Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and magnesite

PTEC-14827; No of Pages 9 Powder Technology xxx (2019) xxx

Contents lists available at ScienceDirect

Powder Technology journal homepage: www.elsevier.com/locate/powtec

Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and magnesite Wenbao Liu, Xiangyu Peng ⁎, Wengang Liu ⁎, Xinyang Wang, Qiang Zhao, Benying Wang School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China

a r t i c l e

i n f o

Article history: Received 5 August 2019 Received in revised form 10 October 2019 Accepted 12 October 2019 Available online xxxx Keywords: Substituent mechanism Magnesite Quartz Steric hindrance Electrostatic attraction

a b s t r a c t The flotation performances of magnesite and quartz were investigated with dodecylamine (DDA), N-dodecyliso-propanolamine (DIPA, containing one iso-propanol substituent group), or N,N-dodecyl-diiso-propanolamine (DDIPA, containing two iso-propanol substituent groups). Different flotation behaviors of the three collectors were obtained and compared. Fourier transform infrared spectroscopy (FTIR) analysis shows that the electrostatic attraction and hydrogen bonding were of crucial importance in the adsorption of three cationic collectors on mineral surfaces. Effect behavior and mechanism of the introducing iso-propanol substituent on amine collectors were investigated by surface tension measurements, molecular dynamics simulation (MDS), and density functional theory. These results show that the introduction of iso-propanol substituents into DDA could weaken the electrical properties of polar groups and enhance the cross-sectional size of polar groups, which can weaken the electrostatic effect and enhance the spatial position barrier effect, thereby enhancing the selectivity of DIPA and DDIPA. Meanwhile, the increasing hydroxyl (O) active site number enhanced the performance of the collector by affecting the strength of hydrogen bonding. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Magnesite (MgCO3), as a typical carbonate hydrophilic mineral, is the essential raw material for highly active magnesium and its compounds. These products are widely used in alloys, refractories, aerospace, catalysts, pharmaceuticals, food, and other fields [1]. Magnesite resources are abundant in China, mainly concentrated in eastern Liaoning Province [2]. Over the past decades, with extensive exploitation and unreasonable utilization of magnesite resources, low-grade magnesite being rich in impurities of silicon, iron, and calcium is more and more abundant [3]. Meanwhile, on account of the inability to satisfy the needs of industrial production, a large number of low-grade magnesite ore have been discarded or accumulated [4,5]. Therefore, efficient and comprehensive utilization of low-grade magnesite has been a new research hotspot [4]. Flotation is one of the effective methods for separating and purifying low-grade complex minerals [6]. It is frequently utilized in the separation of magnesite ore with cationic amine surfactants as collectors. The common cationic collector is the amines collector, which contains at least one hydrophobic long alkyl chain and one polar groupcontaining basic nitrogen atom, such as dodecylamine (DDA) [7]. In recent years, with the development of the cationic flotation, the consumption of amine surfactants is increasing year by year. It is estimated that ⁎ Corresponding authors. E-mail address: [email protected] (W. Liu).

about 5500 tons of amine surfactants are used in flotation in Brazil every year [8]. However, these amine collectors are still conventional DDA-based collectors [2]. In the industrial application process of these amine collectors, some shortcomings which worsen the flotation indexes are exposed, such as low collectability, poor selectivity, large cohesive bubbles, and sensitivity to slime [9]. In view of this, large collectors such as quaternary ammonium salt, ether amine, diamine, and polyamine were developed, and their flotation performance (collecting ability or selective ability) was investigated [7,10,11]. Although DDA and monoether amine have been widely used in the reverse flotation of magnesite ore, few studies of new collectors in magnesite flotation are reported. In recent years, benzyl-dodecyl-dimethyl ammonium bromide (BDDAB, Gemini surfactant), bis(2-hydroxyethyl) dodecylamine (BHDA, tertiary amine collector), and N-dodecyl-isopropanolamine (DIPA, secondary amine collector) have been introduced into magnesite flotation successively, and their flotation behaviors and mechanisms have also been studied [4,9,12]. It is well known that flotation behavior has a close relationship with the molecular structure of the collector. Hence, studying the molecular structure of the collector and its influence on the flotation behavior of magnesite ore is helpful for the design and development of the novel magnesite flotation collector. In this paper, two collectors, N-dodecyl-iso-propanolamine (DIPA, containing one iso-propanol substituent group) and N,N-dodecyldiiso-propanolamine (DDIPA, containing two iso-propanol substituent groups), were designed by introducing different numbers of

https://doi.org/10.1016/j.powtec.2019.10.060 0032-5910/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: W. Liu, X. Peng, W. Liu, et al., Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and ..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.060

2

W. Liu et al. / Powder Technology xxx (2019) xxx

Table 1 Chemical compositions of pure minerals (wt%). Sample

SiO2

MgO

CaO

Fe2O3

Al2O3

P2O5

Magnesite Quartz

0.16 99.42

47.18 0.006

0.40 0.004

0.12 b0.01

0.09 0.178

b0.01 b0.01

isopropanol groups into DDA. And then, these iso-propanol amines as the novel collector were used in the reverse flotation of magnesite ore. Flotation behaviors and mechanisms of three collectors (DDA, DIPA, and DDIPA) to magnesite ore were investigated and analyzed by the flotation tests and Fourier transform infrared spectroscopy (FTIR). Effect behavior and mechanism of the iso-propanol substituent on amine collectors were investigated by surface tension measurements, density functional theory (DFT), and molecular dynamics simulation (MDS). 2. Materials and methods 2.1. Minerals and reagents High-purity samples of quartz and magnesite were obtained from Haicheng in Liaoning Province, China. Mineral samples were crushed and grounded to obtain the −0.074 mm fractions. To further purify the samples, the samples of quartz were leached with dilute hydrochloric acid, and those of magnesite were purified by the magnetic separator and shaking table. The compositions of two samples were analyzed by X-ray diffraction spectra (XRD) and X-ray fluorescence (XRF). The results are presented in Table 1. These results suggest that the purity of magnesite and quartz was 99.07% and 99.42%, respectively. Artificial mixed ore was obtained by mixing the samples of magnesite and quartz proportionally, according to the production indexes of magnesite concentrator in Haicheng area. The MgCO3 grade of two artificial mixed ores was 97.0% (high-quality magnesite ore, HQ) and 90.0% (high‑silicon magnesite ore, HS). DIPA and DDIPA were synthesized in the laboratory and used as collectors with purity above 95.0%. All reagents for experiments were of analytical grade and obtained from Sinopharm Chemical Reagent Co., Ltd. Sodium hydroxide or hydrochloric acid was used for pH adjustment. In all experiments, deionized water (DW) was chosen to exclude the influence of different water quality on the experiment. 2.2. Flotation tests The XFGII flotation machine (5–35 g, Jilin Exploration Machinery Plant, China) was used to study the flotation performances of the single minerals and artificial mixed ores with three collectors (DDA, DIPA, and

Fig. 2. Flotation behavior of pure minerals as a function of pH (DDA = 50.0 mg/L, DIPA = 40.0 mg/L, DDIPA = 50.0 mg/L).

DDIPA) [13]. The mineral samples (5.0 g) mixed with DW (30.0 mL) were poured in a cell volume (40.0 mL) and agitated for 2.0 min at 1620.0 r/min. Then, pH regulators were added into the pulp to regulate the pH values. After agitating for 2.0 min, the pulp added the collector was agitated for another 2.0 min. After artificial scraping for 5.0 min, the obtained products of concentrates and tailings were separately dried, weighted and chemically assayed to calculate recoveries. Each experiment was conducted three times (experimental error 2.0%) at 25 ± 2 °C. And the average of the results was presented in the paper. 2.3. Surface tension measurements Surface tension measurements of three collectors were performed in the JK99B type automatic tensiometer, which was purchased from Shanghai Zhongchen Digital Technic Apparatus Co., Ltd. The platinum plate method was employed in the measurement at 25 ± 2 °C. DW was not only used to prepare the different concentration solutions of three collectors, but also was often used to calibrate the tensiometer. Each value of the surface tension measurements was obtained by the average value of at least three effective measurements (experimental error 2.0%) [14]. 2.4. FTIR spectra analysis A Nicolet 740 FTIR spectrometer was used to measure the FTIR spectrometer of the samples. The measurements with KBr disc pellet method were conducted at 25.0 ± 2.0 °C. 1.0 g of −5.0 μm pure samples and 30.0 mL DW with or without three collectors were agitated for 30 min. Subsequently, the pulp was filtered to obtain solid samples. Then, the samples were washed three times using DW. Finally, it was dried using a vacuum oven at a low temperature (35.0 ± 2.0 °C) [15]. 2.5. Computational simulation

Fig. 1. Flotation behavior of pure minerals as a function of collector concentration.

The molecules and ion structures of DDA, DIPA and DDIPA were built by Gaussian View. Then, the structures in aqueous solutions were optimized and frequency analyzed using DFT/B3LYP/6-311 + G(d,p) basis and IEF-PCM with solvent (water, dielectric constant = 78.36, T = 298 K) [16]. The geometrically optimized molecules corresponding to the global minima were obtained with no imaginary frequencies were observed in the vibrational spectra [17]. To describe the reactivity of collector molecules, the dipole moment, the orbital energies of HOMO, the orbital energies of LUMO, the global electrophilicity index(ω), the absolute chemical hardness(η), the electronic chemical potential(μ), and the atom charge values of three collectors were calculated and analyzed

Please cite this article as: W. Liu, X. Peng, W. Liu, et al., Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and ..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.060

W. Liu et al. / Powder Technology xxx (2019) xxx

3

Table 2 Results of the froth flotation tests. Sample

Collector

Dosage (mg/L)

Yield/%

HQ

DDA DIPA DDIPA DDA DIPA DDIPA

50.00 40.00 50.00 50.00 40.00 50.00

8.05 86.70 95.32 12.50 80.25 84.58

HS

a

MgCO3

Separation Index (SI)a

SiO2

Grade/%

Recovery/%

Grade/%

Recovery/%

98.87 99.02 98.77 96.84 98.61 98.50

8.21 88.51 97.06 13.45 87.93 92.57

0.40 0.13 0.32 3.15 0.51 0.60

1.05 4.93 10.08 3.23 5.55 7.02

2.90 12.19 17.16 2.16 11.13 12.85

Separation index (SI), SI = (ε1 × (100-ε2)/(100-ε1)/ε2)0.5, ε1- MgCO3 recovery in concentrate, ε2- SiO2 recovery in concentrate.

[18]. The obtained structures of DDA, DIPA and DDIPA were introduced into Materials Studio 8.0 software (MS 8.0) and analyzed by the Dmol3 module at the level of local density approximation using PBEsol functions. The electronic eigenstates were expanded by the DNP (double numerical plus polarization) basis set with a global orbital cutoff of 3.7 Å. All core and valence electrons were used in the calculations. The SCF convergence was 1.0 × 10−6 eV/atom. The maximum displacement, the convergence criteria fixed for the maximum force, and energy were 5.0 × 10−3 Å, 2.0 × 10−3 Ha/Å, and 1.0 × 10−5 Ha, respectively. Other parameters of the calculations were derived from the default settings of the software [19]. MDS was often used to study the adsorption of the collector on the minerals. Adsorption models of quartz surface (101) and magnesite surface (101) with 2000 water molecules and a vacuum slab of 60 Å in thickness were constructed [20,21]. Three collectors mainly existed with ionic forms basing on the solution chemical calculation, and fifteen of them were introduced into the mineral-water system (quartz model (29.20 Å × 29.46 Å × 76.36 Å) or magnesite model (28.39 Å × 27.82 Å × 87.54 Å)) to study the adsorption process of three collectors on quartz and magnesite surfaces [9,21]. The overall neutral system of the models was achieved by adding some Cl− or Na+ ions. All the built simulation systems were preliminarily optimized using geometry optimization with a smart algorithm method in periodic boundary conditions. MDS was performed using a Nosé thermostat at NVT (constant-volumetemperature) ensemble in COMPASS II force field. The equations of motion were integrated with a time step of 1.0 fs. The Ewald summation method and the atom-based method were employed to account for the long-range electrostatic interactions and van der Waals interactions, respectively. After 0.5 ns, the sufficient equilibrium of the models was achieved. Subsequently, the further calculation time of 1.5 ns was used to obtain the optimal adsorption configuration of three collectors and mineral. The optimal adsorption configurations obtained by MDS were used to analyze the adsorption characteristics of three collectors and mineral. The interaction energy (ΔE) of three collectors and mineral

Fig. 3. Surface tension of DDA, DIPA, and DDIPA.

was calculated by the formula 1. ΔE ¼ Etotal −E mineral −Ereagent

ð1Þ

In Formula 1, Etotal, Emineral, and Eraegent were regarded as the energy of the optimal adsorption configuration, the mineral surface model, and the collector, respectively. A lower negative of ΔE suggests the more stable adsorption between the collector and the minerals [9]. 3. Results and discussion 3.1. Flotation tests Flotation behaviors of quartz and magnesite conditioned with various concentrations of three collectors in the natural pH (about 7.2) are provided in Fig. 1. It shows that the recoveries of quartz conditioned with three collectors were very high and had little difference as the concentration of three collectors increases from 20.0 to 70.0 mg/L. At 20.0 mg/L the collector, the recovery of quartz was above 90.0% and increased slowly as the concentration increased. No significant change in the floatability was observed and the flotation recovery of quartz fluctuated around 96.0% at higher concentrations. It indicates that three Table 3 The values of various surface parameters. Samples DDA DIPA DDIPA

CMC/mol·L−1 0.0105 0.0002 0.0005

γCMC/mN·m−1 25.88 25.71 25.19

ΓCMC/mol·m−2 −7

1.53 × 10 1.91 × 10−7 2.28 × 10−7

Amin/nm2 10.83 8.72 7.30

Fig. 4. FTIR spectra of quartz untreated and treated with the collector.

Please cite this article as: W. Liu, X. Peng, W. Liu, et al., Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and ..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.060

4

W. Liu et al. / Powder Technology xxx (2019) xxx

Fig. 5. FTIR spectra of magnesite untreated and treated with the collector.

collectors had high collecting ability to quartz. However, there was a significant difference in the recovery of magnesite conditioned with three different collectors. The recovery of magnesite with DDA increased gradually with increasing DDA concentration. When the DDA concentration was above 50.0 mg/L, the recovery fluctuated around 80.0%. The recovery of magnesite increased slowly as DIPA concentration increasing. It was under 15.0% in the whole studied range. Moreover, the recovery of magnesite with DDIPA was under 5.0%. The collecting ability of magnesite was in the sequence of DDA N DIPA N DDIPA, and DIPA and DDIPA exhibited little collecting ability to magnesite. Concluded from the results, the introduction iso-propanol substituents into DDA had little effect on quartz recovery, it could significantly reduce magnesite recovery with the introducing iso-propanol substituent numbers increasing. The flotation performances of quartz and magnesite as a function of the pulp pH at the optimum collector concentration (DDA = 50.0 mg/L, DIPA = 40.0 mg/L, DDIPA = 50.0 mg/L) are shown in Fig. 2. It is shown that the recovery-pH curves of quartz using three collectors had similar variations. With the increase of pH values, the quartz recovery quickly increased (pH b 4) and attained a maximum value of about 96.0%, and

then decreased sharply (pH N 10). However, when the pulp pH value was above 10.0, the recovery reduction speed of DIPA and DDIPA was significantly faster than that of DDA. This phenomenon could be attributed to the existing state of the collector in the solution [4]. It also demonstrates that the recovery-pH curves of magnesite with three collectors had significant variation. As the pH increased, the recovery of magnesite with DDA slowly increased to 82.39%, and then reduced when the pH value was above about 9.5. When DIPA was used as the collector, as the pH increased, the recovery of magnesite slowly increased to a maximum of about 10.0%, and then decreased. However, when DDIPA was used as the collector, the recovery of magnesite tardily fluctuated and was under 4.0%. It could be inferred that flotation behavior of three collectors to quartz and magnesite existed obvious differences. Without depressant, the range pH values from 4.1 to 9.5 were regarded as the optimal pH range to the flotation separation of magnesite and quartz using DIPA or DDIPA as the collector. While DDA used as the collector to realize the separation of quartz from magnesite needed to add a suitable depressant. It also can be concluded that increasing the introducing isopropanol substituent number into DDA would increase its sensitivity to pH, especially in the strong alkaline conditions. When the pulp pH was above 11.32, the recovery of quartz with DIPA and DDIPA reduced to 48.32% and 1.31%, respectively, which were much lower than that with DDA (68.86%). To study the separation efficiency of three collectors to magnesite and quartz, the flotation of two typical artificial mixed minerals was performed. The results are displayed in Table 2. When the optimal dosage of the collector was used, the yield of concentration and the separation index increased as the increase of the introducing isopropanol substituent number into DDA (DDIPANDIPANDDA) for two kinds of the artificially mixed ores [22]. The grades of SiO2 with DIPA and DDIPA as the collector were b0.6%, which was in accordance with the market requirement. The yield of concentration with DDA as the collector was only 8.05%, it reveals that the selectivity of DDA to magnesite ore was extremely weak, and the reasonable depressant should be added in the flotation separation of magnesite ore [9]. When the concentration of the collectors was the same, DDIPA had better desilication and separation efficiency to magnesite ores than DDA. At 40.0 mg/L DIPA as the collector, the grade of MgCO3 was N98.0%, and the recovery was N85.0%. In addition, the removal rate of SiO2 was above 94.0%. Therefore, DIPA and DDIPA, which have high selectivity and good solubility as the collector, have potential application prospects in the utilization of low-grade magnesite ore [7,23]. Meanwhile, the introduction of iso-propanol

Fig. 6. The snapshots of adsorption configuration after equilibrium state of quartz (left) and magnesite (right)with DDA. (The colour representation is as follows: white- hydrogen atoms, gray‑carbon atoms, red‑oxygen atoms, blue‑nitrogen atoms, palegreen‑chlorine atoms, green- magnesium atoms. For clarity, water molecule is hidden.). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: W. Liu, X. Peng, W. Liu, et al., Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and ..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.060

W. Liu et al. / Powder Technology xxx (2019) xxx

5

Fig. 7. The snapshots of adsorption configuration after equilibrium state of quartz (left) and magnesite (right)with DIPA. (The colour representation is as follows: white- hydrogen atoms, gray‑carbon atoms, red‑oxygen atoms, blue‑nitrogen atoms, palegreen‑chlorine atoms, green‑magnesium atoms. For clarity, water molecule is hidden.). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

substituents into DDA would enhance its selectivity. The more the number of iso-propanol substituents is, the better the selecting ability will be. 3.2. Surface activity study Surface tension measurement is usually considered as an effective method to explain the flotation performance of reagents. The surface activity of three collectors was analyzed by surface tension measurements to study the effect of the isopropanol substituent on its performance. The results are shown in Fig. 3. It can be noted that the surface tension value decreased rapidly with the increase of three collectors concentration, and then reached a stable state corresponding to the critical micelle concentration (CMC). Three collectors' ability to reduce air-water interfacial tension followed the order: DIPAbDDIPAbDDA. The CMCs of DIPA and DDIPA decreased from 0.0105 mol/L (DDA) to 0.0002 mol/L and 0.0005 mol/L, and the corresponding surface tension values decreased from 25.88 mN/m to 25.71 mN/m and 25.19 mN/m,

respectively. It indicates that introducing the isopropanol substituent could improve its activity ability and reduce the dosage of the collector. The saturation adsorption (Γmax) and average minimum area per molecule (Amin) were calculated to evaluate the adsorption amount of the collector on the surface of the mineral, and the results are displayed in Table 3 [24]. From Table 3, it shows that Γmax value followed the order: DDIPANDIPANDDA, and Amin value followed the order: DDIPAbDIPAbDDA. It can be concluded that the increasing number of isopropanol substituent to DDA could enhance the surface activity of reagent and reduced the dosage of reagent. These might be due to that the introduction of different numbers of isopropanol substituent into DDA could enhance the hydrophobic association between the hydrophobic chains of the reagents, and weaken the repulsive force between the ionic head groups [25]. Therefore, the arrangement of surfactant molecules at the air-water interface tended to be closer [24,26]. In summary, DIPA and DDIPA had lower dosage and better hydrophobicity than DDA, and thereby being more easily arranged in the gas/liquid interface to reduce the interfacial free energy [27].

Fig. 8. The snapshot of adsorption configuration after equilibrium state of quartz (left) and magnesite (right)with DDIPA. (The colour representation is as follows: white‑hydrogen atoms, gray‑carbon atoms, red‑oxygen atoms, blue‑nitrogen atoms, palegreen‑chlorine atoms, green‑magnesium atoms. For clarity, water molecule is hidden.). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: W. Liu, X. Peng, W. Liu, et al., Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and ..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.060

6

W. Liu et al. / Powder Technology xxx (2019) xxx

Table 4 The adsorption energy between the three collectors and mineral surfaces. Adsorption energy △E/kcal·mol−1

DDA

DIPA

DDIPA

Quartz(101) Magnesite(101) I1,2a/%

−567.69 −470.13 0.17

−577.25 −222.69 0.62

−560.57 −175.63 0.69

a

Selectivity index-I1,2 = (△Equartz − △Emagnesite)/△Equartz.

3.3. FTIR spectra analysis To study the adsorption behavior and mechanism of three collectors on the surface of minerals, FTIR measurements were performed to obtain the information of valence linkage. The results are presented in Figs. 4–5. Fig. 4 shows that new intense peaks around 2853 and 2924 cm−1 appeared on the quartz spectrum treated with three collectors, corresponding to the stretching vibrations of the \\CH2\\ and \\CH3 groups of the collectors [28]. Meanwhile, no other new peaks appeared, indicating that the adsorption of three collectors on the quartz surface occurred and mainly via electrostatic interaction. In addition, the characteristic quartz peak around 1091 cm−1 (the O-Si-O asymmetrical stretching vibrations), 781 cm−1 (the Si\\O symmetrical stretching vibrations), 694 cm−1 (the Si\\O symmetrical bending vibrations), and 461 cm−1 (the Si\\O\\Si asymmetrical bending vibrations) underwent a slight shift to the low frequency direction, it also suggests that these characteristic groups took part in the adsorption process. This might be due to that hydrogen bonding occurred in the adsorption process [29–31]. Fig. 5 shows that the magnesite spectrum treated with DDA appeared the new intense peaks around 2853 and 2924 cm−1 from DDA. And the characteristic magnesite peak around 1401 cm−1 (the CO2– 3 antisymmetric stretching vibrations) and 885 cm−1 (the CO2– 3 out-ofplane bending vibrations) existed a slightly low-frequency shift. These suggest that the characteristic groups took part in the adsorption process, and the adsorption of DDA on the magnesite surface mainly via electrostatic interaction and hydrogen bonding [32]. While no new peaks or no obvious peak shift occurred in the spectra of magnesite

treated with DIPA and DDIPA. These results confirm that the adsorptions of DIPA and DDIPA on the magnesite surface were negligible. Meanwhile, introducing different numbers of iso-propanol substituent into DDA had a significant effect on its adsorption on magnesite, while it shows little effect on that of quartz. The adsorption of three collectors on the surface of quartz was mainly attributed to the electrostatic interaction and hydrogen bonding [33]. Introducing different numbers of iso-propanol substituents into DDA might affect the electrostatic interaction and hydrogen bonding between the collector and the surface of the mineral. 3.4. Molecular dynamics simulation In order to further study the effect of iso-propanol substituents in DDA on the adsorption properties of quartz and magnesite, the adsorption behaviors of three collectors on the surfaces of quartz and magnesite were studied by MDS in MS 8.0 software. Equilibrium adsorption model is utilized to calculate the adsorption energy [34]. Some snapshots of the equilibrium adsorption models are illustrated in Figs. 6–8, in which the water molecules were removed. From Figs. 6–8, it can be concluded that three collectors could be adsorbed vertically on the negatively charged quartz surface, and formed hydrogen bonds with oxygen atoms exposed on the quartz surface. The strong adsorption between three cationic collectors and quartz could be due to electrostatic attraction and hydrogen bonding [23]. While there were significant differences between the adsorption of three collectors and magnesite surface. These show that three collectors were aggregated in the solution, and the micelles formed by DDIPA were more compact than those of DDA but looser than those of DIPA [9], which was consistent with the results of surface tension analysis. This phenomenon might ascribe to the weaker repulsive force between the ionic head groups in DIPA and DDIPA than DDA [35]. Furthermore, these also show that the aggregation of DDIPA was far from the magnesite surface, while those of DIPA and DDA were near the magnesite surface, and the hydrogen bond existed between DDA and the exposed O on the magnesite surface. Based on the above analyses, it could be concluded that

Fig. 9. Optimized geometries of DDA, DIPA and DDIPA at the B3LYP/6-311 + G (d, p) level. (Blue- N, Red-O, Gray-C, White-H). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article as: W. Liu, X. Peng, W. Liu, et al., Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and ..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.060

W. Liu et al. / Powder Technology xxx (2019) xxx

7

Table 5 Calculated obtained chemical parameters of the DDA, DIPA and DDIPA at B3LYP/6–311 + G (d,p) level in aqueous. Samples

DM/Debye

EHOMO/a.u.

ELUMO/a.u.

η/a.u.

μ/a.u.

ω/a.u.

ET/a.u.

Charges/e

DDA DIPA DDIPA

1.91 3.36 4.99

−0.252 −0.233 −0.221

0.005 0.003 0.001

0.129 0.118 0.111

−0.124 −0.115 −0.110

0.059 0.056 0.055

−528.467 −721.681 −914.894

−0.543 −0.445 −0.394

DM-Dipole moment (Debye); EHOMO-The orbital energies of HOMO (a.u.); EHOMO-The orbital energies of LUMO (a.u.); μ-Electronic chemical potential (a.u.); η-Absolute chemical hardness (a.u.); ω-The global electrophilicity index (a.u.); ET-Total Energy (a.u.).

electrostatic interaction and hydrogen bonding played a role in the adsorption of three collectors on the surface of quartz and magnesite [4]. Meanwhile, weak negatively charged magnesite surface could absorb strong positive charges of collectors, and the strong positive metal cations (Mg2+) on the magnesite surface could adsorb anion species and repel cation species [9]. In order to quantify the adsorption capacity between three collectors and mineral surfaces, the interaction energies were calculated [36]. The results of the interaction energy are shown in Table 4. Table 4 shows that the adsorption energies of three collectors on the surface of quartz were much lower than those of magnesite, and all the adsorption energies were negative. It indicates that the adsorption between three collectors and the surface of quartz was easier than that on the magnesite surface [36,37]. It also shows that the adsorption energies of three collectors on the quartz surface were basically the same, but those on the magnesite surface were distinct from each other and followed the following order: DDA b DIPAbDDIPA. The selectivity index (I1,2) of three collectors followed the following order: DDIPANDIPANDDA [38]. The results suggest that the collection performance of three collectors to magnesite followed the following order: DDA N DIPANDDIPA, and the selectivity order of three collectors to magnesite ore was just opposite (DDIPANDIPANDDA), which was in agreement with the floatation results. Furthermore, it also proves that the introduction of isopropanol substituent into DDA mainly affects the collecting ability to magnesite, and thereby improving the selectivity of collectors. 3.5. The characteristics of the collector DFT is often used to evaluate the chemical reactivity and structural reaction relationship between minerals and collectors, which quantifies the relationship between the structure and performance of the collector [16]. Physicochemical parameters of DDA, DIPA, and DDIPA were studied by DFT with B3LYP/6–311 + G(d,p) level in aqueous [39]. Their optimized molecular structures are displayed in Fig. 9, and the obtained chemical parameters are illustrated in Table 5. Fig. 9 shows that the addition of isopropanol substituent into DDA could increase the number of active sites in the collector molecule, which was usually believed to enhance its collection performance [4,10]. The isopropanol substituent has a strong electronegative O atom which could easily form the hydrogen bonds with the mineral surface [23]. Meanwhile, introducing the isopropanol substituent into DDA would increase the space steric hindrance of the collectors (DDIPANDIPANDDA), which could increase the selectivity of the collectors [40]. Mulliken atomic charge values of N atoms for three collectors in the aqueous phase were negative, suggesting the electron-donating capability followed the order of DDIPAbDIPAbDDA [41]. It also can be seen that the mulliken charge of N atom increased with the increasing number of the isopropanol substituent. From Table 5, the dipole moment of DIPA (3.36) was less than that of DDIPA (4.99) and larger than that of DDA (1.91). The increased the dipole moment of the collector could enhance their electrostatic and van der Waals interactions with mineral surfaces in an aqueous system [42]. The HOMO energies of three collectors increased in the following order: DDIPANDIPANDDA. The higher HOMO energy, the stronger the electron-donating ability. The LUMO energies of three collectors increased in the opposite order, suggesting the electronaccepting ability also followed the order of DDIPANDIPANDDA [41]. In

general, if the metal atoms on the surface of minerals have abundant D-orbital electrons, the LUMO of the collector can accept D-orbital electrons from the HOMO of minerals, thus would form a back-donor bond [42]. In the study, there are no d-orbital electrons in Mg2+ and Si4+ ions. So, the HOMO energies order infer that the adsorption ability also followed the same order (DDIPANDIPANDDA). The absolute chemical hardness (η) value and the global electrophilicity index (ω) followed the order (DDIPAbDIPAbDDA), and electronic chemical potential (μ) followed the order (DDIPANDIPANDDA). It can be known that, with the adding number of the isopropanol substituent, the absolute chemical hardness value decreased, electronic chemical potential value increased. The electrophilic ability could directly react with the electrostatic interaction with mineral surfaces in an aqueous system [43]. Meanwhile, combining the adsorption mechanism of three collectors that electrostatic attraction played a major role in the adsorption of three collectors to quartz and magnesite, these indicate that the collecting ability followed the order: DDIPAbDIPAbDDA, which had differences with the previous results [42]. Three cationic collectors mainly existed in the form of cationic ions in the neutral flotation system. Hence, the frontier orbital eigenvalues and polar space structure of three cationic collectors were calculated in the Dmol3 module. The results are shown in Table 6. Frontier orbital theory deems that the energy difference between HOMO and LUMO is inversely proportional to the degree of the interaction of the collector on the mineral [16]. From Table 6, it can be seen that the calculated △E2 was bigger than that of △E1, however, the value of the energy difference is smaller, the interaction is easier to happen, so the interaction mainly occurred between LUMO of minerals and HOMO of reagents [16]. For quartz and magnesite, △E2 of quartz with three collectors was relatively lower, which shows that quartz had a stronger interaction with those collectors. △E2 of quartz with DIPA+ was smaller than that with DDA+ and bigger than that with DDIPA+. The group charges show that the polar group charge of DIPA+ (0.752 e) was greater than that of DDIPA+ (0.678 e) and smaller than that of DDA+ (0.838 e). Meanwhile, mulliken charge of N atom, polar group cross-section size and height increased with the addition of iso-propanol substituent. Consequently, the nonselective electrostatic interaction strength of reagents with quartz

Table 6 Frontier orbital eigenvalues and polar space structure. Parameter

DDA+

DIPA+

DDIPA+

Quartz

Magnesite

EHOMO/eV ELUMO/eV △E1-quartz/eV △E1-magnesite/eV △E2-quartz/eV △E2-magnesite/eV Charges/e

−0.317 −0.186 0.101 0.126 0.243 0.231 N − 0.298

Groups charges/e PG cross-section size/nm2 PG height/nm

0.838

−0.315 −0.147 0.140 0.165 0.241 0.229 N − 0.351 O − 0.500 0.752

−0.313 −0.114 0.173 0.198 0.239 0.227 N − 0.384 O − 0.491 0.678

−0.287 −0.074 – – – – – – –

−0.312 −0.086 – – – – – – –

0.019

0.088

0.289

–

–

0.085

0.460

0.412

–

–

△E1 = ELUMO(reagent) − EHOMO(mineral), △E2 = ELUMO(mineral) − EHOMO(reagent), PG-Polar group.

Please cite this article as: W. Liu, X. Peng, W. Liu, et al., Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and ..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.060

8

W. Liu et al. / Powder Technology xxx (2019) xxx

followed the following order: DDIPAbDIPAbDDA, and the steric hindrance followed the following order: DDIPANDIPANDDA, which indicate that the selectivity of three collectors followed the same order (DDIPAN DIPANDDA) [4,14]. These results agreed with the flotation tests and the results of MDS. Thus, introducing the number of the iso-propanol substituent into DDA could weaken the polar group charge and increase the geometric configuration of the polar group. From the above, with increasing the number of introducing iso-propanol substituent into DDA, the electrostatic interaction strength of the collectors to minerals gradually decreased and its steric hindrance gradually increased, meanwhile, the number of hydroxyl (O) active sites also increased and affected the strength of hydrogen bonding, these eventually lead to the selectivity of the collectors increase. 4. Conclusion Given the flotation tests, FTIR analyses, surface tension measurements, MDS, and DFT analyses, the following conclusions can be obtained: (1) The magnesite ores could be separated at pH 4.1–9.5 with 40.00 mg/L DIPA or 50.00 mg/L DDIPA without depressant, compare to DDA. Introducing the isopropanol substituent group into DDA could affect the collecting ability, and strengthen the selectivity of the collector (DDIPANDIPANDDA). The same rules were further confirmed by MDS. (2) Surface tension tests showed that introducing the different number of isopropanol substituent groups into DDA could enhance its hydrophobic ability and reduce the dosage of reagent. (3) The adsorption of three collectors on mineral was mainly attributed to the electrostatic interaction and hydrogen bonding. Introducing the isopropanol substituent group into DDA could weaken the electrical properties of polar groups and enhance the geometric configuration size of the polar group, which could weaken the electrostatic effect but enhance the spatial position barrier effect, thereby enhancing the selectivity. The effect of increasing hydrogen bond formation sites number was also considered. Declaration of Competing Interest None. Acknowledgments The authors thank the National Natural Science Foundation of China (No.51874074, 51874168), the open fund of Guangdong Provincial Key Laboratory of Development and Comprehensive Utilization of Mineral Resources (2017B030314046), and the young and middle-aged science and technology innovation talent support program of Shenyang (RC170556) for the financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.powtec.2019.10.060. References [1] A.N. Santana, A.E.C. Peres, Reverse magnesite flotation, Miner. Eng. 14 (2001) 107–111. [2] J. Yao, W.Z. Yin, E.P. Gong, Depressing effect of fine hydrophilic particles on magnesite reverse flotation, Int. J. Miner. Process. 149 (2016) 84–93. [3] S.G. Ozkan, Beneficiation of magnesite slimes with ultrasonic treatment, Miner. Eng. 15 (2002) 99–101. [4] W.B. Liu, W.G. Liu, S.J. Dai, T. Yang, Z. Li, P. Fang, Enhancing the purity of magnesite ore powder using an ethanolamine-based collector: insights from experiment and theory, J. Mol. Liq. 268 (2018) 215–222.

[5] N. Luo, D.Z. Wei, S.Y. Bai, C. Han, C.E. Zhang, Elimination of the adverse effect of calcium ion on the flotation separation of magnesite from dolomite, Minerals 7 (2017). [6] L.H. Xu, J. Tian, H.Q. Wu, Z.Y. Lu, Y.H. Yang, W. Sun, Y.H. Hu, Effect of Pb2+ ions on ilmenite flotation and adsorption of benzohydroxamic acid as a collector, Appl. Surf. Sci. 425 (2017) 796–802. [7] W.B. Liu, W.G. Liu, D.Z. Wei, M.Y. Li, Q. Zhao, S.C. Xu, Synthesis of N,N-Bis(2-hydroxypropyl)laurylamine and its flotation on quartz, Chem. Eng. J. 309 (2017) 63–69. [8] J. Kozeny, Über die kapillare Leitung des Wassers im Boden (Aufstieg Versickerung und Anwendung auf die Bewässerung): Sitz. Ber. Akad. Wiss. Wien, Math. Nat. (Abt. IIa) 136a (1927) 271–306. [9] W.B. Liu, W.G. Liu, B. Zhao, L. Zhao, D. Li, P. Fang, W. Liu, Novel insights into the adsorption mechanism of the isopropanol amine collector on magnesite ore: a combined experimental and theoretical computational study, Powder Technol. 343 (2019) 366–374. [10] H. Duan, W.G. Liu, X.Y. Wang, W.B. Liu, X.R. Zhang, Effect of secondary amino on the adsorption of N-Dodecylethylenediamine on quartz surface: a molecular dynamics study, Powder Technol. 351 (2019) 46–53. [11] A.S. Patra, N.V.R. Nulakani, Y. Pavan Kumar, V. Subramanian, J. Dash, A.K. Mukherjee, Design and synthesis of novel polyamine collector to recover iron values from iron ore slimes, Powder Technol. 325 (2018) 180–191. [12] I. Brezáni, J. Škvarla, M. Sisol, Reverse froth flotation of magnesite ore by using (124-12) cationic gemini surfactant, Miner. Eng. 110 (2017) 65–68. [13] W.B. Liu, W.G. Liu, B.Y. Wang, H. Duan, X.Y. Peng, X.D. Chen, Q. Zhao, Novel hydroxy polyamine surfactant N-(2-hydroxyethyl)-N-dodecyl-ethanediamine: its synthesis and flotation performance study to quartz, Miner. Eng. 142 (2019). [14] C. Wang, X.L. Cao, L.L. Guo, Z.C. Xu, L. Zhang, Q.T. Gong, L. Zhang, S. Zhao, Effect of adsorption of catanionic surfactant mixtures on wettability of quartz surface, Colloids Surf. A Physicochem. Eng. Asp. 509 (2016) 564–573. [15] H. Wu, J. Tian, L. Xu, S. Fang, Z. Zhang, R. Chi, Flotation and adsorption of a new mixed anionic/cationic collector in the spodumene-feldspar system, Miner. Eng. 127 (2018) 42–47. [16] X.R. Zhang, Z.B. Qian, G.B. Zheng, Y.G. Zhu, W.G. Wu, The design of a macromolecular depressant for galena based on DFT studies and its application, Miner. Eng. 112 (2017) 50–56. [17] G.Y. Liu, X.L. Yang, H. Zhong, Molecular design of flotation collectors: a recent progress, Adv. Colloid Interf. Sci. 246 (2017) 181–195. [18] X.L. Yang, B. Albijanic, G.Y. Liu, Y. Zhou, Structure–activity relationship of xanthates with different hydrophobic groups in the flotation of pyrite, Miner. Eng. 125 (2018) 155–164. [19] D. Azizi, F. Larachi, Surface interactions and flotation behavior of calcite, dolomite and ankerite with alkyl hydroxamic acid bearing collector and sodium silicate, Colloids Surf. A Physicochem. Eng. Asp. 537 (2018) 126–138. [20] H. Zhang, W.G. Liu, C. Han, D.Z. Wei, Intensify dodecylamine adsorption on magnesite and dolomite surfaces by monohydric alcohols, Appl. Surf. Sci. 444 (2018) 729–738. [21] W.B. Liu, W.G. Liu, B.Y. Wang, Q. Zhao, H. Duan, X.D. Chen, Molecular-level insights into the adsorption of a hydroxy-containing tertiary amine collector on the surface of magnesite ore, Powder Technol. 355 (2019) 700–707. [22] M. Xu, Modified flotation rate constant and selectivity index, Miner. Eng. 11 (1998) 271–278. [23] W.G. Liu, W.B. Liu, X.Y. Wang, D.Z. Wei, B.Y. Wang, Utilization of novel surfactant Ndodecyl-isopropanolamine as collector for efficient separation of quartz from hematite, Sep. Purif. Technol. 162 (2016) 188–194. [24] X. Huang, K. Huang, Y. Jia, S. Wang, Z. Cao, H. Zhong, Investigating the selectivity of a xanthate derivative for the flotation separation of chalcopyrite from pyrite, Chem. Eng. Sci. 205 (2019) 220–229. [25] L. Xu, Y. Hu, J. Tian, H. Wu, L. Wang, Y. Yang, Z. Wang, Synergistic effect of mixed cationic/anionic collectors on flotation and adsorption of muscovite, Colloids Surf. A Physicochem. Eng. Asp. 492 (2016) 181–189. [26] C. Li, Z. Gao, Tune surface physicochemical property of fluorite particles by regulating the exposure degree of crystal surfaces, Miner. Eng. 128 (2018) 123–132. [27] A. Cornellas, L. Perez, F. Comelles, I. Ribosa, A. Manresa, M.T. Garcia, Self-aggregation and antimicrobial activity of imidazolium and pyridinium based ionic liquids in aqueous solution, J. Colloid Interface Sci. 355 (2011) 164–171. [28] S. Fang, L. Xu, H. Wu, K. Shu, Y. Xu, Z. Zhang, R. Chi, W. Sun, Comparative studies of flotation and adsorption of Pb(II)/benzohydroxamic acid collector complexes on ilmenite and titanaugite, Powder Technol. 345 (2019) 35–42. [29] X.B. Li, Q. Zhang, B. Hou, J.J. Ye, S. Mao, X.H. Li, Flotation separation of quartz from collophane using an amine collector and its adsorption mechanisms, Powder Technol. 318 (2017) 224–229. [30] R.M.F. Lima, P.R.G. Brandao, A.E.C. Peres, The infrared spectra of amine collectors used in the flotation of iron ores, Miner. Eng. 18 (2005) 267–273. [31] W.G. Liu, W.B. Liu, S.J. Dai, B.Y. Wang, Adsorption of bis(2-hydroxy-3-chloropropyl) dodecylamine on quartz surface and its implication on flotation, Res. Phys. 9 (2018) 1096–1101. [32] Q. Teng, Y.L. Feng, H.R. Li, Effects of silicate-bacteria pretreatment on desiliconization of magnesite by reverse flotation, Colloids Surf. A Physicochem. Eng. Asp. 544 (2018) 60–67. [33] R. Atkin, V.S.J. Craig, E.J. Wanless, S. Biggs, Mechanism of cationic surfactant adsorption at the solid–aqueous interface, Adv. Colloid Interf. Sci. 103 (2003) 219–304. [34] X. You, M. He, W. Zhang, H. Wei, X. Lyu, Q. He, L. Li, Molecular dynamics simulations of nonylphenol ethoxylate on the hatcher model of subbituminous coal surface, Powder Technol. 332 (2018) 323–330.

Please cite this article as: W. Liu, X. Peng, W. Liu, et al., Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and ..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.060

W. Liu et al. / Powder Technology xxx (2019) xxx [35] Z.Q. Huang, H. Zhong, S. Wang, L.Y. Xia, W.B. Zou, G.Y. Liu, Investigations on reverse cationic flotation of iron ore by using a Gemini surfactant: ethane-1,2-bis(dimethyldodecyl-ammonium bromide), Chem. Eng. J. 257 (2014) 218–228. [36] L.X. Li, H.Q. Hao, Z.T. Yuan, J.T. Liu, Molecular dynamics simulation of sideritehematite-quartz flotation with sodium oleate, Appl. Surf. Sci. 419 (2017) 557–563. [37] H.Q. Hao, L.X. Li, Z.T. Yuan, J.T. Liu, Molecular arrangement of starch, Ca2+ and oleate ions in the siderite-hematite-quartz flotation system, J. Mol. Liq. 254 (2018) 349–356. [38] G.Y. Liu, H. Zhong, T.G. Dai, L.Y. Xia, Investigation of the effect of N-substituents on performance of thionocarbamates as selective collectors for copper sulfides by ab initio calculations, Miner. Eng. 21 (2008) 1050–1054. [39] X. Wang, W. Liu, H. Duan, B. Wang, C. Han, D. Wei, The adsorption mechanism of calcium ion on quartz (101) surface: a DFT study, Powder Technol. 329 (2018) 158–166.

9

[40] C.L. Peng, Y.H. Zhong, F.F. Min, Adsorption of alkylamine cations on montmorillonite (001) surface: a density functional theory study, Appl. Clay Sci. 152 (2018) 249–258. [41] X. Yang, S. Liu, G. Liu, H. Zhong, A DFT study on the structure–reactivity relationship of aliphatic oxime derivatives as copper chelating agents and malachite flotation collectors, J. Ind. Eng. Chem. 46 (2017) 404–415. [42] L.Q. Deng, G. Zhao, H. Zhong, S. Wang, G.Y. Liu, Investigation on the selectivity of N((hydroxyamino)-alkyl) alkylamide surfactants for scheelite/calcite flotation separation, J. Ind. Eng. Chem. 33 (2016) 131–141. [43] G. Liu, J. Xiao, D. Zhou, H. Zhong, P. Choi, Z. Xu, A DFT study on the structurereactivity relationship of thiophosphorus acids as flotation collectors with sulfide minerals: implication of surface adsorption, Colloids Surf. A Physicochem. Eng. Asp. 434 (2013) 243–252.

Please cite this article as: W. Liu, X. Peng, W. Liu, et al., Effect mechanism of the iso-propanol substituent on amine collectors in the flotation of quartz and ..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.10.060