From trash to resource: A green approach to noble-metals dissolution and recovery

A process based on the lixiviant properties of organic mixtures of dihalogen/S,S-ligands, N,N′-dimethyl-perhydrodiazepine-2,3-dithione (Me₂dazdt) and tetraalkylthiuramdisulphide (Et₄TDS) in the presence of diiodine, for gold recovery from the non-ferrous metal fraction of real shredded waste electric and electronic equipment (WEEE), is presented here. Selective dissolution of metals is achieved through a sequence of three steps where the oxidation of different kinds of metals is achieved by using: (1) refluxing water solutions of HCl 1:5 under Ar atmosphere (Sn, Zn, etc.); (2) water solutions of NH₃/(NH₄)₂SO₄mixtures in the presence of H₂O₂ on the resting sample (Cu, Ag); and (3) acetone solutions of Me₂dazdt or Et₄TDS/I₂ mixtures on the final residue (Au). Each step is followed by a further treatment for: (1) metal recovery, in the case of Au, Cu, Ag; and (2) inertization, in the case of heavy metals. As a whole, the process is very promising for effective recovery of gold and other valuable noble-metals and for using non harmful reagents in mild conditions.