Liquid-liquid extraction of aluminum (III) from sodium hydroxide solutions by alkylated hydroxyquinoline

Transcription

1 Vol. 40, No RESEARCH REPORT Liquid-liquid extraction of aluminum (III) from sodium hydroxide solutions by alkylated hydroxyquinoline Taichi SATO* In the extraction of aluminum (III) from sodium hydroxide solutions by 7-(5,5,7,7-tetramethyl-1-octen-3-yl)-8- hydroxyquinoline (Kelex 100, designated as HQ hereafter) in kerosene, the distribution equilibria and kinetics have been investigated under different conditions. From the dependence of the distribution coefficient on the concentrations of aqueous sodium hydroxide solution and Kelex 100, it is deduced that the extraction reaction can be expressed as Al conditions suggest that aluminium (III) is taken up through either formation of two different activated species, [Al (OH)3], OH- or Na+[Al(OH)3], OH-, depending on the concentration of sodium hydroxide in the aqueous phase. Keywords: liquid-liquid extraction, aluminum, alkylated hydroxyquinoline (Kelex 100), sodium hydroxide, sodium aluminate (Received November 18, 1989) 1. Introduction The solvent extraction of metals from aqueous alkaline solutions has not been extensively investigated as that from acid solutions, because most of the metal ions are very easily to precipitate as metal hydroxide from alkaline solutions. However it has already been reported that the application of the solvent extraction technique to the recovery of gallium from sodium aluminate solution from the Bayer process is possible by using 7-alkylated hydroxy-quinoline1)-4) The present author also reported the extraction of zinc (II)5),6) lead (II)5),6), gallium (III)5)-11) and aluminum (III)9),10),12) from sodium hydroxide solutions of by 7- (5,5,7,7-tetramethyl-1-octen-3-yl)-hydroxyquinoline (Kelex 100, HQ), and accordingly found that alkylated hydroxyquinoline is effective to extract metals existing in alkaline solutions as the species M(OH)m-n. Thus this study presents the work performed in order to obtain further informationon the equilibria and kinetics of the extraction of aluminum (III) from aqueous sodium hydroxide solutions by Kelex Experimental 2.1 Reagents Kelex 100 (Schering AG) was purified by a two step under 0.36mmHg7), and diluted with kerosene. In this work, it was preferred to use Kelex 100 without any modifier according to the previous work8). The aqueous aluminate solution was prepared by dissolving aluminum (99.85%) in the sodium hydroxide solution of the selected concentration9)-12). The aluminum concentration was 0.01mol dm-3 except for the loading tests. The other chemicals were of analytical grade. 2.2 Procedures Equal volumes (15cm3) of Kelex 100 in kerosene and aqueous aluminate solution, placed in 50cm3 centrifugal glass tube, were shaken with 340rpm for a required time (preliminary experiments showed that equilibrium was of the shaker was determined in the order that the rate is free from diffusion control. After the mixture was quickly separated by centrifuge, the concentration in both phases * Faculty of Engineering, Shizuoka University, Hamamatsu, Japan and Queen's University, Metallurgical Engineering Department, Kingston, Ontario, Canada.

2 was assayed as follows: aluminum in the organic phase was stripped many times repeatedly with 2mol dm-3 hydrochloric acid on addition of 2-ethylhexyl alcohol; the concentration of metal in the aqueous solution was determined by EDTA titration using Xylenol Orange (XO) as indicator13). The concentration of sodium extracted into organic phase was measured by atomic absorption spectrophotometry using Hitachi Ltd., Model A after stripping into aqueous phase with 2mol dm-3 hydrochloric acid or nitric acid solutions. The concentration of chloride and the water content in the organic phase were examined by Volhard's and Karl-Fisher's methods as indicated with the equilibrium data is the ratio of the aluminum concentration in organic phase to that in the aqueous phase. 2.3 Infrared and Raman spectroscopies The infrared spectra of the organic extracts prepared by evaporation in vacuo of n-hexane used as diluent were measured on the Japan Spectroscopic Co., Ltd. grating models IRA-1 ( cm-1) and IR-F ( cm-1) using a capillary film between thallium halide plates of polyethylene films. The Raman spectra of the organic extracts were measured on a JASCO laser Raman spectrophotometer model R-300 using a source of Ar-laser. 3. Results and discussion Fig. 1 Extraction of aluminum (III) from sodium hydroxide solutions by Kelex 100 in kerosene at from NaOH solutions and mixed 0.1mol dm-3 NaOH/NaClO4 solutions, respectively). 3.1 Extraction isotherms The extraction of aluminum (III) from sodium hydroxide solutions and mixtures of sodium hydroxide and sodium perchlorate with and 0.05mol dm-3 Kelex 100 in Fig. 1. The distribution coefficient decreases with increasing the concentration of aqueous sodium hydroxide solution, and is not affected when a part of the sodium hydroxide is replaced by sodium perchlorate, indicating that the extraction of aluminum (III) proceeds through ion-exchange [NaOH] at constant concentrations of Kelex 100 in kerosene give straight lines with slopes of -0.55, -0.62, and for 0.025, 0.05, 0.1 and 0.2mol dm-3 Kelex 100, respectively. At a constant total sodium concentration, however, as the distribution coefficient is inversely proportional to the hydroxide concentration, those slopes are expected to approach minus unity, taking the mean activity of sodium hydroxide in aqueous solutions into consideration. Further the Raman spectra of aqueous solutions of aluminum (III) in sodium hydroxide exhibit the absorption at 621cm-1, suggesting that the species of aluminium (III) exists in a point group of Td as Al(OH)-415). On the one hand, as reported previously, when the distribution of sodium between sodium hydroxide solutions and kerosene solutions of Kelex 100 in 0.05mol dm-3 is ex- [Kelex 100]/[NaOH] in the organic phase gives 100, 120, 120, 120, 82, 31, 9.1, 2.2 and 1.2 at initial aqueous sodium hydroxide concentrations in 1, 2, 3, 4, 5, 6, 7, 8 and 10mol dm-3 respectively. This indicates that at the concentration of aqueous sodium hydroxide solution below 5mol dm-3 the effect of sodium is negligible for the extraction of metals and Kelex 100 exists as HQ in the organic phase, and that a drastic uptake of sodium occurs at the concentration of aqueous sodium hydroxide solution above 5mol dm-3 owing to the formation of the species NaQ suppressing the extraction of metals. It is thus presumed that th extraction of aluminum (III) from sodium hydroxide solutions by Kelex 100 proceeds by the following cation-exchange reaction in which (a) and (o) denote the aqueous and organic (Fig. 2). According- and the extraction give straight lines with a slope of-3 ly, it can be inferred that n=3 equilibrium can be formulated as This is supported by continuous variation experiments: in the continuous variation of aluminum concentration in the

3 Vol. 40, No Table 1 Infrared spectral data for purified Kelex 100 and the complex of Al (III) with Kelex 100 Fig. 2 Solvent dependence of distribution coefficient for the extraction of aluminum (III) from sodium hydroxide solutions by Kelex 100 in kerosene at itial sodium hydroxide concentrations, mol dm-3). Fig. 3 Variation of aluminum concentration in the organic phase as a function of initial concentration of Kelex 100 at fixed total concentration of 0.05mol dm-3 of initial aqueous aluminum and Kelex 100 in the extraction of aluminum (III) from aqueous solu- organic phase as a function of initial concentration of Kelex 100, using fixed total concentrations of initial aluminum and Kelex 100 in 0.05mol dm-3 each at a constant concen- the organic aluminum concentration exhibits a maximum *vs=very strong, s=strong, ms=medium strong, m=medium, w=weak, vw=very weak, sh=shoulder at a molar fraction of 0.75 ([HQ]init=3[Al]init aq) in either case (Fig. 3). In the extraction of aluminum (III) with 0.05mol dm-3 Kelex 100 in kerosene at a constant concentration of sodium hydroxide in 6mol dm-3, the loading test of aluminum to the organic phase suggests that the molar

4 Table 2 Temperature dependence of the distribution coefficient in the extruction of aluminum (III) from 1mol dm-3 sodium hydroxide solution with 0.1mol dm-3 Kelex 100 in kerosene Fig. 4 Variation in the distribution coefficient as a function of mixing time for the extraction of aluminum (III) from 1mol dm-3 sodium hydroxide solution with 0.1mol dm-3 Kelex 100 in kerosene at Fig. 5 First order rate expression for the uptake of aluminum (III) as a function of mixing time in the extraction from sodium hydroxide solutions with (numerals on lines are sodium hydroxide concentrations, mol dm-3). ratio [Al]/[Kelex 100]/[Na]/[H2O] in the organic phase approaches a limiting value of 1:3:0:0 with increasing initial concentration of aluminum in the aqueous phase consistent with the formation of the species AlQ3, although the organic phase is not yet saturated by aluminum loading at the initial aqueous aluminum concentration in 1mol dm-3. The formation of the species AlQ3 which is in an octahedral arrangement is also confirmed by infrared spectral result of the organic extracts (Table 1): the Al-N and Al-O streching bands appear at 655 and 360cm-1, respectively; the C=N and C-O stretching absorptions which appear at 1580 and 1405cm-1, respectively, for free extractant shift to lower frequencies; the OH stretching and bending bands at 3360 and 1280cm-1, respectively, decrease in intensities in comparison with those for free extractant. From it is deduced that the species formed in the organic phase possesses the structure in which oxine group coordinates to aluminum through oxygen and nitrogen. 3.2 Temperature effect The extraction of aluminum (III) from aqueous solutions containing 0.01mol dm-3 sodium aluminate in 1mol dm-3 sodium hydroxide with 0.1mol dm-3 Kelex 100 in result that the distribution coefficient decreases with rising temperature as indicated in Table 2. The value of heat of estimated to be 43.4kJ mol Extraction rate The extraction of aluminum (III) from sodium hydroxide solutions by Kelex 100 is relative slow to attain to the equilibrium state as illustrated in Fig. 4, and accordingly a kinetic investigation is carried out under non-equilibrium conditions in order to elucidate the mechanism of reaction in this extraction system. For the extraction of aluminum (III) from sodium hydroxide solutions containing sodium aluminate in 0.01mol dm-3 with 0.5mol dm-3 Kelex 100 condition that the concentration of aluminum in the aqueous phase at equilibrium is negligible in comparison with that in the organic phase when Kelex 100 is present in excess, the following rate expression is obtained by assuming a first order reaction with respect to the aluminum concentration: where subscripts i and t indicate the initial state and the non-equilibrium state at mixing time t, respectively, and kobs indicates the apparent rate constant. When the values of ln ([Al]aq,i/[Al]aq,t) are plotted against the mixing time in the extraction of aluminum (III) from sodium hydroxide solutions with 0.5mol dm-3 Kelex

5 Vol. 40, No Fig. 6 Dependence of kobs on sodium hydroxide concentration in the extraction of aluminum (III) from aqueous solutions with 0.5mol dm-3 Kelex 100 in Fig. 8 Dependence of kobs on Kelex 100 concentration in the extraction of aluminum (III) from Fig. 7 Dependence of kobs on sodium concentration in the extraction of aluminum (III) from aqueous solutions containing 0.1mol dm-3 sodium hydroxide and sodium perchlorate with 0.5mol dm-3 first order with respect to the aluminum concentration in aqueous phase (Fig. 5). In addition, with increasing sodium hydroxide concentration, the rate is almost the same at [NaOH]init aq<0.6mol dm-3 and increases at [NaOH]init aq>0.6mol dm-3 (Fig. 6). In the extraction of aluminum (III) from aqueous solutions containing 0.1 mol dm-3 sodium hydroxide and sodium perchlorate with log-log plot of kobs vs. total aqueous sodium concentration suggests that kobs is zero and first orders at low and higher concentrations of sodium hydroxide, respectively, with respect to the sodium concentration (Fig. 7). Also the dependence of kobs on the Kelex 100 concentration in the extraction of aluminum (III) from aqueous solutions containing 0.1 and 1mol dm-3 sodium hydroxide with 0.5mol dm-3 Kelex 100 in kerosene indicates that kobs is zero and first orders at low and higher concentrations of sodium hydroxide, respectively, with respect to the Kelex 100 concentration (Fig. 8). Hence the observed rate in the extraction of aluminum (III) from aqueous solutions at low concentration of sodium hydroxide is expressed as where kobs=kl, and at higher concentration of sodium hydroxide 4. Conclusion As the rate expression is affected by the concentration of aqueous sodium hydroxide solution, it is thought that there are the rate determining steps which occur in parallel with some reactions. Accordingly the theoretical rate expressions derived from assuming various extraction mechanisms are compared with the observed ones in Eqns. (4) and (5) in order to decide the rate determining steps. Since the predominant species of aluminum (III) in aqueous solutions is in Al(OH)-4, the following assumption is made at first: the aluminum (III) combines with Kelex 100 as the species of Al(OH)-4, Al(OH)3, taking off OH- from Al (OH)-4, or Na+[Al(OH)4]-; the Kelex 100 which combines with aluminum (III) exists as the species of HQ, Q- or NaQ. Afterwards, some theoretical rate expressions are obtained by the combination of the species of aluminum (III) and Kelex 100 to each other. Consequently it is presumed that Kelex 100 does not always combine directly with the species Al(OH)-4 in the aqueous solution, but combines with the intermediate species Al(OH)3 and/or Na+ [Al(OH)3], OH-. It is thus inferred that for the extraction

6 of aluminum (III) from sodium hydroxide solutions by Kelex 100 in kerosene the slow rate is mainly attributable to the formation of activated species such as [Al(OH)3], OH- and Na+[Al(OH)3], OH-. In these cases, we suppose that all rate determining reactions take place at the interface, similar to the extraction process of gallium (III)8). On the other hand, when the separation factor of gallium ed on the basis of the distribution data (for the extraction from aqueous solutions containing mol dm-3 gallium or 0.01mol dm-3 aluminum in sodium hydroxide at different concentrations with 0.025mol dm-3 Kelex , 59.0, 82.4, 85.8 and 67.1 at the concentrations of aqueous sodium hydroxide solutions in 0.2, 0.4, 0.6, 1, 2, 4 and 6mol dm-3, respectively. In this case, however, since the mixing times are 8 and 24h in the extraction of gallium (III) and aluminum (III), respectively, the separation between gallium (III) and aluminum (III) is also examined by the difference in the extraction rate. In the extraction from aqueous solutions containing mol dm-3 gallium or 0.01mol dm-3 aluminum in 1mol dm-3 sodium hydroxide with 0.1mol dm-3 Kelex 100 in kerosene at time gives the following result under the non-equilibrium conditions: 1200, 672, 360, 215, 187 and 148 for 1, 2, 4, 8, 12 and 24h, respectively. This suggests that the separation of gallium and aluminum in Bayer liquor using Kelex 100 should be carried out under the non-equilibrium conditions. Acknowledgements The author wishes to thank Dr. K. Sato and Mr. H. Oishi for assistance with part of the experimental work. References 1) A. Leveque and J. Helgorsky: J., Proc. Int. Solvent Extr. Conf., 1977, Toront, Vol. 2, Can. Inst. Mining and Metallurgy, 1977, p ) J. Helgorsky and A. Leveque: Ger. Pat. 2,530,880 (1976), 2,743,475 (1978); Fr. Pat. 2,307,047 (1976), 2,397,464 (1979). 3) E. Uhelemann and M. Mickler: Anal. Chim. Acta, 130 (1981) ) E. Uhelemann and W. Weber: Anal. Chim. Acta. 156 (1984) ) T. Sato, T. Nakamura and H. Oishi: Proc. Int. Solvent Extr. Conf., 1983, Denver, 1983, p. 274; Solvent Extr. Ion Exch., 2 (1984) 45. 6) T. Sato, T. Nakamura, M. Yabuta and H. Oishi: Chem. Lett., (1982) ) T. Sato, T. Nakamura, H. Oishi and M. Yabuta: Proc. Symp. Solvent Extr., 1984, Hamamatsu, 1984, p ) T. Sato and H. Oishi: H., Hydrometallurgy, 16 (1986) ) T. Sato: Chemical Separations, 1st Int. Conf. Separations Sci. Technol., 1985, N. Y., Vol. 1, Litarvan Literatu, Denver, 1986, p ) T. Sato: J. Jpn. Inst. Light Met., 36 (1986) ) T. Sato and H. Oishi: Proc. Symp. Solvent Extr., 1987, Osaka, 1987, p ) T. Sato, K. Sato, H. Oishi and Y. Takeuchi: ISEC'88, Int. Solvent Extr. Conf., 1988, Moscow, Vol. III, 1988, p ) J. Kinnunen and B. Wennerstrand: Chemist- Analyst, 46 (1957) ) E. g., T. Sato and H. Watanabe: Anal. Chim. Acta, 49 (1970) ) E. R. Lippincott, J. A. Psellos and M. C. Tobinm: J. Chem. Phys., 20 (1952) 536.