The Paper machine wet end is the section where Pulp i.e. a slurry of water and fiber is spread into a running sheet, while separation of water fiber occurs. There are few different types of processes however what is common is a feed zone where the slurry is present as the input, the fiber mat that is the wet layer of the paper sheet, the fabric which acts as a barrier or a filter, and lastly the flooding zone where white water flows and it is on the opposite side of the fabric.
Paper Machine forming section
What fabric does is act as a barrier, it retains the fiber on the side and allows water to drain on the other side. The various zones from which fabric passes are natural drainage, air gradient drainage, low vacuum zone, and then high vacuum zone. However, during this formation zone of paper, since, water has to pass through the fabric pores or voids, a lot of small fines of fiber also pass out sometimes due to the small size and sometimes due to the pulling effect of the vacuum.
Paper makers have always been in the dilemma about what is to be done. Though a lot of other external factors also come into play, the most significant point to consider is the right water and fiber chemistry during the formation zone.
Myth amongst mill managements: Chemicals are being used only because of Paper Maker is corrupt, else forming section is a mechanical process only.
Myth amongst Paper Makers: There is no need of chemicals as we are using very good quality fiber and there are no fines.
What the paper-making process requires is optimum retention of fiber on the forming side, which sometimes is called as “first pass retention”. Retention is the ratio of the difference in consistency of the headbox and white water to the consistency of the headbox. Not every machine can have the same FRP value and neither there is a global standard. There are so many variables at play always that the readings may vary. However, for paper machine runnability it is required to have a constant value of FPR for a long duration, and fluctuations are an absolutely avoidable scenario. To ensure fibers are retained on the wet end it is necessary to bring the fine particles together in such a size so that the vacuum is not able to pull the flocs out. This happens with help of retention aids. Retention aids form divalent bonds between fines and attach them together so that they can pass the forming stage What we are talking about is not only the fiber fines but also the particles of desired filler. However, during this process, the bonds are formed in such a manner that the porosity of the fiber mat reduces. This reduced porosity acts as barrier in flow of water as well thus reducing the drainage from the wet end.
Coagulant and flocculant action