Apparatus - Hydrothermal Technology - page 112

calcium source via water; thus, ion species dissolved from the two sources diffuse
into each other in the autoclave to produce xonotlite (hydrated calcium silicate).
Platinum plates are placed in the autoclaves to collect the reaction products formed
by ion diffusion. The design must have originated from the solution growth and gel
growth techniques. This autoclave can work up to a temperature of 250 C and a
pressure of usually 100 bar. However, this design is very useful for the synthesis

of some special compounds through ion diffusion.

,

Autoclaves for the Sampling of Hydrothermal Fluids
The “black box” nature of the hydrothermal system hinders the ready understanding
of the hydrothermal crystallization processes taking place in a given set of
conditions in an autoclave. This is one of the reasons for the slow raise in the popu-
larity of hydrothermal research, especially in the previous century. The earliest auto-
clave designs did not have any provisions for the withdrawal of the sample or fluid
from the autoclave during the experimental run. As the experiment progresses, the
solvent solute interaction yields several metastable and stable phases, accompanied

by a continuous change in the characteristic properties of the media, e.g., the pH and
redox potential. It is possible to understand these changes only if we can withdraw
the hydrothermal solution from the autoclave and subject it to several studies, or by
introducing a desired gaseous pressure in the autoclave through an external pressure
source. Only in the last few decades have such provisions been made in the auto-
clave designs, and they are commercially available. Figures 3.27

3.29 show the
commercially available autoclaves with facilities to draw out the sample or fluid.
These are popularly known as Start Parr autoclaves (Japan and USA) [68] and
Berghoff autoclaves [69] . Also some Japanese manufacturers have come out with
similar designs [70,71] . Several configurations of valves permit the sampling of

Figure 3.27 Commercially available autoclaves.
Source: Photograph courtesy of Parr Instruments Co., IL, USA
[68] .