Separation Of Granular Mixtures Research Articles

К анализу внутрислоевых перемещений при сепарировании сыпучих смесей

К анализу внутрислоевых перемещений при сепарировании сыпучих смесей

On the new numerical solution of corona‐electrostatic electric field model with inductive electrostatic electrode for the industrial separation of granular mixture

PurposeThe purpose of this paper is to study the industrial separation of granular mixture by approximation of combined corona‐electrostatic electric field with feeder and inductive electrostatic electrode.Design/methodology/approachThe original numerical method developed in Caron's Kybernetes paper is used and extended to an industrial construction. The method gives a mathematical model of an industrial process and converts the non‐linear partial derivative system into an iterative system of linear equations. Using the well‐known finite difference approximation, a numerical solution is computed very quickly.FindingsIn order to really obtain a computer‐aided numerical solution, it is necessary to define a really manageable approximation method. The new simulation results are detailed and show better results for the steady state in time processing and finally, to imagine improvements of the industrial processes.Originality/valueThe paper shows the fast numerical solution, which leads to confidence in the numerical approximations to imagine improvements of the industrial processes easily.

Charging of one or several cylindrical particles by monopolar ions in electric fields

In many electrostatic processes (precipitation of dust, separation of granular mixtures, spraying of powders) insulating particles are subjected to unipolar charging in the presence of other bodies. The present paper addresses this problem from both a computational and an experimental point of view, with the aim to extend an original numerical method of unipolar charge computation to the case of two or more particles, relatively close to each other. The charge acquired by cylinders of various dielectric constants was evaluated with a computer program based on the boundary element method of field analysis. The experimental setup simulated the charging conditions of millimeter-size calibrated cylinders of polyvinyl chloride in a roll-type electrostatic separator, the unipolar space charge being generated by a wire-type electrode. The experimental results, which were in good agreement with the theoretical predictions, put forward that the relative spacing between the particles changes the value of the saturation charge, an effect, which is stronger for particles of higher dielectric constant. These facts should be taken into account in the design of any electrostatic technology based on the corona charging of granular matter.

Size and Density Separation in Granular Materials

ABSTRACTWe review mechanisms leading to the separation of granular mixtures according to the size or density of the constituent particles. Besides a general overview, the focus of this article is on separation induced by vertical vibrations of the vessel holding the granular material, and specifically on recent results showing a density-dependence to the separation process. A convenient measure of the separation speed is the rise time of a larger “intruder particle” from some fixed initial depth to the free surface, where it will remain. Recent experiments have shown that for large intruder sizes the rise time depends on the density of the intruder particle. Furthermore, in three-dimensional systems we found that this density dependence is highly nonmonotonic. While there is no detailed theoretical understanding available yet, our experiments demonstrate that this surprising behavior is produced by interactions not only between the large intruder particle and the smaller particles of the surrounding granular bed, but also between the particles and the interstitial air.

Computational estimation of ESD conditions between charged particles and grounded surfaces

The paper examines the conditions in which a discharge occurs between charged spherical particles and grounded surfaces, in air, at atmospheric pressure. The study is of interest to a large group of electrostatic applications which involve the processing of particulate matter: separation of granular mixtures, precipitation of dust, and spraying of powders. At the same time, this simple physical model simulates a class of hazardous situations quite often met in the operation of electronic circuits. The boundary-element method was employed for the computation of the electric field in the gap of length x between a charged conductive spherical particle of radius R and a grounded plate. The data on field distribution represented the input data of an original program which employed Pedersen's criterion for estimating the breakdown conditions of that air-gap. The study was carried out for spheres of various sizes (radii: 0.01-1 mm). Some computations were performed for the case when the grounded plate was covered with an insulating layer of known permittivity. The computational results were in good agreement with the experimental data for both long (x/R>10) and short (x/R<0.1) gaps.

Modeling of conductive particle behavior in insulating fluids affected by DC electric fields

Several electrostatic technologies, such as separation of granular mixtures, flocking, printing, or biological cell manipulation, are based on the accurate control of conductive particle motion in insulating gases or liquids by means of relatively high DC electric fields. This paper is aimed at characterizing the behavior of such particles by numerical modeling of two aspects: (1) particle motion under the action of electric field forces and (2) insulation breakdown triggered by mobile particles. The equations of particle motion were written by taking into account both gravitational and drag forces, as well as the rebound at particle impact with the electrodes. If the particles move in ionized air, their charge varies in time. In that case, the equation of particle charge should be added to the mathematical model. The output data of the programs for numerical simulation of particle behavior are in good agreement with the available experimental results. Particle movements were shown to be influenced by the intensity of the electric field, by the density of the space charge, by size and mass density of the particles, as well as by their coefficient of restitution at impact with the electrodes. The conclusions regarding the behavior of conductive particles in insulating fluids are useful for the development of improved electrostatic separation technologies; they are of particular interest to all manufacturers of high-voltage equipment.

Electrostatic Charges on Clays

Cation charges present on the surfaces of clay minerals are expected to play a major role in industrial processes involving dry electrostatic separation of granular mixtures containing clay particles. A recent promising potential application has been the removal of clay from British Columbia pulverized coals prior to burning. The clay minerals kaolinite and montmorillonite form the main component of the ash in most Canadian coals: kaolinite is eastern coals and montmorillonite in western coals. The electric charges present on the surfaces of the clay mineral kaolinite have been analyzed from an application point of view, namely, for their ability to transfer onto and remain on other surfaces after contact and separation. Kaolinite samples homoionized with calcium and sodium were prepared from natural kaolin, and the charge transfer after contact and separation with coal was compared for various dry densities of the kaolinite. The results show the following. 1) The electric charge transfer from calcium kaolinite is substantially larger than that from both the sodium kaolinite and natural kaolinite. Charge transfer from untreated montmorillonite exceeds that from all kaolinite samples. 2) The charge transfer after 100 contacts is inversely proportional to the dry density of the clay on a semilog scale. The phenomenon is believed to be related to the changes which occur in the total contact area and in the electrical conductivity of the clay.