FI109209B - Procedure for checking delignification and bleaching of a pulp slurry - Google Patents

Description

109209

A method for controlling lignin removal and bleaching of paper pulp slurry

The present invention relates to a process for lignin removal and bleaching of aqueous lignocellulosic pulp under acidic conditions at a pulp mill bleaching plant. The invention also comprises a process for producing paper or pulp from said bleached pulp slurry.

Lignin is an amorphous polymer that, like an adhesive, holds together cellulose fibers in wood.

Some lignin is also present in the fiber wall. In its natural state, wood has a light color of lignin.

The purpose of chemical defibration, such as sulphate cooking, is to dissolve the lignin from the wood material with the help of a broth. During cooking, the lignin decomposes into smaller units and darkens, thus increasing the absorption of light. The dark lignin components, some of which have dissolved and some of the doors still in the fiber, give the pulp a dark color.

Lignin removal and bleaching, referred to in the following text and the appended claims as "bleaching", are chemical refining of fibers in cooked lignocellulosic pulp. The main purpose of acidic bleaching sequences is to remove lignin and other components capable of absorbing visible light, thereby affecting the lightness of the pulp. Preferably, the bleaching process should not significantly affect the fiber, i.e. the carbohydrate components of the lignocellulose.

, Selectivity in bleaching can be defined as the relative reactivity of a given reactive substance with respect to the lignin and carbohydrate components of the pulp in a competitive situation.

. * ·. Competitive initial reactions are believed to consist of the addition of an electrophile to the .1 * aromatic ring or olefinic structures in the lignin and the capture of the hydrogen atom from the carbohydrates.

The bleaching steps contemplated by the present invention include acidic 1 "bleaching steps such as one or more chlorine dioxide (D) steps, peracetic acid (Paa) steps, ozone (Z) steps, as well as multi-chemical steps utilizing bleaching, ·, combinations of substances.

2,109,209

The bleaching plant generally uses bleaching sequences consisting of several bleaching steps, usually separated by the washing steps, although in some cases, washing may be omitted.

An example of a modern ECF sequence for the production of bleached sulphate cellulose pulp is Do (EOP) DED, where D stands for chlorine dioxide bleaching, E stands for extraction and EOP stands for oxygen and peroxide enhanced extraction. In such a system, the unbleached pulp enters the bleaching plant with a basic pH. The chlorine dioxide bleaching is then carried out under acidic conditions and the subsequent extraction is carried out under alkaline conditions.

Many modern bleaching sequences include the ozone bleaching step Z. This step may, for example, be included in the sequence ZQ (PO). The ozone can also be combined with the chlorine dioxide derivative in the (ZD) or (DZ) step. The pH of the pulp slurry should be lowered to about 3 for ozone in the Z-stage to achieve efficient bleaching.

There is also a bleaching step using peracetic acid (Paa) where bleaching is carried out at a pH of about 4-6.

In addition to various sequences containing separate bleaching steps, the pulp can be bleached in a so-called multi-chemical step which has multiple bleaching processes without intermittent washing. Examples of such steps are the (ZD) or (DZ) - mentioned above. in addition to the combination, the (ZPaa) or (AD) step wherein A represents acidic hydrolysis.

The acids commonly used for acidifying pulps for acidic bleaching steps, ···, are mineral acids such as sulfuric acid. It has been proposed to adjust the pH of acidic bleaching steps with carbon dioxide, a gas that reacts with water to produce carbonic acid.

However, there is very little real information on the effect of carbon dioxide on bleaching.

; > · Practical experiments have been carried out at the factory in Finland using carbon dioxide to adjust pH • '': in the enzyme bleaching step. In addition, it has been reported that the use of carbon dioxide in washing. _ in one of the plants in Sweden will provide a better washing effect, ···. reduced consumption of bleaching chemicals (Östberg, G., et al., The World Pulp and Paper Week, June 4-7, 1996, Stockholm, pp. 508-515).

t 3 109209

Carbon dioxide is also disclosed in GB 815 247 for use in combination with sodium hydroxide to provide a pH buffer for in situ chlorine dioxide bleaching to be performed at pH 9-5. Teder, A., et al., TAPPI 61 (12) (1978), p. 59-62 have reported that in two-step pH-adjusted chlorine dioxide bleaching, the pH can be maintained at high pH for extended periods of time using NaHCO3 buffer or NaOH.

The present invention is based on the realization that while carbon dioxide has little or no effect on the final pH of highly acidic bleaching steps, such as Step D, carbon dioxide has an effect on the bleaching result and can be used to control the chemical reactions of bleaching chemicals and various lignin between compounds in the pulp slurry.

Carbon dioxide is a gas that is soluble in aqueous materials under basic conditions, e.g. water or pulp slurry. The dissolved gas forms carbonic acid, H2CO3, which is easily dissociated as shown below: H2O2 CO2 (g) <=> CO2 (d) + H2O <=> H2CO3 <=> H * + HC03 '<=> 2H ++ CO332' (g = gas; d = dissolved)

The carbonic acid produced by the soluble carbon dioxide is a weak acid capable of lowering the pH of the alkaline pulp to neutral and slightly below pH 6. However, according to the present invention, it has been found that although carbon dioxide is capable of lowering the initial pH of the , its pH-lowering effect disappears at lower pH ranges, and contrary to previous beliefs, carbon dioxide cannot be used to 'lower' the pH in the actual (final) bleaching step.

According to the present invention, it has been found that carbon dioxide can be used to control the rate of degradation and degradation of the various components of the aqueous lignocellulosic slurry.

It is therefore an object of the present invention to provide an acid bleaching process in which the degradation of lignin and carbohydrates in the pulp slurry is controlled.

It is another object of the invention to improve the selectivity of bleaching by directing the function of the bleaching: ": the chemical mainly for the reaction with lignin.

4, 109209

It is a further object of the invention to provide a bleaching process in which the degradation process of carbohydrates and / or lignin is effected by attenuating the degradation of said compounds into smaller particles.

The object of the invention is to improve the selectivity of bleaching and thus to obtain a certain amount of bleaching pulp which is lighter in color and / or has a lower kappa number.

Another object of the invention is to improve the selectivity of bleaching and thereby obtain a bleaching pulp of equal light and / or the same kappa number with a smaller amount of bleaching chemical.

Another object of the invention is to reduce the chemical oxygen demand (COD) of the bleaching filtrate.

The present invention is defined in the appended claims. The invention thus relates to a process for bleaching aqueous lignocellulosic pulp under acidic conditions in a pulp mill bleaching plant, said method comprising the steps of: providing an aqueous lignocellulosic pulp slurry in said bleaching plant; adding a bleaching chemical to said pulp slurry and adjusting an acidic pH thereto to cause a reaction between the lignin and said bleaching chemical; adding a carbon dioxide generating agent before or simultaneously thereto to said lignocellulosic pulp slurry to provide carbon dioxide in said acid pulp slurry to control the degradation of lignin and its derivatives in its reactions with said bleaching chemical; and then subjecting said pulp slurry with an "" E "" basic extraction step to dissolve and remove the reacting lignin compounds, · · · from said pulp.

* · I

A preferred carbon dioxide generating agent comprises carbon dioxide in gaseous form.

However, carbon dioxide may also be provided in liquid or solid form. In an acidic aqueous slurry, carbon dioxide can also be produced by compounds which produce carbon dioxide by dissociation and / or degradation at that pH. Such compounds include gases or liquids which are capable of generating carbonate and / or bicarbonate ions in aqueous slurry under basic or neutral conditions. Examples of compounds are alkali metal bicarbonates and carbonates, which decompose at low pH values to release carbon dioxide.

The carbon dioxide generating agent used in accordance with the present invention is preferably a carbon dioxide gas, which is fed directly into the aqueous pulp slurry. However, it may also be an aqueous liquid producing carbon dioxide, such as dilution water containing carbon dioxide, bicarbonate or carbonate. It is not critical to the purpose of the invention how the carbon dioxide enters the slurry, it is only required that it provides carbon dioxide in the aqueous pulp slurry.

i | ! Preferably, carbon dioxide should be added directly to the pulp slurry prior to or immediately after said bleaching step, but after any washing step prior to the bleaching step. If carbon dioxide is added to the scrubbing stage, most of it is removed with the scrubbing water and the bleaching effect is poor. However, carbon dioxide may be added to any dilution water used to dilute the pulp after the pre-washing step.

Preferably, said carbon dioxide generating agent comprises gaseous carbon dioxide injected into said pulp slurry stream just prior to the addition of a D, Z, Paa or multi-chemical bleaching step.

The use of carbon dioxide to control bleaching reactions in accordance with the present invention has several advantages, e.g.: - the pulp can be obtained after leaching with a higher brightness and kappa number as carbon dioxide. directs the action of the bleaching chemical on the initial bleaching reactions with lignin., - increased brightness does not cause a decrease in viscosity because carbon dioxide 11 suppresses reactions between bleaching chemicals and carbohydrates, · ·. an improvement in the COD content of the filtrate can be obtained. * * ·. - improved bleaching efficiency can be used to reduce bleaching chemicals, '1'. consumption - there are no environmental hazards to carbon dioxide.

": The pulp slurry to be treated in accordance with the present invention is not critical. Almost any lignin-containing pulp can be bleached by the processes of this invention. Examples of such pulps are chemical pulps, organosolv pulps, mechanical pulps, chemical-mechanical pulps. pulps, semi-chemical pulps, pulps containing recycled fibers or waste paper, or any mixture thereof of a pulp mill bleaching plant.

6 109209

The pulp may be introduced into the bleaching plant after cooking or may be directed to the bleaching plant through an oxygen-using lignin removal step and possibly other treatments.

The consistency of the pulp may be low, medium or high. Better | for reasons of workability, it is generally preferable to use low to medium consistency pulp. Typical consistency is 1-18%, preferably 3-15%.

j

The present invention also encompasses further processing the bleached pulp at a pulp or paper mill to provide a dried pulp and / or paper containing said bleached pulp. The preparation of the dried pulp and paper from the bleached pulp can be carried out in a conventional manner well known to those skilled in the art.

The pulp slurry entering the bleaching plant is generally alkaline and in a preferred embodiment of the invention the pH is adjusted to an acidic or neutral pH by adding said carbon dioxide generating agent. However, it may sometimes be more economical and also technically sufficient to adjust the pH of said pulp using a combination of carbon dioxide and another acid or base.

In addition, it should be emphasized that carbon dioxide is a much more environmentally acceptable chemical than most other acids. Therefore, using carbon dioxide alone is an environmentally friendly procedure. The final adjustment of the acidic pH for the actual bleaching is preferably done with a bleaching chemical or by adding another, stronger acid to the slurry.

i (

In the case of the first D stage, it is technically advantageous to lower the initial pH with carbon dioxide '.E near neutral and to add chlorine dioxide to obtain the desired pH of about 1.7-4.4, t · ·. taking into account the acidic by-products of the reactions between lignin and chlorine dioxide. Note that the addition of carbon dioxide does not significantly lower the final pH of the bleaching process.

. ' · ·. If desired, another acid such as sulfuric acid, sulfuric acid, alum, waste acid from chlorine dioxide production can be used to achieve the required lower pH.

7 109209

The strongly acidic conditions of the bleaching release carbon dioxide from the carbonates and bicarbonates contained in the solution at a higher pH. The released carbon dioxide affects bleaching by providing a controlled bleaching effect.

According to the present invention, it has surprisingly been found that when an effective amount of carbon dioxide is present in said pulp slurry at the same time as the bleaching chemical, the carbon dioxide affects the balance and / or progression of subsequent reactions between the lignin (or derivatives thereof) and the bleaching chemical (or derivatives) inhibiting the degradation of derivatives.

It has also surprisingly been found that an effective amount of carbon dioxide attenuates the degradation of carbohydrates in the pulp slurry by affecting the balance and / or progression of all carbohydrate degradation reaction chains.

Each of the above functions increases the selectivity of the bleaching chemical for controlled primary reactions with lignin.

Since the primary reactions between the lignin and the bleaching chemical are generally sufficient to render the resulting lignin derivative soluble in the aqueous medium under basic conditions, all other reactions between the lignin derivatives and the bleaching chemical are unnecessary for the bleaching. If the bleaching chemical is allowed to act freely on the lignin derivatives, the lignin reacts with a series of reaction steps that eventually lead to degradation. Among the fragments resulting from such degradation is also carbon dioxide.

In addition, if a bleaching chemical such as chlorine dioxide or ozone is allowed to act freely. ··, the carbohydrate chains of the cellulose fibers are broken down and eventually degraded. Such. ·. the result of the operation can be seen as a decrease in the viscosity of the bleached pulp.

· · The decomposition of lignin and carbohydrates to carbon dioxide can be observed in the bleached pulp • "': the carbon dioxide content of the pulp itself as well as in the gas phase in contact with the pulp, even when no carbon dioxide is introduced into the pulp.

'. According to the present invention, it has surprisingly been found that the carbon dioxide produced by the carbon dioxide generating agent of the present invention suppresses the secondary reactions that disrupt the carbohydrate chains and / or 8109209 lignin derivatives. Thus, the introduction of an effective amount of carbon dioxide into the pulp drives the bleaching chemical to advantageous primary reactions which cause active bleaching.

According to the present invention, it has surprisingly been found that the amount of carbon dioxide in the slurry should be at an effective level and that raising the amount of carbon dioxide much above the optimum level counteracts the beneficial effects of added carbon dioxide by reducing the brightness and / or kappa number of

The cause of the unfavorable effect of too much carbon dioxide on pulp is currently not understood.

However, while teaching the present invention to seek the optimum level of carbon dioxide addition, one skilled in the art will readily find an effective amount of carbon dioxide to be added to the mass. This can be done by monitoring the increase in brightness caused by the addition of the carbon dioxide generating agent and / or the decrease in bleaching chemical consumption until the trend reverses and the brightness begins to decrease or chemical consumption increases. A preferred way to monitor the selectivity of bleaching is to measure the COD content of the pulp filtrate from the bleaching step. The optimum amount of CO2 for a given C102 feed occurs at the same point as the minimum COD content of the filtrate.

In chlorine dioxide bleaching, an effective amount of carbon dioxide has been found to produce in the gas phase of the pulp system a concentration of carbon dioxide close enough to that produced by a similar pulp system bleached under the same conditions. concentration of carbon dioxide without addition of carbon dioxide generating agent.

• 1

It has been found that if a mass less than the effective amount mentioned above is obtained. ' _ the amount of carbon dioxide, the concentration of carbon dioxide in the gas phase is still the same, ie bleaching • », · ·. reactions occurring during the course of time produce carbon dioxide to break down the components of the pulp. · 1. due to breakdown or degradation. Without being bound by any theory, it is believed that when carbon dioxide is actively added to the pulp, carbon dioxide saturation is achieved without decomposing the pulp components. This suppresses the formation of additional carbon dioxide and suppresses reactions'; balance away from cleavage and degradation reactions. Therefore, the bleaching chemical is not '. _ used to cause non-bleaching degradation reactions.

It will be apparent to those skilled in the art that the present invention improves upon a conventional process at a pulp mill and that the end product of the process is bleached paper or 9,109,209 pulp according to the invention. The present invention thus also relates to a process for producing paper or pulp. In such a process, the aqueous unbleached pulp slurry is bleached in one or more acid bleaching steps, assisted by a carbon dioxide generating agent, and subsequently extracted in a bleaching plant. The pulp processing after said bleaching step (s) is then carried out in a conventional manner and the pulp is treated to form a paper or pulp web.

The invention will now be described in more detail with the aid of some examples which are merely illustrative and are not to be construed as limiting the invention in any way.

Example 1

Factory scale experiments with CCV addition in D stage

To evaluate the effect of carbon dioxide addition on the pH of the D stage, experiments were performed on a pulp mill fiber line using the sequence D (EO) (EP) D (EP) D. In the experiments, carbon dioxide was supplied after the MC pumps but before the chlorine dioxide supply. The carbon dioxide doses used were 2 and 5 kg / ADt. The first experiments lasted three hours in the Do and Di stages. Two further experiments were performed in the Di stage with CCh doses of 2 and 5 kg / ADt. Each of these experiments took one week.

The factory had a system that automatically changed chemical consumption to achieve the desired bleaching result.

* * * »· The ability of CC> 2 to stabilize pH was measured by titration of the slurry with base. The effect of carbon dioxide addition (! - t on the initial pH of the bleaching, the final pH and the pH of the incoming pulp •>) was measured.

The effect of carbon dioxide on the pH of the incoming pulp was clear. At a carbon dioxide dose of 2: kg / ADt, the pH dropped from 8.9 to 7.4 and at a dose of 5 kg / ADt to 7.0.

! ! I

However, the initial pH and the final pH of bleaching did not change significantly. When the initial pH was measured after the chlorine dioxide feed, the pH remained constant at 3.3. When the final pH was measured with a? T DO filter, the pH remained constant at 3.0 at a dose of 2 kg / ADt but increased to 3.1 at a dose of 5 kg / ADt.

10 109209

The results clearly showed that although the pH of the incoming mass decreased, the initial pH and end pH did not decrease.

Example 2 T Conditional Scale Experiments in D i Step t i

Carbon dioxide was introduced at the same point as in the experiments of Example 1. Bleaching was followed by EOP- | extraction step. Chemical consumption was again changed according to the automation system. Chemical consumption therefore had to be calculated over the entire fiber line. Also includes sodium hydroxide and hydrogen peroxide the consumption was calculated in addition to the consumption of chlorine dioxide. All of these results were compared to three reference periods before and after these experiments, representing a total of four weeks.

All parameters and test values are shown in Table 1.

Table 1. Chemical consumption in the Di test on the fiber line C102 dose ____ C02 dose, kg / ADt ___ 0__2__5_

Do phase, kg / ADt 31 ^ 6 29 ^ 2 3T4 ~ D 1 phase, kg / ADt 20.0 18.5 19.3 P2 phase, kg / ADt__6.9 7.7__7 ^ 2_

Yhteensä__58,5__55,4__57,9_

NaOH____ CO 2 dose, kg / ADt__0__2__5_: EO phase, kg / ADt 8.4 8.1 7.8 EP phase, kg / ADt 1.2 1.3 1.3 '' E2 phase, kg / ADt__5 ^ 4__4 ^ 2__6 ^ 2_:: Total__15,0__13,6__15,3_. 1. ·. H2O2 ;,; C02 dose, kg / ADt__0__2__5_! EP stage, kg / ADt 4.0 3.8 3.9 E2 stage, kg / ADt__2J .__ 2J) __ 2 ^ 2_

Yhteensä__6J__5JS__6J_

Washing loss ___ CC> 2 dose, kg / ADt__0__2__5_

Dp phase, kg / ADt_ 15.4__13.0__14.8_

The results of the experiments clearly showed that the chemical consumption decreased by the addition of '1; 1 1 using. However, using a lower dose of carbon dioxide seemed to produce better results. It should be emphasized that neither of these results should be considered as absolute values of> s «la» 11 109209. The large variation in chemical consumption at each step was due to automatic change and changes in the lignin removal and bleaching load balance between each step.

Example 3

The carbon dioxide content of the gas phase of the bleached pulp

The pulp from the factory fiber line brown pulp filters was treated to remove lignin in plastic bags at a dose of about 30 kg / ADt chlorine dioxide, as active chlorine, at 60 ° C for 60 minutes. Carbon dioxide was added as dry ice, at doses of about 2 and 4 kg / ADt of carbon dioxide.

After delignification, the pulps were cooled in plastic bags with cold water and transported to a gas laboratory in a bucket. To measure the carbon dioxide content of the gas phases from the plastic bags, gas samples (100 ml) were taken with a needle and analyzed by gas chromatography.

The results showed that large amounts of carbon dioxide are formed when the pulp is delignified in the Do step. The carbon dioxide levels observed in the gas phases ranged from 7.3% to 19.6%. However, there was only a small difference between zero and 2 kg / ADt gas concentrations. The gas concentrations were 9.5% and 10.7%, respectively.

At high doses of carbon dioxide the carbon content increased significantly. A carbon dioxide dose of 4 kg / ADt resulted in a gas phase carbon dioxide concentration of about 16.8%.

• ·! 'Example 4! .T. Do Phase Laboratory Experiment with Extraction, · ·, The pulp was taken after the MC pump before the Do Phase of the pulp mill fiber line. The experiments were carried out in a laboratory with a CRS reactor capable of controlling reaction temperature and pressure. carbon dioxide and chlorine dioxide, and to measure the pH of the filtrate.

I · t I ·

Mass (200 g / air dry) and dilution water were added to the reactor and slightly warmed to the reaction temperature. When the temperature was reached, carbon dioxide was added. Chlorine-1 was then added, the dioxide was added, and the reaction pressure was achieved by moving the piston in the adiabatic cylinder of the device.

There was no fluidization during the reaction, only the temperature was controlled.

12 109209

At the end of the reaction, a gas sample and a filtrate sample were taken and analyzed. The pulp was then washed with 15 liters of water and the extraction step was performed in a plastic bag in a water bath. After extraction, the pulps were again washed with 15 liters of water.

Mass consistency was measured and viscosity, kappa number and ISO brightness were determined.

A total of 12 laboratory experiments were performed during the Do phase. The purpose of evaluating the effect of various process parameters on pulp, filtrate and gas properties was to change temperature, pressure and chlorine dioxide and carbon dioxide dosages. The experiments were performed at two different temperatures (60 and 70 ° C), pressure (4 and 6 bar), chlorine dioxide (26 and 30 kg / ADt) and three different carbon dioxide doses (0, 2 and 4 kg / ADt). The results of these experiments are shown in Table 2.

Table 2. Laboratory Tests Variables and Results TT Ό 9? - TT TT K O Λ I s s g s -g. £ a · 3 £ o o ö g. g-g! ^ g- E s Si i ° f t S I I &% M £ · § I. 1 1 I g- s? § £

1 o: g g§co R O A, S 'S ^ J «Q O

sr S ET & B. s Q g i H Ȥ

«S. ^ a 6 * '° {w S

> ° 3 EL SC „ά t £ FT X- σ ^ ^ ^ § S ^> {^ Π σ s o e o g 8 s v ^ s n QS.> <Fi> σ> ö ~

D

... 1 (5 13 (3 0 (5 5CU NIGHT 6 (5 12 FUNCTION 4 (5 6 (5 Ö (9 5 (6 2 9 6 60) (Ö 29/7 Ϊ '·. ·' "1) 5 15 (2 Ö1 30 (5 0 (5 6 (6 WORK 1287 44 64 0 (3 5 (8 2 9 6 7Ö Ö 33 (Ö 2! Ί! T9 TT Ö1 49 (3 Ö (1 6 Π 1293 5 (1 5 (1 0 (3 5 (3 Ö 9 4 'WORK) (Ö 29 (1 3 T (8 16 (8 0 (1 46/7 Ö T 6 1539 Ö 9 3 (1 1 9 6 6Ö Ö {Ö 29 (8 4; {T (5 15 (2 0 (3,472 5 {i 7 (1 13 1335 Ϊ 9 5 {9 5 (6 52 1 9 6 70) 2Ö {Ö 5 2 T / 7 17 {9 Ö2 31 {1 5 {1 6 (1 5 1309 6 {1 6 {1 5 {2 5 {9 Ö 9 6 7Ö 5 {Ö 30 (5 6; '1: T (9 152 02 52 {3 5 {2 3 {9 1Ö 1533 42 6 {9 5 {3 62 2 9 1 7Ö 5 {Ö 3Ö2 7 T {1 13 {5 Ö {1 49 {7 5 (2 6 / 7 7 Ϊ364 3 {2 7 {2 5 {7 6 (3 1 9 4 6Ö Öfi 29 {8 8 "TU 162 5 {3 31 {3 5 {2 6 (1 6 Γ343 2fl 3 {Ö 0 (5 3 ( 3 4 9 4 60 Ö (Ö 33 (0 9 '' · 3,4 16,5 5 {3,482 5 {1 7 (1 33 Ϊ380 3 {1 3 {1 (l 332 lo ~

Tl Γ9 / 7 ((l 48 (3 ((l (3 9 9 l 9 TT l / t / 3 (3 ((3 9 9 1 70 (ö 33 (ΓΓ ΓΓ "'1' Tl ilj 5 (3 31 (3 02 3 {9 3ö 1373 5 {3 3 2 5 {3 1 (3 2 9 4 ~ 3ö 0 {2 33 {ö i3 ~ II «* I * I> .1111» f. »» 13 109209

Table 2. (continued)

Kappa - Carbon Dose, Viscosity, ml / g Carbon Dose, Number kg / ADt kg / ADt

Pressure 0 2 4 Pressure 0 2 4 4 bar p 5 $ M 4bär '127Ö L35Ö 1350 6 bar 6.6 6.5 7.8 6 bar 1340 1300 1330 Temperature 0 2 4 Temperature 0 2 4 60 ° C 73 ~ <p 1 $ 60 ° C ~ Ϊ380 1340 Ϊ350 70 ° C 6.6 6.2 7.4 70 ° C 1280 1310 1330 C102 Dose 0 2 4 C102 Dose 0 2 4 26 kg / ADt 73 pp 26 kg / ADt Ϊ3Ϊ0 30330 1340 30 kg / ADt 6.4 6.2 7.4 30 kg / ADt 1300 1320 1350 ISO Brightness,% Carbon Dose, Final pH Carbon Dose, kg / ADt kg / ADt

Pressure 0 2 4 Pressure 0 2 4 4 bar 483 52 $ 5Ö6 4bär ~ $ 3/7 6 bar 49.7 50.4 47.0 6 bar 3.0 4.5 3.6 Temperature 0 2 4 Temperature 0 2 4 60 ° C 483 p9 493 60 ° CP p 43 70 ° C 49.6 51.5 47.2 70 ° C 2.9 3.7 2.5

ClOj dose 0 2 4 C102 dose 0 2 4 26 kg / Adt 4M 5L2 4p 26 kg / ADt 33 pp 30 kg / Adt 50.2 51.2 48.2 30 kg / ADt 2.8 3.7 4, 1! COD Content, Carbon Dose, Born C02, Carbon Dose, kg / ADt kg / ADt kg / ADt kg / ADt | Pressure 0 2 4 Pressure 0 2 4; ··· 4 bar 18.1 14.9 15, 4 bar 5.4 4.1 2.6 ·. · 6 bar 17.2 15.8 16.0 6 bar 4 , 7 4.4 1,0 Temperature 0 2 4 Temperature 0 24

v. '60 ° C Ϊ63 Fp Ϊ63 60 ° C P P P

70 ° C 18.1 15.2 15.2 70 ° C 5.6 4.6 1.9: C102 dose 0 2 4 C102 dose 0 2 4 26 kg / Adt lp P9 ΪΡ 26 kg / ADt 4Ä p $ 2 30 kg / Adt 17.3 15.8 16.1 30 kg / ADt 5.8 4.7 1.6

The results show that despite the obvious fluctuations due to chemical additions, temperatures, etc., carbon dioxide improved the bleaching efficiency and selectivity. In addition, the improvement was significantly better with an average increase in CO2 than with a high increase in CO2. The effect of pH on bleaching was negligible.

h 109209

Example 5

Laboratory tests in the Di stage without extraction

Experiments similar to those of Example 4 were performed for the Di step without subsequent basic extraction. The results showed that without the extraction step the same benefits as in Example 4 were not obtained.

Example 6

Laboratory tests in the Z phase

Two experiments are performed in a laboratory reactor. The deoxygenated pulp has a consistency of 35%, a pH of 2.5 and a temperature of 40 ° C.

In the first experiment, only the ozonizing gas is used, in the second experiment, 15% of carbon dioxide is thoroughly mixed with the ozonation gas prior to feeding. The gases are fed into the mass, keeping the total ozone dose at the same level. The reaction is allowed to proceed with the fluidizing stirring for 2 minutes.

After the basic extraction of the pulp, the viscosities of the bleached pulps are measured. The viscosity of the ozone-treated pulp has decreased from 22 mPas to 17.5 mPas, while the final viscosity of the carbon-treated pulp is 18.0 mPas.

»I t * *

Claims (18)

109209 A process for bleaching an aqueous lignocellulosic pulp under acidic conditions in a pulp mill bleaching plant, said process comprising the steps of: providing an aqueous lignocellulosic pulp slurry in said bleaching plant, simultaneously with said lignocellulosic pulp slurry to provide carbon dioxide in said acidic pulp slurry to control the degradation of the lignin or derivatives thereof in its reactions with said bleaching chemical, then performing an alkaline extraction step on said pulp slurry to dissolve the reacted lignin compounds. Process according to claim 1, characterized in that said carbon dioxide generating agent comprises carbon dioxide in gaseous, liquid or solid form. A process according to claim 2, characterized in that said substance comprises gaseous carbon dioxide injected into said pulp slurry stream just prior to the addition of said bleaching chemical. i _ A process according to claim 1, characterized in that said carbon dioxide generating agent comprises a gas or liquid capable of producing carbonate and / or bicarbonate ions in said aqueous slurry under basic or neutral conditions. . · A process according to any one of claims 1 to 4, characterized in that said carbon dioxide generating agent produces in said pulp slurry an effective amount of carbon dioxide capable of affecting the balance and / or propagation of reactions between the lignin or its derivatives and said bleaching chemical or derivatives. that the degradation of said lignin derivatives is prevented. 109209 The process according to any one of claims 1 to 4, characterized in that said carbon dioxide generating agent produces an effective amount of carbon dioxide capable of preventing carbohydrate degradation in said pulp slurry by affecting any balance and / or progression of the carbohydrate degradation reaction chain, in the direction of the reaction. A method according to claim 5 or 6, characterized in that said effective amount of carbon dioxide is one which produces a carbon dioxide content in the gas phase of the pulp system substantially equal to the carbon dioxide produced in the gas phase of the bleached pulp system under the same conditions. Process according to claim 6 or 7, characterized in that the preferred effective amount of carbon dioxide to be added is determined by monitoring the COD value of the bleached and unleached pulp filtrate at various carbon dioxide addition rates and determining the COD minimum of the pulp to be bleached. A process according to claim 1, characterized in that said aqueous lignocellulosic pulp slurry is at an alkaline pH before the addition of said carbon dioxide generating agent. Process according to Claim 9, characterized in that the pH of said pulp slurry is adjusted to a pH of 6.5 to 7.5 by the addition of gaseous carbon dioxide. ! .11. Process according to claim 1, characterized in that said carbon dioxide generating agent is added during and / or after the washing step preceding said bleaching step. I f A process according to claim 1, characterized in that said bleaching step is selected from the chlorine dioxide step (D), the peracetic acid step (Paa), the ozone step (Z), and the multi-chemical step using a combination of bleaching chemicals. 109209 The process according to claim 1, characterized in that said bleaching step comprises a D or Z step and the final bleaching pH of said pulp slurry is adjusted to a pH of 1.5 to 4.4. A process according to claim 1, characterized in that said bleaching step comprises a Paa step and the final bleaching pH of said pulp slurry is adjusted to a pH of 4-6. A process according to any one of claims 1 to 14, characterized in that said aqueous cellulosic pulp slurry comprises chemical pulp, organosolve pulp, mechanical pulp, chemo-mechanical pulp, semi-chemical pulp, pulp containing recycled fibers or paper scrap, or any blend of these pulp mills. - in the establishment. Process according to claim 1, characterized in that said aqueous lignocellulosic pulp slurry comprises pulp from which the lignin has been removed by oxygen. Process according to Claim 1, characterized in that said aqueous lignocellulosic pulp slurry has a consistency of 1 to 18%, preferably 3 to 15%. Process according to one of the preceding claims 1 to 17, characterized in that said pulp slurry is later processed into bleached paper or pulp. t · »» ·, S 109209