PROPERTIES OF HEAT-INSULATING MATERIALS (A REVIEW)
A. L. Yurkov
and L. M. Aksel’rod
Translated from Novye Ogneupory, No. 3, pp. 18 – 22, March, 2005.
Original article submitted September 17, 2004.
Heat-insulating materials for the lining of aluminum electrolysis cells based on diatomite, vermiculite, perlite,
and calcium silicate are briefly reviewed. Data on heat conductivity of these materials over the range of
200 – 800°C are given, with emphasis placed on the behavior in the electrolysis bath. Properties of heat-insu
lating materials available from domestic and foreign manufacturers are discussed in terms of the classification
temperature and safe operating temperature.
Despite the ready availability of a wide range of heat-in-
sulating materials, a mere four or five of them have found ap-
plication for the heat insulation of electrolysis cells. The in-
sulators in question are lightweight chamotte products and
materials based on diatomite, vermiculite, perlite, and cal-
cium silicate. The main reason for the preference given to
these materials is that they are capable of sustaining long-
term mechanical loading without deformation at tempera-
tures as high as 900°C.
The high-grade diatomites contain more than 70% SiO
A microporous structure (pore size 0.005 – 0.01 mm across)
is the main property that provides a low heat conductivity of
diatomite [1, 2]. To further decrease heat conductivity, com
bustible additives (sawdust) are introduced into the material
which, as they burn out during sintering, increase the pore
number. Argillaceous species that make up part of the diato
mite rock impart plasticity to the molding mix and control
the sintering regime and maximum permissible operating
temperature of the material. As a rule, diatomite as-recov
ered from the quarry is used with no other minerals added;
for this reason, heat conductivity and operating temperature
of diatomite products purchased from various manufacturers
may differ appreciably.
The raw material recovered from quarries is averaged to
obtain the required consistency, and sawdust is added to the
mixture. Depending on the mixture consistency, extrusion or
slip-casting techniques are used to prepare green performs of
the needed shape; next, the green performs are sintered in a
tunnel furnace. In recent years, to meet progressively more
stringent size tolerances, the usual practice is to surface-fin-
ish the sintered components using diamond machining tools.
HEAT-INSULATING PERLITE-BASED MATERIALS
Perlite is a high-silica mineral of volcanic origin with a
glass-like structure. By chemical composition, it is close to
granite; it contains more than 1% water. When heated,
perlite, owing to the vigorous evolution of chemically bound
water, expands in volume to form a lightweight porous mate
rial. The swell ratio of perlite, depending on its occurrence,
may be as high as 20. Sharp heating to 800 – 900°C imparts
to perlite a structure with inner isolated pores. For producing
perlite-based heat-insulating materials, firing and nonfiring
technologies have been developed using, for example, clay
or cement binders. The maximum permissible operating tem
perature for insulating materials based on high-quality perlite
may reach 1200°C.
Viewed chemically, vermiculite is hydrated magnesium-
potassium aluminosilicate — a variety of highly hydrated
biotitic mica. Vermiculite, when heated to 650 – 750°C, ex
Refractories and Industrial Ceramics Vol. 46, No. 3, 2005
1083-4877/05/4603-0170 © 2005 Springer Science+Business Media, Inc.
From materials submitted to the 4th International Seminar
“Refractories for the aluminum industry.”
Russian Aluminum Management Joint-Stock Co., Russia;
Magnezit Trading House JSC, Russia.