subject: Stock Prep Systems For Paper Mills [print this page] Paper mills that produce board, linerboard, fluting, whitetop and many other products use recovered fibre as their source material. Once the mill has its supply of recycled fibre / post consumer waste it must sort and process the fibres from that waste, to a point where a viable product can be produced. This involves a blending and sorting process that becomes ever more selective and efficient.
The type of system used depends on the end product produced and will vary from paper mill to paper mill. The first stage in the process is the pulper, normally a low consistency pulper operating at approx. 5%. Throughout the pulping process the contents of the pulper are circulated to remove the heavier and larger impurities. The more contaminants that can be removed at this stage gives less contaminants will need to be removed further down the line.The pulp is normally cycled through heavy reject pumps / de-trashing units (with or without screenplates) and rotating drum screens (trommels), while the bale wires and large plastic impurities are removed using a ragger. The pulp then moves forward to a series of centrifugal cleaners, called Low Density Cleaners. These cleaners used a centrifugal rotation to help separate smaller particles, such as staples, sand, glass from the fibre stock.
A series of pressure screens are also used to help remove more contaminants and tars and glues (stickies). The pressure screens can be low consistency or high consistency screens and can use a combination of slotted baskets or perforated baskets. Pressure screens can be vertical or horizontal, with each type normally using a rotor that rotates and helps generate pulses to push the fibre through the static basket. In recent years there have been many advances in the construction of pressure screen baskets. Many of the major stock prep. machinery suppliers now offer screen baskets manufactured without any welded parts, giving a much more stable screen basket that can withstand the forces involved in modern screening situations. Screening is a specialised process and there are various types used by paper mills.Coarse screening removes the larger, or coarse, contaminants and then the fine screening is used to remove smaller contaminants. Many mills use another level of screening called fractionation. This process will further separate the fibres into short, medium and long. These fibres can then be used to create a blend that gives the best properties to the end product board that the paper machine produces.
