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Flotation behavior of four dodecyl tertiary amines as collectors of diaspore and kaolinite
Mining Science and Technology (China) 21 (2011) 249e253
Contents lists available at ScienceDirect
Mining Science and Technology (China) journal homepage: www.elsevier.com/locate/mstc
Flotation behavior of four dodecyl tertiary amines as collectors of diaspore and kaolinite Changmiao Liu a, b, *, Feng Ansheng a, b, Guo Zhenxu a, b, Cao Xuefeng c, Hu Yuehua c a
Zhengzhou Institute of Multipurpose Utilization of Mineral Resources, CAGS, Zhengzhou 450006, China China National Engineering Research Center for Utilization of Industrial Minerals, Zhengzhou 450006, China c School of Minerals Processing and Bio-engineering, Central South University, Changsha 410083, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2010 Received in revised form 30 September 2010 Accepted 30 October 2010
The flotation of diaspore and kaolinite by one of a series of tertiary amines (DRN, DEN, DPN and DBN) was investigated. The tertiary amines show better floating recovery for kaolinite compared to diaspore. The maximum recovery D-value is 45% over a pH range from 3 to 8. FT-IR spectra confirm the presence of hydroxyl groups on the surface of kaolinite and diaspore. Zeta potential measurements show that the mineral surfaces are negatively charged over a wide pH range. Ionization of hydroxyl groups mainly accounts for the surface charging mechanism. The adsorption of tertiary amines onto the mineral surface is due mainly to electrostatic effects and the difference in electrostatic effect between a collector and the two minerals can explain the flotation separation. Inductive electronic and steric effects from the substituent groups result in different collecting powers for the four tertiary amines. Copyright Ó 2011, China University of Mining & Technology. All rights reserved.
Keywords: Kaolinite Diaspore Amines collectors Froth flotation
1. Introduction China has huge bauxite reserves most of which are of the diasporic type [1,2]. The useful mineral in diasporic bauxites is mainly diaspore and the gangue mineral is mainly kaolinite. A fatal disadvantage of Chinese diasporic ores is their low Al2O3 to SiO2 mass ratio, which generally ranges from 4 to 6. Because the mass ratio of Al2O3 to SiO2 is lower than 8, the minimum requirement for the Bayer process, Chinese bauxites cannot be processed directly by the advanced process [3,4]. Increasing the Al2O3/SiO2 ratio of middle or low grade bauxite requires the separation of diaspore from the gangue minerals by flotation using a suitable amine collector [5,6]. Zhao et al., Jiang et al., and Cao et al. have extensively studied the flotation separation mechanism of diaspore and kaolinite using different types of amines [7e12]. Amines examined included amide amines, primary amines, poly-amines and ether-amines. They found that the separation mechanism is mainly controlled by the electrostatic effect, by hydrogen-bonding and by solution chemistry. Very little information concerning the use of tertiary amines in the flotation separation of diaspore from kaolinite exists, so we designed and synthesized a series of tertiary amines for experimental trials. * Corresponding author. Zhengzhou Institute of Multipurpose Utilization of Mineral Resources, CAGS, Zhengzhou 450006, China. Tel.: þ86 371 68632067. E-mail address: [email protected] (C. Liu).
The collectors used here are N,N-doubly substituted dodecyl amines including DRN, DEN, DPN, and DBN. The substituents of DRN, DEN, DPN, and DBN are methyl (eCH3), ethyl (eC2H5), propyl (eC3H7) and benzyl (eC7H7) respectively. The substituent effects of eCH3, eC2H5, eC3H7 and eC7H7 were expected to play an important role during flotation separation of diaspore from kaolinite. The flotation behavior and mechanism, of four tertiary amines used on kaolinite has been discussed in a paper published several months ago [13]. The aim of this work was to investigate the flotation behavior and mechanism of tertiary amines on separation of diaspore from kaolinite and to determine the influence of the substituent effect on the flotation performance of tertiary amines.
2. Experimental 2.1. Materials Pure diaspore and kaolinite were supplied by the Xiaoyi Mine in Shanxi province of China. The 0.074 mm fractions were used in micro-flotation tests. Chemical analyses of the diaspore and kaolinite are given in Table 1. The flotation collectors were four tertiary amines (DRN, DEN, DPN and DBN). Analytical grade sodium hydroxide and hydrochloric acid were used for pH control. Details for all these reagents are given in Table 2.
1674-5264/$ e see front matter Copyright Ó 2011, China University of Mining & Technology. All rights reserved. doi:10.1016/j.mstc.2010.10.001
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C. Liu et al. / Mining Science and Technology (China) 21 (2011) 249e253
Table 1 Chemical analysis of kaolinite and diaspore (%). Sample
Kaolinite Diaspore
Grade (mass fraction) Al2O3
SiO2
Fe2O3
TiO2
CaO
MgO
K2O
Na2O
H2O
Lost
39.20 80.98
43.67 0.78
0.32 0.29
1.98 2.84
0.01 0.01
0.068 0.046
0.094 0.007
0.028 0.025
13.65 14.06
13.98 14.50
2.2. Methods Micro-flotation tests using diaspore (0.074 mm) were carried out in a 40 mL flotation cell. Each flotation test used 3 g of mineral sample in 40 mL of distilled water. The zeta potential was determined with a zeta potential analyser manufactured by Brookhaven Instruments. The concentration of solids in the mineral dispersion was about 0.01e0.05% (by weight). Infrared adsorption spectra were obtained as KBr pellets using a Fourier transform infrared spectrometer (NEZUS 470-FT-IR). All quantum chemical calculations on the tertiary amines were performed using Gaussian 03 [14]. Energies were corrected by means of the full counterpoise technique. Tertiary amines and tertiary amine cations were initially optimized using MM2 and MP3 methods. These optimized structures were then subjected to DFT optimization and calculations. 3. Results and discussion 3.1. Flotation performance of tertiary amines on pure diaspore and kaolinite Studies on the influence of Hþ on pure mineral flotation were carried out to determine the collecting performance of DRN, DEN, DPN and DBN with regard to both diaspore and kaolinite (Fig. 1). RecoveryepH curves show that DBN has the worst collecting performance for recovery of diaspore and kaolinite. When the pH is less than 9 the recovery using DRN, DEN or DPN on diaspore increased slowly with pulp pH. For pH values greater than 9 the recovery of diaspore using these three tertiary amines decreased with increasing pulp pH. The recovery of kaolinite in acid pulps using DRN, DEN or DPN was higher than recovery from alkaline pulps. In acidic and weakly alkaline pulps the three collectors selectively float kaolinite rather than diaspore. Kaolinite and diaspore recoveries differ significantly; the maximum difference is about 40%. The relationship between collector dose and flotation recovery of diaspore and kaolinite at a pH of 5.5 indicates that recoveries increase with increasing collector levels until a maximum is reached (Fig. 2). Tertiary amine concentrations of 4 104 mol/L give the maximum recoveries of diaspore and kaolinite: 60% and 80% (DRN), 65% and 95% (DEN), 55% and 82% (DPN) and 3% and 58% (DBN) respectively. The three tertiary amines float kaolinite in preference to diaspore. 3.2. Mechanism of adsorption of tertiary amines on the diaspore surface An FT-IR spectrum of diaspore (Fig. 3a) shows characteristic sharp bands near 2112 cm1 and 1981 cm1 due to hydroxyl
stretching vibrations from Al-OH groups. The sharp peaks near 1070 cm1 and 984 cm1 are due to hydroxyl bending vibrations of Al-OH [15]. The FT-IR spectrum of kaolinite (Fig. 3b) shows characteristic sharp bands near 3675 cm1 due to hydroxyl stretching vibrations from Si-OH and Al-OH groups. The sharp peaks around 1200e900 cm1 arise from stretching vibrations of SieOeSi bonds [16]. These FT-IR results confirm the presence of OeH groups on the mineral surface of both diaspore and kaolinite particles. The zeta potentials of diaspore and kaolinite are shown in Fig. 4. The IEP (iso-electric point) of kaolinite and diaspore is 3.2 and 4.8 respectively. The surface of kaolinite and diaspore is positively charged for a pH less than 3.2 and 4.8 respectively and negatively charged under weak acidic and basic conditions. The surface charge of the two minerals may be controlled by two things. The first is the isomorphic exchange of mineral surface ions. Chemical analyses (Table 1) indicate that diaspore and kaolinite are composed of many ions (Al3þ, Fe2þ, Ti2þ, Kþ, Naþ). Al3þ can be replaced by Fe2þ or Ti2þ leading to the formation of a permanent negative charge at the position of some oxygen atoms [17]. Second, ionization of surface AleOeH and SieOeH groups is also responsible for charge on the diaspore surface: this may be represented as:
Al OH5Al O þ Hþ
(1)
Al OH þ OH 5Al O þ H2 O
(2)
Si OH5Si O þ Hþ
(3)
Si OH þ OH 5Si O þ H2 O
(4)
These reactions are determined by the pulp pH. Adsorption or dissociation of Hþ and OH is the dominating factor in the charging mechanism of the diaspore surface. Tertiary amines have an N atom with sp3 hybridization and a lone pair of electrons in a 2s orbital. These atoms bind strongly to protons because of the unoccupied orbital. After bonding with Hþ the tertiary amines become cationic collectors:
H3 C ðCH2 Þ10 CH2 NR2 þ Hþ 5H3 C ðCH2 Þ10 CH2 NR2 Hþ
(5)
Cationic collectors are attracted to the permanent negative surface charge of kaolinite or diaspore, when the pulp pH is less than 3.2 or 4.8. In the pH range from 3.2 to 8 or from 4.8 to 8, the kaolinite or diaspore surface is negatively charged. Now the tertiary ammonium cations react with the two mineral surfaces by electrostatic forces, too. In an alkaline pulp the tertiary ammonium ions
Table 2 Details of reagents used in the flotation tests. Name and code
Molecular formula
Basic properties
Purity (%)
Source
N,N-Dimethyl-dodecyl amine DRN N,N-Diethyl-dodecyl amine DEN N,N-Dipropyl-dodecyl amine DPN N,N-Dibenzyl-dodecyl amine DBN Sodium hydroxide Hydrochloride
C12H25N(CH3)2 C12H25N(C2H5)2 C12H25N(C3H7)2 C12H25N(C7H7)2 NaOH HCl
Yellow liquid Dark-red liquid Brown liquid Yellow liquid White solid Light yellow liquid
>90 >85 >70 >50 Analytical purity Analytical purity
Synthesized in lab. Synthesized in lab. Synthesized in lab. Synthesized in lab. Made in Tianjing Made in Tianjing
C. Liu et al. / Mining Science and Technology (China) 21 (2011) 249e253
react with hydroxyl groups and are expected to form free amine and water. Neutral tertiary amines cannot interact electrostatically with a negatively charged surface. The nitrogen atoms of a neutral tertiary amine do not bond to hydrogen atoms and no hydrogen
a
bonds can thus exist between the collector molecules and the oxygen atoms on the kaolinite or diaspore surface. Kaolinite and diaspore recovery in an alkaline pulp is, therefore, far less than in an acidic pulp.
b
Diaspore
80 DRN DEN DPN DBN Collectors 2.0×10- 4 mol/L
60 40 20 0
2
4
6
8
10
12
)
80 Recovery
)
Kaolinite 100
100
Recovery
251
60 40 20 0
14
2
4
pH
6
8
10
12
14
pH
Fig. 1. Recovery of diaspore and kaolinite as a function of pH using tertiary amines.
a
b
Diaspore
100
Recovery (%)
DRN DEN DPN DBN pH=5.5
60 40 20 2
4 6 8 Dosage (10- 4 mol/L)
10
Recovery (%)
100
80
0
Kaolinite
80 60 40 20
0
2
4 6 8 Dosage (10- 4 mol/L)
Fig. 2. Recovery of diaspore and kaolinite as a function of collector dose using tertiary amines.
Fig. 3. FT-IR spectra of diaspore and kaolinite.
Fig. 4. Effect of DRN, DEN, DPN and DBN on the zeta potential of diaspore and kaolinite.
10
252
C. Liu et al. / Mining Science and Technology (China) 21 (2011) 249e253
Table 3 Calculated charge on the ammonium group of different tertiary amines. Head group Charge (e)
eCH2eNHe(CH2)2 0.8621
eCH2eNHe(CH2CH3)2 0.8644
eCH2eNHe(CH2CH2CH3)2 0.8683
eCH2eNHe(CH2C6H5)2 0.8541
Table 4 Steric indices of the substituent groups in the tertiary amines. Substituent group
eCH3 (Å)
eCH2CH3 (Å)
eCH2CH2CH3 (Å)
eCH2C6H5 (Å)
Length along the carbon chain Diameter perpendicular to the carbon chain
0.65 1.787
2.159 1.987
3.668 2.187
6.28 4.88
Over the whole pH range the zeta potential of kaolinite is lower than that of diaspore (Fig. 3). Therefore, the electrostatic forces between cationic tertiary ammonium ions and the kaolinite surface is expected to be stronger than that with a diaspore surface. Results of zeta potential measurements confirm this. After reacting with tertiary amines (pH > 3.2) the zeta potential of both kaolinite and diaspore increases but the recruitment of kaolinite is much bigger than that of diaspore. This shows the stronger interaction of tertiary amines with kaolinite and explains why recovery of kaolinite exceeds that of diaspore. 3.3. How substituents influence the flotation performance of tertiary amines Diaspore and kaolinite recovery using DRN, DEN, DPN or DBN decreases in the following order DEN > DPN > DRN > DBN. These four tertiary amines differ from each other because of the substituent groups on nitrogen. These different substituents probably cause the difference in flotation behavior of these four amines. After bonding with protons the amines become cationic ammonium ions where the nitrogen atom is positively charged. The different inductive effects of eCH2, C2H5, C3H7 or C7H7 cause the charge distribution of the ammonium group to follow certain rules. Quantum chemistry calculations were done to estimate the positive charge distribution of these ammonium groups (Table 3). The charge on the ammonium group is 0.8621, 0.8644, 0.8683 or 0.8541 for eCH2eNHþe(CH3)2, eCH2eNHþe(CH2CH3)2, eCH2eNHþe (CH2CH2CH3)2, or eCH2eNHþe(CH2C6H5)2, respectively. A more positively charged group would result in a stronger electrostatic effect between a cationic collector and the surface of kaolinite or diaspore. The electrostatic effect between collector and mineral surface increases in the order DPN > DEN > DRN > DBN. Furthermore, the steric effect of the substituent groups should also be considered, in addition to inductive effects. The lengths of eCH3, eC2H5, eC3H7 and eC7H7 are 0.65 Å, 2.159 Å, 3.668 Å and 6.28 Å, respectively. The corresponding diameters are 1.787 Å, 1.987 Å, 2.187 Å and 4.88 Å (Table 4). The steric effect of these groups increases in the order eC7H7 > eC3H7 > eC2H5 > eCH3. A larger steric effect causes a stronger solvent effect, which decreases the ability of the amine to become protonated. A larger steric effect also decreases the possibility of a cationic tertiary ammonium ion adsorbing onto the diaspore surface. Hence, both inductive and steric effects should cause the collecting ability of the four tertiary amines to fall in the order DEN > DPN > DRN > DBN. 4. Conclusions 1) Micro-flotation tests using DRN, DEN, DPN or DBN as collector indicate that kaolinite recovery is higher than that of diaspore over a wide pH range. The maximum recovery difference, comparing kaolinite to diaspore, is about 45%.
2) An increase in collector dose causes recovery of both minerals to increase. The collecting abilities of the four tertiary amines for kaolinite or diaspore fall in the order DEN > DPN > DRN > DBN. 3) FT-IR spectra confirm the presence of hydroxyl groups on the surface of kaolinite and diaspore. Zeta potential measurements show the surfaces of diaspore and kaolinite are negatively charged over a wide pH range. Diaspore and kaolinite have isoelectric points of 4.8 and 3.2, respectively. Both the ionization of hydroxyl groups and isomorphic exchange account for the surface charging mechanism. 4) Both substituent induced inductive and steric effects can influence the flotation performance of these four tertiary amines. Considering both effects, the collecting ability of the four tertiary amines is predicted to fall in the order DEN > DPN > DRN > DBN. Acknowledgements This work was supported by the National Key Fundamental Research and Development Program (No. 2005CB623701), the National Department Public Benefit Research Foundation from Ministry of Land and Resources (No. 201011031), the Foundation for the Author of Zhengzhou Institute of Multipurpose Utilization of Mineral Resources CAGS (No. 2935), for which the authors express their appreciations. References [1] Zhao ZD. Bauxite and alumina industry of world. Beijing: Science Press; 1994 [In Chinese]. [2] Liu GY. A study on flotation and de-silication of diaspore bauxites. Changsha: Department of Chemical Engineering, Central South University of Technology; 1999 [In Chinese]. [3] Papanastassiou D, Csoke B, Solymar K. Improved preparation of the Greek diaporic bauxite for Bayer-process. Light Metals 2002;11:67e74 [In Chinese]. [4] Ma C, Tian X, Liu R, Yang X. Selection of Bayer digestion technology and equipment for Chinese bauxite. Light Metals 1996;4:187e91 [In Chinese]. [5] Wang YH, Hu YH, He PB, Gu GH. Reverse flotation for removal of silicates from diasporic-bauxite. Mineral Engineering 2004;17:63e8. [6] Zhong H, Liu GY, Xia LY. Flotation separation of diaspore from kaolinite, pyrophyllite and illite using three cationic collectors. Mineral Engineering 2008;21:1055e61. [7] Zhao SM, Wang DZ, Hu YH. Flotation of aluminosilicate using N-(2-aminoethyl)-1-naphthaleneacetamide. Minerals Engineering 2003;16:1031e3. [8] Zhao SM, Wang DZ, Hu YH. The flotation behavior of N-(3-amin-opropyl)dodecanamide on three aluminosilicate. Minerals Engineering 2003;16:1391e5. [9] Jiang H, Hu YH, Qin WQ. Interaction and flotation of diaspore with alkylamine hydrochlorides. Transactions of Nonferrous Metals Society of China 2001;11:430e3. [10] Jiang H, Hu YH, Qin WQ. Solution chemistry of flotation separation of diasporic bauxite. The Chinese Journal of Nonferrous Metals 2001;11(1):125e30 [In Chinese]. [11] Cao XF, Hu YH. Synthesis of N-decyl-1, 3-diaminopropanes and its flotation properties on aluminium silicate minerals. Transactions of Nonferrous Metals Society of China 2003;13(2):417e20. [12] Cao XF, Hu YH. Synthesis of g-alkoxy-propylamines and their collecting properties on aluminosilicate minerals. Journal of Central South University 2004;11(3):280e5.
C. Liu et al. / Mining Science and Technology (China) 21 (2011) 249e253 [13] Liu CM, Hu YH, Cao XF. Substituent effects in kaolinite flotation using dodecyl tertiary amines. Mineral Engineering 2009;22:849e52. [14] Frish MJ, Trucks GW, Schlegel HB. Gaussian03, G03RevB.01. Pittsburgh (PA): Gaussian, Inc; 2003. pp. 124e254. [15] Pan ZL. Crystallography and mineralogy. Beijing: Beijing Geology Press; 1994. pp. 164e82 [In Chinese].
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[16] Elbokl TA, Detellier C. Aluminosilicate nanohybrid materials intercalation of polystyrene in kaolinite. Journal of Physics and Chemistry of Solids 2006;67:950e5. [17] Hu YH, Liu XW, Xu ZH. Role of crystal structure in flotation separation of diaspore from kaolinite, pyrophyllite and illite. Mineral Engineering 2003;16(3):219e27.
Contents lists available at ScienceDirect
Mining Science and Technology (China) journal homepage: www.elsevier.com/locate/mstc
Flotation behavior of four dodecyl tertiary amines as collectors of diaspore and kaolinite Changmiao Liu a, b, *, Feng Ansheng a, b, Guo Zhenxu a, b, Cao Xuefeng c, Hu Yuehua c a
Zhengzhou Institute of Multipurpose Utilization of Mineral Resources, CAGS, Zhengzhou 450006, China China National Engineering Research Center for Utilization of Industrial Minerals, Zhengzhou 450006, China c School of Minerals Processing and Bio-engineering, Central South University, Changsha 410083, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 July 2010 Received in revised form 30 September 2010 Accepted 30 October 2010
The flotation of diaspore and kaolinite by one of a series of tertiary amines (DRN, DEN, DPN and DBN) was investigated. The tertiary amines show better floating recovery for kaolinite compared to diaspore. The maximum recovery D-value is 45% over a pH range from 3 to 8. FT-IR spectra confirm the presence of hydroxyl groups on the surface of kaolinite and diaspore. Zeta potential measurements show that the mineral surfaces are negatively charged over a wide pH range. Ionization of hydroxyl groups mainly accounts for the surface charging mechanism. The adsorption of tertiary amines onto the mineral surface is due mainly to electrostatic effects and the difference in electrostatic effect between a collector and the two minerals can explain the flotation separation. Inductive electronic and steric effects from the substituent groups result in different collecting powers for the four tertiary amines. Copyright Ó 2011, China University of Mining & Technology. All rights reserved.
Keywords: Kaolinite Diaspore Amines collectors Froth flotation
1. Introduction China has huge bauxite reserves most of which are of the diasporic type [1,2]. The useful mineral in diasporic bauxites is mainly diaspore and the gangue mineral is mainly kaolinite. A fatal disadvantage of Chinese diasporic ores is their low Al2O3 to SiO2 mass ratio, which generally ranges from 4 to 6. Because the mass ratio of Al2O3 to SiO2 is lower than 8, the minimum requirement for the Bayer process, Chinese bauxites cannot be processed directly by the advanced process [3,4]. Increasing the Al2O3/SiO2 ratio of middle or low grade bauxite requires the separation of diaspore from the gangue minerals by flotation using a suitable amine collector [5,6]. Zhao et al., Jiang et al., and Cao et al. have extensively studied the flotation separation mechanism of diaspore and kaolinite using different types of amines [7e12]. Amines examined included amide amines, primary amines, poly-amines and ether-amines. They found that the separation mechanism is mainly controlled by the electrostatic effect, by hydrogen-bonding and by solution chemistry. Very little information concerning the use of tertiary amines in the flotation separation of diaspore from kaolinite exists, so we designed and synthesized a series of tertiary amines for experimental trials. * Corresponding author. Zhengzhou Institute of Multipurpose Utilization of Mineral Resources, CAGS, Zhengzhou 450006, China. Tel.: þ86 371 68632067. E-mail address: [email protected] (C. Liu).
The collectors used here are N,N-doubly substituted dodecyl amines including DRN, DEN, DPN, and DBN. The substituents of DRN, DEN, DPN, and DBN are methyl (eCH3), ethyl (eC2H5), propyl (eC3H7) and benzyl (eC7H7) respectively. The substituent effects of eCH3, eC2H5, eC3H7 and eC7H7 were expected to play an important role during flotation separation of diaspore from kaolinite. The flotation behavior and mechanism, of four tertiary amines used on kaolinite has been discussed in a paper published several months ago [13]. The aim of this work was to investigate the flotation behavior and mechanism of tertiary amines on separation of diaspore from kaolinite and to determine the influence of the substituent effect on the flotation performance of tertiary amines.
2. Experimental 2.1. Materials Pure diaspore and kaolinite were supplied by the Xiaoyi Mine in Shanxi province of China. The 0.074 mm fractions were used in micro-flotation tests. Chemical analyses of the diaspore and kaolinite are given in Table 1. The flotation collectors were four tertiary amines (DRN, DEN, DPN and DBN). Analytical grade sodium hydroxide and hydrochloric acid were used for pH control. Details for all these reagents are given in Table 2.
1674-5264/$ e see front matter Copyright Ó 2011, China University of Mining & Technology. All rights reserved. doi:10.1016/j.mstc.2010.10.001
250
C. Liu et al. / Mining Science and Technology (China) 21 (2011) 249e253
Table 1 Chemical analysis of kaolinite and diaspore (%). Sample
Kaolinite Diaspore
Grade (mass fraction) Al2O3
SiO2
Fe2O3
TiO2
CaO
MgO
K2O
Na2O
H2O
Lost
39.20 80.98
43.67 0.78
0.32 0.29
1.98 2.84
0.01 0.01
0.068 0.046
0.094 0.007
0.028 0.025
13.65 14.06
13.98 14.50
2.2. Methods Micro-flotation tests using diaspore (0.074 mm) were carried out in a 40 mL flotation cell. Each flotation test used 3 g of mineral sample in 40 mL of distilled water. The zeta potential was determined with a zeta potential analyser manufactured by Brookhaven Instruments. The concentration of solids in the mineral dispersion was about 0.01e0.05% (by weight). Infrared adsorption spectra were obtained as KBr pellets using a Fourier transform infrared spectrometer (NEZUS 470-FT-IR). All quantum chemical calculations on the tertiary amines were performed using Gaussian 03 [14]. Energies were corrected by means of the full counterpoise technique. Tertiary amines and tertiary amine cations were initially optimized using MM2 and MP3 methods. These optimized structures were then subjected to DFT optimization and calculations. 3. Results and discussion 3.1. Flotation performance of tertiary amines on pure diaspore and kaolinite Studies on the influence of Hþ on pure mineral flotation were carried out to determine the collecting performance of DRN, DEN, DPN and DBN with regard to both diaspore and kaolinite (Fig. 1). RecoveryepH curves show that DBN has the worst collecting performance for recovery of diaspore and kaolinite. When the pH is less than 9 the recovery using DRN, DEN or DPN on diaspore increased slowly with pulp pH. For pH values greater than 9 the recovery of diaspore using these three tertiary amines decreased with increasing pulp pH. The recovery of kaolinite in acid pulps using DRN, DEN or DPN was higher than recovery from alkaline pulps. In acidic and weakly alkaline pulps the three collectors selectively float kaolinite rather than diaspore. Kaolinite and diaspore recoveries differ significantly; the maximum difference is about 40%. The relationship between collector dose and flotation recovery of diaspore and kaolinite at a pH of 5.5 indicates that recoveries increase with increasing collector levels until a maximum is reached (Fig. 2). Tertiary amine concentrations of 4 104 mol/L give the maximum recoveries of diaspore and kaolinite: 60% and 80% (DRN), 65% and 95% (DEN), 55% and 82% (DPN) and 3% and 58% (DBN) respectively. The three tertiary amines float kaolinite in preference to diaspore. 3.2. Mechanism of adsorption of tertiary amines on the diaspore surface An FT-IR spectrum of diaspore (Fig. 3a) shows characteristic sharp bands near 2112 cm1 and 1981 cm1 due to hydroxyl
stretching vibrations from Al-OH groups. The sharp peaks near 1070 cm1 and 984 cm1 are due to hydroxyl bending vibrations of Al-OH [15]. The FT-IR spectrum of kaolinite (Fig. 3b) shows characteristic sharp bands near 3675 cm1 due to hydroxyl stretching vibrations from Si-OH and Al-OH groups. The sharp peaks around 1200e900 cm1 arise from stretching vibrations of SieOeSi bonds [16]. These FT-IR results confirm the presence of OeH groups on the mineral surface of both diaspore and kaolinite particles. The zeta potentials of diaspore and kaolinite are shown in Fig. 4. The IEP (iso-electric point) of kaolinite and diaspore is 3.2 and 4.8 respectively. The surface of kaolinite and diaspore is positively charged for a pH less than 3.2 and 4.8 respectively and negatively charged under weak acidic and basic conditions. The surface charge of the two minerals may be controlled by two things. The first is the isomorphic exchange of mineral surface ions. Chemical analyses (Table 1) indicate that diaspore and kaolinite are composed of many ions (Al3þ, Fe2þ, Ti2þ, Kþ, Naþ). Al3þ can be replaced by Fe2þ or Ti2þ leading to the formation of a permanent negative charge at the position of some oxygen atoms [17]. Second, ionization of surface AleOeH and SieOeH groups is also responsible for charge on the diaspore surface: this may be represented as:
Al OH5Al O þ Hþ
(1)
Al OH þ OH 5Al O þ H2 O
(2)
Si OH5Si O þ Hþ
(3)
Si OH þ OH 5Si O þ H2 O
(4)
These reactions are determined by the pulp pH. Adsorption or dissociation of Hþ and OH is the dominating factor in the charging mechanism of the diaspore surface. Tertiary amines have an N atom with sp3 hybridization and a lone pair of electrons in a 2s orbital. These atoms bind strongly to protons because of the unoccupied orbital. After bonding with Hþ the tertiary amines become cationic collectors:
H3 C ðCH2 Þ10 CH2 NR2 þ Hþ 5H3 C ðCH2 Þ10 CH2 NR2 Hþ
(5)
Cationic collectors are attracted to the permanent negative surface charge of kaolinite or diaspore, when the pulp pH is less than 3.2 or 4.8. In the pH range from 3.2 to 8 or from 4.8 to 8, the kaolinite or diaspore surface is negatively charged. Now the tertiary ammonium cations react with the two mineral surfaces by electrostatic forces, too. In an alkaline pulp the tertiary ammonium ions
Table 2 Details of reagents used in the flotation tests. Name and code
Molecular formula
Basic properties
Purity (%)
Source
N,N-Dimethyl-dodecyl amine DRN N,N-Diethyl-dodecyl amine DEN N,N-Dipropyl-dodecyl amine DPN N,N-Dibenzyl-dodecyl amine DBN Sodium hydroxide Hydrochloride
C12H25N(CH3)2 C12H25N(C2H5)2 C12H25N(C3H7)2 C12H25N(C7H7)2 NaOH HCl
Yellow liquid Dark-red liquid Brown liquid Yellow liquid White solid Light yellow liquid
>90 >85 >70 >50 Analytical purity Analytical purity
Synthesized in lab. Synthesized in lab. Synthesized in lab. Synthesized in lab. Made in Tianjing Made in Tianjing
C. Liu et al. / Mining Science and Technology (China) 21 (2011) 249e253
react with hydroxyl groups and are expected to form free amine and water. Neutral tertiary amines cannot interact electrostatically with a negatively charged surface. The nitrogen atoms of a neutral tertiary amine do not bond to hydrogen atoms and no hydrogen
a
bonds can thus exist between the collector molecules and the oxygen atoms on the kaolinite or diaspore surface. Kaolinite and diaspore recovery in an alkaline pulp is, therefore, far less than in an acidic pulp.
b
Diaspore
80 DRN DEN DPN DBN Collectors 2.0×10- 4 mol/L
60 40 20 0
2
4
6
8
10
12
)
80 Recovery
)
Kaolinite 100
100
Recovery
251
60 40 20 0
14
2
4
pH
6
8
10
12
14
pH
Fig. 1. Recovery of diaspore and kaolinite as a function of pH using tertiary amines.
a
b
Diaspore
100
Recovery (%)
DRN DEN DPN DBN pH=5.5
60 40 20 2
4 6 8 Dosage (10- 4 mol/L)
10
Recovery (%)
100
80
0
Kaolinite
80 60 40 20
0
2
4 6 8 Dosage (10- 4 mol/L)
Fig. 2. Recovery of diaspore and kaolinite as a function of collector dose using tertiary amines.
Fig. 3. FT-IR spectra of diaspore and kaolinite.
Fig. 4. Effect of DRN, DEN, DPN and DBN on the zeta potential of diaspore and kaolinite.
10
252
C. Liu et al. / Mining Science and Technology (China) 21 (2011) 249e253
Table 3 Calculated charge on the ammonium group of different tertiary amines. Head group Charge (e)
eCH2eNHe(CH2)2 0.8621
eCH2eNHe(CH2CH3)2 0.8644
eCH2eNHe(CH2CH2CH3)2 0.8683
eCH2eNHe(CH2C6H5)2 0.8541
Table 4 Steric indices of the substituent groups in the tertiary amines. Substituent group
eCH3 (Å)
eCH2CH3 (Å)
eCH2CH2CH3 (Å)
eCH2C6H5 (Å)
Length along the carbon chain Diameter perpendicular to the carbon chain
0.65 1.787
2.159 1.987
3.668 2.187
6.28 4.88
Over the whole pH range the zeta potential of kaolinite is lower than that of diaspore (Fig. 3). Therefore, the electrostatic forces between cationic tertiary ammonium ions and the kaolinite surface is expected to be stronger than that with a diaspore surface. Results of zeta potential measurements confirm this. After reacting with tertiary amines (pH > 3.2) the zeta potential of both kaolinite and diaspore increases but the recruitment of kaolinite is much bigger than that of diaspore. This shows the stronger interaction of tertiary amines with kaolinite and explains why recovery of kaolinite exceeds that of diaspore. 3.3. How substituents influence the flotation performance of tertiary amines Diaspore and kaolinite recovery using DRN, DEN, DPN or DBN decreases in the following order DEN > DPN > DRN > DBN. These four tertiary amines differ from each other because of the substituent groups on nitrogen. These different substituents probably cause the difference in flotation behavior of these four amines. After bonding with protons the amines become cationic ammonium ions where the nitrogen atom is positively charged. The different inductive effects of eCH2, C2H5, C3H7 or C7H7 cause the charge distribution of the ammonium group to follow certain rules. Quantum chemistry calculations were done to estimate the positive charge distribution of these ammonium groups (Table 3). The charge on the ammonium group is 0.8621, 0.8644, 0.8683 or 0.8541 for eCH2eNHþe(CH3)2, eCH2eNHþe(CH2CH3)2, eCH2eNHþe (CH2CH2CH3)2, or eCH2eNHþe(CH2C6H5)2, respectively. A more positively charged group would result in a stronger electrostatic effect between a cationic collector and the surface of kaolinite or diaspore. The electrostatic effect between collector and mineral surface increases in the order DPN > DEN > DRN > DBN. Furthermore, the steric effect of the substituent groups should also be considered, in addition to inductive effects. The lengths of eCH3, eC2H5, eC3H7 and eC7H7 are 0.65 Å, 2.159 Å, 3.668 Å and 6.28 Å, respectively. The corresponding diameters are 1.787 Å, 1.987 Å, 2.187 Å and 4.88 Å (Table 4). The steric effect of these groups increases in the order eC7H7 > eC3H7 > eC2H5 > eCH3. A larger steric effect causes a stronger solvent effect, which decreases the ability of the amine to become protonated. A larger steric effect also decreases the possibility of a cationic tertiary ammonium ion adsorbing onto the diaspore surface. Hence, both inductive and steric effects should cause the collecting ability of the four tertiary amines to fall in the order DEN > DPN > DRN > DBN. 4. Conclusions 1) Micro-flotation tests using DRN, DEN, DPN or DBN as collector indicate that kaolinite recovery is higher than that of diaspore over a wide pH range. The maximum recovery difference, comparing kaolinite to diaspore, is about 45%.
2) An increase in collector dose causes recovery of both minerals to increase. The collecting abilities of the four tertiary amines for kaolinite or diaspore fall in the order DEN > DPN > DRN > DBN. 3) FT-IR spectra confirm the presence of hydroxyl groups on the surface of kaolinite and diaspore. Zeta potential measurements show the surfaces of diaspore and kaolinite are negatively charged over a wide pH range. Diaspore and kaolinite have isoelectric points of 4.8 and 3.2, respectively. Both the ionization of hydroxyl groups and isomorphic exchange account for the surface charging mechanism. 4) Both substituent induced inductive and steric effects can influence the flotation performance of these four tertiary amines. Considering both effects, the collecting ability of the four tertiary amines is predicted to fall in the order DEN > DPN > DRN > DBN. Acknowledgements This work was supported by the National Key Fundamental Research and Development Program (No. 2005CB623701), the National Department Public Benefit Research Foundation from Ministry of Land and Resources (No. 201011031), the Foundation for the Author of Zhengzhou Institute of Multipurpose Utilization of Mineral Resources CAGS (No. 2935), for which the authors express their appreciations. References [1] Zhao ZD. Bauxite and alumina industry of world. Beijing: Science Press; 1994 [In Chinese]. [2] Liu GY. A study on flotation and de-silication of diaspore bauxites. Changsha: Department of Chemical Engineering, Central South University of Technology; 1999 [In Chinese]. [3] Papanastassiou D, Csoke B, Solymar K. Improved preparation of the Greek diaporic bauxite for Bayer-process. Light Metals 2002;11:67e74 [In Chinese]. [4] Ma C, Tian X, Liu R, Yang X. Selection of Bayer digestion technology and equipment for Chinese bauxite. Light Metals 1996;4:187e91 [In Chinese]. [5] Wang YH, Hu YH, He PB, Gu GH. Reverse flotation for removal of silicates from diasporic-bauxite. Mineral Engineering 2004;17:63e8. [6] Zhong H, Liu GY, Xia LY. Flotation separation of diaspore from kaolinite, pyrophyllite and illite using three cationic collectors. Mineral Engineering 2008;21:1055e61. [7] Zhao SM, Wang DZ, Hu YH. Flotation of aluminosilicate using N-(2-aminoethyl)-1-naphthaleneacetamide. Minerals Engineering 2003;16:1031e3. [8] Zhao SM, Wang DZ, Hu YH. The flotation behavior of N-(3-amin-opropyl)dodecanamide on three aluminosilicate. Minerals Engineering 2003;16:1391e5. [9] Jiang H, Hu YH, Qin WQ. Interaction and flotation of diaspore with alkylamine hydrochlorides. Transactions of Nonferrous Metals Society of China 2001;11:430e3. [10] Jiang H, Hu YH, Qin WQ. Solution chemistry of flotation separation of diasporic bauxite. The Chinese Journal of Nonferrous Metals 2001;11(1):125e30 [In Chinese]. [11] Cao XF, Hu YH. Synthesis of N-decyl-1, 3-diaminopropanes and its flotation properties on aluminium silicate minerals. Transactions of Nonferrous Metals Society of China 2003;13(2):417e20. [12] Cao XF, Hu YH. Synthesis of g-alkoxy-propylamines and their collecting properties on aluminosilicate minerals. Journal of Central South University 2004;11(3):280e5.
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