Dehydration of sodium carbonate monohydrate with indirect microwave heating

Article abstract of DOI:10.1016/j.tca.2006.06.020

In this study, dehydration of sodium carbonate monohydrate (Na₂CO₃·H₂O) (SCM) in microwave (MW) field with silicon carbide (SiC) as an indirect heating medium was investigated. SCM samples containing up to 3% free moisture were placed in the microwave oven. The heating experiments showed that SCM is a poor microwave energy absorber for up to 6 min of irradiation at an 800 W of microwave power. The heat for SCM calcination is provided by SiC which absorbs microwave. The monohydrate is then converted to anhydrous sodium carbonate on the SiC plate by calcining, i.e. by removing the crystal water through heating of the monohydrate temperatures of over 120 °C. The calcination results in a solid phase recrystallization of the monohydrate into anhydrate. In the microwave irradiation process, dehydration of SCM in terms of indirect heating can be accelerated by increasing the microwave field power.

Full text of DOI:10.1016/j.tca.2006.06.020

Thermochimica Acta 448 (2006) 31–36
Dehydration of sodium carbonate monohydrate with
indirect microwave heating
Abdullah Seyrankaya∗, Barıs¸ Ozalp
Dokuz Eylul University, Engineering Faculty, Mining Engineering Department, Division of Mineral Processing,
35160 Buca, Izmir, Turkey
Received 17 August 2005; received in revised form 22 June 2006; accepted 23 June 2006
Available online 29 June 2006
Abstract
In this study, dehydration of sodium carbonate monohydrate (Na2CO3·H2O) (SCM) in microwave (MW) field with silicon carbide (SiC) as
an indirect heating medium was investigated. SCM samples containing up to 3% free moisture were placed in the microwave oven. The heating
experiments showed that SCM is a poor microwave energy absorber for up to 6 min of irradiation at an 800 W of microwave power. The heat for
SCM calcination is provided by SiC which absorbs microwave. The monohydrate is then converted to anhydrous sodium carbonate on the SiC
plate by calcining, i.e. by removing the crystal water through heating of the monohydrate temperatures of over 120 ◦C. The calcination results in
a solid phase recrystallization of the monohydrate into anhydrate. In the microwave irradiation process, dehydration of SCM in terms of indirect
heating can be accelerated by increasing the microwave field power.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Mineral processing; Sodium carbonate monohydrate; Drying; SiC; Microwaves
1. Introduction
alternatives:
Na2CO3H2O(s) → Na2CO3(s) + H2O(g)
ꢀH298 = 58.770 kJ/mol H2O
,
Sodium carbonate, soda ash are used as a source of Na2O
in the glass manufacture, in the production of various sodium
chemicals for water treatment, paper production, iron desulphur-
ization, and many other uses. Although, production from brines
and fossil trona deposits is becoming increasingly important,
soda ash is mostly produced synthetically with Solvay pro-
cess which requires common salt and limestone by some 60
solvay plants in the more than 40 countries [1]. The sodium
carbonate is recovered from its aqueous solution as sodium
carbonate monohydrate by atmospheric evaporative crystalliza-
tion. The monohydrate is then converted to anhydrous sodium
carbonate by calcining, i.e. by removing the crystal water
through heating of the monohydrate to temperatures of 120 ◦C
(1)
The drying and dehydration of the monohydrate is carried out
more frequently in steam-tube rotary dryers where the crystals
are dehydrated into anhydrous soda ash at 150–200 ◦C; how-
ever, fluidized bed dryers are also used [3,6]. The final product
made by the monohydrate process is dense soda ash. Both these
dryers have some disadvantages. In rotary dryers, mixing is not
good and gas temperatures are low. In fluidized bed dryers, the
possibility of powder formation and dusting of the product is
quite high as a result of attrition caused by collision of particles
[7,8].
Energy transfer as heat from the surrounding environment
to the wet solid can occur as a result of convection, conduc-
tion, or radiation and some cases as a result of combination
of these effects [9]. With this study a new drying technique
is exposed as indirect drying with MW, heat is transferred by
conduction to the material, through a susceptor. Silicon carbide
was chosen as a susceptor due to its absorption characteris-
tics of microwaves (MWs). Earlier studies on MW heating of
¨
or higher (see Eq. (1)). In addition to this Gartner [2], Garrett
[3], Haynes [4], Aitala and Aitala [5] gave excellent reviews
of the current state-of-the-art processes and feasible process
∗
Corresponding author. Tel.: +90 232 412 7519; fax: +90 232 453 0868.
E-mail address: a.seyrankaya@deu.edu.tr (A. Seyrankaya).
0040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2006.06.020

32
A. Seyrankaya, B. Ozalp / Thermochimica Acta 448 (2006) 31–36
ceramic materials show that SiC absorbs MWs considerably
resulting in greater heating in SiC [10,11]. One of the reasons
to use ceramic in this study is that the ceramics can withstand
high temperature. In addition, the temperature profile within the
ceramic materials is uniform due to higher thermal conductivity
and these materials may be the perfect choices to be used as
supports.
Inrecentyears, microwaveenergyhasfoundincreasingappli-
cation to the thermal processing of various kinds of materials
and products, such as agricultural, chemical, mineral, food, tex-
tile, paper and lumber [7,12–14]. MWs propagate through the
materials and the accompanying transport processes result in
dissipation of electric energy into heat, which lead to the term
‘volumetric heat generation’ due to MW radiations [7]. MWs are
widely preferred over conventional heating mainly for shorter
processing times. During conventional heating the heat is radi-
ated from the burner to the surface of the material and the
material is heated due to surface heat flux whereas during MW
heating, thematerialdielectriclossisresponsibletoconvertelec-
tric energy into heat within the entire volume [10]. The unique
heating mechanisms of microwaves permit dramatic energy sav-
ings in many instances, as well as providing competitive benefits
[9,13].
2.2. Solid phase analysis
As the crystal water of the monohydrate becomes unstable
upon heating above ca. 120 ◦C, it desorbs to the surrounding
air and monohydrate recrystallizes to anhydrate (Eq. (1)). The
accompanying weigh loss of the sample due to desorption of the
water can be used to determine the amount of monohydrate in the
sample. The weigh loss for pure sodium carbonate monohydrate
is
M
H2O
XNa CO ·H O
=
= 0.1453
(2)
2
3
2
M
Na2CO3·H2O
The weigh loss for pure sodium carbonate anhydrate is
XNa CO = 0
(3)
2
3
Whether the dehydration is complete or not is determined by
placing the sample in an oven at 150 ◦C for 24 h. The amount of
crystal water in the samples was calculated from the decrease in
weight. The identification of a crystal phase was, of course, not
only based on the weigh loss of a sample, but also verified either
from light microscopy, scanning electron microscopy (SEM)
images and powder X-ray diffraction (XRD).
Although drying techniques are cornerstones of many indus-
trial applications and also almost universally understood these
techniques have undergone dramatic developments over the last
30 years. In this paper, the results of the experimental work to
dryoffthe crystalwater fromthesodiumcarbonatemonohydrate
powder using microwave energy are presented.
2.3. Temperature measurement
Simultaneoustemperaturemeasurementinacontinuouselec-
tromagnetic field using a thermocouple is a major problem as
the thermocouple–field interaction leads to gross errors [9,15].
Some researches found that a very thin metal sheath surround-
ing the thermocouple could be used to overcome this prob-
lem, but contact between the thermocouple tip and the metal
sheath must be avoided, while, shielding reduces the response
of the thermocouple [15]. To avoid such a problem, the tem-
perature of the irradiated sample was measured by DT-8819
non-contact infrared thermometer immediately after turning off
the microwave power.
2. Experimental
2.1. The indirect microwave heating system
The present system was designed to generate a moderate ther-
mal gradient, providing uniform and sufficient heating of the
sample. In this study a Premier, PMO-20 microwave oven was
modified as an indirect microwave heating system. It has a fre-
quency of 2450 MHz with a maximum output power of 800 W.
Drying experiments were conducted at six different microwave
output powers of 136, 168, 264, 440, 616 and 800 W. The heat-
ing profile of the oven conducted with water only shows an
almost uniform magnetic field. The test was conducted by heat-
ing 100 ml water in a beaker and reading its temperature with
about 5 s delays, i.e. interrupting the power and reading the tem-
perature.
A ␤-SiC plate (15 cm diameter) used as the susceptor mate-
rial, containing the sample forming a layer of 1–2 mm thick-
ness, was placed inside microwave cavity during treatment for
absorption of microwave energy. Dehydration experiments were
performed with 50.08 0.01 g samples (where shows stan-
dard error of the mean, S.E.M.). For the mass determination, a
digital balance of 0.01 g accuracy (Sartorius GP4102-OCE) was
used. A new sample was used at each microwave output power
selected for drying. Each experiment was replicated at least three
times.
2.4. Absorption characteristics of SiC and SCM
TherateofMWenergyabsorptionbyamaterialisdetermined
by its dipole rotation and ionic conductance characteristics [16].
Here SiC is absorbed MW energy that provides heating by dipole
rotation [15]. The amount of thermal energy deposited into a
material due to microwave heating (power absorption density)
is dependent on the internal electric field strength within the
minerals, the frequency of the microwave radiation, and on the
dielectric properties of the material [17]. If the electric field
strength is known the power absorption density per unit volume
for dipolar rotation of the mineral can be approximated from
Eqs. (4) and (5):
Pv = kE2fεꢀ tan δ
or
(4)
Pv = kE2fεꢀꢀ
(5)

A. Seyrankaya, B. Ozalp / Thermochimica Acta 448 (2006) 31–36
33
Table 1
The thermal and dielectric properties of SiC, ZrO2, Al2O3, and sodium carbonate, as collected from [10,11,18]
Material property
Water
SiC
Al2O3
ZrO2
Na2CO3
2533a
0.0418
1042
5.3–8.4
Density, ρ (kg m−3
)
1000
3100
40
3300
26.66
27.99
3750
26
1046
10.8
6100
Thermal conductivity, k (W m−1 K−1
)
0.609
4190
78.1
10.44
2 at 500 ◦C
2468 at 500 ◦C
–
Heat capacity, Cp (J kg−1 K−1
)
Dielectric constant, εꢀ (at 2450 MHz)
Dielectric loss, εꢀꢀ (at 2450 MHz)
0.1566
0.034 at 25 ◦C
–
a
Absolute density at 25 ◦C.
where Pv is the power density (W/m3), f the frequency of the
microwave radiation (MHz) current research is on 2.45 GHz,
εꢀ the relative dielectric constant or relative permittivity, εꢀꢀ the
dielectric loss factor of the mineral, tan δ the loss tangent or
dissipation factor, and E is the electric field strength (V/m).
If a material does not have significant magnetic properties,
only complex permittivity can be considered and defined by the
following equations:
ε∗ = εꢀ − jεꢀꢀ
εꢀꢀ
tan δ =
εꢀ
(6)
(7)
The complex permittivity, ε*, is related to the ability of material
tocoupleelectricalenergyfromamicrowavefield. Thedielectric
constant, εꢀ, is a measure of the ability to store electrical energy
in the material, and the loss factor, εꢀꢀ, is related to the amount
of energy a dielectric material can dissipate in the form of heat,
and j = (−1)1/2. The loss tangent, tan δ, is the ratio of dielectric
loss to dielectric constant.
Fig. 1. Heating behaviour of SCM in the microwave oven at 800 W for 6 min.
depth is defined as
√
ꢀ
ꢁ
ꢃꢄ−1/2
ꢂ
λ0
2
Dp =
εꢀ
1 + (εꢀꢀ/εꢀ)2 − 1
(8)
2π
To explain the influence of microwave heating, the specific
heat, thermal conductivity and dielectric properties of materials
have to be considered. From Table 1, it is clear seen that SiC is
the most suitable susceptor due to its high thermal conductivity
and dielectric loss.
where, DP is the penetration depth, λ0 is the free space wave-
length defined by λo = c/f, c being the velocity of light and f
being the frequency.
In general, if the penetration depth exceeds sample thickness,
the material behaves transparent. Volumetric heating results, if
the penetration depth is comparable with the sample thickness.
At a fixed microwave frequency, only the material proper-
ties ε* and tan δ determine the total power dissipated within the
sample. Both these parameters depend strongly on temperature.
InordertounderstandinteractioncharacteristicsofSCMwith
MW, SCM powder, was spread on the Teflon line sheet, placed
into microwave oven and then irradiated at three power levels of
136, 440 and 800 W. The results of irradiation tests, presented in
Fig. 1, show that the SCM was a poor absorber of the microwave
energy for up to 6 min of irradiation. The sample’s temperature
increased to 55.30 3.56 ◦C during 6 min at the highest power
level of 800 W. A few minutes exposure, even at low power
levels (136 W), the temperature of SiC plate increased rapidly.
The figure clearly indicates the strong absorption of the SiC
plate. Experimental time versus temperature curves for SiC and
SCM are given in Figs. 2 and 3 for increasing power loadings.
2.5. Penetration depth
The dielectric constant and dielectric loss determine the depth
of effective energy penetration into material. The penetration
Fig. 2. Effect of power-dependent microwave absorption on the heating of SiC
plate. Error bar represents S.E.M. (95%, N = 3).

34
A. Seyrankaya, B. Ozalp / Thermochimica Acta 448 (2006) 31–36
Fig. 4. Sodium carbonate ratios after MW irradiation of SCM at different
microwave energy levels.
Fig. 3. Effect of power-dependent microwave absorption on the heating of SCM
on SiC plate. Error bar represents S.E.M. (95%, N = 3).
heating weight loss analysis with heating in an oven at 150 ◦C for
24 h. It was shown that the dehydration began almost immedi-
ately under microwave energy at a bulk temperature of 140 ◦C.
Eighteen percent of SCM was converted within only 2 min at
440 W. At 136 W no dehydration was observed, most probably
because the temperature did not increase significantly.
In an attempt to understand the effect of the different dry-
ing techniques on the quality of the produced sodium carbonate
anhydrate, a comparison was made between dehydration and
drying in a fluidised bed versus indirect microwave heating using
withtheexcellentstudyofChenetal. [6]. Accordingtothisstudy
monohydrate conversion begins approximately 100 s after the
operation commences, a steady-state conversion of about 0.33
is reached after about 104 s. As seen from Fig. 4 in MW heating
complete conversion of SCM was achieved within only 2 min
at 800 W. In addition to this the results of the microwave versus
conventional heating experiments are clearly shown in Fig. 5 as
conversion of the SCM versus time. Moreover, microwave heat-
ing provides great time savings, compared with conventional
drying.
Otherwise, if the sample is too thick, absorption limits to the
surface.
The results of irradiation tests, presented in Figs. 2 and 3,
showed that while penetration depth exceeds the SCM thickness,
it is comparable with the SiC plate. Due to the small penetration
depth in SiC, the power decays rapidly from the faces. The prop-
erty of SiC to rapidly heat in the presence of MWs is exploited
to indirectly heat low loss materials like SCM, with SiC being
used as a susceptor causing heat to be transferred by radiation to
low loss sample. The microwave penetration depth into the SiC
plate also decreases as the temperature rises. Similar findings
were reported in previous studies [15].
3. Results and discussion
The SCM is a relatively poor microwave absorber (Fig. 1)
than the SiC. Since, SiC is an excellent microwave absorber
(Fig. 2), it was used to provide the quick warm-up of the SCM
and the results are shown in Fig. 3. The samples are processed for
6 min at 136, 168, 264, 440, 616 and 800 W. It can be seen that
usingSiCplateprovidedasignificantincreaseinthesampletem-
perature. Thus, a maximum temperature of 385 ◦C was achieved
at 800 W. This strategy may be advantageous for achieving a
greater heating rate.
The extent of the temperature increase can be explained rather
by the thermal runaway phenomenon than by simple thermal
conduction alone. This type of behaviour has been reported by a
number of authors [19–21]. Though thermal conduction occurs
to some extent, the dielectric loss tangent increases rapidly with
temperature and thus steep temperature increases can occur. The
area of the sample which interacts with SiC will heat faster than
the rest of the sample. Then they will transfer their own temper-
ature to the other parts of the sample and this phenomenon will
occuragain. Thermalrunawayisacomplicatedphenomenonand
depends on sample size, geometry and the relationship between
the relative dielectric constant (εꢀ) and temperature [21].
It has been seen that the complete drying or conversion occur
at 440 W and upper power levels as shown in Fig. 4 from the
Fig. 5. SCM conversion of conventional and microwave heating.

A. Seyrankaya, B. Ozalp / Thermochimica Acta 448 (2006) 31–36
35
Fig. 6. XRD pattern of Na2CO3·H2O before MW irradiation.
Fig. 7. XRD pattern of Na2CO3 after MW irradiation of SCM.
This observation was confirmed by the analysis of the XRD-
patterns of the samples. While powder X-ray diffraction of sam-
ple essentially showed peaks corresponding to SCM in Fig. 6,
in Fig. 7 powder X-ray diffraction of the microwave-irradiated
product essentially showed peaks corresponding to anhydrous
sodium carbonate.
SEM photographs are depicted in Figs. 8(a and b)
and 9(a and b). While in the first picture, monohydrate crys-
tals, orthorombic, long needle-shaped crystals are visible (see
Fig. 8(a)), monoclinic structure of the anhydrate crystals can be
seen very clear in Fig. 9(a).
Calcination (i.e. thermal solid-state recrystallization) of SCM
usually results in agglomerates (pseudomorphs) of micron size
anhydrate crystals (Fig. 9(b)). As seen in this image, the shape
of the original crystal is maintained, but it is composed of tiny
anhydrate crystals. The crystal structure of the monohydrate dif-
fers significantly from the anhydrate—so the anhydrate does not
simply form by taking the water out of the monohydrate. The
monohydrate crystals have to dissolve, while anhydrate crystals
have to nucleate and grow. The pseudomorphs form, because
there is no medium, in which the ions (the building blocks of the
crystal) can move easily (like a solution). Therefore, the recrys-
tallization occurs usually in the solid state (short diffusion paths,
high super-saturation, and high degree of nucleation). A solid
phase recrystallization is very unlikely to produce well-shaped,
comparatively large anhydrate crystals.
The surface of anhydrate crystals looks rather rough (see
Fig. 9(a)). The prism-shaped crystals have noticeable pores. For
high calcination temperature, the anhydrate size becomes even
smaller and the pseudomorphs even denser, as the crystallites
sinter together. Also it can be seen that there is no caking forma-
tion occurred from Fig. 9(a). There is also no distinct influence
of the operating conditions in this hybrid system on the bulk den-
sity of the product. It was found that approximately 1000 kg m−3
aftercompletedehydrationwasachievedwhichisthesamevalue
of the product that produced with the conventional drying meth-
ods.
Fig. 8. SEM images of Na2CO3·H2O before MW irradiation: (a) long needle-shaped crystals, (b) small crystallites adhered to the surface of the SCM.

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A. Seyrankaya, B. Ozalp / Thermochimica Acta 448 (2006) 31–36
Fig. 9. SEM images of Na2CO3 after MW irradiation of SCM: (a) anyhydrate crystals, (b) agglomerates (pseudomorphs) of micron size anhydrate crystals.
4. Conclusions
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In many processes, the drying of the finished product has
a major influence on the production duration. To increase the
output of system, however, the production duration must be
reduced. Conventional drying methods offer little scope for
acceleration. Finding applications for the microwave process-
ing methods in these well-established and highly competitive
industries is extremely difficult. However, microwave drying is
fast and gentle on the product. This assures it a place among the
drying techniques of the future. In this investigation, the influ-
ence of microwave heating on the modification of dehydration
properties SCM was experimentally confirmed and the expres-
sive shortening of the exposing time observed in comparison
to the conventional heating. With the susceptor, SCM was con-
vertedtosodiumcarbonatewithindirectmicrowaveheating. The
increase in microwave power and air temperature significantly
reduced the drying time of the SCM crystals.
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The authors gratefully thank Mr. Ismail Ceylan and Mr. Ali
Ugurelli from Eti Soda Production Marketing Transportation &
Electricity Generation Industry & Trade Inc. and Dr.-Ing. Robert
¨
S. Gartner, Laboratory for Process Equipment, Delft University
of Technology, for their helpful discussions.
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