Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/04—Pulping cellulose-containing materials with acids, acid salts or acid anhydrides
  • D21C3/06—Pulping cellulose-containing materials with acids, acid salts or acid anhydrides sulfur dioxide; sulfurous acid; bisulfites sulfites

Definitions

• the present invention generally relates to the release of cellulose fibers from solid cellulosic materials, and more particularly, to an improvement in the neutral to weakly alkaline sulfite process used for extracting cellulose from solid cellulosic material.
• alkaline digestion the most common example of which is the sulfate process.
• This process essentially comprises the use of a strongly alkaline solution consisting essentially of NaOH or other alkalis such as sulfides or polysulfides having a pH value greater than 12.
• sulfate process essentially comprises the use of a strongly alkaline solution consisting essentially of NaOH or other alkalis such as sulfides or polysulfides having a pH value greater than 12.
• sulfides or polysulfides having a pH value greater than 12.
• the cellulose fibers have good strength properties but poor whiteness qualities. Accordingly, expensive and time consuming bleaching processes must be employed in order to obtain a proper whiteness of the cellulose fibers for use in the manufacture of high grade papers.
• cellulose fibers having a greater whiteness use is made of a second group of processes which comprise sulfite processes wherein the main digestive agent is S0 in free and/or combined form. While the cellulose fibers produced by these processes are much whiter than the cellulose fibers produced by the alkalis, the fibers are generally not as strong as the fibers produced I by the alkaline processes.
• the main disadvantage of this process is the relatively poor quality of the pulp product which must invariably be mechanically fiberized in order to separate the fibers.
• the process of the present invention comprises the reduction of logs of wood as supplied to the mill to a consistency of fairly small chips, cooking the chips with a hot aqueous solution, commonly referred to as liquor, cooking the chips and liquor under pressure so that the pulping agent component of the liquor attacks the lignins and other organic material in the wood releasing the cellulose for washing, filtering and refining and making into paper or other uses.
• the required time for the digestion is appreciably lower than for the conventional methods and ranges between 5 and 30 minutes with a usual period of between 8 to 10 minutes.
• the time required for the process of the present invention is less than that required for the production of semichemical pulp or for carrying out the alkaline digestion process at higher temperatures and with a thorough preliminary impregnation.
• the resulting cellulose fibers are of a considerably better quality and no mechanical fiberizing is required.
• the digestion liquors may comprise solutions of alkaline sulfites or bisulfites with additions such as Na CO NaHCO and/or preferably NaOI-I, NaHS, Na S O N328, NAS or other polysulfides.
• the pH value of the digestion liquors at the beginning of the cooking process ranges between 7.0 and 11.5, preferably between 7 to 10.
• the digestion or cooking temperature is between 200 and 280 C., and preferably between 225 and 280 C. This temperature is generally higher than permissible in previously known and conventional processes.
• the thorough preliminary impregnation of the present process can be accomplished by known methods by the use of steam or vacuum, by mechanical or hydraulic pressure, or by agitation, or by any combination of those steps.
• the wood chips Prior to the impregnation of the crude cellulose material with the digestion liquir the wood chips may be treated with steam or subjected to a vacuum.
• the cellulosic raw material may be subjected to an initial comminution to form wood chips having a maximum length of 25 mm., and a maximum thickness of 5 mm. A minimum length of 3 mm. and a minimum thickness of about 0.5 mm. is advisable. This will result in obtaining cellulose fibers having a greater strength.
• While the present invention is essentially directed to the use of wood, other cellulosic materials are not excluded. Such other cellulosic materials having various chip sizes and comprising, for example, waste material from plants, grasses, bamboo, can be used for producing cellulose fibers of a good quality according to the present invention.
• the present invention also has the advantage of making possible the production of cellulose pulp having optimum qualities and good whiteness even by the use of digestion liquors containing substantial quantities of Na S O These quantities may range up to 60% of the total chemical content of the liquor. In the known sulfite processes darker grades of cellulose pulps are produced by such small quantities of Na S O' as 2%. In the process of the present invention the presence of a relatively small quantity of Na S and/ or polysulfides does not have any detrimental effects on the produced cellulose fibers.
• this process has the further advantage of a high production rate.
• Example Comminuted pine wood chips having an average length of 15 mm. and an average thickness of 2 mm. were deposited in an autoclave and steamed for about 2 minutes under a pressure of 6 atmospheres. The wood was accordingly quickly heated and the entrapped air was driven out of thewood. The digestion liquor which was heated to about 250? C. was then introduced into the autoclave. This liquor had the following composition:

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Citations (5)

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• 1961
  • 1961-10-20 DE DEM50639A patent/DE1198666B/de active Pending
• 1962
  • 1962-10-18 US US231558A patent/US3262839A/en not_active Expired - Lifetime

Patent Citations (5)

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