Particle size reduction is the first step in the feed manufacturing process and, in terms of pellet mill operations, has a great influence on pellet quality (between 15% and 20%).

Fine particle structures are favorable for:

- mixing homogeneity,
- agglomeration,
- process ability,
- fat and liquid absorption,
- low wear.

At the same time, high contents of fines are not favorable for:

- healthy nutrition,
- flow ability,
- feed intake,
- dust formation and explosion safety

Grinding, or particle-size reduction, is a major function of pellet manufacturing for several reasons, all of them improving the ease of handling ingredients and their storability:

- clumps and large fragments are reduced in size;
- some moisture is removed due to aeration;
- additives such as antioxidants may be blended.

Within a feed mill two solutions can be adopted: pre-grinding and post-grinding.
Pre-milling occurs when each individual component is milled individually before mixing, instead post-milling takes place when the components are weighed and mixed before being milled.
Pre-grinding allows you to grind better and faster, saving time and energy. However, the product already ground into flour is more complex to handle and requires the use of appropriate silos that facilitate the extraction. For these reasons, now in the most cases the feed industry in Europe is shifting on post-grinding, although this system requires more effort in balancing the ingredients. In modern plants that use post-grinding, a separator is inserted before the mill that bypasses the fine parts, making the process more efficient.

Both the pre-milling and post-milling systems can be:

- direct
- direct with pre-sieving
- with recirculation after milling
- with pre- and intermediate-sieving
- direct with pre-milling and direct with post-milling

Increase of:

Peripheral speed
Grain size analysis: decreases
Productivity of hammer mill: decreases


Amount of hammers
Grain size analysis: decreases
Productivity of hammer mill: decreases


Diameter of holes in net
Grain size analysis: increases
Productivity of hammer mill: increases


Thickness of net
Grain size analysis: decreases
Productivity of hammer mill: decrease

Density of holes in net
Grain size analysis: increases
Productivity of hammer mill: increases


Area of inner shell
Grain size analysis: increases
Productivity of hammer mill: increases


Amount of air
Grain size analysis: increases
Productivity of hammer mill: increases

Hammer mills are mostly impact grinders with swinging or stationary steel bars forcing ingredients against a circular screen or solid serrated section designated as a striking plate. The material is held in the grinding chamber until it's reduced to the size of the openings in the screen.
The number of hammers on a rotating shaft, their size, arrangement or sharpness, the speed of rotation, the wear patterns and the clearance at the tip relative to the screen or striking plate are important variables in grinding capacity and the appearance of the product.
Heat imparted to the material, due to the work of grinding, is related to the time it is held within the chamber and the air flow characteristics. Impact grinding is most efficient with dry, low-fat ingredients, although many other materials may be reduced in size by proper screen selection and regulated intake.

Two types of arrangements exists in the hammer mill:

Horizontal hammer mill: it’s the most common and consists of a horizontal drive shaft, which suspends vertical hammers to crush any friable and fibrous dry materials containing less fat.

Vertical hammer mill: in this mill, the drive shaft is positioned vertically and screens and hammers are positioned horizontally. Material successfully reduced in size to the diameter of screen holes or smaller, are carried by gravity outside the mill and thence by air or conveyor to storage in "make-up" bins. Oversize particles, not easily broken, drop through the mill and may be recycled or discarded. Thus, foreign materials, such as metal and stones, are discharged before they are forced through the screen causing damage.

The productivity of hammer mill can be approximately counted on the basis of the formula below

                                        G (kg/h) = kW * D * JkW

G = productivity of hammer mill in one hour
KW = power of main motor
D = diameter of holes in the net
JkW = coefficient of milling characteristic of each product

JkW coefficients for some products

Oats: 27
Rice bran: 15
Wheat: 40
Corn: 55
Flour of coconut extract: 80
Flour of sunflower extract: 50
Flour of soybean extract: 70
Meat-meal: 50
Fish-meal: 12
Barley: 14
Remains after barley purifying: 5
Beet pulp: 11
Salt (NaCl): 75
Rye: 20

Each product has its own optimal milling speed [m/s].

Millet: 48
Corn: 52
Wheat: 65
Rye: 75
Oats: 88
Barley: 105
Bran: 110
Chaff of oats: 115

Below you can find some coefficients of milling:

Barley: 1
Oats: 2
Wheat: 3
Corn: 4
Flour of soybean extract: 5

Capacity (counted in m3/min) of sucking air for hammer mill should be twice bigger than maximal capacity of hammer mill counted in m3/min.

Decreased airflow can be a reason of:

✔ decreased productivity of hammer mill
✔ finer final product
✔ bigger demand for energy
✔ higher milling temperature
✔ quicker wearing
✔ bigger dusting

In the case of roller mills occurs a combination of cutting, attrition, and crushing.
There are smooth or corrugated rolls rotating at the same speed set at a predetermined distance apart with material passing between the two. A tearing action may be added by operating the rolls at different speeds and by corrugations which are different for each roll (for example, the top roll may have off-radial spiral corrugations and the bottom roll lateral corrugations).
Roll grinding is economical but limited to materials which are fairly dry and low in fat.

The most common grinders are the hammer mills and the roller mills. They have been applied to the task of particle size reduction or grinding in feed milling applications.
Roller mills have been used in the processing of common feed materials for years. The earliest roller mills used in the feed milling were abandoned flour milling roll stands, used primarily to produce coarse granulation of friable materials. Over time, roller mills have been used to perform a wide variety of tasks related to the production of animal feeds.

Hammer mills have traditionally been used to produce the finer grinds for pelleting and many mash (meal or non-pelleted) feed applications as well. The hammer mill is a relatively simple machine and requires a fairly low degree of skill in regard to both the operation and maintenance. However, recent significant changes in the industry have caused many to reassess their approach to particle size reduction. Increasing energy costs, increasing customer awareness of feed quality and environmental concerns all challenge the validity of the hammer mill as the only choice for particle size reduction applications.
The introduction of large diameter hammer mills has limited the noise these machines produce naturally during operations.

HORIZONTAL HAMMER MILL	VERTICAL HAMMER MILL	ROLLER MILL
moderate investment costs	moderate investment costs	high initial cost
universal application (any     friable material and fiber)	less contents of fines, more structure	careful treatment of the product
easy shifting of the particle size (wider range)	easy shifting of the particle size	uniform particle size distribution
simple operation/maintenance	simple operation/maintenance	expensive maintenance
greater particle size variability	aspiration system not necessary (fan integrated in the heavy particle separator)	limited particle entry size
high specific power requirement	lower energy demand	reduced specific power requirement
high throughput rates	higher throughputs can be achieved (depending on product)	reduced heating of the product
robustness and plainness		particle size tend to be irregular in shape
high fine content		no effect on the fibrous product

 

To produce pellets of acceptable quality the particle size of the ground materials must be correct. Finer grinding will result in a better-quality pellet or extruded feed, increases the capacity of the pellet mill or extruder, and reduces wear of the pellet mill or extruder working parts such as dies, rollers and worms. --
Because animal needs vary considerably, the degree of processing for various diets also must vary.
Ruminant animals such as cattle and sheep have rather long, complex digestive tracts and so require a less processed feed material. On the other hand, many of the ingredients used in ruminant feed pellets consist of low protein, high fiber material so fine grinding may be required to achieve a reasonable pellet quality.
Swine have a short, simple digestive system (much like humans) and therefore benefit from a more highly processed feed, while poultry have a short but rather complex digestive system and, depending on the makeup of the diet, can efficiently utilize feed stuffs less highly processed than swine. Although it has been postulated that finer grinding increases substrate availability for enzymatic digestion, there is evidence that coarser grinding to a more uniform particle size improves the performance of birds maintained on mash diets (and in lower but still significant way in pellet diets).
This counter-intuitive effect may result from the positive effect of feed particle size on gizzard and gut development. A more developed gizzard is associated with increased grinding activity, resulting in increased gut motility and greater digestion of nutrients.
The size and the age of the animals also affect the dietary requirements so far as particle size is concerned. Younger animals require a finer, more highly processed feed than do older, more developed livestock. Factors such as moisture content of the grain, condition of the hammers and/or screens (hammer mill) or the condition of the corrugations (roller mills) can produce widely varying results. In addition, the quality of the grain or other materials being processed can have a dramatic impact on the fineness and quality of the finished ground products.

An optimal grinding requires the right amount of intake of air, which is cleaned in the nozzle filters. This also guarantees an economic use of the hammer mill. The air is necessary to quickly transport the fine product from the area of the hammers through the grinding sieve and, at the same time, to cool the product.

By the ventilator, mounted in front of the filter a vacuum is generated and air is sucked into the air inlet funnel of the hammer mill in feed device, passes through the milling chamber and milling screen, thus keeping the screen perforation clear. On its way to the filter, which is of automatic self cleaning type, the aspiration air carries dust along which settles at the outer surfaces of the filter pockets. Compressed air burst is injected into the inside of the filter pockets thus reversing the airflow for a split second and removing the settled dust from the filter cloth.
The intervals of these are controlled by the rotary air distributor which releases the compressed air from the pressure tank via air valves to the injector nozzles mounted in front of the open filter pockets. The dust is collected in a hopper with an airlock or directly dumped into following bin or conveying system.

The filtering surface must be evaluated considering a filtering ratio = 2

This means that the airflow in m3 /min is twice the filtering surface in m2.
For example, in order to filter 90 m3/min of air aspirated from the mill we’ll need a 45 m2 filter.

The volume of the air used to clean the sleeves in Nl/min is equal to 5-8 times the filtering surface in m2

According to the ATEX rules, these are the solutions adopted for the filters for hammer mills:

● A flame catcher must be installed to prevent the effects of a possible explosion inside the building. Sometimes you have to add bottle with CO2, that opens automatically very quickly.

● Another possibility is to built a channel and guide the explosion gas out of the building.