Effect of structure of aromatic ethers on their extraction of Au(III) from acidic chloride media

Hydrometallurgy 183 (2019) 207–212

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Effect of structure of aromatic ethers on their extraction of Au(III) from acidic chloride media

T

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Tatsuya Oshima , Takashi Horiuchi, Kiyoharu Matsuzaki, Kaoru Ohe Department of Applied Chemistry, Faculty of Engineering, University of Miyazaki, 1-1 Gakuen Kibanadai Nishi, Miyazaki 889-2192, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Solvent extraction Aromatic ether Precious metal Gold Hydrochloric acid, 1-Dodecoxybenzene

An aromatic ethereal compound, 1-methoxy-2-octoxybenzene, was recently found to be a good extractant for Au (III) from hydrochloric acid media. The present study therefore investigated the structural factors of ethereal compounds for the extraction of Au(III) under dilute extractant concentrations. In the series of aromatic ethereal compounds used in this study, 1-dodecoxybenzene showed the highest extractability for Au(III). There was a clear correlation between the extractability of Au(III) and the strength of hydrophobicity of the aromatic ethereal compounds. Au(III) was selectively extracted using 1-dodecoxybenzene over other precious- and basemetal cations. Au(III) was quantitatively stripped from this extractant using aqueous thiourea solution.

1. Introduction The use of electronic devices has proliferated in recent years and the quantity of these that require disposal is growing rapidly throughout the world (Widmer et al., 2005). Over 300 t of gold are used annually in electronic components, such as integrated circuits, contacts, and bonding wires, so waste electrical and electronic equipment (WEEE) is recognized as an important secondary gold resource (Kang and Schoenung, 2005; Hagelüken and Corti, 2010). A printed circuit board of a portable computer can contain up to 250 g/t Au, which is significantly higher (25–250-fold) than that of primary gold ores (~1–10 g/t Au) (Tuncuk et al., 2012). Processes for the recovery of gold from WEEE have therefore been developed (Cui and Zhang, 2008). Solvent extraction is one of the most important technologies for the separation and recovery of gold from primary and secondary leaching solutions (Syed, 2012). Oxygen-containing compounds act as extractants for Au(III) in hydrochloric acid media. Methyl isobutyl ketone (4-methyl-2-pentanone; MIBK) is one of the most effective extractants for Au(III) (Cox, 1992; Strelow et al., 1966; Branch and Hutchison, 1986; Kargari and Kaghazchi, 2004). Extraction of Au(III) using MIBK has traditionally been the first step in the analysis of noble metals solutions (Diamantatos, 1981). Narita et al. (2005, 2006) reported extraction of Au(III) from hydrochloric acid media using monoamide compounds: selective extraction of Au(III) from solutions containing less than 3.0 M HCl was achieved. Xiong et al. (2010) reported extraction of Au(III) using vacuum pump oil. In this system, it is likely

that silicon oil, which contains oxygen atoms, acted as the extractant for Au(III). More recently, a newly developed ethereal compound, cyclopentylmethyl ether (CPME), was found to act as an extractant for Au (III) (Oshima et al., 2017). Dibutyl carbitol (bis(2-butoxyethyl)ether; DBC), an ethereal compound, is currently the most popular extractant for Au(III) from hydrochloric acid media (Jung et al., 2009): Au(III) is selectively extracted from many precious- and base-metal cations. Typically, more than 99% gold extraction can be achieved and Au(III) can be concentrated into DBC up to 50 g/L (Javanshir et al., 2011). After washing with hydrochloric acid, gold is directly reduced from the loaded organic phase by heating with oxalic acid (Mironov, 2012). Au(III) extraction using DBC is commercially applied in INCO's (currently Vale) operations (Edward and te Riele, 1983; Javanshir et al., 2011). Mironov (2012, 2013) studied extraction behavior of Au(III) across a wide range of Au(III) concentrations using DBC, suggesting that six water molecules were coextracted for each Au(III). Although DBC is a powerful extractant for Au (III), it is gradually lost from the system when continuously used in commercial operation because of its solubility in the aqueous phase: solubility of DBC in water is 2.7 g/L (Mironov, 2012); therefore, development of new extractants for Au(III) is worthwhile. The authors recently reported extraction behavior of Au(III) using a novel aromatic ethereal compound, 1-methoxy-2-octoxybenzene (oMOB), from hydrochloric acid media (Horiuchi et al., 2018). o-MOB showed higher extractability for Au(III) than DBC under dilute conditions. Additionally, o-MOB showed the highest extractability of the

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Corresponding author. E-mail addresses: [email protected] (T. Oshima), [email protected] (T. Horiuchi), [email protected] (K. Matsuzaki), [email protected] (K. Ohe). https://doi.org/10.1016/j.hydromet.2018.12.017 Received 26 July 2018; Received in revised form 6 December 2018; Accepted 19 December 2018 Available online 21 December 2018 0304-386X/ © 2018 Elsevier B.V. All rights reserved.

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180 mmol) were added. To the solution, a dimethyl furan (DMF) solution that contained 24.9 g (100 mmol) 1-bromdodecane was added dropwise and the mixture was stirred for 24 h at 60 °C. After DMF evaporation in vacuo, a chloroform solution of the residue was washed sequentially with 1 mol/dm3 hydrochloric acid, 1 mol/dm3 aqueous sodium hydroxide, and distilled water. After drying over anhydrous sodium sulfate and filtration, the solution was evaporated in vacuo. The resulting viscous liquid DB product had the following properties: 1H NMR (400 MHz, CDCl3, 25 °C) 0.88 (3H, t, CCH3), 1.26 (16H, m, C–CH2–C), 1.44 (2H, m, C–CH2–C–C–O), 1.75 (2H, m, C–CH2–C–O), 3.90 (2H, t, C–CH2–O–Ar), 6.87 (3H, m, AreH), 7.23 (2H, m, ArH–C–O) (Fig. 2). In order to evaluate the degradation of DB at high acidity, the following test was conducted: An aqueous solution (5.0 cm3) containing 5 mol/dm3 HCl was contacted with DB (1.0 cm3) in a stoppered Erlenmeyer flask. After shaking the mixture at 30 °C for 24 h, DB was recovered and 1H NMR spectra of DB before and after contacting with HCl solution were compared. Additionally, aqueous solubility of DB was investigated by contacting an aqueous solution (10 cm3) containing 5 mol/dm3 HCl was contacted with DB (1.0 cm3) at 30 °C for 24 h. After phase separation, the concentration of BD was determined from absorbance at 272 nm using a UV–vis spectrophotometer (Shimadzu UV-2450).

positional isomers. The aromatic monoether, 1-octoxybenzene (OB), also showed high extractability for Au(III), indicating that the coordination of two oxygen atoms is not essential to its extraction. In the present study, a series of aromatic monoether compounds of different alkyl chain lengths was prepared. To study the structural features influencing the extraction of Au(III) from hydrochloric acid media, extractability under dilute conditions was compared using various compounds: aromatic monoethers, aromatic diethers, diphenylether (DPE), the aliphatic ethers DBC and 1-methoxy-2-octoxyethane (MOE), and dodecylbenzene (DBN), which does not contain an ether group. Additionally, extraction of metal ions using the aromatic monoether compound, 1-dodecoxybenzene (DB), which showed the highest extractability, was studied in detail. Stripping of Au(III) from DB was also studied. Systematic studies for the extraction of Au(III) using aromatic ethers have not been conducted to date. 2. Experimental 2.1. Materials Analytical-grade gold(III), palladium(II), platinum(IV), rhodium (III), iron(III), gallium(III), indium(III), cobalt(II), nickel(II), copper(II), and zinc(II) chlorides (Wako Pure Chemical Ind. Ltd., Japan) were used to prepare solutions of the metal ions for extraction tests. The molecular structures of the organic compounds that were used as extractants for Au(III) are shown in Fig. 1. Analytical-grade DBC, methoxybenzene (anisole; MB), DBN, and DPE (Wako Pure Chemical Ind. Ltd., Japan) were used without further purification. o-MOB, 1-methoxy-3-octoxybenzene (m-MOB), 1-methoxy-4-octoxybenzene (p-MOB), OB, and MOE were synthesized according to the procedures described in our previous paper (Horiuchi et al., 2018). DB, 2-ethylhexoxybenzene (2EHB), 1-hexoxybenzene (HB), and 1-butoxybenzene (BB) were synthesized as described in Section 2.2. All other reagents were of analytical grade and were used as received. The logarithm of the partitioning coefficient between n-octanol and water (logP) is a well-known quantitative indicator of the hydrophilic–lipophilic balance. The logP values of the ethereal extractants were estimated by using MarvinSketch 6.2.1 software (ChemAxon Ltd., Budapest, Hungary) and the KLOP method (Klopman et al., 1994).

2.2.2. 2-Ethylhexoxybenzene 2EHB was prepared using phenol and 1-bromo-2-ethylhexane, in a manner similar to that of DB. The resulting 2EHB product had the following properties: 1H NMR (400 MHz, CDCl3, 25 °C) 0.94 (6H, m, CCH3), 1.33–1.56 (8H, m, C–CH2–C), 1.72 (1H, m, C–CH(C2H5)–C), 3.82 (2H, d, C–CH2–O–Ar), 6.90 (3H, m, AreH), 7.26 (2H, m, ArH–C–O). 2.2.3. 1-Hexoxybenzene HB was prepared using phenol and 1-bromohexane, in a manner similar to that of DB. The resulting HB product had the following properties: 1H NMR (400 MHz, CDCl3, 25 °C) 0.90 (3H, t, CCH3), 1.32 (4H, m, C–CH2–C), 1.46 (2H, m, C–CH2–C–C–O), 1.75 (2H, m, C–CH2–C–O), 3.91 (2H, t, C–CH2–O–Ar), 6.88 (3H, m, AreH), 7.24 (2H, m, ArH–C–O).

2.2. Synthesis of ethereal compounds

2.2.4. 1-Butoxybenzene BB was prepared using phenol and 1-bromohexane, in a manner similar to that of DB. The resulting BB product had the following properties: 1H NMR (400 MHz, CDCl3, 25 °C) 0.96 (3H, t, CCH3), 1.47 (2H, m, C–CH2–CH3), 1.74 (2H, m, C–CH2–C–O), 3.91 (2H, t, C–CH2–O–Ar), 6.88 (3H, m, AreH), 7.24 (2H, m, ArH–C–O).

2.2.1. 1-Dodecoxybenzene DB was prepared as follows using phenol and 1-bromododecane, in a manner similar to that of o-OB (Horiuchi et al., 2018). To 50 cm3 DMF, phenol (11.3 g, 120 mmol) and potassium carbonate (24.9 g,

Fig. 1. Molecular structures and abbreviations of ethereal extractants used. 208

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Fig. 2. Synthetic scheme for 1-dodecoxybenzene (DB).

2.4. Stripping of Au(III)

100

Stripping of extracted Au(III) from DB was studied using hydrochloric acid solution containing thiourea or oxalic acid as follows (Horiuchi et al., 2018). Extraction of 0.1 × 10−3 mol/dm3 Au(III) from 1.0 mol/dm3 hydrochloric acid (100 cm3) using a toluene solution (100 cm3) containing 200 × 10−3 mol/dm3 DB was performed in a manner similar to that described in Section 2.3. The organic phase into which Au(III) was extracted was divided into 5.0-cm3 portions, each of which was contacted with a 5.0-cm3 fresh aqueous solution that contained hydrochloric acid (0–5.0 mol/dm3), thiourea (0 or 1.0 mol/dm3), or oxalic acid (0 or 100 × 10−3 mol/dm3). The phases were mixed and shaken at 30 °C for 24 h. The aqueous stripping solution was separated from the organic phase and the stripping percentage (Stripping [%]) was calculated according to Eq. (2):

Extraction [%]

80

60

40 Au(III) 20

Pd(II) Stripping [%] =

20

40

× 100

(2)

where [Au(III)]org,init represents the initial concentration of Au(III) in the organic phase and [Au(III)]aq,eq is the Au(III) concentration in the aqueous phase after stripping.

0 0

[Au(III)]aq, eq [Au(III)]org, init

60

contact time [min] 3. Results and discussion

Fig. 3. Rate of Au(III) and Pd(II) extraction using 1-dodecoxybenzene (DB) in toluene. [Au(III) or Pd(II)] = 0.1 × 10−3 mol/dm3, [DB] = 100 × 10−3 mol/ dm3 (for Au(III)) or 200 × 10−3 mol/dm3 (for Pd(II)), [HCl] = 1.0 mol/dm3 (for Au(III)) or 0.3 mol/dm3 (for Pd(II)).

3.1. Extraction behavior of Au(III) from hydrochloric acid media using aromatic ethers Fig. 3 shows the extraction rate of Au(III) and Pd(II) from hydrochloric acid using DB. The concentrations of DB and HCl for Au(III) and Pd(II) were different to control the equilibrium extraction percentage: [DB] = 100 × 10−3 mol/dm3 or 200 × 10−3 mol/dm3 and [HCl] = 1.0 mol/dm3 or 0.3 mol/dm3 for Au(III) and Pd(II), respectively. Au(III) extraction using DB reached equilibrium within 10 min, which was similar to that using the ethereal compounds DBC, CPME, and o-MOB (Mironov, 2012; Oshima et al., 2017; Horiuchi et al., 2018). During the operation of extraction, reduction of Au(III) to metallic gold Au(0) was not observed at all. Pd(II) extraction was relatively slower and reached equilibrium within 30 min. For operational convenience, the extraction experiments in this study were conducted by contacting both phases for 24 h. From the result of 1H NMR spectra, DB was not degraded at all after contacting with 5.0 mol/dm3 HCl at 30 °C for 24 h. Additionally, solubility of DB in 5.0 mol/dm3 HCl was 1.3 mg/L (5× 10−6 mol/dm3), which is much smaller than that of DBC in water (2.7 g/L) (Mironov, 2012). Hence leak of DB by contacting with aqueous phase is quite small. Fig. 4 shows the extraction profiles of Au(III) from 1.0 mol/dm3 HCl using DB, o-MOB, and DBC as a function of extractant concentration (Horiuchi et al., 2018). DB shows similar extractability for Au(III) to oMOB. Extraction using 200 × 10−3 mol/dm3 DB was 98.6%. This result showed that the monoethereal DB could extract Au(III) and the presence of two ethereal oxygens is not an essential factor for Au(III) extraction. Under dilute conditions, the extractability of DBC for Au(III) was much lower than that of DB and o-MOB. Extraction using 200 × 10−3 mol/dm3 DBC was only 2.2%. Under dilute conditions, the most popular Au(III) extractant DBC was not effective. It is likely that Au(III) was extracted as hydrogen tetrachloroaurate(III) (HAuCl4) using

2.3. Liquid–liquid extraction tests Liquid–liquid extraction tests were conducted batchwise with a typical procedure as follows (Horiuchi et al., 2018). An aqueous solution containing a 0.1 × 10−3 mol/dm3 metal ion (Au(III), Pd(II), Pt(IV), Rh (III), Fe(III), Al(III), Ga(III), In(III), Co(II), Ni(II), Cu(II), or Zn(II)) was prepared in HCl. The initial concentration of hydrochloric acid was adjusted to 0.1–8.0 mol/dm3. An extracting solution containing a 200 × 10−3 mol/dm3 extractant (DB, 2EHB, HB, BB, MB, or DBN) in toluene was prepared. Equal volumes (10 cm3) of the aqueous and extracting solutions were mixed in a stoppered Erlenmeyer flask and shaken (120 rpm) in a thermostatted water bath at 30 °C for 24 h. After phase separation, the concentration of hydrochloric acid in the aqueous solution was determined by acid–base titration using an automatic potentiometric titrator (Kyoto Electronics Manufacturing AT-700). Metal-ion concentrations in the aqueous solutions before and after extraction were determined using a polarized Zeeman atomic absorption spectrometer (HITACHI Z-2310) or inductively-coupled plasma atomic emission spectrometer (Shimadzu ICPS-8100). The extraction percentage of a specific metal ion was calculated according to Eq. (1):

Extraction [%] =

[M]org, eq [M]aq, init

× 100 =

[M]aq, init − [M]aq, eq [M]aq, init

× 100

(1)

where [M]aq,init and [M]aq,eq represent the initial and equilibrium concentrations of the metal ion in the aqueous phase, respectively; [M]org,eq is the total concentration of metal ion in the organic phase at equilibrium. The results were compared with those using 200 × 10−3 mol/ dm3 of extractants (o-MOB, m-MOB, p-MOB, OB, MOE, DPE, or DBC), as previously reported (Horiuchi et al., 2018). 209

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100

100 Extraction of Au(III) from 1.0 M HCl [%]

DB

Extraction [%]

80

60

40

DB

2000

Fig. 4. Effect of extractant concentration on the extraction of Au(III) using 1dodecoxybenzene (DB), 1-methoxy-2-octoxybenzene (o-MOB), and dibutylcarbitol (DBC) in toluene (Horiuchi et al., 2018): [Au (III)] = 0.1 × 10−3 mol/dm3, [HCl] = 1.0 mol/dm3.

DBC, with co-extracting six water molecules (Mironov, 2012, 2013). Probably the ethereal compounds DB and o-MOB would extract Au(III) as AuCl4− with H+. To study structural features of the extractants, extraction behaviors of Au(III) using 200 × 10−3 mol/dm3 of various organic compounds were compared as a function of hydrochloric acid concentration (Fig. 5). There was a close correlation between the extractability of Au (III) and the alkyl chain length of monoethereal compounds. DB showed the highest extractability for Au(III) of the variety of organic compounds used in this study. Greater than 97% extraction was achieved using DB across all HCl concentrations employed ([HCl] = 0.1–7.7 mol/dm3). In the series of monoethereal compounds, the extractability of OB was second to that of DB. In contrast, the other aromatic monoether compounds tested (2EHB, HB, BB, MB, and DPE) did not extract Au(III) under these conditions. The alkyl chain length of

DB OB 2EHB

80 Extraction [%]

HB BB

60

MB o-MOB

40

m-MOB p-MOB

20

DPE 0 -1.5

MOE -1

-0.5

0

log[HCl]eq

0.5

1

MB o-MOB

40

m-MOB

aromatic diethers

p-MOB 20

DPE MOE

DBC 3

5

7

9

DBN

monoethereal compounds, which significantly influences their hydrophobicity, was therefore found to be one of the most important factors influencing Au(III) extraction. As previously reported (Horiuchi et al., 2018), the extractability of Au(III) by positional isomers of aromatic diether was o-MOB > p-MOB > m-MOB. Under higher HCl concentrations, DB showed higher extractability for Au(III) than o-MOB; hence, more hydrophobic monoether compounds showed higher extractability than diether compounds. The alkylbenzene compound DBN did not extract Au(III) at all, indicating that at least one ethereal oxygen was necessary for the extraction of Au(III). The aliphatic ethereal compounds, MOE and DBC, did not extract Au(III) under dilute conditions (Horiuchi et al., 2018). Because the hydrophobicity of the ethereal extractant is an important factor for Au(III) extraction, this was evaluated using logP as the indicator. The relationship between logP of the extractants and their extent of extraction of Au(III) from 1.0 mol/dm3 HCl is shown in Fig. 6. Relatively less hydrophobic aromatic monoethers did not extract Au (III): 2EHB (logP = 5.08), HB (logP = 4.25), BB (logP = 3.31), and MB (logP = 1.92). The logP values of DB (logP = 7.07) and OB (logP = 5.19), which showed good extractability of Au(III), were relatively high. The extractability of OB was much higher than that of 2EHB, although they have same molecular mass and similar logP values. There seemed to be a critical difference between OB and 2EHB for the extraction of Au(III); however, the mechanism is not yet clear. The electron density on the ethereal oxygen may increase with the extension of alkyl chain. Such the additional factors would influence the extractability of aromatic ethers. The molecular mass (236.4 g/mol) and logP value calculated by the KLOP method (logP = 5.05) for the positional isomers of aromatic diethers o-MOB, p-MOB, and m-MOB were the same, but their extractabilities for Au(III) were quite different: o-MOB (94.7% extraction) > p-MOB (34.9%) > m-MOB (21.5%). On the basis of the results for these monoether and diether compounds, there must be a critical point for the logP value, of around 5, for extraction of Au(III). The number of ether groups is also important: the alkylbenzene DBN was very hydrophobic (logP = 8.2), but did not extract Au(III) at all. o-MOB (logP = 5.05; extraction = 94.7%) showed higher extraction than OB (logP = 5.19; extraction = 85.9%), indicating that the diether was slightly advantageous for Au(III) extraction over the monoether. DPE (logP = 3.62), MOE (logP = 3.54), and DBC (logP = 3.11) are less hydrophobic and did not extract Au(III) under these conditions.

[Extractants] [mM]

100

BB

60

Fig. 6. Relationship between logP of extractant and extraction percentage of Au HCl. [Au(III)] = 0.1 × 10−3 mol/dm3, (III) from 1.0 mol/dm3 [Extractant] = 200 × 10−3 mol/dm3.

0 1500

aromatic monoethers

logP of extractant [-]

DBC

1000

HB

1

20

500

2EHB

0

o-MOB

0

OB 80

DBC DBN

3.2. Extraction behavior of metal ions from hydrochloric acid media using DB

Fig. 5. Extraction behavior of Au (III) using various extractants in hydrochloric acid media (Horiuchi et al., 2018): [Au(III)] = 0.1 × 10−3 mol/dm3, [Extractant] = 200 × 10−3 mol/dm3.

Because DB showed the highest extractability for Au(III) of the 210

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100

1

Au(III) Pd(II) Pt(IV)

0.8

Rh(III) 60

[Au(III)] org.eq [mM]

Extraction [%]

80

Fe(III) Ga(III)

40 In(III) Co(II) 20 Ni(II)

0.6

0.4

Cu(II) 0 -1.5

-1

-0.5

0

0.5

1

0.2

Zn(II)

log[HCl]eq

Fig. 7. Effect of hydrochloric acid concentration on the extraction of metal ions using 1-dodecoxybenzene (DB): [metal] = 0.1 × 10−3 mol/dm3, [DB] = 200 × 10−3 mol/dm3.

0 0

2

4

6

8

10

12

[Au(III)]aq.init [mM]

series of aromatic ethers, its selectivity for various metal ions was studied. Fig. 7 shows extraction profiles of various metal ions using DB as a function of hydrochloric acid concentration. Au(III) was quantitatively extracted under wide range of hydrochloric acid concentrations. In a previous study, extraction of Au(III) using o-MOB decreased with increasing HCl concentration (Horiuchi et al., 2018); in contrast, extraction of Au(III) using DB did not decrease with increasing HCl concentration. The extraction profile of Pd(II) using DB was quite different to that using o-MOB. Extraction of Pd(II) from 0.1 mol/dm3 HCl using DB was 88.0%. Extraction of Pd(II) decreased with increasing HCl concentration; hence, DB can separate Au(III) from Pd(II) only under higher HCl concentrations. In contrast, the extraction of Pd(II) using oMOB was very small (Horiuchi et al., 2018). The extraction of Pt(IV) using DB was also higher than that using o-MOB. The extraction was 68–73% in 0.1–4.76 mol/dm3 HCl and decreased in 6.9 mol/dm3 HCl. The extraction of Rh(III) was small (5.7–10.4%) and independent of HCl concentration. The trivalent metal ions Fe(III) and Ga(III) are present as anionic chloride complexes under higher HCl concentration (Martell and Smith, 1974), so their extraction increased with increasing HCl concentration. The extraction behaviors of the anionic chloride complexes of Fe(III) and Ga(III) were similar to those using ethereal compounds, such as CPME and o-MOB (Oshima et al., 2017; Horiuchi et al., 2018). In(III) was not extracted from 0.1–1.0 mol/dm3 HCl, but was slightly extracted as the HCl concentration increased. The divalent metal ions, Co(II), Ni(II), and Cu(II), were not extracted at all; Zn(II) was slightly extracted from 1–5 mol/dm3 HCl. The extraction capacity of Au(III) using 200 × 10−3 mol/dm3 DB was studied as a function of Au(III) concentration (Fig. 8). The concentration of extracted Au(III) increased with an increase of initial concentration of Au(III) in the aqueous phase, but saturated at 0.43 × 10−3 mol/dm3. The extraction capacity is much smaller than the concentration of DB (200× 10−3 mol/dm3). The small extraction capacity using DB is much smaller to that (3.88 × 10−3 mol/dm3) using o-MOB shown in the previous study (Horiuchi et al., 2018), nonetheless the extractability of DB is higher than that of o-MOB as shown in Fig. 5. The commercially employed extractant DBC can load 50 g/dm3 (250 × 10−3 mol/dm3) Au(III) (Mironov, 2013; Javanshir et al., 2011). Dilute DB showed higher extractability than dilute DBC, as shown in Fig. 4; however, the extraction capacity of dilute DB is low for practical use.

Fig. 8. Effect of Au(III) concentration using 1-dodecoxybenzene (DB) in hydrochloric acid media: [DB] = 200 × 10−3 mol/dm3, [HCl] = 1.0 mol/dm3.

Table 1 Stripping of Au(III) from 1-dodecoxybenzene (DB). (III)]ini = 0.1 × 10−3 mol/dm3, [DB] = 200 × 10−3 mol/dm3.

[Au

Stripping reagent

Stripping [%]

Distilled water 0.01 M HCl 0.1 M HCl 1.0 M HCl 5.0 M HCl 0.1 M HCl + 1.0 M Thiourea 1.0 M HCl + 1.0 M Thiourea 5.0 M HCl + 1.0 M Thiourea 10 mM (COOH)2 100 mM (COOH)2 100 mM (COOH)2 + 0.1 M HCl 100 mM (COOH)2 + 1.0 M HCl

1.8 0.3 0.6 1.4 3.1 100 100 100 4.9 56.9 0 1.24

3.3. Stripping of Au(III) Table 1 shows the stripping of Au(III) extracted using DB for various stripping reagents. Because the extraction efficiency for Au(III) using DB is high, as shown in Figs. 4 and 5, stripping using distilled water and hydrochloric acid solution was quite low. Au(III) was quantitatively stripped from the loaded organic phase by contact with a stripping solution that contained thiourea (Xiong et al., 2010). Oxalic acid is used for reductive recovery of Au(III) extracted using DBC (Mironov, 2012): Au(III) was partly stripped from DB using 100 × 10−3 mol/dm3 oxalic acid (56.9%). 4. Conclusions From the results of this study, Au(III) extraction from hydrochloric acid media improved with increasing hydrophobicity of aromatic ether extractants. The alkyl chain length of the aromatic ether significantly altered the extractability of Au(III). The aromatic diether, o-MOB, was also a good extractant for Au(III), indicating that the presence of two ether groups is not an essential feature of good extractants. DB was the most hydrophobic in a series of aromatic ethereal compounds and showed the highest extractability for Au(III). Under dilute conditions, 211

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DB and o-MOB showed higher extractability than the commercially available extractant DBC. Selective extraction of Au(III) from various metal ions in hydrochloric acid media was possible using DB; however, the extraction capacity for Au(III) using dilute DB was very low. A study of Au(III) extraction in hydrochloric acid media using aromatic ethereal compounds under higher concentration conditions is underway.

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