Thermophoresis of ultra - fine SiO2 particles formed in a flame was experimentally investigated by setting an adhering surface perpendicular to a flame axis. The following results were obtained. 1) Near the adhering surface, the number density of particles is considered to increase up to almost twice that in a flame. This is because the approaching velocity of particles decreased to about half of its original value keeping the cross sectional area of the flowing region constant. 2) The thermophoresis velocity was 1.0∼1.4cm/s. 3) A particle diameter corresponding to the thermophoresis velocity is 0.4∼0.8μm, which is larger than a particle diameter measured in a flame, that is, 50∼60nm. 4) Particles are considered to adhere to each other in the vicinity of the surface. 5) A main mechanism of particle adhesion is thermophoresis, and the effect of inertia can be neglected.
†This report was originally printed in J. Soc. Powder Technology, Japan. 26(8), 558-564 (1989) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Particle size analysis for micro-particles by capillary hydrodynamic chromatography was studied experimentally. Retention times of particles depended on their diameter but were independent of their concentration. The linearity of the peak area with particles concentration was found. It is shown that the particles diameter and their concentration can easily and rapidly be analyzed by capillary hydrodynamic chromatography. Resolution of peaks were calculated from chromatograms of binary-particle samples. Particle diameters of a binary-particle sample for complete peak separation were predicted from the resolution of the peaks. The mechanism of capillary hydrodynamic chromatography was discussed by considering the dependence of the volume flow rate of solvent on the retention time, the resolution of the peak and the peak width.
†This report was originally printed in J. Soc. Powder Technology, Japan. 27(6), 394-398 (1990) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
At present, there is no reliable method available for determining the compaction characteristics of ultrafine particles, the sizes of which ranges from several to one hundred nanometers. In this paper, the authors propose the centrifugal compaction method to evaluate the compaction characteristics of the ultrafine particles by applying the self-compressive stress by centrifugal force in a packed powder bed. With such a system, compressive strains can be determined with good reproducibility as a function of centrifugal force. The relationships between compressive strain and centrifugal force for ten kinds of ultrafine particles, such as Fe, Co, Al, SiO2, Al2O3 and TiO2 were found to be divided into two categories. One category can be written as a linear relationship between the logarithm of compressive strain and the logarithm of centrifugal force, while the other can be described by a linear relationship between compressive strain and the logarithm of the centrifugal force.
†This report was originally printed in J. Soc. Powder Technology, Japan. 26(9), 632-637 (1989) 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 granular medium is idealized here by a model composed of stress/strain field deviations in a random continuum. Total stress-strain relationships for this model are derived by integrating relationships. Further more, the theory is developed to explain of the dispersion of internal microstress, initial elastic strain, and local yield stress on the macroscopic behavior of a granular medium. Predictions of the theory are reported and agree with the results of the experimental work.
†This report was originally printed in J. Soc. Powder Technology, Japan. 27(3), 146-152 (1990) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Adhesive forces of sub-micron particles of limestone were measured by using a split-cell type tensile strength tester under various vapor pressure conditions of water, ethanol and acetone. The adhesive force generated by ethanol vapor or acetone vapor adsorption was smaller than that generated by water vapor adsorption. Agglomerated particles generated from a micro-fluidized bed of the limestone sample with various atmospheric gases were dispersed by a high speed gas stream through a small pipe, and the size distribution of the agglomerates was measured. The agglomerates containing adsorbed ethanol were easily dispersed into the single particles by about half the force required for the agglomerates containing adsorbed water, which qualitatively corresponded to the decrease in the adhesive force of the particles by adsorbing ethanol vapor instead of water vapor.
†This report was originally printed in J. Soc. Powder Technology, Japan. 27(3), 159-164 (1990) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
The property of a shaped body by slip casting in ceramic processing greatly depends on the dispersive state in the slurry. The agglomerate process in a high densed slurry we studied by three dimensional computer simulation. The movement of particles was determined by cohesion probability between particles. The aggregative and this process was discussed from the view points of the aggregation rate, the coordination number and the aggregative ratio by using cohesion probability. The experimental results from the measurement of viscosity and SEM observational dried slurries supported this simulation well.
†This report was originally printed in J. Soc. Powder Technology, Japan. 27(4), 218-224 (1990) 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 α-alumina fine powder was coated by SiO2 ultra-fine particles with a diameter of nano-meters. SiO2 by hydrolysis of ethyl silicate with water adsorbed on the powder was precipitated onto the powder in the hydrophobic liquid which contained 90 vol.% ligroin, 10 vol.% ethyl silicate and 0.015 mol/l stearic acid. The thickness of the coated layer was controllable by changing the amount of the adsorbed water up to 10 wt.% of the powder.
†This report was originally printed in J. Soc. Powder Technology, Japan. 27(3), 170-173 (1990) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
The formation of TiO2 thin film and fine particles by the thermal decomposition of titanium tetraisopropoxide (TTIP)vapor was carried out experimentally using a laminar-flow aerosol reactor (LFAR). TTIP vapor and He gas mixture was introduced into a heated pipe in a laminar flow, and the thin films formed and the particles deposited on the pipe wall were observed using an electron microscope under various supplied concentrations of TTIP vapor, reactor temperatures and operating pressure inside the reactor It has been found that the generation of fine particles is enhanced at higher concentration of TTIP vapor, higher reaction temperature and higher pressure, but the formation of thin films is more enhanced lower pressure. The observed trends agreed qualitatively with the simulation results obtained by numerically solving the aerosol dynamic equation where the particle formation was evaluated by a simplified reaction and coagulation model.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU, 16(3), 535-542 (1990) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
Factors of milling rates and contamination due to abrasion of balls were investigated in order to obtain ultrafine powder of pure piezoelectric ceramics. Higher milling rates and lower contamination from balls were obtained for smaller balls. Calcined piezoelectric powders of 1 μm in mean particle diameter were milled into the fine powders of 0.2 μm for about 7 min. It was estimated that milling rates of this case (0.4 mm PSZ balls) was about 105 times as that of ball mill (ball size 10 mm). Even smaller ultrafine particles of 19 nm in mean particle diameter were obtained by milling for 90 min.
†This report was originally printed in Journal of the Japan Society of Powder and Powder Metallurgy, 37(6), 827-831 (1990) 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.
It is generally known that the results of particle size analyses done in liquid suspension are more reliable than those in a dry system. This stems from the fact that most principles are based on sedimentation or electric conductivity changes in liquids. With the development of laser diffraction instruments for particle size distribution, the dry powder measurement became quite significant. Although we know the difficulties of dispersion of very fine particles in gases, we have to take into account the great importance of sample preparation procedures for the wet technique. This involves the influence of suitable liquid, the proper dispersant and the time of dispersion, The topic of this work is comparison of the results obtained by dry and wet techniques on laser diffraction instruments evaluated by mass balance for air classification processes with a cut size under 10 micrometers. The comparison was carried out with different materials. Wet analyses were made with well-known instruments. The results are supplemented also by other principles of measurement. The accordance of analyses and mass balance for fractions of coarse and fine products with input material is greatly influenced by instrument quality. The differences in the results obtained with the same instrument by wet and dry techniques are relatively small even in the range of the low limit of the instrument. So it wouldn't always be necessary to make use of the wet technique, rather, more attention should be given to the quality and performance of the instruments.
The operation and performance of a novel device, the Electromechanical Valve for Solids (EVS), for control of flow of granular materials are described. The EVS applies a DC electric field to the granular material in a dense-phase state. This induces electrical interparticle forces which retard and can ultimately halt the flow, hence providing an actuating mechanism. The EVS has a very fast response time which makes it very useful for fast flow control. The response characteristics of the EVS and the results of flow control using a simple PID control action are reported.
In the study of locked particles, the recent development of a theoretical understanding of the volume locking profile using textural transforms has been significant. In all derivations, it was assumed that the interfacial surface area was flat. In this paper, the effect of a curved interfacial surface is explored. It was found that as the particle size approached the inclusion size, the volume locking profile became evermore asymmetric. Moreover, as this ratio approached one, there was a large movement of the Ferrara Point. Since the latter is a key parameter in defining the locking profile, its movement is significant.
Computer simulations of the flow fields in centrifugal air classifiers have been undertaken in order to improve both the performance and efficiency of the commercially available Acucut air classifier. Although the simulations have led to a much greater understanding of the nature of the flow likely to prevail in such systems, accurate predictions have been found to be strongly dependent on the assumption of correct inlet flow conditions during the initiation of the simulations. In spite of the fact that these simulations assumed axial symmetry (which is an oversimplification), they have enabled the identification of a number of engineering modifications which have contributed to reducing the cut size, and increasing the efficiency, of an Acucut Laboratory classifier.
Wet classification has been challenged by the needs of the industry to produce finer and finer powders in slurry form. Hydrocyclones have maintained their position as the most important classifiers in wet mineral processing. Due to the recent developments in hydrodynamic modelling, the understanding of the flow phenomena has become rather clear and has allowed the design of hydrocyclones suitable for special purposes. New cyclones have good separation efficiencies and low imperfection values.
High pressure roller mills have been used successfully for grinding brittle materials since 1985. The advantages of this new technology are, less energy consumption, less wear and noise emission and a smaller mill size compared with ball mills. The HP-roller mill achieves comminution in a particle bed stressed by a high pressure. The performance of this mill differs from that of a ball mill with respect to the decoupling of throughput and comminution action. To operate a HP-roller mill, one has to understand the features of interparticle breakage, the interdependence of milling force and energy absorption, the capacity issue and the wear problem.
Preferable operational conditions of gas/solid flows are often situated in the transitional regime between two well defined modes, typical examples being pneumatic transport in a horizontal pipe and circulating fluidized beds. The energetic optimum of horizontal pneumatic transport is situated in the transitional regime between strand flow and fully developed flow in which regimes with different physical factors (wall friction of sliding material or single particle wall impact, respectively) prevail. The circulating fluidized bed may be regarded as the transition from a bubbling fluidized bed to pneumatic transport vertically upwards. Experimental results indicate particle slip velocities which are quite a bit higher than predicted by single particle fall velocity, hence indicating strong phase separation in terms of clusters. In both situations, empirical correlations fail because they ignore the complicated structure of the system in question, i.e. being transitional. On the other hand, even very sophisticated modelling can fail, because it is mostly based on inherent a priori assumptions concerning the state of flow. As an alternative, comparably simple models of two phase flows in horizontal pipes and for circulating fluidized beds are presented. The models originate from very obvious visual observations and from considerations of momentum exchange between the two phases. The evaluation of the respective resulting sets of algebraic equations can be represented in terms of state diagrams for gas/solid flows with partial phase separation.
This article provides an overview on the use of field-flow fractionation (FFF) for particle size analysis and for the characterization of other particle properties such as density, porosity, and the thickness of adsorbed layers. While FFF is a relatively new technology for particle characterization, it is one of the most versatile and powerful techniques now available for characterizing particle populations. The unique features contributing to the effectiveness of FFF include high resolution, relatively high speed, adaptability to different types and sizes of particles, and the ability to collect narrow fractions for further characterization by microscopy and other techniques. For background, the mechanism of FFF is described in two parts, one applicable to particles over 1μm diameter and the other relevant to submicron size particles ranging down to 1nm size. It is shown how particle size distributions are obtained for a variety of particulate materials in both size ranges. The strategies needed for measuring particle properties other than size and size distribution are discussed.