CA1042160A - Alkali/oxygen gas digestion of wood - Google Patents
Alkali/oxygen gas digestion of woodInfo
- Publication number
- CA1042160A CA1042160A CA208,553A CA208553A CA1042160A CA 1042160 A CA1042160 A CA 1042160A CA 208553 A CA208553 A CA 208553A CA 1042160 A CA1042160 A CA 1042160A
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- CA
- Canada
- Prior art keywords
- digestion
- wood
- process according
- fibers
- oxygen
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C3/00—Pulping cellulose-containing materials
- D21C3/22—Other features of pulping processes
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C3/00—Pulping cellulose-containing materials
- D21C3/02—Pulping cellulose-containing materials with inorganic bases or alkaline reacting compounds, e.g. sulfate processes
- D21C3/026—Pulping cellulose-containing materials with inorganic bases or alkaline reacting compounds, e.g. sulfate processes in presence of O2, e.g. air
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- Chemical & Material Sciences (AREA)
- Inorganic Chemistry (AREA)
- Paper (AREA)
- Joining Of Building Structures In Genera (AREA)
- Diaphragms For Electromechanical Transducers (AREA)
Abstract
PROCESS FOR THE PRODUCTION OF CELLULOSE
PULP FROM WOOD BY DIGESTION OF THE WOOD
WITH ALKALI AND OXYGEN
ABSTRACT OF THE DISCLOSURE
A process is provided for the production of cellulose pulp from wood which comprises digesting the wood with oxygen gas and alkali in aqueous solution in a digestion zone; detaching cellulose fibers from the surface of the wood in the course of the digestion and in the presence of digestion liquor; suspending the detached fibers in digestion liquor; removing such detached fibers from the digestion zone, as the digestion of the wood continues; and continuing the digestion of the remaining wood with oxygen and wood with oxygen and alkali in aqueous solution to form cellulose pulp.
PULP FROM WOOD BY DIGESTION OF THE WOOD
WITH ALKALI AND OXYGEN
ABSTRACT OF THE DISCLOSURE
A process is provided for the production of cellulose pulp from wood which comprises digesting the wood with oxygen gas and alkali in aqueous solution in a digestion zone; detaching cellulose fibers from the surface of the wood in the course of the digestion and in the presence of digestion liquor; suspending the detached fibers in digestion liquor; removing such detached fibers from the digestion zone, as the digestion of the wood continues; and continuing the digestion of the remaining wood with oxygen and wood with oxygen and alkali in aqueous solution to form cellulose pulp.
Description
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SPECIFICATION
Most cellulose pulp produced today in commerce is prepared by the sulfate or kra-ft proce3s, in which wood is digested or pulped with aLl{ali and sodium sulfide, and sodium sulfate is used as the make-up 5 chemical to cover losses in the recovery cycle. The greater part of the remainder of the cellulose pulp is produced by the three variants of the sulfite process, in which the active digestion or pulping chemi-cals comprise acid sulfite, bisulfite, or neutral sulfite.
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Some pulp is still produced by the so-called soda process, 10 in which sodium carbonate is used instead of sodium sulfate as the make-up chemical, but this process has a number of serious disadvantages, compared to the kraft process, particularly a low pulp yield and a poor pulp quality, and consequently this process is no longer used to any considerable extent.
The sulfate process also has a number of disadvantages, however, the most serious one, from the standpoint of pollution of the environment, being the discharge of sulfur dioxide, hydrogen sul-fide and other waste gases, as well as the black liquor efnuent, which must be captured, processed for recovery, and recycled to maintain 20 an economical operation. Even with maximum recovery and reutili-zation of waste chemicals, contamination of the atmosphere and of - waterways adjacent a sulfate pulp mill remains a serious problem which has notbeen entirely overcome, and as a result, manykraft pulp mills today are faced with the necessity of developing a work-25 able and practical alternative to the sulfate process. This alterna-tive is not provided by the sulfite or soda processes.
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~.04'~ 0 It h~s b~en suggested that the part played by the sulfide in freeing the cellulose pulp fibers from the wood can be taken by oxygen .
It has been known for several years that wood can be digested to form cellulose pulp by using oxygen gas in the presence of aqueous alkali. The oxygen gas serves as an oxidant which in the presence of the aqueous alkaline liquid phase attacks the lignin o the wood, and converts it into soluble degradation products which dissolve in the aL~aline digestion liquor. However, although the process has many attractive features, it has not received commercial accept-ance because the reaction is quite difficult to control. A non-uniform degradation of the wood produces cellulose pulp which . . . . . .
contains excessively degraded carbohydrate materials. Moreover, access to the interior of the wood is difficult, due to slow mass kansfer, and it is almost impossible to obtain a thorough pulping of the wood, and at the conclusion of the digestion a considerable proportion of the wood remains in the form of slivers, which have to be separated from the pulp.
Harris U. S. patent No . 2, 673, 148, dated March 23, 1954, proposed an oxygen digestion process using quite high oxygen pressùres, of the order of at least 800 psi. This was thought necessary in order to obtain and maintain a sufficiently high oxygen concentration in the digestion liquor. This is one of the serious problems in oxygen diges-tion processes due to the fact that oxygen is a gas which has a low solubility in the digestion liquor. The results obtained in this process were not satisfactory.
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1~34;~1~i0 Granga~rcl ancl Saunders, U.S. patent No. 2, 926,114, dated February 23, l9G0, stated that oxygen prior to 1957 had been tried both at low and at high oxygen pressures. Grangaard et al. proposed a digestion at pH 7 to 9 over at least a m~jor portion of the cooking time,under oxygen pressures of 40 to 250 psi, using conventional batch digesters. The pH is maintained within the desired range by a buffer such as sodium bicarbonate, or by continuous addition of aLkali such as sodium hydroxide or sodium carbonate, to neutralIze free acids formed throughout the digestion. However, the process gives the pulp unacceptable properties. The main improvement is in the spent liquor.
Other disadvantages are:
(1) An extremely high consumption of sodium bicarbonate.
SPECIFICATION
Most cellulose pulp produced today in commerce is prepared by the sulfate or kra-ft proce3s, in which wood is digested or pulped with aLl{ali and sodium sulfide, and sodium sulfate is used as the make-up 5 chemical to cover losses in the recovery cycle. The greater part of the remainder of the cellulose pulp is produced by the three variants of the sulfite process, in which the active digestion or pulping chemi-cals comprise acid sulfite, bisulfite, or neutral sulfite.
.. . ............ . ..
Some pulp is still produced by the so-called soda process, 10 in which sodium carbonate is used instead of sodium sulfate as the make-up chemical, but this process has a number of serious disadvantages, compared to the kraft process, particularly a low pulp yield and a poor pulp quality, and consequently this process is no longer used to any considerable extent.
The sulfate process also has a number of disadvantages, however, the most serious one, from the standpoint of pollution of the environment, being the discharge of sulfur dioxide, hydrogen sul-fide and other waste gases, as well as the black liquor efnuent, which must be captured, processed for recovery, and recycled to maintain 20 an economical operation. Even with maximum recovery and reutili-zation of waste chemicals, contamination of the atmosphere and of - waterways adjacent a sulfate pulp mill remains a serious problem which has notbeen entirely overcome, and as a result, manykraft pulp mills today are faced with the necessity of developing a work-25 able and practical alternative to the sulfate process. This alterna-tive is not provided by the sulfite or soda processes.
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~.04'~ 0 It h~s b~en suggested that the part played by the sulfide in freeing the cellulose pulp fibers from the wood can be taken by oxygen .
It has been known for several years that wood can be digested to form cellulose pulp by using oxygen gas in the presence of aqueous alkali. The oxygen gas serves as an oxidant which in the presence of the aqueous alkaline liquid phase attacks the lignin o the wood, and converts it into soluble degradation products which dissolve in the aL~aline digestion liquor. However, although the process has many attractive features, it has not received commercial accept-ance because the reaction is quite difficult to control. A non-uniform degradation of the wood produces cellulose pulp which . . . . . .
contains excessively degraded carbohydrate materials. Moreover, access to the interior of the wood is difficult, due to slow mass kansfer, and it is almost impossible to obtain a thorough pulping of the wood, and at the conclusion of the digestion a considerable proportion of the wood remains in the form of slivers, which have to be separated from the pulp.
Harris U. S. patent No . 2, 673, 148, dated March 23, 1954, proposed an oxygen digestion process using quite high oxygen pressùres, of the order of at least 800 psi. This was thought necessary in order to obtain and maintain a sufficiently high oxygen concentration in the digestion liquor. This is one of the serious problems in oxygen diges-tion processes due to the fact that oxygen is a gas which has a low solubility in the digestion liquor. The results obtained in this process were not satisfactory.
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1~34;~1~i0 Granga~rcl ancl Saunders, U.S. patent No. 2, 926,114, dated February 23, l9G0, stated that oxygen prior to 1957 had been tried both at low and at high oxygen pressures. Grangaard et al. proposed a digestion at pH 7 to 9 over at least a m~jor portion of the cooking time,under oxygen pressures of 40 to 250 psi, using conventional batch digesters. The pH is maintained within the desired range by a buffer such as sodium bicarbonate, or by continuous addition of aLkali such as sodium hydroxide or sodium carbonate, to neutralIze free acids formed throughout the digestion. However, the process gives the pulp unacceptable properties. The main improvement is in the spent liquor.
Other disadvantages are:
(1) An extremely high consumption of sodium bicarbonate.
(2) An extremely high oxygen consumpt.on.
(3) It is difficult to control the process, and a nonuniform pulp is obtained.
As the Grangaard et al~ patent illustrates, it has not been possible to develop a practical pulping process using oxygen in place of sulfide. A recent investigation o~ the oxygen digestion process by J. CO
Lescot, "Essais de delignification de bois feuillus par l'oxygene en milieu alcalin" (Ph.D. Thesis, Univ. of Grenoble, France, Octo 27, 1967), resulted in the conclusion that aLkaline oxygen digestion was not feasible commercially, since the difficulties of impregnation were important, even when using magnesium oxide as a protectorO
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Nonetheless, work has continued, with the objective of 2~ developing the aL~cali-oxygen gas digestion process to a stage where it is practical commercially.
.'' ' : . '.' '` ' ' '. ' ~' ' U.S. patent No. 3,764,464, patented Octo~er 9, 1973, proposed that celiulose pulp be prepared from wood by alkaline oxygen digestion in the presence of alkali metal carbonate or bicarb~nate. A gas phase including unreacted oxygen gas and carbon dioxide is formed during the digestion, and the carbon dioxide is separated from the gas phase by absorption while oxygen gas from which the carbon dioxide has been separated is recycled for subsequent digestion of wood.
Samuelson stated that in this way the alkaline oxygen digestion of wood can be controlled so as to inhibit formation slivers, as well as to prevent undue degradation, thereby to increase the - uniformity of the cellulose pulp, and improve its color and strength ~
3 properties. In the process, the alkaline digestion liquor comprises ~ ~ -aL~ali metal bicarbonate or carbonate, or both, oxygen gas is pro- ~ -vided under pressure to the reaction system, and carbon dioxide that 3 15 is formed and enters the oxygen phase during the digestion process is separated at least once during the digestion, and preferably either continuously, or from time to time, so as to maintain a high partial pressure of oxygen in the gas phase. The carbon dioxide that is separated can be recovered and used to form alkali metal carbonate, or bicarbonate, or both, and recycled, and since the resulting process i~ more efficient, this results in greater economy both of aLkali and .;
of oxygen.
Samuelson and Noreus, U.S. patent No. 3,769,152, :
patented October 3~, 1973, accordingly proposed to digest wood with a mixture of alkali and oxygen by limiting the amount of alkali at the beginning of the digestion to at most 75%, and preferably from about 5 to about 20%, of the total molar quantity of alkali required for the digestion, and to add the alkali progressively, either continuously or in increments, during the digestion, while maintaining the pH of the .
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lQ`~;~160 digestion liquor in the course of the digestion within the range from about 9 . 5 to al)out 13, and preferably from about 9 . 5 to about 11. 5.
Under these conditions, if the amount of allcali is restricted (unlike prior processes in which all of the alkali required for the 5 digestion is added ab_ itio), condensation of the lignin in the cellulose is avoided, and thus a darkening due to this condensation is prevented, while at the same time the cellulose ~s protected against excess degradation. Moreover, at the lower alkali concentration, the deleterious effect of oxygen in the production of an oxidative de-10 gradation of the cellulose in the wood may also be inhibited to someextent and the method can be used for the production of low-viscosity pulps e.g. rayon pulp.
Attempts also have been made to circumvent the difficulties encountered with aL~ali oxygen gas digestion processes, by first -15 predigesting the wood material, then mec~anically defibrating the wood to provide a semichemical pulp, and then delignifying the pulp with aL~ali and oxygen gas. Such processes have been designated oxygen gas digestion processes, but they are properly designated processes for bleaching semichemical or mechanical pulp. These methods are ex-20 tremely expensive, and the pulp yield is low.
In accordance with the invention, it has been determined that the all~aline oxygen gas digestion of wood in coarse particulate form such as wood chips is facilitated and the properties of the resulting cellulose pulp improved, if during the digestion process, the r 25 wood is treated, either continuously or intermittently, in the presence of the aqueous digestion liquid, so as to detach fibers from the surface of the wood, the detached fibers suspended in the digestion li~uor, and removed from the digestion zone; and then ~. .
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lV42160 - continuing the cligestion of the remaining wood with oxygen gas and alkali -; in aqueous solutioll to form cellulose pulp.
The process according to the present in~rention can be applied both to the continuous digestion of wood and to the batchwise digestion - 5 of wood. The treatment proceæs for detachment of the fibers can be applied only once, or it can be continued throughout the digestlon process, continuously or intermittently, or from time to time. Normally, it is more advantageous to begin the treatment process only after the surface of the wood material has been digested, e. g., after the digestion process has conlinued for a period of time within .~
the range from about 1/2 to about ten ho .rs, corresponding to la propo~tion witbin the range from about 20% to about 70% of the total digestio~ time requirèd. -A variety of known physical te~hniques can be applied 15 to the surface of the wood to detach fibers therefrom. Liquids ormechanical tools can be used. The treatment is applied to the sur-face of the wood only; it is not intended to penetrate deeply into the wood particles or to defibrate or disintegrate the particles in their entirety. A depth of from a fraction of a millimeter, corresponding 20 to one or a few fiber diameters, up to ~ f~w millimeters, e. g. 2 mm is quite sufficient. The treated particles retain their particulate identity, and do not in any way correspond to semichemical pulp.
One way of using a liquid to tear away the surface of the particles is to direct jets or streams of liquid against the wood 25 with such a force that surface fibers are detached. As digestion . . - . - - - . .
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1(~42160 proceeds by attack on the wood particle surface, fibers are fully or partially exposed. Pressurized high velocity streams of fluid forced under pressure through a mass or bed of such wood particles will detach SUCIl fibers. For this purpose, the digestion liquor itself or any liquor obtained from any oxygen gas-aLkali treatment can be used.
It is also possible to dètach surface fibers by directing a gaseous stream,blast,or jet ,preferably of oxygen gas,at elevated - pressure against the wood material or through a bed thereof.
Application of gaseous streams is suitably effected in the presence ` of digestion liquor.
It has been found to be particularly advantageous to -;
effect a simultaneous treatment of the wood material with both liquid and gas. This treament is suitably applied to partially digested wood material in a mass or bed. Irrespective of whether the process is effected solely with liquid, solely withgas, or with both, the height of the mass or bed may, to advantage, be between 0. 5 and 5 meters. -Larger or smaller heights can be used, however. The liquid or gas should be applied with such force that the mass bed is fluffed up or stirred, and the wood particles tumbled or otherwise moved about slowly or rapidly. ;
Another way to detach fibers is to apply mechanical 1 cutting, tearing, abrading or gouging to the wood surface. A
.J mechanical tool is applied with insufficient force to disintegrate the -~ 25 particles. The mechanical tool can be applied together with or insequence totreatment with gas and/or liquid, as described above.
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fibers in the liquor. 10421~0 It is impol~tant that the detached fibers l~e separated from the remaining wood material in the digestion zone The term "digestion zone" refers to the zone or apparatus in which the digestion of the wood is carried out, and includes the actual digester apparatus in which the digestion of wood with o~ygen and alkali takes place, or the zone or zones of an apparatus system in which such digestion ta}~es place. The separated fibers are not subjected to unnecessary de-' composition in the digestion liquor, and can, if desired, be treat.~d separately with aLl~ali and oxygen gas under conditions more appropriate to - fibers. Unexpectedly, and possibly owing to several interrelated -' factors, the continued digestion of the remaining wood material in the digestion zone is greatly facilitated, and the selectivity in the J process (defined as the quotient between the rate of delignification and the rate of cellulose decomposition) noticeably increases. This appears to be the direct result of detaching fibers from the wood material.
After the fiber suspension has been removed from the digestion zone, the fibers are separated from the digestion'liquor.
This can be effected with or without cooling of the suspension. The fibers can be separated from the liquor in one or more stages, using known devices, or combinations thereof. It is particularly advantageous to fractionate the fiber suspension into two or more fractions having different fiber contents, suitably, into a completeb or substantially completely fiber-free liquid fraction, and a fraction which is rich in fibers. In accordance with a preferred ' embodiment, this fractionation is effected by centrifugal separation .
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, Satisractory tools include agitators whicll work the partially digested woocl ma~erial to release the fibers, and/or pump means, such as centrifugal pumps or so-called high-con-centration pumps for pulp pumping, ~d other known devices for 5 detaching exposed fibers from non-digested or poorly digested particulate wood material.
Abrasion methods can àlso be used for detaching fibers in accordance with the invention. The wood material is preferably in the form of chips, and by suitable means the chip pieces- are caused 10 to rub against each other, and thus abrade their surfaces. Abrasion can be effected, for example, by using a rotary digester. This latter method is particularly suitable for application in conjunction with batch digestion processes. Methods based on rubbing or other abrasion techniques to detach the fibers can be, to advantage, com-lS bined with one or more of the previously mentioned treatment methods.
Vibration is a particularly effective method for detaching -fibers. One or more vibrators can be placed at appropriate positions in the digester apparatus. Another suitable vibration method is to pass the partially digested material over a vibrating table or a vibrating 20 screen. The vibration methods can also be combined advantageously with the other previously mentioned methods for detaching fibers during the digestion process from the partially digested wood material.
The detachment of the fibers from the surface of the wood particles is preferably though not necessarily carried out in the 25 presence of the alkali-oxygen gas digestion liquor. The liquor then serves as a suspending medium for the detached fibers. The de-taching technique can aid in dispersing and suspending the detached 'U
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processes, suital~ly by using hydrocyclones and/or separatore, or other continuously oper.lted centrifuges.
Screens or filters can also be used to remove the fibers or a fiber concentrate from the suspension.
For thermal technology reactions and also recovery of the fibers from the suspension as soon as possible, it is often suitable to maintain the fiber suspension at~a high temperature during the separation,su~h as at at most 10 lower than the temperature of the suspension when it was removed from the digestion zone.
Flotation and sedimentation techniques ma~ also be used, in a manner known per se, to effect the fiber separation or as a step therein.
3 It has been found particularly suitable to fractionate the suspension into a fiber-rich suspension and a fiber-lean or fiber-free suspension in an array of hydrocyclones operating at a temperature which approximates the temperature of the digestion liquor. The ~
fiber-rich suspension is then subjected to a further separation in one or more steps, to separate the fibers therefrom, in a manner kno~vn per se, for-example by screening and filtering. At least the last step can be effected to advantage subsequent to lowering the temperature.
This lowe~ing of the temperature can be continued down to a temperature, forexample, of 60-90~C., sothatsaidseparation process can be effected without disadvantage at atmospheric pressure.
The separated liquid fraction, which is completely or sub-stantially free of fibers, can be recycled for use as a digestion liquor in oxygen gas-alkali digestion processes. Thus, the liquor can be re-cycled to the same digestion process as that from which it was re-moYed, or it can be used in a different digestion charge, or it can be ., ', 10 ~:' , , ''' ' ' ' , . - - : . . .
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S' -- . . ~ , . .' ; . : -~4;~160 charged to another cligestion zone. It can also be used as treatment liquid in the hydraulic treatment of wood material Ior releasing fibers in accordance with the invention.
Before recycling, the alkali content and the content of any digestion adjuncts, such as degradation inhibitors, stabilizers, and buffering agents, can be replenished or modified in any way, and the pH adjusted, if necessary, to within the desired range.
Subsequent to separating the fibers, but prior to returning the liquid suspending medium to the oxygen gas digestion process, it may be suitable to heat the liquor, preferably to a temperature which is higher than that prevailing during the digestion process. In con-junction therewith, it is convenient to maintain the liquor in intimate contact with oxygen gas or air, optionally in the presence of a catalyst which expedites the destruction of readily oxidized substances. I
1i 15 The oxygen digestion process of the invention is applicable , to any kind of wood. In general, hardwood such as asp~n and birch can be pulped more easily than softwood, such as spruce and pine, but both types of wood can be pulped satisfactorily using this process. Exemp-lary hardwoods which can be pulped include birch, beech, poplar, ` 20 cherry, sycamore, hickory, ash, oak, chestnut, aspen, maple, alder ' and eucalyptus. Exemplary softwoods include spruce, fir, pine, cedar, ~uniper and hemlock.
In the case of softwood, the processing conditions, including the particle size of the wood fragments, the digestion temperaiNre, the aL~ali concentration, and the oxygen pressure, should be carefully de-termined and controlled during the digestion.
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The wood should be in particulate form. Wood chips having dimensions that are conventionally employed in the sulfate process can bn used. However, appreciable advantages with respect to uniformity of the digestion process under all kinds of reaction conditions within the stated ranges can be obtained if the wood is in the form of nonuni-form fragments of the type of wood shavings or chips having an aver-age thickness of at most 3 mm., e.g. about 0.2 to about 2 mm.
Other dimensions are not critical. Wood slivers and splinters, wood granules, and wood chunks, and other types of wood fragments can also be used.
The process of the invention can be eEfected to advantage on wood which, prior to being digested with oxygen gas and aL~ali, . ~ - . . , . - . .
- has been subjected to a chemical pretreatment process with acid, ~ . . . . . . . .
- neutral and ~ or alkaline aqueous solutions . The pretreatment process5 - i& suitably effected at elevated temperatures, for example, within the range from about 100 C to about 200 C., so that at least partial deacetylatlon of the wood is obtained. Treatment in an acid en-vironment with acid-additives or solely water can-be effected- to ad-vantage if pulps are desired which have a low content of hemi-cellulose, that is to say, pulps with relatively low yield.
When producing paper pulps having a relatively high yield, for example, from 50 to 70 kg unbleached pulp per 100 kg dry wood, it is particularly suitable to pretreat the wood material with a liquid containing basic neutralizing agents at a temperature within the range :
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- -1~14;~160 from about 12U to al:out 2ûO~C., preferably from 130 to 1803C.
The basic neutralizing agent used may be the came alkali used in the oxygen gas-alkali digestion process. It has been found particularly suitable to use sodium bicarbonate, although sodium carl)onate and/or 5 mixtures thereof with sodium bicarbonate have been found to give good results.
It has been found suitable to de-gas carbon dioxide fbrmed during the pretreatment process.
The reaction time during the pretreatment process is 10 suitably adapted so that the alkali added to said process is not totally consumed, so that the solution has a certain buffer effect, so as to avoid acid hydrolysis, unless otherwise desired. An addition of from , 5 to 30% by weight sodium bicarbonate, calculated on the dry weight ~ of the wood, and a tre~tment time of from 0. 5 to ~ hours at from - 15 120 to 180 C is particularly preferred.
As a result of the pretreatment process, the chips are often dark brown or black in color, although this color rapidly disappears during the subsequent oxygen gas aLkali digestion process. In combination with the method of the present invention, the pretreatment 20 process affords surprising advantages with respect to the selectivity of delignification, and, additionally, results in an appreciable shortening of the time required for the oxygen gas-alkali digestion process.
When preparing the pretreatment liquid, it is an advantage 25 to use digestion liquor from the oxygen gas-alkali digestion process, said liquor being removed at the end of a cooking sequence or during the course of such a sequence.
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~ 104Z160 The total amount of alkali that is required for the digestion is determined by the quality and type of the pulp to be produced, and is within the range from about l to about 10 kilomoles per 1000 kg.
of dry wood. It is well known that certain types of pulp are more 5 digested than others. This is entirely conventional, and does not form a part of the instant invention. Cellulose pulps intended to be used in the production of regenerated cellulose fibers, such as vis-cose, acetate and cuprammonium pulps, are quite fully digested, and should have a low content of lignin and hemicellulose. In the 10 production of such pulps, in accordance with the process of the invention, the amount of aLkali can be within the range from about 6 to about 8 kilomoles calculated as NaOH per 1000 kg. of dry wood.
For the production of bright paper pulp, which is readily defibered when the digester is blown, the amount of alXali used in the process of the 15 invention can be within the range from about 2 . 5 to about 5 kilomoles.
Generally, for most of the types of pulps given an intermediate degree of digestion, such as pulps for fine paper~ plastic fillers, and soft paper or tissue paper, the amount of alkali in the process of the invention is within the range from about 2 to a~out 6 kilomoles per 20 1000 kg. of dry wood.
Any alkali metal hydroxide or a3kali metal carbonate can be employed, such as sodium hydroxide, potassium hydroxide, ~
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lithium hydroxide, sodium carbonate, potassium carbonate and lithium carbonate. The sodium carbonate o~tained in the burning of cellulose 25 digestion waste liquors can be used for this purpose. The use of alkali ..... ..
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metal car~onate~ alld/or bicarbonates may be more advanta~eous than the use of alkali metal hydroxides in maintaining the pH of the digestion liquor within the staged range, because of the buffering properties of the carbonate or bicarbonate present or formed in situ. Consequently, 5 mixtures of aL~ali metal hydroxides and a~ali metal carbonates are particularly satisfactory to obtain the advantages of each, and dilute their disadvantages .
It is also possible to use mixtures with all~ali metal hydroxides or carbonates with aLl~ali metal bicarbonates such as sodium bicarbonate and 10 potassium bicarbonate. The aLkali metal bicarbonate in this case serves as a buffer. Other bu~fering agents, compounds of alkali metals with nondeleterious acidic anions, can be employed, such as a~ali metal acid phosphates.
A buffering agent, particularly a bicarbonate, is especially 15 desirable when it is desired to operate at a relatively low pH, for example, -`~ from about 6. 5 to about 10. In this case, bicarbonate or other buffering - agent can be added to advantage even if a~ali metal carbonate is present.
For economic reasons, the sodium compounds are preferred as the alkali metal hydroxide, a~ali metal carbonate and aLkali metal 20 bicarbonate.
All of the aL'cali required for the digestion can be added to the digestion liquor initially. Limiting the amount of alkali metal hydroxide and /or aLkali metal carbonate in the initial stages of the process improves the quality of the cellulose pulp, both if the digestion i8 at a 25 high pN, in excess of 1(1or at a low pH.
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If a mixture o~a~ka;~l me~)al hydroxide and aLlcali metal carbonate is used, it is particularly suitable if the initial charge comprises sodium carbonate, optionally with an addition of sodium bicarbonate as described above, the remainder of the a~kali added as the digestion proceeds being sodium hydroxide. If the aLkali -charge initially is al~ali metal hydroxide, it is usually important in producing pulps having a low lignin content that the initial charge be low, within the range from about 2 to about l 0%, of the total molar quantity of aLkali.
Whether or not the digestion process is carried out contin-uously or as a batch process, the alkali metal hydroxide and/or alkali metal carbonate can be charged continuously or in increments-to the digestion liquor. In a continuous digestion, the wood is caused to ' move through the digester from one end to the other which thereby constitutes a reaction zone In a batch process, the wood, usually in the form of chips, is retained in the reaction vessel throughout the digestion.
The oxygen gas digestion process can be effected, for example, with sodium hydroxide as an active allcali at a pH exceeding 10, wherewith it is advantageous to add the active alkali incrementally as it iS consumed. For the majority of wood types and pulp qualities, s it is more advantageous however to maintain the pH during the major ~ . ....... _ _ .. . .......................... . . . ..
portion of the digestion process with oxygen gas within the range from 3 about 6. 5 to about 10. When producing paper pulp from hardwood, it 25 has been found that much higher mechanical strength properties are obtained if the pH lies within the range from about 7 to about 9.
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.
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, - ' ' ~()4;~160 All pH-values refer to determination at 20 C on a sample withdrawn from the cligester through a cooler. Even in this case it is suitable to - add the active alkali successively as it is consumed. The active aLkali used is pre~erably sodium carbonate and/ or sodium bicarbonate and mixtures therof. The developed carbon dioxide retards the reaction, since it contributes to a decrease in pH. Carbon dioxide should normally be removed, e.g. by bleeding together with oxygen gas, by absorption, cooling or in accordance with other known methods.
Since the oxygen that is employed as an essential compon-10 ent in the digestion process of the invention is a gas, the so-called gas phase digestion procedure can be used to advantage. In this case, - the wood and the film of digestion liquor present on the wood are kept in continuous contact with the oxygen-containing gas. E the wood is completely or substantially immersed in the digestion liquor, it is s 15 important to agritate the wood and/or the gas and/or atomize the gas3 or the liquor. The oxygen should be dissolved or dispersed in the digestion liquor to the greatest extent possible. Dissolution or dis- -? persion of the oxygen in the liquor can take place within the digestion vesseI and/or externally of the same, such as in nozzles, containers or other known devices used for dissolving or dispersing gases in ;~
liquids.
Transfer of oxygen to the wood material impregnated with .
digestion liquor is important in the process, and is controlled by ; ~ -adjusting the oxygen pressure? the digestion tempsrature, and/or the . ~
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~ 0~;2160 propurtion of gas-liquid contact surfaces, includ~ng the wood impreg-nated with digec;tion liquor.
The oxygen is preferably employed as pure oxygen? but mixtures of oxygen with other inert gases c~n be used, such as, for 5 example, mixtures of oxygen with nitrogen and with carbon dioxide and with both, as well as air. Compressed air can also be used, although this complicates the devices for dissolving or dispersing the oxygen in the reaction mixture.
Prior to contact with the oxygen, the wood suitably in the 10 form of chips can be impregnated with an aqueous digestion liquor containing the desired chemicals. The chips are impregnated under vacuum, or under atmospheric pressure or superatmospheric pres-sure, or by other methods conventional in wood digestion processes.
The wood may also be treated with steam before being brought to the 1~ digestion zone.
The temperature employed during the impregnation can be w~thin the range from about 20 to about 120 C., although temperatures within the range from 90 to 120C. would not normally be used except under special circumstances. ~ -The digestion can be carried out at a temperature within the range from about 100 to about 180 C. Usually, it is advantageous if the digestion temperature is permitted to rise during the digestion process from an initial temperature of the order of from 100 to 120 C.
.
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to the maYimum digestion~;emperature, of the order of from 150 or 175~C.
At a maximum temperature of 100 C., the digestion pro-cess proceeds slowly, hut on the other hand, moderate oxygen pressure s 5 and simple technical apparatus can be used.
If a maximum digestion temperature of from 150 to 180 C
is used, the digestion will proceed rapidly. On the other hand, selectivity is relatively poor. At these temperatures, an exceedingly effective transfer of oxygen to the wood from the gas phase is re-quired. This requires intimate contact and high oxygen pressure. By effective control methods, however, all of which are conventional, it is possible to control the digestion within this temperature range, particularly when producing cellulose pulp of moderate yield.
Normally, a digestion temperature during the major part ;~
of the digestion process withinthe range from-120 to 16ûC. is preferred, at which temperature the digestion can take place in a reasonable time using relatively simple apparatus and under moderate oxygen pressure, with good control of pulp quality, and good selectivity. A relatively rapid reaction and a very high degree of selectivity is obtained within ;the temperature range of from- 135 to 150 C. For the majority of wood materials and pulp types, this latter temperature range is con-sidered the optimum range, when the pH is within the range 6. 5 - 10.
A wide range of partial pres~ures of oxygen can be used.
Normally, it is possible at pressures to work within the range oi from about 1 to about 300 bars, although in practice the range of from 5 to 100 bars is more feasible. The optimum pressure range lies in many instances at from 10 to about 50 bars, and very often at from 20 to about 40 bars.
19 :
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I04;~1~0 Tt has been found that an improved degree of selectivity is obtained when using a high partial pressure with respect to oxygen gas. Economic and safety reasons place an upper limit, the position - of which depends, for example, on the raw material and test~ of materials.
Because of the consumption of oxygen in the course of the digestion, and the higher rate at wpich the digestion proceeds at high reaction temperatures, it follows that the higher the reaction temper-ature, the higher the pressure that should be applied during the reac-tion. The optimum temperature and pressure conditions for a given pulp can be determined by digestion sampling procedures, as is well J known. Such trial-and-error experimentation is conventional, and is not a part of this invention.
Pulps for a certain field of use, for example, for use in the production of most types of paper, should have a high strength. In such cases, it is suitable to carry out the digestion in the presence of an inhibitor or mixture of inhibitors which protect the cellulose and hemicellulose molecules against uncontrolled degradation. The effect of the inhibitors is reflected by the viscosity of the pulp, and the degree of polymerization of the cellulose.
The inhibitors can to advantage be charged to the digestion liquor during an early stage of the digestion,prefarably, at the beginning, before the digestion heating is begun. Thus, they can be added to the digestion liguor before combination with the wood, or shortly thereafter. Suitable inhibitors are water-insoluble magnesium -compounds such as magnesium carbonate, waterffoluble magnesiumcompounas, such as magnesium sulphate,and complex magnesium compounds.
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~ 4;~160 It is oEtcn suitable during the digestion to withdraw a portion of the di~ estion liquor, SUCil as by draining, pressing, displacement or f-ltering. This liquor can be returned to the digestion process at a later stage, or to a subsequent batch, and in this event it is advan~age-O~lS to heat the liquid or a part thereof under pressure to an elevated temp~rature of the order of from 110 to about 200C. in intimate con-tact with an o~Tgen-containing gas such as air in order to oxidize organic substances in the liquor. The liquor can be fortified by adding alkali metal hydroxide and/or all~ali metal carbonate and/or inhlbitor before or after pressure-heating.
It may be desirable to wash the wood with water between s the pretreatment stage and the oxygen digestion process. Thi~ wash-ing step may be desirable in the case of any of the pretreatment proc~
esses described above. The washing, however, increases the cost of the processing, and a~o increases the risk of water contamination of the pulp with metal ions and metal compounds, and consequently it may .
often be more practical to omit the washing step A surface-active agent can be added to the digestion liquor, and contributes to a reduction in the resin content of the wood cellu-lose produced from the wood. This also surprisingly contributeæ to a reduction in the lignin content, and a more uniform delignification.
The æurface-active agent is suitably added at the beginning of the digestion process, or during an early stage of the digestion, and may J be present during all or only a part of the digestion. Cationic, anionic, ,.,;. ~ ~ -, ...
.. .
iO4~ 0 and nonionic surf~ce-active agents and mixtures thereof can be used.
If liquor is circulated during the digestion proces;s, it is suitable to use agcnts which do not produce foam. Examples of suitable surface-active agents are polyaL~cylene glycol ethers of fatty alcohols and alkyl 5 phenol polyoxyallcylene glycol ethers. Sulfonated anionic surface-active agents such as the aL'cylbenzene sulfonates can also be used.
The unbleached pulp has a high brightness. With birch pulps a brightness of 52~C according to SCAN is obtained, with a Kappa number of 15 and a total yield of 58%. With a Kappa number 10 of ~, a yield of 56~G and a brightness of 61~/c according to SCAN has been obtained. With spruce, there was obtained a SCAN brightness ;
of 50~ at a Kappa number of 19 and a yield of 54(3;~G by weight.
The pulps can be used directly with no bleaching and are comparable with semi bleached sulphate pulp and unbleached 1 ~i sulphite pulp, and can be used in place of these for different fields of use.
For example, they can be used to advantage directly for producing tissue paper, light cardboard and magazine paper. When a higher degree of brightness is desired, as for fine paper, rayon 20 and cellulose derivatives, the pulp can easily be bleached in accord-ance with known methods by treatment with chlorine, chlorine di-oxide, chlorite, hypochlorite, peroxide, peracetate, oxygen or any combinations of these bleaching agents in one or more bleaching sequences as described in Canadian patent No. 901, 220 issued ~ . , .
1~)4Z160 May 30, 1972. Chlorine dioxide has been found to be a particularly suita~le bleaching agent for the o~ygen digested cellulose pulp obtained in accordance with this invention. The consumption of bleaching chemicals is markedly lower in bleaching oxygen digested pulps of the invention than when bleaching sulfate cellulose.
The chemicals used for the digestion process can be recovered after the waste liquor is burned and subsequent to optionally causticizing all or part of the carbonate obtained when burning the liquor.
- ïo The method is particuiarly suitable for application in the continuous manufacture of wood cellulose pulp, since, owing to the fact that detached fibers are removed, the whole volume of the digestion zone is available for use. To enable the apparatus to be used in the most effective manner, with batchwise digestion processes, 15 ~ it is suitable to interrupt the oxygen gas digestion process whilst a certain quantity of partially digested chips (so-called shives) still remain (for example 5 to lO~o by weight of the wood material), and ~, to separately convert the shives to pulp with exposed fibers, e. g.
by a separate oxygen gas digestion process, optionally subsequent to mechanically treating the shives.
A preferred embodiment of the digestion process of the invention and of the cellulose pulp of the invention is shown in the following Example:
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104~ ;0 Cornmercially available birch chips about 5 mm thick and Z5 mm long were suhjected to a pretreatment process with 20% by weight aqueous sodium bicarbonate at 160 C for 2 hours at a ratio of wood:
liquid of 1:5 (kg/dm~3). The treatment process was carried out in an 5 autoclave. The pressure in the autoclave was lowered every half-hour to almost atmospheric pressure, to remove expelled carbon dioxide.
During the pretreatment process readily soluble hemicellulose was re-moved and the wood deacetylated. Subsequent to this treatment process, the yield was 85%, calculated on the dry weight of the wood, and the chips ` 10 wereblack in color.
The chips were subjected to an aLkali oxygen gas digestion in an autocIave digester in which a circulating aqueous digestion liquor pre- -pared from sodium bicarbonate was sprayed over the chips. The wood:
liquor ratio was 1:14. The oxygen gas pressure was 9 bars and the 15 temperature 140C. During the digestion process, saturated aqueous sodium bicarbonate solution was added, and carbon dioxide was removed continuously, so that the pH measured on cold (20 C) digestion liquid withdrawn from the digester-under cooling was maintained between 7. 5 and and 8. The amount of sodium bicarbonate charged was 1. 8~o by weight 20 during the course of the digestion, calculated on the dry weight of the ingoin~ wood.
After 3 hours digestion, digestion liquor was forced up through the bed of partialb digested chips at a speed such as to cause the chips in the bed to tumble about. In this way, fibers which were liberated during 25 the digestion were detached from the chips, and passed into suspension in the digestion liquor. ~ ~ -` ~
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~ . ' _ . ' ,, . . . ' --" 104;~160 T}l~ liqu~r was contilluously withdrawn from the digester and fractionated in ~n array Or hydrocyclones which separated the suspension into a fiber-lean and fiber-rich fraction. The input suspension had a pulp concentration of 0. 5% by weight, and the fiber-rich suspension had a pulp concentration of 1. 5% by weight.
The fiber lean fraction was recycled by means of a centrifugal pump to the digester and circulated through the bed up-wardly from the bottom of the bed. This hydraulic tumbling of the chip bed was continued for 10 minutes, during which time the digestion apparatus was under 9 bars oxygen gas pressure. ~ ~ -The fiber-rich fraction was passed through a pressurized dewaterer. The filtrate obtained was recycled to the digester. The pulp was washed. The process of forcing the digestion liquor from below through the bed of partially digested chips and the fiber 15 separation steps in accordance with the foregoing were repeated at intervals of 30 minutes, until a total digestion time of 5. 5 hours had elapsed. The pressure was then lowered, and the remaining pulp and partially digested wood were blown out. The pulp was screened and washed. The shives were subjected to continuous oxygen gas 20 digestion.
Laboaratory tests on the resulting birch chips showed that ~ -after an elapsed digestion time of 3 hours, 26. 5% by weight of the original wood was obtained as pulp in the suspension, while 42. ~% was present as incompletely digested chips. After 3. 5 hours, lV42160 the corresponding figures were 38. 9% and 30. 0%. The viscosity of the pulp recovered from the suspension was 910 cm3/g, g according to SCAN. The quantity of alkali-resistant pulp (determined in 5% NaOH according to SCAN (R5 value) was 68. 9%
by weight, and the viscosity of the alkali-resistant plllp was 1268 cm /g.
At the end of the digestion time (i. e. after a total digestion time of 5. 5 hours) the total yield of screened pulp was 54. 9~o. The viscosity of the pulp was 880 cm /g. The R5 value was 70. 0 and the viscosity of the aLt~ali-resistant pulp was 1200 cm /g. In addition, there was ob~ained 6. 2% shives calculated on the original dry wood.
After the shives had been digested for 2 hours, a further 3% of screened pulp was recovered.
;~ The total pulp-yield was thus 57. 9% by weight, calculated .
on the dry, commercially available birch chips. The results from determinations on the viscosity of the pulp extracted with 5% NaO~
æhowed that the method results in but slight attach on the cellulose molecules, as compared wlth other methods of oxygen gas digestion.
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As the Grangaard et al~ patent illustrates, it has not been possible to develop a practical pulping process using oxygen in place of sulfide. A recent investigation o~ the oxygen digestion process by J. CO
Lescot, "Essais de delignification de bois feuillus par l'oxygene en milieu alcalin" (Ph.D. Thesis, Univ. of Grenoble, France, Octo 27, 1967), resulted in the conclusion that aLkaline oxygen digestion was not feasible commercially, since the difficulties of impregnation were important, even when using magnesium oxide as a protectorO
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Nonetheless, work has continued, with the objective of 2~ developing the aL~cali-oxygen gas digestion process to a stage where it is practical commercially.
.'' ' : . '.' '` ' ' '. ' ~' ' U.S. patent No. 3,764,464, patented Octo~er 9, 1973, proposed that celiulose pulp be prepared from wood by alkaline oxygen digestion in the presence of alkali metal carbonate or bicarb~nate. A gas phase including unreacted oxygen gas and carbon dioxide is formed during the digestion, and the carbon dioxide is separated from the gas phase by absorption while oxygen gas from which the carbon dioxide has been separated is recycled for subsequent digestion of wood.
Samuelson stated that in this way the alkaline oxygen digestion of wood can be controlled so as to inhibit formation slivers, as well as to prevent undue degradation, thereby to increase the - uniformity of the cellulose pulp, and improve its color and strength ~
3 properties. In the process, the alkaline digestion liquor comprises ~ ~ -aL~ali metal bicarbonate or carbonate, or both, oxygen gas is pro- ~ -vided under pressure to the reaction system, and carbon dioxide that 3 15 is formed and enters the oxygen phase during the digestion process is separated at least once during the digestion, and preferably either continuously, or from time to time, so as to maintain a high partial pressure of oxygen in the gas phase. The carbon dioxide that is separated can be recovered and used to form alkali metal carbonate, or bicarbonate, or both, and recycled, and since the resulting process i~ more efficient, this results in greater economy both of aLkali and .;
of oxygen.
Samuelson and Noreus, U.S. patent No. 3,769,152, :
patented October 3~, 1973, accordingly proposed to digest wood with a mixture of alkali and oxygen by limiting the amount of alkali at the beginning of the digestion to at most 75%, and preferably from about 5 to about 20%, of the total molar quantity of alkali required for the digestion, and to add the alkali progressively, either continuously or in increments, during the digestion, while maintaining the pH of the .
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lQ`~;~160 digestion liquor in the course of the digestion within the range from about 9 . 5 to al)out 13, and preferably from about 9 . 5 to about 11. 5.
Under these conditions, if the amount of allcali is restricted (unlike prior processes in which all of the alkali required for the 5 digestion is added ab_ itio), condensation of the lignin in the cellulose is avoided, and thus a darkening due to this condensation is prevented, while at the same time the cellulose ~s protected against excess degradation. Moreover, at the lower alkali concentration, the deleterious effect of oxygen in the production of an oxidative de-10 gradation of the cellulose in the wood may also be inhibited to someextent and the method can be used for the production of low-viscosity pulps e.g. rayon pulp.
Attempts also have been made to circumvent the difficulties encountered with aL~ali oxygen gas digestion processes, by first -15 predigesting the wood material, then mec~anically defibrating the wood to provide a semichemical pulp, and then delignifying the pulp with aL~ali and oxygen gas. Such processes have been designated oxygen gas digestion processes, but they are properly designated processes for bleaching semichemical or mechanical pulp. These methods are ex-20 tremely expensive, and the pulp yield is low.
In accordance with the invention, it has been determined that the all~aline oxygen gas digestion of wood in coarse particulate form such as wood chips is facilitated and the properties of the resulting cellulose pulp improved, if during the digestion process, the r 25 wood is treated, either continuously or intermittently, in the presence of the aqueous digestion liquid, so as to detach fibers from the surface of the wood, the detached fibers suspended in the digestion li~uor, and removed from the digestion zone; and then ~. .
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lV42160 - continuing the cligestion of the remaining wood with oxygen gas and alkali -; in aqueous solutioll to form cellulose pulp.
The process according to the present in~rention can be applied both to the continuous digestion of wood and to the batchwise digestion - 5 of wood. The treatment proceæs for detachment of the fibers can be applied only once, or it can be continued throughout the digestlon process, continuously or intermittently, or from time to time. Normally, it is more advantageous to begin the treatment process only after the surface of the wood material has been digested, e. g., after the digestion process has conlinued for a period of time within .~
the range from about 1/2 to about ten ho .rs, corresponding to la propo~tion witbin the range from about 20% to about 70% of the total digestio~ time requirèd. -A variety of known physical te~hniques can be applied 15 to the surface of the wood to detach fibers therefrom. Liquids ormechanical tools can be used. The treatment is applied to the sur-face of the wood only; it is not intended to penetrate deeply into the wood particles or to defibrate or disintegrate the particles in their entirety. A depth of from a fraction of a millimeter, corresponding 20 to one or a few fiber diameters, up to ~ f~w millimeters, e. g. 2 mm is quite sufficient. The treated particles retain their particulate identity, and do not in any way correspond to semichemical pulp.
One way of using a liquid to tear away the surface of the particles is to direct jets or streams of liquid against the wood 25 with such a force that surface fibers are detached. As digestion . . - . - - - . .
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1(~42160 proceeds by attack on the wood particle surface, fibers are fully or partially exposed. Pressurized high velocity streams of fluid forced under pressure through a mass or bed of such wood particles will detach SUCIl fibers. For this purpose, the digestion liquor itself or any liquor obtained from any oxygen gas-aLkali treatment can be used.
It is also possible to dètach surface fibers by directing a gaseous stream,blast,or jet ,preferably of oxygen gas,at elevated - pressure against the wood material or through a bed thereof.
Application of gaseous streams is suitably effected in the presence ` of digestion liquor.
It has been found to be particularly advantageous to -;
effect a simultaneous treatment of the wood material with both liquid and gas. This treament is suitably applied to partially digested wood material in a mass or bed. Irrespective of whether the process is effected solely with liquid, solely withgas, or with both, the height of the mass or bed may, to advantage, be between 0. 5 and 5 meters. -Larger or smaller heights can be used, however. The liquid or gas should be applied with such force that the mass bed is fluffed up or stirred, and the wood particles tumbled or otherwise moved about slowly or rapidly. ;
Another way to detach fibers is to apply mechanical 1 cutting, tearing, abrading or gouging to the wood surface. A
.J mechanical tool is applied with insufficient force to disintegrate the -~ 25 particles. The mechanical tool can be applied together with or insequence totreatment with gas and/or liquid, as described above.
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fibers in the liquor. 10421~0 It is impol~tant that the detached fibers l~e separated from the remaining wood material in the digestion zone The term "digestion zone" refers to the zone or apparatus in which the digestion of the wood is carried out, and includes the actual digester apparatus in which the digestion of wood with o~ygen and alkali takes place, or the zone or zones of an apparatus system in which such digestion ta}~es place. The separated fibers are not subjected to unnecessary de-' composition in the digestion liquor, and can, if desired, be treat.~d separately with aLl~ali and oxygen gas under conditions more appropriate to - fibers. Unexpectedly, and possibly owing to several interrelated -' factors, the continued digestion of the remaining wood material in the digestion zone is greatly facilitated, and the selectivity in the J process (defined as the quotient between the rate of delignification and the rate of cellulose decomposition) noticeably increases. This appears to be the direct result of detaching fibers from the wood material.
After the fiber suspension has been removed from the digestion zone, the fibers are separated from the digestion'liquor.
This can be effected with or without cooling of the suspension. The fibers can be separated from the liquor in one or more stages, using known devices, or combinations thereof. It is particularly advantageous to fractionate the fiber suspension into two or more fractions having different fiber contents, suitably, into a completeb or substantially completely fiber-free liquid fraction, and a fraction which is rich in fibers. In accordance with a preferred ' embodiment, this fractionation is effected by centrifugal separation .
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, Satisractory tools include agitators whicll work the partially digested woocl ma~erial to release the fibers, and/or pump means, such as centrifugal pumps or so-called high-con-centration pumps for pulp pumping, ~d other known devices for 5 detaching exposed fibers from non-digested or poorly digested particulate wood material.
Abrasion methods can àlso be used for detaching fibers in accordance with the invention. The wood material is preferably in the form of chips, and by suitable means the chip pieces- are caused 10 to rub against each other, and thus abrade their surfaces. Abrasion can be effected, for example, by using a rotary digester. This latter method is particularly suitable for application in conjunction with batch digestion processes. Methods based on rubbing or other abrasion techniques to detach the fibers can be, to advantage, com-lS bined with one or more of the previously mentioned treatment methods.
Vibration is a particularly effective method for detaching -fibers. One or more vibrators can be placed at appropriate positions in the digester apparatus. Another suitable vibration method is to pass the partially digested material over a vibrating table or a vibrating 20 screen. The vibration methods can also be combined advantageously with the other previously mentioned methods for detaching fibers during the digestion process from the partially digested wood material.
The detachment of the fibers from the surface of the wood particles is preferably though not necessarily carried out in the 25 presence of the alkali-oxygen gas digestion liquor. The liquor then serves as a suspending medium for the detached fibers. The de-taching technique can aid in dispersing and suspending the detached 'U
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processes, suital~ly by using hydrocyclones and/or separatore, or other continuously oper.lted centrifuges.
Screens or filters can also be used to remove the fibers or a fiber concentrate from the suspension.
For thermal technology reactions and also recovery of the fibers from the suspension as soon as possible, it is often suitable to maintain the fiber suspension at~a high temperature during the separation,su~h as at at most 10 lower than the temperature of the suspension when it was removed from the digestion zone.
Flotation and sedimentation techniques ma~ also be used, in a manner known per se, to effect the fiber separation or as a step therein.
3 It has been found particularly suitable to fractionate the suspension into a fiber-rich suspension and a fiber-lean or fiber-free suspension in an array of hydrocyclones operating at a temperature which approximates the temperature of the digestion liquor. The ~
fiber-rich suspension is then subjected to a further separation in one or more steps, to separate the fibers therefrom, in a manner kno~vn per se, for-example by screening and filtering. At least the last step can be effected to advantage subsequent to lowering the temperature.
This lowe~ing of the temperature can be continued down to a temperature, forexample, of 60-90~C., sothatsaidseparation process can be effected without disadvantage at atmospheric pressure.
The separated liquid fraction, which is completely or sub-stantially free of fibers, can be recycled for use as a digestion liquor in oxygen gas-alkali digestion processes. Thus, the liquor can be re-cycled to the same digestion process as that from which it was re-moYed, or it can be used in a different digestion charge, or it can be ., ', 10 ~:' , , ''' ' ' ' , . - - : . . .
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S' -- . . ~ , . .' ; . : -~4;~160 charged to another cligestion zone. It can also be used as treatment liquid in the hydraulic treatment of wood material Ior releasing fibers in accordance with the invention.
Before recycling, the alkali content and the content of any digestion adjuncts, such as degradation inhibitors, stabilizers, and buffering agents, can be replenished or modified in any way, and the pH adjusted, if necessary, to within the desired range.
Subsequent to separating the fibers, but prior to returning the liquid suspending medium to the oxygen gas digestion process, it may be suitable to heat the liquor, preferably to a temperature which is higher than that prevailing during the digestion process. In con-junction therewith, it is convenient to maintain the liquor in intimate contact with oxygen gas or air, optionally in the presence of a catalyst which expedites the destruction of readily oxidized substances. I
1i 15 The oxygen digestion process of the invention is applicable , to any kind of wood. In general, hardwood such as asp~n and birch can be pulped more easily than softwood, such as spruce and pine, but both types of wood can be pulped satisfactorily using this process. Exemp-lary hardwoods which can be pulped include birch, beech, poplar, ` 20 cherry, sycamore, hickory, ash, oak, chestnut, aspen, maple, alder ' and eucalyptus. Exemplary softwoods include spruce, fir, pine, cedar, ~uniper and hemlock.
In the case of softwood, the processing conditions, including the particle size of the wood fragments, the digestion temperaiNre, the aL~ali concentration, and the oxygen pressure, should be carefully de-termined and controlled during the digestion.
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The wood should be in particulate form. Wood chips having dimensions that are conventionally employed in the sulfate process can bn used. However, appreciable advantages with respect to uniformity of the digestion process under all kinds of reaction conditions within the stated ranges can be obtained if the wood is in the form of nonuni-form fragments of the type of wood shavings or chips having an aver-age thickness of at most 3 mm., e.g. about 0.2 to about 2 mm.
Other dimensions are not critical. Wood slivers and splinters, wood granules, and wood chunks, and other types of wood fragments can also be used.
The process of the invention can be eEfected to advantage on wood which, prior to being digested with oxygen gas and aL~ali, . ~ - . . , . - . .
- has been subjected to a chemical pretreatment process with acid, ~ . . . . . . . .
- neutral and ~ or alkaline aqueous solutions . The pretreatment process5 - i& suitably effected at elevated temperatures, for example, within the range from about 100 C to about 200 C., so that at least partial deacetylatlon of the wood is obtained. Treatment in an acid en-vironment with acid-additives or solely water can-be effected- to ad-vantage if pulps are desired which have a low content of hemi-cellulose, that is to say, pulps with relatively low yield.
When producing paper pulps having a relatively high yield, for example, from 50 to 70 kg unbleached pulp per 100 kg dry wood, it is particularly suitable to pretreat the wood material with a liquid containing basic neutralizing agents at a temperature within the range :
_. . .
- -1~14;~160 from about 12U to al:out 2ûO~C., preferably from 130 to 1803C.
The basic neutralizing agent used may be the came alkali used in the oxygen gas-alkali digestion process. It has been found particularly suitable to use sodium bicarbonate, although sodium carl)onate and/or 5 mixtures thereof with sodium bicarbonate have been found to give good results.
It has been found suitable to de-gas carbon dioxide fbrmed during the pretreatment process.
The reaction time during the pretreatment process is 10 suitably adapted so that the alkali added to said process is not totally consumed, so that the solution has a certain buffer effect, so as to avoid acid hydrolysis, unless otherwise desired. An addition of from , 5 to 30% by weight sodium bicarbonate, calculated on the dry weight ~ of the wood, and a tre~tment time of from 0. 5 to ~ hours at from - 15 120 to 180 C is particularly preferred.
As a result of the pretreatment process, the chips are often dark brown or black in color, although this color rapidly disappears during the subsequent oxygen gas aLkali digestion process. In combination with the method of the present invention, the pretreatment 20 process affords surprising advantages with respect to the selectivity of delignification, and, additionally, results in an appreciable shortening of the time required for the oxygen gas-alkali digestion process.
When preparing the pretreatment liquid, it is an advantage 25 to use digestion liquor from the oxygen gas-alkali digestion process, said liquor being removed at the end of a cooking sequence or during the course of such a sequence.
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.. .
~ 104Z160 The total amount of alkali that is required for the digestion is determined by the quality and type of the pulp to be produced, and is within the range from about l to about 10 kilomoles per 1000 kg.
of dry wood. It is well known that certain types of pulp are more 5 digested than others. This is entirely conventional, and does not form a part of the instant invention. Cellulose pulps intended to be used in the production of regenerated cellulose fibers, such as vis-cose, acetate and cuprammonium pulps, are quite fully digested, and should have a low content of lignin and hemicellulose. In the 10 production of such pulps, in accordance with the process of the invention, the amount of aLkali can be within the range from about 6 to about 8 kilomoles calculated as NaOH per 1000 kg. of dry wood.
For the production of bright paper pulp, which is readily defibered when the digester is blown, the amount of alXali used in the process of the 15 invention can be within the range from about 2 . 5 to about 5 kilomoles.
Generally, for most of the types of pulps given an intermediate degree of digestion, such as pulps for fine paper~ plastic fillers, and soft paper or tissue paper, the amount of alkali in the process of the invention is within the range from about 2 to a~out 6 kilomoles per 20 1000 kg. of dry wood.
Any alkali metal hydroxide or a3kali metal carbonate can be employed, such as sodium hydroxide, potassium hydroxide, ~
. .
lithium hydroxide, sodium carbonate, potassium carbonate and lithium carbonate. The sodium carbonate o~tained in the burning of cellulose 25 digestion waste liquors can be used for this purpose. The use of alkali ..... ..
.
--- IO~
metal car~onate~ alld/or bicarbonates may be more advanta~eous than the use of alkali metal hydroxides in maintaining the pH of the digestion liquor within the staged range, because of the buffering properties of the carbonate or bicarbonate present or formed in situ. Consequently, 5 mixtures of aL~ali metal hydroxides and a~ali metal carbonates are particularly satisfactory to obtain the advantages of each, and dilute their disadvantages .
It is also possible to use mixtures with all~ali metal hydroxides or carbonates with aLl~ali metal bicarbonates such as sodium bicarbonate and 10 potassium bicarbonate. The aLkali metal bicarbonate in this case serves as a buffer. Other bu~fering agents, compounds of alkali metals with nondeleterious acidic anions, can be employed, such as a~ali metal acid phosphates.
A buffering agent, particularly a bicarbonate, is especially 15 desirable when it is desired to operate at a relatively low pH, for example, -`~ from about 6. 5 to about 10. In this case, bicarbonate or other buffering - agent can be added to advantage even if a~ali metal carbonate is present.
For economic reasons, the sodium compounds are preferred as the alkali metal hydroxide, a~ali metal carbonate and aLkali metal 20 bicarbonate.
All of the aL'cali required for the digestion can be added to the digestion liquor initially. Limiting the amount of alkali metal hydroxide and /or aLkali metal carbonate in the initial stages of the process improves the quality of the cellulose pulp, both if the digestion i8 at a 25 high pN, in excess of 1(1or at a low pH.
' .:
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. . .. .
, - - , - .. . .
.. . .
If a mixture o~a~ka;~l me~)al hydroxide and aLlcali metal carbonate is used, it is particularly suitable if the initial charge comprises sodium carbonate, optionally with an addition of sodium bicarbonate as described above, the remainder of the a~kali added as the digestion proceeds being sodium hydroxide. If the aLkali -charge initially is al~ali metal hydroxide, it is usually important in producing pulps having a low lignin content that the initial charge be low, within the range from about 2 to about l 0%, of the total molar quantity of aLkali.
Whether or not the digestion process is carried out contin-uously or as a batch process, the alkali metal hydroxide and/or alkali metal carbonate can be charged continuously or in increments-to the digestion liquor. In a continuous digestion, the wood is caused to ' move through the digester from one end to the other which thereby constitutes a reaction zone In a batch process, the wood, usually in the form of chips, is retained in the reaction vessel throughout the digestion.
The oxygen gas digestion process can be effected, for example, with sodium hydroxide as an active allcali at a pH exceeding 10, wherewith it is advantageous to add the active alkali incrementally as it iS consumed. For the majority of wood types and pulp qualities, s it is more advantageous however to maintain the pH during the major ~ . ....... _ _ .. . .......................... . . . ..
portion of the digestion process with oxygen gas within the range from 3 about 6. 5 to about 10. When producing paper pulp from hardwood, it 25 has been found that much higher mechanical strength properties are obtained if the pH lies within the range from about 7 to about 9.
.
.
.
, - ' ' ~()4;~160 All pH-values refer to determination at 20 C on a sample withdrawn from the cligester through a cooler. Even in this case it is suitable to - add the active alkali successively as it is consumed. The active aLkali used is pre~erably sodium carbonate and/ or sodium bicarbonate and mixtures therof. The developed carbon dioxide retards the reaction, since it contributes to a decrease in pH. Carbon dioxide should normally be removed, e.g. by bleeding together with oxygen gas, by absorption, cooling or in accordance with other known methods.
Since the oxygen that is employed as an essential compon-10 ent in the digestion process of the invention is a gas, the so-called gas phase digestion procedure can be used to advantage. In this case, - the wood and the film of digestion liquor present on the wood are kept in continuous contact with the oxygen-containing gas. E the wood is completely or substantially immersed in the digestion liquor, it is s 15 important to agritate the wood and/or the gas and/or atomize the gas3 or the liquor. The oxygen should be dissolved or dispersed in the digestion liquor to the greatest extent possible. Dissolution or dis- -? persion of the oxygen in the liquor can take place within the digestion vesseI and/or externally of the same, such as in nozzles, containers or other known devices used for dissolving or dispersing gases in ;~
liquids.
Transfer of oxygen to the wood material impregnated with .
digestion liquor is important in the process, and is controlled by ; ~ -adjusting the oxygen pressure? the digestion tempsrature, and/or the . ~
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~ 0~;2160 propurtion of gas-liquid contact surfaces, includ~ng the wood impreg-nated with digec;tion liquor.
The oxygen is preferably employed as pure oxygen? but mixtures of oxygen with other inert gases c~n be used, such as, for 5 example, mixtures of oxygen with nitrogen and with carbon dioxide and with both, as well as air. Compressed air can also be used, although this complicates the devices for dissolving or dispersing the oxygen in the reaction mixture.
Prior to contact with the oxygen, the wood suitably in the 10 form of chips can be impregnated with an aqueous digestion liquor containing the desired chemicals. The chips are impregnated under vacuum, or under atmospheric pressure or superatmospheric pres-sure, or by other methods conventional in wood digestion processes.
The wood may also be treated with steam before being brought to the 1~ digestion zone.
The temperature employed during the impregnation can be w~thin the range from about 20 to about 120 C., although temperatures within the range from 90 to 120C. would not normally be used except under special circumstances. ~ -The digestion can be carried out at a temperature within the range from about 100 to about 180 C. Usually, it is advantageous if the digestion temperature is permitted to rise during the digestion process from an initial temperature of the order of from 100 to 120 C.
.
~ .
.
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to the maYimum digestion~;emperature, of the order of from 150 or 175~C.
At a maximum temperature of 100 C., the digestion pro-cess proceeds slowly, hut on the other hand, moderate oxygen pressure s 5 and simple technical apparatus can be used.
If a maximum digestion temperature of from 150 to 180 C
is used, the digestion will proceed rapidly. On the other hand, selectivity is relatively poor. At these temperatures, an exceedingly effective transfer of oxygen to the wood from the gas phase is re-quired. This requires intimate contact and high oxygen pressure. By effective control methods, however, all of which are conventional, it is possible to control the digestion within this temperature range, particularly when producing cellulose pulp of moderate yield.
Normally, a digestion temperature during the major part ;~
of the digestion process withinthe range from-120 to 16ûC. is preferred, at which temperature the digestion can take place in a reasonable time using relatively simple apparatus and under moderate oxygen pressure, with good control of pulp quality, and good selectivity. A relatively rapid reaction and a very high degree of selectivity is obtained within ;the temperature range of from- 135 to 150 C. For the majority of wood materials and pulp types, this latter temperature range is con-sidered the optimum range, when the pH is within the range 6. 5 - 10.
A wide range of partial pres~ures of oxygen can be used.
Normally, it is possible at pressures to work within the range oi from about 1 to about 300 bars, although in practice the range of from 5 to 100 bars is more feasible. The optimum pressure range lies in many instances at from 10 to about 50 bars, and very often at from 20 to about 40 bars.
19 :
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. ;. ,. . - .. - . . . .. . .. . . .. .
- . .. . .. . .
I04;~1~0 Tt has been found that an improved degree of selectivity is obtained when using a high partial pressure with respect to oxygen gas. Economic and safety reasons place an upper limit, the position - of which depends, for example, on the raw material and test~ of materials.
Because of the consumption of oxygen in the course of the digestion, and the higher rate at wpich the digestion proceeds at high reaction temperatures, it follows that the higher the reaction temper-ature, the higher the pressure that should be applied during the reac-tion. The optimum temperature and pressure conditions for a given pulp can be determined by digestion sampling procedures, as is well J known. Such trial-and-error experimentation is conventional, and is not a part of this invention.
Pulps for a certain field of use, for example, for use in the production of most types of paper, should have a high strength. In such cases, it is suitable to carry out the digestion in the presence of an inhibitor or mixture of inhibitors which protect the cellulose and hemicellulose molecules against uncontrolled degradation. The effect of the inhibitors is reflected by the viscosity of the pulp, and the degree of polymerization of the cellulose.
The inhibitors can to advantage be charged to the digestion liquor during an early stage of the digestion,prefarably, at the beginning, before the digestion heating is begun. Thus, they can be added to the digestion liguor before combination with the wood, or shortly thereafter. Suitable inhibitors are water-insoluble magnesium -compounds such as magnesium carbonate, waterffoluble magnesiumcompounas, such as magnesium sulphate,and complex magnesium compounds.
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,....... ~ . ; ~ .. .... ....
. . ..
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~ 4;~160 It is oEtcn suitable during the digestion to withdraw a portion of the di~ estion liquor, SUCil as by draining, pressing, displacement or f-ltering. This liquor can be returned to the digestion process at a later stage, or to a subsequent batch, and in this event it is advan~age-O~lS to heat the liquid or a part thereof under pressure to an elevated temp~rature of the order of from 110 to about 200C. in intimate con-tact with an o~Tgen-containing gas such as air in order to oxidize organic substances in the liquor. The liquor can be fortified by adding alkali metal hydroxide and/or all~ali metal carbonate and/or inhlbitor before or after pressure-heating.
It may be desirable to wash the wood with water between s the pretreatment stage and the oxygen digestion process. Thi~ wash-ing step may be desirable in the case of any of the pretreatment proc~
esses described above. The washing, however, increases the cost of the processing, and a~o increases the risk of water contamination of the pulp with metal ions and metal compounds, and consequently it may .
often be more practical to omit the washing step A surface-active agent can be added to the digestion liquor, and contributes to a reduction in the resin content of the wood cellu-lose produced from the wood. This also surprisingly contributeæ to a reduction in the lignin content, and a more uniform delignification.
The æurface-active agent is suitably added at the beginning of the digestion process, or during an early stage of the digestion, and may J be present during all or only a part of the digestion. Cationic, anionic, ,.,;. ~ ~ -, ...
.. .
iO4~ 0 and nonionic surf~ce-active agents and mixtures thereof can be used.
If liquor is circulated during the digestion proces;s, it is suitable to use agcnts which do not produce foam. Examples of suitable surface-active agents are polyaL~cylene glycol ethers of fatty alcohols and alkyl 5 phenol polyoxyallcylene glycol ethers. Sulfonated anionic surface-active agents such as the aL'cylbenzene sulfonates can also be used.
The unbleached pulp has a high brightness. With birch pulps a brightness of 52~C according to SCAN is obtained, with a Kappa number of 15 and a total yield of 58%. With a Kappa number 10 of ~, a yield of 56~G and a brightness of 61~/c according to SCAN has been obtained. With spruce, there was obtained a SCAN brightness ;
of 50~ at a Kappa number of 19 and a yield of 54(3;~G by weight.
The pulps can be used directly with no bleaching and are comparable with semi bleached sulphate pulp and unbleached 1 ~i sulphite pulp, and can be used in place of these for different fields of use.
For example, they can be used to advantage directly for producing tissue paper, light cardboard and magazine paper. When a higher degree of brightness is desired, as for fine paper, rayon 20 and cellulose derivatives, the pulp can easily be bleached in accord-ance with known methods by treatment with chlorine, chlorine di-oxide, chlorite, hypochlorite, peroxide, peracetate, oxygen or any combinations of these bleaching agents in one or more bleaching sequences as described in Canadian patent No. 901, 220 issued ~ . , .
1~)4Z160 May 30, 1972. Chlorine dioxide has been found to be a particularly suita~le bleaching agent for the o~ygen digested cellulose pulp obtained in accordance with this invention. The consumption of bleaching chemicals is markedly lower in bleaching oxygen digested pulps of the invention than when bleaching sulfate cellulose.
The chemicals used for the digestion process can be recovered after the waste liquor is burned and subsequent to optionally causticizing all or part of the carbonate obtained when burning the liquor.
- ïo The method is particuiarly suitable for application in the continuous manufacture of wood cellulose pulp, since, owing to the fact that detached fibers are removed, the whole volume of the digestion zone is available for use. To enable the apparatus to be used in the most effective manner, with batchwise digestion processes, 15 ~ it is suitable to interrupt the oxygen gas digestion process whilst a certain quantity of partially digested chips (so-called shives) still remain (for example 5 to lO~o by weight of the wood material), and ~, to separately convert the shives to pulp with exposed fibers, e. g.
by a separate oxygen gas digestion process, optionally subsequent to mechanically treating the shives.
A preferred embodiment of the digestion process of the invention and of the cellulose pulp of the invention is shown in the following Example:
~
,. ,: . . - .. . , . .. ~ . . . .
104~ ;0 Cornmercially available birch chips about 5 mm thick and Z5 mm long were suhjected to a pretreatment process with 20% by weight aqueous sodium bicarbonate at 160 C for 2 hours at a ratio of wood:
liquid of 1:5 (kg/dm~3). The treatment process was carried out in an 5 autoclave. The pressure in the autoclave was lowered every half-hour to almost atmospheric pressure, to remove expelled carbon dioxide.
During the pretreatment process readily soluble hemicellulose was re-moved and the wood deacetylated. Subsequent to this treatment process, the yield was 85%, calculated on the dry weight of the wood, and the chips ` 10 wereblack in color.
The chips were subjected to an aLkali oxygen gas digestion in an autocIave digester in which a circulating aqueous digestion liquor pre- -pared from sodium bicarbonate was sprayed over the chips. The wood:
liquor ratio was 1:14. The oxygen gas pressure was 9 bars and the 15 temperature 140C. During the digestion process, saturated aqueous sodium bicarbonate solution was added, and carbon dioxide was removed continuously, so that the pH measured on cold (20 C) digestion liquid withdrawn from the digester-under cooling was maintained between 7. 5 and and 8. The amount of sodium bicarbonate charged was 1. 8~o by weight 20 during the course of the digestion, calculated on the dry weight of the ingoin~ wood.
After 3 hours digestion, digestion liquor was forced up through the bed of partialb digested chips at a speed such as to cause the chips in the bed to tumble about. In this way, fibers which were liberated during 25 the digestion were detached from the chips, and passed into suspension in the digestion liquor. ~ ~ -` ~
, ,.; . :.
.
r.~ ~ ' , , . ~ ' , ' ' , ~.
~ . . . , - . . . ' . ~
~: . - -`: . ' . ' '' ''''. '- " , '. ~
~ . ' _ . ' ,, . . . ' --" 104;~160 T}l~ liqu~r was contilluously withdrawn from the digester and fractionated in ~n array Or hydrocyclones which separated the suspension into a fiber-lean and fiber-rich fraction. The input suspension had a pulp concentration of 0. 5% by weight, and the fiber-rich suspension had a pulp concentration of 1. 5% by weight.
The fiber lean fraction was recycled by means of a centrifugal pump to the digester and circulated through the bed up-wardly from the bottom of the bed. This hydraulic tumbling of the chip bed was continued for 10 minutes, during which time the digestion apparatus was under 9 bars oxygen gas pressure. ~ ~ -The fiber-rich fraction was passed through a pressurized dewaterer. The filtrate obtained was recycled to the digester. The pulp was washed. The process of forcing the digestion liquor from below through the bed of partially digested chips and the fiber 15 separation steps in accordance with the foregoing were repeated at intervals of 30 minutes, until a total digestion time of 5. 5 hours had elapsed. The pressure was then lowered, and the remaining pulp and partially digested wood were blown out. The pulp was screened and washed. The shives were subjected to continuous oxygen gas 20 digestion.
Laboaratory tests on the resulting birch chips showed that ~ -after an elapsed digestion time of 3 hours, 26. 5% by weight of the original wood was obtained as pulp in the suspension, while 42. ~% was present as incompletely digested chips. After 3. 5 hours, lV42160 the corresponding figures were 38. 9% and 30. 0%. The viscosity of the pulp recovered from the suspension was 910 cm3/g, g according to SCAN. The quantity of alkali-resistant pulp (determined in 5% NaOH according to SCAN (R5 value) was 68. 9%
by weight, and the viscosity of the alkali-resistant plllp was 1268 cm /g.
At the end of the digestion time (i. e. after a total digestion time of 5. 5 hours) the total yield of screened pulp was 54. 9~o. The viscosity of the pulp was 880 cm /g. The R5 value was 70. 0 and the viscosity of the aLt~ali-resistant pulp was 1200 cm /g. In addition, there was ob~ained 6. 2% shives calculated on the original dry wood.
After the shives had been digested for 2 hours, a further 3% of screened pulp was recovered.
;~ The total pulp-yield was thus 57. 9% by weight, calculated .
on the dry, commercially available birch chips. The results from determinations on the viscosity of the pulp extracted with 5% NaO~
æhowed that the method results in but slight attach on the cellulose molecules, as compared wlth other methods of oxygen gas digestion.
' .
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Claims (27)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for the production of cellulose pulp from wood in coarse particulate form which comprises digesting the coarse particulate wood with oxygen and alkali in an aqueous solution constituting a digestion liquor in a digestion zone; then, after the digestion has continued for a proportion within the range from about 20% to about 70% of the total digestion time required, detaching cellulose fibers from the surface of the wood and continuing to detach cellulose fibers from the surface of the wood, in the course of the digestion in the presence of digestion liquor, by application of physical force to the surface of the wood; suspending the detached fibers in digestion liquor; separating such detached fibers from the digestion zone as the digestion of the wood continues; and continuing the digestion of the remaining wood with oxygen and alkali in aqueous solution to form cellulose pulp.
2. A process according to claim 1, which comprises spraying the wood with a liquid under pressure to detach the fibers.
3. A process according to claim 2 in which the liquid is a digestion liquor.
4. A process according to claim 1, which comprises disposing the particulate wood in a bed and forging liquid through the bed to detach the fibers.
5. A process according to claim 4 in which the liquid is a digestion liquor.
6. A process according to claim 1, which comprises applying a stream of gas under pressure against the particulate wood to detach the fibers.
7. A process according to claim 6 in which the gas is an oxygen-containing gas.
8. A process according to claim 1, which comprises disposing the particulate wood in a bed and then forcing a gas through the bed to detach the fibers.
9. A process according to claim 8 in which the gas is an oxygen-containing gas.
10. A process according to claim 1, which comprises disposing the particulate wood in a bed and then forcing liquid and gas through the bed to detach the fibers.
11. A process according to claim 1, which comprises detaching the fibers by application of mechanical force.
12. A process according to claim 1, which comprises rubbing the particulate wood to detach the fibers.
13. A process according to claim 1, which comprises tumbling particulate wood to detach the fibers.
14. A process according to claim 1, which comprises vibrating the particulate wood to detach the fibers.
15. A process according to claim 1, which comprises separating the fibers from the digestion zone as a fiber suspension in digestion liquor.
16. A process according to claim 15, which comprises separating the suspension into a substantially fiber-free fraction and a fiber-rich fraction.
17. A process according to claim 16 in which the suspension is separated by centrifugal force.
18. A process according to claim 15 which comprises screening or filtering the suspension to separate the fibers therefrom.
19. A process according to claim 15, which comprises separating fibers from the suspension at a temperature at most 10° C
lower than the temperature of the fiber suspension upon removal from the digestion zone.
lower than the temperature of the fiber suspension upon removal from the digestion zone.
20. A process according to claim 15, which comprises separating fibers from the suspension, and utilizing the resulting liquor as a digestion liquor in an oxygen gas-alkali digestion process.
21. A process according to claim 15, which comprises separating fibers from the suspension, and utilizing the resulting liquor to detach fibers from the surface of the wood in an oxygen gas-alkali digestion process.
22. A process according to claim 1, which comprises treating the wood prior to the said oxygen gas-alkali digestion with an aqueous pretreating solution.
23. A process according to claim 22 which comprises pretreating the wood with an aqueous solution comprising an alkaline compound at a temperature within the range from about 120 to about 200° C.
24. A process according to claim 24 in which the alkaline compound is sodium bicarbonate or carbonate.
25. A process according to claim 1, which comprises maintaining the pH within the range from about 6. 5 to about 10, during the major portion of the oxygen gas-alkali digestion.
26. A process according to claim 1 which comprises maintaining the temperature within the range from about 120 to about 160° C, during the major portion of the oxygen gas-alkali digestion.
27. A process according to claim 1, which comprises detaching partially or fully exposed fibers on the surface of the wood.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| SE7312302A SE392926B (en) | 1973-09-10 | 1973-09-10 | PROCEDURE FOR ALKALIUM CONNECTION OF WOOD |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1042160A true CA1042160A (en) | 1978-11-14 |
Family
ID=20318482
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA208,553A Expired CA1042160A (en) | 1973-09-10 | 1974-09-05 | Alkali/oxygen gas digestion of wood |
Country Status (12)
| Country | Link |
|---|---|
| JP (1) | JPS5149001B2 (en) |
| AT (1) | AT339723B (en) |
| BR (1) | BR7407491D0 (en) |
| CA (1) | CA1042160A (en) |
| CH (1) | CH589172A5 (en) |
| FI (1) | FI56036C (en) |
| FR (1) | FR2243296B1 (en) |
| GB (1) | GB1455399A (en) |
| IT (1) | IT1020750B (en) |
| NO (1) | NO140991C (en) |
| SE (1) | SE392926B (en) |
| ZA (1) | ZA745653B (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS52121502A (en) * | 1976-04-07 | 1977-10-13 | V Nauchinoopuroizubuodosutobue | Process for producing halfffinished products from ground vegetable material |
| JPS55153664U (en) * | 1979-04-20 | 1980-11-06 |
-
1973
- 1973-09-10 SE SE7312302A patent/SE392926B/en unknown
-
1974
- 1974-08-28 CH CH1175374A patent/CH589172A5/xx not_active IP Right Cessation
- 1974-09-05 ZA ZA00745653A patent/ZA745653B/en unknown
- 1974-09-05 CA CA208,553A patent/CA1042160A/en not_active Expired
- 1974-09-06 NO NO743209A patent/NO140991C/en unknown
- 1974-09-07 JP JP49103406A patent/JPS5149001B2/ja not_active Expired
- 1974-09-09 BR BR7491/74A patent/BR7407491D0/en unknown
- 1974-09-09 FI FI2639/74A patent/FI56036C/en active
- 1974-09-09 IT IT69724/74A patent/IT1020750B/en active
- 1974-09-09 AT AT723874A patent/AT339723B/en not_active IP Right Cessation
- 1974-09-10 GB GB3945974A patent/GB1455399A/en not_active Expired
- 1974-09-10 FR FR7430626A patent/FR2243296B1/fr not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| FI56036B (en) | 1979-07-31 |
| JPS5149001B2 (en) | 1976-12-24 |
| NO743209L (en) | 1975-04-07 |
| NO140991C (en) | 1979-12-19 |
| JPS5053602A (en) | 1975-05-12 |
| ATA723874A (en) | 1977-02-15 |
| SE392926B (en) | 1977-04-25 |
| CH589172A5 (en) | 1977-06-30 |
| GB1455399A (en) | 1976-11-10 |
| FR2243296B1 (en) | 1976-12-31 |
| FI263974A (en) | 1975-03-11 |
| DE2441440B2 (en) | 1976-08-19 |
| ZA745653B (en) | 1975-11-26 |
| SE7312302L (en) | 1975-03-11 |
| NO140991B (en) | 1979-09-10 |
| FI56036C (en) | 1979-11-12 |
| AT339723B (en) | 1977-11-10 |
| BR7407491D0 (en) | 1975-09-09 |
| FR2243296A1 (en) | 1975-04-04 |
| IT1020750B (en) | 1977-12-30 |
| DE2441440A1 (en) | 1975-03-27 |
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