Phosphorus has a detrimental effect on the quality of steel as it reduces its ductility and toughness. In order to ensure a good quality of the final product, the requirements for steel regarding the phosphorus content are very strict. Recently, it has been reported that the levels of phosphorus in iron ores are increasing due to a gradual shortage of low phosphorus iron ores. Thus, the phosphorus content of the hot metal charged into the BOF (Basic Oxygen Furnace) process will be increasing, which will sharpen the challenges associated with meeting the end phosphorus requirements. Steelmakers are therefore exploring the possibilities for refining high phosphorus iron ore and aim at developing solutions that ensure controlling the final phosphorus contents within the target range under those new circumstances.

A detailed study of the thermodynamics of dephosphorization reaction at conditions relevant for the BOF process is necessary in order to point out the potential as well as the limitations of phosphorus removal for different starting conditions and blowing strategies. This is important for deriving the appropriate process solution, for example, in deciding whether a single or a double slag treatment is required, or for the development of innovative process technologies and new control methods.

Even though the quaternary system CaO-FeO_x-SiO₂-P₂O₅ is the fundamental oxide system for low and high phosphorus refining processes, its thermochemical behavior remains unclear. Instead, it is common practice to reduce this system to its ternary sub-systems CaO-FeO_x-SiO₂ and CaO-FeO_x-P₂O₅when studying the thermochemical behavior of low and high phosphorus systems. However, the phase diagrams available in the literature do not always cover the total temperature range of the industrial process. In addition, the effect of the oxidation state of FeO_x and that of minor oxides such as MgO or MnO on the phase boundaries of those ternary systems remains unclear.

By means of carrying out extensive evaluations of the oxide systems relevant for both low and high phosphorus refining processes, this thesis aims at providing a deep understanding of the state of industrial slags in the composition, temperature, and p(O₂) ranges relevant for the entire process. In addition, an assessment of the dephosphorization potential of industrial slags under those conditions is provided. The extent of phosphorus removal from the metal phase is controlled by the dephosphorization potential of the slag phase, which is generally believed to be a strong function of the composition and temperature of the relevant oxide system.

A computational thermodynamics approach is used for the thermodynamic evaluations carried out in the present work, which involves coupling the newly developed thermodynamic database BOFdePhos to the software package FactSage ^TM. The non-ideal associate solution model is used for the description of the Gibbs energy of the liquid phase and the Compound Energy Formalism (CEF) for the description of the Gibbs energy of the solid solution phases while the solid stoichiometric compounds are treated with simple temperature-dependent Gibbs energy functions. This approach has been selected due to the fact that modeling approaches developed according to the CALPHAD (CALculation of PHAse Diagrams) method have a good capability for extrapolating from assessed binary and ternary sub-systems to higher-order systems. Thus, it is possible to generate new phase diagrams using experimental data from lower-order systems.

The thermodynamic evaluation of the oxide system CaO-FeO_x-SiO₂-P₂O₅ and its ternary subsystems were carried out for the total composition, temperature, and p(O₂) ranges relevant for the process. The results were used for the assessment of the state of industrial slags by means of the analysis of plant trial measurements. The assessments indicate that industrial slags are heterogeneous throughout the major part of the process and, in most cases, saturated with the solid C2S_C3P (2CaO.SiO₂_3CaO.P₂O₅) phase. However, most phosphorus distribution approaches (Lp-approaches) reported in the literature were developed for purely liquid slags. Thus, a new Lp-approach has been developed in the present work, which considers the effect of the amount, type, and composition of the different slag phases. The type and amount of phases formed during the process are found to be highly sensitive to the composition, temperature, and oxidation state of FeO_x. The risk involved with simplifying the oxide system to its main components, as well as with the non-consideration of the effect of p(O₂) is underlined in this work.

This work demonstrates how new methods and strategies for enhancing dephosphorization control can be developed based on a combination of thermodynamic, experimental, and industrial evaluations. Those methods include an implementation concept for the thermodynamic database into dynamic models, the specification of a new target slag region for achieving optimal dephosphorization results, as well as the development of a new BOF method for ensuring a flexible, yet accurate, control of the slag composition towards the target region.