FI59434C - DELIGNIFIERING OCH BLEKNING AV CELLULOSA MED SYRE - Google Patents

Description

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Patent · och registerstyrelsen Anaeiun utiagd oeh uti. * KrKt «n pubiicand 30.o4.81 (32) (33) (31) Pjryduttjr« uoMcwi — Begird priority 07.03.73 USA (US) 338862 Toteennäytetty-Styrkt (71) International Paper Company, 220 East 42nd Street, New York,

New York 10017, USA (72) Sunanda K. Roymoulik, New Delhi, India-India (lN),

Kenton J. Brown, Suffern, New York, USA (74) Berggren Oy Ah (54) Delignifiering and bleaching of cellulose with oxygen - Delignifier and cellulose treatment with cellulose

The invention relates to a bleaching and delignification process as set out in the preamble of claim 1.

For many years, pulp has been traditionally delignified and bleached using various chlorination treatments. These are sometimes referred to in the paper industry as CpE for the five-step CpEDED or six-step CpEHDED bleaching scheme steps. The use of chlorine gas is not cheap and the removal of chlorine residues and chlorine-containing by-products from wastewater requires an expensive chemical recovery system to reduce water and environmental pollution problems.

For several years now, it has been proposed to replace chlorine in oxygenation and bleaching with oxygen. Although numerous methods for delignifying and bleaching pulp with oxygen have been disclosed, such as in U.S. Patents 1,860,432, 2,926,114, 3,024,158, 3,274,049, 3,384,533, 3,251,730, 3,423,282 and 3,661,699, in French Patents 1 310 248 and 1 387 853 and in the articles Nikitin et al., Trudy Leningradshoi Lesotekb. Nickeskoi Akad. i.S.M. Korova (Transactions of the Leningrad Academy of Forestry), Vol. 75, p. 145-155 (1956), Vol. 80 pp. 65-75, 77-90 (1958) and Bumazh. Prom., Vol. 35, no. 12, p. 5-7 (I960). None of these are economically viable. Many of these require preservatives to prevent the depolymerization of cellulose, such as 2,594,334 magnesium carbonates, as disclosed in U.S. Patent 3,384,533, and to maintain the viscosity of the pulp. In addition to the fact that the use of such Kemi-cabbages causes deposits and peeling in the process equipment, the use of these substances has a considerable disadvantage because they cause increasing contamination. If contamination is to be avoided, expensive equipment must be installed to remove these protective substances from the wastewater.

The only delignification and bleaching method using oxygen that has been developed to be commercially significant is disclosed in U.S. Patent 3,660,225. However, this method requires the use of high pulp consistency and high pressures, which results in the use of expensive pressure equipment, as well as expensive dewatering and diffuser equipment. Another important disadvantage of this high pressure process, even outweighing the high investment costs, is that it causes considerable damage to the pulp (cellulose), which can only be controlled to some extent by the use of depolymerization preservatives such as magnesium carbonate. As discussed above, cellulose depolymerization inhibitors incur increased costs and require expensive recovery measures if these are to be removed from wastewater.

It is an object of the present invention to provide a useful continuous process for delignifying and bleaching pulp at low cost.

Another object of the invention is to provide a low cost continuous process for bleaching pulp without chlorination.

It is a further object of the invention to provide a process for delignifying and bleaching pulp in which it is not necessary to use preservatives for the depolymerization of cellulose, such as magnesium carbonate.

It is a further object of the invention to provide a low cost continuous process for delignifying and bleaching pulp with oxygen, whereby it is not necessary to use high-pressure equipment.

It is a further object of the invention to provide a process for the continuous delignification and bleaching of pulp using oxygen which can be carried out in normal bleaching towers in use.

The objects of the invention are implemented by a method according to the invention, the main features of which appear from claim 1.

The objects of the invention will become apparent to those skilled in the art from the following description of the drawings.

Fig. 1 shows a flow chart of an embodiment of the invention, 59434 3 Fig. 2 shows another embodiment of the invention, also as a flow chart, Fig. 3 shows a graph between the viscosity of bleached pulp and the Kappa number in bleaching, where the pressure during bleaching is gradually reduced during bleaching according to the invention.

It has been found that if an alkaline fiber stock is bleached at a low, less than 10% consistency with oxygen, and if bleaching is initiated under a slight overpressure, such as less than 9.5 2 kp / cm, the stock contains only a minimum of insoluble oxygen and gradually reduced. excellent bleaching results, expressed by a low Kappa number, without adversely affecting the viscosity. The result was unexpected. It was difficult to believe that delignification and bleaching could be performed satisfactorily without the use of high pressures and without causing an additional decrease in viscosity, which would mean an additional release of cellulose. This is accomplished without the use of cellulose release inhibitors such as magnesium carbonate. The invention thus brings many significant advantages: 1. Delignification and bleaching can be achieved without significantly damaging the cellulose and bleaching can be carried out economically. The process thus provides cleaner and more valuable cellulose for the paper and similar industries.

2. High oxygen pressure is not necessary, and therefore neither high pressure equipment can withstand.

3. Existing equipment can be used satisfactorily for bleaching.

4. The bleach solution can be easily regenerated by adding additional alkali such as sodium hydroxide and circulating the solution. Thus, unused chemicals can be reused in this continuous process and substances released into the environment are eliminated.

The process according to the invention uses an alkaline pulp having a low consistency, for example less than 10%, suitably 2-6% 3, preferably 3 - *>%. The alkali is added so as to raise the pH to between 9 and 14, suitably between 11.5 and 12.5. When sodium hydroxide is used, it is suitable to add 1 to 10 g per liter, or in other words 0.1 to 1.0% by weight of the fiber stock.

The alkaline pulp pulp is preferably mixed in a strong mixer so that no large oxygen bubbles remain in the pulp. It is not desirable for 59434 to have oxygen bubbles with a diameter greater than 1.6 mm in the stock. In the most preferred case, insoluble oxygen is not present in the fiber stock at all. Oxygen is usually added at 0.1-4% by weight of the fiber stock. The preferred maturing rate for conifers is 0.2-0.8% and for deciduous trees the best results are achieved with amounts of 0.2-0.4%. The formation of all oxygen bubbles of significant size should be avoided as they cause channeling which interferes with the flow of stock up through the tower, which in turn results in uneven bleaching with undesirable results. Larger bubbles also tend to accumulate and this must also be prevented. All insoluble bubbles should be broken up so small that they do not accumulate to a significant extent.

All insoluble oxygen bubbles larger than 1.6 mm in size are released from the system before the pulp pulp is taken to the bleach tower.

The mixing of oxygen into the fibrous stock can be performed by any high speed and high shear mixer or gas absorber. Such devices include e.g. Mixing Equipment Co. Inc. mixers Lighten's In-line mixer and Line-Blender. However, any high shear mixer can be used.

Alternatively, the alkaline fiber stock can be briefly subjected to a high-pressure oxygen pretreatment before being fed to a bleaching tower. It is desirable to use momentary pressures of up to 2 to 21 kp / cm, or 2-10 atmospheres. This pretreatment is carried out for a short time in a small pressure vessel, which does not significantly increase the cost of the equipment used.

During the bleaching treatment, the reaction temperature is preferably in the range of 70 to 120 ° C, with temperatures of 90 to 110 ° C being most preferred. If reaction temperatures exceeding 110 ° C are used, low pressure equipment must of course be used. Therefore, the maximum temperature used depends to some extent on the height of the bleaching tower or the initial pressure used. The temperature should not exceed the boiling point of the fiber stock at that pressure.

During bleaching, the pressure of the alkaline fiber suspension is gradually reduced, with a difference in initial and final pressure of at least about one atmosphere, with a maximum difference of about 10 atmospheres. This pressure difference during bleaching can indicate the pressure in the bleaching tower, although any method can be used to reduce the pressure gradually or continuously during bleaching. Thus, 5,59434 bleaches own a height of 91 m, cause an initial pressure of 9 »5 2 kp / cm, and a bleaching tower with a height of 12 m, causes an initial. 2 pressure 1.2 kp / cm. It is desirable not to use a bleaching tower with a height of more than 91 m with a minimum height of approximately 12 m.

The residence time of the fiber stock in the bleaching tower may vary depending on the system pressure and the desired bleaching weapon. Some pulps require stronger bleaching conditions than others. In general, times of 5 to 120 minutes are sufficient. For higher initial pressures achievable with a higher tower, times of 2-60 minutes can be used. When using a 12 meter tower with an initial difference in final pressures of approximately one atmosphere, times of 30-60 minutes are used, preferably approximately 40 minutes is sufficient.

One advantage of the invention is that your bleaching and delignification process does not require any additional equipment. The bleaching tower 8 may be an existing tower previously used in the pulp industry for bleaching using chlorination. These towers are relatively inexpensive because they do not require high pressure resistant structures and are therefore easy to manufacture.

With reference to Figure 1, one form of apparatus for carrying out the process will be explained below. The fiber suspension of the desired consistency is obtained by sufficiently mixing pulp, sodium hydroxide or other alkaline substance such as ammonia, sodium carbonate, etc., and preferably recycled lye due to the alkali mixer 1. This circulating liquor is used to economically utilize the alkali of the bleaching solution. The pump 3 transfers the pulp of the desired density to an oxidizer or mixer 4, which is a tank with a high-speed and high-shear mixer, such as a Lightnin * Mixer, for mixing and decomposing the oxygen into the alkaline pulp. Alternatively, the oxygen can also be first mixed with the hot alkaline circulating liquor from line 2, as shown in Fig. 1 in point 2a, and fed to the alkali mixer. The oxygen-treated pulp is then passed to a heat exchanger 5, where steam is introduced to raise the temperature to the desired value. The heated and oxygen-treated pulp is then alternatively passed to a momentarily pressurized pretreatment vessel 6, where the pressure is momentarily raised by means of oxygen. All insoluble and dissipated oxygen is then removed from the liquid by means of a degassing device 7, after which the gassed oxygen-treated alkaline pulp is transferred to the bottom of the bleaching tower 8. The alkaline and oxygen-treated pulp passes upward through the tower, as shown in the figure, and is given a suitable residence time to allow the desired bleaching and delignification to take place. Mixing of the fiber stock in the tower should be avoided. The initial pressure and the pressure difference during bleaching are determined by the height of the tower 8.

From the tower, the mixture is then transferred via pipe 9 to scrubber 10. Warm residual alkali liquid recovered from the first scrubber is collected in tank 11 and some of it is returned via line 2 to alkali mixer 1. The remainder is recycled to scrubber 10. The fiber is then transferred to scrubber 10. the washed pulp is then passed on to subsequent bleaching steps, such as chlorine dioxide bleaching. The wastewater from scrubber 14 is collected in a tank 15 »part of which is used for washing unbleached pulp and the rest is led via line 16 for use in washing pulp with the first scrubber 10.

The embodiment shown in Fig. 2 differs from that shown in Fig. 1 in that the alkali mixer 1 is provided with a heating jacket for raising the temperature of the pulp and in that the separate heat exchanger 5 and the pre-pressure chamber 6 are removed from the apparatus of Fig. 1. The oxidizer device 4 'in Figure 2 includes a Lighten tube mixer or other mixer that rapidly mixes oxygen with the pulp with its high shear force.

An unexpected advantage of the invention is shown in the drawing in Figure 3. The figure shows the relationship between pulp viscosity and Kappa number in two experiments, one of which continuously reduced the pressure during bleaching according to the invention (indicated by circles in the figure) (shows a pressure drop from 2.8 kp / cm to zero) and the other for comparison Panne is kept constant (shown in the figure in squares), with a constant pressure of 2.8 kp / cm. As shown in Figure 3, the graphs are straight.

Viscosity indicates the average degree of polymerization of the cellulose in the pulp sample, i.e., the average chain length of the cellulose. Thus, a decrease in the viscosity of the cellulose means a decrease in the degree of polymerization and indicates damage to the cellulose during bleaching. Excessive damage to the cellulose must be avoided because it indicates poor properties of the paper made from the pulp.

The kappa number is determined as the amount of potassium permanganate consumed by the pulp sample and indicates the amount of lignin contained in the pulp. The higher the Kappa number, the less pulp has been bleached and 7,594,334 delignified. By comparing the Kappa numbers of the pulp samples before and after the bleaching treatment, an idea of the amount of lignin removal that can be obtained can be obtained.

As shown in Figure 3, in the same Kappa number, the viscosity value obtained with bleaching where the pressure decreased is approximately 2-2.5 cent-poise higher than that obtained with constant pressure bleaching with the same pulp. Thus, bleaching according to the invention with a decreasing pressure causes less harmful degradation of cellulose. This indicates that the process of the invention causes substantially less damage to cellulose at the same degree of bleaching and delignification.

It has also been found that the process of the invention, which gradually reduces the pressure during bleaching and delignification, also causes less pulp loss at the same degree of bleaching and delignification than that normally caused by oxygen bleaching at constant pressure. This means that cellulose and hemicellulose are less damaged than can be expressed by reduced pulp loss. This is due to the fact that the process of the invention is more selective in lignin removal than constant pressure oxygen bleaching. In conclusion, it can be stated that the process according to the invention is more economical than a process which is otherwise similar but which is carried out at a constant pressure.

The following examples are provided to illustrate the invention. The examples are, of course, only to clarify the invention and not to limit it. In the following examples and throughout the presentation, the amounts are given in parts by weight, unless otherwise indicated.

Using the apparatus of Figure 1, except that the high pressure pretreatment vessel 6 was not included, numerous fiber suspensions with a consistency ranging from 2-3% were treated in accordance with the invention and first mixed with 0.1 N sodium hydroxide. The fiber suspension was then heated to boiling point. The oxygen gas was mixed into the heated suspension and pressurized to a pressure of 2.8 kp / cm and stirred vigorously in an oxidizer 4 equipped with a high shear mixer for a period of five minutes. Excess insoluble and immiscible oxygen was gasified by the gasifier 7 and the oxygen-treated pulp was passed to the bottom of the bleaching tower 8, which was approximately 27.5-29 meters high. The stock was allowed to rise without stirring through the tower so that it had a residence time of approximately 60 minutes. As the stock rose through the tower, the pressure on the stock 2 8 59434 decreased from 2.8 kp / cm to atmospheric pressure. When the stock was passed through the tower, no bubbles separated from it on the surface of the stock. The bleaching results are shown in Table 1.

table 1

Kappa- Whiteness Pin-whisk EL cost,

cP

Original unbleached pulp (hardwood) 1 * 1.0 32.4 25.05

Oxygen bleached pulp

Experiment 1 (consistency 2.0%) 7.4 53.2 15.55

Experiment 2 (consistency 2.0%) 8.0 52.5 15.15

Experiment 3 (consistency 3.0%) 8.3 50.5 15.47

The results given in Table 1 show that delignification and bleaching with normal hardwood sulfate pulp to be bleached can be bleached with oxygen in a simple tower, using low consistency in bleaching and without the use of depolymerization preservatives such as magnesium carbonate.

Example 2 The apparatus of Figure 2 was used, in which the pulp was slurried with 0.1 N concentrated sodium hydroxide to pH 11.5 and a consistency of approximately 3%, after which the pulp was heated to approximately 90 ° C. The oxygen was mixed in an oxidizer 4 'equipped with a Lighten tube mixer, model No. 4LBC, operating at a speed of 1700 rpm, to obtain oxygen in the slurry. Residual oxygen that did not decompose or dissolve in the stock. gasified in the apparatus from 7 stocks. The oxidized and alkali-treated pulp was then passed to the bottom of a bleaching tower 8, which was 25 meters high, and which caused an initial pressure of a bottom 2 of approximately 2.5 kp / cm. The fiber stock was allowed to rise slowly through the tower without being stirred, resulting in a total residence time of approximately 42 minutes. During the phase, no bubbles rising to the surface were observed.

The bleaching results are shown in Table 2.

9 59434

Table 2

Dissolving-leaf- Hardwood-paper-pulp pulp

Kappa number

Unbleached 6.2 15.8

Bleached, 3.3 10.4% decrease 47 34

Bleaching (EL)

Unbleached,% 43.2 28.5

Bleached,% 54.6 42.7% increase 26 49

Example 3

The apparatus shown in Figure 1 was applied, both with and without chamber 6. Softwood pulp having a consistency of 3 and pulverized with 0.1 N sodium hydroxide was subjected to the treatment according to the invention. The stock was heated to boiling point at atmospheric pressure. Oxygen gas was mixed into the heated stock and the mixture was pressurized to 2.8 kp / cm and stirred vigorously in an oxidizer 4 equipped with a strong shear mixer. The mixing step lasted about 5 minutes. In those experiments where chamber 6 was in use, the stock was subjected to an oxygen gas pressure of approximately 7 kp / cm for a period ranging from one minute to 30 minutes. Excess insoluble and non-dispersed oxygen was gassed out in the apparatus 7. The oxygen-treated pulp was then passed to a bleaching tower 8 at a height of approximately 27-29 m. The pulp was allowed to rise through the tower without stirring for a residence time of approximately 60 minutes. During the rising phase, the pressure on the fiber stock decreased from approximately 2.8 kp / cm to atmospheric pressure. No gas bubbles were observed at the top of the tower that would have separated from the fiber stock during bleaching. No depolymerization preservatives were used in bleaching. The bleaching results are shown in Table 3 below.

Table 3

Kappa Chapter Whiteness, EL

Original unbleached pulp

Oxygen bleached pulp, test 1 (no high pressure treatment)

Experiment 2 (high pressure treatment) 9.6 46.2 10 59434

Example 4

The experiment of Example 1 was repeated using a pulp consistency of 3% and using two test pulps, hardwood sulfate and softwood sulfate. The results of the experiment are given in Table 4 below.

Table 4

Original unbleached

Pulp Kappa Chapter Whiteness (EL) Viscosity

0.5 CED, cP

Lehtipuusulfaatti

Lot 1 13.7 34.0 25.05

Lot 2 16.2 34.7 36.90

pinewood

Lot 1 31.4 27.7 35.83

Lot 2 21.5 29.9 21.74

oxygen bleached

Pulp Kappa Chapter Whiteness (EL) Viscosity

0.5 CED, cP

Lehtipuusulfaatti

Lot 1 8.3 50.5 15.47

Lot 2 8.7 52.0 21.00

pinewood

Lot 1 15.2 33.2 15.00

Lot 2 11.4 38.4 12.50 The terms and terms used are intended only to describe and not limit the invention, nor are the use of such terms and expressions to exclude an equivalent or part of the method presented and described, but it should be noted that many variations are possible within the requirements.

Claims (15)

A continuous process for bleaching and delignifying cellulose, producing a pulp slurry having a concentration of not more than 10% in alkaline solution and this slurry is treated with oxygen at elevated temperature and at an initial pressure above atmospheric pressure, characterized by dissolving oxygen in the slurry and dispersing it thoroughly in such a way that no agglomerating bubbles ignite, that the slurry is subjected to an initial pressure of less than 9 atmospheres, after which the pressure is reduced slightly without the slurry undergoing any major stirring, a pressure gradient of about 1-10 atmospheres is obtained between the initial pressure and the final pressure. 2. A method according to claim 1, characterized in that the slurry is continuously introduced into the lower part of a vertical elongated tower, that the slurry is fed up through the tower without undergoing any major stirring, maintaining a pressure gradient of 1-10 atmospheres between the slurry. even the entry point in the lower part of the tower and the point in the upper part of the tower at which the slurry is withdrawn, and that treated slurry is continuously withdrawn from the upper part of the tower. 3. A process according to claim 2, characterized in that substantially no soluble acid is present in the slurry as it is introduced into the tower. Method according to claim 2, characterized in that the acidified slurry introduced into the tower is free from