GB2199657A

GB2199657A - Diamond sorting

Info

Publication number: GB2199657A
Application number: GB08726532A
Authority: GB
Inventors: Stuart Hedley Robertson; Carlton Nigel Dodge
Original assignee: De Beers Industrial Diamond Division Pty Ltd
Current assignee: De Beers Industrial Diamond Division Pty Ltd
Priority date: 1986-11-12
Filing date: 1987-11-12
Publication date: 1988-07-13
Anticipated expiration: 2007-11-12
Also published as: AU595660B2; AU8117987A; GB2199657B; BR8706126A; GB8726532D0

Abstract

A mass of particles 10 is separated into a diamond fraction 28 and a gangue fraction 30 by subjecting the particles of the mass to laser irradiation 20 at a frequency known to cause H3 or N3 luminescence in diamond. Particles which luminesce in the wavelength ranges of H3 or N3 luminescence are selected eg by photodiode array 22, processor 24 and fluid blast nozzle 26. <IMAGE>

Description

"DIAMOND SORTING" BACKGROUND TO THE INVENTION THIS invention relates to the sorting of diamonds from gangue.

The vast majority of naturally occurring diamonds contain greater or lesser amounts of nitrogen as an impurity. The nitrogen impurity exists in the form of a few atoms only at what are termed point defects in the diamond crystal, or in the form of so-called platelets. The electrons at the point defects are normally at the lowest possible energy. strate.

It is known that it is possible to excite electrons at these point defects by means of incident radiation, thereby causing the electrons to move temporarily to higher unstable energy states.

The electrons rapidly lose the absorbed energy, which is emitted as light within a well-defined wavelength range i.e. the crystal luminesces.

The luminescence is not an inherent property of the diamond itself, but is the result of the nitrogen related defects in the diamond.

Two luminescence systems associated with diamond are known as the H3 and N3 systems. The luminescence spectra of these two systems contain sharp lines known as the zero phonon line and an associated broad band spectrum known as the vibronic sideband.

Both luminescence systems are caused by nitrogen-related defects.

Diamonds are conventionally classified into types Ia, Ib, IIa and lib. Most natural diamonds are of type Ia and they contain fairly substantial amounts (of the order of 0,1%) of nitrogen as impurity. Type Ib diamonds are fairly rare in nature but almost all synthetic diamonds are of this type. Type Ib diamonds typically contain nitrogen concentrations of the order of 500ppm. Type IIa diamonds have a lower nitrogen concentration and are very rare in nature, while type IIb diamonds have a negligible nitrogen concentration, and are extremely rare in nature.

H3 luminescence has been detected in diamond types Ia, Ib and IIa while N3 luminescence has been detected in diamond types Ia and Ib.

The present invention seeks to use this phenomenon in a diamond sorting method and apparatus.

SUMMARY OF THE INVENTION The invention provides a method for sorting a mass of particulate material into a diamond fraction and a gangue fraction, the method including subjecting the particles of the mass to laser radiation at a frequency chosen to cause H3 luminescence in diamond, at a frequency chosen to cause N3 luminescence in diamond, or at both such frequencies, detecting which particles luminesce in the wavelength ranges of H3 or N3 luminescence at such frequency or frequencies of incident laser radiation, and selecting those particles which luminesce in these wavelength ranges.

The invention also provides for the cooling of the mass of particles prior to the particles being subjected to the laser radiation.

In a preferred form of the method, the particles are subjected individually to laser radiation at both the chosen frequencies either simultaneously or in sequence.

The invention also provides apparatus for sorting a mass of particulate material into a diamond fraction and a gangue fraction, the apparatus including means for irradiating the particles with laser radiation at a frequency chosen to cause H3 luminescence in diamond, at a frequency chosen to cause N3 luminescence in diamond or at both such frequencies, means for detecting which of the particles in the mass luminesce in the wavelength ranges of H3 or N3 luminescence at such frequency or frequencies of incident laser radiation, and means for selecting those particles which do luminesce in these ranges.

BRIEF DESCRIPTION OF THE DRAWING It shows a schematic illustration of apparatus according to the invention.

DESCRIPTION OF AN EMBODIMENT In the drawing, a mass of ore particles 10, composed of diamond particles and unwanted gangue particles, is organised into a stream on an endless conveyor belt 12 passing over a head pulley 14.

As the belt passes around the head pulley 14, it projects the stream of particles 10 in free flight. The particles are subjected to laser radiation at a position 18. Although only a single laser 20 is illustrated in the drawing, the particles are in fact subjected to incident laser radiation at two distinct frequencies, being the frequencies known to cause H3 and N3 luminescence in diamonds. This can be achieved by two separate lasers or by a single laser which is operated in all-lines mode with a suitable filter system, possibly including a rotating filter, the filter system serving to filter out all wavelengths other than those known to cause H3 and N3 luminescence.The preferred laser-wavelengths are about 463nm, at which corresponds to the -frequency at which H3 luminescence is stimulated in diamond, and about 380nm, which corresponds to the frequency at which N3 luminescence is stimulated.

The incidence of the laser radiation causes diamond particles in the stream to luminesce and this is detected by a photodiode array illustrated schematically at 22. Signals from the array are fed to a processor 24 which analyses those signals and which controls the operation of a fluid blast nozzle 26 situated downstream of the array. When luminescence of a particle in the wavelength range of H3 or N3 luminescence is detected, the fluid blast nozzle is actuated at the appropriate time to issue a short duration blast of fluid, usually air. The relevant particle is ejected from the stream of particles and falls into a bin 28.

Other particles which do not luminesce in the H3 or N3 luminescence wavelength range are not deflected from the stream but continue unimpeded into a waste bin 30.

From what was said earlier in this specification, it will be appreciated that the apparatus described above will be useful in sorting type Ia, Ib and IIa diamonds from unwanted gangue because of their higher concentrations of nitrogen. The apparatus is considered to be inappropriate for sorting type IIb diamonds from the gangue, the reason being that the nitrogen content of type 1Ib diamonds is so low that neither H3 nor N3 luminescence can be expected to occur. This is not considered to be a serious flaw in the invention in that type IIb diamonds are extremely rare in nature.

It should be noted that it may be preferred in some cases to have a single laser only which will irradiate the particles with a frequency known to cause H3 luminescence only. Alternatively, the frequency may be that which will cause N3 luminescence only.

Yet another alternative is to have two lasers which will operate in sequence irradiate the particles with laser radiation at the appropriate frequencies. Of course, it will usually be preferable to irradiate with both frequencies so that all types of diamonds (with the exception of type IIb) will be detected and separated.

As stated previously, the preferred laser wavelengths are approximately 463nm for H3 luminescence and approximately 380nm for N3 luminescence. One or more dye lasers can be used to produce incident radiation at these wavelengths. Tests indicate that, as an alternative, an Argon ion laser can be operated at wavelengths of about 5l4nm and 488nm to produce H3 and N3 luminescence in the diamond particles. A krypton laser is yet another possibility which has been shown to produce luminescence at the appropriate wavelengths of incident laser radiation.

As a simple illustration of the principles of the invention, blue luminescence can be stimulated in diamond particles when those particles are subjected to laser radiation at the N3 excitation wavelength. The blue luminescence indicates the presence of nitrogen-related defects giving rise to N3 luminescence.

Similarly, yellow/green luminescence, indicative of H3 luminescence, can be stimulated in diamond particles subjected to the H3 excitation wavelength.

In the practical form of the invention as illustrated, the photodiode array can be replaced by a suitable filter system to filter out luminescent wavelengths outside the N3 or H3 ranges.

The most accurate sort will be achieved when a sharp zero phonon line indivative of N3 or H3 luminescence is detected. Thus the photodiode array or filter system may be arranged for the wavelengths associated with the zero phonon lines of N3 and H3 luminescence to be detected, other wavelengths being excluded.

The accuracy in this case can be increased by cooling the particles down prior to sorting, since a reduced temperature will lead to increased sharpness and detectability of the zero phonon lines for N3 and H3 luminescence.

If the sort is carried out at room temperature the zero phonon lines will not be so sharp and the selection decision can then be made on the basis of an analysis of a broader band spectrum in the range of say 500 to 575nm. In this case, luminescence attributable to N3 and H3 nitrogen related defects as well as luminescence attributable to 3H, S1 and AH3 nitrogen-related defects will also be detected.

Claims

CLAIM5:

1.

A method for sorting a mass of particulate material into a diamond fraction and a gangue fraction, characterized in that the method includes subjecting the particles of the mass to laser radiation at a frequency chosen to cause H3 luminescence in diamond, at a frequency chosen to cause N3 luminescence in diamond, or at both such frequencies, detecting which particles luminesce in the wavelength ranges of H3 or N3 luminescence at such frequency or frequencies of incident laser radiation, and selecting those particles which luminesce in these wavelength ranges.

2.

The method of claim 1 characterized in that the laser radiation has a frequency of about 463nm, a frequency of about 380nm, or both such frequencies.

3.

The method of claim 1 characterised in that the particles are subjected to laser radiation by means of an argon ion laser operated at wavelengths of about 514nm and 488nm.

4.

The method of claim 1 characterized in that a single laser is operated in all-lines mode with a filter system to filter out frequencies other than those chosen to stimulate H3 or N3 luminescence in diamond.

5.

The method of any one of the preceding claims wherein the particles of the mass are cooled before they are subjected to the laser radiation.

6.

Apparatus for sorting a mass of particulate material into a diamond fraction and a gangue fraction, characterized in that the apparatus includes means for irradiating the particles with laser radiation at a frequency chosen to cause H3 luminescence in diamond, at a frequency chosen to cause N3 luminescence in diamond or at both such frequencies, means for detecting which of the particles in the mass luminesce in the wavelength ranges of H3 or N3 luminescence at such frequency or frequencies of incident laser radiation, and means for selecting those particles which do luminesce in these ranges.

7.

The apparatus of claim 6 characterized in that the means for irradiating the particles with laser radiation comprises a single laser operable in all lines mode, and wherein a filter sytem is provided to filter out all frequencies in the laser radiation other than a frequency chosen to cause N3 luminescence in diamond, a frequency known to cause H3 luminescence in diamond, or both such frequencies.

8.

The apparatus of claim 6 or claim 7 characterized in that the means for irradiating the particles is arranged to irradiate the particles with laser radiation having a frequency of about 463nm, a frequency of about 380nm, or both such frequencies.

9.

The apparatus of claim 6 characterized in that the means for irradiating the particles with laser radiation is an Argon ion laser adapted to operate at wavelengths of about 514nm and 488 nm.

10.

The apparatus of any one of claims 6 to 9 wherein the means for detecting luminescence in the particles comprises a photodiode array.

11.

The apparatus of any one of claims 6 to 10 wherein means are provided for cooling the particles of the mass prior to their being subjected to laser radiation.

12.

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

13.

A sorting apparatus substantially as herein described with reference to the accompanying drawing.