Process For Treating Cellulosic Materials From Which Metal Ions Have Been Removed With Alkali And Oxygen In The Presence Of Complex Magnesium Salts

Abstract

Cellulosic materials from which metal ions have been removed by acids or complexing agents is delignified with alkali in the presence of oxygen, and particularly air, and in the presence of magnesium compounds such as, for instance, chelates of magnesium and aliphatic alpha- and beta-hydroxycarboxylic acids. The invention is of particular application to the reduction of lignin content in cellulose pulps without causing deleterious degradation of the cellulose, the magnesium compounds reducing or entirely preventing attack of oxygen on the hemicellulose and cellulose carbohydrates, without appreciably diminishing the oxidation of the lignin and its dissolution in the course of the process. The process is also useful to obtain a controlled dissolution of the hemicellulose.

Parent Case Text

This application is a continuation-in-part of Ser. No. 869,875 filed Oct. 27, 1969.

Description

It is known that chemical and semichemical cellulose pulps can be treated with oxygen in an alkaline medium in order to dissolve lignin. The oxygen treatment is carried out at an elevated temperature, of the order of 100.degree. C., and does not normally require more than 1 hour. The amount of alkali required is of the order of 4 to 5% NaOH based on the dry pulp. It is possible to obtain a good reduction in lignin content in this way, but unfortunately, at the same time hemicellulose is also dissolved, and a significant decomposition of the cellulose takes place, as evidenced by a lower viscosity value. Also, the strength properties of paper manufactured from such treated pulp are poor.

It has been proposed (U.S. Pat. No. 3,384,533 dated May 21, 1968, to Robert et al.) that the process be improved by carrying out the treatment in the presence of a metal carbonate, such as barium carbonate, calcium carbonate, magnesium carbonate or zinc carbonate, in an amount within the range from about 0.5 to 3 percent by weight of the pulp. Of these chemicals, magnesium carbonate gives the best results, when in an amount of approximately 1 percent by weight of the pulp. However, magnesium carbonate is quite expensive, and the treatment is costly. Calcium carbonate, which is cheaper, is much less effective. In the case of all of these salts, the difficulty is that a powdered water-insoluble material must be charged to and mixed with the aqueous cellulose pulp system, and it is accordingly hard to obtain and maintain a homogeneous mixture, with uniform effect.

A further serious disadvantage is that a particularly complicated apparatus is required to carry out the heterogeneous reaction if the process is to be effected in a reasonable length of time.

In accordance with the present invention, a process is provided for treating cellulosic materials with alkali in the presence of oxygen, and in the presence of a magnesium compound, such as a magnesium salt of an organic acid having from two to about 12 carbon atoms and either one carboxylic acid group and an alpha or beta hydroxy group such as an aliphatic alpha- or beta-hydroxycarboxylic acid, or a mixture of a magnesium salt and an aliphatic alpha- or beta-hydroxycarboxylic acid or salt thereof, or two or more carboxylic acid groups and no or from one to 10 hydroxyl groups such as a dicarboxylic acid or a mixture of a magnesium salt and such acid or salt thereof. In this process, the cellulose is subjected to a pretreatment in which catalytically active metals and/or metal compounds are removed or deprived of their catalytic activity, and the concentration of cellulose during treatment with oxygen is from about 30 to about 60 percent, preferably from 32 to 40 percent. Moreover, it is preferred that alkali be added incrementally or continuously, so that, in addition to the amount of alkali present at the beginning of the oxygen treatment, more alkali is added in at least one additional stage during the oxygen treatment.

It has been found that in this process it is possible in one stage to reduce the lignin content by more than 50 percent without causing deleterious degradation of the cellulose, or appreciable loss of hemicellulose. In fact, 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. It has surprisingly been found that the protective effect obtained when adding magnesium compounds to the system can be increased to such an extent that the process can be effected in practice at high-pulp concentrations if, prior to being treated with oxygen, the cellulose is subjected to a pretreatment step in which metals and/or metal compounds which catalyze the attack on the cellulose and hemicellulose are removed or deprived of their catalytic activity, and if, furthermore, alkali is not added in one single charge, but a percentage of the total quantity of alkali charged is initially introduced to the system, and more alkali is added when the alkali previously charged to the system has been either completely or partially consumed, so that a large excess of alkali is not present during any stage of the process.

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, cherry pulp, sycamore pulp, hickory pulp, ash pulp, beech pulp, poplar pulp, oak pulp, and chestnut pulp, or of other ligno-cellulosic material such as bamboo, bagasse straw and reeds. The invention is particularly advantageous 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.

In most cases where the starting cellulose pulp is free of lignin, or where the lignin content is low, either naturally so, or because it has been delignified, 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. In these processes, the magnesium compounds have the property of protecting the cellulose and hemicellulose molecules against uncontrolled degradation.

The process of the invention is particularly advantageous 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 alkaline oxygen gas bleaching. It is also applicable to the controlled dissolution of hemicellulose in cellulose pulps, either during or after delignification.

Tests have shown that it is also advantageous if the cellulose is swelled in the alkali, and that, when an excess quantity of alkali has been used, the surplus is removed in a manner known per se.

The following advantages are gained when applying the process of the invention:

1. Uniform bleaching can be obtained without it being necessary to agitate the cellulose, except possibly when mixing in the alkali, or to subject it to a mechanical treatment other than that necessary in loosening the fibers thereof;

2. The cellulose can be bleached in a continuous process;

3. The cellulose is easily, transported, e.g., by means of screw, scraper or gas-carrier conveyor systems, and sluice valves and other valve arrangements;

4. Liquids or liquid droplets, which cause uneven bleaching an accumulation of cellulose in the bleaching apparatus, are avoided;

5. The costs of the apparatus are much lower than when the process is applied to a wood pulp suspension;

6. Reduced heat consumption;

7. The process is effected at lower oxygen-gas pressure at a given same reaction time;

8. The bleaching chemicals used can be recovered in a high concentration, rendering the process more economical.

One surprising advantage of the process of the invention is that it gives a cellulose pulp having a very low content of extractive substances, "resins," such as fats, sterols, hydrocarbons, waxes and fatty alcohols, in the treated material, even in the case of cellulose material which is very difficult to deresin by other methods. An example of such materials is wood cellulose produced from fresh wood, particularly hardwood, such as birch.

In order to obtain the aforementioned advantages, particularly those under items 1 to 5, it has been found that the concentration of cellulose during the oxygen treatment should not be less than 30 percent, based on the treating solution. At lower concentrations, the cellulose may adhere to the apparatus, and is liable to dry out, creating, among other things, danger of explosion. Tests have shown that the protective effect afforded by additions of magnesium compounds at low cellulose concentrations is completely unsatisfactory. Among the disadvantages observed have been reduction in viscosity and strength of paper produced from the pulp.

The concentration of cellulose is here calculated in weight percent of the material impregnated with the alkaline liquid. The lower concentration limit is important, although to some extent dependent on the nature of the cellulosic material and the apparatus used.

One factor important to the result and to the continuity of the process is that the alkaline liquid must be completely absorbed by the cellulose and retained by the fiber, so that, consequently, no free coherent liquid phase exists. The concentration must be so high that no liquid droplets are forced out of the material whilst it is in, for example, the bleaching tower or other positions in the apparatus where the pulp is not worked mechanically but can be compressed by its own weight.

Test have shown fully unsatisfactory results, both with regard to bleaching and to the transport of the cellulose through the apparatus, when the cellulose concentration falls below 30 percent. The best results have been obtained with a concentration within the range from about 32 to about 40 percent, for example an initial concentration of 38 percent, and a final concentration of 33 percent. The pulp, however, can be worked to advantage within the concentration range of 32 to 50 percent or, if a significant reduction in viscosity is desired, at a concentration as high as 60 percent, i.e., within the range from about 30 to about 60 percent. With regard to the operation and simplicity of the apparatus, it is particularly suitable that the concentration of cellulose during the final stage reaches at least 30 percent.

In accordance with one embodiment, which has been found to afford great practical advantages, the oxygen treatment is effected in two or more stages, wherein the alkali-containing liquid is added to the system between and/or during the treatment stages. In many instances, it is desirable to add the alkaline liquid to the pretreated cellulosic material before the oxygen treatment is begun. The alkaline liquid can suitably be added in excess quantities, thereby obtaining uniform liquid impregnation. The excess alkaline liquid is later removed in a known manner, for example, by pressing, to obtain a concentration within the given limits; after which the pulp is disintegrated, e.g., in a shredder, in a manner to provide a porous pulp having a greater surface area. Accessibility to the cellulose during the oxygen gas treatment is in this way increased. The oxygen gas treatment can be effected to advantage during the disintegration process, although the treatment process may be begun instead after the pulp has been disintegrated.

In accordance with another embodiment, the pretreated cellulosic material, which may be dry or wet, is loosened by mechanical treatment, whereafter alkaline liquid is added. In order to obtain uniform impregnation, it is important that the material be in motion when the alkaline liquid is added, for example is being mechanically treated, or is falling freely, or is moving in a flow of gas and/or steam. In accordance with a further embodiment, which is to be preferred with respect to simplicity of apparatus, the alkaline liquid is added to the pretreated cellulosic material while the material is being disintegrated by mechanical treatment. In this embodiment, the alkaline liquid is added during the oxygen treatment, or simultaneously therewith. With respect to apparatus costs, it is desirable that contemporaneous treatment with oxygen gas be effected during the mechanical treatment process, which in this instance, at the same time, constitutes a first step in the oxygen treatment.

When carrying out the method of the invention, it is important that the oxygen treatment be effected while the cellulosic material is being disintegrated by mechanical treatment or afterwards.

In all instances where alkaline liquid is added to the cellulosic material while the material is in high pulp concentration, it is desirable that the alkaline liquid be added to the system in a finely divided atomized form. The alkaline liquid can be charged to the system by known devices, for instance stationary or rotary jets or nozzles. The liquid can be charged, suitably together with steam and/or with the oxygen necessary for the treatment. The manner in which the liquid is charged is less important when the alkaline liquid is added whilst the cellulosic material is being subjected to mechanical treatment. If the liquid is charged while the material is falling freely, or moving in a stream of gas and/or steam, for example, in a so-called cyclone bed or a blower conduit, atomization of the liquid is desirable, and the liquid is suitably charged to the system in the form of an aerosol.

In one embodiment which has shown very good results, alkaline liquid is supplied to the pretreated cellulosic material, and any surplus of alkaline liquid removed, for instance by pressing, after which the cellulosic material is loosened by mechanical treatment during contemporaneous treatment with oxygen, and atomized alkaline liquid is supplied to the loosened material without or preferably during contemporaneous treatment with oxygen. The alkaline liquid is added to the loosened material suitably in the form of an aerosol, for example while the loosened material is introduced into the upper end of a tower through which it continuously passes, and is subjected to treatment with oxygen.

The quantity of alkali necessary to the process depends on the quantity of lignin and hemicellulose which it is desired to remove. Normally, the total alkali addition, calculated as NaOH is within the range from about 0.5 to about 10 percent, based on the dry weight of the cellulosic material. Charges within the range of 7 to 12 percent alkali are used when it is desired to liberate large quantities of lignin and/or hemicellulose. In pulps of low lignin content, the normal total charge lies within the range of 1 to 7 percent. The alkali normally used is sodium hydroxide, optionally in admixture with sodium bicarbonate. A slurry of calcium hydroxide, so-called lime milk, may also be used as an alkaline medium in the process. This latter medium, however, may cause deposits in the apparatus.

Observations have shown that for a predetermined degree of lignin dissolution, the least amount of cellulose decomposition is obtained when the quantity of alkali present at the beginning of the oxygen treatment is the smallest possible in practice, and when the major portion of the alkali is added incrementally or continuously as the alkali initially charged to the system is totally or partially consumed. When producing strong paper pulps, it is suitable that from about 10 to about 50 percent of the total quantity of alkali charged to the system is present at the beginning of the treatment, and that the remainder of the alkali is either added stepwise, in one or more stages, or continuously, during the oxygen treatment, or whilst the cellulose passes through apparatus in which no oxygen treatment takes place.

The oxygen may be in the form of oxygen and/or air, or mixtures of oxygen with any inert gas, such as nitrogen, argon, helium and krypton. In order to promote a rapid reaction between the cellulose and the charged gas, the partial pressure of oxygen at the beginning of the process should be at least one atmosphere. A practical upper limit for the partial pressure of oxygen is 20 atmospheres above atmospheric; the higher the pressure, the more rapid the chemical reactions. The high-pulp concentration and the looseness of the material facilitates dissolution of the oxygen in the cellulose fibers impregnated with alkaline liquid, whereby lower pressures than these used when treating low pulp concentrations can be applied. Normally, an oxygen gas pressure within the range of from about two to about 10 atoms is preferred. It is often convenient to charge a further quantity of oxygen gas or air during the process, and to release gas mixtures enriched with respect to inert gas during the process.

The reactions take place slowly at low temperatures, e.g., 50.degree. C.. In order to shorten the reaction time, the treatment is normally effected at a temperature within the range from about 80.degree. to about 130.degree. C. However, in those cases where a significant reduction in viscosity of the pulp is required, a somewhat higher temperature can be used, e.g., about 140.degree. C. In the case of sulphate paper pulps, the treatment process is carried out to advantage at temperatures of between 90.degree. and 110.degree. C. A lower temperature is chosen in the case of sulphate paper pulps, provided that a significant reduction in the hemicellulose content is not desired. The temperature may be changed incrementally or continuously during the process. Consequently, it is convenient to start at a low temperature. This is particularly true for pulps containing hemicellulose, which, in nonoxidized state, is attacked by alkali, e.g., sulphite pulps and semichemical pulps.

Very short reaction times, e.g., in the region of 5 minutes, can be used when working with high-oxygen gas pressures and high temperatures. It has been found relatively simple to control the process under these conditions, since the reaction practically stops when the alkali is consumed. To avoid working at high pressures, it is possible instead to use, for example, oxygen gas at atmospheric pressure, wherewith treatment times of 10 hours and more can be used. Normal reaction times range from 10 to 120 minutes.

The magnesium compounds employed in the process of the invention have the important property of reducing or entirely preventing the attack of oxygen on the carbohydrates present in the cellulose and hemicellulose, without to any notably great extend affecting the oxidation of lignin extent 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. Thus, in the presence of the magnesium compounds of the invention, the treated delignified pulp is found to have a higher viscosity than would be obtained in their absence.

The soluble preferred complex or chelate salts contsitute a preerred class of magnesium compounds, because they give most effective protection with a minimum magnesium concentration.

It is known that aliphatic alpha-hydroxycarboxylic acids of the type RCHOHCOOH and the corresponding beta-hydroxycarboxylic acids RCHOHCH.sub.2 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 10 carbon atoms, or a hydroxy-substituted hydrocarbon radical having from one to nine hydroxyl groups, and from one to about 10 carbon atoms.

Exemplary alpha- and beta-hydroxy carboxylic acids are glycolic acid, lactic acid, glyceric acid, .alpha.,.beta.-dihydroxybutyric acid, .alpha.-hydroxy-butyric acid, .alpha.-hydroxy-isobutyric acid, .alpha.-hydroxy-n-valeric acid, .alpha. -hydroxy-isovaleric acid, .beta.-hydroxy-butyric acid, .beta.-hydroxy-isobutyric acid, .beta.-hydroxy-n-valeric acid, .beta.-hydroxy-isovaleric acid, erythronic acid, threonic acid, trihydroxy-isobutyric acid, and saccharinic 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 .beta.-hydroxy-isocrotonic acid.

Also useful are organic acids having two or more carboxylic groups, and no or from one to 10 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, isosaccharic acid, talomucic acid, tricarballylic acid, aconitic acid, and dihydroxy tartaric acid.

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, trisodium-magnesium 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 alkali- or water-soluble degradation products of polysaccharides which are dissolved in such liquors, as well as alkali- or water-soluble degradation products of cellulose and hemicellulose. The chemical nature of these degradation 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.sub.6 -isosaccharinic and C.sub.6 -metasaccharinic acids has been demonstrated, as well as C.sub.4 - and C.sub.5 -metasaccharinic acids. Glycolic acids 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 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 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, and this mixture can be added to the pulp. Preferably, the waste liquor employed as the 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. It is also possible to add the magnesium salt, oxide or hydroxide to the pulp, and then bring the pulp 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.

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 0.005% 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 effect. However, for economic reasons, it is usually desirable to use as little magnesium as possible, and normally an amount within the range from about 0.01 to about 0.5% MgO, calculated on the dry weight of the pulp, is employed.

It has been found that less effective but in many cases adequate protection against decomposition of the cellulose also can be obtained if the alkaline liquid contains a solid magnesium compound, e.g., magnesium carbonate, magnesium oxide and magnesium hydroxide. In this latter instance, however, large amounts of magnesium compounds are required, and the process becomes less satisfactory economically. The addition and distribution of solid compounds also involves technical complications. The amount of magnesium compounds should in this instance be within the range from about 0.5 to 2.0% calculated as MgO on the dry weight of the pulp.

A negligible shielding effect is obtained when practicing the process of the invention using magnesium silicate in solid form. On the other hand, it has been found that an alkaline silicate, for example, sodium silicate, or other compounds which stop free radical reactions, for example, aromatic organic substances such as benzene, can be added to advantage together with magnesium compounds, preferably complex magnesium compounds, and that in this way the protective effect against cellulose decomposition is further increased. The quantity of sodium silicate or benzene introduced to the system is within the range from about 0.05 to about 3.0 percent, estimated on the dry weight of the cellulosic material, preferably from 0.1 to 1.0 percent.

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. 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 pulp.

Some waste liquors are particularly high in magnesium ion because of the nature of the pulp or of the pulping process. For example, waste pulping liquors from unbleached pulps produced by 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 sufficient 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 chloride, magnesium formate, 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, such as magnesium oxide, magnesium hydroxide, and magnesium carbonate, are also advantageous. Since these are water-insoluble, 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 phosphate, magnesium silicate and magnesium sulfide.

The pretreatment of the cellulose can be effected to advantage with an acid aqueous solution containing one or more acids, e.g., mineral acids such as hydrochloric acid, nitric acid, phosphoric acid, or sulphurous acid. The use of sulphurous acid is particularly advantageous in the case of cellulose containing iron compounds, while sulphuric acid is preferred with cellulose containing copper and copper compounds. Progressive treatment with several acid aqueous solutions provides an increased effect if for example, both iron and copper compounds are present in the material. The solution may have a pH of 1 or thereover, for example, from 1.5 to 4, preferably 2 to 3.5. Mixtures of different mineral acids may also be used, for example, mixtures containing hydrochloric acid, sulphuric acid, and organic acids such as formic, acetic, and trichloroacetic acids.

In accordance with a particularly suitable embodiment, the pretreatment is effected with acid bleaching liquids used in the bleaching process, or washing water obtained from acid bleaching stages.

When practicing the pretreatment methods hitherto described, it is suitable to wash the cellulosic material with water before the alkaline liquid is charged to the system, wherewith a more effective removal of the deleterious heavy metal compounds which catalyze cellulose decomposition is obtained, than if the alkali were charged directly subsequent to the acid treatment process.

In accordance with another embodiment, which can be applied in the absence of or in combination with a pretreatment process in an acid environment, the cellulosic material is subjected to a pretreatment process in the presence of a complex builder for catalytically active heavy metal compounds, such as those which contain copper, cobalt, iron and manganese. Examples of such complexing agents are ethylene diaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylene-triaminepentaacetic acid (DTPA), hydroxyethylimindiacetic acid, hydroxyethylethylene-diaminetriacetic acid (HEDTA) and polyphosphoric acid and salts of said acids. Amines, for example, ethylene-diamine, have also been found to give good results, particularly if cobalt is present. The complex builders can be used at pH values of, for example, from 4 to 10, preferably 5 to 8. When treating certain cellulosic materials, it has been found suitable to add complexing agents having a low pH, e.g., from 2 to 5, preferably 3 to 4, and then to raise the pH, e.g., by adding alkali. It is also suitable when practicing this type of pretreatment to wash the material prior to charging to the system the alkali necessary for the oxygen treatment.

The pretreatment step can be carried out at any pulp concentration whatsoever, for example, concentrations within the range from about 1to about 20 percent. For practical reasons, however, a concentration of 5 to 10 percent is preferred. The temperature may be room temperature, although an elevated temperature within the range from 40.degree. to 70.degree. C., for example, gives a more rapid treatment. In the presence of certain metal compounds, the treatment time may be very short, for example, 2 minutes. A longer reaction time from about 10 to about 60 minutes, is often preferred, however, in order that an acceptable result can be obtained. If high temperatures and/or long reaction time are used, a pH of less than 1.5 should be avoided when pretreating with acid solutions.

In order to further improve the process, surface tension reducing substances, for example, nonionic or anionic wetting agents, or mixtures thereof can be used during the oxygen treatment.

The process can also be applied to materials which have previously been treated in a known manner with, for example, chlorine, hydrochlorite or chlorine dioxide or a combination of such bleaching agents. The process can be applied with particular advantage to a weakly chlorinated cellulose pulp.

The pulp treated in accordance with the process of the invention can be further processed in accordance with known methods, as desired. It can, for example, be bleached with chlorine and/or sodium hydrochlorite and/or chlorine dioxide, and it may also be subjected to continued refinements, in accordance with known procedures. In the case of sulphate pulp it has been shown to be particularly advantageous to treat the pulp with a mixture of chlorine and chlorine dioxide, followed by alkali extraction and final bleaching with, for example, chlorine dioxide.

The following example in the opinion of the inventors represent preferred embodiments of their invention.

EXAMPLE 1

Unbleached pine sulphate pulp was used in these experiments. The pulp had the following analysis:

Kappa number 31.4 Viscosity 1192 cm..sup.3 /g. ac- Viscosity cording to SCAN 177 cp. accord- ing to TAPPI

the pulp was pretreated with an acid residual liquid obtained from a bleaching stage in which a mixture of chlorine and chlorine dioxide had been used to bleach an acid alkali-treated pulp having a kappa number of 17. The conditions in this bleaching stage were as follows:

Pulp concentration % 6 Time, hours 3 Temperature .degree.C. 50 Charge % active chlorine (estimated on pulp) 4.3 Distribution Cl.sub.2 /ClO.sub.2 (estimated as active chlorine) 85/15 Residual chlorine % 0.1 Final pH 2.0

the unbleached pulp was treated with the residual acidic liquid, under the following conditions:

Temperature .degree.C. 20 Time, min. 30 Pulp concentration % 6

Subsequent to this treatment step, the residual liquid was removed by pressing, after which the pulp was washed with water. The pulp and an aqueous solution of NaOH and magnesium complex were mixed carefully to a 3 percent pulp concentration. The pulp was then dewatered to a concentration of 34 percent. The content of NaOH in the pulp was then 3.5 percent, calculated on the dry weight of the pulp, and the content of magnesium complex 0.2 percent, calculated as MgO. The magnesium complex was produced by dissolving magnesium sulphate in a liquid containing complexing agents, in this instance, liquor separated from cellulose pulp which had been treated with oxygen gas and alkali was used as a complexing agent. The pulp, which contained NaOH and magnesium complex, was then loosened in peg shredder, after which it was introduced into a pressure vessel which was heated by direct steam contact to 100.degree. C. The pressure vessel was then connected to an oxygen gas tube, and the pressure adjusted to 8 kg./cm..sup.2. After 20 minutes reaction time, the pulp was washed with water. The pulp had the following analysis:

Kappa number 16.4 Viscosity 852 cm..sup.3 /g. ac- cording to SCAN viscosity 50 cp. accord- ing to TAPPI

The same starting pulp, treated with acid residual liquid, was mixed with NaOH and magnesium complex in a similar manner, but with the alkali charge divided so that only 1.5 percent, estimated on dry weight of pulp, was charged at a 3 percent pulp concentration, after which, after 5 minutes reaction time, at 34 percent pulp concentration and 8 kg./cm..sup.2 oxygen pressure, a further addition of 2.0% NaOH in atomized liquid form was made, without magnesium complex. The reaction was continued for 15 minutes more, after which the pulp was washed with water. The pulp had the following analysis:

Kappa number 16.2 Viscosity 1003 cm..sup.3 /g. ac- cording to SCAN Viscosity 81 cp. accord- ing to TAPPI

it is evident that the viscosity becomes very low, if all the alkali is charged simultaneously with the oxygen-alkali treatment at high-pulp concentration. By dividing the alkali charge, it is possible to reduce the initial concentration of alkali, and considerably improve the viscosity of the pulp.

EXAMPLE 2

Unbleached pine sulphate pulp was used in this experiment. The pulp had the following analysis:

Kappa number 34.7 Viscosity 1181 cm..sup.3 /g. ac- cording to SCAN Viscosity 170 cp. accord- ing to TAPPI

the pulp was treated with aqueous ethylene diamine tetraacetic acid solution (EDTA), with agitation, and under the following conditions:

Pulp concentration, % 5 Time, min. 30 Temperature .degree.C. 60 EDTA, % of pulp 0.2

After being treated, the pulp was washed with water, after which a magnesium complex containing alkali was charged successively to the system, and the pulp treated with oxygen gas, as described in Example 1. The pulp obtained had the following analysis:

Kappa number 16.0 Viscosity 993 cm..sup.3 /g. ac- cording to SCAN Viscosity 79 cp. accord- ing to TAPPI

a treatment process according to the invention using complexing agent (EDTA) during the pretreatment thus has an effect which, from the point of view of stabilizing the viscosity, is comparable with a treatment using acidic water. Furthermore, a lower Kappa number on the oxygen-alkali treated pulp was obtained, despite the fact that the starting pulp had a higher Kappa number than the pulp used in Example 1. This can be explained by the fact that an acid-treated pulp consumes more alkali than a pulp which has not been acid treated. Oxygen-alkali treatment of pulp which has not been pretreated with EDTA resulted in the following analysis:

Kappa number 16.3 Viscosity 893 cm..sup.3 /g. ac- cording to SCAN Viscosity 56 cp. accord- ing to TAPPI

a much poorer viscosity thus is obtained if the pulp is not pretreated with EDTA, than if a corresponding oxygen-alkali-treatment is effected on pulp which is pretreated with EDTA.

EXAMPLE 3

The same EDTA-treated pulp as that used in Example 2 was used in this experiment. The pulp was dewatered to a pulp concentration of 34 percent, after which the pulp was enclosed in a pressure vessel, and heated with direct steam contact to 100.degree. C. The pressure vessel was then connected to an oxygen gas tube, and the pressure set at 8 kg./cm..sup.2. A solution containing NaOH magnesium complex according to Example 1 and sodium silicate in a quantity corresponding to 1.5% NaOH, calculated on the quantity of dry pulp, was then charged, in atomized liquid form. The quantity of magnesium complex was 0.1 percent, calculated as MgO, and the quantity of sodium silicate was 0.3 percent, based on the amount of dry pulp. After 5 minutes reaction time, more alkali solution was introduced to the system, corresponding to 2.0% NaOH in atomized liquid form. The reaction was then continued for another 20 minutes, after which the pulp was washed with water. The pulp had the following analysis:

Kappa number 15.7 Viscosity 980 cm..sup.3 /g. ac- cording to SCAN Viscosity 75 cp. accord- ing to TAPPI

good results with regard to Kappa number and viscosity thus can be obtained according to the invention, even if the alkali addition is made after the oxygen gas pressure is connected to the pressure vessel.

EXAMPLE 4

unbleached pine sulfite pulp which after being digested to a pulp concentration of 5 percent was washed with an aqueous solution containing ethylene diamine of pH 8.1, the quantity of ethylene diamine rising to 0.2 percent, estimated on the dry weight of the pulp, was used for this experiment. The pulp had the following analysis:

Kappa number 12.6 Viscosity 1144 cm..sup.3 /g. ac- cording to SCAN Viscosity 142 cp. acord- ing to TAPPI

the liquid was removed by pressing, to obtain a dry content of 50 percent. The pulp was loosened in a peg shredder, after which 2.0% NaOH in atomized liquid form was added to the pulp. The pulp was then placed in a pressure vessel, and heated to 90.degree. C. with direct steam, afterwhich the pressure vessel was connected to an oxygen gas tube. The pressure was set at 4 kg./cm..sup.2. After 5 minutes reaction time, a further 3% NaOH in atomized form was added to the pulp, after the oxygen gas overpressure had been interrupted. The oxygen gas pressure was then reset to 4 kg./cm.sup.2, after which the reaction was continued for another 20 minutes. The pulp was then washed with water. The pulp had the following analysis:

Kappa number 4.3 Viscosity 675 cm..sup.3 /g. ac- cording to SCAN Viscosity 28 cp. accord- ing to TAPPI

the results show that a high-pulp concentration can be used when delignifying viscose pulp according to the invention, and that it is possible to obtain a controlled viscosity reduction, which is very desirable in the manufacture of viscose pulp.

Claims

We claim:

1. In the process for treating cellulosic materials with alkali in the presence of oxygen, and particularly air, the improvement which comprises pretreating the cellulosic material with a compound selected from the group consisting of acids and complexing agents for catalytically active heavy metal compounds to remove or inactivate catalytically active metals or metal compounds, bringing the concentration of cellulosic material to within the range from about 30 to about 60 percent, and then delignifying the cellulosic material with alkali in the presence of oxygen and a soluble complex magnesium compound selected from the group consisting of a magnesium salt of organic acids having from two to about 12 carbon atoms and one carboxylic acid group and an alpha or beta hydroxyl group or at least two carboxylic acid groups and no or from one to about 10 hydroxyl groups, and a magnesium salt of polyphosphoric acids, and charging the alkali in at least two increments during the oxygen treatment, the complex magnesium compound being in an amount sufficient to reduce or entirely prevent attack of oxygen on the hemicellulose and cellulose carbohydrates, without appreciably diminishing the oxidation of the lignin and its dissolution in the course of the process.

2. A process according to claim 1, in which the pretreatment is carried out with an acid solution containing an acid selected from inorganic acids and organic acids.

3. A process according to claim 2, in which the acid is a mineral acid.

4. A process according to claim 2, in which the acid is a sulfurous acid.

5. A process according to claim 2, characterized in that the pretreatment is effected with acid bleaching waste liquor.

6. A process according to claim 1, characterized in that the pretreatment stage is effected in the presence of complexing agents for catalytically active, heavy metal compounds, selected from copper, cobalt, iron and manganese.

7. A process according to claim 1, in which the oxygen-treatment is effected at a concentration of cellulosic material of from about 32 to about 40 percent.

8. A process according to claim 1, in which the oxygen treatment is carried out in at least two stages, and alkali is added between each stage and/or during each stage.

9. A process according to claim 1, in which alkali is added to the pretreated cellulosic material, and then the material is disintegrated by mechanical means prior to or during the oxygen treatment.

10. A process according to claim 1, in which the pretreated cellulosic material is disintegrated by mechanical means, and then alkali is added.

11. A process according to claim 1, in which alkali is added to the pretreated cellulosic material while it is being mechanically disintegrated during the oxygen treatment.

12. A process according to claim 1, in which oxygen treatment is carried out while the cellulosic material is being mechanically disintegrated.

13. A process according to claim 1, in which alkali is added in an atomized condition.

14. A process according to claim 1, in which alkali is added to the pretreated cellulosic material, the material is mechanically disintegrated during oxygen treatment, and atomized alkali is supplied to the disintegrated material during the oxygen treatment.

15. A process according to claim 1, in which the oxygen treatment is carried out in the presence of a silicate compound.

16. A process according to claim 15, in which the silicate is an alkali metal silicate.

17. A process according to claim 1, wherein the magnesium compound is a chelate of magnesium and an aliphatic alpha- or beta-hydroxycarboxylic acid.

18. A process according to claim 1, wherein the magnesium compound is a magnesium salt of a dicarboxylic aliphatic acid.

19. A process according to claim 1, wherein the magnesium compound is a magnesium polyphosphate.

20. A process according to claim 1, wherein the treatment is carried out on lignin-bearing wood cellulose to remove lignin at least in part.

21. A process according to claim 1, wherein the magnesium compound is a chelate of magnesium and the complexing organic acid is contained in waste liquor from a process in which cellulosic material is treated with alkali.

22. A process according to claim 21, wherein a magnesium salt is added to the waste liquor to form the complex magnesium salt in situ in the liquor.

23. A process according to claim 1, wherein the partial pressure of the oxygen at the beginning of the treatment is at least about 1 atm.

24. A process according to claim 1, wherein the treatment is carried out at a temperature within the range from about 80.degree. to about 140.degree. C.

25. A process according to claim 1, wherein the cellulosic material prior to being treated is impregnated with an aqueous solution of a complex magnesium salt or components which in the solution form a complex magnesium salt.

26. A process according to claim 25, wherein a portion of the solution is removed from the pulp prior to the treatment.

27. A process according to claim 1, wherein the quantity of alkali calculated as NaOH is within the range from about 0.5 to about 12 percent based on the dry weight of the cellulosic material.

28. A process according to claim 1 wherein the content of magnesium during the treatment is at least 0.005 up to about 1 percent calculated as MgO on the dry weight of the pulp.

29. A process according to claim 1, wherein the magnesium compound is a chelate of magnesium and

an aliphatic hydroxy acid having from two to about 12 carbon atoms and from one to about 10 hydroxyl groups.

30. A process according to claim 29, wherein the acid is selected from the group consisting of glycolic acid, lactic acid, dihydroxy-butyric acid, aldonic acid, gluconic acid, mannonic acid, tartaric acid and oxalic acid.

31. A process according to claim 1, wherein in addition the delignification is controlled to obtain a controlled dissolution of hemicellulose in cellulose pulp, either during or after delignification, in the presence of the complex magnesium compound.

32. A process according to claim 1, wherein the cellulosic material is an unbleached, partially bleached or bleached cellulose sulfate pulp, sulfite pulp or semichemical pulp derived from wood.

33. A process according to claim 1, wherein the process is controlled to obtain a controlled dissolution of hemicellulose in a low lignin or lignin-free cellulose pulp in the presence of the complex magnesium salt.