FI93746B - Process for the preparation of bleached chemithermomechanical pulp - 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

93746

The present invention relates to the preparation of a bleached chemithermomechanical pulp in which the bleaching takes place with hydrogen peroxide in an alkaline aqueous medium.

To produce the pulp, the cellulosic material, such as wood chips, is subjected to separate or combined mechanical, chemical or thermal treatments.

Mechanical pulps or high yield pulps are prepared by subjecting the lignocellulosic material to mechanical fiberization, for example in a grinder or fiberizer or disc mill.

The chemomechanical pulps of the present invention, hereinafter referred to as CTMP pulps, are prepared by incorporating in the above process a non-decomposable cooking step carried out at 100 ° C or higher under saturated water vapor pressure with sodium sulfite or sodium bisulfite, sodium bisulfite or more generally sulfur dioxide hereinafter referred to as "sulphite without specification", in the presence of.

CTMP pulps have a certain advantage for industry, as they are a compromise between mechanical pulps and actual chemical pulps.

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For example, the yield of CTMP pulp, in terms of dry weight of the pulp relative to the dry weight of the starting material, is generally greater than 85% and most often at least about 90%, bringing them quite close to purely mechanical pulps and expressed herein as non-degradable soup.

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However, the bleaching of CTMP pulps, which is required in this field by the quality requirements of the goods made from it, poses problems. Namely, this bleaching, which is carried out, for example, with hydrogen peroxide in an alkaline medium, still produces too poor results, probably due to the detrimental thermal effects to which the lignocellulosic substance is exposed before bleaching, especially during the cooking process defined above.

The above description is developed, for example, in James P. Casey, "Pulp and Paper Chemistry and Chemical Technology", 3rd edition, vol. I, 1980, pp. 242-250, 654, 655.

It is an object of the present invention to produce CTMP pulps obtained by hydrogen peroxide bleaching, in addition to the specific advantage of a person skilled in the art, to a degree of whiteness generally above 80% or 80 ° when measured at the usual 457 nm wavelength using magnesium oxide as a reference standard. With an Elrepho-type spectrophotometer.

Various solutions have been proposed in the past for the production of pulps with a sufficiently high degree of whiteness by means of sulphite.

For example, FR Patent No. 1,150,451 proposes cooking the pulp at a temperature up to 150 ° C with sulphite or hydrosulphite in an acidic medium. The possible bleaching of the defibered pulp should become easier from this, but no details have been given on this point.

FR Patent No. 1,389,308 notes that better bleaching is expected when treatment with bisulfite ion SO H

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the acid solution is followed by treatment, as well as in an acidic medium and without saturated water vapor pressure with sulfite of sodium borohydride, which is used rather than hydrosulfite. The presence of sodium borohydride in connection with sulphite in the first treatment of n 3 93746 is detrimental and leads to an even worse result than that obtained when sulphite is used alone in both treatments. Bleaching of the fibrous pulp prepared in the recommended manner with H2O2 achieves degrees of whiteness well below 80%.

Finally, U.S. Patent No. 3,981,765, which clarifies and supplements the corresponding FR Patent No. 2,186,984, states that when sodium borohydride is used in an early environment, with or without sulfite, it must necessarily occur in an environment where the starting material -pH is greater than 13 and should be monitored or preferably preceded by the use of an acidic aqueous solution such as SO 2 solution or bisulfite solution. In practice, the described method has an advantage only when using hardwood. There is no mention of the cumulative effect of the described alkali treatment and alkali treatment in the presence of hydrogen peroxide.

It is known from the prior art that in a pulping process carried out before bleaching using sulphite and a substance such as hydrosulphite or borohydride, an acidification step is always considered necessary.

) 'When H 2 O 2 can be used to bleach sulphite-free pulps to very white, the industry has lost interest in the interaction between the latter and substances such as hydrosulphite and borohydride. In the presently known form, this combination at least complicates the process, causes SO2 or! additional bisulfite consumption and increased waste load. The invention avoids the disadvantages of the previous methods while improving the result of the current methods for producing pulps bleached to a high degree of whiteness with hydrogen peroxide.

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The process for the production of bleached CTMP pulp according to the invention is characterized in that (a) the lignocellulosic material from which the pulp is produced is subjected to grinding or mechanical defibering and cooking before, during or after said milling or defibering and in which the sulphite and sodium borohydride, which acts as a reducing agent, is allowed to act simultaneously at a pH of 8 to 12.5 at a temperature of 100 ° C or higher at a saturated vapor pressure so as to obtain a chemithermomechanical pulp in a yield of lignocellulosic pulp. is greater than 85% by dry weight of the material, and (b) bleaching the resulting mass with hydrogen peroxide in an alkaline medium.

The amount of sodium borohydride used as a reducing agent may vary depending on its quality, but is generally used in an amount of 0.01 to 0.5% by weight based on the dry lignocellulosic material. Unless otherwise stated or indicated, this amount, as well as all other amounts in the following description, is expressed as a percentage by weight of the dry weight of the lignocellulosic material. Sodium borohydride is readily used as an aqueous solution, such as a solution containing 12% by weight of sodium borohydride sold by Ventron Corporation under the tradename BOROL®.

According to the invention, the cooking temperature can rise to 200 ° C. In most cases, the temperature is chosen to be about 120-160 ° C.

The pressure is practically the same as the water vapor pressure at the cooking temperature.

During cooking, the amount of molecular oxygen, such as oxygen in the air, especially the oxygen contained in the lignocellulosic substance, must of course be kept to a minimum. This is achieved in a known manner by means of steam or pressure, as mentioned, inter alia, in James P. Casey, cited above, p. 213, which further describes, on pages 5 93746 to 219-229, devices, in particular pressure-resistant devices, suitable for CTMP. for the manufacture of pulps.

The amount of sulphite used is the same as that normally used in processes not covered by the invention and which are known to be suitable for the production of CTMP pulps. It is thus about 0. Ι-ΙΟ%, most often 1-6% expressed as SO 2. As the sulfite, sodium sulfite, sodium bisulfite or a mixture of sulfur dioxide and sodium hydroxide can be used.

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

For example, the cooking of a lignocellulosic feedstock such as wood chips according to the invention may be preceded by mechanical defibering which continues the onset of action or is carried out at low temperature and atmospheric pressure.

For example, wood chips can be cold impregnated in a known manner with an aqueous solution of the reagents needed to carry out the invention and then cooked according to the invention and finally defibered as mentioned in the previous paragraph.

The cooking according to the invention can be started, for example, in a mechanical defibering device or refiner fed with a lignocellulosic feedstock, for example wood chips, or by already defibering the pulverized material, after which it is completed if the need so requires.

The cooking according to the invention can, for example, be carried out on the pulp after defibering and grinding of a lignocellulosic feedstock, for example wood chips.

The consistency of the pulp, the weight percentage of lignocellulosic material calculated as dry matter in the mixture, is not a critical factor for the practice of the invention. In practice and according to the embodiments used, it is about 5-50%, usually 5-25% of the material already fiberized.

The duration of cooking according to the invention depends on other performance parameters, including the type of equipment used. In most cases, it does not exceed 1 hour and is usually from a few tens of seconds to 30 minutes.

The bleaching of the pulp prepared as part of the process according to the invention may comprise washing the pulp to remove sulphite ions and a reducing agent as far as possible and exposing this washed pulp to hydrogen peroxide in an alkaline aqueous medium.

The advantage achieved by the invention, the improvement of bleaching even to a high degree of whiteness, is thus established.

The washing is carried out in a known manner, for example in a repeated or non-repeated series of dilutions of the pulp with water and its re-concentration by compression or filtration.

Reference is made, for example, to the article by H. Kruger, H.U. Suss Pin, 1982, International Sulfite Pulping Conference, Toronto, 20-22. October 1982, 143-148, to evaluate the significance of this washing.

Bleaching is performed, for example, using about 0.5-10%, preferably 2-6% hydrogen peroxide, and about 1-6% sodium silicate solution having a density of 1.33 at a pH of about 9-11, 40-100 ° C at a temperature of about 0.5-2 hours and a consistency of 10-30%. The bleaching solution may contain additives such as mainly one or more complexing or sequestering agents, such as, for example, diethylenetriaminepentaacetic acid and ethylenediaminetetraacetic acid as sodium salts, generally in an amount of about 0.1-1%. Before bleaching, the pulp to be bleached can be treated with complexing agents or sequestering agents of the above-mentioned quality and amount, at a temperature of 20-100 ° C, preferably 50-100 ° C, preferably 50-95 ° C, so as not to have to work under pressure, while maintaining a sufficiently rapid complexation or sequestration time, at a consistency of about 5-30%, with residence times varying from about 5 minutes to 2 hours, followed by a wash, for example by compression.

The cooking according to the invention may itself be preceded by treatment of the lignocellulosic substance with a complexing or sequestering agent of the type described above. The consistency may then be about 5-50% without generally being greater than about 25-30% when the treated material has been pre-mechanically fiberized. Such a treatment further contributes to the inherent effect of the invention on the degree of difficulty obtained after bleaching and the effect of hydrogen peroxide.

The lignocellulosic agent may further be contacted with a complexing or sequestering agent during cooking according to the invention.

The following examples, which are presented to illustrate but not to limit the invention, may be used to evaluate the invention and its advantages.

In these examples: - the amounts are expressed as a percentage by weight of the dry weight of the lignocellulosic substance, unless otherwise stated or obvious, - when treatment with a complexing or sequestering agent, called sequestration treatment, is carried out before cooking, unless otherwise specified in 90 °, With a residence time of 15 minutes at a consistency of 10% in a 0.5% aqueous solution containing 40% by weight of the sodium salt of diethylenetriaminepentaacetic acid, abbreviated as DTPA, 8 93746 - initial pH of the cooking medium depending on the sulfite and reducing agent impregnation solution if necessary, its pH is 7 to 12,5, - a substance called BOROL is used as the sodium borohydride and the amount is mentioned in this description, - silicate means an aqueous solution of sodium silicate with a density of 1,33, - the degree of whiteness is measured (457 nm) ELREPHO- type spectrophotometer manufactured by KARL ZEISS.

Example 1

Fibered softwood chips with a whiteness of 53.7% are boiled in an autoclave under saturated water vapor pressure at 120 ° C for 0.5 hours, at a consistency of 20%, using together 5.9% sodium sulphite Na 2 SO 4, 2.5% sodium hydrosulphite and 2% sodium hydroxide.

The soup is followed by washing with water to remove sulfite ions and reducing agent. The resulting CTMP pulp is then bleached at 90 ° C with a residence time of 2 hours, at a consistency of 20%, 5% hydrogen peroxide, 2% sodium hydroxide, 4% silicate and 0.5% DTPA.

The degree of whiteness thus obtained is 82.8%.

By proceeding as above, with the exception of cooking, which is carried out without hydrosulphite for comparison, using sodium bisulphite 5% and NaOH 1.2%, i.e. the SO 2 / NaOH ratio, which is considered practically optimal in such a case, the whiteness achieved is only 81%, while in addition, consuming 1.05 times more H 2 O 2

Example 2

The same lignocellulosic feedstock as in Example 1 is subjected to sequencing treatment and then washing with water. It is then treated as in Example 1.

The degree of whiteness finally obtained is 83.3% when the soup, which is leached before bleaching, comprises the combined action of hydrosulfite and sulfite. It is only 81% when the cooking conditions used in the comparative experiment defined in Example 1 are used, and in addition the consumption of H 2 O 2 is 1.12 times.

Example 3

Example 2 is repeated by treating the pulp together with hydrosulphite and sulfite, but the cooking temperature is 140 ° C instead of 120 ° C.

The whiteness obtained in this way is still high, i.e. 82.6%.

Example 4

The lignocellulosic feedstock of Example 1 is sequentially subjected to sequestration, water washing, cooking under saturated water vapor pressure at 120 ° C with a residence time of 0.5 hours, 20% consistency, using simultaneously 1.5% BOROL and 5.9% sodium sulfate. and finally washing with water to remove sulfite and reducing agent. The resulting CTMP pulp is then bleached as after the interaction of hydrosulfite and sulfite in Example 1.

The degree of whiteness obtained is 84.5%.

It is still 82% when the bleach is changed for H 2 O 2, NaOH and silicate, which are now 2%, 1% and 3% respectively.

Example 5

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Example 4 is modified to change the amount of BOROL from 1.5% to 1% and to replace sodium sulfite with sodium bisulfite, which is used at 5%.

The obtained CTMP pulp is bleached as after the combined action of hydrosulfite and sulfite in Example 1. The degree of whiteness obtained after bleaching is 82.7%.

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Example 6

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Example 5 is repeated, but using 3% BOROL instead of 1%. The whiteness is then 83.8%.

Example 7 250 kg / h of fibrous softwood chips are treated by a complexation treatment at 90 ° C with 0.5% DTPA, at a consistency of 40%, then washed by compression and autoclaved at saturated water vapor pressure using a combination of 1.5% BOROL and 2.5% sodium bisulphite neutralized with sodium hydroxide. Cooking is started at 140 ° C for 5 minutes and then continued at 100 ° C for 1 hour. After washing with water to remove sulphite ions and reducing agent, the CTMP mass obtained is bleached to a consistency of 28% at 80 ° C with a residence time of 1 hour, using 5.1% Na 2 Na 2 O, 4% silicate and 0.5% DTPA.

After the pulp has been ground in a plate mill and sorted, the CTMP pulp obtained has a whiteness of 80%. The tensile strength, or breaking length, is 3,835 in meters.

By repeating the procedure described above, but excluding g) BOROL, the degree of whiteness is only 77%, but using 5.4% H 2 O 2 and a breaking length of 3 290 m.

The degree of whiteness of the starting chips after any defibering grinding without chemical reagent is 55.5% and the breaking length is 2,755 m.

Example 8

When the process according to the invention is repeated as described in Example 7, but with the cooking according to the invention carried out using together 2.5% sodium hydrosulphite and 5% sodium bisulphite neutralized with NaOH and 1% NaOH, the CTMP mass , bleached as in Example 7, but using only 4.7% H 2, has a whiteness of 78.5% and a breaking length of 3,525 m.

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...... EXAMPLE 9

The same lignocellulosic feedstock as in Example 1 is subjected to a sequestration treatment and then washing with water. It is then boiled according to the invention at 120 ° C for 0.5 hours, at a consistency of 20%, using simultaneously 1% thiourea dioxide and 5.9% sodium sulphite.

After cooking, a water wash is performed to remove sulfite ions and a reducing agent. The resulting washed mass is then bleached as after the combined action of hydrosulfite and sulfite in Example 1, at 90 ° C with a residence time of 2 hours, at a consistency of 20%, using 5%, 2% sodium hydroxide, 4% silicate and 0.5% DTPA: a.

The degree of whiteness achieved in this way is 83.5%.

Es ime r k k i_ _ 10

The fibrous hardwood chips with a whiteness of 59.9% are boiled in an autoclave under saturated water vapor pressure at 120 ° C for 0.5 hours, at a consistency of 20%, using together O) 1.5% BOROL and 5.9% sodium sulphite, followed by washed with water to remove sulfite and reducing agent. The resulting washed mass is then bleached at 90 ° C for a residence time of 2 hours at 20% consistency using 5% 4% silicate and 0.5% DTPA.

The degree of whiteness achieved is 86.6%.

By proceeding as above, with the exception of the soup, which is carried out without comparison with BOROL, the degree of whiteness achieved is only 84.1%, while 1.05 times more h 2 O is consumed.

Example 11 250 kg / h of softwood chips which have been defibered are treated by impregnating them with 0.5% DTPA at 90 ° C at a consistency of 20%, washed with successive dilutions of water and pressing 12 93746 and then boiled at saturated water vapor pressure at 120 ° At C for about 30 seconds in a mechanical fiberizer at a consistency of 28%, using together 1.5% BOROL and 3% SC> 2 mixed with sodium hydroxide to a pH of 8.4, continuing the treatment at 100 ° C for 30 minutes. The pulp, washed after cooking with successive dilutions of water and compression to remove sulphite ions and reducing agents, is finally bleached at 80 ° C for a residence time of 2 hours at a consistency of 26.5% with 5.2% H 2 O 2, 2% NaOH, 4% silicate and 0.5% DTPA.

After grinding and sorting in a plate mill, the bleached pulp has a whiteness of 83.6%.

©

When the procedure is as above, but excluding BOROL, the degree of whiteness of the bleached pulp is only 80.1%, while still consuming 1.05 times more.

Esimerkkil2

Proceed as in Example 11, but start from the hardwood chips and carry out the cooking under saturated water vapor pressure © using together with BOROL 2.45% SC 4 mixed with sodium hydroxide to give a pH of 11.5.

The resulting bleached pulp has a whiteness of 87.3%, a break length in meters of 3,235 and a tear index of 352.

©

When carried out as above, but without the use of BOROL, the degree of whiteness is only 86%, the breaking length in meters is only 2,885 m and the tear index is 310, the degree of grinding being the same.

Example 13

Example 4 is repeated for a fibrous softwood chip with a degree of whiteness of 62%, except that the sequestration treatment is not carried out before cooking, but cooking is carried out using 0.5% DTPA in addition to sulphite and BOROLINO 2.

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The degree of whiteness of the bleached pulp obtained is 86%, whereas instead it is only 84% when treated as above, but without the use of BOROL.

Example 14

After preheating the softwood chips with steam, it is impregnated, immersed and drained, simultaneously with 3% sodium sulphite, 3% sodium bisulphite, 1.5%

Oj ROLIA, 0.5% DTPA and boiled in an autoclave under saturated water vapor pressure for 4 minutes at 190 ° C before being mechanically defibered by sudden removal from the autoclave through a die and grinding in a plate mill.

The pulp thus obtained, after washing to remove sulphite, is bleached at 90 ° C for a residence time of 2 hours at a consistency of 20% with 5% 2% sodium hydroxide, 4% silicate and 0.5% with DTPAra.

The whiteness of the pulp finally obtained in this way is 71% ·> when • * 5 6% instead the whiteness of the ground wood chips was only © aa-

When carried out as before, but without the use of BOROL ·> s, the degree of whiteness obtained is only 66% and the breaking length in meters is less than »3 755 'when it was 3 900 above, the tear index remaining practically unchanged - _from i 5 tuj a

The battery of the invention can not only be used to produce very high CTMP pulps with a high degree of whiteness, but also to retain the properties of conventional bleached pulps and even improve them.

Claims (8)

93746 A process for the production of bleached chemithermomechanical pulp, characterized in that (a) the lignocellulosic material from which the pulp is made is subjected to grinding or mechanical defibering and cooking before, during or after said milling or defibering, wherein the sulphite and the reducing agent, sodium borohydride, is allowed to act simultaneously at a pH of 8 to 12.5, at a temperature of 100 ° C or higher, at a saturated vapor pressure, so as to obtain a chemithermomechanical mass in a yield corresponding to the mass of the resulting mass and the starting material; is greater than 85% by dry weight of the lignocellulosic material, and (b) bleaching the resulting pulp with hydrogen peroxide in an alkaline medium. Process according to Claim 1, characterized in that 0.01 to 0.5% by weight of sodium borohydride, based on the weight of the dry lignocellulosic material, is used. Process according to Claim 1 or 2, characterized in that the cooking temperature does not exceed 200 ° C. Process according to one of Claims 1 to 3, characterized in that the cooking temperature is 120 to 160 ° C. Process according to one of Claims 1 to 4, characterized in that the sulphite is sodium sulphite, sodium bisulphite or a mixture of sulfur dioxide and sodium hydroxide and that the amount of sulphite, expressed as sulfur dioxide, is 0.1 to 10% by weight of the dry lignocellulosic substance. Process according to one of Claims 1 to 5, characterized in that the lignocellulosic substance is treated with a complexing or sequestering agent before or during cooking. Process according to one of Claims 1 to 6, characterized in that the lignocellulosic substance is treated with the sodium salt of diethylenetriaminepentaacetic acid before or during cooking. Process according to one of Claims 1 to 7, characterized in that hydrogen peroxide is used at a pH of between about 9 and 11. Process according to one of Claims 1 to 8, characterized in that hydrogen peroxide is used in an amount of 2 to 6%, based on the weight of the dry lignocellulosic material.