FI85729B - FLERSTEGSPEROXIDBLEKNING AV MECHANICAL MASS. - Google Patents

Abstract

Bleaching of a mechanical pulp, such as chemimechanical pulp (CMP), chemithermomechanical pulp (CTMP), and the like, achieves high brightness. A lignocellulose mechanical pulp is subjected to two different peroxide (P) stages (12, 18), and a hydrosulfite (dithionite) (T) stage (20). A P-P-T sequence is preferred (although a P-T-P sequence also produces good results). Bleaching is typically accomplished in all stages at a consistency of between about 8-30%. Between the peroxide stages, the pulp is preferably pressed (19). Between a peroxide stage and a subsequent hydrosulfite stage the pulp is preferably pressed (19 min ), and the pH reduced to between about 5.5-6.0 when diluting with SO2 water (26). The P-P-T bleaching sequence results in pulp with unexpectedly high brightness, brightness levels of about 83% ISO, and greater, being possible.

Description

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Multistage peroxide bleaching of mechanical pulp

Conventional typical bleaching sequences for bleaching mechanical pulps, such as mechanical pulp, chemimechanical pulp (CMP), hot milling (TMP) and chemical hot milling (CTMP), consist of the sequences peroxide (P), hydrosulphite (T) or P to T.

Conventional peroxide bleaching with sodium or hydrogen peroxide is typically performed at a consistency of about 12-10%. The most common is a one-step peroxide treatment. To sin has also been used for two - stage peroxide bleaching (P - P) for medium - brightness mechanical pulps, ie pulps with a brightness of about 75 to 80% ISO.

15 One-step reductive bleaching with hydrosulfites (dithionites) is also commonly used. However, the bleaching sequence P - T is also used, which achieves a brightness in the range of about 74 to 78% ISO. As the final brightness in the P to T sequence is improved, the proportion of the final stage T brightness generation gradually decreases as the peroxide charge increases.

In accordance with the present invention, it has surprisingly been found that by dividing the peroxide treatment into two or more steps, the T-step substantially improves the bleaching effect, i.e. about 2-4 ISO units. In a typical P - T sequence, the proportion of the hydrosulfite phase is very small (less than 2 ISO units) at 80+ ISO.

According to the present invention, there is provided a process for preparing a high brightness lignocellulosic pulp by subjecting a mechanical lignocellulosic pulp to steps comprising peroxide bleaching and a hydrosulphite bleaching sequence 20, characterized in that the mechanical pulp is then subjected to a first peroxide bleaching 20 for the production of bleached mechanical pulp.

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There may be more than two peroxide steps, and the steps of the above method are preferably performed sequentially. Between steps, the pulp is preferably compressed to partially remove the liquid in the pulp. This removed liquid 5 (filtrate) is normally reused as dilution water from the previous steps or to recover chemicals. At all stages, the consistency of the pulp is about 8 to 50%, although the pulp is typically diluted after compression (e.g., to a consistency of 8 to 15%). In each of the 10 bleaching steps in operation, the consistency is about 8 to 30%.

The invention also relates to a mechanical pulp which has a high brightness and which is produced by means of the above-mentioned steps. According to the invention, a mechanical pulp having a brightness of about 83% ISO (or higher) is produced.

It is a primary object of the present invention to provide a high brightness mechanical pulp. This and other objects of the invention will become apparent from the detailed description of the invention and the appended claims.

Figure 1 schematically shows the steps of an exemplary method according to the present invention; Fig. 2 schematically shows a preferred modification of the method of Fig. 1; Fig. 3 schematically shows an exemplary apparatus used in one peroxide step of the method of Fig. 1; Fig. 4 schematically shows an exemplary apparatus used in the hydrosulfite bleaching step of Fig. 1, and Fig. 5 schematically shows a modification of the method 30 of Fig. 1 in which the T-phase is between two P-steps.

The invention can be used to improve the brightness of all types of mechanical pulps. As used in the specification and claims, the term "mechanical lignocellulosic pulp" refers to conventional mechanical pulps, such as abrasive and mill pulps, e.g., CMP, CTMP, and TMP.

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Figure 1 shows the production of mechanical pulp at a typical mechanical pulp production station 10, which also uses refiners and, if desired, other pulping steps. Once the mechanical lignocellulosic pulp has been prepared at station 10, it is subjected to the first sodium or hydrogen peroxide bleaching Pj, referred to in Figure 1 at 12. The peroxide bleaching takes place in conventional equipment and conventional bleaching conditions used in mechanical pulp peroxide bleaching (e.g. 45-90 °). C; 15-240 minutes). A typical form of such an apparatus is shown schematically in Figure 3, in which the pulp is fed to a mixer 14 and peroxide, sodium hydroxide and buffering and stabilizing agents or the like (e.g. sodium silicate and MgSO 4) are added. After mixing in the mixer 14, the consistency of the pulp can be increased in the screw press 15 if desired and finally the pulp is fed to the bleaching tower 16. The bleaching typically takes place at a consistency of about 8-30%, preferably about 12-25%.

In the present embodiment, after the treatment in step Pj, the pulp is fed to a second peroxide step P2, referred to in Figure 1 at 18. In this step, the pulp is subjected to another typical peroxide bleaching sequence. After step 12, other peroxide bleaching sequences (e.g., P3, P4, etc.) may be attached.

In a preferred embodiment of the invention, the mechanical pulp is compressed between steps P2 and P2. This is shown schematically in Figure 2, where the pressing station 19 is located between the peroxide bleaching steps 12 and 18. At the pressing station 19, the pulp is pressed to a high consistency (e.g. 30-50%) before bleaching liquid is added for the next peroxide step 18. It can also be seen from Figure 2 that after step 18 a pressing station is preferably used: 35 19 '.

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After the second (or next, i.e. last) peroxide bleaching sequence, the pulp is subjected to hydrosulfite bleaching, which is schematically indicated by Figure 20 in Figure 1. The conditions of the hydrosulfite bleaching step 20 are conventional (e.g. 45-90 ° C; 10-60 minutes) for with the difference that the bleaching preferably takes place at a pulp consistency of about 8 to 30%, preferably about 8 to 15%. Hydrosulfite bleaching of mechanical pulps of medium consistency is described in U.S. Patent Application 589,938, filed March 5, 1984; and the apparatus and procedures described in the patent application can be used in bleaching. , e.g. buffering, chelating and sequestering agents. The pulp is fluidized in a mixer 22 and then fed to a conventional upstream flow tower 24.

It is also desirable that the pH of the mechanical pulp be lowered between positions 18 and 20 (e.g. to 5.5 to 6.0). This is typically done by supplying SO2 water. If the addition of SO 2 water lowers the consistency of the pulp at station 20 below the desired level, the pulp may be compressed between stations 18 and 20, preferably before the pH-lowering station 26; see. the pressing station 19 'of Figure 2.

The bleached mechanical pulp prepared according to the sequence shown in Figure 1 is very light and a brightness of about 83% ISO units or can be achieved. . bigger. Almost equal brightness can be achieved by using a peroxide step both before and after the hydrosulfite step. This is shown schematically in Figure 5, where the T-phase 20 'is between the P1 phase 12' and the P2 phase 18 '.

The following examples illustrate the improved balance that can be achieved with the method of the invention.

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Example 1 - Prior art

The P-T sequence was applied to unbleached CTMP pulp with an ISO value of 53.5%, a CSF value of 160/110 (cold / hot) ml (milliliters; Canadian Standard Freeness) 5 and containing 0.4% DCM. extract (resin amount measure), 92 ppm manganese and 9 ppm iron. Prior to the peroxide step, the pulp was pretreated with 0.2% DTPA based on the oven dry pulp at a consistency of 3% and a temperature of 60 ° C. The temperature in both the P and T stages was 80 ° C and the consistency of the pulp in both stages was 15%. The treatment time in the peroxy step was 180 minutes and the peroxide solution was buffered and stabilized with 3% sodium silicate and 0.2% MgSO 4. In step T, the reaction time was 30 minutes and 1% hydrosulfite was used, which was completely consumed. 15 The results were:

Peroxide step Charged H 2 O 2% 5.0

Consumed H 2 O 2% 3.3 20 Charged NaOH% 3.5

Final pH 9.3

Brightness% ISO 77.8

Hydrosulfite Step 25 Final pH 5.5

Brightness% ISO 78.7

Example 2

By carrying out the present invention with the same mass as in Example 1, pretreating in the same way and using the same buffering and stabilizing agents, the same temperatures (i.e. 80 ° C) and consistency (i.e. 15% in all steps) and a reaction time of 180 minutes in each period. in the binding phase and 30 minutes in the hydrosulfite phase • 35 (1%, which was completely consumed), the following results were obtained: 6 85729

Peroxide Step 1 Charged H 2 O 2% 3.0

Consumed H 2 O 2% 2.3

Charged NaOH% 3.0 5 Final pH 9.0

Brightness% ISO 75.4

Peroxide Step 2 Charged H 2 O 2% 5.0 10 Spent H 2 O 2% 2.9

Charged NaOH% 3.5

Final pH 10.5

Brightness% ISO 80.7 15 Hydrosulfite stage

Final pH 6.1

Brightness% ISO 83.5

Between the second peroxide step and the hydrosulfite step 20, the pulp was compressed and the pH was lowered by adding SO 2 water. The pH was lowered to a pH of about 5.5 to 6.0 just prior to the hydrosulfite step, and the pulp was diluted as needed to maintain a consistency of about 15% during all active bleaching steps.

Example 3 In this example, a hydrosulfite step was placed between two peroxide steps (see Figure 5). The mass and general conditions were the same as in Example 2, and the following results were obtained:

Peroxide Step 1 Charged H 2 O 2% 3.0

Worn H 2 O 2 * 2.5

Charged NaOH% 3.0 '35 Final pH 9.3

Brightness% ISO 75.4 7 85729

Hydrosulfite step Final pH 6.3

Brightness% ISO 77.7 5 Peroxide step 2

Charged H 2 O 2% 5.0

Consumed H 2 O 2% 3.2

Charged NaOH% 3.5

Final pH 10.2 10 Brightness% ISO 82.6

The final brightness of the pulp at almost 83 ISO (82.6) was surprisingly high, although not quite as high as the value obtained with the sequence of Example 2.

Thus, it can be seen that according to the present invention, a high-brightness mechanical pulp can be produced and a synergistic effect of improving the brightness of the pulp can be achieved by using several peroxide steps before the hydrosulfite step. Although the invention has been presented and described herein in light of the present knowledge as the most practical and preferred embodiment of the invention, it will be apparent to those skilled in the art that many modifications are possible within the scope of the invention and must be encompassed to include all related methods and products.

Claims (9)

8 85729 A process for preparing a high brightness lignocellulosic pulp by subjecting a mechanical lignocellulosic pulp to steps comprising peroxide bleaching and a hydrosulfite bleaching sequence (20), characterized in that the mechanical pulp is subjected to a first peroxide bleaching (12) and then a second peroxide bleaching (18). and then to a hydrosulfite sequence 10 (20) to produce a bleached mechanical pulp. A method according to claim 1, characterized in that the consistency of the pulp in each step is about 8 to 30%. A method according to claim 2, characterized in that the consistency of the pulp during the hydrosulfite bleaching sequence is about 8 to 15%. A method according to claim 3, characterized in that it comprises, before the hydrosulphite bleaching sequence, an additional step (26) in which the pH of the mechanical pulp is reduced to about 5.5 to 6.0. A method according to claim 1, characterized in that it comprises an additional step (19) between the peroxide bleaching steps, in which the mechanical pulp is compressed. A method according to claim 1, characterized in that the first and second peroxide bleaching steps and the hydrosulfite bleaching sequence are the only bleaching steps of the mechanical pulp treatment. High brightness mechanical lignocellulosic pulp, characterized in that the mechanical lignocellulosic pulp is subjected to the following steps: (a) a first peroxide bleaching sequence (12), (b) a second peroxide bleaching sequence (18) and (c) a hydrosulphite bleaching sequence (20). . * 35 High brightness mass according to Claim 7, characterized in that it has a brightness of approximately 83% ISO. 9 85729