FR2695947A1 - Chlorine-free bleaching process for chemical paper pulp - Google Patents

Abstract

A bleaching process for chemical pulp involves at leastone treatment with chlorine-free bleaching agent, at least twoalkaline treatments and at least one acid treatment between thealkaline treatments, the novelty being that at least one of thealkaline treatments is an alkaline bleaching treatment and thatan ozone treatment is carried out before the first of the twoalkaline treatments. Also claimed are (i) application of theabove process to the bleaching of kraft papermaking pulp fortyping paper; and (ii) chemical pulp which has been bleached bythe above process and which has a degree of whiteness greater than 80.

Description

 PROCESS FOR WHITENING CHEMICAL CELLULOSIC PASTES

 The invention relates to the cellulosic pulp industry and in particular the bleaching of chemical pulps, that is to say pulps obtained by sulphite processes or alkaline processes such as the kraft process.

 The industrial bleaching processes of cellulosic pulps generally use different sequences corresponding to different successive treatments, and always among them at least one treatment using chlorine or chlorine derivatives.

Thus, for example, a known bleaching process uses successively
a) an oxygen delignification treatment,
b) a chlorination treatment (Cl2) in which the degree of substitution of Cl2 by the chlorine dioxide (do2) can reach 50% to 100%,
c) an alkaline extraction treatment with sodium hydroxide (NaOH) often reinforced by an oxygen treatment,
d) a chlorine dioxide treatment,
e) an alkaline extraction treatment,
f) again a treatment with chlorine dioxide.

 Other treatments associated with treatments using chlorinated products or derivatives can be carried out, in particular treatments with hydrogen peroxide (H 2 O 2) and / or treatments with sodium hypochlorite (NaoCl).

In the majority of cases, chlorinated products: Cl2,
ClO 2, NaOCl, are used and with the exception of the process described in the publication FR-A-2,655,668, it is not known in the field of cellulosic pulps, industrial techniques for obtaining bleached chemical pulps having the desired whiteness without the use of these products.

 Recently, increasing environmental constraints have led cellulosic pulp manufacturers to modify whitening sequences.

 In the French patent publication FR-A-2,655,668 cited above, a process for bleaching a chemical cellulosic pulp which avoids the use of chlorinated products has been proposed.

 This process comprises at least one treatment with a bleaching agent containing no chlorine, at least one alkaline treatment and at least one acid treatment, at least one alkaline treatment being preceded by an acid treatment which preferably is between two alkaline treatments .

 Alkaline treatment means either a treatment with sodium hydroxide or equivalent, or a treatment in an alkaline medium, that is to say a treatment combining the action of sodium hydroxide or equivalent with another agent, especially oxygen or hydrogen peroxide.

 Acidic treatment means either a treatment with an acid or a treatment in acidic medium, that is to say combining the action of the acid with another agent, in particular oxygen or hydrogen peroxide. .

 The process described in FR-A-2 655 668 makes it possible to obtain whiteners greater than 70 according to the ISO 2470 standard for both bisulphite pastes and kraft pastes.

 Nevertheless, when it is desired to obtain a whiteness greater than 85, which is generally the desired degree of whiteness for a paste intended for writing printing, the method described is not entirely satisfactory since it then requires an elongated sequence comprising many treatments that result in a high overall cost of blanching.

 The invention provides a bleaching method which allows to obtain a very high degree of whiteness while avoiding the use of chlorine agent and limiting the number of treatments or bleaching stages.

 The process for bleaching cellulosic chemical pulps according to the invention uses a sequence of different treatments comprising at least one acidic treatment interposed between two alkaline treatments and at least one treatment with ozone preceding the first of the two alkaline treatments, of which at least one is an alkaline bleaching treatment. Preferably, the two alkaline treatments are alkaline bleaching treatments.

 By alkali bleaching treatment is meant according to the invention a treatment in an alkaline medium combining the action of sodium hydroxide or an equivalent base with a bleaching agent selected in particular from oxygen (o treatment), hydrogen peroxide (P-treatment), a mixture or combination of hydrogen peroxide and oxygen (PO treatment). By alkaline treatment is meant treatment with sodium hydroxide or equivalent or treatment in an alkaline medium as explained above.

 The sequence according to the invention using a treatment with ozone followed by an alkaline treatment, itself followed by an acid treatment preceding another alkaline treatment, at least one of the alkaline treatments being an alkaline bleaching treatment, makes it possible to surprisingly to obtain a whiteness higher than that obtained by using sequences comprising a higher number of bleaching operations but not including this combination of treatment.

It seems that ozone not only exerts a solubilizing action of lignin allowing it to be extracted from the pulp but that it also has an activating action on lignin which remains present in the pulp and which predisposes it to a pulp. subsequent bleaching with a bleaching agent in an alkaline treatment 0 or P or
PO.

 The acidic treatment following the alkaline treatment for P or PO also predisposes the pulp to further bleaching.

 This acidic interlay treatment may be either a treatment with an acid that can then be called "acid wash", or a treatment in an acid medium, that is to say combining the action of the acid with at least one other agent that can be selected from oxygen, oxygen peroxide, ozone.

According to an advantageous form of the process according to the invention, the treatment with ozone which starts the essential sequence according to the invention is carried out in the presence of a
complexing agent, for example EDTA (ethylene diamine tetraacetic), which, in particular by acting on the heavy metals present in the paste to form complexes, thus prevents these metals from adversely affecting the ozone (decomposition or destruction of the ozone).

 The presence of this agent therefore increases the effectiveness of the treatment with ozone. This efficiency can be further improved by introducing the complexing agent into the paste before the introduction of ozone, preferably at a previous stage which will be referred to as a complexing stage which is advantageously an acid stage.

 According to another preferred characteristic of the invention, the treatment with ozone (treatment Z) which is also an acid stage is carried out on dough with a high consistency, for example on a dough having a dryness greater than 30% . This high consistency notably allows a reaction of ozone at atmospheric pressure.

 Other previous or subsequent treatments can be added to the essential treatments of the sequence according to the invention.

The reactants and the treatments used are therefore chosen in particular from the following: oxygen in an alkaline medium (O), ozone (Z), hydrogen peroxide either in an alkaline medium (P) or in
acid medium (Pa), - soda (e), - combination of oxygen and peroxide, either in medium
alkaline (PO), either in acid phase (POa), - sulfuric acid, sulfur dioxide, or any other
acid not containing chlorine and allowing
obtaining the desired pH, generally less than 5
(a) - complexing agent in an acid medium (aX).

 The bleaching process according to the invention can therefore use, in particular, the following sequences ... Z - (O or P or PO) - (a or Pa or Z) - P ...

 The treatments designated by letters and used in the preceding sequences, are more precisely the following, the concentrations being expressed by weight relative to the weight of dry pasta.

O: Oxygen treatment in the alkaline phase - pulp concentration 6 to 12% - amount of oxygen used 0.5 to 3% by weight of
dry pasta, - quantity of soda or equivalent
work: 0.5 to 4.5% - temperature from 600C to 1200C, - pressure: 1 to 5 bars - reaction time: 15 minutes to 120 minutes a: acid treatment - H2SO4 or SO2 or H3PO4 reagents,
or any other acid that does not contain chlorine and
allowing to obtain an oH from 2 to 5, - paste concentration: 1 to 12% - duration: 5 minutes to 60 minutes, - temperature: 15 C to 800C
P: Treatment hydrogen peroxide (H2 2) in medium
alkaline - amount of peroxide used: 0.5 to 4%, - NaOH or equivalent: 1.0 to 2.5% - DTPA (diethylene triamine pentacetic acid): 0.05 to 0.2%
(or other agent that prevents decomposition of H202)> - paste concentration: 5 to 30% - temperature: 55 to 850C - reaction time: 60 minutes to 180 minutes
PO or P / O: Mixed peroxide-oxygen treatment in alcaled phase - quantity of reagents used
- oxygen: 0.5 to 2.5%
- peroxide: 0 to 2%
- NaOH or equivalent: 0.5 to 4.5%
MgSO 4: 0 to 0.5% - temperature: 0 to 1000 ° C., pressure: 1 to 5 bars, paste concentration: 6 to 12, reaction time: 15 minutes to 120 minutes
POa: Mixed peroxide-oxygen treatment in acid medium - quantity of reagents used
- oxygen: 0.5 to 2.5%
- peroxide: 0.5 to 2.0%
- H2S04 or any other equivalent acid, not containing
of chlorine and not reducing in sufficient quantity to
initial pH 1.5 to 3.5 - temperature: 600C to 1000C, - pressure: 1 to 5 bar - reaction time: 30 minutes to 180 minutes - pulp concentration: 6 to 12% e - Soda treatment NaOH or with equivalent base
in sufficient quantity to bring the dough to a
pH> 10.5.

Pa - Peroxide treatment in acidic medium
This treatment is equivalent to the POa treatment.

Z - ozone treatment - pulp concentration: 6 to 12% average consistency
or 30 to 40% in high consistency, - ozone is prepared from oxygen in an ozonizer
by Corona effect. At the exit the concentration of ozone
in oxygen is about 10%, - ozone is used at 2 to 10kg per tonne of
dough.

aX - complexing stage using EDTA.

 The process according to the invention makes it possible to achieve a degree of whiteness measured according to the ISO 2470 standard of greater than 80 and, where appropriate, greater than 85 for all the chemical pastes treated. It is equally applicable to bleaching of bisulphite pastes and kraft pastes. A particularly advantageous application of the process is the bleaching of a kraft pulp for writing printing.

 Subsequently, several examples of the bleaching process according to the invention are described.

EXAMPLE 1
Kraft pulp obtained from hardwood (oak, chestnut, hornbeam, etc.) is treated by the bleaching process using the following sequence of operation
Z - PO - a - P with - Z treatment:
It is a high-consistency ozone treatment.

The consistency of the dough is thus increased up to 35% by passage on a filter press. The 10% ozone in oxygen is injected into the high consistency mixer and the reaction is carried out in a specific reactor. The paste is then sent to a filter to be washed with water or with a return juice from a later stage.

- PO treatment
The pulp leaving treatment Z is sent to the base of an ascending tower after addition of liquid reagents (sodium hydroxide, peroxide, oxygen).

 The passage time is between 15 and 120 minutes.

 The temperature is between 60 and 95 OC.

 The pulp concentration is about 10%.

 The initial pH is 11.

 The dough is then washed on a filter.

- treatment
The reaction is carried out in a tower after addition of sulfuric acid to the pulp to a pH of 5, the reaction time is about 15 minutes. The dough is then washed with filter.

- treatment P
The dough is concentrated to a consistency of about 40% on a filter press. After introducing the hydrogen peroxide into the paste, the bleaching reaction is carried out in a reactor. The paste is then washed on a filter.

 At the end of this last treatment, the dough has a whiteness of 81.

EXAMPLE 2
A kraft pulp obtained from hardwood is treated by the bleaching process using the following sequence of operation
O - aX - Z - PO - a - P with - treatment 0 (in the presence of soda)
It is an oxygen treatment. After washing the sodium hydroxide to obtain an initial pH of 11 and then kraft pulp on the filter, oxygen is introduced into a mixer and a reaction is carried out in an oxygen reactor at a temperature of 950 ° C. and at a pressure of 5 bars. After returning to atmospheric pressure, the treated pulp is washed on a filter.

- treatment aX
It is a complexing stage which consists of introducing sulfuric acid and EDTA at a rate of 2 kg per ton of pulp and carrying out the reaction in a vat.

 The paste is then washed on a filter.

 The following treatments are similar to the treatments described in Example 1.

 The bleached pulp finally obtained has a whiteness of 88.

EXAMPLES 3 TO 8
In these examples, a hardwood kraft pulp is treated with the following respective sequences
O - aX - Z - O - Pa - P
O - aX - Z - P - a - p
aX - Z - PO - a - P
O - aX - Z - PO - a - P - a
O - aX - Z - PO - Z - P
aX - Z - O - Pa - P
EXAMPLES 9
In these two examples according to the invention, a bisulphite pulp is treated by the implementation of respective sequences
O - aX - Z - PO - a - P
0 - aX - Z - P - Z - P
O - aX - Z - PO - Z - P
COMPARATIVE EXAMPLES
In these examples, a kraft pulp (Comparative Examples 1 to 3) and a bisulphite paste (Comparative Examples 4 to 6) are treated with a bleaching sequence which does not use the following Z-alkaline treatment-acid treatment-alkaline treatment, essential according to the invention.

 Comparative Examples 1 to 3 are carried out according to the respective sequences on kraft pulp.

O - a - PO - a - P
0 - aX - Z - PO - a
O - aX - Z - PO - P - P
Comparative Examples 4 to 6 are made according to the following respective sequences on bisulphite pulp.

O - a - PO - a - P
O - aX - Z - PO - a
O - aX - Z - PO - P - P
The results concerning the whiteness of the preceding examples are grouped together in the following table

<tb> I <SEP> EXAMPLES <SEP> I <SEP> WHITE <SEP> I
<tb><SEP> 1 <SEP> 81
<tb><SEP> 2 <SEP> 88 <SEP>
<tb> 3 <SEP> 87 <SEP>
<tb> 4 <SEP> 86 <SEP>
<tb><SEP> 5 <SEP> 83 <SEP>
<tb> 6 <SEP> 88
<tb> 7 <SEP> 88 <SEP>
<tb><SEP> 8 <SEP> 82
<tb> 9 <SEP> 86 <SEP>
<tb><SEP> 10 <SEP> 1 <SEP> 88 <SEP>
<tb><SEP> It <SEP>
<tb> comparative <SEP> 1 <SEP> 77
<tb>"<SEP> 2 <SEP> 77 <SEP>
<tb>"<SEP> 3 <SEP> 76 <SEP>
<tb>"<SEP> 4 <SEP> 78 <SEP>
<tb>"<SEP> 5 <SEP> 79 <SEP>
<tb>"<SEP> 6 <SEP> 79
<Tb>
This table demonstrates the surprisingly surprisingly good technical effect and advantages due to the use of a Z-treatment alkaline bleaching treatment acid treatment-alkaline bleaching treatment, according to the invention.

Claims (10)

 Process for bleaching chemical cellulosic pulps, in a sequence of different treatments to obtain bleached pulps comprising at least one bleaching agent treatment not containing chlorine, at least two alkaline treatments and at least one treatment intermediate acid between the two alkaline treatments characterized in that at least one of the two alkaline treatments is an alkaline bleaching treatment and the sequence further comprises a treatment with ozone (Z) preceding the first of the two alkalinity treatments.  2. Method according to claim 1, characterized in that the treatment with ozone (Z) is carried out in the presence of a complexing agent.  3. Method according to claim 2, characterized in that the complexing agent is introduced at an acid stage (aX) preceding the action of ozone.  4. Method according to one of claims 1 to 3, carac terse in that it uses at least the following sequence  Z - (P or PO or 0) - acid treatment - P  5. Process according to claim 4, characterized in that the acidic treatment is chosen from the group of the following treatments: a, Z, Pa, POa.  6. Method according to one of claims 4 to 5, carac terse in that it uses the following sequence  O - aX - Z - PO - a - P  7. Method according to one of claims 4 to 5, charac terized in that it uses the following sequence  O - aX - Z - PO - a - P - a.  8. Method according to one of claims 1 to 7, charac terized in that the treatment with ozone (Z) is carried out on a high consistency paste.  9. Application of the method according to one of claims 1 to 8, the bleaching of a kraft paper pulp for printing-writing.  10. Bleached chemical cellulosic pulp by carrying out the method according to one of claims 1 to 9, characterized in that it has a degree of whiteness greater than 80.