Stock (pulp) preparation

webadmin — Sun, 07 Apr 2013 08:42:12 +0000

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.)

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Pulp types and their preparations

webadmin — Fri, 07 Dec 2012 08:41:00 +0000

Rags of cotton and linen were the major source of pulp for paper before wood pulp. Today almost all pulp is of wood fiber. Cotton fiber is used in specialty grades, usually in printing paper for such things as resumes and currency.
Sources of rags often appear as waste from other manufacturing such as denim fragments or glove cuts. Fibers from clothing come from the cotton boll. The fibers can range from 3 to 7 cm in length as they exist in the cotton field. Bleach and other chemicals remove the color from the fabric in a process of cooking, usually with steam. The cloth fragments mechanically abrade into fibers, and the fibers get shortened to a length appropriate for manufacturing paper with a cutting process. Rags and water dump into a trough forming a closed loop. A cylinder with cutting edges, or knives, and a knife bed is part of the loop. The spinning cylinder pushes the contents of the trough around repeatedly. As it lowers slowly over a period of hours, it breaks the rags up into fibers, and cuts the fibers to the desired length. The cutting process terminates when the mix has passed the cylinder enough times at the programmed final clearance of the knives and bed.
Another source of cotton fiber comes from the cotton ginning process. The seeds remain, surrounded by short fibers known as linters for their short length and resemblance to lint. Linters are too short for successful use in fabric. Linters removed from the cotton seeds are available as first and second cuts. The first cuts are longer.
The two major classifications of pulp are chemical and mechanical. Chemical pulps formerly used a sulfite process, but the kraft process is now predominant. Kraft pulp has superior strength to sulfite and mechanical pulps. Both chemical pulps and mechanical pulps may be bleached to a high brightness.
Chemical pulping dissolves the lignin that bonds fibers to one another, and binds the outer fibrils that compose individual fibers to the fiber core. Lignin, like most other substances that can separate fibers from one another, acts as a debonding agent, lowering strength. Strength also depends on maintaining long cellulose molecule chains. The kraft process, due to the alkali and sulfur compounds used, tends to minimize attack on the cellulose and the non-crystalline hemicellulose, which promotes bonding, while dissolving the lignin. Acidic pulping processes shorten the cellulose chains.
Kraft pulp makes superior linerboard and excellent printing and writing papers.
Groundwood, the main ingredient used in newsprint and a principal component of magazine papers (coated publications), is literally ground wood produced by a grinder. Therefore it contains a lot of lignin, which lowers its strength. The grinding produces very short fibers that drain slowly.
Thermomechanical pulp(TMP) is a variation of groundwood where fibers are separated mechanically while at high enough temperatures to soften the lignin.
Between chemical and mechanical pulps there are semi-chemical pulps that use a mild chemical treatment followed by refining. Semi-chemical pulp is often used for corrugating medium.
Bales of recycled paper (normally old corrugated containers) for unbleached (brown) packaging grades may be simply pulped, screened and cleaned. Recycling to make white papers is usually done in a deinking plant, which employs screening, cleaning, washing, bleaching and flotation. Deinked pulp is used in printing and writing papers and in tissue, napkins and paper towels. It is often blended with virgin pulp.
At integrated pulp and paper mills, pulp is usually stored in high density towers before being pumped to stock preparation. Non integrated mills use either dry pulp or wet lap (pressed) pulp, usually received in bales. The pulp bales are slushed in a pulper.

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Stock (pulp) preparation

Pulp types and their preparations