All About Disc Filter

disc filter

What is disc filter? Disc filter is used for pulp concentration and white water recovery. As one of paper pulp machines in the paper pulp plant, disc filter mainly applies for following aspects:

  • Whitewater recovery: white water treatment, pulp recovery, and filler.
  • Pulp concentration: Concentration of various pulps after sieving or before rinsing, especially for pulp with a low concentration of water filtration.
  • The pulp washing: pulp washing can be spray-converted on the sizing layer of the transfer section disk.
  • White liquor filtration: Alkali recovery of caustic white liquor filtration and white mud tray washing.

Working principle of disc filter

The distribution valve of disc filter has several zones, every zone has own working state:

  1. Natural filtration zone: the spindle turns filter disc, when a fan-shaped plate is immersed in the liquid surface, enter the natural filtration zone, and the pulp is adsorbed to the filter to form a fiber mat. In this area, very few short fibers and filtrate are passed through the filter to form a cloudy filtrate. The filtrate is discharged through a drain to the cloudy filtrate pool.
  2. Vacuum filtration zone: The spindle continues to rotate into the vacuum filtration zone. At this time, the fiber mat on the filter disc has reached a certain thickness (ideally, it should be 2-3 mm), which acts as a filter and the fiber in the filtrate is sucked onto the cushion under vacuum suction. Due to cushion, the solids of the filter are greatly reduced; a clear filtrate is formed, which is called a clear filtrate. Discharge through the drain tube to the clear filtrate tank. The fan-shaped plate is turned out of the liquid surface, and the vacuum does not disappear, so that the cushion layer on the filter screen is dehydrated and the concentration is increased.
  3. Pulp peeling zone: The filter disc continues to rotate and enters the pulp peeling zone. At the moment, the peeling nozzles on both sides of the filter disc are sprayed out of the fan-shaped water column to peel off the pulp layer and fall into the receiving hopper. The vacuum disappears and the pulp is sent into screw feeder by the flushing water flows from the flushing pipe, and diluted.
  4. Screen washing zone: The filter disc is finally transferred to the washing network area, and the water column sprayed by the nozzle of the swing washing device is used to wash the net. after regeneration and recovery of filter capacity, the system entered into the next filtration cycle. In this way, the filter disc is continuously rotated, and a total of 160 fan-shaped nets of 8 concentrated screens in different sections are in a different interval, resulting in continuous concentration and cleaning, so that the pulp is concentrated and washed.

Disc filter for pulp concentration

There are some tips of disc filter for pulp concentration operation, specifications as following:

  • Rotation rate of filter: Generally, the filter disc rotation speed is controlled to 0.8-1 minutes according to the pulp quality and production requirements. If the rotation speed is too low, it will affect the output. The rotation speed is too fast, and the pulp layer on the filter disc is difficult to be peeled off, which also affects the yield.
  • Control of pulp concentration: The pulp concentration should be controlled between 0.8-1.3%. If the concentration is too low, the pulp layer of filter is too thick, which will cause the pulp to fall off easily, resulting in concentration of pulp increase in the tank. Finally, the pulp overflows from the tank.
  • Control of the feed flow rate: The feed flow rate should be controlled 250mm above the center line of the filter shaft. Here, a liquid level controller is provided to prevent the pulp from overflowing due to the high slurry volume.

Pulp concentration and whitewater recovery comparison

Like we said before, disc filter can be not only applied to pulp concentration, also applied to whitewater recovery. What’s difference between two applications of disc filter?

  • Filter screen numbers: for the filter screen, numbers of whitewater reach to 80, while, the pulp concentration just need 40-50. Because the fiber length of whitewater is shorter than that of paper pulp, only large screen can ensure that short fiber cannot lose from screens. In the pulp concentration, if the screen numbers beyond 50, the recovery rate of fine fiber and filler is too high, increase the beating degreed of pulp, which directly affect the paper pulp making.
  • Requirement of water log height: the disc filter for whitewater recovery requires higher height of water log, generally more than 10m.
  • Water consumption for peeling or slushing: disc filter for whitewater recovery need less water, because concentration of whitewater is low, not easy to be dehydrated, the pulp on the filter disc has small thickness and low concentration. Because the water filtration of the pulp is better and the concentration of the pulp is higher, the thickness and concentration of the concentrated layer are larger. It is not easy for peeling and slushing. It is necessary to have a large amount of water and pressure to peel off the pulp from the filter disc, and sent it to the screw conveyor. The angle of the stripping nozzle should be adjusted in time according to the peeling condition. In addition, the water pressure of the pulp slushing should not be too large, as long as the peeled pulp can be slipped into the screw conveyor, but the amount of water must be guaranteed.

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