Jet mill

Last updated November 19, 2019

A jet mill grinds materials by using a high speed jet of compressed air or inert gas to impact particles into each other.^[1] 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.^[2] Particles leaving the mill can be separated from the gas stream by cyclonic separation.^[3]

A 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 is used. Rotational effects and gravity are used to separate mixtures of solids and fluids. The method can also be used to separate fine droplets of liquid from a gaseous stream.

Particle size

A jet mill consists of a short cylinder, 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. 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:

In fluid dynamics, a vortex 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 wake of a boat, and the winds surrounding a tropical cyclone, tornado or dust devil.

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 solving the Stokes flow limit for small Reynolds numbers of the Navier–Stokes equations.

V={\frac {2}{9}}{\frac {\left(\rho _{p}-\rho _{f}\right)}{\mu }}g\,R^{2}

,

where V is the flow settling velocity (m/s) (vertically downwards if ρ_p > ρ_f, upwards if ρ_p < ρ_f ), g is the gravitational acceleration (m/s²), ρ_p is the mass density of the particles (kg/m³), ρ_f is the mass density of the fluid (kg/m³), μ is the 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 process, until they are small enough to stay in the center of the mill where the discharge port is located.

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 of the Earth's crust. In industry, it is an important unit operation in mineral processing, ceramics, electronics, and other fields, accomplished with many types of mill. In dentistry, it is the result of mastication of food. In general medicine, it is one of the most traumatic forms of bone fracture.

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

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References

↑ Mohamed Rahaman; Mohamed N. Rahaman (7 August 2006). Ceramic Processing. CRC Press. pp. 41–. ISBN 978-0-8493-7285-8.
↑ John B. Wachtman (28 September 2009). Materials and Equipment - Whitewares Manufacturing: Ceramic Engineering and Science Proceedings, Volume 14. John Wiley & Sons. pp. 264–. ISBN 978-0-470-31618-4.
↑ Oleg D Neikov; I. B. Murashova; Nicholas A. Yefimov; Stanislav Naboychenko (24 February 2009). Handbook of Non-Ferrous Metal Powders: Technologies and Applications. Elsevier. pp. 60–. ISBN 978-0-08-055940-7.

External links

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[RahamanRahaman2006-1] Mohamed Rahaman; Mohamed N. Rahaman (7 August 2006). Ceramic Processing. CRC Press. pp. 41–. ISBN 978-0-8493-7285-8.

[Wachtman2009-2] John B. Wachtman (28 September 2009). Materials and Equipment - Whitewares Manufacturing: Ceramic Engineering and Science Proceedings, Volume 14. John Wiley & Sons. pp. 264–. ISBN 978-0-470-31618-4.

[NeikovMurashova2009-3] Oleg D Neikov; I. B. Murashova; Nicholas A. Yefimov; Stanislav Naboychenko (24 February 2009). Handbook of Non-Ferrous Metal Powders: Technologies and Applications. Elsevier. pp. 60–. ISBN 978-0-08-055940-7.