EP0968328B1 - Bleaching and delignifying cellulose using caroate/caro's acid and production of same - Google Patents

Abstract

The invention relates to a method for bleaching and/or delignifying cellulose in which a) a sodium hydroxide solution and a peroxydisulfate solution are first of all produced by electrolysis of a sulphate, b) the peroxydisulphate solution is converted by hydrolysis into a Caro's acid/caroate solution, c) said Caro's acid/caroate solution is left to cool and d) used immediately as a bleaching solution for bleaching and/or delignifying cellulose, the Caro's acid/caroate solution becoming neutralized upon introduction into the bleaching solution.

Description

The present invention relates to a method for bleaching or / and pulp delignification using a Caro's acid / caroate solution.

Traditionally, chlorine has been used in large quantities as a bleach Paper pulp used. Because chlorinated lignin is environmentally toxic attributed to polychlorinated aromatic compounds has increasingly been tried, the bleach chlorine to be replaced by other bleaching agents. This was initially great success in using chlorine dioxide / chlorate as Bleach achieved. However, there are other efforts also to replace chlorine dioxide / chlorate with other bleaching agents, to get a TCF pulp (TCF = Total Chlorine Free).

The bleach used here included Ozone, oxygen and hydrogen peroxide used to make chlorine-free paper. However, all of these bleaches have drawbacks such as insufficient delignification, insufficient brightness of the bleached pulp or / and costly or / and manufacturing processes associated with hazards.

Caroat / Caro's acid (peroxomonosulfuric acid, H ₂ SO ₅ ) has also been proposed as a further bleaching agent for bleaching and delignifying pulp, in particular for chlorine-free bleaching of fine paper in order to achieve bleaching degrees with a whiteness level of 89 degrees ISO. A disadvantage of using Caroat / Caroscher acid as a bleaching agent, however, is the high effort required to obtain the pure salt as a crystalline product. Furthermore, Caro's acid in its pure form is an extremely corrosive substance, which makes it difficult to store and transport it.

US-A-5,366,593 describes a method for bleaching lignocellulose-containing Material with a monoperoxysulfate as oxygen donor as reactants and a ketone can be used to add a water soluble dioxirane in situ form, which then acts as a bleach. This is when bleaching the pulp recovered sulfate and by electrolysis in peroxydisulfate converted. The direct one use of Caroat / Caro's acid as a bleaching agent is not considered.

US-A-4,144,144 describes the production of sodium persulfate in one Electrolysis cell, the current yields by suitable choice of Anolytes can be increased.

EP-A-0 415 149 describes a method for bleaching and delignifying of lignocellulosic materials using peroxomonosulfuric acid and / or its salts in combination with oxygen and / or one Peroxide. The peroxomonosulfuric acid comes in the form of its salt used or by reacting hydrogen peroxide with concentrated Won sulfuric acid. A similar process using peroxomonosulfuric acid by reacting hydrogen peroxide and sulfuric acid produced in situ is also described in B. Amini et al., Tappi Journal, Vol. 78, No. 10 (1995), 121-133.

US-A-4,049,786 describes a process for the production of peroxymonosulfate by rapid hydrolysis of a peroxydisulfate with concentrated Sulfuric acid, with loss of active oxygen during hydrolysis can be suppressed by a suitable choice of the reaction temperatures.

WO-A-94/05851 describes a method for bleaching lignocellulose-containing Pulp using persulfate.

The object of the present invention was therefore to provide a method to provide with which the advantageous bleaching Delignification effects of Caro's acid / caroate used can be while using the pure product Caro's acid / caroate related difficulties Manufacturing effort and risk of corrosion for storage and Transport devices can be avoided.

According to the invention, this object is achieved by a method for bleaching and / or delignifying pulp according to claim 1.

The method according to the invention enables the use of Caroic acid / Caroat in the form of a solution, which makes the the crystalline pure product of Caro's acid / caroate Disadvantages can be avoided. These disadvantages are, in particular, the high effort required to do this to obtain pure salt as a crystalline product as well as the Corrosiveness of Caro's acid.

It is essential to the invention that a Carosche Acid / caroate solution in or in the immediate vicinity of a pulp or paper mill can be generated, so long Transport routes that separate the water from the solution and formation of the pure product required become. The separation of water and the formation of the pure Caro's acid / Caroat product before long storage or transport over long distances is necessary because on the one hand, the pure product itself is more stable and therefore longer is storable as a solution and secondly by a wide margin Transport routes the water content in the solution transport considerably more difficult and more expensive.

The preparation of a Caro's acid / caroate solution in or in succeed in the immediate vicinity of a pulp or paper mill by first by electrolysis of a Sodium sulfate, Sodium hydroxide solution and a sodium peroxodisulfate solution generated. The peroxodisulfate solution can then by hydrolysis in a Caro's acid / caroate solution be transferred. Safe during this manufacturing process performed near wood, pulp or paper is the conventional manufacture of caroat Oleum and 70% hydrogen peroxide are very risky because every contact of the raw materials with wood, pulp or paper causes a fire. Because of this, it was So far not possible, Caroic acid / Caroat near a To produce pulp or paper mill.

Another advantage of using electrolysis for manufacturing a peroxodisulfate solution and simultaneous production of caustic soda can be seen in the fact that in pulp and paper mills often use cheaper electricity Hydropower and waste sulfate from sulfate pulp is available that can be used cheaply. Prefers is the proportion of waste sulfate from sulfate pulp in the Total amount of sulfate used in step (a) more than 5 % By weight, particularly preferably more than 50% and most preferably more than 90%.

According to the invention, the peroxodisulfate solution produced by electrolysis is then converted into a Caro's acid / caroate solution by hydrolysis. Such a Caro's acid / caroate solution was found to be relatively stable after cooling. By adding H ₂ O ₂ during the hydrolysis, Caro's acid / caroate decay can be further minimized. According to the invention, the Caro's acid / caroate solution obtained is used directly as a bleaching solution for bleaching and / or delignifying pulp, preferably pulp. The Caro's acid / caroate solution can be fed to the bleaching liquor, for example, via a direct line or in the form of bulk transport. According to the invention, it is therefore not necessary to produce Caro's acid / caroate in crystalline form.

When entering the bleaching fleet, the Carosche Neutralized acid / caroate solution according to the invention. Neutralizing at this point it is advantageous because a pH <5 favors the stability of Caro's acid / caroate solution and also the heat of neutralization released in the bleaching liquor is distributed, causing decomposition of the Caroats / the Caro's acid is further reduced.

As already stated above, it is essential to the invention that the method according to the invention in or in the immediate vicinity a pulp or paper mill. Another advantage of such an arrangement is that large paper mills usually already have a chlor-alkali electrolysis system feature that after adjustment to the Persulfate electrolysis requirements can be used can. Furthermore, for example in the kraft pulp process about 5 to 20 kg of sodium sulfate are supplied per ton of paper. This sodium sulfate can be fed into the Circuit of the pulp process through the persulfate electrolysis be smuggled. The one recovered after paper bleaching Sulfate is then used for the pulp process used. This way it becomes for the pulp process anyway needed sodium sulfate before feeding into the Pulp process by the bleaching process according to the invention smuggled, recovered from it and has sufficient Purity in order for the pulp process to be able to be used.

Another advantage of the method according to the invention is in that it is not necessary to add additional chemicals to use, so related problems regarding Disposal and environmental compatibility do not occur.

According to the invention, that generated in electrolysis Sodium hydroxide solution to neutralize the Caro's acid / caroate solution used in the bleaching liquor.

It is also preferred to use a membrane cell for the electrolysis or to use a diaphragm cell, such as, for example in Ullmann, volume 19, page 216.

Hydrolysis of the peroxodisulfate solution, in which a Carosche Acid / caroate solution is formed is preferred at 50 to 90 ° C and particularly preferably at 60 to 80 ° C for 30 to 60 Minutes, preferably 30 to 40 minutes. For hydrolysis a 1 to 4 N sulfuric acid is preferably used. After hydrolysis, the Caro's acid / caroate solution is left which preferably has a pH of <5, to 0 ° C to Cool 50 ° C, preferably to 10 ° C to 30 ° C.

That when bleaching and / or delignifying pulp, Sulfate formed in particular is recovered and can also be fed into a sulfate pulp process, for example become.

The method according to the invention enables pulp, in particular paper with high brightness, to be obtained at a whiteness level of 89 degrees ISO. A preferred bleaching sequence is, for example:
O - Q - P - - X - P *,
wherein O is oxygen, Q is a complexing agent, such as EDTA, PH ₂ O ₂ and X - P *, the Caro's acid / caroate solution. However, other bleaching sequences can also be used.

The invention is further illustrated by the following examples.

Example 1 - Preparation of Caro's acid

Caro's acid was prepared by hydrolysis from a sodium peroxodisulfate solution. For this purpose, 2 mol / l (4n) NPS (sodium persulfate), that is 476 g / l and 0.25 m sulfuric acid (25.5 g / l) were heated to 70 ° C. with stirring. The composition of the solution before and after the hydrolysis is given in Table 1: time NPS H ₂ SO ₅ H ₂ SO ₄ before heating 476 g / l 0 g / l 25.5 g / l after 3 hours 59.9 g / l 107 g / l 210.7 g / l

The yield of H ₂ SO ₅ was 46.9% after 3 hours. After cooling the solution to room temperature, a decrease in the caroate content of only 2.8% was observed in 20 hours (104 g H ₂ SO ₅ / l).

In a further experiment, 2 mol / l (4n) NPS, that is 476 g / l, and 2n sulfuric acid (98 g / l) were heated to 70 ° C. with stirring. The composition of the solution before and after the hydrolysis is given in Table 2. time NPS H ₂ SO ₅ H ₂ SO ₄ before heating 476 g / l 0 g / l 98 g / l after 1 hour 9.5 g / l 162.3 g / l 246 g / l

The yield of H ₂ SO ₅ was 71.2% after 1 hour. After cooling the solution to room temperature and partial neutralization to 0.34N H ₂ SO ₄ , the caroate content only decreased relatively by 1.3%.

Example 2 (not according to the invention) Bleaching and delignification with KMPS (potassium monopersulfate) and hydrogen peroxide compared to caroate from Example 1

Muster 5656 enthält 68,5 % Wasser, einen biociden Zusatz und der Kappawert (Lignin) beträgt 20,3. Muster 5657 enthält 60,9 % Wasser, einen biociden Zusatz und der Kappawert (Lignin) beträgt 12,8. Muster 5657 ist bereits mit Sauerstoff vorbehandelt. Es ist deshalb heller und enthält weniger Lignin.

Two Finnish pulp samples were examined:

Sample 5656 contains 68.5% water, a biocidal additive and the kappa value (lignin) is 20.3. Sample 5657 contains 60.9% water, a biocidal additive and the kappa value (lignin) is 12.8. Sample 5657 is already pretreated with oxygen. It is therefore lighter in color and contains less lignin.

Experiment a)

Pulp sample 5656 (not pre-bleached) was bleached with KMPS. The consistency was 3%. 13% KHSO ₅ atro (29.6 g Curox / 100 g dry cellulose) were used, which corresponds to 10% H ₂ SO ₅ atro. The bleaching effect was visible immediately after addition.

After 30 minutes of treatment at 60 ° C. and pH 2.5, the The sample is sucked off and washed with plenty of water. After careful Drying at 30 ° C overnight was brightness and weight certainly.

Experiment b)

Pulp sample 5656 (not pre-bleached) became hydrogen peroxide bleached.

The consistency was 3%. 2.9% H ₂ O ₂ atro were used. This amount of active oxygen content (corresponds to 1.4% AO atro) is the same as 10% H ₂ SO ₅ atro (experiment a).

After 30 minutes of treatment at 60 ° C and pH 6, the sample became sucked off and washed with plenty of water. After careful Drying at 30 ° C overnight was brightness and weight certainly.

Experiment c)

Pulp sample 5656 (not pre-bleached) was bleached with a solution of 116.4 g H ₂ SO ₅ obtained according to Example 1.

The consistency was 3%. 10% H ₂ SO ₅ atro were used. This amount of active oxygen content corresponds to 1.4% AO atro.

After 30 minutes of treatment at 60 ° C. and pH 1.5 The sample is sucked off and washed with plenty of water. After careful Drying at 30 ° C overnight was brightness and weight certainly.

Experiment d)

Pulp sample 5657 (not pre-bleached) was bleached with KMPS.

The consistency was 3%. 13.3% KHSO ₅ atro (29.6 g Curox / 100 g dry cellulose) were used, which corresponds to 10% H ₂ SO ₅ atro.

After 30 minutes of treatment at 60 ° C. and pH 2.5, the The sample is sucked off and washed with plenty of water. After careful Drying at 30 ° C overnight was brightness and weight certainly.

Tries)

Pulp sample 5657 (pre-bleached) became hydrogen peroxide bleached.

The consistency was 3%. 2.9% H ₂ O ₂ atro were used. This amount of active oxygen content (corresponds to 1.4% AO atro) is the same as 10% H ₂ SO ₅ atro.

After 30 minutes of treatment at 60 ° C and pH 6, the sample became sucked off and washed with plenty of water. After careful Drying at 30 ° C overnight was brightness and weight certainly.

Experiment f)

Pulp sample 5657 (not pre-bleached) was bleached with a solution of 116.4 g of H ₂ SO ₅ obtained according to Example 1.

The consistency was 3%. 10% H ₂ SO ₅ atro were used. This amount of active oxygen content corresponds to 1.4% AO atro.

After 30 minutes of treatment at 60 ° C. and pH 1.5 The sample is sucked off and washed with plenty of water. After careful Drying at 30 ° C overnight was brightness and weight certainly.

Overview of bleaching tests a) to f)

attempt sample bone dry AO consistency brightening Kappa a) 5656 29.6% KMPS 1.4% 3% + 9.2% 18.4 = -12.2% b) 5656 2.9% H ₂ O ₂ 1.4% 3% + 5.2% 0 c) 5656 10% H ₂ SO ₅ 1.4% 3% +7.6% 18.9 = -9.6% d) 5657 29.6% KMPS 1.4% 3% +14.3% 12.9 = -12.4% e) 5657 2.9% H ₂ O ₂ 1.4% 3% +13.9% 0 f) 5757 10% H ₂ SO ₄ 1.4% 3% +13.9% 11.3 = -12.4%

Claims (5)

1. A method of bleaching and/or delignifying cellulose by means of Caro's acid/caroate, in which

   (a) a sodium hydroxide solution and a peroxydisulphate solution are first of all produced by electrolysis of sodium sulphate

   (b) the peroxydisulphate solution is converted by hydrolysis into Caro's acid/caroate solution

   (c) said Caro's acid/caroate solution is left to cool and

   (d) then used immediately as bleaching solution for bleaching and/or delignifying cellulose, the Caro's acid/caroate solution being neutralised upon introduction into the bleaching solution by the sodium hydroxide solution produced in step (a).

   (e) the sulphate formed during bleaching and/or delignification of cellulose is recovered and returned to step (a).
2. The method of claim 1,
   characterised in that
   a membrane cell or a diaphragm cell is used for electrolysis in step (a).
3. The method according to one of the preceding claims,
   characterised in that
   the hydrolysis of the peroxydisulphate solution in step (b) is carried out at 50 °C to 90 °C.
4. The method according to one of the preceding claims,
   characterised in that
   the hydrolysis of the peroxydisulphate solution in step (b) is carried out in 1 to 4 N sulphuric acid.
5. The method according to one of the preceding claims,
   characterised in that
   the Caro's acid/caroate solution is allowed to cool to 0 °C to 50 °C.