electrostatic separation ser
DESCRIPTION
Mineral processing.TRANSCRIPT
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.
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-
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