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Article

Gold and Silver Recovery from a Refractory Pyritic Concentrate by Roasting and Alkaline Pressure Oxidation

by
Ana María Espinoza-Martínez
1,*,
Jesús Leobardo Valenzuela-García
1,
María Mercedes Salazar-Campoy
1,*,
Martín Antonio Encinas-Romero
1,
Guadalupe Martínez-Ballesteros
1 and
José Refugio Parga Torres
2
1
Department of Chemical Engineering and Metallurgy, Universidad de Sonora, Hermosillo 83000, Mexico
2
Department of Materials Science and Metallurgy, Instituto Tecnológico de Saltillo, Saltillo 25280, Mexico
*
Authors to whom correspondence should be addressed.
Minerals 2025, 15(12), 1260; https://doi.org/10.3390/min15121260 (registering DOI)
Submission received: 29 October 2025 / Revised: 19 November 2025 / Accepted: 25 November 2025 / Published: 28 November 2025
(This article belongs to the Section Mineral Processing and Extractive Metallurgy)
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Abstract

Refractory gold and silver ores present significant challenges because precious metals are encapsulated within sulfide matrices, severely limiting extraction by conventional cyanidation. In this study, a pyritic concentrate from the Bacis Mine (Durango, Mexico) was characterized and subjected to two oxidative pretreatments—roasting and alkaline pressure oxidation—before cyanidation. X-ray diffraction confirmed pyrite to be the dominant phase, with quartz and minor carbonates contributing to the material’s refractory character. Roasting at 550 °C achieved gold and silver extraction of 80% and 70%, respectively, which improved to 89% Au and 74% Ag with the addition of hydrogen peroxide. In contrast, alkaline pressure oxidation at 150 °C and 1 MPa O2 yielded the highest extraction of 92% for Au and 76% for Ag at 1 h. Thermodynamic analysis using the Fe–S Pourbaix diagram at 80 °C supported these experimental results, showing the destabilization of FeS2 under oxidizing and moderately alkaline conditions. Overall, this study demonstrates that alkaline pressure oxidation is a technically efficient and environmentally favorable pretreatment for refractory gold ores.

1. Introduction

Refractory gold and silver ores are characterized by the fine dissemination and encapsulation of precious metals within sulfide minerals, mainly pyrite and arsenopyrite. These associations hinder the penetration of cyanide and oxygen, limiting dissolution and making conventional cyanidation processes inefficient, with low recoveries and high reagent consumption [1]. Similar limitations have been recently documented for fine-grained refractory ores containing ultrafine gold inclusions [2].
To overcome these limitations, oxidative pretreatments are commonly applied to decompose the sulfide matrix, liberate the precious metals, and enhance their exposure to cyanide solutions [3]. Nevertheless, traditional oxidative methods such as roasting generate environmentally hazardous byproducts, mainly SO2 and H2SO4, which present serious environmental challenges and require costly abatement systems [4]. Despite these drawbacks, cyanidation remains the predominant leaching method for precious metals worldwide due to its simplicity, cost-effectiveness, and efficiency when applied to non-refractory ores [5]. For refractory ores, however, pretreatments tailored to the mineralogical characteristics are required [5,6]. Various authors emphasize the importance of optimizing pretreatment conditions to improve the liberation of gold locked within sulfide matrices [7,8,9,10,11]. The dissolution of gold and silver in cyanide solutions requires oxygen as an oxidant, leading to the formation of stable aurocyanide and argento-cyanide complexes [5]:
4 Au + 8 NaCN + O2 + 2 H2O → 4 NaAu(CN)2 + 4 NaOH
4 Ag + 8 NaCN + O2 + 2 H2O → 4 NaAg(CN)2 + 4 NaOH
However, in refractory ores, the metals are encapsulated in sulfide or telluride phases, preventing cyanide and oxygen from accessing the mineral surfaces. For this reason, pretreatment is required to oxidize the sulfide matrix prior to cyanidation.
Roasting is one of the oldest and most widely applied pretreatments [12]. It oxidizes sulfide and telluride minerals, releasing the associated gold and silver and increasing their accessibility to cyanide solutions [4]. The Fe–S–O system at 600 °C was constructed using HSC Chemistry 10 Outotec, Espoo, Finland (Figure 1). Pyrite undergoes oxidation first to magnetite and subsequently to hematite under oxidizing conditions [5,12]:
3 FeS2 (s) + 8 O2 (g) → Fe3O4 (s) + 6 SO2 (g)
Although technically effective, roasting has the significant drawback of generating large amounts of SO2 gas, which requires expensive gas-cleaning systems to comply with environmental regulations [4,12]. Additional evaluations of roasting performance have shown significant environmental and operational limitations, particularly in high-sulfur concentrates [13,14,15,16].
As an alternative, pressure oxidation has been widely studied and implemented as one of the most efficient pretreatments for refractory ores, achieving gold recovery rates of up to 99.5% in subsequent cyanidation [6]. The process is conducted in autoclaves under controlled conditions, either in acidic or alkaline media, allowing for the complete decomposition of pyrite, arsenopyrite, and carbonates in short residence times (1–3 h). Alkaline-pressure oxidation (POX) is particularly advantageous for ores containing carbonates or moderate sulfide contents, as it avoids excessive acid consumption while ensuring efficient sulfide oxidation. Under these conditions, pyrite decomposes according to [6]:
2 FeS2 + 7.5 O2 + 7 H2O → 2 Fe(OH)3 + 8 H+ + 4 SO42−
Compared to roasting, alkaline-pressure oxidation is an oxidative pretreatment performed in autoclaves at elevated temperature and pressure, enabling rapid sulfide decomposition and enhanced gold and silver recoveries while minimizing environmental emissions [6,17]. Advances in alkaline POX have demonstrated improvements in sulfur stabilization and reduced acid formation relative to acidic systems [12,18,19].
The stability of iron sulfide phases under varying pH and Eh conditions can be further understood through Pourbaix diagrams. The Fe–S Pourbaix diagram at 80 °C (Figure 2) shows that pyrite (FeS2) is stable under reducing, acidic, and near-neutral conditions, which explains its persistence in natural deposits and its resistance to direct cyanidation. At higher Eh values and moderately alkaline pH, ferric oxides and hydroxides (e.g., Fe2O3, FeOH species) become stable, corresponding to the oxidative breakdown of sulfides [20]. Elevated temperatures narrow the stability field of FeS2 and expand those of ferric oxides and hydroxides, reinforcing the thermodynamic and kinetic roles of temperature in sulfide decomposition and phase transformation [21]. Thermodynamic assessments of the Fe–S–O–H system support these stability trends and provide valuable guidance for pressure-oxidation design [22,23]. These insights are essential for designing effective pretreatments and predicting long-term sulfide stability.
These stability fields help explain the behavior observed in experimental work. Under acidic to near-neutral and low-Eh conditions, pyrite remains thermodynamically stable, which accounts for its resistance to direct cyanidation. Conversely, at higher Eh and moderately alkaline pH, the formation of ferric oxides and hydroxides becomes favorable, promoting sulfide breakdown and increasing the exposure of gold and silver during leaching. This thermodynamic framework provides the basis for understanding the differences in performance between roasted and pressure-oxidized samples.
A refractory pyritic concentrate obtained from the Bacis Mine—an underground operation located in Durango, Mexico—was used in this study. This long-standing mining complex processes approximately 2000 tons per day of argentiferous ore and is recognized for producing sulfide-rich concentrates with significant silver values. The mineralization consists primarily of pyrite, argentiferous pyrite, sphalerite, chalcopyrite, argentite, and electrum hosted in quartz veins. This operational scale and ore type highlight the practical relevance of developing cost-effective pretreatment strategies—such as alkaline pressure oxidation—for improving the extraction of precious metals from refractory pyritic concentrates. The main objective of this study is to evaluate and compare the efficiency of roasting and alkaline pressure oxidation (POX) as pretreatment methods for improving gold and silver recovery from a refractory pyritic concentrate. Emphasis is placed on understanding the mineralogical transformations, thermodynamic behavior, and leaching kinetics that influence extraction performance.

2. Materials and Methods

A representative pyritic concentrate sample containing gold and silver was obtained, then homogenized and ground to produce different particle-size fractions. The particle size ranges −200/+150, −250/+200, and −270/+250 Tyler mesh (104, 74, 61 µm approx.) were selected because they represent typical grinding intervals used in industrial gold concentrators and correspond to conventional classification ranges for sulfide flotation concentrates. These fractions allow assessing the influence of particle size on oxidation behavior and subsequent cyanide leaching performance, with an overall 80% passing size (Figure 3).
The physical characterization of the concentrate was performed using a Phenom ProX desktop scanning electron microscope (Thermo Fisher Scientific, Eindhoven, The Netherlands) equipped with an integrated energy-dispersive X-ray spectroscopy (SEM–EDS) detector, and X-ray diffraction (XRD) PANalytical X’Pert PRO diffractometer (Malvern Panalytical B.V., Almelo, The Netherlands) was used to identify the primary mineralogical phases. The precious metal content was determined by fire assay using a flux mixture of borax, sodium carbonate, and litharge. Additional chemical analyses for copper, iron, and zinc were performed using atomic absorption spectrometry (AAS) with a PerkinElmer AAnalyst 400 Atomic Absorption Spectrometer (PerkinElmer Inc., Waltham, MA, USA). These base metals are relevant because they act as cyanicides, consuming cyanide during leaching and thereby reducing precious metal extraction.
Cyanidation experiments were carried out under ambient conditions (25 °C) using a 0.5 L glass reactor—a slurry containing 20 wt.% solids were prepared by mixing the pretreated material with tap water. Sodium cyanide (NaCN) was added to achieve an initial concentration of 5 g/L. Before the addition of cyanide, lime (CaO) was introduced to adjust and maintain the pulp pH at 11. This alkaline condition was essential to prevent volatilization of toxic hydrogen cyanide gas (HCN) and to ensure optimal stability of free cyanide ions during leaching. The tests were designed to evaluate the kinetics of gold and silver extraction. For this purpose, 25 mL aliquots of the slurry were collected at 1, 2, 4, 8, and 24 h of leaching (Table 1 and Table 2).
Each sample was immediately filtered to separate the liquid and solid phases. The pregnant solutions were analyzed for Au and Ag content using atomic absorption spectroscopy (AAS), and the concentration of free cyanide was also determined to monitor reagent consumption. The corresponding solid residues were dried, homogenized, and analyzed by fire assay to quantify the residual gold and silver values. Overall extraction efficiency and kinetics were assessed by comparing the dissolved metal concentrations with those remaining in the solid residues. A representative example of the leaching profiles is presented in Figure 4.
Roasting experiments were conducted using 150 g of concentrate per test under the conditions summarized in Table 3.
To evaluate the effect of alkaline additives, an additional experiment was conducted by adding 10 g of magnesium oxide (MgO) to neutralize part of the sulfur compounds generated during roasting. The roasting step was performed in a Thermolyne 6000 Laboratory Furnace (Thermo Scientific, Waltham, MA, USA) under oxidizing conditions. After roasting, 50 mL of hydrogen peroxide (H2O2) was added to the calcine before cyanidation. The purpose of this step was to oxidize residual sulfide species and increase calcine porosity, thus enhancing cyanide accessibility to encapsulated precious metals (Figure 5).
Pressure oxidation tests were conducted in a 1 L Parr 4520 series titanium pressure reactor (Parr Instrument Company, Moline, IL, USA) (Figure 6), selected for its corrosion resistance and ability to withstand high temperatures and pressures—a slurry containing 20 wt.% solids were prepared with tap water, and lime (CaO) was added to maintain the pulp pH above 11. This alkaline environment minimized silica dissolution and stabilized iron as hydroxides. The experimental variables included particle size (104, 74, and 61 μm), temperature (80, 120, and 150 °C), and oxygen partial pressure (73, 110, and 145 psi). The particle size fractions represented typical grinding ranges used in mineral processing, while the temperature and oxygen pressures were selected to cover both mild and aggressive oxidation regimes. Before sealing, the reactor vessel was purged with oxygen to eliminate residual air. Once sealed, it was pressurized to the target oxygen pressure and maintained under agitation at 600 rpm using a mechanical stirrer to ensure uniform suspension and efficient gas–liquid mass transfer. Given the highly exothermic nature of pyrite oxidation, the reactor temperature was monitored continuously during POX tests, and external heating was adjusted as needed to maintain stable operating conditions and prevent thermal runaway. Each test lasted 1 h. At the end of the test, the reactor was quenched to room temperature by immersion in a water bath to halt further reactions. The resulting pulp was filtered, and both solid residues and leachates were collected for subsequent chemical and mineralogical analyses. The operating conditions are summarized in Table 4.

3. Results

3.1. Mineralogical Characterization

Microstructural and point-microchemical observations of the untreated concentrate, as shown in Figure 7, reveal angular and irregular particles, which are characteristic of flotation-derived pyrite concentrates. The grains exhibit massive textures with uneven cleavage surfaces and locally brighter microdomains associated with fine inclusions embedded within the pyrite matrix. These features are typical of refractory sulfide materials in which gold and silver occur as submicrometric inclusions or electrum hosted within internal fractures or structural defects. Energy-dispersive spectroscopy (EDS) of the selected spot confirms that the examined area is dominated by iron and sulfur in proportions consistent with pyrite (FeS2) as the principal phase. Peaks of Si and O are attributed to quartz (SiO2), whereas minor Al reflects aluminosilicate gangue. Trace signals of Cu and Zn indicate the presence of small amounts of chalcopyrite or sphalerite, consistent with the bulk chemical composition. The presence of gold, even at low levels, supports its occurrence as finely disseminated inclusions within pyrite, reinforcing the concentrate’s refractory nature and emphasizing the need for oxidative pretreatment to improve precious-metal accessibility during cyanidation.
X-ray diffraction (XRD) patterns of the untreated concentrate (Figure 8) revealed pyrite (FeS2) and quartz (SiO2) to be the main crystalline phases for particle sizes of (a) 104 µm, (b) 74 µm, and (c) 61 µm, respectively. In the finest fraction (61 µm), calcite (CaCO3) was also detected, confirming the refractory nature of the concentrate and indicating high acid consumption if acidic pretreatments were applied. Similar results have been reported for carbonate-bearing ores, where acid consumption reduces efficiency [5,24]. Therefore, alkaline pressure oxidation is a more suitable alternative.
XRD detected pyrite and quartz (and calcite in the 61 µm fraction) because these phases occur in sufficiently high abundance to generate well-defined diffraction peaks. Other sulfide minerals known to be present in the concentrate, such as chalcopyrite and sphalerite, occur at much lower proportions or are finely disseminated and fall below the detection threshold of the technique. This behavior is common in complex sulfide concentrates, where minor phases are either poorly crystalline or too diluted to be resolved by XRD.
To provide a more precise representation of the inferred mineralogical composition from the XRD patterns, mineralogical reconstruction was prepared to summarize the relative proportions of the principal crystalline phases in the untreated concentrate. Table 5 presents the estimated distribution of the main minerals identified by XRD.
After roasting at 550 °C (Figure 9), with particle sizes of (a) 104 µm, (b) 74 µm, and (c) 61 µm, respectively, pyrite and quartz persisted, while hematite (Fe2O3) appeared as a new phase. In the 61 µm sample, pyrrhotite (Fe1−xS) was also identified, reflecting an intermediate oxidation stage. These results agree with Marsden and House 4, who described the transformation of pyrite and pyrrhotite to hematite during oxidative roasting. Similar phase evolution has been documented by Hu et al. [25], Aracena et al. [26], and Zhang et al. [27], who also highlighted the increased porosity of the calcine and the resulting improvement in gold recovery. The presence of hematite is particularly relevant, as it indicates effective sulfide oxidation and reduction of precious metal encapsulation, a key factor for subsequent cyanidation. Previous studies have confirmed that the conversion of pyrite to hematite enhances gold and silver recovery by facilitating cyanide diffusion [28].
Fire Assay and atomic absorption analyses are summarized in Table 6, showing contents of Au and Ag (g/t) and Cu, Fe, and Zn (wt.%). Copper, iron, and zinc act as cyanicides, consuming cyanide and oxygen to form stable complexes Cu(CN)43−, Zn(CN)42−, and Fe(CN)63− that compete with the complexation of gold and silver. This phenomenon increases reagent consumption and reduces leaching kinetics, ultimately extending the residence time required for efficient extraction of precious metals [29,30].
In agreement with Marsden and House [5], the detrimental effects of cyanicides are particularly pronounced in refractory ores, where the simultaneous presence of sulfide minerals further hinders metal extraction. Previous studies [29,32] have reported that even low concentrations of copper and zinc can significantly increase cyanide consumption. At the same time, high iron contents may also promote the precipitation of secondary phases (e.g., jarosite or ferric hydroxide), which encapsulate gold particles. These results confirm that the chemical composition of the concentrate not only defines its refractory character but also emphasizes the need for a practical pretreatment step to minimize cyanide losses and improve gold and silver recovery.

3.2. Conventional Cyanidation Test

Gold leaching under atmospheric conditions showed a slight advantage at smaller particle sizes. For example, the test conducted with 74 µm particles achieved 48% extraction, compared with 46% for 104 µm particles, after 24 h of leaching at a NaCN concentration of 5 g/L (Figure 10).
These results are consistent with the observations reported by Nunan et al. [21], Soto et al. [12], and [33] for gold and copper extraction, where decreasing particle size generally enhances dissolution due to the increased surface area. At 61 µm, recovery dropped to 32%, confirming other reports [34,35] that smaller particles do not always improve leaching. Excessive grinding can lead to unfavorable effects, such as particle agglomeration, poor permeability of the leach solution, or even the sintering of ultra-fine particles, ultimately resulting in marginal or lower-than-expected extraction values. Figure 11 shows the silver leaching results for concentrates without pretreatment, conducted over 24 h using a NaCN concentration of 5 g/L. Higher extractions were observed with decreasing particle size, reaching 67% for the concentrate with a particle size of 61 µm, followed by 60% for the 74 µm fraction and 50% for the coarser 104 µm fraction.
Although the trend confirms that smaller particle sizes generally improve silver dissolution by increasing surface area, the overall recoveries remain relatively low. This behavior reflects the refractory nature of the ore, in which silver is partially encapsulated within sulfide minerals or associated with secondary phases resistant to cyanidation. Similar observations have been reported in previous studies.
Xie and Dreisinger [36] noted that silver dissolution in cyanide solutions is slower than gold, often limited by surface passivation and the formation of intermediate species, such as Ag2S. Adams [37] emphasized that in refractory ores, silver is commonly associated with jarosite, chlorargyrite, or other stable mineral phases that are not efficiently oxidized during direct cyanidation. Furthermore, Deschênes and Prud’homme [28] demonstrated that without an oxidative pretreatment, silver recoveries rarely exceed 60–70%, highlighting the need for processes such as pressure oxidation or roasting to improve leachability.

3.3. Roasting Effect on Extraction

Roasted concentrates at 550 °C were leached under ambient conditions for 24 h using [NaCN] = 5 g/L. For a particle size of 74 µm, 80% extraction was achieved, whereas for a particle size of 104 µm, only 46% extraction was obtained. Additionally, a test with the addition of hydrogen peroxide (H2O2) showed a significant improvement, achieving an 89% extraction, consistent with previous studies [17,21,38] (Figure 12).
According to Hinojosa [24], the particle sizes that lead to favorable results in roasting processes generally range from −75 µm to −150 µm. Therefore, identifying the optimal particle size is crucial to achieve satisfactory oxidation: coarse particles tend to result in incomplete roasting. In contrast, excessively fine particles may undergo sintering, which reduces gold extraction [15]. Silver extraction results are presented in Figure 13, showing maximum recovery of 70% and 62% for particle sizes of 74 µm and 104 µm, respectively.
An additional test incorporating hydrogen peroxide (H2O2) resulted in a 5% increase over the corresponding baseline experiment, achieving 74% silver extraction. This improvement confirms the beneficial effect of intensified oxidation, consistent with previous reports [39]. It is also noteworthy that maximum silver recoveries are generally achieved during the initial hours of leaching, whereas gold requires longer contact times under similar conditions.

3.4. Pressure Effect on Oxidative Pretreatment

The influence of oxygen pressure on gold leaching during oxidative pretreatment was evaluated in an autoclave (Figure 14). The highest extraction (92%) was obtained at 145 psi, confirming the strong pressure dependence of refractory sulfide oxidation. At lower pressures, recoveries decreased to 73% (110 psi) and 72% (73 psi) for a particle size of 61 µm. Overall, increasing oxygen pressure enhanced sulfide oxidation and improved gold exposure and dissolution.
Silver extraction from oxidized concentrates treated under different oxygen pressures is shown in Figure 15. The highest extraction (76%) was obtained at 145 psi, followed by 60% at 110 psi and 53% at 73 psi. These results confirm that higher oxygen pressures promote more effective silver leaching under the evaluated conditions (61 µm particle size, 80 °C). This behavior is consistent with the role of oxygen pressure in accelerating sulfide oxidation [40], thereby releasing encapsulated precious metals. However, silver exhibited lower recoveries than gold under similar pretreatment conditions, which can be attributed to slower cyanidation kinetics [41], surface passivation, and association with secondary minerals. Phases such as argentojarosite or chlorargyrite remain stable under conventional leaching conditions [42], limiting silver recovery in refractory ores.
The influence of temperature on gold extraction during oxidative pretreatment was evaluated in an autoclave, followed by cyanidation with 5 g/L NaCN under ambient conditions (Figure 16).
The concentrate oxidized at 150 °C achieved the highest extraction (92%), compared with 82% at 120 °C and 73% at 80 °C. These results demonstrate that higher temperatures significantly enhance sulfide oxidation, facilitating gold liberation and dissolution. Such behavior aligns with previous studies showing that increasing pretreatment temperature accelerates the oxidation of pyrite and pyrrhotite, producing porous hematite that exposes occluded gold [5,40]. Elevated temperature and pressure promote rapid sulfide breakdown and higher recoveries, whereas suboptimal conditions lead to incomplete oxidation and residual sulfides that hinder gold dissolution. These results confirm that 150 °C provides an optimal balance between reaction kinetics and mineral transformation, enabling efficient sulfide oxidation and improved gold extraction.
Silver leaching from autoclaved oxidized concentrates at different temperatures is shown in Figure 17. The highest extraction (76%) was obtained at 150 °C, where most silver dissolution occurred during the initial hours of leaching, in contrast with the slower behavior at lower temperatures. At 120 °C, extraction decreased to 70%, while the lowest extraction (59%) was observed at 80 °C. These results demonstrate that temperature has a strong influence on the kinetics of silver dissolution. Higher pretreatment temperatures promote more extensive sulfide oxidation, thereby improving silver accessibility during cyanidation [24,43].

3.5. Comparative Evaluation of Oxidative Pretreatments for Gold and Silver Extraction

Gold extraction was carried out under ambient conditions using 5 g/L NaCN, pH ≈ 11, constant agitation (350 rpm), and a residence time of 24 h. Among the four pretreatments, the highest extraction (92%) was obtained from the pressure-oxidized concentrate at 61 µm, treated at 150 °C and 1 MPa O2. Roasting at 550 °C with a particle size of 74 µm resulted in 80% extraction, consistent with Zhang et al. [27], who reported ~85% extraction at 500–600 °C due to porosity generated during calcination. However, at temperatures > 600 °C, hematite tends to sinter, reducing extraction. When hydrogen peroxide (H2O2) was added to the roasted concentrate (550 °C, 74 µm), extraction improved to 89%, attributable to further sulfide oxidation and increased cyanide accessibility. According to Soto et al. [12], particle size reduction significantly enhances recovery. Rusanen et al. [40] demonstrated that elevated temperatures and oxygen pressures strongly promote refractory sulfide oxidation within a short time frame (30–60 min). The different extraction behaviors of gold and silver reflect their mineralogical associations within the concentrate. Gold typically occurs as fine inclusions within pyrite grains, requiring more extensive sulfide oxidation and longer leaching times for full exposure. In contrast, silver is partially hosted in more reactive minerals such as argentite or argentojarosite, which dissolve rapidly under cyanidation conditions. This explains why silver extraction reaches near-complete saturation within the first hours, whereas gold continues to increase over the full leaching period. These factors explain the improved performance observed in this study. In contrast, the untreated concentrate confirmed its refractory nature, yielding only 48% extraction at 74 µm (Figure 18).
Silver extraction by conventional cyanidation at atmospheric pressure was performed using 5 g/L NaCN, a pH ≈ of approximately 11, constant agitation (350 rpm), and a residence time of 24 h. For the roasted concentrate treated at 550 °C with a particle size of 74 µm, extraction reached 70%. When hydrogen peroxide (H2O2) was added under the same roasting conditions, extraction increased to 74%. In contrast, the untreated concentrate yielded only 60% silver extraction, confirming the importance of oxidative pretreatment in enhancing leachability (Figure 19).
The distinct kinetic behavior observed in the gold and silver extraction profiles reflects their different mineralogical associations and the diffusion-influenced nature of cyanidation in partially oxidized sulfide residues. Gold is mainly hosted as ultrafine inclusions within pyrite or as electrum encapsulated in the sulfide matrix, requiring progressive exposure as oxidation products dissolve; therefore, its extraction increases steadily over the full 24 h. In contrast, part of the silver occurs in more reactive and readily accessible phases—such as argentite or electrum located along grain boundaries—which dissolve rapidly, leading to an early plateau in silver extraction within the first hours. This divergence confirms that mass-transfer limitations become increasingly significant as the reaction front advances during cyanidation of oxidized concentrates. Pourbaix diagram of the Fe–S system at 80 °C (Figure 2) provides a thermodynamic framework for interpreting the experimental results. Pyrite (FeS2) remains stable at low Eh values and in acidic to near-neutral solutions, which explains the limited extraction observed during direct cyanidation (≤48% Au, ≤60% Ag). These conditions favor sulfur retention as sulfide, restricting the spontaneous oxidation required to liberate precious metals. At higher Eh and moderately alkaline pH, the diagram predicts the stabilization of ferric oxides and hydroxides (e.g., Fe2O3, FeOH species), consistent with the experimental formation of hematite during roasting at 550 °C. This transformation increases porosity and improves cyanide access to occluded gold and silver, yielding 80% Au and 70% Ag recoveries. The addition of hydrogen peroxide further enhanced the oxidation of residual sulfides, producing up to 89% Au and 74% Ag, in agreement with similar oxidative intensification strategies reported by Nunan et al. [21] and Guzmán et al. [20]. In contrast, alkaline pressure oxidation offered superior performance. Under optimized conditions (150 °C, 1 MPa O2, 61 µm), extractions reached 92% Au and 76% Ag after only 1 h. These values surpass those obtained by roasting and are comparable to those reported by Rusanen et al. [40] and Bidari & Aghazadeh [44] for pyrite concentrates treated under high-pressure conditions. The improvement is attributed to the complete decomposition of sulfides and the stabilization of iron as hydroxides, which prevents excessive acid consumption while ensuring efficient exposure of precious metals. Temperature effects were also evident. Increasing the pretreatment temperature from 80 to 150 °C significantly enhanced gold and silver dissolution, confirming the dual thermodynamic and kinetic role of temperature in accelerating sulfide oxidation and facilitating the formation of porous ferric oxides. The experimental results align with the Pourbaix predictions, which indicate a contraction of the FeS2 stability field at elevated temperature and expansion of ferric oxide domains. Alkaline pressure oxidation is not only technically more efficient than roasting but also environmentally preferable, as it avoids SO2 emissions and minimizes acid generation. The integration of thermodynamic predictions with experimental data underscores its potential as a sustainable pretreatment route for refractory pyritic concentrates.

4. Conclusions

The results demonstrate that oxidative pretreatment is essential for improving the recovery of precious metals from refractory pyritic concentrates. Direct cyanidation proved ineffective, with gold and silver recoveries not exceeding 48% and 60%, respectively, due to the encapsulation of precious metals within sulfide matrices. Roasting at 550 °C partially overcame this limitation, reaching 80% Au and 70% Ag, while the addition of hydrogen peroxide further enhanced extractions to 89% Au and 74% Ag. However, the generation of large volumes of SO2 during roasting remains a critical environmental drawback. In contrast, alkaline pressure oxidation under optimized conditions (150 °C, 1 MPa O2, 61 µm) achieved the highest recoveries of 92% Au and 76% Ag in only one hour, clearly surpassing roasting and aligning with values reported for other high-pressure processes. The thermo-dynamic predictions from the Pourbaix diagram of the Fe–S system at 80 °C support these findings by showing the destabilization of pyrite and the stabilization of ferric oxides under oxidizing, moderately alkaline conditions, which favor metal liberation. Alkaline pressure oxidation is confirmed as a technically efficient and environmentally favorable pretreatment, combining high extraction efficiency with the advantage of avoiding SO2 emissions and minimizing acid generation, thereby representing a sustainable alternative for the processing of refractory gold ores.

Author Contributions

A.M.E.-M.: Methodology, experimental work, data curation, writing—original draft preparation. J.L.V.-G.: Conceptualization, validation, supervision, resources, project administration, writing—original draft preparation, critical review and editing. M.M.S.-C.: Conceptualization, validation, supervision, writing—original draft preparation, critical review and editing. M.A.E.-R.: Involved in methodology, analytical work, and validation. G.M.-B.: Contribution to data analysis, validation, review and editing. J.R.P.T.: resources, conceptualization, methodology, validation, review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

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

Acknowledgments

The authors would like to thank the Departments of Chemical Engineering and Metallurgy and Geology of the University of Sonora for providing laboratory facilities and technical assistance. The authors also thank Bacis Mine for supplying the concentrate samples and for their collaboration in this research. A.M.E.M. gratefully acknowledges the scholarship granted by SECIHTI, which contributed to the completion of this work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Minerals 15 01260 g001
Figure 1. A phase stability diagram for the Fe–S–O system at 600 °C.
Figure 1. A phase stability diagram for the Fe–S–O system at 600 °C.
Minerals 15 01260 g001
Minerals 15 01260 g002
Figure 2. A Pourbaix diagram of the Fe–S system at 80 °C.
Figure 2. A Pourbaix diagram of the Fe–S system at 80 °C.
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Minerals 15 01260 g003
Figure 3. The particle size reduction of the pyritic concentrate sample.
Figure 3. The particle size reduction of the pyritic concentrate sample.
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Minerals 15 01260 g004
Figure 4. A schematic diagram of the cyanidation process under standard conditions.
Figure 4. A schematic diagram of the cyanidation process under standard conditions.
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Minerals 15 01260 g005
Figure 5. Schematic representation of oxidative pretreatment by roasting.
Figure 5. Schematic representation of oxidative pretreatment by roasting.
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Figure 6. Experimental methodology for alkaline pressure oxidation.
Figure 6. Experimental methodology for alkaline pressure oxidation.
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Minerals 15 01260 g007
Figure 7. (a) SEM micrograph of the untreated pyritic concentrate, (b) EDS spectrum acquired from the indicated spot on a representative particle, and (c) corresponding elemental composition (wt.%) obtained by point microanalysis.
Figure 7. (a) SEM micrograph of the untreated pyritic concentrate, (b) EDS spectrum acquired from the indicated spot on a representative particle, and (c) corresponding elemental composition (wt.%) obtained by point microanalysis.
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Minerals 15 01260 g008
Figure 8. X-ray diffraction (XRD) patterns of the concentrate for particle sizes of (a) 104 µm, (b) 74 µm, and (c) 61 µm.
Figure 8. X-ray diffraction (XRD) patterns of the concentrate for particle sizes of (a) 104 µm, (b) 74 µm, and (c) 61 µm.
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Minerals 15 01260 g009
Figure 9. X-ray diffraction (XRD) patterns of roasted concentrates at 550 °C for particle sizes of (a) 104 µm, (b) 74 µm, and (c) 61 µm.
Figure 9. X-ray diffraction (XRD) patterns of roasted concentrates at 550 °C for particle sizes of (a) 104 µm, (b) 74 µm, and (c) 61 µm.
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Figure 10. Gold extraction by conventional cyanidation as a function of particle size.
Figure 10. Gold extraction by conventional cyanidation as a function of particle size.
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Figure 11. Silver extraction by conventional cyanidation as a function of particle size.
Figure 11. Silver extraction by conventional cyanidation as a function of particle size.
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Figure 12. Gold extraction from roasted pyritic concentrates at 550 °C as a function of particle size (74 and 104 µm) in 5 g/L NaCN solution.
Figure 12. Gold extraction from roasted pyritic concentrates at 550 °C as a function of particle size (74 and 104 µm) in 5 g/L NaCN solution.
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Minerals 15 01260 g013
Figure 13. Silver extraction from roasted pyritic concentrates at 550 °C as a function of particle size (74 and 104 µm) in 5 g/L NaCN solution.
Figure 13. Silver extraction from roasted pyritic concentrates at 550 °C as a function of particle size (74 and 104 µm) in 5 g/L NaCN solution.
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Figure 14. Gold extraction from pressure-oxidized concentrates as a function of oxygen pressure (73, 110, and 145 psi) in 5 g/L NaCN solution at pH ≈ 11.
Figure 14. Gold extraction from pressure-oxidized concentrates as a function of oxygen pressure (73, 110, and 145 psi) in 5 g/L NaCN solution at pH ≈ 11.
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Minerals 15 01260 g015
Figure 15. Silver extraction from pressure-oxidized concentrates as a function of oxygen pressure (73, 110, and 145 psi) in 5 g/L NaCN solution at pH ≈ 11.
Figure 15. Silver extraction from pressure-oxidized concentrates as a function of oxygen pressure (73, 110, and 145 psi) in 5 g/L NaCN solution at pH ≈ 11.
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Figure 16. Gold extraction by cyanidation from oxidized concentrates at different temperatures at 5 g/L NaCN solution at pH ≈ 11.
Figure 16. Gold extraction by cyanidation from oxidized concentrates at different temperatures at 5 g/L NaCN solution at pH ≈ 11.
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Figure 17. Silver extraction by cyanidation from oxidized concentrates at different temperatures in a 5 g/L NaCN solution at pH ≈ 11.
Figure 17. Silver extraction by cyanidation from oxidized concentrates at different temperatures in a 5 g/L NaCN solution at pH ≈ 11.
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Minerals 15 01260 g018
Figure 18. Gold extraction as a function of time for different case studies.
Figure 18. Gold extraction as a function of time for different case studies.
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Figure 19. Silver extraction as a function of time for different case studies.
Figure 19. Silver extraction as a function of time for different case studies.
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Table 1. Process variables and parameters for cyanidation after roasting.
Table 1. Process variables and parameters for cyanidation after roasting.
ParameterValue
Particle size, µm104, 74, and 61
Temperature, °Cambient
Pressureatmospheric
NaCN, g/L5
Solids wt.%20
Stirring speed, rpm350
pH≥11
Time (h)24
H2O2, mL50
Table 2. Process variables and parameters for cyanidation after pressure oxidation.
Table 2. Process variables and parameters for cyanidation after pressure oxidation.
ParameterValue
Particle size, µm104, 74, and 61
Temperature, °Cambient
Pressureatmospheric
NaCN, g/L5
CaO, Kg/t45
Solids wt.%20
Agitation speed, rpm350
pH≥11
Time (h)24
Table 3. Process variables and parameters for roasting experiments.
Table 3. Process variables and parameters for roasting experiments.
ParameterValue
Particle size, µm104, 74, 61
Temperature, °C500, 550, 600
Time (h)1
Table 4. Process variables and parameters for pressure oxidation tests in alkaline medium.
Table 4. Process variables and parameters for pressure oxidation tests in alkaline medium.
ParameterValue
Oxygen pressure, MPa0.5, 0.75, and 1
Temperature, °C80, 120, and 150
Particle size, µm104, 74, and 61
CaO, Kg/t44.23
Solids wt.%20
Agitation speed, rpm600
Time (h)1
Table 5. The mineralogical reconstruction of the concentrate.
Table 5. The mineralogical reconstruction of the concentrate.
Compoundswt.%
Pyrite, FeS243.6
Chalcopyrite, CuFeS21.8
Sphalerite, ZnS2.3
Quartz, SiO249.3
Calcite, CaCO33.3
Total100.0
Table 6. The chemical composition of the pyritic concentrate determined by fire assay and atomic absorption spectroscopy (AAS) [31].
Table 6. The chemical composition of the pyritic concentrate determined by fire assay and atomic absorption spectroscopy (AAS) [31].
Au
(g/t)		Ag
(g/t)		Cu
wt. (%)		Fe
wt. (%)		Zn
wt. (%)
25.565654.330.6220.821.54
Note: Precious metals are expressed in grams per ton (g/t), while base metals are reported in weight percent (wt.%).
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Espinoza-Martínez, A.M.; Valenzuela-García, J.L.; Salazar-Campoy, M.M.; Encinas-Romero, M.A.; Martínez-Ballesteros, G.; Parga Torres, J.R. Gold and Silver Recovery from a Refractory Pyritic Concentrate by Roasting and Alkaline Pressure Oxidation. Minerals 2025, 15, 1260. https://doi.org/10.3390/min15121260

AMA Style

Espinoza-Martínez AM, Valenzuela-García JL, Salazar-Campoy MM, Encinas-Romero MA, Martínez-Ballesteros G, Parga Torres JR. Gold and Silver Recovery from a Refractory Pyritic Concentrate by Roasting and Alkaline Pressure Oxidation. Minerals. 2025; 15(12):1260. https://doi.org/10.3390/min15121260

Chicago/Turabian Style

Espinoza-Martínez, Ana María, Jesús Leobardo Valenzuela-García, María Mercedes Salazar-Campoy, Martín Antonio Encinas-Romero, Guadalupe Martínez-Ballesteros, and José Refugio Parga Torres. 2025. "Gold and Silver Recovery from a Refractory Pyritic Concentrate by Roasting and Alkaline Pressure Oxidation" Minerals 15, no. 12: 1260. https://doi.org/10.3390/min15121260

APA Style

Espinoza-Martínez, A. M., Valenzuela-García, J. L., Salazar-Campoy, M. M., Encinas-Romero, M. A., Martínez-Ballesteros, G., & Parga Torres, J. R. (2025). Gold and Silver Recovery from a Refractory Pyritic Concentrate by Roasting and Alkaline Pressure Oxidation. Minerals, 15(12), 1260. https://doi.org/10.3390/min15121260

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