Adrian Mihalcioiu

Robust design of electrostatic separation processes

The aim of this paper is to analyze the robustness of the electrostatic separation process control. The objective was to reduce variation in the process outcome by finding operating conditions (high-voltage level, roll speed), under which uncontrollable variation in the noise factors (granule size, composition of the material to be separated) has minimal impact on the quantity (and the quality) of the recovered products. 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 <5 mm) containing various proportions of copper and PVC. The design and noise factors were combined into one single experimental design, based on Taguchis approach, and a regression model of the process was fitted. The impact of the noise factors could be estimated, as well as the interactions between the design and noise factors. The conditions of industry application of Taguchis methodology are discussed, as well as the possibility of adapting it to other electrostatic processes.

Controlling Particle Trajectory in Free-Fall Electrostatic Separators

The purity and recovery of concentrates obtained in industrial free-fall electrostatic separators can be increased by preventing the impacts between the particles and the electrodes. The aim of this paper is to analyze the possibility to control particle trajectories in such separators by modifying the conditions of particle admission in the interelectrode space. The parametric equations of a charged particle trajectory in the electric field between the electrodes of a free-fall separator served for writing a numerical modeling program in MATLAB. The ten control factors of the free-fall electrostatic separation process were employed as input variables of this program: particle charge and dimension, trajectory start point coordinates, feed input angle and initial velocity, electrode length and inclination, interelectrode spacing, and applied high voltage. A custom-designed laboratory free-fall electrostatic separator (length of the electrodes: 1000 mm; standard interelectrode spacing: 300 mm; and nominal high voltage: 90 kV), provided with a fluidized-bed tribocharger, was employed for validating the conclusions of the numerical modeling. The geometrical data of this separator were taken into account for computing the initial velocity nu0 of the particles entering the electric field zone. Another important parameter of the numerical model, which is the granule charge q, was attributed the value measured at the exit of the fluidized-bed tribocharger. The numerical simulations were performed for the two values of the feed input angle: alpha = 4deg and alpha = 8deg, considering polyethylene therephtalate (PET) and polyvinyl chloride (PVC) particles of different sizes. A good agreement was found between the theoretical predictions and the results of the experiments carried out with two binary mixtures: 50% PET/50% PVC and 10% PET/90% PVC. Both the numerical modeling and the experimental study demonstrated that the feed input angle a influences the outcome of the electrostatic separation to a great extent.

Using Design of Experiments and Virtual Instrumentation to Evaluate the Tribocharging of Pulverulent Materials in Compressed-Air Devices

Tribocharging of pulverulent materials in compressed-air devices is a typical multifactorial process. Quantification of the effects of the factors and of their interactions is a prerequisite for the development of new tribocharging devices for industrial applications. This paper aims at demonstrating the interest of using the design of experiments methodology in association with virtual instrumentation for the study of such processes, in view of their modeling and optimization. A classical 23 full-factorial design followed by a composite design were employed for conducting experiments simulating the tribocharging conditions of starch powder. The response function was the charge/mass ratio of the material collected in a modified Faraday cage, at the exit of the tribocharging device, the factors under investigation being the injection pressure, the dillution pressure, and the vortex pressure. The charge measurements were performed using a digital electrometer connected to a personal computer equiped with a data acquisition system. The data were processed by a custom-designed LabView virtual instrument. By using appropriate design of experiments software, it was possible to estimate the effects of these factors and then derive the model of the process as a quadratic polynomial function.

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.

High-voltage monitoring in electrostatic separators

High voltage is known to be one of the main control variables in any electrostatic-separation process. From this perspective, the aim of this paper is twofold: to develop an effective high-voltage monitoring system and to demonstrate that it can be a useful tool for controlling the overall operating conditions of an electrostatic-separator system. A custom-designed virtual instrument was employed for processing the experimental data provided by a high-voltage probe, the output of which was connected to an electrometer. In several experiments, the output of the high-voltage probe was also connected to a digital oscilloscope, in order to obtain a better understanding of the variation of the electrode potential after a spark discharge. The laps of time without corona discharge and/or with low electric field intensities could thus be accurately determined, and the impact of the spark discharges on the outcome of the separation process evaluated. The dispersion of high-voltage measured values was found to increase in the presence of the material. The statistical analysis of the data revealed a significant correlation between the standard deviation of the high-voltage and the concentration of metal in the processed material. The results of this paper could be helpful for those seeking the optimization of the operating conditions for the electrostatic separation applications, in which the metal content in the feed materials exhibits substantial fluctuation with time

Charge-Decay Characteristics of Granular Materials Forming Monolayers at the Surface of Grounded Electrodes

Laboratory studies and in-field observations have shown that the charge-decay characteristics of the granular materials at the surface of the grounded roll electrode significantly influence the outcome of the electrostatic separation process. This paper validates an indirect method of charge-decay characterization, based on the measurement of the electrical potential at the surface of a monolayer of granular insulating material. The study was performed on three materials - polyvinyl chloride, polyethylene, and rubber - extracted from chopped electric wire wastes. The granules (characteristic size in the range 1-4 mm) were disposed on the surface of a grounded plate electrode (layer area: 100 mm times 100 mm; electrode area: 200 mm times 200 mm). A wire-type corona electrode, energized from a dc high-voltage supply, was employed for charging the granules. The potential due to the charge at the surface of the granular layer was measured with the capacitive probe of an electrostatic voltmeter connected to a personal computer. Data acquisition and processing were done using the LabView environment. The influence of particles characteristics and of ambient factors was studied. The findings enabled a more accurate modeling of discharging phenomena that affect the performances of electrostatic separators. The method can be easily adopted in electrostatic discharge studies for material characterization.

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.

Charge-decay characteristics of granular materials forming mono-layers at the surface of grounded electrodes

Laboratory studies and in-field observations shown that the charge decay characteristics of the granular materials at the surface of the grounded roll electrode significantly influence the outcome of the electrostatic separation process. The present paper validates an indirect method of charge decay characterization, based on the measurement of the electrical potential at the surface of a monolayer of granular insulating material. The study was performed on three materials - PVC, PE and rubber - extracted from chopped electric wire wastes. The granules (characteristic size in the range 1 to 4 mm) were disposed on the surface of a grounded plate electrode (layer area: 100 mm x 100 mm; electrode area: 200 mm x 200 mm). A wire-type corona electrode, energized from a DC high-voltage supply, was employed for charging the granules. The potential due to the charge at the surface of the granular layer was measured with the capacitive probe of an electrostatic voltmeter connected to a personal computer. Data acquisition and processing were done using the LabView environment. The influence of particles characteristics and of ambient factors was studied. The findings enabled a more accurate modelling of discharging phenomena that affect the performances of electrostatic separators. The method can be easily adopted in ESD studies for material characterisation.

Factors that influence the tribocharging of pulverulent materials in compressed-air devices

Tribocharging of pulverulent materials in compressed-air devices is a typical multi-factorial process. This paper aims at demonstrating the interest of using the design of experiments methodology in association with virtual instrumentation for quantifying the effects of various process varaibles and of their interactions, as a prerequisite for the development of new tribocharging devices for industrial applications. The study is focused on the tribocharging of PVC powders in compressed-air devices similar to those employed in electrostatic painting. A classical 2 full-factorial design (3 factors at two levels) was employed for conducting the experiments. The response function was the charge/mass ratio of the material collected in a modified Faraday cage, at the exit of the tribocharging device. The charge/mass ratio was found to increase with the injection pressure and the vortex pressure in the tribocharging device, and to decrease with the increasing of the feed rate. In the present study an in-house design of experiments software was employed for statistical analysis of experimental data and validation of the experimental model.

Charging and discharging of insulating particles on the surface of a grounded electrode

The study of the ionic charging of insulating particles is intimately related to the technological developments in the field of electrostatic precipitation of dusts, deposition of powders, or separation of granular materials. The aim of the present paper is to analyze the corona charging of mm-size insulating disks, as well as their discharging when they are no longer exposed to the action of an external electric field. The experiments were carried out on a roll-type electrostatic laboratory separator, provided with a wire-type corona electrode, simulating the actual charging/discharging conditions in an industrial unit. Particles of various sizes were charged on the surface of the roll electrode, then the high voltage supplied to the corona electrode was turned off and the particles collected in a Faraday pail, connected to an electrometer. The charge measurements were performed at various time intervals from high-voltage turn-off. In this way, the charge decay could be recorded and the discharge process fully characterized. The measured data show that the discharge process depends on the nature, size and shape of the particles, as well as on the contact conditions between the particles and the grounded roll electrode. These data could guide the design of the electrostatic separation experiments that proceed any new industrial application of this technology.