Extraction of Metals from Copper Tailings through Leaching: History
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Lidia Ester Santibáñez-Velásquez

A copper tailing is a residue, product of the flotation of sulfide minerals, which contain a variety of elements that can be valorized. The extraction of metals from copper tailings consist of applying metallurgical techniques, such as acid leaching or magnetic concentration, to obtain a valuable product. Currently, this is an important objective, given that mining operations have increased the generation of tailings. Acid leaching is a process that consists of dissolving a solid material, such as a tailing, by applying an acid solution. This process forms two final products: an insoluble solid, rich in aluminosilicates, and an acid liquid solution with different metal ions. Both products may have different characteristics and can be used for subsequent applications.

Adapted from: https://doi.org/10.3390/met12111924

  • copper tailing
  • leaching
  • valorization

1. Introduction

Mining activity has increased due to worldwide demand, which rises as the world’s population grows. This mineral extraction causes a significant waste generation, such as tailings and slags [1]. Mining production comes from all over the world. China, Australia, India, Brazil, South Africa, Russia, Canada, and Chile, are some countries with significant mining production [2]. Concerning Chile, the South American country sustains a crucial part of its economy in mining activity, mainly in the commercialization of copper obtained from the north of the country. However, with the increase in exploitation, ore grades are getting progressively lower, so mining operations have increased the amount of processed material. Hence, new concentrator plants and the expansion of the existing ones have increased the generation of tailings, which consists of a residue product of the flotation of sulfide minerals [3]. This has turned into a focus of ecological concern, given the delicate global environmental context, incentivizing many efforts to valorize this residue [4].
The valorization of tailings includes the utilization of the mineral matrix, as it contains mainly silica, and the recovery of residual metals, containing metals such as iron, copper, titanium, and many others [5]. Depending on the ore and the process used to treat it, it is possible to recover valuable elements from mine tailings. For example, copper sulfide ores are treated by flotation to recover copper, so the produced tailings contain mainly silicon oxide and a variety of metals [6]. In this regard, tailings may contain minerals such as quartz, orthoclase, magnetite, hematite, pyrite, and chalcopyrite, among others. Additionally, the total iron grade may be around 19%, due to their origin in the north of Chile [7][8]. In addition, given that tailings have already been mined and crushed, the treatment costs are considerably lower than primary ores, turning tailings into an economic raw material [5]. Therefore, studies have been made to use tailings as a construction material such as cement and to recover valuable elements such as iron through magnetic concentration and copper through bioleaching [7][9][10]. This reprocessing of mine tailings by different technologies is being explored to make mining operations more sustainable [11]. Acid leaching is widely used in mineral extraction and the recycling of valuable metals [12]. Sulfuric acid is the best-known and most conventionally used leaching agent; however, some studies recommend hydrochloric acid since the presence of chloride ions is advantageous for copper extraction [9][13][14]. In addition, many studies are focused on copper recovery from slags. Dimitrijević et al. worked with hydrochloric acid concentration below 2 M and combined it with hydrogen peroxide on copper slag samples [15]. Chen et al. performed a similar study using, also with copper slag, an 11 M concentration of hydrochloric acid at temperatures above 80 °C [13].
In the present research, we carried out the characterization of an iron-rich tailing and the extraction of metals through leaching. We focused on some interesting elements such as copper, aluminum, calcium and magnesium. If the tailing is submitted to acid treatment, e.g., with hydrochloric acid (HCl), a liquid solution and an insoluble solid will be generated. Both phases (liquid and solid) represent an opportunity to produce valuable secondary species [16][17].

2. Collection and Characterization of the Tailing Sample

The tailing sample was obtained from a tailing dam placed in the north of Chile (Figure 1a). The sample (Figure 1b) was taken from the wall of the tailing dam using the trial pits technique. Then, this sample passed through a homogenization method considering rolling and quartering to obtain a representative sample. Subsequently, sieving was performed to analyze the size distribution and it was compared with a Laser Diffraction analysis. In addition, SEM-EDX and XRD analyses were performed to determine the tailing composition. Elements were determined with an AAS analysis.
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Figure 1. (a) Copper tailing dam in Vallenar, Chile [18], and (b) the tailing sample obtained.

3. Leaching Procedure

The initial tailing sample was leached using a HCl solution. To perform the leaching, 1 g samples were weighed and deposited in round-bottom flasks, each of them with 250 mL of the prepared acid solution (3 M, 6 M, 9 M or 12 M). The mixture was stirred with a magnetic stirrer at 550 rpm under a controlled temperature, coupling the flask with a laboratory condenser tube. After the leaching time, the solution was vacuum filtered using a porous glass filter. The filtered liquid solution and the insoluble solid left in the filter were reserved for later analysis. This procedure was repeated changing the acid concentration, the reaction time, and the reaction temperature as shown in Figure 2, where “C” is the acid concentration, “t” is time, and “T” is temperature. In addition, all combinations shown in Figure 2 were performed twice. The dissolution times were 6 h, 16 h, and 62 h, a long enough time. The temperatures were 25 °C (considered as room temperature), 50 °C, 60 °C, and 70 °C. After finishing the experiments, the total iron, copper, aluminum, calcium, and magnesium concentrations in the filtered acid solution were determined by AAS. The insoluble solids were dried, at 65 °C, for 24 h to be analyzed with SEM-EDX and XRD.
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Figure 2. Proposed method for acid dissolution.

4. Results

4.1. Tailing Sample Characterization

The particle size P80 was 221 ± 23 µm according to the granulometric analysis by sieving, and 216 ± 0.652 µm according to the Laser Diffraction analysis, meaning that 80% of the sample is approximately below an ASTM 60 mesh, corresponding to 250 µm (Figure 3). The morphology and general composition of the tailing sample were identified by SEM-EDX, as seen in Figure 4. At 100×, the tailing particles are irregular, and the composition is based on silicates and iron oxides. The XRD analysis, shown in Figure 5, was applied to the tailing sample, revealing that the composition was mainly quartz (SiO2), magnetite (Fe3O4), orthoclase (KAlSi3O8), and albite (NaAlSi3O8), confirming the SEM-EDX analysis. Through AAS, it was determined that the tailing contains a total iron grade of 19%.
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Figure 3. Granulometry of the tailing sample using (a) sieving and (b) Laser Diffraction.
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Figure 4. (a) SEM image and (b) EDX mapping of the tailing sample at 100× magnification.
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Figure 5. XRD diffractogram of the tailing sample.

4.2. Acid Solution Characterization

The acid solutions obtained through leaching and posterior filtration were analyzed by AAS, giving the concentrations of total Fe, Cu, Al, Ca, and Mg. The combinations of acid concentration (C), time (t), and temperature (T) for each experiment, along with their results, are shown in Table 1. The concentrations for each element of interest were calculated as the average between the doubled experiments. Therefore, the standard error was calculated.
Table 1. Average concentration and standard error associated of the elements obtained.
1 This combination was tripled.

4.3. Insoluble Solid Characterization

By SEM-EDX analysis, it was found that the solid samples had an irregular morphology composed mainly of silica with presence of several elements. Figure 6 shows one of the insoluble solids obtained from a solution leached with HCl 9 M, at 25 °C, for 16 h. The grains are irregularly shaped and the elements are mostly homogeneously distributed in the sample. Table 2 shows the elemental composition of the tailing sample and the insoluble solid sample obtained under the conditions of HCl 9 M, 25 °C, and 16 h of leaching, by EDX. Results show the weight percent of each element found. Figure 7 shows the XRD of the sample obtained at 9 M, at 25 °C, for 16 h. The main phases in the sample are orthoclase (KAlSi3O8), quartz (SiO2), and albite (NaAlSi3O8), matching the SEM-EDX analysis. In the same analysis, it is possible to observe peaks corresponding to other unidentified crystalline species marked with black circles. For further comparison between the tailing sample and the insoluble solid, an X-ray Fluorescence (XRF) analysis was performed on both samples. The results are shown in Table 3.
Figure 6. (a) SEM image and (b) EDX mapping of an insoluble solid sample at 100× magnification.
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Figure 7. XRD diffractogram of an insoluble solid sample.
Table 2. Elemental composition of the tailing sample and the insoluble solid obtained by EDX.
Table 3. Elemental composition of the tailing sample and the insoluble solid obtained by XRF.

5. Discussion

Mineral extraction brings with it the generation of waste, such as tailings and slags. It is important to find new ways to treat these wastes and extract as many valuable species as possible. The variability of elements in each sample makes leaching a complex process. Hydrochloric acid leaching is an alternative when it is desired to extract more than one element at a time.
Based on the analysis of the tailing sample from northern Chile, it can be observed that an advantage of it is the high concentration of iron, having a total iron grade of 19%. Size distribution analyses by sieving and Laser Diffraction show that P80 is in the order of 220 µm (Figure 3). This fine size distribution eases the leaching process given that the iron particles are released [19]. From SEM analysis it is possible to observe a dispersion of particles in the same size order (Figure 4). The EDX characterization (Figure 4) allows us to observe that iron is present in two forms: iron oxide and iron sulfide. Copper can be observed as an oxide, and aluminum, calcium, and magnesium as silicates. Accordingly, SEM-EDX analysis indicates that copper is homogeneously distributed over the sample surface, unlike titanium, which can be observed as discrete particles. Iron is associated with oxygen and sulfur, showing that there could be species such as magnetite, hematite, or pyrite in the sample. Magnesium, calcium, sodium, chlorine, and copper, are homogeneously distributed in the sample and seem to be linked with oxygen; in addition, sodium seems to be related to aluminum. Discrete particles associated with phosphorus and calcium are observed. Titanium is seen as a bright particle and is also distributed through the image, being possibly related to oxygen. Note that carbon is not part of the sample, but is part of the graphite sheet support. The tailing sample characterization shows an association between oxygen and most elements, such as potassium, aluminum, and silicon, probably forming orthoclase, and possibly forming hematite or magnetite with iron. This is confirmed by the XRD analysis since the main minerals are quartz, magnetite, orthoclase, and albite (Figure 5). Other species may be present, but in concentrations less than 3% by volume, which is the detection limit of the XRD equipment. Having only Kα radiation from Cu and no other tubes, it is challenging to eliminate the fluorescence of Fe. Nevertheless, this composition correlates with the studies conducted on different tailings from Chile [6][7].
Acid leaching is an efficient conventional technology applied to iron extraction, as it has been proven to have good tailing dissolution [5][20]. In this research, the concentration of some metallic elements was monitored from the acid solution and tabulated (Table 1). The average concentration of Fe was plotted as a function of acid concentration, time, and temperature, to analyze its behavior, as shown in Figure 8. It is seen in Figure 8a that the iron concentration increases as the acid concentration grows. This is in agreement with previous studies carried out with hydrochloric acid [13]. Therefore, according to the proposed method (Figure 2), after using 3, 6, 9, and 12 M of HCl concentration at 16 h and 25 °C, the 9 M concentration was selected. This is because the concentrations of the extracted elements are balanced with the increase in acid, e.g., at 9 M, the iron concentration is high and has a variation that can equal the concentration obtained at 12 M (Figure 8a). Concerning acid dissolution, results showed that the tailing contained an important amount of iron, in the range of 752 mg/L when the conditions are 9 M, 16 h, and 25 °C (Table 1). Afterwards, it is seen in Table 1 that time does not influence the iron concentration, as the behavior is practically constant, remaining in the order of 749.40 and 808.50 mg/L (Figure 8b). This can be attributed to the fact that iron leaching is usually achieved within a few hours [21]. According to the proposed method, the time was selected in 16 h as it achieves the best extraction of copper and also magnesium. Following the last section of Table 1, the temperature remains between 749.40 and 808.50 mg/L, which is practically constant as seen in Figure 8c. This can be related to the complementary effect between acid concentration and temperature [22]. Thus, since the HCl concentration selected was high, the temperature required for the metal extraction is low. Therefore, the most convenient is to use room temperature at 25 °C.
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Figure 8. Average concentration of Fe as a function of (a) HCl concentration, (b) time and (c) temperature.
The importance of the other metallic elements contributed to analyze the complexity of the leaching. As seen in Table 1, copper is not affected by acid concentration as it remains between 1.15 and 1.47 mg/L, and aluminum decreases with acid concentration from 26.54 to 16.81 mg/L. Regarding calcium and magnesium, they reach their highest yields at 9 and 6 M, respectively. Thus, the chosen elements are Cu, Al, Ca, and Mg in the same acid solution. Their average concentrations are plotted in a radar chart (Figure 9) showing the variation in front of the HCl concentration, time, and temperature of the reaction. It can be seen that iron has a significant concentration compared to the other elements, e.g., copper had to be converted from mg/L to cg/L to be seen in the radar chart. For copper, the concentration yields at 16 h, while aluminum remains in the range of 20.40 and 22.71 mg/L whether the time is 6, 16, or 62 h. Calcium remains in the range of 14.84 to 16.96 mg/L, and magnesium is between 28.98 and 31.09 yielding at 16 h. Additionally, for copper, aluminum, calcium, and magnesium, the highest concentrations are obtained at 50 °C, whether they do not show a clear tendency. In Figure 9a, it is possible to see that Cu, Ca and Mg tend to have higher concentrations between 6, 9, and 12 M, with 3 M not being enough to extract these elements. On the contrary, Al tends to be higher at lower acid concentrations. With Figure 9b it is seen that a time between 16 and 62 h is suitable for a successful extraction of these elements, being enough with 16 h. Figure 9c shows that 50 °C is enough to obtain high concentrations of Cu, Al, Ca, and Mg, as higher temperatures do not increase the concentration of these elements. However, it was experimentally determined that a room temperature of 25 °C is enough to extract iron (Figure 8c) and to maintain a more stable and efficient energy system.
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Figure 9. Radar chart of Cu, Al, Ca and Mg average concentrations as a function of (a) HCl concentration, (b) time and (c) temperature.
After acid dissolution, the main elements in the products are silica, oxygen, iron, potassium, and aluminum. Iron and aluminum are expected to be in the acid solution, and silica along with potassium is expected to be in the insoluble solid. Regarding the insoluble solid obtained after the leaching process (Figure 6), it was determined that it contains silicates that should be in a purer form, given that the presence of iron is low and it is associated with sulfur. Meaning that silicates associated with potassium, calcium, and sodium persist after leaching with acid (Figure 6), showing that the valuable metals contained in the tailing will be present in the acid solution after the leaching process, and the minerals related to silica will be in the insoluble solid retained in the filter. Table 2 shows that oxygen and iron are the most present elements in the tailing sample. As previously discussed, carbon is not part of the sample, but is part of the graphite sheet support, so it is not part of the tailing. Regarding the insoluble solid sample, it can be seen (Table 2) that a minimal amount of iron stays with the solid retained in the filter, so iron is mainly in the acid solution. Almost all the silicon is present in the solid. Since copper is not detected in the solid, it must have been dissolved in the acid solution. Part of the aluminum is in the solid forming orthoclase together with silicon. Small amounts of calcium and magnesium stay in the solid. It is noticed that silicon, oxygen, aluminum, potassium, and sodium are part of silica, orthoclase, and albite, as shown in the XRD (Figure 7). On the other hand, calcium and magnesium, sodium with chlorine, and iron with sulfur, seem to be related. A minimum amount of titanium is seen as particles possibly related to oxygen. There is also the presence of fluorine as particles on the sample that might be related to fluorosilicates. Table 3 shows the chemical composition of the tailing sample and in the insoluble solid with more detail. Elements such as Cr, V, and Eu, were not detected by the previous techniques. Regarding the main element analyzed, the insoluble solid contains only 5.47% Fe in comparison with the tailing, which contains 17.70% Fe, meaning that most of the iron was dissolved.
Given that the elements of interest were extracted from the copper tailing, future research contemplates the treatment of the acid solution to form a product containing valuable species [23].

6. Conclusions

The tailings of northern Chile contain an important amount of iron and other valuable species, such as copper and aluminum, which can be extracted through leaching with hydrochloric acid. The most relevant condition in this process is the acid concentration, due to its effect on the extraction of metals. Conditions can be selected according to the element that wants to be obtained, e.g., if a high iron extraction is wanted, a high acid concentration should be applied, while on the contrary, if a high extraction of aluminum is wanted, a low acid concentration should be used. The variables time and temperature do not show a relevant influence on the iron concentration, so it can be considered that it presents a constant behavior. Within the ranges studied, the conditions of 9 M acid concentration, 16 h reaction time, and a temperature of 25 °C were selected as the optimum leaching parameters. This process differs from traditional leaching by using hydrochloric acid instead of sulfuric acid; in addition, the raw material is a residue from a metallurgical process instead of mined ore. This proves that valuable elements such as iron, copper, and aluminum, along with calcium and magnesium, can be extracted from copper tailings for further valorization. The silicates obtained in the insoluble portion show low iron concentrations and could be used in other applications such as additives in the construction industry.

This entry is adapted from the peer-reviewed paper 10.3390/met12111924

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