  Steelmaking contributes 8% to the total CO2 emissions globally, primarily due
to coal-based iron ore reduction. Clean hydrogen-based ironmaking has variable
performance because the dominant gas-solid reduction mechanism is set by the
defects and pores inside the mm-nm sized oxide particles that change
significantly as the reaction progresses. While these governing dynamics are
essential to establish continuous flow of iron and its ores through reactors,
the direct link between agglomeration and chemistry is still contested due to
missing measurements. In this work, we directly measure the connection between
chemistry and agglomeration in the smallest iron oxides relevant to magnetite
ores. Using synthesized spherical 10-nm magnetite particles reacting in H2, we
resolve the formation and consumption of w\"ustite (FeO) - the step most
commonly attributed to agglomeration. Using X-ray scattering and microscopy, we
resolve crystallographic anisotropy in the rate of the initial reaction, which
becomes isotropic as the material sinters. Complementing with imaging, we
demonstrate how the particles self-assemble, subsequently react and sinter into
~100x oblong grains. Our insights into how morphologically uniform iron oxide
particles react and agglomerate H2 reduction enable future size-dependent
models to effectively describe the multiscale iron ore reduction.
