CH653720A5 - PROCESS FOR THE TREATMENT OF CHEMICAL PAPER PULP. - Google Patents

Description

The invention relates to a new treatment for chemical pulp.

In the following description, the expression chemical pulps mainly designates unbleached or lightened cellulose pulps, that is to say still containing lignin, obtained by acid, neutral or alkaline sulfite processes, with sulfate (Kraft process), with sodium hydroxide, carbonate or oxygen, with or without the presence of a delignification catalyst.

As we know, the bleaching of cellulosic pulps is the treatment which consists in eliminating, by the action of chemical reagents, the coloring matter associated with cellulose fibers, a large part of which is made up of lignin in a very condensed and deeply modified state during cooking reactions. The bleaching of chemical pulp is mostly done using chlorinated agents, such as chlorine, chlorine dioxide or sodium hypochlorite.

However, these bleaching agents entail significant disadvantages with regard to the fight against pollution. Indeed, recycling in the recovery circuit of chemicals used for pulping wood, used bleaching liquors, intended to reduce the quantity of polluting effluents, faces serious problems (risks of corrosion of the installations , risk of explosion in boilers), due to the very large amount of chlorides present in these used liquors.

In order to eliminate these problems, it has been proposed to use chlorine-free agents for bleaching. Thus, the spent liquors from this treatment can be recovered and returned to the reagent recovery circuit without incurring the risks due to chlorides. Initially, it was proposed to use oxygen as a bleaching agent. This process makes it possible to obtain satisfactory results insofar as it has thus been possible to reduce the pollution due to the bleaching operation without any risk. However, the oxygen treatment requires a very expensive investment, which ultimately explains the limited development of this process.

To overcome this essentially economic disadvantage, it has also been proposed to replace the oxygen with peroxides, in particular hydrogen peroxide. This solution has the advantage of being able to be implemented at atmospheric pressure, therefore in a simple traditional bleaching tower, which represents a much lower investment than that of the oxygen process. The hydrogen peroxide treatment has been developed in Kraft chemical pulp manufacturing units and in some sulfite pulp manufacturing units. However, hydrogen peroxide is a much more expensive chemical reagent than oxygen and has been shown not to achieve the performance of oxygen treatment under acceptable economic conditions.

Indeed, it is known that the metal cations present in chemical pulps can decompose hydrogen peroxide in such a way that the efficiency of the treatment with hydrogen peroxide is reduced. It has been determined that the negative cations to be eliminated are mainly manganese and copper, and perhaps, to a lesser degree, iron.

To improve the efficiency of the hydrogen peroxide treatment, it has been proposed to precede this treatment with a pretreatment with an acid (see in this sense Japanese patent application No. 76/102103 filed on March 5, 1975 by Nippon Pulp Ind. KK, the French patent N ° 77/24131 of the holder published under N ° 2398839 - in particular p. 6, lines 25 and following -, the publication of the inventors appeared in "Paperi ja puu", N ° 4A, 1981, pp. 301 to 308, in particular table p. 305, and the “Tappi Environmental Conference 1981 Prepints”, New Orleans, April 27-29, 1981, pp. 93-101).

According to these various authors, an acid treatment of the paste at room temperature preceding the peroxide treatment makes it possible to significantly reduce the amount of metal cations with a negative effect. One of the preferred acids for this treatment is sulfuric acid. Thus, it has been able to demonstrate that pretreatment with sulfuric acid at room temperature was very effective in reducing the quantity of cations with a negative effect, and in particular manganese, which is recognized as the most harmful metal cation.

However, it has been discovered that, quite unexpectedly, if we refer to the teachings of the known prior art and recalled above, when the acid treatment preceding the peroxide treatment is carried out at a temperature high, the efficiency of the subsequent peroxide treatment is notably improved, while retaining the mechanical characteristics of the pulp thus treated, although the elimination of metal cations is not appreciably improved.

In other words, the efficiency of the treatment of a chemical pulp with peroxide, in particular hydrogen peroxide, can be improved by another means than those described in the prior art and which aim only at eliminating the metal cations with negative effect present in the paste.

The process according to the invention for the treatment of chemical pulp of the type comprising:

- a first stage of treatment with an acid solution,

- a second stage of treatment with an alkaline solution of peroxide,

is characterized in that the first stage of treatment is carried out at a temperature of at least 50; vs.

Advantageously, this treatment is carried out at a temperature between 60 and 80: C. It could theoretically be carried out at a temperature above 100r C, but such a temperature would require operating under pressure, which would entail special and costly installations and especially would risk degrading the pasta.

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In other words, and therein lies the unexpected effect of the invention, it was not imaginable that the application of a known treatment to reduce the concentration of cations with negative effect would allow, when the the treatment temperature is increased, the lignin content of the pulps thus treated is significantly reduced, even though this increase in temperature has no appreciable effect on the rate of elimination of the cations with negative effect contained in these pasta.

In other words, the increase in the temperature of the known acid treatment gives the same result with regard to the lowering of the concentration of cations with negative effect, but also provides a better delignification of the doughs subjected to the treatment according to the 'invention. The residual lignin content of the pasta is conventionally expressed below by the Kappa index (AFNOR standard NFT 12018).

As already said, the chemical pastes capable of being treated in accordance with the invention can be very varied. We can retain Kraft pasta, soda pasta, sulfite pasta, soda / oxygen, soda / anthraquinone, carbonate ...

These pastes can be treated according to the invention to varying consistencies, in particular depending on the material used. If more generally these consistencies are between 3 and 20%, in practice the consistency during treatment is close to 10 to 15%.

A wide variety of acid solutions can be used during acid treatment (see in particular the table on page 305 of the publication "Paperi ja puu", No. 4A, 1981, mentioned above). Advantageously, mineral or even organic acids are used, such as:

- sulfurous acid (H2S03),

- hydrochloric acid (HCl),

- phosphoric acid (H3PO4).

In practice, sulfuric acid (H2SO4) is mainly used.

Organic acids can optionally be used, which have the advantage of being more easily recycled in the factory circuits.

One can also use any acid effluent available in the factory, for example the acid solution coming from the chlorine dioxide preparation workshop and which contains sulfuric acid, the effluent of the treatment with SO2 of the bleached pasta or any acidic bleaching effluent. Unless otherwise indicated, the concentrations of reactants will be expressed, in what follows, by weight of pure product relative to the weight of the treated pulp counted in dry. The acid concentration is adjusted so as to obtain a pH between 1 and 5 and advantageously between 2 and 3. The duration of this acid treatment is in known manner fixed between 30 and 180 min, preferably between 60 and 120 min.

As already said, the temperature of this treatment is at least 50 ° C and advantageously between 60 and 80 ° C.

The treatment according to the invention can be carried out in any conventional pulp-making installation, without additional investment. For example, a ramp of the last dough washing filter can be fed with an acid solution after cooking, then transfer the dough thus impregnated into a conventional tower resistant to the temperature and the action of the acid. The acid-treated pulp is then washed and then treated in a known manner with an alkaline solution of peroxide.

For this second treatment, chemical compounds of general formula R — O — O — X are used as peroxide in which R denotes a radical and X a metal ion or hydrogen. By way of example, mention may be made of hydrogen peroxide or other organic or mineral peroxides such as potassium peroxide, calcium peroxide, sodium perborate, potassium and sodium persulfates, peracetic acid. .. In an industrial embodiment, hydrogen peroxide is preferably used. The amount of peroxide used is advantageously between 0.5 and 10% and preferably between 1 and 5%. As already said, the second treatment solution also contains an alkali. In practice, the treatment liquor contains from 1 to 10%, preferably from 1 to 4% of sodium hydroxide.

This treatment with the alkaline peroxide solution is carried out at a temperature between 50 and 100 ° C, preferably at a temperature between 60 and 80 ° C, and this for a period between 0.5 and 5 h, from preferably between 1 and 3 h.

So, as already said, it is advantageous to operate at atmospheric pressure.

This second treatment may or may not be followed in a conventional manner by washing with water.

The pulp thus treated can then undergo, in the usual manner, additional whitening treatments.

The manner in which the invention can be implemented and the advantages which ensue therefrom will emerge more clearly from the following exemplary embodiments, given by way of indication and without limitation. The processing conditions and the results obtained are summarized in Tables I and II, Table I concerning the treatment of a Kraft pulp of softwood of Kappa index 31, Table II concerning the treatment of a Kraft pulp of wood hardwood (beech) with Kappa index 22.

In Example 1, given by way of reference, a Kraft pulp of softwood with a Kappa index of 31 was treated conventionally, in a single stage, using an alkaline solution of hydrogen peroxide.

In Example 2, the same paste was treated with the same alkaline solution of hydrogen peroxide, but preceding this treatment with an acid treatment implemented according to the conditions described in the prior art, in particular in the Japanese patent application No. 76/102103 filed March 5, 1975 by Nippon Pulp Ind. K.K.

It can be seen that the acid treatment eliminates a large part of the metal cations, in particular of the manganese cations, contained in the starting paste and thus promotes delignification by the peroxide.

(Tables on next page)

In Examples 3 to 10, the same Kraft pulp of resinous wood was treated according to the teachings of the invention by varying the temperature conditions and the acid concentration of the first treatment, as well as the hydrogen peroxide concentration of the second treatment. It is seen, by comparison between examples 3, 4 and 10, that the increase in the temperature of the first treatment has no appreciable effect on the elimination of harmful metal cations, but that it does, on the other hand, provide an improvement net of delignification, without requiring an increase in either the acid concentration of the first treatment or the peroxide concentration of the second treatment.

The comparison between Examples 11 and 12 shows that increasing the temperature of the acid treatment has the same effect on the delignification of a hardwood pulp.

As an indication, to obtain by means of the treatments according to Example 2 a paste with the same Kappa index as that obtained in Example 4, the peroxide concentration of the second treatment should be increased by an additional 2%, which would be very disadvantageous on the economic plan.

Table III brings together the mechanical characteristics of the basic unbleached pulp and of the pulp treated in accordance with the teachings of the invention (example 4), that is to say having undergone:

- firstly, treatment with a 2% aqueous sulfuric acid solution at 70 ° C for 2 h,

- then, after washing, a second treatment using a solution containing 1% hydrogen peroxide and 1.5% sodium hydroxide at 90 ° C for 90 min.

These mechanical characteristics were measured according to AFNOR standards after classification of the doughs in a Weverk laboratory binder (0.15 mm slots) and refining in the Jokro mill up to 40 ° SR. Thus, from this table III it is clear that the pulps treated according to the invention have the same mechanical properties as the untreated untreated Kraft pulps. In other words, the method according to the invention does not modify the mechanical properties

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Table I

- Kappa index paste

1

2

3

4

Examples 5 | 6

7

8

9

10

Kraft softwood pulp 31

- Conditions of the 1st treatment

Consistency of dough (%)

12

12

12

12

12

12

12

12

12

Temperature (: C)

20

50

70

70

70

70

70

70

80

Duration (min)

120

120

120

120

120

120

120

120

120

H2S04 concentration (%)

2

2

2

1

3

2

2

2

2

pH

2

2

2

2.5

1.5

2

2

2

2

- Cation content after

1st treatment (ppm)

Fe

53

30

30

31

31

31

31

30

Cu

8.8

4.0

3.7

3.8

-

3.8

3.8

3.8

4.0

Mn

103

5.0

3.0

3.0

3.0

3.0

3.0

3.0

- Kappa index

after 1st treatment

30.5

30.5

30.5

30.5

- Conditions of the 2nd treatment

Consistency of dough (%)

12

12

12

12

- 12

12

12

12

12

12

Temperature (D C)

90

90

90

90

90

90

90

90

90

90

Duration (min)

90

90

90

90

90

90

90

90

90

90

H2O2 concentration (%)

1

1

1

1

1

1

0.5

2

3

1

NaOH concentration (%)

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

- Kappa index

22

20.6

20.2

18.5

19.1

18.6

21.8

15.9

14.4

17.5

after the 2nd treatment

Table II

Examples

11

12

- Dough to be treated

Kraft beech paste

Kappa index

22

- Conditions of the 1st treatment

Consistency of dough (%)

12

12

Temperature (° C)

20

70

Duration (min)

60

60

H2S04 concentration (%)

2

2

pH

2

2

- Conditions of the 2nd treatment

Consistency of dough (%)

12

12

Temperature (° C)

90

90

Duration (min)

90

90

H2O2 concentration (%)

1

1

NaOH concentration (%)

1.5

1.5

. - Kappa index after the 2nd treatment

13.5

12.0

of this pasta, contrary to what one might have feared by increasing the temperature of the acid treatment.

In Examples 5 and 6, the sulfuric acid concentration of the liquor of the first treatment was varied. In Examples 7, 8 and 9, the peroxide concentration of the liquor of the second treatment was varied.

Example 13: 65

The same Kraft softwood pulp is used as in Example 4 having undergone the same treatment as in Example 4, that is to say a first acid treatment at 70 ° C. followed by a treatment with per-

Table III

Characteristics of Kraft Softwood Pulp

Index

Length

Index

Index of

Burst burst tear kappa

(AFNOR

(AFNOR

(AFNOR

(AFNOR

NF

NF

NF

NF

T 12018)

Q 03 004)

Q 03 014)

Q 01 012)

(km)

(kPa / m2 / g)

(mN / m2 / g)

Before

treatment

31.0

9.75

7.22

9.03

After

18.5

10.03

7.29

9.02

treatment

according to invention

(ex. 4)

oxide. The dough thus treated is then taken up again to subject it again to an identical treatment, that is to say:

- firstly, with a 2% aqueous sulfuric acid solution at 70 ° C for 2 h,

- then, in a second step, with an alkaline solution of peroxide containing 1% by weight of hydrogen peroxide and 1.5% by weight of sodium hydroxide on a paste at a concentration of 12%, and this for 90 min at 90 ° C.

A paste is thus obtained, the Kappa index of which is reduced from 18.5 to 11.0 and having a whiteness of 58% (value measured according to AFNOR standard NF Q 03039) and the mechanical characteristics of which are maintained.

Example 14:

Example 13 is repeated, but replacing the Kraft pulp of softwood with a Kraft pulp of hardwood (beech) identical to that

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used in Example 12. In addition, the nature of the acid is modified by replacing the aqueous sulfuric acid solution with an aqueous solution of a mixture containing 1% sulfuric acid and 2% sulfur dioxide. By way of comparison, the Kappa index is reduced from 12 to 9.0 and a whiteness of 53.5% is obtained. 5

Example 15:

A Kraft pulp of resinous wood of Kappa index 31 is used, treated according to the teachings of the invention at 70 ° C., that is to say, in accordance with Example 4. This pulp is subjected to an additional bleaching treatment. in two stages under the following conditions:

- first stage, with chlorine dioxide:

C102 concentration 1.5% 15

temperature 70 ° C

duration 60 min consistency of dough 6%

- second stage, with hydrogen peroxide:

H202 concentration 1.5%

1% NaOH concentration

sodium silicate concentration at 38 ° B 3%

temperature 70 ° C

duration 180 min consistency of dough 10%

There is thus obtained by a short process (four stages), not comprising a chlorine treatment, a whiteness of 87.5%, which shows that the pulp treated according to the invention has an excellent whitening ability.

As already said, the process according to the invention has numerous advantages, since it makes it possible to obtain, by the simple raising of the temperature of the acid treatment, an unexpected improvement in delignification while retaining the pasta thus treated with good mechanical properties and an excellent ability to bleach, and this under advantageous conditions of implementation, cost and reduction of pollution.

R

Claims (9)

653,720 1. Process for the treatment of chemical pulp, in which we perform: - a first treatment using an acid solution, - then a second treatment using an alkaline peroxide solution, characterized in that the first acid treatment is carried out at a temperature of at least 50 ° C. 2. Method according to claim 1, characterized in that the first treatment is carried out at a temperature between 60 and 80 ° C. 2 3. Method according to one of claims 1 or 2, characterized in that the pH of the acid solution of the first treatment is between 1 and 5, preferably between 2 and 3. 4. Method according to one of claims 1 to 3, characterized in that the duration of the first treatment is between 60 and 120 min. 5. Method according to one of claims 1 to 4, characterized in that the acid solution contains a mineral acid or an organic acid. 6. Method according to claim 5, characterized in that the acid solution contains sulfuric acid. 7. Method according to one of claims 1 to 6, characterized in that the alkaline solution of the second treatment contains from 0.5 to 10% by weight of hydrogen peroxide, preferably from 1 to 2%. 8. Method according to claim 7, characterized in that the alkaline solution of the second treatment also contains from 1 to 10% by weight of sodium hydroxide, preferably from 1 to 5%. 9. Method according to one of claims 1 to 8, characterized in that the treatment is repeated several times by means of two successive solutions, respectively hot acid solution and alkaline peroxide solution.