WO1997015714A1

WO1997015714A1 - Bleaching sequence for chemical paper pulp including sulphonating and oxidising bleaching stages

Info

Publication number: WO1997015714A1
Application number: PCT/SE1996/001295
Authority: WO
Inventors: Petter Tibbling; Erik Nilsson
Original assignee: Kvaerner Pulping Ab
Priority date: 1995-10-26
Filing date: 1996-10-11
Publication date: 1997-05-01
Also published as: SE9503764D0; SE516219C2; SE9503764L

Abstract

Method of bleaching chemical paper pulp, in a manner which is preferably entirely chlorine-free, in a bleaching sequence which includes at least one acidic, sulphite-containing, reducing bleaching stage, which bleaching stage is carried out, under conditions which are in the main sulphonating, to a degree of sulphonation which exceeds 10 %. One or two oxidising bleaching stages follow(s) directly thereafter, without intervening washes. If single, this oxidizing bleaching stage can be acidic or alkaline; if there are two oxidizing bleaching stages, the first can be acidic and the second alkaline.

Description

BLEACHING SEQUENCE FOR CHEMICAL PAPER PULP INCLUDING SULPHONATING AND OXIDISING BLEACHING STAGES

TECHNICAL FIELD The present invention relates to a method of bleaching chemical paper pulp in a bleaching sequence which includes at least one acidic, sulphite- containing, reducing bleaching stage. At least one of the said sulphite-containing stages is carried out under conditions which are mainly sulphonating and is directly followed, without any intervening wash, by one or two oxidizing bleaching stages. The latter can be an acidic or alkaline stage or, if there are two oxidizing stages, firstly an acidic stage and then an alkaline stage. The sulphonating conditions are produced by the system parameters pressure, temperature, time, pH and sulphur charge being optimized such that the maximum proportion of water-soluble lignin is formed. While the bleaching sequence in this context is preferably entirely chlorine-free, one or more complementing chlorine-containing stages can also be included.

STATE OF THE ART AND PROBLEMS

Due to the increasing interest in the environ- ment and comprehension of the ecological cycle in nature, there is a great desire, both among consumers and producers, to decrease the discharge of pollutants arising from human activity. Producers of pulp and paper have often been portrayed as villains in connection with the environment. However, in recent years, vigorous attempts have been made to decrease the discharges from our pulp and paper mills, and major advances have indeed been made. The demand from the market for chlorine-free bleached paper has led to the incipient use of alternative bleaching chemicals such as ozone (Z) , persulphuric acid (Caro' s acid, Ca) , peracetic acid (Pa) and hydrogen peroxide (P) . However, one disadvantage of these bleaching chemicals is that they either require generating equipment, with associated high capital cost, or entail the substantial expenditure involved in purchasing the bleaching chemicals.

Other disadvantages are, for example, that ozone and peracetic acid degrade the pulp. When hydrogen peroxide alone is used for the bleaching, the disadvantage is that the finally bleached pulp contains a residual lignin content corresponding to 2-4 kappa units, which more than doubles the brightness reversion which is induced by heat and UV light. Pulp which is bleached to full brightness, 88-90 ISO, using alkaline peroxide alone also tends to require higher beating energy and give inferior strength properties as compared with sequences using ozone or peracids.

At Kvaerner Pulping Technologies AB (formerly Kamyr AB) , we have developed a method of bleaching chemical paper pulp, preferably in an entirely chlorine-free manner, in a bleaching sequence which comprises at least one acidic, sulphite-containing, reducing bleaching stage and at least one alkaline, oxidizing bleaching stage in arbitrary order.

At least one of the said acidic, sulphite-containing, reducing bleaching stages is carried out under conditions which are mainly sulphonating to a degree of sulphonation exceeding 10%, preferably exceeding 20%, and even more preferably to 40-80%.

It is also previously known through WO 92/07139 to carry out a reducing stage of the sulphite type or bisulphite type between two peroxide stages. However, in this case, the first peroxide stage is acidic while it is only the second stage which is alkaline. Nor is the sulphite stage carried out under sulphonating conditions.

Through SE 461 991, a method is known of improving subsequent oxygen delignification by means of treatment with alkaline sulphite. This treatment with sulphite thus has another purpose and is not carried out, either, under conditions which are appreciably sulphonating.

Through EP 433 138, it is known to have two alkaline stages, for example peroxide, with an intervening acidic stage, for example S0₂, in the bleaching sequence. The acidic stage is not carried out under sulphonating conditions in this case either. In the examples, the temperature is indicated to be between 20°C and 80°C and the pressure is assumed to be atmospheric.

SOLUTION AND ADVANTAGES The present invention results in an improved method in which sulphur compounds which are internally generated, and available, in the pulp mill are used for providing, for example in combination with hydrogen peroxide, pulps which are bleached in a manner which is preferably entirely chlorine-free, which method is characterized by the low chemical cost of ozone bleaching, the pulp of relatively high quality which is derived from peracetic acid/chlorine dioxide bleaching, and the lower capital cost of bleaching purely with peroxide. That the bleaching sequence should preferably yield pulp which is bleached in a manner which is entirely chlorine-free means that it is preferred for the invention to be carried out as a part of a total bleaching sequence which is performed entirely without using any chlorine-containing chemical. However, it is conceivable that it is desirable, in certain cases, for one or more complementing chlorine-containing stages, for example chlorine gas or chlorine dioxide, to be included in the total bleaching sequence. In the method according to the invention, the bleaching sequence includes at least one acidic, sulphite-containing, reducing bleaching stage which is carried out under conditions which are mainly sulpho¬ nating. The method is characterized by an oxidizing bleaching stage following directly thereafter, without any intervening wash. Alternatively, two or more oxidi¬ zing bleaching stages follow, with the first preferably being acidic and the second preferably being alkaline. There is preferably no wash between these oxidizing stages. The oxidizing stage(s) is/are preferably (a) oxygen stage(s) with or without hydrogen peroxide. Alternatively, use is made of pressurized peroxide stages (PO) in accordance with SE 9301960, which implies that they contain peroxide and are carried out at a pressure at the top of the bleaching vessel which exceeds 1 bar, preferably exceeds 1.2 bar and more preferably exceeds 1.5 bar. At the bottom of the bleaching vessel, the pressure should exceed 3 bar, preferably exceed 4 bar and more preferably exceed 5 bar. The temperature in this context should exceed 90°C, preferably be identical to or greater than 100°C, and more preferably be between 100 and 120°C.

Alternatively, a pressurized acidic peroxide stage is used as the oxidizing stage. However, the invention concept includes any oxidizing stage whatever.

Examples of conceivable bleaching sequences are Q (PO) AR*, O, PO) and Q (AR*, 0, PO) , where symbols in brackets, separated by commas, indicate stages without an intervening wash. Further variants of these bleaching stages without any intervening wash are (AR*, 0 + H₂0₂, PO) , (AR*, E + 0 + H₂0₂) or (AR* , PO) .

Sulphonation involves S0₂ being consumed while HS0₃- groups enter the lignin fragments as substituents, in association with which water-soluble compounds of residual lignin are formed in the pulp. This results in a substantial kappa reduction directly after washing. It is easy to verify that sulphonation is taking place from the fact that the pH falls from 4-6 initially down to 3-4 , and from the fact that S0₂ is consumed and the kappa number declines. Expressed as water-soluble lignin, the degree of sulphonation is defined, for a given bleaching stage, as (kappaι-kappa₂) /kappai, where kappai is the kappa number entering the stage and kappa₂ is the kappa number leaving the stage.

In the method according to the invention, the sulphonating, acidic, sulphite-containing reducing bleaching stages are taken to a degree of sulphonation exceeding 10%, preferably exceeding 20% and even more preferably to 40-80%. The reaction time for such a stage is 30 seconds - 120 minutes, preferably 5-120 minutes and even more preferably 30-120 minutes. The sulphur consumption, calculated as sulphur dioxide, during the sulphonating, acidic, sulphite-containing reducing bleaching stages is 1-40 kg per ton of pulp, preferably 2-15 kg per ton of pulp, and even more preferably 3-10 kg per ton of pulp.

The sulphonating conditions are achieved in one or more sulphite stages by means of a pressure at the top of the bleaching vessel which exceeds 1 bar (absolute) , preferably exceeds 1.2 bar and more preferably exceeds 1.5 bar, a pressure at the bottom of the bleaching vessel which exceeds 2 bar, is preferably between 3 and 15 bar, and more preferably between 5-10 bar, and by means of a temperature which exceeds 90°C, is preferably identical to or greater than 100°C, and more preferably is between 100 and 120°C. Such a stage is designated (AR*) and should also contain complexing agents, for example EDTA, for eliminating metals, in accordance with the scheme (S0₂ + Q, E) or (S0₂ + Q) . In this regard, E denotes alkaline extraction. In addition to S0₂, sulphite solutions or bisulphite solutions, such as Na₂S0₃ or NaHS0₃, can be used as a sulphite source. Another possibility is to use, as a sulphur source, a sulphur-contaming process stream from an installation for gasifying and combusting black liquor, for example a so-called Chemrec® reactor (see SE-C-448 173) . Yet another possibility is to generate sulphur dioxide internally by combusting sulphur gases which have been driven off from black liquor to what is in the main sulphur dioxide. This sulphur dioxide can then be absorbed in bleaching department liquid which is then used in the sulphite stage or, alternatively, as an acidifier, for example in association with a complexing agent stage. Anthraquinone is also expediently used in the sulphite stages in order to improve bleaching selectivity still further. Addition of sodium borohydride represents another option for increasing the reducing capacity of the sulphite solution. A sulphite stage which is only acidic and reducing, without being pressurized, and is consequently not appreciably sulphonating, is designated (AR) .

Carrying out one or two oxidizing stages directly after the sulphonating stage, without any intervening wash, eliminates residual S0₂, on the one hand, and provides the possibility, on the other hand, of optimizing the metal profile of the pulp by the choice of pH in the oxidation stage/stages. In addition to this, it can be assumed that there will be a contri- bution to the oxidation potential, thereby permitting a saving in the more expensive hydrogen peroxide. Thus, for example, the sequence Q(P0) (AR*,0,P0) has been found to give a lower final kappa number, a higher viscosity and a lower peroxide consumption as compared

The equipment for carrying out the bleaching sequence according to the invention expediently consists of a combination of pressurized reactors.

Through EP 433 138, it is known to have two alkaline stages, for example peroxide, with an interveninσ acidic stage, for example S0₂, in the bleaching sequence. The acidic stage is not carried out under sulphonating conditions in this case either. In the examples, the temperature is indicated to be between 20°C and 80°C and the pressure is assumed to be atmospheric.

That sulphonating conditions are not achieved by this method is evident from the experiments which are reported below.

EXPERIMENTS

In one experimental series, an isothermally cooked pulp was bleached which, after oxygen delignifj cation, had a kappa number of 12.1, a viscosity of 1020 dm³/kg and a wash carry-over corresponding to 5 kg of COD per ton of pulp. Two bleaching sequences were tested:

1. Q(PO) (PO) : preliminary elimination of transition metals followed by two pressurized peroxide stages, entirely in accordance with known technique (SE 9301960) . The pulp was washed between the three stages indicated.

2. Q(PO) (AR*,0,P0) : as in 1 but with an acidic S0₂ stage at 105°C and under a pressure of 6 bar. There then followed a pressurized acidic 0₂, and then a pressurized alkaline PO. The pulp was washed after Q, after (PO) and after (AR*, O, PO) .

Experimental conditions and results are reported in Table 1.

The two preliminary stages Q(PO) were common to the two experimental sequences. The Q stage was carried out in accordance with the LIGNOX procedure (SE 8902058) .

While H₂S0₄ was thus used in the laboratory experiment for acidifying in the complexing agent stage, it is preferred in a mill to use sulphur dioxide or returned H⁺ ions which have been released in a sulphonating reaction in subsequent stages for acidifying incoming pulp. Alternatively, the acidification stage can be carried out without a complexing agent. Table 1

1. Additives Sequence 1 Sequence 2

2. Conditions Q(PO) (PO) Q(PO) (AR*,0,

3. Results PO)

Q stage (common to 1-2)

1. H₂S0₄, kg ptp¹ 5

EDTA, kg ptp 2

2. Cone. , % 10

Time, min 60

Temp. °C 70

3. Final pH 5.2 wash wash

PO stage (common to 1-2)

1. H₂0₂, kg ptp 18

DTPA, kg ptp 2

MgS0₄, kg ptp 3 =

NaOH, kg ptp 19

2. Cone. , % 10

Time, hours 2 =

Temp., °C 120

Pressure, bar (abs.) 6 =

3. H₂0₂ consumption, kg ptp 14 =

Kappa 4.0 =

Viscosity, dm³/kg 851 =

Brightness, % ISO 82 = wash wash 1 . Additives Sequence 1 Sequence 2

2. Conditions Q(PO) (PO) Q(PO) (AR*,0,

3. Results PO)

AR*

1. S0₂, kg ptp 10

DTPA, kg ptp 2

MgSO«, kg ptp 3 2. Cone. , % 10

Time, hours 60

Temp. , °C 105

Pressure, bar (abs.) 6

Starting pH 5.5 3. Final pH 4.7

S0₂ consumption, kg ptp 6

Kappa 3.3

Viscosity, dm³/kg 845 no wash

O

1. NaOH, kg ptp 18

0₂, kg ptp ~3

Final pH 11.1

Kappa 3.2

Viscosity, dm³/kg 816 no wash

1. Additives Sequence 1 Sequence 2

2. Conditions Q(PO) (PO) Q(PO) (AR*,0,

3. Results PO)

PO stage

H₂0₂, kg ptp 23 18

DTPA, kg ptp 2 -

MgS0₄, kg ptp 3 -

NaOH, ]r.g ptp 22 -

Cone. , % 10 10

Time, hours 2 2

Temp. , °C 115 115

Pressure, bar (abs. ) 6 6

Final pH 11.0 10.4

H₂0 consumption, kg ptp 17 14

Kappa 3.3 2.6

Viscosity, dm³/kg 686 740

Brightness, % ISO 88.0 88.0

Brightness reversion, % ISO² 84.4 84.5

Brightness loss, % ISO 3.6 3.5

Total consumption of H₂0₂ , kg ptp 31 28

1) kg ptp = kilograms per ton of pulp 2) after 3 hours at 105°C, 0% relative humidity

- li ¬ lt can be seen from the experimental results that, after the preliminary bleaching Q(PO), the pulp had a kappa number of 4.0, a viscosity of 851 dm³/kg and a brightness of 82% ISO. In addition, the result shows that, after the concluding PO stage in the two sequences, a substantially inferior result is obtained with sequence 1. In particular, sequence 2 gives a higher viscosity, a lower kappa number and a lower peroxide consumption for achieving the same brightness. The invention is not limited by the embodiment in the experiments and can be varied within the scope of the subsequent patent claims. Thus, it will be readily evident to the skilled person that there are very many possibilities of varying the configuration of the bleaching sequence especially since the oxidizing stage(s) can use any oxidizing bleaching chemical.

Claims

PATENT CLAIMS

1. Method of bleaching chemical paper pulp, preferably in a manner which is entirely chlorine-free, in a bleaching sequence which includes at least one acidic, sulphite-containing, reducing bleaching stage, which bleaching stage is carried out under conditions which are mainly sulphonating to a degree of sulphonation which exceeds 10%. The method is c h a r a c t e r i z e d i n that there then follows at least one oxidizing bleaching stage directly thereafter, without any intervening wash.

2. Method according to Claim 1, c h a r a c t e r i z e d i n that the said sulphonating conditions are produced by a pressure which, everywhere in the bleaching vessel, exceeds the steam pressure of the liquid and of gases dissolved in the liquid and by a temperature which exceeds 90°C, is preferably identical to or greater than 100°C, and more preferably is between 100 and 120°C.

3. Method according to Claim 1 , c h a r a c t e r i z e d i n that the said sulphonating conditions are produced by a pressure at the top of the bleaching vessel which exceeds 1 bar, preferably exceeds 1.2 bar and more preferably exceeds 1.5 bar, a pressure at the bottom of the bleaching vessel which exceeds 2 bar, is preferably between 3 and 15 bar, and more preferably between 5-10 bar, and by a temperature which exceeds 90°C, is preferably identical to or greater than 100°C, and more preferably is between 100 and 120°C.

4. Method according to any one of Claims 1-3, c h a r a c t e r i z e d i n that the said sulphonating, acidic, sulphite-containing, reducing bleaching stage is carried out to a final pH of 1-7, preferably 2-6.

5. Method according to any one of Claims 1-4, c h a r a c t e r i z e d i n that the sulphur consumption, calculated as sulphur dioxide, during the said sulphonating, acidic, sulphite-containing, reducing bleaching stage is 1-40 kg per ton of pulp, preferably 2-15 kg per ton of pulp and even more preferably 3-10 kg per ton of pulp.

6. Method according to any one of Claims 1-5, c h a r a c t e r i z e d i n that the reaction time for the aaid sulphonating, acidic, sulphite-containing, reducing bleaching stage is 30 seconds - 120 minutes, preferably 5-120 minutes and even more preferably 30- 120 minutes.

7. Method according to Claim 1 , c h a r a c t e r i z e d i n that the said oxidizing bleaching stage contains peroxide, preferably hydrogen peroxide, and n that at least one such stage is carried out at a pressure at the top of the bleaching vessel which exceeds 1 bar, preferably exceeds 1.2 bar and more preferably exceeds 1.5 bar, while the pressure at the bottom of the bleaching vessel should exceed 3 bar, preferably exceed 4 bar and more preferably exceed 5 bar. At the same time, the temperature in the bleaching vessel should exceed 90°C, preferably be identical to or greater than 100°C and more preferably be between 100 and 120°C.

8. Method according to Claim 1, c h a r a c t e r i z e d i n that the said oxidizing bleaching stage consists of oxygen delignification with or without hydrogen peroxide.

9. Method according to Claim 1, c h a r a c t e r i z e d i n that after a first oxidizing stage, which is carried out under acidic conditions, there follows, without any intervening wash, a further oxidizing bleaching stage which is carried out under alkaline conditions.

10. Method according to Claim 1, c h a r a c t e r i z e d i n that the said oxidizing bleaching stage is carried out under conditions which are in the main alkaline.

11. Method according to Claim 1, c h a r a c t e r i z e d i n that the said oxidizing bleaching stage is carried out under conditions which are in the main acidic.

12. Method according to Claim 1, c h a r a c t e r i z e d i n that the bleaching sequence is initiated by the pulp being acidified to pH 4-6, preferably in the presence of complexing agents. Use is preferably made, as an acidifying agent, of sulphur dioxide, sulphite, bisulphite, sulphuric acid, effluent from the said acidic, sulphite-containing, reducing bleaching stage, or a combination of one or more of these acidifying agents.

13. Method according to Claim 1, c h a r a c t e r i z e d i n that the consistency of the pulp during the said acidic, sulphite-containing, reducing bleaching stage is between 8 and 16%, preferably between 9 and 11%.