Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Continuous chemical treatment or continuous charging or...
Patent
1995-05-18
1999-07-06
Nguyen, Dean T.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Continuous chemical treatment or continuous charging or...
162 60, 162237, D21C 324
Patent
active
059193379
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The environmental authorities are placing ever more stringent demands on the pulp industry to decrease the use of chemicals which can be harmful to the environment, such as chlorine, for example. Thus, permitted discharges of organic chlorine compounds in the effluent water from bleaching plants and the subsequent cooking process have been successively decreased and are now at such a low level that pulp works have in many cases stopped using organic chlorine compounds as bleaching agents. In addition, market forces are tending successively to increase the demand for paper products which have not been bleached with chlorine.
The pulp industry is therefore seeking methods which permit bleaching of pulp without using these chemicals. As an example of such a method, the lignox method (see SE-A 8902058) can be mentioned, in which, inter alia, bleaching is carried out with hydrogen peroxide. Ozone is another bleaching chemical of interest which is also being used to an increasing extent. It is thus possible, using such bleaching chemicals, to achieve the brightnesses which are demanded for marketed pulp, i.e. 89 ISO and higher, without using chlorine-containing bleaching agents.
However, there is a problem in employing these bleaching chemicals which do not contain chlorine in currently known bleaching processes, namely that these chemicals impair to a relatively large extent the quality of the pulp fibers.
DESCRIPTION OF THE INVENTION
With the aid of experiments which have been conducted under the auspices of Kamyr AB, it has emerged, in a surprising manner, that extremely good results with regard to delignification and strength properties can be achieved if the pulp is cooked at the same temperature level throughout essentially the whole digester, i.e. if essentially the same temperature is maintained in all the cooking zones and a certain quantity of alkali is also added to the lowest zone of the digester, which zone is normally used for washing in countercurrent owing to the fact that essentially the same temperature level is maintained in virtually the whole digester, very extensive delignification can be obtained at a relatively low temperature. In addition, it has emerged that the strength properties are affected in a particularly advantageous manner, that a higher yield of fibre raw material is obtained and that the reject quantity is decreased. These advantages are most clearly evident from the diagrams shown in FIGS. 1 and 2, which diagrams show comparative values between pulp (softwood) which was cooked by the modified conventional cooking technique and pulp which was cooked in accordance with the process according to the invention (in a similar digester, i.e. having a cocurrent upper cooking zone, a middle countercurrent cooking zone and a bottom countercurrent washing zone), in which a constant temperature level of about +155.degree. C. was maintained in the whole digester.
The invention relates to an arrangement, which is advantageous from the point of view of equipment, for effecting a cooking in accordance with the new method, in particular with regard to digesters built according to an older principle, and consisting of an upper cocurrent cooking zone and a lower countercurrent washing zone. Thus, certain practical problems ensue as a consequence of an isothermal cooking process. A first such problem is the difficulty of efficiently reaching and maintaining the temperature in the lower part of the digester, i.e. that part which is normally utilized for washing.
A related problem is that, in order to be able to maintain the high temperature in the digester in the preferred case, the pulp must be taken out of the digester at a temperature exceeding +100.degree. C., implying that, if a blowing off to atmospheric pressure were to take place, a disintegration of an explosive nature would be obtained in direct connection with this, with consequent negative effect on the pulp quality.
In order to avoid the said strength-diminishing disintegration of the cooked pulp, it i
REFERENCES:
patent: 4123318 (1978-10-01), Sherman
patent: 5066362 (1991-11-01), Meredith
patent: 5236554 (1993-08-01), Greenwood
patent: 5328564 (1994-07-01), Jiang et al.
Backlund, E.A., "Extended Delignification . . . Continuous Digester", Tappi Journal, Nov. 1984.
Andtbacka Stig
Backlund .ANG.ke
Jacobsen Finn
Olsson Sven-Erik
Svanberg Johanna
Kvaerner Pulping Technologies AB
Nguyen Dean T.
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