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Metallurgy
- A Preface by "Pyromatic"
Metallurgy, science
and technology of metals, includes the extraction of metals from ores,
the preparation of metals for use, and the study of the relationship
between structures and properties of metals.
Metallurgy is
the study of crystalline structure of metals and alloys and relationship
of this structure to the physical properties of metals and microscopic
examination of suitably prepared specimens.
Metallurgy also
relates to the properties of metals and metallic mixtures, alloys
to their microstructure. Metallurgists make use of measurements of
electrical conductivity, macroscopic density and its variation in
temperature (thermal expansion). Metallurgy is the branch of science
which deals with the general relationship of between the composition,
structure and properties of metals and alloys, as well as the changes
brought about by thermal, chemical, and metallurgical treatment.
Metallurgical
processes consists of two operations: Concentration, separating a
metal or metallic compound from the useless waste rock material, or
gauge, which accompanies it in the ore; and refining, producing the
metal in a pure or nearly pure state suitable for use. Three types
of processes are employed both for concentration and refining: mechanical,
chemical, and electrical. In most cases a combination of these methods
is used.
One of the simplest
methods of mechanical separation is gravity separation. This process
is based on the difference in specific gravity between native metals
and metallic minerals, and the other rock materials with which they
are mixed. When crushed ore or ore concentrates are suspended either
in water or an air blast, the heavier metal or metallic mineral particles
fall to the bottom of the processing chamber, and the lighter gangue
is blown or washed away. The prospector's technique of panning gold
from gold-bearing sand, for example, is a small-scale gravity-wwparation
process. Similarly, by virtue of its higher specific gravity, magnetite,
a mineral of iron, may be separated from the gangue rock in which
it occurs.flotation is the most important present-day method of mechanical
concentration. In its simplest form, flotation is a modified gravity
process in which finely ground ore is mixed, usually with a liquid.
The metal or metallic mineral floats while the gangue sinks, although
the reverse is true in some instances. In most modern flotation processes,
the floating of either the metal or gangue is aided by an oil or other
surface-active agent. By this means, comparatively heavy substances
can be made to float on water. In one typical process, a finely ground
ore containing copper sulfide is mixed with water, to which small
amounts of oil, acid, or other so-called floatation reagents are added.
When air is blown through this mixture, a froth is formed on the surface
that has the property of mixing with the sulfide but not with the
gangue. The latter material settles, and the sulfide is collected
from the froth. Use of the flotation process has made possible the
exploitation of many ore deposits of low concentration, and even of
the wastes from processing plants that used less efficient techniques.
In some cases, by means of differential flotation, different minerals
can be concentrated from one complex ore in a single process.
Ores, such as
magnetite, that have marked magnetic properties are concentrated by
means of electromagnets that attract them, metal but do not attract
the gangue.
Electrostatic separation employs an electric field to separate minerals
of different electrical properties by exploiting the attraction between
unlike charges and the repulsion between like charges.
Chemical separation
methods are, in general, the most important from the economic point
of view. In present-day practice chemical separation often is used
as a second stage after mechanical concentration. A greater tonnage
of refined metal is obtained by smelting than by any other process.
In smelting, the ore, or the concentrate from a mechanical separation
process, is heated with a reducing agent and flux to a high temperature.
the reducing agent combines with the oxygen in a metallic oxide, leaving
pure metal and the flux combines with the gangue to form a slag that
is liquid at the smelting temperature and thus can be skimmed off
or poured away from the metal. The production of pig iron in blast
furnaces is an example of smelting, and the process is also used to
extract copper, lead, nickel, and many other metals from their ores.
Amalgamation
is a metallurgical process that utilizes mercury to dissolve silver
or gold to form an amalgam. This process has been largely supplanted
by the cyanide process, in which gold or silver is dissolved in solutions
of sodium temperature below the melting point of the metal. In the
case of carbonates, carbondioxide is driven off in the process, leaving
a metallic oxide. Whe sulfides are roastes, the sulfur combines with
the oxygen of the air to form gaseous sulfur dioxide, leaving metallic
oxides, which are subsequently reduced by smelting.Agglomeration of
ore fines (fine particles) is accomplished by sintering or pelletizing.
In the sintering process, fuel, water, air, and heat are used to fuse
the ore fine into a porous mass. In pelletizing, moistened fine is
formed into smalll pellets in the presence of limestone flux and then
fired. A number of other processes, of which pyro-metallurgy (high-temperature
metallurgy) and distillation are the most important, are employed
in further refinement stages of a variety of metals. In the process
of electrolysis, the metal is deposited at the cathode from aqueous
solutions or in an electrolytic furnace. Copper, nickel, zinc, silver,
and gold are several examples of metals that are refined by deposition
from aqueous solutions. Aluminium, barium, calcium, magnesium, beryllium,
potassium, and sodium are metals that are processed in electrolytic
furnaces.
The most important
tools of the metallurgist are the microscope & X-ray machine.
Microscopic examination of prepared specimens makes possible the determination
of size, structure, orientation of metal crystals.
By means of
such examinations, metallurgists can frequently identify a metal or
alloy, discover possible impurities, and check on the effectiveness
of heat treatments for hardening or annealing. Metal specimens for
metallographic examination are usually highly polished and then etched
with dilute acids; this treatment brings out the grain structure by
attacking the boundaries between the grains or by attacking one of
the constituents of an alloy. When metals are to be examined under
the high magnification of an electron microscope, a thin electron-transparent
replica or cast of the etched surface can be made, because bulk metals
do not transmit an electron beam. Alternatively, an extremely thin
specimen can be made; the microstructure that is observed is a projection
of that contained within the thin specimen.When X rays are passed
through a specimen of a crystalline substance, diffraction patterns
are produced that can be interpreted to determine the internal structure
of the crystals.
Metallographic
research has shown that as a metal is stretched or otherwise deformed,
minute slippages occur between the layers of atoms that make up the
crystal, permitting the metal to take on a new shape and increasing
its hardness and strength. If the metal is heated after deformation,
it recrystallizes; that is, the atoms rearrange themselves to form
new unstrained crystals. This fact explains why metals become brittle
after bending when cold and why they become soft again after reheating.
The aim of physical
metallurgy, as a science, is to establish the physical laws governing
the structure of an alloy and its properties and to find the best
possible composition, manufacturing techniques, and treatment of the
alloy to obtain the required physical and mechanical properties.
The equipments
relating to the science of metallurgyare specified below
1. Specimen cut-off machine
2. Specimen Mounting Press with digital timer
3. Wet and dry Linisher
4. Polishing Stand
5. Polishing/Lapping machines (Single/Double/Triple disk)
6. Microscope
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