Stock Dosing, Deaeration, Cleaning and Screening - process info

Stock Dosing, Deaeration, Cleaning and Screening - Introduction and background

Different pulps (deinked recycled fiber pulp, TMP pulp, chemical pulp and broke) are stored in different storage towers. A recipe in the control system controls the amount of each pulp dosed to the paper machine mixing chest. Filler and retention agent are dosed to the mixing chest. In this chest all the pulps are mixed and then pumped to the machine chest.

The stock is pumped to/through the wire silo into a special dilution zone. It is important, for operation stability, that the level of the wire silo is constant, which is accomplished by an overflow.

The diluted stock is pumped to the deaerator via the first stage centrifugal cleaner system. The cleaner system removes heavy particles, such as sand, and particles with round shape, such as shives and bark. The stock is led tangential in the upper part of the cleaner and this gives a rotation in the cleaner. Due to the strong rotation heavy particles are separated from light ones. 
Accept leaves the cleaner from the top and reject leaves the cleaner bottom. Reject from the first stage is further diluted and pumped to the second stage, etc.

Accept from the first stage cleaner and deaerator is pumped (at approximately 0,8 % consistency) to the primary machine screen. The deaerator removes air from the stock. The screen removes impurities and those bigger than the screen plate perforation normally will be rejected.

The reject is taken out continuously from the bottom of the screen to a reject tank prior to the second stage screen.