1. INTRODUCTION
Gold is a highly valued metal with excellent chemical and electrical properties, making
it a popular choice for use in jewelry and electronic devices [1,2]. As the demand for gold, it has become increasingly important to recover it from
secondary resources. The first step in the recovery process is to dissolve the gold
present in the scrap. Sine the oxidation potential is very low of gold this step requires
the use of oxidizing agents in HCl solutions. Several separation methods have been
employed to recover Au(III) from the leaching solutions, including solvent extraction
[3,4], ion exchange [5], reduction [6], and precipitation [7]. Among these methods, solvent extraction is the most effective and cost-efficient
method for separating Au(III) from solutions containing other metal ions.
Gold (III) in concentrated hydrochloric acid solutions forms AuCl4− when the concentration of chloride ions is higher than 0.1 M [8]. To selectively separate gold (III) from other metal ions in HCl solutions, various
types of acidic (Cyanex 272 and LIX 63), neutral (TBP, DBC and MIBK), and basic (Aliquat
336) extractants have been used [9-12]. However, the selectivity of TBP, MIBK and Aliquat 336 for gold (III) from concentrated
HCl solutions is not good, resulting in low gold (III) from other metal ions due to
the co-extraction of impurities. In general, noble metal ions present in the etching
solutions of printed circuit boards and electroplating solutions can be separated
from other base metal ions by cementation with copper metal [13]. In this case, it is necessary to separate gold(III) ions from the leaching solutions
of the cemented copper, which contain other noble metal ions. According to our studies
on the solvent extraction of Au(III), Cyanex 272 is the most effective among the above-mentioned
extractants in separating Au(III) from the HCl solutions containing Pt(IV) and Pd(II)
[8,11,14].
Some data on the solvent extraction of Au(III) by Cyanex 272 have been reported but
the extraction reaction has not been identified. McCabe-Thiele diagram is generally
employed to predict the number of stages necessary to extract certain amount of target
metal ions in multi-stage counter-current extraction. The extraction isotherm can
be obtained by either experiments or calculation. In order to calculate the extraction
isotherm, extraction reaction together with its equilibrium constant is necessary.
In this work, the extraction reaction of Au(III) by Cyanex 272 from HCl solutions
was identified and its equilibrium constant was estimated.
For this purpose, the extraction data was obtained by varying the concentrations of
Au(III), HCl and Cyanex 272. Slope analysis method was applied to the extraction data
in order to propose the extraction reaction. Moreover, Fourier-transform infrared
spectroscopy (FT-IR) was compared between fresh and loaded Cyanex 272 to investigate
some change in its chemical structure during extraction. Nuclear magnetic resonance
(NMR) spectroscopy was also analysed to investigate the molecular level interactions
between Cyanex 272 and the extracted Au(III) ions.
2. EXPERIMENTAL
2.1 Chemicals and Reagents
To prepare the synthetic solutions of Au(III), HAuCl4 (Sigma-Aldrich, 30wt% in dilute HCl) was dissolved in hydrochloric acid solutions
(Daejung Co., 35%). Cyanex 272 (Solvay Cytec Industries, 85%) was used without purification,
while reagent grade kerosene (Daejung Co.) and chloroform-D (CDCl3, Sigma-Aldrich, 99.8%) served as diluents. Analytical grade reagents were used for
all experiments. H3PO4 (Daejung Co., 85%) was used as a standard in the analysis of the MNR spectra measurement.
All reagents used were of analytical grade.
2.2 Procedure
The extraction experiments involved shaking organic and aqueous phases in a 100 mL
screwed cap bottle using a Burrell wrist action shaker (model 75, USA) at ambient
temperature (22 ± 1°C) for 30 minutes. The phase ratio of the two phases was fixed
at unity. After the reaction, the mixed solutions were left on a separatory funnel
to separate the aqueous and organic phases. The concentration of Au(III) in the aqueous
solution before and after extraction was measured by ICP-OES (Inductively coupled
plasma-optical emission spectrometry, Spectro Arcos, Germany). The concentration of
Au(III) extracted into Cyanex 272 was calculated using mass balance. The extraction
percentage [15] of Au(III) (%E) was defined by Eq. (1).
where eo and er are the mass of Au(III) ions in the aqueous solution before and after extraction,
respectively. The distribution coefficient (D) of Au(III) by Cyanex 272 was calculated
as the ratio
where [Au(III)]org and [Au(III)]aq are the total concentration of Au(III) in the organic and aqueous phases after extraction.
The FT-IR spectra of Cyanex 272 was analyzed by FT-IR Spectrometer (Vertex 80V, Bruker)
[16] with the method of attenuated total reflectance (ATR). NMR spectra measurements
of Cyanex 272 were performed on an ECZ-500 (Broker, JEOL) at room temperature and
at 600 MHz for 31P. For the 31P measuring, CDCl3 was used as a diluent, while H3PO4 was used as an internal chemical shift reference to indicate the difference in the
resonance frequencies of the atoms [17].
3. RESULTS AND DISCUSSION
3.1 Effect of HCl and Cyanex 272 concentration on the extraction of Au(III)
Au(III) is considered a hard acid and thus has a strong tendency to form complexes
with chloride ions. As a result, in HCl solutions where the concentration of HCl is
higher than 0.1 M, Au(III) is present as Cl4− [8,18]. Various studies have reported that Caynex 272 can selectively extract Au(III) from
concentrated HCl solutions over other metal ions [8,13,14]. Since Cyanex 272 is a weak organic acid, it would not be dissociated in concentrated
HCl solutions. Therefore, Cyanex 272 would take part in the extraction of Au(III)
as a molecule from concentrated HCl solutions.
Slope analysis method is generally employed to identify the extraction reactions.
In order to apply solpe analysis method, equilibrium data on the extraction of the
metal ions are necessary. Therefore, the effect of Au(III), HCl and Cyanex 272 concentrations
on the extraction of Au(III) was investigated. First, Au(III) solutions with concentrations
of 0.1, 0.5 and 1.0 g/L were prepared by varying the concentration of HCl in the solutions
from 1 to 9 M.
Fig 1 represents the variation in the percentage of Au(III) extraction with HCl concentrations,
for several Au(III) concentrations, using 0.05 M Cyanex 272. The extraction percentage
increased quickly when HCl concentration increased from 1 to 5 M, then slowly increased
as HCl concentration increased to 9 M. Additionally, the extraction percentage of
Au(III) was affected by the initial concentration of Au(III), and was at its minimum
when Au(III) concentration was 1.0 g/L. In Fig 2, it is clear that the dependence of Au(III) extraction on Cyanex 272 concentration
was affected by HCl concentration. The extraction percentage of Au(III) became higher
when HCl concentration increased from 1 to 9 M. Furthermore, when HCl concentrations
were 1 and 3 M, the extraction percentage of Au(III) was almost linearly increased
with Cyanex 272 concentration. However, when HCl concentrations were 5, 7 and 9 M,
lower concentration of Cyanex 272 was sufficient to extract most of the Au(III).
Fig 2 clearly indicates that HCl concentration is an important variable in the extraction
of Au(III) by Cyanex 272 from HCl solutions.
3.2 Application of slope analysis method to the extraction data of Au(III) by Cyanex
272
Cyanex 272 would not dissociate in concentrated HCl solutions. Therefore, Cyanex 272
molecule would react with Cl4− from concentrated HCl solutions. According to our previous work [13], Cyanex 272 can completely separate Au(III) from other metal ions such as Pd(II)
and Cu(II) present in an HCl solution. Moreover, Xing et al. [8,9] also reported that Cyanex 272 can selectively extract Au(III) over either Pd(II)
and Pt(IV) or Zn(II) and Ni(II) from the HCl solutions. When Au(III) is extracted
by Cyanex 272 through solvating mechanism, electrically neutral species of Au(III)
can be extracted. There might be two kinds of electrically neutral species of Au(III)
in HCl solutions, AuCl3 and HAuCl4 which can be formed by the interaction between AuCl4 and hydrogen ions. Therefore, the extraction reaction of Au(III) by Cyanex 272 through
solvating mechanism can be represented as the following two equations.
where n represents the number of moles of Cyanex 272 (HA) that takes part in the reaction.
Our extraction results indicated that the extraction percentage of Au(III) was increased
with HCl concentration. Considering that the mole fraction of AuCl3 would be decreased with HCl concentration and hydrogen ions take part in the extraction
reaction of Cl4−, Eq. (4) can better represent the extraction reaction of Au(III) by Cyanex 272 from HCl solution
through solvating mechanism. After inserting the definition of the distribution coefficient
of Au(III) into the equilibrium constant of Eq. (4) and rearrangement of the resulting equation would lead to the following equation.
In Eq. (5), K represents the equilibrium constant of Eq. (4)
and [H+] and [HA] represent the equilibrium concentration of hydrogen ion and Cyanex 272
after extraction. The equilibrium concentrations of the reactants can be calculated
by using the mass balance as follows
In the above equations, subscript initial means the initial concentration. When the
initial concentrations of HCl and Cyanex 272 are in excess of that of Au(III), the
equilibrium concentrations of hydrogen ion and Cyanex 272 might be equal to the initial
concentrations of hydrogen ion and Cyanex 272.
Fig 3 illustrates the logarithmic plot of HCl concentration against the logarithmic distribution
coefficient of Au(III), with a fixed Cyanex 272 concentration of 0.05 M. All three
concentrations of Au(III) had a slope of one, which matches Eq. (5). This result indicates that one mole of hydrogen ions is required to extract one
mole of Au(III). On the other hand, Fig 4 depicts the logarithmic plot of Cyanex 272 concentration against the logarithmic
distribution coefficient of Au(III) for various HCl concentrations, with a fixed initial
concentration of Au(III) at 0.1 g/L. The slopes of the plots were between 2.2 and
2.9, indicating that at least two moles of Cyanex 272 are necessary to extract one
mole of Au(III).
3.3 Determination of the loading capacity of Cyanex 272 for Au(III)
The slopes of the plots in Fig 4 were in the range between 2.2 and 2.9. Therefore, in order to determine the number
of moles of Cyanex 272 participating in the extraction of one mole of Au(III), loading
capacity of Cyanex 272 for Au(III) was measured. For this purpose, 0.01 M Cyanex 272
was consecutively contacted with 0.1 g/L Au(III) solution until the Au(III) in the
loaded organic was saturated. In these experiments, the concentration of HCl was fixed
at 5 M.
Fig 5 shows the cumulative concentration of Au(III) in the loaded Caynex 272 after each
stage of contact. After 7th stage, Au(III) concentration in the loaded Cayenx 272
was saturated. The loading capacity of 0.01 M Cyanex 272 was determined to be 0.07
g/L Au(III). At the loading capacity of Cyanex 272, the molar ratio of Cyanex 272
to Au(III) was about 30, indicating that molecular HCl can be extracted by Cyanex
272. Since the mole fraction of molecular HCl would be proportional to HCl concentration,
it might be said that the loading capacity of Cyanex 272 for Au(III) would be decreased
with HCl concentration.
Based on the slope analysis of the extraction data and loading capacity of Cyanex
272, the following reaction was proposed as the extraction reaction of Au(III) by
Cyanex 272 from HCl solutions.
Eq. (8) indicates that the plot of log [H+] + 2log [HA] vs. log D would result in a straight line with a slope of unity.
Fig 6 represents these plots for several concentrations of Au(III). The values of the slope
of these plots were unity, verifying that Eq. (8) is responsible for the extraction of Au(III) by Cyanex 272 from HCl solution. The
intercept of Fig 6 is equal to the equilibrium constant of Eq. (8). The equilibrium constant of Eq. (8) was estimated to be 2.7 by taking the mean of the intercept values of Fig 6.
3.4 Analysis of FT-IR spectra data
There might be some change in the FT-IR of Cyanex 272 when some reactions occur at
the phosphinic acid of Cyanex 272. In this case, it is necessary to investigate the
specific vibrational characteristic bands of Cyanex 272 before and after the reaction.
Therefore, the FT-IR of fresh Cyanex 272 and loaded Cyanex 272 was compared and is
represented in Fig 7.
Table 1 lists the characteristic vibration bands of the fresh Cyanex 272 and the loaded Cyanex
272. The absorption bands at 2953.8-2870.8 cm−1 are assigned to the hydrogen bond of the dimeric Cyanex 272. The absorption band
at 1465 cm−1 corresponds to the P-O stretching vibration of fresh Cyanex 272. This P-O vibration
band shifts to little higher wave number at 1466 cm−1 for the loaded Cyanex 272. This might indicate the occurrence of coordination reactions
between the P-O group and the extracted Au(III). To investigate this kind of interaction,
NMR spectra will be analysed in next section. Meanwhile, the characteristic peak of
P-O-C in the fresh Cyanex 272 was 1170.6 cm-1, which was nearly same to the P-O-C peak (1170.5 cm-1) of the loaded Cyanex 272. The intensities of the P-O-H stretch band at 957.8 cm-1 of the loaded Cyanex 272 was almost similar to that of the fresh Cyanex 272 (957.9
cm-1) [19-21].
There was negligible change in the characteristic vibration bands of P=O, P-O-C and
P-O-H between fresh and the loaded Cyanex 272, indicating that there was no change
in the chemical structure of Cyanex 272 during the extraction of Au(III) from HCl
solutions. Therefore, it can be said that Cyanex 272 take part in the solvent extraction
of Au(III) as a molecule.
3.5 Analysis of NMR spectra data
A characteristic of organophosphorus acidic extractants like Cyanex 272 is that they
form stable dimer in non-polar solvents [22]. When the concentration of chloride ion is high, metal chloride complexes can be
extracted by organophosphorus acidic extractants through either outer sphere assemblies
or ion pairs as represented in Eq. (8). The phosphinic acid (-PO2) group of Cyanex 272 can form coordinate bond with gold ions. A single Cyanex 272
molecule has several coordination sites, allowing it to coordinate up to four gold
ions simultaneously. This results in the formation of complex with tetrahedral geometry
which is stable due to the optimal overlap of orbitals and efficient sharing of the
electron pairs between Cyanex 272 and the gold ion [23].
The molecules or atoms absorb the electromagnetic waves of certain energy in a specific
external magnetic field, reflecting the structural information of the molecule of
interest [24]. To determine the chemical shifts of the P atoms, 31P NMR spectroscopy was employed.
Figs 8(a) and (b) display the 31P NMR spectra of fresh and loaded Cyanex 272. The spectra of fresh Cyanex 272 shows
large resonances at 50.8 and 60.4 ppm for the phosphorus atom, and small peaks at
35.4, 4.6, and 0.5 ppm, which might be ascribed to the presence of some impurities.
However, when Au(III) was extracted into Cyanex 272, the signals of the phosphorus
atoms at 50.8 and 60.4 ppm disappeared in the complex spectra. This indicates that
Au(III) ions form coordination bonds with the phosphorus atom of Cyanex 272 [25-27]. The FT-IR spectra showed that the chemical structure of Cyanex 272 remained unchanged
during the extraction of Au(III). Based on the results obtained from FT-IR and NMR
spectra, it can be said that the extraction mechanism of Au(III) by Cyanex 272 occurs
through the solvating mechanism, and Au(III) ions form coordinative bonds with the
phosphorus atom of Cyanex 272.
4. CONCLUSIONS
The extraction reaction of gold(III) from hydrochloric acid solution by Cyanex 272
was studied and identified. Among several variables like the concentrations of Au(III)(0.1-1
g/L), HCl(1-9 M) and Cyanex 272(0.01-0.1 M), the effect of HCl concentration on the
extraction of Au(III) was the most pronounced. The extraction percentage of Au(III)
was increased as the concentration of HCl and Cyanex 272 increased. By applying slope
analysis method to the extraction data, the extraction reaction of Au(III) by Cyanex
272 from concentration HCl solution was proposed as
Cl4− (aq) + H+ (aq) + 2HA(org) = [HAuCl4·2HA](org)
This reaction agreed well with the extraction data that the extraction of Au(III)
increased with the increase of HCl concentration. The loading capacity of Au(III)
by 0.01 M Cyanex 272 was determined to be 0.07 g/L, indicating that Cyanex 272 extract
both electrically neutral Au(III) complexes and HCl. FT-IR spectra showed no significant
change in the functional groups of Cyanex 272 during Au(III) extraction. NMR spectroscopy
revealed a change in the characteristic peaks of the phosphorus atom of Cyanex 272,
indicating the formation of a coordinative bond between Au(III) and Cyanex 272.