Physical methods
Electrolytic deposition
• In this method, the processing conditions are so chosen that metals of high purity are precipitated from aqueous solution on the cathode of an electrolytic cell. This method is mainly used for producing copper, iron powders. This method is also used for producing zinc, tin, nickel, cadmium, antimony, silver, lead, beryllium powders.
• Copper powder => Solution containing copper sulphate and sulphuric acid; crude copper as anode
• Reaction: at anode: Cu -> Cu+ + e-; at cathode: Cu+ + e- ->Cu
• Iron powder => anode is low carbon steel; cathode is stainless steel. The iron powder deposits are subsequently pulverized by milling in hammer mill. The milled powders are annealed in hydrogen atmosphere to make them soft.
• Mg powder => electrodeposition from a purified magnesium sulphate electrolyte using insoluble lead anodes and stainless steel cathodes
• Powders of thorium, tantalum, vanadium => fused salt electrolysis is carried out at a temperature below melting point of the metal. Here deposition will occur in the form of small crystals with dendritic shape.
In this method, final deposition occurs in three ways,
1. A hard brittle layer of pure metal which is subsequently milled to obtain powder (eg. iron powder).
2. A soft, spongy substance which is loosely adherent and easily removed by scrubbing.
3. A direct powder deposit from the electrolyte that collects at the bottom of the cell.
Factors promoting powder deposits are, high current density, low metal concentration, pH of the bath, low temperature, high viscosity, circulation of electrolyte to avoid of convection.
Advantages:
1 : - Powders of high purity with excellent sinterability.
2 : -Wide range of powder quality can be produced by altering bath composition.
Disadvantages:
Time consuming process; Pollution of work place because of toxic chemicals;
Waste disposal is another issue; Cost involved in oxidation of powders and hence they should be washed thoroughly.
