Abstract Carbon emissions pose a serious threat to the continued survival of this planet. All sectors of society must, therefore, lower their emissions, this includes the steel industry. The production of steel is based on iron ore reduction by carbon. In an attempt to relieve the steel industry from its inherent fossil dependence an initiative called HYBRIT has been started. It aims to supplant carbon reduction with hydrogen reduction. Currently, there is no economically viable industrial production of steel that uses fossil-free hydrogen as reduction agent. In order to create economic viability for such a process work has to be conducted to innovate and optimize. This study aims to be a part of that optimization process by creating a tool for optimizing the slag system with regards to phosphorus removal. 26843 slag compositions were evaluated using modules written in “Matlab” combined with “Thermo-Calc”. 1583 possible slag compositions were found to be suitable for phosphorus removal. These compositions were then optimized after slag weight in order to minimize slag associated cost. The compositions were tested against two theoretical raw materials with varying initial phosphorus content 250 ppm and 125 ppm. It was found that the initial phosphorus concentration of the raw material has a substantial impact not only on the slag costs but also the slag praxis that should be used.