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Article

Flotation Tailings from Cu-Au Mining (Bor, Serbia) as a Potential Secondary Raw Material for Valuable Metals Recovery

by
Vanja Trifunović
*,
Ljiljana Avramović
,
Dragana Božić
,
Marija Jonović
,
Dragan Šabaz
and
Dejan Bugarin
Mining and Metallurgy Institute Bor, 1 Albert Ajnštajn, 19210 Bor, Serbia
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(9), 905; https://doi.org/10.3390/min14090905
Submission received: 26 July 2024 / Revised: 25 August 2024 / Accepted: 31 August 2024 / Published: 3 September 2024
(This article belongs to the Special Issue Metallurgical Solid Waste: Mineralogy, Chemistry and Application/Treatment, 2nd Edition)
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Abstract

:
The increased exploitation of ores leads to the generation of mining waste, which has a negative impact on the environment and human health. For this reason, it is necessary to take care of it in an adequate way by applying some of the possible treatments. In addition to protecting the environment by applying appropriate treatment, there is also the possibility of making a profit by valorizing useful elements from mining waste. In order to choose the most adequate treatment, it is necessary to perform the characterization of mining waste. This paper contains a detailed characterization of the flotation tailings deposited at the Old Flotation Tailings in eastern Serbia, originating from copper ore processing. Characterization includes physico-chemical analysis, polarizing microscope analysis, X-ray Diffraction analysis (XRD) and Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) analysis analysis. The obtained results indicate that the investigated flotation tailings can be used as a secondary raw material for metal recovery, in this case primarily copper (whose content is about 0.24%), gold (with a content of about 0.43 ppm) and silver (with a content of about 1.7 ppm). Considering that the content of valuable elements is quite low, it is suggested to apply hydrometallurgical treatment for their recovery.

1. Introduction

Given that the demand for valuable metals is constantly growing, and high-quality ore deposits are being depleted, great efforts are being made to develop technologies for extracting metals from lower-quality ores, as well as from secondary raw materials. Every year, billions of tons of mining waste are generated during mining activities [1,2,3,4,5,6,7].
Copper is one of the most important base metals and is widely used in industry. In the Earth’s crust, copper is most often present in the form of copper–iron-sulfide and copper sulfide, such as the minerals chalcopyrite (CuFeS2) and chalcocite (Cu2S). About 80% of copper ore originates from Cu-Fe-S minerals, primarily chalcopyrite as the most abundant mineral that is difficult to dissolve in acid solutions. For this reason, most plants in the world base copper production on pyrometallurgical processes, including the flotation process, followed by smelting and metal refining [3,8,9,10,11,12,13]. In the process of flotation of copper ore, flotation tailings containing metal sulfides are separated and represent a special challenge for the topic of mining waste management. Tailings storage requires a large space in open dumps, which can sometimes exceed the size of mining facilities [1,3,11,14,15,16]. Tailings storage sites, or tailings dumps, are the source of most excess situations in mining. Tailings produced during the exploitation process can be used in the process of filling the excavated space or disposed of in open landfills covered with dams. Dam failure can have catastrophic consequences. Also, air pollution occurs in arid and semi-arid regions due to surface erosion [7]. Due to the increased exploitation of copper ores and the discovery of low-quality deposits, it can be expected that even more tailings will be present in the coming years, which could cause more environmental problems and challenges for their solution [1,3,11,14,15,17,18,19]. Due to the formation of acid mine drainages (AMDs) and the exposure of the tailings to atmospheric oxygen, bacteria and water, there is a dispersion of metals from the tailings into the environment, which directly degrades the quality of surface and underground waters as well as the surrounding soil [3,9,20,21,22,23,24,25].
Due to all of the above, it is important to find a suitable solution for the treatment of flotation tailings and at the same time the immobilization of heavy metals contained in them. In this way, not only will a useful product be extracted from the waste materials but also environmental problems that tailings can cause will be avoided. As Hasan et al. [9] described in their paper, currently represented methods for tailings management include physical treatment, chemical remediation and phytoremediation in order to protect the environment. However, tailings as a secondary source of metals have attracted the attention of many researchers in recent years, who present various proposals for their treatment in their papers [2,17,20,26,27,28,29,30,31,32,33]. Valuable minerals and metals in tailings are found in the form of fine and ultra-fine particles trapped within chains of minerals and free particles, but they cannot be separated by flotation or other concentration methods. With the development of separation and extraction technologies, the economic profitability of tailings treatment is increasingly justified [3,34,35]. The processing of the tailings would ensure the recovery of valuable metals, as well as the reduction of waste and the minimization of the environmental risks that its disposal may carry. The advantages of metal recovery from tailings are multiple: a certain profit is accomplished, the consumption of natural resources is reduced and the creation of waste is minimized. Considering that it is not necessary to excavate the ore, nor to prepare it additionally in order to reduce the particle size, the costs of tailings processing are significantly lower [1,3,6,9,36]. Also, treatment of tailings would reduce the exposure of people and the environment to contaminating substances. Technologies for the treatment of tailings can be applied for the metals recovery, but the tailings can also be used as a supplementary material suitable for use in construction, as they can be used for backfilling mines or in some other secondary industries [3,14,15,23,24,37,38,39,40,41].
Considering the fact that copper recovery from ore is low through the flotation process, it indicates that the flotation tailings contain valuable copper components. In addition, old flotation tailings sometimes have a higher copper content (0.2%–0.4% Cu) than low-grade ores (0.2%–0.3% Cu). The reason for the higher copper content in the tailings located at the old flotation tailings is that at the time when these tailings were disposed of, a different technology of the copper processing process, i.e., the flotation process, was applied, which had less utilization of copper than it has today. The copper content of old flotation tailings is similar to the world average content of about 0.4% copper in mined ore. Taking into account the current price of copper on the stock market and predictions that copper demand will increase, flotation tailings and low-grade copper ores represent a potential future source of copper [18,36,42].
Compared to pyrometallurgical processes, hydrometallurgical treatment has a greater potential for the treatment of complex low-content sulfide ores and concentrates, as well as secondary materials, which results in an increase in the utilization of metals and a reduction in the risk of air pollution. In recent years, research and development of hydrometallurgical processes have intensified as an alternative to pyrometallurgical treatment [3,5,6,18].
Given that the mineralogical and chemical compositions of the tailings are complex and variable, their detailed characterization is necessary to assess the appropriate treatment, followed by the application of appropriate processes adapted to the specific composition of the investigated tailings [3,5,34,43]. In this paper, a detailed characterization of the flotation tailings from the Old Flotation Tailings in Bor, located in eastern Serbia, was carried out to assess the metal content and consider the possibility of their use as a secondary raw material for valuable metals recovery.

2. Materials and Methods

During more than a century of mining and production of copper, gold and silver in the Mining and Smelting Basin in Bor, in the eastern part of Serbia, landfills of mining waste were formed, which, due to the low content of useful components, could not be treated in an economically justified way for a certain period of time. These are disposal sites for overburden from the “old” surface mine, flotation tailings, residual ore in the pit and so on. Flotation tailings from the Bor mining complex in Serbia are a by-product of the ore beneficiation process, where valuable minerals such as copper and gold are extracted from the copper ore. The Bor mining area is located in a region rich in metal minerals. The geological composition of the flotation tailings from Bor is specific. The main minerals present in the tailings include quartz, pyrite, kaolinite and alunite. Such a mineral assemblage came about as a result of various processes of enrichment and alteration in copper production [44,45].
Physically, flotation tailings have a very fine texture. The particle size usually ranges from a few micrometers to a few millimeters. The tailings color can vary from light gray to dark gray, depending on the presence of different minerals and their oxidation stage. The tailings density is generally lower compared to raw ore due to the removal of heavier minerals during the flotation process. Chemically, flotation tailings often show an acidic character, with a pH value usually in the range of 2.5 to 3.0, due to the presence of sulfides such as pyrite (FeS₂). The presence of pyrite can lead to the formation of acid mine drainage water (AMD), which can contribute to the self-leaching of trace heavy metals, including cadmium, lead and arsenic, which can have adverse effects on the environment [44,45,46].
From an environmental point of view, flotation tailings are challenging due to their acidic pH and potential for heavy metal leaching, which can adversely affect the environment. To mitigate environmental impacts, measures such as covering the tailings with inert materials, adding neutralizers (such as lime) to adjust the pH and implementing a continuous monitoring system are implemented [44,45,46].
In terms of uses, flotation tailings are often used in the construction industry, such as road construction material or as fill for foundations and for land reclamation. However, such applications require pre-treatment and stabilization of the material. Tailings management includes strategies for erosion control, pollution prevention and long-term remediation planning to ensure minimal negative impacts on the environment [44,45,46].
Disposal of flotation tailings in the Bor flotation was carried out in the Bor River valley, on the Old Flotation Tailings site [26,47,48]. In the phase of tailings exploitation, the Old Flotation Tailings site is spatially divided into three fields separated from each other by cyclone sand dams. Field 3 is filled with overburden from the surface mine and ash from the thermal power plant so that Fields 1 and 2 of the flotation tailings remain available for possible additional revaluation of useful components [48,49,50]. The flotation tailing itself is located on the border of the urban and industrial parts of the city, and below it, there is a city wastewater collector. Due to its close proximity to the city center, the Old Flotation Tailings in Bor are also one of the sources of negative environmental impact, which is reflected in the spreading of fine dust into the environment during windy periods and the runoff of acidified waters [49].
For the last 6 years, Mining and Metallurgy Institute Bor (MMI Bor) has been preparing annual reports (Elaborates) [44,45,46] on the state of the Old Flotation Tailings in Bor. Writing of the Elaborate for 2024 is in progress. All Elaborates contain geological, mineralogical, chemical and technological investigations of flotation tailings samples from the location of the Old Flotation Tailings in Bor. The calculated reserves of the technogenic deposit amount to about 22 Mt of mineral raw material, with about 12 t of Au, 63 t of Ag and 51,300 t of Cu metal [44,45,46].
Serafimovski et al. [49], as well as other authors [47,48,50], after obtaining the results of mineralogical and geochemical tests of the flotation tailings originating from the Old Flotation Tailings in Bor, concluded that these tailings may be of interest for possible further valorization of certain elements, and then they applied different techniques for their valorization. Based on the above, it can be concluded that significant amounts of copper, gold and silver are present in the flotation tailings and that their valorization would make a significant profit, and at the same time, environmental protection would be achieved by treating the tailings.

2.1. Sampling

The flotation tailings, the characterization of which is presented in this paper, are located in Bor, in eastern Serbia, in the former Mining Smelter Basin in Bor, the current Serbia ZiJin Bor Copper mine. Figure 1 shows the tailings deposited at the Old Flotation Tailings in Bor and their situational plan. The blue points mark the sampling points, i.e., the drillholes from which flotation tailings samples were taken from Field 1 and Field 2. Based on the time dynamics of tailings disposal at the Old Flotation Tailings in Bor, composites of flotation tailings were made from the collected samples and have the following names: COMPOSITE I, COMPOSITE II and COMPOSITE III. In Figure 1, the areas where COMPOSITE I, COMPOSITE II and COMPOSITE III were sampled are marked in red, purple and green, respectively.
The sampling of flotation tailings is presented in Figure 2. Samples of flotation tailings were taken by drilling and forming 18 drillholes, then geological mapping was performed, and the samples were separated by location and depth of sampling into separate bags. Sampling points were strategically selected to capture the most critical outputs and/or inputs after the various mineral processing steps over the long period of flotation tailings disposal at the Old Flotation Tailings in Bor. Seven sampling points are located in the Field 2 zone and eleven are in the Field 1 zone.
After the chemical analysis of samples by drilholes, through the profiles, composite samples of flotation tailings were formed for further investigations.
The samples COMPOSITE I and COMPOSITE III were formed from samples from the part of the tailings that includes the Old French Tailings and Field 1 and were disposed of in the same period, which means that during that period a similar technology of the copper ore processing process was applied and that the content of copper and other elements in that obtained tailings is similar. For that reason, they were mixed so that a composite sample marked COMPOSITE I+III was obtained. The formed COMPOSITE I+III contains an estimated total amount of tailings of about 60% of the total amount of tailings.
COMPOSITE II was formed from samples from the part of the tailings that includes Field 2, which represents about 40% of the total amount of tailings.
The complete characterization of the samples was performed in parallel from the two formed composite samples COMPOSITE I+III and COMPOSITE II.

2.2. Sample Preparation

Composite samples of flotation tailings were homogenized individually, on foil, according to a defined procedure. After homogenization, sampling was performed to select representative samples for physico-chemical and mineralogical characterization using the quartering method.
For physical and chemical characterization, both representative samples of flotation tailings were taken in triplicate, in amounts of 0.5, 1.0 and 2.0 kg, and were dried in a dryer at a temperature of 105 °C until constant mass, for 24 h.
Two dry representative samples were prepared for morphological analysis by immersion in epoxy resin, after which they were ground and polished with silicon carbide, and then polished with diamond slurry. The samples were first analyzed with a polarizing microscope (JENAPOL-U, Carl Zeiss-Jena, Germany), then steamed with gold and analyzed with a scanning electron microscope with energy dispersive spectroscopy (SEM-EDS, JSM IT 300LV, JOEL, Japan).
To prepare the sample for X-ray diffraction analysis, both representative samples of flotation tailings in duplicate (dry samples) were prepared by grinding in an agate mortar.

3. Sample Characterization

3.1. Physico-Chemical Characterization of the Samples

Physical characteristics of representative samples of flotation tailings composites include the determination of sample density, bulk density and sample pH.
Chemical characterization was performed by dissolving the samples in 4 acids (HCL, HNO3, HClO4 and HF), and then the concentrations of the monitored elements were analyzed using the following methods: atomic absorption spectroscopy (AAS) (PerkinElmer PinAAcle 900F, USA), inductively coupled plasma atomic fusion spectrometry (ICP-AES) (Spectro CirosVision, Germany), carbon sulfur analysis (CSA) (Thermo Horiba EMIA-920V2, Japan), gravimetry (G) and fire assay (FA). The pH values were measured with an pH meter (Handheld ICP/pH meter IM-32P DKK-TOA, Japan).

3.2. Granulometric Composition of the Samples

A size analysis was performed to determine the particle size of the samples. The granulometric composition of two samples of the flotation tailings composite was determined by the sieving method on laboratory sieves made of fine mesh, wire and a perforated metal plate (SRPS ISO 2591-1:992). Sieve mesh sizes in the range of above 4 to below 0.038 mm were used.

3.3. Mineralogical Characterization of the Samples

The mineralogical characterization of the two flotation tailings composite samples includes polarization microscope analysis, X-ray diffraction (XRD) analysis and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) analysis.
Qualitative–quantitative mineralogical analysis was performed on a polarization microscope for reflected and transmitted light of the brand “Axioscope 5”, from the company Zeiss (Oberkochen, Germany), with a measuring device. Also, photomicrographic kit with the image acquisition program by Zeiss—Axiocam 105 color system (Oberkochen, Germany)—was used. Qualitative mineralogical analysis of the samples was performed under a polarizing microscope, using the immersion method with qualitative identification of the minerals present.
X-ray diffraction analysis was performed on a “RigakuMiniFlex 600” instrument (XRD, RigakuMiniFlex 600, Japan) with a “D/teXUltra 250” high-speed detector and an X-ray tube with a copper anode. The recording conditions were as follows: angle range 3°–90°, step 0.02°, imaging speed 10°/min, X-ray tube voltage 40 kV and current 15 mA. Mineral identification was performed in the PDXL 2 Version 2.4.2.0 software, and the diffractograms obtained were compared with the data from the Crystallography Open Database (COD). The detection limit of XRD analysis is about 1%. Also, using XRD analysis, a semi-qualitative assessment of the mineral content in composite samples of flotation tailings was performed.
SEM-EDS analysis (SEM-EDS, JSM IT 300LV, JOEL, Japan) was performed on two composite samples of flotation tailings.

4. Results and Discussion

4.1. Physico-Chemical Characterization of the Samples

The results of the physical characterization of representative samples of flotation tailings composites are presented in Table 1.
The pH value of 2.7–2.8 indicates that the investigated samples have an acidic character, and thus the process of self-leaching of tailings would be possible due to the reaction of the sample with atmospheric water, that is, the generation of acidic mine drainage (AMD).
Table 2 shows the chemical characterization of representative samples of flotation tailings composites.
Based on the obtained results of chemical analyses, it can be concluded that both samples of flotation tailings have a similar content of copper (0.25%–0.23%), iron (7.62%–8.90%), calcium (0.78%–0.74%), gold (0.45–0.40 ppm), silver (1.40–2.00 ppm), as well as other tracked elements. Very similar results of chemical analysis were obtained by Özdemir et al. [47] who also used a sample of flotation tailings originating from Bor, Serbia, for their investigations, but their sample was taken from another location than the sample used in this paper.
The copper content in tailings of more than 0.2% has the greatest economic interest. Also, it can be established that the share of oxide copper in relation to the total copper for the investigated composite samples is 72% for the sample COMPOSITE I+III, while for the sample COMPOSITE II, it is 52%, which has a significant impact on the eventual application of the acid leaching process of copper in the application of hydrometallurgical treatment of tailings.
For metals content of <0.5 wt.% in flotation tailings, researchers suggest the application of a hydrometallurgical process (leaching process) to maximize the extraction of target metals. In their paper, Araya et al. [7] state that the chemical composition of flotation tailings in each deposit is different and depends on the type of mineral rocks that are mined and processed. In the geochemical characterization of the Chilean tailings examined in their work [7], a high percentage of silicon oxide species was also observed, as in the tailings from the disposal site in Serbia investigated in this paper, due to the minerals that are processed, and this is also the case mainly in tailings deposits.

4.2. Granulometric Composition of the Samples

The size analysis confirmed that 66.50% of particles present in the sample COMPOSITE I+III have a size smaller than 75 µm (Table 3).
The results of the size analysis of the sample COMPOSITE II showed that 69.10% of the particles of the tested sample were smaller than 75 µm (Table 4).
The obtained results of the size analysis of both samples of flotation tailings (Table 3 and Table 4) suggest the following: (1) it is confirmed that these tailings are a threat to human health because they consist of 61% small particles (particles < 53 μm in size), which can easily be carried by the wind and enter the respiratory system; (2) over 70% of particles in the tailings have a size of <106 μm, which is suitable for the leaching process since there is no need for additional grinding. The same conclusions were reached by Ruiz-Sánchez et al. [43] by examining samples of flotation tailings from mines in Mexico.
Elghali et al. [2] explained in their paper that mineralogical changes occur during tailings oxidation and that the changes are reflected in terms of their chemical, physical and hydrogeological properties and acid generation potential as a function of time. During the oxidation and dissolution of minerals (carbonates, sulfides, silicates), the particle size distribution becomes finer, which also affects the specific surface area, oxygen diffusion and overall particle reactivity. With an increase in the specific surface area and a decrease in the particle size distribution, the reactivity of the sample increases, which represents another advantage for the application of the hydrometallurgical process of flotation tailings treatment to recover valuable metals. Similar results regarding the particle size of flotation tailings are obtained by Hansen et al. [20], Liu et al. [38] and Arunachalam et al. [13].

4.3. Mineralogical Characterization of the Samples

4.3.1. Polarization Microscope Analysis

Polarization microscope analysis revealed that the samples of flotation tailings have a high content of pyrite (about 97%–98% of the sulfide mass) and a very low content of copper sulfide minerals. The sample COMPOSITE II is dominated by digenite, coveline and enargite (about 2% sulfide mass), while the sample COMPOSITE I+III is dominated by enargite and chalcopyrite (about 3% sulfide mass). Free gold minerals (electrum) were determined in the sample COMPOSITE I+III. The minerals of the tailings are silicates and quartz, while carbonates and sulfates are less common. Oxide copper is bound to Cu-limonite.
Of the copper sulfide minerals, covelline, digenite and enargite were found in the samples, which occur in traces. Their content is presented in Table 5. Other copper minerals occur in the subtrace, while pyrite is the most abundant sulfide mineral.
The mineral content found in composite samples of flotation tailings is consistent with the results obtained in the investigation by Velizar et al. [48], who also performed tests on a sample of flotation tailings from the same location as we did.
Qualitative and quantitative microscopic analysis was performed on the sample COMPOSITE I+III. Based on the obtained qualitative microscopic analysis, the presence of the following minerals was determined in the sample: enargite, chalcocite, covelline, chalcopyrite, tetrahedrite, pyrite, native gold, magnetite, hematite, rutile, leucoxene, Cu-limonite and tailings minerals (Table 5). The presence of minerals was confirmed by X-ray diffraction.
The content of sulfide mass in the whole sample is 17.7%, where about 92% of sulfide minerals are free grains. Figure 3 shows the percentage share of the textural composition of all sulfide minerals (97.0% pyrite share), and Figure 4 shows the mutual relationship between the mineral grains of copper sulfide minerals (3.0% share).
Of the copper sulfide minerals, enargite and chalcopyrite were found in the sample, which occur in the trace, while the other copper minerals occur in the subtrace (Table 5). Pyrite is the most abundant sulfide mineral, which occurs in free grains at over 90%. “Visible” gold minerals have been determined, namely a single free grain (2.8 μm) and as an inclusion in pyrite (2.1 μm), presented in photomicrographs in Figure 5. The gold minerals according to their high luster correspond to electrum (an alloy of gold with a little silver). Non-ore minerals (tailings) are represented by silicates, quartz and rarely carbonates and sulfates.
Qualitative and quantitative microscopic analysis was performed on the sample COMPOSITE II. Based on the obtained qualitative microscopic analyses, the following mineral composition was determined: digenite, enargite, chalcocite, covelline, chalcopyrite, tetrahedrite, pyrite, magnetite, hematite, rutile, cassiterite, leucoxene, Cu-limonite, tailings minerals (Table 5). The minerals were confirmed by X-ray diffraction analysis.
The content of sulfide mass in the whole sample is 23.3%, where sulfide mineral grains are free at about 98%. The percentage share of the textural assemblage of all sulfide minerals (pyrite share 98.4%) is presented in Figure 6, and Figure 7 shows the mutual relationship between mineral grains of copper sulfide minerals (share of 1.6%).
Of the copper sulfide minerals, covelline, digenite and trace amounts of rutile were found in the sample. Their content, as well as the content of other copper minerals that occur in the subtrace, is shown in Table 5. Pyrite is the most abundant sulfide mineral, which occurs in free grains at about 98%. Its content is shown in Table 5. “Visible” gold minerals have not been determined, but they are most likely related to the copper and iron sulfides present. Non-ore minerals (tailings) are represented by silicates, quartz, rarely carbonates and sulfates and are shown in photomicrographs in Figure 8.

4.3.2. XRD Analysis

The results of the XRD analysis showed that both investigated samples (COMPOSITE I+III and COMPOSITE II) have similar qualitative mineralogical compositions. The diffractogram of the sample COMPOSITE I+III is presented in Figure 9. In this sample, the following minerals were identified: quartz (SiO2), pyrite (Fe2S), kaolinite (Al2Si2O5(OH)4) and alunite (KAl3(SO4)2(OH)6). Quartz is the most abundant mineral in the sample, kaolinite and pyrite are less abundant, while alunite is the least abundant. The diffractogram of the sample COMPOSITE II is presented in Figure 10, and the following minerals were identified in this sample: quartz (SiO2), pyrite (Fe2S), kaolinite (Al2Si2O5(OH)4) and alunite (KAl3(SO4)2(OH)6). Quartz is also the most abundant mineral in this sample, kaolinite and pyrite are less abundant, while alunite is the least abundant.
A semi-qualitative assessment of the mineral content in the composite samples of flotation tailings was made using XRD analysis. The obtained results are presented in Table 6.
The obtained results of the semi-quantitative diffraction analysis confirmed the results obtained by the quantitative analysis using the polarizing microscope, i.e., the pyrite content in the COMPOSITE I+III sample is 17% and 20% in the COMPOSITE II sample.

4.3.3. SEM-EDS Analysis

To study and analyze the microstructure of two composite samples of flotation tailings, SEM-EDS analysis was performed. Figure 11 shows EDS images with marked places where EDS analyses of the composite sample of flotation tailings COMPOSITE I+III were carried out at two positions and a tabular presentation of the estimated chemical composition of the analyzed spectra by SEM-EDS analysis.
Figure 12 shows an EDS image with marked places where EDS analyses of the composite sample of flotation tailings COMPOSITE II were performed. A tabular representation of the estimated chemical composition of the analyzed spectra by SEM-EDS analysis is also presented in Figure 12.
SEM-EDS analysis of the composite samples of the flotation tailings revealed the presence of quartz and the presence of pyrite with gold admixture in both examined samples, and in the sample COMPOSITE I+III, the presence of minerals from the feldspar group was also determined. The admixture of gold in pyrite is in the range of 1.0–1.5 wt.% in the sample COMPOSITE I+III and in the range of 1.17–1.27 wt.% in the sample COMPOSITE II.

5. Conclusions

To reprocess and recycle mining waste, flotation tailings can be treated to recover valuable metals and to protect the environment. Many researchers have already developed and proposed various technologies with a high degree of success in their experiments. It is certainly most important to first carry out a detailed characterization of the flotation tailings to decide which technology is the most adequate to apply to obtain satisfactory results.
In this paper, a detailed chemical and mineralogical characterization of mining waste, i.e., two composite samples of flotation tailings originating from eastern Serbia, was performed.
XRD analysis showed the presence of four mineral phases in both samples, namely: quartz (SiO2), pyrite (Fe2S), kaolinite (Al2Si2O5(OH)4) and alunite (KAl3(SO4)2(OH)6). Further mineralogical analysis also determined the presence of gold and silver.
Chemical analysis determined a copper content of about 0.24%, gold of about 0.43 ppm and silver of about 1.7 ppm. The flotation tailings that are the subject of this paper were deposited at the Old Flotation Tailings in Bor for a long period of time during which the utilization of copper in the flotation process varied. For this reason, these tailings have a higher content of copper and other elements (Au and Ag) compared to the tailings that are produced today in the flotation process. Considering that low-quality ores have a copper content of 0.2%–0.3%, there is a possibility of using these tailings as a secondary raw material for its recovery.
The characterization carried out in this paper aimed to plan the application of adequate treatment of flotation tailings and valorize the useful components found in it. Based on the obtained results of the complete characterization of the flotation tailings presented in this paper, and based on a comparative review of literature data, it can be concluded that there are indications that they can be used as a secondary material for metals recovery, specifically copper, gold and silver. Considering that the content of these metals is quite low (<0.5 wt.%), the application of hydrometallurgical treatment that would include a combination of leaching processes, the purification of the pregnant leaching solutions and the extraction of metals is suggested. The pyrite content in the tested tailings would favor certain hydrometallurgical processes and would reduce the consumption of sulfuric acid as a leaching reagent. Our further research will be carried out with the aim of recovering valuable metals using hydrometallurgical procedures with the possibility of obtaining a stable solid residue that can be disposed of in a safe manner without a negative impact on the environment.

Author Contributions

Conceptualization, V.T.; methodology, V.T. and L.A.; software, M.J. and D.B. (Dragana Bozić); validation, L.A., D.Š. and D.B. (Dragana Bozić); formal analysis, D.B. (Dejan Bugarin); investigation, V.T., L.A. and D.B. (Dragana Bozić); resources, V.T. and L.A.; data curation, V.T., D.Š. and L.A.; writing—original draft preparation, V.T., M.J. and D.Š.; writing—review and editing, V.T., L.A., D.Š., M.J., D.B. (Dejan Bugarin) and D.B. (Dragana Bozić); visualization, V.T.; supervision, L.A. and D.B. (Dragana Bozić); project administration, M.J. and D.B. (Dejan Bugarin); funding acquisition, L.A. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful to the Ministry of Education, Science and Technological Development of the Republic of Serbia for financial support according to the contract with the registration number 451-03-66/2024-03/200052.

Data Availability Statement

The data are contained within this article. The original contributions presented in this study are included in this article; further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank the Mining and Metallurgy Institute Bor, where all the investigation presented in this paper was carried out.

Conflicts of Interest

The authors declare no conflicts of interest.

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Minerals 14 00905 g001
Figure 1. The appearance of the Old Flotation Tailings in Bor.
Figure 1. The appearance of the Old Flotation Tailings in Bor.
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Figure 2. Sampling of flotation tailings: (a) one of the sampling sites; (b) drilling; (c) geological mapping.
Figure 2. Sampling of flotation tailings: (a) one of the sampling sites; (b) drilling; (c) geological mapping.
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Minerals 14 00905 g003
Figure 3. Textural assembly of aggregates with sulfides in the sample COMPOSITE I+III.
Figure 3. Textural assembly of aggregates with sulfides in the sample COMPOSITE I+III.
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Figure 4. The percentage ratio of sulfide copper minerals in the sample COMPOSITE I+III.
Figure 4. The percentage ratio of sulfide copper minerals in the sample COMPOSITE I+III.
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Figure 5. Photomicrographs of the sample COMPOSITE I+III: (a) Appearance of pyrite grains (Py) with inclusions of chalcopyrite (Ccl); (b) Appearance of free grains of pyrite (Py) and enargite (Eng); (c) Free grain of gold (Au); (d) Free grains of pyrite (Py) and simple fusion of pyrite (Py)–chalcopyrite (Ccl); (e) A single grain of native gold (Au) in pyrite (Py); (f) Free grains of chalcocite (Cc) and pyrite (Py).
Figure 5. Photomicrographs of the sample COMPOSITE I+III: (a) Appearance of pyrite grains (Py) with inclusions of chalcopyrite (Ccl); (b) Appearance of free grains of pyrite (Py) and enargite (Eng); (c) Free grain of gold (Au); (d) Free grains of pyrite (Py) and simple fusion of pyrite (Py)–chalcopyrite (Ccl); (e) A single grain of native gold (Au) in pyrite (Py); (f) Free grains of chalcocite (Cc) and pyrite (Py).
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Figure 6. Textural assembly of aggregates with sulfides in the sample COMPOSITE II.
Figure 6. Textural assembly of aggregates with sulfides in the sample COMPOSITE II.
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Figure 7. The percentage ratio of sulfide copper minerals in the sample COMPOSITE II.
Figure 7. The percentage ratio of sulfide copper minerals in the sample COMPOSITE II.
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Figure 8. Photomicrographs of the sample COMPOSITE II: (a) Appearance of pyrite (Py), quartz (Qz) grains with inclusions of covelline (Cv) and rutile (Rt); (b) Appearance of pyrite grains (Py) chalcopyrite with pyrite (Ccp); (c) Appearance of covelline grains (Cv) with inclusions of pyrite (Py) and appearance of Cu-limonite (lmn); (d) Appearance of a complex fusion of quartz (Qz)–digenite (Dg) with inclusions of rutile (Rt); (e) Appearance of a complex digenite (Dg)–coveline (Cv) fusion with pyrite inclusions (Py); (f) Free grain appearance of enargite (Eng) and rutile (Rt).
Figure 8. Photomicrographs of the sample COMPOSITE II: (a) Appearance of pyrite (Py), quartz (Qz) grains with inclusions of covelline (Cv) and rutile (Rt); (b) Appearance of pyrite grains (Py) chalcopyrite with pyrite (Ccp); (c) Appearance of covelline grains (Cv) with inclusions of pyrite (Py) and appearance of Cu-limonite (lmn); (d) Appearance of a complex fusion of quartz (Qz)–digenite (Dg) with inclusions of rutile (Rt); (e) Appearance of a complex digenite (Dg)–coveline (Cv) fusion with pyrite inclusions (Py); (f) Free grain appearance of enargite (Eng) and rutile (Rt).
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Figure 9. Diffractogram of the sample COMPOSITE I+III.
Figure 9. Diffractogram of the sample COMPOSITE I+III.
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Figure 10. Diffractogram of the sample COMPOSITE II.
Figure 10. Diffractogram of the sample COMPOSITE II.
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Figure 11. SEM-EDS images for sample COMPOSITE I+III (magnification 300×).
Figure 11. SEM-EDS images for sample COMPOSITE I+III (magnification 300×).
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Figure 12. SEM-EDS image for sample COMPOSITE II (magnification 450×).
Figure 12. SEM-EDS image for sample COMPOSITE II (magnification 450×).
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Table 1. Physical characterization of representative composite samples of flotation tailings.
Table 1. Physical characterization of representative composite samples of flotation tailings.
Sample NameSample Density
(g·cm−3)		Bulk Mass
(kg·m−3)		pH Value
COMPOSITE I+III2.7651.1762.82
COMPOSITE II2.6900.9762.77
Table 2. Chemical composition of two representative samples of flotation tailings composites.
Table 2. Chemical composition of two representative samples of flotation tailings composites.
ElementUnitContent
COMPOSITE I+IIICOMPOSITE II
Cutotal%0.250.23
Cuox%0.180.12
Fe%7.628.90
Ca%0.780.74
K%0.540.59
Na%0.230.19
S%9.3412.38
SiO2%52.8849.88
Al2O3%10.7611.00
Auppm0.450.40
Agppm1.402.00
Srppm648642
Asppm122166
Znppm23.649.1
Table 3. Tabular presentation of the granulometric composition of the flotation tailings composite sample COMPOSITE I+III.
Table 3. Tabular presentation of the granulometric composition of the flotation tailings composite sample COMPOSITE I+III.
Sieve Opening Size
d (mm)		Mass Participation
m (%)		Sieve Reflection
R (%)		Sieve Sifting
D (%)
−4+2.367.607.60100.00
−2.36+1.700.207.8092.40
−1.70+0.8500.408.2092.20
−0.850+0.6000.408.6091.80
−0.600+0.4250.308.9091.40
−0.425+0.3006.7015.6091.10
−0.300+0.2121.1016.7084.40
−0.212+0.1504.4021.1083.30
−0.150+0.1066.3027.4078.90
−0.106+0.0756.1033.5072.60
−0.075+0.0535.5039.0066.50
−0.053+0.0385.5044.5061.00
−0.038+0.0055.50100.0055.50
Table 4. Tabular representation of the granulometric composition of the flotation tailings composite sample COMPOSITE II.
Table 4. Tabular representation of the granulometric composition of the flotation tailings composite sample COMPOSITE II.
Sieve Opening Size
d (mm)		Mass Participation
m (%)		Sieve Reflection
R (%)		Sieve Sifting
D (%)
−4+2.360.300.30100.00
−2.36+1.700.100.4099.70
−1.70+0.8500.100.5099.60
−0.850+0.6000.100.6099.50
−0.600+0.4250.100.7099.40
−0.425+0.3000.401.1099.30
−0.300+0.2127.208.3098.90
−0.212+0.1506.3014.6091.70
−0.150+0.1068.6023.2085.40
−0.106+0.0757.7030.9076.80
−0.075+0.0537.8038.7069.10
−0.053+0.0386.2044.9061.30
−0.038+0.0055.10100.0055.10
Table 5. Qualitative mineralogical analysis of the tested samples.
Table 5. Qualitative mineralogical analysis of the tested samples.
MineralCOMPOSITE I+IIICOMPOSITE II
Pyrite (FeS2)17.1522.96
Coveline (CuS)0.020.12
Digenite (Cu9S5)-0.10
Enargite (Cu3AsS4)0.340.08
Tetrahedrite (CuFeSbS)0.050.05
Chalcopyrite (CuFeS2)0.090.02
Chalcocite (CuS2)0.04<0.01
Native gold<0.01-
Magnetite (Fe3O4)0.320.19
Hematite (Fe2O3)0.110.04
Rutile (TiO2)0.210.27
Leucoxene (TiO2)0.310.61
Cassiterite (SnO2)-0.03
Cu-limonite (CuFe2O3∙H2O)0.900.72
Tailings minerals80.4674.81
In total:100.00100.00
Note: - the mineral has not been determined.
Table 6. Semi-quantitative assessment of mineral content in composite samples.
Table 6. Semi-quantitative assessment of mineral content in composite samples.
MineralAssessment of Mineral Content (%)
COMPOSITE I+IIICOMPOSITE II
Quartz (SiO2)4643
Pyrite (Fe2S)1720
Kaolinite (Al2Si2O5(OH)4) 3131
Alunite (KAl3(SO4)2(OH)6)66
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Trifunović, V.; Avramović, L.; Božić, D.; Jonović, M.; Šabaz, D.; Bugarin, D. Flotation Tailings from Cu-Au Mining (Bor, Serbia) as a Potential Secondary Raw Material for Valuable Metals Recovery. Minerals 2024, 14, 905. https://doi.org/10.3390/min14090905

AMA Style

Trifunović V, Avramović L, Božić D, Jonović M, Šabaz D, Bugarin D. Flotation Tailings from Cu-Au Mining (Bor, Serbia) as a Potential Secondary Raw Material for Valuable Metals Recovery. Minerals. 2024; 14(9):905. https://doi.org/10.3390/min14090905

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Trifunović, Vanja, Ljiljana Avramović, Dragana Božić, Marija Jonović, Dragan Šabaz, and Dejan Bugarin. 2024. "Flotation Tailings from Cu-Au Mining (Bor, Serbia) as a Potential Secondary Raw Material for Valuable Metals Recovery" Minerals 14, no. 9: 905. https://doi.org/10.3390/min14090905

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