US2516664A - Hypochlorite bleaching of ground wood - Google Patents

Description

Patented July 25, 1950 HYPOCHLQRITE BLEACHING OF. GROUND WOODv Daniel O.Ad'ams and George B. Hughey, (loving,-

ton, Va., assignors to West Virginia Pulp and" PaperCompany, New York; N; Y., a corporation cf Delaware Ne Drawing. Application October 16,1944, Serial No. 558,952

Claims. 1

Our present invention relates to improvements in the'bleaching of ground wood, more especially ground oak. and other ground woods such as chestnut, chestnut oak, maple, etc., which contain a substantial amount of tannin and other natural color bodies.

In recent years much study has been given to the bleaching of ground wood. Usually and especially for the manufacture of newsprint, ground wood is not bleached. However, the desirable printing characteristics of paper containing a large proportion of ground woodhave led to proposals for its use in high grades of paper such as magazine, book paper and the like. In many instances a base ground wood sheet or one containing a substantial proportion of ground wood is coated with a suitable coating composition. However, to be successful the ground wood must have a reasonably good color of substantial permanence, otherwise only a slight proportion of it may be" used in the furnish; or if used in substantial proportion in the base sheet an undesirably heavy coat must be given it in order to have. a final product of acceptable brightness.

Prior toour discovery the most successful attempts-to bleach ground wood. have been those involving the use of sodium peroxide. We have discovered, however, that under properly controlled conditions even superior results'may be had by hypochlorite bleaching; In applying our improved hypochlorite method-to the bleaching ofiground hard woods, as for example oak and other woods containing appreciable amounts of tannin, it is not practicable to obtain as bright a pulp as with ground spruce, for example. We have discovered, however, that the ground hard woods-such as oak, chestnut, etc., may be made to yield a bleached pulp of comparable brightness with bleached ground spruce or poplar by subjecting the ground hard Wood to a preliminary dilute caustic extraction whereby to remove certain of the incrustations and color bodies associated with the fiber, which extraction is directly followed, after Washing, by the hypochlorite bleach.

After the hypochlorite bleach is had, the brightness may be further increased by the use of a moderate excess of an anti-chlor, preferably sulfurous acid.

Our invention will be best understood by ref- 2 erenc-e to the following detailed examples "illustrative thereof.

Example I.Without caustic extraction A slurry of ground wood preparedfrom redoa-k 'containing'15-.2"grams ofoverr dried (0': D.) pulp at 5% consistencywa-s heated to F. To this slurry 0.5 grams'oi NaOH was added A volume of calcium hypcchlorite solution containing-1.5 grams available chlorine-was added with agitation, given an initial pH of l 0.l. Thereafter during the bleach the" pH was maintained between 10 and 10.5 by additions of NaOH solution as required. The bleach waspermittedto proceedt'o exhaustion of the hypochlorite, which required twenty minutes. As soon as exhaustion had occurred 1.5 gramSSOa were added and the mixture allowed to stand ten minutes. The pulp was thickened on a Buechner funnel and thoroughly washed. This bleaching increased the brightness of the pulp from 43.1% to"57.0'%' asdetefi mined by the General Electric reflection meter using a No. 1 filter. When the hypo'chl'orite was increased to 2.25 grams available chlorine (i.' e., 15% based on the O. D. pulp) the exhaustion period was found to be sixty minutes and a final brightness of 61.2 resulted, with other conditions having remained the same.

Example II.With caustic extraction A 1745 gram sample of ground wood prepared from red. oak was" treated with. 35 grams NaOH in sufficient solution to give a final concentration of .1 gram per liter. This resulted in a pulpslurry of approximately 5% consistency (O. D. basis). The temperature was maintained at 77 F; and at. the end of one'hourthe pulp slurry was: thickened on a filter: and thoroughly washed.

The pulp from the foregoing.extractionfwas then dispersed in water, and sufilcienti calcium hypochlorite solution to contain 17.5 grams available chlorine (equal to 10%- of the. Weight of the unbleached pulp) wasadded to the slurry. The pH was maintained between 10 and IOLSthroughout the bleach. After the bleach liquor was added, the consistency of the slurry was 5.0% 0. D. and the temperature F. The bleach was allowed to proceed until the chlorine was practically exhausted, and then 168 grams of S02 were added. After ten minutes the pulp was 3 thickened on a filter and thoroughly washed. The resulting pulp had a brightness of 67.5 as determined with the General Electric reflection meter.

When the strength of the caustic extraction solution was increased to 5 grams per liter, a final brightness of 71.8 was obtained, other conditions having remained substantially the same.

In the caustic extraction step temperature has not been found to be critical. At 65 F. substantially the same results were obtained as at 140 F. In general no increased benefits were had by increasing the amount of sodium hydroxide beyond 10% based on the dry weight of the pulp.

In carrying out the hypochlorite bleach, brightening of the pulp occurred at a pH slightly above 7, with optimum results between 10.0 and 11.5. Variation in pulp consistency in the hypochlorite bleaching stage between 5 and 10% O. D. was without noticeable effect in the result. However, the brightness of the pulp increased with the concentration of hypochlorite until 10% to 15% available chlorine on unbleached dry pulp basis was added. Further increases in the quantity of bleach added were found to cause a decrease in brightness of resulting pulp. In lieu of calcium hypochlorite other compounds yielding hypochlorite ions may be used, e. g., the alkali metal hypochlorites and other alkaline substances yielding hydroxyl ions may be used in the extraction step, such as sodium carbonate, trisodium phosphate, sodium sulfide, sodium sulfite, sodium silicate, etc., as well as the corresponding salts of the alkali metals or ammonia.

The addition of sulfur dioxide either as such or in the form of sulfurous acid, or as a bisulfite or other salt yielding sulfite ion is made in an amount greater than that needed as an antichlor, and has been found to result in a brightening of the pulp by a number of points. Various bleaching aids may be used if desired, as for example magnesium sulfate, sodium silicate, these salts being added to the bleaching bath. However, we have not thus far found them to be necessary,

excellent results having been obtained without their aid. We claim:

1. The method of hypochlorite bleaching of ground wood of the type rich in tannin and color bodies, to yield a ground wood of a brightness at least equal to that which obtains by the hypochlorite bleaching of ground spruce wood, which comprises first extracting therefrom the tannin and color bodies soluble in dilute aqueous caustic alkali solution at temperatures under the boiling point thereof, and then reacting the extracted ground wood with hypochlorite ion in alkaline solution in an amount not substantially in excess of 15% available chlorine based on the dry ground wood.

2. The method of hypochlorite bleaching of ground wood of the type rich in tannin and color bodies, to yield a ground wood of a brightness at least equal to that which obtains by the hypochlorite bleaching of ground spruce wood, which comprises first extracting therefrom the tannin and color bodies soluble in dilute aqueous caustic alkali solution at temperatures under the boiling point thereof by treating same with a dilute solution of sodium hydroxide in quantity not substantially more than ten per cent. sodium hydroxide based on the dry weight of the pulp, washing the so treated ground wood and then reacting the extracted ground wood with hypochlorite ion in alkaline solution in an amount not substantially in excess of 15% available chlorine based on the dry ground wood.

3. The method of hypochlorite bleaching of oak ground wood to yield a ground wood of a brightness at least equal to that which obtains by the hypochlorite bleaching of ground spruce wood, which comprises first extracting therefrom the tannin and color bodies soluble in dilute aqueous 9 hydroxide based on the dry weight of the pulp,

and then reacting the extracted ground wood with hypochlorite ion in alkaline solution in an amount not substantially in excess of 15% available chlorine based on the dry ground wood.

4. The method according to claim 1, in which the reaction with hypochlorite ion is carried out with a pH in excess of 10.

5. The method of hypochlorite bleaching of ground wood of the type rich in tannin and color bodies, to yield a ground wood of a brightness at least equal to that which obtains by the hypochlorite bleaching of ground spruce wood, which comprises first extracting therefrom the tannin and color bodies soluble in dilute aqueous caustic alkali solution at temperatures under the boiling point thereof, and then reacting the extracted ground wood with hypochlorite ion in an alkaline solution in an amount in excess of ten per cent. but not substantially in excess of fifteen per cent.

,10 available chlorine based on the dry ground wood.

The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 200,085 Patrick Feb. 5, 1878 223,670 Farrell Jan. 20, 1880 263,797 Mitscherlich Sept. 5, 1882 1,052,675 Loomis Feb. 11, 1913 1,203,511 Embree Oct. 31, 1916 1,539,433 Schorger May 26, 1925 1,843,467 Traquair Feb. 2, 1932 2,187,016 Craig Jan. 16, 194.0

2,359,891 Brallier Oct. 10, 1944 FOREIGN PATENTS Number Country Date 101,475 Great Britain Oct. 11, 1917 296,547 Great Britain Sept. 6, 1928 298,333 Great Britain Oct. 11, 1928 OTHER REFERENCES Bard: Efiect of Tannins Tech. Assn.

Papers, Series XXIV, 1941, pages 199-203, page 203 esp. pertinent.

Claims (1)

1. THE METHOD OF HYPOCHLORITE BLEACHING OF GROUND WOOD OF THE TYPE RICH IN TANNIN AND COLOR BODIES, TO YIELD A GROUND WOOD OF A BRIGHTNESS AT LEAST EQUAL TO THAT WHICH OBTAINS BY THE HYPOCHLORITE BLEACHING OF GROUND SPRUCE WOOD, WHICH COMPRISES FIRST EXTRACTING THEREFROM THE TANNIN AND COLOR BODIES SOLUBLE IN DILUTE AQUEOUS CAUSTIC ALKALI SOLUTION AT TEMPERATURES UNDER THE BOILING POINT THEREOF, AND THEN REACTING THE EXTRACTED GROUND WOOD WITH HYPOCHLORITE ION IN ALKALINE SOLUTION IN AN AMOUNT NOT SUBSTANTIALLY IN EXCESS OF 15% AVAILABLE CHLORINE BASED ON THE DRY GROUND WOOD.