Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08B—POLYSACCHARIDES; DERIVATIVES THEREOF
  • C08B1/00—Preparatory treatment of cellulose for making derivatives thereof, e.g. pre-treatment, pre-soaking, activation
  • C08B1/08—Alkali cellulose
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1026—Other features in bleaching processes
  • D21C9/1036—Use of compounds accelerating or improving the efficiency of the processes

Definitions

• a process for treating cellulosic materials with alkali in the presence of oxygen, and particularly air, and in the presence of complex magnesium salts of aminopolycarboxylic acids or alkali metal salts thereof.
• the invention is of particular application to the reduction of lignin content in cellulose pulps and the digestion of wood cellulose without causing deleterious degradation of the cellulose, the complex magnesium salts reducing or entirely preventing attack of oxygen on hemicellulose and cellulose carbohydrates, without appreciably diminishing the oxidation of the lignin and its dissolution in the course of the process.
• a process for treating cellulosic materials with alkali in the presence of oxygen, and in the presence of a complex magnesium salt of an aminopolycarboxylic acid or alkali metal salt thereof, or a mixture of a magnesium salt and an aminopolycarboxylic acid or alkali metal salt thereof.
• a complex magnesium salt of an aminopolycarboxylic acid or alkali metal salt thereof or a mixture of a magnesium salt and an aminopolycarboxylic acid or alkali metal salt thereof.
• the dissolution of hemicellulose can be controlled so as to be insignificant or appreciable, as desired, so that the process is also applicable to hemicellulose dissolution.
• these compounds are inexpensive, and since they are water-soluble, they can be added in solution form, and form a homogeneous aqueous alkaline system in which the cellulose pulp is suspended, and containing solubilized magnesium ion.
• the process of the invention is particulary advantage ous in the alkaline treatment of lignin-containing wood cellulose in the presence of oxygen gas or air, for the purpose of removing lignin.
• This process is referred to in the art as akaline oxygen gas bleaching. It is also applicable to the controlled dissolution of hemicellulose in cellulose pulps, either during or after delignification, as well as to the alkaline digestion or pulping of wood in the presence of oxygen gas or air.
• treating and treatment are inclusive of bleaching, digestion, and like processes, unless otherwise indicated.
• the complex magnesium salts employed in the process of the invention have the important property of reducing or entirely preventing the attack of oxygen on the car bohydrates present in the cellulose and hemicellulose, without to any notably great extent affecting the oxidation of lignin and its dissolution.
• This protective effect is most noticeable with regard to the attack of oxygen on the cellulose molecule, and primarily the attack of oxygen along the anhydroglucose chain of the cellulose molecule, an attack which gives rise to a rapid lowering of pulp viscosity.
• the treated delignified pulp is found to have a higher viscosity than would be obtained in their absence.
• the process of the invention is applicable to unbleached, partially bleached or bleached cellulose pulps, prepared from any cellulose source by any pulping process, for example, sulfate pulp, sulfite pulp and semichemical pulp.
• the invention is especially applicable to cellulose pulps derived from wood, such as spruce pulp, pine pulp, hemlock pulp, birch pulp, fir pulp, maple pulp. alder pulp, aspen pulp, eucalyptus pulp, cherry pulp, sycamore pulp, hickory pulp, ash pulp, beech pulp, poplar pulp, oak pulp, and chestnut pulp.
• the invention is particularly advantageoiis in the preparation of any pulp in which it is especially desired to avoid degradation of the cellulose during processing, such as most grades of paper pulp, and when it is desired to obtain a uniform controlled degradation, such as in the manufacture of viscose pulp of a desired viscosity.
• the process of the invention can be applied to remove hemicellulose, and/or cause oxidation of end groups of the cellulose, with a regulated diminution of the pulp viscosity.
• the complex magnesium compounds have the property of protecting the cellulose and hemicellulose molecules against uncontrolled degradation.
• the oxygen digestion process of the invention is applicable to any kind of wood.
• 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 hard woods which can be pulped include birch, beech, poplar, cherry, sycamore, hickory, ash, oak, chestnut, aspen, maple, alder and eucalyptus.
• Exemplary softwoods include spruce, fir, pine, cedar, juniper and hemlock.
• 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 nonuniform fragments of the type of wood shavings or chips having an average thickness of at most 3 mm., and preferably from about 0.2 to about 2 mm. Other dimensions are not critical. Sawdust, wood flour, slivers, splinters, wood granules and wood chucks and other types of food fragments can also be used. In the case of softwood, the particles should be thin.
• the complex magnesium aminopolycarboxylic acid salts in accordance with the invention are formed from aminopolycarboxylic acids having the formula:
• 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.
• 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 number of acid groups available and converted to alkali metal salt form.
• aminopolycarboxylic acids examples include ethylenediaminetetraacetic acid, nitrilotriacetic acid, diethylenetriaminopentaacetic acid, ethylenediaminetriacetic acid, tetraethylenepentaamineheptaacetic acid, and hydroxyethylethylenediamine triacetic acid, and their alkali metal salts, including the mono, di, tri, tetra and penta sodium, potassium and lithium salts thereof.
• aminocarboxylic acids which can be used to advantage are iminodiacetic acid, 2 hydroxyethyliminodiacetic acid, cyclohexanediaminetetraacetic acid, anthranil N,N diacetic acid, and 2-picolylamine-N,N-diacetic acid.
• complexing acids can be present in rather large quantities, well in excess of the amount of magnesium present, and within the range from about two to about ten times the amount needed to complex with, solubilize, and thus prevent precipitation of magnesium hydroxide or other insoluble magnesium salts during the treatment.
• the magnesium complexes with these acids by forming salts with the acid groups and by chelation with the nitrogen-containing groups or hydroxy groups, if present. Since the complexes are soluble in the alkaline treating liquor, the precipitation of insoluble magnesium compounds is eifectively prevented.
• a particularly effective composition is a combination of the complex magnesium salts of the invention with other magnesium compounds, and especially other complex magnesium compounds.
• n is zero or one.
• the acid is an alpha-hydroxy acid
• n is one
• the acid is a beta-hydroxy acid
• R is the above formula is hydrogen oran 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.
• alphaand beta-hydroxy carboxylic acids are glycolic acid, lactic acid, glyceric acid, 0a,,B-dihYdI'OXY- butyric acid, a-hydroxy-butyric acid, a-hydroxy-isobutyric acid, a-hydroxy-n-valeric acid, a-hydroxy-isovaleric acid, B-hydroxy-butyric acid, fi-hydroxy-isobutyric acid, fl-hydroxy-n-valeric acid, fl-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, gulonic acid, idonic acid, altronic acid, allonic acid, ethenyl glycolic acid, and fi-hydroxy-isocrot
• 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, glutanconic 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.
• oxalic acid, malonic acid, tartaric acid, malic acid, and citric acid ethyl malonic acid
• succinic acid isosuccinic acid, glutaric acid, adipic acid, suberic acid, azel
• the polyphosphoric acids are also good complexing agents for magnesium, and the magnesium salts of these acids are useful in combinations with the complex magnesium aminopolycarboxylic acid salts.
• Exemplary are disodium-magnesium pyrophosphate, trisodium-magnesium tripolyphosphate and magnesium polymetaphosphate.
• acids naturally present in waste liquors obtained from the alkaline treatment of cellulosic materials.
• These acids represent the alkalior Watersoluble degradation products of polysaccharides which are dissolved in such liquors, as well as alkalior watersoluble degradation products of cellulose and hemicellulose.
• the chemical nature of these degradation products are complex, and they have not been fully identified.
• saccharinic and lactic acids are present in such liquors, and that other hydroxy acids are also present.
• 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 sulfite digestion processes, and liquors from sulfate digestion processes, i.e., kraft waste liquor.
• the waste liquors obtained in alkaline oxygen gas bleaching or digestion processesand alkaline peroxide bleaching processes can also be used. lln 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 cellulose pulp. 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, the aminopolycarboxylic acid, hydroxycarboxylic acid, or polyphosphoric acid, or salt thereof, and this mixture can be added to the pulp.
• a waste liquor employed as a source of complexing acid or anhydride or salt thereof can be mixed with a magnesium salt, oxide or hydroxide, before being introduced to the process, or the magnesium salt, oxide or hydroxide can be added to the pulp, and then the pulp brought into contact with the complexing acid or anhydride or salt thereof. It is also possible to combine the complexing acid or anhydride or salt thereof with the pulp, and then add the magnesium salt, oxide or hydroxide, but this method may be less advantageous in practice.
• magnesium is added, whether as salt, oxide, hydroxide, or complex salt, the amount of magnesium is calculated as MgO.
• a noticeable improvement in the treating process is obtained when as little magnesium as 0.005% MgO, calculated on the dry weight of the pulp, is added.
• a high proportion of magnesium, up to 1% MgO, calculated on the dry weight of the pulp, has been employed without disadvantageous eifect.
• magnesium-containing waste liquor Upon conclusion of the alkaline oxygen gas treatment, 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.
• 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 pulp.
• Some wood pulps are particularly high in magnesium ion because of the nature of the pulp or of the pulping process.
• unbleached pulps produced by digestion of wood with magnesium bisulfite or magnesium sulfite usually contain enough magnesium ion so that no addition of magnesium compound need be made.
• Waste liquors from these processes 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.
• 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 or -insoluble magnesium compound can be used, such as, for example, magnesium sulfate, magnesium chloride, magnesium bromide, magnesium chlorate, magnesium potassium chloride, magnesium formate, magnesium acetate, magnesium oxide, 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 foreign 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 are also advantageous. Since these are waterinsoluble, it is desirable, 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 pulp, or before commencing the alkaline oxygen gas reaction. Any other water-insoluble magnesium compounds can be used in this way, for instance, magnesium oxide or hydroxide, magnesium phosphate, magnesium silicate and magnesium sulfide.
• the alkaline treatment of the pulp in the presence of oxygen is carried out in the normal way. Since the processes differ somewhat in the steps and conditions, bleaching (with delignification) and digestion (with delignification) are considered separately, in that order.
• the partial pressure of oxygen at the beginning of the bleaching should be at least one atmosphere.
• lower pressures can be used, when a slower reaction is acceptable.
• the process can be carried out at pressures approximating atmospheric pressure, while if air is used, because of the lower proportion of oxygen, higher pressures, usually superatmospheric pressures, are employed.
• oxygen is used, a practical upper limit is 20 atmospheres, while if air is used, a practical upper limit is 60 atmospheres. The higher the pressure, the more rapid the reaction.
• an oxygen gas pressure within the range from about 2 to about 12 atmospheres is preferred.
• the bleaching is usually carried out at a temperature within the range from about to about 150 C. If it is desired to reduce the viscosity of the pulp, the higher temperatures can be used, of the order of 130 to 140 C.
• a lower temperature is used, if a significant reduction of the hemicellulose content is not desired. If a significant reduction of the hemicellulose is desired, however, then it is desirable to employ a rather high temperature.
• the treatment is carried out advantageously at from to C.
• the temperature can be varied upwardly or downwardly, progressively or continuously, during the process. It is in many cases desirable to begin the reaction at a low temperature, and then to gradually increase the temperature during the reaction. This is particularly true in the case of pulps containing hemicellulose which in an unoxidized condition is attacked by alkali, for example, sulfite pulps, and semichemical pulps.
• the reaction temperature is low while the hemicellulose remains unoxidized, but as it becomes oxidized, in the course of the reaction, the temperature can be increased, thus reducing the total reaction time.
• the concentration of cellulosic material in the reaction mixture can be varied within wide limits, and is in no way critical. Concentrations within the range from about 3 to about 45% are employed. It is, however, preferable to effect the treatment at a concentration in excess of 10%, and preferably within the range from about 15 to about 35%. When high pulp concentrations are treated, the pulp should be shredded mechanically after or at the same time as the reagent chemicals are added to the reaction mixture.
• the cellulosic material is first impregnated with an aqueous solution of the complex magnesium salt, or an aqueous solution of the components which in admixture give rise to the complex magnesium salt, before being treated with air or oxygen.
• the excess of the impregnating solution can then be removed, for example, by filtering and/or by pressing, before the treatment is begun.
• the solution that is removed can, of course, be used for impregnating additional cellulosic material.
• the amount of alkali required in the bleaching depends on the quantity of lignin and/or hemicellulose which it is desired to remove. Normally, the alkali charge (calculated as NaOH) is within the range from about 0.5 to about 12% NaOH, based on the weight of the cellulosic material present. Other alkalis can be used, such as potassium hydroxide or lithium hydroxide, and sodium carbonate, in which event the amounts are changed proportionately. If it is desired to dissolve large quantities of lignin and/or hemicellulose during the process, an alkaline charge within the range of about 7 to about 12% can be used. When bleaching a pulp having a low lignin content, in which case a smaller amount of lignin and/or hemicellulose is to be dissolved, the charge can be within the range from about 0.5 to about 7%.
• the proportion of hemicellulose dissolved decreases as the amount of alkali is reduced, and accordingly, the amount of both the lignin and the hemicellulose dissolved can be regulated by control of the amount of alkali added.
• the alkali attacks the lignin preferentially, and by limiting the amount of alkali present at any given time, it is possible to remove the lignin with a minimum of attack upon the cellulose and hemicellulose in the course of the reaction.
• the desired grade of pulp can thus be controlled by the manner and rate at which the alkali is charged to the system, and the size of the alkali charge, and the reaction time.
• the alkali can be combined with the pulp either before, during, or after combination with the complex magnesium salt, and it can be introduced in whole or in part in this way.
• the mixing with alkali can be effected at the desired reaction temperature, or at a lower temperature, after which the temperature is increased to reaction temperature.
• the reaction time required depends upon the oxygen gas pressure and the reaction temperature. If the oxygen gas pressure is high, and the reaction temperature is high, the reaction can be complete in rather a short time, for example, five minutes. When oxygen gas is employed at atmospheric pressure, reaction times of ten hours and more can be used. Normally, however, in a commercial process, where a high rate of production per hour is desirable, the reaction times will be Within the range from about 10 to about 120 minutes.
• the reaction time is easy to control, since the reaction halts when the alkali is consumed, and thus the reaction time can be increased or shortened, depending upon the amount of alkali added at any given time, for a given gas pressure and temperature of reaction.
• the bleached and delignified pulp can be further processed in accordance with known methods, as desired. It can, for example, be bleached with chlorine and/ or sodium chlorate and/or chlorine dioxide, and it may also be subjected to continued refinements, in accordance with known procedures.
• the digestion process of the invention under controlled conditions and moderate oxygen pressures digests wood with a mixture of alkali and oxygen and obtains in high yield a cellulose pulp having a high brightness and a low lignin content.
• What is essential in order to obtain these results is to limit the amount of alkali at the beginning of the digestion to at most 75%, and preferably from about to about 20%, of the total molar quantity of alkali required for the digestion, and to add the alkali progressively, either continuously or in increments, during the digestion, while maintaining the pH of the digestion liquor in the course of the digestion Within the range from about 9.2 to about 13, and preferably from about 9.5 to about 11.5.
• the total amount of alkali that is required for the digestion is determined by the quality and type of the pulp to be produced, and is within the range from about 1 to about 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.
• 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.
• the amount of alkali used in the process of the invention can be Within the range from about 2.5 to about 5 kilomoles.
• 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.
• alkali metal hydroxide or alkali metal carbonate can be employed, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, 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 of alkali metal carbonates may be more advantageous than the use of alkali metal hydroxides in maintaining the pH of the digestion liquor within the stated range, because of the buffering properties of the carbonate or bicarbonate prescut or formed in situ. Consequently, mixtures of alkali metal hydroxides and alkali metal carbonates are particularly satisfactory, to obtain the advantages of each, and dilute their disadvantages.
• alkali metal bicarbonates such as sodium bicarbonate and potassium bicarbonate.
• the alkali metal bicarbonate in this case serves as a bufler.
• Other buffering agents 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 amount of bulfering agent such as alkali metal bicarbonate is usually within the range from about 1 to about 5 kilomoles per 1000 kg. of dry wood.
• the alkali metal bicarbonate or other buffering agent should be added to the digestion liquor either initially or at an early stage of the digestion.
• the addition of the bicarbonate or other buffering agent increases the buffer capacity of the digestion liquor, thereby assisting in avoiding variations in pH outside the prescribed range during the digestion.
• the buffering agent, particularly a bicarbonate is especially desirable when it is desired to operate at a relatively low pH, for example, from about 9.2 to about 9.5. In this case, bicarbonate or other buffering agent can be added to advantage even if alkali metal carbonate is present.
• buffering agents and particularly bicarbonates
• carbon dioxide may be produced in the course of the digestion as the buifer 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.
• the addition of minor amounts of the buffering agent within the stated range contributes to pulp uniformity because of its assistance 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 as those described in US. Pats. Nos. 3,652,385 and 3,652,386 referred to above. 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 sodium compounds are preferred as the alkali metal hydroxide, alkali metal carbonate and alkali metal bicarbonate.
• Limiting the amount of alkali metal hydroxide and/ or alkali metal carbonate in the initial stages of the process is quite important in obtaining a cellulose pulp of the desired quality. At most, 75% 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 most pulps, 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% of the total molar quantity required for the digestion. The remainder of the alkali is added progressively, either incrementally or continuously, as the digestion continues. When producing bright pulps having a low lignin content, it is satisfactory to charge not more than 20% and suitably from about 5 to about 20% of the alkali at the beginning of the digestion process.
• the initial charge comprises sodium carbonate optionally with an addition of sodium bicarbonate as described above, the remainder of the alkali added as the digestion proceeds being sodium hydroxide.
• 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 of the total molar quantity of alkali.
• the alkali metal hydroxide and/or alkali metal carbonate can be charged continuously or in increments to the digestion liquor.
• the wood is caused to move through the digester from one end to the other which thereby constitutes a reaction zone.
• the wood usually in the form of chips is retained in the reaction vessel throughout the digestion.
• the so-called gas phase digestion procedure can be used to advantage.
• 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 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 wood 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 to 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. Generally, however, it is to advantage if the digestion temperature is allowed to rise during the digestion process, so that normally the temperature during impregnation would not exceed about 60 C.
• 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 or C.
• a digestion temperature of from 90 to 110 C. can be used to advantage when producing semi-chemical pulps, the fibers of which are not fully liberated until after subjection to a mechanical treatment process, such as in a rlefiner, after the digestion process. These are high yield pu ps.
• 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.
• the liquor can be fortified by adding additional alkali metal hydroxide and/or alkali metal carbonate and/or inhibitor before or after pressure-heating.
• the pretreatment can be carried out with such solutions at lower temperatures, from about 30 to about 100 C., preferably using an acid solution such as a 0.1 to 1% aqueous solution of sulfuric acid, nitric acid, or phosphoric acid. These acids can also be used in high temperature pretreatment.
• the oxygen digestion process of the present invention Before carrying out the oxygen digestion process of the present invention, it is particularly suitable to pretreat the wood with an aqueous solution containing-sulfur dioxide, sodium bisulfite and/or sodium sulfite or other alkali metal sulfite such as potassium bisulfite or sulfite.
• the treatment causes some dissolution and modification of the wood material, which has been found to be favorable during the oxygen digestion, particularly in the case of wood material which is diflicult to pulp without any form of pretreatment, such as softwood.
• the pulp By pretreating the wood in this manner with water or aqueous solutions, the pulp can be modified to any desired degree, and by suitably selecting the conditions according to trial-anderror experimentation (which can be carried out on a small sample) to suit the wood used, the treating conditions can be optimized for different fields of use of the pulp product. Generally speaking, the pretreatment causes a reduction in the consumption of alkali during the oxygen digestion process of the invention.
• the pretreatment stage or part thereof is preferably carried out in the presence of a complexing agent for bivalent and/or polyvalent metal ions, such as copper, iron, manganese, cobalt and vanadium.
• a complexing agent for bivalent and/or polyvalent metal ions such as copper, iron, manganese, cobalt and vanadium.
• suitable complexing agents are chelating salts of nitrogen-containing polycarboxylic acids of the class set forth above in conjunction with the magnesium complex as well as polyphosphates and ethylenediamine and ethylenediamine derivatives, although other complexing agents of an inorganic or organic nature can also be used to advantage.
• the effect can be increased if mixtures of different complexing agents are used, since certain complexing agents have more of an affinity for certain polyvalent metal ions than others, and a blend is better capable of chelating a mixture of polyvalent metal ions for this reason.
• the use of complexing agents in connection with the pretreatment has been found to promote uniformity of the pulp during digestion.
• This washing step may be desirable in the case of any of the pretreatment processes described above.
• the washing increases the cost of the processing, and also increases the risk of water contamination of the pulp with metal ions and metal compounds, and consequently it may often be more practical to omit the washing step, unless it can be carried out with deionized water, at 'low cost. Omission of the washing is usually disadvantageous.
• complexing agents for bivalent and/or polyvalent metal ions present during the oxygen digestion process. Any complexing agents which are stable and not deleteriously affected by the digestion liquor can be used. Suitable complexing agents include those mentioned above.
• 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, or disintegrator, or shredder, may be appropriate. After an extensive or more complete pulping or digestion, the wood can be defibrated 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 pulped wood cellulose 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.
• 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 sequences as described in for example US. Pat. No. 3,652,388.
• 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 causticizing all or part of the carbonate obtained when burning the liquor.
• the pulp was finely divided in a peg shredder at a dry content of 30%. Water and sodium hydroxide were added in amounts to give a 3% pulp concentration. The pulp suspension was then vigorously stirred with a propeller agitator, and the additives mentioned below were blended in. The pulp was then separated by filtration, and squeezed to a dry solids content of 27%, after which the pulp was shredded in a peg shredder.
• the oxygen gas bleaching was carried out under an oxygen gas pressure of 8 kg./ :m.
• EXAMPLE 2 Birch chips about 1 mm. thick and having a lignin content of 21.1% were digested in a series of four runs. In the last run, a magnesium complex prepared from MgSO and ethylenediaminetetraacetic acid (EDTA) was added.
• EDTA ethylenediaminetetraacetic acid
• the lignin content of the pulp after the digestion was 1.5%, the pulp yield was 51.5%, and pulp brightness 75% SCAN.
• the plup obtained in (b) had a particularly high brightness.
• the improvement which comprises treating the cellulose material with alkali in the presence of oxygen and in the presence of a complex magnesium salt of an aminopolycarboxylic acid or alkali metal salt thereof, the complex magnesium salt reducing or entirely preventing attack of oxygen on the hemicellulose and cellulose in the course of the process.
• aminopolycarboxylic acid has the formula:
• A is selected from the group consisting of -CH COOM and --CH CH OH, M is hydrogen or an alkali metal, and n is a number within the range from zero to five.
• a process according to claim 2, wherein the complex magnesium salt is a chelate of magnesium and a nitrilotriacetic acid.
• cellulosic material is an unbleached, partially bleached or bleached cellulose sulfate pulp, sufite pulp or semichemical pulp derived from wood.
• a process according to claim 1 wherein the cellulosic material is cellulose pulp and the partial pressure of the oxygen at the beginning of the treatment is at least about 1 atm.
• a process according to claim 1 wherein the cellulosic material is cellulose pulp and the quantity of alkali calculated as NaOH is within the range from about 0.5 to about 10% based on the dry weight of the cellulosic material.
• a process according to claim 1, wherein the source of magnesium is MgSO MgO, MgCl Mg(OH) MgCO or Mg(NO 19.
• the complex magnesium salt includes in addition a complex magnesium salt of an aliphatic aor fi-hydroxycarboxylic acid.
• cellulose pulp produced is a pulp for use in making fine paper, plastic fillers, and soft paper or tissue paper, and the amount of alkali is within the range from about 2 to about 6 kilomoles per 1000 kg. of dry wood.

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• Chemical & Material Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Biochemistry (AREA)
• Materials Engineering (AREA)
• Health & Medical Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Medicinal Chemistry (AREA)
• Polymers & Plastics (AREA)
• Organic Chemistry (AREA)
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• 1970
  • 1970-03-02 SE SE02732/70A patent/SE353749B/xx unknown
• 1971
  • 1971-02-26 ZA ZA711271A patent/ZA711271B/xx unknown
  • 1971-02-26 US US119369A patent/US3701712A/en not_active Expired - Lifetime
  • 1971-02-26 FI FI583/71A patent/FI55531C/fi active
  • 1971-03-01 DE DE2109542A patent/DE2109542C3/de not_active Expired
  • 1971-03-01 FR FR7106969A patent/FR2084031A5/fr not_active Expired
  • 1971-03-01 NO NO758/71A patent/NO133594C/no unknown
  • 1971-03-02 JP JP46010410A patent/JPS5022601B1/ja active Pending
  • 1971-03-02 AT AT178371A patent/AT311169B/de not_active IP Right Cessation
  • 1971-03-02 BR BR1287/71*[A patent/BR7101287D0/pt unknown

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