Geoscience Reference
In-Depth Information
Figure 3.42 (a) Stellite
Tuttle cold-cone-sealed
autoclaves and (b) stainless
steel acid digestion bombs
which ruptured at high
pressure.
rupture disks calibrated to burst above a given pressure. Such rupture disks are
commercially available for various ranges of bursting pressure. Provision should be
made for venting the live volatiles out in the event of rupture. A blow-down tank
has to be commissioned in the case of a large-size hydrothermal laboratory. Proper
shielding of the autoclave is needed to direct the corrosive volatiles away from the
personnel. In the case of a large autoclave, the vessels are placed with proper
shielding in a pit. For safety purposes, when hydrothermal autoclaves are operated
simultaneously at a given temperature, a total static overpressure caused by a
simultaneous rupture of all the operating autoclaves should not exceed 0.5 psi,
which is determined by the strength of the walls and the dimension of the labora-
tory. In several laboratories, make-up pressure units are installed for safety.
The hydrothermal laboratory should have a well-equipped workshop for the proper
maintenance of the autoclaves. After each run, the autoclave should be properly trea-
ted before using it for the next run. The contacts, especially at the sealing sections,
should be kept sharp without any lint, by polishing the surfaces at the contact after
each run to avoid scratches. The gasket must remain flat in its seat without tilting.
The autoclave threads should be maintained carefully and are always painted with
pipe thread lubricant to reduce friction and prevent seizing of the autoclaves at high
temperatures. Some workers even place a light coating on the seal rings to ensure that
they are properly sealed. Graphite grease or graphite in water medium and suspended
molybdenum sulfide or copper lubricants are used. For runs above 200
C, graphite
lubricant is more effective because other lubricants may decompose.
With a new or recently remachined vessel, it is advisable first to clean out the
residual cutting oil with an organic solvent and to heat distilled water in the vessel
one or more times, until the odors of sulfur gases or organic species are no longer
present. For vessels that have been used with H
2
S, many cycles may be required.
Once a vessel has contained reduced sulfur, it should not be used for sulfur-free
experiments unless it has been vacuum cleaned while hot. Such a degassed vessel
previously used for reduced systems can then be heated, at about 350
C open to the
atmosphere, to promote an oxide surface layer for corrosion protection. Proper pre-
caution should be taken while sealing the vessel. In the case of cold-seal equipment,
explosive vessel failure is of paramount concern. Such failure is likely to occur in
the event of rapid quenching of the entire vessel and overheating of the furnace. In
spite the simple, rapid quench it provides, immersing the vessel in water is to be
avoided. Nowadays, numerous remote quenching designs have been developed
