WO1988001379A1

WO1988001379A1 - Laser ablation inspection

Info

Publication number: WO1988001379A1
Application number: PCT/AU1987/000268
Authority: WO
Inventors: Albert Peter Hawkins; Terence Charles Hughes
Original assignee: Cra Services Limited
Priority date: 1986-08-15
Filing date: 1987-08-17
Publication date: 1988-02-25
Also published as: ZA876083B

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 a pulse of laser radiation from a laser generator (14), via a beam expander (15), a mirror (17) and a focusing lens (18), 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 (25) which transmits the radiation to a spectrometer.

Description

LASER ABLATION INSPECTION TECHNICAL FIELD

This invention relates to methods and apparatus fo inspecting material samples to determine their composition or least the presence of a particular substance therein. The invention has particular, but not exclusive, application to t inspection of a continuous material stream in grade control, material classification, ore sorting and drill core logging applications.

There are known material classification and ore sorting systems in which a stream of material is fed through a inspection station where the material is inspected by optical scanning, x-ray fluorescence, radiation detection, magnetic permeability measurements or other inspection technique and th separated into different fractions according to the results of the inspection. Similar inspection techniques are also applie to drill core logging in exploration and mining operations. T present invention provides a novel inspection technique whereb 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 meth of inspecting a sample for presence of a particular substance, comprising subjecting the surface of the sample to a pulse of laser radiation so as to cause ablation of a quantity of material from the surface into a plume and examining the plum for presence of said substance therein.

The plume may be examined by atomic emission spectroscopy. More specifically, the plume may be examined fo spectral emission lines associated with said substance and due to atomic emission generated in the plume by the energy of the laser radiation. In that case, it is preferred that the examination for spectral emission lines should be conducted after a time delay from formation of the plume in order to all the emission spectrum to quieten. Alternatively the plume could be examined by atomic absorption spectroscopy, by atomic fluorescence or by any other convenient technique.

The invention also extends to a material classification or sorting process in which a stream of samples is inspected by the above-described method and subsequently separated into fractions according to the result of the inspection.

The invention also provides apparatus for inspecting a sample for presence of a particular substance comprising a laser generator to generate a pulse of laser radiation; sample presentation means to present a sample to be inspected for exposure of its surface to the pulse of laser radiation whereby to cause ablation of a quantity of material from the surface of the sample into a plume_? and plume examination means to examine the plume for the presence of said substance therein.

The plume examination means may comprise an atomic emission spectrometer.

The invention also extends to material classificatio or sorting apparatus incorporating inspection apparatus of the above kind.

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

A high peak laser pulse focused on the surface of a substance causes material in the region of the irradition zone to be ablated. The ablation, process involves creation of such high temperatures that material breaks down resulting in the excitation of atomic and ionic spectra present in an optical plume of free atoms. Each plume has a population of excited atoms in a mixture representative of the element mixture in t region of the resulting crater, which may typically be about 0.5 to. lmm diameter by 0.5mm deep. By sampling a moving stre of material at a sufficiently high sampling density, an overa assessment of local concentation 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 quiet and thus the useful analytical lines become more prominent.

An optical multichannel analyser with time resoluti 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 whi the spectral lines are read electronically (after initiation o the laser pulse) may be chosen to optimize identification of elements under interes .

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

By the present invention it is possible to remotely sample a stream of material in such a manner as to provide information for a subsequent sorting, classification or contro action. Typically, the optical spectral channel response for each ..aser shot is accumulated and averaged over an area of surface under examination. The extent of this area is chosen 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 classificatio procedure based on the above-mentioned element spectral signatures pre-taught to the processor. In this way the closes 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 (i.e. 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 emission spectroscopy, where selected lines are measured in the spectrum of the atomic plume generated by the high power laser pulse itself. This can be enhanced depending on the elements of interest by careful choic 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 spectra of all individual chemical elements which make up the 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 i 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 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 C) the population of atoms in the ground state alwa far exceeds that of atoms in excited states and this populatio unlike that of excited states is relatively insensitive to min temperature variations. This allows a more reliable quantatative measure to be made of the abundance of a particul 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 i normally carried out at 90° 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° C), as can be obtained by electrical discharges or laser irradiation, higher 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 usin 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 are lead at the 405.8nm energy level and silver at the 302.4 a 304.7nm energy levels. BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully explained, its application to the detection of minerals in 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 analyser for analysing mineral content of drill cores by a method according to the invention;

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 excitation by for example the 2676AO resonance transition to th

2 o 6 P %level. Flourescence measurements would involve, for example, a decay from 6 P°3/2 excited level to ground state wit

2428A° radiation, or the 3123A° radiation to the metastable

Q n n

5d 6s^ TD5/2 state. Such a metastable state can be used as note above for atomic absorption, measuring the absorption of 3123A radiation exciting atoms to the 6^P°3/2 state.

With reference to the lead energy level diagram of

Figure 2, the emission process involved for the detection of lead at 4058A° arises from decay from the P_]_° state to the

₃ metastable P state, the former higher energy state being populated by thermal collision processes due to the high intensity laser-solid interaction. Figure 3 illustrates the application of the present invention to a drill core analyser for analysing mineral conte 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 comput 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 len on the table 13. Lens 18 is carried on a platform 19 disposed above table 13 and movable up and down 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 co beneath platform 19 and produces servo signals to automaticall adjust the height of the platform so as to maintain the laser focusing lens 18 at a fixed distance above the upper surface o 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 tra Typically, the laser generator may produce 50 laser pulses per second, allowing a core travel rate of around 25mm/sec. 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 fro being measured, hence increasing the detection capability of th 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 mean 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 pulverise powder for splitting and making x-ray fluorescence or similar analysis produces low average values of trace minerals, whereas the illustrated apparatus enables precise spacial identificatio 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, n vacuum requirements and automatic operation.

Figures 4 to 8 show plots of response curves derived from the spectrometer of an apparatus of the general kind illustrated in Figure 3 during analysis of certain test materials. Figure 4 illustrates the spectrometer response to irradiation of a solid lead sample. The lines labeled "LASER 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 represents an interval of 20μ sec and each division of the 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 a 3864.1A°and nickel at 3414.7A°. For the lead molybdenum and 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 a the molybdenum and nickel lines.

Figures 5 and 6 show response curves resulting fro scans of core from a base metal deposit containing lead and z sulphides and country rock typically comprised of garnet quartzites. Figure 5 shows the lead line response at 4057.8A and the zinc line response at 3345.0A obtained from a low gr mineralized zone of material and Figure 6 shows the responses obtained from a non-mineralized zone of the same material. I 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 achieve from a relatively high grade ore body. Figure 7 illustrates lead line and zinc line responses obtained from a mineralized zone of the material whereas Figure 8 shows virtually no response for these lines from a non-mineralized zone from the same material.

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

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

Material 31 cut from the mine face is fed via a conveyor 32 onto a short slide plate 33. Size of the material is typically 25mm and in chip form as produced by the hard roc cutter. The throughputs may typically be up to 50tph.

The material leaving slide plate 33 falls freely under gravity and the falling stream is irradiated by pulses o 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 pe line (1000 pulses per second from the laser). Each pulse whic irradiates mineral surface is typically 5MW peak power with about 1 microsecond duration. It has been determined that this is sufficient to remove approximately 100 microns depth by 1mm 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 t be read simultaneously by separate photomultipliers.

Photomultiiplier 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 PCT/AU86/00284) .

A decision will then be made in the processor to either allow the material to continue in the processing stream by passing onto a conveyor 41, or be rejected by actuating flap 42 so that the reject material is removed from further 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 grad ore without significant waste dilution to the subsequent processing circuit, thus saving significant costs such as would have been incurred by hauling waste material to the surface, an 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 separat stage. Use of a continuous cutting automated mining system would as nearly as possible ensure that material spatial association at the mine wall is preserved through to the sort apparatus.

INDUSTRIAL APPLICABILITY

As typified by the illustrated forms of apparatus, the invention has particular application to core logging and sorting or classification operations. However, the invention not limited to these particular applications. For example, apparatus constructed in accordance with the invention could used for major element identification of rock at a mine face during automatic mining procedures in order to direct the progress of the ore gathering. The continual exposure of fres surface at the rock face due to the cutter lends itself ideal to such automatic analysis.

Apparatus in accordance with the invention could a be used to assay directly in real time slag flow from smelters and the results could be fed back to help control the smeltin process. It will also be appreciated that the invention coul also be applied to the evaluation of material on a particle-by-particle or sample-by-sample basis in which case individual samples could be submitted separately for inspectio and there need not be a continuous stream of material for analysis.

Claims

1. A method of inspecting a sample for presence of a particular substance, comprising subjecting the surface of the sample to a pulse of laser radiation so as to cause ablation of a quantity of material from the surface into a plume and examining the plume for presence of said substance therein.

2. A method as claimed in claim 1 wherein the plume is examined for spectral emission lines associated with said substance and due to atomic emission generated in the plume by the energy of the laser radiation.

3. A method as claimed in claim 2, wherein the examination for spectral emission lines is conducted after a time delay from formation of the plume in order to allow the emission spectrum to quieten.

4. A method as claimed in claim 1 wherein the plume is examined by atomic absorption spectroscopy.

5. ^* A method as claimed in claim 1 wherein the plume is examined by detection of atomic fluorescence in the plume.

6. A material classification or sorting process in which a stream of samples is inspected by a method as claimed in any one of the preceding claims and subsequently separated into fractions according to the result of the inspection.

7. Apparatus for inspecting a sample for presence of a particular substance comprising: a laser generator to generate a pulse of laser radiation; sample presentation means to present a sample to be inspected for exposure of its surface to the pulse of laser radiation whereby to cause ablation of a quantity of material from the surface of the sample into a plume; and plume examination means to examine the plume for the presence of said substance therein.

8. Apparatus as claimed in claim 7 wherein the examination means comprises an atomic emission spectrometer.

9. Apparatus as claimed in claim 8 wherein the plume examination means further comprises an optical fibre to receive emissions from the plume and to transmit those emissions to the emission spectrometer.