Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Waste paper or textile waste
Patent
1987-04-23
1988-04-12
Bashore, S. Leon
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Waste paper or textile waste
162 6, 162 29, 162190, 1621812, D21C 502, D21B 132, D21H 368
Patent
active
047372389
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to processes for the delignification of waste paper, particularly waste paper having a high lignin content. More particularly, the present invention relates to the delignification of waste paper, such as corrugated cardboard, which contains chemical high-yield pulp.
BACKGROUND OF THE INVENTION
Waste papers of the aforementioned types often contain aluminum in amounts of approximately 5 to 10 kg/ton of bone-dry pulp. This aluminum has usually been added in the form of alum as a retention agent during the prior papermaking process for these paper products. It is known that the presence of this aluminum creates problems of incrustation in evaporation plants. It is also known that magnesium compounds can be added thereto for the purpose of reducing these problems. Aluminum can thus be precipitated in the form of aluminum-magnesium compounds, and in this manner the amount of dissolved aluminum in the pulp suspension can be decreased.
In view of the relatively high aluminum content in these waste papers, however, such measures are generally not sufficient to make it possible to recycle white water from the delignification step to the evaporation plant. The presence of too great an amount of dissolved aluminum being returned to the evaporation step would in such a case give rise to the aforementioned incrustation problems.
It is desirable and necessary, however, to recycle the white water from the delignification step, particularly in view of the chemical and COD-content of the white water.
SUMMARY OF THE INVENTION
In accordance with the present invention, these and other objects have now been accomplished by the discovery of a process for treating waste paper containing aluminum and having a high lignin content in a method which restores to a certain extent the original properties of the fibers, particularly with respect to flexibility and the degree of swelling, without at the same time increasing the resistance to dewatering thereof. In particular, this process includes producing an aqueous pulp suspension of the waste paper containing aluminum by adding an aqueous solution to the waste paper, screening the aqueous pulp suspension so as to produce a screened aqueous pulp suspension, dewatering the screened aqueous pulp suspension so as to produce a concentrated aqueous pulp suspension and a first white water stream including substantially all of the aluminum from the waste paper, delignifying the concentrated aqueous pulp suspension so as to produce a delignified aqueous pulp suspension, separating a substantial portion of the aqueous liquid from the delignified aqueous pulp suspension so as to produce a separated pulp stream and a second white water stream substantially free of aluminum, transferring the separated pulp stream to a paper mill for the production of paper therefrom, diluting the separated pulp stream with a dilution liquid stream in the paper mill, evaporating the second white water stream, and transferring at least a major portion of the first white water stream to the paper mill for use as a dilution liquid stream therein.
In accordance with one embodiment of the process of the present invention, a portion of the first white water stream is utilized for producing the aqueous pulp suspension. In a preferred embodiment the screened aqueous pulp suspension has a concentration of about 1%.
In accordance with another embodiment of the process of the present invention, the process includes steam treating the concentrated aqueous pulp suspension produced in the dewatering step.
In a preferred embodiment the delignifying step is conducted in the presence of oxygen, and is preferably carried out at a pulp concentration of between about 10 and 12%.
In accordance with another embodiment of the process of the present invention, the process includes producing a third white water stream in the paper mill and recycling that third white water stream as a recycled dilution liquid stream for the separated pulp stream.
In short, by means of the present
REFERENCES:
patent: 2057059 (1936-10-01), Rue
patent: 4115188 (1978-09-01), O'Brien et al.
patent: 4416727 (1983-11-01), Elton
patent: 4595455 (1986-06-01), Mannbro
Bashore S. Leon
Hastings K. M.
Sunds Defibrator Aktiebolag
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