Ciprian Dragan

Tribocharging and electrostatic separation of mixed granular solids in fluidized bed devices

Granular materials, when fluidized by air or other gaseous medium, acquire electrostatic charge by inter-particle and particle-wall collisions. The effectiveness of particle tribocharging achieved with such fluidization process is crucial for establishing the feasibility of electrostatic separation of mixed granular solid wastes in the recycling industry. The present paper aims at a crude evaluation of the effects of the main factors that influence the efficiency of such a tribocharger, in the presence of an electric field orthogonally oriented to the direction of the fluidization air. The experiments were carried out on binary mixtures taking combinations of two of the three polymeric materials in different tests: (a) virgin polyamide, (b) polycarbonate and (c) polyacetal granules. The variables investigated were the duration of the tribocharging process, the level of the high-voltage applied to the electrode and the composition of the mixture. The analysis of the experimental data revealed the effects of each variable on the mass and the charge of the granules collected at the electrodes.

Tribocharging of Mixed Granular Plastics in a Fluidized-Bed Device

The output of any electrostatic separation process is strongly dependent on the effectiveness of particle charging. The aim of the present paper is to evaluate the effects of several factors that might influence the efficiency of a fluidized bed tribocharger, in the presence of an electric field orthogonally oriented to the direction of the fluidization air. The experiments were carried out on several synthetic mixtures of binary granular mixtures. The velocity of the fluidization air, the duration of the tribocharging process, and the composition of the mixture were the three process variables investigated. Whenever possible, the effects of these variables on the output of the process (i.e., the mass and the charge of the granules collected at the electrodes), were evaluated using the experimental design methodology. The results of full- and composite-factorial experimental designs were analyzed with a commercial software (MODDE 5.0, Umetrics). The effectiveness of the charging process was found to depend significantly on each of these factors. The propitious aerodynamic design of the device is a prerequisite for the successful industry application of the technique.

Experimental Study of the Optimum Operating Conditions of a Pilot-Scale Tribo-Aero-Electrostatic Separator for Mixed Granular Solids

Mixed granular insulating materials can be electrostatically separated by tribocharging them in a fluidized bed affected by an electric field orthogonally oriented to the direction of the fluidization air. The aim of this paper is to optimize the operating conditions of a novel pilot-scale tribo-aero-electrostatic separator, i.e., maximize the total mass of the granules collected at the two electrodes that generate the electric field. The mixture to be separated was composed of acrylonitrile butadiene styrene (ABS) and high impact polystyrene (HIPS) originating from the recycling of waste electric and electronic equipment. A set of experiments carried out for two durations (30 and 60 s), at two values of ambient relative humidity ( and ), pointed out that the separation becomes efficient beyond a certain value of the initial mass of the mixtures. The experimental design methodology and commercial software MODDE 5.0 were then employed for investigating the effects of two control variables of the industrial process: the speed of the metallic conveyors that serve as high-voltage electrodes and the humidity of ambient air. The effectiveness of separation was found to depend significantly on the latter factor. Surface-potential-decay measurements performed on charged layers of the two sorts of particles facilitate the interpretation of separation results: At high , in contact with an electrode, the ABS granules lose their charge faster than the less hydrophilic HIPS granules. As a consequence, fewer ABS granules are collected at the exit of the tribo-aero-electrostatic separator.

Factors that influence the efficiency of a fluidized-bed-type tribo-electrostatic separator for mixed granular plastics

Fluidized bed devices have already been used as tribochargers for various industrial electrostatic separation processes. In the present paper, the authors investigate the behaviour of polyamide – polycarbonate granular plastic mixtures in a parallelepiped bed, the height of which is roughly 2 times its length or width, so that the collisions between granules become the prevailing tribocharging mechanism. Two of the opposite walls of the tribocharging chamber consist of metallic plates connected to two DC high-voltage supplies of opposite polarities, so that the charged particles are attracted to the electrodes and separated while still in the fluidized state. The collecting hoppers are designed as Faraday cups connected to two electrometers, thus allowing the instantaneous measurement of the charge carried by the separated particles. Experimental design methodology was employed for the optimization of the tribo-aero-electrostatic separation process, the input variables being the high-voltage applied to the electrodes and the duration of the tribocharging. Higher voltages applied to the electrode system do not necessarily lead to larger quantities of collected products but improve the purity of the concentrates. The composition of the mixture influences the outcome of the process.

Electrostatic Basis for Separation of Wheat Bran Tissues

New fractionation technologies are presently under investigation in order to enhance the wheat bran nutrient value. The explicit aim of this paper is to evaluate the possibility of using electric field forces for the separation of the fine particles generated from each wheat bran tissue during the milling process. Corona-charging and surface-potential-decay experiments were performed on samples of finely ground wheat bran and aleurone tissues (maximum size of 0.3 mm). Ground bran, which has a higher moisture content, more rapidly loses the charge imparted by corona discharge. For both kinds of samples, finer particles charged better than larger particles, and lyophilized samples were found to better preserve their charge than those not subjected to a freeze-drying process. An experiment carried out using a belt-type corona electrostatic separator validated the possibility of sorting these tissues based on the differences in their charge decay characteristics. Tribocharging experiments performed on the same tissues have also led to encouraging results: The charge/mass ratio of ground bran tissues was twice that of aleurone. These differences in the physical properties of the two types of tissues provide the technical basis for the development of electrostatic-separation technologies as part of more efficient bran-cracking strategies.

Triboelectrostatic Phenomena in Suction-type Dilute-phase Pneumatic Transport Systems

Pulverulent and granular materials processed in any suction-type dilute-phase pneumatic transport system tribocharge the pipes they move through. The aim of this study is to refine the understanding of the triboelectrostatic phenomena involved. A laboratory installation was employed for the investigation of the charging and discharging of various zones of an insulating pipe during and after the suction of a well-defined quantity of fine, dust-like, particles. The amount and the sign of the charge that was measured using several custom-designed electrostatic induction probes depended on the nature of the processed powders and varied from one zone of the pipe to another. Surface potential decay measurements confirmed that the insulating pipe retains high levels of charge for several hours after the cease of the suction. These observations point out the need of thorough laboratory modelling of any new pneumatic transport system, in order to avoid the electrostatic hazards related to electric charge build-up in various parts of the installation during operation.

Numerical Modeling of Triboelectric Charging of Granular Materials in Vibrated Beds

The triboelectric charging of granular materials is a complex physical phenomenon, the outcome of which is difficult to predict. The aim of this paper is to validate a numerical model of the tribocharging process taking place in vertically-vibrated beds of granular plastics. The granular dynamics is modeled with the Distinct Element Method that is commonly employed to design pneumatic conveyors or predict hopper emptying. The charge exchanged in granule-to-granule and granule-to-wall collisions is computed by taking into account the physical properties of the respective materials, their area of contact, as well as the effect of the electric field created by a system of high-voltage electrodes and by the charges of the granules themselves. The numerical model simulates also the free-fall motion of the charged granules in the electrostatic field created between two vertical plate electrodes. The outcome of the simulation, i.e. the charge of the particles collected at the two electrodes in a definite laps of time, is in good agreement with the results of an experiment carried out with mixtures of mm-size polyamide and polycarbonate granules in a laboratory vibrated bed.

Experimental study of the effect of ambient air humidity on the efficiency of tribo-aero-electrostatic separation of mixed granular solids

Mixed granular insulating materials can be electro-statically separated by tribocharging them in a fluidized bed affected by an electric field orthogonally oriented to the direction of the fluidization air. The aim of the present paper is to evaluate the effect of ambient air humidity on the outcome of this tribo-aero-electrostatic separation process, i.e. the mass of the granules collected at the two electrodes that generate the electric field. The mixture to be separated was composed of Acrylonitrile Butadiene Styrene (ABS) and High Impact Polystyrene (HIPS) originating from the recycling of waste electric and electronic equipment. A set of experiments carried out for two durations (30 s and 60 s), at two values of ambient relative humidity (RH = 20% and RH = 40%), pointed out that the separation becomes efficient beyond a certain value of the initial mass of the mixtures. The experimental design methodology and commercial software MODDE 5.0 were then employed for investigating the effects of two process variables: the speed of the metallic conveyors that serve as high-voltage electrodes, and the humidity of ambient air. The effectiveness of separation was found to depend significantly on the latter factor. Surface potential decay measurements performed on charged layers of the two sorts of particles facilitate the interpretation of separation results: at higher RH, the hydrophilic ABS granules loose faster their charge when in contact with a grounded plate. As a consequence, they are in less number than the HIPS granules to be collected at the exit of the tribo-aero-electrostatic separation chamber.

Triboelectric Phenomena in Suction-Type Dilute-Phase Pneumatic Transportation Systems for Granular Plastics

The triboelectric phenomena related to particle-to-particle and particle-to-wall impacts are known to affect the efficiency of suction-type dilute-phase transport systems. The aim of this paper is to evaluate the effects of two factors: the granular material feed rate and the aspirating air flow rate. The study was conducted with mm-size ABS and HIPS particles, two granular materials originating from genuine information technology (IT) wastes. The tribocharging processes in polyvinyl chloride (PVC) and aluminum pipes were modeled using the response surface method of experimental design. An induction-type sensor connected to an electrometer has been employed for the measurement of the charge imparted to a well-defined section of the duct, which is equal to the charge transferred to the particles passing through that section of the pneumatic transport system. The measured data were processed by a virtual instrument developed in LabVIEW and then analyzed using commercial software (MODDE 5.0, Umetrics, Sweden). Under the specific conditions of the experiments described in this paper, the charge/mass ratio of the processed particles was found to increase with the aspirating air speed, but was less affected by the granular material feed rate. It was concluded that the appropriate design of the transport system might provide an effective precharging of the granular mixtures of insulating materials that are processed in standard triboelectrostatic separators.

Premises for the Electrostatic Separation of Wheat Bran Tissues

New fractionation technologies are presently under investigation in order to enhance the wheat bran nutrient value. The explicit aim of the present paper was to evaluate the possibility of using the electric field forces for the separation of the fine particles generated from each wheat bran tissue during the milling process. Corona charging and surface potential decay experiments were performed on samples of more or less finely- grinded wheat bran and aleurone tissues. For both kinds of samples, finer particles charged better than larger particles, and lyophilized samples were found to better preserve their charge than those not subjected to a freeze-drying process. An experiment carried out using a belt-type corona-electrostatic separator validated the possibility of sorting these tissues based on the differences in their charge decay charcateristics. Tribocharging experiments performed on the same tissues have also led to encouraging results: the charge/mass ratio of ground bran tissues was twice that of aleurone. These differences in the physical properties of the two types of tissues represent the premises for the development of electrostatic separation technologies as part of more efficient bran cracking strategies.