Hydrogen-based direct reduction is the most promising technology for CO2-neutral steelmaking. The gas-solid reduction reactions characterize these aggregates, producing sponge or direct reduced iron (DRI) consisting of metallic iron, residual iron oxide, and gangue. That is in contrast to the blast furnace (BF), which simultaneously reduces, melts, and, deslags the iron ore. Therefore, a separate melting step is required for sponge iron, whether briquetted or pellet material or unagglommerated fines with particle sizes <8 mm. Nowadays, the electric arc furnace (EAF) is the preferred melting aggregate. Although quantitatively subordinated, this direct reduction reactor (DR) – EAF sequence is considered an industrially proven technology, especially in natural gas-rich regions such as Iran, Saudi Arabia, United Arab Emirates, and Mexico. When comparing it to the integrated BF – basic oxygen furnace (BOF) capacities in Europe, besides the reducing agent, also the iron carriers differ significantly. The latter uses lower grade materials, typically containing 58% < Fetot < 65%; the former rely on so-called DR-grades with Fetot> 67%. They are considered the highest-grade iron ores and an expensive minority compared to sinter or BF-grade concentrate. Further beneficiation decreases the total yield and means additional effort. Therefore, processing lower-grade iron ores via direct reduction is of great importance with regard to economically worthwhile and CO2-neutral steel.
Various aspects of this question are examined in the scope of this thesis. Introductory mass balance calculations are used to define different DRI processing strategies and to compare their strengths and weaknesses. Looking at different cases, it is clear that the EAF is the unit of choice for sponge iron from DR-grade ore. Nevertheless, lower grades demand an alternative two-step melting process, using a smelter to produce hot metal and a BOF to refine it to crude steel.
Secondly, the behavior of phosphorus during direct reduction is investigated. BF hot metal contains phosphorus in reduced condition. Since phosphorus remains bound in apatite during direct reduction, this indicates an advantageous behavior during melting, as a kinetic rephosphorization delay can be expected.
Dipping tests highlight the role of carbon during DRI dissolution in a melt. While carbon-free H2-based sponge iron melts slowly, carbon accelerates softening. Further, a significant difference in the behavior between contact with slag and steel is observed. Both observations correlate with lab-scale melting tests; the carbon-containing material results in a blister-free structure and pronounced separation from gangue and steel. A further test shows an intensive stirring effect in slag and steel induced by the electric arc. The primary conclusion can be made in case of the DRI charging spot. It should be fed directly into the arc, as the stirring increases the chance of contact with the metallic phase, which is essential for rapid DRI melting.
Last but not least, slag foaming is evaluated. Adding a carbon carrier generates gas, resulting from the reduction reactions with FeO. No foaming occurs in blast furnace-like slag, while EAF slag shows extensive foamability.