J. Cent. South Univ. Technol. (2009) 16: 0749-0752

DOI: 10.1007/s11771-009-0124-1

Flotation of kaolinite with dodecyl tertiary amines

CAO Xue-feng(曹学锋), LIU Chang-miao(刘长淼), HU Yue-hua(胡岳华)

(School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China)

Abstract: The flotation of kaolinite using a series of tertiary amines (N,N-dimethyl-dodecyl amine (DRN), N,N-diethyl-dodecyl amine (DEN), N,N-dipropyl-dodecyl amine (DPN) and N,N-dibenzyl-bodecyl amine (DBN)) was investigated. The results show that the maximum recoveries of kaolinite for DEN, DPN and DRN are 93%, 88% and 84%, respectively, but that of DBN is very low. On the basis of zeta potential and FT-IR spectra, the ionization of surface hydroxyl and isomorphic exchange of surface ions account for the charging mechanisms of kaolinite surface. The adsorption mechanism of tertiary amines on kaolinite surface is mainly electrostatic. The isoelectric point (IEP) of kaolinite increases from 3.4 to some more positive points after the interaction of kaolinite with the four tertiary amines. The FT-IR spectra of kaolinite change with the presence of some new sharp shapes belonging to the tertiary amines. The inductive electronic effects and space-steric effects of —CH₃, —C₂H₅, —C₃H₇ and  —C₇H₇ bonding to N atom result in different collecting power of the four tertiary amines.

Key words: kaolinite; tertiary amine; flotation; electrostatic effect; inductive electronic effect; space-steric effect; recovery

                                                                                                           
1 Introduction

Bauxite is the main raw material for alumina industry, and there are plenty of diaspore-type bauxite resources in China. However, because of the fatal disadvantage of the low ratio (4-6) of aluminum oxide to silica mainly coming from kaolinite, pyrophyllite and illite, Chinese bauxite cannot be the qualified materials for the popular and advanced Bayer’s technology to produce alumina [1]. Nowadays, people have made a lot of researches on decreasing silica of bauxite by using the flotation, and researches have been focused on two aspects including collectors and processing technique. The reagents for bauxite flotation were developed so quickly that many new high efficient collectors appeared in the bauxite flotation. ZHANG and FENG [2] made a detailed study on the mechanism between sodium oleate and diaspore, and gave some very good explanations.  ZHAO et al [3], JIANG et al [4], HU et al [5-6], and ZHONG et al [7] made a lot of researches about the separation of kaolinite from diaspore using different kinds of amine collectors including amide amines, primary amines, poly-amines and ether-amines, and they found that the flotation interaction mechanism was mainly controlled by the electrostatic effect, hydrogen-bonding effect and solution chemistry. But there are few reports about the tertiary amine in the separation of kaolinite from diaspore. So in this work, a series of tertiary amines were designed and synthesized. Collectors used here were N,N-double substituent- dodecyl amines, and the substituting groups were —CH₃, —C₂H₅, —C₃H₇ and —C₇H₇. The flotation behavior and mechanism of kaolinite with a series of dodecyl tertiary amines were investigated.

2 Experimental

2.1 Materials

The samples of kaolinite for flotation tests were pure minerals from Xiaoyi Mine in Shanxi Province, China. After being pounded to pieces and hand sorting, kaolinite ores were ground to powder samples with diameter less than 74 μm for flotation tests.

Four tertiary amines were used for flotation collectors, and their code names were N,N-dimethyl- dodecyl amine (DRN), N,N-diethyl-dodecyl amine (DEN), N,N-dipropyl-dodecyl amine (DPN) and N,N-dibenzyl-bodecyl amine (DBN), respectively. And the pH modification reagents were HCl and NaOH. The details of all these reagents are listed in Table 1.

2.2 Methods

Micro-flotation tests with kaolinite having size less than 74 μm were performed in a 40 mL flotation cell. 3 g of mineral sample was used for each flotation test in 40 mL of distilled water.

Zeta potential was determined with a zeta potential analyzer manufactured by Brookhaven Instruments, USA. The mass fraction of solids in the mineral dispersion was 0.01%-0.05%.

Table 1 Details of reagents used in tests

Infrared absorption spectra were obtained through the KBr pellet method by a Fourier transform infrared (FT-IR) spectrometer NEZUS 470-FT-IR.

3 Results and discussion

3.1 Flotation tests

Studies on the influence of H⁺ on pure mineral flotation were carried out to determine the collecting power of DRN, DEN, DPN and DBN on kaolinite (see Fig.1). The curves of recovery—pH show that the four tertiary amines have higher recoveries for kaolinite in the acid pulp than in the alkaline pulp. The curves indicate that a series of tertiary amines have the collecting power for kaolinite in the order of DEN＞DPN＞DRN＞DBN, and the maximum recoveries of DRN, DEN and DPN are about 90%, and that of DBN is only about 75%.

Fig.1 Recovery of kaolinite as function of pH at collector concentration of 0.2 mmol/L

The relationship between collector dosage and flotation recovery of kaolinite at a given pH (5.5) indicates that the recovery increases with increasing collector concentration until a maximum reaches (Fig.2). When the collector concentration is up to 0.4 mmol/L, the maximum recoveries are 80%, 95%, 83% and 60% for DRN, DEN, DPN and DBN, respectively.

Fig.2 Recovery of kaolinite as function of dosage DRN, DEN, DPN and DBN at pH 5.5

3.2 Flotation mechanism

FT-IR spectra of kaolinite under different conditions are shown in Fig.3. The characteristic bands of sharp shape at around 3 670 cm^-1 are due to the stretching vibration of hydroxyl from the Si—OH and Al—OH groups. And the sharp shapes around at 1 200-900 cm^-1 are due to the stretching vibration of Si—O—Si band. It seems that there exist some free water molecules in the minerals because of bend absorptions near 1 625 cm^-1 [8].

Fig.3 FT-IR spectra of kaolinite without and with tertiary amines: (a) Kaolinite; (b) Kaolinite-DRN; (c) Kaolinite-DEN; (d) Kaolinite-DPN; (e) Kaolinite-DBN

The relationship between zeta potential of kaolinite and pH is shown in Fig.4. The zeta potential of kaolinite is negative in a wide pH range with an isoelectric point (IEP) of 3.2, and it decreases when pH is above the IEP point.

Fig.4 Relationship between zeta potential of kaolinite and pH

Based on the FT-IR results of kaolinite, there are plenty of Si—OH and Al—OH on the surface of kaolinite. And the ionization of hydroxyl groups can explain the surface charging mechanism of kaolinite as follows [9]:

Si—OHSi—O^-+H⁺                        (1)

Al—OHAl—O^-+H⁺                       (2)

Si—OH+OH^-Si—O^-+H₂O                  (3)

Al—OH+OH^-Al—O^-+H₂O                 (4)

Adsorption or dissociation of H⁺ and OH^- accounts for the surface charge of kaolinite. And the ionization of hydroxyl groups is dependent on pH, so the surface charge of kaolinite decreases with the increase of pH (Fig.4). Another reason for the surface charging mechanism of kaolinite is the isomorphic exchange of surface ions. Si⁴⁺ can be replaced by Al³⁺ , and Al³⁺ can be replaced by Fe²⁺ or K⁺, resulting in some permanent negative charged oxygen atoms on the surface [10].

In the tertiary amine molecules, N atom belongs to the sp³ hybridization type with lone pair electrons in 2s orbit, and strongly tends to bind with a H⁺ that has the unoccupied orbit as follows:

H₃C—(CH₂)₁₀—CH₂—NR₂+H⁺

H₃C—(CH₂)₁₀—CH₂—NR₂ H⁺    (5)

In the acid and weak alkalic pH range, tertiary amines shift to the cationic collectors after bonding with a H⁺ [11]. As kaolinite surface is negatively charged, the interaction mechanism between tertiary amine cations and kaolinite should be physical adsorption by electric-static power, which is demonstrated by both the FT-IR and zeta potential results (Figs.3 and 4). After DRN, DEN, DPN and DBN are introduced into the kaolinite system, the FT-IR spectra of kaolinite are changed with the appearance of C—N bending vibration peak (2 750 cm^-1) and N—C—H group stretching vibration peak (2 850 cm^-1). The zeta potential of kaolinite increases with the addition of tertiary amines, and the increasing extent of zeta potential is in the order of DEN＞DPN＞DRN＞DBN, which is the same as the flotation ability order. When pH is above 10, tertiary amines are mainly in the form of neutral molecules. There is no electro-static effect between collectors and mineral surface, so the recoveries of kaolinite flotation decrease.

Flotation tests indicate that recoveries of acidic pH range are better than those of alkalic pH range, which are not in agreement with the results of zeta potential. And the more the negative zeta potential of kaolinite in the alkalic pH range, the higher the recovery of the kaolinite flotation. This irregular rule may be explained by two aspects. The first is the adsorption differences of amine reagents on the plane and plane of kaolinite surface. The second is hydrophobic aggregation of chain-chain of amine reagents on the  plane of kaolinite [12-13].

The different collecting power for kaolinite is contributed to the inductive electric effect and space-steric effects of the substituents bonding to N atom in the amine molecule. Generally, with the electron denoting effect of substituents, the electron population of N atom becomes bigger to make N atom more easily bond with H⁺, which allows tertiary amines to become cationic reagents more easily and show a better collecting power. However, the space-steric effect makes an opposite effect on the tertiary amines. The longer the substituent chain is, the bigger the space volume of the substitute has, the more difficultly the N atom bonds with H⁺, the more difficultly the tertiary amine shifts to become a cationic reagent. Meanwhile, the bigger substituetns can cause the cationic tertiary amines to be adsorbed on the kaolinie surface more difficultly. The substituents of DRN, DEN, DPN and DBN are —CH₃, —C₂H₅, —C₃H₇ and —C₇H₇, respectively. The inductive electric effect of these subsituents is in the order of    —C₃H₇＞—C₂H₅＞—CH₃＞—C₇H₇, and the space- steric effect of these subsituents is in the order of     —C₇H₇＞—C₃H₇＞—C₂H₅＞—CH₃. Comprehensive results of the two effects indicate the collecting power a series of amines of in the order of DEN＞DPN＞DRN＞DBN.

4 Conclusions

(1) Flotation tests indicate that a series of dodecyl tertiary amines can recover kaolinite better in acidic pH range than in alkalic pH range. DEN has the best flotation ability with the highest flotation recovery of 93%. The collecting power of a series of dodecyl tertiary amines for kaolinite is in the order of DEN＞DPN＞DRN＞DBN.

(2) On the basis of zeta potential, the surface of kaolinite is mainly negative charged in a wide pH range. With the addition of the four tertiary amines, zeta potential of kaolnite is remarkably increased, and the increasing extent is in the order of DEN＞DPN＞  DRN＞DBN.

(3) FT-IR spectra show that the kaolinite surfaces are negative charged after H⁺ is ionized away from the mineral surface. And a series of dodecyl tertiary amines are adsorbed on the kaolinite surface by the physical adsorption of electric-static force.

(4) Comprehensive results of inductive electric effect and space-steric effect of substituents bonding to N atom are reasoned for the collecting power order of  DEN＞DPN＞DRN＞DBN.

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(Edited by CHEN Wei-ping)

Foundation item: Project(2005CB623701) supported by the National Basic Research Program of China

Received date: 2008-12-19; Accepted date: 2009-03-31

Corresponding author: HU Yue-hua, Professor; Tel: +86-731-88836019; E-mail: hyh@mail.csu.edu.cn