Sodium salts of D2EHPA, PC-88A and Cyanex-272 and their mixtures as extractants for cobalt(II)

Hydrometallurgy, 34 (1994) 331-342

331

Elsevier Science B.V., Amsterdam

Sodium salts of D2EHPA, PC-88A and Cyanex272 and their mixtures as extractants for cobalt (II) N.B. DevP, K.C. N a t h s a r m a a a n d V. C h a k r a v o r t t y b aHydro and Electrometallurgy Division, Regional Research Laboratory (CSIR), Bhubaneswar 751013, Orissa, India bDepartment of Chemistry, Utkal University, Vani Vihar, Bhubaneswar-751 004, Orissa, India (Received February 5, 1993; revised after accepted June 9, 1993 )

ABSTRACT Extraction of cobalt(II) from an acidic sulphate solution has been studied using sodium salts of D2EHPA, PC-88A and Cyanex-272 in benzene; the corresponding pHo.5 values are 6.0, 6.25 and 6.6, respectively. The extracted species appear to be CoA2(HA)2. The electronic spectra of the extracted organic phase support the nature of the species extracted. The loading capacity of 0.1 M extractants in benzene has been determined. The influence of NaC1, NaNO3, Na2SO4 and NaSCN on the extraction systems has been investigated. Synergism has been observed with the binary mixture of all the three extractants used. Of the three extractants, sodium salt of Cyanex-272 has been found to be the best synergist and sodium salt of D2EHPA the least.

INTRODUCTION

Deep-sea manganese nodules, copper-converter slag, spent catalyst, etc., mostly contain copper, cobalt, nickel, iron and manganese besides other impurities. These materials can be treated by sulphuric acid leaching to recover the metal values. Depending on the leaching conditions, the leach liquor contains copper, nickel, cobalt, iron and manganese along with other impurities. After removal of the impurities such as iron and manganese from the leach liquor, the solvent extraction technique can be used to separate and recover copper, nickel and cobalt from the solution. The separation of cobalt and nickel from a sulphate solution is difficult because both the metal ions are extracted as cations. Ritcey et al. [ 1 ] developed a process using di (2-ethylhexyl) phosphoric acid (D2EHPA) as the exCorrespondence to: Dr. K.C. Nathsarma, Regional Research Laboratory, C.S.I.R., Bhubaneswar, 751 013, Orissa, India.

0304-386X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved.

SSDI 0 3 0 4 - 3 8 6 X ( 9 3 ) E 0 0 3 5 - M

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N.B. DEVI ETAL.

tractant which was found to be more selective for cobalt than nickel. Golding and Barcley [2] used the phosphorus-based extractants (phosphonic and phosphinic acid) for the extraction and separation of cobalt and nickel from aqueous solutions. Di- (2,4,4 trimethylpentyl)phosphinic acid (Cyanex-272) is highly selective for cobalt and is useful for treating solutions with a higher proportion of nickel to cobalt. In the case of Cyanex-272 it has been observed that polymerization in the organic phase begins after a certain level of cobalt loading resulting in increase of the organic phase viscosity [3, 4]. Dreisinger and Cooper [ 5 ] investigated the solvent extraction and separation of cobalt and nickel using 2-ethylhexyl-phosphonic acid mono 2-ethylhexyl ester (PC88A). It was observed that the viscosity of the organic phase was greatly increased by high cobalt loading. This effect was not observed in case of D2EHPA. However, in the case of PC-88A and Cyanex-272, the organic phase loading has to be limited to 70% to prevent any large increase in viscosity which ultimately decreases the rate of mass transfer. The Cyanex-272 extraction system should be operated at a maximum of 60% of the loading capacity. If the extractants are pre-equilibrated with sodium hydroxide prior to metal extraction [6 ], the rate of extraction does not decrease. The mass transfer coefficient depends on the proportions of the neutralised extractant molecules [ 7 ]. This paper reports the solvent extraction of cobalt from an aqueous sulphate solution using benzene solutions of commercial D2EHPA, PC-88A and Cyanex-272 after being converted to their sodium salts. These studies have also been carried out with mixtures of the sodium salts of these extractants.

Reagents The commercial extractants D2EHPA, PC-88A and Cyanex-272 were supplied by Mobil, Daihachi and American Cyanamid, respectively. The extractants were used after their conversion into corresponding sodium salts. These salts were prepared by adding a standard and concentrated solution of NaOH to 1 M solutions of the extractants and after thoroughly mixing a single phase was obtained. Benzene (Merck) was used as the diluent. Tri-n-butyl phosphate (TBP) supplied by Merck was used as the phase modifier and its level was maintained at 5 vol%. Cobalt sulphate (COSO4.7H20) and other chemicals used were obtained from BDH. Stock solutions of cobalt (II) sulphate were prepared in distilled water and concentrated H2SO4 was added to the solutions to suppress hydrolysis of the metal ion. Cobalt (II) sulphate solution was analysed by titration with EDTA [ 8 ]. Stock solutions of sodium chloride, sodium nitrate, sodium thiocyanate and sodium sulphate used in the experiments were prepared in distilled water and were of 5 M except the sodium sulphate solution (2.5 M).

SODIUM SALTS OF D2EHPA, PC-88A AND CYANEX-272 AND THEIR MIXTURES

333

Extraction procedure The aqueous phase (10 ml) containing the metal ion ( ~0.01 M) and 0.1 M Na2SO4 was equilibrated with an equal volume of the sodium salts of D2EHPA, PC-88A and Cyanex-272 in benzene and 5 vol% TBP. The pH adjustment of the solution was made before the extraction by addition of dilute H2SO4 or NaOH solution. After equilibration for 5 min at ambient temperature, the phases were allowed to separate and the equilibrium pH of the aqueous phase was measured. Cobalt (II) was analysed by the thiocyanate method [ 9 ], at ).max = 620 nm using a Varian DMS- 100 UV-Visible Spectrophotometer. The concentration of metal ion in organic phase was calculated from the difference between the metal concentration in the aqueous phase before and after extraction. The distribution coefficient (D), and percentage of extraction (%E) were calculated accordingly. RESULTS AND DISCUSSIONS

Influence of equilibrium pH Extraction of cobalt (II) from an aqueous solution by D2EHPA, PC-88A and Cyanex-272 in benzene was poor due to the poor cation exchange property of the extractants with respect to cobalt (II) for which the extraction of the metal ion was carried out using 0.02 M sodium salts of the extractants in benzene containing 5 vol% TBP in the equilibrium pH range 3.0-7.0. Figure 1 presents plots of percentage extraction versus equilibrium pH. The extraction began at equilibrium pH 4.0, increased with increasing pH and attained a maximum value of 60%. The mixture of 60% sodium salt of the extractant and the un-neutralised extractant is probably responsible for extraction up to 60%. In the case of D2EHPA, PC-88A and Cyanex-272, the corresponding pHo.5 values are 6.0, 6.25 and 6.6, respectively. Figure 2 presents the plots of log D versus equilibrium pH and the slopes in case of D2EHPA, PC-88A and Cyanex-272 are 1.6, 1.6 and 2.0, respectively.

Extraction mechanism The pre-equilibrium level is given by: Na~-aq) + 1/2 (HA)z(org) ~-~-NaA(org) +H~-aq )

( 1)

In the case of Cyanex-272, polymerization of the organic phase took place when sodium hydroxide loading in the organic phase was about 70%. Thus, conversion of Cyanex-272 to its sodium salt was maintained at 60%. There was third phase formation in the case of PC-88A during the preparation of sodium salt, for which sodium ion loading was kept at 60%. Though 100%

334

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N.B. DEVI ET AL

70

,

GO

o No02EH.A

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¢ ¢ ¢ I

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I 1

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30 20

10 0

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3.5

4.5

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7.0

Equilibrium pH Fig. 1. Effect of equilibrium pH on the percentage extraction o f cobalt.

I

I 0

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Na D2EHPA

o.21- zx NQPC-OBA

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| o Nocyo,.x- off ~ " t 24272 /-- ~ l[co l:o.m. f /" 2-

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DI-

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Equilibrium pH Fig. 2. Effect of equilibrium p H on the distribution coefficients o f cobalt.

conversion o f D2EHPA to its sodium salt can be obtained, even then the neutralization was maintained at 60% for uniformity. The sodium ion in the extractant was replaced by cobalt ion according to the following reaction:

SODIUM SALTS OF D2EHPA, PC-88A AND CYANEX-272 AND THEIR MIXTURES

2+ --~CoA2(org) + 2Na ~-aq) 2NaA(org) Jr Co(aq)

335 (2 )

Once the sodium ions have been replaced, extraction of cobalt takes place by the following reaction: 2+ CO(aq) + 2 (HA)2(org)-~CoA2 (HA)2(org) + 2H~-aq)

(3)

Equation (3) explains the results obtained from slope analysis.

Influence of extractant concentration The extraction of cobalt(II) from 0.01 M solution by the sodium salts of D2EHPA, PC-88A and Cyanex-272 in the range 0.005-0.08 M (Fig. 3) showed that the percentage extraction increased with the increasing extractant concentration. The extraction becomes quantitative at and above 0.04 M extractant concentration. The plot of log D versus log [ extractant ] shown in Fig. 4 is linear in the range of extractant concentration 0.005-0.03 M and the slope of the line is 2.3. This indicates that for a mole of cobalt ion, two moles of extractant are required. The extractant being a mixture of 60% sodium salt and 40% un-neutralised acid, the extraction is pH dependent. In a set of experiments at a given extractant concentration (0.1 M) when cobalt concentration in the solution was varied from 0.01-0.06 M, the results plotted in Fig. 5 ( [Cobalt]org, k g / m 3 versus [Cobalt], M in the feed) indicate that the loading capacity of 60% converted sodium salts of the three extracrants is 2.00 k g / m 3 against the theoretical loading capacity of 2.95 k g / m 3. Thus, the solvent was loaded to 68% of the theoretical capacity. ....

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[Extroctont], M

Fig. 3. Variationof cobalt extractionwith extractant concentration.

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N.B. DEVI ETAL. 1.4

I

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Na PC-88A

A /

[ ] Na Cyanex-272 0.2 E:I

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%

,,,o~

"

o~

- -0.4

-I.(: -1.6 -2.4

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I

-2.0 -1.6 log [Ext rQ ctctnt]

-1.2

Fig. 4. Plot of log [ Extractant ] versus log D.

I

]

I

I

2.0 1.0

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2EHPA NQ PC-88A

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: o. 1 M I

0.02

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0.03 0.04 0.05 ~'CobGIt]~ M in feed

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0.06

Fig. 5. Loading capacity of 0. l M extractants.

Figure 6 is the plot of log [Cobalt]org versus log [Cobalt ]raff. The plots are linear which indicates that m o n o m e r i c species are extracted into the organic phase.

Effect of salts The effect of salts such as NaC1, NaNO3, NaSCN and Na2SO4 (in the concentration range 0.1-2.0 M ) on the extraction efficiency of 0.02 M extractants was studied for 0.01 M cobalt sulphate solution. The effect of NaC1 on the extractability of the three extractants was almost nil. The influence of NaNO3 on the extraction efficiency of D2EHPA is zero. On the other hand, there was a marginal increase in the percentage extraction in the case of PC-

S O D I U M SALTS O F D2EHPA, PC-88A A N D CYANEX-272 A N D T H E I R M I X T U R E S

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Na C~anex-272

,-.~° 0.3 .,,_. o o

0.1

0 -3.2

1 -2.4

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0.B

log [Cobalt] raft.

Fig. 6. Plot of log [cobalt ]orgversus log [cobalt ] ram

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Na2SO 4 o No D2 EH PA A NQ P C - B B A

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272

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[salt], M Fig. 7. Effect of Na2SO4 and NaSCN on cobalt extraction. 88A and Cyanex-272. As shown in Fig. 7, the percentage extraction of cobalt(II) slowly decreased from 60 to 48, 58 to 45 and 59 to 42 in the case o f D2EHPA, PC-88A and Cyanex-272, respectively with increasing Na2SO4 concentration in the feed. Eventhough the initial p H was maintained at the pHo.5 values for the extractants, the equilibrium pH changed only slightly with

338

N.B. DEVIETAL.

the increasing sulphate ion concentration. When NaSCN concentration was increased in the feed solution, the percentage extraction in the case of D2EHPA decreased substantially from 60 to 22% as shown in Fig. 7. On the other hand, there was a small increase in the percentage extraction in the case of PC-88A and Cyanex-272, respectively.

Electronic spectra Electronic spectra of the blue coloured organic phase (Fig. 8 ) obtained from the extraction of cobalt(II) by sodium salts of D2EHPA, PC-88A and Cyanex-272 showed that the same cobalt complex of tetrahedral species was extracted [ 10 ]. Moreover, the spectra indicated that the nature of the extracted complex was the same for all the extractants as reported by Preston [ 9 ]. Spectroscopic studies support eq. (3).

Synergism The term synergism is used to describe enhancement of equilibrium distribution ratios due to the mixing of extractants. In the present case, synergism is due to the replacement of one extractant in the extracted complex by the other. Synergism was observed in mixtures of any two of the extractants studied. This property of the solvent system has been utilized for advantage in the

I

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I

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I - I ~ D2EHPA I I - Na PC-88A I I I - NoCyanex -272

w 0

0.30C

0.0001 500

J

540

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580

620

~ ~ 660

700

W a v e l e n g t h , nm

Fig. 8. Electronic absorption spectra of cobalt bearing extractants in benzene.

339

SODIUM SALTS OF D2EHPA, PC-88A AND CYANEX-272 AND THEIR MIXTURES TABLE 1

Distribution coefficient values of cobalt using NaD2EHPA as the extractant and NaPC-88A and NaCyanex272 as synergists SI.

DA

No.

NaD2EHPA (0.01 M )

Conc. of NaPC-88A (SI. No. 1-5) and NaCyanex-272 (Sl. No. 6 - 1 0 )

DB NaPC-88A (S1. No. 1-5) and NaCyanex-272 (S1. No. 6 - 1 0 )

DA+ B

0.160 0.316 1.097 2.100 5.330 0.026 0.310 1.041 2.170 3.750

0.551 1.142 4.061 16.140 234.600 0.670 1.200 4.301 13.900 202.000

AD

D(A+B )

S ' C ' = l ° g ~-D A-+ D

(M) 1 2 3 4 5 6 7 8 9 10

0.288 0.288 0.288 0.288 0.288 0.288 0.288 0.288 0.288 0.288

0.005 0.010 0.020 0.025 0.030 0.005 0.010 0.020 0.025 0.030

0.103 0.538 2.676 13.750 229.000 0.356 0.602 2.972 11.440 198.000

0.090 0.277 0.467 0.830 1.621 0.329 0.302 0.510 0.752 1.699

D A = Distribution coefficient of cobalt with NaD2EHPA. DB= Distribution coefficient of cobalt with NaPC-88A (S1. No. 1-5 ) and with NaCyanex-272 (S1. No. 6 10). DA+B = Distribution coefficient of the mixture. AD=DA+B-- (DA+DB).

TABLE2 Distribution coefficient values o f cobalt using NaPC-88A as the extractant and NaD2EHPA and NaCyanex272 as synergists SI. NO.

DA NaPC-88A, (0.01 M )

Cone. of NaD2EHPA (S1. No. 1-5) and NaCyanex-272 (S1. No. 6 - 1 0 )

DB NaD2EHPA (S1. No. 1-5) and NaCyanex-272 (S1. No. 6 - 1 0 )

DA+B

0.150 0.288 1.070 1.840 4.070 0.026 0.310 1.040 2.170 3.75

0.590 1.030 4.300 15.030 100.500 0.570 1.240 4.590 23.360 608.000

AD

S.C.=log

D(A+B)

DA+D~

(M) 1 2 3 4 5 6 7 8 9 10

0.316 0.316 0.316 0.316 0.316 0.316 0.316 0.316 0.316 0.316

0.005 0.010 0.020 0.025 0.030 0.005 0.010 0.020 0.025 0.030

0.124 0.426 2.914 12.870 96.110 0.228 0.614 3.234 20.870 603.900

0.102 0.232 0.492 0.843 1.360 0.222 0.297 0.530 0.973 2.175

Distribution coefficient of cobalt with NaPC-88A. D a = D i s t r i b u t i o n coefficient of cobalt with NaD2EHPA (SI. No. 1-5 ) and with NaCyanex-272 (S1. No. 610). DA + B= Distribution coefficient of the mixture. AD=DA+B- (DA+Da). D A=

340

N.B. DEVI ET AL.

TABLE 3 Distribution coefficient values of cobalt using NaCyanex-272 as the extractant and NaD2EHPA and NaPC88A as synergists S1.

DA

No.

NaCyanex-272 (0.01 M)

1 2 3 4 5 6 7 8 9 10

0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31

Conc. of NaD2EHPA ($1. No. 1-5) and NaPC-88A (Sl. No. 6-10) (M)

DB NaD2EHPA (S1. No. 1-5) and NaPC-88A (S1. No. 6-10)

DA+a

0.005 0.010 0.020 0.025 0.030 0.005 0.010 0.020 0.025 0.030

0.150 0.288 1.070 1.840 4.070 0.160 0.316 1.097 2.100 5.330

0.83 0.97 3.90 19.30 85.85 0.67 1.70 4.38 11.94 303.00

AD

D(A+B )

S.C. =log DA+ D-------~

0.370 0.372 2.520 17.150 81.470 0.200 1.074 2.973 9.530 297.400

0.256 0.210 0.451 0.953 1.292 0.154 0.434 0.493 0.695 1.730

Distribution coefficient of cobalt with NaCyanex-272. Da = Distribution coefficient of cobalt with NaD2EHPA (S1. No. 1-5 ) and with NaPC-88A (S1. No. 6-10 ). DA+a = Distribution coefficient of the mixture. AD=DA+a- (DA+Da). D A=

i

I

I

2.4 - o 0.01M NaD2EHPA A 0.01M NaPC-SBA 2.0

._u O

K3~

1.6

1.2

0.~ 0./~

j I

0.oi

I

I

0.02

0.03

0.04

[No. Cya nex-272 ] ~M

Fig. 9. Plot o f synergistic co-efficient versus [Na Cyanex-272], M for binary mixtures o f Na Cyanex-272 with N a D 2 E H P A and Na PC-88A.

SODIUM SALTS OF D2EHPA, PC-$SA AND CYANEX-272 AND THEIR MIXTURES

341

extraction of cobalt from 0.01 M solution by keeping one extractant at 0.01 M and varying the other one (synergist) in the range 0.005-0.03 M. The synergistic coefficient (S.C.) is defined as: S.C.--log ( Dmix/DA +De,) where Dmixis the distribution coefficient of the mixture at the chosen concentrations and DA + DB is the sum of the individual distribution coefficients at the same concentration. It was found that the synergistic coefficient in all cases increased with increasing concentrations of the synergist at their pHo.5 values. Experimental data for the synergism systems of NaD2EHPA, NaPC88A and NaCyanex-272 are shown in Tables 1, 2 and 3, respectively. The tables show that phosphinic acid is the best synergist and the phosphoric acid is the least useful. Thus, the extractants can be arranged in the order of their synergism as NaD2EHPA < PC-88A < Cyanex-272. The effect of synergism was most pronounced in the mixtures of phosphonic and phosphinic acids ( S.C. = 2.1 ) when phosphinic acid was used as the synergist (Fig. 9). On the other hand, synergism was the lowest in the mixtures of phosphinic acid and phosphoric acid (S.C. = 1.29 ) where phosphoric acid was used as the synergist. CONCLUSIONS

In the extraction of cobalt from sulphate solutions using sodium salts of D2EHPA, PC-88A and Cyanex-272, the extraction coefficient increased with an increase in equilibrium pH. The metal to extractant ratio was 1 : 2 for all the extractants. Extraction of metal ion from 0.01 M solution increased linearly with increasing solvent concentration up to 0.04 M. In all cases, two moles of the extractant were associated with the extracted complex. Moreover, the extracted complex was m o n o m e r i c in all cases. The loading capacity of the 60% neutralized 0.1 M s o d i u m salts was determined to be 2.00 k g / m 3 against the theoretical loading capacity of 2.95 k g / m 3. Sodium chloride and sodium nitrate in the concentration range 0.1-2.0 M had no effect on the extraction coefficient of the extractants. The presence of Na2SO4 in the feed decreased the percentage extraction for all the extractants. The effect of NaSCN lowered the extraction efficiency of NaD2EHPA considerably. On the other hand, use of NaSCN in the systems with PC-88A and Cyanex-272 increased the percentage extraction to a small extent. The electronic spectra showed the nature of extracted complex to be identical in all cases. Mixtures of any two extractants displayed synergism. The effect was most pronounced where Cyanex272 was used as the synergist. On the other hand, the effect was the least where D2EHPA was used as the synergist.

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N.B. DEVI ET AL.

ACKNOWLEDGEMENTS T h e a u t h o r s are t h a n k f u l t o Dr. R.P. Das, H e a d o f the H y d r o - a n d Electrom e t a l l u r g y D i v i s i o n for e n c o u r a g e m e n t a n d Prof. H.S. Ray, D i r e c t o r , Regional R e s e a r c h L a b o r a t o r y , B h u b a n e s w a r for p e r m i s s i o n to p u b l i s h the paper. O n e o f the a u t h o r s , Ms. N.B. D e v i is t h a n k f u l to Dr. R.P. D a s a n d Prof. H.S. R a y for h e r w o r k i n g as a S e n i o r P r o j e c t Assistant w i t h o u t w h i c h it w o u l d n o t h a v e b e e n possible to c a r r y o u t the work.

REFERENCES 1 Ritcey, G.M., Ashbrook, A.W. and Lucas, B.H., Development of a solvent extraction process for the separation of cobalt and nickel. CIM Bull., 68 (753 ) ( 1975 ): I 11-123. 2 Golding, J.A. and Barclay, C.D., Can. J. Chem. Eng., 60 (1988): 970. 3 Komasawa, I., Otake, T. and Ogawa, Y., J. Chem. Eng. Japan, 17 ( 1984), p. 410. 4 Xun Fu and Golding, J.A., Solvt. Extr. Ion Exch., 5 (2) ( 1987): 205. 5 Dreisinger, D.B. and Cooper, W.C., Hydrometallurgy, 12 ( 1984): 1-20. 6 Ritcey, G.M. and Ashbrook, A.W. (Editors), Solvent Extraction Principles and Application to Process Metallurgy. Part 1, 4, Elsevier, Amsterdam (1984). 7 Xun Fu and Golding, J.A., Solvt. Extr. Ion Exch., 6(5) (1988): 889. 8 Vogel,A.I. (Editor), Text Book of Quantitative Inorganic Analysis Including Elementary Instrumental Analysis, 4th ed. (revised), ELBS and Longman, London ( 1978 ). 9 Preston, J.S., Hydrometallurgy, 9 (1982): 115. 10 Cotton, F.A. and Wilkinson, G. (Editors), Advanced Inorganic Chemistry, 4th ed. Wiley, New York, (1980).