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SEPARATIONS Study on Mg2+ Removal from Ammonium Dihydrogen Phosphate Solution by Predispersed Solvent Extraction JianHong Luo, Jun Li,* Yang Jin, Yi Zhang, and DongSheng Zheng Department of Chemical Engineering, Sichuan UniVersity, Chengdu, Sichuan 610065, People’s Republic of China
The predispersed solvent extraction (PDSE) is a new method for separating solutes from aqueous solution, and for its high extraction ratio, the PDSE presents huge potential for the extraction from dilute solution. In the present paper, colloidal liquid aphrons are successfully generated using kerosene as a solvent, di(2-ethylhexyl)phosphoric acid (D2EHPA) as an extractant, sodium dodecyl benzene sulfate (SDBS) as surfactant in the aqueous phase, and polyoxyethylene sorbitol anhydride monolaurate (Tween-20) in oil phase. The extraction of Mg2+ from ammonium dihydrogen phosphate solution is studied by using colloidal liquid aphrons (CLA) and colloidal gas aphrons (CGA) in a semibatch extraction column. To study the extraction ratio and advantages of the PDSE process in the removal of Mg2+, various parameters, initial Mg2+ concentration, D2EHPA volume fraction, reaction temperature, phase volume ratio, mass fractions of dodecyl trimethylammonium bromide (HTAB), mass fractions of SDBS, mass fractions of Tween-20, and initial pH of NH4H2PO4 solution, are studied and optimized. The results show that Mg2+ in NH4H2PO4 solution can be effectively removed by the PDSE process. An extraction ratio of more than 86.3% is attained at the optimized parameters, and superior-grade NH4H2PO4 (MAP) can be obtained by two levels of extraction. Introduction MAP is used for flame retardant and drip-irrigation fertilization, which needs high pure MAP mainly manufactured with thermal-process phosphoric acid in the past. The cost of thermalprocess phosphoric acid is very high. The yellow-phosphorus manufacturers are closed because of the pressure from the energy consumption and environment protection. The cost of yellow phosphorus, as a basic raw material of thermal-process phosphoric acid, is becoming higher and higher. So the low cost of wet-process phosphoric acid (WPA) has gradually gained attention in recent years. However there are some undesirable impurities (Fe3+, Al3+, Mg2+) in WPA. They will lower the quality of MAP products. To get the superior grade MAP, the WPA should be purified. Improving the pH of the solution, often to 4, can remove most of the metal ions, but there still are some Mg2+ which can cause formation of troublesome water-insoluble substances in the following concentration and crystallization process. The main ingredient of water-insoluble substances is magnesium-containing phosphate. Therefore, before concentrating the neutralized MAP solution, the Mg2+ must be removed. Several methods based on solvent extraction1,2 are used to remove Mg2+ in the phosphate industry; however, conventional solvent extraction suffers from two disadvantages. It needs a mixing-setting stage, and it requires a high phase volume ratio of solvent/water below which extraction is poor. Furthermore, because of the nature of the mixing, sometimes there is the undesirable possibility of the formation of a third, colloidal phase which is difficult to eliminate. A new technique of predispersed solvent extraction avoids these problems because it eliminates the need for a mixing-setting stage. Consequently, in this paper, the PDSE process is introduced to extract Mg2+ from the NH4H2PO4 solution. * To whom correspondence should be addressed. E-mail: lijun@ email.scu.edu.cn. Tel: 86-28-85460936. Fax: 86-28-85460936.
The PDSE was first proposed by Sebba,3 which is applicable to treat the dilute solution with a low solvent/water ratio.4,5 Organic solvents including extractant are predispersed to micrometer-sized globules, which form an enormous interfacial area where a transfer of solute from one phase to another can occur very rapidly with a minimum energy requirement. Although the solvent is usually lighter than water and would be expected to rise naturally, this is very time-consuming because of the small size of CLA. The used solvent is usually recovered by flotation using CGAs. When a CGA dispersion is added to an aqueous solution, the CGA rises due to the natural buoyancy force, and a CLA rise velocity is further enhanced after CGAs combining with CLAs. Since CGAs have considerable mechanical strength and a larger surface area than gas bubbles, the CGA is effective for the stripping of solvents after extraction.6 There have been a number of reported applications for PDSE. Lye and Stuckey7 successfully applied CLA to the extraction and stripping of antibiotic erythromycin-A. The kinetics of the extraction and stripping processes has been investigated. Zhang et al.5,8 studied the preparation, characterization of CLAs and CGAs, and flotation of a hydrophobic organic dye from water by PDSE process in semibatch and continuous countercurrent extraction columns. Limb and Stuckey9 used CLAs to mobilize B-galactosidase. Lye and Stuckey10 investigated the structure and stability of colloidal liquid aphrons using a variety of experimental techniques. Their findings support the structure model proposed by Sebba3 who suggested that polyaphron (the aggregate of CLAs) phases resemble a biliquid foam while the individual CLA, dispersed in a continuous aqueous phase, consists of spherical, microsized oil droplets surrounded by a thin aqueous “soapy-shell”. Recently, Kim and Hong12 used CLAs to extract succinic acid; meanwhile, they also12,13 studied the effect of salts and pH on the extraction characteristics of succinic acid. In this study the removal of Mg2+ from ammonium dihydrogen phosphate with PDSE is investigated. The aim of this
10.1021/ie801277t CCC: $40.75 2009 American Chemical Society Published on Web 01/08/2009
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Figure 2. Samples of diluted CLAs generated from SDBS in water and Tween-20 in kerosene examined by a microscopic camera.
Figure 1. Schematic diagram of the experimental setup: 1, PDSE column; 2, extract; 3, CLAs; 4, pumps; 5, valves; 6, raffinates; and 7, CGAs generator.
work is to experimentally study the effects of various factors on the extraction ratio using CLAs and CGAs in a semibatch extraction column. Experimental Section 1. Materials. The solvent used in this work is kerosene. D2EHPA is employed as an extractant produced by Luo yang Zhongda Chemical Company (China) (AR grade). The surfactants used in this work are SDBS (AR grade), HTAB (AR grade), and Tween-20 (AR grade). 2. Preparation of Polyaphrons and CGAs. The oil phase (500 mL) containing nonionic surfactant is gradually added (1.0-1.5 mL/min) into a foaming aqueous ionic surfactant solution under suitable mixing conditions (600 r/min) using a mechanical stirrer. A white creamy dispersion system of CLAs is obtained. The phase volume ratio (PVR) is defined as the volume ratio of the oil phase to the aqueous phase. CGAs are prepared in an apparatus similar to that of Sebba.14 The generator is composed of a 1 L beaker containing surfactant aqueous solution and a central stirrer. A high-speed motor drives a shaft at 6000 r/min. The beaker is initially filled with 175 mL surfactant aqueous solution and is stirred at a high speed until a constant volume of creamy CGAs is generated. In the PDSE experiment, CGAs are generated on demand. 3. Extraction Apparatus and Procedure. PDSE experiments are carried out in an apparatus designed for semibatch operation as shown in Figure 1. The glass column has dimensions of 4 cm inner diameter and 1.2 m height. During the experiment, the column is initially filled with 100 mL ammonium dihydrogen phosphate aqueous solution (pH ) 4.0-4.5) with the initial Mg2+ concentrations 600 ppm. The diluted polyaphrons (colloidal liquid aphrons, CLAs) as shown in Figure 2 are pumped into the column at a rate of 10 mL/min. The system is then settled for 5 min to allow a sufficient time of contact between CLAs and aqueous solution before CGAs as shown in Figure 3 are introduced from the column bottom. The amount of CGAs is 400 mL, and the flow rate of CGAs is 20 mL/min. A thin layer of oil phase with the extracted solute is obtained in the upper layer of the solution. The samples are collected from the raffinate. 4. Extraction Experiment Conditions. Solvent, kerosene; phase volume ratio (PVR is defined as the volume ratio of the
Figure 3. Samples of CGAs generated from HTAB in water examined by a microscopic camera.
dispersed oil phase to the continuous phase), 10:1; stirring speed, 600 r/min; solvent addition rate, 0.001-0.0015 L/min; oil phase surfactant (mass/volume), 2% Tween-20; aqueous phase surfactant, 1 g/L SDSB; phrase ratio, 4:10; CLA, 30% D2EHPA; CLA flow rate:, 0.01 L/min (0.04 L diluted to 0.06 L); CGA, 2.857 g/L HTAB in aqueous solution; CGA flow rate, 0.02 L/min; initial Mg2+concentration, 600 mg/L; initial pH of ammonium dihydrogen phosphate, 4.5; settle time, 15 min; reaction temperature, 30 °C. 5. Parameters that Could Affect the PDSE Process. To study Mg2+ removal efficiency and advantages of the PDSE process, it is necessary to optimize various parameters that could affect the process. The parameters to be optimized are the initial surfactant concentration, the concentration of Mg2+, the D2EHPA volume content, the reaction temperature, the phase volume ratio, the mass contents of HTAB, the mass contents of SDBS, and the initial pH of NH4H2PO4 solution. 6. Analysis. The concentration of Mg2+ is determined by atomic absorption spectrophotometry (GF3000). Result and Discussion The extraction ratio (E) is defined as followed: 2+
E)
Mg Mg - M(r) M(i) Mg M(i)
2+
(1)
2+
2+
Mg E represents the efficiency of PDSE; M(i) the moles of Mg2+ Mg2+ in initial solution, mol; and M(r) the moles of Mg2+in the raffinate, mol. 1. Effect of Volume of CGA. The results of the experiments are presented in Figure 4. The figure indicates that the extraction ratio of Mg2+ is improved with the increase of volume of CGA. Lee and Hong6 pointed out that the surface areas provided by CGAs play an important part in addition to the surface charges of both CGA and CLA. Increasing the amount of CGA
2058 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009
Figure 4. Extraction ratio (E) versus the volume of CGA (mL).
Figure 5. Extraction ratio (E) versus the initial Mg2+ concentration.
Figure 6. Extraction ratio (E) versus the phase ratio (R).
dispersion, the surface area to which CLAs could attach also increases so that the extraction ratio increases. However, when the volume of CGAs increases to a certain value, CGAs absorb most parts of CLAs, and the extraction ratio (E) remains almost unchanged, because the extraction reaction reaches equilibrium. 2. Effect of Initial Mg2+ Concentration. Figure 5 shows that the extraction ratio (E) decreases with the increase of initial Mg2+ concentration, as Wang et al.15 reported that the extraction by CLA is more effective for dilute solution. Since the mount of extractant is given, so the numbers of free extractant molecular taking participate in the extraction reaction are also fixed. 3. Effect of Phase Ratio. Figure 6 shows the effects of phase ratio (solvent phase/water phase): with the increasing phase ratio (R), the extraction ratio (E) increases gradually. The reason15 for this is that increasing phase ratio in CLAs will enhance the amount of solvent and extractant. 4. Effect of D2EHPA Volume Fraction (%). Increasing D2EHPA concentration in CLAs will increase the amount of
Figure 7. Extraction ratio (E) versus the D2EHPA volume fraction (%).
extractant, so the numbers of free extractant that participate in the extraction reaction will also increase. Wang Yundong15 reported the same trend on the removal of phenol from dilute solution. Therefore, the extraction ratio (E) increases as shown in Figure 7. 5. Effect of Mass Fractions of HTAB. The experimental results show that the mass content of HTAB does not affect the extraction ratio (E). The reason is that increasing the HTAB concentration does not affect the mean diameter of CGA, when the HTAB concentration is much higher than the critical micelle concentration (CMC) of HTAB. 6. Effect of Mass Fractions of SDBS. With the increasing mass fractions of SDBS, the extraction ratio (E) increase rapidly first, and then remains nearly invariable. As Wang et al.16 pointed out, when the concentraction of SDBS is lower than its CMC, with the increasing concentraction of SDBS, the mean bubble size of CLAs decreases to the minimum size near the CMC. Then with the rising concentration of SDSB, the average bubble size of CLAs tends to be unchanged. That is in agreement with the fact that there is an inverse relationship between the surface area and the radius of the aphrons.17 7. Effect of Mass Fractions of Tween-20. With the increasing mass fractions of Tween-20, the extraction ratio (E) increases first and then remains nearly invariable. The reason is that the mean bubble size of CLAs decreases with an increase in the concentraction of Tween-20 up to 3.4 g/L and then remains approximately unchanged. Matsushita et al.18 reported the same trend on the removal of dilute products with PDSE. 8. Effect of Initial pH of Ammonium Dihydrogen Phosphate Solution. D2EHPA (HA) contains dissociable H+, so the mechanism of extracting Mg2+ with HA is perhap in accordance with the cation exchange. In general, the extraction reaction can be described as follows 2+ + nqMg(a)
q(s + 2n) (HA)m(o) f (MgnA2n · sHA)q(o) + m [(MgnA2n · sHA)q](o)[H+](a)2nq + K) 2nqH(a) (2) [Mg2+](a)nq[(HA)m](o)q(s+2n)/m
where m is the aggregation number of D2EHPA. Then, the equilibrium constant K is given as [(MgnA2n · sHA)q](o) )
1 [Mg2+](o) qn
(3)
and the distribution ratio of Mg2+ can be expressed as D)
[Mg2+](o) [Mg2+](a)
(4)
Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 2059 Table 1. Composition of the NH4H2PO4 N (%) P2O5 (%) Fe3+ (%) Mg2+ (%) Al3+ (%) mass fraction g12
g61
e0.0003
As (%) F- (%) SO42- (%)
e0.0012
pH
mass fraction e0.0010 e0.0010 e0.0020 4.5-4.8
e0.0003
heavy metal (Pb) % e0.0005
water-insoluble H2O (%) substance (%) e0.2
e0.05
following concentration and crystallization process using the extracted solution. As Table 1 shows, the food grade MAP can be obtained. Figure 8. Extraction distribution ratios (log D) versus the initial pH of ammonium dihydrogen phosphate solution.
Figure 9. Extraction distribution ratios (log D) versus the reaction temperature.
Conclusion On the basis of the results of this research on the removal of Mg2+ from ammonium dihydrogen phosphate solution with PDSE, the following specific conclusions can be drawn: 1. PDSE treatment can be an effective method for the removal of Mg2+ from ammonium dihydrogen phosphate solution. 2. The optimized parameters affecting the process are as follows: volume of CGA:,300 mL; initial Mg2+concentration, 600 mg/L; D2EHPA volume fraction, 30%; reaction temperature, 65 °C; phase volume ratio, 1:3; mass fraction of HTAB, 5 × 10-4; mass fraction of SDBS, 2 × 10-3; and initial pH of NH4H2PO4 solution, 4.5. 3. The mechanism of the extraction of Mg2+ with HA accords with the cation exchange and chelation. 4. The thermodynamic function values of the extraction are ∆H ) 1.95 × 10-2 J · mol-1; ∆G ) 6.08 × 103 J · mol-1; and ∆S ) 20.05 J · mol-1 · K-1. Acknowledgment
q(s + 2n) log [(HA)m](o) + log D ) log K + 2nqpH + log nq + m (nq - 1) log [Mg2+]a (5) The plot of log D-pH as shown in Figure 8 is a straight line with the slope of approximately 0.2477, suggesting 2nq ) 0.25, which indicates the chelate complex of (MgA2 · 2HA) can be obtained. So the mechanisms of the extraction of Mg2+ with HA are in accordance with the cation exchange19 and chelation. The intercept ) -0.5478 also can be obtained, According to eq 4, the extraction equilibrium constant K ) 0.0896 can be obtained. Therefore, the extraction distribution ratios (D) of Mg2+ increase rapidly as the initial pH of ammonium dihydrogen phosphate solution rises in the extraction system. 9. Effect of Reaction Temperature. The distribution ratio (D) increases as the temperature rises. A linear relationship is found between log D and 103 T-1 from the Figure 9. From the famous Van’t Hoff equation,20 d log D/d(1/T) ) -∆H/(2.303R) + const, and the ∆H value is 1.95 × 10-2 J · mol-1 which shows that the extraction of Mg2+ with D2EHPA is endothermic; ∆G ) -RT ln K, and the ∆G value is 6.08 × 103 J · mol-1 (T ) 303K); and ∆G ) ∆H - T∆S, and the ∆S value is 20.05 J · mol-1 · K-1.
Examination A kind of practical wet-process phosphoric acid is neutralized to pH ) 4 with ammonia and filtered. The neutralized solution is then extracted under the above-mentioned optimal technology conditions. The superior grade MAP are produced at the
The authors gratefully acknowledge the financial support of this work by eleventh five year national key technology R&D program (No. 2008BAE58B01) and New Century Excellent Talents of Ministry of Education (NCET-07-0577), the People’s Republic of China. Literature Cited (1) McCullough, J. F. Phosphoric acid purification:comparing the process choices. Chem. Eng. 1976, 83 (26), 101–103. (2) Lo, T. C.; Baird, M. H.; Hanson, C. Handbook of SolVent Extraction; Wiley: New York, 1983. (3) Sebba, F. Foams and Biliquid Foams: Aphrons; Wiley: New York, 1987. (4) Michelsen, D. L.; Ruettimann, K. W.; Hunter, K. R.; Sebba, F. Feasibility study on use of predispersed solvent extraction/flotation technique for removal of organics from wastewaters. Chem. Eng. Commun. 1986, 48, 155–163. (5) Zhang, C.; Valsaraj, K. T.; Constant, W. D.; Roy, D. Studies in Solvent Extraction Using Polyaphrons. II. Semibatch and Continuous Countercurrent Extraction/Flotation of a Hydrophobic Organic Dye from Water. Sep. Sci. Technol. 1996, 31, 1463. (6) Lee, D. W.; Hong, W. H. Removal of an organic dye from water using predispersed solvent extraction. Sep. Sci. Technol. 2000, 35, 1951– 1962. (7) Lye, G. J.; Stuckey, D. C. Extraction of erythromycin-A using colloidal liquid aphrons: Part II. Mass transfer kinetics. Proceeings of ISEC’96, Melbourne, Australia, 1996; Vol. 2, p 1399. (8) Zhang, C.; Valsaraj, K. T.; Constant, W. D.; Roy, D. Studies in Solvent Extraction Using Polyaphrons. I. Size Distribution, Stability, and Flotation of Polyaphrons. Sep. Sci. Technol. 1996, 31, 1059. (9) Lamb, S. B.; Stuckey, D. C. Proceedings of the IChemE 1997 Jubilee; 1997; Vol. 2, p 917 (research event). (10) Lye, G. J.; Stuckey, D. C. Colloids Surf., A 1998, 131, 119. (11) Kim, B. S.; Hong, Y. K. Predispersed solvent extraction of succinic acid aqueous solution by colloidal liquid aphrons in column. Biotechnology and Bioprocess Engineering. 2004, 9 (6), 454–458.
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ReceiVed for reView August 22, 2008 ReVised manuscript receiVed November 21, 2008 Accepted December 1, 2008 IE801277T
