A study of the autogenous Hydrogen-DRI slag and its impact on the dephosphorization of fossil-free steel at different oxygen potentials

Abstract: The present study comprises aspects related to the phosphate capacity, the dephosphorization of fossil-free steel, and the utilization of potential by-products. The focus is mainly given to the functions and impact of the autogenous H-DRI slag in the dephosphorization process and the possibility for future slag valorization.At the outset, the applicability of the phosphate capacity concept on systems containing multivalent species is critically examined. For the examination, the variation in the slag structure depending on the oxygen potential was considered theoretically. To support the theoretical consideration, experiments were conducted to illustrate the dependence and to show the effect on the phosphate capacity. The results demonstrated a significant effect of the oxygen potential on the phosphate capacity. Consequently, the use of the concept for multivalent slags falls under serious question.To better orientate the future steelmaking process, the dephosphorization power of slags related to the autogenous H-DRI slag was investigated experimentally. The CaO-MgO-SiO2-FeO system constituted the fully liquid slags, which were equilibrated with liquid iron at 1873 K. Further, the oxygen potential was fixed by closing the system. The dephosphorization power of the autogenous slag was found to be theoretically sufficient to refine the steel made from H-DRI from phosphorus adequately. Thus, it was concluded that the H-DRI slag could be used as a base for the EAF slag to save energy and material.Due to the industrial novelty of the H-DRI material, little is known about the dephosphorization mechanism. Therefore, to facilitate a more efficient process design, the dephosphorization mechanism for H-DRI with different reduction degrees was studied under two different heat transfer conditions. Firstly, by heating and melting H-DRI in a poor heat transfer situation, i.e., in a gas phase at 1873 K, and secondly, under better conditions where the heat transfer is still insufficient for direct melting, i.e., by heating in a liquid slag at 1923 K. The melting process was found to influence the dephosphorization mechanism significantly. In the poor heat transfer situation, the dissolution of the phosphorus-bearing apatite phase was facilitated by the bulk movement of the autogenous slag, which occurred during the melting of the metal phase. In the better heat transfer situation, the bulk slag penetrated the pore network of the H-DRI, a process that was enhanced by the autogenous slag. Since a greater slag mass was available for dissolution, the steel made from H-DRI was dephosphorized already prior to melting.Lastly, the possibility for vanadium extraction from an especially engineered autogenous H-DRI slag was investigated experimentally at 1873 K. For the production of high-quality ferrovanadium alloy, a feasible vanadium extraction requires the fulfillment of two demands. Phosphorus should be predominantly partitioned to the metal and vanadium to the slag. Thus, the experiments featured an acidic slag of the Al2O3-SiO2-FeOx-VzOy system and liquid iron as the metal phase. Also, to fix the oxygen potential, the system was closed. The dephosphorization power of the acidic slags was very low, within the investigated range, while vanadium was mostly partitioned to the slag. The proposed slag system could, therefore, provide an opportunity to utilize an especially engineered autogenous slag for vanadium extraction.