EP0087553B1 - Process for producing half-bleached cellulose pulps - Google Patents

Abstract

1. A process for the production of half-bleached cellulose pulps by applying excess oxygen pressure, hydrogen peroxide and sodium silicate in an aqueous alkaline suspension, characterised in that for bleaching and partial delignification, oxygen, hydrogen peroxide and sodium silicate are simultaneously allowed to act on the cellulose pulp.

Description

Semi-bleached pulps have whiteness levels that lie between unbleached and highly bleached pulp. Commercial ,. highly bleached pulps have a white content of 88 to more than 90% Elrepho (R457).

Bleached and semi-bleached pulps affect the range between unbleached (20 to 30% Elrepho for sulfate pulps, 50 to 60% Elrepho for sulfite pulps) and approx. 80 to 88% Elrepho.

Commercially available, semi-bleached sulfate pulps have a whiteness of 70 to 75% Elrepho.

Semi-bleached pulps already have a market today. Medium whiteness levels of the pulp are quite sufficient for many paper qualities. These include wood-containing products, such as newspaper and magazine papers, as well as LWC papers (light-weight-coated papers) and other coated wood-containing qualities at the upper end of the scale. Since the pulp is used in these types of paper to increase strength, its share is only between 10 and 50%. The resulting white content of these papers is determined by the predominant wood pulp content. With the Zelistoffweiße the total white content, for example of newsprint, can only be offered to a very limited extent.

The production of semi-bleached pulp via a known bleaching process - e.g. B. C-E-H - however offers hardly any advantages for the producer. The loss of yield in the known bleaching is determined by the chlorine requirement of the first stage and the amount of alkaline extractable material in the subsequent E stage. (Das Papier 35, S.V. 25 ff (1981)).

Incomplete chlorination has no advantages, since the remaining lignin either leads to an increased need for hypochlorite with a correspondingly higher load through this step or to a brown coloring of the pulp when little hypochlorite is used. Whether the pulp is bleached after the C-E stages with a low hypochlorite amount to whiteness levels by 70% remission or with a D-E-D sequence to 90% remission plays only a minor role for the overall yield and wastewater pollution.

There is therefore generally considerable interest in a bleaching process in which, in contrast to the known chlorination, the lignin is partially retained, but a whiteness of 70% or 75% remission can still be achieved.

DE-A2 040 763 discloses a process for bleaching cellulose-containing substances and for removing lignin, in which at least one bleaching stage is used in addition to oxygen-alkali bleaching using water glass stabilized peroxide.

With this method, a very extensive lignin breakdown is achieved, as is evident from the examples based on the changes in the kappa number.

This is associated with a significant loss of substance, so that the bleaching yield sometimes drops to 85-86%.

The application relates to a process for the production of semi-bleached pulps using oxygen excess pressure, hydrogen peroxide and sodium silicate in an aqueous alkaline suspension, characterized in that oxygen, hydrogen peroxide and sodium silicate are allowed to act simultaneously on the pulp for bleaching and partial delignification.

A consistency of 10 to 30%, preferably 10 to 20%, a temperature of 50 to 110 ° C, preferably 60 to 90 ° C, an oxygen pressure of 1 to 10 bar, preferably 2 to 4 bar, an amount of peroxide from 0.25 to 3%, preferably from 0.5 to 2%, an amount of caustic soda from 1.0 to 5%, preferably from 1 to 3% and a quantity of sodium silicate from 1.0 to 6%, preferably from 2 to Comply with 4%.

In one embodiment, after the oxygen has been mixed in until the peroxide has been added, there can be a residence time of up to 60 minutes.

The process according to the invention is particularly suitable for bleaching hardwood and softwood sulfite and sulfate pulp to a white content of 40 to 80% (Elrepho R 457).

In contrast to the use of known bleaching chemicals with the disadvantages described above, the known single-stage bleaching with hydrogen peroxide offers considerable advantages. Whilst the yield is largely retained, whiteness levels of 70 to 80% remission in sulfite pulps are accessible in a single treatment step. The relatively high cost of hydrogen peroxide is disadvantageous for the use of the known method, since up to 3% H ₂ O ₂ has to be used to reach this white area. (Das Papier 35, S v 25 ff (1981)).

Compared to the known peroxide bleaching process, the process according to the invention enables a marked reduction in the use of peroxide with comparable results in terms of whiteness, yield and strength. A pulp is obtained which, in addition to a high lignin content, has a high degree of whiteness, with no reduction in the yield.

A particularly simple type of application on an industrial scale is the combination of an oxygen and alkali mixture with a medium consistency with an optionally directly subsequent mixture of hydrogen peroxide and sodium silicate. The oxygen can be added via a refiner. During the short dwell time of the material flow when pumping up to the bleaching tower in a tube, oxygen becomes a partial delignification and an oxidation of the peroxide-consuming, dissolved organic substances. If the pulp is then left to react in one of the usual downward working towers, considerably less peroxide is required to achieve the same white content as without the addition of oxygen. A temporal separation of the oxygen and peroxide addition is also possible. Peroxide can only be added to the oxygen-pretreated pulp directly before the tower entrance. However, there should not be a washing stage between the two mixing processes, since peroxide consumers are only introduced again through the usual use of circulating water. In the same way, the oxygen mixer can be used to work directly into an upward tower. The working pressure in this application is 2 to 4 bar.

Surprisingly, it is possible with the process according to the invention to bleach pine sulfate pulp and sulite pulp in one step to a whiteness content of approximately 70% Elrepho (R 457).

Examples

The percentages given are based on oven-dry (otro) pulp.

example 1

• a) Nach dem Verfahren der bekannten einstufigen Peroxidbleiche werden dabei folgende Chemikalienmengen eingesetzt :
  
  Festigkeiten bei 20 SR : Reißlänge 5,3 km
  Weiterreißarbeit 1 230 mNm/m
• b) Nach dem erfindungsgemäßen Verfahren werden folgende Chemikalienmengen benötigt :
  • 1,5 % H₂0₂, 1,4 % NaOH und 3,0 % Natriumsilikat-Lösung

A hard spruce sulfite pulp with Kappa 28 and starting whiteness 51.9 and strengths of 5.8 km tear length and 1410 mNm / m tear strength at 20 SR should be bleached to a whiteness of 70% remission (Elrepho R 457).

• a) The following amounts of chemicals are used in accordance with the process of known single-stage peroxide bleaching:
  
  Strengths at 20 SR: length 5.3 km
  Tear tear 1 230 mNm / m
• b) The following amounts of chemicals are required by the process according to the invention:
  • 1.5% H ₂ 0 ₂ , 1.4% NaOH and 3.0% sodium silicate solution

• Abwasserbelastung (CSB) : 51,8 kgO₂/bdt
• Festigkeiten bei 20 SR: Reißlänge 5,6 km

Weiterreißarbeit 1 370 mNm/mThese chemicals are mixed with the unbleached pulp under the simultaneous effect of an overpressure of 3 bar oxygen pressure, oxygen consumption 1.1%. After 2 hours at 75 ° C and a consistency of 14%, the following results are obtained:

• Waste water pollution (COD): 51.8 kgO ₂ / bdt
• Strengths at 20 SR: length 5.6 km

Tear work 1 370 mNm / m

The process according to the invention reduces the peroxide requirement by more than 30%. Since the advantages of bleaching with peroxide, high yield, low wastewater pollution and good strength values are retained, the cost-effectiveness of the process is considerably enhanced by the use of inexpensive oxygen.

Example 2

• a) Die Bleiche wird nach dem bekannten Verfahren der einstufigen Peroxidbleiche durchgeführt mit :
  • 1,25 % H₂0₂, 1,2% NaOH und 3,0 % Natriumsilikat-Lösung
    
    
    Festigkeiten bei 20 SR : 7,3 km Reißlänge
  
  1 360 mNm/m Weiterreißarbeit
• b) Nach dem erfindungsgemäßen Verfahren wird derselbe Zellstoff wie folgt gebleicht :
  • 0,75 % H₂0₂, 1,1 % NaOH und 3,0 % Natriumsilikat-Lösung werden in einer Sauerstoffeinmischerapparatur bei einem Sauerstoffpartialdruck von 2 bar mit dem Zellstoff umgesetzt. Sauerstoffverbrauch 1,2 % bez. auf Zelsltoff. Nach 10 Minuten Reaktionszeit in einem Steigrohr wird drucklos in einem Turm zuendegebleicht.
    
    1 370 mNm/m Weiterreißarbeit

Spruce Magnefite cellulose fabric for use in newsprint should be bleached to a white content of 60. The initial whiteness is 50.7, kappa 32 and the strength at 20 SR with 8.1 km tear length and 1 210 mNm / m tear strength.

• a) The bleaching is carried out according to the known method of single-stage peroxide bleaching with:
  • 1.25% H ₂ 0 ₂ , 1.2% NaOH and 3.0% sodium silicate solution
    
    
    Strengths at 20 SR: 7.3 km tear length
  
  1 360 mNm / m tear propagation
• b) According to the method according to the invention, the same pulp is bleached as follows:
  • 0.75% H ₂ 0 ₂ , 1.1% NaOH and 3.0% sodium silicate solution are reacted with the pulp in an oxygen mixer at an oxygen partial pressure of 2 bar. Oxygen consumption 1.2% on Zelsltoff. After a reaction time of 10 minutes in a riser pipe, the bleaching is completed without pressure in a tower.
    
    1 370 mNm / m tear propagation

Example 3

Pine sulfate pulp should be bleached to a whiteness of> 50 in one stage.

• a) Nach dem bekannten Verfahren der einstufigen Peroxidbleiche werden 2,8 % H₂0₂, 2,2 % NaOH und 3,5% Natriumsilikat-Lösung eingesetzt.
  
• b) Nach dem erfindungsgemäßen Verfahren wird wie folgt gearbeitet:
  • 22 % NaOH werden bei 14 % Stoffdichte, 85 °C und einem Sauerstoffpartialdruck von 3 bar mit dem Zellstoff gemischt. Der Sauerstoffverbrauch liegt bei 1,8 % bez. auf Zellstoff. Nach einer Reaktionszeit von 5 Minuten (= Verweilzeit im Reaktionsrohr) werden in einem zweiten Mischer 1,8 % H₂0₂ und 3,5 % Natriumsilikatlösung zugegeben. Dabei sinkt die Stoffdichte auf 12 %, die Temperatur auf 80 °C. Nach 90 Minuten erhält man folgendes Ergebnis :
    

The initial whiteness is 23.4, the kappa number 37 '

• a) 2.8% H ₂ 0 ₂ , 2.2% NaOH and 3.5% sodium silicate solution are used according to the known process of single-stage peroxide bleaching.
  
• b) The process according to the invention is carried out as follows:
  • 22% NaOH are mixed with the pulp at 14% consistency, 85 ° C and an oxygen partial pressure of 3 bar. The oxygen consumption is 1.8%. on pulp. After a reaction time of 5 minutes (= residence time in the reaction tube), 1.8% H ₂ O ₂ and 3.5% sodium silicate solution are added in a second mixer. The consistency drops to 12%, the temperature to 80 ° C. After 90 minutes you get the following result:
    

Example 4

Pine sulfate pulp should be bleached to a whiteness level of around 70 (R 457) in one step. This is not possible using the known bleaching process with only one peroxide stage. White contents 55 cannot be achieved from the starting whites 23, 4 and with a kappa number of 37.

• Bei einer Stoffdichte von 25 %, einer Temperatur von 100 °C wird der Zellstoff mit 3,0 % NaOH gemischt. Ein Sauerstoffpartialdruck von 7 bar wird während einer Reaktionszeit von 90 Minuten aufrechterhalten (Verbrauch 2,1 %). Direkt nach dem Reaktor wird der Zellstoff mit 1,8 % H₂0₂ und 4,0 % Natriumsilikatlösung gemischt. Eine Stoffdichte von 18 % wird eingestellt. Die Temperatur sinkt dabei auf 83 °C. Nach einer Stufe Verweilzeit ergibt sich folgendes Resultat :
  

According to the method according to the invention, however, the desired white level can be achieved as follows:

• At a consistency of 25%, a temperature of 100 ° C, the pulp is mixed with 3.0% NaOH. An oxygen partial pressure of 7 bar is maintained for a reaction time of 90 minutes (consumption 2.1%). Immediately after the reactor, the pulp is mixed with 1.8% H ₂ 0 ₂ and 4.0% sodium silicate solution. A fabric density of 18% is set. The temperature drops to 83 ° C. After one level of dwell time, the result is as follows:
  

This pulp is ideally suited for use as a pulp component in newspaper, magazines and LWC papers. It was not only in the bleaching in comparison to conventional bleaching processes such. B. CEH achieved a 4% higher bleaching yield, but also a 50% lower waste water pollution. The latter also contains no chlorinated compounds.

Claims (3)

1. A process for the production of half-bleached cellulose pulps by applying excess oxygen pressure, hydrogen peroxide and sodium silicate in an aqueous alkaline suspension, characterised in that for bleaching and partial delignification, oxygen, hydrogen peroxide and sodium silicate are simultaneously allowed to act on the cellulose pulp.

2. A process according to claim 1, characterised in that a material density of from 10 to 30 %, a temperature of from 50 to 110 °C, an oxygen pressure of from 1 to 10 bars, a peroxide quantity of from 0,25 to 3 %, a quantity of sodium hydroxide solution of from 1,0 to 5 %, and a sodium silicate quantity of from 1,0 to 6 % are maintained.

3. A process according to claim 1 or 2, characterised in that after mixing in the oxygen up to the addition of peroxide, a residence time of up to 60 minutes can be maintained.