EP0263040B1 - Process for bleaching lignocellulosic materials - Google Patents

Description

The present invention relates to a process for bleaching lignocellulosic materials.

The term “lignocellulosic materials to which the process of the invention applies” denotes here paper pulps of mechanical, thermomechanical, chemo-mechanical, chemo-thermomechanical origin as well as wood in the form of chips being transformed into one of these types of pulp by treatment. in a machine as described for example in FR-A-2319737, FR-E-2436844, FR-A-2418295 and FR-A-2451963, all from the company CREUSOT-LOIRE.

Such lignocellulosic materials need to be bleached so that the products finally produced from them, such as for example the paper intended for the printing of newspapers and magazines, have the required quality.

The process of the invention relates more particularly to a process for bleaching these materials in which a bleaching treatment using a reducing agent and a bleaching treatment using an oxidizing agent are combined.

It is known to carry out the bleaching treatment by means of a reducing agent such as sodium hydrosulfite or sodium dithionite used as it is or generates from sodium borohydride, sodium borohydride itself, thiourea dioxide or formamidinesulfinic acid.

It is also known to carry out the bleaching treatment by means of an oxidizing agent such as, for example, chlorine dioxide, peracetic acid, ozone and, above all, hydrogen peroxide.

The use of hydrosulfite and that of hydrogen peroxide are for example described by R. R. KINDRON, "PULP & PAPER", Nov. 1980, 54 (11), 127-130.

That of hydrosulfite generated from borohydride is for example described by John W. GERRIE, "PULP & PAPER MAGAZINE of CANADA", vol 75, No 7 / July 1974, 89-92 and by Robert G. GUESS, "PULP & PAPER ", June 1979, 53 (6), 74-81.

That of borohydride is for example described by V. LORAS and N. SOTELAND, "NORSK SKO-GINDUSTRI", 26 (10), 1972, 255-258.

Finally, it is known to carry out bleaching according to a method combining a bleaching treatment by means of an oxidizing agent and a bleaching treatment by means of a reducing agent as it is described for example in the last reference cited above.

In all cases, the use of complexing or sequestering agents is held as necessary to counteract the negative effect of metals, such as iron or manganese, which are always present in practice in the materials to be bleached.

The use of such complexing or sequestering agents is described for example in the German patent application DE-OS 3308298, in the article by Robert G. GUESS already cited, in that of P. WHITING, JM PITCHER, DF MANCHESTER, " JOURNAL of PULP & PAPER SCIENCE ", SEVEN. 1984, 10 (5), J119-J126 and in that of J. Roger HART, "PULP & PAPER", June 1981, 138-140.

As complexing or sequestering agents, for example, sodium tripolyphosphate, sodium tetrapyrophosphate, diethylenetriaminepentaacetic and ethylenediaminetetraacetic acids in the form of sodium salts are used.

The introduction of complexing or sequestering agents can take place either simultaneously or prior to that of the bleaching agent. In the latter case, the material to be bleached is subjected to a pretreatment by the complexing or sequestering agent.

In the case of a bleaching or medium treatment of an oxidizing agent such as hydrogen peroxide, the best results are achieved when the pretreatment by means of the complexing or sequestering agent ends with a thorough washing.

On the contrary, in the case of a bleaching treatment by means of a reducing agent, improvement in bleaching can only be expected according to the art of the technique in a process in which the complexing or sequestering agent accompanies necessarily the reducing agent in its action.

This is apparent, for example, from the work of V. LORAS et al. and those of P. WHITING et al. cited above, another teaching of which is that, in a process combining the action of a reducing bleaching agent, such as dithionite or borohydride, and that of an oxidizing bleaching agent such as hydrogen peroxide, order of intervention of the two bleaching agents which leads to the best results depends on the nature of the reducing agent chosen and therefore cannot be predetermined.

An illustration of the need for the complexing or sequestering agent up to the process of the invention to accompany the action of the reducing agent, is provided by Example VIII of EP-A-0187477 . This example shows that this necessity exists when the dough is pretreated with the complexing or sequestering agent and washed effectively before the action of the reducing agent. It is also taught by V. LORAS et al. in the article already cited, that when the complexing or sequestering agent accompanies the reducing agent in its action a pretreatment has no influence on the result in a process in two stages of bleaching, reducing and then oxidizing.

According again to the nature of the reducing agent, the conditions, such as those especially of pH, under which this reagent exerts its action at best can be essentially different: acid medium, for example for dithionite, alkaline medium, for example for borohydride.

So far, the industry cannot take advantage of a process for bleaching lignocellulosic materials, as defined here, capable of combining the action of an oxidizing bleaching agent and that of a reducing bleaching agent in a manner indifferent to the nature of the reducing agent .

The present invention not only offers such a process but also leads, within the framework of its generality, to whitening effects superior to those achieved to date by known processes based on a combination of oxidative whitening and whitening. reducer.

The method of the invention is characterized in that the lignocellulosic material, after pretreatment by means of at least one metal complexing or sequestering agent, is subjected to a washing of efficiency greater than 96% before successively undergoing a treatment of bleaching using a reducing agent at a pH between 8 and 12, washing to remove the reducing agent and bleaching treatment using an oxidizing agent.

By washing is understood the operation consisting in eliminating more or less completely from the lignocellulosic material the liquid phase present in it, thanks for example to a concentration of the lignocellulosic material for example by pressing on a filter, or thanks to a sequence, repeated or no, dilution, most often with water, then concentration of the lignocellulosic material, for example by pressing on a filter.

By efficiency is meant, expressed in percent, the degree of elimination of the liquid phase present before washing in the lignocellulosic material.

The washing which follows the pretreatment of the lignocellulosic material with the complexing or sequestering agent in the process of the invention is most often carried out in practice by diluting the lignocellulosic material with water and concentration by pressing on a filter, a temperature generally between approximately 20 ° C and 90 ° C, often between approximately 20 ° C and 60 ° C for economic reasons.

In the pretreatment using the complexing or sequestering agent, this is used at a rate of approximately 0.1% to 1% by weight of the weight of lignocellulosic material in the dry state. Unless specified or evident, it is also in percent by weight of the weight of lignocellulosic material in the dry state that the quantities of reactants are expressed in everything that follows.

The complexing or sequestering agent is most often chosen from sodium tripolyphosphate, sodium tetrapyrophosphate, sodium salts of acetic acids, nitrilotriacetic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid.

The pretreatment with the complexing or sequestering agent is most often carried out at a temperature between about 20 ° C and 100 ° C, preferably between 50 ° C and 95 ° C so as not to have to work under pressure while keeping a speed sufficiently rapid complexation or sequestration.

The consistency, content in% of the treatment medium in lignocellulosic material in the dry state, can be equaled in the pretreatment to a value between fairly wide limits, about 5% to 50% but it is generally not higher about 25% in the case of pasta and most often remains between 10% and 40% in the case of chips to reconcile economy and efficiency of subsequent washing.

The duration depends on other parameters and the equipment used. Generally between about 5 minutes and 2 hours in the case of pasta it can be reduced to a few minutes and even a few tens of seconds in the case of chips as defined here.

The bleaching treatment which follows the washing with an efficiency greater than 96% is carried out by means of a reducing agent chosen most often from sodium dithionite, thiourea dioxide, sodium borohydride.

The latter is advantageously used in an amount of about 0.05% to 0.7% in the presence of sodium hydroxide, for example in the form of about 0.4% to 6% of the alkaline solution of sodium borohydride containing 12% by weight of this product and marketed under the name of BOROL ^@ by the company VENTRON Corporation.

Sodium dithionite, to present the best activity in the required pH range, is preferably used at a rate of about 1% to 10% with about 0.5% to 5% of sodium hydroxide and an amount of sodium silicate corresponding to approximately 1% to 4% of an aqueous solution of this product with a density equal to 1.33, so that the weight ratio of sodium dithionite / sodium hydroxide is between approximately 1.5 and 2 , 5 and that the weight ratio sodium silicate solution, density 1.33 / sodium hydroxide is between approximately 1 and 2.

As for thiourea dioxide, still for carrying out the invention in a preferred manner, it is used at a rate of approximately 1% to 10% in the presence of approximately 0.5% to 5% of sodium hydroxide.

The consistency in the reduction whitening treatment is most generally between approximately 5 and 50%, practically between approximately 5% and 25% in the case of pasta.

The reducing bleaching treatment is carried out at a temperature between about 30 ° C and 90 ° C which, although having relatively little influence in the range chosen, is preferably between about 40 ° C and 60 ° C.

The reducing whitening treatment generally lasts between about 0.5 hours and 2 hours.

It can only be practiced, as is known, away from oxygen, in particular oxygen from the air, most often thanks to an atmosphere of nitrogen or water vapor.

At the end of the bleaching treatment using the reducing agent and then washing to remove the reducing agent, the efficiency of which is advantageously greater than 96%, as in the case of the washing which follows the pretreatment, treatment with an oxidizing agent is carried out, the oxidizing agent being the most general ment chosen from chlorine dioxide, peracetic acid, ozone, hydrogen peroxide.

The invention authorizes not to again use a complexing or sequestering agent before or during oxidative bleaching, contrary to the practice currently recommended.

Apart from this particularity, the oxidative bleaching is preferably carried out by means of hydrogen peroxide used for example at a rate of approximately 0.5% to 10%, in the presence of sodium silicate, at a pH between approximately 9 and 11 , at a temperature between about 60 ° C and 90 ° C during most often about 0.5 hour to 2 hours with a consistency generally between about 15% and 30%.

The process of the invention can be carried out in any apparatus combining devices known in the paper industry to be suitable for pretreatment, washing, bleaching operations such as among other mixers, filters, presses.

It was thus noted the possibility of carrying it out, in whole or in part, in a wood chip processing machine described in the CREUSOT-LOIRE patents already cited.

Such a machine essentially consists of two helical surfaces penetrating into one another, wound around parallel shafts, driven in synchronous rotation in the same direction inside a sheath enveloping them, and determining the upstream downstream in the direction of advancement of the material a succession of zones for entraining and braking the material.

The action of the complexing agent with the sequestrant on the shavings having, depending on the type of paste intended, undergone or not the effect of a chemical reagent such as, for example, sulfite, then takes place in a surprisingly rapid manner and effective. The same is true of the washing associated with this action.

The examples below, given as an indication but not limiting, make it possible to judge the interest in the process of the invention.

In these examples, the quantities of reagents used are expressed in percent by weight of the weight of lignocellulosic material in the dry state, the terms of washing and of effectiveness have the meanings indicated pectively above, the term of sodium silicate designates a solution commercial sodium silicate with a density equal to 1.33, the pH during the reducing bleaching is each between 9 and 12 and the degrees of whiteness of the pastes (457 nm) are measured in percent using a KARL ZEIS spectrophotometer of the ELREPHO type.

Example 1

10 g of a mechanical grinding wheel pulp, of softwood which contains 4 g of lignocellulosic material in the dry state and whose degree of whiteness is equal to 56.4% are brought to a consistency equal to 10% to be pretreated in a mixer using 0.5% of a solution containing 40% of sodium salt of diethylenetriaminepentaacetic acid for 15 minutes at 90 ° C before being subjected to a washing carried out by carrying out two dilutions of the paste pretreated using, in total, 0.5 water, each dilution being followed by a concentration by pressing until a consistency of 35%, so that a washing efficiency of 99% be assured.

The dough thus obtained is brought to a consistency of 15% in order to be bleached in a mixer under a nitrogen atmosphere using 1.25% sodium dithionite in the presence of 0.75% sodium hydroxide and 1% of sodium silicate, for 1 hour at a temperature equal to 90 ° C.

After washing to remove the reducing agent, the dough is bleached in a mixer at a consistency equal to 20% using 3% hydrogen peroxide in the presence of 1.5% sodium hydroxide and 4% sodium silicate for 2 hours at 90 ° C.

Finally, the pulp resulting from the above treatments to a degree of whiteness equal to 81% while only 55% of the hydrogen peroxide used is consumed.

Example 2 (comparative)

Example 1 is repeated, omitting to wash the dough after the pretreatment.

The paste finally obtained has a degree of whiteness which is no longer equal to 78% while the consumption of hydrogen peroxide is 1.4 times higher than in Example 1.

Example 3

Example 1 is repeated with the exception that the amounts of reactants involved in the reducing bleaching are 5% sodium dithionite, 2% sodium hydroxide and 3% silicate.

The degree of whiteness finally reached is equal to 83% for a consumption of hydrogen peroxide which only amounts to 56% of the hydrogen peroxide used.

Example 4

10 g of the pulp of Example 1, brought to a consistency equal to 10% to be pretreated as in Example 1, are washed after pretreatment so that the washing efficiency reaches 97% before being subjected in a mixer under a nitrogen atmosphere, to a consistency of 15%, to the action of 1.25% of thioerium dioxide in the presence of 0.8% of sodium hydroxide for 40 minutes at a temperature equal to 90 ° C.

The paste is then washed to remove the reducing agent, then bleached in a mixer for 2 hours at 90 ° C at a consistency equal to 20%, using 10% hydrogen peroxide, in the presence of 3% hydroxide sodium and 4% sodium silicate.

The bleached pulp obtained has an excellent whitening degree since it is equal to 89% while only 40% of the hydrogen peroxide used is consumed.

Example 5

By doubling in Example 4 the amounts of thiourea dioxide and sodium hydroxide but by using only 1% hydrogen peroxide during oxidative bleaching accompanied by 0.5% sodium hydroxide and 2% sodium silicate, the paste, due to the bleaching ability acquired during the process of the invention, has, at the end of it, a degree of whiteness equal to 82%, reached for a consumption of hydrogen peroxide equal to 64% of the hydrogen peroxide used.

Example 6

10 g of a mechanical coniferous wood pulp made in a machine of the type described in FR-A-2436844 and FR-A-2451963, which has a degree of whiteness equal to 53.7% and contains 4 g of lignocellulosic material in the dry state, are brought to the consistency of 10% to be pretreated and then washed like the grinding wheel paste in Example 1.

The bleaching treatment is then carried out on the pretreated and washed dough brought to a consistency equal to 20% in a mixer under nitrogen, using 5% BOROL ^@ in the presence of 5% sodium silicate, for 1 hour at a temperature of 50 ° C.

After washing to remove the reducing agent, the oxidative bleaching is finally carried out at the consistency of 20%, using 3% hydrogen peroxide, 1.5% sodium hydroxide and 4% sodium silicate for 2 hours at a temperature of 90 ° C.

The degree of whiteness of the pulp thus bleached reaches a value equal to 85.3% for a consumption of 53.2% of the quantity of hydrogen peroxide used.

Example 7

600 kg / h of coniferous wood shavings which contain 40% of vegetable matter in the dry state and which lead by simple mechanical defibration to a mechanical paste of degree of whiteness equal to 54%, are introduced continuously into a first machine of the type described in the CREUSOT-LOIRE patents cited above, provided with holes for introducing reagents or washing liquids such as water into entrainment zones and filtering parts as indicated for example in the French patent publishes under the number 2451963.

The pretreatment using 0.5% a 40% sodium salt solution of diethylenetriaminepentaacetic acid takes place for 1 minute on average in the first training zone and in the first braking zone at a temperature of 85 ° C, temperature at which, moreover, the entire process is carried out.

Water is introduced downstream of the first braking zone so as to carry out the washing in the second training zone with a consistency equal to 10%.

The evacuation of the liquid expressed by compression of the paste into the second braking zone brings the consistency to a value equal to 40% downstream of this zone when water is again introduced to bring the consistency to an equal value 10% in the third training zone.

The evacuation of the liquid expressed by compression of the dough into the third braking zone brings the consistency to a value equal to 40% downstream of this zone when 5% of sodium dithionite, 2% of sodium hydroxide are added. and 3.5% of sodium silicate with water to operate the reducing bleaching in the fourth training zone and the fourth braking zone at a consistency equal to 35%.

At the outlet of the first machine the dough stays for 1 hour at the same consistency in a tank under a water vapor atmosphere before being introduced into a second machine of the same design as the first where the reducing agent is eliminated by dilution, compression, filtration in the first three parts of the machine, each part consisting of a drive zone and a material braking zone.

The oxidative bleaching takes place in the fourth part of the machine, then in a tank during a stay of 1 hour, at a consistency of 25%, using 4.7% of hydrogen peroxide, 2% of hydroxide. sodium, 4% sodium silicate and 0.1% of the 40% sodium salt solution of diethylenetriaminepentaacetic acid introduced into the fourth training zone.

The dough is finally passed through a two-disc refiner from BAUER and has a whiteness level of 78.5%.

Example 8 (comparative)

Example 7 is repeated, omitting to carry out the washing after pretreatment in the first machine.

The degree of whiteness of the paste finally obtained is then only 76.5%.

Example 9

10 g of chemothermomechanical sulfite pulp, from softwood, a type of pulp deemed to be difficult to bleach, which contain 4 g of lignocellulosic material in the dry state and which have a whiteness level of 46%, are brought to a consistency of 10% to be pretreated and washed after pretreatment like grinding wheel paste in Example 1.

The paste thus pretreated and washed in a highly efficient manner is subjected in a mixer under nitrogen atmosphere, with a consistency of 15%, to a reducing bleaching treatment using 5% sodium dithionite in the presence of 2% d sodium hydroxide and 3% sodium silicate for 1 hour at 90 ° C.

The dough, washed to remove the reducing agent, is then treated in a kneader using 2% hydrogen peroxide in the presence of 1% sodium hydroxide and 3% sodium silicate, at a consistency equal to 20%, for 2 hours at 90 ° C.

The bleached pulp obtained has a degree of whiteness equal to 72% while 75% of the hydrogen peroxide used is consumed.

Example 10 (comparative)

Example 9 is repeated except that the reducing bleaching treatment follows the pretreatment without washing the pretreated dough.

The bleached pulp finally obtained has only a degree of whiteness equal to 67% for a significant consumption of hydrogen peroxide, reaching 95% of the quantity committed.

The mechanical properties of the bleached pulp in this example are also lower than those of the bleached pulp according to the invention. For example, the breaking length is 1.25 times higher when proceeding according to example 9 than when proceeding according to example 10.

Example 11

By operating as in Example 9, and therefore ensuring in particular a highly effective washing of the dough between the pretreatment and the reducing bleaching, but ending with a bleaching by means of 10% hydrogen peroxide, 3% d sodium hydroxide and 4% sodium silicate, the whiteness of the bleached chemothermomechanical pulp obtained reaches 80% while 75% of the quantity of hydrogen peroxide used is transformed.

Thus, the process according to the invention offers a general method for obtaining milled mechanical type pasta, with a high degree of whiteness and 25% to 35% absolute greater than the degree of whiteness of the original unbleached pasta, conceding, having regard to this strong whitening effect, only reduced consumption of an oxidizing bleaching agent such as hydrogen peroxide.

Claims (14)

1. Process for bleaching a lignocellulosic material in the form of a mechanical, thermomechanical, chemimechanical or chemithermo- mechanical papermaking pulp or of wood chips in the course of conversion into one of these types of pulps by treatment in a machine consisting essentially of two interpenetrating helical surfaces wound around parallel shafts driven in synchronous rotation in the same direction inside a barrel surrounding them and determining, from the upstream toward the downstream in the direction of forward travel of the material, a series of zones for entraining and braking the material, in which there are combined a bleaching treatment by means of a reducing agent and a bleaching treatment by means of an oxidizing agent and comprising a pretreatment of the lignocellulosic material by means of at least one metal-complexing or sequestering agent followed by a washing with an efficiency higher than 96% so that the said agent does not accompany the reducing agent in the bleaching treatment carried out subsequently by means of the latter at a pH of between 8 and 12, a washing to remove the reducing agent and a bleaching treatment by means of an oxidizing agent.

2. Process according to Claim 1, characterized in that the washing after pretreatment is carried out at a temperature of between 20°C and 90°C.

3. Process according to either of Claims 1 and 2, characterized in that the washing after pretreatment is carried out by dilution with water and concentration by pressing on a filter.

4. Process according to one of Claims 1 to 3, characterized in that the complexing or sequestering agent is chosen from sodium tripolyphosphate, sodium tetrapyrophosphate and the sodium salts of citric, nitrilotriacetic, ethylenediaminetetraacetic and diethylenetriaminepentaacetic acids.

5. Process according to one of Claims 1 to 4, characterized in that the complexing or sequestering agent is introduced in a proportion of 0.1% to 1%.

6. Process according to one of Claims 1 to 5, characterized in that the pretreatment is carried out at a temperature of between 20°C and 100°C, the consistency being between 5% and 50%.

7. Process according to one of Claims 1 to 6, characterized in that the bleaching treatment by means of the reducing agent is carried out at a temperature of between 30°C and 90°C, the consistency being from 5% to 50%.

8. Process according to one of Claims 1 to 7, characterized in that the reductive bleaching treatment is carried out by means of sodium dithionite introduced in a proportion of 1% to 10%, in the presence of 0.5% to 5% of sodium hydroxide and of 1% to 4% of an aqueous solution of sodium silicate with a relative density of 1.33, so that the sodium dithionite/sodium hydroxide weight ratio is between 1.5 and 2.5 and that the weight ratio of sodium silicate solution of relative density 1.33 to sodium hydroxide is between 1 and 2.

9. Process according to one of Claims 1 to 7, characterized in that the reductive bleaching treatment is carried out by means of thiourea dioxide introduced in a proportion of 1% to 10% in the presence of 0.5% to 5% of sodium hydroxide.

10. Process according to one of Claims 1 to 7, characterized in that the reductive bleaching treatment is carried out by means of sodium borohydride introduced in a proportion of 0.05% to 0.7% in the presence of sodium hydroxide.

11. Process according to one of Claims 1 to 10, characterized in that the oxidative bleaching treatment is carried out by means of an oxidizing agent chosen from chlorine dioxide, peracetic acid, ozone and hydrogen peroxide.

12. Process according to one of Claims 1 to 11, characterized in that the bleaching treatment by means of the oxidizing agent is carried out at a pH of between 9 and 11, the consistency being between 15% and 30%, by means of 0.5% to 10% of hydrogen peroxide in the presence of sodium silicate.

13. Process according to one of Claims 1 to 12, characterized in that the pretreatment by means of the complexing or sequestering agent is carried out in the machine of Claim 1.

14. Process according to Claim 13, characterized in that the washing after pretreatment is performed in the machine of Claim 1.