Production of Metal Powders

 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.