The grinding aim is to transform all the particles making up a formula to a similar particle size, consuming as low energy as possible. The similar particle size favors thereafter obtaining homogeneous and stable mixtures. The grinding fineness promotes pellet cohesion and the electrical efficiency of the pellet mill.
Unless design constraints of the plant, only raw materials as grains or pellets higher than desired particle sizes are ground. All others powdery raw materials will be added at the mixing step.
The most grinding mill type used in animal feed is the hammer mill.
Other grinding mill types (roller mill, disc mill,…) can be occasionally used in some feed mills but its use remains marginal. In each plant, one or more grinding mills are used depending on the desired rates for this operation. The supply device of each grinding mill includes a silo bin with, at its base, a screw conveyor which carries out the supply device of the grinding mill. Its rotational speed defines the grinding mill flow rate.
At the entrance of the grinding chamber, raw materials are purged of all foreign materials: the metal particles by a ferrous cleaner (a magnetic device) and the denser particles (stones) with a destoner operating with an airflow. The grinding chamber is delimited by a screen, calibrating the desired final particle size distribution. The particles are ground by reversible moving hammers hooked up on discs mounted on a rotor usually in rotation from 1500 to 3000 rpm.
Between the disc and the hammer, fixed counter-hammers may be present.
High vacuum pressure generated by suction into the hopper beneath hammer mill, provided by a large airflow also contributing to the operation of the destoner, extract the particles of the grinding chamber through the screen when they reached the right particle size.
The entrained air with the product is then separated from the meal most often by filtration in a sleeves battery installed on the ceiling of the hopper under hammer mill. The cleaning sleeves are provided by a sequential injection system of compressed air at 6-8 bars.
In its lower part, the hopper under hammer mill is provided with a transfer system (generally a screw conveyor) to ensure the ground products removal. This evacuation is completed by a rotating valve system, that prevents the air received, by the fan, comes to the downstream circuit of the hammer mill.