NOVEL REAGENTS FOR EXTRACTION OF COPPER(II)
FROM AMMONIAC MEDIA
V. Gusev, D. Pashkina, A. Radushev
Institute of Technical Chemistry of Ural Branch of the RAS, Perm, Russian Federation
gusevvyu53@mail.ru
To commercially produce copper, a method termed Solution Extraction–ElectroWinning (SE-EW) is widely applied. The method is
based on transfer of copper(II) ions from aqueous phase to organic phase
with use of a suitable extraction reagent. To extract copper(II) ions from
ammoniac media, two types of reagents, namely oxyoximes and diketones, are currently used [1]. Oxyoximes, being a very efficient extraction reagent due to their chemical structure, form a salt with ammonia [2] which, while in electrolyse solution, deteriorates its parameters.
To remove this salt, additional stage is required to wash organic phase,
to regenerate the reagent and to recycle it. These arrangements complicate processing line. -Diketones do not react with ammonia; however,
they are markedly less efficient extractants for copper(II).
A search for new extractants which, as -diketones, would not
transfer ammonia and, at the same time, would have more efficient extraction properties, was of interest.
Earlier [3, 4], we had studied benzoic acid N, Ndialkylhydrazides. These reagents were shown to not react with ammonia, like -diketones, and to efficiently extract copper(II) from ammoniac solutions. However, these compounds are inferior to -diketones in
effectiveness of copper(II) extraction to some extent; besides, they are
badly soluble in aliphatic hydrocarbon solvents widely used in industry.
This peculiarity appreciably limits their practical application.
Our investigations were aimed at para-tert-butylbenzoic acid N,
N-dialkylhydrazides. Introduction of tert-butyl group to para-position
of benzol ring was expected to enhance hydrophobicity of the molecule
and to result in better solubility in hydrocarbon solvents. In addition, the
+I-effect of this group was thought to enhance strength of copper complexes under formation and, thus, to improve extraction properties of
these compounds, as compared with benzoic acid N, Ndialkylhydrazides.
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The studied reagents can be presented with use of the following
general formula: 4-t-BuC6H4CONHN(R)2, where R stands for Н (1),
CH3 (2), i-С4Н9 (3), С4Н9 (4), C6H13 (5), С8Н17 (6), С10Н21 (7).
Appearance of these reagents is white crystalline substances. Their
solubility was studied with use of gravimetrical method in solvents
widely used in laboratory practice (water, ethanol), or in extraction processes (i-AmOH, hexane, kerosene, o-xylene). The obtained findings are
summarized in Table 1. As is apparent, compound (1) is good soluble
only in aliphatic alcohols, badly soluble in water and in hydrocarbon
solvents. Its N, N-dialkyl-substituted compounds are practically insoluble in water. As the length of N, N-alkyl radicals increases, solubility
of compounds in organic solvents enhances and attains its maximum–in
case with aliphatic hydrocarbons–for compound (6). Further augmentation of radicals length leads to worsening of solubility, due to more intensive intermolecular interaction. Branching of these radicals–as is the
case with compound (3)–leads to worsening of solubility, as compared
with compound (4) with non-branched radicals. Compounds with
RC6H13 are highly soluble in hexane and in kerosene and, hence, promising for extraction processes.
Table 1. Solubility of compounds (1-7) at 25оС
Solvent
Water
EtOH
i-AmOH
o-xylene
hexane
kerosene
(1)
1,0
0,005
263,6
1,37
52,0
0,27
8,6
0,045
0,2
0,001
–
Solubility, g / L
mol / L
(2)
(3)
(4)
0,85
n/s
n/s
0,0039
285,8
142,2
475,2
1,29
0,46
1,56
157,1
182,4
410,4
0,71
0,60
1,35
9,7
116,2
230,0
0,044
0,38
0,76
0,18
6,4
13,7
0,008
0,021
0,045
8,5
16,2
–
0,028
0,053
n/s – non soluble
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(5)
(6)
(7)
n/s
n/s
n/s
–
–
–
437,4
1,21
–
–
–
–
–
>930
>2,23
>1020
>2,83
>1000
>2,40
>1380
>3,32
>450
>0,95
>590
>1,25
Capability of compounds to react or not to react with ammonia is
determined by their acid-base properties, in particular, by their capability
to donate proton. Acid-base properties of compounds were investigated
with use of spectrophotometric method. To determine pH diapasons for
various compounds, dependences of optical density of compounds solutions on pH values and the Hammett function were obtained. Acidity
value was varied with HCl and KOH solutions. Patterns of the optical
density vs. acidity and concentration of alkali for compound (2) are presented in Fig. 1.
А 0,65
0,60
0,55
0,50
0,45
0,40
0,35
12,0
12,5
13,0
13,5
14,0
14,5
15,0
15,5
Н_
Fig. 1. Absorbance (A) of ethanol-water (3:1) solutions of compound (2) vs. pH
value and the Hammett function (H_). С(2)= 10-4 mol/L,  = 237 nm, l =
1 cm.
Patterns for remaining reagents are similarly shaped. As is apparent from Fig. 1, donation of proton occurs in the interval “pH 13 Hammett function 14.5” (1.8 mol/L of KOH). Thus, as pH values of
ammoniac copper-containing solutions are appreciably lower, a conclusion may be drawn that compounds under investigation, just as benzoic
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acid N, N-dialkylhydrazides, will not donate proton under extraction
conditions, i.e. they will not react with ammonia.
An important feature of reagents used in extraction processes is
their distribution between organic and aqueous phases. n-Xylene and
hexane were selected as the organic phase, as the aqueous phase–0.5
mol/L solution of HCl and 1 mol/L solution of NH3. The obtained findings are presented in Table 2 [5]. As is apparent from Table 2, molecular
mass (m.m.) and distribution coefficient of reagents (DHL) vary symbately. Lower values of lg DHL–as is the case with hydrochloric acid–are
related to formation of protonated form of reagents and its better solubility in the aqueous phase. Drag-out of compounds into ammoniac media
happened to be insomuch negligible that–in certain cases–it could not be
measured with use of standard analytical methods and, hence, distribution coefficient value could not be calculated as well. Judging from this
parameter, compounds (6) and (7) outperform commercial copper extraction reagents of the oxyoxime class with lg DHL=3.6-4.0 [6].
Table 2. Distribution coefficients of n-t-BuС6Н4CОNHN(R)2 (HL)
lg DHL
R
HCl 0,5 mol/L
p-xylene
i-C4H9
2,7
C4H9
1,4
C6H13
2,1
C8H17
3,3
C10H21
4,4
n/a – not available
NH3 1 mol/L
p-xylene
hexane
3,1
‒
3,9
3,8
n/a
4,7
n/a
n/a
n/a
n/a
To study extraction equilibriums assisted with these compounds,
investigation of extraction Cu(II), Ni(II), Co(II), and Zn(II) cations with
use of compound (5) dissolved in kerosene, depending on pH values and
on NH3 concentration was performed (Fig. 2) [7].
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E, % 100
80
Cu(II)
Zn(II)
Co(II)
Ni(II)
60
40
20
0
5
3
15
10
pH
8
20
13
25
C NH , моль/л
3
Fig. 2. Extraction degree (E%) of metal ions for compound (5) vs. pH values
and NH3 concentration. С(5) = 0,05 mol/L (kerosene), СМ(II) = 0,005
mol/L, Vо:Va = 1:2,  = 5 min.
Extraction of Cu(II) ions starts at pH2, attains over 99% value at
pH 4 and remains at this level up to CNH3 8 mol/l. As concentration of
NH3 further progresses, its extraction degree decreases and equals 70%
at CNH3 10.5 mol/L. Formation of metal oxide precipitates followed by
their localization at phase interface becomes to be observable simultaneously with extraction: 99% of Co(II) is extracted at pH 9.5-10.8; 95% of
Ni(II) is extracted at pH 9.9-10.6; 98% of Zn(II) is extracted at pH 8.39.7. A broader interval of Co(II) extraction enables fitting conditions to
selectively recovery of copper and to segregate it from satellite elements.
Comparison of compounds under investigation with N, Ndialkylhydrazides of benzoic acid [8] results in obvious idea of that introduction of tert-butyl radical to para-position of benzol ring appreciably broadens interval of efficient extraction of copper(II) to larger values
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of CNH3 concentrations, i.e., these compounds are of more high extraction capacity to copper(II) ions.
Fig. 3 demonstrates how degree of Co(II) extraction is dependent
on alkyl radical’s length in a series of n-t-C4H9C6H4C(O)NHNR2 compounds.
E, %
100
80
60
40
●-5
×-6
20
○-7
0
0
5
10
pH
15
3
20
8
25
С12
NH3, mol/L
Fig. 3. Extraction degree of copper(II) for compounds (5), (6) and (7) (HL) vs.
pH values and NH3 concentration. СHL = 0,05 mol/L (kerosene), СМ(II) =
0,005 mol/L, Vо:Va = 1:2,  = 5 min.
As is apparent, most broad interval of its maximal extraction
(from pH 4.0 to 8 mol/L NH3) is observable during extraction assisted
with compound (5). As the length of alkyl radicals in this series augments, this interval narrows: pH 4.5–4.5 mol/L NH3 for compound (6)
and pH 4.8–4 mol/L NH3 for compound (7). This phenomenon is presumably explained by strengthening of intermolecular bonds resulting in
a narrowed interval of maximal extraction.
Fig. 4 demonstrates how degree of Co(II) extraction is dependent
on the nature of solvents, by the example of compound (7).
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E, %
100
1
3
80
2
60
40
20
0
0
4
8
pH
12
2
16
6
20
10
СNH3, mol/L
Fig. 4. Extraction degree of copper(II) for compound (7) vs. pH values and NH 3
concentration. С = 0,05 mol/L (1 – hexane, 2 – kerosene, 3 - o-xylene),
СCu(II) = 510-3 mol/L, Vо:Va = 1:2,  = 5 min.
As is apparent, most broad interval of its maximal extraction
(from pH 4.5 to 7 mol/L NH3) is observable during extraction assisted
with hexane (pattern 1). The most narrow interval of extraction (from
pH 4.5 to 2 mol/L NH3) is observable for aromatic solvent o-xylene
(pattern 3). Interval from pH  4.5 to 4 mol/L NH3 is observable for kerosene; its narrowing–as compared with hexane–can be explained by
availability of impurities of aromatic nature.
Methods enabling to shift equilibrium, saturation, isomolar series
have evinced that compounds under investigation, just as benzoic acid
N, N-dialkylhydrazides, form with copper(II) a complex of
[Cu(II)]:[reagent]=1:2 ratio. With use of the ion exchange method, the
complex was ascertained to have no charge. Thus, extraction of copper(II) from ammoniac solutions with use of reagents under investigation can be presented as follows:
[Cu(NH3)4]2+(a) + 2HL(о)  CuL2(о) + 2NH3(a) + 2NH4+(a)
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(1)
Here, (a) and (o) stand for the aqueous and organic phases,
respectively.
Extraction constant serves as a quantitative parameter for
extraction capacity of compounds. Extraction constants of copper(II) for
reagents under investigation were calculated by means of equation (1).
The obtained findings are summarized in Table 3. This Table contains
also extraction constant values of copper(II) for a commercial extractant
of -diketone class (Lix 54).
Table 3. Values of lgKex for compounds (3-7) (Р = 0,95).
HL
3
4
5
6
7
Lix 54
Solvent
o-xylene
o-xylene
kerosene
kerosene
kerosene
kerosene
lgKex
-0,5±0,1
3,6±0,1
4,2±0,1
4,5±0,2
4,2±0,1
2,38
n
7
7
7
5
5
-
Р – confidence level, n – number of measurements
As is apparent from the Table, compounds (5), (6), and (7) most
efficiently extract cations of copper(II). Logarithm value of extraction
constant obtained under similar experimental conditions for Lix 54
equals 2.38 [9]. Thus, the compounds under investigation show evidence
of being more efficient reagents to extract copper(II) from ammoniac
media, as compared with Lix 54.
1.
2.
3.
4.
5.
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