NEUTRAL BIFUNCTIONAL PHOSPHONATES AS REAGENTS FOR SOLVENT-EXTRACTION OF AU(III) - SELECTIVE RECOVERY OF GOLD FROM HYDROCHLORIC-ACID SOLUTIONS USING TETRABUTYL 1,3-XYLYLENEDIPHOSPHONATE

One new and three known derivatives of dibutylphosphonic acid, namely, the alpha,alpha'-bis(dibutylphosphonato)-1,3-xylene and bis(dibutylph osphonato)alkanes, commonly known as the tetrabutyl 1,3-xylylenediphos phonate (TBmXDP) and tetrabutyl alkylenediphosphonates (TBADP), respec tively, were synthesized for use in the solvent extraction of precious metals. From 0.1M (1M=1 mol dm(-3)) NaCl solution at pH 3, these bifu nctional reagents substantially extract Au(III), Cu(II) and Sm(III) in 1,2-dichloroethane (DCE), where the extent of extraction as well as t he extraction selectivity is governed by the length of the hydrocarbon chain joining the two P=O groups in the diphosphonate molecule. Au(II I) extraction from hydrochloric acid medium was studied in detail by u sing tetrabutyl 1,3-xylylenediphosphonate because of its superior extr action properties. In hydrochloric acid solutions (0.1 to 5.0 M), the extraction of Au(III) proceeds through formation of a 1:1 solvating co mplex, TBmXDP-HAuCl4 and the equilibrium constant of the extraction re action (K-ex) varies with aqueous HCl concentration. The monofunctiona l P=O containing ligand, tributyl phosphate (TBP) also follows similar stoichiometry in its extraction of Au(III); but with much lower value s of K-ex. On the basis of solubility and phase characteristics of TBm XDP-HAuCl4, system, a mixed diluent is proposed for the recovery of go ld using this reagent. The selectivity of extraction, stripping and ex traction kinetics were evaluated in relation to the recovery of the me tal from acidic waste solutions and the data are compared with those o f TBP.