Model processes for hydrogen plasma and direct reduction of iron ore

Decarbonisation of the steel production to a hydrogen-based metallurgy is one of the key steps towards a sustainable economy. While still at the beginning of this transformation process, with multiple possible processing routes on different technological readiness, we conduct research into the related fundamental scientific questions at the MPIE. 

At the core of these efforts is the development of suitable experimental methodologies to elucidate the underlying physical, chemical, and microstructural phenomena governing the reduction kinetics and boundary conditions. Currently, two processes for the sustainable reduction of iron ore – as the most critical step in a carbon-neutral steel production – are being developed and deployed.

Hydrogen-based direct reduction (HyDR) is a major contender for the future of green ironmaking without a direct release of CO₂ emissions. It relies on a multistep solid-gas reaction, where iron ore in pellet or powder form is subjected to elevated temperatures under a reducing gas atmosphere. The fundamental understanding of such a complex reaction process is necessary from the thermodynamics and kinetics perspectives. The in-house designed thermo-gravimetric analysis (TGA) setup coupled with mass spectrometry at the MPIE ([1]) allows us to precisely measure the reduction kinetics of iron oxides under well-defined gaseous atmospheres (containing e.g. H₂, NH₃, or CO). The infrared furnace enables extremely fast and well-defined ramping up to 10 K/s and a high operational temperature up to 1000 °C. During the HyDR experiments, the in situ weight change of the iron oxides can be continuously tracked by a sensitive thermo-balance with an accuracy of 0.1 μg to quantify the reduction degree (i.e. loss of oxygen) [2]. Furthermore, the gas composition near the sample surface can be analysed using a fused capillary attached to the quadrupole mass spectrometer. Such an additional technique provides valuable information on the alternation of the gas species during HyDR, indicating the reaction paths and kinetics. With the expansion of the research activities in hydrogen-based metallurgical science and technology at the MPIE, new equipment, such as a TGA setup with the capability of mixing different gases and a fluidised-bed setup is under construction. This development will allow for in-depth investigations and further optimisation of complex industrial processes.

However, the fragmented sponge iron coming out of the HyDR process needs to be molten in order to be processed into various steel alloys. This is typically performed in an electric arc furnace (EAF), resulting in a major energy penalty. That motivates the hydro plasma smelting reduction (HPSR) process. Here the iron ore is molten and simultaneously reduced in an EAF with a hydrogen plasma arc, offering thermodynamic and kinetic advantages, but on a yet lower technological readiness level [3]. We study the fundamental aspects of the HPSR process, for example regarding ore composition and plasma parameters, by use of a modified, lab-scale, arc-melting furnace (Fig. 1). While being smaller and less complex than an industrial EAF, it allows us to systematically study the phenomena in the most relevant zone, i.e. where the hydrogen containing plasma interacts with the molten and partially ionised iron oxides. Currently ongoing is the implementation of improved diagnostic devices such as mass spectroscopy of the reactor atmosphere and spectral analysis of the plasma constituents, together with academic and industrial partners, within the group of “Sustainable Material Science and Technology”.