Enhancing the flotation recovery of copper minerals in smelter slags from Namibia prior to disposal

Namibia Custom Smelters (NCS) process a range of copper concentrates in their three furnaces, namely; top submerged lance, copper converter and reverberatory furnaces, in order to produce mattes and fayalitic slags. The copper content of the slags range between 0.8 to 5 wt. % and this is considered too high for disposal to the environment. Currently, the slags are sent to a milling and flotation plant for liberation and recovery of residual copper. The copper recoveries realized in the plant are much lower than expected and it has been postulated that some copper minerals may be occurring in forms that are more difficult to float like oxides or fine disseminations in the gangue matrix. Mineralogical analysis of the slag samples was done using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. The analysis did not reveal the presence of copper oxide minerals, however most scans showed copper sulphide minerals as free grains and some finely disseminated in fayalite gangue. In the first phase of the present experimental studies, the slags were milled to 75% passing 45 microns, which is the degree of milling done in the existing plant mill-float circuit. A range of commercial flotation reagents that include xanthates, dithiophosphates, mercaptobenzothiazole, thionocarbamates, fatty acids, sulphides and sulphates were used in the flotation test-work. The copper recoveries obtained in the mill-float stage were between 70 - 80%. In the second phase of the study, the flotation tailings were further milled to 90% passing 45 microns and floated. The cumulative copper recoveries increased markedly to over 90%, which represents a significant improvement in comparison to the recoveries obtained from the mill-float process. Sodium alkyl dithiophosphate, mercaptobenzothiazole (FC7245) was found to be the secondary flotation reagent that gave the best copper recoveries.