Every year, several hundred thousand tons of LD slag are produced in steel production in Austria alone [1]. In addition to iron, this slag contains about 1 % P2O5 bound within the C2S phase and is separated from the steel to be produced using the Linz-Donawitz process. In order to recycle this slag within the process systematic studies on the selective separation of phosphorus by leaching in citric acid, as well as in comparison with alternative chemical/mechanical processes are being carried out. Therefore, a literature study focusing on BOF slag and citric-acid-leaching was carried out. Several test series with varying parameters, in particular leaching time and citric acid concentration, were realized. Further grinding tests as well as high intensity magnetic separation and settling velocity tests were carried out on promising products. To enable these magnetic separation experiments with constant flow rate and solids volume concentration, the laboratory matrix separator was modified over the course of this work.