- Email: [email protected]
The effect of sodium hydrosulfide on molybdenite flotation as a depressant of copper sulfides
Minerals Engineering 148 (2020) 106203
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
The effect of sodium hydrosulfide on molybdenite flotation as a depressant of copper sulfides Yang Chen, Xumeng Chen, Yongjun Peng
T
⁎
School of Chemical Engineering, The University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Molybdenite flotation Sodium hydrosulfide Copper depressant Edge plane Basal plane
Depressing copper sulfides using sodium hydrosulfide (NaHS) in the flotation of molybdenite is a common practice. However, the effect of NaHS on the flotation of molybdenite is not clear despite a number of studies. In this study, the effect of NaHS concentrations which depressed the flotation of chalcopyrite on the flotation of molybdenite of different particle sizes was investigated. It was found that molybdenite flotation was promoted at a low NaHS concentration. A high NaHS concentration only reduced the flotation kinetics slightly without affecting the final flotation recovery. The effect of NaHS on the flotation of smaller molybdenite particles was more significant. Cryo-XPS analysis was then carried out to understand the underpinning mechanism. It was evident that the flotation behavior of molybdenite in the presence of NaHS was determined by the reactions of NaHS on the edge plane surface of molybdenite. At a low NaHS concentration, NaHS adsorbed on the edge plane surface and formed hydrophobic species such as elemental sulfur and polysulfide. At a high NaHS concentration, the hydrophobic species disappeared with more HS− adsorbed. At both low and high NaHS concentrations, NaHS did not adsorb on the basal plane surface of molybdenite which remained its surface hydrophobicity.
1. Introduction Molybdenite (MoS2) is the main source of molybdenum which plays a significant role in the modern society with its principal use as a ferroalloy in iron and steel industries (Dutta and Lodhari, 2018). Molybdenite is often associated with copper sulfides such as chalcopyrite (CuFeS2) and chalcocite (Cu2S). Conventionally, the beneficiation of copper and molybdenum (Cu-Mo) ores is achieved through two stages of flotation. The first stage recovers copper sulfides and molybdenite as a bulk concentrate with a thiol collector and the second stage selectively recovers molybdenite with an oil collector against copper sulfides (Zanin et al., 2009; Yuan et al., 2019). Over decades, a number of inorganic and organic reagents have been developed to depress copper sulfides in molybdenite flotation (Yuan et al., 2019). However, the most commonly used copper depressants today are still NaHS and Na2S with the former being predominant (Hirajima et al., 2014). The role of NaHS and Na2S in depressing copper sulfides in flotation is associated with the desorption of thiol collectors and the modification of copper mineral surface properties (Gaudin, 1957; Nagaraj et al., 1986). Taking xanthate as an example, it may adsorb on copper sulfides through copper-xanthate complexes. When NaHS or Na2S is added as a depressant, the hydrolyzed or ionized HS− becomes predominant (Poorkani and Banisi, 2005). HS− has a stronger affinity to copper ions ⁎
than xanthate and therefore can replace xanthate on the surfaces of copper sulfides (Bhambhani et al., 2014). In addition, the addition of NaHS or Na2S may decrease the Eh, which prevents the oxidation of copper sulfides to form hydrophobic products such as polysulfide and elemental sulfur on the surfaces (Taheri et al., 2014). Unlike copper sulfides, molybdenite has a layered structure with two orientation surfaces, basal and edge planes. Basal planes are formed by the breakage of weak van der Waals forces between layers (S-S bonds) while edge planes are generated by the rupture of strong covalent Mo-S bonds (Dickinson and Pauling, 1923). As a result, basal planes are hydrophobic responsible for the natural floatability of molybdenite while edge planes are hydrophilic (Chander and Fuerstenau, 1972; Lu et al., 2015). Particle size is an important factor affecting the basal/edge ratio of molybdenite and its flotation behavior. The basal/ edge ratio of molybdenite decreases with a decrease in particle size and therefore smaller molybdenite particles show less surface hydrophobicity and lower floatability (Castro and Correa, 1995; Yang et al., 2014). A number of studies were conducted to investigate the effect of NaHS on molybdenite flotation. Amelunxen and Amelunxen (2009) found that molybdenite floatability was relatively unaffected by NaHS. Similar observations were found recently by Zhao et al. (2018) and Wu et al. (2018). However, Peng et al. (2017) demonstrated that
Corresponding author. E-mail address: [email protected] (Y. Peng).
https://doi.org/10.1016/j.mineng.2020.106203 Received 16 April 2019; Received in revised form 5 December 2019; Accepted 12 January 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
NaHS were of AR grade. DI water was used in preparation of all the reagents, grinding and flotation tests.
molybdenite recovery only remained unchanged at a low concentration of Na2S and a sharp drop of molybdenite recovery occurred when Na2S concentration was higher than 5 mmol/L. In contrast, Yin et al. (2010) observed a higher molybdenite grade and recovery after sodium sulfide was added in flotation. In general, the previous studies disagreed with the effect of NaHS or Na2S on molybdenite flotation and did not consider the context of Cu-Mo separation. The effect of NaHS or Na2S on molybdenite flotation should be governed by the surface modification on its basal and edge planes, but this aspect was not investigated in the previous studies. In this study, the Cu-Mo separation was considered as the context and the range of NaHS concentration which depressed chalcopyrite flotation was examined in the flotation of molybdenite particles at different sizes. Then, the surface changes of molybdenite basal and edge planes before and after the addition of NaHS were characterized by cold stage X-ray Photoelectron Spectroscopy (Cryo-XPS, below −145 °C) and related to molybdenite flotation behavior.
2.2. Methods 2.2.1. Grinding and flotation Chalcopyrite or molybdenite mineral (100 g) was combined with 150 mL water and ground with 5.4 kg stainless steel rods in a laboratory stainless steel rod mill for 4.1 min for chalcopyrite and 20 min for molybdenite to obtain a P80 of 106 μm. To obtain a P80 of 53 μm for molybdenite, 8.9 kg stainless steel rods were used and the grinding time was 20 min. Diesel oil (0.15 g) was added in the rod mill before grinding for better dispersion. After grinding, the pulp was transferred to a 1.5 L JK batch flotation cell and was topped up to 1.5 L by adding DI water. The reagents were added in the following order: pH modifier (NaOH), depressant (NaHS) and frother (MIBC) and the conditioning time for each of them was 3, 3 and 1 min, respectively. The concentration of MIBC in the flotation was 20 ppm. The agitation speed was fixed at 1000 rpm and the air flow rate during the flotation was 3.0 L/min. The flotation froth was scraped every 10 s and four flotation concentrates were collected after cumulative times of 1.0, 3.0, 6.0 and 10.0 min.
2. Materials and methods 2.1. Materials and reagents Molybdenite and chalcopyrite samples were purchased from GEODiscoveries, Sydney and originated from Peru. The chemical compositions of chalcopyrite and molybdenite samples, analyzed by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), are shown in Table 1. By assuming all the copper from chalcopyrite and all the molybdenum from molybdenite, the purity of the chalcopyrite sample was 93.4 wt% and the purity of the molybdenite sample was 97.2 wt%. The chalcopyrite sample was crushed through a jaw crusher and then a roll crusher, and screened to collect +0.6–3.35 mm particle size fractions. The molybdenite sample was cracked using a hammer and the resulting flakes were chopped using a scissor and then a blender. +0.6–3.35 mm molybdenite particle size fractions were collected for further tests. All the processed samples were sealed in polyethylene bags and stored in a freezer at a temperature of −20 °C to minimize further oxidation. For XPS analysis, a high quality molybdenite crystal was selected to prepare basal and edge plane surfaces. A pair of tweezers was used to exfoliate the top layer of the molybdenite crystal to obtain a smooth and flat basal plane surface. The edge plane surface was generated by cutting the crystal perpendicularly to the layered structure using a diamond saw and the crystal was then covered with epoxide resin so that only the desired edge plane surface was exposed (Miki et al., 2017). The basal plane surface was used once and for a different test a new basal plane surface was prepared while the edge plane surface was renewed by polishing with #600 to #2000 emery paper (3M, USA), following the work published in literature (Tan et al., 2015). Diesel oil and MIBC were used as the collector and the frother, respectively, in flotation tests. They are normally used in the flotation of molybdenite against copper sulfides. Both were of industrial grade and used as received. Diesel oil was added in flotation without dilution while MIBC was diluted to 1 wt%. The pH was adjusted by the addition of 1 M NaOH solution instead of lime commonly used in flotation plants to eliminate the effect of Ca2+ on the flotation. Sodium hydrosulfide (NaHS) was added as a depressant in the flotation. Both NaOH and
2.2.2. XPS analysis Cryo-XPS was used to identify the chemical species present on both basal and edge surfaces of molybdenite after NaHS treatment. NaHS solutions with 0 mmol/L NaHS, 3 mmol/L NaHS (low concentration) and 12 mmol/L NaHS (high concentration) determined from flotation tests were prepared. The basal or edge surface was first conditioned in one of the NaHS solutions for 3 min, rinsed with Milli-Q water to remove residual NaHS on molybdenite surface and then dried with pure nitrogen gas, following the procedure published in literature (McLeod et al., 2010; Deng et al., 2013; Huai et al., 2017). The sample was then loaded into the pre-cooled sample chamber at −145 °C using a metal stub with an insulating tape which was submerged into liquid nitrogen for 1 min to cool the sample (Huai et al., 2018). XPS data were acquired using a Kratos Axis ULTRA X-ray photoelectron spectrometer incorporating a 165 mm hemispherical electron energy analyzer. The incident radiation was monochromatic Al Kα X-rays (1486.6 eV) at 150 W (15 kV, 15 mA). Survey (wide) scans were taken at a pass energy of 160 eV and multiplex (narrow) high resolution scans were taken at 20 eV. Survey scans were carried out over the 1200–0 eV binding energy range with 1.0 eV steps and a dwell time of 100 ms. Narrow highresolution scans were run with 0.05 eV steps and 250 ms dwell time. Base pressure in the analysis chamber was 1.0 × 10−9 Torr and during sample analysis was 1.0 × 10−8 Torr (Mu et al., 2017). The resulting spectra were charge-corrected using C 1s as a reference with a binding energy of 284.8 eV. Data processing and fitting were carried out using Casa XPS software. 3. Results and discussion 3.1. Flotation performance 3.1.1. Chalcopyrite flotation Before investigating the effects of NaHS on molybdenite flotation,
Table 1 Chemical compositions of chalcopyrite and molybdenite samples. Species present (wt.%)
Chalcopyrite Molybdenite
Al2O3
Bi
CaO
Co
Cu
Fe
Mo
Pb
S
SiO2
Ti
Zn
0.26 0.13
0.08 0.23
0.86 0.07
0.03 0.01
32.5 0.06
28.8 0.14
– 58.3
0.66 0.06
33.4 39.5
2.09 1.52
0.03 –
1.29 0.01
2
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
Fig. 1. The effect of NaHS addition on chalcopyrite flotation.
Fig. 2. The effect of NaHS addition on the flotation of 106 µm molybdenite.
Table 2 The Eh (mV vs SHE) in chalcopyrite flotation at various NaHS concentrations. NaHS concentration (mmol/L)
0
3
6
9
12
Chalcopyrite
374.1
–233.9
−274.9
−288.0
−329.4
the flotation of chalcopyrite was conducted first at different NaHS concentrations. Flotation results are shown in Fig. 1. It can be seen that chalcopyrite displayed a good floatability in the absence of NaHS and the recovery reached nearly 90% in 1 min flotation and 90.8% at the end of flotation. The addition of NaHS depressed chalcopyrite flotation. At 3 mmol/L NaHS, although no obvious decrease in the final chalcopyrite recovery was observed, the flotation kinetics dropped significantly with about 12.0% chalcopyrite recovery in 1 min flotation. With the further increase of NaHS concentration, the final chalcopyrite recovery decreased by 46.8% and 69.7% in the presence of 6 mmol/L and 9 mmol/L NaHS, respectively. In the presence of 12 mmol/L NaHS, chalcopyrite was almost completely depressed with only 4.4% recovery at the end of flotation. This NaHS concentration was equal to 10 kg/t, which agrees with the scale applied in the industry from several to dozens kg/t of NaHS (Amelunxen and Amelunxen, 2009). The Eh (mV vs SHE) in chalcopyrite flotation at various NaHS concentrations was measured and shown in Table 2. It can be seen that the Eh in flotation was around 375 mV in the absence of NaHS. With the addition of NaHS, the Eh decreased sharply to a range from −230 to −330 mV due to the reduction property of NaHS. The measured Eh value is in line with the previous studies with the addition of NaHS in flotation pulp (Pourghahramani et al., 2017; Wu et al., 2018). Diesel oil, the collector for molybdenite, was used in chalcopyrite flotation. It is known that chalcopyrite is naturally floatable under slightly oxidizing conditions and it is believed that elemental sulfur that forms on chalcopyrite surface is responsible for the natural floatability of chalcopyrite (Heyes and Trahar, 1977; Luttrell and Yoon, 1984). Diesel as an oil collector may physically adsorb on the hydrophobic surface of chalcopyrite to enhance its surface hydrophobicity and floatability (Bos and Quast, 2000). When NaHS is added as a copper depressant, the ionized HS− will decrease the Eh, producing a reducing environment where the chalcopyrite surface is free of hydrophobic species such as S0 (Herrera-Urbina et al., 1999). As a result, diesel cannot adsorb on chalcopyrite surface in the presence of NaHS and chalcopyrite floatability may decrease with an increase in NaHS concentration as observed in this study.
Fig. 3. The effect of NaHS addition on the flotation of 53 µm molybdenite.
was studied, and the results are shown in Fig. 2 and Fig. 3, respectively. From Fig. 2, it can be found that molybdenite recovery reached 90% at the end of flotation in the absence of NaHS. In the presence of 3 mmol/L NaHS, molybdenite recovery even increased to 95% at the end of flotation and the flotation kinetics also increased slightly. The increase of NaHS concentration to 6 mmol/L and 9 mmol/L slightly reduced the flotation kinetics but the final molybdenite recovery was still around 95%. When NaHS concentration was further increased to 12 mmol/L, the flotation kinetics reduced significantly and the final molybdenite recovery dropped back to 90%. In general, the addition of NaHS at the concentration range which depressed chalcopyrite flotation did not have a negative effect on the flotation of 106 µm molybdenite, and a low NaHS concentration enhanced the flotation, instead. Fig. 3 shows that the flotation of 53 µm molybdenite reached 53.5% recovery at the end of flotation in the absence of NaHS, much lower than that from the flotation of 106 µm molybdenite. This is expected because the floatability of molybdenite is related to particle size. With the reduction of particle size, firstly, the overall surface area increases and therefore the coverage of diesel oil on particle surfaces becomes less at the same diesel concentration. In addition, the ratio of basal/ edge decreases with the reduction of particle size. The basal plane surface is known to be hydrophobic while the edge plane surface hydrophilic. A lower basal/ edge ratio corresponds to a lower molybdenite floatability. Fig. 3 also shows that the addition of 3 mmol/L NaHS significantly increased molybdenite recovery at the end of flotation from 53.5% to 76.8%. In the presence of 6 mmol/L NaHS, the flotation kinetics started to decrease slightly, although the final molybdenite recovery was similar. Both flotation kinetics and final molybdenite
3.1.2. Molybdenite flotation Following chalcopyrite flotation, the effect of the same NaHS concentrations on the flotation of 106 µm and 53 µm molybdenite particles 3
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
Table 3 The Eh (mV vs SHE) in molybdenite flotation at various NaHS concentrations. NaHS concentration (mmol/L)
0
3
6
9
12
106 µm molybdenite 53 µm molybdenite
360.2 351.7
−249.0 −278.3
−283.5 −303.2
−294.6 −313.9
−339.7 −347.8
recovery started to decrease at a NaHS concentration higher than 9 mmol/L, however, the final molybdenite recovery was still significantly higher than that in the absence of NaHS. The comparison between Fig. 2 and Fig. 3 shows that NaHS displayed a similar effect on the flotation of 53 µm and 106 µm molybdenite. A low NaHS concentration was beneficial to molybdenite flotation but this benefit was reduced at a high NaHS concentration. In general, the effect of NaHS on the flotation of 53 µm molybdenite was more pronounced. The Eh (mV vs SHE) in molybdenite flotation at various NaHS concentrations was also measured and shown in Table 3. It can be seen that the Eh in molybdenite flotation was around 350 mV in the absence of NaHS. As observed in chalcopyrite flotation, the Eh decreased sharply to a range from −250 to −350 mV after the addition of NaHS in molybdenite flotation due to the reduction property of NaHS. It was noticed that the addition of NaHS increased flotation pH as shown in Table 4. The addition of 3 mmol/L, 6 mmol/L, 9 mmol/L and 12 mmol/L NaHS increased the flotation pH from 7.4 to 9.8, 9.9, 10.1 and 10.5, respectively, in the flotation of 106 µm molybdenite and from 7.2 to 9.5, 10.0, 10.5 and 10.6, respectively, in the flotation of 53 µm molybdenite. In order to identify the effect of modified pH values through NaHS additions on molybdenite flotation, the flotation of 106 µm and 53 µm molybdenite in the pH range from 7 to 11 adjusted by NaOH additions was conducted and the results are shown in Fig. 4. It can be seen that 90% and 54% molybdenite recoveries were achieved at the end of flotation of 106 µm and 53 µm molybdenite, respectively, at pH 7.0 and molybdenite recovery decreased only slightly with an increase in pH value. Lucay et al. (2015) found that molybdenite floatability decreased with an increase in pH value from 4 to 12. Tabares et al. (2006) indicated that the magnitude of zeta potential of molybdenite increased with an increase in pH value and was always negative, resulting from the singly-charged hydromolybdate (HMoO4−) species and double-charged molybdate ions (MoO42−) on the edges. The repulsive force between negatively charged molybdenite particles and bubbles would increase with an increase in pH value, leading to decreased molybdenite flotation (Yuan et al., 2018). Obviously, the beneficial effect of NaHS on molybdenite flotation as shown in Fig. 2 and Fig. 3 is not through the pH change. It may result from the interaction of NaHS with basal and edge planes of molybdenite, which was studied further below.
Fig. 4. Molybdenite recovery as a function of pH adjusted with NaOH solution in the flotation of 106 µm and 53 µm molybdenite. Table 5 The XPS atomic concentration of elements on basal and edge planes of molybdenite with and without NaHS treatment. Molybdenite
Basal-0 mmol/L NaHS Basal-3 mmol/L NaHS Basal-12 mmol/L NaHS Edge-0 mmol/L NaHS Edge-3 mmol/L NaHS Edge-12 mmol/L NaHS
Cryo-XPS analysis was conducted on basal and edge plane surfaces of molybdenite after being conditioned at different NaHS concentrations.
Table 4 The pH value in molybdenite flotation at various NaHS concentrations. 3
6
9
12
106 µm molybdenite 53 µm molybdenite
7.4 7.2
9.8 9.5
9.9 10.0
10.1 10.5
10.5 10.6
Mo
S
O
S/Mo
25.54 20.31 22.87 21.95 16.62 18.15
52.41 43.08 48.00 46.05 38.05 41.54
22.05 35.63 29.13 29.92 42.07 35.41
2.05 2.12 2.09 2.09 2.29 2.29
3.2.2. High-resolution spectra of basal plane and edge plane surfaces High resolution scans were conducted to identify the chemical state of Mo and S on basal plane and edge plane surfaces of molybdenite. Fig. 5 shows the Mo 3d 5/2 and S 2p XPS spectra recorded from the basal plane without and with NaHS treatment. The peak at 230.1 eV is attributed to Mo (IV) in the form of MoS2 (Baltrusaitis et al., 2015). S 2p spectra were fitted using the 2p 1/2 and 2p 3/2 doublet with a fixed intensity ratio of 1:2 and 1.18 eV energy separation (Chen et al., 2014). The peaks at 162.9 eV and 164.1 eV correspond to the S 2p 3/2 and S 2p 1/2 of S2−, respectively (Von Oertzen et al., 2006). The Mo and S species were quantified based on the decoupling of the Mo 3d and S 2p XPS spectra, respectively, as shown in Table 6. It can be seen that the treatment by NaHS did not cause any change to Mo and S species, which indicates that no new species were generated or adsorbed on the basal plane surface after NaHS treatment.
3.2.1. Wide-scan of basal-plane and edge-plane surfaces Table 5 shows the atomic concentrations of the elements on the basal and edge planes of molybdenite with and without NaHS treatment. It can be found that oxygen concentration was higher on the edge
0
Ratio
plane than on the basal plane, which indicates a higher degree of oxidation on edge plane. It agrees well with the findings observed by Lince and Frantz (2001) who demonstrated that the edge plane of molybdenite was more easily oxidized than the basal plane. To further illustrate the chemical compositions of basal and edge planes, the ratio of S/Mo was calculated. The wide–scan of basal plane surfaces demonstrated that the S/Mo ratio of basal plane at 0, 3 and 12 mmol/L NaHS was 2.05, 2.12 and 2.09, respectively. While the S/Mo ratio of edge plane at the corresponding NaHS concentration was 2.09, 2.29 and 2.29, respectively, the difference between treated and untreated basal planes was insignificant. Therefore, the NaHS treatment had little effect on the composition of basal plane surfaces. While the S/Mo ratio of the basal plane slightly decreased after the addition of 12 mmol/L NaHS compared with 3 mmol/L NaHS, the slight difference could not indicate a clear change on the basal plane surface given the detection limit of survey scan. High resolution spectra with a higher precision were used to detect the change on the basal plane surface below.
3.2. Cryo-XPS analysis
NaHS concentration (mmol/L)
Atomic concentration %
4
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
Fig. 6. Mo 3d and S 2p XPS spectra recorded from the edge plane of molybdenite in the absence (A and D) and presence of 3 mmol/L NaHS (B and E) and 12 mmol/L NaHS (C and F).
Fig. 5. Mo 3d and S 2p XPS spectra recorded from the basal plane of molybdenite in the absence (A and D) and presence of 3 mmol/L NaHS (B and E) and 12 mmol/L NaHS (C and F).
designated as S2− species experiencing an electronic potential higher than that of sulfur in MoS2 (162.5 eV and 163.7 eV), indicating the adsorption of HS− or S2− on the molybdenite edge surface (Zhao et al., 2018). The new doublet at 164.4 and 165.6 eV found in the S 2p spectra in Fig. 6 E was due to the formation of polysulfide and elemental sulfur Sn2−/S8 after the edge plane was treated at 3 mmol/L NaHS (Fantauzzi et al., 2015). However, after increasing NaHS concentration to 12 mmol/L, this doublet disappeared and the concentration of HS−/ S2− increased from 18.32% to 29.98%. This indicates that the strong reducing environment at a high NaHS concentration made the edge plane of molybdenite difficult to generate polysulfide and elemental sulfur, and at the same time more sulfide or hydrosulfide ions adsorbed on the surface. Based on the flotation results and XPS analysis, the changes on both two orientation surfaces of molybdenite at low and high NaHS concentrations are summarized and illustrated in Fig. 7. There was no change on the basal plane surface in XPS analysis before and after NaHS treatment, and the change of surface property such as hydrophobicity relies on the change of surface composition. Therefore, the basal plane surface remained its hydrophobicity after NaHS treatment due to its inertness. In the absence of NaHS, the edge surface composition was still stoichiometric MoS2 with Mo (IV) and there was no other oxidation state of Mo found on the edge surface as indicated in the XPS results, which agrees with the previous study by Tan et al. (2015). After NaHS treatment, due to the adsorption and formation of sulfur species on the
Fig. 6 shows the Mo 3d 5/2 and S 2p XPS spectra recorded from the edge plane of molybdenite with and without NaHS treatment. The quantification of Mo and S species is summarized in Table 7. On the edge plane surface, the peak of Mo (IV) was around 229.8 eV and the predominant doublet attributed to S2− for S 2p 3/2 was located at 162.5 eV. Comparing with the basal plane without NaHS treatment in Fig. 5, it is interesting that all peaks in Fig. 6 shifted to lower binding energies due to the electronic effects caused by the rupture of strong covalent bonds (Johnson et al., 2000). In addition, the full width at half-maximum (FWHM) of both Mo 3d and S 2p peaks of the edge plane (0.7 eV) was greater than that of the basal plane (0.5 eV), which is also related to the higher surface energy of the edge plane (Tan et al., 2015). After NaHS treatment, a smaller peak at 229.2 eV with the FWHM of 0.7 eV in the Mo 3d spectra was observed, which corresponds to Mo 3d with an oxidation state less than IV, or so-called reduced Mo (Kondekar et al., 2017). The concentration of so-called reduced Mo increased from 15.45% to 25.86% with the increase of NaHS concentration from 3 to 12 mmol/L. In the S 2p spectra, besides the predominant doublet with the FWHM of 0.7 eV attributed to sulfur in MoS2, a new doublet at 162.0 and 163.2 eV with the FWHM of 0.7 eV was observed after NaHS treatment as shown in Fig. 6 E and F. Another new doublet at 164.4 and 165.6 eV with the FWHM of 1.1 eV was also found in Fig. 6 E. Without the addition of these two doublets, the curves in Fig. 6 E and F cannot be fitted properly. The sub-sulfur shown in Fig. 6 E and F can be
Table 6 The percentage of Mo 3d and S 2p species on the basal plane of molybdenite in the absence and presence of 3 mmol/L and 12 mmol/L NaHS. Basal
Mo
S
Samples
Species
Position
FWHM
At. %
Species
0 mmol/L NaHS
MoS2 Mo 3d 5/2
230.1
0.5
100
3 mmol/L NaHS
MoS2 Mo 3d 5/2
230.1
0.5
100
12 mmol/L NaHS
MoS2 Mo 3d 5/2
230.1
0.5
100
MoS2 MoS2 MoS2 MoS2 MoS2 MoS2
5
S S S S S S
2p 2p 2p 2p 2p 2p
3/2 1/2 3/2 1/2 3/2 1/2
Position
FWHM
At. %
162.9 164.1 162.9 164.1 162.9 164.1
0.5 0.5 0.5 0.5 0.5 0.5
66.68 33.32 66.68 33.32 66.68 33.32
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
Table 7 The percentage of Mo 3d 5/2 and S 2p species on the edge plane of molybdenite in the absence and presence of 3 mmol/L and 12 mmol/L NaHS. Edge
Mo
S
samples 0 mmol/L NaHS
Name MoS2 Mo 3d 5/2
Positions 229.8
FWHM 0.7
At. % 100
3 mmol/L NaHS
Reduced Mo 3d 5/2 MoS2 Mo 3d 5/2
229.2 229.7
0.7 0.7
15.45 84.55
12 mmol/L NaHS
Reduced Mo 3d 5/2 MoS2 Mo 3d 5/2
229.3 229.7
0.7 0.7
25.86 74.14
molybdenite edge plane surface, the electronic environment was different from the stoichiometric MoS2, presenting a nonstoichiometric MoxSy with intermediate oxidation states of Mo (Spevack and McIntyre, 1993; Kondekar et al., 2017). Sn2− /S8 can only form on the edge plane surfaces due to the higher energy and pronounced electrochemical activity of the edge plane surfaces, resulting in faster electron transfer. The formation of Sn2− or S8 on sulfide mineral surface shown in Reactions (1) and (2) needs an appropriate reducing environment and reactive site. For Reactions (1) and (2) to occur, the NaHS concentration must be in an appropriate range. A higher reducing condition may be produced with an increase in NaHS concentration (Pourghahramani et al., 2017). At a low NaHS concentration, HS− may be oxidized through Reaction (1). Then it adsorbs on molybdenite surface, resulting in the formation of element sulfur or polysulfide through Reaction (2). The formation of polysulfide or element sulfur on the edge plane surface will render molybdenite particles more hydrophobic. However, at a high NaHS concentration with a high reducing condition, the molybdenite surface may be free of reactive sites and highly negatively charged as a result of a high adsorption density of hydrosulfide ions and it will be difficult for Reaction (2) to take place so that the formation of Sn2− or S8 on molybdenite surface will be difficult as found in the previous studies (Herrera-Urbina et al., 1999; Yin et al., 2010; Peng et al., 2017). In this case, fine molybdenite particles with a higher edge/ basal ratio would have more Sn2−/S8 species formed on their edge plane surfaces at the same NaHS concentration compared to coarse molybdenite particles, resulting in a more pronounced improvement in flotation recovery after adding NaHS. In general, the floatability of molybdenite in the presence of NaHS is governed by the concentration of Sn2−/S8 formed on the edge plane surface.
xHS − +
Name MoS2 S 2p 3/2 MoS2 S 2p 1/2 MoS2 S 2p 3/2 MoS2 S 2p 1/2 Sn2−/S8 2p 3/2 Sn2−/S8 2p 1/2 HS−/S2− S 2p 3/2 HS−/S2− S 2p 1/2 MoS2 S 2p 3/2 MoS2 S 2p 1/2 HS−/S2− S 2p 3/2 HS−/S2− S 2p 1/2
Positions 162.5 163.7 162.5 163.7 163.8 165.0 162.0 163.2 162.6 163.8 162.0 163.2
FWHM 0.7 0.7 0.7 0.7 1.1 1.1 0.7 0.7 0.7 0.7 0.7 0.7
1 (x − 1) O2 → Sx2 − + H2 O + (x − 2) OH− 2
MS + Sx2 − + H2 O +
1 O2 → MS − Sx + 2OH− 2
At. % 66.68 33.32 51.27 25.62 3.21 1.60 12.21 6.10 46.69 23.33 19.99 9.99
(1) (2)
4. Conclusion This study indicates that molybdenite flotation was improved after the addition of NaHS at a low concentration. A high NaHS concentration only slightly reduced the flotation kinetics, but had little effect on the final molybdenite recovery. The effect of NaHS on molybdenite flotation was more pronounced for smaller particles. The flotation behavior of molybdenite in the presence NaHS was associated with the reactions of NaHS on the basal plane and edge plane surfaces of molybdenite. Due to its inertness, the basal plane surface was immune to NaHS and remained its hydrophobicity regardless of NaHS concentration. Unlike the basal plane surface, the edge plane surface did react with NaHS and changed the surface property. At a low NaHS concentration, elemental sulfur or polysulfide formed on the edge plane, thus increasing the surface hydrophobicity, while at a high NaHS concentration, these hydrophobic species disappeared and more HS− adsorbed on the edge plane surface. CRediT authorship contribution statement Yang Chen: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data curation, Writing - original draft. Xumeng Chen: Conceptualization, Methodology, Software, Validation, Formal analysis, Writing - review & editing, Supervision. Yongjun Peng:
Fig. 7. Schematic diagram of the change on the basal plane and edge plane surfaces of molybdenite at low and high NaHS concentrations. 6
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
Conceptualization, Validation, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition.
Johnson, S.B., Franks, G.V., Scales, P.J., Boger, D.V., Healy, T.W., 2000. Surface chemistry–rheology relationships in concentrated mineral suspensions. Int. J. Miner. Process. 58 (1), 267–304. Kondekar, N.P., Boebinger, M.G., Woods, E.V., McDowell, M.T., 2017. In situ XPS investigation of transformations at crystallographically oriented MoS2 interfaces. ACS Appl. Mater. Interfaces 9 (37), 32394–32404. Lince, J.R., Frantz, P.P., 2001. Anisotropic oxidation of MoS2 crystallites studied by angle-resolved X-ray photoelectron spectroscopy. Tribol. Lett. 9 (3), 211–218. Lu, Z., Liu, Q., Xu, Z., Zeng, H., 2015. Probing Anisotropic Surface Properties of Molybdenite by Direct Force Measurements. Langmuir 31 (42), 11409–11418. Lucay, F., Cisternas, L., Gálvez, E., López-Valdivieso, A., 2015. Study of the natural floatability of molybdenite fines in saline solutions. Effect of gypsum precipitation. Miner. Metall. Process. J. 32 (4), 203–208. Luttrell, G., Yoon, R., 1984. Surface studies of the collectorless flotation of chalcopyrite. Colloids Surf. 12, 239–254. McLeod, K., Kumar, S., Dutta, N.K., Smart, R.S.C., Voelcker, N.H., Anderson, G.I., 2010. Xray photoelectron spectroscopy study of the growth kinetics of biomimetically grown hydroxyapatite thin-film coatings. Appl. Surf. Sci. 256 (23), 7178–7185. Miki, H., Matsuoka, H., Hirajima, T., Suyantara, G.P.W., Sasaki, K., 2017. Electrolysis oxidation of chalcopyrite and molybdenite for selective flotation. Mater. Trans. 58 (5), 761–767. Mu, Y., Li, L., Peng, Y., 2017. Surface properties of fractured and polished pyrite in relation to flotation. Miner. Eng. 101, 10–19. Nagaraj, D., Wang, S., Avotins, P., Dowling, E., 1986. Structure-activity relationship for copper depressants. Trans. Inst. Min. Metall. C 95. Peng, H., Wu, D., Abdalla, M., Luo, W., Jiao, W., Bie, X., 2017. Study of the effect of sodium sulfide as a selective depressor in the separation of chalcopyrite and molybdenite. Minerals 7 (4), 51. Poorkani, M., Banisi, S., 2005. Industrial use of nitrogen in flotation of molybdenite at the Sarcheshmeh copper complex. Miner. Eng. 18 (7), 735–738. Pourghahramani, P., Asqarian, H., Bagherian, A., 2017. Effects of pH and pulp potential on the selective separation of Molybdenite from the Sungun Cu-Mo concentrate. Int. J. Min. Geo-Eng. 51 (2), 147–150. Spevack, P.A., McIntyre, N., 1993. A Raman and XPS investigation of supported molybdenum oxide thin films. 2. Reactions with hydrogen sulfide. J. Phys. Chem. 97 (42), 11031–11036. Tabares, J.O., Ortega, I.M., Bahena, J.R., López, A.S., Pérez, D.V., Valdivieso, A.L., 2006. Surface properties and floatability of molybdenite. In: Proceedings of 2006 ChinaMexico Workshop on Minerals Particle Technology. Taheri, B., Abdollahy, M., Tonkaboni, S.Z.S., Javadian, S., Yarahmadi, M., 2014. Dual effects of sodium sulfide on the flotation behavior of chalcopyrite: I. Effect of pulp potential. Int. J. Miner. Metall. Mater. 21 (5), 415–422. Tan, S.M., Ambrosi, A., Sofer, Z., Huber, Š., Sedmidubský, D., Pumera, M., 2015. Pristine basal- and edge-plane-oriented molybdenite MoS2 exhibiting highly anisotropic properties. Chem. – A Eur. J. 21 (19), 7170–7178. Von Oertzen, G.U., Harmer, S., Skinner, W.M., 2006. XPS and ab initio calculation of surface states of sulfide minerals: pyrite, chalcopyrite and molybdenite. Mol. Simul. 32 (15), 1207–1212. Wu, D., Peng, H., Abdalla, M., 2018. Effect of sodium sulfide waste water recycling on the separation of chalcopyrite and molybdenite. Physicochem. Probl. Miner. Process 54 (3), 629–638. Yang, B., Song, S., Lopez-Valdivieso, A., 2014. Effect of particle size on the contact angle of molybdenite powders. Miner. Process. Extr. Metall. Rev. 35 (3), 208–215. Yin, W.-Z., Zhang, L.-R., Feng, X., 2010. Flotation of Xinhua molybdenite using sodium sulfide as modifier. Trans. Nonferrous Metals Soc. China 20 (4), 702–706. Yuan, D., Cadien, K., Liu, Q., Zeng, H., 2019a. Flotation separation of Cu-Mo sulfides by O-Carboxymethyl chitosan. Miner. Eng. 134, 202–205. Yuan, D., Cadien, K., Liu, Q., Zeng, H., 2019b. Selective separation of copper-molybdenum sulfides using humic acids. Miner. Eng. 133, 43–46. Yuan, D., Xie, L., Shi, X., Yi, L., Zhang, G., Zhang, H., Liu, Q., Zeng, H., 2018. Selective flotation separation of molybdenite and talc by humic substances. Miner. Eng. 117, 34–41. Zanin, M., Ametov, I., Grano, S., Zhou, L., Skinner, W., 2009. A study of mechanisms affecting molybdenite recovery in a bulk copper/molybdenum flotation circuit. Int. J. Miner. Process. 93 (3–4), 256–266. Zhao, Q., Liu, W., Wei, D., Wang, W., Cui, B., Liu, W., 2018. Effect of copper ions on the flotation separation of chalcopyrite and molybdenite using sodium sulfide as a depressant. Miner. Eng. 115, 44–52.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The financial support from the Australian Research Council, Newcrest Mining Limited, Vega Industry-UK and Vega Industry-Middle East through the ARC Linkage Project of LP130100913 is gratefully acknowledged. The first author also thanks the scholarship provided by the University of Queensland and China Scholarship Council (CSC). The authors gratefully acknowledge the XPS facilities and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility (AMMRF) at the Centre for Microscopy and Microanalysis (CMM) at The University of Queensland, Australia. References Amelunxen, P. and R. Amelunxen 2009. Moly plant design considerations. Baltrusaitis, J., Mendoza-Sanchez, B., Fernandez, V., Veenstra, R., Dukstiene, N., Roberts, A., Fairley, N., 2015. Generalized molybdenum oxide surface chemical state XPS determination via informed amorphous sample model. Appl. Surf. Sci. 326, 151–161. Bhambhani, T., Nagaraj, D.R., Gupta, P., Lawrence, A., Peart, M., Zarate, P., 2014. Practical aspects of Cu-Mo separations and alternatives to NaSH and Noke reagent. Bos, J., Quast, K., 2000. Effects of oils and lubricants on the flotation of copper sulphide minerals. Miner. Eng. 13 (14–15), 1623–1627. Castro, S. and A. Correa 1995. The effect of particle size on the surface energy and wettability of molybdenite. In: Proc. 1st UBC-McGill Int. Symposium on Processing of Hydrophobic Minerals and Fine Coal (JS Laskowski and GW Poling, eds.), Met. Soc. of CIM. Chander, S., Fuerstenau, D., 1972. On the natural floatability of molybdenite. Trans. AIME 252, 62–69. Chen, X., Seaman, D., Peng, Y., Bradshaw, D., 2014. Importance of oxidation during regrinding of rougher flotation concentrates with a high content of sulfides. Miner. Eng. 66, 165–172. Deng, M., Karpuzov, D., Liu, Q., Xu, Z., 2013. Cryo-XPS study of xanthate adsorption on pyrite. Surf. Interface Anal. 45 (4), 805–810. Dickinson, R.G., Pauling, L., 1923. The crystal structure of molybdenite. J. Am. Chem. Soc. 45 (6), 1466–1471. Dutta, S.K., Lodhari, D.R., 2018. Molybdenum. extraction of nuclear and non-ferrous metals. Springer, pp. 205–209. Fantauzzi, M., Elsener, B., Atzei, D., Rigoldi, A., Rossi, A., 2015. Exploiting XPS for the identification of sulfides and polysulfides. RSC Adv. 5 (93), 75953–75963. Gaudin, A.M., 1957. Flotation. McGraw-Hill. Herrera-Urbina, R., Sotillo, F., Fuerstenau, D., 1999. Effect of sodium sulfide additions on the pulp potential and amyl xanthate flotation of cerussite and galena. Int. J. Miner. Process. 55 (3), 157–170. Heyes, G., Trahar, W., 1977. The natural flotability of chalcopyrite. Int. J. Miner. Process. 4 (4), 317–344. Hirajima, T., Mori, M., Ichikawa, O., Sasaki, K., Miki, H., Farahat, M., Sawada, M., 2014. Selective flotation of chalcopyrite and molybdenite with plasma pre-treatment. Miner. Eng. 66–68, 102–111. Huai, Y., Plackowski, C., Peng, Y., 2017. The surface properties of pyrite coupled with gold in the presence of oxygen. Miner. Eng. 111, 131–139. Huai, Y., Plackowski, C., Peng, Y., 2018. The galvanic interaction between gold and pyrite in the presence of ferric ions. Miner. Eng. 119, 236–243.
7
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
The effect of sodium hydrosulfide on molybdenite flotation as a depressant of copper sulfides Yang Chen, Xumeng Chen, Yongjun Peng
T
⁎
School of Chemical Engineering, The University of Queensland, St. Lucia, Brisbane, QLD 4072, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Molybdenite flotation Sodium hydrosulfide Copper depressant Edge plane Basal plane
Depressing copper sulfides using sodium hydrosulfide (NaHS) in the flotation of molybdenite is a common practice. However, the effect of NaHS on the flotation of molybdenite is not clear despite a number of studies. In this study, the effect of NaHS concentrations which depressed the flotation of chalcopyrite on the flotation of molybdenite of different particle sizes was investigated. It was found that molybdenite flotation was promoted at a low NaHS concentration. A high NaHS concentration only reduced the flotation kinetics slightly without affecting the final flotation recovery. The effect of NaHS on the flotation of smaller molybdenite particles was more significant. Cryo-XPS analysis was then carried out to understand the underpinning mechanism. It was evident that the flotation behavior of molybdenite in the presence of NaHS was determined by the reactions of NaHS on the edge plane surface of molybdenite. At a low NaHS concentration, NaHS adsorbed on the edge plane surface and formed hydrophobic species such as elemental sulfur and polysulfide. At a high NaHS concentration, the hydrophobic species disappeared with more HS− adsorbed. At both low and high NaHS concentrations, NaHS did not adsorb on the basal plane surface of molybdenite which remained its surface hydrophobicity.
1. Introduction Molybdenite (MoS2) is the main source of molybdenum which plays a significant role in the modern society with its principal use as a ferroalloy in iron and steel industries (Dutta and Lodhari, 2018). Molybdenite is often associated with copper sulfides such as chalcopyrite (CuFeS2) and chalcocite (Cu2S). Conventionally, the beneficiation of copper and molybdenum (Cu-Mo) ores is achieved through two stages of flotation. The first stage recovers copper sulfides and molybdenite as a bulk concentrate with a thiol collector and the second stage selectively recovers molybdenite with an oil collector against copper sulfides (Zanin et al., 2009; Yuan et al., 2019). Over decades, a number of inorganic and organic reagents have been developed to depress copper sulfides in molybdenite flotation (Yuan et al., 2019). However, the most commonly used copper depressants today are still NaHS and Na2S with the former being predominant (Hirajima et al., 2014). The role of NaHS and Na2S in depressing copper sulfides in flotation is associated with the desorption of thiol collectors and the modification of copper mineral surface properties (Gaudin, 1957; Nagaraj et al., 1986). Taking xanthate as an example, it may adsorb on copper sulfides through copper-xanthate complexes. When NaHS or Na2S is added as a depressant, the hydrolyzed or ionized HS− becomes predominant (Poorkani and Banisi, 2005). HS− has a stronger affinity to copper ions ⁎
than xanthate and therefore can replace xanthate on the surfaces of copper sulfides (Bhambhani et al., 2014). In addition, the addition of NaHS or Na2S may decrease the Eh, which prevents the oxidation of copper sulfides to form hydrophobic products such as polysulfide and elemental sulfur on the surfaces (Taheri et al., 2014). Unlike copper sulfides, molybdenite has a layered structure with two orientation surfaces, basal and edge planes. Basal planes are formed by the breakage of weak van der Waals forces between layers (S-S bonds) while edge planes are generated by the rupture of strong covalent Mo-S bonds (Dickinson and Pauling, 1923). As a result, basal planes are hydrophobic responsible for the natural floatability of molybdenite while edge planes are hydrophilic (Chander and Fuerstenau, 1972; Lu et al., 2015). Particle size is an important factor affecting the basal/edge ratio of molybdenite and its flotation behavior. The basal/ edge ratio of molybdenite decreases with a decrease in particle size and therefore smaller molybdenite particles show less surface hydrophobicity and lower floatability (Castro and Correa, 1995; Yang et al., 2014). A number of studies were conducted to investigate the effect of NaHS on molybdenite flotation. Amelunxen and Amelunxen (2009) found that molybdenite floatability was relatively unaffected by NaHS. Similar observations were found recently by Zhao et al. (2018) and Wu et al. (2018). However, Peng et al. (2017) demonstrated that
Corresponding author. E-mail address: [email protected] (Y. Peng).
https://doi.org/10.1016/j.mineng.2020.106203 Received 16 April 2019; Received in revised form 5 December 2019; Accepted 12 January 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
NaHS were of AR grade. DI water was used in preparation of all the reagents, grinding and flotation tests.
molybdenite recovery only remained unchanged at a low concentration of Na2S and a sharp drop of molybdenite recovery occurred when Na2S concentration was higher than 5 mmol/L. In contrast, Yin et al. (2010) observed a higher molybdenite grade and recovery after sodium sulfide was added in flotation. In general, the previous studies disagreed with the effect of NaHS or Na2S on molybdenite flotation and did not consider the context of Cu-Mo separation. The effect of NaHS or Na2S on molybdenite flotation should be governed by the surface modification on its basal and edge planes, but this aspect was not investigated in the previous studies. In this study, the Cu-Mo separation was considered as the context and the range of NaHS concentration which depressed chalcopyrite flotation was examined in the flotation of molybdenite particles at different sizes. Then, the surface changes of molybdenite basal and edge planes before and after the addition of NaHS were characterized by cold stage X-ray Photoelectron Spectroscopy (Cryo-XPS, below −145 °C) and related to molybdenite flotation behavior.
2.2. Methods 2.2.1. Grinding and flotation Chalcopyrite or molybdenite mineral (100 g) was combined with 150 mL water and ground with 5.4 kg stainless steel rods in a laboratory stainless steel rod mill for 4.1 min for chalcopyrite and 20 min for molybdenite to obtain a P80 of 106 μm. To obtain a P80 of 53 μm for molybdenite, 8.9 kg stainless steel rods were used and the grinding time was 20 min. Diesel oil (0.15 g) was added in the rod mill before grinding for better dispersion. After grinding, the pulp was transferred to a 1.5 L JK batch flotation cell and was topped up to 1.5 L by adding DI water. The reagents were added in the following order: pH modifier (NaOH), depressant (NaHS) and frother (MIBC) and the conditioning time for each of them was 3, 3 and 1 min, respectively. The concentration of MIBC in the flotation was 20 ppm. The agitation speed was fixed at 1000 rpm and the air flow rate during the flotation was 3.0 L/min. The flotation froth was scraped every 10 s and four flotation concentrates were collected after cumulative times of 1.0, 3.0, 6.0 and 10.0 min.
2. Materials and methods 2.1. Materials and reagents Molybdenite and chalcopyrite samples were purchased from GEODiscoveries, Sydney and originated from Peru. The chemical compositions of chalcopyrite and molybdenite samples, analyzed by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), are shown in Table 1. By assuming all the copper from chalcopyrite and all the molybdenum from molybdenite, the purity of the chalcopyrite sample was 93.4 wt% and the purity of the molybdenite sample was 97.2 wt%. The chalcopyrite sample was crushed through a jaw crusher and then a roll crusher, and screened to collect +0.6–3.35 mm particle size fractions. The molybdenite sample was cracked using a hammer and the resulting flakes were chopped using a scissor and then a blender. +0.6–3.35 mm molybdenite particle size fractions were collected for further tests. All the processed samples were sealed in polyethylene bags and stored in a freezer at a temperature of −20 °C to minimize further oxidation. For XPS analysis, a high quality molybdenite crystal was selected to prepare basal and edge plane surfaces. A pair of tweezers was used to exfoliate the top layer of the molybdenite crystal to obtain a smooth and flat basal plane surface. The edge plane surface was generated by cutting the crystal perpendicularly to the layered structure using a diamond saw and the crystal was then covered with epoxide resin so that only the desired edge plane surface was exposed (Miki et al., 2017). The basal plane surface was used once and for a different test a new basal plane surface was prepared while the edge plane surface was renewed by polishing with #600 to #2000 emery paper (3M, USA), following the work published in literature (Tan et al., 2015). Diesel oil and MIBC were used as the collector and the frother, respectively, in flotation tests. They are normally used in the flotation of molybdenite against copper sulfides. Both were of industrial grade and used as received. Diesel oil was added in flotation without dilution while MIBC was diluted to 1 wt%. The pH was adjusted by the addition of 1 M NaOH solution instead of lime commonly used in flotation plants to eliminate the effect of Ca2+ on the flotation. Sodium hydrosulfide (NaHS) was added as a depressant in the flotation. Both NaOH and
2.2.2. XPS analysis Cryo-XPS was used to identify the chemical species present on both basal and edge surfaces of molybdenite after NaHS treatment. NaHS solutions with 0 mmol/L NaHS, 3 mmol/L NaHS (low concentration) and 12 mmol/L NaHS (high concentration) determined from flotation tests were prepared. The basal or edge surface was first conditioned in one of the NaHS solutions for 3 min, rinsed with Milli-Q water to remove residual NaHS on molybdenite surface and then dried with pure nitrogen gas, following the procedure published in literature (McLeod et al., 2010; Deng et al., 2013; Huai et al., 2017). The sample was then loaded into the pre-cooled sample chamber at −145 °C using a metal stub with an insulating tape which was submerged into liquid nitrogen for 1 min to cool the sample (Huai et al., 2018). XPS data were acquired using a Kratos Axis ULTRA X-ray photoelectron spectrometer incorporating a 165 mm hemispherical electron energy analyzer. The incident radiation was monochromatic Al Kα X-rays (1486.6 eV) at 150 W (15 kV, 15 mA). Survey (wide) scans were taken at a pass energy of 160 eV and multiplex (narrow) high resolution scans were taken at 20 eV. Survey scans were carried out over the 1200–0 eV binding energy range with 1.0 eV steps and a dwell time of 100 ms. Narrow highresolution scans were run with 0.05 eV steps and 250 ms dwell time. Base pressure in the analysis chamber was 1.0 × 10−9 Torr and during sample analysis was 1.0 × 10−8 Torr (Mu et al., 2017). The resulting spectra were charge-corrected using C 1s as a reference with a binding energy of 284.8 eV. Data processing and fitting were carried out using Casa XPS software. 3. Results and discussion 3.1. Flotation performance 3.1.1. Chalcopyrite flotation Before investigating the effects of NaHS on molybdenite flotation,
Table 1 Chemical compositions of chalcopyrite and molybdenite samples. Species present (wt.%)
Chalcopyrite Molybdenite
Al2O3
Bi
CaO
Co
Cu
Fe
Mo
Pb
S
SiO2
Ti
Zn
0.26 0.13
0.08 0.23
0.86 0.07
0.03 0.01
32.5 0.06
28.8 0.14
– 58.3
0.66 0.06
33.4 39.5
2.09 1.52
0.03 –
1.29 0.01
2
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
Fig. 1. The effect of NaHS addition on chalcopyrite flotation.
Fig. 2. The effect of NaHS addition on the flotation of 106 µm molybdenite.
Table 2 The Eh (mV vs SHE) in chalcopyrite flotation at various NaHS concentrations. NaHS concentration (mmol/L)
0
3
6
9
12
Chalcopyrite
374.1
–233.9
−274.9
−288.0
−329.4
the flotation of chalcopyrite was conducted first at different NaHS concentrations. Flotation results are shown in Fig. 1. It can be seen that chalcopyrite displayed a good floatability in the absence of NaHS and the recovery reached nearly 90% in 1 min flotation and 90.8% at the end of flotation. The addition of NaHS depressed chalcopyrite flotation. At 3 mmol/L NaHS, although no obvious decrease in the final chalcopyrite recovery was observed, the flotation kinetics dropped significantly with about 12.0% chalcopyrite recovery in 1 min flotation. With the further increase of NaHS concentration, the final chalcopyrite recovery decreased by 46.8% and 69.7% in the presence of 6 mmol/L and 9 mmol/L NaHS, respectively. In the presence of 12 mmol/L NaHS, chalcopyrite was almost completely depressed with only 4.4% recovery at the end of flotation. This NaHS concentration was equal to 10 kg/t, which agrees with the scale applied in the industry from several to dozens kg/t of NaHS (Amelunxen and Amelunxen, 2009). The Eh (mV vs SHE) in chalcopyrite flotation at various NaHS concentrations was measured and shown in Table 2. It can be seen that the Eh in flotation was around 375 mV in the absence of NaHS. With the addition of NaHS, the Eh decreased sharply to a range from −230 to −330 mV due to the reduction property of NaHS. The measured Eh value is in line with the previous studies with the addition of NaHS in flotation pulp (Pourghahramani et al., 2017; Wu et al., 2018). Diesel oil, the collector for molybdenite, was used in chalcopyrite flotation. It is known that chalcopyrite is naturally floatable under slightly oxidizing conditions and it is believed that elemental sulfur that forms on chalcopyrite surface is responsible for the natural floatability of chalcopyrite (Heyes and Trahar, 1977; Luttrell and Yoon, 1984). Diesel as an oil collector may physically adsorb on the hydrophobic surface of chalcopyrite to enhance its surface hydrophobicity and floatability (Bos and Quast, 2000). When NaHS is added as a copper depressant, the ionized HS− will decrease the Eh, producing a reducing environment where the chalcopyrite surface is free of hydrophobic species such as S0 (Herrera-Urbina et al., 1999). As a result, diesel cannot adsorb on chalcopyrite surface in the presence of NaHS and chalcopyrite floatability may decrease with an increase in NaHS concentration as observed in this study.
Fig. 3. The effect of NaHS addition on the flotation of 53 µm molybdenite.
was studied, and the results are shown in Fig. 2 and Fig. 3, respectively. From Fig. 2, it can be found that molybdenite recovery reached 90% at the end of flotation in the absence of NaHS. In the presence of 3 mmol/L NaHS, molybdenite recovery even increased to 95% at the end of flotation and the flotation kinetics also increased slightly. The increase of NaHS concentration to 6 mmol/L and 9 mmol/L slightly reduced the flotation kinetics but the final molybdenite recovery was still around 95%. When NaHS concentration was further increased to 12 mmol/L, the flotation kinetics reduced significantly and the final molybdenite recovery dropped back to 90%. In general, the addition of NaHS at the concentration range which depressed chalcopyrite flotation did not have a negative effect on the flotation of 106 µm molybdenite, and a low NaHS concentration enhanced the flotation, instead. Fig. 3 shows that the flotation of 53 µm molybdenite reached 53.5% recovery at the end of flotation in the absence of NaHS, much lower than that from the flotation of 106 µm molybdenite. This is expected because the floatability of molybdenite is related to particle size. With the reduction of particle size, firstly, the overall surface area increases and therefore the coverage of diesel oil on particle surfaces becomes less at the same diesel concentration. In addition, the ratio of basal/ edge decreases with the reduction of particle size. The basal plane surface is known to be hydrophobic while the edge plane surface hydrophilic. A lower basal/ edge ratio corresponds to a lower molybdenite floatability. Fig. 3 also shows that the addition of 3 mmol/L NaHS significantly increased molybdenite recovery at the end of flotation from 53.5% to 76.8%. In the presence of 6 mmol/L NaHS, the flotation kinetics started to decrease slightly, although the final molybdenite recovery was similar. Both flotation kinetics and final molybdenite
3.1.2. Molybdenite flotation Following chalcopyrite flotation, the effect of the same NaHS concentrations on the flotation of 106 µm and 53 µm molybdenite particles 3
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
Table 3 The Eh (mV vs SHE) in molybdenite flotation at various NaHS concentrations. NaHS concentration (mmol/L)
0
3
6
9
12
106 µm molybdenite 53 µm molybdenite
360.2 351.7
−249.0 −278.3
−283.5 −303.2
−294.6 −313.9
−339.7 −347.8
recovery started to decrease at a NaHS concentration higher than 9 mmol/L, however, the final molybdenite recovery was still significantly higher than that in the absence of NaHS. The comparison between Fig. 2 and Fig. 3 shows that NaHS displayed a similar effect on the flotation of 53 µm and 106 µm molybdenite. A low NaHS concentration was beneficial to molybdenite flotation but this benefit was reduced at a high NaHS concentration. In general, the effect of NaHS on the flotation of 53 µm molybdenite was more pronounced. The Eh (mV vs SHE) in molybdenite flotation at various NaHS concentrations was also measured and shown in Table 3. It can be seen that the Eh in molybdenite flotation was around 350 mV in the absence of NaHS. As observed in chalcopyrite flotation, the Eh decreased sharply to a range from −250 to −350 mV after the addition of NaHS in molybdenite flotation due to the reduction property of NaHS. It was noticed that the addition of NaHS increased flotation pH as shown in Table 4. The addition of 3 mmol/L, 6 mmol/L, 9 mmol/L and 12 mmol/L NaHS increased the flotation pH from 7.4 to 9.8, 9.9, 10.1 and 10.5, respectively, in the flotation of 106 µm molybdenite and from 7.2 to 9.5, 10.0, 10.5 and 10.6, respectively, in the flotation of 53 µm molybdenite. In order to identify the effect of modified pH values through NaHS additions on molybdenite flotation, the flotation of 106 µm and 53 µm molybdenite in the pH range from 7 to 11 adjusted by NaOH additions was conducted and the results are shown in Fig. 4. It can be seen that 90% and 54% molybdenite recoveries were achieved at the end of flotation of 106 µm and 53 µm molybdenite, respectively, at pH 7.0 and molybdenite recovery decreased only slightly with an increase in pH value. Lucay et al. (2015) found that molybdenite floatability decreased with an increase in pH value from 4 to 12. Tabares et al. (2006) indicated that the magnitude of zeta potential of molybdenite increased with an increase in pH value and was always negative, resulting from the singly-charged hydromolybdate (HMoO4−) species and double-charged molybdate ions (MoO42−) on the edges. The repulsive force between negatively charged molybdenite particles and bubbles would increase with an increase in pH value, leading to decreased molybdenite flotation (Yuan et al., 2018). Obviously, the beneficial effect of NaHS on molybdenite flotation as shown in Fig. 2 and Fig. 3 is not through the pH change. It may result from the interaction of NaHS with basal and edge planes of molybdenite, which was studied further below.
Fig. 4. Molybdenite recovery as a function of pH adjusted with NaOH solution in the flotation of 106 µm and 53 µm molybdenite. Table 5 The XPS atomic concentration of elements on basal and edge planes of molybdenite with and without NaHS treatment. Molybdenite
Basal-0 mmol/L NaHS Basal-3 mmol/L NaHS Basal-12 mmol/L NaHS Edge-0 mmol/L NaHS Edge-3 mmol/L NaHS Edge-12 mmol/L NaHS
Cryo-XPS analysis was conducted on basal and edge plane surfaces of molybdenite after being conditioned at different NaHS concentrations.
Table 4 The pH value in molybdenite flotation at various NaHS concentrations. 3
6
9
12
106 µm molybdenite 53 µm molybdenite
7.4 7.2
9.8 9.5
9.9 10.0
10.1 10.5
10.5 10.6
Mo
S
O
S/Mo
25.54 20.31 22.87 21.95 16.62 18.15
52.41 43.08 48.00 46.05 38.05 41.54
22.05 35.63 29.13 29.92 42.07 35.41
2.05 2.12 2.09 2.09 2.29 2.29
3.2.2. High-resolution spectra of basal plane and edge plane surfaces High resolution scans were conducted to identify the chemical state of Mo and S on basal plane and edge plane surfaces of molybdenite. Fig. 5 shows the Mo 3d 5/2 and S 2p XPS spectra recorded from the basal plane without and with NaHS treatment. The peak at 230.1 eV is attributed to Mo (IV) in the form of MoS2 (Baltrusaitis et al., 2015). S 2p spectra were fitted using the 2p 1/2 and 2p 3/2 doublet with a fixed intensity ratio of 1:2 and 1.18 eV energy separation (Chen et al., 2014). The peaks at 162.9 eV and 164.1 eV correspond to the S 2p 3/2 and S 2p 1/2 of S2−, respectively (Von Oertzen et al., 2006). The Mo and S species were quantified based on the decoupling of the Mo 3d and S 2p XPS spectra, respectively, as shown in Table 6. It can be seen that the treatment by NaHS did not cause any change to Mo and S species, which indicates that no new species were generated or adsorbed on the basal plane surface after NaHS treatment.
3.2.1. Wide-scan of basal-plane and edge-plane surfaces Table 5 shows the atomic concentrations of the elements on the basal and edge planes of molybdenite with and without NaHS treatment. It can be found that oxygen concentration was higher on the edge
0
Ratio
plane than on the basal plane, which indicates a higher degree of oxidation on edge plane. It agrees well with the findings observed by Lince and Frantz (2001) who demonstrated that the edge plane of molybdenite was more easily oxidized than the basal plane. To further illustrate the chemical compositions of basal and edge planes, the ratio of S/Mo was calculated. The wide–scan of basal plane surfaces demonstrated that the S/Mo ratio of basal plane at 0, 3 and 12 mmol/L NaHS was 2.05, 2.12 and 2.09, respectively. While the S/Mo ratio of edge plane at the corresponding NaHS concentration was 2.09, 2.29 and 2.29, respectively, the difference between treated and untreated basal planes was insignificant. Therefore, the NaHS treatment had little effect on the composition of basal plane surfaces. While the S/Mo ratio of the basal plane slightly decreased after the addition of 12 mmol/L NaHS compared with 3 mmol/L NaHS, the slight difference could not indicate a clear change on the basal plane surface given the detection limit of survey scan. High resolution spectra with a higher precision were used to detect the change on the basal plane surface below.
3.2. Cryo-XPS analysis
NaHS concentration (mmol/L)
Atomic concentration %
4
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
Fig. 6. Mo 3d and S 2p XPS spectra recorded from the edge plane of molybdenite in the absence (A and D) and presence of 3 mmol/L NaHS (B and E) and 12 mmol/L NaHS (C and F).
Fig. 5. Mo 3d and S 2p XPS spectra recorded from the basal plane of molybdenite in the absence (A and D) and presence of 3 mmol/L NaHS (B and E) and 12 mmol/L NaHS (C and F).
designated as S2− species experiencing an electronic potential higher than that of sulfur in MoS2 (162.5 eV and 163.7 eV), indicating the adsorption of HS− or S2− on the molybdenite edge surface (Zhao et al., 2018). The new doublet at 164.4 and 165.6 eV found in the S 2p spectra in Fig. 6 E was due to the formation of polysulfide and elemental sulfur Sn2−/S8 after the edge plane was treated at 3 mmol/L NaHS (Fantauzzi et al., 2015). However, after increasing NaHS concentration to 12 mmol/L, this doublet disappeared and the concentration of HS−/ S2− increased from 18.32% to 29.98%. This indicates that the strong reducing environment at a high NaHS concentration made the edge plane of molybdenite difficult to generate polysulfide and elemental sulfur, and at the same time more sulfide or hydrosulfide ions adsorbed on the surface. Based on the flotation results and XPS analysis, the changes on both two orientation surfaces of molybdenite at low and high NaHS concentrations are summarized and illustrated in Fig. 7. There was no change on the basal plane surface in XPS analysis before and after NaHS treatment, and the change of surface property such as hydrophobicity relies on the change of surface composition. Therefore, the basal plane surface remained its hydrophobicity after NaHS treatment due to its inertness. In the absence of NaHS, the edge surface composition was still stoichiometric MoS2 with Mo (IV) and there was no other oxidation state of Mo found on the edge surface as indicated in the XPS results, which agrees with the previous study by Tan et al. (2015). After NaHS treatment, due to the adsorption and formation of sulfur species on the
Fig. 6 shows the Mo 3d 5/2 and S 2p XPS spectra recorded from the edge plane of molybdenite with and without NaHS treatment. The quantification of Mo and S species is summarized in Table 7. On the edge plane surface, the peak of Mo (IV) was around 229.8 eV and the predominant doublet attributed to S2− for S 2p 3/2 was located at 162.5 eV. Comparing with the basal plane without NaHS treatment in Fig. 5, it is interesting that all peaks in Fig. 6 shifted to lower binding energies due to the electronic effects caused by the rupture of strong covalent bonds (Johnson et al., 2000). In addition, the full width at half-maximum (FWHM) of both Mo 3d and S 2p peaks of the edge plane (0.7 eV) was greater than that of the basal plane (0.5 eV), which is also related to the higher surface energy of the edge plane (Tan et al., 2015). After NaHS treatment, a smaller peak at 229.2 eV with the FWHM of 0.7 eV in the Mo 3d spectra was observed, which corresponds to Mo 3d with an oxidation state less than IV, or so-called reduced Mo (Kondekar et al., 2017). The concentration of so-called reduced Mo increased from 15.45% to 25.86% with the increase of NaHS concentration from 3 to 12 mmol/L. In the S 2p spectra, besides the predominant doublet with the FWHM of 0.7 eV attributed to sulfur in MoS2, a new doublet at 162.0 and 163.2 eV with the FWHM of 0.7 eV was observed after NaHS treatment as shown in Fig. 6 E and F. Another new doublet at 164.4 and 165.6 eV with the FWHM of 1.1 eV was also found in Fig. 6 E. Without the addition of these two doublets, the curves in Fig. 6 E and F cannot be fitted properly. The sub-sulfur shown in Fig. 6 E and F can be
Table 6 The percentage of Mo 3d and S 2p species on the basal plane of molybdenite in the absence and presence of 3 mmol/L and 12 mmol/L NaHS. Basal
Mo
S
Samples
Species
Position
FWHM
At. %
Species
0 mmol/L NaHS
MoS2 Mo 3d 5/2
230.1
0.5
100
3 mmol/L NaHS
MoS2 Mo 3d 5/2
230.1
0.5
100
12 mmol/L NaHS
MoS2 Mo 3d 5/2
230.1
0.5
100
MoS2 MoS2 MoS2 MoS2 MoS2 MoS2
5
S S S S S S
2p 2p 2p 2p 2p 2p
3/2 1/2 3/2 1/2 3/2 1/2
Position
FWHM
At. %
162.9 164.1 162.9 164.1 162.9 164.1
0.5 0.5 0.5 0.5 0.5 0.5
66.68 33.32 66.68 33.32 66.68 33.32
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
Table 7 The percentage of Mo 3d 5/2 and S 2p species on the edge plane of molybdenite in the absence and presence of 3 mmol/L and 12 mmol/L NaHS. Edge
Mo
S
samples 0 mmol/L NaHS
Name MoS2 Mo 3d 5/2
Positions 229.8
FWHM 0.7
At. % 100
3 mmol/L NaHS
Reduced Mo 3d 5/2 MoS2 Mo 3d 5/2
229.2 229.7
0.7 0.7
15.45 84.55
12 mmol/L NaHS
Reduced Mo 3d 5/2 MoS2 Mo 3d 5/2
229.3 229.7
0.7 0.7
25.86 74.14
molybdenite edge plane surface, the electronic environment was different from the stoichiometric MoS2, presenting a nonstoichiometric MoxSy with intermediate oxidation states of Mo (Spevack and McIntyre, 1993; Kondekar et al., 2017). Sn2− /S8 can only form on the edge plane surfaces due to the higher energy and pronounced electrochemical activity of the edge plane surfaces, resulting in faster electron transfer. The formation of Sn2− or S8 on sulfide mineral surface shown in Reactions (1) and (2) needs an appropriate reducing environment and reactive site. For Reactions (1) and (2) to occur, the NaHS concentration must be in an appropriate range. A higher reducing condition may be produced with an increase in NaHS concentration (Pourghahramani et al., 2017). At a low NaHS concentration, HS− may be oxidized through Reaction (1). Then it adsorbs on molybdenite surface, resulting in the formation of element sulfur or polysulfide through Reaction (2). The formation of polysulfide or element sulfur on the edge plane surface will render molybdenite particles more hydrophobic. However, at a high NaHS concentration with a high reducing condition, the molybdenite surface may be free of reactive sites and highly negatively charged as a result of a high adsorption density of hydrosulfide ions and it will be difficult for Reaction (2) to take place so that the formation of Sn2− or S8 on molybdenite surface will be difficult as found in the previous studies (Herrera-Urbina et al., 1999; Yin et al., 2010; Peng et al., 2017). In this case, fine molybdenite particles with a higher edge/ basal ratio would have more Sn2−/S8 species formed on their edge plane surfaces at the same NaHS concentration compared to coarse molybdenite particles, resulting in a more pronounced improvement in flotation recovery after adding NaHS. In general, the floatability of molybdenite in the presence of NaHS is governed by the concentration of Sn2−/S8 formed on the edge plane surface.
xHS − +
Name MoS2 S 2p 3/2 MoS2 S 2p 1/2 MoS2 S 2p 3/2 MoS2 S 2p 1/2 Sn2−/S8 2p 3/2 Sn2−/S8 2p 1/2 HS−/S2− S 2p 3/2 HS−/S2− S 2p 1/2 MoS2 S 2p 3/2 MoS2 S 2p 1/2 HS−/S2− S 2p 3/2 HS−/S2− S 2p 1/2
Positions 162.5 163.7 162.5 163.7 163.8 165.0 162.0 163.2 162.6 163.8 162.0 163.2
FWHM 0.7 0.7 0.7 0.7 1.1 1.1 0.7 0.7 0.7 0.7 0.7 0.7
1 (x − 1) O2 → Sx2 − + H2 O + (x − 2) OH− 2
MS + Sx2 − + H2 O +
1 O2 → MS − Sx + 2OH− 2
At. % 66.68 33.32 51.27 25.62 3.21 1.60 12.21 6.10 46.69 23.33 19.99 9.99
(1) (2)
4. Conclusion This study indicates that molybdenite flotation was improved after the addition of NaHS at a low concentration. A high NaHS concentration only slightly reduced the flotation kinetics, but had little effect on the final molybdenite recovery. The effect of NaHS on molybdenite flotation was more pronounced for smaller particles. The flotation behavior of molybdenite in the presence NaHS was associated with the reactions of NaHS on the basal plane and edge plane surfaces of molybdenite. Due to its inertness, the basal plane surface was immune to NaHS and remained its hydrophobicity regardless of NaHS concentration. Unlike the basal plane surface, the edge plane surface did react with NaHS and changed the surface property. At a low NaHS concentration, elemental sulfur or polysulfide formed on the edge plane, thus increasing the surface hydrophobicity, while at a high NaHS concentration, these hydrophobic species disappeared and more HS− adsorbed on the edge plane surface. CRediT authorship contribution statement Yang Chen: Conceptualization, Methodology, Software, Formal analysis, Investigation, Data curation, Writing - original draft. Xumeng Chen: Conceptualization, Methodology, Software, Validation, Formal analysis, Writing - review & editing, Supervision. Yongjun Peng:
Fig. 7. Schematic diagram of the change on the basal plane and edge plane surfaces of molybdenite at low and high NaHS concentrations. 6
Minerals Engineering 148 (2020) 106203
Y. Chen, et al.
Conceptualization, Validation, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition.
Johnson, S.B., Franks, G.V., Scales, P.J., Boger, D.V., Healy, T.W., 2000. Surface chemistry–rheology relationships in concentrated mineral suspensions. Int. J. Miner. Process. 58 (1), 267–304. Kondekar, N.P., Boebinger, M.G., Woods, E.V., McDowell, M.T., 2017. In situ XPS investigation of transformations at crystallographically oriented MoS2 interfaces. ACS Appl. Mater. Interfaces 9 (37), 32394–32404. Lince, J.R., Frantz, P.P., 2001. Anisotropic oxidation of MoS2 crystallites studied by angle-resolved X-ray photoelectron spectroscopy. Tribol. Lett. 9 (3), 211–218. Lu, Z., Liu, Q., Xu, Z., Zeng, H., 2015. Probing Anisotropic Surface Properties of Molybdenite by Direct Force Measurements. Langmuir 31 (42), 11409–11418. Lucay, F., Cisternas, L., Gálvez, E., López-Valdivieso, A., 2015. Study of the natural floatability of molybdenite fines in saline solutions. Effect of gypsum precipitation. Miner. Metall. Process. J. 32 (4), 203–208. Luttrell, G., Yoon, R., 1984. Surface studies of the collectorless flotation of chalcopyrite. Colloids Surf. 12, 239–254. McLeod, K., Kumar, S., Dutta, N.K., Smart, R.S.C., Voelcker, N.H., Anderson, G.I., 2010. Xray photoelectron spectroscopy study of the growth kinetics of biomimetically grown hydroxyapatite thin-film coatings. Appl. Surf. Sci. 256 (23), 7178–7185. Miki, H., Matsuoka, H., Hirajima, T., Suyantara, G.P.W., Sasaki, K., 2017. Electrolysis oxidation of chalcopyrite and molybdenite for selective flotation. Mater. Trans. 58 (5), 761–767. Mu, Y., Li, L., Peng, Y., 2017. Surface properties of fractured and polished pyrite in relation to flotation. Miner. Eng. 101, 10–19. Nagaraj, D., Wang, S., Avotins, P., Dowling, E., 1986. Structure-activity relationship for copper depressants. Trans. Inst. Min. Metall. C 95. Peng, H., Wu, D., Abdalla, M., Luo, W., Jiao, W., Bie, X., 2017. Study of the effect of sodium sulfide as a selective depressor in the separation of chalcopyrite and molybdenite. Minerals 7 (4), 51. Poorkani, M., Banisi, S., 2005. Industrial use of nitrogen in flotation of molybdenite at the Sarcheshmeh copper complex. Miner. Eng. 18 (7), 735–738. Pourghahramani, P., Asqarian, H., Bagherian, A., 2017. Effects of pH and pulp potential on the selective separation of Molybdenite from the Sungun Cu-Mo concentrate. Int. J. Min. Geo-Eng. 51 (2), 147–150. Spevack, P.A., McIntyre, N., 1993. A Raman and XPS investigation of supported molybdenum oxide thin films. 2. Reactions with hydrogen sulfide. J. Phys. Chem. 97 (42), 11031–11036. Tabares, J.O., Ortega, I.M., Bahena, J.R., López, A.S., Pérez, D.V., Valdivieso, A.L., 2006. Surface properties and floatability of molybdenite. In: Proceedings of 2006 ChinaMexico Workshop on Minerals Particle Technology. Taheri, B., Abdollahy, M., Tonkaboni, S.Z.S., Javadian, S., Yarahmadi, M., 2014. Dual effects of sodium sulfide on the flotation behavior of chalcopyrite: I. Effect of pulp potential. Int. J. Miner. Metall. Mater. 21 (5), 415–422. Tan, S.M., Ambrosi, A., Sofer, Z., Huber, Š., Sedmidubský, D., Pumera, M., 2015. Pristine basal- and edge-plane-oriented molybdenite MoS2 exhibiting highly anisotropic properties. Chem. – A Eur. J. 21 (19), 7170–7178. Von Oertzen, G.U., Harmer, S., Skinner, W.M., 2006. XPS and ab initio calculation of surface states of sulfide minerals: pyrite, chalcopyrite and molybdenite. Mol. Simul. 32 (15), 1207–1212. Wu, D., Peng, H., Abdalla, M., 2018. Effect of sodium sulfide waste water recycling on the separation of chalcopyrite and molybdenite. Physicochem. Probl. Miner. Process 54 (3), 629–638. Yang, B., Song, S., Lopez-Valdivieso, A., 2014. Effect of particle size on the contact angle of molybdenite powders. Miner. Process. Extr. Metall. Rev. 35 (3), 208–215. Yin, W.-Z., Zhang, L.-R., Feng, X., 2010. Flotation of Xinhua molybdenite using sodium sulfide as modifier. Trans. Nonferrous Metals Soc. China 20 (4), 702–706. Yuan, D., Cadien, K., Liu, Q., Zeng, H., 2019a. Flotation separation of Cu-Mo sulfides by O-Carboxymethyl chitosan. Miner. Eng. 134, 202–205. Yuan, D., Cadien, K., Liu, Q., Zeng, H., 2019b. Selective separation of copper-molybdenum sulfides using humic acids. Miner. Eng. 133, 43–46. Yuan, D., Xie, L., Shi, X., Yi, L., Zhang, G., Zhang, H., Liu, Q., Zeng, H., 2018. Selective flotation separation of molybdenite and talc by humic substances. Miner. Eng. 117, 34–41. Zanin, M., Ametov, I., Grano, S., Zhou, L., Skinner, W., 2009. A study of mechanisms affecting molybdenite recovery in a bulk copper/molybdenum flotation circuit. Int. J. Miner. Process. 93 (3–4), 256–266. Zhao, Q., Liu, W., Wei, D., Wang, W., Cui, B., Liu, W., 2018. Effect of copper ions on the flotation separation of chalcopyrite and molybdenite using sodium sulfide as a depressant. Miner. Eng. 115, 44–52.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The financial support from the Australian Research Council, Newcrest Mining Limited, Vega Industry-UK and Vega Industry-Middle East through the ARC Linkage Project of LP130100913 is gratefully acknowledged. The first author also thanks the scholarship provided by the University of Queensland and China Scholarship Council (CSC). The authors gratefully acknowledge the XPS facilities and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility (AMMRF) at the Centre for Microscopy and Microanalysis (CMM) at The University of Queensland, Australia. References Amelunxen, P. and R. Amelunxen 2009. Moly plant design considerations. Baltrusaitis, J., Mendoza-Sanchez, B., Fernandez, V., Veenstra, R., Dukstiene, N., Roberts, A., Fairley, N., 2015. Generalized molybdenum oxide surface chemical state XPS determination via informed amorphous sample model. Appl. Surf. Sci. 326, 151–161. Bhambhani, T., Nagaraj, D.R., Gupta, P., Lawrence, A., Peart, M., Zarate, P., 2014. Practical aspects of Cu-Mo separations and alternatives to NaSH and Noke reagent. Bos, J., Quast, K., 2000. Effects of oils and lubricants on the flotation of copper sulphide minerals. Miner. Eng. 13 (14–15), 1623–1627. Castro, S. and A. Correa 1995. The effect of particle size on the surface energy and wettability of molybdenite. In: Proc. 1st UBC-McGill Int. Symposium on Processing of Hydrophobic Minerals and Fine Coal (JS Laskowski and GW Poling, eds.), Met. Soc. of CIM. Chander, S., Fuerstenau, D., 1972. On the natural floatability of molybdenite. Trans. AIME 252, 62–69. Chen, X., Seaman, D., Peng, Y., Bradshaw, D., 2014. Importance of oxidation during regrinding of rougher flotation concentrates with a high content of sulfides. Miner. Eng. 66, 165–172. Deng, M., Karpuzov, D., Liu, Q., Xu, Z., 2013. Cryo-XPS study of xanthate adsorption on pyrite. Surf. Interface Anal. 45 (4), 805–810. Dickinson, R.G., Pauling, L., 1923. The crystal structure of molybdenite. J. Am. Chem. Soc. 45 (6), 1466–1471. Dutta, S.K., Lodhari, D.R., 2018. Molybdenum. extraction of nuclear and non-ferrous metals. Springer, pp. 205–209. Fantauzzi, M., Elsener, B., Atzei, D., Rigoldi, A., Rossi, A., 2015. Exploiting XPS for the identification of sulfides and polysulfides. RSC Adv. 5 (93), 75953–75963. Gaudin, A.M., 1957. Flotation. McGraw-Hill. Herrera-Urbina, R., Sotillo, F., Fuerstenau, D., 1999. Effect of sodium sulfide additions on the pulp potential and amyl xanthate flotation of cerussite and galena. Int. J. Miner. Process. 55 (3), 157–170. Heyes, G., Trahar, W., 1977. The natural flotability of chalcopyrite. Int. J. Miner. Process. 4 (4), 317–344. Hirajima, T., Mori, M., Ichikawa, O., Sasaki, K., Miki, H., Farahat, M., Sawada, M., 2014. Selective flotation of chalcopyrite and molybdenite with plasma pre-treatment. Miner. Eng. 66–68, 102–111. Huai, Y., Plackowski, C., Peng, Y., 2017. The surface properties of pyrite coupled with gold in the presence of oxygen. Miner. Eng. 111, 131–139. Huai, Y., Plackowski, C., Peng, Y., 2018. The galvanic interaction between gold and pyrite in the presence of ferric ions. Miner. Eng. 119, 236–243.
7