The determination of gold in geologic materials by neutron-activation analysis using fire assay for the radiochemical se

Geological Survey

Page Abstract- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Acknowledgments - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Reagents and apparatus- - - - - - - - - - - - - - - - - - - - - - - - - - - Procedure- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Results and discussion - - - - - - - - - - - - - - - - - - - - - - - - - - - Irradiation and decay time - - - - - - - - - - - - - - - - - - - - - - - - Fire assay - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Interferences - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Precision and accuracy - - - - - - - - - - - - - - - - - - - - - - - - - Sampling - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Application - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - References cited - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

2. Eight replicate determinations of gold in Hawaiian
3. Comparison of gold values obtained by this method with those from other methods - - - - - - - - - - - - - - - - - - - Neutron -activation methods provide exception a 1

sensitivity for the determination of gold and generally eliminate the problem of reagent blanks. Methods developed by Vincent and Smales (1956), Vincent and Crockett (1960), Shcherbakov and Perezhogin (1964), DeGrazia and Haskin (1964), and Baedeker and Ehmann (1965) and those summarized by Emery and Leddicotte (1961) provide for the irradiation of 0.1 to 0.5 g (gram) of rock sample. After a short cooling period, the gold is radiochemically s epa r at e d for gamma counting of the 0.413 Mev photopeak for Au 198 or for beta counting. Generally, the accepted activation-analysis methods involve extensive chemical manipulations to achieve the necessary radiochemical purity.

Washington and Holman (1966) applied activation analysis to the silver bead obtained after the conventional fire-assay procedure. Their detection limit was 10 ppb (parts per billion) gold on a 15-g sample because of the Ag 110m activity and the comparatively large reagent blank. Simon and Millard (1968) improved on this method by irradiating the lead button prior to cupellation with silver. Although this method has the advantage of using a 10- to 20-g sample for analysis, it is not applicable to routine analysis because only a few samples can be irradiated simultaneously and the fire-assay flux must be purified before assay to maintain a low blank.

In the method described here, activation analysis was combined with the classical fire assay for the radiochemical separation of gold. The detection limit for gold (defined as 3a, where a is the square root of the background) is 0.05 ppb if the sample contains less than 10 ppm (parts per million) silver. Irradiation and decay times were adjusted to provide low

The Determination of Gold in Geologic Materials by Neutron-Activation Analysis Using Fire Assay for the Radiochemical Separations

By J. J. Rowe ond F. 0. Simon

INTRODUCTION radioactivity less than 1 me (millicurie) for 20 1-g samples). The fire-assay procedure provides for the rapid radiochemical separation of gold with a minimum of operator attention, so that as many as 40 samples can be processed in 8 hours. ACKNOWLEDGMENTS The authors wish to express their thanks to L. P. Greenland, U.S. Geological Survey, for hismanyhelpful suggestions and review of the manuscript. The assistance of L. P. Harris of the Naval Research Laboratory reactor staff and F. W. Brownofthe U.S. Geological Survey is also gratefully acknowledged.

REAGENTS AND APPARATUS Standard gold-stock solutions: Solution A (1,000 ppm Au)-Dissolve 1.00 g of gold metal in aqua regia and dilute to 1 liter with 2 M HCl. Solution B (10 ppm Au)-dilute 5.00 ml (milll'iiter) of solution A to 500 ml with 2 M HCl. Solution B is stable for several months. Solution C (0.5 ppm Au)-dilute 5.00 ml of solution B to 100 ml with 2 N HCl. Solution C should be prepared each time monitors are prepared. Fire-assay flux: Thoroughly mix 1,000 g PbO, 100 g Na2C03, 20 g Na2B407, and 50 g flour (r e·d u c in g power:lO g Pb per g flour).

Gold carrier: For each sample, place a 20- to 50-mg (milligram) piece of gold metal in a heat-resistant dish, and heat with a torch until the gold fuses into a small bead. Weigh the bead (.±0.1-mg accuracy), and place it in a numbered vial. Counting equipment: A pulse-height-analyzer system, using a four -input mixer -router coupled to four 3- by 3-inch Nai(Tl) well-type crystals, was used to collect four spectra simultaneously.

1. Prepare each standard (flux monitor) by placing approximately 0.2 g of gold-free spectrographically pure silica in a small polyethylene vi a 1. Weigh the silica and vial(:tO.Ol-mgaccuracy); add two drops of solution C, and weigh again. From the weight of solution and its specific gravity, calculate the amount of gold added (approx 50 ng (nanograms)). Dry overnight and seal the vial with an electric soldering iron.
2. Grind each sample to less than 100 mesh and carefully mix and quarter. Weigh and seal 1 g of each sample in a 215-dram polyethylene vial. Irradiate 16 samples plus four monitors for 10 hours in a neutron flux of 5xl012 neutrons per square centimeter per second. Store samples for 10 days to allow decay oftheshort-livedisotopes (Prim a r i 1 y 15-hour Na24 and 2.6-hour Mn56).
3. For each sample, place 80 g of fire-assay f 1 u x into a fire-assay crucible and make a depression in the flux to receive the sample.
4. Transfer the irradiated sample to the depression in the flux and mix.
5. Add one bead of gold carrier; cover the mixture with a layer of borax glass and cover the crucible with a scorifying dish.
6. Place the crucible in a fire-assay furnace preheated to 900°-1, 000°C and fuse for 1 hour.
7. Pour each melt into a steel mold; cover with a piece of asbestos and allow to cool.
8. Break the slag away from each lead button in a plastic bag to prevent scattering of glass. Each button should weigh 2 8±8 g.
9. Cupel each lead button at 800°-900°C.
10. Weigh each final gold bead to determine the chemical yield and place in a plastic vial.
11. Collect spectra. After several days, collect another set of spectra to verify the decay of 2. 7 -day Aul98.
12. Standards (monitors) are treated the same as samples.

130 Calculate the area of each 0.413 Mev photopeak for Aul98 (Covell, 1959) and adjust data to zero decay time.

14. Determine the amount of gold in each sa mp I e using the equation:

where Vi is the weight of gold, A is the activity, and Y is the carrier yield; the subscripts s and m refer to sample and monitor, respectiVely.

After samples are irradiated for 10 hours and then are permitted to decay for 10 days, the residual radioactivity is generally less than 1 me. The Au198 activity is sufficient to provide 300 to 400 counts per minute per ng of gold in the 0.413 Mev photopeak using Covell's (1959) method for calculating areas.

.A§ I Y§ W.§.=Wm ~m I Ym..

RESULTS AND DISCUSSION Irradiation and decay time

Interferences Generally the method is specific for gold, in that the 0.413 Mev photopeak of Aul98 is used for the calculations. Most silver originally in the sam p 1 e will be found in the final bead. AgllOm has gammaphotopeaks at 0.445, 0.619, 0.665, and 0.884 Mev; thephotopeak at 0.445 Mev is very small compared with that at 0,665 Mev. The silver contribution to the gold photopeak is negligible unless the silver in the original sample is greater than 10 ppm. Small amounts of antimony (60- day Sbl24) and cerium (33-hour Cel44) may also be found in the gold bead; however, these do not interfere with the 0.413 Mev photopeak of Aul98.

The neutron flux is monitored by using four standards in each irradiation, placed at different levels in the irradiation bucket. The specific activity of the monitors varies by less than 5 percent from the mean, which can be presumed to be the flux variation. Table 2. -Eight reElicate determinations of gold, in Earts Eer billion,

See table 2.

Millard (unpub. data, 1968), neutron-activation analysis with separation of gold on ion-exchange paper. Millard, Rowe, and Brown (unpub. data, 1967), neutron-activation analysis with radiochemical separation of gold. '~Baedeker (1967), neutron-activation analysis with radiochemical separation of gold, Vincent and Crockett (1960), neutron-activation analysis with radiochemical separation of gold. Hamaguchi and others (1961), neutron-activation analysis, method not described. Shcherbakov and Perezhogin (1964), neutron-activation analysis with radiochemical separation of gold. Simon and Millard (1968), neutron-activation analysis after fire-assay preconcentration. Sarma, Sen, and Chowdhury (1965), emission spectrographic analysis on silver fire-assay bead. in Hawaiian basalts, G-1, and W-1

TLW-67-17 Cone Crater, Hawaii

Basalt------- Granite from Kershaw, N.C-------- ----do------- ----do------- 4.28 4.50 4.08 4.98 4.18 4.38 4.16 4.15

.27 Basaltic glass, Hawaii

3. 0 HK 1955 Kilauea, Hawaii
4. 4' If 4. 9'
5. 6' If 2. 0'
6. 7' G-1

b U.S. Geological Survey standard rocks, G-1, and W-1, and three samples of Hawaiian basalts, one of which was a basaltic glass, were each analyzed eight times. The results are shown in table 2. From the replicate analysis, the relative error is estimated to be about ± 10 percent of the amount of gold present.

The determination of gold by this method is in good agreement with results determined by accepted methods bf activation analysis. Comparison ofresults for six standard rocks and several other rock samples is shown in table 3.

REFERENCES CITED Baedeker, P. A., 1967, The distribution of gold and iridium in meteoritic and terrestrial materials: Lexington, Kentucky Univ. Ph. D. dissert., 104 p.; U.S. Atomic Energy Comm. Tech. Rept. OR0- 2670-17' 120 p. Baedeker, P. A., and Ehmann, W. D., 1965, The distribution of some noble metals in meteorites and natural materials: Geochim. et Cosmochim. Acta, v. 29, p. 329-342. Beamish, F. E., 1966, The analytical chemistry ofthe noble metals: New York, Pergamon Press, p. 162-176. Bugbee, E. E., 1940, A textbook of fire assaying [3d ed. ]: New York, John Wiley & Sons, Inc., 314 p. Chow, A., and Beamish, F. E., 1967, An experimental evaluation of neutron activation, wet assay, and fire assay methods of determining gold in ores: Talanta, v. 14, p. 219-321. Covell, D. F., 1959, Determination ofgamma-ray abundance directly from total a b sorption peak: Anal. Chemistry, v. 31, p. 1785-1790. DeGrazia, A. R., and Haskin, Larry, 1964, On the gold content of rocks: Geochim. et Cosmochim. Acta, v. 28, p. 559-564. Emery, J. F., and Leddicotte, G. W., 1961, The radiochemistry of gold: Nat I. Acad. Sci. -Nat I. Research Council Nuclear Sci. Ser., NAS-NS-3036, 34 p. Hamaguchi, Hiroshi, Kuroda, Rokuro, Tomura, Kenji, Watanabe, Kenju, Yasunaga, Tsutomu, 0 saw a, Masumi, Onuma, Naoki, Hosohara, Kyoichi, and Endo, Tadashi, 1961, Values for trace elements in G-1 and W-1 with neutron activation analysis: Geochim. et Cosmochim. Acta, v. 23, p. 296-299. Sarma, B. D., Sen, B. N., and Chowdhury, A. N., 1965, Platinum and gold contents of granite G-1 and diabase W-1: Econ. Geology, v. 60, p. 373-374. Shcherbakov, Y. G., and Perezhogin, G. A., 1964, Geochemistry of gold: Geokhimyia 6, p. 518-528. Simon, F. 0., and Millard, H. T., 1968,Determinatwn of gold in rocks by neutron activation using fire assay preconcentration: Anal. Chemistry, v. 40, p. 1150-1152. Vincent, E. A., and Crockett, J. H., 1960, Studies in the geochemistry of gold-Part 2, the gold content of some basic and ultrabasic rocks and stone meteorites: Geochim. et Cosmochim. Acta, v. 18, p. 143-148. Vincent, E. A., and Smales, A. A., 1956, Determination of palladium and gold in igneous rocks by activation analysis: Geochim. et Cosmochim. Acta, v. 9, p. 145-160. Washington, R. A., and Holman, R. H. C., 1966, A rapid and sensitive method for determining gold in rocks and other geological materials: Canada Geol. Survey Paper 65-7, 18p. Wilson, A. D., 1964, The sampling of silicate rock powders for chemical analysis: Analyst, v. 89, p. 18-30.

Where this page came from

This page was imported from U.S. Geological Survey. Published by the U.S. Geological Survey and, as a work of the United States government, in the public domain.

Nobody has written it yet — it is the source material at a new address, which is why search engines are asked to skip it and why no one earns from it. It is up for grabs: take it on, and it is yours to rewrite and to earn from.