Technology Development and Industrial Practice of Distinct Low-Cost Heap Bioleaching at Monywa Copper Mine

Abstract

This paper presents a case study on heap bioleaching at the Monywa copper mine in Myanmar. Through mineralogical characterization and leaching tests, specific heap bioleaching technologies were developed and implemented at the mine. These technologies include acidification and start-up of heap bioleaching without external acid addition, ore classification with process optimization, and selective inhibition of pyrite oxidation for acid/iron balance during heap bioleaching. The optimized heap bioleaching technologies implemented at the Monywa copper mine have reduced both capital and operating costs. These advantages are specifically reflected in the use of multi-lift pads for both heap bioleaching and final residue storage, optimized processing based on ore characteristics, and the implementation of a solution closed cycle process without the need for additional acid or neutralization. These findings demonstrate a cost-effective approach to heap bioleaching and provide practical insights for operational optimization in similar copper mines.

1. Introduction

Bio-hydrometallurgy, also known as bio-oxidation or bioleaching, is a process that utilizes specific microorganisms to extract valuable metals from minerals or other materials [1]. This technology is primarily used to exploit low-grade ores, tailings, and waste ores that are difficult or expensive to recover using traditional smelting methods [2,3,4]. One commonly applied method is heap bioleaching, which has been used for processing copper sulfide ores for several decades, mainly focusing on secondary copper sulfide ores [5]. While heap bioleaching exhibits slower leaching kinetics and occasionally lower copper recovery rates than flotation–smelting processes, its substantially lower capital and operational costs make it a more economically viable option for low-grade copper ores [6]. The heap bioleaching process involves piling coarse ore on an impermeable liner and utilizing microorganisms to oxidize sulfide minerals to release metal ions. Commonly utilized microorganisms include acidophilic iron- and sulfur-oxidizing bacteria and archaea [7]. The dissolved copper in the solution is then collected and processed further through well-established solvent extraction–electrowinning techniques.

Although heap bioleaching has been successfully implemented in numerous mines, significant challenges remain in developing and operating efficient bioleaching plants [8]. A primary obstacle is the lack of universally applicable processes due to the inherent variability in ore mineralogy and composition across different mines [9]. Additionally, microorganisms require a certain amount of time to oxidize and dissolve sulfide minerals, so the reaction kinetics of bioleaching are relatively slow [1]. Different ores and leaching periods may necessitate the use of specific types of microorganisms for effective bioleaching. However, finding and cultivating suitable microbial strains for specific ores is a complex and time-consuming task. Bioleaching efficiency is strongly dependent on multiple interdependent parameters including temperature, oxygen supply, pH, and nutrient concentration [7]. Maintaining an optimal balance among these factors presents considerable operational challenges. Furthermore, the concomitant dissolution of gangue minerals during copper leaching introduces impurities into the pregnant leach solution (PLS), which can significantly compromise the subsequent solvent extraction and electrowinning processes [10]. Addressing these difficulties requires continuous research and technological innovation. Previous studies have shown that optimizing microbial selection and improving control over reaction conditions can increase the reaction rate and ultimately enhance extraction efficiency [1].

The Monywa copper mine, located in Myanmar at coordinates 22°07′ N and 95°02′ E, is recognized as one of the largest copper sulfide mines in Asia [11]. Within this mine, the newly operated Letpadaung ore body holds an ore reserve of over 1.5 billion tonnes, with an average copper grade of approximately 0.40%, and geological reserves of over 5 million tons of copper. The Letpadaung copper mine is situated in the Sagaing region of Myanmar. The heap bioleaching–solvent extraction–electrowinning process was designed and employed for processing purposes at this mine [12].

However, similarly to that at other plants, the heap bioleaching process at Letpadaung encountered several operational challenges, such as the permeability problem of the clay ore type, huge acid and iron requirements for heap bioleaching start-up, and difficult control of the acid–iron balance during heap bioleaching [13]. In this study, these challenges were addressed through technological development, and a distinct low-cost heap bioleaching–solvent extraction–electrowinning plant was established at Letpadaung. The specific solutions include the following: (i) Ore was stacked either after primary crushing or by run-of-mine (ROM) stacking using trucks and stackers on the multi-lift permanent heap. (ii) Ore was directly stacked without undergoing pelletization or aeration treatment, and all solutions were recirculated within the plant without sulfuric acid addition, solution neutralization, or pumping outside. This manuscript focuses on the introduction of the principle and technological development of various technologies, including the use of water for acidification and heap start-up, ore classification, and their operational optimization, as well as maintaining a balance of acid and iron during heap bioleaching, along with their industrial application.

2. Mineralogy and Leaching Tests

The mineral composition of the ore was analyzed using Mineral Liberation Analyzer (MLA), and the results are shown in

Table 1. Typical average mineral composition of Monywa Letpadaung ore determined by Mineral Liberation Analyzer (MLA).

Mineral	Formula	Percentage (%)
Chalcocite	Cu2S	0.52
Chalcopyrite	CuFeS2	0.10
Pyrite	FeS2	12.13
Quartz	SiO2	56.46
Alunite	KAl3(SO4)2(OH)6	12.93
Limonite	2Fe2O3∙3H2O	2.10
Sericite–illite–kaolinite–pyrophyllite	K2(AlFeMg)4(SiAl)8O20(OH)4∙nH2O, Al4Si8O20(OH)4∙nH2O, Al4SiO4O10(OH)8 Al2(Si4O10)(OH)2	9.56
Alkali gangue minerals	CaMg(CO3)2, CaCO3, FeCO3	1.2

. The copper minerals in the sample were mainly chalcocite and chalcopyrite, along with trace amounts of bornite, covellite, and enargite. Other metallic minerals included pyrite, with trace amounts of hematite, limonite, and sphalerite. The non-metallic minerals were mainly quartz and alunite, followed by clay minerals (including mainly sericite, illite, pyrophyllite, and kaolinite), occasionally accompanied by apatite, gibbsite, and zircon. The content of acid-consuming minerals, such as dolomite, calcite, and siderite, was low.

As shown in Figure 1, chalcocite primarily occurs in granular and irregular forms, embedded between pyrite and other mineral grains, as well as within fractures. It is commonly observed in fine vein-like occurrences within pyrite fractures, with some instances of chalcocite occurring between pyrite and non-metallic mineral grains. This structural arrangement of chalcocite facilitates copper recovery through hydrometallurgy methods [14]. Occasionally, fine-grained chalcocite is encapsulated by pyrite and other minerals (Figure 1d), making this form susceptible to losses in the leaching residue.

Based on the mineralogy analysis, it is expected that chalcocite in the ore exhibits high leachability, except in cases where it is enclosed within other minerals. Prior to industrialization, several leaching tests were conducted using different types of Monywa ore samples obtained from drill core samples and the mine pit, including shake flask tests, column tests, and pilot heap leaching tests. These tests generally confirmed that chalcocite in Myanmar’s sulfide ores exhibits good leachability, and the copper recovery rates from column leaching tests are presented in

Table 2. Leaching performance of different ore types by column leaching tests.

Ore Type	Ore Crush Size	Leaching Time (Day)	Copper Recovery (%)	H2SO4 Production (kg/kg Cu) *
High clay	P80 = 180 mm	186	85.2	−1.11
	P80 = 50 mm	186	83.3	−1.34
Medium clay II	P80 = 180 mm	206	84.2	1.23
	P80 = 50 mm	206	85.6	1.32
Medium clay I	P80 = 180 mm	206	80.5	1.86
	P80 = 50 mm	206	83.2	2.32
Low clay	P80 = 180 mm	206	72.3	2.98
	P80 = 50 mm	206	80.5	3.07

* Acid balance includes the acid generation during leaching and also the H⁺ back to leaching through raffinate during solvent extraction, calculated as (acid generation during leaching + 1.54 × Cu production)/ore weight. Negative numbers indicate that the ore required H₂SO₄ consumption.

. The column leaching apparatus comprised a custom-made plexiglass column (Φ50 mm × 200 mm). The operating temperature was set at 30 °C, and the pH of the leaching solution was 1.8. Notably, the leaching experiments were conducted solely with sulfuric acid and did not involve microbial inoculation. Copper recovery rates from most ore samples exceeding 80% were achieved over a leaching period of approximately 200 days when using a crush size of P₈₀ = 180 mm and 50 mm, indicating the viability of heap bioleaching for the ore. The leaching performance of different ore types shows that the copper leaching rate of low-clay ores is slightly lower than that of high-clay ores, and the leaching recovery of low-clay ores is significantly affected by ore particle size. This phenomenon does not indicate that a higher clay content leads to a better copper recovery, and the occurrence state of minerals determines the contact efficiency between the leaching solution and copper minerals. MLA analysis reveals that the vein-like distributed copper minerals in Monywa ore are the fundamental reason for the efficient leaching process. Copper distributed in a dispersed manner inside hard ores is more difficult to leach because hard ores are not easy to weather, making it hard for the leaching solution to make contact with this part of the copper. In contrast, chalcocite with a vein-like distribution is more likely to be connected to the out-side, allowing the leaching solution to leach out the copper. Combined with the result that the leaching rate of low-clay minerals is affected by ore particle size, it can be found that for medium- or high-clay ores, this part of minerals may eventually be exposed. However, for low-clay primary zone hard ores, the exposure of this part of ores is relatively difficult, and a finer ore particle size is required to achieve effective leaching. At the same time, the clay content mainly affects the permeability, and this influence is more limited in column leaching tests compared to industrial-scale operations.

The ore from deeper layers of the deposit contained a higher percentage of chalcopyrite, which could potentially result in lower copper recovery during future heap bioleaching processes. As shown in Table 2, the low-clay ore type exhibited lower copper recovery rates, which could be attributed to the presence of chalcopyrite that is less susceptible to weathering during leaching. Additionally, in some of the closed-cycle column leaching tests, the concentration of acid accumulated to over 20 g/L, and the iron concentration increased to over 40 g/L. It is important to consider that the ore contains approximately 12% pyrite, which undergoes oxidation during copper leaching. Furthermore, the gangue minerals in the ore have low acid consumption, potentially leading to excessive iron and acid concentrations in the solution. This poses a risk to the solvent extraction and electrowinning processes [13]. Moreover, certain portions of the ore contain significant amounts of clay minerals, especially in the upper part of the ore body. Although solution percolation was not completely blocked in the column leaching tests, permeability issues may still arise during heap leaching. These factors should be given careful attention during industrial production to ensure optimal results.

3. Process Description

Heap bioleaching–solvent extraction–electrowinning was employed to process the Monywa copper mine (Figure 2). The ore was classified into different types based on geological modeling, including low-clay, medium-clay I, medium-clay II, and high-clay ore. The entire ore deposit was divided into different ore types based on the labeling of drill core samples according to the clay ore type and content. Due to differences in mineral composition and weathering performance, the required crushing fineness for leaching varied for each type of ore. Ore with a lower clay content was crushed and stacked using conveyor belts, while ore with a higher clay content was directly stacked using trucks. The stockpile was a multi-layer permanent heap, serving as both the leaching site and storage for leached residue after leaching. No agglomeration or sieving of the ore was applied before stacking, and no aeration was used in the heaps.

Once the leachate reached a certain copper concentration (usually > 4 g/L), it entered the extraction–electrowinning workshop, and raffinate was returned to the stockpile as the irrigation solution. If the copper concentration did not reach the qualified liquid concentration level during the leaching process, it was used as intermediate solution for circulating irrigation on the heap leaching field. Rainfall in the Monywa area during the rainy season had a significant impact on the water balance of the heap leaching system, with excess solution collected in storm ponds based on the copper concentration and replenished with river water during the dry season. No sulfuric acid was added to the heap bioleaching solution, and all solutions circulated within the system without external discharge.

4. Critical Technology Development of Monywa Heap Bioleaching

4.1. Acidification and Start-Up of Heap Bioleaching Without Acid Addition

Since the pyrite content is high in the ore, a proposal was made to acidify the heap through pyrite oxidation. Pyritic mine waste produces acid mine drainage in some mines worldwide, and column leaching tests showed net acid generation for most column leaching tests of Letpadaung ore, lending confidence to this approach. Batches of column tests combined with solvent extraction (for solution recycling) demonstrated that water irrigation alone could sustain copper leaching while steadily increasing acid and iron concentrations. Once the ore’s pH dropped below 3.0, microbial activity was initiated to enhance leaching. To further amplify pyrite oxidation, a novel strategy was proposed: maintaining ore wetness during transport and heap irrigation to prolong the oxidation time and accelerate the reaction rate. This method initially only used water for acidification treatment to reduce the pH value to 3.0, after which bioleaching microbes were introduced via irrigation to optimize copper recovery.

The implementation of water-based start-up for heap bioleaching significantly reduced the need for acid procurement and temporary storage facilities during the initial production phase, resulting in a decrease in the initial amount of acid required for future excess acid at the Letpadaung copper mine, reducing future neutralization costs. The ore was excavated from the mine pit and stacked in heaps from 2015, with heap bioleaching commencing at the end of the year. The ore was already acidified upon stacking in heaps, with a pH of 3.0–4.0. Under this pH, dissolved Cu²⁺ would not hydrolyze, and copper concentrations could reach as high as 4 g/L (Figure 3c). After solvent extraction of the Cu²⁺, H⁺ would return via raffinate, making the leaching more sustained. Between 2016 and 2017, approximately 35.58 million tonnes of ore were processed, producing 76,000 tonnes of copper, and the mine started production smoothly. By 2018, the acid and iron concentrations in the leaching system had reached 16 g/L and 32 g/L, respectively (Figure 3a,b), meeting the requirements for copper leaching and facilitating the construction of an efficient microbial community for bioleaching, and the copper concentration had reached 5 g/L. With a bacterial concentration of 10⁷ cells/mL, the continuous expansion of heap leaching system was ensured.

Furthermore, 16S rDNA analysis confirmed the growth of functional microbes, showing a microbial community dominated by Acidithiobacillus ferrooxidans (At. ferrooxidans) and other species such as Acidithiobacillus thiooxidans (At. thiooxidans), Acidithiobacillus caldus (At. caldus), Leptospirillum ferriphilum (L. ferriphilum), Leptospirillum ferrooxidans (L. ferrooxidans), Sulfobacillus acidophilus (S. acidophilus), Ferroplasma acidiphilum (F. acidiphilium), and Acidiphilium sp. Most of the microbes were iron-oxidizing bacteria, with a small portion being sulfur-oxidizing bacteria. At. ferrooxidans is usually the dominant species in many heap bioleaching plants, especially when the solution is not acidified and does not accumulate too many salts [7]. At. ferrooxidans is highly active in iron oxidation, and previous studies have shown that the Leptospirillum and Ferroplasma groups are dominant in extreme environments with low pH and high ionic strength [11].

4.2. Ore Classification and Operational Optimization

The Letpaduang deposit employed two different heap leaching fields for production. Pad 1# utilized a belt transportation system to save costs, while Pad 3# relied on truck transportation due to its close proximity to the mining pit. During the construction of the first three cells of Pad 1#, no classification of ore properties was conducted. All ores from the mining area were sent to the crushing system, transported to the heap leaching field via conveyor belts, and stacked. However, it was observed that the primary crushed ore in Pad 1# had a lower copper leaching rate and final recovery compared to the ROM stacking ore in Pad 3# (Figure 4). Copper recovery in Pad 3# consistently exceeded 70%, while in Pad 1# it was only around 55%. Liquid accumulation and channeling were observed on the heap surface during irrigation in Pad 1#, significantly impacting the permeability of the heap field and hindering leaching effectiveness. Despite attempts to address this issue through measures like heap rehandling, the leaching rate and recovery rate remained significantly lower when compared to Pad 3#. After 30 days of leaching, the lowest leaching rate in Pad 1# was only 2%, reaching 10% after 90 days. It took approximately 400 days to achieve a leaching rate of around 55%, which was much lower than the design target. The slow leaching rate and low recovery rate resulted in resource wastage and limited the production capacity of the Letpaduang copper mine.

It was discovered that without ore classification, the different clay ore types were mixed and crushed together. As a result, the medium- and high-clay copper ores were crushed first and subjected to secondary impact and crushing from the low-clay hard ores. This excessive crushing of the clay ore type generated a large amount of fine ore. The high content of fine ore caused frequent blockages in the intermediate ore stacker. Additionally, the funnel of the belt transportation system at the rear end of the ore stacker experienced severe blockages, especially during the rainy season. These issues led to a decrease in production capacity and a much slower heap construction rate than expected. In the copper heap bioleaching process, the permeability of the heap is a critical factor affecting the copper leaching rate and final recovery rate. However, due to the large amount of clay fines in the crushed ore, the permeability of the heap field became very poor. This resulted in lower leaching rates and recovery rates for the medium- and high-clay copper ores after crushing. On the other hand, with ROM ore stacking by truck, copper recovery was faster and higher compared to that with the crushed ore.

After site sampling analysis and laboratory tests were conducted, the ores from the deposit were classified into different types based on their clay content and weathering degree in an acid solution. These types included low clay, medium clay I, medium clay II, and high clay (Figure 5). Following the ore classification, the high-clay ore and medium-clay II ore were not crushed, significantly improving the permeability in Pad 1#. The average copper leaching rate doubled, and the final copper recovery exceeded 75%. Meanwhile, Pad 3# leaching was not affected.

4.3. Select Inhibition of Pyrite Oxidation for Acid/Iron Balance

The oxidation of pyrite in the ore provides Fe³⁺ oxidant and sulfuric acid, which are essential for heap bioleaching [15]. Because of pyrite oxidation, initially, only water irrigation was used to start the heap bioleaching process at a low cost. This approach reduced the use of sulfuric acid and future neutralization costs. As mentioned above, the high pyrite content of ore in Monywa has an acid neutralization potential of about 7.5 kg/t [12]. The leaching–solvent extraction process for chalcocite itself generates net acid. Therefore, around 5% pyrite oxidation could result in excessive sulfuric acid generation and an accumulation of iron. To prevent this, measures should be taken to inhibit pyrite oxidation and thereby control the increase in acid and iron levels. Column tests and industrial practices at the Monywa mine have also shown excessive accumulation of sulfuric acid and iron in the circulating solution. However, in Myanmar, due to political and economic factors, building and operating neutralization plants to decrease acidity and ion concentrations in the heap leaching solution is expensive. Moreover, neutralizing millions of tonnes of acidic solution presents significant operational challenges and carries potential environmental risks.

The chemical oxidation of pyrite is highly sensitive to the environmental redox potential. It has been reported that a 100 mV increase in redox potential leads to a fivefold increase in the pyrite oxidation rate [16,17]. The rest potential of pyrite is approximately 650 mV (vs. Standard Hydrogen Electrode, SHE), meaning that pyrite oxidation barely occurs at lower oxidation potentials [18]. Fortunately, for bioleaching microbes, the gradual accumulation of iron, acid, and other ions (such as magnesium and calcium) in the solution naturally inhibits their activity, leading to a decrease in the oxidation of Fe²⁺ to Fe³⁺ and a subsequent decrease in the redox potential within the heaps. This natural process slows down pyrite oxidation. However, this natural inhibition is not sufficient. By 2018, the acid concentration had already exceeded 15 g/L, and the iron concentration had exceeded 30 g/L, both of which are too high for bioleaching microbes and also have adverse effects on the solvent extraction process [12]. When the acid concentration in the qualified solution reaches around 16 g/L, the extraction efficiency decreases from ~95% to only 60%. As a result, the transfer of copper from the extraction workshop to the electrowinning workshop decreases by about 35%, leading to reduced cathode copper production due to the decreased production capacity of the extraction workshop.

The rate of pyrite oxidation is sensitive to redox potential [18], while chalcocite is not as sensitive [19,20]. This allows for selective inhibition of pyrite oxidation. In the leaching system, the activity of iron-oxidizing bacteria is the key factor determining the redox potential. By inhibiting the activity of iron-oxidizing bacteria with chemicals, the redox potential can be suppressed, thereby achieving the inhibition of the oxidation of pyrite. Therefore, by using a specific reagent, we developed a technology that selectively inhibits the microbial oxidation of Fe²⁺ to decrease the redox potential. Column tests and pilot heap tests showed that under the inhibition conditions, the solution’s redox potential remained low throughout the leaching process, with the potential staying below 450 mV for hundreds of days. In contrast, in the control tests, the solution’s potential increased rapidly, reaching a peak of 700 mV. Under low-redox-potential conditions, pyrite oxidation is significantly inhibited, while copper dissolution is barely affected (Figure 6). These results align with previous reports that pyrite oxidation is strongly influenced by the redox potential of the solution, while chalcocite oxidation is weakly influenced by it. Furthermore, the switch in pyrite oxidation is still determined by the solution’s redox potential in the presence of bacteria [18].

Based on the above investigations, we have developed a technology for selectively inhibiting pyrite oxidation through control of the redox potential during heap bioleaching of copper sulfide ore. Based on iron production estimates, pyrite dissolution was 8.32% in the control tests and only 0.83% in the inhibition tests, resulting in over 80% inhibition of pyrite oxidation.

5. Discussion

Although heap bioleaching has been used for processing copper sulfides, mainly secondary copper sulfides, for several decades, different copper deposits and their various deposit positions present distinct characteristics, which require different operational strategies. As such, optimizing heap bioleaching at different leaching stages is critical to the success of a project. The Letpadaung heap bioleaching practice can provide valuable insights for other plants.

One characteristic of the Letpadaung mine is that the ore body contains different ore types, particularly related to the clay content, which is likely to be true for other mines as well. The clay mineral type and content reflect the natural weathering degree of the rock, which affects copper exposure under acidic solutions and heap permeability [10]. Thus, different operational parameters are required for different ore types. The Letpadaung mine achieved optimized copper recovery through simple ore classification and processing. It was proven that the high-clay ore type can reach copper recovery of over 80% even at the ROM size. In contrast, low-clay ore types may require primary, secondary, or tertiary crushing. Maintaining heap permeability is vital for heap bioleaching as it serves as the basis for reaction and leaching [21,22]. If ore is not planned to be agglomerated, larger ore sizes are preferred to guarantee permeability, with minimal crushing preferred as clay minerals tend to produce fine particles that can block heap porosity. Additionally, low-clay types that are less prone to weathering during heap bioleaching require more crushing to achieve higher copper recovery. The deep ore body of the Letpadaung mine contains a higher percentage of chalcopyrite than recently mined ores. A crushed size of P₈₀ = 20 mm or less is preferred for chalcopyrite ore types [23]. Thus, secondary or tertiary crushing may be required for higher copper recovery of the deep ore body in future operations.

Another characteristic of the Letpadaung mine is that its pyrite content is relatively high at approximately 12%. This made it possible to acidify the heap by only water irrigation, accelerating pyrite oxidation through industrial measures at each step of mining, transportation, stacking, and irrigation [12]. Pyrite oxidation was managed by various kinds of measures to progress from a neutral condition to an acidic microbial condition. This approach not only saved costs associated with sulfuric acid but also did not add any burden to future acid and iron balance. However, the high pyrite content may impact future heap bioleaching and solvent extraction–electrowinning processes. In contrast, other secondary copper sulfide bioleaching mines globally, mainly located in arid desert areas in South America, contain a higher amount of chalcopyrite and lower pyrite content, mostly less than 3% [24]. And many of them use on–off heaps, which means that the pyrite oxidation time is much shorter than that in the permanent heap. These mines exhibit net acid consumption during the leaching–extraction process, requiring 1–2 kg/t of sulfuric acid addition during agglomeration before heap stacking.

In several cases of acid-excess heap leaching operations, environmental pollution incidents have occurred due to the high pyrite content and location in rainy regions. Examples include the Zijinshan mine environmental incident in 2010, where acid water overflow resulted in direct economic losses of hundreds of millions of Renminbi (RMB) and brought shame to Zijin Mining [14,25]; the Talvivaara mine incident in 2014, where leakage from the pregnant solution pond was the final straw that led to the bankruptcy of Talvivaara [26,27]; and the Cananea copper mine, which caused pollution of downstream rivers. The pyrite content in Letpadaung is much higher than that in these mentioned heap leaching plants. The Letpadaung mine’s advantage in acid–iron balance lies in its lower rainfall and higher evaporation rate, ensuring a net water supply system. However, as the mine is located adjacent to main rivers and villages, controlling acid water overflow should be given special attention. The crucial factors to selectively inhibit pyrite oxidation without influencing copper leaching were elucidated, and then the technology was developed for controlling pyrite oxidation through redox potential management under microbial Fe²⁺ oxidation regulation. We have successfully managed the iron and acid balance without neutralization till now, and we also have confidence in the future technology to maintain suitable pyrite oxidation even though the pyrite content in Letpadaung is much higher than that in other mines.

6. Conclusions

The Monywa Letpadaung copper mine has developed a distinct low-cost heap bioleaching technology through integrated process optimization, demonstrating key innovations in ore management, pyrite oxidation control, and solution cycling. Ore classification processing without agglomeration is needed, and a multi-lift permanent heap style can guarantee permeability under a relatively larger ore size. To control the pyrite oxidation rate, no aeration is employed. Through measures promoting and inhibiting pyrite oxidation, the acid–iron balance in leach solutions is maintained, requiring no sulfuric acid addition or acid neutralization. Besides this, the solution cycling system poses limited hazards to the environment. This distinct low-cost heap bioleaching case will also provide valuable insights for other heap bioleaching plants.