METALLURGY , SEM 3
Chapter 14 , “ Mineral processing”, S.K.JAIN, CBS PUBLISHERS &
DISTRIBUTORS P Ltd, N Delhi, 2012.
Chapter 17, “Principles of Mineral Dressing” , A. M. Gaudin, Tata Mc
Graw Hill, 1993 .
MAGNETIC SEPARATION is a process in which magnetically susceptible material is
extracted from a mixture using a magnetic force.
Concentration of certain minerals can be done using MAGNETIC FORCES.
Different minerals have different magnetic properties.
By combining magnetic forces with gravitational or frictional forces, mineral particles can be
Three products are obtained – CONCENTRATES, MIDDLINGS and TAILINGS.
MAGNETIC FIELD is exerted by a magnet around it.
When a material is kept in this field, it experiences
1. Paramagnetic (weakly magnetic)- attracted along the lines of force of a magnet to the
point of greater magnetic field intensity( Mn, Cr Ni, Co., Ce, Ti )
Ferromagnetism (strongly magnetic) - A special case of paramagnetism-(permanent)- minerals
having permanent magnetism if the temperature is below the Curie Temperature-( the
temperature above which a ferromagnetic substance loses its ferromagnetism and
becomes paramagnetic ). These materials will become magnetized when placed in a
magnetic field and will remain magnetic after the external field is removed (REMNANCE).
Eg:- magnetite, hematite, pyrrhotite, ferro-silicon etc.
2. Diamagnetic ( slightly repelled along the lines of force of a magnetic field) .
Metallic Bi faces repulsion in a magnetic field.
Magnetic minerals result from properties that are specific to a number of elements.
Minerals that do not have these elements and thus have no magnetism are called
diamagnetic .Eg:- quartz, calcite, apatite.
Magnetic force F ( dynes ) = ( m1m2 )/ μd2.
m1 , m2 = strength of magnetic poles ,
d = distance between poles ( cm ),
μ = constant depending upon the medium =
Mag . permeability
1 for a vacuum,
Near to 1 for most materials,
very large for ferromagnetic
MECHANISM OF MAGNETIC SEPARATIONThe strongly magnetic material is separated from the weaker one using their degrees of
SUSCEPTIBILITY – Ratio of the intensity of magnetisation to the magnetic field of the object.
It is a dimensionless number - indicates the degree of magnetization of a material in
response to an applied magnetic field.
fe e d
Materials, which are repelled from the magnetic field, are called diamagnetics.
They have negative values of the magnetic susceptibility.
c o n c e n tra te
( par amagnet i c
par t i cl es)
ta i l i n g
( di amagnet i c
par t i cl es)
Particles attracted towards greater intensity of the magnetic field are called paramagnetics
Magnetisation / unit mass = ς = ( B – H ) / πρ .
B = induced flux /unit area ( Gauss ), H = Flux intensity of the existing field, ρ = Sp.gr. Of the
Magnetic susceptibility X = (magnetisation intensity / magnetic field strength) in e.m.u.
= ς / H.
A mineral particle is “ SATURATED “ on Magnetisation.
Ferromagnetic materials are easily saturated.
Non magnetic materials are not saturated.
Degree of saturation of other materials lie in between these two.
Hence they can be easily separated magnetically.
m a g netiza tio n, w H
ferro m a g netics
ferri- a nd a nty ferro m a g netics
true pa ra m a g netics
dia m a g netics
m a g netic field, H
m a g netic susceptibility
Influence of magnetic field on magnetization of materials
ferro m a g n etics
tru e p a ra m a g n etics
Cu rie p o in t
Né e l p o in t
a n ty ferro m a g n etics
d ia m a g n etics
tem pera ture
Influence of temperature on magnetic susceptibility of materials 8
MAGNETIC METHOD IS USED TO SEPARATE
1. “tramp” iron from minerals ( mechanically held impurities) – to safe guard the
2. concentration of Magnetite,
3. concentration of iron minerals made magnetic – roasted hematite ,siderite,
4. remove small quantities of iron from ceramic raw materials,
5. cleaning magnetic material ( Ferro Silicon Fe-Si) used in HMS etc.
REQUIREMENTS FOR EFFICIENT MAGNETIC SEPARATION1.Separation in a moving stream (of particles).
2. The stream moves through consecutive magnetic fields.
3. Polarity of magnets reversed for alternate magnets- the ferromagnetic materials
passing under rotate through 180 0 , thus freed from gangue material.
4. Successive magnetic fields should be increasingly stronger- Removes strongly
magnetic particles first and weaker by subsequent magnets.
5. The material should be spread over- gangue should not mask.
6. Converging field should be provided at each point of separation.
7. The material to be separated should be moisture free.
Edison separator consists of a bar magnet .
The ore as a thin stream falls in front of the
Susceptible particles are deflected inwards.
Nonsusceptible particles continue to fall
This separator was not wholly successful due
inability of control flow of solid in a
thin sheet and
lack of control over the speed of the
MAGNETIC SEPARATORS1. Converging field should be produced.
2. Intensity should be able to be controlled.
3. Uniform feeding arrangement.
4. Speed should be controlled.
5. Non magnetic material should not floc and mask the magnetic poles.
6. Disposing separated particles as concentrates, middlings and tailings.
DESIGN OF MAGNETIC POLESCONVERGING FIELD NEEDED FOR SEPARATION OF MAGNETIC MATERIALS.
1. The field of a magnet having two point poles ( horse shoe magnet) is
STRONGLY CONVERGENT near the poles and
UNIFORM at the equator .
The field near EACH POLE ONLY IS SUITABLE.
2.If there are two flat poles nearby – UNIFORM FIELD – UNSUITABLE.
3. A pointed pole near a flat pole- CONVERGING FIELD- SUITABLE.
DESIGN OF MAGNEIC POLES
A steep magnetic flux gradient is essential for efficient separation of minerals.
WEDGE SHAPED attracting poles are used.
The lines of magnetic force start from poles normal to the surface.
The greatest attraction occurs along the sharp edge of the wedge shaped pole.
Concentrate is deflected
from the ore path at d1
Middlings are deflected
from the ore path at d2
MAGNETIC SEPARATORS1. Electromagnets or permanent magnets ( ALNICO, Ceramics ) can be used.
2. The system may be DRY or WET.
1. DRY MAGNETIC SEPARATORLow and high intensity can be used.
The ROM ore is subjected to coarse comminution.
Rough concentrate is made first.
Subsequent concentration is done by further comminution and magnetic separation.
AdvantagesNo water used – more economical.
Good for separation after dry autogenous grinding.
The concentrate will not get damaged by water under freezing climatic conditions.
DRY MAGNETIC SEPARATORS1.Dry belt magnetic ( WETHERILL ) separator.
A high powered machine.
Material is fed to the feed roller through a hopper.
The material is uniformly spread over the entire width of an
endless conveyor belt running over the feed and discharge
It passes through magnets.
The poles are arranged one above the other.
The poles of the upper magnets have pointed edges .
Lower poles are flat.
Cross belts prevent adherence of magnetic particles to the poles .
They remove the separated particles.
2. Dry drum separator.
It has a series of oppositely charged magnets.
They are held inside a rotating drum.
The magnets are stationary.
The feed is introduced at the top of the drum.
Magnetic particles are attracted and held to the
drum. Non magnetic particles fall down.
The rotating drum brings them to the place where
no magnets are placed.
They are released .
3. High Intensity Dry Magnetic separator.
Used for weakly magnetic fine sized materials.
Two drums made of non magnetic material are
surrounded by fixed , permanent magnets
The rolls are laminated – alternate lamina of
permeable and non permeable materials.
So the Magnetic field is strongly convergent
towards the rolls.
Magnetic materials are separated on the
High Intensity Dry Magnetic separator.
WET MAGNETIC SEPARATORS1. Drum type wet magnetic separator.
A drum with fixed assembly of permanent
magnets inside dips into a tank holding the
Magnetic particles are lifted from the pulp.
Non magnetic particles are left out.
The water current keeps the pulp in
The concentrate is removed from the magnets
by water sprays / flushing water.
A system with TWO DRUMS too can be used.
The concentrates from the first drum are
treated ( Scavenged ) in the second one.
Concentrate, middlings and tailings are
WET MAGNETIC SEPARATORS
ADVANCES IN MAGNETIC SEPARATION.
1. High intensity Wet Magnetic Separation.
Can be used to separate weakly magnetic materials
( hematite, ilmenite, Cr, Mn, Ni,Mo etc ) from non
1. an electromagnetic coil of a series of
2. a rotating ring to converge the
3. feed and wash water entry,
4. discharge chutes for magnetic and non magnetic materials.
High intensity Wet Magnetic Separation.
2. A superconducting magnet
An electromagnet made from coils of superconducting wire.
The superconducting wire has NO ELECTRICAL RESISTANCE, at LOW TEMPERATURES ( Eg:- NbTi in Liquid He at 4.2 K) .
They must be cooled to cryogenic temperatures ( < −150 °C ) during operation.
In its superconducting state the wire can conduct much larger electric currents than ordinary
It creates INTENSE MAGNETIC FIELDS.
Superconducting magnets can produce greater magnetic fields .
It is cheaper to operate as NO ENERGY IS DISSIPATED AS HEAT IN THE WINDINGS.
DEMAGNETISATION OF THE CONCENTRATE.
The Susceptible minerals ( concentrate ) retain
residual magnetism after magnetic concentration.
It hinders the further processing of the
A demagnetiser demagnetises the material.
It has a A.C. Coil.
The AC causes change in direction of magnetic field
induced on the material passing through it.
This removes the remnant magnetism in the
The dry material / wet pulp flows through it.
The coil has gradually reducing diameter.
This results in gradually reducing intensity.