CA2067844A1

CA2067844A1 - Process for bleaching cellulosic material and plant for carrying out the process

Info

Publication number: CA2067844A1
Application number: CA 2067844
Authority: CA
Inventors: Michael Sinner; Karl Schwarzl
Original assignee: Individual
Current assignee: Primetals Technologies Austria GmbH
Priority date: 1991-05-02
Filing date: 1992-05-01
Publication date: 1992-11-03
Also published as: RU2096547C1; DE59207155D1; FI921962A0; ATA91891A; EP0512977A1; EP0512977B1; ES2094337T3; AT395445B; FI921962A

Abstract

ABSTRACT OF THE DISCLOSURE:

There is disclosed a process and a plant for bleaching cellulosic materials using ozone. An ozonic gas is produced in an ozone generator by corona in air or oxygen. The ozonic gas is compressed and an aqueous solution enriched with oxygen (strong water) is recovered in an absorption vessel under pressure. The strong water is depressurized in a depressurizing zone, thus releasing a gas having a high O3 content (strong gas. The strong gas is diluted with an ozone-free or oxygen-poor gas. The gas obtained is mixed with the cellulosic materials to be bleached and is reacted in a bleaching zone. The material suspension bleached is degassed, the solid material is washed and processed.

Description

The invention relates to a process for bleaching cellulosic material, in particular pulp, by means of ozone ( 03 ) -When bleaching cellulose with ozone, an ozonic gas is initially produced by means of an ozone generator.If one departs from oxygen, using corona to form ozone, oxygen concentrations of 100 to 130 g 03/Nm may be attained, the current demand increasing with the 03 content. The ozonic gas in a mixer is admixed to the pulp to be bieached or is employed in the form of an aqueous ozonic solution.
If a cellulose consistency of 1 to 20 ~ is applied, one speaks of low or medium consistency bleaching, if cellulose is used at a higher consistency of more than 20 %, this is termed high consistency - bleaching.
When applying medium consisteny bleaching at consistencies of about 10 ~, which is done most frequently, it is not possible with ozone bleaching to employ any desired amount of 03 per ton of cellulose, not even if the bleaching gas is fed to the mixer under pressure; the maxium amount of ozone used is 3 to 4 kg 03/t cellulose, because the amount of gas to be admixed to the cellulose in a mixer is limited.
As the ozonic bleaching gas acts upon the materials to be bleached, it is not oxygen that reacts, but only the ozone contained therein. The oxygen leaves 2~6784~

the bleaching vessel at the end of the bleaching procedure, containing various impurities, such as carbon monoxide, carbon dioxide as well as hydrocarbons and other undesired components. In this contaminated form, the oxygen comin~ from the bleaching vessel no longer can be used to recover ozone, but either must be discarded or must be subjected to expensive purification procedures.
The mode of operation described involves high oxygen and current demands, the bleaching effect obtained is not always satisfactory, either.
~ he invention aims at avoiding the disadvantages and difficulties described and has as its object 1) to control the overall process: ozone production and bleaching in a manner that the oxygen from which the 2~3 mi~ture i5 produced, does not get into contact with the bleaching liquor such that it can be used for another ozone production immediately upon drying;
2) to reduce the amount of oxygen used considerably as compared to the conventional mode of operation; and 3) to increase the amount of ozone to be introduced into the bleaching liquor per ton of cellulose, if desired.
In this connection, also the bleaching effect is to be improved. In addition, the proc~ss is to be applicable 2~7~4 both to low and to medium and high consistency bleaching.
In accordance with the invention, these objects are achieved by a combination of the following measures:
a) that an ozonic gas is produced in an ozone generator by corona in air or oxygen, b~ that the ozonic gas is compressed and an aqueous solution enriched with oxygen (strong water) is recovered in an absorption vessel under pressure, c) that the strong water is depressurized in a depressurizing zone, thus releasing a gas having a high 03 content (strong sas), d) that the strong gas is diluted with an ozone-free or oxygen-poor gas, e) that the gas obtained is mixed with the materials to be bleached and is reacted in a bleaching zone, f) that the material suspension bleached is degassed, the solid material is washed and processed.
According to the invention, the ozone content in the 02/03-mixture produced under a) may amount to 30 to 130 g 03/Nm . This content depends on the bleaching effect sought and on economic considerations. I~ the amount of ozone to be admixed to the cellulose is to be 25 increased to 4.5 to 8 kg 03/t cellulose, based on the ozone added so far, an ozonic gas will be obtained in the ozone generator at the upper limit of 130 g 03/Nm .

2~7~

If it suffices to keep the addition of ozone low, a gas will be produced in the ozone generator at the lower limit of the range, which has the advantage of requiring less energy.
The compression of the ozonic gas and the preparation of the aqueous solution (strong water) in an absorption vessel may be effected at a pressure of 7 to 10 bar. The strong water may contain 200 to 350 g 03/m3 with a temperature-dependency being involved.
The gas obtained in the depressurizing zone according to c) and having an elevated 03-content contains more than lO0 g 03/m3, usually between 200 and 250 g 03/m3; it is diluted with an ozone-free or oxygen-poor gas, an oxygen-poor gas being a gas that does not contain more oxygen than air. In any event, the dilution must be such that the risk of decomposition, which would occur above 50 g 03/Nm , is avoided; as a rul~, the gas is adjusted to a content of 100 to 180 9 03/Nm , pre~erably of 100 to 150 g 03/Nm .
By the process of the invention, it has become possible to increase the amount of ozone employed in medium consistency bleaching using mechanic mixers and gaseous ozone, to 4 to 8 k~ 03/~ of the material to be bleached.
An embodiment of the process according to the invention, which, in the first place, is suitable for consistencies of from 1 to 20 % still pumpable, will be 2~67~4~

explained in more detail by way of the diagram of Fig.
1.
A modified process for the production of cellulosic material, in particular pulp, to be employed in high consistency bleaching, which is preferred with consistencies of more than 20 %, is characterized by the following measures:
a) that an ozonic gas is produced in an ozone generator by corona in air or oxygen, b) that the ozonic gas is compressed and an aqueous solution enriched with ozone (strong water) is obtained in an absorption vessel under pressure, c) that the strong water is depressurized in a depressurizing zone, gas having a high 03 content (strong gas) being released, d) that the strong gas is diluted with ozo~e-free or oxygen-poor gas, preferably to an ozone content of 40 to 60 g 03/Nm , e) that the gas obtained is mixed with the material suspension to be bleached having a consistency of more than 20 % and is reacted in a bleaching zone, f) that the offgas from the bleaching zone is recycled to the depressurizing zone and ~s used as a dilution gas, g) that the bleached material suspension is washed and processed.

2~8~

Measures a) to e) are identical with those of the first embodiment; however, the 03-content is reduced more intensively in the dilution in order to avoid damage to the fibers. Also, there is no separate degassing station after bleaching, but the offgas from bleaching is recycled directly as a dilution gas. ~his process variant is illustrated in detail in the diagram of Fig. 2.
~ he invention also relates to a plant for carrying out the process, comprising an ozone generator, an absorption column and a drier connected in terms of gas ducts, a depressurizing vessel including a liquid volume and a gas volume, the liquid volume being connected with the absorption column by an ozonic solution feed duct and by a return duct for recycling return water, a bleaching tower or bleaching reactor provided with a supply duct for supplying the material to be bleached and with a discharge duct for the bleached material, which plant is characterized in that the bleaching tower or bleaching reactor is connected with the gas volume of the depressurizing ~essel by an offgas recirculation, through which ozone-free or oxygen-poor gas is feedable to the gas volume as a dilution gas.
According to a preferred embodiment, the bleaching reactor is connected with a degassing and pressure control station.

2~7~

The invention will be explained in more detail with reference to the accompanying diagrams (Figs. 1 and 2~.
In the diagram according to Fig. 1, in which the gas-feed ducts are entered in broken lines and the ducts conducting the aqueous solutions or suspensions are entered in full lines, oxygen is fed to an ozone generator 1 through a duct 2. An O2/O3-mixture obtained and having a content of, for instance, 50 to lOO g O3/Nm oxygen, is compressed to a pressure of 7 to lO
bar and is conducted into the absorption (washing) column 4 through a duct 3. There, the ozone is washed out of the oxygen by means of circulating water and is dissolved.
The non-dissolved oxygen is supplied to the drier 6 through a duct- 5 and from there is returned to the ozone generator through a duct 7. Thus, oxygen in this circuit of the process gets into contact with pure water only and, therefore, will not be contaminated.
The aqueous ozone solution produced under pressure (strong water) is supplied through a duct 8 from the absorption column 4 to the depressurizing vessel 9 working without pressure. Since the solubility of ozone is lower in the pressureless state than under pressure, ozone and oxygen are exhaled from the solution~
Since the solubility of oxygen is lower than the solubility of ozone in water, a highly concentrated 20~784~

ozone gas is formed in the gas volume 9a of the depressurizing vessel as can not be produced by means of presently available ozone generators. The ozone content within the gas volume 9a of the depressurizing vessel may amount up to 250 g 03/Nm . However, above this c~ntent there is the risk of an explosive decomposition such that it must be seen to it that a dilution gas is fed into the gas volume 9a of the depressurizing vessel thIough duct 11 in order to reduce the ozone content, for instance, to a content of 200 g/Nm or less. In any event, the ozone content must amount to more than 100 g/Nm3 in order to be able to realize ~he desired bleaching action. Return water from the depressurizing zone 9 is recycled into the washing column 4 through duct 10.
- The diluted gas- is withdrawn from the depressurizing zone 9 through duct 12 and is fed to a mixer 13. A duct 14 for the material suspension to be bleached leads to this mixer 13, which material suspension is premixed in a mixer 15 and is adjusted to an acidic pH of 4 or less. This premixed suspension is mixed in the mixer 13 with the bleaching gas comins from duct 12. Advantageously, the bleaching gas, at that stage, has a concentration of 100 to 150 g 03/Nm3.

~leaching proper, i.e., the reaction of the bleaching gas with the material suspension, takes place in a bleaching tower 16 at a reaction time of 5 to 10 2~7~

minutes. From the bleaching tower 16, the suspension, i.e., the mixture of material suspension and bleaching gas, via duct 17, gets to the pressure control zone 18, in which ozone-free or ozone-poor gas is released. In this degassing zone, air is introduced through duct 19.
The air/oxygen mixture is fed to the gas volume 9a of the depressurizing zone 9 through ducts 20 and 11 as a dilution gas. In this manner, the oxygen content of the bleaching gas is lowered in the circuit between depressurizing zone, bleaching tower and offgas recirculation; the oxygen saved may amount up to 80 as compared to earlier modes of operation.
The cellulose suspension withdrawn from the pressure control zone is washed in the washing filter 21, the waste water 22 forming is supplied to the duct - 17 conducting the bleached suspension prior to reaching the pressure control zone as a dilution water through duct 23. The bleached cellulose is discharged through duct 24.

In the process diagram according to Fig. 2, identical plant parts are denoted by the same reference numerals, thus, the ozone generator by 1, the oxygen supply duct by 2, the 2/3 duct by 3, the washing column by 4, the oxygen return duct by 5 and the drier by 6 such that this circuit corresponds to that of Fig.
1. Likewise, the ozone solution coming from the washing 2~67~4 column 4 is denoted by 8, the depressurizing zone by 9 and the gas volume of this zone by 9a.
In the same manner as described in respect of Fig.
1, the return water is recycled from the depressurizing zone to the washing column through duct 10. Also this circuit corresponds to that described in Fig. 1.
In the gas space 9a of the depressurizing zone, the exhaled ozone is diluted, the dilution gas corning from duct 11, and is conducted directly into the bleaching reactor 16 through duct 12. As in the first embodiment, oxygen is again saved by supplying a dilution gas.
The path of the material to be bleached leads through a duct 25 to a press 26, where the material is adjusted to the desired concentration, e.g., to a consistency of above 20 ~. Subsequently, the material is supplied to a fluffer 27 to produce fluffs. These fluffs continue to pass through duct 25 into the bleaching reactor, which is provided with appropriate inlet and outlet openings.
The bleached material leaving the bleaching reactor is diluted at the washing filter 21 and conducted to cellulose washing. Air 19 may additionally be fed to the systern via a pressure control system connected to the bleaching reactor 16. However, it is also possible to remove offgas from the system in case the gas required for dilution becomes less than the gas 2~7~4 incurred. Offgas from the bleaching reactor is supplied as a dilution gas through duct 20 and duct 11.
In high consistency bleaching as described with reference to the diagram of Fig. 2, it is usually operated at a lower ozone content of the bleaching gas, preferably at an ozone content of 40 to 60 g O3/Nm oxygen, in order to keep any damage to the fibers low.
When carrying out the process according to the invention, the following quan~titative data are typically obtained:
lS,000 g oxygen are required to produce l,000 g ozone having a concentration of 100 g 03/Nm . Taking into account small losses, 15,500 g oxygen are actually required to produce the 2/3 mixture in the 03-generator in order to produce 1,000 g ozone, the amount - of oxygen being constantly supplemented.
From the absorption column, strong water containing 200 g 03/m is obtained. At the same time, 1,300 g oxygen/m3 dissolve in the strong water under the conditions indicated. With the plant operated continuously, the dissolved gases emerge from the solution during depressurization of the strong water in the depressurizing zone, l,000 g ozone and 1,300 g oxygen thus being released, which corresponds to a ;~ 25 total of 2,300 g oxygen.
In recycling ~o the ozone generator, the oxygen supplied amounts to 15,500 g and the oxygen consumed 2~7~

amounts to 2,300 g. Thus, 13,200 g oxygen remain to be recycled. Since small losses still must be taken into account in processing the oxygen, the amount will be reduced by 5 ~ such that the recycled amount of oxygen is 12,540 g or 80.9 % of the amount supplied.
During continuous operation, 2,960 g fresh oxygen are supplied for the production of 1,000 g ozone. In the depressurizing column, a gas mixture forms which consists of 1,000 g ozone and 1,300 g oxygen. By supplying air or offgas, the ozone content of the gas is adjusted to the desired ozone concentration value.
The oxygen recycled to the ozone generator only needs to be dried, because otherwise it does not contain any impurities.

Claims

1. A process for bleaching cellulosic materials using ozone, which process comprises providing an ozone generator to produce an ozonic gas in said ozone generator by corona in air or oxygen, pressurizing said ozonic gas and providing an absorption vessel to prepare an aqueous ozone-enriched solution under pressure, providing a depressurizing zone to depressurize said aqueous ozone-enriched solution under release of a gas having a high O3-content, diluting said gas having a high O3-content with an ozone-free or oxygen-poor gas so as to obtain a diluted gas, mixing said diluted gas with said cellulosic material to be bleached to obtain a material suspension and providing a bleaching zone for reaction of said material suspension to obtain a bleached material suspension, degassing said bleached material suspension so as to obtain solid material, washing and processing said solid material.

2. A process as set forth in claim 1, wherein ozone-free or ozone-poor offgas is obtained in degassing said material suspension, and further comprising recycling said ozone-free or ozone-poor offgas to said depressurizing zone and using it as a dilution gas.

3. A process as set forth in claim 1, wherein ozone-free or ozone-poor water is obtained in said depressurizing zone, and further comprising recycling said ozone-free or ozone-poor water to said absorption column as return water.

4. A process as set forth in claim 1, wherein an absorption column is provided as said absorption vessel and an ozone-poor oxygen-containing gas incurs in said absorption column, further comprising drying said ozone-poor oxygen-containing gas and feeding it to said ozone generator.

5. A process as set forth in claim l, wherein a washing zone is provided for washing said solid material with washings being obtained, further comprising supplying said washings from said washing zone to said bleached material suspension as diluting water prior to degassing.

6. A process as set forth in claim 1, wherein said gas having a high 03-content is diluted with a dilution gas to a content of from 100 to 180 g/Nm3.

7. A process as set forth in claim 1, wherein said gas having a high O3-content is diluted with a dilution gas to a content of from 100 to 150 g/Nm3.

8. A process as set forth in claim 7 or 8, wherein said dilution gas is offgas.

9. A process as set forth in claim 1, wherein said reaction of said material to be bleached with said ozonic gas takes place for 5 to 10 minutes.

10. A process as set forth in claim 1 to be used in medium consistency bleaching in combination with mechanic mixers, wherein the amount of ozone supplied is up to 8 kg/ton of said material to be bleached.

11. A process as set forth in claim 1, wherein said material suspension is adjusted to a pH of about 4 at most, prior to mixing with said ozonic gas.

12. A process as set forth in claim 1, wherein said material to be bleached has a consistency ranging between 1 and 20 %.

13. A process as set forth in claim 1, wherein a gas containing 30 to 130 g ozone/Nm3 is produced in said ozone generator.

14. A process as set forth in claim 11, further comprising compressing said ozonic gas to a pressure of up to 10 bar.

15. A process for bleaching cellulosic material using ozone, which process comprises providing an ozone generator to produce an ozonic gas in said ozone generator by corona in air or oxygen, pressurizing said ozonic gas and providing an absorption vessel to prepare an aqueous ozone-enriched solution under pressure, providing a depressurizing zone to depressurize said aqueous ozone-enriched solution under release of a gas having a high 03-content, diluting said gas having a high 03-content with an ozone-free or oxygen-poor gas so as to obtain a diluted gas, mixing said diluted gas with said cellulosic material to be bleached to obtain a material suspension having a consistency of above 20 % and providing a bleaching zone for reaction of said material suspension to obtain a bleached material suspension and offgas, recycling said offgas from said bleaching zone to said depressurizing zone and using it as said dilution gas, washing and processing said bleached material suspension.

16. A process as set forth in claim 15, wherein said gas having a high O3-content is diluted to an ozone content of from 40 to 60 g 03/Nm3.

17. A plant to be used for carrying out the process set forth in claim 1 or 15 and of the type including an ozone generator, an absorption column and a drier connected in terms of gas ducts, a depressurizing vessel including a liquid volume and a gas volume, an ozonic solution feed duct arranged to connect said liquid volume with said absorption column and a return duct for recycling return water arranged to connect said liquid volume with said absorption column, a bleaching tower or bleaching reactor provided with a supply duct for supplying said cellulosic material to be bleached and with a discharge duct for discharging said bleached material, the improvement comprising an offgas recirculating means arranged to connect said bleaching tower or bleaching reactor with said gas volume of said depressurizing vessel and to supply ozone-free or oxygen-poor gas to said gas volume as said dilution gas.

18. A plant as set forth in claim 17, further comprising a degassing and pressure control station connected with said bleaching reactor.

19. The process of claim 1 or claim 15, wherein said cellulosic material is pulp.

20. The plant of claim 17, wherein said cellulosic material is pulp.