Towards Circularity in Serbian Mining: Unlocking the Potential of Flotation Tailings and Fly Ash

Abstract

This paper examines sustainable industrial practices in Serbia, particularly in the mining and energy sector, focusing on the potential of flotation tailings and fly ash, as materials with the largest share in disposed waste in Serbia in 2023 (95%). It highlights the environmental challenges of mining waste and explores innovative approaches to waste management within the circular economy framework. The study analyzes the current state of mining waste in Serbia, particularly in copper mining regions in the east of the country. It discusses the potential for metal recovery from waste and its reuse in various industries. The research also investigates the use of fly ash from thermal power plants as a valuable resource in the construction industry and other sectors. The paper reviews existing initiatives and legislation in Serbia in order to promote sustainable mining practices and waste utilization. By presenting case studies and potential applications, the study demonstrates how implementing circular economy principles in the mining sector can contribute to environmental protection, resource conservation, and economic growth in Serbia. The comprehensive overview of the current state in Serbia provides a solid foundation for establishing a higher degree of circularity in the mining and energy sectors.

1. Introduction

Mining is a key factor in the economic development of a country. As a pillar of economic and social development, mining improves living standards but simultaneously causes environmental degradation and increased consumption and depletion of natural resources. According to the global report on resources for the year 2024, published by the UN [1], the exploitation of raw materials has tripled in the last 50 years. The report also predicts that this number will increase by 60% if urgent and coordinated actions are not taken.

Large amounts of mining waste generated during the mining process pose a significant environmental hazard. Mining activities generate over 100 billion tons of solid waste yearly globally, representing a significant environmental challenge requiring appropriate management strategies [2]. It is estimated that the annual global production of metal mining waste is about four billion tons [3]. At the same time, over 1.2 billion tons of residual waste has already been accumulated in the European Union (EU) [4].

Mining waste generated during the exploitation, extraction, and processing of mineral raw materials can be in liquid form, as wastewater or acid mine water, and solid waste as flotation tailing. Additional descriptive waste categories are given in

Table 1. Mining and mineral processing waste and its characteristics [5].

Category	Description	Characteristics and Impacts
Overburden	Material removed to access mineral deposits.	- Consists of soil and rocks, often in large volumes. - Low immediate pollution levels can cause soil erosion and loss of vegetation. - It is used for reclamation, but improper management can disrupt ecosystems.
Waste rock	Material with low concentrations of minerals is not economically viable for extraction.	- When exposed to air and water, it often contains sulfide minerals, leading to acid mine drainage (AMD). - Heaps can destabilize landscapes, causing landslides. - Pollution of nearby water sources is common, affecting aquatic life and human consumption.
Tailings	Finely ground material is generated during the processing of mineral resources.	- Contains residual minerals and flotation chemicals like cyanide or sulfuric acid. - Highly prone to leaching, contaminating groundwater and surface water. - Tailings storage facility failures can result in catastrophic environmental disasters, spreading pollutants over vast areas. - Dam failure.
Mine water	Water generated during mining activities.	- Includes acid mine drainage (AMD) rich in heavy metals and sulfates. - It can affect agricultural land, rendering it infertile. - Long-term pollution persists in water bodies, harming biodiversity and reducing drinking water quality.
Sludge	Sludge produced from mine water treatment in active facilities.	- Dense mixture of solids and treatment chemicals. - It may contain toxic elements such as arsenic, lead, or cadmium, posing significant disposal challenges. - Requires safe containment to prevent leaching into soil and water.
Gaseous waste	Gaseous emissions from mining processes.	- Dust from blasting can cause respiratory illnesses like silicosis. - Emission of gases like sulfur oxides (SOx) and nitrogen oxides (NOx) contributes to acid rain and air pollution. - Flotation and smelting fumes can be toxic to both humans and wildlife.
Fly ash *	Ash is produced during coal combustion in thermal power plants.	- High alkalinity due to its chemical composition. - It contains heavy metals like mercury, chromium, and lead, making it hazardous if improperly handled. - Accumulation in dumpsites causes dust storms and long-term contamination of soil and groundwater.

* Although fly ash belongs to the energy sector, due to the similar problem of depositing large quantities as flotation tailings, its use found its place in this work.

.

Fly ash and flotation tailings have been deposited in large quantities globally, making them important secondary resources [2,6]. The biggest challenges today, which are related to these two raw waste materials, are their proper disposal and the remediation of the consequences of irregular disposal [7].

Using fly ash and flotation tailings as secondary raw materials has numerous benefits, such as reducing the cost of the economic tax for landfilling and maintenance. However, it can have also a negative impact on the environment and human health [7].

Besides reducing the negative impact of fly ash on the environment and human health, their use as secondary raw materials offers numerous advantages, including lower economic costs for disposal and maintenance as well as a reduction in the land area covered by fly ash waste.

Mining waste disposal in its original form destroys fertile soil, forest areas, and surface and groundwater flows [8,9]. Finely dispersed particles from mine tailings and wind-borne fly ash [10] are deposited on crops and households, threatening resources such as soil, water, air, and food production by accumulating toxic substances. Toxic elements also spread vertically, disrupting ecosystems over larger areas and negatively affecting plant and animal communities and their habitats. Also, another of the key challenges associated with flotation tailings is the possibility of a dam bursting, which, regardless of other types of mining waste, represents a significant environmental problem. A potential breach of the flotation tailings dam releases a high-energy flood wave, endangering ecosystems downstream, including human and animal populations [11]. Unfortunately, such incidents are numerous and often occur due to natural disasters or technical defects in the construction of dams [12,13].

The deposition of flotation tailings and fly ash, in addition to causing significant consequences if the deposition is inadequately implemented, also has economic consequences.

Global annual fly ash production varies by source, with estimates ranging from 400 million [14] to over 750 million tons [15]. These figures underscore the substantial amount of fly ash generated worldwide and reflect differences in various data collection methods and the inclusion of different countries. Analyses indicate that the global fly ash market is projected to grow at an annual rate of approximately 4.83% from 2025 to 2033, driven mainly by the construction industry’s demand for fly ash in cement and concrete [16].

In addition to the breach of dams, the deposited sulfide flotation tailings and ore tailings lead to the formation of acid mine drainage (AMD), which in the world literature received the epithet “many-headed beast” due to the wide range of negative impacts they have on the environment [17]. In the case of flotation tailings that have carbonate minerals, they can have buffering capacity and slow down the AMD formation process [5]. The formation of AMD usually, but not exclusively, occurs from iron–sulfide aggregate rocks [18]. Sulfide minerals from the outcrop, primarily pyrite (FeS₂), create acidic mine waters when exposed to oxygen, water, and a series of chemical and biological processes [19]. AMD remediation remains expensive, costing globally approximately USD 1.5 billion annually. In comparison, global environmental liabilities exceed USD 100 billion [20], which creates an additional obligation to remediate the flotation tailings as soon as possible.

Environmental concerns are common in developed and developing countries, and although their specificities may differ, both types of economies face similar challenges on the technical, economic, social, and legislative levels [21]. To better understand and solve these problems, it is crucial to thoroughly study the available data and the existing literature and compare different management strategies.

This paper aims to provide a comprehensive overview of research on this topic, focusing on the specific situation in the Republic of Serbia, a European but non-EU country with a developing economy. The review highlights the considerable potential for sustainable mining practices in Serbia, with a particular focus on flotation tailings and fly ash, management strategies, and current and innovative approaches for mineral mining and processing waste management that can be integrated into the circular economy concept. Also, an integrated system is proposed to increase the economic value of mineral mining and processing waste while reducing its adverse environmental consequences. Lastly, the study presents an overview of regulations concerning the management of extractive waste in the European Union and Serbia, discussing its implementation as a significant step towards sustainable practices in the Serbian mining sector.

2. The Principles and Possibilities of Circular Economy Within Mining Sector

A circular economy (CE) organizes the economy in such a way that materials are used as long and efficiently as possible, the need for new resources is reduced, and the amount of waste and harmful environmental effects are kept to a minimum. The CE concept offers numerous advantages to the mineral mining and processing sector. It enables the exploitation of valuable metals and production of high-value materials for other industries while reducing waste and soil pollution. This approach eliminates the need to use natural areas as landfills for mining and energy sector wastes. To achieve these goals, it is necessary to develop technologies for the recycling and recovery of materials, invest in modern infrastructure, support sustainable markets, and encourage cooperation between industry sectors, scientific institutions, governments, local communities, and consumers. Recent research has highlighted the importance of understanding the drivers and barriers to adopting CE principles to achieve efficient resource utilization [22]. Implementing these principles is closely linked to establishing conditions for the effective implementation of circular strategies. Recently, ten strategies were established as part of the circular economy 9R framework, which includes Refuse, Rethink, Reduce, Reuse, Repair, Refurbish, Remanufacture, Repurpose, Recycle, and Recover, (R0–R9, respectively) [23]. These strategies can be utilized in various mining operations, helping to close material loops and maximize value creation in the sector. Still, this study will focus on three strategies: reduction, reuse, and recycling, as it will also examine the generation and potential uses of mine tailings and fly ash. Recognizing the CE is vital, particularly in the mining, energy, and construction sectors, which are the three most significant drivers of the industry and also the largest sources of waste that can be repurposed as secondary raw materials.

Reduce: Reducing waste in the mining industry starts with mine planning, maximizing the valorization of existing resources (using new methods of ore enrichment, which would reduce the amount of tailings) and alternative sources of metals before opening new ones. In addition to using modern technology, it is necessary to work on reducing the exploitation of raw materials and more efficient use of resources as well as reducing energy needs in mining through advanced machines that consume less energy. The application of modern technologies, ground-penetrating radar (GPR), drones, and artificial intelligence (AI) and the automation of mining processes can significantly reduce waste materials and pollution, especially in tailings generation and AMD formation. The management of AMD is of particular importance, given that it is one of the most challenging environmental problems related to mining activities. Contamination of land and water systems caused by AMD seriously threatens natural ecosystems and human health, making treating these waters a key step towards sustainable mining development [24]. To keep AMD under control, flotation tailings must be disposed of in a controlled manner. Apart from AMD, AI can enable the selection of a more precise location when locating ore.

On the other hand, caution is necessary when using AI in mining. AI can lower the environmental and social costs associated with mining by utilizing advanced mineral processing and waste management technologies. While AI enhances machine efficiency, it also comes with a notable environmental footprint, which includes gas emissions, and the material costs related to its development and training [25].

Reuse: The reuse of mining waste refers to strategies that allow mining waste to be used without further processing or recycling, with the aim of reducing the amount of waste and prolonging its use in various industrial, construction, or environmental projects. Old mine waste dumps represent a significant source of raw materials, with advantages such as easy availability and fine particles that do not require additional processing [26,27,28]. These materials can be transformed into valuable products such as building materials (which will be discussed in more detail in this paper), including cement, bricks, and ceramics, and then fertilizers or materials for the restoration of degraded mining areas, if not characterized as hazardous waste that require further processing. Also, they can be utilized as backfill for mine shafts and closed mines, but care must be taken to dispose of them as an environmentally and mechanically stable waste [29,30].

Flotation tailings and fly ash, although characterized as potentially significant resources for a sustainable future, are still waiting for a breakthrough in implementation on an industrial scale, despite the achieved technological progress in ore processing in recent decades [31].

Recycling: Recycling mining waste enables the recovery of valuable metals, reducing waste and minimizing environmental risks. For example, a significant portion of steel, copper, and aluminum today is recycled from secondary sources. With the development of advanced separation and extraction technologies, the economic viability of tailings [32,33,34] and fly ash [35,36,37], treatment is becoming increasingly justified, opening up opportunities for new business models and contributions to local economies.

Flotation tailings represent an economically profitable source of metals and can be considered as a secondary deposit [38]. Since it is unnecessary to exploit the ore or crush it additionally, the costs of tailing treatment are significantly lower [32,39,40]. Also, the treatment reduces people’s and the environment’s exposure to contaminants. In addition to metal recovery, tailings can also be used as supplementary material in the construction industry, for example, to fill mines or in other industries [32,41,42,43].

After recovering available amounts of copper, gold, and sulfur into a concentrate, these materials can be used in smelting as an energy source or to produce sulfuric acid [44]. At the same time, reprocessing helps minimize the negative environmental impacts associated with acid mining drainage [45].

In agriculture, processed and neutralized tailings can be used as a soil additive or fertilizer containing phosphorus and potassium. This is especially useful in degraded areas, improving soil fertility and increasing agricultural yields.

The energy sector also uses tailings. Carbon-rich materials can be processed into briquettes and pellets as alternative energy sources. In addition, tailings containing rare earth elements are used to produce batteries and components for wind turbines, thus supporting the development of renewable energy sources [32].

Tailings also play a significant role in environmental projects, such as stabilizing riverbanks and rehabilitating mining areas. Mixed with organic matter, they can be used for afforestation and biodiversity conservation, providing that they do not emit toxic elements and are environmentally stable [46].

Industrial tailing applications include producing glass, paint pigments, wastewater treatment, and metal extraction for electronics and automobiles. Its capacity to remove heavy metals from wastewater demonstrates its importance in environmental solutions [47].

New research ideas point to tailings’ potential to produce nanomaterials for medical and electronic applications [48] and to promote high-performance concrete [49]. Such innovations confirm that tailings can greatly enhance sustainable technologies’ development and conserve natural resources when adequately treated.

In addition to industry, flotation tailings can be used to make decorative objects, sculptures, and tiles, supporting local crafts and small businesses. This approach contributes to local economic development and promotes sustainability [50].

The application of flotation tailings and fly ash is presented in

Table 2. The applications of flotation tailings in the construction sector.

Material	Application
Mine tailings (various types)	Recycling and reuse in construction materials [51,52]
Hematite tailings	Production of eco-friendly construction bricks [34]
Coal mining and processing wastes	Brick production and fuel for burning [53,54]
Copper mine tailings	Brick production [55]
	Production of fine-grained concrete [43,56]
	Production of paving stones as aggregates [56]
Iron ore tailings	Recycled aggregate concrete with polypropylene fibers [56,57]
Low-sulfide base-metal tailings	Rendering and masonry mortars [56]

and

Table 3. Applications of fly ash [58].

Applications/Category	Descriptions	Advantages
Construction industry	- Light aggregate in brick and blocks [59] - Cement replacement in concrete (up to 50%) improves durability, strength, and machinability [60] - Stabilize soil and embankments on roads	- Reduction in CO2 emissions - Reducing the exploitation of natural resources - Improving the soil’s bearing capacity and building material durability
Soil reclamation and stabilization	- Soil stabilization for infrastructure projects on soft surfaces - Filling and stabilization of mining pits	- Reducing the risk of erosion - Enabling renewable land use for agricultural or other purposes
Industrial application	- Neutralization of acidic industrial waters - Stabilization of hazardous materials - Production of lightweight aggregates and insulation materials	- Reducing environmental risks - Efficient management of resources
Environmental and energy applications	- Wastewater treatment (adsorbent for heavy metals and pollutants) - Binding of CO2 through carbonization - Stabilization of mining pits	- Reduction in greenhouse gas emissions - Improving wastewater quality - Reducing the impact of mining activities

.

All these applications of fly ash contribute to closing the waste loop and reducing dependence on the exploitation of natural resources. Through its application, industrial waste is diverted from landfills to valuable products, preserving nature and reducing costs in construction and industrial projects. In this way, fly ash becomes an important resource in achieving the goals of the circular economy, which combines environmental, economic, and social benefits to achieve sustainable development [61].

3. Circular Economy Within Mining Sector in Serbia

Mining waste constitutes the most significant portion in Serbia, accounting for nearly 95% of the country’s total waste production in 2023 [62]. This category encompasses waste generated during exploration, extraction from mines or quarries, and physical and chemical treatment of ores.

Regarding total quantities of hazardous and non-hazardous waste, according to Eurostat data [63], in the category of mining and quarrying, from 2010 to 2022, Serbia consistently ranked among the top six countries in Europe for mining waste generation. The country’s position has steadily risen over the years: 2010–2012: sixth place; 2016–2018: fifth place; 2020: fourth place; and 2022: first place. In 2022, Serbia produced an astounding 24,542 kg of mining waste per inhabitant, including hazardous and non-hazardous waste.

Serbia also ranks prominently in the production of hazardous mining waste. From 2010 to 2022, the country consistently placed first or second in Europe for hazardous mining waste generation. The amount of hazardous mining waste per capita in Serbia has increased significantly: 2010–2020: approximately, 2000 kg per capita; 2022: about 4500 kg per capita (which places Serbia just behind Finland, which generated 5000 kg of hazardous mining waste per inhabitant in 2022) [63].

These statistics highlight the urgent need for improved waste management practices and environmental policies in Serbia’s mining sector.

According to data from the Republic Statistical Office [64], the graphs in Figure 1 illustrate the millions of tons of mining waste and waste generated by the energy sector each year. After mining waste, the second largest category in volume is waste produced from electricity, gas, and steam supply processes, with fly ash representing approximately 80% of this industrial waste category [65]. About seven million tons of ash and slag are annually produced in the thermal power plants located in Serbia. Although it is known that ash is a valuable resource that is used all over the world in the construction industry to obtain cement and concrete in a more sustainable manner, in Serbia, only 3% is used for reuse [66].

Figure 1 (left) shows the total amount of mining waste generated over the past 10 years, including both hazardous and non-hazardous waste. According to data from the Statistical Office of Serbia [64], the quantity of disposed mining waste has significantly increased in the last two reporting years compared to earlier years. However, due to the lack of more specific data, we can only speculate about the reasons for this increase. Figure 1 (right) displays the waste produced by the energy sector, with a significant portion coming from fly ash generated by burning coal in thermal power plants.

The annual average amount of mining waste deposited for the previous ten years is about 40 million tons of tailings and about 6.6 million tons of fly ash in the Republic of Serbia [64].

Flotation tailings, especially older tailings, often contain significant concentrations of metal compared to lower-grade ores. Namely, the concentration of copper in tailings can range between 0.2% and 0.4%, while in lower-quality ores, it ranges from 0.2% to 0.3% [67]. This can be explained by the application of outdated flotation technologies that were once in use, which did not enable a high degree of copper extraction. Thanks to technological advances, these methods today are much more effective. Considering the growing demand for copper and its current market prices, flotation tailings, as well as lower-grade ores, are increasingly viewed as a significant source for future exploitation [39,68].

In Serbia, the mining industry produces large amounts of waste, and one of the largest generators is primary copper production with eight active mines. In the Republic of Serbia, which has an area of 88,490 km², there are about 200 active mines and approximately 250 closed mines, according to the data of the Ministry of Environmental Protection of the Republic of Serbia [63]. These sites harbor vast amounts of deposited tailings, some of which have significant metal concentration data, making them potentially profitable for secondary mining. Figure 2 illustrates the geographical locations of the Bor, Majdanpek, Grot, Veliki Majdan, Rudnik, and Lece mines. The copper mines in Bor and Majdanpek belong to copper deposits, while Grot, Veliki Majdan, Rudnik, and Lece are Pb–Zn deposits. The following text describes their flotation tailings, located near the mine, along with a significant number of associated tailings [64].

The locations in Figure 2 present a dual challenge and opportunity. Tailings are an environmental problem, however, with appropriate analyses and technologies, they can be transformed into a resource of great value. This opens the possibility for a significant contribution to the sustainable development of the mining sector and the economically profitable use of secondary raw materials in Serbia.

Due to the significant copper production in Serbia, large amounts of waste material from primary copper processing were also created. The Bor and Majdanpek mines, which have been operating since 1903, have caused significant changes in the environment, serious geological disturbances, contamination of water resources, and disruption of biodiversity. The city of Bor is considered one of the most polluted sites in Serbia [69]. The Bor copper mine consists of three flotation tailings: the Old Flotation Tailings, the RTH Flotation Tailing Dump (hereafter, RTH Tailings), and the Veliki Krivelj Tailings. The Old Flotation Tailings and the RTH Tailings float ore obtained from underground mining, while the Veliki Krivelj Tailings are from surface mining.

The Old Flotation Tailings in Bor are located on the very border with the town of Bor. It consists of two smaller basins (Field 1 and Field 2) formed in the Bor River valley. The tailing dumps have an uncovered area of about 0.57 km², and a smaller part has been partially recultivated (about 0.07 km²) [70]. It is estimated that 22.3 million tons of tailings are disposed of in this tailing dump, which potentially represents a commercially affordable source of gold, silver, and copper based on available data (

Table 4. Estimation of amounts of gold, silver, and copper affordable from disposed tailings [71].

Metal	[g/t *]	[t **]
Au	0.53	118
Ag	2.83	63.1
Cu	2300	51,290

* Share of metal per ton of disposed mining waste. ** Total mass of metal in the observed amount (22.3 million tons) of mining waste.

) [71].

The active flotation tailing pit Veliki Krivelj has a total degraded area of 4.8 km² (of which 3.6 km² has been recultivated). It contains 190–195 million tons of tailings, with a copper content of about 0.10% [72].

The total copper reserves of the ore at Majdanpek were estimated at 800 Mt, containing 0.4–0.8% Cu and 0.25–1.0 g/t Au [73].

Two main flotation tailing ponds that operate within the Majdanpek copper mine are “Valja Fundata” and “Šaški Potok”. By 2021, 350 million tons of tailings have been disposed of at 3 km², with a maximum layer thickness of 140 m, within the “Valja Fundata” tailing dump [71].

According to the data, 104 million m³ of overburden and 65 million tons of gangue were deposited at location in Majdanpek [74]. In comparison, at the mine site in Bor, six million m³ of overburden and two million t of gangue were deposited.

Flotation tailing pit “Grot”, located in the south of the country, contains 550 million tons of tailings that include lead (Pb), zinc (Zn), silver (Ag), and cadmium (Cd); however, precise data regarding composition are not available. According to reports from the “Grot” mine for 2022 and 2023, 20 thousand tons of overburden and 5 million tons of gangue were also deposited at this location [72].

The Lece flotation tailing deposit, located in southern Serbia, belongs to the Pb–Zn ore deposits and contains 2–3 million tons of tailings, which contain gold (1.33 g/t), silver (3.64 g/t), and indium (In) [74].

The flotation tailing pit at Rudnik (Zlokućanski Potok) occupies an area of 400 thousand m². It holds 8.7 million tons of tailings, which have a high silver content of 1.1 g/t, along with aluminum at 1.34 g/t, bismuth (Bi), cadmium (Cd), and other elements that are significant for industrial use [72].

According to recent data, between 2022 and 2023, the Lecemine site received 80 thousand tons of overburden and 400 thousand tons of gangue. In comparison, the Rudnik mine site deposited 1.8 thousand m³ of overburden and 500 thousand tons of gangue during the same period [74].

The Veliki Majdan tailing pond, located in the west of Serbia, spans an area of 8600 m² with an estimated total of over 1.9 million tons of tailings present. Chemical analyses indicate significant concentrations of heavy metals, including lead (Pb), zinc (Zn), copper (Cu), chromium (Cr), cobalt (Co), nickel (Ni), antimony (Sb), arsenic (As), mercury (Hg), as well as iron (Fe). Data from 2022 and 2023 show that 21 thousand tons of overburden and 32 thousand tons of gangue were deposited at the Veliki Majdan mine site [75].

Numerous scientific papers have investigated the possibility of valorization of flotation tailings from Serbia. Antonijević et al. [76] investigated the Cu leaching from the flotation tailings of the Bor copper mine in a glass reactor, adding iron chloride to a 0.1 mol/dm³ solution of sulfuric acid as an oxidizer. The ratio of solid-to-liquid (S/L) phase was 1:5. After 30 h of leaching, a copper yield of 88% was achieved.

Stevanović et al. [77] carried out leaching on the same flotation tailing samples only in columns using sulfuric acid at a concentration of 0.1 mol/dm³, with or without oxidizing agents in an S/L ratio of 1:1. The highest achieved copper yield was 89.87% during acid leaching in the column for a sample taken at a depth of 10 m without the addition of an oxidizing agent.

Stanković et al. [78] used an aqueous solution rich in acidophilic bacteria and iron obtained from the highly acidic lake “Robule” near Bor to leachate copper from the Bor flotation tailings in a rotary mixer. The average copper leaching rate was approximately 80%.

Sokić et al. [79] conducted experiments on leaching copper from oxide–sulfide ore from the Cerovo copper mine, which is also part of the Bor basin. The obtained degree of copper leaching was 73.8% after only three hours of leaching with a sulfuric acid solution with a concentration of 0.3 mol/dm³. Stevanović et al. [80] tested the leaching of copper from the overburden of the Cerovo mine, which mainly contains copper oxide minerals, followed by a solvent extraction process. The total copper utilization throughout the process was 70%, confirming that this process can be applied in industrial conditions with positive economic results.

Recently, Stanković et al. [81] leached copper from the Bor flotation tailings with a solution of sulfuric acid (0.2 mol/dm³). The solid residue after the leaching was further treated by the flotation concentration process. The average degree of copper removal in the leaching stage was 70%, while after re-flotation of the residue and its leaching, this degree increased to 83–85%. At the end of the process, the concentration of total copper was reduced three times compared to the content in the initial sample of Bor tailings before the leaching process [81].

These data indicate a significant potential for the valorization of metals from secondary sources in Serbia, many of which are on the list of critical raw materials of the EU [82].

In general, the basic concepts for disposed mining waste, flotation tailings, and fly ash utilization, following the circular economy principles, are shown in Figure 3. This concept can be divided into three main steps of material processing and residue extraction:

• The primary step (primary extraction) involves

  (a)

  The usage of coal through combustion with residual fly ash obtained. Note: The fly ash, adequate for use in cement and/or concrete production, can only be produced at high combustion temperatures or the combustion of coal results in fluidized ash with an irregular particle structure and unstable properties (often inadequate for mentioned applications) [83].

  (b)

  The processing of ore to obtain concentrate and flotation tailings.

• The secondary step focuses on revaluing metals from (a) fly ash and (b) flotation tailings, providing critical raw materials (CRMs) but with residual waste. Serbia’s ore deposits are recognized as a significant source for obtaining mineral raw materials, including CRM [72].
• Tertiary step: In the context of a circular economy, this step refers to the potential for reducing, reusing, and/or recycling the remaining waste to obtain material for further use. This may include, for example, stabilization processes like desulphurization and the utilization of leftover material within the construction sector.

Fly ash from thermal power plants can be effectively used in construction, applying the reuse and recycling principles of the circular principles. Its use as a binding material in interaction with cement increases the quality of concrete and construction while reducing cement consumption, which fits the reduction principles. This practice saves money and contributes to the circular economy by reducing waste and using resources more efficiently. This would reduce environmental impact and create new economic opportunities, exemplifying the circular economy approach [84].

Aside from illustrating potential uses for fly ash, Figure 3 also provides a comparative overview of mining tailing separation, which is discussed in more detail in the following text.

3.1. Reuse Potential of Mining Waste and Fly Ash in Serbia

Approximately seven million tons of fly ash and slag are produced annually in thermal power plants in Serbia, of which only 3% is used in the cement industry [85].

The Environmental Protection Agency of the Republic of Serbia receives reports from companies about the waste they generate and how it is managed. In 2023, the Agency reported that Serbia produced 7.84 million tons of waste, including 90,000 tons of hazardous waste. Thermal power plants are among the most significant waste producers in the country. Their coal combustion waste, classified as “10 01—Waste from power plants and other combustion plants”, makes up 75% of the total waste, amounting to 5.87 million tons [63].

While this amount is lower than in previous years, it remains an environmental challenge, with most of it classified as non-hazardous waste (5.47 million tons) and stored at plant locations [86]. However, there are opportunities for sustainable management, as fly ash from coal combustion can be used in construction and cement production, contributing to a CE [63].

The iron and steel industry also generates significant amounts of slag processing waste, partly disposed of and reused. The export of non-hazardous waste, such as iron-containing metals, is a significant waste management segment [63].

There are three cement plants in Serbia: Lafarge Serbia (Beočin), CRH Serbia (Popovac, Paraćin), and Titan Cement Plant Kosjerić (Kosjerić). Lafarge and CRH have permits for the co-incineration of hazardous and non-hazardous waste as an alternative fuel, while Titan is in the process of obtaining a permit. These plants use over 300 thousand tons of non-hazardous and hazardous waste annually as an alternative raw material, including fly ash and granular slag from blast furnaces. This contributes to more efficient waste management and reduces environmental impact [63].

Due to its positive characteristics, such as mineralogical composition and high pH value, fly ash from thermal power plants in Serbia can be used for the treatment of hazardous waste. In the work of Radovanović et al. (2016) [87], fly ash was used to stabilize sludge from wastewater treatment in a copper smelter. By optimizing the addition of ash and other agents, stabilization of over 99% of metals (Cu, Pb, Ni, and Zn) and over 90% of As was achieved. The resulting solidified material had the characteristics of a non-hazardous waste suitable for disposal. Štulović et al. (2024) [88] investigated the possibility of flotation tailing geopolymerization using fly ash as a sustainable treatment of this type of mining waste. The resulting fly ash-based geopolymers had excellent structural, thermal, and mechanical properties, suitable for further application as a building material [88]. Ðolić et al. (2021) [89] presented the circular use of fly ash. In their work, fly ash was first impregnated with goethite and then used as an adsorbent to remove As from wastewater. Value was added to this used/spent adsorbent by applying it as an adhesive in building materials with advanced mechanical properties [89].

Also, flotation tailings and fly ash have been successfully used to remediate AMD [90]. Laboratory tests have shown that flotation tailings from copper mine Majdanpek and fly ash from coal-fired power plants in Serbia (“Nikola Tesla” and “Kostolac”) can be used to neutralize the highly acidic water of Lake Robule, situated near Bor in Serbia. When flotation tailings were used for neutralization, more than 99% of aluminum, iron, and copper were successfully removed, along with 98% of lead and 92% of zinc and about 90% of zinc and lead for both types of fly ash. The contribution of the results obtained in this research is an explicit and unambiguous indication that deposited waste, flotation tailings, and fly ash can be used to treat other types of waste, such as AMD. In this way, the negative impact of waste on the environment is reduced. Also, the solid residue after the AMD treatment is enriched with metals and can be subject to further testing in order to extract valuable components. On the other hand, treated AMD has such a chemical composition that it is safe for discharge into the environment. This result has emerged as good practice in using already deposited tailings, a bright spot in mining after long-term mining activity in Bor. This research marks an important step towards implementing sustainable development principles, particularly in terms of minimizing waste, treating it, and finding ways to reuse it within the same industry [90,91].

3.2. Legislation in Serbia

Serbia has taken significant steps towards implementing the CE in the mining sector. The Ministry of Environmental Protection has established a Working Group for the CE, while the Chamber of Commerce has established the CE Sector.

A notable step forward is the development of the Mining Waste Cadaster, financed by the EU, whose goal is to improve waste management. The Mining Water Cadaster also deals with treating and reusing water generated in mining processes. Adopting the by-product regulations allows mining waste to be turned into valuable raw materials for various industries, further contributing to sustainability.

The Mining Waste Cadaster project in Serbia, funded by the EU, was initiated to map and manage mining waste across hundreds of locations. By 2020, 150 landfills containing a total of 24 million m³ of waste had been identified, with detailed examinations conducted on 41 of these landfills, which account for 80% of the total waste. The investigations involved analyzing soil, groundwater, and surface water as well as assessing landslide risks.

However, by 2024, the web application [74] only provided basic information about these locations, and a unified database had not been established. Experts have highlighted the necessity of transparent management in this area; however, an additional issue is the confusion surrounding data. More detailed information, including data on hazardous waste and site remediation, was located on the ministry’s server but is not accessible to the public through the application.

State auditors have pointed out the existence of incomplete and inaccurate waste data, and mining waste continues to be managed without the required permits. Experts emphasize the urgent need for transparent and comprehensive management to minimize environmental risks [74].

This part of the paper covers the entire framework for the management of mineral mining and processing waste with a review of legislators’ efforts to improve environmental laws and reduce the risks associated with mining waste through awareness programs (

Table 5. Legislation in the service of waste reduction.

Law/Regulation
SERBIA	Description
Environmental Protection Act [92]	Establishes an integrated system for environmental protection, sets obligations for entities, and outlines the environmental permitting process.
Waste Management Act [93]	Regulates waste management, including mining waste, focusing on reducing environmental impacts.
Water Act [94]	Governs the protection and use of water resources, including regulations on wastewater discharge from mining operations.
Air Protection Act [95]	Sets air quality standards and measures to reduce industrial pollution, including emissions from mining activities.
Mining and Geological Research Act [96]	Regulates mining and geological activities, emphasizing environmental protection during operations.
Various regulations and decrees (e.g., Decree on Waste Categories and Recycling Standards) [97]	Includes specific regulations, such as the Decree on Waste Categories, detailing recycling standards for mining waste.
The End-of-Waste Status [98,99]	Rulebook on the types of waste for which an application may be submitted, permitted procedures and treatment technologies for types of waste, and other special elements for determining the end-of-waste status (“Official Gazette of the Republic of Serbia”, No. 19/2024 and 47/2024) [98]: This regulation prescribes in detail the types of waste for which an application for end-of-waste status may be submitted, the permitted procedures and treatment technologies, as well as other special elements relevant for the determination of end-of-waste status. Regulation on technical requirements and other special criteria for certain types of waste that cease to be waste (“Official Gazette of the Republic of Serbia”, No. 78/2019) [99]: This regulation prescribes technical requirements and specific criteria for certain types of waste (such as glass, paper, textile, aggregate, and metal), which, after meeting certain conditions, cease to be classified as waste.
European Union	Description
Landfill Directive [100]	It aims to prevent or reduce as much as possible any negative impact of landfills on surface water, groundwater, soil, air, or human health by introducing stringent technical requirements.
Directive 2006/21/EC on the Management of Waste from Extractive Industries [101]	Provides a framework for the sustainable management of mining waste, including preventive measures to minimize environmental impacts.
Directive 2008/98/EC on Waste [102]	Defines general principles for waste management, focusing on prevention, reuse, and recycling within a circular economy.
Raw Materials Initiative (COM(2008)699) [103]	This study is an important step towards understanding the recycling potential of mining waste in the EU. Its results provide the basis for a more sustainable approach to the use of raw materials and enable the development of effective policies that support the circular economy and sustainable development
Directive 2010/75/EU on Industrial Emissions [104]	Covers industrial emissions, requiring the best available techniques (BATs) for environmental protection.
Water Framework Directive (2000/60/EC) [105]	Establishes a framework for water protection and sustainable use, including monitoring and management of water pollution.
Directive 2014/95/EU on Non-Financial Reporting [106]	Requires companies to disclose environmental and social information, promoting transparency and sustainability.
EN 12620:2002+A1:2008; Aggregates for Concrete [107]	The standard specifies the requirements for aggregates in concrete, including natural, recycled, and manufactured types, to ensure quality and performance.
European Green Deal [108]	The European Green Deal is an EU initiative to achieve climate neutrality by 2050 through sustainable resource use, biodiversity protection, pollution reduction, and a just transition across all sectors.
Circular Economy Action Plan [109]	Outlines EU’s strategic approach to circular economy, focusing on reducing waste, increasing recycling, and fostering sustainable industries.
New Construction Products Regulation (CPR) [110]	Establishes harmonized rules for marketing construction products in the EU (replacing regulation (EU) No. 305/20111) in order to improve legal clarity, reduce administrative burdens, and support the EU’s climate and circular economy goals while ensuring safety.

). A key objective of these regulations is to protect the communities living in the vicinity of mines and preserve the environment while encouraging responsible and ethical business.

Table 5 provides an overview of the regulations on managing extractive waste in the EU and Serbia. The table classifies laws that provide guidelines for the reclamation of mining areas and water and air conservation, contributing to sustainable development and environmental protection.

The core principles of a CE are straightforward: preventing waste and pollution, extending the lifespan of materials and products through repair and reuse, and restoring natural ecosystems. To enhance the implementation of the CE in Serbia’s mining sector, it is essential to further develop new technologies and practices for effective waste management. Increasing the efficiency of resource recovery processes, improving waste neutralization methods, and implementing innovative recycling strategies are key to long-term results. Decision makers should provide support through appropriate regulatory and financial frameworks that encourage the recycling and reuse of mining waste, thereby paving the way for more sustainable and environmentally responsible mining. Raising public and industry awareness of the benefits of the CE through educational campaigns and research support can accelerate the broader application of these principles.

For the successful implementation of the CE in the mining waste and fly ash management sector, it is necessary to establish a national register of these materials as an initial source of data for resource recovery projects. This register should contain detailed information on the quantities, compositions, and potential uses of tailings and ash at different locations. In parallel, it is necessary to intensify research and development of technologies for efficient extraction of valuable metals from tailings and for broader application of fly ash in construction and other industries. Strong cooperation between scientific research institutions, industries, and government authorities is crucial to ensure that innovative solutions can find their way to practical application within these sectors. Additionally, it is necessary to develop and implement stricter standards for the characterization and use of these materials, especially their impact on the environment and human health. Finally, it is necessary to create an encouraging economic environment through tax incentives or subsidies for companies that actively apply the principles of the circular economy in managing mining waste and fly ash.

3.3. European Practice and Serbian Perspective for Mining Waste Management

In the European Union countries, many efforts have been made in the mining sector to prevent, reduce, and minimize the negative impacts of the mineral extraction activities and flotation tailing disposal on the environment and nature. These efforts are summarized in the Best Available Techniques (BATs) Reference Document for the Management of Waste from Extractive Industries [111]. Duly tailing management requires a good knowledge of the waste and its behavior over time and depends on various factors such as the mineralogical, chemical, mechanical, and physical properties of the waste. To assess the potential impacts of deposited mining waste over time, the European Commission suggests the EN 15875 test [111], a static test for the determination of acid potential and neutralization of sulfidic waste, as well as the EN 12457 (1–4) [111] test for the classification of waste as inert, non-inert, non-hazardous, or hazardous. This test is also adopted in Serbian legislation as a test for the classification of waste intended for disposal at landfills of inert/non-hazardous/hazardous waste [97]. Mineralogy is mostly the main characteristic of the tailings that determines the method of its treatment, further processing, or disposal. In general, oxide, sulfide, silicate, and carbonate minerals can be distinguished in the waste, which could further undergo fundamental chemical changes (e.g., weathering of sulfides to oxides). It is also important to have a complete mechanical and physical characterization of the tailings including shear strength, particle size distribution (which influences shear strength), density, plasticity, moisture content, permeability (hydraulic conductivity), etc. The main techniques for treating mining waste are (a) pre-sorting and selective handling that separate potentially valuable materials from the waste stream before the waste itself is sent for treatment and/or disposal; (b) physical treatment aiming at phase separation (evaporation, gravity separation, centrifugation, filtration); and (c) chemical treatment with the aim to reduce the negative environmental impacts of the waste (precipitation and separation, neutralization, stabilization/solidification, coagulation and flocculation, desulfurization, removing or destroying cyanides, etc.).

In the management of mining waste, the following waste management hierarchy of actions should be followed: first, the prevention of solid waste generation, then reduction in waste generation, and later, the reuse and recycling of mining waste. In order to prevent the generation of mining waste, techniques, such as pre-sorting and selective handling of materials as by-products/products, return to excavation cavities, and use of mining material for internal or external purposes (for site rehabilitation or for sale on the market as construction products), are used. In order to reduce the generation of mining waste, it is the practice to send the waste off-site for appropriate treatment and/or disposal or to apply appropriate separation of hazardous, non-hazardous, and inert waste with selective handling of each of them. Reprocessing flotation tailings to obtain valuable components is only applicable if it is technically and economically feasible and environmentally acceptable, which depends on the characteristics and quantity of the waste.

In the Republic of Serbia, the implementation of such actions is only in its infancy. In the Bor and Majdanpek districts, with a long history of copper ore extraction, a new stage in the reclamation of tailings on the territory of copper ore mines has only just begun. Grassing and afforestation of the dams of the Veliki Krivelj flotation tailing pond have been carried out, thereby preserving and restoring habitats, improving soil properties, reducing erosion, and enriching the soil with organic matter, as well as protecting other natural resources, such as air and water.

The reuse of fly ash produced by burning coal as a substitute for natural minerals or industrial products reduces the exploitation of raw materials, reducing energy consumption and CO₂ emissions. For example, one ton of fly ash replacing cement can reduce CO₂ emissions by approximately 600 kg [83]. In Serbia, the reuse of fly ash is quite low compared to the EU, where about 70% of fly ash is used in the production of cement, concrete, and concrete products [83]. In order to be used in construction, fly ash must have certain physical and chemical characteristics. For example, for the use in concrete according to EN 206 (European standard for concrete), the following characteristics are important: Loss on Ignition (LOI) and the presence of chlorides and free calcium oxide, while for use in cement, the physical, chemical, and mechanical parameters of cement are determined by the EN 197-1 standard and the values for LOI, sulfates, and chlorides are monitored [83]. In order to obtain fly ash of favorable quality for further use and to reduce the proportion of unburnt carbon, the combustion process has to be optimized to achieve complete combustion conditions [83]. In addition to this combustion optimization, desulphurization methods are also applied to obtain the highest quality of fly ash. Apart from European standard EN 197-1, American standard ASTM C618 for the classification of fly ash in construction is also in force [112]. These combustion optimizations ensure that fly ash of satisfactory quality is obtained, and its further use is made possible. From an economic point of view, as of 2023, the cost of fly ash-blended cement, a traditional and commercially used clinker substitute, ranged from 25 to 30 USD/ton of CO₂. In comparison, natural pozzolana, a sustainable and emerging alternative to clinker, costs 70 to 75 USD/ton of CO₂ [113], making it an attractive option for construction projects. In Serbia, three out of eight thermal power plants have put standardized fly ash on the market, while deposed ash is sold at significantly lower prices [114]. Considering that some EU countries, like Germany and Poland, have reduced coal production while being technologically advanced and already utilizing a large share of fly ash for economic purposes, Serbia could, in the future, benefit by increased utilization at home or by positioning itself as a potential exporter of fly ash. To seize this opportunity, Serbia needs to adjust its legislation policies accordingly.

There are no defined best-available techniques for treating fly ash that has unsuitable characteristics for further use in the construction industry, apart from its safe disposal. One potential solution is fly ash inertization, a process that reduces its toxicity. This allows for its disposal in landfills at lower costs while minimizing environmental risks [115].

While recycling mining waste into construction materials brings environmental and economic benefits, it needs further investigation due to the presence of heavy metals such as mercury, arsenic, and lead [116]. These harmful substances can be released into the air, water, and soil, endangering health through dust inhalation, contact, or contaminated water, especially in residential and industrial buildings where children are particularly vulnerable. Therefore, strict regulations, quality monitoring, and the application of stabilization/immobilization technologies are necessary to reduce risks and ensure the safe use of these materials. Additionally, utilizing fly ash and tailings in construction supports a circular economy model, where waste from one industry is repurposed as a resource for another.

Meanwhile, the construction industry is witnessing significant changes and new challenges. With the new ISO 5900 series of circular economy standards (ISO 59004, ISO 59010, and ISO 59020, introduced in 2024 [117]), significant progress has been made towards adopting circular economy practices globally, including in the construction sector. The sector’s objectives have shifted to using more sustainable materials such as mine tailings and fly ash. Still, companies have been put in a position to avoid accusations of “greenwashing” (a term reflecting misleading investments or false prerogatives about the environmental benefits of products or services) to meet increasing investor expectations. The new EU Construction Products Regulation [111], which entered into force in January 2025, further emphasizes the importance of sustainability and digitalization, promoting recycled materials and innovative techniques.

4. Conclusions

The Republic of Serbia is among the top six European countries for the production of both non-hazardous and hazardous mining waste. In terms of hazardous mining waste specifically, it consistently ranked first or second from 2010 to 2022. This paper indicates significant potential for sustainable mining practices in Serbia, focusing on flotation tailings and fly ash. These materials, often viewed as waste, can be converted into valuable resources within a CE framework.

Flotation tailings, especially from older deposits, have been found to contain high concentrations of valuable metals such as copper, gold, and silver. Advances in processing technologies now allow for the economic reprocessing of these tailings, which reduces environmental impact while recovering valuable resources. Case studies from various mining sites in Serbia—including Bor, Majdanpek, and Veliki Majdan—demonstrate the potential for metal recovery and environmental remediation.

Fly ash, produced in large quantities by thermal power plants in Serbia, presents another significant opportunity for sustainable resource utilization. Data could be more promising, with only about 3% of it utilized in the cement industry, indicating significant underuse. However, fly ash has potential applications in construction, agriculture, and environmental projects. Utilizing fly ash as a secondary raw material could significantly decrease waste accumulation and reduce the need for natural resource extraction.

Implementing CE principles in Serbia’s mining sector faces challenges, including regulatory barriers and the need for technological investment. Recent initiatives, such as the establishment of the Working Group for Circular Economy and the development of the Mining Waste Cadaster, demonstrate a growing commitment to sustainable practices. However, the Mining Waste Cadaster continues to face ongoing issues with data accuracy and accessibility.

In European Union countries, there are accepted reference documents and standards that prescribe a waste management hierarchy, from the separation of hazardous and non-hazardous waste streams to recommendations for treatment and disposal (hazardous waste) and use (non-hazardous) on-site and off-site. To unlock the potential of mining waste and fly ash in Serbia, it is crucial to enhance the legislative framework, invest in research and development, and encourage collaboration between industry, academia, and government. Utilizing advanced technologies, mining waste can be converted into secondary raw materials for the cement and ceramic industries, fostering waste reduction, environmental protection, and new economic opportunities within a circular economy.

The transition to a CE within Serbia’s mining sector needs numerous steps to be fully implemented. However, it represents a potent chance for developing different sectors, obtaining an upgraded level of environmental protection, helping resource conservation, and contributing to economic growth. Serbia can pave the way for a more sustainable and prosperous future in mining and related industries by harnessing the potential of flotation tailings and fly ash.