|1. What is a jet
|2. How does a jet
mill work ?
|3. Industrial jet
Jet mills are used to reduce the particle size, i.e. granulometry, of bulk solids thanks to a high speed gas (air, steam) jet. Those bulk solids can be of different natures, typically foodstuff, pharmaceuticals, metal powders or minerals. In the quite large family of comminution equipment (size reduction equipment), and more particularly of impact mills, jet milling machines have the particularity to have no moving part as the energy is supplied by a gas released at high speed in a milling chamber where coarse particles are also introduced. The milling mechanism is only by impact of the particles in between the particles themselves or against the walls of the milling chamber.
Jet mills have several advantages that can make them attractive to companies :
Due to its technology, based on the injection of a gas (typically air, or an inert gas like nitrogen, argon, however steam can also be used), jet mills have also some limitations to consider :
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The working principle of jet mills is explained in this part.
The gas used in jet mill is usually one of the following :
Suppliers are proposing 2 types of design [Liu] :
Those mills are equipped with vertical, cylindrical, milling chamber in which the material is introduced at the middle of the chamber, and the grinding gas injecting a the bottom. The air and product exhaust is from the top of the chamber, however the outlet is equipped with a classifier allowing to select which particles, below a target size, can leave the mill, and which ones need to be recycled back to the milling chamber. As the gas is injected at the bottom of the chamber, the particles are fluidized with circulation streams establishing, hence the denomination of fluidized bed jet mill.
The other design is simpler, the material is fed into a grinding chamber similar to a disc where the gas is injected at several points. The air and the material milled is then exiting at the center of the grinding chamber.
Figure 2 : spiral jet mill
The feeding system to the milling chamber must act as an airlock in order to avoid that the air escape and prevent the inlet of material.
The milled particles are leaving the mill with a large volume of
gas. It is then required to separate the material of interest,
from the gas. It is typically done thanks to a cyclone, a filter
of a combination of them.