A Pretreatment of Refractory Gold Ores Containing Sulfide Minerals to Improve Gold Leaching by Ammonium Thiosulfate: A Model Experiment Using Gold Powder and Arsenic-Bearing Sulfide Minerals

Abstract

The use of thiosulfate to extract gold from refractory ores is promising because of its non-toxicity and high selectivity. Sulfide minerals (i.e., pyrite, arsenopyrite, chalcopyrite), major gold carriers in refractory gold ores, however, hinder gold extraction due to the high consumption of a lixiviant. In this study, a new method to improve gold extraction from sulfide bearing gold ores is proposed based on the model experiments using a mixture of gold powder and arsenopyrite-bearing sulfide (HAsBS) ore. The effects of HAsBS ore on gold leaching in ammonium thiosulfate solutions were investigated, and it was found that gold extraction in the presence of HAsBS ore was suppressed because of the unwanted decomposition of thiosulfate on the surface of sulfide minerals. To improve gold extraction in the presence of the sulfide minerals, this study investigated the effects of the pretreatment of HAsBS ore using ammonium solutions containing cupric ions and confirmed that HAsBS ore was oxidized in the pretreatment and its surface was covered by the oxidation products. As a result, thiosulfate consumption was minimized in the subsequent gold leaching step using ammonium thiosulfate, resulting in an improvement in gold extraction from 10% to 79%.

1. Introduction

Gold (Au) is an important metal in the livelihood of humans and is currently used for jewelry (53%), financial purposes (i.e., investment and central banks) (39%), and electric and electronic equipment (6%), and the remaining 2% is used for other applications such as medical and pharmaceutical products [1,2,3]. This demand for gold can be sustained by discovering new high-grade gold deposits, but finding the high-grade deposits has become difficult. Wilson et al. (2022) reported that there has been a decrease in the gold grades in global gold deposits from 10 g/t in the 1970s [4] to less than 1.5 g/t [5] currently, of which the deposits being discovered have more unwanted materials that are difficult to process. Hence, the gold mining industry has been driven by the demand for gold to find ways to process lower grade and refractory gold deposits over the past 50 years [6,7]. In these refractory ores, gold commonly occurs as free particles of gold and electrum, with the inclusion of these phases in the sulfide mineral matrix [8]. The encapsulation of gold in sulfide matrices as well as its association with other minerals (e.g., arsenopyrite (FeAsS), chalcopyrite (CuFeS₂), and pyrite (FeS₂)) renders these gold ores refractory. For the past century, cyanide has been the dominant lixiviant for the extraction of gold from its hypergenic ores, but due to its toxicity, environmental protection organizations and governments are pushing for the ban of cyanide usage in gold extraction processes [9,10,11,12]. To address this problem, alternative lixiviants such as halides, thiourea, and thiosulfate have been suggested by many researchers. Among them, thiosulfate is the most promising lixiviant due to its low toxicity, high selectivity to gold, and suitability for use with refractory gold ore containing carbonaceous materials [13,14,15,16]. The industrial application of thiosulfate is, however, still limited due to unacceptable high thiosulfate consumption and the passivation of gold, though it is notable that Goldstrike Mine, Nevada, USA, is using thiosulfate [17,18]. Decomposition of the thiosulfate, as well as the passivation of gold, further accelerates when sulfide minerals (e.g., pyrite and/or arsenopyrite) exist in the refractory ore leaching system [19,20,21]. Moreover, in the case of high arsenopyrite-bearing sulfide ore (HAsBS), it causes acid mine drainage containing arsenic (As) [22], which is harmful to micro- and macro-organisms. In ammonium thiosulfate leaching, HAsBS causes the decomposition of thiosulfate and the formation of a passivation layer (i.e., cuprous sulfide (Cu₂S), sulfur (S)) on the gold surface, which hinders the dissolution of gold by acting as a barrier between thiosulfate ions and gold grains [21]. To enhance the gold extraction in the ammoniacal thiosulfate leaching system, roasting has been the predominantly used technique to oxidize refractory sulfide gold ores before leaching [23]. By roasting, dense sulfide minerals are oxidized and converted to porous oxides, which increases the contact between gold and the lixiviant, and minimizes thiosulfate decomposition, resulting in the enhancement of gold extraction. In the case of HAsBS, however, roasting generates toxic gases containing sulfur oxide gases (SO_x) and arsenic oxides (As₂O₃ or As₂O₅) making it an unsuitable technique to treat HAsBS [24,25]. These toxic emissions can lead to leaks of arsenic and potential incidents of exposure, highlighting the unsuitability of roasting for HAsBS ores [26,27].

To avoid toxic arsenic containing gas formation, arsenic-bearing sulfide minerals in refractory gold ores can be pre-oxidized in low-temperature processes to minimize thiosulfate decomposition in the same manner as high-temperature roasting. Pre-oxidation of sulfide minerals using ammonium solutions containing cupric ions can be considered as a pretreatment process for refractory gold ores at ambient temperature and pressure conditions. In this pretreatment process, sulfide minerals are oxidized by cupric ions (or cupric amine complexes), and associated metal species such as copper in the sulfide minerals are extracted into aqueous phase by forming soluble complexes with ammonium, while iron, the dominant metal in the sulfide minerals, is precipitated as metal oxyhydroxides on the surface of sulfide grain. This may minimize thiosulfate decomposition due to sulfide minerals in the subsequent gold extraction process using ammonium thiosulfate.

This paper proposes the pre-oxidation of sulfides using ammonium solutions containing cupric ions as a low-temperature pretreatment suitable for arsenic-bearing refractory ore for improving gold extraction in the succeeding ammonium thiosulfate leaching. To demonstrate the effects of the proposed pre-oxidation, model experiments were conducted using the mixture of gold powder and HAsBS ore. The effects of the HAsBS ore on gold extraction were investigated to confirm the suppression mechanism of gold extraction with the ore. The effects of the pretreatment using ammonium solutions containing cupric ions on the subsequent gold leaching using ammonium thiosulfate was demonstrated.

2. Materials and Methods

2.1. Materials

The HAsBS sample was supplied by Kitabita Mine, Yamaguchi Prefecture, Japan.

Table 1. Chemical composition of arsenopyrite sample.

Elements	Fe	As	S	O	Mg	Si	Cu	Br	Zn	Sb	Others
wt.%	29.2	25.1	15.4	14.5	11.5	3.9	0.24	0.1	0.03	0.01	0.02

shows the chemical composition of the HAsBS sample determined using the X-ray fluorescence spectrometer (XRF, EDXL300, Rigaku Corporation, Tokyo, Japan). Quantitative X-ray diffraction (XRD, Multiplex, Rigaku Corporation, Tokyo, Japan) analysis result (Figure 1) showed that the HAsBS sample was composed of arsenopyrite, chalcopyrite, pyrite, pyrrhotite, and quartz.

The ore sample was crushed using a jaw crusher (1023-A, Yoshida Manufacturing Co., Ltd., Sapporo, Japan) with an aperture size of 1 mm, and ground using a Retsch RS 200-disc mill (Retsch Inc., Hean, Germany) to sizes of 80% less than 38 µm. Leaching experiments were conducted using gold powder (99.99%), sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O), ammonium sulfate ((NH₄)₂SO₄), ammonia water (NH₄(OH), 31%), and cupric sulfate (CuSO₄ 5H₂O), which were all obtained from Wako Pure Chemical Industries, Ltd., Japan. To characterize the surface of gold during leaching, a gold foil (thickness 0.127 mm and 99.99% purity) obtained from Sigma-Aldrich, Corporation, St. Louis, MO, USA, was used.

2.2. Leaching Experiments

Batch leaching experiments were carried out in 50 mL Erlenmeyer flasks containing a known amount of HAsBS (1 g) and 10 mg of gold powder in 10 mL of ammonium thiosulfate solution consisting of 1 M Na₂S₂O₃·5H₂O, 0.25 M (NH₄)₂SO₄, 0.5 M NH₄OH, and 10 mM CuSO₄·5H₂O. The flasks were shaken in a thermostat water bath shaker at 25 °C for 24 h with a constant shaking amplitude and frequency of 40 mm and 120 min⁻¹, respectively. The effects of leaching time (2–24 h), solid-to-liquid ratio (1–20%), CuSO₄ concentration (0.1–15 mM), Na₂S₂O₃ concentration (0.1–2 M) on gold dissolution were investigated in the leaching experiments. After leaching, the suspensions were filtered using 0.2 µm membrane filters (Sartorius AG, Göttingen, Germany), and the filtrates were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP–AES 9820, Shimazu Corporation Japan (margin of error $\pm$2%)) to measure the concentration of dissolved elements and by ultraviolet-visible spectroscopy (UV–Vis, UV–970, Japan Spectroscopic Co., Ltd., Tokyo, Japan) to analyze the thiosulfate and sulfur species. The residues were washed thoroughly with deionized (DI) water, dried in a vacuum drying oven at 40 °C, and analyzed by XRF, X-ray photoelectron spectroscopy (XPS, JPS–9200, JEOL Ltd., Akishima, Japan), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDX, JSM–IT200, JEOL Ltd., Tokyo, Japan) for chemical and surface characterization, respectively.

To determine the amount of dissolved metal in the leaching experiments of gold powder in ammonium thiosulfate solutions, the metal extraction was calculated using the following equation.

$$\mathrm{M}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=\frac{{\mathrm{C}}_{\mathrm{m}}\times \mathrm{V}}{\mathrm{m}\times \mathrm{M}}\times 100\%,$$

(1)

where C_m is the concentration of dissolved metal in mg/L, V is the volume of leaching solution in L, m is the mass of the sample (g), and M the metal content in the sample (mg/g), respectively. In addition, thermodynamic calculations using Geochemist’s Workbench (GWB) software were carried out for the Fe–As–S–H₂O and Fe–S–H₂O systems to predict the dissolution behavior of the metal species in the HAsBS ore for the conditions used in the leaching experiments.

2.3. Characterization of the Passivation Layer on Gold Surface

Batch leaching experiments were conducted to characterize the passivation layer formed on the gold grains in ammonium thiosulfate solutions. Leaching was conducted using 4 mm × 4 mm gold foil, 1 g HAsBS, and 10 mL of ammonium thiosulfate solution, all of which were put in 50 mL flask and shaken (i.e., a thermostat water bath shaker at 25 °C; amplitude 40 mm; shaking speed 120 min⁻¹) for 24 h. The residues were washed thoroughly with DI water, dried in a vacuum drying oven at 40 °C, and analyzed by XRF, XPS, and SEM–EDX for chemical and surface characterization.

2.4. Pretreatment Experiments Using Ammonium Solution Containing Cupric Ions

Pre-oxidation experiments were carried out by mixing 10 mg gold powder, 1 g HAsBS, and 10 mL of solution containing 0.5 M NH₃, 0.5 M NH₄Cl, and 10 mM CuCl₂ in a 50 mL Erlenmeyer flask and shaking in a thermostat water bath shaker at 25 °C (amplitude 40 mm; shaking speed 120 min⁻¹) for 24 h. After 24 h, shaking was stopped for decantation and 5 mL of supernatant was taken for analysis. Subsequent gold leaching was carried out by adding 5 mL of 2 M Na₂S₂O₃ solution containing 0.5 M NH₃, 0.5 M NH₄Cl, and 10 mM CuCl₂ to the flask and shaking the flask for another 24 h. After gold leaching, solutions and solid residues were collected by filtration, and analyzed by ICP-AES and SEM–EDX, respectively. Control experiments (gold leaching experiments without pre-oxidation) were carried out by shaking 10 mg gold powder, 1 g HAsBS, and 10 mL of solution containing 1 M Na₂S₂O₃, 0.5 M NH₃, 0.5 M NH₄Cl, and 10 mM CuCl₂ in 50 mL Erlenmeyer flask at 25 °C for 24 h.

To evaluate the effect of pretreatment on thiosulfate decomposition, leaching experiments were carried out by mixing 4 g of HAsBS (i.e., pretreated, and untreated) with 40 mL of thiosulfate solution (1 M Na₂S₂O₃, 0.5 M NH₃, 0.5 M NH₄Cl) with and without 10 mM CuCl₂ in a 50 mL Erlenmeyer flask using magnetic stirring. A 2 mL suspension was collected at regular time intervals (20 min) and the filtrate was analyzed by UV–Vis.

3. Results and Discussion

3.1. Effect of HAsBS Ore on Gold Dissolution

The suppression mechanism of gold extraction from refractory ores is classified into two phenomena: one is the physical encapsulation of gold grains in the sulfide matrix, which prevents the contact of the lixiviant with the gold grains. The second is a chemical suppression mechanism that includes thiosulfate decomposition and passivation of the gold grains. The suppressive effect of physical encapsulation can be overcome by ultra-fine grinding to expose the gold grain to the lixiviant, but chemical suppression remains even after ultra-fine grinding is conducted. As a first step in the present study, the chemical suppressive effects of sulfide minerals on gold extraction were evaluated by leaching experiments using the mixture of gold powder and HAsBS ore (Figure 2). In the absence of HAsBS, over 90% of gold was extracted within 8 h, and more than 99% gold extraction was attained after 24 h. This fast-leaching kinetics matches with previous studies [28,29,30].

The dissolution reaction of gold in ammonium thiosulfate has been reported by many authors and is summarized as: (Equation (2)) [27,29,30,31,32,33,34].

$$\mathrm{A}\mathrm{u}+2{{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}+{\mathrm{C}\mathrm{u}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_4}^{2+}\to \mathrm{A}\mathrm{u}{{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2}^{3-}+{\mathrm{C}\mathrm{u}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_4}^{+}$$

(2)

In Equation (2), gold is oxidized by the cupric ammine complex (Cu(NH₃)₄²⁺), and the extracted gold (Au⁺) is stabilized by forming a complex with thiosulfate anions (i.e., Au(S₂O₃)₂³⁻). The cuprous ammine complex (Cu(NH₃)₄⁺) formed in Equation (2) is then oxidized to a cupric ammine complex (Cu(NH₃)₄²⁺) with dissolved oxygen in Equation (3). The overall reaction of gold dissolution from Equations (2) and (3) is summarized by Equation (4).

$${4\mathrm{C}\mathrm{u}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_4}^{+}+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to {4\mathrm{C}\mathrm{u}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_4}^{2+}+4 {\mathrm{O}\mathrm{H}}^{-}$$

(3)

$$4\mathrm{A}\mathrm{u}+8{{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4\mathrm{A}\mathrm{u}{{\left({\mathrm{S}}_2{\mathrm{O}}_3\right)}_2}^{3-}+4 {\mathrm{O}\mathrm{H}}^{-}$$

(4)

In the overall reaction of gold dissolution (Equation (4)), copper ammines (Cu(NH₃)₄²⁺/(Cu(NH₃)₄⁺) do not appear, and they are considered to be a catalyst for gold extraction. In the solution phase, the decomposition of thiosulfate occurs; that is, thiosulfate ions are oxidized by the cupric ammine complex to form tetrathionate (S₄O₆²⁻) as follows [31,32]:

$$2{\mathrm{C}\mathrm{u}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_4}^{2+}+2{{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}\to 2{\mathrm{C}\mathrm{u}{\left(\mathrm{N}{\mathrm{H}}_3\right)}_4}^{+}+{ {\mathrm{S}}_4{\mathrm{O}}_6}^{2-}$$

(5)

Because of this reaction (Equation (5)), thiosulfate ions, needed to extract gold, are decomposed, but as Figure 2 shows, more than 99% of gold was extracted in the absence of the HAsBS ore; this implies that the effects of thiosulfate decomposition can be considered negligible.

In the presence of HAsBS, a maximum of 10% gold extraction was attained after 24 h, which was much lower compared to the absence of HAsBS where over 99% extraction was obtained. There are two possible reasons that can explain the drastic reduction in gold extraction in the presence of HAsBS, which are: (1) the formation of a passivation layer on the surface of gold that retarded the dissolution of gold [21], and (2) the accelerated decomposition of thiosulfate by associated sulfide minerals [19]. In the following sections, the above hypotheses are discussed based on the experimental results.

3.2. Oxidation of HAsBS Ore in Ammonium Thiosulfate Solution

During gold leaching in ammonium thiosulfate solutions, arsenopyrite and pyrite, the major sulfide minerals in HAsBS ore (Figure 1), are oxidized by the cupric ammine complex. Thermodynamic calculations performed using GWB software for the Fe–As–S–H₂O and Fe–S–H₂O systems (Figure 3) showed that iron arsenate (FeAsO₄) and hematite (Fe₂O₃) are the stable phase for the iron and arsenic species at the observed pH (9.8) and Eh (0.15 V) conditions in the leaching experiments, shown as the cross point of the red dotted lines in Figure 3. Based on this, the possible reactions for arsenopyrite and pyrite oxidation are assumed to be:

$$\mathrm{F}\mathrm{e}\mathrm{A}\mathrm{s}\mathrm{S}+{14\mathrm{C}\mathrm{u}{\left({\mathrm{N}\mathrm{H}}_3\right)}_4}^{2+}+8{\mathrm{H}}_2\mathrm{O}\to \mathrm{F}\mathrm{e}\mathrm{A}\mathrm{s}{\mathrm{O}}_4+{{\mathrm{S}\mathrm{O}}_4}^{2-}{+14\mathrm{C}\mathrm{u}{\left({\mathrm{N}\mathrm{H}}_3\right)}_4}^{+}+16{\mathrm{H}}^{+}$$

(6)

$$2\mathrm{F}{\mathrm{e}\mathrm{S}}_2+{12\mathrm{C}\mathrm{u}{\left({\mathrm{N}\mathrm{H}}_3\right)}_4}^{2+}+7{\mathrm{H}}_2\mathrm{O}\to {\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3+4{{\mathrm{S}\mathrm{O}}_4}^{2-}+{12\mathrm{C}\mathrm{u}{\left({\mathrm{N}\mathrm{H}}_3\right)}_4}^{+}+14{\mathrm{H}}^{+}$$

(7)

To confirm the proposed oxidation reactions for arsenopyrite and pyrite, soluble Fe, As, and S concentrations during the leaching experiments with HAsBS were analyzed (Figure 3). The dissolved sulfur concentration increased from 39 to 47 g/L in the first 6 h of leaching, Figure 4b. This is consistent with reaction (6) and (7), where sulfate ions are released by the oxidation of HAsBS. After 6 h, the soluble sulfur concentration started to decrease, and the concentration reached 36 g/L at 24 h. The decline in soluble sulfur concentration indicates that precipitation of the sulfur species occurs during the leaching. This may be interpreted by assuming the decomposition of thiosulfate due to the presence of an excess amount of dissolved oxygen. According to Rodriguez et al. [33], an excess amount of dissolved oxygen favors the oxidation of thiosulfate and its complexation with Cu; oxygen causes the oxidation of Cu(I) to Cu(II), at the same time decomposing thiosulfate to form (S₂O₃)₃⁵⁻, sulfite(SO₃²⁻), sulfate(SO₄²⁻), and elemental sulfur(S) as shown in Equations (8) and (9) [34]. In addition, the rate of oxidation of Cu(I) to Cu(II) is slower than the reduction of Cu(II) by thiosulfate, which may result in the formation of Cu(I)-sulfide precipitates, thus causing a reduction in the sulfur concentration.

$${{3\mathrm{S}}_2{\mathrm{O}}_3}^{2-}+{6\mathrm{H}}_2\mathrm{O}\rightleftharpoons 2\mathrm{S}{\mathrm{O}}_4^{2-}+{4\mathrm{S}}^0+2\mathrm{O}{\mathrm{H}}^{-}$$

(8)

$$3{{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}+6\mathrm{O}{\mathrm{H}}^{-}\rightleftharpoons {4{\mathrm{S}\mathrm{O}}_3}^{2-}+2{\mathrm{S}}^{2-}+{3\mathrm{H}}_2\mathrm{O}$$

(9)

$${{\mathrm{S}}_2{\mathrm{O}}_3}^{2-}\to {{\mathrm{S}\mathrm{O}}_3}^{2-}+{\mathrm{S}}^0$$

(10)

As shown in Figure 4a, the extraction of Fe and As was very low with about 0% and less than 0.4% after 24 h of leaching, respectively. This indicates that both Fe and As species are not stable in the aqueous phase under the leaching conditions, and they are precipitated, probably, as ferric arsenate, hematite, or ferric hydroxides. Such precipitates might affect the gold extraction; they may cover the surface of gold powder, suppressing gold extraction, and this is discussed in detail in the next section, Section 3.3.

3.3. Effects of the Passivation Layer on Gold Extraction

As highlighted in Section 3.2, the second possible reason that can cause the reduction in gold extraction in the presence of HAsBS is the formation of a passivation layer. A passivation layer covers the surface of the gold and prevents interaction between the gold and the lixiviant, resulting in low gold extraction. In a study by Yang et al. (2015), characterization of the passivation layer after ammonium thiosulfate leaching in the presence of arsenopyrite revealed that the passivation layer was comprised of cuprous sulfide (Cu₂S), elemental sulfur (S), iron oxyhydroxide (FeOOH), and iron arsenate (FeAsO₄) species on the surface of gold. To confirm the presence of these materials, this study analyzed the surface of the leached gold foils using SEM–EDX and XPS. Figure 5 shows the results of SEM–EDX analysis for gold foil before and after leaching with HAsBS ores.

The SEM–EDX images (Figure 5) and EDX spectrum (Figure 5(a-1,b-1)) did not detect any sulfur, arsenic, or copper species but showed the presence of oxygen and iron on the surface of gold after leaching. This may be because the amounts of the passivating materials on the gold surface were below the detection limit of SEM–EDS analysis. To detect a small amount of passivating materials, XPS was used to characterize the gold surface in the presence (i) and absence (ii) of HAsBS, (Figure 6).

As shown in Figure 6, in the XPS spectrum, signals from Cu, Fe, As, S, and O were detected on the surface of the gold foil leached in the presence of HAsBS (i). In the deconvolution results for the Cu 2p3/2 spectrum, Figure 6a, a strong peak at 932.5 eV was assigned to Cu(I) bound to a sulfur species as Cu₂S [35], and minor peaks at 934.5 eV and 936.0 eV corresponding to Cu(II)-oxide and Cu(II)-sulfate were detected, respectively [36]. In the deconvoluted spectrum of S 2p3/2, peaks were detected at 163.0 eV and 161.7 eV and were assigned to disulfide and monosulfide sulfur species combined with copper species, respectively [37], and at 165.5 eV corresponding to elemental sulfur. In addition, satellite peaks were observed at 167.3 eV and 167.9 eV, which could be S associated with the sulfate species [38]. In the case of oxygen, the O 1s deconvoluted peaks at 531.9 eV and 534.3 eV were observed and were all attributed to oxygen in hydroxyl ions of Fe(III) hydroxide [39], while another small peak at 536.0 eV was assigned to the oxygen of water molecules. The iron spectrum showed peaks at 706.8 eV and 710.0 eV corresponding to Fe(II)-AsS and Fe(III)-AsS in the bulk FeAsS, respectively. The other iron peaks detected were at 711.7 eV corresponding to Fe(III)–hydroxide and at 713.7 eV attributed to similar Fe(III)-SO₄ species [40]. In addition, there were satellite iron peaks observed at 715.8 eV and 715.1 eV, which were assigned to hydrated Fe(III) oxide [41]. The arsenic spectra exhibited strong peaks at 41.3 eV and 45.5 eV, assigned to arsenic in the bulk FeAsS and As(III)-oxide, respectively. Furthermore, a peak at 46.2 eV was detected and assigned to (As(V)-O) species. The results from the XPS validated the presence of a passivation layer for the gold foil leached in the presence of HAsBS ore and were consistent with the work conducted by Yang et al. (2015) but did not give a clear indication of the source of elemental sulfur that formed on the surface of the gold foil.

To determine the source of the sulfur species observed on the gold foil, XPS characterization was undertaken for the gold foil leached in the absence of the HAsBS (ii) and the results are outlined in Figure 6(aii–eii). The XPS spectrum of Cu 2p3/2 (Figure 6(aii)) showed a peak centered at 933.5 eV assigned to Cu(II) oxide, while peaks observed at 931.9 eV and 935.6 3 eV were assigned to Cu(II) sulfide and hydrated Cu(II) sulfate, respectively [36]. For the sulfur spectrum (Figure 6(bii)), two peaks at 161.9 eV and 164.1 eV were shown and assigned to S²⁻ and elemental sulfur (S), respectively. Oxygen peaks at 533.9 eV and 531.3 eV were assigned to the hydroxide ion (OH⁻), and peaks at 532.4 eV and 536.2 eV were attributed to the oxygen of water and Cu(II) species, respectively [42]. These results show that even in the absence of HAsBS, elemental sulfur was formed on the gold surface. This indicates that the major source of sulfur in the passivation layer may be formed by the decomposition of thiosulfate.

The XPS analysis established that during gold leaching, the major passivating species on the gold surface are elemental sulfur, Cu₂S/Cu₂O, and Fe(III)−O. The following

Table 2. XPS peak parameters for Fe 2p, As 3d, S 2p, O1 s, and Cu 2p spectra.

Spectral Peak	Binding Energy	FWHM	Line Shape	Chemical Species	Content (at. %)	References
Fe 2p3/2 a	706.8	1.2	GL (20)	Fe(II)–AsS	7.8	[22]
Fe 2p3/2 a	710.0	2.4	GL (20)	Fe(III)–AsS	7.3	[40]
Fe 2p3/2 a	711.7	0.9	GL (20)	Fe(III)–O	19.9	[43,44]
Fe 2p3/2 a	713.7	1.3	GL (20)	Fe(III)–SO42−	39.5	[45,46]
As 3d5/2 a	41.3	1.1	GL (20)	As(O)	31.6	[43]
As 3d5/2 a	43.7	0.9	GL (20)	As(I)–O	9.5	[43]
As 3d5/2 a	45.5	1.3	GL (20)	As(III)–O	42.6	[47]
As 3d5/2 a	46.2	0.8	GL (20)	As(V)–O	16.4	[47]
S 2p3/2 a	161.7	1.3	GL (20)	Monosulfide (S2−)	9.3	[43]
S 2p3/2 a	163.0	2.2	GL (20)	Disulfide (S22−)	52.7	[43]
S 2p3/2 a	165.5	2.7	GL (20)	Elemental (S)	27.0	[43]
S 2p3/2 a	167.9	0.7	GL (20)	Thiosulfate (SO32−)	3.5	[43]
S 2p3/2 a	167.3	1.1	GL (20)	Thiosulfate (SO32−)	7.6	[43]
S 2p3/2 b	160.6	1.7	GL (20)	Monosulfide (S2−)	22.8	[43]
S 2p3/2 b	162.3	1.77	GL (20)	Disulfide (S22−)	69.6	[43]
S 2p3/2 b	163.9	0.68	GL (20)	Elemental sulfur (S0)	3.0	[45]
S 2p3/2 b	165.9	0.81	GL (20)	Sulfate (SO42−)	4.52	[43]
Cu 2p3/2 a	932.5	2.3	GL (20)	Cu(I)–S	57.9	[48]
Cu 2p3/2 a	934.5	2.1	GL (20)	Cu(II)–O	33.5	[35,49]
Cu 2p3/2 a	936.0	1.3	GL (20)	Cu(II)–sulfate.	8.8	[36]
Cu 2p3/2 b	931.9	1.9	GL (20)	Cu(I)–S	57.5	[48]
Cu 2p3/2 b	933.5	3.6	GL (20)	Cu(II)–O/Cu(OH)2	26.8	[50,51]
Cu 2p3/2 b	935.9	1.7	GL (20)	Cu(OH)2	6.1	[21,51]
Cu 2p3/2 b	938.4	1.7	GL (20)	Cu(II)–sulfate.	9.5	[36]
O 1s a	531.9	2.5	GL (20)	Hydroxyl oxygen (OH−)	73.9	[22]
O 1s a	534.3	1.6	GL (20)	(O)attached to water	6.5	[38]
O 1s a	536.0	2.6	GL (20)	(O)attached to water	19.6	[38]
O 1s b	533.1	1.9	GL (20	Hydroxyl oxygen (OH−)	36.6	[51]
O 1s b	534	1.6	GL (20)	(O)attached to water	39.7	[51]
O 1s b	535.0	1.7	GL (20)	(O)attached to water	20.1	[51]
O 1s b	536.3	1.1	GL (20)	(O)attached to water	3.5	[51]

Keys: a—gold foil leached with HAsBS, b—gold foil leached without HAsBS.

gives a summary of the assignments used to interpret the XPS spectra for the surface of the gold in the presence and absence of HAsBS.

Using the data in Table 2, the presence of a passivation layer composed of Cu₂S, S, and FeOOH was confirmed on the gold surface in the presence of HAsBS by XPS analysis. The formation of passivation materials (Cu₂S and S) was, however, also observed even without HAsBS, indicating that the major source of sulfur in the passivation materials comes from the decomposition of thiosulfate. Considering these results, the effects of the passivation layer were limited and may not cause low gold extraction in the presence of HAsBS ore as used in this study but could be caused by thiosulfate decomposition.

3.4. Effect of HAsBS on Thiosulfate Consumption

In the previous section, the formation of a passivation layer on gold with HAsBS and its effects were discussed. Another possible mechanism for the low gold extraction is thiosulfate decomposition induced by sulfide minerals. Therefore, the stability of thiosulfate in the presence of HAsBS was evaluated using UV–Vis spectral analysis to confirm thiosulfate decomposition by HAsBS. The UV–Vis spectral analyses for 0.5 M NH₄Cl/0.5 M NH₃ buffer solutions containing 100 µM of sodium thiosulfate or sodium sulfate are shown in Figure 7. Thiosulfate gave a strong absorbance peak at 214–216 nm, which is in line with previous reports [52,53]. The absorbance for sulfate ions was very low in the measured range (190–290 nm), and thus it can be assumed that absorbance at 215 nm (Abs₂₁₅) corresponds to thiosulfate.

The stability of thiosulfate in 0.5 M NH₄Cl/0.5 M NH₃ buffer solutions containing 100 µM of sodium thiosulfate was assessed with and without HAsBS (Figure 8). The effects of HAsBS addition with time on the absorbance at 215 nm (Abs₂₁₅), corresponding to thiosulfate, were measured (Figure 9). The vertical axis of Figure 9 shows Abs₂₁₅ at t min normalized with initial, at t = 0 min, Abs₂₁₅(0), given by the ratio Abs₂₁₅(t)/Abs₂₁₅(0). Without HAsBS, the normalized absorbance slightly decreased with time, indicating that thiosulfate tends to self-decompose [17]. Even with the self-decomposition, the normalized absorbance remained over 0.60 after 80 min, suggesting that about 65% of thiosulfate remains in the experiments. The decreasing rate of the normalized absorbance was strongly affected by the addition of HAsBS. With HAsBS, the normalized absorbance rapidly decreased to 0.20 at 20 min, and it reached 0.05 at 80 min. This suggests that thiosulfate decomposition was accelerated by HAsBS. Xu et al. (2017) reported that thiosulfate decomposition is mainly due to the oxidation on semiconductor sulfide minerals (Figure 10); thiosulfate ions are oxidized on the anodic sites of the mineral surface and electrons are transferred through the semiconductor minerals to a cathodic site where the reduction of oxidizing agents such as oxygen and Cu(NH₃)₄²⁺ occurs. In a study by Chen et al. (2008), it was established that pyrite, pyrrhotite, and arsenopyrite can result in thiosulfate decomposition. Based on the mineralogical analysis conducted in this study, HAsBS contains arsenopyrite, pyrite, and pyrrhotite (Figure 1). From this, it can be assumed that the rapid decomposition of thiosulfate with HAsBS is mainly due to the sulfide minerals in the ore. This allowed us to hypothesize that thiosulfate decomposition in the presence of HAsBS may be the dominant mechanism for the suppression of gold extraction. Based on this assumption, a method to enhance the extraction of gold associated with HAsBS is proposed in the next section.

3.5. Pre-Oxidation of HAsBS and Its Effect on Gold Leaching

As discussed in the previous section, the major problem causing the low gold extraction is the decomposition of thiosulfate due to the presence of sulfide minerals in HAsBS. To solve this problem, the present study proposes a two-step process for gold extraction from high arsenopyrite-bearing gold ores: (1) the pre-oxidation of HAsBS using an ammonia solution containing cupric ions, and (2) ammonium thiosulfate leaching of the pretreated ore to extract gold. From Equation (6) and Equation (7), when arsenopyrite and pyrite are oxidized by the cupric ammine complex, the surface of HAsBS will be covered by a “passivation layer” comprised of oxidation products such as ferric arsenate and ferric oxyhydroxides. This passivating material minimizes the oxidative decomposition of thiosulfate ions on the surface of HAsBS, thus enhancing the gold extraction. In this section, demonstration experiments using a model ore composed of gold powder and ground HAsBS were conducted to evaluate the effect of the pre-oxidation on gold extraction.

3.5.1. Pre-Oxidation of HAsBS Using Ammonia Solution Containing Cupric Ions

For the pre-oxidation, a solution containing 0.5 M NH₃, 0.5 M NH₄Cl, and 100 mM CuCl₂ was used in which the cupric ammine complex was the oxidant for HAsBS. From Equations (6) and (7), it is expected that iron and arsenic species are precipitated during the oxidation of HAsBS. As shown in Figure 11, after 24 h of pretreatment in an ammonium solution containing cupric ions, about 25% of the sulfur dissolved, but almost no iron and arsenic species were detected in the solution as expected. Because the solution used in the pre-oxidation did not contain thiosulfate, gold extraction in the pre-oxidation step was small (about 1%).

The SEM–EDX analysis to evaluate the insulating layer formation on HAsBS after pretreatment for 24 h is shown in Figure 12. The morphology of the HAsBS before and after pre-oxidation shows that the HAsBS surface changed from smooth to “pitted”, and it was also confirmed that precipitates were attached to the HAsBS surface after pre-oxidation. From EDX mapping for the residue, Fe, As, S, and O were evenly distributed on the mineral grain. The intensity of oxygen in the EDX spectrum (Figure 13) was higher in the residues as compared to the HAsBS particles before leaching. The stronger oxygen peak for the residue may be attributed to the formation of FeAsO₄ adsorbed on the ferric oxyhydroxides precipitated from the oxidation of HAsBS (Equation (8)). Weak copper signals were also detected on the HAsBS leach residues, which might indicate the formation of Cu(I)-sulfide precipitates on the mineral surface as shown in Figure 12(b-5).

Figure 14 shows the XPS results for the residue from pre-oxidation. In the Fe 2p_3/2 spectrum, Figure 14a shows two peaks at 708.2 eV and 711.4 eV, and both were assigned to Fe(III)−O [54,55]. In the deconvoluted results of the As 3d_5/2 spectrum, (Figure 14b), two peaks were detected at 45.6 eV and 47.3 eV, corresponding to As(III)−O and As(V)−O, respectively. In the O 1s spectrum, Figure 14d, on the treated HAsBS, a peak at 532.3 eV, which corresponds to the hydroxyl oxygen (OH⁻), was detected. These results are consistent with the hypothesis that ferric arsenate (FeAsO₄) and ferric oxyhydroxide (FeOOH) are present as oxidation products of arsenopyrite and pyrite.

In the deconvoluted S 2p spectrum, Figure 14e, strong peaks at 162.5 eV and 164.1 eV were dominant and assigned to disulfide (S₂²⁻) attached to Cu(I) species [38], S²⁻ attached to Cu(II) species [56], and elemental sulfur (S) [57], respectively. At higher energy levels, satellite peaks for sulfur were observed at 166.5 eV, 168.6 eV, and 169.8 eV, which are due to sulfate species.

The XPS analysis also confirmed that strong Cu signals appeared after pre-oxidation. In the Cu 2p3/2 spectrum for the residue after pre-oxidation (Figure 14c), a dominant peak was observed at 932.3 eV and this was assigned to Cu(I), which could have originated from Cu₂S or Cu(S₂O₃)₃⁵⁻ [58]. This may suggest that during the pre-oxidation, copper ions in the solution phase reacted with sulfide minerals in HAsBS ore and formed copper sulfides on the surface. A peak at 934.7 eV corresponded to Cu(II)-oxide species. In addition, the Cu 2p3/2 spectrum had satellite peaks at higher energy levels; that is, 936.0 eV, 937.0 eV, and 938.6 eV, which were assigned to Cu(II)-sulfate species.

The results of surface analyses (SEM–EDX and XPS) confirm that the HAsBS surface is covered by passivating materials such as FeAsO₄ and FeOOH after the pre-oxidation step.

3.5.2. Effect of Pre-Oxidation of HAsBS on Thiosulfate Decomposition

The effects of pre-oxidation were assessed in the previous section (Section 3.5.1), and it was found that a passivating layer, which comprised of oxidation products, formed on the surface of HAsBS. In this section, the effect of the pretreatment (pre-oxidation) of HAsBS on thiosulfate decomposition was evaluated using UV–Vis analysis (Figure 15). From the UV–Vis spectra of the thiosulfate solution, which had reacted with untreated HAsBS, the adsorption at 215 nm (Abs₂₁₅) decreased drastically with time, while the rate of decrease became slower when the thiosulfate solution was reacted with pretreated HAsBS. The decay of absorbance with pretreated HAsBS was almost the same as that without HAsBS. These results confirmed that pre-oxidation of HAsBS, forming a passivating layer on its surface, was effective as a way to minimize the decomposition of thiosulfate.

3.5.3. Effect of Pre-Oxidation of HAsBS on Gold Extraction

Figure 16 shows the effects of the pre-oxidation of HAsBS on the subsequent gold leaching using ammonium thiosulfate solutions, and the results show that gold extraction was significantly improved after pre-oxidation for 24 h; that is, gold extraction was 10% without pre-oxidation, while it increased to 79% when pre-oxidation was applied. The ICP-AES results indicate that passivation of the HAsBS surface was enough to suppress thiosulfate decomposition in the subsequent gold leaching step. When the surface of the HAsBS is oxidized, the oxidation products cover the surface of the sulfides and prevent the transfer of electrons from the thiosulfate ions to the cupric ammine complex as shown in Figure 10, thus minimizing thiosulfate decomposition.

4. Conclusions

The effects of HAsBS on gold extraction in ammonium thiosulfate were investigated by leaching experiments using a model ore composed of gold powder and ground HAsBS. The results showed that during ammonium thiosulfate leaching of the gold/HAsBS mixture, HAsBS having semiconductor properties facilitated the oxidation of the thiosulfate by the cupric ammine complex, resulting in the decomposition of thiosulfate, which is needed for gold extraction. As a result, gold extraction was suppressed in the presence of HAsBS. The thiosulfate decomposition can be minimized by applying a pre-oxidation treatment of HAsBS in cupric ammonium solutions, which results in passivation of the mineral surface with oxidation products. When the pre-oxidation was applied before ammonium thiosulfate leaching, gold extraction was improved from 10% to 79%. It is further noted that this approach minimizes reagent consumption as the copper (II) ions used in the pretreatment stage can be reused in the subsequent thiosulfate leaching.