The use of stirred-media mills for grinding into the micron and submicron size range is reviewed. Mill performance and energy efficiency are discussed in the context of mill mechanics as related to mill power, material transport and flow, and particle breakage mechanisms. The development and adaptation of general size-mass balance models to fine grinding in stirred-media mills is evaluated. Specific problems in the application of the process models and, especially, in parameter estimation are described. Comparison of breakage rates and breakage distributions with those observed in coarser grinding systems reveals several similarities but some differences, particularly with respect to size-dependence. It is shown that the approach to a grinding limit can have significant effects on grinding rates and product size distributions.

View full abstract

Because of the industrial importance of dispersion and separation technologies, a large number of researches on the floc breakage have been carried out in various fields, but the fundamental mechanism is still poorly understood. In this review, with priority given to our works, fundamental aspects on the breakage of aggregates in fluids are overviewed theoretically and experimentally, using theories and experimental data available at present, and the future works for the better understanding are considered.

View full abstract

With the increase in the importance of dispersed materials (powders, aerosols, emulsions etc.) to trade, there is an increasing awareness of the need to verify that instruments which measure particle properties, particularly size, are operating within defined limits of accuracy. As a minimum, this process requires some form of verification with reference to standard particles whose properties are known in relation ultimately to the international standards of mass and length (so-called traceability chain). In some cases, a formal calibration to establish instrument response in terms of size, shape or concentration may be required. This article reviews the particle standards that are available to establish the performance of measurement equipment, placing most emphasis on particle size, as this is the variable that is generally of most importance to industry. However, secondary properties, such as shape, density and refractive index, influence the response of many types of particle size analyzer. Attempts to provide standard materials that may enable independent assessment to be made of the effect of some of these variables on instrument performance are therefore also considered.

View full abstract

The adsorption of poly (ethylene oxide) (PEO) of 7,500 MW, a hydrogen bonding polymer, onto the surface of monodisperse colloidal silica particles was investigated as a function of volume fraction of solids. Adsorption studies revealed the presence of two plateaus, which was attributed to a change in conformation from a flat, "pancake" type, to a more elongated "brush" type conformation of the adsorbed polymer, where the terminal hydroxyl groups remained bound to the silica surface. Adsorption studies also revealed that, within the concentration range studied, there was no effect upon PEO adsorption as a function of volumefraction.

View full abstract

The effect of surfactant and polymeric additives on the viscosity behavior and stability of aqueous colloidal dispersions of silica particles under extreme conditions (low pH, high ionic strength) has been investigated. The surfactant and polymer used as dispersing agent were C₁₂TAB, a cationic surfactant, and DarvanC, a commercially available polymer. It was found that the surfactant stabilized dispersions show a lower viscosity and a more uniform resistance to flow than the samples stabilized through electrostatic repulsion or polymer induced forces in the system. Stability analysis through turbidity measurements indicated that the state of the dispersion changes from an unstable regime to a stable regime above a critical concentration of C₁₂TAB in the system. Viscosity measurements as a function of temperature indicates that C₁₂TAB dispersing agent can further improve the flowability of the dispersion at higher temperatures.

View full abstract

Thin metal oxide films have been deposited onto various substrates, under ambient pressure, utilizing supercritical CO₂-assisted nebulization of aqueous solutions. In this process, supercritical CO₂ is contacted with an aqueous precursor solution inside a low-dead-volume tee. The resulting emulsion is allowed to expand out of a capillary restrictor, and the fine particle aerosol formed is directed toward heated substrates. Dehydration, pyrolysis and oxidation of the precursors occur at or near the surface of the substrate to form the desired metal oxide film. Thin films of zinc oxide, zinc gallate and manganese-doped zinc gallate have been prepared using this technique.

View full abstract

Challenges in pneumatic conveying have been explored in an attempt to provide the researcher and practicing engineer with a realistic view of the topic. While much has been achieved in the field in both modeling and experimental understanding, one can not rely totally on these predictions for complete design reliability. The most difficult topic is the individual properties of the various materials that are attempted to be conveyed. These material properties place limitations on the designer sometime because of their finest and others because of delicate nature in requiring that no change in the product occurs during the conveying operation. Unusual applications of pneumatic conveying are reviewed including such conditions as electrostatic generation, temperature and pressure effects.

View full abstract

Screening and classification are used to separate particles based on differences in size. Industrially, these processes are done on a continuous basis to produce two (or more) product streams of varying degrees of fineness. Screening involves the separation of particles based on the probability of passage through a series of apertures of uniform size. Variables such as the number of presentations per second, size of the particle relative to aperture size, and retention time on the screen all are important in determining the probability of passage through the screen. In classification, the separations are determined by the movement of particles in a fluid, typically water or air. In this case, the probability of a particle reporting to the coarse or fine stream depends on the relative effect of gravity and fluid drag. This paper describes the factors affecting both screening and classification, the types of devices used in the separations, and the effects of staging to improve process performance.

View full abstract

We report on the effect of alcohols in the acid peptization of aggregated titania nanoparticle produced from alkoxides. Peptization was studied in the presence of each one of the following alcohols: methanol, ethanol, propanol and isopropanol. We find that the final particle size is correlated to the dielectric constant of the peptizing medium. Kinetic measurements reveal that the rate of deaggregation is not affected by the presence of alcohol; however, the tendency for reaggregation of the peptized colloid increases significantly. We conclude that alcohols prevent the full redispersion of the aggregates by decreasing the colloidal stability of the suspension. This conclusion is supported by the measured zeta potential of the peptized particles, which is found to decrease when alcohol is present.

View full abstract

The attrition of particulate materials during their handling and processing may result in losses of material, poor product quality, poor flowability and environmental pollution caused by the generation of dust. It is not surprising therefore that so much research has been devoted to studying the breakage mechanism of particles, in order to reduce the attrition in conveying and handling systems. In this paper, an extensive literature review on the above topics is presented. The literature review also covers the experimental systems that are commonly used to evaluate these phenomena. Moreover, some new experimental results are presented to clarify future trends, to better understand the complex mechanisms at work, to reduce the required number of ‘standard indices' and to enable better engineering design.

View full abstract

Heat transfer between submerged surfaces and gas-fluidized beds depends on fluid mechanics and particle dynamics. Therefore, reliable prediction of the heat transfer coefficient must be based on the observation of particle motion towards and from solid surfaces. Experiments with luminescent particles reveal a rather broad particle residence time distribution at solid surfaces. This broad residence time distribution gives rise to a smearing-out effect of the two different heat transfer mechanisms, namely particle convective and gas convective, respectively. Well-defined residence times can be realized by means of a rudimentary variant of fluidized bed heat transfer in the form of moving bed heat transfer. Experimental and theoretical results obtained from moving bed heat transfer allow the sound modeling of the two "pure" mechanisms (particle convective and gas convective). A predictive equation is derived which may be seen as a safe interpolation between the two extremes. The comparison with a large number of experiments proves the reliability of the prediction with respect to any feature of fluidized bed heat transfer.

View full abstract

Experimental work was carried out to determine the influence of operating parameters on the roll press compaction of lactose and the interactions between feeding and compaction. A laboratory roll press was specially instrumented. The press and instrumentation are described. The first part of the experimental work deals with the adjustment of the roll press parameters in order to produce compacts with good mechanical strength and aspect for several roll press throughput. The measured normal stresses applied to the powder, the roll gap variations and the roll press throughput are correlated with the ratio between the speed of the rollers and the screw feeder speed, also called work coefficient. The roll press throughput is principally governed by the screw feeder speed. The second part of the experimental work deals with characterisation of the influence of the feeding conditions on the compaction. The single screw feeder produces locally a periodic disturbance which is responsible for compact heterogeneity. The normal stress measured on the rollers was correlated with the period of the screw. A minor variation of the feeding pressure produces a great variation of the compaction stress.

View full abstract

In Positron Emission Particle Tracking (PEPT), a single positron-emitting tracer particle is placed within the bulk of particles in the device of interest. The tracer will typically be of the same material as the bulk, or comparable with it in size and density. A positron emitted from the tracer annihilates with an election, leading to the production of two near collinear 'back-to-back' gamma rays. The detection of these and subsequent pairs of gamma rays enable the tracer to be located in three dimensions by simple triangulation. Typically, a particle moving at 1 ms^-1can be located to within 1 mm 500 times a second, whilst at 0.1 ms^-1 the spatial resolution is improved to 0.5 mm for a location calculated 50 times a second. The tracer particles position data can then be processed to yield information such as particle velocity, the residence time distributions within specified zones, and the frequency of entering these zones. A real time visualisation of the tracer motion enables the technique to be used for rapid diagnostic and development studies.
In the current work, PEPT is being applied in the investigation of a wide range of Particle Technology unit operations including mixing, gravity driven flows, rotary kilns and fluidisation. In each case, detailed trajectories have been obtained, enabling new insights into particle behaviour. In mixing, for example, it has been possible to obtain dispersion coefficients relating to each volume element of the bed, so enabling the separate contribution of each part of the mixer to be distinguished.

View full abstract

We report measurements of flow transitions, from avalanching to rolling, for granular material in rotary kilns. In the avalanching mode, the surface slips periodically; in the intervals between avalanches, all particles rotate with the kiln. In the rolling mode, the surface particles slide down continuously; the material underneath the surface rotates with the kiln.
Our measurements give Froude numbers (Rω²/g) for transitions, which are significantly different for sand and TiO₂ powder.
For the avalanching mode, we measured cycle times and deduced t₁₂, the avalanche time; t₁₂ was also measured directly by video photography. For kilns of diameters 0.2-0.5 m, both methods give t₁₂, of order 1-2 sec and it appears to be proportional to √l, l being the chord length of the granular bed, the maximum distance of fall for avalanche material.
Simple theory, assuming the avalanche particles slide down a frictional surface, gives fair estimates of t₁₂ and may be a basis for predicting avalanche-to-rolling transitions in large industrial kilns.

View full abstract

The fundamentals of particulate solids consolidation and flow behaviour using a reasonable combination of particle and continuum mechanics are explained. By means of the model "stiff particles with soft contacts" the combined influence of elastic, plastic and viscoplastic repulsion in particle contacts is derived. Consequently, contact normal force displacement F_N(h_K) and adhesion force models F_H(F_N) are presented to describe the stationary, instantaneous and time consolidation behaviour in particle contacts as well as in the bulk. On this particle mechanical basis, the stationary, instantaneous and time yield loci as well as uniaxial compressive strength σ_c(σ₁), effective angle of internal friction ϕ_e(σ₁), bulk density ρ_b(σ₁) are derived and shown for a very cohesive sub-micron titania powder. Finally, these models in combination with accurate shear cell test results are used as constitutive functions for computer aided silo design for reliable flow and pressure calculations.

View full abstract

This paper presents an overview of recent basic research on flame aerosol reactors for the gas-phase synthesis of nanoparticles. Emphasis is placed on flame reactor technology as it is widely used in industry for the large-scale manufacture of oxide and carbon nanoparticles. The importance of reactant gas mixing, additives and external electric fields in flame technology is highlighted for the control of product particle properties by affecting the chemistry, temperature and collision histories. Laser-induced fluorescence (LIF), Fourier transform infrared spectroscopy (FTIR) and thermophoretic sampling are addressed as some of the promising diagnostic tools in flame aerosol research and even for on-line process control. Recent work on aerosol dynamics modeling is presented, and the growing importance of computational fluid dynamics (CFD) aimed at better understanding the particle formation and growth mechanisms in flames is emphasized, focusing on the synthesis of non-aggregated nanoparticles.

View full abstract

A scanning mobility particle sizer (SMPS, TSI Model 3934) and a white-light optical particle counter (OPC, as described by Umhauer 1983) are combined. With these two instruments, fast and non-intrusive on-line measurements of particle size distributions in a range of 15 nm up to 15 μm are possible. A filter provides the possibility of taking samples in order to analyse the morphology and chemical composition of the aerosol. Both liquid and solid airborne particles in diluted suspensions can be measured. The complete system is able to cope with particle concentrations of up to 10⁷ particles/cm³ without dilution of the aerosol. Since the measuring principles of the combined devices are based on different physical properties, careful calibration is necessary. Both theoretical calculations and experimental methods are used for calibration purposes. The measuring results of different devices are compared during simultaneous characterisation of several aerosols. The performance of the combined measuring system is demonstrated by means of comparison with a common cascade impactor in the laboratory as well as during industrial application.

View full abstract

The authors evaluated the degree of liberation of printed circuit boards ground by impact milling to recover valuable materials. Wastes of two board types were ground with a swing-hammer-type impact mill. Heavy medium separation using tetrabromoethane was employed to separate the ground materials into the sink product, containing mainly copper, and the float product, consisting mainly of epoxy resin and glass fiber. The degree of liberation was evaluated with heavy medium separation. The sink product was larger than the float product. When processing boards whose surfaces are fully plated with copper foil we attained a degree of liberation for copper in the sink of up to 95%, which was higher than the degree of printed circuit board liberation. By contrast, the copper content of the float product was quite low at less than 1%. Thus, non-copper materials are easily liberated, and non-copper materials with few impurities can be obtained from printed circuit board wastes.

^† This report was originally printed in J. Soc. Powder Technology, Japan. 36(6), 479-483 (1999) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.

View full abstract

This paper describes a novel system for continuous monitoring of powder electrostatic charge in pneumatic transportation process. An electrostatic detecting system based on the electrostatic field strength measurement, together with a purge air system for preventing powder adhesion, has been newly developed. Performance of the system has been confirmed in a powder pneumatic transportation process under various kinds of powder materials and operating conditions. Specific charge of powder and the induced current at the transportation pipe were simultaneously measured. The relationship between the electrostatic field strength and the space charge density and the measured induced current were investigated. It is found that the electrostatic field strength has linear relationship between the space charge density and the induced current. This proves that our newly developed system is a very effective and simple device for continuously measure the electrical charge of powder. A model for the powder charge in the pneumatic transportation process has also been proposed here to understand the powder charge mechanism.

^† This report was originally printed in J. Soc. Powder Technology, Japan. 34(10), 778-784 (1997), 35(12), 846-855 (1998)in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.

View full abstract

Room-temperature dechlorination process for polyvinyl chloride (PVC) has been developed. The process employs a mechanochemical (MC) treatment of PVC and CaO mixture followed by washing of mixture with water. MC reaction between PVC and CaO was induced by the treatments with a planetary mill and a new type of mill named TS-type MC reactor, which converted PVC into polyethylene and CaOHCl. The former product essentially remains undissolved in water while the latter dissolves. Addition of excess CaO to PVC accelerates the reaction and enhances the yield of dechlorination to over 95% within 120 minutes for the MC treatment with planetary mill. A scale-up test with a TS-type MC reactor also gave promising results of dechlorination yield over 90% for 10 hours. Therefore the MC treatment of PVC with CaO under atmospheric condition would be promising in the practical application of dechlorination process.

^† This report was originally printed in J. Soc. Powder Technology, Japan. 36(6), 468-473 (1999) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.

View full abstract

The behavior of particles on a vibrating plate was investigated both theoretically and experimentally. Ultrasonic vibration of 39 kHz was applied, and the mass median diameters of test powders were in the range of D_p50=0.5 to 46 μm. Experimental results showed that all particles moved randomly on the vibrating plate. Fine particles, approximately 10 μm or less in mass median diameter, formed many small agglomerates, which also vibrated with their shapes and sizes intact. The size distribution of agglomerates could be represented by a log-normal distribution. Adhesion and separation stresses in an agglomerate were used to theoretically analyze the agglomeration mechanism. The analysis showed that agglomerate diameter was inversely proportional to primary particle diameter. Furthermore, it was shown that compressive breaking load could be estimated from the analysis.

^† This report was originally printed in J. Soc. Powder Technology, Japan. 36(1), 16-22 (1999) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.

View full abstract

Experimental and theoretical studies have been conducted on the classification of fly ash particles using a bench-scale louver separator. A new type of louver blade based on Joukowski's theory demonstrated classification performance superior to that of the plane-type blade. Classification performance changed with the angle of louver blades, the optimum blade angle being 50º for the Joukowski type and 70º for the plane type.
The finite element method was used to solve the Navier-Stokes equations. Calculated partial separation efficiency agreed with the experimental results.
The unburned carbon amount on the classified fine particle decreased about 20 to 30 wt%.

^† This report was originally printed in J. Soc. Powder Technology, Japan. 36(6), 454-461 (1999) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.

View full abstract

Because gas permeation through a powder bed is affected by molecular flow, applicability of the Kozeny-Carman equation, which neglects this effect, is limited to powders coarser than about 10 μm. Φ_B=Φ_V + δΦ_M is a general expression in some permeability equations for finer powders, where Φ_B denotes the permeability of a bed, Φ_V is a viscous flow term, and Φ_M is a molecular flow term. δ is a constant that adjusts for the molecular flow effect. It has been more than 40 years since δ=0.515 was proposed by Rigden, 0.97 by Lea and Nurse, and 1.215 by Carman. It is because of these large differences in δ that the Kozeny-Carman equation is still used. In this paper δ=0.82 is proposed as a reasonable value based on a least squares analysis of Rigden's data and with reference to Knudsen's equation of capillary permeation. The use of δ=0.82 allows the more reasonable measurement of powders down to about 1 μm.

^† This report was originally printed in J. Soc. Powder Technology, Japan. 35(9), 649-654 (1998) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.

View full abstract

Microscopic and macroscopic wettabilities of various silica surfaces modified with trimethylsilyl groups were studied, focusing on the effect of their surface geometrical structures. Micro and macro geometrical structures were investigated by water vapor adsorption. Microscopic wettability was determined by water vapor adsorption, while macroscopic wettability was determined by measuring the contact angle. Microscopic wettability affects the continuous two-dimensional water layer, while macroscopic wettability is influenced by capillary condensation and surface roughness. Surface wettability is essentially governed by microscopic wettability, which is a primary property of various silica surfaces. Capillary condensation and surface roughness enhance wettability as a secondary effect which results from the primary property of microscopic wettability.

^† This report was originally printed in J. Soc. Powder Technology, Japan. 36(7), 528-533 (1999) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.

View full abstract