DETERMINATION OF SILVER, GOLD AND COBALT IN SULFIDE ORES AND PRODUCTS OF THEIR PROCESSING BY ICP-AES

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1 DETERMINATION OF SILVER, GOLD AND COBALT IN SULFIDE ORES AND PRODUCTS OF THEIR PROCESSING BY ICP-AES O.V. Evdokimova, N.V. Pechishcheva, P.V. Zaitceva, K.Yu. Shunyaev IMET UrB RAS, Yekaterinburg, , 101, Amundsena st., ABSTRACT The analytical technique has been developed for the determination of valuable trace components, namely gold, silver and cobalt in materials containing high concentrations of sulfur, copper, iron and nickel by inductively coupled plasma atomic emission spectrometry (ICP-AES). For this purpose, the effect of the major components on the atomic emission of target analytes in solutions after the acid decomposition of the samples was studied. Also the method for calibrating the spectrometer and the optimal operating parameters were chosen, as well as the optimum wavelengths for observing the analytes emission. The quality of the procedure was verified using certified reference materials of copper, copper-nickel, goldsilver sulfide ores, matte and their mixtures. INTRODUCTION Sulfide ores are an important source of nonferrous metals, for example, copper, nickel and cobalt. In most of them, microquantity of precious metals (including gold and silver) are found. In the practice of the analytical chemistry laboratory of Institute of Metallurgy of UrB RAS, it is often necessary to quickly determine the valuable microcomponents of sulfide ores of various compositions and products of their processing. The main components of analyzed samples are nickel, copper, iron, sulfur, and also silicon, calcium, magnesium. The purpose of the present work was the development of a multi-element technique that allows determining microcomponents of sulfide ores and their products, namely gold (with a content of % wt.), silver ( % wt.) and cobalt ( % wt.) simultaneously in the presence in solution of significant quantities (up to 40% wt.) of sulfur, copper, iron and nickel. For the determination of gold and silver in ore materials, various types of assay analysis, variants of photometry, atomic absorption spectrometry (including electrothermal one) and atomic emission spectrometry (usually with preliminary extraction or sorption) are used in laboratory practice; cobalt is also determined by photometry, atomic absorption, atomic emission, and by a potentiometric method (1-5). There are some works about application of X-ray fluorescence analysis, gamma and neutron activation method for determining these elements (5). Most of these methods are time-consuming and allow determining only one component at the same time; they require the separation of the matrix, which increases the duration of analysis significantly and can lead to significant errors. All these circumstances make it difficult to analyze a large flow of samples. Inductively coupled plasma atomicemission spectrometry (ICP-AES) due to the high speed of measurements and the ability to carry out multi-element analysis could be an alternative. We did not find IСP-AES techniques of simultaneous determination of gold, silver, and cobalt in sulfide ores and products of their processing in the literature. 143

2 Taking into account the possible matrix effects from the significant amount of copper, nickel and iron in the sulfide materials on the analytic signal of gold, silver and cobalt, it is necessary to carefully optimize the stage of sample preparation and the operating conditions of ICP-AES analysis. In the paper (6) preliminary experiments for the development of ICP- AES technique of determining the components of sulfide copper and nickel ores and concentrates were carried out. It was shown by thermodynamic simulation and experimentally that the acid digestion by heating the samples in a mixture of HCl : HNO 3 (3:1) followed by separation from the precipitate (in which silicon is main component) is effective in determining gold, silver and cobalt. However in the mentioned work products of processing of non-ferrous sulfide raw materials, for example, matte, were not considered. The tasks of the present work were: - verification of the acceptability of acid digestion of sulfide ore raw materials and products of its processing of various compositions for the determination of gold, silver, and cobalt by ICP-AES method; - investigation of matrix influences from macro components on the atomic emission signal of gold, silver, and cobalt in solutions after decomposition of samples; the choice of optimal wavelengths, the method of calibration of the spectrometer and the optimum parameters of the spectrometer for ICP-AES determination of target analytes. Experimental 1. Objects The objects of the study were reference materials of sulfide raw materials of non-ferrous metals, with certified mass fraction of gold, silver, and cobalt, as well as their mixtures (see Table 1). 2. Equipment Emission measurements were performed by ICP-AES spectrometer Optima 2100 DV Perkin Elmer with a quartz burner. Analytical spectral lines: Au I nm, Au I nm, Ag I nm, Ag I nm, Co II nm, Co II nm, Co II nm. Optimized operational parameters for determining gold, cobalt and silver: Rf power W; carrier argon flow rate L min -1 ; auxiliary gas flow rate- 0.2 L min -1 ; plasma gas flow rate L min -1 ; observation mode - axial; horizontal position of the plasma observation zone - 15 mm; sample uptake rate ml min -1 ; spraying time - 40 s, number of replicas - 2, registration of peak in height, the 1-point background position for Au I nm is nm and for Au I nm is nm. For semi quantitative X-ray fluorescence analysis S4 Explorer (Bruker) spectrometer was used. 144

3 Table 1. The objects of the study. Certified reference materials of the sulfide ores and their processing products Certified value of mass fraction, % Sample Ag Au Co S Cu Ni Fe SiO 2 R34а n/c n/c Copper concentrate (0.08)* (n/d) R35 n/c n/c n/c Copper sulfide ore (0.05) (n/d) (18.9) FShT-30 n/c n/c Matte (2.8) (0.39) RMO-5 n/c n/c Copper-nickel sulfide (21.5) (22.7) ore SО-24 n/c Gold-silver ore (0.05) Mixture No 1 n/c n/c RMО-5 and FShT (12.2) (11.8) (1:1) Mixture No 2 n/c n/c RMО-5 and FShT (17.8) (18.3) (4:1) Mixture No 3 n/c n/c n/c n/c R35 and FShT (0.3) (19.4) (10.9) (18.0) (1:1) * In parentheses here and in the whole table the indicative values obtained by semi quantitative X-ray fluorescence analysis n/c not certified; n/d not detected by X-ray fluorescence 3. Reagents Reagents of analytical purity were used for all experiments. For the spectrometer calibration the reference materials of ions solutions were used: for gold - GSO (MSO 0623:2004), for silver - GSO and for cobalt - GSO , and nickeli, iron and copper solutions prepared from pure metal (not less than 99.9 % wt.). Synthetic mixtures were prepared for studying the matrix effect on the emission signal of the analytes: the concentration of the analytes in solutions was kept constant (0.5 mg L -1 for gold and silver, 1.0 mg L -1 for cobalt); copper, iron and nickel concentrations were varied within the range corresponding to their possible content in solutions after decomposition of sulfide ore materials samples - from 0.05 to 5.0 g L -1. Solutions of matrix elements were obtained by dissolving of Cu(CH 3 COO) 2 H 2 O, FeCl 3 6H 2 O, Ni(NO 3 ) 2 6H 2 O in the water. The operating conditions of ICP-AES analysis were selected using synthetic mixtures prepared on the basis of sulfide ore reference materials after sample preparation and fixed addition of the gold, silver and cobalt solutions (up to their concentration of 1 mg L -1 ). To prepare all the solutions distilled water was refined using a PureLab UHQ (Elga) unit. 145

4 Results and Discussion 1. Investigation of matrix influence Using the acid digestion solutions of the samples R34a, R35, RMO-5, FShT-30, spectra of the most sensitive gold, silver and cobalt emission lines were obtained: Ag I nm, Ag I nm, Au I nm, Au I nm, Co II nm, Co II nm and Co II nm. As a result, the influence of the matrix components, namely, iron, copper, nickel, on the intensity of analyte emission lines was revealed. The examples of the influence are demonstrated in Fig. 1: a) background effect of copper and iron on the Ag I nm, b) spectral overlap of iron on the Au I nm, c) background effect of nickel on the Co II nm. Therefore, it was decided to investigate the effect of these metals on the emission of analytes in more detail. The emission intensity of all the mentioned spectral lines of analytes (Me = Au, Ag, Co) was measured at concentration in the solution 0.5 mg L -1 of Au or Ag and 1.0 mg L -1 of Co in the absence (I(Me)) and in the presence (I(Me) Mt ) of matrix elements (Mt = Fe, Cu, Ni) in different concentrations. For each case, the degree of matrix influence was calculated as γ = I(Me) Mt /I(Me). In Fig. 2, the degree of matrix influence on silver (a), gold (b) and cobalt (c) as a function of the matrix element concentrations is presented. As can be seen, the degree of matrix influence remains insignificant up to 100 mg L -1 of copper, nickel and iron, then it more or less sharply increases. Compared to the Ag I nm line, the Ag I nm line is less influenced by the matrix components, the degree of the influence is decreased in the sequence Fe > Ni > Cu. The presence of nickel does not influence on the intensity of studied Au lines. Copper concentrations, greater than 1000 mg L -1, significantly reduce the intensity of background signal of both Au lines. As for iron, Fe II nm and Fe I nm overlap the Au I nm line, Fe II nm affects Au I nm line. As a result, taking into account the calculated degree of matrix influence, it is possible to recommend the use of Au I nm for analyze of samples containing significant amounts of iron; but if the sample contains less than 500 mg L -1 of iron, Au I nm is preferable. 146

5 a) b) c) Fig. 1. Spectra of the emission lines obtained by ICP-AES analysis of sulfide ore materials after acid digestion: a) Ag I nm (background effect of Cu, Fe), b) Au I nm (spectral overlap of Fe line), c) Co II nm (background effect of Ni). 147

6 a) Ag _Fe Ag _Fe γ(ag) Ag _Cu Ag _Ni Ag _Ni Ag _Cu Log C(Me) b) Au _Fe Υ(Au) Au _Fe Au _Ni Au _Ni Au _Cu Au _Cu Log C(Me) c) 3.6 Co _Ni Co _Fe γ(ag) 2.1 Co _Ni Co _Ni_Cu Co _Fe_Cu Co _Fe_Cu Log C(Me) Fig. 2. The degree of matrix influence of Сu, Ni, and Fe on the intensity of Ag, Au, and Co emission lines vs logarithm of the metal concentration (mg L -1 ) The cobalt line Co II nm was partially overlapped with the Fe II nm line. Therefore, for samples with large quantities of iron, the use of Co II nm and Co II nm is recommend, in other side due to the spectral overlap of the Ni II nm and Ni I nm lines on them the Co II nm line is preferably for analyzing of copper-nickel mattes. The background effect of copper on the cobalt lines is insignificant. Based on the results of the studies, the matrix effect was compensated by adding 2.5 g L -1 of copper and 5 g L -1 of iron to the calibration solutions for gold determination. For the silver determination, addition of 2.5 g L -1 of copper, 2.5 g L -1 of nickel and 5 g L -1 of iron was 148

7 made. As for cobalt determination, 500 mg L -1 of nickel should be added. The blank for the calibration should be a solution containing the acids used to sample pretreatment of investigated materials. 2. ICP-AES instrumental optimization Using the axial plasma observation, the following operating conditions were optimized: Rf generator power (W), the position of the observation zone along the horizontal axis (h, mm), the carrier gas (argon) flow rate (V Ar, L min -1 ). The Rf power varied in the range from 1200 to 1500 W, the position of the observation zone was from 12 to 17 mm, the carrier gas flow rate was from 0.65 to 0.95 L min -1. Maximum of the analyte emission signal at their concentration 1 mg L min -1 was selected as a criterion for optimization. It was found that the maximum values of emission signals (number of measurements n = 5) for Au I nm, Au I nm, Ag I nm, Ag I nm, Co II nm, Co II nm and Co II nm were observed at Rf power = 1500 W, V Ar = 0.70 L min -1, h = 15 mm (see Fig. 3 and Fig. 4). These operating conditions were chosen for further work. Similar studies were performed using radial plasma observation. It was found that the maximum emission signals of the target trace components were observed under the following condition: Rf = 1500 W, carrier gas flow rate 0.80 L min -1, observation zone height above the inductor 14 mm. Comparison of the emission signal RSD values obtained by using two mode of plasma observation under optimal conditions shown that for the axial observation RSD values are smaller. For example, for one of the synthetic mixture emission signal the RSD with axial observation of Au I nm, Co II nm and Ag I nm were 0.5 %, 0.4 %, and 0.9 % correspondingly, whereas with radial observation RSD value were 0.5 %, 1.0 % and 1.2 % (n = 5). Therefore, for the further work, the axial observation of plasma was chosen. 3. Optimized procedure As a result of the study the following procedure was formulated. 2.0 g of sulfide ore material (or product of their processing) sample is heated in a mixture of HCl : HNO 3 (3:1) (40 ml) in a heat-resistant beaker on an electric stove for 15 min, then is filtered through "Blue ribbon" filter. Precipitate is discarded. The solution is transferred into a 100 ml volumetric flask and diluted to the mark by distilled water. A blank sample containing all reagents, except for the material under study, is prepared. Solutions for the calibration of the spectrometer (0.1, 0.5, 1, 2.5 mg L -1 for gold and silver, 1, 5, 20 mg L -1 for cobalt) are prepared by diluting reference materials of ion solutions of the analytes. Aliquots of iron and copper solutions are added to the calibration solutions for gold, so that 2.5 g L -1 of copper and 5 g L -1 of iron are contained in the final volume. For the silver determination addition of 2.5 g L -1 of copper, 2.5 g L -1 of nickel and 5 g L -1 of iron to the calibration solutions is needed, for cobalt determination adding 500 mg L -1 of nickel is required. Then calibration solutions are diluted to the mark by the blank sample. The zero point for the calibration should be the blank. 149

8 Co a) I, cps W Co Co I (Co nm), cps h, mm b) W Ag Ag I, cps h, mm c) I (Au nm), cps W Au Au I (Au nm), cps h, mm Fig. 3. Dependence of the a) Co, b) Ag and c) Au emission intensity on the position of the observation zone for different values of Rf power (from top to bottom for each emission line: 1500 W, 1400 W, 1300 W, 1200 W), V Ar = 0.8 L min -1. Atomic emission of silver, gold and cobalt is measured; mass concentration (mg L -1 ) of the analyte according to the calibration plot is determined. The mass fraction of the analyte, taking into account the dilution and the sample weight is calculated. According to this developed procedure, gold, silver and cobalt were determined in the samples indicated in Table 1. The results are given in Table

9 I(Co), cps I(Au), cps V Ar Fig. 4. Dependence of the emission intensity: 1 - Au I nm, 2 - Au I nm, 3 - Ag I nm, 4 - Ag I nm, 5 - Co II nm, 6 - Co II nm, Co II nm on the carrier gas flow rate V Ar ; Rf = 1500 W, h = 15 mm. Table 2. Results of ICP-AES determination of Au, Ag, Co content in reference materials (n = 5, wt%) Sample RMO-5 Coppernickel sulfide ore R35 Copper sulfide ore R34а Copper concentrat e SО-24 Goldsilver ore FShT-30 Matte Mixture No 1 Mixture No 2 Certified Au Ag Co Found Found Found Certifie Certified d Au Au Ag Ag Not certified Not certified Not certified Co Co Co

10 Mixture No Not certified Conclusion The results obtained according to the procedure developed in this paper are in good agreement with the certified analyte contents in reference materials, but in samples with a significant nickel content the results for silver determination are slightly underestimated and the gold determination results are slightly overestimated. For the determination of silver and cobalt, in general, all tested analytical lines proved to be equally suitable, whereas for gold the best results were mainly obtained by using the Au I line of nm. The study is supported by Program of UB RAS, project References [1]. Zakharov Y.А., Okunev R.V., Hasanova S.I., Irisov D.S., Haibullin R.R.: Atomic absorption determination of gold and silver in rocks and ores using double-stage probe atomization in the graphite furnance. Analitika i kontrol [Analytics and control] (4) [2]. El Wakil A.F.: `Studies on the determination of gold in geological samples without separation by ICP-OES.` Der farma Chemica (12) [3]. GOST Copper concentrates. Methods of analysis. Russian National Standard, [4]. ISO 10378:2016. Copper, lead and zinc sulfide concentrates. Determination of gold and silver. Fire assay gravimetric and flame atomic absorption spectrometric method. [5]. [6]. Maiorova A.V., Pechishcheva N.V., Shunyaev K.Yu.: Theoretical and experimental studies of the effectiveness of sample preparation methods of non-ferrous metals sulfide raw materials for the determination of the micro- and macro- components by ICP-AES. Butlerov Communications (11)