2
66
L. Rongti et al. / Thermochimica Acta 398 (2003) 265–267
Reagent) were used in this study. Magnesia, graphite
powder and transition metal were mixed in a molar
ratio of 1:1:1/2 and the mixture was shaped into a
Combining Eqs. (2) and (3) and taking logarithms,
one obtains
ꢀ
ꢁ
dα
dt
Ea
5
mm diameter pellet at 100 MPa with a cold press.
ln
= ln[Af (α)] −
(4)
RT
The graphite crucible used in this study was 6.4 mm
o.d., 5.8 mm i.d. and 5.5 mm in height.
In Eq. (4), f(α) is determined only by α. When we
choose the data of dα/dt and the absolute temperature,
T, at the same value of α from experiments at differ-
ent heating rates, there should be a linear relationship
between the chosen ln(dα/dt) and 1/T. The slope of
the line gives the activation energy Ea. This method is
called the multi-heating rate method.
The reduction experiments were carried out with a
thermo-balance (NS95, Sinku-riko, Japan) interfaced
to a computer data acquisition and analysis system.
The sample was put into the graphite crucible, which
was positioned on the top of the thermocouple. The
inert atmosphere was maintained with argon at a flow
3
−1
rate of 1.67 cm s . After 30 min, the sample was hea-
ted from room temperature to 773 K and held at this
temperature for 10 min to remove the absorbed gases
and water from the sample. Then the temperature was
raised to 1973 K at different heating rates. To correct
for the effect of the graphite crucible, blank experi-
ments were carried out with blank graphite crucibles.
Blank runs were carried out under the same conditions
as those used for the samples. The sample phases
were analyzed by XRD (Rigaku D/MAX IIIB, Japan).
3. Experimental results and discussion
3.1. Effects of Cu, Ni, and Co on the reduction
ratio
The effects of Cu, Ni and Co on the reduction ra-
tio as a function of temperature are shown in Fig. 1.
The curves show no obvious effect of the transition
metals at temperature lower than 1600 K. At tempera-
ture higher than 1600 K, the reduction ratio increases
with temperature and the catalytic effect of the tran-
sition metals is clearly shown. Nickel is most active
followed by copper and cobalt.
Cu, Ni and Co accelerate the reaction, but do not
decrease the initial reaction temperature. Furthermore,
Cu, Ni and Co are also added as impurities when used
in the steel-making industry.
2
.2. Non-isothermal kinetics
During the thermogravimetric reduction experi-
ment, the mass loss of the sample is monitored as
a function of time. The reduction ratio α at a given
instant is defined as
ꢀ
W
α =
(1)
W0
where W0 represents the initial mass of magnesium in
the sample and ꢀW is the magnesium mass change to
that instant.
For an irreversible reaction, the reaction rate can be
expressed as
dα
−
= k(T )f (α)
(2)
dt
where f(α) is a function of α, the fraction of reaction
or reduction ratio, and k(T) is the rate constant, whose
relation with the temperature can be given by the
Arrhenius equation as
ꢀ
ꢁ
Ea
k(T ) = A exp −
(3)
RT
where A is the pre-exponential constant, R the gas
constant and Ea is the activation energy.
Fig. 1. Effects of Cu, Ni and Co on the reduction ratio.