WO1996009434A1

WO1996009434A1 - Method of bleaching kraft pulp

Info

Publication number: WO1996009434A1
Application number: PCT/FI1995/000506
Authority: WO
Inventors: Kaj Henricson; Olavi Pikka
Original assignee: Ahlstrom Machinery Oy
Priority date: 1994-09-19
Filing date: 1995-09-18
Publication date: 1996-03-28
Also published as: CA2200327C; FI944348A0; FI944348A; SE9700992L; SE9700992D0; CA2200327A1

Abstract

The present invention relates to a method of bleaching sulfate cellulose pulp to a brightness of at least ISO 85 by using at least two of the following bleaching chemicals: oxygen, ozone, and peroxide. Characteristic features of the invention are that the pulp is digested; subjected to a heavy metals removal treatment with at least one of the following treatment agents or methods: acidification, EDTA, and DTPA; delignified in a pressurized two-step oxygen stage; and bleached at least in one acid peroxide stage.

Description

METHOD OF BLEACHING KRAFT PULP

Field of invention

The present invention relates to a method of bleaching sulfate pulp, i.e. so-called kraft pulp, to a brightness of at least ISO 85 by using elemental chlorine free chemicals such as, for instance, oxygen and peroxide.

Background art For environmental reasons, among other things, there has been a tendency to reduce the use of elemental chlorine in bleaching of cellulose pulp. The first attempts led to the use of chlorine dioxide which in some sense is a better alternative in view of protection of the environment than the use of elemental chlorine. Also marketing factors direct the development towards giving up the use of chlorine altogether, both elemental chlorine and chlorine compounds. Very many customers appreciate greatly the fact that the product they use has been produced without using chlorine at all (TCF) . The minimum requirement is that at least no elemental chlorine in gas form has been used in the manufacture of the product (ECF) .

A few years ago when ozone bleaching at medium consistencies had been developed to an industrially applicable mill-scale stage (cf. European patent 0 397 308) most companies developing processes and apparatus for the cellulose industry concentrated almost exclusively on developing ozone bleaching to a form which would be applicable in their own processes.

Ozone, however, has its drawbacks. The most significant of these is perhaps the high price which is mostly due to the fairly difficult production method. A further drawback is its high reactivity which means that bleaching equipment using ozone, as well as the ones using gaseous chlorine, must be kept absolutely gas tight. For the same reason, ozone easily damages cellulose itself if the dose of ozone is not exactly correct or if adequate mixing is not provided.

For the reasons discussed above, both peroxide (P) stages and alkaline oxygen stages activated with peroxide (EOP) , and particularly pressurized stages of this kind, have recently been included in bleaching sequences (cf. article "Medium consistency pulp washer generates superior washing efficiency", TAPPI Journal, June 1990; PCT/FI92/00198; European patent application no. 93301222; PCT/FI93/00222) . However, as may be concluded from the above publications, ozone still has a very significant role in bleaching sequences.

Disclosure of invention

The present invention pays attention to the environment protection values which direct the development away from the use of chlorine chemicals, and to economic aspects which at least to some extent speak against the use of ozone. A bleaching chemical cheaper than ozone, and also as to its bleaching properties very competitive with ozone, is hydrogen peroxide, to name one preferred alternative in the broad family of peroxides.

Until recently, bleaching sequences utilizing peroxide have included as an essential part thereof a so-called Q stage which removes metals and precedes each peroxide stage and is separated by an efficient intermediate washing stage from the peroxide stage. This has been considered necessary also when the sequence has included several P stages. The Q stage may involve mere acidification of the pulp at a low pH (pH approx. 3 - 5) or treatment with a complex former such as EDTA or DTPA at a pH of 4 - 6. In other words, until now it has been considered necessary to wash away prior to the peroxide bleaching stage from the pulp the heavy metals it contains as the metals were believed to decompose the peroxide. It has been found out, however, that in accordance with the method of the present invention it is not always necessary to pretreat the pulp by a Q stage to remove heavy metals but the reaction conditions in the peroxide stage may be made suitable for the peroxide bleaching process irrespective of the presence of heavy metals in the pulp. This kind of an improvement in the peroxide bleaching stage brings a pulp mill remarkable investment savings as the Q stage, which in view of the bleaching process itself is unnecessary, is no longer needed before a peroxide stage whereby also the washer between the Q stage and the P stage is made unnecessary.

The method of the invention has made it possible to bleach pulp to a brightness of 85 - 88 ISO, and even greater, without the use of chlorine and also without the use of ozone if such is considered a risk.

A characteristic feature of the present invention is that pulp is at least partially bleached with peroxide alone, or with peroxide and oxygen, at acidic conditions. This kind of an acidic peroxide method has been suggested for example in Finnish patent application no. 906076 and some older articles. However, both the patent application mentioned above and the articles deal with bleaching of sulfite pulp with peroxide at acidic conditions. In pulp production, one of the most significant differences between the sulfate and the sulfite processes is the acidity of the cook; a sulfate cook is alkaline and a sulfite cook acidic. When the pH of the cook is acidic, heavy metals are efficiently separated to the digestion liquid and most of them are removed during brown stock washing prior to bleaching. In sulfate digestion, heavy metals are not separated into the digestion liquid and thus cannot be removed in the brown stock washing. The characteristic features of the method of the present invention are disclosed by the appended patent claims.

Brief description of drawings The method of the invention is described in detail below, by way of example, with reference to the accompanying drawings of which:

Figs. 1 - 5 illustrate a few bleaching sequences according to preferred embodiments of the invention; and Fig. 6 illustrates graphically the brightness and Kappa numbers obtained by a sequence according to the invention as compared with two other potential sequences.

Preferred embodiments

Fig. l illustrates a bleaching sequence 0-Q-(OP)m-P_aQ-P, preceded by a sulfate digestion either in a continuous or a so-called batch digester to a Kappa number of 30 - 15, preferably to a Kappa number of approx. 20. It is possible to reach even lower Kappa numbers by some advanced digestion methods but in most cases that happens at the expense of the yield. The 0 stage at the beginning of the sequence is a so-called oxygen delignification stage which aims at bringing the Kappa number down to below 15, preferably below 12. The oxygen delignification is followed by a Q stage already described above employed to remove heavy metals from the cellulose pulp by chelating (with EDTA, DTPA or corresponding chelating agents) where metal ions are combined chemically into complexes or by acidifying where metal ions dissolve in the liquid phase so that heavy metals do not cause problems in the following peroxide (P) stage. However, we have found out that a Q stage performed as an acid stage lowers the Kappa number whereby this phenomenon can be taken in use when delignifying of pulp is wanted, and especially in combination with oxygen stages. The Q stage is preferably carried out at a pH range of 3 - 6 and at a temperature of 60 - 100°C and with a treatment time of 20 - 120 min. It is also possible to treat the pulp with enzymes in the same stage to expose the lignin for subsequent delignification stages. The Q stage is ended with an efficient washing stage (=) to wash off the heavy metals separated from the pulp at this stage. Preferably a so-called DD washer manufactured by A. Ahlstrom Corporation and disclosed for example in U.S. patents no. 4,919,158 and 5,116,423, is used as the washing device. The advantage provided by this kind of a fractionating washer is the washing result which is remarkably more efficient and precise than the one obtained by a conventional washer, and the production of filtrate fractions having different concentrations, the utilization of which filtrate fractions gives other advantages to the process.

The subsequent oxygen stage (OP)m in fact comprises one or several, preferably pressurized, oxygen and/or peroxide stages with no intermediate washing between them as is closer described in Finnish patent application no. 934056. In this oxygen and/or peroxide stage having one step or several steps, oxygen or peroxide may be added as supplemental chemical to one step or several steps if desired. In addition to one-step oxygen and/or peroxide stage, also for example the following two-step stages may be used: 0/0, 0/P, P/O, P/P, OP/0, PO/O, OP/P, PO/P, OP/OP, OP/PO, P0/P0, and PO/OP in which the first character in each step stands for the main active bleaching chemical and the possible second character for the supporting bleaching chemical. In other words, the substantially delignifying part of the sequence may, for instance, be O - Q - 0, O - Q - OP, 0 - Q - P, O - Q - O/O, 0 - Q - 0/P, 0 - Q - P/0, O - Q - P/P etc. After the oxygen and/or peroxide stage described above the Kappa number of the pulp has reduced to the range of 8 - 3. The oxygen and/or peroxide stage is followed in the sequence by a peroxide stage connected to the process directly after the washer of the oxygen stage without a metal removal preceding a P stage. The solution allowing operation without a separate metal removal stage (Q) by acidifying or chelating prior to a peroxide stage, is reducing the pH of the peroxide stage acidic to a range of 2 - 6, preferably to 4 - 6. It is also preferable that the acidic peroxide stage P_a is pressurized (the pressure being usually below 10 bar, preferably 3 - 5 bar) and that the temperature is 60 - 130°C, preferably 90

- 110°C. Activators may be added to a peroxide stage of this kind to disperse hydrogen peroxide so as to produce a strong oxidant. For example, the peroxide may be dissolved to water and active oxygen. Pressurizing the stage ensures that the produced gaseous chemical stays close to the fiber.

An acidic peroxide stage P_a may be combined with a Q stage during which for example EDTA or acid is added to the pulp to remove heavy metals. Both steps in the P_aQ stage, or quite as well in a QP_a stage, are performed at a pH of below 7, at the same temperature, preferably within the range mentioned above, in both the steps, and preferably with at least the P_a step being pressurized, which also was described above. If necessary, the pH may be adjusted between the steps if the pH of the P_a step is very low, i.e. below 3. The pH of the Q step should be 3

- 6, preferably 4 - 6. After a P_aQ stage the pulp is washed (=) , preferably with a fractionating washer, to remove the heavy metals treated in the Q step.

The last treatment stage in the sequence described here is an alkaline peroxide stage employed to increase the final brightness to a desired level, preferably over 88. The process conditions in this P stage are: the pH 10 - 12, the temperature 60 - 130°C, preferably 90 - 110°C, the pressure usually below 10 bar, preferably 3 - 5 bar, and the treatment time 30 - 120 min.

Fig. 2 illustrates an embodiment slightly different from the sequence in Fig. 1. The single- or multi-step oxygen/peroxide stage (OP)m now stands for a multi-step oxygen stage intensified with peroxide (OP/OP) .

Fig. 3 illustrates an alternative sequence in which the first step of a multi-step oxygen/peroxide stage has been intensified with peroxide (OP/P) .

Fig. 4 illustrates the one-step oxygen/peroxide stage of Fig. 1 as a multi-step peroxide stage (P/P) .

Fig. 5 illustrates an embodiment in which the multi-step peroxide stage of Fig. 4 has been replaced by a multi- step peroxide stage and in which the first peroxide step has been intensified with oxygen (PO/P) .

The chemical feed rates and the process conditions in the alternative two-step sequences described above are the ones given in the Finnish patent application 934056 mentioned above, if applicable. However, it must be noted that the present invention covers also embodiments in which the bleaching is carried out at its simplest as a one-step process with one chemical whereby thus the second chemical dose, i.e. either oxygen or peroxide dose, is 0.

Figure 6 illustrates results received from comparative laboratory tests performed. The figure illustrates the changes in Kappa number and brightness obtained from tests using three sequences. In addition to comparison of different potential bleaching sequences, the objective of the test was also to find out, by stabilizing the total peroxide volume used in the whole sequence to six per cent irrespective of the number of peroxide stages or steps, which sequence would give the best bleaching result. The sequences used in the tests were Q - PO/P - P, Q - PO/P - P_aQ - PO and for comparison a sequence Q - PO/P - Z - P containing ozone.

The Figure indicates that in all the three tests the PO/P stage reduced the Kappa number from 12 to 6 and increased the brightness from 40 to almost 80. It should be noted that a separate oxygen delignification immediately after the digestion gives a Kappa level of 6 - 9, thus the starting point for further bleaching is quite different from the one with the present invention.

When alkaline peroxide (P) without heavy metals removal is used as the following stage it is found out that the Kappa number does not change much and also the brightness increases only a few per cents. When ozone was used the Kappa number reduced to approx. three and the brightness increased to approx. 84. Acidic peroxide (P_aQ) seems to work the same way as ozone although the Kappa number reduced to about two and the brightness increased to 85.

Two of the sequences were continued by performing the bleaching subsequent to the ozone with an alkaline P stage and the bleaching following the P_aQ stage with an alkaline PO stage. The final Kappa number reached in both these sequences was below 1 and the brightness in the sequence utilizing ozone about 90 and in the sequence utilizing acidic peroxide even about 93.

Yet another interesting and especially promising bleaching method includes a novel pretreat ent with an oxygen delignification stage followed by a Q stage run as an acid stage (A) and an oxygen bleaching stage. In this embodiment, the property of an acid stage to decrease the Kappa number of pulp is used. The acid stage (A) in accordance with this embodiment is different from the Q stage discussed earlier as in A stage the main concern is not to remove heavy metals but to improve bleachability by lowering the lignin content of pulp. In other words, the pulp production begins with the following partial sequence digestion - O - A - 0. The following practical example describes the process in more detail.

Example: laboratory test Pulp was digested in accordance with the so-called Kraft process and thereafter oxygen delignified to a Kappa number of 14 or less. After the oxygen delignification, the pulp was treated in an acid stage at a pH of 3 - 5, preferably 3 - 4, at a temperature of 95°C, the temperature preferably ranging from 90 to 110°C, and for a time period of about 1 - 2 hours. The pulp was further bleached with oxygen whereby a final Kappa number of 5 - 3 was reached. Thus, with a production sequence digestion - O - A - O pulp was produced which is very suitable for further bleaching. The purpose of the acid stage (A) is to improve bleachability, not to remove metals. In the tests performed the pulp was further bleached with partial sequences ZQ - P and P_aQ - P. Both sequences produced pulp having a brightness above 88 ISO, even 90 ISO, or greater.

Thus, pulp having a full brightness was produced with only two peroxide stages in addition to the pretreatment of the present invention, i.e. the entire sequence being digestion - 0 - A - O - P_aQ - P. Another sequence producing corresponding brightness values, i.e. digestion - O - A - O - ZQ - P, used only one P stage but an ozone stage was needed. In applications where only a slight modification of an existing bleaching plant based on using chlorine dioxine is needed, the use of a sequence digestion - O - A - O - DQ - P comes into question which would drastically reduce the use of chlorine dioxide. Performed tests have shown that chlorine dioxide dose of less than 40 kg counted as active chlorine is sufficient, most often the use of chlorine dioxide dose of 10 to 25 kg/BDMT results in excellent brightness. In other words, the final bleaching stage P may be preceded by P_aQ, ZQ, DQ or any other oxidative treatment combined with a Q stage.

As can be understood from the above description, a new environmentally friendly bleaching method has been developed which can replace both the bleaching stages containing elemental chlorine and the bleaching stages containing chlorine compounds and which can reduce the use of expensive ozone. However, it should be understood that only a few preferred embodiments of the bleaching method of the invention have been described above by way of example and these embodiments do not in any way intend to limit the scope of protection of the invention from what is defined by the appended patent claims. Thus, it is clear for example that although all the examples describe only a two-step oxygen and/or peroxide stage, there may be only one step or more than two steps in the stage. Further, also the washes not mentioned separately above may be fractionating washes.

Claims

We claim:

1. A method of bleaching sulfate cellulose pulp with chlorine-free chemicals at least to a brightness of ISO 85, comprising the steps of

- digesting pulp by the so-called kraft process;

- treating the pulp in an acidic peroxide stage (P_a) without a preceding heavy metals removal treatment separated with a wash.

2. A method as claimed in claim 1, wherein between the kraft digestion and the P_a stage

- pulp is subjected to heavy metals removing treatment (Q) with at least one of the following treatment agents or methods: acidification, EDTA, DTPA, enzyme

- pulp is bleached in an oxygen and/or peroxide stage (OP)m.

3. A method as claimed in claim 1, wherein the oxygen and/or peroxide stage (OP)m is a one-step or a multi-step bleaching stage performed without an intermediate wash.

4. A method as claimed in claim 3, wherein the one-step or multi-step oxygen and/or peroxide stage (OP)m contains in addition to the one-step O or P stage at least the following two-step bleaching stages:

0/0, 0/P, P/0, P/P, OP/P, PO/0, OP/P, PO/P, OP/OP, OP/PO, P0/P0, and P0/0P, in which 0 signifies oxygen chemical and P signifies peroxide and in which the first character in each step represents the main active bleaching chemical and the possible second character the bleaching chemical supporting the bleaching reaction.

5. A method as claimed in claim 2, 3, or 4, wherein the oxygen and/or peroxide stage (0P)m is pressurized.

6. A method as claimed in claim 2, wherein a so-called Q stage for removing heavy metals from the pulp has been connected to the acidic peroxide stage (P_a) without an intermediate wash (QP_a or P_aQ) .

7. A method as claimed in claim 2 or 6, wherein the acidic peroxide stage (P_a) is followed by an alkaline peroxide stage (P) after an intermediate wash.

8. A method as claimed in claim 2 or 6, wherein the acidic peroxide stage (P_a) is pressurized.

9. A method as claimed in claim 6 or 8, wherein both the steps of the P_aQ stage are performed at the same temperature and at a pH of below 7.

10. A method as claimed in claim 2, wherein the pulp is subjected to the following treatments between the digestion and the removal of heavy metals: - the pulp is washed;

- the pulp is oxygen delignified (0) ; and

- the pulp is washed.

11. A method as claimed in claim 2 or 6, wherein after the heavy metals removal stage (Q) and the acidic peroxide stage (P_a) the pulp is subjected to an efficient, preferably so-called fractionating wash.

12. A method as claimed in claim 2, wherein the bleaching sequence contains at least as a part of the sequence the following partial sequence:

Q = (0P)m - P_aQ, in which Q signifies removal of heavy metals either by acidifying or by chelating; = signifies an efficient, fractionating wash;

(0P)m signifies a one-step or multi-step oxygen and/or peroxide stage; - signifies a conventional wash; and

P_aQ (or QP_a) signifies an acidic peroxide stage combined with a heavy metals removal.

13. A method as claimed in claim 12, wherein subsequent to the P_aQ stage, pulp is bleached to it final brightness by an alkaline peroxide stage (P) .

14. A method as claimed in claim 13, wherein the alkaline peroxide stage is performed at a temperature of 60 - 130°C, a pressure of less than 10 bar, preferably 3 - 5 bar while the treatment time is 30 - 120 min and the pH 10 - 12.

15. A method as claimed in claim 13 or 14, wherein the alkaline peroxide stage is preceded by an efficient, preferably fractionating wash.

16. A method as claimed in claim 10 and 13, wherein the process used in the manufacture of pulp is

a kraft cook - O - Q = (0P)m - P_aQ = P, in which

O signifies oxygen delignification ; Q signifies heavy metals removal either by acidification or chelating;

(0P)m signifies a one-step or a multi-step oxygen and/or peroxide stage;

P_a signifies an acidic peroxide stage; P_aQ (or QP_a) signifies an acidic peroxide stage combined with a heavy metals removal;

P signifies an alkaline peroxide stage;

- signifies a conventional wash; and

= signifies an efficient, preferably fractionating wash.

17. A method as claimed in claim 2, wherein the sequence used is kraft cook - (0P)m - P_a - (OP)m, kraft cook - (OP)m - QP_a - (OP)m, or kraft cook - (OP)m - P_aQ - (OP)m.

18. A method as claimed in claim 2, wherein at least one of the washing stages, preferably all the washing stages are fractionating washes.

19. A method of bleaching sulfate pulp with chlorine- free chemicals to a brightness of at least 85 ISO, comprising the steps of cooking and oxygen delignifying the pulp to a Kappa number of 20 or below, preferably 14 or below; treating the pulp in an acid stage to improve bleachability which acid stage preferably has a pH of 3 - 5 and a temperature of 90 - 110°C;

- oxygen bleaching the pulp to a Kappa number of 8 - 2, preferably 5 - 3;

- further bleaching the pulp with at least hydrogen peroxide.

20. Method as claimed in claim 19 comprising, before the hydrogen peroxide stage, a delignifying stage combined with a Q stage, said delignifying stage preferably being a P_a stage, a Z stage, D stage or any other oxidative stage.

21. Method as claimed in claim 20, wherein the bleaching sequence is kraft cook - O - A - O - P_aQ - P, kraft cook - O - A - O - ZQ - P or kraft cook - O - A - O - DQ - P.