Mihai Bilici

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.

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.

Tribo-aero-electrostatic separator for ternary mixtures of granular plastics

This paper presents a comprehensive study of an electrostatic separation device for the recycling of granular plastic materials from waste electrical and electronic equipment (WEEE). The peculiarity of this system is the association of two charging techniques: (1) fluidized bed (dynamic charger) and (2) sliding on inclined plates (static charger)-and two superposed separation units, in order to achieve the selective sorting of a granular mixture composed of three different species of insulating materials. The tests were performed on a mixture of high-density polyethylene (HDPE), polyvinyl chloride (PVC), and polycarbonate (PC). A set of tribocharging experiments was conducted for each material, so that to choose the nature of the walls of the static charger. The analysis of the purity of separated products was performed using an image processing program written in MATLAB. High-purity separation (85%, 95%, and 75% for the PVC, PC, and HDPE products, respectively) was obtained at a recovery rate beyond 80%.

Experimental modeling of the tribo-aero-electrostatic separation of mixed granular plastics

A tribo-aero-electrostatic separator can be successfully employed for the selective sorting of granular mixtures of polymers. The tribocharging of the material to be separated occurs in a parallelepiped fluidized bed. Two opposite walls of this device consist of metallic plates connected to two DC high-voltage supplies of opposite polarities, so that the charged granules are attracted to these electrodes and separated while still in fluidized state. The granules are then collected in two recipients each placed in a Faraday pail, to allow the instantaneous measurement of the mass and charge of the separated products. The experiments were carried out on binary mixtures of virgin polyamide and polycarbonate granules both in intermittent and continuous regime Real-time data-acquisition enable the monitoring of the separation outcome, i.e. mass, purity and charge, at various values of the applied voltage, of the mass of material and of the proportions in the granular mixture.

Tribo-Aero-Electrostatic Separator for Ternary Mixtures of Granular Plastics

This paper presents a comprehensive study of an electrostatic separation device for the recycling of granular plastic materials from waste electrical and electronic equipment (WEEE). The peculiarity of this system is the association of two charging techniques: 1) fluidized bed (dynamic charger) and 2) sliding on inclined plates (static charger)—and two superposed separation units, in order to achieve the selective sorting of a granular mixture composed of three different species of insulating materials. The tests were performed on a mixture of high-density polyethylene (HDPE), polyvinyl chloride (PVC), and polycarbonate (PC). A set of tribocharging experiments was conducted for each material, so that to choose the nature of the walls of the static charger. The analysis of the purity of separated products was performed using an image processing program written in MATLAB. High-purity separation (85%, 95%, and 75% for the PVC, PC, and HDPE products, respectively) was obtained at a recovery rate beyond 80%.

Numerical Simulation of Tribo-Aero-Electrostatic Separation of Mixed Granular Solids

Numerical simulation can be a powerful tool in the research and development of any new electrostatic process. The aim of the present work is to introduce a simple mathematical model for simulating the outcome of a recently patented tribo-aero-electrostatic separation process for binary mixtures of granular materials. The process is characterized by the fact that the charging of the granules is produced in a fluidized bed device, in the presence of an electric field. The mathematical model assumed that the probability of a granule to be separated can be expressed as a function of the number of impacts with granules belonging to the other class of materials. When this probability was given by a normal distribution law, the results of the simulations differed significantly from those of an experiment conducted with a granular mixture of polyamide and polycarbonate particles, in a laboratory tribo-aero-electrostatic separator. In order to improve the predictive capability of the simulations, a polynomial function derived from the regression of the experimental data was employed for expressing the same probability. Thus, it was possible to calculate the evolution in time of the mass of granules separated at the electrodes for various compositions of the granular mixture. The computed results were in good agreement with the experiments.

Numerical simulation of tribo-aero-electrostatic separation of mixed granular solids

Numerical simulation can be a powerful tool in the research and development of any new electrostatic process. The aim of the present work is to introduce a simple mathematical model for simulating the outcome of a recently patented tribo-aero-electrostatic separation process for binary mixtures of granular materials. The process is characterized by the fact that the charging of the granules is produced in a fluidized bed device, in the presence of an electric field. The mathematical model assumed that the probability of a granule to be separated can be expressed as a function of the number of impacts with granules belonging to the other class of materials. When this probability was given by a normal distribution law, the results of the simulations differed significantly from those of an experiment conducted with a granular mixture of polyamide and polycarbonate particles, in a laboratory tribo-aero-electrostatic separator. In order to improve the predictive capability of the simulations, a polynomial function derived from the regression of the experimental data was employed for expressing the same probability. Thus, it was possible to calculate the evolution in time of the mass of granules separated at the electrodes for various compositions of the granular mixture. The computed results were in good agreement with the experiments.

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.