CA2743838A1 - Composition and process for bleaching of mechanical wood pulp - Google Patents

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 chelating agent added to a bleaching solution of hydrogen peroxide to increase the pulp brightness of a mechanical wood pulp and to reduce peroxide decomposition. The process is useful in both silicate and silicate-free bleach solutions.

Description

Description Composition and process for bleaching of mechanical wood pulp CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of Canadian Patent No. 2576882.
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 has 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. But the effectiveness of the bleaching liquor is reduced in the absence of stabilisers against decomposition of the peroxide by transition metal ions, e.g. iron and manganese in the cellulosic material or water. In order to avoid the detrimental effect of these ions, a chelating agent is often introduced into the bleaching process or the pulp is pretreated with a chelating agent. The most common chelating agents are EDTA
and DTPA. Sodium silicate solutions normally called water glass have been used in stabilizing hydrogen peroxide solutions, which are used in alkaline peroxide bleaching of mechanical pulps, but this gives rise to other problems e.g. corrosion and scaling.
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. Also there is a need to partly or totally replace sodium silicate in alkaline peroxide bleaching processes 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.

It is known that hydrogen peroxide will decompose very rapidly in an alkaline milieu in the presence of transition metal ions. The most abundant of these ions in pulps are iron and manganese. The copper ion is also very detrimental for alkaline hydrogen peroxide, but normally it can enter the process only via used process waters.

The theory of the function of sodium silicate varies, but one theory is that sodium silicate will deactivate the catalytic surface of iron and other heavy metal ion "precipitates".

If the silicates, e.g. in a white-water closed loop system, enter the paper making process, they will disturb the papermaking process, e.g. by precipitating on hot surfaces, causing holes in the paper reel, etc. Another disadvantage with sodium silicate is that when the bleaching liquors are recycled and ultimately fed into the recovery boiler, where the so-called black liquor from the cooking process after concentration is burned, the silicate will cause severe scaling and thus decrease the heat transfer in the recovery boiler, which in the worst case scenario can cause an explosion of the recovery boiler.

One solution to stabilize alkaline hydrogen peroxide solutions or to avoid sodium silicate is based on the use of antiscalants. U.S. Pat. No. 4614646 discloses the employing of peroxide in a silicate-free system in the presence of alkyleneaminephosphonic acids and polyalkylenepolycarboxylic acids.

Another patent, U.S. Pat. No. 5145558, describes a pulp bleaching system employing peroxide which uses a quaternary amine compound in the stabilized bleach solution.

It is disclosed in EP 0369711B1 a stabilised peroxide composition consisting of hydrogen peroxide and a phosphate stabilising additive.

U.S. Patent Application No. 20080264584 Al discloses the use of a copolymer of AHPS
and an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, maleic acid or itaconic acid, together with a chelating agent and an alkaline earth metal compound, such as magnesium sulphate, either mixed together or added separately, to achieve very good bleaching performance and total replacement of sodium silicate.

Other U.S. patents which employ such copolymers of acrylic acid in a peroxide bleaching system, include U.S. Pat. No.7867357B2 and U.S. Pat. No.4363699.

Based upon the limitations of the above patents, there is an ongoing need for improved but inexpensive mechanical pulp having increased brightness; through efficient bleaching performance with the use of a stabilized peroxide bleaching system, and through enhanced production and utilization of peroxide residuals.

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 peroxide decomposition.

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.

In the pulp and paper industry where chelants are added to enhance peroxide bleaching systems, levels of chelant from 1-6 kg/tonne of pulp are typically used. The chelants referred to above are the carboxylic acid derivatives of amines, e.g.
diethylenetriaminepentaacetic acid (DTPA), which are added at the pretreatment (prebleaching) stage to take metals out of the pulp. The chelant is partially removed in the subsequent dewatering step, but that which remains is rapidly destroyed in the bleaching step when contacted with the peroxide (See U.S. Pat. No.4614646). Bleaching of mechanical pulp has in the past been conducted with hydrogen peroxide, employing sodium silicate as a stabilizer, but this system results in problems when insoluble silicates are deposited upon the fibres and the machinery employed. When deposited on kraft paper fibres the result is a harsher feel of the paper. The fouling of equipment can cause down-time and shortened life of the equipment. Because of this, silicate-free systems have been suggested.

One of the simplest and most effective means of peroxide solution stabilization is by the addition of organophosphoric acid or polycarboxylic acid. Organophosphoric acids and polycarboxylic acids have excellent chelating abilities, low threshold inhibition and lattice distortionability; they can prevent scale formation, calcium carbonate in particular, in water systems. They can chelate with Fe, Cu, and Mn ions to form stable chelating compounds and show excellent scale and corrosion inhibition effects at temperatures of 250 C or below. They also have good chemical stability under high pH values, limit hydrolyzation and limit decomposition under ordinary light and heat conditions .
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, 0% - 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 0.1 % - 1 % chelating agent by weight based on the pulp wherein the chelating agent is selected from the organophosphonic acids group consisting of, A) amino trimethylenephosphonic acid (CAS No. 6419-19-8), B) 1-hydroxy ethylidene-1,1-diphosphonic acid (CAS No. 2809-21-4), C) ethylene diamine tetra(methylene phosphonic acid) (CAS No. 1429-50- 1), D) diethylenetriaminepenta(methylene phosphonic acid) (CAS No. 15827-60-8), E) 2-phosphonobutane-1,2,4-tricarboxylic acid (CAS No. 37971-36-1), F) 2-hydroxyphosphonocarboxylic (CAS No. 23783-26-8), G) hexamethylenediaminetetra(methylenephosphonic acid) (CAS No. 23605-74-5), and H) bis(hexamethylenetriaminepenta(methylenephosphonic acid)) (CAS No. 34690-00-1).
Or, the chelating agent is selected from the organophosphoric salts group consisting of, A) tetra sodium salt of amino trimethylenephosphonic acid (CAS No. 20592-85-2), B) penta sodium salt of amino trimethylenephosphonic acid (CAS No. 2235-43-0), C) potassium salt of amino trimethylenephosphonic acid (CAS No. 27794-93-0), D) monosodium of 1-hydroxy ethylidene-1,l-diphosphonic acid (CAS No. 29329-71-3), E) ethylenediamine tetra(methylenephosphonic acid) sodium (CAS No. 1429-50-1), F) disodium of 1-hydroxy ethylidene- 1, 1 -diphosphonic acid (CAS No. 7417-83-7), G) tetra sodium of 1-hydroxy ethylidene-l,1-diphosphonic acid (CAS No. 3794-83-0), H) potassium salt of 1-hydroxy ethylidene-1,1-diphosphonic acid (CAS No. 67953-76-8), I) pentasodium salt of ethylene diaminetetra(methylenephosphonic acid) (CAS

No. 7651-99-2), J) hepta sodium salt of diethylenetriaminepenta(methylenephosphonic acid) (CAS No. 68155-78-2), K) sodium salt of diethylenetriaminepenta(methylene phosphonic acid) (CAS No. 22042-96-2), L) sodium salt of 2-phosphonobutane-1,2,4-tricarboxylic acid (CAS No. 40372-66-5), M) potassium salt of hexamethylenediaminetetra(methylenephosphonic acid) (CAS No. 53473-28-2), N) partially neutralized sodium salt of bis(hexamethylenetriaminepenta(methylene phosphonic acid)) (CAS No. 35657-77-3).

Or the chelating agent is selected from the polycarboxylic antiscalants group consisting of, A) polyacrylic acid (CAS No. 9003-01-4), B) polyacrylic acid sodium (CAS
No.
9003-04-7), C) hydrolyzed polymaleic anhydride (CAS No. 26099-09-2), D) copolymer of maleic and acrylic acid (CAS No. 26677-99-6), E) acrylic acid-2-acrylamido-methylpropane sulfonic acid copolymer (CAS No. 40623-75-4), F) acrylic acid-2-hydroxypropyl acrylate copolymer (CAS No. 55719-33-0) at a temperature in the range of 65 - 80 C., and preferably in the range of 70 - 75 C. At such temperatures, the reaction may proceed for about one to three hours. The pH range of the method is preferably about 10 - 12.

We have found that residual hydrogen peroxide, after the bleaching tower, can be decreased from 8.5 kg to 4.5 kg by using 10.0 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, 37.5 kg caustic soda per tonne of dry pulp and 6.0 kg chelating agent per tonne of dry pulp to form a silicate-free bleaching solution for use in a mechanical pulp bleaching process.

By using the inventive process, an ISO brightness value of at least 84 is preferably obtained, still more preferably, an ISO brightness value of at least 85.6 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, which 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 chelant is made to a 5% solution.

Laboratory Studies A mechanical softwood pulp sample (from a pulp mill in British Columbia, Canada) had a brightness of 58.6% ISO, and a concentration of 24.51%, treated with DTPA.
Laboratory bleached at 20% concentration pulp, 70 C and 150 minutes of retention time.
From the result shown in TABLE 1, it is evident that TAED and chelant 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
ISO Peroxide Caustic Silicate TAED Chelant Peroxide Brightness Brightness (%) (%) (%) (%) (%) (%) 58.6 5.0 3.75 3.0 0.00 0.00 0.85 79.6 58.6 5.0 3.75 3.0 0.01 0.10 1.05 80.5 58.6 5.0 3.75 2.0 0.50 0.20 0.99 82.3 58.6 5.0 3.75 0.0 1.00 0.60 0.45 85.6 58.6 5.0 3.75 1.0 0.40 0.30 0.78 84.0 58.6 5.0 3.75 0.5 0.30 0.50 0.99 83.5 58.6 5.0 3.75 0.0 0.01 1.00 1.75 81.3 Laboratory Studies A mechanical softwood pulp sample (from a pulp mill in British Columbia, Canada) had a brightness of 52% ISO, and a concentration of 31%, treated with DTPA.
Laboratory bleached at 24% concentration pulp, 70 C and 150 minutes of retention time using a single-stage process.

It is evident from TABLE 2 that increasing the amount of chelant does have a resultant positive effect on pulp brightness. Bleached pulp with ISO brightness of more than 80.5 points have been obtained.

Incoming ISO Peroxide Caustic (%) Silicate (%) TAED Chelant ISO
Brightness %) Brightness 52 4.0 3.6 3.0 0.00 0.00 71 52 4.0 3.6 3.0 0.01 0.30 73 52 4.0 3.6 3.0 0.02 0.50 74 52 5.8 4.2 3.2 0.00 0.00 76 52 5.8 4.2 3.2 0.01 0.30 79 52 5.8 4.2 3.2 0.02 0.50 80.5 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 C and 150 minutes of retention time.

About 20.0 points of brightness were gained with TAED, chelant and hydrogen peroxide, compared to about 14.0 points with hydrogen peroxide. It is further evident that TAED
and chelant with hydrogen peroxide bleaching is more effective in pulp brightening during the second-stage compared to conventional hydrogen peroxide bleaching during the second bleaching stage.

Incoming ISO Peroxide Caustic (%) Silicate (%) TAED Chelant ISO
Brightness M) (W Brightness 66 3.5 2.0 2.5 0.00 0.00 80.0 66 3.5 2.0 1.0 0.50 0.50 86.0 66 3.5 2.0 0.0 0.40 0.60 85.5 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.

Unbleached Bleach Paper ISO Peroxide Caustic Silicate TAED Chelant Tower Machine Brightness (%) (%) (%) (%) (%) ISO ISO
Brightness Brightness 50 6.0 4.5 3.5 0.0 0.0 74.0 72.0 50 6.0 4.5 3.5 0.1 0.2 75.0 74.0 50 6.0 4.5 2.0 0.3 0.5 78.0 78.0 50 6.0 4.5 0.0 0.3 0.6 77.5 77.3 Evidently, TAED, chelant and hydrogen peroxide bleaching gives the best optical properties with about a six point increase in the ISO brightness after the paper machine.
The brightness does not change between the bleaching tower and paper machine, or even after pulp refining, in preparation for papermaking stock. Therefore, it is clear that this new composition and process for bleaching of mechanical wood pulp is more effective in stabilizing pulp or paper when compared with conventional hydrogen peroxide bleaching of pulp, which typically will undergo a brightness reversion of two or more ISO points between the hydrogen peroxide tower and paper machine in a closed white-water system.

Claims (4)

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 pulp brightness of a mechanical wood pulp and to reduce peroxide decomposition, wherein the second chemical component is 0.1 % - 1% chelating agent by weight based on the pulp.

2. A solution as claimed in claim 1 wherein the chelating agent is selected from the organophosphonic acids group consisting of, A) amino trimethylenephosphonic acid (CAS No. 6419-19-8), B) 1-hydroxy ethylidene-1,1-diphosphonic acid (CAS No.

21-4), C) ethylene diamine tetra(methylene phosphonic acid) (CAS No. 1429-50-1), D) diethylenetriaminepenta(methylene phosphonic acid) (CAS No. 15827-60-8), E) 2-phosphonobutane-1,2,4-tricarboxylic acid (CAS No. 37971-36-1), F) 2-hydroxyphosphonocarboxylic (CAS No. 23783-26-8), G) hexamethylenediaminetetra(methylenephosphonic acid) (CAS No. 23605-74-5), and H) bis(hexamethylenetriaminepenta(methylene phosphonic acid)) (CAS No. 34690-00-1).

3. The composition according to claim 1 wherein the chelating agent is selected from the organophosphoric salts group consisting of, A) tetra sodium salt of amino trimethylenephosphonic acid (CAS No. 20592-85-2), B) penta sodium salt of amino trimethylenephosphonic acid (CAS No. 2235-43-0), C) potassium salt of amino trimethylenephosphonic acid (CAS No. 27794-93-0), D) monosodium of 1-hydroxy ethylidene-1,1-diphosphonic acid (CAS No. 29329-71-3), E) ethylene diamine tetra(methylene phosphonic acid) sodium (CAS No. 1429-50-1), F) disodium of 1-hydroxy ethylidene- 1, 1 -diphosphonic acid (CAS No. 7417-83-7), G)tetra sodium of 1-hydroxy ethylidene-1,1-diphosphonic acid (CAS No. 3794-83-0), H) potassium salt of 1-hydroxy ethylidene-1,1-diphosphonic acid (CAS No. 67953-76-8), I) pentasodium salt of ethylene diamine tetra(methylene phosphonic acid) (CAS No. 7651-99-2), J) hepta sodium salt of diethylenetriaminepenta(methylene phosphonic acid) (CAS No.

2), K) sodium salt of diethylenetriaminepenta(methylene phosphonic acid) (CAS
No.
22042-96-2), L) sodium salt of 2-phosphonobutane-1,2,4-tricarboxylic acid (CAS
No.
40372-66-5), M) potassium salt of hexamethylenediaminetetra(methylene phosphonic acid) (CAS No. 53473-28-2), and N) partially neutralized sodium salt of bis(hexamethylenetriaminepenta(methylene phosphonic acid)) (CAS No. 35657-77-3).

4. The composition according to claim 1 wherein the chelating agent is selected from the polycarboxylic antiscalants group consisting of; A) polyacrylic acid (CAS
No. 9003-01-4), B) polyacrylic acid sodium (CAS No. 9003-04-7), C) hydrolyzed polymaleic anhydride (CAS No. 26099-09-2), D) copolymer of maleic and acrylic acid (CAS
No.
26677-99-6), E) acrylic acid-2-acrylamido-2-methylpropane sulfonic acid copolymer (CAS No. 40623-75-4), and F) acrylic acid-2-hydroxypropyl acrylate copolymer (CAS
No. 55719-33-0).