*2.1. Materials*

The natural high purity pyrite specimens used in this investigation were acquired from Ward Scientific Establishment Inc. Scanning electron microscopy (SEM), as well as scattered energy x-ray spectroscopy (EDS) are shown in Figure 1 and was used a Zeiss scanning electron microscopy, model EVA MA-10 (CARL ZEISS Ltd., Oberkochen, Germany) and Oxford X-ray detector (OXFORD Instruments, Oxford, UK).

**Figure 1.** (**A**) SEM micrograph of pyrite sample and (**B**) energy x-ray spectroscopy (EDS) spectrum.

Two different sizes of pyrite were prepared, considering a range between 38 and 65 μm (−65 + 38 μm) for micro flotation tests and −38 μm for flocculation kinetics tests. Seawater was obtained 200 m from the coast of San Jorge Bay (Antofagasta, Chile) and it was filtered by UV filter to eliminate bacterial activity. Before being used, seawater passed through a quartz sand filter (50 μm) and a mechanical polyethylene filter (1 μm) to remove insoluble particles. Chemical analysis is presented in Table 1, which were obtained by different analytical techniques (atomic absorption spectrometry (AAS), Argentometric method, and acid–base volumetry).

**Table 1.** Seawater ion concentration and method of analysis (pH 8).


Flotation reagents were potassium amyl xanthate (PAX) as collector and methyl isobutyl carbinol (MIBC) as frother. Commercial xanthates generally have a purity of between 60 and 90%. They usually contain residual alkali hydroxide or metal carbonate that is intentionally added to slow down the

decomposition of the product during storage. Because of this, PAX was purified by dissolving it in acetone and recrystallizing from diethyl ether. All aqueous solutions were prepared using distilled water. Guar gum G4129 of high purity was supplied by Sigma-Aldrich (see the chemical structure in Figure 2).

**Figure 2.** Chemical structure of guar gum.

#### *2.2. Preparation of Flotation Reagents*

A stock solution of PAX was prepared by adding 750 mg in a 100 mL flask by filling it up with distilled water. From this solution, the required aliquots were taken to achieve the dosages expected in the tests. For the stock solution of MIBC, 0.021 g of the reagent was added to 100 mL of distilled water. From this, 14.3 mL was taken, which corresponded to a concentration of 20 ppm of MIBC. For the stock solution of guar gum (750 mg/L), the powder was stirred in distilled water for 5 h. From this solution, aliquots were taken to reach the corresponding concentration of guar gum.

#### *2.3. Microflotation Tests*

Microflotation tests were carried out in a 150 mL Partridge–Smith glass cell using air at a flowrate of 100 mL/min. Pyrite–water suspensions were prepared using 2 g of pyrite (considering a range between 38 and 65 μm) and 150 mL aqueous solution. The suspensions were stirred in a beaker with magnetic agitation and conditioned at the required pH for 3 min. Afterwards, PAX was added to the suspension at the desired concentration and contacted with the pyrite for 3 min, then with the frothing agent for another 2 min. Finally, the suspension was transferred to a Partridge–Smith cell and flotation was carried out for 3 min, scraping off the froth every 5 s. The pulp level in the microflotation test was kept constant by adding a background solution prepared at the same chemical reagent concentration as the original aqueous solution. To evaluate the influence of the depressant concentration on the floatability of pyrite, guar gum was added before the collector and contacted with the mineral for 5 min. All flotation tests were performed in triplicate. After the microflotation test, the non-floated and floated products were dried and weighed to calculate the pyrite floatability.

#### *2.4. Aggregate Characterization*

The effect of reagent conditioning on aggregate characteristics was evaluated; in this case, the mean chord length using the Focused Beam Reflectance Measurement (FBRM) technique. The instrument model was Particle Track G400 with a measurement of every 2 s. Direct images of particle aggregates were obtained with the Particle Vision Measurement (PVM) technique, using the V819 model. Three-hundred milliliters of seawater and 2 grams of pure pyrite were used. After 4 min of mixing, guar gum was added with the desired concentration, and after 5 min, PAX and MIBC were added.

#### *2.5. Fourier Transform Infrared (FTIR) Spectroscopy*

FTIR studies were performed to provide information on the infrared spectra of pyrite, PAX, guar gum, and the interaction products between the mineral, the collector, and the polysaccharide. A Fourier Infrared Spectrometer (PerkinElmer, Santiago, Chile) that operates in the range of 4000 to 400 cm−<sup>1</sup> was used for the spectroscopic studies.

#### *2.6. Surface Tension*

Four sets of surface tension tests were performed at pH 8 and 23 ◦C: (i) seawater, 20 ppm of MIBC and various guar gum concentrations (0–200 ppm); (ii) seawater and various MIBC levels (0–200 ppm); (iii) seawater and various guar gum concentrations (0–200 ppm); and (iv) seawater, 20 ppm of MIBC, 75 ppm of PAX, and various guar gum concentrations (0–200 ppm). The surface tension was determined by the bubble up technique using a tensiometer Lauda model TD 3 (LAUDA DR. R. WOBSER GMBH & CO, Lauda-Königshofen, Germany).

#### **3. Results and Discussions**

#### *3.1. Microflotations*

Figure 3 presents the recovery of pyrite in seawater as a function of PAX addition in the presence of 20 ppm of MIBC and in the absence of guar gum. The tests were performed at pH 8 to emulate the conditions used in the copper industry that operates with seawater [38]. There is a PAX dosage that maximizes the recovery of pyrite, which was close to 75 ppm. At this PAX addition, the pyrite recovery was 80%. Increasing PAX dosages lead to a little decrease in pyrite recovery.

**Figure 3.** Pyrite recovery as a function of PAX concentration (pH 8, 20 ppm of MIBC).

Dixanthogen formation through the oxidation of the collector ion turns the pyrite surface hydrophobic [39]. This collector oxidation can take place with the reduction of oxygen (Equation (1)). López-Valdivieso et al. [40] proposed that oxidation of the collector ion is coupled with the reduction of surface iron hydroxides (Equation (2)). The pyrite used in this study had iron hydroxides on its surface as will be shown below.

$$\text{RCCS}\_2^- + \frac{1}{2}\text{O}\_2 + \text{H}\_2\text{O} \rightarrow \text{ (ROCS}\_2\text{)}\_2 + 2\text{OH}^-\tag{1}$$

$$2\text{ROCS2}^{\circ} + 2\text{Fe(OH)}\_{3} + 6\text{H}^{+} = \left(\text{ROCS2}\right)\_{2} + 2\text{Fe}^{2+} + 6\text{H}\_{2}\text{O} \tag{2}$$

Figure 4 shows the pyrite flotation recovery as a function of guar gum addition at pH 8 in seawater and in the presence of 75 ppm PAX and 20 ppm MIBC. As the dosage of the polysaccharide increases up to 100 ppm, the recovery of pyrite gradually decreases from 80 to 23%, demonstrating that the polysaccharide is a good pyrite depressant. The OH groups of the guar gum molecules likely adsorbed onto OH sites of ferric hydroxide of the pyrite surface by a similar adsorption mechanism occurring between dextrin and oxidized pyrite [30,41]. The difference between guar gum and dextrin is that two cis-OH groups of the guar gum would link to a ferric hydroxide OH site, whereas, in the case of dextrin, only one OH group would link to a ferric hydroxide site. As the guar gum is a highly hydrophilic big molecule, its adsorption rendered the pyrite surface very hydrophilic. At guar gum dosages greater than 100 ppm, the recovery of pyrite gradually increased reaching 39% at 200 ppm guar gum.

**Figure 4.** Pyrite recovery as a function of guar gum concentration (pH 8, 75 ppm of PAX, 20 ppm of MIBC).

#### *3.2. Characterization of Aggregates*

Figures 5–7 present the characterization of pyrite aggregates throughout their chord length (using the FBRM probe), at various concentrations of guar gum without PAX or MIBC. In a collector and frother free environment, guar gum promoted the growth of aggregates, even at low concentrations. Adding 5 ppm polysaccharide, the aggregates grew from 120 to 160 μm (Figure 5A). At the optimal dose (100 ppm), the maximum chord length was slightly higher than 192 μm (Figure 6A). This value is stabilized at further dosages. At 200 ppm guar gum, the maximum aggregate size was 188 μm, suggesting that the limit zone in which the polysaccharide could act as a flocculating agent was reached. A notable difference in all particle flocculation systems is related to the fragmentation of the floc structure, caused by the hydrodynamic conditions. The FBRM probe provides the particle count classified in different bins according to their size, presenting their size evolution over time. In Figure 5B, it can be seen that after the addition of 5 ppm guar gum, the particle count for flocs smaller than 150 microns radically decreased, with a subtle increase of the flocs having a size between 150 and 300 microns. However, within a few seconds of flocculation, the number of flocs (counts) less than 150 microns increased steadily. When the guar gum dosage was 100 ppm, particles smaller than 150 microns are connected to produce larger stable aggregates, indicating that the aggregates are joined with greater strength. Increasing the dosage of guar gum up to 200 ppm raised the particle count again after a few seconds of flocculation (Figure 7A). The excessive amount of polymer may have saturated the particle surface so the polymer bound a less number of particles. Weak aggregates formed at this dosage.

**Figure 5.** (**A**) Mean square chord length and (**B**) counts evolution of suspension; 5 ppm of guar gum added after 1 min (no frother or collector).

**Figure 6.** (**A**) Mean square chord length and (**B**) counts evolution of suspension; 100 ppm of guar gum added after 1 min (no frother or collector).

**Figure 7.** (**A**) Mean square chord length and (**B**) counts evolution of suspension; 200 ppm of guar gum added after 1 min (no frother or collector).

Figures 8–10 exhibit the outcomes for the size of the aggregates at different concentrations of guar gum and in the presence of 75 ppm PAX and 20 ppm MIBC. A notable effect of the collector and the frother is the redispersing of the particles that are bound into the floc, especially at low polymer dosages. As shown in Figure 8A, with 5 ppm guar gum, there was an instantaneous growth of aggregates. However, after 1 min, following the addition of PAX and MIBC, a significant dispersion of particles with rope length less than 150 μm was observed. This seems to indicate that guar gum desorbed from the pyrite surface due to xanthate adsorption. Lopez-Valdivieso et al. [30] show that dextrin desorbed from pyrite surface by xanthates. Dispersion of the particles favored the recovery of pyrite since dispersed particles are more prone to float than those that make up an aggregate. According to Figure 8B, the dispersion due to PAX and MIBC is attenuated by increasing the dosage of the polysaccharide. At the optimum pyrite depression concentration of 100 ppm guar gum, the average decrease in aggregates is gradual (Figure 9A). In addition, particles smaller than 150 μm were released in a substantially lower quantity than at low guar gum dosages (Figure 9B) and not significant presence of particles smaller than 10 μm was quantified. Above 100 ppm guar gum, at a 200 ppm guar gum, particles redispersing increased again (Figure 10B), which coincides with the increase in pyrite recovery (Figure 5).

**Figure 8.** (**A**) Mean square chord length and (**B**) counts evolution of suspension: 5 ppm of guar gum added after 1 min (MIBC 20 ppm, PAX 75 ppm).

**Figure 9.** (**A**) Mean square chord length and (**B**) counts evolution of suspension: 100 ppm of guar gum added after 1 min (MIBC 20 ppm, PAX 75 ppm).

**Figure 10.** (**A**) Mean square chord length and (**B**) counts evolution of suspension: 200 ppm of guar gum added after 1 min (MIBC 20 ppm, PAX 75 ppm).

Figures 11–13 present images of suspended pyrite particles and their conformation in the absence and presence of guar gum, PAX, and MIBC. In the absence of these reagents (Figure 11), the particles are mostly dispersed and a low number of agglomerates are noted. This was expected, as particles seek to agglomerate in a highly saline environment due to compression of the electrical double layer. The increasing formation of aggregates as the dose of guar gum increases is observed in Figure 12A for 5 ppm and in Figure 13A for 100 ppm. Although the shape of the aggregates is irregular and different from a sphere, it is seen that in both cases the floc structures are compact, and composed of several particles. After the addition of PAX and MIBC, particle redispersion is evident as depicted in Figure 12B (5 ppm), where the aggregates reduce their size but gain in sphericity. At 100 ppm guar gum (Figure 13B), there is no substantial reduction in the extent of the agglomerates. There is an apparent redistribution of the particles changing the shape of the floc structure; however, this is consistent with that reported above for the size of the flocs.

**Figure 11.** Suspended particles of pyrite in seawater at pH 8 (7 min).

**Figure 12.** Suspended particles of pyrite in seawater at pH 8 interacting with flotation reagents: (**A**) 5 ppm of guar gum (7 min) and (**B**) 5 ppm of guar gum, 75 ppm of PAX, and 20 ppm of MIBC (7 min).

**Figure 13.** Suspended particles of pyrite in seawater at pH 8 interacting with flotation reagents: (**A**) 100 ppm of guar gum (7 min) and (**B**) 100 ppm of guar gum, 75 ppm of PAX, and 20 ppm of MIBC (7 min).

#### *3.3. FTIR Analysis*

Figure 14 presents the spectra of guar gum, PAX, pyrite contacted with guar gum, and pyrite contacted with both PAX and guar gum in seawater at pH 8. The bands between 1700 and 3000 cm−<sup>1</sup> and 1200 and 1500 cm−<sup>1</sup> are due to the groups CH2 and CH3 of the collector alkyl (Figure 14A) [42]. The bands between 3000 and 3700 cm−<sup>1</sup> are attributed to the hydrogen bonds of hydroxyl groups, due to hydroxides and oxy-hydroxide iron species [42]. The band at 3450 cm−<sup>1</sup> is due to the stretching vibration of the hydroxyl groups in the guar gum structure. This indicates that hydrogen bonds strongly link to the O–H groups (Figure 14B).

In Figure 14C, the band at 1650 cm−<sup>1</sup> is characteristic of water molecules indicating sulfate hydration [43]. The intense band at 1650 cm−<sup>1</sup> suggests a stretch of the glucopyranose ring. This is indicative that the pyrite surface is more hydrated and hydrophilic. The bands between 900 and 1200 cm−<sup>1</sup> corresponds to the asymmetric oxygen vibration (C-O-C), attributed to the xanthate and may be dixanthogen. The bands between 1000 and 1050 cm−<sup>1</sup> are due to the vibration C=S [44]. Finally, the bands between 900 and 650 cm−<sup>1</sup> correspond to oxidized iron species on pyrite such as the oxyhydroxides goethita (α-FeOOH) and limonite (α-FeOOH·*n*H2O)) [43]. Accordingly, it is confirmed that the pyrite surface is oxidized and this oxidation occurs rapidly. The bands in the ranges 1300 to 850 cm−1, belong to sulfate species, such as iron sulfates (Fe2(SO4)3xH2O between the bands 1000 and 1150 cm<sup>−</sup>1).

The bands between 500 to 750 cm−<sup>1</sup> are due to the stretching of the guar gum ring. As noted, they appeared in the guar gum–pyrite spectrum at 750 and 850 cm−<sup>1</sup> (Figure 14D), indicating Guar Gum on the pyrite surface. In the guar gum–PAX–pyrite spectrum (Figure 14C), they are almost entirely reduced. This is an indication that PAX desorbed some Guar gum and co-adsorption of PAX and guar gum took place on the pyrite.

**Figure 14.** FTIR spectra for (**A**) PAX, (**B**) guar gum (GG), (**C**) pyrite (Py) + GG + PAX, and (**D**) Py + GG (pH 8, 35 ppm of PAX, 20 ppm of guar gum).

#### *3.4. Surface Tension*

Figure 15 shows the surface tension of seawater as a function of MIBC and guar gum concentrations. Both reagents affected the surface tension at low concentrations. At 5 ppm, the surface tension dropped by approximately 10%. In this study, a fixed concentration of MIBC (20 ppm) was used while varying the guar gum concentration from 0 to 200 ppm However, a synergistic effect in the surface tension is not noted by adding the two reagents. This result is not in agreement with others reported elsewhere stating that there is a synergistic effect when polymers are mixed with frothers [45,46]. The synergistic effect would cause an overstabilization of the bubbles, which would increase gangue mechanical entrainment lowering the concentrate quality. Accordingly, if guar gum were used in flotation it would not affect the bubbles characteristics so bubble overstabilization would be avoided.

**Figure 15.** Surface tension as a function of reagents concentrations: (i) varied guar gum (GG) + 20 ppm of MIBC, (ii) varied MIBC, (iii) varied guar gum, and (iv) varied guar gum + 20 ppm of MIBC + 75 ppm of PAX (seawater at pH 8).

#### **4. Discussion**

Pyrite is common in cooper and polymetallic ore deposits. It is floatable and shows a high affinity with the flotation collectors that are used to concentrate the valuable metal sulfides. Therefore, flotation of pyrite is desirable as it impacts the quality of the metal concentrate.

Traditionally, pyrite is depressed at highly alkaline conditions. However, this approach cannot be implemented in seawater as precipitation of calcium and magnesium ions affects the performance of valuable minerals. Besides, there is an excessive consumption of lime, due to the buffer effects, that restricts the sustainability of depressing pH at high pH [15]. The current strategy operates at the slurry's natural pH and applies reagents that can selectively depress pyrite [47]. In this context, the organic reagents has shown promising results in the copper, molybdenum, talc, mica, galena, sphalerite, and pyrrhotite flotation [2]. However, their performance in seawater has not been explored yet, where a highly saline environment modifies the electrostatic interactions between surfaces and alters the structure in which water molecules are organized. It brings significant consequences in mineral processing.

Guar gum, used in this study, presented encouraging results, as FTIR studies indicate that this reagent was able to interact with the pyrite, decreasing its recovery from 80% to 23% in the presence of PAX. Although, the use of polysaccharides in general (guar gum, dextrin, starch, etc.) requires that the surface of pyrite should be oxidized at a certain level to achieve the interaction between the OH groups of the guar gum. The more ferric hydroxide on the pyrite surface, the higher is the adsorption of the polysaccharide [30].

The polysaccharide is expected to act by two mechanisms. On the one hand, the coating of the pyrite surface prevents the formation of ROCS2-M bonds, which maintain the connection between the collector and the pyrite surface. Additionally, the in situ analysis of the particles employing the FBRM and PVM techniques revealed their flocculation. The hydrodynamic conditions exert stress that redisperses the particles, especially for low polymer dosages. At the same time, as the concentration of guar gum increases, the redispersing of the flocs decreases, due to the greater strength of the agglomerate structure. As the aggregates of pyrite are massive, even if the bubbles attach to them, the bubbles will not be able to transport them to the flotation froth so they will not float. Although the polymer is a surface active agent, the surface tension results indicate that there is not a synergistic effect with the frother, as has been reported elsewhere [46]. Overdose of guar gum saturate the surface of pyrite, limiting the size of the agglomerates. This leads to an increase in recovery as less massive aggregates formed, which can be carried to the froth phase by bubbles.

#### **5. Conclusions**

Guar gum was used to promote pyrite depression in flotations with seawater in the presence of the collector propyl xanthate. The tests were performed at natural pH to emulate the operating conditions of copper concentration plants using seawater. The results were promising, and it was found that the polysaccharide efficiently depressed pyrite in a highly saline environment. The polysaccharide adsorbs on the pyrite surface turning it very hydrophilic on top of the hydrophobicity due to adsorbe collector. The depression of pyrite was accompanied by strong flocculation of particles that generated massive aggregates, which are difficult to be transported by bubbles to the froth phase. However, the collector and frother (PAX and MIBC) redispersed the agglomerates making possible their levitation to the froth by bubbles. This can be reduced by increasing the polymer concentration to minimize the redispersion of the agglomerates. However, an overdose of guar gum leads to a re-stabilization of the agglomerates lowering its depression effect for the pyrite.

**Author Contributions:** The manuscript was written through the contributions of all authors. R.I.J. designed the research, C.I.C. and E.C.P. performed the experiments, C.I.C. wrote the first draft, R.I.J., P.R., N.T., and A.L.-V. analyzed the results and wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Conicyt Fondecyt 11171036 and Centro CRHIAM through Project ANID/FONDAP/15130015.

**Acknowledgments:** R.I.J. thanks CONICYT Fondecyt 11171036 and Centro de Recursos Hídricos para la Agricultura y la Minería (CRHIAM) through Project ANID/FONDAP/15130015. Pedro Robles thanks the Pontificia Universidad Católica de Valparaíso for the support provided.

**Conflicts of Interest:** The authors declare no conflicts of interest.

#### **References**


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