ELECTROSTATIC &

HIGH TENSION SEPARATION

Principle

This separation technique uses the differences in electrical conductivity between various minerals.

Suitable for mineral particles in the size range of 75 to 1000 microns. <10 micron cannot be used

Industrial application: Limited application, Beach sand separation, Thermal power plant to catch flying fine particles

Conducting Non-Conducting

Cassiterite

Chromite

Ilmenite

Magnetite

Rutile

Garnet

Monazite

Quartz

Sillimanite

Zircon

Conductivity is surface property unlike sp.gr. and magnetic susceptibility.

Universal concentrating method

Best result with very dry material. (Heating is imp)

Quartz

readily

assumes

-ve ch

arge !

Often a possibility of overlap in the application b/w mag and elec sep zs

Electrostatic Separators

Electro-static(No current flow)

Electro-dynamic or High Tension

(electron flow and gaseous ionization)

Particles are bombarded by high voltage ionizing electrode

Particles receive charge by virtue of being present in an electrical field

Limitations:(1) Perfectly dry feed ( a big constraint)(2) Small throughput - Process requires single layer, one particle deep (same as dry mag sep). (3) Humidity(4) Temperature

Particle Charging Mechanism…..

• Contact of Dissimilar particles

• Ion bombardment

• Induction

Charge development by contacting of dissimilar particles is by electron transfer or by ion transfer. Not a dominant mechanism in electrostatic separation.

Gases are generally insulators. But when high voltage is applied, they actually break-down and gas “discharges”. When particles pass through the corona, they get struck by electrons or anions and hence become charged. This is the dominant mechanism in High Tension Separation.

If a particle on a grounded rotor is placed in an external electric field, the particle will rapidly develop a surface charge by induction. Both conductors and non-conductors become polarized.

?

Particle charging Two non-conductive particles acquire opposite

charges bycontact with each other.Oppositely charged particles can then be separatedunder the influence of an electric field.

+

+

+

+

+

+++

+

+E

lectric Field (-)

Mechanism of Electrostatic Separation by Charge

Induction –

Conductor Particle

Particle develops

equipotential surface

+

+

+

+

+

-+-

-

+E

lectric Field (-)

+

Mechanism of Electrostatic Separation by Charge

Induction –

Non-Conductor Particle

Particle develops

image forces

ELECTROSTATIC SEPARATOR

Typical minerals either pinned / thrown from rotorMinerals thrown from

rotor (Conducting) Minerals pinned to rotor(Non conducting)

CassiteriteChromiteDiamondFloursparGalenaGold …….

ApatiteBariteCalciteCorundumGarnet ……

High tension separators operate on feed size 60 to 500 μ

Conducting fraction often contain coarse non conductors and Non conducting fraction fine conductors. – (can use screens!)

Multi pass is generally practiced in High tension separation.

Magnetics Non Magnetics

Magnetite – TIlmenite – TGarnet – PMonazite – P

T = thrown from high tension separator surface (cond.)

Rutile – TZircon – PQuartz – P

P = Pinned to high tension separator surface (non cond.)

Typical minerals either pinned / thrown from rotor

Laboratory high-tension separator

It uses predominantly

pinning effect with

lesser lifting effect

A corona is a process by which a current, perhaps sustained, develops from an electrode with a high potential (50 kV) in a neutral fluid, usually air, by ionizing that fluid so as to create a plasma around the electrode. The ions generated eventually pass charge to nearby areas of lower potential, or recombine to form neutral gas molecules.When the potential gradient is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive.

CORONA DISCHARGE

MechanismThe mixture of minerals of varying susceptibilities to surface charge are fed on to a rotating drum which is conducting in nature. The drum is earthed.

An electrode assembly is supplied with DC supply of upto 50 kV, usually of negative polarity.

The air between the rotor and the electrode gets ionised due to high voltage of the electrode assembly.

The minerals rolling down the roller receive spray discharge of electricity.

The poor conductors get a high surface charge causing them to get pinned to the rotor.

The high conductors are not held by the rotor as the charge they received is immediately dissipated to the rotor.

-

-

Ionizing Electrode

Static Electrode

Conducting

Non-Conducting

Mechanism of Electrostatic Separation by Ion

Bombardment

The attraction of particle carrying one kind of charge towards an electrode of the opposite charge is known as LIFTING EFFECT.

In the PINNING EFFECT the non-conducting mineral particles having received the surface charge from the electrode, retain this charge and are pinned to the oppositely charged separator surface by positive – negative effect.

HIGH TENSION SEPARATOR

Grounded Rotor

Forces Acting on a Particle on the Grounded Surface

Fg

Fc

Fe

Fi

lCentripetaF

imageF

ctorPinning Fa

lCentripetaF

ElectricalF

rceLifting Fo

Electrical Field

OPERATING VARIABLES• Feed Rate• Rotor Speed/Roll speed• Position of Electrode wire w.r.t electrode tube• Position of Electrode assembly w.r.t roll• Variation of DC Voltage and polarity• Splitter Plate Position• Heating of feed

SYSTEM VARIABLES• Particle size (surface charges on coarse grain are lower in

relation to its mass than on a fine grain.)

a) Conducting fraction + coarser non conductor particles

b) Non conducting fraction + fine conductor particles• Conductivity of mineral

Note: Multi-pass compulsory for good separation

Beach Sand MineralsMinerals size Sp.G Mag Suscept Elect Cond

Ilmenite MF HEAVY Strong Mag Cond

Rutile MF HEAVY Non-Mag Cond

Garnet C LIGHT Mag Non-Cond

Sillimanite C HEAVY Non-Mag Non-Cond

Zircon F HEAVY Non-Mag Non-Cond

Monazite F HEAVY Weak Mag Non-Mag

Quartz C LIGHT Non Mag Non-Cond

Magnetite C HEAVY Strong Mag Cond

• High tension separation as primary roughers then final cleaning in ES (using lifting effect only)

• Plate type ES – separate small non conductor from majority conducting• Screen Type ES – remove small amount of conductor from mainly non

cond.• Heavy minerals (feed grade 2 to 20 %, concentrate 90%)• Wet mag sep. first then HT sep. if Ilmenite (mag) is dominat

Wet Feed

Low Intensity Magnetic Separator

Gravity Preconcentrator

Tailings Quartz &

Garnet

Magnetics Magnetite

Non-Magnetics Zircon & Rutile

Magnetics Ilmenite & Monazite

Garnet

Non-Magnetics

Conducting Rutile

NonConducting Zircon

NonConducting Monazite

Conducting Ilmenite

Heavies Conc

High Intensity Magnetic Separator

High Tension Separator

High Tension Separator

SEPARATION OF BEACH SAND HEAVY MINERALS USING PHYSICAL SEPARATION METHODS

Electro static precipitator (ESP)

Electrostatic precipitation removes particles from theexhaust gas stream of an industrial process.Ex. Power/electric, cement, metal, chemicals, paper industries

(1) Ionization - Charging of particles using discharge electrodes.(2) Migration - Transporting the charged particles to the collectingsurfaces.(3) Collection - Precipitation of the charged particles onto thecollecting surfaces.(4) Charge Dissipation - Neutralizing the charged particles on thecollecting surfaces.(5) Particle Dislodging - Removing the particles from thecollecting surface to the hopper.(6) Particle Removal - Conveying the particles from the hopper to a disposal point.

Schematic of wire-plate ESP

Electro static precipitator (ESP)

Mechanism of ESP

ESP in a

Thermal Power Station

Other electrical-separation processes of theoretical importance

(1) Dielectric separation – suspend mixture of minerals in a non conducting fluid whose dielectric constant is intermediate between that of two groups of minerals. Particles of higher dielectric constant than that of fluid travel in direction of most rapid increase in electrical field.(2) Electrosmosis………..? – slimes suspended in water – similar to electrolysis (slower).

Other Physical beneficiation techniques

(1)Differential resistance to wear (hardness)Quartz – diamond separation.(2) Difference in adhesion to oil