The purpose of the sifting process
The sifting process is used to separate the undesirable undersized portions of the product from the correctly sized material. The undersized product can return into the pellet mill for repelleting and is referred to as “recycle” or “fines”.
When a crumbled product is being made, it is likely that some pellets would not be properly broken to a specific size. These oversized particles are also removed by screening and returned to the original crumbler roll for reprocessing and again they are screened.
If the crumber is used, approximately 25- 30% of fine materials will return through the pellet mill machine for reworking.
The screening equipment is a significant factor for the final pellet quality. It does no good to remove all the fines after the cooler if the pellet is of such poor quality that it cannot withstand the rigors of in-plant handling. Careful attention should be given to a screener’s ability to keep the screens clean or free from binding. Some sort of knocker mechanism is required to provide the impact needed to move fines through the screen wire.
The majority of the screening devices used today are oscillating, vibrating, or gyrating wire or metal screens having the appropriate opening sizes.