Stock (pulp) preparation

Stock preparation is the area where pulp is usually refined, blended to the appropriate proportion of hardwood, softwood or recycled fiber, and diluted to as uniform and constant as possible consistency. The pH is controlled and various fillers, such as whitening agents, size and wet strength or dry strength are added if necessary. Additional fillers such as clay, calcium carbonate and titanium dioxide increase opacity so printing on reverse side of a sheet will not distract from content on the obverse side of the sheet.
Pulp is pumped through a sequence of tanks that are commonly called chests, which may be either round or more commonly rectangular. Historically these were made of special ceramic tile faced reinforced concrete, but mild and stainless steels are also used. Low consistency pulp slurries are kept agitated in these chests by propeller like agitators near the pump suction at the chest bottom.
In the following process, different types of pulp, if used, are normally treated in separate but similar process lines until combined at a blend chest:
From high density storage or from slusher/pulper the pulp is pumped to a low density storage chest (tank). From there it is typically diluted to about 4% consistency before being pumped to an unrefined stock chest. From the unrefined stock chest stock is again pumped, with consistency control, through a refiner. Refining is an operation whereby the pulp slurry passes between a pair of discs, one of which is stationary and the other rotating at speeds of typically 1000 or 1200 RPM for 50 and 60 Hz AC, respectively. The discs have raised bars on their faces and pass each other with narrow clearance. This action unravels the outer layer of the fibers, causing the fibrils of the fibers to partially detach and bloom outward, increasing the surface area to promoting bonding. Refining thus increases tensile strength. For example, tissue paper is relatively unrefined whereas packaging paper is more highly refined. Refined stock form the refiner then goes to a refined stock chest, or blend chest, if used as such.
Hardwood fibers are typically 1 mm long and smaller in diameter than the 4 mm length typical of softwood fibers. Refining can cause the softwood fiber tube to collapse resulting in undesirable properties in the sheet.
From the refined stock, or blend chest, stock is again consistency controlled as it is being pumped to a machine chest. It may be refined or additives may be added en route to the machine chest.
The machine chest is basically a consistency leveling chest having about 15 minutes retention. This is enough retention time to allow any variations in consistency entering the chest to be leveled out by the action of the basis weight valve receiving feedback from the on line basis weight measuring scanner.(Note:Many paper machine mistakenly control consistency coming out of the machine chest, interfering with basis weight control.)