ON-STREAM XRF ANALYSIS OF
HEAVY METALS AT PPM CONCENTRATIONS
G Roach and J TicknerCSIRO Minerals, PMB 5, Menai 2234, NSW, Australia. Email: Greg.Roach@csiro.au
ABSTRACT
An XRF analyser suitable for measuring up to 15 liters of slurry has been tested in the laboratory. A range of slurries consisting of approximately 40 wt% solids and low concentrations of tantalum were obtained and measured. The accuracy of the analyser has been determined to be ±2 ppm (1 sigma). Improvements are planned that will significantly increase the speed and accuracy of the analyser and facilitate its adaptation to online / byline monitoring of mineral process streams.
INTRODUCTION
CSIRO Minerals is developing methods for the on-stream measurement of parts-per-million (ppm) concentrations of heavy metals in process slurries using K-shell X-ray fluorescence (XRF). As part of this ongoing development, an investigation was carried out on mineral slurries containing approximately 100 ppm of tantalum (Tantalum concentrations are reported as tantalum oxide (Ta2O5) as a proportion of the dry solids mass).
MEASUREMENT OF SLURRIES WITH THE CSIRO ANALYSER Sample provision and third-party analysis
CSIRO Minerals was supplied with eight samples from a tantalum production facility. Each sample consisted of at least 10 liters of slurry, containing 31-42 wt% solids. The solids content was determined by drying the samples in an oven at 110°C for at least 2 days and weighing the residue. After each slurry had been dried and sub-sampled for analytical XRF analysis, the remaining dry ore was weighed and reconstituted with water for measurement in the CSIRO analyser.
The original set of only eight slurries was considered too small for multivariate analysis. Therefore, additional samples were prepared based on this set. Firstly, the tantalum concentrations of two of the samples were increased in 5 ppm intervals by the addition of Ta2O5 powder. Secondly, iron oxide (Fe2O3) was added to one sample to increase its iron content to 2, 4 and finally 8 wt%. These additions were made to investigate the effects of changing matrix composition on the Ta measurement. In this way, the core set of 8 samples was expanded to 22 samples suitable for measurement.
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The tantalum content of all samples was measured independently by three separate facilities using differing techniques, namely laboratory XRF analysis, neutron activation analysis (NAA) and ICP. A Grubbs’ [1] intercomparison analysis was performed and the most accurate measurement technique was determined to be the XRF analytical measurements with an estimated repeatability error of 1.6 ppm. The ICP measurements were found to have systematic errors related to the difficulty of completely dissolving all the tantalum into solution, whereas the NAA results showed relatively large statistical errors and hence neither were used in any further analysis.
The CSIRO XRF Analyser
The CSIRO XRF slurry analyzer is illustrated in figure 1. Two counter-rotating mixers are used to ensure slurry uniformity. The rig is designed to allow a large volume of slurry (approximately 10 liters) to be measured. The radiation source is a 37 MBq (1 mCi) Co-57 radioisotope source. The resulting scattered and fluorescent X-rays are transmitted through a polycarbonate window and are detected by a high resolution X-ray detector. The characteristic tantalum K X-rays are used to estimated the tantalum content rather than the L X-rays. The K X-rays are more suitable for two reasons: Firstly, they are more highly penetrating allowing a much larger volume to be analysed; and secondly they are also much less sensitive to matrix and particle size effects.
Slurry sample Collimator X-ray detector in vacuum housing γ-ray source Incident γ-ray Scattered X-ray Mixers Polycarbonate window
Figure 1. Schematic drawing of laboratory rig.
A high-purity, planar germanium detector (Canberra 1000 mm2 LEGe) was used to detect the scattered and fluorescent X-rays, read-out using a Aptec 5004 combination shaping amplifier/ADC/MCA card. This was operated at a modest count rate (limited by the intensity of the source) of approximately 30k c/s with a corresponding dead time of 25%. Each measurement took approximately 5½ hours.
Uniformity of the Slurry During Measurement
The most difficult problem faced in obtaining repeatable measurements was ensuring that the slurry was homogeneous. In particular, it was not sufficient just to lift all the solids off the bottom of the tank if a density profile remained where the larger particles were found near the bottom of the tank. This is important since the rig is more sensitive to material within a few centimeters of the source and detector. In the slurry samples used, the tantalum is not uniformly distributed with respect to particle size – in fact there are systematic variations! These introduce significant errors in tantalum determination if the slurry is not well mixed.
Calculation of Tantalum Concentration and Solids Mass Fraction
Raw XRF Spectra
Figure 2 shows a typical X-ray spectrum for a slurry containing approximately 100 ppm of tantalum. Each channel is 100 eV wide. The broad peak centered at 85 keV is due to Compton scattered source γ-rays, with the smaller peaks at channels 122 and 136.5 keV due to Raleigh scattered source γ-rays.
0 200 400 600 800 1000 1200 1400 0.5 1 1.5 2 2.5 3 3.5 4x 10 6
Channel number (100 eV bins) Co u nts p er c h a n n el
Figure 2. An X-ray spectrum obtained from measurement of a slurry containing approximately 100 ppm of Ta2O5.
Figure 3 is an enlarged view of the spectrum in the vicinity of the tantalum Ka X-ray peaks. The tantalum Ka2 and Ka1 peaks are centered on channels 563 and 575.
540 550 560 570 580 590 600 610 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 x 105
Channel number (100 eV bins) Co u nts p er c h a n n el
Figure 3. An enlarged view of the measured XRF spectrum in the vicinity of the tantalum Ka peaks.
Tantalum Determination
A four term linear regression analysis was used to relate the tantalum content of each sample to features observed in the X-ray spectrum. The terms used were the number of counts in the tantalum peaks, the background in the vicinity of the peaks, the number of counts in the Compton scattering peak, and a small term necessary to correct for a monotonic drift in the electronics with respect to time. To avoid over-fitting on the relatively small data set used, the 22 samples measured were divided into a training set (consisting of 21 points) used to determine the regression coefficients, and a test set (1 point) used to evaluate the performance of the calibration. The calibration/training process was cycled through all 22 of the samples.
The presence of tungsten at 0 – 10 ppm levels was revealed by the NAA data. We were able to correct for any interference satisfactorily by estimating the background as the sum of three different windows – two of which straddled the tungsten Ka1 peak.
Solids Fraction Determination
To determine the solids loading, a multivariate linear regression analysis was used to relate information on the shape of the Compton scattering peak to the solids fraction. The Compton peak shape is expected to vary with solids fraction for two reasons:
1. Doppler broadening of Compton scattered X-rays depends on the atomic number (Z) of the element from which the X-ray scatters, with higher Z elements having broader Compton scattering peaks. Compton profile analysis [2] has been shown to provide
good discrimination between the lightest elements (such as H and O) and heavier elements present in the tantalum ore.
2. The proportion of multiply to singly Compton scattered photons decreases with increasing solids loading fraction due to photoelectric absorption.
This measurement technique has the advantage that no additional apparatus is needed and that the on-line mass measurement is performed over almost an identical region of slurry to which the tantalum concentration is being measured.
RESULTS
Tantalum Concentration
Figure 4 shows a plot of the CSIRO calculated tantalum versus the analytical XRF laboratory determined values. The horizontal error bars represent the uncertainty of the analytical determination, and the vertical error bars on each point are the combined CSIRO and analytical XRF error.
Figure 4. Plot of tantalum concentration for the CSIRO feasibility rig versus the analytical XRF concentration.
The RMS error of 2.3 ppm (1 sigma) is the root mean square error between the CSIRO and analytical XRF analyses. It is a measure of the total error including the estimated 1.6 ppm repeatability error of the analytical analysis. Correcting for this results in an estimated error for the CSIRO analyzer of 1.7 ppm. There is always a degree of uncertainty in apportioning errors between two measurements and a more conservative claim would be that our data is consistent with a 1 standard deviation error of approximately 2 ppm.
The 2.3 ppm RMS error between the analytical XRF results and the CSIRO results is better than the agreement between analytical XRF and either the neutron activation analysis (about 3 ppm) or the ICP (>10 ppm) results.
Whilst our results are very encouraging, it should be noted that they have been obtained on a fairly limited suite of measurements, based on just 8 discrete samples. Further measurements are required to properly evaluate other factors contributing to the analyzer accuracy, notably matrix and particle size effects.
Solids Mass Fraction
The on-line determinations of the solids mass fraction were compared with the true mass fractions, calculated from the masses of solid and water used to reconstitute the slurries. The measurement accuracy for the solid fraction determination was found to be about 0.3 wt%, with a repeatability error of 0.16 wt%. For a typical solids loading of about 40 wt%, this leads to a 0.7% relative error in the determination of the Ta2O5 content, expressed as a fraction of the dry mass.
CONCLUSION
Measurements undertaken by CSIRO Minerals at the Lucas Heights Laboratory have shown that the current XRF rig is capable of accurately measuring the tantalum content of slurries. An accuracy of approximately 2 ppm was obtained for a measurement time of 5½ hours.
Several improvements to the analyzer are planned. These include:
• Using a strong radioisotope source together with a high-speed digital X-ray processor to increase the X-ray counting rate;
• Altering the geometry of the analyzer to measure slurry passing through a pipeline. This improves the signal-to-background ratio for the tantalum X-rays and minimizes the sensitivity of the analyzer to the spatial distribution of the solids fraction.
It is estimated that it will be possible to achieve a statistical error on the measurement of Ta2O5 of approximately 1 ppm with a 20 minute counting time. In principle, the same technique can be used to determine other high Z elements to similar accuracy.
ACKNOWLEDGEMENTS
The authors would like to thank Steve Rainey and Ivan Kekic for assistance in the design and construction of the slurry analyser.
REFERENCES
[1] Grubbs F, 1948, On Estimating Precision of Measuring Instruments and Product Variability, J Amer. Statistical Assoc., 43, 243-264
[2] Tickner J and Roach G, Characterization of coal and minerals using Compton Profile Analysis, to appear in Nuclear Instruments and Methods B
