Solvent extraction of base metals by mixtures of organophosphoric acids and non-chelating oximes

Hydrometallurgy, 10 (1983) 187--204

187

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

SOLVENT EXTRACTION OF BASE METALS BY MIXTURES OF ORGANOPHOSPHORIC ACIDS AND NON-CHELATING OXIMES

JOHN S. PRESTON

Council for Mineral Technology, Randburg 2125 (South Africa) (Received June 5, 1982; accepted in revised form September 27, 1982)

ABSTRACT Preston, J.S., 1983. Solvent extraction of base metals by mixtures of organophosphoric acids and non-chelating oximes. Hydrometallurgy, 10: 187--204. The effect of non-chelating oximes on the extraction of several transition and nontransition metals by solutions of organophosphoric acids ( H : A : ) in xylene has been investigated. Synergistic enhancements of extraction of divalent transition metal ions were found with the oximes of aliphatic aldehydes, the enhancements of extraction increasing in the order VO:* < Cr 2+ < Mn :+ < Fe 5+ < Co :+ < Cu 2+ < V 2+ < Ni ~+. Large synergistic effects were also found for copper(I) and silver(I). Among the divalent non-transition metals studied (Mg, Ca, Zn, Cd, Sn, and Pb), only cadmium showed a synergistic effect. No significant synergism was found for any of the trivalent metal ions studied (Fe, Cr, V, A1, Bi, La, Ce, and Nd). The extracted complexes of copper, cobalt, and nickel were shown to be octahedral in structure, with the compositions Cu(HA 5 )5 (°xime)5, Co(HA~ )5 (oxime)2, and NiA(HA 5 )(oxime)3 , respectively, in which HA T acts as a bidentate ligand. Extraction rates were found to be rapid, even for nickel, and complete stripping of metal-loaded organic phases was effected by contact with 0.5 M mineral acid. Some practical applications, such as the recovery of nickel from acidic leach liquors, are envisaged.

INTRODUCTION During a recent investigation of the solvent extraction of cobalt and nickel by commercial organophosphorus acids [ 1], we observed that the addition of n o n - c h e l a t i n g o x i m e s s u c h as 2 - e t h y l h e x a n a l o x i m e ( E H O ) , o r c h e l a t i n g o x i m e s s u c h as 5 , 8 - d i e t h y l - 7 - h y d r o x y d o d e c a n - 6 - o n e oxime (LIX 63), produced marked synergistic effects, which enabled these metals to be extracted under acidic conditions (pH 0--3). Although examples of mixed extractant systems containing LIX 63 and di(2-ethylhexyl)phosphoric acid (D2EHPA) have been described previously [2--7], their ability to extract cobalt and nickel under such acidic conditions has not been utilised -- presumably on account of the slow extraction rates encountered, particularly for nickel. Accordingly, we cons i d e r e d o u r o b s e r v a t i o n o f t h e i m p r o v e d e x t r a c t i o n r a t e s o b t a i n e d u p o n replacement of LIX 63 by the non-chelating oxime EHO to be of some interest

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© 1983 Elsevier Science Publishers B.V.

188 [1]. Moreover, we found that complete stripping of metal-loaded mixtures of D2EHPA and EHO was effected by contact with 0.5 M mineral acid, in contrast to the solutions of 5 M hydrochloric acid and 8 M sulphuric acid required for the stripping of cobalt and nickel, respectively, from mixtures of D2EHPA and LIX 63 [7]. Further advantage may accrue from the replacement of the chelating oxime by a non-chelating oxime, owing to the now wellknown susceptibility of cobalt(II) complexes of the former towards oxidation to their cobalt(III) analogues, which strongly resist conventional stripping treatments [ 8]. In this paper we report the extraction characteristics of several mixtures of organophosphoric acids and non-chelating oximes that we have studied as potential reagents for the recovery of nickel and cobalt from acidic leach liquors. In view of the novel nature of these reagents, we have also investigated their behaviour towards some other metals of importance in hydrometallurgical processes. EXPERIMENTAL

Reagents The structures of the organophosphoric acids investigated are shown in Fig. 1. The D2EHPA (Ia) used was BDH laboratory reagent grade, and diisodecylphosphoric acid (Ib) was supplied by Daihachi Chemical Industry Co. 0% /OH

0% /OH

C4.H9~HCH2O/ P~"OCH2~HC~.H 9 C2H5 C2H5

C10H210/P~0E10H21

0R /OH P~ ~0/ OCH2~HC&H 9 ~ ~'-~

lie)

([b) t

C2Hs

(]c)

0% /OH

0% /OH

0R /OH

0% /OH

0/

0/

o/P~o

O/P\OCH2CF?

OCH2~HC~.H 9

OCH2~HC4H 9

N02

Ct

(Id)

C8H17

(Ie)

C8H17

{If) f

C8H17 (Ig} #

Fig, 1. Structures of organophosphoric acid extractants. (Ia) di(2-ethylhexyl)phosphoric acid; (Ib) diisodecylphosphoric acid; (Ic) 2-ethylhexyl phenylphosphoric acid; (Id) 2,4-dichlorophenyl 2-ethylhexylphosphoric acid; (Ie) 2-ethylhexyl 4-nitrophenylphosphoric acid; (If) di[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphoric acid; (Ig) 2,2,2-trifluoroethyl 4( 1,1,3, 3-tetramethylbu tyl)phenylphosph oric acid. $C,0H21 = 2,4,6-trimethylheptyl; * C8H~7 = 1,1,3,3-tetramethylbutyl.

189 under the designation DP-10R. Di[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphoric acid (If) was isolated from octylphenyl acid phosphate (Mobil Chemicals) by partition between hexane and ethylene glycol [9]. 2-Ethylhexyl phenylphosphoric acid (Ic), 2,4-dichlorophenyl 2-ethylhexylphosphoric acid (Id), 2-ethylhexyl 4-nitrophenylphosphoric acid (Ie), and 2,2,2-trifluoroethyl 4-(1,1,3,3-tetramethylbutyl)phenylphosphoric acid (Ig) were prepared by the following method, as exemplified for c o m p o u n d (Id): 2,4-dichlorophenol (163 g, 1.00 mol) was heated with potassium chloride (10 g, 0.13 mol) in refluxing phosphoryl chloride (240 ml, 2.62 mol) for 24 hours. The excess phosphoryl chloride was distilled out, toluene (150 ml) was added, and the mixture was cooled to 10°C while 2-ethylhexanol (122 g, 0.94 mol) in pyridine (85 ml, 1.05 mol) was introduced with stirring during 11/2 hours. The mixture was allowed to stand in a stoppered flask at 20°C for 40 hours and was then p o u r e d into water (400 ml). The organic phase was separated and washed with water (2 X 200 ml) to remove the pyridine hydrochloride. The crude acid chloride was then hydrolysed by stirring it with water (200 ml) and adding sodium carbonate until a pH value of 8 was reached. The aqueous layer was discarded and the lower phase, containing the sodium salt of the product required, was extracted with hexane (400 ml and 6 X 150 ml) in order to remove any non-acidic impurities. The solution of the sodium salt was acidified with hydrochloric acid (110 ml), and the free acid was extracted with hexane (100 and 50 ml), washed with water (2 X 400 ml), and dried over anhydrous sodium sulphate. Removal of the hexane by evaporation gave 2,4-dichlorophenyl 2-ethylhexylphosphoric acid as a colourless, viscous oil in 72% yield. Non-chelating oximes were prepared from commercially available carbonyl compounds by heating with hydroxylamine hydrochloride and sodium acetate in aqueous ethanol. The following products were obtained as colourless oils, unless stated otherwise: 2-ethylbutanal oxime, b.p. 89°C at 20 mmHg; 2-ethylhexanal oxime, b.p. 113°C at 20 mmHg; octanal oxime, recrystallised from methanol as thin prisms, m.p. 58°C; 2,2-dimethylpropanal oxime, recrystallised from hexane--diethyl ether as white, volatile crystals, m.p. 41°C; octan-2one oxime, b.p. 122°C at 20 mmHg; octan-3-one oxime, b.p. 118°C at 20 mmHg; 2,4-dimethylpentan-3-one oxime, recrystallised from hexane as white crystals, m.p. 36°C; benzaldehyde oxime, b.p. 127°C at 20 mmHg; and acetophenone oxime, recrystallised from aqueous ethanol as fine, white needles, m.p. 60°C.

Metal distribution equilibria Metal distribution equilibria were determined at 20°C using a method previously described [ 1]. Equilibria were reached rapidly (1 to 10 minutes) except in the case of iron(III) and aluminium(III), for which contact times of 24 hours were used.

190 Aqueous phases contained 0.10 M metal nitrates (except for iron(II), chromium(II), oxovanadium(IV), vanadium(II), and vanadium(III), which were used as sulphates, and tin(II), which was used as chloride) in 1.0 M (NH4,H)NO3 (except for chromium(II), oxovanadium(IV), vanadium(II), vanadium(III), and copper(I), for which 0.33 M (Na, H)2SO4 was used, and tin(II), for which 1.0 M (NH4,H}C1 was used). Metal solutions were prepared from the A.R. grade salts, except for chromium(II), for which the electrolytic-grade metal was dissolved in dilute sulphuric acid; vanadium(II), which was prepared by the electrolytic reduction of oxovanadium(IV) sulphate at a mercury cathode [ 10]; vanadium(III), which was prepared by the addition of an equimolar a m o u n t of oxovanadium(IV) sulphate to vanadium(II) sulphate; and copper(I), which was prepared by the dissolution of copper(II) sulphate and an equimolar amount of copper metal in 0.33 M (Na,H)2SO4 containing 5 vol. % acetonitrile. Experiments with easily oxidised species -- chromium(II), vanadium(II), vanadium(III), tin(II), iron(II), and copper(I) -- were done with deoxygenated solvents under a nitrogen atmosphere. Metal solutions were analysed by standard titrimetric methods [ 11,12], except for chromium(III), oxovanadium(IV), and dioxovanadium(V), which were analysed spectrophotometrically (the last-named species after reduction to oxovanadium(IV) with sulphur dioxide), using the absorption maxima at 575 (Cr 3+) and 765 nm (VO 2+ ). Organic phases contained 0.50 F organophosphoric acid, either alone or with 0.50 M non-chelating oxime, in xylene. (Concentrations of organophosphorie acids are here expressed in terms of formality units (F), defined as the number of gram formula weights of m o n o m e r per litre of solution.) RESULTS AND DISCUSSION Effect o f oxime structure on the extraction o f nickel and cobalt

The effects of various types of non-chelating oximes (0.50 M) on the extraction of nickel and cobalt by 0.50 F solutions of D2EHPA in xylene at 20°C are shown in Figs. 2 and 3, respectively. The pH values at 50% metal extraction (pH0.s) for the different mixtures are shown in Table 1. It can be seen that different degrees of enhancement of extraction are produced by oximes of different structures, and that, for both metals, the enhancements of extraction increase in the following orders: R--CNOH--C2H5 < R--CNOH--CH3 << R--CNOH--H, R2CH--CNOH--CHR2 < Ph--CNOH--CH3 < RCH2--CNOH--CH3, and R3C--CNOH--H ~ Ph--CNOH--H << R2CH--CNOH--H < RCH2--CNOH--H, where R represents an n-alkyl group and Ph a phenyl group. Since these orders closely represent the orders of decreasing steric hindrance about the oxime function, it would appear that this is the dominant factor in determining the

191

extent of the synergistic effect. In contrast, the electron-donating ability of the substituent hydrocarbon groups is approximately the reverse of the above orders and it appears therefore that this factor is of little importance to the synergistic effect. Molecular models show that in the mixed-ligand complex the substituent hydrocarbon groups on the oxime are brought into close proximity with the organophosphoric acid ligands and that severe steric hindrance occurs unless substituent R (Table 1) contains at least one a-hydrogen. 1,

2

~:<~5 6

?5

c o % c~

50

×

25

i

i

J

i

1

2

3

a

Equilibrium pH

Fig. 2. Extraction of nickel(II) by mixtures of D2EHPA and non-chelating oximes. Aqueous phases, 0.10 M Ni(NO 3 )2 in 1.0 M (NH,,H)NO 3. Organic phases, 0.50 F D2EHPA plus 0.50 M oxime in xylene. Oximes: (1) octanal oxime; (2) 2-ethylhexanal oxime; (3) 2,2-dimethylpropanal oxime; (4) octan-2-one oxime; (5) octan-3-one oxime; (6) 2,4-dimethylpentan-3-one oxime. The broken line shows the extraction of nickel by 0.50 F D2EHPA alone.

I

J2

3

75

4 5 /i"

>,l/

Z-

50

25

I

I

I

I

2

3 Equilibrium pH

Fig. 3. Extraction of eobalt(II) by mixtures of D2EHPA and non-chelating oximes. Aqueous phases, 0.10 M Co(NO 3 )2 in 1.0 M (NH 4,H)NO 3. Organic phases, 0.50 F D2EHPA plus 0.50 M oxime in xylene. Oximes: (1) octanal oxime; (2) 2-ethylhexanal oxime; (3) octan-2-one oxime; (4) octan-3-one oxime; (5) 2,4-dimethylpentan-3-one oxime. The broken line shows the extraction of cobalt by 0.50 F D2EHPA alone.

192 TABLE 1 Extraction of nickel and cobalt a by mixtures of D2EHPA (0.5 F) and non-chelating oximes R--CNOH--R' (0.5 M) in xylene at 20°C R

C7H15 C4HgCH(C~H5) C:HsCH(C:H s ) C6H 5 (CH3)3C C6H13 CsHll C6H5 (CH3)2CH

R'

H H H H H CH 3 C~H5 CH 3 (CH3)~CH

PH0.5

~ pH0. s b

Ni

Co

C o - Ni

Ni

Co

1.34 1.58 1.73 3.41 3.46 3.80 3.94 4.03 4.15

1.72 1.99 2.13 3.25 3.50 3.49 3.59 3.71 3.77

0.38 0.41 0.40 -0.16 0.04 -0.31 -0.35 -0.32 -0.38

2.77 1.96 2.53 1.69 2.38 1.55 0.70 0.43 0.65 0.18 0.31 0.19 0.17 0.09 0.08 -0.03 -0.04-0.09

a Initial aqueous phases contained 0.1 M Ni(NO 3)2 or 0.1 M Co(NO 3): in 1.0 M (NH,,H) NO~. bApH0.5 = PH0.s(D2EHPA) - PH0. s (D2EHPA plus oxime).

M o r e o v e r , unless s u b s t i t u e n t R' (Table 1) is also h y d r o g e n (i.e., t h e c o m p o u n d is an a l d o x i m e ) , t h e r e is c o n s i d e r a b l e steric i n t e r a c t i o n b e t w e e n R' and t h e oxi m e h y d r o x y l g r o u p w h e n t h e l a t t e r a d o p t s t h e c o n f o r m a t i o n w h i c h interacts least w i t h t h e o r g a n o p h o s p h o r i c acid ligands. I l l u s t r a t i o n o f t h e e f f e c t o f steric h i n d r a n c e o n t h e e x t e n t o f t h e synergistic e f f e c t is p r o v i d e d b y t h e c h a n g e s b r o u g h t a b o u t in t h e e l e c t r o n i c s p e c t r u m o f t h e c o b a l t - - D 2 E H P A c o m p l e x b y t h e a d d i t i o n o f n o n - c h e l a t i n g o x i m e s o f diff e r e n t s t r u c t u r e s . In t h e a b s e n c e o f o x i m e , t h e e x t r a c t e d c o b a l t c o m p l e x shows a s p e c t r u m c h a r a c t e r i s t i c o f t e t r a h e d r a l s y m m e t r y (Fig. 4, c u r v e 1). T h e addit i o n o f n o n - c h e l a t i n g o x i m e s causes a partial or, in s o m e cases, c o m p l e t e transf o r m a t i o n o f t h e blue, t e t r a h e d r a l species t o a p i n k , o c t a h e d r a l o n e (Fig. 4, curves 2 t o 6). T h e s p e c t r a clearly s h o w t h a t t h e e x t e n t o f t h e t r a n s f o r m a t i o n f r o m t h e t e t r a h e d r a l t o the o c t a h e d r a l c o m p l e x increases with decreasing steric h i n d r a n c e a b o u t t h e o x i m e f u n c t i o n . We h a v e r e p o r t e d t h e s t o i c h i o m e t r y o f t h e t e t r a h e d r a l c o m p l e x p r e v i o u s l y [ 1], a n d t h a t o f t h e o c t a h e d r a l c o m p l e x is discussed in a s u b s e q u e n t section o f this paper. I t is i n t e r e s t i n g t o n o t e t h a t b y far t h e largest synergistic e f f e c t s o c c u r w i t h aliphatic a l d o x i m e s o f t h e p r i m a r y - a n d s e c o n d a r y - c a r b o n chain t y p e s ( R C H 2 - - C N O H - - H a n d R 2 C H - - C N O H - - H , respectively). We e l e c t e d to investigate o n e c o m p o u n d o f t h e l a t t e r t y p e , C4H9CH(C2Hs ) - - C N O H - - H ( E H O ) , in g r e a t e r detail.

193 200

,

,

1

150 'T E i o E

100

!

cl o ~n ~n o

B o

50

I

400

I

I

600

800

Wavelength(nm) Fig. 4. Electronic spectra of the cobalt(II)--D2EHPA complex in the presence of nonchelating oximes. Cobalt concentration 0.01 M in 0.05 F D2EHPA plus 0.05 M oxime in xylene. Oximes: (1) none; (2) 2,4-dimethylpentan-3-one oxime; (3) octan-a-one oxime; (4) octan-2-one oxime; (5) 2-ethylhexanal oxime; (6) octanal oxime. TABLE 2 Extraction of nickel and cobalt a by mixtures of organophosphoric acids (0.5 F) and EHO (0.5 M) in xylene at 20°C Organophosphoric acid b

Ia Ib Ic Id Ie If Ig

PH0. s (acid)

pH0.s(acid + oxime)

ApH0.s c

Ni

Co

Ni-Co

Ni

Co

Co-Ni

Ni

Co

4.11 3.72 2.92 2.71 2.30 2.63 1.99

3.68 3.42 2.85 2.50 2.30 2.53 1.79

0.43 0.30 0.07 0.21 0.00 0.10 0.20

1.58 1.30 0.84 0.66 0.81 0.42 0.35

1.99 1.70 1.23 0.98 1.18 0.79 0.71

0.41 0.40 0.39 0.32 0.37 0.37 0.36

2.53 2.42 2.08 2.05 1.49 2.21 1.64

1.69 1.72 1.62 1.52 1.12 1.74 1.08

aInitial aqueous phases contained 0.1 M Ni(NO3 )2 or 0.1 M Co(NO 3)2 in 1.0 M (NH4,H)NO3. bStructural formulae of organophosphoric acids are shown in Fig. 1. c APH0.s = PHo. 5(acid) - PH0.s(acid plus oxime).

194

Effect of organophosphoric acid structure on the extraction of nickel and cobalt Table 2 shows the pHo.s values for the extraction of nickel and cobalt by several organophosphoric acids (0.50 F), and by their mixtures with EHO (0.50 M) in x y l e n e at 20°C. The data for extraction by the organophosphoric acids alone show that the i n t r o d u c t i o n of electron-withdrawing groups into the e x t r actan t structure (Fig. 1) causes a considerable decrease in the pH0.s values for b o th metals. For example, a decrease of 1.81 pH units is observed between the pH0.s values for the extraction of nickel by D2EHPA (Ia) and 2-ethylhexyl 4-nitrophenylphosphoric acid (Ie). Interestingly, the cobalt--nickel separation is also decreased, from 0.43 to 0.00 pH unit, respectively for the above-named compounds. This appears to be related to a change in the structure of the cobalt complexes. Thus, whereas the cobalt com pl ex of D2EHPA displays tetrahedral s y m m e t r y , the cobalt complexes of the arylphosphoric acids shown in Fig. 1 are of octahedral type, as evidenced by their electronic spectra (e.g., that of e x t r a c t a n t (Id), emax 10.5 1 mo1-1 cm - I at 525 rim, compare emax 168 1 mo1-1 cm -~ at 625 nm for the D2EHPA complex). The nickel complexes are octahedral in all cases (e.g., that of e x t r a c t a n t (Id), emax 2.9 at 660 and 6.9 1 mol -~ cm -1 at 392 n m , compare emax 2.5 at 725 and 6.1 1 tool -1 cm -1 at 395 nm for the D2EHPA complex). The addition of EHO to the organophosphoric acids causes marked enhancem e nt o f metal extraction and, in all cases, brings about a reversal of the selectivity for cobalt over nickel shown by the organophosphoric acids alone. With the ex cep tio n o f extractants (Ie) and (Ig), the synergistic shifts lie in the range 2.05 to 2.53 pH units for nickel, and 1.52 to 1.74 pH units for cobalt. The smaller shifts observed for c o m p o u n d s (Ie) and (Ig) may be attributable to the presence in these extractants of highly polar groups, (NO2 and CF3, respectively), which can act as hydr oge n- bond acceptors for the h y d r o x y l p r o t o n of the oxime, t h e r e b y reducing the effective c o n c e n t r a t i o n of the latter. Table 2 shows t ha t the use of mixtures of organophosphoric acids and nonchelating oximes permits the extraction of nickel and cobalt under remarkably acidic conditions, although in no case was a useful separation of the two metals observed. Nevertheless, practical application of the mixed reagents may be attractive in systems where prior or subsequent separation of cobalt and nickel can be effected, and we shall r epor t in detail on one such example elsewhere [ 13]. In contrast, our preliminary observations on the use of mixtures of carboxylic acids and non-chelating oximes reveal that much improved selectivities for nickel over cobalt can be obtained with these reagents.

Stoichiometries o f extracted complexes The extraction of a divalent metal, M 2+ , by a dimerized organophosphoric acid, H2A2, in the presence of a non-chelating oxime, B, can be represented by the general equation

195 M 2+ + nH2A2 + m B = MA2(HA)2n-2Bm + 2H +

(1)

in which ionic species are p r e s e n t in the a q u e o u s phase and u n c h a r g e d species are p r e s e n t in the organic phase. H e n c e the f o l l o w i n g expression can be derived: l o g D = log Kex + n log [H2A2] + m log [B] + 2 p H ,

(2)

where Kex is the e q u i l i b r i u m c o n s t a n t f o r eqn. (1) and D represents the distrib u t i o n c o e f f i c i e n t o f the m e t a l b e t w e e n the organic and the a q u e o u s phase. I n s p e c t i o n o f eqn. (2) shows t h a t plots o f log D - m log [B] against p H at constant [H2A2 ] s h o u l d consist o f straight lines with slope 2.0, and t h a t plots of log D - rn log [B] against log [H2A2 ] at c o n s t a n t p H should consist o f straight lines with slope n. Similarly, plots o f log D - n log [H2A2 ] against p H at constant [B] and plots o f log D - n log [H2A2 ] against log [B] at c o n s t a n t p H should consist o f straight lines with slopes 2.0 and m, respectively. Such plots for the e x t r a c t i o n o f c o p p e r ( I I ) f r o m 0.20 M a m m o n i u m n i t r a t e solutions by m i x t u r e s o f D 2 E H P A ( 0 . 0 1 - - 0 . 0 8 F ) and E H O ( 0 . 0 0 5 - - 0 . 0 4 M) in x y l e n e at 20°C gave g o o d straight lines in all cases. The slopes o b t a i n e d indicate t h a t the metal d i s t r i b u t i o n shows s e c o n d o r d e r d e p e n d e n c e s o n pH (slopes 1.9), on D 2 E H P A c o n c e n t r a t i o n (Fig. 5, curve 1; slope n = 1.8), and on E H O concentration (Fig. 6, curve 1; slope m = 1.7). H e n c e the e x t r a c t e d c o m p l e x can be

1

i Lu

E

o

I

1

k

1

[

2 [0g [ HzA 2] +

Fig. 5. Ef;ect of D2EHPA concentration on the extraction of copper(II), eobalt(II) and nickel(II) by mixtures of D2EHPA and EHO in xylene. Organic phases, 0.040 M EHO (nickel, cobalt)* or 0.020 M EHO (copper) plus D2EHPA in xylene. Aqueous phases, 0.001 M metal nitrate in 0.20 M NH4NO 3. (i) copper at pH 3.0; (2) cobalt at pH 4.0; (3) nickel at pH 3.0.

196

I

I

o

~ o

0/ I [0g[EHO]+

3

Fig. 6. Effect of EHO concentration on the extraction of copper(II), cobalt(II), and nickel(II) by mixtures of D2EHPA and EHO in xylene. Organic phase, 0.020 F D2EHPA (copper, cobalt) or 0.080 F D2EHPA (nickel) plus EHO in xylene. Aqueous phases, 0.001 M metal nitrate in 0.20 M NH4NO ~. (1) copper at pH 3.5; (2) cobalt at pH 4.0; (3) nickel at pH 3.2. f o r m u l a t e d as CuA2 (HA)2B~ or as t h e e q u i v a l e n t f o r m , in w h i c h t h e D 2 E H P A ligands are r e p r e s e n t e d as b i d e n t a t e dimers, Cu(HA2 )2B2. A n a l o g o u s results were o b t a i n e d f o r t h e e x t r a c t i o n o f c o b a l t ( I I ) f r o m 0.20 M a m m o n i u m n i t r a t e s o l u t i o n s (Fig. 5, c u r v e 2, slope n = 1.8, a n d Fig. 6, c u r v e 2; slope m = 1.8. Also. p l o t s o f l o g D - m log [B] against p H at c o n s t a n t [H2A2] a n d of l o g D - n log [H2A2 ] against p H at c o n s t a n t [B] gave slopes of 1.8 to 2.0). For the extraction of nickel(II) from 0.20 M ammonium nitrate solutions b y m i x t u r e s o f D 2 E H P A ( 0 . 0 2 - - 0 . 3 2 F ) a n d E H O ( 0 . 0 1 - - 0 . 0 8 M) in x y l e n e , a s e c o n d o r d e r d e p e n d e n c e o f m e t a l d i s t r i b u t i o n u p o n p H was again f o u n d (slopes 1.8 to 1.9). In c o n t r a s t , the d e p e n d e n c e o f m e t a l d i s t r i b u t i o n on t h e c o n c e n t r a t i o n s o f D 2 E H P A a n d E H O were f o u n d t o be s o m e w h a t less t h a n s e c o n d o r d e r (Fig. 5, c u r v e 3; slope n = 1.5) and s o m e w h a t greater t h a n s e c o n d o r d e r (Fig. 6, c u r v e 3; slope m = 2.4), respectively. We f o u n d n o dep e n d e n c e o f m e t a l d i s t r i b u t i o n on a q u e o u s nickel c o n c e n t r a t i o n , h o w e v e r , w h i c h indicates [8] t h a t t h e l o w - o r d e r d e p e n d e n c e on D 2 E H P A c o n c e n t r a t i o n d o e s n o t result f r o m t h e p r e s e n c e in the organic p h a s e o f d i m e r i c c o m p l e x e s such as, f o r e x a m p l e , B

B

B

B

197

which, in any event, should result in values of n = 1.5 and m = 2.0 in eqn. (2). It appears, therefore, that for the nickel complex, extensive displacement of one of the neutral D2EHPA oxygen-donor ligands by the stronger nitrogendonor oxime ligand occurs: B

B

H~ A ' x ' ~ Ni I ~A~\ ,

~

/

A/f~A/.H

+

B ~

A " ~ N~i/

H \\A/

A

t ~...B

4-

HA

.

6

That such a displacement occurs for nickel but not for copper or cobalt may result from the greater ligand-field stabilisation energy of the d s ion, which favours the stronger-field ligand. The low experimental value (m = 2.4) compared with the expected m = 3 for the above complex is probably largely attributable to self-association of the oxime in the organic phase, which has been shown to occur to a significant extent for compounds of this type, even at low concentrations [ 14]. Spectrophotometric measurements gave confirmatory evidence on the stoichiometries of the extracted complexes of nickel and cobalt. Figure 7 shows plots of the molar absorptivity of solutions of the D2EHPA complexes of nickel and cobalt in xylene, to which various amounts of EHO were added. For nickel, the measurements were made at the absorbance maximum of the mixed-ligand complex (620 nm), whereas for cobalt the disappearance of the intensely coloured D2EHPA complex was monitored at its absorbance maxT NI

.__.----o100

•T

I I/-

o E

i/I

> o c~

50

C0

2

4

6

8

f'~-

10

12

0

Oxlme to metal rofio

Fig. 7. Spectrophotometric titrations of nickel(II) and cobalt(II) complexes of D2EHPA against EHO in xylene. Nickel at 620 nm, 0.05 M metal, 0.50 F D2EHPA; cobalt at 625 rim, 0.02 M metal, 0.20 F D2EHPA.

198 TABLE 3 Extraction of nickel and cobalt a by mixtures of D2EHPA and EHO in xylene at 20°C [D2EHPA] (M)

0.25 0.05 0.05

[EHO] (M) PH0.s

0.05 0.25 2.00

Co

Ni

Co - Ni

3.10 3.01 2.30

2.81 2.42 1.57

0.29 0.59 0.73

aInitial aqueous phases contained 0.01 M Ni(NO 3)2 or 0.01 M Co(NO 3)2 in 1.0 M (NH4 ,H)NO3. i m u m (625 n m ) as increasing a m o u n t s o f E H O were added. T h e intersections of the rectilinear p o r t i o n s o f these plots, which r e p r e s e n t the idealised f o r m s o f the curves o b t a i n a b l e in the case o f v e r y high f o r m a t i o n c o n s t a n t s o f the mixed-ligand c o m p l e x e s , clearly indicate the f o r m a t i o n o f c o m p l e x e s with E H O - t o - m e t a l ratios o f 3 : 1 for nickel and 2 : 1 f o r cobalt. The l o w - o r d e r d e p e n d e n c e o f nickel d i s t r i b u t i o n on D 2 E H P A c o n c e n t r a tion (n = 1.5) results in a relatively small e f f e c t o f D 2 E H P A c o n c e n t r a t i o n u p o n the pH0.s o f nickel e x t r a c t i o n , whereas the higher-order d e p e n d e n c e on EHO c o n c e n t r a t i o n (m = 2.4) results in a c o r r e s p o n d i n g l y larger e f f e c t o f E H O c o n c e n t r a t i o n u p o n the pH0.s value. These effects can be utilised to optimise the s e p a r a t i o n o f nickel f r o m c o b a l t o b t a i n a b l e with m i x t u r e s o f D 2 E H P A and EHO, since the d i s t r i b u t i o n o f the latter metal displays equal d e p e n d e n c e s u p o n the c o n c e n t r a t i o n s o f b o t h e x t r a c t a n t s (n = m = 1.8). Thus, for e x a m p l e , whereas the selectivity for nickel over c o b a l t o f a solution c o n t a i n i n g 0.25 F D 2 E H P A and 0.05 M E H O in x y l e n e is o n l y 0.29 p H unit, the selectivity is increased to 0.59 p H u n i t for a s o l u t i o n c o n t a i n i n g 0.05 F D 2 E H P A and 0.25 M EHO, and t o 0.73 p H u n i t for a s o l u t i o n c o n t a i n i n g 0.05 F D 2 E H P A and 2.00 M EHO. These results are detailed in Table 3.

Effect of EHO on the extraction of base metals by D 2 E H P A Table 4 shows the pH0.s values for the e x t r a c t i o n o f several transition and n o n - t r a n s i t i o n metals b y 0.50 F D 2 E H P A and b y m i x t u r e s o f 0.50 F D 2 E H P A and 0.50 M E H O in x y l e n e at 20°C. The e x t r a c t i o n curves for some metals o f i m p o r t a n c e in h y d r o m e t a l l u r g i c a l processes are s h o w n in Figs. 8--10. It can be seen f r o m Table 4 that, for the divalent transition metals, the a d d i t i o n o f E H O causes m a r k e d e n h a n c e m e n t o f the e x t r a c t i o n b y D 2 E H P A , and t h a t the ext e n t o f the synergistic e f f e c t lies in the o r d e r (Zn}< Cr
Fe< Co
Ni.

It is interesting t h a t the above order, with the e x c e p t i o n o f Cr 2+, also repre-

199 TABLE 4 Metal extraction by D2EHPA (0.5 F) and by mixtures of D2EHPA (0.5 F) and EHO (0.5 M) in xylene at 20°C Metal ion a

pH0.s (D2EHPA)

pH0.s(D2EHPA + EHO)

ApH0.s b

Cu 2+ Ni 2+ Co :+ Fe ~+ Mn 2+ Cr 2+ V 2+ VO :+

2.90 4.12 3.70 3.56 2.82 2.15 4.15 1.25

1.05 1.60 2.00 2.10 2.02 1.47 2.25 0.96

1.85 2.52 1.70 1.46 0.80 0.68 1.90 0.29

Mg 2+ Ca 2+ Zn :+ Cd :+ Sn 2+ Pb 2+

3.81 2.85 1.42 2.98 -0.15 2.50

3.69 2.97 1.58 1.48 -0.12 2.33

0.12 - 0 .1 2 - 0 .1 6 1.50 - 0 .0 3 0.17

Fe 3+ Cr 3+ V 3+ A13+ Bi 3+ La 3+ Ce 3+ Nd 3+

-0.32 3.12 1.90 1.53 0.29 1.96 1.73 1.46

-0.40 2.98 1.67 1.51 0.37 2.07 1.85 1.61

0.08 0.14 0.23 0.02 -0.08 -0.11 - 0 .1 2 -0.15

<0.00 <0.00 c -0.10 4.80 d

> 3.25 >4.07 1.27 0.04

Ag + Cu + VO~ NH +

3.25 4.07 c 1.17 4.84 d

a Aqueous phase compositions are given in the experimental section. DApH0. 5 = pH0.5(D2EHPA) - PH0.s(D2EHPA + EHO). c Aqueous phases contained 5 vol.% CH3CN. dpH at which the D2EHPA content of the organic phase is half-neutralised when aqueous phase contains 1.1 M NH4NO 3. sents the order of increasing ligand-field stabilisation of octahedral complexes over the c o r r e s p o n d i n g t e t r a h e d r a l c o m p l e x e s [ 15]. In c o n t r a s t , the o r d e r of e x t r a c t i o n in t h e a b s e n c e o f E H O , Ni~

V<

Co<

Fe<

Mn~

Cu<

Cr<

(Zn).

is, w i t h t h e e x c e p t i o n o f c o p p e r , t h e e x a c t r e v e r s e o f t h e o r d e r o f i n c r e a s i n g synergistic effect, and appears to reflect the increasing ability of the metal ions t o a d o p t t h e t e t r a h e d r a l ( o r p o s s i b l y , in t h e c a s e o f Cr 2+ a n d C u ~+, t e t r a g o n a l )

200

configuration favoured by the rather bulky D2EHPA dimer ligands. The synergistic effect of EHO on the extraction of the divalent transition metals by D2EHPA thus clearly reveals its role in the f o rm at i on of the mixed-ligand octahedral complexes. The changes brought a b o u t in the electronic spectra of the extracted nickel and cobalt complexes by the addition of EHO have been illustrated previously [ 1 ]. Marked differences are also apparent between the spectrum of the c o p p e r ( I I ) - - D 2 E H P A c om pl ex (e 29 1 m o l - ' cm -1 at 815 nm) and that of the corresponding mixed-ligand complex (e 38.5 1 mo1-1 cm -~ at 665 nm); the strong displacement of the absorbance m a x i m u m towards higher energies in the presence o f EHO confirms the incorporation of the nitrogen-donor ligands into the c o o r d i n a t i o n sphere of the metal ion. The spectrum of the oxovanadium(IV)--D2EHPA com pl e x comprises two partially resolved bands at 795 (e 21 1 mol - I cm - ~ ) and ~ 690 nm (e 18.5 1 m o l - ' cm - ' ) , which are displaced to 787 (e 20.5 1 mol - ' cm - ~ ) and ~ 675 nm (e 15 1 mol - ] cm - ~ ) respectively, in the presence of EHO. These relatively small spectral changes, as well as the small synergistic effect observed for the VO 2+ ion, probably result f r o m the presence of the 0 2 - ligand in the extracted complex, which leaves only one vacant {or H20- occupied) site available for incorporation of the oxime into the c oor di nat i on sphere of the metal. Among the remaining divalent transition metals, either the absorptions of the complexes in the visible region are t o o weak (Mn 2+ and Fe 2+) or the organic solutions are too susceptible towards oxidation (Cr 2+ and V 2+) for useful spectra to be obtained. The data in Table 4 show that no significant synergistic effects were f o u n d for any o f the trivalent ions studied. This is to be expected, since the requirem e n t for a neutral extracted species dictates the incorporation of three monovalent D2EHPA dimer ligands into the complex, with resultant coordinatively i

i

Cu

75

///

25

i

Co

Cu

/ ; Y f

,

C0

o/

~'°'o

o.o.

o/

/ot

/o" Ni

(/'///,o"'f /

50

Ni

/

?'

/

.d

io ~

,( I 1

/ ,/

/

so/ .o /

I 2

i 3

/

' io JD i 4

Equilibrium pH

Fig. 8. Extraction of copper(II), cobalt(II) and nickel(II) by D2EHPA and by D2EHPA plus EHO in xylene. Organic phases, 0.50 F D2EHPA plus 0.50 M EHO (full lines), and 0.50 F D2EHPA (broken lines). Aqueous phases, 0.10 M metal nitrate in 1.0 M (NH4, H ) NO 3 .

201

Ag

Cd

Pb

zC//

Cd

/

Ag

.0 /

0~

?s o:-'-

Cu

d /° /

50

o //

//

~ "

/

°/ /

o /

/ /"

//

/I 0

/P o/,o,2 /2 ? /o/ J'" .." o'" /%_-f

/

/

-70

10

I

I

I

I

1

2

3

4

Equilibrium

pH

Fig. 9. Extraction of silver(I), copper(I), cadmium(II) and lead(II) by D2EHPA and by D2EHPA plus EHO in xylene. Organic phases, 0.50 F D2EHPA plus 0.50 M EHO (full lines), and 0.50 F D2EHPA (broken lines). Aqueous phases, 0.10 M metal nitrate in 1.0 M (NH, ,H)NO 3 (for copper(I), 0.10 M Cu~SO 4 in 0.33 M (Na,H)~SO 4 plus 5 vol. % CH3CN).

saturated octahedral configurations. Although the trivalent lanthanide ions are commonly eight- or nine-coordinate [ 15,16]., octahedral coordination appears to be favoured in the case of the D2EHPA complexes [ 9,17], presumably because of the steric demands of the bulky ligands. The absence of any synergistic effect with mixtures of D2EHPA and EHO shows that the coordination sphere is not expanded in the presence of the oxime, and confirms the indifference of the lanthanides towards nitrogen-donor ligands [15]. The slight depression of extraction observed for the lanthanide ions, as well as for bismuth(III), in the presence of EHO can be rationalised in terms of associative interactions i

i

Fe'"

i

Cr++ f

i

(r+++ o"

0/

/0 ~

75

o

50 c~ × uo

/?/ o / 25

0

/ /

p-"

F~.U

o

Cr++

i

I

1

2

,of £ / / " " "..o

Equilibrium

I

i

3

z~

pH

Fig. 10. Extraction of chromium(II), chromium(III), iron(II) and iron(III) by D2EHPA and D2EHPA plus EHO in xylene. Organic phases, 0.50 F D2EHPA plus 0.50 M EHO (full lines), and 0.50 F D2EHPA (broken lines). Aqueous phases, 0.10 M metal sulphate (divalent ions) or nitrate (trivalent ions) in 1.0 M (NH 4,H)NO 3 (for chromium(II), 0.33 M (Na,H)~SO4).

202 between D2EHPA and the oxime additive, as discussed previously [ 1]. The small enhancements of extraction found for trivalent aluminium, iron, chromium, and vanadium probably reflect the fact that these small, readily hydrolysed ions may be extracted, in part at least, as hydroxyl-containing species such as FeOH(HAz )2(H20) [18,19], for which some enhancement of extraction may result by replacement of the coordinated water molecule by the oxime ligand. The low degree of interaction of EHO with the D2EHPA complexes of trivalent ions is also evident from the electronic spectra of those species that show absorbance maxima in the visible region. Thus, the spectra of the blueviolet chromium(III) extract (e 18.5 at 410 and 16.5 1 mo1-1 cm -1 at 575 nm) and the lilac neodymium(III) extract (which shows several complex bands) remain substantially unchanged in the presence of EHO. The green vanadium(III) complex (e 14.5 at 440 and 9.0 1 mol -~ cm -1 at 700 rim) becomes very sensitive towards oxidation to the oxovanadium(IV) complex in the presence of EHO. Among the divalent non-transition metals studied, only cadmium displays a strong synergistic effect with mixtures of D2EHPA and EHO. The contrast with zinc, the extraction of which is depressed by addition of EHO, reveals the preference of the smaller ion (Zn 2+, ionic radius 0.069 rim) for tetrahedral coordination, and that of the larger ion (Cd 2+, ionic radius 0.092 nm) for octahedral coordination. The absence of significant synergistic effects among the divalent non-transition metals (e.g., calcium and magnesium) can be ascribed to the preference of these cations towards oxygen-donor ligands [ 15,16]. The large synergistic effects found for silver(I) and copper(I) are noteworthy. These monovalent ions are unusual, however, in that they can be extracted to some extent by solutions of the non-chelating oxime alone. The extraction of silver by EHO is largely independent of pH, but strongly dependent upon the aqueous nitrate concentration. Thus, for example, we found 51--54% extraction from 0.10 M silver nitrate in 1.0 M (NH4,H)NO3 by 0.50 M EHO in xylene, furthermore, the extracted metal could be readily stripped from the organic phase by contacting with water. For copper(I), we found 17% extraction from 0.33 M H2SO4 by 0.50 M EHO in xylene; the extracted complex could be readily oxidised to an insoluble, green copper(II) compound. The synergistic effects found for silver(I) and copper(I) are readily understood, since reaction of these monovalent cations with the single HA~ ligand required for charge neutrality results in species which are coordinatively unsaturated with respect to their preferred tetrahedral configurations. The synergistic effects produced by EHO are considerably larger than those reported for mixtures of phenolic compounds and D2EHPA, both for copper(I) [20] and for silver(I) [21].

Practical applications Although a detailed consideration of possible practical metal separations

203

does n o t fall within the scope o f this paper, we nevertheless draw a t t e n t i o n to some n o t a b l e selectivity reversals a p p a r e n t f r o m Table 4. In particular, the selectivity o f D 2 E H P A alone for calcium and m a g n e s i u m over nickel (1.27 and 0.31 p H units, respectively) c o n t r a s t s m a r k e d l y with t h a t o f m i x t u r e s of D 2 E H P A and E H O for nickel over calcium and m a g n e s i u m (1.37 and 2.09 p H units, respectively). Similar observations apply to the selectivities of m i x t u r e s o f D 2 E H P A and E H O for cobalt, c o p p e r and manganese over calcium and magnesium. Table 4 also shows the p H values at which 50% neutralisation o f D 2 E H P A b y a m m o n i a occurs, b o t h in the absence and in the presence of EHO. It can be seen that, w h e n D 2 E H P A is used t o recover metals such as c o b a l t and nickel f r o m solutions c o n t a i n i n g a m m o n i u m salts, the relatively high pH values r e q u i r e d for efficient metal e x t r a c t i o n result in considerable neutralisat i o n o f the e x t r a c t a n t , and c o n s e q u e n t extensive c o - e x t r a c t i o n of a m m o n i a . This p r o b l e m is m u c h r e d u c e d w h e n m i x t u r e s o f D 2 E H P A and EHO are used, since efficient metal e x t r a c t i o n can be e f f e c t e d in a p H range in which the ext e n t o f neutralisation o f D 2 E H P A is negligible. We have m a d e a detailed evaluation of the use o f m i x t u r e s of D 2 E H P A and E H O for the e x t r a c t i o n of nickel f r o m acidic a m m o n i u m sulphate solutions, and the results will be r e p o r t e d elsewhere [ 1 3 ] . A m o n g the n o n - t r a n s i t i o n metals, interesting selectivity reversals result f r o m the selectivity s h o w n by m i x t u r e s of D 2 E H P A and E H O for c a d m i u m over lead (0.85 p H unit), for silver over lead (> 2.33 pH units), for c o p p e r ( I ) over zinc (> 1.58 p H units), and for c a d m i u m over zinc (0.10 pH unit). These selectivities c o n t r a s t with t h o s e o f D 2 E H P A alone for lead over c a d m i u m (0.48 p H unit), for lead over silver (0.75 p H unit), for zinc over c o p p e r ( I ) (2.65 pH units), and for zinc over c a d m i u m (1.56 p H units), respectively. ACKNOWLEDGEMENT This p a p e r is p u b l i s h e d b y permission of the Council for Mineral Technology.

REFERENCES 1 Preston, J.S., Hydrometallurgy, 9 (1982) 115. 2 Joe, E.G., Ritcey, G.M. and Ashbrook, A.W., J. Metals (AIME), 18 (1966) 18. 3 Ritcey, G.M., in: D. Dyrssen, J.O. Liljenzin and J. Rydberg (Eds.), Solvent Extraction Chemistry, North-Holland Publishing Co., Amsterdam, 1967, p. 648. 4 Ashbrook, A.W., Ritcey, G.M. and Joe, E.G., U.S. Patent 3,455,680 (1969). 5 Metallurgie Hoboken-Overpelt, U.S. Patent 4,088,733 (1978). 6 De Schepper, A. and Van Peteghem, A., Canadian Patent 1,067,704 (1979). 7 De Schepper, A., Coussement, M. and Van Peteghem, A., International Conference on Advances in Chemical Metallurgy, Bombay, 1979, paper 8. 8 Preston, J.S., J. Inorg. Nucl. Chem., 37 (1975) 1235. 9 Peppard, D.F., Mason, G.W., Maier, J.L. and Driscoll, W.J., J. Inorg. Nucl. Chem., 4 (1957) 334.

204 10 Kranz, M., Inorg. Synth., 7 (1963) 94. 11 Vogel, A.I., A Textbook of Quantitative Inorganic Analysis, 3rd edn., Longmans, London, 1961. 12 West, T.S., Complexometry with EDTA and Related Reagents, 3rd edn., BDH Chemicals Ltd., Poole, 1969. 13 Preston, J.S. and Fleming, C.A., Paper to be presented at the 3rd International Symposium on Hydrometallurgy, 112th AIME Annual Meeting, Atlanta, March 1983. 14 Preston, J.S., J. Inorg. Nucl. Chem., 42 (1980) 441. 15 Cotton, F.A. and Wilkinson, G., Advanced Inorganic Chemistry, 4th edn., WileyInterscience, New York, 1980. 16 Phillips, C.S.G. and Williams, R.J.P., Inorganic Chemistry, Oxford University Press, Oxford, 1966. 17 Kolarik, Z. and Pankova, H., J. Inorg. Nucl. Chem., 28 (1966) 2325. 18 Roddy, J.W., Coleman, C.F. and Arai, S., J. Inorg. Nucl. Chem., 33 (1971) 1090. 19 Islam, M.F. and Biswas, R.K., Can. J. Chem., 57 (1979) 3011. 20 Muir, D.M., Benari, M.D. and Preston, J.S., Proc. Int. Solvent Extr. Conf. ISEC '80, Liege, 1980, paper 80-77. 21 Bray, L.A. and Moore, R.L., U.S. Patent 3,414,403 (1968).