Repulping And Screening - Process Info
Repulping and screening - introduction and background
Waste paper, newsprint and magazines, is fed to the drum pulper inlet chute.
The drum pulper gives a continuous repulping of waste paper. Unlike a conventional pulper the drum pulper has no rotor. This gives a gentle repulping.
The defibration is accomplished as the pulp is lifted, with baffles, and falls down time after time. The impurities which are led to the end of the drum pulper are transported to the reject treatment. The repulping part of the drum pulper is unperforated.
The paper is wet with white water to a consistency of 18 % and chemicals as caustic soda, sodium silicate, hydrogen peroxide and soap or a collector starts to react with the paper. To get an efficient repulping the retension time is 20 minutes.
The next part of the drum pulper is the screening part. This part is perforated with 6 – 8 mm holes and white water is supplied to dilute the pulp to 4 % consistency. The added chemicals (caustic soda and sodium silicate) increases the pulp pH-value to release ink. Hydrogen peroxide bleaches the fibres which get yellow by time. Soap or collector makes the repulping more efficient. Soap also helps to remove ink and it collects ink in the flotation later in the process.
Pulp from the drum pulper is pumped to the coarse screening system. The coarse screening consists of a three stage screening system. The coarse screen has a double function. It works as a centrifugal cleaner at the same time as it works as a ordinary screen.
A light reject is taken out from the top of the screen and led to the next screening stage. From each screen is also a heavy reject, containing staples, glass and coarse sand, taken out to the reject treatment. The pulp is fed to the first stage screen.
The accept is led to a reaction chest prior to a pre-screening system.
The reject is led to a reject tank and from there pumped to the second stage screen.
The accept from the second stage screen is forwarded, together with the accept from the first stage screen, to the reaction chest.
The reject is led to another reject tank and pumped to the third stage screen.
The accept from the third stage screen is led in cascade back to the first reject tank, while the reject is led to the reject treatment.
The pulp is diluted with white water in the screen to make the screening easier.