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Effect of Ca selective chelator BAPTA as depressant on flotation separation of magnesite from dolomite
Minerals Engineering 144 (2019) 106050
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Effect of Ca selective chelator BAPTA as depressant on flotation separation of magnesite from dolomite
T
Wanzhong Yina,b,c, Haoran Suna, , Jongsu Honga,d, , Shaohang Caoa, Bin Yanga, Changdok Wond, Myongsong Songe ⁎
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a
School of Resources & Civil Engineering, Northeastern University, Shenyang 110819, China Liaoning Key Laboratory of Mineral Processing Technology, Shenyang 110819, China c Northeastern University Genetic Mineral Processing Research Center, Shenyang 110819, China d Faculty of Resource Probing Engineering, Kim Chaek University of Technology, Pyongyang 1001, Democratic People’s Republic of Korea e Faculty of Mining Engineering, Kim Chaek University of Technology, Pyongyang 1001, Democratic People’s Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Magnesite Dolomite Depressant Chelator BAPTA Flotation
With the depletion of high-grade magnesite mineral resources, improving the grade of magnesite obtained from gangue minerals such as dolomite has been attracting considerable attention from the industry. However, it is difficult to achieve the effective separation of magnesite from dolomite because of their similar chemical and crystal properties. In this study, 1, 2-bis (o-aminophenoxy) ethane-N,N,N′,N′-tetraacetic acid (BAPTA) was used as a depressant for dolomite to improve the separation of magnesite from dolomite. Microflotation tests of the single minerals and artificially mixed minerals using oleic acid as a collector were performed to evaluate the selective depression effect of BAPTA. The results indicated that the BAPTA depressant had a selective effect on dolomite, thereby suggesting that a magnesite grade of 87.5%, recovery of 95.8%, and CaO content of 3.8% can be achieved with a BAPTA dosage of 62.5 mg/L. The depression mechanism of BAPTA was investigated using zeta-potential measurements, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results indicated that BAPTA was selectively adsorbed on the surface of dolomite. The selective depression effect can be attributed to the selective chelating formation of BAPTA on Ca rather than Mg, which not only adsorbed BAPTA decreasing the adsorption sites of oleic acid on the dolomite surface, but also prevented the precipitation of Ca on the surface of the magnesite.
1. Introduction Magnesite often contains carbonate minerals in the form of gangue because of its formation characteristics (Aslani et al., 2010; Botero et al., 2007; Chen and Tao, 2004; Kozhevnikov et al., 1973; Luo et al., 2016). In particular, the effective flotation separation of magnesite from dolomite, which is one of the carbonate minerals, is difficult because of their similar crystal structure, chemical composition, and surface properties (Gence and Ozbay, 2006; Gence, 2006; Hu et al., 2003; Luo et al., 2017). Therefore, various techniques have been employed to improve the selectivity of flotation of magnesite from dolomite; these techniques include flotation systems, using different types of reagents, and changing the addition order of the reagents. The contact angles of magnesite and dolomite were investigated in the presence of oleic acid and dodecylamine (DDA) as a collector
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without a depressant (Gence, 2006; Gence and Ozbay, 2006; Yao et al., 2016). Magnesite and dolomite exhibit small contact angles of 10.4° and 6.6°, respectively, in distilled water, thereby indicating that they are hydrophilic. With the addition of DDA as a cationic collector, there is no significant change in the contact angle of magnesite, but the contact angle of dolomite increases to 88°, thereby indicating that it becomes hydrophobic. Thus, the recoveries of magnesite and dolomite are approximately 5% and 50%, respectively, in reverse flotation for single minerals using DDA. In the presence of sodium oleate as an anionic collector, the contact angles of magnesite and dolomite increase to 79° and 39°, respectively. This suggests that it is possible to introduce reverse flotation using DDA and direct flotation using sodium oleate. Some reagents such as carboxymethyl cellulose and sodium silicate can increase the difference between the contact angles of magnesite and dolomite (Zhang et al., 2018). Additionally, Acintol FA–1 tall oil and SH were used as a collector
Corresponding authors at: School of Resources & Civil Engineering, Northeastern University, Shenyang 110819, China (H. Sun and J. Hong). E-mail addresses: [email protected] (H. Sun), [email protected] (J. Hong).
https://doi.org/10.1016/j.mineng.2019.106050 Received 4 March 2019; Received in revised form 24 September 2019; Accepted 25 September 2019 Available online 27 September 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
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and a depressant, respectively, to achieve effective separation of magnesite (Chen and Tao, 2004; Chen and Tao, 2005; Marinakis and Shergold, 1985; Matis et al., 1988; Matis and Gallios, 1989; Ruan et al., 2018; Wonyen et al., 2018). Although various reagents have been used in the form of a single or mixed depressant, these depressants are insufficient for achieving adequate differential flotation selectivity between magnesite and calcite. Although there are differences in the recovery rates of flotation, the trends are similar (Chen and Tao, 2004, 2005; Gence and Ozbay, 2006; Matis et al., 1988; Matis and Gallios, 1989; Moudgil and Ince, 1991; Yao et al., 2016). Despite significant progress of research based on single minerals, flotation tests for the treatment of mixed minerals indicate an insufficient separation effect, and the reason behind it, has rarely been studied. The main cause of the insufficient separation of magnesite from dolomite was investigated by Luo et al. (2017); the results indicated that the Ca species dissolved from the dolomite surface precipitated on the surface of the magnesite, thereby making the surface of magnesite similar to that of dolomite. In this study, to achieve differential flotation selectivity between magnesite and dolomite using oleic acid as a collector, the Ca-ion selection chelate 1, 2-bis (o-aminophenoxy) ethane-N,N,N′,N′-tetraacetic acid (BAPTA) was employed as a depressant for improving the separation effect of magnesite. It is more selective for Ca than for Mg, and the selective action of the four carboxylic acid functional groups allows binding with Ca ions (Koide and Yamada, 1990; Nagaraj, 1982; Marabini et al., 2007; Sorour et al., 2016; Ricci et al., 1998). The stability constants pKca of BAPTA for binding with Ca and Mg are 6.9 and 1.77, respectively, thereby indicating that BAPTA is far more stable for binding with Ca than with Mg (Baer et al., 1992; John and Emma, 2009; Oiki et al., 1994). In this study, BAPTA was considered as a depressant, and attempts were made to improve the selective separation of magnesite from dolomite. Microflotation tests, zeta-potential measurements, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were performed to investigate the depression mechanism.
Fig. 1. Structure of BAPTA.
2.2. Microflotation tests Microflotation experiments involving single minerals and artificially mixed minerals were performed in a 30-mL microflotation cell of a laboratory XFG self-aeration flotation machine (Jilin Exploration Machinery Plant, China) at a stirring speed of 1800 rpm. Simple comparative experiments on the addition order of the reagents indicated that it was advantageous to inject the depressant before stirring the solution. Deionized water (20 mL) and the sample (2.0 g) were added to the microflotation cell, and after the addition of BAPTA, the microflotation was started. A flowsheet of the microflotation test is shown in Fig. 2. Finally, the concentrate was scraped, dried, and weighed; the recovery of the concentrate and tailings was calculated, and various analyses were performed. Each experiment was conducted at least three times, and the average results were calculated. In accordance with a previous report (Luo et al., 2017), the pH was set as 11 in this experiment. BAPTA is stable at all pH values and is not significantly affected by the pH. In the microflotation test, oleic acid, which is known to be stable under high-pH conditions, was used as a collector. Therefore, the pH was set as 11 for all experiments. In single-mineral experiments, 2.0 g of single-mineral samples or artificially mixed minerals with a magnesite-to-dolomite mass ratio of 1:1 were used. The microflotation experiments were performed to examine the selectivity effect of BAPTA on the flotation of magnesite and dolomite with respect to the BAPTA concentration. The sodium oleate dosage was 120 mg/L, and the experiments were performed at 25 ± 1 °C. The recovery rates, grades, and CaO contents of the concentrate were evaluated at different BAPTA concentrations. To investigate the effect of BAPTA on the floatability of the magnesite single mineral in the presence of Ca, the amount of Ca ions in the solution was adjusted using CaCl2 (with the same number of moles of NaCO3).
2. Materials and methods 2.1. Materials and reagents The samples of magnesite and dolomite used in this study were obtained from Haicheng magnesite mine, Liaoning Province, China. The phase compositions of the two minerals were determined via X-ray diffraction analysis (X’Pert Pro MPDDY2094, PANalytical B.V., Holland), and the chemical compositions were determined via X-ray fluorescence chemical analysis. The magnesite sample consisted of 45.80% MgO, 1.24% CaO, 0.14% Fe2O3, and 1.94% SiO2. The dolomite sample consisted of 21.36% MgO, 30.34% CaO, 0.08% Fe2O3, and 1.50% SiO2. The results indicated that the purities of magnesite and dolomite exceeded 95%. The samples were crushed, dried, ground by a porcelain ball mill, sieved, and classified to desired particle sizes. A particle size of −78 + 37 μm was selected for microflotation experiments and XPS, whereas finer particles of < 37 μm were ground to < 5 μm in an agate mortar for FTIR spectroscopy and zeta-potential measurements. BAPTA (depressant), sodium oleate (NaOL) (collector), and CaCl2 (regulator) were purchased from China National Pharmaceutical (analytical grade). BAPTA was prepared using dimethyl sulfoxide (DMSO) as a solvent and diluted to the corresponding volume with deionized water before the microflotation tests. The structure of the Ca chelator BAPTA is shown in Fig. 1. The pH was adjusted using NaOH and HCl solutions, and deionized water was used for all the experiments.
Fig. 2. Flowsheet of the microflotation test. 2
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2.3. Zeta-potential tests Zeta-potential measurements of magnesite and dolomite were conducted using a Nano–ZS90 zeta-potential analyzer (Malvern Instruments Ltd., Britain). The 20 mg mineral sample was finely ground to −5 μm in an agate mortar, and 50 mL of deionized water was added. CaCl2 was added to the Mg sample solution to adjust the Ca concentration of the solution, and the zeta potential was measured as a function of BAPTA dosage. The collector, oleic acid (120 mg/L), was added, and the pH was adjusted to 11 using HCl and NaOH solutions. The solution was magnetically stirred for 5 min to ensure that the suspension was homogenized. The suspension was transferred to a sample cuvette, and then the zeta potential was measured. Each value was the average of three measurements. 2.4. FTIR spectroscopy The single minerals were ground to −5 μm in an agate mortar. A 1.0 g sample was added to a 20 mL glass cell, followed by Ca ions and BAPTA. Then, the solution was stirred for 2 min, and the pH was adjusted to 11 for 3 min before the solids and liquids were separated. The solid phase was removed via filtration, washed three times with deionized water, and dried under vacuum conditions at 40 °C for 24 h. The infrared spectra of the samples were recorded using a Nicolet 380 FTIR spectrometer at 25 ± 2 °C in the range of 4000–500 cm−1.
Fig. 3. Flotation performance for single minerals with respect to BAPTA dosage (oleic acid concentration = 4 × 10−4 mol/L, pH = 11.0).
2.5. XPS measurements The sample preparation for XPS was identical to that for FTIR spectroscopy. XPS spectra of samples treated with and without BAPTA were recorded using an America Thermo VG ESCALAB 250Xi spectrometer using Al α X-rays (1486.6 eV) as a sputtering source at a power of 150 W (15 kV, 10 mA). Based on a C1s peak from contaminations (around 284.8 eV), the binding energy scale was corrected in accordance with the internal binding-energy standard (Marinakis and Shergold, 1985). First, each sample was scanned extensively to identify the elements constituting the mineral samples. Subsequently, the highresolution scans were used to acquire more data in various elemental energy regions. To analyze the XPS data, including peak fitting and semiquantitative estimation, the Avantage software was used. 3. Results and discussion
Fig. 4. Effect of Ca2+ on the recovery of magnesite (BAPTA dosage of 50 mg/L, oleic acid concentration = 4 × 10−4 mol/L, pH = 11.0).
3.1. Microflotation test results
magnesite was maintained, but when Ca was added prior to BAPTA, the recovery decreased rapidly (Fig. 4). The results are consistent with those reported by Luo et al. (2017); the decrease in recovery was due to the dissolution of dolomite and adsorption of dissolved Ca species, thereby resulting in the presence of Ca on the surface of the magnesite similar to the dolomite surface. Consequently, magnesite was depressed. The results indicate that the addition order of the reagents is very important. In this case, the recovery of magnesite decreased significantly with an increase in the Ca concentration; this was similar to the tendency of depression for the single-mineral flotation test of dolomite. The recovery of magnesite was 76.3% without Ca. It changed when the calcium concentration was higher than 15 mg/L; the recovery decreased to 39.6% at a Ca concentration of 15 mg/L. This tendency was also observed in the flotation tests of the artificially mixed minerals at a pH of 11 with oleic acid as a collector. As shown in Fig. 5, BAPTA had a significant depression effect on dolomite: the recovery of dolomite was 13.8% with a BAPTA dosage of 62.5 mg/L and 76.4% without BAPTA. Conversely, at a BAPTA dosage of 62.5 mg/L, the recovery of magnesite increased from 78.2% to a maximum value of 95.8%. In this case, the grades of magnesite and
Single-mineral flotation tests were conducted to observe the effects of the BAPTA depressant dosage on the flotation of magnesite and dolomite. The results are shown in Fig. 3. Fig. 3 shows that the floatability of dolomite decreased rapidly in the beginning with an increase in the depressant dosage at a pH of 11 and an oleic acid content of 120 mg/L, thereby indicating that BAPTA had a strong depression effect on dolomite. The Ca species dissolved in the solution from the surface of the dolomite in the initial stage of pulp agitation was combined with BAPTA to form a Ca chelate, which was suspended in the solution. Simultaneously, the depressant was adsorbed at the sites of the Ca ions on the surface of dolomite. Consequently, the adsorption of oleic acid on the dolomite surface was hindered. Fig. 3 shows that the flotation recovery of dolomite was 66.6% without BAPTA under the same flotation conditions, but the recovery decreased rapidly with an increase in the BAPTA dosage. At a BAPTA dosage of 50 mg/L, the recovery of dolomite decreased linearly with a recovery of 2.6% and changed very little thereafter. At a BAPTA dosage of 87.5 mg/ L, the recovery of dolomite was 0.98%. The stable constant of the BAPTA chelator with Mg on the surface of magnesite was lower than that with Ca on the surface of dolomite. In the case of the addition of BAPTA followed by Ca, the recovery of 3
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Fig. 5. Recovery and grade of flotation with respect to BAPTA dosage for artificially mixed minerals.
Fig. 7. Zeta potentials of magnesite with and without BAPTA as a function of Ca concentration (BAPTA 50 mg/L).
cation adsorption sites available for adsorption of the negatively charged groups of the anion collector was reduced. Therefore, at a pH of 11, the RCOO– and RCOOHRCOO– species of oleic acid were adsorbed to a greater extent on the magnesite surface than on the dolomite surface. According to Fig. 6, the zeta potential of magnesite was not affected by the BAPTA dosage, but it increased slightly (from −34.05 to −30.15 mV) as the BAPTA dosage increased from 0 to 75 mg/L, thereby indicating that the BAPTA depressed the adsorption of Ca species on the magnesite surface by forming the chelate with the Ca ions of the liquid phase of the pulp. The effect of Ca on the zeta potential of magnesite in the presence of BAPTA is shown in Fig. 7. For this measurement, BAPTA was added prior to solution conditioning to form the Ca chelate in the solution before the adsorption or precipitation of dissolved ions on the magnesite surface. Fig. 7 indicates that in the presence of Ca2+ in the aqueous solution, without the depressant, the zeta potential increased due to the adsorption of Ca species on the surface of magnesite. The zeta potential of magnesite without BAPTA increased by 14.95 mV; it was −19.1 mV at a Ca concentration of 25 mg/L and −34.05 mV without Ca. However, when a BAPTA dosage of 50 mg/L was added followed by Ca, the zeta potential of magnesite did not increase significantly. It shifted only by 3.65 mV in the same Ca-concentration range. In the Ca-concentration range of 0–10 mg/L, the zeta potential of magnesite did not change due to the effect of BAPTA, but it changed slightly at a higher Ca concentration, thereby indicating that the BAPTA dosage depended on the Ca concentration. Importantly, the BAPTA depressant in the aqueous solution significantly reduced the zeta potential of the dolomite but did not significantly affect the magnesite.
Fig. 6. Zeta potentials of magnesite and dolomite at different BAPTA dosages.
dolomite were 87.49% and 12.5%, respectively, and the CaO content was 3.8%. As shown in Fig. 6, because of the selective separation effect of BAPTA on the recovery rate and grade of magnesite and dolomite, the two minerals were effectively separated with the increasing BAPTA concentration. Fig. 5 indicates that the BAPTA acted as a depressant only on the dolomite and functioned as an activator for magnesite, thereby suggesting that the selective separation of the two minerals can be achieved with BAPTA alone.
3.2.2. FTIR spectroscopy results FTIR spectroscopy was performed to elucidate the action mechanism of BAPTA on the surfaces of magnesite and dolomite for the flotation test using oleic acid as a collector. The results are shown in Figs. 8 and 9, respectively. In the NaOL spectrum, bands corresponding to the stretching vibrations of the functional groups eCH2e and eCH3 appear at 2921.31 and 2851.26 cm−1, respectively. The peaks at 1446.49 and 1560.65 cm−1 are the symmetric and asymmetric stretching vibration peaks, respectively, of carboxyl (eCOOe), which is the main functional group of NaOL (Fukami and Maeda, 1998). The peak at 721.89 cm−1 is the in-plane bending vibration adsorption peak of e(CH2)ne. The main functional groups of BAPTA consisted of four carboxyl groups. As shown in Fig. 8a, one of the characteristic peaks in the
3.2. Mechanism study 3.2.1. Zeta-potential results To elucidate the depression mechanism of BAPTA on the surface of the minerals, the zeta potentials at different BAPTA dosages and a pH of 11 were measured. The results are shown in Fig. 6. As shown in Fig. 6, the magnesite and dolomite were both negatively charged. As the BAPTA dosage increased, the zeta potential of dolomite shifted more toward the negative direction. The zeta potential of dolomite without BAPTA was −15.7 mV, but it was shifted by 22.1 mV in the negative direction (to −37.8 mV) at a BAPTA dosage of 75 mg/L. This indicates that owing to the adsorption of the negatively charged BAPTA functional groups on the Ca sites of the dolomite, the surface of the dolomite had a more negative charge, and the number of 4
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Fig. 9. FTIR spectra of untreated dolomite and dolomite treated with BAPTA: (a) transmittance in the range of 4000–400 cm−1; (b) absorbance in the range of 3200–2700 cm−1. Fig. 8. FTIR spectra of untreated magnesite and magnesite treated with BAPTA: (a) transmittance in the range of 4000–400 cm−1; (b) absorbance in the range of 3200–2700 cm−1.
assigned to the symmetrical stretching vibration of CO3, the out-ofplane bending vibration, and the in-plane deformation vibration, respectively, and weak bands appear at 2942.33, 2857.88, and 2536.07 cm−1. Bands at 2921.68 and 2852.50 cm−1 were observed for the magnesite samples treated with NaOL, thereby indicating that NaOL was absorbed on the Mg surface. The small shifts at 2921.31 and 2851.26 cm−1 for NaOL indicate the chemical adsorption of NaOL on the magnesite surface. No bands other than those of NaOL were detected because of the covering of the magnesite bands. The shape, size, and position of the bands corresponding to magnesite treated with BAPTA did not change, thereby suggesting that BAPTA did not significantly affect the adsorption of NaOL on the surface of magnesite. However, the bands at 2921.31 and 2851.26 cm−1 for NaOL were shifted slightly to 2922.06 and 2852.47 cm−1, respectively. The adsorption at 3053.54–2852.47 cm−1 is compared between magnesite + NaOL and magnesite + BAPTA + NaOL in Fig. 8b. The addition of BAPTA prior to the conditioning of the pulp revealed that
BAPTA spectrum was the wide adsorption peak of OeH and CeO of the dimers of the carboxyl group, which was the main functional group of this chelator. This peak appeared weakly in the range of 2500–3300 cm−1. The peak at 1714.69 cm−1 is assigned to the symmetric adsorption band of C]O. The CeO stretching vibration adsorption bands (1320–1210 cm−1) and the CeN vibration adsorption bands of the tertiary amine R1R2R3N (1100–1030 cm−1) are also characteristic bands of BAPTA, and the OeH out-of-plane bending vibration adsorption bands appear at 975.24 cm−1. They are complex and weak. The adsorption band at 749.75 cm−1 is attributed to the out-ofplane deformation vibration of eCH2eCH2e at the center of the BAPTA molecule. The high-frequency vibration band of 1350–1250 cm−1 corresponding to N-benzene is not obvious. For magnesite, the peaks at 1020.26, 886.01, and 748.04 cm−1 are 5
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magnesite surface, thereby implying that the BAPTA was selectively bound to Ca ions in the solution, forming a chelate, and not to Mg on the surface of magnesite. For the surfaces of magnesite and dolomite used in this study, the atomic distribution ratios of Mg1s were 8% and 4.6%, respectively. And the sum of Ca1s and Ca2p (8.73%) on the dolomite surface was about 10.3%. The comparison indicates that the distribution ratio of Ca adsorbed on the surface of magnesite not treated with BAPTA was similar to that of Ca adsorbed on the dolomite surface. In the aqueous solution, Ca species were adsorbed on the magnesite surface in the form of calcium carbonate; thus, the distributions of metal atoms on the magnesite surface were similar to those on the dolomite surface (Luo et al, 2017). However, by using BAPTA, which formed a chelate only with Ca ions in the solution and on the dolomite surface, the selective separation of magnesite from dolomite was achieved. 4. Conclusions
• BAPTA exhibited selective depression for dolomite. A magnesite
Fig. 10. XPS spectra of the untreated magnesite surface and the magnesite surface treated with BAPTA in the presence of Ca ions.
•
the adsorption of oleic acid was higher on the surface of magnesite than on the surface of dolomite, thereby indicating that BAPTA was active toward magnesite. As shown in Fig. 9a, in contrast to the FTIR spectra of magnesite, the peak at 2627.84 cm−1 was clearly observed for dolomite not treated with reagents. The band of dolomite treated with BAPTA was slightly shifted compared with that of dolomite not treated with the BAPTA depressant. The shape of the overall FTIR curve and the position of the bands were similar to those for the magnesite treated with BAPTA. However, as shown in Fig. 9a, the bands at 2921.92 and 2852.41 cm−1 for dolomite treated with NaOL were not observed in the FTIR spectra for dolomite treated with BAPTA + NaOL. In particular, in the adsorption spectra for BAPTA on dolomite at 3020.74–2852.41 cm−1, the characteristic adsorption bands of dolomite were significantly lower than those of dolomite not treated with BAPTA (Fig. 9b). The absorbance magnitude of this band was close to that of dolomite without any reagent treatment. This indicates that BAPTA had a strong depression effect on the adsorption of oleic acid on the dolomite surface, thereby indicating that the BAPTA depressant can improve the flotation selectivity between magnesite and dolomite.
•
concentrate with recovery of 95.8% a grade of 87.5%, and a CaO content of 3.8%, was obtained using microflotation with oleic acid as a collector at a pH of 11 and a BAPTA dosage of 62.5 mg/L. FTIR and XPS results indicated that the depression mechanism of BAPTA included not only the formation of a chelate with Ca species in the pulp but also adsorption on the active sites of dolomite; consequently, oleic acid was not adsorbed on the surface of dolomite and produce an effective depression. BAPTA not only depressed the adsorption of oleic acid on the dolomite surface but also improved the activation of the magnesite surface.
Acknowledgements This work was supported by the General Program of the National Natural Science Foundation of China (Grant No. 51874072), the National Natural Science Foundation of China (Grant No. 51504053), and the Fundamental Research Funds for the Central Universities of China (Grant No. 170107013). References Aslani, S., Samim-Bani-Hashemi, H.R., Arianpour, F., 2010. Beneficiation of Iranian magnesite ores by reverse flotation process and its effects on shaped and unshaped refractories properties. Bull. Mater. Sci. 33, 697–705. Baer, D.R., Moulder, J.F., 1993. High resolution XPS spectrum of calcite (CaCO3). Surf. Sci. Spectra 2, 1–7. Baer, D.R., Marmorstein, A.M., Williford, R.E., Blanchard, D.L., 1992. Comparison Spectra for Calcite by XPS. Surf. Sci. Spectra 1, 80–86. Botero, A.E.C., Torem, M.L.L., De Mesquita, M.S., 2007. Fundamental studies of Rhodococcus opacus as a biocollector of calcite and magnesite. Miner. Eng. 20, 1026–1032. Chen, G., Tao, D., 2004. Effect of solution chemistry on floatability of magnesite and dolomite. Int. J. Miner. Process. 74, 343–357. Chen, G.L., Tao, D., 2005. Reverse flotation of magnesite by dodecyl phosphate from dolomite in the presence of sodium silicate. Sep. Sci. Technol. 39, 377–390. Fukami, Y., Maeda, Y., 1998. Raman and FT-IR studies of photodynamic processes of cholesteryl oleate using IRFELs. Nucl. Instru. Methods Phys. Res. B 144, 229–235. Gence, N., 2006. Wetting behavior of magnesite and dolomite surfaces. Appl. Surf. Sci. 252, 3744–3750. Gence, N., Ozbay, N., 2006. pH dependence of electrokinetic behavior of dolomite and magnesite in aqueous electrolyte solutions. Appl. Surf. Sci. 252, 8057–8061. Hu, Y., Chi, R., Xu, Z., 2003. Solution chemistry study of salt–type mineral flotation systems: role of inorganic dispersants. Ind. Eng. Chem. Res. 42, 1641–1647. John, B., Emma, R., 2009. Calcium in biological systems. Adv. Inorg. Chem. 61, 251–366. Koide, Y., Yamada, K., 1990. Selective flotation of metal ions by using chelating surfactants. J. Jpn. Oil. Chem. Soc. 39, 736–743. Kozhevnikov, E.K., Kropanev, S.I., Baranovskii, N.I., 1973. Beneficiation of dolomites. Raw Mater. 3, 19–21. Luo, N., Wei, D.Z., Shen, Y.b., Han, C., Zhang, C.E., 2017. Elimination of the adverse effect of calcium ion on the flotation separation of magnesite from dolomite. Mineral 7, 150. Luo, X.M., Yin, W.Z., Wang, Y.F., Sun, C.Y., Ma, Y.Q., Liu, J., 2016. Effect and mechanism of dolomite with different size fractions on hematite flotation using sodium oleate as
3.2.3. XPS analysis results To analyze the composition of the untreated magnesite surface and the magnesite surface treated with BAPTA, XPS was performed. The results are shown in Fig. 10. To examine the effect of BAPTA on the precipitation of Ca, 10 mg/L CaCl2 was added to the solution of the magnesite single-mineral system. As shown in Fig. 10, the characteristic Auger peaks of XPS for magnesite were Mg1s, O1s, and C1s, and the bond energies were 1304.29, 531.23, and 284.91 eV, respectively. For the binding-energy peaks of magnesite treated with Ca2+, slight shifts appeared. Additionally, compared with the peaks of magnesite not treated with any reagents, new peaks corresponding to Ca2p and Ca2s appeared at 347.21 and 349 eV, respectively. Ca1s and Ca2p (8.9%) occupied approximately 10.7% of the Mg surface area, whereas Mg1s occupied 3.5%. According to previous reports, Ca2p3/2 and Ca2p1/2 optical peaks indicate the presence of calcite (Baer et al., 1992; Baer and Moulder, 1993). The Ca2p peak at 347.21 eV suggests that Ca existed in the chemical state of calcium carbonate on the Mg surface. As a result of the introduction of BAPTA prior to the addition of Ca, the peaks related to Ca did not appear, thereby indicating that the BAPTA strongly depressed the adsorption of Ca on the surface of magnesite. Moreover, the N atom in the BAPTA molecule was not detected on the 6
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W. Yin, et al. collector. J. Cent. South Univ. 23, 529–534. Marabini, A.M., Ciriachi, M., Plescia, P., Barbaro, M., 2007. Chelating reagents for flotation. Miner. Eng. 20, 1014–1025. Marinakis, K.I., Shergold, H.L., 1985. The mechanism of fatty acid adsorption in the presence of fluorite, calcite and barite. Int. J. Miner. Process. 14, 161–176. Matis, K.A., Balabanidis, T.H.N., Gallios, G.P., 1988. Processing of magnesium carbonate fines by dissolved–air flotation. Colloids Surf. 29, 191–203. Matis, K.A., Gallios, G.P., 1989. Anionic flotation of magnesium carbonates by modifiers. Int. J. Miner. Process. 25, 261–274. Moudgil, B.M., Ince, D.E., 1991. Flotation separation of apatite from dolomite using dodecylamineand sodium chloride. Part. Technol. Surf. Phenom. Pet 191–197. Nagaraj, D.R., 1982. Chelating agents in mineral processing. In: III AIME Annual Meeting, Dallas, pp. 14–18. Oiki, S., Yamamoto, T., Okada, Y., 1994. Apparent stability constants and purity of Ca–chelating agents evaluated using Ca–selective electrodes by the double–log
optimization method. Cell Calcium. 15, 209–216. Ricci, A.J., Wu, Y.C., Fettiplace, R., 1998. The endogenous calcium buffer and the time course of transducer adaptation in auditory hair cells. J. Neurosci. 18, 8261–8277. Ruan, Y., Zhang, Z., Luo, H., Xiao, C., Zhou, F., Chi, R., 2018. Effects of metal ions on the flotation of apatite, dolomite and quartz. Mineral 8, 141. Sorour, Mohame D.H., Hani, Heba A., Hayam, F Shaalan, Mayyada, M.H., El–Sayed,, 2016. Experimental screening of some chelating agents for calcium and magnesium removal from saline solutions. Desalin. Water. Treat. 57, 22799–22808. Wonyen, D.G., Kromah, V., Gibson, B., Nah, S., Chelgani, S.C., 2018. A review of flotation separation of mg carbonates (dolomite and magnesite). Mineral 8, 354. Yao, J., Yin, W., Gong, E., 2016. Depressing effect of fine hydrophilic particles on magnesite reverse flotation. Int. J. Miner. Process. 149, 84–93. Zhang, H., Liu, W.G., Han, C., Hao, H.Q., 2018. Effects of monohydric alcohols on the flotation of magnesite and dolomite by sodium oleate. J. Mol. Liq. 249, 1060–1067.
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Effect of Ca selective chelator BAPTA as depressant on flotation separation of magnesite from dolomite
T
Wanzhong Yina,b,c, Haoran Suna, , Jongsu Honga,d, , Shaohang Caoa, Bin Yanga, Changdok Wond, Myongsong Songe ⁎
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a
School of Resources & Civil Engineering, Northeastern University, Shenyang 110819, China Liaoning Key Laboratory of Mineral Processing Technology, Shenyang 110819, China c Northeastern University Genetic Mineral Processing Research Center, Shenyang 110819, China d Faculty of Resource Probing Engineering, Kim Chaek University of Technology, Pyongyang 1001, Democratic People’s Republic of Korea e Faculty of Mining Engineering, Kim Chaek University of Technology, Pyongyang 1001, Democratic People’s Republic of Korea b
ARTICLE INFO
ABSTRACT
Keywords: Magnesite Dolomite Depressant Chelator BAPTA Flotation
With the depletion of high-grade magnesite mineral resources, improving the grade of magnesite obtained from gangue minerals such as dolomite has been attracting considerable attention from the industry. However, it is difficult to achieve the effective separation of magnesite from dolomite because of their similar chemical and crystal properties. In this study, 1, 2-bis (o-aminophenoxy) ethane-N,N,N′,N′-tetraacetic acid (BAPTA) was used as a depressant for dolomite to improve the separation of magnesite from dolomite. Microflotation tests of the single minerals and artificially mixed minerals using oleic acid as a collector were performed to evaluate the selective depression effect of BAPTA. The results indicated that the BAPTA depressant had a selective effect on dolomite, thereby suggesting that a magnesite grade of 87.5%, recovery of 95.8%, and CaO content of 3.8% can be achieved with a BAPTA dosage of 62.5 mg/L. The depression mechanism of BAPTA was investigated using zeta-potential measurements, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results indicated that BAPTA was selectively adsorbed on the surface of dolomite. The selective depression effect can be attributed to the selective chelating formation of BAPTA on Ca rather than Mg, which not only adsorbed BAPTA decreasing the adsorption sites of oleic acid on the dolomite surface, but also prevented the precipitation of Ca on the surface of the magnesite.
1. Introduction Magnesite often contains carbonate minerals in the form of gangue because of its formation characteristics (Aslani et al., 2010; Botero et al., 2007; Chen and Tao, 2004; Kozhevnikov et al., 1973; Luo et al., 2016). In particular, the effective flotation separation of magnesite from dolomite, which is one of the carbonate minerals, is difficult because of their similar crystal structure, chemical composition, and surface properties (Gence and Ozbay, 2006; Gence, 2006; Hu et al., 2003; Luo et al., 2017). Therefore, various techniques have been employed to improve the selectivity of flotation of magnesite from dolomite; these techniques include flotation systems, using different types of reagents, and changing the addition order of the reagents. The contact angles of magnesite and dolomite were investigated in the presence of oleic acid and dodecylamine (DDA) as a collector
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without a depressant (Gence, 2006; Gence and Ozbay, 2006; Yao et al., 2016). Magnesite and dolomite exhibit small contact angles of 10.4° and 6.6°, respectively, in distilled water, thereby indicating that they are hydrophilic. With the addition of DDA as a cationic collector, there is no significant change in the contact angle of magnesite, but the contact angle of dolomite increases to 88°, thereby indicating that it becomes hydrophobic. Thus, the recoveries of magnesite and dolomite are approximately 5% and 50%, respectively, in reverse flotation for single minerals using DDA. In the presence of sodium oleate as an anionic collector, the contact angles of magnesite and dolomite increase to 79° and 39°, respectively. This suggests that it is possible to introduce reverse flotation using DDA and direct flotation using sodium oleate. Some reagents such as carboxymethyl cellulose and sodium silicate can increase the difference between the contact angles of magnesite and dolomite (Zhang et al., 2018). Additionally, Acintol FA–1 tall oil and SH were used as a collector
Corresponding authors at: School of Resources & Civil Engineering, Northeastern University, Shenyang 110819, China (H. Sun and J. Hong). E-mail addresses: [email protected] (H. Sun), [email protected] (J. Hong).
https://doi.org/10.1016/j.mineng.2019.106050 Received 4 March 2019; Received in revised form 24 September 2019; Accepted 25 September 2019 Available online 27 September 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
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and a depressant, respectively, to achieve effective separation of magnesite (Chen and Tao, 2004; Chen and Tao, 2005; Marinakis and Shergold, 1985; Matis et al., 1988; Matis and Gallios, 1989; Ruan et al., 2018; Wonyen et al., 2018). Although various reagents have been used in the form of a single or mixed depressant, these depressants are insufficient for achieving adequate differential flotation selectivity between magnesite and calcite. Although there are differences in the recovery rates of flotation, the trends are similar (Chen and Tao, 2004, 2005; Gence and Ozbay, 2006; Matis et al., 1988; Matis and Gallios, 1989; Moudgil and Ince, 1991; Yao et al., 2016). Despite significant progress of research based on single minerals, flotation tests for the treatment of mixed minerals indicate an insufficient separation effect, and the reason behind it, has rarely been studied. The main cause of the insufficient separation of magnesite from dolomite was investigated by Luo et al. (2017); the results indicated that the Ca species dissolved from the dolomite surface precipitated on the surface of the magnesite, thereby making the surface of magnesite similar to that of dolomite. In this study, to achieve differential flotation selectivity between magnesite and dolomite using oleic acid as a collector, the Ca-ion selection chelate 1, 2-bis (o-aminophenoxy) ethane-N,N,N′,N′-tetraacetic acid (BAPTA) was employed as a depressant for improving the separation effect of magnesite. It is more selective for Ca than for Mg, and the selective action of the four carboxylic acid functional groups allows binding with Ca ions (Koide and Yamada, 1990; Nagaraj, 1982; Marabini et al., 2007; Sorour et al., 2016; Ricci et al., 1998). The stability constants pKca of BAPTA for binding with Ca and Mg are 6.9 and 1.77, respectively, thereby indicating that BAPTA is far more stable for binding with Ca than with Mg (Baer et al., 1992; John and Emma, 2009; Oiki et al., 1994). In this study, BAPTA was considered as a depressant, and attempts were made to improve the selective separation of magnesite from dolomite. Microflotation tests, zeta-potential measurements, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were performed to investigate the depression mechanism.
Fig. 1. Structure of BAPTA.
2.2. Microflotation tests Microflotation experiments involving single minerals and artificially mixed minerals were performed in a 30-mL microflotation cell of a laboratory XFG self-aeration flotation machine (Jilin Exploration Machinery Plant, China) at a stirring speed of 1800 rpm. Simple comparative experiments on the addition order of the reagents indicated that it was advantageous to inject the depressant before stirring the solution. Deionized water (20 mL) and the sample (2.0 g) were added to the microflotation cell, and after the addition of BAPTA, the microflotation was started. A flowsheet of the microflotation test is shown in Fig. 2. Finally, the concentrate was scraped, dried, and weighed; the recovery of the concentrate and tailings was calculated, and various analyses were performed. Each experiment was conducted at least three times, and the average results were calculated. In accordance with a previous report (Luo et al., 2017), the pH was set as 11 in this experiment. BAPTA is stable at all pH values and is not significantly affected by the pH. In the microflotation test, oleic acid, which is known to be stable under high-pH conditions, was used as a collector. Therefore, the pH was set as 11 for all experiments. In single-mineral experiments, 2.0 g of single-mineral samples or artificially mixed minerals with a magnesite-to-dolomite mass ratio of 1:1 were used. The microflotation experiments were performed to examine the selectivity effect of BAPTA on the flotation of magnesite and dolomite with respect to the BAPTA concentration. The sodium oleate dosage was 120 mg/L, and the experiments were performed at 25 ± 1 °C. The recovery rates, grades, and CaO contents of the concentrate were evaluated at different BAPTA concentrations. To investigate the effect of BAPTA on the floatability of the magnesite single mineral in the presence of Ca, the amount of Ca ions in the solution was adjusted using CaCl2 (with the same number of moles of NaCO3).
2. Materials and methods 2.1. Materials and reagents The samples of magnesite and dolomite used in this study were obtained from Haicheng magnesite mine, Liaoning Province, China. The phase compositions of the two minerals were determined via X-ray diffraction analysis (X’Pert Pro MPDDY2094, PANalytical B.V., Holland), and the chemical compositions were determined via X-ray fluorescence chemical analysis. The magnesite sample consisted of 45.80% MgO, 1.24% CaO, 0.14% Fe2O3, and 1.94% SiO2. The dolomite sample consisted of 21.36% MgO, 30.34% CaO, 0.08% Fe2O3, and 1.50% SiO2. The results indicated that the purities of magnesite and dolomite exceeded 95%. The samples were crushed, dried, ground by a porcelain ball mill, sieved, and classified to desired particle sizes. A particle size of −78 + 37 μm was selected for microflotation experiments and XPS, whereas finer particles of < 37 μm were ground to < 5 μm in an agate mortar for FTIR spectroscopy and zeta-potential measurements. BAPTA (depressant), sodium oleate (NaOL) (collector), and CaCl2 (regulator) were purchased from China National Pharmaceutical (analytical grade). BAPTA was prepared using dimethyl sulfoxide (DMSO) as a solvent and diluted to the corresponding volume with deionized water before the microflotation tests. The structure of the Ca chelator BAPTA is shown in Fig. 1. The pH was adjusted using NaOH and HCl solutions, and deionized water was used for all the experiments.
Fig. 2. Flowsheet of the microflotation test. 2
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2.3. Zeta-potential tests Zeta-potential measurements of magnesite and dolomite were conducted using a Nano–ZS90 zeta-potential analyzer (Malvern Instruments Ltd., Britain). The 20 mg mineral sample was finely ground to −5 μm in an agate mortar, and 50 mL of deionized water was added. CaCl2 was added to the Mg sample solution to adjust the Ca concentration of the solution, and the zeta potential was measured as a function of BAPTA dosage. The collector, oleic acid (120 mg/L), was added, and the pH was adjusted to 11 using HCl and NaOH solutions. The solution was magnetically stirred for 5 min to ensure that the suspension was homogenized. The suspension was transferred to a sample cuvette, and then the zeta potential was measured. Each value was the average of three measurements. 2.4. FTIR spectroscopy The single minerals were ground to −5 μm in an agate mortar. A 1.0 g sample was added to a 20 mL glass cell, followed by Ca ions and BAPTA. Then, the solution was stirred for 2 min, and the pH was adjusted to 11 for 3 min before the solids and liquids were separated. The solid phase was removed via filtration, washed three times with deionized water, and dried under vacuum conditions at 40 °C for 24 h. The infrared spectra of the samples were recorded using a Nicolet 380 FTIR spectrometer at 25 ± 2 °C in the range of 4000–500 cm−1.
Fig. 3. Flotation performance for single minerals with respect to BAPTA dosage (oleic acid concentration = 4 × 10−4 mol/L, pH = 11.0).
2.5. XPS measurements The sample preparation for XPS was identical to that for FTIR spectroscopy. XPS spectra of samples treated with and without BAPTA were recorded using an America Thermo VG ESCALAB 250Xi spectrometer using Al α X-rays (1486.6 eV) as a sputtering source at a power of 150 W (15 kV, 10 mA). Based on a C1s peak from contaminations (around 284.8 eV), the binding energy scale was corrected in accordance with the internal binding-energy standard (Marinakis and Shergold, 1985). First, each sample was scanned extensively to identify the elements constituting the mineral samples. Subsequently, the highresolution scans were used to acquire more data in various elemental energy regions. To analyze the XPS data, including peak fitting and semiquantitative estimation, the Avantage software was used. 3. Results and discussion
Fig. 4. Effect of Ca2+ on the recovery of magnesite (BAPTA dosage of 50 mg/L, oleic acid concentration = 4 × 10−4 mol/L, pH = 11.0).
3.1. Microflotation test results
magnesite was maintained, but when Ca was added prior to BAPTA, the recovery decreased rapidly (Fig. 4). The results are consistent with those reported by Luo et al. (2017); the decrease in recovery was due to the dissolution of dolomite and adsorption of dissolved Ca species, thereby resulting in the presence of Ca on the surface of the magnesite similar to the dolomite surface. Consequently, magnesite was depressed. The results indicate that the addition order of the reagents is very important. In this case, the recovery of magnesite decreased significantly with an increase in the Ca concentration; this was similar to the tendency of depression for the single-mineral flotation test of dolomite. The recovery of magnesite was 76.3% without Ca. It changed when the calcium concentration was higher than 15 mg/L; the recovery decreased to 39.6% at a Ca concentration of 15 mg/L. This tendency was also observed in the flotation tests of the artificially mixed minerals at a pH of 11 with oleic acid as a collector. As shown in Fig. 5, BAPTA had a significant depression effect on dolomite: the recovery of dolomite was 13.8% with a BAPTA dosage of 62.5 mg/L and 76.4% without BAPTA. Conversely, at a BAPTA dosage of 62.5 mg/L, the recovery of magnesite increased from 78.2% to a maximum value of 95.8%. In this case, the grades of magnesite and
Single-mineral flotation tests were conducted to observe the effects of the BAPTA depressant dosage on the flotation of magnesite and dolomite. The results are shown in Fig. 3. Fig. 3 shows that the floatability of dolomite decreased rapidly in the beginning with an increase in the depressant dosage at a pH of 11 and an oleic acid content of 120 mg/L, thereby indicating that BAPTA had a strong depression effect on dolomite. The Ca species dissolved in the solution from the surface of the dolomite in the initial stage of pulp agitation was combined with BAPTA to form a Ca chelate, which was suspended in the solution. Simultaneously, the depressant was adsorbed at the sites of the Ca ions on the surface of dolomite. Consequently, the adsorption of oleic acid on the dolomite surface was hindered. Fig. 3 shows that the flotation recovery of dolomite was 66.6% without BAPTA under the same flotation conditions, but the recovery decreased rapidly with an increase in the BAPTA dosage. At a BAPTA dosage of 50 mg/L, the recovery of dolomite decreased linearly with a recovery of 2.6% and changed very little thereafter. At a BAPTA dosage of 87.5 mg/ L, the recovery of dolomite was 0.98%. The stable constant of the BAPTA chelator with Mg on the surface of magnesite was lower than that with Ca on the surface of dolomite. In the case of the addition of BAPTA followed by Ca, the recovery of 3
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Fig. 5. Recovery and grade of flotation with respect to BAPTA dosage for artificially mixed minerals.
Fig. 7. Zeta potentials of magnesite with and without BAPTA as a function of Ca concentration (BAPTA 50 mg/L).
cation adsorption sites available for adsorption of the negatively charged groups of the anion collector was reduced. Therefore, at a pH of 11, the RCOO– and RCOOHRCOO– species of oleic acid were adsorbed to a greater extent on the magnesite surface than on the dolomite surface. According to Fig. 6, the zeta potential of magnesite was not affected by the BAPTA dosage, but it increased slightly (from −34.05 to −30.15 mV) as the BAPTA dosage increased from 0 to 75 mg/L, thereby indicating that the BAPTA depressed the adsorption of Ca species on the magnesite surface by forming the chelate with the Ca ions of the liquid phase of the pulp. The effect of Ca on the zeta potential of magnesite in the presence of BAPTA is shown in Fig. 7. For this measurement, BAPTA was added prior to solution conditioning to form the Ca chelate in the solution before the adsorption or precipitation of dissolved ions on the magnesite surface. Fig. 7 indicates that in the presence of Ca2+ in the aqueous solution, without the depressant, the zeta potential increased due to the adsorption of Ca species on the surface of magnesite. The zeta potential of magnesite without BAPTA increased by 14.95 mV; it was −19.1 mV at a Ca concentration of 25 mg/L and −34.05 mV without Ca. However, when a BAPTA dosage of 50 mg/L was added followed by Ca, the zeta potential of magnesite did not increase significantly. It shifted only by 3.65 mV in the same Ca-concentration range. In the Ca-concentration range of 0–10 mg/L, the zeta potential of magnesite did not change due to the effect of BAPTA, but it changed slightly at a higher Ca concentration, thereby indicating that the BAPTA dosage depended on the Ca concentration. Importantly, the BAPTA depressant in the aqueous solution significantly reduced the zeta potential of the dolomite but did not significantly affect the magnesite.
Fig. 6. Zeta potentials of magnesite and dolomite at different BAPTA dosages.
dolomite were 87.49% and 12.5%, respectively, and the CaO content was 3.8%. As shown in Fig. 6, because of the selective separation effect of BAPTA on the recovery rate and grade of magnesite and dolomite, the two minerals were effectively separated with the increasing BAPTA concentration. Fig. 5 indicates that the BAPTA acted as a depressant only on the dolomite and functioned as an activator for magnesite, thereby suggesting that the selective separation of the two minerals can be achieved with BAPTA alone.
3.2.2. FTIR spectroscopy results FTIR spectroscopy was performed to elucidate the action mechanism of BAPTA on the surfaces of magnesite and dolomite for the flotation test using oleic acid as a collector. The results are shown in Figs. 8 and 9, respectively. In the NaOL spectrum, bands corresponding to the stretching vibrations of the functional groups eCH2e and eCH3 appear at 2921.31 and 2851.26 cm−1, respectively. The peaks at 1446.49 and 1560.65 cm−1 are the symmetric and asymmetric stretching vibration peaks, respectively, of carboxyl (eCOOe), which is the main functional group of NaOL (Fukami and Maeda, 1998). The peak at 721.89 cm−1 is the in-plane bending vibration adsorption peak of e(CH2)ne. The main functional groups of BAPTA consisted of four carboxyl groups. As shown in Fig. 8a, one of the characteristic peaks in the
3.2. Mechanism study 3.2.1. Zeta-potential results To elucidate the depression mechanism of BAPTA on the surface of the minerals, the zeta potentials at different BAPTA dosages and a pH of 11 were measured. The results are shown in Fig. 6. As shown in Fig. 6, the magnesite and dolomite were both negatively charged. As the BAPTA dosage increased, the zeta potential of dolomite shifted more toward the negative direction. The zeta potential of dolomite without BAPTA was −15.7 mV, but it was shifted by 22.1 mV in the negative direction (to −37.8 mV) at a BAPTA dosage of 75 mg/L. This indicates that owing to the adsorption of the negatively charged BAPTA functional groups on the Ca sites of the dolomite, the surface of the dolomite had a more negative charge, and the number of 4
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Fig. 9. FTIR spectra of untreated dolomite and dolomite treated with BAPTA: (a) transmittance in the range of 4000–400 cm−1; (b) absorbance in the range of 3200–2700 cm−1. Fig. 8. FTIR spectra of untreated magnesite and magnesite treated with BAPTA: (a) transmittance in the range of 4000–400 cm−1; (b) absorbance in the range of 3200–2700 cm−1.
assigned to the symmetrical stretching vibration of CO3, the out-ofplane bending vibration, and the in-plane deformation vibration, respectively, and weak bands appear at 2942.33, 2857.88, and 2536.07 cm−1. Bands at 2921.68 and 2852.50 cm−1 were observed for the magnesite samples treated with NaOL, thereby indicating that NaOL was absorbed on the Mg surface. The small shifts at 2921.31 and 2851.26 cm−1 for NaOL indicate the chemical adsorption of NaOL on the magnesite surface. No bands other than those of NaOL were detected because of the covering of the magnesite bands. The shape, size, and position of the bands corresponding to magnesite treated with BAPTA did not change, thereby suggesting that BAPTA did not significantly affect the adsorption of NaOL on the surface of magnesite. However, the bands at 2921.31 and 2851.26 cm−1 for NaOL were shifted slightly to 2922.06 and 2852.47 cm−1, respectively. The adsorption at 3053.54–2852.47 cm−1 is compared between magnesite + NaOL and magnesite + BAPTA + NaOL in Fig. 8b. The addition of BAPTA prior to the conditioning of the pulp revealed that
BAPTA spectrum was the wide adsorption peak of OeH and CeO of the dimers of the carboxyl group, which was the main functional group of this chelator. This peak appeared weakly in the range of 2500–3300 cm−1. The peak at 1714.69 cm−1 is assigned to the symmetric adsorption band of C]O. The CeO stretching vibration adsorption bands (1320–1210 cm−1) and the CeN vibration adsorption bands of the tertiary amine R1R2R3N (1100–1030 cm−1) are also characteristic bands of BAPTA, and the OeH out-of-plane bending vibration adsorption bands appear at 975.24 cm−1. They are complex and weak. The adsorption band at 749.75 cm−1 is attributed to the out-ofplane deformation vibration of eCH2eCH2e at the center of the BAPTA molecule. The high-frequency vibration band of 1350–1250 cm−1 corresponding to N-benzene is not obvious. For magnesite, the peaks at 1020.26, 886.01, and 748.04 cm−1 are 5
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magnesite surface, thereby implying that the BAPTA was selectively bound to Ca ions in the solution, forming a chelate, and not to Mg on the surface of magnesite. For the surfaces of magnesite and dolomite used in this study, the atomic distribution ratios of Mg1s were 8% and 4.6%, respectively. And the sum of Ca1s and Ca2p (8.73%) on the dolomite surface was about 10.3%. The comparison indicates that the distribution ratio of Ca adsorbed on the surface of magnesite not treated with BAPTA was similar to that of Ca adsorbed on the dolomite surface. In the aqueous solution, Ca species were adsorbed on the magnesite surface in the form of calcium carbonate; thus, the distributions of metal atoms on the magnesite surface were similar to those on the dolomite surface (Luo et al, 2017). However, by using BAPTA, which formed a chelate only with Ca ions in the solution and on the dolomite surface, the selective separation of magnesite from dolomite was achieved. 4. Conclusions
• BAPTA exhibited selective depression for dolomite. A magnesite
Fig. 10. XPS spectra of the untreated magnesite surface and the magnesite surface treated with BAPTA in the presence of Ca ions.
•
the adsorption of oleic acid was higher on the surface of magnesite than on the surface of dolomite, thereby indicating that BAPTA was active toward magnesite. As shown in Fig. 9a, in contrast to the FTIR spectra of magnesite, the peak at 2627.84 cm−1 was clearly observed for dolomite not treated with reagents. The band of dolomite treated with BAPTA was slightly shifted compared with that of dolomite not treated with the BAPTA depressant. The shape of the overall FTIR curve and the position of the bands were similar to those for the magnesite treated with BAPTA. However, as shown in Fig. 9a, the bands at 2921.92 and 2852.41 cm−1 for dolomite treated with NaOL were not observed in the FTIR spectra for dolomite treated with BAPTA + NaOL. In particular, in the adsorption spectra for BAPTA on dolomite at 3020.74–2852.41 cm−1, the characteristic adsorption bands of dolomite were significantly lower than those of dolomite not treated with BAPTA (Fig. 9b). The absorbance magnitude of this band was close to that of dolomite without any reagent treatment. This indicates that BAPTA had a strong depression effect on the adsorption of oleic acid on the dolomite surface, thereby indicating that the BAPTA depressant can improve the flotation selectivity between magnesite and dolomite.
•
concentrate with recovery of 95.8% a grade of 87.5%, and a CaO content of 3.8%, was obtained using microflotation with oleic acid as a collector at a pH of 11 and a BAPTA dosage of 62.5 mg/L. FTIR and XPS results indicated that the depression mechanism of BAPTA included not only the formation of a chelate with Ca species in the pulp but also adsorption on the active sites of dolomite; consequently, oleic acid was not adsorbed on the surface of dolomite and produce an effective depression. BAPTA not only depressed the adsorption of oleic acid on the dolomite surface but also improved the activation of the magnesite surface.
Acknowledgements This work was supported by the General Program of the National Natural Science Foundation of China (Grant No. 51874072), the National Natural Science Foundation of China (Grant No. 51504053), and the Fundamental Research Funds for the Central Universities of China (Grant No. 170107013). References Aslani, S., Samim-Bani-Hashemi, H.R., Arianpour, F., 2010. Beneficiation of Iranian magnesite ores by reverse flotation process and its effects on shaped and unshaped refractories properties. Bull. Mater. Sci. 33, 697–705. Baer, D.R., Moulder, J.F., 1993. High resolution XPS spectrum of calcite (CaCO3). Surf. Sci. Spectra 2, 1–7. Baer, D.R., Marmorstein, A.M., Williford, R.E., Blanchard, D.L., 1992. Comparison Spectra for Calcite by XPS. Surf. Sci. Spectra 1, 80–86. Botero, A.E.C., Torem, M.L.L., De Mesquita, M.S., 2007. Fundamental studies of Rhodococcus opacus as a biocollector of calcite and magnesite. Miner. Eng. 20, 1026–1032. Chen, G., Tao, D., 2004. Effect of solution chemistry on floatability of magnesite and dolomite. Int. J. Miner. Process. 74, 343–357. Chen, G.L., Tao, D., 2005. Reverse flotation of magnesite by dodecyl phosphate from dolomite in the presence of sodium silicate. Sep. Sci. Technol. 39, 377–390. Fukami, Y., Maeda, Y., 1998. Raman and FT-IR studies of photodynamic processes of cholesteryl oleate using IRFELs. Nucl. Instru. Methods Phys. Res. B 144, 229–235. Gence, N., 2006. Wetting behavior of magnesite and dolomite surfaces. Appl. Surf. Sci. 252, 3744–3750. Gence, N., Ozbay, N., 2006. pH dependence of electrokinetic behavior of dolomite and magnesite in aqueous electrolyte solutions. Appl. Surf. Sci. 252, 8057–8061. Hu, Y., Chi, R., Xu, Z., 2003. Solution chemistry study of salt–type mineral flotation systems: role of inorganic dispersants. Ind. Eng. Chem. Res. 42, 1641–1647. John, B., Emma, R., 2009. Calcium in biological systems. Adv. Inorg. Chem. 61, 251–366. Koide, Y., Yamada, K., 1990. Selective flotation of metal ions by using chelating surfactants. J. Jpn. Oil. Chem. Soc. 39, 736–743. Kozhevnikov, E.K., Kropanev, S.I., Baranovskii, N.I., 1973. Beneficiation of dolomites. Raw Mater. 3, 19–21. Luo, N., Wei, D.Z., Shen, Y.b., Han, C., Zhang, C.E., 2017. Elimination of the adverse effect of calcium ion on the flotation separation of magnesite from dolomite. Mineral 7, 150. Luo, X.M., Yin, W.Z., Wang, Y.F., Sun, C.Y., Ma, Y.Q., Liu, J., 2016. Effect and mechanism of dolomite with different size fractions on hematite flotation using sodium oleate as
3.2.3. XPS analysis results To analyze the composition of the untreated magnesite surface and the magnesite surface treated with BAPTA, XPS was performed. The results are shown in Fig. 10. To examine the effect of BAPTA on the precipitation of Ca, 10 mg/L CaCl2 was added to the solution of the magnesite single-mineral system. As shown in Fig. 10, the characteristic Auger peaks of XPS for magnesite were Mg1s, O1s, and C1s, and the bond energies were 1304.29, 531.23, and 284.91 eV, respectively. For the binding-energy peaks of magnesite treated with Ca2+, slight shifts appeared. Additionally, compared with the peaks of magnesite not treated with any reagents, new peaks corresponding to Ca2p and Ca2s appeared at 347.21 and 349 eV, respectively. Ca1s and Ca2p (8.9%) occupied approximately 10.7% of the Mg surface area, whereas Mg1s occupied 3.5%. According to previous reports, Ca2p3/2 and Ca2p1/2 optical peaks indicate the presence of calcite (Baer et al., 1992; Baer and Moulder, 1993). The Ca2p peak at 347.21 eV suggests that Ca existed in the chemical state of calcium carbonate on the Mg surface. As a result of the introduction of BAPTA prior to the addition of Ca, the peaks related to Ca did not appear, thereby indicating that the BAPTA strongly depressed the adsorption of Ca on the surface of magnesite. Moreover, the N atom in the BAPTA molecule was not detected on the 6
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