FI102301B - Method for treating cellulosic pulps - Google Patents

Description

102301

METHOD FOR TREATMENT OF CELLULOSE PULP

The present invention relates to a method 5 for treating cellulosic pulps according to the preamble of claim 1.

The invention also relates to a process according to the preamble of claim 10 for bleaching cellulosic pulps with chlorine-free bleaching chemicals.

10 Today, the bleaching of cellulose pulps is based on the use of chlorine-free bleaching chemicals such as oxygen, hydrogen peroxide and ozone, as well as chlorine dioxide. Prior to these bleaching steps, the metals to be bleached are chelated to remove heavy metals that catalyze reactions detrimental to the quality of the pulp. In cellulose pulps, heavy metals are bound mainly to carboxylic acid groups.

15

There are some important drawbacks to the known bleaching technique. Thus, the consumption of bleaching chemicals, especially non-chlorine bleaching chemicals, is quite high in bleaching, which significantly increases the manufacturing cost of bleached pulp. The chelating agents used for the binding of metals also increase the said costs.

«Il <', (, 20 Furthermore, oxygen- and peroxygen-bleached pulps are characterized by a degree of - I t'.:.: In some cases even strong - post-yellowing.

The object of the present invention is to eliminate the drawbacks associated with the prior art and to provide a completely new solution for the treatment of cellulosic pulps. It is also an object of the invention to provide a process for the production of cellulosic pulps, in particular temporal ones. to bleach cellulosic pulps prepared under The object of the invention is • · · • t · *. it is still easy to obtain, e.g. with oxygen and / or peroxide, to bleach the pulp.

It is known that cellulose pulps contain 4-O-methyl-α-D-glucuronic acid groups':; (Glukuronihapporyhmiä). According to our recent observation, in addition to glucuronic acid groups, sulphate pulps also contain a considerable amount of xylan-bound 4-deoxy-p-L-tertrohex-4-enopyranosyluronic acid groups (hexenuronic acid 2 102301 groups). The number of these groups in some masses is even substantially higher than the number of known glucuronic acid groups.

In pulp bleaching, hexenuronic acid groups have been found to consume electrophilically reactive bleaching chemicals such as chlorine, chlorine dioxide, ozone and peracids (Buchert et al., 3rd European Workshop on Lignocellulosics and Pulp, Stockholm, 28-31.8.1994). On the other hand, the consumption of oxygen and hydrogen peroxide used as bleaching chemicals under temporal conditions is not affected by the hexenuronic acid groups because they do not react with said substances. Thus, in oxygen and / or peroxide bleaching, no decomposition of hex-10 senuronic acid groups occurs. A particular problem with oxygen and / or peroxide bleached pulps, on the other hand, is their relatively low brightness and / or, as already stated above, their tendency to post-yellow.

Based on the above, our invention is based on the idea that by selectively removing hexenuronic acid groups from cellulose pulps during bleaching, the consumption of bleaching chemicals can be reduced. Surprisingly, it has been found that the tendency of the pulp to rise at the same time decreases at the same time. Bleaching also becomes more selective because heavy metals are chelated more efficiently.

According to the invention, said selective removal of hexenuronic acid groups is performed by adjusting the aqueous suspensions of cellulosic pulps to be slightly acidic, typically adjusting the pH to about 2 to about 5, and treating the aqueous suspensions at an elevated temperature.

More specifically, the process according to the invention for treating cellulosic pulps is mainly characterized by what is stated in the characterizing part of claim 1.

• · 1 • · · · · • · ’· ·· The present bleaching method involves treating the cellulosic pulp III ·. 'in the presence of bleaching in the presence of water at a temperature of at least 70 ° C to remove the hexenuronic acid groups contained in the cellulose pulp at a pH of about 2 i' .10 to about 5, 'the invention is characterized by what is stated in the characterizing part of claim 10.

102301 3

By means of the invention, it is possible to obtain an easily bleachable cellulose pulp produced by the sulphate process or a similar alkaline process which produces hexenuronic acids in the pulp. The pulp is characterized in that it contains at most a small amount of hexenuronic acids and can be easily bleached without chlorine (ECF) or chlorine-5 without chemicals (TCF) or even with oxygen gas and / or peroxide alone. Furthermore, it is typical of the pulp that its post-yellowing, expressed as a PC number, is less than 2.

The treatment according to the invention, which is carried out on the pulp in aqueous suspension at an elevated temperature in an acidic environment, is hereinafter also referred to as "acid pretreatment".

The term "in connection with bleaching" means that the acid pretreatment is carried out either before bleaching, during bleaching or at the latest after bleaching. When electrophilically reactive substances such as chlorine, chlorine dioxide, ozone and peracids are used as bleaching chemicals, it is particularly advantageous to carry out a pretreatment before bleaching, since in this way the consumption of these bleaching chemicals can be reduced. It can also be expressed that the treatment is performed on unbleached cellulosic pulp according to its properties, e.g. bleachability. On the other hand, when oxygen gas and / or peroxide is used for bleaching (or bleaching treatment), the pretreatment can be performed both before and after bleaching. In the latter case, the treatment is preferably carried out immediately after bleaching before possible drying of the pulp (i.e. never-dried pulp). The pretreatment can also be carried out between the bleaching steps of the bleaching sequence.

, 25

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".I, Examples of suitable bleaching sequences include: ti * ·

• M

: <··: A (Q) -0-Z-P

A (Q) -0 Pn

: '3: 0 0-A (Q) -Z-P

0 A (Q) -P

O-A-D-E-D

0-A-X-Pn 102301 4 A = acid pretreatment according to the invention at elevated temperature O = oxygen treatment P = peroxide treatment

Pn = several successive peroxide treatment steps 5 E = alkali step Q = complexing treatment D = chlorine dioxide treatment X = enzyme treatment 10 There may be alkali steps between the oxygen bleaching steps. Enzymes known per se, such as cellulases, hemicululases and ligninases, can be used to enhance bleaching.

According to the present invention, the cellulosic pulp is treated in the presence of water at a temperature of at least 70 ° C in the pH range of about 2 to about 5 (typically in the pH range of 2-5) to remove the hexenuronic acid groups contained in the cellulosic pulp. Particularly preferably, the pH of the aqueous suspension of cellulose pulp is maintained at about 3-4 during treatment. Various acids can be used to adjust the pH of the pulp stock, inorganic acids, e.g. mineral acids such as sulfuric and hydrochloric acids, and organic acids such as formic and / or acetic acid. If desired, the acids can be buffered, e.g. with salts of acids such as formates, to keep the pH as constant as possible during the treatment. The temperature can vary over a wide range from 70 ° C »· · !! ' up. Preferably, the temperature is maintained at at least 80 ° C. If operating under non-pressurized conditions, there is a natural upper limit of 100 ° C. Higher temperatures are also possible when pressure issues are used. Thus, for example, the treatment can be carried out in a silent, 1 ', non-volatile pressure of 200 to 500 kPa at a temperature of about 110 to 130 ° C. To prevent excessive degradation of the fiber, the upper temperature limit is generally set at about 180 ° C.

20 The duration of the treatment varies depending on the pH and temperature and the material to be treated. Of course, it also depends on how complete removal of hexenuronic acids is desired. In general, the processing time is about 0.5 minutes to 48 hours, preferably about 1 minute to 24 hours. In the examples below, which have been operated under unpressurized conditions, the processing times are in the order of 1 to 10 hours. At 100 ° C, a typical treatment time is about 0.5 to 4 hours. Under pressure, e.g. at about 120-130 ° C, the treatment can be performed in about 1 minute to 1 hour, typically in about 5-20 minutes.

The aim is to remove as much of the hexenuronic acid as possible, preferably at least about 50% and particularly preferably at least about 75%, preferably at least about 90%. The term "pulp contains at most the minimum amount of hexenuronic acids" means that the amount of hexenuronic acids is at most 50%, particularly preferably at most 25% and most preferably at most about 10% of the amount present in the corresponding untreated pulp after cooking. 90% removal is achieved, for example, by treating the pulp at pH 3.5, at 100 ° C for about 2 hours.

In order to prevent excessive degradation of the carbohydrate material, complete removal of hexenuronic acid groups is generally not sought.

15 The treatment can be carried out continuously (batch treatment) in a flow-through reactor or batchwise (batch treatment). The pulp is treated in the presence of water, i.e. the pulp obtained from the pulp cooking process is pulped in water so that the consistency of the pulp in the pretreatment according to the invention will be about 0.1 to 50%, preferably about 1 to 20%. Pretreatment: The 20 is preferably mixed under. For continuous processing, the following may be used: ': stationary mixers.

• * «··« · .. * · * The solution according to the invention can be applied to pulps prepared by the sulphate process and other temporal methods which contain hexenuronic acid groups. By "sulfate process" is meant a cooking process in which the main cooking chemicals consist of sodium sulfide and sodium hydroxide. Examples of other time-consuming cooking processes include extended soups based on the continuation of conventional sulfate cooking until the pulp has dropped below about 20. These processes typically involve oxygen treatment. cooking methods include, for example, 30 *: extended modified continuous cook (+ AQ), EMCC (extended modified continuous cook), batch • · •: · · i soup, Super-batch / 02, MCC / 02 and continuous cooking / 02. According to our research, hexenuronic acids make up about 0.1 to 10 mol% of the hydrolysis products of the xylanase treatment of softwood pulps produced by said cooking methods. After the pretreatment 102301 6 according to the invention, the content of hexenuronic acids decreases to about 0.01 to 1 mol%.

The pretreatment according to the invention is carried out in the bleaching sequence either before or after the oxygen or peroxide step, but before the chlorine dioxide, ozone or peracid step (e.g. perforic acid or peracetic acid step), in order to reduce the consumption of chlorine dioxide, ozone and / or peracids, respectively. Since pretreatment can improve the bleachability of pulps, the invention can decisively reduce the consumption of said bleaching chemicals and / or eliminate the use of chlorine dioxide, ozone or peracids in bleaching. Many chemical pulping processes involve a delignifying oxygen step as the last 10 steps. The treatment according to the invention can be performed as well before this oxygen step as after it.

According to a preferred embodiment, the bleaching chemical used is a predominantly peroxide-containing substance (usually hydrogen peroxide) to produce a bleached pulp having a post-yellowing tendency, expressed as a PC number, of less than 2.

The peroxide treatment preferably involves oxygen gas pretreatment.

The pH of the oxygen-treated pulp is first adjusted to about 3 to 4 and the temperature is raised to 80 to 130 ° C, where it is maintained for 5 minutes to 10 hours, after which it is treated in an alkaline environment to produce a hydrogen peroxide bleached pulp. As the peroxide-containing substance, in addition to hydrogen peroxide, caronic acid or the like can also be used, which decomposes under suitable conditions (e.g. in an alkaline atmosphere) to form hydrogen peroxide or peroxo ions. The pretreatment according to the invention can be carried out with heavy metal-binding chelates. In the presence of «* ··· * to remove heavy metals bound in its cellulose pulp. Examples of these chelating agents are EDTA and DTP A. In general, chelating agents are dosed to the pulp in an amount of about 0.2% by weight. It should be noted that one particular advantage of the acid pretreatment according to the invention is that it makes the removal of metals quite efficient, even without the treatment with a chelating agent, as can be seen from Example 8.

The acid pretreatment can also be performed on unbleached or bleached pulp to modify its papermaking properties. Thus, by removing the acid groups 102301 7, the water-binding capacity of the pulp can be reduced, whereby a stiffer pulp is obtained which is suitable for use in e.g. packaging boards.

In the following, the invention and its embodiments will be described in more detail by means of a few 5 application examples.

Figure 1 shows graphically the effect of acidity on the hydrolysis rates of the arabinose and hexenuronic acid groups of pine sulphate pulp at 80 ° C. The theoretical curves are fitted to the test points according to Equations 1 and 2 shown in Example 2, respectively.

The pulp numbers have been determined according to SCAN-C 1:77, the viscosity according to SCAN-CM 15:88 and the brightness according to SCAN-C 11:75. The tendency to post-yellowing has been measured by the dry heating method (24 h, 105 ° C). 15 The PC number was calculated from the results.

Example 1 4 · O-Methylglucuronoxylan isolated from hardwood was treated with 1 M sodium hydroxide

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20, in solution at 160 ° C for 2 h. The solution was cooled and xylan precipitated from solution by adjusting the solution to neutral. The precipitated xylan was washed and dried. Xylania ·; treated with endoxylanase. The hydrolyzate was fractionated using anion exchange chromatography and gel filtration. By this procedure, an oligosaccharide fraction was isolated which was found to contain 4-deoxy-β-L-rAreo-hex-4-enuronoxylotriose (80%) and tetraose (20%) by NMR spectroscopy.

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V * A portion of the oligosaccharide mixture was dissolved in 10 mM acetate buffer (pH 3.7) in deuterium oxide.

: * '' R dissa. The solution was transferred to an NMR tube and changes in the solution were monitored at 80 ° C

': ": at 17 h for 1 H NMR spectroscopy.

: 3 O ·.

• · j ··· Degradation of hexenuronic acid groups followed the first order. The conversion was 55% after a reaction time of 17 h. No hydrolysis of xylosidic bonds was observed. Decomposition of the hexenuronic acid groups yielded almost the same number of compounds that were identified as furan-2-carboxylic acid (δΗ3 = 7.08 ppm, δΗ4 = 6.59 ppm, δΗ5 = 7.64 ppm, Jh3 h4 = 3.5 Hz, JH4 H5 = 1.7 Hz, JH3H5 = 0.8 Hz) and formic acid (δΗ = 8.37 ppm). In addition, a small amount of a component was identified which was identified as 2-furaldehyde-5-carboxylic acid (δΗ3 = 7.13 ppm, δΗ4 = 7.52 ppm, ÖCH0 = 9.60 ppm, JH3 H4 = 3.5 Hz).

5

By way of example, hexenuronicidal bonds can be selectively hydrolyzed under mild conditions without significant hydrolysis of xylosidal bonds. Similarly, it can be concluded that the glucosidic and mannosidic bonds of cellulose and glucomannan, which are more resistant to xylosidic bonds of xylan, are stable under these conditions.

10

Example 2

Pine sulphate pulp (number 25.9) was incubated in buffered solutions (pH 1.5-7.8) at different temperatures (25, 50 and 80 ° C) for 2 h. After the treatments, the pulp samples 15 were washed with water. The washed masses were treated with xylanase and the hydrolysates were analyzed as ‘H

By NMR spectroscopy.

Changes in the carbohydrate composition of the pulp were observed only at the highest ';'; at the temperature used (80 ° C). In contrast to the hydrolysis of ordinary glycosides, hex; the hydrolysis of the senuronic acid groups was not directly proportional to the hydronium ion content of the solution (Equation 1) but the pH dependence of the reaction rate clearly showed tt ';' reaction via the free hexenuronic acid group without hydronium ion-induced catalysis (Equation 2, Figure 1).

• · · «· 25 (1) k = * o [H30 +] (2) k = k0 {\ / {\ + ΐς / [Η30 +])} •« <• «· * ♦] * By way of example, the hexenuronic acid groups of the cellulose pulp can be selectively «·· * * removes under weakly acidic conditions (pH> 2) at elevated temperature. Partial hydrolysis of arabinose groups occurs, but the resulting yield loss is very • 30 '; small due to the low arabinose content of cellulose pulps (softwood pulps 1%, •: · ·: hardwood pulps 0%).

102301 9

Example 3

The oligosaccharide mixture (15.5 mg, 0.025 mmol) was added to boiling 0.01 M formate buffer (pH 3.3, 27 mL). The solution was refluxed for 3 h. Samples (0.5 ml) were taken at appropriate intervals and diluted with water (5 ml). Light absorption was measured in the wavelength range of 200 to 500 nm. The formation of furan-2-carboxylic acid (λ ^ = 250 nm) followed the first order (k = 0.44 h'1). The calculated molar absorbance per number of hexenuronic acid groups was 8,700. This absorbance value can be used to determine the hexenuronic acid content of cellulosic pulps.

10

Example 4

The oligosaccharide mixture (2.0 mg, 3.22 pmol) was dissolved in water (4.8 mL). To the solution were added 0.6 ml of 2 M sulfuric acid and 0.6 ml of 0.02 M potassium permanganate (12.0 pmol). After ten minutes, 0.12 ml of 1 M potassium iodide and 100 ml of water were added to the solution.

The iodine content of the solution was determined spectrophotometrically (350 nm, ε = 16,660). Consumption of Permanent Ghanaate was calculated based on Equation 3.

. Y: (3) 2MnO 4 - + 10Γ + 16H + -► 2Mn2 + + 5I2 + 8H 2 O 4 4:20:::: The consumption of permanganate was 7.98 pmol, i.e. 2.5 equivalents per hexenuronic acid group> t'i '. Since the ..li 'unit number determination used to describe the lignin content of cellulosic pulps is performed under exactly the same reaction conditions, the hexane «» »V * senuronic acid groups can cause a significant error relative to the actual lignin content.

• · • Ml • «(* ·’ * Example 5 ··· • ·

Birch sulphate pulp (3 g, kappa number 16.5) was treated in 0.06 M formate buffer (pH: 30 '; 3.2, 250 ml) at 100 ° C for 4 h. Degradation of hexenuronic acid groups was monitored by light absorption by 2-furancarboxylic acid (250 nm, ε = 8.700).

The total amount of hexenuronic acid groups was 70 meq / kg mass. The number of pieces of the treated pulp was 10.6.

102301 10

According to the example, a considerable number of hexenuronic acid groups can be easily removed from the sulphate pulp, as a result of which the number of pieces used to describe the degree of delignification of the pulp is significantly reduced. A similar decrease is expected in the consumption of bleaching chemicals that react with electrophilic hexenuronic acid groups.

5

Example 6

Oxygen and peroxide bleached pine sulfate pulp (9 g, kappa number 5.3) was treated in 0.06 M formate buffer (pH 3.2, 600 mL) at 100 ° C for 2.5 h. Degradation of Hek-10 senuronic acid groups was monitored by light absorption by 2-furancarboxylic acid (250 nm, ε = 8.700).

The total amount of hexenuronic acid groups was 48 meq / kg mass. All hexenuronic acid groups were removed from the pulp after 3030 min of reaction time. The treated mass 15 was filtered on a Biichner funnel and washed with water. Compared to the original pulp, the treated pulp seeped very easily. The number of pieces of the treated pulp was 2.3.

According to the example, the kappa number of the sulphate pulp bleached with oxygen and peroxide is very low after the treatment removing the hexenuronic acid groups. The treatment of Example 20 clearly improves the ability to produce fully bleached TCF pulps without ozone bleaching.

• »·« 'I * Example 7 • f ·· 1 1 * 1 * ♦ · · * <· «25 Oxygen bleached birch sulphate pulp (100 g, chapter number 11.5) was mixed in water (3 L).

. The pH of the suspension was adjusted to 3.4 by adding 2 ml of concentrated formic acid. The suspension thus prepared was incubated at 100 ° C for 4 h. Degradation of hexenuronic acid groups was monitored by UV absorption by 2-furancarboxylic acid (250 nm, ε = 8.700). The amount of hexenuronic acid groups removed was 54 meq / 30 / kg of pulp, which corresponds to about 98% of the total number of hexenuronic acid groups in the pulp.

The number of pieces of the treated mass was 6.2.

Both untreated and treated pulp were chelated with EDTA (0.2% of 102301 11 pulp) at 3.5% consistency. The treatment was performed at 60 ° C for 45 minutes.

After washing, the pulps were subjected to peroxide bleaching (3% hydrogen peroxide by pulp) at a consistency of 10 5%. Magnesium sulphate (0.5% by weight) and sodium hydroxide (1.8% by weight) were used as stabilizers. The temperature was 90 ° C and the bleaching time 180 min. From the washed pulps, the number of pieces, viscosity, brightness and tendency to post-tan (pc number) were determined. The properties of the masses are shown in Table 1.

10 Table 1. Effect of pretreatment (A) on the bleachability of oxygen bleached (O) birch sulphate pulp with peroxide (P) 15 Step Residue H202 Unit number Viscosity Brightness Pc number (% by weight) (ml / g) (% ISO) ό-ΓΤ75-mss-mn- OP 0 9.3 1125 61.0 2.5 20 OA 6.2 1065 49.9 O AP 2.1 3.2 980 76.1 1.1,, It can be seen from the results that the pretreatment strongly influenced the behavior of the pulp in 25 peroxide steps. Peroxide consumption decreased significantly, but nevertheless the increase in brightness was more than double that of the untreated pulp. The post-yellowing tendency of the pretreated pulp, expressed as a PC number, was more than 50% lower than »• 41 ·,:. with untreated pulp.

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9 <30 Example 8 9. 90 • 9 00 '.Tr Pine sulphate mass (100 g, kappa number 25.9) was stirred in water (3 L). The pH of the suspension was adjusted to 3.5 by adding 1.5 ml of concentrated formic acid. The suspension thus prepared was incubated at 100 ° C for 2.5 h. The decomposition of the hexenuronic acid groups was monitored by UV absorption by 2-furancarboxylic acid (250 t i; nm, ε = 8.700). The total number of hexenuronic acid groups removed was 32

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meq / kg pulp, which corresponds to about 95% of all hexenuronic acid groups in the pulp.

102301 12

Both untreated and treated pulp were chelated with EDTA (0.2% of pulp) at a consistency of 3%. The treatment was performed at 50 ° C for 45 minutes. The metal contents of the pulps were determined by atomic absorption spectrophotometry.

5

In particular, the removal of hexenuronic acid groups reduced the iron and manganese contents of the pulp (Table 2). The reduction was considerably greater for iron than that obtained with chelate treatment and for manganese as much as for chelate treatment.

10 Table 2. Effect of pretreatment (A) and chelation (Q) on metal contents of pine sulphate pulp (mg / kg) Treatment Iron Copper Manganese 15 - 22.0 6.5 36.8 A 10.7 5.7 2.4 Q 20.9 0.9 1.8 AQ 10.4 1.3 0.2 20

As iron and manganese are the most harmful metals for TCF bleaching, the use of chelating agents can be replaced either partially or completely by a treatment to remove the hexenuronic acid groups ·. ·. *. If chelating agents are used, it is most advantageous to add them during the treatment to remove hexuric acid groups.

(«· · P ·· • ·» l • · · · • · · · · • · 9 t 11 «I | •« «

Claims (22)

102301 A process for treating cellulosic pulps 5 prepared by a sulphate process or a similar alkaline process, characterized in that the cellulosic pulp is treated in the presence of water at a temperature in the range of about 2 to about 5 ° C to about 80 ° C to 130 ° C for a sufficient time between 5 minutes and 10 hours. to remove atonic acid groups by at least 50% and to reduce the mass number of 10 pulps. Process according to Claim 1, characterized in that the treatment is carried out on the cellulosic pulp in order to change its properties. 15 Process according to Claim 1 or 2, characterized in that the treatment is carried out in connection with bleaching. Method according to one of Claims 1 to 3, characterized in that the treatment is carried out by bleaching. . either before or after the possible oxygen or peroxide stage, but before the chlorine dioxide, ozone or peracid stage with the aim of reducing the consumption of chlorine dioxide, ozone or peracids. • · · « Process according to Claim 4, characterized in that the treatment is carried out in the presence of heavy metal-binding chelates to remove heavy metals bound to the cellulose pulp. Process according to Claim 1, characterized in that the treatment is carried out at a consistency of 0.1 to 50%. 35 t «« · 102301 Process according to one of the preceding claims, characterized in that the treatment is carried out at a pH of from about 3 to about 4. Process according to Claim 1, characterized in that the pH of the cellulosic pulp is adjusted by means of an inorganic or organic acid. 9. A process for bleaching cellulose pulps prepared by the sulphate process or a similar alkaline process with chlorine-free bleaching chemicals, characterized in that the cellulose pulp is treated with bleaching in the presence of water at a pH of about 2 to about 80 to 130 ° C for a sufficient time of 5 minutes. - between 10 hours to remove the hexenuronic acid groups contained in the pulp by at least 50% and to reduce the pulp number of the pulp. Process according to Claim 9, characterized in that oxygen and / or hydrogen peroxide is mainly used as the bleaching chemical and that a white, stripped pulp with a tendency to post-yellowing, expressed as a PC number, of less than 2 is produced. Process according to Claim 9 or 10, characterized in that the treatment is carried out in the bleaching sequence either before or after the oxygen or peroxide step. Process according to one of Claims 9 to 11, characterized in that the treatment is carried out in the presence of heavy metals and binding chelates to remove the heavy metals bound to the cellulose pulp. Process according to one of Claims 9 to 12, characterized in that the treatment is carried out at a consistency of 0.1 to 50%. 102301 Process according to one of Claims 9 to 13, characterized in that the treatment is carried out at a pH of from about 3 to about 4. Process according to one of Claims 9 to 14, characterized in that the pH of the cellulosic pulp is adjusted by means of an inorganic or organic acid. Process according to one of Claims 9 to 15, characterized in that the pulp is pulverized to a consistency of 0.1 to 50%, after which its pH is adjusted to about 3 to about 4 by adding acid, and the pulp is kept at about 80 to 130 ° C, after which the pulp is treated in an alkaline medium to produce a pulp bleached with hydrogen peroxide. 17. Use of an acid treatment in bleaching to remove at least 50% of the hexenuronic acid groups contained in cellulose pulps prepared by a sulfate process or similar alkaline process and to reduce the pulp count, in which the pulp is treated in the presence of water at a pH in the range of about 2 to about 80 to 130 At 5 ° C, sufficient time between 5 minutes and 10 hours to remove the hexenuronic acid groups by at least 50% and to reduce the pulp number. • · · ·· ♦ · Use according to Claim 17, characterized in that the treatment is carried out in the bleaching sequence either before or after the possible oxygen or peroxide step, but before the chlorine dioxide, ozone or peracid step, with the aim of reducing chlorine dioxide, ozone or consumption of peracids. 19. A cellulosic pulp prepared by a sulfate process or similar alkaline process and treated in the presence of water at a pH in the range of about 2 to about 5 ° C to about 130 ° C for a time sufficient to reduce the kappa number and remove hexenuronic acids by at least 50%. by% of the amount present in the corresponding untreated pulp after cooking, which pulp has been bleached after acid treatment. A process for treating cellulosic pulps prepared by a sulphate process or an equivalent alkaline process, characterized in that the cellulosic pulp 10 is treated in the presence of water at a temperature in the range of about 2 to about 5 ° C to about 130 ° C for a sufficient time between 5 minutes and 10 hours. to remove the nitric acid groups by at least 50% and to reduce the bulk density of the pulp, and that the treatment is carried out before the chlorine dioxide, ozone or peracid step in order to reduce the consumption of chlorine dioxide, ozone or peracids. 21. A process for bleaching cellulose pulps prepared by a sulphate process or an equivalent alkaline process with chlorine-free bleaching chemicals, characterized in that the cellulose pulp is treated with bleach in the presence of water at a temperature in the range of about 2 to about 80 to 130 ° C for a sufficient time of 5 minutes. - between 10 hours to remove the hexuronic acid groups contained in the cellulose pulp by at least 50% and to reduce the pulp number of the pulp, to chelate the acid-treated pulp and to bleach the pulp using • · · · as a bleaching chemical mainly oxygen and -30 / or hydrogen peroxide for the production of bleached pulp, • · · having a tendency to post-yellow, expressed as a PC number, of • · · less than 2. Process according to one of Claims 1 to 3, characterized in that the treatment is carried out in a bleaching sequence before the chlorine dioxide, ozone or peracid step in order to reduce the consumption of chlorine dioxide, ozone or peracids. • · · •·· • • · · · · · · · · · · · · • • • • • • • • • • • 102301