FR2650605A1 - PROCESS FOR REDUCING CHLORINE COMPOUNDS PRODUCED DURING BLEACHING BY CHLORINATION OF PAPER PULP - Google Patents

Abstract

A chlorination stage bleaching method is disclosed which comprises suppressing formation of polychlorinated dioxins and furans without resulting in cellulose degradation and poor pulp viscosity by controlling the molecular chlorine:hypochlorous acid ratio by adding the chlorine to the pulp in multiple charges and by adding an alkaline material to the pulp during the chlorination stage such that chlorination stage pH is controlled to a value from above about 1.8 to below about 3.5.

Description

Process for the reduction of chlorine compounds produced during bleaching by chlorination of paper pulp
The present invention relates to a process for reducing the formation of polychlorinated dioxins and furans in chlorine bleaching which is typically part of the chemical delignification and fiber purification process from small wood chips. Of.

more particularly, this invention relates to the bleaching of the pulp with sequential addition of chlorine and at a controlled pH in the chlorination stage, which reduces the production of chlorinated dioxins and furans during the chlorination and alkalinization stages, without sacrificing the delignification of the dough or the viscosity of the dough in the absence of cellulose protectors or other additives.

Concerns have recently been expressed regarding the environmental effects of chlorine compounds formed by the bleaching of chemical pulp. Although research is incomplete - and the debate continues regarding the environmental risk posed by these compounds, special attention has been paid to dibenzo-pdioxins and chlorinated dibenzofurans. Practical means have been sought in the laboratory and in the factory to prevent or reduce the formation of these and related compounds. Certain proposed approaches, such as oxygen determination, or the replacement of chlorine with chlorine dioxide at the chlorination stage, entail significant expenditure, both in terms of investment and treatment costs. , significant time is required to implement these options.

We therefore want an inexpensive and shorter-term solution.

Research into the causes of the formation of chlorine compounds during chlorination bleaching has led to proposing approaches to the reduction of tetrachiorated dioxins (TCDD)
and tetrachlorinated furans (TCDF) mainly concentrated on modifying the chlorination stage of bleaching.

Swedish engineers have claimed, for example, that TCDD / TCDF levels are exponentially related to the multiple of chlorine or Kappa factor; let's just say that the critical factor is the amount of chlorine applied to a certain amount of lignin. (The Influence of Lignin Content and Bleaching Chemicals on the Formation of Chlorinated Dioxins, Dibenzofurans and Phenolics "(Influence of lignin content and bleaching chemicals on the formation of dioxins, dibenzofurans and chlorinated phenolic compounds) by Axegard et al. and "Influence of Oxygen Pretreatment and Chlorine Ratio on the Formation of PCDDs and PCDFs in Pulp Bleaching" By Swanson and coll., presented at the 1988 Dioxin Conference in Umea, Sweden).
US Patent Application Pending Nc-Series 262,534 indicates that the amount of chlorine can actually stay at these conventional levels as long as the chlorine concentration does not exceed a rate at which unwanted chlorinated dioxins and furans are produced at any time any during chlorination bleaching, as a method to reduce the formation of these undesirable compounds.

The chlorine concentration control should according to this
Application to be obtained by fractional addition of chlorinated chemical compounds.

Washing the pulp after chlorination with excess water lowers the levels of 2,3,7,8-TCDD and 2,3,7,8-TCDF in the subsequent stage by only 5 to 108. This suggests that there is little chance of eliminating chlorinated dioxins and furans from the bleached pulp by improving the washing of the bleaching plant.

However, pending US Patent Application Serial No. 07/338 320 describes that washing unbleached softwood pulp with an aqueous alcohol solution reduces the amount of chlorinated dioxins and furans by 80% or more. formed during the subsequent stages of chlorination and alkalinization, by comparison with a control representing the previous process in which there is not this pre-wash stage of chlorination. Of course, washing the dough with organic solvents may be too expensive at present to be used industrially.

Due to the equilibrium relationship between molecular chlorine (C12) and hypochlorous acid (HOC1) in water as seen in Figure 1, it has been postulated that a shift in this equilibrium to reduce the concentration of molecular chlorine to raise the pH in the stage above 1.8 this chlorination compared to the final pH characteristic of this stage, 1.5 to 1.8, reduces the formation of polychlorinated dioxins and polychlorinated furans. The chlorine (C12) present for lower pH values, oxidizes and participates in electrophilic substitution reactions with lignin to form chlorinated organic compounds and does not generally degrade the cellulose. The resulting degradation of the cellulose and the unacceptable viscosity of the pulp would appear to oppose an upward adjustment of the pH in the chlorination stage as a means of reducing the formation of PCDDs and PCDFs during bleaching of the pulp. On the other hand, since hypochlorous acid is a stronger oxidant and serves to oxidize and degrade cellulose, as well as lignin, bleaching at high pH can reduce the mechanical strength of the pulp, as it is put highlighted by the low viscosity values of the dough.

US Patent Application Pending Serial No. 07/351 826 proposes to suppress the formation of polychlorinated dioxins and furans by incrementally adding a pH adjustment base during chlorination and bringing the final pH from the stage of chlorination to a value greater than 2.2 without increasing the degree of degradation of the cellulose and without reducing the viscosities of the pulp.

Canadian Patent No. 1,149,111 proposes chlorination bleaching at even higher pH values, and aims to optimize the economics of replacing chlorine with chlorine dioxide without significantly increasing (rather than seeking to reduce) the quantity of "toxic decomposition products resulting from the use of chlorine. The Patent proposes a minimum pH of 3.5 at the chlorination stage, a minimum fraction of 30% of chlorine dioxide and it is proposed in all if necessary, an addition of chlorine to follow an initial addition of chlorine dioxide.

The invention will be better understood on reading the detailed description, given below by way of example only, of preferred embodiments in connection with the attached drawing in which - FIG. 1 represents the equilibrium relation of the concentration in molecular chlorine stage the formation of hypochlorous acid for increasing pH values. As the pH increases, the relative concentration of chl @ r @ decreases and the concentration of hypochlorous acid increases; - Figures 2 and 3 represent the effects of process changes on the formation of 2,3,7,8-TCDF During bleaching by chlorination of softwood pulp for various degrees of whiteness of the chlorinated pulp: Figure 2 compares a single chlorine charge process with multiple chlorine charge systems; and Figure 3 compares multiple charge systems of chlorine with varying rates of chlorine dioxide replacement and with or without adjustment of the pu by addition of alkali.

It has been found that the greatest reduction in the formation of polychlorinated dioxins and furans is obtained during chlorine bleaching by lowering the concentration of molecular chlorine both by introducing the chlorine into the pulp in at least two increments and by adding an alkaline dough product. In addition, this suppresses the formation of PCDD and PCDF without this resulting in degradation of the cellulose and a low viscosity of the paste.

The viscosity is protected by gradually raising the pH and adjusting it within a range which avoids the overproduction of hypochlorous acid. For example, in one embodiment, the alkaline product (used to adjust the pH upward) is added in several small doses to adjust the final pH of the chlorination to a value greater than −1.8 and less than approximately 3, 5. The ratio of molecular chlorine to hypochlorous acid is less than 20: 8Q at all times according to calculations which assume equilibrium conditions for a measured value of.pH. Figure 1 shows this balance between chlorine and hypochlorous acid for various pH values.

Four ingredients are necessary to make paper (1) raw materials, for example wood from a forest! 9! energy from coal, oil, gas or wood by-products, for example bark or waste from the papermaking process itself; (3) water, many of which is used as a conveyor to transport the product throughout the process and which is recycled, but part of this water is discharged into the atmosphere as steam from driers or. as a liquid after purification in a wastewater treatment plant; and (4) experienced operators and management.

Not all pasta is produced chemically. "Crushed wood" or "mechanical pulp", as the names imply, is produced by grinding wooden logs against abrasive stone surfaces or between rotating steel discs with cutting blades against their faces to obtain fragmented fibers or bundles of fibers.

This paste is used for printing newspapers and papers and the like where high opacity and good print quality are desired, but where mechanical strength is not a primary requirement.

 The preparation of the chemical pulp begins with the fragmentation of the wood into small chips. The shavings are sieved, the rejects being both too large sticks or too small fines, and they are brought to the top of a "scrubber" or of a cooking vessel at high pressure. Chemicals are added and the reaction is carried out for a period of time with a prescribed temperature program so that the lignin from the drink and certain hemicelluloses are dissolved and extracted from the shavings. Then, the cooked product is discharged discontinuously in a batch process or continuously in a blowing tank.

where vapor and other volatiles are removed.

The cooking liquor, which is now a "black liquor" due to the dissolved lignin, is transferred to a recovery cycle for chemical products.

The dough is washed with water to remove the black liquor in, for example, a series of rotating drums coated with metal wires. The washed brown proauit is sieved, diluted and can be transferred to batteries of centrifugal cyclone purifiers to separate the large and heavy "dust", for example silica or metallic particles, before bleaching. Since sieving and purification in cyclones are only effective with dilute suspensions, while bleaching requires higher consistencies for economic reasons, the dough is "thickened" by extracting part of its water, using perforated drums coated with metal wires on which the dough is made to form a mat.

The thickened unbleached pulp is then subjected to a series of bleaching operations. These can vary widely both with regard to the types of chemical agents used and their sequences.

In a characteristic system, called CEDED (Chlorine-Extraction-Chlorine dioxide-Extraction-Chlorine dioxide), the pulp is first delignified with elemental chlorine, then subjected to extraction with sodium hydroxide and finally bleached with two stages of chlorine dioxide, separated by alkalization. As is well known in the art, the first stage of chlorination of bleaching often involves various combinations of chlorine and chlorine dioxide. Chlorine dioxide specifically attacks lignin to a much higher degree than it attacks cellulose, unlike chlorine which is a less specific oxidant; however, chlorine dioxide is more expensive. It is also preferably used for the final stages. After the final bleaching, the white pulp is washed to leave the pulp manufacturing plant and enter the paper manufacturing plant. '
During the chlorination stage, chlorine and chlorine dioxide have similar, but not identical, functions. Various methods for optimizing delignification and minimizing viscosity losses are discussed in the literature. Mixtures of chlorine dioxide and chlorine are often superior to either chemical alone. In addition, the effects of these chemical agents can vary if the sequence of their addition to the paste is changed. Various variants exist, but a common approach consists in first adding chlorine dioxide and, after a short period of retention to allow reaction of chlorine dioxide, chlorine is added. This process of adding the two chemical agents in sequence is often claimed to give the best delignification at the lowest cost. It is particularly applicable when relatively large proportions of chlorine dioxide are used, that is, 50% of the chlorine is replaced by chlorine dioxide on an active chemical basis. replacement chlorine dioxide, the sequence of the additions is reversed to obtain the best conservation of the viscosity of the paste.,
If such amounts of chlorine are to be used, a compromise can be reached between the adverse effects of a high concentration of chlorine leading to the formation of TCDD and TCDF and excessive formation of hypochlorous acid by bleaching at values high pH and causing degradation of cellulose. As will be demonstrated in the following examples, a reduction to at least 50 of the production of TDCC and TCDF is obtained without reducing the total chlorine applied by adjusting the concentration of molecular chlorine. The invention described here applies to all money laundering systems. chlorination where the level of chlorine applied is sufficient to produce concentrations of molecular chlorine in the reaction system resulting in the formation of undesirable levels of TCDD and TCDF. The object of the present invention is to make it possible to have sufficient molecular chlorine to react with la-lignin, but to avoid an excess of this molecular chlorine to limit the formation of TCDD and TCDF. By limiting the formation of hypochlorous acid by adjusting the pH, it minimizes the degradation of the cellulose and the. loss of viscosity. this compromise is obtained by the combination of the incremental addition of chlorine and, addition to the paste of a basic compound to adjust the pH to values greater than approximately 1.8 but less than approximately 3.5, which results in a molecular chlorine / hypochlorous acid ratio of less than 20:80 (Figure 1).

que la concentration en chlore peut être abaissée en (a) ajoutant de l'eau, (b) diminuant H+ (augmentant le pH), ou (c) diminuant C1 . En outre, la constante d'équilibre, K, décrite dans l'équation rHoCll rH4 i rcri
K = [Cl2] [H2O] est connue pour augmenter légèrement lorsque la température augmente. A 250C, K = 4,3 x 10-4, tandis qu'à 450C, K = 6,1 x 10-4; On peut.utiliser cette expression d'équilibre pour calculer les valeurs qui établissent la courbe de la figure 1. Ceci procure une autre opportunité pour réduire ia concentration en chlore, mais l'importance de l'effet est faible.It results from the chlorine-hypochlorous acid relationship

that the chlorine concentration can be lowered by (a) adding water, (b) decreasing H + (increasing the pH), or (c) decreasing C1. In addition, the equilibrium constant, K, described in the equation rHoCll rH4 i rcri
K = [Cl2] [H2O] is known to increase slightly as the temperature increases. At 250C, K = 4.3 x 10-4, while at 450C, K = 6.1 x 10-4; This equilibrium expression can be used to calculate the values which establish the curve of Figure 1. This provides another opportunity to reduce the chlorine concentration, but the magnitude of the effect is small.

The process of the invention is used in pulp bleaching processes which employ a chlorination stage. In one embodiment, the method of the invention is used in the CEDED system described above. As will be appreciated by those skilled in the art, there are many pulp bleaching sequences, and no limitation of the invention is brought about by the following examples.

EXAMPLE 1
To demonstrate the improvements made by the sequential addition of chlorine, a softwood pulp (number
Kappa 30,2) is bleached in the laboratory with chlorine and replaced at 10% with chlorine dioxide. The total active chlorine applied in all cases is calculated by multiplying by 0.24 the Kappa number of the unbleached pulp ("Kappa factor"). Total active chlorine is determined by multiplying the% of chlorine dioxide by 2.63 and adding the percentage of chlorine (all percentages are by weight based on the oven-dried pulp).

Chlorination is carried out on 55 g (on the basis of the oven-dried dough) of dough at 50 ° C. in a glass reactor with a capacity of 2 liters, with constant stirring. The chlorine gas dissolves in cold distilled water. The C12 concentration is determined by titrating a sample of the solution just before addition to the paste. The chlorine dioxide solution is added 15 seconds after the initial charge of C12. The consistency after addition of all the reagents is 2.5%.

After 1 hour, the dough thickens to a consistency close to 158 in a Buchner funnel using a sieve made of 300 mesh stainless steel wire (both being washed beforehand with absolute methanol). The fines are retained with the dough by pouring the initial filtrate through the dough mat.

Extractions are carried out in heat-sealable bags.

The final pH of the extraction is at least 11.0. The consistency of the dough is 12, the reaction time l hour, one.

temperature of 70 C. The extracted paste is diluted to a consistency of 1.5% with distilled water and thickened to a consistency close to 15%.

A sample of each chlorinated paste is analyzed for TCDF. The results are shown in Table 1.

Table 1
Effect of chlorine content on the formation of 2,3,7,8-TCDF
in softwood pulp-at stage C
Kappa 30, Kappa factor 0.24
Processor 2,3,3,8,8-TCDF CEK CE Visc. final pH
Kappa ppt N cp addition at stage C
A C, 74 175 4.5 22.1 1.8 charge
Two 0.24 79 4.5 17.1 1.6 charges
Three 0.24 27 4.7 18.2 1.7 charges (1) Studies have indicated that 2,3,7,8-TCDF can be used as an indicator
the presence of 2,3,7,8-TCDD, which is the corresponding chlorinated dioxin isomer
laying, at lower levels.

In one case, all the chlorine is added to the stirred p at once, in about 5 to 10 seconds. Other experiments are carried out in which the same total amount.

chlorine is divided into two or three separate charges and is added in sequence to maintain low chlorine concentrations in the dough. The amount of 2,3,7,8
TCDF is reduced by around 50% when the chlorine is discharged into two equal charges, at 20-minute intervals. The rate of 2.3,
7.8-TCDF is reduced by approximately 85% when chlorine is added in three (uneven) loads. The second charge is added after 10 min, and the final amount is added after 30 min of total reaction time. In all cases, all the chlorine dioxide is added 15 seconds after the first chlorine load. By adding chlorine in sequence, there is never a chlorine concentration as high - as when it is added in one time. It is thus demonstrated that, if the chlorine concentration is kept low, the formation of TCDD and TCDF can be minimized.

EXAMPLE 2
Studies are being carried out in the paper mill to confirm, on an industrial scale, the benefits of reducing the formation of chlorinated dioxins and furans. Samples are taken in the bleaching plant of both softwood pulp and hardwood pulp, from tests carried out using a single addition of chlorine and several (three additions of chlorine); and the samples were analyzed to determine the amount of 2,3,7,8-TCDD- or TDCF. Finally, tests are carried out, they are sampled and analyzed, these tests combining several additions of chlorine with an addition of alkali to maintain the pH at a value between approximately 1.8 and approximately 3.5, preferably between 2 , 0 and 2.4. This latter technique, as claimed here, more consistently produces acceptable viscosities of dough with a minimum formation of 2,3,7,8-TCDD or TCDF, as shown by
Tables 2 and 3.

Table 2
Analysis of 2,3,7,8-TCDD / F
on samples of hardwood pulp
taken from the bleaching facility
(temperature = 50 C # 2; consistency = 2-3%)
Chlorination stage Final bleaching stage
Conditions of 2,3,7,8- 2,3,7,8- 2,3,7,8- 2,3,7,8 laundering @@@@@@ C @@ TCDF TCD @ @CDF
No. (ppt) * (ppt) * (ppt) (ppt) 1 ND (2.0) 6.4 2.9 12
2 3.3 14 2.4 11 a single 3 7.3 41 8.7 42 addition 4 - - 6.2 49 of Cl2 5 - - 5.9 19
6 5.3 82 1.3 21
7 3 3 20 2.4 21
8 / 8.3 37
9 - - 2.6 5.7
10 - - 6.3 30
11 - - 1.2 10.1 several 12 - - ND (0.8) 8.8 additions 13 - - 0.6 3.0 of Cl2 14 - - ND (0.3) 3.9
15 - - ND (0.9) ND (0.5)
16 @ @ ND (@ @) @@@
16 - - ND (0.9) 8.5
several additions 17 - - ND (2,3) ND (1,5) of Cl2 and additions 18 - - ND (2,3) ND (1,2) of alkali 19 - - ND (0,1) 0, 6 20 - - ND (0.2) 1.0
21 - - ND (0.05) 0.45
22 - - ND (0.1) 0.51
23 - - ND (0.4) 0.4
24 - - ND 0.6 0.86
25 - - ND (0.4) 0.34
26 - - ND (Or7 0151
When no data is indicated, there is no production
Table 3
Analysis of 2,3,7,8-TCDD / F
on samples of softwood pulp
taken from the bleaching facility
(temperature = 50 C # 2; consistency = 3-4.5%)
Chlorination stage Final bleaching stage
Conditions of 2,3,7,8- 2,3,7,8- 2,3,7,8- 2,3,7,8
bleaching TCDD TCDF test TCDD TCDF
No (ppt) (ppt) (ppt) (ppt)
1 31 215 19 87
only one 2 - - 13 105
addition 3 NO (22 *) 188 ND (3.8) 53
of C12 4 40 2g3 4X1 17
- 5 - - 4.6 70
6 - - 5.4 14 several
additions of Cl7 7 - - ND (0.8) @, @
8 - - 3.8 12.4
ç - - ND (1.8) 7.1
several additions @@ - - ND (1.2) ND (0.7)
of C12 and additions 11 ND (0.08) 0.6
of alkali 12 - - 0.9 5.5
13 '- - ND (0.3) 0t53
* Estimated maximum potential concentration
Consequently, the minimization of the formation of 2,3,7,8-TCDD or TCDF is optimally obtained, while maintaining good viscosities of the pulp during chlorination bleaching by adjusting the pH by an addition of 'alkali, combined with sequential addition of chloredansîa paste.

EXAMPLE 3
Factory tests. are carried out to determine the benefits of fractional chlorine fillers and the addition of sodium hydroxide to adjust the pH to reduce the formation of TCDD / TCDF when bleaching softwood pulp. The results are shown graphically in Figures 2 and 3.

Figure 2 presents data from a series of factory tests on softwood pulp. In the curve marked "a charge11, the degrees of whiteness vary when the level of total active chlorine - (C12 + Calo2) in stage C is modified. The level of substitution ClO2 (instead of Cl2) is modified by 9% to 27% in these short-term trials. One of the benefits of higher substitution is that it results in higher pH at stage C.

In the family of curves of FIG. 2 marked "three charges", the degree of substitution by C102 is either 10 or 25%, and caustic soda is used for the adjustment of the pH in both cases, as it is shown in figure 3.

The individual contributions of some of the changes.

of the process can be estimated from Figures 2 and 3. By comparing a charge (with 9 to 27% substitution by chlorine dioxide) with three charges (10 to 25% substitution by chlorine dioxide), for a 28% whiteness, we can predict1 as indicated on the
Table 4, a reduction of. 82 to 92% of 2.3, -7.8-TCDF. Using a base for pH adjustment would reduce
again the percentages of TCDF, to arrive at a 'reduction
more than 99% expected. Given a system with three
mixers, this data would predict a reduction of 83
to 96% of 2,3,7,8-TCDF by adding an alkali for adjustment
pH.

Table 4
Estimated contribution of process changes
to reduce the percentages of 2,3,7,8-TCDF
in softwood pulp at stage C
2,3,7,8-TCDF% reduction Process (ppt) for compared compared to compared to
GE 28. at load 25% ClO2 10% ClO2
no NaOH no NaOH one charge 118 - three charges 250o ClO2, no soda 21.7 '82 10% ClO2, no soda 9.4 92 10% ClO2, addition of soda 1.6 99 93 83 25% ClO2, addition sodium hydroxide 0.8 99 @ 96 92
interpolated from Figure 3.

Based on the reduction in the percentage of 2,3,7,8-TCDF formation by the combination of addition of several chlorine fillers with a 25% fraction of chlorine dioxide and addition of alkali with respect to at the reduction achieved with multiple chlorine charges and a 10% fraction of chlorine dioxide and an addition of alkali, further reductions are insignificant with up to less than about a 30% fraction of chlorine dioxide in the chlorine solution. Also, in view of the reductions obtained with less than a fraction of 30% of chlorine dioxide, any other substitution of chlorine by relatively expensive chlorine dioxide is not economically justified. Consequently, the present invention relates to additions of chlorine up to less than about 30% for the fraction of chlorine dioxide.

Although the invention has been described and illustrated here with reference to various specific materials, procedures and examples, it is understood that it is not limited to these particular materials, combinations of materials and procedures chosen for this purpose. Many variations of such details can be used, as will be appreciated by those skilled in the art.

The chlorine source for the chlorination step can be any chlorine-producing compound. such as chlorine, chlorine dioxide, hypochlorous acid, hypochlorites, chlorine monoxide, monochlorine monoxide and combinations of these products. The basic compound added to adjust the pH from about 1.8 to about 3.5, below is preferably sodium hydroxide; but can alternatively be an alkaline product such as, calcium oxide, magnesium oxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, calcium carbonate and ammonia.

This alkaline product can be added to the pulp before the addition of chlorine, or it can be added in-between multiple charges of chlorine or between several of these charges.

Claims (10)

Claims 1.- Process for reducing the formation of dioxins and polychlorinated furans during bleaching by chlorination of  unbleached pulp, characterized in that the molecular chlorine / hypochlorous acid ratio is adjusted to less than about 20:80 by adding a compound producing chlorine to the pulp in several charges and by adding an alkaline product to the pulp.  2.- Method according to claim 1, characterized in that the chlorine producing compound is a member of the group consisting of chlorine, chlorine dioxide, hypochlorous acid, hypochlorites, chlorine monoxide, monochlorine monoxide and combinations of these products.  3.- Method according to claim 1, characterized in that the alkaline product is chosen from the group comprising  sodium hydroxide, calcium oxide, magnesium oxide  sium, potassium hydroxide, magnesium hydroxide,  calcium hydroxide, sodium carbonate, potassium carbonate, calcium carbonate and ammonia.  4.- Method according to claim-1, characterized in that the alkali product is intrcauit in the dough in several charges.  5.- Method according to claim 1, characterized in that the alkaline product is added between two charges of  chlorine or between several of these charges.  6.- Method according to revenaicat'ion 1, characterized in that the alkaline product is added before the addition of chlorine to the paste.  7.- Method according to claim 1, characterized in that the chlorination stage is the initial stage of chlorination of a sequential bleaching-extraction process of the pulp. 8. A process according to claim 7, characterized in that the chlorination stage is the first stage of a CEDED pulp bleaching system. 9.- Method according to claim 2, characterized in that the chlorine producing compound is the combination of chlorine fractions and chlorine dioxide up to a maximum fraction of chlorine dioxide which can reach about -308 * 10.- Method according to claim 9, characterized in that the fraction of chlorine dioxide is 25%. 11.- Method according to claim 9, characterized in that the fraction of chlorine dioxide is 108.