Geoscience Reference
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reaction occurs in a closed system, which prevents the evaporation of volatile
materials [110] .
Vertical Autoclaves Used in Hydrometallurgy
High-pressure hydrometallurgy has made considerable progress in the last decade
owing to advances in autoclave designs, which are quite large and are on an indus-
trial scale [111] . These autoclaves may be in the form of cylinders, vertically
mounted or horizontally laid, spherical, or in the form of a long horizontal tube.
The agitation in these autoclaves is accomplished with the injection of high-
pressure steam [112] . These steam agitated autoclaves are used mainly for leaching
bauxite by NaOH within the temperature range 140
150 C and pressure
2
2500
3500 kPa. The vessels are usually fabricated from welded steel cylinders
with spherical ends. Diameters vary from 1.5 to 2 m and heights from 6 to 12 m.
Autoclaves of similar designs, but with acid-resistant brick lining, are used for
leaching oxidized ores, e.g., laterites by concentrated H 2 SO 4 at 250 C and
4000 kPa. These autoclaves have undergone an enormous increase in size for pro-
cesses treating pyrite and arsenopyrite concentrates, containing gold particles and
other related sulfide ores. Some autoclaves, especially with spherical shapes, have
provisions for rotation at 8
2
15 rpm. In continuous operations, high-pressure mem-
brane piston pumps are used for introducing pulps into autoclaves. Figure 3.37
shows schematic diagrams of representative autoclaves [113] . The reader can get
more information in the works of Habashi [111,112,114] .
2
Flow Reactors
The supercritical hydrothermal method is an extension of hydrothermal technology.
The supercritical hydrothermal technology deals with the reactions at temperatures
just near or above the critical temperature. Although supercritical fluid technology
SCF (Skp, Cullin, F-box) technology has been in use from late nineteenth century
as a tool to understand the natural mineralization, the actual impetus for this tech-
nology as a tool to process a wide range of materials began in 1980s. With the
invention of Green Chemistry in the early 1990s, there was a surge in the popular-
ity of SCF technology. Green Chemistry is important because supercritical fluids,
particularly supercritical CO 2 (scCO 2 ) and, to a more limited extent, scH 2 O are per-
ceived as environmentally more acceptable replacements for the petroleum-based
solvents, which are currently used in the world's chemical industries [115] .The
supercritical hydrothermal method has witnessed a seminal growth in its applica-
tion to the processing of a variety of materials owing to the availability of a wide
range of solvents. The use of carbon dioxide has lots of advantages and replacing
organic solvents in a number of chemical processes, including nanoparticle fabrica-
tion, food processing (such as decaffeination of coffee beans), chemical
manufacturing, extraction, dry cleaning, semiconductor wafer cleaning, polymer
processing, recycling, waste treatment, organic decomposition of dangerous and
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