ОПТИКА И СПЕКТРОСКОПИЯ, 2014, том 116, № 3, с. 394-396
СТЕКТРОСКОПИЯ АТОМОВ И МОЛЕКУЛ
УДК 543.423
A RAPID ICP-OES STRATEGY FOR DETERMINATION OF GOLD AND SILVER IN BLISTER COPPER BY NITRIC ACID DIGESTION
© 2014 г. Gai Zhang and Min Tian
School of Materials and Chemical Engineering, Xi'an Technological University, Xi'an 710032, China
E-mail: gaizhang@aliyun.com Received July 2, 2013
A rapid strategy for the analysis of gold and silver in blister copper by inductively coupled plasma optical emission spectrometry (ICP-OES) was firstly proposed. Nitric acid was used to digest blister copper instead of commonly used sulfuric acid. This prevented forming the salt of copper sulfate in the filtration process when the volume of the mixture is very small. Thus, the time of filtration was saved. After filtrating, aqua regia was used to digest the residue and acidize the filter liquor. Two parts of gotten solution were directly determined by ICP-OES. The cycle of analysis was shortened compared with sulfuric acid-fire assay. The proposed method was successfully applied to determine gold and silver in blister copper, and the results were in good agreement with those obtained by lead fire assay.
DOI: 10.7868/S0030403414030076
1. INTRODUCTION
Blister copper (copper in the range of 99% purity) is produced by blasting air through the molten copper to remove impurities and drive off most of the sulfur as sulfur dioxide. Blister samples often contain trace amounts of gold and silver, so it is very necessary to accurately determine their contents considering comprehensive utilization of resources. In general, gold and silver in blister copper are analyzed by lead fire assay [1, 2]. However, fire assay has some shortcomings including high energy consumption, great labour intensity, time-consuming and lead pollution.
In this work, a method for the decomposition of sample with aqua regia and the detection of Au and Ag by ICP-OES was developed. Because copper content in blister copper is above 90%, copper maybe interfere determination of gold and silver due to matrix effects and high salt concentration. Before the digested solution was detected, copper must be separated by acid considering its matrix interference. At present, sulfuric acid is commonly used to decompose blister copper [3—5]. The mixture was filtrated and the solution was analyzed by lead fire assay. However, in a practical pre-treatment, it is easy to produce a large amount of solid salts of copper sulfate when the volume of the mixture is very small. The crystal of copper sulfate may influence separation of gold and silver with matrix. We used nitric acid to digest blister copper instead of sulfuric acid, considering better decomposing ability of nitric acid and more soluble for copper nitrate than copper sulfate. The blister copper was decomposed with nitric acid and then the mixture was directly filtrated. The separated solution was used to determine silver by ICP-OES. Gold was reserved in the filter paper and the residues. The filter paper and the residues was
ashed and decomposed with nitric acid. Gotten solution was used to analyze gold content by ICP-OES. The proposed method was successfully applied to determine gold and silver in blister copper, and the results were in good agreement with those obtained by fire assay. The relative standard deviation was less than 3%.
2. EXPERIMENTAL 2.1. Apparatus
A simultaneous inductively coupled plasma optical emission spectrometer (iCAP MFC 6300 Radial, Thermo Fisher Scientific) was used in the study employing a high-resolution echelle optical system and charge injection device (CID) array detector, with an extended spectral range of 166—847 nm and axial plasma view. Appropriate solutions were introduced into the ICP plasma using an ordinary sample introduction system including a quartz nebulizer, a cyclonic spray chamber and a quartz torch. The ICP-OES operating conditions were listed in Table 1.
2.2. Reagents
Superior grade nitric acid and hydrochloric acid were purchased from Xi'an Chemical Reagent Co. The 1.0 g L-1 standard stock solutions of Au and Ag were purchased from China National Standard Material Center. Calibration standards were freshly prepared by appropriate dilution of the stock solution. All solutions were prepared in pure water (distilled and passed through a Milli-Q water purification system from Millipore, Milford, MA, USA).
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Table 1. The operating parameters for Thermo 6300R ICP-OES
Operating parameter Parameter value
Excitation frequency, MHz 27.12
Radio frequency forward power, W 1150
Plasma view Axial
Detector CID- 86 chip
Plasma gas flow rate, L/min 12.00
Auxiliary gas flow rate, L/min 1.00
Carrier gas flow rate, L/min 0.45
Coolant gas flow rate, L/min 14.00
Sample uptake rate, mL/min 2.0
Integration time, s 30 s for low wavelength (166-230 nm) 10 s for high wavelength (230-847 nm)
2.3. Method
5 g powdered blister copper sample was weighed into a 250 mL beaker, wetted with a small amount ofwa-ter. 30 mL nitric acid of superior grade was added to the beaker and then it was heated until the solution became bright. After cooling, the solution was filtrated into the 500 mL flask. Dilute the filtrate with water to 500 mL.
2.3.1. Determination of gold and silver in filter liquor. A 300 mL flask with 100 mL aliquot filtrate was heated at low temperature to a small volume with 10— 15% aqua regia medium and then added to a piece of foam about 0.4 g, shaked in order to absorb gold. After 30 min, the foam was washed out by water and blotted by filter papers and then placed into a 25 mL-tube. Added 12 mL hydrochloric acid-thiourea (1%—1%), and then the tubes were boiled in water bath at 100 °C for 30 minutes. After cooling, the solution was used for the determination of gold. Ag in the aliquot filtrate was directly detected by ICP-OES.
2.3.2. Determination of gold and silver in filter residue. The residue with the filter paper was transferred into the crucible, and then was ashed. After cooling, the residue was decomposed with aqua regia. Gold in the solution was analyzed by the steps in Section 2.3.1.
Another piece of filter residue with filter paper was put into the crucible, and then was ashed. After cooling, add 20 mL hydrochloric acid, after a while, add 5 mL nitric acid. The residue was dissolved at low temperature for half an hour to a volume of 15 mL. After cooling, the solution was diluted to 50 mL and then was used for the determination of silver by ICP-OES.
3. RESULTS AND DISCUSSION
In practical sample analysis, because commercial nitric acid of analytical grade may contain chloride, some gold in filter residue was dissolved, leading to under estimated gold content. The results of gold will become lower. That is why nitric acid of superior grade was using in the next experiments.
Samples with different gold content were pretreat-ed using different methods. If gold content is not high, samples solution will be enriched using foam method considering higher accuracy for sample determination. If gold content is higher, samples solution was directly detected by ICP-OES.
The results of gold and silver in blister copper were determined by this method and compared with ones by sulfuric acid digestion and fire assay. The results were shown in Table 2. As seen from Table 2, all of gold in blister copper was retained in filter residue, while silver was completely dissolved into filtrate. Thus, for gold analysis, we may ash filter residue and filter paper and then dissolved with aqua regia. The solution was used to determine gold. For silver analysis, the filtrate was acidified and then was used to determine silver.
4. CONCLUSION
Blister copper decomposition with nitric acid has allowed to avoid generating solid salt, which have influence on filtration. ICP-OES was used instead of fire
Table 2. The gold and silver contents in blister copper samples*
Au, ^g/g Ag, ^g/g
No. this method fire assay this method fire assay
filtrate filter residue filtrate filter residue
BC-1 <0.05 1.36 1.36 38 <2 41
BC-2 <0.05 29.86 29.92 371 <2 363
BC-3 <0.05 15.66 15.45 221 <2 231
BC-4 <0.05 6.38 6.29 112 <2 108
BC-5 <0.05 3.53 3.44 71 <2 76
* The average results for five determinations.
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GAI ZHANG, MIN TIAN
assay. These improved the analysis efficiency. The results obtained by this method were found consistent with those obtained by lead fire assay.
ACKNOWLEDGMENTS
We gratefully acknowledge financial support of this work by the Foundation of Education Department of Shaanxi Province (2013JK0929), the Foundation of Education Department of Shaanxi Province (2012JK0607) and the Key Disciplines Fund of Shaanxi Province (080503).
REFERENCES
1. Method for chemical analysis of blister copper-determination of gold and silver content, China nonferrous metals standard YS/T 521.3-2006.
2. Shanhai Lin, Chinese J. Metallurgical Analysis 22, 53 (2002).
3. Ming Tao and Bo Chen, Chinese Journal of Spectroscopy Lab. 22, 1306 (2005).
4. Iwao Tsukahara and Minoru Tanaka, Talanta 27(8), 655 (1980).
5. I. Tsukahara, Talanta 24(10), 633-7 (1977).
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