JPS6350466B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a method for bleaching chemical cellulose pulp, particularly cellulose pulp produced by alkaline cooking. Examples of alkaline pulps include sulfate pulp, polysulfide pulp, and soda pulp. In soda pulp,
Contains pulp cooked with sodium hydroxide as the cooking chemical in the presence of various additives. Examples of such additives include redox catalysts such as anthraquinones. The present invention can also be applied to other chemical pulps, such as sulfite pulps. State of Technology Clark (Paper Trade Journal Tappi Sec.
118, 62 (1944)) treated the pulp with nitrogen dioxide in an aqueous suspension at 90° C. for 1 to 1.5 hours;
then at 90°C for 30 minutes at a pulp concentration of 7% and an alkali charge corresponding to 2% NaOH calculated on the dry weight of the pulp, or at 50°C.
We discovered that cellulose pulp could be partially delignified by extracting it for 60 minutes. This treatment causes significant depolymerization of cellulose,
It is reflected by the much lower viscosity of the treated pulp compared to the pulp subjected to chlorination and alkaline extraction. Bouli (French Patent No. 2158873) avoided depolymerization by treating the pulp with nitrogen dioxide at low temperatures, preferably below 20° C., for an extended period of time, followed by alkaline extraction under mild conditions. However, cellulose pulp is only delignified to a very low extent and the process does not help solve existing environmental problems. A two-step process in which cellulose pulp is pretreated with nitrogen dioxide and then bleached with oxygen gas has also been described, but although a high degree of delignification is possible, this process results in a severe decrease in viscosity. DISCLOSURE OF THE INVENTION Technical Problem The problem of the invention of a process by which cellulose pulp can be bleached without the use of chlorine or chlorine-containing bleaching agents or with a minimum use of such bleaching agents is to produce bleached chemical pulp. This is an urgent issue for all countries. Effluents from conventional bleaching plants contain toxic and mutagenic compounds. Oxygen gas bleaching partially solves this problem, but the methods currently applied industrially require a high Only about 50% of the lignin present in cellulose pulp is delignified. The viscosity of cellulose pulp is suitable for use as a measure of degradation. According to the SCAN method, the viscosity of bleached pulp is
It must not be less than 900dm ³ /Kg. Solution The above-mentioned problem is solved in a surprising manner by the present invention. The present invention is characterized in that the pulp is contacted with a gaseous phase containing _NO2 in the presence of water in the activation stage, and oxygen gas is added to utilize the intermediate product NO for activation, after which the pulp is subjected to alkaline treatment. The present invention relates to a method for delignifying cellulose pulp produced by chemical pulping of lignocellulosic material subjected to a process. The method comprises carrying out both the activation step and the alkaline treatment under intense conditions, i.e. at such a high temperature that some decomposition of the cellulose molecules occurs during the activation step and at 95 to 150°C during the alkaline treatment, as appropriate. is 101 or
It is characterized in that it is carried out at 140° C., preferably from 110 to 120° C., and the treatment time at the lowest temperature of each temperature range exceeds 45 minutes, 30 minutes and 15 minutes, respectively. The change in the intrinsic viscosity of the cellulose pulp is here used as a measure of the extent to which the cellulose molecules have been degraded. The values given below were determined without lignin and hemicellulose removal, which is the most reproducible method for pulps with moderate lignin content (e.g. sulfate pulps with Katsupa numbers below 35). . however,
The contribution of lignin and hemicellulose to viscosity is very small compared to the same weight of cellulose molecules, and bleaching reduces the lignin content and removes hemicellulose with little loss of pure cellulose pulp under the conditions employed. It is necessary to keep in mind that the purpose is to When cellulose depolymerization is negligible, the intrinsic viscosity of the pulp will increase. For pine sulfate pulps of the type used in most of the examples, a decrease in Kappa number of 10 units results in an increase in viscosity of about 50 dm ³ /Kg under conditions where cellulose depolymerization is negligible; The corresponding increase for sulfite and sulfate pulps made from hardwoods is significant as a result of a greater loss of hemicellulose. To check that depolymerization of some cellulose molecules has occurred during the activation stage, the viscosity is measured according to SCAN-C15:62, which is carried out on the activated pulp by washing it with water and 35
This can be done by rapid drying at 0.degree. C. and immediately measuring the viscosity. When carrying out the method according to the invention, the intrinsic viscosity of the pulp after activation and water washing is the same or preferably at least 5% lower than the intrinsic viscosity of the pulp before said activation.
Even when the viscosity is lower, the extent to which the pulp is delignified is still greater, for example 8% or more. For most pulps, the extent to which the intrinsic viscosity is reduced during the activation step is advantageously between 2 and 35%, suitably 5%.
25%, preferably 10 to 20%. Activated pulp can be damaged by, for example, using excessive drying times or using excessively high temperatures during drying.
and is easily depolymerized during storage. Therefore, it would not be appropriate for routine analysis to measure the viscosity of pulp immediately after activation. Under suitable conditions, no appreciable cellulose depolymerization occurs during alkaline treatment, but instead an increase in viscosity is obtained as a result of the removal of lignin and some hemicellulose. It is therefore preferred in practice to adapt the conditions during activation such that the pulp after said alkaline treatment has a viscosity lower than that which can be calculated for the removed lignin and hemicellulose. Usually, alkali-treated pulp is 0 to 25%, suitably 5 to 25%, more than unbleached pulp.
It is suitable to have an intrinsic viscosity by SCAN of 20%, preferably 10 to 15% lower. These ranges primarily relate to softwood sulfate pulps. For sulfite pulps and hardwood sulfate pulps, the preferred viscosity reduction is half of the above values. The nitrogen dioxide present in the activation stage can be supplied in substantially pure form _NO2 or allowed to be produced in the reactor after supplying nitrogen monoxide and oxygen thereto. NO ₂ plus NO can also be fed to the reactor. Dinitrogen tetroxide and other polymeric forms are considered nitrogen dioxide _NO2 . 1 mole of dinitrogen tetroxide is 2 moles of nitrogen dioxide
Calculated as moles. Adducts in which nitric oxide is present are calculated as nitrogen oxides in a similar manner. Therefore, dinitrogen trioxide N ₂ O ₃ is calculated as 1 mole of nitrogen monoxide and 1 mole of nitrogen dioxide.
Adducts with oxygen will probably also occur as intermediates. Some amount of oxygen must be introduced into the activation stage, both when supplying nitrogen dioxide NO ₂ and when supplying nitric oxide. In order to obtain the best possible results with the simplest equipment, it is appropriate to supply oxygen in substantially pure form to the activation stage. Liquid oxygen can also be charged to the reactor, which is evaporated, for example, on entry into the reactor in which the activation step is carried out. The amount of oxygen fed into the activation stage is necessary to obtain the desired result, calculated as at least 0.05 moles of O ₂ per mole of NO ₂ fed. per mole of NO ₂ supplied
The best performance in the tested devices was obtained when 0.15 to 0.30 mole O ₂ was supplied to the activation stage. NO ₂ in liquid form is commercially available and can be introduced into the process in this form. Conveniently,
The nitrogen dioxide is vaporized before entering the reactor in which the activation step is carried out, or simultaneously with the introduction of nitrogen dioxide into the reactor. NO ₂ can also be obtained by oxidizing NO with oxygen.
NO can advantageously be produced by catalytic combustion of ammonia, the combustion process being preferably carried out in connection with a bleaching plant in which the method according to the invention is carried out. A gas phase containing NO ₂ can be obtained by reacting oxygen with NO before or during the activation step. supplied
When calculated per mole of NO, the total amount of _O2 supplied is such that both the supplied and interval NO2 are utilized for the activation stage, and that virtually all of the NO and _NO2 generated from NO are used for the activation stage. to be consumed at the end of at least
It should be 0.55 mol O2, suitably 0.6 to 5 mol, preferably 0.65 to 0.80 mol _O2 . A mixture of NO and _NO2 can also be fed to the activation stage. In this case, the amount of oxygen supplied is
The aforementioned precautions for dimers, polymers and adducts are taken, and the aforementioned proportions correspond to the respective amounts of these nitrogen oxides. The amount of the aforementioned nitrogen oxides (NO ₂ +NO) fed to this process is between 3 and 3 per 100 kg of dry cellulose pulp.
300, suitably 10 to 150, preferably 25 to 150
Reaching 75 moles. The total pressure in the activation reactor is suitably maintained below atmospheric pressure, preferably near atmospheric pressure. In order to obtain the best activation results and the lowest possible nitrogen oxide emissions from the reactor, the introduction of as little inert gas as possible, such as air, when introducing the pulp into the reactor is recommended. This is very important. The amount of inert gas entering the activation reactor is
Maximum 50 mol per 1000 kg of bone dry pulp, suitably maximum
It has been found that it should be 20 mol, preferably 0.05 to 5 mol. Temperature is 50 to 90℃
The desired degree of cellulose depolymerization is obtained during the activation stage, when the temperature is suitably maintained between 55 and 80°C. Low temperatures usually require unrealistically long processing times, while excessively high temperatures result in excessive depolymerization of cellulose. For many pulps, the preferred temperature range is 60 to 70°C. The residence time is adapted with respect to the amount of NO ₂ and/or NO charged, the activation temperature, the type of pulp being treated, and the pulp concentration, and is estimated by measuring the viscosity according to the method described above. When working in the preferred temperature range of 50 to 90℃, the activation time is
longer than 10 to 0.2 (t-50) minutes and 660 to 16 (t
−50) minutes or less. In this case t means the temperature in °C. The activation time should be near the lower limit for high pulp concentrations and high NO ₂ and/or NO charges, and is increased for low pulp concentrations and low charges. The pulp consistency during the activation stage must be kept between 16 and 50%, higher concentrations are difficult to reach. Actually the most useful range is 22
40%, preferably 27 to 35%. After the activation step, the pulp is preferably washed with water and/or a suitable aqueous solution. If this washing and steaming step were omitted, a large amount of neutralizing alkaline agent would be consumed in the post-treatment stage of the pulp. Instead of water, or preferably after washing the pulp with water, it is convenient to treat the pulp with an alkaline solution, for example a spent bleach solution or a spent liquor from an alkaline treatment according to the invention. According to a preferred embodiment, the cellulose pulp is prepared in water and/or in dilute aqueous solution after the activation step;
Washing is carried out under conditions such that an aqueous acid solution is obtained, which aqueous acid solution is used to wash the pulp after cooking the pulp, preferably after replacing the cooking liquor from the pulp with the liquor obtained from the alkaline stage. . Regardless of whether the pulp is washed after the activation step with water or an aqueous solution such that an acidic aqueous solution is obtained, or whether this washing step is omitted, the pulp is subjected to an alkaline reaction before extraction. It is suitable to wash with a liquid, preferably at a temperature of 40 to 80°C. The alkali used in the alkaline treatment step is preferably sodium hydroxide. According to a preferred embodiment of the invention, the spent liquor from this alkaline treatment stage can be used in the alkaline treatment stage after being supplemented with fresh alkali. The amount of sodium hydroxide charged will be adapted to the amount of water present during this treatment step, so that the concentration of free sodium hydroxide may range from 2 to 50 g as free NaOH per kg of water present during the alkaline treatment step, as appropriate. amounts to 4 to 30 g, preferably 5 to 20 g. When the effluent is not recycled to the alkaline treatment stage and when the pulp is thoroughly washed with water after activation, the amount of free sodium hydroxide present corresponds to the total amount of sodium hydroxide fed. When the effluent is recycled and the pulp contains residual alkali from the alkaline washing step, or when the pulp contains nitric acid, the concentration of free sodium hydroxide will decrease in the pulp immediately after introducing the alkali. It is defined by the amount of alkali obtained and determined by potentiometric titration to PH9 with hydrochloric acid. Other alkalis can also be used, such as white liquor and oxidized white liquor.
If the charged alkali contains sulfides, the sulfides can be determined by titration before measuring free sodium hydroxide.
removed by precipitation. The pulp consistency during the alkaline treatment stage is usually maintained between 5 and 45%, suitably between 16 and 40%, preferably between 25 and 35%.
A high pulp concentration is preferred for thermoeconomic reasons. Recycling the spent waste liquid to the alkaline treatment stage saves energy and alkali and improves the recovery of organic materials used as fuel in known manner. The treatment time in the alkaline stage was found to have a significant effect on the extent of delignification, other conditions being similar. For pulps with low contents of carbonyl groups and hemicelluloses, such as softwood sulfate pulps, the optimum temperature range is usually 101 to 104°C, and often preferably 110 to 104°C.
The temperature is 120℃. At a temperature of 101°C, the processing time will be at least 30 minutes, and at 110°C a minimum of 15 minutes. Processing time temperature range from 95 to
Significantly improved delignification is obtained when extending to 60 minutes or preferably to 120 minutes at 120°C. An increase to 4 hours results in a further decrease in Katsupa number without any appreciable decrease in viscosity. On the other hand,
The treatment time should be, for example, 15 minutes or less at 150°C. In the temperature range from 120 to 140°C, the residence time is suitably from 15 to 120 minutes, preferably from 30 to 60 minutes. A temperature range of 95 to 101° C. can be preferably employed for pulps with high carbonyl groups and hemicellulose content, such as sulfite pulps and hardwood sulfate pulps. At a temperature of 95°C, processing time will exceed 45 minutes. Longer treatment times should be employed, especially when it is desired to reduce the amount of hemicellulose present. When this temperature range is employed for softwood sulfate pulps, it is extended to at least 1 hour, suitably at least 2 hours, to obtain a high degree of delignification. It is entirely possible to use pulps treated according to the invention, i.e. pulps in which the pulp is first activated in the presence of nitrogen dioxide and then treated in an alkaline stage, without subsequent bleaching. . However, after the pulp has been treated according to the invention, it is preferred to subject it to a final bleaching process, preferably using environmentally friendly bleaching agents such as chlorine dioxide, peroxide and ozone. Benefits Several benefits are obtained when chemical cellulose pulp is subjected to the delignification bleaching process according to the present invention. One feature of the bleaching carried out in accordance with the present invention is that the process is advantageous in that it is environmentally and highly selective. Comparing the process according to the invention with the conventional delignification bleaching of cellulose pulp by chlorine followed by an alkaline stage, the process according to the invention is very advantageous for the environment. When carrying out the process of the present invention, the waste liquid can be recovered, readily evaporated and combusted in a conventional soda recovery boiler in conjunction with the cooking waste liquid. This is not possible with chlorine bleach. Compared to oxygen gas bleaching, the process of the invention exhibits a higher selectivity, ie a higher amount of lignin can be removed from cellulose pulp while maintaining a higher viscosity. Compared to nitrogen dioxide treatment (addition of oxygen gas) of cellulose pulp followed by oxygen gas bleaching, the process according to the invention is often superior in selectivity. Besides, the cost of oxygen gas is avoided and simpler equipment can be used. When carrying out the present invention, compared to the usual alkaline stage following the nitrogen dioxide treatment (oxygen gas addition) of cellulose pulp, i.e. where there are no harsh conditions in either the nitrogen dioxide stage or the alkaline treatment stage, Although the pulp is delignified to a much higher degree, it can maintain a high viscosity. Further, when calculated using the same Katsupa number, an even higher pulp yield can be obtained. Preferred Embodiments of the Invention Several tests according to the invention and comparative tests have been carried out. The method of carrying out the test and the results obtained are clear from the following examples. Example 1 Industrial pine sulfate pulp having a Katsupa number of 31.3 and a viscosity of 1220 dm ³ /Kg calculated according to SCAN-C15:62 was washed with water and pressed to a pulp density of 36.3%. The pulp was loosened using a Petsugu shredder and charged into a reactor at 65°C. A vacuum was applied to remove air from the reactor. As a result, the temperature is 60℃
descended to. Gaseous nitrogen dioxide obtained by evaporation of liquid N ₂ O ₄ was introduced into the reactor for 4 minutes.
This charge corresponds to 86 moles of NO ₂ per 100 kg of pulp. After charging the reactor with nitrogen dioxide, 15 moles of oxygen gas were introduced over a period of 4 minutes. The temperature was maintained at 60°C by rotating the reactor in a water bath. The treatment was carried out for 120 minutes, calculated from the moment the introduction of nitrogen dioxide started. After 120 minutes of treatment, the viscosity of the pulse decreased by 160 dm ³ /Kg. The pulp was then washed with water, pressed, and impregnated with sodium hydroxide by kneading without removing the solution. The pulp concentration was kept at 25% and alkali was added at two levels. The pulp was then subjected to hot alkaline treatment after removing the air by vacuum. Heat was provided by live steam. In one test series the temperature was 101°C and in the other series it was 111°C. As in other tests conducted at lower pulp concentrations, varying conditions in the alkaline stage did not significantly affect the viscosity of the alkaline treated pulp. In a test treated with nitrogen dioxide for 120 minutes,
The viscosity value is 1100dm ³ /Kg. Table 1 shows the pulp number and pulp yield. Table 1 also shows the results obtained for a reference pulp prepared by activating the pulp at 40° C. for 20 minutes, all other conditions being the same. The viscosity of these pulps was slightly higher (1120 dm ³ /Kg) despite the drastically inferior delignification. The results are shown in Table 1 below. TABLE The above table shows that pulps with surprisingly low lignin contents can be produced by prolonged NO ₂ /O ₂ activation followed by alkaline treatment carried out under harsh conditions. The table also shows that by using the conditions according to the invention in the activation stage, higher yields are obtained compared to pulps of the same Kappa number than when applying mild conditions. This was confirmed in another test. Example 2 Spruce sulfite pulp having a Katsupa number of 10.6 and a viscosity of 996 dm ³ /Kg as measured by SCAN-C15:62 was washed with water and the pulp concentration was 36.5%.
I pressed it. The pulp was loosened with a peg shredder and introduced into the reactor at 65°C.
The reactor was evacuated to remove the air within it. This caused the temperature to drop to 60°C. liquid
Gaseous nitrogen dioxide obtained by evaporation of N ₂ O ₄
The reactor was charged for a minute. This preparation has 100 pulp
This corresponded to 43 moles of NO ₂ per kg. After supplying nitrogen dioxide, 8 mol of oxygen gas was charged into the reactor over 4 minutes. The temperature was maintained at 60°C by rotating the reactor in a water bath. The treatment lasted for 120 minutes calculated from the start of the nitrogen dioxide feed. After the activation treatment stage, the viscosity of the pulp is 40dm ³ /Kg
only decreased. Next, the pulp is washed with water, pressed,
By kneading without removing the solution,
Impregnated with sodium hydroxide. Pulp concentration 15%
and the alkali charge was 2.1 g of NaOH per kg of water present. The temperature was 101°C and the treatment time was 60 minutes. The tests carried out according to the invention yielded a pulp with a Kappa number of 2.5 and a viscosity of 1040 dm ³ /Kg. For comparison, the above test was repeated, except that the activation phase was performed for 5 minutes. Thus, this test was not performed in accordance with the present invention. This test yielded a pulp with the same viscosity as the pulp obtained from the tests carried out according to the invention, but with a Kappa number of 4.3. These harsh conditions during both the activation and alkaline stages result in a significant reduction in the lignin content and, in the case of sulfite pulp, a significantly high viscosity.

Claims (1)

[Scope of Claims] 1. Pulp produced by a method for chemical pulping of lignocellulosic materials is brought into contact with a gaseous phase containing NO ₂ in an activation step in the presence of water, and an intermediate is added to the activation of the pulp. In a method for delignifying cellulose pulp comprising supplying oxygen gas to utilize NO and then subjecting the pulp to an alkaline treatment, both the activation step and the alkaline treatment are carried out under intense conditions, i.e. at high temperatures and for long periods of time such that the pulp after activation and water washing during the stage exhibits an intrinsic viscosity of at most the same or preferably at least 5% and at most 35% lower compared to the intrinsic viscosity of the pulp before activation. , and 95 to 150℃ during alkaline treatment, suitably
A process as described above, characterized in that it is carried out at 101 to 140°C, preferably 110 to 12°C, and the treatment time at the lowest temperature of each temperature range exceeds 45 minutes, 30 minutes and 15 minutes, respectively. 2 The temperature during the activation step is maintained between 50 and 90°C, suitably between 55 and 80°C, preferably between 60 and 70°C, and the activation time is between 10 and 0.2 (t-50) minutes, where t is in °C. 2. A method according to claim 1, characterized in that the temperature indicated above is exceeded. 3 Activation time is 660 to 16 (t-50) minutes (but t
3. The method according to claim 2, wherein the temperature is less than or equal to the temperature expressed in degrees Celsius. 4 The pulp concentration during the activation stage is between 16 and 50%;
Suitably 22 to 40%, preferably 27 to 35%
4. A method according to any one of claims 1 to 3, characterized in that: 5 The amount of alkali at the start of the alkaline treatment is calculated as free sodium hydroxide and is calculated as Kg of water present.
5. A method according to any one of claims 1 to 4, characterized in that the amount is 2 to 50 g, suitably 4 to 50 g, preferably 5 to 20 g. 6 During alkaline treatment the pulp density is between 5 and 45.
%, suitably 16 to 40%, preferably 25 to 40%
5. The method of claim 5, wherein the percentage is maintained at 35%. 7. The method according to claim 5 or 6, in which the spent liquid from the alkaline treatment is replenished with fresh alkali and is reused for the alkaline treatment. 8. The method according to any one of claims 5 to 7, wherein the alkali is sodium hydroxide.