Screening and Dewatering - Process Info
Screening and dewatering - Introduction and background
During refining the fibres are ”stressed”, i.e. the fibres are bent and creased. In the latency chest the fibres can ”stress off”, i.e. straighten, during stirring. Pulp from the latency chest is pumped to the pressurised screening system.
The screening consists of a primary/secondary stage screening system. The pulp with 3 – 4 % consistency is diluted to a screening consistency of 1,0 – 1,5 % with
white water prior to the first stage.
The accept from the first stage is led directly to the discfilter to be dewatered.
A reject is taken out from the lower part of the screen. To avoid too high reject consistency the pulp is further diluted in the screen.
The reject is diluted and pumped to the second stage screen.
The accept is forwarded to the disc filter together with the accept from the primary screen.
The reject is led to the reject treatment consisting of dewatering press, reject refiner and further screening.
The pulp from the screening is dewatered to 12 – 14 % consistency on the disc filter. In the disc filter screw conveyor the pulp is diluted to approximately 5 % with white water from PM or bleach plant on its way to the dilution chest. The dilution has two effects, the white water supply from PM to the TMP plant through counter current washing and an extra pulp washing in the dilution chest.
From the dilution chest the pulp is pumped to a storage tower and then to the dewatering press and the bleaching tower.
Two or three filtrates (cloudy, clear and super clear filtrate) are taken out from the disc filter vacuum valve. The filtrates are led to one or more white water tanks to be reused in the plant.
The filter cloth on the discs are cleaned with clear or super clear filtrate through oscillating cleaning showers.