Investigation of Magnetite Oxidation Kinetics at the Particle Scale

Metallurgical and Materials Transactions B - The induration of magnetite pellets is a complex physico-chemical process that involves oxidation, sintering, and heat transfer. The thermal- and...     

Kinetics of Oxidation Reaction for Magnetite Pellets

An experiment for the oxidation process of single magnetite pellet and theoretical analysis based on modified unreacted core shrinking (MUCS) model were carried out, and the controlling mechanisms of the initial and developing reactions were examined, respectively. From the study of the initial reaction, it was found that the chemical reaction of surface is the controlling step of the overall reaction when the temperature is up to about 750 K, while the mass transfer through the gaseous boundary layer dominates the reaction rate when the temperature is above 750 K. As the reaction developing within the pellet, the mass transfer through the produced layer becomes the controlling step. In addition, the effects of reaction conditions (such as oxygen concentration, temperature) on the fractional oxidation of magnetite pellet were determined.     

Mechanism and Kinetics of the Thermal Oxidation of Natural Sphalerite

The oxidation of natural sphalerite on heating in an oxidative medium is studied by thermogravimetry coupled with scanning calorimetry, mass spectrometry of released gases, and X-ray powder diffraction analysis. The mechanism of sphalerite oxidation when the particle surface is equally accessible and sulfur dioxide is removed from the reaction zone is the formation of ZnO, ZnFe₂O₄, and SO₂. The process is found to be one-stage, as determined by a nonisothermal kinetic method. The activation energies are from 293 to 317 kJ/mol depending on the model used. Natural sphalerite is oxidized in the kinetic regime, and the rate-determining steps are the formation and growth of new-phase nuclei.     

TiCl4高温气相氧化过程的动力学研究

采用积分线性回归法和多元线性回归法研究了TICl4氧化过程的动力学，建立了氧化过程的动力学方程。两种方法得到的表观活化能分别为97．39和80．01kJ·mol^-1，频率因子分别为1．0487×10^4s^-1和2．09×10^2mg·Pa^-2·min^-1。实验考察了反应温度和TICl4流量对反应速率的影响，结果表明在1000℃以上时，反应速率随温度升高迅速增加，反应速率与TICl4流量基本呈线性关系。通过对氧化过程的机制分析表明，在反应初期及中期，反应速率常数取决于TICl4浓度；在反应末期，反应速率常数取决于氧气浓度。     

Oxidation Kinetics of AlN Under CO2 Atmosphere

The oxidation kinetics of AlN powder in CO₂ atmosphere was investigated using thermogravimetric analysis. The experiments were carried out in the isothermal mode at different flow rates of CO₂, 20 mL/min, 40 mL/min, 150 mL/min, and 265 mL/min. The results showed that the oxidation rate was affected by temperature as well as the flow rate of CO₂. Based on the experimental data, a kinetic model for predicting the oxidation behavior of AlN powder has been developed, which could simulate both the chemical reaction and diffusion -controlling periods as a function of temperature and carbon dioxide concentration.     

Analysis of the oxidation reactions of CaS

The chemical reactions of dense pellets of solid CaS in oxidizing atmospheres were studied by using continuous thermogravimetric analysis and iodimetric titration of SO₂ in the off-gas. The experiments covered a temperature range of 1223 to 1853 K, and Ar-O₂ mixtures varying from 1 to 100 pct O₂. The oxidation of CaS was found to be a complex process involving the formation of CaO and CaSO₄. Within the experimental conditions stated, three different processes were observed: 1) Weight loss with the formation of CaO and SO₂ which occurred at high temperatures and low partial pressures of O₂. 2) Weight gain which occurred at low temperatures and high partial pressures of O₂. 3) Combination of a) weight gain with the overall formation of CaSO₄ and b) weight loss with the overall oxidation of CaS to CaO and the decomposition of CaSO₄ leading to an oscillatory behavior which occurred at intermediate temperatures and intermediate to high partial pressures of O₂. The boundaries between these three processes are distinct. The rate of oxidation of the sulfide is limited first by the transport of O₂ across the gas boundary layer and later by the diffusion of O₂ through the porous reaction product. From these results and the Ca-S-0 stability diagram, it is possible to predict the experimental conditions which will produce processes of weight gain, weight loss and oscillatory behavior.     

单晶硅氧化动力学及氧化膜结构分析

用气固相反应原理分析了单晶硅热氧化机理。在氧化中期,考虑到热应力对扩散系数的影响,从而得出了硅氧化的一个新模型,即化学反应控速——四次方混合控速——扩散控速模型。用该模型处理了文献[4]中的数据与本实验结果均得到满意的相关系数。另外,用扫描电镜和透射电镜对氧化膜进行了观察,分析了单晶硅的氧化膜生长机理。     

Thermal Oxidation Kinetics of Graphene-Micro Powders in Oxygen Flow

Powder Metallurgy and Metal Ceramics - The oxidizing properties of graphene-micro powders in a purified oxygen flow were measured (vO2 = 5.21 ? 10–4 mole ? sec–1 = const)....     

Oxidation Kinetics of Vanadium Slag Roasting in the Presence of Calcium Oxide

The isothermal and non-isothermal oxidation kinetics of a converter vanadium slag in the presence of calcium oxide was studied using thermal analysis. The isothermal experimental data for the whole oxidation process are described in terms of the equation [1? (1?α)^2/3] = kt with E_a = 20.42 kJ mol^–1 at lower temperatures of 400-500 °C, and described by [(1?α)^–1/3?1]² = kt with E_a = 227.66 kJ mol^–1 at temperature higher than 500 °C. In the nonisothermal oxidation study, heating rate greatly affects the oxidation process. Using a heating rate of 3 °C min^–1 results in overlapping oxidations of vanadium spinel and augite over temperature range of 608-959 °C, which is described by the 3/2 order reaction. Increasing the heating rate to 5 °C min^–1 or 10 °C min^–1, only oxidation of vanadium spinel takes place in temperature range of 657-914 °C and 691-954 °C respectively, both described by the third order chemical reaction. As the slag particle decreases from 250 µm to 48 µm, the kinetic equation for describing the overlapping oxidation process changes from the Anti–Zhuravlev equation with internal diffusion controlling to reaction limiting equations.     

The Redox Kinetics of the Oxidation for CO on EuFeO_3 Catalyst

ＴｈｅＲｅｄｏｘＫｉｎｅｔｉｃｓｏｆｔｈｅＯｘｉｄａｔｉｏｎｆｏｒＣＯｏｎＥｕＦｅＯ_３ＣａｔａｌｙｓｔＺｈａｎｇＲｕｉｑｉｎ（张瑞芹）；ＹｕＱｉｑｕａｎ（俞启全）；ＪｉｎＹｕｎ（金韵）（ＤｅｐａｒｔｍｅｎｔｏｆＣｈｅｍｉｓｔｒｙ，ＰｅｋｉｎｇＵｎｉ?..     

Oxidation Kinetics,Structural Changes and Element Migration during Oxidation Process of Vanadium-titanium Magnetite Ore

The oxidation kinetics,structural changes,and elements migration during the oxidation process of the vanadium-titanium magnetite(VTM)ore were analyzed.Kinetics analysis indicated that the oxidation process was controlled by diffusion control and could be divided into interface diffusion and lattice diffusion with apparent activation energy of 99.69kJ/mol and 144.08kJ/mol in the range of 800-1 000 ℃,respectively.The surface structure changed with the oxidization temperature as follows:dense surface→nano-sized sheets→submicron particles→molten particles.The compact structure changed into porous one because of the elements migration and enrichment.Both Fe and Ti elements migrated in the opposite direction during the oxidation process.The V element in the raw ore stably existed in the form of V~(5+)state,some vanadium migrated and occupied the tetrahedral sites of the hematite during the oxidation process.     

Kinetics and Reaction Mechanisms of High-Temperature Flash Oxidation of Molybdenite

The kinetics and reaction mechanism of the flash oxidation of +35/–53 μm molybdenite particles in air, as well as in 25, 50, and 100 pct oxygen higher than 800 K, has been investigated using a stagnant gas reactor and a laminar flow reactor coupled to a fast-response, two-wavelength pyrometer. The changes in the morphology and in the chemical composition of partially reacted particles were also investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential thermal analysis (DTA), and electron microprobe. High-speed photography was also used to characterize the particle combustion phenomena. The effects of oxygen concentration and gas temperature on ignition and peak combustion temperatures were studied. The experimental results indicate that MoS₂ goes through a process of ignition/combustion with the formation of gaseous MoO₃ and SO₂ with no evidence of formation of a molten phase, although the reacting molybdenite particles reach temperatures much higher than their melting temperature. This effect may be a result of the combustion of gaseous sulfur from partial decomposition of molybdenite to Mo₂S₃ under a high gas temperature and 100 pct oxygen. In some cases, the partial fragmentation and distortion of particles also takes place. The transformation can be approximated to the unreacted core model with chemical control and with activation energy of 104.0 ± 4 kJ/mol at the actual temperature of the reacting particles. The reaction was found to be first order with respect to the oxygen concentration. The rate constant calculated at the actual temperatures of the reacting particles shows a good agreement with kinetic data obtained at lower temperatures. The ignition temperature of molybdenite shows an inverse relationship with the gas temperature and oxygen content, with the lowest ignition temperature of 1120 K for 100 pct oxygen. Increasing the oxygen content from 21 to 100 pct increases the particle combustion temperature from 1600 K to more than 2600 K. A high oxygen content also resulted in a change of the reaction mechanism from relatively constant combustion temperatures in air to much faster transient combustion pulses in pure oxygen.     

高性能钕铁硼粉末氧化和吸附行为动力学

本文对NdFeB粉末的氧化及吸附行为进行了研究,氧化温度为室温。研究发现,NdFeB粉末与空气作用包括两个方面,一是氧化过程,二是吸附过程。未经表面处理的粉末在最初200小时氧化增重迅速,接近0．2％,而经表面处理的粉末在100小时前增重为零。另外NdFeB粉末由于吸附增重可高达0．35％,解吸过程需要长达300小时左右,由此可见,NdFeB粉末的吸附过程对产品性能危害要比其氧化过程还要严重。     

Oxidation of porous nanocrystalline titanium nitride. I. Kinetics

We used the continuous weighing method to study the oxidation kinetics in air for TiN specimens pressed and sintered from nanocrystalline powders with particle size ≤55 nm. Oxidation was carried out at 500–1000 °C for 240 min. By comparing with the oxidizability of compact titanium, we estimated the total reaction surface S of the porous specimens as a function of their oxidation conditions. The mass of absorbed oxygen Δm was calculated from the mass gain ΔP, taking into account the volatile component N₂. We have shown that the maximum mass gain Δm at 600 °C is due to reaction of oxygen with the largest reaction surface. Within 120 min, external pores close up, S decreases, and then a continuous oxide layer forms in which diffusion of oxygen is slowed down. At 700–800 °C, the process of closing up of the pores is activated, and S decreases by an order of magnitude compared to 600 °C. After the first 40–50 min, a continuous oxide film forms and virtually no further mass gain occurs. As the temperature increases, the oxidation rate increases. At 900 °C, the reaction surface becomes equal to the external surface of the specimen, but the thickness of the scale increases linearly. We hypothesize that for T > 850 °C, counterdiffusion of titanium ions is superimposed on diffusion of oxygen. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), pp. 98–103, January–February, 2006.     

Reaction Mechanism and Kinetics of Enargite Oxidation at Roasting Temperatures

Roasting of enargite (Cu₃AsS₄) in the temperature range of 648?K to 898?K (375?°C to 625?°C) in atmospheres containing variable amounts of oxygen has been studied by thermogravimetric methods. From the experimental results of weight loss/gain data and X-ray diffraction (XRD) analysis of partially reacted samples, the reaction mechanism of the enargite oxidation was determined, which occurred in three sequential stages:

1. $4{\text{Cu}}_{ 3} {\text{AsS}}_{ 4} \left( {\text{s}} \right){\text{ + 13O}}_{ 2} \left( {\text{g}} \right){\text{ = As}}_{ 4} {\text{O}}_{ 6} \left( {\text{g}} \right){\text{ + 6Cu}}_{ 2} {\text{S}}\left( {\text{s}} \right){\text{ + 10SO}}_{ 2} \left( {\text{g}} \right) $
2. $ 6{\text{Cu}}_{ 2} {\text{S}}\left( {\text{s}} \right){\text{ + 9O}}_{ 2} \left( {\text{g}} \right){\text{ = 6Cu}}_{ 2} {\text{O}}\left( {\text{s}} \right){\text{ + 6SO}}_{ 2} \left( {\text{g}} \right) $
3. $ 6{\text{Cu}}_{ 2} {\text{O}}\left( {\text{s}} \right){\text{ + 3O}}_{ 2} \left( {\text{g}} \right){\text{ = 12CuO}}\left( {\text{s}} \right) $

The three reactions occurred sequentially, each with constant rate, and they were affected significantly by temperature and partial pressure of oxygen. The kinetics of the first stage were analyzed by using the model X?=?k ₁ t. The first stage reaction was on the order of 0.9 with respect to oxygen partial pressure and the activation energy was 44?kJ/mol for the temperature range of 648?K to 898?K (375?°C to 625?°C).     

柠檬酸盐法脱除SO_2过程中氧化副反应的动力学研究

以低浓度二氧化硫柠檬酸盐吸收后液为对象,研究硫酸根生成速率,考察温度、柠檬酸总浓度、pH等因素的氧化副反应,探索氧化副反应机制和抑制方法。研究表明,氧化速率对柠檬酸浓度是0.5级反应,对pH是0.8级反应,对氧是0级反应;控制柠檬酸浓度为1.5mol/L、pH=4.5、反应温度为95℃的条件下进行吸收解吸操作,可以使氧化副反应最小。     

The CaS solubility in the CaO bearing slags

The CaS solubility and sulfide capacity for the CaO–SiO₂–CaF₂ and CaO–Al₂O₃–SiO₂ systems have been measured at 1300°C and 1400°C, respectively. With the CaO–SiO₂–CaF₂ system, the slag doubly saturated with CaO and 3CaO · SiO₂ has the highest CaS solubility of 12.5 wt.%. On the liquidus the slag always has a higher CaS solubility than when it is not on the liquidus. The sulfide capacity was confirmed to decrease with increasing SiO₂ content. With the CaO–Al₂O₃–SiO₂ system, the CaS solubility was found to depend only on CaO content. A good correlation between the sulfide capacity and the CaS solubility was observed as expected from theory. The temperature dependence of CaS solubility in those systems was also discussed.     

低品位铅冰铜硫酸氧化浸出动力学

采用H₂SO₄-H₂O₂体系对低品位铅冰铜进行了氧化浸出研究,利用ICP、XRF、粒度分析、XRD和SEM-EDS等手段对铅冰铜进行了物质组成研究,并分析了浸出动力学过程。结果表明：在初始硫酸浓度210 g/L、浸出温度70℃、搅拌速度400 r/min、过氧化氢与铅冰铜的质量比5、浸出时间180 min和液固比11的条件下,铜浸出率可达90.69%。动力学分析表明,低品位铅冰铜的硫酸氧化浸出过程符合未反应收缩核模型,反应活化能为9.67 kJ/mol,浸出过程受扩散控制。