Solvent extraction characteristics of thiosubstituted organophosphinic acid extractants

lIydrometa/lurgy, 30 ( 1992) 345-365

345

Elsevier Science Publishers B. Y., Amsterdam

Solvent extraction characteristics of thiosubstituted organophosphinic acid extractants Kathryn C. Sole and J. Brent Hiskey Copper Research Center. University ofArizona 4717 East Fort Lowell Road. Tucson. AZ 85712. USA

(Revised version accepted, December 23, 1991 )

ABSTRACT Sole, K.e. and Hiskey, J.B., 1992. Solvent extraction characteristics of thiosubstituted organophosphinic acid extractants. In: W.e. Cooper and D.B. Dreisinger (Editors), Hydrometallurgy Theory and Practice. Proceedings ofthe Ernest Peters International Symposium. Hydrometallurgy. 30: 345365. Organophosphorus reagents are well known in solvent extraction. Commercial operations for the separation of cobalt from nickel have been successfully carried out using organophosphoric, -phosphonic, and -phosphinic acid extractants. Two new reagents in this class are the mono and dithio analogs of the commercial dialkylphosphinic acid, Cyanex 272. The replacement of oxygen by sulfur in these reagents enables extraction to be carried out at much lower pH. Characterization of Cyanex 272, Cyanex 302 (bis-(2,4,4-trimethylpentyl )-thiophosphinic acid), and Cyanex 301 (bis-(2,4,4-trimethylpentyl)-dithiophosphinic acid) has been undertaken. A comparison of the solvent extraction behavior of metallurgically important first-row transition metal ions from acidic sulfate solution by these reagents is reported. Distribution coefficients shift to lower pH with increasing sulfur substitution and decreasing pK. of the extractant, the greatest effect being observed for nickel. Stoichiometry of the extraction reactions, and the nature of the metal complexes formed have been determined using slope analysis techniques and spectroscopic measurements.

INTRODUCTION

Organophosphorus solvent extraction reagents have been widely studied in the past two decades, particularly with respect to cobalt-nickel separation in weakly acidic sulfate media. Earlier work concentrated extensively on the dialkylphosphoric acid, D2EHPA (di- (2-ethylhexyl )-phosphoric acid), resulting in a number of patents, and the commercial implementation of several processes employing this extractant [1,2]. The subsequent development of phosphonic and phosphinic acid extractants, in particular 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (marketed as PC-88A by Daihachi Correspondence to: Kathryn C. Sole, Copper Research Center, University of Arizona, 4717 East Fort Lowell Road, Tucson, AZ 85712, USA.

0304-386X/92/S05.00 © 1992 Elsevier Science Publishers B.Y. All rights reserved.

346

K.C. SOLE AND J.B. HISKEY

Chemical Co. and SME 418 by Shell Chemicals) [2-4], and bis- (2,4,4-trimethylpentyl )-phosphinic acid (commercially available as Cyan ex 272 from American Cyanamid Co.) [5], led to dramatic improvements in cobalt-nickel separation factors in the order: phosphoric < phosphonic < phosphinic acid. Excellent comparisons of the extraction characteristics of these reagents for cobalt and nickel have been documented by Preston [6,7], Danesi et al. [8], and Yuan et al. [9]. According to the Hard-Soft Acid-Base (HSAB) concept [10], the complexation of a soft Lewis acid, such as Ni (II), Cu (II), Co (II) or Zn (II), with a soft Lewis base should occur with high selectivity. The donor atoms of the most common Lewis bases have electronegativities increasing in the order S < Br < N < CI < 0 < F. Sulfur substitution of organophosphorus reagents should therefore prove beneficial to the extraction of these metal ions. The solvent extraction of several soft acids by various thiosubstituted organophosphorus reagents has been investigated for both analytical and hydrometallurgical applications. Handley and Dean investigated the extraction of a number of metals from H 2S04 and HCI by trialkylthiophosphates [ 11 ], a dialkylthiophosphoric acid [12], and dialkyldithiophosphoric acids [13]. Uranium and rhenium extraction by dialkyldithiophosphoric acids [14,15], and the extraction of Hg (II) [ 16] have also been reported. In the late 1970s, a new extractant, D2EHDTPA (di-(2-ethylhexyl)-dithiophosphoric acid), became available from Daihachi Chemical Co. Replacement of the oxygens of the phosphoric acid by sulfur increased the acidity of the extractant significantly, and selected base metal ions were able to be extracted quantitatively from concentrated (up to 20 N) sulfuric acid [17]. The kinetics of extraction were, however, rather slow, thereby limiting the usefulness of this extractant [ 18-20]. Because of the similarities in the acid properties of the sulfur and oxygen acids of phosphorus, the dialkylthiophosphinic acids exhibit structural tautomerism, as shown schematically below. Equilibrium does not favor either isomer [ 12 ] .

S II R 2 P-OH thiono

0 ~

" R 2 P-SH thiol

Baldwin and Higgins [21 ] carried out infrared studies on di-n-butylthiophosphoric acid. In carbon tetrachloride solvent, 90% of the acid was found to exist in the thiono form. On complexation with silver, however, the thiol form predominated, thereby indicating an equilibrium shift. Although solvent extraction by thiosubstituted organophosphoric acids has been fairly widely studied, less work has been carried out on the phosphinicacid reagents. The extraction of platinum-group metals by diphenyldithio-

347

CHARACTERISTICS OF THIOSUBSTITUTED ORGANOPHOSPHINIC ACID EXTRACTANTS

phosphinic acid has been reported by Kabanova and Usova [22]. The equilibria and kinetics of the extraction of cadmium from phosphoric acid by dialkyldithiophosphinic acids have been studied in some detail [23,24]. A range of solid organometallic complexes of different dialkyldithiophosphinic acids has also been prepared and characterized by Kuchen and co-workers [2530]. More recently, the extractants Cyanex 302 and Cyan ex 301 have become available from American Cyanamid Co. [31]. These extractants are the respective monothio and dithio analogs of Cyan ex 272. The chemical structures of the main components in these reagents may be represented as shown below: o

R

S

R

/

~

/

/

/ R

OH

R

OH

Cyanex 272

Cyanex 302

pR. 6.37

pK. 5.63 CH 3

R -

CH 3

-

I C I

/ p

p

p

where

S

R

R

SH

Cyanex

301

pK. 2.61

CH 3 CH z

-

I

CH -

CH z

-

CH 3

To date, a number of fundamental studies of cobalt and nickel extraction by Cyan ex 272 have been published, and there is substantial agreement on the nature of the extracted metal species with respect to coordination stoichiometry and geometry under various operating conditions [6,32-34]. At low metal loadings, cobalt exists as Co(HA2h, and nickel as Ni(HA 2h(HAh, where (HA h represents the extractant dimer [6,33 ]. These species have also been equivalently represented as CoA 2(HAh and NiA22(HAh, where dimerization of the extractant in the metal complex is not explicitly shown. The extraction reactions are: C02+ +2(HAh ~Co(HA2h +2 H+

(1)

Nj2+ +3(HAh~Ni(HA2h(HAh +2 H+

(2)

where overlining indicates species in the organic phase. Although Cyanex 272

348

K.C. SOLE AND J.B. HIS KEY

is marketed as a cobalt-specific extractant [31 ], it may also be used to extract other transition and lanthanide metals under appropriate conditions [35-37]. Sastre et al. [37] investigated the extraction of cadmium, copper and zinc from nitrate solution by Cyanex 272. The general extraction reaction was given as: (3) The extracted copper complex was found to be CuA2(HA)2. For cadmium and zinc, the formations of both CdA2(HAh and CdA2(HAh, and ZnA 2 (HA) and ZnA 2 (HAh were postulated, the predominant species depending on the pH and on the extractant concentration. These authors gave no indication of the expected coordination geometry for their proposed complexes. Cyan ex 301 was originally developed for the selective extraction of zinc from effluent streams containing calcium, such as those generated in the manufacture of rayon by the viscose process [38,39]. Besides the technical brochures [31 ], little information concerning the extraction behavior of Cyan ex 302 and Cyan ex 301 is yet available. This paper compares and contrasts the extraction characteristics of Cyanex 272, Cyan ex 302, and Cyan ex 301, with particular reference to first-row transition metals commonly encountered in extractive metallurgy. EXPERIMENTAL PROCEDURES

Reagents The extractants Cyan ex 272, Cyan ex 302, and Cyanex 301 were supplied by Cyanamid Canada, Inc., and were used without further purification. All other chemicals were of reagent-grade quality. Aqueous solutions were made up using distilled, deionized water of 18 MQ'cm purity. The diluents used were xylene (ACS grade, mixture of 0- and p-xylene) supplied by EM Science, hexane (ACS grade, mixture of aliphatic isomers) from Fisher Scientific, and IES, an ion exchange solvent supplied by Chevron. IES comprises approximately 54% paraffins, 35% naphthenes, and 11 % xylenes and C s aromatics. Before using a portion of the organic phase for extraction experiments, it was washed of water-soluble components and saturated with the aqueous phase by shaking for 10 min with 0.5 MH 2S0 4 , using an organic: aqueous (O:A) phase ratio of 0.5. The concentrations of extractants quoted in this paper refer to moles per liter of the major extractant component, expressed in monomer units.

CHARACTERISTICS OF THIOSUBSTITUTED ORGANOPHOSPHINIC ACID EXTRACTANTS

349

Metal distribution equilibria The equilibrium distribution of a particular metal cation between the organic and aqueous phases as a function of pH was determined in a conventional manner. Equal volumes (200 ml) of the two phases were contacted by rapid stirring, and the pH adjusted by small additions of concentrated H 2S04 or NaOH. The pH was measured using a calibrated glass combination electrode. The pH of the dispersion did not differ significantly from that of the stationary aqueous phase. Equilibrium was attained rapidly in most cases, usually 2-3 min, although a contact time of 15 min was typically allowed between each pH adjustment. Samples (5 ml) of each phase were then withdrawn, and the metal content of the aqueous phase determined. All experiments were carried out at room temperature (25 ± 1°C). In all cases, phase separation occurred readily, making it unnecessary to add modifiers to enhance coalescence. Initial aqueous phases contained 10- 3 AI of the metal cation, prepared from the sulfate salt. A constant ionic strength was maintained by the addition of 0.5 AI Na2S04. Metal concentrations in the aqueous phase were determined by atomic absorption spectrophotometry. Since the volume changes after contact were negligible, metal contents of the organic phases were determined by mass balance.

Chromatographic and spectroscopic methods The purities of Cyan ex 302 and Cyan ex 301 were determined by gas chromatography (GC), using trioctylphosphine oxide (TOPO) (Aldrich, 99%) as an internal standard [31]. The acidic protons of the thio groups were methylated by reaction at 65°C for 1 h with N,N-dimethylformamide dimethyl acetal (Aldrich, 94%) prior to GC analysis. Separation was achieved using a Supelco SPB-l fused-silica capillary column. A Hewlett-Packard model 5880A instrument with a flame-ionization detector (flO) was used. The temperature was programmed to heat the column from 200 to 300°C at a rate of 15 ° C min - , , and then to hold at 300 °C for 5 min. The values of the relative response factors (RRF) for R 2PSOCH 3 and R 2PS 2CH 3 against TOPO were close to 1. Identification of the impurity species present in Cyanex 302 and Cyanex 301 was carried out using a Hewlett-Packard model 5890 gas chromatograph and model 5970 mass spectrometric detector (GC-MS). Samples were prepared as for the GC-FIO analyses. Each GC peak was scanned across the massspectral range from 30 to 550 mass/charge (m/z) units. Phosphorus-31 nuclear magnetic resonance spectroscopy e'p-NMR) was carried out using a Bruker 250 MHz Fourier-Transform instrument, with 85% H 3 P04 as an external standard. Samples were prepared at concentrations of

350

K.C. SOLE AND J.B. HISKEY

(0)

II

e-

(b)

.

,I

;--

I

j

11 1007

0

(c)

J._

100 90 80 70 60 50 40 30 20 10 0 ppm

Fig. I. 3Ip_NMR spectra of Cyan ex reagents: (a) Cyanex 272; (b) Cyanex 302; (c) Cyanex 301.

100 g 1- I in CDCI 3 • and were scanned across the range of chemical shifts from oto 100 ppm. Electronic spectra of the loaded Cyanex e.xtractants were recorded from 190 to 900 nm using a Milton Roy Spectronic 1201 UV-Visible spectrophotometer. Samples were diluted in xylene to give an absorbance ofless than 1. and were contained in quartz cuvettes with a 10 mm path length. The reference used was the corresponding unloaded organic phase.

CHARACTERISTICS OF THIOSUBSTITUTED ORGANOPHOSPHINIC ACID EXTRACTANTS

351

RESULTS

Characterization a/the reagents Reagent purity The purity of Cyanex 272 was determined by potentiometric titration of the extractant in 75% aqueous propan-2-01 with standard sodium hydroxide [31]. The reagent was found to contain 87-88% of the monoacid, and <0.1 % diacid, depending on the particular batch. From gas chromatographic analysis, Cyan ex 302 was found to be 78-80% pure, and Cyan ex 301, 75-83% pure. The impurities present in these two reagents varied from lot to lot, probably due to the experimental nature of these reagents and the batch manufacturing process.

31p_NMR Spectra The 31p_NMR spectra of Cyan ex 272, Cyan ex 302 and Cyan ex 301 are shown in Fig. 1. The major peaks in the spectrum of Cyanex 272 (Fig. la) Occur at 59.6 and 59.9 ppm, due to the existence of the R 2 P0 2 H dimer and monomer, respectively. Phosphine oxide impurities were detected at resonance frequencies of 46.0 and 34.2 ppm. The dominant feature of the Cyan ex 302 spectrum (Fig. 1b) is the triplet centered at 95.3 ppm, and due to the species R 2 PSOH. The Cyan ex 301 spectrum (Fig. lc) showed peaks due to the R 2 PS 2 H dimer and monomer at 67.0 and 67.1 ppm, respectively. The triplet at 97.0 ppm is due to the presence of the monothio species. Phosphine oxide and sulfide impurities were detected at 34.8 and 46.2 ppm. TABLE 1 Peak assignments for GC spectra Extractant

Relative retention

Species

Cyan ex 302

0.382 0.410 0.471 0.608

R2P0 2H Unidentified R2 PSOCH 3 R3 PO

Cyanex 301

0.471 0.545 0.708

R2 PSOCH 3 R2 PS 2CH 3 R3 PS

TOPO internal standard, conditions as given in text.

352

K.C. SOLE AND J.B. HISKEY

TABLE 2 Compositions of Cyanex 272, Cyanex 302 and Cyanex 301 Extractant

Species

Cyanex 272

R2 P0 2 H R3 PO Unknown

87-88 10 -2

Cyanex 302

R2 PSOH R3 PO R2 P02 H R2 PS 2 H Unknowns

78-80 10-12 2-3 2 -8

Cyanex 301

R2 PS 2 H R3 PS R2 PSOH Unknown

75-83 5-8 3-6 -2

Concentration (%)

GC-A.fS Identification of the impurities 'present in the extractants was obtained by GC-MS analysis. The peaks of the GC spectra of Cyan ex 302 and Cyanex 301 are assigned in Table 1. Analysis of the titration data, NMR spectra, chromatograms, and massspectral data gave the extractant compositions shown in Table 2.

Afetal distribution equilibria Effect ofsulfur substitution in the phosphinic acid The extraction equilibria of copper (II), cobalt (II), nickel (II), zinc (II), and iron (III) by 0.1 .M Cyanex 272, Cyan ex 302 and Cyanex 301 in xylene are shown in Figs. 2-4. The pH at which extraction of each metal occurs, decreases in the order: phosphinic acid> thiophosphinic acid> dithiophosphinic acid The order in which the metals are extracted also changes with increasing sul• fur substitution, as shown below: Cyanex 272: Fe
CHARACTERISTICS OF THIOSUBSTITUTED ORGANOPHOSPHINIC ACID EXTRACTANTS

100

~

O

c .. C -

9

l/

If

/a/

~ 60

a

~ 40

/

')(

w

o

0

x

/

/

j

/

/

/l

'!

a

20

i

ij

2

ti

... x-X...... 1

(VV'V

r!

80

__

d

x I

V

353

/

I....

/l!.

x/,.......

dod

• Co(ll) N,OI) x Cu(11) v Zn(ll) a Fe(1I1) d

/ OI~ _ _~V~~__~__~__~__~____~

o

I

234

567

Equilibrium pH

Fig. 2. Extraction of metals by Cyanex 272. Aqueous phase: I X 1O- 3A1 metal as sulfate, 0.5 AI Na2S04' Organic phase: 0.1 AI Cyanex 272 in xylene

100--x 80

~ ';;" 60

.2

ti

,g

40

)(

w 20

/

rl'

~

/r~'

(

_ . • j

! j I.

•

I

V

•

I

v.'~

00

,. ~

2

.t{"

lJ

I.

/

i

I

• CoOl) N,(l1) x CuOI) V Zn(ll) o FeOl1) l!.

d

d"li.

4 5 3 Equilibrium pH

6

7

Fig. 3. Extraction of metals by Cyan ex 302. Aqueous phase: I X 10- 3 AI metal as sulfate. 0.5 AI Na2S04' Organic phase: 0.1 AI Cyanex 302 in xylene.

100 *~l'q- ~I>"

~8Jrr jooij

#

~ 40

• Co(ll) d Ni(ll) II CuO!) v Zn(lI) o Fe(lI!)

w 20/ it

°0~~--~2~~3~~4--~5~-6~~7~--~ Equilibrium pH

Fig. 4. Extraction of metals by Cyanex 30 I. Aqueous phase: I X 10- 3 AI metal as sulfate, 0.5 AI Na2S04. Organic phase: 0.1 AI Cyanex 30 I in xylene.

354

K.C. SOLE AND J.B. HISKEY

2

,.v'7,.. t;l ,. .•

.~;/

l til

o

g' 0

i

~

'~K"

'/" -I.

i

/v

/0$

" "

x

I

Slope=2 /0.05M • O.IOM A 020 M v 0.50M Cyanex272 ---- Cyanex302 -.-.-. Cyanex 301

l l /

,R 1:/

,vv,' v'v "

,

-2

~

/tt'

x'

./

' , i If '~f/v"

"

11

it.

V

l

'

" I

/'

" xl ,

o

234

567

Equilibrium pH

Fig. 5. Effect of extractant concentration on the extraction of cobalt. Aqueous phase: I X 10- 3 AI Co(ll) as sulfate, 0.5 AI Na2S04' Organic phase: Cyanex reagent in xylene .

,t.

.1

2 I

o

,.I

/.

,Ai;

/'

ll,'~ I~ vftl""

ClO .9 -I

Slope=2

"1'/ ,I ,It. Y' ,',{ •

"

rI ' ,- w:"

'I iI

/

I

.i

-2

o

2

3 4 5 Equilibrium pH

x 005M - OIOM A 020M v 050M - - Cyanex272 ---- Cyanex302 -._.-. Cyanex301

6

7

Fig. 6. Effect of extractant concentration on the extraction of zinc. Aqueous phase: I X 10- 3 AI Zn (II) as sulfate, 0.5 AI Na2S04' Organic phase: Cyanex reagent in xylene.

discriminates effectively against nickel at pH values below 6, but poor copper-cobalt selectivity is attained. Cyan ex 302 rejects nickel at higher pH, and excellent copper-cobalt selectivity may be attained. Calcium and magnesium are rejected completely by Cyanex 302 in this pH range. At low pH, Cyanex 301 shows little selectivity for any of the metals studied, although magnesium, aluminum, and calcium are not extracted in this pH range. The order of metal extraction by Cyanex 301 is the same as that observed for the dialkyldithiophosphoric acid, D2EHDTPA [17], and is consistent with the order of stability constants of di valent metal complexes predicted by the Irving-Williams series [40].

CHARACTERISTICS OF THIOSUBSTITUTED ORGANOPHOSPHINIC ACID EXTRACTANTS

355

Effect ofextractant concentration The capacities of these extractants for the metal ions vary from 70 to 100 mg ml- I. The metal loadings in this work may therefore be regarded as representing equilibrium conditions, with little perturbation of the organic-phase concentrations. The effect of extractant concentration on the pH dependence of the extraction equilibria was investigated by varying the concentration up to 0.5 Af. Typical results for cobalt and zinc, given in terms of the distribution coefficients, are shown in Figs. 5 and 6. Data for the other metals, averaged over a series of experiments, are given in Table 3 as pHso values, defined as the pH at which 50% extraction occurs. For all extractants, an increase in extractant concentration causes a shift to lower pH. For Cyan ex 272 and Cyanex 30 I, increasing the extractant concentration by an order of magnitude (0.05 to 0.5 M) causes a shift of about 1 pH unit. For Cyan ex 302, a pH shift of > 2 units is observed for all metals considered.

Effect ofionic strength The effect of the ionic strength was studied by the addition of sodium sulfate to the aqueous phase. The data for cobalt extraction by Cyanex 272 are shown in Fig. 7. An increase from zero to 1.0 AfNa2S04 shifted the extraction equilibria to higher pH by about 1 pH unit. This observation may be partially explained in terms of the aqueous-phase complex equilibria: TABLE 3 Effect of extractant concentration on the pH-dependence of extraction Extractant

Concn.

pH so·

(AI)

Co (II)

Ni(lI)

Cu(lI )

Zn(lI)

Fe(II1)

Cyan ex 272

0.05 0.10 0.20 0.50

5.5 5.2 4.85 4.45

7.4 7.05 6.6 6.05

5.0 4.6 4.35 4.05

3.4 3.0 2.75 2.05

2.0 1.6 1.25 0.85

Cyanex 302

0.05 0.10 0.20 0.50 .

4.5 4.0 3.2 2.65

6.4 5.8 3.6 3.2

<-I <-I <-I <-I

2.25 1.8 1.3 0.8

1.4 1.0 0.6 0.1

Cyan ex 301

0.05 0.10 0.20 0.50

1.3 0.85 0.6 0.2

1.55 1.3 0.6

<-I <-I <-I <-I

-0.4 <0 <-I

0 0 0

Aqueous phase: 1X 10- 3 AI metal as sulfate, 0.5 AI Na2S04. Organic phase: extractant in xylene. ·Averaged values.

356

K.C. SOLE AND J.B. HISKEY

t!'

100 80

~

-c;

.Q

.§Iw

60

ti

[N0 2S04]

l$

~ 40 )(

lLJ

20

°

I

2

~@a .-Lo/! 3

4

·OOM "O.IM 60.2M v 05M I.OM

a

5

6

7

8

Equilibrium pH

Fig. 7. Effect of ionic strength on cobalt extraction. Aqueous phase: 1.7 X 10- 3 M Co (II) as sulfate. Organic phase: 0.053 M Cyanex 272 in IES.

Na2S04 + H+ +=tNa+ + HNaS04

(4)

Na2S04+H++=t2 Na++HSOi

(5)

These reactions effectively remove free protons from solution, thereby increasing the measured pH, and the pH at which extraction occurs. Sodium is also known to be extracted into the organic phase, and exists as ion pairs and aggregates with the basic form of the extractant [41,42]. Sella and Bauer [42] have demonstrated the existence of the organic species Na+HA2 (or Na+ A - (HA», Na+ A -, and (Na+ A - )p, where p is about 13 for Cyanex 272. Furthermore, this work showed that under conditions where the sodium concentration in the aqueous phase exceeds the extractant concentration of the organic phase, the predominant organic species is (NaA) 13, and the concentration of the dimeric acid, (HA)2, is negligible.

Effect oforganic diluent It is well known that in many SX systems the nature of the diluent may have a marked effect on the metal extraction characteristics. In particular, the aromatic or aliphatic nature of the diluent may be important. The effect of diluent on the extraction of copper and cobalt by Cyanex 272 was examined, using the diluents IES, xylene, and hexane. These experiments indicate that cobalt extraction is independent of diluent; copper extraction was constant in the presence of hexane and IES, but shifted to lower pH in the presence of xylene.

CHARACTERISTICS OF THIOSUBSTITUTED ORGANOPHOSPHINIC ACID EXTRACT ANTS

357

DISCUSSION

Slope analysis and stoichiometry of the extracted species Assuming that the organophosphorus extractants exist predominantly as the dimeric species [43], the extraction reactions may be written as [6]:

Mn+ +m(HAh ~M(HA2)n(HAh(m_n) +nH+

(6)

The equilibrium constant for the reaction is then given by: K= [M(HA 2)n(HAh(m-n)] [H+ ]n [Mn+] [(HAh]m

(7)

The distribution coefficient, D, is given by the ratio of the total metal content in the organic phase to the metal content in the aqueous phase: D= [M] = [M(HA2)n(HAh(m_n)] [M] [Mn+]

(8)

Therefore:

D [H+]n

K = -===="-[(HAh ]m

(9)

and: log K=log D-n pH-m log [(HA)2]

(10)

At constant extractant concentration, a plot oflog D against pH should give a straight line with slope n. Since the extractant concentrations far exceed the metal concentrations under the present experimental conditions, it is assumed that this condition holds. Similarly, at pHso, the pH at which 50% metal extraction occurs, D= 1, and log D = O. Relation ( 10) then becomes:

n pHso = -m log [(HAh] -log K

(11)

A plot of pHso against log [(HAh] will give a straight line of slope - min, and a y-intercept of - (log K) In. These plots are shown in Figs. 8-10. For most of the metal-extractant systems considered, good linear correlation is achieved, and the data are well behaved. Deviations from expected trends are discussed in the following section. Further results of the slope analyses are summarized in Table 4. Figures 5 and 6 and Table 4 show that for cobalt, nickel, copper, and zinc, plots of log D against pH give slopes very close to the expected value of 2 for divalent metal ions in all three extractant systems. (With copper, only Cyanex 272 data could be presented in this manner; Cyan ex 302 and Cyanex

358

K.C. SOLE AND J.B. HISKEY Sr-----------------------------~ -A _ _ A

7

6

5 pH 50 4

3 2

- - A _____ A_

-1-

-11-

-i-B_

-1----II .-

-e_ _lC_

----~-

-3 _ _ -B-

i_

CoUl) Cu(II)

-v- Zn(ll)

-D_

-8-

I

Ni(ll)

FeWI)

o~--~--~--~--~--~~--~--~

-IS

-1.6

-1.4 -1.2 -1.0 Log [(R 2 P0 2 H}z1

-OS

-06

Fig. 8. Plot of pH 50 against log [(R 2P02 Hh1 for Cyan ex 272.

7

-A _ _ _ _ _

6

5 pH 50 4

3 2

A_,

,,

-e_____ ,'"

.-----::~~ Ni(ll)

-v____ - a_ _ _ _

v- - _ _ _ _ _ l1

-

Co(ll)

v- Zn(ll)

a ____a

o ~--'----..l---'----'------'-==-a-Fe (III) -I.S

-16

-14 -1.2 -1.0 Log [(R 2PSOH)21

-OS

-0.6

Fig. 9. Plot of pH 50 against log [(R 2PSOHh1 for Cyanex 302.

4

3 pH 50

2

_._

I

-A _ _ A_

----.--

o

-e_

-._~---- A- Ni(lI)

a V

-I -2~--L---L---~--~--~--~

-IS

-1.6

-14 -12 -10 Log [(R2PS 2H)21

-O.S

Co(ll) Fe WI) Zn(ll)

__~

-0.6

Fig. 10. Plot of pH so against log [(R 2PS 2H h1 forCyanex 301.

359

CHARACTERISTICS OF THIOSUBSTITUTED ORGANOPHOSPHINIC ACID EXTRACT ANTS

TABLE 4 Slope analysis of extraction data Extractant

Parameter

Co (II)

Ni(lI)

Cu(lI)

Zn(lI)

Fe(lII )

Cyanex 272

nmIn m" ( -logK)/n n mIn m ( -logK)/n n mIn m (-log K)/n

1.8 -1.07 2 3.80 1.9 -1.9 4 1.47 1.9 -1.07 2 -0.47

1.8 -1.4 3 5.22 1.8

1.7

-1.06 2 3.30

1.8 -1.2 2.4 1.46

1.7 -1.1 2 0.26 1.8

Cyanex 302

Cyanex 301

1.7

-1.5 3 -0.13 1.8

1.9 -1.4 3 -0.04

-1.3 2 -0.69 2.2

-Averaged values. "Calculated assuming n=2. TABLE 5 Conditional equilibrium constants for the extraction of metals by some organophosphorus extractants (25°C) Extractant

Diluent

Aqueous phase

Metal

LogK

Reference

Cyanex 272

Esso DX3641

0.2MNa2S0. 0.3MNH.N0 3

Isopar-H

0.1 MNaN0 3

xylene

0.5MNa2S0.

-7.58 -9.72 -7.22 -9.80 1.50 1.9 4.5 -7.6 -10.4 -6.6 -2.9

[31 ]

toluene

Co Ni Co Ni Cu ZnZn" Co Ni Cu Zn

Cyanex 302

xylene

0.5MNa2S0.

Co Zn

-2.9 0.26

this work

Cyanex 301

xylene

0.5MNa2S0.

Co Ni

0.94 0.08

this work

[8] [36]

this work

-ZnA 2(HA). "ZnA2(HAh.

301 show complete extraction of copper above pH 0.) In the case of iron, the value of n was closer to 2 than to 3. The aqueous-phase compositions used in this work were modelled as a

360

K.C. SOLE AND J.B. HISKEY

function of pH using the computer program, PHREEQE [44]. This analysis showed that under the high ionic strength conditions used, the predominant species in the aqueous phase for all the divalent metals is the undissociated sulfate salt, MS0 4 (aq). The equilibrium between the ionic species, M2+, and MS0 4 (aq) is maintained as extraction ofM2+ into the organic phase occurs. This analysis also predicted the existence of both Fe2+ and Fe3+ in the iron solutions. Conditional equilibrium constants for some of the extraction reactions were calculated from eq. (11 ). Table 5 compares the values obtained in this work with the available published data. For cobalt and nickel extraction by Cyan ex 272, the results obtained in this work and those of prior investigations are in good agreement. This is the first time that extraction constants for metal complexation with Cyanex 302 or Cyanex 301 have been reported.

Nature afthe extracted species Cyanex 272 The results obtained in this work confirm those of Preston [6] and others with respect to the cobalt and nickel complexes formed by Cyanex 272, i.e., the tetrahedral Co(HA2)2 and the octahedral Ni(HA 2)2(HA)2, respectively. At these extractant concentrations, the sixth coordination position of the nickel complex remains hydrated. The copper complex formed appears to be Cu (HA2) 2, probably existing in a planar configuration. This is in agreement with the complex proposed by Sastre et al. [37], and is consistent with the four-coordination geometry preferred by Cu (II) [45]. The non-integer value of m for zinc extraction by Cyanex 272, as shown in Table 4, indicates the formation of more than one species. This has been explained by Sastre et al. [37] in terms of the formation of Zn(HA2h and ZnA2(HA), the former being the predominant species under these experimental conditions. Due to the uncertainty in the oxidation states of the iron species, their coordination geometry was not determined.

Cyanex 302

•

From the data given in Table 4, it appears that the cobalt complex formed with the monothioacid is Co(HA2h2(HAh. This complex is therefore expected to be predominantly octahedral, having the structure shown below. This configuration was confirmed by the inspection of the electronic spectrum of the loaded organic phase. Both octahedral and tetrahedral cobalt complexes exist, giving rise to absorption bands centered at 330 nm and 680 nm, respectively. The organic phase has a blue-green color, observed because of the much high molar extinction coefficient of the tetrahedral species [45].

CHARACTERISTICS OF THIOSUBSTITUTED ORGANOPHOSPHINIC ACID EXTRACTANTS

361

(HA)2 A H

A

""

"~

A

r"" /

H

A

A

I i

/

/

Co /

A

V

H

Co '1'

r::::

H

"

A

A

(HA)2

tetrahedral coordination

octahedral coordination

(Cyanex 272 and Cyanex 301)

(Cyanex 302)

The zinc complex appears to be Zn(HA2)2(HA)2' Somewhat unusual results were obtained for nickel in the slope analysis, with a large transition in the pHso values occurring between 0.1 and 0.2 MR 2 PSOH (see Fig. 9). The higher extractant concentrations showed conventional straight line behavior in plots oflog D against pH, and formed complexes having a red-purple color. At the lower concentrations, log D-pH plots exhibited unusual shapes, and the extracted complexes were bright yellow. It appears that more than one complex is forming, and that conventional approaches to understanding the extraction chemistry are inadequate for this system. This effect was found to be reproducible, but has not yet been satisfactorily explained. The extractant component under consideration, R 2PSOH, comprises only about 80% of the reagent. It is therefore likely that the impurities also form complexes, and introduce some distortion to the data analysis. The strength of the hydrogen bonding with sulfur is much weaker than that with oxygen [ 46], and it is possible that, unlike other organophosphorus reagents, complete dimerization of the extractant does not occur.

Cyanex 301 It appears that the cobalt complex formed is tetrahedrally coordinated Co(HA2)2, and that nickel forms the octahedral complex Ni(HA 2)2(HA)2, similar to the complexes formed with Cyanex 272. Reliable data for the other elements were not available, since the low pH range in which this extractant operates limits the accuracy of the pH readings, and significant errors are introduced into the data analysis when using the slope analysis technique. The presence of two sulfur ligands on the phosphorus atom is also indicative that the probability of extractant dimerization may be further reduced for this extractant. This requires further investigation. CONCLUSIONS

The new thiosubstituted dialkylphosphinic acid solvent extraction reagents, Cyanex 302 and Cyan ex 301, have been characterized with respect to

362

K.C. SOLE ANDJ.B. HISKEY

compositional analysis. The extraction behavior of these reagents with respect to some first row transition metal ions has been compared with that of the commercially available dialkylphosphinic acid, Cyanex 272. With increasing sulfur substitution, the pKa of the acid decreases, metal extraction at lower pH is possible, and the order of metal extraction changes. Nickel extraction is most affected by increasing thio substitution; the effect on iron extraction is negligible. Both Cyan ex 302 and Cyan ex 301 extract copper completely at pH O. Conventional slope analysis techniques and spectroscopic measurements have been used to characterize the nature and geometry of some of the extracted metal complexes. Cobalt is tetrahedrally coordinated as Co (HA 2h by the major components in Cyan ex 272 and Cyanex 301; in Cyanex 302, both the octahedral species, Co(HA2)22(HA)2, and the tetrahedral species are predicted. Copper forms Cu(HA2h in Cyanex 272. Nickel forms the hydrated octahedral complexes, Ni(HA 2h(HAh, with both Cyan ex 272 and Cyan ex 301. Unusual behavior is observed for nickel extraction by Cyanex 302, and is not yet fully understood. Zinc forms predominantly Zn (HA 2) 2 in Cyan ex 272, and Zn(HA2h(HAh, in Cyan ex 302. Conditional equilibrium constants for the formation of some of the extracted metal species in these systems are also reported. Because of the novelty of these reagents, many questions still remain concerning both their physical properties and their extraction characteristics. Work at this laboratory is currently in progress to clarify the nature of the extracted metal species, investigate the extent of dimerization of the reagents, and to measure the equilibrium constants of the extractant-water systems. The long-term stability of the thiol groups, and the stripping behavior are also under consideration. ACKNOWLEDGEMENTS

Dr. W.A. Rickelton of Cyanamid Canada, Inc. kindly supplied samples of the Cyanex extractants. Dr. K. Christensen carried out the 3Ip_NMR analyses, and M. Malcomson assisted with the GC;MS analyses. Their expertise is gratefully acknowledged. This work was supported by the Copper Research Center at the University of Arizona under the U.S. Defense Logistic Agency, Defense National Stockpile Center, Grant Number DN-004. Financial support (for KCS) from the Foundation for Research Development (South Africa) is also acknowledged.

CHARACTERISTICS OF THIOSUBSTITUTED ORGANOPHOSPHINIC ACID EXTRACTANTS

363

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