Anais do 47º Congresso Brasileiro de Cerâmica
Proceedings of the 47th Annual Meeting of the Brazilian Ceramic Society
672
15-18/junho/2003 – João Pessoa - PB - Brasil
MONITORING OF CLAY MATERIALS. PART 2: GASEOUS RELEASES DURING
THE FIRING PROCESS.
R. Toledo, D. R. dos Santos, R. T. Faria Jr and H. Vargas
Universidade Estadual do Norte Fluminense Darcy Ribeiro – UENF / CCT / LCFIS
Av. Alberto Lamego, 2000, Campos dos Goytacazes, RJ, CEP: 28013-600
E-mail: drsantos@uenf.br.
ABSTRACT
Ceramic materials were produced at different temperatures, and the gases
released during firing were analyzed. Emissions of CO, CO2, NO, N2O, and CH4 were
observed, while NH3 and SO2 were not observed within our detection limits (1 ppm).
The evolution of crystalline phases, open porosity, volumetric shrinkage and rupture
tension were correlated to the amount of gases released at different temperatures,
ranging from 300 C to 1200 C, and these results can be applied to an optimization
of the production process.
Keywords: Gases, traditional ceramics, firing
INTRODUCTION
Clay deposit areas have a high economic potential, depending on fundamental
properties such as soil mineralogical composition, plasticity and porosity. One of the
most important products of clay soils is the ceramics, commonly used in build
constructions as well as in high technological areas.
The clay firing generates a mixture of gaseous compounds, which are released
into the atmosphere. The volatiles commonly evolved are H 2O, O2, CO, CO2, SO2,
NH3, and in some cases fluorine and chlorine, which are environmentally unsafe
emissions and cause serious deterioration of furnace linings.(1) Therefore, the
identification of the soil components that release gases, such as organic matter, clay
minerals, carbonates, sulfides and sulfates, as well as the determination of the
Anais do 47º Congresso Brasileiro de Cerâmica
Proceedings of the 47th Annual Meeting of the Brazilian Ceramic Society
673
15-18/junho/2003 – João Pessoa - PB - Brasil
amount of released gases is essential for a detailed evaluation of the environmental
impact of this industrial activity.
The composition of clay raw materials is important for controlling the production
process, since the constituents determine the firing characteristics and properties of
the end-product, such as degree of vitrification, porosity and crack development,
bloating and black coring. X-rays diffraction and thermal analysis have been widely
used for studying the composition of clay raw materials and for technological control
of the ceramic firing processes. Thermal analysis has also shown that the amount of
gases released from these key phases is strongly correlated to the material
porosity.(1-3)
In this work we present results on the thermal, mechanical and structural
properties of clay deposits localized in northern Rio de Janeiro state in Brazil, where
this kind of soil is abundant and used for production of ceramics, mainly bricks and
roof tiles. We investigate the relationship among open porosity, shrinkage, rupture
tension (strength), and the evolved gases as a function of the firing temperature. In
addition, X-rays diffraction analysis disclosed the amorphous/crystalline phase
transitions that occur in the same temperature ranges. This work is part of a larger
project aiming a complete characterization of the regional clays,(4,5) in order to
enhance the quality of the final products, optimize the production processes and
evaluate the environmental impact of this extraction activity.
EXPERIMENTAL PROCEDURES
The soil samples were dried, grounded and passed through a sieve with
nominal aperture of 840 m. The resulting homogeneous powder was very
representative of the natural soil, which presents 95 % of particles with grain sizes
below 50 m. This powder was extruded in order to obtain bricks with average
dimensions of 100  20  10 mm. The bricks were dried in air at room temperature
during one week and subsequently at 110 C during 24 hours. Thereafter the bricks
were submitted to a slow firing process in a furnace of high heat capacity. The
temperature was slowly raised up to 600 C and kept constant during 60 minutes in
order to avoid cracks owing to a quartz phase transition that occurs at 575 C. Then
the temperature was increased to a firing temperature T F, and kept at this value
during 180 min. Subsequently the sample was cooled at a controlled rate of
Anais do 47º Congresso Brasileiro de Cerâmica
Proceedings of the 47th Annual Meeting of the Brazilian Ceramic Society
674
15-18/junho/2003 – João Pessoa - PB - Brasil
1.5 C/min. Similar procedures were applied for preparing bricks with T F = 300, 400,
500, 600, 700, 800, 900, 950, 1000, 1050, 1100 and 1200 C.
The brick dimensions and weight were measured before and after the firing
process, and the volumetric shrinkage was calculated. The open porosity was
measured by water absorption. The brick resistance to an applied tension was
determined using a device where the sample rests on two cylindrical supports while a
progressive force is applied to its superior surface.
X-rays powder diffraction data were measured at room temperature using CuK
radiation at a conventional diffractometer (Seifert URD65) with a graphite diffracted
beam monochromator. Diffraction data were collected in the angular range
3  2  75 with step 0.03 and counting time of 3 s.
The gases released by the samples were collected directly from the furnace
using a suction pump and a Tedlar bag. Samples were heated in powdered and
extruded forms, weighting 50 mg in both cases. The firing process was carried out
from room temperature to 1100 C with a heating rate of 4 C/min, and the gas
samples were collected after 20 min of wait at different temperatures, namely 300,
400, 500, 600, 700, 800, 850, 900, 950, 1000, 1050 and 1100 C. Gases were
measured in a commercial infrared gas analyzer (URAS 14, from Hartmann &
Braun).(6) In our experimental setup the concentrations of CO, CO2, SO2, NH3, NO,
N2O, and CH4 were simultaneously analyzed for a gas flow of 300 ml/min using
different analyzing cells connected in series. Before each sample analysis the cells
were calibrated using pure standard N2.
RESULTS AND DISCUSSIONS
Fig. 1 shows the evolution of the X-ray diffraction patterns as a function of the
firing temperature TF. The natural powdered soil and the as-extruded samples
presented very similar diffraction profiles. A quantitative phase analysis was
performed for the natural powder samples using the Rietveld refinement method and
reported in a previous work,(5) showing that kaolinite is the main crystalline phase
with 86 wt. %, followed by quartz (5 %), anatase (5 %) and gibbsite (4 %). Iron oxides
such as hematite, magnetite and illmenite were not detected. Very similar
Anais do 47º Congresso Brasileiro de Cerâmica
675
Proceedings of the 47th Annual Meeting of the Brazilian Ceramic Society
15-18/junho/2003 – João Pessoa - PB - Brasil
diffractograms were observed for the samples treated up to 400 C, which showed
the same mineral composition were kaolinite is the major phase.
It can be seen that between 400 and 500 C occurs the transformation from
kaolinite to a non-crystalline phase, metakaolin: Al2Si2O5(OH)4  Al2Si2O7 + 2H2O,
while the other crystalline phases (illite and quartz) remain unchanged up to 900 C.
Treatments at 1000 C or higher cause new structural changes, with the formation of
mullite, crystoballite and hematite phases.
o
1200 C
o
1100 C
o
1050 C
o
1000 C
o
Intensity (a.u.)
950 C
o
900 C
o
850 C
o
800 C
o
700 C
o
600 C
o
500 C
o
400 C
o
300 C
5
10
15
20
25
30
35
40
45
2 ( degrees)
Fig. 1 –X-ray diffraction patterns for bricks fired at different temperatures.
As shown in the gases evolution profiles (Fig. 2) small quantities of CH 4, NO
and N2O gases and large quantities of CO and CO2 gases were observed. The major
quantity of released gases occurred for temperatures up to 800 C. The samples did
not produce SO2 within our detection limits (1 ppm). The volumetric concentration of
CO2 reached 8600 ppm at 400 C, decreasing to 600 ppm at 700 C. Carbon dioxide
is one of the most frequently occurring decomposition products in thermo-analytical
practice. Its large evolution peak is due to organic matter oxidation, while CO profile
Anais do 47º Congresso Brasileiro de Cerâmica
676
Proceedings of the 47th Annual Meeting of the Brazilian Ceramic Society
15-18/junho/2003 – João Pessoa - PB - Brasil
is due to an incomplete oxidation of organic matter, according to the equation
2CO  C + CO2. The interaction of carbon compounds with water vapor during
combustion of organic matter results in less amounts of CO and more CO 2, according
to the following reactions: CO + H2O  CO2 + H2 and C + 2H2O  CO2 + 2H2. There
is also a small concentration of CH4 at 300 C deriving from organic matter on the
surface of the powder grains.
9
Concentration (ppm)
8
NO
N2O
CH4
7
6
5
4
3
2
1
0
300
500
700
900
1100
o
Temperature ( C)
10000
Concentration (ppm)
8000
CO
CO2
6000
4000
2000
0
300
500
700
o
900
1100
Temperature ( C)
Fig. 2 – Gases evolution profiles for powder samples.
Besides the carbonate gases, we observed small fractions of N 2O from 300 C
to 500 C, and NO, whose evolution profile developed between 300 C and 600 C.
These nitrogen compounds are not frequently studied for clay materials. The
Anais do 47º Congresso Brasileiro de Cerâmica
Proceedings of the 47th Annual Meeting of the Brazilian Ceramic Society
677
15-18/junho/2003 – João Pessoa - PB - Brasil
maximum quantity of N2O was observed for 300 C, while NO presents a small peak
of 4 ppm at 400 C.
In order to investigate evolved gas released under conditions that simulate
normal practices of firing, thermal analysis was carried out on extruded pieces, and
the results are presented in Fig. 3.
Concentration (ppm)
10
N2O
CH4
8
6
4
2
0
300
500
700
900
1100
o
Temperature ( C)
Concentration (ppm)
7000
CO
CO2
6000
5000
4000
3000
2000
1000
0
300
500
700
900
1100
o
Temperature ( C)
Fig. 3 – Gases evolution profiles for extruded samples.
Organic matter combustion, as represented by the CO2 evolution profile,
occurred at an approximately constant rate from 300 C to 500 C, then decreased
continuously to 600 ppm at 700 C, and reached zero at 1100 C. As in the case of
powder samples, a small concentration of CH4 was detected at 300 C due to the
removal of organic matter from the brick surface (open pores).
Anais do 47º Congresso Brasileiro de Cerâmica
678
Proceedings of the 47th Annual Meeting of the Brazilian Ceramic Society
15-18/junho/2003 – João Pessoa - PB - Brasil
It is worth to note that a small quantity of N2O was detected from 300 C up to
800 C, while NO was not observed during the complete cycle. This behavior is
different of the observed concentrations for powder samples, where N2O was
produced in higher quantities and at lower temperatures. This result suggests that the
open powder system enhances the interaction with external oxygen, while within the
solid sample the gas is trapped by the pore system and is released at higher
temperatures.
The same situation was observed for the CO2 and CO concentrations. As
expected, the gases for the powder material were released in higher concentrations
than for the solid pieces. In bricks the gas is released as the pore system varies,
which occurs during the expulsion of adsorbed water (about 175 C), dehydroxylation
(from about 400 C to 700 C) and quartz expansion (at 575 C) that helps to close
the cracks.
40
Rupture tension (MPa)
Porosity and shrinkage (% vol)
50
open porosity
volumetric shrinkage
rupture tension
30
20
10
0
300
400
500
600
700
800
900
1000
1100
1200
o
Temperature ( C)
Fig. 4 – Evolution of the physical properties as a function of the firing temperature TF.
Anais do 47º Congresso Brasileiro de Cerâmica
Proceedings of the 47th Annual Meeting of the Brazilian Ceramic Society
679
15-18/junho/2003 – João Pessoa - PB - Brasil
In order to complete the characterization we took into account the physical
properties of the bricks as a function of the firing temperature. Evolution of the open
porosity, rupture tension, and volumetric shrinkage is presented in Fig. 4.
Between 300 and 700 C we observed small variations of these physical
properties, which are associated to the loss of adsorbed structural water and kaolinite
dehydroxylation. Between 700 and 1000 C the open porosity diminishes slowly while
the shrinkage and rupture tension increase. After 1000 C the open porosity
decreases abruptly from 39.4 to 2.5 % vol., while the shrinkage increases from 15.9
to 41.9 % vol. and the rupture tension presents a strong increase from 9.1 to 28.2
MPa (Fig. 4). XRD results showed that these variations are associated to a
recrystallization in the samples, with the formation of mullite, crystoballite and
hematite phases. This behavior suggests that a sintering process occurred, leading
to the improvement of the ceramic properties.
CONCLUSIONS
Evolved gas analysis is a powerful tool for examining clay raw materials for
ceramics as it provides chemical information that can readily be utilized. In the case
of powder clays, for example, this technique provided information on the nature and
quantities of gases released during heating and helped to identify those components
responsible for individual thermal reactions such as dehydroxylation and oxidation.
However, the open system conditions of a loose powder heated in an oxidizing
environment are not analogous to the actual situation within an extruded piece,
where constituent clay particles are tightly packed and a pore system is present.
Even if firing takes place in an oxidizing environment, access of oxygen will be
restricted and volatiles may be confined within the pore system, allowing chemical
reactions with the solid phase.
We would like to point out the good agreement among our thermal, structural
and rheological results. The clay mineral dehydroxylation takes place above 400 C,
simultaneously with the transformation from the kaolinite major phase to a noncrystalline metakaolin phase. The organic matter oxidation, with the formation of
carbon and nitrogen compounds, is active in the range from 200 C to 700 C. In this
temperature range we observed small variations of the physical properties, such as
density, porosity, shrinkage and strength. The observed gases evolutions are indeed
Anais do 47º Congresso Brasileiro de Cerâmica
Proceedings of the 47th Annual Meeting of the Brazilian Ceramic Society
680
15-18/junho/2003 – João Pessoa - PB - Brasil
correlated with the porous system, since the major emissions occurred in the same
temperature range where the main reactions occur and the open porosity presents
small variations. Firing at temperatures equal to 1000 C or higher causes
recrystallization in the samples, with the formation of mullite, crystoballite and
hematite phases. In this temperature range we observed an enhancement of the
ceramic properties: the open porosity diminishes from 39.4 to 2.5 % vol., the
volumetric shrinkage increases from 15.9 to 41.9 % vol. and rupture tension
increases from 9.1 to 28.2 MPa.
The results presented in this work are part of a larger project that comprises the
optimization of procedures adopted for the production of bricks and roof tiles, as well
as the analysis of the mineralogical composition of the raw material and its relation to
the optical and thermal properties of the extruded pieces as a function of the firing
temperature. The Fe content in raw materials, for example, affects the ideal firing
temperature for recrystallization, which is responsible for some effects such as
hardening and shrinkage. Furthermore, we are concerned with the environmental
impact related to the clay extraction and gases emitted during production, which are
also studied and will be reported in another paper.
ACKNOWLEDGEMENTS
The authors acknowledge CNPq and FAPERJ for financial support.
REFERENCES
1. D.J. Morgan, App. Clay Sci. 8, 81 (1993).
2. A.J. Parsons, S.D.J. Inglethorpe, D.J. Morgan, and A.C. Durham, J. Thermal Anal.
48, 49 (1997).
3. O. Delbrouck, J. Janssen, R. Ottenburgs, P. Van Oyen, and W. Viaene, App. Clay
Sci. 8 187 (1993).
4. J. Alexandre, F. Saboya, B.C. Marques, M.L.P. Ribeiro, C. Salles, M.G. da Silva,
M.S. Sthel, L.T. Auler, and H. Vargas, Analyst 124, 1209 (1999).
5. D.R. Santos, R. Toledo, M.S.O. Massunaga, J.G. Carrió, L.T. Auler, E.C. da Silva,
A. Garcia-Quiroz and H. Vargas, Review of Scientific Instruments, 74, 355 (2003).
6. F. Harren and J. Reuss, Enc. of Appl. Phys, 19, 413 (1997).
Anais do 47º Congresso Brasileiro de Cerâmica
Proceedings of the 47th Annual Meeting of the Brazilian Ceramic Society
15-18/junho/2003 – João Pessoa - PB - Brasil
681