Selective Recovery Process for Gold Utilizing a Functional Gel Derived from Natural Condensed Tannin

Abstract

A tannin gel was prepared from condensed tannin (wattle tannin), which is a ubiquitous and inexpensive natural material extracted from the leaves or barks of plants. The tannin gel has the ability to adsorb and reduce the metal ions with a redox potential higher than that of the tannin gel. The selective recovery of Au from a mixed solution of various precious metal ions (Au, Pd, Pt, and Rh) was performed using such features of the tannin gel. The predominant species of each precious metal ion are AuCl4–, PdCl42–, PtCl62–, and RhCl52– at pH 1.0 and pCl 1.0. The redox potential of the AuCl4–/Au couple is much higher than that of the tannin gel, while the other precious metals such as the PtCl62–/PtCl42– couple, PdCl42–/Pd couple, and RhCl52–/Rh couple have redox potentials lower than that of the tannin gel. Under the conditions of pH 1.0 and pCl 1.0, the entire amount of Au in the solution was adsorbed perfectly by the tannin gel, while the other precious metals were hardly adsorbed. The adsorbed Au was reduced to metallic Au on the tannin gel. Thus the selective adsorption and reduction of Au could be achieved by controlling the predominant ions of each precious metal ion in the solution.