Extraction par solvants fondus: i-équilibres de distribution du zinc(ii) entre le thiocyanate de potassium et le phénanthrène fondus (à195 °c)

Abstract

A study of the distribution of cations between a molten salt, potassium thiocyanate and an immiscible molten organic phase, phenanthrene, has been undertaken with Zn2+ ions. The equilibrium temperature was 195 °C. Zinc(II) can be extracted by organic ligands dissolved in phenanthrene ; three organophosphorus compounds - trioctylphosphine oxide (TOPO), triphenylphosphine oxide (TPPO) and hexamethylphosphorotriamide (HMPT) - and two pyridine-type compounds - 2,2'-bipyridyl (bipy) and 1,10-phenanthroline (phen) - can be used as extracting agents. A study of the variation of the distribution coefficient Dzn in terms of ligand concentration in the organic phase has allowed the formulae of the extracted species to be determined as Zn(TOPO)2(SCN)2, Zn(HMPT)2(SCN)2, Zn(TPPO)3 (SCN)2, Zn(bipy)(SCN)2 and Zn(bipy)2(SCN)2. The corresponding extraction equilibria constants have been determined. The value of the distribution coefficient may be lowered by complexing zinc(II) with cyanide ion in the salt phase. The species extracted with organophosphorus compounds and 2,2'-bipyridyl are the same. Two associations, Zn(phen)(SCN)2 and Zn(phen)3(CN)2, have been found under these conditions. The formation constants of zinc(II)-cyanide complexes in molten KSCN have been deduced from distribution coefficient variations.The use of a diagram which allows the results to be compared conveniently is proposed for the distribution equilibria of a metal cation between two liquid phases when an organic ligandL¯and an inorganic complexing agent CN− are varied (log [L¯] = f(log [CN−]), with constant DZn).