Many studies have been made on liquid bridges between particles, but the mechanism of liquid bridge formation and its adhesion force when the bridge is in an equibrium condition with the surrounding humidity has not been theoretically clarified. A study has been made on this subject and a new theory is introduced in this paper. Next, a mechanism to disperse aggregate particles down to primary particles by the acceleration of an air stream is explained and a discussion is made on the dispersion force applied to aggregate particles and the adhesion force between particles (van der Waals force and liquid bridge force).
The need for preparing solid particles with required properties has led to the development of processes allowing control of these properties: high purity, size, size distribution, crystalline form, flowability, packability, or other specific effects. This paper deals with several “new” processes of preparation of solid particles of controlled properties. Most of these processes have been known for a long time but they have not been carried out extensively in production. The potential of such processes is shown.
If exhaust gases are to be cleaned from particulate matter at high flow rates and, simultaneously, at low pressure drops, an electrostatic precipitator (ESP) is a suitable device. ESPs are widely used for removal of fly-ash in coal fired power plants commonly operating at moderate temperatures up to about 200°C and ambient pressure. Today, particle collection at high temperatures and/or high pressures is of current interest because of new concepts for electrical power generation systems. However, the capabilities of electrical precipitators under these extreme conditions are only poorly investigated. Therefore, the paper works out the fundamentals of high-temperature and/or high-pressure electrostatic precipitation. It provides a basis for discussion and gives the interested reader several tools to permit estimating the potential for electrostatic precipitators under extreme conditions.
The grinding method for sample preparation affects the results of X-ray diffraction analysis of minerals, mainly the soft and inelastic components of raw materials and synthetic matrices. This fact is determined by a “disturbance” of crystal order in the surface layer of particles and is more pronounced when a fine comminution is performed and when a high shearing stress is applied in grinding. This effect is remarkable on asbestos minerals. Subsequent to some information on the development of the science of “mechanochemistry”, this paper reports the results of several tests on single, binary and polygenic materials containing chrysotile and crocidolite under different grinding conditions (type of mill and grinding time). The aim of the research is to give suggestions about a sample preparation methodology for XRD quantitative determination of asbestos in bulk materials, suitable for obtaining more accurate results.
Two distinct non-intrusive radiation methods known as scanning gamma ray tomography and single profile photon absorptiometry are presented in this paper as tools for the investigation of interstitial voidage distributions of granular flows in both gases and liquids. Single profile absorptiometry coupled with the consecutive radial transformation of the linear data obtained within seconds of flow time is used as a tomographic tool for dynamic voidage characterization in different flow regimes of air and liquid-based systems, as well as a complete tomographic projection procedure, which requires scan times of the order of minutes to achieve high spatial resolutions over a large cross-sectional area of the vessels, A novel tomographic technique currently under development known as dual photon tomography is proposed to facilitate simultaneous voidage mapping and particle tracking in 3-D granular flows.
The separation of fine particles from gases with particle sizes of about 1μm is still a difficult problem, especially at high solids loadings of the gas stream and high gas temperatures. Practical experiences show that the separation efficiency really obtained with hot gas cleaning is generally less than calculated. Based on new experiments and a better calculation procedure, it is also possible to use cyclone separators for these separation problems. For a successful solution, it is necessary to increase the knowledge about the gas/solids flow and to pay attention to the boundary layer flow within the cyclone, but also to think about new multi-stage separation processes.
Fine and granular products possess significantly different pneumatic conveying characteristics. Fine products usually have smooth conveying characteristic curves from dilute- to dense-phase. Between dilute- and dense-phase there is a pressure minimum curve. However, when granular products are conveyed between dilute- and dense-phase, significant fluctuations in pressure and vibration can occur along the pipeline. To obtain reliable operating conditions for the designed pneumatic conveying system, three semi-empirical correlations are developed in this paper for the calculation of the pressure minimum curve for fine powders and the boundaries between dilute-phase, unstable-zone and dense-phase flow for granular products in terms of particle properties and conveying conditions. Finally, a new procedure is presented for the prediction of total pipeline air pressure drop and the economic design of pneumatic conveying systems with the aim of reducing power consumption and particle damage.
ZnFe2O4 plays an important role as an adsorbent in the process of hot gas desulfurization. The influence of mechanical activation on the capacity S of ZnFe2O4 and on the following oxidation (regeneration) is investigated. The particle size is changed by mechanical activation in a characteristic way, and an inverse spinel structure is formed. With the mechanically induced degree of inversion the decomposition temperature of ZnFe2O4 is strongly diminished. The formation of the inverse spinel structure as well as the breakage of particle size lead to an essential increase of reactivity in the reaction with H2S. The following oxidation of the sulfurized zinc ferrite is also influenced by the primarily performed mechanical activation. For the investigations, ZnFe2O4 which had been treated in a planetary mill was used. The sulfurization was carried out with compressed powders. The characterization was performed mainly by X-ray diffraction (XRD), scanning electron microscopy (SEM), measurement of particle size and chemical analysis.
A method was developed to granulate ceramic particles using metallo-organic soap as a binder. A high shear mechanical mixer (i.e., mechanofusion) was used to coat the soap over the ceramic particle surfaces. Agglomeration of the coated particles was triggered by increasing the mixing temperature to the softening point of the soap and transforming the soap from a crystalline to an amorphous phase. The crystalline soap acted as a solid lubricant and the amorphous phase was a cohesive binder. Granule growth was correlated to the thermomechanical properties of the soap, which depended on temperature, applied pressure and compaction history. The resultant spherical granules were densely packed assemblages of coated particles (i. e., multicore microcapsules). Sequential cladding of layers onto pre-formed core granules resulted in microstructures with hierarchical compositional ordering.
Composite particles are formed using negative and amphoteric monodispersed latices as the core and shell respectively, and the experimental conditions to obtain maximum coverage of the surface of core particles with amphoteric particles and the mechanism of the deposition process are investigated. As results, (1) the experimental conditions for forming the composite particles are clarified; (2) the maximum coverage of the core surface with amphoteric particles is found not to be greater than 0.3; and (3) the coverage is mainly determined by the strength of the repulsive force between deposited particles. The relation between the coverage and the zeta potential of composite particles is also clarified.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 18(5), 637-642 (1992) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
In order to apply hydrodynamic Gelds to the shape separation of fine particles, the dependencies of dynamic properties of particles in a cyclone on the particle shape were discussed. Glass beads, glass powders, silica sands and mica particles in a 10–100 μm size range were classified into five fractions by wet cyclones. The equivalent diameters of classified particles were measured by a free settling method, and the geometric parameters were obtained from SEM microphotos. The Stokes diameters in free settling were in agreement with the mean values which were estimated for an ellipsoid and a thin disc to settle in random orientation. On the other hand, Stokes diameters in the cyclones were smaller than those in free settling. The discrepancy between these diameters was large especially for flaky mica particles, suggesting that a cyclone can be used to separate flaky particles from bulky ones.
†This report was originally printed in J. Soc. Powder Technology, Japan. 29(11), 838-844 (1992) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
A new method has been developed to measure and evaluate the operational powder characteristics for fine particle processing based on the reentrainment phenomena. Experiments on particle reentrainment were carried out using an accelerated air flow for 21 different test powders. Both the reentrainment fluxes, which were measured by an electrostatic method, and the average air velocities were automatically sampled by a computer. The sampled data were processed to obtain the reentrainment profiles as a function of the average air velocity. The cumulative reentrainment efficiencies were also obtained and represented as a function of the wall shear stress. It was found that fine particles having tendencies to form large aggregates were reentrained mainly in the early stage of the measurement when the flow velocity was as low as 5 m·s−1. Also, the mass ratio of the reentrained large aggregates to the total particles was used to obtain information on the particle-particle (cohesion) or particle-wall interactions (adhesion). On the other hand, the cumulative reentrainment efficiency-curves must be applied to determine the operational conditions controlling the amount of adhered particles in various aerosol processes.
†This report was originally printed in J. Soc. Powder Technology, Japan. 29(12), 897-905 (1992) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
A mechanofusion apparatus which can operate at a high vacuum pressure of the order of 10-2 Pa was developed. Using this apparatus, formation of Cu composite with Al2O3 core particles was conducted. The rate of progress of composite formation was low under atmospheric or low vacuum conditions but under a high vacuum or in an Ar displaced atmosphere, such formation was remarkably accelerated. Namely, it was found that composite formation is greatly accelerated by lowering the oxygen partial pressure. Following oxygen analysis of the Cu covering layer of composite particles by AES, it was found that with a higher atmospheric pressure, the oxygen amount became higher and oxygen penetrated into the interior. As a result of microstructure observations of the composite particles to which extended mechanofusion processing and chemical etching had been applied, grain boundaries were observed in the Cu covering layer which may imply that the Cu particle surface becomes exposed to high temperature as more sintering occurs. This microstructure is very similar to the structure of Cu after high temperature annealing. The individual particle surface temperatures during mechanofusion was estimated as being around 1000 K from the experimental results of sintering behavior of cold pressed Cu.
†This report was originally printed in Journal of the Japan Society of Powder and Powder Metallurgy, 39(12), 1124-1128, 1129-1133 (1992) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of Japan Society of Powder and Powder Metallurgy.
Removal of fine particles from wafer surface by air jets was experimentally investigated in order to seek an effective surface-cleaning method which uses no cleaning liquids. Monodisperse polystyrene latex particles with diameter between 0.25 and 3.3 μm were deposited on a silicon wafer by gravitational settling and removed by air jets from a rectangular nozzle. Particles were blown off the moment the air jet struck the wafer surface, and afterwards no particle reentrainment occurred. This suggests that the sequential pulses of air jets are effective for the removal of fine particles. By exposure of wafer surface to sequential pulse air jets, particles with a diameter as small as 0.25μm were almost completely blown off the surface. The experimental results also indicated that the removal efficiency of particles per pulse air jet, which is the ratio of number of particles reentrained during an air jet exposure to that before the exposure, is kept constant for each exposure to sequential pulse air jets.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 19(1), 114-119 (1993) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
Fine Al2O3, MgO and SiO2 powders of various sizes were consolidated into a dense and uniform structure by high-pressure cold isostatic pressing. Although the average particle diameter was less than 21 nm, Al2O3, MgO and SiO2 powder compacts were compressed to 60% of their theoretical density by cold isostatic pressing at 1 GPa, because the open and strong aggregate structure collapsed under the cold isostatic pressure. The pore size of these compacts decreased below the primary particle size. Especially, in the case of MgO powders, the maximum relative density increased to 80% and the ratio of pore size to primary particle size was less than 20%. However, in the case of Al2O3 powders, whose size was more than 100 nm, a slight increase in relative density and decrease in pore size in the compacts occurred with an increase in isostatic pressure up to 1 GPa. In the case of particles larger than 100 nm and high-hardness materials, high cold isostatic pressure was not effective in increasing the packing density. The mean vertical force and compressive stress at the contact point between particles in compacts and the maximum tensile stress in a particle during isostatic pressing were estimated using Rumpf's and Hertz's equation. Because these stresses were smaller than the hardness and tensile strength of Al2O3 or MgO materials, no viscous deformation or fracture of particle took place during compacting. The relations between the ratio of mode pore diameter to particle diameter and the relative density of Al2O3, MgO and SiO2 compacts agreed with those of spherical and monosized particle beds. The main mechanism of densification of compacts during isostatic pressing was the collapse of aggregates and rearrangement of particles.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 19(2), 220-229 (1993) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
A dry powder coating method has been applied to develop high-performance contact-materials for the electrical industry. Using a high-speed/high-shear type of mill, the dispersion and compounding process of particulate Ag and nano-meter-size Ni powder were investigated. The transformation of the particles throughout the process was shown by the torque and temperature curves during the treatment in the mill. The optimum operating conditions for both dispersion of fine particles into the matrix and sintering process were discussed based on the physical properties of bulk powders and sintered composite.
†This report was originally printed in Journal of the Japan Society of Powder and Powder Metallurgy , 40(3), 299-302 (1993) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of Japan Society of Powder and Powder Metallurgy.
Experimental and theoretical studies were conducted on the separation of submicron powder using a revised type of cyclone. The cyclone has a movable inlet guide plate at the inlet section, and 50% cut size is controlled from 0.45 to 0.75 μm by changing the inlet clearance of the guide plate. But the 50% cut size increases again with a very narrow inlet clearance of the guide plate, because the radial velocity of particles decreases and fluid turbulence increases under such condition. The 50% cut size decreases to about 0.4 μm when both the inlet guide plate and the blowdown method are used. The partial separation efficiency with a spiral-inlet cyclone is higher than a cyclone with tangential inlet, and the 50% cut size decreases with a highly polished inside wall surface.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU, 19(3), 476-482 (1993) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.