Kinetics of the oxidation of ilmenite

The oxidation of ilmenite (FeTiO₃) in air and dry oxygen was investigated over the temperature interval 600 to 970°C. Dense platelets of ilmenite crystals as well as powder samples of ilmenite were oxidized. The weight data were recorded employing a thermobalance. The oxidation kinetics of ilmenite platelets were parabolic except for the initial stages during which logarithmic kinetics were observed. For powder samples the logarithmic rate law was followed primarily. The logarithmic rate law was attributed to free penetration of oxygen through cracks and short-circuit paths. The activation energies associated with the logarithmic rate law were nearly one-half of those obtained from parabolic oxidation. The growth morphology of the products of oxidation of ilmenite was observed with a scanning electron microscope. The effect of growth morphology on the kinetics is discussed, and a probable reaction mechanism is suggested for the oxidation of ilmenite.     

钛铁矿（FeTiO3）粉末的氧化过程中的物相转变、微观形貌及其氧化机制

探讨钛铁矿氧化过程中的物相转化、形貌改变及其氧化机理。在700~800°C时,钛铁矿(FeTiO3)转变为赤铁矿(Fe2O3)和金红石(TiO2),当温度高于900°C时,三价铁板钛矿开始形成。原始的钛铁矿粉末呈现顺磁性,但是经过中温(800~850°C)氧化后,氧化产物呈现弱铁磁性。同时,讨论钛铁矿的氧化机理。通过对微结构的观察,发现在中温氧化过程中颗粒表面出现大量微孔,其在氧化过程中能够强化氧的传质。     

印度东部钛磁铁矿的氧化行为和相分析

钛磁铁矿是一种复杂的共生矿石,含有钛铁矿、磁铁矿、铁铝尖晶石和镁铁铝尖晶石等矿物。对从印度东部采集到的钛磁铁矿石进行XRD、WDXRF、SEM和M？ssbauer谱分析。在氧气气氛下,通过TG-DTA分析对矿石的氧化行为进行研究。随后,在氧气和空气气氛下,将样品在不同温度下（873-1473 K）保温不同时间,进行等温氧化实验。观察到在较低的温度下钛铁矿相转变为赤铁矿、氧化钛,而在较高的温度下转变为钛酸亚铁相。将氧化后的矿样与焦炭混合压制成圆柱形球团,在1473 K下进行直接还原,成功地实现了将磁铁矿铁转变为氧化铁和二氧化钛的相变。     

Reduction mechanism of natural ilmenite with graphite

Reduction of Bama ilmenite concentrate containing 49.78% TiO2 and 27.96% total Fe by graphite was studied using thermogravimetric analysis system under argon gas ambient from 850 to 1 400 ℃. The reduction degree of Bama ilmenite is enhanced with increasing temperature and the molar ratio of carbon to oxygen, and the reaction rate varies with temperature and reduction time simultaneously. The phase transformation, chemical composition, microstructure and morphology of reduced samples were investigated by using X-ray diffractometry, scanning electron microscopy, and energy disperse spectroscopy, respectively. The high content of impurities in Bama ilmenite evidently bates the reduction of ilmenite. Forming the enrichment zone of manganese prevents complete reduction of Fe^2＋. The reduction products are mostly reduced iron, rutile, reduced rutiles, Ti3O5 and pseudobrookite solid solution. The reduction kinetics was also discussed. The results show that the reduction temperature is a key factor to control reaction rate.     

Effect of yttrium coating on the oxidation behavior of Ni3Al

Yttrium-coated Ni₃Al with post heat treatment has shown good high-temperature oxidation resistance. To understand the effect of the Y-coating and post heat treatment on the oxidation resistance of Ni₃Al, the specimens were coated with Y by an ion-plating method, and heat treatment was performed at low oxygen level before or after the Y-coating was applied. Performance of the Y-coated Ni₃Al was evaluated by isothermal and cyclic oxidation tests. A simple deposition of Y on Ni₃Al did not change the oxidation kinetics, but the post heat treatment after Y-ion plating significantly decreased the oxidation rate of Ni₃Al. The scale formed on Y-coated Ni₃Al with post heat treatment after Y-ion plating showed a fine and dense structure which was grain refined by the presence of a (Y, Al) O-type oxide in the scale. The coated Y layer becomes a Y-Al compound during heat treatment. The presence of the (Y, Al)O-type oxide in grain boundaries or the lattice of Al₂O₃ modify the diffusion rate of Al and oxygen, and the oxide microstructure during oxidation. Improvement of cyclic-oxidation resistance of Ni₃Al by the Y-coating occurs because the presence of (Y, Al)O-type oxide develops fine-grain oxides which can easily relieve the growth stress.     

A TECHNOLOGICAL INVESTIGATION OF SYNTHETIC RUTILE BY SELECTIVE CHLORINATION OF ILMENITE

本文叙述了以国产钛铁矿为原料,在沸腾炉内通入空气或氧气,用选择氯化法分离钛和铁,制取人造金红石的系统性工艺研究。由钛、铁选择氯化分离的反应过程进行热力学的理论分析,得出选择氯化反应受炉内温度和一氧化碳、二氧化碳分压比的影响关系。在钛铁矿原料中配加适量的过剩炭和在炉内通入适量的空气或氧气,可以解决选择氯化反应自热使之持续进行的关键问题。对反应的各种工艺参数,如温度、配炭量、通氧量以及氯气流量、氯化时间、原料粒度等,均经过实验室、半工业性和工业化生产的条件试验。获得的粗金红石又进行了精选试验。全部试验结果表明:这种选择氯化新工艺,不但流程短,而且产品成本低、质量高,还同时可以生产优质副产品三氯化铁。     

Influence of heat exposure time on isothermal degradation of plasma sprayed CoNiCrAlY coatings

In this work, the thermal degradation behavior of selected CoNiCrAlY coatings has been studied. Emphasis was placed upon the change of oxygen content, porosity, surface roughness, and oxidation behavior. The results show that the isothermal degradation of coatings was considerably influenced by the heat exposure time and powder particle distribution. The values of oxygen content, surface roughness, and TGO scales converge sharply into the stable extremes at the initial stage of heat exposure. The particle size has an effect on the oxygen content in as-sprayed coatings. After heat exposure, the difference of the oxygen content on coating deposited using between the smaller and larger particle was decreased. The cross-sectional porosity decreased at Stage I_P, increased at Stage II_P, and then decreased again. These could be due to the sintering effect at Stage I_P. The surface roughness decreased up to Stage I_R, but over Stage I_R, surface roughness was kept in relatively fixed value or increased slightly. The TGO thickness growth rate and parabolic oxidation rate constant are influenced by the surface roughness of coatings.     

Improvement of nonisothermal oxidation behavior of fe and fe-cr alloys by superficially applied reactive oxide coatings

The nonisothermal oxidation behavior of pure iron and a few iron-chromium alloys in dry air has been studied. The effects of a superficial coating of a reactive oxide, CeO₂, on the oxidation behavior were studied. Linear heating rates of 3 K/min and 6 K/min were maintained up to a final temperature ranging from 1273–1473 K. Coatings were applied either from a slurry or an aqueous bath. The CeO₂ coating has been found to be effective not only in decreasing the nonisothermal oxidation rate but also in improving the scale adherence. Moreover, the coated samples withstood a number of heating cycles without scale rupture. The mass gain of the samples as a function of temperature was recorded by means of a sensitive balance, and the scales have been characterized by SEM, EPMA, and x-ray diffraction analysis.     

A mathematical model for internal oxidation

A mathematical model for internal oxidation kinetics was developed using numerical methods (finite difference) and computer techniques. The flexibility of the model permitted analysis of semi-infinite and finite situations with planar, cylindrical, and spherical geometries for systems with various amounts of local solute enrichment. Graphical results are presented for subscale thickness as a function of time and local enrichment as a function of position in the subscale. The model is also applied to internal oxidation with a discontinuous change in surface oxygen concentration; a graphical solution encompassing a wide range of possible experimental conditions is presented. The use of the model in analyzing nonisothermal internal oxidation problems is demonstrated.     

Influence of nitrogen and water vapor on the oxidation of thorium between 400 and 1000° C

The reaction of thorium with air, oxygen, oxygen-nitrogen, and oxygen-water vapor atmospheres has been studied in the temperature range of 400–1000°C using thermogravimetric analysis and metallographic and x-ray diffraction examination. It was demonstrated that H₂O and N₂ as well as O₂ are involved in the reaction of thorium with air. Oxidation in pure oxygen followed a relatively slow rate. Faster rates and nonisothermal conditions were observed under certain conditions with mixed atmospheres. Above 600° C,N₂ caused accelerated oxidation and scale color changes. The effects of H₂O were similar but occurred below 600°C. The rate laws followed for reaction with air are complex combinations of cubic, parabolic, and linear laws.     

Effect of manufacturing related parameters on oxidation properties of MCrAlY‐bondcoats

The effect of MCrAlY‐bondcoat manufacturing parameters prior to TBC deposition on the bondcoat oxidation behavior and TBC lifetime was studied. The studied material was a NiCoCrAl(Y/Hf) free‐standing coating. It was found that variation of oxygen partial pressure during vacuum plasma spraying and the vacuum heat treatment procedure significantly affects the yttrium and hafnium distribution in the coating. In coatings sprayed at high pO₂, yttrium and hafnium were mainly tied up into oxide precipitates. This effect is apparently responsible for an early alumina scale spallation and failure of the initially studied TBC system during cyclic oxidation. In contrast, the coating sprayed at low pO₂ revealed an overdoping effect, i.e. extensive yttrium and hafnium incorporation into the scale resulting in an accelerated scale growth rate and internal oxidation. It was shown that by variation of the vacuum heat treatment parameters the yttrium and hafnium distribution in the near‐surface regions of the low oxygen coating can be modified. The latter result demonstrates the potential of minimizing the negative overdoping effect on the scale growth in the thermal‐sprayed MCrAlY coatings with low oxygen and/or high reactive element contents by optimization of the vacuum heat treatment procedure.     

The effect of oxygen concentration on the oxidation of low-carbon steel in the temperature range 1000 to 1250°C

This paper describes the oxidation behavior of low-carbon steel samples in binary gas mixtures of oxygen and nitrogen, at oxygen concentrations ranging between 1% and 15% and temperatures ranging between 1000 and 1250°C. Sample weight gains versus time were analyzed, along with measurements and calculations of sample heating rates due to exothermic heat of reaction at the sample surface. It was found that initial rates of oxidation depended on oxygen content in the gasmixture and that these reaction rates were linear up to oxide thicknesses of 0.4 to 0.5 mm. Calculations of linear oxidation rate constants based on equations for mass transport of oxygen in the gas mixture to the sample surface showed good agreement with those measured experimentally, indicating that the initial period of oxidation is controlled by the mass transport of oxygen to the reaction interface. The linear rate constants showed little dependency on temperature, an activation energy of approximately 17kJ/mole being obtained. Measurements of sample surface temperatures have shown that within this linear-oxidation regime, interfacial temperatures of the samples increase with increasing oxygen contents in the gas mixture, owing to exothermic heats of oxidation. Subsequent oxidation kinetics were found to be parabolic. Measured parabolic rates constants were in good agreement with previous investigations, with activation energy values of approximately 127kJ/mole.     

表面溶解对微细粒钛铁矿与钛辉石浮选分离的影响

通过红外光谱(FT-IR)和X射线光电子能谱(XPS)分析,研究钛铁矿与钛辉石的表面溶解行为对其浮选分离的影响.实验结果表明,弱酸性条件下的表面溶解有利于提高钛铁矿与钛辉石的可浮性差异.在弱酸性条件下,由于钛铁矿与油酸钠的作用以Fe为主,而表面溶解有利于其在钛铁矿表面的氧化,使钛铁矿可浮性得到提高;同时,油酸钠与Ca和Mg的作用导致了钛辉石的可浮选,但表面溶解降低了钛辉石表面Ca和Mg的含量,使钛辉石可浮性明显下降.对于原矿TiO2品位为8.41%的攀枝花钛铁矿,经表面溶解处理后浮选可将粗选精矿TiO2的品位由26.7%提高到31.73%.     

High temperature oxidation of silicon carbide based materials

The process of high temperature oxidation of two silicon carbide based materials differing by methods of their production and properties has been studied up to 1500°C in air. The oxidation was performed under the isothermal conditions and at the programmed heat rate of 10° per minute. It was found that the oxidation resistance of the material was the function of the presence of extrinsic metals having close affinity for oxygen. It was also found that under heating up to 1500°C in air phase transitions occurred in the SiC surface layer.     

微细浸染型金矿酸性热压氧化预处理动力学研究

以氧气消耗速率表征氧化速率,研究了微细浸染型硫化物金矿热压氧化预处理过程动力学的影响因素,考察了矿浆pH值、温度、矿样粒度、氧分压、气液界面面积和矿浆浓度对氧化速率的影响,以及氧化率和氧化速率的关系。结果表明,矿浆pH2.5时,保持较高的氧化速率;氧化速率随温度升高、氧分压增加和矿浆浓度降低而增大;增加气液接触面积可提高氧化速率,当矿浆浓度为33%、氧分压1.6 MPa,在优化条件下,气液接触比表面积达到25 m2/m3,40 min可完成氧化预处理。     

锆合金氧化膜在真空退火过程中的氧扩散行为

针对已预腐蚀生成一定厚度氧化膜的Zr-Sn-Nb合金,研究了其在不同温度下进行真空热处理过程中的氧扩散动力学及亚稳相演变行为。结果表明,真空退火后氧化膜变薄,氧在氧化锆基体中的扩散增强,并计算了特定合金中氧的扩散系数。退火后微观化学分析表明亚稳相层厚度增加。固溶氧锆基体(Zr(O))层也明显增厚。针对该现象,讨论了对应氧扩散及亚稳相形成过程：该扩散极为可能是由ZrO₂和(Zr(O))之间存在的氧含量梯度以及锆基体的高氧溶解度造成,受抑制的氧化速率将促进亚稳相的生长。在实际水腐蚀情况下,氧化及氧化膜向基体溶解过程应该是共存的。当氧化速率受限时,氧化膜向基体溶解作用将增强,有利于形成较厚的亚稳层。     

Effect of atmosphere on the synthesis of potassium titanate

Potassium titanate (K4Ti3O8) was synthesized by the reaction between ilmenite and concentrated KOH solution in the atmosphere of nitrogen,still air, and oxygen, respectively. The obtained samples were systematically investigated by X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometer (ICP-OES), and scanning electron microscopy (SEM). XRD results indicate that K4Ti3O8 have been synthesized in different atmospheres. The oxidizing atmosphere could enhance the conversion rate of Ti from ilmenite to K4Ti3O8, and Fe(Ⅱ) is easily oxidized to trivalent iron Fe(Ⅲ) during the reaction. Furthermore, SEM images show that the different atmospheres have significant effect on K4Ti3O8 crystal morphology and particle size. Well shaped K4Ti3O8 crystals are obtained in nonoxidizing atmosphere.     

Influence of Carbon Content on Aluminothermic Reduction of Ilmenite During Hot Pressing

JOM - Al2O3-TiCN-Fe composites were successfully prepared by hot pressing using natural ilmenite, aluminum, and carbon. The process of aluminothermic reduction of ilmenite and the influence of...     

Effect of Oxygen Input Rates in the Decarburization of Chromium Steel

Methods for calculating the heat balance of a furnace during the oxygen blow and for estimating the concurrent change in chromium analysis have been developed. The carbon content at the end of the blow can be estimated by means of these relationships when the initial composition and temperature of the bath, the amount of oxygen injected, and the duration of the oxidizing period are known. The relationships are used to evaluate the effects of oxygen input rate, chromium content, and temperature of the bath before the blow on metallic oxidation, oxygen usage, and temperature at the end of the blow.     

Effect of enamel top coating on oxidation behavior of multi-arc ion plating NiCrAlY coating in oxygen containing water vapor

The oxidation behavior of Ni-Cr-Al-Y coating produced by multi-arc ion plating with and without an enamel coating was investigated in flow oxygen and oxygen containing water vapor at 900 ℃. The results show that Ni-Cr-Al-Y coating exhibits low oxidation rate at 900 ℃ in pure oxygen and the oxidation kinetics follow the parabolic rate law. The presence of water vapor accelerates the oxidation rate of Ni-Cr-Al-Y coating and K38G alloy.The enamel top coating is very effective in protecting the Ni-Cr-Al-Y coating from water vapor corrosion attack, but the corrosion attack beneath the enamel coating is not observed.