Solvent Extraction Research and Development, Japan, Vol. 24, No 1, 23 – 35 (2017)

Purification of Wet Process Phosphoric Acid by Solvent Extraction using Cyclohexanol

Xing LI, Jun LI,* JianHong LUO,* Yang JIN and Da ZOU Department of Chemical Engineering, Sichuan University Chengdu, Sichuan 610065, P. R. China

(Received July 16, 2015; Accepted September 30, 2015)

Solvent extraction using organic solvents is an important technology for the purification of wet process

phosphoric acid (WPA). In this study, the purification process of WPA, using a new type of solvent

extraction system, cyclohexanol diluted in kerosene, was investigated. The equilibrium phase diagram of

the system H3PO4H2Osolvent mixtures at 313.2K was obtained. The effects of extraction time,

phosphoric acid concentration, cyclohexanol concentration, temperature, phase ratio on extraction

efficiency were studied. The extracted species was shown to be 2H3PO4 · 2C6H12O. The extraction process

of H3PO4 is exothermic and the enthalpy change, H, was obtained. Via a cross-current three-stage

extraction, the solvent mixtures have a high efficiency for phosphoric acid purification from the actual

industrial WPA with an H3PO4 extraction yield of 90.2% at room temperature. The study is capable of

enriching theoretical foundation and technical guidance for the phosphoric acid purification process.

1. Introduction

Phosphoric acid is an important mineral acid and commonly manufactured by two routes, thermal

and wet process. Wet process phosphoric acid (WPA) is a more economical and environmental-friendly

method of phosphoric acid production [1-3]. However, WPA contains various undesirable impurities and

must be purified during the downstream production. Many techniques have been investigated to purify

WPA such as crystallization, ion precipitation, solvent extraction and ion exchange. Improving

experimental results have led to growing acceptance of solvent extraction as an efficient, practical and

economical method which is now the most widely-used technique applied in the industrial production

process at present [4-6].

In the solvent extraction method, extensive research has been conducted to find a suitable extractant

which is efficient, of low toxicity with good flowability and selectivity to H3PO4. Many organic solvents

have been investigated, e.g. esters [7-8], ethers [9], ketones [10] and alcohols [11]. Aliphatic alcohols, such

as butanol, hexanol and octanol, have been studied as extractants diluted in kerosene for purification of

WPA. The experimental results show that alcohol-kerosene mixtures have the advantages of favorable

efficiency and selectivity to H3PO4 and are cheap, easy to use and they separate quickly without

emulsification and three phase formation [12]. Compared with these above-mentioned alcohol extractants,

cyclohexanol has good immiscibility with the aqueous solution and physicochemical stability due to its ring

structure. Cyclohexanol has been used for leaching and extraction. The experimental results indicate

favorable extraction efficiency of metallic ions in the ore leaching and good selectivity to iron in

liquid-liquid systems [13-14]. Kerosene is used as the diluent in an appropriate proportion to strengthen the

flowability and decrease the viscosity of the two-phase mixtures. On the other hand, kerosene will lower

the extraction ability in the solvent mixtures to some extent [15-16]. The cyclohexanol-kerosene mixture is

- 23 -

 

an extraction system which has not yet been studied for WPA purification.

The aim of this work was to study the solvent extraction of H3PO4 from industrial WPA with

cyclohexanol diluted in kerosene. In this study, the equilibrium phase diagram of the system H3PO4H2O

solvent mixtures at 313.2K was investigated. The effects of several parameters such as extraction time,

phosphoric acid concentration, cyclohexanol concentration, temperature and phase ratio on the extraction

were investigated. The extracted species and the value of the enthalpy change of the extraction reaction

were determined. A cross-current three-stage extraction as well as scrubbing and stripping of the loaded

organic phase was applied to obtain purified diluted phosphoric acid from industrial WPA.

2. Experimental

2.1 Materials

Cyclohexanol was obtained from Jinshan Chemical Reagent Co. (Sichuan, China). Pure H3PO4 (≥

85%) was obtained from Kelong Chemical Reagent Co. (Sichuan, China). The reagents were both

analytically pure and used without further pretreated. Kerosene was provided by Zhongcui Chemical Co.

(Sichuan, China). Before use, the kerosene was washed with concentrated sulfuric acid then neutralized

with 5% Na2CO3 solution and washed with water until the pH was neutral and finally distilled at

458.2-528.2 K. The industrial raw WPA was supplied by Furui Chemical Plant (Yunnan, China), which was

obtained by decomposing phosphate ore using sulfuric acid. The composition of the industrial WPA is

shown in Table 1.

Table 1. Composition of the industrial WPA supplied by Furui Chemical Plant.

Composition H3PO4 SO42- F- Fe3+ Al3+ Mg2+ Zn2+ Cr3+

wt % 64.51 3.7350 0.3406 0.3550 0.6279 0.8596 0.0410 0.0058

2.2 Procedure

The equilibrium studies were carried out at T= 313.2 K in a water thermostat (± 0.1 K). Known

amounts of diluted pure phosphoric acid and solvent mixtures were mixed and shaken for 2 h and then left

to settle for 2 h to completely separate the two liquid phases. After separating thoroughly, the organic phase

and the aqueous phase were weighed and taken for analysis.

The extraction temperature was controlled and maintained in a water thermostat (± 0.1 K). Known

amounts of wet process phosphoric acid were diluted with deionized water to prepare the aqueous phase

while cyclohexanol was mixed with kerosene in an appropriate proportion to prepare the organic phase.

The prepared phosphoric acid and solvent mixtures were then mixed and stirred for a certain time. After

standing for a sufficient time, the mixtures completely separated into two equilibrated phases. The organic

phase and the raffinate aqueous phase were weighed and taken for analysis. The volume of the liquid

phases was also determined (± 0.1 mL).

The concentration of H3PO4 in the aqueous solution was identified by the phosphorus molybdic acid

quinoline weight method [17]. The water content in the organic phase was determined by the Karl-Fisher

potentiometric titration method using a compact potentiometric titrator (Metrohm 916 Ti-Touch,

Switzerland) [11]. Measurement of Fe3+ concentration in the aqueous solution was performed using a

UV-Visible spectrophotometer (Mapada UV-1100, China) at a wavelength of 510 nm [17]. The fluoride

- 24 -

 

composition in the aqueous phase was determined by an ion meter (Radiometer PHM240, France) using a

fluoride selective electrode (ISE25F) with a saturated calomel electrode as the reference electrode. The

concentration of SO42- in the aqueous solution was identified using a UV-Visible spectrophotometer at a

wavelength of 440 nm [17]. Measurement of the concentration of other metallic ions (Al3+, Mg2+, Zn2+,

Cr3+) in the aqueous solution was performed with an ICP-AES (Thermo elemental iris advantage, USA).

FT-IR spectra (Nicolet 7100, USA) was used for the analysis of organic phase. The concentration of H3PO4

and other components in the organic phase were obtained from material balances. Deionized water was

used throughout the experiments.

The distribution coefficient (D) is calculated as

( )

( )

[ ]

[ ]o

a

cD

c (1),

where ( )[ ] oc and ( )[ ] ac are the molar concentrations of the component in the organic and aqueous phases

at equilibrium, respectively.

The separation factor (β) is calculated as

3 4H PO

i

Dβ =

D (2),

where 3 4

H POD and iD are the distribution coefficients of H3PO4 and the impurity, respectively.

The extraction yield (E %) of H3PO4 is calculated as

Mass of H PO in the organic phase% %

Mass of H PO in the initial solution

E

3 4

3 4

100 (3)

3. Results and Discussion

3.1 Equilibrium studies of the extraction system H3PO4H2Osolvent mixtures

In this extraction system, it is necessary to establish the phase diagram of the system H3PO4H2Osolvent mixtures to reveal the distribution of H3PO4 between the aqueous and solvent phase. The solvent

mixture consists of 75% cyclohexanol and 25% kerosene. The composition of the conjugated phases is

obtained from material balances. All the percentages in this section represent the mass fractions (wt %).

The equilibrium phase diagram of the system H3PO4H2Osolvent mixtures at 313.2 K is shown in

Figure 1. The conjugated phases are connected with tie-lines for each mixture. The plait-point marked in

the phase diagram is established according to the Hand method [18]. The composition data of the initial

mixtures and equilibrium phases at 313.2 K are given in Table 2. As the results show, the pseudo-ternary

system has two completely miscible liquid pairs which are water + H3PO4 and solvent mixtures + H3PO4. It

also has one partially miscible liquid pair, water + solvent mixtures. The plait-point of the system

demonstrates favorable capability of the solvent mixtures for purifying WPA due to the ability of the

organic phase to load high H3PO4 concentrations which is over 60%. The results also show that the solvent

mixtures have low solubility in the aqueous solution. In addition, the solubility of water in the solvent

mixtures increases slowly from 11.23% to 14.18% as the mass fraction of H3PO4 increases from 4.15% to

45.79% in the organic phase. The phase equilibrium study of the system indicates the solvent mixtures can

be considered as favorable candidates for the purification of WPA.

- 25 -

Figure 1. Equilibrium phase diagram of the extraction system H3PO4H2Osolvent mixtures (75%

cyclohexanol +25% kerosene) at 313.2K.

Table 2. Equilibrium data of the extraction system H3PO4 (1) H2O (2) solvent mixtures (75%

cyclohexanol +25% kerosene) (3) at 313.2K. a

Initial mixtures Aqueous phase Organic phase

w1 w2 w3 w1 w2 w3 w1 w2 w3

51.15 15.39 33.46 75.35 20.73 3.92 45.79 14.18 40.03

43.06 17.86 39.08 65.44 30.65 3.91 36.61 13.28 50.11

36.15 20.74 43.11 57.64 38.49 3.87 27.92 12.39 59.69

30.93 23.32 45.75 48.32 47.72 3.96 24.08 11.91 64.01

26.52 25.17 48.31 43.45 52.57 3.98 18.91 11.67 69.42

20.76 28.47 50.77 36.60 59.54 3.86 13.10 11.58 75.32

13.85 31.92 54.23 27.34 68.75 3.91 7.40 11.37 81.23

7.20 35.13 57.67 14.88 81.13 3.99 4.15 11.31 84.54

38.76 61.24 96.05 3.95 11.23 88.77 a w is the mass fraction (%) ; blanks: does not exist or not detected.

3.2 Effect of extraction time

The effect of extraction time, ranging from 3min to 30min, on the extraction efficiency was

investigated (Figure 2). The distribution coefficients of H3PO4 increased rapidly from 0.14 to 0.23 in the

time range of 3-10 min and then it remained almost constant. The results indicate that it only took 10 min to

attain extraction equilibrium. The separation factors of Fe3+ and SO42- stop increasing when the extraction

time was over 15min. The extraction time of 15 min was used in the further experiments to ensure that

extraction equilibrium was achieved. In addition, excessive extraction time will lead to poor yields of

phosphoric acid, poor selectivity and longer phase separation times [19].

- 26 -

 

Figure 2. Effect of extraction time on the distribution coefficient of H3PO4 and the separation factor for

Fe3+ and SO42-; [Cyclohexanol] = 1.90 mol/L, [H3PO4] = 5.07 mol/L, O/A = 4, T = 313.2K: (■)

3 4H POD ; ( )

β for Fe3+; (○) β for SO42-.

3.3 The extracted complex

It is useful to know the stoichiometry of the extracted complex, which can be determined from the

extraction equilibrium studies. Assuming that phosphoric acid transfers into the organic phase in a neutral

form and exists in the organic phase as a complex of the form pH3PO4 · qC6H12O [20]. The extraction of

H3PO4 from the phosphoric acid solution by cyclohexanol can be described as

( ) ( ) ( )C H O C H O(H PO ) ( ) (H PO ) ( )a o op q p q 6 12 6 123 4 3 4 (4),

where the subscripts a and o stand for the aqueous and organic phases, respectively. p and q are the

stoichiometric coefficients of H3PO4 and cyclohexanol in the reaction above, respectively.

The equilibrium constant (Keq) of the extraction reaction in Eq. (4) is

( )

( ) ( )

[ H PO ]

[H

C H O

C H OPO ] [ ]o

eq = p q

a o

p qK

3 4

3 4

6 12

6 12

(5)

The distribution coefficient of H3PO4 can be calculated as

( ) ( )

( ) ( )

[H PO ] [ H PO ]

[H PO

C H O

] [H PO ]o o

a a

p p qD

63 4 3 4

3 4 3 4

12 (6)

An indirect approximation method based on slope analysis was used to verify the stoichiometry of the

extracted species. Inserting Eq. (6) into Eq. (5) and then taking the logarithm of both sides of the resulting

equation leads to the expression

( ) ( )log log ( ) log [H PO ] lo [ ]Hg C Oeq a oD pK p q 3 6 1241 (7)

Inserting Eq. (6) into Eq. (7) gives

( ) ( ) ( )log [H PO ] log log [H PO ] log H[C O]o eq a o pK p q 6 123 4 3 4 (8)

- 27 -

 

After determining the values of p and q in the extraction reaction in Eq. (4), the extracted species can then

be determined.

3.3.1 Effect of H3PO4 concentration

The effect of H3PO4 concentration in the range of 1.63-6.94 mol/L on the extraction was studied.

The concentration of cyclohexanol in the initial organic phase remained 1.90 mol/L and the temperature

was maintained constant at 313.2K. The equilibrium distribution results for pure H3PO4 and the industrial

WPA used in this work (Table 1) were compared. Plots of log [H3PO4](o) versus log [H3PO4](a) are shown in

Figure 3. Using regression analysis, the following relations can be established from the obtained data:

The pure H3PO4cyclohexanol system:

( ) ( )log [H PO ] 1.58 1.96 log [H PO ]o a 3 4 3 4 , R = 0.999 (9)

The industrial WPAcyclohexanol system:

( ) ( )log [H PO ] 1.66 2.01 log [H PO ]o a 3 4 3 4 , R = 0.998 (10),

where R is the correlation coefficient.

The H3PO4 concentration in the organic phase increased as the H3PO4 concentration in the aqueous

phase increased. For the same condition, the [H3PO4](o) of pure H3PO4 was higher than that of the industrial

WPA. This is mainly because the co-extraction of some impurities reduces the extraction efficiency of the

solvent mixtures. The slopes of the two lines are 1.96 and 2.01, respectively. According to the slopes of the

two lines, p in Eq. (4) equals 2 approximately.

Figure 3. Equilibrium isotherms for the analytical concentrations of the organic phase [H3PO4](o) and the

aqueous phase [H3PO4](a) for the extraction; [C6H12O] = 1.90 mol/L, O/A = 4, T = 313.2K: (□) pure H3PO4;

(■) industrial WPA.

3.3.2 Effect of cyclohexanol concentration

The effect of cyclohexanol concentration on the extraction was studied. The cyclohexanol

concentration was varied in the range of 1.64-1.90 mol/L. The concentration of H3PO4 in the initial aqueous

- 28 -

phase remained at 2.85 mol/L and the temperature was maintained constant at 313.2K. As Figure 4 shows,

the distribution coefficients increased with an increase in the cyclohexanol concentration. It is preferable to

have a high extractant concentration for better extraction efficiency. Using regression analysis, the

following relation was established from the obtained data:

log 1.15 1.90 l C H Oog [ ]D 6 12 , R = 0.998 (11)

According to Eq. (11), the slope of the plot of log D versus log [C6H12O] is 1.90, which would give the

number of molecules of [C6H12O] engaged in the reaction (Eq. (4)). Therefore q in Eq. (4) approximately

equals 2.

Figure 4. Effect of cyclohexanol concentration on the distribution coefficient of H3PO4; [H3PO4] = 2.85

mol/L, O/A = 4, T = 313.2K.

3.3.3 IR absorption spectrum of the extracted complex

According to 3.3.1-3.3.2, the extraction mechanism of H3PO4 extraction with cyclohexanol can be

represented as follows:

( ) ( ) ( )2(H PO ) 2( ) 2(H PO ) 2(C H C )O H Oa o o 3 4 3 46 12 6 12 (12)

The possible extracted complex of H3PO4–C6H12O is 2H3PO4 · 2C6H12O.

The IR spectrum of the extracted complex H3PO4–C6H12O (B) is shown in Figure 5, compared with

that of cyclohexanol (A) before extraction. The –OH bond stretching vibration at 3345 cm-1 in

cyclohexanol is shifted to a higher frequency at 3382 cm-1 and the peak is seen to be widened. This is

because of certain amounts of –OH bonds introduced with H3PO4 and H2O after complexation. In addition,

a peak is observed at 1642 cm-1, which is also ascribed to the stretching vibration of the –OH group. The

peak at 1069 cm-1 may be assigned to the stretching vibration of the C–O bond in cyclohexanol. In the

extracted complex, due to hydrogen bond formation, the electron cloud is distributed more averagely and 

the C–O bond stretching vibration frequency is lowered to 1067 cm-1.

- 29 -

 

Figure 5. IR spectrum of cyclohexanol (A) and the H3PO4-cyclohexanol complex (B).

3.4 Effect of temperature

The effect of temperature in the range of 293.2K-338.2K on the extraction was investigated. The

concentration of H3PO4 in the initial aqueous phase and the concentration of cyclohexanol in the initial

organic phase remained as 5.07 mol/L and 1.90 mol/L, respectively.

Plots of log D versus 1/T are shown in Figure 6. Using regression analysis, the following relations

can be established from the obtained data:

H3PO4, in the range of 293.2K-308.2K:

3log 0.72 0.03 [1/ 10 ]D T , R = 0.999 (13)

H3PO4, in the range of 308.2K-338.2K:

3log 1.03 0.12 [1/ 10 ]D T , R = 0.997 (14)

Fe3+, in the range of 293.2K-338.2K:

3log 0.27 0.42 [1/ 10 ]D T , R = 0.997 (15)

SO42-, in the range of 293.2K-338.2K:

3log 0.29 0. 1/31 [ ]10D T , R = 0.998 (16)

According to the Van’t Hoff equation [21]

Δ log Δ

Δ(1 / ) 2.303

D H

T R (17),

the distribution coefficients of H3PO4 decrease with an increase in temperature. In the range of

293.2K-308.2K, the value of the enthalpy change H of H3PO4 extraction is -0.590 kJ/mol and the value of

H is -2.434 kJ/mol when the temperature is over 308.2K, which is calculated from the slope of the linear

relation. This result indicates that the extraction reaction of H3PO4 is exothermic and the thermal effect will

increase at a certain temperatures. The volatilization of the solvent also increases at higher temperature.

- 30 -

 

Increasing temperature has an adverse effect on the extraction efficiency, which is in agreement with the

previous study [10]. On the other hand, the distribution coefficients of Fe3+ and SO42- increase with higher

temperature. According to Eq. (15) and Eq. (16), the value of the enthalpy change for the extraction of Fe3+,

H, is 8.072 kJ/mol and that of SO42- the H is 5.876 kJ/mol, which indicates that the co-extraction

reaction of these two impurities is endothermic and the effect of temperature on the SO42- extraction is

smaller than that for Fe3+. Higher temperature has a negative effect on the H3PO4 extraction selectivity. The

selected optimal extraction temperature is 298.2K. The results show that the extraction system can perform

well at room temperature with low energy consumption and good selectivity so that it has good prospects

for industrial application.

Figure 6. Effect of temperature on distribution coefficients of H3PO4, Fe3+ and SO4

2-; [Cyclohexanol] =

1.90 mol/L, [H3PO4] = 5.07 mol/L, O/A = 4: (■) H3PO4; ( ) Fe3+; (○) SO42-.

3.5 Effect of phase ratio

The effect of phase ratio (O/A) ranging from 1.0 to 5.0 on the extraction was studied. The

concentration of H3PO4 in the initial aqueous phase and the concentrations of cyclohexanol in the initial

organic phase remained as 5.07 mol/L and 1.90 mol/L, respectively. The temperature was maintained

constant at 298.2K.

Figure 7 shows that the extraction yields of H3PO4 increase from 51.0% to 80.2% as the phase ratio

is raised from 1.0 to 5.0. The result indicates that a higher extraction yield is obtained with a higher phase

ratio. This is because when the phosphate content in the aqueous phase is constant, increasing the phase

ratio can lead to higher extraction equilibrium values, thereby increasing the extraction yield. In addition, a

higher phase ratio gives better selectivity over impurities but higher viscosity of the two-phase mixtures

and more solvent consumption [7].

- 31 -

 

Figure 7. Effect of phase ratio O/A on extraction yield of H3PO4 and separation factor for Fe3+ and SO4

2-

impurities: [Cyclohexanol] = 1.90 mol/L, [H3PO4] = 5.07 mol/L, T = 298.2K: (■) E %; ( ) β for Fe3+; (○) β

for SO42-.

3.6 Cross-current multi-stage extraction of H3PO4

In order to predict the number of the theoretical stages required for the extraction process, a diagram

for the extraction by cyclohexanol diluted in kerosene was constructed at 298.2K [22]. Using regression

analysis, the following relations of the distribution of H3PO4 in the organic and aqueous phases were

established from the obtained equilibrium data: 20.003 0.109 + 0.661Y X X , R = 0.996 (18)

where Y and X are the mass fraction of H3PO4 in the organic and aqueous phases, respectively.

From Figure 8, it can be calculated that the cumulative extraction yield of H3PO4 is 54.6%, 78.4%,

90.7% and 97.9%, respectively for each stage. The multi-stage extraction of H3PO4 from the industrial

WPA (Table 1) was conducted with the H3PO4 concentration in the initial aqueous phase and the

cyclohexanol concentration in the initial organic phase kept at 5.07 mol/L and 1.90 mol/L, respectively. The

temperature was maintained constant at 298.2K and the phase ratio, O/A, was 3. The calculated and

experimental values of the mass fraction of H3PO4 for each stage are compared in Table 3. The

experimental values correspond well with the calculated ones. The cumulative extraction yield of H3PO4 is

56.1%, 79.2%, 90.2% and 97.6%, respectively for each stage. Considering the H3PO4 yield and the

consumption of solvent, three-stage cross-current extraction is chosen as the optimal operation condition,

which can obtain a high extraction yield. On the other hand, kerosene used as the diluent in the solvent

mixtures can strengthen the flowability and decrease the viscosity of the two-phase mixtures but will lower

the extraction ability at the same time. Thus a balance between the extraction efficiency and the operating

conditions in the industrial application must be achieved. It can be concluded that cyclohexanol diluted in

kerosene is a good extractant for H3PO4 from industrial WPA.

- 32 -

 

Figure 8. Diagram for the H3PO4 extraction by cyclohexanol and kerosene mixtures: [Cyclohexanol] = 1.90

mol/L, [H3PO4] = 5.07 mol/L, O/A = 4, T = 298.2K.

Table 3. Comparison between the calculated and experimental value of the H3PO4 concentration

of each stage (X for aqueous phase, Y for organic phase).

3.7 Scrubbing and stripping

The organic phase loaded with H3PO4 after extraction still contains impurities including equilibrium

concentrations of some trace metallic impurities (Fe3+, Mg2+, Al3+, et.) and the co-extracted fluoride and

sulfate ions. Scrubbing of the organic phase using deionized water at a 10:1 O/A phase ratio is thus

conducted to remove these undesirable impurities. After scrubbing, the stripping of H3PO4 from the

scrubbed loaded organic phase can be conducted at a 4:1 O/A phase ratio through three stages cross-current

re-extraction using deionized water. The composition of the processed WPA is shown in Table 4. The

scrubbing and stripping operation was carried out at a constant temperature of 313.2K.

As Table 4 shows, scrubbing of the organic phase using deionized water is able to remove the

impurities efficiently after extraction. The loss of H3PO4 in the scrubbing process is less than 0.5% from the

loaded organic phase. By the above purifying process of extraction, scrubbing and stripping, the metallic

impurities are successfully removed to less than 5ppm in the purified diluted PA without any other

purifying technology.

Ion precipitation of sulfate using calcium carbonate or barium carbonate is generally conducted

before or after extraction to remove the sulfate ions further. The fluoride ions remaining in the purified

diluted PA after re-extraction can be efficiently removed in the concentration operation to produce PA of

food and analytical grade [23].

X, Y

(wt%) X1 Y1 X2 Y2 X3 Y3 X4 Y4

Calculated 29.32 6.28 13.94 2.73 5.98 1.41 1.37 0.80

Experimental 28.32 6.49 13.42 2.82 6.32 1.34 1.55 0.75

- 33 -

Table 4. Composition of different types of phosphoric acid.

Composition H3PO4

(wt %)

F-

(ppm)

SO42-

(ppm)

Fe3+

(ppm)

Al3+

(ppm)

Mg2+

(ppm)

Zn2+

(ppm)

Cr3+

(ppm)

Extracted

organic phase 4.81 20.4 114.2 16.2 6.1 5.5 N.D N.D

Scrubbed

organic phase 4.49 15.1 74.7 5.1 1.1 1.3 N.D N.D

Stripped

phosphoric acid 15.76 8.2 23.4 3.0 1.2 1.8 N.D N.D

4. Conclusion

In this work, purification of wet process phosphoric acid by solvent extraction with using

cyclohexanol diluted in kerosene has been studied. The equilibrium phase diagram of the system H3PO4H2Osolvent mixtures (75% cyclohexanol + 25% kerosene) at 313.2K was obtained. The results show that

it takes 10 min to attain extraction equilibrium. Cyclohexanol concentration and phosphoric acid

concentration both have a positive effect on H3PO4 extraction. Therefore, the WPA can be pre-concentrated

to an appropriate degree to increase the purifying efficiency. The extracted species is demonstrated to be

2H3PO4 · 2C6H12O. The H3PO4 extraction process is exothermic with an enthalpy change, H, of -0.590

kJ/mol in the temperature range of 293.2K-308.2K and H is -2.434 kJ/mol when the temperature is over

308.2K. The solvent mixture have a high efficiency for phosphoric acid purification from industrial WPA

with an H3PO4 extraction yield of 90.2% via a cross-current three-stage extraction. Although still

experimental，the solvent mixture has the potential for the extraction system to be mixed with other

solvents that may lead to better immiscibility with the aqueous solution. Based on the low power

consumption and recycling of solvent, environment pollution is expected to be less than for other

techniques for WPA purification. The new process has good prospects for industrial application.

Acknowledgement

Project supported by the National Natural Science Foundation of China (No. 21306116) and the

Technology Commission Foundation of Sichuan Province of China (No. 2014JY0079).

References

1) A.A. El-Asmy, H.M. Serag, M.A. Mahdy, M.I. Amin, Sep. Purif. Technol., 61, 287-293 (2008).

2) N. Boulkroune, A.H. Meniai, Energ. Procedia, 18, 1189-1198 (2012).

3) L. Monser, M.B. Amor, M. Ksibi, Chem. Eng. Process, 38, 267-271 (1999).

4) M. Takahara, Production of phosphoric acid of high purity, US Patent, 3,917,805 (1975).

5) R. Kijkowska, D. Pawlowska-Kozinska, Z. Kowalski, M. Jodko, Z. Wzorek, Sep. Purif. Technol., 28,

197-205 (2002).

6) M.B. Elleuch, M.B. Amor, G. Pourcelly, Sep. Purif. Technol., 51, 285-290 (2006).

7) M.Y. Huang, B.H. Zhong, J. Li, J. Chem. Eng. Data, 53, 2029-2032 (2008).

8) H. Ahmed, H. Diamonta, C. Chaker, R. Abdelhamid, Sep. Purif. Technol., 55, 212-216 (2007).

- 34 -

9) B. Ahlem, M. Abdeslam-Hassen, B. L. Mossaab, Chem. Eng. Technol., 24, 1273-1280 (2001).

10) M. Feki, M. Stambouli, D. Pareau, H.F. Ayedi, Chem. Eng. J., 88, 71-80 (2002).

11) C.W. Ye, J. Li, J. Chem. Technol. Biot., 88, 1715-1720 (2013).

12) M.I. Amin, M.M. Ali, H.M. Kamal, A.M. Youssef, M.A. Akl, Hydrometallurgy, 105, 115-119 (2010).

13) N.M. Dacia, E.M. Hoxhaa, G. Vujičić, Fuel, 64, 520-522(1985).

14) R.K. Mishra, P.C. Rout, K. Sarangi, K.C. Nathsarma, Hydrometallurgy, 108, 93-99(2011).

15) Y. Jin, J. Li, J.H. Luo, D.S. Zheng, L. Liu, J. Chem. Eng. Data, 55, 3196-3199 (2010).

16) R. Sanghani, Procedia Eng., 83, 225-232 (2014).

17) Y. Zhang, G. Lin, Z. Li, Analysis for Ammonium Phosphate Production, first ed., Chengdu University

of Science and Technology Press, Chengdu (1991).

18) L.A. Robbins, R.W. Cusack, Liquid–liquid extraction operations and equipment, R.H. Perry, D.W.

Green, J.O. Maloney (Eds.), Perry’s Chemical Engineers Handbook, seventh ed., Mac Graw-Hill, New

York (1997).

19) H.H. Nejad, M. Kazemeini, Procedia Eng., 42, 1302-1312 (2012).

20) R. Dhouib-Sahnoun, M. Feki, H.F. Ayedi, J. Chem. Eng. Data, 47, 861-866 (2002).

21) U.K. Deiters, Fluid Phase Equilib., 336, 22-27 (2012).

22) Y. Jin, Y.J. Ma, Y.L. Weng, X.H. Jia, J. Li, J. Ind. Eng. Chem., 20, 3446-3452 (2014)

23) L.S. Wang, Z.Q. Long, X.W. Huang, Y. Yu, D.L. Cui, G.C. Zhang, Hydrometallurgy, 101, 41-47

(2010).

- 35 -