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A jet mill
grinds In Ireland, grinds are private tuition; a major industry in Ireland, particularly at secondary school level. In 2012, the Office of the Revenue Commissioners launched an investigation into a perceived failure of some teachers to declare extra ...
materials by using a high speed jet of compressed air or inert gas to impact particles into each other. 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. Particles leaving the mill can be separated from the gas stream by
cyclonic separation Cyclonic separation is a method of removing particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. When removing particulate matter from liquid, a hydrocyclone is used; while from gas, a gas cyclone ...
.


Particle size

A jet mill consists of a short
cylinder A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an ...
, 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 In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in ...
. 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 from the gas as it flows from the nozzles along the wall to the outlet in the center of the mill The drag on small particles is less than large particles, according to the formula derived from
Stokes' law In 1851, George Gabriel Stokes derived an expression, now known as Stokes' law, for the frictional force – also called drag force – exerted on spherical objects with very small Reynolds numbers in a viscous fluid. Stokes' law is derived by ...
, :V = \frac\frac g\, R^2, where ''V'' is the flow settling velocity (m/s) (vertically downwards if ''ρp'' > ''ρf'', upwards if ''ρp'' < ''ρf'' ), ''g'' is the
gravitational acceleration In physics, gravitational acceleration is the acceleration of an object in free fall within a vacuum (and thus without experiencing drag). This is the steady gain in speed caused exclusively by the force of gravitational attraction. All bodie ...
(m/s2), ''ρp'' is the
mass density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
of the particles (kg/m3), ''ρf'' is the mass density of the fluid (kg/m3), ''μ'' is the
dynamic viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
(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 Comminution is the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. In geology, it occurs naturally during faulting in the upper part ...
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

{{reflist, refs= {{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– {{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– {{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–


External links


Cross section diagram

Jet mills
Grinding mills