CA1039908A - Process for the delignification of lignocellulosic material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process is provided for the delignification of lignocellulosic material in raw or in pulp form, which comprises carrying out the de-lignification with oxygen and alkali in the presence of a manganese compound improving the selectivity of the delignification, and increasing the rate of delignification. In a preferred embodiment of the invention the ligno-cellulosic material prior to the delignification is treated to remove copper, cobalt and iron compounds which catalyze the degradation of carbohydrates.

Description

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SPECIFICATION
The con~ersion of raw lignocellulosic material to unbleached and then to bleached pulp requires an extremely complex and intricate series of chemical reactions and physical process, usually requiring two or more stages in which different reactions are involved.
The first is referred to as pulping, and the second as bleaching. Both however include delignification.
Rydholm in Pulping ProcesseS has pointed o~t that ~e com~nQn .~
purpose of all chemical pulping processes is to achieve fiber liberation by delignification, and they can be classified according to their different ways of achieving this. Reactions with the carbohydrates occur at the same time, and dissolution of certain amounts of the carbohydrates and chemical modification of the remainder determine the quality of both dissolYing and paper pulps, alld are therefore controlled accordingly.
Dissolution of the extraneous components of wood is important to pulp quality. Inorganic side reactions occur, which are of importance not only for the regeneration of the pulping chemicals, but indirectly for the reactions with the wood during the cook.
Alkaline delignification results in alkaline hydrolysis OI the phenolic ether bonds, whereby lignin is rendereid soluble in alkali.
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Sulfidation by hydrosulfide in the Kraft process may both accelerate the cleavage of phenolic ether bonds and cause direct cleavage of alkyl ether -bonds, as well as protect alkali-sensitive groups from a condensation which could retard the delignification. Sulfonation of benzyl alcohol and alkyl 2i5 ether groups in the sulfite process renders the lignin water-soluble; the cleavage of the alkyl ether bonds, which keep the initlally formed lignosulfonates bound to the wood, occurs by sulfitolysis or acid hydrolysis.
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At the same time sulfonation of the reactive groups prevents their ~ - , .
partal~ing in condensation reactions. Neutral sulfite pulping, which involves less delignification, utilizes sulfonation of certain groups in the lignin to hydrophilic sulfonates, the dissolution of which is effected by unknown reactions, which may involve bo$h sulfitolysis and hydrolysis. :
Finally, nitration and chlorination of lignin, used in some minor pulping processes, together with some oxidation, as in oxygen-alkali pulpiIlg, cause changes at the aromatic nuclei of lignin, which lead to decomposition of the lignin macromolecules to smaller fragments, soluble in water or `~ ` - ~ . . .
alkali. ~ ~ ~
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In all delignification, one side reaction of lignin is most undesirable, its self-condensation, which occurs in both acid and alkaline .
medium, rendering the lignin less soluble and dark in color~ which darkens . ..
the color of the pulp. Chemical pulping cannot entirely avoid lignin condensation, and the lignin remaining in the pulp after cooking is more . :. ~ . .
or less condensed. The purpose of the bleaching reactions is to cause i:such degradation of these lignin molecules that they can be dissolved, and thus ~mprove the color of the pulp. ~ -Although in most pulp uses lignin is an undesirable or at best inert component of the pulp, no preparation of unbleached pulp aims at complete deligniflcation. This is primarily because of the unavoidable i reactions with the carbohydrates during the delignification. These reactions become particularly serious towards the end of the cook, when the rate of delignification is slow, because of the small amounts o lignin remaining and their high degree of condensation or inaccessibility. When pulps with a high content of hemicellulose are desired, considerable - -amounts of lignin are left in the pulp. For unbleached pulps the upper -~` , . . . ~ . .:
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.. ,; , limits are set by the brightness and brightness stability required, as well as the extent to which lignin can be allowed to impair the beating and strength properties of the pulp. In the case of bleached pulps the cost of bleaching agents is the limiting factor.
The alkaline degradation of carbohydrates starts at the .
aldehydic end groups and proceeds along the chains in a sort of peeling reaction with conversion of the sugar monomers to saccharinic and other hydroxy acids. Thi~ reaction occurs fairly rapidly at 100C and therefore precedes delignification. At higher temperatures there occurs a direct alkaline hydrolysis of the glycosidic bonds, which also affects the more `
cryætalline parts of the carbohydrates. This reaction not only leads to new losses of yield by peeling reactions starting at the freshly formed aldehydic groups~ but also to a shortening of the cellulose chains and a deterioration of the strength properties oi the pulp. Another reaction, involving an intramolecular rearrangement, causes a stabilization of the ., carbohydrate molecules under formation of a carboxyl end group.
The selecti~rity of the pulping chemicals with respect to delignification determines the yield of the pulping process and to some - extent the pulp properties. In the sulfite process, sulfonation and acid `
hydrolysis contri~te to delignification, and acid hydrolysis to the carbohydrate degradation and dissolution. In the Kraft process, ~ulfida~lon and alkaline hydrolysis contribute to delignification, and alkaline peeling and hydrolysis to the carbohydrate degradation. The delignification proceeds more rapidly in the sulfite cook than in the Kraft , .
cook, and lower temperatures can therefore be used in the former, which is fortunate because the hydrolysis of the glycosidic borids of the carbohydrates occurs much more rapidly in acidic than in alkaline medium. Alkaline ..;. ~
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peeling reactions, on the other hand, require lower temperature than the ~ `alkaline delignification, and they unavoidably decrease the carbohydrate yield, to a degree which depends on both chemical and physical changes in their structure. Accessibility phenomena improve the selectivlty of lignin removal It is a consequence of the above phenomena that the rate `
of pulping is governed mainly by the rate of delignification. Of the ` delignification reactions mentioned above, chlorination is most rapid ~-and occurs at a technically acceptable rate also at room temperature.
Nitration is somewhat slower, but can be performed at temperatures below 100C without overlong reaction times. However, the remaining `.`-reactions, which involve the least e~pensive chemicals and are accordmgly the most important, unfortunately require elevated temperatures and -pressures to proceed sufficiently rapidly. This causes an expensive ~
heat consumption, expensive pressure vessel constructions, and difficulties in the construction of continuously operating machinery ` `
because of the problem of feeding chips against a reaction zone of elevated pressure.
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These problems naturally ha~e led to investigation of possible catalysts for the reactions concerned.
Autooxidation reactions are kr~wnto be catalyzed by small , quantities of compounds of the transition metals, such as copper, cobalt and iron. Pradt et al Swedish Utla~gningsskrift No. q3 Ol~i8-2, published August 7, 1973, indicate that the`rate of delignification of wood using oxygen and alkali could be increased in the presence of a copper salt as a catalyst. It has however been demonstrated (Svensk Papperstidning 76 480--485 ~973)) that the addition of copper salts ~ .
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using either wood powder or wood chips results in a severe degradation of the cellulose, which in turn gives a lower viscosity of the cellulose at a given lignin content and a given Kappa number (referred to generally as an impaired selectivity).
In accordance with the invention, it has now been determined that both the rate and the selectivity of the delignification can be improved, if the lignocellulosic material is delignified by oxygen and alkali in the presence of catalytically-active manganese compounds. The manganese compounds should be addeA prior to the start of the delignification, or at an early stage of the delignification, and before dissolution of approximatel~
10 ~C of the lignin content of the starting lignocellulosic material.
Some types of lignocellulosic material contain manganese compounds. At least a proportion of such manganese compounds apparently is locked in, in an inactive noncatalytic form, however, unable to cataly~e delignification to a noticeable extent. The delignification of such manganese-containing lignocellulosic material is also improved, in accordance with the in~ention, by adding catalytically active manganese compounds, i. e. manganese compounds capable of supplying manganese to the delignification reaction in a catalytlc form, in which possibly manganese ion is provlded in s~lution in the alkaline delignification liquor in an active condition. Such added manganese in active form catalyzes the 1-delignification, increasing the rate of delignification, and improves selectivity, as shown by a higher viscosity at a given Kappa number of the resulting pulp, whether bleached or unbleached.
In a preferred embodiment of the invention, the lignocellulosic material prior to the oxygen alkali delignification is treated so as to remove at least a major proportion and preferably substantially all of the ~ 3~
catalytically active metal ion or compounds that may be present with -the material, such as copper, iron and cobalt. Such removal enhances the catalytic activity of the manganese compounds in the course of delignification and results in a synergistic retarding effect of the added manganese on the depolymerization of the cellulose. Surprisingly, the removal even of manganese present with the lignocellulosic material - ~ -ab initio in the course of such a pretreatment improves the selectivity and catalytic effect of manganese compounds added subsequently, and ,, ,.: . . .
prior to or at an early stage of the oxygen-alkali delignification.
The manganese compound can be added initially in a sufficient amount, or incrementally or continuously in the ~ourse of the delignification, together with or separately from incrementally or continuously added alkali. Such supplemental addition of manganese may be desirable in order to maintain a suitable concentration of active manganese compounds throughout the delignification.
It is also suitable to carry out the delignification in one or . ~ , . .
more stages, at varying p~'s in the course of each stage, and active .. ~.
- manganese compounds can be added to the delignification reaction mixture `
in one, or several, or all of these stages.
; 20 The added manganese compounds employed in the process ~;r. ' of the invention provide manganese in catalytically active form to the delignification~ For this purpose, the manganese should be preferably in a form which provides bivalent manganese. The anion with which the added manganese is associated can be inorganic or organic, and the - ~ .
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added manganese ean also be associated in a complex whieh provides a proportion of manganese.
Exemplary bivalent manganese compounds include manganous oxide, manganous chloride, manganous bromide, manganous hydroxide, manganous nitrite, manganous sulfate, manganous ea~bonate, manganous phosphate, manganous ehlorate, manganous acetate, manganous formate, mangallous oxalate, and eomplex salts of manganous ion with ehelating inorga~ie and organie acids.
Aliphatic alpha-hydroxycarboxylie acids of the type RCHOHCOOH and the correspondingbeta-hydroxyearboxylie aeids RCHOHCH2COOH have the property of forming ehelates with manganese.
Exemplary alpha- and beta-hydroxy earboxylie aeids are : glyeolie acid, lactie aeid, glyceric acid, cY "~ -dihydroxybutyrie aeid, a-hydroxybutyrie acid, ol -hydroxyisobutyrie aeid, c~ -hydroxy-n-valerie aeid, o~ -hydro~yisovaleric acid, ,B -hydro~yisobutyric aeid, ,B -hydro2~y-. ,. :
isovalerie aeid, erythronie aeid, threonie aeid, trihydroxyisobutyrie acid, ~; and sugar acids and aldonie aeids, sueh as glueonie aeid, galaetonic aeid, talonie aeid, marlnoic aeid, axabollie acid, ribonie aeid, xylonie aeid, lyxonie aeid, gulonie aeid, idonie aeid, altronic aeid, allonie aeid, ;i ~
ethenyl glyeolie aeid, and ,B-hydroxyisoerotonie aeid. ;

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:i~39 Also useful are organic acids having two or m~re carboxylic groups, and no or from one to ten hydroxyl groups, such as oxalic acid, malonic acid, tartaric acid, malic acid, and citric acid, ethyl malonic acid,succinic acid, isosuccinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid, maleic acid, fumaric acid, glutaconic ~-acid, citramalic acid, trihydroxy glutaric acid, tetrahydi~O~y adipic acid, dihydroxy maleic acid, mucic acid, mannosaccharic acid, idosaccharic - ~;
acid,talomucic acid, tricarballylic acid, aconitic acid, and dihydroxy tartaric acid.
Manganese complexes of nitrogen-containing polycarboxylic acids are especially effective inhibitors. Several important acids belonging to this group have the formula:
HOOCcH2 ~- N- (c2H4~)ncH2c()oH :~

or alkali metal salts thereof, in which A is the group--CH2COOH or --CH2CH2OH, where n is an integer from zero to five. The mono, di, tri, tetra, penta and higher alkali metal salts are useful, according to the available carboxylic acid groups converted to alkali metal salt form.
Examples of such compounds are ethylene diamine tetra-- acetic acid, ethylene diamine triacetic acid, nitrilotriacetic acid, diethylene-triaminopentaacetic acid, tetraethylenepentamine heptaacetic acid, and hydroxyethylene diamine triacetic acid, and their alkali metalsalts, includingthemono, di, tri,tetraandpentasodium, ~
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potassium and lithium salts thereof. Other types of aminocarboxylic ,~ , , .
acids which can he used to advantage are iminodiacetic acid, 2-hydroxy~
ethyliminodiacetic acid, cyclohexanediamine tetraacetic acid, anthranil-N, . .
N-diacetic acid, and 2-picolylamine-N,N-diacetic acid.

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These complexing agents can be present in rather large quantities, within the range from about two to about ten times the amount needed to prevent precipitation of manganese compounds during the impregnation of the lignocellulosic material with manganese. The use of waste pulping or bleaching liquor in combination with compiexing agents of this type is particularly advantageous.
The polyphosphoric acids are also good complexing agents for manganese, and the manganese salts of these acids are useful in the ,~ .. ..
process of the invention. Exemplary are disodium manganous pyrophosphate, trisodium manganous tripolyphosphate and mangaIIous polymetaphosphate. `; ~ -- Especially advantageous from the standpoint of cost are the acids naturally present in waste liquors obtained from the alkaline I; ;
treatment of cellulosic materials. These acids represent the all~ali- or ~, .
water-soluble degradation products of polysaccarides which are dissolved in such liquors, as well as alkali- or water-soluble degradation products ~ ~
of cellulose and hemicellulose. The chemical nature of these degradation ~ ;
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products are complex, and they have not been fully identified. However, it is known that saccharinic and lactic acids are prese~Lt in uch liquors, ~d that other hydroxy acids are also present. The presence OI
C6-isosaccharinic and C6-metasaccharinic acids has been demonstrated, -as well as C4- and C5 metasaccharinic acids. Glycolic acid and lactic acid are also probable degradation products derived from the hemicelluloses, together with beta-gamma -dihydroxy butyric acid.
Carbohydrate acid-containing cellulose waste liquors which can be used include the liquors obtained from the hot alkali treatment of cellulose; liquors from sulfite digestion processes; and liquors from sulfate digestion processes, i. e., Kraft waste liguor. The waste liquors obtained in alkaline oxygen gas bleaching processes, for ~ -. -; , ' 9 '', '"'''''`' ''';

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e~ample, thosediscloseclinU. S. patentsNos. 3,652,385and3,652,386, or alkaline peroxide bleaching processes can also be used. In this instance, the alkaline liquor can be taken out from the process subsequent to complet-ing the oxygen gas delignification or during the actual delignification process.
The complex manganese salts can be formed first, and then added to the Iignocellulosic material. They can also be formed in situ from a water-soluble or water-insoluble manganous salt, oxide or hydroxide, in admixture with the complexing acid, and this mixture can be added to the .
lig~ocellulos-c material. Preferably,the waste liquor employed as the source of complexing acid or lactone or salt thereof can be mixed with ;
a manganous salt, oxide or hydroxide, before being introduced to the ~ -process. It is also possible to add the manganous salt, oxide or hydroxide to the delîgnification liquor, and then bring the liquor into contact wîth the complexing acid or lactone or salt thereof. It is also po=sible to combine the complexing acid or lactone or salt thereof with the liquor and then add the manganous salt, oxide or hydroxîde, but ~his method may be less advalltageous in practice.
Manganese compounds providing manganese ion in a higher -;
valence state, such as trivalent or tetravalent manganese, can he used, but may lead to the production of pulp having an impaired brightness.
Exemplary higher polyvalent manganese compounds înclude manganic chlorlde, manganic nitrite, manganîc sulfate, manganic carbonate, ^-manganic acetate, manganic ormate and manganic o2~alate, and complex salts of manganic ion with any of the chelating acids mentioned above.
It is not understood why the addition of manganese has a different effect upon the course of the delignification than manganese ,,, ~, , .. . . .
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which is already present in the lignocellulosic material. Since however it has been determined that the addition of manganese in one or several .
increments following the start of the delignification is more beneficial than addition of all of the manganese initially, at the start of the delignification, it is suggested that possibly the manganese becomes inactive in some nonionized or nondissociated complex with the lignocellulosic material, which renders the manganese unavailable for catalytic action. Even manganese that is added at the start of the delignification may be consumed in such reactions, and thereby unavailable ;;
catalytically for the delignification reaction system.
It has not been possible to determine the form of catalytic manganese present in the delignification reactioll system, nor has it been possible to distinguish between active manganese and inactive manganese in this system by analytical methods. For this reason, analysis of the `~
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lignoc~llulosic material for manganese content is not revealing. All that is known is that the manganese must be added in a catalytic form, and that it should be freshly added, for optimum effect. Consequently, , throughout the specification and claims, reference to manganese in active ~`
form or in catalytic form is a reference to such manganese compounds.
In whatever form manganese is added, whether as salt, oxide, hydroxide, or complex salt, the amount of manganese is calculated as M~.
The quantity of manganese compounds added to the system !',, ', '~ ' is seIect ed according to the nature of the starting material, and the ;
desired quality of the delignified product. -,- . . . ; ~
Amounts within the range from about 0. Ol to ab~ut l~/c by weight of the dry lignocellulosic material give good results. Beneficial .-.. . . . . . . . .................................. .
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effects may be observed at 0. 001~C by weight of the dry lignocellulosic material. Optimum results have been obtained at amounts within the range from about 0~ 05 to about 0. 5~. Amounts in excess of 1~c up to

2~ may not afford any better effect under normal conditions, and may 5 result in an impaired brightness, but such amounts can be used. ~
The oxygen-alkali delignification process in accordance ; ;
with the invention is applicable to the delignification of any kind of lignocellulosic material, such as bagasse, straw, jute, and particularly wood.
The delignification process of the invention is applicable to any kind of wood. In general, hardwood such as beech and oak can be pulped more easily than softwood~ such as spruce and pine, but both types of wood can be pulped satisfactorily using this process. Exemplary hardwoods which can be pulped include birch, beech, poplar, cherry, sycamore, hickory, ash, oak, chestnut, aspen, maple, alder and eucalyptus. Exemplary softwoods include spruce9 fir, pine, cedar, juniper and hemlock.
The lignocellulosic material should be in particulate form.
Wood chips having dimensions that are conventionally employed in the oxygen a.lkali pulping process can be used. However, appreciable advantages with respect to uniformity of the delignification process under all kinds of reaction conditions can be obtained if th~ wood is in the form of nonuniform fragments of the type of wood shavings or chips having an average thickness of at most 3 mm, and preferably within the range from rl , about 0. 2 to about 2 mm. Other dimensions are not critical. Sawdust, wood flour, wood slivers and splinters, wood granules, and wood chunks, :
and other types of wood fragments can also be used.

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The oxygen-alkali delignification process in accordance -~
with the invention is also applicable to the delignification of unbleached cellulose pulp. The process can be used to advantage with wood pulp of any type, including mechanical pulp, but particularly chemical pulp ~-and semichemical pulp. The chemical pulp can be prepared by any - . - . -pulping process. Oz~ygen-aLkali pulp, sulfate pulp and sulfite pulp are illustrative. The invention is applicable to cellulose pulps derived ; - from any type of wood, such as spruce pulp, pine pulp, hemlock pulp, birch pulp, cherry pulp, sycamore pulp, hickory pulp9 ash pulp, beech ;
pulp, poplar pulp, oak pulp, and chestnut pulp.
The delignification process of the invention can also be carried out in conjunction with the oxygen delignification of, for example, defibrated wood, and wood which has first been subjected to a chemical . . .
treatment, for e~ample a soda cooking operation, and subsequently -~ ~-; 15 defiberi~ed. This latter method is somel:imes referred to as an oxygen cooking process, although the oxygen bleaching of semi-chemical pulp is a , ~ . ..
better designation. Normally, an o~ygen delignification process is : . ; . .. ~
continued, even when concerned with an oxygen cooking process, until the material is readily defiberized. Shives separated after the cooking ., . i.. , .- .
and uncooked material can be returned to the process,or treated separately in accordance with known methods.
Prior to treatment by the process of the inventlon, the lignocellulosic material optionally but preferably is subjected to a pre-treatment with water and/or an aqueous solution in one or mor0 stages ~5 so as to remove metal ions or compounds thereof such as copper, cobalt and iron, and also manganese and any other metal ions which may be present. A pretreatment is especially advantageous in the case of hardwood chips.

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Such metal ions or compounds may have a deleterious effect upon the delignification, and may also increase attack on the carbohydrates in the course of the delignification, due to a catalytic effect on the degradation reactions. Frequently, when such metal ions or compounds are allowed to remain during the delignification process of the invention, the result is a lower viscosity in the treated pulp, or a lower carbohydrate content thereof, or both, either or both of which may well be undesirable.
~-~ The pretreatment accordingly is carried out under 10 cond-tions such that these metal ions or compounds are removed by dissolution in the treating liquor.
It is frequently possible to remove all or part of such metal ions or compounds by washing the lignocellulosic material with water. This results in the removal of water-soluble metal compounds .,. - :
15 by leaching or dissolution. An improved dissolution is obtained at elevated temperatures. The longer the washing time, the greater the ;~
proportion of metal ions or compounds that are extracted.
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A sultable washing treatment is carried out using hot water at a temperature within the range from about 90 to about 160 C
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20 for from 0. l to about 10 hours. In the course of the heat treatment in the presence of water, some of the lignocellulosic material is hydrolyzed to give organic acids which dissolve in the solution, for example acetic acid, and the resulting acid solution has an improved capacity for dissolution of metal ions or compounds present in the lignocelluloslc material.
Aqueous acidic solutions containing organic and inorganic acids can also be used, such as acetic acid, citric acid, formic acid, hydrochloric acid, sulphuric acid, nitric acid, phosphoric acid and ':, , , ~ 3 sulphurous acid. Such solutions can have a pH within the range from ;
about 1 to about 5, suitably from about 1. 5 to about 4, and preferably from about 2 to about 3. 5, with the contact continued for from about 0.1 to about 10 hours. Treatment with acidic aqueous solutions can be carried out al ambient temperatures, i.e., from about 10 to about 30C, but elevated temperatures can also be used, ranging from about 40 to about 1404C. In the case of raw lignocellulosic materials, such as wood, such a treatment may be accompanied by h~drolysis of the cellulose, with the formation of additional acids.
Howe~er, when the delignification process of the mvention is applied to paper pulp, it is usually desirable to avoid hydrolysis of the cellulose. In such cases, the time and temperature of the treatment together with the pH should be adjusted so that depolymerization of the .
carbohydrate material in the pulp is kept to a minimum.

With certain raw lignocellulosic materials, and ~;
particularly wood in particulate form, especially hardwood~ it has been found advantageous to carry out the pretreatment with an aqueous --alkaline solution, such as an alkali rnetal hydroxide or alkali metal ~` -, . . .. .
carbonate or bicarbonate solution, for example, sodium hydroxide, sodium carbonate and sodium bicarbonate solution, the alkaline hydroxide`s or salts being used singly or in admixture. .
Such an alkaline treatment is carried out preferably at an elevated temperature within the range from about 100 to about 200~C, suitably from about 120 to about l90~C, and preferably from about 140 a5 to about 180C, until there has been dissolved in the solution an amount of lignocellulosic material within the range from about 2 to about 40~C
by weight, suitably from about 5 to about 30~C by weight, and preferably . .
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from about 5 to about 20(3tc by weight, based on the dry weight of the lignocellulosic material. The treatment time can be within the range from about 0.1 to about 10 hours, suitably from about 0. 25 to about 4 hours, and preferably from about 0. 5 to about 2 hours.
Any carbon dioxide formed during the treatment is -preferably vented, elther continuously or from time to time. ;
- Chelating or complexing agents for the metal ions to -be removed can also be present. Such solutions have a superior extracting effect for the metal content of the lignocellulosic material. Any of the chelating acids referred to above in connection with the manganese complexes can be used. Exemplary complexing agents include the polyphosphates, such as pentasodium tripolyphosphate, tetrasodium pyrophosphate, and sodium hexametaphosphate; isosaccharinic acid, lactic acid, dihydroxybutyric acid and aldaric acid; and aminopolycarboxylic : 15 acids having the general formula .
~' . MOOCCH2 ~:
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- N--(C2H~H~n--CH2COOM

in which A is CH2COOH or CM2CH2OH and n is a number within the range from 0 to 5, and M is hydrogen, an alkali metal or ammonium.
Suitable chelating acids include ethylene-diamine tetra-acetic acid, nitrilotriacetic acid and diethylene triaminepentaacetic acid, as well as amines, particularly hydroxy alkyl amines such as mono-, di- and tri-ethanolamine, and diamines, triamines and higher polyamines having complexing properties. Mixtures of these complexing and chelating agents can also be used, especially combinations of chelating agents that contain nitrogen with chelating agents that do not contain nitrogen.

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Particularly useful are the metal complexing agents present in waste cellulose pulping, cellulose bleaching and other cellulose processing liquors, which may be either alkaline or acidic. Such -liquors as indicated above in conjunction with the manganese complexes normaily contain complexing agents derived from the cellulose, as well as the complexing agents added for the purpose of the cellulose process !', ~ ' from which the waste liquor is obtained.
~ Suitable waste liquors are for example waste pulping ~ -.. .;- .
liquors, especially those from oxygen alkali pulping processes, and waste bleaching liquors, especially those from oxygen-alkali bleaching , . -.~ .
processes. Particularly advantageous are liquors from oxygen-alkali -delignification processes that contain complexing agents for cellulose ;~;
degradation inhibitors. Used wash water from cellulose treatment processes also can be employed, including wash waters previously used for the pretreatment of earller batches of lignocellulosic material -treated by the process of the invention, as well as waste liquors from the delignification process of the invention.
Pretreatment liquors of different types can advantageously be combined or applied in sequence, as desired, for the greatest possible beneficial effect from different types of liquors. Thus, for example, in a first step a pretreatment may be effected with water containing dissolved . .
sulphur dioxide having a pH of 2, at a temperature of 20C, followed by treatment with an aqueous solution of sodium bicarbonate and sodium carbonate in the ratio of 7:3 (20~C per weight based on dry wood) at ~ ~
160C for two hours in the presence of 0.1~Zc diethylenetriamine pentaacetic -acid, based on the dry weight of the lignocellulosic material. -Air may be injected into the pretreatment liquor under pressure; oxygen may also be introduced.
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~L~3~ 8 Whether or not a pretreatment is applied9 it is desirable .
to wash the lignocellulosic material prior to the oxygen-alkali delignifi~
cation process of the invention. Such washing of a pretreated lignocellulosic material makes it possible to remove not only residual traces of metal ions - 5 or compounds but also traces of the pretreatmerlt liquor. The wash waters from this step can be returned to the pretreatment step.
The conditions under which the oxygen-all~ali delignification proce~s of the invention is carried out a~e selected to accommodate the lignocellulosic m~terial being treated and the purposes for which i~s treatment product is to be used. Since the process is~applicable both to raw lignocellulosic material and to pulped lignocellulosic material, which are chexnically and physically quite different and nonequivalent materials, different delignification conditions may be desirable. -The delignif-ication in the presence of added manganesè
compounds in accordance with the invention can be carried out at a pE within the ran~e from about 6. 5 to about 11, and preferably within the range ~rom ~;
about ~ to about 10. Optimum results are obtained iE the pH is held within the range from about 7 to about 9 . 5 during the major part o the delignificatlon.
It is important that pH be determined by measurements on a ~ ~
deligniEic~tion liquor at ambient temperature i. e., from 10 to 30C. Con- ;;
sequently if the pH of a hot delignification liquor is to be determined, the liquor is cooled to ambient temperature be~ore such pH determination. This ~-~
is necessary in order to obtain accurate and reproductible p~I measurements.
The pH within the range r~ferred to in the clainls is determined at ambient temperatureO
- The total amount of a~kali that is required for the delignifica-tion is determined by the auality and type of the pulp to be produced and is withitl the range from about 1 to ~0 kilomoles per 1, 000 kg. of dry wood.

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(: ellulose pulps intended to be used in the production of regenerated cellulosefibers, such as ~riscose, acetate and cuprammonium pulps, are quite fully delignified~ and should have a low content of lignin and hemicellulose. In the production of such pulps, in accordance with the process of the mvention, the amount of alkali can be within the range from ahout 6 to about 8 kilomoles per 1, 000 kg. of dry wood. Semichemical pulps are given an intensive .,, , : ~, ` mechanieal treatment following their digestion in order to liberate the ~ ~
, ~ .
cellulose fibers, and in the production of such pulps using the process of ~ -the invention, the amount of alkali can be much less, within the range from ^ ~ ~
, 10 about 1 to about 2 kilomoles per 1, OOOkg. of dry wood. For the production ~ .
of bright paper pulp, which is readily defibered when the digester is blown, the amount of alkali used in the process of the 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 -15 fine pap~r, plastic fillers, and soft paper or tissue paper, the amount of alkali in the process of the invention is withîn the range from about 2 to about ,~ ~.. :,. .
; 6 kilomoles per 1, 000 kg. of dry wood. ~
. . .:
Any a~ali metal hydroxide or alkali metal carbonate can be employed, such as sodium hydroxide, potassium hydroxide, lith um hydroxide, .: .
20 sodium carbonate, potassium carbonate and lithium carbonate. The sodium carbonate obtained in the burning of cellulose digestion waste liquors can be used for this purpose. The use o~ alkali metal carbonates may be more ` -. . '!,. ' . ' advantageous than the use of alkali metal hydroxides in maintaining the pH

of the delignification liquor within the stated range, because oE the buffering .. .... .
:, ,' . ;
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:
.:

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?8 properties of the carbonate or bicarbonate present or formed in situ. Con-sequently, mixtures of alkali metal hydroxides and aLkali metal carbonates are particularly satisfactory to obtain the advantages of each, and dilute , their disadvantages. However, if a~ali metal carbonate such as sodium carbonate is the sole all~ali charge, the total amount of sodium is greater, and this imposes a greater load on the sodium recovery system.
The pH range employed in the delignification process of the invention is considerably below the pH range used when sodium hydroxide is used as the active alkali. The pH range in accordance with the invention is therefore obtained using as the alkali an appropriate mixture of alkali metal carbonate and/or bicarbonate, either or both which may be admixed with alkali metal pydroxide in a minor proportion, to give a pM within the stated range. It is thus possible to use mixtures-with alkali metal hydroxides or ca;rbonates with alkali metal bicarbonates such as sodium bicarbonate and potassium bicarbonate. The alkali metalbicarbonate in the case serves as a buffer. Otlier buffering agents, compounds of alkali metals with nondeleterious acidic anions, can be employed, such as alkali metal acid phosphates3 such as potassium dihydrogen pllosphate, Potassium monohydrogen phosphate, sodium dihydrogen phosphate, sodium mono-hydrogen phosphate, as well as the lithium salts of these anions.
,, ... . .: . :. -The amount of buffering agent such as a~ali metal bicarbonate is usually within the range from about 1 to about 5 kilomoles per 1, 000 kg.
of dry wood. The alkali metal bicarbonate or other buffering agent should be added to the delignification liquor either initially or at an early stage of thedelignification. The addition of the bicarbonate or other buffering agent . . , !

~ 39~
increases the buffer capacity of the delignification liquor, there~y assisting in avoiding variations in pH outside the prescribed range during the delign~
` ification.
Large amounts of buffering agents, and particularly bicarbonates, ' 5 should be avoided, however, since the presence of large amounts of additional foreign anions can be undesirable. In the case of bicarbonates, carbon dioxide may be produced in the course of the delignification as the buffer is ., .
consumed. The carbon dioxide dilutes the oxygen, and adds an extra load to the chemical recovery system, and is therefore undesirable in large amounts. However, the addition of minor amounts of the buffering agent within the stated range contribute to pulp uniformity because of their assistance i. . . . .
~¦ ~ in maintaining pH.
. .. . . . . . .
Also useful as a buffer are the base liquors from previous digestions and/or the waste liquors from oxygen bleaching processes, such ~, 15 as those described in U. S. patents Nos. 3, 652, 385 and 3, 652, 386, In this ~; :
! way, better economy is obtained in chemical recovery, which can be 1 effected after evaporating and burning the waste digestion liquor, using ;, lmown methods. ;
., :, .
For economic reasons, the sodium compoun~s are preferred - 20 as the alkali metal hydroxide, alkali metal carbonate and alkali metal bi-carbonate.
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` ~39~
It is also possible to add the additional chemicals normally present in digestion liquors, such as sodium sulfide or other alkali metal ~-~
sulfide. At most~ such chemicals are added in an amount of about l kilomole per 1,000 kg. of dry wood.
Limiting the amount of alkali metal hydroxide and/or alkali metal carbonate in the initial stages of the process may be quite advantageous in obtaining a cellulose pulp of the desired quality. At most, 75 percent of the total molar quantity required of the alkali can be added ab initio, and even this high percentage is only desirable if the pulp to be manufactured is :' a semichemical pulp, or if the wood has been pretreated with sulfur dioxide ` in aqueous solution. For mostpulps, including even the semichemical pulps, a better cellulose pulp is obtained if the initial charge of alkali is within the `; range from about 2 to about 50 ~ercent of the total molar quantity required for the delignification. The remainder of the alkali is added progressiveIy, either ;
incrementally or continuously, as the delignification continues. When producing ; bright pulps having a low lignin content, it is satisfactory to charge llot more -than 20 percent and siiitably from about 5 to about 20 percent of the alkali at the beginning of the delignification process.
~ a mixture of alkali metal hydroxide and alkali 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 alkali added as the delignification proceeds being sodium hydroxide. If the alkali charge initially is alkali 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 10 percent, 5 of the total molar quantity of a~ali.
Whether or not the delignification process is carried out continuously or as a batch process, the alkali metal hydroxide and/or alkali ;~
metal carbonate can be charged continuously or in increments to the delignification liquor. In a continuous delignification, the wood is caused 10 to move through the reactor from one end to the other which thereby con~
stitutes a reaction zone. In a batch process, the wood, usually in the form of chips, is retained in the reaction vessel throughout the delignification. r,~ , ~ , . .
- Since the oxygen that is employed as an essential component in the delignification 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 delignification liquor present on the wood are kept in continuous contact with the oxygen-containing gas. If the wood is completely or substantially immersed in the delignification liquor, it is important to agitate the wood and/or the gas and/or atomize the gas or the liquor. The oxygen should be dissolved or dispersed ïn the delignification liquor to the greatest e~ent possible. Dissolution or dispersion of the oxygen in the liquor can take place within the reactor and/or externally of the same, such as in nozzles, containers or other known devices used for dissolving or dispersing gases in liquids.
In application to wood in chip form, the cooking liquor can be allowed to run continuously or intermittently over the chips during the delignification process. In the case of pulped lignocellulosic material with ~~ lS~3~ 8 ~1le fibres exposed, such as chemical pulp such as sulphate pulp, semi-r. chemical or mechanical pulp, one can impregnate the pulp with a solution containing active alkali, remove excess solution, by draining and/or pressing operations, and then subject the pulp to the delignification process.
The method can also be applied to a slurry of the lignocellulosic material in the delignification liquid, while the material is in intimate conta with c,xygen under pressure.
Transfer of oxygen to the delignification material impregnated with delignification liquor is important in the process, and is controlled by adjusting the oxygen pressure, the delignification temperature and/or the proportion of gas-liquid contact surfaces, including the wood impregnated with delignification liquor .
` The oxygen is preferabb employed as pure oxygen, but mi~tures of oxygen with other inert gases can b0 used, such as, for 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.
The partial pressure of oxygen can be as low as 1 bar, although under normal conditions it is most advantageous to use a pressure of at least ' 20 5 bars. When the method is applied to non-defibrated wood chips or similar types of wood fragments, e.g. sticks or shavings or sliced wood chips, it is ;
- suitable to maintain an oxygen pressure of at least 10 bars. A strong reduction in the shive content and an improvement in the selectivity is obtained at higher oxygen pressures, such as pressures within the range from about 12 , . .
to about 100 bars. The best results at reasonable apparatus costs are obtained within the range from about 20 to about 40 bars, within which range the shive content i9 surprisingly low in comparison with parallel tests at 5 bars pressure.
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- During the major part of the oxygen-alkali delignification process, the temperature should be maintained within the range from about 100 to about 170C. A~temperatur~s within the range of from 100 to 120C., the reaction is slow. The preferred temperature range is from about 120 to 1~0C,still more preferably from about 120 to about 150C. A temperature .
from 120 to 140C is particularly suitable for the treatment of lignocellulosic material having a low lignin content, e. g. wood cellulose of the sulphate pulp type, while a temperature from 130 to 150C is particularly suitable ~or wood chips and other wood fragments with a retained wood structure.
Pulps for a certain field of use, for example, for use in ;

the production of paper, should have a high degree of strength. In such cases, it is suitable to carry out the delignification in the presence of an ;~
inhi~itor or mixture of inhibitors which protect the cellulose and hemicellu- ;
lose molecules against uncontrolled degradation. The effect of the mhibitors ~5 is reflected by the viscosity of the pulp9 and the degree of polymerization ., - of the cellulose.

The inhibitors can to advantage be charged to the delignUication , liquor during an early stage of the deli~nification or, preferably, at the beginning, before the delignification heating is begun. Thus, they can be 20 added to the delignification liquor before combination with the wood, or shortly i~:
thereafter . Suitable inhibitors are water-insoluble magnesium compounds j such as magnesium carbonate. Magnesium carbonate is known, and is ~ ;
disclosed in U.S.patent No. 3, 384, 533 to Robert et al. dated May 21, 1968 as useful in the delignification and bleaching of cellulose pulps with a~ali and oxj~Jgel~, but this is not a digestion of wood. Other water-insoluble magnesium compounds such as magnesium oxide and hydroxide are disclosed ;'' ~
,.

., 1(1;~9~8 in South Africanpatent No. 3771/68 to L'AirLiq-lide, also relating to alkaline oxygen bleaching of cellulose pulps. Also useEul are water-soluble magnesium compounds such as magnesium chloride or magnesium acetate, which form water-insoluble magn~sium coml~ounds in the alkaline digestion liquor such 5 as magnesium hydroxide or magnesium carbonate, and therefore exist as such insoluble compounds after the digestion. These are also ~

- ~ .
disclosed in South Afrlcan patent No. 3771/68 . However~ rnagnesium compounds which are soluble in the digestion liquor in the course of the digestion process are preferred. Such water-soluble magnesium compounds are disclosed in U. S.patents Nos . 3, 652, 385 and 3, 652, 386, both patented March 28, 1972S ~`~
the disclosures of which are hereby incorporated by reference.
After the oxygerl delignification process has been completed, the pulp may optionally be subjected to a mechanical treatment in order to liberate $he fibers. If the pulping is brief or moderate, a defibrator, dis-integratorj or shredder may be appropriate. After an extensive or more complete pulping or delignification, the wood can be defibrated in the same manner as in oth,er conventional cellulose cooking processes9 such as ' ' ' , , ' ' ,,~ ,,,, . . ? ., ' - ' ' , _,................ _. ... __ ~
sul~ate pulping, by blowing off the material from the digester, or by ~ ..', ' :' pumping.
The pulped wood cellulose that is obtained in accordance ~; ;
with the process of the invention is of such whiteness that it can be used to ... . .
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9 5e~8 advantage directly for producing tissue paper, light cardboard and magazine paper. When a higher de~ ree of bri~ htness is desired, as for fine paper, :
rayon and cellulose derivatives, the pulp r:an easily be bleached in accor~ance with known methods by treatment with ch~orine, chloride dioxide, chlorite, ~ `
hypochlorite, peroxide, peracetate, oxygen or any combinations of these ;
bleaching agents in one or more hleaching sequence as described in for exa~;nple ~anadian Patent No. ~01, 220, pa~ented May 30,-;;1972.
Ch~orine dioxide has been found to be a particularly suitable bleachin~ ;
agent for the oxygen digested cellulose pulp obtained in accordance with ` 10 this invention. The consumption o~bleaching chemicals- is generally;~ . ::.
markedly lower in bleaching o~cygen digested pulps oE the invention than when bleaching sulfatq cellulose. ~ ~A"
The chemicals used for the Idigestion process can be re-covered after the waste liquor is burned and subsequent to optionally causticizing all or part of the carbonate obtained when burning the liquor.
Preferred embodimen~:s of the d~lignification process oE
the invention and of the cellulose pulps of the imTention are shown in the ;
following Examples:
E~MPLE 1 ... ... ..
~20 Saw dust from birch wood was treated with five parts of ;, SO2 water havin~ a pH of 3 at 20 ~ for ten minutes. The resulting solution , . . . .
contained copper, manganese, iron, magnesium, calcium and sodium ions, and was allowed to drain off. The operation was repeated twice. The - : ' ,''',:

27 ~ -.
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, . . . , , .. .. , . ~ , . ..
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~re-treatsd powder was then was~Ied with water, and divided into two equal portions.
The first portion was subjected to an oxygen-alkali delign-ification process at an oxygen partial pressure oE 21 bars and a temperature S oE 135~, with sodium bicarbonate as the alkali. In order to eliminate mass `~ transer problems and interEerence by trace metals, the process was effected ~t a low wood-to-liquid ratio (8:700) and with pure oxygen, whlch was caused to bubble through the Teflon-lined reactor ~eflon is poly-t~trafl~oroeth~lene and is a registered- trademark3,; and a low initial concentration~lN~O3 (0.05 mol/litre).~yThe^~:te~ts wereicarried out at difEerent deligniEication-~imés"~ The pulp was-filtered off after termination of the trea~tment~processc, Th~ Kappa number according to SCAN was used to determine tke lignin contentD The intrinsic viscosity was determinsd a~cording to SCAN.
The second portion of the pre-treated birch sawdust was `~ treated in the sam~ manner, but in accordance with the invention with an addition o manganous chloride corresponding to a charge o 0.1% `
Mn based on the dry weight o the wood p~wder. The addition was n~de to the a~ueous sodium bicarbonate solution before beginning the oxygen~
aLkali deligni~icati~n. ;-For comparison purposes, the results obtained with a ``
........ . .
control using the sarne birch sawdust without pre-treatment and without an addition of manganese are also given.
, !' ~. . .
In all tests, the pH at the end of the process was withir~
the range 9.1 - 9,2, the pHbeing measured a~ter rapidly cooling to room -~ ;
temperature.
The results shown in the Table below illustrate that the ;~
delignification takes place more slowly in the case of pre-treated wood than with untreated wood, and that the depolymerization of the cellulose .
~ 28 (decrease in viscosity) is not inhibited by the bleaching with SO2 water.
By adding a manganous salt in accordance with the invention, the delignification rate is increased, so that it is only insignificantly lower !'`, ' ' .
than that obtained with unleached sawdust. In spite of this, a markedly 5 higher viscosity was obtained, compared at the same delignification time and also at the same Kappa number, than in the control made without the addition of manganese.

Reaction time Kappa Viscosity Hour__ Number cm3/g ;
Control without pretreatment 5 39.4 968 7 21. 1 892 ~`~
; .:1. ~.
9 15.2 876 Control leached 7 42.4 892 9 31.3 843 ' , 15 .i 11 21. 5 7q4 `~ Treatment in accordance 7 24.0 965 with the invention 9 17. 6 954 , ' .
11 12. 7 909 . .: ., .

Unbleached birch sulphate pulp having a Kappa number of 20.2 and a viscosity o~ 1236 cm3/g was subjected to a pre-treatment process with ethylene diamine tetraacetic acid (EDTA), Na-salt at room temperature . . ~ .
, for fifteen minutes. The pulp concentration was 3%, and the quantity of EDTA
: ~ , 25 charged to the system was 0.2% by weight based on the dry weight of the pulp.
. :.: ' .
The pulp was then washed and impregnated with an aqueous solution containing different quantities of manganous sulphate.
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~3~
The excess solution was removed by pressing, and an aqueous ,! ' .
sodium bicarbonate solution containing 100 g NaHCO3 per liter was admixed with the pulp so that the pulp consistency was 26% and the quantity of bicar-bonate corresponded to 5% by weight NaHCO3 based on the dry weight of the 5 pulp.
The quantity of added manganese was calculated by analysing the removed excess solution, and determined to be 2. 5, 32 and 320 mg Mn per kg .~ ~
The pulp was delignified (bleached) with oxygen at 120C

in three different test series. The duration of this treatment was between 20 and 90 minutes. The total partial pressure of o~Tgen was seven bars.

The viscosity according to SCAN was studied as a function of the Kappa number. ~; -. . .
With a Kappa number of 13, the viscosity was 1130 to 1150 cm3/g, ~ rans with manganese? while the control without manganese had a ~ `
viscosity of 1090 cm3/g.
With a Kappa number of 11, corresponding viscosities were 1080 - 1090 cm3/g i~ the presence of manganese, an~ 1020 cm /g for the control without manganese. ;
At a Kappa number of 9, the viscosities were 980 to 990 cm /g with manganese and 890 cm /g without manganese.
These results show that the selectivity is greatly improved - i when manganese is present in accordance with the invention. Further, in this case the effect is not significantly influenced by the magnitude of the manganese addition. The Inanganese obviously is more important, the ;
longer the bleaching process is continued. : ~ -s..: ' ' ',',,' ~

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Industrial birch chips were pre-treated by heating at 160C
with an aqueous solution of NaHCO3 at a wood: liquor ratio of 1: 5 for two hours. The bicarbonate solution contained EDTA (Na-salt). The NaHCO3 ^-charge corresponded to 20% by weight, and the EDTA to 0.1% by weight, hoth based on the dry weight of the wood.
The o~ygen cooking process was effected at a partial pressure of oxygen of 21 bars by means of a spraying method, aqueous sodium bi-carbonate solution being circulated over the pre-treated chips for four hours ~;
at 140C. The wood:liquor ratio was 1:14. At the commencement of the cooking operation the bicarbonate charge was 2.1% NaHCO~ based on the dry weight of the wDod . The pH was maintained at 7 . ~ - 8 . 0 during the entire cooking operation, by injecting aqueous sodium bicarbonate solution. -With the addition of 0. 5~/0 Mn as manganous sulfate based on ~he dry weight of the wood, a pulp having a Kappa number 8. 6 and a viscosity of86û cm3/g was obtained. Controls wîthout manganese and without the addition of EDTA during the pre-treatment process gave pulps with a Kappa number `~;;
wa~ 13.2 after the same cooking time, and the viscosity was 880 cm3/g. A
control with a cooking time of 4. 75 hours gave a pulp having a Kappa number of 8. 7, and a viscosity of 800 cm3/ g.
As the results show, the method according to the invention leads to a catalyzed delignification, and to an improved selectivity in the delignification, i. e. a higher viscosity at a given lignin content.

31 ~ , : .
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Claims (22)

Having regard to the foregoing disclosure, the following is claimed as inventive and patentable embodiments thereof:

1. A process for the delignification of raw lignocellulosic material to produce cellulose pulp having a higher viscosity at a given Kappa number which comprises pretreating said lignocellulosic material with a liquid selected from the group consisting of water and aqueous solutions to remove from the lignocellulosic material metal ions and compounds which catalyze the degradation of carbohydrates, and then carrying out the delignification with oxygen and alkali at a pH within the range from about 7 to about 10 in the presence of a catalytically-active manganese compound added in an amount within the range from 0.01 to about 1% by weight Mn based on the dry weight of the lignocellulosic material to improve the selectivity of the delignification and increase the rate of delignification.

2. A process according to claim 1, in which the manganese compound is added prior to the start of the delignification.

3. A process according to claim 1, in which the manganese compound is added at an early stage of the delignification, and before dissolution of approximately 10% of the lignin content of the starting ligno-cellulosic material.

4. A process according to claim 1, in which the manganese compound is capable of supplying manganese ion to the delignification reaction in a catalytic form.

5. A process according to claim 1, in which the manganese compound is added to the lignocellulosic material at the start of the deligniflcation.

6. A process according to claim 1, in which the manganese compound is added incrementally in the course of the delignification.

7. A process according to claim 1, in which the manganese compound is added continuously in the course of the delignification.

8. A process according to claim 1, in which the manganese compound is impregnated into the lignocellulosic material prior to the delignification with oxygen and alkali.

9. A process according to claim 1, in which the manganese compound is a bivalent manganous compound.

10. A process according to claim 9, in which the manganous compound is selected from the group consisting of manganous oxide, manganous chloride, manganous bromide, manganous hydroxide, manganous nitrite, manganous sulfate, manganous carbonate, manganous phosphate, manganous chlorate, manganous acetate, manganous formate, manganous oxalate, and complex salts of manganous ion with chelating inorganic and organic acids.

11. A process according to claim 1, in which the amount of manganese compound is within the range from about 0.05 to about 0.5%
by weight Mn, based on the dry weight of the lignocellulosic material.

12. A process according to claim 1, wherein the pretreating liquid is an aqueous solution which contains a metal complexing agent.

13. A process according to claim 1, wherein the pretreating liquid is water.

14. A process according to claim 1, wherein the pretreating liquid is an aqueous acidic solution.

15. A process according to claim 17 wherein the pretreating liquid is an aqueous alkaline solution comprising at least one alkali selected from the group consisting of sodium carbonate, sodium bicarbonate, and sodium hydroxide

16. A process according to claim 1, wherein the pretreating liquid is a waste liquor from the oxygen-alkali delignification process.

17. A process according to claim 11, wherein after the.
pretreatment but prior to the oxygen-alkali delignification the pretreated lignocellulosic material is washed with a member selected from the group consisting of water and acidic and alkaline aqueous solutions.

18. A process according to claim 1, wherein the oxygen-alkali delignification process is effected at an oxygen partial pressure of at least 5 bars.

19. A process according to claim 1 in which the lignocellulosic material is wood in the form of particles having a wood structure, and the oxygen-alkali delignification is carried out at an oxygen partial pressure of at least 10 bars.

20. A process according to claim 1, wherein the temperature for the major part of the delignification is maintained within the range from about 120 to about 160°C.

21. A process according to claim 1, in which the lignocellulosic material is wood in the form of particles having a wood structure, and the temperature is maintained within the range from about 130 to about 150°C
for the major part of the delignification.

22. A process according to claim 1, wherein a magnesium compound is added as a cellulose degradation inhibitor during the delignification.