In this work within the K1-MET project 1.7 (Valorization of steelmaking slags), the selective leaching of phosphorus from the C2S phase of LD slag with citric acid from previous research work is being pursued further. The aim of the work is to plan a pilot-scale cycle test. The literature section deals with ion exchange processes for the recovery of phosphorus from the liquid phase. Some aspects of precipitation and crystallization reactions are described as well. The experimental section is dedicated to steps that are essential for the process sequence. First and foremost, the grain size dependence of the leaching process is investigated. With regard to an economically feasible process, the solids concentration in the leach should be increased. Furthermore, the solid liquid separation has to be considered at three different process stages. Filtration resistances are determined for this purpose. To close the cycle, the recovery of citric acid is also covered Grinding tests are carried out on a laboratory scaled stirred media mill in order to investigate the grinding behaviour and to elaborate the specific energy input to assist the selection of grinding devices. The findings from the laboratory tests will be used to create a process sequence for a closed-loop test. The leaching process is characterized on the basis of mass balances by calculating the elemental distribution on the various products. The focus is put on the phosphorus distribution to the final and circular product.