Mass Spectrometry to Phase Analysis of Gold in Gold Ores

CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 46, Issue 2, February 2018 Online English edition of the Chinese language journal

Cite this article as: Chinese J. Anal. Chem., 2018, 46(2): e1801–e1809

RESEARCH PAPER

Application of Inductively Coupled Plasma-Atomic Emission Spectrometry/Mass Spectrometry to Phase Analysis of Gold in Gold Ores ZHAO Liang-Cheng1,*, WANG Jing-Gong1, LI Xing1, ZHANG Shuo1, ZHANG Zhao-Fa1, HU Ming-Yue2, LU Feng1, WANG Yi-Dan4, HU Yan-Qiao1, GUO Xiu-Ping1, LIU Qing-Xue1, LIU Hua-Jie3,* 1

Hebei Geological Laboratory, Baoding 071051, China National Research Center for Geoanalysis, Beijing 100037, China 3 College of Chemistry and Environmental Sciences, Hebei University, Baoding 071002, China 4 College of Life Sciences, Hebei University, Baoding 071002, China 2

Abstract:

Inductively coupled plasma-atomic emission spectrometry/mass spectrometry (ICP-AES/MS) is a potentially powerful tool

in chemical phase analysis of gold in batch mode, especially applicable to the low-grade gold ores with gold content of far below detection limit of the other methods, but it has not been used in gold phase analysis of gold ores. In this work, three types of typical gold deposits (altered rock type, quartz vein type, and microscopic disseminated type) and national standard reference materials of gold ores were used to establish and validate a method for gold phase analysis of gold ores using ICP-AES/MS. The optimum conditions of phase analysis were determined, including the sample granularity and preparation procedures, separation absorbent, pretreatment procedures of various phases of gold and optimized instrument parameters. Evaluation of the optimized method showed that this method had acceptable precision (RSD: 1.1%–10.6%) and accuracy (relative error, RE: 0.5%–6.3%), and the detection results of gold in ores were comparable with those obtained using the hydroquinone volumetric method-extraction flame atomic absorption spectrometry (VOL-AAS) and graphite furnace atomic absorption spectrometry (GFAAS) methods. The sum content of gold of the 4 phases (free gold, FAu; linked gold, LAu; sulphide-bearing gold, SAu; and other mineral-bearing gold, AAu) conformed to the total gold content and was consistent with the results of rock-mineral identification. The proposed method had a low detection limit (0.30 ng g–1) and wide linear range (5.0 ng mL–1–20.00 μg mL–1). It is a simple, rapid, and efficient method for gold phase analysis in batch form. Key Words:

Gold phase analysis; Inductively coupled plasma-atomic emission spectrometry; Inductively coupled plasma-mass

spectrometry; Altered rock type; Quartz vein type; Microscopic disseminated type

1

Introduction

Gold exists in nature with very low concentrations and is mostly found in the form of free gold (FAu). The abundance of gold in the Earth’s crust is 1.0 ng g–1, and the average abundance of gold in different sediments and soils is 1.2–2.0 ng g–1 [1]. The occurrence and concentration of gold in ores are closely related to the industrial value of gold deposits, which is an important indicator in geological exploration, deposit

appraisal, and resource utilization. Chemical phase analysis of gold ores, consisting of the study of gold monomers of a particular size and their distribution and content in major deposit carriers, is one of the important means of identifying the existence of gold[2]. There are many related studies on the phase analysis of gold ores, mainly focused on high- or medium-grade gold ores in China and Russia. These studies include the physical observation of microstructures using electron microscopy,

________________________ Received 19 June 2017; accepted 25 November 2017 *Corresponding author. E-mail: [email protected]; [email protected] This work was supported by the China Geological Survey (No. 12120113015000), and the Hebei Bureau of Geology & Mineral Resources Exploration, China (No. 201502). Copyright © 2018, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(17)61070-3

ZHAO Liang-Cheng et al. / Chinese Journal of Analytical Chemistry, 2018, 46(2): e1801–e1809

X-ray diffraction, etc.[3–6], and the chemical methods of separation and quantification of gold phases in ore using selective solvents[7,8]. The phase analysis of gold in gold ore is relatively mature in China. In 2006, value assignments of four phases of gold ores in five gold deposits measured using methods other than Inductively coupled plasma-atomic emission spectrometry/mass spectrometry (ICP-AES/MS) by Wang et al[9] were approved as the national certified reference materials (CRMs) for the chemical phase analysis of gold. The follow-up studies on these CRMs provided important references for relevant scientific research and production[10–12]. In addition, the research on phase separation and the valence state of gold in ore is gradually becoming more sophisticated in China[2,13]. Relative to abundant information on the high-grade or medium-grade gold ores, there is a dearth of phase analysis studies on the low-grade gold ores, and for those that have been conducted, virtually all have focused on methods other than ICP-AES/MS. Previous studies used hydroquinone volumetric titration, hydroquinone volumetric-atomic absorption spectrometry and other methods, and mostly with emphasis on the optimization of selective solvent systems[8,14,15]. Such methods required long and cumbersome procedures, and their detection limit did not meet current requirements of geological exploration and deposit appraisal for low-grade gold ores. For example, as required by the National Multi-purpose Regional Geochemical Survey initiated by China Geological Survey in 1999, the detection limit of Au was 0.0003 μg g–1 [16], which was far below that of methods used in previous studies. In contrast, ICP-AES/MS technique involves simple procedures and a low detection limit. It is widely used in the quantitative analysis of gold in gold ore samples[17–22]. However, to date, there have been no reports on the application of ICP-AES/MS technique to chemical phase analysis of gold in gold ores. The main objective of this study was to establish an accurate, simple, rapid, and low-detection-limit capability for characterizing gold phases in gold ores using ICP-AES/MS. Three representative gold ores were selected: altered rock type gold deposit (from Shihu gold deposit, SH), quartz vein type gold deposit (from Jinchangyu gold deposit, JCY), and microscopic disseminated type gold deposit (from Yuerya gold deposit, YEY) in China[23–25]. Based on rock-mineral identification and related experiments, ICP-AES/MS tests and optimization were performed on the five phases of gold: (a) total gold; (b) free gold (FAu; gold is wholly exposed, visible to the naked eye or under the microscope); (c) linked gold (LAu; gold is partially exposure, connected with other minerals); (d) sulphide-bearing gold (SAu; gold is embedded in sulfide inclusions); and (e) other mineral-bearing gold (AAu; gold is embedded in other mineral inclusions). The optimization parameters were described as following: sample preparation, influence of selective solvent system on phase decomposition and separation, removal of high-concentration selective

solvents that would affect the ICP-AES/MS test, ICP-MS online internal standards, monitoring and correction of analytical signal drift, compensation and correction of the matrix effect. Based on the national CRMs for chemical phase analysis of gold, the optimized ICP-AES/MS method was evaluated. The results from the optimized ICP-AES/MS method were compared with those by hydroquinone volumetric method-extraction flame atomic absorption spectrometry (VOL-AAS) and graphite furnace atomic absorption spectrometry (GFAAS). This study provides new scientific data and technical parameters for phase analysis of gold in gold ores.

2 2.1

Experimental Standard solutions and principal reagents

Gold standard stock solution (GSB04-1715-2004; ρAu = 1000 μg mL–1) was purchased from National Analysis and Testing Center for Non-ferrous Metals & Electronic Materials, China. The gold standard intermediate solution A (ρ(Au) = 20 μg mL–1) was prepared by dilution of the gold standard stock solution with 10% aqua regia. The gold standard intermediate solution B (ρAu = 1.0 μg mL–1) was prepared by further dilution of the standard intermediate solution A with 10% aqua regia. The gold standard working solution for ICP-AES (0.00, 0.20, 0.50, 1.00, 5.00, 10.00, 15.00 and 20.00 μg mL–1) and ICP-MS (0.000, 0.005, 0.010, 0.020, 0.050, 0.100 and 0.200 μg mL–1) was prepare by further dilution of the standard intermediate solution A and B with 10% aqua regia, respectively. Activated carbon with 200 mesh granularity was soaked for four days in 40 g L–1 NH4HF2 and 4% HCl solution separately. Then solution was filtered and the activated carbon was washed with 4% HCl to remove F and Fe ions. And then it was washed with water until neutral and dried for use. The other principle reagents were deionized water (resistivity ≥ 18.0 MΩ·cm), HCl, HNO3, HClO4 and HF (all analytical pure), I2-KI solution (dissolving 7.5 g of I2 and 15.0 g of KI in 100 mL of water), and Foam plastic (commercially available). 2.2

Certified reference materials

Three certified reference materials (CRMs) for the bulk analysis of gold were used: GBW07246, GBW07247, and GBW(E)070012 (provided by the Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences). Five CRMs for chemical phase analysis of gold were used: GBW07189, GBW07190, GBW07191, GBW07192 and GBW07193 (provided by the Central-South Institute of Metallurgical Geology). These CRMs were used for quality control. The standard values of CRMs can be found in the literature[12].

ZHAO Liang-Cheng et al. / Chinese Journal of Analytical Chemistry, 2018, 46(2): e1801–e1809

2.3

Gold ore samples

Gold ore samples were collected from three gold deposits in Hebei province, China: Shihu of Shijiazhuang City (SH), Jinchangyu of Qianxi County (JCY), and Yuerya of Kuancheng County (YEY). The deposit type for SH, JCY, and YEY were post-magmatic hydrothermal quartz vein-fault structural altered rock type, post-magmatic mesothermal quartz compound type, and microscopic disseminated type, respectively[23‒25]. 2.4

Instruments and operation parameters

An Agilent 7700X inductively coupled plasma-mass spectrometer (Agilent Technologies, Tokyo, Japan) was used in ICP-MS analysis. A Prodigy high dispersion full spectrum direct-reading plasma emission spectrometer (Leeman Co., Ltd., Madison, USA) was used in ICP-AES analysis. The latter instrument had off-peak background subtraction software, an Echelle grating system, a charge injection device (CID), a detachable three-layer concentric quartz torch pipe, a high-efficiency cyclonic spray chamber, and a vertical layout. The operating parameters of the two instruments are listed in Table 1. The other instruments were SFS-1 wet-sieve shaker (Tangshan Lukai Scientific and Technological Co., Ltd., Tangshan, China), HY-4 multipurpose speed oscillator (Guohua Electric Appliance Co., Ltd., Changzhou, China), Leica DM4500 reflective microscope (Leica Microsystems, Wetzlar, Germany; for rock-mineral identification), AA220Z graphite furnace atomic absorption spectrometer (Varian Inc., Palo Alto, USA), AFS-3000 dual-channel atomic fluorescence spectrometer (Beijing Kechuang Haiguang Instrument Co., Ltd., Beijing, China), Carl Zeiss two-meter plane grating spectrograph (Carl Zeiss AG, Oberkochen, Germany), and CAAM-2000 multi-purpose atomic absorption spectrometer (Beijing Haotianhui Science and Trade Co., Ltd., Beijing, China). 2.5

Methods

Polished and thin sections of gold specimens were made for rock-mineral identification. The sections were observed using reflective microscopy for determining the characteristics such as components, content, granularity, morphology, relationship, and secondary changes of metallic and non-metallic minerals. These characteristics were used to identify gold phases[9]. Gold sample was prepared by coarse, medium and fine crushing with a jaw crusher, a disc grinding machine or roller machine, and a rod mill, respectively. Elemental concentration was analyzed by the chemical analysis methods (the gravimetric, spectrophotometric and volumetric methods)[22], GFAAS[22], Atomic fluorescence spectrophotometry[22], Semi-quantitative emission spectrometry[22], VOL-AAS analysis[14], and ICP-AES/MS analytical instruments. Total gold concentration of five sampling weights (5.0, 10.0, 20.0, 30.0 and 50.0 g) of prepared sample was compared to determine the best sampling weight. The selective dissolution and separation processes for the studied material were optimized on the basis of published literatures[7,9–10,13]. Related steps and technical parameters regarding sample preparation, dissolution, separation, and measurement of gold phases are detailed in following Section 3.2–3.5.

3 3.1

Results and discussion Occurrence of gold and element composition in gold ores

Typical specimens of three deposit type (SH, JCY and YEY) were selected and ground to prepare polished and thin sections. Rock-mineral identification showed that the main occurrence of gold in these samples consisted of FAu, LAu and SAu (Fig.1). The granularities of FAu in SH, JCY and YEY specimens were0.010–0.080 mm, 0.005–0.030 mm and 0.010–0.100 mm, respectively. The sizes were all in micro- and medium-grain level, and the gold particle surface was clean. Thus, in the follow up phase analysis, five testing phases were selected: FAu, LAu, SAu, AAu and total gold.

Table 1 Operating parameters of ICP-MS and ICP-AES ICP-MS

ICP-AES

Parameter

Set value

Parameter

Set value

RF Power Sample depth Carrier gas flow rate Lifting rate Nebulizer temperature Measurement mode Reading times Integral time Sampling cone type Nebulizer Cooling water temperature Resolution

1300 W 6.7 mm 1.15 L min–1 0.1 rpm 2 ºC Peak jumping 3 0.5 s Ni cone High salt nebulizer 18 ºC < 0.7 u

RF Power Cooling gas flow rate Auxiliary gas flow rate Nebulizer pressure Speed of peristaltic pump Exposure time Reading times Lifting time Observation height

1100 W 19.0 L min–1 0.1 L min–1 248 kPa 1.1 mL min–1 15 s 2 25 s 15 mm

ZHAO Liang-Cheng et al. / Chinese Journal of Analytical Chemistry, 2018, 46(2): e1801–e1809

The sample preparation processes were selected as follows: coarse crushing by a jaw crusher, medium crushing by a disc grinding machine or roller machine, and fine crushing by a rod mill. The best crushing granularity was 74 μm (200 mesh). We prepared 16 gold samples (SH: n = 6; JCY: n = 5; YEY: n = 5) with three different granularities (120 mesh, 150 mesh, and 200 mesh) and measured total gold and FAu using the VOL-AAS method. Contents of total gold and FAu were stable under 200 mesh treatment, indicating a best uniformity and representativeness. A smaller granularity increased the grinding cost and difficulty, and the results varied. The above samples under 200 mesh were used for grinding test (by wet-sieve shaker, grading granularity of 74 μm). The results showed that screening rates of the 74-μm samples were 99.97%–99.99%. After treatment with diluted HNO3, no gold was observed in the +74 μm samples using partial reflecting microscopy and binocular stereomicroscopy. These results indicated that the preparation process and crushing granularity were satisfactory. 3.3

Fig.1 Photographs and micrographs of the gold ore samples from Shihu (a, b), Jinchangyu (c, d) and Yuerya gold deposits (e, f). Au: free gold. Gn: galena. Q: quartz. Py: pyrite. Sp: sphalerite

Because the determination of the selective solvent and dissolving condition for gold phases are related to element composition of the specimen, chemical composition was analyzed using the chemical methods, atomic fluorescence spectrometry, and semi-quantitative emission spectrometry. The results are shown in Table 2.

The total gold in samples of 5 sampling weights (5.0, 10.0, 20.0, 30.0 and 50.0 g) from the three gold deposits was measured by ICP-AES/MS, and the test data were compared. On the basis of the content of influencing elements (Table 2), the amount of selective solvent, and the difficulty in filtration, the optimum sample weight was determined to be 10.0 g. A higher sample weight increased the amount of selective solvent, which resulted in operational difficulties in subsequent analysis. However, a lower sample weight would reduce the sample representativeness and increases analytical errors. 3.4

3.2

Optimization of sample preparation conditions

Optimization of sample amount

Optimization of selective dissolution and separation of gold

Table 2 Mass fraction of some elements in gold ore samples Sample Shihu Jinchangyu Yuerya Shihu Jinchangyu Yuerya

Element concentration (wt %) Si

Al

As

C

Ca

Cl

Cu

Fe

K

23.19 23.18 28.70

3.60 6.16 6.87

0.021 0.0043 0.0020

0.39 1.56 1.13

0.46 3.32 3.71

0.08 0.12 0.03

0.590 0.031 0.037

14.01 7.93 4.17

1.63 1.00 3.71

Mg

Mn

Na

Pb

S

Sb

Zn

2.48 2.27 0.99

0.07 0.11 0.18

0.14 2.87 1.84

2.89 1.11 0.15

10.28 3.86 0.61

0.0005 0.0017 0.0017

3.36 1.21 0.38

Element concentration (μg g–1) Shihu Jinchangyu Yuerya

Cr

Co

Ni

Mo

V

Ba

Cd

Zr

Ag

150 200 20

50 50 15

50 150 10

100 700 150

100 200 100

200 1500 500

500 100 < 100

100 100 100

202 19.0 10.0

Methods

Chemical analysis methods

Semi-quantitative emission spectrometry

Element concentration (ng g–1) Hg Shihu

479

Jinchangyu Yuerya

559 123

Atomic fluorescence spectrometry

ZHAO Liang-Cheng et al. / Chinese Journal of Analytical Chemistry, 2018, 46(2): e1801–e1809

Based on the summarization of references on the selective solvent system and dissolving separation methods for gold phases[7,9–10,13], the selective dissolution and separation processes for altered rock type gold deposits, quartz vein type gold deposits, and microscopic disseminated type gold deposits were optimized as follows. 3.4.1

Dissolution and separation of total gold

The optimized method was as follows: (1) A 10.00 g sample was placed in a porcelain combustion boat and heated in a furnace at 700 °C for 2–4 h to remove confounding elements such as S, As, and C. The sample was taken out and cooled down. (2) The sample was moved into a beaker, and 100 mL freshly prepared 1 + 1 aqua regia was added. The sample was heated to boiling on an electric plate for 1 h. Polyethylene glycol was added and stirred well. Activated carbon was used for dynamic adsorption. (3) The residue was again dissolved in aqua regia + HClO4 + HF for further dynamic adsorption by activated carbon. (4) Both activated carbon papers were transferred into the porcelain crucible and heated to 700 °C until the black carbon particles were gone. (5) The residue was dissolved in aqua regia and transferred into a 100 mL volumetric flask. A suitable dilution extent was selected according to the total gold content. (6) The sample was analyzed by ICP-AES/MS. 3.4.2

Dissolution and separation of FAu

The optimized method was as follows: (1) A 10.00-g sample was placed in a 300 mL Erlenmeyer flask. Water (50–70 mL), 5–8 g Hg (approximately 0.37–0.60 mL), and 20–30 ceramic plates were added and plugged in the cover. The flask was placed in an oscillator and oscillated for 3 h (blow washing the mineral powders on the walls with water every hour). After oscillation, the amalgam was separated and placed in a 300 mL beaker. A 1 + 1 mixture of aqua regia (100 mL) was added, and the beaker was heated until all amalgam was dissolved. The 1 + 1 aqua regia was added to

the remaining 100 mL and heated to boiling for 1 h. The following steps were the same as in Section 3.4.1 (2) to (6). The obtained gold content was the content of FAu. The most important parameters in this method were the amount of Hg and the duration of Hg mixing. Two CRMs (GBW07192 and GBW07193) and four samples (Shihu #1, Shihu #5, Jinchangyu block, and Jinchangyu mixed surface) were treated using different amounts of Hg and different durations of mixing time to determine the optimal amount of Hg and duration of mixing time. Comparison of experimental results indicated that, with a 10.00 g sample weight, FAu could be completely separated with the Hg amount of 5–8 g and the mixing duration of 2 h, as shown in Fig.2. Considering content of S and As (Table 2), the ideal amount of Hg was determined to be 5–8 g and the ideal mixing duration was over 3 h. When the amount of Hg was too small, the reaction of arsenic and sulphide minerals with Hg generated Hg powder and made the recovery of the amalgam more difficult. If an excessive amount of Hg was used, it would create insurmountable problems in subsequent tests. 3.4.3

Dissolution and separation of LAu

The optimized method was as follows: (1) A 10.00 g parallel sample of Section 3.4.2 (1) was placed in a 250-mL Erlenmeyer flask, 50 mL of I2-KI solution (7.5 g I2-15.0 g KI per 100 mL) was added, and the flask was oscillated for 1 h. (2) The solution was filtered using a Buchner funnel (adding some pulp), and the flask and residue were washed with I2-KI (I2-KI was diluted 10 times with liquid extract) three times and then washed with water 8–10 times before transferring the filtered solution into a beaker. (3) The filtered solution was heated on an electric plate, vaporized to 40–50 mL to remove most of the I2. HNO3 was added after cooling down to remove all I2 and allowed to evaporate till dry. (4) A total of 100 mL aqua regia (1 + 1) was added to leach the LAu residue. The subsequent steps were the same as in Section 3.4.1 (2) to (6). The obtained gold amount was the total of FAu and LAu. The LAu amount was obtained by subtracting FAu.

Fig.2 FAu concentration under different Hg amount treatment (A), and Total gold content under different Hg mixing duration (B) Values are mean ± SD. n = 5. SH denotes Shihu gold deposit. JCY denotes Jinchangyu gold deposit

ZHAO Liang-Cheng et al. / Chinese Journal of Analytical Chemistry, 2018, 46(2): e1801–e1809

The key parameters of this method were the determination of the selective solvent system and its concentration, and optimization of the oscillation duration. Two phases (FAu and LAu) should be completely leached without phase crossing or incomplete separation. The most efficient selective solvents for FAu and LAu were I2-KI and I2-NH4I solvents[7,15]. Gold deposits were treated with I2-KI solution with the same volume but different concentrations. The results indicated that the best selective separation solvent was 7.5 g I2-15.0 g KI per 100 mL. When I2-KI solution of the above concentration was used to treat samples (five national CRMs for chemical phase analysis of gold and three gold deposits) under different oscillation durations (0.5–2.0 h), stable results were obtained when the oscillation time was over 1 h. Thus, the best oscillation time was determined to be 1 h. 3.4.4

Dissolution and separation of SAu

The optimized method for SAu was as follows: The residue from Section 3.4.3 (4) was transferred together with the filter paper into a porcelain combustion boat and heated in a muffle furnace to 500 °C for ashing. This temperature was maintained for 1 h until ashing was complete, and then, the sample was taken out and cooled down to room temperature. The subsequent steps were the same as in Section 3.4.3 (1) to (4). The obtained result was the content of SAu. The key parameters of this process were the selection of the burning temperature and burning time. Lower burning temperatures would result in a lower SAu amount because of incomplete exposure and dissolution in the I2-KI solvent system. Conversely, higher burning temperatures would result in difficulties in leaching because of sample caking, and also result in higher content by destroying the structure of AAu. Comparison of the S content among samples with different burning temperatures and burning times, the results indicated that S in a sample could be completely oxidized when the burning temperature was 500°C and the burning time was 1 h. 3.4.5

Dissolution and separation of AAu

The optimized method for AAu was as follows: The residue from Section 3.4.4 was heated to 600–700 °C for ashing. The mineral powder was transferred to a Teflon beaker after cooling, and 100 mL aqua regia (1 + 1), 20 mL HF, and 5 mL HClO4 were added to the beaker in turn. The beaker was heated in a low temperature water bath to boiling for 1 h with frequent stirring. Polyethylene glycol was added after cooling. The subsequent steps were the same as Section 3.4.1 (2) to (6), and the result was the content of AAu. The key parameter in this method was the amount of HF. According to the Si content in gold deposit (Table 2) and the experimental results, 20 mL HF was suitable for the 10.00 g sample.

3.4.6

Selection of separation adsorbent for gold phases

The separated gold cannot be directly used for ICP-MS testing due to the high concentration of metals/metalloids in the selective solvent, especially that of K+ and I–, and that of Si and Fe rich in samples (Table 2). As shown in Fig.3, both a 4 μg mL–1 Si-containing (adding 1 mL of 100 μg mL–1 of Si solution to 1 μg mL–1 gold standard working solution in a 25 mL centrifuge tube) and an 8 μg mL–1 Fe-containing (adding 2 mL of 100 μg mL–1 of Fe3+ solution to 1 μg mL–1 gold standard working solution in a 25 mL centrifuge tube) gold standard working solutions showed a lower signal intensity relative to the Si- and Fe-free gold standard working solution, indicating an inhibitory effect of Fe and Si on measurement of gold. Hence, an adsorbent was used for separating the selective solvent from the gold phase solutions. Foam plastic and activated carbon were often used for this purpose[22,26‒30]. In this study, the adsorption and selectivity of the two materials to gold were compared, and the results indicated that the activated carbon was better than the foam plastic. Thus, activated carbon was used for gathering of different phases of gold. After ashing of the activated carbon papers, aqua regia was used for leaching at a constant volume. The solution was diluted to certain acidity for further testing. 3.5

Optimization of ICP-AES/MS conditions

We first described the selection of the analytical spectral line and background subtraction. There were two recommended gold spectral lines for the ICP-AES instrument: 242.795 and 267.595 nm. Using the ideal working conditions of the instrument (Table 1), the wavelength scanning results of a blank solution, a gold working standard solution, a 4 μg mL–1 Si-containing gold working standard solution, and an 8 μg mL–1 Fe-containing gold working standard solution were obtained, as shown in Fig.3. There was no significant interference near 267.595 nm, but the interference near 242.795 nm was significant. Considering the stability, detection limit and linear range of the measurement process, matrix interference in the gold deposit sample, spectral intensity, spectral shape and background interference, the 267.595 nm spectral line with high intensity, low interference, clear background, and high signal-to-noise ratio was selected as the analytical spectral line. Figure 3 also indicated that although the background position around the analytical signal was fixed, differences still existed in background levels and locations for different solutions. Thus, background signals need to be read near both sides of the signal to reduce their impact on the rest results. Eventually the background reading range was determined to be left 5 and right 4 of 267.595 nm spectral line. The second issue is to remove physical interference by adding an internal standard 187Re online. In ICP-MS analysis, the main sources of interference that affect the measurement

ZHAO Liang-Cheng et al. / Chinese Journal of Analytical Chemistry, 2018, 46(2): e1801–e1809

Fig.3 Wavelength scanning and background correction in ICP-AES

results are isotope interference, polyatomic ion interference, matrix interference, and physical interference[31]. In this method, the first three types of interference have minimal effect. Gold has only a single isotope 197Au in nature[32], and its interference effect can be ignored. The potential disturbance of the polyatomic ions arises primarily from 180 Hf17O, 181Ta16O and 180Hf16O1H[33,34]. However, Ta and Hf usually have very low contents in ores (Ta: 10‒1–101 μg g–1; Hf: 101–102 μg g–1)[26], low dissolution rates in aqua regia and selective solvent, and low concentrations in solution after activated carbon absorption[28,30]. The matrix effect was negligible because of a very small amount of the soluble solid after acid dissolution of the ashes of activated carbon. In the ICP-MS analysis of gold, the physical interference can be removed by adding an internal standard of 187Re or 103 Rh[26-28,30,35]. This addition can effectively monitor and correct short-term and long-term drift of the analytical signal, therefore significantly compensating for the matrix effect. In this study, we found that the online addition of 25 ng L–1 internal standard 187Re or 103Rh could effectively control the instrument signal drift and matrix effect. However, Rh might be symbiotic with noble metals in samples and resulted in inaccurate measurements of gold. 187Re was thus used as the internal standard element. It produced excellent results, and the measurement accuracy was significantly improved. 3.6

Linearity, detection limit, precision, and accuracy of optimized ICP-AES/MS method

The optimized ICP-AES/MS method exhibited excellent linearity and a low detection limit for gold measurements. The standard regression equations for ICP-AES and ICP-MS measurements of gold were Y = 11855X (R2 = 0.9996; based on a calculation using 8 ICP-AES standard working solution concentrations: 0.00, 0.20, 0.50, 1.00, 5.00, 10.00, 15.00 and 20.00 μg mL–1) and Y = 21279X (R2 = 0.9999; based on a calculation using 7 ICP-MS standard working solution concentrations: 0.000, 0.005, 0.010, 0.020, 0.050, 0.100 and 0.200 μg mL–1), respectively. The results indicated that the

measurement of gold by this method exhibited a satisfactory linear relationship in the concentration ranges of 0.20–20.00 μg mL–1 (ICP-AES) and 5.0–200.0 ng mL–1 (ICP-MS). Under the ideal working conditions of the instrument (Table 1), the blank solution was tested in parallel 12 times. The standard deviation of the measurement results was multiplied by 10 and then multiplied by the dilution factor (2.5) to calculate the detection limit of gold, which was 0.30 ng g–1 (ICP-MS) and 0.082 μg g–1 (ICP-AES), respectively. Eight national CRMs (GBW07246, GBW07247, GBW(E)070012, GBW07189, GBW07190, GBW07191, GBW07192 and GBW07193; all n =10) were subjected to chemical phase analysis of gold according to methods described in Section 3.2–3.5. Table 3 shows the relative standard deviation (RSD) and relative error (RE), the results are mostly lower than 5%, indicating an acceptable precision and accuracy of this method. 3.7

Comparison between optimized ICP-AES/MS method and other testing methods

Two gold samples were taken from each of the three deposits. The chemical phase analysis of gold was performed using ICP-AES/MS according to optimized methods in Section 3.2–3.5, and the results were compared with that of GFAAS and VOL-AAS test. The comparison showed that the total gold content and gold content of each phase measured with these methods were essentially identical (Table 4), indicating that the optimized method was consistent and comparable to other methods.

4

Conclusions

The chemical phase analysis of gold in gold ores using the ICP-AES/MS method was carried out. In this method, the ideal sample granularity, fine grinding method, and sample weight were determined. The selective solvent system was established to be the mixture of Hg metal and I2-KI (7.5 g I215.0 g KI per 100 mL). The phase decomposition and separation

ZHAO Liang-Cheng et al. / Chinese Journal of Analytical Chemistry, 2018, 46(2): e1801–e1809

Table 3 Precision and accuracy of the method Method

ICP-MS

ICP-AES

Gold concentration

Certified reference material for gold

Measured value

Certified value

Mean

RSD (%)

RE (%)

GBW07246

21.5 ng g–1

(22.85 ± 2.42) ng g–1

10.6

6.28

GBW07247

50.0 ng g–1

(49.07 ± 1.62) ng g–1

3.3

–1.86

GBW(E)070012

300 ng g–1

(299.3 ± 6.38) ng g–1

2.1

–0.23

GBW07189

3980 ng g–1

(3946 ± 105) ng g–1

2.7

–0.85

–1

–1

3.82

0.50

GBW07189

3.98 μg g

GBW07190

9.00 μg g–1

(9.07 ± 0.19) μg g–1

2.04

0.76

GBW07191

10.7 μg g–1

(10.22 ± 0.49) μg g–1

4.80

–4.49

GBW07192

15.1 μg g–1

(14.98 ± 0.16) μg g–1

1.07

–0.79

GBW07193

–1

(25.90 ± 0.58) μg g–1

2.25

–2.26

26.5 μg g

(4.00 ± 0.15) μg g

Certified values of CRMs can be found elsewhere12. SD denotes standard deviation. RSD denotes relative standard deviation. RE denotes relative error. n = 10.

Table 4 Comparison of analytical results of gold in practical samples measured by different methods Gold concentration (ng g–1)

Sample No.

Method

Total gold

Sum of phases

FAu

LAu

SAu

AAu

SH #1

ICP-AES ICP-MS VOL-AAS/GFAAS

10.5* – 10.6*

9878 – 9854

4386 – 4336

4232 – 4232

– 534 554

– 726 732

SH #2

ICP-AES ICP-MS VOL-AAS/GFAAS

27.7* – 27.4*

27.15* – 26.96*

10.9* – 10.8*

14.9* – 14.8*

332 339

1019 1047

ICP-AES

6.50*

6667

5403

-

-

-

ICP-MS

–

–

–

913

320

30.8

VOL-AAS/GFAAS

6.30*

6631

5360

913

327

30.6

JCY #2

ICP-AES ICP-MS VOL-AAS/GFAAS

– 299 272

– 298.5 303.9

– 206 213

– 81.2 79.3

– 8.86 9.09

– 2.48 2.48

YEY #1

ICP-AES ICP-MS VOL-AAS/GFAAS

– 0.26* 0.24*

– 246.5 252.3

– 24.5 23.2

– 185 190

– 12.0 13.1

– 25.0 26.0

YEY #2

ICP-AES ICP-MS VOL-AAS/GFAAS

11.30* – 11.47*

11.51* – 11.54*

10.53* – 10.60*

– 933 900

– 22.0 24.0

– 19.5 21.3

JCY #1

Values with “*” are in units of μg g–1. n = 10. “-” denotes values were not measured. SH denotes Shihu gold deposit. JCY denotes Jinchangyu gold deposit. YEY denotes Yuerya gold deposit. GFAAS denotes graphite furnace atomic absorption spectrometry. VOL-AAS denotes hydroquinone volumetric method-extraction flame atomic absorption spectrometry.

systems were selected to be aqua regia for decomposition and activated carbon for separation. Technical parameters, such as the selection of spectral line (Au 267.595 nm) and online internal standard (187Re, 25 ng L–1) in ICP-AES/MS measurements, were determined. The evaluation of this method indicated that the results on the CRMs were consistent with the certified values. The measured results of gold were consistent with the results obtained using the GFAAS and VOL-AAS methods. The measured results of all phases in the samples were consistent with the conclusions from rock-mineral identification. FAu, LAu, SAu and AAu could be measured by this method. With its low detection limit and wide linear range, this method also met the precision and accuracy requirements, and therefore could be applied to rapid phase analysis of gold in gold ores. It provided important scientific data and a technical reference for geological

exploration, beneficiation, and comprehensive utilization of gold deposit resources.

References [1]

Chi Q H. Geochimica, 2002, 31(4): 347–353

[2]

Huang B G, Zhang Z Y, Yang L, Gao J, Li Y R, Chen S.

[3]

Osovetsky B. Nat. Resour. Res. , 2016, 25: 241–253

[4]

Borisovna N O, Naumov V A, Osovetskiy B M, Lunev B S,

Chinese J. Inorg. Anal. Chem., 2011, 1(2): 6–12

Kovin O N. Middle-East J. Sci. Res. , 2013, 18(3): 316–320 [5]

Naumov V A, Osovetsky B M, Lithol. Min. Resour., 2013, 48(3): 237–253

[6]

Cook N J, Chryssoulis S L. Can. Mineral., 1990, 28: 1–16

[7]

Wang H J, Quan B, Ning X X. Chem. Anal. Meter., 2013, 22(1): 103–107

ZHAO Liang-Cheng et al. / Chinese Journal of Analytical Chemistry, 2018, 46(2): e1801–e1809

[8] [9]

Guo J B, Liu J B, Cai M, Zhu J G. Anal. Test Technol. Instr.,

and Mineral Analysis: 4th ed. Beijing: Geological Publishing

2014, 20(1): 13–19

House, 2011: Vol. 3, Ch. 63, 614-654; Vol. 4, Ch. 84: 781–877

Wang F, Guo M S, Wang K, Zhou H, Ye Q, Yu K P. Rock Min. Anal., 2006, 25(3): 263–269

[10] Luo X H, Chen Z S, Zhang Y, Wang C S, Song B T. Modern Instruments, 2011, 17(2): 46–48 [11] Luo X H, Chen Z S, Zhang Y, Chen X. Gold, 2011, 32(5): 59–61 [12] Wang Y X, Gu T X, Wang X H, Gao Y S. Practical Handbook of Reference Materials for Geoanalysis (Chinese-English): 2nd ed. Beijing: Geological Publishing House, 2013. [13] Wang H J. Gold Sci. Technol., 2013, 21(2): 55–60 [14] Cheng W C, Hu Y Q, Liu A Q, Yuan Z W, Wang L. Chinese J. Inorg. Anal. Chem., 2014, 4(1): 37–40 [15] Fei F Y, Ma X J, Qi Z J. Gold. , 2015, 36(10): 82–84 [16] Li M, Xi X H, Xiao G Y, Cheng H X, Yang Z F, Zhou G H, Ye J Y, Li Z H. J. Geochem. Explor., 2014, 139: 21–30 [17] Ulrich T, Guenther D, Heinrich C A. Nature, 1999, 399: 676–679 [18] Zwahlen C, Cioldi S, Wagner T, Rey R, Heinrich CA. Econ. Geol., 2014, 109: 1341–1358

[23] Chen C, Niu S Y, Wang B D, Sun A Q, Ma B J, Wang W X, Gao Y C. Geol. China, 2009, 36(6): 1340–1349 [24] Jia S S, Wang E D, Fu J F, Guo Y Y. J. Northeastern Univ. (Nat. Sci.), 2012, 33(10): 1492–1495 [25] Niu S Y, Wang B D, Sun A Q, Chen C, Wang Z L, Ma B J, Wang W X, Jiang X P, Zhao Y L, Gao Y C, Liu H B, Qiu J P. Chinese J. Geochem., 2009, 28(4): 386–396 [26] Yang Z P, Jiu X Z, Huang H L, Wei S T. Chinese J. Anal. Lab., 2006, 25(9): 99–102 [27] Wang R M. Rock Min. Anal., 2011, 30(3): 295–298 [28] Liu X L, Sun W J, Wen T Y, Wang T F, Sun W Z, Li Y X, Guo J. Chinese J. Anal. Chem., 2015, 43(9): 1371–1376 [29] Fang H, Yang X B, Yuan L, Hao H F, Wei X H. Spectrosc. Spect. Anal., 2003, 23(5): 976–978 [30] Wang L, Tang Z Z, Lai X Z, Lian W L, Hu J Z, Zhou L. Rock Min. Anal., 2013, 32(3): 420–426 [31] Shi Y H, Yang Z P, Song C A, Qiu L, Hu S H. Spectrosc. Spect. Anal., 2012, 32(5): 1387–1390

[19] Barefoot R R, Van Loon J C. Talanta, 1999, 49(1): 1–14

[32] Shi Y H, Xiong C X, Huang J H. Gold, 2009, 30(2): 43–45

[20] Ji C H, Li J Q, Huang W J, Bao R, Guo R, Hu J J. Spectrosc.

[33] Chen Y X, Wei X H, Li X R, Lv Z E, Ruan G W. Metall. Anal.,

Spect. Anal., 2010, 30(5): 1396–1399 [21] Sun Z H, Zhu K, Mao Y, Wang W G. Spectrosc. Spect. Anal., 2004, 24(2): 233–235 [22] The Editorial Committee of Rock and Mineral Analysis. Rock

2011, 31(6): 14–18 [34] Zhao W, You Y T, Xu S, Zheng J M, Li J J. Metall. Anal., 2012, 31(10): 41–45 [35] Yu T, Shao K, Li K J. Metall. Anal., 2015, 35(3): 32–36