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The technology of glass and ceramics - Hlavac J.

Hlavac J. The technology of glass and ceramics - Oxford, 1983. - 429 p.
Download (direct link): tehnologyofglass1983.djvu
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with possibly just a brief holding period. Compositions from the system
Li20 -A1203-Si02 and MgO (ZnO, BaO) - A1203 -Si02 are considered suitable
for this manufacturing process (cf. Dusil and Strnad, 1974).
3. GLASS-CERAMICS FROM CHEAP NATURAL AND WASTE RAW
MATERIALS
Most types of technical glass-ceramics are comparatively expensive
materials because their manufacture requires costly raw materials (Li20,
B203, Ti02) and high temperatures for melting. However, for application
in industry and the building industry, it is possible to produce glass-
ceramics based on widely available rocks (the so-called petrositals) and
on industrial wastes, in particular metallurgical slags (the so-called
slagsitals). The melting temperatures are in the range of 1350 to 1500
C. Fused rock products are in fact predecessors of this type of glass-
ceramics, as they crystallize spontaneously during cooling, thus yielding
solids similar to glass-ceramics, even though they are comparatively
coarse-graincd and do not attain the properties of the materials prepared
by controlled crystallization. Because of their kinship to glass--
ceramics, fused rocks will be dealt with in this chapter, although they
are not usually included among glass-ceramics in the narrower sense of
the term.
Fused rocks and petrositals. The fused rock industry (petrurgy)
started before World War II (USSR, Germany); however, substantial
development took place much later, on the basis of systematic research of
(1) the processes involved in fusion and crystallization of various
rocks, and (2) the final properties of the resulting products.
The outstanding property of fused rocks is their high resistance to
abrasion and corrosion. For instance, fused basalt products show a
service life 4-10 times longer than that of steel when employed as
components of equipment used in the transport of particulate materials,
which thus makes them an attractive substitute for metals.
The technological process includes melting of the natural raw
material, usually basalt, at 1350 to 1400 C in a shaft kiln; because of
the steepness of the viscosity curve, the material does not lend itself
to standard glass forming technologies, so that the melt is processed by
casting into sand or metal moulds*. A new technique of dynamic
(centrifugal) casting into preheated rotating metal moulds has been
developed for
* The foundry technology does not admit the manufacture of
dimensional ty precisc products of complex shapes. For this reason,
sintered basak technology has been developed for the manufacture of
nozzles, drawing dies, dies for auger brick machines, etc., where the
materials exhibit a resistance to abrasion still higher than that of cast
basalt. The fused basalt powder is mixed with a plasticiser and pressed;
the pressings are sintered in an electric tunnel kiln at 1120 to 1140 C.
238
the casting of tubes. After casting, the melt crystallizes rapidly in
the mould within 2 to 5 minutes; the product is then removed from the
mould and introduced into the annealing lehr. Crystallization kilns are
sometimes used as an intermediate technological stage. The products
contain more than 80% crystalline phase (mostly pyroxene) and 10% glassy
phase.
The most suitable raw materials are basalt rocks containing 43.5 to 47
wt. % Si02,
11 to 13 % A1203, 4 to 7% Fe203, 5 to 8% FeO, 8 to 11% MgO, 10 to 12%
CaO, 2 to 3.5% 2, 3 to 5.5% Na20 + K20. Basic basalts with a lower Si02
content crystallize rapidly with a resulting coarse-grained structure and
tendency to cracking. Those with high S;02 content crystallize too slowly
and therefore non-uniformly throughout the volume of the ware. The
primary crystalline phase is usually magnetite Fe304 which on further
cooling acts as crystallization nuclei for the prevailing phase of
monoclinic pyroxene*. The optimum properties are exhibited by finely
crystalline materials with a spherolitic structure formed at low
crystallization temperatures.
The high resistance to abrasion of basalt has already been mentioned;
furthermore, crystalline basalt shows an outstanding hydrolytic stability
and at room temperature resists even strong acids; however, it does not
resist them at boiling temperatures. Vitreous basalt shows a resistance
inferior by an order of magnitude. The electrical conductivity of basalt
is of combined ionic and electron type, owing to the presence of iron
oxides or magnetite.
Fused basalt is utilized chiefly in the manufacture of floor tiles,
castings for cyclone and other separators, nozzles, augers and grate
bars; the centrifugal casting technology is used in the production of
conveyor pipings for mines, mill corps, etc. A special case is
represented by the application of fused basalt for the incorporation of
radioactive wastes. Fused rocks are dealt within the extensive series of
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