FI75374C - HYPOKLORITBEHANDLING AV KEMISKA PAPPERSMASSOR. - Google Patents

Abstract

1. Process for bleaching chemical paper pulps by means of an alkaline hypochlorite solution and then hydrogen peroxide, without intermediate washing of the treated pulp, characterized in that the hydrogen peroxide is added when 85% to 95% of the hypochlorite is consumed.

Description

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Hypochlorite Treatment of Chemical Pulps In this specification, the term "chemical pulps" refers primarily to unbleached or "unbleached" cellulosic pulps, i. pulps still containing lignin and obtained by various methods, for example by the acidic, neutral or alkaline sulphite method, the sulphate (force) method, the soda, carbonate or oxygen method, with or without a lignin removal catalyst.

As is known, bleaching of cellulosic pulps is a treatment in which chemical reagents remove a dye associated with cellulosic fibers, much of which consists of lignin, which is in a highly condensed state and is thoroughly modified during refraction reactions. Bleaching of chemical pulps is most often performed with chlorine-containing agents such as chlorine, chlorine dioxide or sodium hypochlorite.

Conventionally, bleaching is performed with a series of steps of the type CE ^ D ^ ^ Dj, i.e. first chlorine treatment (C), then first extraction with lye (E 2), first treatment with chlorine dioxide (D 2), followed by a second extraction with lye (E 2) and finally a second treatment with chlorine dioxide (D 2).

Efforts have been made to reduce the cost of bleaching by reducing the number of steps in the series and / or using other chemical reagents. For example, it has been proposed to add an oxidizing agent to the alkali extraction steps (E) or to add a step using such an agent immediately after the alkali extraction; this agent could be oxygen. However, the process requires oxygen in the form of a gas, which is poorly soluble in temporal liquids, which requires the introduction of expensive mixing systems. In addition, the effect of oxygen is significant only in the first alkali extraction step (E2) and practically non-existent in the second (E2) ·

In parallel, it has already been proposed to feed either sodium hypochlorite or hydrogen peroxide into the alkali extraction steps. Current estimates show that the former compound has important advantages at the economic level (see: TAPPI Journal - August 1983 - Vol. 66, No. 8, page 77 -....). For economic reasons, treatment with sodium hypochlorite (H) in bleaching kits is also of interest. However, it is important to completely adjust the pH of the reaction, as well as the amount of hypochlorite fed and the treatment temperature, to avoid damage to the cellulose by hypochlorite.

In addition, FR Patent 2,255,418 proposes the treatment of a chemical pulp with a mixture of hypochlorite and hydrogen peroxide. The two reagents used then react immediately with each other to form a monoatomic oxygen, which is a very strong oxidizer but also very damaging, so that this treatment is in fact an oxidation of the pulp with monoatomic oxygen.

The invention alleviates these disadvantages. It relates to a process which improves efficiency but is easier and cheaper to carry out, and this without damaging the cellulose, while improving the whiteness of the pulps thus treated, without significantly deteriorating the strength properties of the pulp.

This method of bleaching chemical pulps, using an alkaline hypochlorite solution followed by hydrogen peroxide, is characterized in that, without washing the pulp to be treated beforehand, hydrogen peroxide is added when about 85-95% of the hypochlorite has been consumed.

If the addition of hydrogen peroxide is carried out when less than 85% of the hypochlorite has been consumed, there is the above-mentioned risk of deterioration due to the formation of monoatomic oxygen.

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If the addition of hydrogen peroxide is carried out after more than 95% of the hypochlorite has reacted, it is found that the above-mentioned side effects inherent in the treatment with hypochlorite alone have already practically elapsed.

In practice, hydrogen peroxide is preferably added when about 90% of the hypochlorite has been consumed; this addition is performed during the alkali treatment itself. This achieves not only better lignin extraction, but also greater whitening of the pulp, without compromising the quality of the pulp.

In a further preferred embodiment of the invention, hydrogen peroxide is added in a proportion of 0.1-2%, preferably 0.2-0.5% by weight, based on the dry weight of the pulp, added by an amount of sodium hydroxide of less than 2% by weight of the dried pulp, without modifying the treatment temperature , and sodium or calcium hypochlorite is used as the hypochlorite and the pulp to be treated is either sulphate or sulphite pulp.

In the following, the ratios of the reagents are expressed, as before, as a percentage by weight of the dry weight.

As already mentioned, the pulps treated in this way not only retain their strength properties, but also achieve considerable whiteness without having to resort to an expensive series of processing steps.

Surprisingly, by adding hydrogen peroxide, possibly with the addition of sodium hydroxide, to this reaction medium, which contains at least most of the already reacted hypochlorite and pulp, and this without any washing and without any significant modification of operating conditions, lignin removal from pulp and pulp can be significantly continued. without deteriorating the mass. In fact, prior art teachings might have suggested that an inverse effect would be obtained because: 4 75374 - damage to the mass would be expected from the effect of the monoatomic oxygen thus formed; - the primary effect of peroxide on lignin products solubilized by hypochlorite should be due to the consumption of peroxide by these products; - the treatment is carried out at a relatively low temperature (about 40 ° C) at which the peroxide is generally considered to be relatively inefficient; - finally, part of the peroxide fed should decompose under the action of residual hypochlorite or hypochlorous acid.

As already mentioned, hydrogen peroxide is added when 85-95%, preferably about 90% of the hypochlorite has elapsed, a situation which can be considered to have been reached after the reaction time has elapsed from about the total reaction time normally used.

The manner in which the reaction can be carried out and the advantages which it provides are better illustrated by the following working examples, which are given by way of illustration but not by way of limitation.

Example 1

Perform the test as follows.

Commercial raw power pulp made of resinous wood with a Kappa number of 31 (standard AFNOR NF-T-12018) and a degree of polymerisation (DP) of 1 650 (standard TAPPI-T-230 v. 76) and a whiteness of 27 (standard AFNOR NF-Q-03039), is treated under the following treatment conditions: Chlorination step (C): 6% chlorine, at 20 ° C for 45 minutes, with a pulp consistency of 3.5%; - alkali extraction step (E) with 3% sodium hydroxide solution, at 70 ° C, for 60 min, with a pulp consistency of 10%.

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After the chlor-alkali treatment (CE), the pulp is subjected to a second alkali extraction, this time (H) in the presence of sodium hypochlorite (NaClO), in a ratio of 2%, at 40 ° C, for 2 hours, with a consistency of the pulp of 10%.

After the CEH series, the brightness of the pulp is 56.5 and its degree of polymerization is 1620.

According to the first option, the amount of hypochlorite is increased to 3% instead of 2% and the duration of treatment to 2 1/2 hours instead of 2 hours. The whiteness of the obtained mass is 69.5, with a DP of 1330, which illustrates the deteriorating effect of hypochlorite.

Alternatively, the relative amount of sodium hypochlorite is added to a further 3.5%. The whiteness of the treated pulp is 71.0% and DP 1150.

This perfectly illustrates the limits of treatment with sodium hypochlorite. This treatment greatly improves whiteness, but at the expense of pulp quality. Also, if it is desired to reduce the bleaching cost by replacing the CEDED step with the CEHDED step, i.e. more simply CEHD, the relative amount of hypochlorite used and thus the reduction in the relative amount of chlorine dioxide used is limited.

Example 2

The pulp of Example 1 is subjected to the two chlorotron treatment (CE) steps of Example 1 and then treated with 2% hypochlorite under the conditions of Example 1.

However, after a reaction time of 90 minutes, iodometric determination of the treatment effluent shows that 92% of the sodium hypochlorite has been consumed.

0.5% Hydrogen peroxide to which 1% NaOH has been added is then added to the reaction mixture. The two compounds are added with as little water as possible so as not to substantially modify the treatment conditions, especially the temperature, which remains close to 40 ° C.

After 60 minutes, the treatment (which therefore lasts 2 1/2 hours) is stopped. The degree of whiteness obtained is 72.0 and the degree of polymerization (DP) has remained at 1560.

Thus, except that whiteness is improved, cellulose is not substantially damaged. However, such a result, which cannot be obtained with conventional hypochlorite treatment alone, is achieved in one step, by adding hydrogen peroxide according to the invention at the end of the hypochlorite treatment.

Example 3

For the test, the raw magnesium sulphite pulp made of resinous wood with a Kappa number of 18,7 and DP 1500 is bleached in the usual way, using bleaching series PD1ED2 (where P is the peroxide step), under the following conditions: - P: 1% H2 ° 2 'j ° h ° n is added 1.5% soda, at 90 ° C, 90 minutes, with a pulp consistency of 12%; -: 1% chlorine dioxide, at 78 ° C, 90 minutes, with a pulp consistency of 9.5%; - E: 1% sodium hydroxide solution, at 60 ° C, 40 minutes, the consistency of the pulp being 10%; - 0.2% chlorine dioxide, at 78 ° C, for 180 minutes, with a pulp consistency of 9.5%.

After these four bleaching steps, a pulp with a whiteness of 90 and DP 1450 are obtained.

Example 4

Perform a second test as follows.

Bleaching the same pulp as in Example 3, with a series of PDH, in which the first two steps P and D are carried out under the same conditions as in Example 3.

It is then treated with sodium hypochlorite in a ratio of 0.5% (NaClO), at 36 ° C, for 180 minutes, with a consistency of the pulp of 8.5%.

The whiteness of the pulp thus treated is 87.0% and DP 1451.

If the amount of hypochlorite is increased to 1.5% and the duration of treatment to 210 minutes, the whiteness improves by only one point (88.0), but the DP decreases to 1197.

Thus, although the conventional sodium hypochlorite step allows a satisfactory target to be achieved in whiteness, it causes a considerable deterioration of the mass.

Example 5

The previous example is repeated using 0.5% sodium hypochlorite. But after 135 minutes of hypochlorite treatment, 0.5% hydrogen peroxide and 0.5% NaOH are added to the reaction mixture, using as little water as possible so that the temperature of the treatment bath (38 ° C) does not change.

After a reaction time of 75 minutes, 90 are obtained as whiteness and 1400 as DP.

Thus, if the result obtained in this example is compared with that obtained in the previous examples, the whiteness has improved by three points and it has been possible to reduce the bleaching series to three steps without deteriorating the pulp.

That is, the four-stage, conventional series PDED according to Example 3 can be replaced by a series comprising only three stages PD (HP), where (HP) is the alkaline stage according to the invention.

Thus, the process according to the invention can both reduce the consumption of chlorine dioxide in series CEHDED, or CEDHD, and reduce the number of bleaching steps, as shown in Examples 4 and 5 above.

Examples 6 and 7

Example 5 is repeated for the first time (Example 6) by shortening the reaction time of the hypochlorite so that only 80% is consumed, and for the second time (Example 7) by extending the reaction time so that practically 98% of the hypochlorite is consumed.

The whiteness of the pulp treated according to Example 7 is practically the same as that of the pulp according to Example 5, which in turn is better than the whiteness of the pulp according to Example 6.

Especially in the case of Example 6, as in the case of Example 7, the deterioration of the mass is considerable.

The method according to the invention offers numerous advantages over currently used methods. These include: - the possibility of achieving better or at least equivalent whiteness without causing damage to the pulp; - reduction of the number of bleaching steps, as no washing is required, which results in a significant reduction in bleaching costs.

This method can be successfully applied to all types of chemical pulps, whatever the starting plant (conifer, hardwood, annuals, etc.).

Claims (6)

9 75374 A process for the chemical pulp or semi-alkaline hypochlorite solution and then for the hydrogenated peroxide without washing the treated pulp, characterized in that hydrogen peroxide is added with a residual amount of hypochlorite corresponding to 5-15% of the initial weight of the pulp. Process according to Claim 1, characterized in that hydrogen peroxide is added in an amount of 0.1 to 2% by weight, based on the weight of the pulp. Process according to Claim 1 or 2, characterized in that the addition of hydrogen peroxide is carried out without changing the treatment temperature. Process according to one of Claims 1 to 3, characterized in that sodium hydroxy dia is added to the hydrogen peroxide. Process according to Claim 4, characterized in that the amount of sodium hydroxide added is less than 2% by weight, based on the weight of the pulp. Process according to one of Claims 1 to 5, characterized in that the hypochlorite is sodium hypochlorite or calcium hypochlorite.