Paper making and fiber liberation – Processes and products – Non-fiber additive
Patent
1983-05-20
1985-07-30
Smith, William F.
Paper making and fiber liberation
Processes and products
Non-fiber additive
162190, 210764, 210928, D21F 166
Patent
active
045320071
DESCRIPTION:
BRIEF SUMMARY
In the pulp and paper industry, possibly except the kraft paper industry, as little discolouration as possible is desired, as the finished products should have a high brightness (whiteness). One reason of discolouration may be the brown colouring, which the pulp exhibits at a high temperature due to certain oxidation processes. The reaction products formed can however be degraded by bleaching, chlorine, peroxide or dithionite but also oxygen being used as a bleaching agent.
Now, it has been found that pulp (paper) is discoloured by dark colouring in certain cases. Previously, this discolouration has not been so embarassing that it has been a problem or caused any measures. The tendency of closing the process systems, i.e. reducing the water consumption in the process, seems, however, to have led to an increase of this discolouration.
It has now been shown that the use of oxygen, oxygen containing gases or substances giving off oxygen reduce or inhibit the dark colouring in production of pulp and paper, these substances being added to whitewater systems, especially closed ones in such an amount that aerobic conditions are maintained substantially permanently in at least a part thereof, conveniently at least 50% and preferably the major portion thereof, e.g. 75-90% thereof, or substantially the whole whitewater system, i.e. that the redox potential (Eh) is kept higher than about -100mV or that it does not sink below -120mV more than occasionally or at any rate not below -140 to -160mV, say -150mV. It has been found that a suitable feed is in the form of air or other oxygen containing gases, as well as pure oxygen gas. It has also appeared to be possible to use substances giving off oxygen such as peroxides, e.g. hydrogen peroxide.
In the closed whitewater systems from paper preparation or transport water of pulp, which is completely or partly recirculated (re-used) it has appeared that the best results are obtained if so much oxygen is added that some amount of the oxygen is constantly maintained in the water or that the water is substantially free of oxygen for a time that is not so long. Satisfactory results are achieved if oxygen is added to the water at least once during the circulation in such an amount that a substantial excess of oxygen remains at least immediately after the place of addition, e.g. up to 1 hour after the addition or at least for about 20 min.
It has been found that this addition of oxygen does not act as a bleaching agent, i.e. it does not degrade organic reaction products formed. It seems instead as if the mechanism at this discolouration and its removal lies in the fact that metals, above all iron, which is released from the process apparatus during the circulation of the pulp, stock or the water, react with sulfides, especially hydrogen sulfide, to form metal sulfides, such as iron sulfide. Of course the metals may also come from other sources, e.g. the wood substance. Sulfur is mainly present as sulfate and is mostly added to the process in different forms, e.g. sulfuric acid, sodium bisulfite and aluminum sulfate (alum). All the sulfate will above all form hydrogen sulfide under the influence of anaerobic bacteria, e.g. such ones from the group of desulfovibrio bacteria. Thus, the mechanism is apparently quite another than a thermally conditioned oxidation. This also explains why this discolouration previously has not been embarrasing in a high degree. This closing of the process systems which has now been the consequence i.a. of increased requirements on waste water treatment in factories has however increased the content of dissolved, organic substances in the system. These are easily oxidized and any free oxygen is then consumed, life conditions being obtained for anaerobic bacteria. Moreover, these organic substances are suitable as nutrition for these bacteria.
The life cycle of the anaerobic bacteria seems to be interrupted by the present invention, the formation of dark colouring metal sulfides, especially iron sulfide, being reduced to a large extent.
The invention is ill
REFERENCES:
patent: 3405930 (1968-10-01), Morter
patent: 3514278 (1970-05-01), Brink, Jr.
patent: 3578610 (1971-04-01), Mader et al.
patent: 3876497 (1975-04-01), Hoffman
ABIPO, vol. 51, No. 1, 7/80, #642.
Wenzl, J. H., "Closure of Paper & Board Mill Production Systems and its Effect on Production Conditions", Tappi Annual Meeting, Mar. 1981.
Holmens Bruk Aktiebolag
L'Air Liquide S.A. pour l'Etude et Exploitation des Procedes Geo
Smith William F.
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