GB2305374A - Particle sorting - Google Patents

Abstract

In a method of sorting small particles, typically diamond (10) and gangue (12) particles having a size of 3mm or less, on the basis of differences in their thermal diffusivity the particles are firstly cooled e.g. in liquid nitrogen bath (14) to a low temperature below the freezing point of a liquid. Thereafter the particles are introduced into a body of the liquid such as water (18), the surface tension of the liquid previously having been adjusted such that particles (12) with relatively low thermal diffusivity cause freezing of the liquid with which they are in contact and are accordingly caused to float. Particles (10) with relatively high thermal diffusivity lose heat too rapidly to freeze the liquid and accordingly sink in the liquid. The floating and sunken particles are then recovered separately, in bins (24) and (26).

Description

BACKGROUND TO THE INVENTION THIS invention relates to particle sorting according to thermal properties.

Various proposals have already been made for sorting particles according to their thermal properties, and in particular according to their thermal diffusivities. In one known proposal, particles which are to be sorted are placed on a moving layer of ice. Particles which have high thermal diffusivity rapidly lose heat to the ice which refreezes about them and captures them. Other particles with lower thermal diffusivity lose heat more slowly to the ice and so are not captured within a specific time window.

Captured and uncaptured particles are subsequently removed separately from the ice layer by passing the ice layer through a water bath.

While this technique has been found to operate reasonably well with larger particles, it has been found that inaccurate sorting can take place when smaller particles, typically those in the size range -2+lmm, are involved. It is believed that this is attributable to the fact that the smaller particles are unable, because of their low mass, to adhere properly to the ice layer, particularly if a film of water is present on the ice. Because of their poor adherence, the smaller particles detach from the ice haphazardly in the water bath. Also, the hydrodynamic drag on the smaller particles as they enter the water bath tends to detach them in a haphazard manner.

SUMMARY OF THE INVENTION The invention provides a method of sorting small particles on the basis of thermal diffusivity, the method comprising the steps of cooling particles which are to be sorted to a low temperature below the freezing point of a liquid, introducing the cooled particles into a body of the liquid, the surface tension of the liquid previously having been adjusted such that particles with relatively low thermal diffusivity cause freezing of the liquid with which they are in contact and are accordingly caused to float in the liquid by increased surface tension effects, while particles with relatively high thermal diffusivity lose heat too rapidly to freeze the liquid and accordingly sink in the liquid, and separating the floating and sunken particles.

In this specification, the term "small particles" refers to particles having a size of 3mm or less, preferably in the size range -2+0,5mm.

In a preferred embodiment, the particles are placed, at a feed location, on the surface of a moving body of the liquid and the particles are sorted by means of a weir, situated downstream of the feed location, over which floating particles are carried by the liquid but behind which sunken particles are collected BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail, by way of example only, with reference to the accompanying drawing which diagrammatically illustrates an apparatus used to carry out the method of the invention DESCRIPTION OF A PREFERRED EMBODIMENT In the embodiment now described, the method of the invention is used to sort small diamond particles from small gangue particles.Referring to the drawing, a mass of particles which is to be separated and which contains both diamond particles 10 and gangue particles 12 is cooled in a liquid nitrogen bath 14. The particles are deposited from the bath 14 onto a vibrating feeder tray 16 which distributes them evenly onto the surface of a flowing body of water 18.

The water is typically about 1 0mm deep and its flow is maintained at a constant rate, to the right in the drawing, by means of a feed tank 20 in which a constant head of water is maintained. The surface tension of the water is adjusted to a desired value by controlling its pH and temperature and by addition of appropriate concentrations of selected surfactant(s).

Those particles in the mass which have a relatively high coefficient of thermal diffusivity, i.e. the diamond particles 10 in the present example, gain heaf rapidly from the water and their temperature is soon elevated above the freezing point of water. Such particles accordingly sink almost immediately in the water.

Those particles in the mass which have a relatively low coefficient of thermal diffusivity, i.e. the gangue particles 14, gain heat more slowly from the water and remain at a temperature below the freezing point of water.

They accordingly freeze the water with which they are in contact. A layer of ice accordingly forms on the particles where they are in contact with the water.

The surface tension of the water is initially adjusted such that the interaction between the ice layers on the relevant particles and the immediately surrounding water increases the surface tension effects acting on the particles to a level at which the particles are maintained in a floating condition.

The moving body of water conveys the floating gangue and sunken diamond particles downstream to a weir 22 over which the floating gangue particles pass into a tailings bin 24. The sunken diamond particles, on the other hand, are captured in a diamond concentrate bin 26. The water that overflows into the tailings bin 24 is recirculated to the tank 20 by a pump 28.

Claims (9)

1.

A method of sorting small particles on the basis of thermal diffusivity, the method comprising the steps of cooling particles which are to be sorted to a low temperature below the freezing point of a liquid, introducing the cooled particles into a body of the liquid, the surface tension of the liquid previously having been adjusted such that particles with relatively low thermal diffusivity cause freezing of the liquid with which they are in contact and are accordingly caused to float in the liquid by increased surface tension effects, while particles with relatively high thermal diffusivity lose heat too rapidly to freeze the liquid and accordingly sink in the liquid, and separating the floating and sunken particles.

2.

A method according to claim 1 wherein the particles are placed, at a feed location, on the surface of a moving body of the liquid and the particles are sorted by means of a weir, situated downstream of the feed location, over which floating particles are carried by the liquid but behind which sunken particles are collected.

3.

A method according to claim 2 wherein the body of liquid is supplied from a feed tank in which a constant head of the liquid is maintained so that the body of liquid moves at a constant rate.

4.

A method according to claim 3 wherein the floating particles are collected in a first bin downstream of the weir and the sunken particles are collected in a second bin upstream of the weir, and liquid from the first bin is recycled to the feed tank.

5.

A method according to any one of claims 2 to 4 wherein the particles which are to be sorted are cooled to the low temperature in a bath of liquid nitrogen from which they are located on the surface of the moving body of water by a vibrating feeder tray.

6.

A method according to any one of the preceding claims wherein the particles with a relatively high thermal diffusivity are diamond particles and the particles with relatively low thermal diffusivity are non-diamond particles.

7.

A method according to claim 6 wherein the particles have a size of 3mm or less.

8.

A method according to claim 7 wherein the particles have a size in the range -2+0,5mm.

9.

A method of sorting small particles substantially as herein described with reference to the accompanying drawing.