lec#6 ms - electrostatic separation

8/3/2019 Lec#6 MS - Electrostatic Separation

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MECHANICAL SEPARATION

SS LEAKS

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What is Electrostatics? Why a Charged Comb attracts paper & dry dandruff?

What is Electrostatic Separation?

Principle

Types Of Electrostatic Separations:

Wet Electrostatic Separations

Dry Electrostatic Separations

Technique Of Charging (Charging Mechanism)

Ion BombardmentContact Electrification

Conductive Induction

Applications

Electrostatic Separation Machines

Electrostatic Precipitator

DefinitionConstructionFunctional Description

Types

Wet Electrostatic Precipitator

Dry Electrostatic Precipitator

Applications

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` Electrostatics is the branch of science that deals with the phenomenonarising from stationary or slow-moving electric charge.

Why A Charged Comb Attracts Paper And Dry Dandruff ?

` Suppose the comb ends up positive.

` Then near side of paper ends up slightly negative from attraction ofopposite charges.

` The other side ends up slightly positive due to repulsion of similar charges.

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` Separation of finely pulverized materials by placing them inelectrostatic separators. Also known as high-tension separation.

PULVERIZED MATERIAL

` A substance that is reduce to fine particles, by crushing or

grinding.


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` Electrostatic separation is a method of separation based on differential

attraction or repulsion of charged particles under the influence of an

electric field.

` Applying an electrostatic charge to the particles is necessary step before

particle separation can be accomplished.

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Various techniques can be used for charging these include` Contact electrification

` Conductive induction

` Ion bombardment

Generally, separators of the conductive-induction type have a lower capacityper unit length of electrode than the ion-bombardment.

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When dissimilar material touch each other, there is an opportunity for the transferof electric charges.

` The extend of charge transfer can be such that a significant surface charge of

opposite sign is developed when the materials are later separated.

` High temperatures and low humidity favor the development of high surface chargesthrough the mechanism of contact electrification.

` Rubbing the materials together to increase the area of effective contact can alsolead to high surface charges.

` Particles carrying charges of opposite polarity due to contact electrification will beattracted to opposite electrodes when passing through an electric field and thus canbe separated from each other.

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` The term conductive induction describes the process by which an initially

uncharged particle that comes into contact with a charged surface assumes thepolarity and, eventually, the potential of the surface.

` The charged plate must be balanced by other oppositely charged (or earthed) bodies

to maintain overall neutrality.

` If a conductor particle and a nonconductor particle are just separated from contactwith a charged plate, the conductor particle will be repelled by the charged plate

and the non-conducting particle will be neither repelled nor attracted by it.

A question arises How electrostatic separation be done through this technique?

`

In electrostatic separation, this is usually accomplished by means of a singleelectrode of charge opposite in sign to that of the charged plate.

` The conductor particle is then in the electrical field between the two electrodes and

experiences a net electrostatic force in the direction of the second electrode.

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` The non-conducting particle,

having no net charge,

experiences no electrostatic

force in a uniform electric field.

` Electrostatic separation of the

conductor and nonconductor

particles can be accomplished

by movement of the conductors

in the electric field.

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The most positive and strongest method of charging particles for electrostatic

separation is ion bombardment.

WORKING:

Both particles are bombarded by ions of atmospheric gases generated by anelectrical corona discharge from a high-voltage electrode (usually a finetungsten-alloy wire at 20 to 30 kVwith respect to ground and several

centimeters away from the particles).

When ion bombardment ceases, the conductor particle loses its acquiredcharge to ground very rapidly and experiences an opposite electrostaticforce tending to repel it from the conducting surface.

The non-conducting particle, however, being coated on its side away fromthe conducting surface with ions of charge opposite in electrical polarity tothat of the surface, experiences an electrostatic force tending to hold it tothe surface.

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There are two types of electrostatic separations:

WET ELECTROSTATIC SEPARATIONS

In wet electrostatics separations dust particles are removed from

wet saturated air stream (100% relative humidity).

DRY ELECTROSTATIC SEPARATIONS

In dry electrostatic separation particles are separated from dry

air stream.

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` MINERAL BENEFICIATION

Electrostatic methods are widely used in the processing of ores with mineral

concentrates.

`

Charging by ion bombardment is the technique used in most mineralseparations.

` PLASTIC AND METAL RECYCLING

Electrostatic separation has been increasingly applied to recover nonferrous

metals from industrial plastics (telephone and communication scrap).

` It also is an important step in the recycling of beverage bottles to reject any

remaining nonferrous metals. (Fig. 19-61b).

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There are different types of electrostatic machines which are used in ourindustry:

` Tribo-electric Separators

` Conductive-Induction Machines

`

Ion-Bombardment Machines

But Electrostatic Precipitator has

more importance than all of above

so we only discuss it here.

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` A device for removing small particles, as of smoke, dust, or oil, from agas, as air, by passing the gas first through an electrically charged screenthat gives a charge to the particles, then between two charged plateswhere the particles are attracted to one service.

` When enough dust has accumulated, the collectors are shaken to dislodge

the dust, causing it to fall with the force of gravity to hoppers below.

` The dust is then removed by a conveyor system for disposal or recycling.

` Electrostatic precipitators are highly efficient filtration devices.

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` Depending upon dust characteristics and the gas volume to be treated, there

are many different sizes, types and designs of electrostatic precipitators.

` A shell or house contains the electrodes, exhaust gases and rapper.

` The shell must be well built with a rigid frame to hold the components in

their proper place.

` Hot temperatures inside the shell can vary greatly from the temperatures

outside the shell.

` Such tremendous temperature differences can cause expansion and

contraction to stress joints and welds.

` Often shells are insulated to minimize temperature differences and prevent

gases from condensing into corrosive liquids.

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The major precipitator components that accomplish these activities are asfollows:

` Discharge Electrodes

` Power Components

`

Precipitator Controls` Flue Gas Conditioning

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` Electrostatic precipitator (ESP's) are used to extract airborne dust particlesfrom the air.

` Dust particles are charged by electrodes and adhere to the collector plates

in the electrostatic precipitator.

` Electro-mechanical rappers dislodge the dust particulate from theelectrodes and collector plates (rappers are located external to theelectrostatic precipitator).

` The dust particles fall into a common hopper where they are exited throughan air lock to the residual collecting conveyor system.

` The clean air is evacuated from the electrostatic precipitator into theatmosphere.

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` A dry electrostatic precipitator ( ESP ) is a large box.

` Particulate control begins when the dust laden gases are drawn into one

side of the box.

` Inside, high voltage electrodes impart a negative charge to the particlesentrained in the gas.

` These negatively charged particles are then attracted to a grounded

collecting surface which is positively charged.

` The gas then leaves the box up to 99.9% cleaner than when it entered.

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` For wet electrostatic separations usedwet electrostatic separators.

` WESP uses a vertical cylindrical tube

with centrally-located wire electrode

(gas flowing upward) with watersprays to clean the collected

particulate from the collection surface

(plates, tubes).

` Wet ESPs are used for industrialapplications where the potential for

explosion is high

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` removal of dirt from flue gases in steam plants` removal of oil mists in machine shops and acid mists in chemical process

plants

` cleaning of air to remove fungi and bacteria in establishments producing

antibiotics and other drugs, and in operating rooms

` cleaning of air in ventilation and air conditioning systems` cleaning of blast furnace gases

` recovery of valuable materials such as oxides of copper, lead, and tin

` separation of rutile from zirconium sand

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lec#6 ms - electrostatic separation

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