Washing to High Density Storage
Washout the Black Liquor, Store in the Tank
The Purpose
Washing’s purpose is to remove residual liquor to make sure our pulp is nice and pretty. In addition, it is to recover spent chemicals for the Kraft Recovery Cycle with minimum dilution.
How?
Multi-Stage Counter-Current Washing Sequence. We have three blow tanks, three washer sequences, three washers per sequence except for sequence one which has 4. A Filtrate Tank for each washer in the sequence. All the washers then feed into the High Density Storage Tanks.
The Process
From the Blow Tank/Hot Stock Refiner/Screens, dirty stock is put on the vat side of the first washer in the series of 3 or 4. With the rotary vacuum washer, a thick layer of pulp builds up on the rotating wire while a shower is sprayed onto the rotating pulp from the next filtrate tank. The wash that comes off goes into that washers filtrate tank.
The pulp then rotates over to the mat side and when the rotating cylinder becomes thick enough with pulp, the pulp gets scrapped off and will go to the next washer.
This process happens again as the pulp goes from the vat side to the mat side with cleaner filtrate tank shower spraying the pulp. At the very last washer in the sequence, regular water is used from the Washer Shower Tank.
The first filtrate tank from the first washer in the sequence has the dirtiest black liquor and is sent to the foam tank. From the foam tank, the black liquor is sent to the 50' Weak Black Liquor Storage Tank.
The from the last washer in the sequence, the pulp is sent to a standpipe and then the high density storage tanks. The standpipe is used to prevent cavitation for the pump to the high density tanks.
Variables
- Pulping process
- Stock hardness
- Stock Freeness
- Species
- Temperature
- Distribution
- Show method of application
- Specific loading on wire
- Vat consistency
- Vat rotational Speed
- Dilution Factor
- Air in Stock (foam)
- Fabric fouling/Mesh characteristics
Things that will get you
- Higher dilution factor makes for better washing, but increased evaporation loading
- Let air escape the pulp sheet to prevent foaming which prevents good vacuum to the rotating vat.
- Maximize displacement efficiency by uniformly applying the wash.
