FR2553444A1 - HYPOCHLORITE TREATMENT OF CHEMICAL PAPER PULP - 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

HYPOCHLORITE TREATMENT OF CHEMICAL PAPER PULP

  The invention relates to a new hypochlorite treatment for chemical pulp.

  In the remainder of the description, the expression “chemical pulps” designates mainly unbleached or lightened cellulose pulps, that is to say still containing lignin, obtained by various processes, for example with acid sulfite, neutral or alkaline, sulphate

  (Kraft process), with sodium hydroxide, carbonate or oxygen, with or without the presence of a delignification catalyst.

  As is known, the bleaching of cellulosic pulps is the treatment which consists in eliminating by action of chemical reagents, the dyestuff associated with cellulose fibers, a large part of which is made up of lignin in a very condensed and profoundly modified state cooking reactions The whitening of chemical pasta is done most of the

  time using chlorinated agents, such as chlorine, chlorine dioxide or sodium hypochlorite.

  Conventionally, the bleaching is carried out by sequences of the CE 1 D 1 E 2 D 2 type, that is to say first of all a treatment with chlorine (C), then a first extraction with soda (E 1), then a first treatment with chlorine dioxide (D 1), followed by a second extraction with sodium hydroxide (E 2) and finally a second treatment with

chlorine dioxide (D 2).

  Attempts have been made to reduce the cost of bleaching by reducing the number of stages in the sequence and / or by using other chemical reagents. It has been proposed, for example, to introduce an oxidizing agent into the

  alkaline extraction stages (E) or adding a stage using such an agent immediately after an alkaline extraction; this agent could be oxygen.

  However, this process uses oxygen gas which is sparingly soluble in alkaline liquors, which requires the use of expensive mixing systems. Furthermore, the effect of oxygen is only noticeable in the first stage of alkaline extraction (El), and is

  practically zero in the second (E 2).

  At the same time, it has already been proposed to introduce into alkaline extractions, either sodium hypochlorite or hydrogen peroxide. Current evaluations show that the first compound has significant economic advantages (see publication: TAP Pl Journal August 1983 Volume 66, n 8, pages 77 et seq. Also, for these economic reasons, the treatment with sodium hypochlorite (H) in the bleaching sequences, is gaining interest However, it is important to control perfectly the p H of the reaction, as well as the quantity of hypochlorite introduced and the treatment temperature to avoid degradation of

cellulose by hypochlorite.

  moreover, in French patent FR-A-2 255 418, it has been proposed to treat the chemical pulp using a mixture of hypochlorite and hydrogen peroxide.

  two reactants brought into contact immediately react one on the other to give singlet oxygen, which is a very powerful oxidizing agent, but also very degrading, so that this treatment is in fact an oxidation of the paste by singlet oxygen.

  The invention overcomes these drawbacks. It relates to a process which improves the efficiency of the stages of alkaline treatment of the bleaching sequences of chemical pulps, but which is easier and more economical to use and this without degradation of the cellulose, while by improving the whiteness of the doughs thus treated,

  without significant deterioration in mechanical properties.

  This process for bleaching paper pulp

  chemical with an alkaline solution of hypochlorite 35 is characterized in that, just before the end of this treatment

  hypochlorite, and without prior washing of the

  treated dough, hydrogen peroxide is added to this dough.

  Advantageously in practice: hydrogen peroxide (H 202) is added when at least 90% of the hypochlorite has been consumed; this addition is carried out during the alkaline treatment itself; not only a better lignin extraction is obtained, but also a greater whitening of the dough, without harming the quality thereof; hydrogen peroxide is added with soda; the addition of the peroxide is carried out at the same temperature as the treatment with hypochlorite; the peroxide is added in an amount of 0.1 to 2%, and advantageously from 0.2 to 0.5%; the amount of soda added to the hydrogen peroxide is less than 2%; the hypochlorite is advantageously sodium or calcium hypochlorite; the treated paste is a kraft paste or a sulfite paste. Reagent levels are expressed, as well as in

  what follows, in percent (%) by weight relative to the weight of the dry counted dough.

  As already said, doughs treated in this way only retain all their mechanical properties, but exhibit remarkable whiteness, without

  having to resort to an expensive succession of processing phases.

  It is surprising that by adding hydrogen peroxide, optionally added with sodium hydroxide, in the reaction medium containing the hypochlorite and the paste which has already reacted, at least in part preponderant, and this without any washing or any significant modification of the operating conditions, it is possible to significantly continue the delignification and the bleaching of the dough and this, without degrading it. In fact, the teachings of the prior art led to think that one would obtain an opposite effect, since: one had to expect degradation of the dough by the singlet oxygen thus produced; the preferential action of peroxide on wood products solubilized by hypochlorite should lead to consumption of peroxide by these products; the treatment is carried out at a relatively low temperature (400 ° C.), at which the peroxide is generally considered to be ineffective; finally, part of the peroxide introduced should be decomposed by hypochlorite or residual hypochlorous acid. As already said, we add hydrogen peroxide

  when at least 90% of the hypochlorite has been consumed.

  It can be considered that this 90% threshold has been reached after a reaction time representing at least three quarters of the total reaction time usually applied. 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 by way of non-limiting indication.

Example 1

  A witness is produced by operating in the following manner.

  A conventional unbleached kraft pulp of softwood, Kappa index 31 (AFNOR standard NF-T-12018) and degree of polymerization (DP) 1650 (standard TAPPI-T-230 of 76) is treated by conventional chlorosodation. whiteness of 27 (AFNOR standard NF-Q-03039) The conditions 35 of this treatment are as follows: Chlorination stage (C): 6% chlorine, at 20 C for 45 minutes, with a paste concentration of

3.5%;

  Alkaline extraction stage (E) with 3% sodium hydroxide, at 70 ° C., for 60 minutes, with a paste concentration of 10%. After chlorosodation, (CE), the paste undergoes a second alkaline extraction carried out this time in the presence of sodium hypochlorite (H) (Na Cl O), at a rate of 2%, 10 to 40 C, for two hours, with a concentration in

10% paste.

  After the CEH sequence, the paste has a whiteness of 56.5 and a DP of 1620.

  In a first variant, the hypochlorite level is increased to 3% instead of 2% and the treatment time to 2 hours 30 minutes instead of 2 hours The paste obtained has a whiteness of 69.5 for a DP of 1,330 , what

  illustrates the degrading effect of hypochlorite.

  In another variant, the level of sodium hypochlorite is further increased to 3.5%. The treated pulp has a whiteness of 71.0 and a DP of 1150. This perfectly illustrates the limits of treatment with hypochlorite of

  sodium Admittedly, this treatment significantly improves whiteness, but at the expense of the quality of the dough.

  Also, if we plan to reduce the cost of bleaching by replacing the CEDED sequence with a CEHDED sequence, or more simply CEHD, the level of hypochlorite used, and therefore the reduction in the level of chlorine dioxide,

are limited.

  Example 2 The pulp of example 1 undergoes the two stages of chlorosodation (CE) of example 1, then is treated with 2% of hypochlorite under the conditions of example 1 However, after 90 minutes of reaction, the dosage 35 by iodometry of the treatment effluent indicates that -6

  92% of the sodium hypochlorite was consumed.

  0.5% by weight of hydrogen peroxide added with 1% sodium hydroxide is then added to the reaction medium. These two compounds are added with a minimum of water, so as not to substantially modify the treatment conditions, particularly the temperature, which remains close to 40 C. After 60 minutes, the treatment is stopped (which thus lasts 2 hours 30 minutes) The degree of whiteness obtained is 10 72.0 and the degree of polymerization (DP) has been maintained

at 1,560.

  Thus, not only does it improve the whiteness, but above all it does not appreciably degrade the cellulose. Such a result cannot be achieved by a single conventional treatment with hypochlorite and is nevertheless obtained in a single stage by the addition of peroxide

  of hydrogen and / or soda at the end of the hypochlorite treatment.

Example 3

  By way of control, a unbleached unbleached magnesium bisulphite pulp of resinous wood, of index Kappa 18.7, of DP 1500, is bleached conventionally by a PD 1 ED 2 sequence (P being a peroxide stage), under the following conditions: P: 1% H 202 supplemented with 1.5% soda, at C, for 90 minutes, with a paste concentration of 12%; D 1: 1% of chlorine dioxide at 78 C, for minutes with a paste concentration of 9.5%; E: 1% soda at 60 C for 40 minutes with a paste concentration of 10%; D 2: 0.2% chlorine dioxide at 78 C for

  minutes with a paste concentration of 9.5%.

  At the end of these four stages of bleaching, we

  obtains a whiteness paste 90, the DP of which is 1450.

Example 4

  A second witness is produced by operating in the following manner. The same paste is bleached as in Example 3 with a PDH sequence, the first two stages P and D being carried out under the same conditions as in Example 3. It is then treated with sodium hypochlorite in the right manner. 0.5% (Na CO l), at 36 C for 180 minutes

  with a paste concentration of 8.5%.

  The dough thus treated has a whiteness of 87.0 and a

PD of 1,451.

  If the amount of hypochlorite is increased to 1.5% and the duration of the treatment to 210 minutes, the whiteness is

  only improved by one point (88.0), but the PD drops from 15 to 1,197.

  Thus, the conventional stage with sodium hypochlorite certainly makes it possible to achieve a satisfactory objective of whiteness, but results in a notable degradation.

paste.

  Example 5 The preceding example is repeated with 0.5% sodium hypochlorite. After 135 minutes of treatment with hypochlorite, 0.5% of hydrogen peroxide and 0.5% of sodium hydroxide are added to the reaction mixture. , and this with the minimum of water, so as not to modify the temperature

of the treatment bath (38 "C).

  After 75 minutes of reaction, a whiteness of 90 and a DP of 1,400 are obtained.

  Thus, if we compare the result obtained in this example with those obtained in the previous example, we have

  improved whiteness by three points and the sequence could be shortened to three stages without degrading the dough.

  In other words, the traditional four-stage PDED sequence of Example 3 can be replaced by a sequence comprising only three PD (HP) stages, -8

  in which (HP) is the alkaline stage according to the invention.

  Thus, the process according to the invention makes it possible both to reduce the consumption of chlorine dioxide in the sequences of the CEHDED, CEDHD type, as well as to reduce the number of bleaching stages, as illustrated

examples 4 and 5 above.

  The process according to the invention has many advantages compared to those exploited to date. One can cite: the possibility of obtaining an improved whiteness or at the very least retained, and this without causing degradation of the dough; the reduction in the number of bleaching stages, since there is no intermediate washing, which results in a substantial reduction in the cost of bleaching. This process can be successfully applied to all types of chemical pulp, regardless of the starting plant (softwood, hardwood, plants

annuals, etc.).

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Claims (7)

  1 / Process for bleaching chemical pulp using an alkaline hypochlorite solution, characterized in that, just before the end of the hypochlorite treatment, and without washing the treated pulp beforehand, the following is added to this paste of hydrogen peroxide. 2 / A method according to claim 1, characterized   in that hydrogen peroxide is added when at least 90% of the hypochlorite has been consumed.   3 / A method according to claims 1 and 2, characterized in that the hydrogen peroxide is added soda.   4 / A method according to claim 1, characterized in that the addition of hydrogen peroxide takes place   at the same temperature as the hypochlorite treatment.   / Method according to claim 1, characterized   in that the hydrogen peroxide is added in an amount of 0.1 to 2% by weight of the weight of the dough.   6 / A method according to claim 5, characterized in that the hydrogen peroxide is added in a proportion   0.5% by weight of the weight of the dough.   7 / A method according to claim 3, characterized in that the amount of soda added to the peroxide   of hydrogen is less than 2% by weight of the weight of the pulp.   8 / A method according to claim 1, characterized in that the hypochlorite is a sodium hypochlorite or calcium.   9 / Kraft pulp or sulphite pulp bleached with   method according to one of claims 1 to 8.   DEPOSITOR: Technical Center called "TECHNICAL CENTER OF THE PAPER, CARDBOARD AND CELL INDUSTRY (Law of July 22, 1948) AGENT: Cabinet Michel LAURENT