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.     

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.     

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.     

Process Optimization and Reaction Mechanism of Removing Copper From an Fe-Rich Pyrite Cinder Using Chlorination Roasting

The aim is to remove copper from a pyrite cinder by optimizing the chlorination roasting process using response surface methodology (RSM) and the reaction mechanism of chlorination roasting based on thermodynamic calculation was discussed. A quadratic model was suggested by RSM to correlate the key parameters, namely, dosage of chlorinating agent, roasting temperature and roasting time to the copper volatilization ratio. The results indicate that the model is well consistent with the experimental data at a correlation coefficient (R²) of 0.95, and the dosage of chlorinating agent and roasting temperature both have significant effects on the copper volatilization ratio. However, a roasting temperature exceeding 1170°C decreases the volatilization ratio. The optimum conditions for removing copper from the cinder were identified as chlorinating agent dosage at 5%, roasting temperature at 1155.10 °C and roasting time of 10 min; under such a condition, a copper volatilization ratio of 95. 16% was achieved from the cinder. Thermodynamic calculation shows that SiO₂ in the pellet plays a key role in the chlorine release from calcium chloride, and the chlorine release reactions cannot occur without it.     

锌精矿的粒度对氧化焙烧的影响

研究了硫化锌精矿的粒度大小对氧化沸腾焙烧的影响，然后结合生产实践，提出了针对不同粒度的硫化锌精矿在配料过程中和氧化沸腾焙烧工序所应采取的措施。     

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 and Mechanism of High-Temperature Oxidation in Air of Au - Cu Alloy

We have used thermogravimetry to study the kinetics of high-temperature (up to 800 °C) oxidation of the alloy 58.3 mass% Au - 41.7 mass% Cu with isothermal heating of the specimens. Using petrographic analysis of the oxide layers, we determined the reaction products. We have shown that up to 200 °C, the indicated alloy is not oxidized at all. More rapid oxidation of the alloy is observed at temperatures above 400 °C. Up to 500 °C, an inner layer consisting of Cu₂O predominates in the two-layer scale on the alloy, while the outer CuO layer has a significantly smaller thickness. At 600 °C, the upper layer of scale contains Cu₂O while the lower layer contains Cu₂O and gold. At higher temperatures, all the way up to 800 °C, the scale is two-layer as before but its upper layer contains CuO while its lower layer contains Cu₂O and small gold rods distributed in that oxide. Thus we have established three oxidation regions characterized by different scale phase compositions and different mechanisms for the process, mainly due to transition from an ordered state of the alloy (intermetallic AuCu₃) to a completely disordered solid solution of gold in copper. We used the Arrhenius equation to calculate the apparent activation energy for oxidation: E₁ = 20.4 kJ/mole for the temperature range 400–500 °C and E₂ = 9.5 kJ/mole for 600–800 °C. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 85–91, July–August, 2005.     

Fluidized Bed Selective Oxidation-Sulfation Roasting of Nickel Sulfide Concentrate: Part I. Oxidation Roasting

Two-stage oxidation-sulfation roasting of nickel sulfide concentrate in fluidized bed was investigated to generate water-soluble metal sulfates as an alternative process to smelting of the sulfide concentrate for the recovery of valuable metals. The first stage, i.e., oxidation roasting, was employed to preferentially oxidize the iron before performing sulfation roasting. A batch fluidized bed roaster was constructed for roasting tests. Roasting products from various roasting temperatures and different roasting times were analyzed by SEM/EDS, EPMA, XRD, and ICP-OES to investigate the oxidation roasting behavior of the nickel concentrate as a function of temperature and time.     

Dielectric Properties and Oxidation Roasting of Molybdenite Concentrate by Using Microwave Energy at 2.45 GHz Frequency

Metallurgical and Materials Transactions B - Conversion of electromagnetic energy into heat depends largely on the dielectric properties of the material being treated. Therefore, determining the...     

硅镁型红土镍矿球团焙烧固结机制研究

针对红土镍矿在焙烧过程中物相转变及固结机制问题,对原矿进行了化学成分、X射线衍射(XRD)分析,得知红土镍矿主要以Fe2O3和单斜形蛇纹石矿物为主,另外含有部分十字沸石和利蛇纹石;通过热重(TG)、差热(DTA)测试,得到了在焙烧过程中,自然水、结晶水及羟基分别在195,293和612℃被脱除,在830℃时,部分硅酸盐发生物相转变;并用Factsage软件对红土矿在加热过程中液相的产生量进行了理论计算,当焙烧温度为1220℃时红土矿球团开始产生液相.在实验室中,利用箱式电阻炉进行球团焙烧实验,结果表明,焙烧球团的抗压强度和落下强度随着焙烧温度和时间的增加而增大,焙烧温度低于1200℃时,球团依靠固相反应和再结晶固结,抗压强度及落下强度较低;焙烧温度达到1300℃时,红土矿会产生34％的液相,冷却后使球团固结,抗压强度和落下强度大大增加;当焙烧温度一定时,球团抗压强度和落下强度随焙烧时间的延长而增加,但是球团的强度增加幅度较小.     

钴基合金DZ40M的高温氧化激活能

DZ40M合金是一种定向凝固钴基高温合金.本文测定了该合金在900和1000℃两个温度下的空气氧化增重动力学数据,通过线性处理得到了其不同温度下的氧化速率常数.根据氧化速率常数的Arrhenius关系,即可得出该合金在高温环境中的氧化激活能,其值约为563858 J·mol-1.还在此基础上进一步获得了关于该合金的抛物线氧化速率常数的具体表征.     

Effects of Nitrogen Deficiency on the Kinetics and Mechanism of Electrolytic ZrN x Oxidation

Use has been made of potentiodynamic polarization curves, XRD, and scanning electron microscopy (SEM) in the electrolytic oxidation in 3% NaCl solution for specimens of nitrogen-deficient zirconium nitride (ZrN_0.67, ZrN_0.77, ZrN_0.87, and ZrN_0.97), as well as pure zirconium. In all cases, the anodic polarization curves have several stages which characterize during oxidation both active dissolution of ZrN _x and Zr in the electrolyte as well as the formation of surface layers of ZrOCl₂, ZrN _x O _y , and α‐ZrO₂ of monoclinic form. The corrosion resistance of single-phase ZrN _x specimens in 3% NaCl solution decreases in the sequence ZrN_0.97 → ZrN_0.87 → ZrN_0.77, and the initial stages of interaction between the specimen surface and the electrolyte largely determine the subsequent behavior of specimens. It is found that ZrN _x containing a large number of nitrogen atom vacancies, in particular ZrN_0.77, is closer in corrosion behavior to metallic zirconium than it is to stoichiometric ZrN (the reduction in the corrosion resistance is undoubtedly due to the reduction in the ionic-covalent components of the bonds in ZrN _x ).     

Oxidation Behavior of NiAl-30.75Cr-3Mo-0.25Ho Alloy at High Temperatures

The oxidation behavior of NiAl-30.75Cr-3Mo-0.25Ho alloy from 1300 to 1500 K in air atmosphere was investigated. The results reveal that oxidation resistance of the alloy is improved by the addition of Ho. At 1500 K, the oxidation kinetic curve obeys the parabolic law (n≈0.5), whereas the oxidation kinetic curve of the tested alloy follows the cubic relations (n≈0.3～0.4) from 1300 to 1450 K. An activation energy of about 261 kJ·mol-1 was determined for the tested alloy. It is found that a continuous and compact Al2O3 layer has formed on the surface of NiAl-30.75Cr-3Mo-0.25Ho alloy after oxidation 100 h at various tested temperatures. A rich-Ho solution formed on the boundaries of Cr(Mo) phase. Doped little amount of Ho in NiAl-31Cr-3Mo alloy promotes the transformation from θ-Al2O3 phase to α-Al2O3 phase, and decreases the size of Al2O3 and the crack forming in the oxidation scale prolongs the spalling time of the film at high temperature. The volatilizing oxides of Cr, Mo and the reactive element effects (REEs) make the mass gain lower than that of pure NiAl.     

利用介入法氧化焙烧降低黑钨精矿中的杂质

阐述了介入法氧化焙烧的原理及特点，并详细介绍了利用介入法氧化焙烧在黑钨精矿降硫方面的试验研究。氧化焙烧在黑钨精矿试验方面取得降硫70％～8l％，黑钨精矿基本不损失的良好指标，提高了产品质量。     

Use of Molten Salt Fluxes and Cathodic Protection for Preventing the Oxidation of Titanium at Elevated Temperatures

The current study demonstrates that it is possible to protect both solid and liquid titanium and titanium alloys from attack from air by cathodically polarizing the titanium component using an electro-active high-temperature molten salt flux and a moderate polarization potential. The electrolytic cell used comprises a cathode of either solid titanium or liquid titanium alloy, an electrolyte based on molten calcium chloride or fluoride salt, and an anode consisting of an inert oxygen-evolving material such as iridium metal. The new approach renders possible the processing of titanium at elevated temperatures in the presence of oxygen-containing atmospheres.     

化学动力学机理耦合EDC燃烧模型对湍流扩散火焰的数值模拟

将化学动力学机理耦合燃烧模型进行湍流燃烧数值模拟有重要意义.在分析目前现有燃烧模型的基础上,采用CH4/空气化学反应动力学机理和EDC燃烧模型对燃烧器中的二维湍流扩散燃烧进行模拟,得到了燃烧温度、物种浓度随空间的变化曲线图,经分析发现,计算结果可以较好反应文献中的解析解.由此得出结论:将化学动力学机理耦合EDC燃烧模型可以较好地模拟湍流扩散燃烧的火焰结构,从而为工程实际复杂燃烧情况下的污染物、中间物种和痕迹量物种生成机理的研究提供基础.