The extraction of niobium and tantalum from oxalate solution
has been studied using the esters of 5-hydroxy-l-aryl-4-pyridone-
2-carboxylic acid as extractants. The influence of the concentration
of hydrochloric, sulphuric, hydrofluoric and oxalic acids on the
extraction of niobium and tantalum has been investigated. The
dependence of the extraction of these metals upon the concentration
of the extractant in the organic phase was also studied. A
procedure for the separation of niobium from tantalum directly
from oxalate solution is described. The radionuclides 95Nb and 182Ta
were used for the determination of the distribution coefficients

K. Blažević

M. J. Herak

M. Janko

English

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CCA-542 CR 0 AT I CA CHEM I CA A CT A 41 (1969) 85 542.61 :546.883 :516.882 Original Scientific Paper 5-Hydroxy-1-aryl-4-pyridone-2-carboxylic Acid Esters as Chelating and Extracting Agents. I. Extraction and Separation of Tantalum(V) and Niobium(V) from Oxalate Solutions'~ M. J. Herak, M. Janko, and K. Bl.atevic Institute »Rueter Boskovic« and Institute of Inorganic and Analytical Chemistry, University of Zagreb, Zagreb, Croatia, Yugoslavia Received March 3, 1969 The extraction of niobium and tantalum from oxalate solution has been studied using the esters of 5-hydroxy-l-aryl-4-pyridone--2-carboxylic acid as extractants. The influence of the concentrat-ion of hydrochloric, sulphuric, hydrofluoric and oxalic acids on the extraction of niobium and tantalum has been investigated. The dependence of the extraction of these metals upon the concentrat-ion of the extractant in the organic phase was also studied. A procedure for the separation of niobium from tantalum directly from oxalate solution is described. The radionuclides 95Nb and 182Ta were used for the determination of the distribution coefficients. INTRODUCTION The extraction and separation of Nb(V) and Ta(V) by various organic compounds has been the subject of a number of investigations, as previously r eported1•2. The extraction of Nb(V) and Ta(V) and their separation by organic compounds from the 4-pyridone series is discussed in the present paper. The synthesis of these compounds and their absorption spectra have already been described3- 5. It has been found that compounds similar to 4-pyridones form with metals very stable chelate compounds6- 0. The extraction properties of 4-py ridone compounds are investigated with regard to the Ta(V) and Nb(V) metallic ions. The following esters of 5-hydroxy-1-aryl-4-pyridone-2-carboxylic acids w er e used: 0 Compound R1 R2 HOD C6Hs I I COOR I CH a II C2Hs C6Hs N 1 III C2H5 4-CH3 · C6H4 I IV C2Hs 4-Br · C6H4 R2 Tantalum(V) and niobium(V) were extracted from oxalate solutions in the presence or absence of mineral acids. The conditions for a selective extraction of these metals were studied. * Part of this publication is taken from the thesis submitted by M. Krnjako-vic-Janko to the Faculty of Science, The University of Zagreb, in partial fulfilment of the requirements for the M. Sc. Degree. 86 M. J. HERAK, M. JANKO, AND K. BLA2EVIC EXPERIMENTAL The extraction of metals was performed in Erlenmeyer flasks using equal volumes (2 ml.) of the organic and aqueous phase at approximately 20° C. Shaking time necessary to obtain equilibrium was experimentally determined. After the equilibrium has been attained the systems were centrifuged to speed up the separation of the phases. Radionuclides were used for the determination of the distribution coefficients. The radioactivity of the organic and aqueous phase was measured with a gamma scintillation counter (Nal/Tl). All the measurements were taken after one single extraction. The radionuclide 95Nb was provided as an oxalate complex with excess of oxalic acid from Radiochemical Centre Amersham. 182Ta was prepared by irradiating a solution of tantalic acid in oxalic acid in the nuclear reactor of the »Boris Kidric«, Institute, Vinca, Yugoslavia. The standard solutions of niobium and tantalum were obtained from Nb20 0 and Ta205 (Hopkin and Williams), previously fused with KHS04• The m elt was dis-solved in oxalic acid. The prepared solutions were then precipitated with ammonia, the precipitate washed with 20/o NH4Cl solution, and dissolved in oxalic acid. The concentrations were checked by gravimetric analysis. It is known10 that simple dilution of solutions of niobic or tantalic acids in oxalic acid may bring about hydrolysis which is made evident by the formation of an opalescence or precipitate on standing. Therefore niobic and tantalic acid in oxalic acid solutions were always freshly prepared. Shaking time of 30 min. was found to be sufficient to obtain reproducible results, the equilibrium being attained after about 15 min. RESULTS AND DISCUSSION For the preliminary investigation of the extraction of Nb(V) from oxalate solutions containing different concentrations of hydrochloric acid, 4-pyridone compounds I, II, III and IV were used (Fig. 1). In further investigation, only the compounds II and III were used. I 2 --·-YI'\ '"l"'b --·- ) r\\'\ 1 - 0 - 1 )7~ \~ ,/0 \ ....... ~ y '~-0 0 \i \ I \ ~. u\ 0 2 -2 -1 log concn. HCl [MJ Fig 1. Distribution ratio D of Nb(V) as a function of HCl concentration in the presence of sxio-• M oxalic acid in the aqueous phase. Concentration of niobium lXlO-< M; concentration of organic compounds 3XlO·• M; Compound I (0); compound II (X); compound III (Q); Compound IV (L',). SEPARATION OF TANTALUM AND NIOBIUM 87 The Influence of the Concentration of HCl, H2SO~ and HF on the Extraction of Nb(V) and Ta(V) from Oxalate Solutions by the Compound III The dependen~e of the distribution coefficient, obtained by the extraction of niobium from the oxalate solution, on the concentration of mineral acids is shown in Table I. The concentration of niobium was 1 X 10-4 M and that of TABLE I Dependence of the Extraction of Niobium(V) on the Concentration of HCl, H 2S04, and HF Niobium 1 X 10-4 M, oxalic acid 8 X 10-3 M, compound III 3 X 10-3 M. Concn. D Concn. D Concn. D HCl (M) H 2S04 (M) HF (M) 0.1 2.28 0.5 7.7 0.003 2.13 0.5 8.10 1.0 10.5 0.01 1.32 1.0 15.0 1.5 14.7 0.03 0.171 1.5 20.95 2.0 14.05 0.05 0.045 2.0 18.75 3.0 9.63 0.1 0.008 3.0 6.62 4.0 2.63 0.3 0.003 4.0 1.43 5.0 0.22 0.5 0.002 oxalic acid 8 X 10-a M. A 3 X 10-3 M solution of the compound III in chloroform was used in the extraction. The results indicate that the distribution coefficient is increasing with the increase of the concentrations of HCl or H2S04 reaching a maximum at 1.5 M concentrations of these acids; further increase causes the decrease of the distribution coefficient. By performing similar experiments with Ta(V) results presented in Table II were obtained. TABLE II Dependence of the Extraction of Tantalum(V) on the Concentration of HCl, H2S04 and HF Tantalum(V) 1 x 10·4 M, oxalic acid 8 X 10·3 M, compound III 3 X 10-3 M. Concn. D Concn. D Concn. D HCl (M) H2S04 (M) HF (M) 0.05 1.27 0.05 4.78 0.005 23.25 0.1 0.53 0.1 4.53 0.01 34.7 0.3 1.54 0.3 6.77 0.03 15.9 0.5 1.86 0.5 11.3 0.05 8.33 1.0 3.84 1.0 37.4 0.1 2.22 2.0 25.8 2.0 164 0.3 0.22 3.0 19.7 3.0 175 0.5 0.046 5.0 0.89 5.0 33.8 1.0 0.024 The extraction of niobium and tantalum decreases with the increase of the concentration of HF and is negligible for niobium at 0.1 M HF and for tantalum at 1 M HF (Tables I and II). 88 l\II. J . HERAK, M. JANKO, AND K. BLA2EVIC The Influence of the Oxalic Acid Concentration on the Extraction of Niobium(V) and Tantalum(V) Fig. 2 shows the results of the extraction of Nb(V) and Ta(V) from the oxalate solution as a function of the oxalic acid concentration. The concentrat-ion of the metal was 5 X 10-5 M and that of the organic compounds II and III 5 X 10-3 M. The results obtained show that there is a linear dependence of log D on the log concentration of H 2C2Q 4 of both Ta(V) and Nb(V) extracted 2 '~N 1 \ 0 \l ,, \. LI 0 ~- u'o, \ \\Slop•-1 "-o 'o " \ 0 0 ~ Slope =--l5 \ \ 1 \ \. \ -2 -J - 2 -1 --+ log concn. Hi<? o, (M] Fig. 2. Distributution ratio D of Ta(V) and Nb(V) as a function of oxalic acid concentration in the aqueous phase. Concen tration of tantalum (00 ) : 5X lO-• M; niobium (.) : 5X l0-5 M; com-pound II <O•> 5X 10-• M; compound III (0 ) 5X 10-a M. with compound II. The slopes for niobium and tantalum were -1.5 and - l.O, respectively. The extraction is found to decrease by further increase of the oxalic acid concentration; this being more pronounced for Nb than for Ta. The extraction of tantalum with compound III reveals no linear dependence upon the oxalic acid concentration. The Dependence of the Extraction of Niobium(V) and Tantalum(V) on the Concentration of the Ligands The influence of the concentration of the compound III in the organic phase on the extraction of tantalum is shown in Fig. 3. The total concentration of the compound III used was plotted on the abscissa. When the experiments were performed at oxalic acid concentration of 1.12 X 10-2 M a metal-ligand ratio 1 : 2 was obtained while, at higher concentration, this was found to be 1: 2.5. SEPARATION OF TANTALUM AND NIOBIUM 89 I I ll o/ /J J 2 Slop~ :2 0 0 ~ /;j I Slop~=2S I , ,1 I I ~! / !? I ·~ / 0 ~ o'----+------lt---1<'------l------1 -1 If I I " -2 I -4 -3 -2 -4 -3 -2 - log concn. Compound Ill [Mj I oa concn. Comoound II rMJ Fig. 3. Distribution ratio D of Ta(V) as a function of the compound III concentration in the organic phase. Concentration of tantalum 5X 10-s M and oxalic acid 1.12X l0" M (0) ; 2x 10-2 M (D}; 2x10-1 M (6). Fig. 4. Distribution ratio of Ta(V) as a function of the comp·ound II concentration in the organ; e phase. Concentration of tantalum 5X 10-s M and oxalic acid 1.12 X 10-2 M (0); 2x 10-2 M (6 ); 2x 10-1 M (X ) . 2 1 f I /I Slop~= 2.5 l / / I / I I • 0 ~ 1 I I I -2 i -4 -3 -2 -- log concn. Compound II [MJ Fig. 5. Distribution ratio D of Nb(V) as a fuction of the compound II concentration in the organic phase. Concentration of niobium 5X 10-s M and oxali;! acid 8X10-3 M (0); 2x 10-2 M <•>· 90 M. J. HERAK, M. JANKO, AND K. BLAZEVIC Similar experiments were carried out using compound II (Fig. 4). Parallel curves were obtained at different concentrations of oxalic acid and in all cases the metal-ligand ratio was 1 : 2.5. It has been found that the influence of the concentration of compound II is the same for the extraction of Nb(V) as for Ta(V) (Fig. 5). Two parallel curves were obtained having a slope of 2.5. The extraction of Nb(V) was performed at the oxalic acid concentrations 8 X 10-3 and 2 X 10-2 M, the niobium concentration being 5 X 10-5 M. The extraction mechanism is too complicated to be fully explained by the results presented in this paper. Further investigations are in progress. Direct Separation of Ta(V) and Nb(V) from Oxalic Acid Solution In order to determine the possibilities of separating tantalum from niobium the extraction of both metals has been studied under identical conditions. The concentration of niobium and tantalum was 1 X 10-4 M and that of the compound III 5 X 10-3 M . Fig. 6 gives the results of extraction plotted as the extraction percentage vs. the concentration of the oxalic acid. At low oxalic acid concentrations 93°/o of the niobium present was extracted. However, when the concentration of oxalic acid was increased the extraction decreased and was practically negligible at 5 X 10-1 M oxalic acid. The behaviour of tantalum was found to be different from that of niobium. At low oxalic acid concen-trations up to 980/o of the tantalum was extracted. Its extraction was slightly 1001--- ---+-----+-----+----j 00-~ o-0 ........... Ta ~~ / o'o 601------+-Y<-----<>--t--~--+--4 ~ ,,j ~ 60 1--- ---+--- \+----+--- - - +-----< " \ ! ~ D\ 1" \ ~1------+----~,r+-----+----! I\~ o...._ ............ Ta! t 6 \ \ \ ~o ~Q, ;°/ \~- -!.,I Nb\ ;a. \ 0 ]' i -1 -2 -2 -1 -3 -2 -1 - log concn. H2C204 [M] - log concn. H2 cp4 [M] Fig. 6. Extraction of Nb(V) and Ta(V) from oxalate solution aB a function of oxalic acid con-centration in the aqueous phase. Concentration of niobium ([']) and tantalum (0) 1x10-• M and the compound III sx10-s M. Fig. 7. Distribution ratio D of Nb(V) and Ta(V) as a function of oxalic acid concentration in the aqueous phase. Oxalate ions (H2C20, + K2C20•) were constant 5.6X10-1 M. Concentration of tan-talum (06) and niobium <••> sx10-s M. Compound II <O•> and compound III (6.A.) sx10-• M. SEPARATION OF TANTALUM AND NIOBIUM 91 reduced by increasing the concentration of oxalic acid up to 5 X 10-2 M; how-ever, further increase of the concentration up to 3 X 10-1 M again increased the extraction. At concentrations of oxalic acid higher than 3 X 10-1 M the extraction of tantalum decreased. In order to find out whether this behaviour is caused by the change of the concentration of oxalate ions or H+ ions the con-centration of H 2 C2Q4 + K 2C20 4 = 5.6 X 10-1 M was kept constant. The results (Fig. 7) demonstrate that the extraction of niobium and tantalum strongly depends on the concentration of the H+ ions, indicating that a good separation of tantalum from niobium is possible when the oxalic acid concentration is higher than 3 X 10-1 M. The separation factor is about 104• The purpose of the experiments described in this paper was to ascertain whether the 4-pyridones would be useful for the extraction of niobium and tantalum. As far as we know these compounds were not as yet used in the extraction of metals. Since the oxalato complexes of niobium and tantalum are soluble in water and easily accessible, the investigation of the extraction from oxalic acid solutions seemed to be of particular importance. REFERENCES 1. V. Jago di c, M. M. Her a k, and M. J. Her a k, Radiochimica Acta 6 (1966) 64. 2. M. M. Her a k, V. Jago di c, and M. J. Her a k, Croat. Chem. Acta 39 (1967) 89. 3. V. Hahn and S. Kuk o 1 j a, Croat. Chem. Acta 33 (1961) 137. 4. K. B 1 a z e vi c and V. Hahn, Croat. Chem. Acta 38 (1966) 113. 5. K. B 1 a z e vi c and N. Trina j s t i c, Croat. Chem. Acta 39 (1967) 25. 6. B. Bryant and W. C. Ferne 1 i us, J. Am. Chem. Soc. 76 (1954) 5351. 7. R. L. Pelsok, R. L. Meeker, and I. D. Shields, J. Am. Chem. Soc. 83 (1961) 2081. 8. M. I. Go p j a e v, V. S. Vo 1 k ova, and G. A. To 1st i k o v, Zh. Obshch. Khim. 28 (1958) 2102. 9. G. A. To 1st i k o v, Zh. Obshch. Khim. 29 (1959) 2372. 10. F. Fairbrother, The Chemistry of Niobium and Tantalum, Elsevier Pu-blishing Company, Amsterdam, 1967. IZVOD Ekstrakcija i separacija tantala(V) i niobiuma(V) s esterima 5-hidroksi-1-aril-4-piridon-2-karbonskih kiselina M. J. Herak, M. Janko i K. Blazevic Ispitivana je ekstrakcija niobiuma i tantala iz oksalatnih otopina pomocu estPra 5-hidroksi-1-aril-4-piridon-2-karbonskih kiselina otopljenih u kloroformu. Koefici-jenti ekstrakcije su odredivani pomoeu radionuklida 9'·Nb i 1s2Ta. Ispitan je utjecaj koncentracije klorovodicne, sumporne i fluorovodicne kiseline na ekstrakciju niobiuma i tantala iz oksalatno kiselih otopina. Studirana je ovisnost ekstrakcije niobiuma i tantala o koncentraciji ekstraktanta u organskoj fazi i o koncentraciji oksalne kiseline u vodenoj fazi. Odredeni su uvjeti pod kojima je moguea separacija Ta(V) i Nb(V) direktno iz oksalatno kisele otopine. INSTITUT »RUDER BOSKOVIC« I lNSTITUT ZA ANORGANSKU I ANALITICKU KEMIJU SVEUCILISTA U ZAGREBU Primljeno 3. ofojka 1969. 

5-Hydroxy-1-aryl-4-pyridone-2-carboxylic Acid Esters as Chelating and Extracting Agents. I. Extraction and Separation of Tantalum(V) and Niobium(V) from Oxalate Solutions

5-Hydroxy-1-aryl-4-pyridone-2-carboxylic Acid Esters as Chelating and Extracting Agents. I. Extraction and Separation of Tantalum(V) and Niobium(V) from Oxalate Solutions

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