Geoscience Reference
In-Depth Information
1
Hydrothermal Technology—
Principles and Applications
1.1 Introduction
The hydrothermal technique has been most popular, garnering interest from scien-
tists and technologists of different disciplines, particularly from 1990s. The term
hydrothermal is of purely geological origin. It was first used by the British geolo-
gist, Sir Roderick Murchison (1792
1871), to describe the action of water at an
elevated temperature and pressure in bringing about changes in the earth's crust
leading to the formation of various rocks and minerals
[1]
. The majority of the
minerals formed in the postmagmatic and metasomatic stages in the presence of
water at elevated pressure and temperature conditions are said to be “of hydrother-
mal origin.” This covers a vast number of mineral species including ore deposits. It
is well known that the largest single crystal formed in nature (beryl crystal of
.
1000 g) and some of the largest quantities of single crystals created by man in
one experimental run (quartz crystals of several hundreds of grams) are both of
hydrothermal origin.
An understanding of the mineral formation in nature under elevated pressure and
temperature conditions in the presence of water led to the development of the hydro-
thermal technique. In 1839, the German chemist Robert Bunsen contained aqueous
solutions in thick-walled glass tubes at temperatures above 200
C and at pressures
above 100 bar
[2]
. The crystals of barium carbonate and strontium carbonate that he
grew under these conditions mark the first use of hydrothermal aqueous solvents as
media. It was successfully adopted by Schafthaul
[3]
to obtain quartz crystals upon
the transformation of freshly precipitated silicic acid in Papin's digestor. Thus, the
hydrothermal technique became a very popular means to simulate the natural condi-
tions existing under the earth's crust and synthesize them in the laboratory. These
studies dealing with laboratory simulations have helped the earth scientists to deter-
mine complex geological processes of the formation of rocks, minerals, and ore
deposits. As the subject became more and more popular among geologists, new
branches of geology emerged, such as “Experimental Mineralogy” and “Experimental
Petrology.”
The first successful commercial application of hydrothermal technology began
with mineral extraction or ore beneficiation in the previous century. The use of
sodium hydroxide to leach bauxite was invented in 1892 by Karl Josef Bayer
