Powder preparation - mixing and grinding

However, it is usually quite easy to determine as a loss in weight after heating to a suitable temperature. 

The first step is to weigh out the raw material with due allowance for impurity and moisture content. Many modern weighing machines record each weighing  

Wet ball-milling is faster than dry-milling and facilitates the separation of the milling media from the charge. Its disadvantage is that the liquid must be removed, which is most economically achieved by filtration, but any soluble constituent will be removed with the liquid. If this is undesirable the alternative is to evaporate the liquid which can be effected by spray-drying if the liquid is water. Evaporation can also take place in shallow trays in ovens in this case soluble material may appear as a skin on the surface and heavy coarse particles may settle out. 

Alternatives to ball-mills include vessels with rotating paddles and rotating barrels that contain baffles that ensure that the powder is tossed about. These have no grinding action so that the particle size is unaltered. They also have little effect on aggregates. 

Contamination by the milling media can be largely avoided by the use of fluid energy mills. In these the powder is entrained in two streams of high-velocity air which are made to impinge on one another so that the particles are broken up by impact. The feedstock must already be ground to within a factor of 10 of the final size required and adjustments are necessary to suit particular materials. 

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