CA1129161A - Delignification and bleaching process and solution for lignocellulosic pulp with peroxide in the presence of metal additives - Google Patents
Delignification and bleaching process and solution for lignocellulosic pulp with peroxide in the presence of metal additivesInfo
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- CA1129161A CA1129161A CA319,915A CA319915A CA1129161A CA 1129161 A CA1129161 A CA 1129161A CA 319915 A CA319915 A CA 319915A CA 1129161 A CA1129161 A CA 1129161A
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Abstract
Abstract An improved process and solution for delignifying and brightening lignocellulosic pulp. The solution comprises from about 0.1 to about 5% by weight, based on oven-dried pulp of a peroxide and about 0.5 to about 20 mole percent, based on the moles of peroxide, of a metal ion selected from the group consisting of tungsten, molybdenum, chromium, osmium and selenium. The preferred process of this invention com-prises contacting a lignocellulosic pulp with about 0.5 to about 20 mole percent, based on moles of peroxide, of a metal ion selected from the group consisting of tungsten, molybdenum, chromium, osmium and selenium at a temperature below about 50°C;
adjusting the pH of the resultant pulp slurry to between about 1 and about 7; adding 0.1 to about 5% by weight based on oven-dried pulp of peroxide and effecting pulp delignification and brightening at about 30°C to about 100°C until most of the peroxide has been consumed.
adjusting the pH of the resultant pulp slurry to between about 1 and about 7; adding 0.1 to about 5% by weight based on oven-dried pulp of peroxide and effecting pulp delignification and brightening at about 30°C to about 100°C until most of the peroxide has been consumed.
Description
Background of the Invention This invention r~lates to an improved process for delignifying and brightening lignocellulosic pulp. More particularly, the invention i8 direc~ed at a peroxide-based delignifying and bleaching solution and process for ligno-cellulosic pulp. In this process a lignocellulosic pulp is contacted at a temperature below about 50C with a solution of metal ions selected from the group consisting of tungsten, molybdenum, chromium, osmium and selenium. The pH of the resultant slurry is adjusted to between about 1-7. A peroxide is added and pulp delignification and brightening effected at 30-100C until most of the peroxide has been consumed. The process affords pulp delignification and brightening with minimal destruction of the cellulosic portion of the pulp.
Wood is composed of two main parts -- a fibrous carbohydrate or cellulosic portion and a non-fibrous portion comprising a complex chemical, commonly referred to as lignin.
For use in paper-making processes, wood must first be reduced to pulp, which can be defined as wood fibers capable of being slurried or suspended and then deposited as a screen to form a sheet. The methods employed to accomplish this pulping usually involve either physical or chemical treatment of the wood or some combination of the two processes to alter its chemical form and to give desired paper properties.
In mechanical pulping, the wood is physically ground to a high-yield, lignin retained pulp, most often referred to as groundwood pulp. In chemical pulping, the wood chips are digested with a chemical solution to solubilize a portion of the lignin and effect its removal. The more usual of these " ~
~lZ9~6~
digestive procedures are the sulfi~e, sulfate or Kraft, soda and modified sulfite processes.
After the wood has been digested or mechanically ground, the resulting material is generally a darkly colored cellulosic fiber. The dark color is attributable to the fact that not all of the lignin has been removed during digestion and none has been removed during mechanical pro~
cessing. This dark pulp is commonly referred to as unbleached pulp. Unbleached pulp may pass directly to the paper ma~ing operation only if paper color is unimportant.
More usually, unbleached lignocellulosic pulps are bleached or brightened to a brightness consistent with the planned utilization o~ the pulp, brightness being a measure of pulp reflectivity under standardized conditions. Pulp bleaching is most often a multi-stage process employing various chemicals to remove or alter the lignin of the lignocellulosic pulp such that the resultant pulp is no longer light absorbing or dark in color.
Two classes of compounds generally find use as lignocellulosic brighteners, namely reducing agents and oxidizing agents. Common reducing agents include sulfurous acids, hydrosulphites, borohydrides, amine boranes, and bi-sulfites. Common oxidizing agents include chlorine-based compound~, peroxides, ozone, oxygen, peracids, permaganates and chromates.
~he ready availability and environmentally appeal-ing character of peroxides have made them increasingly popular in recent years as bleachants in the paper-making industry. Such peroxides are used in the "lignin retaining"
bleaching of groundwood and other high yield pulps to provide
Wood is composed of two main parts -- a fibrous carbohydrate or cellulosic portion and a non-fibrous portion comprising a complex chemical, commonly referred to as lignin.
For use in paper-making processes, wood must first be reduced to pulp, which can be defined as wood fibers capable of being slurried or suspended and then deposited as a screen to form a sheet. The methods employed to accomplish this pulping usually involve either physical or chemical treatment of the wood or some combination of the two processes to alter its chemical form and to give desired paper properties.
In mechanical pulping, the wood is physically ground to a high-yield, lignin retained pulp, most often referred to as groundwood pulp. In chemical pulping, the wood chips are digested with a chemical solution to solubilize a portion of the lignin and effect its removal. The more usual of these " ~
~lZ9~6~
digestive procedures are the sulfi~e, sulfate or Kraft, soda and modified sulfite processes.
After the wood has been digested or mechanically ground, the resulting material is generally a darkly colored cellulosic fiber. The dark color is attributable to the fact that not all of the lignin has been removed during digestion and none has been removed during mechanical pro~
cessing. This dark pulp is commonly referred to as unbleached pulp. Unbleached pulp may pass directly to the paper ma~ing operation only if paper color is unimportant.
More usually, unbleached lignocellulosic pulps are bleached or brightened to a brightness consistent with the planned utilization o~ the pulp, brightness being a measure of pulp reflectivity under standardized conditions. Pulp bleaching is most often a multi-stage process employing various chemicals to remove or alter the lignin of the lignocellulosic pulp such that the resultant pulp is no longer light absorbing or dark in color.
Two classes of compounds generally find use as lignocellulosic brighteners, namely reducing agents and oxidizing agents. Common reducing agents include sulfurous acids, hydrosulphites, borohydrides, amine boranes, and bi-sulfites. Common oxidizing agents include chlorine-based compound~, peroxides, ozone, oxygen, peracids, permaganates and chromates.
~he ready availability and environmentally appeal-ing character of peroxides have made them increasingly popular in recent years as bleachants in the paper-making industry. Such peroxides are used in the "lignin retaining"
bleaching of groundwood and other high yield pulps to provide
2~
~Z9161 substantial brightness gains, but not the high brightness attainable with chemical pulps. Such processes are described, for example~ in Hook, "Peroxide Bleaching Opens New Potentials For Groundwood Pulps", Pulp ~ Paper International, 45-48 (June 1975), Vartiainen, "Vtilization of Peroxide in Pulp Bleaching", Papper Och Tra, 51, 277-284 (1969~, U.S. Patent Nos. 2,187,016, 3,023,140, and 2,251,731 and Canadian Patent No. 970,111. Additionally, peroxides are employed as a "capping" or super-bleach stage at the end of common multi-stage bleaching processes for chemical pulps. Exemplifying these processes are those described in Canadian Patent Nos.
966,604 and 970,111, U.S. Patent Nos. 3,193,445 and 3,462,344 and Vartiainen, "Utilization of Peroxide in Pulp Bleaching", PapPer Och Tra, 51, 277-284 (1969), Delattre, "Hydrogen Per-oxide as a Bleaching Agent for Kraft Pulps", Papper Och Tra, 117-127 (1971), Hartler et al., "Peroxide Bleaching of Kraft Pulps", Tappi, 43, 806-813 (1960?, Christensen, "Bleaching Sulphate Pulp with Hydrogen Peroxide", Pulp and Paper Magazine of Canada, 62-66 (1971), Christensen, "Bleaching of Sulphate Pulps with Hydrogen Peroxide", Norsk S~ogindustri, 268-271 (October, 1973), and Mlakar & Peltonen, "Peroxide in the Semi~
bleaching of Kraft Pulp", Papper Och Tra, 11, 629-638 (1968).
However, peroxide bleaching by itself is not able to e~fect sufficient pulp bleaching for most paper-making re~uirements. This is particularly true when the peroxide is employed in an internal stage in a multi-stage bleaching process after initial ozone or oxygen bleaching. PrevioUs variations of the conditions for peroxide-pulp contact have either not remedied this inferior bleaching characteristic or 80 degraded the cellulosic portion of the pulp that it was no longer useful for paper-making processes.
llZ9161 The failure of peroxide bleachants in internal or prebleaching stages is apparently due to the inability of peroxide to delignify lignocellulosic pulps sufficiently to remove the light-absorbin~, dark lignin components and yet" maintain that resultant pulp viscosity and strength necessary for subsequent paper-making processes.
For example, at more alkaline pH's, i.e. 9-11, per-oxide bleaching is usually conducted in the presence of sodium silicate and magnesium sulfate stabilizers to avoid peroxide decomposition by heavy metal contaminants. But, under such conditions substantially no delignification is obtained. At higher pH's auto-decomposition of the peroxide beco~es exces-sive so as economically to disadvantage the process.
Conversely, at acid pH's, i.e. 2-7, the peroxide reaction with the lignin chromophores of the pulp is sluggish and often accompanied by severe viscosity losses. Rapson, "The Role of pH in Bleaching Pulp", Tappi, 39, 284-294 (1956) and U.S. Patent No. 2,112,116. Similar pulp viscosity and strength reductions are adduced by the addition of metals such ~s manganese,cobalt, iron, copper and aluminum to oxidizing bleaching agents such as peroxide. These special oxidants have bee~ used to good advantage in U.S. Patent Nos. 2,975,169 and ~,3~,527 to produce pulp, which while unsuitable for paper ~an~facture, is well constituted for the production of viscose rayoh or other cellulose-like derivatives.
Other additives are likewise ineffective in improving the brightness gain achievable with peroxide solutions. For example, U.S. Patent Nos. 2,119,519 and 2,249,646 disclose the additi~n of titanium having a valence of less than four to ~0 ~idlzlng bleaching agents, including peroxides. This addition is said to activate the alkaline bleaching process so as to reduce both the treatment time and oxidant concentration needed to obtain a given level of brightness gain. Such reductions advantageously benefit pulp strength since vis-cosity losses and other pulp degradation are minimized dueto the abbreviated oxidant-pulp content. However, even this improved process adduces no significant pulp delignification.
Peracids are one class of oxidizing bleaching agents which effect selective lignin removal. These reagents are essentially derivatives of peroxides. However, they are quite expensive and do not lend themselves to easy recovery and regeneration.
Summar of the Invention Y
It is accordingly an object of this invention to provide a process and solution whereby peroxide may be employed to delignify selectively and brighten lignocellulosic pulp yet retard that pulp viscosity loss and degradation caused by cellulose oxidation.
Another object o this invention is to provide a peroxide based delignifying process and solution wherein the conditions of delignification avoid significant auto-decom-posltion of peroxide so as to require none of the conventional peroxide stabilizers.
A further object of this invention is to provide a pulp delignification and bleaching process and solution where-in the spent bleachants may be recovered in conventional recovery systems without scaling.
These and other additional objects and advantages of this invention, apparent from the detailed description and claims which follow, are accomplished in accordance with this invention by a pulp delignifyinc3 and brightening solution com-prising from about 0.1 to about 5% by weight, based on O.D.
5.
(oven-dried) pulp, of a peroxide and about 0.5 to about 20 mole - percent, based on moles of peroxide, of a metal ion selected from the group consisting of tungsten, molybdenum, chromium, osmium and selenium, and sufficient acid to maintain the pH
of the solution between about 1 and about 7. More preferably, the acid is added subsequent to pulp-metal admixture.
In the preferred process of this invention, a ligno-cellulosic pulp is contacted at a temperature below about 50C
with the metal solution. The pH of the resultant slurry is adjusted to between about 1 and about 7 and peroxide is added to the slurry. Pulp delignification and brightening are usually effected at about 30C to about 100C until most of the peroxide has been consumed.
Detailed Description of the Invention This invention relates to a peroxide-based solution and process for selectively delignifying and brightening lignocellulosic pulp. The solution and process avoid attack on the cellulosic portion of the pulp so as substantially to preclude degradation of the pulp and reduction of its viscosity to levels below those acceptable for paper-making processes.
According to this invention, wood pulps and other lignocellu-losic pulps may be utilized to good advantage. By way of exam-ple, preferred lignocellulosic pulps include those wood pulps digested by the well-known sulfite, sulfate or Kraft, soda and modified sulfite processes. The process is particularly use-ful for the bleaching of hardwood Kraft pulps especially those which have been semi-bleached with conventional oxygen bleach-ants. Such semi-bleaching is disclosed, for example, in Soteland, "Bleaching of Chemical Pulps with Oxygen and Ozone", Pulp and Paper Magazine of Canada, 75, T153-158 (1974).
The novel delignifying and brightening solution of this invention comprises a peroxide, said peroxide being pre-sent in an amount effective for bleaching and delignifying 1 6.
~W, lignocellulosic pulps, this effective amount usually being from about 0.1 to about 5~ by weight, based on O.D. pulp, about 0.5 to about 20 mole percent, based on moles of per-oxide, of a metal ion selected from the group consisting of tungsten, molybdenum, chromium, osmium and selenium, and sufficient acid to maintain the pH of the solution between about 1 and about 7. More preferably the acid is added sub-sequent to pulp-metal admixture.
In the preferred process of this invention, a ligno-cellulosic pulp is contacted at a temperature below about 50Cwith such metal solution. The pH of the resultant slurry is adjusted to between about 1 and about 7 and peroxide added to the slurry. Pulp delignification and brightening are effected at about 30C to about 100C until most of the peroxide has been consumed. However, it is to be understood that other sequences which avoid pulp-peroxide contact at low pH and high temperature in the absence of metal ion are within the scope of the invention.
The methodology for carrying out this process in the pulp slurry is conventional in the art. Most commonly, slurries of consistency from about 1% to about 50%, and more preferably 10~ to 35% are employed. The reaction is conducted between 30C and 100C and more preferably between 50C and 80C. The time of reaction is not critical being primarily controlled by the amount of peroxide remaining in the pulp slurry and its effective reaction rate. Usually a reaction time of from 0.5 to about 4 hours is sufficient to consume most of the peroxide, the choice of an actual reaction time being dependent on the pH
and temperature. For example short reaction times are favored in slurries of low pH and high temperature.
The peroxide used in the bleaching solution and process of this invention may be selected from any number of conventional sources. These include inorganic peroxides, such as sodium peroxide, organic peroxides and aqueous solutions 7.
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of hydrogen peroxide itself, the last being preferred. The chosen peroxide or a mixture thereof may be used in an about 0.5 to about 5% by weight, based on O.D. pulp, but preferably is present between about 0.5 and about 2% by weight.
The peroxide stabilizers taught in the art to slow deleterious auto-decomposition of peroxide during bleaching are not needed in the slurries of this invention. Instead, the metallic activators of this invention appear both to stabilize the peroxide and to activate its selective oxidative attack on the lignin of the lignocellulosic pulp.
The metal containing additives, whose metallic por-tions are selected from the group consisting of tungsten, molyb-denum, chromium, osmium and selenium, may be selected from a wide range of such compounds. The selected compounds need only be able to provide the desired metallic ions in solution and not interfere with the pulp brightening process to be acceptable. Examples of useful compounds of this type in-clude osmium tetroxide, selenium dioxide, sodium tungstate, sodium molybdate and sodium dichromate. More preferably, the metallic portion is tungsten and most preferably the tungsten iB in the plus six oxidation state, since tungsten ions of lower oxidation states are more wasteful of peroxide.
Although it is not necessary that the chosen additive be totally soluble under the conditions of this invention, such dissolution is more usual and preferred.
While not wishing to be bound by theory, it is thought that a 2:1 peroxo-tungstate or other metal-peroxide complex functions as the lignin selective oxidant. Its action is catalytic since the metal is returned to the reser-voir of free peroxide to reform the reactive complex. At stoichiometric concentrations of metal ion, the competing metal induced peroxide decomposition appears to slow, the metal complex apparently having a stabilizing effect on the pulp.
As the p~l o~ the slurry is lowered from 6 to about 2, pulp brightening and delignification seem to improve, but pulp viscosity losses become significant at below about pH 3. It is theorized that such improvement is a function of a favored formation of peroxo-metal complex at low pH. However, elevated temperatures may also force this formation at higher pH's. For example, pulp contact at pH 5 and 77C is very effective in delignification of lignocellulosic pulp. Where tungsten is the metal ion employed such conditions are preferred.
Following the selective delignification and brightening of this invention, the lignocellulosic pulp may be washed in a conventional manner or additionally treated in any number of well~recognized paper-making pro-cesses. The filtrate from such washing is amenable to standard recovery operations without scaling. Further, any metal residue remaining on the pulp fiber may serve as a pigment to build opacity.
Due to the relatively high cost of even the small amounts of metal ions used in this invention, it is important that thorough washing and a high degree of recycle be employed to improve the economics of the process of this invention.
In order to describe the present invention so that it may be more clearly understoo~, the following examples are set forth. These examples are primarily for the purpose of llZ9161 illustration and any specific enumeration therein should not be construed as a limitation on the broader concepts of this invention.
Example I
A hardwood Ticonderoga mill pulp was pulped via a conventional Kraft process and semi-bleached by a conven-tional oxygen bleaching process to a 19.7 cp viscosity, 48.6 Elrepho brightness and 6.9 permanganate number. The pulp was contacted with a test solution comprising varying con-centrations of hydrogen peroxide and tungsten metal ion.
The resultant slurry, lO~ consistencyr was treated to adjust the pH to the desired test value and pulp-solution contact continued at 77C for 2 hours. After this hleaching treat-ment, the pulp was filtered and certain pulp properties were monitored. Subsequent to an alkaline extraction at pH
12.0, (70C for 90 minutesl these properties were measured again. The comparative results are displayed in Table I.
, 10.
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Table I
Ini tial Pulp 1 2 3 4 5 6 % H202 (O.D. pulp) -_ __ 1 2 % Tungsten (O.D.
pulp) -- 0.30 0.15 0.15 0.06 -- --pH (initial~ -- 4 . 8 5 . 2 5 . 2 5 . 0 5 . 0 --pH tfinal) -- 5-0 5.3 5~4 5.2 5.3 10.5 Residual H202 (g/l) __ __ 0.007 0.041 0.136 0.666 0 Viscosity ~cp)* 19.7 19.~ 18.1 25.6 16.8 14 . 2 19. 4 Elrepho Brightness* 48.6 51.3 55.5 59.3 57.4 56.7 56.6 Permanganate No.
(25 ml~* 6.9 6.1 4.0 3.0 3.5 5.7 6.0 Viscosity ~cp)** -- 17.4 16.0 17.4 15.7 14.1 --***
Elrepho Brightness** -- 51.2 55.9 60.0 56.7 56.1 --***
Permanganate No.
(25 ml)** -- 5~9 3.5 2.9 3.4 5.7 --***
_ * Before alkaline extraction ** ~fter alkaline extraction *** No reason, to perform extraction This comparative testing demonstrates that in the absence of peroxide tungsten ion does not bleach pulp (Test No. 1). Conversely, in the absence of metal ion in accordance with this invention, residual peroxide content is substantially higher and pulp viscosity significantly inferior to peroxide bleachants including such ions (Test No. 5). Test No~ 6 is the control wherein the pulp is bleached using an alkaline peroxide process common in the art. Stabilizers of 0~05~
- 112~161 magnesium sul~ate and 5.~ sodium silicate are used in such control. The me~allic bleachan~s of this invention provide substantiall~ greater permanganate number reductions yet afford similar brightness gains and viscosity retention than the prior alkaline bleaching process.
Example II
An unbleached hardwood pulp was pulped via a con-ventional Kraft process to a permanganate number of 9.9, viscosity of 28.0 and Elrepho brightness of 33.2. It was contacted with a solution of 2% hydrogen peroxide (based on O.D. pulp) containing 0.15% tungsten ion (based on O.D. pulp).
The pH of the resultant slurry was adjusted to 5 and the bleaching continued at 77C for 2 hours. Subsequent to standard alkaline bleaching, the pulp displayed a perman-ganate number of 4.8, a viscosity of 15.0 and a brightness of 44.7. A control pulp, bleached under identical conditions but lacking tungsten had a permanganate number of 7.6, a - Yiscosity o~ 22~3 and a brightness of 41~5 ExamPle III
A hardwood Ticonderoga mill pulp was oxygen-bleached in the Pfaudler reactor to a permanganate number of 6.2, giVing a 0.5% CED viscosity of 22.4 cp and an GE brightness of 46.5%. Peroxide bleaches were made on lO0 gram O.D. batches of this pulp at 10~ consistency and 77C. The conditions of the peroxide stage are depicted in Table II.
Following peroxide bleaching the pulp was bleached in two ozone stages separated by an extraction stage. In the first ozone stage 0.8~ ozone based on O.D. pulp was applied and in the second 0.4% was employed. The ozone stages were conducted identically at p~I 3, consistency 1.0% and 22C. Extractions were made at pH 7.5, 6.0% consistency and 50C.
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Table I I
.
OP* ZE~OPalkZEZOPaZ13Z
% H202 (O.D. pulp) 1. 1 ol --% Pa (peracetic acid) 2 (O.D. pulp) -- -- 2.23 % Tungsten Applied (O.D.
pulp) 0 3 pH 5 10.5 7.5 Time (hours) 2 2 3 Permanganate Number3 3.9 5.4 3.3 Viscosity (cp)3 22.5 21.8 22.1 Brightness, % GE3 54.4 60.2 67.7 Yield %3 99.0 98.7 98.5 ~ Ozone Consumed, (O.D.
pulp) First stage 0.48 0.53 0.51 Second stage 0.27 0.28 0.28 Viscosity (cp)4 12.5 12.7 11.7 Brightness, % GE4 85.2 83.9 88.6 Overall Yie].d %4 93.2 96.4 96.0 Reverted Brightness4 77.3 80.3 84.0 l. This bleachant included 5% sodium silicate and 0.05%
magnesium sulfate by weight based on O.D. pulp.
2. This is the molar equivalent of 1.0~ H202 (O.D. pulp)
~Z9161 substantial brightness gains, but not the high brightness attainable with chemical pulps. Such processes are described, for example~ in Hook, "Peroxide Bleaching Opens New Potentials For Groundwood Pulps", Pulp ~ Paper International, 45-48 (June 1975), Vartiainen, "Vtilization of Peroxide in Pulp Bleaching", Papper Och Tra, 51, 277-284 (1969~, U.S. Patent Nos. 2,187,016, 3,023,140, and 2,251,731 and Canadian Patent No. 970,111. Additionally, peroxides are employed as a "capping" or super-bleach stage at the end of common multi-stage bleaching processes for chemical pulps. Exemplifying these processes are those described in Canadian Patent Nos.
966,604 and 970,111, U.S. Patent Nos. 3,193,445 and 3,462,344 and Vartiainen, "Utilization of Peroxide in Pulp Bleaching", PapPer Och Tra, 51, 277-284 (1969), Delattre, "Hydrogen Per-oxide as a Bleaching Agent for Kraft Pulps", Papper Och Tra, 117-127 (1971), Hartler et al., "Peroxide Bleaching of Kraft Pulps", Tappi, 43, 806-813 (1960?, Christensen, "Bleaching Sulphate Pulp with Hydrogen Peroxide", Pulp and Paper Magazine of Canada, 62-66 (1971), Christensen, "Bleaching of Sulphate Pulps with Hydrogen Peroxide", Norsk S~ogindustri, 268-271 (October, 1973), and Mlakar & Peltonen, "Peroxide in the Semi~
bleaching of Kraft Pulp", Papper Och Tra, 11, 629-638 (1968).
However, peroxide bleaching by itself is not able to e~fect sufficient pulp bleaching for most paper-making re~uirements. This is particularly true when the peroxide is employed in an internal stage in a multi-stage bleaching process after initial ozone or oxygen bleaching. PrevioUs variations of the conditions for peroxide-pulp contact have either not remedied this inferior bleaching characteristic or 80 degraded the cellulosic portion of the pulp that it was no longer useful for paper-making processes.
llZ9161 The failure of peroxide bleachants in internal or prebleaching stages is apparently due to the inability of peroxide to delignify lignocellulosic pulps sufficiently to remove the light-absorbin~, dark lignin components and yet" maintain that resultant pulp viscosity and strength necessary for subsequent paper-making processes.
For example, at more alkaline pH's, i.e. 9-11, per-oxide bleaching is usually conducted in the presence of sodium silicate and magnesium sulfate stabilizers to avoid peroxide decomposition by heavy metal contaminants. But, under such conditions substantially no delignification is obtained. At higher pH's auto-decomposition of the peroxide beco~es exces-sive so as economically to disadvantage the process.
Conversely, at acid pH's, i.e. 2-7, the peroxide reaction with the lignin chromophores of the pulp is sluggish and often accompanied by severe viscosity losses. Rapson, "The Role of pH in Bleaching Pulp", Tappi, 39, 284-294 (1956) and U.S. Patent No. 2,112,116. Similar pulp viscosity and strength reductions are adduced by the addition of metals such ~s manganese,cobalt, iron, copper and aluminum to oxidizing bleaching agents such as peroxide. These special oxidants have bee~ used to good advantage in U.S. Patent Nos. 2,975,169 and ~,3~,527 to produce pulp, which while unsuitable for paper ~an~facture, is well constituted for the production of viscose rayoh or other cellulose-like derivatives.
Other additives are likewise ineffective in improving the brightness gain achievable with peroxide solutions. For example, U.S. Patent Nos. 2,119,519 and 2,249,646 disclose the additi~n of titanium having a valence of less than four to ~0 ~idlzlng bleaching agents, including peroxides. This addition is said to activate the alkaline bleaching process so as to reduce both the treatment time and oxidant concentration needed to obtain a given level of brightness gain. Such reductions advantageously benefit pulp strength since vis-cosity losses and other pulp degradation are minimized dueto the abbreviated oxidant-pulp content. However, even this improved process adduces no significant pulp delignification.
Peracids are one class of oxidizing bleaching agents which effect selective lignin removal. These reagents are essentially derivatives of peroxides. However, they are quite expensive and do not lend themselves to easy recovery and regeneration.
Summar of the Invention Y
It is accordingly an object of this invention to provide a process and solution whereby peroxide may be employed to delignify selectively and brighten lignocellulosic pulp yet retard that pulp viscosity loss and degradation caused by cellulose oxidation.
Another object o this invention is to provide a peroxide based delignifying process and solution wherein the conditions of delignification avoid significant auto-decom-posltion of peroxide so as to require none of the conventional peroxide stabilizers.
A further object of this invention is to provide a pulp delignification and bleaching process and solution where-in the spent bleachants may be recovered in conventional recovery systems without scaling.
These and other additional objects and advantages of this invention, apparent from the detailed description and claims which follow, are accomplished in accordance with this invention by a pulp delignifyinc3 and brightening solution com-prising from about 0.1 to about 5% by weight, based on O.D.
5.
(oven-dried) pulp, of a peroxide and about 0.5 to about 20 mole - percent, based on moles of peroxide, of a metal ion selected from the group consisting of tungsten, molybdenum, chromium, osmium and selenium, and sufficient acid to maintain the pH
of the solution between about 1 and about 7. More preferably, the acid is added subsequent to pulp-metal admixture.
In the preferred process of this invention, a ligno-cellulosic pulp is contacted at a temperature below about 50C
with the metal solution. The pH of the resultant slurry is adjusted to between about 1 and about 7 and peroxide is added to the slurry. Pulp delignification and brightening are usually effected at about 30C to about 100C until most of the peroxide has been consumed.
Detailed Description of the Invention This invention relates to a peroxide-based solution and process for selectively delignifying and brightening lignocellulosic pulp. The solution and process avoid attack on the cellulosic portion of the pulp so as substantially to preclude degradation of the pulp and reduction of its viscosity to levels below those acceptable for paper-making processes.
According to this invention, wood pulps and other lignocellu-losic pulps may be utilized to good advantage. By way of exam-ple, preferred lignocellulosic pulps include those wood pulps digested by the well-known sulfite, sulfate or Kraft, soda and modified sulfite processes. The process is particularly use-ful for the bleaching of hardwood Kraft pulps especially those which have been semi-bleached with conventional oxygen bleach-ants. Such semi-bleaching is disclosed, for example, in Soteland, "Bleaching of Chemical Pulps with Oxygen and Ozone", Pulp and Paper Magazine of Canada, 75, T153-158 (1974).
The novel delignifying and brightening solution of this invention comprises a peroxide, said peroxide being pre-sent in an amount effective for bleaching and delignifying 1 6.
~W, lignocellulosic pulps, this effective amount usually being from about 0.1 to about 5~ by weight, based on O.D. pulp, about 0.5 to about 20 mole percent, based on moles of per-oxide, of a metal ion selected from the group consisting of tungsten, molybdenum, chromium, osmium and selenium, and sufficient acid to maintain the pH of the solution between about 1 and about 7. More preferably the acid is added sub-sequent to pulp-metal admixture.
In the preferred process of this invention, a ligno-cellulosic pulp is contacted at a temperature below about 50Cwith such metal solution. The pH of the resultant slurry is adjusted to between about 1 and about 7 and peroxide added to the slurry. Pulp delignification and brightening are effected at about 30C to about 100C until most of the peroxide has been consumed. However, it is to be understood that other sequences which avoid pulp-peroxide contact at low pH and high temperature in the absence of metal ion are within the scope of the invention.
The methodology for carrying out this process in the pulp slurry is conventional in the art. Most commonly, slurries of consistency from about 1% to about 50%, and more preferably 10~ to 35% are employed. The reaction is conducted between 30C and 100C and more preferably between 50C and 80C. The time of reaction is not critical being primarily controlled by the amount of peroxide remaining in the pulp slurry and its effective reaction rate. Usually a reaction time of from 0.5 to about 4 hours is sufficient to consume most of the peroxide, the choice of an actual reaction time being dependent on the pH
and temperature. For example short reaction times are favored in slurries of low pH and high temperature.
The peroxide used in the bleaching solution and process of this invention may be selected from any number of conventional sources. These include inorganic peroxides, such as sodium peroxide, organic peroxides and aqueous solutions 7.
llZ916~
of hydrogen peroxide itself, the last being preferred. The chosen peroxide or a mixture thereof may be used in an about 0.5 to about 5% by weight, based on O.D. pulp, but preferably is present between about 0.5 and about 2% by weight.
The peroxide stabilizers taught in the art to slow deleterious auto-decomposition of peroxide during bleaching are not needed in the slurries of this invention. Instead, the metallic activators of this invention appear both to stabilize the peroxide and to activate its selective oxidative attack on the lignin of the lignocellulosic pulp.
The metal containing additives, whose metallic por-tions are selected from the group consisting of tungsten, molyb-denum, chromium, osmium and selenium, may be selected from a wide range of such compounds. The selected compounds need only be able to provide the desired metallic ions in solution and not interfere with the pulp brightening process to be acceptable. Examples of useful compounds of this type in-clude osmium tetroxide, selenium dioxide, sodium tungstate, sodium molybdate and sodium dichromate. More preferably, the metallic portion is tungsten and most preferably the tungsten iB in the plus six oxidation state, since tungsten ions of lower oxidation states are more wasteful of peroxide.
Although it is not necessary that the chosen additive be totally soluble under the conditions of this invention, such dissolution is more usual and preferred.
While not wishing to be bound by theory, it is thought that a 2:1 peroxo-tungstate or other metal-peroxide complex functions as the lignin selective oxidant. Its action is catalytic since the metal is returned to the reser-voir of free peroxide to reform the reactive complex. At stoichiometric concentrations of metal ion, the competing metal induced peroxide decomposition appears to slow, the metal complex apparently having a stabilizing effect on the pulp.
As the p~l o~ the slurry is lowered from 6 to about 2, pulp brightening and delignification seem to improve, but pulp viscosity losses become significant at below about pH 3. It is theorized that such improvement is a function of a favored formation of peroxo-metal complex at low pH. However, elevated temperatures may also force this formation at higher pH's. For example, pulp contact at pH 5 and 77C is very effective in delignification of lignocellulosic pulp. Where tungsten is the metal ion employed such conditions are preferred.
Following the selective delignification and brightening of this invention, the lignocellulosic pulp may be washed in a conventional manner or additionally treated in any number of well~recognized paper-making pro-cesses. The filtrate from such washing is amenable to standard recovery operations without scaling. Further, any metal residue remaining on the pulp fiber may serve as a pigment to build opacity.
Due to the relatively high cost of even the small amounts of metal ions used in this invention, it is important that thorough washing and a high degree of recycle be employed to improve the economics of the process of this invention.
In order to describe the present invention so that it may be more clearly understoo~, the following examples are set forth. These examples are primarily for the purpose of llZ9161 illustration and any specific enumeration therein should not be construed as a limitation on the broader concepts of this invention.
Example I
A hardwood Ticonderoga mill pulp was pulped via a conventional Kraft process and semi-bleached by a conven-tional oxygen bleaching process to a 19.7 cp viscosity, 48.6 Elrepho brightness and 6.9 permanganate number. The pulp was contacted with a test solution comprising varying con-centrations of hydrogen peroxide and tungsten metal ion.
The resultant slurry, lO~ consistencyr was treated to adjust the pH to the desired test value and pulp-solution contact continued at 77C for 2 hours. After this hleaching treat-ment, the pulp was filtered and certain pulp properties were monitored. Subsequent to an alkaline extraction at pH
12.0, (70C for 90 minutesl these properties were measured again. The comparative results are displayed in Table I.
, 10.
llZ916~
Table I
Ini tial Pulp 1 2 3 4 5 6 % H202 (O.D. pulp) -_ __ 1 2 % Tungsten (O.D.
pulp) -- 0.30 0.15 0.15 0.06 -- --pH (initial~ -- 4 . 8 5 . 2 5 . 2 5 . 0 5 . 0 --pH tfinal) -- 5-0 5.3 5~4 5.2 5.3 10.5 Residual H202 (g/l) __ __ 0.007 0.041 0.136 0.666 0 Viscosity ~cp)* 19.7 19.~ 18.1 25.6 16.8 14 . 2 19. 4 Elrepho Brightness* 48.6 51.3 55.5 59.3 57.4 56.7 56.6 Permanganate No.
(25 ml~* 6.9 6.1 4.0 3.0 3.5 5.7 6.0 Viscosity ~cp)** -- 17.4 16.0 17.4 15.7 14.1 --***
Elrepho Brightness** -- 51.2 55.9 60.0 56.7 56.1 --***
Permanganate No.
(25 ml)** -- 5~9 3.5 2.9 3.4 5.7 --***
_ * Before alkaline extraction ** ~fter alkaline extraction *** No reason, to perform extraction This comparative testing demonstrates that in the absence of peroxide tungsten ion does not bleach pulp (Test No. 1). Conversely, in the absence of metal ion in accordance with this invention, residual peroxide content is substantially higher and pulp viscosity significantly inferior to peroxide bleachants including such ions (Test No. 5). Test No~ 6 is the control wherein the pulp is bleached using an alkaline peroxide process common in the art. Stabilizers of 0~05~
- 112~161 magnesium sul~ate and 5.~ sodium silicate are used in such control. The me~allic bleachan~s of this invention provide substantiall~ greater permanganate number reductions yet afford similar brightness gains and viscosity retention than the prior alkaline bleaching process.
Example II
An unbleached hardwood pulp was pulped via a con-ventional Kraft process to a permanganate number of 9.9, viscosity of 28.0 and Elrepho brightness of 33.2. It was contacted with a solution of 2% hydrogen peroxide (based on O.D. pulp) containing 0.15% tungsten ion (based on O.D. pulp).
The pH of the resultant slurry was adjusted to 5 and the bleaching continued at 77C for 2 hours. Subsequent to standard alkaline bleaching, the pulp displayed a perman-ganate number of 4.8, a viscosity of 15.0 and a brightness of 44.7. A control pulp, bleached under identical conditions but lacking tungsten had a permanganate number of 7.6, a - Yiscosity o~ 22~3 and a brightness of 41~5 ExamPle III
A hardwood Ticonderoga mill pulp was oxygen-bleached in the Pfaudler reactor to a permanganate number of 6.2, giVing a 0.5% CED viscosity of 22.4 cp and an GE brightness of 46.5%. Peroxide bleaches were made on lO0 gram O.D. batches of this pulp at 10~ consistency and 77C. The conditions of the peroxide stage are depicted in Table II.
Following peroxide bleaching the pulp was bleached in two ozone stages separated by an extraction stage. In the first ozone stage 0.8~ ozone based on O.D. pulp was applied and in the second 0.4% was employed. The ozone stages were conducted identically at p~I 3, consistency 1.0% and 22C. Extractions were made at pH 7.5, 6.0% consistency and 50C.
112916~
Table I I
.
OP* ZE~OPalkZEZOPaZ13Z
% H202 (O.D. pulp) 1. 1 ol --% Pa (peracetic acid) 2 (O.D. pulp) -- -- 2.23 % Tungsten Applied (O.D.
pulp) 0 3 pH 5 10.5 7.5 Time (hours) 2 2 3 Permanganate Number3 3.9 5.4 3.3 Viscosity (cp)3 22.5 21.8 22.1 Brightness, % GE3 54.4 60.2 67.7 Yield %3 99.0 98.7 98.5 ~ Ozone Consumed, (O.D.
pulp) First stage 0.48 0.53 0.51 Second stage 0.27 0.28 0.28 Viscosity (cp)4 12.5 12.7 11.7 Brightness, % GE4 85.2 83.9 88.6 Overall Yie].d %4 93.2 96.4 96.0 Reverted Brightness4 77.3 80.3 84.0 l. This bleachant included 5% sodium silicate and 0.05%
magnesium sulfate by weight based on O.D. pulp.
2. This is the molar equivalent of 1.0~ H202 (O.D. pulp)
3. Before ozone bleaching
4. After final ozone bleaching and standard washing stages.
These results demonstrate that alkaline peroxide ~leaching and bleachin in accordance with this invention are complimentary systems as interior bleaching stages. Alkaline peroxide provides mainly bleaching an~ the peroxides of this invention mainly delignification. However, the reactive llZ~61 conditions of the bleachants of these inventions are more compatible with subsequent low temperature acidic ozone stages. Therefore, su¢h peroxide solutions are preferred for the interior stage of multi-stage bleaching sequences.
These xesults illustrate the usefulness of a variety of metal additives in accordance with this invention.
While I have hereinbefore presented a number of embodiments of my invention, it is apparent that my basic construction can be altered to provide other embodiments which utilize my invention. Thus, it will be appreciated that the scope of my invention is to be defined by the claims appended hereto rather than the specific embodiments which have been presented hereinbefore by way of example.
1~ .
These results demonstrate that alkaline peroxide ~leaching and bleachin in accordance with this invention are complimentary systems as interior bleaching stages. Alkaline peroxide provides mainly bleaching an~ the peroxides of this invention mainly delignification. However, the reactive llZ~61 conditions of the bleachants of these inventions are more compatible with subsequent low temperature acidic ozone stages. Therefore, su¢h peroxide solutions are preferred for the interior stage of multi-stage bleaching sequences.
These xesults illustrate the usefulness of a variety of metal additives in accordance with this invention.
While I have hereinbefore presented a number of embodiments of my invention, it is apparent that my basic construction can be altered to provide other embodiments which utilize my invention. Thus, it will be appreciated that the scope of my invention is to be defined by the claims appended hereto rather than the specific embodiments which have been presented hereinbefore by way of example.
1~ .
Claims (12)
1. A solution for delignifying and brightening lignocellulosic pulp comprising a peroxide, said peroxide being present in an amount effective for bleaching and delignifying lignocellulosic pulp; about 0.5 to about 20 mole percent based on moles of said peroxide of a metal ion selected from the group consisting of tungsten, molybdenum, chromium, osmium and selenium; and sufficient acid to maintain the pH of said solution between about 1 and about 7.
2. The solution of claim 1 wherein said peroxide concentration is between about 0.5 and about 2% by weight, based on oven-dried pulp.
3. The solution of claim 1 wherein said peroxide is hydrogen peroxide.
4. The solution of claim 1 wherein said metal ion is tungsten.
5. The solution of claim 1 wherein the pH of said solution is maintained between about 3 and about 6.
6. A process for delignifying and brightening lignocellulosic pulp comprising contacting an aqueous pulp slurry of said lignocellulosic pulp with a solution comprising from about 0.1 to about 5% by weight, based on oven-dried pulp, of a peroxide; about 0.5 to about 20 mole percent based on moles of peroxide of a metal ion selected from the group consisting of tungsten, molybdenum, chromium, osmium and selenium; and sufficient acid to maintain the pH of said pulp slurry between about 1 and about 7.
7. The process of claim 6 wherein said peroxide concentration is between about 0.5 and about 2% by weight.
8. The process of claim 6 wherein said peroxide is hydrogen peroxide.
9. The process of claim 6 wherein said metal ion is tungsten.
10. The process of claim 6 wherein the pH of said slurry is maintained between about 3 and about 6.
11. A process for delignifying and brightening lignocellulosic pulp comprising maintaining the temperature of an aqueous pulp slurry between about 30°C and 100°C, said pulp slurry comprising lignocellulosic pulp; from about 0.1 to about 5% by weight, based on oven-dried pulp, of hydrogen peroxide and about 0.5 to about 20 mole percent based on moles of peroxide of tungsten ion; and having a pH
between about 3 and about 6, until most of said peroxide is consumed.
between about 3 and about 6, until most of said peroxide is consumed.
12. The process of claim 11 wherein said pH is 5 and said slurry temperature is 77°C.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US89456178A | 1978-04-07 | 1978-04-07 | |
| US894,561 | 1978-04-07 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1129161A true CA1129161A (en) | 1982-08-10 |
Family
ID=25403249
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA319,915A Expired CA1129161A (en) | 1978-04-07 | 1979-01-18 | Delignification and bleaching process and solution for lignocellulosic pulp with peroxide in the presence of metal additives |
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| US6165318A (en) * | 1994-06-20 | 2000-12-26 | Kemira Chemicals Oy | Delignification of chemical pulp with peroxide in the presence of a silicomolybdenic acid compound |
| US6660128B1 (en) | 1998-04-16 | 2003-12-09 | Pulp And Paper Research Institute Of Canada | Oxidase process for pulp |
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