The invention relates to a multiple-step bleaching process for the bleaching of a chemical cellulose pulp.

The purpose of the bleaching of chemical pulp is to bring to completion, after the digestion, the removal of residual lignin from the pulp. Bleaching is currently often started with oxygen delignification, whereafter further bleaching can be carried out by various methods. In totally chlorine free (TCF) bleaching, delignification can be continued with, for example, ozone, peracetic acid or hydrogen peroxide in acid or alkaline conditions. In elemental chlorine free (ECF) bleaching, chlorine dioxide steps are used, with intermediate alkali steps. In ECF bleaching, increasingly often oxygen chemicals are used, i. e. oxygen, ozone, hydrogen peroxide and peracids, for promoting bleaching. For example, chlorine dioxide can be saved by the use of hydrogen peroxide in an ECF bleaching sequence. On the other hand, also for environmental reasons, efforts are being made to use increasingly smaller doses of chlorine dioxide in bleaching. Furthermore, methods have been developed in which chlorine dioxide and peracetic acid are used in the same step.

Peracetic acid (PAA) is a compound which forms when acetic acid and hydrogen peroxide react in the presence of a catalyst. There are various products commercially available, for example, 38 per cent distilled peracetic acid (dPAA) and an equilibrium mixture of peracetic acid (ePAA), which typically contains peracetic acid approximately 20 % by weight.

US patent publication 3,865,685 discloses a bleaching process which comprises a plurality of alternating steps in which chlorine dioxide and a per-compound are used as the active bleaching agents. In particular, hydrogen peroxide is used as the per- compound in the process.

The applicant's non-public FI patent application 974 221 describes an enhanced bleaching process for chemical pulp wherein chlorine dioxide (C102) or a combination of chlorine dioxide and peracetic acid (C102-PAA) is used electively for the final bleaching.

The conventional combined use of chlorine dioxide and peracetic acid has a disadvantage in the relatively large quantity of peracetic acid required (the optimum C102-PAA ratio is 1: 1). If, on the other hand, the amount of peracetic acid is

reduced, the combined use of chlorine dioxide and peracetic acid in the final bleaching does not significantly improve the brightness of the final pulp as compared with a final bleaching in which only chlorine dioxide is used.

Normally, a chemical pulp can be bleached relatively easily to a degree of brightness of 88-90 % ISO by conventional ECF bleaching processes. Depending on the type of wood and the pulp cooking method, etc., raising the brightness further from this level may increase the cost of chemicals considerably. These last brightness units are, as regards the cost of chemicals, normally the most expensive in proportion to the increase in brightness.

The object of the invention is, in the form of a multiple-step bleaching process, to improve the final bleaching and, in particular, to provide an improvement over conventional final bleaching carried out using chlorine dioxide. By means of the invention, it is possible to achieve an increase in brightness in the final bleaching efficiently and economically.

The principal characteristics of the invention are given in the accompanying claims.

According to the invention, there is thus provided an improved multiple-step bleaching process for the preparation of a chemical cellulose pulp, the process comprising as the last bleaching step an after-bleaching in which a delignified and bleached cellulose pulp is bleached with a percarboxylic acid.

The said percarboxylic. acid may be performic acid, peracetic acid or perpropionic acid. An advantageous percarboxylic acid is peracetic acid, which may be an equilibrium solution of peracetic acid in water or a distilled peracetic acid. Distilled peracetic acid is especially advantageous. It is also possible to use percarboxylic acid in the form of a salt, for example, an alkali metal salt, such as a sodium salt.

Percarboxylic acid is preferably used in an amount of approximately 0.1-5 kg/metric ton of dry pulp, especially preferably approximately 0.5-3 kg/metric ton of dry pulp, indicated as 100 per cent percarboxylic acid.

According to the invention, it has been observed that peracetic acid can be used as an after-bleaching agent to increase the brightness of already bleached pulps, in which case by using only a few kilograms of PAA the brightness of pulp can be raised by 1.5-2 % ISO from a level of brightness of approximately 88-91 % ISO.

The pH value in the said last PAA bleaching step is preferably approximately 4-8.

According to one preferred embodiment of the invention, the said last bleaching step is preceded by a chlorine dioxide step (D-step).

Chlorine dioxide is preferably used in an amount of approximately 5-30 kg/metric ton of dry pulp, calculated as active chlorine.

The pH value in the D-step is preferably approximately 2-5.5.

It is possible, when necessary, to remove the chlorine dioxide residue from the reaction environment after the D-step by acidification, for example, with sulfur dioxide or bisulfite, such as sodium bisulfite.

According to the invention, the procedure is preferably such that, between the last bleaching step and the D-step preceding it, the pulp is not washed.

Thus the process according to the invention, which comprises a D-step and a subsequent percarboxylic acid step, preferably a PAA step, is suitable for use as after-bleaching for ECF pulps. In conventional ECF bleaching, the last step is often a D-step. According to the invention, it is possible in bleaching plants to dose the peracetic acid into the pulp after the D-step without an intermediate wash. In this case, the chlorine dioxide has already had time to be totally consumed, whereafter the pH of the pulp can, when necessary, be raised slightly before the dosing of the peracetic acid. The method described above has, over the combined use of chlorine dioxide and peracetic acid (meaning in the same step), the advantage that both of the chemicals can be used as precisely as possible within their optimal pH ranges.

Furthermore, the procedure leads to the buffering and stabilization of the pulp pH, which is not necessarily as successful in the combined use of chlorine dioxide and peracetic acid.

According to the invention, it has been observed unexpectedly that there is obtained a synergic bleaching effect of peracetic acid and chlorine dioxide, and additionally this is achieved using small peracetic acid doses, for example, approximately 1-3 kg/metric ton of dry pulp, indicated as 100 per cent peracetic acid.

According to another embodiment of the invention, the said last bleaching step is preceded by an alkaline peroxide step (P-step).

The pH value in the alkaline peroxide step may be approximately 8-12, preferably approximately 10-12.

Preferably the procedure is such that no washing of the pulp is carried out between the said last bleaching step and the preceding alkaline peroxide step.

The process last described can be applied to TCF bleaching and ECF bleaching which end in an alkaline peroxide step, which peroxide may be pressurized or unpressurized. Thus the final pH of the peroxide step is alkaline, in which case the peracetic acid can, at the same time, be exploited as the final acidification chemical after the alkaline step.

The process according to the invention is particularly suitable for a chemical cellulose pulp which is a sulfate pulp obtained from hardwood or softwood.

Furthermore, the process according to the invention is well suited for use in the production of market pulp for the bleaching of pulp going to the dryer. The process according to the invention is also suited for use in integrated pulp and paper mills.

In the process according to the invention, the consistency of the pulp is preferably approximately 3-30 %, especially preferably approximately 5-15%. According to the invention, the consistency can be lowered before the PAA step, for example, after the D-step, without, however, carrying out a separate washing step. The lowering or raising of the consistency by adding or removing water does not affect the functioning of the invention.

In the final bleaching, the pulp is usually quite free of various interfering substances detrimental to peracids, such as heavy metals (e. g. Fe and Mn). When necessary, it is, however, possible to add to the final step stabilizing agents, such as complexing agents, alkaline earth metal salts, or other commonly known stabilizers of per- compounds. Possible use of stabilizing agents does not affect the functioning of the invention, since their purpose is only to stabilize the bleaching agent so that it will not be wasted through any decomposition reactions and that the radicals formed in a decomposition reaction will not break down the carbohydrates in the pulp and damage the strength of the pulp.

The brightness of the pulp to be bleached is, before the bleaching step according to the invention preceding a percarboxylic acid bleaching, preferably 70% ISO or higher, the kappa number of the pulp is preferably below 5.

The brightness of the pulp to be bleached is, before the percarboxylic acid bleaching, preferably above 85% ISO, especially preferably 88% ISO or above this.

The kappa number of the pulp is preferably below 4.

The temperature during the bleaching step or steps may be approximately 40- 100 °C, preferably approximately 50-90 °C. The reaction time in the D-step may be approximately 60-300min and in the percarboxylic acid step approximately 1- 300 min.

Certain chlorine dioxide reactors produce chlorine dioxide water which contains a considerable amount of chlorine. Such chlorine dioxide water can be used in the process according to the invention. Thus, in the process according to the invention the chlorine content of usable chlorine dioxide water may vary within a range of 0- 20 % of the total active chlorine content.

The bleaching steps D-PAA and P-PAA according to the invention, described above, may be preceded by bleaching steps known per se, some of them being E (alkaline extraction), H (hypochlorite), D (chlorine dioxide), O (oxygen), P (peroxide), Z (ozone), PAA (peracetic acid), and various combinations thereof.

The invention is described below in greater detail with the help of Examples 1-5. In the examples, bleaching experiments were carried on a delignified and bleached softwood sulfate pulp, which had been taken from an ECF bleaching plant before the last chlorine dioxide step. The pulp is called here basic pulp. The brightness of the basic pulp was 82 % ISO and its kappa was 1.8.

In the examples, there was used a distilled peracetic acid which contained peracetic acid approximately 38 % by weight.

Example 1 The basic pulp was first bleached with chlorine dioxide 5 kg/metric ton of pulp (calculated as active chlorine). The reaction time was 120 min and the reaction temperature was 70 °C. The consistency of the pulp was 10 %. The initial pH of the basic pulp was 5 and after the D bleaching step the pH of the mixture was 4.7. After the D-step the brightness of the pulp was 87.8 % ISO.

The pulp was thereafter transferred, without an intermediate wash, to the after- bleaching step, in which the pulp was bleached with peracetic acid 2 kg/metric ton of pulp (calculated as 100 per cent peracetic acid). The reaction time was 60 minutes and the reaction temperature was 60 °C. The consistency of the pulp continued to be 10 %. The initial pH of the pulp was 6, and after the PAA step the pH was 5.5 and the brightness of the pulp was 89.9 % ISO. The brightness of the

pulp thus rose by 2.1 % ISO during the PAA step and in total by 7.9 % ISO relative to the basic pulp.

Example 2 The basic pulp was first bleached with chlorine dioxide 7 kg/metric ton of pulp (calculated as active chlorine). The reaction time was 120 min and the reaction temperature was 70 °C. The consistency of the pulp was 10 %. The initial pH of the basic pulp was 5 and after the D-step the pH of the mixture was 4.7. After the D- step the brightness of the pulp was 88.2 % ISO.

The pulp was thereafter transferred, without an intermediate wash, to the after- bleaching step, in which the pulp was bleached with peracetic acid 2 kg/metric ton of pulp (calculated as 100 per cent peracetic acid). The reaction time of the second after-bleaching step was 60 minutes and the reaction temperature was 60 °C. The consistency of the pulp continued to be 10 %. The initial pH of the pulp was 6, and after the PAA step the pH was 5.5 and the brightness of the pulp was 90.0 % ISO.

The brightness of the pulp thus rose by 1.8 % ISO during the PAA step and in total by 8.0 % ISO relative to the basic pulp.

Example 3 The basic pulp was first bleached with chlorine dioxide 5 kg/metric ton of pulp (calculated as active chlorine). The reaction time was 120 min and the reaction temperature was 70 °C. The consistency of the pulp was 10 %. The initial pH of the basic pulp was 5 and the pH of the mixture after the D-step was 4.7. After the D- step the brightness of the pulp was 88.0 % ISO.

After the D-step, a sulfur dioxide acidification was carried out on the pulp in order to remove any chlorine dioxide residues.

Thereafter, the pulp was transferred without an intermediate wash to an after- bleaching step, in which the pulp was bleached with peracetic acid 2 kg/metric ton of pulp (calculated as 100 per cent peracetic acid). The reaction time was 60 minutes and the reaction temperature was 60 °C. The consistency of the pulp continued to be 10 %. The initial pH of the pulp was 6, and after the PAA step the pH was 5.5 and the brightness of the pulp was 89.3 % ISO. The brightness of the pulp increased by 1.3 % ISO in the PAA step and in total by 7.3 % ISO relative to the basic pulp.

Reference Example 4 The basic pulp was bleached using a mixture which comprised chlorine dioxide 5 kg/metric ton of pulp (calculated as active chlorine) and peracetic acid 2 kg/metric ton of pulp (calculated as 100 per cent peracetic acid). The reaction time was 120 minutes and the reaction temperature was 70 °C. The consistency of the pulp was 10 %. The initial pH of the basic pulp was 5.5 and the pH of the mixture after the D/PAA step was 4.. 7. After the D/PAA step the brightness of the pulp was 88.2 % ISO, and the increase in brightness relative to the basic pulp was 6.2 % ISO.

The brightness of the final pulp was clearly lower than the final brightness (89.9 % ISO) of the pulp bleached by the process according to Example 1. Bleaching using a mixture of chlorine dioxide and peracetic acid thus produced a pulp having a brightness which was only slightly higher than the brightness of a pulp bleached with only chlorine dioxide.

Reference Example 5 The basic pulp was bleached using a mixture which comprised chlorine dioxide 7 kg/metric ton of pulp (calculated as active chlorine) and peracetic acid 2 kg/metric ton (calculated as 100 per cent peracetic acid). The reaction time was 120 minutes and the reaction temperature was 70 °C. The consistency of the pulp was 10 %. The initial pH of the basic pulp was 5.5 and the pH of the mixture after the D/PAA step was 4.7. The brightness of the pulp after the D/PAA step was 88.3 % ISO, and the increase in brightness relative to the basic pulp was 6.3 % ISO. The brightness of the final pulp was clearly lower than the final brightness (90.0 % ISO) of the pulp bleached by the process according to Example 2. Furthermore, it can be noted that bleaching with a mixture of chlorine dioxide and peracetic acid produced a pulp having a brightness only slightly higher than the brightness of a pulp bleached with only chlorine dioxide.

The results of the examples are summarized in Table 1.

Table 1 Example 1 Example 2 Example 3 Ref. 4 Ref. 5 Step D D D D/PAA D/PAA Time (mm) 120 120 120 120 120 Temperature (°C) 70 70 70 70 70 Consistency (%) 10 10 10 10 10 C102 (kg/ton of pulp; 5 7 5 5 7 active C1) PAA (kg/ton of pulp)---2 2 Initial pu 5 5.5 5.5 Final pH 4.7 4.7 4.7 Brightness (% ISO) 87.8 88.2 88.3 Intermediate treatment No wash No wash S02 treat./ No wash Step PAA PAA PAA Time (mm) 60 60 60 Temperature (°C) 60 60 60 Consistency (%) 10 10 10 PAA (kg/ton of pulp) 2 2 2 Initial pH666 Final pH 5.5 5.5 5.5 Brightness (% ISO) 89.9 90.0 89.3