Electrostatic separator

Electrostatic separators can be selected according to the following funda-menfal equation  

Electrostatic precipitators have been used successfully to eliminate fine dusts in many types of waste gases. They have been employed in settling of fine powders of lead oxide, arsenic, and foundry gases. Other applications include settling of dust from gases produced in cemenf ovens, and in the recovery phosphorous pentoxide from gases produced in phosphorous ovens. 

It may be added here that electrostatic separation suffers from more or less the same disadvantages as dry magnetic separation in that both require a perfectly dry feed, and both have a relatively small capacity for finely divided material. For most efficient operation, it is necessary that the feed be processed in a layer (one particle deep), but this severely restricts the throughput of the electrostatic separation process. 

In general, all electrostatic separator systems contain at least four components (i) a chargingdischarging mechanism (ii) an external electric field (iii) a nonelectrical particle trajectory device and (iv) feed and product collection systems. Depending primarily on the charging mechanism involved, the electrostatic separator systems are classified into three categories (i) free fall separators (ii) high tension separators and (iii) conduction separators. 

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. 

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The heavy mineral sand concentrates are scmbbed to remove any surface coatings, dried, and separated into magnetic and nonmagnetic fractions (see Separation, magnetic). Each of these fractions is further spHt into conducting and nonconducting fractions in an electrostatic separator to yield individual concentrates of ilmenite, leucoxene, monazite, mtile, xenotime, and zircon. Commercially pure zircon sand typically contains 64% zirconium oxide, 34% siUcon oxide, 1.2% hafnium oxide, and 0.8% other oxides including aluminum, iron, titanium, yttrium, lanthanides, uranium, thorium, phosphoms, scandium, and calcium. 

To avoid generation of waste brines and the associated serious problem of brine disposal, the potash industry in the former FRG began converting some operations to electrostatic separation, a dry process for separating potassium salts from other soluble salts (24,25). 

Induced-roU separators have also been used in the concentration and cleaning of heavy minerals found in beach sands. Examples are the mtUe and ilmenite beach sands of Florida and New Jersey. Induced-roU separators are frequently used in combination with high tension or electrostatic separators. 

A variable-speed drive is usually used on the feed and cross-belt drives to exercise control in separator operation, although the speed is not usually changed once the optimum operating condition is estabUshed. Feed rates and the selection of the number of magnetic poles are usually deterrnined by preliminary laboratory tests. The mineral types involved in the feed largely determine the number of poles selected. High intensity cross-belt separators are frequendy used in combination with induced-roU or electrostatic separators. 

Dry Coal Cleaning. Developments in the areas of magnetic and electrostatic separation as a means of cleaning coals in the dry state include high gradient magnetic separation (HGMS), triboelectrostatic separation (TESS), and dry coal purifier (D-CoP). 

Frank Knoll/ M S / Min Proc / President, Carpco, Inc., Jacksonville, EL. (Super Conducting Magnetic Separation and Electrostatic Separation)... 

Solids separation based on density loses its effectiveness as the particle size decreases. For particles below 100 microns, separation methods make use of differences in the magnetic susceptibility (magnetic separation), elec trical conductivity (electrostatic separation), and in the surface wettability (flotation and selec tive flocculation). Treatment of ultrafine solids, say smaller than 10 microns can also be achieved by utilizing differences in dielectric and electrophoretic properties of the particles. 

Ion Bombardment fFig. 19-55c) The most positive and strongest method of charging particles for electrostatic separation is ion bombardment. Use of ion bombardment in charging materials of... 

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

The common types of other industrial electrostatic separators employ charging bv conductive induction anchor ion bombardment. Figure 19-56 illustrates the principles of application,... 

Conductive-Induction Machines Electrostatic separators exploiting the principle of conductive induction will generally use the follovv -ing electrode designs ... 

FIG. 19-57 TriiKK lectric separators, -S tat electrostatic separator for silica removal from industrial minerals, (Courtesy of Carpeo., Inc.)... 

FIG. 19-58 Tril)oelectric separators, Belt-tvpe electrostatic separator for separation of caibon from flv ash, (Coniiesy of Separation Ceehnoloeie.s, Ine.)... 

FIG. 19-59 Condiictivoindiiction platc-tvpc electrostatic separator. (Conr-IcHij of Mincm] TechnoJoi jj, Lid.)... 

Power Supplie.s Iligh-voltage ac and dc power supplies for electrostatic separators are iisiiallv of solid-state construction and feature variable outputs ranging from 0 to 30,()()() for ac wiper transformers to 0 to 60,000 for the dc supply The maximum current requirement is approximately 1,0 to 1,5 rnA/rn of electrode length. Powder supplies for industrial separators are typically oil-insulated, but smaller diw-epoxv-insulated supplies are also available. 

TABLE 19-25 Machine Capacities of Electrostatic Separators for Mineral Applications... 

Plastic and Metals Recycling Electrostatic separation has been inereasinglv applied to recover nonFerroiis metals From industrial plasties (telephone and eornrniinieation scrap). It also is an important step in the reeveling oF beverage bottles to reject anv remaining nonFerroiis metals. Both oF these reeveling applications make use oF roll-type ion-bornbardrnent separators (Fig, 19-61/ ),... 

Typical Operating Conditions Table 19-26 presents some typical values oF important operating conditions For the separation oF several diFFerent types oF Feed materials. In considering candidate processes For a given separation job, the table can sometimes be help-Fiil in showing that materials oF similar properties and/or economic v ahie can be treated bv electrostatic separation. 

TABLE 19-26 Typical Operating Conditions for Electrostatic Separations... 

Magnetic separation for ferrous materials eddy-current separation for aluminum electrostatic separation for glass from wastes free of ferrous and aluminum scrap magnetic fluid separation for uouferrous materials from processed wastes... 

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