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Introduction

Mosely first discovered, in 1913, that thefrequency of emitted X-radiation excited by anelectron beam is a function of the atomicnumber of the analysed element. Thisdiscovery led to the development of spectro-chemical analysis. The forerunners of theelectron microanalysers of today, however,were invented only during the nineteen fifties.The electron microprobe was developed inparallel by Castaing and Guinier in France andby Borovski in Russia, but Castaing’s designwas the basis of modern microanalysers. Theinitial area of analysis was brought down fromover 1 mm2 to less than 2 μm2, as it is today.In 1956 Cosslett and Duncumb invented thescanning electron microprobe, where a beam ofelectrons could be scanned across an area of asample. This, in turn, led to the development

of the electron microprobe with both scanningand point analysis capabilities that we knowtoday.

Essentially, all electron microbeaminstruments operate by the production of abeam of electrons from a filament in a vacuum(i.e. an electron gun). The electron beamleaves the gun and is accelerated and focusedby a series of electromagnetic lenses whilepassing down an evacuated column. Theelectrons eventually strike the surface of thesample to be analysed and may travel througha thin sample (transmitted electronmicroscopy) or react with a thicker sample toproduce several kinds of reflected radiation.

Transmission electron microscopy involvesimaging at high magnification and the use ofhigh accelerating voltages. Although mostcommonly used for medical or biologicalapplications, the transmission electronmicroscope (TEM) is also used for certainmetallurgical applications. These include theexamination of thin foils and surfacesreproduced as replicas. Occasionally energydispersive X-ray analysers are attached to theTEM to allow phase identification (see EDXanalysis).

When an electron beam strikes a solidsample too thick to allow electrontransmission, secondary, backscattered andAuger electrons are excited close to thesurface. In addition, characteristic X-radiationis generated to a depth of approximately 2 µm(depending on the sample composition and theaccelerating voltage of the electron beam).Emitted radiation is shown in Figure 3.

Emitted radiation may be processed inpoint or scanning mode to provide informationon the phase analysed, by imaging and/ormicroanalysis.

The use of electron microbeamtechniques in metallurgical analysisby L. Andrews*

Synopsis

Electron microbeam analysis of metallurgical samples covers abroad field and has applications in the areas of minerals processing,hydrometallurgy, pyrometallurgy, physical metallurgy andcorrosion.

Some idea of the possibilities of microbeam techniques can beillustrated by examining the following example of a run of minegold ore sample. Milling and bulk chemical analysis of the ore willprovide major and minor element analysis as well as the gold grade.More detailed chemistry can also reveal the presence of traceelements that may be advantageous or deleterious to downstreamprocessing. Electron microbeam analysis can provide additionalinformation in the form of size, liberation, association andcomposition of the gold, the percentage of refractory gold, the size,liberation, association and composition of enclosing sulphide orarsenide phases and the levels and association of the trace elementsthat may affect processing. In most cases electron microbeamanalysis complements bulk chemistry, and does not replace it. Bothtechniques should be used in parallel for maximum effect.

This paper touches on the origin and history of electronmicrobeam techniques, as well as the theory behind microbeamanalysis and processing. The more common types of instrumentsand their applications will also be dealt with briefly.

* Anglo Research.© The Southern African Institute of Mining and

Metallurgy, 2007. SA ISSN 0038–223X/3.00 +0.00. This paper was first published at the SAIMMWorkshop, Analytical challenges in Metallugy,23–24 November 2006

79The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 107 NON-REFEREED PAPER FEBRUARY 2007 ▲

The use of electron microbeam techniques in metallurgical analysis

Secondary electron imaging (SEI) is used to producetopographic images, which provide information on surfacecontours or crystal morphology. Backscattered electronimaging (BEI) or ‘compo’ mode reveals the relative density ofthe phases imaged because the BEI intensity is proportionalto the atomic weight of the phase under the beam. Examplesof SEI and BEI scans are shown in Figures 4 and 5.

Auger electron spectroscopy (AES) is a surface techniqueperformed under very high vacuum. Solid samples can beanalysed by removal of thin layers of sample by ionsputtering, electron beam irradiation, and processing ofemitted Auger electrons. This is known as a depth profile.The detection limit of trace elements analysed by thistechnique is very low. Metallurgical applications in this areaare mainly flotation or leaching related.

The characteristic X-radiation produced by electron-specimen interaction can be processed in point mode byenergy-dispersive or wavelength-dispersive X-ray analysis.

Energy-dispersive X-ray (EDX) analysis is the mostcommonly used electron microbeam technique in metallurgyand finds applications in the fields of minerals processing,hydrometallurgy, pyrometallurgy, physical metallurgy,corrosion studies and forensics.

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80 FEBRUARY 2007 VOLUME 107 NON-REFEREED PAPER The Journal of The Southern African Institute of Mining and Metallurgy

Figure 2—The scanning electron microanalyser as produced byDuncumb and Metford in 1958

Figure 3—Radiation emitted from a solid sample when struck by anelectron beam. Adapted from Viljoen and Johnson (1983)

Figure 4—SEI showing crystal form

Figure 5—BEI of chromitite showing relative phase density by intensity

Figure 1—Early electron microprobe developed by Duncumb in 1956

EDX analysis involves the rapid simultaneous acquisitionof all elements present under the beam heavier than lithium.This is the most easily available form of electron microbeamanalysis, and is similar to bulk XRF determination, but phasespecific. EDX detectors are found on most scanning electronmicroscopes (SEM’s) and were historically identified by theattached liquid nitrogen dewar. Modern detectors, however,are liquid nitrogen free. The advantages of EDX analysis arespeed—especially important for quick phase identification,and for automated techniques —and versatility. An SEM (ormicroprobe) can be set up to run qualitative EDX analysis, orquantitative EDX analysis when standards are used. Themain limitations of EDX analysis are relatively poor spectralresolution and high detection limits (see later).

Examples of a SEM and automated instrumentation usingEDX are shown in Figures 6 and 7.

Wavelength-dispersive X-ray (WDX) analysis isgeometry based using reflection of X-rays from a crystal ofspecific d-spacing, which travels on a Rowland circle inside aspectrometer. Light element spectrometers can quantifyoxygen and carbon using gas flow detectors. WDXspectrometers form the basis of the electron microprobe andare sometimes attached to the scanning electron microscope(WD-SEM). The advantages of WDX analysis are goodspectral resolution and lower detection limits when comparedto EDX analysis (see below). WDX analysis involvescalibration on standards, so highly accurate results can beobtained. This, and the fact that element acquisition issequential, however, means that this type of analysis is

relatively slow and phase analysis is usually restricted to amaximum of twelve elements. WDX spectrometers attachedto a microprobe and to a WD-SEM are shown in Figures 8and 9.

Spectral resolution is a measure of peak separation, andthis is generally greater in WDX than EDX analysis. Thebest-known peak overlap in EDX is that of Pb-M with S-K.Although modern EDX software can correct for the overlap,problems are encountered when one of the two elements ispresent in minor to trace amounts. The two peaks areseparate and therefore measurable using WDX techniques.Other overlapping peaks in EDX include Ru with Cl, Mg withAs, Ti with O, Zr with Pt. Peak overlaps occur even in WDXspectra when dealing with the platinum group elements orthe rare earth elements. Quantitative analyses of theseelements should be run on WDX using PHA settings toremove high order peak interference. It may also benecessary to choose β rather than α peaks, and/or to employcertain software corrections.

The detection limit (DL), defined as the minimumdetectable amount of an element at 99% confidence level, isimportant when trace elements have to be measured within agiven phase. Detection limits may be lowered by playing withthe accelerating voltage, the specimen current, and thespectral background settings but the limit is invariably lowerin WDX than in EDX analysis as the following examplesshow:

The use of electron microbeam techniques in metallurgical analysisJournal

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81The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 107 NON-REFEREED PAPER FEBRUARY 2007 ▲

Figure 6—A scanning electron microscope with an EDX detectorattached

Figure 7—A QemSCAN with four EDX detectors. Detector multiplicityallows shorter analysis times

The use of electron microbeam techniques in metallurgical analysis

Palladium in pentlandite ((Fe,Ni)9S8)EDX-DL = 0.5 %WDX-DL = 100 ppm

Vanadium in chromite ((Mg,Al,Cr,Fe)3O4)EDX-DL = 0.3 %EDX-DL = 80 ppm

From the information provided thus far it is evident thatinstruments such as the electron microprobe, which runs inWDX mode, would be the most suitable choice to measuretrace element distribution between phases, for multi-elementquantification and for samples containing peak-overlappingelements. EDX is suitable for qualitative analysis, andquantitative analysis using standards. Most SEM-EDXquantification systems currently in use, however, requirecomplex standards to closely match the unknown to be used,whereas WDX-EMP Quantification can be achieved usingelemental standards or simple sulphides and oxides.

As mentioned earlier, the most valuable features of EDXanalysis are speed and whole spectrum acquisition, and thisis why the technique is used in automated analysis.Instruments such as the QemSCAN (quantitative evaluationof material by scanning electron microscopy) or the MLA(Minerals Liberation Analyser) utilize a combination of BEIand EDX for phase identification, followed by softwareprocessing.

Such instruments are capable of running many sampleswithout operator input and the data can be processed later toproduce accurate statistical information, even from tailingssamples running at <1g/t values. A number of analysismodes may be used, singly or in combination. Phases may beidentified according to their BEI intensity, by centroid EDXanalysis, by linescan EDX or using EDX grids over mineralgrains. Averaged data then define modal analysis, phase sizedistribution, liberation and association. Such information isobviously invaluable for beneficiation by milling andflotation. Less obvious, but extremely valuable, are theapplications in exploration geology and pyrometallurgy.Processing of core samples taken during exploration usingthese techniques may influence mining and beneficiationpractices, as well as financial planning. In smeltingoperations, the use of automated modal analysis incombination with EMP-WDX or SEM-EDX analysis canquantify losses to slag, and predict value recovery fromfurnaces and/or converters. This information can be used forprocess optimization.

The information presented in this paper is intended as anintroduction to the application of electron microbeamtechniques in metallurgical analysis. More detailedinformation, tailored to project requirements, can be obtainedby approaching a microbeam technologist at an electronmicroscopy unit or a research organization.

References

VILJOEN, E.A. AND JOHNSON, J.A. Microbeam techniques in applied mineralogy.

Special Publications of the GSSA 7, 1983. pp. 499–506.

GOLDSTEIN, J.I, NEWBURY, D.E., ECHLIN, P., JOY, D.C., FIORI, C., and LIFSHIN, E.

Scanning electron microscopy and X-ray microanalysis. Plenum Press,

New York and London. 1981. ◆

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82 FEBRUARY 2007 VOLUME 107 NON-REFEREED PAPER The Journal of The Southern African Institute of Mining and Metallurgy

Figure 8—This electron microprobe is fitted with three vertical WDXspectrometers and one EDX detector

Figure 9—The WD-SEM has EDX capability and one horizontal WDXspectrometer (at the rear)