SEPARATIONS

Study on Mg2+ Removal from Ammonium Dihydrogen Phosphate Solution byPredispersed 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. Inthe 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) assurfactant 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 liquidaphrons (CLA) and colloidal gas aphrons (CGA) in a semibatch extraction column. To study the extractionratio 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 dodecyltrimethylammonium bromide (HTAB), mass fractions of SDBS, mass fractions of Tween-20, and initial pHof NH4H2PO4 solution, are studied and optimized. The results show that Mg2+ in NH4H2PO4 solution can beeffectively removed by the PDSE process. An extraction ratio of more than 86.3% is attained at the optimizedparameters, and superior-grade NH4H2PO4 (MAP) can be obtained by two levels of extraction.

Introduction

MAP is used for flame retardant and drip-irrigation fertiliza-tion, which needs high pure MAP mainly manufactured withthermal-process phosphoric acid in the past. The cost of thermal-process phosphoric acid is very high. The yellow-phosphorusmanufacturers are closed because of the pressure from theenergy consumption and environment protection. The cost ofyellow phosphorus, as a basic raw material of thermal-processphosphoric acid, is becoming higher and higher. So the lowcost of wet-process phosphoric acid (WPA) has gradually gainedattention in recent years. However there are some undesirableimpurities (Fe3+, Al3+, Mg2+) in WPA. They will lower thequality of MAP products. To get the superior grade MAP, theWPA should be purified. Improving the pH of the solution, oftento 4, can remove most of the metal ions, but there still are someMg2+ which can cause formation of troublesome water-insolublesubstances in the following concentration and crystallizationprocess. The main ingredient of water-insoluble substances ismagnesium-containing phosphate. Therefore, before concentrat-ing the neutralized MAP solution, the Mg2+ must be removed.Several methods based on solvent extraction1,2 are used toremove Mg2+ in the phosphate industry; however, conventionalsolvent extraction suffers from two disadvantages. It needs amixing-setting stage, and it requires a high phase volume ratioof solvent/water below which extraction is poor. Furthermore,because of the nature of the mixing, sometimes there is theundesirable possibility of the formation of a third, colloidal phasewhich is difficult to eliminate. A new technique of predispersedsolvent extraction avoids these problems because it eliminatesthe need for a mixing-setting stage. Consequently, in this paper,the PDSE process is introduced to extract Mg2+ from theNH4H2PO4 solution.

The PDSE was first proposed by Sebba,3 which is applicable totreat the dilute solution with a low solvent/water ratio.4,5 Organicsolvents including extractant are predispersed to micrometer-sizedglobules, which form an enormous interfacial area where a transferof solute from one phase to another can occur very rapidly with aminimum energy requirement. Although the solvent is usuallylighter than water and would be expected to rise naturally, this isvery time-consuming because of the small size of CLA. The usedsolvent is usually recovered by flotation using CGAs. When a CGAdispersion is added to an aqueous solution, the CGA rises due tothe natural buoyancy force, and a CLA rise velocity is furtherenhanced after CGAs combining with CLAs. Since CGAs haveconsiderable mechanical strength and a larger surface area thangas bubbles, the CGA is effective for the stripping of solvents afterextraction.6

There have been a number of reported applications for PDSE.Lye and Stuckey7 successfully applied CLA to the extraction andstripping of antibiotic erythromycin-A. The kinetics of the extrac-tion and stripping processes has been investigated. Zhang et al.5,8

studied the preparation, characterization of CLAs and CGAs, andflotation of a hydrophobic organic dye from water by PDSE processin semibatch and continuous countercurrent extraction columns.Limb and Stuckey9 used CLAs to mobilize B-galactosidase. Lyeand Stuckey10 investigated the structure and stability of colloidalliquid aphrons using a variety of experimental techniques. Theirfindings support the structure model proposed by Sebba3 whosuggested that polyaphron (the aggregate of CLAs) phases resemblea biliquid foam while the individual CLA, dispersed in a continuousaqueous phase, consists of spherical, microsized oil dropletssurrounded by a thin aqueous “soapy-shell”. Recently, Kim andHong12 used CLAs to extract succinic acid; meanwhile, theyalso12,13 studied the effect of salts and pH on the extractioncharacteristics of succinic acid.

In this study the removal of Mg2+ from ammonium dihy-drogen phosphate with PDSE is investigated. The aim of this

* To whom correspondence should be addressed. E-mail: lijun@email.scu.edu.cn. Tel: 86-28-85460936. Fax: 86-28-85460936.

Ind. Eng. Chem. Res. 2009, 48, 2056–20602056

10.1021/ie801277t CCC: $40.75 2009 American Chemical SocietyPublished on Web 01/08/2009

work is to experimentally study the effects of various factorson the extraction ratio using CLAs and CGAs in a semibatchextraction column.

Experimental Section

1. Materials. The solvent used in this work is kerosene.D2EHPA is employed as an extractant produced by Luo yangZhongda Chemical Company (China) (AR grade). The surfac-tants used in this work are SDBS (AR grade), HTAB (ARgrade), 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 surfactantsolution under suitable mixing conditions (600 r/min) using amechanical stirrer. A white creamy dispersion system of CLAsis obtained. The phase volume ratio (PVR) is defined as thevolume ratio of the oil phase to the aqueous phase. CGAs areprepared in an apparatus similar to that of Sebba.14 Thegenerator is composed of a 1 L beaker containing surfactantaqueous solution and a central stirrer. A high-speed motor drivesa shaft at 6000 r/min. The beaker is initially filled with 175 mLsurfactant aqueous solution and is stirred at a high speed untila constant volume of creamy CGAs is generated. In the PDSEexperiment, CGAs are generated on demand.

3. Extraction Apparatus and Procedure. PDSE experi-ments are carried out in an apparatus designed for semibatchoperation as shown in Figure 1. The glass column has dimen-sions of 4 cm inner diameter and 1.2 m height. During theexperiment, the column is initially filled with 100 mL am-monium dihydrogen phosphate aqueous solution (pH ) 4.0-4.5)with the initial Mg2+ concentrations 600 ppm. The dilutedpolyaphrons (colloidal liquid aphrons, CLAs) as shown in Figure2 are pumped into the column at a rate of 10 mL/min. Thesystem is then settled for 5 min to allow a sufficient time ofcontact between CLAs and aqueous solution before CGAs asshown in Figure 3 are introduced from the column bottom. Theamount of CGAs is 400 mL, and the flow rate of CGAs is 20mL/min. A thin layer of oil phase with the extracted solute isobtained in the upper layer of the solution. The samples arecollected from the raffinate.

4. Extraction Experiment Conditions. Solvent, kerosene;phase volume ratio (PVR is defined as the volume ratio of the

dispersed oil phase to the continuous phase), 10:1; stirring speed,600 r/min; solvent addition rate, 0.001-0.0015 L/min; oil phasesurfactant (mass/volume), 2% Tween-20; aqueous phase sur-factant, 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, 0.02L/min; initial Mg2+concentration, 600 mg/L; initial pH ofammonium dihydrogen phosphate, 4.5; settle time, 15 min;reaction temperature, 30 °C.

5. Parameters that Could Affect the PDSE Process. Tostudy Mg2+ removal efficiency and advantages of the PDSEprocess, it is necessary to optimize various parameters that couldaffect the process. The parameters to be optimized are the initialsurfactant concentration, the concentration of Mg2+, the D2EHPAvolume content, the reaction temperature, the phase volumeratio, 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 byatomic absorption spectrophotometry (GF3000).

Result and Discussion

The extraction ratio (E) is defined as followed:

E)M(i)

Mg2+-M(r)

Mg2+

M(i)Mg2+ (1)

E represents the efficiency of PDSE; M(i)Mg2+

the moles of Mg2+

in initial solution, mol; and M(r)Mg2+

the moles of Mg2+in theraffinate, mol.

1. Effect of Volume of CGA. The results of the experimentsare presented in Figure 4. The figure indicates that the extractionratio of Mg2+ is improved with the increase of volume of CGA.Lee and Hong6 pointed out that the surface areas provided byCGAs play an important part in addition to the surface chargesof both CGA and CLA. Increasing the amount of CGA

Figure 1. Schematic diagram of the experimental setup: 1, PDSE column;2, extract; 3, CLAs; 4, pumps; 5, valves; 6, raffinates; and 7, CGAsgenerator.

Figure 2. Samples of diluted CLAs generated from SDBS in water andTween-20 in kerosene examined by a microscopic camera.

Figure 3. Samples of CGAs generated from HTAB in water examined bya microscopic camera.

Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 2057

dispersion, the surface area to which CLAs could attach alsoincreases so that the extraction ratio increases. However, whenthe volume of CGAs increases to a certain value, CGAs absorbmost parts of CLAs, and the extraction ratio (E) remains almostunchanged, because the extraction reaction reaches equilibrium.

2. Effect of Initial Mg2+ Concentration. Figure 5 showsthat the extraction ratio (E) decreases with the increase of initialMg2+ concentration, as Wang et al.15 reported that the extractionby CLA is more effective for dilute solution. Since the mountof extractant is given, so the numbers of free extractantmolecular taking participate in the extraction reaction are alsofixed.

3. Effect of Phase Ratio. Figure 6 shows the effects of phaseratio (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 theamount of solvent and extractant.

4. Effect of D2EHPA Volume Fraction (%). IncreasingD2EHPA concentration in CLAs will increase the amount of

extractant, so the numbers of free extractant that participate inthe extraction reaction will also increase. Wang Yundong15

reported the same trend on the removal of phenol from dilutesolution. Therefore, the extraction ratio (E) increases as shownin Figure 7.

5. Effect of Mass Fractions of HTAB. The experimentalresults show that the mass content of HTAB does not affectthe extraction ratio (E). The reason is that increasing the HTABconcentration does not affect the mean diameter of CGA, whenthe HTAB concentration is much higher than the critical micelleconcentration (CMC) of HTAB.

6. Effect of Mass Fractions of SDBS. With the increasingmass fractions of SDBS, the extraction ratio (E) increase rapidlyfirst, and then remains nearly invariable. As Wang et al.16

pointed out, when the concentraction of SDBS is lower than itsCMC, with the increasing concentraction of SDBS, the meanbubble size of CLAs decreases to the minimum size near theCMC. Then with the rising concentration of SDSB, the averagebubble size of CLAs tends to be unchanged. That is in agreementwith the fact that there is an inverse relationship between thesurface area and the radius of the aphrons.17

7. Effect of Mass Fractions of Tween-20. With the increas-ing mass fractions of Tween-20, the extraction ratio (E) increasesfirst and then remains nearly invariable. The reason is that themean bubble size of CLAs decreases with an increase in theconcentraction of Tween-20 up to 3.4 g/L and then remainsapproximately unchanged. Matsushita et al.18 reported the sametrend on the removal of dilute products with PDSE.

8. Effect of Initial pH of Ammonium DihydrogenPhosphate Solution. D2EHPA (HA) contains dissociable H+,so the mechanism of extracting Mg2+ with HA is perhap inaccordance with the cation exchange. In general, the extractionreaction can be described as follows

nqMg(a)2++ q(s+ 2n)

m(HA)m(o)f (MgnA2n · sHA)q(o) +

2nqH(a)+ K)

[(MgnA2n · sHA)q](o)[H+](a)

2nq

[Mg2+](a)nq[(HA)m](o)

q(s+2n)/m(2)

where m is the aggregation number of D2EHPA.Then, the equilibrium constant K is given as

[(MgnA2n · sHA)q](o) )1

qn[Mg2+](o) (3)

and the distribution ratio of Mg2+ can be expressed as

D)[Mg2+](o)

[Mg2+](a)

(4)

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).

Figure 7. Extraction ratio (E) versus the D2EHPA volume fraction (%).

2058 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009

log D) log K+ 2nqpH+ log nq+ q(s+ 2n)m

log [(HA)m](o) +

(nq- 1) log [Mg2+]a (5)

The plot of log D-pH as shown in Figure 8 is a straight linewith the slope of approximately 0.2477, suggesting 2nq ) 0.25,which indicates the chelate complex of (MgA2 ·2HA) can beobtained. So the mechanisms of the extraction of Mg2+ withHA are in accordance with the cation exchange19 and chelation.The intercept ) -0.5478 also can be obtained, According toeq 4, the extraction equilibrium constant K ) 0.0896 can beobtained. Therefore, the extraction distribution ratios (D) ofMg2+ increase rapidly as the initial pH of ammonium dihydrogenphosphate solution rises in the extraction system.

9. Effect of Reaction Temperature. The distribution ratio(D) increases as the temperature rises. A linear relationship isfound between log D and 103 T-1 from the Figure 9. From thefamous 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 showsthat 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 20.05J ·mol-1 ·K-1.

Examination

A kind of practical wet-process phosphoric acid is neutralizedto pH ) 4 with ammonia and filtered. The neutralized solutionis then extracted under the above-mentioned optimal technologyconditions. The superior grade MAP are produced at the

following concentration and crystallization process using theextracted solution. As Table 1 shows, the food grade MAP canbe obtained.

Conclusion

On the basis of the results of this research on the removal ofMg2+ from ammonium dihydrogen phosphate solution withPDSE, the following specific conclusions can be drawn:

1. PDSE treatment can be an effective method for the removalof Mg2+ from ammonium dihydrogen phosphate solution.

2. The optimized parameters affecting the process are asfollows: volume of CGA:,300 mL; initial Mg2+concentration,600 mg/L; D2EHPA volume fraction, 30%; reaction tempera-ture, 65 °C; phase volume ratio, 1:3; mass fraction of HTAB,5 × 10-4; mass fraction of SDBS, 2 × 10-3; and initial pH ofNH4H2PO4 solution, 4.5.

3. The mechanism of the extraction of Mg2+ with HA accordswith 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

The authors gratefully acknowledge the financial support ofthis work by eleventh five year national key technology R&Dprogram (No. 2008BAE58B01) and New Century ExcellentTalents of Ministry of Education (NCET-07-0577), the People’sRepublic of China.

Literature Cited

(1) McCullough, J. F. Phosphoric acid purification:comparing the processchoices. 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 techniquefor removal of organics from wastewaters. Chem. Eng. Commun. 1986,48, 155–163.

(5) Zhang, C.; Valsaraj, K. T.; Constant, W. D.; Roy, D. Studies inSolvent Extraction Using Polyaphrons. II. Semibatch and ContinuousCountercurrent Extraction/Flotation of a Hydrophobic Organic Dye fromWater. Sep. Sci. Technol. 1996, 31, 1463.

(6) Lee, D. W.; Hong, W. H. Removal of an organic dye from waterusing predispersed solvent extraction. Sep. Sci. Technol. 2000, 35, 1951–1962.

(7) Lye, G. J.; Stuckey, D. C. Extraction of erythromycin-A usingcolloidal liquid aphrons: Part II. Mass transfer kinetics. Proceeings ofISEC’96, Melbourne, Australia, 1996; Vol. 2, p 1399.

(8) Zhang, C.; Valsaraj, K. T.; Constant, W. D.; Roy, D. Studies inSolvent Extraction Using Polyaphrons. I. Size Distribution, Stability, andFlotation 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. Biotechnologyand Bioprocess Engineering. 2004, 9 (6), 454–458.

Figure 8. Extraction distribution ratios (log D) versus the initial pH ofammonium dihydrogen phosphate solution.

Figure 9. Extraction distribution ratios (log D) versus the reactiontemperature.

Table 1. Composition of the NH4H2PO4

N (%) P2O5 (%) Fe3+ (%) Mg2+ (%) Al3+ (%)heavy metal

(Pb) %

mass fraction g12 g61 e0.0003 e0.0012 e0.0003 e0.0005

As (%) F- (%) SO42- (%) pH H2O (%)

water-insolublesubstance (%)

mass fraction e0.0010 e0.0010 e0.0020 4.5-4.8 e0.2 e0.05

Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 2059

(12) Kim, B. S.; Hong, Y. K. Effect of salts on the extractioncharacteristics of succinic acid by predispersed solvent extraction. Biotech-nol. Bioprocess Eng. 2004, 9 (3), 207–211.

(13) Kim, B. S.; Hong, Y. K. Effect of pH on the extraction character-istics of succinic acid and the stability of colloidal liquid aphrons. KoreanJ. Chem. Eng. 2002, 19 (4), 669–672.

(14) Sebba, F. An improved generator for micron-sized bubbles. Chem.Ind. 1985, (9), 91.

(15) Wang, Y. D.; Cheng, M.; Xu, L. L.; Dai, Y. Y. Removal of Phenolfrom Dilute Solutions by Predispersed Solvent Extraction. Chin. J. Chem.Eng. 2000, 8 (2), 103–107.

(16) Wang, Y. D.; Cheng, M.; Xu, L. L.; Dai, Y. Y. Preparation andcharacteristics of colloidal liquid aphrons and colloidal gas aphrons.Tsinghua Sci. Technol. 1998, 38 (6), 42–45.

(17) Amiri, M. C.; Somunk, J. Effect of gas transfer on separation ofwhey protein with aphron flotation. Sep. Sci. Technol. 2004, 35, 161–167.

(18) Matsushita, K.; Mollah, A. H.; Stuckey, D. C. Predispersed solventextraction of dilute products using colloidal gas aphrons and colloidal liquidaphrons: aphron preparation, dtability and size. Colloids Surf. 1992, 69,65–72.

(19) Van, D. D.; Pinoy, L.; Courtijn, E.; Verpoort, F. Influence of acetateions and the role of the diluents on the extraction of copper (II), nickel (II),cobalt (II), magnesium(II) and iron (II, III) with different types of extractants.Hydrometallurgy 2005, (78), 92–106.

(20) Out, E. O.; Chiarizia, R. Thermodynamics of the extraction of metalions by dialkyl-substituted diphosphonic acids. II. The U(VI) and Sr(II)case. SolVent Extr. Ion Exch. 2001, 19 (6), 1017–1036.

ReceiVed for reView August 22, 2008ReVised manuscript receiVed November 21, 2008

Accepted December 1, 2008

IE801277T

2060 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009