Journal of Nuclear and Radiochemical Sciences, Vol. 5, No. 1, pp. 11-15, 2004

RevisedSulphide Precipitation of Iron and its Effect on the Extraction of Uranium from
Phosphoric Acid Medium
Y. A. El-Nadi and J. A. Daoud*
Hot Laboratories Centre, Atomic Energy Authority, 13759, Cairo, Egypt
Received: March 1, 2004; In Final Form: June 12, 2004
Investigations on the removal of iron from phosphoric acid by precipitation method were carried out using pureand commercial sodium sulphides. The effect of iron on the extraction of uranium from phosphoric acid using a
mixture of di-2-ethylhexyl phosphoric acid (HDEHP) and a commercial trioctyl phosphine oxide (CYANEX 921)
in kerosene has been investigated. The influence of HDEHP, CYANEX 921, Fe(III), sulphide concentrations aswell as temperature on the extraction of U(VI) from phosphoric acid were separately investigated. The results arecompared and optimum conditions for monitoring the effect of iron and sulphide on the extraction of uranium
from phosphoric acid are proposed.

1. Introduction
Phosphoric acid produced by Wet-Process contains small
amounts of uranium together with many kinds of impurities,
such as sulphate, fluorine compounds, iron, aluminum, magnesium, and calcium, as well as organic impurities.1 The presence of such impurities in phosphoric acid adversely affects
the process performance as well as the quality of produced
acid. Further, the dissolved iron has a strong effect on
increasing the acid viscosity and consequently decreasing thefiltration rate. Iron also forms precipitates during concentration, clarification, and storage causing sludge problems accompanied by P2O5 losses. In addition, the P2O5 water-solubility of
the fertilizers produced from high iron acids is low.2
The earliest known purification methods of phosphoric acidare based on the precipitation of sparingly soluble salts. Defluorination by Na2SiF6 or MgSiF6·6H2O precipitations are
traditionally used methods. 3, 4 Some companies combine
precipitation of barium sulphate with iron and the excess ofbarium for sulphate precipitation is removed from the acidusing cationite. 5 The precipitation of heavy metals as
sulphides also constitutes one of the used methods. 6 The
degree of phosphoric acid purification was found to be higher
when extraction technique with organic solvents is applied.7−11
In this context, recovery of uranium from phosphoric acid bysolvent extraction using synergic solvent, di-2-ethylhexyl
phosphoric acid – trioctyl phosphine oxide (HDEHP – TOPO),
diluted in kerosene was exceedingly investigated.12−18 Khorfan
et al.19 showed that Al(III) and Ca(II) have a marked beneficial
effect on uranium extraction from phosphoric acid medium,
while Mg(II), Fe(III), and V(V) have a negligible effect. An
increase in Fe(II) led to a decrease in the extraction of uranium
from the same medium. Stas et al.20 have established twomathematical models representing the effect of five parameters(uranium, iron, HDEHP concentration, P2O5%, and organic/
aqueous ratio) on the selectivity and yield of U(VI) extraction,
allowing the choice of the desired experimental conditions andestimation of the purity of the loaded solvent (U org / Fe org ).
These authors reported that the choice of high selectivity andyield is very difficult since factors that increase the ratio(Uorg / Feorg) decrease the yield of uranium extraction and viceversa.

Since Egyptian commercial phosphoric acid usuallycontains a significant amount of iron as Fe(III) (about 2.5 g/
100 mL), its removal becomes of major interest as the presenceof this ratio affects the extraction of uranium and hence thepurification of the acid. The main objective of this investigation is to study the factors affecting the removal of iron as
Fe(III) from phosphoric acid by precipitation and its effect on
the extraction of uranium. In this context, the precipitation ofiron was carried out with pure and commercial sodium
sulphide in order to remove or decrease its concentration to thelowest possible value and assess the conditions for maximum
extraction of uranium from phosphoric acid.
2. Experimental
2.1. Chemicals and Reagents. Uranyl nitrate was an (AR)Merck product. Orthophosphoric acid (85%) and ferric chloride were obtained from Adwic (Egypt). Pure sodium sulphide
was supplied from Winlab (England), while its commercial
form was a product of Solvay (Italy). HDEHP was a productof Union Carbide Corporation, USA and CYANEX 921,
which is a commercial trioctylphosphine oxide containing 99%
of the active substance,21 was kindly supplied by Cytec Inc.,
USA and used as received.
2.2. Procedure. Iron was spectrophotometrically determined using xylenol orange method.22 This method was modified to work in phosphoric acid medium and the effect of acidconcentration on the absorbance of Fe(III) plotted in Figure 1
indicated that 0.03 M phosphoric acid is the optimum concentration for this method. The absorbance of the complex
reached a maximum value at 550 nm after five minutes andwas stable for two hours. U(VI) was measured at 665 nm by
Arsenazo-III method.23 Both methods gave no interferencewith each other. The precipitation of Fe(III) was carried out
by adding the sodium sulphide solid to the phosphoric acidsolution with continuous stirring followed by centrifugationand filtration to remove the precipitate. Unless otherwise
stated, the aqueous phase contained 100 ppm U(VI) and/or 25
g/L Fe(III) in 5 M phosphoric acid. The extraction procedure
was performed by shaking equal volumes of the aqueous andorganic phases in stoppered glass tubes using thermostatedshaking water bath.

*Corresponding author. E-mail: jadaoud@hotmail.com. FAX:
+202-6352474.

© 2004 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/23/2004

12

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

El-Nadi0.5100

λ max = 550 nm

32

90

0.4

30

80

0.3

0.2

26

60
50

24

40

P2O5 / %Precipitation / %Absorbance

28
70

22

30
20

0.1

20
18

10

0.0

0.001

0.01

0.1

0

1

[H3PO4] / M

0

5

10

15

20

25

30

16
40

35

Na2S / g/100 mL

Figure 1. Effect of H3PO4 concentration on the absorbance of Fe(III)complex with xylenol orange indicator at λ = 550 nm.

Figure 2. Effect of sodium sulphide on P2O5 and the precipitation
percent of Fe(III) from 5 M phosphoric acid.
100

3. Results and Discussion

3+

2Fe + 3S

2−

2FeS↓ + S↓.

(1)

In this concern, precipitation investigations were carried out
using sodium sulphide in its pure and commercial forms. The
results shown in Table 1 indicate that the precipitation percentof iron increases with increasing the sulphide concentration
with higher values in favor of the pure form. The amount of
sodium sulphide added seemed to be much more than the
expected stoichiometric value which may be due to the partial
consumption of sulphide in reaction with phosphoric acid. It
was also found that sodium sulphide has almost no effect on
the precipitation of U(VI) present in phosphoric acid within
the used concentration range.
TABLE 1: Comparison between Pure and Commercial
Sodium Sulphide in the Precipitation of 25 g/L Iron from 5
M Phosphoric Acid Medium
Na2S/ g/100 mL
5
10
15
20
25
30
35

Iron Precipitation / %Pure Form
Commercial Form
15.8
27.6
57.6
73.5
81.8

8.2
19.0
28.8
48.8
62.8
67.2
72.8

3.1.1. Effect of Sodium Sulphide. The precipitation percent
of Fe(III) plotted in Figure 2 as a relation among the amount ofcommercial sodium sulphide in g/100 mL, iron precipitation%, and P2O5% show that the precipitation percent of Fe(III)gradually increases with the addition of sodium sulphidereaching almost 75% precipitation with 35 g of sodium
sulphide per 100 mL phosphoric acid solution. This amount ofsulphide is equivalent to double the stoichiometric ratio necessary to precipitate the total amount of Fe(III) according to theabove reaction (eq 1). More addition of sulphide caused the
agglutination of the solution making the filtration very difficultand giving no significant enhancement in Fe(III) precipitation.
On the other hand, the phosphoric acid concentration
decreased from 5 M (30% P 2 O 5 ) to about 3 M (18% P 2 O 5 )Precipitation / %3.1. Precipitation Process. Sodium sulphide leads to theprecipitation of iron according to24
80

60

40

0

10

20

30

40

50

60

70

Temperature / ˚CFigure 3. Effect of temperature on the precipitation of Fe(III) from 5
M H3PO4 by 25 g sodium sulphide per 100 mL H3PO4.

upon addition of sulphide which may limit the use of large
amounts of sulphide to remove Fe(III). This decrease in theacid concentration may be explained by the reaction between apart of phosphoric acid and sodium sulphide leading to formation of H2S gas.24
3.1.2. Effect of Time and Temperature. The effect of timeon the precipitation of iron from phosphoric acid studied over
the range of 1 – 60 minutes showed that ten minutes are quite
sufficient for complete precipitation. The increase in temperature in the range of 15 – 55˚C indicates that the temperature
has a positive effect on the precipitation of Fe(III) from phosphoric acid, see Figure 3.
3.2. Extraction Process. A mixture of HDEHP and CYANEX
921 (total concentration 1.0 M) was used to investigate theextraction of U(VI) and Fe(III) from phosphoric acid. The
extraction percent (%E) of U(VI) was found to increase withincreasing the molar ratio of the above mixture up to 4:1 thendecreases with further increase, as reported.20, 21 On the other
hand, the extraction of Fe(III) did not exceed 1.4% with thesame extractants as shown in Figure 4. This is in good agreement with the published data which show that, in absence ofchloride, no extraction for Fe(III) takes place from nitric,
sulphuric, perchloric, or phosphoric acid.25 Therefore, the ratio
4:1 (HDEHP:CYANEX 921) was selected for extraction of
U(VI) from 5 M H3PO4 in presence of Fe(III) ions.
3.2.1. Effect of Extractant. Plotting the distribution ratio
values of U(VI) versus HDEHP concentrations in the range of
0.5 – 1.2 M or CYANEX 921 concentrations in the range ofSulphide Precipitation of Iron and its EffectJ. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

[Cyanex-921] / M
1.0
100

0.8

0.6

0.4

13

8.5
0.2

0.0

8.0

7.5

ln KexExtraction Percent (%E )

80

60

7.0
40

6.5
20

U(VI)
Fe(III)

0
0.0

6.0
3.0
0.2

0.4

0.6

0.8

[HDEHP] / M

Figure 4. Effect of synergism of HDEHP and CYANEX 921 on theextraction of U(VI) and Fe(III) from 5 M H3PO4.
[CYANEX 921] / M

0.1
10

1

Distribution Ratio (D)Slope = 1

Slope = 21

HDEHP
CYANEX 921
0.1

1

3.1

3.2

3.3

5

[HDEHP] / M

Figure 5. Effect of HDEHP or CYANEX 921 on the extraction of
U(VI) from 5 M H3PO4.

UO2A2(HA)2X + 2H+,

UO22+ + 2(HA)2 + X(2)where HA and X denote HDEHP and CYANEX 921 molecules, respectively, and bars refer to the species in the organic
phase.
The extraction constant of eq 2 is given byKex =D[H+]2[(HA)2]2 [X],(3)where D is the distribution ratio of U(VI) between organic andaqueous phases.
The calculation of Kex at different [H+], [HA], and [X] using
the previous equation gave an average value of 1920 ± 53 M−1.
3.2.2. Effect of Temperature. The increase in temperature in
the range of 15 – 55˚C was found to decrease the extraction of
U(VI). The extraction constants, Kex, of the extracted species

3.5

3.6

-1

Figure 6. Effect of temperature on the extraction constant of U(VI)from 5 M H3PO4 by 0.8 M HDEHP + 0.2 M CYANEX 921 in kerosene.

calculated by applying eq 3 and plotted in Figure 6 as lnKex
versus the reciprocal of the respective absolute temperatures,
1/T, gave a straight linear relation with a positive slope fromwhich the thermodynamic parameters were calculated byapplying the common thermodynamic equations.26
The enthalpy change (∆H) and the entropy change (∆S)
values are –23.65 kJ mol−1 and 0.14 kJ mol−1 K−1, respectively.
The U(VI)-HDEHP complex usually contains two hydrate
water molecules when extracted and these are released when
CYANEX 921 is added.25 The release of water means that tworeactant molecules (e.g., UO2A2(HA)2·2H2O and X) may form
three product molecules (e.g., UO 2 A 2 (HA) 2 X and 2H 2 O),
leading to a positive ∆S value. Since CYANEX 921 is morebasic than H2O, it is expected to form a stronger bond via an
exothermic reaction (negative ∆H value). Hence, both enthalpy
and entropy changes favor the extraction reaction, resulting inlarge values of lnKex (7.56 M−1 at 25˚C). Therefore, the overall
extraction equation (2) may be rewritten as follows,
UO22+ + 2(HA)2

H2O

UO2A2(HA)2·2H2O + 2H+,UO2A2(HA)2·2H2O + X
0.18 – 0.3 M, Figure 5 gave linear relations with respective
slopes of 2 and 1 suggesting the participation of one moleculeof CYANEX 921 and two molecules of HDEHP in theextracted uranium species.
Based on the experimental results and considering that
HDEHP is found as dimer in aliphatic diluents,25 the extraction
equilibrium is parallel with that reported25 and may be given by

3.4
3

1/T x10 / K

1.0

UO2A2(HA)2X + 2H2O.

(4)
(5)

3.2.3. Effect of Fe(III). The results of the effect of sodium
sulphide given in section 3.1.1. indicate that the maximum
possible amount of sodium sulphide which could be used for
precipitation of Fe(III) from 5 M phosphoric acid did notremove it completely and more than 0.6 g/100 mL of Fe(III)are left in the acid. Therefore, it is important to study theeffect of Fe(III) on the synergistic extraction of U(VI) from
phosphoric acid by HDEHP- CYANEX 921 in kerosene. The
addition of Fe(III) in the acid medium was found to decrease
markedly the extraction of U(VI), and this decrease was nearly
not affected by the increase in Fe(III) concentrations in therange 0.3 -2.6 g/100 mL as shown in Figure 7. This decreasein the extraction of U(VI) may be explained by the presence ofexcess sulphide ions (even in a small amount) which may
retard the extraction of uranium, in addition to the possible
reduction of U(VI) to U(IV) by sodium sulphide. To overcome this problem, hydrogen peroxide was used to oxidize theformed amount of U(IV) to the more extractable U(VI) andsulphide ions to sulphur. In this context, addition of 0.1 M
H2O2 after addition of sodium sulphide was found to enhancethe extraction of U(VI) to 74%. The increase in hydrogen
peroxide concentration increased the extraction process whichreached a maximum value of 88% with 0.3 M then remains
constant with further addition, see Figure 8. The extraction of14

J. Nucl. Radiochem. Sci., Vol. 5, No. 1, 2004

El-NadiNa2S / g/100 mL
5

10

15

20

25

30

100

40

Fe(III)Na2S

80

Extraction Percent (%E)

3580

Extraction Percent (%E )

100

0

60

40

60

40

20

20

0
0.0

0
0.0

0.5

1.0

1.5

2.0

2.5

3.0

both Fe(III) and U(VI) from 5 M phosphoric acid solution
without addition of sodium sulphide (only H2 O 2 ) was also
carried out for comparison and found to be 12.5% and 56%,
respectively. This marked decrease in the percent of U(VI)extraction shows the importance of removal of Fe(III) by
precipitation followed by addition of H2O2 prior to the extraction process to avoid its coextraction and its suppressive effect.
3.3. Application on Commercial Phosphoric Acid. The
applicability of the proposed method for the removal of Fe(III)from commercial phosphoric acid was tested on a pretreatedacid (green acid). In this concern, 10 mL of the acid washeated to 50˚C and 2.5 g of sodium sulphide was added with
continuous stirring. After centrifugation and filtration, thesolution was treated by 0.4 mL H2O2 and shaken with equalvolume of HDEHP – CYANEX 921 mixture (of ratio 4:1) inkerosene at 25˚C. The two phases were separated and theamounts of Fe(III) and U(VI) were determined. It was foundthat iron extraction percent remains as low as obtained previously with the pure phosphoric acid, while the extraction
percent of uranium decreased from 83% to about 66%. Thisdecrease may be explained by the possibility of extraction ofother ions usually found in the commercial acid such as Co,
Ni, Mn, Cd, Zn, and Pb which may compete with uranium inthe extraction process.
4. Conclusions
Sodium sulphide was found to be useful for precipitation of
Fe(III) from phosphoric acid medium and the pure form gave
better results compared with the commercial one. Ten minutes
were found to be quite sufficient for complete precipitation andthe temperature increases the precipitation percent. The
synergic mixture of HDEHP and CYANEX 921 in kerosene
used for the extraction of U(VI) from phosphoric acid showedpoor extraction of Fe(III). The extraction of U(VI) from phosphoric acid by HDEHP – CYANEX 921 mixture in the ratio
4:1 decreased with temperature. The marked decrease in theextraction percent of U(VI) due to the presence of excess
sulphide ions was effectively overcome by addition ofhydrogen peroxide, which leads to the oxidation of U(IV) tothe extractable U(VI) form and sulphide to sulphur.
Application of the proposed precipitation method forremoval of Fe(III) on a commercial green phosphoric acidgave satisfactory results.
Acknowledgments. The authors are deeply indebted toProf. Dr. H. F. Aly for his valuable suggestions and continuous

0.2

0.3

0.4

0.5

[H2O2] / M

Remained Fe(III) / g/100 mL

Figure 7. Effect of Fe(III) and sodium sulphide concentrations on
the extraction of U(VI) from 5 M H3PO4 by 0.8 M HDEHP + 0.2 M
CYANEX 921 in kerosene.

0.1

Figure 8. Effect of H2O2 concentration on the extraction of U(VI)from 5 M H 3 PO 4 by 0.8 M HDEHP + 0.2 M CYANEX 921 inkerosene after precipitation of iron by Na2S.

guidance. Deep thanks are also due to Dr. H. Serag for hiscooperation and support.
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