Selective flotation separation of smithsonite from calcite by application of amino trimethylene phosphonic acid as depressant

Journal Pre-proofs Full Length Article Selective flotation separation of smithsonite from calcite by application of amino trimethylene phosphonic acid as depressant Yanfei Chen, Xuekun Tang PII: DOI: Reference:

S0169-4332(20)30419-0 https://doi.org/10.1016/j.apsusc.2020.145663 APSUSC 145663

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Applied Surface Science

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19 November 2019 20 January 2020 4 February 2020

Please cite this article as: Y. Chen, X. Tang, Selective flotation separation of smithsonite from calcite by application of amino trimethylene phosphonic acid as depressant, Applied Surface Science (2020), doi: https:// doi.org/10.1016/j.apsusc.2020.145663

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Selective flotation separation of smithsonite from calcite by application of amino trimethylene phosphonic acid as depressant Yanfei Chen, Xuekun Tang* School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China.

Abstract: Amino trimethylene phosphonic acid (ATMP), an eco-friendly reagent widely applied in industrial production, was employed as a depressant for flotation separation of smithsonite from calcite for the first time. Micro-flotation test results show that ATMP exhibits selective inhibition effect on calcite flotation. Calcite and smithsonite can be well separated in flotation process by applying ATMP as the depressant and sodium oleate (NaOL) as the collector. Based on a series of measurements including zeta potential, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR), it is found that large amounts of ATMP can be chemically absorbed on the surface of calcite by chelating with surface Ca2+ sites and thus inhibiting the adsorption of NaOL. While, in comparison, much less adsorption of ATMP is taken place on the surface of smithsonite and hardly affects further adsorption of NaOL. Atomic force microscopy (AFM) images of calcite after treated with ATMP indicate that almost the whole surface of calcite is covered with the dense agglomerate adsorption layer. All in all, these results present excellently selective depression ability of ATMP on smithsonite-calcite flotation separation process. Key words: Smithsonite; Calcite; Amino trimethylene phosphonic acid; Chemisorption; Flotation separation.

1. Introduction Zinc (Zn) is one of the most versatile transition metal elements that plays a crucial role in many modern industries, such as battery, alloy, mechanical engineering, electric appliance and food [1-4]. It is rated the 4th most consumed metals in the world, only after iron, aluminum, and copper [5]. Especially in recent ten years, the demand for zinc metal

Corresponding author. E-mail address: [email protected] (X. Tang).

source has steadily grown owing to the fast development of many zinc-consuming industries [5,6]. For a long time, zinc sulphide ores, including sphalerite (ZnS) and marmatite (ZnxFe1-xS), are the main mineral sources (>85%) for extraction of Zn metal in industrial production [7]. While, in recent years, the exploit and utilization of oxide zinc ores has attracted more and more commercial interests due to the continuous depletion of zinc sulphide ores [7-9]. Smithsonite (ZnCO3), a kind of carbonate minerals, is one of the most essential industrially available zinc oxide minerals [8,9]. Flotation, a physico-chemical separation process utilizing different surface hydrophilic/hydrophobic properties of minerals, is the main industrial method for beneficiation of smithsonite from ore deposits [6,8,10]. Longchain fatty acids, e.g. sodium oleate and tall oil, are often employed as collectors in the flotation process [8,11,12]. It can be adsorbed on the surface of smithsonite and thus forming a hydrophobic adsorption layer to greatly improve the floatability of smithsonite [9,12]. Calcite (CaCO3), as a carbonate gangue, is commonly associated with smithsonite in ore deposits [8,13]. Calcite has better floatability than smithsonite due to the chemisorption of fatty acids on surface active Ca2+ sites, leading to difficulty in smithsonite-calcite flotation separation [12,14]. Moreover, calcium ions generated from calcite through surface dissolution may also bring negative effect on the flotation of smithsonite [15,16]. Thus, it is indispensable to add selective depressant to decrease the floatability of calcite in smithsonite flotation process [8,12]. Water glass is a commonly used depressant for flotation separation of smithsonite from calcite in industry [12]. It can inhibit the interaction between calcite and collector by absorbing on the surface of calcite [17,18]. However, this kind of depressant has a significant drawback of high dosage [17]. With high concentration of water glass in pulp, the sedimentation and filtration of concentrate or tailing become very difficult [12,19]. Moreover, it would also bring obvious negative influence on the flotation of smithsonite. Apart from water glass, previous literatures [12,13,18-20] have reported that natural organic macromolecule polymers, such as dextrin, guar gum, xanthan gum, starch and fenugreek gum, can also be used as efficient depressants for calcite. Nevertheless, these depressants also require large consumption and have limited selectivity on flotation 2

separation of smithsonite from calcite. For example, Luo et al [13] found fenugreek gum could be simultaneously adsorbed on the surface of smithsonite and calcite at the same level. There are similar problems when using low molecular organic reagents, e.g. citric acid, tartaric acid and tannic acid as depressants for calcite and smithsonite separation [18,21]. Besides, the weak depression effect of these low molecule organic depressants also limits their application in smithsonite and calcite separation process [18]. Thus, it is eager to explore comprehensive depressants that can realize the flotation separation of smithsonite from calcite. Amino trimethylene phosphonic acid (ATMP), an eco-friendly reagent with formula of N(CH2PO3H2)3, is extensively applied as scale and corrosion inhibitor in treating circulating water and cleaning equipment due to its strong chelating ability with various metallic ions (e.g. Fe2+, Mg2+ and Ca2+) [22-27]. Its molecular structure is presented in Fig. 1. Typically, as a widely applied scale inhibitor, ATMP can inhibit the formation of CaCO3 in circulating water by chelating with Ca2+ to form Ca-ATMP chelate compound [23,24]. Recently, in flotation research area, Liu et al [27] found that ATMP could be used as an efficient depressant to realize the separation of lizardite (A kind of magnesium silicate mineral) from pentlandite. In their research, it is reported that ATMP could be selectively absorbed on the surface of lizardite through chelating with surface Mg2+ sites. Since abundant researches and industrial applications have presented that ATMP has strong chelating ability with Ca2+, ATMP is potentially applied as an efficient depressant for separation of smithsonite from calcite. However, little research was conducted on this. OH

HO P O

N HO

P

O

O P

OH

OH

OH

Fig. 1 The molecular structure of ATMP.

Motivated by the above summary, for the first time, an attempt was made to realize the efficient flotation separation of smithsonite from calcite by utilization of ATMP as depressant in current research. Sodium oleate (NaOL), a typical fatty acid that has been 3

widely applied in industrial flotation process, was employ as the collector for smithsonite. Laboratory-scale micro-flotation tests were carried out to evaluate the effect of ATMP on the flotation performance of smithsonite and calcite. After that, various surface measurement methods, including zeta potential, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy imaging (AFM) were carried out to obtain an in-depth understanding of the interaction between reagents and minerals.

2. Experimental 2.1 Mineral samples

(a)

(b)

S

C

S - Smithsonite

Intesity (a.u.)

Intensity (a.u.)

C - Calcite

S S

10

20

S S 30

40

S S

S 50

60

S SS 70

C C C

C 80

2-Theta-Scale

10

20

30

40

CC 50

C

C 60

70

80

2-Theta-Scale

Fig. 2 The XRD patterns of (a) smithsonite and (b) calcite.

Smithsonite and calcite mineral samples were obtained from Changsha (Hunan province, China) and Chenzhou (Hunan province, China), respectively. The block mineral samples were successively hand sorted, hammer-crushed and ground by a agate mortar into powders. For both mineral samples, particle size fraction of −74 + 38 μm was collected by dry screening to be used for flotation tests. The X-ray diffraction (XRD) spectra of asprepared minerals are shown in Fig. 2. As presented in Fig. 2a and Fig. 2b, they indicate that both mineral samples are of high purity. The chemical compositions of the smithsonite and calcite mineral samples are listed in Table 1, further confirming the high purity of the as-prepared mineral samples.

4

Table 1 Chemical compositions of the as-prepared smithsonite and calcite mineral samples. Sample

ZnCO3

CaCO3

Pb

Fe

Al2O3

SiO2

Smithsonite

99.02

0.05

0.02

0.24

0.04

0.42

Calcite

/

99.09

/

0.13

0.11

0.31

2.2 Micro-flotation tests In the current research, micro-flotation tests were carried out to investigate the influence of ATMP on flotation behavior of smithsonite and calcite. All the flotation tests were carted out in an XFG type flotation machine produced by Jilin exploration machinery plant. The flotation machine is equipped with a hanging 40 mL cell. 50 wt% ATMP solution and analytically pure NaOL purchased from Shanghai Macklin Biochemical Co., Ltd. were used as depressant and collector, respectively. Milli-Q water (18.2 mΩ·cm) was used throughout the flotation tests and surface analyses. For each flotation test, 2 g of mineral samples and 35 mL water were mixed in the cell fixed on flotation machine under agitation to form a uniform pulp. Throughout the flotation process, the pulp was continuously agitated by a build-in impeller at the speed of 1700 rpm. Subsequently, 0.1 mol/L sodium hydroxide (NaOH) solution or 0.1 mol/L dilute hydrochloric acid (HCl) solution was added to adjust the pH value of the pulp. When needed, desired amount of ATMP was added in the pulp. Simultaneously, additional NaOH solution were slowly added in the pulp to maintain the pre-determined pH value. After 5 min stirring, designed amount of NaOL was added in the cell, followed by 3 min conditioning. Successively, flotation was performed and forth product was collected for 4 min via manual scraping. Both concentrate (forth product) and tailing (unfloated product) were separately collected, and then in sequence filtered, dried and weighted. The recovery of smithsonite can be calculated through the weight distributions between the two products. For that of artificially mixed minerals, it can be calculated according to Zn grade of concentrate and feed. All flotation tests were repeated for 3 times. The average of the tests was reported as the final value, and error bars represent one 5

standard deviation around the average value. 2.3 Zeta potential analysis All zeta potential measurements were carried out on a Zeta Potential Analyzer (Coulter Delsa 440sx, Malvern, Instruments Ltd, UK). To prepare samples for zeta potential measurements, mineral samples were firstly ground to finer than 5 μm in an agate mortar. 0.05 g of the above samples were dispersed in 50 mL electrolyte solution (1 ×10-3 M potassium nitrate solution) to form a uniform suspension. The suspension was continuously agitated by a magnetic stirrer at ambient temperature in a beaker. 0.1 M HCl or NaOH solution was then added in the suspension to adjust the pH value. When needed, desired amount of ATMP and NaOL was successively added at an interval of 5 min. In the process, additional HCI or NaOH solution was slowly added in the pulp to maintain the pre-determined pH value. After 10 min agitation, the suspension was standing for another 10 min in the beaker. 2 mL of supernatant in the beaker was pipetted out via an injection syringe. For each sample, zeta potential values were measured as a function of pH in triplicate and taken the mean as the final value. 2.4 FTIR analysis To prepare the samples for FTIR analysis, firstly, the as-prepared mineral powders were further being grinded until the particle size reaches -5 μm. 1 g of the as-prepared samples were completely dispersed in 20 mL water under agitation to form a uniform suspension. Then, designed amounts of ATMP and NaOL were successively added in the suspension at an interval of 5 min. The pH value of the suspension was adjusted to 9.5 by addition of moderate NaOH or HCl solution. After 30 min agitation, the suspension was filtrated, rinsed and dried in a vacuum oven to obtain the final samples for FTIR measurement. The FTIR spectra of the as-prepared samples were recorded on an UV-2350 type infrared spectrometer (Shimadzu Co., Ltd, Japan) from 4000 cm−1 to 2000 cm−1. Sixteen scans were accumulated with a resolution of 4 cm−1 for each spectrum. 2.5 XPS measurement XPS analysis was carried out to detect the chemical state transformations of mineral 6

samples before and after treated with ATMP. All XPS analyses were performed on a KAlpha 1063 type ultrahigh vacuum electron spectrometer (Thermo Fisher Scientific Co., Ltd, UK). Spectra were collected using a monochromatic Al Kα source (1486.7 eV) and a 300 × 500 μm spot size. High-resolution scans were performed at a pass energy of 30 eV and a step size of 0.05 eV. All binding energies were calibrated using contaminant carbon (C1s = 284.6 eV). To prepare samples for analysis, 1 g of as-prepared smithsonite samples consistent with that used in flotation tests were dispersed in 100 mL ATMP solution with certain concentration by continuous agitation to form a uniform suspension. After being agitated for 10 min, the suspension was filtrated, rinsed and dried in a vacuum oven to obtain the final samples for XPS analysis. 2.6 AFM measurement In the current research, AFM measurement was carried out on a MultiMode SPM AFM (Veeco Instruments, Inc., USA) to investigate the surface morphology variations of calcite samples before and after being treated with ATMP. To prepare the samples for AFM measurement, the calcite samples were polished by a semi-automatic polishing machine (Tegramin-25, Struers). The polished calcite was rinsed with ultrapure water for several times. When needed, the freshly cleaved calcite was immersed in the ATMP solution with a concentration of 50 mg/L for 5 min. After that, the surface of the sample was blow-dried by high-purity argon. The whole process was carried out in a glove box to avoid the contamination of the sample. AFM measurement on the as-prepared samples was carried out in a tapping mode. The measurement details were according to the reported literatures [28-30].

3. Results and discussion 3.1 Micro-flotation test results Micro-flotation tests were carried out to investigate the effect of collector dosage on the flotation behavior of smithsonite and calcite. The flotation test results are presented in Fig. 3, illustrating the flotation recovery of smithsonite and calcite as a function of NaOL 7

dosage (Flotation condition: pH=9.5; Without addition of ATMP). As seen in Fig. 3, obviously, for both minerals, they cannot be well recovered at low NaOL dosage due to their bad natural floatability. The flotation recovery of the two minerals significantly increases as the increase of NaOL dosage. For calcite, the flotation recovery barely changes when NaOL dosage reaches above 40 mg/L. While, for smithsonite, it is 90 mg/L. Both minerals can be well recovered with flotation recovery of above 91% with the addition of NaOL, indicating the excellent collecting ability of NaOL to smithsonite and calcite. As can be seen from the variation tendency of recovery, calcite has even much better floatability than that of smithsonite, which reveals that smithsonite cannot be separated from calcite without the addition of depressant. Moreover, the flotation results also suggest that the optimum NaOL dosage for flotation recovery of the current smithsonite should be 90 mg/L.

Fig. 3 Flotation recovery of smithsonite and calcite as a function of NaOL dosage.

Additional micro-flotation tests were carried to research the effect of ATMP on flotation performance of smithsonite from calcite. Fig. 4 presents the effect of pulp pH on the flotation recovery of smithsonite and calcite with/without the addition of ATMP (Flotation condition: NaOL dosage=90 mg/L; When needed, ATMP dosage=50 mg/L). It can be seen that the variation of pH from 7 to 11.5 hardly affects the flotation behavior calcite. For smithsonite, the flotation behavior is significantly influenced by pulp pH. As seen, the flotation recovery increases from 60.21% to 91.13% as the pH value increases from 7 to 9.5, while gradually decreases when pH value excesses 9.5. The results indicate the optimum 8

pulp pH for the flotation of smithsonite should be 9.5. With the addition of ATMP in calcite flotation system, the flotation recovery significantly decreases from above 94% to less than 7% at the pH range of 7-11.5, showing the excellent effect of ATMP on depressing the flotation of calcite at a wide pH range. Compared with smithsonite untreated with ATMP, the recovery of smithsonite with the addition of ATMP decreases by about 3-10% at the same pH value. Especially, at pH 9.5, the decreased degree of recovery is only about 3%. This implies the slightly negative effect of ATMP on smithsonite flotation. Even so, comparatively, the influenced degree of ATMP on calcite flotation behavior is much higher than that of smithsonite. Noteworthy, for smithsonite with the addition of ATMP, the recovery remains high (88.73%) at pH 9.5. All in all, the results show excellent ability of ATMP on selective depressing calcite in flotation process.

Fig. 4 Effect of pulp pH on flotation recovery of smithsonite and calcite treated by different reagents.

Fig. 5 Flotation recovery of smithsonite and calcite as a function of ATMP dosage.

9

Fig. 5 presents flotation recovery of smithsonite and calcite as a function of ATMP dosage (Flotation condition: NaOL dosage=90 mg/L; pH=9.5). As seen in Fig. 5, the flotation recovery of smithsonite slightly decreases from 91.13% to 88.15% as ATMP dosage increases to 70 mg/L. The results indicate slight effect of ATMP on the flotation behavior of smithsonite at pH 9.5. While, for calcite, the recovery significantly decreases from 94.77% to 6.7% when ATMP dosage increases to 50 mg/L, confirming the excellent depression effect of ATMP on calcite. The flotation recovery of calcite barely changes by further raising ATMP dosage to 70 mg/L. Thus, the optimum ATMP dosage for flotation recovery of the current smithsonite should be 50 mg/L. Table 2 Results of flotation of calcite and smithsonite mixed minerals under different conditions Flotation Condition

NaOL: 90 mg/L Pulp pH: 9.5

ATMP: 50 mg/L NaOL: 90 mg/L Pulp pH: 9.5

Zn grade

Zn recovery

(%)

(%)

94.28

24.97

92.98

Tailing

5.72

31.09

7.02

Feed

100.00

25.32

100.00

Concentrate

48.05

44.21

83.90

Tailing

51.95

7.85

16.10

Feed

100.00

25.32

100.00

Product

Yield (%)

Concentrate

Based on the single mineral flotation results, flotation tests on artificially mixed minerals (Smithsonite: Calcite=1:1) were performed to verify the effect of ATMP on the separation performance of smithsonite from calcite. The flotation conditions and results are listed in Table 2. As seen, in contrast flotation test, above 90% of the mixed minerals enter into concentrate product in the absence of ATMP, indicating selective separation of the two minerals is impossible under this condition. In comparison, with the addition of 50 mg/L ATMP, excellent separation performance of the two minerals is realized, and a concentrate with Zn grade of 44.21% and Zn recovery of 83.90% was obtained. In the whole, the results manifest that ATMP can be potentially used as an efficient depressant in flotation separation of smithsonite from calcite.

10

3.2 Zeta potential measurement results Surface charge is an important parameter in flotation process due to its great influence on adsorption behavior of flotation reagents on minerals surface. Thus, in current research, zeta potential measurements were carried out to investigate the influence of ATMP on the surface charge of smithsonite and calcite. The results are illustrated in Fig. 6, presenting zeta potentials of smithsonite and calcite as a function of pH value under different conditions. As presented in Fig. 6a, zeta potentials of calcite are negatively shifted as the increase in pH value, and the isoelectric point (IEP) appears at around pH 8.7, which is basically in accordance with the previous reports [31,32]. ATMP is negatively charged in solution due to the abundant phosphate groups (Fig. 1) [27]. The existence of ATMP in the pulp greatly affects the surface charge of calcite. As seen, in the overall studied pH range (7-11.5), zeta potentials of calcite significantly decrease and the IEP shifts to much lower pH value with the addition of ATMP. This indicates that ATMP can be absorbed on the surface of calcite [33,34]. Moreover, with the addition of NaOL in ATMP-calcite pulp system, relatively minor decrease of zeta potentials can be observed in Fig. 6a, especially at pH range of 8-9.5, indicating little adsorption of NaOL on calcite surface. Clearly, it implies that ATMP initially adsorbed on the surface of calcite would greatly prevent the adsorption of NaOL. This should be the reason that ATMP can depress the floatation of calcite. Fig. 6b shows the IEP of smithsonite appears at around pH 8.1. Similar to calcite, in the overall studied pH range (7-11.5), zeta potentials of calcite with the addition of ATMP decrease and the IEP shifts to lower pH value, which implies that ATMP is absorbed on the surface of smithsonite. This should be the reason that the presence of ATMP can also inhibit smithsonite flotation to a certain degree. Noteworthy, the variation of zeta potentials for smithsonite in the presence of ATMP is much lower than that of calcite. Moreover, with the addition of NaOL in ATMP-smithsonite pulp system, zeta potentials greatly decrease at pH range of 7-11.5, which indicates that large amounts of NaOL are absorbed on the surface of smithsonite. The above flotation test results indicate the optimum pH value for recovery of smithsonite is 9.5. As marked in the Fig. 6a, zeta 11

potential of calcite decreases by 28.39 mV at pH 9.5. In comparison, for smithsonite, it is 9.7 mV. It is reasonable that the adsorption extent of ATMP on calcite is much higher than that of smithsonite. Besides, it can be seen that zeta potentials significantly decrease with the addition of NaOL in ATMP-smithsonite pulp system at pH 9.5. This indicates the presence of ATMP has little impact on the interaction between smithsonite and NaOL, which correspondences with the flotation test results [31].

Fig. 6 Zeta potentials of (a) smithsonite and (b) calcite as a function of pH under different conditions.

3.3 FTIR measurement results FTIR measurements were carried out to further research the influence of ATMP on collector adsorption on minerals. Fig. 7 depicts FTIR spectrum of NaOL applied in the flotation tests. As marked in the figure, band peaks at around 2980 cm-1 and 2851 cm-1 are corresponding to (CH2)n group (carbon chain) [14,33]. Fig. 8 presents FTIR spectra of smithsonite and calcite before and after treated with ATMP or NaOL. As shown in Fig. 8a, for calcite, after interacted with NaOL, the peaks around 2981 cm-1 and 2839 cm-1 should be the stretching bands of (CH2)n group according to Fig. 7, indicating the well adsorption of NaOL on the surface of calcite under this condition [14]. On contrast, for calcite in sequence interacted with ATMP and NaOL, the spectrum is basically same as that of pristine calcite, implying the adsorption of NaOL on the surface of calcite is inhibited by ATMP. Moreover, it is noteworthy that the FTIR spectrum of calcite is barely changed after interacted with ATMP. This may be due to the small molecular weight and low dosage of ATMP (50 mg/L). Under the condition, the adsorption of ATMP on calcite may cannot be 12

detected by FTIR measurement. Fig. 8b presents FTIR spectra of smithsonite interacted with NaOL and smithsonite successively interacted with ATMP and NaOL, which manifests the well adsorption of NaOL on the surface of smithsonite due to the observations of peaks at 2933 cm-1 and 2851 cm-1. Besides, the results also imply the adsorption of NaOL on the surface of smithsonite is little affected by the presence of ATMP. In addition, similarly, the FTIR spectrum of smithsonite is also basically unchanged with the addition of ATMP. This should be attributed to the same reason to that of calcite. In addition, much less adsorption amount of ATMP on smithsonite may also cause this phenomenon. In the whole, FTIR results show the obvious difference for ATMP on affecting NaOL adsorption on the surfaces of calcite and smithsonite.

Fig. 7 FTIR spectrum of NaOL.

Calcite+NaOL Intensity (a.u.)

(b)

Calcite+ATMP+NaOL

Calcite+ATMP

Calcite

4000

Smithsonite+ATMP+NaOL

(CH2)n

Smithsonite+NaOL

(CH2)n Transmittnace

(a)

Smithsonite+ATMP

Smithsonite

3600

3200

2800

2400

2000

3900

3600

3300

3000

2700

2400

-1

-1

Wavenumber (cm )

Wavenumber (cm )

Fig. 8 FTIR spectra of (a) calcite and (b) smithsonite treated with different reagents.

3.4 XPS measurement results 13

2100

XPS analysis was carried out to examine the chemical state of calcite before and after interaction with ATMP, and the results are presented in Fig. 9. Fig. 9a presents the high-resolution spectra of Ca 2p. As seen, for the pristine calcite, the peaks centered at the binding energy of 346.94 eV and 350.44 eV are corresponding to Ca 2p3/2 and Ca 2p1/2 in calcite [18,36-38]. After interacted with ATMP, the two peaks of Ca 2p3/2 and Ca 2p1/2 are shifted to lower binding energy of 346.80 eV and 350.34 eV, respectively. Fig. 9b presents the high-resolution spectra of O 1s. It can be clearly seen that the peak of O 1s spectrum centered at the binding energy of 531.34 eV is shifted by 0.09 eV after interacted with ATMP. For calcite, this peak can be divided into two peaks centered at the binding energy of 531.12 eV and 531.51 eV, which are assigned to CO32- and carbon oxide on the surface of calcite [34]. While, after interacted with ATMP, the two peaks are shifted to 530.95 eV and 531.48 eV. The shifts of the above peaks (Ca 2p and O 1s) should be induced by the chelation reaction between ATMP and Ca2+ sites on the surface of calcite [27,36].

(a)

(b)

Ca 2p

346.80 eV

531.25 eV 530.95 eV

Intensity (a.u.)

Intensity (a.u.)

350.34 eV

Calcite+ATMP 346.94 eV

531.48 eV

Calcite+ATMP 531.34 eV 531.12 eV

350.44 eV

531.51 eV

Calcite 344

346

348

350

352

Calcite

354

528

530

Binding energy (eV)

532

534

Binding energy (eV)

(c)

N 1s

(d)

P 2p 132.46 eV

Intensity (a.u.)

Intensity (a.u.)

133.27 eV

Calcite+ATMP

Calcite+ATMP

Calcite

Calcite

396

398

400

402

130

404

Binding energy (eV)

132

134

Binding energy (eV)

14

136

Fig. 9 The high-resolution XPS spectra of calcite before and after interaction with ATMP: (a) Ca 2p; (b) O 1s; (c) N 1s; (d) P 2p.

Fig. 9c depicts the high-resolution spectra of N 1s. As seen, obviously, N 1s of pristine calcite is much weaker than that of calcite interacted with ATMP, indicating the adsorption of ATMP on the surface of calcite [27]. Fig. 9d depicts the high-resolution spectra of P 2p. Clearly, there is scarcely any P 1s observed in the spectrum of pristine calcite. In comparison, after interacted with ATMP, a remarkable peak at 132.74 eV is detected, further confirming the adsorption of ATMP on the surface of calcite. This peak can be divided into two peaks centered at 136.46 eV and 133.27 eV, which are associated with PO3H in the molecular structure of ATMP and Ca-O-P group on calcite surface [27,39,40]. This confirms that ATMP chemically reacts with Ca2+ sites on the surface of calcite. 3.5 AFM measurement results Fig. 10a illustrates the height and 3D images of pristine calcite. As seen, both the height image (left) and 3D image (right) show that the surface of pristine calcite is flat, uniform and continuous. This implies the surface of as-prepared sample is not contaminated in the preparation process. On contrast, Fig. 10b illustrates the height and 3D images of calcite after being interacted with ATMP. Clearly, compared with pristine calcite, the ATMP treated calcite presents a uniform surface with dense agglomerate adsorption. The 3D image (right) further shows the ATMP treated calcite has much rougher surface than that of pristine calcite owing to the adsorption of ATMP. Almost the whole surface of calcite is covered with the dense agglomerate adsorption layer. 3.6 Conceivable depression mechanism of ATMP Combined with the flotation results and characterization results, a schematic model is proposed in Fig. 11 to illustrate the effect of ATMP on flotation separation process of smithsonite from calcite. The excellent depression effect of ATMP on calcite is originated from its strong chelating ability with Ca2+ sites. Abundant reported researches have revealed that the predominant species on the surface of calcite are Ca2+ and Ca(OH)+ at 15

pH<13 [13,14,41]. Thus, ATMP would be absorbed on the surface of calcite through chelating with surface Ca2+ sites. As illustrated in Fig. 11, the adsorbed ATMP covers the surface of calcite, thus inhibiting further adoption of NaOL. In this way, the flotation of calcite is depressed. In comparison, for smithsonite, there is few ATMP absorbed on the surface (as concluded in zeta potential measurement results). This might be that ZnCO3 (1.46×10-10) has much lower solubility product constant than that of CaCO3 (4.96×10-9) [9,22]. As reported by previous literatures, the predominant species on the surface of smithsonite should be zinc hydroxide [Zn(OH)2] and hydrozincite [Zn5(CO3)2(OH)6)] at the pH range of 8.5-11.5 [31,35,42,43]. Possibly, these species inhibit the adsorption of ATMP on smithsonite surface. Another explication, as reported by Liu et al [12], is the OO distance in -PO3H2 group match much better with the Ca-Ca distance of calcite surface than that of Zn-Zn distance of smithsonite. They consider this probably the reason for the interaction of ATMP with calcite is much higher than that with smithsonite. Even so, more researches require to be carried out in the future to in-depth reveal the selective interaction mechanism of ATMP with calcite [44,45].

a

b

16

Fig. 10 AFM images (left-height; right-3D): (a) bare calcite; (b) calcite interacted with ATMP

Fig.11 The conceivable mechanism of ATMP on separation of smithsonite from calcite.

4. Conclusions In the current research, ATMP was employed as a selective depressant to realize flotation separation of smithsonite from calcite. Micro-flotation test results show the flotation of calcite can be significantly depressed with the addition of ATMP. While, smithsonite flotation is slightly affected by ATMP. Artificially mixed minerals flotation tests present that efficient separation of smithsonite from calcite is realized using ATMP as the depressant and NaOL as the collector at pH 9.5. Zeta potential, XPS and FTIR measurements indicate that ATMP can be adsorbed on both minerals. However, the adsorption degree of ATMP on calcite is much higher than that of smithsonite. The slight adsorption of ATMP on smithsonite surface barely inhibits further adsorption of NaOL. Atomic force microscopy (AFM) images indicate that almost the whole surface of calcite is covered with the dense agglomerate adsorption layer, thus inhibiting the adsorption of NaOL. In the whole, the presence of ATMP induces great disparities in surface properties of smithsonite and calcite to interact with NaOL, thus resulting in flotation separation of the two minerals. 17

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Highlights ATMP used as depressant in smithsonite-calcite flotation process for the first time. The floatability of calcite can be strongly depressed by ATMP. The flotation recovery of smithsonite was slightly affected by the presence of ATMP. The adsorption degree of ATMP on calcite is much higher than that of smithsonite. The surface of calcite covered by ATMP adsorption layer in the flotation process.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Credit Author Statement Yanfei Chen (First author): Methodology, Validation, Investigation, Data Curation, Validation, Writing - Original Draft. Xuekun Tang (Corresponding author): Conceptualization, Formal analysis, Resources, Writing - Review & Editing, Visualization, Supervision.

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