The industrial handling procedures for bulk materials, for example, to store, to convey, to mix, and to fill, etc. often lead to dust emissions. The generated dust is closely related to health hazards and environmental pollution, and is also a cause of fires and explosions. The particle size distribution and concentration determine the risks. The dust liberation property of disperse particle systems – so-called powders – depends on a multitude of variables and also on the method and intensity of stressing. The present paper describes practically all the dust measuring methods published in literature. It also presents related particle sizing and counting techniques and systematically summarises the integrated measurement methods.
Filtration of liquid suspensions is a widely practiced process in many industries. In recent years, membrane filtration of colloidal solutions has attracted a considerable amount of attention. The appropriate control of both the filtration rate and the rejection (or the transmission) of the particles and/or the macromolecules in the filtration process is of great interest in both industry and academia. Such filtration behaviors are strongly affected by the properties of the filter cake formed by the accumulation of the particles and/or the macromolecules on the surface of the filter medium or the membrane. This paper overviewed the author’s own contributions on the recent developments on the behaviors of the filter cake in cake filtration and membrane filtration. The paper will mainly deal with measurements and analysis of the internal structures of the filter cake, the role of the solution environment in the properties of the filter cake, and filtration and fractionation mechanism of mixtures.
The paper deals with the modeling of the agglomeration of crystals during their crystallization. Crystal agglomeration actually consists of two steps, i.e. particle collision and agglomerate strengthening by crystal growth. The expression of agglomeration rates can be written in terms of a collision rate coupled with an efficiency factor. However, the mechanisms and rates of collision and disruption are related to the type of liquid flow that the mother crystals and the agglomerate experience, which in turn are dependent on their respective sizes. In particular, the influences of the absolute and relative sizes of mother particles, of the local energy dissipation and of the fluid viscosity differ according to the three types of motions, i.e. Brownian, laminar, turbulent. Besides this, the rapidity of the crystal growth, which in turn is a function of the supersaturation, plays a major role in the strengthening rate. The question of the limit cases between two regimes is also treated. The method takes into account and unifies previous expressions obtained by other authors in the various regimes. The model is also able to calculate the average agglomeration degrees.The paper is illustrated by one example of crystal agglomeration from our recent work and introduces a general model.
In this paper we studied the aerated discharge of two magnesium carbonate powders differing in their average diameter and particle size distribution. These samples were characterized by means of fluidization experiments and rheology shear tests carried out in a rotating shear cell. In the hopper discharge experiments, besides the discharge rates and the mass of residual solids as a function of the aeration rate, the aggregative behavior was observed by means of photographic techniques. Solids aggregates were actually visible within the aerated beds of solids during the fluidization experiments and in the streams of the discharging solids. Experimental data on the powder flow properties and on fluidization were analysed in order to permit evaluation of the aggregate diameters. These values compared fairly well with the aggregate diameters observed and made it possible to evaluate the voidage values outside the aggregates. These results were used in a modified form of the De Jong and Hoelen  equation to predict the solids discharge rate with a reasonable degree of accuracy.
The simultaneous treatment of dissimilar solids is encountered in a number of processes that exploit fluidization technology (granulation, combustion, pyrolysis, etc.), but a satisfactory description of multicomponent fluidization dynamics is far from being achieved even for the relatively simple case of binary fluidization. This paper discusses the fluidization properties of two-component beds of solids differing either in particle density or diameter. It is shown that although the minimum fluidization velocity of the mixture can be calculated by fully theoretical equations, it has very little meaning if referred to a two-component particle system. Experiments on both kinds of mixtures demonstrate that the binary fluidization process occurs within a characteristic velocity range whose boundaries coincide with the “initial” and the “final fluidization velocity” of the particle mixture. Substitution of the conventional concept of umf by these parameters allows us to recognize that the fluidization dynamics of any binary mixture is determined by the initial arrangement of the fixed bed, as well as by the system composition and size or density ratio of its components.
A great deal of experimental data on the mechanochemical treatment of inorganic oxides and mixtures thereof falls into the simple scheme involving the concurrent manifestation of grinding, particle aggregation, and primary crystallite coalescence, respectively. The properties of resulting powders proved to be determined by the position of dynamic equilibrium among the processes taking place. Exerting an influence on the course of one or several processes makes it possible to shift the position of equilibrium and thus to obtain powders with different extents of aggregation, different particle sizes and size distribution curves, or to carry out the treatment under conditions favorable for mechanochemical synthesis. As a result, a weakly agglomerated 15-nm α-Al2O3 powder was prepared in one case, and either yttrium or calcium-stabilized zirconia formed directly in the mill in the other case.
The study of particle breakage in particulate processing is presented at two principal scales. Single-particle impact results are shown to be useful in deducing and understanding process-scale particle breakage behaviour. Particulate breakage in fluidised bed granulation, high-shear granulation and pneumatic conveying systems are explored with a focus on presenting techniques and results connected with furthering understanding and modelling.
This paper presents a numerical study of the gas fluidization of a mixture of 45,000 cohesive and cohesionless particles (D=100 μm and ρ=1,440 kgm−3) using a Combined Continuum and Discrete Model (CCDM). In such a model, the motion of individual particles is obtained by solving Newton’s second law of motion and flow of continuum fluid by the local averaged Navier-Stokes equations. In this work, the cohesion among particles is caused by van der Waals interactions. The Hamaker constants are used to distinguish the cohesivity among particles so that finite values are assigned to cohesive particles and zero values to cohesionless particles. It is shown that the presence of cohesionless particles in an assembly of cohesive particles can improve their flowability and that sustainable fluidization can be achieved if the amount of cohesionless particles is sufficient.
An approach to quantifying the impact grinding performance of different materials is presented. Based on a dimensional analysis and on fracture mechanical considerations, two material parameters, fMat. and Wm,min, are derived theoretically. fMat. characterises the resistance of particulate material against fracture during impact comminution. Wm,min gives the mass-specific energy which a particle can absorb without fracture. Using this approach, various materials over a wide size range, e.g. different polymers, crystalline substances, glass and limestone, can be characterised quantitatively. The derived material parameters are applied to the systematic modelling of grinding in impact mills. A population balance model is presented and the results of the simulation for an air classifier mill are shown. The developed model permits a clear separation of the influence of material properties, mill-specific features and operating conditions, thus enabling a deeper understanding of the impact grinding process.
In modeling comminution systems, breakage rate (selection) functions are generally influenced more by the comminution machine than are breakage distribution functions, which are controlled by material properties. Linear grinding kinetics can be expressed either in terms of grinding time or specific energy consumption. Nonlinearities are caused by energy transfer mechanisms in the comminution machine whereby coarser particles might be ground preferentially or are protected by fines, by energy dissipation through interparticle friction in compressed bed comminution, and sometimes from heterogeneities produced in the feed particles. This paper discusses modifications of breakage rate functions in the grinding model for a number of situations and compares simulated and experimental results.
Compaction of granulated powder is a common forming process used in the ceramics industry. Glass spheres were used as a model system to investigate granule failure during die compaction. Stresses within an assembly of spheres follow a network of pathways. Results demonstrate the statistical nature of granule failure during compaction, with some granules failing at very low applied pressures while a large fraction persist at even the highest applied loads. At high compaction pressures, size distributions of compacted spheres were seen to approach the Dinger-Funk distribution for maximum packing. In the limiting case of maximum density, the Dinger-Funk equation predicts 33% of the volume of granules will have sizes in the range of the initial size distribution.
Fe nanopowder, derived from microwave plasma synthesis (Materials Modifications Inc., Fairfax, VA), was obtained and characterized for particle size and size distribution. The methods used included dynamic light scattering (DLS), static laser scattering (SLS), surface area and size by Brunauer, Emmett, and Teller (BET) analysis, small angle neutron scattering (SANS), neutron diffraction (ND), x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Based on these methods, it was concluded that the Fe powder was composed of nanosized particles, but in micrometer-sized aggregates. DLS indicated a mean agglomerate size with a single mode distribution of 70 ± 6 nm. In contrast, SLS revealed a wide bimodal distribution ranging from 0.5 to 20 μm. The mean particle sizes that resulted from BET and XRD analyses were 60 nm and 20 nm, respectively. SANS, in combination with ND, determined that the powder had a bimodal distribution of mean size 24 and 64 nm. TEM and FESEM confirmed that the powder is composed of 50–80 nm particles that are found in large, dendritic particle agglomerates that are on the order of micrometers. The information derived from these results indicates that all of the selected methods were helpful in making an accurate and complete characterization of the powder.
Segregation is a widely occurring undesirable phenomenon in industries that store, handle and process particulate materials. Size-segregation induced by the percolation mechanism is observed in several important processes that negatively impacts the product quality and mixing. To quantify size-segregation, a constitutive model based on simultaneous convective and diffusive demixing was developed and validated. The primary segregation shear cell (PSSC) was used to measure the fundamental parameters and validate the convective-diffusive segregation model. Glass beads of size ratios of 10.9:1 (1250:115 μm), 8.7:1 (1000:115 μm), and 5.1:1 (1000:196 μm) were used for model parameter determination; whereas, size ratio of 6.4:1 (1250:196 μm) was used for model validation. As shown in a previous study, an effective segregation direction could be measured and used to validate the convective-diffusive segregation model for percolation. This justified the use of an effective segregation direction to model the percolation of fines. When compared to the normalized measured data for size ratios larger than 8.7:1, the convective-diffusive model resulted in standard deviations of 0.035. However, for size ratios smaller than 6.4:1, diffusive demixing was occurring during shear with the absence of a rapid initial discharge phase, i.e., minimal contribution due to convective component. Estimating the percolation for the 6.4:1 size ratio was accomplished by using the mean data of the 5.1:1 size ratio, which resulted in standard deviation of 0.055. The initial rapid discharge present in 10.9:1 and 8.7:1 size ratios suggests that a critical size ratio exists that differentiates free-fall discharge segregation from random mixing segregation. This can be critical to powder manufacturers that could use this critical limit to define size distribution recommendations during manufacturing.
An experimental investigation was carried out on the production of silver powder based on a vapor phase synthesis process. The main objective of the work was to investigate the feasibility of producing fine metal particles in a lab scale basis, which would lead to a fine grained microstructure of compacts produced from a mixture of these powders with graphite. Using a thermal plasma high temperature reactor, fine silver particles were produced at a rate of 500 g/h, with the thermal plasma power rated at 15 kW. The powders produced were characterized for particle size and size distribution by laser scattering technique, and particle size and morphology using scanning electron microscopy. Sintering experiments of compacted bodies obtained from a mixture of the powders produced and graphite powder demonstrated the feasibility of producing a material presenting a homogeneous microstructure suitable to the fabrication of high performance electrical switch contacts. Metallographic preparations of Ag/C compacts were used for the microstructure analysis using optical microscopy. The experimental apparatus involved the use of a transferred arc plasma evaporator coupled with a tubular cooling section in which the hot gases carrying the metal vapor are quenched to produce fine particles. A study was carried out to provide theoretical support to the experimental investigation. Applied to the tubular gas quenching section, a 2-D model was used to determine the temperature, velocity and species concentration fields. Fluid-dynamics was combined with a model for the nucleation and growth of particles based on the moments of particle size distribution.
Chemical Mechanical Polishing (CMP) process is widely used in the microelectronics industry for planarization of metal and dielectric layers to achieve multi-layer metallization. For an effective polishing, it is necessary to minimize the surface defects while attaining a good planarity with optimal material removal rate. These requirements can be met by controlling the chemical and mechanical interactions during the polishing process, or in other words, by engineering the slurry chemistry, particulate properties and stability. This paper reviews the impact of chemical, inter-particle and pad-particle-substrate interactions on CMP performance. It is shown that for consistently high performing slurries, stability of abrasive particles must be achieved under the dynamic processing conditions by providing sufficient pad-particle-wafer interactions.
The authors investigated the gravity separation of plastic, rubber, and wire harnesses in automobile shredder residue using a gas-solid fluidized bed. Uni-beads (barium silica titanate glass), zircon sand, and glass beads were employed as fluidized particles. Superficial air velocity was changed to see how the plastic, rubber, and wire harnesses floated and sank in the fluidized bed. Wire harnesses were almost completely separated from the other constituents by using uni-beads and zircon sand. Plastic can be separated from rubber by using glass beads, although separation efficiency is relatively low. Precise adjustment of superficial air velocity is essential for attaining high separation efficiency because particle flow and air bubbles in the fluidized bed affect how objects float and sink.
†This report was originally printed in J. Soc. Powder Technology, Japan, 38(10), 702-709 (2001) 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 a powder pneumatic conveying process, powders are remarkably charged due to the collisions and frictions between themselves or against the conveying pipe, which sometimes causes explosion and fire. In order to prevent these troubles and hazards, development of a reliable system for monitoring and control of electrification (electrostatic charge) is strongly required. In this study, an electrostatic charge control system composed of corona discharge neutralizer, electrostatic filed strength sensor and computer control system was newly developed and applied to the powder pneumatic conveying process. Dynamic characteristics of electrostatic charge and its elimination by the corona discharge neutralizer were analyzed. Based on the characteristics, a simplified transfer function composed of first order lag element including dead time was proposed and optimal control parameters for the digital PID control was determined. Performance of the control system was also investigated experimentally under various control parameters. It was found that the electrostatic charge during powder pneumatic conveying process was favorably self-controlled by means of the newly developed control system.
†This report was originally printed in J. Soc. Powder Technology, Japan, 39(7), 496-502 (2002) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Pulverized coals produced by a rounding method have a wide range of particle sizes. Even though the coals have the same Hardgrove Grindability Index (HGI), some coals are difficult to pulverize by the rounding method. There are many pores in the coal. When pore pressure is rapidly reduced, cracks are generated in the pores by the expansion of air and the coal becomes brittle. The purpose of the present work is to investigate the performance of coal pulverization by an improved rounding method specifically by the embrittlement of coal due to the cracks generated in the pores. The experimental results show that pulverized coals for CWM which have a wide range of particle size can be easily produced by the improved rounding method. Furthermore, a discharge pressure at the embrittlement treatment can control the particle size of the pulverized coals.
†This report was originally printed in J. Soc. Powder Technology, Japan, 38(9), 607-611 (2001) 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 wet micro-feeder that utilizes ultrasonic wave force was designed and constructed to feed small particles into a container of wet particulate materials. The particle discharge characteristics from a nozzle with the inside diameter Dn were investigated using spherical and irregular particles of 80 to 180mm in diameter, dp. As a result, the critical ratio for particle blockage of the present feeder, (Dn/dp)c≒3.0, was found to be smaller than the values obtained previously (>4 ∼ 5) in the gravitational field for dry powders. The present feeder was able to achieve stable and continuous discharge of small amounts of particles. Multiple regression analysis proved that the discharge rate of particles, i.e. the number of particles discharged from the nozzle per unit of time, N̄, depended on Dn, dp, the applied voltage V0, and the median value of the particle shape index (surface roughness), y50, and that N̄ was proportional to Dn2.29. Therefore, N̄ could be controlled by V0 and Dn for given particles.
†This report was originally printed in J. Soc. Powder Technology, Japan, 38(9), 617-625 (2001) 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 simulation method for initial-stage sintering was proposed to understand the effect of sintering conditions on the sintering behavior of two particles of different sizes and compositions. This study employed a homogeneous solid solubility system of Cu-Ni. The sintering experiments were performed with two spherical Cu-Ni particles in contact, and the sintered particles were observed by SEM and EDX. It was confirmed that the sintered particles always form a curved neck and copper was the dominant component in the neck. The proposed simulation method, in which the model for two particles of identical composition was modified, involves several mass transport mechanisms: surface diffusion, volume diffusion, grain-boundary diffusion, evaporation-condensation and, newly, grain growth. The sintering behavior of Cu-Ni particles was simulated using the proposed method and the results were highly similar to the experimental results. It was also found that grain growth was not negligible in initial-stage sintering, and that shrinkage and grain growth responded sensitively to sintering conditions. Consequently, by analysis of these simulation results, it is possible to understand the mechanism of sintering two spherical particles of different compositions and to determine the optimum sintering conditions.
†This report was originally printed in J. Soc. Powder Technology, Japan, 28(2), 202-210 (2002) 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 mechanochemical (MC) synthesis of La-modified lead zirconate titanate (PLZT) powder from its constituent oxides was conducted with planetary ball milling. A single-phase perovskite PLZT powder was synthesized by dry-milling the oxide mixture for a relatively short period (36 ks at 400 rpm). Dense PLZT bulk samples with fine-grained microstructures were obtained even from powders milled for a short time (3.6 ks) and at a lower sintering temperature than for the powders prepared by the conventional wet-mixing process. The present study suggests that the mechanochemical synthesis of PLZT comprises the following three stages: First, the pulverization, fine mixing, and granulation of the starting powders occur in the early stage. Second, the particles of the powders increasingly lost their crystallinity due to milling ball impact. Finally, the reaction between PbO and TiO2 produced PbTiO3, whose reaction with the remaining oxides formed (Pb, La)(Zr, Ti)O3 solid solutions.
†This report was originally printed in J. of the Japan Soc. of Powder and Powder Metallurgy, 48(10), 943-949 (2001) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Japan Society of Powder and Powder Metallurgy.