PHYSICO-CHEMICAL FEATURES OF THE SYNTHESIS OF MULLITE
IN MIXTURES OF QUARTZ-PYROPHYLLITE ROCK
WITH A FLUORIDE-FORMING COMPONENT
T. V. Vakalova,
V. V. Kravchenko,
and V. V. Gorbatenko
Translated from Novye Ogneupory, No. 4, pp. 27 – 32, April, 2014.
Original article submitted January 27, 2014.
It is established that the use of an ammonium fluoride treatment on quartz-pyrophyllite rock (45%
pyrophyllite and 53% quartz) at 650°C changes the traditional mechanism by which mullite is synthesized; it
is formed not from metapyrophyllite but from fluoridated pyrophyllite, with the formation of aluminum tri
fluoride and topaz being intermediate stages of the synthesis process. The optimum ratio of quartz-
pyrophyllite rock to ammonium hydrodifluoride is 1:1.6, which at 1300°C increases the yield of acicular
mullite (with particles 20 – 30 mm long and 3 – 5 mm in cross section) from 12 to 68 wt.%. The content of re
sidual quartz in the mullite is no greater than 15 – 18 wt.%.
Keywords: pyrophyllite, ammonium hydrodifluoride, fluoridation, heat treatment, synthesis, phase forma-
tion, mullite, topaz, corundum.
Alumosilicate ceramics with a crystalline mullite phase
have now come into wide use in different areas of science
and industry. The excellent functional properties of these ce-
ramics are determined not only by the total content of mullite
but also by its structural-morphological state (prismatic par-
ticles or acicular particles).
The main problem encountered in attempting to obtain
mullite from the natural alumosilicate raw material is that ef
ficient new low-temperature methods need to be found to
synthesize mullite in order to produce mullite ceramics in
which the content of the glassy phase is low and the mullite
has a non-isometric habit. Together, these characteristics im
prove the service properties of the ceramic: chemical stabil
ity, heat resistance, refractoriness, and mechanical strength at
room temperature and high temperatures.
One method that is used on an industrial scale to obtain
alumosilicate ceramics from natural raw materials and maxi
mize the yield of mullite entails the addition of an alu
mina-bearing component so that the silica which is formed
during thermal degradation of the rock-like minerals (pyro
phyllite, kaolinite, etc.) is bound inside the secondary mullite
of the material. Another approach which might hold promise
as a means of increasing the amount of mullite that is formed
is to enrich the product obtained from firing the natural
alumosilicate raw material by having the silica component
separated out during the process of synthesizing mullite. The
silica component can be obtained either in the form of the
free quartz which is always present in the raw material or as
a structural component of the alumosilicate mineral.
In connection with this, it is interesting to examine the
use of a method which uses fluorine ions to decompose sili
cates [1 – 3]. The fluoride-forming agents that are the most
convenient to use to break down silicate materials are fluo
rides of ammonia. Under normal conditions, these fluorides
are nonaggressive crystalline substances which when heated
react with silica as follows to form ammonium hexafluoro
F·HF = (NH
The ability of the ammonium hexafluorosilicate which is
formed to undergo sublimation at 320° makes it possible to
effectively remove it (and thus eliminate excess silica) from
the alumosilicate matrix, which in turn makes it possible to
increase the yield of mullite. In the investigation being dis
cussed in this article, the natural alumosilicate raw material
which was used was pyrophyllite-bearing rock from the
Kul-Yurt-Tau deposit (in the Republic of Bashkortostan).
Refractories and Industrial Ceramics Vol. 55, No. 2, July, 2014
1083-4877/14/05502-0131 © 2014 Springer Science+Business Media New York
National Research Tomsk Polytechnic University, Tomsk, Russia.