Sustainable Steel Making: Hydrogen Plasma Reduction of Iron Ore

Iron- and steelmaking is the largest single industrial CO 2 emitter, accounting for 6.5% of all CO 2 emis- sions on the planet. This fact challenges the current technologies to achieve carbon-lean steel production and to align with the requirement of a drastic reduction of 80% in all CO 2 emissions by around 2050. Thus, alternative reduction technologies have to be implemented for extracting iron from its ores. The hydrogen-based direct reduction has been explored as a sustainable route to mitigate CO 2 emissions, where the reduction kinetics of the intermediate oxide product Fe x O (wüstite) into iron is the rate- limiting step of the process. The total reaction has an endothermic net energy balance. Reduction based on a hydrogen plasma may offer an attractive alternative. Here, we present a study about the reduction of hematite using hydrogen plasma. The evolution of both, chemical composition and phase transforma- tions was investigated in several intermediate states. We found that hematite reduction kinetics depends on the balance between the initial input mass and the arc power. For an optimized input mass-arc power ratio, complete reduction was obtained within 15 min of exposure to the hydrogen plasma. In such a process, the wüstite reduction is also the rate-limiting step towards complete reduction. Nonetheless, the reduction reaction is exothermic, and its rates are comparable with those found in hydrogen-based direct reduction. Micro- and nanoscale chemical and microstructure analysis revealed that the gangue elements partition to the remaining oxide regions, probed by energy dispersive spectroscopy (EDS) and atom probe tomography (APT). Si-enrichment was observed in the interdendritic fayalite domains, at the wüstite/iron hetero-interfaces and in the oxide particles inside iron. With proceeding reduction, however, such elements are gradually removed from the samples so that the final iron product is nearly free of gangue-related impurities. Our findings provide microstructural and atomic-scale insights into the com- position and phase transformations occurring during iron ore reduction by hydrogen plasma, propelling better understanding of the underlying thermodynamics and kinetic barriers of this essential process.

Annually, 2.6 billion tons of iron ore (mostly hematite) are converted into steel by the integrated blast furnace (BF) and basic oxygen furnace (BOF) route, accounting for approximately 70% of the global steel production. The remaining 30% is realized by melting steel scraps and directly reduced iron (the latter is also referred to as sponge iron) in electric arc furnaces (EAF). On av- erage, about 2.1 tons of CO 2 are produced per ton of crude steel . This number corresponds to about 6.5% of all CO 2 emissions on the planet and makes iron- and steelmaking the largest in- dustrial individual emitter of CO 2. To mitigate global warming, a drastic reduction of 80% in all CO 2 emissions is targeted until 2050. Thus, disruptive technology changes in iron ore reduction must be urgently implemented, already within the next years. 

The use of hydrogen instead of carbon for iron ore reduction is currently explored as an alternative sustainable route to miti- gate the CO 2 emissions. The direct reduction of iron ore pel- lets by pure molecular hydrogen above 570 °C occurs with the intermediate formation of other iron oxide variants, viz. Fe 2 O 3 (hematite) → Fe 3 O 4 (magnetite) → Fe x O (wüstite) → Fe (iron). Although the net energy balance of the overall process is endothermic, studies demonstrate that its reduction kinetics can be reasonably faster than that of commercial direct reduction conducted in shaft furnaces using reformed natural gas (e.g. the Midrex process). This observation motivates the current global investments in hydrogen-based direct reduction pilot plants.

Hydrogen plasma offers a viable alternative for carbon-neutral iron making. Its high energy and enhanced density of H radicals and exited states help to overcome the reaction’s activation barrier and has the potential for enhancing the Fe x O reduction rates by an order of magnitude, enabling iron conversion to reach commercially viable rates. Also, hydrogen plasma-based reduction allows the production of liquid iron in one single step, in which the input fine ores are melted and reduced simultane- ously without the need for intermediate agglomeration or refine- ment processing, as the melting point of iron oxide (1565 °C) only slightly exceeds that of iron (1538 °C).

During the hydrogen plasma reduction (HPR), a plasma arc zone is generated between an electrode and the input iron ore (e.g. hematite). In this zone, the ore can be melted and re- duced by hydrogen in both molecular and plasma states. The lat- ter is composed of vibrationally ionized (H + , H 2 + and H 3 + ), ex- cited (H ∗) and atomized (H) species, which are formed through the mutual elastic and inelastic collisions of hydrogen particles with electrons. Under such conditions, the corresponding degree of hydrogen dissociation is determined by the competing ionization (e.g. e −+ H 2 → H 2 + + 2 e −) and recombination (e.g. H + + e −→ H) events. The high energy carried by the hy- drogen plasma is partially released at the reaction interface, gen- erating a large amount of local heat. This heat diminishes the need for external power supply and promotes high power- efficiency to the process. Thus, the thermo-kinetics advantages observed in HPR depend directly on the concentration of hydrogen plasma radicals that are able to reach the reaction interface.