Sample pretreatment is important for chemical phase analysis of elements. In this study, the geological samples of the Laozuoshan gold mine are chosen to pretreat by ultrasonic centrifugation and cyclotron oscillation, and the content of gold in eight chemical phases (water-soluble, ion exchange and clay adsorption, organic matter bound, iron-manganese oxide bound, naked or seminaked, carbonate bound, sulfide bound, and insoluble silicate states) is determined by atomic absorption spectrometry. The results show that the gold content of water-soluble, ion exchange and clay adsorption, iron-manganese oxide, and naked or seminaked states in the rock and ore samples is low, and some samples have high gold content of insoluble silicate states in the two methods. However, the gold content of organic matter bound, carbonate bound, and sulfide bound states obtained by ultrasonic centrifugation and cyclotron oscillation methods is significantly different. According to the X-ray fluorescence spectrometry data and the actual geological condition, the result given by the cyclotron oscillation method is more reasonable. The gold content of sulfide bound state in sediment samples is the highest and consistent with the mineral information, which could be applied to preliminarily predict the rock and ore conditions in the corresponding mining areas. In contrast with ultrasonic centrifugation, the cyclotron oscillation method has the advantages of simplicity, high efficiency, practicality, and environmental protection, and it can be better used for the determination of gold chemical phase state in geological samples by atomic absorption spectrometry.
Gold is an extraordinary mineral resource with important status and wide application in geology [
Chemical phase analysis [
In order to accurately, quickly, and effectively determine the content of phase gold in geological samples, the continuous improvement and innovation of analytical techniques are particularly important. In the phase analysis process, sample pretreatment [
The main purpose of this study was to explore, contrast, and validate the pretreatment process applicable to the gold chemical phase analysis in geological samples by AAS. Firstly, the geological samples of the Laozuoshan gold deposit were chosen to analyze the gold-related elements by X-ray fluorescence spectrometry (XRF) [
Hydrochloric acid (HCl), nitric acid (HNO3), and hydrofluoric acid (HF) in guaranteed reagent grade and ultrapure deionized water with a resistivity of 18.2 MΩ·cm at 25°C were used for sample preparation. The other reagents were of analytical grade. The main reagents prepared in the experiment were 50 g/L ammonium citrate solution, 4 g/L sodium hydroxide (NaOH)-40 g/L sodium pyrophosphate (Na4P2O7) mixed solution, 100 g/L ammonium citrate-40 g/L hydroxylamine hydrochloride mixed solution (pH ≈ 7 with ammonia), 5 g/L iodine (I2)-15 g/L potassium iodide (KI) mixed solution (pH ≈ 10 with ammonia), 4 mol/L acetic acid (HAc) solution, 5 mL/L bromine (Br2)-100 g/L sodium chloride (NaCl) mixed solution, 250 g/L ferric chloride (FeCl3) solution (with 1% HCl), 10 g/L thiourea solution (with 1% HCl), and aqua regia.
National standard gold single element solution (GSB 04-1715-2004, 1000
Samples were triturated by a planetary ball mill (QM-3SP4, Laibu, China), weighed with an electronic balance (ATY124, Shimadzu, Japan), and ashed in a muffle furnace (SXL-1008, Jinghong, China). The electric heating plate (SB-1.8-4, Shanghai Shiyan, China) was applied in total gold digestion process. The cyclotron oscillator (HY-8A, Jintan Jingda, China), centrifuge (TDL-5A, Anting, China), ultrasonic cleaner (KQ-400KDE, Kunshan, China), and vacuum pump (SHB-III, Great Wall, China) devices were used for phase gold pretreatment. The constant-temperature water bath (HH-S26S, Jintan Instruments, China) was applied to the gold desorption. Some gold-related elements were determined by an X-ray fluorescence spectrometer (EDX 6000B, Tianrui, China). Gold determination was performed on an AAS (A3, Persee, China) instrument. The optimum operating parameters of the XRF, flame, and graphite furnace AAS are summarized in Table
Instrument operating parameters.
Parameter | Setting |
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Initialization element | Ag |
Initialization channel | 2210 |
Tube current | 250 |
Tube voltage | 40 kV |
Counting rate | 1 |
Vacuum time | 25 s |
Measure time | 100 s |
Replicates | 3 |
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|
Element | Au |
Wavelength | 242.8 nm |
Spectral bandwidth | 0.4 nm |
Lamp current | 2.0 mA |
Filter coefficient | 0.6 |
Integration time | 3 s |
Burner height | 6 mm |
Flame type | Air-C2H2 |
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|
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|
Element | Au |
Wavelength | 242.8 nm |
Spectral bandwidth | 0.4 nm |
Lamp current | 4.0 mA |
Filter coefficient | 0.1 |
Integration time | 3 s |
Injection volume | 20 |
Measurement methods | Peak area |
XRF: X-ray fluorescence spectrometry; AAS: atomic absorption spectrometry.
Heating program for graphite furnace.
Step | Temperature (°C) | Heating time (s) | Holding time (s) | Internal gas flow (mL/min) |
---|---|---|---|---|
Dry | 110 | 10 | 10 | 200 |
Ashing | 600 | 10 | 15 | 200 |
Atomization | 1800 | 0 | 3 | 0 |
Exclusion | 1900 | 1 | 2 | 200 |
The collected ore and sediment samples of the Laozuoshan gold deposit were dried, smashed by a crusher, finely grinded to powder by a planetary ball mill, filtered through a 200-mesh sieve, and placed in a desiccator for use.
With reference to the geological and mineral industry standard DZ/T0279.19-2016 of the People’s Republic of China, combined with the actual situation of the laboratory, the leaching experiment of total gold in geological samples was designed as the following two steps: High-temperature ashing-acid digestion: Ten grams of sample were accurately weighed into a porcelain crucible, heated to 700°C in a muffle furnace, and held for 1.5 hours. After cooling, the sample was transferred to a Teflon Erlenmeyer flask (TEF), into which 50 mL 50% aqua regia and 10 mL HF were added. The TEF was placed on a hot plate and heated to dissolve the sample, and the solution was kept slightly boiled, evaporated to half, and then cooled. Gold enrichment and desorption: 80 mL ultrapure deionized water, 3 mL FeCl3 solution, and a gold-absorbing foam (3 × 2 × 1 cm3, ∼0.35 g) were added to the resulting solution of the TEF, which was then placed on a cyclotron and shaken for 30 minutes at a frequency of 200 rpm. Subsequently, the foam was taken out from the flask, the residual acid and residue were rinsed with ultrapure deionized water, the water was drained with the filter paper, and the solution was placed in a 50 mL colorimetric tube in which 25 mL thiourea solution had been added. After 45 minutes in the boiling water bath, the foam was repeatedly pressed and taken out. The resulting solution was cooled to room temperature and to be measured.
At the same time, the same method was used for the blank control experiment.
Methods I and II represent ultrasonic centrifugation and cyclotron oscillation processes, respectively. Method I uses ultrasonic waves to propagate in the form of longitudinal waves within the liquid, cause the liquid molecules to vibrate at their equilibrium positions, and then increase the solubility of the sample in the solvent. When the ultrasonic power is strong enough, the cavitation even instantaneous high temperature and high pressure will generate. This extreme environment is easy to accelerate the internal motion or depolymerization of molecules and cause chemical reaction. Here, the samples were sonicated at the frequency of 40 kHz for 1 hour in an ultrasonic cleaner with the power of 200 W and the temperature of 25°C–30°C, and then centrifuged at 4500 rpm for 15 minutes in a centrifuge. Method II, also called the circumferential oscillation, is a 360° rotation oscillation on the horizontal surface. During the oscillation process, the oscillated liquid will appear in a swirl shape in the container and its turbulence is more intense, which allows the sample to be distributed more evenly throughout the extractant and thereby accelerates the diffusion rate of the extracted gold into the solution. Here the samples were shaken at the frequency of 200 rpm for 1.5 hours on a cyclotron oscillator with the power of 100 W and the amplitude of 20 mm.
Method I: Ten grams of sample and 50 mL ultrapure deionized water were accurately taken into a centrifuge cup, shaken well, and sonicated in an ultrasonic cleaner for 1 hour. The temperature of the ultrasonic cleaner should be maintained at 25°C–30°C during the ultrasound process. Then the sample was centrifuged at 4500 rpm for 15 minutes and filtered through a 0.45
Method II: Ten grams of sample and 50 mL ultrapure deionized water were accurately taken into a conical flask with cover, shaken at 200 rpm for 1.5 hours on a cyclotron oscillator, and filtered through a 0.45
Fifty millilitres of 50 g/L ammonium citrate was added to the remained residue after the gold extraction of WSS, and the subsequent operations were the same as methods I and II of Section
Fifty millilitres of 4 g/L NaOH-40 g/L Na4P2O7 mixed solution was added to the remained residue after the gold extraction of IECAS, and the subsequent operations were the same as methods I and II of Section
Fifty millilitres of 4 g/L 100 g/L ammonium citrate-40 g/L hydroxylamine hydrochloride mixed solution was added to the remained residue after the gold extraction of OMBS, and the subsequent operations were the same as methods I and II of Section
Fifty millilitres of 5 g/L I2-15 g/L KI mixed solution was added to the remained residue after the gold extraction of IMOBS, and the subsequent operations were the same as methods I and II of Section
Fifty millilitres of 4 mol/L HAc was added to the remained residue after the gold extraction of NSNS, and the subsequent operations were the same as methods I and II of Section
Fifty millilitres of 5 mL/L Br2-100 g/L NaCl mixed solution was added to the remained residue after the gold extraction of CBS, and the subsequent operations were the same as methods I and II of Section
Methods I and II: 50 mL of 50% aqua regia and 10 mL HF were added to the remained residue after the gold extraction of SBS, and the subsequent operations were the same as the experimental procedure of total gold above, respectively.
Under the optimal conditions of the instrument in Table
The accuracy and precision requirements of analytical methods can be validated by separately calculating the logarithmic deviation (Δ log
In Tables
Combined with the geological characteristics of the Laozuoshan gold deposit, the main constant elements Al, Ca, Fe, K, Mg, Na, and Si (expressed by oxide) and trace elements Co, Cu, Mn, Ni, Pb, Sn, and Zn of the sample were determined by XRF, and the results are shown in Tables
The content of main constant and trace elements.
Constant element (unit: 10−2) | |||||||
---|---|---|---|---|---|---|---|
Sample | Al2O3 | CaO | Fe2O3 | K2O | MgO | Na2O | SiO2 |
LZY01 | 3.140 | 7.598 | 34.367 | 0.188 | 0.586 | 2.213 | 44.198 |
LZY02 | 8.916 | 14.113 | 9.429 | 1.693 | 2.472 | 0.201 | 58.545 |
LZY03 | 7.945 | 20.719 | 8.677 | 0.166 | 1.746 | 0.668 | 47.335 |
LZY04 | 8.062 | 15.368 | 10.244 | 2.061 | 2.182 | 0.292 | 56.328 |
LZY05 | 12.504 | 9.485 | 3.513 | 1.600 | 1.136 | 1.968 | 61.755 |
LZS01 | 11.318 | 1.524 | 1.620 | 3.443 | 0.431 | 2.457 | 63.061 |
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Trace element (unit: 10−6) | |||||||
Sample | Co | Cu | Mn | Ni | Pb | Sn | Zn |
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LZY01 | 255.089 | 900.836 | 1055.870 | 48.409 | 39.484 | 0.607 | 133.309 |
LZY02 | 19.689 | 120.047 | 2003.167 | 30.235 | 7.462 | 0.826 | 160.777 |
LZY03 | 18.392 | 1856.429 | 1848.804 | 11.438 | 14.410 | 0.742 | 134.596 |
LZY04 | 20.457 | 243.509 | 1973.857 | 20.027 | 4.505 | 0.784 | 143.916 |
LZY05 | 6.162 | 157.230 | 554.794 | 4.761 | 13.303 | 1.757 | 24.905 |
LZS01 | 1.475 | 26.164 | 117.641 | 2.562 | 48.004 | 0.052 | 297.242 |
The measurement results of the total gold in the sample by AAS are shown in Tables
Analysis results of total gold.
Sample | LZY01 | LZY02 | LZY03 | LZY04 | LZY05 | LZS01 |
---|---|---|---|---|---|---|
Content (10−6) | 46.985 | 3.980 | 26.060 | 1.840 | 1.490 | 0.176 |
The gold content of each chemical phase in the sample was determined by AAS, the weighted value (the sum of eight phase gold contents), the percentage (the ratio of the phase gold content to the weighted value), and the leaching rate (the ratio of the weighted value to the total content) were calculated, and all the results are listed in Tables
Analysis results of phase gold.
Sample | Method | WSS | IECAS | OMBS | IMOBS | NSNS | CBS | SBS | ISS | Weighted value | Leaching rate |
---|---|---|---|---|---|---|---|---|---|---|---|
LZY01 | I | 0.211 | 0.222 | 21.856 | 0.024 | 0.213 | 0.215 | 0.272 | 21.585 | 44.598 | 94.92 |
(0.47) | (0.50) | (49.01) | (0.05) | (0.48) | (0.48) | (0.61) | (48.40) | ||||
II | 0.224 | 0.238 | 0.240 | 0.026 | 0.216 | 0.384 | 22.261 | 21.572 | 45.161 | 96.12 | |
(0.50) | (0.53) | (0.53) | (0.06) | (0.48) | (0.85) | (49.29) | (47.77) | ||||
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LZY02 | I | 0.000 | 0.019 | 4.456 | 0.002 | 0.011 | 0.005 | 0.036 | 0.065 | 4.594 | 115.43 |
(0.00) | (0.41) | (97.00) | (0.04) | (0.24) | (0.11) | (0.78) | (1.41) | ||||
II | 0.006 | 0.006 | 0.051 | 0.007 | 0.003 | 0.032 | 3.079 | 0.605 | 3.789 | 95.20 | |
(0.16) | (0.16) | (1.35) | (0.18) | (0.08) | (0.84) | (81.26) | (15.97) | ||||
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LZY03 | I | 0.118 | 0.112 | 12.906 | 0.015 | 0.133 | 0.031 | 0.158 | 11.439 | 24.912 | 95.60 |
(0.47) | (0.45) | (51.81) | (0.06) | (0.53) | (0.12) | (0.63) | (45.92) | ||||
II | 0.125 | 0.194 | 1.163 | 0.014 | 0.115 | 0.792 | 11.153 | 11.817 | 25.373 | 97.36 | |
(0.49) | (0.76) | (4.58) | (0.06) | (0.45) | (3.12) | (43.96) | (46.57) | ||||
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LZY04 | I | 0.000 | 0.012 | 2.105 | 0.001 | 0.000 | 0.003 | 0.110 | 0.109 | 2.340 | 127.17 |
(0.00) | (0.51) | (89.96) | (0.04) | (0.00) | (0.13) | (4.70) | (4.66) | ||||
II | 0.000 | 0.019 | 0.203 | 0.003 | 0.000 | 0.041 | 1.113 | 0.180 | 1.559 | 84.73 | |
(0.00) | (1.22) | (13.02) | (0.19) | (0.00) | (2.63) | (71.39) | (11.55) | ||||
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LZY05 | I | 0.000 | 0.019 | 1.566 | 0.002 | 0.000 | 0.003 | 0.016 | 0.004 | 1.610 | 108.05 |
(0.00) | (1.18) | (97.27) | (0.12) | (0.00) | (0.19) | (0.99) | (0.25) | ||||
II | 0.000 | 0.006 | 0.213 | 0.001 | 0.000 | 0.042 | 1.129 | 0.046 | 1.437 | 96.44 | |
(0.00) | (0.42) | (14.82) | (0.07) | (0.00) | (2.92) | (78.57) | (3.20) | ||||
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LZS01 | I | 0.000 | 0.000 | 0.030 | 0.010 | 0.035 | 0.006 | 0.072 | 0.010 | 0.163 | 92.61 |
(0.00) | (0.00) | (18.40) | (6.13) | (21.47) | (3.68) | (44.17) | (6.13) | ||||
II | 0.000 | 0.006 | 0.022 | 0.002 | 0.000 | 0.023 | 0.096 | 0.044 | 0.193 | 109.66 | |
(0.00) | (3.11) | (11.40) | (1.04) | (0.00) | (11.92) | (49.74) | (22.80) |
I: ultrasonic centrifugation; II: cyclotron oscillation; WSS: water-soluble state; IECAS: ion exchange and clay adsorption state; OMBS: organic matter bound state; IMOBS: iron-manganese oxide bound state; NSNS: naked or seminaked state; CBS: carbonate bound state; SBS: sulfide bound state; ISS: insoluble silicate state. Among the eight phase states, the value outside the parentheses is the content of gold in each phase, and the unit is 10−6; the value in parentheses is the ratio of the phase gold content to the weighted value, and the unit is %. The weighted value is the sum of the eight phase gold contents, and the unit is 10−6; the leaching rate is the ratio of the weighted value to the total content, and the unit is %.
The histogram for the percentage of gold in each chemical phase (I: ultrasonic centrifugation; II: cyclotron oscillation; WSS: water-soluble state; IECAS: ion exchange and clay adsorption state; OMBS: organic matter bound state; IMOBS: iron-manganese oxide bound state; NSNS: naked or seminaked state; CBS: carbonate bound state; SBS: sulfide bound state; ISS: insoluble silicate state).
In addition, the gold content of SBS in sediment samples was the highest, indicating that the sediment system was affected by the primary halo, and its main phase composition was consistent with the mineral information in the mining area. Thus, the gold content and phase behavior of sediments could be used to preliminarily predict the rock and ore conditions in the corresponding mining areas, analyze and deduce the hard-to-obtain medium with the easily obtained sampling medium, estimate the deep mineral information with the aid of shallow source information, and then provide some theoretical guidance for the exploration of concealed gold deposits and the delineation of gold anomalies.
Comparing the gold leaching rates of the two methods in Table
To reasonably analyze the chemical phase of gold in geological samples, good pretreatment and test methods are necessary. In this work, two pretreatment processes of ultrasonic centrifugation and cyclotron oscillation for the extraction of gold from eight chemical phases are compared, and the gold content is determined by AAS. It is found that the experiment steps are cumbersome, and it is easy to introduce more errors and cause a certain degree of noise pollution in ultrasonic centrifugation method. When extracting gold of OMBS, ultrasound accelerates the internal molecular movement and depolymerization and releases the subsequent gold of CBS and SBS in advance, which causes the high gold content of OMBS and the extraction results to be inconsistent with the actual situation. However, the main phase obtained by cyclotron oscillation method is SBS, which is consistent with the XRF analysis and the geological information of the Laozuoshan gold deposit. Moreover, the cyclotron oscillation has the advantages of simple experimental steps, relatively low error, and high gold leaching efficiency. Therefore, the cyclotron oscillation method is an accurate and quick pretreatment method, which meets the development requirements of green chemistry and environmental protection, and can be well suited for the determination of the chemical phase of gold in geological samples by AAS.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
This research was supported by the Chinese Academy of Geological Sciences Project (AS2016P02), Deep-Penetrating Geochemistry Project (2016YFC0600606 and 2016YFC0600600) funded by the State Key Research and Development Program, National Natural Science Foundation of China (51603083), and Mapping Chemical Earth Project (DD20160116).
Table S1: the measured values of main constant and trace elements for CRMs GBW07105 and GBW07309 by XRF. Table S2: the analysis results of total gold for CRMs GBW7247a and GBW07298a by AAS. Table S3: the measured values of main constant and trace elements for six samples by XRF. Table S4: the measured values of total gold for six samples by AAS. Table S5: the measured values of phase gold for six samples by the ultrasonic centrifugation method. Table S6: the measured values of phase gold for six samples by the cyclotron oscillation method.