Peroxy compounds, such as hydrogen peroxide, the derivatives of hydrogen peroxide or peroxy acids, are effective oxidants. They can be used for various purification or bleaching purposes. When such compounds are used, there may arise the problem of their deficient stability. In many cases, detrimental compounds which decompose these compounds pass into the process through chemicals, the material being treated or process waters. These detrimental substances may be either organic or inorganic. The most detrimental substances are heavy metals, such as manganese, iron and copper, which catalyze the decomposition of peroxy compounds. Also, for example, chromium has a detrimental effect. Heavy metals are capable not only of decomposing peroxy compounds, whereupon the consumption of these compounds respectively increases, but also of otherwise damaging the material being treated.

The detrimental effects of metal ions can be decreased by binding them with a complexing agent. The complexing agents used include in particular chelating agents, such as polyaminopolycarboxylic acids. These include in particular ethylenediaminetetraacetic acid (EDTA) and the salts thereof and diethylenetri- aminepentaacetic acid (DTPA) and the salts thereof. Also very effective are certain phosphonic acids, among which the phosphonic acids corresponding to EDTA and DTPA, ethylenediaminetetra (methylenephosphonic acid) (EDTMPA) and the salts thereof and diethylenetriaminepenta (methylenephosphonic acid) (DTPMPA) and the salts thereof are commonly used.

The bleaching agents often used in laundry detergents are perborates, sodium carbonate peroxyhydrate or corresponding solid sources of peroxide together with a suitable activator. The activator used is, for example, tetraacetylethylenediamine (TAED). In this case hydrogen peroxide and peroxy acids are formed in the washing solution. Complexing agents, such as EDTA or phosphonates, are commonly used in laundry detergents to bind detrimental heavy metals and calcium, to stabilize the

washing solution and to provide other effects enhancing the wash. Perborates and sodium carbonate peroxyhydrate with or without an activator have begun to be used also in dishwasher detergents.

Peroxy compounds are commonly used also for the treatment of wood pulps, in particular as bleaching chemicals. Also in these processes, in particular heavy metals are detrimental in decomposing peroxy compounds, and the effect of heavy metals should in general be inhibited. For example, patent application FI-A-942968 describes transition-metal activated bleaching of wood pulp with peroxy compounds in acid conditions. In the method, DTPA, EDTA or DTMPA is used for chelating the heavy metals.

However, even chelates may catalyze the decomposition of peroxy compounds. For example, it is noted in the article Z. Yuan et al.,"The role of transition metal ions during peracetic acid bleaching of chemical pulps,"Pulp & Paper Canada 98: 11 (1997), pp. 24-29, that the manganese complex of DTPA catalyzes the decomposition of both peracetic acid and hydrogen peroxide. The corresponding Mn complex of phosphonate, DTPMPA, catalyzes the decomposition of peracetic acid.

Today the most commonly used complexing agents are either completely non- biodegradable, such as EDTA and the above-mentioned phosphonates, or poorly biodegradable, such as DTPA. Another question of environmental protection is the nitrogen and phosphorus contents of the said compounds. In general it has always been assumed that in the presence of transition metal (heavy metal) ions there is needed a chelating agent which has at least one nitrogen atom. For example, a good result in bleaching wood pulp has so far not been achieved with nitrogen-free complexing agents, at least when they have been used alone.

Description of the invention Now the stabilizing method according to Claim 1 has been invented. Certain preferred applications of the invention are disclosed in the other claims.

According to the invention, a compound according to the following formula is used for the stabilizing of peroxy compounds

where R = H or (CH2)"COOA, where n is 1-3, and A = H, an alkali metal ion such as an Na or K ion, or an equivalent of an earth alkali metal ion such as an Mg or Ca ion.

Of the compounds according to the formula there should be mentioned in particular carboxymethylsuccinic acid (CMOS), in which case R is H, and oxydisuccinic acid (ODS), in which case R is CH2COOH, and the salts thereof.

Stabilization can be carried our by adding to the peroxy compound, or to the mixture containing it, or to the material being treated, a stabilizing agent according to the invention. The stabilizing agent can be added to the mixture also before the peroxy compound.

The suitable amount of stabilizing agent may be, for example, 0.001-50%, such as 0.01-5%, of the weight of the peroxy compound.

It is a particular advantage of the invention that the stabilization can be carried out using a nitrogen-free compound. Also, the stabilizing compound does not contain phosphorus. The method is thus very advantageous in terms of environmental protection. Nevertheless, the stabilization result is very good.

The peroxy compound may be, for example, hydrogen peroxide, a peroxy acid such as peracetic acid, or a peroxy acid such as sodium borate peroxyhydrate or sodium carbonate peroxyhydrate formed with the aid of an activator from hydrogen peroxide or from a hydrogen peroxide derivative such as sodium perborate or sodium carbonate peroxyhydrate.

Stabilization according to the invention may be used, for example, in various purification or bleaching processes or in the preparation and storage of peroxy compounds.

When a stabilizing compound is added to the process, it can be added simultaneously or almost simultaneously with the peroxy compound.

The reason why the said agents work is not known. In any case, it most likely must be that the transition metal complexes of the agents according to the invention do

not catalyze the decomposition of peroxy compounds in spite of being weaker complexing agents for heavy metals than are (poly) aminopolycarboxylates, for example EDTA and DTPA, and the corresponding phosphonates, such as EDMPA and DTMPA. On the other hand, they may activate radicals formed from peroxy compounds.

CMOS, for example, can be prepared through the addition reaction of maleic acid and glycolic acid. The addition reaction occurs in alkaline conditions, and, for example, lanthanides can be used as catalysts in it. ODS can be prepared through the addition reaction of malic acid with maleic acid. Also other compounds according to the formula can be prepared in a corresponding manner, starting from maleic acid or its derivatives. CMOS and ODS are also in the EINECS register, and the preparation of the former has been described in, for example, the article J. van Westrenen et al., Inorganica Chimica Acta 1991, pp. 233-243. The product is easily biodegradable.

One use of the invention consists of various washing processes and the detergent compositions which are used in them and in which peroxy compounds are used. The stabilizing agents according to the invention can be added to the detergent composition or they may be added separately in connection with the washing process itself.

When peroxy compounds, such as perborates or sodium carbonate peroxyhydrate, are used in detergents, for example together with TAED, peracetic acid forms in addition to hydrogen peroxide in the washing solution. It has now been observed, unexpectedly, that, when stabilizing agents according to the invention are used, it is possible to achieve higher peracetic acid contents and to produce solutions more stable as regards the peroxy compounds than when conventional chelating agents, for example EDTA, are used.

The above-mentioned stabilizing agents can be used advantageously also in other detergents in which the activator is not TAED but some other substance which also forms peracetic acid or some other peroxy acid. An example of the latter is iso- nonylbenzene sulfonate (iso-NOBS), which is used as a commercial activator.

When there are both a peroxy acid and hydrogen peroxide present in washing solutions, and then that portion of the hydrogen peroxide which has not been converted to peroxy acid remains stable for a long time, it is clear that the stabilizing agents concerned can also be used in detergents which contain hydrogen

peroxide. The tendency of hydrogen peroxide to decompose increases as the pH rises. For this reason the increasing effect on the stability of hydrogen peroxide is most prominent at a pH above 7. When present-day laundry detergent powders are used, they usually produce a pH which is close to 11.5, although today there is seen a trend towards achieving lower washing pH values.

Stabilizing agents according to the invention can be used advantageously also in diswasher detergents in which peroxy compounds, such as perborates and sodium carbonate peroxyhydrates, are used. In this case an improved stability of the bleaching agent is achieved in addition to the capacity of binding calcium.

Stabilizing agents according to the invention can also be incorporated as stabilizing agents in perborates and sodium carbonate peroxyhydrate. The latter product is most commonly coated in order to improve shelf life, and the stabilizing agents according to the invention can be incorporated into the product itself and/or into the coating.

Another use of the invention is the treatment of wood pulp with a peroxy compound. The heavy metals which hamper the bleaching and delignification of pulps can be rendered ineffective by using a separate treatment or by adding the stabilizing agent to the bleaching solution.

In the peroxy bleaching of a chemical pulp, the binding of all metals cannot usually be carried out in alkaline conditions, because then, for example, iron precipitates in the form of hydroxides, oxides and oxyhydroxides, which strongly catalyze the decomposition of peroxy compounds. For this reason the stabilization preceding peroxy bleaching, for example hydrogen peroxide bleaching, is carried out in acid conditions. In this case, for example, the removal of manganese is important.

Manganese can best be removed at a pH of 4.5-6.5.

In modem methods of bleaching chemical pulps the bleaching is preceded by oxygen delignification. In this process there have been added to the pulp magnesium ions the continued presence of which in the pulp is important when the bleaching is carried out using peroxy compounds, for example, hydrogen peroxide. The best Mg/Mn ratio is achieved at a pH of 4.5-5. For this reason, the stabilization is best carried out at a pH clearly below 6, for example close to pH 5.5, for example at a pH of 5.3.

At the beginning of the bleaching sequence the purpose is to remove lignin, and the actual bleaching is not carried out until towards the end of the bleaching. Since nowadays lignin is more and more removed during the cooking and the subsequent

oxygen delignification, the difference between the concepts has dimmed. However, if the kappa numbers which describe lignin are after oxygen delignification above 10, or even only 10-6, the first bleaching step is in fact a mere delignification step.

Delignification can be carried out quite selectively by using peroxy acids, for example peracetic acid, in which case the delignification is carried out at a pH close to 5. With all delignification or bleaching processes carried out using peroxy compounds it is possible to combine stabilization according to the present invention in order to eliminate the detrimental effects of heavy metals. In this case it is advantageous that the stabilizing agent can be added, for example, directly to that step, whereby simpler apparatus systems can be arrived at, and also the retention of the pulp in the bleaching process can be sped up. In this manner the economy of the process can be further improved.

If peroxy acids are used towards the end of the bleaching sequence, such steps are most commonly carried out at a higher pH than the delignification steps, because the bleaching properties of peroxy acids, for example peracetic acids, are better at an elevated pH, for example a pH of 7-9.

Stabilizing agents according to the present invention can also be used in the bleaching of mechanical pulps. It is possible to use in the bleaching, for example, a peracetic acid-hydrogen peroxide sequence (PAA-P), in which case the bleaching is carried out in the PAA step at a pH close to neutral. The use of peracetic acid introduces the advantage that sodium silicate, which is normally required in the peroxide bleaching of mechanical pulp, need not be used. The substances according to the invention can be used in all such sequences as utilize peroxy compounds, and most preferably so that a separate stabilizing step need not be carried out, the stabilizing agent being added in connection with the bleaching.

One further use of the invention is the stabilization of peroxy compounds or of mixtures containing them during their preparation or storage.

The invention is illustrated below with examples which do not, however, limit the scope of the invention.

Example 1 In order to test the stability of the peracetic acid (PAA) formed from TAED, experiments were performed without stabilizing agents and with ODS. In order to find out the effect of metals, manganese and iron ions were added to the solution so that the concentration of each was 0.2 mg/l. Since the pH has an essential effect on

the results, an experiment was also carried out at a somewhat higher pH. In the experiments a solution was used to which there had been added 1 g of Na2C03/1, the metals, and 140 mg/1 (calculated as a 100% acid) of a stabilizing agent in acid form.

The solution was heated to 50 °C, and the hydrogen peroxide and pure, uncoated TAED were added so that their concentrations were 3 and 2 g/1. TAED was thus used in the experiments below the equivalence ratio of hydrogen peroxide, taking into account that TAED is capable of binding 2 molecules of hydrogen peroxide.

The amounts of hydrogen peroxide and peracetic acid were determined by standard methods; first hydrogen peroxide on ice by using a cerium sulfate solution, and then peracetic acid iodometrically.

Table 1 Stabilizing Time pH H202 PAA H202 % of PAA % of agent/target min g/l g/l original theoretical None 0 7.8 5 7.1 2.335 0.296 82.2 22.2 10 6.8 2.138 0.272 75.4 20.4 15 6.6 2.092 0.257 73.6 19.2 20 6.5 2.046 0.220 71.5 16.5 25 6.4 1.994 0.212 69.6 15.8 30 6.3 1.939 0.200 67.6 15.0 Yes 0 7.8 5 7.0 2.598 0.604 95.6 45.3 10 6.6 2.414 0.587 89.2 44.0 15 6.5 2.329 0.554 85.9 41.6 20 6.4 2.269 0.538 83.7 40.4 25 6.2 2.252 0.539 83.1 40.4 30 6.2 2.186 0.440 79.4 33.0 Yes 0 8.0 5 7.5 2.309 0.664 86.9 49.8 10 7.2 2.176 0.921 86.2 69.1 15 7.1 2.069 0.952 83.2 71.4 20 7.1 2.094 1.076 85.9 80.7 25 7.0 1.981 0.958 80.3 71.9 30 7.0 1.981 0.936 80.0 70.2

The experiments with ODS were carried out using solutions of different concentrations, their total acid concentrations being different. The pH differences in the experiments are due to this.

It can be observed from the table, first, that more peracetic acid forms when ODS is used. Second, it can be seen that a pH rise increases the amount of peracetic acid formed. It can also be observed that the use of ODS improves the total amount of peroxy compounds (calculated as hydrogen peroxide in the table).

Example 2 Next, a new experiment was performed in which the washing conditions at a high pH, approx. 10.5, were simulated. The water used was hardened with 25 g/1 of CaCl2 to a hardness of 12°dH. There were additionally added to the water manganese 0.0045 mg/1, iron 0.08 mg/l, and copper 0.002 g/1, in order that the conditions should correspond to those prevailing when tap water is used in washing.

The experiment was carried out in the same manner as the preceding experiment, but the amount of hydrogen peroxide was 0.66 g/1 and the amount of TAED was 0.6 g/1. Furthermore, 2 g/1 of sodium carbonate was added to the solution. The amount of stabilizing agent was 140 mg/1 in all of the experiments. The experiment was performed at 50 °C.

Table 2 Stabilizing Time pH H202 PAA H202 % of PAA % of agent/target min g/l g/l original theoretical None 0 10.7 5 10.5 0.343 0.212 66.4 53.1 10 10.5 0.303 0.122 54.2 30.4 15 10.5 0.253 0.0578 42.2 14.4 EDTA 0 10.7 5 10.5 0.349 0.182 65.2 45.6 10 10.4 0.239 0.106 43.5 26.6 15 10.4 0.234 0.0732 40.4 18.3 CMOS 0 10.7 5 10.5 0.380 0.227 73.0 56.9 10 10.4 0.295 0.165 55.9 41.3 15 10.5 0.260 0.0816 44.9 20.4

It can be seen from the results that in the system concerned the peroxy compounds are more unstable than in the conditions of the previous experiment. However, CMOS is a better stabilizer than EDTA. A comparison between those experiments of Tables 1 and 2 in which stabilizing agents were not used shows, however, that more peracetic acid is formed at a higher pH, but the system is much more unstable with respect to per-compounds. One partial reason is, at least, that hydrogen peroxide decomposes more rapidly at a higher pH. However, the results show that the use of CMOS improves the yield of peracetic acid and does not detract from the stability of the bleaching solution; on the contrary, it somewhat improves the overall result.

Example 3 In order to find out how the type of the metal ions and, on the other hand, the pH, which was between the pH values used in Examples 1 and 2, affect the results, experiments were performed using different metal ion systems and at different pH values. The experiments were performed by using ODS as a stabilizing agent as in experiment (Series I). In the second series there were used hardened water and metal ion amounts as in Example 2 (Series II). pH control was carried out by adding the same amount of sodium carbonate in the experiments of Series I and the same amount in the experiments of Series II. (Note: not the same amount in Series I and II).

Table 3 Stabilizing Time pH H202 PAA H202 % of PAA % of agent/target min g/l g/l original theoretical None, Series I 0 9.8 5 8.0 2.128 0.200 73.9 15.0 10 7.5 1.990 0.342 71.4 25.6 ODS, Series I 0 9.7 5 8.1 2.374 0.452 85.9 33.9 10 7.5 2.105 0.649 79.8 48.7 None, Series II 0 9.7 5 7.5 2.206 0.191 76.4 14.3 10 7.2 2.112 0.207 73.5 15.6 ODS, Series II 0 9.1 5 7.5 2.469 0.312 87.0 23.4 10 7.2 2.291 0.409 82.4 30.7 CMOS, Series 0 9.9 II 10 7.6 2.214 0.253 77.6 19.0 15 7.3 2.137 0.282 75.4 21.2

It can be seen that, especially in a system which simulates the water used in the washing of laundry and which contains small amounts of iron, manganese and copper, a very clear improvement is achieved when ODS is used as a stabilizing agent and a good result is also achieved when CMOS is used.

Example 4 In the washing of laundry the pH values are in general higher than those shown in Table 3. For this reason, experiments were carried out according to Series II as in Example 3, but the amount of sodium carbonate was increased.

Table 4 Stabilizing Time pH H202 PAA H202 % PAA% of agent/target min g/l g/l of original theoretical None, Series II 0 10.5 5 10.2 1.783 0.177 62.1 13.3 10 10.2 1.222 0.0562 41.6 4.2 ODS, Series II 0 10.5 5 10.2 1.976 0.440 72.4 33.0 10 10.1 1.560 0.151 54.3 11.3 CMOS, Series II 0 10.4 5 10.2 2.131 0.271 75.1 20.4 10 10.2 1.970 0.319 70.4 23.9

Since the conditions in the experiments (Examples 1-4) described above largely correspond to the conditions produced when laundry detergent compositions which contain NKPH and an activator are used, it is clear that the said chelating agents can be used in NKPH or together with it.

Example 5 In order to find out the effect of the stabilizing agents when NKPH is used, the stabilizing agents were experimented with in the same manner as in Expriment 2, but instead of hydrogen peroxide there were used 2.21 g of crystalline NKPH and sodium sulfate coated NKPH. Sodium carbonate was not added in the experiments, and the stabilizing agent amount was 200 mg/l. The original active oxygen concentrations in these NKPH products were 14.4% and 13.7%. Thus the amounts of hydrogen peroxide forming in the solution would theoretically be 0.68 and 0.64 g/1.

Table 5 NKPH/stabilizing Time pH H202 PAA H202 % PAA % of agent/target min g/l g/l of original theoretical Crystalline/none 0 10.6 5 10.4 0.510 0.160 85.9 58.5 15 10.3 0.399 0.0550 62.5 20.1 Crystalline/ODS 0 10.6 5 10.5 0.557 0.202 95.6 73.8 15 10.4 0.456 0.0874 73.2 32.0 Coated/none 0 10.7 5 10.5 0.350 0.199 68.3 73.1 15 10.3 0.312 0.0561 52.4 20.5 Coated/ODS 0 10.6 5 10.3 0.419 0.221 80.5 80.8 15 10.3 0.325 0.0850 56.5 31.1

It can be seen that both the amount of the formed peracetic acid and the total amount of peroxy compounds, calculated as hydrogen peroxide, were higher when ODS was used than when it was not used.

Stabilizing agents such as ODS and CMOS can advantageously also be incorporated into NKPH and/or into its coating, in which case a stabilizing agent need not be used separately.

Example 6 Previously there has been noted the good effect of sodium sulfate in the stabilizing of NKPH if NKPH is tested together with zeolite used in detergents, in warm and moist conditions aimed at simulating the decomposition of NKPH in a detergent in situations in which the detergent comes into contact with warm and moist air. In such tests, NKPH, which is in itself quite stable, decomposes very rapidly, as has been shown in, for example, the applicant's patent application publication WO 98/32831 (NAS-PVP coating).

In the following experiment, the effects of CMPS and ODS were experimented with in such a simulated test.

The coating of NKPH was carried out using an Aeromatic Strea 1 coating apparatus.

400 g of unscreened NKPH granules prepared in a fluidized-bed granulator were used as the raw material. The active oxygen content in the NKPH used as the initial substance was 14.9%. Sodium sulfate was dissolved in water to form an almost saturated solution. When all of the sodium sulfate had dissolved, a stabilizing agent, CMOS or ODS, was added so that its amount was 300 ppm (calculated as acid) of the amount of the sodium sulfate (NAS), and the temperature was raised to 60 °C.

Thereafter the solution was pumped at this temperature to the coating apparatus.

The fluidization temperature was approx. 50-55 °C during the coating, which was continued until the amount of the coating was 10% by weight. The fluidization was continued after the coating until the moisture percentage was below 0.5%. A maximum of 30 minutes is required for this. The active oxygen content was 13.6%.

Stability tests were carried out by mixing first the prepared samples with zeolite 4A at a weight ratio of 50: 50. Approx. 5 g of the sample prepared in the said manner was placed in an open, flat-bottomed vessel with sides. The vessel was placed in a climate chamber in which the temperature was 30 °C and the relative humidity 70%.

The active oxygen content was determined from the samples by titrating the hydrogen peroxide with a potassium permanganate solution. The reference sample was a coated NKPH, prepared earlier in the same manner, in which the coating contained no CMOS or ODS.

Table 6 Sample Decomposition % Decomposition % 1 week 2 weeks NKPH/NAS 9.5 13.3 NKPH/NAS/CMOS 6.4 15.3 NKPH/NAS/ODS 8. 8 15.2 1 It can be seen that the stabilizing agents do not decrease"external"stability obtained by means of only sodium sulfate.

Example 7 Another method of measuring stability is so-called TAM (thermal activity measurement), in which the heat generation produced is measured in controlled conditions. Such a measuring method has been described in patent publication WO 95/15292, p. 16. An isothermal microcalorimeter is used in the tests. From the test

results it can be concluded whether the product can be stored and transported in bulk. In many tests it has often been observed that, even if the external stability is good, this result obtained by TAM is poor. It has not been possible to explain the reason for this difference. It has been assumed that the result obtained using TAM described the internal stability of NKPH, whereas the tests performed in a weather cabinet with mixtures of NKPH and zeolite rather describe the external stability in open storage of the detergent.

The following test was performed in order to determine the effect of CMOS and ODS on results obtained using TAM when they were added to NAS coating solutions.

The tests were performed using a Thermal Activity Monitor type 2277 (Thermometric AB, Järfälla, Sweden), which had been equipped with four calorimeter units and a double vial system. The sample was weighed into the vial to be tested and was closed hermetically. The measurements were carried out at 40 °C, at which humidity corresponding to the critical humidity formed in the vial. The samples containing CMOS and ODS were the same as in the previous example.

Table 7 Time/Sample/NKPH/NAS/CMOS NKPH/NAS/ODS heat generation mW/kg 3 h 1.62 3.37 6 h 2.32 4.93 9 h 2.57 5.48 12 h 2.66 5.89 15 h 2.72 6.28 18 h 2.72 6.49 21 h 2.63 6.59 24 h 2. 82 6.78 It has been observed in practice that the generation of heat should be below 10 mW/kg in a test of a long duration, 24 h, in order that NKPH could be stored and transported in bulk. The results show that in particular CMOS, but also ODS, causes very low generation of heat. The generation of heat reached using CMOS is exceptionally low. NKPHs coated with conventional NAS-coating usually cause a generation of heat between 10 and 15 mW/kg.

The test results presented above show that by using CMOS or ODS, both a good external stability and a good internal stability are achieved.

ODS and CMOS thus promote the formation of peracetic acid and the stability of peroxy compounds in solutions. The above tests show that it is preferable to add the CMOS or ODS already to the coating, in which case a separate adding to the bleaching solution or the detergent is not required. In this case the detergent manufacturer need not mix, for example CMOS or ODS in connection with the detergent manufacture; instead, the manufacturer can purchase stabilized NKPH, which withstands transport and provides the same stabilizing properties as when the said substances are added during the detergent manufacture or during the actual wash. The amounts of the substances added must, however, be sufficient to provide optimal peroxy compound generation and/or stabilization.

CMOS and ODS can also be added during the preparation step of NKPH and/or during the coating of NKPH.

Since in the detergent application of hydrogen peroxide the value of the dissociation constant, pKA, is 11.5 and that of peracetic acid is 8.2, the most optimal pH value in terms of the stability of the bleaching solution and the formation of peracetic acid is probably somewhere between these pH values. In the washing of laundry the bleaching result depends on many factors, such as the type of the stains, the washing temperature, and the time. For this reason the most optimal range is difficult to determine. However, since the stabilizing agents according to the invention, when used, work within the entire washing range, from pH 7 to close to pH 11, in the light of the results it can be noted that they provide advantages in different washing solutions.

The chelating agents according to the invention can also be used in solid washing agents which contain as the bleaching agent perborate or NKPH. Since the latter bleaching agent is usually coated, the chelating agent can be incorporated either into the NKPH itself and/or into its coating.

The stabilizing agents according to the invention can also be used advantageously for improving the stability of the solutions in cleaning agents and disinfectants which contain peracetic acid and/or hydrogen peroxide.

Example 8 An oxygen-delignified softwood pulp which had been cooked in a Superbatch (Xt digester and which had a pulp kappa number of 3.6, a viscosity of 718 dm3/kg and a whiteness of 64.4% ISO was delignified and bleached using a sequence of PAA/Q- P, where PAA/Q means that the chelating agent was added in connection with a peracetic acid bleach and where the dash means that between the steps there was a conventional wash. The conditions in the peracetic acid step were the following in all of the experiments: time 120 minutes, temperature 70 °C, consistency 10% and initial pH 5,5, and the dose of peracetic acid (distilled PAA) was 12 kg/tp (calculated as 100% acid). The corresponding figures in the peroxide step were: 180 minutes, 90 °C, 10% and 10.4, and the hydrogen peroxide dose 20 kg/tp and alkali dosing 10 kg NaOH/tp. The other reaction conditions and results are given in the accompanying table.

Table 8 Step/target PAA/Q PAA/Q PAA/Q PAA/Q PAA/Q Chelating agent None DTPA EDTA CMOS ODS Dose,kg/tp-2 2 2 2 Residual PAA, kg/tp 3.7 0. 6 1. 0 3. 7 3.4 Kappa 2.4 3. 1 2. 8 2. 5 3.0 A-Kappa 1.2 0. 5 0. 8 1. 1 0.6 PAA-consumed, kg/kappa 6.9 22. 8 13. 8 7. 5 14.3 Viskosity, dm3/kg 707 681 680 707 718 A-Viskisity/A-kappa 9.2 74. 0 47. 5 10. 0 0 Whiteness, % ISO 74. 7 72. 1 73. 0 74. 7 74.0 Step/target P P P P P Residual H202, kg/tp 5.0 9. 1 10. 9 14. 6 14.8 H202 consumed, kg/tp 15.0 10. 9 9. 1 5. 4 5.2 Kappa 1.7 1. 9 1. 8 1. 5 1.4 A-Kappa* 1. 91. 71. 82. 12.2 Viskosity, dm3/kg 517 570 582 654 637 A-Viskisity/A-kappa * 118. 2 77. 9 75. 6 42. 7 36.8 Whiteness, % ISO 86.8 84. 6 84. 8 85. 9 85.4 Increase in whiteness, 22.4 20. 2 20. 4 21. 5 21.0 % ISO ** Increase in whiteness, 1.49 1. 85 2. 24 3. 98 4.04 % ISO/kgH202

* total decrease in kappa in both the PAA and P steps ** total increase in whiteness in both the PAA and P steps It can be seen from the results that without a stabilizing agent a good whiteness is indeed achieved in the PAA step, but the viscosity is far too low for the pulp strength to be sufficient. The reason for the viscosity decrease is the lack of stabilizing agent. On the other hand, EDTA and DTPA are good complexers of transition metals, but since the Mn complexes of these substances decompose peracetic acid, the use of these substances is even detrimental. When CMOS is

used, a good whiteness and the highest viscosity are obtained. Furthermore, the consumptions of peracetic acid and hydrogen peroxide are the most advantageous.

The stabilizing agents according to the invention can also be used after the PAA/Q step in a separate stabilizing step in order to enhance the peroxide bleaching. Such a separate treatment may also be replaced by using the stabilizing agents according to the invention directly in the peroxide step.