Al. Iuga

Corona - electrostatic separators for recovery of waste non-ferrous metals

Abstract This paper presents a series of roll-type electroseparators conceived by High-Intensity Electric Fields Research Laboratory of the Polytechnic Institute of Cluj-Napoca in the last twelve years. The multifunctional ILES-1 and ELSEP corona-electrostatic laboratory separators have been used to study the electro-separability of metallics (copper and aluminium) from chopped wire and cable scrap and to optimize the operating conditions. A comparison is made between the current-voltage characteristics of wire-, needles-, and blade-type electrodes. The theoretically-established design criteria (the polarity of the d.c. high-voltage generator, the setting of the active electrodes, the speed and the radius of the roll) were experimentally confirmed and then used for developing ELSIM-series industrial electroseparators, fabricated by “ELECTROMURES” Company of Tirgu Mures.

Factors which influence the insulation-metal electroseparation

Abstract The experimental study of insulation-metal electroseparation as a multifactorial process has been performed by means of a console-type laboratory apparatus, with four high-voltage electrode carriers, interchangeable roll electrodes, and a set of collecting boxes. The paper illustrates how an electroseparator like this can be employed by research-, design- or application-engineers to investigate the role of each parameter or operating variable. It was revealed the influence exerted on the process by: (i) the homogeneity (composition, granulometric distribution) and the superfacial moisture of the feed material; (ii) the positioning of the corona electrodes and the type of their emitting elements. A special study, based on a charge simulation program, was devoted to the shape, size and location of the electrostatic electrode. The interdependence between the electrical and mechanical factors, such as roll speed and roll radius, are distinctively discussed, with reference to an experiment made with a sample of chopped electric wire and cable scrap, containing stranded and massive copper, polyethylene and polyvinyl chloride. Several technical solutions suggested by the authors (corona electrodes with radially-orientated emitting wire segments, non-circular electrostatic electrode) have been adopted by the recycling industry.

Corona–Electrostatic Separation: An Efficient Technique for the Recovery of Metals and Plastics From Industrial Wastes

Better management of secondary resources, such as copper, aluminium and plastics from electric wire and cable waste requires improved methods of processing of reusable materials. This paper presents three typical examples of recycling technologies, each of them using the capability of electric fields to sort metallic and insulating materials from granular mixture. The efficiency of corona–electrostatic separation as a basic operation of a recycling flowsheet has been demonstrated for feedrates not exceeding 200 kg/hour. At the same time, it has been proved that electroseparation can represent a complement to conventional air–gravity separation of metals and insulators from industrial wastes. The in–plant tests provided useful information for a customs–design of corona–electrostatic separators.

High-voltage supplies for corona-electrostatic separators

The selection of the high-voltage supply can play an important role In the optimisation of electrostatic separation processes. The present work aimed to evaluate the influence of the main high-voltage parameters (waveform, polarity, level) on the efficiency of electroseparation, in the case of insulation-metal granular mixtures. A roll-type laboratory electroseparator was employed for the experimental study and the tests were carried out with granular materials prelevated from the technological flowsheet of a recycling plant for electric wire scraps. The experiments shown the existence of a strong interdependence between the level of the operating voltage and the other electrical parameters, Although the full-wave rectifier allows for lower operating voltages than the half-wave rectifier, its general effectiveness in electroseparation processes is superior. The optimum operating voltage of an electroseparator seems to be slightly lower than the level at which the frequency of the spark discharges tends to exceed 60 min/sup -1/. The oscillograms of the voltage and of the current across the separator proved to be of great use for studying the transition from corona to spark discharges. Good insulation-metal electroseparation can be achieved at either positive or negative polarity of the high-voltage supply, but negative electrode energization is recommended for most industry applications.

Factors which affect the corona charging of insulating spheres on plate and roll electrodes

Abstract Corona charging of particulates is employed in many electrostatic applications. Pautheniers equation for the ionic charging of spheres in uniform electric fields cannot be used for particles in the vicinity or in contact with an electrode. Therefore, this paper studies the charging of insulating spheres on plate and roll electrodes affected by monopolar space charge. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (radius: 1.5 mm), when subjected to the corona discharge generated between a high voltage electrode and either a plate or a roll electrode connected to the ground. The spheres were found to be less charged when the grounded electrode was covered by an insulating layer. When the insulating spheres were in contact with the rotating roll, their charge - at a given electric field intensity - was found to reduce with the increase of roll velocity. A similar correlation was found with the initial speed of the particle at the moment of deposing it on the surface of the rotating roll electrode. However, the saturation charge was the same and the increase of the electric field beyond a certain limit determined a diminution of the charge carried by the particles (auto-discharge effect). From a practical point of view, these observations have already served to optimise the operating parameters of roll-type corona electrostatic separators for mixed granular solids.

Modelling of high-intensity electric fields - a method to improve the design of corona-electrostatic separators

Abstract The equations of the corona-electrostatic field in electroseparators, presented in this paper, can not be solved analytical. Therefore, testing different models of either corona or electrostatic electrodes has proven to be the most reliable way to decide the basic structure of new electroseparators. Conductive sheet and electrolytic tank analogues facilitate optimization of electroseparation equipments.

Charge-Neutralization Electrodes for High-Tension Separators

Safe and efficient operation of industrial high-tension separators requires a strict control of residual electric charge of the particles emerging the process. Principles of charge neutralization in a roll-type electroseparator are briefly discussed, in order to establish the general features of the corona electrodes that might accomplish this task. A new electrode design is suggested. Its spacific features: small cross-section (neglectable perturbation of the material stream), low corona on-set voltage, good stability of corona discharge during long-time operation, short maintenance time, have been tested on a laboratory model, prior to industrial application. Several design considerations have been formulated, based on a critical evaluation of the experimental results.

Tribocharging of granular plastic mixtures in view of electrostatic separation

The output of any electrostatic separation process is strongly dependent on the effectiveness of particle charging. A novel laboratory-scale fluidized-bed tribo-charging device has been described in a previous paper. The aim of the present work is to asses the effectiveness of two materials, aluminium and polypropylene, that could be employed for the manufacturing of the tribo-charging chamber of that device. The electrostatic separation experiments designed in this purpose were carried out on three samples of a granular polyethylene/polyvinyl chloride mixture, characterized by different compositions. Aluminium and polypropylene were found to be equally effective for the tribo-electrostatic separation of the 50% PVC, 50% PE sample. For the other two samples (10% PVC, 90% PE; 90% PVC110% PE), the best results were obtained with the polypropylene tribo-charging chamber.

Influence of high-voltage polarity on insulation-metal electroseparation

Abstract A comparative study of the physical phenomena involved in corona-electrostatic separation of insulation-metal granular materials at different polarities has been initiated. The experiments were carried out with a reversible, non-filtered, d. c. high-voltage supply (single-phase step-up transformer / full-bridge rectifier), which energized either a laboratory electroseparator, or a test-cell specially-arranged for the study of corona effects. The electroseparation tests, performed on a sample containing 50% polyvinyl chloride and 50% stranded copper, have demonstrated that both polarities can be employed to ensure a good insulation-metal electroseparation. Subtle differences between corona discharges at positive and negative polarities, in the presence of the materials to be separated, were made evident by several laboratory experiments. Positive corona ensures a wider range of operating voltages, and is more uniform. Negative corona provides greater current at the same voltage levels, and produces a less-intense electric noise. Intense back-corona is generated by the tips of the stranded copper particles.

Particle charge neutralization in roll-type electroseparators

Abstract Safe and efficient operation of industrial electrostatic separators require a strict control of the residual electric charge of the particles emerging from the process. The present paper analyses the efficiency of charge neutralization accomplished by conveniently located electrodes, energized from ac. high-voltage supplies of 1250 Hz and 50 Hz. The tests were carried out on a laboratory electrostatic separator, provided with two wire-type corona electrodes connected to a fully-adjustable d.c. high-voltage supply of positive polarity (0…30)kV peak value. A needle-type corona electrode was employed for the neutralization of the charge. The particulate material used throughout the experiments consisted of mm-size PVC granules, issued from chopping of electric wire insulation. With both the 1250 Hz high-voltage supply and the conventional 50 Hz high-voltage transformer, about 80% of the material is removed from the surface of the roll electrode, following charge neutralization. The tests should continue with a variable frequency high-voltage supply, in order to establish an optimum value of this parameter under various operating conditions.