Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1996-06-12
1998-02-10
Czaja, Donald E.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 19, 162 29, 162237, 162242, 162249, 162251, D21C 700
Patent
active
057164975
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a novel method and device for producing pulp, principally sulphate cellulose, with the aid of a continuous cooking process. The method and the device are principally directed towards exploiting, in a steam/liquid-phase digester, the advantages associated with impregnating the chips with the aid of hot black liquor, in accordance with the displacement principle.
STATE OF THE ART AND PROBLEMS
Experiments on black liquor impregnation are already in progress in connection with the production of sulphate cellulose pulp in continuous cooking. These experiments are demonstrating propounded theories, i.e. that impregnation with the aid of black liquor improves the strength properties of the fibres in the pulp produced in this manner. It appears that the principal reason for the improvement in the strength properties of the fibres is the fibre-sparing effect of the black liquor in combination with the relatively late introduction of a rather large quantity of white liquor, as described in all essentials in our own application EP 91402350.
If, as is customary, a large proportion of white liquor is used in connection with the impregnation, this results in certain fibres being exposed to the aggressive effect of the white liquor, with carbohydrates being broken down in the fibre, whose strength is reduced. The aim of the impregnation is, in the first place, thoroughly to soak each chips so that it becomes susceptible, by diffusion, to the active cooking chemicals which, in the context of sulphate cellulose, principally consist of sodium hydroxide and sodium sulphide. Following diffusion, the active cooking chemicals dissolve the lignins (which bind the fibres together) so that a finished cooked pulp can be obtained.
However, certain of the lignins are more readily dissolved than are others. These readily dissolved lignins can be dissolved both by sodium sulphide and by sodium hydroxide. However, other lignins, which are more difficult to dissolve, require sodium hydroxide as the dissolving agent. The disadvantage of sodium hydroxide is that, as has already been described, it also attacks carbohydrates (as early as the heating-up span 120.degree.-140.degree. C.) resulting in the strength of the fibres being decreased. The sodium sulphide, on the other hand, in the main attacks the lignin rather than the carbohydrates (at cooking temperature, i.e. greater than about 150.degree. C.), and does not, therefore, have the same fibre-weakening effect.
It has long been known that the spent liquor from a sulphate cellulose cook, i.e. black liquor, contains a very small residual quantity of sodium hydroxide but a relatively large proportion of sulphides.
By using black liquor as an impregnation liquid, the advantage is thus achieved that certain readily soluble lignins are dissolved at the same time as the fibre strength is essentially preserved. Over and above this, important gains can be made purely from the point of view of heat economy by utilizing the heat in the hot drawn-off black liquor directly by supplying it for impregnation essentially without any cooling. An additional advantage of black liquor impregnation is that very even soaking of the chips is obtained at a relatively high temperature so that the white liquor, when it is added, can diffuse very rapidly into each individual chips piece. In our above-described application, an account is given of a process in which we have attempted to exploit the abovementioned advantages associated with black liquor impregnation.
However, there are certain disadvantages from the point of view of equipment in the process which we previously proposed, inter alia the existence of central pipes which are arranged at the top and hang downwards.
SOLUTION AND ADVANTAGES
We have now found that by using a two-vessel system, in which the actual cooking vessel is a steam/liquor-phase system, substantial advantages can be gained when cooking sulphate cellulose pulp from a preceding black liquor impregnation when white liquor is supplied
REFERENCES:
patent: 4123318 (1978-10-01), Sherman
patent: 5053108 (1991-10-01), Richter
patent: 5080755 (1992-01-01), Backlund
patent: 5089086 (1992-02-01), Silander
patent: 5192396 (1993-03-01), Backlund
patent: 5346591 (1994-09-01), Henricson
Richter Johan
Richter Ole
Czaja Donald E.
Kvaerner Pulping AB
Leavitt Steven B.
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