Anais do 45º Congresso Brasileiro de Cerâmica
0803201
30 de maio a 2 de junho de 2001 - Florianópolis – SC.
ASPECTOS FLUIDODINÂMICOS E TÉRMICOS DURANTE REMOÇÃO DE ÁGUA
LIVRE EM CONCRETOS REFRATÁRIOS DE ALTA ALUMINA TRATADOS ATÉ
1350ºC
M.D.M. Innocentini, C. Ribeiro, J. Yamamoto, L.R.M. Bittencourt, R.P. Rettore,
V.C. Pandolfelli
Rodovia Washington Luis, km 235, São Carlos, SP, CEP: 13565-905,
Tel: (16) 260-8250 ramal 2067, Fax: (16) 261-5404,
e-mail: pmmi@iris.ufscar.br ou vicpando@power.ufscar.br
Universidade Federal de São Carlos (UFSCar)
Departamento de Engenharia de Materiais (DEMa)
 Magnesita
S.A., Cidade Industrial - 32210-050 - Contagem – MG.
ABSTRACT
This article reports on an experimental investigation of the dewatering process of
cement-free high-alumina refractory castables thermally treated up to 1350ºC.
Simultaneous fluid dynamic, thermal and mass loss effects were investigated during
the removal of physically absorbed water at temperatures of 25 to 700 C. It was
found that the release of steam was decisively affected by the castable’s permeability
level and the heating rate applied. The analysis of fluid dynamics revealed that, at
1C/min, the main bulk of physical water was released as steam under saturated
conditions at 100C. However, at 5C/min, steam was trapped within the pores, with
chaotic water loss and a shift to higher temperatures. A thermal analysis showed that
the endothermic boiling of water may result in a critical thermal shock in the
castable’s structure. Both steam entrapment and thermal shock were more severe
with the reduction in the castable’s permeability level.
Key words: drying, refractory castables, concretes, permeability, particle size
distribution.
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30 de maio a 2 de junho de 2001 - Florianópolis – SC.
INTRODUCTION
The dewatering behavior of refractory castables is highly complex and has been
the subject of many theoretical and experimental studies found in the literature
(1-5).
Much of this complexity lies in the fact that water may vary in content and nature in the
castable prior to dewatering. Physical water, or moisture, refers to the component of
added water that remains chemically uncombined in a castable’s pores after mixing
and curing. Chemical water, on the other hand, includes hydrates and gels of calcium
aluminate cements (CACs) and hydratable alumina binders (HABs) (2).
The relative amount of physical and chemical water depends on the total
amount of water added for mixing and on the content and type of cement.
Nevertheless, the relative importance of each fraction of water on the drying process is
still a controversial question in the literature. The elimination of physical water is the
main cause of weight loss at temperatures approaching 100°C (at ambient pressure).
The decomposition of hydrates and gels, on the other hand, occurs according to
complex kinetics at temperatures ranging from 100ºC to 550C and above, releasing a
specific number of water molecules in proportion to the degraded phase. However,
because temperature plays an important role in the buildup of steam pressure, it is not
yet clear whether boiling at 100C is more deleterious than dehydration at higher
temperatures to the castable’s mechanical strength. Furthermore, thick bodies may
undergo an internal thermal gradient during fast heating programs, causing the release
of steam to shift to higher temperatures and making dewatering even more complex.
Due to experimental difficulties, the literature still shows a lack of simultaneous
fluid dynamic and thermal data on the dewatering process, particularly insofar as the
differences between boiling of physical water and dehydration of binders during the
thermal treatment of high-alumina refractory castables are concerned.
The present work focuses on the optimization of the dewatering process of highalumina refractory castable compositions based on hydratable alumina binders
(HABs). In this context, this report discusses the fluid dynamics, heat transfer and
mass loss involved in the elimination of physical water from refractory castables
subjected to thermal treatment from 25ºC to 700C.
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EXPERIMENTAL PROCEDURE
The castable composition chosen for this study was similar to one previously
described by Innocentini et al.
(6)
and consists of a high-alumina refractory castable
prepared according to Andreasen’s model for particle size distribution (q = 0.21) and
molded in the form of 7.5 cm diameter 2.5 cm thick cylinders. An amount of 2-wt% of
hydratable alumina (Alphabond 300, ALCOA – Pittsburgh, U.S.A.) was used as
binding agent and 4.12 wt% of water was added to the castable mixture. Samples
were cured at room temperature for 24 hours.
Samples were previously calcined in a furnace to decompose all the hydrates
and gels and eliminate the chemical water. Two different temperatures were chosen
(900C and 1350C with a 12-hour dwell time) in order to provide the specimens with
distinct permeability levels
(7).
Heating rates were kept slow (1C/min up to 300C,
2C/min up to 900C and 3C/min up to 1350C.
In order to investigate the fluid dynamic and thermal effects of the removal of
physical water, the open pores of the two pre-fired castable samples were saturated
with water under vacuum.
Dewatering tests were carried out in a hot air permeameter (HAP) described
by Innocentini et al
(8-9).
The test specimen was tightly fixed in a sample-holder and
placed in an electronically controlled electric furnace (7500 W). Dry air supplied by a 2
HP compressor was pre-heated in the furnace according to a preset heating program
and allowed to flow vertically upward through the sample at a constant pressure.
The flow rate was measured at room temperature at the sample’s exit to capture
not only air but also any gas or vapor released by the castable. Temperature and
pressure were monitored at both the sample’s inlet and outlet surfaces exposed to
airflow. The temperature, pressure and flow rate data were electronically recorded by
computer at 1 second intervals throughout the test.
The samples subjected to drying tests were heated at a rate of 1C/min from
ambient temperature to 700C. The air pressure at the sample’s entrance was set at
an absolute value of 1.021x105 Pa (1.10 bar) and kept constant throughout the test.
After this first run, the samples were again saturated with water and subjected to a
further test at a heating rate of 5C/min.
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RESULTS AND DISCUSSION
Figure 1-a shows the primary parameters obtained by a blank test run in the
HAP device with a HAB castable sample pre-treated at 1350C for 12 hours and free
of both moisture and hydrates. The heating rate was 5C/min from 25C to 700C,
with a 100-minute dwell time at 700C. The samples were allowed to cool naturally
inside the furnace.
Figure 1-b displays the flow rate profiles through the blank sample as a
function of the outlet air temperature. The curve obtained at room temperature
indicates a smooth cyclic behavior, with a decrease in flow rate during the heating
phase and an increase during cooling, returning to the initial value at ambient
temperature. As expected, no mass decomposition peak was observed in the
temperature range analyzed. However, considering that air expands volumetrically
with increasing temperature, the original flow rate curve was corrected to the exit
temperature, according to the ideal gas law, to evaluate the actual permeability
changes that occurred in the castable structure
(6).
The corrected curve observed in
Figure 1-b revealed that, at the air pressure applied, thermal expansion caused a
temporary microstructural rearrangement, opening pores and intensifying fluid flow
(8).
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Figure 1. Results of a blank test run with a castable sample pre-treated at 1350C for
12 hours and free of both moisture and hydrates. Heating rate was 5C/min. (a)
Primary parameters obtained by the test. (b) Fluid dynamic curve. (c) Thermal curve.
As shown in Figure 1-c, no detectable local thermal effect was found within the
temperature range analyzed, except for the heat transferred from the air to the
sample (verified by the lower temperature at the sample’s outlet surface).
After the blank test, the same sample was saturated with water and a new test
was run in the HAP device at a heating rate of 1C/min. The results are illustrated in
Figure 2.
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Figure 2. Dewatering behavior of a castable sample pre-treated at 1350C and
saturated with water. The heating rate was 1C/min. (a) Fluid dynamic curve. (b)
Thermal curve.
The flow rate curve in Figure 2-a displayed a single distinct peak around
100C, which coincides with the boiling point of free water under ambient pressure.
The flow rate then slowly decreased during the remainder of the heating stage. The
thermal curve at 1C/min (Figure 2-b) indicates that the sample absorbed energy
during the water loss period, as the temperature at the exit of the sample remained
constant at 100C, while the air temperature at the entrance steadily increased. This
behavior represents the endothermic boiling of free water. The hysteresis in both flow
rate and temperature curves confirmed the occurrence of an irreversible change,
typical of mass loss phenomena in the heating stage.
Extraordinary changes in the dewatering behavior were found when the same
sample was again saturated with water and tested at 5C/min. As shown in Figure 3a, although the main peak again reached its maximum near 100C, the flow rate was
disturbed up to 700C, indicating that steam was generated faster than it could be
eliminated from the castable bulk and its release shifted to higher temperatures. The
thermal curve displayed in Figure 3-b showed a much greater discrepancy between
entrance and exit temperatures than the one observed at 1C/min, with a thermal
Anais do 45º Congresso Brasileiro de Cerâmica
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30 de maio a 2 de junho de 2001 - Florianópolis – SC.
gradient of almost 150C between both castable surfaces during elimination of the
main bulk of free water. The obvious explanation, not covered by traditional
thermogravimetric analyses (TGA), is that liquid water retains all the heat it receives
while it evaporates. The castable bulk is kept at a constant temperature even though
its inlet surface is continuously heated. When the water stops boiling, the resulting
thermal gradient rapidly disappears, causing a thermal shock in the castable
structure.
Figure 3. Dewatering behavior of a castable sample pre-treated at 1350C and
saturated with water. The heating rate was 5C/min. (a) Fluid dynamic curve. (b)
Thermal curve.
In order to evaluate the influence of the castable’s permeability level on the
removal of physical water, a new set of tests was carried out with a sample pretreated at 900C. Prior permeability characterization at room temperature according
to Forchheimer’s equation
(7)
showed that this sample was less permeable than the
one treated at 1350C (k1 = 0.33x10-15 m2 and k2 = 0.36x10-13 m for the sample at
900C and k1 = 1.51x10-15 m2 and k2 = 28.9x10-13 m for the sample treated at
1350C).
Figure 4 shows the fluid dynamic and thermal changes that occurred during
dewatering at 1C/min for the sample pre-treated at 900C. The trends are similar to
Anais do 45º Congresso Brasileiro de Cerâmica
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those shown in Figure 3 for the sample treated at 1350C, indicating that, for the
samples analyzed, permeability plays a minor role in the dewatering of physical water
when slow heating rates are applied. The only perceptible difference was the lower
background flow rate level, reflecting the lower permeability of the sample pre-treated
at 900C.
Figure 4. Dewatering behavior of a castable sample pre-treated at 900C and
saturated with water. The heating rate was 1C/min. (a) Fluid dynamic curve. (b)
Thermal curve.
Figure 5 reveals the severe thermal and mass loss consequences of a
dewatering process in a less permeable castable when the heating rate increased
from 1 to 5C/min. As shown in Figure 5-a, although the main peak was still located
at 100C, steam was intermittently and chaotically released at temperatures up to
700C, and the airflow was even blocked for a few moments. However, Figure 5-b
reveals that the more deleterious effect caused by the decrease in the permeability
level was a noticeable increase of almost 300C in the thermal gradient between both
castable surfaces during elimination of the main bulk of free water.
Anais do 45º Congresso Brasileiro de Cerâmica
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30 de maio a 2 de junho de 2001 - Florianópolis – SC.
Figure 5. Dewatering behavior of a castable sample pre-treated at 900C and
saturated with water. The heating rate was 5C/min. (a) Fluid dynamic curve. (b)
Thermal curve.
From this standpoint, this experiment provided one of the most important
insights to understand the occurrence of explosive spalling in HAB refractory
castables. If a large amount of physical water is retained within the refractory body
when it is subjected to drying at rapid heating rates, the stress caused by the thermal
gradient associated with the high pressure due to the entrapment of steam at higher
temperatures may suffice to overcome the fracture stress, causing failures or even
bursting the structure.
Furthermore, in an actual dewatering process, the absence of airflow through
the structure causes the steam to be released even more slowly and, depending on
the heating rate applied, the amount of physical water retained and the body’s
thickness, the thermal gradient generated between cold and hot faces may be
decisive to the occurrence of failures in the castable structure. Lastly, this scenario is
even more complex if one considers that the permeability level and the mechanical
strength are even lower in a green sample subjected to drying and that the
elimination of physical water may occur concurrently to the cement’s dehydration in
most common castable compositions.
Anais do 45º Congresso Brasileiro de Cerâmica
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CONCLUSIONS
This investigation demonstrated that the steam release that occurs during the
dewatering process of refractory castables pretreated up to 1350ºC is dramatically
affected by the permeability level and heating rate applied. The fluid dynamic
analysis revealed that the main bulk of physical water is released as steam under
saturated conditions at 100C. However, if the castable sample is subjected to a
higher heating rate (5C/min), steam is generated faster than it can be released and
water loss chaotically shifts to higher temperatures. Simultaneous thermal analyses
showed that liquid water retains all the heat it receives while it evaporates. When the
water stops boiling, the resulting thermal gradient rapidly disappears, causing a
thermal shock in the castable structure. The less permeable the castable structure,
the higher the steam entrapment and thermal shock.
ACKNOWLEDGEMENTS
The authors are grateful to the Brazilian research funding institutions FAPESP
and CNPq, and to Alcoa S.A. and Magnesita S.A. for their support of this work.
REFERENCES
(1) W.E. Lee and R.E. Moore, “Evolution of in-situ refractories in the 20th century”, J.
Am. Ceram. Soc., 81 [6], 1385-1410 (1998).
(2) R.E. Moore; J.D. Smith; W.L. Headrick Jr and T.P. Sander, “Monolithic dewatering
theory testing and practice: new challenges”, In: 32nd Annual Symposium on
Refractories, The American Ceramic Society, St. Louis Section, 26p (1996).
(3) W.H. Gitzen and L.D. Hart, “Explosive spalling of refractory castables bonded with
calcium aluminate cement”, Bull. Am. Ceram. Soc., 40 [8] 503-507, 510 (1961).
(4) Z.P. Bazant and W. Thonguthai, “Pore pressure and drying of concrete at high
temperature”. Proc. of the Am. Soc. of Civil Engineers. 104[EM5] 1059-1079 (1978).
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(5) D.L. Hipps and J.J. Brown Jr., “Internal pressure measurements for control of
explosive spalling in refractory castables”. Am. Ceram. Soc. Bull. 63[7] 905-910
(1984).
(6) M.D.M. Innocentini; M.G. Silva; B.A. Menegazzo and V.C. Pandolfelli,
“Permeability of refractory castables at high-temperatures”, J. Am. Ceram. Soc. 84[3],
645-647 (2001).
(7) A.R.F. Pardo, “Permeability of high-alumina self-flow refractory castables”, M.Sc.
Thesis, Federal University of São Carlos, Brazil, 2000 (In Portuguese).
(8) C. Ribeiro; M.D.M. Innocentini and V.C. Pandolfelli, “Dynamic permeability
behavior during drying of refractory castables based on calcium-free alumina
binders”, J. Am. Ceram. Soc., 84[1] 248-250 (2001).
(9) M.D.M. Innocentini; C. Ribeiro; J. Yamamoto; A.E. Paiva; L.R.M. Bittencourt; R.P.
Rettore and V.C. Pandolfelli, “A novel technique to access the drying behavior of
refractory castables”, accepted for publication in the Bull. Am. Ceram. Soc. (2001).