Reduction Behaviors of Hematite to Metallic Iron …
119
Fig. 7 Arrhenius plots of
Fe 2 O 3 → Fe 3 O 4 . (Color
figure online)
The linear regression of the experimental data of −ln(1-X) against t determined
k r . According to the Arrhenius equation, a plot of lnk versus 1/T for the reduction
of Fe 2 O 3 to Fe 3 O 4 is shown in Fig. 7, indicating a good fit to Eq. (6). The activation
energy is estimated to be 17.39 kJ/mol form the slope of the regression line in Fig. 7.
Reaction Kinetics of Fe 3 O 4 → Fe
For the reduction of Fe 3 O 4 → Fe, it is described as a single-step reaction in this study.
Accordingly, the gas flow rate is maintained at 400 mL/min (at NTP) that external
diffusion is considered to be eliminated. Therefore, the reaction rate of the Fe 3 O 4 →
Fe process is controlled by a phase boundary or internal diffusion mechanism. To
obtain the specific mechanism, a generalized Johnson-Mehl-Avrami (JMA) model,
which was developed by Hancock et al. [32], is used, and it has the integral form of
ln[− ln(1 − X )] = n ln k + n ln t
(7)
where n is the kinetic exponent, which depends upon the controlled mechanism of
reduction [33]. The plot of ln[1–ln(1–X)) against lnt is illustrated in Fig. 8. Data in
Fig. 8 show that the slopes of the resulting straight lines increased with increasing
temperature. The value of n range from 1.017 to 1.211 with R
2 for all fits being
greater than 0.99, indicating a phase boundary-controlled mechanism [32]. If this
is the case, the reaction process may be described by a 3D geometrical contraction
models, which can be expressed as
1 − (1 − X )
1 / 3 = kt
(8)
119
Fig. 7 Arrhenius plots of
Fe 2 O 3 → Fe 3 O 4 . (Color
figure online)
The linear regression of the experimental data of −ln(1-X) against t determined
k r . According to the Arrhenius equation, a plot of lnk versus 1/T for the reduction
of Fe 2 O 3 to Fe 3 O 4 is shown in Fig. 7, indicating a good fit to Eq. (6). The activation
energy is estimated to be 17.39 kJ/mol form the slope of the regression line in Fig. 7.
Reaction Kinetics of Fe 3 O 4 → Fe
For the reduction of Fe 3 O 4 → Fe, it is described as a single-step reaction in this study.
Accordingly, the gas flow rate is maintained at 400 mL/min (at NTP) that external
diffusion is considered to be eliminated. Therefore, the reaction rate of the Fe 3 O 4 →
Fe process is controlled by a phase boundary or internal diffusion mechanism. To
obtain the specific mechanism, a generalized Johnson-Mehl-Avrami (JMA) model,
which was developed by Hancock et al. [32], is used, and it has the integral form of
ln[− ln(1 − X )] = n ln k + n ln t
(7)
where n is the kinetic exponent, which depends upon the controlled mechanism of
reduction [33]. The plot of ln[1–ln(1–X)) against lnt is illustrated in Fig. 8. Data in
Fig. 8 show that the slopes of the resulting straight lines increased with increasing
temperature. The value of n range from 1.017 to 1.211 with R
2 for all fits being
greater than 0.99, indicating a phase boundary-controlled mechanism [32]. If this
is the case, the reaction process may be described by a 3D geometrical contraction
models, which can be expressed as
1 − (1 − X )
1 / 3 = kt
(8)