进料位置与风速对旋风分级器颗粒分级效果的影响

根据旋风分级器内气流速度分布特点进行了进料区域划分,运用非稳态离散相模型和分级实验对比了3个代表性进料位置对颗粒运动轨迹及分级精度的影响,分析了1 μm和10 μm颗粒在不同区域内的受力情况。结果表明,边壁区域进料造成粗组分中细粉夹带现象严重,分级精度差;中部进料区域内流场强度大,粗颗粒受离心力强,细颗粒受轴向气流曳力大,有利于减少颗粒在分级区的停留时间,实现粗、细颗粒的快速分级,对改善分级精度有利;中心位置进料延长了粗颗粒的分级运动路程,增加了粗组分跑损的概率,模拟计算15 μm的粗颗粒进入细组分的质量分数达到11.7%。经实验验证,入口气速在10～22 m·s^-1,中部区域进料时分级后粗、细组分粒度分布曲线重合区面积最小,分级粒径比率值平均提高了25.3%,研究结果为离心气流分级设备的进料位置设计提供了一定的指导。     

流场形态对旋风分级器性能的影响

气流分级器性能的优劣很大程度上取决于流场分布,改变常规旋风分级器的切向进风口位置,在分级空间建立不同类型的离心流场,采用数值模拟和分级试验手段分析了分级流场形态对颗粒运动过程和分级性能的影响。结果表明,传统旋风分级器边壁下行流造成粗粉中细颗粒夹带较多,影响分级精度;新型旋风分级器内形成上下两个旋涡,上旋涡均为上行气流,其流量约占总风量的80%,下旋涡携带细颗粒较少,降低了细颗粒进入粗组分的概率;上旋涡可实现对边壁区的细颗粒的轴向淘洗、再分级,提高了分级精度。试验结果表明,入口气速从10m/s增加至22m/s。相较于传统旋风分级器,新型旋风分级器的分级性能明显改善,分级精度指标平均提高27%,压降损失为传统旋风分级器的53%~62%。     

Microstructure and flexure creep behaviour of SiC-particle reinforced Al2O3 matrix composites

The flexure creep behaviour of monolithic Al₂O₃ and 10 vol% SiC-particle reinforced Al₂O₃ matrix composites was investigated in air atmosphere at 1160 to 1400 °C and under a stress of 40 to 125 MPa. Two kinds of SiC particles with different particle sizes and oxygen contents were used in the composites, one having an average size of 0.6 μm with 1.7 vol% SiO₂ impurities and the other of average size 2.7 μm with 3.4 vol% SiO₂ impurities. Compared with the creep behaviour of monolithic Al₂O₃ the strain rate of the composites with 0.6 μm SiC particles did not decrease; however, the composites with 2.7 μm SiC particles exhibited excellent creep resistance. Microstructure analysis showed that the Al₂O₃ grains in the composites with 0.6 μm SiC particles were mainly equiaxed with most of the SiC particles lying at the grain boundaries or triplegrain junctions, whereas the grain features of the composites with 2.7 μm SiC particles were irregular and elongated and most of the SiC particles were entrapped into Al₂O₃ matrix grains. It was revealed that the entrapment of 2.7 μm SiC particles into Al₂O₃ matrix grains was related to the high SiO₂ impurity content on SiC particle surfaces, and the change of grain morphology and the good high-temperature oxidation resistance were responsible for the creep resistance increase of the composites with 2.7 μm SiC particles.     

新型旋风分级器流场特性及颗粒分级性能

针对传统旋风分级器分级效率较低的难题,本文设计了一种中部进风、顶部重力进料式新型旋风分级器,利用数值模拟和试验手段对其流场特征及分级性能进行了研究。模拟结果表明,新型旋风分级器内存在若干旋涡,主气流进入分级器后形成由上、下两个旋涡构成的主分级流场,上部旋涡均为上行气流,下部旋涡为切流返转形式;二次气流形成的细颗粒淘洗旋涡具有近壁面处高转速、中心区快速上升的特点,最大轴向速度达16.5m/s,可强化对边壁处浓集颗粒的剪切分散和淘洗作用,对主分级流场切向速度影响较小,但可使其轴向速度值最大增加100%,这将缩短细颗粒的停留时间;主分级流场与淘洗流场相互作用形成分区流动,具有较明显的动态边界,为粗、细颗粒的定向分离提供了力场基础。试验表明,二次气量占比约20%,主、二次气流气速分别为14m/s和20m/s时,牛顿分级效率可达88%,分级精度指数K值最小为1.84,此时新型旋风分级器具有较高的分级精度。     

On physical properties of aerosol at Ross Island, Antarctica

We report on the physical characteristics of Aitken nuclei and their relation to air mass type at Ross Island, Antarctica. The size distribution spectrum of the particles was derived with a diffusion battery and non-linear mathematical inversion and is considered to be quite accurate over the particle diameter range 0.01 < d < 0.2 μm.

The Aitken nuclei concentration, n_o, decreased at Ross Island by about an order of magnitude from summer (n_o = 600 cm⁻³) to winter (n_o = 100 cm⁻³). Particle size varied with air mass type: Maritime polar air masses had a geometric mean particle diameter d_g = 0.011 μm, while continental Antarctic, cA, air masses possessed somewhat smaller particles (i.e. d_g 0.005 μm). The anomalously small particles associated with cA air suggests the presence of an upper tropospheric or partly stratospheric origin of nuclei perhaps associated with mixing formed by breaking waves over the Ellsworth Range in Marie Byrd Land.     

进气方式和气速对卧轮式分级机性能的影响

传统卧轮式分级机内流场分布比较混乱，分级精度普遍不高。本文基于对分级机内气流运动规律的分析，将传统切向进风方式调整为径向进风，分别设计了百叶窗型和多孔型风筛，试验对比了传统切流风筛和上述两种径向进风方式对分级效果的影响。结果表明，径向进风方式对分级流场形成和细颗粒淘洗更有利，不仅可以提高分级机的粗粉产率，同时还可降低粗组分中的细粉夹带量，提高粗、细颗粒分离的程度，改善颗粒分级效果；百叶窗型风筛分级机的分级效果最优，气流经百叶窗风筛可对粗组分进行多层、充分扬析，减少细颗粒误入粗产品的概率，牛顿分级效率较传统切流风筛分级机平均提高约6%；此外，入口气速也对分级精度有较大的影响，但对分级粒径的影响不明显，存在临界入口气速使得综合分级效果最好。为提升涡轮分级机的颗粒分级性能提供了新思路。     

二次风量对LNJ-36A型气流分级机分级性能的影响

气流分级机二次风量是影响分级机性能的重要因素。通过实验探究二次风量变化对同种物料、不同粒度段及同种粒度段、不同物料分级效果的影响,并提出一种可以直接从粗、细粉粒度分布中得到的分级评价指标--相对分级精度δ,以此来评价分级效果好坏。实验结果表明,不同粒度段的粉煤灰和水泥熟料在相同二次风量的情况下,粒径较小的粉煤灰,其相对分级精度大于粒径较大的粉煤灰;粒径较小的水泥熟料,其相对分级精度大于粒径较大的水泥熟料;同种粒度段的粉煤灰和水泥熟料,在相同二次风量的情况下,相对密度较小的粉煤灰,其相对分级精度大于密度较大的水泥熟料;利用LNJ-36A气流分级机分级粉煤灰和水泥熟料,二次风流量保持在150m³/h、与主气流流量比例保持在0.168时,分级效果最好。     

Kinetics and Mechanism of Formation of Barium Zirconate from Barium Carbonate and Zirconia Powders

The formation of BaZrO₃ from very fine (70–90 nm) ZrO₂ powders and coarser (∼1 μm) BaCO₃ powders has been studied in dry and humid air up to 1300°C using TGA/DTA, XRD, SEM, TEM, and EDS microanalysis. In the temperature range 900°–1100°C, barium is rapidly transported at the surface of the ZrO₂ particles and reacts, forming BaZrO₃. The compound grows as a concentric layer with gradual consumption of the central ZrO₂ particle. The overall formation kinetics of BaZrO₃ is well described by a diminishing core model, and the most likely rate-determining step is a phase-boundary process at the ZrO₂–BaZrO₃ moving interface. The size and shape of the final particles is generally determined by the morphology of the starting ZrO₂ particles and not by that of the BaCO₃. The reaction is faster in humid air than in dry air, and the activation energy decreases from 294 kJ·mol⁻¹ (dry air) to 220 kJ·mol⁻¹ (humid air). When the fraction reacted is >80–90 mol%, the reaction rate rapidly decreases.     

Synthesis of single-phase Ti3SiC2 powder

In this study, free 2Ti/2Si/3TiC powder mixture was heated at high temperatures in vacuum, in order to reveal the possibility for the synthesis of high Ti₃SiC₂ content powder. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the evaluation of phase identities and the morphology of the powder after different treatments. Results showed that almost single phase Ti₃SiC₂ powder (99.3 wt.%) can be synthesized by heat treatment with free 2Ti/2Si/3TiC powders in vacuum at 1210°C for about 3 h. The nucleation and growth of Ti₃SiC₂ within TiC particles was observed. The typical appearance of the formed Ti₃SiC₂ is equiaxed with particle size of 2–4 μm. Effects of temperature and heating time on the morphology and the particle sizes of the synthesized Ti₃SiC₂ powders are not obvious.     

Constitution of mullite glasses produced by ultra-rapid quenching of plasma-sprayed melts

Spherical shaped spray-dried admixtures of chemical pure and very fine -Al₂O₃ and quartz powders with mullite composition (72 wt% Al₂O₃, 28 wt% SiO₂) were used as starting materials. The spray-dried powders (10–100 μm) were melted in a nitrogen plasma flame and subsequently quenched in water thus producing spherical, hollow, and porous particles (≤ 100 μm). The as-quenched spherules consist of mullite glass, some residual -Al₂O₃ and quartz, and a very low amount of newly formed mullite. Double quenching of the material increases the glass content to >90 wt.%. ²⁷Al and ²⁹Si MAS NMR studies show that the rapidly quenched mullite glass is composed of a network of (SiO)-tetrahedra and (AlO)-octahedra, -pentahedra, and -tetrahedra. The frequency distribution of (AlO)-structural units is similar to those in metakaolinite, type I (polymer) mullite precursors, and in other melt-quenched aluminium-silicate glasses suggesting strong structural similarities of these phases. This has been supported by the exothermic mullite crystallization process taking place at ≈ 980 °C in all cases.     

Electrical and mechanical properties of ferroelectric lead zirconate titanate/tungsten oxide ceramics

Ferroelectric PZT/xWO₃ ceramics (when x = 0, 0.5, 1, 3 and 5 vol%) were fabricated from PZT and nano-sized WO₃ powders by a solid-state mixed-oxide method. Phase characterization suggested that the reaction between PZT and WO₃ occurred during the sintering. This reaction seemed more pronounced with increasing the content of WO₃. The maximum density at approximately 97% of the theoretical value was achieved at 1 vol% of WO₃ addition. The grain size was reduced with an addition of WO₃ particles from 7.8 μm for PZT to 1.8 μm for 0.5 vol% WO₃ and 0.8 μm for 1–5 vol% WO₃. Mechanical properties of PZT could be improved with an addition of WO₃ nano-particulates. The addition of 0.5 vol% WO₃ could maintain good electrical properties while increasing WO₃ significantly reduced dielectric and piezoelectric constants of the PZT.     

Sub-micron particle dewatering using hydrocyclones

Hydrocyclones are used for dewatering of solid–liquid suspensions in many industries. Generally, however, large diameter cyclones are used and their application is restricted to large (>25 μm) particles. Small diameter (10 mm) hydrocyclones have the potential to be applied to fine particle (<10 μm) suspensions and, in particular, to collect the sub-micron fraction. This is due to the very small cutsizes that are achieved in these cyclones. In order to apply these small hydrocyclones industrially, knowledge of the range of their classification performance is required. It is found that these cyclones exhibit a fish-hook partition curve, and a high bypass fraction. The very small cutsize (<5 μm) and the relatively large bypass makes the effective collection of sub-micron particles possible. While in most hydrocyclone applications it is found that the bypass fraction equals the water recovery to the underflow, in 10 mm hydrocyclones the bypass fraction is considerably larger than the water recovery. This results in a high particle recovery to the underflow, as well as low water recovery, resulting in a high concentration ratio. Results will be presented to show the separation performance of different hydrocyclone outlet configurations and pressure drops. A general model will be presented that describes the fish-hook and that gives an explanation for its origin. It will be shown that 10 mm hydrocyclones yield a new operating regime for their application to sub-micron solid–liquid separation, as a result of high solids recoveries and low water recoveries.     

Simultaneous SO2/NOx removal by a powder-particle fluidized bed

Simultaneous dry removal of SO₂ and NO_x from flue gas has been investigated using a powder-particle fluidized bed. In a process of flue gas desulfurization by use of solid sorbents such as FeO (dust from a steel plant) and CuO, the smaller the particle size of sorbents, the higher the expected SO₂ conversion. In a powder-particle fluidized bed (PPFB), fine particles less than 40 μm in diameter fed into the bed are fluidized with coarse particles. But only the fine particles are entrained from the bed, and their residence time in the bed is remarkably long.

The reduction of NO_x with NH₃ in the fluidized bed is catalyzed by coarse particles or both coarse and fine particles. In this study, PPFB was applied to simultaneous dry SO₂/NO_x removal process, and several kinds of sorbents or catalysts were evaluated in a PPFB. Using the selected sorbents and catalysts, kinetic measurements were made in the temperature range of 300 to 600°C. SO₂ removal efficiencies were affected by reaction temperature, sorbent/S ratio, and static bed height. NO_x removal efficiencies in excess of 95% were achieved at NH₃/NO_x mole ratio of 1.0. When FeO was used as sorbent, SO₂ conversion increased with increasing temperature and reached 80% at 600°C.     

Effect of Particle Size on the Room-Temperature Crystal Structure of Barium Titanate

The room-temperature tetragonal-to-cubic transformation in BaTiO₃ powders with decreasing particle size has been carefully studied, using materials prepared mainly by hydrothermal methods. Hydrothermal BaTiO₃ powders exhibited a more uniform particle size distribution than oxalate-route powders, with X-ray diffraction and electron microscopy indicating that powders 0.19 μm in size were fully cubic while powders 0.27 μ were completely tetragonal (within a 5% detection limit for cubic material) at room temperature. The tetragonal-to-cubic transformation temperature was also found to lie in the range of 121°± 3°C for BaTiO₃ powders with room-temperature ( c/a ) values > 1.008. No transformation could be detected using differential scanning calorimetry for BaTiO₃ particles with a ( c/a ) > 1.008 at room temperature. BaTiO₃ powder with a particle size just too small (0.19 μm) to be tetragonal at room temperature remained cubic down to 80 K. Different models for the cubic-to-tetragonal room-temperature transformation are discussed. Hydroxyl ions do not appear to greatly affect the cubic-to-tetragonal transformation, which appears to be essentially dependent on particle size. It is concluded that a model based on surface free energy, as previously discussed for the monoclinic-to-tetragonal transformation at room temperature of fine ZrO₂ particles, is consistent with the experimental data.     

Hydrodynamic studies of an advanced fluidized-bed bioreactor for direct interaction with coal

Fine coal particles fluidized by the upflow of a liquid medium containing a dissolved biocatalyst undergo size reduction as the reaction progresses. Three aspects of the design of such a reactor were examined:

1. (1)the use of force balances to describe pressure drop for the segregated bed;

2. (2) measurement of liquid-phase dispersion coefficients; and

3. (3) fluorescent tagging of particles to track size distribution.

Hydrodynamic data were obtained for a liquid-solid fluidized bed of coal particles in the size range 30–150 μm. Illinois No. 6 coal was ball-milled, sieved into four fractions, and suspended in a 0.1% aqueous solution of Tween 80. A sample with a bimodal particle size distribution centred on 49 and 63 μm was placed in a glass column and fluidization and pressure-drop data were compared with a new model developed to describe particle segregation. Measured liquid-phase dispersion coefficients varied from 0.034 to 0.283 cm² s⁻¹ as the flow varied from 0.005 to 0.0159 cm s⁻¹. A technique was also developed for coating coal particles with a fluorescent paint which may allow direct measurement of the change in the fraction of marked particles of known size along the axis of a fluidized bed.     

Preparation of a fine powder composed of oxide–metal composite by an arc-plasma method

Fine oxide–metal composite powders have been prepared under an H₂–Ar gas environment using an arc-plasma irradiation technique. Initial Cu particles with grain sizes between 10 and 30 μm were mixed with TiO₂ powder of submicron size at different weight ratios to form targets by sintering. Mixtures with varying Cu/TiO₂ weight ratios were prepared, but mostly a mixture with the weight ratio at 95:5 was used. The targets were exposed to arc-plasma irradiation to form a fine composite powder. Particles prepared are Cu-fine core particles each coated with a thin film of TiO₂. The thickness of the oxide film ranges from a few nanometers to some tens of nanometers, and is independent of the ambient gas composition and irradiation. The size distribution of prepared composite particles is broad, and most of them range from 20 to 100 nm. H₂ gas in the chamber plays a significant role in forming the composite powder.     

Sintering behavior and mechanical properties of nano-sized Cr3C2/Al2O3 composites prepared by MOCVI process

A process using metal-organic chemical vapor infiltration (MOCVI) conducted in fluidized bed was employed for the preparation of nano-sized ceramic composites. The Cr-species was infiltrated into Al₂O₃ granules by the pyrolysis of chromium carbonyl (Cr(CO)₆) at 300–450 °C. The granulated powder was pressureless sintered or hot-pressed to achieve high density. The results showed that the dominant factors influencing the Cr-carbide phases formation, either Cr₃C₂ or Cr₇C₃, in the composite powders during the sintering process were the temperature and oxygen partial pressure in the furnace. The coated Cr-phase either in agglomerated or dispersive condition was controlled by the use of colloidal dispersion. The microstructures showed that fine (20 –600 nm) Cr_xC_y grains (≤8 vol.%) located at Al₂O₃ grain boundaries hardly retarded the densification of Al₂O₃ matrix in sintering process. The tests on hardness, strength and toughness appeared that the composites with the inclusions (Cr₃C₂) had gained the advantages over those by the rule of mixture. Even 8 vol.% ultrafine inclusions have greatly improved the mechanical properties. The strengthening and toughening mechanisms of the composites were due to grain-size reduction, homogenous dispersion of hard inclusions, and crack deflection.     

Features of corrosion resistance of AlN–SiC ceramics in air up to 1600°C

The kinetics and mechanism of (80% AlN + 20% SiC) and (50% AlN + 50% SiC) powders and ceramics oxidation in air up to 1600°C were studied with the aid of TG, DTA, XRD, EPMA, SEM and metallographic analysis methods. The ceramics samples were obtained by hot pressing fine-dispersion AlN and SiC powders with an average particle size of 1 μm at 1800°C for 2 h. This ensures a fine-grain material structure with a uniform distribution of phase components. It was shown that in a nonisothermal regime, a three-stage oxidation mechanism takes place. It was established that the scale formed consists of three oxide layers. In the inner layer, Al₁₀N₈O₂ oxynitride and β-SiO₂ (cristobalite) phases were observed; in the intermediate layer, β-SiAlON was found for samples with a relatively low SiC content whereas -Al₂O₃ was present in samples with a greater SiC content. The outer layer contains 3Al₂O₃.2SiO₂ (mullite) as a main phase, the latter ensuring highly protective properties of the scale. The materials investigated can be considered as having extremely high resistance to corrosion up to 1550°C.     

Effect of β-Si3N4 seed particles on the property of sintered reaction-bonded silicon nitride

The effect of seeding on the microstructural development and mechanical properties of sintered reaction-bonded silicon nitride was investigated by the use of β-Si₃N₄ particles. Seeding of an appropriate amount of the β-Si₃N₄ particles gave rise to the promotion of density and the resulting increase in fracture strength and hardness. The fracture toughness was also increased due to the development of elongated grains. On the other hand, addition of a large amount of seed particles showed lower strength and fracture toughness owing to the coalescence of large elongated grains. By seeding of 2 wt.% β-Si₃N₄ particles on Si powders of 7 μm, high fracture strength of 1100 MPa and fracture toughness of 7.2 MPa·m^1/2 were obtained in sintered reaction-bonded silicon nitride ceramics.     

Effect of Ultrasonication on the Microstructure and Tensile Elongation of Zirconia-Dispersed Alumina Ceramics Prepared by Colloidal Processing

To obtain dense, fine-grained ceramics, fine particles and advanced powder processing, such as colloidal processing, are needed. Al₂O₃ and ZrO₂ particles are dispersed in colloidal suspensions by electrosteric repulsion because of polyelectrolyte absorbed on their surfaces. However, additional redispersion treatment such as ultrasonication is required to obtain dispersed suspensions because fine particles tend to agglomerate. The results demonstrate that ultrasonication is effective in improving particle dispersion in suspensions and producing a homogeneous fine microstructure of sintered materials. Superplastic tensile ductility is improved by ultrasonication in preparing suspensions because of the dense and homogeneous fine microstructure.