US1768821A - Method of bleaching - Google Patents

Description

Patented July 1, 1930 UNITED STATES PATENT OFFICE LINN BRADLEY, OF MONTCLAIR, NEW JERSEY, AND EDWARD P. MGKEEFE, 0F PLATES- BURG, NEW YORK, ASSIGNOLRS TO BRADLEY-MCKEEFE CORPORATION, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK METHOD OF BLEACHING No Drawing.

This invention relates to improvements in the bleaching and purification of fibrous materials, fabrics or other materials, and especially cellulose from wood for use in the manufacture of pulp and paper products, etc. More particularly the invention relates to the art of bleaching or purifying fibrous materials -in which the bleaching material is added to the fibrous material in successive portions and particularly to such a process in which a reactive chlorine-containing compound is used as a bleaching agent.

This application is a continuation in part of our co-pending applications, and particularly of our U. S. application Serial No. 449,245, filed March 2, 1921, and our U. S. application Serial No. 544,322, filed March 16, 1922.

In heretofore known methods of bleaching with chlorine or chlorine-containing compounds, the bleaching agent, such, for example, as a hypochlorite or bleaching powder, has been used upon the cellulosic material in a single stage. In the treatment of unbleached wood pulp, particularly where the cooking treatment has not removed all non-fibrous matter, as in the case of socalled kraft pulp, the treatment by heretofore known bleaching processes is either insufiicient to produce a good and strong white pulp or it may be so drastic that the cellulosic fibers are weakened and oxidized to a commercially prohibitive extent with a consequent reduction of the yield and quality of the product.

The improved bleaching process of the present invention is a multiple-stage process in which the pulp is subjected in the first stage to the action of a reactive chlorine-containing compound with or Without other reactive bleaching agents, the total amount of reagent or reagents used in the first stage usually being insufficient to complete the bleaching treatment, and in a subsequent stage the bleaching of the pulp is completed by a subsequent treatment with the same and/or different kind of reactive material which may include chlorine-containing compound or compounds.

Application filed March 14, 1928. Serial No. 281,396.

According to the present invention, we avoid certain of the objections to bleaching pulps with the usual chlorine-containing compounds and we are enabled to produce a high white cellulose product of good strength, thus making itsuitable for many purposes for which cellulose bleached by former methods has been found less satisfactory. According to the present invention we employ in the bleaching operation a reactive chlorine-containing compound either alone or in the presence of other suitable compounds. \Ve have found that if such agents or reagents are added in successive stages that an economy in the amount of bleaching reagents may be effectedand a superior grade and yield of pulp may be obtained, particularly when the partially bleached material resulting from the firststage is separated from resulting bleach residues, etc.

The first stage may be carried out by adding a portion of the bleaching material, for example, as a bleaching liquor, and per mitting it to exert its bleaching action upon the fibrous material, such as wood pulp, then removing from the partially bleached fibrous material, soluble compounds resulting from this bleaching operation, by dewatering and/washing, or otherwise. Another portion of the same or another kind of bleaching agent may be used on this fibrous material in subsequent stages.

lVhen only a small or moderate amount of bleaching material is used in such first or preliminary partial bleaching treatment, there appear to be produced compounds which are water-soluble and may be colorless and some of which are not fully oxidized. These compounds can be substantially removed from the fibrous material by the washing or dewatering operation, or otherwise, so that a reduced amount will be present with the fibrous material during the subsequent treatment.

The invention will be further illustrated by the following more detailed description of certain applications thereof, but it will be understood that the invention is not limited thereto,

The improved process of the present invention is of more or less general application. It is particularly advantageous for bleaching chemical pulp from wood that is dillicult to bleach to a high white by a single stage treatment with ordinary bleaching powder or calcium hypochlorite liquor, without commercially objectionable injury to the fibers. The process is of special value as applied to chemical pulp produced by cooking wood with alkaline cooking liquors, such as pulp made by'the so-called soda process or the so-callcd sulfate process or modifications thereof. It is also of special value for tlieblcaching of chemical pulp produced according to our improved processes, set forth in our prior companion applications, Serial No. 434,720, filed January 3, 1921, No. 481,147, filed March 28, 1921, and our U. S. Patent #1,651,665, in which wood is cooked with a cooking liquor made u essentially of normal sodium sultite, or o a mixture of reagents such as caustic soda, sodium sulfide, and/or sodium thiosulfate, and sodium sulfite in varying proportions, with the caustic soda either absent or present in regulated amount such that the fibers are not seriously injured during the cooking operation. \Vhen certain woods, such as pines, are cooked with a cooking liquor so as to produce strong chemical pulp, the reaction of the cooking liquor upon the wood, with resulting conversion of a considerable portion of the wood into soluble compounds,

-= sometimes leaves the unbleached fibrous material in such a condition that a considerable amount of reagent, such as bleaching powder, is necessary to bleach them effectively, while an elevated temperature may also be necessary. The use of a strong alkaline hypochlorite solution upon such material at an elevated temperature may injure the fibers to an objectionable degree.

Pulp may be prepared from jack-pine or loblolly-pine, or pines of a similar character, by cooking the pine wood chips with a cooking liquor containing a preponderating amount of caustic soda and a lesser amount of sodium sulfide, for example, 15 parts by weight of caustic soda and 5 parts by weight of sodium sulfide per 100 partsby weight of wood (figured on bone-dry basis) at a temperature corresponding to saturated steam pressure of 110 lbs., for a period of 6 hours. The resulting pul may be freed from the residual liquor an washed in the usual way. It may be then diluted with sufficient cool water to enable it to be readily handled (e. g, containing 1 to 5 of fibers) and therea ter treated with chlorine by adding a hypochlorite solution (bleaching powder) and liberating chlorine or hypo chlorous acid therefrom b means of added mineral acid, such as by rochloric acid or sulfuric acid. A.moderate amount of chlorine may be thus used, for example, 5 to 10 pounds (more or less) of bleaching powder per 100 pounds of pulp (air-dry basis) together with sutlicient mineral acid to give a suitable acid reaction. Instead of produc ing chlorine or hypochlorous acid in this way, chlorine gas can be directly introduced into the mixture of fibers and water and absorbed by the pulp. This chlorine treatment can be carried out at ordinary atmospheric temperature and requires but a few minutes for its completion, care being taken to uniformly treat the pulp and utilize the chlorine in an economical manner.

After this preliminary acid bleaching treatment, excess liquor is advantageously removed, for example, by means of a suitable pulp-thickening device, and the pulp is preferably washed to remove a considerable part of such dissolved organic matter as may be contained therein. The pulp is then subjected to a further bleaching operation with an alkaline hypochlorite bleach, e. g., using chloride of lime or bleaching powder liquor. The amount of such bleaching agent required for completing the bleaching treatment can usually be materially decreased from that commonly used, when a single-stage bleaching treatment with this reagent is employed. The amount of hypochlorite solution used in such second-stage may, for example, be the equivalent of 10 pounds (more or less) of bleaching powder per 100 pounds of pulp (air-dry basis) and the bleaching can be promoted by heating the'admixed ulp and bleaching liquor to a moderate .dizgree. hen the bleaching operation is thus completed, the pulp may be washed to free it from the residual solution. It is then ready for further use as a bleached pulp.

The bleaching of such pine pulp in the manner above described has been found to provide a good strong white pulp which holds its color satisfactorily even upon prolonged exposure to air. )Vhen the pulp is made with a cooking liquor, such a cooking liquor containing a preponderating amount of normal sodium sulfite, with or without a moderate amount of other digesting agent, such as, for example, caustic soda, which is without seriously objectionable action upon the fibers, the resulting pulp will contain strong fibers which may be thus bleached to a satisfactory white color, without undue injury.

This process is particularly advantageous for bleaching resistant cellulosic fibrous materials which bear a non-fibrous organic content of a nature such that treatment thereof with free chlorine (in the presence of water) renders only a part thereof readily soluble in water and leaves a relatively insoluble part thereof with the fibers, which may, however, be readily oxidized or removed by a suitable subsequent treatment, such as an alkaline hypochlorite bleaching treatment. An active chlorine-containing reagent which is capable of oxidizing some organic constituents and of chlorinating some organic constituents may readily be formed in place when a mixture of unbleached cellulosic fibrous material, water and alkaline hypochlorite is treated with a suitable acid, so as to decompose the alkaline hypochlorite and render the liquor acid to litmus.

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 reduction to be effected in the amount of bleaching reagent required and permits the production of superior bleached products in case of certain pulps, such as those produced from pines, by the above-mentioned sodium sulfite type of digesting liquor. Such bleached pulps are so different from customary pulps as to justify considering. them new commercial products.

f The preliminary treatment of the pulp with chlorine can be effected by introducing free chlorine or chlorinated water or an acid hypochlorite solution into a thin pulp, or by producing such compound therein. The resulting pulp may then be delivered to a thickening device, which isadvantageously of acid-resistant construction, although the liquor may be rendered non-acid after the chlorine has been substantially fully used up and before it reaches the thickening device. Soda or lime may be used for fixing any remaining free acid; The chlorine acts upon unbleached pulp very rapidly even at ordinary temperatures so that its required work (and any subsequent treatment with soda .or lime) can be finished before the chlorine-treated pulp has reached the thickener. The thickening may, for example, be done in a Belmar bleacher (so-called) equipped with a suitable drumwasher, or in other suitable thickening device. By subjecting the pulp to a preliminary chlorine or acid hypochlorite treatment in this way, a special bleaching operation may be avoided, in so far as the thickening and the re moval of the residual acid or non-acid bleach liquor takes place at the same time. The preliminary bleaching treatment and liquor removal appear to alter or remove certain resistant organic matter present so that the subsequent bleaching, for example, with chloride of lime liquor, can be carried out at afnoderate temperature, thereby permitting a saving in steam for heating, while giving an improved bleached pulp which has not been subjected to the injurious action of a stronger alkaline bleach liquor at high temperatures. The bleached pulp accordingly may have an improved color as well as improved properties in other respects, and the bleached pulp obtained, 0. g., from pines, is usually very different from such pulps bleached by the usual single-stage bleaching method with bleaching powder liquor.

According to another application of the invention, the preliminary bleaching operationcan be carried out by adding to and admixing with the pulp chloride of lime solution or sodium hypochlorite solution in amount sufficient to bleach a substantial amount of the pulp but advantageously less than enough for complete bleaching, and thereafter adding and admixing a limited and regulated amount of acid sutlicient to give the pulp a mildly acid reaction and to liberate chlorine or hypochlorous acid, and, after the first bleaching treatment, a predetermined amount of alkali (such as lime or soda) can be added in amount sufficient to give the pulp a mildly alkaline reaction, and also adding enough additional alkaline hypochlorite (e. g., bleaching powder or sodium hypochlorite solution) to finish the bleaching operation, and then moderately heating the pulp and liquor during the latter bleaching treatment.

We consider it more advantageous, however, to subject the pulp to a dewatering and washing operation after the first-stage bleaching treatment and before the subsequent alkaline bleaching treatment, in order to remove certain objectionable organic matters from the pulp and to free it from any excess of acid before adding the hypochlorite liquor for the second bleaching treatment.

Pulp which is difficult to bleach with an alkaline hypochlorite in a single stage (e. g., a solution of bleaching powder) can nevertheless be readily bleached by the improved multiple-stage treatment of the present invention. For example, we have found that kraft pulp, produced from a spruce or pine by the sulfate process (socalled) and which was difficult to bleach to a high white in the ordinary manner with bleaching powder liquor, could be readily bleached to a strong white pulp by the pres ent process. YVehave also found that pulp from Canada jack-pine, Minnesota jackpine, Virginia loblolly pine, yellow pine, etc., (produced by means of a cooking liquor comprising a limited amount oflcaustic soda and lesser amounts of sodium sulfide and sodium sulfite) could be readily bleached by our new method to a high white strong pulp by means of a moderate amount of reactive chlorine'containing bleaching agent.

The latter stage of the multiple-stage bleaching process can be carried out at ordinary temperature if a somewhat increased time is permitted for the bleaching operation, especially with pulp such as poplar pulp made by the soda process and spruce pulp made by the so-ealled (acid) sulfite process, although it is usually better to warm the pulp and admixed liquor to a moderate degree. Bleachedpulpsof excel lent white color-ca'nbejthus obtained, with relatively decreased injury to the pulp, in that the process avoids the severe treatment with strong alkaline hyp'ochlorite liquors at relatively high temperature such as tends to oxidize the pulp'with'rcsulting production of oxy-cellulose or to modify the pulp with resulting production of high amounts of beta. and gamma cellulose, etc. The latter stage of the bleaching processv can be carried out by the addition of sodium perborate, sodium peroxide or by permanganatcs, 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 alkaline permanganate solution is employed ,the pulp preferably.

should be warmed somewhat to facilitate or expedite the bleaching operation. When an lacid solution is used, the bleaching takes place readily at ordinary temperatures although a moderate heating may be cmployed. The manganese compound depos- Zlo ited on the fibers can be readily removed with a solution supplied with sodium sulfite, or bisulfite, and sulfuric acid, or by a solution of sulfurous acid, and the pulp can then be freed from dissolved reagents by as washing.

, Instead of using chlorine or an active chlorine-containing reagent ,alone in the first step, composite bleaching liquors may be employed which containa manganate or permanganate, in addition to such active chlorine-containing reagent. For example, a liquor containing from about'one-fourth part to about three parts, more or less, of sodium permanganate or its equivalent, and

about one-fourth to fifteen or twenty parts of sodium hypochlorite or its chemical equivalent, such amounts of bleaching reagents being based on the air-dry weight of the pulp, such as chemical wood pulp. The

particular ratios of such reagents within the limits named above may be chosen in each case according to the desired degree of bleaching and the material which is to be bleached. A portion of such liquor,-ordinarily about one-half, more or less, may be added to the pul for the first stage of the process and is le t in contact with the pulp until substantially all of the bleaching reagentshave been used up. The spent liquor is then separated from the pulpand the latter preferably washed to remove soluble oxidizable compounds. The remainder of the bleach liquor is then added to the fibrous material and the bleaching carried to the necessary or desired degree. The

hypochlorite in such bleach liquor appears to act in a somewhat different and is supplemented by the action of the hypochlorite so that a lesser amount of permanganate need be used than if it-were used alone. Then a two-stage bleaching treatment is carried out with such composite liquors, they may be either acid or alkaline, but that used in thcggecond stage is advantageously alkaline.

Instead of using the composite liquor in both stages, the first stage may, for example, be conducted with chlorine or a hypochlorite, to bring about a partial bleaching of the material and they bleaching operation can thenbe completed after adding a manganate or permanganate, and to the extent that residual chlorine or active chlorine-containing compounds are present with the pulp when the manganate or permanganate is added, the further bleaching operation may partake of somewhat the same character as 'when a composite bleaching liquor containing permanganate and hypochlorite is used; or, if desired, a composite liquor as described above may be added for the secondstage treatment, after a preliminary treatment wlth actlve chlorine-containing compounds; and in some cases the order may beinverted so that the composite liquor is used in the preliminary treatment and a chlorinecontaining liquor without manganese compounds is added for the final treatment. In suitable cases, the pulp resulting from the first-stage treatment may be dewatered and washed before the next bleaching stage is carried out.

The composite liquors, as described above, may be prepared by chlorinating liquor dontaining a 'mang'anate and an excess of alkali or metallic oxide or hydroxide in the manner described in our co-pending application Serial No. 449,245.

The following example shows another ap lication of the invention;

hort leaf pine was cooked with a cooking liquor containing caustic soda and sodium sulfite for a period of about four hours, the amount of caustic soda being about fifteen parts by weight and the amount of the sodium sulfite 25 arts by weight in 450 parts of liquor used or treating 90-parts by weight of wood (calculated on the bone-dry basis). The pulp obtained was of excellent quality, with long and strong fibers, but it did not bleach readily to a white pulp with a calcium hypochlorite solution used in theordinary manner. It was readily bleached, however, when at least semi-bleached with chlorine in the presence of water at ordinary room temperature for about 15 minutes, then washed and treated with a moderate excess of mildly alkaline calcium hypochlorite (bleaching powder) solution and moderately warmed. Thle resulting pulp was a good strong White Cotton linters, black 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 high .white and strong pulps were obtained.

ond bleaching stage of the process is commenced. The second bleaching stage of the process may be carried out either with the same type or with a different type of bleaching liquor from that which is used in the first stage and may be carried out either at ordinary temperature for a suflicient time or advantageously with moderate warming of the solution. The final bleaching operation can also be carried out with sodium perborate, sodium peroxide, or sodium or potassium permanganate (with or without sulfuric acid) with subsequent treatment of the pulp with sodium bisulfite and sulfuric acid, or a solutionof sulfurious acid followed by a washing operation.

, We have found that the following method of applying the process gives good results. After washing the unbleached pulp to remove substantially all the residual liquor resulting from the cooking of the wood chips, bamboo, jute, cotton linters, etc., the pulp is treated with chlorine gas while being transferred to a washing "or pul thickcning device. The pulp may be sub ected to chlorine gas in any suitable manner, e. g., by adding enough water to the pulp so that it may be readily handled and then pumping the thinned pulp to the top of a baflied tower (preferably acid-resistant) allowing the pulp to pass downward thru the tower and come into contact with chlorine gas introduced preferably near the bottom of the tower, as more fully set forth in our co-pending application, Serial No. 47 0,961, filed May 19, 1921. The major portion of the chlo- L. rine may thus be absorbed by the pulp.

Any chlorine escaping 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 from 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 pulp in one tower andat the same time produce a suitable amount of alkaline hypochlorite bleaching liquor suitable for the second stage of the bleaching process. Sodium hydroxide solution may be used in the second tower instead of milk of lime. The chlorinetreated pulp may be further thinned with water, if desired, and then sent to a thickener (with or without treatment with lime or soda) where the excess liquor is removed. This may then be followed by the secondstage treatment. Instead of passing the pulp thru a tower, it may be treated with chlorine gas with or without thinning by agitating the pulp while the chlorine gas is being introduced, passing any escaping chlorine into milk of lime or a solution of sodium hydroxide. The treated pulp should be dewatered or washed before the second-stage treatment.

Although a number of the examples given above have included an acid treatment in one of the stages of the bleaching process, the invention is in no way limited thereto and in some cases, particularly where the use of acid-resisting apparatus may be undesirable,

both stages of the process may be performed with non-acid bleaching liquors, such, for example, as alkali-metal or alkaline earth metal hypochlorite, or composite liquors containing hypochloritcs as described above.

The following is an example of one manner in which the bleaching operation may be performed with non-acid bleaching liquors. The well-washed and unbleached chemical pulp, e. g., spruce pulp obtained by a sulfate process or an acid-sulfite process, composedof around three to seven pounds of fiber to around 97 to 93 pounds of water, more or less, may be treated in the first stage with a bleaching powder liquor prepared by dissolving calcium hypochlorite in cool water. Such bleaching liquor may advantageously have some calcium hydroxide in solution and even a small amount in suspension in it so as to render it moderately alkaline. The amount of bleaching material added to the pulp for the first-stage treatment may vary somewhat. Thus an amount of bleaching liquor which contains around three to fifteen parts, more or less, of bleaching powder (35% available chlorine) for each 100 parts of fibers, by weight, (air-dry basis) may be added to the pulp and thoroughly mixed therewith; then the mixture may be gradually heated, while it is being well agitated and circulated, by the addition of steam in regulated rate and amount, for example, such as will not overheat one portion to an objectionable temperature, and the temperature of the mixture gradually brought up in a substantially uni- I form manner to around 30 to 60 degrees centigrade, more or less. The treatment may be thus continued until the liquor shows by test that substantially all of its available bleach has been exhausted; then the treated pulp may be subjected to a dewatering operation by means of any suitable device, and the treated pulp may with advantage be given a washing treatment by means of water so as to remove substantially all of the bleach residues and dissolved organic matter from the pulp. Then the dewatered, and preferably washed, pulp is admixed with an additional charge of bleaching li nor of the k nd described above, the amount t us added being sufiicient or somewhat more than sufiicient to carry the bleaching to the desired degree, the mixture of pulp and added bleaching liquors "is circulated and gradually heated and thoroughly mixed, the heating'being continued until a temperature of around 30 to 0., more or less, has been reached. When the fiber has been bleached to the desired degree, the bleach residue is removed in any suitable manner and the bleached fiber then well washed with water. The amount of bleaching liquor added for second-stage treatment may vary somewhat, depending upon the material to be bleached, the degree of whiteness desired, the amount of bleaching reagent utilized in the first-stage treatment, etc. Generally, an amount of bleaching liquor carrying from three to fifteen parts, more or less, of bleaching powder (35% available chlorine per parts of fibres, by weight, is suflicient for the secondstage treatment; and in eneral the larger the amount of bleach use in the first-stage treatment, the lower the amount required for the second-stage treatment. It is advantageous to maintain the liquor (with the pulp) mildly alkaline to litmus until removed.

In case the bleach liquor residue removed from the bleached fibres after the completion of the second-stage treatment in any of the above examples carries a substantial or a rather large amount of unconsumed bleaching reagent, which can be determined by suitable tests, such residue can be utilized, at least in part, for a preliminary bleaching treatment of the unbleached pulp. If desired a regulated additional amount of bleaching material may be added thereto so that the bleaching liquor for the first-stage treatment may be composed (in part) of bleach residue from the second-stage treatment and (in part) of freshly supplied bleaching mater1al. Thus a fair or moderate excess of bleaching liquor, over that actually required for the second-stage bleaching treatment, may be added in the secondstage, and then when the color of the pulp has come up to the desired point. surplus bleach may be removed along with the liquor removed from the bleached pulp and later employed in a preliminary bleaching treatment of unbleached fibrous material. \Vhen operating in the latter manner, care should be taken to avoid heating the liquor in the second-stage to such a temperature than an chemical wood pul is ost from the liquor owing to such heating treatment.

In practice, the bleached pulp and liquor, which ma still contain bleaching material then be'adinixed with unbleached pulp. An rotary filter of the vacuum-type and the pulp may be washed with a regulated amount of water. The stronger liquor, with any desired amount of the washings, may then be admixed with unbleached pulp. An additional amount of bleaching material may be added to such liquor either as a solid or the hypochlorites of alkaline earth metals such as calcium hypoehlorite or chloride of lime.

We claim:

1. The multiple stage process of bleaching which comprises the following steps: (a mixing the unbleached fibrous material with bleach liquor including a hypochlorite of an alkali-forming metal; (6) treating the mixture so as to react a portion of the non-cellulosic organic material with bleaching material comprising a bleaching reagent derived from the said hypochlorite; (c) removing residual liquor containing soluble reaction products from this partially-bleached fibrous material; (d) admixing with this partiallybleaehed fibrous material additional bleach liquor including an oxidizing bleaching reagent, and thereafter treating the fibers so as to convert them into bleached chemical pulp; the amount of available bleaching material supplied to the fibrous material in the first-mentioned mixture being less than would be sutficient to convert the unbleached pulp into high white bleached cheinical pulp in a single-stage bleaching treatment.

2. The process according to claim 1, in which the total amount "of available bleaching material supplied to the fibrous material is less than would be sulficient to convert the unbleached pulp into high white bleached chemical pulp in a single-stage bleaching treatment.

3. The process according to claim 1, in which at least one-half of the total amount of available bleaching material supplied to the fibrous material is supplied to the fibrous material prior to the step c.

4. The process according to claim 1, in which more than one-half of the total obi'ectionable amount of available chlorine amount of available bleaching material supplied to the fibrous material 1s consumed in' the bleaching treatment prior to step cl.

5. The multiple-stage process of bleaching chemical wood pulp,,which comprises the following steps: (a) mixing the unbleached fibrous material with bleach liquor including a hypochlorite of an alkali-forming metal; (b) treating the mixture so as to react a portion of the non-cellulosic organic material with bleaching material comprising a bleaching reagent derived from the said hypochlorite and so as to form a residual liquor which includes dissolved organic matter; (a) removing such residual liquor containing soluble reaction products from the partially-bleached fibrous material; (d) admixing with this partiallybleached fibrous material additional bleach liquor including an oxidizing bleaching reagent and thereafter treating the fibers so as to convert them into bleached chemical pulp, the total amount of available bleaching material supplied to the fibrous material in the process being less than would be sufficient to convert the unbleached pulp into high white bleached chemical pulp in a single-stage bleaching treatment.

, 6. The multiple-stage process of bleaching chemical wood pulp, which comprises the following steps: (a) mixing the unbleached fibrous material with bleach liquor including a hypochlorite of an alkali-forming metal; (1)) treating the mixture so as to react a portion of the non-cellulosic organic material with bleaching material comprising a bleaching reagent derived from the said hypochlorite and so as to form a resid ual liquor which includes dissolved organic matter; (a) removing such residualliquor containing soluble reaction products from the partially-bleached fibrous material; (d) admixing with this partially-bleached fibrous material additional bleach liquor including an oxidizing bleaching reagent and thereafter treating the fibers so as to convert them into bleached chemical pulp, at least one half of the total amount of available bleaching material supplied to the librous material being supplied to the fibrous material prior to step c.

7. The process according to claim (3, m which nearly all of the bleaching material supplied prior to step 0 is exhausted prior to step jid.

8. The process according to claim 1, in whichia mixture including fibrous material and available bleaching material in step d is heated to a moderate temperature.

9. The process according to claim 1, in which a bleach liquor mentioned in step d is an alkaline liquor.

10. The multiple-stage process of bleaching chemical wood pulp, which comprises the following steps: (a) mixing the unbleached fibrous material with bleach liquor including a hypochlorite of an alkali-forming metal; (6) treating the mixture so as to react a portion of the non-cellulosic organic material with bleaching material comprising a bleaching reagent derived from the said hypochlorite and so as to form a residual liquor which includes dissolved organic matter; (0) removing suchresidualhquor containing soluble reaction products from the partially-bleached, fibrous material; ((1) admixing withthis partially-bleached fibrous material additional bleach liquor including an oxidizing bleaching reagent and thereafter treating the fibers so as to convert them into bleached chemical pulp, one of said mixtures being treated with bleach liquor at low consistency.

11. The process according to claim 5, in which the partially bleached fibrous material supplied to step. cl contains a substantial amount of colored products of the prior bleaching treatment which are substantially insoluble in water. s

12. The multiple-stage process of bleaching chemical wood pulp, which comprises the following steps: (a) admixing with the pulp bleaching material comprising prin-- cipally a hypochlorite of an alkali-forming metal; (Z2) treating the pulp in the resulting mixture until substantially all of the available bleaching material has beenexhausted, thereby producing a residual liquor which contains dissolved organic matter derived from the pulp, which organic matter is capable of reacting with chlorinated lime; (c) separating most of the resulting residual liquor from the resulting partially-bleached pulp; ((Z) admixing this partially-bleached pulp with alkaline liquor including bleaching material comprising principally a hypochlorite of an alkali-forming metal, the amount of such bleaching material bein sufficient to convert the partially-bleache pulp into bleached chemical pulp; (a) warming the mixture to a moderate temperature and continuing the bleaching treatment until bleached chemical pulp'has been obtained; the total amount of bleaching material supplied to the pulp being less than would be suiiicient to convert'the pulp into a high white bleached chemical pulp in a single-stage bleaching treatment, and the g CERTIFICATE or CORRECTION.

PatentJNo. 1,768,821. GrantedJuly 1, 19 0, t

LINN BRADLEY ET AL.

it is hereby certified that error appears in the printed specification of the above-numbered patent requiring correction as follows: Page 1, line 80, before the word "washing" insert the word or; page 6, line 29, after "chlorine" insert a parenthesis; same page, line 71, strike out "then be admixed with unbleached pulp. An" and insert in substantial amount, may be treated on a; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 21st day of October, A. D. 1930.

M. J. Moore,

(Seal) Acting Commissioner of Patents.