CHAPTER
VII
THERMOGRAVIMETRIC ANALYSIS OF
MAGNESIUM SILICATES AND BASIC
MAGNESIUM CARBONATES
The term thermobalance seems to have been
introduced by Honda in 19^5 to describe an instrument
that he constructed which continuously measured
weight changes of a substance at gradually varying
temperature95*"96*
In 1926, Saito described an
improved balance and in succeeding years various
other Japanese workers made further improvements
which allowed hundreds of pyrolysis curves to be
obtained.
Prom 1923 to 1928, French workers
Guichard and his students developed thermobalance
instrumentation and studied the scopes and limitations
of the method.
In 1943, Chevenard developed a
thermobalance which bears his name, that was both
rugged and sensitive enough to be employed in an
industrial laboratory.
In 1946, Duvaf7and his
coworkers began studies of the pyrolysis of
analytical precipitate employing the the Chevenard
thermobalance and in the following four years, they
accorded the pyrolysis curves of several hundred
inorganic precipitates.
Thus it can be seen that most of the
early work using thermobalance included in organic
materials and did not involve estimation of kinetic
156
parameters in pyrolysis such as frequency factor, reaction
order and activation energy.
to eliminate
One of the early attempts
these parameters for the pyrolysis of
organic compounds was made in 1951 by Van Erevelen^®
et al.
However,
the method proposed was tedious and
involved many assumptions.
In 1958 Freeman and Carrolf^
developed a widely used method for the determination of
reaction kinetics using thermobalance.
During the past
few years much progress has been made in developing more
suitable methods.
Thermogravimetric analysis
(TGA) is defined as
a continuous process which involves the measurements of
sample weight
as the reaction temperature is changed
by means of a programmed rate of heating.
In the present study the thermogravimetric
analysis was carried out by two different methods;
static and dynamic.
In a static method a b o u t 1 g of sample was
accurately weighed in a silica crucible and introduced
into the furnace maintained at constant temperature.
The range selected was from 100° to 900°C.
The samples
were heated for three hours during which the furnace
temperature was maintained constant.
They were
removed from the furnace,cooled in a desiccator and
weighed.
7.1
The TGA of different samples of magnesium
157
silicate was conducted by the above method.
Their
TGA curves are shown in Pig. 8,9, 10 and 11.
The curves can be divided into two groups.
(1) Those representing the decompositinn of the
products prepared by double decomposition process
(curves I to VI) and (2) those representing the
decomposition of the products prepared by the sol
process (curves VI?,VIII and IX).
The curves I to VI (Pig* 8 and 9) represent
the isothermal decomposition, (dehydration).
The
percentage decomposition is plotted against the tem­
perature of calcination.
In all the cases the de­
composition increases with the temperature of calcin­
ation, reaches the maxim**
value and thereafter remains
constant.
The curves obtained
are nonlinear but they
resemble in shape to the typical Freundlich adsorption
isotherm ( X *
The results were therefore
plotted on a log-log p a p e r ,
A straight line is expected
to be obtained and it was obtained when logarithm of
% decomposition was plotted against logarithm of temper­
ature of calcination.
It was observed that the percentage decomposition
of magnesium silicates ( curves I - VI) is related to
the temperature of calcination by the following equation.
D =» a t ^
158
where D is the percentage decomposition, t is the
temperature (°C) of calcination, a and b are constants.
The values of constants a and b as worked out
from the graphs are given separately in a Table 14.
Table 14 1 Values
of constants a and b of the
..... ....
equation D = at°
" ■
--------------- "
Product
'-
No.
''
^ T"
.........~
-
—
a
b
I
1.244
0.2164
II
6.459
0.2074
III
2.833
0.3469
IV
3.513
0.3102
V
5.112
0.2111
VI
3.867
0.2478
Thus it can be seen from the curves (represent­
ing thermolysis of magnesium silicates I - VI) that
magnesium hydroxide does not exist as an independent
entity in all the above magnesium silicates.
As a
result they behave in a different manner from those
prepared by sol process.
In all the above curves,
water comes out very slowly and it is difficult to
identify the existence of any arbitrary hydrate
or its stability at any temperature within the
selected range.
In the case of the 2nd group curves (curves
Fig- 83
"
D^iiRATUP.P °C
Thermal Dehydration of Magnesium Siiicat
c
0
Noixisodwooaa • %
Ob annivK
©'
„
So.
60
.
7*0
Mg.O 2.16 Si02 1.78-H20
JGf^
MgO 1.064 Si02 1.378 H20
MgO 3.354 Si02.2.473 HgO
THERMAL DEHYDRATION OF MAGNESIUM SILICATES
Vui
-Q-- O
4#
cn
o
7 0 /3
-- 1—
j:
HA
o-- o-
F i g . ; fyv
XI
% DECOMPOSITION
162
.i
Fig. 11
Temp„°C
THERMAL DEHYDRATION OF MAGNESIUM SILICATES.
163
VII - IX ) showing the percentage decomposition of the
products prepared by the sol process, the curves consists
of two portions.
The sudden change taking place at
300°C may be attributed to the decomposition of magnesium
hydroxide.
As seen earlier, in the differential thermal
analysis and X-ray diffraction studies that the compounds,
prepared by the interaction of magnesium chloride and
sodium silicates behave differently when compared to those
obtained by the interaction of magnesium hydroxide and
the corresponding silica sol prepared from the respective
silicates.
This difference in behaviour is further
substantiated with the help of thermogravimetric analysis.
To explain this, it should be assumed in the
first instance, that the products obtained by the sol
process are physical agglomerates of magnesium hydroxide
and silica, then the thermogravimetric curves can be
supposed to be the resultant curve representing the
total effect of the dehydration and decomposition
process of magnesium hydroxide as v/ell as the dehydrat­
ion and the dehydroxylation phenomena of the silica gel.
In order to get more precise information for
fuller understanding of the resultant curve, it appears
essential to discuss separately the thermolysis of
magnesium hydroxide and silica gel separately which
would be helpful in explaining
the behaviour of the
164
mixture of both.
Magnesium hydroxide:
It was prepared by treating
aqueous solution of magnesium chloride with strong
solution of sodium hydroxide s o l u t i o n 10?
The thermo­
lysis curve of magnesium hydroxide descends conti­
nuously until the temperature is reached at 820°C.
The adsorbed water begins to come off at room temp­
erature, the loss is rapid near 62°C, begins to
slow down at 134°C and is complete near 290°C. A n
.
abrupt change in the loss of weight is then observed
and the greater part of the combined water is elimin­
ated at 420°C and the rest of the water then departs
slowly.
Silica gel:
It was prepared by treating aqueous solution
of sodium silicate with solution of hydrochloric acid"
Silica gel loses water upto 150°C which is adsorbed
physically.
Water remaining on silica gel at 115°C
is present as layer of hydroxyl groups on the silica
surface.
This water is termed as the bound water.
Water evolved between 115°C - 600°C comes from the
dehydration of the -surfai'e
hydroxyl groups. Above
600°C there is sintering \d.th loss of silica surface
and simultaneous loss of water.
Now keeping in mind the thermolysis of the
two components viz. magnesium hydroxide and silica
gel TGA curves of the sol products VII,Till and
IX
101
.
165
can be accounted for as follows.
Curve VII (MgO 2.16 Si02 1.78 HgO )
Silica gel loses its physisorbed water upto 150°C
at the sane tine magnesium hydroxide also gives off water
slowly but at 420°C it loses its combined water abruptly.
In the present case abrupt loss of water starts above
300°C.
Curve VIII
(MgO 1.064 Si02 1.378 H 20.)
Water comes out slowly from both silica gel and
magnesium hydroxide as before but at 300°C abrupt change
.
commences and ends at 4
j.
Curve IX
(MgO 3.354
3
5
Si02 2.473 H20 )
Water is evolved from both the components upto
325°C.
At this stage magnesium hydroxide starts losing
water abruptly upto 400°C.
Prom the above it appears that the slow and
gradual losses of water is due to the removal of the
physisorbed water and the abrupt loss of water is due
to the decomposition of magnesium hydroxide.
Slight
changes in the temperature of decomposition o f magnesium
hydroxide is difficult to account for.
However, it
is felt that the changes appearing in the curves VII, VIII
and IX upto 3UO°C may be attributed to
the difference
in the rate o f loss of water from different types of
gels as the raw material (sodium silicate) used have
different composition,
structure and other characteristics.
TGA by dynamic methods :
In this method the thermobalance
\
166
*ade by Linseis KG.SELB/GERMANY
was used for the study
on magnesium silicates.
Procedure:
About 500-600 mg of the sample was weighed
accurately in a weighing bottle and transferred to the
pan of the
thermobalance and the empty weighing bottles
was accurately weighed to get the exact amount of the
material transferred into the pan.
An
possible care
was taken during transferring the powder into the pan
to avoid the l.ss
due to the movements of the air.
The furnace was heated at a uniform rate (10°C /minute)
and the changes in tile weight and temperature were
recorded
on the recorder.
in Fig. 12 and 15*
Curves
Tho TGA curves are shown
The results are discussed below.
I - VI
In all the above curves as the products lose
water gradually it is not possible to identify any
stable hydrate within the selected range.
Curves VII. VIII and IX
All these curves show a loss upto 150°C} thereby
followed by a horizontal upto 300°C.
another loss in weight occurs.
Between 3Q0-400°C
The first loss in
weight is due to the explusion of free water.(Dehydr­
ation of magnesium hydroxide and silica gel) and
therefore tnis loss
upto 150°C is maximum in sample
IX which contain maximum amount of SiOg and minimum
in sample
SiOg.
VIII which contains minimum amount of
The second stage of dehydration corresponds
'
167
to t h e d e h y d ra tio n e f magnesium h y d ro x id e .
T h is f a c t
i s r e f l e c t e d by a maximum l o s s i n w eight f o r sam ple V III
a t th is sta g e .
E x ac t c o r r e l a t i o n s betw een th e JDTA and TGA
r e s u l t s a r e n o t o b ta in e d a s DTA sam p le s were d i l u t e d
w ith th e i n e r t r e f e r e n c e m a t e r i a l (50 mg o f th e sam ple
was d i l u t e d w ith 950 mg o f i g n i t e d alum inium o x i d e ) .
7 .2
T h e rm o g ra v im e tric a n a l y s i s o f heav y b a s ic m agnesium
c a rb o n a te
The TGA o f heavy b a s ic magnesium c a rb o n a te was
a ls o c a r r i e d o u t by two d i f f e r e n t m ethods v i z . s t a t i c
m ethod and th e dynam ic m ethod.
S t a t i c m ethod
P ro c e d u re :
The same p ro c e d u re was a d o p te d a s d e s c r ib e d
e a r l i e r i n th e TGA o f magnesium s i l i c a t e s .
Heavy b a s i c magnesium c a r b o n a te s t a r t s l o s i n g
i t s f r e e w a te r a t 100°C and p a r t o f i t s com bined w a te r .
T h e r e a f t e r , i t g iv e s o u t i n a d d i t i o n to com bined w a te r
c a rb o n d io x id e .
lo ss
T h is f a c t i s m arked o u t by a c o n s id e r a b le
i n w eig h t betw een 200 - 400°C .
TGA by s t a t i c
m ethod g iv e s q u a l i t a t i v e in f o r m a tio n a b o u t th e th e rm a l
s t a b i l i t y o f h eav y b a s ic magnesium c a r b o n a te . T h e r e fo r e ,
TGA by dynamic m ethod was c a r r i e d o u t a l s o .
Dynamic m ethod
P ro c e d u re :
The th e rm o g ra v im e tric a n a l y s i s o f heavy
b a s i c magnesium c a r b o n a te was c a r r i e d o u t on L i n s e i s
> WEIGHT' OF THE SAMPLE
i
WEIGHT OF THE SAMPLE
1
100
Fig 14
200
-
500
'
500
600
700
0
\
...
-J________L
;
&
uo^^iisodHO^ad
o
CM
i
J-
o
Tenperature °C .
400
800
900
THERMAL DECOMPOSITION OF HEAVY BASIC MAGNESIUM CARBONATE
170
ffWWVS SHI 1 0 % ‘ IM
2D *
30
4 o r
50 |
a n
\
F IG . 1 5 .
oot
\08
70
300
4oo
c
Soo
t
TEMPERATURE °C
%
........ ......
TGA CURVE OF HEAVY p*RTC IV.
aoo
8 MgO CO
2
V
----- 3 % C 0 3 Mg(0H)2 2HgO
\ ~ ~ " 3 MgC03 Mg( OH) g 2 . 5 H2 0
009
JoOL
90
— * - 5 MgC03 ,Mg(OH)2 3H2 0
oo)
\
• ••v
•500
&oo
10QQ
1010
171
172
thermobalance.
The same procedure was followed
as described earlier in the TGA o f magnesium silicates.
The thermogravimetric analysis shows that
the decomposition of heavy basic magnesium carbonate
takes place in stages when it is subjected to thermal
treatment at a uniform rate of heating.
stage starts before 100°C;
The first
it corresponds to the
expulsion o f a part of water.
.
This can be represented
by the following equation,
3MgC0j Mg(0H)2 4H20
— >
3MgC03 Mg(0H)2 3 H 2 0
This expulsion of water continues upto 300°C during
which two more molecules of water are lost
3MgC03 Mg(0H)2 3H20
—
3MgCC>3 Mg(OH)2 H g O + 2H2 0
On further heating the heavy basic magnesium carbonate
decomposes giving of carbon dioxide and the residual
water.
The stable phase between 375 - 500°C is 8MgO cOg.
This decomposition can be represented by the follow­
ing equation
2
3MgC03 Mg (OH) 2
H 2 0 - > 8 M g 0 C02 + 5C0g + 4H20
The final product corresponds to MgO.
Exact correlations between the DTA and TGA
results' are not obtained as DTA sample was diluted
with the inert reference material (50 m g of the sample
was diluted with 950 m g of ignited aluminium oxide).