Solvent extraction of some base and precious metals using 1,3,4-thiadiazole-2-nonylmercapto-5-thiol

This investigation is one of a series of studies in which the fundamental chemistry which underlies the extraction and separation of precious metals is considered. The title compound (TNSTH) was used as a solvent extraction reagent and shows some promise for extracting and separating the chloro-anions of Au(III) and Pd(II) from strong hydrochloric acid solutions (ca. 5 M). The distribution coefficients from such media were in the order of 10⁴. The title reagent may be used to separate the precious metals from each other. For example, the separation coefficients (i.e., ratios of distribution coefficients) for mixed solutions are 10⁶ for Au(III)/Rh(III) and Pd(II)/Rh(III); 145 for Au(III)/Pd(II); 180 for Pd(II)/Cu(I or II); 10⁶ for Pd(II)/Pt(IV). The time for half of the Pd(II) to be extracted is approximately 6 min, which is acceptable for a commercial process. The title reagent provides a means of separating precious metals from base metals as the latter, with the exception of copper, are not extracted. In the case of copper, the extraction is as Cu(I) rather than Cu(II). The stoichiometry of the Pd(II) extraction is Pd:TNSTH is 1:1.5. Some additional information concerning the nature of the complexes formed by the title compound and chloro-anions in alcoholic solution indicates that Cu(TNST), Ag(TNST), Pd(TNST)₂, and RhCl₃(TNST)₂ are formed. Most of the metals, with the notable exception of rhodium(III) and iridium(IV), can be stripped using a thiourea/HCl solution. The reagent TNSTH appears to be a chelating agent with donor N and S atoms.