# Jet mill

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{{Short description|Pneumatic powered mill}}
thumb|Illustration of a jet mill.  Red arrow shows material entering the mill.  Blue arrows show compressed air entering and circulating in the mill.  Green arrow shows small particles leaving mill.
A '''jet mill'''  [grinds](/source/grinder_(milling)) materials by using a high speed jet of compressed air or inert gas to impact particles into each other.<ref name="RahamanRahaman2006" />  Jet mills can be designed to output particles below a certain size while continuing to mill particles above that size, resulting in a narrow size distribution of the resulting product.<ref name="Wachtman2009" />  Particles leaving the mill can be separated from the gas stream by [cyclonic separation](/source/cyclonic_separation).<ref name="NeikovMurashova2009"/>

==Particle size==
A jet mill consists of a short [cylinder](/source/cylinder_(geometry)), meaning the cylinder's height is less than its diameter.  Compressed gas is forced into the mill through nozzles tangent to the cylinder wall, creating a [vortex](/source/vortex).  The gas leaves the mill through a tube along the axis of the cylinder.  Solid particles in the mill are subject to two competing forces:

# Centrifugal force created by the particles traveling in circles
# Centripetal force created by the [drag](/source/Drag_(physics)) from the gas as it flows from the nozzles along the wall to the outlet in the center of the mill

[[Image:Stokes sphere.svg|thumb|right|175px|Flow past  a solid particle reacting to centrifugal force: [streamlines](/source/Streamlines%2C_streaklines%2C_and_pathlines), drag force ''F''<sub>d</sub> and force by gravity or centrifugal force ''F''<sub>g</sub>. <br/> ''F''<sub>d</sub> points toward the center of the mill, and ''F''<sub>g</sub> points to the wall.]]

The drag on small particles is less than large particles, according to the formula derived from [Stokes' law](/source/Stokes'_law), 

:<math>V = \frac{2}{9}\frac{\left(\rho_p - \rho_f\right)}{\mu} g\, R^2</math>,

where ''V'' is the flow settling velocity (m/s) (vertically downwards if ''&rho;<sub>p</sub>''&nbsp;>&nbsp;''&rho;<sub>f</sub>'', upwards if ''&rho;<sub>p</sub>''&nbsp;<&nbsp;''&rho;<sub>f</sub>''&nbsp;), ''g'' is the [gravitational acceleration](/source/Earth's_gravity) (m/s<sup>2</sup>), ''&rho;<sub>p</sub>'' is the [mass density](/source/mass_density) of the particles (kg/m<sup>3</sup>), ''&rho;<sub>f</sub>'' is the mass density of the fluid (kg/m<sup>3</sup>), ''μ'' is the [dynamic viscosity](/source/dynamic_viscosity) (kg /m*s), and ''R'' is the radius of the spherical particle (m).

The formula shows that particles will be pulled toward the wall of the mill according to the square of their radius or diameter.  Large particles will continue the [comminution](/source/comminution) process, until they are small enough to stay in the center of the mill where the discharge port is located.

==Typical parameters==
* Diameter of mill: from 0.05 meters to 1 meter (from 2 inches to 42 inches)
* Gas pressure: 8.3 Bar (120 PSI)
* Starting particle size: 800 microns or less, or as limited by the size of the inlet of the feed venturi
* Final particle size: down to 0.5 microns

==References==
<references>

<ref name="RahamanRahaman2006">{{cite book|author1=Mohamed Rahaman|author2=Mohamed N. Rahaman|title=Ceramic Processing|url=https://books.google.com/books?id=GC85CKs7URkC&pg=PA41|date=7 August 2006|publisher=CRC Press|isbn=978-0-8493-7285-8|pages=41–}}</ref> 

<ref name="Wachtman2009">{{cite book|author=John B. Wachtman|title=Materials and Equipment - Whitewares Manufacturing: Ceramic Engineering and Science Proceedings, Volume 14|url=https://books.google.com/books?id=GZK-9dVIExQC&pg=PA264|date=28 September 2009|publisher=John Wiley & Sons|isbn=978-0-470-31618-4|pages=264–}}</ref>

<ref name="NeikovMurashova2009">{{cite book|author1=Oleg D Neikov|author2=I. B. Murashova|author3=Nicholas A. Yefimov|author4=Stanislav Naboychenko|title=Handbook of Non-Ferrous Metal Powders: Technologies and Applications|url=https://books.google.com/books?id=6aP3te2hGuQC&pg=PA60|date=24 February 2009|publisher=Elsevier|isbn=978-0-08-055940-7|pages=60–}}</ref>

</references>

==External links==
*[https://web.archive.org/web/20150626141913/http://www.sturtevantinc.com/assets/Sturtevant_Micronizer_Brochure.pdf Cross section diagram] 
*[https://pdf.indiamart.com/impdf/13267396573/SELLER-238199/jet-mills-fluid-energy-mill-.pdf Jet mills]
Category:Grinding mills

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Adapted from the Wikipedia article [Jet mill](https://en.wikipedia.org/wiki/Jet_mill) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Jet_mill?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.