Gravity Separation Operations

Gravity concentrating operations are characterised by processes that allow particles to be held slightly apart so
that they can move relative to each other and therefore to separate into layers of dense and light minerals. The
mechanisms by which this interparticular spacing is accomplished may be used as a convenient means of
classifying gravity concentrators.

Jigs
Stratification in a bed of particles results from the repeated pulsation of a current of fluid up through the bed. The
particles in the bed are expanded so that when pulsation ceases, the particles are allowed to consolidate under the
influence of gravity. Figure illustrates the expansion and contraction of the bed with the heavier, larger particles
falling under hindered settling conditions.
The expansion and contraction of the bed is repeated in a cyclic operation until the heavy and light particles have
stratified according to their specific gravity. The frequency of pulsations usually varies from 50 - 300 cycles per
minute.

Expansion and contraction of a bed of particles due to jigging action.

A particle settling in a viscous fluid is described by the equation below. As a particle just starts to move from
rest, the particle velocity is small and hence the drag force acting on the particle, FD, is negligible since the drag
force increases with particle velocity relative to the fluid.

Thus:

That is, the initial acceleration of the particles depend only on the specific gravity of the solid and fluid and is
independent of the particle size. Once the particles reach an appreciable velocity the fluid drag force becomes
significant and it opposes the particle's further acceleration to the extent that eventually the particle acceleration
drops to zero and a constant terminal velocity is reached which will dependent on the particle diameter as well as
density.
If the duration of the particle movement under gravity is kept short by having a high cycle frequency then the
total distance travelled by the particles will be governed more by the difference in the initial acceleration
between particles due to their density difference rather than by their terminal velocities which is also influenced
by the particle size. That is, for particles with a similar terminal velocity, such as would be experienced by small
heavy particles and large light minerals, a short jigging cycle would be necessary for separation. However, for
coarser particles, longer strokes with decreased speed is found to give better stratification and hence it may be
preferable to split the feed into closely sized fractions and have a jig optimised for each size fraction. With a long
stroke cycle particles will reach their terminal velocities which will dependent on the particle density and size.
Hindered settling conditions will prevail. By adjusting the upward flow of fluid the settling velocity of the fine
light particles can be overcome and the fine particles will be carried upwards and away from the denser heavier

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particles. A stronger pulsation stroke will then allow only the large heavy particle to settle against the rising
force of fluid. For particles having a similar terminal velocity such as the small heavy particles and the large
light particles, separation by this means would not be possible. Hindered settling is more significant for coarse
particle separations where a slower stroke cycle is used, although with coarser feeds, the larger particles may not
have time to reach their terminal velocities. The parameters that will effect hindered settling during jigging are
particle size, density and shape, the fluid density and viscosity, the percent solids and the separator
characteristics.

The jig is normally used to concentrate relatively coarse material and, if the feed is fairly closed sized (e.g. 3-
10mm), it is not difficult to achieve good separation of a fairly narrow specific gravity range in minerals in the
feed (e.g. fluorite, sp. gr. 3.2, from quartz, sp. gr. 2.7). When the specific gravity difference is large, good
concentration is possible with a wider size range. Many large jig circuits are still operated in the coal, cassiterite,
tungsten, gold, barytes, and iron-ore industries. They have a relatively high unit capacity on classified feed and
can achieve good recovery of values down to 15µm and acceptable recoveries often down to 75µm. High
proportions of fine sand and slime interfere with performance and the fines content should be controlled to
provide optimum bed conditions. In the jig the separation of minerals of different specific gravity is
accomplished in a bed which is rendered fluid by a pulsating current of water so as to produce stratification. The
aim is to dilate the bed of material being treated and to control the dilation so that the heavier, smaller particles
penetrate the interstices of the bed and the larger high specific gravity particles fall under a condition probably
similar to hindered settling.

As the pulsation approaches the top of the stroke the upward velocity of water slows and particles will start to
settle again starting with the particles of higher terminal velocity. The particles will then begin to compact down
as they settle against the jig screen. The large particles pack together leaving large voids between them into
which the smaller particles can continue to settle under gravity. This consolidation trickling will help to bring
the fine heavy particles down into the heavy layer and if allowed to go for too long, will also draw the fine light
particles down into the heavy layer and thus contaminate the heavy fraction.(Shown in figure)

Stratification during the dilation stage is controlled by hindered-settling classification with some modification by
differential acceleration, and during the stage that the bed is compacted, it is controlled by consolidation
trickling. The frequency of the jig cycle and the control of events within each cycle is critical in determining the
behaviour of particles within the jig bed. A minimum cycle time is required to allow each phase of the cycle to
be optimum for a given feed. Any further increase in cycle time would not be optimum or the bed would be in a
compacted state and no further separation would occur during this interval hence affecting the capacity. Cycle
speed adjustment is therefore the most important operating variable.

Baum and Batac Jig

Jigging through the screen

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InLine Pressure Jig
The Inline Pressure Jig (IPJ) is a new application of the jigging principle with a completely enclosed and
pressurised jig with a moveable screen action in a circular bed. The pressurisation of the unit, up to 200 kPa,
allows the Inline Pressure jig to be completely filled with slurry and water which slows the slurry velocity and
eliminates the air-water surface tension for potentially improved recovery. A hydraulic ram pulses the screen in
the water with a jigging through the screen operation.
The advantages of the IPJ are a low water consumption, allowing operation in the recirculating stream of a
grinding circuit, a high mass pull of up to 30% to the heavy fraction, feed capacity up to 110 tph, feed sizes up to
30 mm and low power consumption. When used for treating alluvial deposits of precious metals or gemstones,
the completely sealed unit offers security.

Centrifugal Jig
In 1990, Kelsey introduced the first commercial unit that incorporated a centrifugal force to jigging. The Kelsey
Centrifugal Jig (KCJ) operates at up to 40 times gravitation acceleration in order to extend the separation range
of gravity separation down to less than 40 microns. The Kelsey Centrifugal Jig utilises the Harz design which is
divided into two parts, the top section of heavy mineral (ragging) above a screen, and the jig chamber filled with
water and pulsed by a diaphragm plunger. The screen and hutch arrangement is turned 90° from the horizontal to
the vertical and spun about a vertical axis. Gains achieved with the KCJ include:
1. Operating and maintenance cost savings
2. Improved recovery
3. Improved final concentrate grade, and
4. Simplifying the processing circuit