ANALYST, MARCH 1986, VOL. 111 291 Alkyl Cyanide Medium for the Determination of Precious Metals by Atomic Absorption Spectrometry R. Le Houillier and C. De Blois Ministere de I'Energie et des Ressources, Centre de Recherches Minerales, 2700 Rue Einstein, Sainte-Fo y, Quebec, Canada G I P 3 W8 Concentrations ranging from parts per billion levels up to a few parts per million can be adequately determined for platinum, palladium and gold pre-concentrated in a silver bead, when dissolved in an alkaline cyanide solution, by the use of AAS, with a vanadium buffer to correct for precious metal interference. The same medium is also proposed for the determination of silver, platinum, palladium and rhodium pre-concentrated in a gold bead; however, the rhodium recovery is poor. The difficulty in recovering rhodium is not associated with the medium of the final solution but rather with losses produced by the possible formation of lead rhodium oxide during cupellation and with the acid-insoluble flakes containing lead and rhodium.Keywords: Precious metal determination; atomic absorption spectrometry; fire assay pre-concentration; vanadium buffer; alkaline cyanide solution Various methods for the determination of noble metals by atomic absorption spectrometry (AAS) and fire assay have been published and reviewed.l-5 The classical Tire assay is an excellent technique for collecting and concentrating in a bead noble metals from rocks, ores, minerals and other matrices prior to AAS determination. When the beads so produced are dissolved in acid, the contents of noble metals such as silver, gold and palladium can be determined by AAS with good precision.Indeed, Kallman and Hobart6 reported that solu- tions of 100 ng ml-1 of silver and 300 ng ml-1 of gold and palladium can be analysed by AAS with a precision of k 1%. To achieve such results, hydrochloric acid containing a significant amount of gold must be avoided, owing to the possibility of reducing gold in the capillary of the aspirator.6 Moreover, the solubility of silver chloride in a solution containing less than 25% VlV hydrochloric acid is limited. In addition, any precipitation of silver from such a solution containing gold implies a high risk of loss of gold by coprecipitation. Further, silver salts are less soluble in a small volume of hydrochloric acid (25% VIV) than in the same volume of an alkaline cyanide solution.For this reason, occlusion of gold and other precious metals in silver salts may be lost, which is responsible for poor recoveries when a certain amount of a silver salt containing precious metals is not solubilised. However, stable complexes of silver , gold, plati- num, palladium and, to a certain extent, even rhodium are formed in cyanide solutions. The determination of the noble metals by AAS with the use of such a medium is very attractive as dissolution of silver is more rapid and the risk of losses through coprecipitation is avoided. However, interfer- ences between the noble metals in such a medium have not been thoroughly studied. In this paper, an alkaline cyanide medium is proposed for the determination of gold, silver, palladium, platinum and rhodium by AAS in rocks, ores and minerals from parts per billion levels up to 3 p.p.m., after pre-concentration of the noble metals by the fire assay procedure.Interferences are reported and a buffer is proposed. The accuracy and precision obtained for the procedure are presented. Experimental Standard Solutions Standard solutions of precious metals were prepared from Specpure (Johnson Matthey Chemicals) chloroammonium salts of rhodium and palladium, platinum sponge and metallic gold. Silver solutions were prepared from pure silver nitrate (Aldrich). Prepare standard solutions by dissolving weighed amounts of chloroammonium salts of rhodium and palladium in water. Add HCl and dilute with water to obtain a final solution containing 7.5% VlV of HC1.Dissolve in the same medium platinum and gold salts produced by dissolution of the pure metals in aqua regia and evaporate a given volume of this solution to dryness. Dissolve the salts in water and dilute with a solution containing 5% mlV each of KCN and KOH. Add a solution containing 2% mlV of vanadium to obtain a final medium containing 0.5% mlVof KCN and KOH and 1% mlV of vanadium. For silver solutions, dissolve silver nitrate in water, then follow the same procedure as above. The KOH is necessary both as a safety measure, owing to the health hazard with cyanide, and to dissolve any lead remaining in the bead. Buffer Solution To prepare the buffer solution, dissolve 24.0 g of NaV03 overnight, without heating, in 400 ml of demineralised water.After filtration through a Whatman No. 44 filter-paper introduce the solution into a 500-ml calibrated flask and dilute to volume. This solution contains 2% mlV of vanadium. It is important that the demineralised water be low in oxygen in order to avoid the formation of insoluble vanadium oxide. Fire Assay Follow the standard fire assay procedure for collecting and concentrating the noble metals in a bead. Lead was used as the collector throughout the study owing to its technical sim- plicity. The amount of sample used for fire assay was 15 g. For recovery studies, precious metals in solution are added to 15 g of pure quartz and flux mixture and the whole charge is dried before fusion. In general, the lead button containing the precious metals weighs about 25 g. Silver as a Collector As good practice in the fire assay, add silver (10 mg) to collect gold, platinum and palladium in a bead that can be easily dissolved in an acid solution.Conduct the fusion step at 1000 "C but the cupellation must be completed at ca. 940 "C when platinoids are present in order to obtain a bead of low292 ANALYST, MARCH 1986, VOL. 111 lead content. If the cupellation step is not conducted properly, dissolution of the noble salts in alkaline cyanide solution may be a source of problems, because of lead. For the determi- nation of gold, when only gold is present, the silver bead can be produced at a lower cupellation temperature, i.e., about 870 "C. Gold as a Collector Add 5 mg of gold in the fusion step to collect trace amounts of rhodium, platinum and palladium.Perform the fusion at 1000 "C and the cupellation at 940 "C. For silver analysis only, a gold bead obtained after cupellation at 870 "C is adequate. Bead Treatment Treat a silver bead containing gold, platinum and palladium or a gold bead containing platinum, palladium and rhodium in a 30-ml beaker with 5 ml of hot dilute hydrochloric acid YO VlV) to dissolve any gangue left on the surface of the bead after cupellation. Such gangue must be eliminated prior to the dissolution of noble metal salts in alkaline cyanide medium in order to avoid precipitation and thus metal losses.7 Discard the wash solution and treat the bead with 2 ml of hot nitric acid. When no further reaction is observed, add 6 ml of concentrated hydrochloric acid and keep the solution warm for about 10 min, then evaporate the solution to dryness at a low temperature. One advantage of the cyanide dissolution over the use of an acid medium is that if the salts are dried at too high a temperature, precious metal salts may be decom- posed to the metallic state.The cyanide dissolution will still allow a good recovery of those precious metals susceptible to such a reaction. Alkaline Cyanide Dissolution of Noble Metal Salts Add to noble metal salts 1 ml of 5% mlVKCN - 5% mlVKOH solution and dilute the solution to 5 ml with water. Just before the AAS measurements, add 5 ml of sodium metavanadate solution containing 2% mlV of vanadium to eliminate interferences. This addition is carried out just before the AAS measurements because the final solution has a stability of about 4 h.The final volume of solution is 10 ml and it contains 1% mlV of vanadium and 0.5% mlV each of KCN and KOH. Silver and Gold Beads With a silver bead containing only gold, or vice versa, the washing and dissolution procedure is similar to that described above. However, after evaporation to dryness, dissolve the salts in only 0.5 ml of the 5% mlV KCN - KOH solution, then dilute to 5 ml. No buffer for interference suppression is needed for silver and gold determinations only. The final volume of solution is 5 ml and it contains 0.5% mlV each of KCN and KOH. Atomic Absorption Measurements A Varian AA-875 atomic absorption spectrometer, equipped with automatic gas control, an automated background correc- tion system, an air - acetylene burner and an adjustable barrel nebuliser, was used.The wavelengths used to optimise the instrumental parameters with standard solutions are given in Table 1. Table 1 also gives the sensitivities and detection limits (20) obtained for Pt, Pd and Rh dissolved in 0.5% rnlVKCN - KOH - 1% m/V vanadium solution. For gold and silver, the analytical values are valid for vanadium-free solutions. Results and Discussion Interferences In general, the addition of KOH to a cyanide solution enhances the absorbance of the noble metals. The greatest increases are observed with platinum and rhodium in 2% mlV KOH solution. For 8 pg ml-1 of platinum an absorbance increase of 124% is obtained, whereas for 0.8 pg ml-l of rhodium a 40% enhancement is typical.However, at 0.1% mlV KOH, a marked decrease in absorbance is encountered for silver; the decrease is less pronounced for gold. The absorbance of these metals is re-established at a higher concentration of KOH (0.5% mlV). Small silver, platinum and palladium absorbance enhance- ments are observed with increasing concentration of KCN in solution. However, a 54% increase in gold absorbance is obtained in 1% mlV KCN solution. KCN interferes differently with rhodium. A maximum 24% decrease in the absorbance of rhodium occurs at approximately 0.4% mlV KCN. Increasing the KCN concentration to 2% mlV results in a higher absorbance than before the decrease. Table 1. Instrumental parameters and analytical values Lamp Detection limit */ Wavelength/ Slit/ current/ Background Sensitivity/ Element nm nrn mA corrector pg ml-1 ng g-l Au .. . . . . 242.8 1 .o 4 Yes 0.15 15 Ag . . . . . . 328.1 1 .o 3 Yes 0.027 2 Pt 266.0 0.5 10 Yes 0.81 80 Pd . . . . . . 244.8 0.5 5 Yes 0.11 12 Rh 343.5 0.5 5 No 0.065 7 . . . . . . . . . . . . * Detection limits for Au and Ag are valid for 5 ml of final solution compared with 10 ml for all the other elements. These detection limits refer to the original 15-g sample. Table 2. Interferences detected in 0.5% m/V KCN - KOH solution without vanadium buffer Change of analyte absorbance, YO Concentration/ Au 7 Ag Pt , 7 Rh 9 Interferent pg ml-1 l.Opgmi-~ l.Opgml-~ 8.0pgml-1 0.8pgml-1 0.8pgml-l . . . . . . 0 - 34 0 - 32 Au 1500 - Ag . . . . . . 2000 0 0 0 0 Pt . . . . . . 30 0 0 -36 - 73 - 33 Pd .. . . . . 15 0 0 - 40 Rh . . . . . . 15 0 0 - 56 - 22 - - - -ANALYST, MARCH 1986, VOL. 11 1 293 Table 3. Interferent concentrations in 0.5% m/V KCN - KOH - 1% m/V vanadium solution for which vanadium buffer is efficient Interferent concentration/pg ml- 1 Concentration/ Analyte pg ml-1 Ag Au Pt Pd Rh . . . . . . 30 15 15 Au 1 .o 2000 - Ag . . . . . . 1 .o 2000 30 15 15 . . . . . . 50 5 Pt 8.0 1500 500 - 20 Pd . . . . . . 0.8 1500 1000 20 Rh . . . . . . 0.8 1500 200 10 10 - - - Most of the important absorbance changes of the noble metals mentioned above are markedly decreased when a solution containing the same amounts of both KCN and KOH is used. For example, silver, gold and palladium give an almost constant absorbance when solutions containing &lY0 m/V of both KCN and KOH are used.However, rhodium and platinum still show an absorbance increase. Nevertheless, this is not a problem as the final solution of noble metals is always adjusted to contain 0.5% mlV KCN - KOH. Interferences encountered between precious metals dissolved in alkaline cyanide solutions containing 0.5% m/V KCN - KOH are presented in Table 2. No change in analyte absorbance is observed for gold and silver with the concentrations of interferent and analyte reported. The worst situation occurs between platinum and rhodium. It is worth mentioning that, for 1 pg ml-l of lead in solution, no interference with gold, silver, platinum, palladium or rhodium is observed with the concentration of each analyte reported in Table 2. Interferences are eliminated in such alkaline cyanide solutions by the addition of sodium metavanadate.Table 3 reports the analyte and interferent concentrations for which a buffer solution containing 1% mlVof vanadium eliminates the change in analyte absorbance associated with the action of such interferents. It is worth mentioning that 0.15-1% m/V of vanadium in solution increases the absorbance of 1 pg ml-l of rhodium and platinum by 74 and 135%, respectively. However, vanadium has no releasing action on the absorbance of the same concentration of palladium. When only silver and gold are present in a sample, the addition of vanadium can be omitted as there is no significant interference. Procedure Testing Typical fire assay beads resulting from the complete decompo- sition of 15 g of pure quartz, to which a single addition of different amounts of a precious metal were added in solution, were analysed by AAS.Table 4 gives the over-all recovery obtained with a silver bead. No results are given for rhodium as it is known that silver is not a recommended collector for rhodium. Table 5 gives the recoveries of palladium, platinum and rhodium obtained when 5 mg of gold was used as a collector. Palladium gives the best recovery. Gold and platinum have recoveries that tend to be high, between 0.5 and 3 pg (33-200 p.p.b.). Rhodium shows the worst recovery. For a 5-mg gold bead, the amount of gold in solution exceeds the limit for which vanadium buffer is efficient. However, from 200 to 500 yg ml-1 of gold in solution, the decrease in rhodium absorbance is only 4%, which does not explain the decrease in recovery.It is important to note that the recovery reported refers to both fire assay and AAS analyses. The over-all recovery of rhodium is poor and is associated with incomplete dissolution of rhodium from the gold bead. Small flakes recovered from the dissolution of a 5-mg gold bead containing 5 pg of rhodium were analysed and lead and rhodium were found to be the main constituents, as identified by electron probe X-ray microanalysis. A preliminary X-ray diffraction study indicated that the structure of the flake is similar to that Table 4. determination of gold, palladium and platinum in a 10-mg silver bead. Cupellation temperature, 940 "C. Each determination was performed four times Added/ Found/ Recovery, Element Pg Yg YO Au .. . . . . 0.5 0.6 k 0.2 120 1 .o 1.2 f 0.2 120 2.5 2.4 L 0.3 96 5.0 5.5 k 0.5 110 10.0 10.3 k 0.8 103 15.0 15.0 k 0.1 100 Pd . . . . . . 0.5 1 .o 2.5 5.0 10.0 15.0 Pt . . . . . . 1.5 3.0 7.5 10.0 20.0 30.0 0.5 k 0.1 1.1 k 0.1 2.4 k 0.2 5.0 f 0.1 10.0 f 0.3 15.2 f 0.2 1.6 f 0.5 3.8 L 0.5 7.2 k 0.5 9.4 k 0.6 20.0 k 1.4 27.6 f 1.5 100 110 96 100 100 101 106 127 96 94 100 92 Table 5. Determination of palladium, platinum and rhodium in a 5-mg gold bead. Cupellation temperature, 940 "C. Each determination was performed four times Added/ Element Pg Pd . . . . . . 0.5 1 .o 2.5 5.0 10.0 15.0 Pt . . . . . . 1.5 3.0 7.5 12.5 15.0 25.0 Found/ 0.48 f 0.04 1.03 k 0.04 2.35 k 0.18 4.6 k 0.4 10.2 k 0.5 20.0 f 0.5 1.8 f 0.4 3.8 k 0.4 7.3 k 0.4 11.0 k 1.0 14.8 k 2.0 24.7 k 0.5 Recovery, Yo 96 103 94 92 102 100 120 127 97 88 99 99 .. . . . . Rh 0.5 0.43 f 0.05 86 1 .o 0.30 k 0.08 30 2.5 0.73 k 0.05 29 5.0 1.03 k 0.08 21 Table 6. Determination of silver in a 10-mg gold bead. Cupellation temperature, 870 "C. Each determination was performed four times Ag Ag Pg Pg YO found/ Recovery, added/ 2.4 2.1 f 0.5 87.5 4.8 4.2 k 0.5 87.5 15.0 14.3 k 0.6 95.3 of lead rhodium oxide, which would explain why such flakes are not dissolved by acids or alkaline cyanide solutions. A dark grey coating of the gold beads with high rhodium contents was observed. Such a coating is rich in lead and rhodium, as shown by electron probe X-ray microanalysis.ANALYST, MARCH 1986, VOL. 111 294 Table 7. AAS determination of precious metals pre-concentrated in a 5-mg gold bead from blends of standards and a pure quartz.Each determination was performed four times Taken, p.p.b. Found, p.p.b. Blend Ag Pd Pt Rh Agt Pd Pt Rh 3.0 g SARM-7* + 12 g quartz . . 84 306 748 48 - 329 * 16 822 k 31 3 5 t 5 7.5 g SARM-7 + 7.5 gquartz . . 210 765 1870 120 258 ? 61 784 k 24 1747 k 98 99 f 5 5.0 g SARM-7 + 10 g quartz . . 140 510 1247 80 130 5 85 511 k 12 1289 k 38 64 k 8 15.0gSARM-7 . . . . . . 420 1530 3740 240 - 1530 k 90 3640 k 100 170 k 10 * SARM-7, standard prepared by National Institute for Metallurgy, Republic of South Africa. ?- A 10-mg gold bead was produced after cupellation at 870 "C. Table 8. Determination of precious metals in blends of standards and a pure quartz by fire assay and AAS; a 10-mg silver bead was produced after cupellation at 940 "C.Each determination was performed four times Taken, p.p.b. Found, p.p.b. Blend 3.0 g SARM-7 + 12 g quartz 5.0 g SARM-7 + 10 g quartz 7.5 g SARM-7 + 7.5 g quartz 7.5 g SU-lA* + 7.5 g quartz 4.0 g PTA-1* + 11 g quartz 1 .0 g MA* + 14 g quartz . . 15.0gSARM-7 . . . . Au Pd Pt . . 62 306 748 . , 103 510 1247 . . 155 765 1870 . . 310 1530 3740 . . - 185 205 - 813 . . 1186 - . . - - Au Pd Pt 63 * 9 319 k 26 786 k 114 89 k 16 517 k 6 1152 k 156 140 k 15 765 k 13 1818 k 61 310 5 71 1538 k 18 3270 k 160 80+ 18 205 k 12 211 k 50 767 f 183 63 k 24 - - - 1265 -C 84 * SU-lA, PTA-1 and MA, standards prepared by the Canada Centre for Mineral and Energy Technology. From these findings, it appears that the recovery of rhodium by fire assay is a problem that needs more study.During this investigation, it was noticed that when the gold to platinum ratio in a bead is approximately 10, the recovery of rhodium is about 85%, i.e., better than that without platinum. Recovery studies conducted with rhodium already in solution gave excellent results, and indicated that the rhodium recovery problem lies in the fire assay procedure and the acid digestion step. Determination of silver in a gold bead can be conducted without vanadium buffer as precious metals do not interfere. Further, the cupellation temperature must be decreased in order to minimise silver losses, Therefore, silver determina- tions must be conducted on a bead produced at a lower cupellation temperature. Table 6 shows the silver recovery obtained from an analysis performed in a 0.5% mlV KCN - KOH solution. The detection limit of silver determined by AAS is of the order of a few parts per billion.However, silver contamination from the fluxes used in a fire assay often occurs, and it is not recommended to determine silver in the parts per billion range using fire assay as a pre-concentration and separation method. In this work the silver contamination was 1.9 pg (128 ng g-I) and this value was subtracted from the amount found. Standard and Quartz Blend Analysis The whole method applied to blends of standards and a pure quartz give the results presented in Tables 7 and 8. In Table 7, the precious metal content of a 5-mg gold bead, produced at a cupellation temperature of 940 "C, was determined in 0.5% mlV KCN - KOH - 1% rnlV vanadium solution.Silver results obtained from a 10-mg gold bead produced at a cupellation temperature of 870 "C are also given. The silver determination was conducted without vanadium in solution. When only gold, platinum and palladium are to be determined, a silver collection is adequate. Results obtained with a silver bead are given in Table 8 and show that the recovery of precious metals is generally good. However, the platinum recovery decreases at high platinum concentrations in the silver bead, and can be improved by using a gold collector. Conclusion The acid decomposition of precious metal beads produced by fire assay and the dissolution of the noble metal salts in alkaline cyanide solution is an attractive procedure for the determination of gold, silver, palladium, platinum and rho- dium at parts per billion levels in rocks, ores and minerals.Lengthy separation methods are not required as interferencds are corrected by the vanadium buffer added to the cyanide solution. This method offers several advantages over existing methods by eliminating the possible precious metal losses by coprecipitation in HCl solution by formation of stable complexes. Further, it facilitates the rapid dissolution of significant amounts of silver in a small volume of cyanide solution, in contrast to hydrochloric acid. This method is as simple as the others, but safety precau- tions relating to the use of cyanide must be rigorously followed. Finally, the determination of rhodium presents no difficul- ties once it is in solution. However, the recovery of rhodium by fire assay and by acid decomposition of the bead has to be improved. The flakes observed after acid dissolution of a gold bead containing rhodium has not, to our knowledge, been reported elsewhere. The authors thank A. Tremblay, N. Rheaume and P. Plourde for their contributions to the experiments. 1. 2. 3. 4. 5. 6. 7. References Beamish, F. E., and Van Loon, J. C., "Analysis of Noble Metals," Academic Press, New York, 1977. Gupta, J. G., Miner. Sci. Eng., 1973, 5 , 207. Beamish, F. E. , and Van Loon, J. C. , Miner. Sci. Eng., 1972,4, No. 4, 3. Mallett, R. C., Miner. Sci. Eng., 1970, 2, No. 3, 28. Moloughney, P. E., Tuluntu, 1977, 24, 135 Kallm~w, S . , and Hobart, E. W., Tuluntu, 1970, 17, 845. Le Houillier, R., and RhCaume, N., Can. Mefull. Q., 1984,23, 427. Paper A51205 Received June 1 Oth, I985 Accepted September 23rd, 1985