Smelting Jarosite and Sulphur Residue in a Plasma Furnace

Jarosite and sulphur residues are formed during electrolytic recovery of zinc at Boliden Kokkola. In order to minimize the waste generation as well as promote better recovery of metals, pyrometallurgical options have been studied to treat this material. In addition to recovering more metals, another goal was to obtain clean stable slag that could be used in construction. Pilot test work was carried out using an ArcFume plasma process for directly smelting and fuming several combined residues, i.e. jarosite plus sulphur residue from Boliden Kokkola zinc plant. The tests were carried out in two stages, including a smelting stage and a reduction stage. The obtained slag compositions after smelting were typically 1-2 wt% of Zn and 0.1-0.3 wt% of Pb. The smelting stage was followed by a shorter reducing stage to further decrease volatile metals in the slag. Very low compositions of Zn < 1 and Pb < 0.03 wt% were reached. In the smelting stage, no coke was added. In the reducing stage however, coke was added to ensure sufficient reductive conditions for fuming volatile metals such as Zn, Pb, Ag, In, and Ge and obtain good recovery. Heavy metals and elements such as As, Sb, Cd, and Cl reports into fume oxide as well. Consequently, the overall direct CO2 emissions were lower than expected due to avoiding coke use in the smelting stage. The allowed contents of heavy metals in the slag are determined by legislation and the targeted use of the slag.