FR2754550A1 - Delignification and/or bleaching paper paste - Google Patents

Abstract

A process for the delignification and/or bleaching paper paste comprises at least one treatment step with an organic peroxide. The treatment is carried out in the presence of agents selected from phosphorus mineral acids and their salts.

Description

 The invention relates to a process for improving the selectivity of the delignification and / or bleaching of lignin-containing paper pulp by means of an organic peroxy acid in which the pulp is processed by means of a aqueous solution of this organic peroxy acid in the presence of an auxiliary.

 It is known to apply to paper pulp, in particular the chemical paper pulp obtained by cooking cellulosic materials in the presence of chemical reagents, a series of delignifying and / or whitening treatment stages, during which chemicals are used. oxidants. The first step of a chemical pulp bleaching sequence is generally intended to improve the delignification of the unbleached pulp directly from the cooking stage. This first delignifying step is usually carried out on the unbleached pulp with acid-applied chlorine, chlorine dioxide or a combination of both so as to cause a reaction with the residual lignin of the pulp and give rise to products which can be extracted from this paste by solubilization, during a subsequent treatment step.

 For reasons related to the improvement of the environment, the chlorinated reagents are replaced more and more by non-chlorinated reagents, more favorable to the environment. For example, Japanese Patent Application 55/94811 describes the treatment of kraft pulp by delignification with oxygen, which process involves the use of a peracetic acid step in the presence of additives. such as diethylenetriamine pentaacetic acid (DTPA).

 According to the European Patent Application No. EP-A-578 305, the pulp is treated with a peroxyacid in the presence of a stabilizing agent selected from phosphonic acids and their salts, which has the effect of improving the selectivity of delignification.

 In general, the presence of a chelating and / or stabilizing agent in the delignification step makes it possible to avoid significant degradation of the cellulose chains of the dough, this degradation being responsible for a decrease in its viscosity. Although the effect of these protective agents is undeniable, it does not reach a sufficient level, particularly for the production of high quality pulp.

 After the delignifying step, the pasta is usually blanched by means of one or more oxidizing substances chosen in particular from chlorine dioxide, ozone, hydrogen peroxide or peracetic acid. These bleaching operations are generally carried out in the presence of cellulose-protecting agents, in order to avoid the degradation thereof, such as, for example, magnesium sulphate.

 However, the processes described in the state of the art do not make it possible to obtain pulp of sufficiently high quality, because of a rate of degradation of the cellulose still too high.

 The Applicant has therefore sought to remedy this disadvantage.

 The subject of the present invention is a process for delignifying and / or bleaching a paper pulp comprising at least one treatment step using an organic peroxy acid, characterized in that said treatment is carried out in the presence of one or more several ancillary agents for protecting the viscosity of the dough chosen from inorganic phosphorus acids and their salts.

 By phosphorus mineral acids, are meant acids having no carbon-phosphorus bonds; mention may be made, for example, of phosphorous acid, phosphoric acid, pyrophosphoric acid and polyphosphoric acids.

 By salt of an inorganic acid of phosphorus is meant a phosphorus acid which one or more of these acidic functions is salified, and in particular the alkali metal salts.

 In a first preferred embodiment of the invention, the ancillary agent for protecting the viscosity of the paste is pyrophosphoric acid, sodium pyrophosphate, tripolyphosphoric acid or sodium tripolyphosphate.

 The method which is the subject of the present invention can be implemented with any type of paper pulp, whether it be chemical, mechanical or recycled pulp.

These pasta are obtained from any type of wood including those used for the manufacture of kraft pasta, namely softwoods, hardwoods they are also obtained from annual species such as for example straws wheat or rice pasta.

 By organic peroxy acid is meant the percarboxylic acids or the polycarboxylic acids, at least one carboxylic group is peroxidized to percarboxylic group -C (O) -O-OH As examples of such peroxyacids there may be mentioned in particular the peralkanoic acids comprising from 1 to 10 carbon atoms, monoperoxyalkanedioic acids having 1 to 10 carbon atoms, monoperoxyalkenedioic acids having 1 to 10 carbon atoms, diperoxyalkanedioic acids having 1 to 10 carbon atoms, peroxyaryldicarboxylic acids or diperoxyaryldicarboxylic acids in which the aryl represents a phenyl or naphthyl radical.

Examples of such aromatic acids include benzoic peroxy acid, monoperoxyphthalic acids, diperoxyphthalic acids or peroxynaphthoic acids.

 In a second preferred embodiment of the invention, the organic peroxy acid is selected from performic acid, peracetic acid, 1,6-diperoxyhexanedioic acid, 1,4-monoperoxybutenedioic acid, 1,4-acid, monoperoxybutanedioic acid, and perbenzoic acid.

The peroxyacids are prepared according to the methods known to those skilled in the art. In particular, peracetic acid is either obtained by reaction of an aqueous solution of hydrogen peroxide on acetic acid according to the equilibrium
H292 + CH3CO2HF CH3C03H + H20 and is then designated by the expression "peracetic acid" at equilibrium, is obtained by azeotropic distillation under reduced pressure of an aqueous mixture of hydrogen peroxide and acetic acid in the presence an acid catalyst or an aqueous solution of peracetic acid at equilibrium.

 The peroxyacid used may be either substantially pure or preferably in aqueous solution containing at least 4% by weight of this peroxy acid, optionally in admixture with the corresponding organic acid and / or hydrogen peroxide. Preferably, peracetic acid is used in aqueous solution, either in equilibrium mixture with hydrogen peroxide and acetic acid, or obtained by azeotropic distillation.

 In a third preferred embodiment of the invention, the organic peroxy acid is peracetic acid in aqueous solution at least 4% by weight of said peroxy acid and in particular in equilibrium aqueous solution also comprising hydrogen peroxide such as the H 2 O 2 / peracetic acid weight ratio is greater than 0.5 and especially greater than 1.

Such commercially available solutions contain, for example, by weight

<tb> Acid <SEP> 6.2 <SEP>% <SEP> 4.2 <SEP>% <SEP> 15.2 <SEP>% <SEP> 5.2 <SEP>% <SEP> 15.6 <SEP>% <SEP> 16.5 <SEP>% <SEP> 4.4 <SEP>%
<tb> peracetic
<tb> Peroxide <SEP> 22 <SEP>% <SEP> 28.5 <SEP>% <SEP> 23 <SEP>% <SEP> 14.1 <SEP>% <SEP> 14.6 <SEP>% <SEP> 10 <SEP>% <SEP> 37 <SEP>%
<tb> hydrogen
<Tb>
This peroxy acid can also be generated in situ.

 The peroxyacid solutions may contain at least 0.1% by weight of strong acid such as for example sulfuric acid, nitric acid, phosphoric acid, methanesulfonic acid or a mixture of these acids and at most about 3 % in weight. The peroxy acid can also be prepared by contacting an H 2 O 2 and acidic mixture with a heterogeneous acid catalyst, for example a sulfonated polystyrene.

 According to another embodiment of the invention, sodium silicate and / or magnesium salts or magnesium compounds such as magnesia, magnesium carbonate, magnesium sulfate or magnesium sulfate are added to the pulp. magnesium chloride.

 The treatment with this peroxyacid will preferably be carried out in a temperature range of between 200 ° C. and 14 ° C., in particular between 50 ° C. and 1200 ° C. and preferably between 700 ° C. and 900 ° C., but also under pressure, preferably less than or equal to about 10 bars, and in particular lower at 5 bars. In particular, the reactor can be pressurized with air or with a gas containing more than 21% oxygen, and in particular at least 80% oxygen.

 The pH of the treatment step with the peroxy acid may be acidic, neutral or alkaline, but preferably a pH of between 5 and 9 and more preferably between 7 and 8.5 will be chosen. The duration of the treatment will be less than 4 hours, preferably less than 2 hours and especially less than 60 minutes. In a preferred variant of the invention, the treatment with peroxy acid is carried out firstly at a pH of between 2 and 7, in particular between 4 and 6, for less than 4 hours, in particular less than two hours, and preferably less than one hour, at a temperature between 40 and 1000C, then the paste is alkalinized to a pH between 8 and 12, especially between 9.5 and 12 and left less than 4 hours at a temperature between 60 and 1200C.

 Generally, amounts of at least 3 kg peroxyacid per ton of pulp expressed in dry matter and preferably at least 8 kg per ton of pulp expressed as dry matter will be used.

 The treatment may be carried out in the presence of cellulose-protecting agents such as magnesium salts and alkali and alkaline-earth silicates.

 In general, it is not necessary to precede the organic peroxyacid treatment with a wash or a decontaminant pretreatment step with an acidic aqueous solution. This washing or this stage is intended to extract impurities in the form of metal ions from the pulp, which are usually detrimental to the smooth running of the bleaching and / or delignification operations.

 The delignification according to the invention with the aid of a peroxyacid generally comes in addition to the usual steps of delignification and / or bleaching of a paper pulp such as oxygen delignification with ozone, with chlorine dioxide, etc. It is also possible to use the peracetic acid present in the bleaching liquor (residual peroxyacid) for a treatment upstream of bleaching, for example a delignification step.

 According to a preferred embodiment of the invention, the peroxyacid comes from a washing solution of a subsequent step of the pulp treatment process.

 The invention will be better understood with the aid of the following examples of embodiments, given in a non-limiting manner.

Example 1
A K-raft slurry of initial kappa number of 24.2 and a viscosity expressed as a degree of polymerization (Dp) of 1480 is treated with equilibrium peracetic acid. The charges peracetic acid and hydrogen peroxide are equal to 3% by weight relative to the dry pulp; the paste is left in the presence of 0.5% by weight relative to the ancillary viscosity protection agent paste for 2 hours at 900 ° C. Following this treatment, the paste is washed with water and then treated in basic medium 1 hour at 900 in the presence of 3% relative to the dry pulp of soda, and finally washed again with water. The values of the dough indices thus obtained are as follows:
Table 1

<tb><SEP> Complexing <SEP> IK <SEP> Dp <SEP>(<SEP> degree of
<tb><SEP> polymerization)
<tb><SEP> 13.8 <SEP> 1016
<tb><SEP> EDTA <SEP> 16.1 <SEP> 1009
<tb><SEP> DTPA <SEP> 14.5 <SEP> 1055
<tb> DEQUEST <SEP> 2010 <SEP> 13.4 <SEP> 1350
<tb> pyrophosphate <SEP> 12.9 <SEP> 1300
<Tb>
1) It is found that sodium pyrophosphate allows better delignification of the dough compared to the control test, without significant loss of the physical properties of the dough expressed in Dp.

 2) It is also found that the chelatans of metal cations having an amino carboxylic structure and conventionally used in hydrogen peroxide treatments such as EDTA (ethylenediamine tetraacetic acid), DTPA (diethylenetriamine pentaacetic acid) have an adverse effect on delignification.

 3) Finally, it is found that sodium pyrophosphate can be advantageously used in place of a 40% solution of 1-hydroxyethylideneamino-1-dimethylenephosphonic acid HEDP (such as for example the product sold under the trade name DEQUEST 2010) .

Examples 2 to 5
In the examples below, the following various mineral polyphosphates were used:
Sodium polyphosphate, pyrophosphoric acid,
Pentasodium tripolyphosphate, and sodium hexametaphosphate.

The treated pulp has the following characteristic:
- Kappa index: 24.2
- Degree of polymerization: 1510.

The treatment was carried out as follows with the following solution
3% by weight of peracetic acid,
3.4% by weight of hydrogen peroxide.

 This solution is brought to a temperature of 900 ° C., the treatment taking place for 3 hours with a paste consistency of 10%. Table 2 below summarizes the different values obtained as a function of the percentage of ancillary agent for protecting the viscosity by volume of the kappa index. the degree of polymerization, the percentage of hydrogen peroxide consumed, and the percentage of peracetic acid consumed.

Table 2

<tb><SEP> Additive <SEP>% <SEP> Agent <SEP> I. <SEP> Kappa <SEP> DP "<SEP><SEP>%<SEP> H202 <SEP>% <SEP> APA
<tb><SEP> ancillary <SEP> cons. <SEP> cons.
<Tb>

<SEP> Control <SEP> O <SEP> 10.1 <SEP> 504 <SEP> 1.7 <SEP> 3
<tb> Example <SEP> 2 <SEP> Pyrophosphate <SEP> Na <SEP> 0.1 <SEP> 10.3 <SEP> 984 <SEP> 1.6 <SEP> 3
<tb><SEP> Pyrophosphate <SEP> Fast <SEP> Na <SEP> 0.2 <SEP> 10.4 <SEP> 1228 <SEP> 1.1 <SEP> 3
<tb><SEP> Pyrophosphate <SEP> Na <SEP> 0.4 <SEP> 10.6 <SEP> 1367 <SEP> 0.9 <SEP> 3
<tb><SEP> Pyrophosphate <SEP> Na <SEP> 0.5 <SEP> 10.6 <SEP> 1324 <SEP> 1.2 <SEP> 3
<tb><SEP> Pyrophosphate <SEP> Na <SEP> 0.7 <SEP> 10.3 <SEP> 1318 <SEP> 1.1 <SEP> 3
<tb> Example <SEP> 3 <SEP> Pyrophosphoric <SEP><SEP> 0.5 <SEP> 9.1 <SEP> 1165 <SEP> 1.4 <SEP> 3
<tb> Example <SEP> 4 <SEP> Tripolyphosphate <SEP> Na <SEP> 0.5 <SEP> 10.5 <SEP> 1327 <SEP> 1 <SEP> 3
<tb> Example <SEP> 5 <SEP> ~ <SEP> Hexametaphosphate <SEP> Na <SEP> 0.5 <SEP> 9 <SEP> 1240 <SEP> 1.2 <SEP> 3
<Tb>
Example 6
A treatment is carried out, the conditions of which are summarized in Table 3 below. The treatment is carried out with a load of 3% by weight of peracetic acid and 3.3% of hydrogen peroxide with respect to the dry pulp.

Table 3

<tb><SEP> Conditions <SEP> Processing <SEP> with <SEP> 0.5 <SEP>% <SEP><SEP> Control (Without
<tb><SEP> pyrophosphate <SEP> Na <SEP> pyrophosphate)
<tb><SEP> I. <SEP> Kappa <SEP> Dpv <SEP> I. <SEP> Kappa <SEP> DPv
<tb> 900, <SEP> 3 <SEP> hours <SEP> 10.3 <SEP> 1324 <SEP> 10.1 <SEP> 504
<tb> 900, <SEP> 2 <SEP> hours <SEP> 12.8 <SEP> 1300 <SEP> 13.8 <SEP> 1000
<tb> 1100, <SEP> 1 <SEP> hour <SEP> 14.9 <SEP> 1224 <SEW> 15.6 <SEP> 1127
<Tb>
From the examples shown above, it will be noted that all the polyphosphates tested and which are soluble in the bleaching solution give good results. With regard to sodium pyrophosphate, the optimum amount of this product is between 0.2% and 0.5% by weight relative to the dry pulp.

Example 7
A chemical pulp of initial Kappa number (IK) equal to 24 and degree of polymerization (DP) equal to 1510 is treated at 90 ° C. for 1 hour at pH = 4 in the presence of sodium pyrophosphate, with an aqueous solution at 1 ° C. equilibrium of peracetic acid and hydrogen peroxide, the charges peracetic acid and hydrogen peroxide, are respectively equal to 3% and 3.3% by weight relative to the dry pulp; the charge of sodium pyrophosphate is 0.5%.

 The dough is then divided into two portions.

 The first portion is treated with about 5% sodium hydroxide so that the pH increases to 10.5, then the reaction is continued for 2 hours.

 The second portion (control) is washed with water and then treated for 1 hour at 900C with sodium hydroxide.

We obtain the following results
Table 4

<tb><SEP> Paste <SEP> before <SEP> 1st <SEP> serving <SEP> 2nd <SEP> serving
<tb><SEP> treatment <SEP> (test) <SEP> after <SEP> (control) <SEP> after
<tb><SEP> treatment <SEP> treatment
<tb> IK <SEP> 24 <SEP> 6.2 <SEQ> 14.4
<tb> DP <SEP> 1510 <SEP> 1124 <SEP> 1487
<Tb>
Example 8
By following a procedure analogous to Example 7, and modifying the treatment step of the first part (test) by the additional addition of 0.2% of magnesium sulphate, the following results are obtained:
Table 5

<tb><SEP> Paste <SEP> before <SEP> 1st <SEP> serving <SEP> 2nd <SEP> serving
<tb><SEP> treatment <SEP> (test) <SEP> after <SEP> (control) <SEP> after
<tb><SEP> treatment <SEP> treatment
<tb> IK <SEP> 24.8 <SEP> 8.6 <SEP> 16.7
<tb> DP <SEP> 1450 <SEP> 1195 <SEP> 1409
<Tb>
The results of Examples 7 and 8 demonstrate the additional advantage of treating with peroxy acid first at acidic pH and then raising the pH without an intermediate wash.

This increases delignification while maintaining an acceptable degree of polymerization.

Claims (11)

 1. Process for delignification and / or bleaching of a paper pulp comprising at least one treatment step using an organic peroxy acid, characterized in that said treatment is carried out in the presence of one or more ancillary protective agents viscosity of the paste selected from inorganic phosphorus acids and their salts.  The method of claim 1 wherein the ancillary viscosity protection agent of the paste is pyrophosphoric acid, sodium pyrophosphate, tripolyphosphoric acid or sodium tripolyphosphate.  3. Method according to one of claims 1 or 2 wherein the organic peroxy acid is selected from performic acid, peracetic acid, l, 6-diperoxy-hexanedioïque acid, 1,4 monoperoxybutènedioïque acid, 1 1,4-monoperoxybutanedioic acid, and perbenzoic acid.  4. Method according to claim 3 wherein the organic peroxy acid is peracetic acid in aqueous solution at least 4% by weight of said peroxy acid and especially in equilibrium aqueous solution also comprising hydrogen peroxide such as the H 2 O 2 / peracetic acid weight ratio is greater than 0.5 and especially greater than 1.  5. Method. according to one of claims 1 to 4 wherein is added to the pulp during said step of treating sodium silicate and / or magnesium salts or magnesium compounds such as magnesia, magnesium carbonate, magnesium sulphate or magnesium chloride  6. Method according to one of claims 1 to 5 carried out at a temperature between 200C and 1400C and in particular between 500C and 1200C and, preferably, between 70 "C and 900C.  7. The method of claim 6 implemented at a pressure less than or equal to 10 bar and, preferably, less than 5 bar.  8. The method of claim 7 implemented in a pressurized reactor with air or with a gas containing more than 21% oxygen and in particular at least 80% oxygen.  9. Method according to one of claims 7 or 8 implemented at pH between 5 and 9 and in particular between 7 and 8.5.  10. Method according to one of claims 7 or 8 wherein the treatment with peroxyacid is carried out first at a pH of between 2 and 7, especially between 4 and 6, for less than 4 hours, especially less than two hours, and preferably less than one hour, at a temperature of between 40 and 1000 ° C., and then the paste is alkalinized to a pH of between 8 and 12 and in particular between 9.5 and 12 and left less than 4 hours at a temperature between 60 and 1200C.  11. Method according to one of claims 1 to 10, characterized in that the peroxyacid comes from a washing solution of a subsequent step of the pulp treatment process.