AU641273B2

AU641273B2 - Process of analyzing metal particles

Info

Publication number: AU641273B2
Application number: AU83565/91A
Authority: AU
Inventors: Kristian Hohla; Thomas Loree; Manfred Potzschke; Hans-Peter Sattler
Original assignee: Metallgesellschaft AG
Current assignee: GEA Group AG
Priority date: 1990-09-05
Filing date: 1991-09-04
Publication date: 1993-09-16
Anticipated expiration: 2011-09-04
Also published as: AU8356591A; DE4028102A1; EP0476728A1; MX173822B; TW198752B; JPH04264238A; CA2050608A1

Description

AUSTRALIA

Patents Act 1990 P/00/011 2815/91 Regulation 3.2(2)

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodged: *0 0 0 000 *0 0

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000000 0eS0 S0 S 0000 0 09 00 S Invention Title: PROCESS OF ANALYZING METAL PARTICLES The following statement is a full description of this invention, including the best method of performing it known to us 0600 PROCESS OF ANALYZING METAL PARTICLES Description This invention relates to a process for a classifying analysis of a polydisperse bulk material consisting of various metal particles having a particle size of 1 to 200 mm, preferably 10 to 80 rim, which differ in composition, with reference to the relative concentrations of the most frequently occurring metals, wherein metal is removed at a detecting location from a limited surface portion of each metal particle by means of a pulsed laser to form a corresponding plasma, optionally after the limited surface portion has previously been cleaned by means of a laser and the resulting plasma 1 0 has been ramoved, predetermined spectral lines or spectral line ranges are filtered from the line spectrum of the plasma, a characteristic value is determined according to an algorithm from the radiant intensities at the wavelengths of the spectral lines or spectral line ranges which have been filtered out, and a sorting signal is generated in dependence on the comparison of said characteristic value with predetermined limiting 15 values.

The use of lasers in spectroscopy permits, a local spectroanalysis of materials in minute volumes or on minute surface areas by means of a focused laser beam. The resulting plasma emits a line radiation of discrete spectral lines, which can be detected entirely or in part by means of a spectrometer-diode array and can be 2 0 analyzed by an optical multichannel analyzer so that a qualitative analysis of the metal particle is available a few milliseconds after the plasma has been formed. That process has been described as a Laser-Induced Breakdown Spectroscopy (LIBS) in the periodical EOSD Electro-Optical Systems Design, Vol. 14 (1982), No. 10, on pages 35 to 41 and with use of a pulsed Nd:YAG or CO 2 laser has been used, inter alia, for the sorting of 25 scraps consisting of metal particles differing in composition. The rate at which the metal particles can be analyzed is limited only by the repetition rate of the laser, the velocity of the response of the electric detector and the velocity at which the metal particles can be fed to the measuring location (periodical Applied Spectroscopy, Vol. 4J_, 1987, No. 4, pages 572 to 579).

In an embodiment of such a process disclosed in EP-A-O 293 983 at least metal particles having a particle size of 15 to 65 mm pass through a detecting location, at which a portion of the surface of each metal particle is cleaned by means of a pulsed laser beam, the resulting plasma is removed and the cleaned surface portion is again acted upon by a pulsed laser beam. Certain predetermined wavelengths or wavelength ranges are filtered from the line radiation of the resulting plasma and ratios are 2 determined from the radiant intensities at said wavelengths or wavelengths ranges and are compared with adjustable limiting values for the generation of a sorting signai. In that process, Nd:YAG lasers operating in the near infrared range or C002 lasers operating in the intermediate infrared range are employed and the pulsed laser beam produced by such lasers is directly focused, as a rule, on a point of the surface of the metal particle so that the metal is melted and vaporized. The resulting plasma may be contaminated by impurities which have been removed from the combusted boundary zones. A decisive disadvantage resides in that the contours of the surface and the thickness of the metal particles vary so that the laser beam is not focused on a point of the surface and, as a result, a constant energy density is not achieved on the irradiated surface.

For this reason it is an object of the present invention so to improve the process of the kind described first hereinbefore that the plasmas which are produced emit in addition to the physically inevitable white light only the discrete spectral lines which are specific to the metal particles and said plasmas can be produced to have 15 approximately the same intensity regardless of the thickness of the metal particles.

o:.o o •°That object is accomplished in that consecutive individual metal particles are continuously transported on a line with a spacing preferably of at least 0.75 mm from a S" feeding location through the measuring location and along a fixed path to respective O. discharge locations and are irradiated by an Excimer-laser at the detecting location.

The Excimer laser is an inert gas halide laser, which operates in the ultraviolet range with a short pulse length and distinguishes by having a large beam cross-section in conjunction with a high pulse energy and a plateaulike intensity distribution and a

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high pulse-to-pulse stability. Owing to its relatively large divergence the laser beam can be focussed to have a relatively large depth of field on a large area so that the process .25 by which material is removed can be adapted to the various surface contours and/or thicknesses of the metal particles and a setisfactory analysis of the metal particles is thus enabled. Compared to other pulsed lasers an Excimer laser has also the advantage .O.i that the properties of its beam, pulse length, pulse energy and, above all, beam divergence and, as a result, its focussability, are independent on its pulse repetition frequency. Because energy is introduced in a closely confined area into the material to be removed, an action on the boundary zones will substantially be avoided so that a contamintAior of the resulting plasma will be avoided too. Because the Excimer laser cannot be fired in air, the plasma also cannot be adversely affected by the plasma which would form in air and it is not necessary to subtract the spectrum of an air plasma from the spectrum of the plasma of the metal particle.

The spectral radiation of the plasma produced by means of an Excimer laser is almost independent of the work function of the metals.

Each metal particle entering the detecting location moves through an optical barrier and the signal whichl is thus generated is used in an electronic detector to control the Excimer laser.

To permit the laser to be fired in response to the output signal of the electronic detector, the consecutive metal particles should be spaced at least 0.75 mm apart. A feeding at a higher velocity will require a larger spacing of consecutive metal particles.

The optical barrier consists of a light beam, which is directed to a photocell and 1 0 is used to measure the length of each metal particle.

The optical barrier may alternatively consist of a laser beam or of a combination of a light beam and a laser beam so that each metal particle can spatially be detected.

According to a preferred feature of the process, spectral lines in the wavelength range from 200 to 800 nm are filtered from the spectrum emitted by the plasma.

15 The pulse energy applied by the Excimer laser to the metal particle amounts to 30 to 500 mJ.

The pulse rate of the Excimer laser is 10 to 500 Hz, preferably 100 to 150 Hz, Sand a consistent quality of the laser beam is desirably ensured at different frequencies.

660*°The analysis will be simplified if it covers only a small number of spectral lines or a narrow range of spectral lines. To this end the emitted spectral lines are filtered out during an interval of 0.2 to 50 pas, preferably 1.5 to 50 us, after the laser pulse so that the background noise of the plasma radiation will distinctly be reduced and the S• spectra can be more properly identified.

S•The invention will now be explained in more detail with reference to an illustrative embodiment and an associated drawing.

A conveyor belt 1 moving at a velocity of 2.5 m/s is used to feed metal particles 2 having an average particle size of 30 mm and taken from a polydisperse bulk scrap material consisting, on an average, of aluminum alloy scrap 12% zinc alloy scrap 8% copper alhoy scrap 7% brass scrap 7% special steel scrap 1% lead alloy scrap to a detecting location 3 for spectronanalysis. Each metal particle 2 entering the detecting location 3 is detected by a light beam, which is directed to a photocell, and by a laser beam, which comes from an He/Ne-laser 7 and is directed by a focusing lens 5 and a mirror 6. The resulting output signals generated by the photocell 4 and the He/Ne laser 7 are converted to electronic signals by an electronic detector 8 and both resulting signals are combined and used to trigger the Excimer laser 9. The laser beam from the Excimer laser 9 is directed by a mirror 10, the lens 5 and the mirror 6 onto the metal particle 2. By the first laser pulse the metal particle 2 is cleaned on a portion of its surface and the contaminated plasma which is thus produced is removed by an air 1 0 stream. The second plasma pulse applied to the cleaned surface portion of the metal particle 2 serves to produce the metal plasma, which is analyzed by a spectral detector 11, which is preceded by a spectral filter 12, which filters out only discrete spectral lines or spectral line ranges during an interval of 20 j.s after the laser pulse. The oopening of the time window of the spectral detector 11 is controlled by the electronic 15 detector 8, which determines also measured values in a computer-readable form.

Numerical ratios are derived from said measured values in a microprocessor 13 and are *o compared with predetermined limiting values for a generation of signals, which control a sorting apparatus by means of an electronic triggering system 14, that is started by the output signal of the photodetector 4.

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Claims

1. A process for a classifying analysis of a polydisperse bulk material consisting of various metal particles having a particle size of 1 to 200 mm, preferably 10 to 80 mm, which differ in composition, with reference to the relate., concentrations of the most frequently occurring metals, wherein metal is removed at a measuring location from a limited surface portion of each metal particle by means of a pulsed laser to form a corresponding plasma, optionally after the limited surface portion has previously been cleaned by said pulsed laser and the resulting plasma has been removed, predetermined spectral lines or spectral line ranges are filtered from the line spectrum of the plasma, a characteristic value is determined according to an algorithm from the radiant intensities at the wavelengths of the spectral lines or spectral line ranges which have been filtered out, and a sorting signal is generated in dependence on the comparison of said characteristic value with predetermined limiting values, characterized in that consecutive individual metal particles are continuously transported on a line from a feeding location through the measuring location and along a fixed path to respective discharge locations and are irradiated by said pulsed laser at the measuring location.

2. A process according to claim 1, characterized in that the metal particles are spaced at least 0.75 mm apart. 0

3. A process according to claim 1 or 2, characterized in that each metal V. particle entering the measuring location moves through an optical barrier aod the resulting signals are utilized in a trigger circuit for controlling the pulsed 0* 9 laser. 0 o 4. A process according to claim 3, characterized in that the optical barrier consists of a light beam directed to a photocell. 0 G A process according to claim 3 or claim 4, characterized in that the optical barrier consists of a laser beam.

6. A process according to any one of claims 1 to 5, characterized in that spectral lines in the wavelength range from 200 to 800 nm are filtered from the spectrum emitted by the resulting plasma.

7. A process according to any one of claims 1 to 6, characterized in that the pulse energy applied by the pulsed laser to the metal particle amounts to 30 to 500 mJ.

8. A process according to any one of claims 1 to 7, characterized in that the pulsed laser has a pulse rate of 10 to 500 Hz, preferably of 100 to 150 Hz.

9. A process according to any one of claims 1 to 8, characterized in that the emitted spectral lines are filtered out during an interval of 0.2 to 50 ps, preferably of 1.5 to 50 ps, after the laser pulse A process according to any one of the preceding claims, wherein said .i pulsed laser is an Excimer laser.

11. A process for a classifying analysis of a polydisperse bulk material Ssubstantially as hereinbefore described with reference to the accompanying drawings. DATED this 7th day of July, 1993. METAI.=LLESELLSCHAFT AKTIENGESELLSCHAFT s WATERMARK PATENT TRADEMARK ATTORNEYS i THE ATRIUM 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA AU008356591.WPC ABSTRACT In a process for a classifying analysis of a bulk material consisting of metal particles differing in composition, metal is removed at a detecting location from each metal particle by means of a pulsed laser, whereby a plasma is produced, discrete spectral lines are filtered from the line spectrum of the plasma, a characteristic value is determined according to an algorithm from the radiant intensities of said spectral lines, and a sorting signal is generated by o comparison of the characteristic value with predetermined limiting values. An Excimer laser is used for a removal of metal at a substantially uniform intensity regardless of the thickness of the metal particles. 0* 8@ 0* S e 8@