Lucian Dascalescu

It is well known that micrometre-sized particulate matter from burning boilers represents an important health hazard for humans and animals. In this article, a few designs of cylindrical-type electrostatic precipitators (an element has a 354 mm-long aluminium cylinder with 40 mm inner diameter without and with reduced cross-section; 0.16 mm diameter for copper discharge wire) that can be used at the output of household low-power boilers (up to 50 kW) that burn coal are investigated. Negative DC high-voltage energisation (up to 14 kV) and negative pulsed energisation (up to 15 kV at different frequencies: 120, 200, and 280 Hz, using a mechanical rotary spark-gap) of the designs of electrostatic precipitators was provided to measure current and voltage shapes, current through precipitators at different speeds of gas, and dust collection when single or multiple disperser at the inlet of precipitators. A chemical analysis, electrical resistivity, and radioactivity of the collected dust are investigated. The experimental results about collection efficiency with different types of precipitator supplied with DC high-voltage and high-voltage pulses show the collection efficiency was better (2 to 13%, depending on the type of the precipitator and the input distributors) for precipitators with reduced cross-section of the collecting electrode and when the same type of precipitator in conditions of dust that has high electric resistivity.

The aim of this paper is to evaluate the effect of dielectric barrier discharge (DBD) plasma treatment on the tribo-electrostatic separation of brominated and non-brominated polymers contained in granular plastic wastes. The experiments were carried out on mixtures consisting of equal quantities of mm-size brominated and non-brominated polyethylene (PE) particles. Two triboelectric charging devices were used: a rotating multi-cylinder tribocharger, consisting of an assembly of six cylinders made of polyvinyl chloride (PVC), and a vibrating tribocharger (a container made of acrylonitrile butadiene styrene (ABS) and fixed on a vibrating table). The selective sorting of the granular mixtures was carried out using a roll-type electrostatic separator. Exposing the plastic mixture to a DBD plasma was used to increase the triboelectric charge of the particles and improve the efficiency of the tribo-electrostatic separation. The best results were obtained with the vibrating tribocharger: the recovery of almost 90% of the non-brominated PE particles in a product containing less than 1% of bromine.

Dielectric barrier discharges (DBD) are known to change the surface morphology, the surface chemical composition, and the triboelectric properties of the treated polymers. The work presented in this paper was aimed at studying the effect of long-term exposure of high-levels of ambient humidity on the triboelectric charging of DBD treated granular plastics. The experiments were carried out on four types of polymers, ABS, PP, PE and PS, with an average particle size between 4 mm and 6 mm. To study the effects of relative humidity of ambient air, the granules were stored for 72 h in a climatic chamber, disposing of simultaneous humidity and temperature control. The relative humidity was regulated at 82% ± 4%, for a temperature of 19°C ± 1°C. The water content of “humid” granules was measured with a thermal-balance and compared with that of “normal” granules, kept at ambient humidity (46% to 52%), at a temperature of 18°C ± 1°C. The “humid” and “normal” granules were successively exposed to a dielectric barrier discharge (DBD) for 60 s in a custom-designed reactor then charged by triboelectric effect in a multi-cylinder triboelectric device. As expected, the DBD treatment modified the triboelectric behavior of each polymer. The experiments showed that the charge/mass effect of this treatment is different on “humid” and “normal” particles. The study points out the importance of providing appropriate storage conditions (i.e., humidity levels lower than 70%) for the plastic wastes processed by tribo-electrostatic techniques.

The phenomenon of surface charging caused by electrical discharges has attracted significant attention because of its adverse effects on electrical systems and its industrial applications. Since the surface charging and discharge current are both influenced by the charges generated during the discharge, it is important to study the correlation quantitatively. The primary goal of the study is to analyse the relationship between the discharge current pulse and the corresponding surface charge deposition for positive and negative excitation. To establish the relation, variations were introduced in two key parameters influencing the discharge process: the discharge medium (N₂, CO₂, and dry air) and the pressure (100, 90, 80, 70, 60 kPa) within each medium. The excitation voltage waveform is chosen to ensure the generation of only a single current pulse during the discharge. The positive excitation resulted in a higher pulse magnitude for N₂ and dry air, whereas CO₂ exhibited an opposite trend. The change in the current pulse is found to be directly proportional to the variation in charge deposition. The derived empirical formulas establish a linear correlation between the total charge computed from the current pulse and the deposited surface charge, verified by Pearson’s correlation coefficient which suggests a good correlation strength. Of the three gaseous mediums, CO₂ has shown a lower margin of error and consistent discharge results.

Introduction. The accumulation of electrostatic charges are exploited in various technological and industrial applications, but they can also pose significant challenges, especially due to the accumulation in inappropriate locations that can reach dangerous levels. Problem. The static charges are often considered annoying and constitute one of the main sources of hazards. Thus, their neutralization is more than necessary. The objective of this work is to improve the neutralization rate with equipment that can be easily integrated in the production lines. Novelty. The paper reports a comparative study of the neutralization efficiency of two electrode systems, dual and triode, with different high alternating voltages at the industrial frequency of 50 Hz. The use of the industrial frequency of 50 Hz reduces the elements of the neutralization equipment. By connecting the grid to ground, we aim to impose a zero potential on the surface of the initially charged polypropylene fibrous dielectric and to determine the variation of the neutralization rate as a function of the discharge intensity (voltage amplitude). Methodology. The samples were charged during 10 s using a triode-type corona electrode configuration supplied by negative or positive DC high voltage. After 300 s of the charging process. The neutralization was performed during 4 s, using the dual or the triode systems powered by sinusoidal high voltage. Neutralization efficiency is achieved by non-contact sampling of surface potential profiles before and after neutralization. The results show that neutralization efficiency is proportionate to the discharge current intensity. The neutralization using the triode system is more efficient. The results show the possibility of imposing a desired potential on the charged or uncharged dielectric surface by acting on the potential of the metallic grid and the discharge intensity of the triode system. Practical value. The results demonstrate the proportionality of the neutralization efficiency with the discharge intensity for the triode system. Therefore, an adjustment of the voltage amplitude is necessary in order to optimize its efficiency for the dual system. References 30, figures 7.

Polymers are frequently used as contact materials in industrial applications that exploit the triboelectric effect. The aim of this paper is to investigate, with the help of 3D printing manufacturing technologies, how tribocharging of polymers is affected by their surface morphology. The experiments were performed using a linear tribometer to create the electric charge through friction between two samples. To highlight the correlation of surface texture with the level of triboelectric charge, the Polylactic acid (PLA) samples were manufactured with three different surface geometries, referred to as “lines”, “zigzag” and “waves”. The first series of experiments consisted in charging the surface of the PLA samples with the tribocharging bench for 20 and 40 cycles, with an Aluminum bar. The cartography of the electric potential at the surface was then performed, using a 2D mapping system to entrain the capacitive probe of an electrostatic voltmeter. In the second series of experiments, after tribocharging, the PLA samples were placed under the probe of the electrostatic voltmeter for 10 min to determine the surface potential decay (SPD) curve. The range of surface potential values was smaller for the “lines”-textured samples than for the other two, pointing out that the geometry of the texture significantly influences the outcome of the triboelectric charging. When comparing the SPD curves of the three textures for 20 cycles, it can be noted that ones for PLA with “lines” and “zigzags” are almost the same, but different from those of “waves”-type samples.

Waste electrical and electronic equipment (WEEE) contain heavy metals (mercury, chromium, cadmium, lead, etc.) and halogen materials (bromine, chlorine, etc.), which are toxic for humans and harmful to the environment. WEEE are subjected to shredding and grinding operations, followed by magnetic, eddy-current, optical, and float-sink separation. Electrostatic separation could be added to existing WEEE processing systems to distinctly recover the constituents of such granular waste. The aim was to find viable technical solutions to the problem of electrostatic separation of mixtures of conductive and insulating materials in the recycling industry. The feasibility tests were carried out on representative samples of 200 g of finely-ground materials (size less than 5 mm) originating from the processing of WEEE, using a roll-type electrostatic separator to distinctly recover the conductive and insulating materials. The parameters relevant to the efficiency of the separation were the distance between the electrodes, the level of the high-voltage applied to them, the rotation speed of the drum, and the size of the particles. The quality of the separation was evaluated by measuring the recovery and the purity of the conductive fraction, which in some cases could attain almost 80% and more than 88%, respectively.

Waste minimization is a major way to achieve sustainable development. Electrostatic separation is already used in the recycling industry for processing certain mixtures of shredded plastics originating from waste electric and electronic equipment. Standard tribo-electrostatic separators use electric forces to deflect the trajectories of triboelectrically charged particles in the electric field generated between two vertical plate electrodes connected to high voltage supplies of opposite polarities. However, the efficiency of this device is often limited by the impacts between the particles and the electrodes, which diminish the recovery and the purity of the end product. An innovative electrostatic separator was specifically designed to mitigate this risk. The innovation lies in using two rotating co-axial vertical cylindrical electrodes and assisting the movement of the particles with downward-oriented air flow to reduce their impact on the electrodes and improve the quality of the recovered products. The aim of this study was to optimize the operation of the patented electrostatic separator by using experimental design methodology to obtain quadratic polynomial models of the recovery and the purity of the products as functions of the high voltage applied to the electrode system and of the air flow through the device. The experiments were conducted with a granular mixture composed of 88% polypropylene (PP) and 12% high-impact polystyrene (HIPS) particles, extracted from the recycling process of waste electrical and electronic equipment, and triboelectrically charged in a fluidized bed device. A voltage of 50 kV combined with an air flow rate of 1700 m³/min maximized the recovery and the purity of PP and HIPS products collected at the outlet of the separator. These results open promising prospects for expanding the use of tribo-electrostatic separation for efficient recycling of granular waste plastics.

Tribo-electrostatic separation of granular plastic wastes is a complex phenomenon depending on several parameters, including the relative humidity of ambient air. The work presented in this paper aimed at studying the effect of storage environmental conditions on triboelectric charging and electrostatic separation. The experiments were conducted on four types of mm-size granular polymers, ABS, PS, PE, and PP, placed in a climatic chamber for 48 h to 96 h under controlled humidity. The thermal machine of the climatic chamber allowed humidity to be regulated at 82% ± 4% for a temperature of 19°C ± 1°C. The granules stored under these conditions will be designated as “humid.” Their water content was measured with a thermal balance and compared with “normal” granules, kept at ambient humidity varying between 46% and 52%, at 18°C ± 1°C. The water content of each of the four polymer granules increased by at least 10% after 48 h of exposure to the high relative humidity ambient conditions. The thin water layer formed at the particle's surface can explain their modified triboelectric behavior. The recovery and purity of the products collected after the tribo-electrostatic separation of the ABS/PS and PE/PP mixtures were poorer for the “humid” granules due to the less efficient tribocharging in the fluidized bed device.