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Article

Surface Chemistry and Flotation of Gold-Bearing Pyrite

by
Seda Özçelik
1,* and
Zafir Ekmekçi
2
1
Graduate School of Science and Engineering, Hacettepe University, Ankara 06800, Turkey
2
Department of Mining Engineering, Hacettepe University, Ankara 06800, Turkey
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(9), 914; https://doi.org/10.3390/min14090914
Submission received: 16 August 2024 / Revised: 1 September 2024 / Accepted: 3 September 2024 / Published: 6 September 2024
(This article belongs to the Special Issue Surface Chemistry and Reagents in Flotation)
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Abstract

:
Gold grains are observed in a variety of forms, such as coarse-liberated native gold grains, and ultra-fine grains associated with sulfide or non-sulfide mineral particles, in the form of solid solution in sulfide minerals, mainly pyrite. In the flotation of gold ores, bulk sulfide mineral flotation is generally applied to maximize gold recovery. This approach gives high gold recoveries, but it also causes the recovery of barren sulfide minerals (i.e., sulfide mineral particles with no gold content), which increases concentrate tonnage and transportation costs and reduces the grade sometimes to below the saleable limit (approx. 10 g/t Au). This study addresses the differences between gold-bearing and barren pyrite particles taken from various ore deposits and utilizes these differences for the selective flotation of gold-bearing pyrite. The laboratory scale flotation tests conducted on three pyrite samples having different cyanide soluble gold contents show that a selective separation between gold-bearing pyrite and barren pyrite particles could be achieved under specific flotation conditions. Gold recovery is correlated directly with the cyanide-soluble gold in the ore samples. Electrochemical experiments were conducted to elucidate the differences in surface properties of the two types of pyrite. The barren pyrite particles were more cathodic and prone to cathodic reduction of OH− and depressant ions on the surface, and they could be depressed effectively without significantly affecting the gold-bearing particles.

1. Introduction

Gold can exist in visible or refractory (invisible) forms within gold-bearing sulfide minerals, mainly arsenopyrite, pyrite, tetrahedrite, and chalcopyrite. The visible gold is generally observed as native gold, electrum (Au, Ag), and/or gold tellurides (Calaverite (AuTe2), Sylvanite ((Au,Ag)Te4), Montbrayite (Au2Te3), etc.) associated with the sulfide minerals generally in the forms of nuggets, vein gold, and fracture gold [1,2]). The statistics of 102 types of gold deposits show that approximately 85% of the total gold is extracted from the gold-bearing pyrite in these gold deposits [3]. Therefore, it is important to concentrate pyrite for the recovery of gold.
Pyrite is generally concentrated by flotation, a physico-chemical separation process utilizing the differences in the surface properties of minerals. Pyrite flotation is generally performed at natural or slightly alkaline pH values using a strong xanthate-type collector, such as potassium amyl xanthate (KAX) or sodium isopropyl butyl xanthate (SIBX). Promoters with modified dithiophosphate or thionocarbamate chemistry are generally used to enhance the flotation of gold-bearing minerals and increase gold recovery. Activators, such as CuSO4, NaHS, and even Pb(NO3)2 can also be used according to the type of the sulfide minerals to be floated and the surface state of the sulfide minerals (such as fresh, tarnished, or oxidized) [4,5,6].
Under these flotation conditions, all the sulfide minerals, both gold-bearing and barren (i.e., sulfide mineral particles with no gold content) sulfide minerals in an ore can be floated to the concentrate. This approach may achieve high gold recoveries but also increase concentrate tonnage and transportation costs, and reduce the gold grade of the concentrate. The gold grade of a pyrite concentrate should be a minimum of 10 g/t, and the payability for gold increases with the increasing gold grade of the concentrate. Selective flotation of gold-bearing pyrite from barren pyrite can be highly beneficial considering economic, environmental, and technical factors, particularly for low-grade ores with a low Au/S ratio [7].
In the literature, there are several research studies on separating gold and pyrite using selective collectors [4,8] and depressants [9]. The presence of gold in a pyrite particle, either in visible or invisible forms, can change the electronic structure and, hence, floatability of gold-bearing pyrite [10,11]. Gold-bearing pyrite has higher conductivity, oxidation, and stability compared to barren pyrite. It is claimed that the presence of gold may enhance the formation and the adsorption of dixanthogen on the gold-bearing pyrite surface. Electrum is a very common gold mineral observed in gold-bearing sulfide ore deposits. The presence of Ag in electrum changes the electrochemical and surface characteristics of the pyrite particles. Ag provides active adsorption sites for both xanthate-type strong collectors and also the selective collectors that are generally used as promoters in the flotation of gold-bearing sulfide ores.
The chemical and electrochemical conditions of the flotation pulp can significantly affect the surface characteristics of the sulfide minerals and enhance the differences in the flotation behavior of gold-bearing pyrite and barren pyrite particles [12,13]. Gold is a noble metal and is electrochemically inert, as reflected by its lack of reactivity in air and most aqueous solutions [1]. The presence of dissolved oxygen and redox couples such as Fe2+/Fe3+ ions can alter pyrite surface properties of gold-bearing pyrite and barren pyrite in different ways, and this could allow the separation of these two types of pyrite by flotation [14,15].
Several electrochemical research studies have been conducted using gold and pyrite electrodes to investigate the influence of gold on the electrochemical characteristics of pyrite. In some of the studies, the two electrodes are connected to simulate the gold-bearing pyrite particles. However, the artificial coupling of gold and pyrite electrodes can be different from the real gold-bearing pyrite particles in the ore. This difference becomes extreme if the gold is in the form of electrum, gold tellurides, or any other mineral form. Given the use of gold-bearing particles from the ore itself or flotation concentrates, a comparison with the barren pyrite becomes more important for understanding the surface characteristics and flotation behavior of gold-bearing pyrite and the selective flotation from the barren pyrite particles.
This research aims to explore the differences between gold-bearing and barren pyrite particles and utilize these differences for the selective flotation of gold-bearing pyrite. This allows the production of pyrite concentrates with higher gold grades and makes low-grade sulfide ore deposits economically feasible. This approach would reduce the processing cost, improve resource efficiency, and minimize the environmental impact. Surface chemistry studies and flotation tests were conducted using samples prepared from gold-pyrite ores and a pyrite concentrate with high gold content.

2. Materials and Methods

2.1. Materials

Three samples, having different chemical and mineralogical characteristics, were used in the experiments. Sample 1 is a flotation concentrate produced by bulk sulfide mineral flotation from a Au-Pyrite ore. Sample 2 and Sample 3 are massive sulfide ore samples containing gold as the valuable metal. These samples were selected according to their cyanide-soluble gold (CNSolAu) content from the same ore deposit, because the CNSolAu value is considered an indicator of the exposed gold in pyrite particles.
The chemical analysis of the test samples is given in Table 1. Sample 1 is a flotation concentrate and contains a higher concentration of Au, Ag, Pb, and Zn than the other two samples. The gold grades of Sample 2 and Sample 3 are 1.42 g/t and 2.4 g/t, respectively. These two samples were classified according to their CNSolAu content.
Mineralogical characterization of the samples was performed using QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy). Particle Mineralogical Analysis (PMA) was used for modal mineralogy and liberation/locking analysis. The gold search was conducted using Trace Mineral Search (TMS) mode with Energy Dispersive Spectrometry (EDS). The distribution of the major minerals of the three samples is summarized in Table 2. Sample 1 contains pyrite (44.9%), galena (19.7%), sphalerite (8.47%), and small amounts of arsenopyrite (1.03%) and chalcopyrite (0.7%). The major non-sulfide minerals were quartz, Mn-Ca silicates, Mn-Ca carbonates, and K-feldspar.
The gold-bearing phases detected are essentially gold–silver alloys (electrum) with composition of 50%–77% Au and 50%–23% Ag. Gold is usually locked with pyrite or encapsulated within galena/pyrite, but there are also liberated gold particles at coarse particle sizes (+20 µm). Approximately 36% of gold was in the form of liberated particles. Figure 1 shows back-scattered electron (BSE) images of gold-bearing pyrite particles. The CNSolAu content of Sample 1 was measured as 92%. This means that most of the gold is exposed and can interact with the flotation reagents.
Sample 2 and Sample 3 are from the same deposit and have similar mineral distributions. The samples consist of pyrite, quartz, and calcite as the major mineral phases. The gold search was conducted on the HLS (Heavy Liquid Separation) concentrates due to the low head assays. The gold was mainly in the form of native gold with rare Ag-Au-Te grains. The grain size of gold was 1–13 µ in Sample 2 and 1–5 µ in Sample 3. The gold grains were locked in pyrite. Figure 1 shows BSE images of typical gold grains associated with pyrite particles.

2.2. Experimental Methods

2.2.1. Flotation Tests

Rougher kinetic flotation tests were conducted using a Denver-type flotation machine in a 2.5 L volume flotation cell. The pulp density was adjusted at 30%w/w. The agitation speed and the airflow rate were set to 1550 rpm and 4–5 L/min, respectively. Analytical grade H2O2, FeSO4, NaClO, CaClO, NaHS, Na2S2O5, and dextrin from potato starch were used as pyrite depressants in the tests. Sodium Aerofloat (NaAF, ethyl dithiophosphate) and Aero8045 (dithiophosphate formulation) from Syensqo were used as collectors. F-549 (Syensqo) was used as the frother in all the tests. Lime was used as a pH regulator where needed. The sequence of reagent addition was depressant, collector, and frother. The conditioning time was 2 min for the depressants and collectors, and 1 min for the frother. The concentrates were assayed for Au (fire assay with AAS finish for up to 100 g/t gold content and gravity finish for values over 100 g/t), S (Leco), Ag, As, Pb, Zn, and Fe (ICP-MS).

2.2.2. Electrochemical Studies

Two mineral electrodes were used to demonstrate the differences in electrochemical and surface properties of the gold-bearing pyrite (Au-Py) and barren pyrite (B-Py). The Au-Py electrode was manufactured using the pyrite particles concentrated by heavy liquid separation from a gold-pyrite ore. The B-Py electrode was produced from a high-purity pyrite sample purchased from Wards’ Natural Science. The −212 + 150 µm size fraction was used in both electrodes. The pyrite samples were washed with diluted HCl and acetone sequentially, and then rinsed with distilled water to remove any oxidized and organic species from the surface and dried in a vacuum desiccator. The pyrite particles were attached to a copper plate through a conductive silver epoxy. The copper plate with attached pyrite particles was then immersed in an epoxy resin so that only the pyrite particles were exposed. The surface of the mineral electrode was polished using an aluminum paste before the electrochemical experiments. This method allows using the same electrode several times after surface cleaning with aluminum paste. Figure 2 shows a photo of the mineral electrode surface.
The electrochemical experiments were performed using Gamry Reference 600 Model Potansiostat/Galvanostat. The measurements were performed in a standard three-electrode electrochemical cell with a standard calomel electrode (reference electrode), a platinum wire (counter electrode), and a mineral electrode (working electrode). The open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements were conducted on both pyrite samples. The EIS measurements were performed in the frequency range of 1 × 105 to 1 × 10−2 Hz. The potentials shown in this paper were converted to the standard hydrogen electrode (SHE) scale. A constant polarization potential (100 mV) was applied during EIS measurements. High-purity de-ionized water was used in all experiments. Buffer solution (pH 9.2) of 0.05 M Na2B4O7·10H2O was used as an electrolyte, and all measurements were carried out at room temperature (~25 °C). High-purity nitrogen gas purged through the solution for 15 min to remove the dissolved oxygen in the solution. The pyrite electrodes were polished with alumina and thoroughly rinsed before each experiment.

3. Results

3.1. Flotation Tests

3.1.1. Flotation Concentrate (Sample 1)

Sample 1 is a flotation concentrate containing a high amount of pyrite, galena, and sphalerite. The gold and silver assays were 85 g/t and 211 g/t, respectively. A series of flotation tests was conducted to separate the gold-bearing sulfide minerals from the barren pyrite.
Various types of reagents can be used to depress pyrite in flotation. The type of depressants is selected according to the ore type and mineral content. In the flotation of copper minerals and copper-activated sphalerite, pyrite is generally depressed at high pH (pH = 11.5) using lime. However, high pH could sometimes negatively affect the flotation of gold and gold-bearing pyrite [5]. In these circumstances, specific depressants should be used. In this study, in addition to the high pH, the effects of different types of depressants and flotation conditions (NaHS, FeSO4, and Na2S2O5 (Sodium metabisulfite, MBS), high-temperature conditioning followed by flotation at high pH, and oxidizing reagents such as Ca(ClO)2, NaClO, and H2O2 in combination with dextrin) on the selective flotation of gold-bearing pyrite from the barren pyrite were investigated.
Figure 3 shows the gold recovery as a function of mass pull, gold grade vs. recovery curves, and the gold-pyrite selectivity curves. The floatability of both gold-bearing pyrite and barren pyrite was very low at high pH. As it was mentioned above, Sample 1 is a flotation concentrate and both gold-bearing and barren pyrite particles are covered with collector molecules. In order to remove the residual collector molecules and oxidized species from the surface of the minerals, in one of the tests, the material was pre-treated at high temperature, filtered, repulped, and floated at pH 11.5 [16,17,18]. In another test, NaHS was used for surface cleaning [19]. After conditioning with NaHS, the material was filtered and repulped using fresh water. The flotation was performed at pH 11.5 to depress the barren pyrite particles. The redox potential of the pulp was measured as −47 mV (Ag/AgCl) after repulping with fresh water. The results show that surface cleaning at high temperature improved the gold recovery and the gold/pyrite selectivity to some extent. The use of NaHS, however, depressed both types of pyrite particles. Application of these pre-treatment methods is complicated even in the laboratory scale tests and the flotation performance was not satisfactory. Therefore, alternative reagent schemes were tested using strong pyrite depressants.
The sulfide minerals, particularly pyrite, can strongly be depressed under extreme pulp electrochemical conditions. Pyrite is the most cathodic sulfide mineral [20] and the most affected by changing the pulp electrochemistry. Oxidative conditions form stable and hydrophilic iron oxide/hydroxide species, which inhibit the adsorption of collector molecules on the pyrite surface [16,21]. Surface oxidation can be achieved by pre-aeration or the use of specific oxidizing reagents, such as hydrogen peroxide (H2O2) [9,22], sodium hypochlorite (NaClO), or calcium hypochlorite (CaClO2) [23]. In some of the applications, dextrin (a low-molecular-weight carbohydrate produced by hydrolysis of starch and glycogen) is used as a supporting reagent to improve pyrite depression [23,24]. The dextrin molecules are hydrophilic and adsorb on the iron oxide/hydroxide sites at the surface of the oxidized pyrite. This can prevent collector adsorption even at high dosages and maintain the depression of pyrite particles.
Rougher kinetic flotation tests were performed using H2O2 (15 lt/t, 35% purity), NaClO (15 lt/t), and CaClO2 (750 g/t) as oxidizers and dextrin (300 g/t). The flotation tests were performed at natural pH (pH = 7–8). The redox potential of the pulp increased up to 400 mV (Ag/AgCl) after the addition of the oxidizers and decreased to about 150 mV during the flotation, due to oxidation of pyrite. Both the gold recovery and the selectivity were improved substantially with the use of an oxidizing reagent–dextrin combination. The gold grade of the concentrate increased to 600 g/t with the use of CaClO2 + Dextrin. The three reagent combinations were effective and achieved high selectivity. These results showed that the flotation behavior and, hence, surface chemistry of gold-bearing pyrite particles are different from the barren pyrite particles under oxidative flotation conditions.
Pyrite can also be effectively depressed using sulfur-based reagents, such as Na2S2O5 and Na2SO3 [6]. In this study, Na2S2O5 (sodium metabisulfite, MBS) was used for the depression of barren pyrite particles. After the addition of 3 kg/t MBS, the pulp was pre-aerated for 5 min. The pulp redox potential was about 30 mV (Ag/AgCl) after the pre-aeration stage. Stage addition of collector (NaAF) was applied with 30 g/t addition in each stage. The flotation time was 2 min in each stage and four concentrates were collected in the test. The flotation was conducted at a natural pH (pH 7).
The flotation performance of the MBS test was similar to the tests performed using oxidizers (Figure 3). The gold grade of the first concentrate of the kinetic test was as high as 500 g/t. The silver grade of the concentrate could also be increased up to 900 g/t. At the end of the test, a rougher concentrate was produced assaying 273 g/t Au at 85% gold recovery. The oxidizing conditions produced rougher concentrates with similar gold grade (about 260 g/t) but at lower recoveries (about 70%).
The results showed that MBS can effectively depress the barren pyrite particles without negatively affecting the gold-bearing pyrite particles. The trend of the gold/pyrite selectivity curves is very close in the MBS and oxidizer tests. Both methods can be considered for selective gold flotation from the pyrite concentrates.
The grade and recovery trends of silver were very close to that of gold (Figure 3). The silver content of Sample 1 was about 200 g/t (Table 1) and increased to >600 g/t in the tests performed using the depressants. The use of MBS produced higher silver grades and recovery. This could be attributed to the flotation of electrum particles, and also galena particles, which may also contain silver [25]. There was a direct correlation between the silver grade and gold and lead grade of the concentrates. The silver grade of the concentrates increases linearly with increasing gold and lead grades. The copper (71%) and lead (81%) recoveries followed the same trend, but not the Zn. MBS is also known as an effective sphalerite depressant and, hence, Zn recovery was only 27% in the absence of copper sulfate.
The chemical and mineralogical analyses of Sample 1 show the presence of arsenopyrite in the material. The effects of pre-treatment methods and the use of different types of depressants on the flotation of arsenopyrite are illustrated in Figure 4. The arsenic grade of the material was about 2.5%, which is presumably in the penalty range. The addition of NaHS obviously removed the oxidized species from the surface of arsenopyrite and produced a concentrate with a higher As grade than the other conditions. The arsenic grade was decreased to <0.5% using oxidizers and to 0.7% with the MBS test. The use of oxidizers and MBS for barren pyrite depression can also depress the arsenopyrite particles effectively and decrease the arsenic content of the concentrate substantially.

3.1.2. Massive Pyrite Ore Samples (Sample 2 and Sample 3)

The gold-bearing pyrite particles could successfully be separated by flotation using MBS or H2O2-Dext. Similar depressants were tested on two low-grade massive pyrite ore samples, Sample 2 and Sample 3 (Table 1). Aero8045 produced higher recoveries than that of NaAF and, hence, it was used as the collector in this part of the study. The two samples were prepared from the same ore deposit according to their CNSolAu content to test the hypothesis of this study. The CNSolAu value indicates the exposed gold at the surface of pyrite particles. The higher the CNSolAu, the larger the gold exposed at the surface of the particles. The exposed gold grains may change the electrochemical behavior of these particles and provide active sites for collector adsorption. Figure 5 shows that the highest gold grade and recovery values were obtained with MBS for both samples. The gold recovery of Sample 2 was higher due to its higher CNSolAu content (62.72%).

3.2. Electrochemical Experiments

Two mineral electrodes were manufactured using gold-bearing pyrite (Au-Py) and barren pyrite (B-Py) particles. OCP and EIS measurements were performed to demonstrate the influence of gold on the surface characteristics of pyrite. The measurements were performed at pH 9.2.
OCP measurements of the two samples were performed for comparison of their electrochemical reactivity. Figure 6 shows the OCP values measured for 5 min. The measurements performed in an alkaline solution show the response of the pyrite electrodes to oxidation by OH− ions. B-Py has a higher OCP than Au-Py. This indicates that B-Py is more cathodic and electrochemically more reactive to OH− ions [26].
The surface characteristics of the two pyrite samples are also compared based on the EIS measurements. The results of the EIS experiments are given in the form of Bode-magnitude and phase angle plots in Figure 7. The results show clearly that there are significant differences between the surface characteristics of the pyrite samples. The surface coatings of iron oxy/hydroxide and sulfur species are expected on pyrite surface in alkaline solutions. The presence of such coatings increases the resistance to the charge transfer of the surface. Zmod values of B-Py were higher, indicating higher resistance to charge transfer than Au-Py. The differences between the two samples were attributed to the presence of gold in Au-Py.
The EIS spectrum of each sample was fitted to an equivalent electrical circuit model to compare the electrochemical properties of the two pyrite electrodes based on conductance and resistance (Figure 8). The proposed model is used to simulate the impedance behavior of the pyrite/electrolyte interface [15,27]. The model parameters were calculated by using Gamry Echem Analyst software Version 6.33. In the electrical circuit model, Rs is the electrolyte resistance and Rct is the charge transfer resistance, representing the resistance of charge transfer through the iron-deficient layer. The pore resistance (Rp) is a measure of the charge transfer in the pores of the product layer, i.e., the Fe-Hydroxide/oxide layer. Csl corresponds to the capacitance of the surface layer consisting of the initial pyrite oxidation species. The double-layer capacitance (Qdl) is represented in terms of the constant phase element that indicates a nonhomogeneous surface. The fitted model parameters and the goodness-of-fit values are given in Table 3. The Chi-squared test gave low values indicating the proposed equivalent electrical circuit simulated the electrical double layer. The “n” is the phase angle and represents the surface roughness of the electrode, usually varying between 0.5 and 1.
The net rate of electrochemical reactions on electrode surfaces is inversely proportional to the charge transfer resistance (Rct) [28]. Rct and Rp of B-Py were much higher than that of the Au-Py electrode, indicating the formation of a higher amount of oxidation products (iron oxides, iron sulfates, and polysulfides), and resulting in a more electrochemically passive surface layer. The presence of gold obviously changed the electrochemical characteristics of pyrite, which can be utilized for the selective separation of Au-Py particles from B-Py particles.

4. Discussion

The presence of impurities, such as As, Au, Ag, Ni, Co, etc., changes the electrical properties and oxidation behavior of pyrite. The pyrite is classified as p-type and n-type based on its electrical properties. The p-type pyrite is iron deficient (S/Fe > 2) and may contain significant quantities of As. The n-type pyrite is sulfur deficient (S/Fe < 2) and may be rich in Co and Ni. The conductivity of the p-type is much lower than that of the n-type.
In addition to the presence of fine grains of native gold and/or electrum, in pyrite, particles may change their electrical properties and flotation behavior. Gold is the most noble metal and a galvanic interaction may occur between pyrite and gold. This may change the surface properties of gold-bearing pyrite and barren pyrite, and allow their separation by flotation [14].
In this study, the electrochemical properties of gold-bearing pyrite (Au-Py) and barren pyrite (B-Py) particles were investigated by using the natural material obtained from different ore types. The electrochemical characterization of the Au-Py and B-Py particles shows that B-Py is more cathodic and prone to reduction of OH− and any other depressant ions on its surface. This resulted in higher charge transfer resistance as shown by the EIS measurements. The Au-Py, on the other hand, is more anodic and allows the adsorption of the collector on the surface. The presence of Ag in electrum may also enhance the adsorption of collector molecules due to the high stability of Ag-X compounds.
Figure 9 shows the galvanic interaction models for a single gold-bearing pyrite particle and for the case when a gold-bearing is in contact with a barren pyrite particle. Huai, et al. [14,15] have studied the single-particle model in detail under ambient and oxygen-enriched conditions. In the single-particle model under oxygen-enriched conditions, the oxygen is reduced at the surface of gold, and the electrons flow from pyrite to gold, which increases the oxidation rate of pyrite (Figure 9a).
In the flotation of gold-bearing pyrite particles, however, barren pyrite particles exist in large quantities affecting the surface overall galvanic interaction mechanism and, hence, the surface properties of the gold-bearing pyrite and barren pyrite particles. Figure 9b shows the galvanic interaction model between a gold-bearing pyrite particle and a barren pyrite particle. The OCP and EIS measurements show that the barren pyrite particles should be the cathodic component in the galvanic interaction model. The electrons flow from Au-Py to B-Py leads to anodic oxidation of Au-Py and oxygen reduction at the surface of B-Py. In addition to the iron-deficient sulfide and polysulfide species, active sites for collector adsorption are formed at the surface of Au-Py. The presence of silver in the electrum particles may enhance the adsorption of collector molecules and increase the degree of hydrophobicity of the Au-Py particles.
The differences in the surface properties of the two-pyrite type were utilized for the selective separation of gold-bearing pyrite from the barren pyrite. The flotation tests show that high pH could not sufficiently expose this difference, and the selective flotation was poor. However, the use of strong oxidizing agents or sulfur-based depressants can successfully enhance the differences in surface properties of the pyrite particles and allow selective separation by flotation.
The flotation results showed that there is a direct relationship between CNSolAu content and gold recovery (Figure 10). The gold recovery increases linearly as a function of CNSolAu. High CNSolAu values indicate the ratio of free gold, i.e., the area of gold/electrum exposed on the surface of pyrite particles. The exposed gold sites are less affected by the depressants compared to the barren pyrite and, hence, the collector molecules can selectively adsorb on these sites.

5. Conclusions

Laboratory scale flotation tests were conducted on three ore samples containing both gold-bearing pyrite (Au-Py) particles and barren pyrite (B-Py) particles. The selective flotation of Au-Py from B-Py was achieved by using strong pyrite depressants and a selective gold collector. MBS (Sodium Metabisulfite) and a dithiophosphate formulation collector were found to be the most effective reagent combination.
There was a direct relationship between CNSolAu and gold recovery, indicating the contribution of free gold on the selective flotation.
Electrochemical studies showed that the presence of gold/electrum in a pyrite particle changes the electrochemical characteristics and surface of the gold-bearing pyrite particles.
According to the galvanic interaction model, the barren pyrite particles are the cathodic component and are prone to cathodic reduction of OH− and depressant ions on the surface. The Au-Py, on the other hand, goes through anodic oxidation-forming iron-deficient sulfur and polysulfide species, as well as active sites for collector adsorption. Consequently, the B-Py particles can be depressed effectively without significantly affecting the Au-Py particles under specific flotation conditions.
This study shows that low-grade gold-pyrite ores can be enriched by flotation depending on the form of gold in the ore. This allows production of pyrite concentrates with a saleable gold grade and makes the low-grade sulfide ore deposit economically feasible.
Further studies should be performed to investigate the surface properties of pyrite particles containing gold as a solid solution and the possibility of separating these particles by flotation in refractory gold ores.

Author Contributions

Conceptualization, S.Ö. and Z.E.; methodology, S.Ö. and Z.E.; validation, S.Ö. and Z.E.; formal analysis, S.Ö.; investigation, S.Ö. and Z.E.; resources, Z.E.; writing—original draft preparation, S.Ö. and Z.E, supervision, Z.E.; project administration, Z.E. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

This study is part of authors’ ongoing Ph.D. Thesis. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. BSE Images of the gold-bearing particles from the test samples: (a) and (b) Sample 1; (c) Sample 2; and (d) Sample 3.
Figure 1. BSE Images of the gold-bearing particles from the test samples: (a) and (b) Sample 1; (c) Sample 2; and (d) Sample 3.
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Figure 2. Optical microscope image of polished mineral electrode surfaces of gold-bearing pyrite (Au-Py).
Figure 2. Optical microscope image of polished mineral electrode surfaces of gold-bearing pyrite (Au-Py).
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Figure 3. Flotation of gold-bearing pyrite particles from the barren pyrite at different chemical conditions in Sample 1: (a) shows the gold recovery as a function of mass pull; (b) gold grade vs. recovery curves; (c) gold-pyrite selectivity curves; and (d) silver grade vs. recovery curves.
Figure 3. Flotation of gold-bearing pyrite particles from the barren pyrite at different chemical conditions in Sample 1: (a) shows the gold recovery as a function of mass pull; (b) gold grade vs. recovery curves; (c) gold-pyrite selectivity curves; and (d) silver grade vs. recovery curves.
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Figure 4. The arsenic grade and recovery of the concentrates produced in the presence of oxidizers and MBS in Sample 1.
Figure 4. The arsenic grade and recovery of the concentrates produced in the presence of oxidizers and MBS in Sample 1.
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Figure 5. The flotation performance of gold-bearing pyrite particles under different flotation conditions in Sample 2 and Sample 3: (a) shows gold grade vs. recovery curves and (b) gold recovery vs. pyrite recovery curves.
Figure 5. The flotation performance of gold-bearing pyrite particles under different flotation conditions in Sample 2 and Sample 3: (a) shows gold grade vs. recovery curves and (b) gold recovery vs. pyrite recovery curves.
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Figure 6. Open Circuit Potential (OCP) of the pyrite samples as a function of time.
Figure 6. Open Circuit Potential (OCP) of the pyrite samples as a function of time.
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Figure 7. EIS results of Au-Py and B-Py pyrite samples obtained at pH = 9.2 and at 100 mV polarization potential. (a) Bode magnitude (b) Bode-Phase angle plot.
Figure 7. EIS results of Au-Py and B-Py pyrite samples obtained at pH = 9.2 and at 100 mV polarization potential. (a) Bode magnitude (b) Bode-Phase angle plot.
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Figure 8. Equivalent electrical circuit for modeling the EIS data [27].
Figure 8. Equivalent electrical circuit for modeling the EIS data [27].
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Figure 9. Schematic illustration of the galvanic interaction between gold and pyrite: (a) in a gold-bearing single particle, and (b) when a gold-bearing particle is in contact with a barren pyrite particle.
Figure 9. Schematic illustration of the galvanic interaction between gold and pyrite: (a) in a gold-bearing single particle, and (b) when a gold-bearing particle is in contact with a barren pyrite particle.
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Figure 10. The relationship between CNSolAu and gold recovery.
Figure 10. The relationship between CNSolAu and gold recovery.
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Table 1. Chemical analysis of the samples used in the tests.
Table 1. Chemical analysis of the samples used in the tests.
CuFePbZnSAuAgAsCNSolAu
%%%%%g/tg/t%%
Sample 10.5225.810.410.5533.8852112.592.0
Sample 20.123.630.050.1225.191.427.00.0962.72
Sample 30.0725.820.060.0626.542.4012.00.0231.93
Table 2. Mineral distribution of the samples used in the tests.
Table 2. Mineral distribution of the samples used in the tests.
Mineral GroupSample 1Mineral GroupSample 2Sample 3
Pyrite44.9Pyrite52.6357.28
Arsenopyrite1.03Galena0.060.11
Galena19.7Sphalerite0.150.07
Sphalerite8.47Chalcopyrite0.050.16
Chalcopyrite0.70Quartz15.7912.90
Cerussite1.28Feldspar1.590.71
Coronadite and other Pb2.91Muscovite4.461.60
Quartz7.97Si-Al Clays1.951.41
K-feldspar1.62Calcite17.0515.63
Muscovite/biotite/illite0.73Rhodochrosite0.005.35
Mn-Ca-silicates3.30Sulfide/Silicate Texture2.671.52
Mn-Fe-(Ca)-carbonates1.85Other Minerals3.63.26
Mn-Ca-carbonates1.42Total100.00100.00
Other Minerals4.12
Total100.00
Table 3. The Equivalent electrical circuit model parameters of Au-Py and B-Py samples.
Table 3. The Equivalent electrical circuit model parameters of Au-Py and B-Py samples.
Rs
(Ω)		Rp
(Ω)		Rt
(kΩ)		Csl
(F)		Qdl
(S*sn)		n (°)Chi-Squared
χ2
Au-Py126.619.3974.211.650 × 10−72.87 × 10−50.572.47 × 10−3
B-Py238.8315.291.045.24 × 10−71.38 × 10−50.765.84 × 10−3
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Özçelik, S.; Ekmekçi, Z. Surface Chemistry and Flotation of Gold-Bearing Pyrite. Minerals 2024, 14, 914. https://doi.org/10.3390/min14090914

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Özçelik S, Ekmekçi Z. Surface Chemistry and Flotation of Gold-Bearing Pyrite. Minerals. 2024; 14(9):914. https://doi.org/10.3390/min14090914

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Özçelik, Seda, and Zafir Ekmekçi. 2024. "Surface Chemistry and Flotation of Gold-Bearing Pyrite" Minerals 14, no. 9: 914. https://doi.org/10.3390/min14090914

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