Rotary kiln incineration combined with the disposal of residual slag on landfills defined by German landfill regulations is the most acceptable treatment for hazardous waste with respect to the environment. The objective of research is to optimize the elution behavior of HWI-slag. Quality-improved slags may be disposed off on cheaper landfill sites still applying to the landfill regulations in order to save disposal costs. For that purpose, the hazardous waste is mixed with additives which lower the melting temperature of slag and initiate chemical reactions with heavy metals in the rotary kiln. The heavy metals are immobilized in the slag by these effects. Limestone, soda and calciumfluoride are used as additives.