chemical engineering research and design 8 8 ( 2 0 1 0 ) 712–717

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Chemical Engineering Research and Design

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Extraction of magnesium from phosphoric acid usingdinonylnaphthalene sulfonic acid

Jing Yua,b, Daijun Liua,∗

a School of Chemical Engineering, Sichuan University, 24, South Section 1, Chengdu 610065, Sichuan, Chinab College of Resources and Environment, Chengdu University of Information Technology, Chengdu 610225, Sichuan, China

a b s t r a c t

The extraction of magnesium ions from phosphoric acid using dinonylnaphthalene sulfonic acid (DNNSA) was stud-

ied. The effects of the extraction time, diluent, phosphoric acid concentration, DNNSA concentration and temperature

were examined. The H3PO4 concentration had a negative effect on magnesium ion extraction, while the DNNSA con-

centration had a significant positive effect. The DNNSA easily forms reverse micelles in the organic phase. The

experimental results were analyzed mathematically by nonlinear regression to determine the stoichiometry of the

complex formed in extraction. It was found that magnesium was extracted in the form of the complex with HA

representing the free acid form of DNNSA. The equilibrium constant of the extraction reaction was calculated to be

59.6.

© 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Reverse micelles; Extraction; Magnesium ions; Dinonylnaphthalene sulfonic acid; Phosphoric acid

author showed that DNNSA can be used as an extractant to

1. Introduction

In a global context, phosphoric acid is the second most pro-duced acid after sulfuric acid. Phosphoric acid is used as araw material in many applications such as in the productionof detergents, food products, alimentary supplies for cattle,toothpastes, and fertilizers. About 90% of the phosphoric acidproduced worldwide is obtained from the wet-process phos-phoric acid (WPA) derived from sulfuric acid digestion. WPAis characterized by its low cost and the presence of highcontent of cationic impurities (Fe, Mg, Al, Cd, and Ca) thataffect its properties and limit its application. Magnesium isone of the most troublesome impurities since the presenceof magnesium ions leads to sludge formation and increasesthe H3PO4 viscosity (Cate and Deming, 1970). WPA has to befurther purified with respect to magnesium ions. In the pre-liminary work, the initial concentration of magnesium ionsin Jinhe crude phosphoric acid is 2.5 wt.% (P2O5 = 24 wt.%).China standard Q/HF21-2004 provides magnesium ion con-tent is no more than 0.005 wt.% in the industrial phosphoricacid.

WPA purification is mainly carried out using solvent extrac-tion. Several methods based on the bulk solvent extraction

∗ Corresponding author. Tel.: +86 84595204; fax: +86 85405854.E-mail address: liudj721@126.com (D. Liu).Received 29 August 2008; Received in revised form 8 October 2009; Ac

0263-8762/$ – see front matter © 2009 The Institution of Chemical Engidoi:10.1016/j.cherd.2009.11.008

of H3PO4 are used industrially to purify WPA (McCullough,1976; Baird et al., 1983; Kijkowska et al., 2002; Feki et al.,2002). Reverse osmosis and nanofiltration have also been usedto purify industrial phosphoric acid (Gonzalez et al., 2002).By applying nanofiltration, it is possible to treat solutionswith high H3PO4 concentrations and obtain a good result. Inthese applications the bulk of the phosphoric acid is extractedinto the organic phase, leaving the impurities in the aqueousphase. There are also a few methods that use the removal ofcations for the purification of WPA (Williams and Stern, 1972;Bradford and Ore, 1977; Daifullah et al., 2002; Monser et al.,1999; Nazari et al., 2004; Mohammad and EI-khaiary, 1997).

In the published literature, limited information is availableon the application of a single extractant for the removal of iron,aluminium, and magnesium from WPA. Dinonylnaphthalenesulfonic acid (DNNSA), a sulfonic acid-based compound, hasproperties that are in line with such a single extractant. Itremains in the ionic form reacting with metal ions at lowpH values; and is not selective with respect to the metalions extracted. Preliminary experimental investigation by the

cepted 5 November 2009

remove iron, aluminium, and magnesium ions from phospho-ric acid.

neers. Published by Elsevier B.V. All rights reserved.

chemical engineering research and design 8 8 ( 2 0 1 0 ) 712–717 713

Nomenclature

Caq molar concentration of metal ion in aqueousphase (M)

Co molar concentration of metal ion in organicphase (M)

D distribution coefficientDwp distribution coefficient between these water

pools and the aqueous phaseHD dinonylnaphthalene sulfonic acid[(HD)m](o)total initial concentration of the extractantO/A volume ratio between organic phase and aque-

ous phaseK equilibrium constant of the reactionT extraction temperaturetextr time of extraction (min)ε polarity of diluent (F/m)ϕwp volume fraction of the water pools in the

organic phase and a constant[ ] concentration of species in brackets (M)

Subscriptsaq aqueous phaseo organic phasem aggregation number of the HD micellesp aggregation number of the complex

outD

pd

2

2

TFnKbusouw

2

OkpwaIhrn

Current literature on the topic lacks fundamental studyn the extraction of magnesium ions from phosphoric acidsing DNNSA. The aim of this study is to examine the dis-ribution of magnesium ions in the system Mg2+–H3PO4–NNSA.

The objective of the work is to develop a commercialrocess for magnesium extraction based on the technologyeveloped. Test work related to this is also reported.

. Experimental

.1. Materials

he extractant DNNSA was a gift from Suzhou Chemicalactory, Jiangsu, P.R. China. A local company supplied mag-esium sulfate heptahydrate of a minimum purity of 99%.erosene was used as the organic diluent although a num-er of others were tested, and the aromatic content of thesed kerosene was 8–15 wt.% [IBP at 760 Torr = 180 ◦C, den-ity = 840 kg/m3, kinematic viscosity at 40 ◦C = 1.0–2.0 cSt]. Allther reagents were of analytical grade. All chemicals weresed without any further purification. Double distilled wateras used in all aqueous preparations.

.2. Experimental procedures

rganic solutions were prepared by diluting DNNSA inerosene. Extractions were carried out as follows. The organichase and the aqueous phase containing magnesium ionsere pipetted into a beaker. The beaker was stirred at 200 rpmnd allowed to stand until phase separation was complete.n all experiments, the beaker was placed in a thermostat

eating mantle to maintain the required temperature. A 722aster spectrophotometer was used for measuring the mag-esium ion concentration in the aqueous phase (Wang and

Fig. 1 – Effect of time on the extraction of magnesium ions.

Chen, 2001). Errors in magnesium analysis were within ±3%.The equilibrium magnesium ion concentration in the organicphase was calculated by mass balance with the aqueousphase. Initially kinetic tests were performed to determine thetime for equilibration. The samples were obtained at differentintervals from the reaction solution.

The loaded organic phase was stripping by 1.0 M sulfuricacid. The stripping phase ratio was O/A = 1, T = 293 K, and strip-ping time = 20 min.

3. Results and discussion

3.1. Effect of extraction time

The kinetics of magnesium ion extraction is illustrated inFig. 1. The percentage extraction of magnesium ions wasincreased from 48.13 to 68.65% with a time range of 10–40 min.Since the plateau region of extraction versus time (Fig. 1)started at a contact period of 30 min, an equilibration periodof 40 min was used in further experiments at 200 rpm.

3.2. Effect of diluent

Four organic solvents were tested as diluents for the extractionof magnesium ions by 0.5 M DNNSA. They ranged in polarity (ε)from n-hexane with the lower polarity ε = 1.89 F/m to kerosene,phenoxin and chloroform with a polarity ε = 4.8 F/m. Extractiondata are compared in Table 1. The lower the polarity the bet-ter the extraction performance of the solvent. Kerosene usedextensively as diluent in commercial application showed sim-ilar performance compared with n-hexane and was thereforechosen for further experiments.

3.3. Effect of the H3PO4 concentration

In the WPA production, WPA concentration is up toP2O5 = 47 wt.% using concentrating phosphate, and using low-grade phosphate rock the WPA concentration is generallyP2O5 = 24 wt.%. The effect of the H3PO4 concentration on mag-nesium ion extraction was studied at two different H3PO4

concentrations, namely, 24 wt.% P2O5 and 47 wt.% P2O5, and ata constant DNNSA concentration of 0.5 M. The isotherm curveof magnesium ions is shown in Fig. 2.

An increase in the H3PO4 concentration is seen to havea significant depress on magnesium ion extraction. Theseresults are in agreement with others for similar systems

714 chemical engineering research and design 8 8 ( 2 0 1 0 ) 712–717

Table 1 – Effect of diluent on the extraction of magnesium (conditions: CHD(o) = 0.5 M; P2O5 = 24 wt.%; CMg2+(aq) = 0.05 M;textr = 40 min; O/A = 1:1; 303 K; 200 rpm).

Diluent n-Hexane Kerosene Phenoxin Chloroform

ε, F/m 1.89 2.0 2.238 4.806E, % 56.61 54.11 47.53 43.03Distribution coefficient 1.219 1.017 0.9056 0.7651

Fig. 2 – Distribution of magnesium between the raffinateand organic phases at different phosphoric acid Fig. 4 – Effect of extractant concentration on the

concentrations.

(Mohammad and El-Khaiary, 1997; Gaonkar and Neuman,1984; Kunugita et al., 1985).

3.4. Effect of the DNNSA concentration

Magnesium ion extraction from H3PO4 using different DNNSAconcentrations was studied.

Fig. 3 shows that an increase in the DNNSA concentrationhas a significant positive effect on magnesium ion extraction.These results are in agreement with others for similar systems(Mohammad and El-Khaiary, 1997; Gaonkar and Neuman,1984; Kunugita et al., 1985).

Fig. 4 shows that the distribution coefficient (D) definedas organic concentration divided by aqueous concentration

increases with an increase in the DNNSA concentration.DNNSA has a low critical micelle concentration of 10−5 M (Miki

Fig. 3 – Distribution isotherm curve for magnesium withvarious extractant concentrations.

distribution coefficient of magnesium.

et al., 1997). This compound easily forms reverse micellesin the organic phase, and it is possible that these reversemicelles may solubilize water and magnesium ions in internalwater pools. The water content was 7.3–8.3 (water to surfac-tant molar ratio) (Miki et al., 1997). If all the magnesium ionsextracted into the organic phase are present only in the inter-nal water pools, the distribution coefficient between thesewater pools and the aqueous phase can be calculated fromthe following equation (Miki et al., 1997):

Dwp = CMg(org)

�wpCMg(aq)= D

�wp,

where Dwp is the distribution coefficient between these waterpools and the aqueous phase, ϕwp is the volume fraction ofthe water pools in the organic phase and a constant, CMg2+(aq)

is the magnesium concentration in the aqueous phase, andCMg2+(org) is the magnesium concentration in the organicphase.

If the mechanism of magnesium extraction in this systemis simple solubilization in the water pools, the value of Dwp

should be constant. However, the value of Dwp increased withincreasing DNNSA concentrations, indicating that the mech-anism is more complex.

3.5. Effect of temperature

The effect of temperature on magnesium ions extraction in asystem consisting of 0.6 M DNNSA + kerosene was studied inthe temperature range 298–313 K.

Fig. 5 shows that increasing temperature is propitiousto extraction. When the temperature increases, the organicphase viscosity decreases. Consequently, the permeation rate

increases. However, it is difficult to increase the environ-mental temperature beyond 313 K. On the other hand, whenthe temperature increases, the volatilization and dissolu-

chemical engineering research and design 8 8 ( 2 0 1 0 ) 712–717 715

Ft

tir

3

Bdmoia

tpTr

wataam

b

K

Table 2 – Parameters for Eq. (6) determined by nonlinearregression.

Parameters

K m n p s d m

(HA)

gd+](

were as follows (Table 2), and the average deviation is 0.002295(Table 3). These values suggest that the MgA2·6HA complex is

ig. 5 – Distribution isotherms for magnesium at differentemperature.

ion of DNNSA increase, and extractant degradation alsoncreases. The optimal extraction temperature was in theange 298–303 K.

.6. Stoichiometric study of the extraction reaction

ecause the primary objective of the present work was toevelop a novel extractant for magnesium ions, detailedechanism investigations were not performed. However, in

rder to understand the novel extractant better, some insightnto the possible extraction mechanisms was provided by thelready available data.

The reaction stoichiometry is important for determininghe extraction mechanism. In the solvent extraction systemresented here, HA is probably present as micellar aggregate.he reaction for magnesium extraction can generally be rep-esented as follows:

npMg(aq)d+ + p(s + dn)

m(HA)m(org)

= (MgnAdn · sHA)p(org) + dnpH(aq)+ (1)

here the subscripts org and aq denote the organic phasend aqueous phase, respectively. HA denotes dinonylnaph-halene sulfonic acid (DNNSA); the subscript m represents theggregation number of the HA micelles; the subscript p is theggregation number of the complex; and s is the extractantolecule number being unbound Mg in reverse micelle.The equilibrium of the extraction reaction shown in (1) can

e characterized by

K =pn[Mgd+](aq)

np([

[Mgd+](org) =Kpn[M

=[(MgnAdn · sHA)p]

(org)[H+](aq)

dnp

[Mgd+](aq)np[(HA)m](org)

p(s+dn)/m. (2)

Value 59.6 8.0 1.0 1.0 6.0 1.9999 8

The above equation contains two unknown equilibriumconcentrations, namely, the equilibrium extractant con-centration and the complex concentration. These may besubstituted with formulae that contain only measurable con-centrations as shown below:

[(HA)m](org) = [(HA)m](org)total −(

(d + (s/n))m

)[Mgd+](org) (3)

and

[(MgnAdn · sHA)p](org)

=[Mgd+](org)

pn, (4)

where [(HD)m](o)total is the initial concentration of the extrac-tant. By substituting Eqs. (3) and (4) into Eq. (2), the followingresult is obtained:

[Mgd+](org)[H+](aq)

dnp

m](org)total − (1/m)(d + (s/n))[Mgd+](org))p(s+dn)/m

. (5)

Upon rearrangement, this can be represented as

aq)np

([(HA)m](org)total − ([Mgd+](org)(d + (s/n))/m))p(s+dn)/m

[H+](aq)dnp

. (6)

Nonlinear regression was used to determine the equilib-rium constant K and the unknown formation coefficient in thereaction model of Eq. (6). A computer program (1st0pt) basedon the Levenberg–Marquardt and Universal Global Optimiza-tion algorithm was used to apply nonlinear regression. Thevalues of the parameters determined by nonlinear regression

Fig. 6 – Comparison of the calculated and experimentalvalues of the magnesium concentration.

716 chemical engineering research and

Table 3 – Calculate results of the average deviation.

Analytical Calculate Deviation (absolute)

0.00474 0.00249 0.002250.00552 0.00852 0.0030.00583 0.00667 0.000840.00584 0.00284 0.0030.00608 0.00508 0.0010.01662 0.01395 0.002670.01799 0.01518 0.002810.01828 0.01945 0.001170.01836 0.01801 0.000350.01854 0.02196 0.003420.01927 0.01604 0.003230.02088 0.02296 0.002080.0216 0.0246 0.0030.02359 0.01852 0.005070.02372 0.01983 0.003890.02495 0.02394 0.001010.02511 0.0238 0.001310.02558 0.02227 0.003310.0272 0.02422 0.002980.02741 0.02889 0.001480.02843 0.03115 0.002720.03176 0.03352 0.001760.03449 0.03483 0.000340.03514 0.03754 0.0024

Average deviation 0.002295

Fig. 7 – McCabe–Thiele diagram for Mg extraction fromDNNSA.

formed according to the following reaction:

Mg(aq)2+ + (HA)8(org)

= (MgA2 · 6HA)(org) + 2H(aq)+ (7)

Fig. 6 graphically illustrates the results of the mathematicalanalysis. In this figure, the experimental values are plottedagainst the calculated values of the magnesium concentrationin the organic phase. From the figure, it can be seen that themodel fits well with the experimental data.

3.7. Commercial application of magnesium extractionfrom WPA with DNNSA

After studying the influence of various parameters on theextraction, it was found that the temperature effects on the

rate of magnesium ion extraction. Increasing temperaturecan improve extraction equilibrium. There was a qualitativeobservation that a high DNNSA concentration has no nega-

design 8 8 ( 2 0 1 0 ) 712–717

tive effect on phase separation in the given extraction system.The loaded organic phase was stripping by 1.0 M sulfuric acid.The percentage stripping of magnesium ions in the organicphase is 94.08%.

The isotherm curve at 303 K for the extraction of mag-nesium ions in H3PO4 by the use of 0.5 M DNNSA wasconstructed. The McCabe–Thiele diagram estimates the num-ber of theoretical stages needed to be four stages for extractionof about 97% magnesium. The results are presented in Fig. 7.

4. Conclusions

The following conclusions can be drawn from this study.

(1) HD can be used as an extractant for removing magnesium(+2) from H3PO4. Sulfuric acid can be used for magnesiumstripping. The extraction reaction of magnesium by HD isan endothermal reaction.

(2) Chemical reactions occur between magnesium ions andthe reverse micelles, and magnesium is extracted in theform of the MgA2·6HA complex.

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