Selective flocculation is a potential process for solid-solid separations in the fine particle size range. The critical barriers to further commercial applications of this technique are the loss in predicted single mineral selectivity in mixed mineral systems, extrapolation of mixed mineral selectivity to natural systems and scale up from bench to plant scale. A comprehensive review of the recent advances in selective flocculation technology which facilitate to overcome these barriers is presented. These include identification/ design of selective reagents based on the surface chemistry of particles, use of site blocking agents to reduce heteroflocculation, and tailoring the polymer characteristics to achieve the desired separation efficiency.
Attrition is commonly found in the clearances of equipment where a mechanical device moves relative to a wall. Particles are trapped and broken in these regions, hence promoting attrition. This work concerns a cell where a blade rotates parallel to a wall. A cone cell design allowed for various gap sizes, particle velocities and blade tip speeds. The breakage rate of particles changed dramatically with gap size. Breakage commenced at a gap size close to a half particle diameter but had a definite minimum between one and one-and-half particle diameters. Little breakage was experienced if the gap much exceeded two particle diameters. When the other conditions of cell are changed, firstly the blade speed and secondly the flow rate of particles through the cell, the effect of gap size on breakage pattern is not changed. The product size distribution can be quite complex and is not simply related to the gap size. Segregation and a changed packing structure may each influence the breakage of a mixture.
Aerogels are highly porous sol-gel derived solids. Their properties, which are extremely fascinating as well as very promising for a large number of applications, are due to their nanostructure. Aerogels can be produced from a variety of materials, whereby silica aerogels are the most thoroughly studied ones. Silica aerogels with densities ranging from a few kg/m3 to a few hundred kg/m3 have been made as monoliths, in granular form or as powders. This paper describes the fundamentals of the sol-gel process, drying techniques and the basic aerogel properties. Special attention is given to their nanostructure and the resultant very low thermal conductivity. Most of the applications so far are based on the thermal, mechanical and optical properties of silica aerogels which are described.
An excellent electrorheological (ER) suspension is invented with composite particles consisting of polymer core and inorganic shell. When the composite particles are subjected to jetstream agitation, the ER effects are strikingly enhanced. Since the ER effects can be attributed to the shell layers on the polymer particles, the surface of inorganic particles must be barely exposed to the surrounding oil. The importance of surface conditions is demonstrated by a scaling analysis. The creep curves at low stresses for suspensions in electric fields are composed of instantaneous elastic and retarded elastic regions. But the suspensions show no elastic recovery after the removal of stresses. The creep and recovery behavior is purely plastic. The dipole-dipole interactions cause chainlike structures of particles. The development of Bingham yield stress can be derived from the ideal chain model in which the particles all align into chains of single-particle width and equal spacing. However, the model cannot predict the solidlike deformation without recovery. The thick column formed by several chains may be responsible for purely plastic responses. The particle concentrations in column are increased in nonuniform electric fields. Since the increase in particle concentration of column lead to high yield stresses, the ER performance of suspension as an overall response can be improved by the electrode design.
Dust explosions in the process industries most often start inside process equipment such as mills, dryers, mixers, classifiers, conveyors, and storage silos and hoppers. For any given type of dust, the ease with which the dust cloud ignites, and the rate with which it burns, vary considerably as a function of parameters well known in powder science and technology. The most important are the primary particle size distribution of the dust, the degree of deagglomeration of the dust particles in the cloud, the dust concentration distribution in the cloud, and the cloud turbulence. The last three properties are entirely dependent on the actual process situation in which the dust cloud is generated. The paper discusses the influences of the four parameters on ignition sensitivity and explosibility, and on the design of mitigatory measures such as venting, isolation and automatic suppression. The role of powder science in understanding the development and propagation of secondary dust explosions is also outlined.
The generation of aerosols has always been an important concern in studies of the hazards of inhaled materials. In recent years the need to generate aerosols for test purposes has spread into many areas of technological endeavour. Literature dealing with the problems associated with the various techniques of aerosol generation has spread into a diversity of technical areas and the relevant literature is not always easy to retrieve, especially since the term "Aerosol Generation" rarely appears in the title of an article. For example, a study of the use of a diffractometer based method of measuring a size distribution would not state that efficient generation of aerosols is a major concern in the accuracy of the data. In this review article, the diverse literature is focused to facilitate the retrieval of appropriate information for scientists who must generate aerosols as part of a larger investigation.
Powders and other granular media are composed of discrete particles and force transmission through a system of particles can only occur via the interparticle contacts. The distribution of contact orientations affects the magnitude of the contact forces which are not uniformly distributed. Since the contact stiffness and contact area are functions of the contact force, the manner in which the contact forces are distributed determines the shear strength, compressibility and conductivity characteristics of granular media. The paper reviews the current understanding of force transmission in granular media as gleaned from both physical experiments and numerical simulations and discusses the mechanical implications.
The importance of stirred media mills in various industries as well as in research is steadily increasing because the quality requirements of comminution products, e.g. the product fineness, rise continuously. This paper gives a survey of different mill designs and of the influence of various operating parameters on the specific energy consumption. The specific energy necessary for a certain product fineness depends above all on the stress intensity, which is among other things a function of the grinding media size and the circumferential stirrer speed. At similar stress intensities, the influence of the stirrer and grinding chamber geometry is small and scale-up using the specific energy is usually possible. Moreover residence time behaviour, wear of grinding media and autogenous comminution are discussed.
Based on a systematic experimental investigation of the mechanical properties of alumina powder a new general 3-D constitutive model for compaction of powders has been developed. A significant feature of the model is that it describes the time effects on compaction. These effects have been revealed by creep and relaxation phenomena and by the influence of the strain rate. Although the theory is applicable to general 3-D conditions all the parameters involved in the model can be derived from the results of a few number of conventional triaxial drained compression tests. The procedure to identify the model parameters from data is general. Unlike the models available in the literature, the model developed in the present paper accounts for both compressibility and dilatant behavior of powders. Thus it provides a realistic description and a better understanding of the mechanisms of powder pressing. The theory is able to model important aspects of powder behavior with a degree of accuracy which appears to make it useful in FEM analyses of stress and density distribution in a pressed compact.
Mechanofusion (MF) is a unique high energy powder processing technique, in which powder characteristics can be significantly modified. The effects of MF on powdered materials depend on MF operating conditions and the starting powders. This paper proposes a preliminary MF modeling concept to develop quantitative relationships among MF variables and their effects. In the modeling, an inner piece "action zone" is defined. It is believed that most MF effects are generated when the particles pass through the action zone. From the model, the pressure imposed on the powders by MF can be described via the geometry of the device, operation speed and material properties. In addition, some basic particle kinematics variables can be estimated. The results derived from the modeling are compared to reported experimental results.
Surface modified particulates have many potential industrial applications ranging from new technologies such as rechargeable batteries, flat-panel displays, etc. to a wide range of unit operation processes such as dispersion, transport and handling, and separation of particulate systems. Due to environmental constraints, there has been a strong interest in the development of dry methods (chemical and/or water free) for particulate coatings. In this paper, we report the feasibility of novel dry method based on a magnetically-assisted impaction coating (MAIC) process for synthesis of composite particulates. In the MAIC process, magnetic particles are accelerated in the chamber using an alternating electromagnetic field. The magnetic particles in turn collide with the core and with submicron sized fine particles (secondary particles) to form composite particles. The adhesion of the secondary particles on the core particles was found to be dependent on several factors including particle size, particle hardness, etc. and a number of processing parameters. Experiments were conducted on a wide variety of particles systems such TiO2/PMMA, Alumina/PMMA, Ag/PMMA, TiO2/ Al2O3, etc. to understand the effect of these parameters. PMMA were preferred to use as a core (primary) particle because of its smooth surface which minimizes surface roughness effects. The composite particles were characterized using standard materials techniques such as scanning electron microscopy, energy dispersive X-ray microanalysis, etc. The results show that MAIC process significantly improved the surface coverage compared to the coverage obtained from standard blending methods. High frequency ultra-sonication of composite particles was also conducted so that weakly adherent particles could be removed from the surface. The efficiency of the coating process was found to be decreased with increasing secondary particle size. High surface coverage was achieved for composite particle with soft cores (e.g. PMMA). To understand the effect of particle hardness on the adhesion process, atomic force microscopy (AFM) studies were conducted as a function of the particle hardness. Based on experimental observations, a model for the particle coating process has been developed.
Coating or blending of fine particles on the surface of primary powder particles is often applied to improve and control the flow behavior of the powder. The effect of coating primary powder particles with fine particles on the cohesion force between primary powder particles is quantitatively examined. The JKR theory is extended to include the effect of particle coating on the force-displacement relationship due to surface energy and elastic deformation. It is shown that the cohesion force between two primary particles in the presence of a fine coating particle is directly proportional to the size ratio of the coating particle to the host powder particle and results in a drastic reduction in the cohesion force. Through discrete element simulation of powder flow, which uses the force-displacement relationship based on the extended JKR theory, the improved flowability is demonstrated. Competing effects of the coating and shear induced migration on the macroscopic behavior of the power flow are discussed. The effect of coating on improving the flowability is also experimentally demonstrated by comparing the measured angles of repose at a static condition and the flow rates of the gravity driven flow through a funnel for powders with and without particle coating.
A system utilizing a coherent sheet of light obtained by a pulsed laser and appropriate optical components was developed to monitor a charged particle stream as it passes through an electric field. The deflection patterns of narrowly sized quartz particles (74×53 μm) were photographed and analyzed by a densitometer. The deflection and dispersion of a spot, representing a group of particles, can be used to determine the average charge and charge distribution in the particle stream. The results demonstrate the efficacy of this technique for determining the charge distribution of a particle stream. Such a device has the potential for monitoring particle charge in continuous airborne particulate streams.
A method for the preparation of stable colloidal suspensions containing discrete molecular sieve crystals is presented. The hydrothermal synthesis of the all-silica tetrapropylammonium silicalite-1 molecular sieve illustrates the preparation. The discrete colloidal crystals ( < 150 nm) are aggregates of plate-like primary particles and they are stabilized in aqueous solution via steric forces that arise as a result of the strong adsorption of the tetrapropylammonium cation on the siliceous surface. The silicalite-1 surface charge is a function of pH as shown by electrophoretic measurements. These properties may be utilized for fabricating advanced materials. An example of the application of the colloidal materials is illustrated by the preparation of a thin silicalite-1 film on a non-charged gold substrate. Knowledge of the surface chemistry of the participating interfaces and the properties of the colloidal crystals allows for the preparation of microporous films with a thickness less than 300 nm.
Kawashima developed the spherical crystallization technique to produce shaped and sized particles with improved properties. He presented two methods: Spherical Agglomeration (SA) and Emulsion Solvent Diffusion (ESD). It was established that ESD proceeds from a spontaneous emulsion of the drug solution into a non-solvent liquid, whereas the solvent and the non-solvent are miscible. We investigated the general pharmaceutical case of a drug substance (DS) which is highly soluble in acetone (S), and moderately soluble in water (NS). The phase diagram DS/S/NS confirms that a concentrated DS/S solution poured into an NS will generate a biphasic liquid/ liquid system (DS/S emulsion into NS), which then moves into a triphasic liquid/liquid/solid one (DS into DS/S emulsion into NS) by counter diffusion S ↔ NS. Droplet size and mixing conditions are major determinants of where and at which speed solid growth will occur in the droplet. In large droplets, the solid growth takes place on the surface and extends in layers to the center. In small droplets, a homogeneous texture is obtained. This article presents a complete kinetic model of these mechanisms, which show how a specific texture can be selected and the whole process controlled. Two kinds of grains were produced and compared according to the micromeritic criteria required for direct tableting. The question of tablet cohesion is not critical with the drug we tested, and both grains are effective with regard to flowability whereas the powder produced from the standard crystallization technique is not suitable. With other drugs, a particular texture may be preferred, depending on cohesion and dissolution rate.
Both conical and cylindrical cyclones are used in the size classification and density separation of particles. Conical hydrocyclones have a more general use and have been thoroughly studied concerning both flow velocity profiles and their classification performance. Cylindrical cyclones with peripheral underflow discharge have been used less and studied less, but there has recently been renewed interest. This concerns particularly density separation as well as size classification in two-stage cylinder-cyclone systems. Tangential velocity profiles differ in conical and cylindrical cyclones. In conical cyclones, they approach the theoretical free vortex profile (with a forced vortex region confined near the axis) ; in cylindrical cyclones with peripheral underflow discharge they approach the forced vortex profile in a large part of the vessel. These results have been obtained by making Laser Doppler Velocimetry (LDV) measurements on 100-mm diameter cylindrical cyclones with peripheral and axial discharges under different flow conditions and with different ratios between overflow and underflow flow rates. It has been demonstrated that the occurrence of the one or the other vortex type does not depend on the vessel shape alone but on the inward radial flow patterns inside the vessel. This depends on how the fluid is fed and discharged as well as on the vessel shape. Finally, the paper discusses the influence that the different tangential and axial velocity profiles in conical and cylindrical cyclones have on separation sharpness, in both size classification and density separation processes.
Using the mechanical processing method called mechanofusion to prepare composite particles, the authors conducted a study on the preparation of metal-ceramic composite particles in a vacuum. The major principle in forming composite particles is the application of mechanical forces to a mixed powder in the apparatus used. The measurement of those forces, however, has seldom been attempted. Here we used strain gauges in an attempt to quantitatively measure the forces acting on a powder bed. An experiment using glass beads as the raw material showed that (1) if the drum's rotational speed is high and the powder charge is large, great compressive and frictional forces are generated, and (2) compressive force is several times larger than frictional force. These forces were measured during the preparation of composite particles using Cu-Al2O3 mixed powder under reduced pressures, and both forces appeared to be higher when atmospheric pressure was reduced.
†This report was originally printed in J. Soc. Powder Technology, Japan. 32(12), 866-873 (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.
For better understanding of gas and particle motion in a fluidized bed, we numerically solved the locally averaged three-dimensional Navier-Stokes equations and the Lagrangian equations for particle motion while taking into account the collisions between particles, and gas-particle mutual interaction. Finite difference methods were used. This simulation obtained locally averaged variables, such as gas velocities, by averaging point variables over the very small region of one computational cell. Small stress terms caused by fluctuations in locally averaged values were ignored at the present stage. In calculating collisions, particles were regarded as hard spheres with a collision time of zero, meaning that collisions occur between only two particles. Results show that the hard sphere collision model can be used to simulate flow fields in fluidized beds whose particle concentrations are relatively dilute.
†This report was originally printed in Kagaku Kogaku Ronbunshu, 22(1), 67-75 (1996) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
Dynamic shape factors for regular-shaped agglomerates, such as rectangular parallelepipeds, V- and W-shapes, and others, have been calculated from their measured terminal settling velocities in the Stokes regime. Experimental results have been analyzed by using a new correlation based on two-sphere hydrodynamic resistance as presented in a previous work. Estimated and experimental dynamic shape factors are in good agreement, with deviation smaller than 5%. The correlation can estimate the dynamic shape factor of a double-sized symmetrical agglomerate from its components, and also in a direction with an unstable settling orientation.
†This report was originally printed in Kagaku Kogaku Ronbunshu, 22(1), 98-105 (1996) 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 analyze the flow behavior of particles by the application of continuum mechanics, a new constitutive equation is derived from considerations of energy dissipation during the flow of the particles. This dissipation mechanism was investigated by computer simulation of the flow behavior of particles on an inclined trough. The energy of flowing particles is dissipated by friction and collision with neighboring particles. The constitutive equation was derived by considering both dissipation mechanisms of energy. The velocity distribution of flowing particles on the inclined trough was estimated by the application of a constitutive equation. The estimated velocity distribution agrees well with the measured one.
†This report was originally printed in J. Soc. Powder Technology, Japan. 33(2), 95-101 (1996) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
We examined the effect of packing conditions on the pressure of a fine powder in a bin. Results of experiments on filling fine powders through sieves of different mesh sizes confirmed that the powder pressure and the charge of a fine powder in a bin are influenced by packing conditions. This is because the mechanical properties of a fine powder change considerably in accordance with packing conditions, i.e., normal stress acting on the bed, state of agglomeration, packing structure, filling rate, and the like. The pressure and charge of a fine powder in a bin can be estimated by Janssen's equation using mechanical properties that depend on packing conditions.
†This report was originally printed in J. Soc. Powder Technology, Japan. 33(2), 102-107 (1996) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
In the preparation of fine particles of metal sulfide such as zinc sulfide and cadmium sulfide using the spray pyrolysis method, the electrostatic spray method was applied to atomize the starting solutions. The optimum operation conditions where the droplets could be generated from a Taylor cone were examined experimentally. It was shown that the use of the electrostatic spray pyrolysis method is available for preparing the several dozens nanometer-sized fine particles, in comparison with a typical ultrasonic spray pyrolysis method that produced the several hundreds nanometer size.
†This report was originally printed in J. Soc. Powder Technology, Japan. 33(3), 192-198 (1996) 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 rotary intrusion rheometer with a conical rotor was improved to detect the intruding stress as well as the shearing torque at a cell bottom. The characteristic curves of shearing torque versus a rotor's depth of intrusion in a powder bed, and normal stress versus depth of intrusion were measured simultaneously under various operating conditions. Two experimental coefficients C and m were introduced to characterize the relationship between the shearing torque and the depth of intrusion. The coefficient m indicates the compressibility tendency of the powder bed during rotor intrusion. The friction property of powders is discussed based on the coefficient C measured at m =1 (without the compressive condition).
†This report was originally printed in J. Soc. Powder Technology, Japan. 33(6), 510-515 (1996) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.