metals-12-01924.pdf

Citation: Santibáñez-Velásquez, L.E.;
Guzmán, A.; Morel, M.J. Extraction of
Iron and Other Metals from Copper
Tailings through Leaching. Metals
2022, 12, 1924. https://doi.org/
10.3390/met12111924
Academic Editor: Mark E. Schlesinger
Received: 18 October 2022
Accepted: 6 November 2022
Published: 10 November 2022
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metals
Article
Extraction of Iron and Other Metals from Copper Tailings
through Leaching
Lidia E. Santibáñez-Velásquez 1,* , Alexis Guzmán 1 and Mauricio J. Morel 2,*
1 Departamento de Ingeniería en Metalurgia, Facultad de Ingeniería, Universidad de Atacama,
Copiapó 1531772, Chile
2 Departamento de Química y Biología, Facultad de Ciencias Naturales, Universidad de Atacama,
Copiapó 1531772, Chile
* Correspondence: lidia.esv@gmail.com (L.E.S.-V.); mauricio.morel@uda.cl (M.J.M.)
Abstract: Currently, mining operations have increased the generation of tailings, which contain a vari-
ety of elements that can be valorized. In this research, tailing samples were leached with hydrochloric
acid of concentrations greater than 3 M, considering the monitoring of iron, copper, aluminum,
calcium and magnesium, as relevant elements of the leached solution. Time and temperature were
also studied. The original tailing sample was taken by trial pits, and a size distribution analysis
was performed. The process generated an insoluble solid, rich in aluminosilicates, and an acid
liquid solution with different metal ions. Elemental analyses were performed on liquid samples by
Atomic Absorption Spectroscopy (AAS), and solid samples by Scanning Electron Microscopy-Energy
Dispersive Spectroscopy (SEM-EDX) and X-ray Diffraction (XRD). Results showed an increasing
trend of the iron concentration as a function of the acid concentration. However, copper is not affected
by the change in acid concentration, but by time and temperature. Aluminum decreases with acid
concentration, keeps constant with time, and yields at 50 ◦C. In the range of the studied parameters,
calcium and magnesium showed a variation without a clear trend. The elements in the acid solution
prepared with a tailing from northern Chile can be recovered for subsequent applications.
Keywords: 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
Metals 2022, 12, 1924. https://doi.org/10.3390/met12111924 https://www.mdpi.com/journal/metals

Metals 2022, 12, 1924 2 of 11
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. Materials and Methods
2.1. Characterization Equipment
For the sieving process, the sample was deposited into the following sieves set: #40,
#50, #70, #100, #140, #200, #270 and #400 ASTM. This set was shaken for 10 min using a
Labtech Hebro Ro-Tap. For the Laser Diffraction analysis, a Malvern Mastersizer 2000 was
employed with distilled water as dispersing agent.
For SEM-EDX, the sample was deposited on a graphite sheet and the measurements
were taken at 100× using a Zeiss EVO MA10 equipment (Carl Zeiss Pvt. Ltd., Oberkochen,
Germany) with Oxford Instruments EDX detector model X-MaxN 20 SDN (Oxford Instru-
ments, Abingdon, UK). The XRD analysis was performed on a Shimadzu LabX XRD-6100
equipment (Shimadzu, Kyoto, Japan), using Cu Kα radiation (λ = 1.5406 Å) with graphite
monochromator. The samples were mounted using acetone. The parameters were 30 kV of
voltage and 20 mA of current. The scanning was performed using a theta–2theta drive axis,
with a scan range from 10 to 120 degrees, at a speed of 2 degrees per minute. The sampling
pitch was 0.02 degrees, and the preset time was 10 s for the tailing sample and 3 s for the
insoluble solid.
2.2. 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.
2.3. 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

Metals 2022, 12, 1924 3 of 11
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.
Metals 2022, 12, x FOR PEER REVIEW 3 of 11
(a) (b)
Figure 1. (a) Copper tailing dam in Vallenar, Chile [18], and (b) the tailing sample obtained.
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 la-
boratory 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 concen-
tration, 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.
(a) (b)
The initial tailing sample was leached using a HCl solution. To perform the leachin
1 g samples were weighed and deposited in round-bottom flasks, each of them with 25
mL of the prepared acid solution (3 M, 6 M, 9 M or 12 M). The mixture was stirred with
magnetic stirrer at 550 rpm under a controlled temperature, coupling the flask with a l
boratory condenser tube. After the leaching time, the solution was vacuum filtered usin
a porous glass filter. The filtered liquid solution and the insoluble solid left in the filt
were reserved for later analysis. This procedure was repeated changing the acid concen
tration, 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 combination
shown in Figure 2 were performed twice. The dissolution times were 6 h, 16 h, and 62
a long enough time. The temperatures were 25 °C (considered as room temperature), 5
°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 b
AAS. The insoluble solids were dried, at 65 °C, for 24 h to be analyzed with SEM-EDX an
XRD.
Figure 2. Proposed method for acid dissolution.
Figure 2. Proposed method for acid dissolution.
3. Results
3.1. 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

Metals 2022, 12, 1924 4 of 11
(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%.
3. Results
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 compo-
sition 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),
of 19%.
(a) (b)
Figure 3. Granulometry of the tailing sample using (a) sieving and (b) Laser Diffraction.
(a) (b)
Figure 4. (a) SEM image and (b) EDX mapping of the tailing sample at 100× magnification.
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 compo-
sition 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),
of 19%.
(a) (b)
(a) (b)

Metals 2022, 12, 1924 5 of 11
Figure 5. XRD diffractogram of the tailing sample.
3.2. 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 calcu-
lated 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.
Experimental Conditions Average Concentration ± Error (mg/L)
C (M) t (h) T (°C) Fe Cu Al Ca Mg
3 16 25 297.60 ± 149.34 1.15 ± 0.03 26.54 ± 0.70 5.23 ± 0.20 27.22 ± 1.02
6 16 25 687.00 ± 38.18 1.24 ± 0.07 23.00 ± 0.05 10.51 ± 0.43 39.33 ± 7.73
9 1 16 25 752.08 ± 68.74 1.29 ± 0.10 21.09 ± 0.57 14.84 ± 2.23 31.09 ± 2.12
12 16 25 783.12 ± 22.06 1.47 ± 0.17 16.81 ± 2.88 13.23 ± 1.33 31.10 ± 1.11
9 6 25 776.20 ± 1.41 0.94 ± 0.07 20.40 ± 0.61 15.76 ± 0.36 28.98 ± 0.36
9 16 25 752.08 ± 68.74 1.29 ± 0.10 21.09 ± 0.57 14.84 ± 2.23 31.09 ± 2.12
9 62 25 779.10 ± 7.50 1.14 ± 0.12 22.71 ± 0.84 16.96 ± 0.90 30.85 ± 0.75
9 16 25 752.08 ± 68.74 1.29 ± 0.10 21.09 ± 0.57 14.84 ± 2.23 31.09 ± 2.12
9 16 50 808.50 ± 23.33 3.56 ± 1.80 42.81 ± 21.58 26.61 ± 8.70 41.64 ± 10.35
9 16 60 749.40 ± 23.59 3.30 ± 0.01 25.11 ± 0.27 7.92 ± 3.62 38.78 ± 2.95
9 16 70 780.38 ± 31.99 2.73 ± 2.08 25.97 ± 4.84 17.65 ± 2.56 36.11 ± 6.57
1 This combination was tripled.
3.3. Insoluble Solid Characterization
By SEM-EDX analysis, it was found that the solid samples had an irregular morphol-
ogy 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 insol-
uble 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
Figure 5. XRD diffractogram of the tailing sample.
3.2. 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.
Experimental Conditions Average Concentration ± Error (mg/L)
C (M) t (h) T (◦C) Fe Cu Al Ca Mg
3 16 25 297.60 ± 149.34 1.15 ± 0.03 26.54 ± 0.70 5.23 ± 0.20 27.22 ± 1.02
6 16 25 687.00 ± 38.18 1.24 ± 0.07 23.00 ± 0.05 10.51 ± 0.43 39.33 ± 7.73
9 1 16 25 752.08 ± 68.74 1.29 ± 0.10 21.09 ± 0.57 14.84 ± 2.23 31.09 ± 2.12
12 16 25 783.12 ± 22.06 1.47 ± 0.17 16.81 ± 2.88 13.23 ± 1.33 31.10 ± 1.11
9 6 25 776.20 ± 1.41 0.94 ± 0.07 20.40 ± 0.61 15.76 ± 0.36 28.98 ± 0.36
9 16 25 752.08 ± 68.74 1.29 ± 0.10 21.09 ± 0.57 14.84 ± 2.23 31.09 ± 2.12
9 62 25 779.10 ± 7.50 1.14 ± 0.12 22.71 ± 0.84 16.96 ± 0.90 30.85 ± 0.75
9 16 25 752.08 ± 68.74 1.29 ± 0.10 21.09 ± 0.57 14.84 ± 2.23 31.09 ± 2.12
9 16 50 808.50 ± 23.33 3.56 ± 1.80 42.81 ± 21.58 26.61 ± 8.70 41.64 ± 10.35
9 16 60 749.40 ± 23.59 3.30 ± 0.01 25.11 ± 0.27 7.92 ± 3.62 38.78 ± 2.95
9 16 70 780.38 ± 31.99 2.73 ± 2.08 25.97 ± 4.84 17.65 ± 2.56 36.11 ± 6.57
1 This combination was tripled.
3.3. 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

Metals 2022, 12, 1924 6 of 11
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.
orthoclase (KAlSi3O8), quartz (SiO2), and albite (NaAlSi3O8), matching the SEM-EDX anal-
ysis. In the same analysis, it is possible to observe peaks corresponding to other unidenti-
fied crystalline species marked with black circles. For further comparison between the
tailing sample and the insoluble solid, an X-ray Fluorescence (XRF) analysis was per-
formed on both samples. The results are shown in Table 3.
(a) (b)
Figure 6. (a) SEM image and (b) EDX mapping of an insoluble solid sample at 100× magnification.
Table 2. Elemental composition of the tailing sample and the insoluble solid obtained by EDX.
Element C O Fe Si Al K S Ca Na Mg Cl Ti Cu P F
Tailing sample
Wt. (%)
42.0 29.7 13.6 6.8 2.3 1.4 1.3 1.1 0.6 0.6 0.2 0.2 0.1 0.1 -
Insoluble solid
Wt. (%)
53.8 29.4 0.9 8.8 1.9 1.5 0.1 0.6 0.5 0.2 0.1 0.1 - - 1.9
Tailing sample
Wt. (%)
42.0 29.7 13.6 6.8 2.3 1.4 1.3 1.1 0.6 0.6 0.2 0.2 0.1 0.1 -
Insoluble solid
Wt. (%)
53.8 29.4 0.9 8.8 1.9 1.5 0.1 0.6 0.5 0.2 0.1 0.1 - - 1.9
orthoclase (KAlSi3O8), quartz (SiO2), and albite (NaAlSi3O8), matching the SEM-EDX anal-
ysis. In the same analysis, it is possible to observe peaks corresponding to other unidenti-
fied crystalline species marked with black circles. For further comparison between the
tailing sample and the insoluble solid, an X-ray Fluorescence (XRF) analysis was per-
formed on both samples. The results are shown in Table 3.
(a) (b)
Tailing sample
Wt. (%)
42.0 29.7 13.6 6.8 2.3 1.4 1.3 1.1 0.6 0.6 0.2 0.2 0.1 0.1 -
Insoluble solid
Wt. (%)
53.8 29.4 0.9 8.8 1.9 1.5 0.1 0.6 0.5 0.2 0.1 0.1 - - 1.9
Figure 7. XRD diffractogram of an insoluble solid sample.

Metals 2022, 12, 1924 7 of 11
Table 3. Elemental composition of the tailing sample and the insoluble solid obtained by XRF.
Element Fe Si Al Mg Cu Ca S K Cl Ti Eu Ba Pd Rb Zr
Tailing sample
Concentration (%)
17.700 12.460 3.320 1.706 0.160 2.149 0.783 2.503 0.334 0.299 0.188 0.074 0.032 0.027 0.026
Insoluble solid
Concentration (%)
5.470 33.460 6.110 0.000 0.310 3.867 1.450 6.491 2.253 0.690 0.128 0.030 0.009 0.032 0.050
Element Sr Mn Cr V Br Y Ag Zn Mo Hf Sc Sm Ni Other Total
Tailing sample
Concentration (%)
0.021 0.014 0.013 0.021 0.004 0.004 0.004 0.015 0.000 0.009 0.002 0.000 0.000 58.120 99.988
Insoluble solid
Concentration (%)
0.047 0.031 0.022 0.029 0.017 0.018 0.003 0.000 0.007 0.018 0.005 0.070 0.003 39.380 100.000
4. 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

Metals 2022, 12, 1924 8 of 11
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.
°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). Con-
cerning 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 concen-
tration 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.
(a) (b) (c)
Figure 8. Average concentration of Fe as a function of (a) HCl concentration, (b) time and (c) tem-
perature.
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 re-
mains 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 cop-
per, 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 de-
termined that a room temperature of 25 °C is enough to extract iron (Figure 8c) and to
maintain a more stable and efficient energy system.
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.

Metals 2022, 12, 1924 9 of 11
(a) (b) (c)
Figure 9. Radar chart of Cu, Al, Ca and Mg average concentrations as a function of (a) HCl concen-
tration, (b) time and (c) temperature.
After acid dissolution, the main elements in the products are silica, oxygen, iron, po-
tassium, 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 insolu-
ble 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 so-
dium persist after leaching with acid (Figure 6), showing that the valuable metals con-
tained 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 fluorosili-
cates. 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 re-
search contemplates the treatment of the acid solution to form a product containing valu-
able species [23].
5. Conclusions
The tailings of northern Chile contain an important amount of iron and other valua-
ble 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
Figure 9. Radar chart of Cu, Al, Ca and Mg average concentrations as a function of (a) HCl concen-
tration, (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].
5. 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

Metals 2022, 12, 1924 10 of 11
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.
Author Contributions: Conceptualization, M.J.M. and L.E.S.-V.; methodology, M.J.M. and L.E.S.-V.;
software, L.E.S.-V.; validation, M.J.M., L.E.S.-V. and A.G.; investigation, L.E.S.-V.; resources, M.J.M.
and A.G.; data curation, L.E.S.-V.; writing—original draft preparation, L.E.S.-V.; writing—review
and editing, M.J.M. and A.G.; visualization, M.J.M.; supervision, M.J.M.; project administration,
M.J.M.; funding acquisition, M.J.M. All authors have read and agreed to the published version of
the manuscript.
Funding: This research was funded by ANID grant number 77190056 and the APC was funded by
Project PAI-LIBAM.
Data Availability Statement: Not applicable.
Acknowledgments: The authors thank Bruno Zazzali, Marisela Navea, Danny Guzmán and IDICTEC
for their support in the characterization. The authors thank Mario Wilson and Evelyn Cárdenas for
their support in the translation of this research.
Conflicts of Interest: The authors declare no conflict of interest.
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