WO1997030210A1

WO1997030210A1 - Process for bleaching of a high yield pulp

Info

Publication number: WO1997030210A1
Application number: PCT/FI1997/000108
Authority: WO
Inventors: Ilkka Renvall; Elias Suokas; Reijo Aksela; Aarto Paren
Original assignee: Kemira Chemicals Oy
Priority date: 1996-02-19
Filing date: 1997-02-19
Publication date: 1997-08-21
Also published as: FI960758A0; FI960758L; FI115641B; AU1797597A

Abstract

A process for the bleaching of a mechanical pulp with peroxides, in which process the pulp is treated with a chelating agent in order to bind into a chelate complex the heavy metals present in the pulp. The chelating is carried out with a mixture of a nitrogen-containing chelating agent and one or more compounds according to formula (I): R1CnHm(OH)p(COOH)qR2 in which formula n is 1-8, m is 0-2n, p is 0-n, q is 0-2, R1 is COOH, and R2 is H, CH2OH or COOH.

Description

Process for bleaching of a high yield pulp

The invention relates to a process for the bleaching of a high yield pulp, wherein the pulp is pretreated before peroxide bleaching and/or is treated in connection with peroxide bleach¬ ing with a chelating agent in order to bind into a chelate complex the heavy metals present in the pulp.

By high yield pulps are meant above all mechanical pulps the bleaching of which differs from the bleaching of chemical pulps in that the aim is to avoid delignification. The yield of pulp is maintained at a high level. The aim in bleaching is to ren¬ der colorless the color-producing components (chromophoric groups) present in lignin. The chelating carried out in connec¬ tion with the bleaching of mechanical pulps differs from the pretreatment carried out in connection with the bleaching of chemical pulps. In the latter case the complexes formed in the chelating are removed by a wash. In the case of mechanical pulps, no wash is carried out in the pretreatment; instead, the pretreated pulp is compressed to the consistency used in bleaching, in which case only a small proportion of the metal complexes are removed from the pulp.

Within the scope of the invention, mechanical pulps also com¬ prise recycled fibers, from which only printing ink is removed, not lignin.

Mechanical pulps can be divided into two principal categories, pure mechanical pulps and chemi-mechanical pulps. These pulps can further be divided into subcategories so that mechanical pulps include stone groundwood (SGW), pressure groundwood (PGW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP) and others such as TRMP and PRMP. Respectively, the chemi-mechanical pulps include low-sulfonated pulps (chemi- thermomechanical pulp CTMP and BCTMP), chemically modified pulps (OPCO) and high-sulfonated pulps (CMP, UHYS).

Mechanical pulps are used in the making of, for example, news¬ print, magazine paper and porous paper grades (tissue papers). Certain highly bleached chemi-mechanical pulps (BCTMP grades) are also used in bleached printing papers to replace chemical pulp.

Hydrogen peroxide is used for the bleaching of mechanical pulps and of recycled fiber which contains paper fiber made from mechanical pulps. In order for the bleaching to be successful, the bleaching conditions must be rendered such that the hydro¬ gen peroxide will not dissociate.

Heavy metals catalyze the dissociation of hydrogen peroxide and peroxy compounds. The ions the most detrimental in terms of bleaching are manganese (Mn) , iron (Fe) and copper (Cu). Also other heavy metals, such as chromium ions (Cr), etc., have a detrimental effect on the consumption of peroxy compounds. Detrimental heavy metals originate in the pulp, the treatment waters and the pulp-treatment apparatus.

In the bleaching with peroxy compounds, heavy metals are bound by using agents which chelate metal ions, for example poly- aminocarboxylic acids. These include in particular ethylene diamine tetra-acetic acid and its salts (EDTA) and diethylene triamine penta-acetic acid and its salts (DTPA), as well as diethylene triamine pentame hylenephosphonic acid (DTMPA) and its salts.

The bleaching of mechanical pulps and recycled fiber is common¬ ly carried out with hydrogen peroxide at a high pH. Magnesium sulfate and waterglass are added during the bleaching step in order to stabilize the peroxide. Many hypotheses have been presented regarding the stabilizing effect of the above- mentioned compounds. The stabilizing effect of waterglass is probably based on waterglass surrounding the hydroxides, oxy- hydroxides and oxides formed under alkaline conditions when iron precipitates, thus inactivating the catalytic surfaces of these solids.

Mechanical pulps are made in paper mills and not in separate plants as are chemical pulps. Thus the effluents formed during bleaching end up in the other effluents of the paper mill. Therefore it would be highly desirable to use biodegradable chelating agents in connection with bleaching.

To avoid environmental loads it would be preferable to replace poorly biodegradable chelating agents at least in part with biodegradable chelating agents which do not contain nitrogen.

The bleaching of recycled fibers can often also be carried out using reducing agents such as dithionite, and peroxide bleach¬ ing is not always necessary. Hydrogen peroxide is used more in the slushing of recycled pulp than in actual bleaching. For this reason the treatment of recycled fiber involves the same problems as regards the stability of hydrogen peroxide.

EDTA and DTMPA are regarded as non-biodegradable. DTPA is poor¬ ly biodegradable.

Previously the hydrogen peroxide bleaching of mechanical pulps was carried out in one step. At present the bleaching is car¬ ried out in two steps, since the aim is a higher pulp bright¬ ness and since for example BCTMP can be used in printing papers. The peroxide remaining after two-step bleaching is recycled to the first step. Thus the residual peroxide is high¬ ly significant for the economy of bleaching.

In those bleaching processes of mechanical pulps in which a high brightness is the aim it has become common to use chelat¬ ing agents for a pretreatment at a pH lower than that at which the actual bleaching takes place. In the publication W.C. Fross et al., Tappi 1992 Pulping Conference, pp. 899-915, "Factors affecting mechanical pulp brightening when peroxide liquor is recycled," a DTPA treatment at a pH of 5.5-6.0 is described as an example.

Biodegradable complexing agents have been developed for deter¬ gent builders. They must at the same time have softening action on water, i.e. they must bind calcium ions and magnesium ions. One such sequestering agent is ethylenediamine disuccinic acid (EDDS). This compound has three stereoisomers.

Another known biodegradable complexing agent is 2,2'-imino- disuccinic acid (ISA). The use of this compound in alkaline detergents is disclosed in EP patent application 509 382. The patent application mentions the use of 2,2'-iminodisuccinic acid as a stabilizer of peroxide compounds, in particular in alkaline detergent compositions which contain hydrogen peroxide and its derivatives.

DE patent application 4 340 043 discloses the use of ISA as a bleaching agent in the bleaching of groundwood. According to the publication, the purpose of ISA is the stabilization of hydrogen peroxide, and the examples show that at a pH of 10 it is a better stabilizer of hydrogen peroxide than is DTPA. There is no demonstration in the publication of the action of ISA on heavy metals, nor of any bleaching results.

In the bleaching and delignification processes according to the present invention, for example biodegradable ethylenediamine disuccinic acid (EDDS) or 2,2'-iminodisuccinic acid (ISA), and their alkali metal salts, can be used successfully as a chelat¬ ing agent.

The object of the present invention is to eliminate the adverse effects of heavy metals in the bleaching of high yield pulps. The aim is to provide for use a biodegradable chelating agent which yields a good bleaching result.

The applicants have observed, surprisingly, that an equally good high yield pulp bleaching result is achieved even if a portion of the above-mentioned nitrogen-containing chelating agent (e.g. EDTA; DTPA; EDDS; ISA) is replaced with biodegrad¬ able nitrogen-free chelating agents, such as carboxylic acids, hydroxycarboxylic acids, polyhydroxycarboxylic acids and hyrdoxypolycarboxylic acids.

It is quite surprising that the above-mentioned hydroxy¬ carboxylic acids can be used successfully as chelating agents together with nitrogen-containing chelating agents in bleach¬ ing. Hydroxycarboxylic acids are not efficient binders of heavy metals. On the other hand, they chelate well calcium and mag¬ nesium. Citric acid, for example, has been used instead of phosphates in phosphate-free detergents and cleansing agents, which are required to bind calcium and magnesium. The usability of the chelating process according to the invention is based specifically on the joint effect of chelating agents of dif¬ ferent types.

When DTPA is used as a chelating agent before peroxide bleach¬ ing of a mechanical pulp, the pH the most preferable in terms of bleaching is approx. 5-5.5. When aminopolycarboxylic acids are replaced with the above-mentioned hydroxycarboxylic acids, the chelating can be carried out at a higher pH. When the che¬ lating agent mixtures according to the invention are used, the usable pH range is 6-8, most preferably pH 6.5-7.5.

The procedure according to the invention is advantageous, since the nitrogen-containing chelating agent can in part be replaced with a biodegradable chelating agent which does not contain nitrogen. The characteristics of the invention are given in the accom¬ panying claims.

According to the invention, nitrogen-containing chelating agents can be replaced with compounds, known per se, having the general formula I

R₁C_nH_πι(OH)_p(COOH)_gR₂ (I)

where n is 1-8, is 0-2n, p is 0-n, q is 0-2,

R-^ is COOH, and

R₂ is H, CH₂OH or COOH.

The detrimental nitrogen load in bleaching effluents can thus be reduced. Conventional carboxylic acids, hydroxycarboxylic acids, polyhydroxycarboxylic acids and hydroxypolycarboxylic acids according to Formula I, such as citric acid, tartaric acid, lactic acid, pimelic acid, glutamic acid, glucoheptonic acid, ascorbic acid, glycolic acid, glutaric acid, adipic acid, succinic acid or malonic acid, can be used as replacement che¬ lating agents.

The process according to the invention can be used in the pre¬ treatment of mechanical pulps and/or in the actual bleaching. The process is suitable for the treatment of all mechanical pulps, such as stone groundwood, pressure groundwood, refiner mechanical pulp, thermomechanical pulp, chemi-mechanical refiner pulp and chemi-mechanical pulp. The process is also suitable for the bleaching of recycled fiber and textile fiber. The process can be used both in single-step and in two-step peroxide bleaching of mechanical pulp. The treatment is also suitable for mechanical pulp bleaching in which dithionite or formadine sulfinic acid bleaching is used instead of peroxide in some step. In the case of mechanical pulps which are sul¬ fonated with sulfite or bisulfite, the compounds according to Formula I may be added already to the impregnation liquor, whereby the efficacy of the bleaching can be improved.

The treatment may be performed on pulps obtained from various fiber raw materials, such as softwood or hardwood.

The pH control of the acid chelating step can be carried out using conventional mineral acids, such as sulfuric acid, sulfur dioxide or an aqueous solution thereof, carbon dioxide, or organic acids such as formic acid and acetic acid.

In the process according to the invention, nitrogen-containing, phosphorus-free chelating agents are preferably used, whereby environmental problems due to phosphorus are avoided. Such chelating agents include ethylenediamine-N,N'-disuccinic acid (EDDS), its various isomers and its alkali metal salts such as sodium and potassium salts, and its earth-alkali metal salts such as calcium and magnesium salts, 2,2'-iminodisuccinic acid (ISA), its various isomers and its alkali metal salts such as sodium and potassium salts, and its earth-alkali metal salts such as calcium and magnesium salts. Usable chelating agents also include, among polyaminocarboxylic acids, ethylenediamine tetra-acetic acid (EDTA) and its salts, and diethylenetriamine penta-acetic acid (DTPA) and its salts. The functioning of nitrogen-containing chelating agents such as these in the in¬ vention is demonstrated in the embodiment examples hereinafter.

Chelating agents within the scope of the invention, but less recommendable, are those which in addition to nitrogen also contain phosphorus, such as polyaminomethylene phosphonic acids or biodegradable aminoalkane diphosphonic acids, the use of which in the bleaching of pulp is known per se from WO applica¬ tion publication 95/12029. Some specific examples of these chelating agents are, among aminoalkane phosphonic acids, 4- morpholinomethylene-1,1-diphosphonic acid (MMDPA) and its salts, among alkane diphosphonic acids, 1-hydroxyethylene di¬ phosphonic acid (HEDPA) and its salts, and, among aminopoly- methylene phosphonic acids, aminotrimethylene phosphonic acid and its salts, ethylene diamine tetramethylene phosphonic acid and its salts, and diethylene triamine pentamethylene phos¬ phonic acid (DTMPA) and its salts.

The invention is described below in examples, which, however, do not limit the invention.

To investigate the transfer of heavy metals, washing experi¬ ments were first performed on a chemical pulp. The success of bleaching is not dependent solely on the efficacy of the che¬ lating of heavy metals, but the chelating results can be re¬ garded as giving a guideline. The best pH range for metal removal can be sought by means of washing experiments. With the help of washing experiments carried out on the same pulp at the same pH it is possible to compare the efficacies in the chelat¬ ing of metal ions by different chelating agents.

Example 1

To investigate the chelating of heavy metals and earth alkali metals, a mechanical pulp was washed with aqueous solutions containing EDDS. The metal contents of the washing solution were analyzed after the wash. Thereby the transfer of iron (Fe), manganese (Mn) , calcium (Ca) and magnesium (Mg) into the washing waters was investigated. The transfer of iron and man¬ ganese into the washing solutions is advantageous for bleach¬ ing. In contrast, the transfer of calcium and magnesium into the washing solutions is disadvantageous for bleaching. In the reference tests the pulp was washed with DTPA or EDTA solu¬ tions. The chelating agent concentrations and the pH during the wash are indicated in Table 1. Table 1

Softwood sulfate pulp Chelating conditions

Kappa number 16.9 Time (t) 60 min

₃ Viscosity 963 dm /kg Temperature (T) 70 °C

Brightness 39.6 % ISO Consistency (CS) 12 % pH 6-7

Chelate Dose Metal contents in the filtrate (ppm) kg/tp PH Fe Mn Mg Ca

No chelate 0 6.0 0.3 0.3 6 22

Na₅DTPA 1 6.5 1.2 2.9 4 17

Na₅DTPA 2 6.5 2.0 2.8 17 48

H^EDDS + Na₃ citrate 0.5+1 7.5 1.3 2.4 8 18

H₄EDDS + Na₃ citrate 0.75+1 5.7 2.2 1.6 17 58

H^EDDS + Na₃ citrate 1+1 7.4 1.4 2.7 10 23 EDDS 1.5 7.1 1.9 2.3 13 37

H₄EDDS + Na gluconate 1+1 6.7 1.8 2.6 9 29

Na ISA 1.5 5.8 1.1 1.9 2 50

Na₄ISA 1.5 8.9 0.0 1.3 25 26

Na^ISA + Na citrate 1+1 7.7 0.5 2.2 15 26

When the dose of DTPA was reduced from a rate of 2.0 kg/tp, the chelating of the heavy metals was incomplete. A chelating result comparable to the best chelating result obtained with DTPA was achieved when EDDS was used at a rate of 1.5 kg/tp. Equally good chelating results were obtained even when the dose of EDDS was reduced to as low as to one-half of this when Na citrate or Na gluconate was used at a rate of 1.0 kg/tp along¬ side it. Likewise, it was possible to reduce the dose of ISA without the chelating result being significantly worsened when Na citrate was used together with it as a chelating agent. It is to be noted that in this experiment the pH of the pulp was considerably high.

Table 1 shows that a water wash is without effect as regards the chelating of metals. Likewise, citric acid used alone does not remove heavy metals. Citric acid chelates only earth-alkali metals, which is not desirable in terms of bleaching. This indicates that a good chelating result is achieved through the joint effect of nitrogen-containing chelating agents and, for example, citric acid.

It is to be noted that a conclusion regarding the bleaching result cannot be drawn directly from washing experiments such as described above. For this reason the effect of corresponding chelating steps on alkaline hydrogen peroxide bleaching was investigated. The residual peroxide content describes the bleaching result relatively well. For this reason, the bright¬ ness of the pulp was not determined in all of the experiments.

Example 2

In order to find out how the various chelating agents would function alone when used in a pretreatment, reference experi¬ ments were first carried out.

A commercial TMP having an initial brightness of 55 % ISO was bleached with peroxide by pretreating it first with chelates for 30 minutes at a consistency of 10 % and a temperature of 60 °C. The dose of chelates was 1.5 kg/metric ton of pulp. The pH in the pretreatment was 6.5. The peroxide bleaching was carried out at a temperature of 70 °C and a consistency of 15 % for 60 minutes. The doses of waterglass and alkali are shown in accompanying Table 2. Table 2

Chelating: T = 70 °C, t = 30 min, CS = 10 %, pH = 6-6.5

Chelating Not P-step-: chelated DTPA EDDS ISA

T, C 70 70 70 70 t, min 60 60 60 60

CS 15 15 15 15 pH, initial 10.2 10.2 10.2 10.2 pH, final abt. 8 .5 abt. 8.5 abt. 8.5 abt. 8.5

H₂0₂, kg/tp 20 20 20 20

Waterglass, kg/tp 20 20 20 20

NaOH, kg/tp 15 15 15 15

Residual H₂0₂, kg/tp 0.5 7.5 4.8 7

Residual H₂0₂, 2.5 37.5 24 35

Brightness, % ISO 65.4 69.6 69.2 69.3

It can be observed that a very low residual peroxide indicates a poor bleaching result.

Since residual peroxide is a good indicator of the success of bleaching and since its measuring is rapid as compared with the measuring of pulp brightness, the following experiments were carried out by comparing only the residual peroxide contents.

Example 3

Refiner mechanical pulp was bleached by pretreating it first at the same consistency, at the same temperature and for the same time as in Example 2. The dose of the chelating agents was 2 kg/metric ton of pulp. The pH in the pretreatment was approx. 6-6.5. The conditions of the peroxide bleaching are shown in Table 3. The final pH of the bleaching was approx. 8.5 in all the experiments. Residual peroxide was determined from the bleaching filtrate.

Table 3

DTPA, EDDS and ISA, dose calculated as Na salt

Chelating

No. 1 2 3 4 5 6 7 8 9 t, min 30 30 30 30 30 30 30 30 30

T, C 60 60 60 60 60 60 60 60 60

CS, % 10 10 10 10 10 10 10 10 10 pH 6-6.5

DTPA, kg/tp 2

EDDS, kg/tp 2

ISA 2

DTPA+citr., kg/tp 1+1 Na gl.+DTPA, kg/tp 1+1 Na gl.+EDDS, kg/tp 1+1 Na gl.+ISA, kg/tp 1+1 ISA+citr., kg/tp 1+1

Peroxide bleaching l I I i i + . i i t, min 60 60 60 60 60 60 60 60 60

T, C 70 70 70 70 70 70 70 70 70

15 15 15 L

CS, % 15 15 15 15 15 15 pH, initial 10.4 10.3 10.5 10.6 10.5 10.5 10.4 10.5 10.5 pH, final 8.4 8.4 8.4 8.5 8.5 8.5 8.5 8.6 8.6

H₂0₂, kg/tp 25 25 25 25 25 25 25 25 25 Waterglass, kg/tp 25 25 25 25 25 25 25 25 25

NaOH, kg/tp 20 20 20 20 20 20 20 20 20

Residual H₂0₂, kg/tp 6.5 11.4 10.5 10.2 10.8 10.9 9.9 9.7 9.9 Residual H₂0₂, kg/tp 26 45.6 42 40.8 43.2 43.6 39.6 38.8 39.6

It can be seen that without chelating the amount of residual peroxide was quite low, whereas DTPA, EDDS and ISA, alone or diluted with citric acid or gluconic acid, yielded a good and almost the same result.

Example 4

Refiner mechanical pulp was bleached with hydrogen peroxide by pretreating it firεt at a consistency of 10 % at 60 °C. In all of the experiments, the chelating agents were dosed at a rate of 2 kg/metric ton of pulp, both in the pretreatment and in the bleaching. In the pretreatment the pH was 6-6.5. The peroxide bleaching was carried out at 70 °C, at a consistency of 15 %, for 60 min. The results of the experiments are shown in Table 4.

Table 4

Chelating

No. 1 2 3 4 5 6 t, min 30 30 30 30 30 30

T, C 60 60 60 60 60 60

CS, % 10 10 10 10 10 10 pH 6-6.5 6-6.5 6-6.5 6-6.5 6-6.5 6-6.5

DTPA, kg/tp 2 1

EDDS, kg/tp 2 1

Citric acid, kg/tp 1

Na gluconate , kg/tp 1 1 1

Peroxide bleaching - X 4- 4. X . t, min 60 60 60 60 60 60

T, C 70 70 70 70 70 70

CS, % 15 15 15 15 15 15 pH, initial 10.5 10.5 10.5 10.5 10.5 10.5 pH, final 8.5 8.5 8.5 8.5 8.5 8.5

H₂0₂, kg/tp 25 25 25 25 25 25

Waterglass, kg/tp 25 25 25 25 25 25

NaOH, kg/tp 20 20 20 20 20 20

DTPA 2 1

EDDS 2 1 1

Citric acid, kg/tp 1

Na gluconate, kg/tp 1 1

Residual H₂0₂, kg/tp 6.5 11.8 10.4 9 9.9 8.9

Residual H₂0₂, % 26 47.2 41.6 36 39.6 35.6

After the bleaching, the residual peroxide was determined from the liquors. The residual peroxide level in experiments in which mixtures of chelating agents were used was comparable to results obtained with DTPA or EDDS. When mixtures of chelating agents were used, the bleaching results were clearly better than in a reference experiment in which chelating agents were not used. Example 5

Refiner mechanical pulp was bleached with hydrogen peroxide at 70 °C, at a consistency of 15 %, for 60 min. The results of the experiments are shown in Table 5.

Table 5

Direct peroxide bleaching

t, min 60 60 60 60

T, C 70 70 70 70

CS, % 15 15 15 15 pH, initial 10. 5 10. 5 10.5 10.5 pH, final 9 8. 7 8.8 8.7

H₂0₂, kg/tp 25 25 25 25

Waterglass, kg/tp 25 25 25 25

NaOH, kg/tp 20 20 20 20

DTPA 2 1

EDDS 2 1

Na gluconate, kg/tp 1 1

Residual H₂0 , kg/tp 7. 6 4. 4 6.9 5

Residual H₂0₂, % 30. 4 17. 6 27.6 20

When mixtures of chelating agents were used, the bleaching result was better than without any chelating agents and nearly the same as when only DTPA or EDDS was used.

Claims

1. A process for the bleaching of a high yield pulp with peroxide, in which process the pulp is treated with a chelating agent in order to bind into a chelate complex the heavy metals present in the pulp, characterized in that the chelating of the pulp is carried out with a mixture of a nitrogen-containing chelating agent and one or more compounds according to Formula

I or a salt thereof

R^H^OH⁾ _p ⁽COOT⁾^ (I)

where n is 1-8, m is 0-2n, p is 0-n, q is 0-2, ^ is COOH, and

R₂ is H, CH₂OH or COOH.

2. A process according to Claim 1, characterized in that the compound according to Formula I is lactic, citric, tartaric, gluconic or glucoheptonic acid.

3. A process according to Claim 1 or 2, characterized in that the nitrogen-containing chelating agent is ethylenedi- amine-N,N'-disuccinic acid, an alkali metal salt or earth- alkali metal salt thereof, 2,2'-iminodisuccinic acid, an alkali metal salt or earth-alkali metal salt thereof, ethylenediamine tetra-acetic acid, an alkali metal salt or earth-alkali metal salt thereof, or diethylenetriamine penta-acetic acid or an alkali metal salt or earth-alkali metal salt thereof.

. A process according to any of the above claims, charac¬ terized in that the peroxide bleaching comprises a bleaching with alkaline peroxide and its combinations with oxygen, per- acids or ozone.

5. A process according to any of the above claims, charac¬ terized in that the chelating treatment is carried out before the peroxide bleaching or in connection with it.

6. A process according to Claim 5, characterized in that the chelating treatment taking place before bleaching is car¬ ried out at a pH of 4-8, preferably at a pH of 5.0-7.5 and most preferably at a pH of 6.5-7.5.

7. A process according to Claim 5 or 6, characterized in that the chelating before bleaching is carried out once or several times.

8. A process according to any of the above claims, charac¬ terized in that the peroxide bleaching is carried out at a pH of 6-12, preferably at a pH of 7-11, and most preferably at a pH of 8-10.5.

9. A process according to any of the above claims, charac¬ terized in that the peroxide bleaching is carried out once or several times.

10. A process according tp any of the above claims, charac¬ terized in that the peroxide bleaching of the pulp is carried out simultaneously with a peroxide and a peracid.

11. A process according to any of the above claims, charac¬ terized in that the peroxide bleaching of the pulp is carried out with hydrogen peroxide, a mixture of hydrogen peroxide and oxygen gas, or an organic per- or perhydro-compound.

12. A process according to any of the above claims, charac¬ terized in that the peracid treatment of the pulp is carried out with peracetic, performic, perpropionic or caron acid, with a transition-metal activated peracetic, performic, perpropionic or caron acid, or with a combination of these.

13. A process according to any of the above claims, charac¬ terized in that the chelating agent is used at a rate of 0.1- 5 kg/metric ton of dry pulp.

14. A process according to any of the above claims, charac¬ terized in that the weight ratio of the compound according to Formula I to the nitrogen-containing chelating agent is between 1:4 and 3:1.