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Article

Thermodynamics of Methylamine and Ammonia Synergy in Copper-Catalyzed Thiosulfate Gold Leaching

by
Heng He
,
Yongbin Yang
*,
Lin Wang
*,
Guangliang Wu
,
Dan Wang
,
Qian Li
,
Yan Zhang
,
Shichao He
and
Tao Jiang
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China
*
Authors to whom correspondence should be addressed.
Metals 2026, 16(3), 323; https://doi.org/10.3390/met16030323
Submission received: 8 February 2026 / Revised: 6 March 2026 / Accepted: 11 March 2026 / Published: 13 March 2026
(This article belongs to the Special Issue Metal Leaching and Recovery)
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Abstract

Thiosulfate leaching is considered a promising alternative to cyanidation for gold extraction because it can be achieved at a low cost. However, existing leaching systems struggle to balance leaching efficiency with thiosulfate consumption. Herein, a novel synergistic Cu-CH3NH2-NH3 leaching system was proposed, balancing thiosulfate consumption and gold leaching efficiency through a mixed-ligand strategy. Thermodynamic analysis revealed that the steric hindrance and electron-donating effects of methylamine effectively block the oxidative decomposition pathway of thiosulfate by Cu(II), significantly reducing thiosulfate consumption. However, this also reduced the dissolution rate of gold. By introducing ammonia to adjust the Cu(II) coordination environment, the system achieved a gold leaching rate of 88.6% with a thiosulfate consumption of 14.2 kg/t-ore, significantly outperforming the traditional Cu-NH3 system. In this system, the gold leaching process mainly is catalyzed by the mixed-ligand complex Cu(NH3)x(CH3NH2)4−x2+. Within the coordination sphere, the methyl group of CH3NH2 inhibits the axial attack of S2O32− on Cu(II) via electron-donating and steric hindrance effects, thereby blocking the redox pathway of S2O32−; simultaneously, NH3 provides active sites to promote the gold oxidation. This study provides a vital theoretical basis and technical support for developing green, low-cost, and high-efficiency gold leaching processes.

1. Introduction

Cyanidation has long been the primary process for gold production due to the low cost of cyanide [1]. However, cyanidation struggles to treat complex and carbonaceous-type ores, and environmental protection requirements also limit its application. This has led to the development of various non-cyanide lixiviants, including thiourea, halides, thiocyanates, and thiosulfate. Among these, the thiourea process is restricted by high reagent costs and potential carcinogenicity [2]. Halide leaching faces challenges related to severe equipment corrosion and toxicity [3]. The thiocyanate system is constrained by its instability in the presence of ferric (Fe3+) ions [4]. Consequently, thiosulfate leaching is regarded as the most promising non-cyanide alternative, owing to its nontoxicity, lower cost, and capability to treat complex and carbonaceous-type ores [1,5,6,7].
The conventional ammonia system (Cu2+-NH3-S2O32−) is composed of thiosulfate, copper ions (Cu2+), and ammonia (NH3). In this system, thiosulfate coordinates with gold (Au+) ions to form the stable Au(S2O3)23− (logK = 26.4) [3], while ammonia acts as a ligand coordinating with Cu(II) to generate the Cu(NH3)42+ complex. The core reaction during the gold extraction process is described in Equation (1).
Au + 5 S2O32− + Cu(NH3)42+ → Au(S2O3)23− + 4 NH3 + Cu(S2O3)35−
In this system, thiosulfate will inevitably undergo oxidation because the reduction potential of the Cu(NH3)42+/Cu(S2O3)35− couple (+0.192 V) is significantly higher than that of the S4O62−/S2O32− couple (+0.022 V) [8]. Thiosulfate gradually decomposes into polythionates (S3O62−, S4O62−) and sulfite (SO32−), eventually forming sulfate (SO42−), as shown in Equations (2)–(5) [9]. This redox behavior follows an inner-sphere mechanism (Figure 1) [10]. The square-planar Cu(NH3)42+ complex possesses open space at the axial positions, allowing S2O32− to easily attack these sites and undergo axial coordination, thereby generating redox intermediates that promote electron transfer. The inherent defects of the conventional copper-ammonia system hinder its industrial application, specifically: (i) the strong oxidizing power of Cu(II) leads to excessive thiosulfate consumption; (ii) the oxidative decomposition products of S2O32− tend to adsorb onto the gold surface [11], forming a passivation layer that hinders gold leaching; and (iii) the high volatility of NH3 causes system instability and environmental issues. To address these limitations, various novel oxidation systems have been proposed to replace the traditional copper-ammonia thiosulfate system. Regarding metal ion oxidants, Fe(III), Co(III), and Ni(II) have been explored as alternatives to Cu(II). While these systems can reduce thiosulfate consumption, they also have significant drawbacks. Co and Ni are expensive and lack widespread availability in minerals [12], and Fe(III) tends to form hydroxide precipitates in alkaline media and exhibits sluggish leaching kinetics [13,14]. In addition, the substitution of NH3 with organic ligands, such as ethylenediamine, tartrate, citrate, and malate, has garnered significant attention. Although these ligands effectively suppress thiosulfate consumption, they are generally constrained by sluggish leaching kinetics [15,16,17]. For example, the malate system requires heating to 60 °C during leaching. Therefore, it is urgent to develop a novel gold leaching system featuring both low thiosulfate consumption and rapid kinetic characteristics.
2 Cu(NH3)42+ + 8 S2O32− → 2 Cu(S2O3)35− + 8 NH3 + S4O62−
4 S4O62− + 6 OH− → 5 S2O32− + 2 S3O62− + 3 H2O
2 S3O62− + 6 OH− → S2O32− + 4 SO32− + 3 H2O
2 SO32− + O2 → 2 SO42−
As the simplest organic amine, methylamine (CH3NH2) features a methyl group (-CH3) acting as an electron donor. This electron-donating effect enhances the electron density on the nitrogen atom. Fábián [18] reported that the chemical reactivity of Cu-CH3NH2 complexes is approximately 3–4 times lower than that of Cu-NH3 complexes. This reduction is attributed to the steric hindrance introduced by the methyl group, which effectively shields the Cu(II) center from attack by S2O32−, thereby inhibiting inner-sphere redox reactions. However, a trade-off exists regarding leaching kinetics. Chandra et al. [19] observed that while the methylammonium ion facilitates the anodic oxidation of gold, its effect is weaker than that of the ammonium ion, suggesting that the steric hindrance of the methyl group also impedes gold oxidation rates. From a physical perspective, methylamine exhibits a saturated vapor pressure of 350.6 kPa under ambient conditions (25 °C, 1 atm), which is significantly lower than that of ammonia (1003.9 kPa) [20,21]. This lower volatility presents a distinct advantage in mitigating environmental risks and reducing reagent consumption.
While current research highlights the potential of methylamine to reduce thiosulfate consumption, its limitations regarding gold dissolution kinetics are equally apparent. Furthermore, a systematic understanding of the leaching performance and underlying reaction mechanisms of methylamine within thiosulfate systems remains lacking. In this study, we compared the thermodynamic differences between ammonia and methylamine systems to identify the predominant Cu(II) complexes. Additionally, leaching experiments using pure gold were conducted to elucidate the mechanisms and kinetic characteristics of the methylamine system. To overcome the performance bottlenecks associated with single-ligand systems, a mixed-ligand Cu-CH3NH2-NH3 system was developed, and its synergistic leaching mechanism was elucidated. Ultimately, this work aims to provide a theoretical basis for the development of novel, low-consumption, and high-efficiency thiosulfate gold leaching processes.

2. Materials and Methods

2.1. Materials

The run-of-mine gold ore samples used in this study were obtained from China National Gold Group Co., Ltd. (Beijing, China). As indicated by the X-ray diffraction (XRD) analysis (Figure 2), the ore is primarily composed of quartz, anorthite, augite, and aluminum silicon oxide. The ore was ground to a fineness of 80% passing −35 μm and stored in sealed plastic bags to minimize oxidation prior to leaching and analysis. Table 1 presents the multi-element chemical analysis, revealing a gold grade of 1.33 g/t, with Fe, S, and Cu contents of 5.47%, 1.14%, and 0.001%, respectively. Additionally, high-purity gold foil (99.999%, 10 × 10 × 1 mm) was utilized for the pure gold leaching experiments. The methylamine aqueous solution was purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Other reagents, including sodium thiosulfate (Na2S2O3·5H2O), ammonium hydroxide (NH3·H2O), and copper sulfate (CuSO4·5H2O), were obtained from Aladdin Chemical Reagent Co. (Shanghai, China). Ultrapure water was used throughout the experiments.

2.2. Experimental Procedure and Method

Prior to the pure gold leaching experiments, the gold foils were polished using 4000-grit sandpaper, followed by ultrasonic cleaning in ethanol and rinsing with ultrapure water. Both pure gold and gold ore leaching experiments were conducted in 250 mL semi-open reactors equipped with mechanical stirring set at 300 rpm. Leaching solutions were prepared by adding predetermined amounts of CuSO4·5H2O, NH3·H2O, aqueous CH3NH2, and Na2S2O3·5H2O to 100 mL of water, in accordance with the experimental design. The pH was adjusted using dilute H2SO4 or NaOH. The experiments were initiated by either adding 25 g of gold ore or suspending the gold foil in the center of the solution. All tests were maintained at 25 °C using a water bath. Over a total reaction period of 8 h, slurry samples were collected at intervals of 1, 2, 4, 6, and 8 h. The collected samples were centrifuged and filtered prior to subsequent analysis. The dissolution performance of pure gold was expressed as the mass of gold dissolved per unit area (g/m2).

2.3. Analytical and Characterization Methods

The mineralogical phase composition of the samples was analyzed using an X-ray diffractometer (XRD, Bruker D8-Advance, Bruker, Billerica, MA, USA) [22,23]. The multi-elemental composition of the ore was determined via inductively coupled plasma optical emission spectrometry (ICP-OES, SPECTRO BLUE, SPECTRO, Kleve, Germany), while the gold content was independently quantified using the fire assay method. Speciation diagrams of Cu(II) complexes were generated using the Medusa-Hydra software (version 2.0). For the leaching solutions, the concentrations of sulfur-oxy anions were measured immediately upon sampling using high-performance liquid chromatography (HPLC, Shimadzu LC-20A, Shimadzu, Kyoto, Japan), and dissolved gold concentrations were determined by atomic absorption spectroscopy (AAS, Thermo iCE3500, Thermo Fisher Scientific, Waltham, MA, USA). Furthermore, to investigate the structural influence of ammonia (NH3) on the Cu(II)-methylamine complexes, the solid compounds were characterized using XRD and Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA). In addition, the speciation of Cu(II) in solution was characterized by UV-vis spectrophotometry (UV-vis, Shimadzu UV2600, Shimadzu, Kyoto, Japan). Complementing the experimental characterization, theoretical calculations were performed using the Gaussian 16 software package [24]. Geometry optimizations and energy calculations were conducted at the B3LYP functional level [25] in conjunction with the def2svp basis set [26] to describe the electron wave functions of all atoms. To account for solvation effects, the Polarizable Continuum Model (PCM) [27] was employed with water specified as the solvent. Vibrational frequency analyses were performed to verify the stationary points, and the absence of imaginary frequencies confirmed that the optimized structures correspond to local minima on the potential energy surface.

3. Results and Discussion

3.1. Thermodynamics on Comparison of Methylamine to Ammonia in Cu-Complexes as Catalysts for Gold Leaching with Thiosulfate

During the gold leaching process, ligands primarily influence the oxidative decomposition of thiosulfate by altering the coordination environment of Cu(II). In general, greater electrochemical stability of Cu(II)-complexes leads to less thiosulfate oxidation. Methylamine shares similar coordination chemistry with ammonia due to the presence of a lone pair of electrons on the nitrogen atom [28]. However, the methyl group changes the basicity and steric structure of the methylamine molecule, significantly affecting its behavior in the gold leaching system. The speciation distribution of ammonia and methylamine at different pH values (Figure 3) shows that methylamine (pKa = 10.5) is more basic than ammonia (pKa = 9.25) [29,30]. Therefore, at the same pH, methylamine is more readily protonated. To identify the predominant Cu(II) complexes under practical leaching conditions, the speciation of Cu complexes in both the methylamine (Figure 4) and ammonia (Figure 5) systems was simulated across varying reagent concentrations and pH ranges. As shown in Figure 4a, within the optimal pH range for gold leaching (10–11) in the methylamine system, Cu(II) exists primarily as Cu(CH3NH2)42+, while Cu(I) is present as Cu(S2O3)35−. These species constitute the Cu(CH3NH2)42+/Cu(S2O3)35− redox couple responsible for catalyzing gold dissolution. In contrast, the optimal leaching pH for the ammonia system typically lies between 9.5 and 10.5 (Figure 5a). This shift is attributed to the higher basicity of methylamine compared to ammonia. Therefore, a higher pH is required. Under these conditions, molecular CH3NH2 predominates over the protonated CH3NH3+ ion.
Further investigation was conducted to examine the effects of copper (Figure 4b), methylamine (Figure 4c), and thiosulfate (Figure 4d) concentrations. The results show that Cu(CH3NH2)42+ and Cu(S2O3)35− consistently remain the thermodynamically dominant species within the standard reagent concentration ranges. This thermodynamic prediction was experimentally corroborated by UV-Vis absorption spectroscopy (Figure 6). Under varying pH levels and methylamine concentrations, a characteristic absorption peak at 600 nm was consistently observed, which aligns with the reported value for Cu(CH3NH2)42+ [31]. A comparison with the copper speciation in the ammonia system reveals that Cu(NH3)42+ and Cu(S2O3)35− are the corresponding predominant species. Notably, the Cu speciation distributions in the methylamine and ammonia systems exhibit a high degree of similarity. This resemblance arises because both CH3NH2 and NH3 form their most stable Cu(II) complexes in a four-coordinate geometry. Furthermore, the stability constants for Cu(CH3NH2)42+ (logK = 12.08) [29] and Cu(NH3)42+ (logK = 12.7) [32] are nearly identical.
Thermodynamic speciation analysis indicated that Cu(CH3NH2)42+ is the dominant Cu(II) complex in the methylamine system. The electronic structure of Cu(II) complexes determines their electrochemical stability and their ability to oxidize thiosulfate. Density Functional Theory (DFT) calculations were employed to determine the lowest-energy optimized structures and Frontier Molecular Orbitals (FMOs) for both Cu(CH3NH2)42+ and Cu(NH3)42+ (Figure 7). As illustrated in Figure 7a, steric hindrance introduced by the methyl groups causes Cu(CH3NH2)42+ to deviate from the square planar geometry typical of Cu(NH3)42+. This distorted spatial configuration effectively hinders the axial attack of S2O32−. Table 2 presents the Cu-N bond lengths and the Mulliken charge distribution for both complexes. Typically, a higher positive Mulliken charge correlates with increased electrophilicity, making the species more susceptible to nucleophilic attack. The calculated results show that the central Cu(II) ion in Cu(NH3)42+ has a higher Mulliken charge (0.594) than that in Cu(CH3NH2)42+ (0.557), indicating that the Cu(II) center in the ammonia complex is more susceptible to nucleophilic attack by S2O32−. Furthermore, the nitrogen atoms in Cu(CH3NH2)42+ exhibit a higher absolute negative Mulliken charge, confirming that the electron-donating effect of the methyl group enhances the electron density on the nitrogen atoms. However, this electron-donating effect does not result in bond shortening. Instead, steric hindrance from the methyl groups prevents the ligands from approaching the Cu center more closely, offsetting the electrostatic attraction. This structural characteristic also accounts for the observation that the stability constant of Cu(CH3NH2)42+ is slightly lower than that of Cu(NH3)42+. Considering that chemical reactivity is governed by the HOMO-LUMO energy gap (ΔEg) [33,34], the comparison in Figure 7b is significant. The larger energy gap of Cu(CH3NH2)42+ (ΔEg = 3.76 eV) relative to Cu(NH3)42+ (ΔEg = 3.53 eV) indicates superior electrochemical stability. This finding provides theoretical substantiation for the suppressed oxidative decomposition of thiosulfate observed in the methylamine system.

3.2. Comparison of Methylamine to Ammonia in Cu-Complexes on Gold Dissolution Behavior in Thiosulfate Solutions

Since the conventional ammonia system is the most widely used method for thiosulfate gold leaching [35], it was used as a benchmark to evaluate the gold dissolution behavior and thiosulfate stability of the methylamine system. To eliminate interference from ore impurities, the leaching behavior of pure gold foils was investigated under the respective optimal leaching conditions determined for each system. As illustrated in Figure 8, the amount of dissolved gold in both systems exhibits a linear increase over time. However, the gold dissolution rate in the methylamine system is slightly lower than that in the ammonia system. This kinetic disparity can be attributed to the reaction mechanism, in which small-molecule ligands must adsorb onto the gold surface to form an intermediate species, such as Au(S2O3)(NH3)2−, to facilitate electron transfer [36], as described in Equations (6)–(8). In the case of methylamine, the methyl group introduces steric hindrance during this adsorption process, thereby reducing reactivity. Conversely, the methylamine system demonstrates a significant advantage regarding thiosulfate conservation. Specifically, thiosulfate consumption in the ammonia system reached 31.3%, whereas it was limited to only 17.7% in the methylamine system.
Figure 9 presents the distribution of thiosulfate oxidation products for the ammonia and methylamine systems. In both systems, tetrathionate (S4O62−) and trithionate (S3O62−) were identified as the primary oxidation intermediates. In the ammonia system, tetrathionate and trithionate are rapidly further oxidized into other sulfur-containing species. In contrast, the oxidation rate of these intermediates is significantly retarded in the methylamine system, resulting in their higher accumulation in the solution. This disparity indicates that the steric hindrance exerted by methylamine not only suppresses the initial decomposition of thiosulfate but also inhibits the subsequent Cu(II)-catalyzed oxidation of intermediate species [10]. This suggests that methylamine, functioning as a ligand for Cu(II), provides comprehensive inhibition of sulfur species oxidative decomposition throughout the entire reaction process.
Au + S2O32− + NH3 → Au(S2O3)(NH3)2−
Au(S2O3)(NH3)2− → Au(S2O3)(NH3)− + e−
Au(S2O3)(NH3)− + S2O32− → Au(S2O3)23− + NH3
Based on the thermodynamic analysis in Section 3.1 and the comparative study of pure gold leaching in this section, the methylamine system exhibits stronger inhibition of thiosulfate decomposition than the traditional ammonia system, although the gold leaching efficiency is slightly lower. This disparity arises from the different Cu(II) coordination environments: the steric hindrance and electron-donating effect of the methyl group inhibit nucleophilic attack by S2O32− while also hindering gold oxidation. Previous studies have established that gold dissolution in the ammonia system is catalyzed by the Cu(NH3)42+/Cu(S2O3)35− redox couple [37]. Drawing upon this established mechanism, the proposed electrochemical catalytic mechanism for gold leaching in the methylamine system is presented in Figure 10. At the anodic zone on the gold surface, Au0 first undergoes adsorption to form the intermediate species Au(S2O3)(CH3NH2)2−, followed by electron transfer, ultimately generating Au(S2O3)23−. Simultaneously, in the cathodic zone, Cu(CH3NH2)42+ acts as the electron acceptor for the electrons generated at the anode and is reduced to Cu(S2O3)35−. Finally, dissolved oxygen in the solution serves as the oxidant, regenerating Cu(CH3NH2)42+ from Cu(S2O3)35−. The primary reactions within the system are described in Equations (9)–(13).
Au + S2O32− + CH3NH2 → Au(S2O3)(CH3NH2)2−
Au(S2O3)(CH3NH2)2− → Au(S2O3)(CH3NH2)− + e−
Au(S2O3)(CH3NH2)− + S2O32− → Au(S2O3)23− + CH3NH2
Cu(CH3NH2)42+ + 3 S2O32− + e− → Cu(S2O3)35− + 4 CH3NH2
2 Cu(S2O3)35− + 8 CH3NH2 + 1/2 O2 + H2O → 2 Cu(CH3NH2)42+ + 6 S2O32−+ 2 OH−

3.3. Comparison of Methylamine to Ammonia in Cu-Complexes on Gold Leaching with Thiosulfate from a Gold Ore

Comparative leaching experiments on pure gold showed that the electron-donating and steric effects of the methyl group suppress thiosulfate consumption in the methylamine system, although the steric hindrance also slows gold leaching kinetics. However, in actual ore leaching, gold occurrence and impurity elements in the mineral matrix strongly influence both gold dissolution and thiosulfate oxidation. Therefore, to evaluate the practical leaching performance of the methylamine system and its applicability to the ore, the effects of pH and reagent concentration on gold extraction and thiosulfate consumption were systematically investigated in both the ammonia and methylamine systems. The results are presented in Figure 11 and Figure 12.
As illustrated in Figure 11a and Figure 12a, solution pH exerts a significant influence on gold dissolution, with both excessively high and low pH levels proving detrimental to the process. At low pH levels, the ligands in the system predominantly exist in their protonated forms. This shift causes copper ions to exist primarily as Cu(S2O3)35−, subsequently reducing both the mixed potential of the system and the gold extraction rate. Furthermore, acidic conditions induce the hydrolysis of thiosulfate, as described in Equation (14), resulting in substantially increased thiosulfate consumption. Conversely, an excessively high pH promotes the formation of CuO precipitates [38], thereby diminishing the concentration of effective copper complexes in the solution. It is noteworthy that the optimal pH for the methylamine system (10.5) is slightly higher than that for the ammonia system (10.0). This difference is attributed to the higher pKa of methylamine (10.5) compared to ammonia (9.25). Consequently, a higher alkalinity is required to ensure a sufficient concentration of free ligands to maintain the stability of Cu(II) complexes.
4 S2O32− + O2 + 4 H+ → 2 S4O62− + 2 H2O
The copper ion concentration is a pivotal factor determining the redox potential of the system. An excessively low redox potential hinders gold dissolution, whereas an excessively high copper concentration leads to the over-oxidation of thiosulfate, resulting in substantial consumption [39]. Furthermore, the generated CuS and S4O62− can form passivation films on the Au surface, thereby impeding dissolution [11,32]. As shown in Figure 11b and Figure 12b, both systems exhibit the phenomenon where high copper concentrations exacerbate thiosulfate decomposition and inhibit gold leaching. However, the optimal catalyst dosage differs markedly between the two systems: the ammonia system requires a copper concentration of 20 mM, whereas the methylamine system requires only 2 mM. This indicates that the methylamine system requires a significantly lower Cu concentration to facilitate gold dissolution. Figure 11c and Figure 12c illustrate the influence of ligand concentration on leaching performance. The results indicate that excessively low ligand concentrations hinder gold dissolution because insufficient ligands destabilize Cu(II), accelerate thiosulfate oxidation, and promote the formation of a passivation layer on the gold surface. As the ligand concentration increases, the gold extraction rate improves. However, beyond a certain threshold, the improvement becomes marginal, and in the methylamine system, a slight declining trend in extraction rate is observed. This is attributed to the fact that higher ligand concentrations enhance the stability of Cu(II), which lowers the reduction potential of the cathodic half-cell reaction (Equation (12)), thereby reducing the gold oxidation rate. Additionally, elevated ligand concentrations stabilize Cu(NH3)42+ and Cu(CH3NH2)42+, minimizing the presence of highly oxidizing Cu(II) species in the solution and thus reducing thiosulfate consumption. Notably, the optimal ligand concentration for the methylamine system is 1.5 M, only half that of the ammonia system (3.0 M), accompanied by a significant reduction in thiosulfate consumption.
From a thermodynamic perspective, thiosulfate ions significantly lower the oxidation potential of gold in aqueous media, promoting the formation of Au(S2O3)23− [37]. As shown in Figure 11d and Figure 12d, increasing the thiosulfate concentration within a certain range favors gold leaching. However, when the concentration is excessively high, the gold extraction rate remains plateaued or even decreases. This is attributed to the formation of passivation films resulting from the oxidation of excess thiosulfate, which hinders gold dissolution.
In summary, the optimal process conditions for the ammonia system were determined to be pH 10.0, 20 mM Cu, 3.0 M NH3, and 0.3 M Na2S2O3. Under these conditions, the system achieved a gold extraction rate of 87.2% with a thiosulfate consumption of 35.9 kg/t-ore. In contrast, the optimal conditions for the methylamine system were: pH 10.5, 2 mM Cu, 1.5 M CH3NH2, and 0.3 M Na2S2O3. This system significantly reduced reagent dosages and lowered thiosulfate consumption to 10.1 kg/t-ore. However, its gold extraction rate of 69.8% remained notably lower than that of the ammonia system. Consequently, in practical ore leaching scenarios, while the methylamine system demonstrates remarkable efficacy in minimizing thiosulfate consumption, it currently faces the limitation of slightly lower leaching efficiency.

3.4. Building of the Methylamine-Ammonia Synergistic System for Cu-Complex Catalysts

While the steric hindrance and electron-donating effects of the methyl group effectively suppress thiosulfate oxidation, they simultaneously increase the steric difficulty of ligand coordination with the gold surface. This hinders gold dissolution and attenuates the anodic oxidation process, ultimately resulting in lower leaching efficiency. Existing literature indicates that ammonia can effectively elevate the mixed potential of the system and enhance the anodic half-reaction of gold, thereby promoting oxidative dissolution [40,41,42]. Consequently, a synergistic leaching system combining NH3 and CH3NH2 was built in this study, aiming to leverage the high reactivity of ammonia to compensate for the kinetic limitations of the methylamine system. While maintaining a constant total ligand concentration (1.5 M), the effect of the NH3 proportion on the synergistic leaching performance was investigated, and the results are shown in Figure 13. The results indicate that as the proportion of NH3 increased from 0% to 50%, the gold extraction rate increased significantly from 69.8% to 88.6%. This suggests that NH3 significantly accelerated the gold leaching kinetics. Meanwhile, thiosulfate decomposition remained nearly unchanged due to the presence of methylamine. However, when the ammonia dosage exceeded 50%, a slight decline in the gold extraction rate was observed. This is likely because intensified thiosulfate decomposition produces oxidation products that passivate the gold surface.
As the proportion of NH3 increased, the fraction of Cu(NH3)42+ in the solution also increased. This led to intensified thiosulfate decomposition, with consumption increasing from 12.9 kg/t-ore to 25.4 kg/t-ore. Notably, the product distribution analysis in Figure 12b reveals that the degree of oxidation of S3O62− and S4O62− in the leachate was higher than that in the pure gold foil system. From this, it can be inferred that other metal ions present in the mineral matrix (e.g., Fe3+, Al3+) catalyzed the conversion of S3O62− and S4O62− into SO42−.

3.5. Mechanisms of the Synergistic Effects of Composite Cu-Complexes

Both NH3 and CH3NH2 influence the gold leaching kinetics and thiosulfate consumption of the system by altering their coordination modes with Cu(II). To verify the influence of NH3 on the formation of Cu(II) complexes within the methylamine system, the characteristic absorption peaks of the different systems were characterized using UV-Vis spectrophotometry, as shown in Figure 14. The Cu(CH3NH2)42+ complex exhibits a characteristic peak at 600 nm, which aligns with the wavelength reported by Agarwala [31] and is distinct from the characteristic peak of Cu(NH3)42+ at 605 nm [43]. Notably, the addition of NH3 induces changes in the absorption peak of Cu(CH3NH2)42+, characterized by a decrease in intensity and a slight redshift. Similar phenomena have been reported previously and interpreted as the formation of mixed-ligand Cu(II) complexes [41,44]. Upon the addition of S2O32−, a new characteristic peak appears at 335 nm, as shown in Figure 14b. This has been previously reported as the characteristic absorption peak of the mixed Cu-NH3-S2O32− intermediate [45]. A comparison reveals that the peak intensity at this wavelength in the methylamine system is significantly lower than that in the ammonia system. This indicates that the steric hindrance caused by the methyl group in Cu(CH3NH2)42+ significantly impedes the coordination of S2O32− with the complex, thereby inhibiting the inner-sphere electron transfer reaction between Cu(II) and S2O32− [10]. Furthermore, the absorption intensity at 335 nm for the mixed system (Cu-CH3NH2-NH3) lies between those of the methylamine and ammonia systems. This trend is consistent with the thiosulfate consumption trends observed in the mineral experiments (Ammonia > Synergistic > Methylamine).
To elucidate the specific coordination structure of Cu(II) in the Cu-NH3-CH3NH2 system, solid complexes of Cu-NH3, Cu-CH3NH2, and Cu-NH3-CH3NH2 were precipitated via ethanol-induced crystallization. After freeze-drying, the products were characterized using XRD and FTIR, with the results presented in Figure 15. The XRD results indicate that the solid Cu-NH3-CH3NH2 complex retains the high-intensity characteristic diffraction peaks of the solid Cu-NH3 complex, suggesting that both possess a similar crystal configuration. Notably, a shift of certain diffraction peaks toward lower angles was observed. This indicates that the larger CH3NH2 molecules partially substituted NH3 molecules, leading to lattice expansion within the crystal structure [46]. FTIR spectral analysis further corroborates this conclusion. In the FTIR spectrum of the solid Cu-NH3-CH3NH2 complex, an absorption peak observed at 1280 cm−1 is attributed to the bending vibration of C-H bonds [47], while the strong single absorption peak at 1643 cm−1 and the band at 3200 cm−1 correspond to the N-H deformation and stretching vibrations of the primary amine in CH3NH2 [48]. This confirms the formation of the mixed-ligand Cu-NH3-CH3NH2 molecule. Combining these results, it is inferred that the primary Cu(II) complex in the synergistic system is Cu(NH3)x(CH3NH2)4−x2+. In this synergistic ligand environment, the strong electron-donating effect and steric hindrance of CH3NH2 effectively inhibit the formation of the Cu(II)-S2O32− redox intermediate, thereby reducing thiosulfate consumption. Meanwhile, the NH3 ligands retain reactive sites that facilitate electron transfer between Cu(II) and Au, thus ensuring a rapid leaching kinetics. Figure 16 compares and summarizes the distinct ligand mechanisms and leaching performances of the Cu-NH3 system, the Cu-CH3NH2 system, and the synergistic Cu-CH3NH2-NH3 system.

4. Conclusions

In thiosulfate gold leaching, balancing leaching efficiency with thiosulfate stability remains a core challenge for achieving a green and efficient gold extraction process. This study proposed and constructed a novel synergistic Cu-CH3NH2-NH3 leaching system. This system overcomes the limitations of single-ligand systems, achieving excellent leaching efficiency while significantly reducing thiosulfate consumption. The main conclusions are as follows:
(i)
A comparison of the thermodynamic speciation and stability of Cu complexes in the methylamine and ammonia systems indicates that the dominant complex of Cu(II) with CH3NH2 is Cu(CH3NH2)42+. Furthermore, Cu(CH3NH2)42+ (Δ Eg = 3.76 eV) exhibits higher electrochemical stability than Cu(NH3)42+ (Δ Eg = 3.53 eV), and the steric hindrance caused by the methyl group effectively prevents the axial attack of S2O32− on the Cu(II) center.
(ii)
Comparative studies of leaching behavior and thiosulfate stability confirm that using methylamine as the ligand for Cu(II) effectively suppresses thiosulfate oxidation throughout the process. However, its gold leaching efficiency is lower than that of the traditional ammonia system. This reveals the conflict between gold leaching efficiency and thiosulfate stability in single-ligand systems.
(iii)
Application studies on gold ore leaching demonstrate that while the gold extraction rate of the methylamine system (69.8%) is lower than that of the traditional ammonia system (87.2%), its thiosulfate consumption is only 10.1 kg/t-ore. This represents a 71.9% reduction compared to the ammonia system (35.9 kg/t-ore), validating its effectiveness in reducing reagent consumption.
(iv)
The construction of the synergistic methylamine-ammonia system successfully overcomes the limitations of single-ligand systems. The optimized synergistic system achieves a gold extraction rate of 88.6%, surpassing that of the traditional ammonia system, while maintaining a reagent consumption of only 14.2 kg/t-ore. This achieves a balance between high leaching efficiency and low thiosulfate consumption.
(v)
UV-Vis spectroscopy and solid-phase characterization confirm the formation of Cu(NH3)x(CH3NH2)4−x2+. Its characteristic absorption peak lies between those of the pure ammonia and pure methylamine systems, and the intensity of the characteristic peak for the Cu(II)-S2O32− active intermediate is significantly reduced. This demonstrates that CH3NH2 within the coordination sphere effectively modulates the redox activity of the copper center, thereby reducing the generation of passivation products; simultaneously, NH3 provides active sites that promote the oxidation reaction of gold, thus ensuring leaching efficiency. Additionally, the lower vapor pressure of methylamine mitigates pollution caused by volatilization. This system provides a new theoretical basis and technical route for developing green, low-cost gold ore leaching processes.

Author Contributions

Conceptualization, H.H. and Y.Y.; methodology, T.J. and Y.Y.; software, G.W. and L.W.; validation, Y.Z. and L.W.; formal analysis, S.H. and G.W.; investigation, Y.Z.; resources, T.J.; data curation, D.W. and S.H.; writing—original draft preparation, H.H. and L.W.; writing—review and editing, Q.L. and T.J.; visualization, Q.L. and D.W.; supervision, Y.Y.; project administration, Y.Z.; funding acquisition, S.H. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support from the National Natural Science Foundation of China (Grant No. 52404310) and the Scientific and Technological Project of Yunnan Precious Metals Laboratory (YPML-20240502095) is gratefully acknowledged.

Data Availability Statement

The original contributions presented in the study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Metals 16 00323 g001
Figure 1. Oxidation of SO32− by Cu(II) producing S4O62−.
Figure 1. Oxidation of SO32− by Cu(II) producing S4O62−.
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Metals 16 00323 g002
Figure 2. XRD patterns of the gold ore.
Figure 2. XRD patterns of the gold ore.
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Metals 16 00323 g003
Figure 3. Species distribution diagram of CH3NH2 and NH3 under different pH values. (0.1 mol/L CH3NH2 or NH3, Eh = 0.25 V vs. SHE, and 298 K).
Figure 3. Species distribution diagram of CH3NH2 and NH3 under different pH values. (0.1 mol/L CH3NH2 or NH3, Eh = 0.25 V vs. SHE, and 298 K).
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Metals 16 00323 g004
Figure 4. Effects of (a) Eh, (b) Cu, (c) CH3NH2, (d) S2O32− on the speciation of Cu complexes in Cu-CH3NH2-S2O32−-H2O system at 25 °C and 1 atm. (Eh 0.1 V, pH = 10, 0.1 M S2O32−,1 M CH3NH2, 0.01 M Cu).
Figure 4. Effects of (a) Eh, (b) Cu, (c) CH3NH2, (d) S2O32− on the speciation of Cu complexes in Cu-CH3NH2-S2O32−-H2O system at 25 °C and 1 atm. (Eh 0.1 V, pH = 10, 0.1 M S2O32−,1 M CH3NH2, 0.01 M Cu).
Metals 16 00323 g004
Metals 16 00323 g005
Figure 5. Effects of (a) Eh, (b) Cu, (c) NH3, (d) S2O32− on the speciation of Cu complexes in Cu-NH3-S2O32−-H2O system at 25 °C and 1 atm. (Eh 0.1 V, pH = 10, 0.1 M S2O32−,1 M NH3, 0.01 M Cu).
Figure 5. Effects of (a) Eh, (b) Cu, (c) NH3, (d) S2O32− on the speciation of Cu complexes in Cu-NH3-S2O32−-H2O system at 25 °C and 1 atm. (Eh 0.1 V, pH = 10, 0.1 M S2O32−,1 M NH3, 0.01 M Cu).
Metals 16 00323 g005
Metals 16 00323 g006
Figure 6. (a) UV-Vis spectra at different pH values for a solution of 0.01 M Cu and 1 M CH3NH2. (b) UV-Vis spectra at different CH3NH2 concentrations for a solution of 0.01 M Cu at pH = 10.
Figure 6. (a) UV-Vis spectra at different pH values for a solution of 0.01 M Cu and 1 M CH3NH2. (b) UV-Vis spectra at different CH3NH2 concentrations for a solution of 0.01 M Cu at pH = 10.
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Metals 16 00323 g007
Figure 7. (a) Structures of optimized Cu(NH3)42+ and Cu(CH3NH2)42+ complexes and (b) their FMO distributions and energy values.
Figure 7. (a) Structures of optimized Cu(NH3)42+ and Cu(CH3NH2)42+ complexes and (b) their FMO distributions and energy values.
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Metals 16 00323 g008
Figure 8. Comparison of (a) gold dissolution kinetics and (b) thiosulfate consumption in the methylamine and ammonia systems.
Figure 8. Comparison of (a) gold dissolution kinetics and (b) thiosulfate consumption in the methylamine and ammonia systems.
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Metals 16 00323 g009
Figure 9. Distribution of sulfur-containing species during the leaching process in (a) the ammonia system and (b) the methylamine system.
Figure 9. Distribution of sulfur-containing species during the leaching process in (a) the ammonia system and (b) the methylamine system.
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Metals 16 00323 g010
Figure 10. Electrochemical catalytic mechanism of gold leaching in the methylamine system.
Figure 10. Electrochemical catalytic mechanism of gold leaching in the methylamine system.
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Metals 16 00323 g011
Figure 11. The effect of (a) pH, (b) Cu(II), (c) NH3·H2O and (d) Na2S2O3 concentration on gold leaching amount and thiosulfate consumption. (20 mM CuSO4, 3 M NH3·H2O, 0.3 M Na2S2O3, pH 10, 25 °C, 8 h).
Figure 11. The effect of (a) pH, (b) Cu(II), (c) NH3·H2O and (d) Na2S2O3 concentration on gold leaching amount and thiosulfate consumption. (20 mM CuSO4, 3 M NH3·H2O, 0.3 M Na2S2O3, pH 10, 25 °C, 8 h).
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Metals 16 00323 g012
Figure 12. The effect of (a) pH, (b) CH3NH2, (c) Cu(II) and (d) Na2S2O3 concentration on gold leaching amount and thiosulfate consumption. (2 mM CuSO4, 1.5 M CH3NH2, 0.2 M Na2S2O3, pH 10, 25 °C, 8 h).
Figure 12. The effect of (a) pH, (b) CH3NH2, (c) Cu(II) and (d) Na2S2O3 concentration on gold leaching amount and thiosulfate consumption. (2 mM CuSO4, 1.5 M CH3NH2, 0.2 M Na2S2O3, pH 10, 25 °C, 8 h).
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Metals 16 00323 g013
Figure 13. Effect of ammonia concentration on (a) gold leaching percentage and thiosulfate consumption, and (b) the distribution of sulfur-containing species in the Cu-CH3NH2-NH3 synergistic system (2 mM CuSO4, 1.5 M CH3NH2 and NH3, 0.3 M Na2S2O3, pH 10, 25 °C, 8 h).
Figure 13. Effect of ammonia concentration on (a) gold leaching percentage and thiosulfate consumption, and (b) the distribution of sulfur-containing species in the Cu-CH3NH2-NH3 synergistic system (2 mM CuSO4, 1.5 M CH3NH2 and NH3, 0.3 M Na2S2O3, pH 10, 25 °C, 8 h).
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Metals 16 00323 g014
Figure 14. (a) Visible and (b) UV absorption spectra of the solution species in the Cu-NH3, Cu-CH3NH2, and synergistic Cu-NH3-CH3NH2 systems.
Figure 14. (a) Visible and (b) UV absorption spectra of the solution species in the Cu-NH3, Cu-CH3NH2, and synergistic Cu-NH3-CH3NH2 systems.
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Metals 16 00323 g015
Figure 15. (a) XRD patterns and (b) FTIR spectra of the solid Cu(II) complexes from the Cu-NH3, Cu-CH3NH2, and synergistic Cu-NH3-CH3NH2 systems.
Figure 15. (a) XRD patterns and (b) FTIR spectra of the solid Cu(II) complexes from the Cu-NH3, Cu-CH3NH2, and synergistic Cu-NH3-CH3NH2 systems.
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Metals 16 00323 g016
Figure 16. Action mechanisms of different ligands and gold leaching performance of the systems.
Figure 16. Action mechanisms of different ligands and gold leaching performance of the systems.
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Table 1. Chemical composition of the gold ore/%.
Table 1. Chemical composition of the gold ore/%.
ElementAu (g/t)AlFeCaKMgNaCuPbAsS
Content1.336.405.473.733.302.262.740.0010.00180.00741.14
Table 2. Bond lengths and Mulliken atomic charges of Cu(CH3NH2)42+ and Cu(NH3)42+ complexes.
Table 2. Bond lengths and Mulliken atomic charges of Cu(CH3NH2)42+ and Cu(NH3)42+ complexes.
Bond/AtomCu(CH3NH2)42+Cu(NH3)42+
Bond Lengths (Å)Cu-N12.0722.050
Cu-N22.0622.050
Cu-N32.0682.049
Cu-N42.0652.050
Mulliken ChargesCu0.5570.594
N1−0.247−0.171
N2−0.244−0.171
N3−0.251−0.171
N4−0.248−0.171
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He, H.; Yang, Y.; Wang, L.; Wu, G.; Wang, D.; Li, Q.; Zhang, Y.; He, S.; Jiang, T. Thermodynamics of Methylamine and Ammonia Synergy in Copper-Catalyzed Thiosulfate Gold Leaching. Metals 2026, 16, 323. https://doi.org/10.3390/met16030323

AMA Style

He H, Yang Y, Wang L, Wu G, Wang D, Li Q, Zhang Y, He S, Jiang T. Thermodynamics of Methylamine and Ammonia Synergy in Copper-Catalyzed Thiosulfate Gold Leaching. Metals. 2026; 16(3):323. https://doi.org/10.3390/met16030323

Chicago/Turabian Style

He, Heng, Yongbin Yang, Lin Wang, Guangliang Wu, Dan Wang, Qian Li, Yan Zhang, Shichao He, and Tao Jiang. 2026. "Thermodynamics of Methylamine and Ammonia Synergy in Copper-Catalyzed Thiosulfate Gold Leaching" Metals 16, no. 3: 323. https://doi.org/10.3390/met16030323

APA Style

He, H., Yang, Y., Wang, L., Wu, G., Wang, D., Li, Q., Zhang, Y., He, S., & Jiang, T. (2026). Thermodynamics of Methylamine and Ammonia Synergy in Copper-Catalyzed Thiosulfate Gold Leaching. Metals, 16(3), 323. https://doi.org/10.3390/met16030323

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