Chemical Methods:
Precipitation:
The principal of precipitating a metal from its aqueous solution by the addition of a less noble metal which is higher in the electromotive series has been applied in numerous metallurgical processes. Ag powder is produced in quantity from its nitrate solution by adding copper or iron according to the reaction:
AgNO3 + Fe --> Ag + FeNO3
This method is used for producing metal powders of Ag, Sn, Pt and iron particles coated with copper. Precipitation is also synonymous with the term electrolytic precipitation to coat metals with a corrosion resistance film. The produced powder take the form of spongy mass which crashed into a hard and brittle powder.
Carbonyl Method:
Of the group of thermally decomposed powders, those produced by thermal decomposition of carbonyls are the most important. Both iron and nickel are produced by decomposition of the respective carbonyls. Carbonyls are obtained by passing carbon monoxide over spongy metal at specific temperatures and pressures according to the reaction:
Fe + 5CO --> Fe(CO)5
This reaction can be controlled by changing temperature and pressure so the following reaction will take place:
Fe + (CO)5 --> Fe powder + 5CO
The chemical purity of the powders can be very high (over 99.5%), with the principal impurities being carbon, nitrogen, and oxygen. Particle size can be controlled very closely. Iron carbonyl powder is usually spherical in shape and very fine (less than 10 m) used for the production of magnet cores, while the nickel powder is usually quite irregular in shape, porous, and fine but the production process is of high cost.
