Selective tribocharging is a suitable way for separating mineral constituents from heterogeneous mixtures using an electrostatic field. Though contact charging mechanisms have been studied intensively, understanding is far from clear and complete, due to the variability of material properties and to the influence of ambient conditions, which are often difficult to control. After outlining the scientific fundamentals of contact electrification, the paper illustrates and discusses some significant cases of selective charging of minerals under different experimental conditions, pointing out the effect of operational variables. Separators of practical interest are briefly described, the performance of which is assessed on the basis of beneficiation results obtained for a variety of ores and coal both on a laboratory and industrial scale.
The application of dynamic light scattering for the determination of particle size distributions in the submicron range is reviewed. First, the basic principles and assumptions used in this application are presented. The practical performances are illustrated with results obtained from round-robin studies. A short comparison with particle sizing by static light scattering is included. Finally, new developments for on-line or in-situ characterization of concentrated and opaque dispersions are briefly presented.
High-performance ceramics have attracted much interest during the last decade, but a broad market introduction has not taken place so far due to the inherent disadvantages of ceramic components in applications with high mechanical loadings. The high brittleness and the low reliability as compared to metals, as well as the high production costs are severe obstacles in an area like the automotive engine. Typical defects of ceramic components may be introduced by the powders themselves, but in a like manner, sources of defects also lie in the forming and the densification of the components. The production technologies used for high- performance ceramics are mostly the same that are applied in traditional ceramics. It is demonstrated exemplarily for the materials silicon carbide, zirconia, alumina and silicon nitride, exactly what the main sources of defects are and how they could be avoided. From this, the desired powder properties can be derived. It is expected that improvements in powder processing and forming technologies will improve the reliability of ceramic components and lower the costs, thus leading to the remarkable market growth which has been predicted for more than ten years.
Rigid ceramic filters have emerged in the last decade as the most promising technology for particulate removal from process gases at temperatures up to 1000ºC. Granular and fibrous forms of media have been developed and both are commonly employed in the form of cylindrical “candles” which are periodically cleaned by application of a reverse gas pulse. Research has focused on this cleaning process, which governs the long-term performance of the filter. The problem is two-fold: to develop an expression for the intrinsic dust cake “detachment stress” from knowledge of the dust particle properties and cake structure, and to model the propagation of the cleaning pulse which is applied to remove it. The results of this research are summarized and experimental methods for investigation of filter cleaning briefly described. The implications for design and further development of ceramic filters are discussed.
Increasing computer power has made discrete particle simulation a practical means for predicting particle motion in industrial processes. Simulation techniques for various particle motions, from dilute to dense phase flows, are explained here. In simulations of this kind, the most important problem is how to model the interactions of particles with walls, with other particles and with the fluid. Calculations based on various models for these interactions are discussed. Particle segregation phenomena, particle mixing in a rotating vessel, dense phase pneumatic conveying, and fluidized bed are shown as examples of simulations which have been made in the author's laboratory.
Today, tablets are still the most desired form of dosage due to the convenient administration, the chemical and physical stability of the drug in a solid state, and the high production rate of a tabletting machine based on the compaction of powders or granules. Because direct compression vehicles are still in the minority, pharmaceutical powders have to be processed in order to obtain a tailored particle size distribution exhibiting a good flowability, compressibility, compatibility, wettability, lack of dust, etc. Thus, the size enlargement by wet agglomeration and drying of pharmaceutical powders is an important unit operation in a batch-wise production of granules. For an optimal quality assurance, the starting materials (drug substance, excipients) and the process technologies applied need to be characterized and defined in an appropriate way. Special attention has to be paid to scale-up processes and to monitoring the amount of granulating liquid added to the powder system. Although a batch-wise production is the method of choice from the point of view of a quality assurance department, as a batch can be accepted or rejected, the trend towards lean and just-in-time production focuses the attention on the feasibility of continous processes such as a wet agglomeration and drying technique. Besides the subsequent production of tablets, spherical pharmaceutical granules called pellets with a mean particle size in the range of 0.4 mm to 1.4 mm are usually filled into hard gelatin capsules. The controlled production of pellets with an appropriate mean size and narrow size distribution is highly desired. Due to environmental protection and safety, the use of distilled or demineralized water as a granulation liquid for the wet agglomeration process is the first choice. However, the design of innovative novel dosage forms, i.e. the modification of powders, may make the use of an appropriate organic solvent may be a prerequisite. Thus, the production of novel dosage forms, of specially tailored properties of granules, may need a special process technology with a solvent recovery system, i.e. a correlation can be established between novel product properties and the use of novel process technologies, such as the vacuum fluidized bed granulation and drying method.
In this work we analyze, in the light of their physical foundation, the different methods of thickener design that have been proposed in the literature. We distinguish three types of methods: those based on macroscopic balances, those based on kinematic models and those based on dynamic models. This classification permits the analysis of thickener design procedures with a clear perspective of their applicability and limitations.
The characterization of electrostatic charge and aerodynamic size properties of particles and powders is of critical importance in many electrokinetic processes and research applications. Some of the major particle and powder technology processes that employ electrokinetics of particles are: 1) electrophotography, including laser printing and ink jet printing, 2) powder coating and electrosprays, 3) electrostatic beneficiation of coal and minerals and electrostatic precipitators, and 4) lung deposition of inhaled pharmaceutical aerosols. In these processes, dispersion, transport, and deposition of particles depends significantly upon the electrokinetic behavior of the particles, determined by (1) the aerodynamic diameter – a parameter that includes the particle size, shape, and density, (2) electrostatic charge – both the magnitude and polarity, and (3) fluid flow and electrical fields surrounding the particles. There are a number of instruments that can be used to characterize the aerodynamic size distribution of particles. Likewise instruments are available to estimate the net average electrostatic charge of particles sampled. However, choice of instruments for real-time simultaneous measurements of aerodynamic diameter and electrostatic charge distributions of particles on a single particle basis is limited. The Electrical Single Particle Aerodynamic Relaxation Time (E-SPART) analyzer can be used for simultaneous characterization of particle size distribution in the range from submicron to 100μm and particle charge distribution in the range from 0 to their saturation charge levels. We present a brief description of the principles of operation of the analyzer, its operational range, its advantages, and limitations. Application of the E-SPART analyzer to some powder technology processes is also briefly discussed, citing current research needs in these processes with examples of experimental investigations. In many of these processes, the E-SPART analyzer can be used to optimize the particle and powder technology processes involving electrokinetic properties of particles.
Plug flow of polyethylene pellets was studied in a 50 mm i.d horizontal pipe. The evolution of plug length and velocity along the pipe was deduced from relationships based on mass balances at the front and back of the plug. The average pressure profile along the pipe was analysed with the help of the Ergun equation which governs interstitial gas flow in the plugs. In the case of low pressure gradients, it was demonstrated that the ratio of the plug length over void length, Lb/Lv , is constant along the pipe. How this ratio is correlated with gas flowrate was established through solids hold-up measurements.
A large number of complicated catalyst geometries are produced by extrusion of plastic ceramic materials. The demands for high precision in the forming process and adequate formability of the materials are extremely stringent. As a first approximation, plastic ceramic materials can be treated as ordinary concentrated suspensions. Rheometric methods, in particular capillary rheometry, are especially suitable for testing of these materials. However, the flow processes occurring during extrusion are very complex, with many special effects such as wall slip, shear-thinning, shear hardening and high entrance pressure loss. As a result, apparent viscosity functions are not material functions. In spite of these difficulties, capillary rheometry, when critically applied, is an advantageous tool in the development of easily extrudable ceramic materials. The correlation of rheometrical test results with the extrusion process during production of honeycomb geometries is presented using aluminium oxide ceramics as an example.
This paper provides an experimental and theoretical examination of the use of standpipes to increase the limiting flowrate by gravity from mass flow bins. The results indicate that the effect of a standpipe attached under the hopper outlet is greater as the particle size becomes smaller and the length of the standpipe becomes longer, provided the standpipe remains full of solids.
The frictional behaviour of dry ceramic materials in the form of spherical beads, sand and spray dried powders has been studied. Measurements were made using standardized flow time, shear cell methods and die pressing. Flow time was found to provide a poor measure of internal friction for coarse material. Internal friction increased with decreasing particle size and deviations from sphericity. Powders with high specific surface area had low apparent and tap density and flowed poorly if at all under gravity loading conditions. However, shear cell and die press experiments showed the coefficient of internal friction measured at load was not sensitive to this parameter. The coefficient of internal friction calculated from shear cell measurements in which the maximum stress was 0.4 MPa was in good agreement with that deduced from die stresses during die pressing to 300 MPa, even when particle fracture occurred at the higher pressures, provided the morphology did not change appreciably.
The solution of the dynamic population balance model (PBM) with transport is problematic. The fact that the dynamic PBM model equation solution to the Inverse Problem for grinding systems is degenerate or underspecified is demonstrated. Two numerical solution approaches to the Inverse Problem are used. These are: 1) providing additional constraints on breakage selection functions or 2) performing the Arbiter-Bhrany (or other) normalization of the selection functions. Actual experimental anthracite batch grinding data is used to demonstrate the non-unique functionality of the batch dynamic mill selection and breakage functions for a real physical system. The Levenberg-Marquardt algorithm for systems of constrained non-linear equations is used to solve the batch dynamic PBM grinding equations to obtain the grinding selection and breakage rate functions. Different solutions to the same PBM transport equations are provided. The mill was modeled as a CSTR operating at various retention times. Batch dynamic PBM data was used to provide the mill kinetic and breakage selection function data. Two different solutions were obtained depending on the numerical solution approach. The severity of the non-uniqueness problem for dynamic grinding is demonstrated. Each solution approach to a dynamic PBM with transport, while giving the same prediction for a single batch grinding time, gives different solutions or predictions for mill composition for other grinding times. This fact makes dynamic nodal analysis and control problematic. The fact that the constraint solution approach gives a solution may suggest that normalization is not necessary.
An effective image-analysis system for use in the study of particulate mineralogical material is described. The available commercial image-processing and image-analysis software systems do not usually include adequate algorithms for the effective analysis of multiphase mineralogical textures. Filtering algorithms are usually inadequate for the accurate removal of image noise without compromising the integrity of phase edges. Although most systems offer good algorithms for the analysis of binary images, algorithms for ternary and higher order images are almost non-existent and these are essential for the analysis of mineralogical material in the particulate state. Existing systems make no provision for effective stereological analysis of image data nor for the interpretation of microstructure using models based on stochastic and integral geometry. This paper describes the development of an image analysis system based on an SEM equipped with secondary electron and back-scattered electron detectors, an image memory for the storage of digital images captured at slow scan speeds and a SUN workstation for image processing and image analysis. The algorithms and procedures described include a full range of linear-intercept analysis routines used in quantitative mineralogy.
Effects of grinding conditions and fractional ball filling on the behavior of balls and their collision characteristics in a vibration mill were investigated by image analysis with a video recorder as well as by a numerical calculation method. It was made clear that an increase in fractional ball filling causes an increase in frequency but a decrease in average intensity of ball collisions, and that it results in acceleration of the circulation speed of the balls in the reverse direction to that of circular vibration of the mill. A good correlation independent of vibration conditions and fractional ball filling was obtained between the rate constant of grinding determined by grinding experiments with glass beads as a feed material and the effective breaking collision frequency of balls calculated by the simulation and defined as the frequency of ball collisions whose intensity is greater than the strength of feed particles. From these results, it was made clear that there is an optimum fractional ball filling, depending on the strength of the feed materials and vibration conditions of the mill, that maximizes the rate of grinding.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU, 17(5), 1026-1034 (1991) and 18(1), 78-86 (1992) 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 flow of granular materials is important in the field of material handling. However, there are many unknown factors associated with the analysis of these flows. Constitutive relations (stress and strain rate relations) are needed to describe the flow dynamics of granular materials. We experimentally obtained the constitutive relations of granular materials by using a chute-type shear apparatus. The chute in which the granular material is contained and covered by a top board, can be moved on straight rails, while the top board is loaded vertically and kept immobile. The shear stresses, normal stresses and strain rates measured by using the chute-type shear apparatus give the constitutive relations of the granular materials. The experimental results describe well the flow characteristics (e.g. the stress ratio of shear to normal stress is proportional to the strain rate, etc.) under the condition in which the friction forces between granular materials dominate.
†This report was originally printed in J. Soc. Powder Technology, Japan. 28(10), 632-637 (1991) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
To clarify the dispersion mechanisms of aggregate particles in air, the dispersion of aggregate particles by acceleration in an air stream, which is a typical dispersion mechanism, was examined. In the experiment, aggregate PSL (polystyrene latex) particles (primary spherical particle diameters are: 5.2 and 2μm) were fed into an ejector, where aggregates were dispersed into smaller sizes by acceleration in a high-speed and high pressure (critical pressure higher than atmospheric pressure) air stream. Dispersed PSL particles were introduced into a sampling chamber and sampled on several slide glasses set on the bottom of the chamber by gravitational deposition. PSL particles thus sampled were observed with a microscope. This method allowed exact evaluation of the numbers of primary particles consisting of aggregates. It was suggested from the experiment that aggregate particles consisting of about 5μm primary particles were almost entirely dispersed into primary particles by acceleration of the particles in the ejector. The dispersion force acting on aggregate particles in an air stream at high Reynolds number was theoretically analyzed for a model aggregate particle and compared with the existing van der Waals adhesive force. It was found that the experimental results could be well explained by the theoretical comparison between the dispersion and adhesion forces.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 18(2), 233-239 (1992) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
Expressions to describe the initial collection efficiency of electret filters for both charged and neutral particles were derived covering a wide range of Coulombic force and induced force parameters including the interceptional effect. The prediction equations were in good agreement with the experimental data. Further, the electret charge stability in humid air and in exposure to liquid and solid particles was studied by measuring the time dependency of the particle penetration. It was found that the electret charge is quite stable in humid air but that it is gradually attenuated when exposed to organic droplets because the collected organic droplets spread over the fibers and weaken the electric field around them.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 18(2), 240-247 (1992) 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 pressure inside a silo, especially the dynamic pressure during discharge is a very important factor in its structural design. However, numerical simulations of this problem have seldom been performed because of its complicated behaviors. In this report, the distinct element method (DEM) which is suitable for discontinuous media is used for the analysis of the static state of granular materials in silos, and the possibility of its use to analyze the silo problem is considered. Also, the characteristic distribution of pressures on walls is studied using the DEM simulation, and the influence of friction coefficients and particle arrangement is examined. The result of this study is that the pressure distributions obtained from the DEM simulation are similar to the theoretical or experimental ones. This result of static analysis is used for the initial condition of dynamic analysis, and it is expected to be used to simulate the complicated behaviors of granular materials during discharge in silos.
†This report was originally printed in J. Soc. Powder Technology, Japan. 29(2), 86-94 (1992) 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, dry powders are classified using mainly a vortex centrifuge or sieving equipment, in which the lowest 50% cut size is still greater than one micrometer. Impactor-type classifiers are capable of operating in the submicron range, but have low throughput and high energy consumption. Our new free vortex cyclones are capable of producing excellent sharp classification of dry powders in submicron cut sizes, especially with the use of a 5% to 15% blow-down flow rate from the dust collector chamber. A specified cyclone has been experimentally studied under various operating conditions using several kinds of test powders.
†This report was originally printed in J. Soc. Powder Technology, Japan. 29(5), 351-355 (1992) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Recently, composite particles covered with different kinds of fine particles have been fabricated using various mechanical methods in a dry phase. However, the quantitative analysis of the composite process has not yet been conducted systematically. In this paper, a mechanical processing method called MECHANOFUSION was used to investigate the composite process of glass beads and titanium dioxide fine particles. As a first trial, the ratio of fine particles fixed onto core particles and the BET specific surface area of the processed particles were measured as a function of the processing time. Consequently, the composite process was described as the following two steps: The first is the adhering step whereby fine particles adhere to the core particles' surface, and the second is the compacting step of the fine particle layers. Furthermore, it was found that the BET specific surface area of the processed powder was correlated with the energy consumption per weight of material in the apparatus.
†This report was originally printed in J. Soc. Powder Technology, Japan. 29(6), 434-439 (1992) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.