US2178649A - Process for bleaching wood pulp - Google Patents

Description

Nw.. 393g, J, SCHUER 78,649

PROCESS FOR BLEACHING WOOD'PULP Fl'ed Nov. lO, 1938 BNVENTOR ATTORNEY Patented Nov.. 7, 1939 UNITED STATES PATENT OFFICE PROCESS FOR BLEACHING WOOD PULP Application November 10, 1938, Serial No. 239,763

comme.

This invention relates to a process for bleaching wood pulp.

Bleaching processes involving the following separate stages have been proposed; chlorination, wherein the pulp in aqueous suspension is treated with elemental chlorine to chlorinate the lignin and other impurities, making them in part soluble in water or in alkali; caustic extraction, wherein a part of the chlorinated impurities are dissolved from the fibers by digestion with a caustic solution; and hypochlorite bleaches in one or more stages, wherein the pulp suspension is subjected to an alkaline hypochlorite solution to oxidlze the remaining insoluble impurities, making them soluble in water or in alkali.

In bleaching pulps containing considerable proportions of diillcultly soluble impurities, such as kraft pulp or any under-cooked pulps, it has been suggested to apply alkaline hypochlorite bleaches in at least two stages after the initial chlorine bleach in order to solubilize substantially completely the greater amount of insoluble impurities present in such pulps. Since the iirst hypochlorite treatment partially oxidizes the residual lignin chloride and other insoluble color bodies remaining in the fibers after chlorination and alkali extraction and the impurities are thereby rendered partially soluble in water, and soluble to a greater extent in alkali, it has been" proposed to remove these oxidized impurities by a caustic extraction stage prior to subjecting the pulp to further hypochlorite treatment.

They addition of alkali/during the hypochlorite treatment so asf'to avoid the vdevelopment of VV acidity` which-would cause deterioration of the pulp being treated has been mentioned in the-1.

hypochlorite treatment, g, This caustic soda,v -it isV stated, may be added from time to time 'tomamv'tain the desired alkaline conditions throughout sired bleach. The exhaustion'of the alkalinity of the bleach solution may be determined by adding phenolphthalein to a sample portion of the solution; if no color develops this indicates that the alkalinity has been exhausted.

I have made the surprising discovery that bleaching of pulp can be accomplished efficiently and a pulp of high strength, purity and whiteness obtained by maintaining alkaline conditions within a pH range from about 8.3 to 10.0, preferably about 8.5, during one or more of the bleach steps to which the pulp is subjected, in which steps the pulp is treated to remove such impurities as require an oxidation treatment (e. g. hypochlorite treatment) for their removal and, when the bleach in the step in question is completely or almost completely consumed, adding alkali to increase the pH to a value not greater than 13.0, preferably about 12.0. One method oi determining exhaustion of hypochlorite bleach is to titrate a sample to which potassium iodide has been added with a standard thiosulfate solution. From this maybe calculated the actual concentra-` tion of available chlorine in the solution and in turn the degree of exhaustion of the hypochlorite. In actual bleaching practice, however, a simpler method is used, namely, a starch potassium iodide paper which is discolored by chlorine in solution.

An experienced operator can ascertain the approximate extent of exhaustion of the bleach solution -by observing the rate of color developmention the test paper.

It is important to note that in the bleaching :step in which my invention is employed, i. e.` prior toA exhaustion or substantially complete exhaustion ofthe bleach. .in the step in question, an alkalinity of from about 8.3 to 10.0 is maintained and thereafter alkali is added to raise the pH to higher levels. With some pulps 1% sodium hydroxide on the basis of bone dry pulp is suilcient for such alkali addition. With other pulps larger quantities up to 5% and 10% caustic soda may be required, depending upon the type of pulp and the bleached product desired. Thel caustic soda serves to extract dyes which, if allowed to remain, detract from the whiteness of the finished A product. I have found that if a concentration of alkali suflicient to perform this extraction is present throughout the bleaching step, bleaching is not properly conducted, as above pointed out,

in that the alkali slows down the bleaching to such an extent as to affect adversely the quality of the pulp.

In carrying out the bleaching of pulp in accordance with this invention, an aqueous suspension of the wood pulp, which may have undergone previous treatments such as chlorination and caustic extraction, is treated, in order to oxidize and thereby solubilize the difilcultly soluble lignin and chloro-lignin bodies and other colored impurities sun remaining in the puip, with a. bleachv solution such as calcium hypochlorite or sodium hypochlorite, chlorine or hypochlorous essary dependingy on the pulp being bleached and conditions under which the bleaching is carried out. As hypochlorite is the most common reagent used at the present time in the paper industry the invention will hereinafter be described in connection with the use of this reagent; it be understood, however, that the invention is not confined to its use but other bleach solutions such as those mentioned may be used in lieu of hypochlorite.

Under the conditions of pH dening the invention, the hypochlorite bleach solution exerts its maximum oxidizing effect on the impurities retained by the fibers without attacking the fibers themselves appreciably. When the hypochlorite has become substantially consumed, sumcient alkali is added to the mixture to dissolve from the fibers the impurities oxidized by the hypochlorite. A pH of 12 has been found advantageous for this purpose. After solution of impurities by the alkali, the pulp may be washed and subjected to further bleaching operations.

It is important that the solution be maintained alkaline during the hypochlorite treatment because the presence of hydroxyl ion exerts a catalytic eect promoting oxidation by the hypochlorite. Moreover, in acid solution the hypochlorous acid and hypochlorite present in the solution attack the cellulose fibers rapidly, resulting in a great loss in ber strength. It is important that the upper pH limit of 10.0 be observed because at a pH above 10.0 the presence of the hydroxyl ion has so great an adverse effect on the equilibrium of the reactions involved that the rate of oxidation of impurities by the hypochlorite is considerably reduced, with a resultant attack upon the cellulose fibers by the alkaline solution during the necessarily longl time of contact and the loss in vfiber quill-1t? and strength caused thereby. Thelnaintenance of a pH range of 8.5 to 9 during the bleach is particularly advantageous because. under such conditions, I have found the hypochlorite solution exerts its maximum oxidizing effect on the impurities present with the least possible amount of injury to the pulp fibers.

The addition of alkali directly to the mixture of pulp and substantially exhausted hypochlorite solution for the purpose of dissolving oxidized impurities eliminates a water-washing operation found necessary in prior art processes, and eilects a saving in equipment and handling costs over such prior art processes.

One example of the practicer of the process of this invention is hereinbelow described in connection with the accompanying drawing forming n a part of this specincation. This drawing shows diagrammatically, for purposes of exempliilcation, a preferred klayout used in the bleaching of spruce kraft pulp. This layout was designed for a four-stage treatment as follows:

(1) chlorination:

(3) Caustic extraction;

(3) Hypochlorite bleach combined with caustic extraction;

(4) A second hypochlorite bleach.

Referring to the drawing, the unbleached pulp from the storage tank I was led into the chlorinator 2 in the form of an aqueous suspension of 3.5% consistency. The pulp was chlorinated at 65 F. with a weighted amount of gaseous chlorine yrepresenting 60% of the chlorine demand of the pulp. The time of chlorination was 45 minutes. The pulp was then thoroughly ywashed in the vacuum washer I.

'Ihe pulp was brought to a consistency of 15% and was digested at a temperature of 110 F. with about 1% of sodium hydroxide based on the dry weight oi' the pulp. After 45 minutes the maximum solubilizlng efect on the chlorinated impurities had been reached and the pulp was washed in the vacuum washer 5 until no longer alkaline to phenolphthalein.

In vthe bleaching tank i the pulp at a consistency of about was treated with calcium hypochlorite in accordance with my invention in amount equivalent to about 30% of the chlorine demand of the pulp, the solution during this treatment having a ing out this example it was found unnecessary to add alkali to maintain the pH` constant. The temperature was maintained at about 95 F. After 30 minutes about 90% of the hypochlorite had been consumed. At this point caustic soda was added in amount equal to about 1% NaOH based on the dry pulp and the temperature was raised to 110 F. The caustic extraction was allowed to proceed for 45 minutes and the pulp was then thoroughly washed in the vacuum washer 1.

In the bleaching tank 8 the washed pulp at a consistency of about 10% was treated for about two hours at a temperature of about 95 F. with an `amount of calcium hypochlorite equivalent to about 10% of the chlorine demand of the pulp. The pH was maintained at about 8.5. After thorough washing in the vacuum washer 9 the bleached pulp was stored in the tank i0.

The above treatment resulted in a bleached pulp of high white color retaining the desirable strength characteristics oi the unbleached pulp. For example, kraft pulp bleached by the above four-stage process embodying my invention, when tested in the standard General Electric brightness tester, showed a more brilliant white color than the same pulp bleached by a fourstage process involving chlorination, caustic extraction, and two hypochlorite bleaches in which during the hypochlorite bleaches usual pH conditions were maintained and not the conditions of' this invention. Moreover, in addition to more effective bleaching, as shown by better color, the tearing resistance of paper made from the above pulp bleached according to my invention was more than 16% higher than the tearing resistance of paper made from the other pulp referred to above.

I'he time 'of treatment, temperature, pulp consistency, and other conditions mentioned in the above example will vary as the invention is applied to the bleaching of other pulps.

In the caustic extraction stage following chlorination, the amount of caustic soda normallyrequired for solution of the alkali-soluble residues varies from 0.5% to 1.5% of the dry pulp. With easy bleaching sulilte pulp 0.5% is usually all that is necessary, whereas hard-bleaching kraft pulp may consume the entire 1.5% added in the extraction stage. Increased caustic concentrapH of about 8.5. In carrytion and temperatures in the caustic extraction stage introduce proportional solution of hemicellulose, with increase in the lalpha-cellulose content of the pulp. The amount of alkali added to dissolve oxidized impurities when the hypochlorite has been consumed in the rst hypochlorite stage may be as high as 5% to 10% of the dry pulp in some cases, depending on the characteristics of the pulp itself and the type of product desired.

During the hypochlorite oxidation, acidic products are sometimes formed. It is necessary in such cases to add small amounts of caustic from time to time during either or both of the hypochlorite treatments in order to maintain the desired pH in the solution during oxidation. In a great many cases, however, it is unnecessary to add any caustic at all during the hypochlorite oxidation, the alkalinity of the calcium hypochlorite, together with the alkali carry-over from the previous caustic extraction, being suicient to maintain the proper pH until the bleach is exhausted.

Substantially equivalent reagents may be substituted for those specically mentioned above. Thus, caustic potash, sodium carbonate, lime, or other strong alkali may be used in place of caustic soda for alkali extraction.

In applying this invention to the bleaching of pulps containing more easily solubilized impurities, one or more of the stages enumerated above may be omitted. Thus, in some cases the bleaching process may be carried out by treating the pulp initially with hypochlorite solution, with addition of caustic when the hypochlorite is consumed, in accordance with the invention, the pulp thereafter being washed and subjected to a second hypochlorite treatment; or the initial treatment may consist of chlorination, followed by one or more hypochlorite stages, one or more of which is combined with caustic extraction in accordance with the invention.

When it is desired to bleach a very impure pulp to a high degree of whiteness, additional stages may be included in the bleaching process. Thus a second chlorination may precede either the rst or the second hypochlorite stages described in the example. In some cases the bleaching process may include three hypochlorite stages, the first two combined with caustic extraction in accordance with the invention. For some pulps it may be advantageous to substitute for the caustic extraction stage following chlorination a combined hypochlorite oxidation and caustic extraction stage.

It is to be understood that this invention is not restricted to the present disclosure otherwise than defined by the appended claims.

I claim:

1. A process for bleaching wood pulp which comprises mixing an aqueous suspension of the pulp with a solution of a hypochlorite, maintaining the alkalinity of the mixture at a pH between 8.3 and `10 until the hypochlorite has been substantially completely consumed and thereafter increasing the pH of said mixture.

2. A process for bleaching wood pulp which comprises mixing an aqueous suspension of the pulp with a solution of a hypochlorite of a metal from the group comprising alkaline earth and alkali metals, maintaining the pH of the mixture at a value between 8.5 and 9 until the hypochlorite has been substantially completely con- 3. In a process for bleaching wood pulp, the step which comprises contacting an aqueous suspension of the pulp with a solution of a hypochlorite of a metal from the group comprising alkaline earth and alkali metals, maintaining the mixture at a pH of about 8.3 to 10 until the hypochlorite has been substantially completely consumed and thereafter increasing the alkalinity of the mixture by the addition of caustic soda to dissolve the alkali-soluble impurities.

4. A process for bleaching wood pulp requiring a plurality of bleaching treatments, which comprises treating an aqueous suspension of the pulp with hypochlorite of a metal from the group comprising alkaline earth and alkali metas to oxidize impurities in the pulp, maintaining a pH of 8.3 to 10 in the digestion mixture until about 90% of the hypochlorite has been consumed, thereafter treating said mixture with caustic soda in amount suiiicient to raise the pH to effect dissolution of the alkali-soluble oxidation products, washing the pulp to remove substantially all the caustic soda, and treating the pulp with a further quantity of hypochlorite solution while maintaining the pH above 8.3.

5. A multi-stage process for bleaching kraft pulp comprising the following separate stages of treatment: (1) chlorinating an aqueous suspension of the pulp with elemental chlorine; (2) digesting the pulp suspension with a solution of an alkali to dissolve the alkali-soluble chlorinated impurities; (3) mixing the pulp suspension with a. solution of a hypochlorite, maintaining a pH between 8.3 and 10 in the mixture until the hypochlorite has been substantially completely consumed, and thereafter adding an alkali to said mixture to increase the pH to dissolve the impuriu ties rendered alkali-soluble by hypochlorite oxidation; and (4) mixing the pulp suspension with a further quantity of hypochlorite to oxidize the residual impurities in the pulp, while maintaining a pH between 8.3 and 10 in the mixture.

6. A multi-stage process for bleaching kraft pulp comprising the following separate stages of treatment: (1) chlorinating an aqueous suspension of pulp with an amount of gaseous chlorine representing 60% of the chlorine demand of the pulp; (2) digesting the pulp suspension with sodium hydroxide in the amount of about 1% based on the dry weight of the pulp to dissolve the alkali-soluble chlorinated impurities; (3) mixing the pulp suspension with a solution of calcium hypochlorite in an amount equivalent to at least of the chlorine demand of the pulp, maintaining a pH between 8.5 and 9 in the mixture until about 90% of the hypochlorite has been consumed, and thereafter adding sodium hydroxide to said mixture in the amount of about 1% based on the dry pulp to increase the pH to about 12 and to dissolve the impurities rendered alkalisoluble by hypochlorite oxidation; and (4) mixing VAthe pulp suspension with an amount of hypochlo- 7. rIhe step in the process for bleaching pulp v which comprises treating an aqueous suspension of the pulp with a chlorine bleach solution, maintaining during said treatment a pH of from about 8.3 to 10 until the bleach has been substantially completely exhausted and thereafter increasing the alkalinity of the mixture.

J OHN SCHUBER.

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