This invention relates to a method at the bleaching of chemical cellulose pulp, more precisely a method of treating the pulp by a reducing agent in alkaline environment.

Traditionally chemical pulp is bleached by means of chlorine and chlorine chemicals, for example hypochlor- ite and chlorine dioxide. Chlorine reacts with the modified lignin structures, which are formed at the cooking of the wood material by substitution, addition or oxidation, whereby the structures are depolymeris- ated and partially are solved. Hereby chlorinated org¬ anic substances are formed, which are difficult to recover and destruct, and which, besides, in many cases are toxic, lipophile and relatively stable. The discharges from the bleaching department, thus, are an obvious environment problem, and it is, there¬ fore, very important to reduce the use of primarily chlorine, but also of other chlorine chemicals at the bleaching of pulp.

In the course of years several proposals have been made to overcome this problem. One such proposal, which at present has become an established technique, com¬ prises the step of after cooking to subject the pulp to pre-bleaching with oxygen. The lignin amount re¬ maining after the cooking can hereby be reduced by half. Bleaching with oxygen, however, also involves disadvant¬ ages. Under unfavourable conditions also the cellulose is attached, which results in lower viscosity. Of other proposals for reducing the amount of chlorine chemicals the technique can be mentioned, which has been developed in recent years, and which comprises the step of adding oxygen' at the extraction step of the final bleaching, so-called intensified alkali extraction. The number of bleaching steps has been reduced hereby.

It also has been proposed since a long time ago to bleach pulp by means of ozone. It was found at exper¬ iments, however, that thereby heavy attacks on the cellulose occur, and there exists today no technique for avoiding these attacks, though they can be de¬ creased slightly by using expensive stabilizers. This method, therefore, is not used on a great technical scale.

The present invention is based on the idea, that the a oresaid problem of reduced- use of chlorine chemicals could be solved in an economical way and simultaneously the pulp properties could be improved by arranging a special treatment step in the bleaching sequence. The step implies that the pulp there is treated with an alkaline solution of a sulphide. The sulphide can consist of sodium sulphide and/or a polysulphide. The exact reaction mechanisms at the special step acc¬ ording to the invention are not known. The addition of sulphide, however, gives rise to a reducing envir¬ onment. This is believed to have such an effect on the remaining residual lignin, that it becomes more reactive by the reducing treatment, partly in the special step and partly in the subsequent oxidizing delignification/ bleaching steps. The reducing environment also seems to have a stabilizing effect on the carbohydrates, which implies a higher pulp viscosity (better pulp quality) of the bleached pulp.

The special step according to the invention preferably is arranged after an oxidative process step, for example an oxygen step or a chlorine-chlorine dioxide step. When the step is arranged in this way, the treatment can take place either directly after the oxidizing step or after an intermediate washing.

At the method according to the invention preferably a sodium hydroxide solution is used as alkaline solution.

The sulphide should be added in an amount of 1-30, suit¬ ably 2-15, preferably 4-12 kg per ton of pulp, calcul¬ ated as sodium sulphide.

According to an especially important embodiment of the method according to the invention, the treatment in the special step is carried out with white liquor, for example from the chemical system of a sulphate mill.

The special reducing step should be arranged after an oxidizing step, for example an oxygen bleaching step. If this is the case, the pulp can be finally bleached after the-step with chlorine chemicals, for example chlorine, hypochlorite or chlorine dioxide. The reducing step also can be arranged after a con¬ ventional chlorine step, chlorine-chlorine dioxide step, chlorine dioxide step or an ozone step. The treatment can be carried out with or without preceding washing.

The special step according to the invention preferably is carried out at a pulp concentration of 3-30$, at a temperature of 80-150 C and during a time of 10-180 minutes. The alkali should be added in an amount of 2-60, preferably 5-40, suitably 8-30 kg effective alkali per ton of pulp, calculated as sodium hydroxide. The spent liquor from the step can be recycled in known manner to the chemical recovery system of the mill. It is, however, also possible to recycle the liquor to the step.

According to a special embodiment of the method accord¬ ing to the invention, after a certain reaction time the reducing treatment can be completed with an oxid¬ ative phase.

This is commenced in such a case after a time of 10-180 minutes and is carried out by adding an oxidizing agent, for example oxygen or peroxide. The term "phase" is to be understood so that the two different treatments are carried out without intermediate washing. The

oxidative phase is carried out at a pulp concentration of 3-30$, a temperature of 50-120°C and during a time of 2-120 minutes.

One way of conventionally bleaching cellulose pulp is shown in the accompanying Pig. 1.

In the Figures, designations for the different bleach¬ ing steps are used which are recommended by TAPPI (Technical Association of the Pulp and Paper Industry!), and which are now generally applied, viz. 0 ? oxygen step D : chlorine dioxide step

(D,C+D) : sequential batching, first chlorine dioxide, thereafter without intermediate washing chlorine and chrBrine dioxide in mixture E : alkali extraction

(EO) : alkali extraction intensified with oxygen

and in addition for the special step' according to the invention is used

(ER) : reducing treatment step

In the conventional bleaching department shown in Fig. 1 the unbleached pulp enters at 1 and is passed after washing in 8 to oxygen bleaching designated 0. Thereafter follows washing in 9. The washing arrangements are here supposed to be fourdrinier wire washers, and washing liquor is supplied in usual manner at 4. Liquor drawn off in washer 9 is passed in counterflow to the pulp to the washer 8, and spent liquor is removed from the system through conduit 3 and runs via evaporation to the recovery system of the mill. The prebleached pulp then is finally bleached in a number of steps comprising sequential chlorine dioxide-chlorine (D,C+D) with one subsequent intensified extraction step 0E and two chlorine dioxide steps D with intermediate alkali extraction E. Bewteen the steps the pulp is washed on

washing filters 10,11,12 and 13, the washing liquor being passed in counterflow. The bleached pulp finally is washed in the washing arrangement 14 and taken out at 2. Washing liquor is supplied through conduits 5 and 6, and spent liquor is removed through conduit 7 and runs either directly or via external waste cleaning to the recipient.

The invention is described in greater detail in the following by way of some embodiments and.with reference to the accompanying drawings, in which Fi . 2 shows a bleaching department with an (ER)-step located directly after oxygen prebleaching, and Fig. 3 shows an (ER)- step located after an oxidizing treatment with chlorine dioxide and chlorine.

In the examples according to the invention an oxygen prebleached softwood sulphate pulp with a kappa number 19,1 and a viscosity 9 ^β 5 is used. The same applies to the pulps used at the reference tests. To get an idea of the chemical costs, the following actual prices have been used:

Oxygen SE 0,685 per kg

White liquor SEK 0,23 per kg NaOH 1)

White liquor " 2)

NaOH chlorine

Chlorine dioxide active chlorine

1) The costs for white liquor have been calculated for the case that the spent liquor from the (ER)-step via washing is recycled to the chemical recovery system of the mill. This applies to example 1, Fig. 2 and example 3.

2) The costs for white liquor here have been calculated for the case, that the spent liquor from the (ER)-step passes to the discharge, because it has been contamin-

ated with chloride from preceding chlorine chemical steps. The sodium loss, therefore, is substituted with NaOH. This case applies to example 2, Fig. 3 and example 4.

EXAMPLE 1

According to the example, a softwood sulphate pulp was used, and the -special ER-step according to the invention was located between the oxygen step and the (D,C+D)-step. The arrangement appears from Fig. 2. In the Figure, for similar arrangements the same reference characters have been used. As reference,the bleaching sequence 0(D,C+D)(EO)DED was carried out. The results are shown in Table 1.

TABLE 1

Invention Reference

Conditions in (ER)-step:

White liquor, sulphidity % 32

White .liquor,batch NaOH kg per ton ^' 10

Pulp concentration % 12

. Temperature °C 140

Time min 120

Analysis after (ER)-step

Kappa number 13,9

Viscosity dm /kg 9 0

Chemical consumption

Oxygen kg per ton 20 20

White liquor kg NaOH per ton 10

NaOH kg per ton 21 23

Chlorine " " . " 10 10

Chlorine dioxide,actice Cl " . " . 25,3 θjQ

Total active Cl /ton/kappa number 2,54 2,62

Analysis after (EO)-step

Kappa number 2,9 3,5

Viscosity 910 935

Analysis of bleached pulp

Diffuse blue reflectance factor 90,0 90,0

Viscosity 86l 823

Analyis of spent liquor

COD 29,2 40,1

T0C1 1,35 1,62

Chemical costs, SEK per ton 115,30 153,46

As appears from the analysis results, by arranging the special step according to the invention surprising technical effects are obtained by

- substantial lower lignin content (lower kappa number) before chlorine bleaching department without simultaneous lowering of the viscosity, which would have been the case if corresponding

^" kappa number lowering would have been carried out with, for example, extended oxygen bleaching substantially lower chlorine chemical consumption to diffuse blue 'reflectance factor equal to that of the reference

- higher viscosity (better pulp quality) of bleached pulp substantially lower C0D-(oxygen consuming substance) and T0Cl-(chlorinated organic substance) emission from bleaching department, and substantially reduced chemical costs at comparable and desired diffuse blue reflectance factor.

EXAMPLE 2

At experiments according to this example the afore¬ mentioned oxygen prebleached softwood sulphate pulp also was used. As appears from Fig. 3, the (ER)-step according to the invention was placed between a (D,C+D)- step and an (EO)-step. A bleaching according to the sequence 0(D,C+D)(EO)DED was carried out as reference. The results obtained are shown in Table 2.

TABLE 2

Invention Reference

Conditions in (ER)-step

White liquor,sHlphidity % 32

White liquor,batch NaOH kg/ton pulp 18

Pulp concentration % 12

Temperature ^W C 130

Time min 120 Analysis after (ER)-step

Kappa number 3,1

Viscosity dm ³ /kg 970 Conditions in (EO)-step

NaOH kg/ton pulp 10 18

Oxygen pressure MPa 0,2 0,2

Pulp concentration 12 12

Temperature 60 70

Time mm 10 120 Analysis after (EO)-step

Kappa number 2,5 3,5

Viscosity dπrYkg 990 935 Chemical consumption

Oxygen kg/ton pulp 20 ^"~ 20

White liquor kg NaOH/ton pulp 18

NaOH kg/ton pulp 15 23

Chlorine " " " 10 10

Chlorine dioxide,active Cl " ^"' " 33,0 40,0

Total active Cl kg/ton/kappa number 2,?5 2,62 Analysis of bleached pulp

Diffuse blue reflectance factor #IS0 90,0 90,0

Viscosity dm /kg 921 823

Analysis of waste liquor

COD kg/ton ^' pulp 40,2 40,1

T0C1 ^" " " " 1,49 1,62

Chemical costs SEK/ton ^' pulp 160,50 153,46

Although the arrangement of the (ER)-step according to the invention in this position yielded ^' a slightly higher chemical cost, considerable advantages have been gained in respect of the viscosity of the pulp, the reduced amount of TOCl in the bleaching department waste liquor and the reduction in chlorine chemical consumption. There exist, besides, possibilities of reducing the chemical cost, a.o. by internal circul¬ ation of waste liquor within the (ER)-step, a fact which has not been paid regard to in the economical calculation.

EXAMPLE 5

According to this example, which shows the invention applied in a future bleaching sequence where elementary chlorine entirely has been replaced by chlorine dioxide, the special reducing step according .to the invention had been placed between an oxygen step and a chlorine dioxide step. The experiments were carried out accord¬ ing to Table 3 below. The sequence 0D(E0)DED was used as reference.

TABLE 3

Invention Reference

Conditions in (ER)-step

White liquor, sulphidity

White liquor,batch NaOH

Pulp concentration

Temperature

Time Analysis after (ER)-step

Kappa number

Viscosity Chemical consumption

Oxygen 20

White liquor kg Na

NaOH kg/ton pulp 19 21 Chlorine " " "

Chlorine dioxide,active Cl " " 51,1 72,2

Total active Cl kg/ton/kappa number 3,68 3,78

Analysis after (EO)-step

Kappa number 4,6 8,-

Viscosity 922 944

Analysis of bleached pulp

Diffused blue reflectance factor #IS0 90,0 90,0

Viscosity dm ³ /kg 737 680

Analysis o£ waste liquor

COD kg/ton pulp 29,1 39,9

TOCl " " " 0,92 1,30

Chemical costs SEK/ton pulp 170,14 224,94

Also in this situation, which is difficult ^' from a cost and chemical point of view, it was found, that the arrangement of (ER)-step according to the invention yielded considerable advantages in respect of lower chlorine dioxide consumption, lower chemical costs, higher pulp viscosity, lower COD-emissicnand additional reduction of the already very low TOCl-emissions.

EXAMPLE 4

According to a further experiment, the step accord¬ ing to the invention was arranged after a (D,C+D)-step as replacement for a conventional (EG)-step. The sequence 0(D,C+D)(E0)DED was used as reference. The results are shown in Table 4 below.

I ι

11

TABLE 4

Invention Reference

Conditions in (ER)- and, respectively, (EO)-step

White liquor, sulphidity

White liquor,batch NaOH

NaOH

Oxygen pressure

Pulp concentration

Temperature

Time

Analyses after (ER)- and,respective y, (EO)-step

Kappa number

Viscosity Chemical consumption

Oxygen kg/ton pulp

White liquor kg NaOH/ton pulp

NaOH kg/ton pulp

Chlorine " " "

Chlorine dioxide,act.Cl " "

Total act.Cl kg/ton/^appa number Analysis of bleached pulp

Diffused blue reflectance factor %IS0

Viscosity dm /kg Analysis of waste liquor

COD kg/ton pulp TOCl II II 11 Chemical costs SEK/ton

Compared with a conventional (EO)-step, the arrange¬ ment of the (ER)-step according to the invention in this position resulted in advantages as lower chlorine dioxide consumption, higher pulp viscosities and red¬ uced TOCl-emission.

By the special reducing step according to the pres¬ ent invention, a very selective delignification is obtained, i.e. the delignification occurs substantially without carbohydrate attack. Compared with conventi¬ onal oxygen prebleaching, the delignification can also be driven further prior to the final bleaching with chlorine chemicals. This is of great importance from an environment point of view - reduced emission of chlorinated organic substances - as well as from an economical point of view - reduced con¬ sumption of expensive chemicals, which cannot be regenerated.

In cases when the reducing alkalic treatment step according to the invention is placed before the chlorine chemical bleaching department, the step easily can be formed so as to fit in the chemical recovery system of a mill. It is hereby possible to use cheap chemicals, which can be regenerated.In a sulphate process, white liquor can advantageously be used, which contains both alkali and sulphide and, respectively, polysulphide.

The invention is not restricted to the embodiments shown and described above, but can be varied within the scope of the invention idea.