Attrition causes material loss and environmental hazards in powder processing. In fluidised beds, the jetting region is the main contributor to attrition. The present paper reviews the recent investigations of the effects of the interaction between single particle properties and jet hydrodynamics. A model of attrition in the jetting region of fluidised beds is presented, based on the impact attrition propensity of single particles and on modelling of the particle flow patterns in the jetting region. The experimental work, carried out for the purpose of evaluation of the model, focused especially on the effects of orifice gas velocity and diameter. The experiments involve measurements of impact attrition of single particles and measurements of particle velocities and solids concentrations in fluidised bed jets. The test materials are fluid cracking catalyst and common salt, both representatives of widely used classes of composite and crystalline materials, respectively. Significant effects of orifice gas velocity and diameter were predicted, which corroborated the experimental data. Hence, the model successfully establishes a link between single particle properties and bulk behaviour in a fluidised jet.
In addition to the process variables, the quality of the powder metallurgical products depends to a large extent on the materials, bulk properties and morphological characteristics of the powders. Material characteristics are relatively well established but the same is not the case with the other two variables because unequivocal definitions of some of the morphological characteristics like particle shape, size and texture on which the bulk properties are strongly depended are yet to be evolved. Among the morphological characteristics, particle shape, though one of the most investigated area remains the least understood. In spite of the complexity of the problem, the necessity for obtaining a better insight of concerned branches in which powders are extensively used have yielded many dimensional and dimensionless parameters for characterizing the powders based on particles shape. The present paper classifies the available literature on the particle shape into four main categories, briefly describes their salient features, critically assesses the merits and demerits of each class and thus makes the perspective more clear for proper and judicious application of one or more methods, of course, depending on the level of accuracy required for charactering the powders.
The tendency of particulate materials to separate into various zones depending on their various physical characteristics is called segregation. The phenomenon is widely encountered in nearly all materials handling situations and leads, quite often, to significant variations in quality. This paper reviews the mechanisms which contribute to segregation and demonstrates, by means of laboratory scale tests, that the particular segregation pattern likely to be encountered in a given situation is a function of both material and process-related variables.
The compaction behavior of ceramic powders is reviewed from the viewpoint of the factors involved and how these relate to green microstructure. The basis for the observed linear relationship between the logarithm of compaction pressure and relative density is discussed, and five generic types of compaction behavior are presented. The relation of granule properties to compaction behavior, green strength and green microstructure are discussed. Die friction, springback, die filling and green microstructure evolution are reviewed, and the needs for a better understanding of pressing are discussed.
It is the aim of this paper to show that it is possible in principle to obtain submicron cut sizes with a counterflow centrifugal classifier, provided that a forced vortex is used in the classification chamber and the circumferential velocities are greater than 100 m/s. These findings can be generalized and are not limited to the geometrical dimensions of the classifier used in the experiments. With a new impeller or deflector wheel classifier, cut sizes as small as 0.3 μm have been obtained with feed mass flow rates of 5 kg/h and higher.
In conventional biotechnology, bioparticles, such as cells or DNA are treated as uniform suspensions or solutions, and one can neither specify a particular particle, nor a particular position on the particle. On the other hand, recent advances in micromachining have enabled the fabrication of μm-scale structures, which, in combination with appropriate bioparticle manipulation method, is expected to introduce "spatial resolution" to the biotechnology, where individual bioparticles are handled in more efficient and automated manner. The author's group has been engaged in the development of electrostatic micromanipulation of bioparticles in microfabricated structures, using dielectrophoresis, electrostatic orientation, electrorotation, etc. as the actuation principle. This article summarizes our experimental results on cell and DNA manipulation/processing, together with the theoretical background.
Solids mixing refers to the blending of particulate components differing in one of their properties. To compare mix and mixer theoretical standard deviation σ is defined as the mixing quality. To simulate the stochastic mixing process the Fokker-Planck-Equation is used. Similar simulations described in the literature up to now, in which the transport coefficient is determined from the throughput, are not generally valid. It could be shown with the modified method that the mixing results depend only on the relationship between the residence time and the fluctuation time of the feed material streams. A model, based on a suggestion of Schulze, is useful to simulate deterministic silo mixers. The experiments confirm the results of the simulations.
The subject of the paper is the application of pulsating flow to the particles heat treating. Pulsation influence mechanisms are segregated by the author into two groups: quasi – steady – state (when, at every point in time, an instantaneous value of the heat transfer rate may be calculated with the equations obtained for steady-state conditions) and unsteady – state which includes the remainder. It is shown that, depending on the process parameters, pulsations can increase, not influence, or even reduce heat transfer. Resonance phenomena and large size particles, for which the influence of gravity is substantial, receive special attention. The heat transfer intensification may be achieved due to increases in both heat transfer coefficient and particle residence time. Longitudinal pulsations (when the pulsation and main flow velocities have the same direction) may be preferable in the unsteady-state range of parameters. Transverse pulsations (when the pulsation velocity is perpendicular to the main flow) can be more effective in the quasi – steady – state range. Nonuniformity of heat treating in pulsating flow, influence of the main flow turbulence, trigger effects, and different methods of pulsation generation, including pulsating combustion, are discussed. In conclusion some recommendations are given for designing practical processes.
Particles of diatomaceous earth and sand were coated with metallic hydroxides. The coating increased the concentration of metals associated with the particles, their zeta potential, and their ability to adsorb microorganisms in water. The maximum adsorption of microorganisms was achieved using particles coated with a combination of ferric hydroxide and aluminum hydroxide.
Sol-gel techniques are interesting for the synthesis and processing of fine particles if the problems of stabilization of colloidal particles, their agglomeration and the fabrication of green bodies with high density are solved. A new approach for the fine particle fabrication from solution was realized by controlling nucleation, growth and surface reactivity by surface modification through selected organic molecules. A variety of nanopowders (e.g. ZrO2, Al2O3, PZT) have been synthesized, and a scaling-up process has been established. Ceramic green bodies show substantially reduced sintering temperatures.
This paper reports a historical description of the "Brazilian Method For The Determination Of Airborne Inorganic Fibres Concentration (including asbestos fibres) -Membrane Filter Method" since the creation of the Technical Group Organization for the study of the several existing methods in 1987, until the publication of the definitive method by "ABNT - Associação Brasileira de Normas Técnicas " (Brazilian Association for Technical Standards) in 1994. The Brazilian Method is very similar to the methods from AIA-Asbestos International Association, to NIOSH-National Institute for Occupational Safety and Health, and ISO- International Standards Organization in collecting samples, preparation and analysis. The main differences are the counting criteria which do not consider the fibre aggregate particulate matter.
This paper reviews techniques for obtaining and processing images of particles and objects (irradiation, response, surveying, storage and analysis). It presents the methodologies by which images can be classified and recognised (detection of domains, boundaries, shapes and textures) with a view to developing correlations between the information provided by the images and the physical and chemical properties of the examined particles. It discusses procedures for recognising the boundaries of images of particles, for analysing their properties (geometrical, Fourier series, fractals, etc.) and for recognising the structures and textures of multi-component particles. The instruments used to acquire and store the images and the hardware needed to process the digitized information are described and assessed. The software needed to analyse the data (loading, colour-level correction, enhancement, filtering, thresholding and labelling) are presented. Procedures for classifying vector structures that may be used to characterise images of individual particles (pattern vectors) or classes of particles (feature vectors) are discussed, and those which are used to recognise particles are analysed. Applications of these methodologies and several case studies concerning mineral grains (free and associated), polished sections of minerals, macerals, inorganic and organic materials are described.
The shape and texture of construction aggregates are important parameters that have a direct bearing on the strength and durability of their asphalt and concrete end products. Although shape standards may vary throughout the world, nearly every country characterizes shape in terms of elongation and flatness. Typically, a given batch of material is rejected if more than a specific percentage of particles have elongation and flatness ratios which exceed some limit. Present procedures for determining these ratios rely on manual techniques which are tedious and tend to limit the number of samples that can be analyzed. Researchers at Virginia Tech have recently developed a rapid shape analysis system which can determine elongation and flatness ratios for a standard batch of 100 particles in under 10 minutes. The system consists of an image analyzer constructed around a personal computer. Results obtained indicate an excellent agreement between the rapid analysis system and standard manual techniques. In addition, the system is capable of providing two quantitative measures of particle roughness. The development and validation of the analyzer and its measurement procedures are discussed.
Previous work by the author1-4) on particle size segregation in bins has been extended to consider the effect of the concentration of coarse particles. At low coarse concentrations a simplification can be achieved by plotting the non-dimensional concentration of coarse material C/CF to summarise the profile. The present study reveals that at high values of CF, the coarse feed concentration, the segregation pattern is highly dependent on CF. The addition of small amounts (less than 1 %) of water to feed mixtures has a profound influence on segregation behaviour in many cases smoothing what would otherwise be severe segregation patterns with dry materials. Previous study contained speculation that segregation was reduced because the water caused an increase in cohesion of the fines content of the feed which in turn slowed the progress of coarse material relative to fine. Further discussion of this phenomenon is provided here.
When mixed with air many natural or artificial types of dust may explode with an energy several times higher than the equivalent mass of TNT. The origin of the dust and its basic properties, which are important from the point of view of a dust explosion, are discussed in the paper. Special attention is focused on the problems of mixture formation, ignition and the subsequent flame propagation. Methods of evaluation of the structure and burning velocities for laminar and turbulent flames are given. Conditions for flame acceleration and transition to detonation are also discussed. Finally, the influence of both inert and reactive particles on homogeneous and dust mixture explosions is presented.
Microcapsule toners were prepared in two steps: fine core particles and the wall of the capsule were prepared in the first and the second steps, respectively. The core material was stearic acid in which carbon black was dispersed. The wall material was the fine polystyrene beads prepared by the soap-free polymerization. In the experiment, operating conditions, such as the volume fraction of stearic acid and the impeller speed and the mixing time in the preparation of the first O/W dispersion, were changed stepwise. It was investigated that how the operating conditions affected the properties of microcapsule toners, such as the size distribution, the mean size and the contents of ferrite powder and the wall material. The mean size of the toner decreased with the increase of the impeller speed and the mixing time (DAV∝Nr1-1.2, DAV∝t1-0.48). The content of the wall material increased with the polymerization time.
†This report was originally printed in Journal of the Japan Society of Colour Material 67(10), 624-631 (1994) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Japan Society of Colour Material.
The effect of electrostatic repulsion force arising from the surface potential ψ and the double layer I/χ around a particle on dispersion or de-agglomeration of agglomerated particles in water is discussed. Several suspensions of PSL (polystyrene latex) particles in an agglomerated state where Ψ and 1/χ are respectively controlled by the pH and the electrolyte concentration Ce were prepared. They were accelerated in a convergent nozzle to give an external force on particles and agglomerated particles were dispersed into primary particles. PSL particles thus dispersed were observed with an optical microscope. It was found that the dispersion of agglomerated particles were enhanced with an increase of Ψ and Ce under a constant external force or a constant flow rate in the nozzle. It is well understood that the dispersion of particles with higher surface potential ψ is much easier. It is, however, intuitively curious that the dispersion of agglomerated particles in higher electrolyte concentration Ce is much easier, since coagulation is usually enhanced in such a higher Ce suspension. The experimental results may be applied to control the dispersion of agglomerated particles in water.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 21(2), 372-377 (1995) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
Composite particles of oxide superconductor were prepared using two kinds of aerosol processes, which were one used an electrostatic force and the other used a vibro-fluidized bed CVD reactor. The composite particle was constructed of an oxide superconductive core particle coated with fine particles and/or a thin film, of which secondary phases acted as magnetic flux pinning centers and sintering aids, etc. The microstructure control of composite particle and the enhancement of the pinning properties of sintered bodies were discussed.
†This report was originally printed in J. Soc. Powder Technology, Japan. 32(3), 144-150 (1995) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Grinding tests were carried out using a model agitation bead mill with a single disc. Four different types of disc were used to examine the effect of the motion of the grinding media in the axial direction. The median diameter of the ground product decreased with increasing axial movement of the grinding media under the same degree of fitting of the grinding media and rate of rotation. Contamination in the product from the agitating element and grinding vessel increased with increasing energy input, irrespective of the shape of the agitating disc. Contamination from the grinding media increased with an increase in the axial movement of the grinding media which promoted mutual collision and hence abrasion of the grinding media. KAGAKU KOGAKU RONBUNSHUVol. 21 (1995) No. 3 P 502-507
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 21(3), 502-507 (1995) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
The influence of particle size (0.019-0.24 μm) of TiO2 on UV-ray shielding capability was studied. UV-ray shielding capability of TiO2 depended on the wavelength. That is, the smaller the primary particle size was, the greater its shielding capability against around 300 nm of UV-B range was. On the contrary, for the shielding capability against around 350 nm of UV-A range, an optimum primary particle size was found. Furthermore, the optimum primary particle size varied, depending on dispersion strength of the particles in a paint.
†This report was originally printed in Journal of the Japan Society of Colour Material 68(4), 203-210 (1995) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Japan Society of Colour Material.