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Reverse froth flotation of magnesite ore by using (12-4-12) cationic gemini surfactant
Minerals Engineering 110 (2017) 65–68
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Short communication
Reverse froth flotation of magnesite ore by using (12-4-12) cationic gemini surfactant
MARK
⁎
Ivan Brezáni , Jiří Škvarla, Martin Sisol Department of Mineral Processing, Institute of Earth’s Resources, Faculty of Mining, Ecology, Process Control and Geotechnology, Technical University of Košice, Letná 9, 042 00 Košice, Slovakia
A R T I C L E I N F O
A B S T R A C T
Keywords: Froth flotation Reverse flotation Magnesite ore Cationic collector Gemini surfactant
The cationic gemini surfactant 1,4-bis(dodecyl-N,N-dimethylammoniumbromide)butane (BDDAB) was tested as a possible collector in the reverse froth flotation of magnesite ore. This novel type of collector was compared with its conventional monomeric equivalents: primary dodecylamine (DA) and tertiary dimethyldodecylamine (DDA). Electrokinetic measurements documented a strong effect of the synthesized gemini surfactant on the zeta potential of quartz. The effect of the three collectors declined in the following order: BDDAB > DA > DDA. Laboratory froth flotation tests confirmed an improved collecting ability of the gemini surfactant compared to both the conventional collectors. The desilication efficiency declined in the following order: BDDAB > DA > DDA. The results obtained with the lowest concentration of the gemini surfactant were better in terms of SiO2 content than those obtained with four times higher dosages of the conventional collectors.
1. Introduction Reverse froth flotation using primary ammonium collector, dodecylamine (DA) has been proven to be efficient beneficiation method to obtain high grade magnesite concentrate (low SiO2 content) (Yao et al., 2016). DA is one of the most used collectors in the reverse froth flotation of silicates from bauxite (Liu et al., 2015) and is also suitable for the reverse cationic flotation of high silica iron ore (Fillipov et al., 2014). Dodecyl tertiary amines (Liu et al., 2009) and quaternary amines (Wang et al., 2004; Jiang et al., 2014) can also be used as collectors of silicate minerals with good results. Recent findings have however proven, that cationic gemini surfactants have stronger collecting power than their monomeric equivalents (Xia et al., 2009). Dimeric or gemini surfactants are compounds with two hydrophobic chains and two hydrophilic head groups, covalently connected through a spacer chain. Gemini surfactants, when compared to their monomeric equivalents are more efficient at lowering the surface tension of water and have lower critical micellization concentrations, resulting in better hydrophobization of minerals and better flotation performance (Xia et al., 2009). Gemini surfactants have also been proven to be more efficient in studies of the reverse froth flotation of iron ore (Huang et al., 2014) and bauxite (Xia et al., 2010). To our knowledge, however, the use of quaternary ammonium gemini surfactants in the reverse froth flotation of magnesite ore has not yet been described. Therefore, our interest is to verify whether the
⁎
gemini surfactants can be efficiently used in the reverse flotation of magnesite ore by a direct comparison of results of laboratory froth flotation using dimeric ammonium based gemini surfactant and similar conventional surfactants which contain one hydrophobic group in the molecules. 2. Experimental 2.1. Minerals and reagents Ground magnesite ore (95% passing 265 µm) and hand-picked samples of magnesite and quartz were acquired from the company Gemerská nerudná spoločnosť, Hnúšťa-Mútnik, Slovakia. Phase composition characterization of finely powdered ore was done using X-ray diffraction (XRD) analysis on D8 Bruker 2θ diffractometer using CuKα radiation. JCPDS PDF 2 database was used for interpretation of the measured spectra. The 12-4-12 gemini quaternary ammonium salt 1,4-bis(dodecylN,N-dimethylammoniumbromide)butane (BDDAB) surfactant was synthesized by refluxing 0.1 mol of 1,4-Dibromobutane (99%, ACROS Organics, Belgium) with 0.21 mol of N,N-Dimethyldodecylamine (95 %, ACROS Organics, Belgium) in dry acetone (99.5%, mikroCHEM, Slovakia) for 20 h. The solvent and the reagents were used as received without further purification. Solid, that precipitated upon cooling was then filtered, washed with dry acetone and recrys-
Corresponding author. E-mail address: [email protected] (I. Brezáni).
http://dx.doi.org/10.1016/j.mineng.2017.04.013 Received 19 December 2016; Received in revised form 12 April 2017; Accepted 21 April 2017 0892-6875/ © 2017 Published by Elsevier Ltd.
Minerals Engineering 110 (2017) 65–68
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3. Results and discussion
tallized from dry acetone to obtain the desired gemini cationic surfactant in form of a pure white powder. The yield of the reaction was 88.4% of the theoretical amount. 1 H and 13C NMR spectra were recorded in DMSO-d6 on a Varian VNMRS spectrometer operating at 599.87 MHz (1H) and 150.84 MHz (13C) at room temperature, using an internal standard tetramethylsilane (0.00 ppm). The spectra, which are included as a supplementary material, confirmed a high purity of the sample which was better than 99%. The methyl and methylene proton signals showed expected chemical shifts and integral intensities. The HSQC experiment allowed assignment of the 13C NMR signals. Signals of the methyl and methylene protons and carbons attached to protonated nitrogens were deshielded and adequately shifted downfield. 1 H NMR spectrum (ppm): δH = 0.86 (6H, t, J = 7.2 Hz, 2 CH3), 1.22–1.35 (36H, m, 18 CH2), 1.63–1.70 (8H, m, 4 CH2), 3.01 (12H, s, 4 CH3N(+)), 3.22–3.27 (4H, m, 2 CH2N(+)), 3.28–3.33 (4H, m, 2 CH2N(+)). 13 C NMR spectrum (ppm): δC = 13.93 (2 CH3), 18.99, 21.71, 22.06, 25.81, 28.53, 28.68, 28.84, 28.94, 29.00, 29.00, 31.26 (11 × 2 CH2), 50.02 (4 CH3N(+)), 62.00 (2 CH2N(+)), 63.18 (2 CH2N(+)). Synthesis route and resulting structure of the BDDAB surfactant is shown in Fig. 1.
3.1. Ore analysis The XRD measurement confirmed the presence of magnesite as a valuable mineral, which is accompanied with talc, chlorite, quartz and dolomite in intergranular layers. The chemical composition of the ore used in flotation tests was 47.8% LOI, 43.1% MgO, 4.0% SiO2, 2.4% Fe2O3 and 0.4% CaO, characterized by the d80 value of 190 µm and only 5.0 wt.% of fine particles below 45 µm. 3.2. Investigation of collector properties Both conventional (Liu et al., 2015, 2009; Fillipov et al., 2014; Jiang et al., 2014) and gemini (Xia et al., 2009; Huang et al., 2014) cationic collectors are physically adsorbed on the surface of minerals by an electrostatic attraction. Adsorption of these collectors on individual minerals can be controlled by the adjustment of pH of the system. The point of zero charge (PZC) of the minerals is thus the most important property (Fuerstenau and Pradip, 2005). The PZC of present silicate minerals, talc (Fuerstenau and Pradip, 2005) and chlorite (Tan et al., 2013), and that of quartz (Yao et al., 2016; Huang et al., 2014; Fuerstenau and Pradip, 2005) lies in the acidic range, below pH 3.0. The PZC values of magnesite and dolomite were measured at pH 6.7 and 6.2, respectively (Yao et al., 2016). At pH 6.5, chosen for the batch flotation tests, the surface charge of magnesite should be positive, while that of the silicate minerals and quartz should be negative. Neutral, up to slightly acidic pH was also found to provide the best flotation recovery for silicate minerals using dodecylamine (Guo, 2010), dodecyl tertiary amines (Liu et al., 2009), quaternary amines (Wang et al., 2004; Jiang et al., 2014) and also ammonium gemini surfactant (Xia et al., 2009). Decreasing collecting abilities of amine collectors in alkaline pulps can be attributed to the formation of their basic amine forms and the simultaneous decrease (especially at pH > 8) of their presence in protonated ammonium forms. pKa values found in specifications of chemical properties of the surfactants are 10.63 and 9.97 for DA and DDA, respectively. In protonated forms, the nitrogen cations of the collectors are electrostatically attracted to the negatively charged surfaces of minerals. The nitrogen atom of a neutral amine present in alkaline pulp environments does not interact with negatively charged surfaces and there is, therefore, a drop in the achieved recoveries. The gemini collector does not have protonation states, because the addition of a fourth alkyl substituent to nitrogen results in a permanent cationic
2.2. Zeta potential analysis Zeta (ζ) potential was determined using SurPASS (Anton-Paar, Austria) electrokinetic analyzer equipped with an automatic titration unit and calculated by Attract software from streaming potential measurements using the Fairbrother - Mastin equation.
2.3. Laboratory froth flotation Froth flotation tests were done in a 3-litre flotation cell. After the slurry (density of 300 g L−1) was transferred to the agitated flotation cell (1600 rpm) and agitated for 3 min, pH was adjusted to 6.5 using an HCl solution. The surfactant solution was then added and the slurry was agitated for 15 min. After the conditioning period, 65 g t−1 of Methyl Isobutyl Carbinol (MIBC) frother was added and agitated for another 1 min. The air was then introduced with 4 L·min−1 volumetric flowrate and 10 min flotation period was conducted. Three tests were run at each condition and average values are reported.
Fig. 1. Synthesis route of the gemini surfactant BDDAB.
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Table 1 Results of the laboratory froth flotation tests. Dosage of collector
Cell product
DA (g t−1)
DDA (g t−1)
BDDAB (g t−1)
Yield (%)
SiO2 (%)
0 100 200 0 0 0 0 0
0 0 0 100 200 0 0 0
0 0 0 0 0 50 100 200
97.2 96.5 96.2 96.9 96.4 96.6 95.9 93.1
2.8 2.2 2.0 2.5 2.4 1.7 1.3 1.0
content of fines minimized the entrainment effect, resulting in froth products with relatively high SiO2 contents and high yields of the magnesite concentrate. Desilication efficiency of the investigated collectors declines in the following order: BDDAB > DA > DDA. Gemini surfactant showed very strong collecting ability towards quartz and silicates in the ore and the dosage of 50 g t−1 of the BDDAB resulted in lower achieved SiO2 contents in magnesite concentrate, than when compared to four times higher concentrations of the other two conventional collectors. The dosage of 200 g t−1 of the BDDAB surfactant resulted in SiO2 content < 1.0%, with LOI about 49.8% and yield of the product > 93%.
Fig. 2. Effect of investigated collectors on ζ potential of quartz.
center of nitrogen. 3.3. Zeta potential evaluation of the adsorption effect The adsorption efficiency of the synthesized quaternary ammonium gemini surfactant BDDAB and its comparison with that of DA and DDA collectors was investigated by determining ζ potential of pure quartz samples in collector solutions. To compensate the advantage of the gemini surfactant with two hydrocarbon chains and doubled molar mass, the effect of monomeric surfactants on the ζ potential was determined at doubled molar concentrations. The results of the ζ potential calculations are shown in Fig. 2. Comparing the adsorption of the three surfactants on the ζ potential of quartz, it can be seen that BDDAB is the most effective surfactant in terms of raising the ζ potential and that the efficiency declines in following order: BDDAB > DA > DDA. The difference in the peak locations for DDA and DA (by ca. 0.5 pH unit) correlates well with the difference of the reported pKa values and pH values at which the basic forms of the collectors appear. DA (Huang et al., 2014), DDA (Liu et al., 2009) and BDDA (Huang et al., 2014) molecules also interact with OH- ions in aqueous solutions through the formation of hydrogen bonds, weakening the hydrogen bonding of the collector with oxygen atoms in quartz surfaces, resulting in a decrease of measured ζ potential at higher pH. Because of a strong correlation between the adsorption density of surfactants, ζ potential and flotation recovery (Fuerstenau and Pradip, 2005), a decrease in the measured ζ potential values is usually also associated with the decrease in mineral recoveries. For DA, this decrease was measured at pH > 10 on a quartz sample (Huang et al., 2014; Fuerstenau and Pradip, 2005). Although measured ζ potential of quartz in BDDAB decreases with increasing pH, no drop in recoveries were measured over the pH range 6–12 (Huang et al., 2014).
4. Conclusion The flotation activity of cationic gemini surfactant 1,4-bis(dodecylN,N-dimethylammoniumbromide) butane synthesized in our laboratory was compared with conventional monomeric cationic collectors dodecylamine and dimethyldodecylamine in reverse froth flotation of magnesite ore. Streaming potential measurements revealed that BDDAB is the most effective surfactant in terms of the ζ potential and that the efficiency declines in the following order: BDDAB > DA > DDA. Laboratory froth flotation tests suggested that the SiO2 content in the magnesite concentrate can be lowered without using any collector due to the presence of naturally floatable talc particles. Use of cationic collectors is crucial in obtaining magnesite concentrate with a low silica content. The desilication efficiency of the studied collectors declined in the following order: BDDAB > DA > DDA. The gemini surfactant showed a superior collecting ability when compared to conventional cationic collectors. The results obtained with 50 g t−1 BDDAB were better in terms of the SiO2 content in the magnesite concentrate than those obtained with four times higher concentrations of the conventional collectors. BDDAB is a very promising surfactant for reverse froth flotation of magnesite ore.
Acknowledgement This work was supported by the research grant project VEGA, no. 1/ 0843/15. This publication is the result of the project implementation OPVaV2009/2.1/01-SORO “Research Excellence Centre on Earth's sources extracting and treatment (Project No. 26220120017)” supported by the Research & Development Operational Programme funded by the ERDF.
3.4. Laboratory froth flotation tests Direct comparison of the three investigated collectors was done using the laboratory froth flotation tests. The design of experiment matrix and the results of the tests are summarized in Table 1. N-way ANOVA (Analysis of variance) test of the presented results confirmed statistical significance (α = 0.01) of all the three investigated factors (dosage of the three collectors) on both magnesite concentrate yield and SiO2 content in the concentrate – cell product. Results suggest that even without using any collector, SiO2 content can be lowered from 4.0% to 2.8%. This can be attributed to natural floatability of talc particles. Low
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mineng.2017.04.013. 67
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58–65. http://dx.doi.org/10.1016/j.mineng.2015.04.009. ISSN 0892-6875. Tan, Hui, Skinner, William, Addai-Mensah, Jonas, 2013. pH-mediated interfacial chemistry and particle interactions in aqueous chlorite dispersions. Chem. Eng. Res. Des. 91 (3), 448–456. http://dx.doi.org/10.1016/j.cherd.2012.08.011. ISSN 02638762. Wang, Y., Hu, Y., He, P., Gu, G., 2004. Reverse flotation for removal of silicates from diasporic-bauxite. Miner. Eng. 17 (1), 63–68. http://dx.doi.org/10.1016/j.mineng. 2003.09.010. ISSN 0892-6875. Xia, Liu-yin, Zhong, Hong, Liu, Guang-yi, Huang, Zhi-qiang, Chang, Qing-wei, Li, Xingang, 2009. Comparative studies on flotation of illite, pyrophyllite and kaolinite with Gemini and conventional cationic surfactants. Trans. Nonferr. Met. Soc. China 19 (2), 446–453. http://dx.doi.org/10.1016/S1003-6326(08)60293-9. ISSN 1003-6326. Xia, Liuyin, Zhong, Hong, Liu, Guangyi, Huang, Zhiqiang, Chang, Qingwei, 2009. Flotation separation of the aluminosilicates from diaspore by a Gemini cationic collector. Int. J. Miner. Process. 92 (1–2), 74–83. http://dx.doi.org/10.1016/j. minpro.2009.02.012. ISSN 0301-7516. Xia, Liu-yin, Zhong, Hong, Liu, Guang-yi, 2010. Flotation techniques for separation of diaspore from bauxite using Gemini collector and starch depressant. Trans. Nonferr. Met. Soc. China 20 (3), 495–501. http://dx.doi.org/10.1016/S1003-6326(09)601680. ISSN 1003-6326. Yao, J., Yin, W., Gong, E., 2016. Depressing effect of fine hydrophilic particles on magnesite reverse flotation. Int. J. Miner. Process. 149, 84–93. http://dx.doi.org/10. 1016/j.minpro.2016.02.013. ISSN 0301-7516.
References Fillipov, L.O., Severov, V.V., Filippova, I.V., 2014. An overview of the beneficiation of iron ores via reverse cationic flotation. Int. J. Miner. Process. 127, 62–69. http://dx. doi.org/10.1016/j.minpro.2014.01.002. ISSN 0301-7516. Fuerstenau, D.W., Pradip, 2005. Zeta potentials in the flotation of oxide and silicate minerals. Adv. Colloid Interface Sci. 114–115, 9–26. http://dx.doi.org/10.1016/j.cis. 2004.08.006. ISSN 0001-8686. Guo, Jian, 2010. A novel depressor useful for flotation separation of diaspore and kaolinite. Min. Sci. Technol. (China) 20 (2), 292–295. http://dx.doi.org/10.1016/ S1674-5264(09)60200-3. ISSN 1674-5264. Huang, Zhiqiang, Zhong, Hong, Wang, Shuai, Xia, Liuyin, Zou, Wenbo, Liu, Guangyi, 2014. Investigations on reverse cationic flotation of iron ore by using a Gemini surfactant: Ethane-1,2-bis(dimethyl-dodecyl-ammonium bromide). Chem. Eng. J. 257, 218–228. http://dx.doi.org/10.1016/j.cej.2014.07.057. ISSN 1385-8947. Jiang, Hao, Sun, Zhongcheng, Xu, Longhua, Hu, Yuehua, Huang, Kai, Zhu, Shusheng, 2014. A comparison study of the flotation and adsorption behaviors of diaspore and kaolinite with quaternary ammonium collectors. Miner. Eng. 65, 124–129. http://dx. doi.org/10.1016/j.mineng.2014.05.023. ISSN 0892-6875. Liu, Changmiao, Hu, Yuehua, Cao, Xuefeng, 2009. Substituent effects in kaolinite flotation using dodecyl tertiary amines. Miner. Eng. 22 (9–10), 849–852. http://dx. doi.org/10.1016/j.mineng.2009.03.005. ISSN 0892-6875. Liu, Jing, Wang, Xuming, Lin, Chen-Luh, Miller, Jan D., 2015. Significance of particle aggregation in the reverse flotation of kaolinite from bauxite ore. Miner. Eng. 78,
68
Contents lists available at ScienceDirect
Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Short communication
Reverse froth flotation of magnesite ore by using (12-4-12) cationic gemini surfactant
MARK
⁎
Ivan Brezáni , Jiří Škvarla, Martin Sisol Department of Mineral Processing, Institute of Earth’s Resources, Faculty of Mining, Ecology, Process Control and Geotechnology, Technical University of Košice, Letná 9, 042 00 Košice, Slovakia
A R T I C L E I N F O
A B S T R A C T
Keywords: Froth flotation Reverse flotation Magnesite ore Cationic collector Gemini surfactant
The cationic gemini surfactant 1,4-bis(dodecyl-N,N-dimethylammoniumbromide)butane (BDDAB) was tested as a possible collector in the reverse froth flotation of magnesite ore. This novel type of collector was compared with its conventional monomeric equivalents: primary dodecylamine (DA) and tertiary dimethyldodecylamine (DDA). Electrokinetic measurements documented a strong effect of the synthesized gemini surfactant on the zeta potential of quartz. The effect of the three collectors declined in the following order: BDDAB > DA > DDA. Laboratory froth flotation tests confirmed an improved collecting ability of the gemini surfactant compared to both the conventional collectors. The desilication efficiency declined in the following order: BDDAB > DA > DDA. The results obtained with the lowest concentration of the gemini surfactant were better in terms of SiO2 content than those obtained with four times higher dosages of the conventional collectors.
1. Introduction Reverse froth flotation using primary ammonium collector, dodecylamine (DA) has been proven to be efficient beneficiation method to obtain high grade magnesite concentrate (low SiO2 content) (Yao et al., 2016). DA is one of the most used collectors in the reverse froth flotation of silicates from bauxite (Liu et al., 2015) and is also suitable for the reverse cationic flotation of high silica iron ore (Fillipov et al., 2014). Dodecyl tertiary amines (Liu et al., 2009) and quaternary amines (Wang et al., 2004; Jiang et al., 2014) can also be used as collectors of silicate minerals with good results. Recent findings have however proven, that cationic gemini surfactants have stronger collecting power than their monomeric equivalents (Xia et al., 2009). Dimeric or gemini surfactants are compounds with two hydrophobic chains and two hydrophilic head groups, covalently connected through a spacer chain. Gemini surfactants, when compared to their monomeric equivalents are more efficient at lowering the surface tension of water and have lower critical micellization concentrations, resulting in better hydrophobization of minerals and better flotation performance (Xia et al., 2009). Gemini surfactants have also been proven to be more efficient in studies of the reverse froth flotation of iron ore (Huang et al., 2014) and bauxite (Xia et al., 2010). To our knowledge, however, the use of quaternary ammonium gemini surfactants in the reverse froth flotation of magnesite ore has not yet been described. Therefore, our interest is to verify whether the
⁎
gemini surfactants can be efficiently used in the reverse flotation of magnesite ore by a direct comparison of results of laboratory froth flotation using dimeric ammonium based gemini surfactant and similar conventional surfactants which contain one hydrophobic group in the molecules. 2. Experimental 2.1. Minerals and reagents Ground magnesite ore (95% passing 265 µm) and hand-picked samples of magnesite and quartz were acquired from the company Gemerská nerudná spoločnosť, Hnúšťa-Mútnik, Slovakia. Phase composition characterization of finely powdered ore was done using X-ray diffraction (XRD) analysis on D8 Bruker 2θ diffractometer using CuKα radiation. JCPDS PDF 2 database was used for interpretation of the measured spectra. The 12-4-12 gemini quaternary ammonium salt 1,4-bis(dodecylN,N-dimethylammoniumbromide)butane (BDDAB) surfactant was synthesized by refluxing 0.1 mol of 1,4-Dibromobutane (99%, ACROS Organics, Belgium) with 0.21 mol of N,N-Dimethyldodecylamine (95 %, ACROS Organics, Belgium) in dry acetone (99.5%, mikroCHEM, Slovakia) for 20 h. The solvent and the reagents were used as received without further purification. Solid, that precipitated upon cooling was then filtered, washed with dry acetone and recrys-
Corresponding author. E-mail address: [email protected] (I. Brezáni).
http://dx.doi.org/10.1016/j.mineng.2017.04.013 Received 19 December 2016; Received in revised form 12 April 2017; Accepted 21 April 2017 0892-6875/ © 2017 Published by Elsevier Ltd.
Minerals Engineering 110 (2017) 65–68
I. Brezáni et al.
3. Results and discussion
tallized from dry acetone to obtain the desired gemini cationic surfactant in form of a pure white powder. The yield of the reaction was 88.4% of the theoretical amount. 1 H and 13C NMR spectra were recorded in DMSO-d6 on a Varian VNMRS spectrometer operating at 599.87 MHz (1H) and 150.84 MHz (13C) at room temperature, using an internal standard tetramethylsilane (0.00 ppm). The spectra, which are included as a supplementary material, confirmed a high purity of the sample which was better than 99%. The methyl and methylene proton signals showed expected chemical shifts and integral intensities. The HSQC experiment allowed assignment of the 13C NMR signals. Signals of the methyl and methylene protons and carbons attached to protonated nitrogens were deshielded and adequately shifted downfield. 1 H NMR spectrum (ppm): δH = 0.86 (6H, t, J = 7.2 Hz, 2 CH3), 1.22–1.35 (36H, m, 18 CH2), 1.63–1.70 (8H, m, 4 CH2), 3.01 (12H, s, 4 CH3N(+)), 3.22–3.27 (4H, m, 2 CH2N(+)), 3.28–3.33 (4H, m, 2 CH2N(+)). 13 C NMR spectrum (ppm): δC = 13.93 (2 CH3), 18.99, 21.71, 22.06, 25.81, 28.53, 28.68, 28.84, 28.94, 29.00, 29.00, 31.26 (11 × 2 CH2), 50.02 (4 CH3N(+)), 62.00 (2 CH2N(+)), 63.18 (2 CH2N(+)). Synthesis route and resulting structure of the BDDAB surfactant is shown in Fig. 1.
3.1. Ore analysis The XRD measurement confirmed the presence of magnesite as a valuable mineral, which is accompanied with talc, chlorite, quartz and dolomite in intergranular layers. The chemical composition of the ore used in flotation tests was 47.8% LOI, 43.1% MgO, 4.0% SiO2, 2.4% Fe2O3 and 0.4% CaO, characterized by the d80 value of 190 µm and only 5.0 wt.% of fine particles below 45 µm. 3.2. Investigation of collector properties Both conventional (Liu et al., 2015, 2009; Fillipov et al., 2014; Jiang et al., 2014) and gemini (Xia et al., 2009; Huang et al., 2014) cationic collectors are physically adsorbed on the surface of minerals by an electrostatic attraction. Adsorption of these collectors on individual minerals can be controlled by the adjustment of pH of the system. The point of zero charge (PZC) of the minerals is thus the most important property (Fuerstenau and Pradip, 2005). The PZC of present silicate minerals, talc (Fuerstenau and Pradip, 2005) and chlorite (Tan et al., 2013), and that of quartz (Yao et al., 2016; Huang et al., 2014; Fuerstenau and Pradip, 2005) lies in the acidic range, below pH 3.0. The PZC values of magnesite and dolomite were measured at pH 6.7 and 6.2, respectively (Yao et al., 2016). At pH 6.5, chosen for the batch flotation tests, the surface charge of magnesite should be positive, while that of the silicate minerals and quartz should be negative. Neutral, up to slightly acidic pH was also found to provide the best flotation recovery for silicate minerals using dodecylamine (Guo, 2010), dodecyl tertiary amines (Liu et al., 2009), quaternary amines (Wang et al., 2004; Jiang et al., 2014) and also ammonium gemini surfactant (Xia et al., 2009). Decreasing collecting abilities of amine collectors in alkaline pulps can be attributed to the formation of their basic amine forms and the simultaneous decrease (especially at pH > 8) of their presence in protonated ammonium forms. pKa values found in specifications of chemical properties of the surfactants are 10.63 and 9.97 for DA and DDA, respectively. In protonated forms, the nitrogen cations of the collectors are electrostatically attracted to the negatively charged surfaces of minerals. The nitrogen atom of a neutral amine present in alkaline pulp environments does not interact with negatively charged surfaces and there is, therefore, a drop in the achieved recoveries. The gemini collector does not have protonation states, because the addition of a fourth alkyl substituent to nitrogen results in a permanent cationic
2.2. Zeta potential analysis Zeta (ζ) potential was determined using SurPASS (Anton-Paar, Austria) electrokinetic analyzer equipped with an automatic titration unit and calculated by Attract software from streaming potential measurements using the Fairbrother - Mastin equation.
2.3. Laboratory froth flotation Froth flotation tests were done in a 3-litre flotation cell. After the slurry (density of 300 g L−1) was transferred to the agitated flotation cell (1600 rpm) and agitated for 3 min, pH was adjusted to 6.5 using an HCl solution. The surfactant solution was then added and the slurry was agitated for 15 min. After the conditioning period, 65 g t−1 of Methyl Isobutyl Carbinol (MIBC) frother was added and agitated for another 1 min. The air was then introduced with 4 L·min−1 volumetric flowrate and 10 min flotation period was conducted. Three tests were run at each condition and average values are reported.
Fig. 1. Synthesis route of the gemini surfactant BDDAB.
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Table 1 Results of the laboratory froth flotation tests. Dosage of collector
Cell product
DA (g t−1)
DDA (g t−1)
BDDAB (g t−1)
Yield (%)
SiO2 (%)
0 100 200 0 0 0 0 0
0 0 0 100 200 0 0 0
0 0 0 0 0 50 100 200
97.2 96.5 96.2 96.9 96.4 96.6 95.9 93.1
2.8 2.2 2.0 2.5 2.4 1.7 1.3 1.0
content of fines minimized the entrainment effect, resulting in froth products with relatively high SiO2 contents and high yields of the magnesite concentrate. Desilication efficiency of the investigated collectors declines in the following order: BDDAB > DA > DDA. Gemini surfactant showed very strong collecting ability towards quartz and silicates in the ore and the dosage of 50 g t−1 of the BDDAB resulted in lower achieved SiO2 contents in magnesite concentrate, than when compared to four times higher concentrations of the other two conventional collectors. The dosage of 200 g t−1 of the BDDAB surfactant resulted in SiO2 content < 1.0%, with LOI about 49.8% and yield of the product > 93%.
Fig. 2. Effect of investigated collectors on ζ potential of quartz.
center of nitrogen. 3.3. Zeta potential evaluation of the adsorption effect The adsorption efficiency of the synthesized quaternary ammonium gemini surfactant BDDAB and its comparison with that of DA and DDA collectors was investigated by determining ζ potential of pure quartz samples in collector solutions. To compensate the advantage of the gemini surfactant with two hydrocarbon chains and doubled molar mass, the effect of monomeric surfactants on the ζ potential was determined at doubled molar concentrations. The results of the ζ potential calculations are shown in Fig. 2. Comparing the adsorption of the three surfactants on the ζ potential of quartz, it can be seen that BDDAB is the most effective surfactant in terms of raising the ζ potential and that the efficiency declines in following order: BDDAB > DA > DDA. The difference in the peak locations for DDA and DA (by ca. 0.5 pH unit) correlates well with the difference of the reported pKa values and pH values at which the basic forms of the collectors appear. DA (Huang et al., 2014), DDA (Liu et al., 2009) and BDDA (Huang et al., 2014) molecules also interact with OH- ions in aqueous solutions through the formation of hydrogen bonds, weakening the hydrogen bonding of the collector with oxygen atoms in quartz surfaces, resulting in a decrease of measured ζ potential at higher pH. Because of a strong correlation between the adsorption density of surfactants, ζ potential and flotation recovery (Fuerstenau and Pradip, 2005), a decrease in the measured ζ potential values is usually also associated with the decrease in mineral recoveries. For DA, this decrease was measured at pH > 10 on a quartz sample (Huang et al., 2014; Fuerstenau and Pradip, 2005). Although measured ζ potential of quartz in BDDAB decreases with increasing pH, no drop in recoveries were measured over the pH range 6–12 (Huang et al., 2014).
4. Conclusion The flotation activity of cationic gemini surfactant 1,4-bis(dodecylN,N-dimethylammoniumbromide) butane synthesized in our laboratory was compared with conventional monomeric cationic collectors dodecylamine and dimethyldodecylamine in reverse froth flotation of magnesite ore. Streaming potential measurements revealed that BDDAB is the most effective surfactant in terms of the ζ potential and that the efficiency declines in the following order: BDDAB > DA > DDA. Laboratory froth flotation tests suggested that the SiO2 content in the magnesite concentrate can be lowered without using any collector due to the presence of naturally floatable talc particles. Use of cationic collectors is crucial in obtaining magnesite concentrate with a low silica content. The desilication efficiency of the studied collectors declined in the following order: BDDAB > DA > DDA. The gemini surfactant showed a superior collecting ability when compared to conventional cationic collectors. The results obtained with 50 g t−1 BDDAB were better in terms of the SiO2 content in the magnesite concentrate than those obtained with four times higher concentrations of the conventional collectors. BDDAB is a very promising surfactant for reverse froth flotation of magnesite ore.
Acknowledgement This work was supported by the research grant project VEGA, no. 1/ 0843/15. This publication is the result of the project implementation OPVaV2009/2.1/01-SORO “Research Excellence Centre on Earth's sources extracting and treatment (Project No. 26220120017)” supported by the Research & Development Operational Programme funded by the ERDF.
3.4. Laboratory froth flotation tests Direct comparison of the three investigated collectors was done using the laboratory froth flotation tests. The design of experiment matrix and the results of the tests are summarized in Table 1. N-way ANOVA (Analysis of variance) test of the presented results confirmed statistical significance (α = 0.01) of all the three investigated factors (dosage of the three collectors) on both magnesite concentrate yield and SiO2 content in the concentrate – cell product. Results suggest that even without using any collector, SiO2 content can be lowered from 4.0% to 2.8%. This can be attributed to natural floatability of talc particles. Low
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mineng.2017.04.013. 67
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