BACKGROUND Jigging is widely used to separate mineral materials into fractions of different densities. A typical jig consists of a jig chamber with a screen bottom. Feed apparatus introduces material to be separated at one end of the screen and discharge means removes the stratified fractions at the other end. A space underneath the screen is called a hutch in which apparatus generates a pulsation in water that fills the hutch and the stratifying space above the screen.

The method is to feed the material onto the feed end of the screen to form a bed on the screen, while causing a pulsation in the water. The upwards and downwards movement of the pulses cause the bed to stratify with denser particles in lower strata and lighter particles in higher strata, as the material moves over the screen towards the discharge end. The lower fractions are removed by discharging through a discharge gate or gates located low down or pumped from below the screen and the lighter fractions, commonly the tailings, are discharged over a weir, thus effecting the separation.

On the bed jigging is typically used for separation of course particles. The apertures of the screen are finer than the finest particles of the material to be separated thus preventing the particles from falling into the hutch. The discharge gate located at or close to the screen, is adjusted to control the rate of discharge of the denser fraction, according to the degree of stratification as the pulsing of the water proceeds. The adjustment is usually automated by

mechanically or electronically tracking the height of the interface between the denser and lighter fractions. Severe problems often arise in this control function, due to obstruction of the gate movement caused by particles in the gate. Also the pulsation through the bed in the vicinity of the gate tends to result in a flow of less dense material from the upper strata of the bed through the gate. This results in a poorer separation despite adequate stratification ahead of the gate.

Through the bed jigging is typically used where only slight density differences occur and is usually more suitable for finer particles. In through the bed jigging the apertures in the screen are larger than the particles of the material to be separated. A layer of particles, which are larger than the apertures is located on the screen and commonly referred to as ragging. The ragging particles are just fractionally less dense than the denser particles to be separated. While the pulsing of the water proceeds, the denser materials work their way through the ragging into the hutch, from which they are normally removed by pump.

A problem encountered in both types of jigging, is that the upward phase of the pulsation is followed by a downward phase, commonly called a propulsion phase and a suction phase. The repeated propulsion phases of the pulsation produce a dilation of the bed, which enhances the stratification of the bed, but the interposed suction phases produce a settling of the bed. The stratification of the bed tends to be counteracted during the settling phases when the less dense particles tend to be sucked back between the denser particles.

In through the bed jigging the less dense materials tend to be sucked through the ragging. This problem occurs even when a so called neutral hutch water pressure is maintained, by introducing water into the hutch to compensate for water losses, which arise when removing the denser fraction from the hutch chamber.

Many different pulsation wave forms have been tried to reduce the suction through the bed. In some designs rotary valves have been installed to pump water into the hutch chamber at timed intervals when the suction stroke is taking place. The energy transfer efficiency of centrifugal pumps, however, is very much lower than the efficiency of energy transfer from reciprocating diaphragms usually used to produce the pulsation. Complex control systems, often pneumatic control systems, have been used. Where the suction phases have been made less energetic, the settling phases still remain interposed between the propulsion phases.

German Patent 887 181 issued in 1953, despite being an early proposal and using mechanical cranks to reciprocate pistons, is interesting in proposing a compensation of the suction effect that can be an exact volumetric displacement or an under-or over-compensation (page 3, lines 16 to 27). The action according to this invention is thus a series of positive pulses (propulsions) interposed by quiescent stages (wherein the suction is exactly compensated).

German Publication Document DE 33 19 535 A1 published in 1984 proposes an hydraulic actuation that produces the propulsion phase, via a rather unusual tubular membrane, but it achieves no better resolution of the suction problem than a slow settling or a pause phase followed by a short, sharp suction phase (page 8, lines 6, 7, 17, 22, 23 and 27). The proposal is, however, interesting in that the membrane, together with a diffuser, provides a uniform flow rate distribution across the bed (page 6, lines 12 to 14 and 17).

PCT patent application PCT/ZA01/00008, published 1 August 2001, by one of the present inventors proposes an hydraulically driven displacement member that forms a base of the settling vessel (i. e. hutch) above and an expansion chamber below (page 3, lines 21-23). The displacement member has a lower diaphragm that is longer than an upper one and one-way valves, so that on downward movement of the member, water is propelled from the expansion chamber to the hutch and a propulsion pulse through the bed is produced

(page 12, lines 25 to 33). On upward movement of the displacement member a propulsion pulse through the bed is also produced and an ongoing sequence of propulsion pulses is produced in the bed.

However, due to engineering disadvantages development work has diverted away from this design.

THE INVENTION An apparatus for wet jigging in accordance with this invention, in which particulate material to be separated is fed to a feeder end of a jigging screen forming a bottom of a jigging chamber and stratified material is removed at the other end of the screen and/or from a hutch chamber below the screen, comprises a positive displacement mechanism including one selected from two actuators or a single double-acting actuator executing reciprocating strokes in at least two pumping chambers adjacent the hutch chamber, with one-way valve arrangements communicating between the pumping chambers and the hutch chamber, to deliver successive positive pulses in water into the hutch chamber and thence upwardly through the jigging screen, the pulse velocity following a wave form exclusively in an upward direction, varying cyclically between zero and a maximum and one-way valves for drawing water into the pumping chambers.

The actuator of the apparatus can conveniently comprise a diaphragm separating two pumping chambers and reciprocating so as to alternately enlarge and decrease the chambers on either side of it. The chambers each have inlet valves and outlet valves, the former drawing water into the chambers and the latter delivering water to the hutch below the screen to create to repetitive upwards water pulses through the screen.

The diaphragm can be a rigid plate with flexible sealing edges, which is a construction that allows a large diaphragm area and relatively short stroke, giving a structure, which is economical to build and operate.

Reciprocating piston and cylinder arrangements are technically possible, within the scope of the invention, for example a double acting piston working in a cylinder with inlet and outlet valves at both ends, but probably more costly, both to manufacture and maintain.

As mentioned, the positive displacement mechanism has a high efficiency of transfer of driving energy to the pulse energy.

The use of two pumping chambers allows the pulses produced in them to be 180 degrees out of phase with each other, so that while one pauses between delivering its pulses, the other supplies pulses. This achieves a wave form for the upwardly directed pulses through the screen which has no extended quiescent periods, during which the dilated bed may settle significantly. For example, if the actuator is moved in essentially in simple harmonic motion, the sine wave form of the combined pulses will have a brief moment at zero, between positive pulses.

If more than two pumping chambers is used, the advantage can be achieved of a combined wave-form in which there is no moment at which upward flow velocity is zero. An oscillating flow velocity is realised which never touches zero, for example where there are three or four chambers. Increasing the number of chambers provides an increasingly smooth profile to the combined wave form.

In this way a method according to the invention is realised, in which there is no reverse or suction flow through the screen, eliminating the adverse effects of suction flow on the stratification process.

Furthermore, the important advantage is realised that the rate of stratification is enhanced since the quiescent periods encountered in the prior art, during which the bed settles are eliminated or severely curtailed. Pulsating bed

dilation and active stratification is maintained essentially continuously, greatly increasing the rate of stratification, in comparison with the art.

The problems associated with the gate or gates for the heavier fractions in on the bed jigging, are solved in accordance with the invention. According to the invention, the gate (s) is/are set at a level, according to the height of the interface between the heavier fraction and the lighter fractions in the stratified bed. This height can be monitored during jigging and the gate height is only adjusted to accommodate any change in this height, the gate is not used to control the discharge of the heavier fraction. According to the invention, the control of discharge is effected by means of an underflow chute that communicates with the gate and provides a delivery channel for delivery of the separated heavier fraction. The underflow channel can be closed off from the channel for delivery of the fraction. When closed off a"dead space"is created communicating with the gate, that is, the pulses near the discharge end of the bed cannot extend through the gate since the underflow channel is closed and full of water. This has the result that no discharge through the gate occurs. When the underflow channel is opened, the dead space is opened and the pulses divert through the gate, bringing about discharge of the heavier fraction through the gate into the underflow channel to the delivery channel.

A method of wet jigging in accordance with this invention, in which particulate material to be separated is fed to a feeder end of a jigging screen forming a bottom of a jigging chamber and stratified material is removed at the other end of the screen and/or from a hutch chamber below the screen, comprises generating successive positive flow pulses in water by positive displacement of water in a pumping chamber or chambers, transmitting the pulses through one-way valves from the pumping chamber (s) into a hutch chamber, directing the pulses thence upwardly through the jigging screen, the pulse velocity following a wave form exclusively in an upward direction, varying cyclically between a lower value and a higher value.

There is thus no pulse flow in a downward direction, or suction, according to the invention. The adverse effects of suction on the bed are eliminated, according to the invention.

In one embodiment of the invention, the lower value can be close to or at zero, for a very brief period, between pulses. In a preferred embodiment, the lower value is at an intermediate value, between zero and the upper value.

The rest periods between pulses can be made very brief, a very small fraction of the period occupied by the pulses. This reduces the quiescent periods between pulses, a pulsing dilation of the bed is maintained and the rate of stratification of the bed is enhanced, with significant economic benefits.

THE DRAWINGS The invention will be more fully described by way of non-limiting example, with reference to the accompanying drawings, in which:- figures 1 and 2 are simplified schematic drawings of apparatus in accordance with a preferred embodiment of the invention, figure 1 being a side elevation, and figure 2 being a plan view, figure 1A is an enlarged portion of a preferred screen for the apparatus, figure 1 B shows a preferred one-way valve, figure 3 is a side elevation of an installation for screening which includes the apparatus shown in figures 1 and 2,

figure 4 is a plan view of the apparatus shown in figure 3, figure 5 is an end elevation of the apparatus shown in figure 3, figure 6 is an isometric view of a jigging apparatus according to the invention, figure 7 is an isometric view of pumping chambers and hutch of the apparatus shown in figure 6, figure 8 is an isometric view of pumping chambers and hutch of the apparatus shown in figure 7, with a wall removed for convenience of illustration, figure 9 is side elevation of a section of the pump chambers and hutch of the apparatus shown in figure 6, figure 10 is an isometric view of the actuator used in the apparatus shown in figure 6, figures 11 and 12 are graphs of a wave form implemented by the apparatus. and figure 13 shows an alternative positive displacement pump.

THE PREFERRED EMBODIMENTS As shown in figures 1 and 2, the apparatus 1 is of the wet jigging type defining a jigging chamber 2, a screen 3 which forms the bottom of the jigging chamber and a hutch 4 below the screen. Particulate material to be separated is fed onto the screen at the position indicated at 5. If the apparatus is set up for on the bed jigging, the denser fraction of the stratified material is removed with water by means of a gate or gates at the other end indicated by the

arrow 6. The tailings and water pass over the weir 7. If the apparatus is set up for through the bed jigging, the heavier fraction of the stratified bed is collected from the hutch by means of an auger 8 located in a sump or collection trough 9 of the hutch. It will be clear that separated material collected in the trough 8 could be removed by other means, such as a pump, for example.

According to this embodiment of the invention, two pump chambers 10 and 11 are provided in a housing 12 which has an actuator in the form of a rigid diaphragm 13 that has a flexible surround 14 joining it to the housing. The diaphragm is reciprocated by suitable mechanical or hydraulic equipment, connected to the diaphragm by a rod 28. Each chamber has an inlet one-way valve 15 and 16 respectively, communicating with an inlet manifold 17, which supplies water to the pumping chambers of the apparatus. Each chamber also has a one-way outlet valve 18 and 19 respectively, delivering pulsed water to a pre-chamber 20 under the hutch.

The one-way valves are conveniently made in the form of flap valves, using a rubber flap over a perforated wall. Figure 1A shows a preferred form, according to the invention, of these flap valves. Each of the valves comprises a hole 36 (or series of holes) in the steel plate, which forms the wall between the pump chambers and the pre-chamber. A rubber flap 38 is fixed to the wall at one edge of the hole (s) and resiliently closes onto the wall around the edges of the hole. The flap opens for flow from the pump chambers into the pre-chamber but close to prevent reverse flow. This arrangement of an upper edge fixed to the wall gives a directed flow downwardly and with minimally accelerated velocity, to avoid forming a current in the water that would cause an aggravated disturbance in the bed on the screen.

The diaphragm is double acting in that it is an actuator for both chambers, enlarging one as it decreases the other. The delivery of pulses from the two chambers is thus 180 degrees out of phase with each other (taking one

complete cycle as represented by 360 degrees). While one is not delivering, but drawing water in from the manifold, the other is delivering, and vice versa.

The pre-chamber is divided from the hutch by an inclined sieve 21. The pre- chamber and inclined sieve 21 are designed to deliver the pulsed water flow evenly and free of turbulence or focused currents to the screen 3. The aim is that at any given moment an essentially constant upward water flow is engendered over the whole area of the screen 3. It will be clear that alternative mechanical arrangements could be used for this purpose, such as baffles, for example.

Water from the delivery end of the apparatus is recycled to the pump chambers, suitably processed and supplied with make up water as necessary.

The two pump chambers can be duplicated by the same arrangement alongside them. The diaphragm in the second pair of pump chambers can be reciprocated 90 degrees out of phase with the reciprocation of the diaphragm in the first pair of pump chambers.

Figure 1A shows a preferred sieve for the division between the pre-chamber and the hutch. It is an angled wedge wire screen with a fine aperture, for example an aperture size less than 1 mm, such as 0,4mm. The angling of the edge wire results in the water pulses being directed at about 20 degrees to the right of the orthogonal direction to the sieve surface, as indicated at 22 and the arrows 23. This distributes the pulses evenly over the jigging screen 3 and without turbulence.

This type of angled wedge wire can also be used for the jigging screen, for on the bed jigging. The screen wedge wires are angled towards the discharge end of the screen, which gives a component of direction of the impulses towards that end, to move the particulate material of the bed towards the discharge.

The discharge is controlled by an underflow chute 24 which can be opened or closed, leading to a delivery chute 25, which in this example leads to the sump or collection trough. The gate 26 is kept at a selected height according to the height of the interface between the heavier fraction and the lighter fractions. Although the gate can be adjusted if this height varies and it establishes an upper limit to the maximum rate at which discharge can occur, it plays no role in starting or stopping the discharge. The starting or stopping of the discharge is controlled, according to the invention, by a valve 27 which allows water to discharge into the underflow chute or not. When not allowed, the space at the gate valve becomes"dead", i. e. there is no water movement and so the discharge of particles is stopped. This is achieved without closing the gate. When allowed to discharge, the particles and water fall down the discharge chute and into the collection trough where the auger removes them. At the bottom of the discharge chute there is a one-way valve 35, to prevent an upward flow of water up the chute, which could occur as a result of the water pulses. No make-up water therefore need be added during this discharge. Control of discharge in this way, without lowering the gate, eliminates the severe problems encountered with discharge control up to now.

Figures 3 to 5 show the apparatus 29 of the invention applied in an installation, which comprises a receiving bin 30, a vibratory feeder 31 which feeds into the feed end 32 of the apparatus. The tailings pass to a dewatering screen 33 to separate the water and the tailings thence to a delivery chute 34.

The tailings could be very small compared with the volume of water, like diamonds separated from the ore, for example.

Figures 6 to 10 show a further embodiment of the invention. The apparatus 60 for wet jigging is incorporated in process equipment that supplies material to be separated at a feed end 61 and delivers separated product at 62. The jigging screen is indicated in figure 9 by broken lines 63, located in a housing, the space above the screen forming the jigging chamber 64. Stratified

material is removed at the end 62 of the screen and/or from a hutch chamber 65 below the screen. There are two pumping chambers, acting in unison, each comprises a positive displacement mechanism including a single double acting actuator 66 (figure 10) executing reciprocating strokes. The two pumping chambers are adjacent the hutch chamber, with one-way valves located in holes 67 and 68 communicating between the pumping chambers and the hutch chamber, to deliver successive positive pulses in water into the hutch chamber and thence upwardly through the jigging screen.

The actuator of the apparatus comprising the diaphragm separates each of the two pumping chambers and while reciprocating alternatingly enlarges and decreases the chambers above and below it. The chambers each have inlet valves at holes 69 and 70, drawing water into the chambers and the valves at holes 67 and 68 delivering water to the hutch below the screen to create to repetitive upwards water pulses through the screen.

The diaphragm 66 can be a rigid plate (figure 10) with flexible sealing edges in the form of rubber strips (not shown), which is a construction that allows a large diaphragm area and relatively short stroke, giving a structure, which is economical to build and operate. A boss 72 allows connection of the driving piston of hydraulic equipment that reciprocates the diaphragm.

As mentioned, the positive displacement mechanism has a high efficiency of transfer of driving energy to the pulse energy.

The use of two pumping chambers could allow the diaphragms to be single acting, the pulses produced in them to be 180 degrees out of phase with each other, so that while one pauses between delivering its pulses, the other supplies pulses. This example, however, has two double acting diaphragms acting in unison, as mentioned, having two reduces the forces required for each diaphragm.

The wall 71 divides the jigging chamber and the hutch into two, side by side, each served by one of the two pumping chambers.

In this way a method according to the invention is realised ; in which there is no reverse or suction flow through the screen, eliminating the adverse effects of suction flow on the stratification process.

Furthermore, the important advantage is realised that the rate of stratification is enhanced since the quiescent periods encountered in the prior art, during which the bed settles are eliminated or severely curtailed. Pulsating bed dilation and active stratification is maintained essentially continuously, greatly increasing the rate of stratification, in comparison with the art.

Figures 11 and 12 show a set of graphs which are useful to assist describe the wave pattern of pulses supplied in accordance with one embodiment of the method of this invention. They also help in making a comparison with the prior art pulsing.

Figure 11 shows the displacement of water through the bed, in the vertical axis. The horizontal axis is proportional to time, indicated for convenience by degrees related to each cycle and applies to both graphs.

Figure 12 shows the rate or speed of water flow through the bed, in the vertical axis, being the first differential of the displacement.

Successive positive pulses upwardly through the jigging screen can be indicated by the wave forms 41 and 42, over one and a half complete cycles nominally indicated by the degrees 0 to 360 plus 180. (If the pulsing is provided by a rotating crank which moves the actuator, the degrees indicated can relate to the crank rotational position. If the actuator is hydraulically driven the degree indication is nominal but the cycle indicated is repeated).

The wave form shown is sinusoidal which will apply approximately if the actuator is driven by a crank and connecting rod. However, different wave forms can be employed in the implementation of the invention, if the actuator is driven e. g. by hydraulic systems where the controls are suitably designed to provide a desired wave form.

The vertical axis on the graph shows that there are no"negative"or suction phases to the pulsation.

All these pulses are delivered through the jigging bed, but the pulses are cumulative, so they can be more correctly indicated by the graph line 41-43- 44 so that the total pulsation effected through the screen and bed is as indicated by the combined or cumulative wave form of successive pulses.

The pulses according to the wave forms 41 and 42 can be provided by the pair of chambers and single diaphragm shown in the preceding drawings (chambers 10 and 11 and diaphragm 13 in figure 1).

The dashed-line pulses 45 could be provided by a duplicated pair of pumping chambers, which could be provided alongside the existing chambers, operated 90 degrees out of phase (this is not shown in the drawings). This would provide a smoother force demand from the motor that drives the diaphragm in the pump chamber or other means of generating the pulses.

The corresponding energy pulses are shown by broken lines 54 in figure 7.

The graph 46,47 in figure 12 correspond to the graph 41,42 respectively in figure 11, on the same time scale. Figure 12 shows the speed of the pulses through the bed, as mentioned. Since the energy of the pulses is proportional to the square of the speed, the graphs 46,47 can also serve to indicate the energy of the pulses, on an appropriate scale.

In figure 11, the pulses according to the prior art are indicated by the dotted lines 48. It will be seen that after the positive (upward) pulse over the period

49, there is a rest period 50 followed by a negative (downward or"suction") pulse 51. The suction pulse is unavoidably long, to minimise the adverse effects of the suction pulse.

By comparison, the invention eliminates negative pulses altogether, but there is another critical advantage, namely that three positive pulses are put into the bed in the time that the prior art puts in only one. The rest periods 54 and 55 according to the invention are much shorter. So the rate of pulsing and hence of stratification of the bed is increased as well as the throughput of a separating machine.

Seen as energy graphs in figure 12, it will be seen that three positive inputs 46,47, 46 of energy (which is invested in bed stratification) are provided by the invention, but only one shown by dotted lines 52 by the prior art followed by a negative energy 53 (during the suction phase), in the same period of time.

Merely to confirm the point that there are alternative mechanical devices for the positive displacement means, figure 13 shows a double acting piston and cylinder arrangement is shown, with one-way inlet and outlet valves, which draw water in from a manifold and deliver it out to the hutch. This would provide the graphed wave shape 42 and a tandem double acting piston and cylinder with its crank 90 degrees out of alignment would provide the wave form 43.

REFERENCE NUMERALS 1 wet jigging apparatus 2 jigging chamber 3 jigging screen 4 hutch 5 feed end 6 discharge end 7 overflow weir 8 extraction auger 9 sump or collection trough 10 pump chamber 11 pump chamber 12 pump housing 13 diaphragm 14 flexible edges of diaphragm 15 one-way inlet valve 16 one-way inlet valve 17 water manifold 18 one-way outlet valve 19 one-way outlet valve 20 pre-chamber 21 sieve 22 angle of water jets from screen 23 direction of water jets 24 underflow chute 25 delivery chute 26 discharge weir 27 discharge control valve 28 rod driving diaphragm 29 separating system 30 receiving bin 31 vibratory feeder

32 feeder end of jigging apparatus 33 dewatering screen 34 delivery chute 35 one-way valve at bottom of discharge chute 36 hole in plate forming valve 37 38 rubber flap 39 40 41 wave form of pulses from one chamber 42 wave form of pulses from other chamber 43 cumulative wave form of pulses from two chambers 44 next cycle of ongoing cumulative wave form of pulses 45 optional 90 degrees out-of-phase pulses 46 speed of pulses or energy input of pulses 47 speed of pulses or energy input of pulses 48 prior art pulse 49 upward phase of prior art pulse 50 rest period of prior art pulse 51 suction phase of prior art pulse 52 speed/energy of prior art upward phase of pulse 53 speed/energy of prior art suction phase of pulse 54 rest period of pulse according to invention 55 rest period of pulse according to invention 60 jigging apparatus 61 feed end 62 delivery end 63 screen 64 jigging chamber 65 hutch 66 actuator 67 holes for valves 68 holes for valves 69 holes for valves 70 holes for valves 71 wall 72 boss on diaphragm