Geoscience Reference
In-Depth Information
(1871
1908) as a process for obtaining pure aluminum hydroxide which can be
converted to pure A1
2
O
3
suitable for processing to metal
[4]
. Even today, over 90
million tons of bauxite ore is treated annually by this process
[5]
. Similarly, ilmen-
ite, wolframite, cassiterite, laterites, a host of uranium ores, sulfides of gold, cop-
per, nickel, zinc, arsenic, antimony, and so on, are treated by this process to extract
the metal. The principle involved is quite simple, very effective, and inexpensive,
as shown below, for example:
OH
2
!½
Þ
2
2
1
Al
ð
OH
Þ
3
1
AlO
ð
OH
H
2
O
2
OH
2
!½
AlOOH
AlO
ð
OH
Þ
1
The above process is easy to achieve and the leaching can be carried out in a few
minutes at about 330
C and 25,000 kPa
[6]
.
Further importance of the hydrothermal technique for the synthesis of inorganic
compounds in a commercial way was realized soon after the synthesis of large sin-
gle crystals of quartz by Nacken
[7]
and zeolites by Barrer
[8]
during late 1930s
and 1940s, respectively. The sudden demand for the large size quartz crystals dur-
ing World War II forced many laboratories in Europe and North America to grow
large size crystals. Subsequently, the first synthesis of zeolite that did not have a
natural counterpart was carried out by Barrer
[8]
in 1948 and this opened altogether
a new field of science, viz., molecular sieve technology
[9]
. The success in the
growth of quartz crystals has provided further stimuli for hydrothermal crystal
growth
[10]
.
Today, the hydrothermal technique has found its place in several branches of sci-
ence and technology, and this has led to the appearance of several related techniques
with strong roots attached to the hydrothermal technique. So we have: hydrothermal
synthesis, hydrothermal growth, hydrothermal alteration, hydrothermal treatment,
hydrothermal metamorphism, hydrothermal dehydration, hydrothermal decomposi-
tion, hydrothermal extraction, hydrothermal sintering, hydrothermal reaction sintering,
hydrothermal phase equilibria, hydrothermal electrochemical reaction, hydrothermal
mechanochemical, microwave hydrothermal, hydrothermal sonochemical, and so on.
This involves: materials scientists, earth scientists, materials engineers, metallur-
gists, physicists, chemists, biologists, and others. Although the technique has attained
its present high status, it has passed through several ups and downs owing to the
lack of proper knowledge pertaining to the actual principles involved in the
process. Hence, the success of the hydrothermal technique can be largely attributed
to the rapid advances in the apparatus involved (new apparatus designed and fabri-
cated) in hydrothermal research and also to the entry of a large number of physical
chemists who have contributed greatly to the understanding of hydrothermal chemis-
try
[11]
. Further, the modeling and intelligent engineering of the hydrothermal pro-
cesses have also greatly enhanced our knowledge in the field of hydrothermal
research
[12,13]
.
In recent years, with the increasing awareness of both environmental safety
and the need for optimal energy utilization, there is a case for the development of
nonhazardous materials. These materials should not only be compatible with
