Slag cleaning is important from both an economical and environmental perspective. Dump slags from copper smelting prove to be a valuable source of base metals, but the abundance of iron in the slag leads to significant amounts of iron reporting to the alloy phase during carbothermic reduction, which poses a significant cost to remove in subsequent hydrometallurgical processes. A carbothermic reduction of nonferrous smelter slag was conducted under the influence of three different slag modifiers to investigate the recovery of cobalt (and copper) versus iron recovery. The test work was performed in two campaigns; one campaign was performed under a constant oxygen potential of 4.33x10(-9) atm, while the other set of test work was performed using graphite under a pure argon atmosphere, It is shown in both cases that, out of these slag modifiers used, TiO2 had a more selective recovery of cobalt compared to CaO and CaF2. It appears that TiO2 reduces the activity coefficient of the FeO in slag due to its strong affinity for FeO in the slag phase. (C) 2002 Elsevier Science Ltd. All rights reserved.