Dr. Dmitri Kopeliovich
The following powder preparation methods are used:
Atomization is one of the most effective industrial powder preparation methods.
This method involves desintagration (atomizing) of liquid metal by means of high speed medium (air, inert gas, water) striking the melt streaming through a nozzle.
Comminution is a mechanical method of powder preparation involving breaking solid particles in mills (ball, vibratory, hummer).
This method is generally applied for the preparation of powders of brittle materials.
The most widely used communition machine is ball mill, utilizing the energy of steel steel balls, which move in a rotating drum and crush the metal pieces into fine particles.
Maximum grinding effect is achieved at optimal rotation speed of the mill (N).
N = 0.6-0.8*Ncr
Ncr may be determined from the expression:
Ncr = (g/2D)1/2/π (revolutions per second)
g - acceleration due to gravity;
D - drum diameter.
If N>0.8*Ncr then the balls “stick” to the drum wall and no grinding ocurrs.
If N<0.6*Ncr then the balls roll without falling. The grinding effect in this case is low (only rubbing).
If N = 0.6-0.8*Ncr then the balls go up together with the rotating drum and then fall down and roll providing grinding by combined action of strike and rubbing.
Metal powders may be prepared by reduction of the metal oxides (carbides) followed by mechanical crushing or precipitation of metals from solution (hydrometallurgical method).
Powder may be also prepared by electrolytic deposition of a metal at the cathode followed by mechanical crushing of the deposited material. This method is used for fabrication of copper powders.
These methods are called chemical powder preparation methods.
Particle may be spherical (gas atomization), irregularly shaped (water atomization), porous (reduced oxides), dendrite like (electrolytic deposition).
Apparent density is a function of porosity of the particles, regularity of their shapes and variety of their sizes.