Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Vacuum treatment
Patent
1991-11-12
1993-05-04
Hastings, Karen M.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Vacuum treatment
162 62, 162246, D21C 110
Patent
active
052078701
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a process and an equipment for pretreatment of chip-formed wood or other cellulosic raw-material by impregnation. The invention is in particular concerned with the penetration of the fibrous cell system, taking place as the first step in the impregnation. As the penetration solution, it is possible to use a solution of chemicals or water.
In the technique for the production of paper fibers, the object of the impregnation of the wood chips is to provide a desired amount of solution as uniformly distributed in the fiber walls in the chips. In this way, the detaching of the fibers while they remain as intact as possible and of suitable quality or the dissolution of lignin and of other encrust materials in a later stage of the process is promoted. The factors affecting the impregnation are indefinite and difficult to control, which may result, e.g., in an overdosage of time and chemicals when attempts are made to guarantee an acceptable final result.
In the technical literature in this field, it is suggested that impregnation takes place as a joint effect of two physical factors, penetration and diffusion, which starts immediately after the solution has reached contact with the chips.
The penetration begins primarily from the cut faces of a chip, from which the solution penetrates into the fiber cavities. Owing to the counter-pressure caused by the air present in the cavities, the penetration slows down and stops soon. Almost the only factor that is mentioned as a factor that promotes the penetration is an increase in the difference in pressure between the solution and the air present in the fiber cavities by reducing the component pressure of the air or by increasing the pressure of the liquid.
Compared to penetration, diffusion is very slow. In diffusion the ions are transferred into the fiber walls in the solution because of differences in the concentrations of chemicals. The diffusion proceeds along liquid connections formed by the water which has penetrated into the fibrous cell system or which was present in said cell system.
Raising of the temperature of the penetration solution promotes the mobility of the ions. However, the chemical concentration of the penetration solution has a decisive significance for the progress of the diffusion into the interior parts of the chips, because the diffusion is always retarded when the moisture contained in the chips dilutes the solution and when its chemical content is reduced as chemicals are absorbed into the cell walls.
Thus, the amount and the coverage of the penetration have a highly essential importance for the initiation and progress of the diffusion as well as for the sufficiency and uniformity of the dosage of chemicals in the fiber walls, which is the ultimate goal.
It has been suggested that the amount of air contained in the cell systems in chips be reduced by means of vacuum produced mechanically or by means of steaming. However, only steaming is employed in the industry.
Steaming impregnation is a simple, but not adequate or controlled process if a good and uniform penetration of the chips is desired. For example, it has been proved that, as a result of absorption with the cooking liquor, the chemical concentrations in different parts of a chip particle varied within wide limits and that a considerably long diffusion time with preheating is required before a concentration necessary for the chemical reactions can be obtained in the inner parts.
The use of a vacuum in order to remove air has been suggested, e.g., in the Finnish Patents 11,987 and 30,091, and in the Swedish Patent 135,529.
The publication SE 135,529 deals with a continuous process and with equipment for impregnation of the chips with liquid. The impregnation takes place so that chips are fed continuously into the top portion of a tower, where it enters into a vacuum. The lower end of the tower, whose top end is closed, is placed in water, so that the chip column sinks in the tower slowly downwards and is submerged in the water, being impregnated at the same
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Aho Antti
Aho Osmo
Aho Osmo
Hastings Karen M.
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