Various different types of waterborne sorting systems exist for the waterborne presentation and sorting of particulate ore, and in particular the sorting of diamond particles from gangue particles. For example, South African patent 90/4531 discloses a method and apparatus for sorting particulate ore material in which a vertical duct is connected below a header tank, and particulate ore material is fed into the header tank to flow under gravity through the duct. The particulate ore is irradiated by a laser source as it travels through the duct, with diamondiferous particles causing a Raman spectral activation. A downstream ejector is used to eject the activated diamond particles from the falling stream.

One of the problems experienced with a wet presentation system of this type is that the particles are relatively difficult to detect and recover accurately within the turbulent stream. In addition, the ore particles tend to contact the sides of the chute, resulting in increased abrasive wear.

SUMMARY OF THE INVENTION According to a first aspect of the invention a method of presenting particles in a liquid stream for sorting purposes includes the steps of conveying liquid and particles in a first particle conveying stream and introducing a second substantially particle free stream to surround the first particle conveying stream thereby to form a combined stream having a central particle conveying portion and an outer substantially particle free portion.

The method preferably includes the steps of radiating the particles in the combined stream, detecting the response of the particles, and sorting the particles based on their response.

The method preferably includes the steps of disagglomerating the particles upstream of the first particle conveying stream.

The method preferably includes the step of accelerating and separating the particles in the first particle conveying stream.

According to another aspect of the invention particle presentation apparatus for the presentation of particles in a liquid stream for sorting purposes includes a first conduit for conveying liquid and particles in a first particle conveying stream, and a second conduit for introducing a second substantially particle free stream to surround the first particle conveying stream thereby to form a combined stream having a central particle conveying portion and an outer substantially particle free portion.

The first conduit preferably includes a venturi pipe which terminates within the second conduit.

The second conduit preferably includes an outlet conduit with an outlet, the venturi pipe terminating within the outlet conduit short of the outlet.

The apparatus preferably includes radiating, detecting and ejecting means for radiating the particles within the second conduit, for detecting specific particles within the second conduit, and for ejecting the specific particles from the combined stream when they are discharged from the second conduit.

The apparatus preferably includes substantially constant head feed means for the first conduit. The substantially constant head feed means may be a steady head tank.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic pictorial view of a waterborne particle sorting system of the invention; Figure 2 shows a cross-sectional side view on the line 2-2 of Figure 1; Figure 3 shows a cross-sectional top plan view on the line 3-3 of Figure 1; and Figure 4 shows a cross-section on the line 4-4 of Figure 1.

DETAILED DESCRIPTION OF THE DRAWINGS Referring first to figure 1, a waterborne particle sorting system 10 comprises, at an upstream end, a feed hopper 12 filled with agglomerated diamondiferous particulate ore 14. An overhead spray bar 16 partly disagglomerates the particulate ore 14 in the feed hopper 12. The particulate ore 14 is fed under gravity into a vibrating feed tray 18 where the partly disagglomerated ore 20 is flooded under water 22 to assist in further disagglomeration. The further disagglomerated particulate ore 14 is fed into a steady head tank 24 which has a water inlet 26 and a siphon 28 for maintaining the water level constant within the tank 24 so as to provide a constant water head of 0.5m, as is shown at 30.

A feed pipe 32 extends from the conical base of the steady head tank 24. The partly disagglomerated particulate diamondiferous ore is further disagglomerated within the tank 24 into discrete particles, as is shown at 34, and exits the tank 24 under a constant pressure and resultant constant velocity.

A venturi feed pipe 36 is connected to the free end of the steady head tank feed pipe 32. The venturi feed pipe 36 passes through a transversely extending water inlet manifold 38. The water inlet manifold 38 is formed with a pair of water inlet ports 40 and 42, and a jacket pipe 44 surrounds the venturi feed pipe 36 where it exits from the water inlet manifold 38. The venturi feed pipe 36 terminates at an outlet 46 just downstream of the jacket pipe termination 44. An outlet pipe 48 terminating in a flattened presentation nozzle 50 extends from the jacket pipe 44, and similarly encloses the venturi pipe 36.

The pressure head 30 from the tank 24, together with the venturi pipe 36, have the effect of accelerating and separating the discrete ore particles 52 as they pass through the tank feed pipe 32 and the venturi feed pipe 36. The water flow past the end 46 of the venturi feed pipe 36 results in a pressure drop which further accelerates the stream of water. As is clear from figure 3, water flows through the inlet manifold 38 in the direction of arrows 54 through an annular space 55 between the venturi feed pipe 36 and the outlet pipe 48.

Figure 4 shows how the stream of water passing through the outlet pipe 48 includes a central particle laden substantially laminar stream 56 jacketed by an outer annular substantially particle free stream 58. Eddies 60 in the outer stream 58, adjacent the inner surface of the outlet pipe 48, serve to keep the particles centralized within the central stream 56. This has the effect of reducing wear on the outlet pipe, and the increased acceleration of the particle-laden stream has the effect of still further separating the particles from one another so that they are discretely presented for sorting.

The separated particles flow past a glass window 60 in the roof of the flattened presentation nozzle 50. The inner surface of the window 50 is located flush with the inner surface of the nozzle so as to minimize turbulence. The particle- laden stream is irradiated by means of X-rays from an X-ray tubes 62, which may be positioned above or below the nozzle. Dual light wavelength discrimination (XST) may also be used effectively to allow the detection system to distinguish between luminescent diamond particles and luminescent gangue particles. The detection system 64, which typically includes a fibre optic link, terminates in a lens arrangement 66 which interfaces directly with the window 60. Refraction is minimized owing to the direct interface between the water and the window 60. However, the lens 66 need not be in direct contact with the window 60, as the distance between the lens 66 and the window may be adjusted to meet the optical requirements. A light splitter (not shown) may be positioned above the lens arrangement 66 to send light to two detectors (also not shown).

An ejector arrangement 68 is located just downstream of the outlet from the presentation nozzle 50 where the flow is predominantly laminar. A vertical air jet 70 ejects diamond particles 72 into a chute 74. The ejected diamond particles 72 go to concentrate, and the gangue particles 76 are disposed in the tailing trajectory 78. Masking of the luminescent particles is reduced owing to the diffusion of the emitted light from these particles through the water. Prior to separation, the feed may be separated into multiple channels. This serves to halt cross talk between channels and minimizes false ejections of the sort. In the particular embodiment, at the point of exiting, the velocity of the water stream or jet was found to be 1.8m/s at 60E per minute per nozzle. Naturally, the water velocity may be varied if and when required to enhance the sort.

The water is preferably optically clear, but does not have to be pure in order to function optimally. A scrubbing system may be provided upstream of the feed hopper 16 to remove clay and fine grits from the diamondiferous gravel to prevent turbidity or contamination of the clear water.

The invention is not confined to the waterborne sorting of diamonds, but may be used to sort any particulate material or present such particulate material for sorting within a liquid stream. The invention has a number of advantages over prior art systems. First, the eddy currents in the outer particle free stream have the effect of shielding the particles from the walls of the outlet pipe 48 and nozzle 50. This reduces wear on the inner surfaces of the outlet pipe and nozzle, and also ensures that the particles follow a relatively predictable linear path within the central essentially laminar stream and are not caught within the turbulent eddies adjacent the wall of the outlet pipe, where they would be subjected to irregular movement and may be caused to ricochet against the walls which would have the effect of reducing the presentability of the particles for subsequent detection and sorting purposes.