Table 11. Simultaneous Determination of Phosphate and Silicate Concentration of mixtures found, ppm" Concentration of mixtures taken, ppm Re1 Re1 Phoserror, Phoserror, Silicon phorus Silicon phorus 0.0 5.0 ... ... ... ... 2.5 10.0 2.57 $2.8 10.04 +0.4 5.0 5.0 4.94 -1.2 5.02 +0.4 5.0 50.0 4.55 -9.0 ... 10.0 2.5 9.82 -1.8 2.48 -0.8 5.0 0.0 ... ... ... ... 50.0 5.0 ... ... 5.04 +0.8 Based on 5-ppm silicon standard and 5-ppm phosphorus standard: integration time, 40 sec; premeasurement time, 45 sec.
z
z
.
.
I
(1
45-second premeasurement time is sufficient for the reaction to form 12-MPA to come to equilibrium. Once standards have been run, 5 determinations of silicate and phosphate can be made in about 15 minutes. For the highest phosphate concentration shown in Table I1 a negative error is evident in the determination of silicate. This error is not attributed to a synergistic effect, but to a "concentration depletion effect," which results because the formation of 12-MPA requires 12 moles of molybdenum for every mole of phosphate. Thus, a t high phosphate concentrations, the molybdate concentration after reaction with phosphate is substantially lower than the initial concentration. Since the rate of formation of P-12-MSA is dependent o n the molybdate concentration, the depletion of Mo(V1) results in a lower rate and errors in silicate determinations in the presence of large amounts of phosphate. Preliminary results obtained at lower acid and molybdate concentrations were satisfactory for pure silicate solutions. However, the concentration depletion effect became important a t lower phosphate concentrations. Higher molybdate concentrations were
tried to reduce the concentration depletion effect, but the high densities of these solutions prevented efficient mixing and caused noisy reaction rate curves. Silicate determinations are not so accurate as phosphate determinations as shown by the data in Table 11. There are several possible reasons for the lower accuracy in silicate determinations. The concentration depletion effect mentioned above can give low results if large amounts of phosphate (>25 ppm) are present. Positive errors can result if the phosphate reaction is not complete before the rate of formation of P-12-MSA is measured. The errors due to this latter effect also increase with increasing phosphate concentration. Thus, the silicate procedure is recommended only for phosphate concentrations in the range of 0-25 ppm of P. Possibly this procedure can be extended to wider concentration ranges. Complete kinetic and equilibrium studies of the formation of the heteropolymolybdates and the heteropoly blues should yield information which can be used to optimize all reaction conditions. Such studies are now in progress in several laboratories. Also simultaneous determinations of phosphate and silicate can probably be made using only one reaction by employing a stopped-flow apparatus to measure the rate of formation of both 12-MPA and P-12-MSA. Initial rates measured during the first few milliseconds can be related to the phosphate concentration, while after 30 seconds a second reaction rate measurement can be used to determine silicate. Work in this area is in progress. RECEIVED for review July 23, 1970. Accepted October 5 , 1970. Presented at 160th National Meeting, ACS, Chicago, Ill., Sept. 1970. One of us (J. D. Ingle, Jr.) gratefully acknowledges a National Science Foundation traineeship and a n ACS Analytical Division Fellowship. This work was partially supported by NSF Grant No. GP-18123.
Radioisotopic X-Ray Analysis of Silver Ores Using Compton Scatter for Matrix Compensation P. G . Burkhalter' U . S . Department of the Interior, Bureau of Mines, College Park, Md. 20740 Radioisotopic X-ray analysis with semiconductor detectors was evaluated for determining silver in ores. X-ray and scatter peak intensities were obtained using computer processingfor curve fitting backgrounds under the Ag Ka peaks and unfolding overlapping spectra. The discrete Compton and coherent scatter peaks obtainable with monoenergetic excitation Were evaluatedas internalstandards for matrix compensation. The Ag Ka-to-Compton ratio provides compensation for X-ray absorption and a single, linear calibration curve can be for silver determination in both uniform and variable matrix ores. For a set of samples taken from a drill core, silver was determined to within *lo% for concentrations above and the detection limit was 0.3 oz/ton (10 ppm). For variable matrix ores, silver can be determined within 120%.
'Presently with U. S. Naval Research Laboratory, Code 7680, Washington, D. C. 20390. 10
SEMICONDUCTOR DETECTORS combined with radioisotopic are being used for nondiffractive X-ray analysis (1-31, Laboratory studies at College Park in the area of mineral technology have emphasized delineation of ore deposits ( 4 , 5 ) such as field analvsis of drill cores. Semiconductor detectors offer advantages portability for field usage and have cient stability so that the sensitivity for silver (about 10ppm) is comparable to that obtained by conventional X-ray fluorescent
bf
(1) A. P. Langheinrich and J. W. Forster, Adcan. X-Ray Anal., 11,
275 (1968). (2) J , R. Rhodes, ORNL-llC-10 (v,ll, 442 (1967). (3) J. D. Frierman, H. R. Bowman, I. Perlman, and C. M. York, Science, 164,588 (1969). (4) . , P. G. Burkhalter, Symp. Nucl. Techniques Mineral Resources, IAEA, Vienna, STIlPUBjl98, p 365 (1969). ( 5 ) P. G. Burkhalter and H. E. Marr 111, Int. J . Appl. Radiat. Isotopes, 21 (7), 395 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
analysis using diffracting crystals. Radioisotopic excitation replaces a bulky 75- to 100-keV X-ray generator with a compact source that can easily be placed on the detector housing and removed for storage when not in use. The ease in exciting the K spectra of high atomic number (2)elements with radioisotopes is important because greater X-ray penetration can lessen difficulties of sample heterogeneity and surface roughness that are encountered in field analysis. Although X-ray analysis provides adequate sensitivity for silver detection in ores, successful field application requires a method t o compensate for variable matrix effect. The silver intensity must be corrected for the variable X-ray absorption due to the matrix to obtain linear intensity us. concentration curves. Of the various X-ray techniques to correct for matrix effects, the internal scatter method is suggested because of the large, discrete scatter peaks in ore spectra excited by monoenergetic X-rays from a n electron-capture radioisotope such as I Z S I . Andermann and Kemp (6) were first t o propose the use of the fluorescence-to-scatter ratio to reduce the effects of variable X-ray absorption in X-ray emission spectrography. I n particular, they obtained good results for Ni K a and P b La intensities in complicated ores using a scatter intensity in the background continuum at an arbitrary energy: Recently, Taylor and Andermann (7) determined that compensation by the internal scatter method is strongly dependent on the choice of scatter energy because of the overlapping Compton and coherent scatter intensities using continuum excitation. They obtained good compensation for Ca K a in a light matrix using the Compton scatter of the primary C r K a radiation from a chromium target X-ray tube. In recent years, monoenergetic excitation with radioisotopes has been used for exciting high-Z elements in ores. Large Compton scatter peaks are observed in the ore spectra, and the internal scatter method using the Compton peak has been found to provide good compensation for light matrix ores. However these researchers (8-10) found that generally the method undercompensates in heavier matrices, necessitating the use of monographs. The purpose of this work was a study of nondiffractive X-ray analysis of silver ores. Computer methods were developed for obtaining silver intensities and the use of scatter for matrix compensation was investigated with synthetic silver standards. Two sets of silver ores were used for silver determinations : variable-matrix ores collected from various mining regions in the United States and samples of a relatively constant-matrix ore obtained from a drill core. X-ray intensities were obtained by computer processing of spectral data collected with a multi-channel analyzer. The Ag K a X-ray peak excited by lz5Iwas superimposed o n a rapidly varying scatter background. A computer program BACKGROUND was used t o calculate the background under the Ag K a peaks. A program BANDFIT written by Fraser and Suzuki (11) was used for unfolding X-ray interferences arising when cadmium or antimony was present in the silver ore. EXPERIMENTAL Instrumentation. Silicon and germanium semiconductor detectors were used in this investigation. A 3-mm thick by (6) G. Andermann and J. W. Kemp, ANAL.CHEM., 30, 1306 (1958). (7) D. L. Taylor and G. Andermann, Adcan. X-Ray Anal., 13, 80 (1970). (8) P. Martinelli and P. Blanquet, Symp. Radiochem. Methods, IAEA, Vienna, STI/PUB/88, p 451 (1965). (9) B. Holynska and L. Langer, Anal. Chim. Acta, 40,115 (1968). (10) A. Lubecki, M. Wasilewskia, and L. Gorski, Spectrochim. Acta, 23A, 831 (1967). (11) R. D. B. Fraser and E. Suzuki, ANAL.CHEM., 38, 1770 (1966).
C O M P T O N OR C O H E R E N T S C A T T E R -. \
/
'\
Te Ko PRIMARY X-RAYS
SOURCE
COLLAR
S I OR G e CRYSTAL
\I
HOUSING,^,
CRY OS T A T
1
Be WINDOW
cl
I
b---Figure 1. Sample, radioisotopic source, and semiconductor detector geometry 50-mm2 area Si(Li) detector manufactured by T M C (formerly Technical Measurements Corp.) was used to measure X-ray intensities at high count rate from the set of variable-matrix ores. Operating a t 100,000 counts per second the pulse width a t half-maximum intensity (FWHM) for Ag K a was 1080 eV. The electronics used at high count rate were a T M C preamplifier Model 336 having pole-zero cancellation, a T M C linear amplifier Model 341 using bipolar amplification with 0.2 psec time constants, and a Canberra biased amplifier and pulse stretcher model 1460. The data for the synthetic silver samples and the set of drill-core samples were accumulated at low count rate with a Ge(Li) detection system manufactured by ORTEC. With a 5-mm thick by 80-mm2 germanium crystal, the system had a pulse resolution of 660 eV for Ag K a at 5000 counts per second. All data were accumulated in a SCIPP 400 channel pulseheight analyzer (PHA) manufactured by Victoreen. The data readout was o n paper tape which was converted to cards for processing with the University of Maryland UNIVAC 1108 computer. FORTRAN programs were used that smoothed the spectral data, located the center of the X-ray and scatter peaks, provided background curves under the Ag K a peak, unfolded overlapping peaks, and calculated X-ray intensities. The computer output was printed intensities, peak-to-background ratios, and spectral curves using a CalComp plotter. Excitation. 10dine'~shas been found to be a good radioisotope for exciting Ag K a radiation (4). This electron-capture isotope emits Te K a (27.5 keV) and Te K P (31.0 keV) X-rays plus a low intensity gamma ray at 35.5 keV. The term monoenergetic is used in this paper in respect to 1 2 6 1 in the sense that most of the silver radiation is excited by Te K a . Other X-rays that could be used for efficient excitation of silver in ores are Sb K and I K radiation. These X-rays could be produced by source-target assemblies but would require source strengths about three orders of magnitude larger than with a n electron-capture source. A source strength of 40 mCi of 1251was used. Iodine125 has a short half-life of only 56 days which might require frequent replacement in a continuous operation. However, the cost of IZ5Iis only a few dollars per mCi and by ordering a much stronger source than necessary and shielding with a low-Z metal foil, a practical source life of over 6 months can be obtained. F o r this study a 50-mCi annular source was purchased from the Radiochemical Centre. The radioactive material was prepared as a thin annular ring 1-mm thick by 5.7-cm diameter. A metal collar was used to hold the source on the detector housing and excite samples in a backscatter geometry as shown in Figure 1. Samples were positioned about 2.5 cm from the detector window, thereby irradiating a sample area of about 7.6-cm diameter.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
11
Compton
scotter
(Te Ka)
Coherenl scatter (Te KO)
P
Pb L m
Zr K
Ba K
Coherent scatter [ Y -ray)
1
38 I 34 36 ENERGY, k e v
Figure 2. Characteristic X-ray lines, scatter peaks, and absorption edges The Te K a primary radiation is very efficient for exciting silver as the Ag K-edge is a t 25.5 keV. Other X-rays that are readily excited in silver ores by 1 2 5 1 are the K spectra of iron, copper, and zinc and the L spectra of lead. Barium which has an absorption edge a t 37.4 keV is not excited by 1251, Figure 2 shows the positions of the absorption edges, fluorescent X-rays, Compton peaks, and coherent scatter peaks of importance in determining silver in ores. The relative intensities for the X-ray lines and scatter peaks correspond to those of a high-quartz matrix ore containing 26.3 oz/ton of silver. For a back-scatter geometry of 128 degrees, the large Compton scatter peak of Te K a occurs at 25.3 keV, just below the energy of the Ag K edge. The Compton peak has a FWHM pulse resolution only 44% broader than the Ag K a X-rays. The Compton peaks are narrow because the angular scatter spread is reduced when using a small detector crystal and a radioactive source in the form of a narrow ring. COMPTON SCATTER FOR MATRIX COMPENSATION
T o obtain silver intensities that vary uniformly with concentration, the measured Ag K a intensity must be compensated for variations in ore matrix. Scattered radiation whose energy is near that of the desired X-ray but whose intensity is independent of Z would experience X-ray absorption proportional t o the characteristic X-ray intensity; and therefore the ratio of characteristic X-ray-to-scatter intensity would be largely independent of X-ray absorption variations. With monoenergetic excitation, distinct Compton and coherent scatter peaks are obtained at higher excitation energies (Eo) using a backscatter geometry. In a light matrix ore, Compton scatter dominates a t higher Eo because coherent scatter decreases as E - 3 . For heavier matrix ores, coherent scatter peaks are large even at higher Eo because coherent scatter varies as approximately Z 3 in the backscatter geometry. On the other hand, Compton scatter is nearly independent of Z but varies as the number of electrons per atomic mass of 12
sample traversed. It is readily apparent that unless the Compton scatter intensity is measured distinctly from coherent scatter that the characteristic X-ray-to-Compton scatter ratio would under-compensate for heavier matrix ores. T o estimate possible success for compensation in silver ores, the X-ray absorption coefficients for Ag K a and the Compton scatter energy together with the variation in Compton scatter intensity with matrix were examined. As the fluorescent Ag K a X-rays and Compton scattered radiation traverses the ore matrix before emerging from the sample, the lower energy silver radiation is more strongly absorbed. However, the ratio of the mass absorption coefficients for Ag K a compared to the Compton scatter energy is nearly constant for the high-Z elements commonly found in silver ores. This ratio of mass absorption coefficients for iron, copper, zinc, zirconium, antimony, barium, and lead varies from 0.690 to 0.705. The same ratio for silica is 0.685. The calculated ratio for a heavy matrix ore consisting of 40% high-Z elements such as barium or lead is 0.693 ; only about 1 higher than for a quartz-type ore with 15 iron which has a ratio of 0.686. It is because of this constancy of the absorption coefficient ratio that the Compton scatter method can be expected to provide compensation for variable matrix. The change in matrix, however, will slightly affect the Compton scatter intensity because the Compton scatter varies as Z/A. The value o f Z / A varies slowly from 0.499 for quartz t o a value of 0.396 for pure lead. Therefore the Compton intensity would be smaller for a heavy matrix ore. The Z / A values were calculated for several of the silver ores used in this work. The Z / A values estimated for light-matrix silver ores such as Crescent or Escalante is 0.494. For several of the heavy matrix ores the Z / A value was lower; about 2 for the Mammoth ores, 4.5 % for Baker Property, and 6 z for Bunker Hill. This reduction in Compton intensity because of Z / A would over-compensate the fluorescent-to-Compton ratio by
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
z
Table I. Intensity Measurements of 1000-ppm Silver Standards Sample matrix Si02 Si02 5% Si02 5% Si02 5% Si02 5%
+ +
+ +
Fe Zr Ba
Pb
Sample Escalante Bunker Hill Mammoth 14 Baker Property
Crescent Mammoth Raise Mammoth 5 Taylor 2 Taylor 1 Mammoth 6 Robinson Orphan Girl Flathead Badger
Ward * Values not available.
I A g ita,
Icampton,
CPS
CPS
Icoherent,
CPS
IAg Ka/Compton
I AK~a l l o o h e r e n t
IcoherentllCompton
200. 150. 90.1 139. 83.8
1890. 1420. 854. 1440. 754.
642, 510. 350. 547. 369.
0.106 0.106 0.106 0.096 0.111
0.31 0.29 0.26 0.25 0.23
0.34 0.36 0.41 0.38 0.49
Table 11. Composition of Variable Matrix Silver Ores Weight per cent - Oz/ton Ag Cd Fe Cu Zn Sb Ba Pb SiOp Gangue minerals 26.3 * 3.9 0.05 3 . 9 * 0.35 0.50 79. Feldspar lo%, calcite 5 %, clay 5 % * 24.1 25.6 7.0 0.05 3 . 6 * 28. 32. Cerussite 20%, chlorite 20%, clay 15%, dolomite 10% 19.3 * 4 . 4 0.09 0.20 3.5 17. 1 . 3 69. Barite 25%, siderite lo%, dolomite 5% 18.3 * 3 . 6 0.10 0.40 * 27. Barite 40%, feldspar lo%, pyrite 5% 0.40 35. * 17.6 * 16. 0.34 0.05 0 . 2 0.05 53. Siderite 40%, sericite 10% 15.1 * 5.0 0.08 0.10 3 . 0 15. Barite 20%, limonite lo%, dolomite 5% 2.7 68. 14.0 * 33. 2.4 1 . 5 0 . 2 2. 0.45 16. Pyrite SO%, dolomite 15%, barite 5% * Dolomite 50 %, calcite 40 % 0.25 0.06 1 . 2 1 . 5 0.40 1 . 2 13.4 6 . 2 * 1 . 2 0.38 0.70 3 . 0 1 . 2 52. Calcite 40%, dolomite 5 % 13.2 4 . 0 10.9 * 21. 2.1 1 . 4 0 . 2 3. 3.6 28. Pyrite 15%, siderite 25% dolomite 10% * 4.2 5.7 * * 0.05 * 1 . 3 83. Feldspar 10Z, calcite 5 % * * 4.8 * 2.3 0.05 2.0 Cerussite lo%, clay 5 % 4 . 8 75. 3.9 * 2 . 8 0.06 0.05 * Calcite 5%, chlorite lo%, mica 10% 0.80 78. 0.3 * * 0.40 61. 3.9 0.26 4 . 6 Feldspar 15%, mica 1 0 2 , pyrite 10% 2.7 4 . 2 * * 21. Limonite 40 %, calcite 15%, cerussite 10% 2.6 5 . 3 0.06 1.0 6 . 2 36.
2 to 6x for heavy matrix silver ores. The absorption coefficient ratio difference of 1.O between light and heavy ores would also cause over-compensation. For most silver ores we could expect the Compton scatter ratio t o provide compensation within a few per cent and that a slight over-compensation may exist for heavy matrix ores. To test this argument that the Ag Ka-to-Compton scatter ratio will provide compensation, five synthetic silver standards were prepared at the 1000-ppm level in matrices of silica and silica plus 5 additions of several high-2 elements commonly found in silver ores. Intensities were obtained for the silver standards from three separate runs with 5-minute accumulations. The silver and scatter intensities together with the Ag Ka-to-Compton ratios are given in Table I. The 5 % high-2 elements in silica standards would correspond in absorption t o light matrix silver ores and we find greater than 50% absorption of the Ag K a intensity in the zirconia and lead containing samples compared t o a silica matrix. Exact compensation was found for the iron and zirconia standards using the Ag Ka-to-Compton scatter ratio and a 5 % overcompensation for the heavier lead matrix as predicted. The barium containing standard has lower silver-to-Compton and coherent-to-Compton ratios indicating too high a Compton scatter intensity, but no explanation can be given for the lack of compensation for the barium sample. The Ag Ka-tocoherent scatter ratio tends to seriously under-compensate as high-Zelements are added to the matrix. ANALYSIS OF SILVER ORES
Composition. A set of 21 silver ore samples with silver content ranging from 2.6 to 26.3 oz/ton (1.0 oz/ton equals 34.3 ppm), was collected from mining regions throughout the United States. These samples are classified as variablematrix ores because of the wide variation in concentration of high-2 elements. I n this paper, high-2 is used to designate
those elements with atomic number greater than 24 to distinguish from ore matrices composed of principally low-Z elements such as aluminum, silicon, and calcium. Table I1 lists the concentration of the high-Zelements and the gangue minerals for 15 of these ores. Iron, copper, zinc, barium, lead, and silicon dioxide were determined chemically; cadmium and antimony concentrations were estimated from relative X-ray intensities measured in an X-ray spectrograph. The minerals forming the gangue were qualitatively determined by powder X-ray diffraction and by optical microscopy. Several of these ores such as Escalante, Robinson, and Flathead are light-matrix quartz-type with only 5 to 15% of elements such as iron but small amounts of heavy elements such as barium or lead. Other ores had heavy matrices with abundances of high-Z elements such as Ward with 21 iron and 6 % lead, Baker Property with 27% barium, and Bunker Hill with 28% lead. The four Mammoth ores have similar matrices with as much as a 40% high-Z content. The two Taylor ores have light matrices but low silica content. Fire-assay was used to determine the silver content of the ores. To ensure homogeneity, the ores were ground to 100 mesh and thoroughly mixed. Three of the ores were analyzed in duplicate and the silver determinations agreed within 1 2 %. A set of 24 silver-ore samples were taken from sections of a drill-hole core from the Westcliffe District, Col. The silver content varied from 0.32 to 41.9 oz/ton. The Westcliffe District ore samples had a relatively uniform matrix consisting of over 50z quartz with 15 to 25% microcline or albite type feldspar and 10 to 15 . . kaolin. Semiquantitative optical spectrographic analysis gave the following range of concentration for high-2 elements: 1 to 10% manganese,
