LT3393B - Process for bleaching of cellulose excluding the use of chlorine - Google Patents

Description

Ozone bleaching is usually combined with bleaching of other formulations, which makes it very difficult to achieve uniform bleaching and good quality performance when using high concentrations (HC). In addition, there is a need for additional investment in expensive water removal facilities. Ozone reacts with cellulose in two stages. The reaction is rapid and complete. The disadvantage of this method without the expense of water removal equipment is that at high concentrations of cellulose in the HC reaction zone, ozone enters and distributes unevenly, resulting in a rapid reaction that may not be uniform and may damage the cellulose structure, especially when initial Kapa values are low. . Kapa value (Kapa number) LT 3393 B is an indicator of lignin content (hardness). In this way it characterizes the ability of cellulose to bleach. This indicator shall be determined separately for each cellulose species. The value of kappa is a dimensionless dimension. It is determined in a standard manner according to ISO 302-81 Pulps-determination of Kapa number. There is an opinion in the literature that HC ozone should not be bleached if Kapa values are below 10 [Lindholm C.-A. Effect of pulp consistency and pH in ozonbleaching (Part 4); Paper and Pun-Paper and Timber, 2/1989; Lindholm C.-A. Effect of pulp consistency and pH in ozonbleaching, (part 2); 1987; Int. Oxygen Delingnification Conference, San Diego, 7-11 June 1987, Abstracts, p. 155; Lindholm C.-A. Effect of pulp consistency and pH in ozonbleaching (part 3); Nordic Pulp and Paper Research Journal N 1 (1988)].

The structure of cellulose is even more vulnerable if it is also bleached with oxygen prior to ozone bleaching.

Known technical solution that does not require the use of environmentally harmful chlorine compounds - ozone bleaching by LC. Compared to HC bleaching, this process consumes more ozone, requires more electricity for mixing, has more complex technology, and has a higher reactivity content, and therefore more dirt.

For bleaching cellulose, the low consistency (concentration) of the reactant mass is generally considered to be up to 5 or 6%. However, there is a general consensus on the use of ozone in bleaching that the desired results can only be achieved with a consistency of no more than 1% (maximum 2%).

Here is U.S. Pat. 4 216 054 specifies a consistency / concentration limit up to and including 0,7%. Performing a process at this consistency requires additional capital injections to ensure a closed water recirculation cycle. This patent describes an LC technology for bleaching kraft cellulose. It has been found that the reaction between cellulose and ozone is inhibited by two barriers: the transition of the ozone from the gas to the liquid phase and its transition from the liquid phase to the solid (to bleached cellulose). According to this patent, the minimum power required for mixing is 11 kW / m ³ . The second step is decisive.

The LC method is also protected by U.S. Pat. No. 4 080 249. Claim 5 of the invention states that the mixing of the cellulose suspension usually requires no more than 18 kW / ton of electricity. For gas containing ozone, the bubble size shall not exceed 3 mm. The 1-2% concentration indicated in all examples of the invention confirms that the LC method is patentable. Apparently, the concentration ranges up to and including 10% are included in the patent, in order to avoid competition, but a concentration lower than 3% is preferred in Claim 6 of the invention. We believe that at concentrations above 3% only unsatisfactory results can be obtained.

The closest known technical solution is disclosed in U.S. Pat. 4,372,812, which indicates a concentration ranging from 1 to 40% with ozone bleaching. However, in the examples provided in the description of the invention (Table 1), the reaction is carried out only in the LC range, namely at a concentration of 1% by weight. The patent describes a multi-stage bleaching method, but ozone is used only in some bleaching stages and not completely throughout the process.

The object of the invention is to produce high quality bleached cellulose with less investment in equipment, energy and environmentally friendly. SUMMARY OF THE INVENTION It is an object of the present invention to adapt a known method to a range of average consistencies (concentrations) (MC) and to achieve better results compared to the reaction in the LC range.

The object is achieved by bleaching softwood pulp wood pulp with an initial Kapa value of 15-1, preferably 4-1, or by bleaching ozone pulp wood pulp with a starting value of up to 30 inclusive, preferably in a known pulp bleaching process. up to 10, wherein a cellulose suspension having a pH of 1-8, preferably 2-3, is heated to 15-80 ° C, preferably to 40-70 ° C, with an ozone gas mixture containing 20300 g / m 2 under vigorous stirring or stirring, preferably 50-150 g / m ozone based on the absolute dry weight of the cellulose, the density of the cellulose suspension during the reaction is 3-20% by weight, preferably 5-20% by weight, in particular 7-15% by weight, to achieve the target ozone-containing gas mixture in the cellulose the suspension is supplied at elevated pressures of, for example, 1.1 to 15 bar, preferably 1.1 to 10 bar, using high shear forces for high agitation or agitation or blender.

The volume ratio of gas to liquid used in the reaction is from 1 + 0.5 to 1 + 8, preferably from 1 + 1 to 1 + 6.

For compressing ozone-containing gases, chilled compressors are used, preferably pumps with an aqueous refrigeration circuit.

High shear mixers are known and used for a variety of purposes, such as in the preparation of paint dispersions (DE patent A-406 430), polyvinyl chloride resin powder (US patent A LT 3393 B).

775,359), thick emulsions (U.S. Patent A-3,635,834), and a suspension of cellulose (JP Patent A-6,309,9389). The mixer of this design has raised blades at certain distances, so that the entire mass is not rubbed but carefully mixed.

The bleaching operations are performed sequentially, repeated several times and, if necessary, bleached in combination with an alkaline extraction. Alkaline extraction can be done using oxygen and peroxide. These reactions accomplish part of the reactor's multiple iterations of cellulose hydrogen by mass return from the reactor to high shear mixer.

Good results are achieved when the cellulose suspension is treated with oxygen and / or hydrogen peroxide before bleaching with ozone, alkaline with hydrogen peroxide, or after treatment with alkali or hydrogen peroxide after bleaching with ozone. A mixture of hydrogen peroxide and oxygen may be used in parallel. After bleaching, ozone can be further treated with peroxide or extracted with alkali solution.

It is expedient to partially return the aqueous filtrate formed after bleaching to a slurry prior to treatment with ozone-depleting cellulose containing acid, since this saves gas and the sulfuric acid solution of the filtrate is acidic to obtain the desired pH value of the effluent. returning it to the cellulosic suspension can be acid and on the other hand, since this water does not enter the drain and therefore does not pollute the environment.

When bleaching softwood pulp with a Kapa value of up to 30 (usually up to 10) according to the patented technology, this figure falls below 10 (correspondingly less than 5).

For bleaching hardwood pulp, the initial Kapa value of 15-1 (usually 4-1) in a patentable manner decreases to 12.0-0.5 (usually 1.5-0.5, respectively). If the whiteness of the unbleached cellulose is 50-80% (usually 70-80%), after the bleaching operations in accordance with the invention, this index increases to 65-90% (usually 75-90%).

Bleaching with medium consistency cellulose suspension (MC mode) uses lower process capacities compared to low consistencies (LC) and on the other hand does not require expensive dewatering devices compared to the HC process, therefore the present invention selects optimum capacities.

Intensive mixing or mixing and co-pressing with this MC bleaching process unexpectedly produces a positive effect: a homogeneous, gentle yet effective reaction of cellulose with ozone. Blending consumes less energy and the ozone reaction with cellulose is more homogeneous than LC bleaching. Measured by viscosity and DP distribution, even at very low Kapa values, the damage to the structure of the cellulose macromolecules is clearly less than in the case of HC bleaching and can at least be compared to that obtained by LC bleaching. Comparative ozone consumption (O ₃ per Kapa unit) is clearly lower than in LC. It is easy to reconstruct the existing process equipment, since in addition to the acid solution for pH control, which would be needed for LC and HC bleaching methods as well, it would be capital cost to install MC pump and MC mixer.

Because it is possible to recycle waste water into the process and reuse the ozone-containing reaction gas, this method does not pollute the environment.

the cost and benefits of mixing energy and ozone as well as the cost of technological equipment are obvious.

Further advantages of the patented process are emphasized in place of single-stage ozonation with multi-stage bleaching. Since most of the stages, when the bleaching process does not use ozone but uses a different reagent such as oxygen, bleaching usually takes place in the MC field, the cellulose concentration between the individual process steps does not need to be changed as by other methods and is more cost effective.

It is true that it is known under Austrian patent no. 380496 A process for bleaching cellulose under ozone pressure wherein a suspension of cellulose at a concentration of 2.5-4.5% by weight (LC bleaching process) is removed under high pressure with ozone. The excess cellulose is then removed from the reaction mass.

Cellulose dewatering mass (4 bar) by gas. increasing the water concentration to 10-30% at a time and for at least 20 min. it is then maintained at the same pressure and temperature. This is followed by an additional reaction caused by close contact between cellulose and ozone-containing gases during LC bleaching.

The patented process has shown that MC bleaching can be successfully treated directly with ozone-containing gas as it is fed at elevated pressure with vigorous agitation and agitation of the reaction mass. This eliminates the need to dilute the cellulose suspension and then remove excess water.

The technological schemes of the non-chlorinated cellulose bleaching process of the present invention are shown in Figs. la and lb.

Main technological devices:

- Cellulose suspension feed line

- supply of acid for pH adjustment

- MC pump

- MC high shear mixer

- reactor

- throttle

- supply of ozone gas mixture

- compressor

- recirculation line

- bleaching column

- Waste gas treatment plant

- water for dilution

- filter for washing

- acidic filtrate line

- return of the filtrate

- degassing unit

After the first bleaching step (eg oxygen bleaching or oxygen peroxide blending), the cellulose suspension is fed through the feed line 1 for bleaching with ozone. The acidic effluent filtrate that remains after the ozone bleaching step is fed into the cellulose suspension via line 14 to adjust the suspension concentration and pH. The additional amount of acid needed to adjust the pH is fed through tube 2. The MC pump 3 directs the cellulose slurry into the MC high-shear mixer 4, which is also fed through line 7 with compressed ozone-containing gas. The suspension and gas mixture are thoroughly and quickly mixed. The reaction takes place under elevated pressure in reactor 5 which has a tubular shape. A recirculation line 9 on the side of the reactor, consisting of a tube and a pump for recirculation, which is provided by the patented process.

The gas-treated cellulose suspension enters the conventional bleaching column 10. It is not necessary according to the invention. FIG. There is a difference in la and lb at this point in the technology, since the cellulose suspension can be fed to this column from the bottom up (la) or from the top down (lb).

When feeding the cellulose suspension from the bottom up (la), it is pressurized through the throttle 6 or bypassing the bleaching column and further reaction may occur. At the outlet of the reacted mass, the pressure is reduced and the waste gas is discharged through line io

11. The cellulose suspension is mixed under normal pressure with a diluent water and enters a flush filter 13. The effluent filtrate formed here is returned through line 15 to the start of the reaction.

1.

From the top down (lb) of the cellulose slurry, the ozone-containing cellulose slurry from the reactor 5 is directed through the throttle 6 to the degassing unit 16, whereupon the pressure is reduced to atmospheric and then fed to the bleaching column 10, quantity.

The trace gas from the ozone depleted gas passes through the trap gas treatment plant 11 (for example, a catalytic or thermal decomposition unit for ozone), from which the oxygen can be used in the oxygen bleaching step. The excess oxygen is purified and returned to the ozone generator. If oxygen is not used in the oxygen bleaching stage, it shall be purified and collected in appropriate containers.

Recirculation of wastewater and waste gas can save a certain amount of energy, especially at high temperatures.

In the reactor 5 and the bleaching column 10, the cellulose suspension is always maintained for up to 3 hours, usually less than 1 hour, preferably even less than 5 minutes.

The invention is illustrated by various examples of the use of the method. The stages of the technological process are marked as follows:

u

EOP - first bleaching step with oxygen O ₂ or a mixture of oxygen O ₂ and hydrogen peroxide H ₂ O ₂ ;

Z - second stage of bleaching with ozone O ₃ ;

PE - third stage of bleaching with hydrogen peroxide H ₂ O ₂ and alkali;

P - peroxide bleaching;

EO - Alkaline oxygen bleaching.

Examples 1-4

Tests are carried out on bleaching hardwood pulp, which is used to produce fiber for the production of book paper. Bleaching begins with the first step of EOP by treating cellulose with a mixture of oxygen and hydrogen peroxide according to known technology. After that it is bleached with proprietary settings, initial

All technological bleached cellulose table. 3 and 4. The characteristics of the bleached cellulose in Example 1 should be interpreted as meaning that the reaction took place at different temperatures and at different pH values. Therefore, it follows that the pH indicator can characterize the viscosity.

Further tests are carried out on the bleaching of sulphite softwood pulp used in paper making.

The cellulose properties were determined by the following methods:

Break length, m - ONORM 1-1114

Fracture resistance WRA, mNm / m - DIN 53115

Viscosity, mP.10 - Zellcheming IV / 30/62

Raw material for testing with the following characteristics:

Kapa value (Tappi 2300S-76) 20.4 Viscosity, mP.10 1500 Whiteness (Elreplo),% 49.7 fracture length (24o SR), m 8900 fracture length (41o SR), m 9200 fracture resistance (24oSR) ), mNm / m 1143 fracture resistance (41oSR), mNm / m 1010 indentation resistance (24oSR), kg / cm - 4.4 resistance to pressure (41oSR), kg / cm - Table 4.2

Technological regimes of deciduous cellulose bleaching and 10 characteristics of cellulose

Indicators Examples 1 2 3 4 Initial cellulose Kapa meaning (unwashed) 2.1 2.9 The meaning of the tomb (the lungs) 1, 9 1.9 1.9 1.9 whiteness,% (Elrepho) 76 76 76 76 viscosity, mP 255 255 255 255 wastewater CSB g / kg abs. dry cel. 5 5 Bleaching parameters pressure, bars 5.2 5, 0 5.0 5, 0 mass concentration,% 10th 9.5 10th 10.7 temperature 'C 47 50 50 23rd pH 2, 3 2.5 5, 0 2.5

continuation of the table

Indicators Examples 1 2 3 4

0 ₃ cost, g / kg O ₃ actual cost, g / kg O ₃ almost. 2 mg / norm.1 O _{3 in} almost exhaust gas, mg / norm.1 reaction time, s mixing time, sg / v _r high shear mixer revolutions per minute

Bleached Cellulose Kapa Meaning of abs. Kapa error 0 ₃ cost (abs. Kapa error) whiteness,% absolute whiteness error viscosity, mP abs. viscosity error

1, 82 1, 60 1.50 1, 60 1, 69 1.57 1.13 1.2 76, 8 79.7 78 83.2 5.2 1.3 17th 21st 120 120 120 120 20th 20th 120 120 1 / 3.2 (5.2 baro) 1 / 2.6 (5 bars) 1 / 2.6 (at 5 bars) 1 / 2.6 (5 bars) 1700 3200 3200 3200 0.9 1.25 1.1 0, 60 1.85 1, 65 0.9 1.3 0, 91 0, 95 1.25 0, 91 83.5 82.5 82 86.3 7.5 6, 5 6, 0 10.3 214 227 218 228 41 28th 37 27th

Example 5

Bleaching was performed in the following sequence of operations:

EOP-Z ₁ -PE ₁ -Z ₂ -PE ₂

(a) The EOP bleaching step is performed in a high shear mixer in the following mode:

NaOH consumption in absolute dry cell,% H ₂ O ₂ consumption in abs. dry cell%

0 ₂ pressure, bars slurry concentration,% reaction time, hours.

temperature, ° C

Indicators of incoming cellulose

Kapa value whiteness,% viscosity, mP.10 breaking length (24oSR), m breaking length (37oSR), m breaking strength (24oSR), mNm / m breaking resistance (37oSR), mNm / m indentation resistance (24oSR), kg / cm resistance to indentation (37oSR), kg / cm

6, 6 75.5 1498 7800 8300

810

1057

3, 3

3.5

This bleached cellulose is further bleached according to the sequence Z ₁ -PE ₁ -Z ₂ -PE ₂ . Three different assays are carried out with a marking V _lr V ₂ and V _3, which in this example is shown as a bleaching operations are performed by repeating them. Table 2 shows the bleached cellulose rates and all-stage regimes.

table

Coniferous Cellulose Bleaching Modes and Indicators

Indicator Testing Vi v ₂ V ₃ V ₄ V ₅

Z, stage

Mass concentration,% Pressure, bars temperature, ° C PH mixing time, s reaction time, s

8.5 8.2 9.0 12th 12th 5.6 5.6 5.6 6.2 6.2 20th 31st 44 24th 24th 2.5 2.5 2.5 2.5 2.5 15th 15th 15th 15th 15th 120 120 120 120 120

continuation of the table

Indicator Testing Vx v ₂ v ₃ V ₄ V ₅

speed of the mixer,

rpm 3200 3200 1500 1700 1700 0 ₃ input, kg / t 1.85 1.78 1.94 2.62 2.62 0 ₃ actual cost, kg / t 1.80 1.70 1.86 2.37 2.37 Vi / Vg (at 5.6 bar pressure) 3.1 2.87 2.61 2.56 2.56 Cellulose indicators Meaning of Kapa 4.9 4.5 4.0 3.7 3.7 0 ₃ cost abs. A grave for error 0.94 1.20 1.40 1.22 1.22 whiteness,% 73 73.4 73.2 75.7 75.7 viscosity, mP.10 1048 971 976 771 771 PE! stage Useful cost of NaOH (abs. dry cell),% 1.0 1.0 1.0 2.5 2.5 H ₂ -,% 0.7 0.7 0.7 1.5 1.5 mass concentration,% 10th 10th 10th 10th 10th reaction time, hours. 2 2 2 3 3 temperature, ° C 65 65 65 65 65 Cellulose indicators Meaning of Kapa 3.2 3.2 2.7 whiteness,% 83.5 84.3 85.2 viscosity, mP.10 1047 981 972 Z Stage ₂ mass concentration,% 8th 8th 8th pressure, bars 5.6 5.6 5.6 temperature, ° C 21st 33 45 pH 2.5 2.5 2.5 duration of mixing, s 15th 15th 15th reaction time, s 120 120 120 speed of the agitator, rpm 3200 1800 3200 0 ₃ input, kg / t 2.7 2.38 2.34 0 ₃ actual cost, kg / t 2.06 1.85 1.91 Vi / Vg (at 5.6 bar pressure) 2.5 2.6 2.5

continuation of the table

Indicator

Testing

Vi v ₂ v ₃ v ₄ v ₅ Cellulose indicators Meaning of Kapa 1.24 1.19 1.19 0 ₃ cost abs. A grave for error 0.95 1.08 0.79 whiteness,% 82.3 83, 9 83.5 viscosity, mP.10 679 581 671 Stage _{2 of} PE NaOH is useful cost / abs. dry cell,% 0.7 0.7 0.7 HA * 0.5 0.5 0.5 mass concentration,% 10th 10th 10th temperature, ° C 65 65 65 Indicators of finished cellulose Meaning of Kapa 0.6 0.6 0.6 2.0 1.6 whiteness,% 90.6 90, 6 90.0 86.2 87.0 viscosity, mP.10 650 583 577 904 714 breaking length (oSR), m 7600 7900 7500 7900 7500 (20) (21) (20) (23) (21) breaking length (oSR), m 8000 8200 8000 8200 8100 (34) (36) (35) (34) (35) fracture resistance 1043 ( 1080 1060 1020 1030 (oSR) mNm / m 20) (21) (20) (23) (21) fracture resistance 1100 1040 1047 - - (oSR) mNm / m (34) (36) (35) resistance to embossing 3.13 3.30 3.27 3.4 3.3 (OP), kg / cm ² (20) (21) (20) (23) (21) resistance to embossing 3.50 3.37 3.43 (OP), kg / cm ² (34) (36) (35)

example

The same as in Example 5, the sulphite softwood pulp is used for papermaking, bleached in the EOP-Z-PE sequence. In this case, there are lower whiteness requirements (over 85%) in an effort not to reduce the cellulosic strength.

The technological parameters of the EOP stage are the same as in Example 5.

Step Z is performed once and the subsequent PE step is not repeated. All data for this test are shown in columns 5 and 6 of Table 2.

example

The test is carried out on bleaching of sulphite softwood pulp for the paper industry.

First, it was bleached using conventional oxygen. The cellulose indicators after this stage are as follows:

Meaning of Kapa 16th Whiteness,% 37.7 Break length (o SR), m 10500 (17) Break length (o SR), m 10700 (21) Fracture resistance, mNm / m 1666 (17) Fracture resistance, mNm / m 1620 (21)

The subsequent bleaching steps are performed in the sequence E _r -EO-Z ₂ -P or Ζχ-ΕΟΡ-Ζ ₂ -Ρ. Ozone Bleaching (Ζχ and Z ₂ ) Mode:

mass concentration,% pressure, bars temperature, ° C

PH mixing time, s reaction time, s mixer revolutions / min. ozone concentration, g / Nm ozone equivalent useful cost, kg / t ozone consumption,%

V / V _g (ZJ, 7 (Z ₂ )

120

3200

100

ACB (ZJ

1.69 (ZJ, 1.96 Z ₂ )

Alkaline oxygen bleaching (EO) or oxygen-peroxide alkaline blending (EOP) mode:

mass concentration,% temperature, ° C NaOH consumption,%

H ₂ O ₂ consumption,%

0.5-2.5

0.5-2.0

The properties of the pulp after these steps are as follows:

Indicator

After the Z _x stage

Capacity value whiteness,%

Before Z Stage ₂

Meaning of Kapa

Ζχ-ΕΟ-Ζ ₂ -Ρ

Ζχ-ΕΟΡ-Ζ ₂ -Ρ

whiteness,% 62 65 After Z Stage ₂ Meaning of Kapa 4 3.5 whiteness,% 78 80 Bleached cellulose (after Stage P) indicators: whiteness,% 85 87 length of rupture, m 980 (15) 970 (15) length of rupture, m 1090 (23) 1060 (22) resistance for breakage, mNm / m 1816 (15) 1780 (15) resistance for breakage, mNm / m 1580 (23) 1610 (22)

Example 5 (Comparative) 5 is performed by bleaching sulphite coniferous cellulose (as in Example 7) in a known manner with the active component chlorine according to Z ₁ -EO-Z ₂ -D bleaching with chlorine dioxide).

EO and

Sequence testing of wood (D 10

In the example of bleaching stages Z _x .

Chlorine bleach Stage D mass concentration,% temperature, ° C active chlorine content,%

Cellulose characteristics

After Z Stage ₃ Kapa Value Whiteness%

Z ₂ modes the same as mode 7:

10-12 after the individual stages were:

Up to Z Stage ₂

Kapa value 7 whiteness,% 62 After Z _{Step 2}

Kapa value 4 whiteness,% 78

Characteristics of bleached cellulose (after all stages):

whiteness,% 89 breaking length, m 980 (15) breaking length, m 1100 (24) breaking resistance, mNm / m 1820 (15) breaking resistance, mNm / m 1520 (22)

Patented chlorine-free bleaching of cellulose and mid-range control of the cellulose suspension (MC mode) saves energy by not requiring additional temperature increase, diluting and concentrating the reactant mass, etc., and can work with smaller volume machines. The main indicator of cellulose quality and the purpose of the invention is whiteness - this method provides at least 87% (Fig. 6), and additional ozone bleaching can achieve as much as 90% whiteness. Compared to chlorine bleaching (Fig. 8), this indicator is no worse and the environment is not polluted by toxic chlorine compounds. The viscosity of cellulose remains low, while the strength and hardness ratios remain unchanged compared to chlorine bleaching.

Claims (6)

DEFINITION OF INVENTION A process for bleaching cellulose free of chlorine by bleaching ozone pulp for pulpwood pulp with an initial Kapa value of 15-1 (preferably 4-1) or with ozone for pulpwood pulp pulp of up to and including 30 (preferably up to 10) containing 20-300 g / m ³ (preferably 50-150 g) of cellulose suspension having a pH of 1-8 (preferably 2-3) heated to 15-80 ° C (preferably 40-70 ° C). / m ³ ) ozone gas mixture not exceeding 2% (preferably 0.05-0.5%) by weight of absolute dry cellulose, characterized in that the concentration of the cellulose suspension in the reaction is maintained at 3-20% (preferably 5-20%, especially 7-15%), pressurizes the mixture of ozone-containing gases into a cellulose suspension up to 1-15 bar (preferably 1.1-10 bar) and uses high-shear mixers for intensive mixing and mixing. 2. Cellulose bleaching process according to claim 1, characterized in that the gas / liquid volume ratio of the reaction is from 1: 0.5 to 1: 8 (preferably 1: 1 to 1: 6). 3. A process for bleaching cellulose according to claims 1 and 2, characterized in that refrigeration compressors, preferably pumps with a water cooling circuit, are used to compress the ozone-containing gas. 4. The process for bleaching cellulose according to claims 1-3, characterized in that the bleaching operations are performed sequentially and repeatedly, and the bleaching can be combined with an alkaline extraction. 5. A process for bleaching cellulose according to claims 1-4, characterized in that it is carried out after bleaching with oxygen, oxygen-hydrogen peroxide mixture or hydrogen peroxide-alkali mixture. 6. A process for bleaching cellulose according to claims 1-5, characterized in that after bleaching with ozone, it is treated with hydrogen peroxide or extracted with an alkali solution. Cellulose bleaching process according to claims 1-6, characterized in that the resulting cellulose is partially returned to the effluent filtrate by treating the suspension of the ozone-rich aqueous filtrate with a gaseous suspension and feeding a solution of an acid such as sulfuric acid together with the effluent filtrate.