State of the art in nickel smelting: direct Outokumpu nickel technology

Experience of the direct Outokumpu nickel (DON) smelting process gained over the first ten years of operation is reviewed, including the significant environmental impacts. In 1995, production capacity at Harjavaltawas increased from 17 000 to 52 000 t/year, by introducing DON technology, additional refinery capacity being based on hydrogen pressure reduction, producing nickel powder and briquettes. In the DON process, high grade nickel matte of low iron content is produced in the flash smelting furnace directly without subsequent converting. The metal values in the smelting slag are recovered in an electric furnace as an iron containing Ni matte. The mattes produced differ from conventional nickel mattes in many respects, and therefore focused R&D was required to adapt to their melting points, fluidities and settling properties. The low melting points of the high grade nickel mattes produced in the DON furnace, particularly those low in copper, have challenged furnace design, in particular in the hearth and its thermal engineering. Other key issues of DON technology are smelting of high magnesia concentrates, and the recovery of nickel and copper, as well as those of platinum group metals and cobalt. These factors can be affected by proper selection of process concept and operational conditions in the processing vessels. Selected results of thermodynamic modelling of DON and electric furnace mattes and high magnesia slags are reviewed, with reference to the mattes produced. The design principles of the flash furnace and electric furnace are also discussed.