Staged reaction kinetics and characteristics of iron oxide direct reduction by carbon

Staged reduction kinetics and characteristics of iron oxide direct reduction by carbon were studied in this work. The characteristics were investigated by simultaneous thermogravimetric analysis, X-ray diffraction (XRD), and quadrupole mass spectrometry. The kinetics parameters of the reduction stages were obtained by isoconversional (model-free) methods. Three stages in the reduction are Fe₂O₃→Fe₃O₄, Fe₃O₄→FeO, and FeO→Fe, which start at 912 K, 1255 K, and 1397 K, respectively. The CO content in the evolved gas is lower than the CO₂ content in the Fe₂O₃→Fe₃O₄ stage but is substantially greater than the CO₂ contents in the Fe₃O₄→FeO and FeO→Fe stages, where gasification starts at approximately 1205 K. The activation energy (E) of the three stages are 126–309 kJ/mol, 628 kJ/mol, and 648 kJ/mol, respectively. The restrictive step of the total reduction is FeO→Fe. If the rate of the total reduction is to be improved, the rate of the FeO→Fe reduction should be improved first. The activation energy of the first stage is much lower than those of the latter two stages because of carbon gasification. Carbon gasification and Fe_xO_y reduction by CO, which are the restrictive step in the last two stages, require further study.     

Phase transformation behavior of titanium during carbothermic reduction of titanomagnetite ironsand

The reduction of titanomagnetite (TTM) ironsand, which contains 11.41wt% TiO₂ and 55.63wt% total Fe, by graphite was performed using a thermogravimetric analysis system under an argon gas atmosphere at 1423–1623 K. The behavior and effects of titanium in TTM ironsand during the reduction process were investigated by means of thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. During the reduction procedure, the titanium concentrated in the slag phase, where the phase transformation followed this sequence: FeO + FeTiO₃ → Fe₂TiO₄ → FeTiO₃ → FeTi₂O₅ → TiO₂. The calculated results for the reduction kinetics showed that the carbothermic reduction was controlled by the diffusion of ions through the product layer. Furthermore, the apparent activation energy was 170.35 kJ·mol-1.     

EFFECT OF ELECTRODEPOSITED Cr2O3 OXIDE FILM ON THE OXIDATION OF Fe3Al

A uniform, dense and defect free Cr₂O₃ thin film, which is amorphous at ambient temperature, was applied on the surface of intermetallic Fe₃Al by electrodeposition reaction sintering, and the effect of this film on the oxidation of Fe₃Al at 900℃ in air was studied. The films and the oxide scales were analyzed by TEM, EDAX,SEM and XRD.It is proved that, by surface applied Cr₂O₃ thin film,a continuous, protective,fine grained α-Al₂O₃ scale was formed on Fe₃Al. As a result, the adherence of the scale and oxidation resistance of Fe₃Al were improved.     

A Self-propagating high-temperature synthesis process for the fabrication of Fe(Cr)-Al2O3 nanocomposite

Self-propagating high-temperature synthesis (SHS) was used to fabricate a Fe(Cr)-Al₂O₃ nanocomposite. The composite was fabricated by the reactions between the powders of Fe, Fe₂O₃, Cr₂O₃, and Al. The effect of blending ratio and mechanical activation of the initial powders and the precursor compressing pressure on the microstructure of the final product was studied by optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The significance of the effect of each of the aforementioned parameters on the quality of the composite (assessed by measuring the compressive strength and wear resistance) was determined using a full-factorial design of experiments method. The results showed that the best molar powder ratio that produced the most homogeneous product through a sustainable SHS reaction was Fe:Fe₂O₃:Cr₂O₃:Al=10:1:1:4. A lower Fe content caused the Fe(Cr) phase to melt and separate from the rest of the materials.     

Carbothermal reduction process of the Fe-Cr-O system

Understanding the reduction behaviors and characteristics of the end products of Fe-Cr-O systems is very important not only for maximizing the recovery of metals from stainless steel dust but also for the subsequent reuse in metallurgical process. The present work first predicted the possible products thermodynamically when FeCr₂O₄ was reduced by C. The reduction behaviors by graphite of three kinds of Fe-Cr-O systems, i.e., FeCr₂O₄, Fe₂O₃+Cr₂O₃, and Fe+Cr₂O₃, were then investigated in 1350–1550℃. Further, the microstructures of final products and element distribution conditions were examined. The results suggest that, thermodynamically, the mass of products for the carbothermal reduction of FeCr₂O₄ is a strong function of temperature, and the initial carbon content is used. More Fe-Cr-C solution and less residual carbon content are obtained at higher temperatures and lower n_C:n_O ratios (the initial molar ratio of C to O in the sample). Experimental data show that the sample amount tends to affect the reduction rate, and the residual carbon content strongly depends on n_C:n_O. With regard to the phases present in products during the reaction process, metal carbides tend to form in the initial stage, whereas Fe-Cr-C solution forms when the degree of reduction is sufficiently high.     

Micro-analysis of high-temperature oxidation-resistance of a new kind of heat-resistant grid plate in grate-kiln

To further improve the oxidation-resistance of materials and reduce the cost of grid plates in grate-kiln, a new kind of heat-resistant grid plate was developed. The microstructure of this grid plate with a life more than 18 months was studied by XRD, SEM and EDS techniques. The results show that high hardness, high intensity and good impact property make the new kind of heat-resistant grid plate and its oxide film have a higher resistance to deformation and abrasion at 900-1000℃ Besides, small grain size is beneficial to form a complete protective oxide film. The oxide film composed of SiO₂ layer, Cr₂O₃ layer and Fe₂O₃ layer is rather thin and bonds closely with the backing. The forming of the chemical stable nickel-rich layer increases the density of Cr₂O₃ layer.     

烧结铁矿中FeO含量对其在富氢高炉中还原行为的影响

Japan started the national project “COURSE 50” for CO₂ reduction in the 2000s. This project aimed to establish novel technologies to reduce CO₂ 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–CO₂–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°C in a CO–CO₂–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 ferrite-derived 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.     

Effect of Fe2O3 on the size and components of spinel crystals in the CaO-SiO2-MgO-Al2O3-Cr2O3 system

size of spinel crystals in the CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃ system was investigated using lab experiments carried out in a carbon tube furnace. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD) were used to analyze the microstructure, components, and the mineral phases of synthetic slags. FactSage 7.1 was used to calculate the crystallization process of the molten slag. The results showed that the addition of Fe₂O₃ promoted the precipitation of spinel crystals and inhibited the formation of dicalcium silicate. The size of spinel crystals increased from 2.74 to 8.10 μm and the contents of chromium and iron in the spinel varied as the Fe₂O₃ addition was increased from 0 to 20wt%. Fe₂O₃ thermodynamically provided the spinel-forming components to enhance the formation of FeCr₂O₄, MgFe₂O₄, and Fe₃O₄. The addition of Fe₂O₃ increased the fraction of liquid phase in a certain temperature range and promoted diffusion by decreasing the slag's viscosity. Therefore, Fe₂O₃ is beneficial to the growth of spinel crystals in stainless steel slag.     

18wt%Cr–8wt%Ni不锈钢中增塑性MnO–Al2O3–SiO2基非金属夹杂物的演变及其浸泡性能

The properties of MnO–Al₂O₃–SiO₂-based plasticized inclusion are likely to change during soaking  process due to its low melting point. In this study, the evolution of the MnO–Al₂O₃–SiO₂-based inclusion of 18wt%Cr?8wt%Ni stainless steel under isothermal soaking process at 1250°C for different times was investigated by laboratory-scale experiments and thermodynamic analysis. The results showed that the inclusion population density increased at the first stage and then decreased while their average size first decreased and then increased. In addition, almost no Cr₂O₃-concentrated regions existed within the inclusion before soaking, but more and more Cr₂O₃ precipitates were formed during soaking. Furthermore, the plasticity of the inclusion deteriorated due to a decrease in the amount of liquid phase and an increase in the high-melting-point-phase MnO–Cr₂O₃ spinel after the soaking process. In-situ observations by high-temperature confocal laser scanning microscopy (CLSM) confirmed that liquid phases were produced in the inclusions and the inclusions grew rather quickly during the soaking process. Both the experimental results and thermodynamic analysis conclude that there are three routes for inclusion evolution during the soaking process. In particular, Ostwald ripening plays an important role in the inclusion evolution, i.e., MnO–Al₂O₃–SiO₂-based inclusions grow by absorbing the newly precipitated smaller-size MnO–Cr₂O₃ inclusions.     

Effect of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on the oxidation induration mechanism of Hongge vanadium titanomagnetite pellets

As part of a research project to develop a novel clean smelting process for the comprehensive utilization of Hongge vanadium titanomagnetite (HVTM), in this study, the effect of Cr₂O₃ addition on the oxidation induration mechanism of HVTM pellets (HVTMPs) was investigated in detail. The results showed that the compressive strength of the HVTMPs was greatly weakened by the Cr₂O₃ addition, mainly because of a substantial increase in the porosity of the HVTMPs. The Cr₂O₃ addition marginally affected the phase composition but greatly affected the microstructural changes of the HVTMPs. Increased amounts of Cr₂O₃ resulted in a decrease in the uniform distribution of the hematite grains and in an increase in the Fe-Cr solid solutions (Fe_1.2Cr_0.8O₃ and Fe_0.7Cr_1.3O₃) embedded in the hematite grains. Moreover, the compact hematite was destroyed by forming a dispersed structure and the hematite recrystallization was hindered during the oxidation induration, which adversely affected the compressive strength. On the basis of these results, a schematic was formulated to describe the oxidation induration mechanism with different amounts of added Cr₂O₃. This study provides theoretical and technical foundations for the effective production of HVTMPs and a reference for chromium-bearing minerals.     

不同形态Fe对Cr2 O3还原的影响

通过1350~1550℃下Fe-Cr2 O3、Fe2 O3-Cr2 O3和FeCr2 O4的碳还原实验,结合X射线衍射和扫描电子显微镜考察不同形态铁( Fe、Fe2 O3和FeO)对Cr2 O3还原的影响.同一温度下最终还原度及还原速率均呈现Fe2 O3-Cr2 O3-C>FeCr2 O4-C>Fe-Cr2 O3-C的趋势,三种样品的还原都经历了氧化物→碳化物→Fe-Cr-C合金的过程;低碳碳化物的产生以及较早形成金属液相使Fe2 O3-Cr2 O3还原更充分,合金液相中碳溶解量低导致FeCr2 O4的还原率偏低,而碳化物偏多、合金液相偏少阻滞了Fe-Cr2 O3还原率的提高.实验得到Fe-Cr2 O3-C、FeCr2 O4-C和Fe2 O3-Cr2 O3-C体系的表观活化能分别为142.90、111.84和128.9 kJ·mol-1.     

菱铁矿在煤基直接还原条件下的转化过程

为研究菱铁矿在强还原气氛下加热过程中铁矿物的转化过程和规律,采用热重分析、X射线衍射和扫描电镜等手段研究了嘉峪关某菱铁矿石在煤基直接还原过程中菱铁矿的热行为和不同条件下焙烧产物中铁矿物的存在形式等.结果表明,菱铁矿在煤基直接还原条件下转化为金属铁的历程为FeCO3→Fe3O4→FeO→Fe.转化过程分为菱铁矿分解和铁氧化物还原两个阶段;热分解阶段在556.6℃时基本结束,最终产物为Fe3O4;铁氧化物的还原阶段在556.6℃以后、1200℃时完全结束,最终产物为金属铁.     

CaO-SiO2-Al2O3-FeO-CaF2-La2O3-Nb2O5-TiO2渣系的活度计算模型

根据熔渣结构的分子离子共存理论,建立了CaO-SiO2-Al2O3-FeO-CaF2-La2O3-Nb2O5-TiO2八元渣系的活度计算模型.利用该活度模型,计算和分析了此渣系中铌、稀土、钛相关组元的活度变化规律.实验结果表明,模型计算结果与实际测量值能较好吻合;在本实验渣系条件下,含铌组元主要为FeO·Nb2O5,含...     

Effects of smelting parameters on the slag/metal separation behaviors of Hongge vanadium-bearing titanomagnetite metallized pellets obtained from the gas-based direct reduction process

Smelting separations of Hongge vanadium-bearing titanomagnetite metallized pellets (HVTMP) prepared by gas-based direct reduction were investigated, and the effects of smelting parameters on the slag/metal separation behaviors were analyzed. Relevant mechanisms were elucidated using X-ray diffraction analysis, FACTSAGE 7.0 calculations, and scanning electron microscopy observations. The results show that, when the smelting temperature, time, and C/O ratio are increased, the recoveries of V and Cr of HVTMP in pig iron are improved, the recovery of Fe initially increases and subsequently decreases, and the recovery of TiO₂ in slag decreases. When the smelting CaO/SiO₂ ratio is increased, the recoveries of Fe, V, and Cr in pig iron increase and the recovery of TiO₂ in slag initially increases and subsequently decreases. The appropriate smelting separation parameters for HVTMP are as follows: smelting temperature of 1873 K; smelting time of 30–50 min; C/O ratio of 1.25; and CaO/SiO₂ ratio of 0.50. With these optimized parameters (smelting time: 30 min), the recoveries of Fe, V, Cr, and TiO₂ are 99.5%, 91.24%, 92.41%, and 94.86%, respectively.     

原位反应TiCP/3 Crl 3复合材料的抗氧化性能研究

研究了原位反应法制备的TiCp/3Crl3钢基复合材料在850℃和950℃的抗 氧化性能．结果表明：与基体合金相比,在850℃条件下,由于TiC颗粒的引入,复合 材料在氧化初期具有短路扩散效应,稳定阶段它的氧化动力学曲线符合对数增长规 律,氧化膜由棱角多面体状的Cr2O3组成,复合材料具有理想的氧化膜结构和优良的 抗氧化性能;在950℃条件下,氧化膜由粗大的FeO·Cr2O3组成,氧化膜致密度下降, 复合材料的抗氧化性能提高不多．,The oxidation resistance of TiC paniculate reinforced 3Cr13 matrix composites prepared by in-situ process at a temperature of 850 ℃ and 950 ℃ has been investigated, the results showed that, com pared to 3Cr13 matrix, the short path diffussional effect is caused by TiC phase at initial oxiding stage, the cruves for oxiding kinetics of comoposite agrees with the law of logarithmic growth. The oxide film consists of polyhedron-like Cr2O3, the composite has an ideal structure of oxide film and excellent oxida tion resistance at 850 ℃. The oxidation resistance of composite increases a little at 950 ℃ due to the de crease in compactivity of oxide film and the increase in size of (FeO· Cr2O3) oxide.     

烧结矿应用于化学链燃烧的反应特性

本文通过热重实验研究了烧结矿作为载氧体的H2还原反应特性,将其与通过溶解法制备的Fe2 O3/Al2 O3载氧体进行了氧化还原反应性比较,在500~1250℃范围内研究了温度对于烧结矿还原反应过程的影响,在950℃下进行了30次循环反应实验,采用四种模型进行了反应动力学分析.结果表明,烧结矿的H2还原转化率大于80%,可以完全再氧化,并具有良好的循环反应性能.在500~950℃范围内,随温度升高还原反应速率及最终转化率都显著增加;而当温度高于1100℃时,在反应后期还原反应速率和最终转化率有下降的趋势.在500~950℃范围内,对烧结矿的还原过程第一反应阶段( Fe2 O3-Fe3 O4/FeO,还原转化率<25%)可采用二阶反应模型( M2)拟合,得到表观活化能为E=36.018 kJ·mol-1,指前因子为A0=1.053×10-2 s-1;第二反应阶段(Fe3O4/FeO-Fe,还原转化率>25%)采用收缩核模型(M4)拟合,得到的表观活化能为E=51.176 kJ·mol-1,指前因子为A0=1.066×10-2 s-1.     

2CaO·SiO2-3CaO·P2O5固溶体形成机理

在1300℃下,通过向CaO-SiO2-FetO-P2O5系熔渣中浸入固相CaO的方法,分析了CaO-SiO2-FetO-P2O5渣系与固相CaO间的反应过程,利用分子离子理论计算了炉渣中各组元的活度,研究了2CaO·SiO2-3CaO·P2O5固溶体的形成机理.研究结果表明,CaO-SiO2-FetO-P2O5渣系中组元主要是以2CaO·SiO2和3FeO·P2O5化合物的形态存在;当固相CaO和炉渣接触时,渣中的2CaO·SiO2首先在固相CaO表面析出并形成2CaO·SiO2固相层;随着CaO组元和3FeO·P2O5化合物向反应界面的不断扩散,生成了2CaO·SiO2-3CaO·P2O5和CaO-FeO固溶体.     

Synthesis and characterization of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@SiO<Subscript>2</Subscript> magnetic composite nanoparticles by a one-pot process

Fe₃O₄@SiO₂ core–shell composite nanoparticles were successfully prepared by a one-pot process. Tetraethyl-orthosilicate was used as a surfactant to synthesize Fe₃O₄@SiO₂ core–shell structures from prepared Fe₃O₄ nanoparticles. The properties of the Fe₃O₄ and Fe₃O₄@SiO₂ composite nanoparticles were studied by X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, and Fourier transform infrared spectroscopy. The prepared Fe₃O₄ particles were approximately 12 nm in size, and the thickness of the SiO₂ coating was approximately 4 nm. The magnetic properties were studied by vibrating sample magnetometry. The results show that the maximum saturation magnetization of the Fe₃O₄@SiO₂ powder (34.85 A·m2·kg–1) was markedly lower than that of the Fe₃O₄ powder (79.55 A·m2·kg–1), which demonstrates that Fe₃O₄ was successfully wrapped by SiO₂. The Fe₃O₄@SiO₂ composite nanoparticles have broad prospects in biomedical applications; thus, our next study will apply them in magnetic resonance imaging.