WO2003048450A1

WO2003048450A1 - Recycled fibre bleaching

Info

Publication number: WO2003048450A1
Application number: PCT/GB2002/005273
Authority: WO
Inventors: Barrie Coles; Neil Anthony Turner; Alison Jane Mathews; Christopher Baum; Isabel Elizabeth Chadwick
Original assignee: Warwick International Group Limited
Priority date: 2001-11-28
Filing date: 2002-11-22
Publication date: 2003-06-12
Also published as: GB0128513D0; AU2002343071A1

Abstract

The present invention provides a process for decolourising cellulose pulp containing coloured fibres, comprising the steps of treating the cellulose pulp with a peroxyacid and subsequently treating the pulp with a reducing agent. The pulp may comprise recycled paper furnish. The peroxyacid is generated in situ by reaction of a peroxygen bleaching agent, such as hydrogen peroxide, with a bleach activator, such as an acylated amine, especially tetracetylethylenediamine.

Description

RECYCLED FIBRE BLEACHING Field of the Invention

The present invention relates to the preparation of cellulosic pulp from recycled waste paper. More particularly, the present invention is directed to the decolorisation of contaminating coloured fibres in recycled paper pulp. Background of the Invention

Cellulosic pulp obtained from recycled paper is a valuable raw material in the paper industry. The manufacture of certain grades of paper, such as tissue and toweling grades, is fairly tolerant of contaminants. In contrast, for production of "printing and writing grade" paper from recycled pulp, it is desirable that the pulp have a high degree of brightness, a homogeneous appearance and include minimal contamination by undesirable fibres, inks, adhesives, dirt and other contaminants. Accordingly, it is preferred that the furnish used to prepare printing and writing grade recycled pulp include exclusively clean office waste paper. One of the primary concerns when attempting to recover usable material from recycled paper is the uncontrollable variability of the quality and cleanliness of the furnish (waste paper) supplied. Paper collected by waste collectors and delivered to recycling mills is rarely screened to provide a uniform quality of furnish, and the cost of screening the paper manually or by automated sorter can significantly increase the price of the waste paper. The problem with the variability of the furnish is that it directly affects the quality of the pulp produced, and the cost of chemicals, used in the bleaching processes discussed below, is directly related to the cleanliness of the furnish. Depending upon the quality (typically its brightness or kappa number) of the furnish different bleaching chemical charges are necessary. Existing sorting processes are unable to remove substrates containing coloured fibres, for example, brown lignin-containing fibres present in non-bleached paper and paperboard products, such as unbleached kraft envelopes, unbleached file folders, and corrugated boxes. In the case of office waste paper, for example, it is common for bales of office waste including clean office grade paper, to be intermixed with these unbleached grades of paper. When such unbleached grades of papers are repulped in a recycling plant together with office grade paper, the resultant brown fibre-containing pulp is less valuable than pulp made without such coloured fibres. It would be highly desirable to provide a process for making printing and writing paper grade recycled pulp which does not require a separate sorting step to remove paper sources containing brown fibres.

Rather than removing the coloured fibres from the waste paper, one solution to the problem is to decolourise the coloured fibres so they are not visible in the finished paper. One method of decolourising coloured fibres requires the use of a bleaching sequence similar to that conventionally used to bleach virgin kraft and sulfite pulps. Such methods typically require several bleaching stages to satisfactorily decolourise coloured fibres. Kraft fibres are particularly difficult to bleach because the lignin is modified by the kraft pulping chemicals. Some of the bleaching chemicals commonly used are oxygen, chlorine, sodium hydroxide, chlorine dioxide, sodium hypochlorite, hydrogen peroxide, and ozone. Such conventional bleaching sequences are able to delignify and decolourise coloured fibres. However, such delignification and decolourisation processes involve high capital and operating costs. The other fibres present in the pulp (those which were previously subjected to bleaching before recycling) are also subjected to this harsh bleaching sequence and are also weakened and partially dissolved. The result is a substantial yield loss.

US 5,733,412 discloses a process wherein coloured paper pulp is first treated with a reducing agent or oxidizing agent. A second agent is introduced to said pulp after introduction of the first agent. The second agent is an oxidizing agent when the first agent is a reducing agent, and a reducing agent when the first agent is an oxidizing agent.

It would therefore be desirable to provide a method for decolourising the coloured fibre in office waste (and similar waste paper sources such as "junk mail" and newsprint collected from households) without the disadvantages of the conventional bleaching technology. It would also be desirable to decolourise mixtures of material, for example, where waste paper sources are mixed with pure ground wood, liner- board and the like.

Ideally, such a method should be non-delignifying and result in low yield losses, and use relatively mild non-chlorine bleaching agents to reduce chemical handling hazards and effluent concerns. Summary of the Invention

, The present invention provides a process for decolourising cellulose pulp containing coloured fibres, comprising the steps of treating the cellulose pulp with a peroxyacid and subsequently treating the pulp with a reducing agent. The present invention is particularly suitable for decolourising pulp in which at least a portion of the furnish is recycled paper. Where recycled paper is the substrate, the recycled paper usually undergoes an initial sorting process to remove wire, plastic and other heavy contaminants. The paper is then treated by conventional methods to produce a pulp suitable for undergoing a bleaching process. This generally involves, cleaning, mechanical re-pulping and consistency modification.

It is essential that the bleaching sequence of the present invention is carried out using the peroxyacid treatment first, followed by treatment with the reducing agent. There may, however, be intermediate treatment steps between the peroxyacid step and reductive step. The treatment of the present invention is essentially an initial oxidative treatment followed by a reductive treatment. This has been found to be particularly effective for decolourising coloured fibres, such as paper substrates containing inks, and including difficult to bleach brown kraft fibre. Without being bound by theory, it appears that when the lignin and other coloured bodies found in the pulp are initially reacted with an oxidising agent, they become more susceptible to reaction with a reducing agent, resulting in excellent decolourising results.

More particularly, it is suggested that the use of a peroxyacid provides a more enhanced pre-treatment for subsequent bleaching with a reductive agent than that by peroxides. Without wishing to be bound by theory, it is proposed that peroxyacids exhibit both electrophilic and nucleophilic characteristics, especially when said bleaching is performed around the neutral to alkaline pH range. On the other hand peroxide only exhibits nucleophilic character. This means that the peroxyacid can chemically modify a greater range of entities in the pulp than peroxide (i.e. those which are susceptible to nucleophilic attack, for example, carbonyl groups in the lignin structure, and those which are susceptible to electrophilic attack, for example, hydroxylation of aromatic rings in the lignin structure).

The preferred peroxyacids are selected from organic peroxyacids, forexample, peracetic acid and performic acid, most preferably peracetic acid. Mixtures of peroxyacids may be used, and mixtures of peroxyacids with other oxidising agents may be used.

It is particularly preferred to use peroxyacid precursors, commonly known as bleach activators, with a peroxide compound. Examples of bleach activator compounds falling in this group are acylating agents, such as acetylating agents. The acylating agent may have the general formula RCOL where L is a leaving group joined via a nitrogen or oxygen atom to the co carbon atom, the conjugate acid of which (LH) has a pK_a in the range 4 to 13, preferably 7 to 11 , more preferably 8 to 11 , and R is an alkyl, aryl or aralkyl group having up to 24 carbon atoms and optionally being substituted. Preferably L has at least one and preferably more than one R CO substituents joined to L via an oxygen or nitrogen atom where the or each R¹ is independently selected amongst the same groups as R. Preferably R and any R¹ groups represents C,..^ alkyl, more preferably methyl. Some specific examples of activators are: (a) N,N-diacetylaniline and N-acetylphthalimide;

(b) N-acylhydantoins, such as N,N'-diacetyl-5,5-dimethylhydantoin;

(c) polyacylated alkylene diamines, such as N,N,N',N'-tetraacetyl ethylene diamine (TAED) and N,N,N',N'-tetraacetyl methylene diamine (TAMD) as disclosed in British Patent No. 907,356; (d) acylated glycolurils, such as tetraacetyl glycoluril (TAGU) as disclosed in

British Patent No. 1 ,246,338; and

(e) alpha-Acyloxy-(N , N')-polyacyl malonamides, such as alpha-acetoxy-(N,N')diacetyl malonamide as disclosed in U.S. Patent No. 3,183,266. The activator is preferably an acyl organoamide, which generally has the structural formula

R²CONR³R⁴, in which R² is selected from C_1-12 alkyl, C_2-12 alkenyl and C_2-12 alkynyl groups, R³ is a second acyl radical and R⁴ is selected from C_1-12 alkyl, C_2-12 alkenyl and C_2-12 alkynyl groups, any of which may be substituted with one or more mono-or di- acyl amino groups. Preferably, the activator comprises two amide groups linked through an alkanediyl group.

In a particularly preferred embodiment, the acyl organoamide has the structural formula (II)

wherein R⁵ is a C_1-6 alkanediyl group, preferably an ethanediyl group, and each R⁶ is individually selected from a C_1-6 alkyl group, preferably methyl, ethyl or propyl. Preferably all R⁶ are the same.

Most preferably, the acyl organoamide is N,N,N',N'-tetracetylethylene-1 ,2 diamine (TAED). The above mentioned activator compounds are advantageously incorporated into a composition, hereinafter referred to as a bleaching composition, which, in addition to the bleach activators, contains a bleaching agent, such as hydrogen peroxide. In addition to, or alternatively to hydrogen peroxide, a percompound which, under alkaline conditions, generates hydrogen peroxide may be used. The composition may be formulated as a pre-mixed composition and may be provided as a concentrated or ready-to-use formulation. The composition may be in solid (including powdered or tablet presentations) or liquid form.

Examples of suitable, commonly used percompounds include alkali metal percarbonate, perborate, persilicate, perphosphate, persulphate and perpyrophosphate. A preferred bleaching agent is hydrogen peroxide.

The reaction of the bleach activator and the hydrogen peroxide generates the peroxyacid utilised in the process of the present invention.

By this method, the use of, for example, distilled peroxyacids is avoided. These can be dangerous to handle and transport. The bleaching agent can be present in the composition in amounts of from

0.1-99% by weight of dry pulp.

Generally, the activator is present in an amount of 0.01 to 50% by weight, preferably 0.025 to 10% by weight, more preferably 0.03 to 5% by weights most preferably 0.05 to 0.5% by weight of dry pulp. The bleaching composition may additionally comprise a pH regulator. The pH regulators are preferably selected from the group consisting of sodium hydroxide, sodium silicate of commercial grade No. 3, sodium sulphite and the like. Preferably, the peroxyacid is generated by providing a bleaching composition comprising 1 to 40 g/l TAED, 1 to 80 g/l hydrogen peroxide (50%), 0.5 to 3 g/l chelant, 1 to 6 g/l sodium hydroxide, made up to 1 litre with water.

In a particularly preferred embodiment, the peroxyacid is generated, by providing a bleaching composition containing 15 grams per litre of a granule containing 80% active tetra acetyl ethylene diamine, 21 grams per litre of 50% hydrogen peroxide, 1.5 grams per litre of phosphonate chelant, 5.5 grams per litre of

50% sodium hydroxide, water made up to 1 litre.

The peroxyacid composition is generally added to a pulp furnish in a conventional manner.

Suitable reducing agents are preferably selected from the group consisting of formamidine sulfinic acid (FAS), sodium hydrosulfite (sodium dithionate) and sodium borohydride. Mixtures of reducing agents may be used. Sodium hydrosulfite is a particularly preferred reducing agent. The amount of reducing agent used is preferably in the range of 0.1 to 50% by weight, preferably 0.2 to 5% by weight, most preferably 0.25 to 0.8% by weight of dry pulp.

The pH value of the bleaching composition during the peroxyacid treatment step is preferably be in the range from 7 to 12, the highest bleaching effect being attained, in particular, in the pH range of 8 to 10.5, most preferably at a pH of about 8.

The pH value ofthe bleaching composition during the reducing agent treatment step is preferably be in the range from 6 to 11 , preferably to 7 to10.5, most preferably at a pH of about 7.5 to 8. The temperature of the peroxyacid treatment is preferably in the range 40°C to 200°C, more preferably 50°C to 170°C, most preferably in the range of 60°C to 100°C.

The temperature of the reducing agent treatment step is in the range of 40°C to 200°C, more preferably 120°C to 170°C, most preferably in the range of 60°C to 100°C.

Treatment times for the peroxyacid treatment step are preferably in the range of 10 minutes to 360 minutes, preferably 20 minutes to 240 minutes.

Treatment times for the reducing agent treatment step are preferably in the range of 10 minutes to 360 minutes, preferably 20 minutes to 240 minutes. It is particularly preferred to incorporate a chelating agent into the unbleached paper pulp composition. This is usually introduced prior to treatment with the peroxyacid. The chelating agent prevents or reduces the decomposition of the hydrogen peroxide. The hydrogen peroxide can be decomposed bythe presence of metal ions. The chelating agent chelates such metal ions, thereby reducing decomposition of the hydrogen peroxide. ,

The chelating agent is preferably chosen from among aminocarboxylic acids, hydrσxycarboxylic acids, phosphonic acids and their salts. Preferably, the chelating agent is selected from the group consisting of alkyl amine di(acetic acids) such as ethylenediaminetetra-acetic acid (EDTA) and diethylenetriaminepenta-acetic acid (DTPA), citric acid, lactic acid, tartaric acid, aldonic acids, uronic acids, alkyl amino di)methylene phosphonic acids), such as N,N,',N¹-ethylene-1 ,2-diamine tetra(methylene phosphonicacid) and diethylenetriaminepentamethylene-phosphonic acid (DTMPA), the salts of these acids and/or their mixtures. Phosphonic acids are particularly preferred chelating agents as they function better than the alternative chelants listed above, under the mildly alkaline conditions used in the preferred embodiments of the invention. The amino acetic acids are also of value and are relatively cost-effective.

The amount of chelating agent is preferably at least 0.05% by weight expressed with respect to dry pulp. Generally, it will not exceed 5%. Most preferably, the amount of chelating agent is in the range 0.1-0.4% by weight of dry pulp.

There may be an intermediate treatment between the peroxyacid treatment step and the reducing agent treatment step. This may be a wash step, a neutralising step, a pH modifying step, a second oxidation step and/or treatment with a chelating agent, or mixtures of these steps. Preferably the intermediate treatment step is treatment with a neutralising agent. In this context, neutralising means quenching the oxidising chemicals in the oxidation step. Preferably the neutralising agent is a bisulphite, most preferably sodium bisulphite.

The neutralising agent is employed to reduce the amount of residual peroxide or peroxyacid after the initial treatment with peroxyacid. This is because the peroxyacid, and in particular, hydrogen peroxide, can reduce the effectiveness of the reducing agent by reaction therewith. It is, however, difficult to ensure that no residual peroxyacid or hydrogen peroxide remain in the composition after the peroxyacid treatment step merely by calculating the amount of peroxyacid/hydrogen peroxide that will be required to effectively bleach the pulp, while not using an excess. Thus,^, it is a particularly preferred embodiment of the present invention to utilise an intermediate neutralising step between the peroxyacid step and the reduction step.

In a particularly preferred embodiment, there is a wash treatment step between the peroxyacid treatment step and the reducing agent treatment step.

Washing typically constitutes washing in water, without the addition of further chemicals. Washing can be performed using conventional pulp washing methods including dilution/extraction or displacement washing.

Adjuvants may be employed to enhance the decolourising action or to lower the required dosage of the oxidizing or reducing agents. Such adjuvants include without limitation, stabilising agents, protective agents such as sodium silicate and magnesium salts, catalysts, and other additives. Examples

According to various particularly preferred embodiments of the present invention, there are a number of bleaching sequences including QP_A/B-Y, (QP_A)-Y, (QP_AP)-Y and P_A-Y. This notation is in accordance with that of the technical literature in the field of the bleaching of paper pulps. The symbols employed above and in the following examples and comparative examples have the following meanings: Q: Treatment with a chelating agent, P_A: Stage with peracetic acid,

P: Stage with Hydrogen peroxide, B: Stage with sodium bisulphite, (): Treatment carried out in the same tower, Wash step between two stages, /: No wash step between two stages,

Y: Sodium hydrosulphite.

A composition was made up composition containing 15 grams per litre of a granule containing 80% active tetra acetyl ethylene diamine, 21 grams per litre of 50% hydrogen peroxide, 1.5 grams per litre of phosphonate chelant, 5 grams per litre of 50% sodium hydroxide, water made up to 1 litre. This was used to provide the peroxyacid treatment in the following examples which are tabulated in tables 1, 2, 3, 4, and 5. As the solution is added in an amount to provide a % TAED per weight of dried pulp, an appropriate amount of solution to be added to the pulp is calculated. Thus, for a 0.1% TAED solution, 6.66 mis of the above solution is added to 100 g of dried pulp.

' The dosage levels of peracetic acid are always quoted in terms of percentage granulate per dry weight of pulp. Thus, the phrase "0.1% Pa addition" means that a 0.1% amount equivalent of the granule as described above (containing 80% active ^■- tetra acetyl ethylene diamine) is being added to dry weight of pulp.

The brightness measurements indicated are a measurement of the degree of reflectivity of a sheet of pulp or paper for light measured under specified standard conditions as set out in the International Standards Organization standard of measurement (ISO).

Standard bleaching towers and apparatus are used for all of the examples. Example 1

This example was formulated to compare the treatment sequence QP_A/ B-Y versus QP/B-Ϋ on recycled fibre pulp. The amount of chelant (DTPA) used was 0.1 % by weight of dry pulp. The amount of sodium bisulphite was 0.25% by weight of dry pulp. The results are shown in table 1. This clearly shows that substituting Pa for P in the sequence results in a higher % ISO.

Example 2

This example was formulated to compare the treatment sequence (QP_A)-Y and (QP_AP) versus P-Y on an unbleached typical recycled pulp furnish. The temperature for all tests was 85 °C and the treatment time was 20 minutes. The amount of chelant (DTPA) used was 0.1 % by weight of dry pulp. The results are shown in table 2.

Example 3

This example was formulated to compare the treatment sequence (QP_A)-Y and (QP_AP) versus P-Y on an unbleached typical recycled pulp furnish. The temperature for all tests was 85 °C and the treatment time was 60 minutes. The amount of chelant (DTPA) used was 0.1% by weight of dry pulp. The results are shown in table 3.

Example 4

This example was formulated to compare the treatment sequence QPa-Y and QPa/B-Y versus QP/B-Y on unbleached recycled pulp. The amount of sodium hydrosulphite used is 0.7% by weight of dry pulp. The amount of chelant (DTPA) used is 0.2% and the amount of Pa is 0.75%. The results are shown in table 4. L,a,b values are the tri-stimuli measurements of colour. The 'L' scale is from 1 to 100, whereby a value of 100 is equal to maximum lightness, a value of 1 is maximum darkness. The 'b' value is the blue/yellow hue. A lower 'b' value indicates a shift in hue from yellow to blue. Most applications require pulps with a hue which is shifted towards the blue end of the spectrum. The 'a' value is the red/green hue. Reverted brightness gives an indication of how much brightness a pulp sample loses with ageing. The test that used exposes the pulp to high humidity and temperature for one hour. The method used is SCAN-CM 11 :95.

Example 5

This example was formulated to compare the quantity of residual oxidant in a series of treatment sequences. The results are shown in table 5. The amount of peroxide used was 0.75% by weight of dry pulp. The amount of peracetic acid used was 0.75% by weight of dry pulp and the amount of chelant used was 0.2 % DTPA by weight of dry pulp.

Table 1

Table 3

Table 4

Table 5

Claims

I . A process for decolourising cellulose pulp containing coloured fibres, comprising the steps of treating the cellulose pulp with a bleaching solution comprising a peroxyacid and subsequently treating the pulp with a reducing agent.

2. The process according to claim 1 , wherein the peroxyacids are organic peroxyacids, preferably selected from peracetic acid and performic acid, most preferably peracetic acid.

3. The process according to claim 1 or claim 2, wherein the peroxyacid is generated by the reaction of a bleach activator with a bleaching agent.

4. The process according to claim 3, wherein the bleach activator is a compound of the general formula RCOL where L is a leaving group joined via a nitrogen or oxygen atom to the co carbon atom, the conjugate acid of which (LH) has a pK_a in the range 4 to 13, preferably 7 to 11, more preferably 8 to 11 , and R is an alkyl, aryl or aralkyl group having up to 24 carbon atoms and optionally being substituted.

5. A process according to claim 4 wherein the activator is N,N,N',N'-tetraacetyl ethylene diamine (TAED).

6. The process according to claim 5 wherein the TAED is used in an amount in the range 1 to 40 g per litre of bleaching solution.

7. The process according to any of claims 3 to 6, wherein the bleaching agent is hydrogen peroxide.

8. The process of claim 7 wherein hydrogen peroxide is used in an amount in the range 0.5 to 40 g per litre of bleaching solution.

9. The process according to any preceding claim, wherein the reducing agent is selected from the group consisting of formamidine sulfinic acid, sodium hydrosulfite and sodium borohydride, preferably, sodium hydrosulphite.

10. The process of any preceding claim wherein the reducing agent is used in an amount in the range 0.2 to 5.0% by weight of dry pulp.

I . The process according to any preceding claim, wherein the pH value of the bleaching solution is in the range from 7 to 12, preferably 8 to 10.5, most preferably about 8.

12. The process according to any preceding claim, wherein the treatment with peroxy acid is carried out in the presence of a chelating agent, preferably selected from the group consisting of amino carboxylic acids, hydroxy carboxylic acids and phosphonic acids.

13. The process of claim 12 wherein the chelating agent is selected from ethylenediaminetetra-aceticacid (EDTA), diethylenetriaminepenta-aceticacid (DTPA), citric acid, lactic acid, tartaric acid, aldonic acids, uronic acids, ethylene diamine tetra(methylenephosphonic acid (EDTMPA) , diethylenetriamine pentamethylene-phosphonic acid (DTMPA), the salts thereof and/or their mixtures, preferably EDTA, DTPA or DTMPA.

14. The process according to claim 12 or claim 13 in which the chelating agent is used in an amount in the range 0.05 to 5.0% by weight of dry pulp preferably

0.1 to 0.4%

15. The process according to any preceding claim, wherein there is a neutralisation step between the peroxyacid treatment step and the reducing agent treatment step in which a neutralising agent is added to the pulp.

16. The process according to claim 15, wherein the neutralising agent is sodium bisulphite.

17. The process according to any preceding claim, wherein there is a wash step between the peroxyacid treatment step and the reducing agent treatment step.

18. The process according to any preceding claim in which the pulp comprises fibres from recycled paper.

19. A process for making paper, comprising forming paper from cellulose pulp which has been decolourised in a process as defined in any preceding claim.