Geoscience Reference
In-Depth Information
high-pressure value as an oxidant. Figure 3.34 shows the schematic diagram of the
microautoclave for using high-pressure gas. An Al 2 O 3 stirring ball of diameter
9.0 mm was added to the autoclave and oxygen gas (2.0 MPa) was charged through
the pressure valve as the oxidant. This setup has been used by the designers to
study the oxidation behavior of lead metal, and this autoclave is very effective for
the treatment of lead. As the oxidation of sprayed liquid lead is carried out, usually
at high temperature (250
400 C), by conventional processes, it results in vapori-
zation of the lead leading to an inevitable environmental pollution problem. In this
connection, this novel equipment comes as a helping device.
2
Hydrothermal Autoclaves for Visual Examination
The hydrothermal process cannot be monitored by “looking” into a steel auto-
clave. Systematic errors appear in the analysis of the crystallization conditions
because the post-factum information is difficult to interpret and is mainly based
on consecutive approximations which are only sufficient if the crystal growth is
reproducible. Nevertheless, in this case, it is difficult to point out a parameter that
controls the entire process because the greater the number of nonmeasured para-
meters in the method, the more difficult it is to optimize the solution. In order to
facilitate the direct observation and control of dissolution of solids, their synthesis,
recrystallization, and growth on seeds, several hydrothermal autoclaves with
quartz and sapphire windows have been designed both in the West and in Russia.
However, their potential has not been fully exploited as the designs are highly
complex, especially the earliest designs of Turlakov et al. [101] and Kuznetsov
and Lobachev [102] , which are quite cumbersome to operate. The objective of
these earlier authors was to study the physicochemical properties of hydrothermal
solutions, the states of crystalline substances in them, the solution chemistry, and
so on, by direct observation. Their studies also included investigation of the con-
vection currents in the autoclave. For this purpose, the walls of the autoclaves are
drilled with two diametrically opposite holes, which are covered with transparent
crystals, or windows made of quartz or sapphire. If the interior of the autoclave is
illuminated through one such window, one can observe what is happening in the
autoclave through the other. By replacing the eye with a camera, movie camera,
or video camera, one can record the processes taking place inside the autoclave;
and by using a spectrograph, the state of the solution can be investigated.
Figure 3.35 shows the schematic diagram of the autoclave with transparent win-
dows [103] . Several problems arose while using these autoclaves under moderate
to higher pressures and temperatures due to the differential expansion of steel and
sapphire, which can be partially overcome by machining the cap from a material
with a coefficient of thermal expansion similar to that of the windows. For exam-
ple, niobium has been successfully used to about 750 C in combination with
sapphire windows [104] . The experiments showed that when the autoclave is not
under higher pressure
temperature conditions, the structure could easily support
an internal pressure of up to 1 kbar. However, it did not always remain sealed
when subjected to high temperatures and high pressures of and to an active soda
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