Silico-ferrite Of Calcium Research Articles

Effects of the Bulk Fe<sub>2</sub>O<sub>3</sub> Concentration on the Controlled Solidification of “Fe<sub>2</sub>O<sub>3</sub>”–CaO–SiO<sub>2</sub> Liquids in Air

The solidification of FeO-CaO-SiO liquids in air at a controlled cooling rate of 2.0 K/s for a range of FeO concentrations in the bulk has been investigated. The compositions investigated were selected such that the bulk compositions were within the hematite primary phase field and had a CaO/SiO ratio of 3.46 wt/wt. Non-equilibrium phase assemblages were formed for all bulk compositions investigated. Specifically, the silico ferrite of calcium (SFC) phase was not formed on cooling. The microstructures and proportions of the phase assemblages formed were found to vary with the FeO concentration in the bulk.

Effect of Cooling Rate on the Controlled Solidification of “Fe<sub>2</sub>O<sub>3</sub>”–CaO–SiO<sub>2</sub> Liquids in Air in Synthetic Iron Ore Sinter

The cooling rate of the liquid oxide can be controlled in industrial sintering processes through the draft pressure and has the potential to influence microstructure formation. The solidification of a liquid within the hematite primary phase field in the ternary FeO-CaO-SiO system in air was undertaken at different cooling rates to determine the impact of cooling rate on the formation of product microstructures. Samples with a bulk composition of 72.7 wt% FeO and a CaO/SiO ratio of 3.46, were cooled from 1623 K (1350°C) at 2 K/s, 0.5 K/s, 0.1 K/s and 0.01 K/s and quenched at 5 K temperature intervals from 1533 K (1260°C) to 1453 K (1180°C). During cooling, four stages of phase assemblage formation were consistently observed at all cooling rates; in order of formation these are, Liquid +hematite (I), Liquid+hematite+dicalcium silicate(CS)(II), Liquid+CS+calcium diferrite (CF)(III) and CS+CF+calcium ferrite (CF)(IV). An intergrowth of silico-ferrite of calcium and aluminium-I (SFCA-I) and CaFeFeO was observed to form in some conditions in regions free of hematite, present in liquids solidifying at 0.5 K/s and 0.1 K/s. The sizes and shapes of microstructures were observed to systematically change with cooling rate, with a slower cooling rate typically resulting in coarser coupled microstructures and larger individual crystals. A larger proportion of coupled microstructures are observed at slower cooling rates, this appears to be related to the degree of undercooling prior to the nucleation of new phases. The equilibrium silico-ferrite of calcium (SFC) phase was not observed at any of the cooling rates investigated.

Effect of Basicity on Sintering Behavior and Metallurgical Properties of High-Chromium Vanadium-Titanium Magnetite

Basicity has an important effect on the behavior of high-chromium vanadium-titanium magnetite (HCVTM) sinter. The effect of basicity on sintering, reduction, and softening-melting properties was investigated in an experiment-scale sinter pot. The results showed that with the basicity increasing from 1.7 to 2.5, the vertical sinter speed, the productivity, the particle size, the reduction disintegration index (RDI), and the reducibility index (RI) increased. The yield increased first and then decreased, while the tumble index (TI) had the opposite trend. The perovskite content increased first and then stabilized, and the silico-ferrite of calcium and aluminum (SFCA) increased from the basicity of 2.3. The HCVTM sinter had a better tendency of the softening behavior with the basicity increase. In addition, the melt droplet comprehensive index S increased, which indicated that the increase in basicity negatively affected the droplet performance. As the basicity increased, the comprehensive index value increased. Considering the adverse effect of basicity on softening-melting properties, the best recommended value is 2.3.

Reaction Characteristics Between Sinter and Serpentine

In recent years, adding an appropriate amount of serpentine into a blast furnace (BF) is considered to be a popular countermeasure to cope with the usage of iron ores with high Al2O3 content; this is because MgO in serpentine can improve the fluidity and desulphurization ability of BF slag with high Al2O3. Aimed to better understand the reaction characteristics between sinter and serpentine, a series of experiments were carried out between 1100 °C and 1300 °C under air atmosphere with a sinter to serpentine ratio between 100:1 and 100:6. The results indicated that higher temperatures and greater serpentine additions led to increased magnesioferrite formation. Samples with a sinter-to-serpentine ratio of 100:1 exposed to temperatures below 1300 °C did not have sufficient activation energy for diffusion to occur between the sinter and serpentine under the conditions to form magnesioferrite. But at 1300 °C, large amounts of magnesioferrite and silicate phases could be observed, and the major phases are magnesioferrite phase. At higher serpentine additions, where the sinter-to-serpentine ratio exceeded 100:3, the magnesioferrite reaction could be completed after reaching 1200 °C. Also, the serpentine first reacted with hematite followed by the reaction with SFCA (Silico-Ferrite of Calcium and Aluminum). The SiO2 in the serpentine formed silicate phases with CaO/Fe2O3 in the sinter. The findings from this work may provide guidelines for adding serpentine into BF and improve the operational performances of BF.

Structural elucidation of triclinic and monoclinic SFCA-III - killing two birds with one stone.

A part of the system CaO-SiO2-Al2O3-Fe2O3-MgO which is of relevance to iron-ore sintering has been studied in detail. For a bulk composition corresponding to 10.45 wt% CaO, 5.49 wt% MgO, 69.15 wt% Fe2O3, 13.37 wt% Al2O3 and 1.55 wt% SiO2 synthesis runs have been performed in air in the range between 1100 and 1300°C. Products have been characterized using reflected-light microscopy, electron microprobe analysis and diffraction techniques. At 1250°C, an almost phase-pure material with composition Ca2.99Mg2.67Fe3+14.58Fe2+0.77Al4.56Si0.43O36 has been obtained. The compound corresponds to the first Si-containing representative of the M14+6nO20+8n polysomatic series of so-called SFCA phases (Silico-Ferrites of Calcium and Aluminum) with n = 2 and is denoted as SFCA-III. Single-crystal diffraction investigations using synchrotron radiation at the X06DA beamline of the Swiss Light Source revealed that the chemically homogenous sample contained both a triclinic and monoclinic polytype. Basic crystallographic data are as follows: triclinic form: a = 10.3279 (2) Å, b = 10.4340 (2) Å, c = 14.3794 (2) Å, α = 93.4888 (12)°, β = 107.3209 (14)° and γ = 109.6626 (14)°, V = 1370.49 (5) Å3, Z = 2, space group P{\overline 1}; monoclinic form: a = 10.3277 (2) Å, b = 27.0134 (4) Å, c = 10.4344 (2) Å, β = 109.668 (2)°, V = 2741.22 (9) Å3, Z = 4, space group P21/n. Structure determination of both modifications was successful using diffraction data from the same allotwinned crystal. A description of the observed polytypism within the framework of OD-theory is presented. Triclinic and monoclinic SFCA-III actually correspond to the two possible maximum degree of order structures based on OD-layers containing three spinel (S) and one pyroxene (P) modules (〈S3P〉). The existence of SFCA-III in industrial iron-ore sinters has yet to be confirmed. Polytypism is likely to occur in other SFCA-members (SFCA, SFCA-I) relevant to sintering as well, but has so far been neglected in the characterization of industrial samples. Our results shed light on this phenomenon and may therefore be also helpful for better interpretation of the powder diffraction patterns that are used for phase analysis of iron-ore sinters.

Controlled Solidification of Liquids Within the SFC Primary Phase Field of the “Fe2O3”-CaO-SiO2 System in Air

To provide fundamental information on the phases and microstructures formed during sintering, a liquid with a bulk composition within the silico ferrite of calcium (SFC) primary phase field in the ternary “Fe2O3”-CaO-SiO2 system in air was solidified at a controlled rate. Samples of a bulk composition with a CaO/SiO2 ratio of 4.00 and 69.24 wt pct Fe2O3, were cooled from 1623 K (1350 °C) at 2 K/s, with samples quenched at temperatures between 1513 K (1240 °C) to 1453 K (1180 °C). The silico ferrite of calcium and aluminium I (SFCA-I) and Ca7.2Fe 0.8 2+ Fe303+O53 phases were observed to form an intergrowth (‘SFC-I’) rather than the anticipated SFC phase. Solidification was found to occur in three stages, Liquid + ‘SFC-I’, Liquid + ‘SFC-I’ + C2S + CF2, and C2S + CF2 + CF, where C2S denotes dicalcium silicate, CF denotes calcium ferrite and CF2 denotes calcium diferrite. The phases formed and the solidification sequence differ from those predicted under equilibrium and Scheil–Gulliver Cooling. Although not directly applicable to industrial operations, this research clearly shows that the formation of both the SFCA and SFCA-I phase in iron ore sinters is controlled by kinetic processes rather than equilibrium conditions.

Microstructure Evolution During Controlled Solidification of “Fe2O3”-CaO-SiO2 Liquids in Air

The principal chemical components in iron ore sintering are Fe2O3, CaO, and SiO2. This sintering process consists of three key steps: heating, holding at peak temperature, and cooling. During the cooling stage, a liquid oxide solidifies to form the final sinter microstructures. To investigate the fundamental processes taking place during the cooling of sinters, a new experimental technique has been developed that allows the stages of solidification to be determined in isolation, rather than inferred from the final microstructures. Fe2O3-CaO-SiO2 oxide samples of a bulk composition having a CaO/SiO2 mass ratio of 3.46 and 73.2 wt pct Fe2O3 were cooled in air from 1623 K (1350 °C) at 2 K/s, quenched at 5 K temperature intervals from 1533 K to 1453 K (1260 °C to 1180 °C), and analyzed using Electron Probe Micro X-Ray Analysis (EPMA). During cooling, four distinct stages were observed, consisting of the phase assemblages Liquid + Hematite (I), Liquid + Hematite + C2S(II), Liquid + C2S + CF2(III), and C2S + CF2 + CF (IV). This solidification sequence differs from that predicted under equilibrium and Scheil–Gulliver Cooling. Importantly, no Silico-Ferrite of Calcium (SFC) phase was observed to form on solidification of the liquid. Based on the microstructures formed and liquid compositions, measured by EPMA, it was demonstrated that kinetic factors play a major role in determining the phases and microstructures formed under the conditions investigated.

Investigation of structural and electrical properties of silico-ferrite of calcium (SFC) in the Fe2O3–CaO–SiO2 system synthesized by solid-state reaction

Silico-ferrite of calcium (SFC) was synthesized through a solid-state route. The phase purity of the sample was characterized by means of X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). X-ray absorption fine structure (XAFS) spectroscopic analysis was used to determine the structure, and the results showed that SFC possesses more Fe–O tetrahedral structure than similarly structured silico-ferrites of calcium and aluminum (SFCA) and verified the existence of a small amount of Fe2+ in SFC. Ac impedance spectroscopy was used to evaluate the electrical properties of SFC, and the results showed that SFC had the properties of a negative temperature coefficient (NTC) thermosensitive material, and indicated that Si4+ donor doping leads to free electrons during the synthesis of SFC. The correlated barrier hopping (CBH) mechanism was proposed for the ac conduction mechanism of SFC, and Si4+ donor-doped SFC exhibited high electronic conductivity at temperatures greater than 102 °C.

Thermodynamic Modelling of Iron Ore Sintering Reactions

Silico-ferrite of calcium and aluminum (SFCA) is one of the most commonly-produced phases in fluxed iron-ore sintering, and has long been regarded as an important bonding phase in industrial sinters. It is thus considered to have a significant effect on sinter quality. In this study, a solid solution model and database has been developed for the SFCA phase, and has been incorporated into the thermodynamic software, Multi-Phase Equilibrium (MPE). MPE calculations were compared with the in situ X-ray powder diffraction (XRD) observations of the formation of SFCA phase during sintering. The effects of the raw material composition, temperature and the oxygen partial pressure on the formation of mineral phases in the sinter, as well as the viscosity of the melt formed during sintering under equilibrium conditions, were modelled using MPE. The results show that the formation of SFCA phase can be promoted by increasing oxygen partial pressure and basicity of the raw material. Increases of Al2O3 and MgO content have no significant effect on the SFCA formation under equilibrium condition. The increase of oxygen partial pressure (10−3 atm or above) and basicity also leads to a decrease in melt viscosity, which enhances the fluidity of the melt, and hence, the assimilation of the sinter. However, increases of Al2O3 and MgO result in the increase of melt viscosity.

A Phase Equilibrium of the Iron-rich Corner of the CaO–FeO–Fe<sub>2</sub>O<sub>3</sub>–SiO<sub>2</sub> System in Air and the Determination of the SFC Primary Phase Field

Experimental studies of the CaO-FeO-Fe2O3-SiO2 system in air from 1 200 degrees C to 1 260 degrees C have been carried out in the Fe -rich region with particular focus on the phase equilibria associated with silico-ferrite of calcium (SFC) phase. The measurements have been made possible through the use of a modified experimental technique involving high temperature equilibration and rapid quenching followed by electron probe X-ray microanalysis (EPMA). Isothermal sections for 1 220 degrees C, 1 240 degrees C, 1 255 degrees C and 1 260 degrees C are reported providing information on the limits of stability of the SFC solid solution and the liquidus in the SFC primary phase field for the first time.

Effect of Gas Atmosphere on the Phase Chemistry in the CaO–FeO–Fe<sub>2</sub>O<sub>3</sub>–SiO<sub>2</sub> System Related to Iron Ore Sinter-making

Experimental phase equilibria studies have been carried out to investigate the phase chemistry in the CaO Fe2O3-SiO2 system related to the iron ore sinter making in 1 atm CO2 at 1 200-1 230 degrees C. The measurements have been performed using a modified experimental technique involving high temperature equilibration and rapid quenching followed by electron probe X-ray microanalysis (EPMA). In contrast to the equilibria in air, the experimental results show that no silico-ferrite of calcium (SFC solid solution) is formed in 1 atm CO2 within the temperature range investigated; that is, the SFC phase is not stable at liquidus temperatures. The liquidus for the iron -rich corner of the CaO Fe2O3-SiO2 system in CO2 atmosphere for the spinel (magnetite Fe3O4) and dicalcium silicate (C2S = Ca2SiO4) primary phase fields has been proposed.

Effect of Cooling Methods on the Strength of Silico-ferrite of Calcium and Aluminum of Iron Ore Sinter during the Cooling Process

In the iron making process, a high mechanical strength is favorable for iron ore sinters in the blast furnace, and the bonding phase is regarded as one of the key components that determines the quality of the iron ore sinter, in which the silico-ferrite of calcium and aluminum (SFCA) is one of the typical phases. In this study, synthesized samples with different SFCA mass fractions were prepared to study the effect of different cooling methods on the strengths of the SFCA samples. The results showed that the strength of a sample could be improved by increasing the SFCA content during a temperature change. Further, the test results for the compressive strength suggested that the SFCA had a positive effect on the strength of the iron ore sinter during cooling, with slow cooling being significantly effective at preventing the generation of thermal stress. Moreover, the Biot number was introduced to normalize all of the cooling methods. The results showed that higher mechanical strengths for iron ore sinters will be obtained with higher SFCA content and lower Biot numbers, which will guide the evaluation of mechanical strength of iron ore sinter after the cooling process in industry.

Combined effect of MgO and basicity varied by different dolomite and burnt lime addition on sintering performance of magnetite concentrates

ABSTRACT In this paper, the combined effects of MgO and basicity (CaO/SiO2), varied by changing the ratio of dolomite and burnt lime to sinter mix, on sintering of magnetite concentrates were investigated in pilot scale. Mineralogy of sinter with various MgO and basicity was analyzed by light microscope and scanning electron microscope (SEM). The results show that increasing MgO led to lower sinter strength and higher fuel consumption, while increasing basicity behaved in the opposite way. Regardless of MgO content, minimum sinter strength occurred in basicity 1.2–1.6, which should be avoided in sinter production. Less SFCA (silicoferrites of calcium and aluminum) and more magnetite were observed in sinter with higher MgO content. Aside from hematite and magnetite, the dominant phases in sinter with low (0.8) and high (2.2+) basicity were glass and SFCA respectively. Sinter with basicity 1.6 had highest crack density and this would inevitably reduce its cold strength.

The Macroscopic Flow direction and Microscopic Distribution of Mg in Sintered Products and Its Influence

Magnesium flux is an indispensable part of sintering materials. Its distribution and reaction behavior have an important influence on the quality of the sinter. Studying the macroscopic flow direction and microscopic distribution of MgO in sintered products is important for guiding the matching of ore for sintering and adjusting sintering technical parameters. The macroscopic flow direction situation of MgO in all sintered products was studied using X-Ray diffractometer and X-Ray fluorescence analyzer. In addition, the microscopic distribution characteristics of Mg element in the main sintered products were studied using scanning electron microscope and energy dispersive spectrometer. The results of this research are as follows. The Mg contents of sintered products are in the following increasing order: sinter ore, sinter internal return ore, and blast furnace return ore. The Mg content of the 1 to 2 mm grain size is the highest in the sinter internal return ore and blast furnace return ore, which is related to the granular distribution of magnesium flux. Moreover, the Mg content of sintered minerals are in the following decreasing order: magnetite; silico-ferrites of calcium and aluminium (SFCA); and silicate and hematite, and the Mg dissolving in the SFCA has a tendency to make the SFCA morphology coarse. The diffusion range of Mg in a large dolomite particle is much narrower, and it mainly stays at the position where the dolomite is located. In the actual sintering process, Mg increases the viscosity of the sintering liquid phase and prevents the oxidation of magnetite, which is not conducive to the improvement of the strength of the sinter.

Flow and penetration behaviours of liquid phase on iron ore substrate and their effects on bonding strength of sinter

ABSTRACTThe flow and penetration behaviours of the liquid phase on nuclei ore play crucial roles in the bonding of the sinter. In this study, the flow and penetration tests of the liquid phase and simulation tests were conducted to study the effects of SiO2 and Al2O3 contents of the adhering fines on the flow area (FA) and maximum penetration depth (MPD) through the micro-sintering method. The effects of the FA and MPD on the bonding strength of the sinter were investigated by a bonding test of quasi-particles. The results show that during the assimilation between the primary liquid phase and haematite ore, the gangues dissolve in the primary liquid phase, promoting the transformation of quasi-calcium ferrite into quasi-calcium silicate liquid. After solidification and precipitation, the two liquids turn into the silico-ferrite of calcium and aluminium and Fe2O3-bearing silicates. Increasing SiO2 or Al2O3 content decreases the FA by raising the viscosity of the primary liquid phase. However, it increases the MPD by reducing the quaternary basicity of the adhering fines and elevating the wettability of the liquid phase. The FA has a positive effect on the bonding strength of the sinter, whereas the effect of the MPD may be indirectly negative.

Effect of Aluminum Dissolved in Hematite on Formation of Calcium Ferrites at 1473 K

Silico-ferrite of calcium and aluminum (SFCA) is a primary bonding phase, which determines the quality of high basicity sinter used widely as raw materials in blast furnace ironmaking. Calcium ferrite containing aluminum (CFA) is a key precursor during the formation process of SFCA and is influenced by Al2O3 content in iron ore fines. In this work, the experiment was conducted to investigate the effect of the aluminum occurrence state on the formation of calcium ferrite by the simulating sintering process of different types of aluminum-bearing hematite at 1473 K. X-ray diffraction, optical microscope, scanning electron microscopy, and energy-dispersive spectroscopy were used to characterize the mineral change in sintered samples. The result shows that the formation of CFA was promoted by both granular Al2O3 wrapped in Fe2O3 and aluminum dissolved in Fe2O3 solid solution, especially for the latter, due to the formation of the CFA directly from the reaction between Fe2O3-Al2O3 solid solution and CaO, and the aluminum content in CFA formed was influenced obviously by the solid solution amount of alumina into Fe2O3 as raw materials.

Study of the enrichment characteristics of sinter by alkali metal vapors

The existence forms and distributions of potassium (K) and sodium (Na) in sinter were studied by simulating the actual situations of K and Na cycle and enrichment in the blast furnace (BF). The results show that the K and Na vapor reacted with the main phase of the sinter and formed two “enrichment layers” in periphery, then the sinter samples were analyzed by means of XRD and SEM. The analysis results indicate that the main phases in light “enrichment layer” are K and Na compounds (ferrite, silicates and aluminates), iron oxide (Fe2O3) and silico-ferrite of calcium and aluminum (SFCA); while the dark “enrichment layer” mainly exists the potassium and sodium oxides. In addition, the adsorption of the sinter on K and Na vapor is the mixed adsorption of chemical and physical, and the chemical adsorption is the main adsorption method.

Basic characteristics of Australian iron ore concentrate and its effects on sinter properties during the high-limonite sintering process

The basic characteristics of Australian iron ore concentrate (Ore-A) and its effects on sinter properties during a high-limonite sintering process were studied using micro-sinter and sinter pot methods. The results show that the Ore-A exhibits good granulation properties, strong liquid flow capability, high bonding phase strength and crystal strength, but poor assimilability. With increasing Ore-A ratio, the tumbler index and the reduction index (RI) of the sinter first increase and then decrease, whereas the softening interval (ΔT) and the softening start temperature (T10%) of the sinter exhibit the opposite behavior; the reduction degradation index (RDI+3.15) of the sinter increases linearly, but the sinter yield exhibits no obvious effects. With increasing Ore-A ratio, the distribution and crystallization of the minerals are improved, the main bonding phase first changes from silico-ferrite of calcium and aluminum (SFCA) to kirschsteinite, silicate, and SFCA and then transforms to 2CaO·SiO2 and SFCA. Given the utilization of Ore-A and the improvement of the sinter properties, the Ore-A ratio in the high-limonite sintering process is suggested to be controlled at approximately 6wt%.

Effects of Gibbsite, Kaolinite and Al-rich Goethite as Alumina Sources on Silico-Ferrite of Calcium and Aluminium (SFCA) and SFCA-I Iron Ore Sinter Bonding Phase Formation

Effects of Gibbsite, Kaolinite and Al-rich Goethite as Alumina Sources on Silico-Ferrite of Calcium and Aluminium (SFCA) and SFCA-I Iron Ore Sinter Bonding Phase Formation

Effect of Addition of Mill Scale on Sintering of Iron Ores

Iron-rich (65 to 70 pct total Fe) mill scale generated during processing by steel mills can be recycled by using it as a ferrous raw material in the sintering process. The effect of mill scale addition on the phase formation of sintered specimens from an industrial sinter blend containing 0 to 15 wt pct mill scale was examined, and the mineral phases formed during sintering under various conditions (T = 1523 K to 1598 K [1250 °C to 1325 °C] and gas compositions of pO2 = 0.5, 5 and 21 kPa) were quantitatively measured. For samples sintered in air (pO2 = 21 kPa), there was negligible effect of mill scale addition on the phases formed. The oxidation of the mill scale was complete, and phases such as Silico-Ferrite of Calcium and Aluminum (SFCA), SFCA-I, and hematite dominated. Under lower oxygen partial pressures (pO2 = 0.5 or 5 kPa), and throughout the temperature range examined, the mill scale was converted to magnetite, with the extent of reaction controlled by the hematite-magnetite conversion kinetics. When sintered in the gas mixture with pO2 = 5 kPa, an increase in the mill scale content from 0 to 15 wt pct resulted in a decrease of hematite and total SFCA phases and a corresponding increase in the amount of magnetite which formed. The oxidation of wustite in mill scale to magnetite decreased the local partial pressure of O2 and increased sintering temperature, which promoted the decomposition of hematite.