The production of pig iron and in further consequence of crude steel is linked with the concurrent output of residual materials like slags, sludges and dusts. These residual materials usually contain high levels of iron and other, valuable heavy metals. Steel mill dusts for example are produced in substantial quantities and with high iron and zinc content, but even today most of them are deposited worldwide. In times of rising raw materials prices and landfill costs, however, it becomes more and more important to recover and to utilize the contained valuable substances. A promising recycling process, with high potential to recover both primary valuable substances in the steel-mill dusts side by side, is the so-called RecoDust-Process, or in a simplified form it is also named Flash-Reactor. It is a carbothermic process with a selective separation and accumulation of the valuable substances. The Flash-Reactor pilot plant in the laboratory of the Chair of Thermal Processing Technology at the University of Leoben is the result of this research project and based on the function principle of the RecoDust-Process. In a single-step process the steel mill dusts and other residual materials can be melted down instantly under reducing conditions and high temperatures up to 1.800°C, wich are generated by a gas/oxygen flame. After the melting the less volatile components in the dust, like iron, are collected at the bottom of the combustion chamber as a liquid slag. The volatile components, such as zinc for example, are evaporated and flow towards the waste gase. Following an afterburning of the zinc into zinc oxide, as a solid form, a zinc-rich dust can be separated by filtration. The aim is a successful, stable operation of the plant and to manufacture an iron concentrate on the one hand and a zinc oxide concentrate on the other hand, containing small amounts of harmful by-elements. The iron-rich slag should be usable as a high-quality iron source in the iron and steel industry and the zinc-rich dust should be recyclable in the zinc industry. This research paper includes the development, planning, construction and start-up of the Flash-Reactor plant. The results and findings of the start-up form the basis for further experiments and optimization works with the pilot plant.