GB2278440A

GB2278440A - Particle classification based on thermal properties

Info

Publication number: GB2278440A
Application number: GB9410457A
Authority: GB
Inventors: Edwin Harm Roos
Original assignee: De Beers Industrial Diamond Division Pty Ltd
Current assignee: De Beers Industrial Diamond Division Pty Ltd
Priority date: 1993-05-26
Filing date: 1994-05-25
Publication date: 1994-11-30
Anticipated expiration: 2014-05-25
Also published as: ZA943378B; GB9410457D0; GB2278440B; AU6328294A; CA2124172A1

Abstract

Particles are classified according to their thermal properties by means of a method in which the first step is that of deriving a first matrix composed of first emitted heat energy values for the particles (16, 18) with the particles at a uniform temperature. This is achieved using a first thermal camera (22) to derive a thermogram which is analysed by a CPU (24). Next, the CPU calculates emissivity equalisation factors which equalise the values in the first matrix when the values are adjusted by the factors. Then the particles are heated or cooled, typically by an infra-read beam (26). A second matrix composed of second emitted heat energy values is then derived, by the CPU, for the heated or cooled particles, using a second thermal camera (28). The CPU adjusts the values in the second matrix by the earlier derived emissivity equalisation factors, thereby producing adjusted heat energy values for the particles. Finally, the particles are assigned classifications according to the magnitude of their adjusted heat energy values. The invention is applicable to recovering diamonds (16) from gravel (18). <IMAGE>

Description

CLASSIFICATION BASED ON THERMAL PROPERTIES THIS invention relates to a system for classifying particles according to their thermal properties.

One application of the invention is in the recovery of diamonds from associated gangue particles in a diamond bearing gravel recovered in diamond mining or exploration activities.

According to a first aspect of the invention, there is provided a method of classifying particles according to their thermal properties, the method comprising the steps of: deriving a first matrix composed of first emitted heat energy values for the particles with the particles at a uniform temperature, calculating emissivity equalisation factors which equalise the values in the first matrix when the values are adjusted by the factors, heating or cooling the particles, deriving a second matrix composed of second emitted heat energy values for the heated or cooled particles, adjusting the values in the second matrix by the earlier derived emissivity equalisation factors to produce adjusted heat energy values for the particles, and classifying the particles according to their adjusted heat energy values.

A second aspect of the invention provides apparatus for classifying particles according to their thermal properties, the apparatus comprising: means for deriving a first matrix composed of first emitted heat energy values for the particles with the particles at a uniform temperature, means for deriving emissivity equalisation factors which equalise the values in the first matrix when the values are adjusted by the factors, means for deriving a second matrix composed of second emitted heat energy values for the particles after heating or cooling thereof, means for producing adjusted heat energy values for the particles by adjusting the values in the second matrix with the earlier derived emissivity equalisation factors, and means for classifying the particles according to their adjusted heat energy values.

The method and apparatus summarised above may form part of a sorting system in which the particles are subsequently sorted into fractions distinguished by the thermal properties of the particles.

The method and apparatus of the invention may be used to classify and sort diamond particles.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a diagrammatic illustration of an apparatus of the invention; and Figures 2 to 8 are matrices used to illustrate the method of the invention.

Figure 1 illustrates diagrammatically the basic components of an apparatus 10 of the invention. The apparatus 10 includes a feed chute 12 from which diamond-bearing gravel 14, containing diamonds 16 and gangue particles or rocks 18, is fed onto the upper run of an endless conveyor belt 20. The particles of the gravel 14 are, at this stage, at a uniform temperature.

The particles pass beneath a first thermal camera 22 which is connected to a central processing unit (CPU) 24. The particles are then heated up by a focused beam 26 of infra-red radiation. Thereafter, the particles pass beneath a second thermal camera 28 which is also connected to the CPU 24.

The particles fall off the belt 20 in free flight. During such flight, the particles pass next to a fluid blast ejection apparatus 30 which is controlled by the CPU 24 and which operates to issue a short duration blast of air whenever a diamond 16 arrives. The air blast deflects the diamonds 16 out of the normal trajectory so that they fall into a collection bin 32, while the undeflected rocks 18 continue along the normal trajectory and fall into a collection bin 34.

The manner in which certain particles are classified as diamonds or rocks, enabling the subsequent sort to be made, is described below.

Figure 2 is a diagrammatic plan view illustrating a small part of the gravel stream on the belt 20. The small portion of the belt which is represented in Figure 2 is shown divided arbitrarily into four quadrants Q, to Q4, each of which contains one particle of the gravel feed stream.

For illustrative purposes, it is assumed that the quadrants Q1 to Q4 contain a diamond Dl, a rock Rl, a diamond D2 and a rock R2 respectively.

The thermal camera 22 views the quadrants Ql to Q4 and transmits to the CPU 24 a thermal image indicative of the heat energy emanating from the quadrants. The CPU 24 generates a thermogram representative of the thermal output of the portion of the gravel feed stream under scrutiny. By way of example it is assumed that the heat energy transmitted by the four quadrants Ql to Q is 20, 20, 5 and 5 units of heat respectively, as indicated in the corresponding matrix seen in Figure 3.

It will be appreciated that the thermal camera 22 alone is unable to distinguish reliably between the diamonds and the rocks, despite the fact that diamond has thermal properties, such as thermal conductivity and diffusivity, which are markedly different from those of the rocks with which diamond is commonly associated in nature. The inability of the thermal camera alone to make the required classification is attributable to variances in thermal emissivity of the particles, particle size and so forth. So, for instance, some rocks may possess thermal emissivity properties of the same or nearly the same magnitude as diamond, while different diamond types may themselves have different thermal emissivities.

It is known from Stephan-Boltzmann's law that the quantity of heat energy emitted by a body is proportional to the product of the emissivity of the body and its temperature to the fourth power. In order to eliminate the effects of emissivity on the heat energy emitted by a body, the CPU 24 performs an algorithm to equalise the energy emitted by each quadrant Q1 to Q4. The CPU derives a matrix of emissivity equalisation factors, in this case as illustrated in Figure 4.

When the energy values in the matrix of Figure 3 are adjusted by multiplying them with the factors in the matrix of Figure 4, equal values are achieved in the four quadrants as indicated in the equalised matrix of Figure 5. The Figure 4 matrix of factors derived by the CPU is stored in a memory of the CPU.

When the particles pass through the infra-red beam 26, they are heated up to different temperatures, depending on their thermal properties and in particular their thermal diffusivities. Diamonds, having the higher thermal diffusivity, are heated up to a higher temperature by the beam than the rock particles.

It should be noted that an infra-red beam is only one way to apply heat rapidly to the particles. In other embodiments of the invention, other heating techniques, for instance a blast of hot air, could be used. It should also be noted that instead of applying heat to the particles, they could be cooled down, for instance by means of a blast of cold air. In each case, the objective is produce particles with differentiated temperatures dependent on thermal properties.

The second thermal camera 28 now views the particles at their nonuniform temperatures and a thermal image, which corresponds to that seen in Figure 2 and which is illustrated in Figure 6, is generated. An exemplary matrix of heat energy values for the quadrants Ql to Q4 is shown in Figure 7.

The Figure 4 matrix of adjustment factors is retrieved from the CPU memory and the values in the matrix of Figure 7 are adjusted by multiplication with the corresponding factors. This produces the Figure 8 matrix of adjusted heat energy values. Referring to Figure 8, it will be seen that the highest adjusted heat energy values are present in quadrants Q1 and Q3, i.e. those quadrants containing diamonds.

The CPU determines which quadrants are the source of adjusted heat energy values higher than a predetermined value and classifies those quadrants as diamond-containing quadrants.

Of course, in a version of the invention where the particles are cooled down rather than heated up, the CPU would be programmed to recognise those particles for which the adjusted heat energy values are lower than a predetermined value.

Referring again to Figure 1, after the particles have fallen from the end of the conveyor belt 20, the fluid blast ejection apparatus acts under the control of the CPU to issue an air blast to remove particles from those quadrants which have been identified as containing diamond.

Although mention has been made of using two separate thermal cameras 22 and 28 it would be possible in other embodiments to use a single camera twice, once before and once after heating or cooling. Also, although specific mention has been made of classifying and sorting diamonds, the principles of the invention are equally applicable to the classification and sorting of other particles which are distinguishable on the basis of thermal properties.

Figures 2 to 8 illustrate the principles of the invention in a very much simplified manner. In practice, thermograms would be generated for substantial areas of the belt, with areas such as the areas Q1 to Q4 in fact being very small areas only of the belt surface. Thermal viewing of substantial belt areas would enable a rapid throughput to be obtained.

In the embodiment described above, the fluid blast ejection apparatus removes particles from the quadrants identified as diamond-containing.

It would of course be equally possible to remove particles from those quadrants which are identified as containing no diamonds, so that a diamond rich fraction collects in the bin 34 while rocks collect in the bin 32. A decision as to whether to remove the diamonds or the rocks would in each case depend on the expected concentrations of the respective particles.

Claims

1.

A method of classifying particles according to their thermal properties, the method comprising the steps of: a) deriving a first matrix composed of first emitted heat energy values for the particles with the particles at a uniform temperature, b) calculating emissivity equalisation factors which equalise the values in the first matrix when the values are adjusted by the factors, c) heating or cooling the particles, d) deriving a second matrix composed of second emitted heat energy values for the heated or cooled particles, e) adjusting the values in the second matrix by the earlier derived emissivity equalisation factors to produce adjusted heat energy values for the particles, and f) classifying the particles according to their adjusted heat energy values.

2.

A method according to claim 1 wherein, in steps a) and d) respectively, the first and second matrices are derived using one or more thermal cameras.

3.

A method according to claim 2 wherein particles to be classified are conveyed sequentially past a first thermal camera which views the particles for the purposes of derivation of the first matrix of emitted heat energy values, past a source of heat which heats up the particles, and then past a second thermal camera which views the particles for the purposes of derivation of the second matrix of emitted heat energy values.

4.

A method according to claim 3 wherein the particles are heated up in step c) by means of an infra-red beam.

5.

A method according to any one of the preceding claims and comprising the further step of sorting the particles into fractions in accordance with their classifications.

6.

A method according to any one of the preceding claims when used to sort diamond particles from non-diamond particles.

7.

Apparatus for classifying particles according to their thermal properties, the apparatus comprising: a) means for deriving a first matrix composed of first emitted heat energy values for the particles with the particles at a uniform temperature, b) means for deriving emissivity equalisation factors which equalise the values in the first matrix when the values are adjusted by the factors, c) means for deriving a second matrix composed of second emitted heat energy values for the particles after heating or cooling thereof, d) means for producing adjusted heat energy values for the particles by adjusting the values in the second matrix with the earlier derived emissivity equalisation factors, and e) means for classifying the particles according to their adjusted heat energy values.

8.

Apparatus according to claim 7 comprising at least one thermal camera for use in the derivation of the first and second matrices of emitted heat energy values.

9.

Apparatus according to claim 8 comprising a first thermal camera arranged to view the particles for the purposes of derivation of the first matrix of emitted heat energy values, a source of heat operative to heat up the particles, a second thermal camera arranged to view the particles for the purposes of derivation of the second matrix of emitted heat energy values, and means for conveying particles which are to be classified past the first thermal camera, the heat source and the second thermal camera in sequence.

10.

Apparatus according to claim 9 wherein the conveying means comprises an endless conveyor belt.

11.

Apparatus according to either one of claims 9 or 10 wherein the source of heat is arranged to direct a beam of infra-red radiation at the particles.

12.

Apparatus according to any one of claims 7 to 11 and comprising means for sorting the particles into fractions in accordance with their classifications.

13.

Apparatus according to any one of claims 7 to 12 when used to sort diamond particles from non-diamond particles.

14.

A particle classification and sorting method substantially as herein described with reference to the accompanying drawings.

15.

A particle classification and sorting apparatus substantially as herein described with reference to the accompanying drawings.