Smelting Jarosite and Sulphur Residue in a Plasma Furnace
393
Fig. 2 Boliden Kokkola roasting, leaching, electrowinning (RLE) flowsheet
230,000 dmt/a (jarosite 160,000 dmt/a and sulphur residue 70,000 dmt/a). Typical
analysis of the combined residue shows following compositions: Zn 2.9%, Pb 2.7%,
Fe 15.2%, Ca 3.2%, S 31.7%, Si 2.0%, Na 1.6%, Cu 1.3%, As 0.7%, Al 0.6%, Cd
0.1%, etc. The chemical composition of elements can vary significantly based on the
feed mixture into the zinc processing (Fig. 3).
Iron residues are formed deliberately in electrolytic production of zinc because the
amount of iron needs to be controlled and removed from the solution. Precipitation
of iron removes many impurities that are detrimental for zinc electrolysis such as
As, Sb, F, Hg, In, Ge, and Al. The unrecoverable portion of Zn, Pb, and Ag ends up
in the landfill.
Iron removal is a crucial part of solution purification prior to electrolytic recovery
of 99,995% pure zinc metal. Typically, half of the amount of concentrate feed ends up
Fig. 3 Landfilled combined residue at Boliden Kokkola