Thermal Decomposition, Relative Humidity Study and X-ray
Diffraction on Aluminum Sulfate Hydrate Granules
Rickard Johansson
Department of Chemical Engineering, Lund University, P. O. Box 124, SE-221 00 Lund, Sweden January 2011
Abstract
Different samples of granulated aluminum sulfate 14-hydrate granules (ALG) produced by Kemira
Kemi AB underwent several experiments where they were exposed to an increasing temperature and
different relative humidities to determine its dehydration derivative and hygroscopicity. X-ray
diffraction (XRD) was used to establish if the ALG product is a mixture of different hydrates rather
than a homogenous 14-hydrate compound. This was done to determine how the chemically bound
water in the ALG interacts with the surrounding air under different circumstances. The results show a
fast dehydration in all samples even at low temperatures (313 K). A control set of samples together
with the relative humidity exposure tests suggest that the granules produced are very much
hygroscopic with a capacity of absorbing >12 weight-% water from surrounding air at a relative
humidity of 93 % at room temperature (298 K). XRD analyses were unable to identify other hydrates
than aluminum sulfate 17-hydrate in the granules but show that the granules are predominantly of an
amorphous nature. Results from the XRD analysis indicate that the ALG produced is a mixture of
different hydrates.
Keywords: Granulation, Thermal decomposition, Aluminum sulfate hydrate, hygroscopicity, X-ray diffraction,
XRD, amorphous, degree of crystallinity
Introduction
The aim of this study is to determine how the
ALG produced at Kemira Kemi AB react on a
phased temperature increase, different relative
humidities and to determine what different
hydrates are present in the ALG.
Aluminum sulfate hydrate is a widely used
flocculating agent in the paper industry as well
as the waste water treatment (Ornek et al,
1998, Cigli G.K and Cetisli H, 2009) It is
produced by mixing Aluminum hydrate
hydroxide with sulfuric acid and water under
pressure at a temperature of 383 K. The
product is a highly viscous melt known as
aluminum sulfate hydrate melt. The reaction is
presented below (Ornek et al, 1998)
2 (
) + 3
+ 12
→
(
This study of the ALG product is to some
extent based on studies made by Ornek,
Gurkan and Oztin on aluminum sulfate 18hydrate produced by Seydishehir Aluminum
Sulfate Factory in Turkey.
) ∙ 18
Ornek, Gurkan and Oztin observed, during a
Thermogravimetric analysis (TGA) run that
Al2(SO4)3·16 H2O is already unstable at room
temperature and is partially dehydrated to
Al2(SO4)3·14 H2O. A dehydration reaction that
release 2 moles of water. The Al2(SO4)3·14
H2O was further dehydrated as the temperature
was increased. (Ornek et al, 1998)
At Kemira Kemi AB this aluminum sulfate
hydrate melt is mixed in a rotating granulation
drum with coalescence crystals (granulated
aluminum sulfate 14-hydrate, ALG) to create
granules with a diameter of 0.5 to 3 mm.
After granulation is completed the granules are
cooled down in a counter current cooling drum
from temperatures of around 338 K to 321 K.
Air from outside surroundings is used as
cooling media with a temperature of 268 K to
288 K and a relative humidity of 40 – 95 %
(depending on the season).
If higher hydrates than 14 is present in the
mixture they will most likely undergo a
dehydration reaction at higher temperatures
than 298 K and release water. The dehydration
1
reaction, presented by Ornek, Gurkan and
Oztin is illustrated below.
(
) ∙ 16
→
(
) ∙ 14
(
) ∙ 14
→
(
) ∙ 8
The final set of tests was an X-ray diffraction
analysis. Four different samples were analyzed
using an X-ray diffractometer. Before analysis,
the fresh samples were ground into a fine
powder and the intensity was measured
between Bragg angle (2θ) 5 and 450. When the
tests were completed, the intensity peaks were
compared to known aluminum sulfate hydrate
peaks to determine what hydrates the ALG
product samples were composed of.
+2
+6
Materials and Methods
The experimental procedure is divided into
three different tests; phased temperature
increase, controlled relative humidity variation
(hygroscopicity test) and X-ray diffraction
analysis.
Results and Discussion
The results from the phased temperature
increase test are presented in Figure 1.
The phased temperature increase test was made
on three different samples of ALG from
different days in operation. The samples were
stored in room temperature in sealed plastic
containers for 2-5 days before the test.
10 grams of each sample was weighed and put
in a Heraeus VT 5050 EK vacuum drying
cabinet and exposed to an initial temperature
of 313 K. The weights of the samples were
logged on a regular basis during the entire test
period. The temperature was increased further
to 333, 373 and 398 K when the dehydration
curves reached constant values and no more
water was evaporated from the granules.
A set of three control samples was
concurrently exposed to a temperature of 296
K with varying uncontrolled relative humidity.
Figure 1. Amount of evaporated water as a function
of time with a phased temperature increase
As seen in Figure 1, the evaporation rates for
the three different samples have a similar
appearance and differ just slightly. After 76
hours in 313 K 0.1897 grams of water had
evaporated and the temperature was increased
to 333 K. During the following 246 hours,
additional 0.7891 grams of water was
evaporated before the dehydration curve
reached a constant value. The temperature was
once again increased from 333 K to 373 K and
1.4806 grams water was evaporated during a
period of 118 hours. The final increase in
temperature from 373 K to 398 K, 0.4068
grams of water was evaporated during 403
hours and the tests were terminated after 891
hours.
To study the ALG:s hygroscopicity, the
granules were exposed to different relative
humidities at room temperature (298 K). The
tests were performed on four different samples
from the aluminum sulfate hydrate granule
factory. The different samples were exposed to
the different relative humidities within an hour
after production. The sample weight ranged
between 5.5 to 8 grams of product. Six
different relative humidity environments were
tested (8, 33, 42 56, 82 and 93 %). The relative
humidity was kept constant inside desiccators
by using saturated salts with an affinity for
water that regulates the water vapor pressure in
the air surrounding the granules. The samples
were weighed every day for 11 days and the
weight difference was plotted against the time
to create a water absorption/desorption curve
over time. This hygroscopicity test was based
on tests performed by Ornek et al.
Cigli and Citisli concluded that the activation
energy values increases as the amount of
hydrate decreases in the aluminum sulfate
hydrate. This suggests that the appearance of
the graph in Figure 1, where the dehydration
curves reach constant values, can be explained
2
by a phased dehydration of the hydrates in the
granules. The dehydration curve reaches a
constant value when the energy supplied by the
surroundings is lower than the activation
energy value to initiate the next dehydration
reaction.
At lower relative humidities water is desorbed
from the granules to the surrounding air and
can be considered constant at 33 % relative
humidity. Below 33 % relative humidity all
samples desorb water to the surroundings.
The control samples, exposed to a temperature
of 296 K with varying relative humidity is
presented in Figure 2.
Figure 3.Absorption of water (weight-%) in hygroscopicity test at room temperature (298 K)
Figure 2. Total weight of sample 1,2 and 3 as a
function of time
The X-ray diffraction analysis is presented in
Figure 4.
The samples illustrated in Figure 2 show an
increase in total weight where sample 1 shows
the highest water absorption potential. This
sample was the most recent sample with
shortest storage time of 2 days. Sample 3 show
the second largest water uptake, this sample
was stored the longest (5 days). Sample 2 has
the lowest water uptake and was stored for 4
days.
The weight curves have a similar appearance
and varies with every weighing but the
tendency is clear; the salt absorbs and release
water to the surrounding air.
Figure 4. X-ray diffraction diagram
The X-ray diffraction diagram indicates that
the ALG produced is to a large extent
amorphous. The XRD as an analysis tool can
only detect crystalline structures but was still
able to detect higher aluminum sulfate hydrates
in the samples. Due to the low crystallinity of
the product, the displayed intensity from the
run cannot be used to determine the total
amount of a specific hydrate in the granules.
Whether the time of storage in sealed
containers affect the hygroscopicity cannot be
determined from this set of control samples as
the results are inconclusive.
The hygroscopicity test (controlled relative
humidity) is presented in Figure 3.
The only hydrate that could be found in ALG
with any degree of certainty was aluminum
sulfate 17-hydrate.
As Figure 3 suggest, the absorption of water
from the surrounding air increase with an
increase in relative humidity at a certain
temperature.
Calculations made on newly produced
aluminum sulfate 14-hydrate granules show
that the mean hydrate is 14.2 rather than 14
3
which suggest a mixture with hydrates at least
as high as, proven by XRD analysis, 17hydrate.
The ALG produced at Kemira Kemi AB has an
amorphous internal crystalloid structure with
low degree of crystallinity.
At the current mode of operation the ALG is
exposed to as high temperatures as 338 K,
which, according to the phased temperature
increase tests, can initiate a dehydration
reaction. When this occur, water is released to
the surrounding air and higher hydrates will be
partially or completely dehydrated to lower
hydrates depending on the exposure time.
References
Cigli G.K and Cetisli H. (2009) Thermal
decomposition kinetics of aluminum sulfate
hydrate Journal of Thermal Analysis &
Calometry, vol 98, pp 855-861
Ornek D., Gürkan T., Oztin C. (1998) Physical
and Chemical Properties of a Highly Viscous
Aluminum Sulfate Melt, Industrial &
engineering chemistry research, American
Chemical society, 37 (7), pp 2687–2690
This is supported by what Ornek et al.
observed in their TGA where higher hydrates
are partially dehydrated down to lower
hydrates.
If aluminum sulfate 17-hydrate is the only
crystalline hydrate in the mixture cannot be
determined by the XRD analysis and what
other hydrates are present in the amorphous
fraction is impossible to speculate about when
no further analysis methods are used.
Conclusions
The tests show that the ALG product is
dehydrated rather quickly at quite low
temperatures from a higher hydrate to lower
and release water in the process. The ALG
product contain a mixture of different hydrates
with some hydrates as high as 17. The product
has a mean hydrate value of 14.2 but the total
amount and what other hydrates are present
could not be determined using solely XRD as
an analysis tool. To be able to identify the
different hydrates hidden in the amorphous
fraction of the crystals another analysis method
than XRD has to be applied. A closer
decomposition of the ALG product study has
to be made.
The hygroscopicity test and the control set of
samples strongly suggest that the ALG is
hygroscopic and has the capacity to absorb
over 12 weight- % of water from the
surroundings in an environment with a relative
humidity of 93 % at room temperature (298 K)
over a period of 11 days. The hygroscopicity
tests show that ALG exposed to an
environment with 33 % relative humidity at
room temperature has an almost constant
weight and therefore release or take up very
little water from the surroundings. Below 33 %
relative humidity at 298 K the ALG product
absorb will water from the surrounding air.
4