CA2230315A1 - Process for increasing mechanical wood pulp brightness in a refiner - Google Patents

Abstract

A pulp refining and bleaching process for increasing the pulp brightness of a mechanical wood pulp prepared from softwood chips wherein the pulp is treated in one or more stages in refiner by adding sodium perborate or peracetic acid generated in a bleach generating unit by reacting sodium perborate or hydrogen peroxide with an activating agent such as tetraacetylethylenediamine in the presence or absence of caustic solution and the pulp discharged from the refiner is subjected to another bleaching stage in a bleach tower by separate addition of sodium perborate or hydrogen peroxide with or without a bleach activator. The process is preferably carried out by passing the pulp through a primary refiner at elevated pressure and optionally to a bleaching tower, a solution of sodium perborate or in-situ generated peracetic acid from a mixture of sodium perborate or hydrogen peroxide and an activator, tetraacetylethylenediamine being fed in the refiner and sodium perborate or hydrogen peroxide with an activator, tetraacetylethylenediamine, being fed in the bleaching tower. The pulp after refining has improved brightness and pulp brightness after an optional tower bleaching has exceeded 75 ISO% points.

Description

BACKGROUND OF THE INVENTION

In accordance with the invention ;3 bright mechanical pulp is produced from lignocellulosic 24 material, for example wood chips , by subjecting the material to a process in which it is steamed, impregnated with a chelating agent, sodium perborate or peracetic acid, preheated 26 and then subjected to mechanical force in a process called "refining" to defibre the ' impregnated wood chip in the form of pulp and the pulp is discharged from the refiner at a pH
28 above 5. 7'he process produces a bright pulp of at least 14 to 20 ISO
points gain in the refiner stage only. Higher brightness has been achieved by carrying out the process preferably by 2 passing the pulp successively through a primary refiner at elevated pressure and then a bleach tower, a solution containing oxidising chemicals being fed to each.
4 The usual way to prepare mechanical pulp is by defibrating wood chips in a refiner under steam pressure at high temperature without significant loss of chemical components in 6 wood chips by optionally adding small quantity of hydrogen peroxide and alkali. The resulting pulps, while quite strong, are highly coloured probably due to the colored 8 chromophores in lignin. Bright papers from this pulp is prepared by modifying the colored chromophores in a process called "tdeaching". This process when carried out in a refiner is called "refiner bleaching" and the same process when carried out in a tank is called "tower bleaching". Such bleaching process is usually carried out in a bleaching tower after refining 12 by treatment with hydrogen peroxide: and /or other oxidative or reductive chemicals, which changes the chromophores to non-coloured products.
14 Recently alternative processes to improve brightness of pulp during the refining and subsequent tower bleaching by treating with oxidising and reducing agents have been 16 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 18 be of particular interest which are intended to brighten pulp efficiently in presence of alkali.
In refining; process the usual temperature is very high (temperature greater than 110°C for a short exposure time) which favorisea decomposition of low temperature reactive oxidants such as peracetic acid and hydrogen peroxide and efficient brightening reaction of generated 22 perhydroxyl ion with chromophore is difficult to attain without a significant loss of these chemicals by decomposition.
24 Addition of hydrogen peroxide and. sodium hydrosulfite during pulping process in a refiner for lignocellulosic material have been experimented with in the past but, as far as is known, 26 none of those chemicals could show improved brightness efficiency over conventional tower bleaching. One of the reasons may be their fast decomposition reactions catalyzed by the 28 presence of heavy metals such as manganese, iron and copper. For this reason in all conventional tower bleaching process the bleaching temperature used are usually less than 2 90°C and even lower in case of mechanical pulp. In refiner bleaching where the reaction conditions such as temperature, time, chemical composition etc. are different from that used 4 for conventional tower bleaching, it is not unexpected to have difference in the efficiency of the same hleaching chemicals in the two different processes. The prior art has not recognised 6 the importance of addition of sodimm perborate in refiner. Sodium perborate which is an effective ,generator of hydrogen peroxide is very effective at higher temperature (usually 8 above 80~~C and preferably above 100°C). Oxidative refiner bleaching may, therefore, be conducted) under the pulp refining condition which is usually at high temperature and pressure (usually above 120° C) for a shorter cycle than presently employed in the conventional tower bleaching..
12 As far a.s is known sodium perborate has never been used in refiner bleaching of pulp. It has two excellent advantages for application in refiner bleaching which are the high 14 decomposition temperature of perb~orate and that it does not require alkali to generate hydrogen peroxide. It has been found that under the condition of refining where the 16 temperature is above 100°C, incorporating sodium perborate without alkali is effective in brightening pulp and it improves pulp brightening efficiency over hydrogen peroxide for an 18 equivalent: concentration of oxidant.
Peracetic acid generation using sodium perborate or hydrogen peroxide and activators is also a well known art and the process is in commercial operation in the detergent industry.
Main adv;~ntage of using bleach activators is to reduce bleaching temperature to as low as 22 20°C or even lower by generating active bleaching chemicals in a solution containing sodium perborate or hydrogen peroxide and an activator. This reduction in the severity of the 24 conditions employed in bleaching reduces tendency towards cellulose degradation and increases efficiency of pulp brightening. Several such activators are known.
They are acetyl 26 derivatives of nitrogenous organic compounds such as tetraacetylglycolurile, diacetyldimethylurea, tetraacetylethelenediamine, triazine derivatives as well as glucoacetyl 28 compound~.s such as pentaacetylglucos,e.

One wa.y to improve the pulp brightening efficiency of hydrogen peroxide or sodium 2 perborate is the addition of these activators in the bleach liquor either in the refining stage or in a bleach tower. When an inorganic persalt or hydrogen peroxide is used alone in pulp 4 bleaching it provides a satisfactory bleaching effect above 60°C, but at a lower temperature, such as a.t 40°C, its bleaching efficiency is extremely low. Among the known bleaching 6 activators the TAED (tetraacetyl-ethylenediamine) is one of the most efficient one for detergent applications. It has been found however that under the normal conditions of pulp 8 bleaching in bleach tower where the temperature is higher than 60°C, a lowering of pulp brightness has been obtained by adding TAED with hydrogen peroxide or sodium perborate.
It has been discovered that a change in the bleaching conditions and more particularly by lowering the bleaching temperature enables improved pulp bleaching to high ISO
brightness 12 and this brightness gain is possible in a alkali-free bleach liquor when sodium perborate is used as oxidising chemical.

CITATION OF THE RELEVANT ART

The most pertinent publications in this area of which applicants are aware are two recent 18 publications, five US patents and one European patent. These are European Pat. No. EP 0 437 329 A1 (1992); Foret, R., "Particulate Bleach composition" US patent no.
4283302; Dugenet, Y., "Process for bleaching household laundry in a wash cycleBleach" US patent no. 4775382;
Sanderson, W.R., "Particulate sodium perborate monohydrate containing adsorbed activator"
22 US patent no. 4545784; Varennes S. et al., " Sodiurr~ perborate bleaching of TMP", Tappi J.
vol. 79, no.3,p. 245 ( 1996) Sain, IVI. M. et al, "Activated bleaching of thermomechanical 24 pulp"; J. Can. Chem. Eng., 1997, vol. 75 (1) pp. 62; Joachimides, T.;
"Process for bleaching mechanical wood pulp with sodiurr~ hydrosulfite and sodium hydroxide in a refiner", US
26 patent no. 5129987; Bengtsson, G.; "Method of manufacturing bleached chemimechanical and semichemical pulp by means of a one-stage impregnation process; US patent no.
28 4756799.

In the first four of these documents use of an activator for increasing bleaching efficiency 2 of sodium perborate in detergent industry is taught. Dugenet specifically teaches that perborate in combination with TAED activator is efficient at low temperature for washing 4 clothes in a pH range 8 to 12. Dugenet did not work on bleaching of mechanical wood pulp.
Thus no teaching or suggestion is provided by this author that perborate can suitably bleach 6 pulp in refiner in presence or absence of TAED activator. What is probably the reference of most interest in this group is European pat. No. EP 0 437 329 A 1 ( 1992) which teaches the 8 use of various peroxyacid bleach precursors in lowering the washing temperature in detergent applications. Tetraacetylethylenediarnine (TAED) is one of the bleach precursors used to improve llow temperature efficiency for washing. However, this reference does not also concern the refining and bleaching of mechanical wood pulp and in situ generation of 12 peracetic .acid by adding TAED with hydrogen peroxide in a bleach tower.
Another article, which considered the use of different bleach activators on the generation 14 of peracetic acid, is published in J. Prakt. Chem. Vol. 334 ( 1992) pp. 293-297. Authors reported that both sodium perborate; and hydrogen peroxide can generate peracetic acid by 16 reacting with tetraacetyl at a relatively low temperature and the activator efficiency is maximum between 10 to 30 min of addition of the activator. This reference strictly teaches 18 the gener;~tion of peracetic acid by reacting TAED with sodium perborate in aqueous alkali.
No bleaching of wood pulp was suggested and the teaching does not concern with refining of wood puhp with peracetic acid.
One at~ticle published in Tenside Surf. Det. Vol 27, no.3, page 187 ( 1990) considered the 22 use of TAED in peroxide solution to generate a strong oxidant such as peracetic acid in an alkaline medium. The article also reported that the peracetic acid can be generated at a 24 temperature below 60°C. The teaching of which is incorporated in this work as reference.
However, this reference strictly teaches the generation of peracetic acid. No bleaching of pulp 26 in refiner or in bleach tower was suggested and the teaching does not concern with refiner pulp bleaching.

In wood pulp bleaching and refining the most relevant documents are two papers from our 2 group and two patents. An U.S. patent number 5129987 to Joachimides et al.
discloses an addition of sodium hydrosulfite and sodium hydroxide in a refiner to improve mechanical 4 wood pulp brightness. Another U.S. patent number 4756799 to Bengtsson et al.
disclosed that an impregnation of wood chips with ;~ solution containing alkali and peroxide and subsequent 6 preheatin~; and refining the chips below 100°C in two stages improved brightness of pulp.
These references strictly concern with reductive and oxidising agents which are now being 8 used industrially for refiner bleaching of pulp rather than process to use sodium perborate in refiner bleaching of mechanical wood pulp.
In one of the recent publication co-authored by S. Varnenes we have shown that perborate can bleach mechanical wood pulp in a conventional bleach tower. The teaching of which is 12 taken here: as a reference. This paper did not recognise the use of perborate in a refiner to bleach wood pulp during refining. Tlms no proposition or recommendation was made in this 14 paper to bleach pulp in-situ during rc;fining in a system containing perborate with or without the presence alkali. In our another p;~per coauthored by M. Parenteau we have demonstrated 16 the use of TAED as an activator in mechanical pulp bleaching in a perborate solution. It has been found that perborate reacted with tetraacetylethylenediamine to generate peracetic acid 18 which was an effective bleaching agent for thermomechanical pulp. This paper did not mention the use of TAED to increase the wood pulp bleaching efficiency of hydrogen peroxide.
Applicants consider that given substantial differences between perborate promoted in-situ 22 refiner bleaching of pulp without addition of alkali followed by tower bleaching of the pulp by in-situ generated strong oxidant from perborate or peroxide and tetraacetylethylenediamine 24 under alkaline condition and, conventional peroxide or perborate or peracetic acid tower bleaching of pulp that above references singly or in combination provide no teaching enabling 26 one to carry out perborate bleaching of pulp in a refiner followed in bleach tower with hydrogen peroxide or sodium perborate and TAED added together in a temperature range 28 between ~;5 to 60°C.

It is accordingly an object of the present invention to provide a perborate pulp bleaching 4 process which gives pulp of improved brightness without the need of addition of alkali or the need to increase either refining energy or the overall amount of oxidant used.
6 In accordance with the present invention bright mechanical wood pulps are produced from lignocellulosic material, for example wood chips, by subjecting the material to a process in 8 which it is steamed for about 10 min., then pre-heated in a digester (preheater) under pressure at a tempE;rature range between 115 to 145°C for about 6 to 8 min., impregnated first with a heavy metal chelating agent such as DTPA (diethylenetriaminepentaacetate) in preheater and then mixed with an oxidising agent selected from peracetic acid, generated in a tank by 12 separately premixing an activator suc h as TAED with hydrogen peroxide or sodium perborate in 0.3: 1 molar ratio, and alkali-free: sodium perborate , a sodium silicate stabiliser and a 14 cellulose protective agent such as magnesium sulphate at the eye of the rotating refiner plates and finally refined and discharged at a pulp consistency of about 20% on dry (od) pulp basis.
16 It is also an object of this invention to improve brightness of above refined pulp further by subjecting it to tower bleaching process by adding a mixture of sodium perborate or hydrogen 18 peroxide, TAED, sodium silicate, magnesium sulphate and alkali being used only with peroxide apt a temperature range 30 to 60°C for a time period between 30 to 90 min.
The tangible embodiments produced by the process aspect of the present invention possess the inherent physical characteristics of being relatively bright pulps when tested by 22 standard brightness methods, and having superior pulp brightness to pulps bleached by hydrogen peroxide in refiner and in bleach tower under the conditions employed in prior art 24 processes, thus being useable for all standard uses of lignocellulosic pulp based paper.
In yet another embodiments of this invention the bleaching process provides a mean to 26 brighten pulp with an oxidant such as sodium perborate in absence of alkali.

Special mention is made of embodiments of the invention wherein the method has 2 advantage over peroxide bleaching process for better optical property achievement.
Special mention was also made of the embodiments of the invention wherein the tower 4 bleaching with perborate or peroxide and tetraethylenediaminetetraacetate reduces the bleaching temperature from about 70''C to about 40°C and the bleaching time from about 120 6 min to about 60 min. thus saving energy cost for pulp bleaching.
The bleaching of mechanical wood pulp earned out by refining softwood chips first with 8 sodium pe:rborate in refiner in a concentration range between 2 to 15 parts per 100 parts of dry wood chips, 3 parts of sodium silicate: per 100 parts of wood chips within a temperature range 115 to 14'i°C for the refining followed by tower bleaching of refined pulp with 2 to 5 parts of hydrogen peroxide or 6 to 15 parts of sodium perborate, with a peroxide/alkali ratio between 12 0 to 1.5 b:y weight and TAED/ peroxide molar ratio 0.1 to 1 within a temperature range 30 to 70°C and time range 30 to 120 min. is. within the scope of this invention.

The wood chips used in the present study was softwood origin. The process of the 16 invention is an unique process for brightening pulp by adding an efficient oxidising agent in refiner and by adding a bleach activator in bleach tower. To practice the process of the 18 invention, the lignocellulosic material, conveniently softwood thermomechanical pulp prepared from wood chips having two different ageing times, conveniently one month and six months.
IN-REFINER BLEACHING
22 For in-refiner bleaching wood chips were pre-steamed under atmospheric pressure for few minutes, conveniently 10 min, may be introduced into a pressurized pre-heated digester for 6 24 to 8 min. conveniently with a plug screw where the wood chips were heated to a temperature conveniently about 130"C where the said preheated wood chips were impregnated with a chelating agent such as DTPA in small amount, conveniently 0.2% by weight of dry wood 2 chips before being fed directly to a refiner eye wherein the chips are treated with a bleaching solution having an effective amount of oxidising agent selected 10.5% by weight of dry 4 wood chips, in absence of sodium hydroxide and to which also have been added a stabiliser such as sodium silicate, conveniently about 3 % by weight of dry wood chips to prevent 6 degradation of bleach oxidant generated in the process and a cellulose protector such as magnesium sulphate in trace amount.. conveniently 0.05% by weight of dry wood chips. The 8 pulping and brightening of pulp cm be obtained by passing the impregnated wood chips through a refiner which contains rotating plates, preferably flat disc and conical disc types with a pre-adjusted plate gap, conveniently between 0.3 to 1.65 mm to defibre the wood chips and thereby facilitating defibered wood chips to react with impregnated bleaching solution.
12 After imI>regnation with chelating agent, oxidising agent, cauticising agent, if any, and stabilizing chemicals in standard fashion, the mixture of wood chips and added chemicals 14 may be refined in a conventional CD 300 Sund-defibrator pilot-scale refiner, conveniently at a production rate 0.55 kg/min with a measured discharge, conveniently about 20% of dry 16 pulp, the resulting pulp being stored in a plastic bag for further reaction conveniently for 30 to 45 min. to produce a pulp conveniently with a freeness of about 200 ml and brightness about 18 68% ISO. The resulting pulp, may be. further bleached in a bleach tower or it may be formed directly to paper. The resulting paper obtained from refiner has improved mechanical strength over conventional thermomechanical pulp.
POST BLEACHING
22 In a preferred embodiment the refined pulp subjected to further bleaching to practice the process of the invention, is treated with a solution containing bleach effective amount of 24 hydrogen peroxide, conveniently 3.5% by weight of pulp, sodium silicate , conveniently 3%
by weight, sodium hydroxide, conveniently 2.7 % by weight and to which has been added a 26 bleach activator, conveniently tetraacetylethylenediamine in a molar ratio of activator/peroxide 0.3, a chelating agent, conveniently DTPA and a cellulose protective agent, conveniently magnesium sulphate. Refined pulp, conveniently 12% by weight , 2 impregnated with bleach solution, has been subjected to heat in a bleach tower conveniently at 40°C for a short time period , conveniently about 60 min. to obtain a bright pulp of ISO
4 brightness above 75.
In another preferred embodiment the refined pulp subjected to further bleaching to 6 practice the process of the invention, is treated with a solution containing bleach effective amount of sodium perborate, conveniently 10.5% by weight of pulp, sodium silicate , 8 conveniently 3% by weight, and to which has been added a bleach activator, conveniently tetraacetylethylenediamine in a molar ratio of activator/peroxide 0.3, a chelating agent, conveniently DTPA and a cellulose protective agent, conveniently magnesium sulphate.
Refined pulp, conveniently 12% by weight , impregnated with bleach solution, has been 12 subjected to heat in a bleach tower conveniently at 40°C for a short time period , conveniently about 60 min. to obtain a bright pulp of ISO brightness above 75.
14 A series of trials were carried out both for in-refiner bleaching and post bleaching operation to establish improved process conditions of the invention. More details of these 16 experimental trials are as follows:
As used herein and in the appended claims the term "aging" means physical and chemical 18 transformation of components in wood chip due to longer storage time under ambient condition.
As used herein and in the appended claims the term "refining" means defibrating wood chips into individual fibres and to develop good mechanical properties of these fibres by 22 applying heat, pressure and mechanical force.
As used herein and in the appended claim the term "bleaching" means modification of 24 coloured chromophores present in pulp lignin in order to improve the brightness of mechanical pulp.

The term "bleach effective amount" of perborate oxidant means a concentration of oxidant 2 such as perborate in solution of from about 6% to about 15% by weight and "bleach effective amount" of peroxide means from about 2% to about 5% by weight.
4 As used herein and in the appended claims, the term "activator" contemplates water soluble tetraacetylethylenediamine.
6 The term "chelation" as used herein and in the appended claim means a chemical capable of reactin;; with heavy metals such as~ iron, manganese, copper to reduce their freed amount in 8 pulp, thereby, reducing the degradation of oxidising agent in pulp.
The term "stabiliser" contemplates water soluble sodium silicate compound and the term "cauticiser" contemplates a solution of sodium hydroxide in a concentration range 1.0 to 5.0%
by weight. The term "cellulose protective compound" contemplates water soluble magnesium 12 sulphate.
The :presence of trace metals in wood catalytically decompose the bleaching chemicals.
14 TMP from hardwood and softwood reveals itself to be effective for brightening, it is pretreated with a chelating agent such as DTPA. In refining and bleaching, the pretreatment 16 process, i.e., a process according to which the pulp is treated with a chelating agent DTPA, and consequently, a chelate complex with free metal in pulp is formed. In practice of the 18 invention, just prior to the addition of bleaching chemicals in refiner, the pulp is treated with 0.2% by weight of DTPA. In refiner bleaching DTPA is added just after the plug screw in digester prior to addition of bleaching chemicals at the refiner eye at the point of rotating disc while the pulp consistency is maintained at about 20%.
22 In one: of the preferred embodiments of the present invention, the chemical composition used during refining to brighten the pulp is either sodium perborate or peracetic acid obtained 24 by mixing; TAED and hydrogen peroxide in a tank at 0.3:1 molar proportion, sodium silicate, magnesium sulphate and alkali, if any. The balance was water to make up the desired pulp 26 consistency. The pre-heating temperature in digester is varied between 115 to 145°C. Once the refining is completed pulps have been stored for 30 to 45 additional minutes in a plastic 2 bag, then diluted to 1 % consistency before being produced in different freeness levels. The pulp is then diluted and neutralized at pH 5.5 with sodium metabisulfite, thickened, washed 4 and pressed to a consistency of about 20% by weight and handsheets are made for brightness and mechanical testing according to CPPA E.1 and CPPA D.12.
6 The initial pH of the refiner bleach liquor may vary from 10 to 13 . For refining with perborate the preferred pH range is bcaween 10 and 11 and for peracetic acid the prefereed pH
8 range is between 10.5 and 11.5. The final pH of the spent liquor after refining may be in the range between 5 to 7.5. The preferred pH range in between 5.5 to 6.5.
A preferred chelating agent is DTPA. Other chelating agents such as EDTA and phosphonates may be used. The preferred concentration of chelating agent is between 0.2 to 12 0.4% by weight of dry pulp. A higher concentration may be used but without additional advantage in pulp brightness gain. A lower concentration is less effective and is not included 14 in the present invention.
In refiner bleaching it is advantageous to add the chelating agent prior to addition of 16 bleaching chemicals. The benefit is c;helation of trace metals prior to addition of oxidants which could render oxidant more stable during the chromophore modification reaction with 18 lignin in pulp. Chelating agent can be added together with other chemical in the refiner eye, but generally efficiency would be less.
A preferred alkali is sodium hydroxide, but other alkali metal salts could also be used which could maintain a pH range of about 10 to 11 during refining and bleaching, a preferred 22 alkali/oxidant ratio being 0.8 to 1Ø Lower concentrations may be used, but the efficiency of process will depend on the nature of oxidant. Usually for peroxide and peracetic acid a higher 24 concentration is preferred and for perborate a bleach liquor without alkali could be used to obtain a significant gain in pulp brightness . A higher concentration of alkali/oxidant ratio of 26 about 1.5/1 is not worthwhile for hydrogen peroxide or peracetic acid because generally it reduces the brightness gain and increases cost. Perborate without alkali is preferred because it 2 reduces the chemical cost without sacrificing pulp brightness. More preferred weight ratio of alkali/peracetic acid is 0.8/1 for peroxide and TAED based system wherein the refined pulp 4 has improved brightness and good mechanical properties.
In refiner bleaching of pulp, the preferred pulp of about 20% consistency is bleached in-6 situ during refining by sodium perborate or peracetic acid. During bleaching at least two intermediate products are formed: perhydroxyl ion and hydrogen peroxide. The mechanism 8 by which perborate and peracetic acid is not known in detail but it is expected that perborate and perac:etic acid could generate an unknown intermediate strong oxidant which then reacts with colored chromophores in lignin of pulp to chemically modify them and improve pulp brightness. Independent of the correct explanation of the pulp brightening mechanism it has 12 been found that a surprising increase in pulp brightness could be achieved by adding perborate in refiner even in the absence of alkali.
14 The temperature of the preheated digester prior to introduction of pulp to refiner was varied between 115 to 145°C. For optimum pulp brightness, the refiner pre-heating temperature is an 16 important factor and should be kept low. The extent of pulp brightening was good in a temperature range between 115 - 130°C. A higher temperature has no practical advantage 18 because it decomposes the oxidants and the brightness gain is low. Below 115°C. the efficiency of the process is generally too low to be worthwhile.
The sodium perborate concentration may range from about 5 % by weight of dry pulp to about 15 % by weight of dry pulp, although a lower concentration down to about 2% by 22 weight of dry pulp may be used with reduced beneficial effect. Similarly, improvements of the brightness of the pulp are not very significant with sodium perborate concentration above 24 12% by weight of dry pulp, while additional cost is not justified by the small additional improvement. Generally, therefore, a sodium perborate concentration range between about 26 6% by weight to 12% by weight on dry pulp, preferably around 11 % by weight of dry pulp is used.

For refiner pulp brightening with peracetic acid generated in a separate tank by reacting 2 hydrogen peroxide and TAED in a molar ratio 1:0.3, the concentration of hydrogen peroxide may vary from 2% to 5% by weight of dry chips. It is preferred to carry out refiner bleaching 4 with peracetic acid generated from 4 wt% of hydrogen peroxide and 0.3:1 molar ratio of TAED and hydrogen peroxide. A lower concentration of generated peracetic acid may be 6 used, but the efficiency of the process reaches a plateau at this concentration and no advantage is usually gained by the use of higher concentration of generated peracetic acid.
8 Wood species from softwood sources may be used. Both fresh and aged wood chips may be used. The preferred wood chips for refiner bleaching is aged chips. The reason for this improved brightness gain with aged chips is not known. A possible explanation may be modification of colored chromophorea in lignin to easily oxidisable states during aging.
12 Whatever may be correct mechanism of aged wood chips brightening process, a high brightness gain has been achieved with aged chips over fresh chips.
14 The refiner plate design and plate gap are also important to obtain homogeneous reaction and optimum defibration of wood chips during refiner bleaching process. Flat disc plate and 16 conical disc plate are preferred plate configurations. Other design also can be used but with less efficiency. The preferred plate gap in refiner for flat disc is between 0.3 to 1.0 mm, more 18 preferred range is between 0.4 to 0.8 mm. A higher plate gap may be used but, the pulp freeness will be very high. A low gap will decrease the pulp freeness to an unacceptable level.
For conical disc type refiner plate the preferred gap is between 1.2 to 2.4 mm. However, for optimum results a suitable combination of nip gaps of flat type and conical type is preferred 22 which may be any suitable gap betwc;en 0.4 to 0.8 mm for flat type and 1.2 to 2.4 mm for conical type plates.
24 Surprisingly and unpredictably it has been found that the specific refining energy of pulp refined with perborate has not been increased over standard thermomechanical pulp, 26 TMP, process or pulp developed with alkaline peroxide in refiner although the brightness gain and mechanical strength of perborate treated pulp is superior to both TMP
and alkaline 2 peroxide treated mechanical pulp, APMP.
The in-refiner bleaching with perborate has additional advantage over conventional tower 4 bleaching in that it reduces the amount of bleach effluent because of the high consistency of refiner pulp which is usually 20% and above compared to about 10 to 12%
consistency for the 6 tower bleaching.
It is also an object of the present invention to provide a tower bleaching method of the 8 aforesaid refined pulp of the present invention with peroxide and perborate which avoids the disadvantages of the prior art processes and which provides advantageous conditions of pulp brightening. Other additional objects and advantages will become apparent from the following description.
12 In a I>referred embodiment of the present invention the further brightening performance of various kind refined pulps including TMP and APMP is being compared with pulp refined 14 with perborate. It has been found that perborate and peracetic acid in-refiner bleached pulp may be bleached further with the same efficiency as compared to APMP under a standard 16 bleaching condition which is usually 70-80° C. and 2-3 hours of residence time in a bleach tower.
18 Also, surprisingly it has been found that pulp refined with perborate or peracetic acid may be fizrther brightened to a ISO brightness point above 75 in a single conventional bleaching stage by treating the pulp together with alkaline peroxide or perborate and TAED
activator in a bleach tower at a relatively low temperature ranging between 20 to 60°C. for a 22 suitable time period eg., 0.25-1.5 hours, but other chemicals and conditions are variable as described in a prior art in the form c>f publication by Daneault and Varennes and included 24 herein as a reference. Successful bleaching in the process of the invention was obtained by using 6 to 15% by weight of perborate or 1 to 5% by weight of peroxide and a 26 TAED/perborate or TAED/peroxide molar ratio between 0.05 to 1. An alkali/perborate weight ratio 0 to 0.3 has been found to be optimum with no practical advantage in using a 2 higher weight ratio. The alkali/peroxide weight ratio for TAED/peroxide combination was between 0.8 to 1.5. Surprisingly, the brightness gain after the treatment of the refined pulp of 4 the present invention with hydrogen peroxide or sodium perborate and TAED
under the above low temperature and short bleaching time conditions was superior to the brightness gain 6 obtained after bleaching the same pulp under a standard industrial bleaching condition, which is usually 70°C and 120 min. at an equal concentration of hydrogen peroxide dosage.
8 The concentration of perborate in bleach tower may vary from 2 to 15% by weight of dry pulp. The preferred concentration is between 8 to 12% by weight. A lower concentration is not preferred because the brightening efficiency is very low. The optimum concentration of peroxide for TAED/peroxide bleaching system is between 2 to 5% of peroxide. A
higher 12 concentration is not useful because the brightness gain is small because a plateau is reached near this concentration level. A lower concentration below 1 % is also not efficient for 14 achieving; high pulp brightness.
The efficiency of the process is increased by increasing TAED/perborate and 16 TAED/peroxide molar ratio, and a molar ratio of 0.02 to 1 may be used, and a molar ratio between 0.05 to 0.3 has been found to be optimum for TAED-peroxide combination and a 18 molar ratiio of 0.1 to 0.5 was found to be optimum for TAED-perborate combination. A higher molar radio may be used, but the brighntess gain is generally very marginal to be worthwhile.
The ratio of alkali/perborate is an important factor for achieving optimum optical and mechanical properties during the tower bleaching process. Bleaching without alkali is 22 preferred for maximising cost reduction. However, an alkali/perborate weight ratio of 0 to 0.5 may be used to achieve higher brightness gain and improved mechanical properties. The 24 optimum alkali/perborate weight ratio is between 0 and 0.3. A ratio above 0.7 is less efficient because it results in pulp yellowing: and reduces brightness of pulp. The optimum ratio of 26 alkali/peroxide in TAED/peroxide bleaching is between 0.8 to 1.5. A ratio lower than 0.6 significantly decreases the bleaching efficiency and a higher ratio above 1.5 increases 2 chemical cost and develops pulp yellowness.
Temperature is the most important factor in the present invention of tower bleaching.
4 Surprisingly, a high brightness gain has been achieved by using a low temperature range between 20 to 50°C. and optimum temperature range was between 30 and 50°C. A
6 temperature lower than 20°C. is also possible but the efficiency is low. A higher temperature above 60°C is not useful because the efficiency of peroxide-TAED or perborate-TEAD
8 oxidation potential towards pulp decreases significantly.
Preferred bleaching time in tower with TAED-peroxide and TAED-perborate systems is between :30 to 75 minutes. A shorter time than 25 minutes may be used with a marginal loss in brightness gain. A higher time above 90 minutes is not suitable because it decreases the 12 pulp brightness and increase energy cost.
In yet another preferred embodiment of the present invention the refined pulp obtained 14 from the aforesaid refining process may be pre-treated with a chelating agent. A preferred chelating agent is DTPA, but other chelating agents may be used, a preferred concentration 16 being 0.2 to 0.4% by weight of dry pulp. A lower concentration may be used, but efficiency is less and no advantage is usually gained by the use of more concentrated alkali.
18 The process of invention provides a number of advantages over prior art processes as well as the possibility to improve the quality of product discussed above. The in-situ perborate bleaching during refining is cheap because it eliminates partly conventional bleaching stage in bleach tower or reduce bleach chemical requirements in conventional 22 bleaching stage and presents no undue problem of handling and treating a large quantity of bleach effluent. The product gives significant brightness gain and better mechanical properties 24 over all prior art in-refiner bleaching and conventional bleaching processes. TAED activated peroxide bleaching of perborate-refined pulp gives an unique possibility to reach high brightness for a mechanical pulp in a single after-refiner bleaching stage at a low temperature 2 and with a very short reaction time as discussed above.
Although described herein with reference to softwood species it is expected that the 4 process of the invention will be applicable to in-refiner bleaching and tower bleaching of a wide range of natural products including hardwood and nonwood species to yield useful 6 bright pulp for papermaking and nonwoven applications.
The following examples further illustrate the best mode comtemplated by the inventors for 8 the practice of their invention.
Example 1 Aged softwood chips from spruce (75%) and Balsam(25%) were first pre-steamed for 10 min. under atmospheric pressure and then impregnated with 0.2% of DTPA in digester for 12 about 6 minutes to 130°C and conveyed to the refiner eye where it was refined with or without injection of bleaching solution. All bleaching solutions contained 3.0% sodium 14 silicate, 0.05% magnesium sulphate, 3.0% sodium hydroxide except where sodium perborate is used and other oxidising chemicals as given in Table below. The pulp after refiner was 16 stored for 30 min. in a plastic bag and then diluted and neutralized at pH
5.5 with sodium metabisulfite. The pulp freeness range is between 100 to 200 ml. The handsheets made from 18 those pulps show optical properties:

In-refiner oxidisingWt%(dry wood Brightness, Opacity, %
agent %ISO

chi Sodium perborate 11.7 69.8 90.5 Peroxide a uivalence)(4.0) Peracetic acid 8.9 60.7 90.8 from eroxide*

Peracetic acid 8.9 63.4 91.6 from erborate*

H dra en eroxide 4.0 63.4 90.2 None 0 49.6 97.5 *Prepared in a separate tank by reacting 4.0 g of hydrogen peroxide with 8.05 g of TAED; **
2 Prepared in a separate tank by reacting 11.7 g of perborate with 8.05 g of TAED.
It is evident that in-refiner bleaching with sodium perborate gives maximum brightness gain 4 and optical properties are superior to in-refiner hydrogen peroxide brightened pulp. About 20 points gain with perborate was obtained compared to about 14 points with hydrogen peroxide.
6 It is further evident that peracetic acid generated from a reaction mixture of perborate and TAED is more effective in pulp brightening during refining compared to that generated from S peroxide and TAED.
Example 2 The example shows the effect of different in-refiner oxidising agents on mechanical properties and their energy consumption during refining. All conditions are the same as in 12 Example 1.

In-refiner oxidisingWt%(dry wood Specific refiningTensile index, agent Nm/g chi ener , MJ/k Sodium perborate 11.7 9.0 40.0 (Peroxide a uivalence)(4.0) Peracetic acid 8.9 7.3 38.4 from eroxide*

Peracetic acid 8.9 7.3 35.2 from erborate*

Hydrogen peroxide 4.0 7.3 36.1 APMP

None (TMP) 0 7.3 34.7 *Prepared. in a separate tank by reacting 4.0 g of hydrogen peroxide with 8.05 g of TAED; **
2 Prepared i.n a separate tank by reacting 11.7 g of perborate with 8.05 g of TAED.
It is evident that specific refining energy does not change very much during in-refiner 4 bleaching with perborate and peracetic acid compared to hydrogen peroxide.
The tensile index of in-refiner perborate bleached pulp has improved properties over TMP
and the one 6 obtained by peroxide in-refiner bleaching.
Example 3 This example compares the in-refiner bleaching of unaged or fresh wood chips.
Fresh softwood chips from spruce (75%) and Balsam(25%) were first pre-steamed for 10 min. under atmospheric pressure and then impregnated with 0.2% of DTPA in digester for 12 about 6 minutes to 130°C and conveyed to the refiner eye where it was refined with or without injection of bleaching solution. All bleaching solutions contained 3.0% sodium 14 silicate, 0..05% magnesium sulphate, :3.0% sodium hydroxide and other oxidising chemicals as given in Table below. The pulp after refiner was stored for 30 min. in a plastic bag and then 16 diluted, neutralized at pH5.5 with sodium metabisulfite and obtained in a freeness level of 200 ml. The handsheets made from those pulps show optical properties:

In-refiner oxidantSodium perborateHydrogen peroxideNone (Peroxide (APMP) (TMP) a uivalence Wt% (dry wood 10.5 3.5 0 chip) 3.5 Bri htness, %ISO 67.9 65.0 53.6 O Tacit , % 90.0 89.3 94.6 Sp. Refining energy,9.7 11.4 11.7 MJ/k Tensile index, 37.2 38.4 35.2 Nml From results showed in above table it is evident that perborate in-refiner bleaching is 4 superior to peroxide in-refiner bleaching with respect to optical properties and refining energy requirement. The mechanical properties of perborate pulp is comparable to peroxide pulp and 6 are superior to TMP. As mentioned before a chelating agent was used before addition of bleach liquor. Chelating agent, DTPA, was added after plug screw at higher concentration 8 than usual for tower bleaching because of the alkaline pulp condition.
Phosphonate type chelant may be used in lesser amount because of their improved chelation stability at higher pH condition.
Example 4 12 This example illustrates the effect of sodium perborate concentration and refining temperature during in- refiner bleaching stage.
14 Fresh softwood chips from spruce (75%) and Balsam(25%) were first pre-steamed for 10 min. under atmospheric pressure and then impregnated with 0.2070 of DTPA in digester for 16 about 6 minutes to 130°C and conveyed to the refiner eye where it was refined with or without injection of bleaching solution. All bleaching solutions contained 3.0% sodium 18 silicate, 0.05% magnesium sulphate. The pulp after refiner was stored for 30 min. in a plastic bag and then diluted, neutralized at pH 5.5 with sodium metabisulfite and obtained in a 2 freeness level of 200 ml. The handsheets made from those pulps show optical and mechanical properties:
Perborate. % 6.0 15.0 6.0 15.0 Tem erature,oC 115 115 145 145 Bri htness,% 67.1 72.2 63.2 65.0 ISO

b* 14.1 12.4 15.7 15.4 O acit , % 90.6 88.6 91.3 89.8 Tensile index.,30.1 33.8 40.5 39.0 Nm/

Burst index., 2.30 2.49 2.52 2.44 kPa* m2/

Tear index, 9.78 9.79 11.07 9.55 mN*m2/

Sp. Refining 7.65 11.18 12.96 11.58 energy, MJ/k Fiber len th, 1.69 1.76 1.87 1.77 mm Perborate 4.42 8.89 5.11 12.58 consum tion, %

It is evident from above results that increasing perborate concentration has a positive effect 6 on pulp brightness and mechanical properties as well. A in-refiner perborate bleached pulp with ISO brightness more than 72 points has been obtained at higher concentration. Increasing 8 temperature to 145°C from 115°C. ha.s a detrimental effect on pulp brightness. Pulp brightness decreased by about 7 points ISO by increasing the refining temperature from 115°C to 145°C.
for a perborate concentration of 15% by weight. The same trend was found at a lower concentration of perborate. The b* value further indicates that pulp is more yellow at higher 12 refiner temperature.
Example 5 14 This example illustrates the effect of alkali-free in-refiner bleaching with hydrogen peroxide, sodium perborate and peracetic acid where peracetic acid has been generated by 16 reacting sodium perborate and TAED ex-situ and then added to the refiner during in-refiner bleaching on optical and mechanical properties. Aged softwood wood chips from spruce 18 (75%) and Balsam(25%) were first pre-steamed for 10 min. under atmospheric pressure and then impregnated with 0.2% of DTPA in digester for about 6 minutes to 130°C and conveyed to the refiner eye where it was refined with or without injection of bleaching solution. All bleaching solutions contained 3.0% sodium silicate, 0.05% magnesium sulphate, and 11.7%
2 perborate and peracetic acid was obtained by reacting 11.8 g of perborate with 8.05 g of TAED separately in a tank and then injecting it at the refiner eye . The pulp after refiner was 4 stored for 30 min. in a plastic bag and then diluted, neutralized at pH 5.5 with sodium metabisulfite and obtained in a freeness level of 200 ml. The handsheets made from those 6 pulps show optical properties:
In-refiner oxidisingISO Brightness,Opacity,Burst index,Tensile agent % % kPa.m2/ index, Nm/

Sodium erborate 69.8 90.5 2.5 40.0 Peracc~tic acid 63.4 91.6 2.1 35.2 from erborate**

Peroxide 63.4 90.2 2.6 36.1 No bleachin (TMP) 49.6 ~ 97.5 2.1 ~ 34.7 ~

8 ** Prepared in a separate tank by reacting Evidenl:ly, in-refiner perborate bleaching gives best optical properties with about 20 points increase in the ISO brightness points. The mechanical properties of in-refiner perborate bleached pulp was superior to TMP. In-refiner peroxide bleaching, however, did not brighten 12 the pulp very much. Therefore, it is clear that perborate is unique in in-refiner bleaching of woodchips even without the addition of free alkali.
14 Example 6 This example compares the efficiency of perborate in-refiner bleaching with perborate 16 tower bleaching at same concentration of oxidant.

In-refiner Tower bleachin Perborate, % TMP 6.0 15.0 Max. TMP 6.0 15.0 Max.
Gain/ initial Gain/
loss loss Bri htness,% 56.3 67.1 72.2 15.9 59.1 71.5 73.6 14.5 ISO

b* 14.8 14.1 12.4 2.4 - - _ _ O acit , ~0 94.6 90.6 88.6 -6.0 - - - _ Tensile index.,35.2 30.1 33.8 -5.1 42.8 - 45.3 2.5 Nm/

Burst index., 2.18 2.30 2.49 0.31 3.30 - 3.09 0.06 kPa*m2/

Tear index, 10.3 9.78 9.79 -0.53 12.02 - 11.85 -0.17 mN*m2/

Sp. Refining 11.69 7.65 11.18 0.51 - - _ _ energy, MJ/k Perborate - 4.42 8.89 - - 3.1 5.8 -2 consum tion, %

Constant conditions:
4 Refiner: Steaming 10 min Discharge consistency = 20%
6 Preheater temperature= 115°C.
Pulp freeness: 200 ml.
8 Bleach tower:
Consistency = 12%
Temperature = 70°C.
Time = 120 min.
12 In another trial, the improvement of optical and mechanical properties of a in-refiner bleached pulp was compared with that of the tower bleached pulp. In both cases a 14 thermomechanical pulp (TMP) made in the same refiner under similar condition as used for this trial in in-refiner and tower bleaching were used as a reference. It has been clear from 16 above Table that in-refiner bleaching has better efficiency in achieving brightness gain over in-tower bleaching. This demonstrates that oxidative bleaching earned out in the refining 18 zone with sodium perborate is more efficient than conventional medium consistency bleaching, suggesting that continuous fracturing of wood chips exposes chromophores that are readily accessible to oxidation by perhydroxyl ion. Again because sodium perborate has higher decompostion temperature it is expected to be more efficient at higher temperature 2 which is in this case again can be achieved during refining.
However, it has been found that mechanical property improvement was more in tower 4 bleaching which is not unexpected because higher refining temperature could have a more severe degradative effect on lignocellulosics than that at a lower tower bleaching temperature.

8 This example compares the optical and mechanical properties of four different types of pulps prepared in the refiner. For chemithermomechanical pulp the concentration of sodium sulfite was 3% on od pulp, for peroxide process (APMP) peroxide was 3.5% and for perborate process sodium perborate (SPBMP) was 10.5% which is equivalent to 3.5% of hydrogen 12 peroxide. 'lfie refiner temperature was 130°C.
Pro erties TMP CTMP APMP SPBMP

Brightness, 56.3 58.3 65.0 67.9 ISO

b* 14.8 13.7 15.0 14.1 Opacity, 94.6 92.8 89.3 90.0 Tensile index..35.2 41.1 38.4 37.2 Nm/

Burst index.. 2.18 2.70 2.54 2.41 kPa*m2/

Tear index. 10.3111.35 10.80 10.08 mN*m2/

Energy. 11.698.70 11.41 9.74 MJ/k Fiber len th. 1.73 1.96 1.84 1.73 mm Consum lion. --- --- 2.98 8.49 %

14 From above results it is evident that brightness gain is much superior for SPBMP
compared to CTMP but only with a marginal less mechanical strength. Again.
SPBMP has 16 superior optical properties than APMP at a comparable mechanical properties and also has superior optical and mechanical properties to TMP. In refiner bleaching with perborate is an 18 unique way to make bright pulp.
The following examples illustrate the effect of post-refiner bleaching of perborate pulp using TAEl~ as an activator.

2 Example 8 illustrates the effect of low temperature post-bleaching of TMP
obtaining by above refining process with two different oxidising systems. which are peroxide and TAED
4 acivated peroxide. During the bleaching process DTPA chelated pulp of about 10 g was placed in a plastic bag where bleach chemicals were added. Beside peroxide or TAED and 6 peroxide mixtures. other bleach chemicals were 3% Na-silicate. NaOH in an alkali peroxide ratio between 0.8 to 1Ø The pulps were bleached for 60 min at 40°C.
Hydrogen peroxide. Brightness.Peroxide consumed.% ISO gain wt% ISO% wt%

0 59.1 - 0 2 71.3 0.85 12.2 3 72.1 1.59 13.0 4 73.1 2.12 14.0 73.8 2.67 14.7 Activated peroxide* Brightness.Peroxide consumed.% ISO gain wt% of ISO% wt%
peroxide(molar ratio TAED :
peroxide = 0.3 :1) 2.0 71.9 - 12.8 3.0 ?4.2 - 15.1 4.0 75.3 - 16.2 5.0 75.7 I - 16.6 Results from the above Table indicates that refined pulp can achieve a brightness of 75% and above at 40°C. only by using activated bleaching process. This results show the unique advantage of using activated bleaching with a peroxide and TAED at a low bleaching 12 temperature. A clear two points gain in the brightness level indicates the advantage of using activated bleaching. In pulping industry it has a direct implication on the energy cost. The 14 improved bleaching efficiency of peroxide-TAED mixture can be attributed to in-situ generation of a strong oxidant during the bleaching process which improved the bleaching 16 efficiency.

18 Example 9 illustrates the effect of low temperature post-bleaching of in-refiner perborate bleached pulp with two different oxidising systems. which are peroxide and TAED acivated peroxide. During the bleaching process DTPA chelated pulp of about 10 g was placed in a plastic bag where bleach chemicals were added. Beside peroxide or TAED and peroxide 2 mixtures. other bleach chemicals were 3% Na-silicate. NaOH in an alkali peroxide ratio between 0.8 to 1Ø The pulps were bleached for 60 min at 40°C.
Hydragen peroxide.Brightness% ISO Perborate, wt% Brightness,% ISO
gain wt% ISO% gain %ISO

0 66.6 0 - - -3.5 70.1 3.5 10.8 68.7 2.1 Activated peroxide*Brightness% ISO Activated perborate*Brightness,% ISO
gain wt% of peroxide(molarISO% gain wt% of ISO%
ratio TAED : perborate(molar peroxide = ratio 0.3 :1 ) TAED : peroxide =

0.3 :1) 3.5% 72.1 5.5 10.8 71.2 4.6 Results from the above Table indicates that in-refiner perborate bleached pulp can achieve a 6 brightness of 72% and above at 40°C. only by using activated bleaching process. This results show the. unique advantage of using activated bleaching with a peroxide and TAED at a low 8 bleaching temperature. A clear two points gain in the brightness level indicates the advantage of using activated bleaching. In pulping industry it has a direct implication on the energy cost.
The improved bleaching efficiency of peroxide-TAED mixture can be attributed to in-situ generatian of a strong oxidant during the bleaching process which improved the bleaching 12 efficiency.
claim:
14 1. A process simultaneously refining wood pulp in a refiner to produce pulp with improved brightness mechanical properties comprising concurently introducing 16 chelating agent. bleach liquor an ood chips or pulp in one or more stages including a primary refiner at high temperatures achiev by increasing steam pressures and where 18 the bleach liquor comprises a solution of sodium per~e without any alkali or ex-situ generated peracetic acid from peroxide or perborate and TAE~th NaOH including other chemicals such as sodium silicate and magnesium suphate. said solut~having an alkaline pH of from pH about 10 to about 12 and wherein not more than about 1

Claims (22)

1. A process for simultaneously refining wood pulp in a refiner to produce pulp with improved brightness and mechanical properties comprising concurently introducing chelating agent. bleach liquor and wood chips or pulp in one or more stages including a primary refiner at high temperatures achieved by increasing steam pressures and where the bleach liquor comprises a solution of sodium perborate without any alkali or ex-situ generated peracetic acid from peroxide or perborate and TAED with NaOH
including other chemicals such as sodium silicate and magnesium suphate. said solution having an alkaline pH of from pH about 10 to about 12 and wherein not more than about 15% by weight of sodium perborate or weight equivalent amount of hydrogen peroxide and 0.3:1 molar ratio of TAED generated peracetic acid. based on the total weight of the dry pulp or wood chips. is added. and the pulp is discharged from the refiner at a pH less than 7.0 and whereby the primary refiner bleaching process produces a brightness gain of at least 16 ISO points in the refiner and improved the mechanical properties of pulp.
wherein the pulp obtained after refining is optionally subjected to another tower bleaching

2. A process as claimed in claim 1 where pulp leaving said pressurized refiner is subjected to further bleaching in a bleaching tower to produce pulp with ISO brightness 75 points and above in a single post bleaching stage comprising concurrent treatment of unbleached refined pulp or in-refiner bleached pulp with a chealting agent and then with a bleach liquor at a temperature not above 50°C and for a time period not exceeding 90 min. and where the bleach liquor comprises either a solution of sodium perborate and TAED with sodium silicate or a solution of hydrogen peroxide and TAED with NaOH and sodium silicate. said solution having an alkaline pH of from 9.6 to 11.2 and wherein not more than 12% by weight of sodium perborate and 8% of TAED or 5% of perxide and 10%
of TAED. based on the total weight of the pulp. is added and the pulp is discharged from the tower with a pH not below 7.0 and where tower bleaching of refiner pulp produced a pulp with ISO brightness above 75%.

3. A process in claim 1 where no alkali is added with sodium perborate or peracetic acid generated from sodium perborate and TAED.

4. A process in claim 2 where no alkali is added with sodium perborate or sodium perborate and TAED mixture.

5. A process in claim 1 wherein a perborate concentration more than 5% by weight of dry wood chips have been added to the refiner.

6. A process is claim 1 where peracetic acid has been generated ex-situ of the refiner by treating separately hydrogen peroxide with TAED or sodium perborate and TAED.

7. A process in claim 1 wherein a refiner temperature between 115 to 145°C. has been used by adjusting the steam pressure.

8. A process in claim 1 wherein the chelating agent is selected from the group comprising of ethylenediamine tetraacetic acid (EDTA). dipentamethylenetetramine pentaacetate (DTPA) and dipentamethylenetriaminephosphonates (DTPMPA).

9. A process in claim 1, wherein the liquor solution has a pH of about pH 9.8 to 12.

10. A simultaneous wood pulp refining and bleaching process which comprises the following steps of: feeding wood chips to a primary presurrized refiner and milling said wood chips at elevated temperature using steam pressure to produce a high concentration pulp;
feeding to said presurrized refiner, during milling, a bleach liquor of sodium perborate.
Sodium silicate and magnesium sulphate solution having a pH of from about 9.8 to about 12 and discharging the said high concentration pulp from the refiner at a pH
not above about 7.0 and optionally bleaching the said refined pulp in a bleaching tower at atmospheric pressure;

11. A process in claim 2 wherein the bleach liquor is a mixture of hydrogen peroxide and tetraacetylethylenediamine (TAED).

12. A process in claim 2 wherein the molar proportion of peroxide to TAED is from about 0.05 to about 0.5.

13. A process in claim 2. wherein the concentration of hydrogen peroxide is from about 2.0%
to about 5% by weight of dry pulp.

14. A process in claim 2 wherein the bleach liquor is a mixture of sodium perborate and tetraacetylethylenediamine (TAED).

15. A process in claim 2 wherein the molar proportion of peroxide to TAED is from about 0.05 to about 0.5.

16. A process in claim 2. wherein the concentration of sodium perborate is from about 6.0%
to about 15% by weight of dry pulp.

17. A process in claim 2. wherein no alkali has been added with perborate and TAED.

18. A process in claim 2. where the pH of the bleach solution is from about 9.8 to about 11.

19. A process in claim 2. wherein the bleaching temperature does not exceed above 50°C.

20. A process in claim 2. wherein the bleaching time does not exceed above 90 min.

21. A process in claim 2. wherein a chelating stage has been introduced before bleaching.

22. A process in claim 2. wherein the chelating agent is selected from the group comprising of ethylenediamine tetraacetic acid (EDTA). dipentamethylenetetramine pentaacetate (DTPA) and dipentamethylenetriaminephosphonates (DTPMPA).