EP1062387A1

EP1062387A1 - Method for bleaching and/or delignification of paper pulp

Info

Publication number: EP1062387A1
Application number: EP99914514A
Authority: EP
Inventors: Eric Chauveheid; Johan Devenyns; Thierry Van Beckhoven
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 1998-03-10
Filing date: 1999-03-06
Publication date: 2000-12-27
Also published as: ZA991903B; AU3330599A; BE1011785A3; WO1999046441A1

Abstract

The invention concerns a method for bleaching and/or delignification of paper pulp using one or several peroxidized oxidants, which consists in pre-treating the paper pulp to a pH higher than 8 by means of one or several chelating agents selected among aspartic acid and its derivatives substituted on the nitrogen atom.

Description

- 1 -

Process for bleaching and / or delignifying paper pulp

The invention relates to a process for bleaching and / or delignifying pulp using one or more peroxide oxidizing agents, in which the pulp is pretreated using a chelating agent. Pulp belongs to different categories. By chemical paper pulp is intended to denote the pulps which have undergone a delignifying treatment in the presence of chemical reactants such as sodium sulphide in alkaline medium (kraft or sulphate cooking), or by ^other alkaline processes.

By mechanical pulp is meant paper pulp which has undergone a mechanical pulping treatment.

By recycled pulp is meant all the pulp whose raw material consists of recovered paper. These recovered papers can include papers made from chemical pulp, mechanical pulp, or recycled pulp. It is generally known to bleach paper pulps with peroxide oxidants such as ozone, peracids or hydrogen peroxide. In these bleaching treatments, it is useful to remove from the paste certain harmful metal ions. These are transition metal ions including, among others, manganese, copper and iron which catalyze decomposition reactions of peroxide oxidants. They degrade the peroxide oxidants used for delignification and bleaching via radical mechanisms and thus increase the consumption of these products while reducing the mechanical properties of the paper pulp.

Removal of metal ions can be achieved by acid treatment. However, these treatments in an acid medium remove not only the harmful metal ions but also the ions of alkaline earth metals such as magnesium and calcium which have a stabilizing effect on the peroxide reagents used and a beneficial effect on the optical qualities. and mechanical pulp. One known way to solve this problem is to selectively remove harmful metal ions by chelation of these ions by means ^of chelating agents. Since the pulp itself has - 2 -

sequestering properties for transition metal ions, powerful aminocarboxylic type chelating agents are used such as ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTP A) in an appreciable amount. The use of these chelating agents poses problems in terms of environmental protection. Since they are only slightly biodegradable, they prove difficult to destroy in conventional water treatment plants, and some of the chelating agents end up in rivers where they can then dissolve heavy metals such as mercury and the cadmium contained in the sediments of these rivers and introduce them into the food chain.

One solution to this problem is to use biodegradable chelating agents as in patent applications WO 97/30208 and WO 97/30210 which respectively describe the treatment of chemical pulp and mechanical and recycled pulp using chelating agents such as for example ethylenediamine-N, N'-disuccinic acid (called EDDS). This known treatment is carried out at a pH of 4 to 8. This treatment has the disadvantage of leading to a level of whiteness substantially lower than that obtained with conventional chelating agents such as for example EDTA. The present invention aims to remedy the aforementioned drawbacks by providing a process for bleaching and / or delignification of paper pulps by means of peroxide oxidizing agents which makes it possible to selectively remove the transition metal ions without leading to problems at the level of the environment and which makes it possible to obtain a high level of whiteness.

To this end, the invention relates to a process for bleaching and / or delignification of paper pulps by means of one or more peroxide oxidizing agents, in which the paper pulp is pretreated to a pH greater than 8 by means of one or more chelating agents chosen from aspartic acid and its derivatives substituted on the nitrogen atom.

One of the essential characteristics of the invention resides in the combination of a particular biodegradable chelating agent with a pH greater than 8. In fact, it has been observed that when the biodegradable chelating agent according to the invention is used at a pH greater than 8, not only the advantages linked to the biodegradability of the chelating agent and the alkaline conditions of the pretreatment are obtained, but also high whiteness which can reach the same level as that obtained with conventional non-biodegradable chelating agents. In addition, the method according to the invention has the advantage of avoiding pH jumps since the bleaching and / or delignification treatments using peroxide oxidizing agents are often carried out under alkaline conditions such as the treatment using the chelating agent according to the invention. The possibility of performing chelation in an alkaline medium facilitates the recycling of effluents.

In the process according to the invention the chelating agent can in particular be chosen from N-carboxymethylaspartic acid, N- (1,2-dicarboxyethyl) - aspartic acid, N- (1,2-dicarboxy acid -2-hydroxyethyl) -aspartic and the compounds of formula

COORi COOR3

(CH ₂ ) _n R5 Rό (CH ₂ ) _p

HC - N - (CH ₂ CH ₂ N) _m - CH

COOR4

in which n is from 1 to 3, m is from 0 to 3, p is from 1 to 3, Ri, R ₂ , R3 and R4 are H, Na, K, Ca or Mg, R5 and R are H, CH ₂ OH, CH ₂ CH ₂ OH or CH ₂ O (CH ₂ CH ₂ O) ι _ιoCH ₂ CH ₂ OH, and mixtures thereof. The chelating agent can for example be ethylenediamine-N, N'-disuccinic acid (EDDS), one or more of its isomers and one or more of its alkali or alkaline-earth metal salts. It can also be 2,2'-iminodisuccinic acid, one or more of its isomers and one or more of its alkali or alkaline earth metal salts. Preferably, the chelating agent is chosen from ethylenediamine-N, N'-disuccinic acid (EDDS), its isomers and its salts. By using a single chelating agent one can already obtain good results. In particular, the chelating agent can be used in the absence of a chinating agent of aminocarboxylic or hydroxycarboxylic type. It is of course possible to use several chelating agents. For example, several chelating agents according to the invention can be combined. As a variant, it is possible to combine a chelating agent according to the invention with a conventional chelating agent such as the aminocarboxylated (eg EDTA) or hydroxycarboxylated (eg glucaric acid) chelating agents. In the process according to the invention, the pretreatment by means of the chelating agent is generally carried out at a pH of at least 8.05, in particular at least - 4 -

minus 8, 1, values of at least 8.2 being usual. The pH is most often less than or equal to 12, in particular less than or equal to 10, values less than or equal to 9 being common. Good results have been obtained at a pH of 8, 1 to 12, in particular from 8.2 to 10. In the process according to the invention, the pretreatment by means of the chelating agent is generally carried out at a temperature of at least 10 ° C, in particular at least 30 ° C, temperatures of at least 40 ° C being recommended. The temperature is most often less than or equal to 90 ° C, in particular less than or equal to 75 ° C, temperatures less than or equal to 60 ° C being recommended for chemical pulps. Good results are obtained by carrying out the pretreatment at a temperature of 10 to 90 ° C, preferably from 30 to 75 ° C.

The pretreatment of the process according to the invention is generally carried out in the presence of water at a consistency of at least 0.1% by weight of dry paste, in particular at least 1% by weight of dry paste, the consistencies of '' at least 2% by weight of dry pulp being common. The consistency is usually less than or equal to 20% by weight of dry pulp, in particular less than or equal to 15% by weight of dry pulp, consistencies less than or equal to 10% by weight of dry pulp being common. Good results are obtained by carrying out the pretreatment at a consistency of 0.1 to 20% by weight of dry paste, preferably from 1 to 10% by weight of dry paste.

In the process according to the invention, the chelating agent is generally used in an amount of at least 0.02% by weight, in particular of at least 0.05% by weight, the amounts of at least 0.1% by weight being recommended. The amount of chelating agent is most often less than or equal to 2% by weight, in particular less than or equal to 1.5% by weight, the amounts less than or equal to 1% by weight being common. Good results are obtained by using an amount of chelating agent from 0.02 to 2% by weight, preferably from 0.1 to 1% by weight. The duration of the pretreatment is not critical.

The peroxide oxidizing agent which is used in the process according to the invention is generally chosen from hydrogen peroxide, organic peracids such as peracetic acid, inorganic peracids such as Caro acid, ozone and oxygen. The peroxide oxidizing agent is preferably hydrogen peroxide. This is advantageously used in an alkaline medium and at temperatures greater than or equal to 70 ° C. Generally, a water washing step is carried out between the pretreatment and the bleaching and / or delignification step using a peroxide oxidizing agent.

Examples Example 1

A chemical paper pulp initially having a whiteness of ISO 48.9 ° was subjected to a delignification and bleaching treatment QW P. Q represents a pretreatment using a chelating agent, W represents a washing with water and P represents a treatment with hydrogen peroxide in an alkaline medium. In Examples 1, 3 and 5 a conventional chelating agent (EDTA) was used and in Examples 2, 4 and 6 a chelating agent according to the invention (EDDS) was used. In examples 1 and 2 step Q is carried out at a pH of approximately 8.5 and in examples 3 and 4 step Q is carried out at a pH of approximately 10, as in the invention. In examples 5 and 6, step Q is carried out at a pH of around 6.5 as in international patent application WO 97/30208.

The results are shown in Table 1 below. The chelation Q was carried out at 50 ° C, for 30 minutes, at a consistency of 4% paste and with an amount of 0.4% by weight of chelating agent. The treatment with hydrogen peroxide P was carried out at 90 ° C for 120 minutes and at a consistency of 12% by weight of paste. 2 g of hydrogen peroxide per 100 g of dry paste and 1.3 g of NaOH per 100 g of dry paste were used in step P.

Table 1

Example Chelating agent used in Q pH in Q Final whiteness (° ISO)

1 EDTA 8.3 67.6

2 EDDS 8.5 67.2

3 EDTA 10.1 66.1

4 EDDS 9.8 65.0

5 EDTA 6.4 68.5 6 EDDS 6.6 61.8

EDTA = ethylenediaminetetraacetic acid EDDS = ethylenediamine-N, N'-disuccinic acid Example 2

A mechanical paper pulp initially having a whiteness of 62.0 ° ISO was subjected to a bleaching treatment Q P. Q represents a pretreatment using a chelating agent and P represents a treatment using hydrogen peroxide in the medium alkaline. In Examples 7, 9 and 11 a conventional chelating agent (DTP A) was used and in Examples 8, 10 and 12 a chelating agent according to the invention (EDDS) was used. In examples 7 and 8 step Q is carried out at a pH of approximately 8.5 and in examples 9 and 10 step Q is carried out at a pH of approximately 10, as in the invention. In examples 1 1 and 12, step Q is carried out at a pH of around 7, as in international patent application WO 97/30210.

The results are shown in Table 2 below. The chelation Q was carried out at 75 ° C, for 10 minutes, at a consistency of 3% paste and with an amount of 0.8% by weight of chelating agent. The treatment with hydrogen peroxide P was carried out at 70 ° C for 120 minutes and at a consistency of 15% by weight of paste. 2 g of hydrogen peroxide per 100 g of dry paste, 1.5 g of NaOH per 100 g of dry paste, 0.05 g of MgSθ4.7H ₂ O per 100 g of dry paste and 3 g of sodium silicate per 100 g of dry paste was used in step P. Table 2

Example Chelating agent used in Q pH in Q Final whiteness (° ISO)

7 DTPA 8.4 72, 1

8 EDDS 8.3 72.7

9 DTPA 9.9 71.2

10 EDDS 9.8 71.1

1 1 DTPA 7.0 71.9 12 EDDS 7.0 71.8

DTPA = diethylenetriaminepentaacetic acid

Claims

RE VE N D I C A T I O N S

1 - Process for bleaching and / or delignification of paper pulps by means of one or more peroxide oxidizing agents, in which the paper pulp is pretreated to a pH greater than 8 by means of one or more chelating agents chosen from aspartic acid and its substituted derivatives on the nitrogen atom.

2 - Process according to claim 1, in which the chelating agent is chosen from N-carboxymethylaspartic acid, N- (1,2-dicarboxyethyl) - aspartic acid, N- (1,2-dicarboxy acid -2-hydroxyethyl) -aspartic and the compounds of formula

COOR] COOR3

(ÇH ₂ ) _n R ₅ R _Ô (ÇH ₂ ) _p

HC - N - (CH ₂ CH ₂ N) _m - CH

COOR ₂ COOR4

in which n is from 1 to 3, m is from 0 to 3, p is from 1 to 3, R \, R ₂ , R3 and R4 are H, Na, K, Ca or Mg, R ₅ and Rg are H, CH ₂ OH, CH ₂ CH ₂ OH or CH ₂ O (CH CH ₂ O) ₁ . ₁₀ CH ₂ CH ₂ OH, and mixtures thereof.

3 - Process according to claim 2, wherein the chelating agent is chosen from ethylenediamine-N, N'-disuccinic acid, its isomers and its salts.

4 - Process according to any one of the preceding claims, in which the pretreatment by means of the chelating agent is carried out at a pH of 8.1 to 12.

5 - Process according to claim 4, in which the pretreatment by means of the chelating agent is carried out at a pH of 8.2 to 10.

6 - Process according to any one of the preceding claims, in which the chelating agent is used in the absence of a chelating agent of aminocarboxylic or hydroxycarboxylic type. -8-

7 - Process according to any one of the preceding claims, in which the pretreatment by means of the chelating agent is carried out at a temperature of 10 to 90 ° C, preferably from 30 to 75 ° C.

8 - Process according to any one of the preceding claims, in which the pretreatment by means of a chelating agent is carried out in the presence of water at a consistency of 0J to 20% by weight of dry paste, preferably from 1 to 10 % by weight of dry dough.

9 - Process according to any one of the preceding claims, in which the chelating agent is used in an amount of 0.02 to 2% by weight, preferably from 0.1 to 1% by weight.

10 - Process according to any one of the preceding claims, in which the peroxide oxidizing agent is chosen from hydrogen peroxide, organic peracids such as peracetic acid, inorganic peracids such as Caro acid, l 'ozone and oxygen.