Reprocessing of zinc galvanic waste sludge by selective precipitation

Multi-step selective precipitation method was tested for processing zinc waste galvanic sludge. This method involved acid leaching of sludge in sulphuric acid, purification of sulphate leach liquors using a sequence of hydroxide, sulphide, and fluoride precipitation to remove trivalent metals, Cu, Cd, Ca, Mg, and Si, oxidative precipitation to remove Mn, and finally precipitation of Zn as zinc carbonate. The solids were separated from leach liquors after each precipitation step, and their phase composition was determined by X-ray powder diffraction and elemental composition by AAS. The flowsheet proposed yielded relatively pure Zn-precipitate, suitable for possible production of raw materials for ceramic, metallurgical, or pigment industry, however due to incomplete leaching of the sludge and co-precipitation of Zn to hydroxides of trivalent metals and Cu, Cd-sulphides, the overall recovery efficiency was only 63-65%.