EP0293309B1 - Process for producing chemithermomechanical pulps - Google Patents

Abstract

Process for the manufacture of CTMP pulps, in which a mechanical grinding of the lignocellulosic material is added to a cooking of the said material at a temperature equal to or higher than 100 DEG C, under saturated steam pressure, with the aid of Na2SO3 or NaHSO3 or SO2NaOH and of a reducing agent more electronegative than the SO3<=> ion, used together and in a medium of initial pH between 7 and 12.5, and bleaching of the manufactured pulp with the aid of H2O2 in an alkaline aqueous medium.

Description

The present invention relates to a process for the manufacture of chemothermal mechanical pulps and more particularly of chemothermal mechanical pulps having a high ability to be bleached using hydrogen peroxide in an alkaline aqueous medium.

To obtain a pulp, cellulosic material, like wood in the form of chips, is subjected to separate or associated actions of mechanical, chemical or thermal origin.

Mechanical type pulps or high yield pulps are produced by mechanical defibration of the lignocellulosic material, for example in a grindstone shredder or a disc shredder or refiner.

The chemothermal-mechanical pastes whose manufacture is covered by the invention, designated in all that follows by CTMP pastes, are obtained by adding to the above process a non-destructive cooking of material carried out at a temperature equal to or greater than 100 ° C. under pressure saturated water vapor in the presence of sodium sulfite, or sodium bisulfite or more generally a mixture of sulfur dioxide SO₂ and sodium hydroxide NaOH, indistinctly referred to in all that follows by sulfite, unless otherwise specified .

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

For example, the yield of CTMP pulp, weight of pulp in the dry state relative to the weight of starting material in the dry state, is generally greater than 85% and most often at least equal to about 90%, this which makes them very close to the pastes of purely mechanical origin and which is translated here by the expression non-destructive cooking of the material.

However, the bleaching of CTMP pasta, which is necessary because of the quality required in this field of market products derived therefrom, poses a problem. Indeed, this bleaching, for example by means of hydrogen peroxide in an alkaline medium, leads to results which are still insufficiently high, probably because of harmful thermal effects previously suffered by the lignocellulosic material, in particular during the cooking defined above.

The above discussion is developed for example in the work of James P. CASEY, "Pulp and Paper Chemistry and Chemical Technology", 3rd ed. vol.I, 1980, p.242-250, 654, 655.

The present invention aims to obtain CTMP pastes which bleaching using hydrogen peroxide brings, except in special cases of interest understandable by those skilled in the art, to a degree of white generally greater than 80%, or 80 °, when it is measured in the usual way today at the wavelength of 457 nm with magnesium oxide as standard of reference using the spectrophotometer of the General Electric or Elrepho type.

Various solutions have already been proposed for obtaining pastes with a sufficiently high degree of whiteness with the aid of sulfite.

For example FR-A-1150451 recommends baking at a temperature of up to 150 ° C., using sulphite and hydrosulphite, in an acid medium. Any bleaching of the defibrated pulp would be facilitated, but no details are provided in this regard.

FR-A-1389308 notes that an improvement in whitening can be expected when a treatment is followed by means of an acid solution of the bisulphite ion SO₃H ^_ , of a treatment, also in acid medium, and without pressure of saturated steam, using sulfite in the presence of sodium borohydride which is preferred to hydrosulfite. The presence of sodium borohydride with the sulphite in the first treatment is harmful and even leads to a lower result than that obtained when the sulphite is used alone in the two treatments. Bleaching by H₂O₂ of a refined paste obtained according to the recommended method leads to whiteness levels well below 80%.

Finally US-A-3981765, which specifies and completes FR-A-2186984, teaches that when the intervention of sodium borohydride takes place in an alkaline medium, with or without sulfite, it is necessarily carried out in medium of initial pH greater than 13 , and should be followed or, preferably, preceded by that of a solution aqueous acid such as a solution of SO₂ or bisulfite. The process described is of interest in practice only for hardwoods. No indication is given on what could be the effect on the dough of the combination of the alkaline treatment described and an alkaline treatment in the presence of hydrogen peroxide.

It is apparent from the known technique that, in a process for preparing a pulp before bleaching which comprises the intervention of the sulfite and an agent such as hydrosulfite or borohydride, an acidification phase is held so constant for a necessity.

Since the use of H₂O₂ makes it possible to whiten pulps obtained with the aid of sulfite to a high level of white, the industry has lost interest in the combined action of the latter with agents such as hydrosulfite. or borohydride. As known, this causes in fact at least a complication of the process, an additional consumption of SO₂ or bisulfite, an increase in the pollutant load. The invention avoids the drawbacks of past methods while improving the result of current methods of making bleached pasta up to a high level of white using hydrogen peroxide.

The invention is first of all a process for manufacturing CTMP pastes in which sodium sulfite or sodium bisulfite or a mixture of sulfur dioxide and sodium hydroxide are involved, and a reducing agent which is more electronegative than ion. sulfite, which consists in adding to a mechanical defibration of the cellulosic material a non-destructive cooking of said material at a temperature equal to or greater than 100 ° C. under saturated steam pressure, using said sulfite, bisulfite or mixture and of said reducing agent, characterized in that said sulfite, bisulfite or mixture, and said reducing agent intervene on the lignocellulosic material jointly and in a medium of initial pH between 7 and 12.5, to lead to a paste made capable of bleaching at using hydrogen peroxide in an alkaline aqueous medium.

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

We can refer for example to the thesis defended on December 6, 1985 at the University Claude Bernard-Lyon-1 by Odile CHALMIN LOUIS-ANDRE: "Reducing properties of sodium dithionite under phase transfer conditions" to judge the electronegativity of dithionite and thiourea dioxide.

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. This quantity, like any quantity in all that follows, is expressed in percent by weight relative to the weight of lignocellulosic material in the dry state, unless indicated or evident. 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 cooking temperature according to the invention can reach 200 ° C. It is most frequently chosen between around 120 ° C and 160 ° C.

The pressure is practically equal to that corresponding to the pressure of the steam at the cooking temperature.

During cooking, the amount of molecular oxygen, such as oxygen in the air, especially that included in the lignocellulosic material must obviously be minimized. This is carried out in a known manner for example using steam or by compression as indicated inter alia the already cited work of James P. CASEY, p 213, which further describes, p 219-229 , equipment, in particular pressure resistant, suitable for the production of CTMP pastes.

The committed amount of sulphite is that usually used in processes which do not incorporate the invention and which are known to be suitable for the preparation of CTMP pastes. It is therefore between approximately 0.1% and 10%, most often between 1% and 6% by expressing it in SO₂.

The cooking according to the invention can be carried out before, during or after a defibering or mechanical refining action.

For example, cooking according to the invention of the starting lignocellulosic material, such as wood chips, can precede a mechanical defibration which continues the action started or which is carried out at low temperature at atmospheric pressure.

For example, wood shavings can be impregnated in a known manner when cold using an aqueous solution of the reagents necessary for carrying out the invention, then cooked according to the invention and finally defibrated as indicated in the preceding paragraph.

For example, cooking according to the invention begins in the defibrator or mechanical refiner supplied by the starting lignocellulosic material, for example wood shavings, or said material already formed in the form of a paste by defibration, to be completed at- beyond if necessary.

For example, the cooking according to the invention can be applied to a paste after defibering-refining of the starting lignocellulosic material, for example wood chips.

The consistency, percentage by weight of lignocellulosic material counted in the dry state in the medium, is not a critical factor for the implementation of the invention. In practice, and depending on the methods of execution adopted, it is between approximately 5% and 50%, in general between 5% and 25% in the case of a material already defibrated.

The duration of cooking according to the invention depends on the other performance parameters including the type of equipment used. It usually does not exceed approximately 1 hour and is generally between a few tens of seconds and 30 minutes.

The invention is also the bleaching of the pulp produced as described just above, which comprises washing said pulp in order to remove therefrom the sulphite ions and the reducing agent and the action on this washed pulp of peroxide. hydrogen in an alkaline aqueous medium.

The advantage of which the invention is the cause, the improvement of the whitening ability up to a high white level, is thus observed.

Washing is carried out in a known manner, for example in a repeated or non-repeated sequence of diluting the dough with water and reconcentrating the dough by pressing or filtration.

We can refer, for example, to the article by H. Kruger, H.U.SUSS _ Tappi, 1982 International Sulfite Pulping Conference, Toronto, October 20-22, 1982, 143-148 _ to judge the importance attributed to this washing.

Bleaching is for example carried out by using an amount of hydrogen peroxide of between approximately 0.5% and 10%, preferably between 2% and 6%, in the presence of approximately 1% to 6% of a solution of sodium silicate of density 1.33, at pH between approximately 9 and 11, at a temperature between 40 ° C and 100 ° C for approximately 0.5 hour to 2 hours, with a consistency between 10% and 30% . The bleaching solution may contain additives such as essentially one or more complexing or 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%. The pulp to be bleached can also be subjected before bleaching to a treatment with, in kind and in quantity, the complexing or sequestering agents as above, at a temperature between 20 ° C and 100 ° C, preferably between 50 ° C and 100 ° C, preferably between 50 ° C and 95 ° C so as not to have to work under pressure while keeping a sufficiently rapid complexing or sequestration rate, at a consistency of between approximately 5% and 30% during periods of time can vary between about 5 minutes and 2 hours, and preferably followed by washing, for example by pressing.

The cooking according to the invention can itself be preceded by a treatment of the lignocellulosic material using a complexing or sequestering agent, of the type described above. The consistency can then be between approximately 5% and 50% without generally being greater than approximately 25% to 30% when the treated material has previously been subjected to a mechanical defibration action. Such treatment contributes to further increasing the intrinsic effect of the invention on the degree of white after bleaching and on the action efficiency of hydrogen peroxide.

It is also possible to put the lignocellulosic material in the presence of the complexing or sequestering agent at the time of cooking according to the invention.

The following examples, given as an indication but not limiting, make it possible to judge of the invention and of its advantages.

• les quantités sont exprimées en pour cent en poids par rapport à la matière lignocellulosique à l'état sec, sauf précision ou évidence,
• quand, avant cuisson, est appliqué un traitement à l'aide d'un agent complexant ou séquestrant, désigné par traitement séquestrant, il est réalisé sauf exception précisée, à 90°C durant 15 minutes, à une consistance de 10%, à l'aide de 0,5% d'une solution aqueuse à 40% en poids de sel de sodium de l'acide diéthylènetriaminepentaacétique, désignée par DTPA
• le pH initial du milieu de cuisson selon et, le cas échéant, celui de la solution aqueuse d'imprégnation contenant le sulfite et l'agent réducteur, est compris entre 7 et 12,5,
• 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 (457 nm) à l'aide du spectrophotomètre de type ELREPHO, fabriqué par KARL ZEISS.

In these examples:

• the quantities are expressed in percent by weight relative to the lignocellulosic material in the dry state, unless specified or evident,
• when, before cooking, a treatment is applied using a complexing or sequestering agent, designated by sequestering treatment, it is carried out, unless otherwise specified, at 90 ° C for 15 minutes, at a consistency of 10%, at using 0.5% of a 40% by weight aqueous solution of sodium salt of diethylenetriaminepentaacetic acid, designated by DTPA
• the initial pH of the cooking medium according to and, where appropriate, that of the aqueous impregnation solution containing the sulphite and the reducing agent, is between 7 and 12.5,
• 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 (457 nm) using the ELREPHO type spectrophotometer, manufactured by KARL ZEISS.

Example 1

Defibrated softwood shavings with a white degree of 53.7% are autoclaved under saturated steam pressure at 120 ° C for 0.5 hours at a consistency of 20% , in the presence, jointly, of 5.9% of sodium sulfite Na₂SO₃, 2.5% of sodium hydrosulfite and of 2% of sodium hydroxide.

Cooking is followed by washing with water to remove sulfite ions and the reducing agent. The CTMP paste obtained is then bleached at 90 ° C for 2 hours, at a consistency of 20%, using 5% hydrogen peroxide H₂O₂, 2% sodium hydroxide, 4% silicate and 0.5 % of DTPA.

The degree of white thus obtained is equal to 82.8%.

By proceeding as above with the exception of the cooking which is carried out, for comparison purposes, without hydrosulfite, using an amount of sodium bisulfite equal to 5% and an equal amount of NaOH at 1.2%, ie a SO₂ / NaOH ratio considered to be practically optimal in such a case, the degree of whiteness achieved is only 81%, further consuming 1.05 times more H₂O₂.

Example 2

The same starting lignocellulosic material as in Example 1 is subjected to the sequestering treatment and then to washing with water. It is then treated as in Example 1.

The degree of white finally obtained is 83.3% when the cooking before blanching involves the joint action of the hydrosulfite with the sulfite. It is only 81% by adopting the cooking conditions defined for comparison in Example 1 with in addition a consumption of H₂O₂ being multiplied by 1.12.

Example 3

Example 2 is repeated in the case of the joint action of hydrosulfite with sulfite but with a cooking temperature of 140 ° C instead of 120 ° C.

The degree of whiteness thus achieved is always high and equal to 82.6%.

Example 4

The lignocellulosic starting material of Example 1 is successively subjected to the sequestering treatment, to washing with water, to baking carried out under saturated steam pressure at 120 ° C. for 0.5 hour, the consistency being 20%, in the presence, jointly, of 1.5% of BOROL® and of 5.9% of sodium sulfite, finally to a washing with water to remove the sulfite and the reducing agent. The CTMP paste obtained is then bleached as after the joint action of the hydrosulfite with the sulfite in Example 1.

The degree of white achieved then is equal to 84.5%.

It is still 82% when the bleaching is modified as regards the amounts of H₂O₂, NaOH and silicate which become equal to 2%, 1% and 3% respectively.

Example 5

Example 4 is modified in that the amount of BOROL® goes from 1.5% to 1% and in that the sodium bisulfite, used at the rate of 5%, replaces the sodium sulfite.

The CTMP paste obtained is bleached as after the joint action of the hydrosulfite with the sulfite in Example 1. The degree of white achieved after bleaching is equal to 82.7%.

Example 6

Example 5 is repeated but with 3% of BOROL® instead of 1%. The degree of white is then equal to 83.8%.

Example 7

250 kg / h of defibred softwood chips are subjected to a complexing treatment at 90 ° C. using 0.5% DTPA, the consistency being 40%, then washed by pressing and cooked in an autoclave under pressure of saturated steam by joint action of 1.5% of BOROL® with 2.5% of sodium bisulphite neutralized with sodium hydroxide. Cooking is started at a temperature of 140 ° C for 5 minutes and then completed at a temperature of 100 ° C for 1 hour. After washing with water to remove the sulphite ions and the reducing agent, the CTMP paste obtained is bleached at a consistency of 28%, at 80 ° C. for 1 hour using 5.1% H₂O₂, 2% NaOH, 4% silicate and 0.5% DTPA.

After refining in a disc refiner and classifying the degree of white of the CTMP paste obtained is equal to 80%. The tensile strength, or breaking length in meters is 3835.

By repeating the process described above but omitting the BOROL®, the degree of whiteness is no longer equal to only 77%, yet engaging 5.4% of H₂O₂, and the rupture length only to 3,290 m.

The starting chips have, after defibering-refining in the absence of any chemical reagent, a degree of whiteness equal to 55.5% and a breaking length equal to 2755 m.

Example 8

By repeating the process according to the invention as it is described in Example 7 but by cooking according to the invention by the joint action of 2.5% sodium hydrosulfite with 5% sodium bisulfite neutralized using NaOH and 1% NaOH, the CTMP pulp bleached as in Example 7 but using only 4.7% of H₂O₂ has a degree of whiteness equal to 78.5% and a breaking length of 3525 m.

Example 9

The same starting lignocellulosic material as in Example 1 is subjected to the sequestering treatment and then to washing with water. It is then subjected to cooking according to the invention at 120 ° C. for 0.5 h, at a consistency of 20%, in the joint presence of 1% of thiourea dioxide and 5.9% of sodium sulfite.

Cooking is followed by washing with water to remove the sulfite ions and the reducing agent. The washed paste obtained is then bleached as after the joint action of the hydrosulfite with the sulfite in Example 1, at 90 ° C for 2 hours, at a consistency of 20%, using 5% H deO₂, 2% d sodium hydroxide, 4% silicate and 0.5% DTPA.

The degree of white thus obtained is equal to 83.5%.

Example 10

Defibrated hardwood shavings with a whiteness level of 59.9% are subjected to autoclaving under saturated steam pressure at 120 ° C for 0.5 hours at a consistency of 20% , in the presence, jointly, of 1.5% of BOROL® and 5.9% of sodium sulfite, followed by washing with water to remove the sulfite and the reducing agent. The washed paste obtained is then bleached at 90 ° C for 2 hours, at a consistency of 20%, using 5% H₂O₂, 4% silicate and 0.5% DTPA.

The degree of white achieved is equal to 86.6%.

By proceeding as above except for the cooking which is carried out, for the purpose of comparison, without BOROL®, the degree of whiteness achieved is only 84.1% by consuming 1.05 times more H₂O₂ .

Example 11

250 kg / h of defibred softwood chips are treated by impregnation with 0.5% DTPA at 90 ° C at a consistency of 20%, washed with water by dilutions and successive pressings, then subjected to cooking under saturated steam pressure at 120 ° C. during the stay of a duration of about 30 seconds in a mechanical shredder at a consistency of 28% by joint action of 1.5 % of BOROL® and 3% of SO₂ in a mixture of pH 8.4 with sodium hydroxide, extending at 100 ° C for 30 minutes. The dough, washed with water after cooking by successive dilutions and pressings to remove the sulfite ions and the reducing agent, is finally bleached at 80 ° C for 2 hours at a consistency of 26.5% using 5.2% of H₂O₂, 2% NaOH, 4% silicate and 0.5% DTPA.

After refining in a double disc refiner and grading, the whiteness of the bleached washed pulp is 83.6%.

By operating as above but omitting the BOROL® in cooking, the degree of whiteness of the bleached dough is only 80.1% by consuming 1.05 times more H₂O₂.

Example 12

The procedure is as in Example 11, but starting from hardwood chips and cooking under saturated steam pressure with, together with BOROL®, 2.45% SO₂ in a mixture of pH 11.5 with sodium hydroxide.

The degree of white in the bleached pulp obtained is equal to 87.3%, the length of the break, in meters, equal to 3,235, and the tear index to 352.

By operating as above but without using BOROL®, the degree of white is only 86%, the length of rupture, in meters, is equal to only 2885 m and the tear index at 310, with the same degree of refinement.

Example 13

Example 4 is repeated on defibred softwood shavings with a degree of white equal to 62% except that the sequestering treatment is not carried out before cooking but that the latter is carried out in the presence, in addition to sulfite and BOROL ®, 0.5% DTPA.

The degree of white of the bleached pulp obtained is equal to 86% while it is only 84% by operating as above but without using BOROL®.

Example 14

After preheating with steam, coniferous wood shavings are impregnated, by immersion and draining, simultaneously with 3% sodium sulfite, 3% sodium bisulfite, 1.5% BOROL®, 0.5 % of DTPA and are autoclaved under saturated steam pressure for 4 minutes at 190 ° C before undergoing mechanical defibration by sudden ejection from the autoclave through a die and refining in a disc refiner.

The dough thus obtained is bleached, after washing to remove the sulphite and, at 90 ° C for 2 hours at a consistency of 20% using 5% H₂O₂, 2% sodium hydroxide, 4% silicate and 0 , 5% DTPA.

The degree of whiteness of the paste thus finally obtained is equal to 71% while the refined starting chips had only a degree of whiteness equal to 56%.

By operating as above but without using BOROL®, the degree of white achieved is only equal to 66% and the rupture length, in meters, is lower, 3755 instead of 3900, for a rupture index practically unchanged.

The invention makes it possible not only to manufacture CTMP bleached pulps of high white level from both softwood and hardwood with reduced consumption of H₂O₂ but also to conserve and even improve the mechanical properties of the pulps. usual bleached.

Claims (14)

1. Process for the manufacture of CTMP pulps which involves sodium sulphite or sodium bisulphite or a mixture of sulphur dioxide and sodium hydroxide, and a reducing agent which is more electronegative than the sulphite ion, which consists in adding to a mechanical grinding of the lignocellulosic matter a nondestructive cooking of the said matter at a temperature equal to or higher than 100°C under saturated steam pressure, with the aid of the said sulphite, bisulphite or mixture and of the said reducing agent, characterized in that the said sulphite, bisulphite or mixture and the said reducing agent act jointly on the lignocellulosic matter in a medium of initial pH of between 7 and 12.5, to produce a manufactured pulp suitable for bleaching with the aid of hydrogen peroxide in an alkaline aqueous medium.

2. Process according to Claim 1, characterized in that the reducing agent is chosen from thiourea dioxide, sodium borohydride and sodium dithionite.

3. Process according to Claim 2, characterized in that the thiourea dioxide or sodium hydrosulphite are introduced in a proportion of 0.1% to 5%.

4. Process according to Claim 2, characterized in that sodium borohydride is introduced in a proportion of 0.01% to 0.5%.

5. Process according to any one of Claims 1 to 4, characterized in that the cooking temperature does not exceed 200°C.

6. Process according to Claim 5, characterized in that the cooking temperature is between 120°C and 160°C.

7. Process according to any one of Claims 1 to 6, characterized in that the quantity of sodium sulphite or of sodium bisulphite or of a sulphur dioxide-sodium hydroxide mixture is such as to make it between 0.1% and 10%, expressed as sulphur dioxide.

8. Process according to any one of Claims 1 to 7, characterized in that the lignocellulosic matter is treated with the aid of a complexing or sequestering agent before cooking.

9. Process according to any one of Claims 1 to 7, characterized in that the lignocellulosic matter is brought into contact with a complexing or sequestering agent at the time of cooking.

10. Process according to either of Claims 8 and 9, characterized in that the complexing or sequestering agent is DTPA.

11. Process according to any one of Claims 1 to 10, characterized in that the cooking is performed before, during or after the mechanical grinding.

12. Bleaching of the pulp manufactured according to one of Claims 1 to 11, which comprises washing the said pulp to remove the sulphite ions and the reducing agent therefrom, and then the action of hydrogen peroxide in an alkaline aqueous medium on this washed pulp.

13. Process according to Claim 12, characterized in that the hydrogen peroxide is introduced in a medium of pH of between 9 and 11.

14. Process according to either of Claims 12 and 13, characterized in that the hydrogen peroxide is introduced in a proportion of 2% to 6%.