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Flotation of diaspore and aluminosilicate minerals applying novel carboxyl hydroxamic acids as collector
Hydrometallurgy 104 (2010) 112–118
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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Flotation of diaspore and aluminosilicate minerals applying novel carboxyl hydroxamic acids as collector Yu-Ren Jiang a,⁎, Bin-Nan Zhao a,b,⁎, Xiao-Hong Zhou a, Li-Yi Zhou a a b
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, People’s Republic of China Hunan Suntown Technology Group Co., Ltd, No.109 Jinxin Road, Suntown Industrial Park, Changsha, Hunan 410200, China
a r t i c l e
i n f o
Article history: Received 10 December 2009 Received in revised form 3 March 2010 Accepted 14 May 2010 Available online 20 May 2010 Keywords: Flotation reagent Diaspore Aluminosilicate Carboxyl hydroxamic acid Adsorption mechanism
a b s t r a c t Three novel carboxyl hydroxamic acids including ortho-carboxyl tetrachlorobenzohydroxamic acid (OCB), ortho-carboxyl hexahydrobenzohydroxamic acid (OHB) and ortho-carboxyl tetrahydrobenzohydroxamic acid (OTB), were synthesized and tested as collectors for flotation of diaspore, kaolinite and illite contained in diasporic bauxite from China. Subsequently, their flotation mechanism to diaspore and aluminosilicate minerals was investigated by zeta potential measurements and FT-IR spectrum checking. The results of flotation experiments show that by using carboxyl hydroxamic acid as collectors, the pulp pH value has significant influence on their collecting performance as the floatability of either diaspore or aluminosilicates varies sharply with their change, and the appropriate pH value for the flotation of diaspore gets close to neutral condition where diaspore presents good floatability while kaolinite and illite exhibits poor performances. Additionally, the floatability of diaspore and aluminosilicates is in the descending order of diaspore, kaolinite, and illite in the presence of three collectors, and their collecting capacity to three minerals is in the ascending order of OTB, OHB and OCB. Of three synthesized carboxyl hydroxamic acids, OCB has the strongest collecting capability to diaspore while relatively weak to aluminoscilicate minerals, whose good selectivity for the flotation between diaspore and aluminosilicates is possibly suited for direct flotation desilication of diasporic bauxite. Moreover, the optimum pH value for diaspore flotation associated with FT-IR spectrum and zeta potentials indicate that the adsorption interaction between the synthesized collectors and diaspore is dominantly a kind of chemical bonding one in the form of three cycle chelate rings due to the coordination of carboxyl and hydroxamate to the metal aluminum atoms, where the oxygen atoms contained in carboxyl and hydroxamate of the polar group have the stereo conditions to form five to seven membered rings. By contrast, the adsorption interactions of the carboxyl hydroxamic acid on the surfaces of aluminosilicate minerals are mainly dominated by means of hydrogen bonds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Bauxite is the major raw material for alumina production, and Bayer process is widely applied in alumina technology with mass ratio of Al2O3 to SiO2 (A/S) more than 8 (Xu et al., 2004) on account of the advantage of short flow, small investment, stable product quality and low energy consumption. Although there are plenty of bauxite resources in China, most of them (at least 90%) are characteristic of high silica, high alumina and low mass ratio A/S (A/S = 4–6) (Huang et al., 2005). High-grade bauxite with mass ratio of A/S greater than 10 is necessary to be processed directly by the Bayer process (Ma et al., 1996; Papanastassiou et al., 2002; Zhong et al., 2008). Due to this reason, majority of the bauxites in China cannot meet the requirement of the advanced Bayer process in alumina technology. Therefore, it is
⁎ Corresponding authors. Tel.: +86 731 88836834. E-mail addresses: [email protected] (Y.-R. Jiang), [email protected] (B.-N. Zhao). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.05.006
highly desirable to increase the mass ratio of A/S by flotation desilication before Bayer process. Flotation is known to be a highly versatile separation technology and has been widely used for industrial mineral processing (Yoon and Shi, 1989). Many researches on direct flotation desilication have been shown to be an efficient method for the desilicating of diasporic bauxite (Feng et al., 1998; Liu, 1999; Lu et al., 2002). Flotation reagents are the critical technique in the flotation separation process. The use of traditional reagents and their combinations has been reported a lot in the field of direct flotation desilication (flotation of diaspore and depression of aluminosilicates) such as oleic acid, tall oil, sodium dodecylbenzene sulfonate, oxidized paraffin soap, 733, styrene phosphate, aliphatic hydroxamic acid, and aromatic hydroxamic acid and their mixtures (Wang et al., 2003; Hu et al., 2004; Hu et al., 2005); Jiang et al. (2001a) synthesized a new type of collector called COBA, which has a strong collecting ability to diaspore, weaker capability to kaolinite, and higher selectivity than salicylhydroxamic acid. The difference of whose collecting performance was due to the difference of polar group, like electronegativity,
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topological index, cross-section size and hydrophobicity. Recently, the research on novel flotation reagents for reverse flotation desilication (flotation of aluminosilicates and depression of diaspore) has been soaring in China. Liu et al. (2003) discovered a kind of cationic polyacrylamide, which had little influence on kaolinite flotation, possessed the ability to inhibit diaspore at pH range of 5.5–8.5. The research work of Li et al. (2001) showed that the modified starch/ polyacrylamide, absorbed on mineral surfaces through electrostatic force, and chemical/hydrogen bonding, were superior to starch in the inhibition of diaspore. Hu et al. (2003a) synthesized the cationic collector N-dodecyl-1,3-diaminopropane, which excelled over lauryl amine in the collecting ability to kaolinite, pyrophyllite and illite with the flotation recoveries over 80%. Zhao et al. (2003a,b, 2004) also synthesized N-(3-aminopropyl) dodecanamide, N-(2-aminoethyl) dodecanamide and N-(3-diethylaminopropyl) fatty amide for flotation of aluminosilicates. It was proved that N-(3-aminopropyl) dodecanamide had relatively strong collecting ability to kaolinite, pyrophyllite, and illite. N-(2-aminoethyl) dodecanamide presented very strong collecting ability to pyrophyllite with the flotation recovery of 97.7%; while relatively weak collecting ability to kaolinite and illite with the flotation recoveries no more than 82%. N-(3diethylaminopropyl) fatty amide possessed very high collecting ability to diaspore with flotation recovery of 99.9%; but the data of flotation of aluminosilicates was not given to understand the selectivity of these collectors. From the aspect of reported flotation reagents for desilication, no matter direct or reverse flotation of bauxite, collectors and depressants have achieved certain developments. However, due to its short history in research, plenty of difficulties and problems still remain. First, the selection of flotation reagents mostly focuses on the traditional ones and their combinations; few studies have reported the designing and synthesis of novel structures. In addition, the present reagents are characteristic of low selectivity, and highly efficient reagents for the separation of bauxite have not been found. For instance, although direct flotation desilication for diasporic bauxite has been employed for several years in China, the recovery of an acceptable bauxite concentrate was not yet very high in commercial scale (Liu and Liu, 2005). Hydroxamic acid is extensively applied in the flotation of rare earth minerals as collector (Yang et al., 1992) because it has nitrogen and oxygen atoms which contain lone pair electrons to coordinate with metal atoms. The self-made COBA (Jiang et al., 2001a) containing carboxyl group and hydroxamate group in the same molecule was employed in the flotation of diaspore and kaolinite as collector showing that it has much higher selectivity than salicylhydroxamic acid, which indicates that the introduction of carboxyl group has greatly enhanced the selectivity of the collector. The present work is to try to design a type of new compounds for the flotation of diaspore against aluminosilicate minerals by putting a carboxyl group and a hydroxamate group into one molecule based on our previous experiment achievements. Using chemical approach, three novel carboxyl hydroxamic acids, ortho-carboxyl tetrachlorobenzohydroxamic acid (OCB), ortho-carboxyl hexahydrobenzohydroxamic acid (OHB) and ortho-carboxyl tetrahydrobenzohydroxamic acid (OTB), were synthesized as collectors for the flotation of diaspore and aluminosilicates. The investigations of the flotation experiments and the adsorption mechanism illustrated that the synthesized carboxyl
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hydroxamic acids was a new type of selective collectors, especially OCB possesses high selectivity for the flotation between diaspore and aluminosilicates and possibly suites the direct flotation desilication of diasporic bauxite. 2. Experimental 2.1. Mineral samples and reagents Diaspore, kaolinite, illite were all were obtained from Henan, China, which were handpicked and then crushed and ground in a porcelain mill. The fraction of minus 0.076 mm was used in flotation. The results of mineralogical analysis, chemical analysis and X-ray diffraction showed that the three minerals were all at least 90% pure. Three novel carboxyl hydroxamic acid compounds including ortho-carboxyl tetrachlorobenzohydroxamic acid (OCB), ortho-carboxyl hexahydrobenzohydroxamic acid (OHB) and ortho-carboxyl tetrahydrobenzohydroxamic acid (OTB) were synthesized in our laboratory. The pH modifiers employed were analytical grade hydrochloric acid or sodium hydroxide, the frother used was chemical grade 1, and 3-dimethylbutanol and distilled water made in our laboratory was used in all tests. 2.2. Synthesis procedures of carboxyl hydroxamic acids Novel carboxyl hydroxamic acids used in the flotation tests were synthesized by reaction of hydroxylamine hydrochloride with the corresponding dicarboxylic esters referring to the method described for the general synthesis procedures (Fatih and Veysel, 2001). The structures of synthesized compounds were identified and consistent with the data of elemental analysis and the IR spectra. Scheme 1 illustrates the general synthetic method of the carboxyl hydroxamic acids. 2.2.1. General synthetic procedures for carboxyl hydroxamic acids 1 Equiv. of respective dicarboxylic acid in ethanol was mixed with 0.4 equiv. of 98% sulfuric acid, and then the solution was refluxed with stirring for 6 h. The excess ethanol was eliminated on a rotary evaporator and sulfuric acid was neutralized with an aqueous solution 5% sodium bicarbonate until formation of carbon dioxide ceased. The formed diester was washed with water and separated in a separatory funnel. 1 Equiv. of hydroxylamine hydrochloride in methanol was added to a methanol solution containing 1 equiv of potassium hydroxide. Potassium chloride precipitated from the solution was removed by suction filtration, and then a methanol solution with 1 equiv of potassium hydroxide was added to the filtrate. Subsequently, the synthesized diester dissolved in methanol was added to the hydroxylamine solution and stirred for 8 h at room temperature. A color change to orange red occurred during the reaction. The volume of the resulting solution was reduced by evaporating at 40 °C and acetone was added to the solution to precipitate potassium salt of carboxyl hydroxamate. The precipitate was recrystallized in water and acidified to pH 5.5 by using aqueous solution 5% hydrochloride acid to give the desired product carboxyl hydroxamic acid. Ortho-carboxyl tetrachlorobenzohydroxamic acid (OCB). Yield: 72.5%. Found (%): C, 30.72; H, 1.04; N, 4.30. Calculated for C8H3Cl4NO4 (%): C,
Scheme 1. General synthetic method of carboxyl hydroxamic acids.
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30.13; H, 0.95; N, 4.39. IR (KBr, νmax/cm− 1): 3175.09vs (NH), 2848.13b (OH), 1667.25vs (CO) and 1569.53s (CN). Ortho-carboxyl hexahydrobenzohydroxamic acid (OHB). Yield: 68.3%. Found (%): C, 52.01; H, 7.23; N, 7.26. Calculated for C8H13NO4 (%): C, 51.33; H, 7.00; N, 7.48%. IR (KBr, νmax/cm− 1): 3500.70vs (NH), 2869.30b (OH), 1714.74vs (CO) and 1562s (CN). Ortho-carboxyl tetrahydrobenzohydroxamic acid (OTB). Yield: 67.8%. Found (%): C, 52.01; H, 6.03; N, 7.32. Calculated for C8H11NO4 (%): C, 51.89; H, 5.99; N, 7.56; IR (KBr, νmax/cm− 1): 3439.46vs (NH), 2923.67b (OH), 1703.42vs (CO) and 1538.85s (CN). 2.3. Flotation performance of carboxyl hydroxamic acids Flotation tests were performed with a XFG-35 flotation machine having 30 mL effective cel1 volume, where the impeller speed was fixed at l650 r/min. In each test, 2.0 g mineral samples and decent amount of distilled water were used, and the dosage of frother used was fixed at 5.0 × 10− 4 mol/L. The general procedures for the flotation tests were as follows: the mineral sample and distilled water were added in the flotation cell, and the suspension was agitated for 1 min. After the desired pH value was adjusted by mass ratio 5% aqueous solution hydrochloric acid or aqueous solution sodium hydroxide, a desired amount of collector and frother were added; and then the flotation was carried out and maintained for 5 min. The floated products and the unfloated ones were collected, dried, and weighed. The flotation recovery was calculated based on the percentage mass ratio of the floated products in the summation of floated and unfloated products. 2.4. FT-IR spectroscopy In order to characterize the nature of the interaction between the collectors and the minerals, the infrared spectra of collectors as well as samples with or without collectors pretreated are measured by the KBr technique. Model FTIR-750 Infrared spectrophotometer from Nicolet CO. in USA was used to obtain the IR spectra. The mineral samples were ground to be less than 5 μm in an agate mortar before being conditioned with 2 × 10− 2 mol/L collectors. 2.5. Zeta potential measurement Zeta potential was measured on a Delsa-440SX zeta potential instrument (Brookhaven Corporation, USA). The mineral sample was further ground to minus 5 μm in an agate mortar. The mineral suspension containing 0.01% (mass fraction) solid was dispersed in a beaker for 15 min and the pH value was measured. 1×10− 3 mol/L KNO3 solution was used as a supporting electrolyte. The measurement error was found to be within ±5 mV after at least three measurements in each condition. 3. Results and discussion 3.1. Flotation behaviors of diaspore and aluminosilicates with carboxyl hydroxamic acids as collectors The flotation recoveries of diaspore, kaolinite and illite with 2 × 10− 4 mol/L of OCB, OHB and OTB as collectors of different carbon types are shown in Fig. 1. It can be seen that by using carboxyl hydroxamic acid as collectors, the pulp pH value has significant influence on their collecting performance as the floatability of either diaspore or aluminosilicates varies sharply with its change, especially when below 6 and above 8. The appropriate pH value for the flotation of diaspore gets close to neutral condition where diaspore presents good floatability while kaolinite and illite exhibits poor performances. Additionally, the floatability of the three minerals is in the descending order of diaspore, kaolinite and illite in the presence of three collectors with the same concentration.
Fig. 1. Effect of pulp pH value on flotation of diaspore and aluminosilicates using OCB, OHB and OTB as collectors: (a) diaspore; (b) kaolinite; (c) illite.
Their collecting capability to three minerals is in the ascending order of OTB, OHB and OCB in the presence of the same concentration. The flotation responses of the diaspore and aluminosilicates as functions of the concentration of OCB, OHB and OTB at pulp pH 7 are presented in Fig. 2. As it can be concluded from Fig. 2, the recoveries of the diaspore and aluminosilicates rise with the increase of collectors' dosages. For diaspore flotation, the collecting capacity of OCB remains at a very high level with the flotation recoveries above 95%. At the
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collecting ability grows correspondingly. For aluminosilicates flotation, although the recoveries rise with the growth of collectors dosage and there exists difference of recovery between kaolinite and illite, the three carboxyl hydroxamic acids still exhibit poor collecting ability to illite and kaolinite as none of their recoveries can reach up to 40%. Integrated results from Figs. 1 and 2, the novel synthesized collectors all present better collecting abilities to diaspore than that to illite and kaolinite, the collecting capacity of them to mineral flotation is in the order of diaspore N kaolinite N illite, and the collecting capacity of them is in the order of OCB N OHB N OTB. Of the three synthesized carboxyl hydroxamic acids, OCB has very strong collecting capacity to diaspore while relatively weak to aluminoscilicate minerals, since the flotation recovery of diaspore above 97% while those of kaolinite and illite below 40% with relatively low OCB dosage in a relatively wide pH range, so OCB is possibly suited for the direct flotation desilication of diasporic bauxite. 3.2. Zeta potential of diaspore and aluminosilicates with or without carboxyl hydroxamic acid
Fig. 2. Effect of OTB, OCB and OHB concentration on flotation of diaspore and aluminosilicates (a) diaspore; (b) kaolinite; (c) illite.
concentration of 2 × 10− 4 mol/L OCB, the recovery of diaspore is 97.4%, but for OHB and OTB, the collecting abilities are relatively poor, and the flotation recovery of diaspore is 67.6% and 45.5%, respectively. The maximum flotation recoveries of diaspore using OCB, OHB and OTB in the tested dosage are 99.7%, 89.2% and 73.0%, respectively. The collecting ability of the three collectors to diaspore is in the descending order of OCB, OHB and OTB, illustrating that with the increase of the nonpolar group in carboxyl hydroxamic acid its
Since it exhibits good selectivity in the flotation between diaspore and aluminosilicate minerals, OCB was selected as a representative compound to investigate its effect on the zeta potential of kaolinite, illite and diaspore. Fig. 3 shows the relationship between zeta potential of minerals and pH values in the absence and presence of OCB. The results present that isoelectric point (IEP) determined is 4.3, 3.4 and 6.3 for kaolinite, illite and diaspore, respectively, which are generally consistent with the previous reported literatures (Fuerstenau and Fuerstenau, 1982; Saada et al., 1995; Hussain et al., 1996; Jiang et al., 2001b; Qin et al., 2003; Xia et al., 2009). Compared with kaolinite and illite, diaspore has a relatively higher IEP. In the pH region below the isoelectric point, the negative zeta potential increases slightly. When pH N IEP, the zeta potential increases rapidly with the negative growth of pH values, while pH reaching 10 for diaspore or 8 for kaolinite and illite, it has no significant change and keeps approximate invariance. However, no matter without or with OCB, the zeta potential of diaspore, kaolinite and illite presents almost the same variance trend. Based on the zeta potential of diaspore and aluminosilites without OCB, qualitatively, the higher IEP value of diaspore is related to a greater number of surface Al–O sites, while the lower IEP value of aluminosilicates is attributed to a greater number of surface Si–O sites. The structure of diaspore is significantly different from aluminosilicates. The comminution destroys ionic/covalent Al–O bonds, resulting in a surface of unsaturated or ionic nature. The aluminosilicates are negatively charged in the pH range 2–10. This is attributable to isomorphic exchange of surface ions and surface ionization of the hydroxyl group. Si4+ can be replaced by Al3+ ions, leading to the formation of negatively charged oxygen surfaces. This permanent negative charge is independent of pH value. Al3+, K+, Na+ ions may be dissolved out of the basal plane of aluminosilicates and contribute to the negative charge of the mineral surfaces. This dissolution process and AlO− results in negatively charged SiO2− 3 2 groups left on the surface. Furthermore, the ionization of the surface hydroxyl group gives rise to the charge on the surface of diaspore and aluminosilicate as: Al–OH ⇌ Al–O− + H+ and Si–OH ⇌ Si–O− + H+. The extent of surface ionization is a function of the pulp pH value. Adsorption or dissociation of H+ and OH− accounts for their surface charges. When collector OCB is introduced into the pulp in the pH range around 7, its polar group is mainly in the form of carboxyl anion (–COO−) and neutral hydroxamate group (–COCNHOH) or hydroaxamic anion (–CONHO−) since pKa1 of –COOH around 4.5 and pKa2 of –CONHOH around 8.5. Therefore, there will be no electrostatic forces between OCB and mineral surfaces. However, around this pH value, better floatabilities of mineral are observed than those at other pH
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OCB and minerals (kaolinite, illite and diaspore); On the other hand, as the polar group of OCB, the oxygen atoms of NC O and –OH in –COOH group, the oxygen atoms of NC O and –NHOH in –CONHOH group all have lone-pair electrons. Since the negative value of net charge of these oxygen atoms is relatively high and aluminum atoms on the mineral surface is absent of electrons, those oxygen atoms in OCB act as Lewis base and may form covalent bond with aluminum atom (acting as Lewis acid) on the mineral surface. OCB possessed poor collecting capacity to aluminosilicates and very high collecting
Fig. 3. Relationship between zeta potential and pulp pH with or without OCB (a) diaspore; (b) kaolinite; (c) illite.
value, especially for diaspore nearly getting to 100% recovery, indicating that OCB around this pH value should be adsorbed on the surfaces of minerals by other force such as chemical bond or hydrogen bond. Analyzed from the polar group of OCB at pH value around 7, on one hand, the NNH and –OH in the –CONHOH group act as donator of hydrogen bond, and ≡ Al–O− and (or) ≡ Si–O− on the mineral surface act as receptor. Accordingly, when OCB contacts with the particles of minerals, hydrogen bond may take place between the above groups of
Fig. 4. FT-IR spectra of diaspore and aluminosilicates with and without collector (a) diaspore; (b) kaolinite; (c) illite.
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capacity to diaspore. These evidences indicate that the adsorption of OCB on aluminosilicates (either kaolinite or illite) is hydrogen bond while the adsorption of OCB on diaspore surface is mainly chemical bond (perhaps including hydrogen bond). 3.3. FT-IR spectra analysis of diaspore and aluminosilicates with or without carboxyl hydroxamic acid To further investigate the interaction mechanism of carboxyl hydroxamic acid on minerals, OCB was also chosen for analyzing its influence on the FT-IR spectra of diaspore, kaolinite, and illite. Fig. 4 presents the FT-IR spectra of diaspore, and aluminosilicates treated with or without OCB. For the IR spectra of free OCB, a broad band at center of 3448.13 cm− 1 is assigned to the vibrating absorption of OH in the carboxyl and hydroxamate group with the strong hydrogen bond. The typical N–H stretching band appears at a medium band of 3000–3200 cm− 1, and a strong absorption at 1667.25 cm− 1 is attributed to the C O vibration of the carboxyl and hydroxamate carboxyl group. For the IR spectra of the three minerals untreated with OCB, a band in 1984–1821 cm− 1 of Al O or Si O stretching absorption were observed due to the relaxation and reconstruction of the planes, which are in great accordance with those reported in previous literatures (Hu et al., 2003b; Xia et al., 2009). When the three minerals treated with OCB, characteristic absorption peaks of OCB were not observed on these minerals surfaces except NC O absorption on diaspore, where such result is possibly caused by the concentration of OCB which is too low to be detected by the FT-IR spectrophotometer. However, it can be observed from Fig. 4 that OCB led the shift of absorption peaks of the minerals in many aspects. Observed from Fig. 4a, diaspore is an amphoteric oxide mineral, so there are a wide stretching and vibrating absorption of OH at 2800 and 3000 cm− 1. The bands of 2117.23 cm− 1 and 1985.32 cm− 1 are the inside and outside swing absorption of OH, 962.37 cm− 1 and 1066.72 cm− 1 are the inside and outside bending and vibrating absorption of OH, and 747.01 cm− 1 is the stretching and vibrating absorption of Al–O. When diaspore is treated with OCB, absorption of C O at 1667.25 cm− 1 in OCB redshifts to 1656.90 cm− 1 with the
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decreased intensity; the stretching and vibrating peak of Al–O in diaspore at 747.01 cm− 1 blueshifts to 766.24 cm− 1 with the decreased intensity. These results illustrate that probably there are chemical bonds between diaspore and the collector; and such results are confirmed by and in agreement with the result of zeta potential mentioned above. Seen from Fig. 4b, the weak absorption at 1633.80 cm− 1 in the spectrum of kaolinite attributing to the bending mode of H–O–H indicates the existence of little free water (Zhao et al., 2003c; Guan et al., 2009). After conditioned by OCB, spectrum of kaolinite exhibits a peak at 1636.80 cm− 1. This may be caused by the formation of the N– H⋯O hydrogen bond between the group of NHOH in OCB and the oxygen of kaolinite surface (Liu et al., 2007); therefore, the N–H bond is enhanced and leads the absorption shift to a higher wavenumber and intensity. In Fig. 4c, similar to kaolinite, after treated with OCB the absorption of 1637.72 cm− 1 in illite shifts to a higher wavenumber of 1639.31 cm− 1 with higher intensity, so, the hydrogen bond is also formed for illite. This result is also confirmed by an agreement with the result of zeta potential mentioned above. To summarize, the adsorption between OCB and diaspore is mainly by means of chemical bonding while for kaolinite and illite is through hydrogen bond. 3.4. Interaction mode of carboxyl hydroxamic acid on mineral surface In the polar groups of the synthesized collectors, –OH of the carboxyl group and –NHOH of the hydroxamate group may dissociate to oxygen anions possessing strong coordination capacity for the formation of covalent bonds to aluminum atom on diaspore surface. Additionally, the oxygen atom of NC O in the carboxyl group –COOH, the oxygen atom of NC O in the hydroxamate group all have lone-pair electron whose negative value of net charge is relatively high, which presents that these atoms all have the possibility of bonding with diaspore. Meanwhile, these atoms have the natural conditions to form five to seven numbered rings. Combined with the analysis of zeta potential and IR spectra as well as the facts of flotation experiments, the interaction mechanism of the synthesized collectors and diaspore
Fig. 5. The suggested interaction mode of carboxyl hydroxamic acid on mineral surface (a) carboxyl hydroxamic acid on diaspore surface through chemical bonding; (b) carboxyl hydroxamic acid on aluminosilicates surface through hydrogen bond.
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can be concluded that oxygen atoms and the surface of the mineral form three-membered chelate ring through chemical bonding (Fig. 5a), meanwhile hydrogen bond may also be formed by the formation of the N–H⋯O and O–H⋯O between the group of COOH and CONHOH in the collector molecule and the oxygen of aluminosilicates surface (Fig. 5b). 4. Conclusions 1. Three carboxyl hydroxamic acid compounds of OCB, OHB and OTB are developed as collectors for the flotation of diaspore, kaolinite and illite. The floatability of diaspore and aluminosilicate minerals is in the descending order of diaspore, kaolinite and illite in the presence of three collectors; and their collecting capacities to three minerals are in the ascending order of OTB, OHB and OCB. 2. The floatability of diaspore and aluminosilicates varies sharply with the variation of pH value of the pulp by using OCB, OHB, and OTB as collectors, and the appropriate pH value for the flotation of diaspore gets close to neutral condition. 3. Collector of carboxyl hydroxamic acid is adsorbed on the surface of diaspore through chemical bonding in the form of three cycle chelate rings; its adsorptions on aluminosilicate minerals are mainly dominated by the way of hydrogen bonds. 4. Of three carboxyl hydroxamic acids, OCB has very strong collecting capacity to diaspore with a recovery nearly close to 100% while relatively weak to aluminoscilicate minerals with recoveries no more than 40%. The good selectivity of OCB for the flotation between diaspore and aluminosilicates is possibly suited for direct flotation desilication of diasporic bauxite. Acknowledgements The research was supported by a grant from the National Natural Science Foundation of China (20876180), for which the authors express their appreciations. Additionally, thanks are given to Ms Jiang Hao for her assistance in the part of flotation tests and to Dr. David Cheng from Case Western Reserve University for the assistance in English correction of this paper. References Fatih, Y., Veysel, T.Y., 2001. The phthalhydroxamate ligand and its divalent transition metal complexes: synthesis characterization and spectral and thermal studies. Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry 31 (8), 1421–1432. Feng, Q.M., Liu, G.Y., Lu, Y.P., 1998. The 90's research and outlook of bauxite on impurity removing by mineral processing. Light Metal 4, 9–13 (in Chinese). Fuerstenau, D.W., Fuerstenau, M.C., 1982. The flotation of oxide and silicate minerals. In: King, R.P. (Ed.), The Principles of Flotation. Chapter 7. Guan, F., Guan, F., Zhong, H., LIU, G.Y., Zhao, zS.G., Xia, L.Y., 2009. Flotation of aluminosilicate minerals using alkylguanidine collectors. Trans. Nonferrous Met. Soc. China 19, 228–234. Hu, Y.H., Cao, X.F., Li, H.P., Jiang, Y.R., 2003a. Synthesis of N-decyl-1,3-diaminopropanes and its flotation properties on aluminium silicate minerals. Trans. Nonferrous Met. Soc. China 13 (2), 417–420. Hu, Y.H., Liu, X.W., Xu, Z.H., 2003b. Role of crystal structure in flotation separation of diaspore from kaolinite, pyrophyllite and illite. Minerals Engineering 6, 219–227.
Hu, Y.H., Sun, W., Jiang, H., 2005. The anomalous behavior of kaolinite flotation with dodecylamine collector as explained from crystal structure considerations. Int. J. Miner. Proc. 76 (3), 163–172. Hu, Y.H., Sun, W., Li, H.P., Xu, Z.H., 2004. Role of macromolecules in kaolinite flotation. Minerals Engineering 17 (9–10), 1017–1022. Huang, C.B., Wang, Y.H., Chen, X.H., Lan, Y., Hu, Y.M., Yu, F.S., 2005. Review of research on desilication of bauxite ores by reverse flotation. Metal Mine 6, 21–24 in Chinese. Hussain, S.A., Demirci, S., Ozbayoglu, G., 1996. Zeta potential measurements on three clays from Turkey and effects of clays on coal flotation. J. Colloid Interface Sci. 184, 535–541. Jiang, Y.R., Hu, Y.H., Cao, X.F., 2001a. Synthesis and structure–activity relationships of carboxyl hydroxidoxime in bauxite flotation. The Chinese Journal of Nonferrous Metals 11 (4), 702–706 in Chinese. Jiang, H., HU, Y.H., Qin, W.Q., Wang, Y.H., 2001b. Mechanism of flotation for diaspore and aluminium-silicate minerals with alkyl-amine collector. The Chinese Journal of Nonferrous Metals 11 (4), 688–692 in Chinese. Li, H.P., Hu, Y.H., Jiang, Y.R., 2001. Interaction mechanism between modified starches and aluminum-silicate minerals. The Chinese Journal of Nonferrous Metals 11 (4), 697–701 in Chinese. Liu, G.Y., 1999. A study on flotation and desilication of diaspore bauxites. Master Thesis, Central South University of Technology, Changsha, Hunan. (in Chinese). Liu, G.Y., Li, Y.P., Dai, T.G., 2003. Reverse flotation with cationic polyacrylamide (CPAM) polymers to separate kaolinite from diaspore. Metal Mine 2, 48–50. Liu, G.Y., Zhong, H., Hu, Y.H., Zhao, S.G., Xia, L.Y., 2007. The role of cationic polyacrylamide in the reverse flotation of diasporic bauxite. Minerals Engineering 20, 1191–1199. Liu, J.R., Liu, X.M., 2005. Application of treating middle and low grade bauxite by oredressing Bayer process in alumina production. Light Metals 4, 11–14 in Chinese. Lu, Y.P., Zhang, G.F., Feng, Q.M., Ou, L.M., 2002. A novel collector RL for flotation of bauxite. Journal of Central South University of Technology 9 (1), 21–24. Ma, C., Tian, X., Liu, R., Yang, X., 1996. Selection of bayer digestion technology and equipment for Chinese bauxite. Light Metals: Proceedings of TMS Annual Meeting, pp. 187–191. TMS. Papanastassiou, D., Csoke, B., Solymar, K., 2002. Improved preparation of the greek diasporic bauxite for Bayer-process. Light Metals: Proceedings of TMS Annual Meeting, pp. 67–74. TMS. Qin, W.Q., Qiu, G.Z., Hu, Y.H., 2003. Flotation of kaolinite and its interaction with hexadecylam m onium brom ide. Trans. Nonferrous Met. Soc. China 13 (3), 679–682. Saada, A., Siffert, B., Papirer, E., 1995. Comparison of the hydrophilicity/hydrophobicity of illite and kaolinites. J. Colloid Interface Sci. 174, 185–190. Wang, Y.H., Hu, Y.H., Chen, X.Q., 2003. Aluminum-silicates flotation with quaternary ammonium salt. Trans Nonferrous Met Soc China 13 (3), 715–719. Xia, L.Y., Zhong, H., Liu, G.Y., Huang, Z.Q., Chang, Q.W., 2009. Flotation separation of the aluminosilicates from diaspore by a Gemini cationic collector. Int. J. Miner. Process 92, 74–83. Xu, Z.H., Plitt, V., Liu, Q., 2004. Recent advances in reverse flotation of diasporic ores—a Chinese experience. Mineral Engineering 17, 1007–1015. Yang, Z.Q., Zhang, Q., Zheng, X.P., Wang, G.M., 1992. Separation of cerite calcium and rare earths-bearing barium fluorophlogopite. Nonferrous Metals 44 (1), 46–53 in Chinese. Yoon, R.H., Shi, J., 1989. Processing of kaolin clay. In: Samsoundaran, P. (Ed.), Advances in Mineral Processing. SME, pp. 366–379. Zhong, H., Liu, G.Y., Xia, L.Y., 2008. Flotation separation of diaspore from kaolinite, pyrophyllite and illite using three cationic collectors. Minerals Engineering 21, 1055–1061. Zhao, S.M., Hu, Y.H., Wang, D.Z., Xu, J., 2003a. Flotation of aluminosilicates using N-(3aminopropyl)-dodecanamide. Trans. Nonferrous Met. Soc. China 13 (5), 1273–1276 in Chinese. Zhao, S.M., Hu, Y.H., Wang, D.Z., Xu, J., 2003b. Investigation on the flotation of aluminosilicates using N(2-Aminoethyl)-dodecanamide. Acta., Phys. Chim. Sin. 19 (6), 573–576 in Chinese. Zhao, S.M., Wang, D.Z., Hu, Y.H., Xu, J., 2003c. Flotation of aluminosilicates using N-(2aminoethyl)-1-naphthaleneacetamide. Minerals Engineering 16, 1031–1033. Zhao, S.M., Wang, D.Z., Hu, Y.H., 2004. Froth flotation of diaspore with N-(3diethylaminopropyl)-fatty amide in Chinese Journal of Nonferrous Metals 56 (2), 84–87.
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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Flotation of diaspore and aluminosilicate minerals applying novel carboxyl hydroxamic acids as collector Yu-Ren Jiang a,⁎, Bin-Nan Zhao a,b,⁎, Xiao-Hong Zhou a, Li-Yi Zhou a a b
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, People’s Republic of China Hunan Suntown Technology Group Co., Ltd, No.109 Jinxin Road, Suntown Industrial Park, Changsha, Hunan 410200, China
a r t i c l e
i n f o
Article history: Received 10 December 2009 Received in revised form 3 March 2010 Accepted 14 May 2010 Available online 20 May 2010 Keywords: Flotation reagent Diaspore Aluminosilicate Carboxyl hydroxamic acid Adsorption mechanism
a b s t r a c t Three novel carboxyl hydroxamic acids including ortho-carboxyl tetrachlorobenzohydroxamic acid (OCB), ortho-carboxyl hexahydrobenzohydroxamic acid (OHB) and ortho-carboxyl tetrahydrobenzohydroxamic acid (OTB), were synthesized and tested as collectors for flotation of diaspore, kaolinite and illite contained in diasporic bauxite from China. Subsequently, their flotation mechanism to diaspore and aluminosilicate minerals was investigated by zeta potential measurements and FT-IR spectrum checking. The results of flotation experiments show that by using carboxyl hydroxamic acid as collectors, the pulp pH value has significant influence on their collecting performance as the floatability of either diaspore or aluminosilicates varies sharply with their change, and the appropriate pH value for the flotation of diaspore gets close to neutral condition where diaspore presents good floatability while kaolinite and illite exhibits poor performances. Additionally, the floatability of diaspore and aluminosilicates is in the descending order of diaspore, kaolinite, and illite in the presence of three collectors, and their collecting capacity to three minerals is in the ascending order of OTB, OHB and OCB. Of three synthesized carboxyl hydroxamic acids, OCB has the strongest collecting capability to diaspore while relatively weak to aluminoscilicate minerals, whose good selectivity for the flotation between diaspore and aluminosilicates is possibly suited for direct flotation desilication of diasporic bauxite. Moreover, the optimum pH value for diaspore flotation associated with FT-IR spectrum and zeta potentials indicate that the adsorption interaction between the synthesized collectors and diaspore is dominantly a kind of chemical bonding one in the form of three cycle chelate rings due to the coordination of carboxyl and hydroxamate to the metal aluminum atoms, where the oxygen atoms contained in carboxyl and hydroxamate of the polar group have the stereo conditions to form five to seven membered rings. By contrast, the adsorption interactions of the carboxyl hydroxamic acid on the surfaces of aluminosilicate minerals are mainly dominated by means of hydrogen bonds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Bauxite is the major raw material for alumina production, and Bayer process is widely applied in alumina technology with mass ratio of Al2O3 to SiO2 (A/S) more than 8 (Xu et al., 2004) on account of the advantage of short flow, small investment, stable product quality and low energy consumption. Although there are plenty of bauxite resources in China, most of them (at least 90%) are characteristic of high silica, high alumina and low mass ratio A/S (A/S = 4–6) (Huang et al., 2005). High-grade bauxite with mass ratio of A/S greater than 10 is necessary to be processed directly by the Bayer process (Ma et al., 1996; Papanastassiou et al., 2002; Zhong et al., 2008). Due to this reason, majority of the bauxites in China cannot meet the requirement of the advanced Bayer process in alumina technology. Therefore, it is
⁎ Corresponding authors. Tel.: +86 731 88836834. E-mail addresses: [email protected] (Y.-R. Jiang), [email protected] (B.-N. Zhao). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.05.006
highly desirable to increase the mass ratio of A/S by flotation desilication before Bayer process. Flotation is known to be a highly versatile separation technology and has been widely used for industrial mineral processing (Yoon and Shi, 1989). Many researches on direct flotation desilication have been shown to be an efficient method for the desilicating of diasporic bauxite (Feng et al., 1998; Liu, 1999; Lu et al., 2002). Flotation reagents are the critical technique in the flotation separation process. The use of traditional reagents and their combinations has been reported a lot in the field of direct flotation desilication (flotation of diaspore and depression of aluminosilicates) such as oleic acid, tall oil, sodium dodecylbenzene sulfonate, oxidized paraffin soap, 733, styrene phosphate, aliphatic hydroxamic acid, and aromatic hydroxamic acid and their mixtures (Wang et al., 2003; Hu et al., 2004; Hu et al., 2005); Jiang et al. (2001a) synthesized a new type of collector called COBA, which has a strong collecting ability to diaspore, weaker capability to kaolinite, and higher selectivity than salicylhydroxamic acid. The difference of whose collecting performance was due to the difference of polar group, like electronegativity,
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topological index, cross-section size and hydrophobicity. Recently, the research on novel flotation reagents for reverse flotation desilication (flotation of aluminosilicates and depression of diaspore) has been soaring in China. Liu et al. (2003) discovered a kind of cationic polyacrylamide, which had little influence on kaolinite flotation, possessed the ability to inhibit diaspore at pH range of 5.5–8.5. The research work of Li et al. (2001) showed that the modified starch/ polyacrylamide, absorbed on mineral surfaces through electrostatic force, and chemical/hydrogen bonding, were superior to starch in the inhibition of diaspore. Hu et al. (2003a) synthesized the cationic collector N-dodecyl-1,3-diaminopropane, which excelled over lauryl amine in the collecting ability to kaolinite, pyrophyllite and illite with the flotation recoveries over 80%. Zhao et al. (2003a,b, 2004) also synthesized N-(3-aminopropyl) dodecanamide, N-(2-aminoethyl) dodecanamide and N-(3-diethylaminopropyl) fatty amide for flotation of aluminosilicates. It was proved that N-(3-aminopropyl) dodecanamide had relatively strong collecting ability to kaolinite, pyrophyllite, and illite. N-(2-aminoethyl) dodecanamide presented very strong collecting ability to pyrophyllite with the flotation recovery of 97.7%; while relatively weak collecting ability to kaolinite and illite with the flotation recoveries no more than 82%. N-(3diethylaminopropyl) fatty amide possessed very high collecting ability to diaspore with flotation recovery of 99.9%; but the data of flotation of aluminosilicates was not given to understand the selectivity of these collectors. From the aspect of reported flotation reagents for desilication, no matter direct or reverse flotation of bauxite, collectors and depressants have achieved certain developments. However, due to its short history in research, plenty of difficulties and problems still remain. First, the selection of flotation reagents mostly focuses on the traditional ones and their combinations; few studies have reported the designing and synthesis of novel structures. In addition, the present reagents are characteristic of low selectivity, and highly efficient reagents for the separation of bauxite have not been found. For instance, although direct flotation desilication for diasporic bauxite has been employed for several years in China, the recovery of an acceptable bauxite concentrate was not yet very high in commercial scale (Liu and Liu, 2005). Hydroxamic acid is extensively applied in the flotation of rare earth minerals as collector (Yang et al., 1992) because it has nitrogen and oxygen atoms which contain lone pair electrons to coordinate with metal atoms. The self-made COBA (Jiang et al., 2001a) containing carboxyl group and hydroxamate group in the same molecule was employed in the flotation of diaspore and kaolinite as collector showing that it has much higher selectivity than salicylhydroxamic acid, which indicates that the introduction of carboxyl group has greatly enhanced the selectivity of the collector. The present work is to try to design a type of new compounds for the flotation of diaspore against aluminosilicate minerals by putting a carboxyl group and a hydroxamate group into one molecule based on our previous experiment achievements. Using chemical approach, three novel carboxyl hydroxamic acids, ortho-carboxyl tetrachlorobenzohydroxamic acid (OCB), ortho-carboxyl hexahydrobenzohydroxamic acid (OHB) and ortho-carboxyl tetrahydrobenzohydroxamic acid (OTB), were synthesized as collectors for the flotation of diaspore and aluminosilicates. The investigations of the flotation experiments and the adsorption mechanism illustrated that the synthesized carboxyl
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hydroxamic acids was a new type of selective collectors, especially OCB possesses high selectivity for the flotation between diaspore and aluminosilicates and possibly suites the direct flotation desilication of diasporic bauxite. 2. Experimental 2.1. Mineral samples and reagents Diaspore, kaolinite, illite were all were obtained from Henan, China, which were handpicked and then crushed and ground in a porcelain mill. The fraction of minus 0.076 mm was used in flotation. The results of mineralogical analysis, chemical analysis and X-ray diffraction showed that the three minerals were all at least 90% pure. Three novel carboxyl hydroxamic acid compounds including ortho-carboxyl tetrachlorobenzohydroxamic acid (OCB), ortho-carboxyl hexahydrobenzohydroxamic acid (OHB) and ortho-carboxyl tetrahydrobenzohydroxamic acid (OTB) were synthesized in our laboratory. The pH modifiers employed were analytical grade hydrochloric acid or sodium hydroxide, the frother used was chemical grade 1, and 3-dimethylbutanol and distilled water made in our laboratory was used in all tests. 2.2. Synthesis procedures of carboxyl hydroxamic acids Novel carboxyl hydroxamic acids used in the flotation tests were synthesized by reaction of hydroxylamine hydrochloride with the corresponding dicarboxylic esters referring to the method described for the general synthesis procedures (Fatih and Veysel, 2001). The structures of synthesized compounds were identified and consistent with the data of elemental analysis and the IR spectra. Scheme 1 illustrates the general synthetic method of the carboxyl hydroxamic acids. 2.2.1. General synthetic procedures for carboxyl hydroxamic acids 1 Equiv. of respective dicarboxylic acid in ethanol was mixed with 0.4 equiv. of 98% sulfuric acid, and then the solution was refluxed with stirring for 6 h. The excess ethanol was eliminated on a rotary evaporator and sulfuric acid was neutralized with an aqueous solution 5% sodium bicarbonate until formation of carbon dioxide ceased. The formed diester was washed with water and separated in a separatory funnel. 1 Equiv. of hydroxylamine hydrochloride in methanol was added to a methanol solution containing 1 equiv of potassium hydroxide. Potassium chloride precipitated from the solution was removed by suction filtration, and then a methanol solution with 1 equiv of potassium hydroxide was added to the filtrate. Subsequently, the synthesized diester dissolved in methanol was added to the hydroxylamine solution and stirred for 8 h at room temperature. A color change to orange red occurred during the reaction. The volume of the resulting solution was reduced by evaporating at 40 °C and acetone was added to the solution to precipitate potassium salt of carboxyl hydroxamate. The precipitate was recrystallized in water and acidified to pH 5.5 by using aqueous solution 5% hydrochloride acid to give the desired product carboxyl hydroxamic acid. Ortho-carboxyl tetrachlorobenzohydroxamic acid (OCB). Yield: 72.5%. Found (%): C, 30.72; H, 1.04; N, 4.30. Calculated for C8H3Cl4NO4 (%): C,
Scheme 1. General synthetic method of carboxyl hydroxamic acids.
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30.13; H, 0.95; N, 4.39. IR (KBr, νmax/cm− 1): 3175.09vs (NH), 2848.13b (OH), 1667.25vs (CO) and 1569.53s (CN). Ortho-carboxyl hexahydrobenzohydroxamic acid (OHB). Yield: 68.3%. Found (%): C, 52.01; H, 7.23; N, 7.26. Calculated for C8H13NO4 (%): C, 51.33; H, 7.00; N, 7.48%. IR (KBr, νmax/cm− 1): 3500.70vs (NH), 2869.30b (OH), 1714.74vs (CO) and 1562s (CN). Ortho-carboxyl tetrahydrobenzohydroxamic acid (OTB). Yield: 67.8%. Found (%): C, 52.01; H, 6.03; N, 7.32. Calculated for C8H11NO4 (%): C, 51.89; H, 5.99; N, 7.56; IR (KBr, νmax/cm− 1): 3439.46vs (NH), 2923.67b (OH), 1703.42vs (CO) and 1538.85s (CN). 2.3. Flotation performance of carboxyl hydroxamic acids Flotation tests were performed with a XFG-35 flotation machine having 30 mL effective cel1 volume, where the impeller speed was fixed at l650 r/min. In each test, 2.0 g mineral samples and decent amount of distilled water were used, and the dosage of frother used was fixed at 5.0 × 10− 4 mol/L. The general procedures for the flotation tests were as follows: the mineral sample and distilled water were added in the flotation cell, and the suspension was agitated for 1 min. After the desired pH value was adjusted by mass ratio 5% aqueous solution hydrochloric acid or aqueous solution sodium hydroxide, a desired amount of collector and frother were added; and then the flotation was carried out and maintained for 5 min. The floated products and the unfloated ones were collected, dried, and weighed. The flotation recovery was calculated based on the percentage mass ratio of the floated products in the summation of floated and unfloated products. 2.4. FT-IR spectroscopy In order to characterize the nature of the interaction between the collectors and the minerals, the infrared spectra of collectors as well as samples with or without collectors pretreated are measured by the KBr technique. Model FTIR-750 Infrared spectrophotometer from Nicolet CO. in USA was used to obtain the IR spectra. The mineral samples were ground to be less than 5 μm in an agate mortar before being conditioned with 2 × 10− 2 mol/L collectors. 2.5. Zeta potential measurement Zeta potential was measured on a Delsa-440SX zeta potential instrument (Brookhaven Corporation, USA). The mineral sample was further ground to minus 5 μm in an agate mortar. The mineral suspension containing 0.01% (mass fraction) solid was dispersed in a beaker for 15 min and the pH value was measured. 1×10− 3 mol/L KNO3 solution was used as a supporting electrolyte. The measurement error was found to be within ±5 mV after at least three measurements in each condition. 3. Results and discussion 3.1. Flotation behaviors of diaspore and aluminosilicates with carboxyl hydroxamic acids as collectors The flotation recoveries of diaspore, kaolinite and illite with 2 × 10− 4 mol/L of OCB, OHB and OTB as collectors of different carbon types are shown in Fig. 1. It can be seen that by using carboxyl hydroxamic acid as collectors, the pulp pH value has significant influence on their collecting performance as the floatability of either diaspore or aluminosilicates varies sharply with its change, especially when below 6 and above 8. The appropriate pH value for the flotation of diaspore gets close to neutral condition where diaspore presents good floatability while kaolinite and illite exhibits poor performances. Additionally, the floatability of the three minerals is in the descending order of diaspore, kaolinite and illite in the presence of three collectors with the same concentration.
Fig. 1. Effect of pulp pH value on flotation of diaspore and aluminosilicates using OCB, OHB and OTB as collectors: (a) diaspore; (b) kaolinite; (c) illite.
Their collecting capability to three minerals is in the ascending order of OTB, OHB and OCB in the presence of the same concentration. The flotation responses of the diaspore and aluminosilicates as functions of the concentration of OCB, OHB and OTB at pulp pH 7 are presented in Fig. 2. As it can be concluded from Fig. 2, the recoveries of the diaspore and aluminosilicates rise with the increase of collectors' dosages. For diaspore flotation, the collecting capacity of OCB remains at a very high level with the flotation recoveries above 95%. At the
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collecting ability grows correspondingly. For aluminosilicates flotation, although the recoveries rise with the growth of collectors dosage and there exists difference of recovery between kaolinite and illite, the three carboxyl hydroxamic acids still exhibit poor collecting ability to illite and kaolinite as none of their recoveries can reach up to 40%. Integrated results from Figs. 1 and 2, the novel synthesized collectors all present better collecting abilities to diaspore than that to illite and kaolinite, the collecting capacity of them to mineral flotation is in the order of diaspore N kaolinite N illite, and the collecting capacity of them is in the order of OCB N OHB N OTB. Of the three synthesized carboxyl hydroxamic acids, OCB has very strong collecting capacity to diaspore while relatively weak to aluminoscilicate minerals, since the flotation recovery of diaspore above 97% while those of kaolinite and illite below 40% with relatively low OCB dosage in a relatively wide pH range, so OCB is possibly suited for the direct flotation desilication of diasporic bauxite. 3.2. Zeta potential of diaspore and aluminosilicates with or without carboxyl hydroxamic acid
Fig. 2. Effect of OTB, OCB and OHB concentration on flotation of diaspore and aluminosilicates (a) diaspore; (b) kaolinite; (c) illite.
concentration of 2 × 10− 4 mol/L OCB, the recovery of diaspore is 97.4%, but for OHB and OTB, the collecting abilities are relatively poor, and the flotation recovery of diaspore is 67.6% and 45.5%, respectively. The maximum flotation recoveries of diaspore using OCB, OHB and OTB in the tested dosage are 99.7%, 89.2% and 73.0%, respectively. The collecting ability of the three collectors to diaspore is in the descending order of OCB, OHB and OTB, illustrating that with the increase of the nonpolar group in carboxyl hydroxamic acid its
Since it exhibits good selectivity in the flotation between diaspore and aluminosilicate minerals, OCB was selected as a representative compound to investigate its effect on the zeta potential of kaolinite, illite and diaspore. Fig. 3 shows the relationship between zeta potential of minerals and pH values in the absence and presence of OCB. The results present that isoelectric point (IEP) determined is 4.3, 3.4 and 6.3 for kaolinite, illite and diaspore, respectively, which are generally consistent with the previous reported literatures (Fuerstenau and Fuerstenau, 1982; Saada et al., 1995; Hussain et al., 1996; Jiang et al., 2001b; Qin et al., 2003; Xia et al., 2009). Compared with kaolinite and illite, diaspore has a relatively higher IEP. In the pH region below the isoelectric point, the negative zeta potential increases slightly. When pH N IEP, the zeta potential increases rapidly with the negative growth of pH values, while pH reaching 10 for diaspore or 8 for kaolinite and illite, it has no significant change and keeps approximate invariance. However, no matter without or with OCB, the zeta potential of diaspore, kaolinite and illite presents almost the same variance trend. Based on the zeta potential of diaspore and aluminosilites without OCB, qualitatively, the higher IEP value of diaspore is related to a greater number of surface Al–O sites, while the lower IEP value of aluminosilicates is attributed to a greater number of surface Si–O sites. The structure of diaspore is significantly different from aluminosilicates. The comminution destroys ionic/covalent Al–O bonds, resulting in a surface of unsaturated or ionic nature. The aluminosilicates are negatively charged in the pH range 2–10. This is attributable to isomorphic exchange of surface ions and surface ionization of the hydroxyl group. Si4+ can be replaced by Al3+ ions, leading to the formation of negatively charged oxygen surfaces. This permanent negative charge is independent of pH value. Al3+, K+, Na+ ions may be dissolved out of the basal plane of aluminosilicates and contribute to the negative charge of the mineral surfaces. This dissolution process and AlO− results in negatively charged SiO2− 3 2 groups left on the surface. Furthermore, the ionization of the surface hydroxyl group gives rise to the charge on the surface of diaspore and aluminosilicate as: Al–OH ⇌ Al–O− + H+ and Si–OH ⇌ Si–O− + H+. The extent of surface ionization is a function of the pulp pH value. Adsorption or dissociation of H+ and OH− accounts for their surface charges. When collector OCB is introduced into the pulp in the pH range around 7, its polar group is mainly in the form of carboxyl anion (–COO−) and neutral hydroxamate group (–COCNHOH) or hydroaxamic anion (–CONHO−) since pKa1 of –COOH around 4.5 and pKa2 of –CONHOH around 8.5. Therefore, there will be no electrostatic forces between OCB and mineral surfaces. However, around this pH value, better floatabilities of mineral are observed than those at other pH
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OCB and minerals (kaolinite, illite and diaspore); On the other hand, as the polar group of OCB, the oxygen atoms of NC O and –OH in –COOH group, the oxygen atoms of NC O and –NHOH in –CONHOH group all have lone-pair electrons. Since the negative value of net charge of these oxygen atoms is relatively high and aluminum atoms on the mineral surface is absent of electrons, those oxygen atoms in OCB act as Lewis base and may form covalent bond with aluminum atom (acting as Lewis acid) on the mineral surface. OCB possessed poor collecting capacity to aluminosilicates and very high collecting
Fig. 3. Relationship between zeta potential and pulp pH with or without OCB (a) diaspore; (b) kaolinite; (c) illite.
value, especially for diaspore nearly getting to 100% recovery, indicating that OCB around this pH value should be adsorbed on the surfaces of minerals by other force such as chemical bond or hydrogen bond. Analyzed from the polar group of OCB at pH value around 7, on one hand, the NNH and –OH in the –CONHOH group act as donator of hydrogen bond, and ≡ Al–O− and (or) ≡ Si–O− on the mineral surface act as receptor. Accordingly, when OCB contacts with the particles of minerals, hydrogen bond may take place between the above groups of
Fig. 4. FT-IR spectra of diaspore and aluminosilicates with and without collector (a) diaspore; (b) kaolinite; (c) illite.
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capacity to diaspore. These evidences indicate that the adsorption of OCB on aluminosilicates (either kaolinite or illite) is hydrogen bond while the adsorption of OCB on diaspore surface is mainly chemical bond (perhaps including hydrogen bond). 3.3. FT-IR spectra analysis of diaspore and aluminosilicates with or without carboxyl hydroxamic acid To further investigate the interaction mechanism of carboxyl hydroxamic acid on minerals, OCB was also chosen for analyzing its influence on the FT-IR spectra of diaspore, kaolinite, and illite. Fig. 4 presents the FT-IR spectra of diaspore, and aluminosilicates treated with or without OCB. For the IR spectra of free OCB, a broad band at center of 3448.13 cm− 1 is assigned to the vibrating absorption of OH in the carboxyl and hydroxamate group with the strong hydrogen bond. The typical N–H stretching band appears at a medium band of 3000–3200 cm− 1, and a strong absorption at 1667.25 cm− 1 is attributed to the C O vibration of the carboxyl and hydroxamate carboxyl group. For the IR spectra of the three minerals untreated with OCB, a band in 1984–1821 cm− 1 of Al O or Si O stretching absorption were observed due to the relaxation and reconstruction of the planes, which are in great accordance with those reported in previous literatures (Hu et al., 2003b; Xia et al., 2009). When the three minerals treated with OCB, characteristic absorption peaks of OCB were not observed on these minerals surfaces except NC O absorption on diaspore, where such result is possibly caused by the concentration of OCB which is too low to be detected by the FT-IR spectrophotometer. However, it can be observed from Fig. 4 that OCB led the shift of absorption peaks of the minerals in many aspects. Observed from Fig. 4a, diaspore is an amphoteric oxide mineral, so there are a wide stretching and vibrating absorption of OH at 2800 and 3000 cm− 1. The bands of 2117.23 cm− 1 and 1985.32 cm− 1 are the inside and outside swing absorption of OH, 962.37 cm− 1 and 1066.72 cm− 1 are the inside and outside bending and vibrating absorption of OH, and 747.01 cm− 1 is the stretching and vibrating absorption of Al–O. When diaspore is treated with OCB, absorption of C O at 1667.25 cm− 1 in OCB redshifts to 1656.90 cm− 1 with the
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decreased intensity; the stretching and vibrating peak of Al–O in diaspore at 747.01 cm− 1 blueshifts to 766.24 cm− 1 with the decreased intensity. These results illustrate that probably there are chemical bonds between diaspore and the collector; and such results are confirmed by and in agreement with the result of zeta potential mentioned above. Seen from Fig. 4b, the weak absorption at 1633.80 cm− 1 in the spectrum of kaolinite attributing to the bending mode of H–O–H indicates the existence of little free water (Zhao et al., 2003c; Guan et al., 2009). After conditioned by OCB, spectrum of kaolinite exhibits a peak at 1636.80 cm− 1. This may be caused by the formation of the N– H⋯O hydrogen bond between the group of NHOH in OCB and the oxygen of kaolinite surface (Liu et al., 2007); therefore, the N–H bond is enhanced and leads the absorption shift to a higher wavenumber and intensity. In Fig. 4c, similar to kaolinite, after treated with OCB the absorption of 1637.72 cm− 1 in illite shifts to a higher wavenumber of 1639.31 cm− 1 with higher intensity, so, the hydrogen bond is also formed for illite. This result is also confirmed by an agreement with the result of zeta potential mentioned above. To summarize, the adsorption between OCB and diaspore is mainly by means of chemical bonding while for kaolinite and illite is through hydrogen bond. 3.4. Interaction mode of carboxyl hydroxamic acid on mineral surface In the polar groups of the synthesized collectors, –OH of the carboxyl group and –NHOH of the hydroxamate group may dissociate to oxygen anions possessing strong coordination capacity for the formation of covalent bonds to aluminum atom on diaspore surface. Additionally, the oxygen atom of NC O in the carboxyl group –COOH, the oxygen atom of NC O in the hydroxamate group all have lone-pair electron whose negative value of net charge is relatively high, which presents that these atoms all have the possibility of bonding with diaspore. Meanwhile, these atoms have the natural conditions to form five to seven numbered rings. Combined with the analysis of zeta potential and IR spectra as well as the facts of flotation experiments, the interaction mechanism of the synthesized collectors and diaspore
Fig. 5. The suggested interaction mode of carboxyl hydroxamic acid on mineral surface (a) carboxyl hydroxamic acid on diaspore surface through chemical bonding; (b) carboxyl hydroxamic acid on aluminosilicates surface through hydrogen bond.
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can be concluded that oxygen atoms and the surface of the mineral form three-membered chelate ring through chemical bonding (Fig. 5a), meanwhile hydrogen bond may also be formed by the formation of the N–H⋯O and O–H⋯O between the group of COOH and CONHOH in the collector molecule and the oxygen of aluminosilicates surface (Fig. 5b). 4. Conclusions 1. Three carboxyl hydroxamic acid compounds of OCB, OHB and OTB are developed as collectors for the flotation of diaspore, kaolinite and illite. The floatability of diaspore and aluminosilicate minerals is in the descending order of diaspore, kaolinite and illite in the presence of three collectors; and their collecting capacities to three minerals are in the ascending order of OTB, OHB and OCB. 2. The floatability of diaspore and aluminosilicates varies sharply with the variation of pH value of the pulp by using OCB, OHB, and OTB as collectors, and the appropriate pH value for the flotation of diaspore gets close to neutral condition. 3. Collector of carboxyl hydroxamic acid is adsorbed on the surface of diaspore through chemical bonding in the form of three cycle chelate rings; its adsorptions on aluminosilicate minerals are mainly dominated by the way of hydrogen bonds. 4. Of three carboxyl hydroxamic acids, OCB has very strong collecting capacity to diaspore with a recovery nearly close to 100% while relatively weak to aluminoscilicate minerals with recoveries no more than 40%. The good selectivity of OCB for the flotation between diaspore and aluminosilicates is possibly suited for direct flotation desilication of diasporic bauxite. Acknowledgements The research was supported by a grant from the National Natural Science Foundation of China (20876180), for which the authors express their appreciations. Additionally, thanks are given to Ms Jiang Hao for her assistance in the part of flotation tests and to Dr. David Cheng from Case Western Reserve University for the assistance in English correction of this paper. References Fatih, Y., Veysel, T.Y., 2001. The phthalhydroxamate ligand and its divalent transition metal complexes: synthesis characterization and spectral and thermal studies. Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry 31 (8), 1421–1432. Feng, Q.M., Liu, G.Y., Lu, Y.P., 1998. The 90's research and outlook of bauxite on impurity removing by mineral processing. Light Metal 4, 9–13 (in Chinese). Fuerstenau, D.W., Fuerstenau, M.C., 1982. The flotation of oxide and silicate minerals. In: King, R.P. (Ed.), The Principles of Flotation. Chapter 7. Guan, F., Guan, F., Zhong, H., LIU, G.Y., Zhao, zS.G., Xia, L.Y., 2009. Flotation of aluminosilicate minerals using alkylguanidine collectors. Trans. Nonferrous Met. Soc. China 19, 228–234. Hu, Y.H., Cao, X.F., Li, H.P., Jiang, Y.R., 2003a. Synthesis of N-decyl-1,3-diaminopropanes and its flotation properties on aluminium silicate minerals. Trans. Nonferrous Met. Soc. China 13 (2), 417–420. Hu, Y.H., Liu, X.W., Xu, Z.H., 2003b. Role of crystal structure in flotation separation of diaspore from kaolinite, pyrophyllite and illite. Minerals Engineering 6, 219–227.
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