US2364177A - Manufacture of bleached kraft pulp - Google Patents

Description

De.5, 1944. -SDWELLS v u "2,364,177

MANUFACTURE OF BLEACHED KRAFT PULP Filed May 1, 1942 L o//er Uff/ace an 5mg/fer bsor er f/V 0 j (061) v a /7 Y J5( 2 l Z? y j!! l efo/'f I Washer I d Z l' l g'ed Absorber Patented Dec. 5, 1944 4llrurrl) s'mfrssN PATENT OFFICE E Sidney n. Weils, 1communi Locks. wis., signor to Th'e Institute of Paper ChemistrylAppleton,`

facture -ofHbleached-,kraft pulp, and more partlculariyfrto- ,an-improved process of preparing bleachedkraft; .nber using salt (NaCl), lime (qaO) and sulfur (S).

lnfthe manufacture of bleached lkraft pulp from'coniferouswoods the wood chips are first cooked in an aqueous bath containing lsodium sulfide (NazS) and sodium hydroxide (NaOH), andthen after washing are subjected to multistage bleaching:4 The cooking operation carried outfin a' digester, ordinarily follows the` conventionalprocedure used in i'the manufacture of unbleached pulps except for more drastic cooking conditions,` longer duration of: cook, more care-` ful `washing,etc. lfThe bleaching operation ordinarllyfcalls for laninitial chlorination step followed by a. subsequent alkaline hypochlorite step such-:asfsdescribed in the bleaching Vprocess of Wells Patent No. 1,883,193.

`It ismniversalr practice, due to the high cost of chemicals; to 4recover Athe sodium base from the black liquoryremaining after the cooking treatment. 'With modern recovery units the as well asl chlorine and caustic soda used inl bleaching, onthe open market. These materials are 4relatively expensive and their use (particularly the 1f ton of salt ca'ke needed for 8 tons oflpulp), hasgdeterminedz to-alarge extent the final cost of kraft pulp. Many suggestions have been made` with reference to chemical sources but up tothe` time ofthe zpresent invention at tempts-.toprovide a balanced kraft operating system have beenfunsuccessful.

The principal' object Vof the present invention -Wis., a corporation of Wisconsin Y n Appucau'on May 1, 1942, serial Nn. 441,258k

' connus. (ci. sia- 19) present -inventionrelatesf.to.the manu-, sulting in the production of electrolytic hydrogen and chlorine and sodium hydroxide. The chlorine produced `in this process is suiiicient to provide for both the chlorination and the hypochloritc treatments, and the 'total sodium hydroxide issuiiicient to provide for makeup caustic soda, alkaline extraction, maintain alkalinity during bleaching, etc. The sodium hydroxide is also sufficient when used with sulfur and the by-product hydrogen to provide for make-up sodium sulfide. This last step, which ls described in-detail below makes it unnecessary to use any salt cake in the process of the present invention.v l

The drawings, which are in the form of now diagrams, will serve to illustrate the present invention. Figure 1 illustrates` a complete system, while Figure 2 illustrates a modification of a portion thereof. In Figure 1 the wood chips are first cooked in the conventional manner in digesters l0 in the presence of aqueous sodium is to provide afwellbalanced` operation adapthydroxide and sodium sulde. After cooking the usual washing operation is carried out in washers Il and the digested unbleached pulp .passed on by way of screens and rilers I2 to `chlorinator' i3. l

The black liquor-obtained from washers Il may be treated as shown to recover the sodium base used for digestion. This process includes concentration of the black liquor in concentrator 20, treatment of the concentrate in boiler furnace and smelter 2l, treatment of the resultving smelt in dissolver 22 and formation of caustic soda from the green -liquor containing the smelt water-solubles in causticizer 23. The sodium compounds in the smelt coming from smelter 2l ordinarily contain small amounts of sodium thiosulfate (NazSzOa) and sodium sulfite (NaaSOa), somewhat larger amounts of sodium hydroxide and sodium sulfate, about 15-20% sodium sulfide and approximately 75% sodium carbonate (NazCOa). The sodium hydroxide formed in the causticizer 23 by treatment with quick lime may be illustrated by the following formula:

The insoluble calcium carbonate (CaCOs) sludge formed during this treatment may be removed after settling, and the water solution or fresh i white liqucrcontaining the desired proportion The process-of the present invention which includes sodium hydroxide` formation by causticizing black liquorfsmelt,` calls for the electrolyzing of common salt, i. e., sodium chloride, re-

of caustic soda passed to digesters I0 for a repetition of the cycle.

The electrolyzing of the salt` may be carried out in electrolytic cells 30. Part of the chlorine washed in washer i6 and is then passed to j bleacher Il for treatment with the calcium hypochlorite formed in absorber 3|. During this treatment the desired alkalinity maybe maintained in bleacher Il by use of 'a portion of the caustic soda formed during electrolysis. The bleached pulp is then subjected to a final washing in washer I8 to obtain the bleached fibers in the desired condition for use in paper making.

The sodium hydroxide obtained from electro? lytic cells 30 is passed to concentratorv32 and the resulting concentrate passed on to absorber 33. Portions of the caustic soda concentrate are also passed td alkaline extractor l and bleacher l1 as indicated above. The hydrogen from electro-y lytic cells 30 is passed t0 retort 34 where it reacts with (gaseous) sulfur heated to a temperature around 390 C. and forms hydrogen sulfideA (HzS). 'Ihe hydrogen sulfide is then passed to absorber.33 which may include scrubbing towers where it reacts with the electrolytic caustic soda to form the desired sodium sulfide. These reactions may be illustrated -by the following formulas:

H2S-I-2NaOH-Na2S-i-2H2O The sodium. sulfide solution is then passed .to digesters I0 where it is mixed with the caustic soda solution obtained. from causticizer 23 and is ready for a. repetition of thevcycle with fresh wood chips.

An alternative procedure in a portion of the system is illustrated by Figure 2. In this modification the sodium hydroxide or a predetermined portion thereof coming from concentrator 32 .is

-conveyed as fresh white liquor to digsters I0 for use as make-up caustic soda. The sodium hydroxide liquorforxned in causticizer 23 is passed to absorber 33' where -it reacts as indicated above with the hydrogen sulde coming from retort 34 to form make-up sodium sulde. If desired, a portion of the electrolytic sodium hydroxide'may be passed as indicated at 35 to absorber 33' in order to enrich the causticized white liquor and provide additional sodium hydroxide for sodium sulfide formation.

The division of the relative amounts of work accomplished between the pulping, purification and bleaching steps is quite iiexible. If more alkali isused in the pulping less chlorine is required for chlorination and bleaching. Also the division of what remains to be done after pulping is to 'a large extent optional. Ordinarily in the preparation of one ton of bleached kraft pulp wherethe causticized liquor is used for make-up caustifc' soda, about 200 pounds ofY chlorine and about-225 pounds of caustic soda are used. With these amounts about 65% of the chlorine is used to chlorinate the unbleached pulp and the remaining (e. g., about 35%) chlorine is used to form calcium'hypochlorite; and about 65% of the caustic soda is used to form sodium sulfide and the remaining (e. g., about 35%) caustic soda is used for alkaline extraction and maintaining alkalinity amount around three-quarters of the 35% caustic soda (e. g., about 26-27% or 60 pounds) is used in the alkaline extractor and around one-fourth of the 35% caustic soda (e. g., about 89% or 20 pounds) is used in the bleacher. A'portion of the chlorinemay also be used if desired in the water system to prevent slime formation.

It will be understoodthat the processes outlined in the flow diagrams are merely illustrative and thatmodiflcations may be made therein as` desired. For example, the active caustic soda in :calcium carbonate `sludge after treatment with lime in the causticizing operation. It will also be understood that an additional hypochlorite bleaching stage may be usedand that additional washers may also be used as desired. In the hypo-l chlorite treatments alkalinity. (e. g., as shown by the phenolphthalein end point) ismaintained by additionof small amounts of caustic` soda as the bleach is exhausted. It will also be understood `by those skilled in the art-that various other modifications may be employed including the recycling` of wash waters, the passing of uncombined yhydrogen from absorber 33 to retort 34 for combining with sulfur, `formation of greenV liquor in dissolver 22 `by use of wash from causticizer 23,;etc.1

In practice the salt (NaCl) impurity in the elec'- trolytic liquor is separated by crystallization in concentrator 32 and returned to the cells as indicated at 36. The various apparatus used inthe process ofthe present invention may be ofthe invention may also be combined with various other procedures including typical kraft mill. di-

gesting, bleaching, recovery, etc., systems to provide a well balanced operation of the type desired. The scope of the present invention should be determined by the claims annexed hereto.. l

I cl im:

1. balanced system for use in the preparation of bleached kraft pulp which comprises preparing unbleached pulp by cooking wood in a caustic soda-sodium sulfide digesting liquor, preparing a sodium salt smelt by heating the black liquor resulting from the cooking treatment, preparing a.

green liquor by dissolving the water solubles of.l

the sodium salt smelt, preparing a fresh sodium hydroxide white liquor by a lime causticizing treatment of the green liquor, preparing electrolytic` hydrogen and electrolytic chlorine and additional fresh sodium hydroxide white liquor by sodium sulfide liquor by reacting the electrolytic hydrogen with sulfur and absorbing the resulting'hydrogen sulfide in sodium hydroxide white liquor, and subjecting the unbleached pulp to -chlorinating and hypochlorite bleachingA treatments, the chlorine for` said treatments being electrolytic and being obtained by said electrolyi sis.

2. In ,the preparation of bleached kraft pulp the process which includes the following steps (l) cooking wood chips in an aqueous sodium hydroxlde-sodium sulfide bath to form unbleached pu1p, (2) smelting the black liquor resulting from step (1) to form a sodium salt smelt, (3) electrolyzing aqueous sodium chloride to form chlorine, sodium hydroxide and hydrogen, (4) chlo-` rinating the unbleached pulp of step (l) with part of the chlorine obtained in step (3), (5) subjecting the chlorinated pulp of step (4) to a1- kaline extraction with part of the sodium hydroxide obtained in step (.3), 6) forming calcium hypochlorite by reacting calcium hydroxide with chlorine obtained in step (3), (7) bleaching the alkalinized pulp of step by treatment with the calcium hypochlorite obtained in step (6), (8) forming hydrogen sulde by reacting sulfur with the hydrogen obtained in step 3), (9) forming a sodium sulde liquor by absorbing the hydrogen sulfide of step (8) in sodium hydroxide obtained in step (3), (10) forming a sodium hydroxide liquor by causticizing the smelt of step (2) by treatment with calcium hydroxide, and (11) mixing the sodium sulfide and sodium hydroxide liquors of steps (9) and 10) respectively for a repetition of the cycle with fresh wood chips.

3. In the preparation of bleached kraft Dull? the process which includes the following steps l) cooking wood chips in an aqueous sodium hydroxide-sodium sulfide bath to form unbleached pulp, (2) smelting the black liquor resulting from step (l.) to form a sodium salt smelt, (3) electrolyzing aqueous sodium chloride to form chlorine. sodium hydroxide and hydrogen, (4) chlorinating the unbleached pulp of step (1) with part of the chlorine obtained in step (3). (5) subjecting the chlorinated pulp of step (4) to alkaline extraction with part of the sodium hydroxide obtained in step (3), (6) forming calcium hypochlorite by reacting calcium hydroxide with chlorine obtained in step 3), (7) bleaching the alkalinized pulp of step (5) by treatment with the calcium hypochlorite obtained in' step (6),

, (8) forming a white liquor containing sodium hydroxide by causticizing the smelt of step (2) by treatment with calcium hydroxide, 9) forming hydrogen sulfide by reacting sulfur with the hydrogen obtained in step (3), (10) forming additional sodium sulde by absorbing the hydrogen sulde of step (9) in the white liquor obtained in step (8)- and (l1) mixing electrolytic sodium hydroxide obtained in step` 3) with the sodium sulfide liquor obtained in step for a repetition of the cycle with fresh wood chips.

4. In the preparation of bleached kraft pulp the process which includes the following steps (l) cooking coniferous wood chips in an aqueous sodium hydroxide-sodium sulfide bath to form unbleached pulp, (2) washing the digested pulp of step (1) (3) concentrating and smelting black liquor resulting from steps (l) and 2) to form a sodiumA salt smelt, (4) electrolyzing aqueous sodium chloride to form chlorine, sodium hydroxide and hydrogen, (5) chlorinating the washed pulp of step (2) with part of the chlorine obtained in step (4), (6) Washing the chlorinated pulp of step (5), (7) subjecting the washed pulp of step (6) to alkaline extraction with part of the sodium hydroxide obtained in step (4), (8) washing the pulp of step (7), (9) forming calcium hypochlorite by reacting calcium hydroxide with chlorine obtained in step (4), (10) bleaching the washed pulp of step (8) by treatment 'with the calcium hypochlorite obtained in step (9), (11) maintaining an alkaline condition during the bleaching treatment by addition of part of the sodiumhydroxide obtained in step (4), (12) washing the bleached pulp of step (10), (13) forming hydrogen sulde by reacting sulfur with the hydrogen obtained in step (4), (14) forming a sodium sulfide liquor by absorbing the hydrogen sulfide of step (13) in sodium hydroxide obtained in step (4), (l5) forming a sodium hydroxide liquor by causticizing the smelt of step (3) by treatment with calcium hydroxide, and (16) forming a fresh cooking liquor by mixing the sodium sulfide and sodium hydroxide liquors of steps 14) and (15) respectively.

5. In the preparation of bleached kraft pulp the process which comprises forming unbleached diuml in the preparation of the green liquor, forming electrolytic chlorine and hydrogen and sodium hydroxide by electrolysis of aqueous sodium chloride, chlorinating the Washed unbleached pulp with electrolytic chlorine, Washing the chlorinated pulp, subjecting the Washed chlorinated pulp to alkaline extraction with electrolytic sodium hydroxide, washing the alkaline extracted pulp, forming calcium hypochlorite by rea-cting electrolytic chlorine `with calcium hydroxide, bleaching the Washed alkaline extracted pulp by treatment with the calcium hydochlorite, maintaining the calcium hypochlorite liquor in an alkaline condition during the bleaching treatment r by addition of electrolytic sodium hydroxide,

Washing the bleached pulp, forming hydrogen sulde by reacting the electrolytic hydrogen with sulfur, forming additional sodium sulfide by abi sobbing the hydrogen sulfide in the sodium hyfill droxidc white liquor, and forming a fresh cooking liquor by mixing electrolytic sodium hydroxide with the sodium sulde liquor.

6. In a balanced system adaptable for use in the preparation of bleached kraft pulp, the process which comprises preparing unbleached pulp by cooking wood in a caustic soda-sodium sulfide digesting liquor, preparing sodium hydroxide by causticizing a sodium saltsmelt obtained by heating the black liquor resulting from the cooking treatment, 'preparing chlorine and hydrogen and additional sodium hydroxide by electrolysis of sodium chloride, and preparing a fresh sodium sulfide liquor by the reaction of the hydrogen with sulfur and the reaction of the resulting hydrogen sulfide with sodium hydroxide, said process providing sufficient chlorine for chlorination and bleaching of the pulp and sufficient caustic soda and sodium sulfide for making up fresh cooking liquors.

SIDNY D. WELLS.

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