Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Chemical treatment after start or completion of mechanical...
Patent
1991-10-29
1992-11-17
Chin, Peter
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Chemical treatment after start or completion of mechanical...
162 17, 162 19, 162 52, 162 57, 162 68, 162242, 162243, 162246, 162248, 162249, 162261, D21B 116
Patent
active
051640422
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of producing high-yield pulp from pulp chip material containing lignocellulose, the material being steam treated in a steam treating station for driving air out of the material and heating of the latter, steam treated material being mixed in a mixing station with liquid containing chemicals, the mixture of steam treated material and said liquid being transferred from the mixing station to an impregnation station for impregnating the material by means of said chemicals, and impregnated material being supplied to a refining station for refining the material.
In a conventional plant for producing high-yield pulp the impregnation station comprises an impregnation container, to which the steam treated chip material is supplied by means of a specially formed feed screw, which is arranged to compress the chip material strongly during feeding of the latter into the lower part of the impregnation container. In the impregnation container, which contains a solution of said chemicals, e.g. sodium sulphite, sodium bisulphite, caustic soda and/or a mixture of said chemicals, the chip material is expanded so that chemical solution is sucked into and distributed in the latter. The chip material in the impregnation container rises in this during a period of time, which is adjusted so that the chip material will be saturated with chemical solution. Saturated chip material is discharged through an outlet in the upper part of the impregnation container and is conveyed via a reaction vessel to the refining station. Fresh chemical solution is supplied to the lower part of the impregnation container in a corresponding amount per time unit as used chemical solution is discharged from the impregnation container together with discharged saturated chip material.
The above described impregnation of the chip material by means of a chemical solution, the chip material being reacted on by the chemicals, provides the advantage that the mechanical separation of the fibres of the chip material is facilitated and improved when refining.
However, in this conventional plant the chemical solution is mixed relatively unevenly with the chip material. To ensures that the chip material as completely as possible reacts with the chemicals, a chemical solution of substantially higher chemical concentration, e.g. 10%, than is theoretically necessary, usually 0.5%, is maintained in the impregnation container. The treatment of the chip material with said high chemical concentration results in the drawback that the energy consumption for refining such treated chip material into pulp will be substantially higher than the refining of optimally chemically treated chip material would require. The high chemical concentration, thus required for conventional production of high-yield pulp, also results in the drawback that the printability properties of paper produced by such pulp will be deteriorated. In addition, the use of such solution of high chemical concentration will mean greater difficulties in and increased costs for cleaning of excess liquid that results from production of paper from such pulp.
Because of the above described compression of the chip material during feeding of the latter into the impregnation container by means of said feed screw, the chip material is worked on mechanically very strongly. This results in the drawback that the strength of the pulp and the paper produced therewith will be detrimentally affected.
In another known plant for producing high-yield pulp, according to SE 157 050, cold chemical liquid is mixed with hot steam treated chip material, existing steam bubbles in the pores and cavities of the chip material being thereby condensed so that liquid is sucked into the latter. The result will be that the liquid is distributed evenly in the chip material. This known plant does not utilize any plug screw of the kind above described, but a screw conveyor takes care of the transportation of liquid treated chip material through an impregnation station.
However, the known plant according to
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Klinga Leif
Larsen Hans
Celleco AB
Chin Peter
LandOfFree
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