PJL-21

Pulp preparation: the preliminare operation to the creation of a paper sheet

Pulp is comprised of fibers that can be virgin or recycled, water and eventual chemical products that are added to increase or modify the organoleptic and mechanical characteristics of the paper fibers in order to better adapt them to the use and the scope that the final product requires.

A main component of pulp is water – an element that enhances the cellulose fibers and serves as a transport means that conducts and uniformly distributes the pulp at the initial phase of the creation of the sheet.


Perini Journal


Pulp is comprised of fibers that can be virgin or recycled, water and eventual chemical products that are added to increase or modify the organoleptic and mechanical characteristics of the paper fibers in order to better adapt them to the use and the scope that the final product requires. A main component of pulp is water – an element that enhances the cellulose fibers and serves as a transport means that conducts and uniformly distributes the pulp at the initial phase of the creation of the sheet.


THE USE OF WATER ALLOWS TO INCREASE THE FIBROUS RAMIFICATIONS OF THE CELLULOSE: by penetrating inside the fiber, it makes them “explode”, thus forming tentacles that, in turn, lead to better mechanical bonding between the fibers (see the section on refiners). This agglomeration and cohesion phenomenon is enhanced and aided also through the use of cationic chemical products; these agents allow perfect bonding with the negative charge possessed by the fibers themselves.

The pulp fibers are then transported onto a wire1 that, thanks to appropriate drainage characteristics, allows to gradually eliminate the water content from the pulp, allowing the fibers to deposit, thus giving rise to the sheet. Any residual water is subsequently eliminated through a felt2, through special vacuum pumps and through contact with the external surface of the Yankee3.


AT THE PULP PREPARATION PHASE, NON ONLY WATER BUT ALSO DIFFERENT CHEMICAL PRODUCTS ARE USED. The eventual resin substances still remaining, which have not been eliminated through the previous wood processing phase and present in the cellulose, are eliminated through the help of talcum, an element which can prevent the formation of clots, on the wire as well as on the felt, that could give rise to imperfections (holes or breaks) on the resulting paper sheet.

In order to enhance bonding between fibers and to make the sheet more resistant and clearer when looked at against the light, chemical products having a cationic (positive) charge are used, which facilitate bonding with the negatively charged cellulose fibers (hydroxils OH-).

The quantity of these chemical products is determined by a well-balanced proportion rule and must respect precise values determined with an instrument that detects the charges present in the pulp solution.

The proper balance of these charges is important in order to obtain water drainage on the wire, for the recovery of the fibers lost and for an effective refluent water purification of the entire process.


THE TYPE OF CHEMICAL PRODUCTS USED DEPENDS ON THE TYPE OF FINAL PRODUCT TO BE MADE.

Natural glues and synthetic resins are used to make the paper more wet-resistant and resistant to tearing – qualities necessary for products such as kitchen towels, printing paper, drawing paper, nautical documents, etc. Softeners and lotions are used to enhance hygienic-sanitary products both en masse (within the pulp during the production process) and also directly on the final product during the packaging phase.

The use of further chemical products is also finalized at reducing or eliminating potential problems and defects caused during the production process. Anti-slime, anti-foam, anti-bacterial, coagulant and flocculent agents enter the production process when required.

Anti-slime and anti-bacterial agents are adopted to kill any fungi or filament agglomerations that may be present due to the organic decomposition of the cellulose fibers. Coagulants and flocculants are used to improve sheet formation on the wire and also to clarify process waters (purification).


PULP PREPARATION IS OBTAINED THROUGH THE USE OF THE FOLLOWING COMPONENTS:


- Pulper

- High density pulp cleaners

- Screeners

- Refiners

- Fan Pump

- Selectors

- Flow box


PULPERS AND HIGH DENSITY PULP CLEANERS.

The raw material is introduced in the pulper4 through a conveyor. The pulper has been previously filled with water, and here begins the preparation cycle.

Inside the pulper, the water and the cellulose are transformed into a paste. It is at this phase of the cycle that the pulp is actually born, and some of the chemical products mentioned above are added. The time required for this phase depends on the type of raw material used and on eventual modifications made to the characteristics of the fibers. Subsequently, the pulp will have to undergo an initial purification process in order to eliminate any foreign objects which may have been accidentally introduced and which could reveal themselves dangerous for the proper functioning of the production process. The purification process is carried out through high density pulp cleaners which, using the various specific weights of the objects, separate the foreign components from the pulp.


SCREENERS

The role of the screeners is to dissolve fibers or knots which have not been completely opened in the pulping phase. Fixed disks which rotate at very high speeds conduct the pulp through fixed paths which become more and more narrow, thus dissolving the knots.


REFINERS AND FAN PUMP

The subsequent phase is carried out by refiners that break the fibers, thus facilitating the passage of water inside the fibers themselves, allowing an increase in the contact and cohesion points (formation of tentacles). The refiners’ disks can be of several types, according to the material used (steel, cast iron, basalt) or to the different sizes and types of blades. The difference in the materials and in the conformation of the blades has an influence on the type of product to be made because one can act on the fiber characteristics, modifying their resistance and length.

In this phase, it is very important to control the consistency of the pulp and the surface speed of the disks, so as to obtain an optimal level of refinement that will influence the mechanical characteristics of the product.

For very resistant products, the level of refinement will be very high, while for soft products, the level will be averagely low. In order to better distribute the fibers on the wire, a Fan Pump5 is necessary; by increasing the percentage of water in the pulp (from 96% to 99.8%), it allows a better fiber distribution on the wire.


SELECTORS AND FLOW BOX

The Selectors carry out the definitive and final phase of purification by eliminating any particles or slag that might still be found inside the pulp. The main phases of pulp preparation are thus completed, and this cycle closes with the distribution of the pulp, in very diluted and substantially purified form, on the wire through the Flow Box6.

The distribution uniformity of the pulp is given by the particular conformation of the flow box, appropriately studied to obtain a constant pulp concentration along its entire width. The subsequent phase of sheet production consists in the elimination of water from the pulp and from the drying process of the sheet.•


RAW MATERIALS USED

For the production of toilet rolls, about 60% short fibers and 40% long fibers are employed. Wet resistant papers (kitchen towels and industrial rolls): comprised of about 60% long fibers and 40% short fibers. Very soft papers, high percentage of short fibers.

The mix between short fibers and long fibers is likewise used to obtain a homogeneous and uniform paper sheet because the use of exclusively short fibers or exclusively long fibers does not result in a satisfactory quality.

The different resistance and quality of the paper is given both by the different use of raw materials and by their elaboration, therefore, it is easy to see how complicated and wide-ranging the production of paper is.


1 Wire: A fibrous mantle in polyamide and polyethylene, appropriately intertwined to allow drainage of water from the pulp composition.

2 Felt: Another fibrous mantle, totally comprised of synthetic fibers, to allow further water drainage and necessary in order to withstand high linear mechanical pressures caused by the contact of the presses with the Yankee.

3 Yankee: A cast iron and/or steel cylinder, void inside, where vapour circulates at high temperatures to completely eliminate any residual moisture from the pulp.

4 Pulper: A machine inside which cellulose sheets and other types of raw materials are inserted, before mincing them and mixing the fibers thus obtained with water.

5 Fan Pump: A double-rotor centrifugal pump that pushes the pulp mixture through the feeding circuit of the PM.

6 Flow Box: A machine that uniformly distributes the pulp mixture on the forming wire. It bears and adjustable opening. •

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