Extraction of tetravalent metals with di-(2-ethylhexyl) phosphoric acid—II

J. inorg, nucl. Chem., 1967.Vol. 29, pp. 1307 to 1315. PergamonPress Ltd. Printedin Northern Ireland

EXTRACTION OF TETRAVALENT METALS WITH DI-(2-ETHYLHEXYL) PHOSPHORIC ACID--II ZIRCONIUM P. H. TEDESCO, V. B. DE RUMI and J. A. GONZ~,LEZQUINTANA Facultad de Quimica y Farmacia, Universidad Nacional de La Plata, Argentina

(First received 12 September 1966; in revised form 15 November 1966) Abstract--The extraction of zirconium from sulphuric and nitric acid solutions by di-(2-ethylhexyl) phosphoric acid (DEHPA) dissolved in kerosene has been studied. To interpret extraction at different anion, hydrogen and D E H P A concentrations some equations are proposed.

INTRODUCTION

IN A PREVIOUSwork tl~ we have studied thorium extraction with kerosene solutions of di-(2-ethylhexyl) phosphoric acid (DEHPA) from sulphuric and nitric aqueous solutions at metal concentration level of 10-2 M (--~3 g/l). In this work we study zirconium extraction at the same conditions and at tracer level, using in this case zirconium 95. We did not find papers on zirconium extraction with alkyl phosphoric acids from sulphuric medium; from nitric medium the only papers we found were those of PEPPARD a n d FERRARO (2) a n d o f HARDY a n d SCARGILL. (3) EXPERIMENTAL All chemicals used were analytical grade. D E H P A solutions were prepared as in the case of thorium, Extractions of macroamounts of zirconium were performed by agitating both phases for 2 min in a separating funnel. The determination of hydrogen in the aqueous phase before and after the extraction could not be made by complexing zirconium with fluoride as it recommended t4~ because it was observed that when a neutral solution of sodium fluoride is added to an acidic solution of zirconium the pH of the solution rises, perhaps according to a reaction of this type: ZrO 2+ + 6F- + H20 ~ ZrFe ~- + 2HOSo it was decided to complex zirconium with a solution of ethylendiaminetetraacetic acid (EDTA) previously neutralized to the phenolphtalein, titrating hydrogen with sodium hydroxide 0.1 N. The organic phase was stripped with a solution composed of 90 ~ sulphuric acid 1 M and 10 ~ a saturated sodium fluoride solution. Zirconium was determined by gravimetry as zirconium dioxide, previous removal of fluoride. Nitrate was determined by spectrophotometry (reaction with phenoldisulphonic acid). Saturation tests were performed as in the case of thorium. Zirconium-95, gamma and beta emitter, with carrier at 10-5 M was used in experiments at tracer levels. In sulphuric medium it was necessary to evaporate to white fumes the stock nitric solution of zirconium-95, in order to obtain a nitrate free solution. ~1~ p. H. TEDESCO,V. B. DE Ruvrt and J. A. GONZ~,LEZ QUINTANA,J. ¢.llorff. nucL Chem. 28, 3027 (1966). t2) D. F. PEPPARD and J. R. FERRARO,J. inorff, nucl. Chem. 10, 275 (1959). lal C. J. HARDY and D. J. SCARGILL,J. inorff, nucl. Chem. 17, 333 (1961). ta~ L M. KOLTHOFFand P. J. ELVING, Treatise on Analytical Chemistry. Part 11, Vol. 5, p. 97. Interscience, New York (1961). 1307

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P.H. TF,DESCO, V. B. DE RUM[ and J. A. GONZ.~LEZQUINTANA

Aqueous solutions were prepared to get a gamma activity of about 20,000 counts/min over a 4-ml aliquot measured with a well-type scintillatingcounter. In these cases extractions were done by agitating 10 ml of organic and 10 ml of aqueous phase in a tube and then centrifugating and measuring the activity over 4 ml of the original and of the equilibrated phases. The sum of these latter was coincident with the former within 10 per cent. In order to avoid contamination with the organic phase from above, special care was needed to take samples of the aqueous phase.

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Results In Fig. 1 the zirconium partition coefficient E°x is represented as a function of sulphate concentration in the aqueous phase, in experiments done at tracer level. There is a continuous decrease of E°x with the increasing sulphate concentration, so we can assume that cationic complexes with sulphate are not fixed by the organic phase. M°re°ver, in equilibrium and saturati°n tests with macr°am°unts °f zirc°nium, sulphate was never found in the organic phase. So, ions as Zr(SO4)~- that, according to CONNICKand McVEY, m~there exists in sulphuric aqueous solutions, were excluded. In Fig. 2 E°A vs. concentration of the dimeric form of DEHPA (HX)= in the organic phase is represented in logarithmic scale. The aqueous phase had zirconium-95 and carrier at 1 0 s M. It can be observed that the slope of the straight line in 1"9. The results of the hydrogen titrations of the aqueous phase before and after the extractions in tests with different zirconium, sulphate and hydrogen concentrations are shown in Table 1. As it was stated, in these titrations EDTA neutralized to the phenolphtalein was used, which releases 2 m-mole of hydrogen per m-mole of complexed zirconium. It can be seen that, no matter what the amount of fixed zirconium, the same volume of sodium hydroxide for the hydrogen titration was used before and after the extraction. The results of saturation tests at different pH and sulphate concentration are given in Table 2. It can be seen that the ratio "m-mole DEPHA/m-mole zirconium" in the saturated organic phase varies between 1.8 and 2.1. <5>R. E. CONNICKand W. H. McV~y, J. Am. chem. Soc. 71, 3182 (1949).

Extraction of tetravalent metals with di-(2-ethylhexyl) phosphoric acid--II

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[ (HX)e 1 M FIG. 2.--Influence of [DEHPA] in the extraction of Zr in sulphuric medium.

([Zr]10-' M. Aqueous solutions ][SO4S-]0 .1 M; Phases ratio (O/A): ¼. ~[H+]0"05 M.

TABLE 1.--EXCHANGE Z r - H

BETWEEN PHASES IN SULPHURIC MEDIUM

Aqueous phase before equilibrium

[SO,Z-](M)

Zr (g/l)

H ÷ in 40 ml (as ml N a O H 0.1 N)

0-05 0.05 0"2 0-2 1.0 1"5

2"6 1.3 2.2 2'2 1'2 0"9

7"2 13"8 14"6 66"0 26.8 27.2

Equilibrated phases H+ in 40 ml aqueous phase (as ml N a O H 0.1 N)

Z r i n 10ml org. phase (rag)

7.1 13.9 14.7 66.0 26.8 27-1

60 27 45 22 14 12

* Phase ratio: 40 ml aqueous solution/10 ml D E H P A solution. D E H P A 0"15 M in kerosene.

TABLE 2.--SATURATION TESTS IN SULPHURICMEDIUM [DEHPA](M)

[SO42-](M)

pH

m-mole DEHPA/m-mole Zr

0"12 0"12 0.12 0.12 0.~9 0.099 0.099 0.10 0.I0

0.1 0.1 0.1 0.1 0-1 0.5 0.5 0'5 0.5

1.0 1.0 2-0 2-0 0"9 1-4 1-8 1"0 2-0

1.8 1.9 2.1 2.1 2.1 2'1 1.9 1"8 2'0

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P.H. TEDESCO,V. B. DERUMIand J. A. GONZ,Id2ZQUINTANA

Discussion

Having proved that complexes with sulphate are not fixed in the organic phase, one can assume these types of reaction for zirconium extraction: Z # + + 4(HX) 2~ ZrH4X a + 4H + ZrO 2+ + 4(HX)~ ~ ZrH4X s + H20 + 2H +

(1) (2)

ZrO ~+ + 2(HX)z ~-ZrOH~X 4 + 2H +

(3)

(Zr(OH)z) ~+ + 2(HX)2 ~ ZrOH2X 4 + H~O + 2H + (Zr(OH)a) z+ + 2(HX)~ ~+-Zr(OH)2HzX4 + 2H +

(4) (5)

(HX)z is the predominant form of DEPHA in the tested solutions. Evidently Equation (1) must be excluded, at least as an important contributing mechanism because, as it is seen in Table 1, in all tests the same volume of hydroxide solution is used before and after the extraction; this means that, since in the complexing of zirconium EDTA releases 2 m-mole of hydrogen per m-mole of complexed zirconium, this must be the amount of hydrogen given by DEPHA per each m-mole of fixed zirconium. The equilibrium constant of Equation (2) is k---- [ZrH4Xs][H÷]2 [ZrO2+][(HX)2] From this we obtain E°x = k" [(HX)214 " [H+]Z ' so if Equation (2) is accomplished, the plot of E°x against [(HX)2 ] in logarithmic scale must be a straight line with a slope of 4. Figure 2 shows a slope of 1"9, which proves that Equation (2) is not a predominant contributing mechanism. Equation (5) must be excluded too because the oxhydrile band did not appear in the i.r. spectra of the organic solutions. Equations (3) and (4) are practically the same because they represent the same exchange zirconium-hydrogen and the same composition of the organic phase; with our measurements we cannot differentiate both reactions. In the saturation tests of Table 2 the ratio "m-mole DEHPA/m-mole zirconium" in the saturated organic phase is 1"8-2"1 ; this corresponds to a complex of composition (ZrOX2). The organic phase increasing viscosity at saturation level is an indication that at these conditions the complex exists under polymeric forms. Summing up, according to the obtained results, extraction of zirconium from sulphuric medium by di-(2-ethylhexyl) phosphoric acid may be represented by the following equations: at tracer level: ZrO ~+ + 2(HX)z ~ ZrOHzX4 + 2H + at macroamounts level: nZrO z+ + n(HX)2 ~- (ZrOX2) + 2nil+ NITRIC MEDIUM Results

The plot of E°x vs. [(DEHPA)2] in logarithmic scale corresponding to experiments in which aqueous nitric solutions of zirconium at 10.5 M were used, is seen in Fig. 3.

Extraction of tetravalent metals with di-(2-ethylhexyl) phosphoric acid--II

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Equilibrium [(DEHPA)~] was calculated for the amount complexed with the zirconium assuming that the relation DEHPA/zirconium is 3/1. It can be seen that the slope is 1"5. ~0 I0.0 i

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Fla. 4.--Influence of [NOs-] in the extraction of Zr in nitric medium. [Zr] in aqueous solutions 10-5 M [DEHPA] 0.1 M. Phases ratio (O/A):½.

E°a as a function of the aqueous phase nitrate concentration is shown in Fig. 4 and Table 3 at tracer and macroamounts levels respectively. In both cases it is seen that E°x is almost independent of nitrate concentration. In Table 4 results obtained in two tests are shown, in which samples of 75 ml of DEPHA 0.097 M in kerosene were equilibrated with 75 ml of zirconium solutions at nitrate concentrations 0"5, 1.0, 3.0 and 6.0 M, respectively. After separation, aliquots of 10 ml of the organic phases were taken and the bulks were again equilibrated with fresh portions of 75 ml of the aqueous solutions. Phases were again separated and

P . H . Tm)r.sco, V. B. DE RVMI and J. A. GONZ~tLEZQUINTANA

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other 10 ml aqueous were taken; the rest of the organic solutions were saturated by means of repeated contacts with fresh aliquots of the aqueous solutions. Ten-minilitre aliquots of the saturated organic phases and those separated after the first and second contact were analyzed for zirconium and nitrate. In Table 4 we can see that the relation "m-mole nitrate/m-mole zirconium" in the organic phase varies in the TABLE 3.--EFFECT OF NITRATECONCENTRATIONIN NITRIC MEDIUM* [NOs-](M) in aqueous phase

m-mole Zr in 10 ml org. phase

E°A

m-mole NOs- in 10 ml org. phase

m-mole NOs-/m-mole Zr in org. phase

0.2

0.24

29

0"07

0.3

0.2

0"23

20

0.07

0-3

0-5

0"22

16

0"08

0"3

0.5

0-23

20

0-09

0"4

1-0

0"22

16

0.12

0-5

1.0

0-22

16

0"13

0"6

2.0

0"18

10

0"25

1"3

2.0

0.21

14

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1"1

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14

0.32

1"5

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0"22

16

0"29

1"3

6.0

0.23

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1"6

6"0

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first contact from 0-4-0.5 to 1.3 when nitrate concentration in the aqueous phase varies between 0.5 and 6"0 M. At the same range of nitrate concentration in the second contact this relation varies between 0.9-1.0 and 1"5 and at saturation, no matter the nitrate concentration in the aqueous phase, the relation DEHPA: NO3-: Zr is constant and equal to 1"6-1-7:1"2-1.3:1.0. It must be pointed out that in a series of experiments nitric solutions without zirconium were contracted with DEPHA at the same conditions that in tests with zirconium and in no case was nitrate found in the organic phase. DISCUSSION

HARDY and SCARGILL(3) have interpreted zirconium extraction by di-n-butylphosphoric acid (HDBP) from nitric solutions according to the equation ZrO 2+ q- 2 NO 3- + 2 (HDBP) a --~ (Zr(NO3-)2.(DBP.HDBP)~) + n 2 0 In saturation tests using DEHPA and zirconium nitrate solutions Pn'PARD et al. found that the relation "m-mole nitrate/m-mole zirconium" in the organic phase was between 1.8 and 2.2. As a consequence they proposed that the complex in the saturated organic phase had the structure [ZrOo.5(NO3-)(DEHPA)]. The resultswe have obtained in thiswork cannot be explainedby an equation such as that proposed by HARDY and SCAROILL for H D B P or by the complex structurewith D E H P A assumed by PEPPARD. {s)

Extraction of tetravalent metals with di-(2-ethylhexyi) phosphoric acid--lI

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P.H. T~esco, V. B. DE RUMI and J. A. GONZkLEZQUIm'AIqA

In order to interpret zirconium extraction by DEHPA from nitric solutions the following equations can be proposed: ZrO ~+ + 2(HX)2 ~-- ZrOH~X, + 2H + / nZrO ~+ + n(HX)~ ~- (ZrOX~),~ + 2nH+j

(6)

(ZrNO3) 3+ + 3(HX)z~ ZrNOsHsXs 3H + / n(ZrNOa) 3+ + 3n/2(HX)~ ~ (ZrNO3X3). + 3nH+j

(7)

ZrO(NO3) + + (HX)z ~--ZrONOsHX ~ + H+ / nZrO(NOs)+ + n/2(HX)2~_- (ZrONOaX)n +nil+ j

(8)

ZrO(NOa) + + 3(HX)z ~---ZrNOaHzXe + H~O + H+ nZrO(NOa) + + 3n/2(HX)z~ (ZrNO3X3),, + nHzO +nH+J

(9)

Zr(NOa)z 2+ + 2(HX),--~ Zr(NOa),H2X4 + 2H+ I nZr(NOa)2~+ + n(HX)~ ~ (Zr(NOa)~X2) + 2nil J z r o ~+ + 2 N O ~ + 2(HX)2 ~ Zr(NO3)2HzX4 + H20 / nZrO ~+ + 2nNO~ + n(HX)~ ~--(Zr(NOa)~X~. + nH~OJ

(10) (11)

The first equation of each pair corresponds to the reaction at tracer level and the second at saturation level; in the latter case the high viscosity of the organic phase indicates polymeric forms. Our results do not agree with any of these equations separately. According to IRVING and EDGINGTON(6) a log E°x 0 log [NO3-] = n0 -- ha

where t~o and ria are the ligand number in the organic and aqueous phase respectively, it is possible to calculate ~i0 if t~ and the effect of nitrate concentration in the aqueous phase on E°x are known. Figure 4 and Table 3 show that a log E°x =0 a log [NO3-] so ~0 = ~ . Since in the assayed nitrate concentrations ranges ~a calculated from the stability constants ~7~varies from less than 1 to a few more than 1, the same must occur in the organic phase and, as a consequence, the fixed complexes must change. This results too from inspection of Table 4 where it is seen that in the first contact when nitrate concentration in the aqueous phase changes from 0"5 to 6.0 M the relation "m-mole nitrate/m-mole zirconium" in the organic phase varies from 0.4-0.5 to 1-3. Reaction (6) must be one of the competitive mechanisms, at least at nitrate concentrations up to I M, otherwise the relation "m-mole nitrate/m-mole zirconium" could not be less than 1. Reaction (8) must be produced too in a certain amount because it is the only explanation for the fact that at saturation the relation "m-mole DEHPA/m-mole zirconium" can be less than 2. This agrees with Fig. 3 where the plotting of E°x vs. (DEHPA) in logarithmic scale gives a straight line of slope 1"5. As te~ H. IRVINGand D. N. EDGINGTON,J. inorg, nucl. Chem. 10, 306 (1959). t*) Stability Constants: Inorganic Ligands. London Chemical Society (1958).

Extraction of tetravalent metals with di-(2-ethylhexyl) phosphoric acid--II

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a consequence Equations (7) and (9) must be excluded because they would produce a relation DEHPA/Zr higher than 2. At certain conditions a relation NOa-/Zr higher than 1 is obtained, as it is seen in Tables 3 and 4, so one must assume that Equations (10) or (11) are produced We think that Equation (10) is more probable than (11) because in zirconium nitric solutions LISTER and McDONALDt81 have identified the Zr(NOa)22+ ion. Summing up, our results can be interpreted by the competitive reactions (6), (8) and (10) which would lead to nitrate and zirconium concentrations in the organic phase proportional to the concentrations of the complex ions that contain them in the aqueous phase; this explains that ti0 = ~a. In the saturated organic phase the relation DEHPA :NO3-:Zr has a constant value no matter what the nitrate concentration in the aqueous phase, It is seen in Table 4 that when nitrate concentration is 0.5 M the relation NOz-/Zr in the organic phase is 0.4-0.5 in the first contact, 1"5 in the second and 1.2-1.3 at saturation. It can be assumed that near saturation, ions are fixed in the organic phase according to their affinity (displacement of some ions by others ?) and this is possibly greater for complexes with nitrate. The relation DEHPA:NOa-:Zr in the saturated organic phase is 1"6-1.7: 1"2-1.3:1"0 which does not fit with any definite complex. For this relation PEPPARD found the values 1.8-2.2:0.6-1.2: l'0. In our tests using an aqueous phase with nitrate 0.5 M this relation was less than 1"2 only in the first two contacts and the relation DEHPA/Zr never was higher than 1.7. tl~ B. A. J. LISTERand L. A. McDoNALD, or. chem. Soc. 1201 (1949).