The basic principles of particle size reduction. Presentation includes a comparison of gravity and pneumatic discharge hammer mills, how finished particle size is determined, and an explanation of particle size distribution.

1. How Does a Hammer Mill Work?
The basic principles of particle size reduction

2. Crush, grind, pulverize, shred… Size reduction is amazing!
But how does it work?

3. Hammer mills work in the principle that most materials will crush, shatter, or pulverize upon impact.

4. The Process:
Material is fed into the mill grinding chamber through the feed chute, typically by gravity.
It repeatedly is struck by ganged hammers which are attached to a shaft which rotates at high speed inside the mill chamber.
The material is crushed or shattered by a combination of repeated hammer impacts, collisions with the walls of the grinding chamber, and particle on particle impacts.
Perforated metal screens or bar grates covering the discharge opening of the mill retain coarse material for further grinding, while allowing properly sized materials to pass as finished product.

5. Gravity Discharge
Pneumatic Discharge
Mill Evacuation

6. Gravity Discharge
Mill Evacuation
Ideal for hard, heavy, and/or abrasive materials such as :
•Glass
•Stone
•Metals
•Aggregates

7. Pneumatic Discharge
Mill Evacuation
Ideal for light, low density material such as :
•Planer shavings
•Paper
•Carpet
•Biomass

8. Finished Particle Size
Regardless the evacuation method, the same factors determine finished particle size:
Varying the:
Screen Size
Shaft Speed
Hammer Configuration
can dramatically alter the finished particle size of the material being processed.

9. Finished Particle Size
Regardless the evacuation method, the same factors determine finished particle size. Generally speaking:

10. Particle Size Distribution
Appropriate screen size is determined by the desired finished particle size, and the properties of the material being processed. Characteristics such as friability and moisture content have an effect on how easily a material is reduced. As a result, using the same screen to process materials of different properties will result in a range of different particle sizes.

11. Particle Size Distribution

Glass is very friable, and will shatter easily upon impact.

Green wood chips are fibrous, with a moisture content up to 50%, both effecting the ease at which they are reduced.

Computer hard drives are very hard, comprised mostly of metals. As a result, they are reduced to a size just slightly smaller than the screen size installed.
Example:

12. Thank you.
We welcome your questions and comments:
Contact Us
Request a Quotation
www.hammermills.com
1-800-447-4634
716-855-1555
61 Depot Street
Buffalo, New York 14206
©2014 Schutte-Buffalo Hammermill, LLC
For more information, download our free ebook: