FI95292C - Pulp bleaching method - Google Patents

Description

x 55292

Pulp bleaching pulp 'Sätt att bleka pulp ·

The present invention relates to a method for bleaching pulp made from cellulosic fibrous material in the production of chemical pulp, wherein the incoming pulp is continuously fed in a bleaching line and bleached with a plurality of bleaching agents containing chlorine dioxide and ozone.

Chlorine is one of the most widely used bleaching agents in the bleaching of cellulosic fibrous material, especially chemical pulp. However, chlorine, which forms chlorinated organic compounds, is considered to be harmful to the environment and its use has thus met with increasing resistance. This has resulted in modified and lower emission limit values for chlorinated substances, defined in kilograms of adsorbed organic halogen (AOX) per tonne of pulp. Until the turn of the century, the level of 1.0 ... 2.0 kg AOX per tonne of dry pulp is likely to be approved by environmental authorities. Thereafter, levels of less than 1.0 kg AOX per tonne of mass shall be taken into account, or possibly earlier if technology allows.

In order to meet the increasingly stringent requirements for the use of chlorine, several methods have been proposed, such as reducing the lignin content of the pulp before chlorine bleaching by prolonging the cooking and performing a preliminary lignin removal with oxygen gas. Other methods have been to reduce the addition of chlorine or to replace chlorine with chlorine dioxide.

It is also known that, from an environmental point of view, ozone is completely superior to chlorine and chlorine dioxide in the bleaching of cellulose fiber compared to commercially used bleaching agents. However, given the high requirements for the quality of the pulp, in particular its brightness, purity and strength, it is currently not possible to completely exclude chlorine dioxide. However, the commercial use of ozone has so far not been possible due to the high cost of ozone production and the substantially weaker selective nature of ozone in the bleaching methods used so far. By weaker selectivity is meant that in addition to lignin, ozone significantly degrades cellulose. The number of pieces of pulp is a measure of lignin content, while its viscosity is a measure of the average chain length of cellulose and an indication of its strength. Bleached softwood pulp with a brightness of 90 ISO and good strength properties should have a viscosity of more than 800 SCAN units (dm3 / kg).

When continuing to bleach oxygen-bleached pulp with chlorine dioxide and / or chlorine, it is necessary to carry out four or five separate steps to bleach the pulp to a brightness that meets market requirements, ie about 90 ISO. By separate step is meant that the washing of the pulp precedes and follows the step. The equipment for washing the pulp forms the most capital-intensive part of the bleaching plant. investment.

It is a main object of the present invention to provide an improved method of bleaching a cellulosic fibrous pulp which significantly reduces the amount of adsorbable organic halogen and which is more efficient than previously used bleaching processes.

Another object of the invention is to provide an improved way of bleaching a cellulosic fibrous pulp which reduces the use of chlorine and chlorine dioxide.

Another object of the invention is to provide an improved way of bleaching a cellulosic fibrous pulp using a smaller number of bleaching steps to obtain a bleached pulp with market brightness and strength.

It is a further object of the invention to increase the selectivity of lignin removal 3 95292 so that a sufficient viscosity can be achieved after the ozone treatment to allow additional separate bleaching steps to be applied to the pulp in order to achieve market brightness.

These objects are fulfilled by the present invention, which is mainly characterized in that the pulp is bleached with chlorine dioxide and ozone in the same step, starting with chlorine dioxide followed by ozone, and that chlorine dioxide and ozone are added at separate points and at each point in the bleaching line. at the same stage, there is no washing between said chlorine dioxide and ozone addition points.

In a preferred embodiment of the invention, ozone is added at two or more points in said step, which can be called a preliminary step, in which case a stirrer is used.

: Ozone is suitably fed after 10 to 99% of the chlorine dioxide has reacted.

Ozone is fed by a carrier gas containing about 2-13% by weight of ozone. The pulp is preferably treated with ozone at a pressure exceeding 1 bar overpressure. Sequence bleaching with chlorine dioxide and ozone or alternatively vice versa is performed at a temperature of 25 to 70 ° C, preferably 45 to 65 ° C.

Treatments with chlorine dioxide and ozone are followed by one or two base extracts, one or both of which may be reinforced with oxygen gas and / or hydrogen peroxide, the first base extraction being performed in a sequence of chlorine dioxide and ozone treatments without intermediate washing, or in a separate step before washing.

The pulp is bleached with chlorine dioxide in one or more 4

So292 stage, plain blue in two stages.

The incoming and treated pulp is preferably medium-concentrated, about 6 to 15%, and preferably consists of pulp in which lignin removal has been carried out with oxygen gas, the pulp being continuously boiled according to a modified process called MCC or a standard process. The pulp may also have been treated with ozone.

The invention will be explained in more detail below with reference to the drawings, in which Fig. 1 schematically shows a bleaching plant for carrying out an embodiment of the method according to the invention, and Fig. 2 shows another bleaching plant for carrying out a second embodiment of the method according to the invention.

The bleaching plant shown in Fig. 1 comprises a storage tank 1, a blowing tank 2, a diffuser washing apparatus at the top of the first upstream bleaching tower 3, a downcomer 4 and a second upstream bleaching tower 5 at the top. The medium pulp (about 6-15%) is fed from the storage tank 1 to the blow tank 2 via the pipe 6 and from the blow tank 2 further to the bleaching tower 3 via the pipe 7. The bleaching tower 3 is connected to the downcomer 4 by a line 8, the downcomer being connected to the bottom of the bleaching tower 5 by a line 9 with a suitable pump 10 arranged close to the downcomer 4. The bleached pulp is taken from the top of the bleaching tower 5 via line 11. Washing liquid is led to the top of the bleaching tower 3 and the bleaching tower 5 via line 12 and line 13, respectively. A carrier gas, such as oxygen gas, nitrogen gas or a mixture thereof, is taken from the top of the blast tank 2 via line 14 and fed to suitable equipment for possible recovery.

The pulp is fed from the storage tank 1 by a feed device 15 arranged at its bottom and from the blow tank 2 by a similar feed device 16 arranged at its bottom. 95292 5 16 is specially designed for feeding medium-concentrated pulp. In addition, the line 6 has two mixers 17 and 18 arranged in succession, i.e. devices for uniformly mixing the treatment agent into the mass passing through them. The mixer 17 is connected to the mixer on the downstream side by a cable piece 6a of a predetermined length. A similar mixer 19 is arranged in a line 7 extending between the blow tank 2 and the first bleaching tower 3. All mixers 17, 18, 19 have liquefaction members e.g. for vigorous and uniform mixing of different treatment agents into the pulp. Such a Blender may advantageously be a "Kamyr MC Mixer", i.e. a mixer specifically designed to provide a very efficient mixing of the treating agents into a medium mass.

In connection with the outlet of the storage tank 1, an acidifying agent, such as sulfuric acid, is added via line 20, if necessary, to acidify the pulp to a pH below 5, for example to a pH of 2-4. Chlorine dioxide is added to the high agitation first agitator 17 via line 21, while ozone-containing carrier gas is added via line 22 to the next high agitation agitator 18. A line such as sodium hydroxide is added via line 23 to the outlet of blower tank 2. 24 through. In addition, a line 25 for connecting hydrogen peroxide, and a second line 26 for supplying high-pressure steam to the pulp are connected to the third high-mixing mixer 19. At the outlet of the downcomer 4, the pump 10 has a line 27 for supplying chlorine dioxide. In addition, a reaction vessel 28 may be suitably provided in the conduit portion 6b between the second mixer 22 and the blow tank, the reaction vessel and conduit portion 6b being sized to provide a residence time of about 0.1 to 10 minutes, preferably 0.5 to 5 minutes, for the mass passing therethrough. . If the conduit portion 6b is sufficient to achieve the necessary residence time of the pulp, the reaction vessel 28 may be omitted.

For example, the pulp acidified to pH 4 is uniformly mixed with chlorine dioxide in a first mixer 17, after which, after a relatively short period of time, an ozone-containing carrier gas is mixed with it. This short time interval can be from 10 seconds to a few minutes or more, e.g. 10 minutes. It is suitable that 10% to up to 99% of the chlorine dioxide has reacted with the pulp before the ozone in gaseous form is fed. Ozone may be present in an amount of about 2 to 13% by weight of the ozone-containing carrier gas by current ozone production methods, but may be added in the future by improved methods, e.g., up to about 20% by weight. The amount of ozone fed can be adjusted by selecting the appropriate gas pressure, because the higher the gas pressure when mixing into the pulp, the more ozone can be mixed into the pulp. Since the carrier gas is not consumed, it is separated via line 14 for possible recovery. After the blowing tank 2, sodium hydroxide is fed to neutralize the pulp and raise the pH to about 11 to 12, and possibly also a small amount of oxygen gas to oxidize the easily oxidizable compounds of the pulp, so that hydrogen peroxide is used more efficiently. The latter bleach is added in a small amount in the next third mixer 19, to which high-pressure steam can also be fed in a sufficient amount so that the temperature of the pulp rises to about 60-90 ° C. Up to the point where high pressure steam is added, and especially before the blow tank, the temperature of the pulp is adjusted to a relatively low value of about 25 to 70 ° C, preferably 45 to 65 ° C. After the blowing tank 2 and up to the washing equipment in the bleaching tower 3, in connection with the various additions, an oxygen gas and peroxide-reinforced mother extract takes place, which is thus interrupted at the top of the bleaching tower 3 when the pulp is washed with washing liquid. The pulp thus washed is then bleached in the bleaching tower 5 with chlorine dioxide, which is added at a pump 10 on the upstream side in a predetermined amount. The bleaching process described thus consists of a bleaching sequence with only two bleaching steps separated by a washing step at the top of the bleaching tower 3. The bleaching sequence can thus be illustrated as follows: (DZEOP) D.

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Figure 2 shows a modified bleaching plant which, compared to Figure 1, has been supplemented to carry out a washing step between the ozone treatment and the base leaching. In both figures, the substantially corresponding parts and elements are provided with the same reference numerals and will therefore not be explained again. As can be seen in Figure 2, the blow tank 2 at the supply device 16 is provided with a line 29 connected to a diffuser-type washing device 30 and comprising a pump 31 for supplying a medium pulp to the washing device 30, which in turn is supplied with washing liquid via a line 32. From the washing device 30, the pulp is led via line 33 to a downcomer 34, which Intetty bleaches to its own bottom 3 via line 7. The line has a suitable pump 35 arranged close to the downcomer 34. The pump 35 has two lines 23, 24 for supplying a basic substance such as sodium hydroxide and oxygen gas, respectively, with oxygen gas being added if desired. Furthermore, line 7 includes a mixer 19 with high mixing power. The mixer has a line 25 for adding hydrogen peroxide and a line 26 for supplying steam to the pulp when desired. Thus, in an alternative embodiment of the bleaching process according to the invention, which is carried out in the bleaching plant according to Figure 2, a washing step is added before the base addition so that the bleaching sequence will consist of three bleaching steps, namely (DZ) (E0P) D. This implementation represents a higher investment, but provides a lower consumption of sodium hydroxide compared to the embodiment described first.

The following example further illustrates the invention and demonstrates its unexpected results compared to two comparative experiments performed under similar conditions.

Example

Three bleaching process experiments were carried out by simulating on a laboratory scale mainly the bleaching plant of Figure 1, which, however, had been supplemented for treatment with chlorine dioxide in the final bleaching step at the end.

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However, in Experiment 2, no addition of chlorine dioxide was made in the first mixer (which was bypassed). In Experiment 3, the first chlorine dioxide bleaching was performed as a conventional pre-bleaching step.

The experiments used a standard type of softwood sulphate pulp bleached with oxygen gas and washed in the usual way. The sulphate mass fed to the storage tank 1 had a kappa number of 18, a viscosity of 1020 dm 3 / kg and a concentration of 10%. This concentration was then maintained throughout the bleaching process in all three experiments.

Said three experiments were performed according to the following bleaching sequences.

Koe 1 Koe 2 Koe 3

Bleaching sequence (DZEOP) DD (ZEOP) DD D (EOP) DD

Number of bleaching steps 33 4

The first bleaching step with chlorine dioxide (D) in Experiment 3 was performed, as mentioned, in a conventional manner at pH 3-4, at 60 ° C and for 60 minutes, followed by a washing step and a bleaching step with oxygen gas and peroxide-enhanced base extraction (EOP). whereby the concentration of sulphate mass was the same (10%) as in the other two experiments, as mentioned above.

In the ozone treatments of Experiment 1 and Experiment 2, the temperature of the sulfate pulp fed from the storage tank was 28 ° C in each case, with chlorine dioxide being vigorously mixed with the pulp in the first stage of Experiment 1 (DZEOP) according to the present invention. The pressure of the added ozone-containing carrier gas was 4.8 bar. The pH of the sulphate mass was between 3.5 and 4, which was adjusted by adding sulfuric acid at the outlet of storage tank 1. The ozone treatment took place for 5 minutes and was stopped by the addition of base, in which case essentially all of the ozone had had 9 95292 effects. In Experiment 1, ozone was fed after 95% of the chlorine dioxide was used, which corresponds to an interval of 20 seconds between additions.

Oxygen gas and peroxide-enhanced base extraction was performed in all three experiments at 70 ° C and 60 minutes, lowering the pressure from 3 bar to atmospheric pressure. The base extraction was stopped by feeding the washing liquid to the top of the bleaching tower 3, followed by two bleaching steps with the addition of chlorine dioxide at a previously set temperature of 70 ° C and 180 minutes in each case and in the same way in all three experiments.

The additions of the different chemicals, the final pH of the base extracts, and the partial and total results in these three experiments are shown in the table below. In the table and other explanation, ADMT means "Air Dry Metric Ton," i.e., tons of air-dry pulp, and AOX means "Adsorbable Organic Halogens."

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Table

Koe 1_Koe 2_Koe 3

Bleaching sequence (DZEOP) DD (ZEOP) DD D (EOP) DD

Step 1 Step 1 C102, kg / ADMT 20 - 40

Step 1 O3, kg / ADMT 10 15

Step 2

NaOH, kg / ADMT 35 20 20 02, kg / dm3 33 3 H 2 O 2, kg / dm3 33 3

Number of pieces 1.9 4.1 2.5

Viscosity, dm3 / kg 810 720 980

Step 2 Step 2 Step 3 CIO2, kg / ADMT 20 45 35

Step 3 Step 3 Step 4 CIO2, kg / ADMT 10 10 10

Brightness,% ISO 89.7 88.9 89.7

Viscosity, dm3 / kg 810 720 930

Total CIO2, kg / ADMT 50 55 85

Total AOX, kg / ADMT approx. 0.1 - 1.5

It can be seen from the table that the 4-step bleaching sequence D (EOP) DD according to Experiment 3 requires a total addition of chlorine dioxide calculated as active chlorine of about 85 kg / ADMT to achieve a brightness of about 90 ISO. The amount of AOX removed from the bleaching step was 1.5 kg / ADMT.

It is further seen that the 3-stage bleaching sequence (ZEOP) of Experiment 2 with initial stage ozone treatment at the beginning of DD and a total addition of chlorine dioxide calculated as active chlorine of 55 kg / ADMT pulp produces even lower brightness (88.9 ISO), but above all viscosity is too low, with consequent deterioration in strength. Due to these low values, AOX measurement is not of interest.

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Finally, the table shows that the 3-step bleaching sequence (DZEOP) according to DD Experiment 1 (invention) with subsequent ozone treatment in the first step with immediate continuous addition of chlorine dioxide surprisingly results in a sufficiently high brightness level and viscosity within acceptable limits (above 800 dm3 / kg). The selectivity of lignin removal has thus improved. In addition, it is found that Experiment 1 results in a surprisingly low amount of AOX, about 0.1 kg / ADMT. From this environmental point of view, the mechanism underlying the particularly advantageous result has not been fully elucidated, but the probable explanation is that ozone decomposes most of the amount of AOX initially formed in the chlorine dioxide treatment.

In the above example, softwood sulphate pulp, leached with oxygen gas and continuously boiled by a standard method, was used, thus requiring two separate chlorine dioxide steps. Coniferous sulphate mash, for which lignin removal has been carried out with oxygen gas and which has been continuously cooked by a modified process called MCC (Modified Continuous Cooking) with a chapter number of 12 to 14 and a viscosity of 1000 to 1100 dm3 / kg, it is usually sufficient to carry out only one final bleaching step with chlorine dioxide to achieve the brightness required by the market. Hardwood pulp may also be considered. The invention can, of course, also be applied to a sulphate or sulphite pulp which has not undergone lignin removal with oxygen gas.

Claims (15)

A method for bleaching a cellulosic fibrous material in the production of a chemical pulp, wherein the incoming pulp is continuously fed in a bleaching line and bleached with a plurality of bleaching agents containing chlorine dioxide and ozone, characterized in that the pulp is bleached with chlorine dioxide and ozone in the same step as chlorine dioxide. it follows from ozone that chlorine dioxide and ozone are added at separate points (21, 22) and each at at least one point in the bleaching line (21 or 22), and that at the same stage there is no washing between said chlorine dioxide and ozone addition points, that the incoming pulp 3a is concentrated, about 6 to 15%, and that ozone is added by means of an ozone-containing carrier gas. A method according to claim 1, characterized in that ozone is added after chlorine dioxide and that ozone is added at two or more points in said same step. A method according to claim 1 or 2, characterized in that ozone is added after two or more different addition points of chlorine dioxide. • «< Process according to one of Claims 1 to 3, characterized in that ozone is added after 10 to 99% of the chlorine dioxide has reacted. The method according to claims 1 to 4, characterized in that the ozone-containing carrier gas contains about 2 to 13% by weight of ozone. Method according to one of Claims 1 to 5, characterized in that the pulp is treated with ozone at a pressure of 55292 - which exceeds 1 bar of overpressure. Ί. Tata according to one of Claims 1 to 5, characterized in that chlorine dioxide 11a and ozone are directly exposed to 25 to 73 ° C, preferably 43 to 55 ° C. 33533. A method according to any one of claims 1 to 7, characterized in that the sequencing treatments are chlorine dioxide. ozone is followed by vxsi or xaxsi base extraction, of which at least v g o .. a nappiXaasu ^. a and / or hydrogen perouside is hydrogenated. A process according to claim 3, characterized in that said base extraction is carried out in a sequence of chlorine dioxide and ozone treatments without intermediate washing. The method according to claim 8, characterized in that the new base is carried out in a separate step, which is preceded by washing. Method according to one of Claims 1 to 10, characterized in that the pulp is treated with chlorine dioxide in one or more stages. Process according to one of Claims 1 to 11, characterized in that the incoming pulp is a concentrated pulp in which the lignin removal has been carried out with oxygen gas and which has been continuously boiled according to a modified process called MCC or according to a standard process. A method according to any one of claims 1 to 12. characterized in that chlorine dioxide is added in said same step in an amount of 5 to 6C kg / ÄMMT, preferably 10 to 30 Kg / ADMT, calculated as active chlorine. • · 95292 -τ Ion according to Claims 1 to 13, characterized in that ozone is added in an amount of 1 to 20 kg / ADMT, preferably 5 to 10 kg / g / A2MT. = i -Sr 10 - 3C kr / ADMT. preferably 15 to 35 xg / AOMT, 1 li · 55292