Production of Metal Powders
Metallic powders possessing different
properties can be produced easily. The most
commonly used powders are copper-base and
iron-base materials. But titanium, chromium,
nickel, and stainless steel metal powders
are also used. In the majority of powders, the size
of the particle varies from several microns
to 0.5 mm. The most common particle size of
powders falls into a range of 10 to 40
microns. The chemical and physical properties of metals
depend upon the size and shape of the
powder particles. There are various methods of
manufacturing powders.The commonly used
powder making processes are given as under.
1. Atomization
2. Chemical reduction
3. Electrolytic process
4. Crushing
5. Milling
6. Condensation of metal vapors
7. Hydride and carbonyl processes.
The above mentioned metallic powder making
techniques are discussed briefly as under.
1. Atomization
In this process, the molten metal is forced
through an orifice and as it emerges, a high
pressure stream of gas or liquid impinges
on it causing it to atomize into fine particles. The
inert gas is then employed in order to
improve the purity of the powder. It is used mostly
for low melting point metals such as tin,
zinc, lead, aluminium, cadmium etc., because of the
corrosive action of the metal on the
orifice (or nozzle) at high temperatures. Alloy powders
are also produced by this method.
2. Chemical Reduction Process
In this process, the compounds of metals
such as iron oxides are reduced with CO or H2
at temperatures below the melting point of
the metal in an atmosphere controlled furnace.
The reduced product is then crushed and
ground. Iron powder is produced in this way
Fe3O4 + 4C = 3Fe + 4CO
Fe3O4 + 4CO = 3Fe + 4CO2
Copper powder is also produced by the same
procedure by heating copper oxide in a
stream of hydrogen.
Cu2 + H2 = 2Cu + H2O
Powders of W, Mo, Ni and CO can easily be
produced or manufactured by reduction
process because it is convenient,
economical and flexible technique and perhaps the largest
volume of metallurgy powders is made by the
process of oxide reduction.
3. Electrolytic Process
Electrolysis process is quite similar to
electroplating and is principally employed for the
production of extremely pure, powders of
copper and iron. For making copper powder, copper
plates are placed as anodes in a tank of
electrolyte, whereas, aluminium plates are placed in
to the electrolyte to act as cathodes. High
amperage produces a powdery deposit of anode
metal on the cathodes. After a definite
time period, the cathode plates are taken out from the
tank, rinsed to remove electrolyte and are
then dried. The copper deposited on the cathode
plates is then scraped off and pulverized
to produce copper powder of the desired grain size.
The electrolytic powder is quite resistant
to oxidation.
4. Crushing Process
The crushing process requires equipments
such as stamps, crushers or gyratory crushes.
Various ferrous and non-ferrous alloys can
be heat-treated in order to obtain a sufficiently
brittle material which can be easily
crushed into powder form.
5. Milling Process
The milling process is commonly used for
production of metallic powder. It is carried out
by using equipments such as ball mill,
impact mill, eddy mill, disk mill, vortex mill, etc.
Milling and grinding process can easily be
employed for brittle, tougher, malleable, ductile and
harder metals to pulverize them. A ball
mill is a horizontal barrel shaped container holding
a quantity of balls, which, being free to
tumble about as the container rotates, crush and
abrade any powder particles that are
introduced into the container. Generally, a large mass
to be powdered, first of all, goes through
heavy crushing machines, then through crushing
rolls and finally through a ball mill to
produce successively finer grades of powder.
6. Condensation of Metal Powders
This process can be applied in case of
metals, such as Zn, Cd and Mg, which can be boiled
and the vapors are condensed in a powder
form. Generally a rod of metal say Zn is fed into
a high temperature flame and vaporized
droplets of metal are then allowed to condense on
to a cool surface of a material to which
they will not adhere. This method is not highly
suitable for large scale production of
powder.
7. Hydride and Carbonyl Processes
High hardness oriented metals such as tantalum,
niobium and zirconium are made to
combine with hydrogen form hydrides that
are stable at room temperature, but to begin to
dissociate into hydrogen and the pure metal
when heated to about 350°C. Similarly nickel and
iron can be made to combine with CO to form
volatile carbonyls. The carbonyl vapor is then
decomposed in a cooled chamber so that
almost spherical particles of very pure metals are
deposited.
