The effect of acid soluble copper concentrates on the operations of the flash smelting and slag cleaning furnaces at Nchanga smelter of Konkola copper mines plc

Abstract

Konkola Copper Mines’ (KCM) Nchanga smelter utilises a modern technology of direct to blister flash smelting process designed by Outotec, Finland. Slags from the direct to blister flash smelting unit are carbothermically reduced through a two stage electric slag cleaning furnaces (ESCF) to produce blister copper of 98.25% Cu and a cobalt recoverable slag containing 3 – 5% Cu and 0.78% Co which is processed in the cobalt recovery furnace (CRF). The smelting units are designed to smelt concentrates containing a maximum of 2.5% ASCu, however operationally the ASCu content in concentrates has gone as high as 5%. The behaviour of ASCu in the flash smelting furnace (FSF) and the subsequent slag cleaning furnace (SCF) operations has not been fully established. Therefore, the aim of the research was to assess the effects of ASCu on the FSF and SCF operating efficiencies. Material balance calculations for the smelting operations were done based on the actual plant data collected for a period of 8 months when the level of ASCu was greater than 1% in feed sulphide concentrates and for another 8 months period when the level of ASCu was less than 1%. The distributions of Cu, Co and Fe in the slag and blister phases were analysed based on the material balances. The degree of oxidation was assessed and compared for the two periods by analysing the sulphur levels in the blister products. The consumption of metallurgical coke in the reduction furnaces was also analysed based on the calculations of coke specific consumption per tonne of copper produced. At less than 1% ASCu in concentrate blends, Cu-in-FSF slag was found to have a negligible dependence on ASCu with the values of Cu-in-FSF slag fluctuating below 19.5%. Cu-in-FSF slag was ranging from 19.6% to 21.2% for ASCu in feed concentrates in the range 1.5% to 4.8%. The degree of oxidation was found to be very sensitive to variations in concentrate blends. Increases in ASCu lower the degree of oxidation which was evidenced by high values of S in blister at 2200 ppm with a subsequent deportment of Co to blister at 170 ppm against a set target of 69 ppm. High Cu losses to FSF slag has a tendency of reducing the loss of cobalt to blister by Cu-Co co-reduction, but however negatively impacts on coke consumption. Per tonne of Cu produced, the electric slag cleaning furnaces’ coke consumption increases with increasing %Cu-in-FSF slag feed. Losses of Co in both FSF and SCF operations lowered the total cobalt recovery in the smelting operations. Further work on thermodynamically assessing the effects of ASCu on the flash and slag cleaning operations was recommended.