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Determination of carbonate, bicarbonate and phosphate in uranium leach liquors containing KMnO4 as oxidant
Talanta ELSEVIER
Talanta 42 (1995) 945-948
Determination of carbonate, bicarbonate and phosphate in uranium leach liquors containing KMn04 as oxidant D . S . R . M u r t y *, A. T h a n g a r a j , R. R a d h a m a n i , R. R a n g a s w a m y Atomic Minerals Division, Department of Atomic Energy, Nagarbhavi, Bangalore - 560 072, India
Received 4 November 1994; revised 16 January 1995; accepted 19 January 1995
Abstract
A simple, rapid and economic method has been developed for the reduction of potassium permanganate in alkaline carbonate uranium leach liquors to a colourless solution. The pink colour of permanganate, if not reduced, seriously impedes the determination of carbonate, bicarbonate and phosphate by titrimetry and spectrophotometry. Out of 17 reductants investigated, commercial sugar has been found to be the most effective, rapid, simple and interference free for further estimations. To the best of the authors' knowledge, this is the first time that sugar as a reductant has been made use of in analytical investigations.
1. Introduction
Tetravalent uranium present in primary and secondary uranium ores does not dissolve in sulphuric acid or carbonate solution at discernible rates, and requires the addition of an oxidant for its oxidation to the hexavalent state. Potassium permanganate is an effective oxidant and hence accepted as an additive in alkaline carbonate leaching for uranium [1]. Because hydroxyl ions are formed during carbonate leaching, it is imperative also to add bicarbonate to prevent the precipitation of leached uranium by providing the required amount of hydrogen ions. The potassium permanganate added imparts a pink colour to the uranium leach liquors, thus vitiating the methods of estimation of carbonate and bicarbonate by titration and phosphate by spectrophotometry. The determination of these three anions in alkaline uranium leach liquors is of immense use for the economic exploitation of leaching procedures, and hence investigations were carfled out to overcome the interference of the * Corresponding author. Fax: (91)80-335-1511.
pink colour of the permanganate in the estimation procedures. These experiments were carfled out on uranium leach liquors of phosphatic siliceous calcite-dolostone samples from Cuddapah Basin, Andhra Pradesh, India. The results are presented in this paper.
2. Experimental
21. R e ~ e n ~ 20% sugar solution is prepared by dissolving 20 g of commercial sugar in water and making up to 100 ml. The solution is filtered through Whatman No. 40 filter paper. Fresh solution is prepared every day as the sugar solution develops bacterial growth due to fermentation on standing, making it unsuitable for pipetting. All other chemicals used are of Analar quality.
2.2. Instrumentation A Shimadzu UV-240 model double beam spectrophotometer was used for the absorption measurements.
0039-9140/95/$09.50 O 1995 Elsevier Science B.V. All fights reserved S S D I 0039-9140(95)01530-0
946
D.S.R. Murty et al. / Talanta 42 (1995) 945-948
Table 1 Percentage recoveryof carbonate, bicarbonate and phosphate No.
l 2 3 4 5
Carbonate
Bicarbonate
Phosphate
Added (rag)
Found (rag)
Recovery Added F o u n d Recovery Added F o u n d (%) (mg) (nag) (%) (rag)
Recovery (mg)(%)
3.0 6.0 9.0 10.0 12.0
3.0 5.8 8.8 9.6 11.7
100 97 98 96 98
93 94 99 97 96
5.0 10.0 15.0 20.0 25.0
4.9 9.7 15.2 20.1 25.0
2.3. Procedure
A 50.0 ml aliquot of the carbonate leach liquor of uranium containing potassium permanganate as an oxidant is taken in a 100 ml beaker. Freshly prepared 20% sugar solution (10.0 ml) is added and stirred with a glass rod. The permanganate in the liquor is reduced to MnO2xH20 and the precipitate settles to the bottom within 30 rain, leaving a clear, colourless supernatant liquid. Three aliquots, 10.0 ml each of the supernatant solution, are taken into three separate breakers for analysis. From the first two beakers, carbonate and bicarbonate are determined by titration with standard HC1 solution [2], using phenolphthalein and bromocresol green as indicators. In the third aliquot, phosphate is determined spectrophotometrically using a vanadomolybdo reagent [3]. A 10.0 ml aliquot of the 20% sugar solution is taken as a blank for each determination.
3. Results and discussion
Because of the presence of dolomitic limestone in Cuddapah basin samples only carbonate leaching for uranium extraction is to be adopted, as the acid consumption in acid leaching is very high. Under alkaline carbonate leaching parameters, uranium present in association with phosphates and Fe-Ti mineral complexes is not solubilised [4]. Potassium permanganate is one of the oxidants studied for oxidising tetravalent uranium to the hexavalent state. As little as 0.01 ml of 0.02 M permanganate imparts a pale pink colour to 100 ml of water [2], so it has to be decolourised to avoid interferences in the determination of carbonate, bicarbonate and phosphate. 17 reductants were tried to reduce the pink coloured permanganate ion either to the colourless
98 97 101 100 100
3.0 5.0 8.0 10.0 14.0
2.8 4.7 7.9 9.7 13.4
manganous ion or to the precipitation of manganese dioxide. All reductants studied are able to decolourise the alkaline leach liquor, but each reductant except sugar has its own advantages and disadvantages for uniform applicability. Sodium chloride or hydrochloric acid decolourises the leach liquors only on heating, whereas hydroxylamine hydrochloride and stannous chloride reduce even in cold conditions. The resulting solution can be used for phosphate estimation only, but carbonate and bicarbonate cannot be estimated quantitatively. Sodium thiosulphate and hydrogen sulphide decolourise, but the resulting solution is very turbid and hence not useful for further estimations. Sodium sulphate and sulphurous acid also reduce permanganate. Although there is no interference for the phosphate estimation, the same cannot be assumed for other estimations. In the presence of potassium iodide the liberated iodine imparts an unwanted brown coiour to the solution rendering it useless for further estimations. When ferrous iron is used for reduction, the iron precipitates and interferes in all the estimations. Ethyl alchohoi or ascorbic acid also give colourless solutions; they not only give turbidity to the leach liquors, but also reduce the molybdovanadate reagent used for phosphate estimation. If hydrogen peroxide is used, the excess should be boiled off. Oxalic acid reduces only on heating, disturbing the carbonate/bicarbonate system. The reducing effect of activated charcoal on permanganate is very slow and also leads to inaccurate values for carbonate and bicarbonate. The fact that permanganate is inherently unstable in the presence of manganese(II) ions, particularly in an alkaline medium [2], has been made use of for decolourisation. In the result-
947
D.S.R. Murty et al. / Talanta 42 (1995) 945-948 Table 2 Analytical data of carbonate, bicarbonate and phosphate in uranium leach liquors No.
l 2 3 4 5 6 7 8 9 l0
Sample
AMD-I AMD-2 AMD-3 AMD-4 AMD-5 AMD-6 AMD-7 AMD-8 AMD-9 AMD-10
Carbonate (g I - ')
Bicarbonate (g I- ~)
Phosphate (g I - ~)
Present method
pH titration
Present method
pH titration
Present method
Emission method
6.3 14.5 41.4 8.2 18.3 9.1 11.2 6.5 16.1 4.6
6.5 14.3 41.4 8.5 18.3 9.3 I 1.4 6.3 16.0 4.6
2.4 4.8 23.2 2.7 5. l 3.1 1.0 1.2 5.1 1.0
2.3 4.9 23.1 2.5 5.3 3.0 1.0 1.4 5.3 l.l
0.2 6.0 5.3 2.1 30.4 17.9 1.6 35.6 4.9 0.6
0.1 6.1 5.4 2.1 30.8 18.1 1.5 35.9 4.9 0.5
ing solution, phosphate can be determined; however, the carbonate and bicarbonate values are not accurate. Although sucrose, the commercial sugar is a non-reducing sugar [5], as far as the reduction of Fehling solution is concerned, because of the linkage of the aldehyde group of the glucose part with the ketonic group of the fructose part, its addition to the pink coloured alkaline leach liquors causes the reduction of Mn 7+ to MnO2xH20. This may be due either to the presence of monosaccharide (glucose/fructose) which is a reducing sugar or to the high redox potential of the permanganate [5]. In the supernatant solution of leach liquors after decolourisation by the addition of sugar solution, phosphate is determined by the molybdovenadate method [3], and carbonate and bicarbonate are determined by titration with standard HC1 [2] using phenolphthalein and bromocresol green as indicators without any interference. Glucose also reduces the pink colour of permanganate. However, in the case of pure glucose, the only advantage is that the reduction takes place within 10-15 min as opposed to the 30 rain necessary for commercial sugar. However, this time factor is outweighed by the cost of glucose with respect to commercial sugar, to provide an economic method. Percentage recovery values calculated by adding known amounts of phosphate, carbonate and bicarbonate and determined by the present procedure are given in Table 1. The recovery values obtained vary from 93 to 101%. The values of phosphate, carbonate and bicarbonate in some alkaline permanganate leach liquors of uranium after decolourisation with sugar solution are presented in Table 2. TA~. 4~-7-F
The values of carbonate and bicarbonate obtained by pH titration with a pH meter [2] and phosphate values obtained by a plasma emission technique [6] are also presented in this table. The values are in fairly good agreement, reflecting the accuracy of the method.
4. Conclusion The pros and cons of various reagents for the reduction of permanganate show that sucrose is the best reductant to obtain a clear and colourless solution for the estimation of carbonate and bicarbonate by titrimetric, and phosphate by absorptiometric methods without any interference. The advantages of this procedure are simplicity, rapidity, economy, and freedom from interference to give uniform applicability.
Acknowledgements The authors are thankful to Sri.KK. Dwivedy, Director, AMD and to Sri.M. Vasudeva Rao for permission to publish the paper, and for encouragement. The inspiration in the form of continuous guidance from Sri.K.P. Cheria is gratefully acknowledged. The authors are also thankful to R. Anupama for her technical assistance.
References [1] Rober C. Merritt, The Extractive Metallurgy of Uranium, prepared under contract with the United States
948
D.S.R. Murty et al. / Talama 42 (1995) 945-948
Atomic Energy Commission (USAEC), Washington, 1971, pp. 85, 104. [2] A.I. Vogel, Text Book of Quantitative Chemical Analysis, 5th edn. (revised), ELBS Longrnan, London, 1991, pp. 299, 369, 279. [3] L. Shapiro, Rapid analysis of silicate, carbonate and phosphate rocks, revised edn., Geological Survey Bulletin No. 1401, Washington, 1975, p. 37.
[4] Ravi Kaul, R.N. Shankaran, Y. Lakshminarayana and Minati Roy Exploration and Research for Atomic Minerals, Vol. 4, AMD, Hyderabad, India, 1991, p. 197. [5] I.L. Finar, Organic Chemistry, Vol. l, ELBS, Longman, London, 1964, pp. 461,439. [6] R.K. Winge, C.A. Fassel, V.J. Peterson and M.A. Floyd, Inductively Coupled Plasma-Atomic Emission Spectroscopy, Elsevier, Amsterdam, 1987.
Talanta 42 (1995) 945-948
Determination of carbonate, bicarbonate and phosphate in uranium leach liquors containing KMn04 as oxidant D . S . R . M u r t y *, A. T h a n g a r a j , R. R a d h a m a n i , R. R a n g a s w a m y Atomic Minerals Division, Department of Atomic Energy, Nagarbhavi, Bangalore - 560 072, India
Received 4 November 1994; revised 16 January 1995; accepted 19 January 1995
Abstract
A simple, rapid and economic method has been developed for the reduction of potassium permanganate in alkaline carbonate uranium leach liquors to a colourless solution. The pink colour of permanganate, if not reduced, seriously impedes the determination of carbonate, bicarbonate and phosphate by titrimetry and spectrophotometry. Out of 17 reductants investigated, commercial sugar has been found to be the most effective, rapid, simple and interference free for further estimations. To the best of the authors' knowledge, this is the first time that sugar as a reductant has been made use of in analytical investigations.
1. Introduction
Tetravalent uranium present in primary and secondary uranium ores does not dissolve in sulphuric acid or carbonate solution at discernible rates, and requires the addition of an oxidant for its oxidation to the hexavalent state. Potassium permanganate is an effective oxidant and hence accepted as an additive in alkaline carbonate leaching for uranium [1]. Because hydroxyl ions are formed during carbonate leaching, it is imperative also to add bicarbonate to prevent the precipitation of leached uranium by providing the required amount of hydrogen ions. The potassium permanganate added imparts a pink colour to the uranium leach liquors, thus vitiating the methods of estimation of carbonate and bicarbonate by titration and phosphate by spectrophotometry. The determination of these three anions in alkaline uranium leach liquors is of immense use for the economic exploitation of leaching procedures, and hence investigations were carfled out to overcome the interference of the * Corresponding author. Fax: (91)80-335-1511.
pink colour of the permanganate in the estimation procedures. These experiments were carfled out on uranium leach liquors of phosphatic siliceous calcite-dolostone samples from Cuddapah Basin, Andhra Pradesh, India. The results are presented in this paper.
2. Experimental
21. R e ~ e n ~ 20% sugar solution is prepared by dissolving 20 g of commercial sugar in water and making up to 100 ml. The solution is filtered through Whatman No. 40 filter paper. Fresh solution is prepared every day as the sugar solution develops bacterial growth due to fermentation on standing, making it unsuitable for pipetting. All other chemicals used are of Analar quality.
2.2. Instrumentation A Shimadzu UV-240 model double beam spectrophotometer was used for the absorption measurements.
0039-9140/95/$09.50 O 1995 Elsevier Science B.V. All fights reserved S S D I 0039-9140(95)01530-0
946
D.S.R. Murty et al. / Talanta 42 (1995) 945-948
Table 1 Percentage recoveryof carbonate, bicarbonate and phosphate No.
l 2 3 4 5
Carbonate
Bicarbonate
Phosphate
Added (rag)
Found (rag)
Recovery Added F o u n d Recovery Added F o u n d (%) (mg) (nag) (%) (rag)
Recovery (mg)(%)
3.0 6.0 9.0 10.0 12.0
3.0 5.8 8.8 9.6 11.7
100 97 98 96 98
93 94 99 97 96
5.0 10.0 15.0 20.0 25.0
4.9 9.7 15.2 20.1 25.0
2.3. Procedure
A 50.0 ml aliquot of the carbonate leach liquor of uranium containing potassium permanganate as an oxidant is taken in a 100 ml beaker. Freshly prepared 20% sugar solution (10.0 ml) is added and stirred with a glass rod. The permanganate in the liquor is reduced to MnO2xH20 and the precipitate settles to the bottom within 30 rain, leaving a clear, colourless supernatant liquid. Three aliquots, 10.0 ml each of the supernatant solution, are taken into three separate breakers for analysis. From the first two beakers, carbonate and bicarbonate are determined by titration with standard HC1 solution [2], using phenolphthalein and bromocresol green as indicators. In the third aliquot, phosphate is determined spectrophotometrically using a vanadomolybdo reagent [3]. A 10.0 ml aliquot of the 20% sugar solution is taken as a blank for each determination.
3. Results and discussion
Because of the presence of dolomitic limestone in Cuddapah basin samples only carbonate leaching for uranium extraction is to be adopted, as the acid consumption in acid leaching is very high. Under alkaline carbonate leaching parameters, uranium present in association with phosphates and Fe-Ti mineral complexes is not solubilised [4]. Potassium permanganate is one of the oxidants studied for oxidising tetravalent uranium to the hexavalent state. As little as 0.01 ml of 0.02 M permanganate imparts a pale pink colour to 100 ml of water [2], so it has to be decolourised to avoid interferences in the determination of carbonate, bicarbonate and phosphate. 17 reductants were tried to reduce the pink coloured permanganate ion either to the colourless
98 97 101 100 100
3.0 5.0 8.0 10.0 14.0
2.8 4.7 7.9 9.7 13.4
manganous ion or to the precipitation of manganese dioxide. All reductants studied are able to decolourise the alkaline leach liquor, but each reductant except sugar has its own advantages and disadvantages for uniform applicability. Sodium chloride or hydrochloric acid decolourises the leach liquors only on heating, whereas hydroxylamine hydrochloride and stannous chloride reduce even in cold conditions. The resulting solution can be used for phosphate estimation only, but carbonate and bicarbonate cannot be estimated quantitatively. Sodium thiosulphate and hydrogen sulphide decolourise, but the resulting solution is very turbid and hence not useful for further estimations. Sodium sulphate and sulphurous acid also reduce permanganate. Although there is no interference for the phosphate estimation, the same cannot be assumed for other estimations. In the presence of potassium iodide the liberated iodine imparts an unwanted brown coiour to the solution rendering it useless for further estimations. When ferrous iron is used for reduction, the iron precipitates and interferes in all the estimations. Ethyl alchohoi or ascorbic acid also give colourless solutions; they not only give turbidity to the leach liquors, but also reduce the molybdovanadate reagent used for phosphate estimation. If hydrogen peroxide is used, the excess should be boiled off. Oxalic acid reduces only on heating, disturbing the carbonate/bicarbonate system. The reducing effect of activated charcoal on permanganate is very slow and also leads to inaccurate values for carbonate and bicarbonate. The fact that permanganate is inherently unstable in the presence of manganese(II) ions, particularly in an alkaline medium [2], has been made use of for decolourisation. In the result-
947
D.S.R. Murty et al. / Talanta 42 (1995) 945-948 Table 2 Analytical data of carbonate, bicarbonate and phosphate in uranium leach liquors No.
l 2 3 4 5 6 7 8 9 l0
Sample
AMD-I AMD-2 AMD-3 AMD-4 AMD-5 AMD-6 AMD-7 AMD-8 AMD-9 AMD-10
Carbonate (g I - ')
Bicarbonate (g I- ~)
Phosphate (g I - ~)
Present method
pH titration
Present method
pH titration
Present method
Emission method
6.3 14.5 41.4 8.2 18.3 9.1 11.2 6.5 16.1 4.6
6.5 14.3 41.4 8.5 18.3 9.3 I 1.4 6.3 16.0 4.6
2.4 4.8 23.2 2.7 5. l 3.1 1.0 1.2 5.1 1.0
2.3 4.9 23.1 2.5 5.3 3.0 1.0 1.4 5.3 l.l
0.2 6.0 5.3 2.1 30.4 17.9 1.6 35.6 4.9 0.6
0.1 6.1 5.4 2.1 30.8 18.1 1.5 35.9 4.9 0.5
ing solution, phosphate can be determined; however, the carbonate and bicarbonate values are not accurate. Although sucrose, the commercial sugar is a non-reducing sugar [5], as far as the reduction of Fehling solution is concerned, because of the linkage of the aldehyde group of the glucose part with the ketonic group of the fructose part, its addition to the pink coloured alkaline leach liquors causes the reduction of Mn 7+ to MnO2xH20. This may be due either to the presence of monosaccharide (glucose/fructose) which is a reducing sugar or to the high redox potential of the permanganate [5]. In the supernatant solution of leach liquors after decolourisation by the addition of sugar solution, phosphate is determined by the molybdovenadate method [3], and carbonate and bicarbonate are determined by titration with standard HC1 [2] using phenolphthalein and bromocresol green as indicators without any interference. Glucose also reduces the pink colour of permanganate. However, in the case of pure glucose, the only advantage is that the reduction takes place within 10-15 min as opposed to the 30 rain necessary for commercial sugar. However, this time factor is outweighed by the cost of glucose with respect to commercial sugar, to provide an economic method. Percentage recovery values calculated by adding known amounts of phosphate, carbonate and bicarbonate and determined by the present procedure are given in Table 1. The recovery values obtained vary from 93 to 101%. The values of phosphate, carbonate and bicarbonate in some alkaline permanganate leach liquors of uranium after decolourisation with sugar solution are presented in Table 2. TA~. 4~-7-F
The values of carbonate and bicarbonate obtained by pH titration with a pH meter [2] and phosphate values obtained by a plasma emission technique [6] are also presented in this table. The values are in fairly good agreement, reflecting the accuracy of the method.
4. Conclusion The pros and cons of various reagents for the reduction of permanganate show that sucrose is the best reductant to obtain a clear and colourless solution for the estimation of carbonate and bicarbonate by titrimetric, and phosphate by absorptiometric methods without any interference. The advantages of this procedure are simplicity, rapidity, economy, and freedom from interference to give uniform applicability.
Acknowledgements The authors are thankful to Sri.KK. Dwivedy, Director, AMD and to Sri.M. Vasudeva Rao for permission to publish the paper, and for encouragement. The inspiration in the form of continuous guidance from Sri.K.P. Cheria is gratefully acknowledged. The authors are also thankful to R. Anupama for her technical assistance.
References [1] Rober C. Merritt, The Extractive Metallurgy of Uranium, prepared under contract with the United States
948
D.S.R. Murty et al. / Talama 42 (1995) 945-948
Atomic Energy Commission (USAEC), Washington, 1971, pp. 85, 104. [2] A.I. Vogel, Text Book of Quantitative Chemical Analysis, 5th edn. (revised), ELBS Longrnan, London, 1991, pp. 299, 369, 279. [3] L. Shapiro, Rapid analysis of silicate, carbonate and phosphate rocks, revised edn., Geological Survey Bulletin No. 1401, Washington, 1975, p. 37.
[4] Ravi Kaul, R.N. Shankaran, Y. Lakshminarayana and Minati Roy Exploration and Research for Atomic Minerals, Vol. 4, AMD, Hyderabad, India, 1991, p. 197. [5] I.L. Finar, Organic Chemistry, Vol. l, ELBS, Longman, London, 1964, pp. 461,439. [6] R.K. Winge, C.A. Fassel, V.J. Peterson and M.A. Floyd, Inductively Coupled Plasma-Atomic Emission Spectroscopy, Elsevier, Amsterdam, 1987.