Determination of Iron in High-Grade Iron Ore and of Lead in Lead Concentrate by Compton Scattering of 60-keV y-Rays from Americium-24 1 R. A. Fookes, V. L. Gravitis, and J. S. Watt Australian Atomic Energy Commission Research Establishment, Lucas Heights, N. S. W., Australia
Determination of the concentration of an element in a matrix of elements of considerably lower atomic number is often required in the mineral industry. The analyses must frequently be undertaken in the harsh environment of an industrial plant or in the field. Under such conditions, the analysis equipment should be simple to operate and sample preparation minimal. Radioisotope techniques based on X-ray fluorescence, X-ray preferential absorption, and Xray backscatter have all been used in such applications (1), but the particular technique to use should be chosen to fit the analysis problem and the conditions in the field or plant. Measurement of the intensity of X-rays Compton-scattered from “infinitely” thick samples is recognized as an approximate method of analysis for a mineral in ores (2, 3 ) , but its potential for highly accurate analysis in specific applications has not been widely realized. In this note, a description is given of determination of the concentration of iron in high grade iron ores and lead in lead concentrates using Compton scatter of 60 keV y-rays from americium241. Iron in high grade iron ores (-90% by weight Fez03) was determined with an absolute rms error of 3~0.13%by weight iron for finely ground and dried samples and f0.9% by weight iron for coarse drill chippings from blast holes. Lead in lead concentrate (-75% by weight Pb) was determined to f0.56% by weight lead.
The intensity I , of X-rays Compton scattered by an infinitely thick sample can, for the geometries commonly used in radioisotope probes, be approximated (6, 7 ) by:
where p and p’ are the mass absorption coefficients in the sample of primary and Compton scattered y-rays, respectively, C is the concentration of an element (wt/wt), a a constant, and subscripts E and i refer, respectively, to the element being determined and to the i t h element of matrix of the sample. Equation 1 assumes that the angle of incidence of the primary y-rays equals the angle of emergence of the Compton scattered y-rays. I , is independent of sample density to a first approximation for radioisotope probes (e.g., Figure 1) because of the broad beam and little collimated geometry used and because the path length of Xrays in the sample is small compared to the path length of source-sample-detector. When the absorption of X-rays in the sample is predominantly by the high atomic number element being determined, Equation 1 can be rewritten: CE
N
az
+ a3
THEORY
Is
The intensity of X-rays backscattered from an “infinitely” thick sample is proportional to the ratio of the probability of a scattering interaction occurring per unit weight of sample and the absorption of X-rays within the sample. The two main types of scattering interaction occurring within the sample are Compton and Rayleigh. Consider monoenergetic X-rays being scattered by the sample a t a fixed angle. The probability of Compton scattering depends on the ratio of atomic number to atomic weight and, hence, is approximately constant for all elements except hydrogen ( 4 , 5 ) . Both the probability of Rayleigh scattering and absorption of X-rays in the sample increase with atomic number and, for the applications being considered in this paper, are mainly due to the high atomic number element whose concentration is being determined. The probability of Rayleigh scattering varies between Z 2 / A and Z 3 / A depending on angle of scatter ( 4 , 51, where Z and A are, respectively, the atomic number and atomic weight of the element being determined. The absorption of X-rays in the sample is essentially due to the photoelectric effect and varies approximately with Z4/A ( 5 ) . Hence the intensity of X-rays Compton scattered from the “infinitely” thick sample depends approximately on l/Z3 whereas the Rayleigh intensity depends only on I/Z to 1/Z2. The Compton scattered intensity is thus more sensitive to changes in the high atomic number constituent of the sample, and its intensity decreases with increase in concentration of this element.
where a 2 and a 3 are calibration constants determined for each application from measurements on typical samples whose element concentrations C E have been obtained by conventional techniques for absolute assay. The energy of the X-rays to be used is chosen to ensure that the absorption of X-rays by the element being determined is relatively high compared with absorption by the sample matrix. Apart from this restriction, the energy is chosen to be as high as possible to ensure that the X-rays penetrate deep within the sample so that analysis is averaged over a large sample weight. Sixty-keV y-rays (from 241Am)was a suitable energy for applications described in this paper, and detected Compton scattered y-rays had,an energy of about 50 keV.
EXPERIMENTAL Apparatus. The geometric arrangement of radioisotope source, sample, and scintillation detector is shown in Figure 1. The source of 60-keV y-rays was 241Am (100 mCi, Radiochemical Centre, U.K., code AMCG). For the iron analysis, a 27 mg/cm2 aluminum filter was used to absorb the iron K X-rays. The Rayleigh scattered 60-keV gamma rays constituted only 2.5-3% of the total count rate and so had negligible effect on the sensitivity of iron determination. A 150 mg/cm2 thulium filter was used for the lead concentrate analysis. This filter reduced the Rayleigh scattered component from 20 to about 5% of the total scattered count rate, and effectively absorbed the lead L X-rays also. These filters ensured t h a t essentially only Compton scattered y-rays were detected, and hence no energy analysis was required. Procedure. The iron ore samples used to test the Compton
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
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II
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Figure 1. Scintillation detector head assembly used for Compton backscattered y-ray method of elemental analysis scatter technique were supplied by Mount Newman Mining Co., W.A., and the lead concentrates by the Zinc Corporation Ltd., Broken Hill, Australia. Sample preparation techniques and counting times were as follows: i) Finely Ground Iron Ore (2.5-mm diam, and --2O%, >4.5-mm diam.) and could not easily be made into briquets. Counts were taken on samples loosely packed into a container with a I-mm thick polystyrene window between sample and radioisotope source. Approximately 20% of the detected count rate resulted from y-rays scattered from the polystyrene window but, in this particular application, the resulting loss in sensitivity did not significantly increase the error in analysis. This thick and rigid window has advantages in field work. T o ensure that a measurement representative of the sample was obtained, ten 40-second measurements of count rate were made with the sample repacked between each of them. iii) Lead Concentrates. Samples taken from streams of two mineral concentrators were dried, and then briquetted and measured as in (i). They were not ground after being taken from the plant stream. Assays of each sample were supplied by the mining companies. The rms error in determination of element concentration was calculated by a least square fit of the measured count rates and supplied assays using Equation 2 as calibration equation.
RESULTS AND DISCUSSION Finely Ground I r o n Ore. In Figure 2, count rate of backscattered y-rays is plotted us. iron concentration of the samples. The rms error in determination of iron was f0.13% iron by weight. Reproducibility of measurements of count rate repeated on the same sample was about 0.05% and, hence, count rate statistics and equipment instability were not a significant cause of error in analysis for iron. The expected rms error, which was calculated using Equation 1 and supplied assays for Fe203, SiO2, and A1203, and loss on ignition for each sample, was 0.07% iron by weight. The difference between expected and experimentally determined errors could be due to errors in the sup590
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64.000
56
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I
60
64
ASSAY IRON
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Figure 2. Determination of iron in finely ground and dried samples of
iron ore plied assays, slight non-uniformities in the sample briquets, or the presence of small quantities of heavy elements in the samples. The errors found in analysis for iron ore are so small that this Compton scatter method appears to have potential for grade control in blending operations for high grade iron ore as found in the Pilbara region of Western Australia and in Brazil. Although concentrations of interfering elements may change gradually over the mine face and hence increase errors in analysis for iron, compensation for this effect could be made on a routine basis, e.g., weekly recalibration of the Compton scatter technique using assays by conventional methods. These samples were also analyzed by X-ray fluorescence using 23sPu excitation, balanced filters, and energy analysis. The rms error was 0.50% iron by weight, reducing to 0.39% when a measurement was made to correct for changes in matrix absorption. This error is three times greater than obtained using the Compton scatter method. I r o n O r e Drill Chippings. Results of determination of iron in drill chippings are shown in Figure 3. The rms error of f0.9% iron by weight was much larger than the error for the finely ground samples because of variations of moisture content, packing density, and particle size and composition of the coarse samples. This technique of analysis for iron in drill chippings could be undertaken using portable equipment a t the blast hole. The simplest approach would appear to be to put the probe directly onto a smoothed surface of chippings and obtain the average count rate from about ten different samples. The moisture content of iron ore a t the mining site a t Mt. Newman Mining Co. normally varies in the range 0.4 to 1%by weight, and this variation will cause only a small error in determination of iron on a dry weight basis.
7900h
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65.030
40
60
50 ASSAY IRON
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)
Figure 3. Determination of iron in iron ore drill chippings
Lead in Lead Concentrates. The y-ray backscatter measurement determined lead with a rms error of f0.56% lead by weight ( i e . , 0.74% of the mean lead concentration). See Figure 4. In this application, there was more potential for errors due to matrix element variations as zinc in the samples used varied from 2 to 7% by wt. The mass absorption coefficient of lead a t 60 keV is 2.7 times that of zinc and so it would be possible for the variations in zinc to cause errors of f0.996 lead by weight in lead determination. This larger error does not occur in practice because there is a rough correlation between lead and zinc concentrations owing to the characteristics of the flotation process. The most likely causes of errors for this set of samples would be errors in supplied assays and non-uniformities in lead concentrate-paraffin wax mixture which is difficult to make completely uniform because of the high density of galena compared with paraffin. This technique has potential for routine measurement of lead in lead concentrates, both in the plant laboratory and also continuously, by radioisotope probes immersed directly into the plant slurry stream (9). The in-stream probe would measure lead per unit weight of slurry, and to determine lead on a wt/wt basis in the slurry solids, a further probe measuring slurry density would be required. These samples were also analyzed for lead by L shell Xray fluorescence. The rms error obtained was 0.70% lead by weight compared with 0.56% lead by weight for the Compton scatter method. CONCLUSIONS The examples described above show that, for appropriately chosen applications, the Compton scatter technique is an accurate method for determination of high concentrations of a high 2 element in a low 2 matrix.
71001 68
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Figure 4. Determination of lead in lead concentrate samples
The advantages of the Compton scatter technique compared with radioisotope XRF techniques are larger sample weights analyzed, less grinding of sample required, and for the examples described above, a lower error in element determination. Compared with X-ray preferential absorption analysis, sample preparation is simpler because "infinitely" thick samples can be used.
ACKNOWLEDGMENT The authors thank R. A. Greig, AAEC for assistance with some of the experimental measurements, and Mt. Newman Mining Co. and the Zinc Corporation Ltd. for supply of assayed samples used in the tests.
LITERATURE CITED (1) "Radioisotope X-Ray Fluorescence Spectrometry," IAEA Technical Report, Series No. 115, Vienna, 1970, (2) J. F. Cameron and C. G. Clayton, "Radioisotope Instruments," Part I, Pergamon Press, Oxford, 1971, p 126. (3) E. Dziunikowski and J. Niewodniczanski, in "Nuclear Techniques and Mineral Resources," IAEA, Vienna, 1969, p 343. (4) C. A. Ziegler, L. L. Bird, and D. J. Chleck, Anal. Chem., 31, 1794 (1959). (5) J. H. Hubbell, "Photon Cross Sections from 10 keV to 100 GeV." NSRDS-NBS29 (1969). (6) K. P. Champion, J. C. Taylor, and R. N. Whittem. Anal. Chem., 38, 109 (1966). (7) D. J. Taylor and G. Andermann. Advan. X-ray Ana/., 13, 80 (1970). (8) K. G. Carr-Brion, Analysf (London),89, 346 (1964). (9) J. S. Wan, At. Energy Ausf., 16 (4), 1 (1973).
RECEIVEDfor review May 7, 1974. Accepted September 23, 1974.
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