FI73019B - FOERFARANDE FOER DELIGNIFIERING / BLEKNING AV CELLULOSAMASSA. - Google Patents

Description

1 73019

A method of removing lignin from a cellulosic pulp to bleach the pulp

The present invention relates to a process for removing lignin from cellulose pulp and / or bleaching the pulp. By cellulose pulp is meant a cellulosic material made from a lignocellulosic material such as wood, straw, bagasse, flax and cotton. The invention can be advantageously applied to a chemical cellulose pulp, i.e. a pulp prepared by alkaline cooking or sulphite cooking of a lignocellulosic substance. Examples of alkali-cooked pulps are sulphate pulp, polysulphide pulp and soda pulp (i.e. pulp developed with the aid of sodium hydroxide and possibly with the addition of oxidation-reduction catalysts). The invention can also be applied to pulps with a higher yield than a chemical cellulosic pulp, for example a semi-chemical pulp.

The lignin removal / bleaching of the cellulosic pulp (hereinafter referred to as bleaching) in several steps is already known. The treatment carried out in the first part of the bleaching series (lignin-removing bleaching) is intended to remove most of the remaining lignin and does not significantly increase the brightness. The speed of processing (brightness-enhancing bleaching) results in an increase in the actual brightness of 25. Lignin-removing bleaching is performed using chlorine, chlorine dioxide or mixtures thereof as bleaching chemicals, while oxidizing chemicals such as hypochlorite, chlorine dioxide and peroxide are used in brightness-enhancing bleaching.

30 The production of chlorine is considerably cheaper than that of chlorine dioxide in terms of energy demand per amount of active chlorine, but chlorine reacts with the carbohydrate moiety of lignin and cellulose pulp and produces toxic substances such as chlorinated hydrocarbons and chlorinated lignin, which are discharged into the water. Chlorine dioxide reacts only with lignin without significant formation of organochlorine compounds and effect on cellulose and hemicellulose. Mixing chlorine dioxide with chlorine reduces the effect on the carbohydrate portion of the cellulose pulp.

In so-called displacement bleaching, a bleaching chemical, e.g. chlorine, is displaced after a certain time with water or an alkali solution, after which this liquid is in turn displaced by another bleaching chemical solution, e.g. containing chlorine dioxide, which simplifies the apparatus.

All bleaching chemicals have both advantages and disadvantages.

Until now, chlorine and chlorine dioxide have not been able to be used optically in the prior art bleaching chemicals, i.e. in such a way that the advantage of the bleaching chemical in question is exploited while its disadvantages are eliminated.

The present invention describes the optimal utilization of the above bleaching chemicals. The invention relates to a process for removing lignin from cellulose-20 pulp and / or for bleaching the pulp with chemicals chlorine and chlorine dioxide, wherein chlorine dioxide is first added to the cellulose pulp in one and the same lignin removal / bleaching step and chlorine after some time. The process according to the invention is characterized in that chlorine is added to the cellulose pulp successively in one or two steps during the lignin removal bleaching process and that the acidity of the cellulose pulp before and / or during the lignin removal / bleaching step is adjusted so that the pH of the cellulose pulp is 3.5-6.0 suitably 4.5-5.5, preferably 4.75-5.25.

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The bleaching agent according to the invention can be placed at any point in the bleaching series, i.e. at the beginning, in the middle or at the end thereof. It is particularly advantageous to place the bleaching step according to the invention at the points where; in which chlorine dioxide steps have previously been used.

It is also possible to replace the chlorine steps with a bleaching step according to the invention, e.g. as the first bleaching step in a bleaching series. The process according to the invention is applicable to cellulose pulps of varying precipitation. However, the cellulosic pulp should contain water 10 and preferably be surrounded by water. According to a preferred embodiment of the invention, the cellulose pulp is in the form of a suspension with a consistency of 2-30%, preferably 2.5-12%.

The cellulosic pulp usually enters and leaves the wash filter black as a uniform pulp web with a consistency of 11-16%. The pulp web is usually allowed to fall down into the screw where it is comminuted.

The pulp suspension is then introduced into an apparatus where chlorine dioxide is mixed as an aqueous solution to give a consistency of about 6-12%. The total amount of active chlorine as well as the amount of chlorine dioxide added to the pulp suspension are primarily dependent on the lignin content of the cellulose pulp and may therefore vary considerably; considerably. Usually, the total contribution of active chlorine, i.e. chlorine and chlorine dioxide, is 0.2-10%, based on 25 absolute dry mass. The amount of chlorine dioxide to be added to the above equipment is 30-90%, suitably 40-85%, preferably 50-70% of the total amount of active chlorine added to the step. The pulp suspension is allowed to react with chlorine dioxide for a period of time. This residence time can be achieved by passing the pulp suspension through a line of a certain length and / or storing the pulp suspension in some kind of container, e.g. a small bleaching tower. The pulp is then transported to a chemical mixing point, where 35 chlorine, usually in aqueous solution, is added. The time between the addition of chlorine dioxide and the first addition of chlorine can vary considerably, e.g., depending on the temperature of the pulp suspension. However, it is - preferred that the first partial addition of chlorine is carried out within 30 minutes, suitably within 15 minutes, preferably within 5 minutes after the addition of chlorine dioxide. The pulp suspension is then passed on to another chemical mixing point where more chlorine is added. The period between the two chlorine additions may be the same as those mentioned above, but need not be the same.

According to a preferred embodiment of the invention, the pulp suspension is conveyed to still other chemical mixing points where chlorine is added. The chlorine addition is thus divided into at least two points, and the amount of chlorine to be added at each point 10 must be 10-25%, suitably 15-25%, preferably 20-24% of the amount of active chlorine accompanying the pulp suspension and not used at the point of addition. The amount of unused active chlorine in the pulp suspension is determined by analysis according to one of the known methods. Because the consumption of bleaching chemicals is higher at the beginning of the bleaching stage, most of the chlorine is added during the first half of the bleaching stage.

The temperature of the bleaching stage is linked to and depends on several parameters, for example the location of the bleaching stage according to the invention, the total addition of active chlorine and the time. Usually it is 20-90 ° C, suitably 30-70 ° C, preferably 40-60 ° C.

The pH of the pulp suspension when introduced into the bleaching step according to the invention varies mainly depending on the previous treatment. For example, the bleaching step according to the invention is not always preceded by a washing step, but the cellulose pulp can be transported directly from the chemical treatment step to the bleaching step according to the invention. Therefore, it is extremely important for a good bleaching result according to the invention that the acidity of the incoming cellulose pulp is monitored and adjusted so that the pH of the cellulose pulp at the end of the bleaching step according to the invention is between 3.5-6.0, suitably 4.5-5, 5 and preferably 35 4.75-5.25.

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Although the above is a preferred embodiment of the invention, it is also possible to monitor and adjust the acidity of the cellulosic pulp during the bleaching stage. If the addition of alkali is necessary, the addition can be carried out at one or more points 5 before the bleaching step and / or at an early stage of the bleaching step.

The invention is characterized, as can be seen from the above, in that chlorine dioxide is first added to the cellulose pulp and then chlorine in succession at two or more points in the casting step. Chlorine dioxide means not only pure chlorine dioxide but also mixtures of chlorine dioxide in which chlorine dioxide is the predominant chemical. The production of pure chlorine dioxide is known to be difficult. Technical chlorine dioxide 15 usually contains a certain amount of chlorine, and although the proportion of chlorine is always about 30%, this mixture is included in the term chlorine dioxide. Similarly, in addition to pure chlorine, chlorine also means mixtures of chlorine in which chlorine is the predominant chemical.

The addition of chlorine dioxide and chlorine to the pulp suspension is as follows.

Chlorine dioxide in aqueous solution is passed to a mixing plant where it is mixed into a pulp suspension. The mixing equipment is called a mixer. These can be divided into two types, namely a dynamic mixer containing one or more moving parts and of the pin mixer type and a static mixer containing no moving parts. Both types of mixers can be used to mix chlorine dioxide into a pulp suspension in accordance with the invention.

Chlorine can be added to the pulp suspension as a whole gas or aqueous solution. Aqueous chlorine is more preferred. The equipment described above can also be used for mixing chlorine into a pulp suspension. It is important that the mixture of chlorine and mas-35 sasuspension is strong. According to a preferred embodiment of the invention, the chlorine is mixed into the pulp suspension by means of a long tube through which the pulp suspension is pumped. The tubes are provided on the inside with fixed spacers placed at a certain longitudinal distance from each other. Immediately in connection with the spacers, more specifically shortly thereafter, wires for the supply of chlorine are connected in the conveying direction of the mass-5 suspension. When the pulp suspension is pumped through the tube, it is subjected to a strong vortex motion as it passes the spacers. It is at these points that chlorine is added as water-soluble. It is particularly advantageous if the chlorine is mixed with the cellulose pulp into the next aqueous solution by separating into part the cellulose pulp and the liquid. This can be achieved, for example, by placing narrower pipes with slits or other holes in the jacket surface in a thicker longitudinal pipe at the points where the chlorine supply lines are connected. A liquid but not a cellulosic pulp is pressed through these holes, and a chlorine solution is introduced into this liquid, after which the cellulosic pulp and the liquid are again mixed after passing the previously mentioned spacers.

Bleaching of cellulose pulp according to the invention provides a number of advantages.

Much of the chlorine dioxide previously used in the bleaching of the various bleaching steps can be replaced by the cheaper bleaching chemical chlorine without increasing the amount of organically bound chlorine (i.e., the most toxic substance) in the bleaching effluent. This means that the economically very important link between the use of chlorine and sodium hydroxide in the bleaching step can be advantageously observed. Too little use of chlorine 30 results in low sodium hydroxide production relative to demand in the pulp industry, which in turn leads to more expensive sodium hydroxide and higher total manufacturing costs for bleached 17,701 9 pulp.

The amount of chlorine dioxide added is better utilized, probably because the addition of chlorine according to the invention acts against the decomposition of HClO 2 to HClO and HClO 2. The treatment temperature and time can be reduced compared to conventional chlorine dioxide bleaching due to the higher efficiency (high reaction rate) of the bleaching process of the invention. The reduction in temperature means an energy saving of 10 and a shorter time means that, for example, the residence volumes in the form of bleaching towers become smaller, which reduces investment and operating costs.

Figure 1 shows a flow chart of an embodiment of the method according to the invention.

Experiments have been carried out with the method according to the invention, and the results obtained are apparent from the following working examples.

Example 1 A pine sulphate pulp with a kappa number of 3.0 and a viscosity of 995 dm / kg was bleached partly with chlorine dioxide according to conventional techniques and partly with chlorine dioxide and chlorine according to the invention.

. : When bleached with chlorine dioxide alone (O-test), 417 g of a pulp suspension with a consistency of 12% were weighed in 25 bleaching vessels. 778.7 g of water were added. The mixture was heated in a water bath to 40 ° C. 54.3 ml of a chlorine dioxide solution with a concentration of 15 g of active chlorine / liter were added. The bleach was mixed into the pulp suspension by means of a propeller mixer for 2 minutes from the moment of addition.

30 bleach cans were kept in a water bath for 30 minites.

The bleaching was then stopped, and the pH and residual chlorine content of the pulp suspension were determined. Sheets were made from the washed cellulosic pulp for brightness determination.

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In the bleaching according to the invention, 417 g of 12% cellulose pulp were also weighed in a bleaching vessel, and 720.8 g of water were added thereto,

The mixture was tempered in a water bath to 40 ° C. 0.1 g of alkali was added to the pulp suspension in order to raise its pH somewhat. With vigorous stirring with a propeller stirrer, 33.3 ml of a chlorine dioxide solution having a concentration of 15 g of active chlorine / liter was added. Time measurement 10 was started immediately after the addition of chlorine dioxide.

After 2 minutes, 28.8 ml, after 4 minutes 26.3 ml and after 6 minutes 23.8 ml of a chlorine solution with a concentration of 4.0 g of active chlorine / liter were added with constant stirring. Stirring was continued for another 2 mi-15 minutes before being stopped. Bleaching continued until a total of 30 minutes had elapsed.

The pH and residual chlorine content of the pulp suspension were then determined. Sheets were made from the washed cellulosic pulp for brightness determination.

20 Conditions and results are shown in Table 1 below.

Table I O-test according to the invention

Pulp consistency,% 4.0 4.0 Temperature, ° C 40 40: 25 Time, minutes 30 30

Addition, chlorine dioxide,% active chlorine 1.63 1.0)) 1.63: Addition, chlorine,% 0.63): End pH 2.9 4.8 Residual chlorine,% active chlorine 0.48 0.13 30

Brightness,% ISO 68.8 75.0 3

Viscosity, dm / kg 965,950

Organically bound chlorine in bleaching sludge, mg / kg organic matter 13 14 9 7301 9

The addition of bleaching chemicals was calculated as a% of the absolute dry mass, the brightness was determined according to SCAN-C 11:75, the viscosity was determined according to SCAN-15: 62 and the amount of organically bound chlorine was determined after extraction with petroleum ether.

As can be seen from the table above, the bleaching process according to the invention is considerably more efficient than conventional chlorine dioxide bleaching, which is manifested in a much higher brightness of the cellulose pulp and a much lower residual chlorine content in the pulp suspension. Despite the large difference in brightness between the O-test and the pulps according to the invention, the viscosity of the pulp according to the invention is only negligibly lower than the viscosity of the pulp according to the O-test. It is further shown that the amount of organically bound chlorine in the bleach waste liquor is essentially the same in both experiments, interrupted by passing the pulp through line 19 to a washing filter (not shown). The temperature of the pulp suspension was 42 ° C throughout the bleaching step. After the last chlorine addition, the consistency of the suspension was 4.2%.

The acidity precipitate of the pulp suspension coming through line 1 was adjusted so that the pH of the pulp suspension at the end of bleaching was 5.2.

; The distribution of chlorine addition is shown in Table 2 below.

25

Time since start of operation Calculated amount of chlorine added. ... of absolute dry mass in minutes 0 '6 3 0,180 4 0,135 5 0,095 6 0,075 30 7. 0.064 8 0.052 9 0.042 10 0.037 11 0.030 12 0.027 13 0.023 14 0.021 15 0.020 10 7 301 9

In both bleaching experiments, the amount of unused active chlorine remaining after bleaching was determined, and the washed pulp was made into sheets, the brightness of which was determined according to SCAN-C II: 75. The measured values are shown in Table 3 below.

5 O-test * According to the invention

Residual chlorine residue,% 0.53 0.10

Brightness ISO,% 69.2 75.3

The above values show that the bleaching process according to the invention is much more efficient than conventional chlorine dioxide bleaching, which is understandable from the lower residual chlorine content of the pulp suspension and the higher brightness of the pulp.

^ -5 Example 3

The same birch sulphate pulp as in Example 2 was reprocessed according to the invention and according to the flow chart shown in Figure 1.

In these experiments, the addition of alkali to the pulp suspension coming through line 1 2 ° was varied so that a pulp suspension was reached at the end of bleaching. five different pH values.

The bleaching conditions are shown in Table 4 below.

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Added chlorinated

Time from start of bleaching: f8 * ® in minutes of absolute dry mass%

Table 4 4.2 5 Mass consistency,% Temperature, ° C 42

Time, minutes 30

The acidity of the pulp suspension is 2.7, 3.5, 4, G

at the end of bleaching, pH 5.0 and 6.0

Addition of chlorine dioxide, active chlorine,% 1/0 10

Chlorine addition,% 0.6

Chlorine was added according to the following scheme with respect to time from the start of bleaching in minutes and amount in% 3 0.160 A 0.120 5 0.080 ,, 6 0.060 10 1 0.049 8 0.037 9 0.027 10 0.022 11 0.015 1.1-- - ------ —12— - - & 7o: 2 “~ 13 0.008 20 14 0.006 15 0.005

After bleaching, the amount of unused active chlorine remaining in the pulp suspension was determined, and the washed pulp was made into sheets, the brightness of which was determined according to SCAN-C11: 75. The measured values are shown in Table 5 below.

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PH of the pulp suspension at the end of bleaching 2.5 3.5 4.6 5.0 ¢ .0

Residual amount of active chlorine% 0.20 0.22 0.23 0.24 0.24

Brightness ISO, °° 70.0 73.0 75.0 75.6 75.9 5 As can be seen from the above values, the residual chlorine content is low in all experiments. The brightness of the pulp is particularly high if the pH of the pulp suspension at the end of bleaching is between 3.5 and 6.0.

Example 4 A birch sulphate pulp with a kappa number of 4.5 and a brightness of 50.9% ISO was treated partly according to conventional chlorine dioxide bleaching, i.e. by adding all the chlorine dioxide at the beginning of the bleaching step, and partly according to the invention, i.e. according to the flow chart shown in Figure 1.

The purpose of the experiments was to achieve the same brightness in the bleached pulps. In the experiment using conventional chlorine dioxide bleaching (Experiment 0), the consistency of the pulp was 6%, the temperature 70 ° C and the time 180 minutes. The amount of chlorine dioxide added was 2.8% of active chlorine, calculated on the absolute dry mass. PH of the pulp suspension; at the end of bleaching was 2.3.

In the experiment according to the invention, the consistency of the pulp was 4.2%, the temperature 50 ° C and the time 30 minutes. The pH of the pulp suspension at the end of bleaching was 5.8. At the beginning of the bleaching, chlorine dioxide was added in an amount of 0.93% of active chlorine, calculated on the absolute dry mass. Chlorine was added at 0.8% of the absolute dry mass. Chlorine was added according to the scheme shown in Table 6 below.

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Table 6

Time from start of bleaching Amount of chlorine added in minutes on the basis of absolute dry mass% 2 0,210 5 3 0,162 4 0,188 5 0,096 6 0,084 7 0,052 8 0,042 10 9 0,037

After bleaching, the amount of unused active chlorine remaining in the pulp suspension was determined, and the washed pulp was made into sheets, the brightness of which was determined according to SCAN-C11: 7-5. The measured values are shown in Table 7 below.

Table 7 _. ......

O-test according to the invention

Active residue,% 0.20 0.08 2q Brightness ISO% 77.3 77.5

As can be seen, the same brightness has been achieved in bleached pulps. This has been achieved despite the fact that the bleaching conditions have been very different. In the process of the invention V 25, the addition of chemicals has been lower, the temperature lower and the time shorter compared to the conventional chlorine dioxide bleaching process. This means vastly better economy. The low acquisition and operating costs are true, although the savings are somewhat affected by the acquisition of equipment similar to, for example, indicated by the number 5 in Figure 1, for example, in a small volume, which means low acquisition and operating costs.

Example 5

Birch sulphate pulp with a kappa number of 4.5 and a brightness of 50.9% ISO was treated in three different ways, partly according to conventional chlorine dioxide bleaching (O-test), partly by adding chlorine dioxide at the beginning of the bleaching stage, partly by adding chlorine dioxide at the beginning of the bleaching stage followed by chlorine addition (D / C), and finally according to an embodiment of the invention.

In Experiment 0, the consistency of the pulp was 4.2%, the temperature 50 ° C and the time 30 minutes. The amount of chlorine dioxide added at the beginning of bleaching was 2.1% of active chlorine, calculated on the absolute dry mass.

In an experiment in which chlorine dioxide was added first and then chlorine (D / C), the consistency of the pulp was 4.2%, the temperature was 50 ° C and the time was 30 minutes. The amount of chlorine dioxide added at the beginning of bleaching was 1.9% of active chlorine, calculated on the absolute dry mass. After a casting time of 2 minutes, 0.21% of chlorine, based on the absolute dry mass, was added.

In the experiment according to the invention, the consistency of the pulp was 4.2% at a temperature of 50 ° C and a time of 30 minutes. The amount of chlorine dioxide added at the beginning of the bleaching was 1.13% of active chlorine based on the absolute dry mass. After a bleaching time of 2 minutes, 0.21% of chlorine based on the absolute dry mass was added, and after a further bleaching time of 1 minute, 0.16% of chlorine based on the absolute dry mass was added. The pH of the pulp suspension in bleaching was adjusted to 5.0 by adding a relatively small amount of alkali.

At the end of bleaching, the amount of unused active chlorine remaining in the pulp suspension was determined in various experiments, and sheets were prepared from the washed pulp, the brightness of which was determined according to SCAN-C 11:75.

Table 7 shows the bleaching conditions and the results obtained.

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Table 3 O-test D / C According to the invention Number of starting mass 4.5 4.5 4.5 "brightness, ISO% 50.9 50.9 50.9

Mass consistency,% 4.2 4.2 4.2 5 Temperature ° C 50 50 50

Total bleaching time, minutes 30 30 30

Chlorine dioxide addition at the beginning 2.10 1.90 1.13% Akti ivichlor

Chlorine added after 2 min,% - 0.21 0.21 10 "3 min% - - 0.16 pH at the end of bleaching 2.3 2.2 5.0 Residual amount of active chlorine,% 1.20 0.80 0.20

Brightness, ISO% 67.0 69.0 74.0 15 Active chlorine used,% 0.90 1.31 1.30

As can be seen from the results shown in Table 8, the method according to the invention has a special position in terms of improving the brightness of the pulp. If the D / C-bleaching, i.e. the addition of chlorine dioxide at the beginning of the bleaching and the subsequent addition of chlorine, is compared with the O-test, i.e. the conventional chlorine dioxide bleaching, a brightness difference of 2 units is observed in favor of the D / C method. However, this improvement has occurred at the expense of significantly increased active-chlorine consumption. If the process according to the invention is examined, it is found that the brightness is still improved by 5 units, despite the fact that the consumption of active chlorine does not exceed the consumption according to the D / C method. recovered in a completely different way than in the other two methods.

Claims (5)

16 7301 9 A method for removing lignin from cellulose pulp and / or bleaching the pulp with chemicals chlorine and chlorine dioxide, wherein chlorine dioxide is first added to the cellulose pulp in one and the same lignin bleaching step and chlorine after some time, characterized in that chlorine is added to the pulp pulp in one or more and that the acidity of the cellulose pulp before and / or during the lignin removal / bleaching step is adjusted so that the pH of the cellulose pulp at the end of the lignin removal / bleaching is 3.5-6.0, suitably 4.5-5.5, preferably from 4.75 to 5.25. Process according to Claim 1, characterized in that the cellulose pulp is in the form of a suspension with a pulp consistency of 2 to 30%, preferably 2.5 to '12%. Process according to Claims 1 to 2, characterized in that the proportion of chlorine dioxide is 30 to 90%, suitably 40 to 85%, preferably 50 to 70%, of the total addition of active chlorine. Process according to Claims 1 to 3, characterized in that the first partial addition of chlorine takes place 30 minutes, suitably 15 minutes, preferably 5 minutes after the addition of chlorine dioxide and that most of the chlorine is added to the cellulose pulp during the first half of the lignin removal / bleaching step. Process according to Claims 1 to 4, characterized in that the amount of chlorine added to the cellulose pulp at each addition point is 10 to 25%, suitably 15 to 25%, preferably 20 to 24% by weight of the amount of active chlorine at the addition point.