In integrated iron and steel mills molten blast furnace slag is produced as a by-product with around 1500°C. According to the state of the art this slag is cooled down with water in wet granulation plants. This process produces granulated blast furnace slag with latent hydraulic properties used in the cement and construction material industry. However, the method discussed causes an energy content loss of 1.5 GJ/t with regard to the blast furnace slag. This represents a large unexploited energy resource of the iron and steel industry. The dry slag granulation, based on the Rotating-Cup Atomizer, is an appropriate method to retrieve this energy. Simultaneously it is also possible to produce high quality granulated blast furnace slag. In order to meet this gap in knowledge, Siemens VAI in cooperation with the chair of Thermal Processing Technology for Research and Development at the University of Leoben decided to establish a new testing plant. The first part of this thesis comprises information about fundamentals of blast furnace slag, the development of dry granulation and the competing processes. The second part deals with the energy balance of the individual trials and the resulting thermal efficiency.