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Article

Combined Bacterial and Pressure Oxidation for Processing High-Sulfur Refractory Gold Concentrate

by
Anna Boduen
1,
Maxim Zalesov
1,2,
Vitaliy Melamud
3,
Victoria Grigorieva
1 and
Aleksandr Bulaev
3,*
1
RIVS Group of Companies, 199155 Saint Petersburg, Russia
2
Faculty of Mineral Raw Material Processing, Saint-Petersburg Mining University, 21st Line 2, 199106 Saint Petersburg, Russia
3
Research Center of Biotechnology, Russian Academy of Sciences, 119071 Moscow, Russia
*
Author to whom correspondence should be addressed.
Processes 2023, 11(11), 3062; https://doi.org/10.3390/pr11113062
Submission received: 16 September 2023 / Revised: 7 October 2023 / Accepted: 18 October 2023 / Published: 25 October 2023
(This article belongs to the Special Issue Advances in Microbial Biotechnology and Bioengineering Processes)
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Abstract

:
Microbially assisted bio-oxidation of sulfide concentrates in stirred-tank reactors (stirred-tank reactor bio-oxidation (STRB)) and acid pressure oxidation (POX) are widely used to pretreat refractory sulfide concentrates and increase gold extraction via cyanidation. Continuous STRB requires a comparatively long residence time; however, in some cases, it cannot effectively oxidize some sulfide minerals. POX enables oxidation in a short residence time. At the same time, if a processed concentrate contains a large amount of sulfur, it decreases the ratio of the solid mineral phase to liquid (pulp density) during POX and limits its economic attractiveness. In the present work, experiments were performed to investigate the problems associated with both processing methods for refractory sulfide concentrates. The experiments combined both treatments (STRB and POX) based on the example of a pyrite–arsenopyrite gold-bearing concentrate. The gold recovery from the untreated concentrate via cyanidation reached 58%. Continuous STRB for 2, 4, and 6 days oxidized 43, 74, and 79% of the sulfide sulfur (Ss), respectively. The gold recovery rates from the bio-oxidation residues were 68, 82, and 88%, respectively. The pressure oxidation of both the concentrate and STRB residues increased Ss oxidation by 97–99% and gold recovery by 96–97%. For 2 days, STRB decreased the Ss content and increased the possible liquid-to-solid ratio for POX. The combined processes result in a new promising direction because the POX stage allows high gold recovery, whereas combining STRB and POX provides products for further POX in terms of Ss content and increases POX productivity.

1. Introduction

Highly effective novel technologies for processing low-grade and refractory materials are needed for gold extraction from refractory sulfide ores, which form a significant part of gold resources [1,2,3,4]. Refractory gold ores cannot be processed effectively via direct cyanidation since the gold particles in this material type are highly dispersed in sulfide minerals, commonly pyrite (FeS2) and arsenopyrite (FeAsS). In order to increase gold recovery from 10 to 60% using direct cyanidation, pretreatment is required to destroy the crystal lattice of sulfide minerals and release fine gold particles, thus making them more accessible for cyanide leaching [1,2,3,4,5,6].
Hydrometallurgical pretreatment techniques, including stirred-tank reactor bio-oxidation (STRB) and pressure oxidation (POX), are used on an industrial scale worldwide [1,3,4,7,8,9,10]. Another method of treating refractory gold materials is oxidative roasting, which is an effective and well-investigated process. However, roasting is accompanied by gaseous emissions released into the atmosphere, mostly sulfur dioxide SO2, mercury, CO, NOx, arsenic, and other components, which significantly influence the environment and require off-gas cleaning systems [5,6,7,8,11,12].
The bio-oxidation of the sulfide minerals in flotation concentrates obtained from sulfide ores is performed by acidophilic microorganisms. These microorganisms oxidize ferrous iron and different reduced sulfur compounds. Bio-oxidation is usually conducted in continuous mode in stirred-tank reactors (with m3 working volumes in the range of tens to thousands) connected in series at certain pH values and temperatures, which are held constant. Depending on the oxidation rates achieved and the sulfide sulfur (Ss) content in the concentrate, including the mineral composition, the pulp density and residence time are usually in the range of a 10–20% solid-to-liquid concentration and 4–6 days, respectively. These parameters were established to achieve Ss oxidation and gold recovery values exceeding 90–95% [3,4,9,10,13,14]. On an industrial scale, STRB has been successfully applied since 1986 to improve gold recovery from refractory concentrates via the cyanide-leaching process [3,4,9,10,13,14].
POX is an intensive method that allows for the near-complete oxidation of sulfide minerals and makes gold particles accessible for further leaching processes. It is conducted at 190–230 °C with an oxygen overpressure of 350–700 kPa. POX is performed in autoclave reactors (or autoclaves) designed to oxidize sulfide minerals at an elevated temperature and pressure. The residence time for POX of pyrite–arsenopyrite concentrates is usually 30–60 min; therefore, it is several orders of magnitude lower than that required for STRB [7,15].
Both STRB and POX play important roles in processing gold-bearing sulfide concentrates containing arsenopyrite. Leaching reduces the formation of toxic gaseous emissions with arsenic oxides, while dissolved arsenic can be removed from pregnant solutions via inert scorodite (FeAsO4·2H2O) formation after neutralization [1,2,3,4,7,16,17,18,19].
Thus, STRB requires a comparatively long residence time compared with POX. In some cases, STRB does not enable the complete oxidation of some sulfides at a high pulp density, despite some options that have improved bio-oxidation rates [8,13,14,20,21,22,23,24,25,26].
POX enables almost complete sulfide mineral oxidation in a short residence time. However, its efficiency decreases when treating certain types of raw materials. It has been shown that POX performance is directly influenced by the sulfur content in the processed concentrate [27]. Sulfur excess in the autoclave feed (>6%) requires cold water to dilute the autoclave pulp, thus decreasing the solid-to-liquid ratio and increasing the volume of the autoclave and, in turn, capital and maintenance costs. In addition to the risk of equipment overheating, an excess of Ss in the concentrate causes the passivation coating of the concentrate particle surface by elemental sulfur formed as an intermediate in sulfide mineral oxidation. Products with a low Ss content require additional heat in the form of steam for effective oxidation [1,28,29]. Conway and Gale calculated how the solid phase content for the POX of a pyrite concentrate performed at 200 °C depends on the sulfur content in the feed [27] (Figure 1). This dependence can be expressed as in Equation (1):
Wt . %   Solids = 1 ( 29.49 S wt + 0.825 )
where Wt.% Solids is the weight percentage of solids (pulp density), and Swt is the weight percentage of sulfur in the feed solids placed in the autoclaves.
Figure 1 is a graphic representation of the dependence of solids versus the sulfide content determined in Equation (1).
The presence of carbonates in the concentrate results in CO2 emission during POX and, consequently, pressure instability in the autoclaves and security issues. Therefore, the presence of carbonates requires additional preprocessing for the sulfide concentrate prior to POX, since carbonates should be removed from the concentrate as impurities, which pose problems due to gas emissions (contrasting with other components of sulfide concentrates, including sulfide minerals and silicates, the transformation of which during POX does not result in dangerous gas formation and pressure instability). Sulfide concentrates should be subjected to acid pretreatment to remove carbonates (decarbonization) [1,30].
In refractory gold concentrate processing, a combination of bio-oxidation and POX technologies is a promising solution. This hybrid method involves two main stages: firstly, sulfide minerals are oxidized by microorganisms to reach a sulfur content acceptable for treatment in autoclaves. Then, the bio-oxidation residue is treated with POX to complete the sulfide oxidation and prepare the residue for further cyanidation.
The POX of bio-oxidation residues allows the complete oxidation of sulfide sulfur as well as an increase in the gold recovery rate via cyanidation [31,32]. In addition, a similar approach was used at the Sao Bento deposit in Brazil to process pyrrhotite containing pyrite–arsenopyrite concentrates [32]. Our results indicate that two-stage oxidation uses bio-oxidation to obtain a product with a composition meeting the requirements of POX, while POX makes it possible to perform the oxidation of sulfide minerals and increase the gold extraction level. Furthermore, the total residence time (total time in the autoclave and bio-oxidation reactors) can be reduced compared with that required for the one-stage bio-oxidation process [32].
The literature information cited in the Introduction of the present article proposed that hydrometallurgical processes, including STRB and POX stages, may be developed to cope with the disadvantages of both the STRB and POX processes, which decrease their efficiency on an industrial scale. Therefore, our goals were to test STRB and POX, as well as their combination, including STRB and POX stages for processing concentrates of gold-bearing pyrite–arsenopyrite ore, and to evaluate the possibility of POX optimization using the preliminary STRB of the studied concentrate. Despite the processes used in the present work (POX and STRB) being well known, many of the peculiarities of conducting these processes are poorly understood. This study provides data on process performance and determines the possibilities of combining processes to increase gold extraction efficiency in industrial processes. The data in the present study have scientific and practical novelty since they can help increase the efficiency of industrial processes and describe the peculiarities of both STRB and POX, which are poorly understood.

2. Materials and Methods

2.1. Materials

The refractory gold-bearing flotation concentrate sample used in this study and the data on its composition were provided by the RIVS company (Saint Petersburg, Russia). The concentrate sample was obtained from a sulfide ore from the Bestobe deposit (the Republic of Kazakhstan). Similar concentrates obtained from ore samples from the same deposit were also used in previous works, all of which contained pyrite and arsenopyrite as the main sulfide minerals [26,33].
The concentrate was analyzed according to Filippova [34] and contained (mass percentages) 24.4% iron, 21.3% sulfide sulfur, 11.1% arsenic (Table S1), and 40.0 g/t Au. The iron and arsenic contents were determined via sample decomposition with a mixture of concentrated HNO3 and HCl and measurements of elemental concentrations using a Kvant-2A absorption spectrometer (Cortec, Moscow, Russia). The sulfide sulfur content was determined via sample decomposition with a mixture of concentrated HNO3 and HCl and further gravimetric determination with barium chloride. The sulfate sulfur content was determined via sodium carbonate extraction and the gravimetric determination of the sulfate content with barium chloride. The S0 content was determined via carbon tetrachloride extraction and spectrophotometric measurements [34]. Pyrite (FeS2) and arsenopyrite (FeAsS) were the main sulfide minerals (Figure S1). The pyrite content in the concentrate was 24.1%, and the arsenopyrite content was 32.8%. The fraction of gold associated with the sulfide minerals was approximately 58% according to gold phase analysis (or diagnostic leaching) [35] (Table 1). Thus, the main fraction of gold was bound to sulfide minerals, whereas gold bound to carbonaceous matter was not detected. Gold phase analysis or diagnostic leaching includes methods based on the sequential treatment of ores, concentrates, and other mineral samples with different reagents to deliberate the gold associated with mineral fractions for further cyanide leaching [35,36,37]. In the present work, we performed gold phase analysis according to the manual in [35].

2.2. Methods

In the present study, we aimed to determine the effect of the concentrate’s oxidative leaching (STRB or POX), as well as combined leaching (including STRB and POX stages), on gold recovery via cyanidation. Therefore, the concentrate was subjected to pretreatment with oxidative leaching (Figure 2 and Figure 3).
The initial concentrate, as well as the residues obtained after STRB, POX, and combined oxidation, were subjected to cyanidation to evaluate the effect of pretreatment on gold recovery.
Different variants of the experiment are shown in Figure 2 and Figure 3. Detailed descriptions of the experiments are provided in Section 2.2.1, Section 2.2.2, Section 2.2.3 and Section 2.2.4.

2.2.1. STRB Experiments

Continuous STRB was performed in 3 1-L laboratory-scale reactors connected in series under the following conditions: a stirring rate of 500 rpm, an aeration rate of 5 L/min, a temperature of 40 °C, and a pulp density of 15% (150 g of the concentrate per 1000 mL of liquid medium). Air was used as a source of oxygen for the microorganisms performing STRB. The residence time in each reactor was 2 days, and the total residence time was 6 days (Figure 3). The residence time was 6 days, reflecting industrial practice [3]. The total duration of the experiment in continuous mode was 50 days. The collection of STRB residues was conducted from the 26th to the 50th day.
During bio-oxidation, the parameters of the pulp liquid phase (pH, Eh, ferric/ferrous ions, and arsenic concentrations) were measured to control bio-oxidation. Liquid samples (of approximately 10 mL) were collected daily to perform measurements using the reactor outlets. We used 1 to 2 mL of the samples to determine iron and arsenic concentrations. Following pH and Eh measurements, the samples were returned to the reactors to avoid significant volume losses that could affect process performance. Since STRB processes do not require aseptic conditions, returning the samples was not dangerous for the reactors’ microbial populations. The stirred-tank reactors were equipped with TW-2.03 circulating water baths (Elmi, Riga, Latvia) and U-shaped titanium heat exchangers to maintain the required temperature, as well as RW20 overhead stirrers (IKA, Staufen, Germany) for stirring.
To perform STRB, a mixed culture of acidophilic microorganisms containing Acidithiobacillus caldus, Leptospitillum ferriphilum, Sulfobacillus spp., Ferroplasma acidiphilum, and Acidiplasma sp. was used. Table 2 presents the main characteristics of the microorganisms [38] detected in the culture and used as the inoculum. This mixed culture was previously used to bio-oxidize sulfide gold-bearing concentrates, and its composition was determined via metabarcoding V3–V4 16S gene fragments [26,33]. The pulp sample from the reactors, where previous experiments were performed, was used as the source of inoculum for the present work. The initial cell number in the pulp of the reactors after inoculation was approximately 1 × 108 cells/mL.
We used a liquid mineral nutrient medium for STRB containing the following salts (Chemically Pure, Rushim, Moscow, Russia) (g/L): (NH4)2SO4—0.750, KCl—0.050, MgSO4·7H2O—0.125, K2HPO4—0.125, and 1.0 L of distilled water (distilled water was obtained using a PHS AQUA 25 aquadistiller (TZMOI, Tyumen, Russia), which provided electrical conductivity to the resulting distillate of 3.0–4.0 µS/cm) [26,33]. A similar medium containing the same components was used in previous works, including experiments with sterile controls. However, the medium components did not provide sulfide mineral oxidation [39]. Thus, in the STRB experiment performed in the present work, the oxidation of sulfide minerals was provided via bio-oxidation using a microbial culture. The pH value was measured daily, maintained in a range of 1.0–1.2 by adding either concentrated H2SO4 (Chemically Pure, Laverna, Moscow, Russia) or CaCO3 (Chemically Pure, Rushim, Moscow, Russia) to the pulp to avoid inhibiting microbial activity and forming secondary iron hydroxide precipitation.

2.2.2. POX Experiments

The STRB residues, as well as the untreated concentrate, were subjected to autoclave oxidation using a Series 4532 titanium autoclave (Parr Instrument Company, Moline, IL, USA) at a volume of 1.9 L. POX was conducted under the following conditions: a temperature of 200 °C, oxygen pressure of 7 bar, residence time of 1 h, and pulp density (solid-to-liquid ratio) of 1:4. Technical oxygen (99.5% purity, according to the state standard GOST 5583-78 [40]) was used as an oxidant.

2.2.3. Cyanidation Experiments

The STRB residues, STRB residues subjected to POX, and an untreated concentrate and residue of concentrate POX (i.e., subjected to POX alone with no STRB) were used for gold extraction via cyanidation. Cyanidation was performed under the following conditions: a duration of 48 h, pulp density of 25%, and NaCN (Practically Pure, Korund, Dzerzhinsk, Russia) concentration of 2 g/L in a 1 L stirred-tank reactor. The cyanide concentration was determined using AgNO3 (Chemically Pure, Sibproekt, Moscow, Russia) titration to adjust its concentration to a level of 2 g/L during leaching [35].

2.2.4. Sampling and Analysis

The pH and redox potential (Eh) values were measured using a pH-150MI pH meter (Izmeritelnaya Tekhnika, Moscow, Russia). The concentrations of ferrous and ferric ions and arsenic were determined using trilonometric and iodometric titration, respectively, using a 10 mL Automatic Graduated Glass Burette (Khimlaborpribor, Klin, Russia) [41,42].
The solid residues of STRB and POX were separated via filtration using paper filters (Ekros, Saint-Petersburg, Russia) and a LABOPORT® KNF N 840.3 FT.18, 34 vacuum pump (KNF Neuberger GmbH, Freiburg, Germany) from the pulp, dried using a RedLine RF 53 drying and heating chamber (Binder, Tuttlingen, Germany) at 70 °C, and analyzed to assess the extent of Ss oxidation via phase analysis according to Filippova [34]. Averaged samples of STRB solid residue obtained from each reactor were used for further analysis and experiments. Elemental contents were determined via phase analysis as described above (Section 2.1), according to Filippova [34].

2.2.5. Data Processing

The results were processed using MS Excel 2013 15.0.459.1506 software (Microsoft, Redmond, WA, USA). The average values of the parameters are shown. For the STRB experiments, Table 3 shows the average values of liquid characteristics for 25 days. The cyanidation tests were performed in duplicate.

3. Results

Table 3 and Table 4 present the STRB experiment results.
The liquid-phase parameters corresponded to known patterns of bio-oxidation for pyrite–arsenopyrite gold-bearing concentrates [3]. The bio-oxidation of the main sulfide minerals of the concentrate (arsenopyrite and pyrite) is described with simplified overall reactions [3]:
2FeAsS + 7O2 + H2SO4 + 2H2O→2H3AsO4 + Fe2(SO4)3
4FeS2 + 15O2 + 2H2O→2Fe2(SO4)3 + 2H2SO4
Thus, the bio-oxidation of arsenopyrite and pyrite leads to the dissolution of iron and arsenic contained in the minerals. It also contributes to the formation of sulfuric acid due to the oxidation of the sulfur moiety. Thus, during bio-oxidation, ferric ions and arsenic concentrations in the liquid increase, and the pH value usually decreases due to sulfuric acid formation, whereas the Eh increases mainly due to ferric ion accumulation.
These patterns were observed in the STRB experiments performed in the present work.
The ferric iron and arsenic concentrations as well as Eh increased from the first to third reactors (Figure 2, Table 3). H2SO4 was added to the pulp of the first reactor, and its consumption was 38 kg/t of concentrate. In the first reactor, the bio-oxidation of the sulfide minerals by the microbial consortium probably failed to provide the required pH decrease due to the short residence time (2 days). In the second and third reactors, the pH values of the liquid phase were low, and CaCO3 was added to the pulp. Their consumptions were 25 and 36 kg/t of concentrate, respectively. Since the pH levels in the pulp of the second and third reactors were low, bio-oxidation in the first reactor (2 days) likely provided almost complete removal of carbonates.
The sulfide sulfur (Ss) oxidation extent is one of the key parameters that influence gold recovery from refractory gold-bearing concentrates [3]. Table 4 presents the results of Ss oxidation. It was shown in the present work that bio-oxidation significantly decreased the Ss content in STRB residues compared with the initial concentrate. As seen in Table 3, STRB oxidation decreased the sulfur content from 21.3% in the initial concentrate to 6.20%. Thus, despite significant Ss oxidation rates, Ss oxidation was not complete. The difference in Ss oxidation between the second and third reactors was not considerable compared with the first reactor. Thus, an STRB duration exceeding 4 days did not significantly increase Ss oxidation.
The POX of the STRB residues decreased the Ss content in the STRB residues (Table 5).
Further cyanidation demonstrated that POX also increased the gold recovery from the bio-oxidation residues (Table 5). The highest rate of gold recovery via bio-oxidation was achieved in 6 days, while both two-stage leaching and POX of the initial concentrate exceeded the gold recovery rate by 95%. Despite the high gold recovery from the concentrate subjected to POX, the cyanide consumption in this experiment was the highest. By contrast, the cyanide consumption was the lowest in variants with the POX of STRB residues, which can reduce the cost of reagents for cyanidation. Based on the results obtained, we cannot definitively explain the difference in cyanide consumption between the experiments when leaching STRB residues subjected to POX, as similar levels of Ss oxidation were reached in these variants (Table 5). Probably, this phenomenon may be explained by the difference in elemental sulfur content in the residues (Table S1), since elemental sulfur may react with cyanide, increasing its consumption due to thiocyanate formation [1]. In all STRB residues, the elemental sulfur content was higher than that in the residues obtained after POX treatment.
The optimal pulp densities for the POX of the concentrate and STRB residues calculated according to Equation (1) are shown in Table 6 and Figure 4.
Thus, even 2-day oxidation helped increase the optimal pulp density, while 4-day STRB doubled the pulp density.
Therefore, conducting SRPB bio-oxidation first and then POX pressure oxidation resulted in the highest gold recovery rate compared with all other studied process alternatives. Although the hybrid technology requires a longer processing time compared with pressure oxidation, having or resulting in similar gold extraction rates, bio-oxidation significantly reduced the sulfur content in the autoclave feed.

4. Discussion

The combined hydrometallurgical processing of pyrite–arsenopyrite concentrate using STRB and POX in sequence helped reach both high Ss oxidation rates and an increase in gold extraction via cyanidation. Both the biological or autoclave oxidative destruction of the main sulfide minerals (i.e., the disruption of the crystal lattice and dissolution of sulfur and metal moiety of minerals) made gold available for further cyanide leaching. According to the experience of practical STRB application on an industrial scale, this process needs a comparatively long residence time (several days) and, in some cases, does not provide high levels of destruction for some sulfide minerals at high pulp densities [3,4,9,10,13,14]. Comparatively low rates of sulfide mineral oxidation are the main disadvantage of STRB compared with other technologies (POX or roasting) [1,3,4,7,9,10]. In the present work, STRB for 4 days provided significantly higher Ss oxidation levels compared with STRB for 2 days. In the same time interval, the Ss oxidation levels after 4- and 6-day bio-oxidation did not differ significantly (73.8 and 78.7%, respectively). Thus, an STRB duration exceeding 4 days did not significantly increase Ss oxidation. This finding suggests that in these experiments, bio-oxidation could not provide complete Ss oxidation, while the increase in the STRB duration could not significantly increase sulfide mineral oxidation. Furthermore, gold recovery did not significantly increase when the STRB duration exceeded 4 days.
In contrast with STRB, POX provides high rates of Ss oxidation despite a short duration (approximately 1 h). At the same time, POX performance at high temperatures increases capital and operating costs compared with STRB due to the high corrosion rates of the equipment [7]. Moreover, to reach a high POX efficiency, concentrates with certain parameters (sulfide mineral and carbonate contents) should be processed [7,26].
The initial STRB of the concentrate, under conditions corresponding to industrial practices (residence time, temperature, and pulp density), did not provide high levels of gold extraction alone (<90%). Despite this, combined hydrometallurgical processing, including the STRB and POX of STRB residues, was a more effective method for treating refractory pyrite–arsenopyrite concentrates since it produced higher gold recovery compared with STRB alone and obtained partially oxidized and decarbonized sulfide products appropriate for POX treatment. A 4-day STRB treatment doubled the pulp density for POX. STRB, which lowered the Ss content compared with the initial concentrate, provided products with features appropriate for POX treatment.
Attempts to increase STRB efficiency in different sulfide concentrate processes by means of introducing additional stages to the flow sheet have been described in the literature [43,44,45,46,47,48,49]. The approaches proposed include two-stage bio-oxidation at different temperatures [43,44], the preliminary high-temperature chemical leaching of the concentrate [45,46,47,48], and an oxidation stage of the bio-oxidation residue [32,49].
In the work of [44], the bio-oxidation of pyrite–arsenopyrite concentrate was performed at temperatures of 30 and 42 °C in the first and second stages, respectively. The two-stage process helped increase the rate of sulfide mineral oxidation compared with the one-stage process at 30 °C. At the same time, introducing the first stage (30 °C) helped increase the pulp density by up to 20%, whereas one-stage bio-oxidation at 42 °C was unstable for a high pulp density.
The MesoTHERM process includes two bio-oxidation stages [44]. In the first stage, bio-oxidation is performed at 38–42 °C, while the second one is performed using thermophilic microorganisms, which perform oxidation in a range of 63–68 °C. Introducing the second stage increased the sulfide mineral oxidation rate and decreased cyanide consumption.
The authors of [45] proposed using the preliminary chemical oxidation stage at 50–80 °C with ferric sulfate solution prior to bio-oxidation. Preliminary chemical oxidative leaching increased both the sulfide mineral oxidation rate and gold recovery compared with one-stage STRB. Similar results were obtained with several gold-bearing concentrates in these works [46,47,48].
In a previous study, ferric leaching at 90 °C was also used in combination with bioleaching to increase the copper and zinc extraction rates from copper–zinc sulfide concentrate [49]. Compared with the works of [44,45,46,47,48], chemical leaching was performed after the bio-oxidation stage, which used unfavorable conditions for the microorganisms (a high temperature and salinity) to increase copper and zinc extraction.
Thus, approaches to improve STRB technology based on additional leaching stages prior to and after bio-oxidation have been actively developed and studied using various sulfide concentrates. At the same time, most of the methods proposed were studied only at a laboratory scale, except for the MesoTHERM [44] process and applying STRB in combination with POX [3,32].
In the present work, we studied the combination of two hydrometallurgical processes that are actively used in industry and have certain advantages and disadvantages. The combination of these two different methods mitigated the disadvantages to some extent, which may increase the total efficiency of the studied concentrate treatment. Since various concentrates differ in sulfide mineral content and mineral composition, the parameters for processing each specific concentrate may differ. With this understanding, the results of the present work may be used as a basis for further trials on treating refractory gold-bearing concentrates.

5. Conclusions

Stirred-tank reactor bio-oxidation (STRB) and acid pressure oxidation (POX) increased the gold recovery from the studied refractory pyrite–arsenopyrite concentrate. The bio-oxidation of this initial concentrate under conditions corresponding to those used in industrial practice (residence time, temperature, and pulp density) did not provide a high gold extraction rate. However, combined hydrometallurgical processing incorporating both STRB and POX was an effective treatment method for sulfide concentrates. This method increased gold recovery more than the STRB process and produced partially oxidized and decarbonized products appropriate for POX. In 4 days, STRB bio-oxidation followed by POX doubled the pulp density and increased the gold recovery. Thus, our results present novel information on the STRB and POX processes and can be used as a basis for the further treatment of refractory gold-bearing concentrates to increase the efficiency of industrial processes and develop new combined hydrometallurgical processes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr11113062/s1, Figure S1: X-ray diffraction pattern of the concentrate (XRD analysis was performed using D2 PHASER (CuKα) difractometer (Bruker, Billerica, USA). 1—mica; 2—chlorite; 3—fieldspar; 4—quartz; 5—arsenopyrite; 6—pyrite; 7—dolomite; Table S1: Main element content in the concentrate and STRB and POX residues (mass %); Table S2: Share of different size fractions of the concentrate (mass %) analyzed using VP50 sieve analyzer (Vibrotechnik, Saint-Petersburg, Russia) and gold content in size fraction (share of the total gold, %).

Author Contributions

Conceptualization, A.B. (Anna Boduen) and A.B. (Aleksandr Bulaev); methodology, A.B. (Anna Boduen) and A.B. (Aleksandr Bulaev); investigation, A.B. (Anna Boduen), M.Z., V.M., V.G. and A.B. (Aleksandr Bulaev); writing—original draft preparation, A.B. (Anna Boduen) and A.B. (Aleksandr Bulaev); writing—review and editing, A.B. (Anna Boduen), M.Z., V.M., V.G. and A.B. (Aleksandr Bulaev). All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Ministry of Science and Higher Education of the Russian Federation.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 11 03062 g001
Figure 1. Dependence of POX pulp density on sulfur content in sulfide concentrates according to Equation (1) [27].
Figure 1. Dependence of POX pulp density on sulfur content in sulfide concentrates according to Equation (1) [27].
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Processes 11 03062 g002
Figure 2. Variants of the experiment: (a) cyanidation of the initial concentrate; (b) STRB of the concentrate and cyanidation; (c) POX of the concentrate and cyanidation; and (d) combined oxidative pretreatment of the concentrate (STRB and POX) and cyanidation.
Figure 2. Variants of the experiment: (a) cyanidation of the initial concentrate; (b) STRB of the concentrate and cyanidation; (c) POX of the concentrate and cyanidation; and (d) combined oxidative pretreatment of the concentrate (STRB and POX) and cyanidation.
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Figure 3. Flowchart of the process studied in the present work.
Figure 3. Flowchart of the process studied in the present work.
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Processes 11 03062 g004
Figure 4. Dependence of POX pulp density on sulfur content in the concentrate and STRB residues [27]. Inverted triangles indicate sulfide sulfur contents in the initial concentrate, STRB residues, and corresponding POX pulp densities.
Figure 4. Dependence of POX pulp density on sulfur content in the concentrate and STRB residues [27]. Inverted triangles indicate sulfide sulfur contents in the initial concentrate, STRB residues, and corresponding POX pulp densities.
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Table 1. The results of gold phase analysis.
Table 1. The results of gold phase analysis.
Gold PhaseShare of the Total Gold, %
Free gold (recoverable via direct cyanidation)32.38
Alkaline-soluble mineral-bound gold (recoverable via cyanidation after alkaline pretreatment)4.36
Iron–manganese oxide-bound gold (recoverable via cyanidation after hydrochloric acid pretreatment)1.14
Sulfide-mineral-bound gold (recoverable via cyanidation after nitric acid pretreatment)58.41
Silicate mineral gold (recoverable via cyanidation after HF pretreatment)3.71
Table 2. Main characteristics of microorganisms in the inoculum [38].
Table 2. Main characteristics of microorganisms in the inoculum [38].
MicroorganismCharacteristic
Oxidized
Inorganic
Compounds		Carbon
Nutrition		Temperature
Characteristic
Acidithiobacillus spp.RISC 1AutotrophModerate thermophile
Leptospitillum spp.Fe2+AutotrophMesophile
Sulfobacillus spp.Fe2+, RISCMixotrophModerate thermophile
Ferroplasma spp.Fe2+HeterotrophMesophile
Acidiplasma spp.Fe2+HeterotrophModerate thermophile
1 RISC—reduced inorganic sulfur compound.
Table 3. Liquid-phase parameters in STRB reactors (average values for 25 days of continuous experimentation) (average values ± SD).
Table 3. Liquid-phase parameters in STRB reactors (average values for 25 days of continuous experimentation) (average values ± SD).
ReactorTotal Residence Time, dpHEh, mVConcentration, g/L
Fe3+Fe2+∑FeAs
121.18 ± 0.06724 ± 713.88 ± 1.360.12 ± 0.0914.00 ± 1.323.70 ± 0.33
241.09 ± 0.07756 ± 1422.16 ± 1.940.13 ± 0.1022.30 ± 1.874.38 ± 0.42
361.10 ± 0.01775 ± 1125.13 ± 0.72Trace concentrations25.31 ± 0.724.29 ± 0.58
Table 4. STRB results.
Table 4. STRB results.
ReactorTotal Residence Time, dSs Content in the Residue, %Residue Yield, %Ss Oxidation, %
1216.473.743.4
248.664.773.8
366.273.678.7
Table 5. Cyanidation results.
Table 5. Cyanidation results.
VariantSs Oxidation, %Au Recovery via
Cyanidation, %		Consumption of
Cyanide, kg/t
STRB residue2 days43.465 ± 518
4 days73.882 ± 121
6 days78.787 ± 225
POX residuePOX of initial concentrate9998 ± 128
POX of 2-day bio-oxidation residue9797 ± 116
POX of 4-day bio-oxidation residue9896 ± 16
POX of 6-day bio-oxidation residue9997 ± 14
Concentrate-58 ± 17.3
Table 6. Optimal pulp density for POX calculated based on Equation (1) for the initial concentrate and STRB residues.
Table 6. Optimal pulp density for POX calculated based on Equation (1) for the initial concentrate and STRB residues.
ProductInitial ConcentratePOX of 2-Day STRB
Residue		POX of 4-Day STRB
Residue		POX of 6-Day STRB
Residue
Ss content, %21.316.48.66.2
Pulp density (Wt.% Solids), %14.117.729.837.7
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Boduen, A.; Zalesov, M.; Melamud, V.; Grigorieva, V.; Bulaev, A. Combined Bacterial and Pressure Oxidation for Processing High-Sulfur Refractory Gold Concentrate. Processes 2023, 11, 3062. https://doi.org/10.3390/pr11113062

AMA Style

Boduen A, Zalesov M, Melamud V, Grigorieva V, Bulaev A. Combined Bacterial and Pressure Oxidation for Processing High-Sulfur Refractory Gold Concentrate. Processes. 2023; 11(11):3062. https://doi.org/10.3390/pr11113062

Chicago/Turabian Style

Boduen, Anna, Maxim Zalesov, Vitaliy Melamud, Victoria Grigorieva, and Aleksandr Bulaev. 2023. "Combined Bacterial and Pressure Oxidation for Processing High-Sulfur Refractory Gold Concentrate" Processes 11, no. 11: 3062. https://doi.org/10.3390/pr11113062

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