Study of the Preparation of Zinc(II) Ferrite and ZnO from Zinc- and Iron-Containing Industrial Wastes

A mixture of ZnFe2O4 and ZnO can easily be produced by heating fresh or sintered hot-dip galvanizing sludges at 1000 °C for 5 h. Ammoniacal leaching of this mixture with a concentrated ammonia solution at room temperature by applying a 60-fold molar excess of ammonia over the whole amount of zinc for 24 h (or using a 48-fold excess of ammonia for 72 h) leads to an almost complete recovery of ZnO (about 44% of the total zinc content) upon separation from ZnFe2O4. Treatment of the formed ammoniacal tetraamminezinc(II) hydroxide solution with sulfide ion (1 mol % related to the Zn content) precipitates all of the dissolved heavy metals with a slight loss of zinc (0.7%). Heating of the purified solution at 100 °C leads to the precipitation of pure ZnO and regeneration of the ammonia.