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How pellet mill impact pellet quality

By Jack Huang    Feb 25, 2014

Pellet mills are pellet processing machines. A pellet mill is one of the factors that influence the pellet quality directly. The parameters of a pellet mill are inherent to the pellet making machine itself, such as, die dimensions, die speed and the gap between the roller and die.

The pellet dies impact the durability of the pellets. The pellet dies are annular matrix of perforations, characterized by the length and the diameter ratios. The die length means the depth of the die and the diameter refers to the diameter of the perforations or the die holes. Generally, pellet durability increases with depth/diameter ratio due to increased shear forces resulting from increased friction between feed stock and die. However, the ratio between the die hole depth to the diameter is too large, the feed stock will block the die and eventually damage the mill. So it is means the soft wood pellets production needs thicker dies with deeper die holes than the hardwood pellet production does.

pellet die with pellets in the die holes

Pellet die with pellets in the die holes

Variable die speeds are used for the production of pellets with different sizes. Die speed of a pellet mill refers to the tangential velocity of the rollers in the process of pelleting. Generally, the high die speeds around 10 m/s are used to process small pellets with the diameter from 3mm to 6 mm whereas the lower die speeds are applied to mold larger pellets with the diameter from 6mm to 7 mm. What’s more, the low die speeds , around 4 to 5 m/s , are also used to process low densities materials to produce high density pellets by expelling the air out of the biomass materials.

The gab between the rollers and die influences the hardness and durability of the pellets. The roller-die gap of a pellet mill is the space between the annular matrix and the rollers. Generally, the narrower the gap is, the higher the pellets hardness and durability will be. However, the the rollers can not touch the surface of the die directly or both of the rollers and die will be broken down by the powerful friction.

gap between the rollers and  the die of a pellet mill

The gap between the rollers and the die of a pellet mill

A pellet mill can impact the pellet hardness and durability in the pelleting process. The die dimensions, die speed, and gap between the roller and die are the process variables which are controllable and adjustable by the selection of quality pellet mills and adjustment in the processing.