  Iron- and steelmaking cause ~7% of the global CO2 emissions, due to the use
of carbon for the reduction of iron ores. Replacing carbon by hydrogen as the
reductant offers a pathway to reduce emissions. However, production of hydrogen
using renewable energy will remain a bottlenecks, because making the annual
crude steel production of 1.8 billion tons sustainable requires a minimum
amount of ~97 million tons of green hydrogen per year. Another fundamental
aspect to make ironmaking sector more sustainable lies in an optimal
utilization of green hydrogen and energy, thus reducing efforts for costly
in-process hydrogen recycling. We therefore demonstrate here how the efficiency
in hydrogen and energy consumption during iron ore reduction can be
dramatically improved by the knowledge-based combination of two technologies:
partially reducing the ore at low temperature via solid-state direct reduction
(DR) to a kinetically defined degree, and subsequently melting and completely
transforming it to iron under a reducing plasma (i.e. via hydrogen plasma
reduction, HPR). Results suggest that an optimal transition point between these
two technologies occurs where their efficiency in hydrogen utilization is
equal. We found that the reduction of hematite through magnetite into wustite
via DR is clean and efficient, but it gets sluggish and inefficient when iron
forms at the outermost layers of the iron ore pellets. Conversely, HPR starts
violent and unstable with arc delocalization, but proceeds smoothly and
efficiently when processing semi-reduced oxides. We performed hybrid reduction
experiments by partially reducing hematite pellets via DR at 700{\deg}C to 38%
global reduction (using a standard thermogravimetry system) and subsequently
transferring them to HPR, conducted with a gas mixture of Ar-10%H2 in an
arc-melting furnace, to achieve conversion into liquid iron.
