Geoscience Reference
In-Depth Information
1. Push rod
2. Nut for pushing
gland packing
3. Nut for pushing cone
Cross section of a-a
a
a
4. Gland packing
1. Piston
5. Cone
6. Space for water
retreat
2. Inner case
a
a
7. Sample
3. Sample
4. Outer case
8. Well for thermocouple
5. Piston
(a)
(b)
Figure 3.36 Schematic diagram of (a) die and (b) autoclave for HHP
[107]
.
can obtain sintered compacts of pure oxides or inorganic powders which it is not
possible to obtain by other ordinary sintering processes due to the transformation.
Silicate powders such as borosilicate and silica glass can easily form a solid body
by HHP technology with compression at 20
30 MPa when the proper amount of
water or alkaline solution is added as a mineralizer.
In this technique, a starting powder containing water is continuously compressed
from outside an autoclave under hydrothermal conditions. The powder is hydrother-
mally treated at autogeneous pressure while it is compressed at much higher
pressure than the vapor pressure inside the autoclave.
Figure 3.36
shows the sche-
matic diagram of an autoclave for HHP. It is a cylinder made of steel with a cylin-
drical chamber 1 cm in diameter. The term reaction pressure means compressive
pressure from outside the autoclave. The effective pressure for samples in the auto-
clave should be smaller than the reaction pressure because of vapor pressure in the
autoclave. The piston had a space into which water from the starting sample was
released by the HHP treatment. The gland packing made of Teflon between the
piston and the push rod prevented leakage.
This technology has been used successfully for solidification of various indus-
trial wastes such as sludge ash
[108]
, radioactive wastes
[107]
, and concrete
wastes
[109]
. One of the advantages of HHP technology is that the hydrothermal
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