Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Treatment with particular chemical
Patent
1989-02-13
1990-04-24
Chin, Peter
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Treatment with particular chemical
8110, 25218822, 423515, D21C 304
Patent
active
049197550
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for bleaching of materials such as wood pulps, clay etc. using a dithionite solution. Particularly the invention concerns a process for such bleaching where a borohydride solution of a relatively low alkalinity is reacted with a bisulphite solution, after which the dithionite solution thus obtained is added to the material to be treated.
It is known to use dithionite, usually sodium dithionite, Na.sub.2 S.sub.2 O.sub.4, for treatment of materials such as clay, wood pulps and textiles. When bleaching wood pulps, especially mechanical pulp, dithionite is used when a lower brightness is desired, while hydrogen peroxide bleaching is used when the requirements of brightness are high. Different dithionite systems are known and can be used. Dithionite in powder form, which is dissolved before use, is favourable from an economical point of view for the user. The handling of powder is, however, unfavourable from work environment point of view and in working with this method losses of active substance is also obtained when preparing the bleach solution. An alternative to the handling of powder is to use a dithionite solution supplied ready for use. However, dithionite solution is instable and in order to avoid losses of active substance during storage resource demanding investments are required for installation of cooling systems and protective gas systems. The supply of ready-prepared dithionite solution also implies a short transport distance between producer and user because of the low amount of active substance. U.S. Pat. No. 3,290,161 describes bleaching of clay by adding directly at the bleaching both alkali metal bisulfite in a solid form and alkali metal borohydride in a solid form. The handling and dissolution of two different solid components is, however, time consuming and troublesome and further increases the disadvantages with handling of powder.
Because of the disadvantages shown by the above mentioned forms of dithionite, an "in situ"-preparation of a dithionite solution at the user has gained an increased importance. The systems hereby used are three-component-systems, based on commercial sodium borohydride solutions, which react with sulphur dioxide and sodium hydroxide, see e.g. DE-A No. 3,427,297. Commercial sodium borohydride solutions, e.g. those sold under registered trademarks Borol and Borino, have a relatively low amount of sodium borohydride and a very high alkalinity, and they contain about 12 percent by weight of NaBH.sub.4, 40 percent by weight of NaOH and 48 percent by weight of water. Preparation of dithionite solution in connection with use thereof starting from such a borohydride solution is favourable as such with respect to high stability for the borohydride solution used, but because of the use of three components the method gives high investment costs for e.g. storage and dosage equipment. The commercial borohydride solutions show disadvantages in tending to crystallize at temperatures of about +13.degree. C., and therefore .heating is often required when handling these solutions. The handling of sulphur dioxide, which normally exists in a liquid form under pressure, also brings disadvantages, from economical point of view as well as from environmental point of view. This has resulted in a limited use of this type of "in situ"-preparation of dithionite solution, and it is only possible for users having access to the equipment and the know-how required for handling of liquid sulphur dioxide and when big annual consumptions exist.
Another type of "in situ"-preparation is known from U.S. Pat. No. 3,167,515, according to which a solid mixture of alkali metal borohydride and alkali metal bisulfite immediately before use is dissolved in water and pH of the solution is regulated to strong alkalinity. However, also here disadvantages concerning the handling of powder exist, which is unfavourable for work environment and which brings losses of active substance at the preparation of the solution. Moreover the relation between bisulfite to borohydr
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Tappi, vol. 53, No. 12, Dec. 1970, "Two-Stage Bleaching of High-Yield Pulp at Blandin", C. A. Richardson et al.
"Bright News: Lower-Cost Method for TMP and Groundwood Bleaching", R. G. Guess, Pulp & Paper, 53(6), Jun. 1979, pp. 74-77, 81.
Chin Peter
Eka Nobel AB
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