EP0351330A1 - 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 chemothermal mechanical pulps, designated in all that follows by CTMP pastes, is meant the pastes which result from the application to a lignocellulosic material, generally wood in the form of chips, of a treatment using one or more chemical agents combined with heating and mechanical defibration operations.

CTMP pastes are of certain industrial interest because they achieve an interesting compromise between 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 starting material in the dry state, generally greater than 85%, most often at least equal to 90%, which makes this very close to purely mechanical pasta.

In the combination of heating, treatment and defibration, mentioned above, the 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 Na₂SO₃, or a bisulfite, in practice sodium bisulfite NaHSO₃, or more generally a mixture of sulfur dioxide SO₂ and sodium hydroxide NaOH, at a temperature equal to or higher than 100 ° C., under saturated steam pressure, said sulfite, bisulfite or mixture being indifferently designated by sulfite in all following. 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 between approximately 120 ° C. and 160 ° C.

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

The consistency, concentration by weight of 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 the treatment depends on the choice of other parameters but generally does not exceed 1 hour.

Expressed as SO₂, 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 at the dry state, as will be the case, unless otherwise specified or evident, for the percentages of material in all that follows.

Chemical agents can be used in the treatment at the same time as the sulphite, such as for example complexing or sequestering agents such as diethylenetriaminepentaacetic acid (DTPA) or ethylenetriaminetetraacetic acid (EDTA) used in the form of sodium salts in an amount generally between approximately 0.1% and 1%.

The combination, as said above, of the treatment with the heating and refining operations generally ends in practice with two successive refining operations in order to make the pulp usable in a safe manner in the paper industry.

One can refer for what concerns obtaining CTMP pasta for example to 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 the United States of America n ° 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 H₂O₂ in an alkaline medium.

In the known technique for manufacturing bleached CTMP doughs, the dough, before being bleached with hydrogen peroxide, must be as far as possible free of chemical agent (s) such as sulphite ions, which it is known that they must cause consumption of H₂O₂ harmful to money laundering 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 being bleached, and this washing is conventionally carried out, for example by virtue of a repeated or not a sequence of dilution and reconcentration of the dough.

When a classification operation of the refined pulp is carried out, which is practically the general case, the dilution must bring the consistency to values comprised within limits as low as 0.5% to 2% approximately. For washing and classification, reference may be made to the work by James P. CASEY already cited, pages 228-231, 363-365,447-452 or to the work already cited also PULP & PAPER MANUFACTURE - in particular chapters XIII - XVIII.

Reconcentrating the whitening dough to a consistency at least equal to approximately 10%, starting from values which are very low, is an operation which the known technique requires not only to effectively eliminate the chemical agent or agents considered to be undesirable in bleaching. but also so that the hydrogen peroxide, in the very absence of these compounds, has a satisfactory effectiveness.

The whitening of the CTMP pulp using hydrogen peroxide in an alkaline medium is usually carried out by using an amount of hydrogen peroxide of between approximately 0.5% and 10%, in the presence of approximately 1% to 6 % of a sodium silicate solution of density 1.33, at a 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 included between about 10% and 30%. The bleaching bath can contain additives such as mainly one or more sequestering or complexing agents, such as for example DTPA, in an amount generally between approximately 0.1% and 1%.

After bleaching the dough is preferably treated with an acid such as SO₂ 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, storage and use in stationery.

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.

The aqueous effluents, coming mainly from washing operations, classification, separation of the final dilution water before or during the manufacture of the paper, depending on whether or not this manufacture is integrated into the manufacture of pulp, which contain practically no more sulphite but a still high pollutant load, are necessarily recycled and serve in particular as a pulp washing agent to remove the sulphite before bleaching with hydrogen peroxide.

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

Despite the effective washing of the pulp using this clear industrial water, the white level of the bleached CTMP pulp which is produced remains significantly lower than that which would be achieved by using water of demineralized natural origin. and not recycled, unfortunately unfortunately not possible to use only from an economic point of view.

The present invention makes it possible to avoid the above drawbacks of the known industrial technique for manufacturing bleached CTMP pastes, without the white level of these pastes being felt, on the contrary.

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

It consists first of all in a process for manufacturing bleached CTMP pulps which comprises a treatment in which the lignocellulosic material is present with the sulphite at a temperature equal to or greater than 100 ° C. under saturated steam pressure, and a bleaching using hydrogen peroxide in an alkaline medium, characterized in that solid or liquid material is not removed from the pulp 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 end of a defibering operation, can be separated from the dough between the start of the treatment and the end of bleaching, for example in a known manner 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 SO₃ ^- and designated in all that follows by reducing agent acts at the same time as sulfite in the treatment. 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 hydrolsulfite or dithionite, or sodium borohydride.

The amount of reducing agent used can vary depending on the nature thereof. It is generally understood, for the first two cited between about 0.1% and 5% and between about 0.01% and 0.5% for the third that is conveniently used in the form of an aqueous solution as for 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 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 SO₂ 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 directly at the outlet of the bleaching. This refining is carried out at atmospheric pressure. The advantage reported by the patent of the United States of America n ° 4718980 in the case only of mechanical doughs and thermomechanical doughs is thus preserved in the case of bleached CTMP doughs without then having to carry out a thickening of the dough.

The invention is therefore also a process for manufacturing bleached CTMP doughs, according to what has already been presented of said invention, characterized in that the bleached CTMP dough 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 treatment until the end of refining after bleaching.

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

The method of the invention, in which there is no need to reheat the dough, makes it possible to dispense with a supply of calories from a source external to the system provided that the conservation of the calories of the system is sufficiently well ensured .

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

The invention applies to softwoods or softwoods, such as hardwoods or hardwoods.

• La figure 1 est un schéma d'une suite de telles opérations selon un mode qu'on peut tenir pour préféré.
• La figure 2 est un schéma qui illustre un exemple de la technique connue telle qu'elle est communément adoptée.

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

• FIG. 1 is a diagram of a series of such operations according to a mode which can be considered to be preferred.
• Figure 2 is a diagram illustrating an example of the known technique as it is commonly adopted.

Referring to FIG. 1: the wood shavings, usually washed, are conveyed from the feed hopper 101 to 102, a chamber for preheating the shavings with steam introduced by 103, from which they come out to pass with the solution of the reagents introduced by 104, into the refiner 105 then into the cyclone 106 from which water vapor is separated by 107. At the outlet 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 leaving 110 is directly brought into the refiner at atmospheric pressure 111 from which it leaves to be classified in 112 after dilution by clear industrial water introduced by 113. Refusals of classification, sufficiently reconcentrated, are refined separately (circuit not shown) or recycled by 114 at the entrance of the refiner 111. After 112 the dough is acidified ée in a known manner and sent to storage 115 and / or to the manufacture of paper by 116.

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

The following examples are given as an indication but not limiting to illustrate the invention and to judge its interest.

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,
- by silicate is designated 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 ELREPHO type manufactured by KARL ZEISS.

Example 1:

Softwood shavings are refined under saturated steam pressure at 120 ° C with 2.75% Na₂SO₂ to yield a paste with a white degree of 57 °.

This paste is bleached, without removing any liquid or solid material, using H₂O₂: 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 white degree 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 carried out using pure demineralized water in place of industrial white water, the degree of whiteness of the bleached pulp, admitted to be as high as possible under the bleaching conditions adopted, is only 1.5 ° higher than that achieved by proceeding according to the invention.

Example 2:

A mechanical softwood pulp pulp, 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% Na₂SO₃ and 0.5% of 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 industrial white water 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 which it includes, is repeated with the difference that 1% of BOROL® is present with the 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 degree of white of the bleached pulp is only 1 ° higher than that achieved by proceeding according to the invention.

Claims (10)

1. Process for the manufacture of bleached chemothermal-mechanical pulps which comprises a treatment in which the lignocellulosic material is present with the sulfite at a temperature equal to or greater than 100 ° C. under saturated steam pressure, and bleaching using hydrogen peroxide in an alkaline medium, characterized in that it is not removed from solid or liquid material from the pulp from the start of said treatment until the end of said bleaching. 2. Method according to claim 1, characterized in that the quantity of sulphite is between 0.5% and 3% expressed as sulfur dioxide. 3. Method according to one of claims 1 or 2, characterized in that an agent more electronegative than the sulfite ion is present in the treatment at the same time as the sulfite. 4. Method according to claim 3, characterized in that the agent more electronegative than the sulfite ion is chosen from thiourea dioxide, sodium borohydride or sodium dithionite. 5. Method according to claim 4, characterized in that the amount of thiourea dioxide or sodium dithionite is between 0.1% and 5%. 6. Method according to claim 4, characterized in that the amount of sodium borohydride is between 0.01% and 0.5%. 7. Method according to one of claims 1 to 6, characterized in that in the treatment the initial pH is between 6 and 12.5. 8. Method according to one of claims 1 to 7, characterized in that the amount of hydrogen peroxide in the bleaching is between 3% and 10%. 9. Method according to claim 8, characterized in that the amount of hyrogenic peroxide is between 4% and 6%. 10. Method according to one of claims 1 to 9, characterized in that directly after bleaching, the dough is refined without elimination of solid or liquid matter from the dough from the end of bleaching until the end of refining after bleaching .

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