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Publication numberUS3764464 A
Publication typeGrant
Publication dateOct 9, 1973
Filing dateJun 18, 1971
Priority dateJun 22, 1970
Also published asCA935958A, CA935958A1, DE2130164A1, DE2130164B2, DE2130164C3
Publication numberUS 3764464 A, US 3764464A, US-A-3764464, US3764464 A, US3764464A
InventorsSamuelson H
Original AssigneeMo Och Domsjoe Ab
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Process for preparing cellulose pulp by alkaline oxygen digestion of wood in the presence of alkali metal carbonate or bicarbonate
US 3764464 A
Abstract  available in
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Description  (OCR text may contain errors)

Oct. 9, H. o. SAMUELSON PROCESS FOR PREPARING CELLULOSE PULP BY ALKALINE OXYGEN DIGESTION OF WOOD IN THE PRESENCE OF ALKALI METAL CARBONATE OR BICARBONATE Flled June 18, 1971 United States Patent M 3,764,464 PROCESS FOR PREPARING CELLULOSE PULP BY ALKALINE OXYGEN DIGESTION OF WOOD IN THE PRESENCE OF ALKALI METAL CARBON- ATE OR BICARBONATE Hans Olof Samuelson, Goteborg, Sweden, assignor to M0 och Domsjo AB, Ornskoldsvik, Sweden Filed June 18, 1971, Ser. No. 154,486 Claims priority, application Sweden, June 22, 1970, 8,589/70 Int. Cl. D21c 9/10 U.S. Cl. 162-65 33 Claims ABSTRACT OF THE DISCLOSURE Cellulose pulp is prepared from wood by alkaline oxygen digestion in the presence of alkali metal carbonate or bicarbonate. 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.

It has been known for several years that wood can be digested to form cellulose pulp by using oxygen gas in the presence of strong aqueous alkali such as sodium hydroxide. The oxygen gas serves as an oxidant which in the presence of the aqueous alkaline liquid phase attacks the lignin of the wood, and converts it into soluble degradation products which dissolve in the alkaline digestion liquor. However, although the two-phase process has many attractive features, it has not received commercial acceptance because the reaction is quite difiicult to control. In part, this is due to the two-phase reaction system. A nonuniform 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 the two-phase system, 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. Pat. No. 2,673,148, dated Mar. 23, 1954, proposed an oxygen digestion process using quite high oxygen pressures, of the order of at least 800 p.s.i. This was thought necessary in order to obtain and maintain a sufiiciently high oxygen concentration in the digestion liquors. This is one of the serious problems in oxygen digestion processes due to the fact that oxygen is a gas and not capable of being solubilized in the digestion liquor by the expedients employed up to now. However, the results obtained in this process were not satisfactory.

Grangaard and Saunders, U.S. Pat. No. 2,926,114, dated Feb. 23, 1960, stated that oxygen prior to 1957 had been used both at low and at high oxygen pressures. However, at low pressures, the pulping was inadequate, and the process had to be used only as a single stage in a multiple stage pulping process, using more conventional pulping chemicals to complete the pulping. At the high pressures, the pressures are so high, large volume batch digesters cannot be readily constructed to withstand them. Grangaard et al. proposed a digestion at pH 7 to 9 over at least a major portion of the cooking time, ranging up to 9.4 at the end of the cook, under oxygen pressures of 40 to 250 p.s.i., using conventional batch digesters. The

3,764,464 Patented Oct. 9, 1973 ICC pH is maintained within the desired range by a butter 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 disadvantages are:

(1) An extremely high oxygen consumption.

(2) It is difiicult to control the process, and a nonuniform pulp is obtained.

(3) A high consumption of sodium bicarbonate, and no method is suggested for the recovery of bicarbonate.

As the Grangaard et al. patent illustrates, it has not been possible to develop a practical pulping process using oxygen. A recent investigation of the oxygen digestion process by I. C. Lescot, Essais de Delignification de Bois Feuillus par lOxygene en Milieu Alcalin (Ph.D. Thesis, Univ. of Grenoble, France, Oct. 27, 1967), resulted in the conclusion that alkaline oxygen digestion was not teasible, since the difficulties of impregnation were important. The author reported that a pre-digestion with other chemicals was necessary before the delignification with oxygenalkali.

In accordance with the invention, a process is provided for the alkaline oxygen digestion of wood which can be controlled so as to inhibit formation of slivers, as well as to prevent undue degradation, thereby to increase the uniformity of the cellulose pulp, and improve its color and strength properties. In the process of the invention, the alkaline digestion liquor comprises alkali metal bicarbonate or carbonate, or both, oxygen gas is provided under pressure to the reaction system, and carbon dioxide that 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 is more eflicient, this results in greater economy both of alkali and of oxygen.

The Figure shows a digestion reaction system for use in continuous carbon dioxide separation in the process of the invention.

The alkali metal carbonate or bicarbonate is preferably sodium carbonate or bicarbonate, but potassium or lithium carbonate or bicarbonate can also be used. The carbonate or bicarbonate can serve as the sole source of alkali, or can be combined with alkali metal hydroxide, such as sodium or potassium hydroxide.

The carbon dioxide can be removed from the oxygencontaining gaseous efiiuent from the digestion by absorbing the carbon dioxide with an alkaline absorption liquid. For this purpose, at least once during the digestion the gas phase is withdrawn from the digestion liquor, and contacted with the alkaline absorption liquid. The resulting carbonated liquid can be used as the digestion liquid in the pulping process, or it can be used for another pulping process, or for other purposes. If it is used to prepare digestion liquid, the carbonated liquid is recycled to the pulping process of the invention.

The alkine absorption liquid is preferably an aqueous alkaline solution containing alkali metal hydroxide, or carbonate, or both. Such a solution is capable of absorbing carbon dioxide to form alkali metal carbonate or bicarbonate, or both. An aqueous solution containing, for example, sodium or potassium carbonate can be obtained from the combustion of waste liquor obtained from a wood pulping process such as the sulfate or sulfite pulping process, or the pulping process in accordance with the invention. Preferably, a waste liquor from a pulping process is concentrated by evaporation to a solids content above about 50% by weight, and then subjected to combustion to form a solid combustion residue or ash comprising substantially sodium or potassium carbonate. It is also possible to make use of the so-called wet combustion process, using oxygen or air under high pressure. In this case, a concentrated liquor is obtained, and the evaporation process may be omitted entirely.

Other examples of waste liquors include waste liquors obtained from hot alkali treatment of, for example, sulfite cellulose pulps, or from cold alkali treatment of sulfite or sulfate cellulose pulps, as well as waste liquors obtained from alkaline oxygen bleaching of chemical or semichernical wood pulp, the bleaching step being carried out in the presence of oxygen ad alkali, preferably under elevated pressure. In the case of such liquors, available and conventional combustion or wet combustion recovery processes can be used to obtain an aqueous concentrated alkali metal carbonate absorption liquor. Mixtures of liquors or solutions containing sodium carbonate obtained from the combustion of ditTerent types of waste liquors can be used.

The process of this invention can be used to advantage to make so-called chemical pulp, that is, a pulp of low lignin content which is readily defibrated. The pulps of the invention can be defibrated merely by blowing from the digestion vessel, or by a mild mechanical treatment. The process of the invention can also be used to make semichemical pulp, a pulp which requires forceful mechanical treatment, such as refiners or other defibrating equipment, in order to liberate the fibers.

The chemical reactions that take place when wood is digested with alkali in the presence of oxygen are not fully understood. Reactions which take place in the presence of alkali, as well as oxidation reactions due to the presence of the oxygen, both occur to a considerable extent. Experiments have shown that the dissolution of the lignin as well as the yield of the pulp and the degradation of the cellulose are influenced considerably by the composition of the digestion liquid as well as by the temperature and the oxygen pressure. At present, no explanation can be given of the favorable results obtainable in accordance with the process of the invention. However, the advantages obtained are certainly due not only to the fact that removal of the carbon dioxide increases the partial pressure of the oxygen. Experiments have shown that advantages are obtained also in the reduction of the proportion of slivers formed from the wood. The process is also more readily controlled, possibly due to the fact that the alkalinity and the buffering capacity of the alkaline digestion liquor can be controlled in a simple manner throughout the digestion process.

The alkaline absorption liquor employed in the process of the invention can be prepared by dissolving the alkali in water. Combustion residues consisting essentially of sodium carbonate are preferably used. For greater economy of operation, instead of Water there can be used a liquid containing sodium salts and organic substances such as waste pulping liquor or digestion liquor withdrawn from the digestion operation, or washing liquids derived from the alkaline treatment of cellulosic materials such as liquids derived from the digestion process of the invention, from alkaline refinement of cellulose, and/ or from the bleaching of wood pulp.

Any alkali metal carbonate can be employed, such as sodium carbonate, potassium carbonate and lithium carbonate, alone or admixed with alkali metal hydroxide such as sodium hydroxide, potassium hydroxide, or lithium hydroxide. The use of alkali metal carbonates is more advantageous than the use of alkali metal hydroxides in maintaining the pH of the digestion liquor within the desired range, because of the buifering properties of the carbonate or bicarbona e p t or formed in situ. Mixtures of alkali metal hydroxides and alkali metal carbonates can also be used. The alkaline digestion liquor employed in the process of the invention can be prepared from mixtures of alkali metal hydroxides or carbonates with alkali metal bicarbonates such as sodium bicarbonate and potassium bicarbonate. The alkali metal bicarbonate in this case serves as a buffer. Other buffering agents, compounds of alkali metals with nondeleterious acidic anions, can be employed, such as alkali metal acid phosphates, and alkali metal acid or bisulfites, such as potassium dihydrogen phosphate, potassium monohydrogen phosphate, sodium dihydrogen phosphate, sodium monohydrogen phosphate, sodium acid sulfite, and potassium acid sulfite, as well as the lithium salts of these anions.

The pH of the alkaline liquor during the digestion can be from about 7 to about 10, preferably from about 8 to about 9.5.

If the pH falls below 7 for an appreciable portion of the digestion process, the quality of the pulp is deleteriously affected. With certain types of pulp, particularly viscose pulps and other dissolving pulps, it is possible to permit the pH to drop below 8, down to about 7, during the final stages of the process without seriously affecting the quality of the pulp. Apparently, brightness is affected most by chemical reactions occurring during the initial and major part of the digestion reactions.

The total amount of alkali (defined as the total amount of alkali metal ions as alkali hydroxide, alkali carbonate and alkali bicarbonate) 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 1 to about 10 kilomoles per 1000 kg. of dry wood. It is well known that certain types of pulp are more 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 viscose, acetate and cuprammonium pulps, are quite fully digested, and should have a low content of lignin and hemicellulose. In the 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 per 1000 kg. of dry wood. Semichemical pulps are given an intensive mechanical 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 about 1 to about 2 kilomoles per 1000' kg. of dry wood. For the production of bright paper pulp, which is readily difibered 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 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 about 6 kilomoles per 1000 kg. of dry wood.

It is also important to restrict the concentration of bicarbonate and other additions so that the added amounts are soluble in the digestion liquor, so as to avoid precipitation within the wood being digested.

Spent liquors from previous digestions and/or the waste liquors from oxygen bleaching processes, such as those described in US. applications Ser. Nos. 869,875, now US. Pat. No. 3,652,386, and 36,670, now US. Pat. No. 3,652,385, can be used in the preparation of the digestion liquor. In this way, better economy is obtained in chemical recovery, which can be effected after evaporating and burning the waste digestion liquor, using known methods.

The digestion process can be carried out continuously or as a batch process. The alkali metal hydroxide and/or alkali metal carbonate can be charged all at once, at the start of the digestion, or only part can be added initially, and the remainder added 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.

Since the oxygen that is employed as an essential component 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. If the wood is completely or substantially immersed in the digestion 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 in the digestion liquor to the greatest extent possible. Dissolution or dispersion of the oxygen in the liquor can take place within the digestion vessel 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 temperature and/or the proportion of gas-liquid contact surfaces, including the wood impregnated with digestion liquor.

The oxygen is preferably employed as pure oxygen, but mixtures of oxygen with other inert gases can be used, such as, for example, mixtures of oxygen with nitrogen, as well as air. Mixtures with carbon dioxide are formed in the course of the process.

It is, however, important that the partial pressure of oxygen in the gas phase of the digestion reaction system be maintained at at least one bar, and preferably within the range from about 3 to about bars, for a uniform and acceptable digestion of the wood. Since carbon dioxide is formed, and reduces the partial pressure of oxygen as it enters the gas phase, while the oxygen also diminishes because of the consumption of oxygen during the digestion, it is a feature of the invention that carbon dioxide is separated from the gas phase and optionally oxygen is also added to replenish that consumed by reaction, so as to maintain the oxygen partial pressure above at least one bar. This can be done by continuously or from time to time circulating the gas phase of the digestion system through an absorption system where the carbon dioxide is absorbed in an alkaline absorption liquid, and the remaining gas, oxygen plus any inert gases not absorbed, recycled to the digestion system.

Prior to contact with the oxygen, the wood suitably in the 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 pressure, or by other methods conventional in wood digestion processes. The wood may also be treated with steam before being brought to the digestion zone.

The temperature employed during the impregnation can be within the range from about 20 to about 120 C., although temperatures within the range from 90 to 120 C. would not normally be used except under special circumstances. In the latter case, the highest temperature during the digestion may be the same as the impregnating temperature, as well as the initial digestion temperature.

The digestion can be carried out at a temperature within the range from about 60 to about 175 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 60 to 90 C. to the maximum digestion temperature of the order of from 110 to 150 or 175 C.

At a maximum temperature of C., the digestion process proceeds slowly, but on the other hand, moderate oxygen pressure and simple technical apparatus can be used. A digestion temperature of from 90 to C. can be used to advantage when producing semichemical pulps, the fibers of which are not fully liberated until after subjection to a mechanical treatment process, such as in a refiner after the digestion process. These are high yield pulps.

If a maximum digestion temperature of from to C. is used, the digestion will proceed rapidly. On the other hand, in this case, an exceedingly effective transfer of oxygen to the wood from the gas phase is required. 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 maximum digestion temperature within the range from 110 to 150 C. 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, irrespective of whether semichemical pulps are being produced or cellulose pulps whose fibers can be liberated without intensive mechanical treatment, or are simply liberated when the cooker or digestion vessel is blown.

The partial pressure of oxygen during the digestion process should be within the range from about 1 to about 20 atmospheres, preferably from about 3 to about 20 atmospheres. Higher pressures should not be used, from the standpoint of safety, and are definitely unnecessary. At lower pressures, the digestion proceeds more slowly, and such pressures are not economically practical. Normally, a pressure within the range from about 3 to about 12 atmospheres is preferred.

Because of the consumption of oxygen in the course of the digestion, and the higher rate at which the digestion proceeds at high reaction temperatures, it follows that the higher the reaction temperature, the higher the pressure that should be applied during the reaction. The optimum temperature and pressure conditions for a given pulp can be determined by digestion sampling procedures, as is well 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 paper, should have a high degree of 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, preferably, at the beginning, before the digestion heating is begun. Thus, they can be added to the digestion liquor before combination with the wood, or shortly thereafter. Suitable inhibitors are water-insoluble magnesium compounds, such as magnesium carbonate. Magnesium carbonate is known, and is disclosed in US. Pat. No. 3,384,- 533 to Robert et al. dated May 21, 1968 as useful in the delignification and bleaching of cellulose pulps with alkali and oxygen, but this is not a digestion of wood. Other water-insoluble magnesium compounds such as magnesium oxide and hydroxide are disclosed in South African Pat. No. 3,771/ 68 to lAir Liquide, also relating to alkaline oxygen bleaching of cellulose pulps. Also useful are water-soluble magnesium compounds such as marnesium chloride or magnesium acetate. These are also disclosed in South African Pat. No. 3,771/68. Magnesium compounds which are soluble in the digestion liquor in the course of the digestion process are preferred. Such are the Water-soluble complex magnesium compounds, which are soluble over a wide range of pH values.

Aliphatic alphahydroxycarboxylic acids of the type RCHOHCOOH and the corresponding beta-hydroxy-carboxylic acids RCHOHCH COOH have the property of forming chelates with magnesium. These chelates are of the type:

In the above formula, n is zero or one. When n is zero, the acid is an alpha-hydroxy acid, and when n is one, the acid is a beta-hydroxy acid.

R in the above formula is hydrogen or an aliphatic radical, which may be a hydrocarbon radical having from one to about ten carbon atoms, or a hydroxy-substituted hydrocarbon radical having from one to nine hydroxyl groups, and from one to about ten carbon atoms.

Exemplary alphaand beta-hydroxy carboxylic acids are glycolic acid, lactic acid, glyceric acid, a,fl-dihydroxybutyric acid, lx-hydroxy-butyric acid, whydroxy-isobutyric acid, a-hydroxy-n-valeric acid, a-hydroxy-isovaleric acid, fi-hydroxy-butyric acid, fl-hydroxy-isobutyric acid, fi-hydroxy-n-valeric acid, ,e-hydroxy-isovaleric acid, erythronic acid, threonic acid, trihydroxy-isobutyric acid, and sugar acids and aldonic acids, such as gluconic acid, galactonic acid, talonic acid, mannonic acid, arabonic acid, ribonic acid, xylonic acid, lyxonic acid, gluonic acid, idonic acid, altronic acid, allonic acid, ethenyl glycolic acid, and B- hydroxy-isocrotonic acid.

Also useful are organic acids having two or more 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, tetrahydroxy adipic acid, dihydroxy maleic acid, mucic acid, mannosaccharic acid, idosaccharic acid, talomucic acid, tricarballylic acid, aconitic acid, and dihydroxy tartaric acid.

Magnesium complexes of nitrogen-containing polycarboxylic acids are especially effective inhibitors. Several important acids belonging to this group have the formula:

HOOCCH:

or alkali metal salts thereof, in which A is the group CH COOH or CH CH OH 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 ethylenediaminetetra acetic acid, ethylene diamine triacetic acid, nitrilotriacetic acid, diethylenetriaminopentaacetic acid, tetraethylenepentamine heptaacetic acid, and hydroxyethylethylenediaminetriacetic acid, and their alkali metal salts, including the mono, di, tri, tetra and penta sodium, potassium and lithium salts thereof. Other types of arninocarboxylic acids which can be used to advantage are iminodiacetic acid, Z-hydroxyethyliminodiacetic acid, cyclohexanediaminetetraacetic acid, anthranil-N,N-diacetic acid, and 2- picolylamine-N,N-diacetic acid.

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 magnesium hydroxide and magnesium carbonate during the digestion. The use of waste digestion liquor in combina tion with complexing agents of this type is particularly advantageous.

The polyphosphoric acids are also good complexing agents for magnesium, and the magnesium salts of these acids are useful in the process of the invention. Exemplary are disodium-magnesium pyrophosphate, trisodiummagnesium tripolyphosphate and magnesium polymetaphosphate.

Especially advantageous from the standpoint of cost are the acids naturally present in waste liquors obtained from the alkaline treatment of cellulosic materials. These acids represent the alkalior water-soluble degradation products of polysaccharides which are dissolved in such liquors, as well as alkalior water-soluble degradation products of cellulose and hemicellulose. The chemical nature of these degradation products are complex, and they have not been fully identified. However, it is known that saccharinic and lactic acids are present in such liquors, and that other hydroxy acids are also present. The presence of C isosaccharinic and C -metasaccharinic acids has been demonstrated, as well as C.,- and c -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 sul'fite digestion processes; and liquors from sulfate digestion processes, i.e., kraft waste liquor. The waste liquors obtained in alkaline oxygen gas bleaching processes, for example, those disclosed in Ser. Nos. 869,875 and 36,670, or alkaline peroxide bleaching processes can also be used. In this instance, the alkaline liquor can be taken out from the process subsequent to completing the oxygen gas treatment stage, or during the actual treatment process.

The complex magnesium salts can be formed first, and then added to the digestion liquor. They can also be formed in situ from a water-soluble or water-insoluble magnesium salt, oxide or hydroxide, in admixture with the complexing acid, and this mixture can be added to the digestion liquor. Preferably, the waste liquor employed as the source of complexing acid or lactone or salt thereof can be mixed with a magnesium salt, oxide or hydroxide, before being introduced to the process. It is also possible to add the magnesium salt, oxide or hydroxide to the digestion liquor, and then bring the liquor into contact with the complexing acid or lactone or salt thereof. It is also possible to combine the complexing acid or lactone or salt thereof with the liquor and then add the magnesium salt, oxide or hydroxide, but this method may be less advantageous in practice.

In whatever form the magnesium is added, whether as salt, oxide, hydroxide, or complex salt, the amount of magnesium is calculated as MgO.

A noticeable improvement is obtained when as little magnesium as 0.01% MgO, calculated on the dry weight of the wood, is added. A high proportion of magnesium, up to 1% MgO, calculated on the dry weight of the wood, has been employed without disadvantageous effect. However, for economic reasons, it is usually desirable to use as little magnesium as possible, and preferably an amount within the range from about 0.05% to about 0.5% MgO, calculated on the dry weight of the wood, is employed.

Upon conclusion of the alkaline oxygen gas digestion, it is possible to separate the magnesium-containing waste liquor and recycle it for reuse. The consumption of magnesium salts is negligible, and usually it is not even necessary to replenish the magnesium content before recycling. However, additional magnesium compound can be added before recycling, if necessary, to restore the magnesium content, as MgO, and maintain a high enough level, for instance, to prevent oxidative degradation of the cellulose or hemicellulose. The consumption of magnesium salt has been noted to be particularly low when waste liquor from a part of the alkaline oxygen gas treatment process is employed as the source of complexing acid, and recycled for continued treatment of new batches of wood.

Some waste liquors are particularly high in magnesium ion because of the nature of the pulp or of the pulping process. For example, liquors from the cooking of wood with magnesium bisulfite or magnesium sulfite usually contain enough magnesium ion so that no addition of magnesium compound need be made. Such waste liquors can be used per se, in the process of the invention, inasmuch as they already contain the complexing acids, and a suflicient proportion of magnesium ion as well.

As a source of magnesium, one may add any magnesium salts, oxide or hydroxide, either to regenerate a spent treatment liquor, or to prepare a waste liquor or other material for use in the process. Any water-soluble magnesium compound can be used, such as for example, magnesium sulfate, magnesium chloride, magnesium bromide, magnesium chlorate, magnesium potassium chlorate, magnesium formate, magnesium oxide, magnesium acetate, magnesium hydroxide, and magnesium nitrate. If it is desired to recover the liquor after the treatment, then it is usually preferable to employ magnesium sulfate, so as to avoid the introduction of corrosive anions into the system. Magnesium compounds which have no deleterious anion or which have an anion which is destroyed in the course of the process, such as magnesium oxide, magnesium hydroxide, and magnesium carbonate, are also advantageous. Since these are water-insoluble, it is desired, however, to combine these with the complexing agent in the presence of water, and await their dissolution, indicating that the complex has been formed, before combining with the digestion liquor, or before commencing the alkaline oxygen gas digestion. Any other water-insoluble magnesium compounds can be used in this way, for instance, magnesium phosphate, magnesium silicate and magnesium sulfide.

It is often suitable during the digestion to withdraw a portion of the digestion liquor, such as by draining, pressing, displacement or filtering. This liquor can be returned to the digestion process at a later stage, or to a subsequent batch, and in this event it is advantageous to heat the liquid or a part thereof under pressure to an elevated temperature of the order of from 110 to about 200 C. in intimate contact with an oxygen-containing gas such as air.

A surface-active agent can be added to the digestion liquor, and contributes to a reduction in the resin content of the wood cellulose produced from the wood. This also surprisingly contributes to a reduction in the lignin content, and a more uniform delignification. The surfaceactive agent is suitably added at the beginning of the digestion process, or during an early stage of the digestion, and may be present during all or only a part of the digestion. Cationic, anionic, and nonionic surface-active agents and mixtures thereof can be used. If liquor is circulated during the digestion process, it is suitable to use agents which do not produce foam. Examples of suitable surface-active agents are polyoxyalkylene glycol ethers of fatty alcohols and alkyl phenol polyoxyalkylene glycol ethers. Sulfonated anionic surface-active agents such as the alkylbenzene sulfonates can also be used.

Also suitable are nonsurface-active quaternary ammonium lower alkyl and/or lower alkanol and/or polyoxyalkylene alkanol salts which have the formula:

In the above formula, from one to four of R R R and R are saturated aliphatic hydrocarbon radicals having from one to about four carbon atoms; and/or from one to four of R R R and R are hydroxylalkyl or polyoxyalkylene radicals terminating in a hydroxyl group,

and havin a formula selected from the group consisting of wherein m is an integer from zero to five, p is an integer from zero to five, and q is an integer from zero to two; and mixtures of two or more thereof. Thus, all of the R radicals are either saturated lower aliphatic hydrocarbon radicals or hydroxyalkyl or hydroxyalkylene polyoxyalkylene radicals of these types.

X is an inorganic anion, and is preferably selected from the group consisting of H50 CH SO C H SO Cl and Br. The nature of X is not critical, provided it is inert in the cellulose pulping liquor.

It will be evident that these compounds are quaternary lower hydrocarbon amines having methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl and tert-butyl groups, in any combination of the same and different groups; quaternary alkanol amines, and quaternary hydroxyalkylene amines having polyoxyethylene, polyoxypropylene,

and/ or polyoxybutylene groups; or quaternary hydrocar-' bon alkanol amines having mixed combinations of such groups.

Exemplary quaternary ammonium compounds are tetramethyl ammonium chloride, trimethylethyl ammonium bromide, monopropyl dimethyl ethyl ammonium chloride and ammonium dibutyl methyl monomethyl sulfate.

The preferred compounds are quaternary methyl triethanolamines having the formula:

H0 CzH4-N CHaSOl HO CzH4 CH;

The quaternary hydrocarbon amines are known, and are available commercially. Where not available, they are readily obtainable by known methods.

The quaternary triethanolamines, tripropanolamines and tributanol amines also are known, and they and their polyoxyalkylene derivatives can be prepared in known manner by the condensation of ethylene oxide, propylene oxide or butylene oxide with ammonia or a mono, di or trialkanolamine. The reaction mixture thus can contain mixtures of mono-, diand trialkanolamines, together with higher polyoxyalkylene derivatives. This mixture can be subjected to quatemization, but it is preferable to distill the product, so as to remove the trialkanolamine fraction in the form of a product having from 98 to 99% trialkanolamine. The quaternary ammonium nonsurface active resin control agent can be prepared from the alkanolamine such as 9899% triethanolamine by quaternizing with the quaternizing agent, such as dimethyl sulfate, in known manner.

It is important to obtain an intimate contact between the oxygen-containing gas and the alkaline digestion liquor. Various expedients can be used to obtain this result. Atomization of the alkaline digestion liquor into the gas phase by means of nozzles or other known devices can be used, as well as gas-phase digestion in which the alkaline digestion liquor is sprayed onto the Wood material during a part of or during the entire digestion period. It is also possible to disperse oxygen-containing gas in the alkaline digestion liquor. This may be done in the digestion vessel or in a different place from which the oxygencontaining alkaline digestion liquor is flowed to the digestion vessel.

The Wood material being digested can be kept stationary during the process or it can be circulated either concurrently or countercurrently to the digestion liquid. The invention can also be applied to advantage to the continuous pulping of wood material wherein Wood is continuously fed to the digester, and pulp continuously withdrawn therefrom by means of suitable devices such as are known, for example, in continuous sulfate pulping processes with the wood having a transit time through the system corresponding to the desired digestion time.

If the digestion system includes a plurality of batch digesters, the oxygen-containing gas phase withdrawn from all the digesters can be circulated to a common carbon dioxide absorption system from which the fortified oxygen-containing gas phase can be returned to all of the; digesters. Similarly, the used absorption liquor also can be returned to all of the digesters in the system.

The absorption system for carbon dioxide can be any system which ensures good contact between the gas-phase effiuent from the digester system and the absorption liquid. Gas scrubbers in which a stream of liquid passes countercurrently downwardly against an upward flow of gas are quite satisfactory. Spray scrubbers or towers, perforated plate towers, wetted wall columns, bubble-cap plate towers, sieve plate towers, packed towers, turbo-gas mixers, orifice-column mixers, injectors, jet mixers, turbogas absorbers, cascade towers, and bubble columns can be used.

The digestion system shown in the Figure is a batch system that includes a digester A fitted with a wood-chipsupporting plate 1 and a plurality of digestion liquor spray nozzles 2. At the top of the digester A are connected a gas effluent line 3 and a digestion liquor recycle line 4. Line 4 leads from a heat exchanger C, fed by line 5 and pump B connected by the line 5 to the bottom of the digester A. Line F leads from a storage or liquid feed reservoir (not shown) for digestion liquor.

Line 3 connects to the carbon dioxide scrubber D, in which gaseous effiuent from line 3 flows countercurrently upward against the downward flow of alkaline absorption liquid sprayed from nozzles 6 at the top of the scrubber. The nozzles are fed from line 7, and liquor is pumped through the line to the nozzles by pump G. A feed line F connects line 7 to a storage or liquid feed reservoir (not shown) for absorption liquid.

Line 8 at the top of the scrubber D draws off scrubbed oxygen gas and returns it to the digester A, impelled by the blower E, where it is distributed by bubble heads 9. Feed line F provides replenishing oxygen to the line 8.

The oxygen digestion 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 softwood s include spruce, fir, pine, cedar, juniper and hemlock.

In the case of softwood, the processing conditions, including the particle size of the wood fragments, the digestion temperature, the alkali concentration, and the oxygen pressure, should be carefully determined and controlled during the digestion.

The wood should be in particulate form. Wood chips having dimensions that are conventionally employed in the sulfate process can be 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 thin fragments of the type of wood shavings or chips having an average thickness of at most 3 mm., and preferably within the range from 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. It is important, particularly in the case of softwood, that the wood fragments be thin, since otherwise the digestion may be nonuniform, and the process may be more difiicult to control.

After the oxygen digestion process has been completed, the pulped wood may optionally be subjected to a mechanical treatment in order to liberate the fibers. If the pulping is brief or moderate, a defibrator, disintegrator,

or shredder may be appropriate. After an extension or more complete pulping or digestion, the wood can be defiberated in the same manner as in other conventional cellulose cooking processes, such as sulfate pulping, by blowing off the material from the digester, or by pumping.

The wood cellulose pulp that is obtained in accordance with the process of the invention is of such whiteness that it 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 and cellulose derivatives, the pulp can easily be bleached in accordance with known methods by treatment with chlorine, chlorine dioxide, chlorite, hypochlorite, peroxide, peracetate, oxygen or any combinations of these bleaching agents in one or more bleaching sequence as described in, for example U.S. application Ser. No. 882,812. Chlorine dioxide has been found to be a particularly suitable bleaching agent for the oxygen-digested cellulose pulp obtained in accordance with this invention. The consumption of bleaching chemicals is generally 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 cauticizing all or part of the carbonate obtained when burning the liquor.

Preferred embodiments of the digestion process of the invention and of the cellulose pulps of the invention are shown in the following examples.

EXAMPLE 1 The apparatus used is shown in the drawing. The digestion vessel A was charged with parts of the birch wood chips, each chip being approximately 3 x 6 x 20 mm. The chips wre supported in the vessel on the perforated plate 1. 500 parts of alkaline digestion liquor at 60 C. was introduced via lines F and 5 at the bottom of the digestion vessel. This liquor was prepared from 450 parts of aqueous 14% sodium bicarbonate solution and 50 parts of spent liquor containing saccharinic and other magnesium complexing acids from a previous digestion process, to which had been added a quantity of magnesium sulfate such that the amount of magnesium (calculated as MgO) complexed with saccharinic and other acids corresponded to 0.2%, based on the dry wood, and the amount of sodium bicarbonate in the digestion liquor introduced was 53%. Enough digestion liquor was introduced so that the chip bed was completely immersed in the liquor, and the wood chips were impregnated with the liquor for 30 minutes. Then, part of the digestion liquor was drawn off through line F to lower the liquid level below the bottom of the chip bed. The quantity of sodium bicarbonate in the volume of liquor remaining after withdrawal corresponded to 20% NaHCO based on the weight of dry wood. The withdrawn liquor was used to prepare another batch of alkaline liquor for a subsequent digestion.

The system was then put under an oxygen pressure of 10 bars. This pressure was maintained during the process by supplying additional oxygen as required. The digestion liquor was circulated through the vessel A by means of the circulation pump B via lines 5 and 4 from the bottom of the digestion vessel to the atomizers 2 at its top, and the alkaline digestion liquor was heated to C. as it passed through the heat exchanger C en route to the vessel. Continuous circulation of the digestion liquor was maintained for one hour, until all of the liquor (at 60 C., initially) had reached 120 C. The digestion was then continued in this way at 120 C. for 24 hours. During the digestion, the atomizing nozzles 2 atomized the liquor, providing a large contact area with oxygen, so that dissolution of oxygen in the liquor was facilitated. The oxygen-containing digestion liquor passed continuously downwardly over the chips, so that they were in continuous contact with a film of digestion liquor which was 13 progressively renewed throughout the digestion. A high rate of circulation of the digestion liquor promotes the pulping of the wood, and in this case, the rate of circulation was 200 liters per minute per 100 kg. of dry wood.

During the digestion, oxygen was consumed and carbon dioxide was evolved. The oxygen admixed with carbon dioxide was circulated continuously between the digestion vessel A and the gas scrubber D via the lines 3 and 8. Circulation was effected by the blower E in line 8, suitable for conveying gases under pressure. In place of the blower, an ejector device or any other apparatus for conveying gases under pressure can be used. The absorption apparatus D was a pressure gas scrubber, but any known type of absorption apparatus can be used. Aqueous absorption liquid containing sodium carbonate and prepared by dissolving combustion residues from the combustion of waste liquor from the process was introduced to line 7 via line F and circulated by the pump G and line 7 to the top of the scrubber D, where it was atomized by means of a series of nozzles 6, so that an efiicient absorption of carbon dioxide was obtained, with simultaneous formation of sodium bicarbonate. The liquor was recirculated via line 7 and pump G until nearly saturated with sodium bicarbonate. From time to time, a portion of the absorption liquid, when saturated with sodium bicarbonate, or nearly so, was withdrawn from the absorption system via line F to a storage (not shown), and used to prepare fresh digestion liquor, and replaced with a fresh supply of 10% sodium carbonate solution.

As the hydrogen carbonate circulating through digester A was consumed during the digestion process, fresh sodium bicarbonate was added from time to time through the line F (desirably, from the storage from line F so that the total charge of sodium bicarbonate amounted to based on the dry wood.

After the digestion had been carried out at 120 C. with recirculation of the liquor for 24 hours, pulping was completed. The oxygen was collected, to be used for a subsequent digestion process, and the digester was emptied by blowing the pulp, the air being admitted through line 8 from blower E.

In this series of experiments, a pulp yield of 60% was obtained, based on the dry wood and dry pulp. The kappa number of the pulp was 15, and its viscosity according to TAPPI was 40. The pulp brightness, measured according to SCAN was 62.

EXAMPLE 2 The procedure of Example 1 was repeated, except that the digestion temperature was 140 C., and the digestion time 6 hours. The pulp yield was 58%, the brightness of the pulp according to SCAN was 58, and its kappa number was 21.

EXAMPLE 3 The process of Example 1 was repeated, using a digestion temperature of 148 C. and a digestion time of 6 hours. The pulp yield was the brightness SCAN was 66, and the kappa number was 9.

EXAMPLE 4 The procedure of Example 3 was repeated, but without addition of magnesium sulfate. The pulp yield was 50.5%, and the brightness SCAN was 70. The kappa number of the pulp was 5. These results show the advantage of having magnesium ion in the digestion liquor during the digestion.

EXAMPLE 5 The digestion vessel A shown in the Figure was charged with 100 parts of sawdust from birch wood and 1500 parts of alkaline digestion liquor containing 1000 parts of spent liquor from an earlier digestion, carried out under the same conditions, and 500 parts of wash water obtained from washing of the pulp produced in the earlier digestion. The cooking liquor contained 10 grams per liter of sodium bicarbonate, and was heated from 70 C. to 140 C. over one hour. The temperature was then kept at 140 C. for 6 hours while the sodium bicarbonate liquor was recirculated continuously, and oxygen at 6 bars pressure was introduced at the bottom of the digestion vessel and passed upwardly through the suspension of sawdust and liquor.

The gas phase of carbon dioxide and oxygen was drawn off at the top of the digestion vessel and passed to and through the pressure scrubber D in the Figure, in which the gas was washed continuously with aqueous 10% sodium carbonate solution, in which the carbon dioxide was absorbed. The result was a sodium bicarbonate solution saturated to about with sodium bicarbonate. The purified oxygen from the pressure scrubber was then returned to the bottom of the digestion vessel A via the compressor E and line 8.

The sodium bicarbonate content in the digestion liquor was determined analytically during the heating period, and during the first five hours at the final temperature level, and the concentration was kept at 8 to 12 grams per liter by introducing nearly saturated sodium bicarbonate solution into the digestion vessel A via lines F and 5. The sodium bicarbonate solution was drawn from storage supplied from the pressure scrubber D, where it was prepared from carbon dioxide evolved during the digestion, and sodium carbonate produced by burning spent liquor after previous evaporation to a 50% concentration.

After 6 hours of digestion at C., the gas pressure was released, and the pulp suspension was blown out, and washed on filters. The spent digestion liquor was evaporated and burned, and the residue used to prepare digestion liquor for a subsequent batch.

The pulp yield Was 53%, calculated as dry pulp on dry wood. The kappa number of the pulp was 19, and the pulp brightness SCAN was 60.

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

1. In the process for the digestion of wood in a digestion liquor comprising a mixture of aqueous alkali and oxygen, the improvement which results in control of the alkaline oxygen digestion of wood to inhibit formation of slivers, as well as to prevent undue degradation, thereby to increase the uniformity of the cellulose pulp, and improve its color and strength properties, comprising digesting the wood, in a reaction system, in an aqueous alkaline digestion liquor which comprises a carbonate selected from the group consisting of alkali metal carbonate, alkali metal bicarbonate, and both; supplying oxygen gas under pressure to the reaction system; separating a gas phase including unreacted oxygen and carbon dioxide that is formed and enters the gas phase during the digestion process to maintain the oxygen partial pressure in the gas phase above at least 1 bar during the digestion; separating carbon dioxide from the gas phase by absorption, and recycling oxygen gas from which carbon dioxide has been separated to the reaction system.

2. The process of claim 1, in which carbon dioxide is separated continuously from the gas phase during the digestion so as to maintain a partial pressure of oxygen in the gas phase within the range from about 3 to about 20 bars.

3. The process of claim 1, in which carbon dioxide is separated from time to time from the gas phase during the digestion so as to maintain a partial pressure of oxygen in the gas phase within the range from about 3 to about 20 bars.

4. The process of claim 1, in which the carbon dioxide that is separated is recovered and used to form alkali metal carbonate, or bicarbonate, or both, and recycled.

5. The process of claim 1, in which the alkali metal carbonate or bicarbonate is selected from the group consisting of sodium carbonate and bicarbonate, and potassium carbonate and bicarbonate.

6. The process of claim 1, in which the carbonate or bicarbonate serves as the sole source of alkali.

7.. The process of claim 1, in which the carbonate or bicarbonate is combined with alkali metal hydroxide.

8. The process of claim 1, in which the carbon dioxide is removed from the oxygen-containing gaseous effluent from the digestion by absorbing the carbon dioxide with an alkaline absorption liquid, and the carbon dioxide-containing alkaline absorption liquid is used in the preparation of aqueous alkaline digestion liquor for another batch of wood.

9. The process of claim 8, in which the alkaline absorption liquid is an aqueous alkaline solution containing an alkali selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, and both.

10. The process of claim 9, in which the alkali is alkali metal carbonate recovered after burning a waste liquor obtained from a wood pulping or pulp bleaching process.

1.1.. The process of claim 9, in which the alkali is recovered from a waste liquor from a pulping process concentrated by evaporation to a solids content above about 50% by Weight.

12. The process of claim 9 in which the alkali metal carbonate used in the absorption liquid is recovered after burning waste liquor obtained from oxygen alkali digestion of wood by the digestion process of claim 1.

13. The process of claim 1, in which the partial pressure of oxygen during the digestion process is maintained within the range from about 3 to about atmospheres, by removal of carbon dioxide.

14. The process of claim 1, in which the digestion of the wood is in the presence of an inhibitor or mixture of inhibitors, which protect the cellulose and hemicellulose molecules against uncontrolled degradation.

15. The process of claim 14, in which the inhibitor is a magnesium compound.

16. The process of claim 15, in which the magnesium compound is a magnesium complex selected from the group consisting of an inorganic or organic acid complex of polyhydroxy acids, organic acids containing at least two carboxylic acid groups, and polyphosphoric acids.

17. The process of claim 16, in which the organic acid is an aliphatic aor ,B-hydroxy carboxylic acid.

18. The process of claim 16, in which the organic acid is an aminopolycarboxylic acid that is soluble in the digestion liquor.

19. The process of claim 1, in which the wood is pretreated with water or an aqueous acidic, neutral, or alkaline solution before the digestion.

20. The process of claim 1, in which a surface-active substance is present during at least a part of the digestion process.

21. The process of claim 1, in which the wood is a hardwood selected from birch, beech, poplar, aspen, maple, alder, and eucalyptus.

22. The process of claim 1, in which the wood is in the form of particles having an average thickness of at most 3 mm.

23. The process of claim 1, in which the amount of alkali added initially is within the range from about 5 to about of the total molar quantity of alkali required.

24. The process of claim 1, in which alkali is added continuously during the digestion.

25. The process of claim 1, in which the alkali is added in increments during the digestion.

26. The process of claim 1., in which the pH is within the range from about 7 to about 13 during the first stages and the major portion of the digestion process.

27. The process of claim 1, in which the total amount of alkali is within the range from about 1 to about 10 kilomoles per 1000 kg. of dry wood.

28. The process of claim 27, in which the cellulose pulp produced is a viscose, acetate or cuprammonium pulp, and the amount of alkali is within the range from about 6 to about 8 kilomoles per 1000 kg. of dry wood.

29. The process of claim 27, in which the cellulose pulp produced is a semichemical pulp, and the amount of alkali is within the range from about 1 to about 2 kilomoles per 1000 kg. of dry wood.

30. The process of claim 27, in which the cellulose pulp produced is a paper pulp, and the amount of alkali is within the range from about 2.5 to about 5 kilomoles per 1000 kg. of dry wood.

29. The process of claim 27, in which the cellulose pulp produced is a pulp for use in making fine paper, plastic fillers, and soft paper or tissue paper, and the amount alkali is within the range from about 1 to about 2 kilomoles per 1000 kg. of dry wood.

32. The process of claim 1, in which the alkali metal carbonate. is sodium bicarbonate, in an amount within the range from about 1 to about 5 kilomoles per 1000 kg. of dry wood.

33. The process of claim 1, in which the digestion temperature is within the range from about to about C.

References Cited UNITED STATES PATENTS 3,654,070 4/1972 Pradt et al 162-65 X 2,926,114 2/1960 Grangaard et al. 162-65 X 2,243,050 5/1941 Plant 162-65 X 3,560,329 2/1971 Nelson et al. 162-30 3,384,533 5/1968 Robert et al. 162-65 2,487,114 11/1949 Dreyfus 162-90 X 3,013,933 12/1961 Briggs 162-90 X 3,652,385 3/1972 Noreus et al 162-65 X 3,652,386 3/1972 Noreus et al 162-65 FOREIGN PATENTS 2,022,866 12/1970 Germany 162-65 OTHER REFERENCES Rydholm, Pulping Processes, pp. 1044-1045.

S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner US. Cl. X.R. 8-111 G16 142 Po-wso UNiTEl) SEATES PATENT oFFlcE Fm e CERTIFICAlE @F QGRRECNQN;

latent No. i, Dated October 9, 1973 Inventor(s) Hans Olof Samuelson It is certified that error appears: the above-identified patent and that said Letters Patent are hereby corrected as shown below olumn 2, line 60 "alklne should be alkaline Column 3, line 18 2 "ad" should be and I I i Column 9, line 25 desired should be desirable C0lumn 9, lines should be ER? 1. X I X L R4 Column 10. line 31 HGC E' 4 HO C H N CH SO should be HOC2H4 CH3 THoc H HO C Pl $11 I Signed and sealed this 18th day of February 1975.

(SEAL) Attest:

- c mRsHAiL .DANN RUTH C. MASON Comi jgoeeg gg ggggents Attesting Officer @223? Y UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No.- 3,764,464 Dated 0ctober$,197-3 'iians Olof Samuelson p 2 It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby correcteci as shown below:

Column 12, line 35 "wre" should be were-- Column 16; line 26 29" should be 31 line 28 insert of after "amount" .line 29 "1 to about 2" should be 2 to about 6 Signed and sesied this 18th day of February 1975.

(SEAL) Attest: 4 3

. I I C. MARSHALL DANN RUTH C. MASON- Commissioner of Patents Attesting Officer and Trademarks

Referenced by
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US4050981 *Jun 11, 1975Sep 27, 1977Mo Och Domsjo AktiebolagProcess for the delignification of lignocellulosic material by maintaining a concentration of carbon monoxide in the presence of oxygen and alkali
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Classifications
U.S. Classification162/65, 8/111
International ClassificationD21C11/00, D21C11/06
Cooperative ClassificationD21C11/06
European ClassificationD21C11/06