EP0801169A1 - Process for the delignification and bleaching of chemical paper pulp - Google Patents

Abstract

The delignification and bleaching of a chemical paste comprises a pretreatment with a complexing or sequestering transition metal complex followed by treatment with hydrogen peroxide at a temperature T greater than 100 degrees C, a pressure greater than 1.5 times the pressure of saturated water vapour at temperature T and in the presence of a polymer with monomer units of formula -ÄC(R1)(R2)-C(OH)(COOM)Ü- (I) and/or (II) and in the presence of a compound A selected from potassium hydroxide, sodium hydroxide and alkali(ne earth) metal carbonates. In the formulae, R1, R2 = optionally different, H, 1-3C alkyl-group, M = H, ammonium group or alkali metal or alkaline earth metal.

Description

The present invention relates to a process for the delignification and bleaching of chemical paper pulps.

Chemical paper pulps or chemical pulps are those obtained by cooking lignocellulosic materials, in particular wood, in the presence of chemical agents such as sodium hydroxide for KRAFT pulps, sulfite or bisulfite.

All types of wood can be used. By way of example, there may be mentioned softwoods such as the various species of pines and firs, hardwoods such as birch, poplar, beech and eucalyptus.

In general, the chemical pulps obtained by cooking are subjected to several stages of delignifying treatments and / or bleaching. The first steps of perfecting the delignification resulting from cooking are followed by those of bleaching.

At the end of these delignifying and bleaching treatments, the pastes must generally have a high level of whiteness, a very low Kappa index while retaining good mechanical properties, that is to say without significant degradation of the cellulose. . This degradation is detectable by measuring the degree of polymerization (DP) of the dough. The PD should be kept as high as possible.

Thus in application WO95 / 31598 a method of delignification and bleaching is described comprising a step of treatment with hydrogen peroxide in the presence of the alkali metal silicate at a temperature T greater than 100 ° C and at a pressure greater than 1 , 5 times the pressure of the saturated water vapor at temperature T. As indicated in Table I of this application, the presence of silicate is necessary to obtain a delignified paste having both a whiteness level and a DP high.

Furthermore, the article by MM. Bertel STROMBERG and Richard SZOPINSKI entitled "Pressurized hydrogen peroxide bleaching for improved TCF bleaching" and presented at the conference "1994 International Pulp Bleaching Conference" shows that bleaching by hydrogen peroxide under pressure leads to substantial degradation of the cellulose.

Despite these prejudices, the Applicant treated the chemical pulps under pressure, with hydrogen peroxide and in the absence of silicate.

Indeed, the Applicant has discovered a new process for delignification and simultaneous bleaching with hydrogen peroxide of a chemical pulp, making it possible to obtain a pulp which is highly bleached and which has retained a good degree of polymerization.

dans lesquelles R₁ et R₂, identiques ou différents, représentent chacun un atome d'hydrogène ou un groupement alkyle comprenant de 1 à 3 atomes de carbone et M représente un atome d'hydrogène, un groupement ammonium ou un métal alcalin ou alcalino-terreux, et en présence d'un composé A choisi parmi l'hydroxyde de potassium, l'hydroxyde de sodium, les carbonates de métal alcalin ou alcalino-terreux.This process is characterized in that after pretreatment with a complexing or sequestering agent of transition metals, in particular manganese, it comprises one or more stage (s) in which the dough undergoes a treatment with hydrogen peroxide to a temperature T greater than 100 ° C., at a pressure greater than 1.5 times the saturated steam pressure at temperature T, in the presence of a polymer comprising units of formula (I) and / or (II ):

in which R ₁ and R ₂ , identical or different, each represent a hydrogen atom or an alkyl group comprising from 1 to 3 carbon atoms and M represents a hydrogen atom, an ammonium group or an alkali or alkaline metal earthy, and in the presence of a compound A chosen from potassium hydroxide, sodium hydroxide, alkali or alkaline earth metal carbonates.

Advantageously, unsubstituted poly-α-hydroxyacrylic acid, the corresponding polylactone and / or the salts of unsubstituted poly-α-hydroxyacrylic acid (with R ₁ and R ₂ = H) are used. Among these salts, the potassium, sodium, magnesium and calcium salts are advantageously chosen.

The average molecular weight of the polymer comprising units of formula (I) and / or of formula (II) is generally between 1,000 and 800,000 and preferably between 2,000 and 100,000.

The polymer comprising units of formula (I) and / or of formula (II) is known as a stabilizing agent for peroxide solutions (GB 1 524 013, FR 2 601 025). It can be prepared using the methods described in patents FR 2 237 914, FR 2 237 916 and FR 2 628 745.

Compound A is advantageously chosen from carbonates, such as sodium carbonate, potassium carbonate, magnesium carbonate and calcium carbonate, since the process according to the present invention offers in this variant the advantage of not produce no liquid effluent (TEF, Totally Effluent Free). Thus after evaporation of the washing water (washing is described below) and incineration of organic materials, the carbonate of alkali or alkaline earth metal is easily regenerated, without the need for a lust caustification step. Preferably, sodium carbonate is used.

In the following, the quantities of the products and reagents, unless otherwise specified, are always expressed in percent by weight relative to the weight of the dry matter of the paste.

The consistency of the dough is expressed in percent by weight of dry matter relative to the total weight of the dough.

The amount of polymer used is generally between approximately 0.05% and approximately 1.5%, preferably between approximately 0.1% and approximately 1% and more particularly between approximately 0.2% and approximately 0.5%.

Depending on the initial paste used and the amount of hydrogen peroxide involved, compound A is used at a rate of 1 to 15% and preferably 6 to 10% for the carbonates.

The amount of hydrogen peroxide used can vary from 0.5 to about 10%. Preferably, an amount of hydrogen peroxide is used between about 1 and about 4% and very particularly between about 1.5% and about 2.5%.

During the treatment step with hydrogen peroxide, it is also possible to use a sequestering agent such as DTPA (sodium diethylenetriaminepentaacetate) or EDTA (sodium ethylenediaminetetraacetate) and preferably in an amount less than 0.2% .

According to the present invention, the paste, before treatment with hydrogen peroxide, can be subjected to one or more step (s) of delignification with ozone and / or chlorine dioxide and / or organic or mineral peracids and / or oxygen, known from the paper industry. Preferably oxygen is used.

After the delignifying treatment, the dough can be washed one or more times with hot or cold water.

Any pasta with a Kappa index (SCAN C1-59 standard) not exceeding 17 before the treatment with hydrogen peroxide is particularly suitable. MCC (modified continuous cooking), EMCC (extended modified continuous cooking) and "Super Batch", whose Kappa index after cooking can reach values as low as 15-18 for softwoods and 13-15 for hardwoods are advantageously used.

The transition metal complexing or sequestering agent used in the pretreatment can be chosen from DTPA, EDTA, phosphonic acids or the salts of phosphonic acids. Several agents can also be combined to increase the efficiency of the pretreatment with regard to a larger number of metals.

The amount of complexing or sequestering agent is generally between approximately 0.05% and approximately 1%. It is preferred to use an amount between about 0.1% and about 0.5%.

The pretreatment temperature is generally from 20 to 100 ° C and preferably between about 60 and about 90 ° C.

The duration of the pretreatment with the complexing agent is generally from 1 to 30 minutes and preferably from 5 to 15 minutes.

The consistency of the dough during pretreatment can vary within limits from 1 to 25%. A consistency of between 5 and 15% is preferred.

Although the pretreatment with the complexing agent can be carried out in an acid pH medium, it is preferred to operate at basic pH. Advantageously, the pH is greater than 7 and less than or equal to 12.5. A pH between 8 and 10 is particularly preferred.

The alkaline pH during the pretreatment can be obtained either by the residual alkalinity of the pulp at the end of the oxygen treatment, or by the alkalinity of the complexing or sequestering agent, or even by the addition of a base like NaOH.

For most pastes, the residual alkalinity of the pulp combined with that of DTPA makes it possible to obtain a pH close to 9 without the addition of sodium hydroxide.

Preferably, the manganese content of the paste before the treatment with hydrogen peroxide does not exceed 5 ppm by weight relative to the weight of the dry matter of this same paste.

After the complexing pretreatment, the paste is washed with water. Washing can be carried out according to techniques known in the paper industry with hot or cold water.

To the paste resulting from the complexing pretreatment, hydrogen peroxide, compound A, the polymer comprising units of formula (I) and / or (II) and optionally water are added so as to obtain the chosen consistency, then the mixture is subjected to a pressure greater than 1.5 times the pressure of the saturated water vapor at treatment temperature T and then the mixture is brought to temperature T. The reactants are added to the paste preferably at room temperature or at a temperature below about 60 ° C.

According to a second variant, it is possible at first to increase the pressure then mix the reagents with the paste and simultaneously increase the temperature.

Preferably one operates according to the first variant.

The devices generally used in the paper industry for cooking pulp and making it possible to keep the pulp impregnated with the aqueous solution of hydrogen peroxide, of compound A and of the polymer at a high pressure and at a high temperature for the chosen period can suitable for carrying out the treatment step with hydrogen peroxide, according to the invention.

After this treatment, the dough is decompressed, optionally cooled and then washed with water so as to remove all of the soluble organic and mineral materials. The washing water can then be concentrated by evaporation and then incinerated in a boiler according to the usual techniques of the paper industry.

The ash obtained consists mainly of recyclable carbonate after purification.

The effluents from this treatment stage containing only organic matter and metallic carbonates, free from chloride and silicate, can also be treated with those from the KRAFT pulp unit (black liquor).

The consistency of the paste, during treatment with hydrogen peroxide is generally between about 4 to 35%. The process can be carried out efficiently at low consistency of about 4 to about 10% and the reaction medium can be easily transferred by pumping.

A consistency of the dough of between approximately 15 and approximately 25% makes it possible to obtain high levels of whiteness and delignification while saving heating energy. A consistency of between approximately 8 and approximately 20% is advantageously chosen since it makes it possible to optimize the yield of the process.

The pressure to which the dough is subjected generally reaches a value greater than 1.5 times the pressure of the saturated steam at the temperature T of the treatment with hydrogen peroxide before the temperature of the medium exceeds 100 ° C. Preferably the pressure is greater than 2 times the pressure of the saturated steam at the treatment temperature T.

Advantageously, a pressure of between 5 and 200 bar absolute is used. For practical reasons of implementation, the pressure is preferably between 5 and 50 bars absolute. A pressure of between 5 and 20 bar absolute is economically preferred.

The pulp can be pressurized by any appropriate means allowing a pressure greater than 1.5 times the pressure of the saturated water vapor to be obtained at the treatment temperature T. Thus this pressure can be established using a compressed gas such as air or nitrogen. It can also be obtained by pumping the dough with a centrifugal or volumetric high pressure pump in a closed enclosure.

The reaction temperature T is most often between 110 and 180 ° C and advantageously from 120 to 150 ° C.

The treatment with hydrogen peroxide generally lasts from 1 minute to 3 hours. The duration varies in opposite direction to the increase in temperature. Preferably the duration is from 15 minutes to 1 hour. These relatively short durations allow the hourly output of the manufacture of delignified and bleached pulp to be increased.

At the end of the treatment with hydrogen peroxide, the paste can undergo a second stage of treatment under the same conditions as previously or under the usual conditions (temperature below 90 ° C., atmospheric pressure, alkaline medium in the presence of either sulfate magnesium or sodium silicate) or it can undergo treatment with chlorine dioxide under conditions known to the paper industry.

• Blancheur : norme ISO 2470
• Indice Kappa : norme SCAN C1-59
• Degré de Polymérisation (DP) : norme Scan SC 15-12.

The definitions of the terms used above and thereafter meet the following standards:

• Whiteness: ISO 2470 standard
• Kappa index: SCAN C1-59 standard
• Degree of Polymerization (DP): Scan SC 15-12 standard.

EXPERIMENTAL PART General procedure a) Pretreatment with the complexing or sequestering agent

The paste, after cooking and optionally after delignification with oxygen, is suspended at 10% consistency with 0.5% of a commercial solution at 40% by weight of DTPA and heated for 15 minutes at 90 ° C. The final pH is 8 to 10 depending on the paste chosen.

The dough is then filtered and washed with demineralized water.

b) Treatment with pressurized hydrogen peroxide

The aqueous solution of hydrogen peroxide, the compound A, the poly-α-hydroxyacrylic acid or the corresponding polylactone and the demineralized water necessary to obtain the chosen consistency, are added to the paste collected in a). Then the reaction medium thus obtained is placed in a stainless steel autoclave. The completely filled autoclave is pressurized with compressed air and then heated to the temperature T chosen for the duration chosen. In order to maintain the pressure at the selected reaction value, the degassing valve of the autoclave is intermittently opened.

After reaction, the autoclave is cooled then decompressed, the paste is collected on a filter and washed with demineralized water. The whiteness, the Kappa index and the DP are then measured according to the standards of the paper industry.

The washing water can be concentrated by evaporation and then incinerated. Ashes made mainly of sodium carbonate can be recycled.

In all the examples, the amounts of reactants are expressed in percent by weight relative to the weight of the dry matter of the paste and the pressures are, unless otherwise indicated, expressed in relative pressure.

• Blancheur = 34,8° ISO
• Indice Kappa = 15,2
• DP = 2100

In Examples 1 to 13, a KRAFT hardwood pulp (KF1) of industrial origin is used, obtained by baking and having the following characteristics:

• Whiteness = 34.8 ° ISO
• Kappa index = 15.2
• DP = 2100

EXAMPLE 1

• Consistance = 15 %
• Température = 140°C
• Durée = 20 minutes
• Pression = 10 bars
• H₂O₂ = 2 %
• Na₂CO₃ = 10 %
• Polylactone de l'acide poly-α-hydroxyacrylique (SPHA) = 1 %

The paste (KF1) is subjected to the pretreatment (a) with the sequestering agent and then treated with hydrogen peroxide under the following conditions:

• Consistency = 15%
• Temperature = 140 ° C
• Duration = 20 minutes
• Pressure = 10 bars
• H ₂ O ₂ = 2%
• Na ₂ CO ₃ = 10%
• Polylactone of poly-α-hydroxyacrylic acid (SPHA) = 1%

The pH of the dough after the pretreatment is 9.5. At the end of the treatment 99% of the hydrogen peroxide was consumed and the final pH is 9.4.

The degree of whiteness of the dough is 73.7 ° ISO, the Kappa number is 7.4 and the degree of polymerization (DP) is equal to 1500.

EXAMPLE 2 (comparative)

The procedure is identical to Example 1 but in the absence of the corresponding poly-α-hydroxyacrylic acid and / or polylactone. The degree of whiteness is then equal to 60.8 ° ISO, the Kappa index is 8 and the DP is 1100.

EXAMPLES 3 to 8

The procedure is identical to Example 1 except that the amount of polylactone varies.

EXAMPLES 9 to 13

The procedure is identical to Example 1 except that the amount of sodium carbonate varies.

Table I summarizes the characteristics of the paste obtained at the end of the tests of Examples 1 to 13. Figures 1-3 show the beneficial effect of sodium carbonate and polylactone on the whiteness and the degree of polymerization of the paste KF1. Table I Example Na ₂ CO ₃ % Polymer% White ° ISO IK DP 1 10 1 73.7 7.4 1500 2 10 0 60.8 8 1100 3 10 0.1 62.7 7.9 1125 4 10 0.3 68.4 7.9 1300 5 10 0.4 71.4 7.8 1460 6 10 0.5 71.7 7.6 nd 7 10 0.75 73.4 7.6 nd 8 10 1.5 73.6 7.6 nd 9 1.5 1 60.6 nd nd 10 2 1 61.5 nd nd 11 4 1 69.1 nd nd 12 6 1 70.3 nd nd 13 8 1 73.2 nd nd na = not determined

EXAMPLES 14 to 19

• Blancheur = 50,1° ISO
• Indice Kappa = 9,7
• DP = 1400

après cuisson et délignification à l'oxygène, a été utilisée dans les exemples 14 à 19.A KRAFT (KF2) hardwood pulp with the following characteristics:

• Whiteness = 50.1 ° ISO
• Kappa index = 9.7
• DP = 1400

after cooking and delignification with oxygen, was used in Examples 14 to 19.

After pretreatment with the complexing agent, the pulp is bleached under the conditions listed in Table II.

The characteristics of the paste at the end of the hydrogen peroxide treatment are also given in Table II.

In Example 18, 8% of a commercial sodium silicate solution (density 1.33) is used in place of the carbonate and the poly-α-hydroxyacrylic acid.

Example 19 is not in accordance with the invention, since a pressure equal to the saturated water vapor pressure at the treatment temperature was applied.

EXAMPLES 20 to 26

• Blancheur = 34,5° ISO
• Indice Kappa= 12,4
• DP = 1 100

après cuisson et délignification par l'oxygène est soumise au traitement séquestrant a) en milieu basique (pH final = 9,3) puis traitée par le peroxyde d'hydrogène dans les conditions mentionnées dans le tableau III.A KRAFT softwood pulp (KR1) having the following characteristics:

• Whiteness = 34.5 ° ISO
• Kappa index = 12.4
• DP = 1100

after cooking and delignification with oxygen is subjected to the sequestering treatment a) in basic medium (final pH = 9.3) then treated with hydrogen peroxide under the conditions mentioned in table III.

Example 21, not in accordance with the invention, was carried out in the absence of polymer. Examples 25 and 26 are comparative tests with the use of sodium hydroxide and silicate or sodium hydroxide and magnesium sulfate in place of sodium carbonate and polylactone.

EXAMPLES 27 to 41

• Blancheur = 31,9° ISO
• Indice Kappa= 15,2
• DP = 1 600

après cuisson et délignification par l'oxygène est soumise à un prétraitement par l'agent complexant a) en milieu basique puis est traitée par le peroxyde d'hydrogène dans les conditions opératoires reportées dans le tableau IV.A KRAFT hardwood pulp (KF3) of industrial origin, having the following characteristics:

• Whiteness = 31.9 ° ISO
• Kappa index = 15.2
• DP = 1600

after cooking and delignification with oxygen is subjected to a pretreatment with the complexing agent a) in basic medium and then is treated with hydrogen peroxide under the operating conditions reported in Table IV.

Examples 35 and 40 are not in accordance with the invention.

Claims (12)

Process for delignification and bleaching of a chemical pulp, characterized in that after pretreatment with a complexing or sequestering agent of transition metals, it comprises one or more step (s) in which the pulp undergoes treatment with hydrogen peroxide at a temperature T greater than 100 ° C, at a pressure greater than 1.5 times the saturated water vapor pressure at temperature T, in the presence of a polymer comprising units of formula (I) and / or of formula (II):

in which R ₁ and R ₂ , which are identical or different, each represent a hydrogen atom or an alkyl group comprising from 1 to 3 carbon atoms and M represents a hydrogen atom, an ammonium group or an alkali or alkaline earth metal , and in the presence of a compound A chosen from potassium hydroxide, sodium hydroxide, alkali or alkaline earth metal carbonates. Process for delignification and bleaching of a chemical pulp, without effluent, characterized in that after pretreatment with a complexing or sequestering agent of transition metals, it comprises (i) one or more step (s) in which paste, undergoes treatment with hydrogen peroxide at a temperature T greater than 100 ° C., at a pressure greater than 1.5 times the saturated water vapor pressure at temperature T, in the presence of a polymer comprising units of formula (I) and / or of formula (II):

in which R ₁ and R ₂ , which are identical or different, each represent a hydrogen atom or an alkyl group comprising from 1 to 3 carbon atoms and M represents a hydrogen atom, an ammonium group or an alkali or alkaline earth metal , and in the presence of a compound A chosen from alkali or alkaline-earth metal carbonates, (ii) one or more washing of the pulp with water at the end of this treatment (iii) and incineration of the wash water. Process according to Claim 1 or 2, characterized in that before treatment with hydrogen peroxide, the paste is subjected to one or more delignification stage (s). Process according to Claim 3, characterized in that the delignifying agent is chosen from oxygen, ozone, an organic or mineral peracid. Process according to any one of Claims 1 to 4, characterized in that the complexing agent is DTPA. Process according to Claim 5, characterized in that the pH of the medium, during the pretreatment, is between 8 and 10. Process according to any one of Claims 1 to 6, characterized in that the compound A is sodium carbonate. Process according to Claim 7, characterized in that the quantity of sodium carbonate used is between 1 and 15% relative to the dry matter of the dough. Process according to any one of Claims 1 to 8, characterized in that the polymer is poly-α-hydroxyacrylic acid or polylactone or the corresponding salts. Process according to Claim 9, characterized in that the average molecular weight of the polymer is between 2000 and 100,000. Process according to Claim 9 or 10, characterized in that the quantity of the polymer used is between 0.1 and 1% relative to the mass of the dough in the dry state. Process according to any one of Claims 1 to 11, characterized in that the pressure of the medium has reached a value greater than 1.5 times the pressure of the saturated water vapor at temperature T before its temperature exceeds 100 ° vs.

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