US1768820A - Method of bleaching pulp - Google Patents

Description

No Drawing Patented July 1, 1930 UNITED STATES PATENT OFFICE mmv BRADLEY, or Mormonism, New JERSEY, AND EDWARD r. MoKEELE'E, or new YORK, 1:. Y., A'ssrenons T0 BRADLEY-MCKEEEE CORPORATION, or new Yonx,

N. Y., A CORPORATION OF NEW YORK This invention relates to an improved method of bleaching. chemical pulp procommonly effected with a solution of bleaching powder or calcium hypochlorite, but this method of bleaching, as commonly practised, not only requires a considerable amount of the bleaching agent, but may also require an elevated temperature to bring about effectivebleaching. In order to bleach the pulp satisfactorily in this way, the action of the bleaching agents upon the fibre may be sufficiently severe to injure the fibres, with production of oxidation products, e. g., oxy-cellulose, or with more or less profound modification of part-of the cellulose of the fibres, e. g., with resulting increase in beta and gamma cellulose, etc.

The improved bleaching process of the present invention is a two-stage process in which the pulp is subjected first to the action Y of chlorine or hypochlorous acid in the presence of sufficient mineral acid togive the pulp an acid reaction, andin which the bleaching operation is completed by a subsequent treatment with another bleaching solution, e. g., a solution of bleaching powder or calcium hypochlorite, or of sodium hypoohlorite, orof sodium peroxide.

'This process is particularly advantageous for treating and more specifically for bleaching those-resistanhfiflllllQsic fibrous materials which bear a non-fibrous organic content of a nature such that treatment thereof with free chlorine renders only a part theretreatment.

of readily soluble in water and leaves a relatively insoluble part thereof with, the fibres, which may, however, be readily bleached by. a suitable subsequent treatment, such as an alkaline hypochlorite bleaching the so-called sulphate special value for the b caching of pulp pro- Application filed March 116, 1922. Serial N0. 544,322.

fibres. The invention is also of special value as applied to pulp made by the cooking of wood with alkaline cooking liquors, such as pulp made by the so-called soda process or rocess. It is also of duced according to our improved process, set forth in certain prior companion applications Serial Nos. 481,146 and 481,147, filed June 28, 1921, in which the wood is cooked with a cooking liquor made up essentially of normal sodium sulphite, or of a mixture of caustic soda and sodium sulphite in varying proportions, with the caustic soda present in regulated amount such that the fibres, are not injured to any considerable degree during the cooking operation. When wood is cooked with such an alkaline cooking liquor,

and particularly in the case of certain woods,

the reaction of the cooking liquor upon the wood, with resulting conversion of a very considerable proportion of the wood into soluble compounds, sometimes leaves the fibres in such a condition that a considerable amount of bleaching powder is necessary to bleach them effectively, while an elevated temperature may also be necessary. The use of an alkaline bleach solution at an elevated temperature may injure the fibres to an ob.- jectionable degree.

According to the present invention, however, the use of an alkaline bleaching solution is preceded by an acid chlorine or hypochlorite bleaching treatment so that the pulp is subjected to an acid bleaching treatment follpwedby a different bleaching treatment.

In the claims the term an active chlorine reagent is particularly intended to include material, which is capable of furnishing chlorine in gaseous or nascent, state. Such a material may readily be formed in place when a mixture of resistant cellulpsic fibrous material, water and alkaline hypochlorite is treated with a suitable acid so as to decom pose the alkaline hypochlorite.

Chemical pulps derived from, for instance,

jack pine or loblolly pine, are impaired in ing operation is soconducted'withcooking liquor carrying a large amount of caustic 'tain'in 1 to 5 of "ous acid in this way, chlorine soda as to produce pulps readily bleached to a high w ite in a single-stage bleaching operation with the usual bleaching powder liquors. The process of the present invention enables us to produce from such woods finished pulps of high white with less im pairment.

The invention will be further illustrated by the following more detailed description of certain ap lications thereof, but it will be understoo that the invention is not limited thereto.

A pulp prepared from jack-pine or lobloll -pine, or pines of a similar character by coo ing the inewood chips with a cooking liquor containing a preponderating amount of caustic soda and a lesser amount of sodium sulphite, for example parts by weight of caustic soda and of sodium sulphite per 100 parts b of wood, (fi ured on an air dried a saturate steam pressure of 110 lbs. and a corresponding temperature for' a period of 6 hours was freed from the residual liquor and washed in the usual way. It was then diluted with sufficient water to enable it to be readil weight handled (e. g., conbres) and was then treated with chlorine by using a hy ochlorite solution (bleaching powder) an liberating the cholrine or hypochlorous acid with a mineral acid, such as sulfuric acid. Instead of producing the chlorine or hypochoras can be directly introduced and absorbe by the pulp. Only a moderate amount of chlorine is necessa for example, 16 lbs. of bleaching pow er per 100 lbs. of pulp (air dried basis) together with sufficient mineral acid to give an acid reaction. This chlorine treatment can be carried out at ordinary atmospheric tern erature and requires but a few minutes device, and the for its completion, ordinar care being taken to uniformly treat thepu p and utilize the chlorine in an economical manner.

After this preliminary acid bleaching treatment the excess liquor is removed, for example, by means of a suitable thickening ulp is preferably washed to remove a consi erable part of the dissolved organic matter that may be contained therein. The pulp is then subjected to a further bleaching operation with an alkaline hypochlorite bleach, e. g., chloride of lime or bleaching wder, although the amount of such bleac ing agent can be materially decreased from that commonly used, when a single bleaching treatment with this reagent is relied upon. The amount of hypochlorite' solution may, for example, be the equivalent of 10 lbs. of bleaching owder per 100 lbs. of pulp (air dried basisj and the bleaching can be promoted by heating the pulp to a moderate degree. \Vhen the bleaching operation is thus completed, the pulp is washed -with' a cooking liquor,

5 parts by weight asis) at to free it from the residual solution, and is then ready for further use as a bleached pulp.

The bleaching of such pine pul in the manner above described has been ound to give'a good strong white pulp which holds its color satisfactoril even upon prolonged exposure to air. W' ion the pulp is made such as a cooking liquor containing a prcponderating amount of normal sodium sulphite with a moderate amount of caustic soda, which is without seriousl objectionable action upon the.

fibres, t e resulting pulp will contain fibres of a substantially uninjured condition such that the pulp well as fibres which may be thus bleached to a satisfactory white color, without undue inury.

The improved process of the present invention makes it possible to economize in the amount of steam required for the bleaching operation and also enables a substantial saving to be effected in the amount of bleaching reagent required, and permits the production of su rior which are so difi ie rent as to justify considering them new commercial products.

The reliminary treatment of the pulp with chlorine can be effected by introducing free chlorine or chlorinated water or an acid hypochlorite solution into the pulp or producing such compound therein on its wa to the thickening device preferably of acid proof construction. The chlorineacts very rapidly even at ordinary temperatures so that its required work will be finished by the time the pul has been thickened in the thickener. T e thickening may, for example, be done in a Belmar bleacher socalled e nipped with a suitable drumwasher, or in ot er suitable thickening device. By subjecting the pulp to a preliminary chlorine or acid hypochlorite treatment in this way, a special bleaching operation is avoided, in so far as the thickenin and the removal of the residual acid bleach iquor takes place at the same time. The preliminary acid bleaching operation appears to remove certain objectionable organic matter or otherwise-dissolve or change certain of the constituents present so that the subsequent bleaching, for example, with chloride of lime, can be'earried'out at a reduced temperature and with saving in time and in steamfor heating, while giving an improved bleached pulp which is not subjected to the injurious action of a stronger alkaline bleach.

The bleached will contain strong fibres as.

bleached products,

tained from pines will be very difierent from such pulps bleached by the usual method.

According to another application of the 'kaline hypochlorite alkaline hypochlorite solution alone (e.

tion. For example,

invention, the preliminary bleaching operation can be carried out by adding chloride of lime solution or sodium hypochlorite solution, together with a limited and regulated amount of acid sufficient to give the pulp an acid reaction and to liberate chlorine or. hypochlorous acid, and, after the first bleaching treatment, a predetermined amount of alkali can be added sufiicient to give the pulp an alkaline reaction together with enough additional alkaline hypochlorite (e. g., bleaching powder or sodium hypochlorite solution) to finish the bleaching operation, the pulp being moderately heated during the second alkaline bleaching treatment.

We consider it more advantageous, however, to subject the pulp to a washing operation after the preliminary acid bleaching treatment and before the subsequent alkaline bleaching treatment, in order to wash out certain objectionable organic matters from the pul and free it from the excess of acid, and 0 compounds dissolved by the acid bleaching liquor before adding the alfor the second bleaching treatment.

Pulp which is diificult to bleach with an a solution of bleaching powder) can nevertheless be readily bleached by the improved two-stage treatment of the present invenwe have found that kraft paper, produced from spruce by the sulphate process so-called and which was difiicult to bleach with bleaching powder alone, could be readily'bleached to a strong white pulp by the present process. 'lVe have also found that pulp from Canada jack-pine, Minnesota jack-pine, Virginia loblolly-pine, yellow pine, etc. (produced by means of a cooking liquor made up essentially of a limited amountof caustic soda and a lesser amount of sodium sulphite) couldbe readily bleached by our new method to a beautiful strong bleached pulp with a moderate amount of bleaching agent. I

The second stage of-the two-stage bleaching process can be carried out at ordinary temperature if a somewhat increased timeis permitted for the bleaching operation, especially with pulp such as poplar pulp made by the soda process-and spruce pulp made by the so-called (acid) sulphite process, although it is better to warm the liquor to a moderate degree. Bleached pulps of excellent white color can be thus obtained, with relatively decreased injury to the pulp in that the process avoids the severe treatment with alkaline hypochlorites at an excess temperature such as tends to oxidize the pulp with resulting production of oxycellulose or to modify the pulp with a resulting production of high amounts of beta and gammacellulose, etc. The second stage of the bleaching process can be carried out by the addition of sodiumperborate, sodium peroxide or by permanganates, for example, by sodium or potassium permanganate, using either-an alkaline solution or one which is made acid by the addition of a suitable acid in sufficient amount. When an alkalinepermanganate solution is emplo ed the pulp preferably should be warme some what to facilitate or expedite the bleachin o eration. When an acid solution is use t e bleaching takes place readily at ordinary temperatures although a moderate heating may be employed. The manganese compound deposited on the fibres can be readily removed with a solution containing sodium sulphite, or bisulphite, and sulphuric acid, or by asolution of sulphurous acid, and the pulp can then be freed from dissolved reagents by washing.

The following example shows another application of the invention:

Short leaf pine was cooked with a cooking a liquor containing caustic soda and sodium su phite for a period of about four hours, the amount of caustic soda being about 15 parts by weight and the amount of the sodium sulphite 25 parts by weight in 450 parts-of liquor used for treating parts by weight of bone dry wood. The pulp obtained was of excellent quality, with long and strong fibres, but it did not bleach readily to a white pulp with a calcium hypochlorite solution. It was readily bleached, however, when treated with chlorine in the presence of water at ordinar room temperature for about 15 minutes, t en washed and treated with calcium hypochlorite (bleachin powder) solution andmoderately warme The resulting pulp was a ood strong white pulp. Cotton linters, blac r gum, red gum, tupelo gum, chestnut, white oak, black oak, red oak, hickory, birch, beech, maple, bamboo, jute and other pulps have been bleached by our new process and in each instance beautr ful white and strong pulps were obtained in an economical and practical manner.

It will thus beseen that the present invention comprises a two-stage bleaching process in which the firststage is an acid bleaching operation with chlorine or hypochlorite bleaching liquor andin which the acid bleach liquor is preferably and advantageously re moved, for the most part, from the pulp before the second stage of the process. The preliminary stage can be carried out with.

the use of chlorine gas added to the pulp or ried out with sodium perborate, sodium peroxideor with sodium or potassium permanganate and with or without sulphuric acid with subsequent treatment of the pulp with sodium bisulphite and sulphuric acid, or a solution of sulphurous acid, followed by a washing operation.

We have found that the followingmcthod of applying the process gives good results. After washing the pulp to remove substantially all residual liquor resulting from the cooking of the wood chips, bamboo, jute, cotton linters, etci, the pulp is treated with chlorine gas while being transferred to a washing or pul thickening device. The pulp may be sub ccted to chlorine gas in any suitable manner, e. g. by adding enough water to the ulp so that it may be readily handled and then pumping the thinned pulp to the top of a ballied tower (preferably acidproof) allowing the pulp to pass downward through the tower and come into contact with chlorine gas introducedpreferably near the bottom of the tower. The major portion of the chlorine may thus be absorbed by the pulp. The chlorine which escapes from near the top of the tower may then be conducted to a tower in which milk of lime is circulated so as to absorb this portion of the chlorine which escaped the first tower. In this manner, by regulating the flow and amount of pulp and of chlorine, the desired amount of chlorine can be absorbed by the )ulp in one tower and at the same time pro uce a suitable amount of alkaline h pochlorite bleaching liquor suitable for t e second stage of the bleaching process. Sodium carbonate solution may be used in the second tower instead of milk of lime. The treated pulp may be further thinned with water if desired and thensent to a thickener where the excess liquor is removed, followed by the second stage treatment. Instead of passing the pulp through a tower, it may be treated with chlorine gas without thinning by agitating the pulp while the chlorine gas is being introduced, passing the escapin chlorine into milk of lime or a solution of sodium carbonate. The treated pulp should be washed before the second stage treatment.

We have used the term alkaline hypochlorite in this specification and the accompanying claims as including hypochlorites of alkali metals such as sodium hypochlorite and the hypochlorites of alkaline earth metals such as calcium hypochlorite-or chloride of lime.

We claim:

1. The method of bleaching chemical wood pulp which comprises subjecting the pulp to a preliminary treatment with an acid chlorine-containing bleach liquor and subsequently, without intermediate high temperature cooking of the pulp, completing the an acid chlorine-containin bleach liquor and subsequently treating t e pulp with an alkaline hypochlorite solution.

4. The method of bleachingpulp containing cellulosic fibres which comprises subject ing the pulp first to an acid hypochlorite bleach solution and subsequently to an alka line hypochlorite bleach solution.

5. The method of bleachin pulp which.

comprises subjecting the pulp first to an acid by )ochlorite bleach solution, freeing the pulp from a considerable part of the solution and subsequently treating the pulp with an alkaline hypochlorite solution.

6. The method of bleaching chemical wood pulp which comprises subjecting the pulp first to an acid bleach liquor and later, with= out intermediate cooking of the pulp, to an alkaline oxidizing bleach liquor.

7. The method of bleaching pulp resulting from the cooking of wood with a nonacid cooking liquor containing monosulfite,

which comprises subjecting the pulp to a two-stage bleaching operation, the first stage being with an acid chlorine-containing bleach liquor and thereafter subjecting the resulting fibrous product to a bleaching treatment with an oxidizing bleach liquor.

8. The method of bleaching pulp resulting from the cooking of wood with a nonacid cooking liquor containing monosulfite, which comprises subjecting the pulp to'a two-stage bleaching operation, the first stage being with an acid chlorine-containing bleach liquor, and thereafter subjecting the resulting fibrous product to a bleachin treatment with an alkaline oxidizing bleac liquor.

9. The method of bleaching pine pulp produced by the cooking of pine wood with a non-acid cooking liquor containing sodium monosulfite, which comprises subjecting the puip to a preliminary bleaching treatment with an acid chlorine-containing bleach liquor and subsequently completing the bleaching operation with a bleach liquor of an alkaline character.

10. As an article of manufacture, pulp from wood fibers not easily bleached, said pulp being characterized by the strength and high white color of the bleached fibers, and having been bleached by subjecting the pulp to a preliminary treatment with an acid the mixture acid, with an amount of an acchlorine-containing bleach liquor and subsequently to an alkaline bleach liquor of an oxidizing character, the fibers of the pulp being substantially uninjured.

11. As an article of manufacture, a pulp consisting essentially of pine fibers of high strength and high white color, said fibers having been bleached by subjection to a preliminary treatment with an acid chlorinecontaining bleach liquor followed by treatment with an alkaline bleach liquor of an oxidizing character, substantially as described.

12. As an articleef manufacture, a pulp consisting essentially of pine fibers of high strength and high white color, said fibers resulting from the cooking of pine wood with a non-acid cooking liquor containing a tive chlorine reagent sufiicient to render the resulting fibrous material readily bleachable to a high white by a bleaching liquor consisting essentially of a solution of a hypochlorite such bleaching liquor containing an amount of available base sufficient to give it a non-acid reaction, and thereafter subjecting the resulting fibrous material to a bleaching treatment with an oxidizing bleaching liquor.

14. The method of treatin unbleached cellulosic fibrousmaterial, w 'ch includes treating the fibrous material, in the presence of an acid, with an amount of an active chlorine reagent sufiicient to render the resulting fibrous material readily bleachable to a high white by a bleaching liquor consisting essentially of a solution of a hypochlorite such bleaching liquor containing an amount of available base sufficient to give it a non-acid reaction, and thereafter subjeotin the resulting fibrous material to a bleaching treatment with an oxidizing bleaching liquor including an amount of available base sulficient both to neutralize such uncombined acid as may then be present and to facilitate bleaching of the resulting fibrous materialto a high white.

15. The method of treating unbleached cellulosic fibrous material, which includes treating the fibrous material, in the presence of a sufficient amount of suitable acid to maintain an acid reaction throughout the treatment, with an amount of an active chlorine reagent adapted to render the re sulting fibrous material readily bleachable to a high white by a bleaching-liquor consisting essentially of a solution of a hypochlorite such bleaching liquor containing an amount of available basesufiicient to give it anon-acid reaction, and thereafter subjecting the resulting fibrous material to a bleaching treatment with amoxidizing bleachin liquor containing an amount of available base sufficient to effect an alkaline condition in the bleach liquor during the latter treatment.

16. The method of treating unbleached cellulosic fibrous material, which includes treating the fibrous material with an active chlorine reagent to renderthe resulting fibrous material readily bleachable by an alkaline bleaching liquor, thereby producing a product including uneombined mineral acid, and thereafter subjecting the resulting fibrous material to a bleaching treatment with an alkaline oxidizing bleaching liquor.

17. The method of treating unbleached cellulosic fibrous material, which includes treating the material with an amount of an active chlorine reagent sufficient to render the resulting fibrous material bleachable by an alkaline oxidizing bleaching liquor in the presence of water and an amount of mineral acid which will the pulp andtherea er bleaching the resulting fibrous material with an alkaline oxidizing bleaching liquor.

ive an acid reaction to:

18. The method of treating unbleached cellulosic fibrous material, which includes treating the material with an amount of an active chlorine reagent sufficient to render the resultin fibrous material bleachable by an alkaline bleaching liquor in the presence of water and an amount of mineral acid which will give an acid reaction to the nip and thereafter bleaching the resu ting fibrous material with. an oxidizing bleaching liquor including an amount of available base suflicient both to facilitate bleaching of the fibrous material to a high white and to neutralize such uncombined acid as may then he present.

19. The method of treatin unbleached cellulosic fibrous material, w 'ch includes treating the material with an amount of an active chlorine reagent and. an amount of mineral acid sufiicient to render the resulting fibrous material readily bleachable by analkaline oxidizing bleachin liquor, and thereafter bleaching the resu ting fibrous material by means of an oxidizing bleaching material.

20. The method of treati unbleached cellulosic fibrous material, WllCll includes treating the material, in the presence of Water and a mineral acid, with such an amount of an active chlorine reagent as is required to convert part of the organic constituents into compounds readily soluble in water and a substantial part of the organic into intimate contact with an active chlorine reagent in acid solution, under such conditions as to convert part of the nonfibrous organic compounds into constituents readily soluble in water and to convert a substantial part of the non-fibrous organic compounds into chemically altered compounds which are not readily soluble in water but which are readily removable by an alkaline oxidizing bleaching treatment, and thereafter bleaching the resulting fibrous material with an alkaline bleaching liquor.

22. The method of bleaching cellulosic fibrous material to a high white, which includes mixing with the pulp a sufficient amount of alkaline hypochlorite in solution and thereafter adding a sufiicient amount of acid to render the liquor acid, and mixing the ingredients so as to promote intimate contact between the fibres and the active chlorine material, thereby preparing the fibrous material for the later bleaching treatment, and thereafter bleachin the prepared fibrous material to a high white by means of an alkaline bleachin liquor.

23. The method of leaching cellulosic ulp to a high white, which includes preparmg a mixture of such pulp and water, incorporating with such mixture a regulated amount of water-soluble hypochlorite, rendering the aqueous mixture sufficiently acid to decompose any alkaline hy ochlorite then present, thereby obtaining a brous product which includes some non-fibrous constituents not readily soluble in water, separating the fibrous material from part of the resulting liquor, and thereafter bleaching the pulp to a high white by means of an oxidizing bleaching liquor.

24. The method of bleaching cellulosic pulp to a high white, which includes preparing a mixture of such pulp and water, incorporating with such mixture a regulated amount of hypochlorite of an alkaline earth metal, rendering the aqueous mixture sufliciently acid to decompose such a hypochlorite, thereby obtaining a prepared fi brous product, separating the fibrous material from part of the resulting liquor, and thereafter bleaching the pulp to a high white with a bleach liquor including a compound capable of oxidizing colored nonfibrous organic constituents then present with the fibrous material.

25. The method of producing bleached pulp of high white from pulps, derived from coniferous woods,- which are difiicult to bleach to a high white by single-stage treatment with usual bleaching owder liquors, which includes treating sue pulp w1th a hypochlorite bleaching liquor under conditions to convert into compounds readily soluble in water a substantial portion of the non-fibrous organic constituentsand to leave with the resulting fibrous product some nonfibrous constituents which are not readily soluble in the liquor then present, separating the resulting fibrous material from a part of the liquor, and thereafter bleaching the'fibrous material to a high white by treatment with an oxidizing bleaching liquor.

26. The cellulosic fi rous material, which includes treating such fibrous material in aqueous suspension with a regulated amount of a bleaching solution containing an alkaline h pochlorite, insufiicient to complete the b caching operation, adding a suitable acid to decompose some of the then remaining alkaline hypochlorite, and subsequently treating the resulting fibrous material wit an oxidizing bleaching liquor.

27. The process of treatin unbleached cellulosic fibrous material, wiiich includes treating such fibrous material in aqueous suspension with a regulated amount of a bleaching solution containing an alkaline hypochlorite, insufiicient to complete the bleaching operation, rendering the solution acid while it contains some unconsumed bleach. separating the resulting fibrous material from a portion of the resulting liquor, and thereafter bleaching the resulting fibrous material by treatment with a bleaching liquor including a hypochlorite.

28. The multiple-step process of producing a high yield of high white fibre from wood, which includes mildly digesting the wood and thus producing a pulp containing rocess of treating unbleached fibres encumbered with non-fibrous organic matter, subsequently treating the fibres with a mild liquor including chlorine in such a chemical condition as to alter encumbering matter, thereby obtaining an insufficiently bleached pulp, and subsequently bleaching the fibres to a high white with an oxidizing bleach liquor. 29. The multiple-step process of producing high white fibre, which includes mildly digesting wood and thus producing a pulp containing fibres encumbered with nonfibrous organic matter, subsequently treating the fibres with a liquor including a hypochlorite so as to alter encumbering organic matter, thereb obtaining a pulp which is only partially bleached and separating some of the resulting liquor from the partially bleached fibrous material, and subsequently bleaching the fibres to a high white with an alkaline bleaching 1i uor containing a reagent capable of solubilizing colored encumterial to a partial bleaching treatment by means of bleaching liquor including a hypochlorite, removing bleach residues from the partially bleached fibrous material, and subsequently bleaching the fibrous material to a high white b means of alkaline bleaching liquor supplie rite, at least one half of the total amount of hypochlorite supplied to the fibrous material being utilized in the partial bleaching treatment.

31. The process of obtaining high white chemical pulp, comprised essentially of strong fibres, which comprises subjecting long fibered wood to a mild digesting treatment b means of a digestive liquor containing so ium hydroxide and an alkaline sulphur bearing compound of sodium as its principal digesting a ents, thereby obtaining strong unbleached c emical pulp, washing such pulp, subjectin the washed unbleached pulp to a partial leaching treatment by means of bleaching liquor supplied with a,

hypochlorite, removing bleach residues from the partially bleached pulp, and subsequently bleaching the pulp to a high white by means ofdistinctly alkaline bleaching liquor supplied with an alkaline hypochlorite, at least one half of the total hypochlorite being utilized in the partial bleaching treatment.

32. As a new product, bleached chemical wood pulp containing fibres, such product having been obtained by a process comprising the following steps :-(a) treating wood so as to produce unbleached chemical wood pulp; (b) washing the unbleached chemical wood pulp; (a) partially bleaching the chemical wood ulp by means of bleaching material inclu ing a chlorine-containing bleaching agent; (d) separatingliquor including bleach residues from the vpartiallybleached chemical wood pulp; (e) subjecting the partially-bleached chemical wood pulp to a regulated bleaching treatment by'means of an alkaline liquor; (f) washing the bleached chemical wood pulp.

'33. As a new product, bleached chemical wood pulp as described in claim 32, such pulp having good strength and being, of a high white color and which substantially holds its color even upon prolonged exposure to air.

34. As a new product, bleached chemical wood pulp as described in claim 32, such with an alkaline hypochlopulp being of excellent white color and bein further characterized by the relatively small amounts of oxy-cellulose and the absence of high amounts of beta and gamma celluloses.

35. As a new product, bleached chemical wood pulp containing fibres characterized CF presence 0? by high White color, such product having been obtained by a process comprising the following steps:-(a) digesting wood by means of digestive matter including a sulphur-bearing digestant under conditions regulated so as to obtain unbleached chemical wood pulp=without undue injury to the p fibres thereof; (b) washing the unbleached chemical wood pulp; (c) admixing a liquor including an alkaline h ochloritewith the unbleached chemical woo pulp and partially bleaching the same by means of bleaching material including, a chlorine-containing bleaching reagent;(d) se arating liquor in-.

cluding organic matter rom the partiallybleached chemical wood ulp; (e) bleaching the partially-bleached c emical wood pulp to a high white by means of a bleaching liquor supplied with an alkaline hypochlorite; (f) washing the bleached chemical wood.

pulp.

36. As .a new product, bleached chemical wood pulp as described in claim 35, the step (a) of the process described in claim 35 including a digesting treatment of the wood by means of a sulphite.

37. As a new product, bleached chemical wood pulp as described in claim 35, the step (a) of the process described therein including a digesting treatment of the wood by means of digestive matter including a normal sulphite.

38. As a new product, bleached chemical wood pulp as described in claim 35, the bleaching liquor mentioned in bleaching step (6) being maintained distinctly alkaline during such bleaching step.

In testimony whereof we afiix our signa-- tures.

LINN BRADLEY. EDWARD P. McKEEFE.