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The effect of whey protein on the surface property of the copper-activated marmatite in xanthate flotation system
Applied Surface Science 479 (2019) 303–310
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The effect of whey protein on the surface property of the copper-activated marmatite in xanthate flotation system
T
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Xianzhong Bu, Fanfan Chen, Wei Chen , Yihao Ding School of Resources Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flotation Whey protein Depressant Copper-activated marmatite
In this paper, the depressant effect of the whey protein, a chelating agent for copper ions, on the flotation of copper-activated marmatite was investigated by micro-flotation, zeta potential, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). The results showed that the formation of a complex through the interaction between the copper ions and the whey protein effectively prevented the activation of marmatite from copper ions, and the flotability of the copper activated marmatite was remarkably decreased. The finding can potentially offer new choices for developing new reagents for depressing the copperactivated marmatite in flotation separation of the polymetallic sulfide ores.
1. Introduction Generally, lead sulfide ore and zinc sulfide ore are closely associated with each other or with copper-containing minerals such as chalcopyrite and copper oxides in most polymetallic sulfide ore deposits [1,2]. At present, flotation is the most commonly used method to recover these valuable minerals [3]. Xanthate is commonly used as the collector for these sulfide minerals in the polymetallic sulfide ore flotation plants. Usually, copper sulfide ore, lead sulfide ore and zinc sulfide ore are preferred to be floated together by bulk flotation and then were separated step by step. In the process, the zinc sulfide is the last to be recovered and therefore, the depression of the zinc sulfide is needed [4]. However, the large amount of Cu2+, mainly due to the flotation activator (copper sulfate) added in the bulk flotation and the dissolution of copper-containing minerals in the size-reduction processes including crushing and grinding [5], often exists in the flotation pulp. The Cu2+ could easily dissolve in flotation pulp and then activate the flotation of zinc sulfide minerals, such as sphalerite and marmatite [6–8]. The Cu2+ activated zinc sulfide ore usually exhibit similar flotability with the copper sulfide minerals [9] and therefore, the flotation selectivity is lowered [10]. In the flotation of the polymetallic sulfide ore, the depression of copper-activated zinc minerals has long been a problem. Depressants, used to selectively depress activated-zinc minerals in polymetallic sulfide flotation, have long been studied [4]. Some inorganic depressants, such as sulfur compounds and cyanide [11,12], were reported to have good depressant effect on the copper activated zinc sulfide minerals in the separation process under weak alkaline
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condition, however, they are toxic and could cause environmental pollution [13]. In recent years, some organic reagents, which were able to form metal complexes, were developed as new depressants in the process [14,15]. Qin et al. introduced DMDC (sodium dimethyldithiocarbamate) in copper sulfide flotation for depressing the Cu2+ activated sphalerite. They found that DMDC can prevent Cu2+ from activating sphalerite and marmatite by forming stable complex with Cu2+ in the flotation pulp [16]. Huang et al. used chitosan to chelate the Cu2+ on the surface of sphalerite and successfully realized the depression of sphalerite in galena flotation [17]. V. Malysiak et al. proposed that the DETA has the ability to remove the Cu2+ from the pyroxene surface, and the polyphosphate could significantly reduce the Cu2+ adsorption on the minerals [18]. Kelebek and Tukel et al. introduced TETA (triethylenetetramine) as an effective chelator for the metal ions in the deactivation/depression process of xanthate-induced flotation of sulfide ores, and found that the combined use of TETA and SMBS (sodium metabisulphite) produced an excellent separation of the polymetallic sulfide ores [19,20]. The key point of the depressants selection in the process is their reactivity to the adsorbed Cu2+ on the zinc minerals. However, the complicated structures of the complexing agent involved in the existing research mostly were of high cost and unfriendly to the environment. The need for the highly efficient and pollution-free depressant for the copper-activated marmatite remains challenging. Proteins are widely seen in the nature for its easy preparation, nontoxic and non-polluting properties, and have the chelating characteristics with many metal ions. Therefore, some proteins have the potential to act as the depressant for the copper-activated marmatite if they could
Corresponding author. E-mail address: [email protected] (W. Chen).
https://doi.org/10.1016/j.apsusc.2019.02.113 Received 17 December 2018; Received in revised form 28 January 2019; Accepted 12 February 2019 Available online 14 February 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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react with Cu2+ or Zn2+. Wang C et al. reported that the peptides in the proteins could react with zinc ions and produce chemically stable peptide zinc chelates [21]. Stanila A el al. found that the copper ions and amino acids can form complexes in aqueous solution under specific conditions, and the carbonyl (C]O), subamino (NeH) were the key factors for the formation and stability of the protein metal chelates [22]. In addition, the amino acid chelated copper has long been widely used in agricultural feed [23]. However, there were little reports referring to using proteins as the selective depressant for the copper-activated marmatite. The aim of the study was to introduce the whey protein (WP) as the selective depressant for the copper-activated marmatite. Micro-flotation test were conducted both in the absence and presence of the WP, when butyl xanthate (BX) was used as collector. Surface analysis, including zeta potential measurement, XPS measurement and FT-IR spectra, were done using mineral samples with and without treatment of the depressant. The depression mechanism was discussed.
Table 1 Chemical composition of the marmatite sample used in this research. Chemical composition
S
Zn
Fe
Pb
SiO2
Content (%)
32.60
49.57
14.20
0.05
3.58
2.2. Micro-flotation tests Micro-flotation tests were conducted in a XFD type laboratory flotation machine (Jilin Prospecting Machinery Factory, China) [24] equipped with a 40 mL plexiglass cell at a stirring rate of 1800 rpm. For each test, 2.0 g of −150 + 74 μm-size marmatite sample was obtained by grinding (using a ceramic ball mill) and wet-sieving the stored pure crystals. The flotation pulp was prepared by mixing and dispersing the samples in 40 mL of deionized water in the cell. After stirring for 1 min, the pH regulator, activator, depressant (if needed) and collector were added into the cell in sequence. Conditioning time for each reagent was 3 min. The flotation began 0.5 min after frother (10 mg/L for all flotation tests) was added and the flotation time was 3 min. After flotation, the froth and sink products were separately collected, filtered, dried, weighed and the recovery was determined based on the weighing data of the two products. Each test was repeated at least 3 times and the mean value was taken as the final recovery data. The standard deviation was calculated and presented as the error bars.
2. Materials and methods 2.1. Mineral samples and reagents The marmatite sample was obtained from Xitieshan Mine, Qinghai, China. A chunk of the big marmatite crystal was crushed through a jaw crusher. The pure marmatite crystal was hand-picked and further crushed in a roll crusher and dry-sieved. The −3 + 1 mm-size fractions was stored for subsequent grinding, flotation, zeta potential measurements, XPS measurements and FT-TR study. The purity of the mineral sample was 96.37%, according to its X-ray diffraction results (see Fig. 1) and chemical analysis (see Table 1). Butyl xanthate (BX) and Pine oil used in this research were purchased from Zhuzhou Flotation Reagents Factory, Hunan, China. They were used as collector and frother, respectively. Copper nitrate (Cu (NO3)2) was bought from Tianjing Kermil Chemical Reagents Development Centre, Tianjin, China and was used as copper ion induced activator for marmatite. The whey protein (WP) used in this research was purchased from LAIYANG WANJIWEI BIO-ENGINEERING CO., LTD, Shandong, China. The whey protein is a kind of white powder and easily dissolves in water. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used as pH regulators. All reagents were analytical pure. Deionized water (Resistivity = 18.2 MΩ·cm) was used for all tests.
2.3. Zeta potential measurements Zeta potentials of marmatite samples were measured in 1 × 10−3 mol/L KNO3 background electrolyte solutions with a Delsa 440sx Zeta Potential Analyzer (Malvern Instruments Ltd., United Kingdom). Fresh marmatite samples (−2 μm) for each measurement was prepared through grinding the stored crystals using an agate mortar. For each measurement, the mineral suspension with 0.01% solid concentration (mass fraction) was conditioned in a beaker and added with desired concentrations of the flotation reagents (c (Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L). The pH of the resultant suspension was adjusted using 0.1 mol/L HCl and 0.1 mol/L NaOH stock solutions. The conditioning time for each reagent was set at 10 min at 25 °C. After a standing of 10 min, the fine mineral particles at the top of the resultant suspension were sucked out for measurement. Each measurement was conducted at least 3 times independently, with a typical variation of ± 5 mV. The average value and of these measurements was taken as the final results. 2.4. XPS measurements The marmatite samples for XPS measurements were prepared by grinding (using a ceramic ball mill) and wet-sieving the stored crushed products. The conditioning operations of the mineral samples for XPS measurement was conducted in 100 mL aqueous solution in a beaker by dispersing 2.0 g samples (−106 + 74 μm). The pulp pH was adjusted using 0.1 mol/L HCl and 0.1 mol/L NaOH stock solutions. The solution was then added with flotation reagents as necessary and conditioned for 30 min. After conditioning, the samples were collected, filtrated, and stored for XPS analysis. XPS measurements were carried out with a Perkin-Elmer Physical Electronics Division (PHI) 5100 spectrometer (Thermo Scientific ESCALAB, USA) [25] with an Mg Kα X-ray source at 200 W and pass energy of 75 eV. The energy scale was calibrated using the Fermi edge and 3d5/2 line (BE = 367.9 eV) for silver, whilst the retardation voltage was calibrated using the position of the C 1s peak (BE = 284.6 eV). The measurements were performed at a take-off angle of 45°. The samples were first examined in survey mode to identify all the elements present, and then the various elemental regions were scanned in order
Fig. 1. XRD spectra of the marmatite sample. 304
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Fig. 2. The effect of copper ion on the flotation recovery of marmatite under different copper ion concentration (a, pH = 7.0, c(BX) = 2 × 10−5 mol/L) and pulp pH (b, c(BX) = 2 × 10−5 mol/L, c(Cu2+) = 3 × 10−5 mol/L).
Fig. 3. The effect of WP the flotation recovery of copper-activated marmatite under different WP concentration (a, pH = 7.0, c(Cu2+) = 5 × 10−5 mol/L, c (BX) = 2 × 10−5 mol/L) and pulp pH (b, c(Cu2+) = 5 × 10−5 mol/L, c(BX) = 2 × 10−5 mol/L, c(WP) = 60 mg/L).
to extract information on chemical bonding and oxidation stages. The obtained XPS spectra were handled with peak fitting and separation. The relative elements concentration was calculated after removing a contamination component via a MultiPak Spectrum software. 2.5. FT-IR spectra measurements The marmatite samples for FT-IR measurements were prepared by grinding the stored marmatite crystals using an agate mortar to get the −2 μm particles. For each measurements, the marmatite slurry was prepared by ultrasonically dispersing 30 mg pure samples (−2 μm) in 100 mL deionized water. Then the flotation reagents were added in sequence to a desired concentration (c(BX) = 2 × 10−5 mol/L, c (WP) = 60 mg/L). The resultant mineral pulp was conditioned with a magnetic stirrer at 25 °C, and the conditioning time for each reagent was 40 min. After conditioning, the mineral pulp was rinsed 2 or 3 times using the corresponding pH stock solutions and was centrifuged. The precipitation was carefully washed 2 times using deionized water and was stored in a vacuum desiccator at room temperature. The powder samples were collected for FT-IR reflection spectra measurement. The FT-IR spectra of marmatite samples treated with different reagent schemes were recorded with an IRAffinity-1 Fourier Transform Infrared Spectrometer (shimadzu, Japan) by KBr reflection method as
Fig. 4. The zeta potential of the marmatite with different reagent treatment (c (Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L).
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Figs. 2 and 3, respectively. Fig. 2 shows the effect of the copper ion on the flotation recovery of marmatite under different copper ion concentration and pulp pH. When pulp pH was controlled at 7.0 and collector concentration at 2 × 10−5 mol/L, the flotation recovery of marmatite increased with the increase of copper ion concentration, as shown in Fig. 2a. When the copper ion concentration increased from 0 to 5 × 10−5 mol/L, the recovery of marmatite increased from 25.60% to 81.78%. The remarkable increase in recovery suggests that the copper ion has good activator effect on the marmatite flotation at pH 7.0. Fig. 2b exhibits the flotability of marmatite under different reagent conditions in the pH range of 4–13. Marmatite showed poor flotability with recoveries keeping at around 10% across the pH range tested, when no reagent was used. When 2 × 10−5 mol/L BX, a commonly seen collector dosage in the flotation separation of copper sulfide ores and zinc sulfide ores, was used, the flotability only was slightly improved with recoveries increasing to about 20–30%. The poor flotability of the marmatite would be very beneficial for the separation process of the Pb-Zn sulfide ores. However, the copper ion is very common in flotation pulp of the symbiotic sulfide ores. The copper ion could significantly activate the marmatite and would result in low selectivity in the separation process, as supported by the increased recoveries from 20 to 30% to 50–70% when 3 × 10−5 mol/L copper ion was introduced. The high recovery across the whole pH range tested also indicates that the activator effect of copper ion on the marmatite flotation is quite strong and it is hard to depress the copper activated marmatite only by adjusting pulp pH. Fig. 3 shows the flotation recovery of the copper-activated marmatite when the WP was introduced as the depressant. In the presence of 5 × 10−5 mol/L copper ion, the marmatite recovery met remarkable drops when the WP concentration increased, as shown in Fig. 3a. The marmatite recovery decreased from over than 80% to < 30%, as the WP concentration increased from 0 to 100 mg/L. The big decrease in recovery suggests that the WP is able to depress the copper-activated marmatite when BX is used as collector at pH 7.0. Fig. 3b shows the
Table 2 The atomic concentrations of the main elements on marmatite surfaces treated with different reagent schemes (c(Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/ L). Samples
Marmatite Marmatite + Cu2+ Marmatite + Cu2+ + WP
Atomic concentrations (%) S 2p
Cu 2p
Zn 2p
Fe 2p
N 1s
18.31 17.37 10.36
0 2.82 0.45
8.62 6.23 4.70
0.23 0.25 0.03
0 0 7.35
Table 3 The valence bond morphology and distribution of sulfur (S) in marmatite with different reagent schemes (c(Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L). Sample
Marmatite Marmatite + Cu2+ Marmatite + Cu2+ + WP
Relative content of sulfur (S) Polysulfide (S2− n)
Sulfide ion (S2−)
23.50% 39.15% 13.53%
76.50% 60.85% 86.47%
before [26]. The wave number range of the spectra was 400–4000 cm−1. Each spectrum was recorded with 30 scans measured at 2 cm−1 resolution.
3. Results and discussion 3.1. Micro-flotation results The activator effect of the copper ion on the marmatite flotation was investigated, and then the depressant effect of the WP on the copperactivated marmatite flotation was studied. The results are shown in
Fig. 5. The broad scan XPS spectra of marmatite with the treatment of different reagent schemes (c(Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L). 306
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Fig. 6. The XPS spectra of Zn 2p (a), Cu 2p (b) and S 2p (c) of marmatite with the treatment of different reagent schemes (c(Cu2+) = 5 × 10−5 mol/L, c (WP) = 60 mg/L).
particles with corresponding reagent treatment were measured to reveal the interactions between the reagents and the mineral in the process. The results are shown in Fig. 4. For bare marmatite, it was negatively charged across the pH range tested and the isopotential point was not observed. As the pH increased, the zeta potential of marmatite decreased from −13.97 mV (pH 8.0) to −32.57 mV (pH 10.0). With treatment of 5 × 10−5 mol/L copper ion, the zeta potential of marmatite was positively charged (about 20 mV) in the whole pH range studied. The big rise in the zeta potential suggests the strong adsorption of the copper ion on the marmatite surface and reveals the activation mechanism of copper ion on marmatite flotation. With treatment of 5 × 10−5 mol/L copper ion +60 mg/L WP, the surface of the marmatite was negatively charged again. Different from the bare marmatite, the zeta potential of the marmatite with both copper ion and WP treatment kept unchanged in the entire pH ranges, indicating that the WP could adsorb on the marmatite through preadsorption of copper ion and then stably cover the particle surface. The zeta potential results demonstrate that the WP could strongly interact with the copper-activated marmatite, which is of great importance for explaining the depressant effect of the WP on the copper-activated marmatite and defining the depression mechanism.
influence of pulp pH on the depressant effect of WP. It was noted that the marmatite recovery decreased to different degrees when 60 mg/L WP was used as depressant, indicating that the depressant effect could exist stably across the whole pH range investigated. With WP + BX scheme, the copper-activated marmatite was well depressed with recovery at around 10%. Therefore, the optimum reagent scheme for the ideal depression of copper-activated marmatite is deemed to be WP + BX in weakly alkaline conditions. The key to the depression of copper-activated marmatite is to make its particle surface hydrophilic through preventing the adsorption of collector or pre-adsorption of hydrophilic inhibitor on its particle surface. Based on the flotation results under different reagent schemes, it is very likely that the WP might adsorb on the surface of copper-activated marmatite and then decrease the subsequent collector adsorption. To define the interactions of the WP with marmatite, zeta potential, XPS and FT-IR of the mineral surface with Cu2+, WP or BX treatment were measured.
3.2. Zeta potential measurement results The flotation results above have confirmed that 5 × 10−5 mol/L copper ion could fully activate the flotation of marmatite and 60 mg/L WP could completely depress the copper-activated marmatite in weakly alkaline pH ranges. Therefore, the zeta potentials of the marmatite 307
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Fig. 7. S 2p XPS spectra of marmatite (a), marmatite + Cu2+ (b) and marmatite + Cu2+ + WP (c) (c(Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L).
3.3. XPS results To gain a better understanding of the adsorption behavior of the WP on the copper-activated marmatite, the X-ray photoelectron spectroscopy of marmatite samples treated with different reagent schemes was measured and the results are shown in Tables 2–3 and Figs. 5–7. For the surface of bare marmatite, the Cu and Zn atomic concentrations were 0 and 8.62%, respectively (see Table 2). With the treatment of 5 × 10−5 mol/L copper ion, the Cu atomic concentration significantly increased to 2.82%, indicating that the Cu2+ could adsorb on the marmatite surface. Meanwhile, the Zn atomic concentration met a slight decrease from 8.62% to 6.23%. The mole ratio of the decreased Zn and the increased Cu was very close to 1:1, showing a big possibility of Cu-Zn replacement in the activating process [7]. With the WP treatment after Cu2+, N atomic concentration met a big rise (from 0 to 7.35%), suggesting the adsorption of the WP on the copper activated marmatite surface. At the same time, other elements including O, S, Cu, Zn and Fe all decreased to different degrees, probably due to the coverage of WP on the particle surface. The changes in the atomic concentrations demonstrate the adsorption of the WP on the copper-activated marmatite surface and are consistent with the broad scan XPS spectra results (see Fig. 5) and the flotation results. To make clear of the above changes in the atomic concentration in
Fig. 8. The FTIR spectra of marmatite treated with different reagent schemes (c (Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L).
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Fig. 9. The schematic diagram of the adsorption behavior of the WP on the copper activated marmatite surface in water.
surface of marmatite. The reduction of the redox reaction might form the basis of the depressant effect of the WP on the copper-activated marmatite.
the process, the XPS spectra of Zn 2p, Cu 2p and S 2p of marmatite with treatment of different reagent schemes was picked out for further analysis and the results are shown in Fig. 6. It was first noted that the peak height of the energy spectrum of Zn and S decreased with the copper ion treatment, and the characteristic peaks of the energy spectrum of Cu appeared simultaneously. With the copper ion + WP treatment, the peak height of the energy spectrum of all atoms was greatly attenuated, showing that the WP treatment greatly impaired the interaction between the copper ion and the marmatite surface. The calculated results of the relative contents of Table 2 also confirmed the atomic energy peak attenuation in Fig. 6. As shown in Fig. 6a, the Zn 2p3/2 XPS band appeared at 1021.5 eV, which belongs to the Zn2+ and was either zinc sulfide or zinc hydroxide [11,12]. From Fig. 6b, marmatite with the copper ion treatment exhibited two vibration peaks at 934.4 eV and 932.3 eV, which were attributed to the appearance of the Cu2+ [9,27].The appearance of Cu2+ and the decrease of Zn2+ on marmatite surface prove that the activator function of the copper ion on marmatite occurs by replacing the zinc ions on the original marmatite surface [28]. With the copper ion + WP treatment, the Cu2+ vibration peak (at 932.5 eV) disappeared. In addition, the other peak (at 934.4 eV) intensity was also lower than the spectra of the bare marmatite or the marmatite with the copper ion treatment. In this case, it could be concluded that the WP treatment eliminated the Cu2+ on the marmatite surface and therefore, efficiently eliminated the reaction point of the copper-activated marmatite towards the collector BX. The asymmetric S2p orbital peak at 162e/V in Fig. 6c attributed to polysulfide or S2− is hard to determine [27,29]. To gain more information on the effect of the WP on the surface phase of copper-activated marmatite, the S 2p XPS spectra of in Fig. 6c was further peakdifferentiated and the relative contents of different valence states S was calculated. The results are shown in Fig. 7 and Table 3. In Fig. 7, the S 2p XPS band appeared mainly at 161.5 eV, and was assigned to S2−, which belonged to the Zn-S or Cu-S. The S 2p XPS bands at 162.5 eV and 163.2 eV belonged to the polysulfide (S2− n, n ≥ 2) [11,29]. With the copper ion treatment, the polysulfide band intensity increased remarkably, indicating that the redox reaction of the Cu2+ towards marmatite not only generated Cu+, but also formed S2− n(n ≥ 2) on the particle surface. With the copper ion + WP treatment, the polysulfide band intensity decreased, and was even lower than that of the bare marmatite, showing that the WP treatment might have eliminated the activator effect of the Cu2+ towards marmatite. According to the S 2p XPS peak fitting results in Table 3, the relative content of S2− decreased from 76.50% to 60.85%, and the relative content of the polysulfide increased from 23.50% to 39.15% with the copper ion treatment. However, with the copper ion + WP treatment, the relative content of S2− increased to 86.74%, and the relative content of the polysulfide decreased to 13.53%, showing that the WP could depress the redox reaction between the copper ion and the S2− on the
3.4. FT-IR results The zeta potential measurements and XPS analysis have demonstrated the strong interaction between the WP and the copper-activated marmatite. In order to define the adsorption mechanism of the WP and the surface species, the FT-IR spectra of marmatite with the treatment of different reagent schemes were measured and the results are shown in Fig. 8. For bare marmatite, its typical absorption bands appeared at 1630.9 cm−1 and 1435.1 cm−1. With the copper ion treatment, no new bands were observed and the typical absorption bands of marmatite did not shift. However, with the copper ion + WP treatment, the new adsorption bands appeared at 1770.0 cm−1, 1573.1 cm−1 and 1407.2 cm−1. The band at 1770.0 cm−1 is attributed to the C]O stretching vibration, belonging to the eCOOR of amino acids in the WP [30]. Band at 1573.1 cm−1 and 1407.2 cm−1 were corresponded to the eNH2 amino acids in the WP [31,32]. The original absorption band at 1435.1 cm−1 was partially covered by the new band at 1407.2 cm−1, and the typical absorption band of the bare marmatite at 1630.9 cm−1 were not observed. The new bands indicate that the adsorption of the WP on the copper-activated marmatite was chemical in essence. 3.5. The mechanism of the depressant effect of the WP on the copperactivated marmatite flotation On the basis of the flotation results and the surface analysis, it can be concluded that the WP could adsorb on the copper-activated marmatite surface. Since the amino acids are able to react with the divalent copper ions [33], the possible reaction equation between the WP and the copper-activated marmatite surface is shown as follows:
Cu2 + + 2H3 N+CHRCOO− ⇔ 2H+ + (H3 N+CHRCOO−)2 Cu where R represents the alkyl groups. The WP could easily dissolve in water and broke down into amino acids under alkaline conditions, which are tightly bound to the metal ions in the solid-water interface. The copper ion on the marmatite surface has the strongest binding force to the amino acid among the all metal ions [34] and therefore, could chelate with the amino acid released by the WP. The chelating reaction is stronger than the redox reaction between copper ion and the sulfur ion on the marmatite surface. The WP combined with the two kind of the copper ions in mineral pulp and therefore the copper ions were not available for the collector BX in the water-solid interface, as shown in Fig. 9. As a result, the flotation of the copper-activated marmatite was depressed. 309
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4. Conclusions [13]
The whey protein was proposed as the depressant for the copperactivated marmatite when xanthate was used as collector. A reagent scheme, i.e., 60 mg/L of WP and 2 × 10−5 mol/L of xanthate was proved to be able to realize the depression of the marmatite with activation of 5 × 10−5 mol/L copper ion in the weakly alkaline pH ranges. Copper ions could react with the sulfur ions on the surface of marmatite, forming polysulfide compounds on particle surface. The reaction could remarkably improve the reactivity of marmatite towards xanthate collectors and result in the activation of marmatite flotation. The whey protein could generate the amino acids under alkaline conditions and chelate with the right copper ions that activated the marmatite surface. The chelation could interfere with the reaction between the copper ions and the sulfur ion on the marmatite surface and result in the depression of the copper-activated marmatite.
[14]
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[16]
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Acknowledgements
[20]
This work was supported by the China Postdoctoral Science Foundation Funded Project (Grant No. 2018M640964), the Special Research Project of Shan Xi Education Department (Grant No. 18JK0473), the Natural Science Project of Shan Xi Education Department (Grant No. 028155386) and the talent science and technology fund of Xi'an University of Architecture and Technology (Grant No. RC1820).
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[23]
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Full length article
The effect of whey protein on the surface property of the copper-activated marmatite in xanthate flotation system
T
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Xianzhong Bu, Fanfan Chen, Wei Chen , Yihao Ding School of Resources Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Flotation Whey protein Depressant Copper-activated marmatite
In this paper, the depressant effect of the whey protein, a chelating agent for copper ions, on the flotation of copper-activated marmatite was investigated by micro-flotation, zeta potential, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). The results showed that the formation of a complex through the interaction between the copper ions and the whey protein effectively prevented the activation of marmatite from copper ions, and the flotability of the copper activated marmatite was remarkably decreased. The finding can potentially offer new choices for developing new reagents for depressing the copperactivated marmatite in flotation separation of the polymetallic sulfide ores.
1. Introduction Generally, lead sulfide ore and zinc sulfide ore are closely associated with each other or with copper-containing minerals such as chalcopyrite and copper oxides in most polymetallic sulfide ore deposits [1,2]. At present, flotation is the most commonly used method to recover these valuable minerals [3]. Xanthate is commonly used as the collector for these sulfide minerals in the polymetallic sulfide ore flotation plants. Usually, copper sulfide ore, lead sulfide ore and zinc sulfide ore are preferred to be floated together by bulk flotation and then were separated step by step. In the process, the zinc sulfide is the last to be recovered and therefore, the depression of the zinc sulfide is needed [4]. However, the large amount of Cu2+, mainly due to the flotation activator (copper sulfate) added in the bulk flotation and the dissolution of copper-containing minerals in the size-reduction processes including crushing and grinding [5], often exists in the flotation pulp. The Cu2+ could easily dissolve in flotation pulp and then activate the flotation of zinc sulfide minerals, such as sphalerite and marmatite [6–8]. The Cu2+ activated zinc sulfide ore usually exhibit similar flotability with the copper sulfide minerals [9] and therefore, the flotation selectivity is lowered [10]. In the flotation of the polymetallic sulfide ore, the depression of copper-activated zinc minerals has long been a problem. Depressants, used to selectively depress activated-zinc minerals in polymetallic sulfide flotation, have long been studied [4]. Some inorganic depressants, such as sulfur compounds and cyanide [11,12], were reported to have good depressant effect on the copper activated zinc sulfide minerals in the separation process under weak alkaline
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condition, however, they are toxic and could cause environmental pollution [13]. In recent years, some organic reagents, which were able to form metal complexes, were developed as new depressants in the process [14,15]. Qin et al. introduced DMDC (sodium dimethyldithiocarbamate) in copper sulfide flotation for depressing the Cu2+ activated sphalerite. They found that DMDC can prevent Cu2+ from activating sphalerite and marmatite by forming stable complex with Cu2+ in the flotation pulp [16]. Huang et al. used chitosan to chelate the Cu2+ on the surface of sphalerite and successfully realized the depression of sphalerite in galena flotation [17]. V. Malysiak et al. proposed that the DETA has the ability to remove the Cu2+ from the pyroxene surface, and the polyphosphate could significantly reduce the Cu2+ adsorption on the minerals [18]. Kelebek and Tukel et al. introduced TETA (triethylenetetramine) as an effective chelator for the metal ions in the deactivation/depression process of xanthate-induced flotation of sulfide ores, and found that the combined use of TETA and SMBS (sodium metabisulphite) produced an excellent separation of the polymetallic sulfide ores [19,20]. The key point of the depressants selection in the process is their reactivity to the adsorbed Cu2+ on the zinc minerals. However, the complicated structures of the complexing agent involved in the existing research mostly were of high cost and unfriendly to the environment. The need for the highly efficient and pollution-free depressant for the copper-activated marmatite remains challenging. Proteins are widely seen in the nature for its easy preparation, nontoxic and non-polluting properties, and have the chelating characteristics with many metal ions. Therefore, some proteins have the potential to act as the depressant for the copper-activated marmatite if they could
Corresponding author. E-mail address: [email protected] (W. Chen).
https://doi.org/10.1016/j.apsusc.2019.02.113 Received 17 December 2018; Received in revised form 28 January 2019; Accepted 12 February 2019 Available online 14 February 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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react with Cu2+ or Zn2+. Wang C et al. reported that the peptides in the proteins could react with zinc ions and produce chemically stable peptide zinc chelates [21]. Stanila A el al. found that the copper ions and amino acids can form complexes in aqueous solution under specific conditions, and the carbonyl (C]O), subamino (NeH) were the key factors for the formation and stability of the protein metal chelates [22]. In addition, the amino acid chelated copper has long been widely used in agricultural feed [23]. However, there were little reports referring to using proteins as the selective depressant for the copper-activated marmatite. The aim of the study was to introduce the whey protein (WP) as the selective depressant for the copper-activated marmatite. Micro-flotation test were conducted both in the absence and presence of the WP, when butyl xanthate (BX) was used as collector. Surface analysis, including zeta potential measurement, XPS measurement and FT-IR spectra, were done using mineral samples with and without treatment of the depressant. The depression mechanism was discussed.
Table 1 Chemical composition of the marmatite sample used in this research. Chemical composition
S
Zn
Fe
Pb
SiO2
Content (%)
32.60
49.57
14.20
0.05
3.58
2.2. Micro-flotation tests Micro-flotation tests were conducted in a XFD type laboratory flotation machine (Jilin Prospecting Machinery Factory, China) [24] equipped with a 40 mL plexiglass cell at a stirring rate of 1800 rpm. For each test, 2.0 g of −150 + 74 μm-size marmatite sample was obtained by grinding (using a ceramic ball mill) and wet-sieving the stored pure crystals. The flotation pulp was prepared by mixing and dispersing the samples in 40 mL of deionized water in the cell. After stirring for 1 min, the pH regulator, activator, depressant (if needed) and collector were added into the cell in sequence. Conditioning time for each reagent was 3 min. The flotation began 0.5 min after frother (10 mg/L for all flotation tests) was added and the flotation time was 3 min. After flotation, the froth and sink products were separately collected, filtered, dried, weighed and the recovery was determined based on the weighing data of the two products. Each test was repeated at least 3 times and the mean value was taken as the final recovery data. The standard deviation was calculated and presented as the error bars.
2. Materials and methods 2.1. Mineral samples and reagents The marmatite sample was obtained from Xitieshan Mine, Qinghai, China. A chunk of the big marmatite crystal was crushed through a jaw crusher. The pure marmatite crystal was hand-picked and further crushed in a roll crusher and dry-sieved. The −3 + 1 mm-size fractions was stored for subsequent grinding, flotation, zeta potential measurements, XPS measurements and FT-TR study. The purity of the mineral sample was 96.37%, according to its X-ray diffraction results (see Fig. 1) and chemical analysis (see Table 1). Butyl xanthate (BX) and Pine oil used in this research were purchased from Zhuzhou Flotation Reagents Factory, Hunan, China. They were used as collector and frother, respectively. Copper nitrate (Cu (NO3)2) was bought from Tianjing Kermil Chemical Reagents Development Centre, Tianjin, China and was used as copper ion induced activator for marmatite. The whey protein (WP) used in this research was purchased from LAIYANG WANJIWEI BIO-ENGINEERING CO., LTD, Shandong, China. The whey protein is a kind of white powder and easily dissolves in water. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used as pH regulators. All reagents were analytical pure. Deionized water (Resistivity = 18.2 MΩ·cm) was used for all tests.
2.3. Zeta potential measurements Zeta potentials of marmatite samples were measured in 1 × 10−3 mol/L KNO3 background electrolyte solutions with a Delsa 440sx Zeta Potential Analyzer (Malvern Instruments Ltd., United Kingdom). Fresh marmatite samples (−2 μm) for each measurement was prepared through grinding the stored crystals using an agate mortar. For each measurement, the mineral suspension with 0.01% solid concentration (mass fraction) was conditioned in a beaker and added with desired concentrations of the flotation reagents (c (Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L). The pH of the resultant suspension was adjusted using 0.1 mol/L HCl and 0.1 mol/L NaOH stock solutions. The conditioning time for each reagent was set at 10 min at 25 °C. After a standing of 10 min, the fine mineral particles at the top of the resultant suspension were sucked out for measurement. Each measurement was conducted at least 3 times independently, with a typical variation of ± 5 mV. The average value and of these measurements was taken as the final results. 2.4. XPS measurements The marmatite samples for XPS measurements were prepared by grinding (using a ceramic ball mill) and wet-sieving the stored crushed products. The conditioning operations of the mineral samples for XPS measurement was conducted in 100 mL aqueous solution in a beaker by dispersing 2.0 g samples (−106 + 74 μm). The pulp pH was adjusted using 0.1 mol/L HCl and 0.1 mol/L NaOH stock solutions. The solution was then added with flotation reagents as necessary and conditioned for 30 min. After conditioning, the samples were collected, filtrated, and stored for XPS analysis. XPS measurements were carried out with a Perkin-Elmer Physical Electronics Division (PHI) 5100 spectrometer (Thermo Scientific ESCALAB, USA) [25] with an Mg Kα X-ray source at 200 W and pass energy of 75 eV. The energy scale was calibrated using the Fermi edge and 3d5/2 line (BE = 367.9 eV) for silver, whilst the retardation voltage was calibrated using the position of the C 1s peak (BE = 284.6 eV). The measurements were performed at a take-off angle of 45°. The samples were first examined in survey mode to identify all the elements present, and then the various elemental regions were scanned in order
Fig. 1. XRD spectra of the marmatite sample. 304
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Fig. 2. The effect of copper ion on the flotation recovery of marmatite under different copper ion concentration (a, pH = 7.0, c(BX) = 2 × 10−5 mol/L) and pulp pH (b, c(BX) = 2 × 10−5 mol/L, c(Cu2+) = 3 × 10−5 mol/L).
Fig. 3. The effect of WP the flotation recovery of copper-activated marmatite under different WP concentration (a, pH = 7.0, c(Cu2+) = 5 × 10−5 mol/L, c (BX) = 2 × 10−5 mol/L) and pulp pH (b, c(Cu2+) = 5 × 10−5 mol/L, c(BX) = 2 × 10−5 mol/L, c(WP) = 60 mg/L).
to extract information on chemical bonding and oxidation stages. The obtained XPS spectra were handled with peak fitting and separation. The relative elements concentration was calculated after removing a contamination component via a MultiPak Spectrum software. 2.5. FT-IR spectra measurements The marmatite samples for FT-IR measurements were prepared by grinding the stored marmatite crystals using an agate mortar to get the −2 μm particles. For each measurements, the marmatite slurry was prepared by ultrasonically dispersing 30 mg pure samples (−2 μm) in 100 mL deionized water. Then the flotation reagents were added in sequence to a desired concentration (c(BX) = 2 × 10−5 mol/L, c (WP) = 60 mg/L). The resultant mineral pulp was conditioned with a magnetic stirrer at 25 °C, and the conditioning time for each reagent was 40 min. After conditioning, the mineral pulp was rinsed 2 or 3 times using the corresponding pH stock solutions and was centrifuged. The precipitation was carefully washed 2 times using deionized water and was stored in a vacuum desiccator at room temperature. The powder samples were collected for FT-IR reflection spectra measurement. The FT-IR spectra of marmatite samples treated with different reagent schemes were recorded with an IRAffinity-1 Fourier Transform Infrared Spectrometer (shimadzu, Japan) by KBr reflection method as
Fig. 4. The zeta potential of the marmatite with different reagent treatment (c (Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L).
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Figs. 2 and 3, respectively. Fig. 2 shows the effect of the copper ion on the flotation recovery of marmatite under different copper ion concentration and pulp pH. When pulp pH was controlled at 7.0 and collector concentration at 2 × 10−5 mol/L, the flotation recovery of marmatite increased with the increase of copper ion concentration, as shown in Fig. 2a. When the copper ion concentration increased from 0 to 5 × 10−5 mol/L, the recovery of marmatite increased from 25.60% to 81.78%. The remarkable increase in recovery suggests that the copper ion has good activator effect on the marmatite flotation at pH 7.0. Fig. 2b exhibits the flotability of marmatite under different reagent conditions in the pH range of 4–13. Marmatite showed poor flotability with recoveries keeping at around 10% across the pH range tested, when no reagent was used. When 2 × 10−5 mol/L BX, a commonly seen collector dosage in the flotation separation of copper sulfide ores and zinc sulfide ores, was used, the flotability only was slightly improved with recoveries increasing to about 20–30%. The poor flotability of the marmatite would be very beneficial for the separation process of the Pb-Zn sulfide ores. However, the copper ion is very common in flotation pulp of the symbiotic sulfide ores. The copper ion could significantly activate the marmatite and would result in low selectivity in the separation process, as supported by the increased recoveries from 20 to 30% to 50–70% when 3 × 10−5 mol/L copper ion was introduced. The high recovery across the whole pH range tested also indicates that the activator effect of copper ion on the marmatite flotation is quite strong and it is hard to depress the copper activated marmatite only by adjusting pulp pH. Fig. 3 shows the flotation recovery of the copper-activated marmatite when the WP was introduced as the depressant. In the presence of 5 × 10−5 mol/L copper ion, the marmatite recovery met remarkable drops when the WP concentration increased, as shown in Fig. 3a. The marmatite recovery decreased from over than 80% to < 30%, as the WP concentration increased from 0 to 100 mg/L. The big decrease in recovery suggests that the WP is able to depress the copper-activated marmatite when BX is used as collector at pH 7.0. Fig. 3b shows the
Table 2 The atomic concentrations of the main elements on marmatite surfaces treated with different reagent schemes (c(Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/ L). Samples
Marmatite Marmatite + Cu2+ Marmatite + Cu2+ + WP
Atomic concentrations (%) S 2p
Cu 2p
Zn 2p
Fe 2p
N 1s
18.31 17.37 10.36
0 2.82 0.45
8.62 6.23 4.70
0.23 0.25 0.03
0 0 7.35
Table 3 The valence bond morphology and distribution of sulfur (S) in marmatite with different reagent schemes (c(Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L). Sample
Marmatite Marmatite + Cu2+ Marmatite + Cu2+ + WP
Relative content of sulfur (S) Polysulfide (S2− n)
Sulfide ion (S2−)
23.50% 39.15% 13.53%
76.50% 60.85% 86.47%
before [26]. The wave number range of the spectra was 400–4000 cm−1. Each spectrum was recorded with 30 scans measured at 2 cm−1 resolution.
3. Results and discussion 3.1. Micro-flotation results The activator effect of the copper ion on the marmatite flotation was investigated, and then the depressant effect of the WP on the copperactivated marmatite flotation was studied. The results are shown in
Fig. 5. The broad scan XPS spectra of marmatite with the treatment of different reagent schemes (c(Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L). 306
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Fig. 6. The XPS spectra of Zn 2p (a), Cu 2p (b) and S 2p (c) of marmatite with the treatment of different reagent schemes (c(Cu2+) = 5 × 10−5 mol/L, c (WP) = 60 mg/L).
particles with corresponding reagent treatment were measured to reveal the interactions between the reagents and the mineral in the process. The results are shown in Fig. 4. For bare marmatite, it was negatively charged across the pH range tested and the isopotential point was not observed. As the pH increased, the zeta potential of marmatite decreased from −13.97 mV (pH 8.0) to −32.57 mV (pH 10.0). With treatment of 5 × 10−5 mol/L copper ion, the zeta potential of marmatite was positively charged (about 20 mV) in the whole pH range studied. The big rise in the zeta potential suggests the strong adsorption of the copper ion on the marmatite surface and reveals the activation mechanism of copper ion on marmatite flotation. With treatment of 5 × 10−5 mol/L copper ion +60 mg/L WP, the surface of the marmatite was negatively charged again. Different from the bare marmatite, the zeta potential of the marmatite with both copper ion and WP treatment kept unchanged in the entire pH ranges, indicating that the WP could adsorb on the marmatite through preadsorption of copper ion and then stably cover the particle surface. The zeta potential results demonstrate that the WP could strongly interact with the copper-activated marmatite, which is of great importance for explaining the depressant effect of the WP on the copper-activated marmatite and defining the depression mechanism.
influence of pulp pH on the depressant effect of WP. It was noted that the marmatite recovery decreased to different degrees when 60 mg/L WP was used as depressant, indicating that the depressant effect could exist stably across the whole pH range investigated. With WP + BX scheme, the copper-activated marmatite was well depressed with recovery at around 10%. Therefore, the optimum reagent scheme for the ideal depression of copper-activated marmatite is deemed to be WP + BX in weakly alkaline conditions. The key to the depression of copper-activated marmatite is to make its particle surface hydrophilic through preventing the adsorption of collector or pre-adsorption of hydrophilic inhibitor on its particle surface. Based on the flotation results under different reagent schemes, it is very likely that the WP might adsorb on the surface of copper-activated marmatite and then decrease the subsequent collector adsorption. To define the interactions of the WP with marmatite, zeta potential, XPS and FT-IR of the mineral surface with Cu2+, WP or BX treatment were measured.
3.2. Zeta potential measurement results The flotation results above have confirmed that 5 × 10−5 mol/L copper ion could fully activate the flotation of marmatite and 60 mg/L WP could completely depress the copper-activated marmatite in weakly alkaline pH ranges. Therefore, the zeta potentials of the marmatite 307
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Fig. 7. S 2p XPS spectra of marmatite (a), marmatite + Cu2+ (b) and marmatite + Cu2+ + WP (c) (c(Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L).
3.3. XPS results To gain a better understanding of the adsorption behavior of the WP on the copper-activated marmatite, the X-ray photoelectron spectroscopy of marmatite samples treated with different reagent schemes was measured and the results are shown in Tables 2–3 and Figs. 5–7. For the surface of bare marmatite, the Cu and Zn atomic concentrations were 0 and 8.62%, respectively (see Table 2). With the treatment of 5 × 10−5 mol/L copper ion, the Cu atomic concentration significantly increased to 2.82%, indicating that the Cu2+ could adsorb on the marmatite surface. Meanwhile, the Zn atomic concentration met a slight decrease from 8.62% to 6.23%. The mole ratio of the decreased Zn and the increased Cu was very close to 1:1, showing a big possibility of Cu-Zn replacement in the activating process [7]. With the WP treatment after Cu2+, N atomic concentration met a big rise (from 0 to 7.35%), suggesting the adsorption of the WP on the copper activated marmatite surface. At the same time, other elements including O, S, Cu, Zn and Fe all decreased to different degrees, probably due to the coverage of WP on the particle surface. The changes in the atomic concentrations demonstrate the adsorption of the WP on the copper-activated marmatite surface and are consistent with the broad scan XPS spectra results (see Fig. 5) and the flotation results. To make clear of the above changes in the atomic concentration in
Fig. 8. The FTIR spectra of marmatite treated with different reagent schemes (c (Cu2+) = 5 × 10−5 mol/L, c(WP) = 60 mg/L).
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Fig. 9. The schematic diagram of the adsorption behavior of the WP on the copper activated marmatite surface in water.
surface of marmatite. The reduction of the redox reaction might form the basis of the depressant effect of the WP on the copper-activated marmatite.
the process, the XPS spectra of Zn 2p, Cu 2p and S 2p of marmatite with treatment of different reagent schemes was picked out for further analysis and the results are shown in Fig. 6. It was first noted that the peak height of the energy spectrum of Zn and S decreased with the copper ion treatment, and the characteristic peaks of the energy spectrum of Cu appeared simultaneously. With the copper ion + WP treatment, the peak height of the energy spectrum of all atoms was greatly attenuated, showing that the WP treatment greatly impaired the interaction between the copper ion and the marmatite surface. The calculated results of the relative contents of Table 2 also confirmed the atomic energy peak attenuation in Fig. 6. As shown in Fig. 6a, the Zn 2p3/2 XPS band appeared at 1021.5 eV, which belongs to the Zn2+ and was either zinc sulfide or zinc hydroxide [11,12]. From Fig. 6b, marmatite with the copper ion treatment exhibited two vibration peaks at 934.4 eV and 932.3 eV, which were attributed to the appearance of the Cu2+ [9,27].The appearance of Cu2+ and the decrease of Zn2+ on marmatite surface prove that the activator function of the copper ion on marmatite occurs by replacing the zinc ions on the original marmatite surface [28]. With the copper ion + WP treatment, the Cu2+ vibration peak (at 932.5 eV) disappeared. In addition, the other peak (at 934.4 eV) intensity was also lower than the spectra of the bare marmatite or the marmatite with the copper ion treatment. In this case, it could be concluded that the WP treatment eliminated the Cu2+ on the marmatite surface and therefore, efficiently eliminated the reaction point of the copper-activated marmatite towards the collector BX. The asymmetric S2p orbital peak at 162e/V in Fig. 6c attributed to polysulfide or S2− is hard to determine [27,29]. To gain more information on the effect of the WP on the surface phase of copper-activated marmatite, the S 2p XPS spectra of in Fig. 6c was further peakdifferentiated and the relative contents of different valence states S was calculated. The results are shown in Fig. 7 and Table 3. In Fig. 7, the S 2p XPS band appeared mainly at 161.5 eV, and was assigned to S2−, which belonged to the Zn-S or Cu-S. The S 2p XPS bands at 162.5 eV and 163.2 eV belonged to the polysulfide (S2− n, n ≥ 2) [11,29]. With the copper ion treatment, the polysulfide band intensity increased remarkably, indicating that the redox reaction of the Cu2+ towards marmatite not only generated Cu+, but also formed S2− n(n ≥ 2) on the particle surface. With the copper ion + WP treatment, the polysulfide band intensity decreased, and was even lower than that of the bare marmatite, showing that the WP treatment might have eliminated the activator effect of the Cu2+ towards marmatite. According to the S 2p XPS peak fitting results in Table 3, the relative content of S2− decreased from 76.50% to 60.85%, and the relative content of the polysulfide increased from 23.50% to 39.15% with the copper ion treatment. However, with the copper ion + WP treatment, the relative content of S2− increased to 86.74%, and the relative content of the polysulfide decreased to 13.53%, showing that the WP could depress the redox reaction between the copper ion and the S2− on the
3.4. FT-IR results The zeta potential measurements and XPS analysis have demonstrated the strong interaction between the WP and the copper-activated marmatite. In order to define the adsorption mechanism of the WP and the surface species, the FT-IR spectra of marmatite with the treatment of different reagent schemes were measured and the results are shown in Fig. 8. For bare marmatite, its typical absorption bands appeared at 1630.9 cm−1 and 1435.1 cm−1. With the copper ion treatment, no new bands were observed and the typical absorption bands of marmatite did not shift. However, with the copper ion + WP treatment, the new adsorption bands appeared at 1770.0 cm−1, 1573.1 cm−1 and 1407.2 cm−1. The band at 1770.0 cm−1 is attributed to the C]O stretching vibration, belonging to the eCOOR of amino acids in the WP [30]. Band at 1573.1 cm−1 and 1407.2 cm−1 were corresponded to the eNH2 amino acids in the WP [31,32]. The original absorption band at 1435.1 cm−1 was partially covered by the new band at 1407.2 cm−1, and the typical absorption band of the bare marmatite at 1630.9 cm−1 were not observed. The new bands indicate that the adsorption of the WP on the copper-activated marmatite was chemical in essence. 3.5. The mechanism of the depressant effect of the WP on the copperactivated marmatite flotation On the basis of the flotation results and the surface analysis, it can be concluded that the WP could adsorb on the copper-activated marmatite surface. Since the amino acids are able to react with the divalent copper ions [33], the possible reaction equation between the WP and the copper-activated marmatite surface is shown as follows:
Cu2 + + 2H3 N+CHRCOO− ⇔ 2H+ + (H3 N+CHRCOO−)2 Cu where R represents the alkyl groups. The WP could easily dissolve in water and broke down into amino acids under alkaline conditions, which are tightly bound to the metal ions in the solid-water interface. The copper ion on the marmatite surface has the strongest binding force to the amino acid among the all metal ions [34] and therefore, could chelate with the amino acid released by the WP. The chelating reaction is stronger than the redox reaction between copper ion and the sulfur ion on the marmatite surface. The WP combined with the two kind of the copper ions in mineral pulp and therefore the copper ions were not available for the collector BX in the water-solid interface, as shown in Fig. 9. As a result, the flotation of the copper-activated marmatite was depressed. 309
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4. Conclusions [13]
The whey protein was proposed as the depressant for the copperactivated marmatite when xanthate was used as collector. A reagent scheme, i.e., 60 mg/L of WP and 2 × 10−5 mol/L of xanthate was proved to be able to realize the depression of the marmatite with activation of 5 × 10−5 mol/L copper ion in the weakly alkaline pH ranges. Copper ions could react with the sulfur ions on the surface of marmatite, forming polysulfide compounds on particle surface. The reaction could remarkably improve the reactivity of marmatite towards xanthate collectors and result in the activation of marmatite flotation. The whey protein could generate the amino acids under alkaline conditions and chelate with the right copper ions that activated the marmatite surface. The chelation could interfere with the reaction between the copper ions and the sulfur ion on the marmatite surface and result in the depression of the copper-activated marmatite.
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Acknowledgements
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This work was supported by the China Postdoctoral Science Foundation Funded Project (Grant No. 2018M640964), the Special Research Project of Shan Xi Education Department (Grant No. 18JK0473), the Natural Science Project of Shan Xi Education Department (Grant No. 028155386) and the talent science and technology fund of Xi'an University of Architecture and Technology (Grant No. RC1820).
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