Geoscience Reference
In-Depth Information
temperature. Likewise, the anion-water distances, for example, as in the case of
iodide-oxygen (water) bond lengths, indicate that the solvation shell expands slightly
with increasing temperatures. Similarly, molecular dynamics simulation studies on the
alkali metals hydration in high temperature water up to 380
C have been carried out
recently, demonstrating a small expansion of the first hydration shell around the chlo-
ride with increasing temperature
[79]
. More or less complete data are available on the
formation of simple, neutral ion pairs for dilute alkali metal halide solutions at HPHT
conditions. However, the understanding of the formation of polynuclear species is still
not clearly understood. Some workers have studied the ion pairing and cluster forma-
tion in a 1 M NaCl solution at 380
C and near critical pressure
[80]
. These studies
indicate the presence of simple monocationic ions and ion pairs together with triple
ions such as Na
2
Cl
2
and NaCl
2
as well as the Na
2
Cl
2
and more complicated polynu-
clear species. Similar studies on other solutions are available in the literature. All these
studies have greatly contributed to the understanding of the geochemical system
wherein the metal-complexing by other ligands is the most important aspect.
However, there is a major lack of overall experimental data pertaining to the metal
complex equilibria in supercritical aqueous systems as well as in binary solvent sys-
tems such as H
2
O
CO
2
. Such data are of enormous importance to the understanding
of the geochemistry of element transport by hydrothermal fluids active in the earth's
crust.
During 1994, a new concept, viz., geothermal reactor, introduced by Japanese
workers, is slowly catching the attention of hydrothermal engineers
[81,82]
. The
principles of geothermal reactors include the direct use of geothermal energy as a
heat source or driving force for chemical reactions. It helps to produce hydrothermal
synthesis of minerals and a host of inorganic materials, extraction of useful chemical
elements contained in crustal materials such as basalt, and use them as raw materials
for hydrothermal synthesis. Thus, the concept of geothermal reactor leads to the
construction of a high-temperature and high-pressure autoclave underground. This
has several advantages over the conventional autoclave technology.
Figure 1.2
shows the schematic sketch of a typical geothermal reactor for min-
eral synthesis
[82]
. The major advantages of the geothermal reactor are:
a
1. Synthesis of ceramic materials by hydrothermal reaction is possible without using fuel or
electricity as main energy source.
2. The system does not discharge the used heat.
3. Outer tube with a slit in the bottom must be strong enough to keep the inner space of the
tube against the pressure by the wall of formation, but inner double tube does not need
the strength against the inner pressure as usual autoclaves.
4. Area of the plant is small because the vertical long reactor is under the ground.
5. It is highly useful to study the alteration of various rocks occurring in the earth's crust in
the presence of fluids of various compositions. Such studies have been carried out for
granite and basalts
[75]
.
Such geothermal reactors are in use in the Miyagi prefecture of Japan, and the
researchers are involved in the studies related to the rock
hydrothermal fluid interac-
tions and also synthesis of some materials (K. Nakatsuka, personal communication).
