Extraction of gold from alkaline cyanide solution by the tetradecyldimethylbenzylammonium chloride/tri-n-butyl phosphate/n-heptane system based on a microemulsion mechanism

Abstract

The mechanism of extraction of gold by tetradecyldimethylbenzylammonium chloride (TDMBAC)/tri-n-butyl phosphate (TBP)/n-heptane solution from an aqueous alkaline cyanide solution was studied by means of extraction equilibrium, Karl Fischer titration, electrical conductivity, FTIR spectroscopy and dynamic laser scattering (DLS). When the gold concentration is lower than 3 g L⁻¹ and the volume percentage of TBP is less than 10%, the plots of the extraction percentage of gold against the molar ratio of [TDMBA⁺] to [Au(CN)₂⁻] and logD–log[TBP]_(o) plot indicated that the stoichiometry of the extracted species is a 1 ∶ 1 ∶ 4 complex, TDMBA⁺ ∶ Au(CN)₂⁻ ∶ TBP. Karl Fischer titration showed that 4 H₂O molecules participate in the formation of such a species. Electrical conductivity measurements confirmed its ionic character. Fourier self-deconvolution of the O–H stretching bands revealed 4 different kinds of water molecules contained in the organic phase, some of which were bound to TBP via hydrogen bonding. A supramolecule [TDMBA⁺] · [Au(CN)₂⁻] · 4H₂O · 4TBP is proposed for the extracted species. Two TBP molecules are bound to [Au(CN)₂⁻] by two H₂O bridges through hydrogen bonding, forming a [(RO)₃PO⋯H–O–H⋯NC–Au–CN⋯H–O–H⋯O = P(OR)₃]⁻ moiety. Two hydrated TBP molecules, (RO)₃PO⋯H–O–H, surround [TDMBA⁺] by ion–dipole interaction. The bulky anion and cation form a lipophilic supramolecule. The possible structure of the supramolecular anion was calculated with an ab initio molecular orbital (MO) method. The DLS study showed that mixing of TDMBAC and Au(CN)₂⁻ in the aqueous phase led to the formation of micelles. When an organic phase containing TBP was added to this aqueous phase, the complexes transferred into the organic phase and reversed micelles or a microemulsion (W/O) were formed when the gold concentration reached a certain limiting value.