FI111387B - Method for chlorine-free bleaching of pulp with acetic acid as an acidifying agent - Google Patents

Description

111387

The present invention relates to a process for the chlorine-free bleaching of a pulp of a cellulosic fibrous material having an 14 or less, preferably 12 or less, and most preferably 10 or less, is continuously fed in a bleaching line and treated with a complexing agent in the presence of at least one acid to adjust the pH to less than 7 and then bleached in a multiple step sequence consisting of one or more organic fly-post and peroxide.

The stringent environmental requirements for the use of chlorine-containing bleaches in pulp bleaching have led to extensive research to find other bleaching agents that are not harmful to the environment such as chlorine-containing bleaching agents that form chlorinated organic compounds. Thus, it has been proposed to bleach pulp using ozone and peroxide in various sequences preceded by treatment with a complexing agent as described, for example, in SE-A-9101300-3. However, the use of ozone has several disadvantages. Ozone has a disintegrating effect on cellulose, and the equipment for producing and feeding ozone in the bleaching process is bulky and expensive. US-A-3,867,246, corresponding to SE-B-25 405130, discloses a second bleaching process that does not employ chlorine which has a sequence of peroxide, peracetic acid and peroxide preceded by treatment with a complexing agent in the presence of an acid to adjust the low pH. . The acid used is usually sulfuric acid. But, like ozone, peracetic acid has a strong disintegrating effect on cellulose.

2, 111387

It is an object of the present invention to overcome the drawbacks associated with bleaching pulp without chlorine by using peracetic acid together with peroxide in the bleaching sequence PPaP preceded by treatment with a complexing agent (Q) in the presence of an acid to obtain a bleached pulp of acceptable viscosity. and strength properties and whiteness 85-90% ISO required by the market.

The process according to the invention is characterized in that during the treatment with the complexing agent, the pulp is acidified with acetic acid.

10

The incoming pulp is preferably prepared by a low Kappa extraction process and then subjected to delignification with oxygen gas so that the pulp has a Kappa number of about 12 or less and a viscosity of at least about 1000 dm 3 / kg.

The peroxide used is preferably hydrogen peroxide.

The second bleaching step, which follows the first peroxide step, is carried out using permic acid, peracetic acid or perpropionic acid, preferably peracetic acid or a mixture of at least two of these peracids.

According to a preferred embodiment, the acetic acid is added in an amount such that the pH is less than 6, preferably less than 5.

Temperatures at all bleaching stages should be 80 ° C or higher, preferably 90 ° C.

Each peroxide step is preferably carried out in an alkaline solution.

3, 111387

The peracetic acid step is preferably carried out in an acidic solution. The acetic acid and peracetic acid, or other suitable peracids, are preferably prepared locally.

The consistency of the pulp in the various treatment steps is about 8-16%, preferably 9-11%.

The following example further illustrates the invention and shows its surprising results.

10 Example

A number of experiments were performed using coniferous sulphate pulp delignified with oxygen gas and washed. The delignified mass had a Kappa of 12.1, a whiteness of 33.7% ISO and a viscosity of 1020 dm 3 / kg. It was fed to the whitening line at a consistency of 10% and maintained during EDTA treatment of the complexing agent and at all bleaching stages. In Experiments 1 and 2, sulfuric acid was used as the acidifying agent in the Q stage, while acetic acid was used in Experiments 3 and 4 according to the present invention.

20 Tests 1 - 4 1. Additives Bleaching Sequence

2. Conditions OPPaP OPPaP

3. Results_1 2-3 4-Step 25 1. EDTA, kg / BDMT 2.0 2.0 H2SO4, kg / BDMT 6.2

Acetic acid, kg / BDMT - 15.0 2. Initial pH 4.7 4.8

Time, min. 60 60 30 Temperature, ° C 70 70 4 111387 P Step 1. H 2 O 2, kg / BDMT 35 35

Experiments 1-4 continue

MgSO 4, kg / BDMT 3.0 3.0 5 NaOH, kg / BDMT 25 25 2. Time, min. 240 240 Temperature, ° C 90 90

Final pH 10.9 11.0 10 1. Additives Bleaching sequence

2. Conditions QPPaP OPPaP

3. Results _ 1 2_3_4 3. H 2 O 2 consumed, kg / BDMT 18 16

Kappa number 5.1 4.9 Viscosity, dm3 / kg 910 911

Whiteness,% ISO 77.3 77.0

Pa Step 1. Peracetic Acid Calculated Total H 2 O 2, kg / BDMT 20 30 20 30 2. pH, buffer solution (sodium acetate) 4.3 4.3 4.3 4.3

Time, min. 240 240 240 240 Temperature, ° C 90 90 90 90 25 3. Acetic acid consumed as H2O2, kg / BDMT 11.6 17.9 9.9 14.2

Kappa number 1.1 0.7 1.2 0.9

Viscosity, dm3 / kg 842,814,852,841

Whiteness,% ISO 84.4 87.4 84.4 86.8 5 111387 P Phase 1. H 2 O 2, kg / BDMT 4.0 4.0 4.0 4.0

MgSO 4, kg / BDMT 3.0 3.0 3.0 3.0

NaOH, kg / BDMT 7.0 7.0 7.0 7.0 5 2. Time, min. 180 180 180 180 · Temperature, ° C 90 90 90 90

Final pH 10.3 10.4 10.6 10.5 3. Consumed H 2 O 2, kg / BDMT 1.5 1.3 0.9 1.1

Capacity 0.8 0.5 1.1 0.7 10 Viscosity, dm3 / kg 808 778 834 828

Whiteness,% ISO 88.1 89.3 87.8 89.2

The results show that the pretreated and bleached pulps according to the invention have a significantly better viscosity, which in turn exhibits better strength properties compared to the pretreated and bleached pulps according to Examples 1 and 2. By using equal amounts of peracetic acid (Pa) in the second bleaching step, 20 kg in Examples 1 and 3 and 30 kg in Examples 2 and 4, the use of acetic acid as sulfuric acid in the Q step results in an increase in viscosity of 26 and 50 units.

As mentioned above, acetic acid and peracetic acid or other suitable peracids are preferably prepared locally. Acetic acid, for example, can be recovered from black liquor in quantities of up to 50 kg / tonne when 25 pulps are produced. For example, peracetic acid can be produced from acetic acid recovered in a known manner. Thus, the method according to the invention can be implemented in a very economical way.

Claims (7)

111387 A process for de-chlorinating a pulp of cellulosic fibrous material in the preparation of a chemical pulp, wherein the inlet pulp 5 treated with oxygen gas and having a kappa number of about 14 or less, preferably 12 or less, and preferably 10 or less is continuously fed to the bleaching line complexing agent in the presence of at least one acid to adjust the pH to less than 7 and then bleaching in a sequence of several steps consisting of peroxide, one or more organic peracids and peroxide, characterized in that, in the treatment with the complexing agent, the pulp is acidified with acetic acid. Process according to Claim 1, characterized in that hydrogen peroxide is used as the peroxide. Process according to claim 1 or 2, characterized in that one or more organic peracid acids are permic acid, peracetic acid or perpropionic acid, preferably peracetic acid or a mixture of at least two of said peracids. Process according to one of Claims 1 to 3, characterized in that the acetic acid is added in an amount such that the pH value is below 6, preferably below 5. Process according to one of Claims 1 to 4, characterized in that each peroxide step is carried out in an alkaline solution. Method according to one of Claims 1 to 5, characterized in that the consistency of the pulp in said treatment with the complexing agent and in said bleaching sequence is about 6-16%, preferably 9-11%. 111387 Use of acetic acid as an acidifying agent in treating a chemical pulp with a complexing agent in chlorine-free bleaching of the pulp, wherein the incoming pulp treated with oxygen gas having a kappa number of about 14 or less, preferably 12 or less and most preferably 10 or less, is continuously fed in the presence of acetic acid to adjust the pH to less than 7, and then bleached in a sequence of several steps consisting of peroxide, one or more organic peracids and peroxide. 9 111587 kommande mass, som har behandlats med syrgas och uppvisar et kappatai av omkring 14 eller därunder, företrädesvis 12 eller där under och mest före-draget 10 eller därunder, mat Continuerligt i en bleklinje och behandlas med komplexbildaren 1 närvaro av The pH account is color-5 to under 7, and the bleaches are in the form of a peroxide, an eller flera organic persyror and a peroxide.