EP0351330B1 - Process for manufacturing bleached chemithermomechanical pulps - Google Patents

Abstract

Process for manufacturing bleached CTMP pulps comprising a treatment of the lignocellulosic stock with sulphite at a temperature of at least 100 DEG C under saturated steam pressure, and a bleaching operation with the aid of H2O2 in alkaline medium without removal of solid or liquid matter from the pulp from the start of the treatment until the end of the bleaching.

Description

The invention relates to the manufacture of bleached chemothermal-mechanical pulps.

By chemical thermomechanical pastes, designated in all that follows by CTMP pasta means pasta which result from application to a lignocellulosic material, generally wood in the form of chips, treatment using one or more chemical agents combined with heating and mechanical defibration operations.

CTMP pasta is of definite industrial interest because they make an interesting compromise between pasta mechanical and chemical pulps themselves.

For example, they are obtained with a yield, weight of dough in the dry state relative to the weight of material dry start, generally greater than 85%, the most often at least equal to 90%, which makes them very close to purely mechanical pasta.

In the combination heating, treatment, defibration, mentioned above, treatment can take place before, during or after defibration.

By treatment is meant here and in all that follows, the operation during which the lignocellulosic material is present with a sulfite, in practice sodium sulfite Na ₂ SO ₃ , or a bisulfite, in practice sodium bisulfite NaHSO ₃ , or more generally a mixture of sulfur dioxide SO ₂ and sodium hydroxide NaOH, at a temperature equal to or greater than 100 ° C., under saturated steam pressure, said sulfite, bisulfite or mixture being indifferently designated by sulfite in all that follows. The treatment includes, where appropriate, a conventional impregnation of the lignocellulosic material using a solution of the selected reagents.

The temperature at which the treatment is carried out generally does not exceed 200 ° C and is preferably included between about 120 ° C and 160 ° C.

The treatment medium generally has an initial pH preferably chosen between 6 and 12.5.

Consistency, weight concentration in paste expressed in the dry state in the medium, is, in the treatment, for example between 10% and 40%, most often between 15% and 30%.

The duration of treatment depends on the choice of others parameters but generally does not exceed 1 hour.

Expressed as SO ₂ , the amount of sulfite is for example between about 0.1% and 10%, most often between 0.5% and 3%, the percentages being given by weight relative to the weight of the lignocellulosic material considered. in the dry state, as will be the case, unless specified or obvious, for the percentages of material in all that follows.

Chemical agents can be used in the treatment at the same time as sulfite, such as complexing or sequestering agents such as diethylenetriaminepentaacetic acids (DTPA) or ethylenetriaminetraacetic (EDTA) used in the form of sodium salts in quantity generally between about 0.1% and 1%.

The combination, as said above, of the treatment with the heating and refining operations concludes usually in practice by two refining operations successive steps to make the dough safe to use in the paper industry.

We can refer to the issue of obtaining CTMP pasta for example from the work of James P. CASEY, PULP & PAPER CHEMISTRY & CHEMICAL TECHNOLOGY, 3rd edition, vol.I, 1980, in particular pages 241-245, 213-219-229, to the work PULP & PAPER MANUFACTURE, VOL. 2, 1987 - MECHANICAL PULPING -, in particular chapters VIII D and XI, or even to the patent of United States of America No. 4718980 in particular Figure 1.

By bleached CTMP pulps is meant here the CTMP pulps defined above, after they have been bleached using hydrogen peroxide H ₂ O ₂ in an alkaline medium.

In the known technique for manufacturing bleached CTMP doughs, the dough, before being bleached with hydrogen peroxide, must as far as possible be free of chemical agent (s) such as sulfite ions, which it is known that they must cause consumption of H ₂ O ₂ harmful to bleaching, as indicated for example by H.KRUGER, HUSUSS, TAPPI PROCEEDINGS, 1982 INTERNATIONAL SULFITE PULPING CONFERENCE, 143-148.

In practice the dough is carefully washed before to be bleached and this washing is conventionally carried out by example thanks to a repeated or not a sequence of dilution and reconcentration of the dough.

When a refined pulp classification operation is performed, which is practically the general case, the dilution must bring consistency to values understood within limits as low as about 0.5% to 2%. We may refer to regarding washing and filing to the work of James P. CASEY already cited, pages 228-231, 363-365,447-452 or to the aforementioned book PULP & PAPER MANUFACTURE - in particular chapters XIII - XVIII.

Reconcentrate the whitening dough to a consistency less scab around 10%, from values that are very weak is an operation which the known technique requires not only to effectively remove the agent (s) chemicals considered undesirable in bleaching but also so that hydrogen peroxide, in the very absence of these compounds, has a satisfactory effectiveness.

Bleaching of CTMP pulp with peroxide of hydrogen in an alkaline medium is usually carried out and engaging an amount of hydrogen peroxide between approximately 0.5% and 10%, in the presence of approximately 1% to 6% of a 1.33 density sodium silicate solution, including pH between about 9 and 11, at a temperature between about 40 ° C and 100 ° C for about 0.5 to 2 hours, with a consistency between about 10% and 30%. The bath of bleaching may contain additives such as mainly a or more sequestering or complexing agents, such as example DTPA, in an amount generally between about 0.1% and 1%.

After bleaching, the dough is preferably treated with an acid such as SO ₂ to stabilize its whiteness, before being diluted with water to a very low consistency, for example of the order of 1%, to allow its transfer. , its storage and its use in stationery.

• la production d'une quantité très importante d'effluents aqueux, environ 100 tonnes par tonne de papier,
• la reconcentration nécessaire de la pâte avant blanchiment, qui implique l'usage d'un appareillage coûteux, filtre pour pâte ou presse par exemple, au moyen duquel il reste malgré tout difficile d'atteindre une consistance élevée dont il est connu qu'elle favorise l'action du peroxyde d'hydrogène.

The current technique for manufacturing bleached CTMP pulp described above and considered in connection with the manufacture of paper has significant drawbacks in terms of economy and / or pollution:

• the production of a very large quantity of aqueous effluents, around 100 tonnes per tonne of paper,
• the necessary reconcentration of the pulp before bleaching, which involves the use of an expensive apparatus, filter for pulp or press for example, by means of which it remains nevertheless difficult to achieve a high consistency which it is known that it favors the action of hydrogen peroxide.

Aqueous effluents, mainly from washing, classification, water separation operations final dilution before or during papermaking depending on whether this manufacturing is integrated into manufacturing or not of dough, which contains practically no more sulfite but a still high polluting load, are necessarily recycled and in particular serve as a washing agent for the paste to remove sulfite before bleaching with hydrogen peroxide.

These aqueous effluents are designated in all that follows by industrial clear water.

Despite the effective washing of the dough using this industrial clear water, the white level of the dough Bleached CTMP which is manufactured remains significantly less than that which the use of water of natural origin would achieve demineralized and not recycled, unfortunately no use already possible from an economic point of view.

The present invention avoids the disadvantages above industrial technique known for make bleached CTMP pasta, without the level of white of this pasta is felt, on the contrary.

It is based on the unexpected finding that a white level at least equal to that achieved by operating according to the known industrial technique is obtained when the dough is not removed before laundering of the agent (s) chemicals believed to interfere with bleaching using hydrogen peroxide.

It consists first of all in a process for the manufacture of chemothermal mechanical pulps (CTMP) bleached from a lignocellulosic material, generally wood in the form of chips, this material being subjected to defibration and to a chemical treatment to give a CTMP pulp. , said treatment possibly taking place before, during or after defibration and consisting of the action of a sulfite chosen from sodium sulfite, sodium bisulfite, a mixture of sulfur dioxide and sodium hydroxide, at a temperature equal to or greater than 100 ° C., under saturated steam pressure, the CTMP paste thus obtained is then bleached with hydrogen peroxide in an alkaline medium, characterized in that no solid matter or any material liquid are eliminated from the start of said treatment until the end of said bleaching.
All that has been stated above applies to the definition of the invention, with the exception of what, after refining, affects, prior to bleaching, washing operations, classification and reconcentration of the dough. which no longer take place. When the treatment is carried out during the refining operation, a lag time of between 5 and 30 minutes at the outlet temperature of the refiner allows the completion of the treatment.

Only water vapor, for example at the outlet of a defibration operation, can be separated from the dough between the start of treatment and end of bleaching, e.g. known way in a cyclone.

It has been found that the advantage obtained by the process of the invention is still observed and is even surprisingly increased, when a chemical agent more electronegative than the sulphite ion SO ₃ and designated in all that follows by reducing agent acts. together with sulfite in processing.
It is even then the preferred embodiment of the treatment in the method of the invention.

The reducing agent is most often chosen from thiourea dioxide or formamidinesulfinic acid, sodium hydrosulfite or dithionite, and sodium borohydride.

The quantity of reducing agent used may vary depending on the nature of it. It is generally understood, for the first two cited between approximately 0.1% and 5% and between about 0.01% and 0.5% for the third which is conveniently used in the form of an aqueous solution as by example the solution containing 12% by weight of sodium borohydride and marketed under the name of BOROL® by the company VENTRON Corporation.

The invention finds all its interest in practice when it is aimed at obtaining CTMP pulps bleached up to a high degree of white, for example 80 ° and more when it is measured in the usual way today at the length d wave of 457 nm with magnesium oxide as a reference standard using the spectrophotometer of the General Electric or Elrepho type. The amount of sulfite expressed as SO ₂ 2 is then preferably between approximately 0.5% and 3% and the amount of hydrogen peroxide preferably between 3% and 10%, most often between 4% and 6% .

Another object of the invention is a method according to the invention in which the bleached CTMP pulp is refined straight out of laundering. This refinement is performed at atmospheric pressure. The advantage reported by the U.S. Patent No. 4,718,980 in Case only mechanical and thermomechanical pastes is thus preserved in the case of CTMP pulp whitened without then have to thicken the dough.

The invention is therefore also a manufacturing process bleached CTMP pasta, as previously presented from said invention, characterized in that the bleached CTMP pulp is refined directly after bleaching and therefore in that it is not removed from solid or liquid matter from the dough from the start of processing to the end of refining after bleaching.

The classification can thus be carried out on dough bleached and refined. Dilute with clear industrial water and recycle classification refusals to the pulp refiner bleached is then done without inconvenience.

The method of the invention, in which there is no to reheat the dough, allows to dispense with a contribution of calories from a source outside the system provided that the conservation of calories in the system is sufficient well insured.

The process of the invention, compared to known techniques, therefore saves mechanical as well as thermal energy.

The invention applies to soft wood or coniferous, like hardwood or hardwood.

The different operations linked according to the invention are each carried out in conventional equipment as to their structure and function respective.

Figure 1 is a diagram of a sequence of such operations according to a mode which one can hold as preferred.

Figure 2 is a diagram illustrating an example of the known technique as it is commonly adopted.

Referring to Figure 1: wood chips, usually washed, are conveyed from the feed hopper 101 to 102, chip preheating chamber at the water vapor introduced by 103, from where they exit to pass with the solution of the reagents introduced by 104, in the refiner 105 then in cyclone 106 from where water vapor is separated by 107. At the exit of 106 the dough passes through the latency zone 108 before being intimately mixed with the bleaching reagents supplied by 109, to be bleached in the bleaching tower 110. The bleached pulp coming out of 110 is directly fed into the refiner at atmospheric pressure 111 where it comes from to be classified in 112 after dilution with clear industrial water introduced by 113. Classification refusals, sufficiently refocused, are refined separately (circuit not shown) or recycled by 114 to the entrance to the refiner 111. After 112 the dough is acidified with known manner and sent to storage 115 and / or to the papermaking by 116.

Referring to Figure 2, the means indicated with 2 in a hundred correspond in structure and function to those of figure 1 subscripted with 1 in hundred. The additional means 217 is a thickening, reconcentration equipment dough between its classification in 212 and its bleaching in 210. The aqueous effluent collected from 217 by 218 contributes to form clear industrial water which is introduced par 213 to serve as a paste washing agent.

The following examples are given for information only but not limiting to illustrate the invention and judge its interest.

• les quantités, comme cela a déjà été signalé, sont exprimées en % en poids par rapport à la matière lignocellulosique prise à l'état sec, sauf précision,
• par DTPA est désignée une solution aqueuse à 40 % en poids de sel de sodium de l'acide diéthylènetriaminepentaacétique et la quantité de DTPA est celle de cette solution,
• par lavage est désignée l'opération combinant dilution et pressage de la pâte,
• le borohydrure de sodium est appliqué sous forme de BOROL® et la quantité indiquée est celle de cette présentation,
• par silicate est désignée une solution aqueuse de silicate de sodium de densité égale à 1,33,
• le degré de blanc est mesuré à la longueur d'onde de 457 nm avec l'oxyde de magnésium comme standard de référence à l'aide d'un spectrophotomètre de type ELREPHO fabriqué par KARL ZEISS.

In these example:

• the quantities, as already mentioned, are expressed in% by weight relative to the lignocellulosic material taken in the dry state, unless otherwise specified,
• by DTPA is designated an aqueous solution at 40% by weight of sodium salt of diethylenetriaminepentaacetic acid and the amount of DTPA is that of this solution,
• by washing is designated the operation combining dilution and pressing of the dough,
• sodium borohydride is applied in the form of BOROL® and the quantity indicated is that of this presentation,
• the term “silicate” denotes an aqueous solution of sodium silicate with a density equal to 1.33,
• the degree of white is measured at the wavelength of 457 nm with magnesium oxide as a reference standard using a spectrophotometer of the ELREPHO type manufactured by KARL ZEISS.

Example 1:

Softwood shavings are refined under saturated steam pressure at 120 ° C with 2.75% Na ₂ SO ₃ to yield a paste with a white degree of 57 °.

This pulp is bleached, without removing any liquid or solid material, using H ₂ O ₂ : 5%, NaOH: 2%, silicate: 4%, DTPA: 0.5%, for 2 hours at 90 ° C at a consistency of 15%.

The bleached pulp obtained has a degree of whiteness of 77.9 °.

If, before being bleached under the above conditions the dough is washed by dilution to a consistency of 1.25% using industrial white water taken from an industrial plant with reconcentration by pressing up to a consistency of 20% so that 95% of the sulphite is removed before adding the bleaching reagents in aqueous solution, its degree of white after bleaching is 77.4 °. It is therefore lower than that achieved by conforming to the invention.

When the above washing is done using pure demineralized water instead of industrial white water, the degree of white of the bleached dough, admitted to be the highest possible under bleaching conditions adopted, is only 1.5 ° higher than that reached in proceeding according to the invention.

Example 2 :

A mechanical pulp of a softwood wheel, the degree of white of which is equal to 53.7 °, is subjected to a pressure treatment of saturated steam at 120 ° C with 2.75% Na ₂ SO ₃ and 0, 5% DTPA for 30 minutes at a consistency of 20%, before being bleached directly, without elimination of solid or liquid material, as in Example 1.

The bleached pulp obtained has a degree of white equal to 78.7 °

If the dough is bleached after washing with water industrial white as in example 1 its degree of white is 78.6 ° and is still lower, at most equal to that achieved by operating according to the invention.

Example 3 :

Example 2 and the comparison it includes, is repeated with the difference that 1% of BOROL® is present with sulfite.

By proceeding according to the invention, the degree of white of the bleached pulp is equal to 82.2 ° while it is only 80.9 ° in the comparison.

When, in the latter, demineralized pure water is used in place of industrial white water, the whiteness of the bleached dough is only 1 ° to that achieved by proceeding according to the invention.

Claims (10)

1. Process for the manufacture of bleached chemithermomechanical pulps (CTMP) from a lignocellulose material, generally wood in the form of chips, this material being subjected to a defibering and to a chemical treatment in order to give a CTMP pulp, the said treatment, which can take place before, during or after the defibering, consisting of the action of a sulphite chosen from sodium sulphite, sodium bisulphite or a mixture of sulphur dioxide and sodium hydroxide, at a temperature equal to or greater than 100°C, under saturated vapour pressure of water, the CTMP pulp thus obtained subsequently being bleached using hydrogen peroxide in alkaline medium, characterized in that no solid material or liquid material is discharged from the beginning of the said treatment to the end of the said bleaching.
2. Process according to Claim 1, characterized in that the amount of sulphite, expressed as sulphur dioxide, by weight with respect to the weight of the lignocellulose material, taken in the dry state, is between 0.5% and 3%.
3. Process according to Claim 1 or 2, characterized in that a more electronegative agent than the sulphite ion is present in the treatment at the same time as the sulphite.
4. Process according to Claim 3, characterized in that the agent is chosen from thiourea dioxide, sodium borohydride and sodium dithionite.
5. Process according to Claim 4, characterized in that the amount of thiourea dioxide or of sodium dithionite is between 0.1% and 5% by weight with respect to the weight of the lignocellulose material taken in the dry state.
6. Process according to Claim 4, characterized in that the amount of sodium borohydride is between 0.01% and 0.5% by weight with respect to the weight of the lignocellulose material taken in the dry state.
7. Process according to one of Claims 1 to 6, characterized in that, in the treatment, the initial pH is between 6 and 12.5.
8. Process according to Claim 1 to 7, characterized in that the amount of hydrogen peroxide in the bleaching is between 3% and 10% by weight with respect to the weight of the lignocellulose material taken in the dry state.
9. Process according to Claim 8, characterized in that the amount of hydrogen peroxide is between 4% and 6% by weight with respect to the weight of the lignocellulose material taken in the dry state.
10. Process according to one of Claims 1 to 9, characterized in that the bleached CTMP pulp is refined directly after the bleaching, without any solid material or liquid material being discharged from the beginning of the said treatment to the end of the post-bleaching refining.

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