XX-th ARS SEPARATORIA Szklarska Poręba, Poland 2005

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1 PREPARATIONS AND EXTRACTION PROPERTIES OF MICROCAPSULES CONTAINING EXTRACTANTS K. SHIOMORI 1, S. KIYOYAMA 2, H. YOSHIZAWA 3, Y. HATATE 4, Y. KAWANO 1 1 Department of Applied Chemistry, University of Miyazaki, Miyazaki Japan 2 Department of Chemical Science and Engineering, Miyakonojyo NCT, Miyazaki Japan 3 Department of Environmental Chemistry and Materials, Okayama University, Okayama Japan 4 Department of Applied Chemistry and Chemical Engineering, Kagoshima University, Kagoshima Japan INTRODUCTION Microencapsulation is a unique technique for enclosing active reagents in a porous polymeric membrane, and has been extensively studied in regard to the elongation of the sustained release of core materials [3, 5], the protection of the encapsulating active-reagents [1], and the extraction of metal ions and acids [2,4,5]. The separation processes of target chemicals from aqueous media can be roughly divided into two methods, which are solvent extraction and adsorption. Solvent extraction has the possibility of being developed into an efficient process by means of enhancing the molecular design and synthesis of extractants. However, solvent extraction still has some problems in that the phase separation of organic and aqueous phases is difficult and that extractants and/or organic solvents are lost by being dissolved into the aqueous phase. Adsorption using resins such as ion exchange resins and chelating resins is useful for the treatment of dilute solutions and is frequently used for hydrometallurgy, and for the recovery of valuable materials and the removal of harmful substances from aqueous wastewater. These resins are, however, very expensive due to the difficulty of their preparation. Thus, the separation technique using extractants-impregnated microporous resins [7,8] and Levextrel resins [8] have gathered much attention in these fields. The extractantimpregnated macroporous resins have many advantages from the standpoint of preparation and operation. However, the following disadvantages have been recently pointed out: all molecules of the impregnated extractant are unreacted by the steric hindrance and the selectivity for the target substance decreased due to the inactivation of the extractant [7]. The microcapsules with porous membrane that encapsulate extractants are expected to be adaptable for separation technology because of the high separation property of the extractant encapsulated, which has been clarified in the solvent extraction, and because of the high capacity of the extracted chemicals utilizing their internal core [2, 4, 6]. 14

2 In this work, we would like to discuss the preparation of microcapsules having highly porous poly divinylbenzene (DVB) membranes enclosing tri-n-octylamine (TOA) by in situ polymerization accompanied by the solvent evaporation, the extraction characteristics of precious metals from an acidic aqueous solution and the column separation of precious metals using the prepared microcapsules. EXPERIMENTAL An organic dispersed phase consisted of DVB, TOA, ADVN as an initiator and toluene as a diluent. An aqueous continuous phase was composed of distilled water containing 2.0 wt% of gum Arabic. The continuous phase was put into the reactor and then the organic phase was poured to the continuous phase. The solution was stirred at 6.67 s -1 to form O/W emulsion. The reactor was then maintained at 343 K for 28.8 ks. under a nitrogen atmosphere in order to prepare the poly-dvb microcapsules by in situ polymerization. All experiments regarding extraction equilibrium were carried out batchwise. The microcapsules were pretreated with aqueous HCl solution in order to change all TOA molecules to an ammoniumchloride salt. The pretreated microcapsules were immersed in the aqueous HCl solution containing precious metals. The concentration of precious metals in the aqueous solution was determined by means of an atomic adsorption spectrometer. The concentration of the metals extracted into the microcapsules was estimated based on the mass balance of the metals in the aqueous solution before and after the extraction. Column operations of extraction and elution of precious metals using the microcapsules, which were pretreated with HCl, were investigated. For the breakthrough experiments, an aqueous HCI solution containing precious metals was fed into the top of the column at a constant flow rate. For the elution experiments, prior to use in the column operation, the microcapsules were immersed in an aqueous HCI solution containing precious metals and attained the extraction equilibrium. The microcapsules extracting the precious metals were packed into the column. The eluents was fed to the column. RESULTS AND DISCUSSION The microcapsules were prepared by changing the concentrations of TOA, toluene and DVB in the dispersed organic phase. The preparation conditions and the results are shown in Table 1. The microcapsules prepared with toluene, MC-2, is shown in Fig.1. The microcapsules were spherical with a relatively large distribution of diameters. The average diameter of the microcapsules tend to increase with the toluene extent. The 15

3 surface of MC-1 prepared without toluene was smooth and uniform, and no pores were observed on the surface. The membrane of MC-1 was a uniform and dense structure. On the other hand, many pores of a few m in diameter were observed on Table 1. Preparation conditions and results of DVB microcapsule Fig. 1. Microphotograph of poly-dvb microcapsules prepared with toluene, MC-2. the surface and the cross section of MC-2 prepared with toluene. The effects of the chloride ion concentration on the distribution of palladium (II) at various initial concentrations of palladium (II) in the aqueous phase and TOA in the microcapsules are shown in Fig. 3. The distribution ratio of palladium (II), D, was plotted against the concentration of chloride ion. D decreased with an increase in the concentrations of chloride ion and palladium (II) in the aqueous solution and increased with an increase in the concentrations of TOA in the microcapsules. 16

4 (A) (B) Fig. 3. Effect of the chloride ion on the distribution ratio of palladium(ii), D Pd, at various initial concentrations of palladium(ii) in the aqueous phase, (A), and TOA in the microcapsules, (B). In the solvent extraction of palladium (II) using TOA ammonium salts from the aqueous solution, PdCl 2 4 is extracted into the organic phase, and the extraction reaction is expressed as follows [28]: 2 2BHCl + PdCl 4 (BH) 2 PdCl 4 + 2Cl ; K Pd. Eq. (1) By considering the equilibrium of Eq. 1 and the distribution ratio for palladium (II), following relation, Eq (2) is derived: D Pd = K Pd {C BHCl,MC /C Cl,aq } 2. Eq. (2) In this equation, the concentration of free ammonium chloride salt in the core solution encapsulated in the microcapsules was calculated from the mass balance of the chemical species related to the extraction reaction. All experimental data shown in Fig. 3 were analyzed based on the Eq. (2). The values of D were plotted against C BHCl,MC /C Cl,aq on the logarithmic scale shown in Fig. 4. All experimental data except key C Pd,aq,0 C B,MC, C BHCl,MC /C Cl,aq [Š] Fig. 4. Plot of the distribution ratio, D Pd, against C BHCl,MC /C Cl,aq 17

5 for the data at very high values. D values were plotted on a single straight line having a slope of 2.0. This result shows that the extraction of palladium (II) with TOA enclosing in the microcapsules is expressed by the reaction of Eq. (1). The intercept of the straight line shown in Fig. 4 gives the equilibrium constant, K Pd, which was determined to be 1.2x10 dm 3 /kg-mc. The calculated results using K Pd are presented as the solid lines in Fig. 3. The calculations agreed closely with the experimental results. A typical breakthrough curves for palladium (II), gold (III), and platinum (IV) obtained at a 0.2 mol/dm 3 aqueous HCI solution containing 5.0 mmol/dm 3 precious metal is shown in Fig. 5. The order in the delay of the breakthrough point for each precious metal agreed with the order of the equilibrium constant of the reaction between precious metal and ammonium salt of TOA obtained in the solvent extraction system [7, 8, 9]. The extraction of precious metals can be also achieved in the column operation. Fig. 5. Breakthrough curves of palladium (III), gold (III), and platinum (IV) By using an 8.0 mol/dm 3 aqueous HCl solution for palladium, a 0.1 mol/dm 3 aqueous HCI solution containing 0.1 mol/dm 3 thiourea for gold, and a 0.1 mol/dm 3 aqueous NaOH solution containing 0.5 mol/dm 3 ethylenediamine for platinum as eluent solutions, the extracted precious metals can be successfully eluted from the column. The mutual separation of precious metals by selective elution, which is carried out by means of a step-wise feed of the suitable eluent for each precious metal in the above order, was carried out. The elution curves of palladium, gold, and platinum are shown in Fig. 6. Palladium, gold and platinum were successfully separated by selective elution from the column by coupling the desired elution solution with each precious metal. 18

6 Fig. 6. Elution curves of palladium, gold and platinum in ternary system REFERENCES 1. G. M. O'Shea, M. F. A. Goosen, Biophys. Acta 1984, 804, H. Yoshizawa, Y. Uemura, Y. Kawano, Y. Hatate, J. Chem. Eng. Japan 1993, 26, Y. Hatate, K. Kasamatsu, Y. Uemura, K. Ijichi, Y. Kawano, H. Yoshizawa, J. Chem. Eng., Japan 1994, 27, S. Nishihama, N. Sakaguchi, T. Hirai, I. Komasawa, Hydrometallurgy 2002, 64, T. Kondo, J. Oleo Sci. 2001, 50 (3), E. Kamio, M. Matsumoto, K. Kondo, J. Chem. Eng., Japan 2002, 35, S. Akita, H, Takeuchi, J. Chem. Eng., Japan 1990, 23, M. Takahashi, Y. Yamashita, S. Kosaka, Nihon Kagaku Kaishi 2000, K. Inoue, Y. Baba, Y. Sakamoto, H. Egawa, Sep. Sci. Technol. 1987, 22, Y. Kawano, S. Osada, K. Shiomori, Y. Baba, K. Kondo, Y. Yoshizawa, Y. Hatate, J. Chem. Eng., Japan 1995, 28, H. Yoshizawa, K. Shiomori, S. Yamada, Y. Baba, Y. Kawano, K. Kondo, YHatate. Solv. Extra. Res. Develop., Japan 1997, 4, K. Shiomori, S. Yamada, Y. Baba, Y. Kawano, H. Yoshizawa, Y. Hatate, K. Kondo, K, Proc. The Fourth Japan-Korea Symp. Sep. Tech., 1996,