This PhD thesis investigates two pyrometallurgical treatment ways for basic Waelz slag. In the first case the Waelz slag is smelted together with retort residues in an autothermic converter process. During the smelting process most of the Zn, Pb and S from the input materials is fumed into a flue dust, so that the remaining slag shows an excellent leaching behaviour. Due to the fact that this treatment route is completely new, the experimental investigations mainly focused on the principal feasibility of the process with the aim to prove the autothermic process operation and define the necessary parameters for it. In addition to this a mass and energy balancing of the process was done. The second treatment process deals with the reduction of Waelz slag on a carbon containing iron bath. Within the associated laboratory trials the reduction behaviour of the various slag components was characterized as a function of time as well as during a continuous slag reduction. Based on these results a mass and energy balance for the whole process chain covering both the Waelz kiln and the slag reduction unit was calculated with the aim to identify possible fields for optimization. In this context it turned out that the installation of a slag reduction unit requires major changes in the operation mode of the kiln as for example a stop of the SDHL process, a reduced CaO addition, a carbon source which is almost free from sulphur and the installation of an additional burner at the kiln end.