Regularities governing the two-stage synthesis of cast Cr2O 3/Al2O3 under conditions of the autowave synthesis

Article abstract of DOI:10.1134/S1070427209090353

The study of the chemical, roentgen phase and structural properties of the products of combustion of system Cr₂O₃-Al-Fe ₂O₃ was carried out. The analysis of the results shows the presence in the system of a set

Full text of DOI:10.1134/S1070427209090353

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2009, Vol. 82, No. 9, pp. 1716−1720.  Pleiades Publishing, Ltd., 2009.
Original Russian Text  K. Sviderskii, 2009, published in Khimicheskaya Promyshlennost’, 2009, Vol. 86, No. 4, pp. 163−167.
TECHNOLOGY OF ORGANIC
AND INORGANIC CHEMISTRY
Regularities Governing the Two-Stage Synthesis
of Cast Cr O /Al О under Conditions
2
3
2
3
of the Autowave Synthesis
K. Sviderskii
Pavlodar State Pedagogical Institute, Pavlodar, Kazakhstan
Received March 4, 2009
Abstract—The study of the chemical, roentgen phase and structural properties of the products of combustion of
system Cr O AlFe O was carried out. The analysis of the results shows the presence in the system of a set of
2
3
2
3
solid solutions. The results of the study make it possible to establish the optimum regime of the heat treatment
of the studied system for creating the new refractory material according to self-propagating high-temperature
synthesis (SHS).
DOI: 10.1134/S1070427209090353
The contemporary concept of the development of
fireproof industry consists in a use of the production of
the resource saving fireproof materials of new generation
which are characterized by the increased environmental
safety and wear resistance. At present the intensification
of technological processes and the innovation ways of the
development of the acting branches of the metallurgical,
chemical, petroleum refining industry, power engineering
and other fields impose ever more stringent requirements
to the fireproof articles utilized for the brick lining of
high-temperature aggregates (fire-bricks, printed masses,
masonry mortar, etc). The application to these purposes
of a method of the self-propagating high-temperature
synthesis characterized by high temperatures and speeds
of chemical transformations (autowave synthesis) makes
it possible to synthesize the materials of the specific
phase composition and structure that govern chemical
and mechanical properties [1, 2].
In this paper we presented the results of studies of the
fireproof materials synthesized by SHS from the charge
which consists of chromite ore and aluminum with
a change in the weight fraction (%) of the chromium(III)
oxide and aluminum content from 15 to 24%. For the
synthesis we used Al of a grade ASD-1 of 99.8% purity.
Chrome ore (a concentrate of the Kempirsaysk deposit)
according to the chemical analysis data had the following
composition: Cr O 51.3%, Al O 8.3%, MgO 15.0%,
2
3
2
3
FeO 17.8%, Fe O 2.0%, SiO 4.2%, CaO 0.14%, S 0.1%.
2
3
2
Also in the experiments the powders of oxides (Al O ,
2
3
Cr O , Fe O ) were used as the initial reagents.
2
3
2
3
Combustion in such systems has two principally
different stages: reduction of elements from the oxides
and futher interaction of the elements with each other
and with the additives. The finely dispersed dry mixtures
Cr O + Al + Fe O + MgO, SiO + Al, and others can
2
3
2
3
2
serve as the typical examples of such systems. However,
use along with SHS of energy of the redox reactions
between aluminum and iron(III) oxide assumes the
presence in the products of the synthesis of the inclusions
of metallic iron which reduce the acid resistance of the
material. The presence of vitreous phase decreases its
mechanical properties.
The chromium oxide Cr O is a very promising
2
3
effective fireproof compound for the production of
different special fireproof material. The wide application
of the chromium oxide fireproof materials in the
metallurgical furnaces is limited by their high cost and
complexity of the technology of their synthesis. In view
of this the chromium oxide fireproof materials are used
in the limited amounts [13].
The basis of the concentrate comprises chromite (Fe,
.
MgO)O (Cr,Al, Fe) O [3] according to the results of RFA
2
3
1
716

REGULARITIES GOVERNING THE TWO-STAGE SYNTHESIS
1717
having parameter of crystal lattice a = 8.312 ± 0.005 Å.
Also small amount of serpentine was found in this
concentrate. We studied the influence of the conditions for
the preparation of samples and for conducting synthesis,
of the heating and cooling rates on the phase composition,
the structure and the properties of the end product of the
reaction.
The analogous phase composition of the products of
the aluminothermic reaction of the reduction of metallic
chromium from the chromium concentrate is given in the
work [4] and testifies about the dissolution into Al O to
2
3
3.9% of Cr O . This material is classified as ruby and
2
3
is characterized by high hardness [5]. As experiments
showed reaching 100%-fullness of the chemical reactions
under the given conditions is not possible. Depending on
the efficiency of the phase separation the products are
the cermet, in which metallic component is distributed
in the form of particles in an oxide matrix, or a gradient
material (divided) in which the part of the metallic phase
is devided into the layers, and other part is distributed in
the oxide matrix.
Conditions for preparation of the charge: the samples
of the studied composition (85% of chrome ore, 15% of
aluminum) were made in the form of pressed tablets
of a diameter 20 mm and a height 20 mm from the dry
mixture also with the addition 8% water. A part of the
samples was dried for 3 hours, and other part, for 20 hours
in air. After drying and mixing the finished charge was
loaded into the refractory reaction forms and was placed
into the reactor and synthesis under the pressure of gas
With an increase in the weight part (b) of Cr O in
2
3
the initial mixture (to 3540%) the rate of combustion
and the fullness of dispersion decrease and the limit of
combustion is attained. Upon approaching to the limit
the combustion of the mixtures becomes nonstationary,
the front of the combustion is bent. With the large b the
product takes the form of the cake in which oxide and
metallic phases are mixed between themselves. With the
growth b the weight of the oxide layer increases. At the
analysis of the products the primary attention was paid
to the region b in which occurs the clear separation of
oxide and metallic phases.
(nitrogen, argon) 4.08.0 MPa was conducted.
The chemical analysis, RFA, local X-ray spectral
and metallographic methods were used for investigating
the products of synthesis in the course of the synthesis.
In the experiments we determined: the average linear
rate of combustion, the fullness of the output of oxide
and metallic phases, the depth of the spread of product
in the course of combustion. The dynamics of a change
in the temperature with the heating and in the process
of combustion were determined on the setup that
consisted of a chamotte body, silicon-carbide heating
elements, system of the registration of combustion which
included two tungsten thermocouples VR-5/20 and
loop oscillograph NO71-6M. Petrographical studies of
samples were performed with the aid of the microscope
MW -71V4.2 with 50- and a 500- multiple increase. RFA
was carried out on the diffractometer DRON-3M, CoK
radiation. The compressive strength of the samples was
determined on the hydraulic press PSU-10.
On the basis of the data of chemical and metallographic
analyses and of RFA of the synthesized products it
was established that at the presence of Cr O in the
2
3
composition of the starting material the oxide layer
contains 2 phases: (1) Al O /Cr O /Fe O spinel and (2)
2
3
2
3
2
3
Al O /Cr O solid solution.
2
3
2
3
The chemical analysis of metallic and oxide phases
showed that in the mixtures (Fe O +2Al) + bCr O with
2
3
2
3
scarce Al at the combustion occur competing chemical
transformations. The first appears as a result of the
The analysis of the results evidences the presence in
the system of the set of solid solutions, the content of
Cr O in some of them changes from 0 to 17%. The visual
reaction of aluminum with iron(III) oxide
2
3
Fe O + 2Al → 2Fe + Al O .
inspection of the combustion products showed that with
the low values of the weight part of the chromium oxide
the combustion products took the form of two clearly
divided cast layers (upper was oxide, lower, metallic).
The presence of the solid solutions became possible
because of the oxidation of aluminum with the formation
of Al O . Certain formed surplus of Al O contributes to
2
3
2
3
This process increases the temperature in the
combustion zone to 1200°C initiating further oxidation
of aluminum in the reactions with the chromium oxides
and partially silicon oxide resulting in a temperature
growth to 2000°С.
2
3
2
3
the dissolution of Cr O , and its different content in the
solution is determined by the gradient of crystallization
temperature after the combustion reaction.
2
3
Cr O + 2Al → 2Cr + Al O
3
2
3
2
3SiO + 4Al → 3Si + 2Al O
3
2
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 9 2009

1718
SVIDERSKII
The high temperature developed in the combustion
on the basis of the aluminothermic reaction of Cr O3
2
zone contributes to the separation of metallic and slag
fractions. Chromium drops into the lower layers of the
sample while in the slag part near the metal is located
corundum, and already behind it follow solid solutions
with the maximum concentration of Cr O in the outer
with aluminum at different content in the charge of the
latter showed that in the product, besides the metallic
chromium are formed the solid solutions of Cr O and
2
3
Al O of different concentrations and consequently,
2
3
with different melting points varied within the limits
of the melting points of Cr O and Al O [6]. As it is
2
3
surface layer. This is completely confirmed by the results
2
3
2
3
of RFA of different sections of the sample.
known [5] the hardness of the composing components
of these solid solutions are equal to 8.5 and 9.0 units,
respectively, according to Mohs scale. An increase in the
hardness number is possible with the formation of the
solid solutions [7]. The obtained results on the hardness
of the studied materials agree with literature data [8].
At a decrease in amount of aluminum in the charge
to 24%, the pure corundum is absent in the phase
composition of the reaction product but two solid
solutions on the basis ofAl O are fixed even more clearly.
2
3
According to the dependence of a change in the lattice
parameters of solid solutions on the concentration of the
components obtained for the hardened samples [6, 7] the
concentrations of the above-mentioned solid solutions of
In this stage of the studies of aluminum the composition
with 15% of aluminum which is a basis for preparing
refractory material utilized in practice is of the greatest
interest. Some of its properties were studied. It is known
Cr O are 30 and 15%, respectively. The X-ray reflections
2
3
in this case overlap; moreover the first of them is widened
that indicates larger dispersiveness and nonequilibrium
of this phase.
[
6] that the density of the solid solutions of Cr O Al O
2 3 2 3
determined according to X-ray and a bottle method data
varies from 4.00 g cm for Al O to 5.25 g cm for
3
3
2
3
At a decrease in the content of aluminum in the charge
to 21% in the course of the synthesis already not two but
one solid solution which contain about 57% of Cr O
Cr O .
2 3
A density value of the studied composition get by the
bottle method equals to 4.75 g cm , and for the initial
charge, 4.05 g cm . Thus, we synthesized a material
3
2
3
is formed, and metallic chromium is separated. In the
samples with 18 and 15% of aluminum after the synthesis
also two solid solutions are detected, but now on the basis
of Cr O . The content of Cr O in these solid solutions
3
whose density grows by 17.5% in comparison with the
initial material. Compaction of the material as a result of
the synthesis leads to the appearance of a large amount
of the pores which merge inside the sample to form
significant cavities. We carried out an evaluation of their
apparent porosity and density, water absorption and of
bulk density. The results of measurement for five samples
are presented in the table.
2
3
2
3
comprises respectively 53 and 66%, and 55 and 73%.
This regularity in the change in the phase composition
with a decrease in amount of the reacting aluminum is
reasonable if we evaluate the state of material at high
temperatures by the diagrams of states CrCr O and
2
3
Al O Cr O [6],as we have done earlier. Formation of
2
3
2
3
The scattering of the indices given in the table
taking into account identical conditions of the synthesis
two solid solutions with the large difference in the lattice
parameters can be caused by the presence of stratification
in the system Cr–Cr O and dissolution in these two
2
3
phases of different amounts of Al O . At the content
Experimental data on the samples with 15% content of alu-
minum
2
3
in the charge of 18% aluminum the temperature of the
warming-up of the sample as a result of the combustion
will be higher than at 15%, and the state of the material
will correspond to the upper peak of stratification. Only
one solid solution in the sample can be synthesized
when the composition of the charge is such that in the
process of the synthesis of the material, it, according to
the diagram, corresponds to the area of eutectic from
the side of chromium, and its crystallization occurs
at one temperature, i.e., in the absence the gradient of
temperatures and concentrations.
Thus, RFA analysis of the materials synthesized
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 9 2009

REGULARITIES GOVERNING THE TWO-STAGE SYNTHESIS
1719
(a)
(b)
Microstructure of the (a) upper and (b) top sections of the samples resulted in the charge combustion with 15% content of aluminum
under the different conditions of the heat removal.
is explained by a form of the samples rather then by
a difference in the characteristics of the product. The
material of the samples is sufficiently dense (the bulk
density reaches 3.88), poorly porous (the apparent
porosity can be about 2.3%), and weakly absorbs water
and the internal part of the samples. In the area of lower
temperature the chromium isolations are more dispersed,
and the solid solutions form the light and dark sections
of different hardness. Thus, from the charge of one
composition it is possible to form different materials
changing only the reaction temperature. The phase
composition in this case can be forecasted according to
the diagram of state. The dark sections are crumbled in
the course of the surface grinding (see the figure).
(~0.6%). At the same time the apparent porosity can be
4
0% which indicates the presence inside the samples of
extensive cavity that is connected with the environment
by means of thin cracks or small t number of capillary
channels. Such samples have higher apparent porosity,
and higher water absorption. If the internal cavity is
completely closed the apparent porosity can be small
thereby the apparent density and the water absorption, for
example, for the sample no. 3, decrease. The significant
porosity (discontinuity) of the samples in spite of the high
hardness of material leads to the low strength indices
CONCLUSIONS
According to RFA data the parameters of these solid
solutions are: a = 4.841 ± 0.005 Е, c = 13.390 ± 0.005Е;
c = 13.807 ± 0.005 Е, i.e., somewhat greater than at a
higher temperature of the synthesis. Thus, on the basis
of the results of the conducted investigation it is possible
to make the following conclusions:
(-compression about 6 MPa).
Thus, the material synthesized in the course of the
combustion of the charge with 15% content of aluminum
whose a phase composition is characterized by the
presence of two solid solutions with the high content
(
1) It is shown a possibility of using the SHS
method for producing the refractory material from the
charge which consisting of chrome ore and aluminum
at achange in the content of the aluminum from 15% to
the stoichiometric one with intermediate values 18, 21,
and 24 wt %.
3
of Cr O , has the high density (4.75 g cm ) but in the
2
3
samples in this case the significant porosity is formed.
It is necessary to note that the combustion temperature
is effected by an influence of the heat losses caused by
the specific conditions of heat withdrawal, and exerts an
essential effect on the properties of the studied samples.
The experiments with the cylindrical sample prepared
from the charge with 15% content of aluminum placed
into a furnace and heated to 1000С in such a way that
the heat withdrawal of an upper and lower surfaces
essentially differs causing the gradient of temperature,
leading to the same results.
(
2) It is shown that the studied mixtures have a wide
range of combustion, melting and phase separation. The
chemical composition of oxide solid solutions can be
changed over wide ranges varying the ratio of reagents
in the initial mixture, the temperature of initial mixture,
and the gas pressure.
(
3) The chemical analysis of the metallic and oxide
phases shows that in the mixtures poor in Al proceed
competing chemical transformations: reduction of iron
oxides and of chromium by aluminum.
Petrographic analysis and RFA showed a difference
in the micro-compositions on the external end surface
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 9 2009

1720
SVIDERSKII
po innovatsionnomu razvitiyu nauki i tekhniki (Young
REFERENCES
Int. Conf. on Innovations in Science and Enginnering),
Chernogolovka, 2005, pp. 33–35.
1
. Merzhanov, A.G., Samorasprostranyayushchiisya
vysokotemperaturnyi sintez. Sovremennye problemy.
Fizicheskaya khimiya (Self-Propagating High-Temperature
Synthesis. Contemporary Problems. Physical Chemistry),
Moscow: Khimiya, 1983.
5. Povarennykh, A.S., Tverdost’ mineralov (Hardness of
Materials), Kiev: Naukova Dumka, 1963, 304 p.
6
7
8
. Diagrammy sostoyaniya silikatnykh sistem. Spravo-chnik
(Diagrams of State of Silicon Systems), Toropova, N.A.
2
. Levashov, E.A., Rogachev, A.S., and Yukhvid, V.I., and
Borovinskaya, I.P., Fizikokhimicheskie i tekhnologicheskie
sostavy SVS (Physicochemical and Technological
Compositions of SHS), Moscow: “Binom”, 1999.
et al., Eds., Leningrad: Nauka, 1969.
. Plinner, Yu.A. and Ignatenko, G.F., Vosstanovlenie okislov
metallov alyuminiem (Reduction of Metal Oxides by
Aluminum), Moscow: Metallurgiya, 1967.
3
. Deer, W.A., Howie, R. A. and Zussman, J., Rock-Forming
Minerals, John Wiley & Sons, 1962.
. Ismailov, M.B., Leonov, A.N., and Satbaev, B.N. In
Issledovanie protsessov tekhnologicheskogo goreniya
(Investigation of Technological Combustion), Alma-Ata:
Izd Kaz. Gos. Univ., 1988, pp. 76–80.
4
. Tarasov, A.G., Gorshkov, V.A., and Yukhvid, V.I.,
Abstracts of Papers, Molodezhnaya mezhd. shkoly-konf.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 9 2009