The recycling of iron containing residues is often limited or prohibited through impurities. Especially the compound of zinc and iron is a long term problem within the iron and steel industry. Zinc coated scrap brings a significant amount of zinc into the production route. In order to recycle steel mill dusts and residues internally, a zinc limit is set by the blast furnace operators. Simultaneously, a cost neutral or profitable external treatment is only manageable for zinc contents over 20%. Unfortunately, the zinc contents of these dusts are often between the limits. The Flash-Reactor offers a possibility of an on-site treatment to gain two separate usable fractions. The regained iron is used on the blast furnace route and the extracted zinc can be reused within the primary zinc production as a secondary resource. The RecoDust-Process separates iron and zinc at temperatures of about 1700 °C through selective volatilization. In the reaction vessel under-stoichiometric conditions are provided by an oxygen-natural gas burner. Through a special melting cyclone the dusts are directly fed into the burner flame and the metal oxides are melted within fractions of a second. All nonvolatile components gather at the bottom and are discontinually tapped, while the volatiles leave the reaction chamber with the flue gas. In the following gas cleaning the flue gas is oxidized by post combustion and a baghouse filter separates the solid gas components. Within this thesis the thermo-chemical processes in the reaction vessel are analyzed. The pilot plant is further optimized and a detailed evaluation of the performed smelting campaigns is presented. Additionally, a collective examination of all campaigns regarding the product quality has been performed.