CA2168290A1 - Method and borate-magnesium salt compositions for enhancing peroxide bleaching - Google Patents

Abstract

Pulp bleaching processes employing peroxides and/or oxygen are improved by usingbleaching additives, preferably before the application of the peroxide and/or oxygen.
The bleaching additives contain at least one alkali metal borate such as a sodium borate and a soluble magnesium compound such as a halide, preferably magnesium chloride.

Description

, METHOD AND BORATE-MAGNESIUM SALT COMPOSITIONS FOR
ENHANCING PEROXIDE BLEACHING

The present invention relates to the discovery of synergistic blends of ingredients and a process to enhance peroxide and oxygen and combinations of the two bleaçhing 10 processes used for the production of paper pulps and textiles. Ble~ching activity is enh~nce~l beyond the additive effects of the individual ingredients.

15 Ble~ching of lignocellulosic materials can be divided into lignin ret~ining and lignin removing ble~hing operations. In the case of ble~ing high yield pulps like Groundwood, Thermo-Mechanical Pulp and Semi-Chemical pulps, the objective is to brighten the pulp while all pulp components including lignin are retained as much as possible. This kind of ble~hinp is lignin retAining Common lignin ret~ining 20 bleaching agents used in the industry are alk~line hydrogen peroxide and sodium dithionite (hydrosulfite).

Hydrogen peroxide decG~ oses into oxygen and water with incleasillg pH, telllpc~ e, heavy metal concentrations, etc. The decomposition products, radicals 25 like HO- and HOO., lead to lower yields by oxidation and degradation of lignin and polyoses. Therefore, hydrogen peroxide is stabilized with sodium silicates and chelating agents when mechanical pulps (high yield pulps) are bleached.

The ble~ching effect is achieved mainly by the removal of conjugated double bonds 30 (chromophores), by oxidation with hydrogen peroxide (P), or reduction with hydrosulfite (Y). Other ble~ching chemicals more rarely used are FAS (Formamidine Sulfinic Acid), Borohydride (NaBH4), Sulfur dioxide (SO2), Peracetic acid, and Peroxomonoslllf~te under strong ~Ik~lin~ conditions.

P~ edLI-lent including electrophilic reagents such as elemental chlorine, chlorine dioxide, sodium chlorite and acid H2O2 increase the ble~ching efficiency of hydrogen 5 peroxide ble~ching In the case of bleaching chemical pulps like kraft pulp, sulfite pulps, NSSC, NSSC-AQ, soda, organosolv, and the like, that is to say with lignocellulosic material that has been subjected to delignifying treatments, ble~.~hing includes further lignin red~lcing 10 (delignifying) reactions. Ble~ching of chemical pulps is performed in one or more subsequent stages. Most common modern ble~ching sequences are CDEDED, 02CDEDED, O2DEopDEP, CDEoDEpD, CEHD, CEHDED, CEDED. (C chlorination, E caustic extraction, H alkaline hypochlorite, D chlorine dioxide, 2 oxygen delignification, CD chlorination substituted with chlorine dioxide, Eo pressurized 15 extraction with oxygen, Eop pressurized extraction with oxygen and peroxide and Ep extraction with peroxide.) In all of these ble~hing sequences, the first two stages are generally considered as the "delignification stages". The subsequent stages are called the "final ble~ching". This 20 terminology describes the main effects that can be seen by the specific chemical ~ G~ S

While in the first two stages the most appale.ll effect is the reduction of residual lignin, in the subsequent stages the most distinguishable effect is the increased 25 brightn~ss.

Hydrogen peroxide, oxygen, and combinations of the two ble~ching colllpounds have been used in ble~.~hing paper pulp and textiles for a number of years. Environmental pre~ e on chlorine based bleaching and the effect it has on effluent from the 30 m~nllf~ctllring process has accelerated the use of chlorine free ble~rhing processes to _, reduce the amount of dioxins and AOX, absorbable organic halides, in the effluent and bleached paper or textiles.

Oxygen ble~chin~ is contlucted under ~Ik~line pH conditions at elevated telllpel~ e 5 and pressure, with the process generating some peroxide ln-~i~ during the reaction.
Peroxide ble~ching is also conducted under ~Ik~line pH conditions, normally at elevated te.llpeldlure. Oxygen stages are being enhanced with the addition of peroxide. There is a trend in caustic extraction stages (wash out lignins) to pressurize the stage and add oxygen or peroxide and sometimes both to enh~n~e bleaching 10 p~ Ir~ llance. Ozone ble~ching is beginning to make an impact. All ofthese alternative methods are being installed or enh~nced in mills to allow the reduction or elimin~tion of the dependency on chlorine based stages.

It is well known that peroxide ble~ching compounds, particularly hydrogen peroxide, 15 require stabilization to prevent the rapid breakdown into water and oxygen inducefl by heavy metals. Iron, copper, and m~ng~n~se ions, either in process water or bound to the bleachable material, have a catalytic effect on the breakdown of the peroxide, especially at higher alkalinity levels. This results in a loss of peroxide and a lower ble~ching efficiency. ChPl~ting agents, such as EDTA (ethylene~ mine tetraacetic20 acid), DTPA (diethylenetriamine pent~cetic acid), gluconic acid, glucoheptanoic acid, tartaric acid, citric acid, polyphosphates, hydroxyalkanephosphonic acid, and ~mino~lk~nephosphQnic acids, along with their collespollding ~lkali metal salts, are well known to prevent the breakdown of peroxide by forming complexes with the metals, r~n~içring them harmless to the peroxide. Chelating agents have been used 25 directly in bleach liquor to stabilize the peroxide. Chelation or Q stages have also been used recently as a low pH washing stage in paper pulp ble~hing to remove metals from the pulp prior to peroxide, oxygen, or ozone ble~çhing Magnesium sulfate, m~nesium chloride, and magnesium oxide have a stabilizing 30 effect on the perhydroxyl ion formed in ~Ik~line peroxide ble~ching H2O2 + NaOH > Na+ + OOH + H2O

Magnesium salts also retard the depolym~ri7~tion of cellulose, which causes loss in strength, that can occur in oxygen or peroxide blç~ching stages. Alkali metal silicates 5 are also used in stabilizing peroxide bleach, but pose a significant risk in the formation of insoluble silicate scale later in the process.

U.S. Patent No. 4,938,842 discloses a peroxide ble~çlling process employing magnç~ium sulfate, sodium silicate and a chPl~ting agent.
U.S. Patent No. 3,716,447 discloses an ~Ik~line oxygen blç~rhing process in which a cellulose pulp is pre-treated with m~,~n~siurn chloride.

U.S. Patent No. 4,849,053 discloses a peroxide blç~ching process in which pulp is 15 pre-treated with stabilizing chemicals including magnesium salts and chelating agents such as EDTA.

U.S. Patent No. 4,619,663 discloses stabilizing compositions (and process) for peroxide textile bleaches comprising metal chelating agents (such as 20 diethylçnetri~minepent~cetic acid) and sodiurn tetraborate decahydrate.

U.S. Patent No. 4,128,495 discloses ble~ching/dete1~ent compositio~c compri~ing phthaloyl peroxide, preferably clesP~.~;Li~ by contact with a diluent such as m~ ;ulll sulfate, and optionally contain sodium pell,o~ e. U.S. Patent No.
25 4,154,695 discloses similar compositions employing diacyl peroxides.

U.S. Patent No. 2,820,690 discloses a peroxide textile ble~ching process in which peroxide is stabilized with orthophosphate plus m~gnesium ions and the solution is optionally buffered with alkali metal borate.

s U.S. Patent No. 3,332,882 discloses a process for activating a peroxygen ble?~çhing compound selected from group including m~gnesium peroxide, metal perborates et al., the activator being a triacyl triazine.

5 U.S. Patent No. 4,086,175 discloses peroxide bleach compositions with a cyanamide activator, m~gn~ium compound and burr~ling agent. The bleach can be a perborate.

U.S. Patent No. 3,384,533 discloses processes for bleaching cellulose pulps withoxygen in ~lk~line media, wherein the pulp is treated with a single catalyst selected 10 from a group including m~g,.~ c~l.onate and alkali metal borates.

There is a continual dçm~nrl for improved chlorine-free bleaclling compositions and processes, particularly those which produce increased brightness in delignified pulps.

An object of this~invention is to provide improved blearhing compositions and processes involving peroxides. Another object is to stabilize the hydrogen peroxide in such bleaching compositions by preventing the catalytic effect of heavy metals which 20 may be present. A further object of the invention is to inc~ease the bleaching effects of hydrogen peroxide.

In accordance with the present invention, these and other objects of the invention are achieved by employing small but effective amounts of a peroxide bleach additive 25 composition comprising at least one alkali metal borate such as sodiurn borate in conjunction with a m~gn~sium salt such as a halide.

These ingredients are used in proportions which are effective to produce synergistic bleaching effects, i.e. effects which are qualitatively or qn~ntit~tively greater than 30 would be expected from the additive effects of the individual ingredients. Although the ranges of p~GI lions may vary with total dosage, the m~t~ri~l to be ble~rhe~l or operation conditions, the propollions can range from about 2:8 to about 8:2.
Preferably both ingredients are added to the pulp or other material to be bleached before the introduction of the peroxide(s).
s Thus, the invention further encol,lpasses a ble~ching process wherein an additive comprising at least one alkali metal borate and at least one m~n~cium salt is added to a pulp or other material to be bleached, then adding at least one peroxide, said alkali metal borate and said m~gnesiurn salt being present in quantities and p~ ol lions 10 effective to produce synergistic ble~hing effects.

Synergism, activitv beyond normal e~ecl~lion with blends of ingredients, has been found with combinations of sodium borates and m~gn~cium salts, preferably chloride.
Each individual colllpollent enhances the ble~ching ability of hydrogen peroxide, 15 either through stabilization of the perhydroxyl ion or chelation of heavy metals. The results with the mixtures are beyond e~e~ lion of the activity of the individualcomponents. Not all ratios of these active ingredients show synergism. Some ratios of actives are merely additive and others are actually antagonistic, where performance is subst~nti~lly below that expected.
These synergistic combinations are further enh~noed by the process of adding said i~lules to paper pulp prior to the addition of hydrogen peroxide or of an ~lk~line peroxide liquor where the llli~ e is in the pulp during the ble~ching process.
Improved bleach response is seen with pulp addition as compared to addition of the 25 mixture to the bleach liquor. Enhanced p~ .~ollllance is measured by increased in pulp brightness or reductions in KAPPA number of the pulp. Activity is also better when the mixtures are added to a pulp prior to the addition of bleach rather than addition in a chelation or Q stage, with the intent of washing heavy metals out of the fiber prior to bleaching. The reason for this difference appears to relate to the available m~gnesium 30 concentration during the peroxid~ ble~chin~ process. Magnesium stabilizes peroxide 21682gO

only when it is soluble.

Other objects and advantages of the invention will be ap~a~ellt from perusal of the following detailed description, including the figure and the appended claims.

The invention will be further understood with reference to the acco,llpa~ying drawing, wherein:
FIG. 1 is a plot illustrating the relative effects upon bri~htn~ss of bleached pulp of various quantities and proportions of the active ingredients.

Lignocellulosic m~teri~lc such as untreated wood, wood chips and annual plants like corn stalks, wheat straw, kenaf and the like can be used in accordance with the invention. Especially suitable is material that has been defiberized in a mechanical, chemical processes or a combination of mechanical and chemical processes such as20 GW, TMP, CTMP, kraft pulp, sulfite, pulp, soda pulp, NSSC, organosolv and the like.
It is the kind of m~t~ri~l in an aqueous ~u~pe~-~;on, hereinafter referred to a pulp, which is treated in accordal~ce with the present invention with the specified additives and subsequently in a follow on stage subjected to an oxygen and/or peroxide stage.
In addition to such pulp, the invention can be applied to any bleachable fibrous25 cellulosic m~t~r~

The present invention can be considered as providing a core process forrned of two stages in a sequence; namely, a step of tre~tmçnt with ble~-hing additives and afollow on stage of oxygen and/or peroxide tre~tment This core sequence can be 30 systematically leplesellled as X--OX; viz, the i.x~ symbolizing the additives step and - 21682!~0 "OX" syrnbolizing the oxygen/peroxide step. The core sequence as defined herein can be followed by one or more additional conventional pulp h~n~ling stages such as washing and additional oxidation, peroxide tre~tm~nt ,teps as well as steps involving treatment with bleaclling additives. Sirnilarly, the core sequence can be preceded by 5 one or more conventional steps such as those mentioned above.

The core sequence, X--OX, should not be hlte.,.lpted by a washing cycle. Et is p[efc.led that the order of the core sequence be X--OX; that is, the additive and pulp followed by at least one oxidation stage (oxygen and/or peroxide). The hl,poll~lce of 10 having the additive treatment precede an OX step resides in the fact that subsequent delignification/oxidation results are unexpectedly enhanced while ret~ining desirable viscosity pr~,l,c.~ies.

The scope of the variations in the overall methods of treating pulp including the 2-15 stage sequence of the invention is very wide and can be illustrated by the following possible r~lesP~ ive sequences.

As used herein, the symbol R rel)resell~ unbleached, brown stock, A is a transition metal removing tre~tm~nt P is any peroxide compound treatment step, O is any 20 oxygen and X--OX is the core process of the invention:

R-X-OX
R-A-X-OX
R-O-X-OX
R-A-O-X-OX
R-A-X-OX-X-OX
R-P-X-OX
R-A-P-X-OX

-g The above is merely illustrative and is not considered limiting.

The conci~t~ncy of the pulp in the ble~ing additive lre~ n~ step can range from 0.01% to 60%, preferably from 5% to 2S%.

It is customary that a chemical base such as NaOH, MgO, or other suitable ~lk~line material be added to the pulp in order to control the acidity at a desired pH level. Any suitable ~lk~line material can be used to control acidity provided it does not adversely effect the process or product. Any sequence of chemical addition of pH controlling 10 alkali and additives in the first step, including the cimnlt~neous addition, can be carried out. The starting pH is not narrowly critical. The starting pH can be 1 to 11.
Preferably, the starting pH of the pulp for the X stage (after addition of caustic and addition of peroxomonosulfuric acid and/or its salts) is between 7 and 11. It is to be noted that the pH profile over the course of the X stage has been clet~rmin~od to be lS subject to wide variation and is not narrowly critical.

Trials have shown that the X-stage tre~tm~nt (additive stage) is very little affected by tel,lpelal~e; that is, the reaction is not very telll~ dlllre depenrlent Thus, the ble~c~ling additive treatment step is effective at low tenlpelalllres such as S degrees C.
20 as well as at ltl~cldlures of up to 100 degrees C. Preferable tellll)~-dlules forthe additive tre~ nt are in the range of 40 degrees C. to 70 degrees C.

Depending on telllp~ .~lure, pH and chernical charge the residence time le~luiled is typically between 1 second up to 10 hours, frequently I minute to 2 hours, although 25 the upper time limit is not critical. Thus, for example the retention time varies as to how long the pulp takes to pass through the conventional ble~cl-ing tower, high intensity mixing zone or the like. Some parts of the pulp may move through rapidly;
e.g. 1/2 hour, while other parts of the pulp may take 24 hours or longer to passthrough. Accordingly, the process of the invention is not dependent on a narrow 30 range of time parameters. Uniform distribution of the additive is hll~. .ati~e to treat all of the fiber to obtain the best results. Addition of dilution water with the additive prior to addition to pulp followed by high shear mixing, such as in a centrifugal stock pump, gives the best results.

5 It is to be noted that the ble~l~hing additive stage can be applied to any kind of treated (bleached) or untreated (e.g. brown stock) pulp. Advantageously, one or more heavy metal and organic co.~ --in~ s elimin~ting process steps can be initially carried out at plet~e~ ent to favorably impact the delignification efficiency of the aforesaid stage.
Ple..~ e conditions for the X-stage can vary for this process as is conventional in pulp operations. Typically, from atmosphere to 0.5 MPa, is suitable.

The tre~tment stage in which ble~c-hing additives are used can be dç~ign~ted by the 15 symbol "X". The new process which is the subject of this invention fea~ s a combined application of the X stage with any other kind of oxygen and/or peroxide stage, generally~described by the symbol (OX). The new process can be abbreviated by "X--(OX)" whereby "(OX)" can stand for O (oxygen dçlignification), Eo, Ep, Eop, Eoh (extraction stages reinforced with oxygen, peroxide, oxygen and peroxide as well 20 as oxygen and hypochlorite respectively), and P (peroxide stage). Although hypochlorite has been mentioned as a possible optional stage that can be used incombination with the X--OX process of the invention after the OX stage, efforts are being made in the industry to elimin~te the use of chlorine chemicals whenever possible.
The process of the invention can be used repeatedly and in combination with the ble~ching stages commonly used in order to delignify and bleach to required levels.
The two tre~tment~, step X and step (OX) should be con~lcte~1 without intermediate washmg.

-It is an object of this invention to employ at least two bleAclling additives A and B in quantities and proportions effective to produce synergistic bleAclling effects that is, to produce h1c~eases in brightn~-cc or other measures of bleAchin~ effects which are more than the combined expected effects of the sepa~dle additives. The optimum total S quantities will vary with the type of pulp or other stock to be bleached, op~ l~ling conditions, etc. but generally the total lu~lti~y will be a small but e~.;live amount in the range of fron about 0.02 to 0.4 weight percent of the pulp. Preferably, the amount ranges from 0.04 to 0.12 weight percent. For a given total quantity the propollions are those which produce a synergistic bleAching effect, preferably IllAxilll;~ g said 10 effect. Such proportions of A:B can range from about 1:1 to 9:1, preferably from about 2:8 to 8:2, and most preferably from about 3:7 to about 7:3.

Additive A is preferably an alkali metal borate such as sodium borate, with lithium and potassium borates also being useful. Ammonium borates can be used if there are 15 no subsequent chlorine-based stages. Various borate components such as sodium metaborate and s ~dium tetraborate, Na2B4O~; potassium pentaborate, K4B50~o, and ammonium borate can be used.

Additive B is a soluble mAgn~sium compound, preferably a halide such as mAg~ium 20 chloride. Magnesium fluoride or bromide may also be used, as well as the sulfate, oxide, hydroxide. carbonate, nitrate and citrate.

FX~MPLES

25 The invention is further illustrated by the following non-limiting examples.

r.AROR~TORY MFTHODS

All laboldtoly bl~aching tests were run on a softwood kraft pulp, obtained from a mill 30 in the southern United States, which had been partially bleached through an oxygen - 2l6s2sn -deligrufication stage followed by a chlorine dioxide stage. All samples were taken from a single batch of pulp. Weighed pulp samples were treated with the appropliate dosage of the synergistic bleach enh~n~çr mixtures of the base m~t~ , mixed at high shear for good distribution, caustic (NaOH at 2.20% on 100% active basis) and 5 peroxide (H2O2 at 1.5% on 100% active basis) added, mixed at high shear again for good distribution, sealed in a polyethylene bag, and placed in an ultrasonic bath at COnSl;ln~ te.l.p~ re for continuous mixing throughout the ble~ching process. Tests were run at 80 degrees C for 60 minlltes The samples were then removed from the bath, and 3.0 g h~n~l~heets were pr~ paled from the pulp using a British sheet mold.
10 The hand sheets were pressed according to standard TAPPI methods and air dried ov~rnight Brightneec measurements on the fini~hçd h~n-lch~ets were determin~d on an Elrepho 2000 Datacolor system. Bri~htn~ was measured at a wavelength of 457nm and is 15 reported in all cases as % ISO bri~htness The reported brightnPss value is an average of 5 replicates on each sheet. The entire laboratory process is quite reproducible with the standard deviation on 6 replicates of the bleaçlling and measurement process at 0.25% ISO measured to two decimal places.

20 This labo.~lo.~ met_od has been shown to produce excellent correlation to actual results in mill conditions with the same chemical dosages.

SYNF.l~GI~M CALCULATION

25 Samples of the combinations were tested in the following ratios of component A to component B: 0:10, 1:9, 3:7, 5:5, 7:3, 9:1, 10:0. The total active solids content was kept con~ at each indicated dosage (0.02% to 0.16% by weight of dry fiber) in the TABLES.

- 21682!~
-NOM~CT ~TU~F.

3:7 at 0.02% means in this ~1iccuccion that a total of 0.02% active solids (excludes waters of hydration) are used to treat the pulp, and components A and B are combined 5 in a ratio of 3 parts of A to 7 parts of B. The brightnPss gains provided by co~llponellt A alone (10:0) and col.lponellt B alone (0:10) at 0.02% active solids are used as rer..cllce points to determine if synergism between actives is genuine.

Gain(actual) - Flopollional Gain(Coll.polle.,l A) - Proportional Gain(Col~,pol1elll B) =
10 Di~.ellce from Expected.

Gain(actual) - 30% Gain(Component A) - 70% Gain(Coll.ponelll B)= Diffèrence fromExpected.

15 If the difference from expected was positive, the synergism between actives was considered genuine and the performance better than expected. If the di~lellce was zero, the performance was merely additive. If the dirrelellce was a negative number, there was antagonism bclweell the actives and pclro~lnance was worse than expected.

20 Table I sets forth the p~l)ollions of sodium borate (A) and m~.~P~ l chloride (B) employed in ~lcces-si /e trials at various dosage levels. Each propol lion ratio is leci~ted a numbered example, with letters ~ccignPd to each dosage level for that pro~llional ratio. The measured brightne~cs gains for these examples are also pre3ellted in Table I. Table II prcsellls the results of calculations to ~et~rmine the 25 brightness difference which each example leple3ellts in co~..p~ison to the expected additive effects of the sodiurn borate and m~)esiu~ chloride.

TABLE, 1 .

C D 8 ~ C D R
~1~ ~ O~lp. Il.lp. Il.lp. 118~ ~rlp.Il-l~h ~hll~ ~l~hb Ibl~h a~ 11~1~ RdJh N~ Iblb 0.~12S ~.04S h~ O.~S ~d. O.llS ~ o l-S Ikl. O I~S Ad. OD~S Ad. 01~45 A~L a~ 0 I~S

IblO ~.~ 70~ n 70 ~ 71.7 1.4 IA 4.1 1~ 44J

S~b:U~ ~ ~.7 aJ aJ a.~ 70~ 41 I.~ 1.7 ~.4 Sd~ ~ ~ ~:7 ~19J 69J 7~4 n.2 71.2 2 ~ W ~- 4~ 4J
4 50~ ~b:~ 5 ~,~ 70~ 71J 71.4 71.1 1~ 3.~ 4. 4~ 4J
5 s~ ~ ~P~ 7~ 70~ 71.5 n~ n.- I ~.4 4.~ ~.7 So~ 4~ aA aA a.~ .2 1.7 1.7 I.~ 11 2~
7 s~ ~b~ l~o a.l 69.~ .4 .5 l.l 2.7 1.

TABLE II

A C D tl A O C D
11~1~ 1~_ ~ hh~l a~ ~1 a~- ~ ~.d_l ~ ~d~l 1~ Dilt. tr_ * W. t~ S Dltt. t~ S DUt. t~ S Dltt. t~o Y~ 0 02S hL ~ 121~ 102% ~L 0.04s ~L o~S ~ 0.12S ~kL 0.16S
Qoll~
QlO 0 0 0 0 0 OS OS OS OS OS
~ 1:7 ~IA -IS .2 ~IA ~I.I IUS -~65 -~lS ~49S -24S
SD~ 7 ~ ,Q4 ~o.2~.4 0,5IWS ~I~S ~4S '- 4 S 12S
4 Sd~ ~p, 5:5 ~.~ Q4 ~,~ 2 I.l l~3S 14S 4 S OS 34S
Sdl~ 7,3~.7 ~.4 I.~ 3.113 S l~S ~IS l-~S 123S
50~ ~ :1 0.5 -I.l 4.- 0 0.43 .3S~3~S ~31S 24S S
~ U~ ~Q,~ o o o o o OS OS OS OS OS

While the present invention has been set forth in terms of specific embodiments thereof, it will be understood in view of the instant disclosure, that n~ ous variations upon the invention are now enabled to those skilled in the art, whichvariations yet reside within the scope of the present te?ching Accordingly, the 5 invention is to be broadly construed and limited only by the scope and spirit of the claims now appended thereto.

Claims (12)

WHAT IS CLAIMED IS:

1. An improved pulp bleaching process wherein bleaching additives comprising at least one alkali metal or ammonium borate and at least one magnesium salt areadded to the pulp in advance of its treatment with at least one bleach selected from peroxides and oxygen, said additives being provided in quantities and proportions effective to produce synergistic bleaching effects.

2. The process of Claim 1 wherein said borate is a sodium borate and said magnesium salt is a halide.

3. The process of Claim 2 wherein said magnesium halide is a chloride.

4. The process of Claim 1 wherein said borate is an ammonium borate.

5. The process of Claim 1 wherein the total quantity of said additives is in therange of from about 0.02 to about 0.4 weight percent of said pulp.

6. The process of Claim 5 wherein said total quantity of additives ranges from 0.04 to 0.12 weight percent of said pulp.

7. The process of Claim 1 wherein the ratio of said additives is in the range offrom about 1:9 to about 9:1.

8. The process of Claim 7 wherein the ratio of said additives is in the range offrom about 2:8 to about 8:2.

9. A pulp bleaching additive composition for a process of bleaching pulp with atleast one bleach selected from peroxides and oxygen, said composition consistingessentially of an alkali metal or ammonium borate and a magnesium halide in proportions in the range of from about 2:8 to about 8:2.

10. The additive composition of Claim 9 wherein said alkali metal borate is a sodium borate and said magnesium halide is magnesium chloride.

11. The additive composition of Claim 9 wherein said borate is an ammonium borate.

12. The additive composition of Claim 9 wherein the ratio of the additive components is in the range of from about 3:7 to about 7:3.