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Solvent extraction of zirconium(IV) from chloride media by D2EHPA in kerosene
Hydrometallurgy 63 (2002) 149 – 158 www.elsevier.com/locate/hydromet
Solvent extraction of zirconium(IV) from chloride media by D2EHPA in kerosene R.K. Biswas*, M.A. Hayat Department of Applied Chemistry and Chemical Technology, Rajshahi University, Rajshahi 6205, Bangladesh Received 29 June 2001; received in revised form 9 November 2001; accepted 9 November 2001
Abstract The apparent equilibrium of extraction of zirconium(IV) from chloride media by D2EHPA in kerosene is found to depend on ageing of the aqueous phase. When the aqueous phase, just after preparation, is used in the extraction study, reproducible results are not obtained. With increasing ageing time, the apparent distribution ratio (D) is increased for a particular set of experimental parameters. The system has been thoroughly investigated for 1- and 30-day ageing. In both cases, the apparent equilibration time is 15 min. D is decreased with increasing Zr(IV) concentration in the aqueous phase but the nature depends on ageing. In both cases of ageing, logD vs. log[HCl] plots are peculiar in nature. For 1-day ageing, the slope is f 1 above 3 M HCl and also below 0.3 M HCl, and is zero within 0.3 – 3.0 M HCl. In contrast, for the 30-day aged system, the slope is f 2 above 2 M HCl, f 1 below 0.5 M HCl, and zero between 0.5 and 2.0 M HCl. The extractant dependencies are 2 and 4 for 1- and 30day aged, systems respectively. Chloride ion has a large effect on D for the 1-day aged system, but no effect on D for the 30-day aged system. From the temperature dependence data, DH values have been determined. The possible extraction equilibrium reactions have been suggested and supported by the loading tests, as well as by the molecular weight, Zr/P ratio, Zr/Cl ratio and infrared spectral data of the extracted species. D 2002 Published by Elsevier Science B.V. Keywords: Extraction equilibrium; Zr(IV); Chloride medium; D2EHPA
1. Introduction Di-2-ethylhexylphosphoric acid (D2EHPA, H2A2) is a commercial extractant and is used widely in extractions of U(VI), V(IV), Th(IV). Ti(IV), Fe(III), Be(II), Zn(II), Mo(VI), Hf(IV), Ln(III), etc. (Sekine and Hasegawa, 1977; Ritcey and Ashbrook, 1979). Previously (Biswas and Hayat, 1985; Kletenik and Bykhovskaya, 1965; Tedesco et al., 1967; Sato, 1970; Yagodin and Tarasov, 1971; Sato and Nakamura, 1971), it has been
*
Corresponding author. Fax: +88-0721-750-064. E-mail address: [email protected] (R.K. Biswas).
reported that D2EHPA can be used for the extraction of Zr(IV) from various acid solutions. However, most of those studies have been done in narrow aqueous acidity regions and may be considered as very preliminary, particularly those using chloride media. In most cases, the peculiarity of this extraction system has been disregarded and no proper explanation for the extraction data has been given because of inadequate knowledge of the aqueous Zr(IV) species. Very recently, the compositions of Zr(IV) species in Cl media ([Cl ] or [HCl] = 0.1 – 1.0 M) have been reported by Singhal et al. (1996). In 1 M Cl medium, the only species present is partly hydrolysed [Zr4(OH)8(H2O)16Cl6]2 + , and this species is changed
0304-386X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 3 8 6 X ( 0 1 ) 0 0 2 2 0 - 1
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totally to [Zr8(OH)20(H2O)24Cl12] in 0.1 M HCl medium. In these complexes, Zr(IV) is 8-coordinated. In [Zr4(OH)8(H2O)16Cl6]2 + , four Zr atoms take the square planar arrangement and each Zr(IV) has four – OH bridgings and carries four H2O as coordinated ligands; whereas six Cl ions take their positions in the secondary coordination sphere of the complexes. In this complex, OH /Zr4 + ratio is 2. But in [Zr8 (OH)20(H2O)24Cl12], two [Zr4(OH)8(H2O)16Cl6]2 + sheets are joined together stackwise through four Zr – OH – Zr bridging with the elimination of four H3O + . In this chargeless polynuclear complex, 12 Cl ions exist in the secondary coordination sphere of the complexes. In the octameric complex, the OH /Zr4 + ratio is 2.5. The species existing in the higher concentration of HCl (>1 M) is still not reported definitely. However, according to Hannane et al. (1990), the addition of concentrated HCl to ZrOCl2 solution leads to the formation of one or several monomeric species of composition [Zr (H2O)8 n Cln](4 n) + . This paper discusses the liquid – liquid extraction characteristics of Zr(IV) from HCl solution of wide concentration range by D2EHPA dissolved in kerosene and the extraction equilibrium reactions have been proposed considering the existence of the above species in the aqueous phase and supported by the loading test, molecular weight, Zr/P ratio, Zr/Cl ratio and infrared spectral data.
2. Experimental 2.1. Reagents D2EHPA was procured from BDH. It had a purity of 98% and was used as such. As a source of Zr(IV), ZrOCl28H2O (M.C. & Bell, 98%) was used. Kerosene was collected from the local market and redistilled to collect the fraction distilling over 220 – 260 C. It was colorless and mostly aliphatic in nature. All other chemicals were of reagent grade and used without further purification. 2.2. Analytical The concentration of Zr(IV) in the aqueous phase was determined by the EDTA – pyrocatechol violet
method (Charlot, 1964) at 590 nm using a WPA S104 spectrophotometer. For pH adjustment of the aqueous Zr(IV) solutions (pH 5.2) required in the above method, a Mettler Toledo 320 pH meter having a combination electrode and an autotemperature compensator was used. For determining the molecular weight of the extracted complexes, the Cryoscopic method using cyclohexane as solvent was used. For determining Zr/P and Zr/Cl atom ratios in the extracted solid complexes, a definite amount of the solid complexes was decomposed by concentrated H2SO4, heated until all carbon was oxidised to make the solution clear and colourless. It was then diluted to 100 mL carefully with water. This solution was analysed for the contents of Zr(IV) (Charlot, 1964), P in the form of PO4 3 by the Molybdate –Hydrazine sulphate (molybdenum blue) method (Vogel, 1978) and chloride by the AgNO3 – K2CrO4 titration method (Vogel, 1978) and verified by the AgCl precipitation method. The infrared spectra of the isolated solid Zr –D2EHP complexes were taken in Nujol mull, as well as in the KBr disc form by courtesy of the Chemical Division of Fisons Bangladesh, Tongi, Dhaka. The spectroscope was a double beam SP3-300 Pye Unicam Infrared spectrophotometer, Cambridge, England. 2.3. Extraction procedure Aliquots of 25 mL of each phase were taken in a 125-mL separating funnel. It was stoppered tightly and shaken for about 30 min in a thermostatted water bath operated at 30 C. The aqueous phases containing definite amounts of Zr(IV) and (H + , Na + ) Cl were prepared and left for 1 or 30 days before extractions. Freshly prepared aqueous solutions were also used for some preliminary experiments. After mixing, the phases were allowed to settle and the aqueous phase was separated. It was analysed for its Zr(IV) content. The difference of the concentrations of Zr(IV) in the aqueous phase between the initial and final states gives the concentration of Zr(IV) in the organic phase. On dividing the Zr(IV) concentration in the organic phase by that in the aqueous phase at equilibrium, the value of apparent distribution ratio (D) was obtained. In all cases, the average of at least three sets of experimental data is reported and the reproducibility is within 5%.
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3. Results and discussion
Fig. 1. Dependence of extraction on contact time. [H2A2] = 0.01 M, [HCl] = 0.3 M, [Cl ] = 3 M, [Zr(IV)](ini) = 330 mg/L. Ageing time: (.) 0 day; (E) 1 day; (x) 30 days.
Initially, many experiments were repeated to obtain reproducible results and it was thought that the reproducible results could not be obtained due to the continuous change in composition of Zr(IV) species in the aqueous phase. The change can be seen in Fig. 1. In all three cases of solution conditioning, it is found that the value of logD is increased with increasing phase contact time up to 15 min. On further increasing the contact time, no change in logD is observed. Therefore, the apparent equilibration time is 15 min, but 30 min has been allowed subsequently to ensure equilibration for various experimental parameters. It can be noticed that logD is increased considerably with the time allowed for ageing (D is increased 10-fold when freshly prepared solution is aged for 30 days). Fig. 2 shows logD vs. ageing time plots for two experimental parameters. In both cases, logD is increased with increasing ageing time. Within the first day, the variation is large and then with further increase in ageing time, the variation is slowed down. In the subsequent studies, data obtained for 1- and 30day aged solutions were obtained. The variation of D
2.4. Procedure for loading and isolating solid complexes For these purposes, 100 mL of 0.1 M D2EHPA in kerosene was taken in a 500 mL elongated type separating funnel and to it equal volume of Zr(IV) solution (2 kg m 3) of definite acidity (0.1, 1.0 and 5.0 M in HCl) was added. The two phases were mixed on a horizontal shaker, disengaged and the aqueous phase was analysed for its Zr(IV) content. The organic phase was repeatedly contacted with fresh aqueous solutions until the saturation of the organic phase with Zr(IV) was obtained. The organic phase saturated with Zr(IV) was transferred into a beaker. Kerosene was allowed to evaporate out under suction at around 80 C. Yellowish transparent solid or semisolid masses were obtained. Adhered free D2EHPA (if any) from these masses was eliminated by washing with 50% alcohol. The complexes were then dried under vacuum and finally in an oven at 90 C for 72 h and stored in a CaCl2 desiccator.
Fig. 2. Dependence of extraction on ageing time. [H2A2] = 0.01 M, [Cl ] = 3 M, [Zr(IV)](ini) = 492 mg/L, [HCl]: (.) 0.3 M, (E) 3 M.
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Fig. 3. Dependence of the extraction on Zr(IV) concentration in the aqueous phase of 1-day ageing. [HCl] = 1 M, [Cl ] = 3 M, [H2A2]: (6) 0.01 M; (.) 0.20 M; (D) 0.03 M; (E) 0.05 M.
with [Zr(IV)]ini as log –log plots is shown in Fig. 3 for 1-d aged system. The slopes of the plots do not change with the variation of [D2EHPA] used in extraction. In all cases, logD is decreased almost linearly with log [Zr(IV)]ini, at least up to 11 mM Zr(IV). With further increase in Zr(IV) concentration in the aqueous phase, logD is little varied though Zr(IV) concentration in the organic phase is increased considerably. The value of logD should be independent of [Zr(IV)] provided the equilibrium pH and [D2EHPA] are kept constant but only if mononuclear complexes of Zr(IV) are present which seems not to be the case. Since the aqueous acidity is high, the change in logD value may be regarded, in the first instance, to be due to the change in equilibrium [D2EHPA]. But when the data are corrected for an presumed constant equilibrium extractant concentration, the independence of D on initial [Zr(IV)] is not noticed. The arrowheaded lines in Fig. 3 represent the variation of D with [D2EHPA] (for these lines, the abscissa represents log[D2EHPA]ini with similar divisions). It is found that the extractant dependence depends on the [Zr(IV)]ini in the aqueous phase. Up to [Zr(IV)] = 0.01 M, the extractant dependence is
f 2 and it is decreased to f 1 for 0.033 M Zr(IV). The variation of the extractant dependence on [Zr (IV)] indicates the variation of the composition of Zr (IV) species in either of the phases with increasing concentration in the aqueous phase. Fig. 4 represents the Zr(IV) dependence data for 30-day aged system. For all three D2EHPA systems, the shape of the plots is greatly changed compared to those given in Fig. 3. For less concentrated solutions of Zr(IV), D is increased, whereas for highly concentrated solutions of Zr(IV), D is decreased appreciably on increasing ageing time. The nonconstancy of logD with the variation of log[Zr(IV)]ini is not solely due to the small variations of equilibrium HCl and D2EHPA concentrations. The slopes of the arrowheaded lines representing the extractant dependence depend extensively on the [Zr(IV)]ini in the aqueous phase. The dependence is 2 for 2.3 mM Zr(IV); it is increased to 4 for 10.9 mM Zr(IV) and again decreased to 1 at 43.5 mM Zr(IV). This variation supports again the change in composition of the aqueous Zr(IV) species with its initial concentration in the aqueous phase. This is the main reason for nonconstancy of logD in Fig. 4, as
Fig. 4. Dependence of extraction on Zr(IV) concentration in the aqueous phase of 30-day ageing. [HCl] = 1 M, [Cl ] = 3 M, [H2A2]: (.) 0.01 M; (E) 0.02 M; (x) 0.03 M.
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well as in Fig. 3. In the subsequent experiments, f 0.5 g/L (5.4 mM) Zr(IV) solution has been used. The variation of D with [H + ] at [Cl ] 3 M is shown in Figs. 5 and 6, respectively, for 1- and 30-day aged systems as log ( log) plots. When log [HCl] < 0.48, then the plots actually represent the logD vs. log[HCl] plots. For all systems of 1-day ageing, the plots are of identical shape. Within 3 – 7 M HCl and 0.3– 0.1 M H + ([Cl ] 3 M), logD is decreased with decreasing acidity and the limiting slopes of the lines in these regions are 1. Therefore, in these acidity regions, H + or HCl is used up in forming the extractable species. On the other hand, within 0.3– 3 M [H + ], logD remains unchanged indicating that the reaction involved in this acidity region does not consume or liberate any H + or HCl. In the case of the 30day aged system in Fig. 5, the limiting slope in the higher acidity region is 2, in lower acidity region is 1 and in the intermediate region (0.5 –2.0 M) is zero. Therefore, 2 mol of HCl (instead of 1 mol for 1-day ageing system) are used up by one mole of existing Fig. 6. Dependence of extraction on [HCl] in the aqueous phase of 30-day ageing. [Zr(IV)](ini) = f 485 mg/L, [Cl ][3 M, [H2A2]: (.) 0.01 M; (E) 0.02 M; (x) 0.03 M; (6) 0.05 M.
Fig. 5. Dependence of extraction on [HCl] in the aqueous phase of 1-day ageing. [Zr(IV)](ini) = f 500 mg/L, [Cl ][3 M, [H2A2]: (6) 0.01 M; (.) 0.02 M; (D) 0.03 M; (E) 0.05 M.
Zr(IV) species to form the extractable species in the higher acidity region. The extractant dependence of D is shown in Figs. 7 and 8 as log –log plots for 1- and 30-day aged systems, respectively. For 1-day aged system, the least squares slopes of the logD vs. log[H2A2]ini plots are 2.00, 2.03 and 2.04 for 0.1, 1 and 7 M HCl systems, respectively. On the other hand, for 30-day aged system, the least squares slopes are 3.91, 3.95 and 4.00 for 0.1, 1 and 5 M HCl systems, respectively. On calculating the equilibrium D2EHPA concentration using the relationship: [D2EHPA]eq=[D2EHPA]ini n[Zr(IV), M](0), where n = 2 for 1-day ageing and 4 for 30-day ageing, the curves for logD vs. log [H2A2]eq plots in Figs. 7 and 8 are obtained. At lower [D2EHPA]eq, slopes approach zero for all cases, whereas, at higher [D2EHPA]eq slopes approach 2 for 1-day ageing (Fig. 7) and 4 for 30-day ageing (Fig. 8). It is, therefore, concluded that the solvation number varies with the [D2EHPA] region and it increases as [D2EHPA] increases. Fig. 9 represents logD vs. log[Cl ] plots for both 1- and 30-day aged systems in three distinct aci-
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[H + ] = 1 M. The respective values are 49 and 46.5 kJ mol 1 when [H + ] = 5 M. The variation of the extraction characteristics of Zr(IV) from HCl medium by D2EHPA with ageing period is summarized in Table 1. Table 2 represents the values of extraction equilibrium constant, Kex calculated from the intercepts of the lines in Figs. 5 –9. For the 1-day aged system, logKex values are 5.02, 4.06 and 3.40 with respective standard deviations of 0.08, 0.05 and 0.25 for 0.1, 1 and 5 M HCl systems, respectively. On the other hand, for the 30-day aged system, logKex values are 9.44, 9.32 and 8.75 (S.D. 0.24, 0.13 and 0.02, respectively) for 0.1, 1 and 5 M HCl systems, respectively. The results for the loading of D2EHPA in the organic phase by Zr(IV) are shown in Fig. 11 for [H + ] of 0.1, 1 and 5 M and for 1-day aged system only. It is noticed that Zr(IV) loading in D2EHPA depends greatly on the aqueous phase acidity.
Fig. 7. Dependence of extraction on extractant concentration for 1day ageing. [Zr(IV)](ini) = f 492 mg/L, [Cl ][3 M; [HCl]: (.) 0.1 M, s = 2.00, I = 4.61; (E) 1 M, s = 2.03, I = 5.03; (x) 7 M, s = 2.04, I = 5.39.
dity regions. For the 30-day aged system, D is independent of Cl concentration irrespective of the aqueous acidity used. But in the case of 1-day aged system, the slopes of the plots are 2 for 1 and 4 M HCl systems. However, for 0.1 M HCl system, a curve is obtained which has a slope of f 2 and 0.5 in higher and lower concentration regions of Cl , respectively. These results indicate that chloride ion is not involved in the extraction reactions of 30-day aged system. But for 1-day aged system, during extraction the involvements of 2 and of 1/2 g ions of Cl /mol of Zr(IV) occur in the higher and lower concentration regions of chloride ion, respectively. The temperature dependencies of D in three distinct acidity regions for both 1- and 30-day ageings are displayed in Fig. 10. For 0.1 M H + system, DH values are 30 and 43 kJ mol 1 for 1- and 30-day aged systems, respectively. Values are 46 and 48 kJ mol 1 for 1- and 30-day aged systems, respectively, when
Fig. 8. Dependence of extraction on extractant concentration for 30day ageing. [Zr(IV)](ini) = f 485 mg/L, [Cl ][3 M; [HCl]: (.) 0.1 M, s = 3.91, I = 8.30; (E) 1 M, s = 3.95, I = 9.20; (x) 5 M, s = 4.00, I = 9.90.
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For HCl concentration of 0.1 M, the saturated Zr (IV) concentration in 0.1 M organic phase is as high as 36.4 kg m 3. It is 18 and 4.22 kg m 3 for 1 and 5 M HCl systems, respectively. These results give that the loading capacities of D2EHPA for Zr(IV) are 56.52, 27.95 and 6.55 g/100 g D2EHPA for 0.1, 1 and 5 M HCl systems, respectively. At lower aqueous acidity, the high loading capacity indicates the strong affinity of Zr4 + for D2EHPA. A volume of 1 L of 0.1 M D2EHPA can extract as high as 36.4 g Zr(IV) from 0.1 M HCl medium; in the saturated complex, the Zr/D2EHPA (dimeric) ratio is 4. This ratio is 2 and 0.46 when extracted from 1 and 5 M HCl media, respectively. The molecular weight of the isolated complexes for 1-day aged system has been determined by the cryoscopic method using cyclohexane as solvent. The molecular weights of the complexes isolated from 0.1, 1.0 and 5 M HCl medium are 3110 F 150, 2160 F 70 and 1630 F 50 (average of five experiments in each case are given). The respective Zr/Cl and Zr/P ratios in the Fig. 10. Dependence of extraction on temperature. [H2A2] = 0.01 M; [Cl ][3 M; (.) [HCl] = 0.1 M, s = 1560; (E) [HCl] = 1 M, s = 2410; (x) [HCl] = 5 M, s = 2550; (6) [HCl] = 0.1 M, s = 2260; (D) [HCl] = 1 M, s = 2510 and (w) [HCl] = 5 M, s = 2410. [Zr(IV)](ini) (., E, x) = f 492 mg/L; (6, D, w) f 485 mg/L; ageing: (., E, x) 1 day; (6, D, w) 30 days.
isolated complexes are 0.615 F 0.015 and 2.02 F 0.02, 0.50 F 0.03 and 1.00 F 0.02 and 0.20 F 0.01 and 0.243 F 0.008 (average of five individual results). Important infrared absorption bands of pure D2EHPA and the isolated extracted species for both 1- and 30day aged systems are given in Table 3. Since it is now known (Singhal et al., 1996) that the aqueous Zr(IV) species at around 0.1 M HCl is [Zr8(OH)20(H2O)24Cl12] and at around 1 M HCl is [Zr4 (OH)8 (H2O)16 Cl6]2 + , the following two equations are suggested for the extraction reactions for the 1-day aged system: ½Zr8 ðOHÞ20 ðH2 OÞ24 Cl12 þ HCl þ 2H2 A2ðoÞ Fig. 9. Dependence of extraction on coexisting chloride ion concentration. (.) [H + ] = 0.1 M, s = 0.50, I = 0.8; (E) [H + ] = 1 M, s = 1.80, I = 1.09; (x) [H + ] = 4 M, s = 2.14, I = 1.06; (6) [H + ] = 0.1 M; (D) [H + ] = 1 M; w[H + ] = 2 M. [Zr(IV)](ini): (., E, x) f 492 mg/L; (6, D, w) f 485 mg/L; ageing: (., E, x) 1 day; (6, D, w) 30 days.
WH3 Aþ2 ½Zr8 ðOHÞ20 ðH2 OÞ24 Cl13 H2 A2 ðoÞ
ð1Þ
½Zr4 ðOHÞ8 ðH2 OÞ16 Cl6 2þ þ 2Cl þ 2H2 A2ðoÞ W½Zr4 ðOHÞ8 ðH2 OÞ16 Cl8 2H2 A2 ðoÞ
ð2Þ
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Table 1 Extraction characteristic for 1- and 30-day aged systems Variable
Dependence for 1-day aged system
Dependence for 30-day aged system
Remarka
H+
+1 0 +1 +1 +2 +2 +2 0.5 – 2.0 30 46 49
+2 0 +1 +1 +4 +2 0 0 40 48 46.5
In In In In In In In In In In In
H2A2
Cl Temperatureb
hcr of H + icr of H + lcr of H + hcr of Zr(IV) icr of Zr(IV) lcr of Zr(IV) hcr and icr of H + lcr of H + lcr of H + icr of H + hcr of H +
a Abbreviations: hcr — high-concentration region; icr — intermediate-concentration region; lcr — low-concentration region. b DH values in kJ mol 1.
In the first case, the extracted species is a solvated ion pair and this equation satisfies the H + , H2A2 and Cl dependencies of + 1, + 2 and + 1 (slope of logD vs. log[Cl ] varies from 2 to 0.5 and the tangential slope at Cl concentration of 3 M is 1), respectively. The composition of the extracted species in Eq. (1) is supported by the loading result [Zr/ D2EHPA (dimeric) ratio = 4], Zr/P atom ratio in the isolated complex (theoretical, 2; experimental, 2.02 F 0.02), Zr/Cl atom ratio in the isolated complex (theoretical, 0.62; experimental, 0.615 F 0.015) and the molecular weight of the isolated complex (theoretical, 3252; experimental, 3100 F 150). Moreover, the infrared spectra of the isolated complex show that the PMO stretching band at 1225 cm 1 (Peppard and Ferraro, 1959) in pure D2EHPA is split into two bands at 1245 and 1210 cm 1 which shows that no chelate complex is formed (Peppard and Ferraro, 1959; Islam and Biswas, 1981); rather this is due to the presence of D2EHPA in both anion and cation of the extracted ion pair in Eq. (1). The absence of a band in the vicinity of 380 cm 1 (Buchler et al., 1961) indicates the absence of Zr to Cl direct bonding and so all chloride ions exist in the secondary coordination sphere of the anions of the extracted anion. The distinct shoulder at 1090 cm 1 in the complex (absent in D2EHPA) is due to Zr –OH – Zr bridging (McVicker and Margan, 1970; Lippincott et al., 1961; Fischer and Fischer, 1965).
On the other hand in Eq. (2), the extracted species is formed by the addition of a Cl and two H2A2 molecules in the secondary coordination sphere of the existing aqueous species. Eq. (2) satisfies the experimental H + , Cl and H2A2 dependencies of 0, 2 and 2, respectively. The composition of the extracted species is supported by the loading result [Zr/D2EHPA (dimeric) = 2], molecular weight (theoretical, 2361; exper-
Table 2 Evaluation of extraction equilibrium constants Seta Fig [H2A2] no. (M)
[H + ] (M)
[Cl ] (M)
Intercept log Kex
Average S.D. log Kex
1
5
4.06
0.05
3
7 9 5
3.40
0.25
4
7 9 6
9.44
0.24
5
8 9 6
9.32
0.13
6
8 9 6
1.48 2.10 2.53 2.90 4.60 0.82 1.00 1.54 2.00 2.40 5.03 1.09 0.40 1.00 1.50 1.92 5.39 1.06 1.70 2.40 3.50 4.20 8.30 1.70 1.30 2.30 3.43 4.21 9.92 2.59 0.80 1.70 2.90 3.40 9.90 2.60
0.08
2
3.0 3.0 3.0 3.0 3.0 – 3.0 3.0 3.0 3.0 3.0 – 3.0 3.0 3.0 3.0 7.00 – 3.0 3.0 3.0 3.0 3.0 – 3.0 3.0 3.0 3.0 3.0 – 3.0 3.0 3.0 3.0 5.0 –
5.02
7 9 5
– – – – 0.10 0.10 – – – – 1.00 1.00 – – – – 7.00 4.00 – – – – 0.10 0.10 – – – – 1.0 1.0 – – – – 5.0 2.0
8.75
0.02
8 9
0.01 0.02 0.03 0.05 – 0.03 0.01 0.02 0.03 0.05 – 0.03 0.01 0.02 0.03 0.05 – 0.03 0.01 0.02 0.03 0.05 – 0.02 0.01 0.02 0.03 0.05 – 0.02 0.01 0.02 0.03 0.05 – 0.02
5.00 5.02 5.10 5.02 5.12 4.86 4.04 3.98 4.09 4.04 4.07 4.13 3.45 3.45 3.59 3.57 2.85 3.50 9.70 9.20 9.60 9.40 9.30 9.50 9.30 9.10 9.53 9.41 9.10 9.29 8.80 8.50 9.00 8.60 8.50 9.10
a Set 1 for 1 day and 0.1 M HCl system. Set 2 for 1 day and 1 M HCl system. Set 3 for 1 day and 5 M HCl system. Set 4 for 30 days and 0.1 M HCl system. Set 5 for 30 days and 1 M HCl system. Set 6 for 30 days and 5 M HCl system.
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sence of direct Zr– Cl bonding and so in this complex also, all chloride ions exist in the secondary coordination sphere of the solvated uncharged complex. The Zr – OH – Zr bridging band in this complex appears at 1098 cm 1. In the higher-acidity region (5 M HCl), the tetrameric species may be converted into monomeric species by the attack of H + to –OH – bridges and the monomeric species may have the composition of the type [Zr(H2O)8-yCly](4 y) + (Hannane et al., 1990). On choosing y = 3, the experimental results obtained for 1-day ageing can be justified by the occurrence of the following reaction in 5 M HCl medium: ½ZrðH2 OÞ5 Cl3 þ þ Hþ þ 2Cl þ 2H2 A2 WH3 Aþ 2 ½ZrðH2 OÞ3 Cl5 H2 A2 ðoÞ
Fig. 11. Loading of Zr4 + in the organic phase. [H2A2](ini) = 0.1 M, [Cl ][3 M, (.) [HCl] = 0.1 M, [Zr(IV)](ini) = 42.7 mM, (E) [HCl] = 1 M, [Zr(IV)](ini) = 33.7 mM, (x) [HCl] = 5 M, [Zr(IV)](ini) = 11.4 mM.
imental, 2160 F 70), Zr/P atom ratio (theoretical, 1; experimental, 1 F 0.02) and Zr/Cl atom ratio (theoretical, 0.50; experimental, 0.50 F 0.03). Moreover, the infrared band at 1225 cm 1 in D2EHPA remains unaltered in the isolated complex and so no chelation through PMO group of D2EHPA in the extracted complex occurs. Further, the absence of a band in the vicinity of 380 cm 1 in the complex indicates the ab-
ð3Þ
Here the extracted species is an ion pair with both ions solvated. Eq. (3) satisfies the experimental H + , Cl and H2A2 dependencies of 1, 2 and 2, respectively. Moreover, the loading result ([Zr/D2EHPA (dimeric) = 0.46; theoretical, 0.5), molecular weight (theoretical, 1612; experimental, 1630 F 50), Zr/P atom ratio (theoretical, 0.25; experimental, 0.243 F 0.008) and Zr/Cl atom ratio (theoretical, 0.20; experimental, 0.20 F 0.01) support the composition of the above extracted species. The infrared band at 1225 cm 1 for PMO vibration in pure D2EHPA is split into bands at 1245 and 1210 cm 1 indicating that both ions of the ion pair are probably solvated. Moreover, a strong infrared band at 376 cm 1 appears in this case which indicates the direct coordination of Cl to Zr(IV); and the band in the vicinity of 1090 cm 1 assigned for Zr–
Table 3 Important infrared absorbtion bands (cm 1) of D2EHPA and Zr(IV) – D2EHPA complexes D2EHPA
3570(w)
1225(s) — — —
Complexes of 1 day – 0.1 M HCl
1 day – 1 M HCl
1 day – 5 M HCl
30 days – 0.1 M HCl
30 days – 1 M HCl
30 days – 5 M HCl
3620(sh) 3390(s) 3190(sh) 1245(s) 1210(s) 1090(sh) 422(s) 353(s)
3620(sh) 3370(s) 3250(sh) 1225(s)
– 3380(s) 3200(sh) 1245(s) 1210(s) – 424(s) 376(s)
3620(sh) 3390(s) 3190(sh) 1245(s) 1210(s) 1090(sh) 422(s) 353(s)
3620(sh) 3380(s) 3250(sh) 1225(s)
– 3380(s) 3190(sh) 1245(s) 1210(s) – 424(s) 376(s)
1098(sh) 422(s) 351(m)
Abbreviations: sh: shoulder, s: strong and m: medium.
1098(sh) 422(s) 351(m)
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OH – Zr bridging is absent for this complex which supports the contention that the extracted species is monomeric. For the 30-day aged system, the experimental results obtained for 0.1, 1 and 5 M HCl systems can be best explained if the following changes during ageing are considered:
occurs via the formation of solvated ion pair or solvated uncharged species depending on the aqueous acidity. The ageing and the acidity of the aqueous phase alter the extraction behaviour.
½Zr8 ðOHÞ20 ðH2 OÞ24 Cl12 þ Cl
One of the authors (M.A. Hayat) is grateful to the UGC (Dhaka) for granting a PhD fellowship and to the Ministry of Education, the People’s Republic of Bangladesh, for granting study leave. A part of the work was presented in the International Seminar on NFMM with satellite symposium on Aluminium held at NML, Jamshedpur, India, February 2000.
! ½Zr8 ðOHÞ20 ðH2 OÞ24 Cl13
ð4Þ
½Zr4 ðOHÞ8 ðH2 OÞ16 Cl6 2þ þ 2Cl ! ½Zr4 ðOHÞ8 ðH2 OÞ16 Cl8
ð5Þ
and
References
½ZrðH2 OÞ5 Cl3 þ þ 3Cl ! ½ZrðH2 OÞ2 Cl6
Acknowledgements
2
þ 3H2 O
ð6Þ
The resultant aged species can be extracted as follows: ½Zr8 ðOHÞ20 ðH2 OÞ24 Cl13 þ Hþ þ 4H2 A2ðoÞ WH3 Aþ 2 ½Zr8 ðOHÞ20 ðH2 OÞ24 Cl13 3H2 A2 ðoÞ
ð7Þ
½Zr4 ðOHÞ8 ðH2 OÞ16 Cl8 þ 4H2 A2ðoÞ W½Zr4 ðOHÞ8 ðH2 OÞ16 Cl8 4H2 A2 ðoÞ
ð8Þ
and ½ZrðH2 OÞ2 Cl6 2 þ 2Hþ þ 4H2 A2ðoÞ 2 Wð2H3 A2 Þ2þ ½ZrðH2 OÞ2 Cl6 2H2 A2 ðoÞ
ð9Þ
Eqs. (7) – (9) explain well the experimental H + , Cl and H2A2 dependencies of extraction ratio, D and the infrared spectral data.
4. Conclusions The solvent extraction of Zr(IV) from Cl medium by D2EHPA is complicated by the slow change in the composition of aqueous Zr(IV) species. The extraction
Biswas, R.K., Hayat, M.A., 1985. Nucl. Sci. Appl. 16 and 17B, 119. Buchler, A., Muttuck, J.B.B., Dugre, D.H., 1961. J. Chem. Phys. 34, 2202. Charlot, G., 1964. Colorimetric Determination of Elements. Elsevier, Amsterdam, p. 432. Fischer, E.O., Fischer, H., 1965. J. Organomet. Chem. 3, 181. Hannane, S.A., Bertin, F., Bouix, J., 1990. J. Bull. Soc. Chim. Fr. 127, 43. Islam, F., Biswas, R.K., 1981. J. Bangladesh Acad. Sci. 5 (2), 19. Kletenik, Yu.B., Bykhovskaya, I.A., 1965. Zh. Prikl. Khim. 20, 567. Lippincott, E.R., Xavier, J., Steele, D., 1961. J. Am. Chem. Soc. 83, 2262. McVicker, G.B., Margan, G.I., 1970. Spectrochim. Acta 26A, 23. Peppard, D.F., Ferraro, J.R., 1959. J. Inorg. Nucl. Chem. 10, 275. Ritcey, G.M., Ashbrook, A.W., 1979. Solvent Extraction Principles and Applications in Process Metallurgy, Part 2. Elsevier, Amsterdam. Sato, T., 1970. Anal. Chem. Acta 52, 183. Sato, T., Nakamura, T., 1971. J. Inorg. Nucl. Chem. 33, 1081. Sekine, T., Hasegawa, Y., 1977. Solvent Extraction Chemistry— Fundamentals and Applications, 1st edn. Marcel Dekker, New York, pp. 451 – 458. Singhal, A., Toth, L.M., Lin, J.S., Affhofter, K., 1996. J. Am. Chem. Soc. 118, 11529. Tedesco, P.H., Rumi, D.V., Quintana, J.A., 1967. J. Inorg. Nucl. Chem. 29, 1307. Vogel, A.I., 1978. A Text Book of Inorganic Quantitative Analysis, 3rd edn. ELBS-Longman, London, (a) p. 218 (b) p. 756. Yagodin, G.A., Tarasov, V.V., 1971. Radiokhimiya 12, 664.
Solvent extraction of zirconium(IV) from chloride media by D2EHPA in kerosene R.K. Biswas*, M.A. Hayat Department of Applied Chemistry and Chemical Technology, Rajshahi University, Rajshahi 6205, Bangladesh Received 29 June 2001; received in revised form 9 November 2001; accepted 9 November 2001
Abstract The apparent equilibrium of extraction of zirconium(IV) from chloride media by D2EHPA in kerosene is found to depend on ageing of the aqueous phase. When the aqueous phase, just after preparation, is used in the extraction study, reproducible results are not obtained. With increasing ageing time, the apparent distribution ratio (D) is increased for a particular set of experimental parameters. The system has been thoroughly investigated for 1- and 30-day ageing. In both cases, the apparent equilibration time is 15 min. D is decreased with increasing Zr(IV) concentration in the aqueous phase but the nature depends on ageing. In both cases of ageing, logD vs. log[HCl] plots are peculiar in nature. For 1-day ageing, the slope is f 1 above 3 M HCl and also below 0.3 M HCl, and is zero within 0.3 – 3.0 M HCl. In contrast, for the 30-day aged system, the slope is f 2 above 2 M HCl, f 1 below 0.5 M HCl, and zero between 0.5 and 2.0 M HCl. The extractant dependencies are 2 and 4 for 1- and 30day aged, systems respectively. Chloride ion has a large effect on D for the 1-day aged system, but no effect on D for the 30-day aged system. From the temperature dependence data, DH values have been determined. The possible extraction equilibrium reactions have been suggested and supported by the loading tests, as well as by the molecular weight, Zr/P ratio, Zr/Cl ratio and infrared spectral data of the extracted species. D 2002 Published by Elsevier Science B.V. Keywords: Extraction equilibrium; Zr(IV); Chloride medium; D2EHPA
1. Introduction Di-2-ethylhexylphosphoric acid (D2EHPA, H2A2) is a commercial extractant and is used widely in extractions of U(VI), V(IV), Th(IV). Ti(IV), Fe(III), Be(II), Zn(II), Mo(VI), Hf(IV), Ln(III), etc. (Sekine and Hasegawa, 1977; Ritcey and Ashbrook, 1979). Previously (Biswas and Hayat, 1985; Kletenik and Bykhovskaya, 1965; Tedesco et al., 1967; Sato, 1970; Yagodin and Tarasov, 1971; Sato and Nakamura, 1971), it has been
*
Corresponding author. Fax: +88-0721-750-064. E-mail address: [email protected] (R.K. Biswas).
reported that D2EHPA can be used for the extraction of Zr(IV) from various acid solutions. However, most of those studies have been done in narrow aqueous acidity regions and may be considered as very preliminary, particularly those using chloride media. In most cases, the peculiarity of this extraction system has been disregarded and no proper explanation for the extraction data has been given because of inadequate knowledge of the aqueous Zr(IV) species. Very recently, the compositions of Zr(IV) species in Cl media ([Cl ] or [HCl] = 0.1 – 1.0 M) have been reported by Singhal et al. (1996). In 1 M Cl medium, the only species present is partly hydrolysed [Zr4(OH)8(H2O)16Cl6]2 + , and this species is changed
0304-386X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 3 8 6 X ( 0 1 ) 0 0 2 2 0 - 1
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totally to [Zr8(OH)20(H2O)24Cl12] in 0.1 M HCl medium. In these complexes, Zr(IV) is 8-coordinated. In [Zr4(OH)8(H2O)16Cl6]2 + , four Zr atoms take the square planar arrangement and each Zr(IV) has four – OH bridgings and carries four H2O as coordinated ligands; whereas six Cl ions take their positions in the secondary coordination sphere of the complexes. In this complex, OH /Zr4 + ratio is 2. But in [Zr8 (OH)20(H2O)24Cl12], two [Zr4(OH)8(H2O)16Cl6]2 + sheets are joined together stackwise through four Zr – OH – Zr bridging with the elimination of four H3O + . In this chargeless polynuclear complex, 12 Cl ions exist in the secondary coordination sphere of the complexes. In the octameric complex, the OH /Zr4 + ratio is 2.5. The species existing in the higher concentration of HCl (>1 M) is still not reported definitely. However, according to Hannane et al. (1990), the addition of concentrated HCl to ZrOCl2 solution leads to the formation of one or several monomeric species of composition [Zr (H2O)8 n Cln](4 n) + . This paper discusses the liquid – liquid extraction characteristics of Zr(IV) from HCl solution of wide concentration range by D2EHPA dissolved in kerosene and the extraction equilibrium reactions have been proposed considering the existence of the above species in the aqueous phase and supported by the loading test, molecular weight, Zr/P ratio, Zr/Cl ratio and infrared spectral data.
2. Experimental 2.1. Reagents D2EHPA was procured from BDH. It had a purity of 98% and was used as such. As a source of Zr(IV), ZrOCl28H2O (M.C. & Bell, 98%) was used. Kerosene was collected from the local market and redistilled to collect the fraction distilling over 220 – 260 C. It was colorless and mostly aliphatic in nature. All other chemicals were of reagent grade and used without further purification. 2.2. Analytical The concentration of Zr(IV) in the aqueous phase was determined by the EDTA – pyrocatechol violet
method (Charlot, 1964) at 590 nm using a WPA S104 spectrophotometer. For pH adjustment of the aqueous Zr(IV) solutions (pH 5.2) required in the above method, a Mettler Toledo 320 pH meter having a combination electrode and an autotemperature compensator was used. For determining the molecular weight of the extracted complexes, the Cryoscopic method using cyclohexane as solvent was used. For determining Zr/P and Zr/Cl atom ratios in the extracted solid complexes, a definite amount of the solid complexes was decomposed by concentrated H2SO4, heated until all carbon was oxidised to make the solution clear and colourless. It was then diluted to 100 mL carefully with water. This solution was analysed for the contents of Zr(IV) (Charlot, 1964), P in the form of PO4 3 by the Molybdate –Hydrazine sulphate (molybdenum blue) method (Vogel, 1978) and chloride by the AgNO3 – K2CrO4 titration method (Vogel, 1978) and verified by the AgCl precipitation method. The infrared spectra of the isolated solid Zr –D2EHP complexes were taken in Nujol mull, as well as in the KBr disc form by courtesy of the Chemical Division of Fisons Bangladesh, Tongi, Dhaka. The spectroscope was a double beam SP3-300 Pye Unicam Infrared spectrophotometer, Cambridge, England. 2.3. Extraction procedure Aliquots of 25 mL of each phase were taken in a 125-mL separating funnel. It was stoppered tightly and shaken for about 30 min in a thermostatted water bath operated at 30 C. The aqueous phases containing definite amounts of Zr(IV) and (H + , Na + ) Cl were prepared and left for 1 or 30 days before extractions. Freshly prepared aqueous solutions were also used for some preliminary experiments. After mixing, the phases were allowed to settle and the aqueous phase was separated. It was analysed for its Zr(IV) content. The difference of the concentrations of Zr(IV) in the aqueous phase between the initial and final states gives the concentration of Zr(IV) in the organic phase. On dividing the Zr(IV) concentration in the organic phase by that in the aqueous phase at equilibrium, the value of apparent distribution ratio (D) was obtained. In all cases, the average of at least three sets of experimental data is reported and the reproducibility is within 5%.
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3. Results and discussion
Fig. 1. Dependence of extraction on contact time. [H2A2] = 0.01 M, [HCl] = 0.3 M, [Cl ] = 3 M, [Zr(IV)](ini) = 330 mg/L. Ageing time: (.) 0 day; (E) 1 day; (x) 30 days.
Initially, many experiments were repeated to obtain reproducible results and it was thought that the reproducible results could not be obtained due to the continuous change in composition of Zr(IV) species in the aqueous phase. The change can be seen in Fig. 1. In all three cases of solution conditioning, it is found that the value of logD is increased with increasing phase contact time up to 15 min. On further increasing the contact time, no change in logD is observed. Therefore, the apparent equilibration time is 15 min, but 30 min has been allowed subsequently to ensure equilibration for various experimental parameters. It can be noticed that logD is increased considerably with the time allowed for ageing (D is increased 10-fold when freshly prepared solution is aged for 30 days). Fig. 2 shows logD vs. ageing time plots for two experimental parameters. In both cases, logD is increased with increasing ageing time. Within the first day, the variation is large and then with further increase in ageing time, the variation is slowed down. In the subsequent studies, data obtained for 1- and 30day aged solutions were obtained. The variation of D
2.4. Procedure for loading and isolating solid complexes For these purposes, 100 mL of 0.1 M D2EHPA in kerosene was taken in a 500 mL elongated type separating funnel and to it equal volume of Zr(IV) solution (2 kg m 3) of definite acidity (0.1, 1.0 and 5.0 M in HCl) was added. The two phases were mixed on a horizontal shaker, disengaged and the aqueous phase was analysed for its Zr(IV) content. The organic phase was repeatedly contacted with fresh aqueous solutions until the saturation of the organic phase with Zr(IV) was obtained. The organic phase saturated with Zr(IV) was transferred into a beaker. Kerosene was allowed to evaporate out under suction at around 80 C. Yellowish transparent solid or semisolid masses were obtained. Adhered free D2EHPA (if any) from these masses was eliminated by washing with 50% alcohol. The complexes were then dried under vacuum and finally in an oven at 90 C for 72 h and stored in a CaCl2 desiccator.
Fig. 2. Dependence of extraction on ageing time. [H2A2] = 0.01 M, [Cl ] = 3 M, [Zr(IV)](ini) = 492 mg/L, [HCl]: (.) 0.3 M, (E) 3 M.
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Fig. 3. Dependence of the extraction on Zr(IV) concentration in the aqueous phase of 1-day ageing. [HCl] = 1 M, [Cl ] = 3 M, [H2A2]: (6) 0.01 M; (.) 0.20 M; (D) 0.03 M; (E) 0.05 M.
with [Zr(IV)]ini as log –log plots is shown in Fig. 3 for 1-d aged system. The slopes of the plots do not change with the variation of [D2EHPA] used in extraction. In all cases, logD is decreased almost linearly with log [Zr(IV)]ini, at least up to 11 mM Zr(IV). With further increase in Zr(IV) concentration in the aqueous phase, logD is little varied though Zr(IV) concentration in the organic phase is increased considerably. The value of logD should be independent of [Zr(IV)] provided the equilibrium pH and [D2EHPA] are kept constant but only if mononuclear complexes of Zr(IV) are present which seems not to be the case. Since the aqueous acidity is high, the change in logD value may be regarded, in the first instance, to be due to the change in equilibrium [D2EHPA]. But when the data are corrected for an presumed constant equilibrium extractant concentration, the independence of D on initial [Zr(IV)] is not noticed. The arrowheaded lines in Fig. 3 represent the variation of D with [D2EHPA] (for these lines, the abscissa represents log[D2EHPA]ini with similar divisions). It is found that the extractant dependence depends on the [Zr(IV)]ini in the aqueous phase. Up to [Zr(IV)] = 0.01 M, the extractant dependence is
f 2 and it is decreased to f 1 for 0.033 M Zr(IV). The variation of the extractant dependence on [Zr (IV)] indicates the variation of the composition of Zr (IV) species in either of the phases with increasing concentration in the aqueous phase. Fig. 4 represents the Zr(IV) dependence data for 30-day aged system. For all three D2EHPA systems, the shape of the plots is greatly changed compared to those given in Fig. 3. For less concentrated solutions of Zr(IV), D is increased, whereas for highly concentrated solutions of Zr(IV), D is decreased appreciably on increasing ageing time. The nonconstancy of logD with the variation of log[Zr(IV)]ini is not solely due to the small variations of equilibrium HCl and D2EHPA concentrations. The slopes of the arrowheaded lines representing the extractant dependence depend extensively on the [Zr(IV)]ini in the aqueous phase. The dependence is 2 for 2.3 mM Zr(IV); it is increased to 4 for 10.9 mM Zr(IV) and again decreased to 1 at 43.5 mM Zr(IV). This variation supports again the change in composition of the aqueous Zr(IV) species with its initial concentration in the aqueous phase. This is the main reason for nonconstancy of logD in Fig. 4, as
Fig. 4. Dependence of extraction on Zr(IV) concentration in the aqueous phase of 30-day ageing. [HCl] = 1 M, [Cl ] = 3 M, [H2A2]: (.) 0.01 M; (E) 0.02 M; (x) 0.03 M.
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153
well as in Fig. 3. In the subsequent experiments, f 0.5 g/L (5.4 mM) Zr(IV) solution has been used. The variation of D with [H + ] at [Cl ] 3 M is shown in Figs. 5 and 6, respectively, for 1- and 30-day aged systems as log ( log) plots. When log [HCl] < 0.48, then the plots actually represent the logD vs. log[HCl] plots. For all systems of 1-day ageing, the plots are of identical shape. Within 3 – 7 M HCl and 0.3– 0.1 M H + ([Cl ] 3 M), logD is decreased with decreasing acidity and the limiting slopes of the lines in these regions are 1. Therefore, in these acidity regions, H + or HCl is used up in forming the extractable species. On the other hand, within 0.3– 3 M [H + ], logD remains unchanged indicating that the reaction involved in this acidity region does not consume or liberate any H + or HCl. In the case of the 30day aged system in Fig. 5, the limiting slope in the higher acidity region is 2, in lower acidity region is 1 and in the intermediate region (0.5 –2.0 M) is zero. Therefore, 2 mol of HCl (instead of 1 mol for 1-day ageing system) are used up by one mole of existing Fig. 6. Dependence of extraction on [HCl] in the aqueous phase of 30-day ageing. [Zr(IV)](ini) = f 485 mg/L, [Cl ][3 M, [H2A2]: (.) 0.01 M; (E) 0.02 M; (x) 0.03 M; (6) 0.05 M.
Fig. 5. Dependence of extraction on [HCl] in the aqueous phase of 1-day ageing. [Zr(IV)](ini) = f 500 mg/L, [Cl ][3 M, [H2A2]: (6) 0.01 M; (.) 0.02 M; (D) 0.03 M; (E) 0.05 M.
Zr(IV) species to form the extractable species in the higher acidity region. The extractant dependence of D is shown in Figs. 7 and 8 as log –log plots for 1- and 30-day aged systems, respectively. For 1-day aged system, the least squares slopes of the logD vs. log[H2A2]ini plots are 2.00, 2.03 and 2.04 for 0.1, 1 and 7 M HCl systems, respectively. On the other hand, for 30-day aged system, the least squares slopes are 3.91, 3.95 and 4.00 for 0.1, 1 and 5 M HCl systems, respectively. On calculating the equilibrium D2EHPA concentration using the relationship: [D2EHPA]eq=[D2EHPA]ini n[Zr(IV), M](0), where n = 2 for 1-day ageing and 4 for 30-day ageing, the curves for logD vs. log [H2A2]eq plots in Figs. 7 and 8 are obtained. At lower [D2EHPA]eq, slopes approach zero for all cases, whereas, at higher [D2EHPA]eq slopes approach 2 for 1-day ageing (Fig. 7) and 4 for 30-day ageing (Fig. 8). It is, therefore, concluded that the solvation number varies with the [D2EHPA] region and it increases as [D2EHPA] increases. Fig. 9 represents logD vs. log[Cl ] plots for both 1- and 30-day aged systems in three distinct aci-
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[H + ] = 1 M. The respective values are 49 and 46.5 kJ mol 1 when [H + ] = 5 M. The variation of the extraction characteristics of Zr(IV) from HCl medium by D2EHPA with ageing period is summarized in Table 1. Table 2 represents the values of extraction equilibrium constant, Kex calculated from the intercepts of the lines in Figs. 5 –9. For the 1-day aged system, logKex values are 5.02, 4.06 and 3.40 with respective standard deviations of 0.08, 0.05 and 0.25 for 0.1, 1 and 5 M HCl systems, respectively. On the other hand, for the 30-day aged system, logKex values are 9.44, 9.32 and 8.75 (S.D. 0.24, 0.13 and 0.02, respectively) for 0.1, 1 and 5 M HCl systems, respectively. The results for the loading of D2EHPA in the organic phase by Zr(IV) are shown in Fig. 11 for [H + ] of 0.1, 1 and 5 M and for 1-day aged system only. It is noticed that Zr(IV) loading in D2EHPA depends greatly on the aqueous phase acidity.
Fig. 7. Dependence of extraction on extractant concentration for 1day ageing. [Zr(IV)](ini) = f 492 mg/L, [Cl ][3 M; [HCl]: (.) 0.1 M, s = 2.00, I = 4.61; (E) 1 M, s = 2.03, I = 5.03; (x) 7 M, s = 2.04, I = 5.39.
dity regions. For the 30-day aged system, D is independent of Cl concentration irrespective of the aqueous acidity used. But in the case of 1-day aged system, the slopes of the plots are 2 for 1 and 4 M HCl systems. However, for 0.1 M HCl system, a curve is obtained which has a slope of f 2 and 0.5 in higher and lower concentration regions of Cl , respectively. These results indicate that chloride ion is not involved in the extraction reactions of 30-day aged system. But for 1-day aged system, during extraction the involvements of 2 and of 1/2 g ions of Cl /mol of Zr(IV) occur in the higher and lower concentration regions of chloride ion, respectively. The temperature dependencies of D in three distinct acidity regions for both 1- and 30-day ageings are displayed in Fig. 10. For 0.1 M H + system, DH values are 30 and 43 kJ mol 1 for 1- and 30-day aged systems, respectively. Values are 46 and 48 kJ mol 1 for 1- and 30-day aged systems, respectively, when
Fig. 8. Dependence of extraction on extractant concentration for 30day ageing. [Zr(IV)](ini) = f 485 mg/L, [Cl ][3 M; [HCl]: (.) 0.1 M, s = 3.91, I = 8.30; (E) 1 M, s = 3.95, I = 9.20; (x) 5 M, s = 4.00, I = 9.90.
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155
For HCl concentration of 0.1 M, the saturated Zr (IV) concentration in 0.1 M organic phase is as high as 36.4 kg m 3. It is 18 and 4.22 kg m 3 for 1 and 5 M HCl systems, respectively. These results give that the loading capacities of D2EHPA for Zr(IV) are 56.52, 27.95 and 6.55 g/100 g D2EHPA for 0.1, 1 and 5 M HCl systems, respectively. At lower aqueous acidity, the high loading capacity indicates the strong affinity of Zr4 + for D2EHPA. A volume of 1 L of 0.1 M D2EHPA can extract as high as 36.4 g Zr(IV) from 0.1 M HCl medium; in the saturated complex, the Zr/D2EHPA (dimeric) ratio is 4. This ratio is 2 and 0.46 when extracted from 1 and 5 M HCl media, respectively. The molecular weight of the isolated complexes for 1-day aged system has been determined by the cryoscopic method using cyclohexane as solvent. The molecular weights of the complexes isolated from 0.1, 1.0 and 5 M HCl medium are 3110 F 150, 2160 F 70 and 1630 F 50 (average of five experiments in each case are given). The respective Zr/Cl and Zr/P ratios in the Fig. 10. Dependence of extraction on temperature. [H2A2] = 0.01 M; [Cl ][3 M; (.) [HCl] = 0.1 M, s = 1560; (E) [HCl] = 1 M, s = 2410; (x) [HCl] = 5 M, s = 2550; (6) [HCl] = 0.1 M, s = 2260; (D) [HCl] = 1 M, s = 2510 and (w) [HCl] = 5 M, s = 2410. [Zr(IV)](ini) (., E, x) = f 492 mg/L; (6, D, w) f 485 mg/L; ageing: (., E, x) 1 day; (6, D, w) 30 days.
isolated complexes are 0.615 F 0.015 and 2.02 F 0.02, 0.50 F 0.03 and 1.00 F 0.02 and 0.20 F 0.01 and 0.243 F 0.008 (average of five individual results). Important infrared absorption bands of pure D2EHPA and the isolated extracted species for both 1- and 30day aged systems are given in Table 3. Since it is now known (Singhal et al., 1996) that the aqueous Zr(IV) species at around 0.1 M HCl is [Zr8(OH)20(H2O)24Cl12] and at around 1 M HCl is [Zr4 (OH)8 (H2O)16 Cl6]2 + , the following two equations are suggested for the extraction reactions for the 1-day aged system: ½Zr8 ðOHÞ20 ðH2 OÞ24 Cl12 þ HCl þ 2H2 A2ðoÞ Fig. 9. Dependence of extraction on coexisting chloride ion concentration. (.) [H + ] = 0.1 M, s = 0.50, I = 0.8; (E) [H + ] = 1 M, s = 1.80, I = 1.09; (x) [H + ] = 4 M, s = 2.14, I = 1.06; (6) [H + ] = 0.1 M; (D) [H + ] = 1 M; w[H + ] = 2 M. [Zr(IV)](ini): (., E, x) f 492 mg/L; (6, D, w) f 485 mg/L; ageing: (., E, x) 1 day; (6, D, w) 30 days.
WH3 Aþ2 ½Zr8 ðOHÞ20 ðH2 OÞ24 Cl13 H2 A2 ðoÞ
ð1Þ
½Zr4 ðOHÞ8 ðH2 OÞ16 Cl6 2þ þ 2Cl þ 2H2 A2ðoÞ W½Zr4 ðOHÞ8 ðH2 OÞ16 Cl8 2H2 A2 ðoÞ
ð2Þ
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Table 1 Extraction characteristic for 1- and 30-day aged systems Variable
Dependence for 1-day aged system
Dependence for 30-day aged system
Remarka
H+
+1 0 +1 +1 +2 +2 +2 0.5 – 2.0 30 46 49
+2 0 +1 +1 +4 +2 0 0 40 48 46.5
In In In In In In In In In In In
H2A2
Cl Temperatureb
hcr of H + icr of H + lcr of H + hcr of Zr(IV) icr of Zr(IV) lcr of Zr(IV) hcr and icr of H + lcr of H + lcr of H + icr of H + hcr of H +
a Abbreviations: hcr — high-concentration region; icr — intermediate-concentration region; lcr — low-concentration region. b DH values in kJ mol 1.
In the first case, the extracted species is a solvated ion pair and this equation satisfies the H + , H2A2 and Cl dependencies of + 1, + 2 and + 1 (slope of logD vs. log[Cl ] varies from 2 to 0.5 and the tangential slope at Cl concentration of 3 M is 1), respectively. The composition of the extracted species in Eq. (1) is supported by the loading result [Zr/ D2EHPA (dimeric) ratio = 4], Zr/P atom ratio in the isolated complex (theoretical, 2; experimental, 2.02 F 0.02), Zr/Cl atom ratio in the isolated complex (theoretical, 0.62; experimental, 0.615 F 0.015) and the molecular weight of the isolated complex (theoretical, 3252; experimental, 3100 F 150). Moreover, the infrared spectra of the isolated complex show that the PMO stretching band at 1225 cm 1 (Peppard and Ferraro, 1959) in pure D2EHPA is split into two bands at 1245 and 1210 cm 1 which shows that no chelate complex is formed (Peppard and Ferraro, 1959; Islam and Biswas, 1981); rather this is due to the presence of D2EHPA in both anion and cation of the extracted ion pair in Eq. (1). The absence of a band in the vicinity of 380 cm 1 (Buchler et al., 1961) indicates the absence of Zr to Cl direct bonding and so all chloride ions exist in the secondary coordination sphere of the anions of the extracted anion. The distinct shoulder at 1090 cm 1 in the complex (absent in D2EHPA) is due to Zr –OH – Zr bridging (McVicker and Margan, 1970; Lippincott et al., 1961; Fischer and Fischer, 1965).
On the other hand in Eq. (2), the extracted species is formed by the addition of a Cl and two H2A2 molecules in the secondary coordination sphere of the existing aqueous species. Eq. (2) satisfies the experimental H + , Cl and H2A2 dependencies of 0, 2 and 2, respectively. The composition of the extracted species is supported by the loading result [Zr/D2EHPA (dimeric) = 2], molecular weight (theoretical, 2361; exper-
Table 2 Evaluation of extraction equilibrium constants Seta Fig [H2A2] no. (M)
[H + ] (M)
[Cl ] (M)
Intercept log Kex
Average S.D. log Kex
1
5
4.06
0.05
3
7 9 5
3.40
0.25
4
7 9 6
9.44
0.24
5
8 9 6
9.32
0.13
6
8 9 6
1.48 2.10 2.53 2.90 4.60 0.82 1.00 1.54 2.00 2.40 5.03 1.09 0.40 1.00 1.50 1.92 5.39 1.06 1.70 2.40 3.50 4.20 8.30 1.70 1.30 2.30 3.43 4.21 9.92 2.59 0.80 1.70 2.90 3.40 9.90 2.60
0.08
2
3.0 3.0 3.0 3.0 3.0 – 3.0 3.0 3.0 3.0 3.0 – 3.0 3.0 3.0 3.0 7.00 – 3.0 3.0 3.0 3.0 3.0 – 3.0 3.0 3.0 3.0 3.0 – 3.0 3.0 3.0 3.0 5.0 –
5.02
7 9 5
– – – – 0.10 0.10 – – – – 1.00 1.00 – – – – 7.00 4.00 – – – – 0.10 0.10 – – – – 1.0 1.0 – – – – 5.0 2.0
8.75
0.02
8 9
0.01 0.02 0.03 0.05 – 0.03 0.01 0.02 0.03 0.05 – 0.03 0.01 0.02 0.03 0.05 – 0.03 0.01 0.02 0.03 0.05 – 0.02 0.01 0.02 0.03 0.05 – 0.02 0.01 0.02 0.03 0.05 – 0.02
5.00 5.02 5.10 5.02 5.12 4.86 4.04 3.98 4.09 4.04 4.07 4.13 3.45 3.45 3.59 3.57 2.85 3.50 9.70 9.20 9.60 9.40 9.30 9.50 9.30 9.10 9.53 9.41 9.10 9.29 8.80 8.50 9.00 8.60 8.50 9.10
a Set 1 for 1 day and 0.1 M HCl system. Set 2 for 1 day and 1 M HCl system. Set 3 for 1 day and 5 M HCl system. Set 4 for 30 days and 0.1 M HCl system. Set 5 for 30 days and 1 M HCl system. Set 6 for 30 days and 5 M HCl system.
R.K. Biswas, M.A. Hayat / Hydrometallurgy 63 (2002) 149–158
157
sence of direct Zr– Cl bonding and so in this complex also, all chloride ions exist in the secondary coordination sphere of the solvated uncharged complex. The Zr – OH – Zr bridging band in this complex appears at 1098 cm 1. In the higher-acidity region (5 M HCl), the tetrameric species may be converted into monomeric species by the attack of H + to –OH – bridges and the monomeric species may have the composition of the type [Zr(H2O)8-yCly](4 y) + (Hannane et al., 1990). On choosing y = 3, the experimental results obtained for 1-day ageing can be justified by the occurrence of the following reaction in 5 M HCl medium: ½ZrðH2 OÞ5 Cl3 þ þ Hþ þ 2Cl þ 2H2 A2 WH3 Aþ 2 ½ZrðH2 OÞ3 Cl5 H2 A2 ðoÞ
Fig. 11. Loading of Zr4 + in the organic phase. [H2A2](ini) = 0.1 M, [Cl ][3 M, (.) [HCl] = 0.1 M, [Zr(IV)](ini) = 42.7 mM, (E) [HCl] = 1 M, [Zr(IV)](ini) = 33.7 mM, (x) [HCl] = 5 M, [Zr(IV)](ini) = 11.4 mM.
imental, 2160 F 70), Zr/P atom ratio (theoretical, 1; experimental, 1 F 0.02) and Zr/Cl atom ratio (theoretical, 0.50; experimental, 0.50 F 0.03). Moreover, the infrared band at 1225 cm 1 in D2EHPA remains unaltered in the isolated complex and so no chelation through PMO group of D2EHPA in the extracted complex occurs. Further, the absence of a band in the vicinity of 380 cm 1 in the complex indicates the ab-
ð3Þ
Here the extracted species is an ion pair with both ions solvated. Eq. (3) satisfies the experimental H + , Cl and H2A2 dependencies of 1, 2 and 2, respectively. Moreover, the loading result ([Zr/D2EHPA (dimeric) = 0.46; theoretical, 0.5), molecular weight (theoretical, 1612; experimental, 1630 F 50), Zr/P atom ratio (theoretical, 0.25; experimental, 0.243 F 0.008) and Zr/Cl atom ratio (theoretical, 0.20; experimental, 0.20 F 0.01) support the composition of the above extracted species. The infrared band at 1225 cm 1 for PMO vibration in pure D2EHPA is split into bands at 1245 and 1210 cm 1 indicating that both ions of the ion pair are probably solvated. Moreover, a strong infrared band at 376 cm 1 appears in this case which indicates the direct coordination of Cl to Zr(IV); and the band in the vicinity of 1090 cm 1 assigned for Zr–
Table 3 Important infrared absorbtion bands (cm 1) of D2EHPA and Zr(IV) – D2EHPA complexes D2EHPA
3570(w)
1225(s) — — —
Complexes of 1 day – 0.1 M HCl
1 day – 1 M HCl
1 day – 5 M HCl
30 days – 0.1 M HCl
30 days – 1 M HCl
30 days – 5 M HCl
3620(sh) 3390(s) 3190(sh) 1245(s) 1210(s) 1090(sh) 422(s) 353(s)
3620(sh) 3370(s) 3250(sh) 1225(s)
– 3380(s) 3200(sh) 1245(s) 1210(s) – 424(s) 376(s)
3620(sh) 3390(s) 3190(sh) 1245(s) 1210(s) 1090(sh) 422(s) 353(s)
3620(sh) 3380(s) 3250(sh) 1225(s)
– 3380(s) 3190(sh) 1245(s) 1210(s) – 424(s) 376(s)
1098(sh) 422(s) 351(m)
Abbreviations: sh: shoulder, s: strong and m: medium.
1098(sh) 422(s) 351(m)
158
R.K. Biswas, M.A. Hayat / Hydrometallurgy 63 (2002) 149–158
OH – Zr bridging is absent for this complex which supports the contention that the extracted species is monomeric. For the 30-day aged system, the experimental results obtained for 0.1, 1 and 5 M HCl systems can be best explained if the following changes during ageing are considered:
occurs via the formation of solvated ion pair or solvated uncharged species depending on the aqueous acidity. The ageing and the acidity of the aqueous phase alter the extraction behaviour.
½Zr8 ðOHÞ20 ðH2 OÞ24 Cl12 þ Cl
One of the authors (M.A. Hayat) is grateful to the UGC (Dhaka) for granting a PhD fellowship and to the Ministry of Education, the People’s Republic of Bangladesh, for granting study leave. A part of the work was presented in the International Seminar on NFMM with satellite symposium on Aluminium held at NML, Jamshedpur, India, February 2000.
! ½Zr8 ðOHÞ20 ðH2 OÞ24 Cl13
ð4Þ
½Zr4 ðOHÞ8 ðH2 OÞ16 Cl6 2þ þ 2Cl ! ½Zr4 ðOHÞ8 ðH2 OÞ16 Cl8
ð5Þ
and
References
½ZrðH2 OÞ5 Cl3 þ þ 3Cl ! ½ZrðH2 OÞ2 Cl6
Acknowledgements
2
þ 3H2 O
ð6Þ
The resultant aged species can be extracted as follows: ½Zr8 ðOHÞ20 ðH2 OÞ24 Cl13 þ Hþ þ 4H2 A2ðoÞ WH3 Aþ 2 ½Zr8 ðOHÞ20 ðH2 OÞ24 Cl13 3H2 A2 ðoÞ
ð7Þ
½Zr4 ðOHÞ8 ðH2 OÞ16 Cl8 þ 4H2 A2ðoÞ W½Zr4 ðOHÞ8 ðH2 OÞ16 Cl8 4H2 A2 ðoÞ
ð8Þ
and ½ZrðH2 OÞ2 Cl6 2 þ 2Hþ þ 4H2 A2ðoÞ 2 Wð2H3 A2 Þ2þ ½ZrðH2 OÞ2 Cl6 2H2 A2 ðoÞ
ð9Þ
Eqs. (7) – (9) explain well the experimental H + , Cl and H2A2 dependencies of extraction ratio, D and the infrared spectral data.
4. Conclusions The solvent extraction of Zr(IV) from Cl medium by D2EHPA is complicated by the slow change in the composition of aqueous Zr(IV) species. The extraction
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