How Zinc Concentrates Are Processed At The Outokumpu Zinc Plant In Kokkola

An electrolytic zinc plant with a capacity of 90,000 tons/year went on stream in Kokkola, Finland, in late 1969. It is a standard electrolytic zinc process plant; however, it has been subjected to several process improvements and enlargements during its time of operation. A fluidized bed roaster treats 500 tons/day of zinc concentrates which also contain mercury. Since the mercury evaporates completely in the roasting process, a mercury removal unit has been added to prevent, contamination of sulphuric acid produced in the neighbouring acid plant. The same unit also removes selenium from the roaster gases. In a separate unit, about 6 tons a year of metallic 99.999% mercury is recovered. The neutral leaching, originally a batch process, has been converted to a continuous operation, and a residue treatment system comprising hot acid leaching and jarosite precipitation has been added. Purification comprises a "hot arsenic-zinc dust" step and a cadmium removal step with zinc dust, both operated batchwise. Purification residues are used to make about 200 tons/ year of 99.99570 cadmium in a process developed by Outokumpu Oy. The electrolytic precipitation of zinc carried out at medium current density gives zinc of 99. 995% purity.