SE445122B - PROCEDURE FOR DELIGNIFICATION OF CELLULOSAMASSA IN THE PRESENTATION OF NITROGEN Dioxide - Google Patents

Description

15 20 25 8OÛ8874 ° = 3 extraction under mild conditions. However, the delignification will be very small and the method does not provide a solution to current environmental problems.

A two-step method involving pretreatment with nitrogen dioxide accompanied by oxygen bleaching has also been described, but this method, although it enables a far-reaching delignification, causes a sharp reduction in viscosity.

Disclosure of the Invention Technical Problem The problem of finding a method for far-reaching delignification without the use of chlorine or chlorine-containing bleaches or with minimal use of such appears to be one of the most current in countries producing bleached chemical cellulose pulp. Emissions from conventional bleachers contain compounds that are toxic and mutagenic. Oxygen bleaching allows a partial solution to the problems, but as it is applied technically today, it allows a delignification to approx. 50% of the lignin present after boiling in the pulp mass without the cellulose decomposing to the limit where the pulp gives too low a strength to the paper produced by the pulp. As a measure of the decomposition, the viscosity of the cellulose pulp is suitably used.

According to the SCAN method, the viscosity of the finished bleached pulp should not be less than 900 dms / kg.

The Solution The present invention solves the above problems in a completely surprising manner. The invention relates to a process for delignifying cellulose pulp, prepared by chemically digesting lignocellulosic material, wherein the pulp in an activation step in the presence of water is brought into contact with a gas phase containing NO 2 and oxygen is supplied, so that intermediate w! year! 8008874-3 formed NO is used for activation, after which the pulp is subjected to an alkali treatment without intentional supply of air or oxygen, characterized by the combination that both the activation step and the alkali treatment are carried out under drastic conditions, ie. at such a high temperature and for such a long time during the activation step that the mass after activation and water washing has at most the same and preferably at least 5% and at most 35% lower intrinsic viscosity than before activation and at a temperature during the alkali treatment of 95-150 ° C, 1o1-14o ° r, preferably 11o-120o ° c, the treatment time at the lower temperature within the respective temperature range exceeding 45, 30 and 15 minutes, respectively.

As a measure of the degradation of the cellulose molecules, changes in the intrinsic viscosity of the cellulose pulp, determined according to SCAN, were used here. The values given below have all been determined without removing lignin and hemicellulose, which is the most reproducible method for pulps with a moderate lignin content (for example for sulphate pulps with kappa numbers below 35). It must be noted, however, that lignin and hemicellulose make a very small contribution to the viscosity compared to the same amount by weight of cellulose molecules and that the bleaching is intended to reduce the lignin content and also give a release of hemicellulose while the loss of pure cellulose is very small. terms of use. Therefore, in cases where the depolymerization of the cellulose is negligible, the intrinsic viscosity increases. In the case of tallow sulphate mousses of the type used in most of the examples in this specification, a kappa number reduction of 10 units results in a viscosity increase of cu. 50 dms / kg under the conditions where the depolymerization of the cellulose can be neglected, while the corresponding increase in sulphite pulp and sulphate pulp of hardwood is significantly greater due to a greater loss of hemicellulose.

As a check that a certain decomposition of the cellulose molecules is obtained during the activation step íšüüßßwå-rš, viscosity measurements according to SCAN-Cl5: 62 were used, which can be made on the activated mass after washing with water and rapid drying at SSOC and immediate determination of the viscosity. In the process according to the invention, the pulp after activation and water washing, which appears from the above, has the same or preferably at least 5% lower intrinsic viscosity than before the activation. Even further delignification is obtained if the viscosity reduction 10 is higher, for example above 8%. In most pulp types, the viscosity reduction during the activation step can advantageously amount to 2-35%, preferably 5-25%, preferably 10-20 ”. The activated pulp can be easily depolymerized, for example when using too long a drying time, too high a temperature during drying and during storage. For this reason, direct determinations of the viscosity after activation are unsuitable for routine work. Under preferred conditions, no appreciable depolymerization of the cellulose takes place during the alkali treatment, but here a viscosity increase is normally obtained due to the removal of lignin and a certain amount of hemicellulose. The viscosity determinations can be made with great precision.

It is therefore more practical in practice to adjust the conditions during activation so that the pulp after the alkali treatment has a viscosity which is lower than that which can be calculated with regard to released lignin and hemicellulose.

In normal cases, it is suitable that the alkali-treated pulp has a self-viscosity according to SCAN which is 0-25%, preferably 5-20, preferably 10-15% lower than the unbleached pulp. These limits relate primarily to softwood pulp sulphate pulps. In the case of sulphite pulps and hardwood sulphate pulps, the preferred viscosity reduction is half of these values. The nitrogen dioxide present in the activation step is either fed as substantially pure NO 2 or allowed to form in the reactor after the addition of nitric oxide and oxygen. NO2 plus NO can also be added.

Nitrogen dioxide (NO2) and other forms of polymers are also counted as nitrogen dioxide (NO2). One mole of nitrous oxide is counted as two moles of nitrogen dioxide. Addition products, which contain nitric oxide, are counted in the same way as nitric oxide. Nitrogen dioxide (N203) is thus counted as one mole of nitric oxide and one mole of nitrogen dioxide. Oxygen addition products are likely to occur as intermediates.

Both with the addition of nitrogen dioxide (NO 2) and nitric oxide (NO), a certain amount of oxygen must be added to the activation step.

In order to obtain the best possible result with the simplest possible apparatus, it is suitable that the oxygen is supplied to the activation step as essentially pure oxygen. Liquid oxygen can also be added and gasified, e.g. upon introduction into the reactor in which the activation process is carried out. The amount of oxygen added to the activation step should be at least 0.05 moles calculated as O 2 per mole of added N02. In many cases a larger amount, suitably 0.1-5 mol O 2 per mol of NO 2 added, is used in order to obtain the desired result. The best results in tested apparatus have been obtained, when the amount added was 0.15-0.30 moles of Oz per mol of NO2 added. NO2 in liquid form is a commercial product which can be added to the process in this form. Preferably, the nitrogen dioxide is gasified before or in connection with the introduction into the reactor for the activation step. NO 2 can also be obtained by oxidation of NO with oxygen. For the production of NO, it is advantageous to use catalytic combustion of ammonia, which can advantageously take place in connection with the bleaching plant where the process according to the invention is carried out. The gas phase containing NO 2 can be produced by reaction between oxygen and NO before or during the activation step. Calculated per mol of added NO, the total amount of added OZ should amount to at least 0.55 mol, suitably 0.6-5, preferably 0.65-0.80 mol O 2 in order for both added and intermediate-formed NO to be used for the activation process and that in the activation step end practically all NO and NO 2 formed by NO should be consumed. g A mixture of NO and NO 2 can also be added to the activation step. In this case, the amount of oxygen is adapted to the amount of each of these nitrogen oxides according to the proportions given above, taking into account dimers, polymers and adducts in the manner defined above. The amount of nitrogen oxides (NOZ + NO) defined above is added to the process in total amounts to 3-300, preferably 10-150, preferably 25-75 gram moles calculated on 100 kg of dry cellulose pulp.

The total pressure in the activating reactor is advantageously kept lower than the atmospheric pressure, preferably close to the atmospheric pressure.

In order to achieve the best results from the activation and the least possible emissions of nitrogen oxides from the reactor, it is important that the supply of inert gas, e.g. air, when feeding the mass into the reactor is as low as possible. It has been found that the amount of inert gas introduced into the activation reactor should amount to a maximum of 50 gram moles, suitably a maximum of 20 gram moles, preferably 0.05-5 gram moles per 1000 kg of dry-solid mass.

A desired depolymerization of the cellulose during the activation step is achieved if the temperature is maintained at 50-9000, preferably at 55-8000. A lower temperature normally requires an unrealistically long treatment time, while an excessively high temperature leads to an excessive depolymerization of the cellulose. For many pulps, the preferred temperature range is 60-70 ° C. The residence time is adapted with regard to the charge of NO 2 and / or NO, the activation temperature, the type of pulp and to the pulp concentration and is tested by determining the viscosity according to the methods given above.

Within the preferred temperature range of 50-90 ° C, the activation time should exceed 10-0.2 (t-50) minutes and be less than 660-166 (t-50) minutes, where t is the temperature in 0 ° C. At high pulp concentration and high investment of NO2 and / or NO, one should be close to the lower limit, while low pulp concentration and low investment means that the activation time is increased.

The pulp concentration during the activation step should be kept at 16-50%. The higher limit can be difficult to achieve. The most useful range in practice is 22-40%, preferably 27-35%.

After the activation step, the mass is suitably washed with water and / or a suitable aqueous solution. If this washing is discontinued, the consumption of alkaline neutralizing agents for the subsequent treatment will increase sharply. Instead of water or preferably after water washing, it is advantageous to treat the pulp with an alkaline-reacting solution, for example bleach liquor or liquor from the alkali treatment according to the invention.

According to a preferred embodiment, the cellulose mass is washed after the activation step with water and / or dilute aqueous solution under such conditions that an acidic solution is obtained, which is used to wash the mass after boiling, preferably after the cooking liquor is displaced with lye from any alkaline step.

Regardless of whether the pulp after the activation step is washed with water or such an aqueous solution that an acidic solution is obtained or if such washing is not carried out, it is convenient to also wash the pulp with an alkaline reacting solution, suitably at a temperature of 40-80 ° C, before the extraction step.

Sodium hydroxide is preferably used as the alkali in the alkali treatment. According to a preferred embodiment of the invention, effluent from the alkali treatment can be used in the alkali treatment after freshening with newly added alkali. The charge of sodium hydroxide is adapted to the amount of water present during this treatment step so that the concentration of free sodium hydroxide amounts to 2-50, preferably 4-30, preferably 5-20. grams of free NaOH per kg of water present during the alkali treatment. When no effluent is returned from the alkali treatment and when the mass has been thoroughly washed with water after the activation step, the amount of free sodium hydroxide corresponds to the total amount of sodium hydroxide added.

When effluent is recycled and when the pulp contains residual alkali from alkaline washing or when the pulp contains nitric acid, the concentration of free sodium hydroxide is defined as the amount of alkali obtained in the pulp immediately after the addition of alkali and determined by potentiometric titration. ring with hydrochloric acid (HCl) to pH 9. Other alkali can also be used, for example white liquor and oxidized white liquor. When sulphide is present in the alkali addition, it is removed by precipitation before the determination of free sodium hydroxide by titration.

The pulp concentration during the alkali treatment is normally kept at 5-45%, suitably at 16-40%, preferably at 25-35%. High pulp concentration is preferred with regard to heat economy. Recycling of spent liquor to the alkali treatment results in a saving of both energy and alkali and leads to an improved recovery of organic matter, which is used as fuel in a known manner.

The treatment time in the alkali step has been shown to have a large effect on the degree of delignification under otherwise equal conditions. For pulps with a low content of carbonyl groups and hemicellulose, for example sulphate pulp of hurrvcd, the most suitable temperature turpentine wool is unmolded at 110 DEG-140 DEG C. and often still at 110 DEG-120 DEG C. at 10 ° C the time should be at least 30 minutes and at 110 ° C at least 15 minutes. A markedly improved delignification is obtained if the time within the temperature range is extended to 60 ° C, preferably 120 minutes. An increase to 4 hours gives a further decrease in the kappa number without a detectable decrease in the viscosity. At 15,000, on the other hand, the time should be short, for example less than 15 minutes. For the temperature range 12U-14000, the residence time is suitably 15-120, preferably 30-60 minutes.

The temperature range 95-110 ° C can be used preferably for pulps with a high content of carbonyl groups or hemicellulose, for example for sulphite pulps and sulphate pulp of hardwood. The time at 95 ° C must exceed 45 minutes. Longer time should be used especially if it is desired to reduce the content of hemicellulose. When this temperature range is used for sulphate pulp of softwood, the time should preferably be increased to at least 1 hour, preferably at least 2 hours, in order to achieve a strong delignification.

It is quite possible to use the pulp treated in accordance with the invention, i.e. that the pulp is first activated in the presence of nitrogen dioxide and then treated in an alkali step, without further bleaching of the pulp. However, it is preferred that the pulp after the treatment according to the invention is subjected to final bleaching using preferably such bleaching agents which are environmentally friendly, for example chlorine dioxide, peroxide and ozone.

Advantages By subjecting chemical cellulose pulp to delignifying bleaching in accordance with the invention, several advantages are obtained.

Characteristic of bleaching in accordance with the invention is that the process is both environmentally friendly and extremely selective.

If a comparison is made between the process according to the invention and conventional delignifying bleaching of the cellulose mass by means of chlorine followed by an alkali step, it is found that the process according to the invention is considerably more environmentally friendly. In the process according to the invention, the effluents can be recovered and without worries evaporated and burned together with the boiling liquor in a conventional recovery boiler. This is not the case with chlorine bleaching.

In comparison with oxygen bleaching, the process according to the invention shows a significantly higher selectivity, ie. significantly more lignin can be removed from the cellulose pulp while maintaining high viscosity.

In comparison with nitrogen dioxide treatment of the cellulose pulp (with oxygen inlet) followed by an oxygen bleaching step, the selectivity in the process according to the invention often seems to be better. In addition, the cost of oxygen is avoided and the equipment becomes simpler.

In comparison with nitrogen dioxide treatment of the cellulose mass (with oxygen inlet) followed by a conventional alkali step, ie. without drastic conditions in either the nitrogen dioxide stage or the alkali stage, the delignification can be carried out further in the process according to the invention while maintaining a high viscosity.

Furthermore, a higher mass yield is obtained measured at the same kappa number.

Best Mode Several embodiments in accordance with the invention and comparative experiments have been made. How these experiments were performed and the results obtained are shown in the following execution examples.

Example 1 A technical sulphate pulp of pine with a kappa number of 31.3 and an intrinsic viscosity of 1220 dms / kg measured according to SCAN-Cl5: 62, washed with water and pressed so that the pulp concentration was 36.3%. The pulp was fluffed in a shredder and introduced into a reactor at 0500.

The reactor was evacuated so that the air was removed.

The temperature dropped to b0 ° C. Nitrogen dioxide in gaseous form, obtained by gasification of liquid N 2 O 4, was introduced into the reactor for 4 minutes. The addition corresponded to 86 gram moles of N02 per 100 kg of pulp. In the course of four minutes after the addition of nitrogen dioxide, 15 gram moles of oxygen were introduced. The temperature was maintained at 60 ° C by rotating the reactor in a water bath. The treatment lasted for 120 minutes from the time when the supply of nitrogen dioxide began.

After the treatment for 120 minutes, the viscosity of the pulp had decreased by 160 dms / kg. The mass was then washed with water, pressed: and impregnated with sodium hydroxide while kneading without removing any solution. The pulp concentration was maintained at 25% and the alkali addition at two levels. The mass was then subjected to a hot alkali treatment after removal of air by evacuation. Heating was done with direct steam. The temperature was 110 ° C in one series of experiments and 11 ° C in another.

As with other experiments performed at lower mass concentrations, the variation of the conditions in the alkali step did not have a significant effect on the viscosity of the alkali-treated pulp. In the experiments with nitrogen dioxide treatment for 120 minutes, the values were at 1000 dm3 / kg. The kappa numbers of the masses as well as the mass yields are shown in Table 1. This also includes the results with reference masses prepared by activation for 20 minutes at 40 ° C under otherwise unchanged conditions. The viscosity of these was unobtrusively higher (1120 dms / kg) despite the dramatically reduced delignification.

The results are shown in Table 1 below. 10 15 20 sooaa74-3 H Table 1 Activation at Temp. in NaOH in alkali- Kappa- Yield 60 ° C - 40 ° C alkali- step number% min someone, step g / kg water ° c - 20 101 3.3 17.9 96.9 ~ 20 lll 3.3 16.9 95.4 120 - 101 3.3 11.3 95.3 120 - 101 33 8.2 93.0 120 - 111 3.3 10.6 94.8 120 - 111 33 7.8 _ 92.5 The table shows that pulps with a surprisingly low lignin content can be produced by activation with NO 2 / OZ for a long time followed by alkali treatment under drastic conditions. The table indicates that, compared with the same kappa number of the pulp, a higher yield is obtained when conditions according to the invention are used in the activation step than under mild conditions. This is confirmed by separate experiments.

Example 2 A sulphite pulp of spruce with a kappa number of 10.6 and an intrinsic viscosity of 996 dms / kg, measured according to SCAN-Cl5: 62, was washed with water and pressed so that the pulp concentration was 36.5%. The pulp was fluffed in a shredder and introduced into a reactor at 6500. The reactor was evacuated so that the air was removed. The temperature dropped to 60 ° C. Nitrogen dioxide in gaseous form obtained by gasification of liquid N 2 O 4 was introduced into the reactor for 4 minutes.

The additive corresponded to 43 gram moles of NO2 per 100 kg of pulp. In the course of 4 minutes after the nitrogen dioxide addition, 8 gram moles of oxygen were introduced. The temperature was maintained at 60 ° C by rotating the reactor in a water bath.

The treatment lasted for 120 minutes from the time when the supply of nitrogen dioxide began. 10 15 20 8008874-3 13 After the activation treatment, the viscosity of the pulp had dropped by 40 dms / kg. The mass was then washed with water, pressed and impregnated with sodium hydroxide while kneading without removing any solution. The pulp concentration was maintained at 15% and the alkali addition was 2.1 grams of NaOH per kg of water present. The temperature was 10 ° C and the treatment time was 60 minutes.

In the experiments described above, which were carried out in accordance with the invention, a pulp with a kappa number of 2.5 and a viscosity of 1040 dms / kg was obtained.

For comparison, the experiments described above were repeated with the only difference that the time in the activation step was reduced to 5 minutes. This experiment was thus not in accordance with the invention.

In this experiment a pulp with the same viscosity as in the pulp was obtained in the test according to the invention, but with a kappa number of 4.3. Drastic conditions during both the activation step and the alkali step thus entail a marked reduction in the lignin content even for sulphite pulp, while maintaining a remarkably high viscosity.

Claims (8)

8008874-3 14 PATENT REQUIREMENTS A process for delignifying cellulose pulp prepared by chemical digestion of lignocellulosic material, wherein the pulp in an activation step in the presence of water is brought into contact with a gas phase containing NO 2 and oxygen gas, so that intermediate formed NO is used for activation, after which the pulp is subjected to an alkali treatment without intentional addition of air or oxygen, characterized by the combination that both the activation step and the alkali treatment are carried out under drastic conditions, i.e. at such a high temperature and for such a long time during the activation step, that the pulp after activation and water washes have at most the same and preferably at least S 1 orh at most 35% lower intrinsic viscosity than before activation and at a temperature during the alkali treatment of 95-150 ° C, preferably 101-140 ° C, preferably 110-12006, the treatment time at the lower temperature within each temperature range exceeds 45, 30 and 15 minutes respectively. Process according to Claim 1, characterized in that the temperature during the activation step is mocked at 50 DEG-0 DEG C., preferably ss-50 DEG C., preferably 60 DEG-70 DEG C. and that the activation time exceeds 10-0.2 ° C. 50) minutes, where t is the temperature in OC. 3. A method according to claim 2, characterized in that the activation time is less than 660-16 (t-50) minutes, where t is the temperature in OC. 4. A method according to claims 1-3, characterized in that the pulp concentration during the activation step is poured at 16-50%, preferably 22-40%, preferably 27-35%. 8008874-3 15 Process according to Claim J ~ 4, characterized in that the amount of alkali at the beginning of the alkali treatment amounts to 2-50, preferably 4-30, preferably 5-20 g per kg of water present, calculated as free sodium hydroxide . Process according to Claim 5, characterized in that the pulp concentration during the alkali treatment is kept at 5-45%, preferably 1b-40%, preferably 25-35%. 7. A process according to claims 5-6, characterized in that effluent from the alkali treatment after refining with freshly added a] ka] i is used in the alkali treatment. 8. A process according to claims 5-7, characterized in that alkali is constituted by sodium hydroxide.