EP0141138B1 - Process for the treatment of cellulosic materials with oxidising agents - Google Patents

Abstract

Cellulosic materials and more particularly wood shavings, pulps, and cellulosic textile fibres are exposed to the combined action of microwaves and at least one oxidizing agent. In particular, the process makes it possible to improve the brightness of the end product.

Description

The present invention relates to a process for the treatment of cellulosic materials and more particularly to paper pulps, wood chips and cellulosic textile fibers with oxidizing agents.

Cellulosic materials intended for the manufacture of paper pulp are subjected to a large number of treatments, some of which are carried out in the presence of oxidizing agents, in particular with a view to improving their whiteness or reducing their lignin content. The methods known to date do not however make it possible to obtain very high whiteness gains or very extensive delignification in a single treatment. We therefore frequently use sequential treatments which generally include a very large number of stages. In addition, the duration of each particular stage is often very long.

Likewise, the cellulose fibers intended for textile applications are also subjected to certain treatments which can be carried out in the presence of oxidizing agents such as bleaching, scouring to remove the impurities, mercerizing to improve the appearance and the solidity thereof. and reduce the tendency to shrink or spray to impregnate them with reagents before maturation. All of these techniques require fairly long treatment times.

The present invention aims to provide a process for the treatment of cellulosic materials with oxidizing agents which makes it possible to increase the efficiency of the oxidizing agent and in particular to improve the whiteness of the finished product. The method according to the invention also makes it possible to significantly reduce the duration of the treatment with the oxidizing agent. The method according to the invention also makes it possible to use significantly less solvent than the known methods, which simplifies the subsequent drying steps and reduces the effluent discharges and the consumption of solvent. Finally, the method according to the invention makes it possible to reduce, or even eliminate, any mechanical agitation in the treatments which required it. This is particularly the case when processing textile fibers and the manufactured products derived therefrom. One can thus avoid the harmful effects of mechanical agitation on fragile textiles such as jerseys, tulle, etc.

The present invention relates to a process for the treatment of cellulosic materials with oxidizing agents according to which the cellulosic materials are subjected to the joint action of microwaves and at least one oxidizing agent.

Microwaves are electromagnetic waves that have a wavelength from about 0.1 to about 100 cm, or a frequency from about 300,000 MHz to about 300 MHz. Good results have been obtained with microwaves with a frequency of 100,000 MHz to 500 MHz.

Various oxidizing agents can be used. In general, they are chosen from peroxidized compounds, oxygen, ozone, permanganate as well as compounds capable of releasing active chlorine such as molecular chlorine, chlorine dioxide, hypochlorous acid, hypochlorites, chlorites and organic substances capable of releasing active chlorine. Peroxidized compounds and compounds capable of releasing active chlorine are well suited. Good results have been obtained with the peroxidized compounds. The peroxide compounds can be chosen from hydrogen peroxide, metal peroxides and more particularly alkali or alkaline earth metal peroxides such as sodium peroxide, inorganic persalts such as perborates, percarbonates and persulfates, inorganic peracids such as persulfuric acid, organic peracids and more particularly those containing from 2 to 7 carbon atoms such as peracetic and perpropionic acids as well as their salts and organic hydroperoxides and peroxides. Good results have been obtained with hydrogen peroxide, sodium peroxide, persulfuric acid, persulfates such as sodium persulfate, peracetic acid and peracetates such as sodium peracetate. The best results have been obtained with hydrogen peroxide. One or more oxidizing agents of the same types or of different types can be used.

The doses of oxidizing agents can vary within very wide limits. In general, they are used in doses of 0.001 to 10% of the weight of dry cellulosic materials (M.S.) and most often of 0.1 to 5% of the weight of dry cellulosic materials. In the case of peroxide compounds, generally 0.1 to 3% of peroxide compounds calculated in hydrogen peroxide equivalents relative to the weight of dry cellulosic materials are used. When using oxygen, the partial pressure of oxygen is generally at least 100 kPa and more often than not at least 300 kPa. In general, the pressure does not exceed 20,000 kPa and in general not more than 10,000 kPa. In the case of compounds capable of releasing active chlorine, these are used in doses of 0.1 to 8% of the weight of dry cellulosic materials.

Various cellulosic materials can be treated according to the method of the invention. In general, these are cellulosic materials containing at least 30 and most often at least 50% by weight of compounds chosen from polysaccharides and their derivatives. These include cellulose and hemicelluloses as well as their derivatives obtained by various chemical treatments such as the acetate process, the viscose process, the ammoniacal copper process, etc. It is thus possible to apply the method according to the invention to the treatment of paper pulps of all types such as mechanical, thermomechanical, semi-chemical, chemical and mechanochemical pulps as well as to recovery pulps, at any stage of their manufacture, including the stages of pulping, bleaching and treatments prior to the manufacture of the sheet of paper or cardboard. You can also use the app quer au bois, or all wood particles such as wood chips. The method is thus suitable for the treatment of wood as well as wood chips or other wood particles intended to be used for the manufacture of paper pulps such as mechanical, thermomechanical or mechanochemical pulps. It can also be applied to the treatment of cellulosic materials other than wood intended for the manufacture of paper pulps such as straw, reeds, bagasse and bamboo.

The cellulosic materials according to the invention can also be chosen from natural cellulosic textile fibers such as linen, cotton, hemp, ramie, jute and sisal and artificial textile fibers such as rayon, viscose rayon, cuproammoniac rayon and cellulose acetate as well as manufactured textiles.

The method according to the invention is particularly suitable for the treatment of paper pulps and more especially for the treatment of chemical pulps such as sulphate, sulphite or bisulphite pulp. It is also well suited for the treatment of wood particles and more particularly wood chips, in particular for their pretreatment before a mechanical pulping process. Good results have been obtained in the treatment of possibly semi-bleached sulphate doughs.

The process according to the invention is generally carried out in the presence of a solvent. The solvent is most often water. The amount of solvent at the start of the treatment according to the invention can vary within wide limits. It is most often at least 0.1% and generally does not exceed 99.5% of the total weight of the mixture subjected to the action of microwaves and which essentially comprises cellulosic materials, solvent, oxidizing agents and any additives. When the solvent is water, the amount of water at the start of the treatment according to the invention is generally at least 0.5% and most often at least 1% of the total weight of the mixture; it generally does not exceed 95% most often not 90%, and preferably not 85% of the total weight of the mixture.

When treating paper pulp of all origins, virgin or recycled, the density at the start of the treatment according to the invention is generally at least 5% and most often at least 8%. It does not generally exceed 99% and most often not 98%.

(The density, which we are talking about here, expresses the consistency or dryness of the paper pulp and is measured in%, ie g dry pulp / 100 g mixture pulp + water + reagents).

The process according to the invention can be carried out in the presence of other additives. Thus, when the oxidizing agent is not itself capable of imparting the proper pH to the mixture, it is possible to add compounds with an alkaline character, that is to say capable of imparting to the water a pH equal to or greater than 7 or compounds with an acidic character, that is to say capable of imparting a pH of less than 7 to water, or alternatively pH regulators such as buffers. The alkali compounds can be hydroxides or carbonates of alkali metals or ammonium, and more particularly sodium hydroxide, or sodium silicate. The alkali metal or ammonium bicarbonates can be chosen as pH regulators. Sulfuric acid can be chosen as an acidic compound. The process according to the invention is generally carried out in the presence of a compound of alkaline character. The dose of compound with an alkaline character is generally 0.1 to 20% of the weight of dry cellulosic materials.

In the case of the treatment of wood shavings, the bleaching of paper pulp or textile fibers, in particular by peroxidized compounds or by compounds capable of releasing active chlorine and preferably by peroxidized compounds such as hydrogen peroxide , the pH is generally equal to or greater than 7, more particularly from 7 to 13 and most often from 8 to 12.

It is also possible to use stabilizers of the oxidizing agent when the latter is likely to deactivate during the treatment. This is particularly the case when the oxidizing agent is a peroxide compound. In general, they are used in amounts of 0.01 to 5% of the weight of dry cellulosic materials.

Various other additives can also be used depending on the particular applications. Among these are sequestering agents, surfactants, agents capable of protecting the cellulosic chains to prevent their depolymerization, wetting agents, activating agents, anticorrosion agents, antistatic agents, desizing agents, optical brighteners , dispersing agents, anti-scaling agents, foaming agents and collecting agents. These additives are generally coated at doses of 0.01 to 10% of the weight of dry cellulosic materials.

The cellulosic materials subjected to the treatment according to the invention can have, before this treatment, very diverse temperatures. They can thus have, before the treatment according to the invention, temperatures ranging from ambient temperature, that is to say approximately 10 to 25 ° C., up to temperatures of 200 ° C. In general, the temperature of the cellulosic materials before the treatment according to the invention depends on the existence or not of a previous treatment. Most often the temperature of the cellulosic materials before the treatment according to the invention is the ambient temperature or that which they have acquired during the possible previous treatment. In most cases, the temperature of the celulosic materials before the treatment according to the invention is from 10 to 90 ° C.

The duration of the treatment according to the invention is variable. In general, it is 0.1 to 120 minutes and most often 0.2 to 30 minutes. It is generally shorter than that of the corresponding treatments using oxidizing agents without the intervention of microwaves.

The process according to the invention can be carried out continuously or batchwise.

The precise conditions for carrying out the process according to the intervention, as well as the nature of the other possible additives used, can vary within wide limits depending on the type of cellulosic material to be treated and the particular purpose of the treatment concerned. In general, these treatments are carried out by soaking the cellulosic materials in aqueous solutions of oxidizing agents or by impregnating them by means of such solutions.

The method according to the invention can be implemented in various industries and in particular in the paper industry and in the textile industry.

Thus, loars of the manufacture of mechanical paper pulps, it is possible to treat, according to the method of the invention, wood particles such as shavings by means of oxidizing agents such as peroxidized compounds, before their pulping in devices such as refiners. This can be done at the same time as or after the "presteaming". The process according to the invention can also take place after passing through a refiner or between two passing through refiners.

The method according to the invention is also well suited for bleaching paper pulps of all types already formed. It can thus be applied at various stages of bleaching consecutive to pulping or cooking. It is particularly suitable for the treatment of unbleached pasta, especially the alkaline pasta collected after washing after cooking, mainly in the case of chemical pasta such as kraft pasta, or for the treatment of pasta before entering the towers. extraction. It is also well suited for bleaching semi-bleached pasta.

The method according to the invention is well suited for the treatment of pressed or dried pasta. The pasta can be pressed or dried by means of various devices suitable for these uses and known by themselves. It is thus possible to use cylinder, screw or belt presses or conventional dryers or “flash-dryers”. Pasta can come in various forms such as leaves or "flocks".

The method according to the invention can also be applied to the various treatments with oxidizing agents to which the cellulosic textile fibers are subjected. This is the case with scalding, mercerizing, bleaching and impregnation such as spraying before maturation. These impregnations can be done in various devices known by. themselves operating either continuously such as "pad-steam", "J-box" and "U-box" and continuous pressurized vaporizers, or semi-continuous such as "pad-roll", or finally discontinuously such as turnstile boats, autoclaves, spinners, "Jigger" and "kier". The method according to the invention is well suited for the pretreatment of the fibers before they are sent to one or other of the abovementioned treatments.

In order to illustrate the invention without limiting its scope, practical examples of implementation are given below. Example 2R was carried out for comparison.

Example 1 and 2R

A semi-bleached softwood kraft pulp according to the CEH sequence of initial whiteness 63.6 ° ISO (standard ISO 2470) was used. The dough was previously "fluffed" (reduced to fluff) in a household mixer.

The dry paste is introduced into a polyethylene bag where it is moistened by spraying with an aqueous solution of hydrogen peroxide. It is then exposed to a microwave beam of a frequency of approximately 2450 MHz in a household microwave oven of TOSHIBA brand ER-649 ET-S with rotary table set in the "DEFROST" position for five minutes (test 1) or in an oven maintained at 100 ° C for five minutes (test 2R).

The whiteness of the dough was measured relative to the whiteness of BaS0 ₄ measured by means of an ELREPHO reflectometer (ZEISS) equipped with the R457 filter and a gloss trap (ISO 2470 standard).

The consumption of hydrogen peroxide was measured by assaying the residual hydrogen peroxide in the presence of paste.

The operating conditions and the results obtained are given in Table I below.

Examples 3 to 5

Three tests were carried out at different acid (tests 3 and 4) and basic (test 5) pH under conditions close to those of Example 1.

The same semi-bleached pulp as that used for the production of Examples 1 and 2R was subjected to prior defibration in the presence of sulfuric acid (tests 3 and 4) or sodium hydroxide (test 5) so as to adjust pH 5, 7 and 9 respectively.

The dough is then wrung, "fluffed" and dried in a ventilated oven at 40 ° C. The dry paste is then subjected to the same treatment as in Example 1.

The operating conditions and the results obtained are given in the table below.

Examples 6 and 7R

The same semi-bleached kraft pulp as that used for the production of Examples 1 and 2R was treated with hydrogen peroxide under the same conditions as in Example 1 (test 6) and in Example 2R (test 7R) except, however, the density which has been adjusted to 15%.

The operating conditions and the results obtained are given in Table I below.

Examples 8.9R, 10 and 11 R

The same semi-bleached paste as in Examples 1 and 2R was treated with sodium hypochloride. Tests 8 and 10 were carried out under the same conditions as test 6, tests 9R and 11 R, under the same conditions as test 7R.

The operating conditions and the results obtained are given in Table IV below.

Examples 12 and 13R

The same semi-bleached paste as in Examples 1 and 2R was treated with chlorine dioxide. Test 12 was carried out under the same conditions as test 6, test 13R under the same conditions as test 7R.

The operating conditions and the results obtained are given in Table V below.

Claims (10)

1. Process for the treatment of cellulose materials by means of oxidizing agents, characterized in that the cellulose materials are subjected to the combined effect of microwaves, the frequency of which ranges between 300 MHz and 300 000 MHz, and of at least one oxidizing agent.

2. Process according to claim 1, characterized in that the oxidizing agent is taken from the compounds peroxides, oxygen, ozone, permanganate and those compounds capable of splitting off active chlorine.

3. Process according to claim 2, characterized in that the oxidizing agent is taken from the compounds of peroxides.

4. Process according to claim 3, characterized in that the oxidizing agent is taken from the group of hydrogen peroxide, sodium peroxide, persulfuric acid, sodium persulfate, peracetic acid and sodium peracetate.

5. Process according to claim 4, characterized in that the oxidizing agent is hydrogen peroxide.

6. Process according to one of claims 1 to 5, characterized in that the oxidizing agent is used at an amount of 0.001 to 10 weight percent of dry cellulose materials.

7. Process according to one of claims 1 to 6, characterized in that it is carried out in the presence of water.

8. Process according to claim 1, characterized in that it is carried out in the presence of 1 to 90% water with respect to the weight of the mixture containing the cellulose materials, water, the oxidizing agent and possible other additives.

9. Process according to one of claims 1 to 8, characterized in that it is carried out in the presence of a compound of alkaline nature.

10. Process according to one of claims 1 to 9, characterized in that the cellulose materials are taken from paper pulp, wood chips and cellulose textile fibers.