Editing Paper machine (section)

== Stock (pulp) preparation ==

Stock preparation is the area where pulp is usually refined, blended to the appropriate proportion of [[hardwood]], [[softwood]] or recycled fibre, and diluted to as uniform and constant as possible consistency. The [[pH]] is controlled and various fillers, such as whitening agents, [[sizing|size]] and [[wet strength]] or dry strength are added if necessary.  Additional [[filler (materials)|fillers]] such as [[clay]], [[calcium carbonate]] and [[titanium dioxide]] increase [[opacity (optics)|opacity]] so printing on reverse side of a sheet will not distract from content on the obverse side of the sheet.  Fillers also improve printing quality.<ref>{{cite book
|title= Wet End Operations Short Course Notes
|last=Technical Association for the Pulp and Paper Industry
|author2=Various
|year= 2005|publisher= TAPPI Press}}</ref>

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.  Because fibre and fillers are denser than water and tend to settle out quickly and also fibres are attracted together to form clumps called floc, 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 1,000 or 1,200 RPM for 50 and 60&nbsp;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 fibres, causing the [[fibril]]s of the fibres 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 from the refiner then goes to a refined stock chest, or blend chest, if used as such.

Hardwood fibres are typically 1&nbsp;mm long and smaller in diameter than the 4&nbsp;mm length typical of softwood fibres. Refining can cause the softwood fibre 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 levelling chest having about 15 minutes retention. This is enough retention time to allow any variations in consistency entering the chest to be levelled out by the action of the [[grammage|basis weight]] valve receiving feedback from the on line basis weight measuring scanner. (Note: Many paper machines mistakenly control consistency coming out of the machine chest, interfering with basis weight control.)<ref group=notes>Results from dynamic material balance sensitivity analysis: The timing for the basis weight control loop is much slower than that of a consistency loop.  Also, varying pressure of the consistency control dilution water will introduce swings in consistency.  This can be and should be verified for any particular system using dynamic material balance software. Run model by creating a sharp consistency change ≈½% and observe system stability.</ref>