Fluidized beds are ideal reactors for non-blast furnace ironmaking
due to their excellent heat and mass transfer efficiency, coke-free
ironmaking and direct use of fine iron ore potential [1,2]. However,
the serious defluidization problem during the reduction of fine iron
ore, caused by the sticking of newly-formed metallic iron at elevated
temperatures, always induces abrupt shut down of the equipment and
hinders the stability of continuous operation in industry [3–5]. Among
various defluidization prevention measures, carbon-coating is simple,
effective, economical and draws more and more attention from recent
researchers [6–10].
Generally for this method, a two-step reduction process is adopted,
where pre-reduction process is conducted at relatively low temperatures
for coating enough carbon on the surface of fine iron ore particles,
to prevent the defluidization during the subsequent high temperature
reduction process. One of the main problems lies in the fact that the direct
reduction iron (DRI) obtained by such process contains a high carbon
content (16.5–22.3 wt.%) as reported by previous studies [9,10],
which is obviously too high to be used as the feedstock of following electric
furnace steelmaking process. This can be attributed to two reasons:
(1) the critical carbon content value (Ccritical) needed to prevent the high
temperature defluidization is too high, e.g., after pre-reduction process,
the carbon content of a pre-reduced iron ore needs to exceed 13–