Abstract
Gold extraction from the rock is generally carried out using mercury. However, the high toxicity of mercury has a very dangerous impact on the environment and health. Various efforts have been made to reduce the use of mercury in gold extraction, one of which is the leaching method using thiosulphate and thiourea solution because they have low toxicity and are environmentally friendly. This study aimed to determine the results of gold extraction with thiosulphate and thiourea solution and determined the optimum concentration and time of extraction. The yield of the gold extract with thiosulphate solution was greater than that of thiourea solution. The thiosulphate solution had an optimum concentration of 0.3 M and an optimum time of 2 h. While the thiourea solution had an optimum concentration of 0.2 M and an optimum time of 3 h. The results of the Friedman test on the leaching time and concentration parameters show that leaching time has a significant effect on the Au leaching process, and the concentration parameter does not affect the Au leaching process from solid samples.
1 Introduction
Gold processing carried out by various industrial sectors including small-scale gold mining generally uses mercury with amalgamation techniques. The use of mercury in the process of separating gold ore is because mercury has properties and characteristics that can be used to bind gold by forming amalgam compounds (Au2Hg3) [1,2]. The properties and characteristics of mercury include having a very high attractiveness to the element gold [3]. The advantages of this amalgamation technique are that it is easy to perform and fast [4]. However, mercury has the highest level of toxicity compared to other metals such as silver, arsenic, lead, nickel, and zinc [3,5,6].
Another method that is also used to separate gold from ore in industry and small-scale gold mining is the cyanide leaching method [7,8]. Leaching is the process of separating a solid that is dissolved with certain reagents [9,10,11]. The cyanide leaching process generally uses reagents such as NaCN and KCN to bind gold ions to form gold complex ions or aurocyanide (AuCN−) [12]. The cyanide method has low operating costs and high extraction yields [13,14]. However, this process also has a high level of toxicity and can pollute the environment [15].
The high impact and danger posed by the amalgamation method and the cyanide method commonly used by gold miners attract attention to be able to seek the development of alternative methods that are safe for the environment and health. Moreover, many gold mining activities are carried out because they can significantly increase people’s income [11]. The government has made various efforts to reduce the impact of the dangers, one of which is by issuing Presidential Regulation No. 21/2019 concerning the National Action Plan for Mercury Reduction and Elimination. Based on the regulation, by 2025, it is targeted to eliminate 100% of mercury, or in other words, mercury is no longer used in gold ore processing activities [16,17,18]. One more environmental friendly method in the recovery of gold from soil and rocks is phytomining. Phytomining is the use of plants to absorb metals in soil and rocks [19,20,21]. However, this method has drawbacks even though it is considered the most environmentally friendly. The disadvantage of phytomining is that the time or the process of taking metals, especially gold takes a relatively long process. This method is usually used to recover metals with relatively low grades to reduce the cost of mining these metals, which are deemed unprofitable due to low rates. The phytomining method uses a leaching process to extract the accumulated metals in the plant, so a reasonably efficient and effective method is to use environmentally friendly reagents. Another alternative method for processing gold ore is using more environmentally friendly reagents. Thiosulphate and thiourea are currently an alternative to cyanide reagents because they show the ability to extract gold and have a smaller negative impact on the environment and health [22,23,24,25,26]. The thiosulphate solution can form an anionic complex [Au(S2O3)2]3− with gold under alkaline conditions, while thiourea solution can form cationic complex Au[CS(NH2)]2 + in acidic conditions [27]. The advantages of these two solutions are the high dissolution rate of gold compared to cyanide solutions and fewer environmental problems [23,28,29]. In this study, the leaching process using thiosulphate and thiourea solutions is expected to increase the recovery of gold from ore by varying the contact time, varying the concentration, and determining the effect of using variations of reagent for leaching.
2 Method
2.1 Materials
The materials used include rock samples from the area mining West Sumbawa-Nusa Tenggara Barat, Indonesia. Chemicals consist of nitric acid (HNO3), hydrochloric acid (HCl), sodium thiosulphate (Na2S2O3), thiourea (CS(NH2)2), sulfuric acid (H2SO4), ferrous(iii)chloride (FeCl3·6H2O), copper(ii)pentahydrous sulfate (CuSO4·5H2O), and ammonium hydroxide (NH4OH), which were purchased from Merck-Millipore. The instruments used were scanning electron microscopy–energy-dispersive X-ray (SEM–EDX) and atomic absorption spectrophotometer (AAS) (Perkin Elmer PinAAcle 900T).
2.2 Preparation and pre-treatment
The gold rocks obtained from West Sumbawa Regency were crushed until it becomes powder. After that, it was sifted with a 100 mesh sieve (150 µm). The sampled in sieve were weighed as much as 100 g. Samples of sifted rocks were roasted for 45 min at a temperature of 500°C [30]. Rock samples were analysed using SEM–EDX to determine the morphology and the elemental content.
2.3 Analysis of gold (Au) content in rocks
A total of 2.5 g of rock samples (after pre-treatment) was placed into a beaker glass, followed by the addition of 25 mL of aqua regia solution. Aqua regia was made by mixing HCl and HNO3 (3:1) v/v [31]. The mixture was stirred and let stand for a while until all the metal dissolves. Then the solution was separated using the filter paper. The obtained filtrate was analysed using AAS to determine the Au content.
2.4 Preparation of leachate solution
2.4.1 Preparation of thiosulphate leachate solution
About 100 mL of sodium thiosulphate solution (Na2S2O3) for each concentration was placed into a 250 mL beaker glass, and then, 0.5 M NH4OH was added until the pH of the solution reached 10. Then, 20 mL of 0.03 M CuSO4·5H2O was added and stirred until a homogeneous solution was produced.
2.4.2 Preparation of thiourea leachate solution
About 100 mL of CS(NH2)2 solution was put into a 250 mL beaker. Then the pH was adjusted to 2 by adding 1 M H2SO4. After that, 30 mL of 0.025 M FeCl3 solution was added and stirred until homogeneous.
2.5 Leaching of Au with thiosulphate solution
2.5.1 Leaching process with contact time variations
A total of 2.5 g of samples were weighed and placed into a 100 mL beaker glass. Then, 25 mL of 0.2 M thiosulphate leachate solution was added. The mixture was sorted with variations in leaching time for 1, 2, 3, and 4 h. The leaching results were separated using filter paper, and the filtrate obtained was analysed using an AAS.
2.5.2 Leaching process with concentration variations
A total of 2.5 g of pre-treatment rock samples were weighed and placed into a 100 mL beaker glass. Then, a 25 mL of thiosulphate leachate solution with a concentration variation of 0.1 M, 0.2 M, and 0.3 M. The mixture was stirred for 3 h. The leaching results were filtered, and the filtrate obtained was analysed using an AAS.
2.6 Leaching of Au with thiourea solution
The same method presented earlier for leaching of Au with thiosulphate solution was followed for leaching of Au with thiourea solution.
3 Results and discussion
The rock samples used in this study were brownish-yellow obtained from the West Sumbawa mining area, West Nusa Tenggara. Rock samples are first carried out by crushing, smoothing, and sifting (Figure 1). The sample was crushed using a stone crusher, then smoothed using a set mortar and pestle, and then sifted. This process aims to obtain a smaller particle size. The small particle size will enlarge the surface area to improve the contact between solids and solution during the extraction process.
Gold extraction scheme.
Preliminary analysis of rock samples using the SEM–EDX instrument was aimed to determine the morphology of the surface as well as the composition contained in the rock. The results are shown in Figure 2, and the composition of the rocks is presented in Table 1. Based on these results, the surface of the rock appears relatively rough as well as large particles. The rock is dominantly made up of oxygen, silica, and carbon by 62.55, 15.12, and 10.91%, respectively. From these results, the composition of the rocks does not indicate the presence of the mineral gold (Au). In this case, gold particles are likely to be excluded from the dominant minerals [32]. The mineral can be pyrite (FeS2). This is in line with Munganyika’s research (2022), which stated that a significant iron content indicates a source of gold in pyrite [29].
The gold rocks after roasting at 500°C.
Chemical composition of rocks
| Element | % |
|---|---|
| O | 62.55 |
| Si | 15.12 |
| C | 10.91 |
| Al | 4.78 |
| Fe | 4.19 |
| Mg | 1.80 |
| K | 0.26 |
| Zn | 0.24 |
| Ca | 0.15 |
To determine the gold content contained in the rock sample, a preliminary analysis was carried out using an aqua regia solvent. The use of this solvent is based on the national gold testing standard contained in SNI 8880:2020. This is also in line with several previous studies such as those conducted by Piotr et al. [33] and Mooki et al. [34], which use aqua regia solvents to determine the initial content of gold and silver metal in rock samples [33,34]. Aqua regia solvents are made by mixing 37% HCl and 65% HNO3 in a ratio of 3:1. The mixture has obtained a solution of yellow colour. Furthermore, the gold leaching process is carried out for 10 min. The reaction that occurs in the process of leaching gold with the solvent of aqua regia is shown in equation (1) [23].
The dissolution result is in the form of a filtrate that has been separated from its residue and analysed using the AAS instrument. Thus, the gold content contained in the rock sample is known to be 0.0015%.
Thiosulphate is known to have the ability to dissolve gold by forming a complex [Au(S2O3)2]3−. Thiosulphate leachate solution is prepared by mixing the Na2S2O3 solution as the main solvent, CuSO4 solution as a catalyst, and ammonia solution, which is used to regulate the pH of the solution to pH 9–11 as well as acts a stabilizer of copper ions. The golden leaching system with thiosulphate occurs under alkaline conditions aimed at preventing the decomposition of thiosulphate and copper ions becomes ineffective when it is below pH 9. This mixture has obtained a solution of blue colour. Furthermore, the leaching process is carried out by adding a leachate solution to the pre-treatment rock sample. The leaching process is carried out based on a pre-determined time. Then the mixture is filtered using filter paper. The filtrate obtained from the filtering results is in the form of a colourless, and clear solution, which is then analysed using AAS to determine the concentration of gold, that has been obtained. The reaction of the formation of a gold-stable complex with thiosulphate occurs through two stages, which is indicated in equations (2) and (3).
The addition of ammonia and copper ions serves as a catalyst that accelerates the rate of leaching. Ammonia and copper ions in the solution form a complex [Cu(NH3)4]2+. This complex plays a role in oxidizing Au to Au+. Then there was a substitution of NH3 with S2O3 2−, so that a more stable [Au(S2O3)2]3− complex was formed. However, in the cathode region, the resulting [Cu(NH3)4]2+ is reduced to [Cu(S2O3)3]5− and [Cu(S2O3)3]5−, which is oxidized again rapidly by dissolved oxygen forming [Cu(NH3)4]2+ [35].
So that the total reaction of dissolution of gold in thiosulphate solution is as follows:
Thiourea is also known to have the ability to dissolve gold by forming the cationic complex [Au(CS(NH2)2)]2+. The thiourea leach solution is made by mixing thiourea solution, FeCl3 solution as an oxidizing agent, and H2SO4 solution as a pH regulator, so that the pH becomes 1–2 because in that pH range, the gold leaching process with thiourea solution is more efficient. If the solution does not have an acidic pH, thiourea becomes unstable and decomposes easily. The results of the mixture obtained a slightly yellowish and clear solution. After that, the leaching process was carried out by adding a leach solution to the pre-treatment rock samples. The leaching process is carried out based on predetermined time. The products obtained after leaching were filtered using filter paper to separate the filtrate and residue. The filtrate obtained was a colourless and clear solution which was then analysed using AAS to determine the concentration of gold that was successfully extracted with thiourea solution.
The reaction for the formation of the gold-thiourea cationic complex consists of two steps. The first step is that thiourea is converted to formamidine disulphide (FDS) by an oxidizing agent, namely Fe3+. Then FDS will react with gold in excess thiourea to form a gold-thiourea complex. The reactions that occur are presented as equations (5) and (6):
The total reaction is shown in equation (7).
Fe(iii) present in the solution from the addition of FeCl3 solution plays an important role in oxidizing thiourea to FDS (NH(NH2)CSSC(NH2)NH). Without the addition of an oxidizing agent, the leaching rate will be slower. Fe(iii) was chosen to be an effective oxidizing agent [36,37,38]. This is also supported by the value of the high standard reduction potential of Fe(iii), which is 0.77 V. FDS from thiourea oxidation can oxidize gold in solution, so that it can help increase the leaching speed.
Research on the effect of thiosulphate and thiourea concentrations on gold extraction was also carried out with a concentration variation of 0.1, 0.2, and 0.3 M using a solid sample of 2.5 g, 25 mL of leachate solution, and a leaching time of 3 h. The results of the leaching in the form of filtrate and residue were separated by filtration, and the filtrate was analysed using an AAS to determine the percentage of gold solubility obtained. The obtained gold extraction is due to leaching with varying concentrations of thiosulphate and thiourea, as shown in Figure 3 and Table 2.
Effect of thiosulphate and thiourea concentration on Au extraction from 2.5 g sample in 25 mL of leachate solution.
Au fraction dissolved in thiosulphate and thiourea leachate solutions
| The concentration of leaching agent (M) | Fraction dissolved | |
|---|---|---|
| Thiosulphate | Thiourea | |
| 0.1 | 0.574 | 0.670 |
| 0.2 | 0.584 | 0.979 |
| 0.3 | 0.436 | 1.000 |
Figure 3 and Table 2 show the results of gold extraction against variations in thiosulphate concentrations. An increase in gold extraction was linearly observed with thiosulphate concentrations of 0.1–0.3 M. Gold extraction at a thiosulphate concentration of 0.3 M reached 100%. From these results, it is known that at the high concentration of thiosulphate, the gold extraction will also increase. The optimum concentration of thiosulphate can accelerate the rate of leaching. However, if the thiosulfate concentration is too low, it can lead to gold deposition [39].
Maximum gold extraction was achieved at 0.2 M thiourea concentration with a yield of 58.4% and decreased at 0.3 M concentration with an extraction percentage of 43.6%. Higher concentrations result in lower gold extraction. This occurs due to the increased decomposition of thiourea into FDS ((CS(NH2)NH)2), which then decomposes irreversibly into cyanamide (NH2CN) and elemental sulphur [38]. The sulphur then forms a passivation layer on the surface of the gold, so that it inhibits the extraction process [40,41,42]. The decomposition of thiourea into formamidine disulphide is shown in equation (8), and the reaction for the formation of cyanamide and elemental sulphur is shown in equation (9).
Thiosulphate solutions are better at extracting Au from solids than thiourea solutions (Table 3). This is due to Fe(iii) in the thiourea solution as an oxidizing agent. Fe(iii) will reduce to Fe(ii). As the leaching time increases in the extraction at pH 2, there is a continuous oxidation of Fe(ii) to Fe(iii), while thiosulfate remains consistently flow throughout the process (Figure 4). This triggers the reduction of Au(i) to Au(0), which is not identified during analysis with AAS. This is relevant to the previous research, which stated that Fe(ii) will undergo an oxidation reaction with O2 to become Fe(iii) through the formation of ferric superoxide (Fe(iii)–(O2˙−)) [43,44].
Effect of leaching time on Au extraction from 2.5 g sample in 25 mL of leachate solution (thiosulphate 0.3 M and thiourea 0.2 M).
Au fraction dissolved in thiosulphate and thiourea leachate solution at leaching time
| Time (h) | Fraction dissolved | |
|---|---|---|
| Thiosulphate | Thiourea | |
| 1 | 0.958 | 0.462 |
| 2 | 1.000 | 0.462 |
| 3 | 0.979 | 0.584 |
| 4 | 0.962 | 0.381 |
Based on Table 4, the results of the data normality test use the Kolmogorov–Smirnov and Shapiro–Wilk approach models. The Kolmogorov–Smirnov approach model is usually used for small sample data (<50 samples), and the Shapiro–Wilk approach model is usually used for more sample data (>50 samples) [45]. In Table 4, the Kolmogorov–Smirnov approach method does not show a significant value. It can be said that the Kolmogorov–Smirnov approach model can provide an overview for testing the normal distribution of data properly compared to the Shapiro–Wilk approach model. This is relevant to the previous research, which stated that the Kolmogorov–Smirnov approach model is less good at determining the normal distribution of data than the Shapiro–Wilk approach model [46]. Table 4 reveals that data obtained from different parameters, including leaching time and concentration, exhibit a normal distribution. However, there is one data point for the leachate solution with thiosulfate on the time parameter that is not normally distributed, as indicated by a p-value of 0.044.
Normality test results
| Variable | Kolmogorov–Smirnov | Shapiro–Wilk | |||||
|---|---|---|---|---|---|---|---|
| Statistic | df | Sig. | Statistic | df | Sig. | ||
| Thiosulphate | Time | 0.358 | 4 | 0.000 | 0.756 | 4 | 0.044 |
| Concentration | 0.268 | 3 | 0.000 | 0.950 | 3 | 0.571 | |
| Thiourea | Time | 0.299 | 4 | 0.000 | 0.926 | 4 | 0.569 |
| Concentration | 0.364 | 3 | 0.000 | 0.799 | 3 | 0.113 | |
The mean difference is significant at the 0.05 level.
Because there are data that are not normally distributed, the Friedman test was carried out to determine the parameters that affect the Au leaching process. Friedman’s test is a nonparametric test for data that are not normally distributed. The results of the Friedman test on the leaching time and concentration parameters presented in Table 5 show that leaching time has a significant effect on the Au leaching process. This is evidenced by the results of calculating the p ≤ 0.050, which means that H0 is accepted (the parameter of leaching time) and has a significant effect. It could be that the concentration parameter does not affect the Au leaching process from solid samples because the p > 0.050 value means that H0 is rejected. This research is relevant to the study by Rui et al. [47], which state that the time parameter is significant in the gold recovery process in a leaching agent [47]. The longer the leaching time, the greater the gold collected. The leaching time parameter plays a more critical role than concentration because there are conditions where less gold will be recovered at a specific concentration. As in the thiosulphate solution, the greater the thiosulphate concentration, the higher the gold recovery results. However, in the thiourea solution, the greater the thiourea concentration, the lower the gold recovery.
Friedman test results
| Friedman test | p Value | Conc. |
|---|---|---|
| Leaching time | 0.046 | Significant |
| Concentration | 0.083 | Not significant |
The mean difference is significant at the 0.050 level.
Using a thiosulphate solution has advantages over using a thiourea solution in extracting gold from the solid. In addition to the effectiveness of thiosulphate in extracting gold, it is also influenced by operational costs and the resulting impact (Table 6). The effectiveness of thiosulphate in extracting gold from solids is due to the strong interaction between thiosulphate and Au by producing a stable [Au(S2O3)2]3− complex. The strong S–Au–S bond results in a stable [Au(S2O3)2]3− complex [39].
Comparison of leaching with thiosulphate and thiourea
| No. | Solution | Ref. | |
|---|---|---|---|
| Thiosulphate | Thiourea | ||
| 1. | [Au(S2O3)2]3− complex is more stable | Au[SC(NH2)2]2 + complex is less stable | [39,48] |
| 2. | More selective for Au | Less selective for Au | [48] |
| 3. | Base system | Acid system | [48] |
| 4. | Non-corrosive to equipment | Corrosive to equipment | [49,50] |
| 5. | Inexpensive | Expensive (more reagents) | [51,52,53] |
4 Conclusions
The gold content contained in the rock sample is 0.0015%. The yield of the gold extract with thiosulphate solution was greater than that with the thiourea solution. Gold extraction at 0.3 M thiosulphate reached 100% and at 0.2 M thiourea reached 58%. The thiosulphate solution had an optimum concentration of 0.3 M and an optimum time of 2 h, while the thiourea solution had an optimum concentration of 0.2 M and an optimum time of 3 h. The results of the Friedman test on the leaching time and concentration parameters show that leaching time has a significant effect on the Au leaching process, and the concentration parameter does not affect the Au leaching process from solid samples. Thiosulphate solution is more effective and environmentally friendly than thiourea solution for the recovery of Au in solids.
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Funding information: This work was not supported by any funding.
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Author contributions: All authors contributed equally.
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Conflict of interest: The authors declare no conflicts of interest.
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Ethical approval: The conducted research is not related to either human or animal use.
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Data availability statement: The datasets generated during and/or analysed during the current study are available from the corresponding author upon reasonable request.
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