EP0446110B1 - Process for producing bleached high-yield pulps - Google Patents

Abstract

Process for the manufacture of high-yield pulps bleached with the aid of hydrogen peroxide in alkaline medium, which consists in subjecting the pulp to be bleached successively:   . to a pretreatment with the aid of a sequestering agent for metal ions, followed by a washing,   . to a treatment with sulphite and a reducing agent which is more electronegative than the sulphite ion which function together and in a medium with an initial pH of between 7 and 12.5, followed by a washing to remove the sulphite ions and the reducing agent,   . to a bleaching treatment with the aid of hydrogen peroxide in alkaline medium in the presence of a quantity of silicate of between 0 % and 1 %.

Description

The present invention relates to a process for manufacturing high-yield bleached pasta.

High-yield pulps are produced by subjecting the starting lignocellulosic material, such as wood in the form of chips, to an adion of the mechanical type, combined or not with actions of the chemical and thermal type.

High yield pulps are of definite interest for the industry because they achieve a valid compromise between mechanical pulps and chemical pulps themselves.

By high yield is meant a yield, weight of dough in the dry state relative to the weight of initial material in the dry state, generally equal to at least 85% and often even to 90%.

The bleaching of high-yield pulps, which requires the quality required in this field for paper and similar products, is carried out using hydrogen peroxide in an alkaline medium. Such money laundering is still a problem both technically and economically.

Indeed, to reduce the loss of efficiency of hydrogen peroxide due to its instability in alkaline medium, sodium silicate is used in quantity such that it is the cause of multiple and serious drawbacks during the elaboration of the pulp and that of the paper, such as for example the formation of solid deposits on the equipment and on the lignocellulosic fiber, or the production of a rough touch paper. Reference is made, for example, to C.W. KUTNEY, Pulp & Paper Canada, 86:12, (1985), 182-189.

Furthermore, the quantity of sodium silicate cannot be reduced without consequently affecting the result of bleaching in an unacceptable manner.

• . à un prétraitement à l'aide d'un agent séquestrant des ions métalliques suivi d'un lavage,
• . à un traitement par le sulfite et un agent réducteur plus électronégatif que l'ion sulfite qui agissent conjointement et en milieu de pH initial compris entre 7 et 12,5, suivi d'un lavage pour éliminer les ions sulfite et l'agent réducteur,
• . à un traitement de blanchiment à l'aide de peroxyde d'hydrogène en milieu alcalin en présence, en une quantité d'environ 1 % à 6 %, d'une solution de silicate de sodium de densité 1,33.

Patent application EP-A-0293309 discloses a process for the production of high-yield pulps bleached using hydrogen peroxide in an alkaline medium which consists in subjecting the pulp to be bleached successively:

• . to a pretreatment using a sequestering agent of metal ions followed by washing,
• . to a treatment with the sulphite and a reducing agent more electronegative than the sulphite ion which act jointly and in a medium of initial pH between 7 and 12.5, followed by washing to remove the sulphite ions and the reducing agent,
• . to a bleaching treatment using hydrogen peroxide in an alkaline medium in the presence, in an amount of about 1% to 6%, of a sodium silicate solution of density 1.33.

In order for this result to remain sufficiently high, it is also possible to subject the material to be bleached, for example, to two stages of bleaching with H₂O₂ instead of one or seeks to substitute for the silicate an agent of analogous efficacy, such as diethenetriaminepentamethylenephosphonic acid (DTMPA). whose equivalence with sodium silicate would only be verified in special cases according to Robert W. ALLISON, Appita, Vol.36, N ° 5, March 1983, 362-370. In any case, such solutions are not economically satisfactory.

The process of the present invention makes it possible to keep only one bleaching step using hydrogen peroxide in an alkaline medium, to eliminate the drawbacks due to sodium silicate, not to burden the economy of the operation. , while ensuring the bleached dough a degree of whiteness and high mechanical qualities.

• . à un prétraitement à l'aide d'un agent sequestrant des ions métalliques suivi d'un lavage,
• . à un traitement par le sulfite et un agent réducteur plus électronégatif que l'ion sulfite qui agissent conjointement et en milieu de pH initial compris entre 7 et 12,5, suivi d'un lavage pour éliminer les ions sulfite et l'agent réducteur,
• . à un traitement de blanchiment à l'aide de peroxyde d'hydrogène en milieu alcalin, et qui se caractérise en ce que le traitement de blanchiment est effectué en l'absence de silicate de sodium.

The present invention consists of a process for manufacturing high-yield pulps bleached using hydrogen peroxide in an alkaline medium, which consists in subjecting the pulp to be bleached successively:

• . to a pretreatment using a sequestering agent of metal ions followed by washing,
• . to a treatment with the sulphite and a reducing agent more electronegative than the sulphite ion which act jointly and in a medium of initial pH between 7 and 12.5, followed by washing to remove the sulphite ions and the reducing agent,
• . to a bleaching treatment using hydrogen peroxide in an alkaline medium, and which is characterized in that the bleaching treatment is carried out in the absence of sodium silicate.

In the definition of the process of the invention given above, as in all that follows, the quantity of a compound is, unless specified or evident, expressed in% by weight relative to the weight of dough in the dry state .

• par pâtes : les pâtes telles que définies en début de ce texte et aussi la matière lignocellulosique sous forme de copeaux de bois. Une machine de type imprégnateur mécanique, comme celle décrite par exemple dans le brevet français n°2319737, le certificat d'addition français à ce brevet, n°2436844, les brevets français n°2418295 et 2451963, ou comme celle de type Impressafiner® décrite par exemple dans l'ouvrage Pulp & Paper Manufacture 3ème Edition, vol.2, Mechanical pulping, pages 98, 99, peut être utilisée pour réaliser en totalité ou en partie le procédé de l'invention à partir de toute pâte tel qu'il est entendu ici. Autrement le procédé de l'invention peut être réalisé dans tout équipement combinant des appareils connus dans l'industrie papetière pour convenir aux opérations de prétraitement, de lavage, de traitement de blanchiment, tels qu'entre autres malaxeurs, filtres, presses, tours de blanchiment.
• par prétraitement de la pâte au moyen d'un agent sequestrant des ions métalliques : l'opération qui consiste à soumettre ladite pâte à l'action d'un composé comme par exemple le tripolyphosphate de sodium, le tétrapyrophosphate de sodium, les sels de sodium des acides acétique, nitrilotriacétique, éthylènediaminetétraacétique, diéthylènetriaminepentaacétique (DTPA).

In the definition of the process of the invention given above as in all that follows, we mean:

• by pasta: pasta as defined at the beginning of this text and also lignocellulosic matter in the form of wood chips. A machine of the mechanical impregnator type, such as that described for example in French patent n ° 2319737, the French certificate of addition to this patent, n ° 2436844, the French patents n ° 2418295 and 2451963, or like that of the Impressafiner® type described for example in the book Pulp & Paper Manufacture 3rd Edition, vol.2, Mechanical pulping, pages 98, 99, can be used to carry out all or part of the process of the invention from any pulp such as it is heard here. Otherwise the method of the invention can be carried out in any equipment combining devices known in the paper industry to be suitable for pretreatment, washing and bleaching treatment operations, such as among other mixers, filters, presses, bleaching towers.
• by pretreatment of the paste using a sequestering agent of metal ions: the operation which consists in subjecting said paste to the action of a compound such as for example sodium tripolyphosphate, sodium tetrapyrophosphate, sodium salts acetic, nitrilotriacetic, ethylenediaminetetraacetic, diethylenetriaminepentaacetic (DTPA) acids.

The sequestering agent is used at a rate of approximately 0.1% to 1%.

• par lavage : l'opération permettant d'éliminer de la pâte, de façon plus ou moins complète, la phase liquide présente en elle, par exemple en concentrant la pâté par pressage sur filtre, ou grâce à une séquence répétée ou non, de dilution de la pâte le plus souvent par de l'eau puis de concentration de la pâte par exemple par pressage sur filtre : l'efficacité du lavage est traduite par le degré d'élimination, exprimé en %, de la phase liquide présente avant lavage.

The pretreatment with the sequestering agent is most often carried out at a temperature between about 20 ° C and 100 ° C, preferably between 50 ° C and 95 ° C so as not to have to work under pressure. The consistency, content of lignocellulosic material in the dry state in the dough, can here be chosen between fairly wide limits, for example between about 5% and 50%.

• by washing: the operation making it possible to eliminate, more or less completely, the liquid phase present in it, for example by concentrating the pâté by pressing on a filter, or by means of a repeated or not sequence, of dilution paste most often with water and then concentration of the paste, for example by pressing on a filter: the washing efficiency is expressed by the degree of elimination, expressed in%, of the liquid phase present before washing.

• par sulfite : le sulfite de sodium, Na₂SO₃, ou le bisulfite de sodium, NaHSO₃, ou plus généralement un mélange de dioxyde de soufre SO₂ et d'hydroxyde de sodium : la quantité de sulfite engagée est celle habituellement mise en oeuvre dans les procédés qui n'incorporent pas l'invention mais qui sont connus pour convenir à l'élaboration de pâte chimicothermomécaniques (pâtes CTMP), cette quantité, exprimée en SO₂, est comprise en conséquence le plus souvent entre environ 0,5 % et 6 %.
• par agent réducteur plus électronégatif que l'ion sulfite SO₃H : un composé, désigné dans tout ce qui suit par agent réducteur, choisi le plus souvent parmi le dioxyde de thiourée ou acide formamidinesulfinique, le borohydrure de sodium, l'hydrosulfite de sodium ou dithionite de sodium.

The washing which follows the pretreatment with a sequestering agent, such as the washing to remove the sulphite ions and the reducing agent, is most often carried out using the dilution-concentration sequence, at a temperature generally between approximately 20 ° C. and 90 ° C, often between 20 ° C and 60 ° C for economic reasons. The effectiveness of such washes is preferably greater than 90%.

• by sulphite: sodium sulphite, Na₂SO₃, or sodium bisulphite, NaHSO₃, or more generally a mixture of sulfur dioxide SO₂ and sodium hydroxide: the quantity of sulphite involved is that usually used in the processes which do not incorporate the invention but which are known to be suitable for the preparation of chemothermal-mechanical pulp (CTMP pulps), this quantity, expressed in SO₂, is therefore most often between approximately 0.5% and 6%.
• by reducing agent more electronegative than the sulphite ion SO₃H: a compound, designated in all that follows by reducing agent, chosen most often from thiourea dioxide or formamidinesulfinic acid, sodium borohydride, sodium hydrosulfite or dithionite sodium.

The amount of reducing agent used can vary depending on the nature thereof. It is generally between 0.1 and 5% in the case of thiourea dioxide or dithionite. Sodium borohydride is used in an amount of about 0.01% to 0.5%. This reducing agent is conveniently used in the form of an aqueous solution such as for example the solution containing 12% by weight of sodium borohydride marketed under the name of BOROL® by the company VENTRON Corporation.

The joint action of the sulfite and the reducing agent on the dough, the sulfite and the reducing agent being present together in contact with the dough, is exerted at a temperature of between approximately 20 ° C. and 200 ° C. It is preferably chosen to be between 60 ° C and 140 ° C. When the temperature is equal to or higher than 100 ° C, the joint action of the sulfite and the reducing agent is exerted under saturated steam pressure.

A sequestering agent, in nature and in quantity as in the pretreatment, can sometimes be advantageously present with the sulphite and the reducing agent.

• par traitement de blanchiment à l'aide de peroxyde d'hydrogène en milieu alcalin : l'opération réalisée en engageant une quantité de peroxyde d'hydrogène comprise entre environ 0,5 % et 10 %, de préférence entre 2 % et 6 %, en l'absence de silicate, à pH compris entre environ 9 et 11, à une température comprise entre 40°C et 100°C durant environ 0,5 heure à 6 heures, avec une consistance comprise entre 10 % et 30 %. La solution de blanchiment peut contenir des additifs comme essentiellement un ou plusieurs agents sequestrants tels que par exemple les acides diéthylénetriaminepentaacétique et éthylènediaminetétraacétique sous forme de sels de sodium en quantité généralement comprise entre environ 0,1 % et 1 %.

The consistency during the joint action of the sulfite and the reducing agent is normally between approximately 5% and 50% depending on the embodiment of the operation, and generally between 10% and 30%. The duration of the operation depends on the other execution parameters, including the type of equipment used. It normally does not exceed 1 hour and is generally between a few tens of seconds and 30 minutes. A very short time corresponds to carrying out the operation in a wood chip processing machine to which reference was made above.

• by bleaching treatment using hydrogen peroxide in an alkaline medium: the operation carried out by using an amount of hydrogen peroxide of between approximately 0.5% and 10%, preferably between 2% and 6%, in the absence of silicate, at pH between about 9 and 11, at a temperature between 40 ° C and 100 ° C for about 0.5 hour to 6 hours, with a consistency between 10% and 30%. The bleaching solution may contain additives such as essentially one or more sequestering agents such as for example diethylenetriaminepentaacetic and ethylenediaminetetraacetic acids in the form of sodium salts in an amount generally between approximately 0.1% and 1%.

• par silicate : une solution aqueuse de silicate de sodium de densité égale à 1,33 (examples comparatifs)

The amount of sodium hydroxide present with hydrogen peroxide in the bleaching treatment is generally between 1% and 6% and most often between 2% and 3%. The optimal amount depends essentially on the nature of the wood.

• by silicate: an aqueous solution of sodium silicate with a density equal to 1.33 (comparative examples)

The following examples, given as an indication but not limiting illustrate the invention. Examples are given by way of comparison of the process of the invention with known processes.

• la pâte de haut rendement blanchie à l'aide de peroxyde d'hydrogène a été obtenue en soumettant au procédé de l'invention ou à un procédé connu, une même pâte mécanique de meule. Cette pâte d'origine contient en mélange du bois de résineux et du bois de feuillus dans la proportion 75/25 et son degré de blanc, mesuré par réfractométrie à 457 nm selon la norme ISO, est égal à 60,6° ISO.
• les quantités de matière sont exprimées, comme auparavant, sauf précision ou évidence, en % en poids par rapport au poids de pâte à l'état sec,
• le DTPA est utilisé sous forme de sel de sodium en solution aqueuse à 40 % en poids et la quantité indiquée est celle de cette solution,
• le borohydrure de sodium est appliqué sous forme de BOROL® et la quantité indiquée est celle de cette présentation,
• le silicate de sodium est engagé comme il a été dit sous forme d'une solution aqueuse de silicate de sodium de densité 1,33 et la quantité indiquée est celle de cette solution (example comparatif),
• le magnésium est mis en oeuvre sous forme de sulfate de magnésium, et sa quantité est exprimée en Mg,
• le peroxyde d'hydrogène est compté en 100 %, comme d'ailleurs auparavant,
• l'efficacité des lavages après le prétraitement par un agent sequestrant et après l'action du sulfite avec l'agent réducteur, est supérieure à 90 %,
• la quantité d'hydroxyde de sodium présente dans le traitement de blanchiment par H₂O₂ en milieu alcalin est celle qui, compte-tenu des autres paramètres, conduit au degré de blanc le plus élevé,
• le degré de blanc est mesuré sur la pâte blanchie amenée à pH 6 à l'aide de SO₂, par réfractométrie à 457 nm selon la norme ISO et il est exprimé en degré ISO (°ISO).

In all the examples:

• the high-yield pulp bleached using hydrogen peroxide was obtained by subjecting the same method of grinding wheel to the process of the invention or to a known process. This original paste contains a mixture of softwood and hardwood in the proportion 75/25 and its degree of white, measured by refractometry at 457 nm according to the ISO standard, is equal to ISO 60.6 °.
• the quantities of material are expressed, as before, unless specified or evident, in% by weight relative to the weight of dough in the dry state,
• DTPA is used in the form of the sodium salt in aqueous solution at 40% by weight and the quantity indicated is that of this solution,
• sodium borohydride is applied in the form of BOROL® and the quantity indicated is that of this presentation,
• the sodium silicate is used as it has been said in the form of an aqueous solution of sodium silicate of density 1.33 and the quantity indicated is that of this solution (comparative example),
• magnesium is used in the form of magnesium sulfate, and its quantity is expressed in Mg,
• hydrogen peroxide is counted in 100%, as elsewhere before,
• the effectiveness of the washes after the pretreatment with a sequestering agent and after the action of the sulfite with the reducing agent is greater than 90%,
• the quantity of sodium hydroxide present in the bleaching treatment with H₂O₂ in an alkaline medium is that which, taking into account the other parameters, results in the highest degree of white,
• the degree of white is measured on the bleached paste brought to pH 6 using SO₂, by refractometry at 457 nm according to the ISO standard and it is expressed in ISO degree (° ISO).

Example 1 :

The original paste is first subjected to a treatment using 0.5% DTPA for 15 minutes at 90 ° C at a consistency equal to 10%, then is washed and then subjected to the joint action of 5% Na₂SO₃ and 1% BOROL® for 0.5 hour at 90 ° C at a consistency equal to 20%.

The paste thus resulting from the succession of operations of pretreatment using the sequestering agent of metal ions, washing and treatment with the sulfite of the reducing agent jointly, is washed and finally subjected to a bleaching treatment at using hydrogen peroxide in an alkaline medium for 5 hours at 60 ° C at a consistency equal to 20% with 4% H₂O₂, 4% NaOH, 0.5% DTPA in the absence therefore of silicate.

The bleached pulp thus obtained has a degree of white equal to ISO 83.9 °.

Example 2 (comparative):

Example 1 is repeated except that the bleaching treatment using hydrogen peroxide in an alkaline medium is carried out in the presence of 1% silicate.

The bleached pulp obtained has a white degree equal to 84.7%.

Examples 3 and 4 (comparative):

Example 1 is repeated except that the bleaching treatment using hydrogen peroxide in an alkaline medium is carried out in the presence of 2% of silicate in example 3, of 4% of silicate in example 4.

It is noted that the bleached pulp resulting from these two tests has a degree of white equal to 84.9% and therefore practically that achieved in Example 2.

Examples 5, 6, 7, 8 (comparative)

Example 1 is repeated, eliminating the treatment of the pulp with the sulfite and the reducing agent together and keeping only the pretreatment with the sequestering agent, the washing after said pretreatment and the bleaching treatment with peroxide. hydrogen in an alkaline medium.

In the latter, the silicate is absent in Example 5 and is present at a rate of 1%, in Example 6, 2% in Example 7, 4% in Example 8.

The dough has a degree of white equal, for example 5, to 76%, for example 6, to 78.2%, for example 7, to 80.3% and for example 8 to 82 , 5%.

Example 9 (comparative)

After the pretreatment with the sequestering agent and the subsequent washing carried out as in Example 1, the washed pulp is subjected to a first bleaching treatment with H₂O₂ in an alkaline medium in the absence of silicate, at 60 ° C. and at a consistency of 20% as in Example 1 but for 1.5 hours with 1.44% H₂O₂ and 1.3% NaOH then washing and finally a second bleaching treatment with H₂O₂ in an alkaline medium under the same conditions of temperature and consistency as the first but with 2.56% H₂O₂, 3.1% NaOH and 0.05% Mg.

The bleached pulp obtained has a degree of white equal to 82.5 ° ISO.

The above examples show the advantage of the process of the invention compared to a single bleaching treatment with hydrogen peroxide in an alkaline medium, however carried out with an amount of silicate corresponding to the optimal level of silicate (4%). , and compared to two successive bleaching treatments with hydrogen peroxide in an alkaline medium without silicate.

Claims (10)

1. Process for the manufacture of high-yield pulps bleached with the aid of hydrogen peroxide in alkaline medium, which consists in subjecting the pulp to be bleached successively:
      to a pretreatment with the aid of a sequestering agent for metal ions, followed by a washing operation,
      to a treatment with sulphite and a reducing agent which is more electronegative than the sulphite ion, which act together and in a medium of initial pH between 7 and 12.5, followed by a washing operation to remove the sulphite ions and the reducing agent,
      to a bleaching treatment with the aid of hydrogen peroxide in alkaline medium, characterized in that the bleaching treatment is carried out in the absence of sodium silicate.
2. Process according to Claim 1, characterized in that the sequestering agent is chosen from sodium tripolyphosphate, sodium tetrapyrophosphate and the sodium salts of acetic, nitrilotriacetic, ethylenediaminetetraacetic and diethylenetriaminepentaacetic acids.
3. Process according to Claim 2, characterized in that the pretreatment is carried out at a temperature of between 20°C and 100°C, at a consistency of between 5 % and 50 %, with the aid of a quantity of sequestering agent of between 0.1 % and 1 %.
4. Process according to one of Claims 1 to 3, characterized in that the sulphite and the reducing agent which is more electronegative than the sulphite ion act together at a temperature of between 20°C and 200°C.
5. Process according to Claim 4, characterized in that the temperature at which the sulphite and the reducing agent act together is between 60°C and 140°C.
6. Process according to one of Claims 1 to 5, characterized in that the quantity of sulphite, expressed as sulphur dioxide SO₂, is between 0.5 % and 6 %.
7. Process according to one of Claims 1 to 6, characterized in that the reducing agent which is more electronegative than the sulphite ion is chosen from thiourea dioxide, sodium hydrosulphite and sodium borohydride.
8. Process according to Claim 7, characterized in that the quantity of thiourea dioxide or of hydrosulphite present with the sulphite is between 0.1 % and 5 %.
9. Process according to Claim 7, characterized in that the quantity of sodium borohydride present in the sulphite is between 0.01 % and 0.5 %.
10. Process according to one of Claims 1 to 9, characterized in that the bleaching with the aid of hydrogen peroxide in alkaline medium is carried out with a quantity of the said peroxide of between 0.5 % and 10 %, at a pH of between 9 and 11, at a temperature of between 40°C and 100°C, and at a consistency of between 10 % and 30 %.