EP0019963B1 - Process for the delignification and bleaching of chemical and semichemical cellulose pulps - Google Patents

Abstract

A process for the delignification and bleaching of chemical and semi-chemical cellulosic pulps in which the pulp is subjected to a treatment with oxygen and a subsequent treatment with peroxide. The effluent from the treatment with peroxide is at least partially recycled to the treatment with oxygen.

Description

The present invention relates to a sequential process for the delignification and bleaching of chemical and semi-chemical cellulosic pulps by the action of oxygen, further comprising a subsequent bleaching treatment with peroxide.

The bleaching of chemical and semi-chemical cellulose pulps intended in particular for the manufacture of paper generally requires several successive bleaching treatments, possibly interspersed with washing, dilution and / or concentration steps, in order to reach the level of residual lignin and to the desired whiteness.

Various types of bleaching agents are commonly used for the treatment of cellulosic pulps. It has thus been proposed to subject the chemical and semi-chemical pastes to the action of oxygen or air in an alkaline medium, and then to bleaching treatments comprising a peroxide treatment. However, the elimination of lignin and the gain in whiteness are accompanied by significant changes in the physical characteristics of the cellulose pastes and more particularly by a significant reduction in the viscosity of the pastes, which translates into depolymerization of the cellulosic chains. At the industrial level the yield of the processes is reduced. In addition, the resistance of the finished products is reduced.

The object of the invention is to provide a process which makes it possible to overcome the drawbacks of the known processes cited above and in particular to avoid excessive depolymerization of the cellulose in chemical and semi-chemical pastes. The method according to the invention also makes it possible to produce paper pulps with good yields. The pasta obtained also has a good degree of whiteness.

To this end, the invention relates to a process for the delignification and bleaching of chemical and semi-chemical cellulosic pulps comprising an oxygen treatment in an alkaline medium and a subsequent treatment using peroxides according to which the effluents from the peroxide treatment are at least partially recycled for oxygen treatment.

In general, only part of the effluents from the peroxide treatment, preferably not exceeding 80% of their weight, is recycled to the oxygen treatment (hereinafter also called stage O). Good results have been obtained by recycling to the oxygen treatment from 5 to 70% of the weight of the effluents from the peroxide treatment (also called stage P below).

The oxygen treatment is generally carried out using an oxygen-containing gas such as oxygen or air. Other mixtures of oxygen-containing gases can also be used. The partial pressures of oxygen can vary within wide limits. They are generally between 100 kPa and 2 MPa. The temperatures are generally between 353 and 423 K and preferably between 373 and 503 K.

The oxygen treatment is carried out in the presence of an alkaline aqueous solution. Various types of alkalis can be used for this purpose. In general, they are chosen from hydroxides, bicarbonates and carbonates of alkali metals and ammonium and their mixtures. Good results are obtained by using an alkali containing sodium hydroxide. The amount of alkali used can vary within wide limits. It is generally between 0.1 and 20%, and preferably between 1 and 15% of the weight of dry pulp.

The density of the paste used in stage 0 is generally between 3 and 50% and preferably between 5 and 40%.

Various additives can be used in stage 0, without this being essential, such as cellulose protecting agents, sequestering agents, pH regulators, etc. The total content of various additives generally does not exceed 10% of the weight of the dry paste.

The protective agents are generally chosen from derivatives of alkaline earth metals. These derivatives are most often alkaline earth metal salts and more particularly magnesium salts. It is thus possible to use during the oxygen treatment of carbonate, sulphate, oxide or magnesium hydroxide.

The oxygen treatment can be carried out in any device known per se suitable for delignification and capable of withstanding the pressures used. Various types of autoclaves can thus be used.

After the oxygen treatment, the dough is degassed. It can be sent as is to subsequent whitening treatment. The dough can also be subjected to one or more washing or dilution, and possibly concentration, stages before sending it to the subsequent bleaching treatment. These various steps can be carried out in various types of apparatus known in themselves. The concentrations can thus be carried out in various types of apparatus allowing the extraction of the dough such as rotary filters or various types of press. Any effluents from these washing, dilution and concentration stages subsequent to stage 0 can be recycled to stage 0 and / or subjected to incineration so as to regenerate the alkali. In general, at least 10% of the weight of these effluents is sent to recovery boilers for incineration.

The pulp obtained can then optionally undergo intermediate bleaching treatments before being sent to the peroxide treatment. It is thus possible to subject the paste obtained from stage O, optionally after the washing, dilution or concentration steps mentioned above, to at least one intermediate treatment chosen from treatments with carboxylic peracids and ozone. However, such intermediate bleaching treatments are not essential. Good results have been obtained by sending the pulp from the oxygen treatment to the peroxide treatment, without any intermediate bleaching treatment.

The peroxide treatment is carried out in the presence of peroxidized compounds of various types. It is thus possible to use hydrogen peroxide, alkali metal peroxides, other inorganic peroxides, and organic peroxides, hydro-peroxides or peracids. In general, a peroxide compound chosen from hydrogen peroxide, sodium peroxide and their mixture is used. Good results are obtained using hydrogen peroxide. The amount of peroxide compounds used calculated as pure hydrogen peroxide is generally between 0.1 and 10% and preferably between 0.2 and 5% of the weight of dry pulp.

The mixture subjected to stage P is maintained at a basic pH, generally between 9 and 13 and more often between 10 and 12.5. To do this, alkalis are added to the mixture, which can be of very diverse natures. In general, hydroxides or carbonates of alkali metals, alkaline earth metals or ammonium, and more particularly sodium hydroxide, are added to the mixture. The amount of alkali used depends on the type of peroxide compound used and more particularly on its possible alkaline character. When the peroxidized compound is hydrogen peroxide, generally 0.2 to 15% by weight of alkali is added relative to the weight of dry pulp. Good results are obtained using 0.5 to 10% alkali. These amounts are reduced when the peroxidized compound contains sodium peroxide.

The densities of the pastes subjected to the peroxide treatment are generally between 2 and 25%. The temperatures are generally between room temperature and the boiling temperature of the mixture. Good results are obtained at temperatures between 333 and 353 K.

Various additives can be used in stage O, without this being essential, such as organic or inorganic stabilizers, surfactants, complexing agents, pH regulating agents, etc.

As stabilizers, compounds generally chosen from magnesium salts such as magnesium sulfate and silicates such as water glass are used.

As complexing agents, agents generally chosen from organic compounds containing nitrogen and / or phosphorus such as nitrilotriacetic, ethylene-diamine-tetraacetic, diethylenetriamine-pentaacetic acids and their salts, polyhyroxycarboxylic acids and their salts such as sodium poly-hydroxyacrylate and hydroxycarboxylic acids and their salts such as sodium gluconate.

The peroxide treatment can take place in various types of apparatus known per se, such as towers or batteries.

The paste resulting from the peroxide treatment is then subjected to one or more concentration stages, possibly after one or more prior dilutions.

Concentration can be done according to various techniques known in themselves such as spinning in a rotary filter or in various types of press. This concentration brings the dough to a density generally greater than 10%.

The effluents from the concentration stages following the peroxide treatment are recycled at least in part to stage 0 according to the invention. They can be returned directly to the device where the oxygen treatment is carried out, or even mixed with stage O effluents intended to be recycled there.

The part of the effluents of stage P which is not recycled in stage 0 is advantageously recycled in stage P or also used to dilute the pulp resulting from stage P before its concentration.

A particularly suitable method consists in diluting the paste resulting from the peroxide treatment, in concentrating it on one or more apparatuses suitable for wringing, in collecting the effluents from these wrings which constitute the effluents of the method according to the invention, in using 30 to 95% of these effluents for the step of diluting the pulp resulting from the peroxide treatment and recycling the balance of these effluents to the oxygen treatment.

The devices used for wringing the dough from the peroxide treatment can also be supplied with a small amount of fresh water or an aqueous solution. In this case, the quantity of water or aqueous solution used is substantially equal to the quantity of effluents from the oxygen treatment sent to the recovery boilers.

Stage P of the process according to the invention can, depending on the desired degree of whiteness, be subjected to one or more additional bleaching treatments. These are generally chosen from treatments with peroxide, chlorine dioxide (also called stages D), hypochlorite (also called stages H), carboxylic peracids (also called stage P _{/ J} and chlorine (called also stages C) or alkaline extractions (also called stage E) Certain specific sequences of subsequent treatments have given good results: these are the CED, CEH, CHH, CPD, CPH, DED, DHD and DPD sequences.

In order to illustrate the invention, a practical example is given below. Example 2 was carried out by way of comparison in the absence of recycling.

Example 1: Bleaching of a pine kraft pulp

A pine kraft paste (initial whiteness 27.6 ° ISO, kappa index 39.6) was subjected to a treatment in five stages comprising a first oxygen treatment and a second peroxide treatment with recycling of the effluent from the peroxide stage to the oxygen stage. The operating conditions are given below. The percentages of the reactants are given relative to the dry paste.

The final viscosity of the paste from the last step measured according to the TAPPI T 230 standard is 12.0 mPa.s.

Example 2: Whitening of a pine kraft pulp

The same pine kraft pulp as that used for carrying out test 1 was subjected to a treatment in five stages comprising a first treatment with oxygen and a second treatment with peroxide without recycling of the effluent from stage with peroxide in the oxygen step. The other operating conditions are identical to those of test 1.

The final viscosity of the paste at the end of the last step is 10.8 mPa.s. The cellulose pulp therefore underwent a greater depolymerization than when the method according to the invention was used.

Claims (14)

1. Process for the delignification and bleaching of chemical and semi-chemical cellulose pulps by an oxygen treatment in an alkaline medium, also comprising a subsequent bleaching treatment by means of peroxides, characterised in that at least part of the effluents from the peroxide treatment is recycled to the oxygen treatment.

2. Process according to Claim 1, characterised in that the proportion of the effluents from the peroxide treatment recycled to the oxygen treatment is between 5 and 70 % of their total weight.

3. Process according to one or other of Claims 1 and 2, characterised in that the oxygen treatment is carried out under an oxygen partial pressure of between 100 kPa and 2 MPa.

4. Process according to any one of Claims 1 to 3, characterised in that from 1 to 15 % of alkali, relative to the weight of the dry pulp, is added during the oxygen treatment.

5. Process according to any one of Claims 1 to 4, characterised in that the alkali used in the oxygen treatment is sodium hydroxide.

6. Process according to any one of Claims 1 to 5, characterised in that the oxygen treatment is carried out on a pulp having a density of between 5 and 40 %.

7. Process according to any one of Claims 1 to 6, characterised in that from 0.2 to 5 % of a peroxide compound, calculated as 100 % strength hydrogen peroxide, is added during the peroxide treatment.

8. Process according to any one of Claims 1 to 7, characterised in that the peroxide used in the peroxide treatment is chosen from sodium peroxide and hydrogen peroxide.

9. Process according to any one of Claims 1 to 8, characterised in that the pH is kept at a value of between 9 and 13 during the peroxide treatment.

10. Process according to any one of Claims 1 to 9, characterised in that the peroxide treatment is carried out on a pulp having a density of between 2 and 25 %.

11. Process according to any one of Claims 1 to 10, characterised in that the effluents from the peroxide treatment are mixed with the effluents from the oxygen treatment before being recycled to the oxygen treatment.

12. Process according to any one of Claims 1 to 11, characterised in that the pulp is subjected to an oxygen treatment in an alkaline medium and then to a peroxide treatment, without intermediate bleaching treatment.

13. Process according to any one of Claims 1 to 11, characterised in that the pulp is subjected to an oxygen treatment in an alkaline medium and then to a series of treatments comprising at least one intermediate treatment chosen from amongst treatments with percarboxylic acids and with ozone, and then a peroxide treatment.

14. Process according to Claim 12 or 13, characterised in that, after the peroxide treatment, the pulp is subjected to at least one additional treatment chosen from amongst treatments with peroxide, with chlorine dioxide, with hypochlorite, with percaboxylic acids and with chlorine, and alkaline extractions.