At voestalpine's Austrian steelworks, around 650,000 tons of LD slag are produced annually during steel production. In Moaveni's PhD thesis (Chair of Mineral Processing, Montanuniversitaet Leoben, 2019), it was shown that the main P content, which interferes with the steel process, is concentrated in the C2S phase (2CaO*SiO2). Selective digestion of the P carrying phase can be achieved in CO2 atmosphere (48h/16 bar). In the subsequent HGMS, an iron yield of 75 % in the magnetic product with a halving of the P2O5 content to 0.54 % was possible. In order to improve results, A. Böhm developed a high-pressure stirred mill (HD-RWKM) in cooperation with Cemtec, in which pressure levels of up to 50 bar are possible. The reactor shall serve to combine mechanical surface scrubbing and the chemical reaction. Within this thesis the mill was successfully put in operation and a safety concept was developed. The tests showed that a higher CO2 uptake, which according to Moaveni is an indicator of digestion success, is possible with shorter test times (1h/40 bar or 2h/30 bar). The need for constant free surfaces and higher CO2 sequestration in the fine fractions was also demonstrated. Despite the success of higher CO2-uptake, an iron yield of approx. 75 - 80 % was associated with a phosphorus yield of approx. 60 - 70 % in the magnetic product. The data analysis of the previous tests showed that no grinding effect can be observed yet at the selected low setting of stirred speed (80 rpm) in the HD-RWKM.