Selective recovery of Au(III), Pt(IV), and Pd(II) from aqueous solutions by liquid–liquid extraction using ionic liquid Aliquat-336
Graphical abstract
Introduction
In recent years, precious metals (PMs) including gold, platinum, palladium, silver, and rhodium have been widely used in many industrial areas such as jewelry, corrosion-resistant materials, chemical catalysts, and various chemical processes, owing to their specific properties such as lustrous, ductile, noncorrosive and high stability [1], [2], [3], [4], [5]. However, the limited resource and endless demand of PMs in industrial spheres have led to the urgent recovery of these noble metals from related PMs-loaded wastewaters. A variety of technologies such as solvent extraction [6], [7], adsorption [8], [9], ion exchange [10], precipitation [11], and electrowinning [12] have been extensively used in the recovery of PMs from their aqueous effluents. Among them, the solvent extraction process has been receiving considerable attention and eliciting widespread interest in the past few decades owing to its fast extraction rate and high loading property [7], [13], [14], [15], [16], [17]. Nonetheless, the process has a difficulty of simultaneously separating and purifying PMs from multi-metal mixtures owing to their similar physical and chemical properties [1], [18]. Thus, developing effective extraction processes for the recovery and separation of PMs to obtain high purity metal solutions is necessary.
A key point for the effective application of the solvent extraction for selective recovery of PMs is to find an appropriate extractant and to design suitable separation procedures. For this purpose, ionic liquids (ILs) have recently attracted attentions instead of traditional extractants for recovery of metals owing to their specific characteristics such as negligible vapor pressures with high thermal stabilities and tunable viscosities, and their good properties towards metals [4], [15], [19]. For example, a model IL Aliquat-336, tricaprylylmethylammonium chloride, (a hydrophobic quaternary ammonium salt ([R3NCH3]+ Cl−)) showed good extraction capacity for metals. Nonetheless, most studies focused on the single metal extraction by using Aliquat-336 [20], [21], [22], [23], [24], [25]. It must be noted that in some studies Aliquat-336 had been combined with other extractants in order to separate PMs [14]. However, the more extractants use, the higher separation cost is. To date, less effort was made to selectively extract and separate targets PMs solely by using Aliquat-336.
In this study, therefore, IL Aliquat-336 was chosen as a model extractant and its selective extraction performance was investigated in a ternary-metal solution containing Au(III), Pt(IV), and Pd(II). Furthermore, an effective sequential extraction process only with one extractant of Aliquat-336 was successfully designed to separate Au(III)/Pt(IV)/Pd(II) and obtain high purity of each metal solution. Therefore, the present study suggested a new possibility to separate Au(III), Pt(IV), and Pd(II) using appropriate Aliquat-336 concentrations through the designed flow sheet.
Section snippets
Materials
Multi-metal solutions were prepared by dissolving certain amounts of H2PtCl6 ⋅ 5.5H2O (Kojima Chem., Korea), PdCl2 (Kojima Chem., Korea) and HAuCl4 ⋅ 3.6H2O (Kojima Chem., Korea) in 0.1 M hydrochloric acid. Aliquat-336 (Sigma-Aldrich, Korea) and benzene (Samchun Pure Chem., Korea) were used as the solvent extractant and diluent, respectively. All other chemicals were of analytical grades, and purified water supplied by deionized water equipment (Millipore/direct-Q3UV, USA) was used throughout the
Factors affecting the selective extraction of Au(III)
For optimizing the selective extraction process, the effects of parameters such as phase contact time, pH, concentration of Aliquat-336, and initial metal concentration were investigated. The extraction efficiency (E) and selectivity coefficient (αsel) were used as the measures of the relative fitness of selective performance towards Au(III), and the results are discussed as follows.
Conclusions
In this study, IL Aliquat-336 was investigated for selective extraction of Au(III) from multi-metal solutions. The results of this study indicated that Aliquat-336 had a good selective extraction performance to Au(III) under the optimum extraction conditions. Furthermore, a process flow sheet was developed for the separation and recovery of Au(III), Pt(IV), and Pd(II) from their mixtures, and high purity of metal solutions were obtained by the designed process. Therefore, these results
Acknowledgments
This work was supported by the Korean Government through NRF (2014R1A2A1A09007378) grant.
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