FR2572431A1 - PROCESS FOR THE PREPARATION OF CHEMICAL PAPER PULP FOR BLEACHING. - Google Patents

Abstract

1. Process for preparing chemical paper pulps in order to bleach them, of the type in which the said pulp is treated with chlorine, then with at least one oxygen compound, characterized in that the pretreatment with chlorine is performed with a pulp having a consistency between 5 % and 15 %, at a temperature between 70 degrees C and 90 degrees C, with a chlorine concentration between 0.8 % and 1.5 % by weight with respect to the weight of the pulp.

Description

PROCESS FOR THE PREPARATION OF CHEMICAL PAPER PULP

QUESTIONS FOR MONEY LAUNDERING.

  The invention relates to a process for the preparation

  tion of chemical pulp for whitening

chastise.

  As we know, delignification is an operation

  ration which consists in removing the maximum of lignin

  contained in the fibers. To remove lignin

  dual and / or degraded by this treatment, it is necessary

  trim the dough for proper bleaching.

  To date, essentially, three types of process

dice have been proposed.

  In a first type of process, it was suggested to treat the dough with chlorine. Unfortunately, this process does not make it possible to recycle the effluents in the factory circuits, due to their high chloride content. In addition, the increase in the temperature of the chlorine treatment, inevitable in the case of intensive recycling, would cause significant degradation of the cellulose (see the publication "Pulp

  and Paper Magazine of Canada, June 1974 --Vol.75 N 6 -

pages 67 and following).

  A second type of process consists in treating the dough by means of oxygen in an alkaline medium. However, if this process makes it possible to recycle the effluents, on the other hand, in practice, one hardly exceeds a lignin extraction rate close to 50%, in order to avoid oxidation and therefore the degradation of the cellulose by l 'oxygen. (see TAPPI-June 1981 Vol. 64

N 6- pages 91 and following).

  In a third type of process, it has been suggested to treat the dough by means of peroxides, in particular of

  hydrogen peroxide. However, in practice, this process

  dice is only effective at temperatures above 100 ° C., in particular in the vicinity of 120 ° C. Unfortunately -2- as we know, by these temperature conditions, we

  causes significant degradation of the dough.

  In French patent FR-A-2 018 562, it was pro-

  posed to treat the dough first with an aqueous suspension of chlorine, then then with o

  gaseous xygene in an alkaline medium at a higher pressure

  higher than atmospheric pressure. In an advantageous embodiment, the pulp treated in aqueous suspension has a consistency of between 3 and 6%, at room temperature, with 2 to 8% of active chlorine by weight relative to the pulp, especially in the form

  hypochlorite, then treat this aqueous suspension

  has a consistency of 3 to 12% with oxygen A no-

  especially in peroxide form in an alkaline medium at a pressure of at least two bars. We thus obtain a

  pulp leading to good papermaking characteristics

  res. Unfortunately, due to the large quantity of hypochlorite introduced, the effluents cannot be recycled. In this way, this process was hardly

developed.

  In short, all the solutions proposed to date for preparing chemical pulp for bleaching have the drawback either of causing significant degradation of the cellulose, or of not allowing the recycling of the effluents.

  unless additional stages of milking are used

  is lying. The invention overcomes these drawbacks. It relates to an improved process for the preparation for bleaching by means of a compound containing chlorine

  active then using a compound containing oxygen

  ne, which is more economical to implement, and which makes it possible to achieve a delignification rate and

improved whiteness.

  This process for treating pulp pulp chi-

  -3 mics in several stages, of the type in which said paste is treated first with active chlorine and then then with an oxygenated compound, is characterized in that the treatment with active chlorine is carried out: - at a temperature between 50 and 100 C, preferably between 70 and 80 C; - on a dough whose consistency = is between 5 and 15%, preferably between 8 and 12%; - and finally, at an active chlorine concentration of between 0.5 and 2%, preferably in the vicinity of 1 to 1.5% by weight relative to the weight of

dough.

  Thus, the choice of these specific operating conditions

  specifically allows, in a completely unexpected way, to be able to recycle the effluents, which did not allow

  not the treatments offered until then while obtaining

  with such good whiteness results and without degreasing

of cellulose.

  Advantageously, in practice: - the pretreatment with active chlorine is carried out with chlorine gas for a period of between five and thirty minutes, preferably between ten and twenty minutes;

  - the subsequent treatment with oxygen is carried out

  killed by hydrogen peroxide at a higher temperature

  lower than 100 C, advantageously close to 120 C.

  Thus, chlorine pretreatment inhibits the degradation

  This normally occurs during peroxide bleaching. So, quite unexpectedly,

  the method according to the invention makes it possible to reach an ex-

  significant degree of pre-whitening without, however, degrading

cellulose.

  It was observed that if the chlorine concentration was less than 0.5% by weight of the weight of the pulp, the inhibitory effect became insufficient. Likewise, if this concentration exceeds 1.5%, this would lead to an unnecessary consumption of chlorine and above all a greater difficulty in recycling effluents. As already said, in practice, the chlorine concentration must be between 0.8 and 1.5% by weight of the weight of the paste. Similarly, if the temperature of the pretreatment at

  chlorine was less than 50 C, this would cause

  practical problems, especially in order to

  pel with recycling effluents that are at least

  at this temperature. Similarly, if this temperature ex-

  yielded 0l0C, it would be necessary to use special devices and a calorie intake, which would make

  pretreatment difficult to achieve.

  As already said, advantageously in practice, the temperature of this chlorine pretreatment must

  be between 70 and 90C. If the temperature

  passing 90 C, there is a tendency to degrade the cellulose without appreciably improving the pre-bleaching. If this temperature is below 70 "C, the treatment loses

of its effectiveness.

  With regard to the consistency of the pulp, it has been observed that if this consistency was less than 5%, this would involve handling a large amount of water, which would make recycling of effluents uneconomical. Similarly, if the consistency was higher

  at 15%, handling the dough from-

would come more difficult.

  As already said, the process according to the invention resides

  of essentially in a particular selection of the operating conditions of a known process of treatment in several phases, one with chlorine, the other using an oxygenated compound. Thanks to this selection under the operating conditions, this pre-bleaching unexpectedly ensures the protection of the cellulose while allowing recycling of the effluents after

  treatment with the oxygen compound. The chlora- stage

  tion being carried out hot, the recycling of these effluents containing very little chlorides in washing the unbleached pulp does not in fact pose any problem.

  The patent FR-A-2,018,562 already cited, which teaches

  hindered the two-phase treatment by means of all a-

  the edge of an aqueous suspension of chlorine and then by means of gaseous oxygen in an alkaline medium, used significant amounts of chlorine, of at least 2%, which made the recycling of effluents impossible. resulting increase in the temperature of chlorination would cause degradation of the

  cellulose. In this way, it was necessary to operate cold.

  The process according to the invention has overcome this prejudice because it could not be expected that by using a similar but different chlorine concentration on a paste having a particular consistency and at an appropriate temperature, the degradation phenomena are more observed. caused by both chlorination

  and oxidation by the oxygenated compound.

  The manner in which the invention can be realized and the advantages which result therefrom will become more apparent from the

  examples of embodiments which follow, given for information only

dicative but not limiting.

  In the following, the reagent levels indicated will be expressed in percent (%) by weight relative to

  by weight of the dough counted in dry.

Example 1:

  A witness is made by operating in the following manner

boasts.

  We treat a commercial kraft pulp, of wood-

  sinuous Kappa 30 index (standard NF-T-12018) whose de-

  option of cellulose polymerization (DP) is 1600 (TAPPI-T-230 of 76) with oxygen in an alkaline medium -6- under the following conventional and industrial conditions: - oxygen pressure (measured when cold ): 5 bars, - quantity of soda: 2%, - treatment temperature: 110 C, - paste concentration: 25%, - treatment time: 45 min 0.5% of magnesium sulphate has been introduced

as a protector of cellulose.

  After washing under usual conditions, the

  this Kappa is reduced to 14.0 and the DP to 1400.

  As we know, we cannot increase the delignifying action of oxygen without degrading in parallel

  cellulose. So if instead of using 2%

  of, we used 3, then 4%, the Kappa index fell

  rait respectively to 12.1, then to 10.2 and the DP falls-

  rait parallel to 1280, then 1130.

Example 2:

  The same kraft paste is used as in Example 1.

  This is treated, prior to treatment with o

  xygene, with 1% chlorine gas, at 70 C, for ten

  minutes, the paste concentration being 10%.

  After washing, the oxygen treatment is carried out.

  dried with 3% sodium hydroxide solution, the other conditions

  remaining those of example 1. The Kappa index ob-

  held is 9.7 and the DP, instead of dropping to 128

  as in Example 1, remains at 1350.

  Thus, although the delignification obtained is greater (lower Kappa index), the degree of polymerization of the cellulose remains at a good level, which makes it possible to preserve the mechanical properties of

dough.

Example 3:

  We repeat example 2 by simply modifying the

  temperature of the pretreatment with chlorine which is

  -7- at 90 C. The Kappa index obtained is 8.5 and the DP

from 1450.

  This example perfectly illustrates the favorable effect

  ble of the increase in the temperature of the pretreatment-

is lying.

Example 4:

  Example 2 is repeated, increasing the rate to 4%

  of soda during the oxygen treatment.

  The Kappa index obtained is 7.9 and the DP drops

only at 1300.

  As an indication, the Kappa indices obtained in Examples 3 and 4 are of the same level as those obtained by the conventional chlorosodation process.

  (CE) in which about 6% chlorine is used.

Example 5:

  Example 2 is repeated while removing the oxygen.

  The Kappa index initially of 30, instead of being reduced to 9.7 as in Example 2, is only 24.

  This perfectly illustrates the delignifian-

  te of oxygen in the process according to the invention.

  In example 5, the use of oxygen has been described as an agent for the delignification treatment. It is also possible to use other oxygenated compounds,

such as hydrogen peroxide.

Example 6:

  The kraft paste of Example 1 is delignified (index

  Kappa 30, DP 1600) under the following conditions.

  - hydrogen peroxide: 1%, - soda: 3%, - treatment temperature: 120 C, - treatment time: 90 min.,

- paste concentration: 12%.

  After washing, the Kappa index is reduced to 17.1 and the DP to 1250o - 8 -

Example 7

  Example 6 is repeated, subjecting the dough to a

  chlorine pretreatment under the following conditions

  -: chlorine level: 1.5%, - temperature: 70 C, - duration: 15 min.,

- paste concentration: 12%.

  After the peroxide treatment under the same conditions as in Example 6, the Kappa index is reduced to 13 instead of 17.1 but the DP only drops to 1470 at

place of 1250.

  These two examples 6 and 7 also illustrate, for example

  actually the favorable effect of chlorine pretreatment

  on the effectiveness of delignification and on its selection

activity.

  The results of these examples 1 to 7 are collected

  for convenience in the attached table.

  If we examine this table, in particular the values obtained by known methods, that is to say in which an oxygen treatment is carried out (example

  1) or a chlorine treatment (example 5), we can see clearly-

  the progress made by the invention (examples 2, 3 and 4). Indeed, it is well known that the oxygen-delignifying treatment and the chlorine treatment

  are two treatments that degrade cellulo-

  It was therefore by no means obvious that by combining

  these two treatments known separately and in con-

  In particular editions, delignification by oxygen would also become selective, since it would only affect lignin without causing depolymerization of the cellulose. In other words, it is surprising that the combination of two treatments known to degrade cellulose lead to an operation which not only

  does not degrade cellulose, but improves delineation

  -9- fication. In addition, the method according to the invention requires

  low reagent consumption. Indeed, the com-

  parison with conventional chloroso- treatment

  donation (CE) (see publication "Pulp and Paper Magazine

  of Canada "June 1974, vol. 75 N 6, pages 67 et seq.

  tes, recalled in the preamble), shows that in perfor-

  almost identical mances, the chlorine level used in the process according to the invention does not represent

  that a quarter, even a sixth of that of this treaty-

  ment (CE). Admittedly, the oxygen consumption in the

  process according to the invention (2 to 3% maximum) is particularly

  ble, but the cost of this consumption is low compared to the savings made on chlorine, so that the economic balance of the process is largely positive. Other advantages of the process according to the invention come from the fact that very small amounts of chlorine are used. The amount of chlorinated organic products formed during the pretreatment with chlorine is therefore very low, which considerably reduces the pollution problem posed by these products, the mutagenic nature of which is known. In addition, the rate of

  chlorides in the chlorination or deligni-

  oxygenation is also very low, which makes it possible to consider favorably the recycling of these effluents in the regeneration circuits. This therefore results in a significant reduction in pollution

  water caused by bleaching effluents,

  duction greater than that authorized by delignifica-

conventional oxygen.

  The delignification process according to the invention is applicable to any type of chemical pulp, regardless of the starting plant (resinous wood or

hardwoods, annual plants, etc.).

T A B L E A U I

  EXAMPLE. 1 i 1 2 3 4 5 6 7 Pre-treatment No No No Yes Yes Yes Yes No Yes Percent C 2 - - - - 1 1 1 1.5 Temperature OC 70 90 70 70 70 Duration in minutes 10 10 10 10 - 15 Concentration in block 10 10 10 10 12 Treatment 02 or H202 Yes yes YES Yes Yes No Yes Yes Oxygen pressure I

in bars 5 5 5 5 5 5 - - -

  Percent H202 - _ _ _ 1 1 NaOH in percent - 2 3 4 3 3 4 3 3 3 Kappa Index 30.0 14.0 12.1 10.2 9.7 8.5 7.9 24.0 17.1 13.0

  DP 1600 1400 1280 1130 1350 1450 1300 - 125C 1470

-

-... 1

- 11 -

Claims (5)

  1 / Process for the preparation of paper pulp   chemical res with a view to their bleaching, of the type in   which said paste is treated with chlorine and then   following by means of an oxygenated compound,; - characterized in that   that the chlorine pretreatment is carried out on a   te having a consistency between 5 and 15%, at a temperature between 50 and 100 C, with a chlorine concentration between 0.5 and 2% o 2 / A method according to claim 1, characterized in that the chlorine pretreatment is performed at a   temperature between 70 and 90 C.   3 / Method according to claim 1, characterized   in that the duration of the chlorine pretreatment is   taken between five and thirty minutes, preferably between ten and twenty minutes.   4 / A method according to claim 1, characterized   in that the chlorine concentration during the pre-   treatment is between 0.8 and 1.5% by weight per relative to the weight of the dough.   / Method according to claim 1, characterized   in that during the chlorine pretreatment, the   dough strength is between 8 and 12%.   6 / A method according to claim 1, characterized   in that the oxygenated compound is oxygen.   7 / A method according to claim 1, characterized   in that the oxygenated compound is hydrogen peroxide-   only react to a temperature close to C. DEPOSITOR: Industrial Technical Center called "CENTER   TECHNIQUE OF THE PAPER INDUSTRY, CAR-   TONS ET CELLULOSES "(Law of 07/22/1948) REPRESENTATIVE: Cabinet Michel LAURENT