The source and control of paper machine deposits

Modern papermaking is, from a mechanical point of view, a very well refined process utilizing what might well be the most advanced process control used in any industry. The art of preparing fibres from trees and forming a sheet of paper from these fibres has reached a degree of sophistication which, for an unknowledged outside observer, must appear phenomenal. Hundreds of tonnes of a leaf thin sheet (e.g. newsprint) can be produced each day at a speed of 90 km/h (equivalent to 1500 m/min) so a chapter within the above title seems trivial in this context. However, the modern papermaking process is very sensitive to disturbances, and the formation of deposits is one of the most important factors limiting the productivity on a paper machine today. Further, paper defects (such as breaks, holes and spots very often caused by deposits) represent one of the most common complaints of a user/buyer of paper, but still deposit control does not usually get high attention and on the contrary, deposit control is often considered unnecessary in the ultimate papermaking process which all papermakers strive to reach. However, it is often forgotten that trees represent a living material made not only of cellulose fibres but also, like any living organism, of a number of substances produced by the tree so that it can live and that recycled waste paper often contains large quantities of non-fibrous material causing a deposit problem. Further, it is only natural that microorganisms (that potentially can cause problems in the papermaking process) are present in a papermaking system, as these represent a natural part of life with presence just about everywhere on the earth. This chapter reviews the source and control of paper machine deposits and the alternative methods of application of chemicals to minimize/eliminate these troublesome deposits.