AU683116B2

AU683116B2 - Method and apparatus for sorting particulate material according to the thermal properties of the particles

Info

Publication number: AU683116B2
Application number: AU12329/95A
Authority: AU
Inventors: Charlie Maurice Levitt; Victor Emul Ross; Paul Verhufen
Original assignee: De Beers Industrial Diamond Division Pty Ltd
Current assignee: De Beers Industrial Diamond Division Pty Ltd
Priority date: 1994-02-21
Filing date: 1995-02-20
Publication date: 1997-10-30
Anticipated expiration: 2015-02-20
Also published as: CA2142759A1; GB9503258D0; BR9500706A; GB2286546B; GB2286546A; RU95103035A; ZA95785B; AU1232995A

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: C* C

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C* C

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Name of Applicant: De Beers Industrial Diamond Division (Proprietary) Limited Actual Inventor(s): Charlie Maurice Levitt Paul Verhufen Victor Emul Ross t C

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Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: METHOD AND APPARATUS FOR SORTING PARTICULATE MATERIAL ACCORDING TO THE THERMAL PROPERTIES OF THE PARTICLES Our Ref 401713 POF Code: 1503/78726 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1- -2- *BACKGROUND TO TE INVENTION THIS invention relates to a method and apparatus for sorting particulate material according to the thermal properties of the particles.

Numerous different methods of sorting diamonds from rock particles, based on differences in thermal properties, have already been proposed.

Many of the known thermal sorting techniques have required large volumes of ice to be formed or are otherwise expensive to operate in practice. A further disadvantage suffered by some prior proposals is the possibility of sorting inaccuracies resulting from a somewhat less than secure adherence of certain particles to the substrate which conveys them.

An object of the present invention is to provide a novel sorting method and apparatus.

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3 SUMMARY OF THE INVENTION A first aspect of the invention provides a method of sorting particles according to their thermal properties, the method the steps of: thermally treating the particles in such a manner that particles of a first type have a temperature which is below the freezing point of a liquid and lower than that of particles of a second type, locating the particles on a perforated conveying surface and causing the conveying surface to pass through a body of the liquid, arranging the relevant parameters such that particles of the first class become firmly secured to the conveying surface as a result of a body of frozen liquid being created on those particles and extending on both sides of the conveying surface, while particles of the second type are not secured to the conveying surface in this manner, and separately recovering secured and unsecured particles from the conveying surface.

The particles may be thermally treated by initially cooling them to a temperature substantially below the freezing point of the liquid and then allowing them to gain to heat, at rates dependent on their thermal diffusivities, from the liquid.

-4- The particles may be allowed to gain heat from the liquid through which the conveying surface passes.

Conveniently, the secured particles are recovered from the conveying surface by the application of heat to those particles. This may be achieved by water sprays or hot air directed at the particles.

In a preferred version of the method, the conveying surface is a perforated cylindrical surface, and the method comprises the steps of irotating the cylindrical surface about its axis so that it passes through a bath of the liquid, and of placing the particles on the inside of the conveying surface either during or prior to passage of the conveying .:oo.i S"surface through the bath of liquid.

The method may be used to sort diamond particles from rock particles.

In this case, the particles are initially cooled and the relevant parameters S. are selected such that the rock particles retain a sufficiently low temperature to create, when located in the liquid, a surrounding body of frozen liquid which extends on both sides of the perforated conveying surface to secures the rock particles firmly to the surface while the diamond particles, on location in the liquid, are able to gain sufficient heat from the liquid to ensure that they do not create a securing body of frozen liquid.

Another aspect of the invention provides an apparatus for sorting particles according to their thermal properties, the apparatus eomalp== a perforated conveying surface, 5 means for causing the conveying surface to move through a body of a liquid, means for locating the particles on the conveying surface after they have been thermally treated in such a manner that particles of a first type have a temperature which is below the freezing point of the liquid and lower than that of particles of a second type, with the result that particles of the first type become firmly secured to the conveying surface as a result of a body of frozen liquid being created on those particles and extending on both 'sides of the conveying surface, while particles of the second type are not secured to the conveying surface in this manner, and means for separately recovering secured and unsecured particles from the conveying surface.

As indicated previously, the perforated conveying surface may be provided by a cylindrical wall of mesh material, the apparatus then comprising drive means for rotating the cylindrical wall about its axis so that successive portions of the cylindrical wall pass through a liquid bath. In a particularly preferred version of the invention, the mesh material is a silk mesh material.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings.

6 In the drawings: Figure 1 diagrammatically illustrates an apparatus of the invention; Figure 2 diagrammatically illustrates the operation of the scoop; and Figure 3 diagrammatically illustrates the manner in which rock particles are firmly secured to the drum wall.

DESCRIPrION OF AN EMBODIMENT Figure 1 shows a hollow, round cylindrical drum 10. The cylindrical wall of the drum is defined by a mesh material 12 which is held in the correct round cylindrical shape by a rigid frame (not shown in Figure 1) across which it spans. In this particular example, the silk has a mesh aperture of approximately 40 micron.

The frame of the drum rests on rollers 16 and 18 and a motor 20 is provided to drive the roller 16. This in turn causes the drum 10 to rotate about its central axis. At the lowest point of the drum, the silk wall passes through a body of water 22 in a water bath 24. Typically, the wall 12 passes through the water at a maximum depth of about 20mm to 7 The numeral 24 indicates means for feeding particles which are to be sorted onto the inner surface of the mesh wall 12 at the position where the silk wall passes through the water 22. In the diagrammatically illustrated version, the feed means 24 is depicted as being a gravity-feed hopper, but in practice any conventional type of feed arrangement, such as a vibratory spiral elevator, could be used.

The mesh wall 12 may be made of a conventional screen printing mesh.

Prior to their deposition on the inner surface of the drum wall, all the 3 particles are cooled to a very low temperature well below the freezing point of water. In a typical example, the particles are precooled by immersing them in liquid nitrogen which brings them to an extremely low temperature of, say -100°C. In addition, the water 22 is preferably maintained at a temperature not far from its freezing point. The temperature of the water may, for instance, be about 5 0

C.

In the present example, the particles which are to be sorted include diamonds 26 and rocks 28. When the cold diamonds enter the water 22 their high thermal diffusivity results in their gaining heat rapidly from the water. The speed of rotation of the drum, water temperature and depth, particle temperature and other relevant parameters are set such that the diamonds 26 gain heat so quickly from the water that they are unable to freeze the water in their immediate vicinities. Accordingly, the diamonds merely rest loosely on the inner surface of the silk wall 12.

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-8- The rock with which diamonds are normally associated in nature has a very much lower thermal diffusivity than the diamonds. Thus the rocks 28 gain heat from the water very much more slowly than the diamonds.

They retain a temperature low enough for a body of ice to be created around them. As a result of the perforations in the silk against which the rocks are resting, ice 27 forms on both sides of the silk as indicated in Figure 3. The rocks adhere to the ice which in turn adheres to the mesh wall. The coherent body of ice which extends on both sides of the mesh anchors the rocks firmly to the mesh material.

The rocks are carried out of the water bath by the mesh wall 12. At a high point water sprays 29 are directed against the mesh wall, both from the outside and the inside. The sprayed water, which is preferably at an elevated temperature, rapidly melts the ice which secures the rocks to the mesh wall, and allows the rocks to become detached from the mesh and to fall into a collection bin As explained above, the diamonds merely rest loosely on the mesh. If, as a result of friction applied to them by the mesh wall, the diamonds are conveyed out of the water, gravity will soon cause them to roll back into the water. Referring to Figure 2, there is shown a portion 32 of the circular drum frame at each end of the drum. Spanning between the portions 32 is a rigid flap 34. The ends of the flap are pivoted to the drum frame portions 32 at respective pivots 36.

-9- A helical tension spring 38 acts between the flap and a frame portion 32 and serves to maintain the flap at an inclination to the radial. At this orientation, the flap serves as a scoop. Each time the flap passes through the water bath, it picks up the diamond particles which are lying loosely on the mesh in the bath. The flap then carries the diamond particles to a location 40 where there is an abutment 41 (Figure 2) which is arranged to act on the flap as the flap moves past. The flap is therefore deflected, against the bias of the spring 38, in the direction indicated in Figures 1 and 2 by the arrows 42. This deflection of the flap allows the diamonds to drop off the flap and into a collection bin 44.

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Various alternative constructions are within the scope of the invention.

For instance, heat could be applied to melt the securing bodies of ice 27 by means other than water sprays. For instance, hot air could be blown onto the drum wall at the appropriate position. Also, it may happen that a diamond is carried along mechanically by one or more rocks which are secured by ice to the drum wall. To overcome this potential problem, it may be necessary to apply period or continuous vibrations to the drum, or alternatively to apply periodic, sharp air blasts to the drum wall.

Although the use of a cylindrical drum leads to a compact apparatus, it would also be possible to use a perforated conveying surface in the form of a conveyor belt passing over idlers which guide it through a water or other liquid bath.

Instead of the particles being dropped directly into the water, they could be dropped onto an upstream portion of the conveying surface.

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10 It is within the scope of the invention for all the particles initially to be heated or cooled to uniformly high or low temperature with a subsequent thermal treatment designed to differentiate the temperatures of the particles. The type of thermal treatment which is employed will be seleted in accordance with the other parameters and the nature of the particles which it is desired to recover.

Although specific mention has been made of a mesh material in the form of screen printing mesh material, it will be appreciated that many other types of perforated material could also be used to form a surface to which selected particles are adhered.

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Claims

1. A method of sorting particles according to their thermal properties, the method omp the steps of: thermally treating the particles in such a manner that particles of a first type have a temperature which is below the freezing point of a liquid and lower than that of particles of a second type, locating the particles on a perforated conveying surface and causing the conveying surface to pass through a body of the liquid, arranging the relevant parameters such that particles of the first gia class become firmly secured to the conveying surface as a result of a body of frozen liquid being created on those particles and extending on both sides of the conveying surface, while partices of the second type are not secured to the conveying surface in this manner, and separately recovering secured and unsecured particles from the conveying surface. IUP~I~ 12 .000 0 6 5

2. A method according to claim 1 wherein the particles are thermally treated by initially cooling them to a temperature substantially below the freezing point of the liquid and then allowing them to gain to heat, at rates dependent on their thermal diffusivities, from the liquid.

3. A method according to claim 2 wherein the particles are allowed to gain heat from the liquid through which the conveying surface passes.

4. A method according to claim 2 or claim 3 wherein the secured particles are recovered from the conveying surface by the application of heat to those particles. A method according to claim 4 wherein the secured particles are recovered from the conveying surface by water sprays or hot air directed at the particles.

6. A method according to any one of the preceding claims wherein the onveying surface is a perforated cylindrical surface, the method 4 4 S 4 ~I 13 "oer iiaghe steps of rotating the cylindrical surface about its axis so that it passes through a bath of the liquid, and of placing the particles on the inside of the conveying surface either during or prior to passage of the conveying surface through the bath of liquid.

7. A method according to any one of the preceding claims when used to :ort diamond particles from rock particles.

8. A method according to claim 7 wherein the particles are initially cooled and the relevant parameters are selected such that the rock particles retain a sufficiently low temperature to create, when located in the liquid, a surrounding body of frozen liquid which extends on both sides of the perforated conveying surface to secures the rock particles firmly to the surface while the diamond particles, on location in the liquid, are able to gain sufficient heat from the liquid to ensure that they do not o* S create a securing body of frozen liquid.

9. An apparatus for sorting particles according to their thermal properties, the apparatus eo isin. a perforated conveying surface, 14 means for causing the conveying surface to move through a body of a liquid, means for locating the particles on the conveying surface after they have been thermally treated in such a manner that particles of a first type have a temperature which is below the freezing .point of the liquid and lower than that of particles of a second type, with the result that particles of the first type become firmly secured to the conveying surface as a result of a body of frozen liquid being created on those particles and extending on both sides of the conveying surface, while particles of the second type are not secured to the conveying surface in this manner, and means for separately recovering secured and unsecured particles from the conveying surface. 9 An apparatus according to claim A!kwherein the perforated conveying surface is provided by a cylindrical wall of mesh material, the apparatus comprising drive means for rotating the cylindrical wall about its axis so that successive portions of the cylindrical wa] 1 pass through a liquid bath. 15

11. An apparatus according to claim 10 wherein the mesh material is a silk mesh material.

12. An apparatus according to any one of claims 9 to 11 wherein the means for recovering secured particles from the conveying surface as means for applying heat to the secured particles.

13. An apparatus according to claim 12 wherein the means for recovering secured particles from the conveying surface compfr one or more water sprays or hot air blowers directed at the secured particles.

14. A method according to any one of claims 9 to 13 when used to sort diamond particles from rock particles. II 16 An apparatus for sorting particles according to their thermal properties substantially as herein described with reference to the accompanying drawings. DATED: 20th February, 1995 PHILLIPS ORMONDE FITZPATRICK Attorneys for: DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) LIMITED V, ABSTRACT The invention concerns a method of sorting particles according to their thermal properties. In the method, the particles (26, 28) are thermally treated in such a manner that particles of one type, typically diamonds, have a temperature which is below the freezing point of a liquid and lower than that of particles of a second type, typically rock particles associated with diamonds. The thermally treated particles (26, 28) are located on a perforated conveying surface and the conveying surface is caused to pass through a body of the liquid. The relevant parameters, such as time and temperature, are selected such that particles of the first type become firmly secured to the conveying surface as a result of a body of frozen liquid being created on those particles and extending on both sides of the conveying surface. Particles of the second class are not secured to the conveying surface in this manner. Thereafter, the different ;types of particles, which are respectively secured and unsecured to the conveying surface, are separately recovered from that surface. II