EP0874082A1 - Continuous process for delignification and/or bleaching of virgin or recycled chemical pulp - Google Patents

Abstract

Continuous delignification and/or bleaching of pristine or recycled chemical paste paper is claimed. It consists of: (a) preparing a suspension of paste paper (b) pre-treating the suspension with oxygen then with peroxide compound (c) recovering of the suspension of the treated paste papers (d) separating the liquid phase of the recovered suspension (e) recycling part of the separated liquid which is introduced in step (a).

Description

The present invention relates to a continuous delignification and / or for bleaching virgin or recycled chemical pulp.

Traditionally whitening and delignification processes for pasta consists of subjecting a suspension of paper pulp to a sequence of several consecutive treatments. These treatments are often performed using chlorinated reagents. In order to protect the environment, many processes have been developed which no longer use chlorinated reagents. The environment can be further protected by reducing or even eliminating effluent discharges. This is practically achieved, in a continuous process, by withdrawing, after a treatment, part of the aqueous medium in which the paste paper was treated and putting this aqueous medium into operation upstream of this treatment.

In the patent application CA 2,132,057 pulps are bleached using hydrogen peroxide. This treatment is carried out at temperature and elevated pressures and may be preceded by ozone therapy in presence of oxygen. No indication is given regarding the recycling of effluents.

The present invention aims to provide a continuous process for bleaching and / or pulp delignification process in which effluent discharges are reduced even deleted while maintaining good performance in terms of delignification and bleaching. Another object of the present invention is to reduce the accumulation of organic compounds in the pulp suspension paper to be treated with a peroxidized compound, which results in a reduction in the chemical oxygen demand.

(a) on prépare une suspension de pâte à papier,

(b) on soumet la suspension à un prétraitement au moyen d'oxygène suivi d'un traitement au moyen d'un composé peroxydé,

(c) on recueille une suspension de pâte à papier traitée,

(d) on sépare au moins une partie du liquide de la suspension recueillie, et

(e) on recycle au moins une partie du liquide séparé que l'on introduit dans l'étape (a) de préparation d'une suspension de pâte à papier.

Consequently, the invention relates to a continuous process for delignification and / or bleaching of virgin or recycled chemical paper pulp according to which:

(a) a suspension of paper pulp is prepared,

(b) the suspension is subjected to a pretreatment with oxygen followed by a treatment with a peroxidized compound,

(c) a suspension of treated pulp is collected,

(d) at least part of the liquid is separated from the suspension collected, and

(e) at least part of the separated liquid which is introduced in step (a) of preparation of a pulp suspension is recycled.

The method according to the invention comprises steps (a) to (e). he can possibly include other intermediate steps.

The method according to the invention can be an integral part of a manufacturing of virgin or recycled chemical pulp which consists of a sequence of more complex treatments. The process according to the invention can therefore be carried out at start of such manufacture of chemical pulp, i.e. immediately after baking the paper pulp. As a variant, the method according to the invention can be carried out before and after other treatments of the paper pulp. In another variant, the method according to the invention can be carried out at the end of the making paper pulp so that step (e) is the final step in manufacturing.

One of the essential characteristics of the invention resides in step (e) that part of the liquid separated from the treated suspension is recycled in step (a).

The different stages of the process according to the invention are described below.

Step (a) of the process according to the invention consists in preparing a suspension pulp by mixing the pulp with a liquid. In general, it it is an aqueous liquid. The consistency of the suspension obtained is usually greater than or equal to 8%, in particular greater than or equal to 10%. The consistency is generally less than or equal to 35%, in particular less than or equal to 20%. Step (a) can be carried out in any device for washing or mixing of paper pulps known to those skilled in the art. She can in particular be carried out on a rotary filter or in a washing press.

Step (b) of the method according to the invention comprises a first treatment with medium oxygen. This pretreatment with oxygen consists in putting the pulp suspension in contact with gaseous oxygen.

Pretreatment with oxygen can be carried out at a higher pH or equal to 7, preferably greater than or equal to 8. The pH is generally lower or equal to 12. The pH is generally adjusted by adding a source of alkalinity. Sodium hydroxide is preferably used.

Pretreatment with oxygen is usually carried out at a temperature greater than or equal to 80 ° C, more precisely 90 ° C, the values greater than or equal to 100 ° C being preferred. The temperature is generally less than or equal to 140 ° C, more precisely 130 ° C, the values lower or being 120 ° C being the most common. The temperature is usually set by adding an adequate quantity of water vapor to the suspension to be treated.

Pretreatment with oxygen is most often carried out at a oxygen pressure greater than or equal to 1 bar, preferably 2 bars. The oxygen pressure is generally less than or equal to 10 bars, more precisely at 8 bars.

Pretreatment with oxygen is commonly performed at a consistency greater than or equal to 8%, in particular greater than or equal to 10%. The consistency is generally less than or equal to 35%, in particular less than or equal to 20%.

The duration of pretreatment with oxygen is most often longer or equal to 5 min, in particular 10 min. The duration is generally shorter or equal to 120 min, preferably 60 min.

The pretreatment with oxygen is advantageously carried out at a pH from 7 to 12, a temperature from 80 to 140 ° C, an oxygen pressure from 1 to 10 bars, a consistency of 8 to 35% and for a period of 5 to 120 min.

It is optionally possible, during the pretreatment with oxygen, to add stabilizers. These stabilizers can be chosen from magnesium salts, phosphates, phosphonates and carboxylates. Magnesium salts are preferred. Magnesium sulphates are especially preferred, in particular magnesium sulphate of formula MgSO ₄ .7H ₂ O.

In a particular embodiment of the method according to the invention, the pretreatment with oxygen is carried out in the presence of peroxide hydrogen. The amount of hydrogen peroxide added is most often greater than or equal to 0.05% by weight relative to the dry paste. The quantity is usually less than or equal to 2% by weight relative to the dry dough.

Pretreatment with oxygen is followed by treatment with of a peroxidized compound. This peroxide compound can be chosen from hydrogen peroxide, peracids and ozone. Preferably the hydrogen peroxide in an alkaline medium.

The treatment with a peroxide compound is generally carried out at a pH greater than or equal to 7, preferably greater than or equal to 9. The pH is usually less than or equal to 12. The pH is generally adjusted by addition a source of alkalinity in the suspension to be treated. We prefer to use sodium hydroxide.

The treatment with a peroxide compound can be carried out at a temperature greater than or equal to 80 ° C, more precisely 90 ° C, the values greater than or equal to 100 ° C being preferred. The temperature is generally less than or equal to 140 ° C, more precisely 130 ° C, the values lower or being 120 ° C being the most common. The temperature is usually set by adding an adequate quantity of water vapor to the suspension to be treated. he can be interesting to operate the treatment with a peroxide compound at the same temperature as that of the pretreatment with oxygen.

Treatment with a peroxide compound can be carried out under pressure atmospheric. In a preferred variant, it is produced under pressure in adding gaseous oxygen. The pressure is generally greater than or equal to 1 bar, preferably 2 bars. Oxygen pressure is generally lower or equal to 10 bars, more precisely to 8 bars. It can be interesting to operate the treatment using a peroxidized compound at the same pressure as pretreatment with oxygen. In another interesting variant step (b) does not include a decompression step between the pretreatment at oxygen and treatment with the peroxide compound.

Treatment with a peroxide compound is commonly performed at a consistency greater than or equal to 8%, in particular greater than or equal to 10%. The consistency is generally less than or equal to 35%, in particular less than or equal to 20%.

The duration of treatment with a peroxide compound is most often greater than or equal to 30 min, in particular 60 min. The duration is generally less than or equal to 240 min, preferably 120 min.

The amount of peroxide compound used is generally greater or equal to 0.05%, in particular to 1% by weight relative to the dry paste. She is usually less than or equal to 8%, especially 2.5% by weight per compared to dry dough.

The treatment with a peroxide compound is advantageously carried out at a temperature of 80 to 140 ° C, a pressure of 1 to 10 bar, a consistency from 8 to 35%, a pH from 7 to 12, for a period of 30 to 240 min and putting using an amount of peroxide compound less than or equal to 8% by weight compared to dry dough. Treatment with a peroxide compound is preferably carried out at a temperature of 100 to 120 ° C, a pressure of 2 to 8 bars, a consistency of 10 to 20%, a pH of 9 to 12, for a period of 60 to 120 min and using an amount of peroxide compound from 1 to 2.5% by weight compared to the dry dough.

It is optionally possible, during the treatment with a peroxidized compound, to add stabilizers. These stabilizers can be chosen from magnesium salts, phosphates, phosphonates and carboxylates. Magnesium salts are preferred. Magnesium sulphates are especially preferred, in particular magnesium sulphate of formula MgSO ₄ .7H ₂ O.

It turned out to be advantageous, during step (b), not to modify so drastic conditions of temperature, pressure, pH and consistency between oxygen pretreatment and treatment with peroxide compound.

Step (b) of the process according to the invention may also contain other treatments. However, good results are obtained when step (b) does not no intermediate washing between pre-treatment with oxygen and treatment with a peroxide compound. Good results are also obtained when step (b) does not include a decompression step between pre-treatment with oxygen and treatment with a peroxidized compound. Preferably, step (b) does not include washing intermediate or intermediate decompression. The best results are obtained when step (b) does not include any intermediate step between the pretreatment with oxygen and treatment with a compound peroxide.

By washing, we mean a step in which the liquid of the pulp suspension is replaced by another liquid that does not contain bleaching agent.

Step (d) of the process according to the invention consists in separating the suspension from paper pulp which has been treated in step (b) and collected in step (c), two parts, namely the liquid on the one hand and the pulp on the other. This separation can be carried out by filtration so as to modify the consistency of the pulp suspension. It can alternatively be carried out by washing in replacing the suspension liquid with another liquid. In this variant, the consistency of the suspension may vary or may be kept constant. The other liquid is usually fresh water not yet used in a any pulp treatment.

Step (d) can be carried out in any washing device or mixture of paper pulp known to those skilled in the art. It can in particular be carried out on a rotary filter or in a washing press.

The liquid separated in step (d) contains fragments of lignin, hemicellulose and cellulose which are dissolved during the treatments of step (b). These soluble organic compounds cause increased demand liquid oxygen and would consume part of the compound peroxidized when the liquid is used in a subsequent treatment of bleaching and / or delignification using a peroxide compound.

In step (e) of the process according to the invention, the liquid separated from the suspension of the pulp already treated to mix it with a pulp untreated paper and thereby form a suspension of pulp treat. The fact that the recycled liquid is first brought into contact with oxygen in oxygen pretreatment, in which a large part of the compounds soluble organic matter is broken down, before being used in treatment with peroxide compound, increases the efficiency of this peroxide compound and allows maintain a high rate of liquid recycling without reducing performance at the delignification and / or bleaching level.

A particular embodiment of the method according to the invention is shown diagrammatically in FIG. 1, the description of which follows. Step (a) of the process according to the invention is carried out in the tank 1 which is supplied with a suspension of paper via line 2 and liquid via line 3. The suspension prepared in the tank 1 is transferred via the conduit 4 into a first container 5 in which the suspension is subjected to pretreatment with oxygen. The conduit 4 is supplied with oxygen, possibly with other reagents and possibly with water vapor via line 6. The pretreated suspension is withdrawn from reactor 5 via line 7 and transferred to a second container 8. In this second container, the pretreated suspension is subjected to treatment with a peroxidized compound. The peroxidized compound, possibly other reagents and possibly water vapor is introduced into the conduit 7 via the conduit 9. The suspension pretreated with oxygen and treated with the peroxide compound is then withdrawn from the container 8 via the conduit 10 and transferred to the tank 11. In this tool, part of the liquid is separated from the paper pulp. The liquid thus separated is withdrawn from the tank 11 via the conduit 3, part of which is recycled to the tank 1 to prepare another suspension to be treated. Part of the recycled liquid can be purged via line 12. The paper pulp is withdrawn from tank 11 via the conduit 13.

The method according to the invention can be applied to any paper pulp virgin or recycled chemical. By chemical paper pulp is meant pulp having undergone a minor treatment in the presence of chemical reagents. We can cite to title of examples of chemical reagents sodium sulfide in alkaline medium (kraft or sulphate cooking) or magnesium, calcium or sulphite ammonium or any other organic solvent.

Example 1, the description of which follows illustrates the method according to the invention. Example 2 is given for comparison.

The kappa index was measured according to ISO R302 and the whiteness according to the ISO 3688 standard.

Example 1 (according to the invention)

A paper pulp with an initial whiteness of 46.8 ° ISO and a kappa index of 9.9 has been subjected to a whitening and O-PO delignification with recycling of effluent from the PO stage upstream of the stage O.

Stage O was carried out at 10% consistency, 6 bar pressure, a temperature of 100 ° C for a period of 30 minutes and adding 1.5% NaOH compared to dry dough. After 30 minutes, 2% peroxide hydrogen was added to the dough without the dough being depressurized and then the PO stage begins. The duration of the PO stage was set at 90 minutes. After 90 minutes, the dough leaving the PO stage was filtered and the filtrate was returned to a new paste as dilution water in the preparation of the paste for the stage at oxygen.

This cycle has been repeated a sufficient number of times (6) to arrive at a system stationary in which the concentration of impurities in the stage filtrate (PO) is stable. This represents a laboratory simulation by a process batch of a continuous delignification system with recycling of the effluent.

The results are given in Table 1. The values obtained with fresh water are also given in Table 1 for comparison. Whiteness, ° ISO kappa index initial 46.8 9.9 O-PO with fresh water 78.3 2.0 O-PO with recycled water 77.5 2.2

Example 2 (given for comparison)

A paper pulp with an initial whiteness of 44.5 ° ISO and a kappa of 7.8 has been subjected to a bleaching and delignification treatment PO with recycling of effluent from PO stage upstream from PO stage.

The PO stage was carried out at 12% consistency, 6 bar pressure, a temperature of 110 ° C for a period of 60 minutes and adding 1.5% NaOH and 2% hydrogen peroxide compared to the dry pulp. After 60 minutes, the dough leaving the PO stage was filtered and the filtrate was put back on a new paste as dilution water in the preparation of the paste for the PO stage.

This cycle has been repeated a sufficient number of times (6) to arrive at a system stationary in which the concentration of impurities in the stage filtrate (PO) is stable. This represents a laboratory simulation by a process batch of a continuous delignification system with recycling of the effluent.

The results are given in Table 2. The values obtained with fresh water are also given in Table 2 for comparison. Whiteness, ° ISO kappa initial 44.5 7.8 PO with fresh water 75.6 4.4 PO with recycled water 70.8 5.1

Claims (8)

Continuous process of delignification and / or bleaching of virgin or recycled chemical pulp according to which: (a) a suspension of paper pulp is prepared, (b) the suspension is subjected to a pretreatment with oxygen followed by a treatment with a peroxidized compound, (c) a suspension of treated pulp is collected, (d) at least part of the liquid is separated from the suspension collected, and (e) at least part of the separated liquid which is introduced in step (a) of preparation of a pulp suspension is recycled. The method of claim 1, wherein step (b) does not include intermediate wash between the pre-treatment with oxygen and the treatment with a peroxide compound. A method according to claim 1 or 2, wherein step (b) does not does not include an intermediate decompression step between pre-treatment with oxygen and treatment with a peroxidized compound. A method according to any of claims 1 to 3, wherein the pretreatment with oxygen is carried out at a pH of 7 to 12, a temperature from 80 to 140 ° C, an oxygen pressure from 1 to 10 bars, a consistency from 8 to 35% and for a period of 5 to 120 min. A method according to any of claims 1 to 4, wherein the pretreatment with oxygen is carried out in the presence of peroxide of hydrogen in an amount less than or equal to 2% by weight relative to the dry dough. A method according to any of claims 1 to 5, wherein the treatment with a peroxide compound is treatment with hydrogen peroxide in an alkaline medium. A method according to any of claims 1 to 6, wherein the treatment with a peroxide compound is carried out at a temperature of 80 at 140 ° C, a pressure of 1 to 10 bar, a consistency of 8 to 35%, a pH of 7 to 12 and for a period of 30 to 240 min, and the peroxidized compound is put in work in an amount less than or equal to 8% by weight relative to the dough dried. The method of claim 7, wherein the treatment with a peroxidized compound is produced at a temperature of 100 to 120 ° C, a pressure of 2 to 8 bars, a consistency of 10 to 20%, a pH of 9 to 12 and for a period from 60 to 120 min, and the peroxidized compound is used in an amount of 1 to 2.5% by weight relative to the dry paste.

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