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Submitted on 1 Jan 1984
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USE OF THE X-RAY MICROANALYSER AS AN IMAGE ANALYSER
M. Jones
To cite this version:
M. Jones. USE OF THE X-RAY MICROANALYSER AS AN IMAGE ANALYSER. Journal de
Physique Colloques, 1984, 45 (C2), pp.C2-211-C2-214. �10.1051/jphyscol:1984246�. �jpa-00223959�
USE OF THE X-RAY MICROANALYSER AS AN IMAGE ANALYSER
M.F. Jones
Mineral Technology Department, Imperial College, London, U.K.
Résumé - Cette contribution décrit l'utilisation en analyseur d'images d'un appareil Camebax-Micro. La configuration initiale utilisait quatre spectro- mètres à cristaux. On y a inclu le signal dû aux électrons rétrodiffusés.
On décrit la technique d'adaptation et on présente quelques exemples d'ap- plications .
Abstract - This paper describes how the Camebax-Micro has been adapted for use as an automatic image analyser. Initially, the system was based on the simultaneous use of four wavelength-dispersive spectrometers but the system has been developed to incorporate the back-scattered electron signal. De- tails are provided of the way the microanalyser was converted and examples are given of the results that are being produced.
Introduction
Mineralogists and metallurgists demand large amounts of accurate data concerning rocks, alloys, ceramics, etc. The required information includes: (1) the proport- ions of the various phases, (2) the grain size distributions of selected phases, (3) the compositions of discrete particles, etc.'*'. Experience has shown that automatic image analysis is often the best method of collecting these data. How- ever, most image analysers rely on optical signals but these have only a limited ability to distinguish minerals because (a) the optical properties of many minerals overlap, (b) optical properties can vary with crystal orientation, and (c) the optical signals are often confused by the presence of polishing artefacts such as pits, scratches and boundary effects. A Camebax-Micro x-ray microanalyser, when used as an image ananlyser, can overcome some of the problems encountered by the optical systems.
Modifying the Camebax-Micro
The Camebax-Micro was adapted primarily to study mineralogical materials. These materials are often very inhomogeneous and specimen surfaces a few square centi- metres in area or more, must be examined to characterise features such as grain size distributions and mineral proportions. The time needed to carry out an area analysis of such large surfaces, (using element concentration maps), is often unacceptably long. Consequently, the new image analysing system has been designed, from the start, as a linear analyser. Details of the specimen surface are examined by a linear traverse made up of a large number of analysed poinds. The mineral at each point is identified as rapidly as possible and the specimen is then moved so that a contiguous '[point" is positioned beneath the (stationary) electron beam.
Mineral discrimination is based on the presence, or absence, of various elements and an identification procedure can usually be completed in 10 ms. For many mineralogical purposes linear traverses up to 1 metre in length are needed to provide statistically reliable results. Such a traverse, made up of 5 x 10->
contiguous small areas, each 2 um in diameter, takes about one and a half hours.
A design study of the instrumental factors needed to convert a multi-spectrometer electron probe microanalyser into an automatic linear image analyser showed that high-speed, real-time control could only be achieved by using a computer network, where the instrumental control functions, the data collection facilities, and the mathematical operations are shared between two or more microprocessors.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984246
JOURNAL DE PHYSIQUE
The Camebax-Micro is especially suitable for conversion since it incorporates a small computer network for routine point analysis. A Motorola 6802 micro-processor controls the movement of the four spectrometers, drives the three specimen stage motors, and measures the x-ray count rates. An external mini-computer, PDP 11/23, carries out all mathematical operations, records the data, and communicates with peripheral devices. The Motorola 6802 and PDP 11/23 were originally connected through series interface circuits but these were too slow for real-time control.
A new interface was designed in collaboration with Cameca, this allows the micro- processor and the mini-computer to communicate at 19200 bytes sec-l.
The PDP 11/23 transmits instructions to the Motorola 6802 concerning, (1) the linear movement of the specimen, giving direction and distance to be travelled, (2) selection of spectrometer crystals and wavelength settings, (3) x-ray counting periods, (4) operation of gear-box back-lash routines, etc. The raw x-ray count rates (and, more recently, the back-scattered electron current value) are passed to the PDP 11/23 and used to identify the mineral at each measured point. The mini-computer uses "filtering" algorithms to distinguish polishing artefacts and boundary effects from real phases. The adjusted data are stored on disc and can be presented in many forms.
Much of the original Cameca software was used in the image analysing system and new read-only-memory boards further reduced the amount of software that had to be written. The x-ray signals from each spectrometer are compared against pre-set threshold levels held in computer memory. Values below the threshold are assigned to category zero, whilst values above the treshold are assigned to category one.
Thus, mineral 1010 produces positive signals from spectrometers Nos. 1 and 3, and signals below threshold levels from spectrometers Nos. 1 and 4. Combinations of two signal levels from 4 spectrometers can discriminate up to 16 phases. However, it is unlikely that more than 6 or 8 of these phases will exist in any single specimen, and 0000 always indicates "unmeasured phases". If a specimen is very complex it may be necessary to carry out repeated traverses in order to identify and measure all of the minerals.
Information collected during a traverse is condensed so that only the lengths of the intercepts across successive features are permanently stored. Linear proport- ions of the various phases, intercept length distributions across these phases, particle compositions, etc. are derived from the condensed data.
Typical Results
The image analyser has been used to measure (a) ores, (b) the products of minerals treatment operations, and (c) metallurgical slags.
Table 1 shows the result from the analysis of a solid ore. Table 2 gives the results of the analysis of a particulate material mounted in an epoxy matrix. The results give the modal analysis, and provide information on particle compositions and on the sizes and the associations of the grains that make up those particles.
The instrument can also search for rare phases using long traverses lasting many hours. Thus, a 2.5 metre traverse on a finely-ground material from a gold ore treatment process found two fragments of gold, each a few micrometres across.
Each fragment was automatically relocated under the electron beam and the spatial relationships with the other minerals were studied by conventional microanayser technqiues such as point analysis and element concentration photographs.
ion to its normal function, it acts as a linear image analyser. In the image analysing mode it relies on x-ray signals from wavelength dispersive spectrometers to discriminate the various phases from one another. The discrimination time, at any point, is about 1 0 milliseconds. The system provides modal analyses, intercept length distributions, and information on particle composition and textures.
Although the new system is capable of further developments, in speed of analysis and in phase descrimination capability, it already provides a great deal of miner- alogical information that cannot readily be obtained by any other means.
References
1 . Jones. M.P.. Automatic mineralogical measurement in mineral processing.
-
-.Proc. XI11 Intern. Min. Proc. Congr. Warsaw 1979. Elsevier, Amsterdam, 1 9 8 1 , 533-565.
Table 1
Phase identification, Intercept length Distributions and Modal analysis of a Solid Specimen of Pyritic
Copper Ore (EMMA 1 5 4 7 )
Intercept length ( L ) Pyrite Arsenopyrite Chalcopyrite Sphalerite Others um
2 249 28 436 503 78
2
-
4 164 33 338 26 7 384 - 6 164 16 204 132 9
6
-
10 256 18 163 8 2 1710
-
16 324 1 105 36 91 6
-
25 269 1 38 1 1 325
-
40 398 1 22 12-
4 0
-
63 267-
4 1 1-
63
-
100 248-
2 9-
100 - 160 114
- -
2-
160 48
- - - -
Total no. of grains 2501 98 1312 1066 154
Total intercept 84412 508 8074 6310 696
length um
Volume
-
percent 84.4 0.5JOURNAL DE PHYSIQUE
Table 2
Analysis of broken fragments of ore: EMMA 1555 -75
+
53urn
a. Particle Composition Analysis: Intercept Length Distributions of "single phase" Particles
Intercept Length urn Sphalerite Pyrite Chalcopyrite Arsenopyrite
2 22 187 6 0
2 - 4 17 118 2 0
4 - 6 8 7 2 4 0
6
-
10 3 112 2 110
-
16 4 125-
1 6 - 25 5 105 1
2 5
-
40 2 190-
40
-
63 1 180 16 3
-
100 55100
-
160 2Total no. of particles 62 1146 16 1
CL vm 462 25778 134 1 0
b. Intercept Length Distributions of Minerals in Composite Particles Intercept Length urn PIS P/C P/A P/S/A P/C/A P/S/C S/P/A/C CIS
0 - 2
- - - - -
-2 - 4 1 3
-
- --
4
-
6 5 3- -
--
6
-
10 16 4 1- -
110
-
16 28 19 5 1-
11 6 - 25 43 27 2 4 3 2
25
-
40 110 85 10 3 5 44 0
-
63 160 126 21 1 4 7 1 46 3
-
100 8 4 75 7 6 6 16100
-
160 5 2-
1 1 1Total particles 452 344 46 30 22 39 Total L um 19904 16086 2054 1602 1164 2262
P = pyrite, S = shpalerite, A = arsenopyrite, C = chalcopyrite.
c. (Proportion of each mineral occurring as particles of different linear composition)
Linear Grade % Pyrite Sphalerite Chalcopyrite Arssenopyrite
0
-
10-
10.4 18.8 22.010
-
20 0.1 19.4 25.5 25.520
-
30 0.2 15.9 15.5 18.830
-
40 0.3 13.7 11.6 6.74 0
-
50 0.6 8.0 8.0 10.050
-
60 1.7 7.3 6.4 5.36 0
-
70 3.7 4.4 3.5 10.270
-
80 6.5 4.2 4.3 -8 0
-
90 17.6 4.3 1.8 -90
-
100 27.1 3.0-
-Apparently
single phase 42.2 9.4 4.6 1.5
