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The magnetic separation process is complex in many respects
due to the varying magnetic susceptibility of the different ores, the amount
of middlings, and the particle size distribution of the processed ore.
The magnetic force acting on a specific particle is
dependent upon these factors as well as other properties such as magnetic
flux and magnetic field gradient, which are created by the magnetic system
in the separator. The product of field time’s gradient, also called the
magnetic force index, varies between different magnetic system designs and
is a factor when selecting the most suitable magnetic system for a
particular process.
As in all applications, the magnetic force is competing
with other forces such as gravity and hydraulic drag; therefore, the feed
volume and tonnage must be balanced to obtain a suitable level of
performance.
Generally speaking, the smaller the particle size that is
being processed results in a lower feed capacity of the equipment. By
selecting a higher magnetic field strength (magnetic flux) the feed capacity
can be increased considerably, in many cases the recovery of the fines
increases significantly with the field strength.
The concurrent style of magnetic separator
features:
·
Feed box with serrated weir overflow for even distribution of the feed
slurry
·
Feed entry section to improve on feed pulp distribution thus ensuring full
width feed to drum;
·
Short pickup zone, which reduces the risk of coarse material settling on
tank bottom.
·
Exchangeable outlet spigots in tank bottom to allow coarse
material to discharge trouble free; suited for processing of coarse ore up
to 6 - 8 mm (3 mesh) and, with special arrangement, up to 15 mm (5/8 inch).
The pulp density recommended for this type of tank is 30 to 50 percent
solids by weight.
CR – Counter-rotation
The counter-rotation style of magnetic separator features
·
Feed box with feed tubes;
·
Feed entry section to improve on feed pulp distribution thus ensuring full
width feed to drum;
·
Long pick up zone;
·
Spigot-less, full-width weir with adjustable (manual) overflow discharge of
the effluent for pulp level control allowing surges in feed flow;
·
Feed tubes are wear protected on the outside by molded polyurethane saddles
and on the inside by polyurethane inserts;
The CR tank is suited for processing medium coarse ore up to 3-8 mm (4 mesh)
at medium to high densities (30 to 50% solids). The CR tank is also
available in an economy version with rubber-lined feed tubes and a fixed
concentrate overflow weir.
CTC
- Countercurrent
·
The countercurrent style of magnetic separator features:
·
Feed box with serrated weir overflow for even distribution of the feed;
·
Feed entry section to improve on feed pulp distribution with full width feed
to drum;
·
Medium long pick-up zone;
·
Full-width effluent overflow weir for pulp level control;
·
The CTC tank design is suited for processing fine to medium-sized ore up to
0.8 mm (20 mesh). The pulp density range recommended for this type of tank
is
·
25 to 45 percent solids by weight.
DWHG Counter-rotation
· Basically counter -rotation tank design
· Extremely long pick-up zone;
· Entry chamber designed to allow for entrapped air to escape and to improve
concentrate drainage;
· Longer magnet assembly arc to compensate for disturbances due to entrapped
air in the pulp;
· High gradient style magnetic assembly for recovery of fine to extremely fine
magnetic material;
Full-width weir with adjustable
(manual) overflow discharge of the effluent for pulp level
control
allowing surges in
feed
flow.
·
This
tank
design
is
especially
intended
for
recovery
of
magnetic matter from
dilute
flotation
tailings with
entrapped
air.
Particle
size
limits
are
dependent
on
flow
rate
but up
to
1 mm
material
will normally
not cause
any
concern.
Magnetic
System
The heart of the magnetic separator is the magnet assembly. There are
basically two different assemblies: High Capacity and High Gradient, (HG).
The High Capacity assembly is the standard magnetic system. The main
differences between the two magnet assemblies are pole pitch, pole sizes,
and number of poles.
The Magnet assemblies are similar in design with both having a number of
main poles and a number of inter- mediate cross poles for flux control and
enhancement of the magnetic performance. The magnet poles are attached to a
rigid steel yoke, which, in turn, is attached to the drum shaft.
The High Capacity assembly is comprised of six main poles and four
intermediate poles, while the High Gradient Assembly has twelve main poles
and eleven cross poles. A special magnet assembly is the DWHG, which
consists of fourteen main poles and thirteen cross poles.
The High Capacity assembly has a higher magnetic flux rating but due to its
inherently lower field gradient the magnetic attraction force is actually
lower when com- pared with the High Gradient type.
The High Gradient version is hence more capable of recovering material,
which is finer or less magnetic; however, its capacity is reduced as its
magnetic force drops off faster as the distance increases from the magnets.
The arc of the High Capacity magnetic system is 124° and has a rating of 120
mT at 50 mm from the magnet surface while the High Gradient magnetic system
has an arc of 113° and a rating of 60 mT. The magnetic system of the DWHG
has an arc of 133° and a rating of 60 mT at 50 mm.
Magnetic Drum
The magnetic assembly is mounted inside of a revolving drum consisting of a
stainless steel shell with aluminum heads. The magnetic assembly is
stationary but is adjustable peripherally.
The drum shaft is supported by spherical roller bearings. All drum bearings
are lubricated from the non- rotating shaft end.
Wear Protection
The feed slurry to the separators is at times very abrasive and hence all
tanks, boxes and launders are lined inside with rubber. The revolving drum
is lined with either rubber or stainless steel and its ends are fitted with
specially designed end collars.
At some applications these wear protection measures need to be reinforced by
increase of liner thickness or replacement by even tougher material such as
ceramics or polyurethane.
Adjustment of magnet and drum position
The magnetic drum and magnetic assembly can easily be adjusted to obtain the
best process performance. The adjustment possibilities include
· Magnet assembly positioning in relation to concentrate discharge weir.
· Horizontal positioning of drum
·
Vertical positioning of drum
Feed Boxes
The feed system for primary distribution of the pulp to the
feed boxes that are supplied with the separators is normally not part of the
equipment supply.
All feed boxes are normally made from mild steel and rubber
lined. Other features of the feed boxes are:
· For models CC and CTC the overflow weir is serrated for more even
distribution
· The feed box for the CR model discharges through a number of steel tubes
into the separator tank.
Drum Drive System
The drum on any magnetic separator has a drive shaft, which
can be adapted to any type of drive. The position of the drive components is
very favourable, since they are located outside of the tank away from pulp
splashes and water. Our experience shows that the drum speed is very seldom
altered and hence most of our separators have been delivered with
direct-drive gearboxes. This system has proven to be safe, efficient, clean,
and quiet and requires very little maintenance.
The older drive system with an integrated V-belt drive is
also available and is recommended for users who want to maintain the
possibility of modifying the drum speed (by shifting drive sheaves).
The normal peripheral speed of the drum is 1.2 m/s unless
another drum speed has been agreed upon.
Concentrate discharge and collection
An overflow weir is provided for the magnetic concentrate
discharge. This weir, manufactured of HDPE or, optionally, in polyurethane,
is adjustable to obtain optimum discharge conditions.
The launders for collection of the concentrate that is
discharged over the weir are made of a rubber- lined combination of mild and
stainless steel and are bolted to the separator tank frame. Depending on the
installation situation, standard launders or, optionally, custom tailored
launders are provided.
Effluent (tailings) discharge and collection
The tailings effluent stream is normally discharged into a
trough beneath the magnetic separators. This trough is not part of the
standard equipment and is designed and provided for by the customer. The
separators may, if so ordered, be directly connected by steel pipes or
rubber slurry hoses to the customer’s effluent system thus eliminating the
need of troughs under the tanks. This is possible with the CR and CTC models
and with special arrangements, also for the CC.
Application Guide Lines
Absolute guidelines for model selection and dimensions are
not available due to the widely varying nature of iron ores; hence, the data
shown in the table below are only indicative and, when in doubt of the
properties of a specific ore, the lower feed rate should be used. Testing in
a laboratory, followed by on site testing is always advisable especially
when planning for larger installations. Sizing of full-size machines using
only laboratory data is normally not sufficient to determine the number of
magnetic separators that are required.
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