Triboelectrification of particulate flows on surfaces: Part I — Experiments

Triboelectrification is a ubiquitous feature of dry particulate processes. It generates hazards in many industrial systems, but is exploited in several other important applications, including electrophotographic toner charging and triboelectric separation. Unfortunately, the charging of continuous particulate flows during interaction with solid surfaces is poorly understood, and design of devices to maximise or minimize triboelectrification is usually qualitative or based on trial and error. This paper presents the results of an experimental case study of the relationship between the accumulated charge and the mechanical attributes of a particulate flow, including contact time, velocity, and the mode of contact. Silica particles of ∼ 1 mm diameter and a stainless steel slide tribocharger were used for this case study.     

废旧塑料摩擦荷电机理及摩擦电选技术研究进展

塑料由于材质轻、化学性质稳定、成本低、耐磨性及耐腐蚀性好等优点被广泛应用于工程建设、食品安全、交通运输以及医疗等领域,如果处理不当会对环境造成严重的污染,废塑料污染防治已成为全球关注的环境问题。目前处理废旧塑料的常用方法有风选法、浮选法、静电分离法和光选法等,摩擦电选作为一种新型干式分选方法越来越受到研究者们的重视,其具有工艺简单、污染小、投资少、成本低等优点。本工作针对废旧塑料的分选回收利用,详细介绍了摩擦电选的荷电机理、影响因素、荷电装置和分选设备的研究现状,指出了目前通过摩擦电选回收废旧塑料的技术问题,并对摩擦电选技术未来的发展趋势和应用进行了展望。     

Triboelectric charging of fungal spores during resuspension and rebound

The triboelectric charging of fungal spores was experimentally characterized during rebound and resuspension. A fungal spore source strength tester (FSSST) was used as a primary aerosol generator for spores of three fungal species and two powders (silicon carbide and silver). The critical velocity of rebound was determined using a variable nozzle area impactor (VNAI), and the charging state of particles after resuspension and rebound was measured using the FSSST, different impactor setups, electrometers, and optical particle counters. In the impactor setups and the FSSST, five different surface materials relevant for indoor environments were used (steel, glass, polystyrene, paper, and polytetrafluoroethylene). The critical velocity of rebound was determined to be 0.57 m/s for fungal spores, which is relatively low compared to silicon carbide and previous results for micron-sized aerosol particles. Based on the rebound impactor measurements, we were able to define the crucial parameters of charge transfer for different particle–surface material pairs. A contact charge parameter, which describes the triboelectric charging during rebound, was found to have a negative correlation with the charging state of the particles after the resuspension from an impactor. This connects the triboelectric charging during rebound and resuspension to each other. Based on the contact charge parameter values, quantified triboelectric series could be formed. The results of this work show that fungal spores can be charged both positively and negatively during rebound and resuspension depending on the fungal species and surface material.

Copyright © 2016 American Association for Aerosol Research     

Triboelectric Charging and Separation of Fine Powder Mixtures

For increasingly finer powders, the material-specific separation at high loadings is a challenging task, for instance in recycling processes. Here, a combination of triboelectric charging and electrostatic separation was investigated for powder mixtures of talcum and calcite. The dependencies of the triboelectric charge on the mass loading, the gas velocity, and the mixture ratio were investigated. While higher charge levels were achieved with increasing gas velocity, the mass loading had an opposite effect on the net charge. Although bipolar charge distributions were observed within pure materials and mixtures, electrical neutralization did not occur in the mixtures. Therefore, already in a non-optimized setup, a decent degree of material enrichment (of up to 53 %) was found on the separating electrodes.     

The Adhesion‐Enhanced Contact Electrification and Efficiency of Triboelectric Nanogenerators

In the present work, the contact electrification of polymers that differ in adhesion strength is studied. Electrical current is measured along with adhesion in macroscale contacting‐separation experiments. Additionally, local adhesion and roughness are studied with atomic force microscopy to get deeper insight into relations between surface properties and electrification. Measurements reveal that higher surface charge is formed on more adhesive surfaces, thus confirming covalent bond cleavage as a mechanism for contact electrification of polymers. Investigated materials possess enhanced contact electrification making them attractive candidates for the conversion of mechanical energy to electrical in triboelectric nanogenerator devices.     

Triboelectric charging of powders: A review

Particles are often electrostatically charged by frictional contact during powder-handling operations. This phenomenon is called ‘triboelectric charging’ or ‘contact electrification’. The charged particles cause problems such as particle deposition and adhesion. In addition, if particles are excessively charged, an electrostatic discharge may occur, which can pose a risk of fire and explosion hazards; thus, to mitigate the adverse effects, it is important to elucidate the underlying triboelectric charging mechanisms. The electrostatics is, on the other hand, very useful in a number of applications that have been developed using the principles. In this review, the basic concepts and theories of charge transfer between solid surfaces are summarized, and chemical factors depending on materials and environmental effects are described. To theoretically analyze the process of particle charging, relevant models are discussed. Using the models, particle charging by repeated impacts on a wall is formulated. To experimentally evaluate particle charging, measurement and characterization methods are outlined. Furthermore, important applications and computer simulations are described.     

A hybrid approach to computing electrostatic forces in fluidized beds of charged particles

In particulate flow devices particles acquire electric charge through triboelectric charging, and resulting electrostatic forces can alter hydrodynamics. To capture this effect, the electrostatic force acting on individual particles in the device should be computed accurately. Electrostatic force is calculated using a hybrid approach consisting of: (1) long‐range contributions from an Eulerian electric field solved using the Poisson equation (2) short‐range contributions calculated using a truncated pairwise sum and (3) a correction to avoid double counting. Euler‐Lagrange simulation of flows incorporating this hybrid approach reveals that bed height oscillations in small fluidized beds of particles with monopolar charge decreases with increasing charge level, which is related to lateral segregation of particles. A ring‐like layer of particles, reported in experimental studies, forms at modestly high charge levels. Beds with equal amounts of positively and negatively charged particles are fluidized in a manner similar to uncharged particles. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2282–2295, 2016     

Influence of the Impaction Angle on the Triboelectric Charging of Aerosol Nanoparticles

A low pressure impactor is used to measure triboelectric charging behavior of metallic nanoparticles. Ag nanoparticles, produced by spark discharge, were impacted onto Pt sputtered targets. The influence of the impaction angle and impaction velocity on the triboelectric charging was investigated. While for perpendicular impaction the charge transfer behavior of previous work was confirmed, the oblique impaction revealed new phenomena. Additional charge transfer was observable, which increases with obliqueness. The possibility of mass transfer between particle and target due to the high-energy collisions was also investigated. SEM characterization and Auger spectroscopy indicate mass transfer from the particle to the target surface.     

3D Conformal Surface Engineering of Continuous Fibers with Porous Microstructures for 1D Advanced Functional Materials

One-dimensional (1D) continuous advanced functional materials and devices with inherent flexibility for complex deformations facilitate a broad range of applications in wearable technology. This communication presents a new electrostatic self-assembly strategy for controllable assembly of nanomaterials to fabricate 1D continuous materials with customizable functions based on a kind of continuous fiber fully surface-engineered with 3D conformal porous microstructures (F@3CPMs) by a unique self-assembly approach of breath figure using water microdroplet arrays. Through gently rubbing the modified fibers with suitable triboelectric materials, either positively or negatively charged F@3CPMs can be rationally prepared with adjustable triboelectric charge intensity. Besides showing superiority in incorporating desired components, such kind of F@3CPMs are demonstrated to have general applicability and enhanced performance in controllable self-assembly of polymeric, metal, and carbon nanomaterials for customizable functionalizations. Moreover, taking advantages of continuous fibers that can deform largely, functional F@3CPMs are further applied for development of 1D flexible motion sensing devices by twisting directly, which can be either used as 1D freestanding devices for straightforward integration with conventional fabrics or woven as a fabric structure integrity for a kind of self-powered interactive textiles without additional battery as power resources to detect and monitor the body motions of human beings.     

提高粉末涂料磨擦带电性

比较了粉末涂装中摩擦和电晕两种方法的优势，简要介绍了摩擦带电的机理，并从摩擦带电剂、粉末表面改性和粉末粒径控制等方面探讨了提高粉末涂料摩擦带电性的方法。     

Performance-Enhanced and Washable Triboelectric Air Filter Based on Polyvinylidene Fluoride/UiO-66 Composite Nanofiber Membrane

Applying triboelectric nanogenerators (TENGs) in air filtration systems to generate electric charges through friction is a major advancement in air cleaning technology. The performance of triboelectric air filter strongly depends on the properties of triboelectric materials. In this work, a better triboelectric material, polyvinylidene fluoride (PVDF)/UiO-66 composite nanofiber membrane (P6-NFM), is designed and fabricated through electrospinning technology by doping UiO-66 into PVDF matrix. As the weight ratio of UiO-66 increases to 1%, PVDF/UiO-66 composite nanofiber-based TENG (P6-TENG) achieves the maximum current, voltage, and triboelectric charge of 4.29 µA, 52.8 V, and 22.02 nC, which are 6.5 times, 5.1 times, and 8.0 times as large as those of pure PVDF-based TENG (P-TENG). Therefore, the triboelectric air filter based on P6-NFM can be easily charged by slapping the fiber membrane and spun-bond fabric. After charging, the removal efficiency of P6-NFM is 92% for PM_0.5 and 98% for PM_2.5, which are 2.8 and 1.2 times those of the uncharged one. More importantly, the filtration efficiency of this air filter keeps stable after the membrane is washed four times. This method of loading UiO-66 on the triboelectric fiber material shows tremendous potential in self-charging and reusable air purification applications.     

不同充放汽条件下的锅炉-蓄热器系统动态特性

以锅炉-蓄热器系统为研究对象,采用集总参数法建立了相应的数学模型,并通过仿真与实验相结合的方式,对两种不同充放汽条件下的系统动态特性进行了研究分析。结果表明所建模型能够正确地反映出系统的动态特性,可为系统的设计优化与安全运行提供一定的参考。进一步分析可知,连续充放汽过程蓄热器从压力下限充到上限每次需要57 s,而间断充放汽过程则需要67 s,时间明显变长;同时对于某些需要周期性间断或瞬间大负荷用汽量的场合,在消除供汽锅炉负荷的较大波动、稳定供汽压力、提高锅炉效率等方面,连续充放汽的效果更为显著。     

Die elektrostatische Aufbereitung von Kali-Rohsalzen

Electrostatic processing of raw potash salt . The paper traces historical development of the electrostatic separation process. The process development by the German potash industry is definitely based on contact charging. Intensity and sign of charge imparted to the individual salt minerals can be precisely influenced by treatment of the salt with chemicals and, at the same time, also with air of a defined relative humidity. The development of efficient free-fall separators was of importance for the technical realization of the process. These separators contain electrodes about 2 m long which consist either of circulating, specially coated rubber bands or of a row of tubes positioned vertically next to each other which rotate continuously around their own axes. Charging of the salt minerals is very probably effected by electron transition. The paper describes tests carried out in high vacuum on KCl (100) and NaCl(100) surfaces for interpretation of the charging operation.     

Triboelectric charging of monodisperse particles in fluidized beds

To investigate the interplay between particle charging and hydrodynamics in fluidized beds, models for triboelectric charging and electrostatic forces were built into a computational fluid dynamics‐discrete element method model. Charge transfer was governed by the difference in effective work function between contacting materials as well as the electric field at the point of contact. Monodisperse particles were fluidized with an effective work function difference between the particles and the conducting walls. For smaller work function differences, hydrodynamics were not changed significantly as compared with an uncharged case. In these simulations, the average charge saturated at a value much lower than the value anticipated based on the work function difference, and a unimodal distribution of charges was observed. For larger work function differences, particles stuck to walls and bed height oscillations due to slugging were less pronounced. For these cases, a bimodal distribution of charges emerged due to effects from strong electric fields. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1872–1891, 2017     

Fractoemission from Failure of Metal-GIass Interfaces

Electron and photon emissions accompanying the failure of a metal/glass interface were studied. Experimental evidence is given in support of a previously presented model involving microdischarges accompanying fracture which result from charge separation occurring between the metal and glass surfaces. The role of contact charging at the metal/glass interface in establishing this charge separation is discussed.     

Electrostatic charging during the flow of grains from a cylinder

Electrostatic charging is ubiquitous in granular processing, leading to problems of safety, jamming and unwanted material segregation. To better understand the mechanics of granular charging, we focus here on flow through a metal cylinder, where we can isolate charging regions near the cylinder walls from noncharging regions further away. We confirm that monodisperse grains charge in proportion to the area of contact between grains and the cylinder walls, and so in large cylinders, most particles remain almost uncharged. Those particles that do charge reach a plateau charge density after filling the cylinder and flowing past the walls a distance of less than one and a half centimeters. For bidisperse granular blends, the net charge produced by the mixture is dominated by the component that comes into contact with the walls of the apparatus. This is found to be caused by segregation effects as well as the coating of the larger particles by the smaller ones. We make use of these results to predict the charge generated in mass flow hoppers, and we test these predictions. Finally we examine the effect of grounding the experimental apparatus, and we find that paradoxically, grounding does not prevent charge accumulation.     

A Method to Measure Static Charge on a Filter Used for Gravimetric Analysis

Static charge present on a filter contributes substantial error to low-level mass measurements. The measurement and sources of static charge are not well understood and this article presents a fundamental method of static charge measurement. As predicted by other researchers, triboelectric charging was found to generate significant static charge on a filter when using 2007 Diesel particulate matter (PM) measurement protocols. Measurements made using our method indicate that PM filters are rapidly and effectively neutralized by ²¹⁰ Polonuim ( ²¹⁰ Po) sources. The neutralization of charge occurred exponentially with characteristic time constants (the time it takes for the charge on the filter to decay by 63%) of 0.4 and 0.7 s, depending on the strength of the source. The experimental neutralization time constants were consistent with theory. The charge remaining on highly charged 47 mm Teflon and TX40 filters after 5 s of neutralization with year old ²¹⁰ Po sources biased the true filter mass by <1 μg when the filters were weighed with a 1 μg resolution Cahn microbalance.     

Triboelectrostatic separation of pvc materials from mixed plastics for waste plastic recycling

This study covers the triboelectrostatic separation of Polyvinylchloride (PVC) materials from mixed plastics such as polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), and polystyrene (PS). The PVC material generates hazardous hydrogen chloride gas resulting from the combustion in the incinerators. The laboratory scale triboelectrostatic separation system consists of a fluidized-bed tribocharger, a separation chamber, a collection chamber and a controller. Negative and positive surface charges can be imparted to the PVC and PET particles, respectively, due to the difference of triboelectric charging series between the particles in the fluidized-bed tribocharger. They can be separated by passing through an external electric field. A highly concentrated PVC (91.9%) can be recovered with a yield of about 96.1% from the mixture of PVC and PET materials in a single stage of processing. For the removal of PVC from the two-component mixed plastics such as PVC/PET, PVC/PP, PVC/PE or PVC/PS, separation results show the recovery of 96–99% with the pure extract content in excess of 90%. The triboelectrostatic separation system using the fluidized-bed tribocharger shows the potential to be an effective method for removing PVC from mixed plastics for waste plastic recycling.     

Improving the efficiency of coal triboelectric separation by chemical conditioning

A study of fine coal beneficiation in a laboratory triboelectric separation system was carried out to provide a fundamental understanding of improving the separation efficiency of thermal coal powder by chemical conditioning. Separation results of untreated and chemically treated fine coal were investigated. The experimental results demonstrated that the charge–mass ratios of clean coal and minerals increased greatly with ethanol serving as the modification chemical. The optimal deash efficiency of 85.46% was obtained when the coal was treated by 2.0 kg/t ethanol. However, excessive dosage of chemical agents had a bad effect on the separation results.     

Vibrated bed contact electrification and charge dissipation

Some characteristics of contact electrification and charge dissipation in a vibrated bed have been studied. The experimental variables of interest were the bulk particle resistivity, the amplitude of vibration and the system electrode geometry. Theoretical models have been proposed relating the measured variables of conduction current, and induced potential in the base plate, to the charging and dissipation processes. Air breakdown has been found to limit insulating particle charge levels, following a linear increase in bed charge with time after vibration. This behaviour has been predicted theoretically. The functional dependence of the induced potential and the conduction current on the vibrational amplitude predicted theoretically for conducting particles, is in agreement with observation.Charge production and dissipation in the vibrated bed simulate the electrification characteristics of many practical bulk particle handling situations. This allows conclusions of general relevance to be deduced from the present study.