Volatilization of fluorine in a CaO–SiO2–CaF2-based system with the substitution of Na2O with K2O

Cuspidine-based systems are used to control the crystallization in mold fluxes, which is enabled by CaF₂ additions. However, excess CaF₂ increases the corrosion of casting machines. Therefore, Na₂O and K₂O are added to the mold flux system to ensure an optimized crystallization and lubrication ability of the flux with the CaF₂ content. This study investigated the effect of substituting Na₂O with K₂O on the volatilization of fluorine in a CaO–SiO₂–CaF₂-based slag system at high temperatures. The substitution of Na₂O with K₂O was performed at 5 mol% intervals. The volatilization was observed by thermogravimetric analysis under several isothermal conditions. The mass loss was measured at a heating rate of 5, 10, and 20 K/min. As the temperature increased, the volatilization of the mixed samples increased. The activation energy was calculated using the Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose methods. A kinetic analysis of the volatilization of fluorine was conducted using the calculated parameters and several known kinetic models. Consequently, the volatilization of the Na-rich sample was controlled by chemical reactions and that of the K-rich sample was identified to be controlled by a phase-boundary-controlled reaction. These results suggest that the addition of mixed alkali oxide promote the volatilization during the early stages of the reaction. From the post-experimental composition analyses, the remaining Na and K in the samples suggested a different mechanism for the Na and K volatilization. The volatilization of Na increased with time, whereas K volatilized easily during the beginning of the reaction.     

Water vapor volatilization and oxidation induced surface cracking of environmental barrier coating systems: A numerical approach

A numerical model is developed for surface crack propagation in brittle ceramic coatings, aiming at the intrinsic failure of rare-earth silicate environmental barrier coating systems (EBCs) under combustion conditions in advanced gas turbines. The main features of progressive degradation of EBCs in such conditions are captured, including selective silica vaporization in the top coat due to exposure to water vapor, diffusion path-dependent bond coat oxidation, as well as crack propagation during cyclic thermal loading. In light of these features, user-defined subroutines are implemented in finite element analysis, where surface crack growth is simulated by node separation. Numerical results are validated by existing experimental data, in terms of monosilicate layer thickening, thermal oxide growth, and fracture behaviors. The experimentally observed quasi-linear oxidation in the early stage is also elucidated. Furthermore, it is suggested that surface crack undergoes rapid propagation in the late stage of extended thermal cycling in water vapor and leads to catastrophic failure, driven by both thermal mismatch and oxide growth stresses. The latter is identified as the dominant mechanism of penetration. Based on detailed analyses of failure mechanisms, the optimization strategy of EBCs composition is proposed, balancing the trade-off between mechanical compliance and erosion resistance.     

A kinetic model for air-steam reforming of bio-oil over Rh–Ni/γ-Al2O3 catalyst: Acetol as a model compound

A new kinetic model is proposed for catalytic reforming of acetol to synthesis gas over a Rh–Ni/γ-Al₂O₃ catalyst. Acetol is one of the most important bio-oil model compounds formed under reactive flash volatilization reaction conditions. The model was implemented in the Aspen Plus simulation package and used to predict the product gas composition at different reaction temperatures and steam and oxygen ratios. The contributions of the reactions both in the reactor freeboard and the catalytic bed were assessed using CSTR and PFR reactor models, respectively. The reaction scheme included decomposition, steam reforming, and water-gas shift reactions. The results from the model predicted the product distribution within an acceptable degree of tolerance. This study confirms that thermal decomposition and partial oxidation of acetol precede the catalytic reactions involving steam. The effects of temperature, oxygen concentration in the feed, the volume of the freeboard, and the catalyst bed height can all be evaluated with this new kinetic model. This work suggests that bio-oil decomposed into different fractions of molecules like acetol can be successfully modelled by a series of decomposition reactions followed by partial oxidation and catalytic steam conversion. The heat transfer within the catalyst bed is found to be critical for achieving a good match with the experimental results.     

顶空-气相色谱法在车用氯化聚氯乙烯制品总碳挥发量测试中的应用

采用顶空-气相色谱法测定氯化聚氯乙烯(CPVC)制品中有机挥发物总碳挥发量。通过对平衡温度、平衡时间以及颗粒大小的探索性试验,确定了本方法的最佳测试条件。以丙酮为外标物得到的标准曲线的线性相关系数为0.999 8,样品测试结果的相对标准偏差为1.36,加标回收率可达99.8%。     

石墨炉原子吸收光谱法测定奶粉中的六价铬

基于Cr(Ⅲ)与噻吩甲酰三氟丙酮(TTA)生成的络合物在石墨炉中的挥发性,建立一种前处理简单、灵敏度高的测定奶粉中六价铬的分析方法。对样品的消化方式、Cr(Ⅲ)与TTA络合的酸度、试剂的加入量、超声波水浴的温度和时间、静置时间与石墨炉挥发Cr(Ⅲ)的原子化条件进行探讨。结果表明,Cr(Ⅵ)在0.002~0.010mg·L-1浓度内呈良好线性关系(r=0.9995)。奶粉样品中Cr(Ⅵ)不同加入量的平均回收率为78%~93%,其相对标准偏差为3.8%~9.4%,检出限为0.4 mg·L-1。方法具有分析速度快、灵敏度高、干扰少、操作简便等特点,应用于奶粉中六价铬的测定获得了满意的结果。     

The relationship between crystallization and break temperature of mould flux

ABSTRACT

Many researchers believe that crystallization is responsible for drastic viscosity changes near the break temperature of mould flux; however, there are no reasonable experimental data to support this view. In this study, the complete crystallization process is recorded in situ by inhibiting the volatilization of fluoride, and the relationship between viscosity change and crystallization near the break temperature is studied. Mould flux with a basicity of 0.65 had no break temperature, and no crystallization occurs within the temperature range of the viscosity test. The other three mould fluxes begin to crystallize before the break temperature, but the crystal volume fractions at the break temperature are different. The break temperature of mould flux is the crystallization temperature with a critical crystal volume fraction, which is related to the viscosity before the break temperature. The higher the viscosity, the smaller the critical crystal volume fraction. Therefore, break temperature?≤?initial crystallization temperature.     

燃煤过程中水蒸气对砷释放特性的影响

为了研究水蒸气对煤燃烧过程中砷释放特性的影响,在800~1 300℃下对庆华煤进行燃烧试验,并结合XRD分析了水蒸气对砷释放的作用机理。气氛中有水蒸气时,可以明显加快煤的失重;相同温度下,气氛中存在水蒸气时,煤中砷的逃逸有一定的增加,随着水蒸气浓度的增加,逃逸率增加趋势放缓;随着温度的增加,水蒸气对砷的逃逸率的提升效果减弱。在水蒸气气氛下灰的矿物质组成结构发生变化,石膏等硫酸盐增多,主要是水蒸气促进了石膏的形成,缩减了高活性CaO的停留时间,抑制了CaO对砷的捕捉,造成砷吸附率降低,逃逸率增加。     

Hydration Model for Evaluation of Thermodynamic Activities in Nitric Acid-Nitrate-Water System

Modeling was carried out on activities of components in the nitric acid-nitrate-water ternary system in order to examine details of the chemical reactions in reprocessing solutions. To modify our earlier “Hydration Model”, the ternary system was regarded as the mixed solution of two kinds of cluster molecules which were dominant associated molecules in the nitric acid-water system and nitrate-water system, respectively. Activities in the ternary system were calculated from those in the two binary systems taking into account the change of entropy and enthalpy during mixing. The entropy term was calculated assuming ideal mixing. The enthalpy term was calculated assuming that the nitric acid cluster would be influenced by one nitrate cluster as if there were m _s /m _na numbers of nitric acid clusters, where m _s and m _na were the numbers of water molecules in nitrate and nitric acid cluster molecules, respectively.

The model was able to predict activities of nitric acid, water and nitrate. Its use was verified for the activities of nitric acid and water provided phase separation did not occur. The verification for nitrate activity was not carried out, however, the model should be suitable.     

钢结硬质合金烧结中的金属挥发问题研究

基于生产实践,针对钢结硬质合金生产过程中的金属挥发问题进行研究,明确了TM52钢结硬质合金真空烧结过程中Mn为主要挥发元素,指出Mn元素大量流失是造成产品脱碳、异常断裂等的原因之一。对减少Mn元素的挥发提出了适合生产实践的解决方案,通过降低烧结温度、缩短保温时间,降低锰元素熔点附近的升温速度,降低最高烧结温度时的真空度等减少Mn元素挥发。     

含金高砷高硫精矿综合回收有价金属

采用二段焙烧—氯化挥发工艺从含金高砷高硫精矿中综合回收有价金属。结果表明,一段弱氧化700℃,二段强氧化850℃条件下,砷和硫的脱除率分别到97.6%和99%;在烧渣磨矿粒度-0.038mm占73%、配5%CaCl2、2%膨润土制团后在1 250℃氯化焙烧2h的条件下,金、银、铅、锌的挥发率分别为96.5%、73.3%、94.6%和75.3%,球团铁品位在60%以上,球团强度2 500N。