M. Mihailescu

Charges and forces on conductive particles in roll-type corona-electrostatic separators

Mathematical modelling is a powerful tool in the optimization of electroseparation technologies and equipment. This paper proposes the use of a numerical method of field analysis to evaluate more accurately the charges and forces on millimeter-size conductive particles of virtually any shape in contact with an electrode. A program based on the boundary-element method was tested for this purpose, and provided the input data to an algorithm for the computation of particle trajectories. The results were in good agreement with the observations made on a laboratory electroseparator. The effect of space charge on the behavior of conductive particles was studied with a three-electrode system (a grounded grid, between a corona electrode and a metallic plate, energized by two high-voltage supplies of opposite polarities). The accuracy of numerical simulations of electroseparation processes is expected to increase when a mathematical model of particle motion in DC corona fields will be incorporated in the existing computer programs. >

Optimization of electrostatic separation Processes using response surface modeling

The efficiency of electrostatic separation processes depends on a multitude of factors, including the characteristics of the granular mixtures to be sorted, the feed rate, the configuration of the electrode system, the applied high voltage, and the environmental conditions. The possibility of optimizing the operation of industrial electrostatic separators using rather simple computed-assisted experimental design techniques has already been demonstrated. The aim of the present work is to analyze the peculiarities of application of a more sophisticated group of response surface experimental design techniques that make use of quadratic functions for modeling the electrostatic separation process. One unique contribution to this work is to consider the economic value of the process in addition to the technical result. The 11 electrostatic separation tests, corresponding to a central composite design, were carried out on samples of chopped electric wire wastes. The CARPCO laboratory roll-type electrostatic separator employed for this study enabled a rigorous control of two factors: the applied high-voltage level and the speed of the rotating roll electrode. The objective was to maximize the benefits from the recycling of both constituents of the binary copper-polyvinyl chloride granular mixture. The optimum operating conditions computed with the quadratic model derived from the experimental results were in good agreement with the data of pilot-plant tests. Thus, the highest extraction of useful materials was obtained at high voltage and low speed, while the optimum conditions for greatest economic value were found to be high voltage and high speed. The response surface methodology can be easily applied to most of the industrial applications of electrostatic separation technologies.

Electrostatic Technologies for the Recycling of Non-Ferrous Metals and Plastics from Wastes

Electrostatic separation is extensively used for the selective sorting of granular mixtures, by means of the electric forces, which act on charged or polarized bodies. The roll-type separator with combined corona-electrostatic field has been proved to be the most advantageous solution when the purpose is to isolate conductive particles form nonconductive ones. The paper presents the contributions of the High Intensity Electric Fields Laboratory of the Technical University of Cluj-Napoca to the development of this technique for various applications in the recycling industry. A multitude of factors inffluence the efficiency of the separation process and the authors show how the results of numerical modeling guided the design of new installations and the optimization of their operating conditions. Laboratory and pilot plant brought evidence of the effectiveness of the proposed technology.

A linear-interaction model for electrostatic separation processes

Previous studies based on Box-Wilson and response surface methods have proven that the design of experiments is a powerful tool in improving electrostatic separation performances by controlling the two main parameters of the process: the high-voltage level and the roll speed. The aim of the present paper is to analyze the possibility of deriving a mathematical model capable to reflect the effects of a larger number of factors, as well as their main interactions. At first, the main variables of the process were listed and classified in accordance with the ease of controlling them. Then two experimental designs were chosen, in accordance with Taguchis methodology. The objective was to minimize the middling fraction. The experiments were carried out on a laboratory roll-type electrostatic separator, provided with a corona electrode and a tubular electrode, both connected to a DC high-voltage supply. The samples of processed material were prepared from genuine chopped electric wire wastes (granule size >1 mm and <2 mm) containing 25% copper and 75% PVC. The first experiment consisted of 16 tests, which enabled the derivation of a linear-interaction model comprising 7 variables and 8 interactions. The second experiment consisted of only 8 tests, as the corresponding model took into consideration 4 variables and 3 interactions. A good agreement was obtained between the two models.

Electrostatic separation processes

Optimization of electrostatic separation processes demands the control of a multitude of factors, including the characteristics of the granular mixtures to be sorted, the feed rate, the configuration of the electrode system, the applied high-voltage and the environmental conditions. The Taguchis experimental designs presented in this article clearly prove that the linear-interaction models of the electrostatic separation processes can reflect the effects of the main factors in a manner that is satisfactory to most case of the practical interest. The Taguchis experimental designs are based on special matrices called orthogonal arrays.

Influence of conductive particles on the breakdown conditions of air gaps in the presence of corona

The operating conditions of corona-electrostatic separators, of various electroprecipitators, and of other similar devices are influenced by the size and shape of the particles which pass through the inter-electrode air-gap. The presence of corona affects the movements and the charge of the particles, and hence modifies the breakdown threshold. The numerical analysis of the electric field using a charge simulation program, enabled a crude evaluation of the effects. Nevertheless, experiments with various spherical and cylindrical particles were necessary to fully-understand the phenomena. The electrode arrangement consisted of a matrix of high-voltage corona-emitting points situated above a grounded plate electrode. Both the computational results and the experimental data demonstrated that the presence of particles in air-gaps affected by corona discharges has a more important influence on the dielectric strength than in the case of a charge-free electric field.<<ETX>>

Modeling of conducting particle charging on plate electrodes affected by non-uniform electric fields

Analytical formulae are available for the estimation of the charge acquired by conductive bodies of regular shape (spherical, hemi-spherical, hemi-ellipsoidal, cylindrical) in contact with a plate electrode affected by a uniform electric field. The electrode arrangements employed for charging the particles in some electrostatic processes generate nonuniform electric fields for which no such formulae can be derived. The authors employed a computer program based on the boundary element method for the numerical analysis of the electric field and the computation of the charge carried by cylindrical bodies in contact with a plate electrode. The study was carried out for an electrode arrangement that simulates the charging conditions in plate-type electrostatic separators. It enabled the evaluation of the effect of field nonuniformity on the amount of charge acquired by single particles, in various positions on the surface of the plate electrode. In most of the actual applications, several particles are simultaneously in contact with the charging electrode. Therefore, the study was extended to the situation of several equally-spaced cylindrical bodies in contact with the plate electrode. Several simple experiments confirmed the theoretical predictions. From a practical point of view, the results reported in this paper will serve as a basis for the development of a computer program for the numerical simulation of particle behavior in plate-type electrostatic separators.