Wood paper machines

Closed-Mill Concept. The closed-mill concept, or water circuit closure, has been studied by the pulp and paper industry for many years. In some parts of the paper manufacturing process, up to 98% of the water is recycled within the process, eg, the wet end of the paper machine. However, in the pulp mill, especiaUy kraft mills, effluents are produced owing to the need to purge from the system various metals that come in with the wood, as weU as organic by-products from the pulping process, additives, and especially chloride ions that originate in the bleach plant. 

Refining and Fractionation. These processes are used to alter and select cellulose properties so the final sheet has the desired properties (51). Properties of recycled fibers differ from those of fibers prepared directly from wood. For example, recovered chemical fibers have lower freeness, an apparent viscosity leading to different water drainage characteristics on paper machines. Recovered fibers also have iacreased apparent density, lower sheet strength, iacreased sheet opacity, inferior fiber—fiber bonding properties, lower fiber sweUiag, lower fiber flexibiUty, lower water reteatioa, reduced fiber fibrillatioa, and much lower internal fiber delamination. 

Pitch Control. Resinous constituents of wood cause problems in paper machines by sticking to the rollers and causing spots or holes in the paper the worst cases cause paper webs to mpture. Costly stoppages, wastages, and quaUty problems because of these resinous substances can be avoided by using Upases (84). 

Triglycerides are important constituents of resin. In softwood, the triglycerides account for 20—40% of total resin content, and in hardwood, 40—50%. The paper industry uses the term pitch for resins that create problems in paper machines. Traditionally, pitch is controlled or reduced by aging the wood, by use of chemicals to avoid deposits on the roUs, or by intensive washing of the pulp. AH these methods add to the cost of paper production. An alternative is to add a Upase to the pulp in a reaction lasting about one hour with the help of agitation. Results from Japanese paper mills show substantial... 

Manufacture. Sodium sulphate is used for making wood pulp when the lignin is removed by boiling. The pulp is then put through the paper. machine. 

Calcium carbonate scale is even more commonplace in Kraft pulp mill systems such as heat exchangers, where, compared to paper machines, it often forms a thick deposit leading to loss of performance of the heat exchanger. Calcium comes into the system with either the wood or from poor recovery liquor clarification and carbonate is formed from degradation of sugars in a side reaction during pulping conditions. Dissolution of filler (calcium carbonate) will increase the concentration of both calcium and carbonate in the system. 

One cause for paper machine foam problems is residual carry-over from the pulp mill or bleach plant. As much as possible the raw materials used should be as uncontaminated as possible. Black liquor is just one example of a contaminant that can cause foam on a paper machine. Another potential foam contributor is recycled fibre, used as part of a furnish. The recycled fibre contains all the ingredients used in paper manufacture and converting coatings, sizing, inks, adhesives and binders, wood components, etc. that impact foam creation and stabilisation. 

In high-speed modem paper machines the floe is subjected to high shear, which may tear the floe apart. The trend toward reduced water usage in the production of paper increases the amounts of soluble anionic wood polymers and electrolytes in the stock, which will also affect the retention on the wire. 

Soure From Touzinsky, G.F. Laboratory paper machine runs with Kenaf thermochemical pulp, TAPPI, 1980, 63(3), 109 Touzinsky, G.F. Kenaf, In Pulp and Paper Manufacturing, Vol. 3., Secondary fibers and non-wood pulping. Chapter 8, TAPPI Press, Atlanta, GA 1987,106. 

Mechanical pulping aqueous environments are similar to paper machine white waters. In chemi-thermo mechanical pulping, however, the addition of sulfur ions can increase corrosivity. Pressurized presteaming of wood chips from seaborne logs can result in environments conducive to chloride SCC and also to the evolution of wood acids that can cause corrosion in the vapor phase. Wear is a more significant problem than corrosion in mechanical pulping environments. 

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