AU711672B2 - Process for oxygen delignification of a paper pulp - Google Patents

Description

1 "PROCESS FOR OXYGEN DELIGNIFICATION OF A PAPER PULP" The present invention relates to a process for oxygen delignification of a paper pulp containing cellulose, in which a paper pulp containing cellulose is formed and then oxygen is injected in one or more stages into the said pulp so as to reduce the proportion of lignin before bleaching of the said pulp, a process comprising at least one pretreatment stage before at least one of the stages of oxygen injection into the pulp.

The effluents originating from the papermaking industry have recently become the subject of environmental concerns. Studies have shown, in fact, that organochlorine compounds generated during the first stage of delignification with chlorine (symbol C) and extracted during the alkaline extraction (symbol that is to say during the sequence C-E, accumulate in the receiving medium.

It is also known that the organochlorine contents (symbolized by AOH and TOC, that is to say Adsorbable Organic Halide and Total Organic Chlorine, respectively) are linearly proportional to the usage of elemental chlorine, which itself depends on the kappa number, representing the lignin content present in the unbleached pulp.

A considerable reduction in the emissions of organochlorine products has been brought about through the use of oxygen delignification, which has made it possible to produce pulps which have lower kappa numbers. These pulps can then be bleached with a lower multiple chlorine number or else with a partial replacement of chlorine with chlorine dioxide, generating smaller quantities of organochlorine products. Oxygen delignification makes it possible to reduce the kappa number of a chemical pulp by more than relative to the initial number of the unbleached pulp. However, this rate of delignification cannot _1IX___III~X~~_llj__-il Il~-Xlltl 2 exceed 50 without causing a considerable degradation of the cellulose, which is seen as a reduction in the mechanical properties of the pulp. This degradation is expressed as a loss of viscosity of the pulp.

It is also known to add magnesium salt into the pulp in order to protect the cellulose against this degradation, but despite this addition it is not possible to go beyond 45 to 50 delignification of the pulp without substantially degrading the cellulose.

In order to overcome this disadvantage, the residual lignin must be activated in order to make the oxygen delignification on the one hand more effective and, on the other hand, more selective. A number of processes have already been proposed for this purpose: It is known, in fact, to employ nitrogen dioxide to modify the residual lignin in order to make it more reactive during an oxygen delignification stage. When kraft pulps are treated with nitrogen dioxide, as described, for example, in the paper entitled "Pretreatment of Kraft Pulp with Nitrogen Dioxide before Oxygen Bleaching", by K. Abramhsson, L.

Lowendahl and O. Samuelson, Svenk Papperstidn. 84 (18): R152, R158 (1981) or with sodium nitrite, as described, for example in the paper entitled "Chlorination Stage Elimination by Using Nitrosation Pretreatment before Oxygen Delignification, Oxygen Reinforced Extraction" by P.W. C Ku, J.S. Hsieh, D. Jayawant and L.L. Houle, TAPPI Journal, 75 146-151 (1992), oxygen delignification reduces their kappa number by more than 50 without altering the viscosity values or the physical properties of the pulps. For example, a coniferous kraft pulp with an initial kappa number of 30 can be delignified to a kappa number of 8 to 10 without significant loss of the pulp viscosity. This corresponds to approximately 65 to 73 delignification. However, this process, besides its cost, is liable to give rise to nitrogen oxides NO x when the effluents are recycled to the boiler. To overcome this problem, other treatments have been described more i~rr~ il-lr~l~lllllllll~ t-X-~IIII-^Y~.~1I~- 3 recently; the most effective of these treatments consists in the use of a chlorination stage preceding an oxygen stage (xO process) or between the two oxygen delignification stages (OxO process). It is known from the following various papers: D. Lachenal and C. De Choudens, "High Efficiency Oxygen and Peroxide Delignification", Cell.

Chem. Tech. 20 553-557 (1986), D. Lachenal, C. De Choudens, L. Bouson -and R.

Lachapelle, "Full Bleaching of Chemical Pulp with Low Charges of Chlorine", Paperi ja Puu 70 616-619 (1988), D. Lachenal and M. Muguet, "Reducing TOC1 with the OxO Process", Pulp and Paper Can. 92 T297- T301 (1991), D. Lachenal, L. Bourson, M. Muguet and A.

Chauvet, "Lignin activation Improves Oxygen and Peroxide Delignification", Cell. Chem. Tech. 24 593-601 (1990), D. Lachenal, C. De Choudens, L.

Boursin, and R. Lachapelle, 1987 International Oxygen Delignification Conference, San Diego, Proceeding, 69, to employ chlorine (Cl1) or chlorine dioxide (CO) or a mixture of these two reactants to increase the oxygen delignification, with results which are comparable to nitrogen dioxide. A hypothesis to account for the high selectivity of this process is the introduction of phenolic groups into the residual lignin. However, the introduction of a chloride (Cl-) ion into the effluents from the oxygen stage prohibits their treatment in a boiler in a subsequent stage, bearing in mind the problems of corrosion of the materials to which they give rise. This represents a major disadvantage for reaching the low pollution levels posed by the new regulations in various countries. In addition, this process, employing chlorine, chlorine dioxide or a mixture of these two products, is incompatible when the paper pulp manufacturing plant wishes to produce so-called TCF 4 pulp ("Total Chlorine Free" or else pulp without employing elemental chlorine).

It is also known, from the paper entitled "Hydrogen-peroxide-reinforced oxygen delignification of Southern pine kraft pulp and short sequence bleaching", by V.R. Parthasarathy, R. Klein, V.S.M. Sundaram, H.

Jameel and J.S. Gratzl, published in TAPPI Journal 177- 187 in 1990, to employ hydrogen peroxide as an activator of an oxygen stage. The hydrogen peroxide has an effect which is additive to that of oxygen: the introduction of a small charge of hydrogen peroxide (0.2 to 0.5 into an oxygen delignification stage, Op sequence, makes it possible to obtain pulps which have lower kappa numbers and viscosities that are higher by approximately 1.5 centipoise. In the said paper it is also described that by using a process of the Op-Op type it is possible to obtain 73 delignification, against only 61 in the case of a process comprising two consecutive oxygen stages (0-0 sequence). At a comparable kappa number, the pulps produced according to the Op-Op process exhibit higher viscosities and better physical properties.

It is also known, from the paper "dimethyldioxirane a Selective Bleaching Agent for Chemical Pulps, dimethyldioxirane used as the interstage treatment in an OTO sequence", by K. Hutn and Lee, Journal of Pulp and Paper Science, 21 J263-J267 (1995), to employ dimethyldioxirane as a source of active oxygen. Dimethyldioxirane has been tested as pretreatment between two oxygen delignification stages (OTO); the results obtained show that dimethyldioxirane is as effective as chlorine dioxide in terms of reduction in the kappa number and of retention of the viscosity. On the other hand, dimethyldioxirane makes it possible to obtain pulps which have superior brightness values. However, although attractive, this process has the major disadvantage of employing dimethyldioxirane, a reactant that is difficult and dangerous to produce on site, ii IIII_1I^_ ~-YlliT~-. 5 since it requires acetone and permonosulphuric acid or one of its salts. This process is therefore difficult to employ on an industrial scale.

It is also known, from G. Fossum and A.

Marklund, TAPPI Journal, 71 79 (1988), to employ a pretreatment with hydrogen peroxide in acidic medium, which has been found to be more effective than a treatment with hydrogen peroxide in a neutral medium.

One of the explanations could be the "in. situ" formation of small quantities of permonosulphuric acid in acidic medium.

It is also known from the paper "Treatment of Softwood Kraft Pulps with Peroxymonosulfate Prior to Oxygen Delignification", E.L. Springer and J.D.

McSweeny, 1992 International Pulp Bleaching Conference Proceeding, TAPPI Press, Boston, 537-545, that a treatment of coniferous kraft pulp with the aid of Caro's acid, carried out before an oxygen delignification stage is as effective as a pretreatment with chlorine, but on the condition that the transition metals present in the pulp are removed by an appropriate treatment. However, the disadvantage of such a process is that it employs large quantities of chelating agents and large quantities of Caro's acid to obtain the same performance as a treatment with chlorine.

It is also known, from the papers entitled "Improved Oxygen Delignification with Peroxyacid Treatment", K.G. McGrougher and R.W. Allison, Appita 47 238-242 (1994), "Improved Oxygen Delignification with Interstage Peroxymonosulfuric Acid Treatment. Part 2. Effects of hydrogen Peroxide", R.W. Allison, K.

McGrouther, D. Lachenal, C. De Choudens and R.

Angelier, 1994 International Pulp Bleaching Conference Proceedings, TAPPI Press, Sans [sic] Diego, 521-529, and "Improved Oxygen Delifgnification [sic] with Interstage Peroxymonosulfuric Acid treatment", R.W.

Allison and K. McGrouther, TAPPI Journal, 78 134- 142 (1995), to employ Caro's acid in order to activate 6 a second oxygen delignification stage (OxO sequence) where x denotes the treatment with Caro's acid. Thus the treatment of a coniferous kraft pulp with 0.6 9 to by volume of permonosulphuric acid allows the delignification and the selectivity to be substantially improved, again on condition that the transition metals are removed. In the said papers it is also described that a pretreatment with chlorine dioxide remains more effective in terms of delignification and retention of the viscosity.

It is known from the paper entitled "Totally Chlorine-Free Bleaching of Kraft Pulps from Australian Eucalypt Woods", P.J. Nelson, C.W.J. Chin, S.G. Grover and H. Ryyn.nn, 8th, IPWCP, Helsinki, p. 331-336 (1995) to carry out an enzyme treatment with xylanase as intermediate treatment, which is found to be an effective means for reducing the kappa number of eucalyptus pulp to a value lower than that obtained using two consecutive oxygen stages. An improvement of the order of 15 is observed on incorporation of an enzyme treatment between the two oxygen stages.

However, the reduction in the kappa number does not exceed 56 Moreover, a higher loss in wood yield is observed with an enzyme treatment.

Furthermore, the use of chlorine-containing reactants, Cl 2 and C10 2 gives rise to organochlorine compounds which are toxic to the environment, and this prevents the recycling of the effluents to the boiler and is incompatible with the production of so-called TCF pulp.

Processes employing oxidizing agents not containing chlorine also exhibit disadvantages, among which may be mentioned: lack of effectiveness in the case of the hydrogen peroxide process, need to pretreat the pulp with a chelating agent or an acid treatment in the case of the Caro's acid process, 7 need to employ large reactant charges in the case of the dimethyldioxirane and Caro's acid (HSO 5 process, high capital cost and increased safety hazard in the case of the dimethyldioxirane process, due to the formation of acetone peroxides.

As for the enzyme process, the percentage of delignification obtained still remains relatively low, when compared with processes employing oxidizing reactants. In addition, the observed losses in yield make this process incompatible with the economy requirements of a paper pulp plant.

All these processes have a certain number of disadvantages.

It is also known, from the paper by Li et al., entitled "Activation of a two-stage oxygen delignification", 8th ISWPC, Helsinki, June 6-9, 1995, p. 337-342, to treat paper pulp with peracetic acid at the equilibrium obtained by reaction with hydrogen peroxide in aqueous solution with acetic acid in an acetic acid:H 2 0 2 ratio 1.7, between the two stages of oxygen delignification. This treatment with peracetic acid at equilibrium is necessarily preceded by a pretreatment aimed at removing the metal ions present in the pulp, a pretreatment consisting of a chelation stage aimed at solubilizing the metal ions, followed by a washing which allows them to be extracted from the pulp. This pretreatment is essential in order to avoid the detrimental effects of the metals on the physical properties of the pulp during the peracetic acid stage.

After pretreatment with peracetic acid the pulp is washed before the second stage of oxygen delignification.

However, such a process has a number of disadvantages, including especially a high peracetic acid charge, retention times of more than an hour, and the presence, besides this peracetic acid treatment stage, of a chelation and washing pretreatment and of a washing post-treatment.

8 The invention makes it possible to avoid the disadvantages of the pretreatment processes of the prior art. The process according to the invention is characterized in that the said pretreatment stage consists in treating the said pulp, or at least a proportion thereof, with an aqueous solution of an organic peroxyacid without any previous chelation stage, and in then going on to the injection of oxygen.

The treatment with an organic peroxyacid according to the invention can be applied before an oxygen stage or else between two oxygen stages according to the xO or OxO or xOxO sequences, and the like. The organic peroxyacid is preferably applied according to the OxO sequence. Without wishing to be bound by any theory, the Applicant company thinks that when two successive oxygen delignification stages are carried out, the second stage is much less effective than the first one because of the large decrease in the phenolic groups of the residual lignin in the pulp after the first oxygen treatment. According to the invention the use of an organic peroxyacid appears to make it possible to reactivate the residual lignin by the introduction of new phenolic groups. A pretreatment with peracetic acid, a hydroxylating reactant, would make it possible to reintroduce such groups onto the residual lignin, thus making it more reactive towards a second oxygen delignification stage.

The quantity of oxygen which is injected in the first and second stages between which the injection of peracetic acid takes place will be preferably substantially equal. However, priority can be given to the injection of oxygen in the first stage, with a slightly smaller injection of oxygen in the second stage, or vice versa.

The organic peroxyacid is chosen from peracetic acid, performic acid, perpropionic acid, monoperoxysuccinic acid, perbenzoic acid and/or monoperoxyphthalic acid.

_L_1 9 The organic peroxyacid will be preferably distilled peracetic acid (obtained by azeotropic distillation) in solution in water or else peracetic acid at equilibrium, that is to say a mixture of peracetic acid, acetic acid and hydrogen peroxide, which may also contain a strong acid such as sulphuric acid, phosphoric acid, nitric acid or methanesulphonic acid employed as catalyst for the reaction: CHCOH H 2 O, CH 3

CO

3 H H20, with a quantity of strong acid 3 and preferably 1 by weight.

When peracetic acid at equilibrium is employed, it is necessary to employ one or more ancillary agents for protecting the viscosity of the pulp, as defined below.

On the other hand, when peracetic acid obtained by distillation is employed, it is not necessary to employ such ancillary agents for viscosity protection, although this is preferable. The ancillary agents for viscosity protection will be preferably inorganic acids of phosphorus and their salts, in particular their alkali metal salts, of the pyrophosphoric acid, sodium, potassium, etc. pyrophosphate type, and the ammonium salts. The magnesium salts, in particular magnesium sulphate heptahydrate, can also be employed advantageously as ancillary agents for viscosity protection. The mixtures of these various products can also be employed.

The treatment according to the invention applies to all the pulps based on cellulose (containing lignin), in the case of which a treatment of delignification with oxygen can be carried out.

The pretreatment with peracetic acid is preferably applied to a pulp which is delignified and washed according to the O-w-x sequence, where 0 denotes a stage of delignification with oxygen, w denotes a washing stage and x the treatment with peracetic acid.

There is therefore no stage of chelation of the pulp before the treatment with peracetic acid. The second stage of treatment with oxygen is preferably carried iii 10 out next, directly after the injection of peracetic acid, without washing between the two.

According to an alternative form of the invention one or more ancillary agents for viscosity protection or else one or more cellulose protector(s) may be added not later than during the injection of peracetic acid into the pulp. Similarly, the pretreatment with peracetic acid may be carried out under pressure of air, oxygen or of any other gas containing oxygen or an equivalent product.

The OxO process according to the invention, which makes it possible to obtain more than approximately 70 delignification of a cellulose-based pulp can be incorporated very simply into an industrial scheme. It comprises preferably the following stages: a) first stage 01 of delignification with oxygen: oxygen is injected at a pressure of 1 to bar, preferably 2 to 5 bar, into the pulp which has been taken to a temperature of between 70 0 C and 1200C, preferably between 80 0 C and 1000C. The pH of the pulp is preferably between 11 and 12 and the retention time (time of oxygen injection) between 30 and 180 min, preferably in the region of 60 minutes. At the end of this stage a washing of the pulp is performed, which brings the pH of the latter approximately between 8 and 11, more particularly between 9 and 10. It is equally possible not to perform this washing stage after the 01 stage. In this case the pH preferably remains between 10.5 and 11.5.

b) this first stage is followed by an injection of peracetic acid at equilibrium or obtained by azeotropic distillation, with incorporation of ancillary agents for pulp viscosity, as defined above, if necessary or appropriate, in the following preferred conditions: the peracetic acid charge is from 1 to kg per ton of paper pulp, expressed as dry material, and an initial pH of the pulp between 5 and 9 and preferably approximately between 7 and 8.5. The 11 addition of the sufficient quantity of peracetic acid to the pulp which has a pH after washing of between approximately 8 and 11, preferably between 9 and makes it possible in general to reach directly a pH which is suitable for this treatment with peracetic acid, that is to say a pH of between 5 and 9. In the case of the pulps which are not washed after the 01 stage it is generally also possible directly to reach a suitable pH (between 5 and 9) by the addition of peracetic acid (or another suitable organic peroxyacid). If it is wished preferably to have a pH of between 7 and 8.5, then an alkaline product must generally be added in order to adjust the pH to the intended value: soda or any other product of this type is suitable. The temperature of this treatment 'is preferably between 50 0 C and 1400C and more preferably between 60 0 C and 1100C, and the treatment time is preferably between 1 and 60 min, preferably less than min and more preferably at most 15 min. One or more ancillary agents for viscosity protection are added if necessary (in general necessary with peracetic acid at equilibrium or similar products generally unnecessary but preferable with peracetic acid obtained by azeotropic distillation), for example 0.5 by weight of MgSO 4 -7HO 2 0 and/or a similar quantity of a phosphoric product as defined above.

c) After the stage b of injection of peracetic acid, without washing, preferably, the second stage 02 of oxygen delignification is carried out in conditions which are similar to those described for stage a, it being possible for the pH to be slightly lower than 12, but preferably between 11 and 12. For this purpose an alkaline agent such as soda is added at the end of stage b and before oxygen injection, in sufficient quantity to return the pH of the pulp to a value which is preferably between 11 and 12.

Thus, according to the invention, unexpectedly, it is possible not only to avoid the chelation before the injection of peracetic acid and not to wash the i 1)__LC~~s__lli__I~-~L-II-_XX~^IIIIX I~~IIX~ 12 pulp after pretreatment with peracetic acid and before the injection of oxygen, but also to carry out this treatment with peracetic acid at a pH close to the neutral pH, preferably slightly alkaline (between 5 and According to another alternative form of the invention the peracetic acid employed consists of peracetic acid obtained by azeotropic distillation, containing from 5 by weight to 55 by weight of peracetic acid.

According to yet another alternative form of the invention the peracetic acid employed consists of an equilibrated solution containing from 10 to 40 by weight of peracetic acid and from 1 to 25 by weight of hydrogen peroxide, the peracetic acid being for the purpose of pretreatment of the pulp before the following oxygen delignification stage, during which the hydrogen peroxide has a bleaching effect on the pulp.

The invention will be understood better with the aid of the following examples of embodiment, which are given without any limitation being implied, together with the figures which show: Figure 1, an illustration of the OwQw(xO,)w process, Figure 2, an illustration of the O 1 w(xO,)w process according to the invention, Figure 3 illustrates an alternative form of Figure 2.

An O 1 wQw(x0 2 )w sequence is shown diagrammatically in Figure 1, in which the pulp undergoes a chelation sequence Q according to the prior art (although the complete sequence does not form part of the prior art). Oxygen is injected at point 1 of the circuit for treatment of a paper pulp containing lignin, and not yet delignified. The reactor 2 allows this first stage 01 of delignification with oxygen of the pulp which is next taken out into the pipework 3 and conveyed into a washer 4, from which the a~_ 13 washing effluent 5 (which is generally removed) is recovered. Thus washed, the pulp is conveyed into the pipework 6. It is next chelated by injection of the chelating agent 7 which reacts with the pulp in the reactor 8. The pulp recovered in the pipework 9 is next washed in the washer 10 and conveyed in the pipework 11 towards the injection of peracetic acid in the mixer 12 by reaction with the pulp in the "pretube" 13. The second stage of oxygen delignification takes place immediately by injection of oxygen into the mixer 14, by delignification reaction in the reactor 15 and then washing in the washer 16. In this sequence there is no washing between the injection of peracetic acid and the second stage of delignification with oxygen.

An ,0 1 w(xO,)w sequence not comprising, according to the invention, any chelation stage is shown diagrammatically in Figure 2. In this figure the same components as those in Figure 1 bear the same references. After washing in the washer 4, peracetic acid is injected via the mixer 12 and reacts with the pulp in the pretube 13. This stage is directly followed by the injection of oxygen into the mixer 14 and the delignification with oxygen 02 in the reactor 15. The process according to the invention thus makes it possible to eliminate the chelation and the washing which must follow, and this not only simplifies the process but considerably reduces the capital costs.

Figure 3 shows an alternative form of Figure 2, in which the same components bear the same references.

In a stage, generally of subsequent washing of the pulp (for example after a stage of bleaching of the pulp with peracetic acid) the washing effluents containing peracetic acid, generally diluted to more than 0.1 I by weight, are recovered to be reinjected at 11 at the mixer 12. If the quantity of peracetic acid originating from these effluents is not necessary, peracetic acid (pure or dilute) may be added to them in sufficient quantity to obtain the desired quantity per ton of pulp, according to the invention. It is possible, of 14 course, to recycle only a proportion of these effluents, to concentrate them if necessary, or to subject them to any suitable treatment before they are injected into the pulp.

Example 1: This example makes it possible to show, by comparison, the advantage of not providing the chelation stage before treatment with peracetic acid, according to the invention.

The table below summarizes the results which we have obtained: OwQwxw Ow wxw Whiteness iso) 49.7 49.0 Kappa number 9.1 9.1 Viscosity (dm 3 /kg) 874 843 Q: 0.2 DTPA, initial pH 6, 10 consistency, 700C, 60 min.

x: 1 dPAA, 0.5 MgSO, 7HO, initial pH consistency, 70 0 C, 60 min.

The same degree of delignification, the same whiteness and a viscosity of the same order of magnitude are observed without chelation (right-hand column).

Example 2: This example compares the effects of dilute peracetic acid and peracetic acid at equilibrium.

The operating conditions are the following: Paa: 1 PAA, 10 consistency, initial pH 5.3, 80 0

C.

02: 2 NaOH, 0.5 MgSO4-7HO, 2.5 bars 02, consistency, initial pH 11.7 to 12.1, 90°C, 60 min.

w: washing with deionized water by dilution to 3 consistency and then concentration to 35-40 consistency, corresponding to 95 effectiveness.

15 Particular conditions of the Paa stage: dPAA 1.0 ePAA 1.0

H

2 0 2 0.06 0.10 0.98 0.96 NaOH 0.18 0.93 1.07 Na 2

H

2 P20O 0.5 Time (min) 90 90 90 Initial pH 5.3 5.4 5.3 5.4 dPAA: Distilled peracetic peracetic acid containing viscosity protection.

ePAA: Equilibrated peracetic peracetic acid containing viscosity protection.

acid; dPAA': an ancillary distilled agent for acid; ePAA': equilibrated an ancillary agent for Composition of the PAA solutions PAA 6.2 34.0

H

2 0, 6.0 0.2 AcOH 12.0 TAO 4.1 7.2 AcOH:

TAO:

Acetic acid Total active oxygen The results after the second stage of delignification with oxygen are the following: -XXIIXII~~ ni-I ~--n~--~LX-ilil--ll~IC--~I~YII.-II-~IIl 16 Initial pH 12.2 11.7 12.1 11.7 12.0 Whiteness iso) 45.2 51.5 56.1 56.8 57.0 Kappa number 8.2 5.7 5.0 5.4 4.7 Viscosity (dm/kg) 826 834 863 759 832 Delignification 61.0 72.9 76.2 74.3 77.6 Viscosity loss 8.3 7.4 4.2 15.8 7.7 Characteristics of the pulp after Olw: Whiteness :40.3 iso Kappa number :10.5 Viscosity :901 dm 3 /kg The above tests show that: Virtually no viscosity is lost after the second 02 stage, despite a total delignification rate higher than 70 The lowest drop in viscosity is obtained with dPAA. The order of selectivity is the following: dPAA' dPAA ePAA ePAA The highest rate of delignification is observed in the case of ePAA'. The PAAs are classified by increasing delignifying power: ePAA' dPAA ePAA dPAA However, ePAA benefits from a higher oxidizing power than dPAA, on account of its higher total active oxygen. In fact, in the case of a PAA charge of 1 the TAO is 0.67 and 0.21 respectively for equilibrated PAA and distilled PAA.

Given this latter point, the highest whitenesses are observed in the case of ePAA.

ePAA ePAA [sic] dPAA Example 3: This example makes it possible to compare a chelated pulp and an unchelated pulp and what are the effects of the pH.

Samples of the same pulp are subjected, in the first case, to a chelation treatment followed by

~I~

17 washing after oxygen delignification and washing and before treatment with peracetic acid and, in the second case, to the same treatment but without chelation and washing. The results obtained are the following: 1. OlwQwPaaw SEQUENCE (chelated pulp) Operating conditions: Q 0.2 DTPA (as 100 pH6, 10 consistency, 70 0 C, 60 mm [sic].

Paa: 1 distilled peracetic acid (dPAA) (as 100 0.5 MgSO4-7H O, pH 3 to 6, 10 consistency, 700C, 70 0 C, 60 min.

w Washing with deionized water by dilution to 3 consistency followed by concentration of the pulp to 35-40 consistency, corresponding to 95 effectiveness.

The pH of the various samples is varied, all having been treated beforehand according to the same Olw sequence, that is to say delignification with oxygen and then washing with water.

Results obtained: Whiteness iso) 47.9 48.6 50.2 50.6 Kappa number 8.8 8.5 8.5 8.2 Viscosity (dm 3 /kg) 825 857 866 866 Delignification 58.1 59.5 59.5 61.0 before the second oxygen stage O0 Characteristics of the pulp after Olw: coniferous kraft pulp (Superbatch cooking) delignified with oxygen Whiteness :39.8 iso Kappa number :11.9 Viscosity :867 dm 3 /kg

I

18 The percentage of delignification obtained is after the first stage 01 of delignification with oxygen.

The viscosity is not altered after the treatment with distilled peracetic acid. In these operating conditions the whiteness and the delignification rate increase with the pH.

pH 5 is obtained directly and distilled peracetic acid is mixed into this pulp.

2. OlwPaaw SEQUENCE (unchelated pulp) Operating conditions: Paa (peracetic acid): 1 dPAA, 10 consistency, Initial pH: 5 to 9, 900C, 30 min.

Results obtained: Whiteness iso) 48.9 50.1 52.4 52.9 Kappa number 7.7 7.7 7.6 7.9 Viscosity (dm 3 /kg) 875 871 889 891 Delignification 63.3 63.3 63.8 62.4 (before the 2nd oxygen stage) Characteristics of the pulp after Olw (the Olw treatment is carried out in all the examples as described above): Whiteness :41.1 iso Kappa number :10.5 Viscosity :915 dm 3 /kg In these operating conditions the best compromise in terms of whiteness, delignification and viscosity is obtained with pH 8.

In addition to better results, there is another advantage in performing stage x at a slightly alkaline pH. By carrying out stage x at a pH 7, the jumps in pH when changing from 0 to x and x to 0, are thus reduced to a minimum.

EC

19 This implies: a better pH profile, smaller usage of acid or of soda for adjusting the pH, effluents of the same kind (in terms of pH).

Example 4: This example enables the effect of the charge of peracetic acid to be determined at various temperatures.

Study of the OlwPaaw sequence Study at a temperature of Operating conditions: Paa: 0.3 0.6 and 1.0 dPAA, initial pH 10 consistency, 70 0 C, 60 min.

w: washing with deionized water by dilution to 3 consistency followed by concentration to 35-40 consistency corresponding to 95 effectiveness.

Results: Whiteness iso) 45.7 47.7 49.1 Kappa number 9.3 8.6 7.8 Viscosity (dm 3 /kg) 886 881 878 Delignification 55.7 59.0 62.9 before the second oxygen stage O, Viscosity loss 3.2 3.7 Characteristics of the pulp after Olw (as described above): Whiteness :41.1 iso Kappa number :10.5 Viscosity :915 dm/kg 20 Study at a temperature of Operating conditions: Paa: 0.9 and 1.0 dPAA, 10 consistency, initial pH 5, 900C, 10 to 30 min.

w: washing with deionized water by dilution to 3 consistency followed by concentration to 35-40 consistency, corresponding to 95 effectiveness.

Results: Gain in whiteness 7.8 8.6 iso) Kappa number 7.8 7.6 Viscosity (dm 3 /kg) 842 837 Delignification 62.1 63.8 before the second oxygen stage 02 Viscosity loss 6.6 7.1 Time (min) 10 Characteristics of the pulp after Olw: Whiteness :41.2 iso Kappa number :10.5 Viscosity :901 dm 3 /kg As the results show, the application of 0.9 of dPAA allows the same gains in whiteness, the same delignification rates and the same viscosity to be attained. In addition, such a dPAA charge allows retention times,lower than 15 minutes to be attained.

Example Effect of washing between the Paa and 02 stages 1. COMPARISON OF THE O1wPaawO2 and Olw(PaaO2)w

SEQUENCES

Operating conditions: Paa: 0.9 dPAA, 10 consistency, initial pH 5, 90 0 C, 10 min.

21 02: 2.0 NaOH, 0.5 MgSO-7HO, 2.5 bars, consistency, initial pH 11.5 to 12.1, 90 0 C, 60 min.

w: washing with deionized water by dilution to 3 consistency followed by concentration to 35-40 consistency, corresponding to 95 effectiveness.

Results: Whiteness iso) 45.2 54.8 54.4 Kappa number 8.2 5.3 5.4 Viscosity (dm 3 /kg) 826 805 778 Delignification 61.0 74.8 74.3 Viscosity loss 8.3 10.6 13.6 Characteristics of the pulp after Olw: Whiteness :41.2 iso Kappa number :10.5 Viscosity :901 dm 3 /kg The elimination of the intermediate washing between the PAA and 02 stages allows the same levels of whiteness and of delignification to be attained.

Example 6: Delignification of a coniferous kraft pulp according to the sequences OlwQwePaawO2w Olw (dPaaO02)w and Olw(Paae+02)w 1. Operating conditions: Q: 0.2 DTPA (as 100 10 consistency, initial pH 6, 900C, 30 min.

dPaa': 0.9 dPaa, 0.5 Na 2

H

2

P

2 10 consistency, initial pH 8.0, 900C, 10 min.

ePaa+: 0.9 ePaa, 0.5 Na 2

H

2

P

2 0 7 10 consistency, initial pH 8.1, 90 0 C, 10 min.

ePaa: 0.9 ePAA, 10 consistency, initial pH 5.4, 90 0 C, 10 min _I~_i 22 02: 2.5 NaOH, 0.5 MgSO4-7H,O, 2.5 bar 0,, consistency, initial pH 11.5 to 12.0, 90 0

C,

min.

w: washing with deionized water by dilution to 3 consistency followed by reconcentration to consistency, corresponding to 95 effectiveness.

2. Results: The results are given after the 02 stage Sequence OlwO2w OlwQwePaawO2w Olw(dPaa0O2)w Olw(ePaa'02)w Initial pH of the 5.4 8.0 8.1 Paa stage Initial pH of the 12.0 11.9 11.5 11.5 02 stage Whiteness 44.7 56.0 57.9 61.9 iso) Kappa number 8.2 5.8 5.8 5.3 Viscosity 813 844 839 835 (dm 3 /kg) Delignification 61.0 72.4 72.4 74.8 Viscosity loss 7.8 4.3 4.9 5.3 Total delignification obtained after the OxO sequence according to the invention.

Initial characteristics of the pulp: Coniferous kraft pulp predelignified with oxygen originating from a Superbatch cooking (kappa number after cooking: 21) Whiteness iso): 40.6 Kappa number: 10.5 Viscosity (dm/kg): 883 Example 7: Delignification of a coniferous kraft pulp according to the 01w(dPaaO2)w sequence: effect of the pH during the Paa stage 1. Operating conditions: dPaa': 0.9 dPAA, 0.5 NaHPO,, 10 consistency, initial pH 5.3 and 8.0, 900C, 10 min.

23 02: 2.5 NaOH, 0.5 MgSO4-7HO, 2.5 bar 02, consistency, initial pH 11.5 to 12.0, 900C, min.

w: washing with deionized water by dilution to 3 consistency followed by reconcentration to consistency, corresponding to 95 effectiveness.

2. Results: The results are given after the 02 stage Sequence Olw02w 01w(dPaa*02)w 01w(dPaa'02)w Initial pH of the 5.3 Paa stage Initial pH of the 02 12.0 11.5 11.6 stage Whiteness iso) 33.6 42.8 46.0 Kappa number 13.7 10.8 10.6 Viscosity (dm/kg) 845 850 826 Delignification 63.0 70.8 71.4 Viscosity loss 9.3 8.8 11.4 Total delignification obtained after the OxO sequence Initial characteristics of the pulp after the Olw stage: Coniferous kraft pulp originating from a conventional cooking (kappa number after cooking: 37), then predelignified with oxygen (Olw) Whiteness iso): 31.7 Kappa number: 18.5 Viscosity (dm/kg): 932 Example 8: Delignification of a coniferous kraft pulp according to the sequences Olw dPaa02w and 01w(dPaaO2)w: effect of the washing between the Paa and 02 stages.

1. Operating conditions: Paad': 0.9 dPAA, 0.5 Na 2

,HP

2 0 7 10 consistency, initial pH 8.0, 90 0 C, 10 min.

I_ i ji~i __LIX~_ 24 02: 2.5 NaOH, 0.5 MgSO 4 -7HO, 2.5 bar 02, consistency, initial pH 11.5 to 12.0, 90 0

C,

min.

w: washing with deionized water by dilution to 3 consistency followed by reconcentration to 35-40 consistency, corresponding to 95 effectiveness.

2. Results: The results are given after the 02 stage Sequence OlwO2w Olw dPaa02w 01w(dPaa'O2) w Initial pH of the 8.0 Paa stage Initial pH of the 12.0 11.9 11.5 02 stage Whiteness iso) 44.7 57.7 57.9 Kappa number 8.2 5.6 5.8 Viscosity (dm/kg) 813 823 839 Delignification 61.0 73.3 72.4 Viscosity loss 7.8 6.7 4.9 Total delignification obtained after the Ox0 sequence according to the invention.

Initial characteristics of the pulp after the Olw stage: Coniferous kraft pulp originating from a Superbatch cooking (kappa number after cooking: 21) and then predelignified with oxygen.

Whiteness iso): 40.6 Kappa number: 10.5 Viscosity (dm 3 /kg) 882

Claims (8)

1. 2. Process according to Claim 1, characterized in that the pretreatment stage is carried out on a pulp which has previously undergone a treatment of delignification with oxygen.
2. 3. Process according to Claim i, characterized in that the organic peroxyacid is either peracetic acid obtained by azeotropic distillation or peracetic acid at equilibrium.
3. 4. Process according to either of Claims 1 and 2, characterized in that the pulp is not washed after the pretreatment with peracetic acid and before the treatment with oxygen. Process according to one of Claims 1 to 4, characterized in that the peracetic acid employed consists of peracetic acid obtained by azeotropic distillation, containing from 5 by weight to 55 by weight of peracetic acid.
4. 6. Process according to one of Claims 1 to 4, characterized in that the peracetic acid employed consists of an equilibrated solution containing from to 40 by weight of peracetic acid and from 1% to 25 by weight of hydrogen peroxide, the peracetic acid being for the purpose of pretreatment of the pulp before the following stage of delignification with 26 oxygen, during which the hydrogen peroxide has a bleaching effect on the pulp.
5. 7. Process according to one of Claims 1 to 6, characterized in that the pretreatment with the organic peroxyacid is performed at a pH of between 5 and 9, preferably between 7 and
6. 8. Process according to one of Claims 2 to 7, characterized in that the quantity of peracetic acid is approximately between 1 and 10 kg per ton of paper, expressed as dry material.
7. 9. Process according to Claims 1 to 8, character- ized in that the quantity of organic peroxyacid which is added is sufficient directly to reach the pH necessary for the pretreatment of the pulp using the said organic peroxyacid. Process according to one of Claims 1 to 9, in which peracetic acid is employed, characterized in that at least one ancillary agent for viscosity protection, preferably chosen from the inorganic acids of phosphorus and their alkali metal salts, and magnesium salts, separately or as mixtures, are thus injected into the pulp.
8. 11. Process according to one of Claims 1 to characterized in that the peracetic acid employed originates from the effluents from a stage situated downstream of the stage of injection of the organic peroxyacid.