Abstract:
Direct Reduced Iron (DRI) is used as an alternative feedstock in electric arc furnaces,
making up 50% or more of the total iron charge. DRI produced with coal based
reductants (for example in rotary kilns) make up roughly 25% of DRI produced in
the world. It was found that SL/RN DRI samples from a kiln cooler had high
nitrogen contents (50-250ppm, depending on particle size), higher than DRI from
gas-based reduction. The higher nitrogen content of SL/RN DRI would increase the
levels of nitrogen of liquid steel produced in the EAFs. The problem is exacerbated
by the fact that the SL/RN DRI contains virtually no carbon (which would aid in
preventing nitrogen pickup). The proposed mechanism of nitrogen pick-up by the
SL/RN DRI is one where nitrogen present within the atmosphere of the rotary cooler
(where hot DRI, discharged at 1000°C from the rotary kiln, is cooled to
approximately 100 °C in ca. two hours) penetrates the solids bed and nitrides DRI
particles. Possible rate-determining steps for nitriding in the cooler have been
evaluated. Nitriding of DRI particles is predicted to be rapid: the most plausible
location for rapid nitrogen pickup is the first 5 meters of the rotary cooler, where the
high temperature, nitrogen-rich gas atmosphere and rapid solids bed mixing are
conducive to nitriding; solid-state and pore diffusion of nitrogen into DRI particles
are predicted to be rapid too. The most plausible rate determining step for nitriding
of DRI particles is that of nitrogen dissociation on the DRI surface, which can be
further retarded by the presence of sulphur. A strong correlation was found between
the amount of “melt-in” carbon in the liquid steel and the final tap nitrogen content,
with 0.3% C resulting in nitrogen levels as low as 50 ppm (80 ppm or less is desired
on the plant in question) at tap, even with DRI material that is high in nitrogen and
contains virtually no carbon. Proposals to increase the melt-in carbon are included.
