Mixed-metal carboxy late complexes - Science Direct

HydrometaUurgy, 13 (1985) 317--326

317

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

MIXED IRON--NICKEL COMPLEXES IN VERSATIC 10 SOLUTIONS

F.M. DOYLE-GARNER* and A.J. MONHEMIUS

Department of Metallurgy and Materials Science, Royal School of Mines, Imperial College of Science and Technology, London SW7 2BP (Great Britain) (Received February 26, 1984; accepted in revised form September 20, 1984)

ABSTRACT Doyle-Garner, F.M. and Monhemius, A.J., 1985. Mixed iron--nickel complexes in Versatic 10 solutions. Hydrometallurgy, 13: 317--326. Iron and nickel Versatic solutions were prepared by solvent extraction with 33% Versatic 10 in Escaid 110. Mixed iron--nickel Versatic solutions with different iron:nickel ratios were prepared by mixing single metal solutions. The infrared spectra Of the single metal and mixed metal Versatic solutions were recorded at 25 and 160°C. Iron Versatic showed absorption bands at 1685, 1575 and 1418 cm -1, probably due to solvating acid, bidentate chelation and bidentate bridging, respectively. Nickel Versatic absorbed at 1665 and 1590 c m - ' ; the 1590 cm -1 band has been assigned to monodentate coordination. All mixed iron--nickel solutions showed absorption bands at 1610 and 1585 c m - ' . The relative intensities of the bands changed with solution composition, but not the frequencies. The mixtures showed none of the absorption bands of iron-only or nickel-only Versatic solutions; thus all the metal ions in solution are assumed to be present as mixed complexes. The average Versatic to total metal charge ratio in the complexes at 160°C was 1.1--1.3 for relatively high metal concentrations. Thin layer chromatography tests were carried out at 25°C; the retention factors of all mixed iron--nickel solutions were the same, and different from those of iron-only or nickel-only Versatic. This gives further evidence for the existence of mixed-metal complexes.

INTRODUCTION

Mixed-metal carboxy late complexes The formation of mixed-metal complexes during solvent extraction with carboxylic acids has been reported previously. Mixed complexes are generally undesirable because when they form, the separation factor for a pair of metals is lower than the theoretical value predicted from the individual distribution coefficients of the metals. Nakasuka et al. assumed that after coextraction with capric acid, the organic phase contained both single-metal *Present address: University of California,Berkeley, Department of Materials Science and Mineral Engineering, Berkeley, C A 94720, U.S.A.

0304-386X/85/$03.30

© 1985 Elsevier Science Publishers B.V.

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and mixed-metal species [1--3]. They determined the composition of the mixed-metal complexes by slope analysis and mass balance, using equilibrium constants previously obtained for extraction from single-metal solutions. The following species were identified: NiCoR4.4HR, ZnNaR3.5HR, CdNaR3.5HR, CdNaR3.7HR and A13Ga3R,2(OH)6, where R represents the caprate anion. Miihl and co-workelTs measured the distribution coefficients of Cu, Ni, Cr and Zn for solvent extraction with n-caprylic acid in the absence and presence of iron [4, 5]. At low pH the distribution coefficients of the nonferrous cations were very much higher for solutions containing iron than for single-metal solutions. The distribution coefficient of iron itself was higher than that of the divalent metals b u t was only slightly increased by the presence of the other ions; thus the enhanced extraction of the other metals lowered their separation factor from iron. Coextraction was attributed to the formation o f mixed-metal complexes. UV--visible spectroscopy indicated an iron to nickel ratio of 3:1 in the mixed complexes.

Infrared spectrophotometry of carboxylate solutions Ashbrook reported infrared spectrophotometric studies of metal-loaded Versatic 911" solutions [6--8], and found that nickel Versatic shows an absorption band at 1130 cm -1, but otherwise gave a spectrum identical to that of unloaded Versatic 911 [7]. Crabtree and Rice studied cobalt Versatic solutions using derivative spectroscopy [9]. The secondary effects of coordination can be detected from the infrared spectra, because the carbonyl vibration frequency of a carboxylate group is determined by its coordination. Carboxylic acid may be present in an organic solution as monomers, hydrogen-bonded dimers or as solvating acid. The carbonyl (C=O) vibration gives a strong absorption band in the range 1800--1740 cm -1 for the m o n o m e r [10]. The dimer has symmetric and antisymmetric vibration modes, b u t only the antisymmetric mode is IR active. The C=O bond is weaker in the dimer than in the m o n o m e r because of hydrogen bonding, so the absorption frequency is lower, typically at 1740--1680 cm -~ [10]. When a carboxylic acid molecule solvates a complex, the hydrogen bonding of the OH group is stronger than in dimers, so the C=O bond is further weakened, and absorbs at a lower frequency than the dimer carbonyl. Crabtree and Rice observed absorption bands at 1700 and 1675 cm -~ for cobalt Versatic extracts, and assigned these bands to the Versatic acid dimer and solvating acid, respectively [9]. Carboxylate anions can bond to metals in any of the following ways [11] :

./oI

R

c

R ]I

/O=M ~I

*Versatic acids are tertiary carboxylic acids marketed by Shell Chemical Co.

I~

319

Structure I contains free ions, and is unlikely in solutions of Versatic acid in hydrocarbon diluents, which have low dielectric constants. Structures II, III and IV are m o n o d e n t a t e coordination, bidentate chelation and bidentate bridging, respectively. The bidentate chelation and bridging structures (III and IV) are symmetric, and therefore have symmetric and antisymmetric vibration modes for the O---C--O group. The strong antisymmetric vibration generally occurs at 1650--1550 cm -1 and the weaker symmetric vibration at 1440--1360 cm -1 [10]. The exact frequency is determined by the strength of the C--O bonds, which depends on the t y p e of carboxylic acid and the strength of the metal-ligand bonds. In bidentate chelation the electron cloud of the O--C---O system is more dense than in bidentate bridging, so has a higher bond order and frequency. Carboxylate groups involved in monodentate coordination (structure II) have absorption bands for both C=O and C---O vibrations. These bands correspond to the antisymmetric and symmetric vibration modes of the symmetric O---C---O group in structures III and IV. Only the C=O vibration occurs at a frequency within the carbonyl range. Because of the high mass of the metal M, relative to hydrogen, the carbonyl frequency of a monodentate complexing group is lower than that of the corresponding acid. However, it is larger by about 100 cm -1 than the antisymmetric vibration mode of structure IV. In their studies on cobalt Versatic solutions, Crabtree and Rice noted a peak at 1580 cm -~ and a band at 1480 cm -1 , which was ascribed to bidentate carboxylate groups. Addition of free acid to the extract introduced a new band at 1595--1602 cm -~ at the expense of the peak at 1580 cm -1. A band which appeared at 1425 cm -~ was thought to be due to bidentate carboxylate groups. Stefanakis and Monhemius [ 12, 13 ] observed bands for iron Versatic solutions analogous to the 1595, 1580 and 1425 cm -~ bands reported by Crabtree and Rice for cobalt Versatic. Bands at 1600, 1575 and 1415 cm -~ were tentatively assigned to m o n o d e n t a t e complexing, bidentate chelation and bidentate bridging, respectively. Both studies indicated that metal Versatic complexes contain carboxylate anions in different modes of coordination. This paper reports infrared spectrophotometric and thin-layer chromatographic studies carried out with Versatic solutions containing iron and nickel. This work was part of a project to study hydrolytic stripping of mixed-metal carboxylate solutions. The solutions were prepared not by coextraction, but instead by mixing single-metal Versatic solutions. EXPERIMENTAL PROCEDURE

Preparation of organic solutions Nickel Versatic solution was prepared by extraction of nickel from nickel sulphate solution with 33% v/v (1.75 M) Versatic 10 in Escaid 110, using

320 sodium hydroxide solution as a neutralising agent. Iron Versatic solution was prepared by exchange extraction with calcium Versatic: calcium oxide was dissolved in 33% Versatic 10 in Escaid 110; then the calcium Versatic solution was shaken vigorously with aqueous ferric chloride solution for about 2 minutes in a separating funnel. The overall reaction can be represented as 2 FeC13 + 3 CaV2 = 2 FeV3 + 3 CaC12

(1)

where a bar above a species indicates that it is present in the organic phase, and V denotes the Versatic anion. The iron Versatic solution was washed with 0.1 M HC1 to a steady pH of 3.0 to remove any residual calcium. Both types of organic solutions were washed three times with distilled water and were then passed through phase-separating paper to remove any physically entrained water. The iron and nickel Versatic solutions were analysed by contacting portions with equal volumes of 1:1 HC1, to strip the metal cations from the organic phase. The aqueous strip solutions were analysed by atomic absorption spectrophotometry. Mixed iron--nickel Versatic solutions with various compositions were prepared by mixing appropriate volumes of iron Versatic, nickel Versatic and 33% Versatic acid.

Infrared spectrophotometry Samples of Versatic acid, iron Versatic, nickel Versatic and mixed iron-nickel Versatic solutions were diluted with Escaid 110, generally to 10% of the original concentration. The infrared spectra of these samples were recorded at 25, and 160 or 165°C using a Perkin--Elmer 577 grating infrared spectrophotometer equipped with a heated optical cell.

Thin-layer chromatography Iron Versatic, nickel Versatic and iron--nickel Versatic solutions were spotted on silica gel coated plates. Four eluting systems were used: ethyl acetate, ethanol, and 10:1 and 1:1 mixtures of ethyl acetate and ethanol. After elution the sample spots were detected by their colour; the retention factor was determined for each sample, where the retention factor, Rf, is defined as Rf = (distance moved by sample spot)/(distance moved by solvent front) RESULTS AND DISCUSSION

Infrared spectrophotometry The infrared spectrum of a 2.5% solution of Versatic 10 at 25°C and 165°C is shown in Fig. 1. At 25°C the acid is present mainly as the dimer and there

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is a strong absorption band at 1700 cm -1 for the carbonyl vibration in the dimer. At 165°C there is partial dissociation to the m o n o m e r form; the m o n o m e r carbonyl absorption band at 1750 cm -1 is very much stronger than at 25°C, whereas the dimer carbonyl band at 1700 cm -1 is weaker. At 165°C the narrow band of the OH vibration in the m o n o m e r is seen at 3540 c m -1 .

I

1

I

I

I

I

, a2

',.t,.

OO

/i

'~

.,I i ij

....

,.o 4000

25C

~65"C I

It

I

/

3000

Ii il II tr h

] I

I

2000 Wovenumber

5 C=O i dimer I 1800 1600

S I

]

1400

120o

Cm "|

Fig. 1. Infrared spectrum of 2.5% Versatic 10 in Escaid 110 at 25°C and 165°C.

The only change observed in the absorption spectra of the metal Versatic solutions at 25°C and 160°C is that the free m o n o m e r carbonyl peak appears at 1750 cm -I at 160°C. Figure 2 shows the carbonyl region of the spectra of 0.38 M iron Versatic and 0.34 M nickel Versatic, both diluted to 10% v/v with Escaid. Compared with the same region in Fig. 1, the additional peaks are due to C--O vibrations in the carboxylic acid and carboxylate anions which are coordinated to metal cations. Iron Versatic shows an inflection at 1685 cm -1, a main peak at 1575 cm -1 and an inflection at 1418 cm -1. The inflection at 1685 cm -1 is due to solvating Versatic acid [9] and the bands at 1575 and 1418 cm -1 have been ascribed to bidentate chelation and bidentate bridging, respectively [12, 13]. Nickel Versatic shows a peak at 1590 cm -1 which is presumed to correspond to the peak at 1600 cm -1 in iron Versatic, assigned to m o n o d e n t a t e coordination in Refs. [12, 13]. Monodentate coordination has also been reported in lower nickel carboxylates [14]. The peak at 1665 cm -I in nickel Versatic has n o t been satisfactorily identified; the optical density is very much higher than that for the solvating acid band seen at 1685 cm -1 in iron Versatic. If there was significantly more solvating acid in nickel complexes than in iron complexes the ligand--metal bonding would be weaker: this would strengthen the C=O bond and increase the carbonyl absorption frequency rather than decrease it by 20 cm -1. It is therefore unlikely that the band at 1665 cm -I is due to solvating Versatic acid.

322 I

I

i

FeV3

~D

rT~O1700 1590

I

1800

I

1

1600 Wovenumber

l

L

I

14.00 C m "1

Fig. 2. Infrared spectrum of 10% solutions of 0.38 M iron Versatic and 0.34 M nickel Versatic at 160°C.

Figure 3 shows the carbonyl region of the spectra of 10% v/v solutions of iron--nickel Versatic samples originally containing (a) 0.2 M Fe and 0.05 M Ni, (b) 0.1 M Fe and 0.05 M Ni, and (c) 0.1 M Fe and 0.1 M Ni. None of the absorption bands of iron-only or nickel-only Versatic solutions appear, which indicates that there are no single-metal complexes in the solutions. All the metal ions must therefore be present as mixed iron--nickel Versatic complexes. The spectra show two principal peaks at 1610 and 1585 cm-1; these are probably due to monodentate coordination and bidentate chelation, respectively. These peaks are at slightly higher frequencies than the peaks for the same type of coordination in single metal complexes, indicating stronger C - O bonding within the ligand, and thus weaker metal--ligand bonding in the mixed-metal complexes. Bidentate chelation, which occurs in iron Versatic complexes, predominates in the sample with the highest iron concentration (Fig. 3(a)). Monodentate coordination, which is characteristic of nickel Versatic complexes, predominates in the mixture with the highest nickel concentration (Fig. 3(c)). The frequency of each peak, hence the strength of the metal--ligand bond, does not vary with solution composition. The average number of carboxylate groups associated with each metal ion in the complexes was found by determining the amount of free Versatic in

323

(a)

I

J OD

16101585

- -

[Fe](M)

[Ni] (M)

(a) (b)

0.2 O. I

0.05 0.05

(c)

O. I I

1800

f

O.t J

I

t _ _

1600 1400 Wovenumber cm"1

Fig. 3. Infrared spectra of iron--nickel Versatic at 160°C (10% v/v solutions of samples with the stated compositions diluted in Escaid 110).

samples with known iron, nickel and total Versatic concentrations. The monomer and dimer peaks were shown to o b e y Beer's Law, and the extinction coefficients of each were obtained from standard solutions. The optical densities of b o t h peaks were then used to calculate the amount of free Versatic acid in samples. The amount of Versatic associated with the metals in solution, either as solvating acid or as carboxylate anions, was then obtained by difference. An empirical correction [15] was used to allow for desolvation, which occurred on dilution. The different valencies of Fe(III) and Ni(II) were taken into account by defining the total metal charge concentration [Mtc] as [Mtc] = 3 [Fe 3+] + 2 [Ni 2+]

(2)

For each sample the ratio of concentration of Versatic in the complexes to the total metal charge concentration, [V]/Mtc ] , is thus given b y IV]/[Mtc ] = ([V] total-- [HV] f)/(3 [Fe 3+] + 2 [Ni 2+] )

(3)

where [V] total is the total molar concentration of Versatic in the sample and [HV] f is the free Versatic concentration, found from the optical densities of the monomer and dimer peaks. Figure 4 shows a plot of [V]/[Mtc] against [Mtc] for samples of various composition at 160°C; there is only a narrow

324

band of possible complex compositions for a given total metal charge concentration. The amount of Versatic associated with each metal charge in the organic complexes decreases rapidly with increasing metal concentration, and a steady Versatic to metal charge ratio of 1.1--1.3 is reached at high metal concentration. The samples had four different Versatic concentrations, as shown in Fig. 4, and three different iron to nickel ratios (4, 2 and 1), but the Versatic to metal ratio in the complexes appears to be largely independent of both parameters. However, [V]/[Mtc ] was higher for nickel-only Versatic solutions and lower for iron-only Versatic solutions than for mixed solutions of comparable [Mtc ] .

I

t

I

l

[vl, M • • e •

1.31 1,75 2.36 2.62

'-U 2

oNi

eFe

C) l 0

I

I 0.5

t

I 1,0

rMtc ] (M) Fig. 4. Versatic to total metal charge ratio in metal c a r b o x y l a t e complexes, as a f u n c t i o n of total metal charge c o n c e n t r a t i o n at 160°C.

The decrease in associated Versatic with increasing total metal charge concentration probably results from desolvation. The solvating acid is assumed to be in equilibrium with the free acid in solution. At low metal concentrations there will be high concentrations of free acid, and thus of solvating acid. At higher metal concentrations the free acid concentration will fall, hence the concentration of solvating acid in equilibrium with the free acid, and the overall Versatic to metal ratio in the complexes, will also fall.

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Thin-layer chromatography The r e t e n t i o n f a c t o r s o b t a i n e d f r o m the thin layer c h r o m a t o g r a p h y tests are r e p o r t e d in Table 1. In all f o u r eluting s y s t e m s iron Versatic left a cont i n u o u s streak o n t h e plate, and nickel Versatic gave a well