minerals Article
An Investigation of Reverse Flotation Separation of Sericite from Graphite by Using a Surfactant: MF Yangshuai Qiu 1 , Yongfu Yu 1,2, *, Lingyan Zhang 1,2 , Yupeng Qian 1,2 and Zhijun Ouyang 1 1
2
*
School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, China;
[email protected] (Y.Q.);
[email protected] (L.Z.);
[email protected] (Y.Q.);
[email protected] (Z.O.) Hubei Province Key Laboratory for Processing of Mineral Resources and Environment, Luoshi Road 122, Wuhan 430070, China Correspondence:
[email protected] or
[email protected]; Tel./Fax: +86-27-8788-2128
Academic Editor: Massimiliano Zanin Received: 7 April 2016; Accepted: 7 June 2016; Published: 23 June 2016
Abstract: In this paper, a surfactant, atlox4862 (formaldehyde condensate of methyl naphthalene sulfonic sodium salt) (MF), was introduced as a depressant for reverse flotation separation of sericite from graphite. Natural flake graphite has a strong hydrophobic property. After interacting with MF, the graphite became moderately hydrophilic. The flotation results showed that MF had a depressing ability for both sericite and graphite and that the flotation separation of sericite from graphite was attributed to the different declining levels of recovery between graphite and sericite with increased MF concentration. For a pulp pH of 8 and a MF concentration of 250 mg/L, the recovery rates of sericite and graphite were 89.7% and 11.3%, respectively. The results of the FTIR spectra and zeta potential measurements demonstrated that the interaction of MF with graphite and sericite is mainly through electrostatic attraction. MF was preferred to adsorb on the surface of graphite, decreasing its zeta potential and improving its hydrophilicity more than that of sericite. Keywords: surfactant; reverse flotation; graphite; sericite; adsorption
1. Introduction Graphite is a type of widely used non-metallic material; large, flaky graphite is an irreplaceable raw material in many industries [1]. Natural graphite is found in three commercial varieties: crystalline flake, microcrystalline or amorphous, and crystalline vein or lump graphite [2]. Among them, crystalline flake graphite has the best floatability, followed by crystalline graphite; amorphous graphite has the poorest floatability. Due to the strong floatability of flake graphite, flotation is the most commonly used means of graphite beneficiation. In conventional processing of large flaky graphite ores, a multistage grinding-flotation process is used to prevent large flakes from being destroyed during regrinding in graphite concentrate [3]. However, high-quality graphite resources that contain large graphite flakes are becoming increasingly scarce; moreover, fractional flaky graphite ores do not contain any large flakes and have relatively finely disseminated grain sizes. To increase the recovery of graphite, these graphite ores must be ground to a much smaller size. However, by doing so, gangue minerals are also ground finely. These fine/ultrafine gangue mineral particles not only affect sub-flotation processes in recovering valuable minerals based on true flotation but also lead to grade reduction through mechanical entrainment. Thus, the quality of final graphite concentrate is often greatly reduced due to the recovery of gangue minerals [4]. The separation efficiency between the valuable mineral and fully liberated and dispersed gangue is dependent on the degree of entrainment [5]. Unlike true flotation, which is selective, both gangue Minerals 2016, 6, 57; doi:10.3390/min6030057
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and valuable mineralsminerals alike canalike be recovered by entrainment. There are many control gangue and valuable can be recovered by entrainment. There arefactors many that factors that the entrainment of hydrophilic gangue such as the properties of gangue, viscosity of the slurry, control the entrainment of hydrophilic gangue such as the properties of gangue, viscosity offroth the structure [6], particle mass and shape entrainment has aentrainment detrimental effect the grade slurry, froth structure [6], [7], particle mass[8]. [7],Because and shape [8]. Because has a on detrimental of the concentrate, of studies have been carried out tohave research effect on the gradea number of the concentrate, a number of studies beenentrainment carried outmechanisms, to research identify factors affecting entrainment and develop models with an objective of predicting entrainment entrainment mechanisms, identify factors affecting entrainment and develop models with an in a flotation cell [4]. entrainment in a flotation cell [4]. objective of predicting Sericite is is one one of of the minerals in in graphite graphite ore ore and and is in Sericite the main main gangue gangue minerals is also also the the main main contaminant contaminant in graphite flotation flotation concentrate. concentrate. Moreover, Moreover, thorough the concentrate concentrate graphite thorough entrainment entrainment commonly commonly reports reports to to the in certain base metal ores [9]. The entrainment behavior of sericite in microcrystalline graphite flotation in certain base metal ores [9]. The entrainment behavior of sericite in microcrystalline graphite was studied by Hongqiang Li, who found that the entrainment of sericite leads to poor flotation flotation was studied by Hongqiang Li, who found that the entrainment of sericite leads to poor selectivityselectivity of commercial microcrystalline graphite ore [10]. ore [10]. flotation of commercial microcrystalline graphite Although studies on the entrainment mechanism have been conducted conducted for for 30 30 years, years, no no effective effective Although studies on the entrainment mechanism have been method has been found to control entrainment in graphite flotation. Thus far, there are two method has been found to control entrainment in graphite flotation. Thus far, there are two main main methods to in flotation [11].[11]. The first is ameliorating the flotation methods tocontrol controlentrainment entrainment in flotation The method first method is ameliorating the machine, flotation such as optimizing the structure of flotation or flotation second machine, such as optimizing the structure of cell flotation cell or column flotation[12–14]. column The [12–14]. Themethod second is optimizing the flotation technology and reagent system such such as using polymer depressant for method is optimizing the flotation technology and reagent system as using polymer depressant hydrophobic flocculation flotation. The use of a polymer depressant that has flocculability in flotation for hydrophobic flocculation flotation. The use of a polymer depressant that has flocculability in can not only the minerals must be depressed become hydrophilic but also increase the flotation can cause not only cause thethat minerals that must be to depressed to become hydrophilic but also particle these minerals. Hence, bubbleHence, entrainment mechanicaland entrainment in entrainment flotation can increasesize the of particle size of these minerals. bubbleand entrainment mechanical be weakened [15]. in flotation can be weakened [15]. impossible to avoid the entrainment of sericite graphite During graphite graphiteflotation, flotation,it is it almost is almost impossible to avoid the entrainment of into sericite into concentrate. Bartrum Bartrum introduced the use of as a depressant for graphite in lead zinc ore graphite concentrate. introduced thenigrosine use of nigrosine as a depressant for graphite in lead processing technology [16]. Carboxyl methylated cellulose (CMC) and dextrin can also be used as zinc ore processing technology [16]. Carboxyl methylated cellulose (CMC) and dextrin can also be depressants of graphite [17]. However, few researchers have studied reverse of graphite used as depressants of graphite [17]. However, few researchers have the studied theflotation reverse flotation of in flotation graphite the objective mineral. Thismineral. study used (formaldehyde condensate of graphite in when flotation whenisgraphite is the objective ThisMF study used MF (formaldehyde methyl naphthalene sulfonic sodiumsulfonic salt) as asodium depressant during flotation recovery condensate of methyl naphthalene salt) for as graphite a depressant forthe graphite during the of sericite. MF is actually a dispersant andaadispersant type of surfactant whose full name is formaldehyde flotation recovery of sericite. MF is actually and a type of surfactant whose full name is condensate of condensate methyl naphthalene sodium salt. The formula and formula name ofand MF name is shown in formaldehyde of methylsulfonic naphthalene sulfonic sodium salt. The of MF Figure 1. It the formaldehyde condensate of methyl naphthalene sulfonate [18]. The lipophilicity is shown inisFigure 1. It is the formaldehyde condensate of methyl naphthalene sulfonate [18]. The naphthalene naphthalene core of MF can adsorb on can the nonpolar of particles withof theparticles sulfonate radical lipophilicity core of MF adsorb onsurface the nonpolar surface with the pointing to the solvent. The silfonate radical is hydrophilic; therefore, an energy barrier was formed sulfonate radical pointing to the solvent. The silfonate radical is hydrophilic; therefore, an energy aroundwas the dispersed particles, leading to particles, mutual exclusion between The between dispersedthem. particles barrier formed around the dispersed leading to mutualthem. exclusion The were then particles dispersedwere in the solvent uniformly dispersed then dispersed in the[19]. solvent uniformly [19].
CH 3
CH 3 CH 2
NaO3S
SO3 Na
n
Figure 1. The The formula formula and and name name of of the depressant agent formaldehyde condensate of methyl naphthalene sulfonic sodium salt (MF).
To investigate the the depressing depressing effect effect of of MF MF on on graphite, graphite, batch batch flotation flotation tests tests and and contact contact angle angle To investigate measurements of single minerals and artificial mixtures composed of sericite and graphite single measurements of single minerals and artificial mixtures composed of sericite and graphite single mineral contact angle angle measurements measurements were were carried carried out. out. In end, Fourier Fourier transform transform infrared infrared mineral and and contact In the the end, spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and zeta potential measurements spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and zeta potential measurements were were performed to explain the adsorbing and depressing mechanism of MF. performed to explain the adsorbing and depressing mechanism of MF.
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2. 2. Materials Materialsand andMethods Methods 2.1. 2.1. Test Test Samples Samples and and Reagents Reagents The The graphite graphite single single mineral mineral sample sample was wasobtained obtainedfrom fromYichang Yichangcity cityin in Hubei Hubeiprovince, province,China. China. Large were selected selected first. first. The The lumps lumps were were then then crushed crushed to to ´2 −2 mm Large lumps lumps of of graphite graphite with with high high purity purity were mm by by aa roll roll crushing crushing mill mill followed followed by by wet wet grinding grinding in in aa conical conical ball ball mill mill with with zirconia zirconia balls balls as as grinding grinding media; media; finally, finally, the the milled milled mineral mineral particles particles were were wet wet screened screened using using 200 200 and and 325 325 mesh mesh US US standard standard sieves, sieves, and and the the −74 ´74to to+45 +45μm µmsize sizefraction fractionwas wascollected collectedand andused used in in batch batch flotation flotation tests. tests. According According to to the the standard standard method method (GB/T (GB/T3521-2008) 3521-2008)[20] [20]in in China, China, the the fixed fixed carbon carbon content content (FC) (FC) of of graphite graphite single mineral was analyzed. The FC of graphite single mineral was 97.75% with an ash single mineral was analyzed. The FC of graphite single mineral was 97.75% with an ash content content of of 2.25%. is the the X-ray X-ray diffraction diffractionpattern patternofofgraphite graphitesingle singlemineral, mineral,combined combined with result 2.25%. Figure Figure 2a 2a is with thethe result of of FC analysis, and indicates that the content of graphite in the sample was higher than 96 wt %. FC analysis, and indicates that the content of graphite in the sample was higher than 96 wt %. G 3.3733
(a)
G: Graphite
1.6766
Intensity(Counts)
G
6.6516
G
10
20
30
40
50
60
70
2-Theta( degree)
(b)
3.3334
Q
Intensity(Counts)
10.0409
S
Q: Quartz S: Sericite
1.9864
Q 5.0065
S
3.2092 2.9976 2.8679 2.7964
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20
30
2.4981
S S SS S
40
50
60
70
2-Theta( degree)
Figure Figure 2. 2. X-ray X-ray diffraction diffraction (XRD) (XRD) patterns patterns of of single single minerals. minerals. (a) (a) Graphite Graphite single single mineral; mineral; (b) (b) sericite sericite single mineral. single mineral.
The sericite single mineral sample was obtained from Xianning city in Hubei province, China. It The sericite single mineral sample was obtained from Xianning city in Hubei province, China. was processed by the same method as graphite. X-ray fluorescence (XRF) analysis indicated that it It was processed by the same method as graphite. X-ray fluorescence (XRF) analysis indicated that it contained 9.17 wt % K2O, 1.09 wt % Na2O, 37.04 wt % Al2O3, and 48.51 wt % SiO2. Figure 2b shows contained 9.17 wt % K2 O, 1.09 wt % Na2 O, 37.04 wt % Al2 O3 , and 48.51 wt % SiO2 . Figure 2b shows the X-ray diffraction pattern of sericite, combined with the results of XRF analysis, and indicates that the X-ray diffraction pattern of sericite, combined with the results of XRF analysis, and indicates that the content of sericite in the sample was higher than 93 wt %. the content of sericite in the sample was higher than 93 wt %. Dispersant MF was used as the depressant of graphite in the tests and was purchased from Dispersant MF was used as the depressant of graphite in the tests and was purchased from Shandong Xiya Chemical Industry Co., Ltd. in Jinan, China. Dodecylamine acetate salt was used as Shandong Xiya Chemical Industry Co., Ltd. in Jinan, China. Dodecylamine acetate salt was used as the collector, and a frother for the experiments was purchased from Sinopharm Chemical Reagent the collector, and a frother for the experiments was purchased from Sinopharm Chemical Reagent Co., Co., Ltd. in Shanghai, China. All regents were of analytical grade and used without Ltd. in Shanghai, China. All regents were of analytical grade and used without further purification. further purification.
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2.2. X-ray Powder Diffraction Analysis The crystalline structure and mineral composition of single mineral samples were determined by X-ray diffraction using a D8 Advance model X-ray powder diffractometer (Bruker Corporation, Stuttgart, Germany), with Cu Kα radiation (λ = 1.5406 Å) operated at a tube current of 40 mA and a voltage of 40 kV. Data of those samples were collected over 2θ values from 3˝ to 70˝ at a scan speed of 1˝ /min. 2.3. FTIR Spectroscopic Measurements and X-ray Photoelectron Spectroscopy (XPS) For the infrared studies, 2 g of single mineral were suspended in 200 mL of reagent solutions for 25 min at pH 7. The pulp was then centrifuged and washed at least three times with distilled water and dried under natural conditions. The infrared spectra of minerals were recorded using a Model Nicolet IS-10 spectrometer (Thermo Fisher Scientific, San Jose, CA, USA) through KBr disks in the range of 400–4000 cm´1 . X-ray photoelectron spectroscopy (XPS) of samples were carried out on photoelectron spectrometer Multilab 2000 (Thermo VG Scientific, East Grinstead, UK)using an Al Kα X-ray source operated at 200 W with 20 eV pass energy. The vacuum pressure ranged from 10´9 to 10´8 Torr, and the takeoff angle was set at 90˝ . 2.4. Contact Angle Measurement The assessments of wettability of single minerals were carried out on GBX MiniLab ILMS (GBX Scientific Instruments, Romans, France). The operating principle of the instrument is a washburn technique [21]. Each value recorded was the average of five separate determinations. 2.5. Zeta Potential Measurement A suspension solution containing 0.1 wt % graphite single mineral particles that had been grounded to ´2.0 µm in an eccentric vibration mill were prepared in a 5 ˆ 10´3 mol/dm3 NaCl solution and dispersed by MF for 10 min. A certain amount of the suspension solution was then taken for zeta potential measurement. The zeta potentials were measured using a Malvern Zetasizer Nano ZS90 (Malvern Instruments, Malvern, UK) equipped with a rectangular electrophoresis cell. The suspension solution pH was adjusted to a desirable value by HCl or NaOH solutions. The conductivity and pH value of the suspension solution were monitored continuously during the measurement, and the environmental temperature was maintained at 25 ˝ C. The results presented were the average of five independent measurements with a typical variation of ˘4 mV. 2.6. Batch Flotation Tests of Single Mineral The flotation test was performed in an XFD flotation cell whose volume was near 140 mL. First of all, the slurry was prepared by adding 2 g of graphite or sericite sample to 120-mL solutions. Second, the depressant MF was added and conditioned for 5 min at an agitation speed of 1000 rpm to allow MF adsorption. Following conditioning the collector, dodecylamine acetate salt was added and conditioned for 3 min at an agitation speed of 1400 rpm. The dosage of dodecylamine acetate salt was 200 mg/L. Flotation was then performed for 3 min. Finally, the floated and unfloated products were collected, and the flotation recovery was calculated based on the solid weight of products. 2.7. Batch Flotation Tests of Artificial Mixture Sericite is one of the main gangue minerals in graphite ore. To investigate the effect of separating the sericite in graphite by reverse flotation, batch flotation tests of artificial mixture samples composed of sericite and graphite single mineral were carried out, and the mass ratio of graphite to sericite was 4:1. First of all, the slurry was prepared by adding 2.5 g of the artificial mixture sample to 120 mL of solution. Second, the depressant MF was added and conditioned for 5 min at an agitation speed of 1000 rpm to allow MF adsorption. Followed the collector dodecylamine, acetate salt was added and
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added and conditioned for 3 min at an agitation speed of 1400 rpm. The pulp pH was adjusted to a conditioned for 3 min at an agitation speed of 1400 rpm. The pulp pH was adjusted to a desirable pH desirable pH value by HCl or NaOH solutions before the collector was added. Flotation was then value by HCl or NaOH solutions before the collector was added. Flotation was then performed for performed for 3 min. Finally, the floated and unfloated products were collected. The recovery of 3 min. Finally, the floated and unfloated products were collected. The recovery of graphite and sericite graphite and sericite in floated products can be calculated by Equations (1) and (2). in floated products can be calculated by Equations (1) and (2). 2(m2 − m1 Ag ) ˙ Rˆ 1− s = m1 Ag×q100% 2pmA2 ´ s − Ag Rs “ 1 ´ ˆ 100%
(1) (1)
As ´ A g ˆ ˙ m2m2 mm11 A As s−´ 1 R = − × R g “ g 1 ´ 2( A − A ) 100% ˆ 100% 2pAss ´ gA g q
(2)(2)
recoveryof ofsericite sericitein infloated floatedproduct product (%); (%); s —therecovery RRs—the R —the recovery of graphite in floated product (%); Rgg—the recovery of graphite in floated product (%); m11—the —theweight weightof ofunfloated unfloatedproduct product(g); (g); m m 2 —the ash weight of unfloated product m2 —the ash weight of unfloated product(g); (g); AAss—the —theash ashcontent contentof ofsericite sericitesingle singlemineral mineral(%); (%); AAgg—the ash content of graphite single mineral —the ash content of graphite single mineral(%). (%).
33.Results and Discussion Results and Discussion 3.1.The TheWettability WettabilityofofGraphite Graphiteand and Sericite Sericite 3.1. Froth flotation of separating valuable minerals from gangues by exploiting differences Froth flotationisisa process a process of separating valuable minerals from gangues by exploiting in their surface wettability. assess theTo wettability of wettability sericite andof graphite, angles single differences in their surface To wettability. assess the sericite the andcontact graphite, the of contact minerals were measured. Themeasured. effect of MF concentration on the wettability ofwettability sericite andofgraphite angles of single minerals were The effect of MF concentration on the sericite powder is shown in Figure 3. in Figure 3. and graphite powder is shown
50 45 40
(b)
20 Powder contact angle (°)
55 Powder contact angle (°)
22
(a)
60
18 16 14 12 10 8
35
0
500
1000
1500
Concentration of MF (mg/L)
2000
0
500
1000
1500
2000
Concentration of MF (mg/L)
Figure Figure3.3.Powder Powdercontact contactangle angleof ofgraphite graphite(a) (a)and andsericite sericite(b) (b)as asaafunction functionof ofMF MFconcentration. concentration.
The initial contact angle of sericite powder was 20.0°.˝ This indicated that the surface of sericite The initial contact angle of sericite powder was 20.0 . This indicated that the surface of sericite was hydrophilic, which agreed with the result of Gao [22]. The initial contact angle of graphite was hydrophilic, which agreed with the result of Gao [22]. The initial contact angle of graphite powder powder was 60.1°, suggesting that it was a hydrophobic mineral. was 60.1˝ , suggesting that it was a hydrophobic mineral. From Figure 3, when the MF concentration was increased from 0 to 2000 mg/L, the contact angle From Figure 3, when the MF concentration was increased from 0 to 2000 mg/L, the contact angle of graphite decreased from 60.1°˝ to 38.9°,˝ suggesting that graphite became moderately hydrophilic. of graphite decreased from 60.1 to 38.9 , suggesting that graphite became moderately hydrophilic. Compared with graphite powder, the contact angle of hydrophilic sericite powder decreased from Compared with graphite powder, the contact angle of hydrophilic sericite powder decreased from 20.0°˝to 9.5°˝slightly. 20.0 to 9.5 slightly. 3.2. 3.2.Batch BatchFlotation FlotationofofSingle SingleMineral Mineral To To investigate investigate the the depressing depressing effect effect of of MF MF concentration concentration on on sericite sericite and and graphite, graphite, batch batchtests testsof of single minerals were conducted, and the test results are shown in Figure 4. single minerals were conducted, and the test results are shown in Figure 4.
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(a)
80 60 40
pH=7
20 0
0
500 1000 1500 Concentration of MF (mg/L)
2000
(b)
80 Recovery of Graphite (%)
100 Recovery of Sericite (%)
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40 pH=7 20
0
0
500 1000 1500 Concentration of MF (mg/L)
2000
Figure Figure4. 4.Recovery Recoveryof ofsericite sericite(a) (a)and andgraphite graphite(b) (b)as asaa function function of of MF MF concentration. concentration.
Sericite recovery decreased sharply with an increasing MF concentration from 50 to 800 mg/L, Sericite recovery decreased sharply with91.6% an increasing MFThe concentration from 50continuously to 800 mg/L, and the sericite recovery decreased from to 18.2%. sericite recovery and the sericite recovery decreased fromconcentration 91.6% to 18.2%. The sericite recovery decreased decreased slightly to 10.99% as the MF further increased to 2000continuously mg/L. slightly to 10.99% as the MF concentration further increased to 2000 mg/L. The graphite recovery sharply decreased from 78.2% to 11.0% with an increasing MF The graphite recovery sharply 78.2% 11.0%200 with an increasing MFconcentration concentration concentration from 50 to 200 mg/Ldecreased and thenfrom leveled offtoabove mg/L. As the MF from 50increased to 200 mg/L and mg/L, then leveled off above 200 mg/L. As the little MF concentration further increased further to 2000 the recovery of graphite changed and was approximately 10%. to 2000 mg/L, the recovery of graphite changed little and was approximately 10%. The foregoing results showed that the recovery of both graphite and sericite decreased as MF The foregoing results showed that the recovery of both graphite and sericite decreased as MF concentration increased. However, the recovery of graphite decreased more sharply than that of concentration increased. However, the recovery of graphite decreased more sharply than that of sericite. When the concentrate of MF increased to 200 mg/L, the sericite recovery decreased to 81.3%, sericite. When the concentrate of MF increased to 200 mg/L, the sericite recovery decreased to 81.3%, whereas the recovery of graphite had already decreased to 11.0%. With a continued increase in the whereas the recovery graphite had already decreased to 11.0%. With continuedthe increase in the MF concentration, the of graphite recovery remained at approximately 10%;a however, recovery of MF concentration, the graphite recovery remained at approximately 10%; however, the recovery of sericite continuously decreased. The difference in recovery between graphite and sericite was due to sericite continuously decreased. The difference in recovery between graphite and sericite was due to the difference in floatability between graphite and sericite after interaction with MF. This difference the difference in floatability between graphite after interaction with MF. This difference creates opportunities for separating the sericite and fromsericite graphite. creates opportunities for separating the sericite from graphite. 3.3. Batch Flotation of Artificial Mixtures 3.3. Batch Flotation of Artificial Mixtures To determine the effect of MF concentration and pulp pH value on the separation of sericite and To determine the effect of MF concentration and pulp pH value on the separation of sericite and graphite, batch flotation tests of artificial mixtures were carried out. graphite, batch flotation tests of artificial mixtures were carried out. 3.3.1. 3.3.1. Effect Effect of of MF MF Concentration Concentration From the test testresults results single minerals, sericite and graphite weredepressed both depressed by MF. From the of of single minerals, sericite and graphite were both by MF. However, However, the effect of the depression was affected by the concentration of MF. The recovery of the effect of the depression was affected by the concentration of MF. The recovery of graphite and graphite and sericite showed a general declining trend as increased. MF concentration increased. To fully sericite showed a general declining trend as MF concentration To fully separate sericite from separate sericite from graphite, the difference in recovery between graphite and sericite should be graphite, the difference in recovery between graphite and sericite should be sufficiently large. When the sufficiently large. When the MF concentration was 50–400 mg/L, the recovery of sericite was MF concentration was 50–400 mg/L, the recovery of sericite was 62.3%–91.6%, whereas the recovery 62.3%–91.6%, whereas the recovery ofrange graphite wasconcentration, 11.5%–78.2%;the in this range of concentration, of graphite was 11.5%–78.2%; in this of MF difference inMF recovery between the difference in recovery between graphite and sericite was significant. Hence, tests were conducted graphite and sericite was significant. Hence, tests were conducted at different MF concentrations at different MF50concentrations ranging from with a pulp pH ofin7.Figure The results were as ranging from to 600 mg/L with a pulp pH50ofto7.600 Themg/L results were as shown 5. shown in Figure 5. It can be seen from Figure 5 that the recovery of both graphite and sericite decreased as MF concentration increased, whereas the difference in the recovery between graphite and sericite increased firstly and then began to decrease above 250 mg/L. As MF concentration increased, graphite recovery showed a sharper decline than did sericite. When the MF concentration increased to 250 mg/L, the difference in recovery between graphite and sericite reached a maximum difference of 70.4%. Simultaneously, the recovery of graphite and sericite was 10.6% and 81.0%, respectively. When the concentration of MF was further increased, the graphite recovery did not change, and the sericite recovery continued to decrease to 31.0%. The difference in recovery between graphite and sericite
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began to decrease with a further increase of MF concentration above 250 mg/L. Hence, to separate sericite from6, graphite more effectively, an MF concentration of 250 mg/L was selected as optimum Minerals 2016, 57 7 of 13 for the separation. 80
(a)
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Difference of recovery between Difference of recovery between sericite and graphite (%)sericite and graphite (%)
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Recovery of sericite (%) Recovery of sericite (%)
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Recovery of sericite Recovery of graphite
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(a)
80
80 20 60
20
0 60
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200 300 400 500 Concentration of MF (mg/L)
600
0 40
Recovery of sericite
Recovery of graphite (%)Recovery of graphite (%)
began to decrease with a further increase of MF concentration above 250 mg/L. Hence, to separate 80 60 60 concentration of 250 mg/L was selected as optimum sericite from graphite more effectively, an MF for the separation. 40 80
(b)
20 60 0 40
100
200
300
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Concentration of MF (mg/L)
Recovery of graphite graphite (a) and the difference in recovery between graphite and Figure 5. Recovery Figure 5. Recovery of of sericite sericite and and graphite (a) and the difference in recovery between graphite and 40 sericite (b) as as aa function function of of MF MF concentration. concentration.20 20 sericite (b)
60
50 20
(a)
40 10 30
2
4
Recovery of graphite 6Recovery8of sericite10
pH value
80 50 70
12
40 60
Difference of recovery between Difference of recovery between graphite and sericite (%)graphite and sericite (%)
Recovery of graphite (%)Recovery of graphite (%)
Recovery of sericite
30
Recovery of sericite (%) Recovery of sericite (%)
3.3.2.20Effect of pH 0 It0 can 100 be seen and sericite decreased as 0 graphite 100 200 300 400 500 600MF 200 from 300 Figure 400 5 that 500 the 600 recovery of both Concentration of thereby MF (mg/L) Concentration of MF (mg/L) The pulp pH affected the surface potential of mineral particle surface, affecting the concentration increased, whereas the difference in the recovery between graphite and sericite increased adsorption of depressant MF on and the surface of particles. Hence, the between depressing effect on the firstlyFigure and then began to above 250 mg/L. concentration increased, graphite recovery 5. Recovery ofdecrease sericite graphite (a)mineral andAs theMF difference in recovery graphite and mineral is affected by the pulp pH. The effects of pulp pH on the separation of sericite from graphite showed a sharper decline than did sericite. When the MF concentration increased to 250 mg/L, sericite (b) as a function of MF concentration. were studied by tests. pulpreached pH varied from 2 to difference 12 with a 250 mg/L the difference in artificial recoverymixture betweenflotation graphite andThe sericite a maximum of 70.4%. MF concentration. The tests results were as shown in Figure 6. and 81.0%, respectively. When the 3.3.2. Effect of pH Simultaneously, the recovery of graphite and sericite was 10.6% From Figure 6, the graphite decreased sharply with increasing pulp pH from 2 concentration of MF was furtherrecovery increased, the graphite recovery did not change, andvalue the sericite The pulp pH affected the surface potential of mineral particle surface, thereby affecting the to 8 and then increased strongly above pH 8. The sericite recovery increased with increasing pulp pH recovery continued to decrease to 31.0%. The difference in recovery between graphite and sericite adsorption of depressant MF on the surface of mineral particles. Hence, the depressing effect on the from todecrease 8 and then varied slightly aboveofpH The difference in recovery between graphite and began2to with a further increase MF8.concentration above 250 mg/L. Hence, to separate mineral is affected by the pulp pH. The effects of pulp pH on the separation of sericite from graphite sericite from increased sharply with an increasing pulp pH value from 2 to 8 and then decreased slightly graphite more effectively, an MF concentration of 250 mg/L was selected as optimum for were studied by artificial mixture flotation tests. The pulp pH varied from 2 to 12 with a 250 mg/L above pH 8. The separation of sericite from graphite was optimal at a pulp pH of 8 with the recovery the separation. MF concentration. The tests results were as shown in Figure 6. of graphite and sericite of 11.3% and 79.0%, respectively. 3.3.2.From EffectFigure of pH 6, the graphite recovery decreased sharply with increasing pulp pH value from 2 to 8 and then increased strongly above pH 8. The sericite recovery increased with increasing pulp pH affected surface potential mineral particle surface, between thereby affecting the 80 fromThe 2 topulp 8 andpH then variedthe slightly above pH 8. of The difference in recovery graphite and 50 adsorption of depressant MF on the surface of mineral particles. Hence, the depressing effect on (b)the sericite increased sharply with an increasing (a) 80pulp pH value from 2 to 8 and then decreased slightly mineral is affected by the pulp pH. The effects of pulp pH on the separation of sericite from graphite above pH 8. The separation of sericite from graphite was 60 optimal at a pulp pH of 8 with the recovery 40 were studiedand by artificial flotation tests. The pulp pH varied from 2 to 12 with a 250 mg/L MF of graphite sericite ofmixture 11.3% and 79.0%, respectively. 70 Recovery of graphite concentration. The tests results were as shown in Figure 6. 40 80 20
(b)
60 0 40
2
4
6
pH value
8
10
12
20 6. Recovery of sericite and graphite (a) and the difference in recovery between graphite and sericite (b) as a function of pulp pH value. 50
Figure 20 10
0
3.4. FTIR Spectra of Minerals Before and After Interaction with MF 40 2
4
6
8
10
12
2
4
6
8
10
12
pH value pH value To study the mechanism of MF in the flotation separation of sericite from graphite, the IR (infrared) spectra of these two minerals before and after interacting with reagents were measured, Figure 6. 6. Recovery Recovery of of sericite sericite and and graphite graphite (a) (a) and and the the difference difference in in recovery recovery between between graphite graphite and Figure and and the results area shown and 8. sericite (b) as functionin ofFigures pulp pH7value.
sericite (b) as a function of pulp pH value.
3.4. FTIR Spectra of Minerals Before and After Interaction with MF From Figure 6, the graphite recovery decreased sharply with increasing pulp pH value from 2 to 8 To study the strongly mechanism of pH MF8.inThe thesericite flotation separation of sericite from graphite, IR and then increased above recovery increased with increasing pulp pHthe from (infrared) spectra of these two minerals before and after interacting with reagents were measured, and the results are shown in Figures 7 and 8.
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2 to 8 and then varied slightly above pH 8. The difference in recovery between graphite and sericite increased sharply with an increasing pulp pH value from 2 to 8 and then decreased slightly above pH 8. The separation of sericite from graphite was optimal at a pulp pH of 8 with the recovery of graphite and sericite of 11.3% and 79.0%, respectively. 3.4. FTIR Spectra of Minerals Before and After Interaction with MF To study the mechanism of MF in the flotation separation of sericite from graphite, the IR (infrared) spectra of these two minerals before and after interacting with reagents were measured, and the results are shown Figures 7 and 8. Minerals 2016,in 6, 57 8 of 12
3500 3500
3000 3000
2500
2000
1598.77 1598.77
Sericite+MF Sericite+MF Sericite Sericite MF MF
894.85 894.85 833.13 833.13
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3054.85 3054.85 2921.92 2921.92 2852.32 2852.32
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1500
2500 2000(cm-11500 Wavenumbers ) Wavenumbers (cm-1)
1000 1000
500 500
4000 4000
3500 3500
3000 2500 2000 1500 -1 3000 Wavenumbers 2500 2000 (cm-1) 1500
831.17 831.17 752.10 752.10
Graphite Graphite
1183.80 1183.80 1135.87 1135.87 1035.59 1035.59
Graphite with MF Graphite with MF 1604.48 1604.48
2925.48 2925.48 2854.13 2854.13
3052.76 3052.76
Figure 7. FTIR spectra of sericite before and after interaction with MF. Figure 7. 7. FTIR FTIR spectra spectra of of sericite sericite before before and and after after interaction interaction with with MF. MF. Figure
1000 1000
500 500
Wavenumbers (cm )
Figure 8. FTIR spectra of graphite before and after interaction with MF. Figure Figure 8. 8. FTIR FTIR spectra spectra of of graphite graphite before before and and after after interaction interaction with MF.
After interaction with MF, the stretching bands of –CH3 and –CH2 groups in MF molecules After interaction with MF, the stretching bands of –CH3 and –CH2 groups in MF molecules −1 on Afteratinteraction with2922 MF, and the 2852 stretching bands of surfaces –CH3 and groups MF molecules appeared approximately cm−1 on sericite and–CH 29262 and 2854incm graphite −1 on graphite appeared at approximately 2922 and 2852 cm−1 on sericite surfaces and 2926 and 2854 cm ´ 1 ´1MF appearedrespectively at approximately 2922stretching and 2852bands cm of on=CH sericite surfaces 2926 and nucleus 2854 cmin on surfaces, [23]. The 2– groups on and naphthalene surfaces, respectively [23]. The stretching bands−1of =CH2– groups on naphthalene nucleus in MF −1 graphite surfaces, respectively [23]. The stretching of =CHsurfaces – groups on naphthalene nucleus in molecules appeared at approximately 3055 cm bands on sericite and 3053 cm on graphite 2 −1 on graphite molecules appeared at approximately 3055 cm−1 ´on sericite surfaces and 3053 cm´ 1 on 1 on MF molecules appeared at approximately 3055 sericite andsurfaces 3053 cmand surfaces, respectively. New bands appeared at cm 895 and 833 cm−1surfaces on sericite at graphite 831 and surfaces, respectively. New bands appeared at 895 and 833 cm´−11 on sericite surfaces and at 831 and 752 cm−1 respectively. on graphite surfaces dueappeared to the bending vibrations solitary hydrogen groups on and the surfaces, New bands at 895 and 833 cm of on sericite surfaces and at 831 −1 752 cm´1on graphite surfaces due to the bending vibrations of solitary hydrogen groups on the naphthalene in surfaces MF molecules the vibration bands ofhydrogen the naphthalene 752 cm onnucleus graphite due to[24]. theMoreover, bending vibrations of solitary groupsnucleus on the naphthalene nucleus in MF molecules [24]. Moreover, the vibration bands of the naphthalene nucleus skeleton in MF molecules appeared at approximately 1599 cm−1 on sericite surfaces and 1605 cm−1 on −1 skeleton in MF molecules appeared at approximately 1599 cm on sericite surfaces and 1605 cm−1 on graphite surfaces. The bands that appeared at 1184, 1136 and 1035 cm−1 on the graphite surfaces were −1 graphite surfaces. The bands that appeared at 1184, 1136 and 1035 cm on the graphite surfaces were the adsorption bands of –SO3 groups in MF molecules [24]. The results of FTIR spectra demonstrated the adsorption bands of –SO3 groups in MF molecules [24]. The results of FTIR spectra demonstrated that, after MF treatment, no new adsorption peaks appeared on sericite and graphite surfaces except that, after MF treatment, no new adsorption peaks appeared on sericite and graphite surfaces except for MF’s adsorption bands, which implied that MF might adsorb onto the two minerals without the for MF’s adsorption bands, which implied that MF might adsorb onto the two minerals without the
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naphthalene nucleus in MF molecules [24]. Moreover, the vibration bands of the naphthalene nucleus skeleton in MF molecules appeared at approximately 1599 cm´1 on sericite surfaces and 1605 cm´1 on graphite surfaces. The bands that appeared at 1184, 1136 and 1035 cm´1 on the graphite surfaces were the adsorption bands of –SO3 groups in MF molecules [24]. The results of FTIR spectra demonstrated that, after MF treatment, no new adsorption peaks appeared on sericite and graphite surfaces except for MF’s adsorption bands, which implied that MF might adsorb onto the two minerals without the formation of new complexes. As an alternative to FTIR, the XPS analysis of samples was carried out. The XPS survey spectra of graphite and sericite after interaction with MF was shown in Figure 9. It can be seen from Table 1 that S and Na elements appear on the surface of graphite and sericite. Meanwhile, the changes in electron binding energy (∆EB ) of C, O and S elements after treatment with MF were relatively small, which Minerals 2016, 6, 57 9 of 12 is less than the error value of experiment equipment. That means the interaction between MF and graphite or sericite adsorption formation of new complexes [25].graphite XPS spectra spectra agree wellwas withphysical FTIR results and without confirm the physical adsorption occurred on and agree well with FTIR results and confirm physical adsorption occurred on graphite and sericite surface. sericite surface. 10 C 1s
Count(1E+05 S)
8
6
K 2p
Na 1s MF+Sericite
4
O 1s MF
2
S 2p
MF+Graphite
0
1000
800
600
400
200
0
EB(eV)
Figure 9. 9. X-ray after Figure X-ray photoelectron photoelectron spectroscopy spectroscopy (XPS) (XPS) survey survey spectra spectra of of graphite graphite and and sericite sericite after interaction with MF. interaction with MF. Table 1. X-ray photoelectron spectroscopy (XPS) analysis results of graphite and sericite after Table 1. X-ray photoelectron spectroscopy (XPS) analysis results of graphite and sericite after interaction interaction with MF. with MF.
Electron Binding Energy, EB/eV Electron Binding Energy, C1s O1s S2p Na1s EB /eV K2p Sample C1s O1s 168.62S2p1071.68 Na1sMF 284.62 531.68 MFMF + Sericite 284.62 531.68 168.24 168.62 284.64 531.80 1072.081071.68 293.23 MF + Sericite 284.64 531.80 −0.38168.240.40 1072.08 ΔEB/eV 0.02 0.12 ∆EB /eV 0.12 ´0.38 MF + Graphite 0.02 284.62 532.44 169.35 1072.21 0.40MF + Graphite 284.62 532.44 169.35 1072.21 0.76 0.73 0.73 0.53 0.53∆E /eVΔEB/eV 00 0.76 Sample
B
K2p 293.23 -
The mechanism of graphite reaction with MF is shown in Figure 10. The figure illustrates that The mechanism of graphite reaction with MF is shown in Figure 10. The figure illustrates that the hydrophobic naphthalene core could adsorb onto the surface of graphite particles preferentially the hydrophobic naphthalene core could adsorb onto the surface of graphite particles preferentially with the sulfonate radical pointing to the water. The surface tension of particles decreases due to the with the sulfonate radical pointing to the water. The surface tension of particles decreases due to the association between the sulfonate radical of the MF molecule and the hydroxyl in the water [26], so association between the sulfonate radical of the MF molecule and the hydroxyl in the water [26], so hydrophobic graphite particles become hydrophilic. The contact angle of graphite decreased after the hydrophobic graphite particles become hydrophilic. The contact angle of graphite decreased after the reaction with MF. In flotation, the depressing effect of MF on graphite was achieved by transforming reaction with MF. In flotation, the depressing effect of MF on graphite was achieved by transforming the surface of graphite particles from hydrophobic to hydrophilic, and then the graphite cannot float. the surface of graphite particles from hydrophobic to hydrophilic, and then the graphite cannot float. The adsorption of MF onto sericite surface also improves its hydrophilicity, and the pre-adsorption of MF interferes with the adsorption of dodecylamine onto sericite surface [27]. This is the mechanism of MF’s depressing effect on sericite in the flotation. In flotation, the sericite and graphite did not have sufficient adsorption of MF on their surfaces when the MF concentration was comparatively small. Hence, the recovery of graphite and sericite did not have a significant decline. As MF concentration increased, due to the sufficient adsorption
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The adsorption of MF onto sericite surface also improves its hydrophilicity, and the pre-adsorption of MF interferes with the adsorption of dodecylamine onto sericite surface [27]. This is the mechanism of MF’s depressing Minerals 2016, 6, 57 effect on sericite in the flotation. 10 of 12
hydrophilic sulfonate radical
Solvent
Solvent Dispersion lipophilicity naphthalene core
molecule MineralsMF 2016, 6, 57 bipolar structure diagram
10 of 12
Graphite particles Dispersed graphite particles
Figure Figure 10. 10. The The MF MF molecule molecule adsorption adsorption at at the the interphase interphase of of graphite-solvent. graphite-solvent. hydrophilic sulfonate radical
Solvent
Zeta potential (mv)
3.5. Zeta Potential Measurement In flotation, the sericite and graphite did not have sufficient adsorption of MF on their surfaces Solvent Zeta potentials of minerals before and aftersmall. interaction MF are shown in Figures 11 and did 12. when the MF concentration was comparatively Hence,with the recovery of graphite and sericite Dispersion not have a significant decline. As MF concentration increased, due to the sufficient adsorption and lipophilicity naphthaleneof coreMF on selective adsorption 10the surface, graphite was intensely depressed and the graphite recovery pH on the surface was not adequate enough to sharplyMFdecreased. Meanwhile, the adsorption of MF molecule bipolar 0 Graphite particles structure diagram lead to the intensely depressing effect on sericite. Therefore, the recovery of sericite was much higher 2 4 6 8 10 12 Dispersed graphite particles than that of graphite. With-10 the further increase in MF concentration, sericite floated worse due to interference with the adsorption of dodecylamine from the sufficient adsorption of MF on the surface -20 Figure The MFofmolecule at the interphase of graphite-solvent. of sericite. Hence, the 10. recovery sericite adsorption similarly had a sharp decrease. -30
3.5. Zeta Potential Measurement -40
Graphite
Zeta potentials of minerals before and after interaction with MF are shown in Figures 11 and 12. Graphite+MF -50
10
Figure 11. Zeta potentials of graphite before and after interaction with MF (concentration = 250 mg/L). pH
Zeta potential (mv) Zeta potential (mv)
0 -100
2
4
6
2
4
6
-20 -10 -30 -40 -20
pH
8
10
12
8
10
12
Graphite Graphite+MF
-50 -30
Sericite
Sericite+MF Figure before and after after interaction interactionwith withMF MF(concentration (concentration==250 250mg/L). mg/L). Figure11. 11.Zeta Zetapotentials potentials of of graphite graphite before and -40
Figure 12.Zeta potentials of 0sericite before and after interaction with MF (concentration = 250 mg/L). 2
4
6
pH
8
10
12
Zeta potential (mv)
The surface of graphite and sericite varied from positively charged to negatively charged as pH -10 value increased. The isoelectric points (IEPs) corresponded to pH values of approximately pH 2.0 for sericite and pH 4.9 for graphite. The surface of graphite was slightly positive due to the adsorption -20 of H+, OH−, or some other ions between pH 2 and 5. The zeta potential of graphite and sericite showed a pronounced shift towards more negative zeta potentials in the presence of MF, indicating that the -30 anionic Gemini molecules adsorbed onto graphite and sericite [28]. Moreover, after interacting with Sericite Sericite+MF MF, the zeta potential of graphite was much more negative than that of sericite, demonstrating that MF was preferred to adsorb-40 on graphite surfaces. The FTIR results implied that MF might adsorb onto the two minerals without the formation of new complexes. The negatively charged MF adsorb
Zeta p
-30 -40
Graphite Graphite+MF
-50
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Figure 11. Zeta potentials of graphite before and after interaction with MF (concentration = 250 mg/L).
Zeta potential (mv)
0
2
4
6
pH
8
10
12
-10
-20
-30
Sericite Sericite+MF
-40
Figure 12.Zeta potentials of sericite before and after interaction with MF (concentration = 250 mg/L). Figure 12. Zeta potentials of sericite before and after interaction with MF (concentration = 250 mg/L).
The surface of graphite and sericite varied from positively charged to negatively charged as pH surface The of graphite andpoints sericite varied from positively charged charged as pH valueThe increased. isoelectric (IEPs) corresponded to pH values to of negatively approximately pH 2.0 for value increased. The isoelectric points (IEPs) corresponded to pH values of approximately pH 2.0 for sericite and pH 4.9 for graphite. The surface of graphite was slightly positive due to the adsorption sericite and pH 4.9 for graphite. The surface of graphite was slightly positive due to the adsorption of of H+, OH−, or some other ions between pH 2 and 5. The zeta potential of graphite and sericite showed + ´ , OH , or some ionsmore between pH 2 zeta and potentials 5. The zetainpotential of graphite sericite that showed aHpronounced shiftother towards negative the presence of MF,and indicating the a pronounced shift towards more negative zeta potentials in the presence of MF, indicating thatwith the anionic Gemini molecules adsorbed onto graphite and sericite [28]. Moreover, after interacting anionic molecules adsorbed onto graphite sericite [28]. Moreover, after interacting with MF, the Gemini zeta potential of graphite was much more and negative than that of sericite, demonstrating that MF, the zeta potential of graphite was much more negative than that of sericite, demonstrating that MF was preferred to adsorb on graphite surfaces. The FTIR results implied that MF might adsorb MF was preferred to adsorb on graphite surfaces. The FTIR results implied that MF might adsorb onto the two minerals without the formation of new complexes. The negatively charged MF adsorb onto the two minerals without the formation of new complexes. The negatively chargedthe MFbenzene adsorb on the negatively charged graphite byhydrophobic/van der Waals interaction between on the negatively charged graphite byhydrophobic/van der Waals interaction between the benzene rings of MF and the graphite hydrocarbon rings at pH > IEP [29]. However, the negatively charged MF adsorb on the negatively charged sericite by hydrogen bond at pH > IEP [30,31]. Obviously, the hydrophobic/van der Waals interaction energy between the nonpolar benzene rings of MF and the nonpolar graphite hydrocarbon rings are much stronger than the hydrogen bond energy between MF and sericite. Therefore, the MF appears to be preferentially selective to the graphite surface relative to the sericite surface.This can explain why the recovery of graphite decreased more sharply than did sericite as MF concentration increased. 4. Conclusions Batch flotation experiments of single mineral and artificial mixtures of flaky graphite–sericite and contact angle measurements were conducted to verify the feasibility of reverse flotation separation of sericite from graphite by using a Gemini surfactant: MF. FTIR spectra and zeta potential measurements were conducted to confirm the mechanism of MF reaction with sericite and graphite. The results of contact angle measurements indicate that MF can improve the hydrophilicity of graphite and sericite. The flotation results of single minerals indicated that MF displayed a depression of both graphite and sericite, but the recovery of graphite declined more sharply than sericite as MF concentration increased. The results of artificially mixed mineral flotation demonstrated the preferential flotation separation of sericite from graphite with a pulp pH at 8 with an MF concentration of 250 mg/L; the recovery rates of sericite and graphite were 89.7% and 11.3%, respectively. Sericite can be separated from graphite by reverse flotation separation. The interaction of MF with graphite and sericite surfaces was dominantly hydrophobic/van der Waals interaction and hydrogen bond, which was confirmed by FTIR spectra, X-ray photoelectron spectroscopy. MF was preferred to adsorb onto the surface of graphite, decreasing its zeta potentials and improving its hydrophilicity relative to sericite. This was confirmed by zeta potential measurement. The reverse flotation separation of sericite from graphite can dramatically reduce the infusion of sericite in the flotation concentrate. However, in this paper, all test samples were single minerals. Tests should be conducted with practical ore samples to evaluate the effectiveness of the MF reagent.
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Acknowledgments: The authors would like to acknowledge the financial support from the Fundamental Research Funds for Central Universities of China (No. WUT: 2014-IV-071) and technical support from the Center for Analysis of Wuhan University of Technology. Author Contributions: Yangshuai Qiu conceived and designed the experiments, performed the experiments, and wrote the paper under the supervision of Yongfu Yu and Lingyan Zhang. Zhijun Ouyang analyzed the data. Yupeng Qian and Yangshuai Qiu revised the manuscripts and approved the manuscripts. Conflicts of Interest: The authors declare no conflict of interest.
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