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.     

Premises for Statistic Control of a Tribocharging Process for Granular Materials

This article aims to identify the appropriate sampling duration for a tribocharging process on a vibratory feeder device in order to compute the capability indexes and set up a statistical control procedure. The outcome of the process is evaluated as the ratio between the charge and the mass of the granules that exit the tribocharging device during a given laps of time. A virtual instrument developed in LabWiew was used in conjunction with a Faraday cage connected to an electrometer and with an electronic scale, to simultaneously measure the charge and the mass of tribocharged granular plastics, for fixed sampling durations.     

Milling and Classification of Printed Circuit Boards for Material Recycling

Recycling of waste electric and electronic equipment (WEEE) is an emerging issue due to its hazardous nature. It is important to identify an appropriate environment-friendly process to recover the valuables and for safe disposal. The present work deals with the two-stage crushing process followed by a circulating air classifier for the separation of metals and nonmetals from the printed circuit boards (PCB). The two-stage crushing process is deployed to liberate the valuables for an appropriate progeny size distribution. The metal content decreases as the particle size decreases below 0.5 mm. However, it increases metal content above 500 µm up to 1,800 µm. It is concluded that the metals primarily enriched in the size range of ?1.8 + 0.5 mm. The amount of metals and plastics present in each fraction is estimated. Among the classifier parameters, air flow velocity played a dominant role in metal enrichment. The material feed rate and rotating guide vane angle have no a significant effect on the enrichment of metals and nonmetals. The air flow velocity found was to be one of the crucial parameters for enrichment of metals. The superficial air flow velocity is optimized for efficient separation of metals and nonmetals of PCBs.     

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.     

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.     

Discrete element numerical simulation of fly ash triboelectrostatic separation in a nonlinear electric field

Fly ash is solid waste produced by thermal power generation, and its carbon content is a key factor affecting its recycling. Due to the large difference in fly ash quality and insufficient tribocharging, the parallel plate electric field with constant electric field strength cannot meet the practical needs of efficient decarbonization of fly ash particles with wide charge range or small charge to mass ratio (CMR). Therefore, a nonlinear electric field structure is proposed. The separation process of fly ash particles in the nonlinear electric field is explored through the establishment of geometric model and the application of CFD-DEM coupled calculation method, and the main influencing factors of fly ash electrostatic dry separation are studied. The results show that the nonlinear electric field structure is feasible to achieve high efficiency decarbonization of fly ash. With the increase of air flow velocity, the loss on ignition of positive electrode first increases and then decreases. The loss on ignition (LOI) of positive electrode products is directly proportional to the voltage and the CMR of the input, but inversely proportional to the feed quantity. Air flow velocity of 20 m/s, voltage of 30 kV, charge-mass ratio of 1.1–1.2 nC/g and feed quantity of 5000/s are suitable conditions for efficient decarbonization of fly ash. Compared with parallel plates, hyperbolic nonlinear electric field has higher decarbonization efficiency and lower energy consumption in experiment.     

Micronized plastic waste recycling using two-disc tribo-electrostatic separation process

Triboelectrostatic separation of millimeter-size granular mixtures is nowadays widely used in the recycling industry. However, the separation of micronized particles of an average granulometric size of 100?µm is still inefficient. This paper is aimed to carry out an experimental investigation of a triboelectrostatic separation process based on a fluidized bed tribocharging system produced between a pair of rotating aluminum disks supplied by two high-voltage DC supplies of opposite polarities. The granular mixture used in this work is composed of micronized white pure virgin PolyVinyl Chloride particles (WPVC) and gray PolyVinyl Chloride particles that contain a small percentage of carbon (GPVC) of average size 100?μm. Moreover, a homemade method was developed to estimate the purity of the separated products. It was deduced that the separation outcome, in terms of recovery and purity, is efficient and depends on several factors: the high-voltage level, the rotating speed of the disks, the fluidization rate, the total mass of the fluidized bed and the composition ratio of the granular mixture.     

Tribo-electrostatic separation of a quaternary granular mixture of plastics

Many tribo-electrostatic separation studies of binary mixtures of millimeter-sized plastic particles have been performed. The objective of this work is an experimental investigation for separating a quaternary mixture comprising four different plastic types issued from waste electrical and electronic equipment. The feasibility of the separation of such quaternary mixtures by the sliding mode tribocharging with a metal wall was demonstrated. The separation of a mixture comprising PA, PC, high impact polystyrene, and polyvinyl chloride granules is better in terms of both purity and recovery when charging the particles by sliding contact with the metal wall, then in the case of a fluidized bed device.     

A new two-roll electrostatic separator for recycling of metals and nonmetals from waste printed circuit board

The electrostatic separation is an effective method for recycling waste electrical and electronic equipment (WEEE). The efficiency of electrostatic separation processes depends on the ability of the separator. As a classical one, the roll-type corona-electrostatic separator has some advantages in recycling metals and plastics from waste printed circuit board (PCB). However, its industry application still faces some problems, such as: the further disposal of the middling products of the separation process; the balance of the production capacity and the good separation efficiency; the separation of the fine granular mixture and the stability of the separation process. A new "two-roll-type corona-electrostatic separator" was built to overcome the limitation of the classical one. The experimental data were discussed and the results showed that the outcome of the separation process was improved by using the new separator. Compared with the classical machine, the mass of conductive products increases 8.9% (groups 2 and 3) and10.2% (group 4) while the mass of the middling products decreases 45% (groups 2 and 3) and 31.7% (group 4), respectively. The production capacity of the new machine increases, and the stability of the separation process is enhanced.     

Optimization of key factors of the electrostatic separation for crushed PCB wastes using roll-type separator

For the electrostatic separation process, the separator is most crucial. As a classical one, the roll-type corona-electrostatic separator has some advantages in recycle of waste electrical and electronic equipment (WEEE). Some researches have been done in this field and shown that there was a complex correlation between its configuration and the efficiency of the separation. In this paper, a fractional factorial design (2v1-5) was built and 32 tests were performed on a roll-type corona-electrostatic separator. The sample of granular mixture got from crushed PCB wastes (size 0.3-0.45 mm, containing 25% metal and 75% nonmetal). The experimental data were discussed and used to analyze the factors' main effect, interaction and optimization of the process. Three liner-interaction mathematical models were derived to describe the mass of middling fraction (M), conductor fraction (C) and Nonconductor fraction (NC), respectively. The results show that the efficiency of the PCB waste electrostatic separation process has a significant correlation with not only factors' main effects, but also the interaction between them.     

Decrease of Cl contents in waste plastics using a gas–solid fluidized bed separator

In order to decrease Cl content in waste plastics, dry density float-sink separation of Cl-contained and Cl-free plastics was explored using a semi-continuous rotating-type gas–solid fluidized bed separator with silica sand. The separator has two distinctive features: (1) the plastics can be fed at a middle height of the sand bed, and (2) when the plastics are recovered with the sand from a container after the float-sink, the recovery height of the sand bed can be changed to designate the plastics as floaters or sinkers. The waste plastics of Cl content = 5.4 wt% were used in this study. The separation was investigated by changing the experimental conditions. As a result, the float-sink of the plastics was affected by the air velocity for fluidization, the float-sink time and the feed amount of plastics. The possible causes of the effects were discussed by focusing on the apparent density of fluidized bed, the fluidization intensity, the size segregation of fluidized particle, the shape of the plastics, and the interactions between the plastics during the float-sink. When the recovery height was changed at the adjusted conditions, the Cl content in the floaters was successfully decreased to be 0.4–0.85 wt%, at which the recovery of the Cl-free plastics was 40–60%.     

Testing steady and transient velocity scalings in a silo

Gravity-driven discharge experiments were performed in a perspex 3D flat bottomed silo which was filled with a granular material, and had a variable discharge orifice size. The granular material used was amaranth seed with an average diameter of 1?mm. Particle Image Velocimetry (PIV) analysis was performed on a high-speed video recording of the discharge, and used to quantify the velocity field within the silo both at steady state and during the development of flow. We verified not only that the steady-state velocity of the granules in the silo scales with the flow rate, but, additionally, the transition to a steady-state regime is also rate-controlled by the volumetric discharge. We present evidence that, away from the discharge orifice, the flow behaves identically, regardless of the orifice diameter, in a scaled time. We discuss these results with reference to the physics and mathematical modelling of granular flows.     

振动裁切的机理与应用研究

从运动学方面对比分析了普通裁切与振动裁切过程中裁刀与纸叠间的相对运动关系。通过分析知,实现振动裁切效果的前提条件是裁切速度小于裁刀的最大振动速度,二者的相对关系决定了一个振动裁切周期内的裁切时间。裁切速度越小,振动频率越高,每一振动裁切周期内的裁切时间越短。振动裁切过程中产生的脉冲裁切力与纸张剪应力、裁刀振动周期、裁切时间以及裁切厚度有关。振动裁切优于普通裁切效果的原因是,脉冲裁切力的细微分割效果减小了一次性裁切面积,从而降低了裁切抗力,提高了裁切质量。     

A note on the velocity of granular flow down a bumpy inclined plane

The velocity distribution of granular flow down a bumpy inclined plane is theoretically studied. The characteristic length scale of local transient cluster plays an important role in determining the flow rheology. After discussing the factors influencing the cluster size, we reproduce all observed velocity distributions successfully.This research was supported by the National Key Basic Research and Development Foundation of the Ministry of Science and Technology of China No. G2000048702.     

Use of plate bending waves for elastic property determination of polymers

A novel method is described for determining elastic properties of both reinforced and isotropic plastics through measurement of the velocity of plate bending (Lamb) waves of frequency around 100 kHz. The method is applicable to test pieces or fabricated articles in the form of a plate or shell of uniform thickness. Lamb waves may be launched and received at a single surface and travel with a velocity which is sensitive to in-plane tensile properties. There is therefore scope for using the method non-destructively for measuring the tensile stiffness in the direction of reinforcement in fibre reinforced plastics. The capabilities and limitations of the technique are demonstrated through measurements made on unreinforced plastics, an aligned carbon fibre reinforced material and two carbon fibre reinforced cross-ply laminates.     

PVC removal from mixed plastics by triboelectrostatic separation

Ever increasing oil price and the constant growth in generation of waste plastics stimulate a research on material separation for recycling of waste plastics. At present, most waste plastics cause serious environmental problems due to the disposal by reclamation and incineration. Particularly, polyvinyl chloride (PVC) materials among waste plastics generates hazardous HCl gas, dioxins containing Cl, and so on, which lead to air pollution and shorten the life of incinerator, and it makes difficultly recycling of other plastics. Therefore, we designed a bench scale triboelectrostatic separator for PVC removal from mixed plastics (polyvinyl chloride/polyethylene terephthalate), and then carried out material separation tests. In triboelectrostatic separation, PVC and PET particles are charged negatively and positively, respectively, due to the difference of the work function of plastics in tribo charger of the fluidized-bed, and are separated by means of splitter through an opposite electric field. In this study, the charge efficiency of PVC and PET was strongly dependent on the tribo charger material (polypropylene), relative humidity (below 30%), air velocity (over 10 m/s), and mixture ratio (PET:PVC=1:1). At the optimum conditions (electrode potential of 20 kV and splitter position of -2 cm), PVC rejection and PET recovery in PET products were 99.60 and 98.10%, respectively, and the reproducibility of optimal test was very good (+/-1%). In addition, as a change of splitter position, we developed the technique to recover high purity PET (over 99.99%) although PET recovery decreases by degrees.     

Interfacial Processes and Tribocharging: Effect of Plasma Surface Modification and Physisorption

The tribocharging characteristics of polymer powders have a significant impact on several industrial processes such as electrostatic powder coating and polymer powder processing. The nature of chemical species on the surface of powder influences the triboelectric charging characteristics. An atmospheric-pressure helium plasma was used to modify tribocharging properties of polystyrene microspheres and acrylic powders. Plasma surface modification reduced the magnitude of tribocharging for the test polymer powders. When tribocharged against stainless steel, the net charge-to-mass ratio (Q/M) for both polystyrene microspheres and acrylic powders was less than that for the control samples. Plasma surface modification also reduced the tribocharging of mannitol, a pharmaceutical powder, against stainless steel. Further, Mars simulant dust (JSC Mars 1) was used as a test sample to study the effect of carbon dioxide physisorption on tribocharging. The Q/M of the sample with physisorbed CO₂ was less than that of the control in all size ranges.     

Parameter study and optimization on filtration and resistance characteristics of granular bed filter

In this paper, experimental studies were carried out to investigate the filtration and resistance characteristics of a fixed granular bed filter (GBF) during the process of dust particles removal for hot industrial flue gas. The effects of geometric and operating parameters (including filtration superficial velocity, granules size, bed height, dust particles concentration and flue gas temperature) were examined according to the orthogonal array design method. The experimental results show that the filtration superficial velocity has the most significant effect on filtration efficiency, which is followed by granules size, bed height, flue gas temperature and dust particles concentration respectively; for pressure drop, the order of the factors are granules size, filtration superficial velocity, bed height, flue gas temperature and dust particles concentration respectively. Based on the experimental results, correlations of filtration efficiency and pressure drop with these parameters were established, which have good prediction accuracy with most of the deviations within 15%. The optimized parameters combinations of the maximum filtration efficiency and the minimum pressure drop of the GBF were gained which agree well with the experimental results. The correlations are significant for design and evaluation of GBF in practical applications.     

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.     

Automatic measurement of velocity and length of moving plate usingself-mixing laser diode

We propose and demonstrate to automatically measure both a velocity and a length of a moving plate, employing a novel laser speckle velocimeter using a self-mixing laser diode (SM-LD). We derive two empirical equations including velocity of length of a moving plate made of white paper or plastic. After determining a pair of constants fs(0) and β in advance calibration, we can automatically obtain velocity or length from the measured mean speckle signal frequency and/or the number of speckle pulses counted during the measuring time. The investigated range of velocity and length is 100 mm/s to 950 mm/s, and 10 mm to 100 mm, respectively. The measurement accuracy of the velocity and the length is approximately 1% and a few percent, respectively