JPS6360158B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a method for bleaching chemically produced pulp, especially alkaline digested pulp.
Examples of alkaline-cooked pulps include sulfate pulp, polysulfide pulp, and soda pulp. The term soda pulp includes pulp digested with sodium hydroxide in the presence of various additives. Examples of additives include redox catalysts such as anthraquinones. The present invention can also be applied to other chemical cellulose pulps, such as sulfite pulps. Clarke (Paper Trade Journal Tappi
Sect. 118, 62 (1944)) is treated with nitrogen dioxide for 1 to 1.5 hours in an aqueous pulp suspension, then treated with nitrogen dioxide for 7 hours.
The cellulose pulp was partially delignified by extraction at 90 °C for 30 min or at 50 °C for 60 min in an alkaline charge corresponding to 2% NaOH at % pulp concentration and calculated on pulp dry weight. I found that I can get it. This treatment results in a high degree of degradation of the cellulose, which is reflected in the pulp whose viscosity is very low compared to that of the pulp subjected to chlorination and alkaline extraction. Bourit (French Patent Specification No. 2158873) is depolymerized by a delignification process consisting of a prolonged treatment with nitrogen dioxide at low temperatures, preferably below 20°C, followed by alkaline extraction under mild conditions. avoided. However, the degree of delignification is very low and this method does not provide a solution to current environmental problems. A two-step process has also been described in which the pulp is pretreated with nitrogen dioxide followed by oxygen gas bleaching, but although this process results in a high degree of delignification, it results in a considerable reduction in viscosity. TECHNICAL PROBLEM DISCLOSURE OF THE INVENTION For environmental reasons, it is necessary to find an economical method to eliminate the need for chlorinating cellulose pulp in the production of bleached chemical pulp. When bleaching pulp with oxygen gas, usually very high oxygen gas pressures are often used throughout the process. Lower oxygen gas pressures lead to lower delignification rates and lower selectivity, defined as the viscosity at a given Kuppa number after the oxygen gas bleaching step. Delignification during treatment at given conditions in the oxygen gas stage can be improved if the treatment is preceded by an activation step in which the pulp is treated with nitrogen dioxide. Improved selectivity is also achieved by such activation, but without a relatively extensive depolymerization of cellulose when using the methods described so far, and without the use of very expensive equipment. Without its use, it was only possible to obtain a limited degree of delignification. According to other previously proposed methods, activation with nitrogen dioxide is followed by hot alkaline treatment. This requires a very high alkaline charge and leads to high carbohydrate losses if low lignin content is to be obtained with this two-step process. Solution The aforementioned problems are solved by the present invention. The present invention provides that pulp produced by chemically digesting lignocellulosic material undergoes an activation stage.
Oxygen gas is supplied to the activation stage in such a way that the NO ₂ -containing phase is contacted and the intermediately formed nitric oxide is utilized for the activation of the pulp, and the activation stage is carried out under harsh conditions i.e. cellulose A process for delignification of cellulose pulp carried out at high temperatures such that some decomposition of the molecules occurs, wherein the process is followed by subjecting the pulp to an oxygen gas/alkali treatment in a pulp/water mixture and during the oxygen gas/alkali stage. The partial pressure of oxygen gas is 0.05
to 0.18 MPa (megapascal = international unit of pressure = 1 x 10 ⁶ newtons per square meter, the same elsewhere), and the partial pressure of oxygen gas is maintained below the partial pressure during any part of the process. It is characterized by being maintained so as not to fall below the lower limit of . A partial pressure of 0.18 MPa for oxygen gas during most of the oxygen gas/alkali treatment process achieves fast delignification and good selectivity, but 0.005 MPa
Delignification occurs very slowly. Partial pressures below 0.005 MPa will harm the whiteness of the pulp. The pulp obtains a gray color, while the pulp treated under high oxygen gas pressure obtains a pure yellow color.
Furthermore, the pulp yield is reduced. Improved pulp brightness and improved selectivity at a given Kuppa number are obtained when the partial pressure is between 0.015 and 0.09 MPa.
Obtained when the value is within the range of . When delignifying unbleached sulfate pulp, especially made from softwood, the best results in terms of selectivity and brightness are
Obtained when applying a range of 0.03 to 0.09MPa. When practiced within this range, reasonable distillation times in the oxygen gas/alkali treatment step are obtained even when the treatment step is at moderate temperatures. As a measure of the extent to which cellulose molecules are degraded, the change in intrinsic viscosity of cellulose pulp measured by the SCAN standard method is used here. All values given hereafter were determined without removal of lignin and hemicellulose, and are also reproducible in the case of pulps with moderate lignin content (e.g. in the case of sulfate pulps with Kappa numbers below 35). It's a method. However, lignin and hemicellulose only contribute slightly to viscosity compared to the same weight of cellulose molecules, and bleaching aims to reduce the lignin content and release hemicellulose, while the loss of pure cellulose is used It should be noted that in the conditions there are very few. Therefore, if cellulose depolymerization is negligible, the intrinsic viscosity will increase. For pine sulfate pulps of the type used in most of the examples herein, the drop in Kappa number of 10 units is approximately 50 dm ³ / under conditions where cellulose depolymerization is negligible.
Kg, but in the case of sulfite and hardwood sulfate pulps, the corresponding viscosity increase is significantly higher due to the large amount of hemicellulose lost. The viscosity measurement method according to SCAN-C15:62 is used to check that some decomposition of cellulose molecules occurs during the activation stage, and this measurement is performed on the activated pulp by washing the pulp with water and This can be done by rapidly drying at 35°C and immediately measuring the viscosity. In the process according to the invention, the pulp after activation and washing it with water exhibits the same intrinsic viscosity as it had before the activation step, or preferably at least 5% less than the original intrinsic viscosity. % lower intrinsic viscosity. If the viscosity is lowered by a high degree, for example by 8% or more, an even higher degree of delignification can be obtained. For most types of pulp, the viscosity is preferably reduced to 2 during the activation stage.
from 5 to 35%, preferably from 5 to 25%, preferably
It can be reduced by 10 to 20%. Depolymerization of activated pulp easily occurs, for example, when excessively long drying times, excessively high temperatures are applied during the pulp drying process and during storage. For this reason, it is not appropriate to measure the viscosity immediately after activation in routine work. In practice, it is often preferable to adapt the conditions during activation so that the pulp exhibits the desired viscosity after the oxygen gas/alkali treatment process. In general, oxygen gas/alkali treated pulp is preferably 0 to 25% lower than unbleached pulp.
%, suitably 5 to 20%, preferably 10 to 20%
Shows 15% lower intrinsic viscosity by SCAN. These limits primarily relate to softwood sulfate pulp. For sulfite pulps and hardwood sulfate pulps, the preferred viscosity reduction is half of the above value. The nitrogen dioxide present in the activation stage can either be charged to the stage alone in the form of substantially pure NO ₂ or allowed to form in the reactor after charging with nitric oxide and oxygen. You can also prepare NO ₂ plus NO. Nitrogen dioxide, the word NO ₂ is dinitrogen tetroxide,
Also includes N ₂ O ₄ and other forms of polymers. One mole of dinitrogen tetroxide should be considered equal to two moles of nitrogen dioxide. Adducts in which nitric oxide is present are likewise calculated as nitric oxide. Thus, dinitrogen trioxide N ₂ O ₃ should be considered to be the same as 1 mole of nitrogen monoxide and 1 mole of nitrogen dioxide. Adducts with oxygen are probably also present as intermediates. A given amount of oxygen gas must be fed into the activation stage both when nitrogen dioxide NO ₂ and nitrogen monoxide NO additions are used. However, in order to obtain the best possible results with the simplest possible equipment, it is suitable to supply the oxygen to the activation stage in the form of substantially pure oxygen gas. Liquid oxygen can also be fed to the activation stage and is evaporated, for example, on entering the reactor in which the activation stage is carried out. The amount of oxygen charged into the activation stage must amount to at least 0.05 mole O ₂ calculated per mole of NO ₂ charged. In many cases, large amounts are used to obtain the desired results, preferably from 0.1 to 5 moles of O ₂ per ₂ moles of NO charged. Best results in the devices tested were between 0.15 and 0.30 mole O ₂ per mole NO ₂ charged. Liquid nitrogen dioxide is commercially available and can be fed into the process in that form. Nitrogen dioxide is preferably evaporated before or at the time it is charged to the reactor in which the activation step is carried out. Nitrogen dioxide can also be obtained by oxidizing nitrogen monoxide with oxygen. Nitrogen monoxide can advantageously be produced by catalytic combustion of ammonia, which can advantageously be carried out in conjunction with the bleaching plant in which the process according to the invention is carried out. The NO2 _- containing gas phase can be obtained by reacting oxygen with nitric oxide before or during the activation step. Calculated per mole of nitrogen monoxide charged, charged O ₂
The total amount of nitric oxide charged and the nitric oxide produced as an intermediate are utilized in the activation stage, and virtually all of the nitric oxide and nitrogen dioxide produced by nitric oxide are used in the activation stage. at least 0.55 mol, preferably 0.6 to 5 mol, preferably 0.65 mol to be consumed at the end of
and 0.80 mol _O2 . A mixture of NO and NO ₂ can also be fed into the activation stage. In this case, the amount of oxygen charged is adapted to the amount of each of these nitrogen oxides, taking into account dimers, polymers and adducts in the manner described above, and according to the proportions described above. The amount of said defined nitrogen oxides NO ₂ +NO charged into the process amounts to a total of 3 to 300, preferably 10 to 150, preferably 25 to 75 mol per 100 kg of dry cellulose pulp. The total pressure of the activation reactor is advantageously maintained at a sub-atmospheric level, preferably at a level close to atmospheric pressure. In order to obtain the best activation results and to ensure the smallest possible nitrogen oxide emissions from the reactor, as little inert gas as possible, e.g. air, should enter the reactor when feeding the pulp. is important. It has been found that the amount of inert gas entering the activation reactor should be at most 50 mol, preferably at most 20 mol, preferably from 0.05 to 5 mol per 1000 kg of bone-dry pulp. The desired activation effect and depolymerization of cellulose during the activation stage is achieved when the temperature is maintained between 42 and 100°C, preferably between 50 and 90°C. Lower temperatures usually require unrealistically long processing times;
On the other hand, excessively high temperatures lead to excessive depolymerization of cellulose. For many pulps, the preferred temperature is
The temperature is between 55 and 70°C. Residence time is the charged NO ₂
and/or is related to the amount of NO, activation temperature, pulp type and pulp concentration and is established by measuring the viscosity according to the method given earlier. When this process is carried out within the preferred temperature range of 50 to 90°C, the activation time is between 10 and 90°C.
Over 0.2 (t-50) minutes, and 660 to 16 (t-50)
Must be less than minutes. Here, t is the temperature at that time expressed in degrees Celsius. At high pulp concentrations and high NO ₂ and/or NO charges, this time should be near the minimum limit, and for low pulp concentrations and low charges, the activation time should be increased. Pulp consistency during the activation stage should be kept between 16 and 50%. Maximum limits are difficult to achieve. A practical useful range is 22 to 42%, preferably 27 to 36%. After the activation step, the pulp is preferably washed with water and/or a suitable aqueous solution. If this washing step is omitted, more alkaline neutralizing agent will be consumed in the subsequent treatment process. Instead of washing the pulp with water, or preferably after washing the pulp with water, it is advantageous to treat the pulp with an alkaline reagent, such as a bleach waste liquor or a waste liquor obtained from an oxygen gas/alkali treatment stage. According to a preferred embodiment, after the activation step the pulp is washed with water and/or a dilute aqueous solution under conditions such that an aqueous acid solution is obtained, which after cooking the pulp preferably converts the cooking liquor into one of the alkaline stages. It can be used for cleaning after replacing with the liquid obtained from. Irrespective of whether the pulp is washed with water or an aqueous solution after the activation step to obtain an aqueous acid solution, or whether such washing is omitted, the pulp is treated with an alkaline reacting solution with oxygen gas. /
Washing, preferably at a temperature of 40 to 80°C, is appropriate before the alkaline treatment step. In principle, the pulp can have a low concentration during the oxygen gas/alkali treatment stage, for example 3 to 8%. This requires a large return of effluent from this stage and results in a correspondingly large amount of wet combustion. Excess free liquid may e.g.
Bleaching of high consistency pulps in which excess liquid is removed by pressing to a pulp consistency of 22% or higher, such as 27 to 35%, or even higher, can also be carried out. This is particularly suitable when the necessary equipment is available, for example when using the method according to the invention in existing oxygen gas bleaching plants or other processes for treating high consistency pulps. Furthermore, good selectivities have been obtained for pulps of average consistency, ie between 8 and 22%, using relatively simple equipment. Although at the upper end the pulp was sticky, highly positive results were obtained at pulp concentrations of 10 to 20%. Laboratory experiments conducted in a rotating autoclave have shown that the preferred range for pine sulfate pulp is 12 to 18%. Although the conditions during the activation stage are harsh, the present invention uses surprisingly mild conditions in the oxygen gas/alkali treatment stage and improves the selectivity and therefore the paper produced from the pulp in simple and inexpensive equipment. It is still possible to obtain a high degree of delignification without impairing the properties. As shown by the examples, the delignification can continue for more than 60%, suitably more than 70%, throughout both stages, and the intrinsic viscosity of the pulp during the oxygen gas/alkali stage is less than 20% or relaxed. When using conditions
It only decreases to less than 10%. An essential requirement in this regard is that the oxygen gas pressure, alkali charge and temperature are adapted such that delignification does not occur too quickly. The residence time in the oxygen gas/alkali treatment stage can advantageously be longer than is customary in modern plants.
For delignification to more than 60%, preferably more than 70%, the time should be from 30 to 300, suitably from 45 to 240, preferably from 60 to 180 minutes. pulps that have low demands on the degree and selectivity of delignification or that can be easily delignified;
For example, in the case of sulfite pulp, a shorter time can be used. Long processing times increase equipment cost, but increase selectivity. The alkali used in the oxygen gas/alkali treatment process is preferably sodium hydroxide. According to a preferred embodiment of the invention, the waste liquor containing lignin residues can be used in the oxygen gas/alkali treatment process after being supplemented with freshly supplied alkali. The amount of sodium hydroxide charged is determined by the concentration of free sodium hydroxide per kg of water present during this alkaline treatment process.
The amount of water present during this treatment step corresponds to 1 to 30, preferably 3 to 20, preferably 4 to 10 g of NaOH. If the effluent from this alkaline treatment stage is not returned and if the pulp is thoroughly washed with water after the activation stage, the free sodium hydroxide present corresponds to the total amount of sodium hydroxide charged. When the pulp contains residual alkali from the alkaline washing step 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 feeding the alkali, which amount is determined by potentiometric titration to PH9 with hydrochloric acid. The alkali used may be different from those mentioned above and may be, for example, white liquor or oxidized white liquor. If sulfides are present in the alkaline charge, they can be removed by precipitation before determining the concentration of free sodium hydroxide by titration. The treatment temperature in the oxygen gas/alkali treatment step was found to be a less critical variable than time, oxygen gas pressure, and alkali charge.
For pulps with low contents of carbonyl groups and hemicelluloses, such as softwood sulfate pulps, the suitable temperature range is 80 to 140°C, and 90 to 120°C is often preferred. 85 to 85 for pulps with a high content of carbonyl groups and hemicelluloses, such as sulfite pulps and hardwood sulfate pulps.
A temperature range of 101° C. can be used advantageously. In one particular application of the method according to the invention,
Oxygen gas is introduced into the system in the usual manner during the oxygen gas/alkali treatment process, i.e. 1 during this stage.
It can be fed once, for example at one location during a continuous process. When pulp is bleached in a bleach tower at high pulp concentrations, such as 25 to 36%, the oxygen gas concentration is equalized and the oxygen gas is brought into constant contact with the pulp, and as the oxygen gas is consumed there be carried to. When bleaching pulp at low concentrations, e.g. on the order of 3 to 22%, the pulp/water mixture may be bleached during all or part of the oxygen gas/alkali treatment process to improve the transfer of oxygen to the liquid phase. It is often necessary to keep in motion. This movement of the pulp/water mixture is achieved by advancing means, such as stirring means, screws and scrapers, and/or by moving the bleaching device, for example by rotating the device. Oxygen gas is also mixed intensively with the pulp, preferably by introducing the oxygen gas into a rapidly rotating mixer, after which the pulp/water mixture is passed through a bleaching tower (e.g. can be reacted with pulp in between (inside up). In order to avoid oxygen gas starvation, supplying oxygen gas at two or more intervals or throughout the oxygen gas/alkali treatment process is appropriate and often necessary in the case of high delignification. be. In order to obtain a treated pulp with a high degree of selectivity in this process and at the same time with a high degree of whiteness, the oxygen gas pressure during the first stage of the oxygen gas/alkali treatment process can be adjusted to It is appropriate to maintain the pressure at a level below that of It is often appropriate to have a partial pressure of oxygen gas in the final stage of the process that is higher than the pressure during most of the first stages of the process. e.g. during the first stage of an oxygen gas/alkali treatment process, e.g.
Air at atmospheric pressure can be used for 30 minutes, after which oxygen gas is reduced to its partial pressure in the gas phase, e.g.
One way is to mix it into the pulp/water mixture to a pressure of 0.18 MPa, preferably 0.09 MPa or less. With these methods, the time required to carry out the oxygen gas/alkali treatment process can be significantly reduced compared to treating the pulp with air at atmospheric pressure for the entire duration of the oxygen gas/alkali treatment process. . The aforementioned pressure at this stage refers to the pressure in the actual apparatus in which the oxygen gas/alkali treatment process is carried out. For pumps, high-power mixers and mixing equipment with pulp residence times of less than 60 seconds, or preferably less than 30 seconds, even higher oxygen gas pressures can be used without compromising selectivity and other benefits achieved by this process. is acceptable. Similarly, the process can continue with continuous oxygen gas bleaching at conventional conditions at high oxygen gas pressures. This is also applicable when the equipment for carrying out such a bleaching process is easy to install and the demands on the strength properties of the pulp are lower than the environmental demands. In many cases, it may be appropriate to subject the pulp to a hot alkaline treatment process after the activation step, without intentionally introducing oxygen-containing gas, before subjecting the pulp to said oxygen gas/alkali treatment process. found. This method is particularly suitable when the pulp is subjected to a further bleaching process after the oxygen gas/alkali treatment process. When the pulp is to be delignified to a high degree, especially when the pulp is subjected to an oxygen gas/alkali treatment process without washing after the hot alkaline treatment process.
The invention allows the use of simple equipment. Oxygen gas savings are also achieved when the pulp is not washed. Additional oxygen savings can be made when waste liquor is removed after the hot alkaline stage, for example by pressing and/or washing the pulp, but generally the pulp/water mixture is free of oxygen gas/alkali from the hot alkaline treatment stage. This variation had a less positive effect than the preferred embodiment, which was sent directly to the processing stage. The thermal alkali treatment step is at approximately the same temperature as the subsequent oxygen gas/alkali treatment step, preferably between 95 and
It is carried out at 125°C, and preferably between 101 and 120°C. Introduction of this hot alkaline treatment results in increased selectivity. In order to obtain good selectivity and at the same time high yield and high brightness of delignified pulp after oxygen gas/alkali treatment process,
The extent to which the pulp is delignified in the oxygen gas/alkali treatment step is at least 20%, preferably at least 25%, preferably 30%, calculated on the amount of lignin before said oxygen gas/alkali treatment process.
It should normally be between 80% and 80%. In order to reduce the extent to which cellulose is depolymerized and to minimize the loss of hemicellulose during the oxygen gas/alkali treatment process, alkali is progressively fed in batchwise or continuously as the process continues. It is appropriate to do so. This operating method is suitable for pulp density between 8 and 22.
%, suitably 10 to 20%, preferably 12 to 20%
It was found to be highly effective for specific cases held at 18%. In this case at least 50% of the alkali required for the delignification process is supplied before the start of the oxygen gas/alkali treatment process, or this percentage of alkali is
Suitably, it is allowed to exist for a period of minutes. Mixing of the alkali during the process can be carried out during or after mixing the oxygen-containing gas with the pulp/water mixture. Regardless of whether or not alkali is supplied during the process, the pH of the bleaching waste solution can be adjusted to between 9.0 and 12.5 by checking the pH continuously or intermittently.
Suitably 9.5 to 11.5, preferably 10.0 to 11.5
11.2 should normally be maintained within the range. This value relates to measurements carried out at room temperature. Pulp treated according to the invention can be used directly for paper production. However, it is often preferred to subject the pulp treated according to the invention to a final bleaching step using environmentally harmless bleaching agents such as chlorine dioxide, peroxide and ozone. Benefits A number of benefits are achieved by delignifying chemical cellulose pulp in accordance with the present invention. A significant factor of the invention is that the process according to the invention is environmentally friendly and highly selective, making it possible to save energy compared to conventional bleaching methods. . When comparing the process according to the invention with the conventional delignification process in which the cellulose process is bleached with chlorine followed by an alkaline step, it will be seen that the process according to the invention is more environmentally friendly. When applying the method according to the invention, the waste liquid can be recovered, evaporated and the cooking waste liquid can be burned in one place in a common soda recovery boiler without any trouble. This is not possible with chlorine bleaching. When compared to oxygen gas bleaching without activation with nitrogen oxides, the method according to the invention exhibits even higher selectivity, i.e. it is possible to remove larger amounts of lignin from cellulose while maintaining a high viscosity. Make it. Compared to a hot alkaline treatment process followed by treatment of cellulose pulp with nitrogen dioxide, while maintaining a high pulp yield and using less alkali when applying the method according to the invention,
It is possible to delignify the pulp to an even higher degree. Thus, while pine sulfate pulp with a Kappa number of 31 can be delignified in one step according to the present invention to a Kappa number of 8 with a carbohydrate loss of 2 to 2.5%, alternative nitrogen dioxide treatment processes and The thermal alkaline process loses about twice as many carbohydrates in one step to reach the same Katsupa number. When applying the method according to the invention, the whiteness according to ISO at this Katsupa number
55 is obtained, but when the pulp is treated with nitrogen dioxide in the same manner and subjected to a subsequent hot alkaline treatment process, an ISO brightness of 35 is obtained. The pulp treated according to the invention was yellow in color, while the pulp treated according to the reference method was gray in color. This is reflected in the color measurements performed by Tappi standard method T524om-79. When implementing the invention, values (CIE) L* = 86.6, a* = -2.0, b* = 27.0 were obtained at Katsupa number 10, while the reference method gave values 74.2, 1.4 and 24.0. One particular advantage achieved by the present invention over current technology is that the oxygen gas/alkaline delignification process is carried out in simple equipment at low oxygen gas pressures, and that the selectivity is higher than that of common conventional oxygen gas. This is much higher than using a gas partial pressure of 0.5 to 0.8 MPa. Bleaching the pulp to a lower lignin content than previously possible, despite the use of low oxygen gas pressures, without the cellulose being degraded to an extent that would significantly affect the properties of the paper made from the pulp. What can be done is especially surprising. One advantage achieved by the method according to the invention is that air can be used fully or partially during the oxygen gas/alkali treatment process, which is not possible with other techniques. or in any case inappropriate. In techniques other than those taught by the present invention, high purity oxygen gas produced by the Linde process is commonly used. The present invention uses low grade oxygen gas, e.g.
It contains from 15% to 15% of inert gas and makes it possible to advantageously use oxygen gas produced by other inexpensive methods. BEST MODE FOR CARRYING OUT THE INVENTION Several experiments according to the present invention were carried out in parallel with a number of comparative experiments. The manner in which these experiments were carried out and the results obtained will be apparent from the Examples below. Example 1 Industrial pine sulfate pulp having a Katsupa number of 32.1 and an intrinsic viscosity of 1215 dm ³ /Kg according to SCAN-C15:62 was washed with water and pressed to a pulp density of 36%. The pulp was fluffed in a Petshu radar and charged to the reactor at 60°C. The air in the reactor was vented therefrom. The temperature thereby dropped to 55°C. Gaseous nitrogen dioxide obtained by evaporation of liquid N ₂ O ₄ was charged to the reactor over a period of 4 minutes. This addition is per 100Kg of pulp.
This corresponded to 75 moles of _NO2 . After preparing nitrogen dioxide,
15 moles of oxygen gas was charged over 4 minutes. Reduce the temperature to 55 °C by rotating the reactor in a water bath.
I kept it. This treatment process continued for 120 minutes, calculated from the time the nitrogen dioxide feed was started. After activating the pulp for 120 minutes, the viscosity of the pulp decreased by 190 dm/Kg. The pulp was then washed with water, pressed, and impregnated with magnesium sulfate and sodium hydroxide while kneading. The oxygen gas/alkali treatment process was carried out in a rotating autoclave at a pulp concentration of 11% and a temperature of 105°C.
The experiment was carried out using a charged amount of NaOH of 8% and a charged amount of magnesium sulfate corresponding to 3% of Mg. The amount charged is calculated based on the dry weight of the pulp. When the gas atmosphere in the autoclave contained air (initial oxygen gas pressure 0.025 MPa and final value 0.018 MPa), a Katsupa number of 9.0 was obtained after 60 minutes.
The intrinsic viscosity of the pulp was 1039 dm ³ /Kg after treatment, compared to the value 1040 dm ³ /Kg obtained before the treatment process. In comparative tests using the same batch of activated pulp, the oxygen gas pressure was
Increased to 0.8MPa. The result was that a cutoff number of 9 was already obtained after 20 minutes. In this case the viscosity dropped to 970dm ³ /Kg. This experiment shows that the use of air at near atmospheric pressure during the oxygen gas/alkali treatment step resulted in better selectivity than treatments carried out with oxygen gas at pressures above atmospheric, as is usual in practice. Indicates that the Examples 2 to 4 The temperature in the activation stage was lowered to 52°C, and the pulp concentration and alkali charge amount in the oxygen gas/alkali treatment process were each reduced to 16%.
The pulp was treated in the same manner as described in Example 1, except that the When the gas atmosphere contained air, a Katsupa number of 9.0 was obtained after 30 minutes. 1160 after the activation step, whereas the viscosity is
It was 1105. When the oxygen gas partial pressure was 0.12 MPa, this Katsupa number was reached in 15 minutes. The viscosity was 1030. In a reference experiment conducted at an oxygen gas partial pressure of 0.7 MPa, the viscosity obtained at a Katsupa number of 9 was 956 dm ³ /Kg. The reaction time was 10 minutes. When the amount of NaOH charged was reduced to 3.0%, it took 120 minutes to obtain a Katsupa number of 9.0 at an oxygen gas pressure of 0.12 MPa. The viscosity was 1100. This experiment shows that the method according to the invention achieves higher selectivity than the reference method. No difference was observed in carbohydrate yield. It has been shown that the amount of alkali charged can be reduced without impairing selectivity by slowly increasing the oxygen gas pressure and the reaction time. Examples 5 and 6 A polysulfide pulp having a kappa number of 34.7 and a viscosity of 1272 dm ³ /Kg prepared from equal weight parts of spruce chips and pine chips was activated in the same manner as described in Example 1. The pulp was washed with water and at a pulp concentration of 29%, a temperature of 105° C., 0.3% Mg added as magnesium hydroxide and an alkali in an amount corresponding to 1.5% NaOH calculated on the dry weight of the pulp. , and subjected to an oxygen gas/alkali treatment process. When the pulp was treated with air, the partial pressure of oxygen gas decreased by about 20%, but in the reference experiment with pure oxygen gas, the oxygen gas pressure was not significantly affected. The results are shown in Table 1 below.

[Table] (Reference)
This experiment shows that oxygen gas/alkali treatment of pulp with air at low pressures has better selectivity than carrying out this step with oxygen at pressures that are actually normal, even at high pulp concentrations. Show giving. Examples 7 to 11 Katsupa number 34.9 and intrinsic viscosity made from spruce (20%) and pine mixed with hardwood (10%)
Sulfade pulp with 1231 dm ³ /Kg was treated with nitrogen dioxide and oxygen gas by the same method described in Example 1, except that the pulp concentration was 32%, the temperature was 53° C., and the total processing time was 55 minutes. It was processed with. After the activation step, the pulp had a viscosity of 1170 dm ³ /Kg. Washing and mixing of chemicals to a pulp concentration of 11% were carried out as described in Example 1, but the amount of alkali charged was lowered to 4.3%. The temperature was maintained at 114℃. The initial oxygen gas pressure was 0.085 MPa, and the final pressure was 0.07 to 0.08 MPa. 12%
Oxygen gas containing inert gases (nitrogen and noble gases) was used. In Examples 7, 8 and 10, oxygen gas was present at 114° C. for 1, 2 and 3 hours, respectively. In Examples 9 and 11, a thermal alkali treatment process in the absence of oxygen for 1 hour at the same temperature was included before the oxygen gas/alkali treatment step.
No washing was performed between alkaline stages. The inert gas in the reactor was replaced with oxygen gas, after which heating was continued directly. A and B have the same pulp density,
Reference experiment at temperature and alkali charge with only alkali treatment in the absence of oxygen gas and without addition of magnesium sulfate. The results obtained are shown in Table 2 below.

[Table] Table 2 shows that when carrying out the method according to the present invention,
Compared to the combination of NO ₂ /O ₂ activation and hot alkaline treatment, a significant reduction in Katsupa number is obtained, and the viscosity reduction is shown to be moderate compared to conventional oxygen bleaching. Furthermore, the table shows that thermal alkaline treatment immediately before the oxygen gas/alkali treatment process reduces the time required in the oxygen gas stage to obtain the same degree of delignification, and that for the same degree of delignification, compared to It shows that higher viscosities are obtained than when oxygen is present during the entire process. Pressure measurements showed that the hot alkaline treatment reduced consumed oxygen gas by at least 30%.
When comparing the carbohydrate loss using the same number of cutlets, there was not much difference between methods that did not include hot alkali treatment. This experiment shows that embodiments of the present invention in which the pulp is subjected to a hot alkaline treatment process after the activation stage and before the oxygen gas/alkali treatment stage achieve significant practical and economic benefits.

Claims (1)

[Claims] 1. Pulp produced by chemically digesting lignocellulosic material is in an activation stage.
Oxygen gas is supplied to the activation stage in such a way that the NO2 _- containing gas phase is contacted and the intermediately formed NO is utilized for the activation of the pulp, and the activation stage is carried out under harsh conditions, i.e. cellulose A process for delignification of cellulose pulp carried out at high temperatures such that some decomposition of the molecules occurs, wherein the pulp is then subjected to an oxygen gas/alkali treatment in a pulp/water mixture and during the oxygen gas/alkali stage. The method characterized in that the partial pressure of oxygen gas is maintained between 0.005 and 0.18 MPa, and the partial pressure of oxygen gas is maintained not to fall below a lower limit of the partial pressure during any part of the treatment process. . 2. The oxygen gas partial pressure is maintained between 0.015 and 0.09 MPa during the oxygen gas/alkali treatment, and the oxygen gas partial pressure is maintained such that it does not fall below a lower limit of the partial pressure during any part of the treatment process. The method according to claim 1, characterized in that the method comprises: 3. The oxygen gas partial pressure is maintained between 0.03 and 0.09 MPa during the oxygen gas/alkali treatment, and the oxygen gas partial pressure is maintained such that it does not fall below a lower limit of the partial pressure during any part of the treatment process. The method according to claim 1, characterized in that the method comprises: 4. During said activation step the temperature 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 more than 0.2 (t-50).
The method according to any one of claims 1 to 3, characterized in that the method is shorter than 660 to 16 (t-50) (where t is the temperature at that time expressed in degrees Celsius). 5. Claims 1 to 4, characterized in that during the oxygen gas/alkali treatment the pulp concentration is maintained between 8 and 22%, suitably between 10 and 20%, preferably between 12 and 18%. Either way. 6. In order to avoid a shortage of oxygen gas in the liquid phase, oxygen gas is charged twice or more during the oxygen gas/alkali treatment process, or throughout the entire period. Either way. 7. Claims 1 to 6, characterized in that in the first stage of the oxygen gas/alkali treatment, the oxygen partial pressure is kept lower than in the later and/or final part of said stage. Either way. 8 After the activation stage, the pulp is heated between 80 and 135°C;
Suitably 95 to 125°C, preferably 101 to 125°C
subjected to hot alkaline treatment at a temperature of 120°C,
8. A method according to any one of claims 1 to 7, characterized in that it is then subjected to an oxygen gas/alkali treatment process without intermediate cleaning. 9. The present invention is characterized in that the alkali necessary for delignification is supplied batchwise or continuously during the oxygen gas/alkali treatment to reduce cellulose depolymerization and minimize hemicellulose loss. Any method from range 1 to 8. 10. The method according to any one of claims 1 to 9, characterized in that the oxygen gas/alkali treatment is carried out in the presence of a waste liquid containing lignin residue.