AU608247B2 - Laser ablation inspection and sorting - Google Patents

Description

Pcr A-AI-787 12 87 WORLD INTCLLCCTU 'I'm 'L O0 41 Intcr L- INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 4 (11) International Publication Number: WO 88/ 01379 CGON 21/71 Al (43) International Publication Date: 25 February 1988 (25.02.88) (21) International Application Number: PCT/AU87/00268 (74) Agent: CLEMENT HACK CO.; 601 St. Kilda Road, Melbourne, VIC 3004 (AU).

(22) International Filing Date: 17 August 1987 (17.08.87) (81) Designated States: AT (European patent), AU, BE (Eu- (31) Priority Application Number: PH 7491 ropean patent), CH (European patent), DE (European patent), FR (European patent), GB (European (32) Priority Date: 15 August 1986 (15,08,86) patent), IT (European patent), JP, LU (European patent), NL (European patent), SE (European patent), (33) Priority Country: AU US.

(71) Applicant (for all designated States except US): CRA Published SERVICES LIMITED [AU/AU]; 55 Collins Street, With international search report, Melbourne, VIC 3000 (AU).

(72) Inventors; and O.J.P. 3 1 MAR 1988 Inventors/Applicants (for US only) HAWKINS, Albert, Peter [AU/AU]; 36 Neerim Road, Caulfield, VIC 3162 HUGHES, Terence, Charles [AU/AU]; Unit 16, AUSTRALIAN 19 Mcllwraith Street, Carlton North, VIC 3054 8 iMAR 1988 PATENT OFFICE This document contains theATENT amendments made under Section 49 and is correct for printing (54) Title: LASER ABLATION INSPECTION 0~P CzQ- OC\ Q (57) Abstract A method and an apparatus of inspecting a sample (12) for presence of a particular substance, in which the surface of the sample is subjected to pulse of laser radiation from a laser generator via a beam expander a mirror (17) and a focusing lens and so as to cause ablation of a quantity of material from the surface into a plume. The plume is then examined for presence of the substance therein by collecting radiation emitted from the plume using an optical fibre which transmits the radiation to a spectrometer.

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I LASER ABLATION INSPECTION AND SORTING TECHNICAL FIELD This invention relates to methods and apparatus for inspecting material samples and sorting thum according to their composition or at least the presence of a particular substance therein.

There are known material sorting systems in which a stream of material is fed through an inspection station where the material is inspected by optical scanning, x-ray fluorescence, radiation detection, magnetic permeability measurements or other inspection technique and then separated into different fractions according to the results of the inspection. The present invention provides a sorting method and apparatus in which the material is inspected by a laser ablation technique whereby the presence of a particular substance within the material inspected can be determined to a high degree of accuracy.

DISCLOSURE OF THE INVENTION According to the invention there is provided a method of sorting material according to the presence of a particular substance therein, comprising feeding the material through an inspection station, directing pulses of laser radiation onto the material as it passes through the inspection station so as to cause ablation cf material from localized regions of the material surface into plumes, examining the plumes for the presence of said substance therein, and subsequently separating the material into fractions according to the extent of the presence of said particular substance therein indicated by the inspection made at the inspection station.

The material may be fed and inspected on a piece by piece basis or in bulk depending on the particular application of the invention.

The plumes may be examined by atomic emission spectroscopy. More specifically, the plume may be examined for spectral emission lines associated with said substance and due 2 to atomic emission generated in the plume by the energy of the laser radiation.

Alternatively the plumes could be examined by atomic absorption spectroscopy, by atomic fluorescence or by any other convenient technique.

The invention also provides material sorting apparatus for sorting material according to the presence of a particular substance therein, comprising material feed means operable to feed the material to be sorted through an inspection station; inspection means to inspect the material as it passes through the inspection station, the inspection means comprising a laser generator to direct pulses of laser radiation onto the material as it passes through the inspection station so as to cause ablation of material from localized regions of the surface of the material into plumes and plume examination means to examine the plumes for the presence of said substance therein; S and material separator means effective to separate the material into fractions in response to the output of the plume examination means.

see*The plume examination means may comprise an atomic Semission spectrometer.

.ioe o The basis of the inspection technique in the method and apparatus of the invention is the laser ablation of a small sample volume of a material sample. The technique can be .applied to inspection of moving stream of particulate material S for sorting purposes and the ablation and inspection procedure can be carried out at high repetition rates in order to maximize o stream sample coverage.

A high peak laser pulse focused on the surface of a substance causes material in the region of the irradiation zone to be ablated. The ablation process involves creation of such high temperatures that material breaks down resulting in the excitation of characteristic atomic and ionic spectra present in an optical plume of free atoms. Each plume has a population of Rj IRI A 1$ C' i tr 3

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excited atoms in a mixture representative of the element mixture in the region of the resulting crater, which may typically be about 0.5 to 1mm diameter by 0.05mm deep. By sampling a moving stream of material at a sufficiently high sampling density, an overall assessment of local concentration can be inferred by optical spectroscopy techniques.

A time delay between formation of a plume and spectral line measurement is desirable in order to avoid the measurement of high generally featureless continuum. The measurement at later times allows the spectrum to become quieter and thus the useful analytical lines become more prominent.

An optical multichannel analyser with time resolution capability (such as a diode array or polychromator) able to resolve spectral lines of interest in the elemental mixture under investigation may be used to provide a simultaneous multi-element signature for each laser pulse. The time at which the spectral lines are read electronically (after initiation of the laser pulse) may be chosen to optimize identification of elements under interest.

A self learning automated classification routine may be applied to the multiple spectral outputs of the polychromator system in order to indicate the best mineral species match in a taught. library for the unknown sample. This can then lead to multiway material separation depending on the physical characteristics of the separation device.

Typically, the optical spectral channel response for each laser shot is accumulated and averaged over an area of surface under examination. The extent of this area is chosen to be consistent with subsequent control action such as the expulsion of waste portions of material detected in ore streams or control of an ore cutting head consistent with the "bite" size of the cutter. The averaged element-related spectral contributions may then be subjected to a classification procedure based on the above-mentioned element spectral signatures pre-taught to the processor. In this way the closest 4 mix of elements to that contained in a stored "library" mix of elements is selected and so attributed to the area of surface under examination. Action can then be taken on the basis of that selection, The above procedure enables identification on the basis of combinations of element responses working with ratios of spectral wavelength intensity values instead of absolute levels) which, being normalized for intensity, reduces the sensitivity to changes in absolute intensity of each plume.

Although other spectrographic techniques are possible, the simplest and most practical technique for examination of the plume is atomic emissicn spectroscopy, where selected lines are measured in the spectrum of the atomic plume S generated by the high power laser pulse itself. This can be enhanced depending on the elements of interest by careful choice S of time of plume interrogation after initiation of the laser pulse. For example, in some species the resident time of excited atoms in higher orbital metastable states is long compared with the laser excitation pulse duration and this fact may be used in characterizing such species.

The light coming from the emission is the sum of o Sspectra of all individual chemical elements which make up the S ablated sample. Thus, the separation of different wave lengths characteristic of specific elements is carried out by a spectrometer. As mentioned above, separation may be enhanced in certain circumstances by using excitation lifetime information.

It is also possible to enhance the sensitivity and selectivity by using a second light source such as a tuned dye S: laser or high intensity hollow cathode lamp carefully directed through the plume generated by the high powered pulsed laser.

This measurement is a form of atomic absorption spectroscopy which conventionally allows the measurement of radiation absorbed by free atoms or ions in the ground state.

For low temperature atomic vapours and plumes (1000-2500 0 C) the population of atoms in the ground state always

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5 far exceeds that of atoms in excited states and this population, unlike that of excited states is relatively insensitive to minor temperature variations. This allows a more reliable quantatative measure to be made of the abundance of a particular element as is practised in conventional atomic absorption spectroscopy.

A third possible technique for carrying out quantatative measurement of element abundance is by atomic fluorescence spectroscopy, with essentially the same equipment as the atomic absorption system. In this case the detection is normally carried out at 900 to the incoming direction of the absorbed beam of the tuned laser (or high intensity hollow cathode lamp) at a different elemental characteristic wave length.

At higher temperatures (above 3000 oC), as can be S obtained by electrical discharges or laser irradiation, higher a ee: energy excited states of the constituent atoms present can be populated to a significant extent. When these excited atoms decay, they often produce metastable species which are stable for sufficient time to allow time resolution spectroscopy using se either atomic absorption or atomic fluorescence techniques to obtain analytical measurements. These metastable states occur when an atom has low lying excited states with the same electronic parity as the ground state and hence are usually depopulated with difficulty under normal conditions. Examples S are lead at the 405.8nm energy level and silver at the 302.4 and 0 304.7nm energy levels.

600 BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more fully explained, its application to the sorting of mineral ores will now be described in some detail with reference to the accompanying drawings, in which:- Figures 1 and 2 are energy level diagrams for gold and lead; Figure 3 illustrates diagrammatically a drill core 6analyser for analysing mineral content of drill cores; Figures 4 to 8 are plots of response curves obtained in the inspection of various rock samples using an analyser of the general kind illustrated in Figure 3; and Figure 9 illustrates diagrammatically a bulk ore sorter constructed in accordance with the invention.

BEST MODES OF CARRYING OUT THE INVENTION Figures 1 and 2 of the accompanying drawings are energy level diagrams for two elements of potential interest, viz gold and lead, which illustrate how atomic absorption and atomic fluorescence techniques may be applied in a process according to the invention. With reference to the gold diagram in Figure 1, a simple absorption process -would involve direct eoe S excitation by for example the 2676A' resonance transition to the 6 2 P' level. Flourescence measurements would involve, for S example, a decay from 6 0

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0 3/2 excited level to ground state with *e S 2428A radiation, or the 3123AI radiation to the metastable 9 6s 2 2D5/2 state. Such a metastable state can be used as noted S above for atomic absorption, measuring the absorption of 3123A radiation exciting atoms to the 6 2 pO3/2 state.

With reference to the lead energy level diagram of Figure 2, the emission process involved for the detection of lead at 4058Ao arises from decay from the Pi. state to the S metastable 32 state, the former higher energy state being populated by thermal collision processes due to the high intensity laser-solid interaction.

Figure 3 illustrates a drill core analyser for analysing mineral content of drill cores. In this apparatus a standard core tray 11 containing lengths of diamond drill core 12 is placed on a horizontal table 13 which is movable horizontally under computer control. A laser generator 14 fitted with a beam expander 15 generates a laser beam 16 which is directed by a mirror 17 vertically downwards and through a focusing lens 18 onto the surface of the drill core 12 being transported beneath the lens on the table 13. Lens 18 is i -7 0S

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carried on a platform 19 disposed above table 13 and movable up and Uown on the main frame 21 of the apparatus by servo mechanisms 22 in response to servo signals derived from an optical height sensor 23 also carried on platform 19. The optical height sensor contains a lens system which focuses on the upper surface of the drill core beneath platform 19 and produces servo signals to automatically adjust the height of the platform so as to i 1 ,,aintain the laser focusing lens 18 at a fixed distance above the upper surface of each drill core passing beneath it so as to maintain proper focusing of the laser beam regardless of variations in core size.

The laser generator 14 produces a pulsed laser beam and the table moves under computer control so as to cause a practically continuous line of laser pulses to successively impinge on the upper surface of the cores contained in the tray.

Typically, the laser generator may produce 50 laser pulses per second, allowing a core travel rate of around Impingement of each laser pulse causes ablation of a small quantity of the core surface material into a plume 24.

Radiation emitted from the plume is collected by an optical fibre 25 through which it is transmitted to a spectrometer for analysis. The fibre view direction is transverse to the plume, thus avoiding direct viewing of the material surface being ablated. This reduces the non-analytical optical continuum from being measured, hence increasing the detection capability of the system.

The apparatus may be controlled and monitored by an appropriate computer system which logs positions of selected element activity above a predetermined threshold. By this means it is possible to identify thin vein regions of, for example, precious metals or indicator minerals, flagging their presence and position in the core suite. The traditional method of taking relatively large lengths of core and producing pulverised powder for splitting and making x-ray fluorescence or similar analysis produces low average values of trace minerals, whereas

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i 8 the illustrated apparatus enables precise spacial identification of fine mineral occurrence which can in many cases serve to identify geological forms of broader interest. The apparatus has the added advantage of no preparation time, no vacuum requirements and automatic operation. It can readily be adapted to automatically sought the inspected material by incorporating a two-way or multi-way material separator at the end of table 13 downstream from the inspection position in order to separate the material into fractions according to the results of the inspection. The separator may be of a kind well known in sorting systems and may be under the direct control of the computer system of the apparatus.

Figures 4 to 8 show plots of response curves derived oio S from the spectrometer of an apparatus of the general kind o illustrated in Figure 3 during analysis of certain test S materials. Figure 4 illustrates the spectrometer response to irradiation of a solid lead sample. The lines labeled "LASER S OUTPUT" shows the response of an infra red detector viewing the reflected energy from the exciting laser pulse derived from a Nd:YAG laser of about 1 Joule output energy without Q-switching.

For this line each division of the horizontal time axis 0@@S represents an interval of 20 sec and each division of the .me.

vertical axis represents a spectrometer output of 1 volt. The line labelled "Pb" shows the response for lead at 4057.8A and the lines labelled Mo and Ni show the response for molybdenum at 3864.1A and nickel at 3414.7 0 For the lead molybdenum and S nickel lines the horizontal time scale is the same as for the laser response line but the vertical scale divisions each represent an output voltage of 2 volts rather than 1 volt and the curves are plotted from a different base to provide better discrimination between the lines. The large (saturated) lead response is apparent whereas there is virtually zero response at the molybdenum and nickel lines.

Figures 5 and 6 show response curves resulting from scans of core from a base metal deposit containing lead and zinc -9sulphides and country rock typically comprised of garnet quartzites. Figure 5 shows the lead line response at 4057.8A 0 and the zinc line response at 3345.0A 0 obtained from a low grade mineralized zone of material and Figure 6 shows the responses obtained from a non-mineralized zone of the same material. It will be seen that there is a massive response from the mineralized zone but virtually no response from the non-mineralized zone.

Figures 7 and 8 illustrate similar results achieved from a relatively high grade ore body. Figure 7 illustrates the lead line and zinc line responses obtained from a mineralized Szone of the material whereas Figure 8 shows virtually no response for these lines from a non-mineralized zone from the oo same material.

In Figures 4 to 8 each division of the horizontal 0 off scale represents a time interval of 50p sec and each division of the vertical scale represents a spectrometer output of 2 volts.

Figure 9 illustrates a bulk ore sorter constructed in accordance with the invention. This ore sorter may for example be a base metal sulphide sorter located at the cutting face in an ornamated hard rock underground mining plant.

Material 31 cut from the mine face is fed via a S conveyor 32 onto a short slide plate 33. Size of the material S is typically 25mm and in chip form as produced by the hard rock cutter. The throughputs may typically be up to The material leaving slide plate 33 falls freely under gravity and the falling stream is irradiated by pulses of light from a Q-switched or pulse-pumped Nd:YAG laser 34, directed via focusing lens 35 and fixed mirror 36 onto a scanning polygon mirror 37 so that the material is covered by scanning at approximately 10 lines per second and 10 pulses per line (1000 pulses per second from the laser). Each pulse which irradiates mineral surface is typically 10GW/cm peak power with about 10 nanosec duration. It has been determined that this is sufficient to remove approximately 100 microns depth by imm AI p 1 10 diameter of material in the form of an ionized plasma of atomic vapour. This plasma generally contains characteristic optical emission lines of the elements ablated from the rock fragments.

A number (typically 10) of these emission lines are introduced via an input collection lens 38 into a polychromator 39 having a set of slits chosen to allow selected element optical lines to be read simultaneously by separate photomultipliers.

Fhotomultiiplier outputs are sampled at an appropriate time after initiation of each pulse plume and the computer processor 40 classifies the pulse according to its mix of element spectral intensities (matching to the closest of a pretaught library of spectral signatures in an identical way to that described in our International Patent Application S PCT/AU86/00284).

A decision will then be made in the processor to 0oo S either allow the material to continue in the processing stream ,ee by passing onto a conveyor 41, or be rejected by actuating flap o 42 so that the reject material is removed from further f processing via a conveyor 43.

In a typical system, the classification decisions from several proximally located pulse plumes may be averaged together to achieve a composite grade for lumps of material of typically 2kg mass, this being the minimum flap actuating response time.

By the above means, it is possible to feed high grade ore without significant waste dilution to the subsequent S processing circuit, thus saving significant costs such as would

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have been incurred by hauling waste material to the surface, and feeding it to mill concentrators.

As the above process does not act on each separate rock, but rather at approximately 2kg "parcels" of material, it is important to ensure that the material is not mixed to any extent from being won from the oreface through to the separating stage. Tse of a continuous cutting automated mining system would as nearly as possible ensure that material spatial 11 association at the mine wall is preserved through to the sorting apparatus.

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Claims (7)

1. 2. A method as claimed in claim 1, wherein the plumes 00 are examined for spectral emission lines associated with said o g substance and due to atomic emission generated in the plumes by SO the energy of the laser radiation.
2. 3. A method as claimed in claim i, wherein the plumes are examined by atomic absorption spectroscopy.
3. 4. A method as claimed in claim i, wherein the plumes e are examined by detection of atomic fluorescence in the plumes.
4. 5. Material sorting apparatus for sorting material 00 according to the presence of a particular substance therein, *0 comprising material feed means operable to feed the material to be sorted through an inspection station; oinspection means to inspect the material as it passes through the inspection station, the inspection means comprising a laser generator to direct pulses of laser radiation onto the material as it passes through the inspection station so as to cause ablation of material from localized regions of the surface of the material into plumes and plume examination means to examine the plumes for the presence of said substance therein; and material separator means effective to separate the material into fractions in response to the output of the plume examination means. i: PA 13
5. 6. Apparatus as claimed in claim 5, wherein the plume examination means comprises an atomic emission spectrometer.
6. 7. Apparatus as claimed in claim 5, wherein the plume examination means comprises an atomic absorption spectrometer.
7. 8. Apparatus as claimed in claim 5, wherein the plume examination means comprises means for detection of atomic fluorescence in the plumes. S 0 S.. S 0 SS S DATED THIS 24TH DAY OF DECEMBER, 1990. CRA SERVICES LIMITED By its Patent Attorneys GRIFFITH HACK CO. Fellows Institute of Patent Attorneys of Australia OSSS 0 SS SS S S S S 5500 55 S S S S.