Diffusion coefficient of Ti^4+ in calcium ferrite/calcium titanate diffusion couple

This study investigated the interdiffusion of calcium ferrite/calcium titanate system in the time range of 0-120 min by the diffusion couple method in a CO/N2 reducing atmosphere at 700℃.The results show that after the diffusion reaction occurred,no longitudinal agglomerations were present on the substrate surface on the calcium titanate side.When the diffusion time was increased to 105 min,a net vacancy flow from calcium titanate to calcium ferrite might have occurred,causing the surface of the calcium ferrite substrate to collapse.The thickness of the diffusion layer of the calcium ferrite/calcium titanate system was about 17-48μm,which conforms to the parabolic law of diffusion.The diffusion coefficient and the Ti^4+concentration in the calcium ferrite/calcium titanate system are related.This shows an increase in the diffusion coefficient with the increase of Ti^4+concentration,and the diffusion coefficient value was in the range of 10^−12-10^−11 cm^2·s^−1.

Calcium ferrite formation characteristic during iron ore sintering with different oxygen atmospheres

It is generally known that the large formation amount of calcium ferrite is favorable for the iron ore sintering. The effects of sintering temperature and O2 content of inlet gas on the calcium ferrite formation characteristic of typical iron ores, including hematite, limonite, specularite and magnetite, were investigated. And the effect of O2 content on the microstructure of the roasted briquettes was also studied in detail. The results show the amount of calcium ferrite initially increases then decreases with the increase of the sintering temperature. The temperature of maximum calcium ferrite generation amount is determined as follows： for hematite and limonite it is 1275 ~C, whereas for specularite and magnetite, 1250℃. The maximum contents of calcium ferrite for hematite, limonite, specularite and magnetite under the optimal sintering temperature are 73%, 82%, 67% and 63%, respectively. Increasing O2 content of the airflow is advantageous to the formation of calcium ferrite. Relatively, the effect of O2 content on the calcium ferrite formation of magnetite is the most pronounced, while O2 content of inlet gas has little effect on the calcium ferrite formation of limonite.

Effect of Al_(2)O_(3)/SiO_(2) ratio on morphology of complex calcium ferrite

Al_(2)O_(3) and SiO_(2) greatly influence the formation of complex calcium ferrite,which is the main bonding phase in high basicity sinters.The effects of Al_(2)O_(3)/SiO_(2) ratios on the morphology of complex calcium ferrite were studied.The main mineral phases in the samples with different Al_(2)O_(3)/SiO_(2) ratios were CaFe_(2)O_(4) with a solid solution of Si or Al atoms and the silico-ferrite of calcium and aluminum.The results showed that the morphology of the complex calcium ferrite changed from lumpy to plate-like and acicular with increases in the SiO_(2) content and the Al_(2)O_(3)/SiO_(2) ratio.When the content of SiO_(2) was 4 wt.%,the main calcium ferrite morphology was acicular,and the number of macropores in the samples increased with the Al_(2)O_(3)/SiO_(2) ratio increasing.The first-principles analysis of the calcium ferrite crystal structure showed that adding SiO_(2) and Al_(2)O_(3) changed the growth mechanism of the CaFe_(2)O_(4) crystal,promoting the formation of platy and acicular complex calcium ferrite.The size of calcium ferrite was significantly smaller due to the increase in CaO-Fe_(2)O_(3)-SiO_(2)-Al_(2)O_(3) viscosity with increasing the Al_(2)O_(3)/SiO_(2) ratio.

Formation Characteristics of Calcium Ferrite in Low Silicon Sinter

Influence of sintering temperature, basicity and MgO content on the formation characteristics of calcium ferrite in low silicon sinter of Baotou Iron and Steel Company was studied by means of mini-sintering test and mineralographic microscope analysis. In addition, the suitable sintering parameters such as temperature and basicity were explored. The results found that optimum temperature for the formation of calcium ferrite is 1 280℃, the basicity of 2.5-2.8 is helpful to the development of acicular or columnar calcium ferrite, and MgO content in the low silicon sintering raw materials should be lower than 2.8 % because MgO can intensively inhibit the formation of calcium ferrite. And calcium ferrite in the sinter belongs to calcium ferrite with low calcium, which is different from that in ordinary sinter at home and abroad. So, it provided theoretical basis for promoting formation of calcium ferrite in low silicon sinter and improving properties of sinter.

Reduction kinetics of MgO-doped calcium ferrites under CO-N2 atmosphere

The effect of magnesia on calcium ferrite(CaO.Fe2O3)reduction by CO was examined by isothermal thermogravimetry.Samples of calcium ferrite added with 0,2,4,and 8 wt.%magnesia(abbreviated as CF,CF2M,CF4M,and CF8M)were prepared.Phase composition was analyzed by X-ray diffraction,and the results indicated that CF2M and CF4M are reduced to lower reduction degree and with lower apparent activation energy than CF;and CF8M with more MgO.Fe2O3 is reduced to a lower degree and with more difficulty compared with CF.Reduction rate analysis revealed that CF,CF2M,CF4M,and CF8M reductions are all typical two-step reactions with the order of CF→CWF(CaO.FeO.Fe2O3)→Fe.The apparent reduction activation energies of CF,CF2M,CF4M,and CF8M are 46.89,37.30,17.30,and 29.20 kJ/mol,respectively.Sharp analysis depicted that CF2M,CF4M,and CF8M reductions are all described by 2D Avrami–Erofeev(A–E)equation(A2)in the whole process,while CF reduction is first expressed by A2 and then by 3D A–E equation(A3).Different from shrinking core model,a new kinetic model for powdery samples reduction was proposed to illustrate the relationship among reduction rates,reduction routes,and model functions.

Effects of Compound Silicate Gangue on Formation of Complex Calcium Ferrite During Sintering Process

In order to improve the quality of sinter produced by Baiyunebo iron ore concentrate,the effects of compound silicate gangue containing kalium and natrium on formation of complex calcium ferrite were studied using mini-sintering test device,optical microscope（OM） analysis and scanning electron microscope（SEM）/energy dispersive X-ray spectroscopy（EDX） analysis.The results show that with the increasing of K2O and Na2O content in sintering samples,K2O and Na2O highly enriched in the glassy phases,the contents of complex calcium ferrite and hematite in sinter decreased and the porosity increased;the mineral microstructure exhibited large cavities and thin framework and became inhomogeneous.The compound silicate gangue containing kalium and natrium significantly inhibited the generation of complex calcium ferrite（SFCA）.When the content of K2O and Na2O was higher in samples,a part of K2O and Na2O entered into complex calcium ferrite,which caused the fine grains of SFCA with acicular and columnar to agglomerate into coarse grains,and its bonding effect to iron oxides reduced.The content of K2O and Na2O in sinter had effects on quantity,composition and morphology of complex calcium ferrite.

Preparation of calcium ferrite by flue gas desulfurization gypsum

At present,the continuous accumulation of the flue gas desulfurization(FGD)gypsum in steel plants leads to the serious environmental issues and resource waste.To achieve green and sustainable development for the steel industry,it is significant to improve the usage of by-product gypsum.Employing the sintering FGD gypsum,ferric oxide,and graphite carbon as raw materials,the effects of the carbon content,reaction time,and molar ratio of CaO to Fe_(2)O_(3)on the desulfurization rate of gypsum were studied based on the orthogonal experiment.The results show that the order of the three influencing factors on the desulfurization rate of FGD gypsum is:molar ratio of CaO to Fe_(2)O_(3)>reaction time>carbon content.Under the optimal conditions of 20 wt.%carbon content,4 h reaction time,and 1:1 molar ratio of CaO to Fe_(2)O_(3),the desulfurization rate of desulfurization gypsum is 95.79%,and 97.57 wt.%of calcium ferrite appears in the solid product,which can be used as sintering additive to increase the economic benefits of enterprises and realize the green ecological development mode of internal generation and internal digestion of solid waste in iron and steel enterprises.

Effect of FeO concentration in sinter iron ore on reduction behavior in a hydrogen-enriched blast furnace

Japan started the national project“COURSE 50”for CO_(2) reduction in the 2000s.This project aimed to establish novel technologies to reduce CO_(2) emissions with partially utilization of hydrogen in blast furnace-based ironmaking by 30%by around 2030 and use it for practical applications by 2050.The idea is that instead of coke,hydrogen is used as the reducing agent,leading to lower fossil fuel consumption in the process.It has been reported that the reduction behavior of hematite,magnetite,calcium ferrite,and slag in the sinter is different,and it is also considerably influenced by the sinter morphology.This study aimed to investigate the reduction behavior of sinters in hydrogen enriched blast furnace with different mineral morphologies in CO-CO_(2)-H2 mixed gas.As an experimental sample,two sinter samples with significantly different hematite and magnetite ratios were prepared to compare their reduction behaviors.The reduction of wustite to iron was carried out at 1000,900,and 800℃ in a CO-CO_(2)-H2 atmosphere for the mineral morphology-controlled sinter,and the following findings were obtained.The reduction rate of smaller amount of FeO led to faster increase of the reduction rate curve at the initial stage of reduction.Macro-observations of reduced samples showed that the reaction proceeded from the outer periphery of the sample toward the inside,and a reaction interface was observed where reduced iron and wustite coexisted.Micro-observations revealed three layers,namely,wustite single phase in the center zone of the sample,iron single phase in the outer periphery zone of the sample,and iron oxide-derived wustite FeO and iron,or calcium ferritederived wustite'FeO'and iron in the reaction interface zone.A two-interface unreacted core model was successfully applied for the kinetic analysis of the reduction reaction,and obtained temperature dependent expressions of the chemical reaction coefficients from each mineral phases.

Effects of Mineral Composition and Microstructure on Crack Resistance of Sintered Ore

Vickers indentation test was used to study the effects of mineral composition and microstructure on crack resistance of sintered ore, and the initiation and propagation of cracks in different minerals contained in sintered ore were examined. The results indicate that the microstructure of calcium ferrites is a major factor influencing crack resistance of sintered ore. Finer grain size of calcium ferrite will lead to higher cracking threshold and better crack resistance of sintered ore. The formation of calcium ferrite with fine grain size during sintering process is favorable for crack resistance of sintered ore.

Amelioration of the mineral compositions and microstructures of the special ore sinters

Mineral compositions and microstructures of fluorine bearing magnetite concentrate and vanadium titanomagnetite concentrate are studied. The results show that a boride and a catalytic oxidation agent have great effects on ameliorating the mineral compositions and microstructures of these sinters, and the catalytic agent is more effective. Comparing these two kinds of ores, the performance of fluorine bearing magnetite concentrate is better. Addition of 0.01% catalyst into fluorine bearing magnetite concentrate can increase calcium ferrite content from 15% 20% up to 34% 39%, and its microstructure changes from large cavities and thin framework into intermediate cavities and thick framework, and the microcracks disappear.

Determination method of high-temperature characteristics of iron-ore sintering based on n(Fe2O3)/n(CaO)

The high-temperature characteristics of iron ores play important roles in optimizing ore proportion of sintering,which are tested by using iron-ore fines and analytical reagent CaO as raw materials.Two calculation methods of CaO addition amount based on binary basicity(basicity method)and n(Fe2O3)/n(CaO)(mole ratio method),respectively,were employed to evaluate the liquid phase fluidity(LPF)and the capability of calcium ferrite formation(CCFF)of iron ores.The results show that the rule of LPF of iron ores under the mole ratio method is different from that with basicity method.The LPF measured by basicity method has a linear positive correlation with the SiO2 content,and there is no linear relationship between LPF and Al2O3 content or mass loss on ignition,which are inconsistent with the results of the previous study.And the results of CCFF with low SiO2 content(<3 wt.%)or high SiO2 content(>7 wt.%)based on basicity method cannot reflect the true CCFF.The mole ratio method could successfully solve this problem by reducing the effect of CaO addition amount changes caused by SiO2 content of iron ores.

Primary slag formation behavior during reduction process of SFCA-I and SFCA

Because the formation behavior of primary slag which decomposed from complex calcium ferrite(SFCA-I and SFCA)is not quite clear,the migration behavior of CaO and Al_(2)O_(3) derived from high basicity or high alumina sinter is always worth studying.The reducibility of three representative sinter samples and the formation behavior of primary slag during reduction process were investigated via X-ray diffraction,scanning electron microscopy,and energy-dispersive spectroscopy characterization.The results show that the reducibility of high basicity sinter is superior to that of high alumina sinter.Minerals with poor reducibility like hercynite and brownmillerite and with large-grained particles like free alumina and silica form in sinters with basicity of 2.4 and Al_(2)O_(3) content of 4 wt.%,respectively.The appearance of these minerals can well explain the reduction stagnation phenomenon occurring in these sinter samples.The migration behavior of CaO and Al_(2)O_(3) during slag formation process is different.CaO can easily combine with SiO2 to form silicate phase or firstly form calcium-rich ferro-aluminate solid solution and then transform to silicate phase,while Al_(2)O_(3) firstly combines with CaO and FeO to form solid solution and then,gradually combines with SiO2 to form calcium aluminum silicate phase.

Production of phenolic compounds in catalytic pyrolysis of bagasse lignin in the presence of Ca_(0.5)Pr_(0.5)FeO_(3)

Herein,sodium dodecylbenzene sulfonate(SDBS)was used as a template to control the synthesis of Ca_(0.5)Pr_(0.5)FeO_(3).Its microstructure,composition,and morphology were detected via X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR),and scanning electron microscopy(SEM).The properties of catalyzing bagasse lignin(BL)pyrolysis were determined by thermogravimetric analysis(TG)test and evaluation of a fixed bed alumina microreactor.The results show that the sample Ca_(0.5)Pr_(0.5)FeO_(3)regulated by SDBS has a cubic crystal phase,and the addition of SDBS does not cause phase transition.Moreover,when the SDBS concentration is 0.10 mol/L,the particle size is 200-500 nm and the specific surface area is 11.26 m^(2)/g.The yields of gas,liquid,and solid products in the BL catalytic pyrolysis are 39.58 wt%,26.76 wt%and 32.36 wt%,respectively.The contents of CO_(2)and CO decrease from 54.07%and 4.98%to 45.29%and 3.23%,respectively.The liquid products are mainly guaiacol,syringol,and phenol,and the total selectivity of phenols is 83.67%,accompanied by a small amount of non-aromatic oxygen-containing compounds(five-membered ring(furan)or ester).Compared with the BL pyrolysis and Ca_(0.5)Pr_(0.5)FeO_(3)catalytic pyrolysis products,the selectivity of guaiacol compounds increases by43.26%,while those of syringol compounds and phenylketones decrease by 30.08%and 3.39%,respectively.The selectivity of 2,6-dimethoxyphenol is 28.37%.After five catalytic pyrolysis-regeneration cycles,the characteristic peaks of the catalyst do not change significantly and the particles are uniform,suggesting that the catalyst has good crystal phase stability and regeneration stability.