Metal Sulfide Nanoparticles as Sphalerite Surface Activators to Improve Zinc Recovery Through Flotation Process

Abstract

CuS nanoparticles (Np) were synthesized and deposited on synthetic sphalerite (S_P) using two different methods. Two nanoparticle products were obtained on the surface of S_P, Np1 and Np2, resulting in two active materials (Np1-S_P and Np2-S_P) with specific characteristics. Nanoparticles and active materials were characterized by TEM, XRD, SEM, and XPS. The collectors PAX and SIPX were adsorbed on Np1-S_P and Np2-S_P to determine the adsorption capacity. Method 1 provides a higher quantity of nanoparticles on S_P, which allows for the adsorption of a higher amount of SIPX. Method 1 was used to deposit nanoparticles on two natural sphalerites (S_N) with different iron contents. S_N, unlike S_P, can be used to test nanoparticle activation results in microflotation experiments. S_N was activated with nanoparticles (Np1-S_N) and using the traditional method (Cu-S_N). The recovery of 75% of zinc using the microflotation process suggests that the hydrophobicity of Np1-S_N is higher than that of Cu-S_N. Nanoparticles improve the hydrophobicity of S_N compared to the traditional activation used in the mining industry. These results suggest that using nanoparticles is an excellent option to activate minerals in flotation processes, decreasing the consumption of reagents and helping to mitigate negative impacts on the environment.

1. Introduction

Zinc metal has a high commercial value. It is used in several industrial applications, such as the manufacture of electric batteries and alloys and the galvanizing processes of iron and different types of steel. With the increase in the global population, manufacturing activities have increased, making zinc one of the most used raw materials; for this reason, it is in great demand in the industrial market [1]. According to an international database, although China is the leading zinc-producing country in the world, Mexico ranks sixth globally and second in Latin America, with 690 M metric tons [2].

The concentration of zinc involves a process known as froth flotation (Figure S1). This is regarded as one of the best methods to separate valuable species from aqueous solutions, due to its flexibility, simplicity, low energy consumption, sludge generation, selectivity, and efficiency for industrial applications [3]. To run an efficient flotation process, one key factor is the flotation reagents: collectors, frothers, and modifiers (Figure S2).

Zinc metal can concentrated through flotation from many sources, including sphalerite (chemical formula: ZnS) [4]. Although sphalerite has inherent flotation, it is insufficient to achieve high zinc concentrations. To improve this process, collectors such as xanthates are used (Figure S3), which increase mineral hydrophobicity; however, the interaction between them is not always strong enough. For this reason, sphalerite requires a prior activation stage [5]. Activators (Figure S2) react directly with the surface of the mineral, providing conditions to produce a stronger interaction between the mineral and collector [6]. For example, Aikawa et al. [7] reported the activation of mineral ZnS with PbSO₄ to improve the floatability of the mineral, because the PbS species formed on the ZnS surface has a high affinity for xanthate collectors.

Many metallic ions have been reported to activate sphalerite (e.g., Cu²⁺). This activation promotes the formation of CuS species on the surface of the sphalerite through the exchange of Zn²⁺, which is an ion abundant in sphalerite [8]. In this conventional activation process, the precursor of Cu²⁺ is the copper sulfate pentahydrate salt, which is the most expensive auxiliary reagent in the flotation circuit. The amount of copper salt used usually varies between 200 and 500 g/t [9]. The quantity is high, and not all Cu²⁺ is adsorbed on the surface of the sphalerite, thus ending up in wastewater. Additionally, this salt is potentially corrosive, decreasing the useful life of the infrastructure. A basic pH value is needed for the activation process (between 10 and 11). Great quantities of lime are used for this purpose, which is a disadvantage [10]. Furthermore, when traditional activation occurs, copper ions are generated, and hydrophilic species (Cu(OH)₂) can be formed, which are also adsorbed on the surface of the sphalerite and do not contribute to improving the flotation. Furthermore, the efficiency of the process tends to decrease [11].

After the activation of sphalerite, the second stage is the adsorption of collector molecules (Figure S3) [12]. The interaction between the polar head (collector) and the activated sphalerite leaves the non-polar tail of the collector oriented towards the bulk solution, increasing the hydrophobicity [13]. The latter is an essential requirement for flotation to occur efficiently [14]. In the literature, xanthate collectors can favor the flotation process to recover zinc. Kondrat’ev and Gavrilova [15] reported that the newly formed copper xanthate can attach to the sphalerite mineral surface more strongly than zinc xanthate, because the first compound has a lower solubility product value (Kps Cu-xanthate = 5.2 × 10^–20) than the second (Kps zinc-xanthate = 4.9 × 10^–9). They also reported that the chain length changes the hydrophobic response [16].

The activation stage can be improved with new technologies such as nanotechnology. Nanoparticles (Nps) on the surface of sphalerite can act as activators, increasing the potential of the mineral to adsorb the collector.

Nanoparticles are also an alternative to reduce the excessive use of common activators and pH controllers. Many researchers have reported the use of nanoparticles as collectors rather than activators to improve the flotation process. Hruzová et al. [9] reported organosolv lignin hydrophobic nanoparticles as collectors for copper recovery. Panagiotis et al. [17] used the same nanoparticles mixed with an SIPX collector in different proportions to float sphalerite and pyrite/arsenopyrite from sulfide minerals. They demonstrated an increase in Zn grade recovery compared to the standard flotation methods. However, they used large quantities of CuSO₄ as an activator. Mallampati et al. [18] synthesized Fe/Ca/CaO nanoparticles to recover Au, Ag, Pt, etc., through flotation. Other authors reported polymeric nanoparticles as collectors [19].

Although nanoparticles have been used as collectors and stabilizers, no work has used metallic sulfide nanoparticles as activators for sphalerite or reported their interaction with collectors.

Copper sulfide (CuS) nanoparticles can be synthesized via several methods in various shapes and sizes [20]. By setting the experimental conditions of the synthesis, CuS can be obtained with different stoichiometric ratios [21].

CuS nanoparticles as an activating agent in sphalerite flotation could offer different advantages: more efficient, selective, and environmentally sustainable activation. This is because the Nps have a greater surface/volume ratio than the same material in bulk; therefore, the contact with the surface of the sphalerite would be greater. These Nps, due to their size, can cover the surface of the sphalerite more uniformly, promoting greater adsorption of the collector, increasing the hydrophobicity of the mineral and the recovery of the zinc metal. Additionally, CuS Nps can release copper ions in a controlled manner. They foster a more uniform activation compared to conventional activation methods. This prevents the formation of passivated phases on the surface of the sphalerite.

The greatest impact and novelty of using nanoparticles as activators are that their small size can optimize the process, reducing reagent consumption and minimizing the negative impact on the environment (Table S1).

Therefore, this work aims to study two CuS nanoparticle synthesis methods, their potential as activated agents of sphalerite, and the adsorption mechanism of two collectors (xanthate-type). This study is completed by validating the activation process supporting nanoparticles on natural sphalerite with iron impurities. Microflotation experiments were conducted with this last material calculating the Zn recovery in this process.

2. Materials and Methods

2.1. Materials

Synthetic sphalerite, also named zinc sulfite or ZnS (S_P), with 97% purity was used. Sphalerite mineral (S_N) was obtained from the Peñoles mine in Zacatecas, Mexico. Two types of S_N were used: a low-iron mineral sphalerite (S_NL) and another medium-iron mineral (S_NM). The particle size of S_NL and S_NM was 0.074–0.037 mm. Copper chloride dihydrate (CuCl₂·2H₂O), sodium sulfide (Na₂S), thiourea (CH₄N₂S), copper acetate (Cu(CH₃COO)₂), and sodium hydroxide (NaOH) were used for nanoparticle synthesis. All reagents were purchased from Sigma Aldrich in ACS grade. The collector’s sodium isopropyl xanthate (SIPX) and potassium amyl xanthate (PAX) were acquired from CYTEC Company, México. SIPX and PAX molecules are shown in Figure S3.

2.2. Activation of Sphalerite with CuS Nanoparticles (Np1-S_P and Np2-S_P)

Two methods were proven for the synthesis and deposition of CuS nanoparticles on SP surface.

The first method (M1) consisted of dripping off Na₂S solution into 0.01 M of CuCl₂·2H₂O. The mixture was stirred at 400 rpm for 30 min at 70 °C, and the formation of CuS nanoparticles (Np1) can be observed. Then, 1 g of S_P was added and stirred for 30 min; the reaction was stopped, filtered, and washed several times to eliminate the remaining impurities. The active sphalerite (Np1-S_P) was dried in an oven at 80 °C for 24 h (Figure 1a).

The second method (M2) is a precipitation one, where Cu(CH₃COO)₂ and CH₄N₂S solutions were mixed (0.2 M and 2 M, respectively). While stirring, the mixture changed from blue to green over time, pointing out the presence of CuS nanoparticles (Np2). Subsequently, 5% NaOH and 1 g of S_P were added and stirred for 2 h at room temperature. Finally, the mixture was filtered and washed with distilled water and acetone several times. The active sphalerite (Np2-S_P) was dried in an oven for 2 h at 80 °C (Figure 1b).

2.3. Characterization

pH at zero-point charge (pH_zpc) was determined for S_P, Np1-S_P, and Np2-S_P. The procedure was described by Blanco-Flores et al. [22]. The phases in the S_P were identified by X-ray powder diffraction (XRD) in a Bruker D8 Advance X-ray diffractometer equipped with a CuKα radiation source and SOL-X solid-state detector. Analyses were performed in a 2θ range from 10° to 90° including 2θ steps at 0.3 s intervals. The surface morphology was performed using scanning electron microscope (SEM) using a JEOL JSM-6510LV SEM microscope operated at 20 kV. The size of Np1 and Np2 was determined by analyzing the micrographs obtained in a transmission electron microscopy (TEM) JEOL 2100 microscope operated at 200 kV accelerating voltage. To obtain statistical information on the particle size distribution, the length of about 200 particles was measured, employing ImageJ 1.54f software.

To investigate the interaction between nanoparticles, the collectors, and the sphalerite surface, X-ray photoelectron spectroscopy (XPS) was performed on an X Thermo Scientific model K-Alpha using Al Ka radiation (1486 eV) and a pass energy of 25 eV generated at 120 W. All binding energies were calibrated for the C1s peak (285 eV) arising from adventitious carbon. An argon ion source neutralizes surface electrostatic charges on the samples mounted on Al tapes. The energy spectra were collected and fitted using Gaussian curves. The FWHM widths selected for XPS deconvolution were based on those reported by Crist [23].

Atomic absorption spectrometry in Pinnacle 500 (Cu) and Perkin Elmer Analyst 200 (Zn) equipment determined the S_P and S_N chemical composition and the percentage of nanoparticles on Np1-S_P, and Np2-S_P.

The H₂O contact angle of S_NL, Cu-S_NL, and Np1-S_NL was determined. For this purpose, a drop of H₂O was deposited on the material polished and exposed in the test tube. Subsequently, the drop was illuminated with diffused light to produce a sharp-edged image. Finally, the photo was captured, and the contact angle was measured.

2.4. Adsorption Kinetics and Isotherms

The equilibrium time was established with the adsorption kinetics of both collectors (SIPX and PAX). With the adsorption capacity, the best synthesis method was determined. That one was used to activate the surface of two different types of natural sphalerite (S_N), which will be described later.

Thus, 0.1 g of Np1-S_P or Np2-S_P was put in contact with 10 mL of one collector solution at an initial concentration of 9.48 × 10⁻⁵ mol/L at room temperature. For each interval, one sample was withdrawn to determine the amount of non-adsorbed collector. For this, a UV-Vis spectrophotometer (Velab) was used at 301 and 302 nm wavelengths for PAX and SIPX, respectively. Equation (1) was used to calculate the adsorption capacity at each time interval (q_t in mg/g).

(1)${\mathrm{q}}_{\mathrm{t}}\mathrm{o}\mathrm{r}{\mathrm{q}}_{\mathrm{e}}=\left({\displaystyle \raisebox{1ex}{${(\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{t}}\mathrm{o}\mathrm{r}{\mathrm{C}}_{\mathrm{e}})$}\!\left/ \!\raisebox{-1ex}{$\mathrm{m}$}\right.}\right)\cdot \mathrm{V}$

where C_i is the initial concentration of collector solutions (mg/L), C_t is the non-adsorbed concentration of collectors in each time interval (mg/L), C_e is the non-adsorbed concentration of collectors in equilibrium time (mg/L) used in the adsorption experiments, m is the doses of sphalerite (g) used in each experiment, and V is the volume of collector solution (L) used in each experiment.

The equilibrium time was identified in the graph obtained by plotting q_t vs. t, corresponding to the adsorption kinetics. Pseudo-First-Order, Pseudo-Second-Order, Modified Ritchie Second-Order, and Avrami kinetic adsorption models (Supplementary Information) were applied to the experimental data to determine the best model for describing the kinetic adsorption process [1,24]. These results were compared to Cu-S_P. This last material is S_P activated using a traditional method, which consists of adsorbing Cu²⁺ on the surface of S_P. This is the standard and conventional way in the mining industry [25]. The procedure is simple: S_P was placed in contact with a solution of copper sulfate pentahydrate during the equilibrium time, maintaining the pH of the medium between 10 and 11. Subsequently, the supernatant solution was discarded, and the activated material (Cu-S_P) was dried.

The adsorption isotherms were obtained with 0.1 g of Np1-S_P or Np2-S_P in contact with 10 mL of collector solutions with different initial concentrations (1 × 10⁻⁵–1 × 10⁻⁴ mol/L). The mixtures were kept under agitation (400 rpm) until the equilibrium time was reached. The mixture was filtered, and the concentration of the non-adsorbed collector (mg/g) was determined by UV-vis spectrophotometry. The adsorbed amount (q_e) was calculated using Equation (1).

The experimental data were used to apply nonlinear isotherm adsorption models: Henry, Elovich, Fowler–Guggenheim, and Langmuir [26].

The two samples of S_N with different percentages of Fe (S_NL and S_NM) validate the efficiency of Np1 as an activating agent. S_NL with Np1 named Np1-S_NL, and medium-iron SNM with Np1 named Np1-S_NM. S_NL and S_NM were activated in the traditional way described above. The obtained materials were labeled as Cu-S_NL and Cu-S_NM.

For microflotation studies, 1 g of Cu-S_NL, Cu-S_NM, Np1-S_NL, and Np1-S_NM was added to the Hallimond cell and then 120 mL of SIPX collector was added and conditioned for 6 min. Finally, it was stirred at 400 rpm and floated using a nitrogen flow of 20 mL/min for 2 min. After flotation, the concentration and tailings were collected to quantify zinc. This was achieved by filtering the recovered mineral with subsequent drying. Quantification was performed through atomic absorption, and the percentage of zinc recovery (% rec.) was reported.

3. Results and Discussion

3.1. Activation of S_P with CuS nanoparticles

Na₂S in M1 is a strong electrolyte (Na⁺ and S^2-). The presence of free ions promotes the quick formation of Np1. This statement was discussed by Li et al. [27] previously.

Thiourea in M2 was hydrolyzed to release S^2- to Np2. This reaction is slow, allowing for the formation of the aqua complex ([Cu(H₂O)₄)]²⁺) and the precipitation of Cu(OH) in addition to Np2 [28].

Another important aspect is the room temperature in M2. The deposition of nanoparticles has two stages: Np2 formation (first) and its diffusion in the activated sites of the material (second). The last stage depends on the temperature; increasing temperature helps particles to diffuse and improve their deposition [29]. M1 was carried out at 70 °C, unlike M2; this explains why the deposition in M1 is better than in M2.

3.2. Characterization

pH_zpc for S_P, Np1-S_P, and Np2-S_P was 6.61, 6.21, and 6.51, respectively, confirming that the S_P surface was modified with the deposition of nanoparticles.

The XRD pattern of S_P (Figure 2) shows characteristic peaks at 28.63°, 47.58°, 56.16° and 76.6°, which correspond to (111), (220), (311), and (331) planes of the cubic phase of ZnS [30]. When nanoparticles were deposited, new diffraction peaks at 26.83°, 30.56°, 39.56°, 51.83°, 73.10° associated with (100), (102), (105), (108), and (208) planes of the hexagonal phase of CuS can be observed. There are two possible peaks (28.38° and 56.21°) that are associated with this same phase and correspond to (101) and (201) planes, but they overlap with the peaks of S_P. This information is confirmed by Harish et al. [31] and Yadav et al. [32]. The slight difference in the intensities of the peaks could be related to the number of nanoparticles deposited on the surface.

The intense peaks are related to the growth over the crystallographic planes, which can be associated with the specific morphology of CuS nanoparticles [33]. It is logical to expect small peaks for nanoparticles of both methods due to their deposition percentage not exceeding 16%. This was confirmed by the atomic absorption technique. With M1, it was possible to deposit more nanoparticles (12%) than with M2 (2%). The shape of the main peaks suggests that the nanoparticles obtained by M2 are smaller. This is logical, considering that thiourea is a capping agent that limits their growth. The crystallinity of the species obtained by M2 was lower than by M1.

SEM micrographs of Np1-S_P and Np2-S_P are shown in Figure 3. In Figure 3a,b, it can be observed that the particles were deposited on the grain boundaries of the S_P material. A high degree of agglomeration with flake-like irregular particles can be observed on both materials (Np1-S_P and Np2-S_P). The agglomeration is due to the high amount of precursors, considering that the growth rate is proportional to sulfur concentration. According to the Cu/S ratio, the largest particle size was obtained by M1 (Cu/S = 0.44) instead of M2 (Cu/S = 0.08) [34].

On Np2-S_P, particles (Figure 3e,f) were deposited in a smoother manner, unlike on Np1-S_P (Figure 3c,d). These characteristics could make a difference in the collectors’ adsorption process. Xie et al. [35] state that the agglomeration of particles in the sphalerite surface can benefit Zn’s flotation process and recovery when this mineral is activated with ammoniacal copper. Therefore, the agglomeration of particles could promote significant adsorption of the collectors. At the same time, this enhances the hydrophobic response of the material, which binds more strongly to microbubbles during flotation, increasing the recovery of Zn.

EDS analysis shows the presence of copper and sulfur on S_P. The (Cu+Zn)/S ratio in Np1-S_P (Figure 4a) was 1.82 and 1.72 in Np2-S_P (Figure 4b). These results confirm that the particle deposition is favored when the sulfur source comes from Na₂S instead of from thiourea. Pal et al. [9] reported that the low composition of copper is characteristic of the CuS phase, although other copper phases may form (Cu(OH)₂). Another important factor is temperature; when this variable increases, the exchange rate between Cu²⁺ and Zn²⁺ is favored, like in Np1-S_P. The percentage of copper increased when synthesis was achieved at high temperature (70 °C), indicating that the deposition of copper ions is favored when the temperature increases from 25 °C (M2) to 70 °C (M1). This behavior is also reported by Gao et al. [32]. According to chemical mapping, it is observed that the deposited particles were uniformly distributed over the entire surface of the S_P material.

Figures show TEM images of Np1 (Figure 5a) and Np2 (Figure 5c). It seems that the most significant spheres (greater than 100 nm) are formed by the aggregation of small nanoparticles, whose average size ranges from 11.18 nm to 10.29 nm for Np1 (Figure 5b) and Np2 (Figure 5d), respectively. Large structures such as nanolayers contain small nanoparticles inside (Figure 5a), and the morphology is spherical in Figure 5c.

For M1, the experimental conditions favored the nucleation process because the release rate was fast. Many CuS nuclei formed the nanoparticles, which were then self-assembled to obtain larger nanoparticles and layer shapes. This suggests that the aggregation speed should have been favored under these reaction conditions concerning the nucleation speed in the nanoparticle formation mechanism. It can be inferred that the nanosheets are made up of CuS nanoparticles or nanolayers. The latter were assembled and agglomerated with each other. According to Kundu et al. [36], the nanosheets were joined together through van der Waals forces or weak covalent bonds.

However, M2 had thiourea as a sulfur source with a slow-release rate. In this sense, Yadav et al. [30] states that slow nucleation may result in abnormal growth in different directions, resulting in irregular polyhedral morphology. Van Oversteeg et al. [37] associate the different morphologies with copper sulfide species, which is also probable in the present work. The smaller size was obtained for Np2-S_P (10.29 nm), followed by Np1-S_P (11.18 nm). The counter-ion used in the M2 was less polarizable than the counter-ion used in the M1, according to Yadav et al. [30], and a larger size was expected for less polarizable copper salt counterion.

In Figure 5b, there are fringes or fingers; they are related to Moiré patterns [38], which are characteristics of materials with optical properties and when two similar patterns overlap with different orientations. For the nanoparticles in the present work, these patterns are the product of networks in different orientations or networks with defects. This could be because, in the first case, the growth of crystals at different speeds causes them to orient in different directions, and in the second case, the high concentration of the sulfur source could promote vacancies. The nucleus growth was interrupted by the growth of other particles. Defects such as dislocations could be generated, and consequently, the growth continues to occur with a specific adjustment in the direction of the crystalline structure. This effect is mainly observed in Np2-S_P (Figure 5c). This phenomenon probably prevented the deposition of a larger amount of S_P on the surface.

Figure S4a shows the survey XPS spectrum of S_P, in which zinc and sulfur were mainly found; carbon and oxygen were detected with very few intense peaks, denoting their low presence in the material. The oxygen peak may be attributed to a slight surface oxidation of the material to which the surface of materials is always exposed, and the formation of Zn-O is promoted. The presence of the C1s peak is probably due to contamination.

The deconvolution of the Zn (the FWHM used was 1.5 ± 0.1) spectrum shows two peaks in Zn2p^3/2 and Zn2p^1/2 (Figure S4b), both with two internal peaks characteristic of the Zn-S (1021 eV and 1044 eV) and Zn-O (1023 eV and 1046 eV) interactions. This last interaction is characteristic of minerals since, during their processing (crushing, grinding, and contact with water or air), the surface tends to oxidize [39].

The S2p spectrum (deconvoluted with FWHM of 1 ± 0.1) shows one peak because S2p^3/2 and S2p^1/2 overlap, forming a single wide peak. When this peak is deconvoluted, four peaks are obtained, two for each mirror spin orbital [40] (Figure S4c). The interactions attributed to these binding energies are S-O (161.8 eV, 163.9 eV) and S-Zn (163 eV and 165 eV). This behavior is normal for sulfur since it is a chemical element with different valencies or oxidation states [41].

3.3. Kinetic and Isotherm of Two Collectors

Kinetics adsorption experiments were carried out with the collectors SIPX and PAX onto Np1-S_P and Np2-S_P compared to Cu-S_P. In all cases, the adsorption of the collectors is higher in the materials with nanoparticles (Np1-S_P, Np2-S_P) than in the ones activated in the traditional way (Cu-S_P); the adsorption capacity increases almost twice. The shortest equilibrium time was achieved for SIPX adsorption kinetics for Np1-S_P (8 min) and Np2-S_P (12 min). With traditionally activated sphalerite (Cu-S_P), for one collector or the other, the adsorption kinetics required a longer time to reach equilibrium (Figure 6a,b). This result is an additional advantage of nanoparticle activation. In the mining industry, the reduction in time impacts not only the technical process but also the economic one. The material with a shorter equilibrium time and greater adsorption capacity was Np1-S_P (qeq = 8.91 × 10⁻⁶ mol/g). Therefore, Np1-S_P has a higher nanoparticle size than Np2-S_P. This could influence the collector’s diffusion time into the adsorption sites. The characteristic roughness of the Np1-S_P surface favorably influenced the higher adsorption of both collectors. The growth of Np2 crystals on S_P in different directions did not benefit the higher adsorption of the collectors; the steric factors probably prevented achieving this objective with Np2-S_P. The chemical structure of SIPX and PAX also influences the process (Figure S3). In conclusion, the size, surface irregularity, and structure of the collectors are acting synergistically.

To prove that SIPX adsorption increases because the nanoparticles act as surface-activating agents, SIPX adsorption was performed on Np1-S_P, Np2-S_P, Np1, Np2, and Cu-S_P (Figure 6c). It is clear that Np1 and Np2 adsorb less collector than when they are supported on the S_P surface. In either case (Np1-S_P, Np2-S_P, Np1, and Np2), the adsorbed collector is greater than the amount on Cu-S_P. In other words, the proposal in this project, at least in the behavior of the collector adsorption, is marked. The other important thing to highlight is that Np1 adsorbs a greater quantity of the collector compared to Np2, alone or supported on S_P as Np1-S_P. With this new activation method, it is not necessary to use Ca(OH)₂, which is a reagent only used in the traditional activation process to increase the pH.

Kinetics adsorption processes were better described using a Modified Ritchie Second-Order Model and Avrami fractional order model (Table 1). For Np1-S_P, SIPX was adsorbed on the active surface sites, but the process cannot be limited to a surface reaction because n_A (0.59) is less than unity. A value between 0.2 and 0.8 points out that the collector should be distributed uniformly and homogeneously on the surface [42,43]. For PAX adsorption, the value of n_A (0.08) decreases, which suggests that the structure of the collector must cause some change in the binding mechanism, probably related to some diffusion-controlled process. Similar results were obtained by Knezevic et al. [44]. The interaction between the surface sites of materials and the SIPX collector was electrostatic. Comparing the pH of the solution after collector adsorption (pH_Np1-Sp-SIPX = 6.12) with pH_zpc determined by Np1-S_P material (6.21), the surface charge is positive; therefore, the electrostatic interaction between Cu²⁺ and the functional group of collectors (sulfur ion) is viable.

When the nanoparticle deposition method changes and the collector type does too, the adsorption process shows different results. The PAX adsorption process on Np1-S_P, Np2-S_P, and Cu-S_P, and SIPX on Np2-S_P was better described by the Modified Ritchie Second-Order Model (Table 1), where a collector molecule may be bound to more than one surface active site. This is because the polar group of both collectors presents elements that should bind via electrostatic attraction or free electron interaction with Cu ions. This statement could be validated by analyzing the values of the β parameter, where it increases concerning the SIPX adsorption for Np2-S_P.

Boyd’s external model and Weber and Morris internal diffusion model [45] were applied to the experimental data of the material that adsorbed the largest amount of collector SIPX, which is Np1-S_P. The best fit was achieved with Boyd’s external model, assuming then that the slow step of the process was given by the diffusion of the SIPX molecules through the liquid film surrounding the Np1-S_P. Using the value of the constant C of the Weber and Morris internal diffusion mode, the initial adsorption factor (R_i) was calculated (0.22). According to Wang et al. [46], the initial process can be classified as strong initial adsorption. This result agrees with the analyses of the kinetic process of SIPX on Np1-S_P.

According to the adsorption isotherm experiments (Figure S5a,b), the adsorbed amount increased rapidly at low initial concentrations. At higher values, the tendency is exponential. This behavior can be explained by different surface interactions, for example, electrostatic or hydrophobic, considering the adsorption sites of Np1-S_P and Np2-S_P and the functional groups of SIPX and PAX.

Adsorption isotherm experiments were determined at lower initial concentrations. The isotherm shows a linear behavior characteristic of multilayer adsorption of molecules like surfactants. The collector molecules are like surfactants. The polar part is frequently oriented toward the surface, while the nonpolar hydrocarbon chain (tail) can generate other types of interactions (hydrophobic) [47].

Although the most common mathematical models to apply for the experimental data of adsorption isotherms are fundamentally Langmuir and Freundlich, their selection is subject to several factors, such as initial concentration. Furthermore, the shape of the experimental isotherm is another factor that helps decide which models for the experimental data need to be analyzed. According to the abovementioned information, the mathematical models selected were Henry, Elovich, Fowler–Guggenheim, and Langmuir [48]. The first model is used when the adsorbed amount and the concentration in the equilibrium are proportional. The Elovich isotherm describes an adsorption process that increases exponentially with initial concentration. This model is based on kinetic aspects and considers multilayer adsorption. The Langmuir model is applied when the material has a homogeneous surface, and a monolayer adsorption process could occur; it is a reference model that gives an idea of the maximum adsorption capacity [23]. This value must be considered if the statistical parameters indicate a good mathematical fit. On the other hand, the Fowler–Guggenheim model is based on lateral interactions between adsorbate molecules on the surface of the adsorbent [49].

Table 2 shows the values of adsorption variables obtained from applied mathematics. The model with the best mathematical adjustment was Henry’s for SIPX on Np1-S_P, and PAX on Np1-S_P and Np2-S_P. However, the Fowler–Guggenheim model fits the experimental data of SIPX on Np2-S_P. The physical meaning of Henry’s model is that since the initial concentrations are so small, there are no lateral interactions between the adsorbate molecules. Since w is greater than zero, the interaction between the adsorbed molecules is attractive. The adsorption capacity is reported using the Elovich model. In Np2-S_P, the adsorption of the PAX was higher than that of SIPX. The preference of the Np1-S_P and Np2-S_P for PAX may be because the former molecules are linear, allowing a greater probability of occupying the greatest number of adsorption sites, unlike SIPX, which has a little branching by the “isopropyl” chain.

3.4. Adsorption Mechanism

An XPS survey scan of Np1-S_P (Figure 7a) shows the presence of primary elements (C, O, Zn, Cu, and S).

Zinc was deconvoluted with an FWHM of 1.5±0.1. Four states were found (Figure 7b): for Zn2p^3/2: Zn-S (1021.9 eV), Zn-O (1023.1 eV), Zn-Cu2+ (1023.9 eV) and Zn-Cu⁺ (1025.1 eV); for Zn2p^1/2: Zn-S (1045.2 eV), Zn-O (1046.5 eV), Zn-Cu²⁺ (1047.6 eV) and Zn-Cu⁺ (1049.1 eV). These values coincide with those reported by Zhu et al. [50]. Comparing the Zn2p spectra of S_P and Np1-S_P (Figure 7c), the energy values of Zn-S and Zn-O are similar. New peaks appear at binding energy values that correspond to more oxidized chemical species. It is important to reiterate that the deposition is carried out in an aqueous medium that promotes surface oxidation, so it is normal to expect Zn-O-type interactions but also Cu-O in different oxidation states.

High-resolution XPS spectra of S in the 2p region, deconvoluted with an FWHM = 1 ± 0.1, are shown in Figure 7d. In this case, several peaks associated with different interactions found, for S2p^3/2, S-O (162.3 eV) and S-Zn (163.2 eV). This can also coincide with S²⁻, S⁻, and S_n²⁻, associated with the crystalline structure of CuS [51], S-Cu²⁺ (164.1 eV) and S-Cu⁺ (165.1 eV). For S2p^1/2: S⁴⁻ (168.8 eV), S-O (169.5 eV); regarding this last sign, it could also coincide with a simultaneous coordination of S for both Cu and Zn, according to Zhang et al. (refer to [41]). The S-O signal could correspond to the presence of SO₄^2-, which can be attributed to a small oxidation of the material. Other signals associated with S are S-Zn (170.3 eV), S-Cu²⁺ (171.0 eV), and S-Cu⁺ (171.6 eV). A slight shift of the peaks can be noticed, which is related to the difference in electronegativity between zinc (1.6 eV) and copper (1.9 eV), which induces the exchange of electrons between them [52].

When comparing the S2p spectra for the S_P and Np1-S_P (Figure 7e) materials, it is observed that there is a separation in the S peaks in the latter material. This is precisely due to the interaction of Cu (in the form of CuS nanoparticles) with the ZnS surface. As the chemical environment changes, there is a shift in the characteristic S-O and S-Zn peaks because S is bound to other more oxidized species.

If both spectra are analyzed (Figure 7b,d), the most intense peaks correspond to the interactions of Zn and S with “O” and promote the appearance of more oxidized states of the chemical species involved. This was not the case for Cu, where the synthesis conditions favored the appearance of nanoparticles with Cu-S interactions (Figure 7f).

Cu was deconvoluted with an FWHM of 1.2 ± 0.1. The signals that come out after the deconvolution are associated with Cu2p3/2 Cu-S (932.3 eV), Cu₂-S (933.6 eV) and Cu-O (935 eV). For Cu2p^1/2, these come out to Cu-S (952.2 eV), Cu₂-S (953.5 eV) and Cu-O (954.9 eV). These results confirm the presence of CuS deposited onto ZnS (Figure 7f). The presence of the copper ion in 1+ and 2+ oxidation states is confirmed, as well as the presence of mixed crystalline phases. The non-presence of satellite peaks at 942.8 and 962.7 eV indicates that the presence of the Cu₉S₅ phase is not evident [53].

The intensity of the peaks shows that the species are in equal proportion. However, another hypothesis has been reported: it can be assumed that the majority phase of all the metal sulfides must be CuS since Cu⁺ is unstable in an aqueous solution and in an oxygen atmosphere, which is why it quickly transforms into Cu²⁺, which precisely forms CuS (known as covellite) [54]. The hypothesis is related to the dual-oxidation state for Cu in the covellite compound because this has a hexagonal crystalline structure, where trigonal and tetragonal sites appear; the first included a Cu⁺ metallic center around three sulfur groups, whereas the tetrahedral Cu²⁺ center bound to four sulfur groups [33,55]. The Cu⁺ is more exposed superficially, so it is logical to think it would be an available link center for xanthate-type collectors.

When the O was deconvoluted with an FWHM of 1.4±0.1, four peaks (Figure 7g) were obtained for O1s: O-S (531.7 eV), O-Zn (532.7 eV), O-Cu²⁺ (533.9 eV) and O-Cu⁺ (534.9 eV). It should be noted that, due to the aqueous medium, surface oxidation occurs. Consequently, interactions of the O-Zn and O-S type, as well as Cu-O in different oxidation states, are expected to be observed.

These results allow us to infer that the deposition was due to another mechanism like electrostatic attraction. This should be through interaction between Cu²⁺ from Np1 deposited and sulfur atoms from the S_P structure. In addition, another hypothesis should be the migration of Cu²⁺ to Zn²⁺ vacancy sites in the crystalline structure of S_P. Both alternatives usually use nanoparticles when they are deposited on the surface of materials: surface interaction forces or chemical affinity, according to Escorcia-Díaz et al. [56].

Hamad et al. [57] refer to all semiconductor materials, such as metal sulfides, that present a relaxed surface in the crystallographic planes (220) of the cubic phase and (100) and (102) of the hexagonal phase. This means that the anions in the structure relax or move towards the surface and the cations towards the internal part of the structure. These are fixed through electrostatic interactions between the surface S²⁻ atoms of S_P and the Cu⁺ of the trigonal sites (more exposed in the crystalline structure of CuS). In this way, a layer of nanoparticles is on the surface. Subsequently, the collector’s adsorption is promoted through the same adsorption phenomenon, which agrees with the results obtained when applying the adsorption kinetics models. Figure 8 shows the proposed interaction mechanism described. This applies to both types of collectors.

In conclusion, this is a more efficient alternative that can replace the traditional activation method of sphalerite in the mining industry.

3.5. Microflotation Test with S_N Activated by Np1

After S_N activation, sphalerite must have the necessary properties to adsorb a collector and finally concentrate metallic zinc through flotation experiments (Figure S6). To validate the efficiency of the activation proposal with CuS nanoparticles, a low-iron sphalerite (S_NL) with an iron content of 0.22% and another with a medium-iron (S_NM) content of 3.76% were used in the microflotation test.

The contact angle values obtained S_NL in an aqueous medium (S_NL+H₂O), traditionally activated sphalerite (Cu-S_NL), and Np1-S_NL were 74.13°, 105.81°, and 139.8°, respectively. This means that by depositing nanoparticles, it is not only possible to adsorb a greater quantity of both collectors, especially SIPX, but also the hydrophobicity of the mineral increased to almost double the value of natural sphalerite (S_NL); this is another relevant contribution of this research.

The results of microflotation of all materials show that when sphalerites were activated with nanoparticles (Np1-S_NL and Np1-S_NM), the percentage of Zn recovery was higher (Figure 9). When sphalerites were activated with nanoparticles, the recovery percentage (% rec.) was between 1.5- and 1.6-times higher than the percentage obtained when activating traditionally (Cu-S_NL and Cu-S_NM).

These results validate the hypothesis initially proposed. Activating with nanotechnology has better results than the traditional activating process in the mining industry.

4. Conclusions

The formation of nanoparticles using two methods occurs through the nucleation of small particles taking advantage of vacancies in the sphalerite structure. Nanoparticles enhance the adsorption of collector molecules compared to the traditional activation method. Equilibrium time and adsorption capacity are specific for each nanoparticle system deposited on the sphalerite and depend on xanthate-type collectors. According to the results from mathematical models and XPS deconvolutions, the deposition of the nanoparticles was due to specific interactions and to the slight deformation or relaxation of the crystalline structure on the surface to facilitate the union of the deposited CuS phase.

The larger amount of collectors improves the hydrophobicity of the sphalerite. Results with synthetic sphalerite can be reproduced on natural sphalerite with different iron contents.

The mineral with the greatest hydrophobic response concentrates more in the microflotation processes. Therefore, concentrating zinc (contained in the sphalerite mineral) by activating its surface with nanoparticles is a technically feasible and environmentally friendly alternative.

Author Contributions

D.M.Á.-M.: investigation; A.B.-F.: writing—original draft, conceptualization, supervision, project administration; H.P.T.-J.: writing—review and editing; M.G.T.: formal analysis, writing—original draft; A.R.V.-N.: investigation, formal analysis; I.A.R.D.: review and editing, resources, project administration; R.d.A.G.: formal analysis, methodology. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author because the research is still ongoing and being tested in industrial applications.

Acknowledgments

The authors would like to thank the Institute of Metallurgy, for the Autonomous University of San Luis Potosí and for Technological Institute of Higher Studies of Tianguistenco financial and other support.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Activation process with the deposition of CuS nanoparticles synthesized by (a) M1 and (b) by M2.

Figure 2. XRD patterns of SP and Np1-SP and Np2-SP.

Figure 3. SEM micrographs of Np1-SP (c,d) and Np2-SP (a,b,e,f).

Figure 4. EDS spectra of Np1-SP using Na2S (a) and Np2-SP using thiourea (b).

Figure 5. TEM micrographs of Np1-SP (a) and particle size histogram (b) and Np2-SP (c) and particle size histogram (d).

Figure 6. Adsorption kinetics of (a) SIPX and (b) PAX collectors onto Np1-SP, Np2-SP, Cu-SP and (c) adsorption of the SIPX collector onto Np1, Np2, Np1-SP, Np2-SP, Cu-SP.

Figure 7. XPS survey spectra of Np1-SP (a), spectra of Zn 2p (b), Zn2p comparation for SP and Np1-SP (c), S 2p (d), S2p comparation for SP and Np1-SP (e), Cu 2p (f) and O1s (g).

Figure 7. XPS survey spectra of Np1-SP (a), spectra of Zn 2p (b), Zn2p comparation for SP and Np1-SP (c), S 2p (d), S2p comparation for SP and Np1-SP (e), Cu 2p (f) and O1s (g).

Figure 7. XPS survey spectra of Np1-SP (a), spectra of Zn 2p (b), Zn2p comparation for SP and Np1-SP (c), S 2p (d), S2p comparation for SP and Np1-SP (e), Cu 2p (f) and O1s (g).

Figure 8. Mechanism to deposition of Np1 and collector adsorption onto Np1-SP.

Figure 9. Microflotation results of traditionally activated materials and CuS nanoparticles.

Table 1

Values of kinetic adsorption models parameters for SIPX on Np1-S_P, PAX on Np1-S_P, SIPX on Np2-S_P, PAX on Np2-S_P, and Cu-S_P.

Adsorption Model Parameters	Materials
	Np1-SP		Np2-SP		Cu-SP
	SIPX Collector Adsorption	PAX Collector Adsorption	SIPX Collector Adsorption	PAX Collector Adsorption	SIPX Collector Adsorption	PAX Collector Adsorption
qe-experimental (mol/g)	8.91 × 10−6	8.77 × 10−6	8. 71 × 10−6	7.60 × 10−6	1.50 × 10−6	4.42 × 10−6
Pseudo First Order Model
qe-calculated (mol/g)	8.91 × 10−6	8.77 × 10−6	8.16 × 10−6	7.59 × 10−6	1.49 × 10−6	4.32 × 10−6
k1 (min−1)	1.06	2.48	3.12	2.55	0.70	0.49
R2	0.9404	0.4511	0.3485	0.4418	0.5679	0.7591
Pseudo Second Order Model
qe-calculated (mol/g)	9.24 × 10−6	8.78 × 10−6	8.80 × 10−6	7.60 × 10−6	1.58 × 10−6	4.66 × 10−6
k2 (g/mol min)	3.35 × 105	5.53 × 106	2.06 × 107	7.31 × 106	8.46 × 105	1.80 × 105
R2	0.9706	0.8884	0.8119	0.8744	0.9113	0.9419
Modified Ritchie Second Order Model
qe-calculated (mol/g)	9.10 × 10−6	8.80 × 10−6	8.17 × 10−6	7.62 × 10−6	1.65 × 10−6	4.78 × 10−6
β	4.58	46.30	191.09	66.87	2.38	1.52
kRM-2 (min−1)	5.99	23.49	56.55	17.66	0.63	0.60
R2	0.9974	0.9283	0.8982	0.9652	0.9647	0.9494
Avrami Fractional Order Model
qe-calculated (mol/g)	8.99 × 10−6	8.79 × 10−6	8.18 × 10−6	7.63 × 10−6	n/a	n/a
nA	0.59	0.14	0.08	0.09	n/a	n/a
kAV (min−1)	1.38	4.11	5.29	4.19	n/a	n/a
R2	0.9987	0.9165	0.8746	0.9558	n/a	n/a
Boyd’s External Model	Kb = 0.732 min−1 R2 = 0.9553
Weber and Morris Internal Diffusion Model	kWM = 7.32 × 10−7 g/min1/2C = 6.96 × 10−6R2 = 0.8494Ri = 0.22

Table 2

SIPX and PAX adsorption parameters for the Np1-S_P and Np2-S_P.

Materials/Parameters Model	Np1-SP	Np2-SP	Np1-SP	Np2-SP
	SIPX Collector Adsorption		PAX Collector Adsorption
Henry model
KHE (L/g)	2.058	2.120	6.409	0.752
R2	0.9454	0.9136	0.9965	0.9893
Elovich model
q0 (mol/g)	3.21 × 10−6	4.38 × 10−6	4.92 × 10−6	2.48 × 10−5
KE (L/mol)	28,871.48	56,701.27	131,183.45	45,261.72
R2	0.9029	0.8731	0.9220	0.7714
Fowler–Guggenheim model
w (kJ/mol)	10.257	5.165	5.926	0.102
KFG (L/mol)	1.18 × 10−3	4.05 × 10−5	2.36 × 10−5	0.6590
R2	0.8690	0.9967	0.9812	0.8782
Langmuir model
qm (mol/g)	0.018	0.245	0.285	1.472
KL (L/mol)	1.19 × 10−5	9.86 × 10−6	3.16 × 10−6	9.90 × 10−6
R2	0.7704	0.7098	0.7621	0.8078

Where: K_HE: Henry’s adsorption binding constant of the adsorbate on the adsorbate surface, q₀: apparent adsorption capacity, K_E: equilibrium constant, w: interaction energy between absorbed molecules, K_FG: Fowler–Guggenheim equilibrium constant, q_m: maximum adsorption capacity and K_L: Langmuir constant.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/separations11120358/s1, Figure S1. Stages of the flotation process of sulfide minerals; Figure S2. Flotation reagents for the concentration and recovery of metals; Figure S3. Chemical structure of SIPX (left) and PAX (right) molecules of collectors; Figure S4. Survey XPS spectrum of SP (a) Zn2p (b), and S2p (c); Figure S5. Experimental adsorption isotherm of Sipx and Pax onto Np1-SP (a) and Np2-SP; Figure S6. Process diagram of zinc concentration by microflotation as a method of concentration of valuable species; Table S1. Comparison between sphalerite activation with CuS nanoparticles and CuSO₄; Table S2. Adsorption kinetics models. References [58,59,60,61,62] are citied in the Supplementary Material.

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