WO2001056701A2

WO2001056701A2 - Jig for density based separation of matter

Info

Publication number: WO2001056701A2
Application number: PCT/ZA2001/000008
Authority: WO
Inventors: Cicero Combrinck
Original assignee: Cicero Combrinck
Priority date: 2000-02-01
Filing date: 2001-02-01
Publication date: 2001-08-09
Also published as: AU4371901A; WO2001056701A3

Abstract

The invention provides a jig (10) for separating particles with different densities from a feed material. The size of the jig may vary depending on the application, and in some embodiments the jig is portable and is bolted to a metal frame (12). Typically a discharge chute (14) discharges a particulate material onto a pair of parallel conveyor belts (16). The conveyor belts discharge the feed material into a feed end (18) of the jig. The slurry moves from the feed end towards a discharge end (20) in the direction indicated by the arrow 'A'. The separation is achieved by pulsing a fluid medium perpendicularly to the direction arrow 'A' at a desired frequency while maintaining fluid flow in an upward direction at all times.

Description

JIG FOR DENSITY BASED SEPARATION OF MATTER

Field Of The Invention

This invention relates to a jig.

Background Of The Invention

A jig is commonly used for separating a mixture of mineral materials having differing densities or specific gravities.

A typical jig includes a feed end where particulate feed material is introduced into the jig, and a discharge end where a separated fraction of the feed material is removed. Known jigs are typically divided into a jig chamber and a lower chamber, known as a "hutch", by means of a screen.

Reciprocating pulses are generated in the slurry, within the jig chamber, in order to stratify the feed material according to density and form a bed on the screen. Material can be removed from the jig by "on the bed" jigging or "through the bed" jigging. On the bed jigging is normally used for separation of relatively coarse particles whereas through the bed jigging is more suitable for relatively fine particles.

In on the bed jigging, the screen has an aperture size which is smaller than the average smallest particle in the feed material. No feed material can therefore move into the hutch. Under the influence of the reciprocating pulses, dense material settles onto the screen. The dense material is typically removed from the jig chamber via discharge gates located above the screen at the discharge end. The remaining fraction of material is removed from the jig chamber via an overflow weir at the discharge end.

By varying the amplitude and/or frequency of the reciprocating pulses, aperture size in the screen, flowrate of the feed material, height and aperture size of the discharge gates, and height of the overflow weir, it is possible to obtain a separation of the feed material.

In through the bed jigging, the screen has an aperture size which is larger than the particles in the feed material. The bed is prevented from falling into the hutch by a layer of particles known as "ragging". The ragging particles are selected to be larger than the size of the apertures in the screen, and have a density slightly less than that of the material which is to be separated. The ragging forms a ragging bed and, under the influence of the reciprocating pulses, dense material settles onto the ragging bed. The material which has a higher density than the ragging settles through the ragging bed into the hutch where it is removed. The less dense material forms a bed on top of the ragging and is removed from the jig chamber via the overflow weir at the discharge end.

By varying the amplitude and/or frequency of the reciprocating pulses, aperture size in the screen, density of the ragging, flowrate of the feed material, height and aperture size of the discharge gates, and height of the overflow weir, it is possible to obtain a separation of the feed material.

As can be seen above, jigs are dependent on the formation of a distinctly stratified bed. The stratified bed is achieved by means of the reciprocating pulses generated in the slurry. Each pulse comprises a propulsion stroke and a suction stroke, relative to the screen. Under the influence of gravity the particles in the feed material tend to settle with the relatively heavier particles tending to settle more quickly than the relatively lighter particles. The propulsion stroke retards the settling velocities of the particles. The heavier particles have a higher momentum and thus the settling velocity of the heavier particles will be less retarded by the propulsion stroke than that of the lighter particles. The propulsion stroke therefore amplifies the difference in the settling velocities of the particles and thus enhances the stratification of the bed.

A problem with known jigs is that the suction stroke tends to reverse this effect. The lighter particles have a lower momentum and the settling velocities are thus more effected by the suction stroke. Accordingly, the lighter particles will tend to settle more rapidly under the influence of the suction stroke than the heavier particles. The suction stroke therefore reduces the difference in the settling velocities of the particles and thus reduces the stratification of the bed. This leads to poor stratification of the bed and compaction of the bed. In through the bed jigging it can also cause the bed to collapse into the hutch.

The problems identified above tend to occur even when make-up water is introduced into the jig in order to compensate for water loss due to the removal of the heavier particles from the hutch. In some known jigs, make-up water is introduced directly into the jig, either into the jig chamber (i.e. above the screen) where it is referred to as "top water", or into the hutch (i.e. below the screen) where it is referred to as "back water". However, controlling the amount of make-up water introduced directly into the jig, and the timing of the introduction of the make-up water to coincide with the suction stroke, requires complicated valves and control apparatus

In addition, some known jigs use either a piston or a paddle arrangement to create the propulsion stroke in the jig A problem with known jigs which operate using the piston arrangement is that the piston is located adjacent the jig chamber and takes up a portion of the surface area of the jig This reduces the effective area of the jig and increases the footprint of the jig making these jigs large and cumbersome A problem with known jigs which operate using the paddle arrangement is that the paddle typically induces horizontal strokes in the fluid which must be converted, by means of baffle plates, into vertical strokes This tends to create turbulence which can disrupt the settling process

It is therefore an object of this invention to provide a jig which addresses these problems and can be used for on the bed and through the bed jigging

Summary Of The Invention

According to a first aspect of the invention there is provided a jig for separating particles with different densities from a feed material, the jig comprising - a settling vessel defining a settling chamber adapted to receive the feed material, a displacement member in substantially sealed engagement with the settling vessel thereby defining at least a part of a base of the settling vessel, an expansion formation in sealed engagement with the displacement member and defining and expansion chamber adapted to receive a fluid and in fluid communication with the settling chamber, wherein the displacement member is movable between an extended position wherein the settling chamber defines a contracted volume and the expansion chamber defines an expanded volume, and a retracted position wherein the expansion chamber defines a contracted volume and the settling chamber defines an expanded volume and a fluid reservoir in fluid communication with the expansion chamber, such that in use when the displacement member moves from the retracted position to the extended position fluid from the expansion chamber is transferred to the settling chamber, and when the displacement member moves relative to the expansion formation from the extended position to the retracted position fluid is introduced into the expansion chamber from the fluid reservoir Preferably the displacement member comprises at least one transfer valve having an open condition wherein the settling chamber is in fluid communication with the expansion chamber, and a closed condition wherein the settling chamber is substantially sealed from the expansion chamber, and wherein the transfer valve is movable from the open condition to the closed condition corresponding to movement of the displacement member from the retracted position to the extended position.

Typically the fluid reservoir comprises at least one make-up valve having an open condition wherein the fluid reservoir is in fluid communication with the expansion chamber, and a closed condition wherein the fluid reservoir is substantially sealed from the expansion chamber, and wherein the make-up valve is movable from the open condition to the closed condition corresponding to movement of the displacement member from the extended position to the retracted position.

Conveniently the displacement member comprises a particle collection formation. Ideally the particle collection formation comprises the at least one transfer valve.

The jig may comprise further a discharge mechanism comprising fluid separation means, wherein the fluid separation means is in fluid communication with the fluid reservoir.

According to a second aspect of the invention, there is provided a jig for separating particles with different densities from a feed material, the jig comprising: a settling vessel defining a settling chamber adapted to receive the feed material; a displacement member in substantially sealed engagement with the settling vessel thereby defining at least a part of a base of the settling vessel; an expansion formation in sealed engagement with the displacement member and defining an expansion chamber adapted to receive a fluid and in fluid communication with the settling chamber; and wherein the displacement member is movable between an extended position wherein the settling chamber defines a contracted volume and the expansion chamber defines an expanded volume, and a retracted position wherein the expansion chamber defines a contracted volume and the settling chamber defines an expanded volume; such that the change in volume in the settling chamber when the displacement member moves between the extended position and the retracted position is smaller than the change in volume in the expansion chamber for the corresponding movement of the displacement member. According to a third aspect of the invention, there is provided a jig of the type having a bed on which particulate materials of different densities are stratified and advanced and from which a fraction denser material is extracted, the stratification being performed in a fluid medium which is pulsed substantially perpendicularly to the passage of the particulate material over the bed, the pulsing being effected by a piston-diaphragm arrangement in which the suction stroke is eliminated and replaced by a hindered settling phase.

The piston-diaphragm arrangement may be provided with a hydraulic actuator for the piston-diaphragm arrangement to permit separate adjustment of the upstroke and the down stroke i.e. the pulsion phase of the pulse and the settling phase of the pulse.

The hydraulic actuator may include at least two hydraulic pistons and a hydraulic fluid accumulator, wherein the accumulator provides damping of the down stroke and shortening of the up stroke. Furthermore, the upstroke piston may have a smaller surface area than the down stroke piston thereby to provide for a difference between the upstroke and the down stroke.

Typically, the pulse can be adjusted by the hydraulic flow to between 60% and 70% bed dilation i.e. increased bed depth in relation to a compacted bed.

The pulse frequency may typically be from 0.75 Hz to 5 Hz, usually 1 Hz to 2 Hz, however, 1.5 Hz has been found to be particularly useful for ferrochrome separation.

The hydraulic actuator may be a hydraulic motor driving a crankshaft which is operatively connected to the piston-diaphragm arrangement.

The speed of the hydraulic motor may be adjusted during a single cycle to provide upward and downward strokes of differing characteristics thereby to provide any desired stroke wave pattern. The amplitude of the wave may be adjusted by adjusting the stroke on the crankshaft and thus the upward and downward strokes may be set accurately.

The hydraulic actuator may be in the form of one or more hydraulic cylinder, in which case the amplitude adjustment is achieved by oscillating the cylinder movement between adjustable limit switches. The adjustment of the jig stroke wave pattern permits optimisation of the jig for all size and density ranges of materials to be separated.

The piston-diaphragm arrangement may include two resiliently deformable diaphragm elements spaced to define a cavity in which the displacement member which forms part of the piston-diaphragm arrangement operates.

The diaphragms may be of different surface area. Typically the lower diaphragm has a surface area from 0,1 % to 20% larger than the upper diaphragm, usually from 5% to 15%, and in one particular embodiment, 10%.

Thus, when the jig is in use the difference in diaphragm surface areas permits a constant flow of fluid in the jig in the upward direction, even during the down stroke. The magnitude of the flow in the upward direction during the downstroke may be related to the difference in surface area between the diaphragms.

Alternatively, additional fluid may be recirculated during the down stroke so as to effect the constant upward flow even during the downstroke.

The jig may include a screen which operates below the fluid level, the screen being made from L-shaped members with the apex facing upwards, the L-shaped members being spaced apart by from 0.25 to 1.5 of the plan view width of adjacent L-members in the screen.

The L-shaped members may be spaced apart by 50% of the plan view width of adjacent L-members in the screen so that the gap between the L-members is half the width of any one L-member.

The jig may have plurality of screens located at different levels in the fluid level.

In particular the jig may include an upper screen and a bottom screen.

According to a further aspect of the invention, a fraction material is allowed to flow from the chamber of the jig, as described above, on or above the level of the bottom screen of the jig chamber into a cylindrical compartment. The cylindrical compartment may be situated at the weir and form part of the weir and be connected to the jig chamber by passageways of sufficient size to allow particles of a desired size to pass therethrough

A screw manufactured from a suitable material is fitted into the cylindrical compartment to form a screw conveyor form conveying material deposited into the compartment to a suitable container

The screw may be manufactured from a plastics material The screw may be of polyurethane

A balanced water level may be maintained in the container to equalise the water pressure

The rate of extraction may be controlled by controlling the screw conveyor speed

When the screw conveyor is shut down then the passageways leading to the screw conveyor are blocked

Typically, the extraction rate is controlled by means of a float detector, or electronically by a management system

Brief Description Of The Drawings

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which

Figure 1 shows a perspective schematic view of a jig according to a first embodiment of the invention,

Figure 2 shows a plan view of the jig according to the embodiment shown in Figure 1 ,

Figure 3 shows a partial cross-section schematic side view of the jig taken along line Ill-Ill in Figure 2,

Figure 4 shows a partial cross-section schematic end view of the jig taken along line I V-IV shown in Figure 2, Figure 5 shows a plan view of a jig according to a second embodiment of the invention,

Figure 6 shows a partial cross-section schematic side view of the jig taken along line VI-VI in Figure 5,

Figure 7 shows a partial cross-section schematic end view of the jig taken along line VII- VII in Figure 5, and

Figure 8 shows, in schematic side view, the hydraulic actuator arrangement of the jig

Embodiments of the invention are described in detail in the following passages of the specification which refer to the accompanying drawings The drawings, however, are merely illustrative of how the invention might be put into effect, so that the specific form and arrangement of the features shown is not to be understood as limiting on the invention

Description Of The Embodiments

Figure 1 shows a jig 10 for separating particles with different densities from a feed material The size of the jig may vary depending on the application, and in the embodiments shown in the Figures the jig is portable and is bolted to a metal frame 12 Typically a discharge chute 14 discharges a particulate material onto a pair of parallel conveyor belts 16 The conveyor belts discharge the feed material into a feed end 18 of the jig The material moves from the feed end towards a discharge end 20 in the direction indicated by the arrow "A" The embodiments described below relate to through the bed jigging, but the jig can also be used for on the bed jigging

The jig comprises a rectangular settling vessel 22 which has four steel side walls 24 and an open bottom and top The side walls define a settling chamber 26 The settling vessel is mounted on the frame 12 by means of supports 28 The settling vessel includes a plurality of parallel steel cross members 30 which provide structural stability and support a screen 32 The screen is not shown in Figures 1 , 2 and 5, but is shown in Figures 3, 4, 6 and 7 It should be understood that the screen could be supported by other means, including clips conn4cted to the walls of the settling vessel The screen 32 divides the settling chamber 26 into a jig chamber 36 and a hutch 38 The size of the apertures in the screen is selected according to the particular application The screen can be easily removed and replaced by a screen with another aperture size. The settling vessel and cross members could be made from another suitable material including a thermoplastics material.

In use, the screen supports ragging 34 which is again shown only in Figures 3, 4, 6 and 7. In the embodiment shown in these Figures, there is a homogeneous bed ragging. It has been found that for the separation of particles of differing densities, optimum separation can be achieved by dividing the ragging bed into sections. In one embodiment the ragging bed is divided into three sections (not shown). The first section, closest to the feed end of the jig, comprises no ragging particles and is used for pre-stratification of the bed. In this first section, a second screen having a very small aperture size may be positioned above the screen 32 in order to support the pre-stratified bed of particles. The second section comprises a ragging bed having a relatively high density, this allows the relatively heavy particles to settle through the ragging bed in this second section. The third section, closest to the discharge end of the jig, comprises a ragging bed having a relatively low density, this allows relatively light particles to settle through the ragging bed in this third section. The ragging bed may be divided into the sections by means of spacers positioned above the screen 32. The length of the sections can be varied depending on the application. If the screen is in fact supported below the cross members 30, as described above, then the cross members themselves may provide the spacers. The manner in which the settled particles are collected and removed from the jig is described in more detail later in the specification.

As can be seen more clearly in Figures 4 and 7, located below the settling vessel 22 is a displacement member in the form of a rectangular steel tank 40 having four side walls 42, an open bottom and top wall defining a particle collection formation. In the embodiment shown in Figures 1 to 4 the particle collection formation is in the form of an undulating sheet 44 which define parallel peaks 46 and troughs 48. The number of undulations will depend on the size of the jig. In an alternative embodiment shown in Figures 5 to 7, the particle collection formation is in the form of six recesses 50 in a sheet. Various embodiments of the particle collection formation are described in more detail later in this specification.

The steel tank 40 is shaped to be received in the hutch 38 and is movable vertically in the hutch. The top edges of the side walls 42 of the steel tank are connected to a rectangular, rubber upper diaphragm 52 which defines a frame around the steel tank and, in turn, is connected to the bottom edges of the side walls 24 of the settling vessel. The upper diaphragm therefore ensures that the displacement member is in sealed engagement with the settling vessel and is movable vertically in the hutch The particle collection formation effectively defines at least a portion of the base of the settling vessel The steel tank 40 is pivotally connected to the settling vessel 22 by means of pivot arrangement 54 The pivot arrangement is in turn connected to a hydraulic actuator (not shown) which causes the steel tank to move relative to the settling vessel in a reciprocal direction indicated by the arrow "B"

The frame includes a fluid reservoir in the form of a sump 56 which is made from metal and is typically full of water Surplus water from the discharge end 20 is recycled to the sump as is described in more detail later in this specification The sump includes support struts 58 welded across the sump in order to provide structural support for the sump and the frame Holes in the struts allow water to circulate through the sump As can be seen more clearly in Figures 3, 4, 6 and 7, in a preferred embodiment of the invention the reservoir comprises also risers in the form of eight hollow metal cylinders 60 which are located in the sump There could be any number of cylinders provided, depending on the application and size of the jig The lower ends 62 of the cylinders are open to allow fluid to enter the cylinders The top ends 64 of the cylinders are welded to the top wall 66 of the sump Located in the top wall of the sump, and within the top end of each of the cylinders, is a make-up valve in the form of a non-return valve 68 It should be understood that the sump could operate without the risers

An expansion formation in the form of a rectangular, rubber lower diaphragm 70 is connected to the bottom edges of the side walls 42 of the steel tank 40 The lower diaphragm 70 defines a frame around the steel tank and, in turn, is connected to the top wall 66 of the sump As can be seen more clearly in Figures 3 and 6, the lower diaphragm 70 has a length which is greater than that of the upper diaphragm 52 The effect of this additional length is described in more detail in this specification

The expansion formation defines an expansion chamber 72 between the bottom of the particle collection formation and the top wall 66 of the steel tank In use the expansion formation is full of water The non-return valves 68 allow water to move from the sump 56 into the expansion chamber 72, and prevent water from moving in the opposite direction

The particle collection formation will now be described in more detail Referring firstly to the embodiment shown in Figures 1 to 4, the troughs 48 are designed to collect the material which settles through the screen 32 and into the hutch 38 This is shown more clearly in Figure 4 Each peak 46 includes a transfer valve in the form of a flap-valve 74 located along its length The flap-valves allow water to move from the expansion chamber 72 into the hutch 38, and prevent water from moving in the opposite direction

In each of the four troughs 48 there is located a first extraction auger 76 and a second extraction auger 78 The first extraction augers 76 extend longitudinally inwards from the feed end 18 of the jig The second extraction augers 78 extend longitudinally inwards from the discharge end 20 of the jig The extraction augers are designed to extract the particles which collect in the troughs Typically the heavier particles which settle in the troughs closer to the discharge end are extracted from the jig by means of the second extraction augers 78 The length of the extraction augers 76 and 78 can thus be varied in order to achieve the desired split of settled particles The first extraction augers 76 discharge the settled particles into a first discharge tube 80 A first discharge auger (not shown) is located in the first discharge tube and withdraws the settled particles from the jig Similarly, the second extraction augers 78 discharge the settled particles into a second discharge tube 82 A second discharge auger (not shown) is located in the second discharge tube and withdraws the settled particles from the jig

Referring now to the embodiment of the particle collection formation shown in Figures 5 to 7, the six recesses 50 in the sheet are designed to collect the material which settles through the screen 32 and into the hutch 38 This is shown more clearly in Figures 6 and 7 Each recess includes a plurality of transfer valves in the form of flap-valves 74 located at regular intervals over the sheet The flap-valves allow water to move from the expansion chamber 72 into the hutch 38, and prevent water from moving in the opposite direction

At the lowest point of each recess 50 there is an orifice The particles which settle through the hutch collect in the recesses and are discharged through the orifice The orifice is connected to a T-piece which in turn is connected to a pipe (not shown) The pipes are connected to a discharge pump (not shown) which pumps the particles which settle in the recess out of the jig Just as with the arrangement of first and second augers described above, each orifice could be connected to a separate pump thus allowing the particles which collect in each recess to be removed from the jig separately

The jig comprises further a discharge mechanism in the form of a water wheel 84 which rotates in the direction indicated by the arrow "C" The water wheel is located at the discharge end 20 of the jig In use material flows from the feed end 18 towards the discharge end 20 and overflows a weir 86 in the settling vessel. The material which overflows the weir is caught in buckets 88 on the water wheel. Each bucket includes a screen (not shown) which allows water to drain from the bucket and return to the sump. The particles are retained in the bucket and are discharged into a launder 90 from which they are transported.

The operation of the jig will now be described with reference to the embodiment shown in Figures 1 to 4. As can be seen most clearly in Figure 4, in a first step the displacement member moves into an extended position within the hutch 38 so that the hutch defines a contracted volume and the expansion chamber 72 defines an expanded volume. The flap-values 74 are closed and, accordingly, the movement of the undulating sheet 44 causes an upward movement of the water in the settling vessel in the form of a propulsion stroke. The non-return valves 68 are open and, accordingly, make-up water from the sump is introduced into the expanded volume of the expansion chamber.

In a second step (not shown) the displacement member moves fromteh extended position to the retracted position such that the hutch defines an expanded volume and the expansion chamber defines a contracted volume. In this second step, flap-valves 74 are open and the non-return valves 68 are closed. As the undulating sheet 44 moves downward towards the top wall 66 of the sump, the water which was in the expansion chamber is transferred into the settling vessel. Therefore, although the displacement member moves through a reverse stroke, no suction stroke is created in the settling vessel as there is an inflow of water.

It should be noted that because the lower diaphragm 70 is longer than the upper diaphragm 52, the change in volume in the hutch when the displacement member moves between the retracted position and the extended position (and vice versa) is smaller than the change in volume in the expansion chamber for the corresponding movement of the displacement member. As a result, the volume of water which is introduced into the hutch from the expansion chamber when the displacement member moves from the extended position to the retracted position (i.e. the second step described above), is larger than the volume created within the hutch by the movement of the displacement member. Accordingly, even in the second step, there is a resultant upward movement of water in the settling vessel. Although it is not shown here, it is possible to include a valve on the side of the displacement member which allows a certain volume of the water in the expansion chamber to be discharged from the expansion chamber so that the amount of water which is introduced into the hutch can be controlled. In Figure 8, reference numeral 100 indicates a jig having a hydraulic actuator arrangement in the form of a hydraulic cylinder 101 , a hydraulic accumulator cylinder 102 and a hydraulic accumulator (not shown).

The hydraulic cylinder 101 has an inlet port 109 through which allows hydraulic fluid to be induced into said cylinder thereby urging lever 106 to its lowermost position, simultaneously lever 5 is drawn down by connecting link 103 and during this operation the hydraulic oil in cylinder 102 is transferred through inlet port 110 to the accumulator.

The hydraulic oil circuit is switched by adjustable limit switches 111 and oil is induced to port 8 thereby moving the levers 5, 6 to their uppermost position. This fast movement is aided by the accumulated oil from the accumulator through port 10.

The amplitude of the movement of the levers 5,6 can be adjusted by adjustment of the limit switches while the frequency of the movement can be adjusted by changing the oil flow rate through the cylinder 101.

Power is transferred to the displacement member 112 of the piston-diaphragm arrangement 113 by connecting members 107.

Claims

1 A jig for separating particles with different densities from a feed material, the jig comprising a settling vessel defining a settling chamber adapted to receive the feed material, a displacement member in substantially sealed engagement with the settling vessel thereby defining at least a part of a base of the settling vessel, an expansion formation in sealed engagement with the displacement member and defining and expansion chamber adapted to receive a fluid and in fluid communication with the settling chamber, wherein the displacement member is movable between an extended position wherein the settling chamber defines a contracted volume and the expansion chamber defines an expanded volume, and a retracted position wherein the expansion chamber defines a contracted volume and the settling chamber defines an expanded volume, and a fluid reservoir in fluid communication with the expansion chamber, such that in use when the displacement member moves from the retracted position to the extended position fluid from the expansion chamber is transferred to the settling chamber, and when the displacement member moves relative to the expansion formation from the extended position to the retracted position fluid is introduced into the expansion chamber from the fluid reservoir

2 A jig as claimed in claim 1 , wherein the displacement member includes at least one transfer valve having an open condition wherein the settling chamber is in fluid communication with the expansion chamber, and a closed condition wherein the settling chamber is substantially sealed from the expansion chamber, and wherein the transfer valve is movable from the open condition to the closed condition corresponding to movement of the displacement member from the retracted position to the extended position

3 A jig as claimed in claim 1 or claim 2, wherein the fluid reservoir includes at least one make-up valve having an open condition wherein the fluid reservoir is in fluid communication with the expansion chamber, and a closed condition wherein the fluid reservoir is substantially sealed from the expansion chamber, and wherein the make-up valve is movable from the open condition to the closed condition corresponding to movement of the displacement member from the extended position to the retracted position.

4. A jig as claimed in any one of the preceding claims, wherein the displacement member includes a particle collection formation.

5. A jig as claimed in claim 4, wherein the particle collection formation includes the at least one transfer valve.

6. A jig as claimed in any one of the preceding claims, including a discharge mechanism including fluid separation means, wherein the fluid separation means is in fluid communication with the fluid reservoir.

7. A jig for separating particles with different densities from a feed material, the jig comprising: a settling vessel defining a settling chamber adapted to receive the feed material; a displacement member in substantially sealed engagement with the settling vessel thereby defining at least a part of a base of the settling vessel; an expansion formation in sealed engagement with the displacement member and defining an expansion chamber adapted to receive a fluid and in fluid communication with the settling chamber; and wherein the displacement member is movable between an extended position wherein the settling chamber defines a contracted volume and the expansion chamber defines an expanded volume, and a retracted position wherein the expansion chamber defines a contracted volume and the settling chamber defines an expanded volume; such that the change in volume in the settling chamber when the displacement member moves between the extended position and the retracted position is smaller than the change in volume in the expansion chamber for the corresponding movement of the displacement member.

8. A jig of the type having a bed on which particulate materials of different densities are stratified and advanced through the jig for extraction of a desired density fraction, the stratification being performed in a fluid medium which is pulsed substantially perpendicularly to the passage of the particulate material over the bed, the pulsing being effected by a piston-diaphragm arrangement in which the suction stroke is eliminated and replaced by a hindered settling phase.

9 A jig as claimed in claim 8, wherein the piston-diaphragm arrangement is provided with a displacement member driven by a hydraulic actuator to permit separate adjustment of the upstroke and the down stroke of the piston-diaphragm arrangement

10 A jig as claimed in claim 9, wherein the hydraulic actuator includes at least two hydraulic pistons and a hydraulic fluid accumulator, wherein the accumulator provides damping of the down stroke and shortening of the up stroke

11 A jig as claimed in claim 10, wherein the upstroke piston has a smaller surface area than the down stroke piston thereby to provide for a difference between the upstroke and the down stroke

12 A jig as claimed in any one of claims 8 to 1 1 , wherein the pulse is adjusted by the hydraulic flow to between 60% and 70% bed dilation thereby to increase bed depth in relation to a compacted bed

13 A jig as claimed in any one of claims 8 to 12, wherein the pulse frequency is from

0 75 Hz to 5 Hz

14 A jig as claimed in any one of claims 8 to 12, wherein the pulse frequency is from

1 Hz to 2 Hz

15 A jig as claimed in any one of claims 8 to 12, wherein the pulse frequency is 1 5 Hz in the case of ferrochrome separation

16 A jig as claimed in any one of claims 9 to 15, wherein the hydraulic actuator is a hydraulic motor driving a crankshaft which is operatively connected to the piston- diaphragm arrangement

17 A jig as claimed in any one of claims 8 to 16, wherein the speed of the hydraulic actuator is adjustable during a single cycle to provide upward and downward strokes of differing characteristics thereby to provide any desired stroke wave pattern

18 A jig as claimed in claim 17, wherein the amplitude of the pulse wave is adjusted by adjusting the stroke on the crankshaft and thus the upward and downward strokes may be set accurately

19. A jig as claimed in claim 17 or claim 18, wherein the hydraulic actuator is in the form of one or more hydraulic cylinder, in which case the amplitude adjustment is achieved by oscillating the cylinder movement between adjustable limit switches.

20. A jig as claimed in any one of claims 9 to 19, in which the piston-diaphragm arrangement includes two resiliently deformable diaphragm elements spaced to define a cavity in which the displacement member which forms part of the piston-diaphragm arrangement operates.

21. A jig as claimed in claim 20, wherein the diaphragms are of different piston equivalent surface area.

22. A jig as claimed in claim 21 , wherein the lower diaphragm surface area is from 0,1% to 20% larger than the upper diaphragm surface area.

23. A jig as claimed in claim 21 , wherein the lower diaphragm surface area is from 5% to 15% larger than the upper diaphragm surface area.

24. A jig as claimed in claim 21 , wherein the lower diaphragm surface area is 10% larger than the upper diaphragm surface area.

25. A jig as claimed in any one of claims 8 to 24, including a screen which operates below the fluid level, the screen being made from L-shaped members with the apex facing upwards, the L-shaped members being spaced apart by from 0.25 to 1.5 of the plan view width of adjacent L-members in the screen.

26. A jig as claimed in any one of claims 8 to 24, including a screen which operates below the fluid level, the screen being made from L-shaped members with the apex facing upwards, the L-shaped members being spaced apart by 50% of the plan view width of adjacent L-members in the screen so that the gap between the L-members is half the width of any one L-member.

27. A jig as claimed in claim 25 or claim 26, wherein the jig has a plurality of screens located at different levels in the fluid level. 28 A jig as claimed in claim 27, wherein the jig includes an upper screen and a bottom screen

29 A method of operating a jig, as described in anyone of the preceding claims, wherein a fraction of material is allowed to flow from the chamber of the jig on or above the level of the bottom screen of the jig chamber into a cylindrical compartment.

30 A method as claimed in claim 29, wherein the cylindrical compartment is situated at the weir and form part of the weir and be connected to the jig chamber by passageways of sufficient size to allow particles of a desired size to pass therethrough

31 A method as claimed in claim 30, wherein a screw manufactured from a suitable material is fitted into the cylindrical compartment to form a screw conveyor form conveying material deposited into the compartment to a suitable container

32 A method as claimed in claim 31 , wherein the screw is manufactured from a plastics material

33 A method as claimed in claim 32, wherein the screw is made of polyurethane

34 A method as claimed in claim 31 , wherein a balanced water level is maintained in the container to equalise the water pressure in the jig

35 A method as claimed in claim 31 , wherein the rate of extraction is controlled by controlling the screw conveyor speed

36 A jig, substantially as herein described and illustrated with reference to the drawings

37 A method, substantially as herein described and illustrated

38 A new jig or a new method, substantially as herein described