EP0578305B1 - Process of delignification of a chemical pulp - Google Patents

Description

The invention relates to a method of delignification of a chemical paper pulp.

It is known to apply to unbleached chemical paper pulps obtained by cooking cellulosic materials in the presence of chemical reagents a sequence of processing steps delignifying and whitening implying the implementation of oxidizing chemicals. The first step in a sequence conventional chemical pulp bleaching aims to perfect the delignification of the unbleached dough as it is present after the cooking operation. This first step delignifying is traditionally carried out by treating the dough unbleached by chlorine in an acid medium or by a chlorine association - chlorine dioxide, in mixture or in sequence, so as to cause a reaction with the residual lignin in the pulp and give rise to chlorolignins which can be extracted of the paste by solubilization of these chlorolignins in the medium alkaline in a subsequent processing step.

For various reasons, it is useful, in some situations, to be able to replace this significant first step by a treatment which no longer uses a reagent chlorine.

We have already proposed to treat a kraft paste with a first oxygen step followed by a peracetic acid step at 70 ° C in the presence of diethylenetriaminepentaacetic acid (DTPA) (patent application JP-A-57 021591 in the name of MITSUBISHI GAS CHEMICAL). In this known process, the presence of stabilizer DTPA avoids significant degradation of the chains cellulose. However, the protective effect of the stabilizer does not reach not yet sufficient level required for pasta production High quality.

The invention aims to remedy the drawbacks of the methods known by providing a process which achieves delignification effective unbleached chemical pulp which allows obtaining pasta with high intrinsic qualities in a wide temperature range.

To this end, the invention relates to a method for improving the selectivity of the delignification of a dough chemical paper using an organic peroxyacid that treating the unbleached dough from the cooking operation by an aqueous solution of this organic peroxyacid in the presence of a stabilizer of said peroxyacid comprising at least one compound selected from the class of phosphonic acids and their salts, the pH of the aqueous peroxyacid solution in contact with the dough being acidic and preferably from 3.5 to 6.5.

According to the invention, by chemical paper pulp is meant the pulp which has already undergone a delignifying treatment in the presence of chemical reagents such as sodium sulfide in alkaline medium (kraft or sulfate cooking), sulfur dioxide or a metallic salt of sulfurous acid in an acid medium (cooking with sulfite or bisulfite). According to the invention, it is also intended to denote by chemical paper pulp, the pulps called in the literature "semi-chemical pulps" such as those where the cooking was carried out using a salt of sulfurous acid in neutral medium (cooking with neutral sulfite, also called NSSC cooking), as well as the pastes obtained by processes using solvents, such as the ORGANOSOLV, ALCELL ^(R) , ORGANOCELL ^(R) and ASAM pastes described in Ullmann's Encyclopedia of Industrial Chemistry , 5th Edition, Vol A18, 1991, pages 568 and 569.

The invention is particularly intended for pasta which has undergone kraft cooking. All types of wood used for production of chemical pulps suitable for processing of the process of the invention and, in particular those used for kraft pulp, i.e. softwood such as, for example, various species of pines and firs and deciduous woods such as, for example, beech, oak, eucalyptus and charm.

According to a first variant of the invention, the peroxyacid organic is usually selected from acid performique and aliphatic carboxylic peroxyacids comprising a single percarboxylic group and a chain saturated linear or branched alkyl of less than 11 atoms carbon. Chain aliphatic carboxylic peroxyacids linear saturated alkyl with less than 6 carbon atoms are preferred. Examples of such peroxyacids are acid peracetic, perpropanoic acid, n-perbutanoic acid and n-perpentanoic acid. Peracetic acid is particularly preferred because of its efficiency and relative simplicity of its preparation methods.

In a variant of the process according to the invention, the peroxyacid organic is selected from diperoxycarboxylic acids comprising a linear or branched alkyl chain of less than 16 carbon atoms and two percarboxylic groups substituted on carbon atoms located in position alpha-omega relative to each other. Examples of such peroxyacids are 1,6-diperoxyhexanedioic acid, acid 1,8-diperoxyoctanedioic acid and 1,10-diperoxydecanedioic acid, and 1,12-diperoxydodecanedioic acid.

In another variant of the process according to the invention, the organic peroxyacid is selected from peroxyacids aromatics comprising at least one percarboxylic group by benzene nucleus. Preferably, we will choose peroxyacids aromatics which have only one percarboxylic group by benzene nucleus. An example of such an acid is acid peroxybenzoic.

Another variant of the process according to the invention consists in choose an organic peroxyacid substituted by one or more halogen atoms or any other functional substituent organic. By any other organic functional substituent, intends to designate a functional group such as the group carbonyl (ketone, aldehyde or carboxylic acid), the group alcohol, nitrogen-containing groups such as nitrile, nitro, amine and amide groups, the groups containing sulfur such as the sulfo and mercapto groups.

The peroxyacid used can be an aqueous solution commercial comprising at least 10% by weight of peroxyacid, equilibrium with at least 12% by weight of organic acid corresponding and at least 1.5% by weight of hydrogen peroxide most often in the presence of a small amount of catalyst in the form of at least 0.3% by weight of a strong acid, usually an inorganic acid. An example of composition commercial organic peroxyacid which is well suited is a concentrated aqueous solution of peracetic acid comprising about 34% by weight of peracetic acid, about 44% by weight acetic acid, about 5% by weight of hydrogen peroxide and about 1% by weight of sulfuric acid. It is also possible to prepare the peroxyacid immediately before use by reaction under the appropriate conditions of acetic acid at 50 to 100% by weight with a concentrated aqueous solution of peroxide hydrogen from 30 to 85% by weight, in the presence of a small amount of an inorganic acid as catalyst.

The peroxyacid can indifferently be used at the state of an aqueous solution of peroxyacid or in the form an ammonium, alkali metal, or alkaline earth metal salt of this peroxyacid.

The peroxyacid treatment according to the invention can take place over a wide range of temperatures. In general, we perform peroxyacid treatment at a temperature of at least minus 2 ° C and preferably at least 20 ° C. Likewise, this temperature generally does not exceed 98 ° C and, preferably, step 95 ° C. The process according to the invention is particularly well suitable for the use of high temperatures, i.e. at least minus 50 ° C and preferably at least 75 ° C.

Generally, the treatment is carried out with peroxyacid organic at atmospheric pressure. The duration of this treatment depends on the temperature and the type of wood used prepare the dough, as well as the efficiency of the baking which has preceded. Duration between approximately 120 minutes and about 360 minutes is fine.

The pH of the treatment stage with peroxyacid is in the acid pH range. In practice, we prefer to fix the pH at a value of at least 3.5. It will also be most often not to exceed a pH value of 6.5.

The treatment according to the invention can be done in any type of suitable equipment for the treatment of paper pulp using acid reagents. The retention tank for the unbleached dough present in all bleaching and playing facilities the role of buffer tank between the wood cooking unit and the dough bleaching unit is particularly suitable for carrying out the method according to the invention. The dough can thus be treated there during its storage without requiring the investment of an expensive specific device.

The consistency of the dough in the processing step with the organic peroxyacid will generally be chosen to be at least 5% of dry matter and preferably at least 10% matter dry. Most often, the consistency will not exceed 40% of dry matter and preferably not 30%.

In the process according to the invention, the amount of peroxyacid organic used is chosen according to the rate of residual lignin in the dough as well as the average duration of the treatment. Generally, amounts of at least 0.1% and, of preferably at least 1% by weight of peroxyacid relative to dry dough is fine. Most often, we will use a quantity of peroxyacid not exceeding 10% by weight per compared to dry dough and preferably not more than 5% of this weight.

According to the invention, the stabilizer used belongs to the class of phosphonic acids and their salts. Preferably, it will be selected from 1-hydroxyethylidene-1,1-diphosphonic acid (HEDPA), ethylenediaminetetra acid (methylenephosphonic) (EDTMPA), diethylenetriaminepenta acid (methylenephosphonic) (DTMPA), triethylenetetraminehexa acid (methylenephosphonic) (TTHMPA), acid pentaethylenehexamineocta (methylenephosphonic) (PHOMPA), cyclohexanediaminetetra (methylenephosphonic acid) (CDTMPA) and nitrilotri (methylenephosphonic) acid (NTMPA). DTMPA and its salts have given excellent results.

The total amounts of stabilizer to be used depend on the type of wood and the cooking process used. In as a general rule, it is recommended to implement a amount of stabilizer of at least 0.05% by weight relative to the dry matter and preferably at least 0.2% by weight. Amounts of stabilizer not exceeding 3% by weight per dry matter and preferably not more than 2% by weight are generally sufficient.

Treatment with organic peroxyacid according to the invention can also be performed in the presence of several stabilizers comprising at least one phosphonic acid or one of its salts and / or sodium silicate. It can also be interesting to associate with at least one phosphonic acid and / or with the silicate of sodium a water-soluble magnesium salt such as sulphate magnesium.

Alternatively, it may be interesting to precede the treatment with organic peroxyacid by at least one washing or a decontaminating pretreatment step using a solution acidic aqueous. The purpose of this washing or this step is to extract the paste the impurities present in the form of metal ions which are detrimental to the proper conduct of laundering operations and / or delignification. All inorganic acids or organic used in aqueous solution, alone or in mixture agree. Strong inorganic acids such as, for example, sulfuric acid or hydrochloric acid are well suited.

It is advantageous that washing or pretreatment acid decontaminant is also carried out in the presence of an agent complexing metal ions. To this end, mixtures of strong inorganic acids mentioned above with acids organic from the class of aminopolycarboxylic acids or aminopolyphosphonic acid or their alkali metal salts are particularly suitable. Examples of acids suitable aminopolycarboxylics are diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid (EDTA), cyclohexanediaminetetraacetic acid (CDTA) and acid nitrilotriacetic (NTA). Diethylenetriaminepentaacetic acid (DTPA) is preferred. Examples of aminopolyphosphonic acids are diethylenetriaminepenta (methylenephosphonic acid) (DTMPA), ethylenediaminetetra acid (methylenephosphonic) (EDTMPA), cyclohexanediaminetetra acid (methylenephosphonic) (CDTMPA) and nitrilotri acid (methylenephosphonic). DTMPA is preferred. The amounts of complexing agent to be used depend on the effectiveness of the complexing agent selected and the metal content of the pulp to be treated. In practice, we put generally at least 0.01% by weight of agent complexing compared to dry dough and, more often, to minus 0.05%. Similarly, we generally do not exceed 1% in weight of complexing agent relative to the dry paste and, most often not 0.25%.

The operating conditions of the decontaminating pretreatment acid are not critical. They must be determined in each particular case depending on the type of pulp and the apparatus in which the treatment is carried out. Of a in general, the choice of acid and the quantity used to impose on the medium a pH below 7, for example between approximately 1 and approximately 6.5. PH especially advantageous are those between about 3.0 and about 6.0. Temperature and pressure are not critical, ambient temperature and atmospheric pressure suitable generally fine. The duration of the pretreatment can vary within large proportions depending on the type of equipment used, the choice acid, temperature and pressure, for example from About 15 minutes to several hours.

It can also be interesting, in certain circumstances depending on the type of wood used and the type baking used to prepare the dough, to interpose between the cooking operation and the peroxyacid treatment a or more additional dough delignification steps using chemical reagents. By delignification by means of chemical reagents, we mean both reagents non-oxidizing agents such as an alkaline reagent such as hydroxide or sodium, magnesium or calcium carbonate as reactants oxidizers in an acid medium such as chlorine, dioxide chlorine, ozone, an inorganic peroxyacid such as acid peroxymonosulfuric, hydrogen peroxide in an acid medium, and oxidizing reagents in an alkaline medium such as peroxide of hydrogen in an alkaline medium, sodium hypochlorite or calcium, molecular oxygen or ozone. It is also possible to combine two or more in a single treatment step of these reagents.

In a variant of the process according to the invention, it is possible, if you want to obtain high levels of whiteness, forward treatment with a peroxyacid by a sequence of stages of traditional bleaching using chemical reagents whether or not involving chlorinated reagents. Examples of such stages are as follows: stages with gaseous oxygen or ozone, alkaline hydrogen peroxide steps in the presence or no gaseous oxygen, chlorine dioxide steps or sodium hypochlorite, alkaline soda extracts caustic.

According to a preferred variant of the process according to the invention, follows the peroxyacid treatment with a step of bleaching with hydrogen peroxide in an alkaline medium. This step with hydrogen peroxide in an alkaline medium can advantageously be done using hydrogen peroxide which generally accompanies peroxyacid: after treatment with the peroxyacid, an alkali is added to the paste and then proceeds with bleaching with hydrogen peroxide, without perform intermediate washing between steps with peroxyacid and alkaline hydrogen peroxide. If necessary, we will do supplementing with additional hydrogen peroxide so as to reach the total quantity required to achieve a effective whitening.

The process according to the invention applies to the delignification and bleaching of all types of dough chemical. It is well suited for delignifying and blanching pasta kraft and sulfite pastes. It is particularly good suitable for processing kraft pasta.

The following examples are given for the purpose of illustrating the invention, without limiting its scope.

Examples 1R and 2R (not in accordance with the invention)

A sample of baked hardwood pulp kraft (initial whiteness 33.7 ° ISO measured according to the standard ISO 2470, kappa index 12.4 measured according to the SCAN C1-59 standard and degree of polymerization 1370 expressed in number of units glucose and measured according to SCAN C15-62) has been delignified by means of a two-step sequence comprising a first step with peracetic acid (Paa) and a second step alkaline extraction with sodium hydroxide. The dough has been washed with demineralized water between the two stages.

The peracetic acid used was an equilibrium aqueous solution containing 240 g / l of CH ₃ CO ₃ H, 420 g / l of CH ₃ COOH, 100 g / l of H ₂ O ₂ and 7 g / l of H ₂ SO ₄ .

After delignification, the treated dough was made whiteness, kappa index and degree of determination polymerization.

The operating conditions were as follows: 1st stage : peracetic acid stage (Paa stage): CH ₃ CO ₃ H content, g / 100g dry paste 1.0 40% DTPA or 100% EDTA content, g / 100g dry paste 0.5 temperature, degrees C 90 duration, min 240 consistency,% by weight of dry matter 10 2nd stage : stage with Na hydroxide (stage E): NaOH content, g / 100g dry paste 2.0 temperature, degrees C 90 duration, min 45 consistency,% by weight of dry matter 10

The results obtained are given in the following table: Example No Nature of stabilizer pH init step. Paa late. Final whiteness ° ISO Final kappa index Final RFP 1R DTPA 4.35 4.20 48.0 7.73 750 2R EDTA 4.35 4.30 48.9 7.46 740

Example 3 : (according to the invention)

Examples 1R and 2R were reproduced, replacing the stabilizer DTPA or EDTA with 0.5 g of the heptasodium salt of diethylenetriaminepenta (methylenephosphonic acid) (DTMPNa ₇ ) per 100 g of dry paste.

The results obtained were: Example No Nature of stabilizer pH init step. Paa late. Final whiteness ° ISO Final kappa index Final RFP 3 DTMPNa ₇ 4.35 4.40 48.2 7.63 1250

Example 4 : (according to the invention)

A softwood pulp having undergone kraft cooking with a whiteness of 30.5 ° ISO, a kappa index of 26.7 and a degree of polymerization of 1510 was bleached using a sequence completely free of chlorine reagents in 4 steps 0 QP Paa under the following operating conditions: 1st stage: oxygen stage (0) pressure, bar 5.5 NaOH content, g / 100 g dry paste 4.0 MgSO ₄ .7H ₂ O content, g / 100 g dry paste 0.5 temperature, degrees C 120 duration, min 60 consistency,% by weight of dry matter 14 2nd stage: stage with a sequestering acid DTPA content 40%, g / 100 g of dry paste 0.5 H ₂ SO ₄ for initial pH of 5.00 temperature, degrees C 55 duration, min 30 consistency,% by weight of dry matter 4.0 3rd stage: H ₂ O _{2 stage} (P) H ₂ O _{2 content} , g / 100 g of dry paste 2.0 NaOH content, g / 100 g of dry paste 1.5 temperature, degrees C 90 duration, min 120 consistency,% by weight of dry matter 10 4th stage: stage with peracetic acid (Paa) Paa content, g / 100 g of dry dough 3.0 DTMPNa ₇ content, g / 100 g of dry paste 0.5 temperature, degrees C 90 duration, min 240 consistency,% by weight of dry matter 10

The results obtained were as follows: Example No Nature of stabilizer pH step Paa Final whiteness ° ISO Final kappa index Final RFP init. end. 4 DTMPNa ₇ 3.6 3.4 68.8 4.61 1070

Examples 5R, 6R and 7R (not in accordance with the invention) and 8, 9 and 10 (in accordance with the invention)

A sample of kraft softwood pulp (initial whiteness 30.5 ° ISO measured according to ISO 2470, kappa index 26.7 measured according to standard SCAN C1-59 and degree of polymerization 1510 expressed in number of glucose units and measured according to the standard SCAN C15-62) has been delignified using a two-part sequence stages comprising a first stage with peracetic acid (Paa) and a second stage of alkaline extraction with sodium hydroxide. The dough was washed with demineralized water between the two stages.

The peracetic acid used was an equilibrium aqueous solution containing 240 g / l of CH ₃ CO ₃ H, 420 g / l of CH ₃ COOH, 100 g / l of H ₂ O ₂ and 7 g / l of H ₂ SO ₄ .

After delignification, the treated dough was made whiteness, kappa index and degree of polymerization determinations.

The operating conditions were as follows: Reagents : Example No. Step 1 2nd step CH ₃ CO ₃ H content g / 100g ps DTMPNa content ₇ g / 100g ps silicate content 36 ° Be g / 100g ps MgSO content ₄ .7H ₂ O g / 100g ps NaOH content g / 100g ps 5R 3 0 3 0 2 6R 3 0 0 1 2 7R 3 0 3 1 2 8 3 0.5 3 0 2 9 3 0.5 0 1 2 10 3 0.5 3 1 2 where DTMPNa ₇ symbolizes the heptasodium salt of diethylenetriaminepenta acid (methylenephosphonic acid). In all of the examples 5R, 6R and 7R and 8, 9 and 10, the same operating conditions were carried out as follows: Step 1 2nd step - temperature, ° C 90 90 - duration, min 240 45 - consistency,% by weight of dry matter 10 10 After treatment, the determinations of whiteness, kappa index and degree of polymerization were carried out on the paste.

The results obtained were as follows: Example No. Final whiteness, ° ISO Final kappa index Final RFP 5R 38.1 13.8 820 6R 38.1 13.3 870 7R 37.9 13.4 630 8 40.7 14.4 1350 9 40.5 14.6 1380 10 41.4 14.4 1360

Claims (10)

1. Process for improving the selectivity of the delignification of a chemical paper pulp by means of an organic peroxyacid, in which the unbleached pulp originating from the cooking operation is treated with an aqueous solution of this organic peroxyacid in the presence of a stabiliser for the said peroxyacid, characterised in that the stabiliser comprises at least one compound selected from the class of phosphonic acids and their salts and in that the pH of the aqueous solution of the organic peroxyacid in contact with the pulp is acid and preferably from 3,5 to 6,5.
2. Process according to Claim 1, characterised in that the organic peroxyacid is peracetic acid.
3. Process according to Claim 1 or 2, characterised in that the treatment with organic peroxyacid is performed at a temperature of between 50 and 98 °C.
4. Process according to any one of Claims 1 to 3, characterised in that the phosphonic acid is diethylenetriaminepenta(methylenephosphonic) acid.
5. Process according to any one of Claims 1 to 4, characterised in that the stabiliser additionally comprises sodium silicate.
6. Process according to any one of Claims 1 to 5, characterised in that the stabiliser for the peroxyacid is a combination of a phosphonic acid or of one of its salts with a water-soluble magnesium salt.
7. Process according to any one of Claims 1 to 6, characterised in that at least one washing operation or a stage of treatment with an acidic aqueous solution is carried out before the treatment with organic peroxyacid.
8. Process according to any one of Claims 1 to 7, characterised in that one or more stages of delignification of the pulp by means of chemical reactants is (are) inserted between the cooking operation and the treatment with the peroxyacid.
9. Process according to any one of Claims 1 to 8, characterised in that the treatment with the peroxyacid is followed by a stage of bleaching with hydrogen peroxide in alkaline medium.
10. Application of the process according to any one of Claims 1 to 9 to the delignification and bleaching of kraft pulps, of sulphite pulps and of the pulps obtained by cooking processes employing solvents.