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
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds
  • D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides

Definitions

• This invention relates to the bleaching of paper pulp and. more particularly, to the bleaching of ground wood pulp.
• ground wood pulp either alone or together with other paper pulps, is employed extensively in the manufacture of paper. It is widely used in the manufacture of paper of the grade employed for printing newspapers, and in fact. most newsprint paper consists almost entirely of ground wood pulp. In many papers intended for printing purposes a high content of ground wood pulp very desirable.
• any increase in brightness is not a permanent characteristic of the pulp since there is a tendency to revert to the original darker color when the pulp or paper produced therefrom is subjected to the action of light and air.
• This tendency "to develop a yellowish coloration upon standing has constituted a serious detriment to the commercial utilization of papers prepared from ground Wood pulp or containing substantial quantities of ground wood pulp in the manufacture of the higher qualities of paper.
• the use of ground wood pulp is restricted principally to the manufacture of relatively cheap papers, such as paper used for printing purposes including newsprint, and for book pap rs of the cheaper grades.
• Ground wood pulp as distinguished from chemical wood pulp or pulp which has been subjected to a chemical treatment such as in the sulfite pulp process, soda pulp process, or sulfate pulp process (kraft). is inherently a less expensive raw material for paper manufacture. If the pulp could be bleached in such a manner that paper prepared therefrom or paper containing relatively large quantities of the ground wood pulp would possess good permanent whiteness, ground wood pulp would undoubtedly be of much greater commercialiimportance in the paper manufacturing industry. Generally speaking, ground wood pulp The previous attempts to bleach pulps, consisting and soda pulp have many characteristics in common and.
• ground wood pulp could be satisfactorily bleached to yield a paper of satisfactory whiteness and satisfactory whiteness retentivity
• the resulting product would be competitive with bleached soda pulp and would be utilized commercially to a far greater extent in the manufacture of papers generally than under the present circumstances.
• Ground wood pulp is, however, a difficult material to bleach, and it is especially diflicult to bleach this product in such a manner that the tendency to yellow upon standing, in both the resulting pulp and the paper produced therefrom. is reduced to a satisfactory degree.
• the essential keynote of our improved process is the critical control of the conditions maintained during the bleaching process wherein the ground wood pulp is subjected to the bleaching action of an oxidizing bleaching agent such as sodium peroxide or an alkaline solution of hydrogen peroxide.
• an oxidizing bleaching agent such as sodium peroxide or an alkaline solution of hydrogen peroxide.
• ground wood pulp can be satisfactorily bleached in an economical and efficient manner under conditions wherein the whiteness retentivity is increased to a remarkable extent by the use of sodium peroxide (NazOz) as the bleaching agent.
• sodium peroxide NazOz
• equivalent agents such as an alkaline solution of hydrogen peroxide, or a solution of a per compound such as sodium perborate, may also be utilized.
• sodium peroxide as our active bleaching agent. The following description of our preferred procedure will therefore refer specifically to sodium peroxide as the bleaching agent.
• sodium peroxide as the bleaching agent we have found that it is desirable to utilize an amount of sodium peroxide equivalent approximately to 5% of the weight of the dry ground wood being treated.
• amounts of chemical agents utilized ordinarily the practice of the paper pulp industry is followed,the amounts being expressed as the percentage based on the weight of the dry ground wood pulp being treated. For example, bleaching with 5% sodium peroxide means that there will be present in the bleach five pounds of sodium peroxide for every one hundred pounds of ground wood being treated. Similarly, bleaching with 2% sodium peroxide means that the bleach liquor contains two pounds of sodium peroxide per one hundr'ed pounds of dry ground wood present in the slurry being treated. This method of expression is customary in the industries utilizing the various paper pulps and will be followed here.
• pH of the mixture of ground wood pulp suspension and bleaching agent Another essential factor which must be strictly regulated in order to secure the desired degree of bleaching and the desired whiteness retentivity is the pH of the mixture of ground wood pulp suspension and bleaching agent.
• the regulation of this pH during the bleaching operation within a very narrow range is essential if a product of commercial value is to be secured.
• the pH must fall strictly within the range 10.0 to 11.0 if results of commercial acceptability are to be obtained.
• relatively higher pH values such as a pH value up to 11.0 can be maintained without the danger of excessive injury to the pulp.
• a pH of 10.5 is the upper limit and that if the pH exceeds 10.5 there will practically always result unsatisfactory bleaching.
• the pH value may go up as high as 11.0 without interfering with the desired bleaching. If the pH value is allowed to go too high, not only is the bleaching not satisfactory but there is a tendency to cause a distinct yellowing in the color of the pulp, which yellow color is very difficult to bleach out or remove therefrom.
• pH values below 9.5 will result in inconsequential and unsatisfactory bleaching and will not produce a pulp of the desired brightness. For most purposes, therefore, it is essential to maintain a pH of not below about 10.0 in order to secure satisfactory bleaching.
• pH values within the range 9.5 to 10.0 while generally not the proper values to attain the highest degree of bleaching, may sometimes be utilized in commercial practice if only a very slight degree of lightening in color is required, especially under those circumstances where the higher temperatures or temperatures of close to 120 F. are utilized.
• sodium silicate in the slurry of ground wood pulp and bleaching agent is very desirable if not essential as it provides the necessary alkalining agent and moreover acts to some extent as a buffering material to maintain the pH value within the narrow range 10.0 to 10.5.
• the amount of sodium silicate which should be present for satisfactory bleaching depends largely on the concentration of sodium peroin'de utilized.
• sodium peroxide i. e. one to two pounds of sodium peroxide per one hundred pounds dry weight of the ground wood, it is desirable to have present at least 6% of sodium silicate.
• sodium silicate concentrations When the sodium peroxide concentrations are higher than 2% we have found. that there is also a definite minimum percentage of sodium silicate which should be present if maximum brightness is to be secured.
• the sodium silicate concentration should be about 10% or ten pounds per one hundred pounds of ground wood.
• the presence of 10% sodium silicate results in a brightness of 68% while with 2% sodium silicate concentration the brightness is only 62.5%.
• the sodium silicate concentration in the bleach bath will ordinarily not exceed 10%, even for the higher concentrations of peroxide, i. e. concentrations up to 10.0% NazOz.
• concentrations of at least 6% sodium silicate are essential, even for very low concentrations of sodium peroxide, concentrations as low, for example, as 0.50% or 0.25%.
• the sodium silicate range extends from 6% sodium silicate to 10% sodium silicate, the particular amount selected being chosen, as previously specified, in accordance with the concentration of sodium peroxide or equivalent agent utilized.
• the sodium silicate is valuable in the bleach bath for protecting metallic equipment against corrosion by alkaline compounds present in that bath.
• the silicate is efiective in maintaining the pH within the range given and to some extent serves both as alkalining agent and buffering agent.
• Example I A solution of sodium peroxide of 1.5% concentration, i. e. 1.5 pounds of sodium peroxide per one hundred pounds dry weight of ground wood pulp being treated, was prepared. This solution was neutralized by the addition of sulfuric acid until the pH fell within the range 10.0 to 10.5.
• the temperature of the mixture of pulp and bleach liquor was maintained at approximately 100 F. for approximately four hours. Care was taken to insure the maintenance at all times of a temperature above 80 F'., and the temperature was also carefully regulated so that it did not exceed 120 F. at any time. At the end of four hours the pulp was removed, washed with water, and used subsequently in the manufacture of paper. The ground wood pulp had been bleached to a very satisfactory degree of brightness and the paper resulting therefrom was of a high whiteness and high degree of whiteness retentivity.
• washing at the conclusion of the bleaching process is not always necessary but may be carried out with advantage in some cases.
• Example [I A solution of sodium peroxide of approximately 4.0% concentration, i. e. 4.8 pounds of sodium peroxide per hundred pounds dry weight of the ground wood to be treated, was prepared. The ground wood pulp of approximately consistency was introduced into solution of sodium peroxide. The pH of the mixture was carefully adjusted by the addition of sulfuric acid so that a pH of 10.5 was secured. To the solution sodium silicate in amount approximately 10%, i. e. ten pounds of silicate per hundred pounds of ground wood, was added. The resulting pH was approximately 10.5.
• the brightness of the pulp as determined by the General Electric Companys Brightness Meter was found to be 69.0. As the original pulp had a brightness of 59.5, it is evident that there was an increase of 9.5 points in brightness.
• Example [H The same ground wood pulp treated in Example II was bleached in a 2.0% sodium peroxide solution which contained approximately 6% sodium silicate. The bleaching was carried out at 90 F. and required five hours. The pH of the mixture of bleach bath and ground wood pulp during the greater portion of this bleaching process was approximately 10.2.
• a process of bleaching ground wood pulp which comprises contacting said pulp with an alkaline solution containing a peroxide at a temperature within the range 80 to 120 F., the pH of the mixture during the major portion of the bleaching process being maintained within the range 10.0 to 11.0.
• a process of bleaching ground wood pulp which comprises contacting said pulp with a solution of an alkali metal peroxide, said mixture being maintained throughout the bleaching at a temperature within the range 80 to 120 F. and the pH during the major portion of said bleaching being maintained within the range 10.0 to
• a process of bleaching ground wood pulp which comprises contacting said pulp with a solution of sodium peroxide, there being present from 0.25 pound to 10 pounds of sodium peroxide per 100 pounds of ground wood present-in said pulp wing treated, the temperature being at all times maintained within the range 80 to 120 F., and the pH of said bleaching liquor during the major portion of the bleaching step being maintained within the range 10.0 to 11.0.
• a process of bleaching ground wood pulp which comprises subjecting a pulp of 4 to 10% consistency to the action of sodium peroxide, said sodium peroxide being present in amounts ranging from 0.25 pound to 10 pounds per 100 pounds dry weight of ground wood, the temperature of the bleaching liquor during the bleaching process being maintained within the range 80 to 120 F., and the pH of said bleach bath being adjusted to fall within the range 10.0 to 11.0 during the major portion of said bleaching.
• a process of bleaching ground wood pulp which comprises subjecting said pulp to the action of sodium peroxide in a bleach bath containing sodium silicate, there being present from 0.25 to 10.0 pounds of sodium peroxide per 100 pounds dry weight of the ground wood pulp being bleached, the pH of said bleach liquor being maintained during the major portion of the bleaching operation within the range 10.0 to 11.0, and the temperature at which said bleaching is carried out being maintained between the range 30 to 120 F.
• a process of bleaching ground wood pulp which comprises subjecting said pulp to the action of an alkaline solution of a peroxide in a bleach bath containing sodium silicate, the temperature of said bleach bath being maintained within the range 80 to 120" F and the pH of said bath being regulated so as to fall within'the range 10.0 to 11.0 during the major portion of said bleaching process.
• a process of bleaching ground wood pulp which comprises subjecting said pulp to the action of an alkali metal peroxide in a solution containing sodium silicate, the alkali metal peroxide concentration ranging from 0.25 pound to pounds per 100 pounds dry weight of the ground wood being treated, said sodium silicate concentration ranging from approximately 6 pounds per 100 pounds dry weight of ground wood, to 10 pounds per 100 pounds dry weight of ground wood, the pH value of said bleach bath being maintained within the range 10.0 to 11.0 during the major portion of the bleaching process and A the temperature at which said bleaching is car: ried out being regulated so that it falls within the range 80 to 120 F. during the major portion of said bleaching.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)

Priority Applications (3)

Applications Claiming Priority (1)

Publications (1)

Family

ID=23145067

Family Applications (1)

Country Status (3)

Cited By (5)

Families Citing this family (1)

• 1939
  • 1939-09-29 US US297150A patent/US2199376A/en not_active Expired - Lifetime
• 1947
  • 1947-06-11 GB GB15419/47A patent/GB632967A/en not_active Expired
  • 1947-07-03 FR FR949704D patent/FR949704A/fr not_active Expired

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