STRIPPING OF COPPER FROM BIS(2,4,4-TRIMETHYLPENTYL)
PHOSPHINODITHIOIC ACID EXTRACT WITH THIOUREA-HYDRAZINE-
SODIUM HYDROXIDE SOLUTION MIXTURE
S. Facon, F. Adekola + , G. Cote*
Laboratoire d’Electrochimie et de Chimie Analytique
Ecole National Supérieure de Chimie de Paris-ENSCP
UMR 7575 -CNRS- ENSCP-Université Pierre et Marie Curie-Paris 6
11, Rue Pierre et Marie Curie, 75231 Paris Cedex, France.
+ On leave from Department of Chemistry, University of Ilorin, Ilorin, Nigeria.
* Corresponding author; gerard-cote@enscp.fr
(G. Cote); Fax +33143251876
Abstract
The stripping of copper from the organic extract of a thio-organophosphinic extractant,
Cyanex ® 301 using a mixture of thiourea, hydrazine and sodium hydroxide aqueous solution has been investigated. The optimal concentrations of the constituent components of the aqueous solution were found to be 1 M, 0.05 M and 5 M respectively. The stripping procedure led to 95% stripping of copper from Cyanex ® 301 extract, and with concomitant regeneration of the extractant. The characterisation of the stripped copper product was done using a combination of microanalyses, cyclic voltammetry and XRD. The product was madeup of two non-stoechiometric copper sulphides, Cu x
S with x =1.60 and 1.77, and with Cu
1.60
S as the major product. As for the stripping mechanism, hydrazine appears to play the role of a reducing agent for the reduction of the disulphide specie of the extractant, R2P(S)S-S(S)PR2, formed during the extraction step back into Cyanex 301; whereas, thiourea plays the role of an oxidizing agent as well as a source of sulphur in the formation of the stripping products.
Keywords : Stripping; Copper; Bis(2,4,4-Trimethylpentyl)Phosphinodithioic acid,
Cyanex ® 301; Copper sulphide; Hydrazine; Thiourea, Sodium hydroxide, Extraction

1.
Introduction
Solvent extraction is one of the efficient hydrometallurgical techniques used for the purification, separation or concentration of metals from aqueous media. It has found extensive application in mineral processing. The organothiophosphorus ligands have attracted a great attention and some review works on their extraction properties are available in the literature
(Cote and Bauer, 1986 and 1989; Sole and Hiskey, 1992; Cote et al., 2002). Bis(2,4,4trimethylpentyl)Phosphinodithioic acid, also known as Cyanex® 301, belongs to the family of the organothiophosphorus ligands. It has been reported by several authors to be of interest for the extraction of a wide range of metals. Among several applications, we can cite our previous studies: Facon et al. (1991) on the extraction of Sb(III), Bi(III), Pb(III) and Sn(III) from hydrochloric acid solutions, and Avila-Rodiguez et al.(1992) on the extraction of In(III),
As(III), Zn(III), Cu(II) and Cd(II) from mixed sulphuric- hydrochloric acids. Studies by other authors include Rickelton and Boyle (1990) on the extraction of zinc; Sole and Hiskey (1995) on the extraction of Cu(II); Tait (1992 & 1993) and Tsakiridis et al. (2004) on the extraction of Co(II) and Ni(II). The nickel and cobalt extraction with Cyanex® 301 is also a key feature of the Inco’s GORO hydrometallurgical process for laterites (Bacon and Mihaylov, 2002), and the same process is also being investigated for an australian laterite (Cheng and Urbani,
2005). Recently, bis(2,4,4-trimethylpentyl)phosphinodithioic acid and its various homologues have also been considered for the selective extractions of actinides (III) (e.g. Am
3+
) against lanthanides (III) (e.g. Eu
3+
) in acidic media in the frame of the studies dedicated to the reprocessing of irradiated nuclear fuels (Zhu, 1995; Zhu et al., 1996; Chen et al., 1998 and
2002; Hill et al., 1998; Modolo and Odoj, 1998; Wang et al., 2001 & 2002; Liang and Li,
2005).

The stripping of metals previously extracted into an organic phase is an important step in industrial application. It enables the recovery of extracted metals on one hand and regeneration or recycling of the extractant for subsequent use on the other hand. This is necessary from the point of view of economics and also to ensure minimal wastes disposal.
The stripping of species such as Ni(II), Pb(II), Sn(II), Sn(IV), Bi(III), Sb(III), etc. from
Cyanex 301 extract remained generally difficult, and whenever possible high concentrations of HCl, for instance, 6-12 M were used for their recovery (Cote and Bauer, 1989; Rice and
Gibson, 1996; Mihaylov et al., 2000). The stripping of copper on the contrary remained particularly difficult even at extremely high concentrations of HCl. These extreme conditions often result in the degradation of the extractant. Sole and Hiskey (1995) demonstrated this in a comparative study on the stripping of copper from organic phase extracts of Cyanex® 301,
Cyanex® 302 and Cyanex® 272; while quantitative stripping of copper from Cyanex® 302 extract was obtained with 13.5M H
2
SO
4
, the use of concentrated (18M) H
2
SO
4
did not strip any copper from Cyanex® 301. Furthermore, Sole and Hiskey (1995) noted a significant disadvantage in the use of such concentrated acids, as they observed discoloration and increased viscosity of the organic phase. The problem was further compounded as a result of significant deterioration in the phase separation when the organic phase extract was contacted with 18 M H
2
SO
4
. Along this line, an experiment carried out on the stripping of copper from
Cyanex® 301 organic phase using an aqueous solution containing 12 M HCl and 1 M CaCl
2 for enhancing the complexing ability of chloride led to only 2% recovery (Facon, 1993). The same difficulty for stripping copper from organic extract was previously reported with the dialkylphosphorodithioic acids (Kholkin et al., 1988). In both cases, i.e., with dialkylphosphinodithioic and dialkylphosphorodithioic acids, oxidation-reduction of extracted copper(II) complex results in the formation of copper(I) and disulphide species.

Thus, Sole and Hiskey (1995) have suggested the formation of a multinuclear oligomeric structure of the copper(I) complex with the disulphide, R
2
PS
2
-
ligand in which the ligands bridge between metal centres. This enhanced stability of the copper extract of Cyanex 301 is the main reason for the troublesome stripping of copper from the organic phase. It was therefore desirable to search for an effective and relatively simpler method for the stripping of copper from Cyanex® 301 extract, which at the same time would not lead to the degradation of the extractant. Binary extractant systems consisting of mixtures of Cyanex® 301 with basic extractants (Primene® JM-T, Amberlite® LA-2, Alamine® 336 and Aliquate® 336) have been shown to exhibit a large synergistic effect on metal stripping in comparison to Cyanex®
301 alone (Jakovljevic et al., 2004; Bourget et al., 2005). More particularly, Cyanex® 301 combined with Aliquate® 336 demonstrated that cobalt and nickel could be much more easily stripped with sulphuric acid or hydrochloric acid as compared to Cyanex® 301 alone.
Although such an approach is highly interesting, the case of copper was not considered. On the other hand, Kholkin et al., (1988) have shown that the use of binary extractants – salts of dialkyldithiophosphoric acid and of quaternary ammonium bases – provide favourable conditions under which copper can be stripped out by cation bonding in aqueous phase. The stripping could be either by solid phase stripping via settling of copper hydroxide or by reaction with ammonia. Indeed, copper(II) is extracted into the binary extractants system according to equation (1):
Cu 2+ aq + SO
4
2 aq + 2R
4
NLorg

(R
4
N)
2
SO
4 org + CuL
2 org (1) and can be stripped from the organic phase, for instance, according to equation (2):
CuL
2 org + (R
4
N)
2
SO
4 org + 4NH
4
OHaq

Cu(NH
3
)
4
2+ aq + SO
4
2aq + 2R
4
NLorg + 4H
2
0 (2)
Thus, in the case of mixture of di(2-ethylhexyl)dithiophosphoric acid and tetraoctylammonium salt, more that 90% of copper is stripped for three countercurrent steps with 10% aqueous ammonia solution at Vo/Vaq = 3.1. It is of interest that no changing of

extractant capacity and other extraction properties was observed in 10 extraction – stripping cycles. We are currently re-visiting this approach with Cyanex® 301 (to be published).
In the present work, we have decided to investigate another route by considering the stripping of copper from Cyanex® 301 organic phase extract via copper sulphide precipitation in the presence of a reductant to reduce the disulphide formed during the extraction step and then to regenerate Cyanex® 301. The study also involved electrochemical characterization of the stripping products by carbon paste electrode technique. The carbon paste electrode technique was chosen because it provides the advantage of working directly on the solid precipitate without prior dissolution. It also enables both quantitative and qualitative determination of the various oxido-reduction species, including the non-stoechiometric ones present in the solid under investigation (Brage et al., 1979; Vitorge et al., 1979; Adekola et al., 1992).
2. Materials and methods
2.1. Reagents
The extractant Cyanex® 301 was kindly supplied by the American Cyanamid Company (now available from Cognis) and was used without further purification. The reagent contained 85%
(w/w) bis(2,4,4-trimethylpentyl)phosphinodithioic acid. Other information on the reagent’s composition and its physico-chemical properties have been previously described (Facon et al.,
1991 and Avila-Rodiguez et al., 1992). All other chemicals used were of analytical grade and were mostly products of Prolabo or Merck. Kerosene (Fluka) containing 10% (v/v) of ndecanol (Aldrich) was used as diluent throughout the study.
2.2. Extraction and Stripping studies
In all stripping experiments, the organic phase was previously loaded in copper as follows: the organic phase constituted of 0.1 M Cyanex® 301 in kerosene (90%) and ndecanol (10%) (v/v) was equilibrated during 15 min with an equal volume of aqueous phase

initially containing 0.096 M Cu(II) in 2 M HCl.. After separation, the loaded organic phase was then contacted with the stripping aqueous solution. An initial stripping aqueous solution was prepared from 2.06 M hydrazine, 1 M thiourea and 5 M NaOH. After agitation in a shaker for 15 minutes, suspension of solid particles in the aqueous phase was obtained, resulting from the settlement of solid product containing copper stripped out of the organic phase. The aqueous phase suspension was then filtered through 0.45µm nylon membrane filter and copper analysed both in the stripped solid and in the filtrate. The recovered organic phase was thereafter put into contact with a fresh copper aqueous solution so as to investigate the possibility of the extractant’s re-use after stripping. A study on the optimization of composition of the aqueous phase used for stripping was also undertaken. This was done in two steps. Firstly, hydrazine concentration was varied between 0.005 M and 2.06 M while the concentrations of sodium hydroxide and thiourea were kept constant at 5M and 1 M, respectively. Secondly, the concentration of thiourea was varied from 0.052 M to 1.0 M, while keeping the concentration of hydrazine and sodium hydroxide at 0.05 M and 5 M, respectively. The mixture of thiourea-hydrazine-sodium hydroxide stripping solution will hereafter be noted THN. In all experiments, both aqueous and organic phases were fixed at an
A/O phase ratio of 1 and at a contact time of 15 mins. All experiments were carried out at 20
± 2°C. Copper concentration in the aqueous phase was determined by Inductively coupled
Plasma - Atomic Emission Spectrometry with an ICP 1500 Plasma Therm. Inc. equipment at
λ = 324.75 nm. The metal concentration in the organic phase was deduced by difference.
2.3. Characterization of the stripped copper precipitates
The precipitates obtained after stripping with THN were characterized by
Microanalysis, Cyclic voltammetry and XRD measurement.

2.3.1. Microanalysis
The microanalysis of the precipitates was carried out at the Laboratoire Centrale d’Analyses of CNRS, Lyon, France. The precipitates were washed with ethanol and then airdried before analysis.
2.3.2. Cyclic Voltammetry with carbon paste electrode technique
The details of the constitution of carbon paste electrode and the experimental set-up have been described elsewhere (Adekola et al., 1992). The electrode paste was constituted by mixing about 50 mg of ultra-pure graphite powder (Johnson-Matthey), ca.1.0 mg of stripped copper precipitate and 40-60 μL of 1.8 M sulphuric acid. The current-potential curves were obtained with the aid of a synchronous motor (Pilonium)-driven potentiometer (Tacussel PRT
20-2) and a recorder (Tacussel EPL 3). All potentials were referred to a saturated calomel electrode (sce). A low potential scan rate of 0.1 mV/s was used for all experiments so as to achieve a complete oxidation or reduction of the studied compound. The total charge consumed at each particular electrochemical reaction was determined by evaluating the area under the corresponding peak of the current-potential curve and from which the number of exchanged electrons was deduced. All the experiments were carried out at 20 ± 2 °C.
3. Theoretical considerations
Following Kholkin et al. (1988), Sole and Hiskey (1995) proposed that the mechanism of extraction involved simultaneous oxidation-reduction of copper(II)-bis(2,4,4trimethylpentyl)phosphinodithioate resulting in the formation of polymeric copper(I) complex and disulphide, (R
2
P(S)S-S(S)PR
2
. Copper(II) was responsible for the oxidation of the extractant. Under the present conditions, the concentration of copper in the organic phase at saturation corresponds to a global 1:2 - metal:Cyanex® 301 molar stoichiometry, as could

be expected from the formation of the Cu
II
L
2
complex. However, the global 1:2 metal:Cyanex® 301 molar stoichiometry also corresponds to the state of the organic phase after disproportioning of the Cu(II) complex and formation of disulphide as expressed by equation(3):
2 n

Cu
II

R
2
P ( S ) S

2


2

Cu
I

R
2
P ( S ) S
 
+ nR
2
P ( S ) S

S ( S ) PR
2
(3)
In reality, the presence of a mixture of Cu
II

R
2
P ( S ) S

2
, Cu
I

R
2
P ( S ) S

and
R
2
P ( S ) S

S ( S ) PR
2
in the Cyanex® 301 organic phase saturated with copper(II) cannot be excluded. This suggests that the stripping of copper as copper sulphides and the regeneration of Cyanex® 301 may involve various types of reactions as listed below:
Cu II

R
2
P ( S ) S

2 , org
+ S 2
 aq

CuS
( s )
+ 2 R
2
P ( S ) S
 org
(4)
2

Cu
I

R
2
P ( S ) S
  n , org
+ nS
2
 aq
 nCu
2
S
( s )
+ 2 nR
2
P ( S ) S
 org
(5)
2

R
2
P ( S ) S

S ( S ) PR
2
 org
+ NH
2

NH
2

4 R
2
P ( S ) SH org
+ N
2
(6)
Thus, the presence of a source of sulphide ions and that of a reductant allowing the reduction of the disulphide, R
2
P(S)S-S(S)PR
2
into dialkylthiophosphinodithioate, R
2
P(S)S
-
are necessary. The thiourea serves as source of sulphide, as it dissociates in the alkaline medium in the presence of copper catalytic surface according to equations 7 and 8 to produce bivalent sulphide:

NH

2 2
CS + OH
-

CH
2
N
2
+ H
2
O + HS

(7)
HS

+ OH
-

S
2-
+ H
2
O (8)
As a result, a combination of thiourea as a source of sulphide and hydrazine as a reductant in alkaline media was chosen for investigating the stripping of copper from Cyanex® 301 extract.

4. Results and discussion
4.1. Stripping with a mixture of hydrazine, thiourea and sodium hydroxide
An initial trial experiment carried out using a mixture of 2.06 M hydrazine, 1 M thiourea and
5 M NaOH led to the stripping of 95% of copper from Cyanex® 301 extract and within 15 minutes. The results obtained in respect of the re-use of regenerated Cyanex® 301 after stripping were summarized in Table 1. It shows that Cyanex® 301 retained its extractive capacity after recovery in agreement with expected reversibility of Eq.(6).
4.2. Optimization of stripping aqueous phase solution
The optimization of stripping (aqueous phase) solution is important for economic reason and also for good understanding of mechanism involved.
Table 2 shows the results of stripping when concentration of hydrazine or thiourea was varied.
If one assumes that only hydrazine plays the role of reductant towards the disulphide and according to equation (6), one mole of hydrazine should enable reduction of two moles of
R
2
P(S)S-S(S)PR
2
yielding four moles of R
2
P(S)SH. However, the experimental results
(Table 2) show that a quantitative stripping would always be obtained even with about onetenth of the stoechiometric amount of hydrazine. This observation reveals the complex role of hydrazine. On the other hand, the results of concentration variation in respect of thiourea
(Table 2) show a decrease in the percentage of copper stripped with decrease in thiourea concentration. As for the effect of the third constituent, that is NaOH, there was a rapid decrease in the percentage of copper stripped with decrease in NaOH concentration. In effect, the stripping yield fell to zero when NaOH concentration was adjusted below 3 M. Based on the above results, the following composition of aqueous phase can be inferred for maximum stripping of copper from Cyanex® 301 extract: 1 M thiourea; 0.05 M hydrazine; 5 M NaOH; and for volumes ratio of 1. For comparative purpose, the stripping was also carried out with aqueous solution of Na
2
S (1 M) in NaOH (1 M), and by replacing hydrazine in THN with

hydroxylamine (0.2 M) as reductant. All other conditions were kept the same. The percentage of copper stripped out of the organic phase was 90% and 78% respectively. The former, although quantitative, was not very attractive as a result of poor decantation of the suspended solid particles. This was unlike THN mixture which gave suspended solid particles with very high settling speed.
4.3. Characterization of copper precipitate (product of stripping)
4.3.1. Characterization by cyclic voltammetry with carbon paste electrode
Characterization of the precipitate obtained from the stripping of copper is useful for better understanding of the mechanism of copper stripping from Cyanex® 301 extract. The results of microanalysis ( % Cu: 70.02; S: 21.02; N: 0.41; C: 2.57; P<0.2 ) show that no copper-thiourea compound was formed. The isolated product contained mainly copper and sulphur, while the % of carbon, phosphorus and nitrogen are very low. These results show that a non-stoechiometric copper sulphide, Cu x
S (x=1.68) was formed during the stripping process. In order to validate the hypothesis of formation of a non-stoechiometric copper sulphide precipitate, the electrochemical behaviour of the product was investigated using carbon paste electrode technique. The current potential curves obtained when the stripped copper precipitate was incorporated into the carbon paste electrode are shown in figures 1 and
2. Figure 1 refers to the current -potential curves with an initial cathodic potential scan, while figure 2 corresponds to current-potential curves with an initial anodic potential scan.
The current-potential curves obtained under the same conditions were then compared with some previously reported works on a commercial copper (I) sulphide, Cu
2
S (Brage et al.,
1979; Vitorge and Lamache, 1979) in order to assign the recorded peaks to their corresponding characteristic electrochemical reactions. For commercial Cu
2
S product, five anodic and two cathodic peaks were observed. All the peaks recorded when the commercial

Cu
2
S was incorporated into the carbon paste electrode and their corresponding electrochemical reactions are listed below:
Anodic (A) and Cathodic (C) peaks (Brage et al., 1979; Vitorge and Lamache, 1979):
Peak A
1
/C
1
: Cu
2
S

Cu
1.92
S + 0.08Cu
2+
+ 0.16e
-
; 0.105V
Peak A
2
/C
2
: Cu
1.92
S

Cu
1.77
S + 0.15Cu
2+
+ 0.30e
-
; 0.20V
Peak A
3
/C
3
: Cu
1.77
S

Cu
1.60
S + 0.17Cu
2+
+ 0.34e
-
; 0.255V
Peak A
Peak C
4
5
:
:
Cu
1.60
S

Cu
1.31
S + 0.29Cu
2+
+ 0.58e
-
; 0.285V
Peak C
4
: Cu
1.31
S + 0.69Cu
2+
+1.38e

Cu
2
S; 0.10V
Peak A
5
: Cu
1.31
S

CuS + 0.31Cu
2+
+ 0.62e
-
; 0.40V
CuS + 0.31Cu
2+
+ 0.62e

Cu
1.31
S ; 0.24V
These peaks correspond to different stages of oxidation of Cu
2
S into CuS. The oxidation reactions represented by A
4
and A
5
were irreversible, while in most cases, peaks A
3
and A
4 were inseparable and did occur as a single peak due to the closeness of their oxidation potentials. These authors also observed a shift in the oxidation-reduction potential to higher value according to the extent of crystalline network of the product used.
The stripped precipitate has a characteristic equilibrium potential of 0.230 V/ sce , indicating that it is a mixed product. This is evidenced from figures 1 and 2 showing both oxidation and reduction peaks with the latter being relatively very small, when potential was scanned starting with either anodic or cathodic direction respectively.
In figure 1, a small cathodic peak noted C
2
at 0.21 V/ sce was obtained followed by two important anodic peaks A
4
and A
5
during the first scan. The peaks A
2
and A
3
were not visible due to the expected very small area of these peaks. They appeared to be occurring as shoulders on the peak A
4
. Peaks C
4
and C
5
were correspondingly obtained during the second cathodic scan. The small cathodic peak C
2
would most probably correspond to the reduction of Cu
1.77
S into Cu
1.92
S at reduction wave potential of 0.21V/ sce .

The anodic peaks A
4
and A
5
produced during the first anodic scan would most probably correspond to an initial oxidation of Cu
1.60
S into Cu
1.31
S, followed by final oxidation of the latter into CuS respectively. The stripped copper precipitate seems therefore to be present principally in the form represented by A
4
, that is, Cu
1.60
S. On the other hand, the currentpotential curves shown in figure 2 corresponding to an initial anodic scan, produced mainly peaks A
4
and A
5
during the first anodic scan and peaks C
4
and C
5
during the reverse(cathodic) scan. Peaks A
1
, A
2
, although absent during the initial anodic scan (fig. 2), were produced in addition to peaks A
4
and A
5
during the second anodic scan. Additionally, only the peak A
4 was obtained during the limited anodic scan from 230 mV to 320 mV (represented by broken line in figure 2); while the peak C
4
was obtained during the reverse cathodic scan from
320mV to 0mV. It is important to note the near-absence of the cathodic peak C
5
at this time.
All the anodic peaks (A
1
, A
2
, A
3
, A
4
& A
5
) were however produced during the extended second anodic scan from 0 mV to 720mV; and cathodic peaks (C
4
and C
5
) during the second cathodic scan from 720mV to 0mV. The very steep or near-vertical rise of the initial anodic scan (fig. 2) with a small shoulder at about 0.24 V/sce would correspond to A
3
, that is, oxidation of Cu
1.77
S into Cu
1.60
S. This corroborates the initial observation concerning the presence of a small peak, C
2
also attributed to Cu
1.77
S, when the initial potential scan was cathodic. The cyclic voltammetric results therefore suggest that the stripped product would most likely be a mixture of two non-stoechiometric copper sulphide products, Cu
1.77
S and
Cu
1.60
S. Cu
1.77
S has been identified with the small reduction peak C
2
and it is occurring as a minor product, while Cu
1.60
S associated with the large oxidation peak A
4
constitutes the major product. The concentrations of the two products have been calculated from the area of their respective peaks. The area of the small peak C
2
(figure 1) was 1.7 x 10
-2
C and that of peak A
4
(figure 2) was 0.57 C. These gave 5.6% of Cu
1.77
S and 94.4% of Cu
1.60
S respectively. The mole ratio Cu:S which is 1.60 from the cyclic voltammetric study, is close to the empirical

value of 1.68 calculated from microanalysis of the copper sulphide precipitate discussed earlier. These results are also supported by the works of various authors on the chemical bath deposition of copper sulphide using thiourea as source of sulphide and in basic media (Fatas et al., 1985; Sartale and Lokhande, 1999; Mane and Lokhande, 2000). In these various studies, the formed compound contained mixture of non-stoechiometric Cu x
S (1.83 ≤ x ≤
1.96) solid phases.
4.3.2. Characterization by X-ray diffractometry (XRD)
The copper precipitate obtained from stripping was also subjected to XRD investigation in order to confirm the existence of Cu
1.77
S and Cu
1.60
S. The characteristics bands extracted from the XRD spectra are summarized in Table 3. The characteristics bands earlier reported by
Vitorge & Lamache (1979) for Cu
1.77
S are also indicated for comparison.
From Table 3, all the XRD major peaks reported for Cu
1.77
S were also found in this work. All other peaks present could therefore be attributed to the second non-stoechiometric solid,
Cu
1.60
S.
The preceding results show that the precipitate obtained from stripping is a mixture of Cu
1.60
S
(94.4%) and Cu
1.77
S(5.6%), which suggests that the disproportioning of Cu
II
[R
2
P(S)S]
2 according to Eq.(3) is not complete.
5. Conclusions
It has been shown in this work that the stripping of copper from Cyanex® 301 extract is possible by using aqueous solution composed of thiourea, hydrazine and sodium hydroxide.
The optimal concentrations of the constituting reagents were established to be 1 M thiourea;
0.05 M hydrazine; 5 M NaOH; and for aqueous and organic volumes ratio of 1. The nondegradability or re-use of the extractant, Cyanex® 301 after stripping of copper was also demonstrated in this study.

The stripping yield was 95%. The resulting copper sulphide precipitate was characterized by microanalysis and cyclic voltammetry; and found to be a mixture of two non-stoechiometric copper sulphides. These are Cu
1.77
S and Cu
1.60
S at concentrations of 5.6% and 94.4% respectively. Mechanistically, hydrazine enabled the reduction of copper(II) while thiourea served as a source of sulphur for the eventual formation of non-stoechiometric copper sulphide. It is therefore evident that sulphur used for producing copper sulphide was sourced from thiourea and not Cyanex® 301, as the extractive capacity of Cyanex® 301 remained practically intact after a complete extraction-stripping cycle. Finally, the fact that the precipitate obtained from stripping is a mixture of Cu
1.60
S and Cu
1.77
S indicates that the disproportioning of copper(II)-Cyanex® 301 complex is not complete in the organic phase.
Acknowledgements
The authors thank Prof. D. Bauer for helpful discussion; while F. Adekola thanks the
Government of France for the award of a research fellowship.
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TABLES
Table 1
Summary of results of a complete cycle of extraction and stripping of copper
Experimental step
Initial Initial Final Final aqueous phase organic phase aqueous phase organic phase
Remarks
Extraction with fresh
Cyanex® 301
0.096 M Cu;
2 M HCl
0.1 M HL; kerosene-ndecanol
(90-10% v/v)
0.046 M Cu 0.05 M CuL
2
100%* extraction
Stripping 2.06M
Hydrazine;
0.05 M CuL
2
1 M Thiourea;
5 M NaOH copper precipitate
0.0026 M
CuL
2
94.6% stripping
Extraction with recovered
Cyanex® 301
0.096 M Cu;
2 M HCl
0.0026 M
CuL
2
0.048 M Cu (0.048 + 100%*
0.0026) =
0.05 M CuL
2 extraction
*calculated with respect to HL (HL stands for Cyanex® 301); stirring time = 15 mins.

Table 2
Variation of percentage of copper stripped with thiourea or hydrazine concentration
[NaOH]=5M; [CuL
2
] in organic phase=0.05 M; contact time=15min.
Thiourea concentration (M) Hydrazine concentration (M)
1.00
0.51
0.05
0.05
0.20
0.10
0.05
0.05
0.05
0.05
Stripping yield (%)
94.6
85.3
84.9
79.0
77.8
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.2
0.3
0.5
1.0
2.0
0.005
0.01
0.02
0.05
0.1
95.0
94.8
94.6
94.0
95.0
82.0
82.0
85.0
95.0
94.5

Table 3
Major bands from the XRD Spectra of the stripped copper sulphide precipitate
Assignment d (nm)
(this study) relative intensity
(this study) d(nm) for Cu
1.77
S relative intensity for
(Vitorge&
Lamache, 1979)
Cu
1.77
S (Vitorge &
Lamache, 1979)
0.37692
0.31936
0.30105
0.27714
0.25889*
0.24988
0.21762
0.19602
0.19536*
0.16668
5.2
42.6
3.6
41
15.9
3.7
5.8
82.8
100
13
--
0.319
0.302
0.277
--
0.252
0.2158
0.1963
--
0.1767
0.13867
0.13789
0.13747
0.13684
2
2.8
2.9
2.4
*bands assigned from this study to Cu
1.60
S
0.1674
0.1603
0.1388
--
--
35
14
40
--
4
12
100
--
6
25
4
10
--
Cu
1.77
S
Cu
1.77
S
Cu
1.60
S
Cu
1.77
S
Cu
1.60
S

Illustrations:
Fig. 1. Current-potential curves for the stripped copper precipitate in 1.8 M H
2
SO
4
. graphite powder, 48.5mg; stripped Cu precipitate, 1.0 mg; H
2
SO
4
, 60 µL; scan rate, 0.10mVs
-1
.
Starting with cathodic potential scan.

Fig. 2. Current-potential curves for the stripped copper precipitate in 1.8 M H
2
SO
4
. graphite powder, 48.9 mg; stripped Cu precipitate, 1.43 mg; H
2
SO
4
, 60 µL; scan rate, 0.10mVs
-1
.
Starting with anodic potential scan (broken line = limited scan; full line = extended scan).
FIGURE CAPTIONS:
Figure 1. Current-potential curves for the stripped copper precipitate in 1.8 M H
2
SO
4
. graphite powder, 48.5mg; stripped Cu precipitate, 1.0 mg; H
2
SO
4
, 60 µL; scan rate, 0.10mVs
-1
.
Starting with cathodic potential scan.
Figure 2. Current-potential curves for the stripped copper precipitate in 1.8 M H
2
SO
4
. graphite powder, 48.9 mg; stripped Cu precipitate, 1.43 mg; H
2
SO
4
, 60 µL; scan rate,

0.10mVs
-1
. Starting with anodic potential scan (broken line = limited scan; full line = extended scan).
LIST OF TABLES:
Table 1: Summary of results of a complete cycle of extraction and stripping of copper
Table 2: Variation of percentage of copper stripped with thiourea or hydrazine concentration
[NaOH]=5M; [CuL
2
] in organic phase=0.05 M; contact time=15min.
Table 3: Major bands from the XRD Spectra of the stripped copper sulphide precipitate