Fluidized Bed Device for Plastic Granules Triboelectrification

The study of the triboelectrification process is the key factor to the successful industrial application of electrostatic separation in the field of plastics recycling. Therefore, the aim of the present work is to evaluate an original device for the laboratory study of this process. The original design of the fluidized bed type triboelectrification device has the following distinctive features: (i) transparent walls to observe the fluidization process and the particles moving in the triboelectrification module; (ii) air distributors with different mesh apertures correlated with granule size; (iii) quick replacement of the triboelectrification Plexiglas chamber with other metallic (aluminium, copper) or nonmetallic chambers (polyvinyl chloride—PVC, polyethylene—PE, and polyethylene terephthalate—PET); (iv) possibility of independent operation, for granule charge measurement or integrated operation, for the electrostatic separation experiments using a free-fall electrostatic separator. The results of charge measurement experiments pointed out the different triboelectric behaviour of two types of granules (polystyrene and low-density polyethylene) in contact with the walls of the device. The electrostatic separation experiments carried out on a binary mixture of mm-size polystyrene/polyethylene granules confirmed the effectiveness of the triboelectrification device.     

Influence of Material Moisture on the Tribocharging Process of Plastic Granules

The control of the triboelectrification factors is the key for a successful application of electrostatic separation to the recycling of mixed plastics waste. This article focuses on the influence of material moisture on the tribocharging process of three granular materials: polyamide (PA), polyvinyl chloride (PVC), and polyethylene terephthalate (PET).

Different values of moisture content were obtained by immersing the material in water or drying it in a laboratory oven. Afterward, the granular material was tribocharged on the tray of a vibratory feeder and the accumulated charge was measured by an electrometer.

The maximum charge/mass ratio was obtained for the three plastic materials at different values of moisture content: 0.27% for PA, 0.12% for PET 0.1% for PVC. Series of 5 consecutive tribocharging experiments performed on the same sample, show that the first contributes with the greater amount of granules charge while the other four only slightly increment this value. The graphical representation of the evolution of the charge/mass ratio versus number of tribocharging experiment conducts to the conclusion that: i) the charge of the granules tends to saturate; ii) it is useless to excessively increase the duration of the tribocharging process.     

Triboelectrostatic separation of granular plastics mixtures from waste electric and electronic equipment

The efficiency of the electrostatic separation of insulating granular mixtures depends on the electric charge carried by the particles. The first objective of this work was to characterize the tribocharging properties of various plastic materials in the composition of waste electric and electronic equipment: polyvinyl chloride (PVC), “high-impact” polystyrene (HIPS), acrylonitrile butadiene styrene (ABS), “high-density” polyethylene (HDPE), polycarbonate (PC), polyamide (PA). Thus, in a first series of experiments, 15 samples composed of binary mixtures of above plastics were initially charged in a fluidized bed device, then separated using a free-fall electrostatic separator. The six plastics could be ordered in a specific triboelectric series: PVC, HIPS, ABS, PEHD, PC, PA. The second objective was to evaluate the efficiency of three tribocharging devices: a static charger, a fluidized bed, and a fan-type device. The triboelectrostatic separation experiments performed with four different binary mixtures (PVC/PA, PC/PA, HIPS/PA, and HIPS/ABS) showed that the fluidized bed is the most effective.     

Comparative Experimental Study of Triboelectric Charging of Two Size Classes of Granular Plastics

The free fall triboelectrostatic separation is widely used for the selective sorting of plastics from granular industrial waste. The electric charge per mass ratio of the granules is a critical parameter influencing the purity of recycled plastics and the efficiency of the electrostatic separation process. The aim of the present study is to validate an experimental procedure for the optimization of the vibratory-type tribocharging device for granular plastics. This tribocharger is composed of a metal plate covered with a thin PET (polyethylene terephthalate) layer. The amplitude of the vibratory motion of the plate can be adjusted using a potentiometer. The length of the vibratory feeder, the flow rate, and the velocity, at which the granules move on its surface, are the variables that can be controlled in order to optimize the tribocharging process. As particle size is an important physical factor influencing the charging process, the study was focused on two size classes (1 to 2 mm and 2 to 5 mm) of ABS (acrylonitrile butadiene styrene) granules originating from the recycling process of waste electrical and electronic equipment. The optimum operating conditions, obtained by using the response surface modeling methodology, differ between the two size classes of granules. Both the length of the vibratory tray and the velocity of the granules on its surface have a significant effect on the outcome of the tribocharging process.     

Experimental and Numerical Modeling of a New Tribo-Electrostatic Separation Process for Granular Plastics Mixtures

The paper aims at optimizing the operation of a free-fall electrostatic separator equipped with a novel tribo-charging device for the processing of granular plastic mixtures. Experiments were performed with a mixture of polycarbonate and polyamide granules (cylinders of 1.5 mm in diameter and 3 mm in length). The samples of granules to be charged were introduced in the gap between the two co-axial cylinders of the tribo-charging device, and were put into turbulent motion for 30 s by two oppositely oriented air jets (pressure: 2 bar). Then, the charged particles were left to fall freely between the vertical plate electrodes of an electrostatic separator. A composite experimental design enabled the modeling and optimization of the tribo-electrostatic separation process as a function of the following control factors: applied high voltage, inter-electrode distance, and the inclination angle of the feeding device. Under optimal conditions, the purity obtained is close to 99%. A numerical model of particle trajectories confirms the conclusions of the experimental study. The simulations point out the effect of each control variable on the outcome of the separation process.     

Granule attrition by coupled particle impact and shearing

A novel device has been developed for continuous shearing and repeated impact of granules in order to simulate granule attrition and dust formation under realistic plant conditions of mechanical stresses, shear strains and strain rates. The device subjects the granules to multiple impacts at a range of velocities prevailing in typical process plants, and to shear deformations using two rollers with an adjustable gap to simulate the level of shear stresses and strains experienced during bulk motion, e.g. discharge from silos onto conveyor belts, etc. In this paper, the device operation and tests carried out to determine the settings required for attaining a desired impact velocity and shear strain rate are described. Subsequently, the extent of breakage of the granules is determined for the specified settings and the results are compared with data obtained by more established methods, e.g. annular shear cell and single particle impact tests.     

On the composite characteristics of small binary granules formed in a continuous rotating conical vessel with grinding media

In order to produce small composite granules having binary components of CaCO₃ and SiC powders, experiments were carried out by the simultaneous operation of granulation, grinding, and separation in a single continuous rotating conical vessel with grinding media. To characterize these composite granules, SiC agglomerates contained in a granule were separated by chemical methods, and the size distributions of granules and SiC agglomerates were measured by an image analyzer through a microscope. The structure of a composite granule was characterized by comparing the size of a composite granule with that of SiC agglomerates in the granule. It was found that composite structures of the granules could be classified into six kinds of structural models, and that the composite structure depended on the size of the granules and the volume fraction of the SiC component in the granule.     

Sunil de Silva-A Scientist and a Visionary

The objective of this work was to analyze the behavior of good conducting granules in a newly designed plate-type electrostatic separator. Commercial software was employed for the numerical analysis of the electric field generated by an original electrode configuration. This enabled the evaluation of the electric charge and forces on conducting particles. The trajectory of a single particle was calculated over a set of 1300 points. The simulated results show that the trajectories depend on the applied voltage, the radius, and the mass density of the granules. They were found in good agreement with the experiments carried out on a laboratory electrostatic separator.     

PARTICLE TRIBOELECTRIFICATION AND ITS USE IN THE ELECTROSTATIC SEPARATION PROCESS

Triboelectrification of solids is known since ancient times. Empirical experiments allow materials to be arranged in so-called “triboelectric series”. Reproducibility, however, is poor in most cases, and the basic mechanisms of the charge transfer are still a subject of speculation. The difference between tribocharging of conductors and insulators is discussed in this paper, and the basic equations for these effects are presented. Triboelectrification of solids has two aspects, one beneficial and one hazardous. The hazardous aspect manifests itself in electrostatic discharges (ESD), which can damage computer equipment, as well as causing fires and explosions during transport and storage of solids and liquids. The beneficial aspect triboelectrification is widely used in many industrial applications, such as the Xerox copying technique, powder coating, pre-charging of fabric filters, electrostatically enhanced cyclone separators, oil mist filtration, aerosol particle collection, and finally, the electrostatic separation of materials. The second part of this paper focuses on industrial electrostatic separation techniques using tribocharging of particles; the separation of potassium minerals, coal beneficiation, and polymer separation for waste processing are presented.     

金属材料静电起电理论分析及实验验证

目的减小包装材料在运输过程中产生的静电对周围或包装内电子产品的影响。方法对金属包装材料与空间粒子的摩擦起电机理进行理论分析,以固体接触分离起电为基础推导出摩擦起电电位的计算公式。为验证理论分析的正确性,以铝系金属材料为例进行摩擦起电实验。结果通过理论推导得到材料静电电位随时间以负指数规律增加,并逐步趋近饱和值,该饱和值与材料的性质有关,并与摩擦速度和接触面积成正比。在材料摩擦起电电位测试实验中,所得的静电电位变化曲线与理论分析相同,且连续测试后发现材料起电电位逐次降低;根据不同材料和不同摩擦速度时静电电位饱和值的变化情况,证实了金属材料摩擦起电电位与材料摩擦系数和接触面积的对应关系。结论通过理论分析和材料摩擦起电实验,得到了摩擦起电电位的增长规律,对减小静电危害起到指导意义。     

The effect of foam polystyrene granules on cement composite properties

Crumbled recycled foam polystyrene waste as well as spherical large and fine blown polystyrene waste is used to produce the filler for a light thermo-insulating composite, the matrix of which is light foam cement. For better cohesion, fillers are hydrophilizated with foam cement surfactant solution.Polystyrene granules and foam cement concrete interaction schemes are discussed. The investigation of foam cement concrete and polystyrene granule contact zone showed that the contact of these two materials is very close, without any fractures or microcracks. Adherence of the two components depends on the size and shape of granules used.When a polystyrene granule is ripped out of foam cement concrete, the emerged “hole” closely repeats the structure of the granule and there is some polystyrene residue left in it. This proves the fact that foam cement concrete contact zone is stronger than the polystyrene granule material. When fine polystyrene granules are used, it disintegrates along the contact zone. Such composite has the lowest adhesion strength, however, it is stronger in comparison with a composite, made with different foam polystyrene granules, provided by better macrostructure. Strength and thermal conductivity of the composite depend on its density, the filler, its sort and amount used, and is defined by regression equations.     

Electrostatics of granules and granular flows: A review

In past decades, the electrostatics of granules and granular flows has obtained more and more attention due to many industrial problems and the associated development of new technologies. Granule-wall collision causes electrification, where charge transfer can be characterized by work function, electron transfer, ion transfer, and material transfer. Electrification is affected by many factors and increases with granule processing, and the charge amount can reach a saturated state where electrification no longer increases, which has been confirmed by single granule and granule conveying systems. In addition, the presence of electrostatic charges has profound influences in relevant areas, including chemistry, chemical engineering, energy, pharmaceuticals, and so on. The measurement technology of electrostatics used in granule conveying systems has been improved with the continuous progress of industry. Furthermore, electrostatics of granules and granular flows will be developed into a more accurate area together with other subjects as an interdisciplinary problem to be concerned. In addition, in the pneumatic conveying system, granule-wall and granule-granule collision or friction can cause material transfer due to material breakage. The working mechanism of the material transfer due to collision or friction has never been fully understood. Such problems will be solved gradually in the future.     

基于静态箱法的PM2.5切割器捕集效率评价及拟合曲线优化研究

研究了基于静态箱法的PM2.5切割器捕集效率评价系统。通过设计并搭建评价系统,使用标准规定的8种粒径的聚苯乙烯小球进行雾化发尘,在静态箱中产生符合测量要求的气溶胶环境,使用空气动力学粒径谱仪对切割前后的颗粒物进行计数测量、最终获取切割器的捕集效率曲线,并选择最佳插值拟合算法进行数据拟合。通过对国产以及进口的切割器分别进行评价实验,获取了切割器的捕集效率。该研究结果有助于规范切割器的生产与使用,为后续开展其他类型切割器的评价提供参考。     

Electrostatic Separation of Peeling and Gluten from Finely Ground Wheat Grains

Sieving and air classification are not efficient enough for the extraction of the high-nutritional-value constituents of wheat. The aim of this article is to validate a simple electrostatic separation method of peeling and gluten, which are two such nutriments contained in finely ground wheat grains. The electrostatic separator is composed of metallic grounded belt conveyer and a rotating roll electrode connected to a high voltage supply. The electrostatic behavior of peeling and gluten powders was characterized using surface potential decay and direct charge measurements. These first set of experiments pointed out the conductive behavior of these powders: in contact with a grounded electrode, they lose their charge in less than 10 s. In a second set of experiments, mixtures of 50% peeling and 50% gluten powders were processed by electrostatic separation. Experimental design methodology was used to model the outcome of the separation process as function of two control variables: the high-voltage applied to the roll electrode and the speed of the belt conveyor electrode. In this way, it was possible to determine the optimal operational conditions for the recovery of high-purity peeling and gluten fractions.     

Miniaturized flow fractionation device assisted by a pulsed electric field for nanoparticle separation

Electric field flow fractionation (EFFF) is a powerful separation technique based on an electrical field perpendicular to a pressure-driven flow. Previous studies of microelectric field flow fractionation (micro-EFFF) indicate that separation performance was limited due to a weak effective electric field caused by polarization layers on the electrode surfaces. In this work, we report on a micro-EFFF device that uses a pulsed voltage scheme to overcome these limitations. The device was fabricated in indium tin oxide (ITO)-coated glass with ITO as electrodes. The effective electric field for pulsed voltage operation was found to be 50-fold stronger when compared with constant voltage operation. A strong influence of pulsed voltage frequency on nanoparticle retention times was observed. Using pulsed voltage, improved separation of polystyrene particles of different surface charge and particle size is demonstrated. Pulsed voltage also offers more parameters compared to the constant voltage mode, e.g., pulse frequency, duty cycle, and waveform to optimize the retention behavior of analytes.     

Book Review

Triboelectrification affects particle adhesion and agglomeration and hence the formulation, manufacture, and use of dry powder inhaler (DPI) devices. Electrostatic charge measurement of two component mixes of spray-dried or crystalline lactose fine particles (<10 μm) 0, 5, 10, 15, 20, and 30% w/w with spray-dried or crystalline lactose 63–90 μm, respectively, has been undertaken using a system incorporating pneumatic transport of the mixed powders to a stainless steel cyclone charging device. The magnitude of charge on the mixes was shown to decrease with increased fine particle content, and there was no significant difference in charge for each concentration between spray-dried and crystalline lactose. Both the variation of charge and powder adhesion to the cyclone surface increased with increase in fine particle content. The proportion of fine particles in carrier systems in DPIs may thus have an important role where triboelectrification is involved.     

Modification of electrostatic charge on inhaled carrier lactose particles by addition of fine particles

Triboelectrification affects particle adhesion and agglomeration and hence the formulation, manufacture, and use of dry powder inhaler (DPI) devices. Electrostatic charge measurement of two component mixes of spray-dried or crystalline lactose fine particles (< 10 microns) 0, 5, 10, 15, 20, and 30% w/w with spray-dried or crystalline lactose 63-90 microns, respectively, has been undertaken using a system incorporating pneumatic transport of the mixed powders to a stainless steel cyclone charging device. The magnitude of charge on the mixes was shown to decrease with increased fine particle content, and there was no significant difference in charge for each concentration between spray-dried and crystalline lactose. Both the variation of charge and powder adhesion to the cyclone surface increased with increase in fine particle content. The proportion of fine particles in carrier systems in DPIs may thus have an important role where triboelectrification is involved.     

Investigation of granular surface roughness effect on electrostatic charge generation

Electrostatic charge generation is a multivariable and complex issue whose working mechanism has never been fully understood. The objective of this paper is to investigate the effect of granule surface roughness on electrostatic charge generation. Two kinds of granule material, Polyvinyl chloride (PVC) and polypropylene (PP) were used with the granule size of 4 mm diameter, 2 mm height and the shape was cylinder or semi-cylinder. The working surfaces were grounded and roughness ranged from 0.140 to 8.600 μm. It was found that uneven surfaces tended to give rise to voids between two solids, where air stored in the voids was able to accelerate discharging. With the same roughness, PVC tended to generate more electrostatic charge than PP by one order of magnitude. For both materials, electrostatic charge generation first increased with surface roughness and then decreased. The maximum electrostatic charge generated was found to occur when the effects of interaction, contact area and voids discharging were at equilibrium. With the combined effect of humidity, surface roughness and contact area, highest electrostatics generation occurred near the mid-roughness tested in this work. Humidity had more effect on electrostatic charge generation as the granule working surface had lower roughness.     

Electron Transfer in Nanoscale Contact Electrification: Photon Excitation Effect

Contact electrification (CE) (or triboelectrification) is a well‐known phenomenon, and the identity of the charge carriers and their transfer mechanism have been discussed for decades. Recently, the species of transferred charges in the CE between a metal and a ceramic was revealed as electron transfer and its subsequent release is dominated by the thermionic emission process. Here, the release of CE‐induced electrostatic charges on a dielectric surface under photon excitation is studied by varying the light intensity and wavelength, but under no significant raise in temperature. The results suggest that there exists a threshold photon energy for releasing the triboelectric charges from the surface, which is 4.1 eV (light wavelength at 300 nm) for SiO₂ and 3.4 eV (light wavelength at 360 nm) for PVC; photons with energy smaller than this cannot effectively excite the surface electrostatic charges. This process is attributed to the photoelectron emission of the charges trapped in the surface states of the dielectric material. Further, a photoelectron emission model is proposed to describe light‐induced charge decay on a dielectric surface. The findings provide an additional strong evidence about the electron transfer process in the CE between metals and dielectrics as well as polymers.     

Effect of contact- and sliding-mode electrification on nanoscale charge transfer for energy harvesting

The process of charge transfer based on triboelectrification (TE) and contact electrification (CE) has been recently utilized as the basis for a new and promising energy harvesting technology, i.e., triboelectric nanogenerators, as well as selfpowered sensors and systems. The electrostatic charge transfer between two surfaces can occur in both the TE and the CE modes depending on the involvement of relative sliding friction. Does the sliding behavior in TE induce any fundamental difference in the charge transfer from the CE? Few studies are available on this comparison because of the challenges in ruling out the effect of the contact area using traditional macro-scale characterization methods. This paper provides the first study on the fundamental differences in CE and TE at the nanoscale based on scanning probe microscopic methods. A quantitative comparison of the two processes at equivalent contact time and force is provided, and the results suggest that the charge transfer from TE is much faster than that from CE, but the saturation value of the transferred charge density is the same. The measured frictional energy dissipation of ～11 eV when the tip scans over distance of 1 Å sheds light on a potential mechanism: The friction may facilitate the charge transfer process via electronic excitation. These results provide fundamental guidance for the selection of materials and device structures to enable the TE or the CE in different applications; the CE mode is favorable for frequent moderate contact such as vibration energy harvesting and the TE mode is favorable for instant movement such as harvesting of energy from human walking.

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