EP0535741B1 - Process for improving the selectivity in the delignification of a chemical pulp - Google Patents

Abstract

A process for improving the selectivity of delignification of a chemical paper pulp, in which the pulp is treated with an aqueous solution of an organic peroxy acid at 2 - 47 degrees C., followed by a water wash and a supplementary stage of alkaline extraction in the presence of hydrogen peroxide.

Description

The invention relates to a method of delignification of a chemical paper pulp. It particularly concerns a process for improving the selectivity of such an operation delignification.

It is known to treat unbleached chemical pulp obtained by cooking cellulosic materials in the presence of chemical reagents through a sequence of processing steps delignifying and whitening implying the implementation of oxidizing chemicals. The first step in a sequence conventional chemical pulp bleaching aims to perfect the delignification of the unbleached dough as it is present after the cooking operation. This first step delignifying is traditionally carried out by treating the dough unbleached by chlorine in an acid medium or by a chlorine association - chlorine dioxide, in mixture or in sequence, so as to react with the residual lignin from the pulp and give rise to chlorolignins which can be extracted from the pulp by solubilization of these chlorolignins in an alkaline medium in a step further processing.

Thus, according to FR-A-1 014 536 a chemical wood pulp is treated first with a hypochlorite and then with a peracid at low temperatures.

For various reasons, it is useful, in some situations, to be able to replace this significant first step by a treatment which no longer uses a reagent chlorine.

The bleaching described in FR-A-1 014 536 can optionally be carried out in a single step with the peracid.

It has also been proposed to treat chemical pulps with a first step with peracetic acid at higher temperatures at 50 ° C and pH between 3 and 9 (Bailey C.W. and Dence C.W., "Peroxyacetic Acid Bleaching of Chemical Pulps", Tappi, January 1966, Vol. 49, No. 1, pages 9 to 15). In this known process, it appears however that the acid treatment peracetic gives rise to pastes of viscosity and properties mechanical not as good as pasta delignified by a step traditional with chlorine in an acid medium due to selectivity less delignification which results in a more marked attack on the cellulose chains.

JP-A-57 21591 discloses a bleaching process in which a Paa step is carried out, and sodium hydroxide is added to the medium resulting from this step to activate the residual peroxide, without washing between the Paa and Ep steps .

The invention aims to remedy the drawbacks of the methods known by providing a process which achieves delignification efficiency of unbleached chemical pulp with selectivity which allows the production of pasta with high intrinsic qualities.

To this end, the invention relates to a method for improving the selectivity of the delignification of a dough chemical paper that the unbleached dough from the cooking operation is subjected to a treatment by an aqueous solution of an organic peroxyacid at a temperature between 2 and 47 ° C, then washing with water only we follow with a step additional processing consisting of an extraction alkaline in the presence of peroxide hydrogen.

According to the invention, by chemical paper pulp is meant designate pasta that has undergone a minor treatment by presence of chemical reagents such as sodium sulfide in alkaline medium (kraft or sulphate cooking), anhydride sulfurous or a metallic salt of sulfurous acid in the middle acid (sulphite cooking). Semi-chemical pastes such as those where cooking was carried out using an acid salt sulfurous in neutral medium (cooking with neutral sulfite again called NSSC cooking) or those obtained by cold caustic soda followed by mechanical defibration can also be bleached and / or delignified by the process according to the invention.

This is particularly intended for pasta that has undergone kraft cooking and the residual lignin content after cooking is in the range of kappa indices between 8 and 40 (or even having a permanganate number between 5 and 30) according to the type of wood species from which they come and the efficiency of the cooking process. All types of wood used for the production of chemical pulp suitable for the implementation of the process of the invention and, in particular those used for kraft pulp, namely softwood like, for example, the various species of pines and firs and hardwoods such as, for example, eucalyptus, beech, oak and charm.

According to the invention, the organic peroxyacid is selected among the aliphatic carboxylic peroxyacids comprising a only percarboxylic group and a saturated alkyl chain linear or branched with less than 11 carbon atoms. The aliphatic carboxylic peroxyacids with saturated alkyl chain linear with less than 6 carbon atoms are preferred. Examples of such peroxyacids are performic acid, peracetic acid, perpropanoic acid, acid n-perbutanoic and n-perpentanoic acid. Peracetic acid is particularly preferred because of its effectiveness and its good biodegradable properties.

In a variant of the process according to the invention, the peroxyacid organic is selected from diperoxyacids carboxylic comprising a linear or branched alkyl chain of less than 16 carbon atoms and two percarboxylic groups substituted on carbon atoms located in position alpha-omega relative to each other. Examples of such peroxyacids are 1,6-diperoxyhexanedioic acid, acid 1,8-diperoxyoctanedioic acid and 1,10-diperoxydecanedioic acid, and 1,12-diperoxydodecanedioic acid.

In another variant of the process according to the invention, the organic peroxyacid is selected from peroxyacids aromatics comprising at least one percarboxylic group by benzene nucleus. Preferably, we will choose peroxyacids aromatics which have only one percarboxylic group by benzene nucleus. An example of such an acid is peroxybenzoic acid.

Another variant of the process according to the invention consists in choose an organic peroxyacid substituted by one or more halogen atoms or any other functional substituent organic. By any other organic functional substituent, intends to designate a functional group such as the group carbonyl (ketone, aldehyde or carboxylic acid), the group alcohol, nitrogen-containing groups such as nitrile, nitro, amine and amide groups, the groups containing sulfur such as the sulfo and mercapto groups.

The peroxyacid can indifferently be used at the state of an aqueous solution of peroxyacid or in the form an ammonium, alkali metal or alkaline earth metal salt of this peroxyacid.

Preferably, treatment with organic peroxyacid takes place at a temperature between 35 and 45 ° C. Generally, the treatment is carried out with peroxyacid organic at atmospheric pressure. The duration of this treatment depends on the temperature and the type of wood used prepare the dough, as well as the efficiency of the baking which has preceded. Duration of approximately 120 minutes to approximately 240 minutes agree well, and it is even possible to exceed this limit of about 100% without risk of substantial degradation mechanical strength properties of the dough. When the treatment temperature is high, i.e. above 45 ° C, it is possible to use shorter durations from about 45 minutes to about 120 minutes. The treatment according to the invention can be done in any type of suitable equipment for the treatment of paper pulp using acid reagents. The retention tank for the unbleached dough present in all bleaching and playing facilities the role of buffer tank between the wood cooking unit and the dough bleaching unit is particularly suitable for carrying out the method according to the invention. The dough can thus be treated there during its storage without requiring the investment of an expensive specific device. The faculty of be able to safely exceed the normal duration of treatment is particularly well suited to the role of storage buffer for the retention tank.

The consistency of the peroxyacid treatment step organic will generally be chosen between 2 and 40% of materials dry and, most often, between 10 and 40%. We prefer high consistencies, for example those between 20 and 40%.

In the process according to the invention, the amount of peroxyacid organic used is chosen according to the rate of residual lignin in the dough as well as the average duration of the treatment. Generally, amounts between 0.5 and 4 X by weight of peroxyacid relative to the dry paste agree well. Most often, a quantity of peroxyacid not exceeding 3% by weight with respect to the paste dry and greater than 1% of this weight.

Alternatively, it may be interesting to precede the treatment with organic peroxyacid by a step of decontaminating pretreatment with an aqueous solution acid. The purpose of this step is to extract the dough from the impurities present in the form of metal ions which are harmful to the proper conduct of laundering operations and / or of delignification. All inorganic or organic acids used in aqueous solution, alone or as a mixture are suitable. Strong inorganic acids such as, for example, acid sulfuric or hydrochloric acid are suitable.

It is advantageous that the acid decontaminating pretreatment is also carried out in the presence of an ion complexing agent metallic. To this end, mixtures of strong acids inorganic mentioned above with organic acids of the class of aminopolycarboxylic or aminopolyphosphonic acids or their alkali metal salts are suitable particularly well. Examples of aminopolycarboxylic acids suitable are diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid, cyclohexanediaminetetraacetic acid and nitrilotriacetic acid. Diethylenetriaminepentaacetic acid (DTPA) is preferred. Of examples of aminopolyphosphonic acids are diethylenetriaminepentamethylenephosphonic acid (DTMPA), ethylenediaminetetra (methylenephosphonic acid), cyclohexanediaminetetramethylenephosphonic acid (CDTMPA) and nitrilotri acid (methylenephosphonic). DTMPA is preferred.

The operating conditions of the decontaminating pretreatment acid are not critical. They must be determined in each particular case depending on the type of pulp and the apparatus in which the treatment is carried out. Of a in general, the choice of acid and the quantity used to impose on the medium a pH below 7, for example between approximately 1 and approximately 6.5. PH especially advantageous are those between about 2.0 and about 5.0. Temperature and pressure are not critical, ambient temperature and atmospheric pressure generally well suited. The duration of the pretreatment can vary widely depending on the type of equipment used, the choice of acid, temperature and pressure, by example from about 15 minutes to several hours.

You can also replace the decontaminating pretreatment by incorporation into the peroxyacid delignification stage itself of one or more complexing agents of metal ions. The latter are chosen from the same agents complexing agents than those described above for the decontaminating treatment.

In another variant of the process according to the invention, it is can, if one wishes to obtain high levels of whiteness, make follow the treatment with a peroxyacid and the extraction in the presence of hydrogen peroxide, by a sequence traditional bleaching steps, whether or not involving chlorinated reagents. Examples of such steps are: following: steps with gaseous oxygen or ozone, steps with alkaline hydrogen peroxide in the presence or absence of gaseous oxygen, chlorine dioxide or sodium hypochlorite steps, alkaline extractions with caustic soda.

The following examples are given for the purpose of illustrating the invention, without limiting its scope.

Examples :

• Indice kappa : TAPPI Method T236, CM85.
• Viscosité : TAPPI Method T230, OM89.

The measurements carried out in the examples described below were carried out according to the following standards:

• Kappa index: TAPPI Method T236, CM85.
• Viscosity: TAPPI Method T230, OM89.

A sample of Ponderosa pine kraft paste kappa index of 28.4 and viscosity of 32.1 mPs was subjected to a step of delignification using an aqueous acid solution peracetic followed by an alkaline extraction step in the presence hydrogen peroxide (Paa Ep sequence).

Four tests were carried out, two under the conditions of temperature and duration of the prior art, noted examples 1R and 2R, and two under the conditions of the invention (noted examples 3 and 4). Example 1R, unlike the other three examples, did not not involve washing the dough with water between the peracetic acid treatment and extraction step alkaline. The hydrogen peroxide present in the step alkaline extraction of Example 1R also comes from residual hydrogen peroxide from the peracetic acid stage which preceded. In Example 4, according to the invention, we have made precede the peracetic acid step with an acid pretreatment using diethylenetriaminepentaacetic acid or DTPA (step Q).

In each of these tests, 2.0 g of peracetic acid were used per 100 g of dry paste. The peracetic acid employed was an aqueous solution of commercial quality and contained hydrogen peroxide in equilibrium with peracetic acid (2.6 g of H ₂ O ₂ per 100 g of dry paste).

The operating conditions and the results obtained are shown in the table below. Ex. No Step Chemical reagents,% Min duration Temperature ° C Kappa index Viscosity mPs PAA H ₂ O ₂ NaOH DTPA 1R Paa 2.0 2.6 60 70 24.0 Ep 2.0 5.4 60 60 17.8 19.5 2R Paa 2.0 2.6 60 70 24.0 Ep 0.5 3.0 60 75 16.5 19.4 3 Paa 2.0 2.6 240 38 22.9 Ep 0.5 3.0 60 75 16.4 24.4 4 Q 0.3 15 45 Paa 2.0 2.6 120 45 25.3 Ep 0.5 2.0 60 75 15.2 29.3

Compared to the unbleached pulp, the results of delignification and loss of viscosity after the alkaline extraction step are as follows: Example Delignification,% Viscosity loss,% 1R 37.3 39.2 2R 41.9 39.6 3 42.2 24.0 4 46.5 8.7

Claims (10)

1. Process for improving the selectivity of the delignification of a chemical paper pulp according to which the unbleached pulp originating from the cooking operation is submitted to a treatment with an aqueous solution of an organic peroxy acid, at a temperature of between 2 and 47 °C, then to a washing with water which is followed by a supplemental treatment consisting of an alcaline extraction in the presence of hydrogen peroxide.
2. Process according to Claim 1, in which the temperature of the organic peroxy acid treatment is between 35 and 45 °C.
3. Process according to Claim 1 or 2, in which the organic peroxy acid is selected from aliphatic peroxycarboxylic acids containing a single percarboxyl group and a saturated linear or branched alkyl chain of less than 11 carbon atoms.
4. Process according to Claim 1 or 2, in which the organic peroxy acid is selected from diperoxycarboxylic acids containing a linear or branched alkyl chain of less than 16 carbon atoms and two percarboxyl groups substituted on carbon atoms located at the alpha,omega-position with respect to one another.
5. Process according to Claim 1 or 2, in which the organic peroxy acid is selected from aromatic peroxy acids containing one percarboxyl group per benzene ring.
6. Process according to any one of Claims 1 to 5, in which the organic peroxy acid is employed in the form of one of its ammonium, alkali metal or alkaline earth metal salts.
7. Process according to any one of Claims 1 to 6, carried out in the storage chest for the unbleached pulp which feeds the bleaching unit.
8. Process according to any one of Claims 1 to 7, in which the treatment with the organic peroxy acid is preceded by a step of decontaminating pretreatment by means of an acidic aqueous solution.
9. Process according to Claim 8, in which the acidic aqueous solution contains at least one compound which sequesters metal ions.
10. Process according to any one of Claims 1 to 7, in which a compound which sequesters metal ions is incorporated with the peroxy acid treatment.