Study on Synergistic Strengthening of Gold Extraction with Copper Ethylenediamine Thiosulfate Using Pyrite and Nickel Ions

Abstract

Gold leaching using the copper–ethylenediamine–thiosulfate (Cu²⁺-en-S₂O₃²⁻) system, which contains copper–ethylenediamine complexes, instead of the use of copper–ammonia catalysis, is environmentally friendly and cost-effective. In this study, pyrite and Ni²⁺ were added to the Cu²⁺-en-S₂O₃²⁻ system to clarify their individual and combined influence on gold leaching. The result obtained showed that when pyrite and Ni²⁺ were separately added to the system, the dissolution of gold was significantly inhibited. However, the disappearance of the negative impacts of these two substances when they were simultaneously added to the system revealed that they exhibited a synergistic effect on gold dissolution. Notably, Ni²⁺ weakened the promotional effect of pyrite on the formation of a Cu-containing passivation layer on the gold surface. Furthermore, the separate addition of Ni²⁺ and pyrite increased the corrosion potential of gold; thus, gold dissolution was inhibited. However, when added together, they brought about a decrease in the corrosion potential of gold, while increasing its dissolution rate. These findings provide a reference for the efficient extraction of pyrite-associated gold, which can be applied to improve the green extraction process of gold.

1. Introduction

Currently, due to process simplicity and cost efficiency, gold is primarily extracted using the cyanide method. However, this method generates large amounts of hazardous wastes and poses a serious health risk to humans [1]. In the past few decades, the traditional Cu²⁺-NH₃-S₂O₃²⁻ gold-leaching method has been investigated as one of the most promising alternatives to cyanidation. This method involves the use of Cu(II) as a catalyst to leach gold from its ore [2,3,4], and it offers several advantages, such as non-toxicity, high efficiency, low cost (the price per ton is cheaper than that associated with the cyanide method), strong adaptability to the ore, and decreased equipment corrosion. Additionally, it is also suitable for the leaching of several carbonaceous gold ores as well as complex gold sulphide ores [5,6]. Therefore, since its advent, its industrial application as a gold extraction method has been extensively studied.

The minerals that are associated with gold ores can affect thiosulphate stability as well as gold-leaching behaviour [7,8]. Usually, gold is symbiotic or is associated with sulphide minerals, especially pyrite (FeS₂) [9], which influences its leaching in thiosulphate solution by accelerating the decomposition of thiosulphate and partially dissolving in the ammonia–thiosulphate solution [5]. Therefore, strategies aimed at mitigating the harmful effects of pyrite are essential for the realization of the thiosulphate leaching of gold. In recent years, many studies have been focused on the use of various additives to reduce reagent consumption and enhance gold extraction from sulphide minerals [10,11].

In addition to being influenced by Au-containing sulphide ores as well as associated ores, the dissolution of metal ions in the thiosulphate system can also be promoted or hindered by a variety of metal impurities, given that these metal impurities can consume thiosulphate or bring about changes in the reaction on the gold surface and affect the recovery of gold from the leaching solution [12,13,14]. Additionally, Arima [15] and Xu [16] observed that the use of Ni instead of Cu as a catalyst can reduce the consumption of thiosulphate and accelerate gold ore leaching. Notably, the effect of metal ions on gold dissolution depends on the ionic species of the metals as well as their concentrations, the chemical properties of the solution, and the reaction that takes place on the gold surface.

In the early stage of our research, to form the Cu²⁺-en-S₂O₃²⁻ gold-leaching system, we proposed the use of ethylenediamine as an alternative to ammonia in the conventional Cu²⁺-NH₃-S₂O₃²⁻ system and conducted a series of related experimental studies [17,18]. The Cu²⁺-en-S₂O₃²⁻ system was found to favour the dissolution of gold and the reduction of thiosulphate consumption to a greater extent, and it showed applicability to fine-grained coated carbonaceous gold ores. However, the effect of sulphide minerals containing different metals on this leaching system remains unclear.

Herein, we investigated the individual and synergistic effects of pyrite and Ni²⁺ on gold leaching in the Cu²⁺-en-S₂O₃²⁻ system. These findings have guiding significance and can serve as a reference for the promotion of the commercial application of the non-cyanide extraction of gold from pyrite-associated ores.

2. Experimental Setup

2.1. Minerals, Reagents, and Characterisation

The pure pyrite samples used in this study were purchased from the Chinese Museum. The pure pyrite was crushed and ground to 100% passing through a 75 µm sieve and stored in a nitrogen-filled airtight plastic bag to prevent further oxidation [7]. Furthermore, the surface area of the gold foil (purity, 99.99%; thickness, 0.1 mm) [8] that was used in the leaching experiments in this study was 1.0 cm². Analytically pure copper sulfate pentahydrate, sodium thiosulfate, ethylenediamine, hydrogen peroxide (30% w/v), sodium chloride, and hydrochloric acid were used in the experiments. Metal stock solutions of nickel were made in deionised water with NiCl₂·6H₂O (chemical grade). For all the reagents used to prepare solutions of desired concentrations in all the experiments in this study, deionised water was used.

2.2. Experimental, Analytical, and Electrochemical Procedures

Experiments were performed in a 250 mL volumetric flask using a stirrer at a rotating speed of 250 rpm. The gold foil (99.99% Au) surfaces were polished by 2000-grid sandpaper, washed with acetone twice, and rinsed 8 times with deionised water before each test. The gold foil was suspended in the upper part of the leaching reactor with a nylon thread, ensuring no contact with the volumetric flask wall during leaching. The leaching system was a mixture of 0.1 mol/L Na₂S₂O₃, 0.005 mol/L CuSO₄, and 0.01 mol/L en (conditions: pH = 10, temperature = 25 °C). The specific leaching experimental methods used in this study were as previously described [17]. Detailed descriptions of the analysis methods for the determination of the concentration of gold in the prepared solutions have been described in a previous study [17]. Thiosulfate concentration was analysed via iodometry [18]. All of the experimental procedures and sample analysis procedures in this article are detailed in the Supplementary Material at the end of this article.

A PHI5000 Versa Probe II (ULVAC-PHI, Japan, Binzhou Chuangyuan equipment machinery manufacturing Co., Ltd. agent) with an Al Kα X-ray source was used to identify the elemental composition of the leached gold foil samples as well as that of the pyrite surface. Thus, Cu 2p, Fe 2p, and S 2p spectra were obtained. To determine the atomic ratio of the sample under analysis, each spectrum was then fitted and analysed using Multi Pak Spectrum software version 2.1. The C 1s spectrum of 284.8 eV was used as a calibration spectrum for binding energy. The Pro X model scanning electron microscope (Phenom, Rotterdam, The Netherlands) was used to observe the surface micro-morphology of the leached gold foils as well as that of the pyrite samples.

To perform the electrochemical test, a three-electrode system, with gold foil (15 mm × 15 mm × 0.1 mm) as the working electrode, platinum foil (10 mm × 10 mm × 0.2 mm) as the counter electrode, and the saturated calomel electrode (SCE) as the reference electrode, was used. All the electrochemical measurements were performed on a CHI600E workstation at room temperature. Before implementing the electrochemical method, the open-circuit potential (Eocp) of the Cu²⁺-en-S₂O₃²⁻ gold-leaching system (0.1 M Na₂S₂O₃ + 0.005 M CuSO₄ + 0.01 M en) was measured so as to ensure the stability of the system during the electrochemical measurement. The open circuit voltage of the system was −0.04 V, and during the experiment, a mixed solution of 0.1 M Na₂SO₃ and 5 mM [Cu(en)₂]²⁺ was used as the electrolyte, and a solution of Ni²⁺ (8 mg/L) or 1 g of pure pyrite was added to the system. The Tafel curve corresponding to the system was obtained at a scan rate of 1 mV/s in the open circuit potential (OCP) within the voltage range of −0.04 ± 0.5 V/SCE.

3. Results and Discussion

3.1. Mineral Characterisation

The mineralogical composition of the pyrite samples was analysed via X-ray fluorescence spectroscopy (XRF). The analysis results are shown in

Table 1. Chemical composition of pyrite (%).

Component	FeS2	SiO2	As2O3
% (w/w)	98.6	0.86	0.54

.

3.2. Effects of Pyrite on Gold Leaching in the Cu²⁺-en-S₂O₃²⁻ System

3.2.1. Effects of Pyrite on Gold Dissolution and Thiosulphate Decomposition

When mineral particles are added to the leaching system, the mutual friction between gold foil and minerals will also enhance the dissolution of gold leaf in the system during the continuous stirring process. In this case, it is not possible to determine whether the final result is determined by the effect of friction on gold dissolution or the effect of pyrite on gold dissolution. Therefore, in order to eliminate the effect of continuous friction between mineral particles and gold foil on gold dissolution in the system, only the specific influence of pyrite on gold dissolution is determined. An amount of 1g quartz was selected as the control group for the adding pyrite [7,19,20,21].

The effects of pyrite on gold foil dissolution and thiosulphate consumption in the Cu²⁺-en-S₂O₃²⁻ system were investigated using quartz as the blank control. Typically, 1 g of pyrite or 1 g of quartz was added to the leaching system, and samples were taken at pre-defined intervals for the analysis of the gold content of the leaching solution (Figure 1). In the case of quartz, the amount of dissolved gold increased with time in an approximately linear manner, whereas pyrite strongly inhibited gold dissolution.

The presence of pyrite brought about a significant increase in thiosulphate consumption within 34 h of leaching (Figure 2). However, in the presence of quartz, only 14.6% of thiosulphate was degraded within 34 h of leaching, indicating that this mineral has no significant effect on thiosulphate decomposition. This observation is consistent with the known chemical inertness of quartz in ammonia–thiosulphate solutions [22]. Specifically, when pyrite was added to the leaching solution, there was a sharp increase in thiosulphate consumption, which reached 36.52% after 34 h of leaching. This is significantly higher than that observed in the presence of quartz. This implies that pyrite significantly accelerated thiosulphate decomposition [5]. Some researchers think that because of its semiconducting properties, pyrite can be used as a conductive band for electron transfer between thiosulphate and oxygen, with the related reaction given by XU and SCHOONEN [23].

4S₂O₃²⁻ + O₂ + 4H⁺ → 2S₄O₆²⁻ + 2H₂O

(1)

However, the rate of thiosulphate consumption in the leaching solution decreased with increasing leaching time. This observation was attributed to the oxidation of pyrite by Cu(I/II) species during leaching to form S₂O₃²⁻, bringing about an increase in the thiosulphate content of the leaching solution [23,24].

In the presence of Cu(II) ions, thiosulphate can be oxidised to tetrathionate and trithionate [25], of which trithionate can be further decomposed to release thiosulphate and sulphite. Additionally, polythionate can reduce Cu(II) ions, thus, bringing about a decrease in the rate of gold leaching [26]; this accounts for the inhibitory effect of pyrite on gold dissolution. Additionally, thiosulphate decomposition can induce the formation of a passivation layer consisting of elemental S and copper sulphide, which prevents the thiosulphate solution from diffusing to the gold surface [27,28,29]. Thus, the increased thiosulphate consumption in the presence of pyrite may result in a greater degree of passivation.

3.2.2. Effects of Ni²⁺ on Gold Dissolution and Thiosulphate Decomposition

Figure 3 shows the effect of different Ni²⁺ concentrations on the dissolution of gold in the Cu²⁺-en-S₂O₃²⁻ system. It can be seen from the figure that when the nickel ion concentration is 8 mg/L, the gold concentration after 34 h is 5.85 mg/L, and when the [Ni²⁺] increases, the amount of gold dissolved will be further reduced. The gold concentration in the blank control group (i.e., 0 mg/L Ni²⁺) was 8.59 mg/L. Therefore, it is determined that different concentrations of Ni²⁺ can inhibit the dissolution of gold, and as [Ni²⁺] increases, the inhibitory effect becomes stronger. Because Ni²⁺ freely competed with Cu²⁺, it reduced the concentration of [Cu(en)₂]²⁺ and brought about a decrease in the amount of gold dissolution. Conversely, a further increase in the concentration of Ni²⁺ significantly promoted the formation of NiS [8]. Therefore, the presence of nickel ions only in the Cu²⁺-en-S₂O₃²⁻ system passivates the gold surface during leaching.

Figure 4 shows that under standard conditions (([S₂O₃²⁻] = 0.1 mol/L, [Cu²⁺] = 0.005 mol/L, [en] = 0.01 mol/L), thiosulphate consumption increased with increasing [Ni²⁺]. This observation could be attributed to the ability of these metal ions to complex thiosulphate [12].

3.3. Effects of Ni²⁺ and Pyrite on Thiosulphate Decomposition and Gold Dissolution

Figure 5 illustrates that in the presence of pyrite, thiosulphate consumption increased with increasing [Ni²⁺]. In addition, as [Ni²⁺] increased from 0 to 8 mg/L, thiosulphate consumption increased from 36.52 to ~40.17%. Furthermore, when [Ni²⁺] exceeded 24 mg/L, the rate of thiosulfate consumption was greater, and in the presence of a higher dose of Ni²⁺ (24 mg/L), the rate of thiosulfate consumption further increased to ~43.84%, which is ~7.32% higher than that observed in pyrite systems without Ni²⁺.

The increased thiosulphate consumption observed in the presence of Ni²⁺ and pyrite is possibly due to the promotional effect of Ni²⁺ ions adsorbed on pyrite on thiosulphate decomposition as well as their strongly thiophilic nature, which allows them to be complexed by thiosulphate; hence, the consumption of large amounts of sodium thiosulfate. In addition, when adsorbed onto the surface of pyrite (especially at high-energy defect sites and grain boundaries), the above cations may partly replace Fe²⁺. This results in the combination of Fe²⁺ and oxygen, which accelerates pyrite surface oxidation and increases thiosulphate consumption. Moreover, the adsorption of Ni²⁺ may also change the semiconductor properties of the surface of pyrite. Therefore, the addition of Ni²⁺ and pyrite can bring about a sharp increase in thiosulphate consumption.

Figure 6 shows the effect of [Ni²⁺] on the concentration of gold during leaching using the Cu²⁺-en-S₂O₃²⁻ system in the presence of pyrite. This figure reveals that an increase in [Ni²⁺] resulted in a significant increase in Au solubility, and the range of [Ni²⁺] that was most favourable for gold dissolution was 8–24 mg/L. Furthermore, as [Ni²⁺] increased from 0 to 8 mg/L, the amount of gold dissolution increased from 12.43 to 22.01 mg/L·cm², i.e., to a value much higher than that observed in the quartz slurry without Ni²⁺ (12.5 mg/L·cm², Figure 1). Even though Au solubility was lower than 22.01 mg/L·cm² at [Ni²⁺] = 24 and 32 mg/L, it was still above the value obtained in the presence of pyrite only. Therefore, the presence of pyrite in the leaching system alongside small amounts of Ni²⁺ enhanced gold dissolution.

3.4. Scanning Electron Microscopy (SEM) Imaging

3.4.1. SEM Imaging of Leached Gold Foil

To study the effects of Ni²⁺ and pyrite on the leaching of gold using the Cu²⁺-en-S₂O₃²⁻ system, the morphologies of pyrite and gold foil after leaching were probed via SEM (Figure 7). Thus, it was observed that the surface of the gold foil that was leached in the presence of both additives (Ni²⁺ and pyrite) showed a different morphology and corrosion degree compared with those leached in the presence of pyrite only.

The surface of the gold foil leached in the presence of Ni²⁺ (8 mg/L) and 1 g of pyrite showed numerous black spots (Figure 7a) as well as scarring features, such as rough pitting, which resulted in the easy corrosion of the gold surface around the affected areas. Additionally, some adsorbed particles were also observed on the gold foil surface. In the presence of pyrite only, the leached gold foil showed only minimal signs of corrosion with no obvious corrosion traces (Figure 7b). However, a thick passivation film was formed on the surface of the gold foil, preventing it from contacting the leaching liquid. This finding supported the hypothesis that gold leaching is severely hindered in the presence of pyrite. According to Feng and van Deventer, colloidal metal particles formed during leaching adhere to the surface of gold, deactivating it and preventing its dissolution [12].

3.4.2. SEM Imaging of Leached Pyrite Surface

Figure 8a shows that numerous fine particles were adsorbed onto the surface of pyrite after leaching in the presence of Ni²⁺ (8 mg/L) and 1 g of pyrite. Additionally, several small pyrite particles were agglomerated, and this reduced the contact area between pyrite and the leaching solution, hindering the formation of a Cu-containing passivation layer on the surface of the gold foil, and facilitating gold dissolution. Figure 8b shows that the pyrite that was added to the leaching system in the absence of Ni²⁺ had a light, smooth, and compact surface, without the agglomeration of fine particles.

3.5. XPS Analysis

3.5.1. XPS Analysis of Gold Surface after Leaching

The Cu 2p, S 2p, and Fe 2p spectra of the gold surface are shown in Figure 9a,b, Figure 10a,b and Figure 11a,b. Notably, hardly any nickel was present on the surfaces of the gold foil and pyrite after leaching in the presence of both pyrite and Ni²⁺.

Table 2. Atomic concentration of gold leached using the Cu²⁺-en-S₂O₃²⁻ leaching system in the presence of Ni²⁺ (8 mg/L) and pyrite (1 g).

Element	O	S	Fe	Ni	Cu	Au
Concentration (%)	22.9	17.4	3.00	0	0.50	56.2

shows the surface elemental contents of the gold foil leached in the presence of both Ni²⁺ and pyrite, while

Table 3. Atomic concentration of gold leached using the Cu²⁺-en-S₂O₃²⁻ leaching system in the presence of pyrite (1 g) only.

Element	O	S	Fe	Cu	Au
Concentration (%)	19.8	12.1	2.97	8.48	56.7

shows the elemental contents obtained when the gold foil was leached in the presence of pyrite only.

Figure 9a,b and

Table 4. The composition and proportion of substances in Figure 9a.

Binding Energy (eV)	Content (%)	Total Content (%)	Species
932.93	50.0	75.0	Cu2S
952.83	25.0
935.02	16.7	25.0	CuS/Cu(OH)2
954.92	8.3

and

Table 5. The composition and proportion of substances in Figure 9b.

Binding Energy (eV)	Content (%)	Total Content (%)	Species
932.70	14.8	22.2	Cu2S
952.60	7.42
935.06	34.2	51.3	CuS/Cu(OH)2
954.96	17.1
940.59	7.28	26.5	O2/Cu/Fe interface product
960.49	3.64
943.78	10.4
963.68	5.2

show the Cu 2p spectra of the surface of the gold foil that was leached in the Cu²⁺-en-S₂O₃²⁻ system containing pyrite with and without Ni²⁺, respectively, which were mainly Cu₂S [30] and CuS/Cu(OH)₂, respectively [31,32]. By comparing Table 2 and Table 3, it is found that the copper element content on the surface of the gold foil after leaching in the system containing Ni²⁺ and pyrite is much smaller than that when only pyrite is leached. Therefore, Ni²⁺ is likely to weaken the promoting effect of pyrite on the deposition of copper on the gold surface [5].

As shown in Table 3, after 34 h of leaching in the pyrite-only system, many Cu-containing substances were deposited on the gold surface; thus, its Cu content reached 8.48 at%. After fitting, it was found that Cu mainly existed in the form of a CuS passivation layer. The oxidation of pyrite by dissolved oxygen and copper (II) during gold leaching in the Cu²⁺-NH₃-S₂O₃²⁻ system is expressed using the following simplified formula (based on the HSC 6.0 database).

$$\begin{array}{c}2\mathrm{Fe}{\mathrm{S}}_2+\left(13/2\right){\mathrm{O}}_2+4\mathrm{Cu}(\mathrm{N}{\mathrm{H}}_3{)}_4^{2+}+7{\mathrm{H}}_2\mathrm{O}+12{\mathrm{S}}_2{\mathrm{O}}_3^{2-}=2\mathrm{FeO}·\mathrm{OH}\\ +4(\mathrm{N}{\mathrm{H}}_4{)}_2\mathrm{S}{\mathrm{O}}_4+4\mathrm{Cu}({\mathrm{S}}_2{\mathrm{O}}_3{)}_3^{5-}+8\mathrm{N}{\mathrm{H}}_3+4{\mathrm{H}}^{+}\\ (∆{\mathrm{G}}^{\mathsf{\theta } }=-616.1 \mathrm{kcal}/\mathrm{mol})\end{array}$$

(2)

The equilibrium constant logK of the Cu(S₂O₃)₃⁵⁻ complex is greater than that of Cu(en)₂⁺ (

Table 6. The equilibrium constants (logK^A) for the complexation of Ni²⁺, Cu⁺, Cu²⁺ with en, and S₂O₃²⁻: ionic strength (I = 0), 25 °C.

Metal Ion	Complexation	LogK
Ni2+	Ni(en)2+	7.52
	Ni(en)22+ NiS2O3	13.84 2.06
Cu+	Cu(en)2+ Cu(S2O3)35−	10.8 13.84
Cu2+	Cu(en)22+	20

A LogK taken from Lange’s Handbook of Chemistry, 15th ed. [33].

). Therefore, the Cu⁺ generated via the above formula more easily cooperates with S₂O₃²⁻ to produce Cu(S₂O₃)₃⁵⁻. which resulting in a decrease in the content of Cu(en)₂²⁺, thereby inhibiting the dissolution of gold in the system. In the copper–ethylenediamine–thiosulfate system, Cu(en)₂²⁺ is the substance that can effectively dissolve gold. The free Cu²⁺ in the system reacts with S₂O₃²⁻ and OH⁻ according to Equation (3), resulting in an increased adsorption of CuS on the surface of the gold foil. It can also be known from the literature that pyrite does indeed facilitate the formation of a passivation layer containing copper [7]. Therefore, it can be concluded that the addition of pyrite to the system will impede the dissolution of gold.

$$C{u}^{2+}+S_2{O}_3^{2-}+2O{H}^{-}=CuS+S{O}_4^{2-}+H_{2}O$$

(3)

When nickel ions and pyrite are added simultaneously, the copper content on the surface of the gold foil is significantly reduced to only 0.5%, primarily existing in the form of Cu₂S. This is because, after the addition of nickel ions and pyrite simultaneously, the Ni(en)₂²⁺ complex is formed by nickel ions in the system and en. Its equilibrium constant of 13.84 is much higher than that of Cu(en)₂⁺ (10.8), making Ni(en)₂²⁺ easier to form and more stable in the system. The formation of Ni(en)₂²⁺ promotes the reduction of Cu(en)₂²⁺ to Cu(S₂O₃)₃⁵⁻-, producing more Cu(S₂O₃)₃⁵⁻. However, with the consumption of S₂O₃²⁻, the Cu⁺, which originally formed Cu(S₂O₃)₃⁵⁻ complex with en is reduced. Therefore, the resulting small quantity of free Cu⁺ combines with the decomposition products of thiosulfate in the system to form Cu₂S (according to Equation (4)). Since the total amount of copper adsorbed on the gold surface in this system is only 0.5%, and the content of the copper-containing passivation layer is lower, the presence of nickel ions reduces the formation of the copper-containing passivation layer on the gold surface, resulting in faster dissolution of gold compared to when only pyrite is added. Thus, it can be observed that Ni²⁺ effectively weakens the promoting effect of pyrite on the formation of a copper-bearing passivation layer on the gold surface. Therefore, it is concluded that nickel ions and pyrite have a synergistic promoting effect on the dissolution of gold.

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

(4)

Figure 10a shows the S 2p spectrum corresponding to the surface of gold after leaching in the presence of 1 g of pyrite and Ni²⁺ (8 mg/L). It can be seen from the peak area in Figure 10 that after adding pyrite and Ni²⁺ for leaching, the main sulfur-containing species on the surface of the gold foil is sulfate. The main substance on the surface of the gold foil after leaching only containing pyrite is Cu₂S [32,34]. By comparing

Table 7. The composition and proportion of substances in Figure 10a.

Binding Energy (eV)	Content (%)	Total Content (%)	Species
161.68	12.0	18.0	CuS/Cu2S
162.86	6.0
162.86	12.5	18.8	FeS2
163.87	6.25
164.98	3.87	5.90	Na2SO3
166.15	1.93
168.69	38.2	57.3	SO42−
169.87	19.1

and

Table 8. The composition and proportion of substances in Figure 10b.

Binding Energy (eV)	Content (%)	Total Content (%)	Species
161.63	27.0	40.5	CuS/Cu2S
162.81	13.5
163.97	11.2	16.8	S
165.15	5.60
165.13	3.47	5.20	Na2SO3
166.31	1.73
168.75	25.0	37.5	SO42−
169.93	12.5

, it is found that the Cu₂S content of the sulphide on the surface of the gold foil after leaching in the system containing Ni²⁺ and pyrite is much smaller than that when only pyrite is leached. At the same time, when only pyrite is contained, the surface of the gold foil after leaching is covered with 16.8% of the elemental sulfur [35,36].

The above findings and the fact that thiosulphate consumption in the co-presence of pyrite and Ni²⁺ was much greater than that in the presence of pyrite only or Ni²⁺ only suggest that the S-containing substances are mainly produced via thiosulphate decomposition. Moreover, the increase in the amounts of sulfate deposited on the surface of the gold foil leached in the co-presence of Ni²⁺ and pyrite indicate that Ni²⁺ not only hinders the deposition of Cu on the gold surface, but also accelerates the decomposition of thiosulphate to sulphate.

Combined with Figure 11a,b and

Table 9. The composition and proportion of substances in Figure 11a.

Binding Energy (eV)	Content (%)	Total Content (%)	Species
706.91	2.6	3.9	FeS2
720.51	1.3
710.50	7.07	10.6	Fe2O3/Fe3O4
724.10	3.53
711.90	35.6	53.4	FeOOH
725.50	17.8
720.03	21.4	32.1	O2/Cu/Fe interface product
733.63	10.7

and

Table 10. The composition and proportion of substances in Figure 11b.

Binding Energy (eV)	Content (%)	Total Content (%)	Species
706.94	3.44	5.16	FeS2
720.54	1.69
710.87	49.0	73.5	Fe2O3/Fe3O4
724.47	24.5
717.60	10.7	16.1	O2/Cu/Fe interface product
731.20	5.40
719.92	3.49	5.24
733.52	1.75

, it is found that the iron-containing material on the surface of the gold foil after leaching in the system containing Ni²⁺ and pyrite is mainly FeOOH [5,37]. However, the gold surface after leaching in the system containing only pyrite contains a large amount of Fe₂O₃ [38,39], implying that the surface of the gold foil that was leached in the Cu²⁺-en-S₂O₃²⁻ system containing pyrite was covered with Fe₂O₃/Fe₃O₄, possibly due to the oxidation of pyrite and its dissolved Fe species.

Based on the above findings, the following conclusions were drawn. In the system containing only pyrite, the passivation film on the gold surface is mainly composed of Cu₂S, S₀, Fe₂O₃, etc. In the presence of both pyrite and Ni²⁺, the Cu species on the gold surface were no longer dominant and were largely replaced by Fe- and S-containing compounds, indicating that Ni²⁺ alleviated the deposition of Cu-containing materials on the surface of the gold foil. This greatly hindered the passivation of the gold foil and facilitated its contact with the leaching fluid to accelerate gold dissolution.

3.5.2. XPS Analysis of the Pyrite Surface after Leaching

Table 11. Atomic concentration of pyrite (1 g) leached in the Cu²⁺-en-S₂O₃²⁻ system in the presence Ni²⁺ (8 mg/L).

Element	O	S	Fe	Ni	Cu	Au
Concentration (%)	77.0	13.4	7.70	0.26	1.50	0.14

and

Table 12. Atomic concentration of pyrite (1 g) leached in the Cu²⁺-en-S₂O₃²⁻ system without Ni²⁺.

Element	O	S	Fe	Cu	Au
Concentration (%)	60.4	27.8	11.8	0	0

show the elemental composition of the pyrite surface after leaching in the presence of both Ni²⁺ and pyrite and pyrite only, respectively. Compared with the elemental composition of the pyrite surface obtained in the pyrite-only case, that obtained in the presence of both Ni²⁺ and pyrite contained small amounts of Au and Cu as well as surface-adsorbed Ni.

Figure 12a,b show the fitted Fe 2p spectra of pyrite recorded after leaching in the presence of 1 g of pyrite and Ni²⁺ (8 mg/L) and 1 g of pyrite only, respectively. The iron-containing substance on the pyrite surface after leaching in the presence of Ni²⁺ is FeOOH [12,40]. The surface of the pyrite leached in the Ni-free system showed mainly FeS₂ and Fe₂O₃/Fe₃O₄ (as show

Table 13. The composition and proportion of substances in Figure 12a.

Binding Energy (eV)	Content (%)	Total Content (%)	Species
706.51	4.26	6.4	FeS2
720.11	2.14
711.50	62.4	93.6	FeOOH
725.10	31.2

and

Table 14. The composition and proportion of substances in Figure 12b.

Binding Energy (eV)	Content (%)	Total Content (%)	Species
706.36	17.6	26.4	FeS2
719.96	8.8
710.87	28.4	42.6	Fe3O4
724.47	14.2
711.80	14.3	21.45	FeOOH
725.40	7.15
719.56	5.19	7.79	O2/Cu/Fe interface product
733.16	2.60
721.93	1.17	1.76
735.53	0.59

). Obviously, when Ni²⁺ and pyrite were present in the leaching system, FeOOH was formed on the surface of the leached pyrite. According to the literature, at pH values in the range 9–10, the oxidation potential of pyrite is as low as −0.4 to −0.3 V [8], and during gold leaching in the ammonia–thiosulphate system, pyrite can be oxidised to FeOOH by dissolved O₂ and Cu (II) (according to Equation (2)).

3.6. Tafel Curves

The electrokinetic polarisation of gold foil was studied in different systems within a scan range of −0.5–0 V and at a scan rate of 1 mV/s. The thus-obtained Tafel curves (Figure 13) were used to determine the Au corrosion potential (E_corr) as well as corrosion current density (j_corr) (

Table 15. Tafel parameters corresponding to gold foil in different solution systems.

System	Ecorr (V)	Jcorr (A)
Cu2+-en-S2O32−	−0.29	$1.27 \times { 10}^{-3}$
Cu2+-en-S2O32−-Ni2+	−0.08	$4.03 \times {10}^{-4}$
Cu2+-en-S2O32−-pyrite	−0.12	$4.14 \times {10}^{-4}$
Cu2+-en-S2O32−-pyrite-Ni2+	−0.37	$1.03 \times {10}^{-3}$

).

In the Cu²⁺-en-S₂O₃²⁻ system, a 1.5 cm × 1.5 cm piece of gold foil was used as the working electrode, and for blank controls, E_corr and j_corr were determined as −0.29 V and 1.27 × 10⁻³ A, respectively. In the presence of Ni²⁺ (8 mg/L) only, E_corr increased to −0.08 V, while j_corr decreased to 4.03 × 10⁻⁴ A, indicating a weakening of the corrosion of the gold foil surface. Furthermore, in the presence of pyrite only, the Fe-containing substances resulting from pyrite oxidation formed a precipitate and attached to the electrode surface, further hindering the oxidation of Au. Therefore, compared with the blank Cu²⁺-en-S₂O₃²⁻ system, the electrode corrosion potential increased, while the current density decreased. When both Ni²⁺ (8 mg/L) and pyrite were added to the leaching system, E_corr decreased, indicating that the gold foil became more prone to corrosion. This finding is consistent with the results of the leaching experiments, indicating that the synergistic effect of Ni²⁺ and pyrite on gold dissolution could be ascribed to the decrease in E_corr and the increase of j_corr. Considering the results of XPS analysis obtained for leached gold foil as well, it became evident that the addition of both Ni²⁺ and pyrite significantly hindered the adhesion of Cu₂S or CuS on the gold surface, and under these conditions, the main compound that was deposited on the Au foil surface during leaching was identified as FeOOH. Furthermore, SEM imaging revealed that these precipitates that were deposited on the gold foil surface after leaching were loose and amorphous; hence, their inhibitory effect on gold foil corrosion was limited.

4. Conclusions

In this paper, the effects of nickel ions and pyrite on the amount of reagent and the amount of gold extraction in a copper-thylenediamine-thiosulfate leaching system were studied. The synergistic effect of nickel ions and pyrite on gold dissolution in a Cu²⁺-en-S₂O₃²⁻ leaching system was discussed in detail, and the related mechanism was studied. The following conclusions were drawn:

• The addition of Ni²⁺ or pyrite to the Cu²⁺-en-S₂O₃²⁻ gold-leaching system could significantly inhibit the dissolution of gold and promote the decomposition of thiosulfate.
• When both pyrite and Ni²⁺ were simultaneously present in the leaching system, they synergistically promoted gold dissolution. The synergistic action of these two substances could increase the concentration of gold to 22.5 mg/L·cm².
• In the pyrite-only Cu²⁺-en-S₂O₃²⁻ gold-leaching system, the surface of the gold foil was found to contain a large amount of copper-containing sulphide layers after leaching. However, hardly any Cu was detected on the surface of the gold foil that was leached in the co-presence of pyrite and Ni²⁺. Thus, Ni²⁺ ions effectively weakened the promotional effect of pyrite on the formation of the Cu-containing passivation layer on the gold surface.
• Tafel polarisation curves showed that the addition of Ni²⁺ or pyrite only increased the corrosion potential of Au; thus, gold dissolution was inhibited. However, in the presence of both Ni²⁺ and pyrite, the corrosion potential of Au decreased, resulting in an acceleration of the amount of gold dissolution.