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Using Hammer Mills for Particle Size Reduction

Using Hammer Mills for Particle Size Reduction
When making pellets from any material, size reduction is an important part of the process. The hammer mill is generally the final stage of size reduction before the material is sent to the pellet mill. when making animal feed pellets a whole range of materials are added into the feed. Each ingredient needs to be added at set amounts and then processed through the hammer mill.
Size Reduction: Particle size should be no more than 1/3 of the diameter of the pellet. Over-grinding however, could lead to more consumption of energy, less capacity and destruction of nutritional compounds in the feed. Research data show that vitamin A in the pellet after repeated grind would decrease by 40%. Size reduction is used to mproves absorption in the digestive tract and modify the physical characteristics of ingredients resulting in improved mixing, pelleting and in some instances, handling or transport. The greatest benefits from particle size reduction in manufacturing processes are related to:

1. Greater surface area for faster digestion
2. Improving ease of handling of some ingredients
3. Improving mixing characteristics of ingredients
4. Increasing pelleting efficiency and pellet quality
5. Meeting customer performance for feeds
Hammer mills reduce the particle size of materials by impacting a slow moving target, such as a cereal grain, with a rapidly moving hammer. The transfer of energy that results from this collision fractures the grain into many pieces. Sizing is a function of hammer-tip speed; hammer design and placement; screen design and hole size; and whether or not air assistance is utilized.

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Because impact is the primary force used in a Hammer mill to reduce the size of the particles, anything that; increases the chance of a collision between a hammer and a target, increases the magnitude of the collision, or improves material take-away, would be advantageous to particle size reduction. The magnitude of the collisions can be escalated by increasing the speed of the hammers. Anderson (1994) stated that when drive speed and screen size were kept constant, the increased hammer-tip speed obtained from increased rotor diameter produced particles of smaller mean geometric size.Particles produced using a hammer mill will generally be spherical in shape with a surface that appears polished. The distribution of particle sizes will vary widely around the geometric mean such that there will be some large-sized and many small-sized particles.