Host: The Society of Chemical Engineers, Japan
A synergistic extraction system of trivalent gold ion (Au(III)) with an acidic extractant D2EHPA (di(2-ethylhexyl)phosphoric acid) and a hexadentate ligand TPEN(N,N,N',N'- tetrakis(2-pyridylmethyl)- 1,2-ethylenediamine) was investigated and its applicability to the simultaneous recovery process of noble metals, Pd(II), Pt(II) and Au(III), from waste electric devices was discussed. Since Au(III) is present stably as an anionic complex, AuCl4-, in aqueous chloride solution, Au(III) and noble metals cannot be recovered simultaneously by an acidic extractants such as D2EHPA. However, by the addition of TPEN in amounts equimolar with Au(III) in the aqueous phase, the extractability of Au(III) with D2EHPA was improved drastically. AuCl4- is complexed strongly with TPEN in the aqueous phase and a cationic complex, Au(TPEN)3+ is formed by the ligand exchange between chloride ion and nitrogen donor. The formation constant of Au(TPEN)3+ was evaluated as about 1010 by the potentiometric titration technique. From these results, both the stoichiometry for the synergistic extraction of Au(III) with TPEN and D2EHPA (HR) and its extraction constant were determined as
AuCl4- + TPEN + 3(HR)2 ↔ Au(TPEN)R3(HR)3 + 3H+ + 4Cl- Kex=105.7TPEN acts as a powerful synergist for the extraction of Au(III) with D2EHPA. Thus, by introducing the proposed synergistic extraction system, Au(III) and other noble metals can be recovered simultaneously and effectively from aqueous chloride solution.