Importance and optimization of particle size in animal feed production

A sustainable feed industry is constantly looking for the best way to process feed, to have the best yield with use of lowest amount of energy and lowest amount of raw materials. In this article we focus on the ways how to optimize the particle size in the grinding process of the feed mill. However, there is still much to learn in order to understand the animal biology and the effect of particle size on its nutrition to be able to define what the most optimal particle size.

Arthur vom Hofe
Segment Manager Feed & Oilseeds
CPM Europe

Particle size distribution plays a decisive role in feed production. On the one hand it greatly influences the energy consumption and physical quality of the feed, but on the other hand it also has a great influence on its nutritional value. However, there is no specific size to define because the optimal size varies depending on the species, feed application and even the age of the animal. Therefore the feed miller needs to be able to control and adjust the particle size depending on the type of feed or maybe even depending on the produced formulation.

In next standing figure a course indication is shown of optimal particle size per specie.

For pelleted feed the desired particle size is greatly related to the used pelleting hole diameter but also to the digestive system of the animal. A finer grind adds to the conditioning system of the Pellet Mill and is beneficial for the pelleting process and physical pellet quality.

Important for mash feed is that the particle size distribution is very even. Meaning no fines and no oversized. Fines are not consumed, so they are lost. Particles size variation will cause segregation of the mix and larger particles will not be digested optimally. This means that mash feeds are especially suited for species which allow courser particles.

ROLLER MILL
With the introduction of the CPM Roskamp Roller Mill, the industry has the possibility to process optimal mash feeds with minimal fines and oversizes at minimal energy consumption. The machine uses two or three pairs of rolls. Rotating with slightly different speeds and an adjustable gap between the rolls. The product is cut between the rolls, not smashed like in traditional Hammer Mill technology. Fines and fibers are not touched and passing through the roll gap. On top of the extreme good “quality” of produced mash, this process is also most energy efficient. The gap between the rolls determines the fineness of the grind and is remotely adjustable. This means that the particle size can be automatically adjusted depending on customer’s desire, formulation or application. The Roller Mill technology is optimally suited when course grinding relative brittle products like grains and seeds. But it’s also sometimes used to produce a relatively fine meal that still contains fibers. The advantages of the Roller Mill are reduced when grinding finer. This is why for finer grinding applications a Hammer Mill is used.

HAMMER MILL
A Hammer Mill consists of a fast-rotating rotor with swinging hardened hammers. Product entering the grinding chamber is reduced in particles size by the impact of the rotating hammers. The particles are leaving the chamber through a screen with small holes. It is understandable that the breaking of the particle is not as precise as with a Roller Mill. And the first impression is that the particle size is not that easy adjustable. Ok, you could consider an expensive, maintenance sensitive automatic screen changing system but there are more options. The final particle size leaving the Hammer Mill is not only relying on the screen hole diameter, there are more parameters.

Tip Speed
Important to realize is that the particle size reduction is depending on the speed differential between the product and the hammer tip. The hammer tip speed is simply a factor of the mill diameter and motor RPM. Selecting a large diameter Hammer Mill driven by a variable frequency drive provides the opportunity to grind with a large range of hammer tip speeds. And with the frequency controller the speed is also easy and quickly adjustable without human interference.

Depending on the application, an ideal tip speed can be selected. For more efficient fine grinding fibrous materials, a high tip speed should be selected. While course grinding brittle products asks for a lower tip speed.

The figure underneath shows the relation between the average particle size and the Hammer Mill tip speed.

When changing speed with a frequency controller on a 3,2 mm screen you achieve a grind of 500-600 micron at 127 m/s, while when grinding on only 76 m/s the average particle size is 700-800 micron.

On a traditional machine with 102 m/s you will see the same particle sizes with 2,8 and respectively a 4 mm screen. This means that with a frequency control you can influence the achieved particle size. And when a large diameter Hammer Mill is used you have the possibility to adjust the particle size over a wide range providing maximum flexibility.

But there is more! With a higher tip speed (larger diameter Hammer Mill) a finer grind can be achieved at a certain screen hole size compared to lower tip speeds (smaller diameter Hammer Mills). It is not hard to imagine the benefits of grinding with larger diameter holes towards energy consumption and screen wear (operational costs). And energy consumption is the number one consideration when looking at a Hammer Mill. The fact is that a Hammer Mill can use >50 times its purchasing price in energy during its lifetime.

Screen area & motor power
The whole idea of efficient fine grinding is that the particle size reduction is done by the impact of the hammers. With a larger screen area the product escapes more effectively from the grinding chamber. Smaller screen surfaces keep the product longer in the grinding chamber causing increased energy consumption (heat) and wear. A typical design range is 120cm²/kW total screen surface or an “open hole area” of >34 cm²/KW installed motor power.

The Hammers
The shape of the hammer also greatly influences the achieved particle size. To produce a constant quality grind and to be able to control this you must minimize hammer wear. Traditional (or low cost) hammers may seem attractive but result in a nearly daily changing grind quality. This is reducing the controllability. Also, it is important to realize that energy consumption of the mill increases drastically when the hammers are reaching the end of their serviceable life. With the CPM Champion extreme long-life hammers (10x traditional) a constant consistent grind at lowest energy consumption and minimal maintenance intervals are achieved. These tungsten carbide hard face single hole hammers are generally preferred to maintain balance of the rotor and minimize the potential for catastrophic hammer failure. The long-lasting hammers not only save on labor and energy costs but also provide a constant grind, day in day out.

Hammer pattern
Hammer patterns (the number and distribution of the hammers on the rotor) and positions (setting the hammer closer to or further from the screen) have a profound effect on the performance of any Hammer Mill. Because different materials grind differently, the ideal number of hammers and clearance to the screen will need to be adjusted according to each application.

For course grinding it is most efficient to grind with a limited number of hammers, but for fine grinding applications it is best to use an extra heavy hammer pattern to achieve the very fine finished products desired. If the rotor is equipped with a larger number of pins (12), the total number of hammers is increased significantly. This without putting an excessive number of hammers on (four or eight) pins, which could lead to high stress and the possible failure of the rotor plates.

Grinding chamber shape
Finally, we do have to realize that the speed differential between the product and the hammer determines the impact, which is required to reduce the particle size of the incoming product. A tear-drop shaped Hammer Mill chamber will maintain the speed differential better than traditional circular chambers. This is because the rotation of the product in the chamber didn’t escape after the first hit is effectively reduced.

STEP GRINDING
The most flexible grinding solution is a system with a Roller Mill in combination with a large diameter speed controlled Hammer Mill. The feed miller can basically produce every desired particle size. From a course mash feed with minimal fines to a very fine grind, all from the same high capacity grinding line.

The Roller Mill will produce course mashes or will prebreak the materials before they are processed with the Hammer Mill. Pre-breaking the material with the Roller Mill before going to the Hammer Mill saves over 25% on total energy consumption. This also means that when placing a Roller Mill before an existing Hammer Mill, the capacity is increased with 30-40%. A positive side effect is that the lifetime of the Hammer Mill wear parts (and with that the consistency of the grind) is enormously extended (x4 or more) due to the pre-breaking action of the Roller Mill.

If in a step grinding system a traditional Hammer Mill is used instead of a CPM Champion teardrop shape, it is necessary to use screens between the two machines to experience the benefits. Sifting provides additionally the possibility to remove the oversized fibrous husks from the Roller Mill ground mash and grind them with the Hammer Mills.

A sustainable feed industry is constantly looking for the best way to process feed, to have the best yield with use of lowest amount of energy and lowest amount of raw materials. In this article we focused on the ways how to optimize the particle size in the grinding process of the feed mill. However, there is still much to learn in order to understand the animal biology and the effect of particle size on its nutrition to be able to define what the most optimal particle size.

About Arthur vom Hofe
Arthur vom Hofe is CPM Europe’s Segment Manager Feed & Oilseeds. He has been already involved in the front line since the early nineties when representing the company for pelleting and particle size reduction equipment. Throughout this time, he has gained valuable experience of feed production processes across the world.