Insights into the selective adsorption mechanism of a novel flotation

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Insights into the selective adsorption mechanism of a novel flotation reagent 4-Amino-5-mercapto-1,2,4-triazole on chalcopyrite surface: an experimental and computational study Zhigang Yin, Yuehua Hu, Wei Sun, Chenyang Zhang, Jianyong He, Zhijie Xu, Jingxiang Zou, Changping Guan, Chen hu Zhang, Qingjun Guan, and Shangyong Lin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03975 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Adsorption mechanism of 4-amino-5-mercapto-1,2,4-triazole as flotation agent on chalcopyrite Zhigang Yina, Yuehua Hua, Wei Suna,∗, Chenyang Zhanga,∗, Jianyong Hea, Zhijie Xua, Jingxiang Zoub, Changping Guana, Chenhu Zhanga, Qingjun Guana, Shangyong, Lina a

School of Minerals Processing and Bioengineering, Central South University, Changsha, Hunan

410083, China. b

School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of

Ministry of Education, Institute of Theoretical and Computational Chemistry, Nanjing University, Nanjing 210023, People’s Republic of China

Abstract: In the present study, a novel compound 4-Amino-5-mercapto-1,2,4-triazole was first synthesized and its selective adsorption mechanism on the surface of chalcopyrite was comprehensively investigated using UV-vis spectra, Zeta potential, Fourier Transform Infrared Spectroscopy(FTIR), X-ray Photoelectron Spectroscopy Measurements(XPS), and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and first principle calculations. The experimental and computational results consistently demonstrated that AMT would chemisorb onto the chalcopyrite surface by the formation of a five-membered chelate ring. The first principle periodic calculations further indicated that AMT would prefer to adsorb onto Cu rather than Fe due to the more negative adsorption energy of AMT on Cu in the chalcopyrite (001) surface, which was further confirmed by the coordination bonding energies of AMT-Cu and AMT-Fe based on the simplified cluster models at a higher accuracy level (UB3LYP/Def2-TZVP). The bench scale results indicated that the selective index improved significantly when using AMT as a chalcopyrite depressant in Cu-Mo flotation separation. Keywords: Chalcopyrite, Triazole derivative, Adsorption mechanism, Density functional theory, First principle calculations

1. Introduction Froth flotation is one of the most important processes to separate valuable minerals from gangue by taking advantage of differences in the physical chemistry of their surface properties1, particularly in their surface hydrophobicity. Hydrophobicity ∗ Corresponding author. Tel:+8673188830482 Email address: [email protected] ∗ Corresponding author. Tel:+8673188830482 Email address: [email protected] 1

ACS Paragon Plus Environment

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differences between valuable and gangue minerals could be regulated by adding different chemical additives (surfactants)2, such as collectors and depressants. It could be conducted economically to selectively separate and enrich the complicated and/or poor ores, by adopting appropriate surfactants. For example, the beneficiation of molybdenite usually relies on a two-stage process to realize flotation separation of molybdenite from other unwanted sulphide ores3. In the first step, a bulk molybdenite concentrate is produced by utilizing kerosene/diesel oil and pine oil/alcohols as collector and frother, respectively. Second, sodium hydrosulphide, sodium cyanide, ammonium sulphide, or Nokes reagent are adopted as depressants in the separation stage. However, these depressants are usually toxicant and cannot meet the requirements of environmental protection. A large number of efforts have been taken to develop alternative environmentally-friendly depressants4-9. Although some novel and environmental-friendly depressants have been reported to achieve good performance in Cu-Mo flotation separation at a laboratory scale, there are still some challenging problems, including toxicity, high price or poor performance in practice, which severely limit their practical applications in Cu-Mo flotation separation on a commercial scale. Considerable attention has been attracted by the derivatives of triazoles and their metal complexes in the recent decades due to their variety of biological and pharmacological activities10-19. In addition to these important applications, derivatives of triazoles or their metal complexes are also of great value and utility in other research fields, such as spectroscopic properties20, radio-protective effects21, electrochemical

properties22,

photoluminescence

properties23

and

corrosion

inhibition24-26. The wide applications of these heterocyclic molecules as various potential candidates should be attributed to their special molecular structures such that a soft σ-donor and potential π-acceptor can coexist27. For example, the thione-thiol tautomerism of 4-amino-4H-1,2,4-triazole-3-thiol and its derivatives could be coordinated with metal ions as unidentate donor ligands (N or S), bidentate donor ligands(N and S) or tridentate donor ligands (N, N and S) in different coordination modes27. Furthermore, 1,2,4-triazole derivatives have been adopted as corrosion inhibitor of copper in recent years

24-25

. The effectiveness of these compounds as

copper corrosion inhibitors should be ascribed to their powerful chelating ability with copper, which usually results in a monolayer formation of Cu-ligand complexes on the copper surface. The monolayer could obstruct the copper dissolution or the 2


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chemical reaction of copper with environmental substances

28

. Previous reports

revealed that when the 3-substituted-4-amino-5-mercapto-1,2,4-triazole Schiff base reacted with Cu(II) ions, its thione functional group could rearrange to the thiol form and allow the S and N atom to bidentately be coordinated with Cu(II) ions and form a stable five-membered ring14. Similar interesting findings were reported

29-31

that the

4-amino-3-(aryl or alkyl)-1,2,4-triazole-5-thione molecule could be coordinated with copper ions and other transition metals (including Ni, Co, Mn and Zn) by the formation of a stable five-membered ring based on the amine and thione-thiol tautomerism32. It was found that 3-hexyl-4-amino-1,2,4-triazole-5-thione (HATT) might chemisorb on the malachite surface by formation of Cu-S and Cu-N bonds with the breakage of S-H bonds in the HATT molecule. The malachite surface was changed from hydrophilicity to hydrophobicity due to the formation of HATT-Cu complexes on the malachite surface, thus the floatability of malachite was significantly improved. The potential capacity of a compound to be coordinated with metal ions makes such a compound become a potential surfactant, which could regulate the surface properties of a given mineral and make the surface of the mineral hydrophobic or hydrophilic to selectively float or depress them. It could be expected that the derivatives of triazole containing an S=C-N-N unit could be adopted as either collector or depressant in the flotation separation of minerals, which depends on how the hydrophilic or hydrophobic groups of a given candidate molecule are balanced. However, there are only a few reports available that describe the synthesis, characterization and application of triazole derivatives in the field of mineral processing32. Particularly, to the best of our knowledge, there are almost no intensive investigations of interaction mechanism of the triazole derivatives as flotation reagents with the involved mineral surface in Cu−Mo separation. The main objective of this paper is to systematically and thoroughly explore the adsorption

behavior

of

a

novel

derivative

of

1,2,4-triazole,

called

as

4-Amino-5-mercapto-1,2,4-triazole (AMT) and its micro-mechanism on the chalcopyrite

surface.

Accordingly,

a

derivative

of

1,2,4-triazole,

called

4-Amino-5-mercapto-1,2,4-triazole (AMT) containing an S=C-N-N unit was first designed and synthesized by the reaction of thiocarbohydrazide (based on the reaction of hydrazine hydrate with carbon disulphide) with formic acid. Then, experimental characterizations were performed with UV-vis spectra, zeta potential, FTIR, XPS, ToF-SIMS to study the surface structure of chalcopyrite and the adsorption 3


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mechanism of AMT on chalcopyrite. In addition, the interaction mechanism at the molecular level was further investigated by first principle calculations. The experimental and computational results consistently demonstrated that AMT would chemisorb onto the chalcopyrite surface by the formation of a five-membered chelate ring, which strongly implied that AMT could become a promising candidate to depress chalcopyrite in flotation practice.

2. Materials and methods 2.1. Materials Chalcopyrite samples were obtained from Dexing Copper Mine, China. The samples were crushed in a porcelain mortar and sieved. The X-ray diffraction pattern confirmed that the chalcopyrite had a purity of over 90%, which met the requirements of the research7. All the used reagents and solvents were analytical reagents, purchased from local suppliers and used without further purification. Copper dichloride (Aladdin Chemical Reagent Co., Ltd.), hydrazine hydrate (Xilong Chemical Co., Ltd.), formic acid and ethanol (Sinopharm Chemical Reagent Co., Ltd.), and carbon disulphide (Kelong Chemical Co., Ltd.) were used. Distilled water was used for UV-vis spectra, zeta-potential, and sample preparation for FTIR, XPS and ToF-SIMS measurements. The mineral surface wetting reagent AMT was synthesized according to the known methods for the preparation of triazole derivatives described in Liang25, and the structure of AMT was confirmed by Infrared spectrometer (IRAffinity-1 Shimadzu, Kyoto, Japan), 1H NMR (Bruker VANCE III 600M), 13C NMR (Bruker VANCE III 600 M) and MS (Agilent 1260+6120 ESI-MS) spectra. AMT: while solid, yield 83.75%, m.p : 167-168°C. Elemental analysis calc. (%) for C3H4N4S (116.12 g/ mol): C 20.69, H 3.45, N 48.28, S 27.59; found. (%): C 21.27, H 3.26, N 48.52, S 26.88; 600 MHz 1H NMR (DMSO) δH: 5.62 (2H, NH2), 8.37(1H, CH), 13.6 (1H, NH); 13C NMR (DMSO) δC: 142.1 (CH=N), 165.7 (C=S); HRMS (ESI+): calculated for C3H4N4S 116.12; found 117.1 [M+H]. 2.2. Zeta potential measurement The zeta potential measurements were conducted using a zeta potential analyzer from Malvern Zeta Sizer Nano Series (England) with electrolyte solution of 0.01 M of KNO3 in the presence or absence of 1×10-4 mol/L AMT. First, 100 mg of 100% passing 5µm chalcopyrite samples were placed into a 100 mL beaker with a certain 4


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amount of electrolyte solution. Then, the pulp was adjusted with diluted HCl or NaOH solutions to the desired pH values and agitated for 5 min. After another 5 min for sedimentation, the supernatant was sampled for the zeta-potential measurement. In each case, the results presented were the average of three independent measurements with a typical variation of ±2 mV. 2.3. UV-Vis spectra of AMT react with Cu2+ ions First, 1×10-4 mol/L Cu2+ and 1×10-4 mol/L AMT solution were prepared by adding certain amount of copper dichloride or AMT to a flask with volume of one liter, respectively. After the preparation of Cu2+ and AMT solutions, 50 mL of each solution was sampled and mixed together in a beaker. After a half hour of oscillation (SHA-82A, China), the samples were left standing for 24 hours, and the supernatant was sampled by a size of Millipore syringe filter with a size of 13 mm×0.45 µm(for filtration) to measure the absorbance. 2.4. FTIR spectra measurement The sample was prepared by adding 100 mg of minerals to 50 mL solution with the depressant concentration of 10-2 mol/L at pH 8. Fourier transform infrared (FTIR) spectra were adopted to characterize AMT, AMT-Cu2+ complexes and the samples of chalcopyrite before and after adsorption of AMT. The infrared spectra of samples were recorded by an IR Affinity-1 (Shimadzu, Japan) FT-IR spectrometer at a resolution of 4 cm-1 in the range of 400 cm-1 to 4000 cm-1 through KBr pellets at room temperature (25±1℃). 2.5. X-ray photoelectron spectra measurements The preparation of samples for XPS measurements were conducted as described in section 2.4. The X-ray photoelectron spectra (XPS) of mineral particles before and after adsorption of AMT were recorded with a K-Alpha 1063 (Thermo Scientific Co., USA) spectrometer with Al Kɑ as the sputtering source at 12 kV and 6 mA, with pressure in the analytical chamber at 1.0×10–12 Pa. All binding energies were referenced to the neutral C 1 s peak at 284.6 eV to compensate for the surface-charging effects. XPS Peak 4.1 software was used to fit the XPS peaks. 2.6. ToF-SIMS measurement The preparation of samples for ToF-SIMS measurements were the same as that of FTIR and XPS. The chemical composition of chalcopyrite surfaces after adsorption of AMT was carried out with ION-TOF IV instruments (Model 2100 Trift II, Physical 5


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Electronics, USA) equipped with a Bi primary ion source. The primary ion beam and acceleration voltage were 15 keV and 5 keV, respectively. The mass range was set at 0-200 amu with one mass unit resolution. The positive-ion spectra were calibrated by 63

Cu+,

65

Cu+, CH3+, and C2H5+, and the negative spectra were calibrated with CH-,

C2H-, and Cl- peaks. 2.7. Computational methods A periodic model of chalcopyrite and molybdenite bulk phases were built from the XRD crystal structure obtained from the American Mineralogist Crystal Structure Database33. All periodic calculations were performed by utilizing the Vienna Atomistic Simulation Package (VASP) in conjunction with an 600 eV energy cutoff and a projector augmented wave (PAW) treatment of core electrons34-35. The optimized super cell (2×2×1) of chalcopyrite bulk structure was used to generate a surface slab model by cleaving Cu−S and Fe-S bonds crossing a specified crystal plane [001].

The optimized super cell (4×4×1) of molybdenite bulk structure was

adopted to obtain a (001) crystal surface of molybdenite with a comparable size of chalcopyrite (001) surface. Slab model of (001) surface with a vacuum region of at least 20 Å was optimized for the calculation of adsorption energies both in vacuum and in the presence of water molecules. To generate a well-distributed water layer, the water layer with 200 water molecules was firstly placed on the surface of chalcopyrite (001) (molybdenite (001) ) to build a water- chalcopyrite (001) (water- molybdenite (001) )interface system. Then, forcite program 36 was adopted to perform molecular dynamic simulation of the interface system with NVT ensemble for two nanoseconds to make the surface water molecules well relax within the totally fixed chalcopyrite (001) surface (molybdenite (001)). The adopted universal force field has been reported to be moderately accurate for predicting geometries for metal complexes37. To save computer storage and computing time, only the 8 water molecules closest to the surface were included in the following DFT calculations. An adsorbate molecule (AMT) was introduced into the optimized slab surface during the adsorption energy calculation in the absence and presence of water molecules. Then, the newly generated interface structures were optimized until the changes in forces and energy less than 0.05 eV/Å and 1.0 × 10-5 eV/Å in two successive iterations. The Monkhorst–Pack scheme was used for k-point sampling in the first irreducible Brillouin zone (BZ) for all structures. Semiempirical dispersion 6


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corrections have also been considered in all the calculations via the DFT-D3 method38. The simplified cluster models of AMT-Cu2+ and AMT-Fe2+ have been used to study the adsorption reaction energies of the novel reagent AMT with Cu2+ and Fe2+. All the cluster (AMT, Cu2+, Fe2+, AMT-Cu2+, AMT-Fe2+) optimization calculations were conducted at B3LYP/Def2-TZVP level by using D01 version of GAUSSIAN 09 programm

39-40

. Solvation effect was included in cluster model calculations with

IEF-PCM polarizable continuum solvation model41. All the figures of the proposed periodic and cluster models are rendered by VESTA 3 42. 2.8 Bench scale flotation tests To evaluate the potential of AMT as a selective depressant to decrease chalcopyrite hydrophobicity, there were two sets of batch flotation data (experiment with or without depressant) obtained from a Cu-Mo bulk concentrate flotation separation in a 1.5 L laboratory machine. The detailed information about the samples and the experimental setup were described in a previous publication43. The selectivity index is a measure of the grade-recovery performance and could be used to characterize the efficiency of flotation separation between any two minerals44. In this section, the classical first-order flotation model was adopted to calculate the flotation rate constant and the selectivity index45.

3. Results and discussion 3.1. Zeta potential measurement As well known, the change in the charge on the mineral surface is closely related to the adsorption of flotation reagents, which is helpful for elaborating the adsorption mechanism of reagents on the active sites of the mineral surface46. The results of zeta potential measurements for chalcopyrite as a function of pH, in the presence and absence of AMT 1×10-4 mol/L, were presented in Figure 1. The zeta potential of original chalcopyrite had almost constant negative values under the tested pH range of 6-11. It is clear that the isoelectric point (IEP) of reagent-free chalcopyrite was about pH value of 5.00, which was very close to previously reported values47-48. After adsorption of AMT on chalcopyrite surface, the zeta-potential became more negative in the tested pH range. The difference in the original chalcopyrite and the AMT treated chalcopyrite in zeta potential increased with the increase in pH, indicating that 7


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there was higher adsorption capacity on chalcopyrite surface under alkaline condition, which should be attributed to two reasons: one is the stronger affinity of AMT adsorption onto copper ion than OH- ions; another reason is that the “OH-” ion could react with the ATM functional group of “S=C-NH-NH2” or “SH-C=N-NH2” (rearrangement from S=C-NH-NH2”) in aqueous solution to promote AMT adsorption onto chalcopyrite. The predicted molecular schematics of the adsorption of AMT onto chalcopyrite were demonstrated in Figure 1 (b). Therefore, the wettability of chalcopyrite might be significantly enhanced due to the adsorption of hydrophilic reagent AMT.

Figure 1. (a) Zeta potential of chalcopyrite as a function of pH in the presence and absence of AMT (C= 1×10-4 mol/L); (b) the predicted molecular schematics of the adsorption of AMT onto chalcopyrite.

3.2. UV spectroscopic analysis In this section, the solution of synthesized ligand was mixed with that of copper ions to evaluate its potential as a chelating agent. Therefore, it could be used as the surface wettability reagent of chalcopyrite if chemical reaction could be observed. The UV-vis spectra of AMT and the mixture of 1×10-4 mol/L AMT ions reacting with 1×10-4 mol/L Cu2+ ions with the same volume at room temperature (25°C) were listed in Figure 2. It is obvious that after interacting with Cu2+ ions, the residual concentration of AMT was much lower than that of the original solution. The changes indicated that Cu2+ ions had a great influence on the absorbance of AMT in wavelengths from 280 to 190 nm. Furthermore, when 1×10-2 mol/L AMT solution was mixed with 1×10-2 mol/L Cu2+ ions in the same volume (50 mL), the brown precipitation appeared immediately, and the pH of the solution reduced from 4.53 to 2.65, indicating that AMT might release the H+ ions into aqueous solutions after 8


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reacting with Cu2+ ions. Compounds containing an S=C-NH-N unit could partly rearrange to SH-C=N-N in aqueous solution14, coordinated with Cu

2+

32

. Therefore, the AMT could be

through the S atom (thiol group -SH or C=S) and N atom

(primary amine N-NH2) through releasing H ions (partly dissociated from thiol group -SH or primary amine N-NH2) into the aqueous solutions49.

Figure 2. UV spectra of AMT in the presence and absence of Cu2+ ions (C=1×10-4 mol/L).

3.3. FTIR spectra measurement The FTIR spectra of AMT, AMT-Cu2+ complexes, and chalcopyrite before and after AMT adsorption were presented in Figure 3. The C-H stretching vibrations of AMT and AMT-Cu2+ complexes appeared at approximately 2931 cm -1. A band at 1608 cm-1 in the spectra of AMT was assigned to the group frequency of υ (N=CH), which remained at the same position in the complexes indicating no coordination between the N atom of the Schiff base and copper ions. The multi bands between 3024 cm -1 and 3170 cm-1 were due to the N-H stretching vibrations of AMT, which weakened or even disappeared in AMT-Cu2+ complexes, demonstrating that AMT was coordinated with Cu2+ through the N atoms of the primary amine. The new bands in the region of 517-538 cm-1 and 441-471 cm-1 were ascribed to Cu-N stretching of ATM-Cu2+ complexes, which further supported the coordination with the N atom14, 18. Some intense bands at approximately 1495 cm-1 and 1564 cm-1 were assigned to the complex vibrations of the N-C(=S)-N-N group of AMT molecule shifted by 41 cm-1 and 62 cm-1 to lower frequency in the AMT-Cu2+ complexes, respectively, indicating that AMT was coordinated with the copper atom through the S and N atoms32. This finding was further confirmed by the appearance a of new band in the range of 678-743 cm-1 in AMT-Cu2+ complexes due to υ (C-S) 50. Therefore, the difference in 9


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FTIR spectra of AMT and AMT-Cu2+ complexes demonstrated that the AMT might react with copper ions through the N atom (primary) and S atom, which was in good agreement with the previous reports7, 51. After adsorption of AMT, the N-H and C-H stretching vibration bands were present on chalcopyrite surface at 3154 cm-1–3279 cm-1 and 2995 cm-1, respectively. Furthermore, the medium bands at 1050-1200 cm-1 in the spectra of AMT treated chalcopyrite were attributed to the C=S stretching vibration18, which confirmed the coordination of AMT with the chalcopyrite surface via the S atom. The far infrared spectra showed weak bands in the region of 437–485 cm-1 that were assigned to Cu-N stretching AMT-Cu2+ complexes. Furthermore, some new peaks at approximately 1510 cm-1, 1334 cm-1, 1210 cm-1 and 883 cm-1 due to AMT-Cu2+ complexes also appeared on the chalcopyrite surface, demonstrating that AMT chemisorbed on the chalcopyrite surface. Therefore, it could be expected that AMT might chemisorb on chalcopyrite surface through the N atom (primary amine) and S atom by formation of AMT-Cu2+ complexes.

Figure 3. The FTIR spectra of AMT, AMT-Cu2+ complexes and chalcopyrite with and without AMT. All the samples were prepared at pH 8.

3.4. XPS measurements To identify the surface species that are most likely responsible for AMT adsorption on chalcopyrite, the full range XPS spectra of AMT and chalcopyrite before and after adsorption of AMT were shown in Figure 4 and the surface atomic concentrations of the elements C, N, O, S, Fe and Cu were listed in Table 1. Clearly, the surface concentration of carbon and nitrogen increased and the corresponding concentration of oxygen, sulphur and copper decreased after the adsorption of AMT, 10


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indicating that the coverage of AMT might reduce the oxidation of chalcopyrite or that the oxide products have been replaced by the adsorbed AMT52.

Figure 4. The full range XPS spectra of AMT and chalcopyrite before and after adsorption of AMT. All the samples were prepared at pH 8.

Figure 5. (a) X-ray photoelectron spectra of S2p ssignals of AMT, AMT-Cu2+ complex and chalcopyrite before and after adsorption of AMT; (b) X-ray photoelectron spectra of N1s signals of AMT, AMT-Cu2+ complexes and chalcopyrite before and after treatment;(c) X-ray photoelectron spectra of Fe2p signals of the chalcopyrite before and after AMT adsorption;(d) X-ray photoelectron spectra of Cu2p signals of CuCl2, AMT-Cu2+ complexes, and chalcopyrite before and after adsorption of AMT. All the samples were prepared at pH 8. 11


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Table 1. The elements atomic contents in samples before and after adsorption of AMT. Atomic concentration of elements (atomic %) Samples C

O

S

Cu

Fe

N

Chalcopyrite

17.36

22.48

30.82

21.31

8.03

0.00

Chalcopyrite + AMT

28.19

14.04

19.79

10.35

8.11

19.52

∆

10.83

-8.44

-11.03

-10.96

0.08

19.52

AMT

30.18

c

13.99

55.82

∆c is defined as the value of the sample after adsorption of AMT minus that before adsorption of AMT.

High resolution X-ray photoelectron spectra of S2p signals of AMT, AMT-Cu2+ precipitate and chalcopyrite in the presence and absence of AMT were presented in Figure 5 (a), which demonstrated that the S2p signal of original chalcopyrite could be resolved into their components, whose binding energies were found at 161.45, 162.56, 163.60 and 169.54 eV, respectively. The strong peaks centred at 161.45 eV and 162.56 eV were attributed to the bulk mono-sulphide and di-sulphides, respectively, which was very close to the results of previous reports of chalcopyrite leached by moderate thermophiles and mesophiles53-54 and the chalcopyrite surface analyses55-56. The other two peaks at higher binding energies of 163.60 and 169.54 eV might represent the characteristic peaks of Sn2-/S0 and SO42-, respectively, indicating that the chalcopyrite surface was mildly oxidized 57-58. The X-ray photoelectron spectra of the S2p signal of AMT showed two peaks at approximately 162.37 and 164.05 eV attributed to the thione sulphur of the C=S group and the thiol sulphur of the C-S-H group, respectively32. The S2p photoelectron spectra of AMT-Cu2+ complex were mainly composed of two components at 162.59 and 163.88 eV, which should be ascribed to the thione sulphur (C=S) specie of AMT-Cu2+ complexes

32, 59

and the

oxidation product of AMT32, respectively. After AMT adsorption, the X-ray photoelectron spectra of S2p signal of chalcopyrite displayed three distinct peaks at approximately 161.34 eV, 162.49eV and 163.68 eV, which should be due to bulk monosulphide, thione sulphur of C=S group and the oxidation product of AMT, respectively, demonstrating that the AMT-Cu2+ complexes and oxidation of AMT might appear on the AMT-treated chalcopyrite surface. Furthermore, the vanished 12


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characteristic peak of sulphate species (SO42-) indicated that the oxidation of chalcopyrite was hindered or replaced by the adsorbed AMT molecules. The X-ray photoelectron spectra of the N1s signal of AMT, AMT-Cu2+ complex and chalcopyrite after adsorption of AMT were presented in Figure 5 (b). It is clear the N1s spectra of AMT could be resolved into two components, whose binding energies were 400.64 eV (C=N-NH-C) and 401.66 eV (C-N-NH2), which was similar to the previously reported adsorption mechanism on chalcopyrite32. The X-ray photoelectron spectra of the N1s signal of AMT-Cu2+ complexes were composed of three components with binding energies located at 399.78 eV, 400.72eV and 401.68eV. The peak at approximately 399.78 eV was attributed to the C-N-N-C group32, 59. The other two peaks of 400.72 and 401.68 eV should be assigned to the Cu-N and C-N-C groups, respectively. These findings suggested the formation of new bond of Cu2+ with the N atom of AMT, which was consistent with the previously reported results by Liu52. After the adsorption of AMT on the chalcopyrite surface, the X-ray photoelectron spectra of the N1s signal showed three peaks at 399.47 eV, 400.36 eV and 401.42 eV, which were similar to the corresponding values for AMT-Cu2+ complexes (399.78 eV, 400.72 eV and 401.68 eV). These results consistently confirmed the formation of the Cu-N bond on the AMT-adsorbed chalcopyrite surface, which was in agreement with previous reports of the adsorption mechanism of HATT on the chalcopyrite surface31. The X-ray photoelectron spectra of Fe2p signals of the original chalcopyrite and the AMT adsorbed chalcopyrite were shown in Figure 5 (c). The peak at approximately 708.66 eV could be attributed to Fe(III)-S, which should be related to fully coordinated Fe atoms on chalcopyrite surface and agreed with the reported values of 707.7 eV60 and 707.5 eV57. Another peak centred at 711.71 eV could be ascribed to Fe(III)-O-OH, which indicated the formation of jarosite or iron (III) oxyhydroxides on the chalcopyrite surface58, 61. After the adsorption of AMT on the chalcopyrite surface, the X-ray photoelectron spectra of Fe2p signals remained at nearly the same position (located at 708.63 and 711.66 eV, respectively), indicating that the Fe atoms were not the dominant sites for adsorption of AMT. As shown in Figure 5 (d), X-ray photoelectron spectra of Cu2p signal of copper dichloride presented two peaks at 933.89 eV and 953.99 eV. Chemical shifts of -1.86 eV and -2.06 eV were observed for the corresponding peaks of AMT-Cu2+ complexes. It should be due to the increased electron density of copper ions from the electron 13


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

donation of S and N atoms. High resolution X-ray photoelectron spectra of the Cu2p signal of the original chalcopyrite showed two peaks located at 932.36 eV and 952.24 eV attributed to Cu in sulphide lattice53 and Cu in oxide products56, respectively, which was in agreement with the findings of characteristic peaks of Sn2-/S0 and SO42discussed above. After adsorption of AMT, the X-ray photoelectron spectra of Cu 2p3/2 and Cu 2p1/2 signals presented peaks located at 952.01 and 932.09 eV, respectively. The chemical shifts were almost the same as those for AMT-Cu2+ complexes. Furthermore, compared with copper dichloride and the original chalcopyrite, the reduced intensity of the Cu3/2 band of both AMT-Cu2+ complexes and AMT adsorbed chalcopyrite might suggest the possibility of reduction of cupric to cuprous during the reaction of AMT with copper ions52. Unfortunately, we could not provide more detailed information to support this hypothesis with the present investigation. It was reported that the coordination mode of the ligand with metal ions not only depended on the pH and metal/ligand ratio but was also affected by the temperature52, 62

. Even for the same functional group, the coordination of ligand with copper (II) has

been reported to form different complexes with different valence states. Cingi found that 4-amino-3-methyl-1,2,4-∆2-triazoline-5-thione was coordinated with cupric to form cuprous complexes in water solution62, Lanfranchi63 found that this process could be realized by adjusting the solution pH to under 5. However, Clark found that 4-amino-3-methyl-1,2,4-triazole-5-thione was coordinated with copper (II) ions to form a five membered ring Cu(II)(AET)2 complex in ethanol solutions29, which was further confirmed by Kajdan31. Based on these findings and the references cited, one could see that the coordination model of 4-Amino-5-mercapto-1,2,4-triazole with copper(II) ions is very complicated. Therefore, these findings from XPS measurement can only confirm that AMT might chemisorb onto the chalcopyrite surface through formation of AMT-Cu2+ complexes through N (-NH2) and S (-SH or -C=S) atoms. 3.5. ToF-SIMS results Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a highly sensitive analytical technique that could provide chemical characterization of the surfaces of materials. The adsorption mechanisms of some flotation reagents on mineral surfaces have been well investigated by ToF-SIMS spectra54, 64-65. ToF-SIMS spectrometry was used to analyse the variability in surface components of AMT-adsorbed chalcopyrite, and the positive-ion and negative-ion (m/z 0-200) 14


Page 14 of 30

Page 15 of 30

TOF-SIMS were given in Figure 6. The high intensity fragments of all the elements were recorded to elaborate the adsorption mechanism of AMT on the chalcopyrite surface. In the positive-ion spectra, the possible organic, inorganic and complex ions detected in the m/z range of 0-200 were CH3+ (m/z 15), C2H3+ (m/z 27), C2H5+ (m/z 29),

C3H5+

(m/z

63

C 4 H 7 + (m/z 55),

C3H7+

41),

Cu+ (m/z 63),

65

43),

CH3.00x10

5

O-

7.50x104

CNC3H5

C2H3+

5.00x104

C2H5+

Io n in te n sity

Io n in te n s ity

(m/z

Cu + (m/z 65), (63 Cu + )C 2 H4 N4 S(m/z179) and

1.00x105

+

OH-

2.25x105

S-

C-

1.50x105

C3H7+

C2H-

SH-

2.50x104

CH3

7.50x104

+

CN2CHN2-

0.00

0.00 0

10

20

30

40

50

0

M/Z

40

4

65

Cu+

6.0x104

C4H7

50

N(65CuS)CNC-

2.0x104

Io n in ten sity

Io n in te ns ity

30

+

9.0x10

4

20

4

Cu

1.2x105

3.0x10

10

M/Z 2.5x10

63

1.5x105

1.5x10

4

C2N265Cu1.0x104

C2N263Cu-

+

5.0x10

0.0

3

N(63CuS)CNC-

0.0

50

60

70

80

90

100

M/Z

100

110

120

M/Z

130

140

150

4x104

500

C2H4N4S Cu+ 63

M-Cu

300

3x104

Io n in te n sity

400

Io n in ten sity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

C2H4N4S65Cu+ M-Cu

200

2x104

N(63CuS)SCNCM-Cu N(65CuS)SCNCM-Cu

1x104 100

0 140

0 150

160

170

180

190

200

M/Z

150

160

170

M/Z 180

190

200

Figure 6. Positive ions (Left) and negative ions (right) ToF-SIMS spectra of chalcopyrite surface after adsorption of AMT in the operational mode of surface spectroscopy. All the samples were prepared at pH 8.

(65Cu+ )C2H4N4S(m/z 181). The possible negative ions in the m/z range of 0-200 were C-(m/z 12), CH-(m/z 13), O- (m/z 16), OH- (m/z 17), C2H- (m/z 25), CN- (m/z 26), S15


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(m/z 32), SH- (m/z 33), CN2- (m/z 40), CHN2-(m/z 41), C2N2(63Cu)-(m/z 115), C2N2(65Cu) (m/z 117), N (63CuS)CNC-(m/z 147), N (65CuS)CNC-(m/z 149), N (63CuS)SCNC-(m/z 179) and N (65CuS)SCNC-(m/z 181). It is obvious that after AMT adsorption, the surface of chalcopyrite was dominated by organic fragments of AMT and AMT-Cu fragments such as CN2- (m/z 40), CHN2-(m/z 41), C2N2(63Cu)

-

(m/z

115), C2N2(65Cu) (m/z 117), N (63CuS)CNC-(m/z 147) and N (65CuS)CNC-(m/z 149). However, the fragments of AMT-Fe were not presented in the spectra, indicating that Fe was not the active site for the adsorption of AMT, which was consistent with the X-ray photoelectron spectra of the Fe2p signal of the AMT-adsorbed chalcopyrite surface. More interestingly, all the fragments of AMT-Cu2+ complexes, C2H4N4S (63Cu+ 179), C2H4N4S (65Cu+ 181), N (63CuS) SCNC-(m/z 179) and N (65CuS) SCNC-(m/z 181), have been detected on the AMT-adsorbed chalcopyrite surface, which adequately demonstrated that AMT was adsorbed on chalcopyrite and formed AMT-Cu complexes, consistent with the findings from the XPS introduced in the previous section.

3.6. First principle Calculations 3.6.1. Periodic model in the vacuum The newly cleaved (001) surface structures of chalcopyrite and molybdenite was shown in Figure 7 (a) and (c), and their further optimized structures at PBE-D3 level was presented in Figure 7 (b) and (d).

Figure 7. The (001) surface structures of chalcopyrite before (a) and after (b) optimization at 16


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Langmuir

PBE-D3 theory level and the (001) surface structures of molybdenite before (a) and after (b) optimization at the same theory level.

It should be emphasized that there is no significant cleavage in all directions of chalcopyrite mineral crystal, which could be explained by the bulk structure of chalcopyrite mineral crystal. In the bulk phase of chalcopyrite, every one S atom was coordinated with four metal atoms and every one metal atom has four ligands of S atoms66. The (001) surface was indicated by KLAUBER as the most adequate surface to explain his XPS data under inert conditions55. Other surfaces such as (111), (101), (110), and (112) have been pointed out as relevant for chalcopyrite with a similar structure66-68, chalcopyrite was considered to have no preferential cleavage. Thus, in this work, the typical (001) surface was generated by cleaving Cu-S and Fe-S bonds crossing the same plane in [001] direction, respectively. Thus, the surfaces could be Cu-terminated, Fe-terminated and S-terminated. One could see from Figure 7, the new generated (001) surface experienced a reconstruction to form a more stable surface structure, which was in good agreement with the previously-reported findings69. On the optimized (001) surface, the average bond lengths of Cu-S and Fe-S changed from 2.302 Å to 2.099 Å and 2.257 Å to 2.149 Å, respectively. On the other hand, due to the natural layered structure of molybdenite, its (001) surface is the most common cleavage surface. As shown in Figure 7 (c) and (d), the average bond length of Mo-S before and after optimization are almost the same, just slightly changing from 2.407 Å to 2.413 Å. It suggests that the (001) surface of molybdenite did not experience an obvious reconstruction, and the (001) surface structure of molybdenite is very similar to its bulk structure. Table 2. The calculated adsorption reaction energies (Eads) of the novel reagent AMT at Cu and Fe active sites in (001) surface of chalcopyrite in the vacuum and in the presence of water. The adsorption reaction energies of AMT on (001) surface of molybdenite with and without water, and the adsorption reaction energies of water molecules on (001) surfaces of molybdenite and molybdenite are given as comparison. The corresponding results obtained at PBE theory level without dispersion correction are also listed in the parenthesis for comparison. Unit is kcal/mol. Systems

Eads (Vacuum)

Eads (8H2O)

AMT@CuFeS2-(001) (Fe) -119.1 (-176.6) -60.3 (-25.9) AMT@CuFeS2-(001) (Cu) -225.4 (-284.9) -152.3 (-158.7) AMT@MoS2-(001) -14.1(8.9) -33.7 (-18.7) H2O@CuFeS2-(001) -15.7 (-21.9) α α H2O@MoS2-(001) -1.5 (3.2) α The value is the average adsorption energy of eight water molecules on the (001) surface of chalcopyrite (CuFeS2) or molybdenite (MoS2), the calculation equation of average adsorption 17


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energy is expressed as Eads = 1 [ Esurf +8 H O − ( Esurf + E8 H O )] . 2 2 8

Based on the optimized (001) surface, AMT was placed on to the corresponding active sites and further optimization calculations at PBE-D3 theory were adopted to obtain the adsorption configurations of AMT on (001) surface of chalcopyrite and (001) surface of molybdenite. The optimized adsorption configurations of AMT on Fe

and Cu active sites in chalcopyrite (001) surface, which were denoted as “AMT@CuFeS2-001(Fe)” and “AMT@CuFeS2-001(Cu)”, were presented in Figure 8 (I-b)

and (I-c), respectively. And the optimized adsorption configuration of AMT on the (001) surface of molybdenite, which was expressed as “AMT@MoS2-001”, was displayed in Figure 8 (II-b) On the (001) surface of molybdenite. AMT could adsorb on the Cu or

Fe site in chalcopyrite (001) surface, the adsorption reaction energy was -225.4 or -119.1 kcal/mol as shown in Table 2, respectively. While the adsorption reaction energy of “AMT@MoS2-001” was only -14.1 kcal/mol. The adsorption energy was given by the equation: Eads = Esystem − ( Eslab + E AMT ) , where Esystem is the total energy of the complex system of chalcopyrite (001) slab with AMT, Eslab is energy of (001) slab of chalcopyrite or molybdenite, and EAMT is the corresponding energy of AMT molecule. Because chalcopyrite was considered to have no preferential cleavage and the surface structures of all the crystal faces were similar in the mineral particles after grinding. AMT molecules preferred to adsorb onto the Cu active sites rather than on Fe active sites in the (001)-like surfaces, which could be attributed to the more negative adsorption energy on Cu, smaller than that on Fe by 106.3 kcal/mol, and even dramatically smaller than that on MoS2-001 by 221.3 kcal/mol. As shown in Figure 8 (I-c), the S atom and N atom in the adjacent NH2 group were coordinated with a Cu atom to form a stable five-membered ring. The Cu-S and Cu-N bond lengths of “AMT@CuFeS2-(001) (Cu)” in Figure 8 (I-c) are 2.280 Å and 2.496 Å, which are shorter than Fe-S and Fe-N bond lengths of “AMT@CuFeS2-(001) (Fe)” in Figure 8 (I-b) by 0.110 Å and 0.620 Å, respectively. On the other hand, as displayed in Figure 8 (II-b), AMT could almost not form a chemical bond with a surface atom in MoS2-001 surface. These finding were also well consistent with the results from ToF-SIMS in the last section. Furthermore, it could be easily found that a qualitatively-consistent tendency judgment could be derived from the calculated results (-284.9 < -176.6 < 8.9) at PBE 18


Page 18 of 30

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Langmuir

theory level without dispersion correction. However, the adsorption energy of a given system at PBE is significantly different from that at PBE-D3. Particularly, the adsorption energy for “AMT@MoS2-001” was -14.1 kcal/mol at PBE-D3, which was much more negative than that of 8.9 kcal/mol at PBE. The qualitative inconsistency indicated the importance of dispersion correction.

Figure 8. The optimized (001)- surface structures of chalcopyrite in the absence (I-a) and in the presence of water (I-d), the adsorption configurations of AMT molecule on Cu (I-b) and Fe (I-c) active sites in the absence of water and the corresponding adsorption configurations on Cu (I-e) and Fe (I-f) in the presence of water on the (001)-surface of chalcopyrite. Similarly, (II-a) and (II-c) are the optimized (001)-surface structures of molybdenite in the absence and in the presence of water, respectively. (II-b) and (II-d) represent the adsorption configurations of AMT molecule on (001)-surface of molybdenite in the absence and in the presence of water, respectively.

3.6.2. Periodic model in the presence of water molecules Since water molecules usually play a crucial role in the floatation process, it is obligatory to take the water molecules into account to more accurately describe the adsorption behaviors of AMT on the chalcopyrite (001) surface70-72. In our model, the water layer containing 200 water molecules were placed onto the chalcopyrite (001)) surface (molybdenite (001) surface) at first, after a relaxation of molecular dynamic simulation for two nanoseconds, only the closest 8 water molecules to chalcopyrite (001) surface (molybdenite (001) surface) were kept for the following calculation. Figure 8 (I-d) displayed the optimized chalcopyrite (001) surface with eight adsorbed water molecules. One could easily find the surface Cu-O and Fe-O bonds between the adsorbed water molecules and the surface Cu and Fe atoms with bond lengths of 19


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Page 20 of 30

2.004 Å (Cu-O), 2.045 Å (Fe-O) and 2.162 Å (Fe-O), respectively. In addition, as shown in Figure 8 (I-d), only three water molecules adsorbed on the surface and formed Cu-O (one water) and Fe-O (two waters) bonds, the remaining five water molecules almost did not adsorb on the surface. Because the Cu/Fe has open unsaturated bonding ability, the water molecules are completely-likely to be attached to these active sites, which was supported by the adsorption energy of water molecule on CuFeS2-001 (-15.7 kcal/mol), as listed in Table 3. Besides, Chandler and Walker had reported that the surface hydrophobicity of chalcopyrite should be ascribed to the exposed copper-sulfur (Cu-S)-rich and S-rich layer resulted from the oxidation of chalcopyrite surface.73-74 This issue has not been included in this work and would be further considered in future. In comparison to Figure 8 (I-a), it is obvious that the chalcopyrite (001) surface experienced a new reconstruction due to the adsorbed water molecules.

Based on

the optimized (001) surface in presence of water molecules, AMT was placed on to the corresponding active sites (Cu or Fe atoms) and further structure optimizations were performed to obtain the adsorption configurations of AMT on chalcopyrite (001) surface in presence of water molecules. The optimized adsorption configurations were exhibited in Figure 8 (I-e) and (I-f). On the chalcopyrite (001) surface, as shown in Table 2, the adsorption energies of AMT on the Cu and Fe sites were -152.3 and -60.3 kcal/mol, respectively, which were much more positive due to the hydration effect in comparison to the case without water molecules. On the other hand, as demonstrated in Figure 8 (II-c), water molecules almost not adsorb on the MoS2-(001) surface, the calculated adsorption energy of -1.5 kcal/mol supported this point. AMT also could not form a chemical bond with a surface atom in MoS2-(001) surface in presence of water molecules. The calculated adsorption energy of -33.7 kcal/mol should be attributed to the interaction of AMT with water molecules and the dispersion effect between AMT and MoS2 -(001) surface. The adsorption energy of AMT on Cu is more negative than that on Fe by 92.0 kcal/mol, which approximates to the adsorption energy of 106.3 kcal/mol in the absence of water molecules. These results have indicated that solvation effect had a major impact on the adsorption energy of AMT on chalcopyrite (001) surface, but subtle influence on the adsorption selectivity of AMT on the surface. It further confirmed the finding of previous subsection that the AMT molecules preferred to adsorb onto the Cu active sites rather than on Fe active sites in the (001)-like surfaces. 20


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Langmuir

The

adsorption

(

energy

was

calculated

by

the

equation:

)

Eads = Etotal system − Eslab+8 H2O + EAMT , where Etotal system is the total energy of the complex system of surface slab with AMT in presence of eight water molecules,

Eslab +8 H 2O is energy of surface slab with eight water molecules and EAMT is the corresponding energy of AMT molecule. Rigorously, the selective adsorption phenomenon could be further explained by the statistic theory named as Boltzmann distribution (also called Gibbs distribution75. The principle of Boltzmann distribution is that the probability of a certain state is expressed as a function with respect to state’s energy and temperature of the system to which the distribution is applied.76 The resulted ratio of probabilities for state I (AMT adsorbed on Fe atom) and J (AMT adsorbed on Cu atom) is presented as pI = e( EJ − EI )/ RT = e−169.6 ≈ 0 (The resulted ratio of probabilities for state K (AMT pJ adsorbed on MoS2-001) and J (AMT adsorbed on Cu atom) is expressed as pK = e( EJ − EK )/ RT = e −218.6 ≈ 0 ), which well supported the observed phenomenon that pJ AMT-Fe fragments (AMT@MoS2-(001) fragments) were absent on AMT-adsorbed chalcopyrite surface in ToF-SIMS measurement.

3.6.3. Cluster model The simplified cluster model AMT-Cu2+ and AMT-Fe2+ were built to study the adsorption reaction energies of the novel reagent AMT with Cu2+ and Fe2+. Figure 9 displayed the optimized geometry structures of AMT-Cu2+ and AMT-Fe2+ complexes at UB3LYP/Def2TZVP level in gas phase and liquid phase with PCM solvation model. In the gas phase, the bond lengths of Cu-S and Cu-N for AMT-Cu2+ were 2.270 Å and 2.245 Å, respectively, which became shorter in liquid phase where the bond lengths were 2.244 and 2.029, respectively. For AMT-Fe2+, the bond lengths of Fe-S and Fe-N were 2.137 Å and 2.030 Å, while in liquid phase, the corresponding bond lengths became 2.273 Å and 2.025 Å, respectively. The significant difference of coordination bond length in gas phase from that in liquid phase (for Cu-N chemical bond of AMT-Cu2+ or Fe-S chemical bond of AMT-Fe2+) indicated that solvation effect would play an important role in the coordination reaction of AMT with Cu2+ or Fe2+. 21


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Page 22 of 30

Table 3. The calculated coordination reaction energies of the novel reagent AMT with Cu2+ and Fe2+ using UB3LYP functional and Def2-TZVP basis set in gas phase and aqueous solution with PCM solvation model. Here, G is thermal Free Energies, Zero-point energy correction is included in GZPE; Temperature is 298.15k with 1 atm pressure. Liquid

Gas

Reactions AMT+Cu2+ AMT+Fe2+

∆G

∆GZPE

∆G

∆GZPE

-259.9 -223.8

-258.2 -221.2

-107.7 -43

-105.9 -40.7

As shown in Table 3, the coordination reaction energy in thermodynamic function G (Gibbs free energy) were significantly negative, which indicated that the coordination reaction of the novel reagent AMT with Cu2+ and Fe2+ were much favorable in thermodynamics. It should be emphasized that the present coordination energy (corresponding to the adsorption energy of periodic model) is given by the

(

)

equation: ∆Gcoor = Gcomplex − GCu2+ / Fe2+ + GAMT , where Gcomplex is the total Gibbs energy of the complex system of AMT with Cu2+ or Fe2+, GCu 2+ / Fe2+ is Gibbs energy of Cu2+ or Fe2+ and GAMT is the Gibbs energy of AMT molecule. Furthermore, the Gibbs free energy difference ( ∆Gcoor , coordination reaction energy) of AMT with Cu2+ was less than that with Fe2+ by about 37.0 kcal/mol at UB3LYP/Def2-TZVP theory level in the gas phase, the corresponding value was 65.2 kcal/mol in the liquid phase at same theory level, which also confirmed that the solvation effect played a crucial role in the coordination of Cu2+ or Fe2+ with AMT. One could expect that AMT would prefer to adsorb onto Cu2+ rather than Fe2+ due to the more negative coordination energy of AMT with Cu2+. The calculated results of simplified cluster models well supported the results obtained by periodic slab model. The consistent results of the simplified cluster model and periodic model demonstrated that the simplified cluster models were good approximations for the adsorption of AMT on chalcopyrite (001) surface, which was also in good agreement with the ToF-SIMS results.

22


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Langmuir

Figure 9. The optimized geometry structures of AMT-Cu2+ and AMT-Fe2+ complexes at UB3LYP/Def-2TZVP level in gas phase are displayed in (a) and (c), and the corresponding optimized geometry structures in liquid phase with PCM solvation model are shown in (b) and (d), respectively.

3.7. Bench scale flotation tests As stated in the above sections, AMT has been shown to adsorb on the chalcopyrite surface thus making the surface change from hydrophobic to hydrophilic. However, the pure mineral tests could not demonstrate the competitive adsorption of AMT on the two minerals. In the current investigation, selectivity index (SI) analysis was applied to chalcopyrite and molybdenite flotation separation, which is the most important challenge in processing copper-molybdenum sulphide ores. Figure 10 showed the recovery of chalcopyrite and molybdenite as a function of flotation time in the presence and absence of a depressant. It is easy to see that the model fits the experimental data very well. The parameters obtained from the model were summarized in Table 4, including rate constant, R∞, modified rate constant and selectivity index of Mo/Cu. As shown in Table 4, the calculated Mo/Cu selectivity index in the absence of a depressant was only 1.45, while it was 5.19 in the presence of AMT. It is clear that Mo/Cu selectivity in the presence of AMT was much better than that without AMT (blank test) due to the much-reduced chalcopyrite flotation rate. The difference in the Mo/Cu selectivity index greatly contributes to the difference in concentrate grade since the hydrophobicity of chalcopyrite has been modified to be hydrophilic due to the adsorption of AMT molecules on the chalcopyrite surface. Meanwhile, the recovery of molybdenite was not affected by the addition of depressant, indicating that AMT had no obvious effect on modifying the hydrophobic properties of molybdenite surface. These results demonstrated that, on 23


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Page 24 of 30

the bench scale, the selective index improved significantly while using AMT as a chalcopyrite depressant in Cu-Mo flotation separation.

Figure 10. Recovery vs. flotation time with and without AMT (pH=8).

Table 4. Model parameters of the model fitted Parameters R∞ K KM S.I. (Mo/Cu)

Control test

AMT

Molybdenite

Chalcopyrite

Molybdenite

Chalcopyrite

78.8 0.96 0.76

77.84 0.67 0.52

78.65 1.05 0.83

25.89 0.6 0.16

1.45

5.19

4. Conclusions In this investigation, a novel compound 4-Amino-5-mercapto-1,2,4-triazole was synthesized and its adsorption mechanism on chalcopyrite was systematically investigated by UV, zeta potential, FTIR, XPS and ToF-SIMS measurements and first principle calculations. The results of UV-vis spectra, zeta potential and FTIR demonstrated that AMT might chemisorb on chalcopyrite through N and S atoms to form a five-membered chelate ring with the copper atom, which was also supported by XPS measurements. The fragments of C2N2(65Cu)-(m/z 117), C2N2S(63Cu)-(m/z 147), C2N2S(65Cu)-(m/z 149)，C2N2S2(63Cu)- (m/z 179) and C2N2S2(65Cu)- (m/z 181), in the negative-ion ToF-SIMS of chalcopyrite, clearly showed that the adsorbed AMT was coordinated with copper ions rather than iron ions, which agreed with the results of XPS. The first principle periodic computational investigations in the presence of water molecules further suggested that the adsorption reaction energy for AMT on Cu 24


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was more favorable than that on Fe on the (001) surface, which was further confirmed by the simplified cluster models at a higher accuracy level (UB3LYP/Def2-TZVP). This study is the first comprehensive investigation of the hydrophobicity regulation onto the chalcopyrite surface using a novel reagent AMT based on a combined experimental and computational study. The bench scale flotation results strongly suggested that AMT could become a promising candidate to depress chalcopyrite in Cu-Mo flotation practice.

Acknowledgements This work was supported by Natural Science Foundation of China (No. 51704330; No. 51374247), the National 111 Project (No. B14034), the Collaborative Innovation Center for Clean and Efficient Utilization of Strategic Metal Mineral Resources; the Innovation Driven Plan of Central South University (No. 2015CX005); the National Science and Technology Support Project of China, and the Startup Fund of Central South University for Young Teachers (502044001). This work was carried out in part using hardware and/or software provided by a Tianhe II supercomputer at the National Supercomputing Center in Guangzhou, and the High-Performance Computing Centers of Central South University and Nanjing University. The staff from the Supercomputing Center and High-Performance Computing Centers and the engineers from Beijing Paratera Technology Co., Ltd. provided effective support, made the computation progress smooth, and they thus deserve our sincere gratitude.

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Insights into the selective adsorption mechanism of a novel flotation

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