CA2576882C

CA2576882C - Composition and process for bleaching mechanical wood pulp

Info

Publication number: CA2576882C
Application number: CA2576882A
Authority: CA
Inventors: Mengjun Li
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-01-25
Filing date: 2007-01-25
Publication date: 2011-05-03
Anticipated expiration: 2027-01-25
Also published as: CA2576882A1

Abstract

A method and compositions using tetra acetyl ethylene diamine (TAED) added to a bleaching solution of hydrogen peroxide to increase peroxide efficiencies and using a fluorescent whitening agent (FWA) added to a bleaching solution of hydrogen peroxide to increase the pulp brightness of a mechanical wood pulp and to reduce brightness reversion of high brightness mechanical pulps. The softwood pulp brightness after the bleaching tower has exceeded 86 ISO% points.

Description

Description Composition and process for bleaching of mechanical wood pulp BACKGROUND OF THE INVENTION

Chemical pulps have good strength properties and a high brightness value.
These attributes, however, are obtained at the cost of low yields and the highly negative effect produced on the environment by the effluent from the bleaching process.

This had led in recent years to intensive development work aimed at producing mechanical pulps in high yields to about 90%, and high brightness values, and with strength properties approaching those of the chemical pulps, while at the same time retaining the opacity and bulk properties unique to the mechanical pulps. The resulting pulps, while quite strong, are highly coloured probably due to the coloured chromophores in lignin.

Bleaching of mechanical wood pulps; such as ground pulp (GP), refiner ground pulp (RGP), thermo-mechanical pulp (TMP), bleached chemical thermo-mechanical pulp (BCTMP), chemical ground pulp (CGP) and deinked recycled newspaper pulp has been in wide use, as have peroxides, such as hydrogen peroxide, sodium percarbonate and so on.

Hydrogen peroxide has environmental benefits over chlorine-based bleaches and it is effective for changing chromophores to non-coloured products in bleaching mechanical pulp. Hydrogen peroxide bleaching liquors conventionally employed have, in general, additives inclusive of sequestering agents, for example, a metal chelating agent, such as sodium diethylenetriaminepentaacetate (DTPA), tetra- and disodium-ethylenediaminetetraacetate (EDTA) and so on, and magnesium sulfate, sodium silicate, for preventing useless decomposition of the peroxide under the bleaching condition due to the presence of contaminant trace metals of natural occurrence or delivered from waste paper.

Caustic soda is also employed, in general, due to its low price as a pH
regulator since peroxides exhibit their bleaching action in an alkaline condition. It has been known that sodium silicate also buffers and stabilizes the pH of a bleaching solution.

In terms of the typical parameters of conventional tower bleaching of pulp with a bleaching solution in which hydrogen peroxide is used as the peroxide, the conditions for bleaching include, in general, a bleaching duration of 1-4 hours at 40-80 degrees C. and a solution pH of 10-11.

Although mechanical pulps have environmental benefits and are high-efficiency pulps, they suffer from some performance disadvantages, which become particularly important when an existing chemical pulp/paper manufacturing plant is being converted to operate with mechanical pulp. One problem is that the mechanical pulp brightness may be lower compared with the chemical pulp brightness and this may significantly reduce the value of the mechanical pulp. A second problem is brightness reversion of mechanical pulps.
Recently alternative processes to improve brightness of mechanical pulp during subsequent tower bleaching, by treating with oxidizing and reducing agents, have been suggested to reduce energy cost and improve optical properties of pulp. For a number of reasons, well known to those in the art, hydrogen peroxide and peracetic acid have proven to be of particular interest, which are intended to brighten pulp efficiently in the presence of an alkali.

JP05186989 proposes an alkaline process using a bleach activator such as TAED
to give a brighter pulp when it is used with oxygen and optionally also with hydrogen peroxide.
Very few details of the process steps are given and hydrogen peroxide is not used in the examples. It appears that the TAED is mixed as a solid with dry pulp at the start of the bleaching step.

In W09521290 there is described a process in which per-acid is produced in situ by reaction of a bleach activator such as tetraacetylethylenediamine and hydrogen peroxide at a pH less than the pKa of peracetic acid formed from the reaction of these chemicals. It is stated that in a preferred process, the TAED is first dissolved in hot water and then added to the hydrogen peroxide before the reacting mixture is dosed to the pulp. A
sequestrant may be added to the pulp before the dosing takes place. It is also stated that conditions must be optimized to ensure that all of the TAED is consumed. The chemistry must be carefully controlled to achieve consistent results when using such powerful bleach as peracetic acid.

In EP456032 there is described a similar pulp bleaching process using alkaline TAED
and hydrogen peroxide. Bleaching of the pulp is done in plastic bags and no detail is given of how a scaled up process should be operated.

CA2041468 proposes use of TAED activated hydrogen peroxide to bleach mechanical wood pulps at lower temperatures within a short period of time.

W09418298 describes a bleaching process where an N-acyl bleach activator is reacted with a source of hydrogen peroxide under acid conditions. The product of this reaction may be used in many bleaching and disinfection applications, including pulp and paper bleaching. The activator and other components may be in the form of particles and these particles may be provided by techniques similar to those used in the laundry detergent industry. For instance by spray drying liquid slurries; by granulation techniques using binders, for instance synthetic or natural polymers (or derivatives); or by melt blending followed by extrusion or other techniques. A composite product including a bleach activator may also include other additives, especially heavy metal sequestrants, and it may include surfactants to act as wetting agents and inorganic salts to act as a diluent or to increase the rate of disintegration or dissolution of the product. The composite product should also include the source of the hydrogen peroxide as well as the bleach activator when it includes the wetting agent. Only two granulated activator particles are exemplified in this document; both contain carboxymethyl cellulose as a binder and neither is used for pulp bleaching.

W09725402 proposes the use of bleach activators such as TAED for various applications including pulp bleaching. The preferred form of the TAED is a granule, but no details are given of the composition of the granule.

CA2230315 describes a refiner bleaching process where the TAED as a bleach activator is reacted with sodium perborate or hydrogen peroxide at elevated pressure.
The pulp after refining has improved brightness and pulp brightness after optional tower bleaching has exceeded 75 ISO% points.

Fluorescence whitening agents (FWA's) have been used to reduce the brightness reversion of high brightness mechanical pulps, as described in Brightness Reversion of Mechanical Pulps XIV: Application of FWA's for High Brightness, High Yield Pulps, IPST Technical Paper Series number 747, A.J. Ragauskas, L. Allison, and C. Li, Tappi Journal (1998). However, this reference describes using an FWA with a BCTMP or BCTMP-kraft furnish to reduce the photoyellowing properties of paper containing high yield pulp, and does not suggest that FWA's can be used in a BCTMP bleaching process.
There is an ongoing need for improved but inexpensive mechanical pulps having improved brightness; and with greater stability of the optical properties, i.e. decreased reversion.

SUMMARY OF THE INVENTION

The present invention relates to a novel, low-energy method of producing high yield mechanical pulp having a final brightness value not previously achieved, and reducing brightness reversion of bleached mechanical pulps containing lignin.

Theoretically calculated, one mole of TAED will be required to react with two moles of hydrogen peroxide. Since the molecular weight of hydrogen peroxide is 34 and that of TAED is 228; 3.35 grams of TAED will react with 1 gram, calculated as pure substance, of hydrogen peroxide employed for bleaching the pulp. However, it may be reasonable to employ hydrogen peroxide, which has a lower price than TAED, in an excess amount, in order to further increase the rate of reaction of hydrogen peroxide with TAED
and to improve the bleaching effect. In addition, more hydrogen peroxide becomes effectively active in the bleaching of pulps by the addition of TAED, so that the amount of hydrogen peroxide actually employed can be reduced.

The major obstacle to the use of mechanical pulps in high-grade paper products is their tendency to photoyellow. This yellowing phenomenon, also known as brightness reversion, occurs primarily as a result of exposure to light and is attributable to the photo-oxidation of lignin. It is generally accepted that the basic mechanism of photoyellowing involves a variety of pathways including: direct absorption of near-UV light by conjugated phenolic groups to form Phenoxy-radicals; abstraction of phenolic hydroxyl hydrogen by aromatic carbonyl triplet excited state. One of the simplest and most effective means of photo-stabilizing mechanical pulps is by the addition of a fluorescent whitening agent (FWA) that prevents light in the 300-400nm ranges from interacting with lignin. FWA's can absorb ultraviolet and invisible radiation at 300-400 nm and transform that radiation into visible blue light at 400-500 nm. The use of FWA's provides a two-fold benefit for mechanical pulps, as these additives enhance the initial brightness of pulp and act as a UV-screen. In this process, FWA's absorb harmful near-UV light and remit light in the visible range giving the mechanical pulp the appearance of higher brightness.
According to the present invention there is provided a method for bleaching mechanical pulp. The method comprises treating (digesting) said pulp in an aqueous hydrogen peroxide solution containing about 1%-6% hydrogen peroxide by weight based on the pulp, 1%-5% caustic soda by weight based on the pulp, 1%-5% sodium silicate by weight based on the pulp, 0.01 %-1 % TAED by weight based on the pulp, which liberates nascent oxygen upon reaction with hydrogen peroxide to increase the peroxide's efficiency, and 0.01 %-1% FWA by weight based on the pulp wherein the fluorescent whitening agent is selected from the group consisting of; A) 4,4'-Bis(6-anilino-l,4-bis(2-hydroxyethyl)amino)-1,3,5-triazin-2-yl)amino)stilbene-2,2-disulfonic acid(CAS
No.
4404-43-7), B) disodium 4,4'- bis(6-anilino-1,4-bis(2-hydroxyethyl)amino)-1,3,5-triazin-2-yl)amino)stilbene-2,2-disulphonate (CAS No. 4193-55-9), C) potassium sodium 4,4'-bis[6-anilino-4-[bis(2-hydroxyethyl)amino]-1,3,5-triazin-2-yl]amino]stilbene-2,2'-disulphonate (CAS No. 70942-01-7), D) 2,2'-Stilbenedisulfonic acid, 4,4'-bis((4-anilino-6-((2- hydroxyethyl)methyl amino)-s-triazin-2-yl)amino)-,disodium salt (CAS No. 13863-31-5), E) disodium 4,4'-bis[(4-anilino-6-morpholino-1,3,5-triazin-2-yl)amino]stilbene-2,2'-disulphonate (CAS No. 16090-02-1), F) tetrasodium 4,4'-bis[[4-[bis(2-hydroxyethyl)amino]-6-(4-sulphonatoanilino)-1,3,5-triazin-2-yl)amino]stilbene-2,2'-disulphonate] (CAS No. 16470-24-9), G) tetrasodium 4,4'-bis[[4-[bis(2-hydroxypropyl)amino]-6-(4-sulphonatophenyl)amino]-1,3,5-triazin-2-yl]amino]-stilbene-2,2'-disulphonate] (CAS No. 67786-25-8), and H) 2,2'-stilbenedisulfonic acid, 4,4'-bis[[4-[(2-carbamoylethyl)(2-hydroxylethyl)amino]-6-(p-sulfoanilino)-s-triazin-2-yl]amino]-tetrasodium salt (CAS No.29637-52-3) at a temperature in the range of 65 to 80 degrees C., and preferably 70-75 degrees C. At such temperatures, the reaction may proceed for about one to three hours. The pH
range of the method is preferably about 10 to about 12.

We have found that residual hydrogen peroxide, after the bleaching tower, could be decreased by 2.3 kg (kilogram) to 9.9 kg using 0.1 kg TAED per tonne of dry pulp in a pre-reaction with 50 kg hydrogen peroxide (calculated at 100% hydrogen peroxide) per tonne of dry pulp and 37.5 kg caustic soda per tonne of dry pulp, 30 kg sodium silicate per tonne of dry pulp and 0.1 kg FWA per tonne of dry pulp to form a bleaching solution for use in a mechanical pulp bleaching process.

By using the inventive process, an ISO brightness value of at least 86 is preferably obtained, still more preferably an ISO brightness value of at least 92 is obtained.
The invention provides several advantages over conventional hydrogen peroxide bleaching: 1. Lower cost benefits due to the amount of hydrogen peroxide, alkali and silicate actually employed that can be reduced while still achieving the same, or greater, ISO brightness values. 2. Increased brightness values for mechanical pulp over conventional bleaching. This increases the number and variety of applications for the product, significantly expanding its marketability. 3. Environmental benefits due to the amount of hydrogen peroxide, alkali and silicate actually employed that can be reduced.
The invention and its advantages will be illustrated in more detail by the examples below which however, are only intended to illustrate the invention without limiting the same.
The percentages and parts stated in the description, claims and examples, refer to percent by weight and parts by weight, respectively, unless otherwise stated.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be further described with reference to the following non-limiting examples:

Pulp brightness (ISO brightness) is measured with a brightness meter, which determines the brightness of a split sheet at a wavelength of 457 nm (ISO D65 Standard Method) Using a bleaching solution of hydrogen peroxide, TAED is made to a 0.5%
solution and FWA is made to a 5% solution.

Laboratory Studies A mechanical softwood pulp sample (from a pulp mill in British Columbia, Canada) has a brightness of 58.6% ISO, and a concentration of 24.51%, treated with DTPA.
Laboratory bleached at 20% concentration pulp, 70 degrees C and 150 minutes of retention time.

From the result shown in TABLE 1, it is evident that TAED and FWA bleaching with hydrogen peroxide provides maximum brightness gain and minimum residual hydrogen peroxide, and is superior to conventional hydrogen peroxide bleaching.

Residual Unbleached Hydrogen ISO Peroxide Caustic Silicate TAED FWA Peroxide Brightness (%) (%) (%) (%) (%) (%) ISO Brightness 58.6 5.0 3.75 3.0 0.00 0.00 1.22 77.5 58.6 5.0 3.75 3.0 0.01 0.10 1.15 83 58.6 5.0 3.75 3.0 0.03 0.20 0.99 86 58.6 5.0 3.75 3.0 1.00 0.01 0.51 84 58.6 5.0 3.75 3.0 0.10 0.30 0.78 89 58.6 5.0 3.75 3.0 0.03 0.50 0.99 90 58.6 5.0 3.75 3.0 0.01 1.00 1.15 92 Laboratory Studies A mechanical softwood pulp sample (from a pulp mill in British Columbia, Canada) has a brightness of 52% ISO, and a concentration of 31 %, treated with DTPA.
Laboratory bleached at 24% concentration pulp, 70 degrees C and 150 minutes of retention time using a single-stage process.

It is evident from the TABLE 2 result that increasing the amount of FWA does have a positive effect on pulp brightness. Bleached pulp with ISO brightness of more than 84 points has been obtained.

Incoming ISO Peroxide Caustic Silicate TAED FWA
Brightness (%) (%) (%) (%) (%) ISO Brightness 52 4.0 3.6 3.0 0.00 0.00 71 52 4.0 3.6 3.0 0.01 0.09 76 52 4.0 3.6 3.0 0.02 0.18 80 52 5.8 4.2 3.2 0.00 0.00 76 52 5.8 4.2 3.2 0.01 0.09 81 52 5.8 4.2 3.2 0.02 0.18 84 Laboratory Studies Bleaching studies were performed on partial two-stage process bleached mechanical softwood pulp samples, which had been bleached in the first stage to have a brightness of 66% ISO, and a concentration of 39.5%, treated with conventional hydrogen peroxide bleaching. Laboratory bleached at 24% concentration pulp, 70 degrees C and 150 minutes of retention time.

About 26 points of brightness were gained with TAED, FWA and hydrogen peroxide, compared to about 14 points with hydrogen peroxide. It is further evident that TAED and FWA with hydrogen peroxide bleaching is more effective in pulp brightening during the second-stage bleaching system compared to conventional hydrogen peroxide bleaching during the second-stage bleaching system.

Incoming ISO Peroxide Caustic Silicate TAED FWA
Brightness (%) (%) (%) (%) (%) ISO Brightness 66 2.0 1.0 2.0 0.00 0.00 74 66 2.0 1.0 2.0 0.01 0.09 78 66 2.0 1.0 2.0 0.02 0.18 81 66 3.5 2.0 2.5 0.00 0.00 80 66 3.5 2.0 2.5 0.02 0.18 89 66 3.5 2.0 2.5 0.03 0.27 92 Mill-Trial A plant test was performed at a pulp and paper mill in British Columbia, Canada.
Mechanical softwood pulp was bleached using a single-stage bleaching process.
The pulp, after the bleaching tower, was diluted and neutralized at pH 5.6 with a sulphur dioxide solution in preparation for papermaking stock, was then run through a pulp refiner and finally made into paper at the paper machine.

Bleaching Unbleached Tower ISO Peroxide Caustic Silicate TAED FWA ISO Paper Machine Brightness (%) (%) (%) (%) (%) Brightness ISO Brightness 50 6.0 4.5 3.5 0.000 0.000 76.4 76.2 50 6.0 4.5 3.5 0.010 0.090 81.9 81.8 50 6.0 4.5 3.5 0.013 0.117 82.7 82.5 50 6.0 4.5 3.5 0.020 0.18 86.0 85.2 Evidently, TAED, FWA and hydrogen peroxide bleaching gives the best optical properties with about a 9-point increase in the ISO brightness points after the paper machine. The brightness does not change between the bleaching tower and paper machine, and even after pulp refining, in preparation for papermaking stock.
Therefore, it is clear that FWA's are effective in stabilizing pulp or paper and have a high affinity for pulp or paper essential.

Claims

1. A bleaching solution of hydrogen peroxide for the bleaching of mechanical wood pulp composed of, as additional components;

a) A first chemical component, which liberates nascent oxygen upon reaction with hydrogen peroxide to increase the peroxide's efficiency, wherein the first chemical component is 0.01 %-1 % tetra acetyl ethylene diamine (TAED) by weight based on the pulp;

b) And a second chemical component for increasing the brightness and reducing the brightness reversion of said pulp, wherein the second chemical component is 0.01 %-1% fluorescent whitening agent (FWA) by weight based on the pulp.

2. A solution as claimed in claim 1 wherein the fluorescent whitening agent is selected from the group consisting of; A) 4,4'-Bis(6-anilino-1,4-bis(2-hydroxyethyl)amino)-1,3,5-triazin-2-yl)amino)stilbene-2,2-disulfonic acid(CAS No. 4404-43-7), B) disodium 4,4'- bis(6-anilino-1,4-bis(2-hydroxyethyl)amino)-1,3,5-triazin-2-yl)amino)stilbene-2,2-disulphonate (CAS No. 4193-55-9), C) potassium sodium 4,4'-bis[6-anilino-4-[bis(2-hydroxyethyl)amino]-1,3,5-triazin-2-yl]amino] stilbene-2,2' -disulphonate (CAS
No.
70942-01-7), D) 2,2'-Stilbenedisulfonic acid, 4,4'-bis((4-anilino-6-((2-hydroxyethyl)methyl amino)-s-triazin-2-yl)amino)-,disodium salt (CAS No. 13863-31-5), E) disodium 4,4'-bis[(4-anilino-6-morpholino-1,3,5-triazin-2-yl)amino]stilbene-2,2'-disulphonate (CAS No. 16090-02-1), F) tetrasodium 4,4'-bis[[4-[bis(2-hydroxyethyl)amino]-6-(4-sulphonatoanilino)- 1,3,5 -triazin-2-yl] amino]
stilbene-2,2' -disulphonate] (CAS No. 16470-24-9), G) tetrasodium 4,4'-bis[[4-[bis(2-hydroxypropyl)amino] -6-(4-sulphonatophenyl)amino] - 1,3,5 -triazin-2-yl]
amino] -stilbene-2,2'-disulphonate] (CAS No. 67786-25-8), and H) 2,2'-stilbenedisulfonic acid, 4,4'-bis[[4-[(2-carbamoylethyl)(2-hydroxylethyl)amino]-6-(p-sulfoanilino)-s-triazin-yl]amino]-tetrasodium salt (CAS No.29637-52-3).