As
the water content is higher than 20%, the changing curves of
water
content decline rapidly because of the easy removal of free
water.
Then, water content decreases slowly till to the endpoint
resulting
from the compulsory removal of crystal water. These
changing
trends were linked with the temperature changes. As
seen,
the temperature experiences a linear increase below 100
C,
and
then a sluggish rise at the range of 100120
C, and followed
with
a rapid increase above 120
C. It corresponded with relatively
rapid,
most rapid and slow decline of water contents in microwave
heating
process. The linear increase of temperature below 100
C is
because
that free water absorbs large amounts of microwave
energy.
Then, a sluggish rise in temperature occurs as the boiling
point
of water (100
C) is reached, accompanying with the fastest
dehydration
rates, due to the massive loss of vaporization heat.
Subsequently,
the rise started at about 120
3
H(SO
4
)
2
(%) NaHSO
C and sharply
increased
to desired temperature. According to previous studies
[24,25],
the elevated temperature velocity has a positive correlation
with the loss tangent (tan d) of material. tan d reflects the
ability
of a material or a medium to convert electromagnetic
energy
into heat at a given frequency, which is relevant to the
temperature
of materials. High tan d implies that the material can
couples
well with a microwave field and leads to rapid heating. It is
supposed
that the sharp rise should be ascribed to the increased
tan
d of heated substance [25], mainly NaHSO
, after the
temperature
reaches about 120
4
H
2
O (%) NaHSO
(%)
40
85.3 14.2 0.3
20
6.4 87.4 6.2
10
0.9 75.3 23.8
3
0.0 17. 8 82.2
4
C