Electrostatic separation contact electrification

Types The primaiy cause of electrostatic charging is contact electrification, which takes place when two different materials are brought into contact and separated. Other causes include induction charging, the formation of sprays, and impingement of charged mist on an ungrounded conductor. 

Charging by contact electrification is an active mechanism whenever dissimilar particles make and break contact with each other, or whenever they slide over a chute or an electrode. This charging mechanism is most frequently used to charge selectively and obtain an electrostatic separation of two species of dielectric materials as realized in a free fall electrostatic separator. 

General Principles Electrostatic separation (of particles), also commonly known as high-tension separation, is a method of separation based on the differential attraction or repulsion of charged particles under the influence of an electrical field. Applying an electrostatic charge to the particles is a necessary step before particle separation can be accomplished. Various techniques can be used for charging. These include contact electrification, conductive induction, and ion bombardment. 

Electrostatic-Separation Machines The first electrostatic machines to be used commercially employed the principle of contact electrification. These were free-fall devices incorporating large vertical plates between which an electrostatic field was maintained. Tribo-electric separation (contact charging) has experienced an increase in applications due to advances in mechanical self-cleaning and electrical design as well as the development of efficient precharging techniques. 

Charge transfer across an interface is a microscopic process, which must occur in essentially every two-phase system. This is the process of contact electrification, and it is closely related to the most ancient of electric phenomena, tribo-electricity. (It is not to be confused with ionization of atoms or molecules.) Once the charge separation has been accomplished, there will be an electrostatic contribution to the mechanical strength of the system. However, it is difficult to measure the charge transfer without separating the two phases and so there has been little success, to date, in quantitative measurement or prediction of the charge separation or of its direct contribution to adhesion. As a result, discussion of this component of adhesion must be somewhat speculative. The remarks here will be confined to the role of the interface and in this regard, matters are fairly clear. 

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