AU632947B2

AU632947B2 - Sorting method and apparatus

Info

Publication number: AU632947B2
Application number: AU80164/91A
Authority: AU
Inventors: Matthias Kofler; Charlie Maurice Levitt; Krzysztof Zygmunt Wieckiewicz
Original assignee: De Beers Industrial Diamond Division Pty Ltd
Current assignee: De Beers Industrial Diamond Division Pty Ltd
Priority date: 1990-07-06
Filing date: 1991-07-04
Publication date: 1993-01-14
Anticipated expiration: 2011-07-04
Also published as: AU8016491A; GB2247195A; GB9114562D0; GB2247195B

Description

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AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION NAME OF APPLICANT(S): De Beers Industrial Diamond Division (Proprietary) Limited ADDRESS FOR SERVICE: DAVIES COLLLS-N- Patent Attornys R!,n,0 4 Little Collins Strcct, Melbourne, 3000.

4 C 4 *C88 ~4O*A 8 A A *8 A A 1 A It INVENTION TITLE: Sorting method and apparatus The following statement is a full description of this invention, including the best method of pe:forming it known to me/us:-

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-2- BACKGROUND TO THE INVENTION THIS invention relates to a sorting method and apparatus. In one application of the invention, it may be used to sort diamondiferous particles from non-diamondiferous particles.

It is known that diamonds have a higher thermal conductivity than other particles with which they are normally associated in nature. The present invention seeks to use this difference in thermal properties in a particle sorting method and apparatus.

THIS invention relates to a sorting method and apparatus. In one application of the I I ~I SUMMARY OF THE INVENTION According to the present invention there is provided a method of sorting particles on the basis of different thermal characteristics, the method comprising the steps of: wetting the particles which are to be sorted with a wetting liquid, bringinc the wetted particles into contact with a surface which is at a temperature lower than the freezing point of the wetting liquid, allowing the wetting liquid to freeze to the surface to adhere all the particles to the surface, passing the adhering particles through a heating zone where heat is applied externally to the particles so that particles of higher thermal conductivity, which accept heat more rapidly than particles of lower thermal conductivity, are released first from the surface by melting the frozen wetting liquid, and 20 separately recovering from the surface the particles of higher thermal conductivity which have been released from the surface and the particles of lower thermal conductivity which are still adhered to the surface.

3563Z 2a I- SUMMARY OF THE INVENTION According to a first aspect of the invention, there is rovided a method of sorting particles on the basis of different thermal char eristics, the method comprising the steps of wetting the particles which ar to be sorted with a liquid, bringing the wetted particles into contact with urface which is at a temperature lower than the freezing point of the liquid aintaining such contact for a period of time sufficient for all the particles be adhered to the surface and thereafter independently separating fro fthe surface different classes of particles which are distinguished from o another by differences in thermal characteristics.

In the preferred embodiment, the particles are wetted with water and are brought S into contact with the surface of a body of ice, typically the surface of a layer of ice formed on a cooled substrate. The layer of ice may be formed on a rotating, o o 2 °internally cooled, hollow drum.

Conveniently, heat is applied to the adhered particles so that particles of a first class, having a higher thermal conductivity than other particles of a second class, are released from the surface of the ice layer while particles of the second class are not a released. The heat may be applied by passing the layer of ice, with the particles adhered, through a body of water. Preferably, the body of water is contained in a water bath and particles of the first class are collected in the water bath.

Particles of the second class may then be removed from the surface of the layer of ice after passage of the layer of ice through the body of water. This can be achieved by means of a scraper.

In another embodiment of the method, the wetted particles are broutght into direct contact with the surface of an internally cooled, hollow, rotating drum.

_I I 't -4t I I I I A second aspect of the invention provides apparatus for sorting particles on the basis of different thermal characteristics, the apparatus comprising a surface, means for maintaining the surface at a temperature lower than the freezing point of a liquid with which the particles have been wetted, means for bringing the wetted particles into contact with the surface for a sufficient period of time for all the particles to be adhered to the surface, and means thereafter for independently separating from the surface different classes of particles which are distinguished from one another by differences in thermal characteristics.

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 shows a diagrammatic side view of an apparatus according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS In the illustrated embodiment, diamonds and diamondiferous particles are sorted from gangue or rock particles.

The drawing shows a hollow drum 10 which is made of copper or other highly thermally conductive material and which has a round cylindrical wall 12 and circular end plates 14 defining an internal, round cylindrical cavity 16. The drum is internally cooled, for instance by evaporation therein of a suitable refrigerant. The degree of cooling is such that the wall of the drum is maintained at a low temperature substantially less than the freezing point of water. The drum is caused to rotate about its axis 18 in the direction of the arrow 20 by a suitable drive motor I r ;i -esws~c-U- (not shown).

Located above the high point of the drum is a feed chute 22 that is supplied by a vibratory feeder with particulate material which is to be sorted. In this case, the particulate material comprises diamonds and diamondiferous particles 24 and gangue particles or rocks 26 which have previously been wetted with water.

Located beneath the drum is a water bath 28 containing water at a temperature substantially above freezing. In this embodiment of the invention a layer of ice is formed on the surface of the wall 12 and is constantly replenished as the wall 12 passes through the water bath 28 and water from the bath freezes onto the drum.

Further around on the path of travel of the drum is a scraper 32 which scrapes over *oo the surface of the ice layer SThe vibratory feeder supplies wet particles 24 and 26 to the chute 22 which causes the particles to drop onto the surface of the ice layer as illustrated. In the present diamond recovery process, no independent prior wetting step is required, with the particles being naturally wet as a result of the prior steps in the recovery process.

In the wet particle feed, the particles are disagglomerated, the feed is smooth and there is little danger of inter-particle collisions once the particles contact the ice layer In an experimental apparatus, the cavity 16 was maintained at a temperature in the range -92°C to -194C, the thickness of the ice layer was maintained at between 0,3mm and 0,5mm and the external temperature of the ice was maintained at The thickness of the ice layer ca- be maintained by means of external heaters, scrapers or the like (not shown). i he temperature of the water in the water bath 28 was maintained at 34°C.

I r (L As the wet particles contact the ice layer, water on the particles is rapidly cooled down and caused to freeze with the result that the particles 24 and 26 are adhered to the ice layer. The adherence of the particles to the ice layer is promoted by the fact that the warmer particles lose heat to the ice which melts in the immediate vicinities of the particles and then, because of the extremely low temperature of the ice layer, refreezes to embed 'the particles at least partially. The diamond and diamondiferous particles will, because of their higher thermal conductivity than the rocks, lose heat more rapidly and will be adhered first to the ice layer. However, the dimensions and speed cf rotation of the drum are chosen so that, irrespectivof the thermal properties of the particle, all particles are adhered to the ice layer before they can fall off under gravity.

As the ice layer passes through the water bath 28, the diamond and diamondiferous Sparticles accept heat more quickly from the warmer water. The heat is imparted to the surrounding ice which melts and permits these particles to separate first from the ice layer. The diamond and diamondiferous particles 24 therefore collect in the bottom of the water bath. Rock particles, which do not accept heat so quickly from the warmer water, remain adhered to the ice layer for a longer period of time. The rock particles are still adbered to the ice layer when the ice layer moves out of the water bath 28, and are subsequently scraped off the ice layer by the scraper 32. The rock particles fall into a bin 34.

t t As the particles 24, 26 separate from the ice layer 30, they leave small craters in the surface of the ice layer, but these are subsequently repaired as water from the bath 28 freezes onto the surface of the ice layer. In some cases it may be desirable to have a thicker ice layer which can be more rapidly repaired. An added advantage of a thick ice layer is the fact that ice is a good insulator. This in turn will promote easier separation of the diamond and diamondiferous particles, since these particles will be less capable of imparting heat through the ice layer to the wall 12 of the I -7drum 10. Instead, the heat which the diamond and diamondiferous particles acquire from the water in the bath 28 will be transferred to the immediately surrounding ice which will melt to permit the particles to drop off into the bath.

As an alternative to the use of a warm water bath, the diamonds and diamondiferous particles can be separated from the ice layer by spraying the ice layer with warm water. The water spray would also serve to repair craters in the surface of the ice layer. Also the rocks could subsequently be parted from the ice layer using hot steam jets, a hot water spray or flames directed at the surface of the ice layer.

*o oIt is important to note that an ice layer is not essential. The sorting process could also work with the wetted particles merely being dropped directly onto the wall 12 of the drum, with the water on the particles then freezing to adhere the particles to the drum surface. The remaining steps of the process remain as described above.

It is antitipated that the embodiment referred to in the '"nmediately preceding paragraph will have the added advantage that the water on the particles will freeze j ivery quickly to adhere the particles to the drum surface, since it is not insulated from the drum surface by the ice layer. Quicker adherence of the particles to the drum is advantageous in that higher drum speeds will be possible, and higher throughputs obtainable.

When compared to dry sorting systems, the wetted state of the particles is advantageous in that there is less danger of particle clogging and the capability of higher throughputs.

I I rrra -ruUCPi~cU~ V~l -8- A major benefit of the embodiment described above is the fact that so-called "black" and "perfect" diamonds can be recovered separately from rock particles. A "black" diamond is one in which the diamond itself is covered with material which is opaque to light. Such diamonds cannot be detected using optical sorting methods such as X-ray sorting in which the sorting criterion is luminescence as a result of incident X-radiation. A "perfect" diamond is one which has no inclusions and which therefore has no luminescent response to incident X-radiation. The present invention relies on the inherent thermal characteristics of diamonds and diamondiferous particles, and not on any optical response.

Claims

11.1_ i -9- STIIHE CLAIMS DEFININC THE INVENTION ARE AS FOLLOWS: 1. A method of sorting particles on the basis of different thermal characteristics, the method comprising the steps of: wetting the particles which are to be sorted with a wetting liquid, bringing the wetted particles into contact with a surface which is at a temperature lower than the freezing point of the wetting liquid, allowing the wetting liquid to freeze to the surface to adhere all the particles to the surface, i i passing the adhering particles through a heating zone where heat is applied externally to the particles so that particles of higher thermal conductivity, which accept heat more rapidly than particles of lower thermal conductivity, are released first from the surface by melting the frozen wetting liquid, and separately recovering from the surface the particles of higher thermal conductivity which have been released from the surface and the particles of lower thermal conductivity which are still adhered to the surface. r. to 10 2. A method according to claim 1 wherein the particles are wetted with water and are brought into contact with the surface of a body of ice. 3. A method according to claim 2 wherein the wetted particles are brought into contact with the surface of a layer of ice formed on a cooled substrate. 4. *44, *D 4 4 44 4 a a 4 4* a, *4 4444 10 A method according to claim 3 wherein the wetted particles are brought into contact with the surface of a layer of ice formed on a rotating, internally cooled, hollow drum. 15 A method according to claim 4 wherein heat is applied to the particles in the heating zone by passing the layer of ice, with the particles adhered thereto, through a body of water. 6. A method according to claim 5 wherein the body of water is contained in a water bath and the particles of higher thermal conductivity are allowed to fall off the layer of ice for collection in the water bath. 11 7. A method according to claim 5 or claim 6 wherein the adhering particles of lower thermal conductivity are scraped from the surface of the layer of ice after passage of the layer of ice through the body of water. 8. A method according to any one of the preceding claims when used to separate diamond and dian'ondiferous particles from gangue particles. 9. A method of sorting particles substantially as herein described with reference to the accompanying drawing. 1' DATED: 10 July 1992 PHILLIPS ORMONDE FITZPATRICK Attorneys for: S. DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) i LIMITED It I A I