JP5478610B2 - Pulp bleaching method using ozone, chlorine dioxide and peroxide - Google Patents

Description

  The present disclosure relates generally to a final bleaching process for pulp. In particular, the present invention relates to a cost-effective method of bleaching oxygen delignified pulp by ozone bleaching to a final brightness exceeding 88% ISO.

  In the bleaching process of both softwood and hardwood pulp, the pulp is usually delignified in one or more oxygen stages and then bleached by various sequences including chlorine dioxide stage, extraction stage, peroxide stage, etc. The

Hardwood pulp differs from softwood pulp in that it contains a large amount of hexeneuronic acid (HexA). The amount of HexA depends on the raw materials used and the cooking conditions. In modern cooking methods that use relatively low cooking temperatures, the HexA content is usually high. Oxidation of HexA with potassium permanganate (KMNO ₄ ) contributes to the kappa number. In hardwood pulp with a kappa number of 10, 50 to 70% of the kappa number can result in HexA, and lignin and other compounds can be as little as 30 to 50%.

During bleaching, HexA can be reduced by oxidation using bleaching compounds such as chlorine dioxide and ozone. One economical method is the degradation of HexA by acid hydrolysis at high temperatures. Therefore, high temperature chlorine dioxide steps (D _HT ) are often employed in modern bleach plants to perform acid hydrolysis with chlorine dioxide bleaching. By performing the high temperature followed by the extraction stage D _HT, can be reduced, for example, the kappa number from 10.5 to 2.5.

  As mentioned above, the pulp can also be bleached with ozone. Ozone is a very effective delignification chemical and reacts very quickly with pulp, requiring only a short residence time. Ozone bleaching is usually performed at temperatures below 60 ° C. because the environment becomes very aggressive. Ozone bleaching is generally used in conjunction with subsequent extraction steps.

  Final bleaching is usually carried out in two separate steps, either in the form of two chlorine dioxide bleaching stages, or in the form of a peroxide bleaching stage followed by a chlorine dioxide bleaching stage. Optionally, the two chlorine dioxide bleaching steps are performed without any intermediate washing. If the peroxide bleaching stage is performed after the chlorine dioxide bleaching stage, these stages are carried out at different pH, usually pH 3.5 to 4 in the chlorine dioxide stage and pH 10 to 11 in the peroxide stage. Is generally performed with an intermediate wash.

Under special circumstances, it has been reported that it is possible to combine a chlorine dioxide bleaching step and a peroxide bleaching step without any intermediate washing, see Suess et al. , Short sequence bleaching without penalties-options for Eucalyptus pull, Appita 2005, p. Reference is made to 461-468. This involves bleaching hardwood pulp using a high temperature chlorine dioxide bleaching stage ( _DHT ) followed by an integrated extraction and peroxide bleaching stage, resulting in only 5 to 7.5 kg active Cl. This can be done when low kappa numbers are obtained which are only required in the chlorine dioxide bleaching stage. In this special case, bleaching using a sequence involving the integration of the chlorine dioxide bleaching stage and the peroxide bleaching stage (ie without any intermediate washing) and the washing after the chlorine dioxide bleaching stage. And it is possible to obtain almost the same results when bleaching using a sequence comprising a peroxide bleaching step.

Suess et al. Also discloses another method that involves a direct combination of ozone, hydrogen peroxide and extraction followed by a combination of chlorine dioxide bleaching and peroxide bleaching. The combination of ozone with hydrogen peroxide and direct extraction is stated to provide relatively high whiteness (69% ISO) and low kappa number (<3) after the extraction stage. However, it is stated that final bleaching requires a large input of chlorine dioxide. A total charge of 5 kg O ₃ , 20 kg ClO ₂ , 8 kg H ₂ O ₂ , and 19 kg NaOH was required to achieve whiteness above 90% ISO. As a result of the large amount of chlorine dioxide required, Suess et al. Conclude that it is impossible to exclude a washer between the chlorine dioxide bleaching stage and the peroxide bleaching stage.

  Suess et al. Therefore suggest that a kappa number of less than 2 is achieved after extraction by including a chlorine dioxide stage prior to the ozone bleaching stage. This alleviates the requirement for chlorine dioxide in the later chlorine dioxide stage and develops into a polishing stage instead of a delignification step. The reduced demand for active chlorine in the chlorine dioxide stage after the extraction stage makes it possible to combine the chlorine dioxide stage with the final peroxide stage without any intermediate washing. The proposed bleaching sequence includes 5 stages and a total of 3 washers. However, the chlorine dioxide bleaching step prior to ozone bleaching results in the presence of chloride in the filtrate after the extraction step, so that the filtrate can be reused for washing after the oxygen stage and / or the filtrate is incinerated. Can be more difficult.

Suess et al. , Short sequence bleaching without penalties-options for Eucalyptus pull, Appita 2005, p. 461-468

  The main objective of the present invention is to provide a cost-effective ozone bleaching process for pulp, which can achieve a final whiteness of over 88% ISO.

  The object is achieved by a pulp bleaching method according to independent claim 1. Preferred embodiments are provided in the dependent claims.

  The pulp bleaching method according to the present invention comprises one less washing step since the peroxide bleaching step is integrated with the preceding chlorine dioxide bleaching step. Therefore, the investment cost is substantially reduced. Furthermore, with the method according to the invention, the handling of the extract obtained in the extraction stage after the ozone bleaching stage can be carried out more efficiently since the extract does not contain chloride.

According to one preferred embodiment, MgSO ₄ is added to the pulp immediately before or during the chlorine dioxide bleaching step. This addition results in higher whiteness at the same chlorine dioxide consumption as compared to when MgSO ₄ is added in the peroxide bleaching stage. Thus, such addition of MgSO ₄ to the pulp can also reduce the amount of chlorine dioxide required for the desired final whiteness.

  The process according to the invention is intended for bleaching hardwood pulp, but is also considered suitable for bleaching other pulps such as softwood pulp.

  In this disclosure, the term integration should be construed as having no intermediate cleaning immediately after the preceding step.

  The process according to the invention comprises performing an ozone bleaching step of the pulp which has been subjected to oxygen delignification and washing, followed by extraction. In contrast to the bleaching method proposed by Suess et al., The method according to the present disclosure does not include chlorine dioxide bleaching prior to ozone bleaching. Therefore, there are no problems associated with reuse of the filtrate obtained in the extraction stage and burning of the filtrate due to incineration, which becomes a problem when the filtrate contains chloride. Chloride in the filtrate makes the filtrate aggressive, increasing the risk of corrosion.

  The ozone bleaching used in the bleaching method of the present disclosure is preferably performed on pulp having a pulp concentration of 30 to 45%, more preferably 35 to 40%, also referred to as high concentration pulp. This reduces the requirement for chlorine dioxide in the subsequent chlorine dioxide bleaching step as compared to when ozone bleaching is performed on wet pulp such as pulp having a concentration of 10 to 12%.

  The ozone bleaching step is followed by an extraction step integrated with ozone bleaching. Preferably, no peroxide is added during these stages as it is not considered necessary to achieve the desired result.

  After the extraction stage, the pulp is washed, followed by a chlorine dioxide bleaching stage. Chlorine dioxide bleaching is suitably 5 to 25 kg active Cl / ton at a pH of 2.5 to 4.5, preferably 3 to 4.5, and a temperature of 70 to 90 ° C, preferably 75 to 85 ° C. It is preferably carried out by adding chlorine dioxide corresponding to 10 to 25 kg active Cl / ton, more preferably 15 to 22 kg active Cl / ton. The time required for chlorine dioxide bleaching depends on the aforementioned factors and can be easily determined by one skilled in the art.

  After the chlorine dioxide bleaching, a peroxide bleaching stage is performed, and the peroxide bleaching stage is integrated with the chlorine dioxide bleaching stage, i.e. the washing of the pulp between the chlorine dioxide bleaching stage and the peroxide bleaching stage Not done. Since no cleaning between these two steps is necessary, the investment cost is substantially reduced as a result of one less washer than the prior art.

  In contrast to the view expressed by Suess et al., It has been found that an effective bleaching sequence can still be realized while integrating the chlorine dioxide bleaching step and the peroxide bleaching step. Even if relatively high chlorine dioxide additions are made during the chlorine dioxide bleaching stage, the peroxide bleaching stage is still effective and can increase the final whiteness of the pulp by about 2 to 3% ISO.

The peroxide bleaching stage can be carried out at pH 9 to 12, preferably pH 9.5 to 11, at substantially the same temperature as the chlorine dioxide stage. The amount of H ₂ O ₂ can suitably be 1 to 8 kg / ton pulp, preferably 2 to 6 kg / ton. The time required for the peroxide bleaching step can be readily determined by one skilled in the art.

It is generally known that magnesium in the form of MgSO ₄ can be added to the pulp to stabilize the stage during the peroxide bleaching stage. The addition of MgSO ₄ can increase the whiteness and viscosity of the pulp. Furthermore, the amount of residual peroxide after the bleaching stage is much more stable than in the case of peroxide bleaching without such addition.

Surprisingly, however, it has been found that adding MgSO ₄ to pulp already in the chlorine dioxide bleaching stage can further improve the bleaching sequence for the bleaching process of the present invention. The same result was not observed in the case of a bleaching sequence having a washing stage between the chlorine dioxide bleaching stage and the peroxide bleaching stage.

Thus, according to a preferred embodiment of the pulp bleaching method according to the present disclosure, MgSO ₄ is added to the pulp immediately before or during the chlorine dioxide bleaching stage, such that the addition of MgSO ₄ is in the peroxide stage. It was found that about 0.5% higher whiteness can be obtained with the same chlorine dioxide consumption as compared with the case of the above. When compared at the same whiteness, for example 90% ISO, this means that about 2 kg active Cl / ton pulp can be saved with such addition.

Suitably 0.5 to 4 kg / ton of MgSO ₄ can be added to the pulp in the chlorine dioxide bleaching stage, preferably 1 to 3 kg / ton is used.

  The hardwood pulp was oxygen delignified and subsequently bleached to 3.3 kappa using HC-ozone. The pulp data after cooking, oxygen delignification stage, and ozone bleaching stage are shown in Table 1.

After that, bleaching was carried out using the sequence S _Inv1 which, after the chlorine dioxide bleaching step according to the invention, carries out a peroxide bleaching step without any intermediate washing. For comparison, the pulp was also bleached with the conventional standard sequence S _Ref1 , which includes a chlorine dioxide bleaching stage, a washing stage, followed by a peroxide bleaching stage. These sequences and the resulting data are shown in Table 2.

  The same whiteness and viscosity were obtained with both the bleaching sequence according to the invention and the conventional bleaching sequence. However, for the bleaching sequence according to the invention, an additional 2 kg NaOH / ton was required to achieve the same final pH.

  These results show that the use of the bleaching method according to the invention eliminates the need for an intermediate washing step and consequently reduces the investment costs by reducing one washing machine.

  Furthermore, the measured color return (Δ whiteness) is much smaller in the case of the bleaching sequence according to the invention. Although the measured delta may be considered somewhat uncertain in some cases, improved Δ whiteness indicates that it can be achieved using the bleaching method according to the present invention.

  Oxygen delignification of hardwood pulp was performed in the mill and bleached in the laboratory to a copper number of 3.3 using HC-ozone. The viscosity was 520 ml / g.

The pulp was then subjected to two different final bleaching sequences, one of which was the sequence S _Inv2 according to the invention and the other was the reference sequence S _Ref2 . S _Inv2 includes a peroxide bleaching stage integrated with the chlorine dioxide bleaching stage after the chlorine dioxide bleaching stage, while the reference sequence S _Ref2 comprises a chlorine dioxide bleaching stage, a washing stage followed by a peroxide bleaching stage. Includes stages. Experiments were performed with the addition of magnesium sulfate (MgSO ₄ ) at different stages of the bleaching sequence.

Table 3 shows the data of 6 experiments A to F of the reference sequence S _Ref2 , while Table 4 contains the data of 6 experiments G to L of the sequence S _{Inv2 according to} the invention.

From the above results, it can be seen that in the case of the reference sequence S _Ref2 , when MgSO ₄ was added in each of the chlorine dioxide stage and the peroxide bleaching stage, almost the same white pieces were obtained with the same chlorine dioxide consumption. However, if the sequence according to the present invention, i.e. in the case of no cleaning between chlorine dioxide bleaching stage and the peroxide bleaching stage, addition of MgSO ₄ to the chlorine dioxide bleaching stage, the peroxide addition of MgSO ₄ Compared to the bleaching stage, a substantially higher whiteness was obtained with a specific chlorine dioxide consumption (except in the case of 10 kg active Cl / ton, where approximately the same whiteness was achieved).

Thus, as a result of the sequence according to the invention, it can be seen that adding MgSO ₄ to the chlorine dioxide bleaching step reduces the amount of chlorine dioxide required for the bleaching when compared at the same whiteness.

From these results, it can also be seen that the difference in viscosity depending on when MgSO ₄ was added was negligible. Furthermore, in the case of the sequence according to the invention, the consumption of peroxide was maximized when MgSO ₄ was added to the peroxide stage.

Claims (7)

A method for bleaching hardwood pulp comprising:
(I) a step of performing an extraction stage after performing an ozone bleaching stage of the oxygen delignified and washed pulp, and without any intermediate washing after the ozone stage, and before the ozone bleaching stage, A step of washing after the extraction stage without performing the bleaching of
(Ii) performing a chlorine dioxide bleaching step on the pulp from step (i) by adding chlorine dioxide corresponding to 5 to 25 kg active Cl / ton ;
Against pulp (iii) chlorine dioxide bleaching, the not having any intermediate wash after chlorine dioxide bleaching, saw including a step of carrying out the peroxide bleaching stage,
A process wherein MgSO ₄ is added to the pulp immediately before or during the chlorine dioxide bleaching stage .   The process according to claim 1, wherein ozone bleaching is carried out so that the pulp has a kappa number of 3 to 4 after the extraction stage.   The process according to claim 1 or 2, wherein the pulp has a concentration of 30 to 45% during ozone bleaching.   4. A process according to claim 1, wherein no peroxide is added in the ozone bleaching stage. The method according to any of claims 1 to 4, wherein MgSO ₄ is added in an amount of 0.5 to 4 kg / od t .   MgSO ₄ 6. The method of claim 5, wherein is added in an amount of 1 to 3 kg / odt. The method according to claim 1 , wherein the hardwood pulp is Eucalyptus-based pulp.