The present invention relates to a process for bleaching lignocellulose material, hereinafter denoted "pulp", wit bleaching agents containing some kind of peroxide. The term "pulp" primarily includes bleached and unbleached cellulose v/ith low lignin content, i.e. so-called chemical pulps produced in accordance v/ith the sulfite, sulfate, soda or oxygen process, but also cellulose pulps v/ith high lignin content, i.e. pulps produced in accordance with the mechanical, thermo-mechanical or chemimechanical methods, where the fibres are exposed by a mechanical process v/ith or without treatment by heat and/or chemicals, as well as pulps produced from recycled fibres.

The bleaching of chemical pulps is generally carried out today v/ith bleaching agents containing chlorine, such as chlorine (Cl ), chlorine dioxide (CIO ) and hypochlorite (EfaClO). A reduction of environmentally disturbing effluents from bleaching plants is desirable. One way of providing this is to recover spent liquor from the bleaching plant together vith spent liquors from digesting. Serious corrosion problems occur when using bleaching agents contain¬ ing chlorine, however, because of the large amount of chlorides which are recycled to the chemical recovery equipment. A second way of providing a reduction of the environmentally disturbing substances is to introduce separate purification of the spent liquors from the bleaching plant before they are discharged into the receiving body of water, although this entails considerable expense as well as other disadvantages. A third way is to use chlorine-free bleaching agents during bleaching. One such bleaching agent is oxygen, which has been increasingly used in recent times. It has been possible to reduce the discharges from bleaching plants by more than 50 β>, using

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an alkaline oxygen step as the introductory bleaching step during bleaching of pine sulphate pulp, for example, because the spent liquors from the oxygen bleaching do not contain chloride and are recoverable. After an oxygen bleaching step, there remains about 50 ^ of the lignin found in the pulp after digestion which must still be dissolved out of the pulp v/ith bleaching agents containing chlorine.

Other types of bleaching chemicals, which are conceivable from the point of view of recovery, are peroxides, e.g. inorganic peroxides such as hydrogen peroxide and sodium peroxide, and organic peroxides such as peracetic acid. Among the peroxides mentioned, it is principally hydrogen peroxide (H_0 _? ) which is used in the cellulo industry for the time being.

Bleaching chemical pulps with hydrogen peroxide is usually carried out in the final part of the bleaching process, i.e. when the majority of the environmentally disturbing substances have alrea been dissolved out of the pulp. The idea of using peroxide in the final step of a bleaching cycle is to obtain an improvement of the brightness stability of the final-bleached pulp. Purther ore, a certain reduction of undesirable extractive substances in the final pulp is obtained.

The use of hydrogen peroxide in the first step of a bleach¬ ing cycle is not applied in practice to any great extent, due to the large amount thereof which has to be added to provide the required dissolution of lignin. To achieve a release of lignin corresponding to that obtained in oxygen bleaching of sulfate pine pulp, an additi of hydrogen peroxide of about 80 kg H _p 0_ per ton pulp is required, which represents a cost of about 300 Swedish Crowns per ton pulp, calculating with today's price for hydrogen peroxide, and this shoul be compared with the price of oxygen bleaching, which is about 25 Sw. Crowns per ton pulp.

The mentioned conventional peroxide bleachings are carried out at a pH of about 10 - 11, measured at the start of bleaching. Bleaching tests with hydrogen peroxide at a pH-value lower than 7 ar to be found described in an article in Tappi, volume 39, number 5,

1956, page 284 - 295. It is apparent from this article that bleachin with hydrogen peroxide at low pH values, especially at pH 0.5, result in substantially the same increase in brightness as for alkaline pH, in spite of lower hydrogen peroxide consumption for acidic pK. Howev

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eavy de eriora on o e pu p v scos y was o a ned a e same time, i.e. the hydrogen peroxide not only attacked the lignin but also the cellulose. This results in deterioration of the mechanical strength properties of the pulp. The present invention constitutes a solution to the problem presented above, and relates to a process for bleaching and the extraction of material containing lignocellulose, using a peroxide- containing bleaching agent to remove lignin in an acidic environment, and is characterized in that at least one step in the bleaching process comprises treatment of the lignocellulose material with a bleaching agent containing peroxide at a pH from -2 to +7, pre erably from -0.5 to 3.0, in the presence of 0,01-5, preferably 0,1-0.5 g/l of an organic or inorganic complexing agent, this treatment being followed immediately by aLkaline extraction of dissolvable lignin, without intermediate washing.

The combined peroxide and extraction step characteristic of the invention can be introduced anywhere in a bleaching cycle, i.e. at the beginning, in the middle or at the end of it, although it is .preferred that the combined peroxide and extraction step is used as the first step in a bleaching cycle. It is furthermore quite possible to use the combined step repeatedly in a bleaching cycle, e.g. as the introductory and terminal steps in such a cycle.

Pulp for processing according to the invention can thus be either unbleached or bleached in a previous step. The pulp consistency is not critical, but can vary between 1-50 _t although a consistency of 8-22 is preferred. Depending on the consistency of the pulp when it is introduced into the bleaching step according to the invention, the pulp is dewatered or diluted so that the desired consistency is obtained. A press is preferably used for dewatering. After possible adjustment of the pulp consistency, the pulp suspension is provided, e.g. in a mixer, with peroxide-contain¬ ing bleaching agent, acid and complexing agent. The acid may either be an inorganic one, e.g. sulfuric acid or nitric acid, or the acidic solution obtained as the residue in chlorine dioxide manufacture, or an organic acid such as oxalic acid. The acid is added to such an amount that the pH of the pulp suspension will be from -2 to 7, preferably from -0.5 to 3.0, The quantity of coiplexinng agent which is to be added is 0.01-5 g/l, preferably 0.1-0.5 g/l _° The amount of peroxide bleaching agent that is added can vary heavily,

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partly depending on the lignin content of the incoming pulp, and partly on the desired lignin content of the pulp after the bleach¬ ing step according to the invention. The suitable amount of peroxide containing bleaching agent is generally 0,1-4 ^c j calculated on the weight of absolutely dry pulp. After the addition of the chemicals mentioned above, the bleaching itself takes place, e.g. in a bleach¬ ing tower. The total bleaching time can vary between 1 and 300 minu¬ tes and the bleaching temperature between 20 and 100 C. A bleaching time of 60-180 minutes and a bleaching temperature of 60-90 C are preferred however. The pulp suspension is subsequently taken to a further mixing apparatus (mixer) and without being washed is provided with alkali, e.g. ammonia, sodium carbonate, sodium hydrogen carbonat sodium hydroxide or oxidized white liquor so that a pH of 7.0-12.0 preferably 9,0-11.0 is obtained, whereon extraction of the pulp takes place, e.g. in a tower. The extraction is 'carried out at a pulp consistency of 1-50 ^, preferably 8-22 o, and the temperature is kept at 20-100°C, preferably 50-80°C, The time for the extraction is 15-300 minutes, preferably 60-180 minutes. Since the pulp is not washed between the peroxide bleaching step and the extraction step, continued bleaching of the pulp takes place at the same time as the extraction, by means of peroxide transferred from the peroxide bleach ing step and not consumed there. This bleaching takes place in an alkaline environment, contrary to the previous acidic environment. At the end of the reaction, the pulp is dewatered, e.g. by means of a press, or washed, whereafter it can be bleached further, e.g. with a bleaching agent containing chlorine, preferably chlorine dioxide. During the bleaching step according to the invention, a considerable amount of delignification takes place, i.e. the content of lignin in the pulp is reduced considerably, while the brightness of the pulp is increased. It has been surprisingly found that during the acidic peroxide bleaching step, no delignification proper takes place, for if the process is broken off after this step, and the lignin content of the pulp is checked, it will be found to be approximately the seme as before bleaching was started. Seduction of the lignin first takes place at the immediately subsequent alkaline extraction. A probable explanation of this is that the lignin has been modified during the acidic peroxide treatment so that it can easily be extracted by the alkali.

According to a preferred a bodiment of the invention, the pulp suspension is dewatered after the acidic peroxide bleaching so that the pulp consistency is increased to 18-50 / ^c o, preferably 25-35 %• Dewatering can be done by means of a press. The bleaching liquor pressed out usually contains unconsumed peroxide, and is therefor recycled to the mixer co ing before the bleaching tower, this mixer also being charged vith fresh peroxide. In accordance with this embodiment, the pulp suspension must be provided with diluting liquid, (apart from alkali) before the extraction step, e.g. water, so that the desired pulp consistency is obtained in the extraction step.

In a similar way, the bleaching liquor pressed out at the extraction step can be recycled to the mixer co ing before the extraction step. This mixer also being supplied with alkali and possibly diluting liquid, as mentioned above.

A large number of both organic and inorganic chemicals can be used as complexing agents, It is preferred that the complexing agent is one of the group of pol carboxylie acids, nitro en-cont in¬ ing polycarboxylic acid and polyphosphates. As examples can be mentioned nitrilotriaminoacetic acid (XJTA), diethylenetria ine pentaacetic acid (DTPJL), ethylenedia ine tetraacetic acid (EDTA), citric acid, tartaric acid and sodium tripolyphosphate (STP?).

It has been found that improvement of the viscosity stabilizing effect of the complexing agent can be obtained by charging it together v/ith chemicals containing magnesium, e.g. magnesium salts, such as magnesium carbonate, sulfate, hydroxide, oxide, an especially suitable magnesium compound is magnesium sulfate (LigSO. ). The amount of charged magnesium compound is 0.01-5 g/1, preferably 0.1-0.5 g/l. As is apparent hereinbefore, the bleaching process accord¬ ing to the invention enables successful delignification of the pulp. However, it has been completely surprisingly found possible to use the bleaching process according to the invention for adjusting the final viscosity of the pulp. In producing papermaking pulp, as high a viscosity as possible is striven for, but in the production of viscose pulps it is striven to lower the viscosity of the pulp to certain definite levels, these levels depending on what the pulp is going to be used for in the viscose industry. The technique usual today for controlling the viscosity of the pulp to the desired level

is to use hypochlorite, e.g. sodium hypochlorite (iTaCIO) in one of the bleaching steps. V/ith the help of temperature and pH as well as the quantity of hypochlorite charged in the bleaching step, viscosity can be controlled to the desired level. As stated above, it has been found possible to replace the hypochlorite step v/ith the bleaching process according to the invention. The -pulp viscosity is guided to the desired level by varying the charge of complexing agents, when this method is used. Pulp viscosity is directly dependent on the amount of complexing agent added, i.e. a low addi- tion of complexing agent gives low viscosity, while a larger amount of complexing agent gives higher viscosity to the pulp.

The process according to the present patent application has important advantages. One of these is that conventional bleaching steps with chlorine-containing bleaching agents can be replaced by the bleaching process according' to the invention. The gain here is that the spent bleaching liquors can easily be recovered, which is not the case with liquors from steps using bleaching agents contain¬ ing chlorine. The quantity of environmentally disturbing substances which must be discharged to the receiving body of water can thus be reduced considerably. The process according to the invention, compared with previously known peroxide bleaching processes, further more leads to a considerable reduction of bleaching chemical costs. In addition, a pulp having good quality characteristics is obtained, e.g. high viscosity with a specified lignin content and very high purity.

The advantages of the process according to the invention are illustrated by the following working examples. Example 1 unbleached birch sulfate pulp vith a lignin content of 17.3, measured as a kappa number according to the SCAϊT-standard, and a

^• z viscosity of 1214 dm /kg was charged with a bleaching solution containing hydrogen peroxide in such a quantity that it corresponded to 1.0 , calculated on the weight of absolutely dry pulp. The pulp consistency was adjusted to 12.0 by adding water. The pulp was divided into sample A and sample B, Sulphuric acid was added to sample A so that a pH of 2,5 was obtained, and sodium hydroxide was added to sample B so that a pH of 11.0 was obtained. After thorough blending in glass vessels, both samples were put into a water both at a temperature of 65 C. The vessels containing the samples were

a owe o s an n e wa er a or ours, w erea er e samp es were dewatered in a centrifuge to 30 Jo pulp consistency. Diluting liquid (water ) was then added to both samples so that the pulp consistency was once again 12 fo. Using sodium hydroxide , the pH of the sample was adjusted to 1 1 ,0, whereafter they were once again placed in the water bath at 65 G. After a 2-hour period in the bath, the process was interrupted and the samples washed with distilled water. After washing, the samples were anlyzed v/ith respect to kappa number in accordance v/ith SCAU-C 1 : 59, viscosity in accordance vith SCAIT-C 15 : 62 and brightness in accordance with SCAH-C 11 : 75. Iodine titration was used to determine the amount of hydrogen peroxide consumed. The table below shows the analytical data obtained for the pulps, including the amount of hydrogen peroxide (H _? 0„ ) consumed. Table 1 pH in peroxi de Kappa Viscosity Brightness % H ₂ 0 ₂

Sample step number ^• / ^c o ISO consumed

A 2.5 12.0 61 9 46.0 0.7 B 1 1 .0 15.0 941 41 .1 1 .0-

As will be seen from Table 1 , a better delignification of the pulp (lower kappa number) was obtained vith hydrogen peroxide at a pH of 2.5 than at pH 11.0. There was also a disastrous deterioration of the pulp viscosity at the same time, however.

The above experiments were repeated, although with the difference that both samples, i.e. sample A., and sample B ₁ , v/ere given an addition of 0.1 diethylenetriaminepentaacetic acid (DTPA) and 0.1 fo agnesiu sulfate (llgSO.), calculated on the weight of absolutely dry pulp, in the first reaction step, i.e. the peroxide step. The same analyses as previously performed v/ere executed nov/ with the following results. Table 2 pH in peroxide Kappa Viscosity Brightness JS ^H _? ^c _? Sample step number dm5/kg ^c / ISO consumed

A ₁ 2.5 12.6 989 46.3 0.4

B ₁ 11.0 15.1 988 41.5 1.0

As is apparent, sample A.. , obtained in accordance with τhe invention, gave a better delignification and brightness in spite of a considerably lower consumption of peroxide compared with bleaching at

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pH 11.0, although this pH is-.,usual in conventional peroxide bleaching. Purthermore, the same viscosity for the process according to the invention was obtained as with bleaching at a pH of 11.0, in spite of a lower kappa number. Exa nle 2

Unbleached sulfite pulp, digested in two steps, kappa number 12.1 and viscosity 1147 dm /kg was treated in the same way as for example 1, Sample A was bleached at a pH in the peroxide step of 2.5 and sample B was bleached at a pH of 11.0 in the peroxide step. In both cases the experiments were carried out with, and without, an addition of 0.1 Ji diethylenetriaminepentaacetic acid (DTPA) and 0.1 Jo magnesium sulfate (lύgSO ). Results of the an&.lyses carried out may be seen from Table 3»

Table 5 pH in peroxide Kappa Viscosity Brightness Jo K_0_ Sample step number dr /kg - ISO consumed

A

Without DTPA

+ xgso ₄ 2.5 4.5 539 61.2 0.9

With - " - 2.5 6.9 10G5 69.8 0.4

B

Without DTPA

+ MgS0 ₄ 11.0 8.5 1005 70.3 1.0

With - " - 11.0 8.8 1064 72.9 1.0

As will be noted, the process according to the invention also works in a similar way for a sulfite pulp. The process according to the invention, i.e. as for sample A with an addition of DTPA + HgSO , gives considerably better delignification for a substantially lower peroxide consumption compared with conventional peroxide bleaching at alkaline pE. The viscosity of the pulp was even somewhat higher compared with the conventionally bleached, pulp, in spite of a lower kappa number. If samples A are compared at the same kappa number, the

•z for ? = 7.0 there is obtained a visco sity of 780 dm /kg for a peroxid consumption of 0.8 JJ, without DTPA + HgSO . . Example 3

An unbleached spruce sulfite pulp, digested in two steps, and with a kappa number of 1 3.4 and visco sity 1 180 was treated wit sulfur dioxide ( S0 _? ) dissolved in water, with the intention of re ov-

in heavy metals from the pulp. The pulp consistency was 3.5 Jo and the treatment was carried out at room temperature for a period of 1 hour with a solution having a content of sulfur dioxide such that the total charge was 2.0 j SO calculated on the weight of absolutely dry pulp. After this treatment the pulp was washed with distilled water and dewatered in a centrifuge to 30 / ^c pulp concentration. The pulp thus treated only contained traces of heavy metals such as iron, copper and manganese.

The pulp was treated according to the mode apparent from Example 1. In these experiments as well, the charge of hydrogen peroxide was 1.0 ^c /o, calculated on the weight of absolutely dry pulp, while the sul ric acid was charged to the pulp in such a quantity that the pH became 2.0,

Two experiments were carried out, sample 1 without, and sample 2 with 0.1 o diethylenetriaminepentaacetic acid (DTPA). The same analyses as in the previous examples were carried out v/ith the following results.

Table 4 pH in peroxide Kappa Viscosity Brightness % H 0 Sample step number d /kg % ISO consumed

1 2.0 6.9 743 61 .9 0.67

2 2.0 7.4 982 74.6 0.32

It will be noted that in the experiment according to the invention, i.e. with sample 2, a considerably higher viscosity and greater brightness v/ere obtained in comparison with the experiment with sample 1, where no complexing agent was charged. In spite of the pulps being delignified approximately to the same extent in both experiments, the hydrogen peroxide consumption in the experiment according to the invention, i.e. v/ith the addition of complexing agent, was only half the consumption for the experiment without the addition of complexing agent.

This example points to a condition which is both remarkable and important, i.e. in spite of the heavy metals having been removed from the pulp by means of an SO wash, the complexing agent has a decisive effect on the pulp viscosity. The conclusion can be drawn from this that the complexing agent, in the process according to the invention, affects the bleaching reaction in a way, not so far investigated, so that the peroxide does not attack the cellulose to

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any notable extent. This is surprising, because complexing agents are generally used in bleaching techniques precisely for complexing heavy metals in order to prevent their injurious effect on the bleaching process. Example 4

An unbleached viscose pulp with a kappa number o ^' f 7.9 and

^• 2 viscosity 787 dm /kg, digested according to the acidic sulfite metho was treated in the way set forth in Example 1. The hydrogen peroxide charge was 0.5 Jo, calculated on the weight of absolutely dry pulp, and sulfuric acid was charged so that the pulp suspension was given a pH of 2.0. The hydrogen peroxide step was followed by an alkali extraction at a pH of 11.0. A series of experiments were made using diethylenetriaminepentaacetic acid (DTPA) as a complexing agent, the quantity of the agent in the hydrogen peroxide step being varied. Por a charge of 0.1 J DTPA, calculated on the weight of absolutely dry pulp, an experiment was made vith an addition of 0.1 J magnesium sulfate (MgSO.), calculated on the weight of absolutely dry pulp. After terminating the process, the viscosities of the pulps v/ere determined. Table 5 pH in peroxide Addition of Addition of Viscosity

Sample step DTPA, MgS0 ₄ , dm3/kg

1 2.0 0 0 445

2 2.0 0.05 0 680

3 2.0 0.1 0 730

4 2.0 0.1 0.1 747

As apparent from the results, it is quite possible to guide the viscosity of a viscose pulp to the desired level by varying the charge of the complexing agent. This means that a hypochlorite (EfaCl step, which is customarily used for this purpose, can be exchanged for the bleaching step according to the invention, enabling recyclin of spent bleaching liquors to the chemical recovery plant.

It is also apparent from the results that it is the conplex- ing agent (DTPA) which has a dominating effect on the pulp viscosity and the addition of magnesium only improves the viscosity marginally Example 5

A pine sulfate pulp v/ith a kappa number of 29 _* 9 and a

^• z viscosity of 1 135 dm /kg was oxygen-bleached so that the kappa numbe

was owere o . an e v s s m g. e oxygen- bleached pulp was treated in the way as described for Example 1 and in accordance v/ith the invention, at a pH of 2.2 in the hydrogen peroxide step, followed by an alkali extraction at a pH of 11.0 (sample A) and also according to conventional technique, with hydrogen peroxide at a pH of 10.9 during the beginning of bleaching (sample B). In both experiments 0.1 J diethylenetriaminepentaacetic acid (DTPA) was added. The same analyses (except for brightness) as in the previous examples v/ere carried out, with the follov/ing results. Table 6 pH in peroxide Kappa Viscosity spent H _^ O , % Sample step number dur/kg

A 2.2 8.7 943 0.51

B 10.9 8.3 947 1 .50

It will be seen that there is the same degree ^' of delignifica¬ tion and viscosity for both samples. In the experiment according to the invention, only one third of the hydrogen peroxide quantity required according to conventional technique v/as consumed. This example shows that it is possible to apply the bleaching step according to the invention in a bleaching cycle, e.g. as a second bleaching step after an initial oxygen bleaching step, and obtain a continued delignification of the pulp at a reasonable cost. Example 6 \

Spruce wood chips were digested according to the sulfite method in a laboratory digester, the chips having an admixture of 5 bark for producing a pulp with low purity, i.e. a pulp with many impurities in the form of specks.

This pulp was subsequently bleached vith the following bleach¬ ing cycles: 1 = Alkali, chlorine, hypochlorite, chlorine dioxide = ECHD

2 = Alkali, chlorine dioxide, alkali, chlorine dioxide = EDED

3 = Peroxide, chlorine dioxide, alkali, chlorine dioxide = PDSD

4 = according to the invention, chlorine dioxide, alkali, chlorine dioxide = UDBD - The conditions in the respective bleaching step are apparent from the table below.

Table 7

ECHD

Temp., G me, h

E 65 2 12 E 65 2 12

C 30 3/4 3 D 60 5/4 3

H 40 4 6 . E 60 2 12

D 75 3 0 D 75 3 6

E 60 2 12 D 60 5/4 3

D 75 3 6 Ξ 60 2 12

D 75 3 6

:) ₁ PH = 11.0 x)o pH = 2. ,0

The charges of chemicals in all the bleaching cycles v/ere adjusted so that a final brightness of the pulp of 91- 0.5 o ISO was obtained.

In order to form an impression of the purity of the pulp, a speck count according to a method developed by ISO (international organization for standardization) with the denotation ISO/TC 6/SG 5/w'G- 7 "Dirt and Shives in Pulp" was adopted. The speck count was ma on unbleached pulp (control), on pulp after the tv/o initial steps in the bleaching cycle, as well as on finally bleached pulp. The result are apparent from the table below. Table 8 Pulp Number of specks and speck area

Group 2 . Group 3 _?, Group 4 ₂ Group 5 2 area (mm' ^" ) area (mm area (mm ) area (mm )

= 1.0-4. ■ 99 = 0.40-0, .99 0.15-0.39 0.04-0.14

Unblei ached 54 84 242 322

EG 18 58 128 414

ΕD 1 20 61 127

PD 2 23 57 118

U 1 10 23 51

ECHD 3 21 26 171

ΞDΞD 0 7 5 51

PDΞD 1 4 6 46

UDED 0 1 3 19

As is shown by the Table, the most superior result was ob¬ tained with the bleaching cycle containing a bleaching step accord¬ ing to the invention. This shov/s that the bleaching process accord¬ ing to the invention, apart from the advantages previously mentioned, also enables the production of a very pure pulp.