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Determination of low levels of uranium in solutions obtained by acid attack on phosphate rock
Talonto.Vol 24, pp 117-120 Pergamon Press, 1977 Prmted m Great Bntam
DETERMINATION OF LOW LEVELS OF URANIUM IN SOLUTIONS OBTAINED BY ACID ATTACK ON PHOSPHATE ROCK FLORIN T. BUNUS
Institute (Received
of Atomic
Physics,
1975. Rewed
31 March
Bucharest,
Romania
11 June 1976. Accepted 15 September 1976)
Summary-The uranium present in the leach liquors obtained by attack on phosphate rock with sulphuric acid can be extracted wtth dt(2-ethylhexyl)phosphoric acid and TBP after oxtdation of any iron(H). . and then strioned at 65” with iron(H) in 8.6M phosphoric acid. The uranium is finally determined with arsenazo %. I.
In recent
years
\
the phosphate
fertilizer
industry
,
haviour of UO$+ and U4+ in phosphoric acid, and then the effect of the other impurities was examined. When the most suitable parameters had been determined, the method was applied to industrial phosphoric acid solutions containing all the impurities obtained in the phosphate rock acid process.
has
the amount of uranium dissolved in wet-process phosphoric acid has become important. An increased demand for uranium on the world market has led to the need to pay more attention to this important source. The phosphate rocks (Florida, North Africa, Israel, etc.) have a uranium content of 0.014.02%, and phosphoric acid solutions 0.1-0.2 g/I. The recovery of uranium as a by-product in the wet phosphoric acid process results in a much cheaper uranium concentrate than that usually obtained from ores. The phosphoric acid solutions obtained in the first stage are 4_5M, and several other elements are present at much higher concentrations than the uranium, typical concentrations (g/l.) after sulphuric acid attack being Fe’+ 0.5-1.0, Fe3+ 2-5, A13+ 1-2, Ca2+ 1-3, SO:15-25, F- 20. Organic matter (humic acids) is also almost always present when the phosphate rock is not calcined.‘*’ For nitric acid attack, prior calcination of the rock eliminates this inconvenience, but some of the uranium is rendered insoluble. In determination of the uranium, many problems are to be expected because of the composition of the sample solution. Any iron(I1) present after sulphuric acid treatment of the rock might lead to reduction of U(V1) to U(IV), because of complexation of Fe(II1) by the phosphoric acid increasing the reducing power of the Fe(I1). In this study, it was intended to separate uranium by solvent extraction and determine it spectrophotometrically. The choice of extractant also poses problems related to the medium. It is well known that liquid cation-exchangers such as di(2-ethylhexyl)phosphoric acid (DEHP) are very effective for actinide extraction, but to attain high distribution coefficients a synergic system involving a neutral phosphorus ester such as tributyl phosphate (TBP), was sought. Kerosene was found to be the most suitable diluent. The extraction system chosen was based on the bedeveloped
rapidly
and
at the same
time
EXPERIMENTAL
All reagents were of analytical grade. For studies on the effect of temperature, separating-funnels fitted with a water-jacket connected to a thermostat were used, several minutes being allowed for temperature equilibration. The funnels were shaken by hand for 5 min, more than enough for equilibrium to be attained (preliminary experiments showed that 2 min are sufficient). The uranium was determined spectrophotometrically as the arsenazo III complex or by the X-ray fluorescence of liquid or solid samples. In the case of liquid samples the intensity of the signal is affected by the uranium concentration. The calibration curves are linear but the slopes depend on the phosphoric acid concentration, which therefore has to be taken into account. In most of the experimental work. 0.1-g.‘]. uranmm(V1) solutions were used, prepared by calcinmg ti02(N03),’ 6Hz0 to the oxide to elimmate nitric acid, and dissolvmg the oxide in cont. phosphoric acid. In the extraction studies iron(I1) or iron(II1) was also present at a concentration of 25 mg/ml. Pure uranium(IV) solutions not containing iron(I1) were prepared by dissolving UOz in hydrochloric actd (containing traces of fluoride) or, more frequently, by dissolving UO, in 2M hydrochloric actd. followed by reduction in a Jones reductor. The effluent from the reductor was passed through a column of Dowex 50 x 8 cationexchange resin and any uranyl(V1) ions were eluted with 1M hydrochloric acid, followed by elution of uranium(IV) with 4-6M hydrochloric acid.3 The same operation was possible with sulphuric acid. The uranium(IV) solution was diluted to a uranium concentration of 0.1 g/l., with enough phosphoric acid to give 4.3M concentration. The final hydrochloric acid concentration was O.O4M, which was negligible. It should be remembered that industrial phosphoric acid solutions have a residual acidity due to sulphuric, hydrochloric or nitric actd, depending on the processing of the phosphate rock. Iron(B) and (III) solutions in phosphoric acid were prepared by dissolving the sulphates m the desired medium. 117
118
FLORINT. BUNUS
Iron was determined by permanganate titration. The aqueous phase was titrated with alkali before and after extraction, and it was thus established that the extraction did not alter the phosphoric acid concentration. Uranium was determined directly in the stripping solution of 4.3 or 8.6M acidity by reduction with zinc and ascorbic acid followed by addition of arsenazo III (aqueous solution). Comparisons were made with standards which had been analysed by X-ray fluorescence. Uranium(V1) in pure phosphoric acid solutions was directly determined colorimetritally. by treatment with arsenazo III. Finally the method was checked with industrial phosphoric acid solutions but in some cases known amounts of U(VI) were also added because of the low concentration of uranium. Some studies were performed in the presence of various concentrations of nitric acid. Iron(I1) and uranium(IV) were generally oxidized by passing air through the solution, or more surely with sodium chlorate.
D
Procedure
Take 10 ml of raw phosphoric acid (about 4.3M), previously treated with enough sodium chlorate to oxidize the iron(I1) present (determined separately), then extract uranium with three 7-ml portions of 1.2M DEHP/O.lSM TPB at 25-30”. Strip the uranium with three lo-ml portions of 8.6M phosphoric acid containing 5-15 mg of Fe(I1) per ml, at 65”. Take 1 ml of the combined strippings and stir for 5 min with several granules of zinc, then add 2 ml of 23/, ascorbic acid solution, 15 ml of 0.006°/0arsenazo III solution in 0.6M acetic acid +0.5M sodium acetate buffer, and leave for 15 min for complex formation. Measure at 665 nm against a reference solution consisting of 1 ml of 8.6M phosphoric acid treated in exactly the same way with zinc etc., and prepare a calibration curve with standards similarly treated (same acidity etc.). RESULTS The first set of experiments on U(IV) and U(V1) in pure 4.3M phosphoric acid solutions, at 25”, the extractant being DEHT + TBP in kerosene, is represented in Fig. 1, where the values of log D are plotted as a function of log [DEHP]. On the straight lines are given the concentrations of the TBP. All concentrations are expressed in mole/l. and D is the distribution coefficient defined as the ratio of the uranium concentrations in the organic and aqueous phases at equilibrium. Plots of log D vs. log [HsPO,] gave straight lines of slope -2 for 0.054.3M TBP + 0.3-1.8M DEHP, the value of D being higher at higher DEHP concentration. The role of temperature is shown in Fig. 2 for 4.3M phosphoric acid and U(V1) or U(IV) (0.1 g/l.). The choice of extractant was generally based on the earlier results but other systems were also tried. Owing to the important role played by iron(I1) and (III) as major constituents in industrial phosphoric acid solutions, its behaviour during the extraction and its influence on the uranium recovery were investigated. It was added as pure Fe(I1) or Fe(II1) to 4.3M phosphoric acid solutions with and without U(V1). Distribution coefficients were plotted as log D LX log [DEHP] at constant TBP concentrations of 0.05, 0.15 and 0.3M. Figure 3 shows the results only for Fe(I1). The D-values for Fe(II1) are only a tenth of those for Fe(II), and there is practically no Fe(II1) in the industrial solutions.
06 08 IO M,
2
HEHP
Fig. 1. Distribution coefficient as a function of DEHP concentration, for U(VI) and U(IV) and 4.3M H3P04. Numbers by the lines refer to molarity of TBP. Since the presence of Fe(I1) leads to the reduction of U(V1) to U(IV) in phosphoric acid solutions, the log D us. log [DEHP] functions are expected to be the same as those represented for U(IV) in Fig. 1. This is indeed so at medium and higher concentrations of DEHP, but at lower concentrations the effect of surplus Fe(I1) is more marked (Fig. 3). In the presence of only Fe(II1) the behaviour of U(V1) is practically the same as that shown in Fig. 1.
Fig. 2. Distribution coefficient W. temperature. U(V1): I-1.8M DEHP; 2-1.8M DEHP + 0.15M TBP; 3-1.8M DEHP + 0.30M TBP: 4--1&W DEHP + 0 15M TBP. 5-1.2M DEHP: ti.6M DEHP + 0.30M TBP; U(IV) 7---1.5&f DEHP + O.OSMTBP.
119
Low levels of uranium
02
I 02
06
04
06
1.0
2
M, HEHP Fig. 3. Distribution coefficient rs. DEHP concentration. U(N) in the presence of Fe(II): Ia.OSM TBP; 2--0.30M TBP. Fe(B): 3--0.05M TBP; 4&Ql5M TBP; 5G1.30M TBP. Increase in temperature generally reduces the value of D. The presence of Fe(I1) at a concentration of 5 mg/ml in 8.6M phosphoric acid decreases D to O.l(M.15 at 5&65’. Thus a stripping solution of this composition is efficient for uranium recovery. Finally the method was applied to industrial phosphoric acid solutions and the results compared with those of X-ray fluorescence (Table 1). DISCUSSION
The fresh industrial phosphoric acid solution obtained by sulphuric acid attack on phosphate rock contains a significant amount of Fe(B) which reduces U(V1) to U(IV), which is less extractable at temperatures below 45” (Fig. 2). At higher temperatures this behaviour changes slightly and it is therefore desirable to convert all the uranium into U(V1). The iron can be oxidized by bubbling air through the solution at 70-W but the process is slow. The speed of reaction increases with temperature but the solubility of oxygen also decreases. It is better to add a reagent such as sodium chlorate. Sometimes the inTable 1. Results from X-ray fluorescence and extractionspectrophotometric methods
Sample
X-ray fluorescence, g/l.
1 2 3 4 5 6 7
0.231 0.240 0.261 0.376 0.188 0.204 0.181
Extractionspectrophotometry, s/l. 0.228 ( f 2”‘)* 0.240 ( + 17:) 0.254 (k 3%) 0.364 (k 5%) 0.192 (*40/d) 0.200 ( * 2%) 0.178 ( + 2%)
* Errors are the mean value from several determinations.
terval between sampling and analysis is long enough to ensure a partial oxidation of iron. The dependence of uranium extraction on the concentration of DEHP and TBP in kerosene at 25” is given in Fig. 1. The slopes decrease from 2.6 for pure DEHP to 1 for 0.3M TBP in the case of U(V1). For U(IV) the values of D aremuch lower (at 25O) and no linear dependence appears. The presence of TBP in the organic phase exerts a synergic effect on the U(V1) extraction up to 1M DEHP concentration but at higher concentration an antagonistic effect is present. However the values of D always increase with DEHP concentration, therefore better uranium recoveries are obtained by extraction with 1.2-1.8M DEHP from 4.3M phosphoric acid at 25”. It is better to add TBP at lower concentrations of DEHP. A family of straight lines of slope -2 was obtained for plots of log D us.log [H,PO,] for various DEHP concentrations and 0.05-0.3M TBP. D should be given by D=
Dll 1 + C,K,[H,PO;-J”
where D refers to the distribution coefficient for uranium(V1) in presence of phosphoric acid and D,, to the distribution in the absence of phosphoric acid. K, is defined by UO;+
+ nH3P0,
K.
[U02(H2P04),](i
-“)+ (2)
The literature on UO:+ complexes in phosphoric acid medium mentions the existence of ions such as U02H3PO:+, U02H2PO:, U02(H2P04)H,PO:, U02(H2P0& .k7 In dilute phosphoric acid UO:+ predominates but at higher acid concentrations various complexes are formed. Over the phosphoric acid concentration range studied, the high value of D at lower acid concentrations supports the supposition that UO:+ is the major extractable species. The behaviour of DEHP (denoted by HX, and dimerized in the organic phase) in the extraction is described by the equation 2+ UOza,
+
W-Wz.,,
=
UOzXtHz.,g
+
=C,
(3)
and corresponds to an ion-exchange mechanism, and to the slope of -2 for log D vs. log [H,PO,]. The mechanisms for other systems are described in the literature. ’ _ l2 In the phosphoric acid system it is likely that there is a similar mechanism in the region where synergism exists. The antagonistic effect seems to play a minor role at concentrations higher than 1M DEHP and it is therefore rather difficult to believe that water is present in the organic phase when the synergic species is destroyed. The extraction coefficient becomes smaller when the TBP concentration is increased, because of a stronger interaction between HX and TBP through hydrogen-bonding, and a consequent reduction in the concentration of free HX. The reaction mechanism chosen leads to a mathematical treatment like that of Sator for UO,X,.2Y
120
FLORIN T. BUNUS
in sulphuric acid medium. From the slopes in Fig. 1 it is possible to assume that this mechanism is partly valid but other contributions are also present to a lower or higher degree, depending on the system studied. The value of D for U(V1) and various DEHP-TBP mixtures decreases with temperature, Fig. 2, which explains the advantage of extraction at low temperature, say 2@-25” and stripping at 65” with 4.3M phosphoric acid (or, even better, 86M). Both extraction and stripping should be done in three steps. The D value increases with DEHP concentration but the presence of TBP makes it less dependent on temperature. From this point of view it is better to use 1.2M DEHP in kerosene than 1.2M DEHP + 0.15M TBP. The D values in both cases are the same at 25” but stripping at 65” is favoured if TBP is absent. The behaviour of U(N), as shown in Fig. 2, might partly be explained by oxidation but it is unlikely that this is the only process responsible for the increased extraction at temperatures higher than 45”. It is this behaviour that makes it necessary to oxidize Fe(I1) and U(IV) before the extraction. However, it was found that stripping at 65” with 8.6M phosphoric acid is more efficient in presence of Fe(II), as U(IV) has much the lower extraction coefficient, -0.1. The role of iron in the extraction of uranium depends on its oxidation state. In the case of Fe(II), Fig. 3, the D values for iron are much lower than those of U(IV) and an increase of TBP concentration from 0.05 to 0.3M appreciably reduces the Fe(I1) extraction. When uranium(V1) at a concentration of 0.1 mg/ml was extracted from 4.3M phosphoric acid in the presence of Fe(B) at a concentration of 25 mg/ml, the values of D (Fig. 3) indicated an extraction comparable with that of U(IV) in the absence of iron. It is thus clear that U(V1) was reduced by Fe(I1) to U(IV) which then behaved as such. It seems that Fe(II), with lower D but higher amount extracted, say 7 mg/ml, exerts little
influence at 1M DEHP concentration on the value of D for U(IV) from the point of view of reducing the concentration of active HX groups. Increasing the TBP concentration markedly reduces the extraction yield both for U(IV) and Fe(II). The values of D for Fe(II1) extraction are a tenth of those for Fe(I1) under the same conditions. Therefore U(V1) will not be practically affected. From the results of all these experiments, the choice of extractant depends on the case under study but sometimes it is preferable to employ pure DEHP in kerosene at say 1.2M concentration, because of the greater influence of temperature on D. Uranium must be in the sexivalent form, therefore oxidation of iron is required. At the same time, lowering the phosphoric acid concentration greatly increases the value of D. Stripping is best done in the presence of Fe(I1) with 4.3-8.6M phosphoric acid. There will then be no organic matter in the stripping solution and hence no interference in the analysis. REFERENCES 1. F. J. Hurst,
2. 3. 4. 5. 6.
D. J. Crouse and K. B. Brown, Ind. Eng. Chem. Process, 1972, 1, 122. F. J. Hurst and D. J. Crouse, ibid., 1974, 3, 286. F. T. Bunus, J. Inorg. Nucl. Chem., 1974, 36, 917. F. Habashi, ibid., 1960, 13, 125. C. F. Baes Jr., J. Am. Chem. Sot., 1954, 76, 354. B F. Thames. ibid., 1957, 79, 4298.
7. Y. Marcus, Proc. 2nd Intern. Geneva Con& 1958, P/1605. 8. C. A. Blake Jr., C. F. Baes Jr.. C. B. Brown, C F. Coleman and J. C. White, ibid., 1958, 28, 289. 9. J. Kennedy and A. M. Deane, J. Inorg. Nucl. Chem.. 1961, 19, 142. 10. A. M. Deane, J. Kennedy and P. G. Sammes, Chem. Ind. London, 1960, 443. 11. D. Dyrssen and L. Kuca, Acta Chem. Stand., 1960, 14, 1945. 12. D. F. Peppard, G. W. Mason and R. J. Slronen, J. Inorg. Nucl. Chem., 1959, 10, 117. 13. T. Sato, ibid., 1964, 26, 311.
DETERMINATION OF LOW LEVELS OF URANIUM IN SOLUTIONS OBTAINED BY ACID ATTACK ON PHOSPHATE ROCK FLORIN T. BUNUS
Institute (Received
of Atomic
Physics,
1975. Rewed
31 March
Bucharest,
Romania
11 June 1976. Accepted 15 September 1976)
Summary-The uranium present in the leach liquors obtained by attack on phosphate rock with sulphuric acid can be extracted wtth dt(2-ethylhexyl)phosphoric acid and TBP after oxtdation of any iron(H). . and then strioned at 65” with iron(H) in 8.6M phosphoric acid. The uranium is finally determined with arsenazo %. I.
In recent
years
\
the phosphate
fertilizer
industry
,
haviour of UO$+ and U4+ in phosphoric acid, and then the effect of the other impurities was examined. When the most suitable parameters had been determined, the method was applied to industrial phosphoric acid solutions containing all the impurities obtained in the phosphate rock acid process.
has
the amount of uranium dissolved in wet-process phosphoric acid has become important. An increased demand for uranium on the world market has led to the need to pay more attention to this important source. The phosphate rocks (Florida, North Africa, Israel, etc.) have a uranium content of 0.014.02%, and phosphoric acid solutions 0.1-0.2 g/I. The recovery of uranium as a by-product in the wet phosphoric acid process results in a much cheaper uranium concentrate than that usually obtained from ores. The phosphoric acid solutions obtained in the first stage are 4_5M, and several other elements are present at much higher concentrations than the uranium, typical concentrations (g/l.) after sulphuric acid attack being Fe’+ 0.5-1.0, Fe3+ 2-5, A13+ 1-2, Ca2+ 1-3, SO:15-25, F- 20. Organic matter (humic acids) is also almost always present when the phosphate rock is not calcined.‘*’ For nitric acid attack, prior calcination of the rock eliminates this inconvenience, but some of the uranium is rendered insoluble. In determination of the uranium, many problems are to be expected because of the composition of the sample solution. Any iron(I1) present after sulphuric acid treatment of the rock might lead to reduction of U(V1) to U(IV), because of complexation of Fe(II1) by the phosphoric acid increasing the reducing power of the Fe(I1). In this study, it was intended to separate uranium by solvent extraction and determine it spectrophotometrically. The choice of extractant also poses problems related to the medium. It is well known that liquid cation-exchangers such as di(2-ethylhexyl)phosphoric acid (DEHP) are very effective for actinide extraction, but to attain high distribution coefficients a synergic system involving a neutral phosphorus ester such as tributyl phosphate (TBP), was sought. Kerosene was found to be the most suitable diluent. The extraction system chosen was based on the bedeveloped
rapidly
and
at the same
time
EXPERIMENTAL
All reagents were of analytical grade. For studies on the effect of temperature, separating-funnels fitted with a water-jacket connected to a thermostat were used, several minutes being allowed for temperature equilibration. The funnels were shaken by hand for 5 min, more than enough for equilibrium to be attained (preliminary experiments showed that 2 min are sufficient). The uranium was determined spectrophotometrically as the arsenazo III complex or by the X-ray fluorescence of liquid or solid samples. In the case of liquid samples the intensity of the signal is affected by the uranium concentration. The calibration curves are linear but the slopes depend on the phosphoric acid concentration, which therefore has to be taken into account. In most of the experimental work. 0.1-g.‘]. uranmm(V1) solutions were used, prepared by calcinmg ti02(N03),’ 6Hz0 to the oxide to elimmate nitric acid, and dissolvmg the oxide in cont. phosphoric acid. In the extraction studies iron(I1) or iron(II1) was also present at a concentration of 25 mg/ml. Pure uranium(IV) solutions not containing iron(I1) were prepared by dissolving UOz in hydrochloric actd (containing traces of fluoride) or, more frequently, by dissolving UO, in 2M hydrochloric actd. followed by reduction in a Jones reductor. The effluent from the reductor was passed through a column of Dowex 50 x 8 cationexchange resin and any uranyl(V1) ions were eluted with 1M hydrochloric acid, followed by elution of uranium(IV) with 4-6M hydrochloric acid.3 The same operation was possible with sulphuric acid. The uranium(IV) solution was diluted to a uranium concentration of 0.1 g/l., with enough phosphoric acid to give 4.3M concentration. The final hydrochloric acid concentration was O.O4M, which was negligible. It should be remembered that industrial phosphoric acid solutions have a residual acidity due to sulphuric, hydrochloric or nitric actd, depending on the processing of the phosphate rock. Iron(B) and (III) solutions in phosphoric acid were prepared by dissolving the sulphates m the desired medium. 117
118
FLORINT. BUNUS
Iron was determined by permanganate titration. The aqueous phase was titrated with alkali before and after extraction, and it was thus established that the extraction did not alter the phosphoric acid concentration. Uranium was determined directly in the stripping solution of 4.3 or 8.6M acidity by reduction with zinc and ascorbic acid followed by addition of arsenazo III (aqueous solution). Comparisons were made with standards which had been analysed by X-ray fluorescence. Uranium(V1) in pure phosphoric acid solutions was directly determined colorimetritally. by treatment with arsenazo III. Finally the method was checked with industrial phosphoric acid solutions but in some cases known amounts of U(VI) were also added because of the low concentration of uranium. Some studies were performed in the presence of various concentrations of nitric acid. Iron(I1) and uranium(IV) were generally oxidized by passing air through the solution, or more surely with sodium chlorate.
D
Procedure
Take 10 ml of raw phosphoric acid (about 4.3M), previously treated with enough sodium chlorate to oxidize the iron(I1) present (determined separately), then extract uranium with three 7-ml portions of 1.2M DEHP/O.lSM TPB at 25-30”. Strip the uranium with three lo-ml portions of 8.6M phosphoric acid containing 5-15 mg of Fe(I1) per ml, at 65”. Take 1 ml of the combined strippings and stir for 5 min with several granules of zinc, then add 2 ml of 23/, ascorbic acid solution, 15 ml of 0.006°/0arsenazo III solution in 0.6M acetic acid +0.5M sodium acetate buffer, and leave for 15 min for complex formation. Measure at 665 nm against a reference solution consisting of 1 ml of 8.6M phosphoric acid treated in exactly the same way with zinc etc., and prepare a calibration curve with standards similarly treated (same acidity etc.). RESULTS The first set of experiments on U(IV) and U(V1) in pure 4.3M phosphoric acid solutions, at 25”, the extractant being DEHT + TBP in kerosene, is represented in Fig. 1, where the values of log D are plotted as a function of log [DEHP]. On the straight lines are given the concentrations of the TBP. All concentrations are expressed in mole/l. and D is the distribution coefficient defined as the ratio of the uranium concentrations in the organic and aqueous phases at equilibrium. Plots of log D vs. log [HsPO,] gave straight lines of slope -2 for 0.054.3M TBP + 0.3-1.8M DEHP, the value of D being higher at higher DEHP concentration. The role of temperature is shown in Fig. 2 for 4.3M phosphoric acid and U(V1) or U(IV) (0.1 g/l.). The choice of extractant was generally based on the earlier results but other systems were also tried. Owing to the important role played by iron(I1) and (III) as major constituents in industrial phosphoric acid solutions, its behaviour during the extraction and its influence on the uranium recovery were investigated. It was added as pure Fe(I1) or Fe(II1) to 4.3M phosphoric acid solutions with and without U(V1). Distribution coefficients were plotted as log D LX log [DEHP] at constant TBP concentrations of 0.05, 0.15 and 0.3M. Figure 3 shows the results only for Fe(I1). The D-values for Fe(II1) are only a tenth of those for Fe(II), and there is practically no Fe(II1) in the industrial solutions.
06 08 IO M,
2
HEHP
Fig. 1. Distribution coefficient as a function of DEHP concentration, for U(VI) and U(IV) and 4.3M H3P04. Numbers by the lines refer to molarity of TBP. Since the presence of Fe(I1) leads to the reduction of U(V1) to U(IV) in phosphoric acid solutions, the log D us. log [DEHP] functions are expected to be the same as those represented for U(IV) in Fig. 1. This is indeed so at medium and higher concentrations of DEHP, but at lower concentrations the effect of surplus Fe(I1) is more marked (Fig. 3). In the presence of only Fe(II1) the behaviour of U(V1) is practically the same as that shown in Fig. 1.
Fig. 2. Distribution coefficient W. temperature. U(V1): I-1.8M DEHP; 2-1.8M DEHP + 0.15M TBP; 3-1.8M DEHP + 0.30M TBP: 4--1&W DEHP + 0 15M TBP. 5-1.2M DEHP: ti.6M DEHP + 0.30M TBP; U(IV) 7---1.5&f DEHP + O.OSMTBP.
119
Low levels of uranium
02
I 02
06
04
06
1.0
2
M, HEHP Fig. 3. Distribution coefficient rs. DEHP concentration. U(N) in the presence of Fe(II): Ia.OSM TBP; 2--0.30M TBP. Fe(B): 3--0.05M TBP; 4&Ql5M TBP; 5G1.30M TBP. Increase in temperature generally reduces the value of D. The presence of Fe(I1) at a concentration of 5 mg/ml in 8.6M phosphoric acid decreases D to O.l(M.15 at 5&65’. Thus a stripping solution of this composition is efficient for uranium recovery. Finally the method was applied to industrial phosphoric acid solutions and the results compared with those of X-ray fluorescence (Table 1). DISCUSSION
The fresh industrial phosphoric acid solution obtained by sulphuric acid attack on phosphate rock contains a significant amount of Fe(B) which reduces U(V1) to U(IV), which is less extractable at temperatures below 45” (Fig. 2). At higher temperatures this behaviour changes slightly and it is therefore desirable to convert all the uranium into U(V1). The iron can be oxidized by bubbling air through the solution at 70-W but the process is slow. The speed of reaction increases with temperature but the solubility of oxygen also decreases. It is better to add a reagent such as sodium chlorate. Sometimes the inTable 1. Results from X-ray fluorescence and extractionspectrophotometric methods
Sample
X-ray fluorescence, g/l.
1 2 3 4 5 6 7
0.231 0.240 0.261 0.376 0.188 0.204 0.181
Extractionspectrophotometry, s/l. 0.228 ( f 2”‘)* 0.240 ( + 17:) 0.254 (k 3%) 0.364 (k 5%) 0.192 (*40/d) 0.200 ( * 2%) 0.178 ( + 2%)
* Errors are the mean value from several determinations.
terval between sampling and analysis is long enough to ensure a partial oxidation of iron. The dependence of uranium extraction on the concentration of DEHP and TBP in kerosene at 25” is given in Fig. 1. The slopes decrease from 2.6 for pure DEHP to 1 for 0.3M TBP in the case of U(V1). For U(IV) the values of D aremuch lower (at 25O) and no linear dependence appears. The presence of TBP in the organic phase exerts a synergic effect on the U(V1) extraction up to 1M DEHP concentration but at higher concentration an antagonistic effect is present. However the values of D always increase with DEHP concentration, therefore better uranium recoveries are obtained by extraction with 1.2-1.8M DEHP from 4.3M phosphoric acid at 25”. It is better to add TBP at lower concentrations of DEHP. A family of straight lines of slope -2 was obtained for plots of log D us.log [H,PO,] for various DEHP concentrations and 0.05-0.3M TBP. D should be given by D=
Dll 1 + C,K,[H,PO;-J”
where D refers to the distribution coefficient for uranium(V1) in presence of phosphoric acid and D,, to the distribution in the absence of phosphoric acid. K, is defined by UO;+
+ nH3P0,
K.
[U02(H2P04),](i
-“)+ (2)
The literature on UO:+ complexes in phosphoric acid medium mentions the existence of ions such as U02H3PO:+, U02H2PO:, U02(H2P04)H,PO:, U02(H2P0& .k7 In dilute phosphoric acid UO:+ predominates but at higher acid concentrations various complexes are formed. Over the phosphoric acid concentration range studied, the high value of D at lower acid concentrations supports the supposition that UO:+ is the major extractable species. The behaviour of DEHP (denoted by HX, and dimerized in the organic phase) in the extraction is described by the equation 2+ UOza,
+
W-Wz.,,
=
UOzXtHz.,g
+
=C,
(3)
and corresponds to an ion-exchange mechanism, and to the slope of -2 for log D vs. log [H,PO,]. The mechanisms for other systems are described in the literature. ’ _ l2 In the phosphoric acid system it is likely that there is a similar mechanism in the region where synergism exists. The antagonistic effect seems to play a minor role at concentrations higher than 1M DEHP and it is therefore rather difficult to believe that water is present in the organic phase when the synergic species is destroyed. The extraction coefficient becomes smaller when the TBP concentration is increased, because of a stronger interaction between HX and TBP through hydrogen-bonding, and a consequent reduction in the concentration of free HX. The reaction mechanism chosen leads to a mathematical treatment like that of Sator for UO,X,.2Y
120
FLORIN T. BUNUS
in sulphuric acid medium. From the slopes in Fig. 1 it is possible to assume that this mechanism is partly valid but other contributions are also present to a lower or higher degree, depending on the system studied. The value of D for U(V1) and various DEHP-TBP mixtures decreases with temperature, Fig. 2, which explains the advantage of extraction at low temperature, say 2@-25” and stripping at 65” with 4.3M phosphoric acid (or, even better, 86M). Both extraction and stripping should be done in three steps. The D value increases with DEHP concentration but the presence of TBP makes it less dependent on temperature. From this point of view it is better to use 1.2M DEHP in kerosene than 1.2M DEHP + 0.15M TBP. The D values in both cases are the same at 25” but stripping at 65” is favoured if TBP is absent. The behaviour of U(N), as shown in Fig. 2, might partly be explained by oxidation but it is unlikely that this is the only process responsible for the increased extraction at temperatures higher than 45”. It is this behaviour that makes it necessary to oxidize Fe(I1) and U(IV) before the extraction. However, it was found that stripping at 65” with 8.6M phosphoric acid is more efficient in presence of Fe(II), as U(IV) has much the lower extraction coefficient, -0.1. The role of iron in the extraction of uranium depends on its oxidation state. In the case of Fe(II), Fig. 3, the D values for iron are much lower than those of U(IV) and an increase of TBP concentration from 0.05 to 0.3M appreciably reduces the Fe(I1) extraction. When uranium(V1) at a concentration of 0.1 mg/ml was extracted from 4.3M phosphoric acid in the presence of Fe(B) at a concentration of 25 mg/ml, the values of D (Fig. 3) indicated an extraction comparable with that of U(IV) in the absence of iron. It is thus clear that U(V1) was reduced by Fe(I1) to U(IV) which then behaved as such. It seems that Fe(II), with lower D but higher amount extracted, say 7 mg/ml, exerts little
influence at 1M DEHP concentration on the value of D for U(IV) from the point of view of reducing the concentration of active HX groups. Increasing the TBP concentration markedly reduces the extraction yield both for U(IV) and Fe(II). The values of D for Fe(II1) extraction are a tenth of those for Fe(I1) under the same conditions. Therefore U(V1) will not be practically affected. From the results of all these experiments, the choice of extractant depends on the case under study but sometimes it is preferable to employ pure DEHP in kerosene at say 1.2M concentration, because of the greater influence of temperature on D. Uranium must be in the sexivalent form, therefore oxidation of iron is required. At the same time, lowering the phosphoric acid concentration greatly increases the value of D. Stripping is best done in the presence of Fe(I1) with 4.3-8.6M phosphoric acid. There will then be no organic matter in the stripping solution and hence no interference in the analysis. REFERENCES 1. F. J. Hurst,
2. 3. 4. 5. 6.
D. J. Crouse and K. B. Brown, Ind. Eng. Chem. Process, 1972, 1, 122. F. J. Hurst and D. J. Crouse, ibid., 1974, 3, 286. F. T. Bunus, J. Inorg. Nucl. Chem., 1974, 36, 917. F. Habashi, ibid., 1960, 13, 125. C. F. Baes Jr., J. Am. Chem. Sot., 1954, 76, 354. B F. Thames. ibid., 1957, 79, 4298.
7. Y. Marcus, Proc. 2nd Intern. Geneva Con& 1958, P/1605. 8. C. A. Blake Jr., C. F. Baes Jr.. C. B. Brown, C F. Coleman and J. C. White, ibid., 1958, 28, 289. 9. J. Kennedy and A. M. Deane, J. Inorg. Nucl. Chem.. 1961, 19, 142. 10. A. M. Deane, J. Kennedy and P. G. Sammes, Chem. Ind. London, 1960, 443. 11. D. Dyrssen and L. Kuca, Acta Chem. Stand., 1960, 14, 1945. 12. D. F. Peppard, G. W. Mason and R. J. Slronen, J. Inorg. Nucl. Chem., 1959, 10, 117. 13. T. Sato, ibid., 1964, 26, 311.