In the extraction of silver from cyanide process (also Mac-Arthur Forrest process), the siliver compound dissolve in NaCN solution forming a soluble complex salt, in presence of air, then silver is precipitated from this complex salt by the addition of Zn, because Zn being more reactive displaces the Ag. Ag2S+4NaCN⇌2Na[Ag(CN)2]+Na2S↓ Na2S+2O2→Na2SO4 2Na[Ag(CN)2]+Zn→Na2[Zn(CN)4]+2Ag↓