NZ198925A - Method for bleaching cellulose pulp using nitrogen oxides and oxygen followed by an alkali treatment - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number 1 98925 f « 198925 PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATION "A METHOD FOR BLEACHING CELLULOSE PULP" ty We, M0 0CH DOMSJO AB, a company incorporated in Sweden, of Fack S-891 01 Ornskoldsvik, Sweden, hereby declare the invention for which I / we pray that a patent may be granted toXMe/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- (followed by page la) 198925 I c* " •;s v» v'C7 A method for bleaching cellulose pulp J A Technical field < ^ / The present invention relates to a method for bleaching chemical cellulose pulp, and more particularly for bleaching cellulose pulp prepared by alkaline , 5 cooking. Examples of alkaline pulps include sulphate pulp, polysulphide pulp and soda pulp. In the term soda pulp is included pulps digested with sodium hydroxide as the cooking chemical, in the presence of different additives. Examples of such additives axe redox catalysts, 10 such as anthraquincne. The invention can also be applied to other chemical cellulose pulps, for example sulphite pulp.

State of the art Clarke (Paper Trade Journal Tappi Sect. 118, 62 15 (1944)) has found that cellulose pulp can be partially delignified by treating the pulp in an aqueous suspension for from 1-1.5 hours at 90°C with nitrogen dioxide, followed by extraction at 90° C for 30 minutes, or at 50°C for 60 minutes at a 71 pulp consistency and an 20 alkali charge corresponding to 21 NaOH calculated on the dry weight of the pulp. The treatment results in a severe depolymerization of the cellulose, which is reflected in a very low viscosity by the treated pulp compared with pulp subjected to chlorination and alkali 25 extraction.

Bourit (French Patent Specification No. 2 158 873) avoids depolymerization, by applying a delignification process in which the pulp is treated with nitrogen dioxide at low temperature, preferably a temperature 30 below 20°C, and for a long period of time, followed by an alkali extraction under mild conditions. The cellulose pulp is only delignified to a very small extent, however, and the method does not afford any solution to existing environmental problems. 198925 i 2 A two-stage method in which the cellulose pulp is pre-treated with nitrogen dioxide followed by oxygen-gas bleaching has also been described, but although enabling a high degree of delignification the method causes a drastic lowering of the viscosity.

Disclosure of the invention The technical problem »■ The problem of finding a method by which cellulose pulp can be delignified to a high degree, without using chlorine or chlorine-containing bleaching agents, or with the minimum use of such agents, is one of the most urget in countries producing bleached chemical cellulose pulp. The effluent from conventional bleach plants contains toxic and mutagenic compounds. Although oxygen-gas bleaching partially solves the problem, the manner in which it is at present applied technically results in an approximately 50a delignif ication of the lignin present in the cellulose pulp after the cook, without degrading the cellulose to the limit at which the pulp affords an altogether low strength of the paper produced from the pulp. The viscosity of the cellulose pulp is suitably used as a measurement of degradation.

According to the SCAN-method the viscosity of the 3 bleached pulp should not be below 900 dm /kg.

Solution The aforementioned problems are solved by the invention in a totally surprising manner. The invention relates to a method for delignifying cellulose pulp produced by chemically pulping lignocellulosic material, whereat the pulp is contacted in an activating stage in the presence of water with a gas. phase containing and oxygen gas is supplied in an amount of 0.05-5 mole per mole of NC>2 supplied and 0.55-5 mole per mole of NO supplied, whereafter the pulp is subjected to an alkali treatment process, 3 1989 characterized by the combination of effecting both the activating stage and the alkali treatment process .under drastic conditions, i.e. at such a temperature and for such a time in the activating stage that a degradation of the cellulose molecules corresponding to a decrease in intrinsic viscosity of 2-35% is obtained, and at a temperature of 95-150°C and a time exceeding 45 minutes at the lower temperature limit in the alkali treatment process.

Changes in the intrinsic viscosity of the cellulose pulp are here used as a measurement of the extent to which the cellulose molecules have been degraded. The values given hereinafter have all been determined without removing lignin and hemicellulose, which is the most reproducible method for pulps with a moderate lignin content (for example with sulphate pulps having a kappa number below 35). It is necessary, however, to ensure that the extent to which lignin and hemicellulose contribute to the 'viscosity is very small in comparison with the same amount by weight of cellulose molecules, and that bleaching is intended to provide a decrease in the lignin content, and also to release hemicellulose, while losing very little of pure cellulose under the conditions employed. When depolymerization of the cellulose is negligible, the intrinsic viscosity of the pulp will increase. In the case of pine sulphate pulps of the kind, used in the majority of the following examples, a lowering of the kappa number by 10 units 3 results in an increase in viscosity of about 50 dm /kg under those conditions at which the depolymerization of the cellulose can be* ignored, while the corresponding viscosity increase with sulphite pulp and sulphate pulp produced from hardwood is noticeably greater, as a result of losing more hemicellulose.

- 'V * -1 JUWJ584 In order to check that some degree of degradation of the cellulose molecules has occurred during the activation stage, viscosity is measured in accordance with SCAN-C15:62, which measurements can be made on the activated pulp after washing the same with water and rapidly drying said pulp at 35°C, and then immediately determining the viscosity.

When carrying out the method according to the invention, the intrinsic viscosity of the pulp after activating said pulp and washing the same with water is the same as, or preferably at least 51 lower than the intrinsic viscosity of the pulp prior to said activation. The extent to which the pulp is delignified is still greater when the viscosity is lowered further, for example by more than 8%. For the majority of pulps, the extent to which the intrinsic viscosity is lowered during the activating stage may, to advantage, be from 2-35%, suitably 5-25%, preferably 10-20%. The activated pulp is readily depolymerized, for example, when employing excessive drying times, or excessively high temperatures during the drying period, and during storage. Consequently, it may not be suitable to determine the viscosity of the pulp immediately after activation as a routine analysis. No noticeable depolymerization of the cellulose takes place during the alkali treatment process under preferred conditions, there being obtained instead an increase in viscosity as a result of the removal of lignin and a certain amount of hemicellulose. The viscosity can be determined to a high degree of accuracy.

Consequently, it is suitable in practice to adapt the conditions during activation so that the pulp, subsequent to said alkali treatment process, has a viscosity which is lower than that which can be calculated with respect to the lignin and hemicellulose released.

In normal cases it is suitable for the alkali 198925 treated pulp to have an intrinsic viscosity according to SCAN which is lower than that of the unbleached pulp by 0-25%, suitably 5-20%, preferably 10-15%. These ranges primarily refer to softwood sulphate pulps. In the case of sulphite pulps and hardwood sulphate pulps, the preferred viscosity drop is half of the aforegiven values.

The nitrogen dioxide present in the activating stage is supplied either in the form of substantially »• pure NC>2, or is allowed to form in the reactor subsequent to supplying nitric oxide and oxygen thereto. NC^ plus NO can also be supplied to the reactor. Dinitrogen tetroxide is regarded as nitrogen dioxide (NC^) as are other forms of polymers. One mole of dinitrogen ■-15 tetroxide is calculated as two moles of nitrogen dioxide. Adducts in which nitric oxide is present are calculated in the same manner as nitrogen oxide. Thus, dinitrogen trioxide (^0^) is considered as one mole nitric oxide and one mole nitrogen dioxide. Adducts 20 with oxygen probably occur as intermediaries.

A certain amount of oxygen gas must be charged to the activating stage, both when supplying nitrogen dioxide (NO2) and when supplying nitric oxide (NO). In order to obtain the best possible results with the 25 simplest apparatus possible, it is suitable to supply oxygen to the activating stage in the form of a substantially pure oxygen gas. Liquid oxygen may also be charged to the reactor, said liquid oxygen being vapourized, for example, when entering the reactor in 30 which the activating process is carried out. The amount of oxygen charged to the activating stage should be at least 0.05 mole calculated as O2 per mole of NO2 supplied. In many cases a greater amount is used, suitably 0.1-5 mole O2 per mole of NO2 supplied, in 35 order to obtain the desired result. The best results #98925 in tested apparatus have been obtained when the amount of oxygen charged to the activating stage is 0.15-0.30 mole of C>2 per mole of NO2 supplied. NC^ in liquid form is a commercial product, which can be supplied to the process in this form. Conveniently, the nitrogen dioxide is vapourized prior to entering the reactor in which the activating stage is carried out, or in conjunction with the entry of the nitrogen dioxide into said reactor. NO2 can also be obtained by oxidizing NO with oxygen. NO can be produced to advantage by cat&lytical combustion of ammonia, this combustion process being effected, to advantage, in connection with the bleaching plant in which the method according to the invention is carried out. The gas phase containing NO2 can be obtained by reaction between oxygen and NO, prior to or during the activating stage. When calculated per mole of NO supplied, the total amount of O2 supplied should be at least 0.55 mole, suitably 0.6-5 mole, preferably 0.65-0.80 mole of O2, in order for both supplied and intermediate NO to be utilized for the activating process, and so that practically all NO and NO2 formed from NO has been consumed at the end of the activating stage., A mixture of NO and NO2 can also be supplied to the activating stage. The amount of oxygen supplied is, in this case, adapted to the amount of each of these nitrogen oxides according to those proportions given in the aforegoing, attention being paid to dimers, polymers and adducts in the aforedefined manner. The amount of the aforedefined nitrogen oxides (N02+N0) supplied to the process reaches in total to 3-300, suitably 10-150,. preferably 25-75 gram-mole calculated on 100 kg of dry cellulose pulp.

The total pressure in the activating reactor is suitably kept below atmospheric pressure, preferably close to atmospheric pressure. 7 198925 In order to obtain the best activating result and the least possible emission of nitrogen oxides from the reactor, it is important that as little inert gas, for example air, as possible is introduced when feeding the pulp into the reactor. It has been found that the amount of inert gas entering the activating reactor should be at most 50 gram-mole, suitably at most 20 gram-mole, preferably 0.05-5 gram-mole per 1000 kg of bone dry pulp. A desirable degree of cellulose depolymerization is obtained during the activating stage When the temperature is maintained at 50-90°C, suitably at 55-80°C. A lower temperature normally requires and un-realistically long treatment time, while an excessively high temperature results in excessive depolymerization of the cellulose. In the' case of many pulps, the preferred temperature range is 60-70°C. The retention time is adapted with respect to the amount of NO^ and/or NO charged, the activating temperature, the type of pulp being treated, and to the pulp concentration, and is estimated by determining the viscosity in accordance with the aforegiven methods. When working within the preferred temperature range of 50-90°C, the activation time should be greater than 10-0.2 (t-50) minutes and less than':660-16 (t-50) minutes, where _t is the temperature in °C. The activation time should lie in the vicinity of the lower limit with high pulp concentration and a high charge of N0£ and/or NO and is increased with low pulp concentrations and low charges. stage should be held at 16-501, although the higher limit may. be difficult to reach. The range most useful in practice is 22-40%, preferably. 27-35%. is washed, suitably with water and/or suitable aqueous solution. If this washing stage is excluded, much more The pulp consistency"' during the activation Subsequent to the activation stage, the pulp 8 198925 alkaline neutralizing agent will be consumed in the subsequent treatment of the pulp. Instead of water, or preferably subsequent to washing the pulp with water, it is convenient to treat the pulp with an alkaline 5 solution, for example spent bleach liquor.or spent liquor from the alkali bleaching according to the invention.

According to a preferred embodiment, the cellulose pulp is washed after the activating stage with watejr 10 and/or a diluted aqueous solution under conditions such as to obtain an acid solution, which is used to wash the pulp after the cook, preferably subsequent to displacing cooking liquor from the pulp with liquor obtained from an alkaline stage.

Irrespective of whether the pulp is washed after the activating stage with water or an aqueous solution, such that an acid solution is obtained, or whether this washing stage is omitted, it is suitable to also wash the pulp prior to the extraction stage 20 with an alkaline reacting solution, suitably at a temperature of 40-80°C.

The alkali used in the alkali treatment stage is preferably sodium hydroxide. According to a preferred embodiment of the invention, spent liquor from 25 the alkali treatment stage can be used in said alkali treatment stage, subsequent to replenishing said spent liquor with fresh alkali. The amount of sodium hydroxide charged is adapted to the amount of water present during said treatment stage, so that the concentration of free 30 sodium hydroxide reaches to 2-50, suitably 4-30, pre ferably 5-20 grams of free NaOH per kg of water present during the alkali treatment stage. When no waste liquor is recycled from the alkali treatment stage, and when the pulp is thoroughly washed with water after the 35 activating stage, the amount of free sodium hydroxide present corresponds to the total amount of sodium 198925 hydroxide supplied. When waste liquor is recycled, and when the pulp contains residual alkali from the alkaline washing stage, 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 introducing the alkali, and is determined by potentiometric titration with hydrochloric acid (HC1) to pH 9.

Other alkali can also be used, for example white liquor and oxidized white liquor. When the alkali charged contains sulphide, the sulphide is removed by precipitation prior to determining the free sodium hydroxide by titration. The pulp consistency, during the alkali treatment stage is normally held at 5-45%, suitably at 16-40%, preferably at 25-35%. High pulp concentrations are preferred with respect to heat economy. Recycling of used waste liquor to the alkali treatment stage results in a saving of both energy and alkali, and improves the recovery of organic substances, which are used as a fuel in a known manner.

The treatment time in the alkali stage has been found to have a great effect on the degree of deligpi-fication under otherwise similar conditions. In the case of pulps having a low content of carbonyl groups and hemicellulose, for example softwood sulphate pulp, the most suitable temperature range is normally 101-140°C, and often preferably 110-120°C. At a temperature of 101°C, the treatment time shall be at least 30 minutes, and at 110°C at least 15 minutes. A markedly improved delignification is obtained when the treatment time is extended within the temperature range 95-120°C to 60 minutes, or preferably 120 minutes. An increase to 4 hours results in a further lowering of the kappa number without any appreciable lowering of the viscosity. On the other hand, the treatment time should be short at 150°C, for example less than 15 minutes. In the temperature range 120-140°C, iJUNWMJ 198925 the retention time is suitably 15-120 minutes, preferably 30-60 minutes.

The temperature range of 95-101°C can be employed preferably with pulps having a high content of carbonyl-groups or hemicellulose, for example with sulphite pulps and hardwood sulphate pulp. At a temperature of 95°C, the treatment time shall exceed 45 minutes. Longer treatment times should be employed, particularly when wishing to reduce the amount of »• hemicellulose present. When this temperature range is employed with softwood sulphate pulp, the treatment time is preferably increased to at least 1 hour, suitably to at least 2 hours, in order to obtain a high degree of delignif ication!...

It is fully possible to use pulp which has been treated in accordance with the invention, i.e. in which the pulp is first activated inithe presence of nitrogen dioxide and then treated in an alkali stage without continued bleaching of the pulp. It is preferable, however, to subject the pulp, subsequent to treating said pulp in accordance with the invention, to a final bleaching process using preferably bleaching agents which are favourable to the environment, for example chlorine dioxide, peroxide and ozone.

Advantages Several advantages are obtained when subjecting chemical cellulose pulp to a delignifying bleaching process in accordance with the invention.

One characteristic of the bleaching process carried out in accordance with the invention is that the process is both favourable to the environment and extremely selective.

When making a comparison between the method according to the invention and conventional delignifying bleaching of cellulose pulp by means of chlorine, followed by an alkali stage, it is found that the method 198925 according to the invention is much more favourable to the environment. When practicing the method according to the invention, the waste liquors can be recovered and readily evaporated and burned together with cooking waste liquor in a conventional soda recovery boiler.

This is not_the case with chlorine bleaching.

When compared with oxygen gas bleaching, the method according to the invention exhibits a much higher degree of selectivity, i.e. much more lignin can be » removed from the cellulose pulp while maintaining a high viscosity.

When compared with the nitrogen dioxide treatment of cellulose pulp (with an oxygen gas charge) followed by an oxygen gas bleaching stage, the selectivety of the method according to the invention would often 3eem to be better. In addition hereto the cost of oxygen gas is avoided, and apparatus of simpler design can be used.

When compared with the nitrogen dioxide treatment of cellulose pulp (with an oxygen gas charge) followed by a conventional alkali stage, i.e. in the absence of drastic conditions in either the nitrogen dioxide stage or the alkali stage, the pulp can be delignified to a far greater extent when practicing the method according to the invention while maintaining a high viscosity. Further, there is obtained a higher pulp yield calculated at the same kappa number.

Preferred embodiment of the invention A plurality of tests according to the invention and comparison tests have been made. The manner in which these tests were carried out and the results obtained are made evident in the following examples.

Example 1 A technical, pine sulphate pulp having a kappa number 198925 3 31.3 and an intrinsic viscosity of 1220 dm /kg calculated according to SCAN-C15:62 was washed with water and pressed to a pulp concentration of 36.3%. The pulp was fluffed in a peg shredder and charged to a reactor at 65°C. The reactor was evacuated, to remove the air therefrom. In doing so, the temperature fell to 60°C.

Gaseous nitrogen dioxide, obtained by vapourizing liquid was introduced into the reactor for four minutes.

This charge corresponded to 86 mole of NC^ per 100 kg of pulp. Subsequent to charging nitrogen dioxide to*the reactor, 15 mole of oxygen gas were introudced for four minutes. The temperature was held at 60°C, by rotating the reactor in a water bath. The treatment was carried out for 120 minutes, calculated from the moment at which the supply of nitrogen dioxide was commenced. After being treated for 120 minutes, the viscosity of the pulp had fallen by 160 dm /kg. The pulp was then washed with water, pressed and impregnated with sodium hydroxide by kneading, without any solution being removed. The pulp concentration was held at 251 and the alkali charge at two levels. The pulp was then subjected to a hot alkali treatment, subsequent to removal of air by evacuation.

The heat was supplied by direct steam. In one test series the temperature was 101°C, and in another 111°C.

As in other tests carried out at lower pulp concentrations, variation in the conditions in the alkali stage had no significant effect on the viscosity of the alkali treated pulp. In tests in which the pulp was treated with nitrogen dioxide for 120 minutes, the 3 values lay at 1100 dm /kg. The kappa number of the pulps, and the pulp yields are given in Table 1. Table 1 also shows the results obtained with reference pulps produced by activation said pulps for 20 minutes at 40°C, with all other conditions unchanged. The viscosity of these pulps was only slightly higher (1120 dm /kg) despite the dramatically impaired delignification. 13 198925 The results are given in Table 1 below.

Table 1 Activation at min 60°C 40°C NaOH in the alkali Kappa Yield stage number % g/kg water 120 120 120 120 20 111 111 101 101 111 101 3.3 3.3 3.3 33 3.3 33 17.9 96.9 16.9 95.4 11.3 95.3 8.2 93.0 .6 94.8 7.8 92.5 The table shows that pulps having a surprisingly low lignin content can be produced by activation with NO2/O2 over a long period of time, followed by an alkali treatment process carried out under drastic conditions. The table also shows that compared with pulps of the same kappa number a higher yield is obtained when employing conditions according to the invention in the activating stage than when applying mild conditions. This was confirmed in separate tests.

Example 2 A spruce sulphite pulp having a kappa number 10.6 3 and an intrinsic viscosity of 996 dm /kg, measured according to SCAN-C15:62, was washed with water and pressed to obtain a pulp concentration of 36.5%. The pulp was fluffed in a peg shredder and introduced to a reactor at 65°C. The reactor was evacuated to remove the air therein. In doing so, the temperature fell to 60°G.

Gaseous nitrogen dioxide, obtained by vapourizing liquid ^O^, was charged to the reactor for a period of 4 minutes. This charge corresponded to 43 gram-mole of NO2 per 100 kg of pulp. Subsequent to supplying said

Claims (7)

14 198925 nitrogen dioxide, 8 gram-mole of oxygen gas were charged to the reactor over a period of 4 minutes. The temperature was held at 60°C, by rotating the reactor in a water bath. The treatment was continued for ' 120 minutes, 5 calculated from the moment at which the supply of nitrogen dioxide,was commenced. After the activation treatment process, the 3 viscosity of the pulp had fallen by 40 dm /kg. The pulp was then washed with water, pressed and impregnated 10 with sodium hydroxide by kneading, without any solution being removed. The pulp concentration was held at 15% and the alkali charge was 2.1 grams of NaOH per kg of water present. The temperature was 101°C and the treatment time 60 minutes. 15 From the aforedescribed tests, carried out in accordance with the invention, there was obtained a pulp having a kappa number 2.5 and a viscosity of 1040 3 dm /kg. For comparison, the aforedescribed tests were repeated, with the difference that the time over which 20 the activation step was carried out was lowered to 5 minutes. Thus, this test was not carried out in accordance with the invention. From this test there was obtained a pulp having the same viscosity as the pulp obtained in the test 25 carried out according to the invention, but with a kappa number of 4.3. Thus, drastic conditions during both the activating stage and the alkali stage result in a marked decrease in the lignin content, also in the case of a sulphite pulp, while retaining a noticeably 30 high viscosity. 15 CLAIMS Q^YVE CLAIM IS: 19892

1. A method for delignifying cellulose pulp produced by chemically pulping lignocellulosic material, in which method the pulp is contacted in an activating stage in the presence of water with a gas phase containing NCL, and oxygen gas is supplied in an amount of 0.05-5 mole 0^ per mole of NO^. .supplied and'D.55--5 mole per mole of NO supplied, whereafter the pulp is subjected to an alkali treatment process, characterized by the combination of effecting both the activating stage and the alkali treatment process under drastic conditions, i.e. at such a temperature and for such a time in the activating stage that a degradation of the cellulose molecules corresponding to a decrease in intrinsic viscosity of 2-351 is obtained, and at a temperature of 95-150°C and a time exceeding 45 minutes at the lower temperature limit in the alkali treatment process.

2. A method according to claim 1, characterized in that the temperature during the activating stage is held at 50-90°C and that the activating time lies between 10-0.2(t-50) and 660-16(t-50) minutes, where t is the temperature in °C.

3. A method according to claims 1-2, characterized in that during the activating stage the pulp consistency is maintained at 16-50%.

4. A method according to claims 1-3, characterized in that the amount of alkali at the beginning of the alkali treatment process is 2-50 g per kg of water present, calculated as free sodium hydroxide. 16 198925

5. A method according to claim 4, characterized in that the pulp consistency during the alkali treatment process is held at 5-45%.

6. A method according to claims 4-5, characterized in that spent liquor from the alkali treatment process is replenished with fresh alkali and then used in the alkali treatment process.

7. A method according to claims 4-6, characterized in that the alkali is sodium hydroxide. By Uis/thsir authorised Agents., A. J. PARK ^ SON. CLtdLLasnVQ