematite-rich ores. They are composed of four main phases: hematite, magnetite, silicoferrite of calcium and aluminium (SFCA) and silicates. Hematite corresponds either to a relict phase (pure Fe 2O 3) or to a restructured mineral; the relict phase can be, not rarely, related to specific fabrics of each iron ore mix employed in the industrial sintering process, while the restructured hematite represents particles grown by coalescence and/or partially reequilibrated (Al 2O 3-bearing). Magnetite is associated to SFCA, which can replace it, and it is also related to MgO-bearing silicate cores, which are favourable sites to develop it. SFCA is an important phase in sinters and, in controlled amount, it contributes to a good mechanical strength of these agglomerates. SFCA common types are massive, columnar and acicular. Pseudomorphs of massive SFCA after magnetite have been observed. The columnar type occurs at the relict hematite edges and in association with magnetite. Intergrowths of thin SFCA and silicates are identified too. Silicates composition ranges from Ca-rich (dicalcium silicate) to Fe–Ca-bearing, both with significant phosphorus content. These microstructural aspects play an important role in evaluating industrial sinters quality such as mechanical strength parameters and behaviour during reduction stages.

Crystal structure of calcium and aluminium silicoferrite in iron ore sinter

The crystal structures of real and synthetic iron ore sinters were compared. In the Fe2O3-CaO-Al2O3- SiO2 system, the crystal structure of a synthetic solid solution of CaO.2Fe2O3 (CF2) was studied by X-ray powder diffraction. As the Al2O3 content in pure CF2 increased, the formation of hemicalcium ferrite increased. On the other hand, CF2 content decreased and hematite increased as the SiO2 content increased. The crystal structure of CF2 with Al2O3 and SiO2 (calcium and aluminium silicoferrite, also known as silicoferrite of calcium and aluminium, SFCA) changed from monoclinic to triclinic, and the unit cell volume decreased when the Al2O3/SiO2 ratio increased. The solubilities of Al2O3 SiO2 in CF2 were 5-7 wt-% and 2-4 wt-%, respectively at 1250°C.

Stability of SFC (silico-ferrite of calcium): solid solution limits, thermal stability and selected phase relationships within the Fe2O3-CaO-SiO2 (FCS) system

Stability of SFC (silico-ferrite of calcium): solid solution limits, thermal stability and selected phase relationships within the Fe2O3-CaO-SiO2 (FCS) system

Stability of SFC (silico-ferrite of calcium) solid solution limits, thermal stability and selected phase relationships within the Fe2O3-CaO-SiO2 (FCS) system

Quenching experiments have been performed to investigate the thermal stability, solid solution limits, and selected phase relationships of SFC (silico-ferrite of calcium) within the Fe2O3-CaO-SiO2 (FCS) system. Experiments were performed in air over the temperature interval 1050–1260°C using a combination of synthetic oxide mixtures and SFC compositions which had been pre-synthesized at 1200°C. SFC forms a solid solution along a trend line between the theoretical end-members CF3 and C4S3. The maximum solid solution range occurs between compositions containing approximately 7.0 through to 11.7 wt% C4S3 component. The solution range is valid between 1060°C and 1240°C. Above 1240°C the compositional range narrows until the liquidus is reached. The maximum liquidus temperature for SFC is composition dependent with the highest melting point (T = 1252°C) recorded from a sample containing 9.0 wt% C4S3. Determination of ferrous iron content in SFC shows a range between 0.24 −0.37 wt% at 1200°C compared to 0.40-0.64 wt% at 1250°C. The absolute Fe2+ content is both temperature and composition dependent, with higher ferrous iron values measured at high temperature and high C4S3 contents. EPMA data, combined with the ferrous iron measurements, indicate a coupled substitution mechanism in SFC represented by the reaction 2(Fe3+) = (Ca2+, Fe2+) + Si4+. Data obtained in the present investigation combined with those available in the literature enable the construction of a series of isothermal sections showing phase relationships within the broader FCS system. These diagrams may be used as a guide to improving the understanding of fundamental sintering phase relations in the high iron corner of the FCS ternary system, as well as providing some insight into the compositional and thermal conditions required to maximize the stability of SFC phase in iron ore sinter.

Assimilation and Mineral Formation during Sintering for Blends Containing Magnetite Concentrate and Hematite/Pisolite Sintering Fines.

Blends for sintering in China commonly contain domestic magnetite concentrate and imported hematite or/and pisolite sintering fines. Interactions between these ores will, no doubt, have an important impact on sintering performance and sinter quality of these blends. Studies have been carried out to look at assimilation of sintering fines and mineral formation during sintering of such blends. The results show that at given mix composition, pisolite ores are easier to be assimilated than hematite and magnetite ores while magnetite shows the least assimilation. For the same ore, the assimilation depends on melt level and ore porosity. For this reason, higher assimilation was observed for porous ores or ores with high gangue content or when an easy-fusing ore was present in the blend. Studies on mineral phase formation of blends containing magnetite, hematite and pisolite indicate that good sinter mineralogy with increasing SFCA (silico-ferrite of calcium and aluminium) content is obtained with increasing Ore E (hematite). Substitution of Ore J (pisolite) for various hematite ores did not lead to significant changes in sinter mineralogy. The only change was the reduced level of primary hematite owing to the high reactivity of Ore J.