FI76129C - Process for the preparation of chemical pulp and thus mainly aged pulp - Google Patents

Description

! 76129

Method of production of chemical pulp and pulp obtained therefrom

The invention relates to a process for the production of chemical pulps, in which the delignification of the chips is carried out in two steps; it also applies to the masses obtained by the method.

As is known, the classical alkaline methods of preparing chemical pulps consist schematically of the following steps: the wood chips are mixed with an alkaline liquid containing a sulfur-containing compound, the temperature of the mixture is gradually raised to between 150 ° C and 180 ° C, then the mixture is boiled until the fiberization point, the time taken for which depends on the nature of the starting wood.

In fact, the fiberization point is reached when the amount of residual lignin corresponds to a Kappa number of 30-35 for conifers and a Kappa number of 15-25 for deciduous trees.

There are two main drawbacks to the use of these widespread classical methods.

First, these methods significantly pollute the atmosphere by releasing malodorous sulfur compounds such as mercaptans, SC 2 and I 2 S. In the case of bleached pulps, chlorine-containing compounds are released with the effluent from the bleaching plant. This problem can be partially solved, however, on the condition that decontamination systems are used, which, as is well known, are unfortunately quite expensive.

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Then, and in addition, the pulp yield of these conventional methods is quite low, being, for example, about 40% for softwoods, which means that only 40 kg of dry pulp per 100 kg of dry chips is obtained. In the case of the sulphate method, in order to improve the yield, it has been proposed to pre-treat the chips before cooking in order to stabilize the carbohydrates, e.g. Unfortunately, these pretreatments require complex gases, which in turn require special equipment that is hardly developed at all.

Sulfate soups have also been proposed in which sulfur, for example, is introduced into the cooking liquor to form "in situ" polysulfides which, by oxidation, stabilize carbohydrates by preventing their decomposition by alkali. Although the yield is improved, the addition of sulfur increases without contamination, so that this method is hardly yet developed at all.

French patent publication 77 37 226, published under number 2,373,637, also proposes the addition of small amounts of anthraquinone (less than 0.1% by weight of dry wood) to the cooking liquid. Contrary to the results shown in the tables in that patent, however, industrial experiments have shown that the improvement in yield remains small and is often accompanied by a significant deterioration in some strength properties of the pulps obtained (see in particular in "antinquinone sulphate application" - K. GOEL, AM AYROUD & B. BRACH - Pulping Conference Proceedings, 24-26 September 9, 1979, Seattle, USA 213 et seq.).

In short, currently known improvements to increase the yield of classical alkaline methods of making chemical pulps either degrade the strength properties of these pulps or increase air pollution. Despite the various experiments carried out, 3,761 29 problems were always reached, while the protection of the environment is becoming increasingly severe and the market is demanding ever stronger masses.

The invention thus relates to an economical process which is carried out in conventional equipment and which, while considerably improving the yield, is not more polluting and improves some of the strength properties of these pulps.

This process for the production of chemical pulp, in which wood chips are boiled at an elevated temperature in an alkaline liquid containing a sulfur compound, is characterized in that this boiling at an elevated temperature is stopped when the ratio of residual lignin in the chips corresponds to a Kappa number of:. Between 45 and 100 for conifers, and. Between 30 and 50 for hardwoods, the fact that the chips thus treated are defibered, and finally that the cooking of these defibered chips is carried out in an alkaline solution containing 0.1 to 1.5% by weight of peroxide on the dry weight of the defibered material. calculated.

In other words, the invention comprises that the cooking of the chips is stopped before the conventional Kappa numbers of the raw chemical masses are reached, and then that the chips thus treated are mechanically defibered in a manner known per se before their cooking is completed with an alkaline solution containing peroxide.

It has been found that the best results are obtained: 4 76129 if the first stage of boiling at elevated temperature is interrupted when the relative amount of residual lignin corresponds to a Kappa number (measured according to AFNOR T 12 018) which is:. Between 45 and 60 for conifers in the case of bleached pulps, between 80 and 100 for conifers in the case of liner pulps,. Between 30 and 40 for deciduous trees (bleached pulps); if the cooking liquor of this first cooking stage contains not more than 0,05% (by weight, calculated on the dry weight of the chips) of hydrogenated or non-hydrogenated anthraquinone or an anthraquinone or anthracene diol, such as those covered by Belgian Patent 877 401 and those derivatives, which are the subject of French Patent Publication No. 2,435,457 to Produits Chimiques Ugine Kuhlmann; if the defibering is performed mechanically (in a disc mill, pulper, etc.); if the liquid of the second cooking stage contains, in addition to the peroxide, 1 to 5%, preferably 2 to 4% of sodium oxide, based on the dry weight of the fibrous material; if the second stage of cooking is operated at a temperature of 20 to 120 WC, preferably between 70 and 100 ° C and the cooking time is 30 to 180 minutes, preferably about 120 minutes; and if the peroxide treatment is carried out in the presence of additives which either have the effect of slowing down the known decomposition reactions of peroxides or of limiting the decomposition of cellulose; these additives may be, for example, magnesium sulphate, sodium carbonate, sodium phosphates or organic compounds.

As the peroxide, known compounds are used, such as those of the general formula R-O-O-X, in which R represents a radical of-76129 and X represents a metal ion or hydrogen. These include hydrogen peroxide, which is economically preferred, and sodium, potassium or calcium peroxide, perborates and persulfates.

The pulp obtained after the second cooking step is washed with water in a known manner. It can then possibly be bleached by conventional bleaching methods.

The manner in which the invention may be practiced and the advantages thereof will become more apparent from the following embodiments, which are given by way of illustration but not limitation. For practical reasons, the starting points of these examples as well as their results are summarized in the tables below.

In the tables: active alkali is expressed as a percentage of the dry weight of the starting wood, sulphidity is expressed as a percentage of the active alkali, sulphite ratio is expressed as a percentage of the dry weight of the starting wood, - durations are expressed in minutes, anthraquinone is expressed as a percentage of dry weight,

The kappa number is measured according to AFNOR T 12 018, the yield of the first cooking stage is expressed as% of the dry weight of the starting wood, the amounts of hydrogen peroxide, sodium oxide and magnesium sulphate in the second cooking stage are expressed as% of the dry weight of the defibering material, the final yield is expressed as% dry weight, the strength properties are measured at 40 ° SR according to the following AFNOR standards:. breaking length in meters according to Q 03 004,. burst factor according to standard Q 03 014,. burst factor according to Q 01 011.

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In the examples in Table 1, wood chips from European pine have been used as starting material.

In Examples II and IV, between the first and second stages, mechanical defibering of the chips obtained from the first cooking stage is performed with a disc mill.

Example I shows a conventional method for producing a pulp.

Example II shows a first embodiment of the invention in which the first cooking step is interrupted after 45 minutes instead of 60 minutes, so that after the second cooking step a Kappa number of the same order of magnitude as in Example I is obtained, i. 36.0 and 37.0, respectively.

Example III shows a prior art embodiment in which 0.05% anthraquinone is added to the cooking liquid; again, for ease of comparison, the same Kappa number as in Example I (37.0) is sought.

Example IV shows an improved embodiment of the invention in which anthraquinone is added to the broth of the first cooking step in a dose of about half the dose of Example III.

Comparing these different examples, the following can be seen: comparing Examples I and II, when the Kappa number is the same, the yield increases from 43.5 to 45.8% (the amount of mass obtained increases by the order of 5%), which when taking into account actual production capacities (for example, the production capacity of an ongoing production unit is in the order of 500 to 600 tonnes per day) represents a significant improvement. In addition, this improvement in yield is associated with the stability of the tear and burst coefficients, but in addition with a considerable improvement in the break length, which was completely unexpected and contrary to conventional teachings; comparing the results of Examples II and IV, the addition of anthraquinone to the first stage broth makes it possible to further increase the final yield (46.5% instead of 45.8%), improving the breaking length, which at the same time increases the value of Example II by 13% IV to 34%. In the prior art (Example III), the addition of anthraquinone alone produced an improvement of only the order of 6%, although the amount of anthraquinone used was almost double that used according to the invention.

Furthermore, in either case, the embodiment according to the invention, (Examples II and IV) does not cause any greater contamination than the solutions according to the prior art (Examples I and III).

The examples in Table II reproduce the conditions in Table I by applying them in a manner known per se to the treatment of hardwood chips (birch chips).

Example V shows the conventional preparation of sulphate pulp.

Example IV shows a first embodiment of the invention in which the first cooking step is performed using a lower amount of active alkali (18% instead of 19.5%), whereby after the second cooking step a Kappa number of the same order of magnitude as in Example V is obtained, namely 20 compared to Kappa to 21.2.

Example VII shows a prior art embodiment in which anthraquinone is added to the cooking liquid in a ratio of 0.05%.

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Example VIII shows an improved embodiment of the invention in which 0.03% of anthraquinone is added to the cooking liquid of the first cooking step.

A comparison of these different examples reveals the same advantages (improved yield and concomitant improvement of some strength properties) as in the examples of conifers in Table I.

In the examples in Table 3, hardwood (beech) chips have been treated with another modification of the chemical pulp production process using an alkaline cooking compound containing a sulfur compound.

Example IX shows a conventional method for preparing a sulphite pulp.

Example X shows an embodiment of the invention in which the first cooking step is stopped after 90 minutes, so that the Kappa number after the second cooking step will be of the same order of magnitude as in Example IX (28 instead of 27).

Comparing these two examples, the same advantages as above can be seen, namely the simultaneous improvement in yield and strength properties.

Applicant's French patent publication 77 24 131, published under number 2,298,839, and applicant's supplementary patent 78 20,715, published under number 2,430,475, describe a process for the preparation of a chemical pulp in which lignocellulosic materials are treated in two cooking steps, first sodium hydroxide solution and then, after intermediate defibering, by a second cooking in an alkaline peroxide solution. In this way, masses were obtained whose properties were close to those of power masses while the pollution was significantly reduced.

9 76129

At the same time, as already mentioned, it was known that all attempts to increase the yield of power pulps caused either a deterioration in the strength properties of these pulps or a significant increase in pollution. Thus, it could not be foreseen that a specific application of the two-stage cooking treatment, where the second cooking is performed in an alkaline dilute peroxide solution and when the first cooking stage is performed under special conditions, could produce such contrary to prior art teachings. In fact, the process according to the invention makes it possible to improve the yield quite considerably, without increasing contamination, and substantially preserving, even improving, some of the strength properties (in particular the breaking length) of the pulp obtained.

The Kappa number of chemical raw pulps prepared in this way is the same as that of conventional chemical raw pulps. They can thus be bleached using conventional bleaching methods.

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HANDLING BEECH CHIPS Eg IX Eg X

First phase

Na 2 SO 3,% 22 22

Na 2 CO 3,% 55

Forced cooking liquid / wood 3 3

Cooking temperature, ° C 175 175

Lifting time to cooking temperature, min 90 90

Time at cooking temperature, min 120 90

Chapter 27 45

Yield,% 51.5 55.5

Second step H 2 O 2,% 0.5

NaOH,% 3.0

MgSO 4, 7 H 2 O,% 0.5

Mass consistency,% 20

Cooking temperature, ° C 90

Duration, min 120

Final Kappa Chapter 27 28

Final yield,% 51.5 53.5

Strength properties (40 ° SR)

Breaking length, m 6340 7200

Outbreak factor 4.4 4.6

Tear coefficient 600 600

Claims (8)

76129 A process for the production of chemical pulps, in which wood chips are boiled at an elevated temperature in an alkaline cooking liquid containing a sulfur-containing compound, characterized in that the boiling at an elevated temperature is stopped when the residual lignin content of the chips corresponds to a Kappa number of. Between 45 and 100 in the case of conifers, and. Between 30 and 50 in the case of hardwood, that the chips thus treated are then mechanically defibered and that the cooking of these defibered chips is finally carried out in an alkaline solution containing 0.1 to 1.5% by weight of peroxide, based on the dry weight of the defibered material. Process according to Claim 1, characterized in that, in the production of softwood pulps to be bleached, the first stage of cooking is interrupted when the residual lignin content corresponds to a Kappa number between 45 and 60. A method according to claim 1, characterized in that in the production of softwood liner pulps, the first stage of cooking is interrupted when the residual lignin content corresponds to a Kappa number between 80 and 100. Process according to Claim 1, characterized in that, in the production of hardwood pulps, the first stage of cooking is interrupted when the residual lignin content corresponds to a Kappa number between 30 and 40. Method according to one of Claims 1 to 4, characterized in that the cooking liquid of the second cooking stage also contains a stabilizing peroxide agent. Process according to one of Claims 1 to 5, characterized in that a maximum of 0.05% (by weight, based on the dry weight of the chips) of a compound selected from the group consisting of anthraquinone is included in the alkaline cooking medium based on sodium oxide and sulfur compound of the first cooking stage. and hydrogenated or non-hydrogenated derivatives of anthraquinone or anthracene diol. Process according to one of Claims 1 to 6, characterized in that the cooking liquid of the second cooking stage contains 1 to 5%, preferably 2 to 4%, by weight of sodium oxide, based on the dry weight of the defibered material. Process according to one of Claims 1 to 7, characterized in that the second cooking step is carried out at a temperature between 20 and 120 ° C, preferably between 70 and 100 ° C, and that the cooking time is 30 to 180 minutes, preferably about 120 minutes. 1. 76129 gft-tgntKr & v