Chip washing, impregnation and refining - Process Info
Chip washing, impregnation and refining - introduction and background
In the TMP plant chips will be supplied from the wood handling system to a chip silo. In a following chip washing system consisting of a chip washer, screw drainer, cleaner and bow screen impurities will be removed and taken to landfill. In order to soften the chips prior to refining low pressure steam will be supplied to the chip silo and the intermediate silo. In case of TMP production chips from the intermediate silo will be fed directly to the first refiner stage. In case of CTMP production (shown in the flow sheet) chips will be treated with chemicals (normally sulphite) and steam to further soften the fiber material.
Chips from the reaction silo feeds a plug screw and will be transported to the pressurized system prior to the primary refiner. In the pressurized system the chips will be heated with steam.
Into the refiner wood chips are fed between two circular rotating discs. The chips are defibrated between the patterned discs and blown to a mechanical steam separator prior to the second stage refiner. The mecanical steam separator is more effective to separate the steam than previous used cyclone.
In the secondary refiner the chips are further grounded between discs. From the secondary refiner the pulp will be blown to a pressure cyclone where generated steam will be separated.
The steam generated in the primary and secondary refiners will be led to a heat recovery system consisting of a reboiler and a scrubber. High pressure steam will be used in the reboiler to generate clean steam (i.e. to be used in the dryer section of a paper machine) and low pressure steam will be used in the scrubber to generate warm water.
In the primary and secondary refiners the pulp consistency will be About 45%. After the secondary refiner the pulp will be diluted to 3-5% consistency. In order to improve pulp strength many mills have installed a LC (low consistency) refining stage as a third refining stage. Low consistency in this case means 3-5%.
The refined pulp falls into a latency chest. 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.
After the latency chest the pulp will be consistency controlled and pumped to a screening system.