CA1062858A - Oxygen bleaching of pulp in presence of high-silica protector - Google Patents
Oxygen bleaching of pulp in presence of high-silica protectorInfo
- Publication number
- CA1062858A CA1062858A CA234,066A CA234066A CA1062858A CA 1062858 A CA1062858 A CA 1062858A CA 234066 A CA234066 A CA 234066A CA 1062858 A CA1062858 A CA 1062858A
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- Canada
- Prior art keywords
- pulp
- bleaching
- oxygen
- semi
- protector
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
- D21C9/10—Bleaching ; Apparatus therefor
- D21C9/1026—Other features in bleaching processes
- D21C9/1036—Use of compounds accelerating or improving the efficiency of the processes
Abstract
OXYGEN BLEACHING OF PULP IN PRESENCE
OF HIGH-SILICA PROTECTOR
Abstract of the Disclosure The disclosed colloidal high silica protector materials protect carbohydrates in chemical cellulosic pulps from oxidative attack during oxygen bleaching in alkaline media at elevated temperatures and superatmospheric pressure. These protector materials are useful in processes which produce high yields of semi-bleached and fully bleached pulps. The preferred bleach sequences for fully bleached pulps begin with an O stage, e.g.
O-D-H, O-D, ODED, and OH, as well as OCEDED, OCED, OCDEDED, etc.
A fully bleached kraft pulp of this invention compares favorably in virtually all respects to a fully bleached kraft pulp obtained by the environmentally objectionable chlorine bleach sequences.
The preferred protectors are high silica water glass compositions wherein the silica/sodium oxide ratio is at least 1.6, preferably at least about 2.0 on a weight/weight basis.
OF HIGH-SILICA PROTECTOR
Abstract of the Disclosure The disclosed colloidal high silica protector materials protect carbohydrates in chemical cellulosic pulps from oxidative attack during oxygen bleaching in alkaline media at elevated temperatures and superatmospheric pressure. These protector materials are useful in processes which produce high yields of semi-bleached and fully bleached pulps. The preferred bleach sequences for fully bleached pulps begin with an O stage, e.g.
O-D-H, O-D, ODED, and OH, as well as OCEDED, OCED, OCDEDED, etc.
A fully bleached kraft pulp of this invention compares favorably in virtually all respects to a fully bleached kraft pulp obtained by the environmentally objectionable chlorine bleach sequences.
The preferred protectors are high silica water glass compositions wherein the silica/sodium oxide ratio is at least 1.6, preferably at least about 2.0 on a weight/weight basis.
Description
106Z8S~
M&G-323.3/USA
. _ _ OXYGEN BLEACEIING OF PULP IN PRESENCE
O~ HI~H-SILICA PROTECTOR
Field of the Invention This invention relates to so-called oxygen bleaching of chemical and semi-chemical cellulosic pulps. An aspect of this invention relates to a protector system for protecting the carbohydrates against degradation during the oxygen bleaching ~ of the pulp.
Description of the Prior Art Most of the cellulosic pulp manufactured at the present time can be considered to be a chemical or semi-chemical type, the commonest chemical pulps being kraft or sulfate pulp, sulfite pulp, and soda pulp. These pulps are typically obtained thxough the digestion of wood chips or the like. graft or sulfate pulp has presently achieved a pre-eminent place in chemical pulp manu-facture for a variety of reasons, including the ability of the kraft process to digest almost any kind of wood, coniferous or deciduous. Unbleached kraft pulps do tend to be very dark in color, however, which can be a severe drawback with respect to many of their potential uses. To overcome this drawback, the colored impurities are removed by washing and bleaching. A-single bleaching stage is ordinarily insufficient and uneconomical where a fairly high level of brightness is desired. ~ypicall~, multi-stage bleaching sequences are used, these sequences being gener-ally referred to by an abreviated nomenclature in which "C"
stands for a chlorination stage, "H" stands for a hypochlorite stage, "D" stands for a chlorine dioxide bleach, '!E" stands for an extraction step, etc. Washing steps are typically indicated
M&G-323.3/USA
. _ _ OXYGEN BLEACEIING OF PULP IN PRESENCE
O~ HI~H-SILICA PROTECTOR
Field of the Invention This invention relates to so-called oxygen bleaching of chemical and semi-chemical cellulosic pulps. An aspect of this invention relates to a protector system for protecting the carbohydrates against degradation during the oxygen bleaching ~ of the pulp.
Description of the Prior Art Most of the cellulosic pulp manufactured at the present time can be considered to be a chemical or semi-chemical type, the commonest chemical pulps being kraft or sulfate pulp, sulfite pulp, and soda pulp. These pulps are typically obtained thxough the digestion of wood chips or the like. graft or sulfate pulp has presently achieved a pre-eminent place in chemical pulp manu-facture for a variety of reasons, including the ability of the kraft process to digest almost any kind of wood, coniferous or deciduous. Unbleached kraft pulps do tend to be very dark in color, however, which can be a severe drawback with respect to many of their potential uses. To overcome this drawback, the colored impurities are removed by washing and bleaching. A-single bleaching stage is ordinarily insufficient and uneconomical where a fairly high level of brightness is desired. ~ypicall~, multi-stage bleaching sequences are used, these sequences being gener-ally referred to by an abreviated nomenclature in which "C"
stands for a chlorination stage, "H" stands for a hypochlorite stage, "D" stands for a chlorine dioxide bleach, '!E" stands for an extraction step, etc. Washing steps are typically indicated
- 2 -- ( 106Z8513 by hyphens. Among the various bleaching sequences which have been used are the follo~Jing: C-E-II-E-DI C-E/iI-D, C-E/H, C-E-D-E-D, and even some more complex sequences such as CEIIDED. Bleaching steps `involving the use of other bleaching agents such as peroxides have also been used, but the most con~on bleaching agents at the present tîme are generally chlorine, hypochlorites (e.g. Ca or Na hypochlorite), and chlorine dioxide. Unfoxtunately, the use o some of these bleaching sequences, particularly the chlorination and extraction stages, can result in the creation of an effluent from the process which has a polluting effect upon natural bodies of water. The polluting effect can be evidenced by, for example, water with increased chemical oxygen demand ~COD) biological oxygen demand (BOD), alkalinity, or acidity, any of which can be harmful to aquatic life.
The development of oxygen bleaching of chemical pulp has opened up the possiblity of relatively non-polluting bleach sequences such as O, ODED, OH, CDOD, OCDEDED, OCEDED, OCED, etc.
wherein O is the oxygen bleach stage, and C, D, E, and H are as defined previously. The symbol CD represents a chlorination stage with some chlorine dioxide. It presently appears that the C stage can be eliminated from many of these sequences. ~any of the significant advantages of oxygen bleaching are described in an article by Gordon Rowlandson in the TAPPI Journal, 54 (No. 6), pages 962 - 967 (~une, 19713. The article describes the possibility o decreased BOD and COD, decreased chemical consumption, adequate pulp strengths, and, perhaps most important, markedly decreased chloride content in the effluent from the bleaching sequences.
~Iowever, the picture presented by currently available data concerning oxygen bleaching of brownstock (e.g. from kraft 10628SI~
pulping) is not uniformly encouraging. The number of variables to be contended ~Jith in producing a good bleached or semi-bleached pulp is formidable. These variables are typically controlled to achieve certain desired properties of the hleached pulp, e.g.
brightness, color, stability to~ard color reversion, opacity, pulp viscosity, pulp strength, lignin content, etc. One factcr which has received attention in the literature is perhaps one ~ o~ the most important from a practical standpoint: the yield of bleached or semi-bleached pulp. (Yield is rarely discussed in the patent literature regarding oxygen bleaching, however.~
Traditionally and practically the term "pulp yield" includes both ino~ganic and organic substances in the pulp.
This much is presently known: oxygen gas treatment of pulp does have a degradative'effect, resulting in bleached or semi-bleached pulp with less mechanical strength, lower pulp viscosity, and lower pulp yield.
Much of the prior art data relating to the oxygen bleach process is concentrated in the measurement of properties such as brightness, color reversion, physical strength values and the like. Efforts have been made to up-grade some or all of these- -properties by, in effect, protecting the useful carbohydrateæ
in the pulp fxom oxidative degradation. It has been found that certain inorganic compounds appear to provide this protective function; see U.S. Patent Nos. 3,657,065 (Smith et al)', issued April 18, 1972; 3,725,194 (Smith et alj, issued April 3, 1973;
and 3,7~0,310 (Smith et al), issued June 19, 1973. The protector compounds are typically inoryanic salts, oxides and hydroxides o~ al};ali or alkaline earth me.als, although silica and zinc and titanium compounds have received some mention. The primar~
emphasis in the patent literature, the scientific literature, ` 1062858 and in commercial practice is on the use of magnesium salts such as magnesium carbonate, magnesium silicate, magn~sium oxide, magnesium sulfate (e.g. epsom salt), and the like. The use of certain siliceous materials other than magnesium silicate has been suggested, i.e. sodium metasilicate, sodium orthosilicate, silica, and silicic acid. In this regardt there appears to be preference in the prior art for water soluble sodium metasilicate.
The use of "protectors' has advanced the state of the oxygen bleaching art, but further improvement in protection against oxidative degradation is needed. The goal of developin~
an oxygen bleach method or sequence which produces bleached or semi-bleached pulp with characteristics fully equivalent to, for example, the conventional CEHED or CEDED sequence is still generally unreached.
There has also been some recent research on the subject of protecting useful carbohydrates during the pulping process itself, e.g. during soda-oxygen pulping. ~owever, the situations, reactions, and mechanisms of oxygen pulping are considered by many scientists to be different from oxygen bleaching. Further-more, many scientists believe that inhibitors or pxotectors ingeneral give relatively insignificant benefits in oxygen pulping as compa~ed to oxygen bleaching. Accordingly, the answers to the problems discussed previously will apparently have to develop out of the art of oxygen bleaching.
Summary of the Invention It has now been discovered that the alkali metal colloidal silicates having a molar ratio of silica to alkali metal o~ide o at least 1.6:1 (preferably the high-silica wa~er glasses), used as protectors in oxygen bleaching, can provide surprising improvcments, compared to conventional protectors or no protector at all, e~g. in semi-bleached or bleached pulp yield. The increase in yield is obtained with no significant loss of brightness, stability to color reversion, viscosity, strength, opacity, delignification, and the like; on the contrary, some of these properties (such as viscosity, strength, opacity and oxygen stage brightness) are enhanced, using the same standards for compari-~ son. Perhaps the most surprising observation is that a high-silica protector of this invention (e.g. a water glass with a silicaj sodium oxide weight/weight ratio of 3.25:1) has particularl~ marked superiority when compared to sodium metasilicate, i.e. the compound variously written Na2SiO3 or Na2O SiO2, wherein the silica/sodium oxide mole ratio is 1:1 and tne weight ratio is 0.97~ ield data indicate that an addition of water glass to the pulp, expressed as SiO2 on a weight/weight basis, of 0.1 - 4% can produce an increase in the semi-bleached oxygen pulp yield of about 1~
or more. Even if allowances are ~ade for the increase in yield attributable to increased inorganic residue in the pulp, the organic yield increase is still very significant from a cost standpoint. In any event, the ash-free pulp yield would not tell the entire story~ It can be very significant that both the inor-ganic and organic yield are increased according to the present invention, since it is desirable for the protector to re~ain with the pulp throughout the entire bleaching sequence.
The colloidal, high-silica alkali metal silicate protectors of this invention can be substituted for the conventional magnesium salt protectors of the present co~mercial oxygen bleach processes ~-ith improved results. Among the preferred o~ygen bleach sequences of this invention are O-D, O-D-r~, O-D-Æ-D, and other sequences starting with an o stage. It has also been found that the spent liquor effluent from the oxygen stage can be recycled to the oxygen reactor by blending it t`7ith as little as 40 - 60~ by weight of a fresh water glass protector medium. If kraft pulp is made in-plant, the caustic in the spent liquor from oxygen bleaching can be recovered in the conventional manner through the kraft mill recovery system. It has been disclosed that sodium silicate has no adverse effect upon the chemical recovery equipment and may even have a beneficial anti-corrosive effect.
A further benefit of this invention is that the biological oxygen demand and chemical oxygen demand levels in the spent bleach liquor tend to be even less when compared to those of the conven-tional oxygen bleach effluent. This beneficial result is appar-ently related to the increased pulp yield, i.e. the reduced level of dissolved organic solids in the bleach liquor.
In the preferred practice of this invention, a commercial water glass te.g. 35 - 59 Be.) is diluted with water to 5 - 50~
by weight of solids and is thoroughly blended with the unbleached pulp, together with a suitable caustic medium. The resulting pulp mixture is then bleached with oxygen gas at elevated temperatures (e.g. 95 - 135~C.) under superatmospheric pressure (e.g. 70 ~
225 psig), and the resulting semi-bleached pulp is used as such or conveyed to the subsequent bleaching stages. A high-silica water glass is the preferred protector, i.e. a water glass with an SiO2/Na2O mole ratio in the range o about 2 to 4~
A less preferred practice of the invention is to use a protector with an SiO2/Na20 weight ratio of up to about 100:1.
Although these higher ratio materials are operative in the invention, they are not, strictly speaking, water glasses and can be more accurately described as surface-hydroxylated colloidal sols.
1 Scope of the Invention This invention is a method of bleaching chemical or semi-chemical cellulosic pulp which involves subjecting the pulp to the bleaching action of oxygen gas in an oxygen gas bleaching zone under superatmospheric pressure and tempera-tures elevated above room temperature in the presence of an alkaline medium and a yield-increasing amount of a protector of this invention, the protector consisting essentially of an alkali metal colloidal silicate having a molar ratio of silica to alkali metal oxide of at least 1.6:1. For example, the protector can consist essentially of a water glass having an SiO2/Na2O ratio within the range of about 2:1 to 4:1 or a surface-hydroxylated silica sol, which can have a much greater SiO2/Na2O ratio, e.g. up to 100:1. The type of pulp most commonly used in this invention is a kraft or sulfate pulp.
A further aspect of the method of this invention is that the semi-bleached pulp recovered from the oxygen bleaching zone can be further bleached in at least one additional bleaching step with a bleaching agent which agent preferably is chlorine, chlorine dioxide, hypochlorite, or peroxide, the most preferred bleaching sequence for producing fully bleached pulp being oxygen-chlorine dioxide-hypochlorite (O-D-H)~
When water glass is used as the protector, the amount of addition of water glass to the pulp, expressed as SiO2 on a weight basis, is preferably 0.1-4~ on pulp. It is also preferred to combine the spent liquor effluent from the oxygen bleaching zone with fresh water glass and recycle the resulting mixture to the oxygen bleaching zone.
~0628513 1 The Unbleached Pulp Oxygen bleaching improves the brightness and stability of virtually any type of pulp, particularly chemical pulps such as soda, sulfite, or sulfate (kraft) pulp. Of the chemical pulps, kraft pulp is the most significant commer-cially and also tends to be particularly dark in color.
(A typical kraft pulp is referred to as "brownstock", and, if the pulp is made into paper without bleaching, the resulting paper has the characteristic brown color of heavy wrapping paper.) Both hardwoods and softwoods can be utilized in the kraft process. Thus, although much of the experience with this invention relates to pine kraft pulp and aspen kraft pulp, this invention is not limited to the use of pulp derived from any particular hardwood or softwood.
Chemical pulps can be produced in pulp mills (e.g.
kraft pulp mills) and obtained for use in a separate oxygen bleach facility. Alternatively, an oxygen bleach facility can be installed right at the pulp mill, so that the pulp or brownstockcan be produced in-plant.
In this case, the protector which remains in the spent liquor effluent from the oxygen stage can be either . . .
- 8a -' ~
106Z85~3 recovered in the pulp mill recovery system and/or recycled for use with fresh protector in the oxygen bleaching step.
The Oxygen Bleach System The alkaline Medium charged to the oxygen reactor typically comprises: (a) a caustic liquor containing about 1 to a~out 5%
basic material (expressed as sodium hydoxide), (b) the bleaching - agent, i.e. oxygen gas, and (c) the protector medium. In commercia~
practice, of course, the oxygen gas is not pure. However, commer-cially available oxygen gas can be substantially free of carbon dioxide and hydrogen and can contain negligible amounts of carbon monoxide and nitrogen. Thus, the partial pressure of oxygen in the bleaching zone can be at least 85 or 90% of the total pressure. Commercial supplies of oxygen with a maximum of 0.5~
impurities are presently available. As is known in th art, oxygen gas as a bleaching agent is readily distinguishable in its behavior and properties from chemically combined sources of oxygen, e.g.
the peroxides. For example, oxygen gas has a rather strong delig-nifying effect under conventional oxygen bleach conditions. Unfor-tunately, as pointed out previously, there can be no ~uestion that bleaching of pulps with oxygen in alkaline media results in significant losses in yield and viscosity. These losses are believed to be primarily due to severe degradation of cellulose and hemicelluloses. The protector system of this invention is designed to reduce or eliminate these losses without sacrificing the other desirable features of the oxygen bleach process.
The protector of this invention can be considered to be an essentially aqueous medium containing a dispersed colloidal silicate having a molar ratio of silica to alkali metal oxide of at least 1.6~ colloidal sodium silicate made by ( 106285~f ( interacting silica sand and sodium carbonate at high tempexatures to produce a product with a silica/sodium oxide mole ratio of at least 1.6:1 and then dispersing the product in water can be referred to as a sodium water glass. A surface-hydroxylated sol is another type of colloidal system which will be discussed subsequently. ~s poin-ted out in Shreve, The Chemical Process Industries, second edition, ~cGraw-Hill, New York, 1956, pages _ 280-282, silicates that have a silica/sodium oxide ratio of 1.6 1 up to 4:1 are called colloidal silicates, and at the present time there are over 40 varieties of commercial sodium silicates, each with a specific use. When the silica/sodium oxide weight ratio is, for example, 3.2:1, 43 Be. is the upper limit of concentration (40% solids). For a 2:1 silica/sodium oxide ratio (also by weight), the upper limit is 60 Be. or 54 weight % solids. Concentrations up to about 65 weight % solids are known, e.g. in the case of the l.6:1 (wt./wt.) silica/sodium oxide ratio. For the 3.75:~
weight ratio, the maximum concentration is 35 Be. or less than 33 weight % solids. Because of handling difficul~ies, the extremely high Baume values are not preferred, and 35-59 Be. is the typical range of concentrations utilized in this invention.
It is ordinarily preferred to dilute the commercial 35-59 Be.
water glass with water prior to combining it with the pulp and the alkaline medium. Excessive dilution (e.g. to 5~ or lessj can create dispersion stability problems, however. As a convenient rule of thumb, O.5 - 2:1 (by volume) dilution of the commercial water glass with water is suitakle. As will be explained subse-quently, dilution with recycled s~ent oxygen bleach liquor is particularly desirable from a cost standpoint; preferred solids content for the diluted water glass is 15-35% by weight.
-- 1 o 106Z85~
~ otassium silicates with higll silica/potassium oxide ratios are known', e.g. about 2 to 3:1 on a weiyht/weight basis or about 3 to 4:1 on a mole/mole basis. These potassium water glasses (e.g. the 2.5:1 weigllt ratio material) perform in about the same manner as sodium water glasses of similar weight ratio. Baume values for the potassium water glasses tend to be lower than those of the colloidal sodium silicates. Since potassium silicates _ generally are consiaerably more expensive than sodium silicates, they are not preferred for use in this invention.
A variety of silica/sodium oxide ratios ~hroughout the 1.6 - 4:1 range are presently available as commercial water glass materials, e.g. from Philadelphia Quartz Company, Diamond Shamrock Company, and the duPont Company. Among these ratios are the following (all on a weight/weight basis): 1.6, l.90, 1.95, 2.0, 2.20, 2.38, 2.4, 2.55, 2.87, 3.22, 3,25, 3.40, and 3.75, As the weight ratio increases above 3.9:1 or the mole ratio increases above 4:1, water glass materials tend to acquire greater and greater sensitivity towards carbon dioxide. Carbon dioxide absorbed from the air can neutralize the alkalinity of the silicate, ultimately resulting in precipitation and/or gellation of the silica. This phenomenon is sometLmes,referred to as "reversion'l.
The more alkaline silicates with silica/sodium oxide ratios below 4:1 (on a moie basis) are more difficult to neutralize and hence moxe stable towards carbon dioxide. Furthermore, a sodium carbonate/silica reaction product with an excessively high silica content can be difficult to disperse in water, making its commer-cial production in the water glass form less practical.
Stable sols or aqueous dispersions characterized by extremely high silica/sodium oxide ratios can be obtained by surface hydroxylating discrete spherical particles of silica. For example, 106285~
"Ludox" (trademark of the duPont Company) is a stable colloidal system with an SiO2/Na2O ratio (by weight) of 50:1 or more, and ratio of about 100:1 and even above 200:1 have been achieved in practice.
The "Ludox" systems are moderately alkaline (p~ above 8 but less than 10). Thus, "Ludox" can be distinguished from an aqueous dispersion of colloidal silica in that the pH of "Ludox" is much higher; furthermore, silanol groups on the surface of the "Ludox"
~particles have been converted to anionic sites, thereby resulting in surface charges which can be different from pure silica, e.g.
Na+ OSi--In short, it is a complex matter to draw relationships between "Ludox" and water glass, though similarities can be found~
The low-silica water glasses can approach a pH of 13, and high-silica water glasses have a degree of alkalinity roughly inter-mediate between "Ludox" and the low-silica water glasses. Both "Ludox" and water glasses have anionic character at least in alkaline media, and the anionic character can persist even when the pH is deliberately manipulated downward. Cationic or electropositive character is not observed until the p~I is well into the acid range. Like high-silica water glasses, "Ludox" ~IS, LS, SM, and TM can be converted to gels through acidification. It has been found that "Ludox" has, in the context of this invention, no sig~
nificant advantages over suitably selected water glasses. Some disadvantages have been noted, however.
Despite the sensitivity to acidification characteristic of sodium silicates, data obtained according to this invention gener-ally indicate that significant advantages can be obtained by increasing silica/sodium oxide ratio of the protector sys~em beyond abou, 2:1 and up to the limits of colloidal stability for water glass (e.g. 4:1 on a mole basis or 3.9:1 on a weight basis) (` 1062~358 There is a slight loss in brightness with the higher silica protec-tors, but this loss is insignifica~t when weighted against the ad-,vantages. ~sing surface-hydroxylated colloidal silica sols, data have been obtained for a silica/sodium oxide ratio ~by weight) of almost 100:1. These data indicate the,possibility of a slight yield increase obtainable at this high ratio, but even this slight increase is obtained with significant sacrifices, e.g. higher protector cost, lower pulp viscosity, and lower brightness as compared to some water glass protectors, e.~. those containing more than 75 weight % silica.on a dry basis.
.
'The Process Once the pulp has been obtained and the protector system has been formulated, the most effective way to protect the pulp is to add the protector system of this invention to the unbleached pul~
before it is introduced into the oxygen bleaching zone. The pro-tector system is preferably not introduced directly into the reactor, i.e. oxygen bleaching zone. Since the oxygen bleaching zone is typically a closed reactor under superatmospheric pressure, the more effective procedure is also the simpler one.
The semi-bleached pulp which is obtained from the,oxygen bleaching step has suf'ficient brightness and adequate color for use in a variety of papers ~hich do not require a high level of brightness. Alternativel~ ~and moxe typically) the semi-bleached pulp is brought to a fully bleached state ~ith one or more additional bleaching steps involving chlorine, chlorine dioxide, hypochlorite salts (e.g. sodium hynochlorite), peroxides (hydrogen peroxide, alkali metal and alkaline earth metal peroxides, etc.), and the like. ~ny of the known oxygen bleach sequences can be used, including O, ODED, O~, OCED, OCEDED, etc.
I~ is preferred not to use chlorine as such in any of these subse~uent bleaching stages, since this ~iould defeat some of the relatively non-polluting characteristics of these ~xygen bleach sequences.
The mechanism by which the advantages of the protector system of this invention are obtained is complex and not fully understood. Although this invention is not bound by any theory, it is presently believed that at least three factors are involved:
(a) retention of inorganic substances, (b) reduction in carbohydrate degradation, and (c) re~oval of colcred or color-forming impurities.
Observations made in connection with the practice of this invention suggest that the relatively high silica-to-sodium oxide xatio and the colloidal nature of the silicate (e.g. water glass) are related in some manner to the achievement of these improvements ~or example, both pulp yield and pulp viscosity increase with increasing silica/sodium oxide ratios, at least for water glass.
The pulp viscosity obtained with a "~udox" HS-40 (trademark) protector is less than for water glasses with silica/sodium oxide weight ratios above 3.25:1, however. Stated another way, excellent overall performance can be obtained when the silicate solids comprise about 75 80 wt. % silica ~e.~ 3.25:1 - 4:1 silica-to-sodium oxide). Studies have been conducted in connection with this invention to investigate the increase in pulp yield in an effort to analyze this increase into inorganic and organic components. The results of these studies point to the conclusion that the increased yield is due in part to ~he retention of inorganic substances and in part to the reduced ioss of carbo-hydrates. Both of these yield increase factors are believed to be significant. The greater retention of carbohydrates is believed ( 106Z85~3 to indica-te that cellulose, hemicelluloses, and the like are given a higll level of protection protection during the oxygen bleaching step. The increased inorganic component of the yield is believed to indicate that some part of the protector (llence the resulting protective effect) is retained and is not lost or carried away in the e~fluent from the oxygen bleaching stage, washing stages, extraction stages, or the like. ~vailable data indicate that, in the typical practice of this invention, about half of the protector material is taken up by the pulp prior to or during oxygen bleaching, and at least part of this uptake remains with the pulp throughout any additional processing steps, including subsequent bleaching stages. As pointed out previously, the protector system which is not taken up by the pulp and goes into the spent liquox from the oxygen reactor or subsequent effluents is recoverable fron and/or recycleable in the process.
Improvements in factors such as yield and viscosity have been noted in comparisons with systems containing prior art pro-tectors and systems with no protector at all. It is believed to be p~rticularly surprising that very significant improvements have been noted with respect to the performance of other types of alkali metal silicates, e.g. sodium metasilicate. Although this invention is not bound by any theory, it is believed that the hydro~yl groups of the colloidal micelles of silica bond with the hydroxyl groups of the cellulose in the pulp, thus forming some type o~ chemical bond between the protector and the cellu-lose chain. It is this portion of the protector (i.e. the portion chemically linked up with the cellulose molecule) which is believed to account mainly for the increased inorganic component of the yield. That is, the chemically bound portion of the protector system is apparently not removed by washing or subsequent blcaching and shoJs up as increased ash content in the fully bleached pulp.
106Z858 ( - For example, it has been found that the ash content of a semi-bleached aspen krat pulp o~tained from the oxygen bleach stage was 2.7~ by weight. That of the same oxygen pulp taken through subsequent chlorine dioxide and hypochlorite bleaching stages (i.e. the entire 0-D-~I sequence) was 1.0~. The same aspen pulp bleached with the conventional CE/IID sequence had an ash content of only 0.3%. These facts are believed to support a ~ hvpothesis that hydroxyl groups of the silica bond to or with h~droxyl groups of the cellulose in the pulp. Further~ore, the chemical b~nd thus formed is presently beleived to block oxidation sites on the cellulose. A further hypothesis is that high-silica colloidal silicates can s~abilize or complex with organic intermed-iates produced by the reaction of carbohydrates with oxygen, thereby stopping or slowing down the conversion of the intermediates to the degradation products.
Protector systems of this invention also improve brightness and color reversion properties of oxygen bleached pulps. The hypo-thesis for this phenomenon is that metal ions, resins, methyl group-containing fragments, and other colored impurities are absorbed by the colloidal silicates. Apparently, these colored impurities are carried away by chemically unbound protector when the pulp is washed or are transformed into colorless compounds.
In any event, the superiority o protector systems of this invention as compared to sodium metasilicate is believed to provide some insight into the mechanism of protective action provided by this invention. It is theorized that the lower silica content and higher water solubility of sodium metasilicate prevents formation of polymeric micelles of silica capable of bonding reactions or other chemical effects with carbohydrates which reduce loss o carbohydrates. This conclusion is evidenced by the yields of the metasilicate-treated pulps (Table I, Example C).
lO~Z85fl The process of this invention can be carried out ~Jithin the broad framewor}~ of the preceding discussion. There are, of course, preferred procedures and process conditions which maximize the effectiveness of a protector system of this invention. ~ typical detailed step-by-step procedure is as follows:
(1) Commercial water glass (e.g. 35 - 59 Be.) is mixed _ with a roughly equal amount of water ~e.g. 50 - 200 parts per hundred by volume, depending upon the concentration of dissolved or dispersed solids in the water glass, the desired pH, etc.).
A praticularly suitable medium for diluting the commercial water glass is recycled spent liquor effluent from the oxygen bleaching staga.
(2) The diluted water glass protector system is added together or in sequence with kraft cooking liquor, i.e. white liquor ~or some other suitable al~aline medium such as an aqueous solution of 1 - 10 wt. % NaOH) evenly onto an unbleached pulp which has been obtained from either a separate or an in-plant pulp mi~l.
The development of oxygen bleaching of chemical pulp has opened up the possiblity of relatively non-polluting bleach sequences such as O, ODED, OH, CDOD, OCDEDED, OCEDED, OCED, etc.
wherein O is the oxygen bleach stage, and C, D, E, and H are as defined previously. The symbol CD represents a chlorination stage with some chlorine dioxide. It presently appears that the C stage can be eliminated from many of these sequences. ~any of the significant advantages of oxygen bleaching are described in an article by Gordon Rowlandson in the TAPPI Journal, 54 (No. 6), pages 962 - 967 (~une, 19713. The article describes the possibility o decreased BOD and COD, decreased chemical consumption, adequate pulp strengths, and, perhaps most important, markedly decreased chloride content in the effluent from the bleaching sequences.
~Iowever, the picture presented by currently available data concerning oxygen bleaching of brownstock (e.g. from kraft 10628SI~
pulping) is not uniformly encouraging. The number of variables to be contended ~Jith in producing a good bleached or semi-bleached pulp is formidable. These variables are typically controlled to achieve certain desired properties of the hleached pulp, e.g.
brightness, color, stability to~ard color reversion, opacity, pulp viscosity, pulp strength, lignin content, etc. One factcr which has received attention in the literature is perhaps one ~ o~ the most important from a practical standpoint: the yield of bleached or semi-bleached pulp. (Yield is rarely discussed in the patent literature regarding oxygen bleaching, however.~
Traditionally and practically the term "pulp yield" includes both ino~ganic and organic substances in the pulp.
This much is presently known: oxygen gas treatment of pulp does have a degradative'effect, resulting in bleached or semi-bleached pulp with less mechanical strength, lower pulp viscosity, and lower pulp yield.
Much of the prior art data relating to the oxygen bleach process is concentrated in the measurement of properties such as brightness, color reversion, physical strength values and the like. Efforts have been made to up-grade some or all of these- -properties by, in effect, protecting the useful carbohydrateæ
in the pulp fxom oxidative degradation. It has been found that certain inorganic compounds appear to provide this protective function; see U.S. Patent Nos. 3,657,065 (Smith et al)', issued April 18, 1972; 3,725,194 (Smith et alj, issued April 3, 1973;
and 3,7~0,310 (Smith et al), issued June 19, 1973. The protector compounds are typically inoryanic salts, oxides and hydroxides o~ al};ali or alkaline earth me.als, although silica and zinc and titanium compounds have received some mention. The primar~
emphasis in the patent literature, the scientific literature, ` 1062858 and in commercial practice is on the use of magnesium salts such as magnesium carbonate, magnesium silicate, magn~sium oxide, magnesium sulfate (e.g. epsom salt), and the like. The use of certain siliceous materials other than magnesium silicate has been suggested, i.e. sodium metasilicate, sodium orthosilicate, silica, and silicic acid. In this regardt there appears to be preference in the prior art for water soluble sodium metasilicate.
The use of "protectors' has advanced the state of the oxygen bleaching art, but further improvement in protection against oxidative degradation is needed. The goal of developin~
an oxygen bleach method or sequence which produces bleached or semi-bleached pulp with characteristics fully equivalent to, for example, the conventional CEHED or CEDED sequence is still generally unreached.
There has also been some recent research on the subject of protecting useful carbohydrates during the pulping process itself, e.g. during soda-oxygen pulping. ~owever, the situations, reactions, and mechanisms of oxygen pulping are considered by many scientists to be different from oxygen bleaching. Further-more, many scientists believe that inhibitors or pxotectors ingeneral give relatively insignificant benefits in oxygen pulping as compa~ed to oxygen bleaching. Accordingly, the answers to the problems discussed previously will apparently have to develop out of the art of oxygen bleaching.
Summary of the Invention It has now been discovered that the alkali metal colloidal silicates having a molar ratio of silica to alkali metal o~ide o at least 1.6:1 (preferably the high-silica wa~er glasses), used as protectors in oxygen bleaching, can provide surprising improvcments, compared to conventional protectors or no protector at all, e~g. in semi-bleached or bleached pulp yield. The increase in yield is obtained with no significant loss of brightness, stability to color reversion, viscosity, strength, opacity, delignification, and the like; on the contrary, some of these properties (such as viscosity, strength, opacity and oxygen stage brightness) are enhanced, using the same standards for compari-~ son. Perhaps the most surprising observation is that a high-silica protector of this invention (e.g. a water glass with a silicaj sodium oxide weight/weight ratio of 3.25:1) has particularl~ marked superiority when compared to sodium metasilicate, i.e. the compound variously written Na2SiO3 or Na2O SiO2, wherein the silica/sodium oxide mole ratio is 1:1 and tne weight ratio is 0.97~ ield data indicate that an addition of water glass to the pulp, expressed as SiO2 on a weight/weight basis, of 0.1 - 4% can produce an increase in the semi-bleached oxygen pulp yield of about 1~
or more. Even if allowances are ~ade for the increase in yield attributable to increased inorganic residue in the pulp, the organic yield increase is still very significant from a cost standpoint. In any event, the ash-free pulp yield would not tell the entire story~ It can be very significant that both the inor-ganic and organic yield are increased according to the present invention, since it is desirable for the protector to re~ain with the pulp throughout the entire bleaching sequence.
The colloidal, high-silica alkali metal silicate protectors of this invention can be substituted for the conventional magnesium salt protectors of the present co~mercial oxygen bleach processes ~-ith improved results. Among the preferred o~ygen bleach sequences of this invention are O-D, O-D-r~, O-D-Æ-D, and other sequences starting with an o stage. It has also been found that the spent liquor effluent from the oxygen stage can be recycled to the oxygen reactor by blending it t`7ith as little as 40 - 60~ by weight of a fresh water glass protector medium. If kraft pulp is made in-plant, the caustic in the spent liquor from oxygen bleaching can be recovered in the conventional manner through the kraft mill recovery system. It has been disclosed that sodium silicate has no adverse effect upon the chemical recovery equipment and may even have a beneficial anti-corrosive effect.
A further benefit of this invention is that the biological oxygen demand and chemical oxygen demand levels in the spent bleach liquor tend to be even less when compared to those of the conven-tional oxygen bleach effluent. This beneficial result is appar-ently related to the increased pulp yield, i.e. the reduced level of dissolved organic solids in the bleach liquor.
In the preferred practice of this invention, a commercial water glass te.g. 35 - 59 Be.) is diluted with water to 5 - 50~
by weight of solids and is thoroughly blended with the unbleached pulp, together with a suitable caustic medium. The resulting pulp mixture is then bleached with oxygen gas at elevated temperatures (e.g. 95 - 135~C.) under superatmospheric pressure (e.g. 70 ~
225 psig), and the resulting semi-bleached pulp is used as such or conveyed to the subsequent bleaching stages. A high-silica water glass is the preferred protector, i.e. a water glass with an SiO2/Na2O mole ratio in the range o about 2 to 4~
A less preferred practice of the invention is to use a protector with an SiO2/Na20 weight ratio of up to about 100:1.
Although these higher ratio materials are operative in the invention, they are not, strictly speaking, water glasses and can be more accurately described as surface-hydroxylated colloidal sols.
1 Scope of the Invention This invention is a method of bleaching chemical or semi-chemical cellulosic pulp which involves subjecting the pulp to the bleaching action of oxygen gas in an oxygen gas bleaching zone under superatmospheric pressure and tempera-tures elevated above room temperature in the presence of an alkaline medium and a yield-increasing amount of a protector of this invention, the protector consisting essentially of an alkali metal colloidal silicate having a molar ratio of silica to alkali metal oxide of at least 1.6:1. For example, the protector can consist essentially of a water glass having an SiO2/Na2O ratio within the range of about 2:1 to 4:1 or a surface-hydroxylated silica sol, which can have a much greater SiO2/Na2O ratio, e.g. up to 100:1. The type of pulp most commonly used in this invention is a kraft or sulfate pulp.
A further aspect of the method of this invention is that the semi-bleached pulp recovered from the oxygen bleaching zone can be further bleached in at least one additional bleaching step with a bleaching agent which agent preferably is chlorine, chlorine dioxide, hypochlorite, or peroxide, the most preferred bleaching sequence for producing fully bleached pulp being oxygen-chlorine dioxide-hypochlorite (O-D-H)~
When water glass is used as the protector, the amount of addition of water glass to the pulp, expressed as SiO2 on a weight basis, is preferably 0.1-4~ on pulp. It is also preferred to combine the spent liquor effluent from the oxygen bleaching zone with fresh water glass and recycle the resulting mixture to the oxygen bleaching zone.
~0628513 1 The Unbleached Pulp Oxygen bleaching improves the brightness and stability of virtually any type of pulp, particularly chemical pulps such as soda, sulfite, or sulfate (kraft) pulp. Of the chemical pulps, kraft pulp is the most significant commer-cially and also tends to be particularly dark in color.
(A typical kraft pulp is referred to as "brownstock", and, if the pulp is made into paper without bleaching, the resulting paper has the characteristic brown color of heavy wrapping paper.) Both hardwoods and softwoods can be utilized in the kraft process. Thus, although much of the experience with this invention relates to pine kraft pulp and aspen kraft pulp, this invention is not limited to the use of pulp derived from any particular hardwood or softwood.
Chemical pulps can be produced in pulp mills (e.g.
kraft pulp mills) and obtained for use in a separate oxygen bleach facility. Alternatively, an oxygen bleach facility can be installed right at the pulp mill, so that the pulp or brownstockcan be produced in-plant.
In this case, the protector which remains in the spent liquor effluent from the oxygen stage can be either . . .
- 8a -' ~
106Z85~3 recovered in the pulp mill recovery system and/or recycled for use with fresh protector in the oxygen bleaching step.
The Oxygen Bleach System The alkaline Medium charged to the oxygen reactor typically comprises: (a) a caustic liquor containing about 1 to a~out 5%
basic material (expressed as sodium hydoxide), (b) the bleaching - agent, i.e. oxygen gas, and (c) the protector medium. In commercia~
practice, of course, the oxygen gas is not pure. However, commer-cially available oxygen gas can be substantially free of carbon dioxide and hydrogen and can contain negligible amounts of carbon monoxide and nitrogen. Thus, the partial pressure of oxygen in the bleaching zone can be at least 85 or 90% of the total pressure. Commercial supplies of oxygen with a maximum of 0.5~
impurities are presently available. As is known in th art, oxygen gas as a bleaching agent is readily distinguishable in its behavior and properties from chemically combined sources of oxygen, e.g.
the peroxides. For example, oxygen gas has a rather strong delig-nifying effect under conventional oxygen bleach conditions. Unfor-tunately, as pointed out previously, there can be no ~uestion that bleaching of pulps with oxygen in alkaline media results in significant losses in yield and viscosity. These losses are believed to be primarily due to severe degradation of cellulose and hemicelluloses. The protector system of this invention is designed to reduce or eliminate these losses without sacrificing the other desirable features of the oxygen bleach process.
The protector of this invention can be considered to be an essentially aqueous medium containing a dispersed colloidal silicate having a molar ratio of silica to alkali metal oxide of at least 1.6~ colloidal sodium silicate made by ( 106285~f ( interacting silica sand and sodium carbonate at high tempexatures to produce a product with a silica/sodium oxide mole ratio of at least 1.6:1 and then dispersing the product in water can be referred to as a sodium water glass. A surface-hydroxylated sol is another type of colloidal system which will be discussed subsequently. ~s poin-ted out in Shreve, The Chemical Process Industries, second edition, ~cGraw-Hill, New York, 1956, pages _ 280-282, silicates that have a silica/sodium oxide ratio of 1.6 1 up to 4:1 are called colloidal silicates, and at the present time there are over 40 varieties of commercial sodium silicates, each with a specific use. When the silica/sodium oxide weight ratio is, for example, 3.2:1, 43 Be. is the upper limit of concentration (40% solids). For a 2:1 silica/sodium oxide ratio (also by weight), the upper limit is 60 Be. or 54 weight % solids. Concentrations up to about 65 weight % solids are known, e.g. in the case of the l.6:1 (wt./wt.) silica/sodium oxide ratio. For the 3.75:~
weight ratio, the maximum concentration is 35 Be. or less than 33 weight % solids. Because of handling difficul~ies, the extremely high Baume values are not preferred, and 35-59 Be. is the typical range of concentrations utilized in this invention.
It is ordinarily preferred to dilute the commercial 35-59 Be.
water glass with water prior to combining it with the pulp and the alkaline medium. Excessive dilution (e.g. to 5~ or lessj can create dispersion stability problems, however. As a convenient rule of thumb, O.5 - 2:1 (by volume) dilution of the commercial water glass with water is suitakle. As will be explained subse-quently, dilution with recycled s~ent oxygen bleach liquor is particularly desirable from a cost standpoint; preferred solids content for the diluted water glass is 15-35% by weight.
-- 1 o 106Z85~
~ otassium silicates with higll silica/potassium oxide ratios are known', e.g. about 2 to 3:1 on a weiyht/weight basis or about 3 to 4:1 on a mole/mole basis. These potassium water glasses (e.g. the 2.5:1 weigllt ratio material) perform in about the same manner as sodium water glasses of similar weight ratio. Baume values for the potassium water glasses tend to be lower than those of the colloidal sodium silicates. Since potassium silicates _ generally are consiaerably more expensive than sodium silicates, they are not preferred for use in this invention.
A variety of silica/sodium oxide ratios ~hroughout the 1.6 - 4:1 range are presently available as commercial water glass materials, e.g. from Philadelphia Quartz Company, Diamond Shamrock Company, and the duPont Company. Among these ratios are the following (all on a weight/weight basis): 1.6, l.90, 1.95, 2.0, 2.20, 2.38, 2.4, 2.55, 2.87, 3.22, 3,25, 3.40, and 3.75, As the weight ratio increases above 3.9:1 or the mole ratio increases above 4:1, water glass materials tend to acquire greater and greater sensitivity towards carbon dioxide. Carbon dioxide absorbed from the air can neutralize the alkalinity of the silicate, ultimately resulting in precipitation and/or gellation of the silica. This phenomenon is sometLmes,referred to as "reversion'l.
The more alkaline silicates with silica/sodium oxide ratios below 4:1 (on a moie basis) are more difficult to neutralize and hence moxe stable towards carbon dioxide. Furthermore, a sodium carbonate/silica reaction product with an excessively high silica content can be difficult to disperse in water, making its commer-cial production in the water glass form less practical.
Stable sols or aqueous dispersions characterized by extremely high silica/sodium oxide ratios can be obtained by surface hydroxylating discrete spherical particles of silica. For example, 106285~
"Ludox" (trademark of the duPont Company) is a stable colloidal system with an SiO2/Na2O ratio (by weight) of 50:1 or more, and ratio of about 100:1 and even above 200:1 have been achieved in practice.
The "Ludox" systems are moderately alkaline (p~ above 8 but less than 10). Thus, "Ludox" can be distinguished from an aqueous dispersion of colloidal silica in that the pH of "Ludox" is much higher; furthermore, silanol groups on the surface of the "Ludox"
~particles have been converted to anionic sites, thereby resulting in surface charges which can be different from pure silica, e.g.
Na+ OSi--In short, it is a complex matter to draw relationships between "Ludox" and water glass, though similarities can be found~
The low-silica water glasses can approach a pH of 13, and high-silica water glasses have a degree of alkalinity roughly inter-mediate between "Ludox" and the low-silica water glasses. Both "Ludox" and water glasses have anionic character at least in alkaline media, and the anionic character can persist even when the pH is deliberately manipulated downward. Cationic or electropositive character is not observed until the p~I is well into the acid range. Like high-silica water glasses, "Ludox" ~IS, LS, SM, and TM can be converted to gels through acidification. It has been found that "Ludox" has, in the context of this invention, no sig~
nificant advantages over suitably selected water glasses. Some disadvantages have been noted, however.
Despite the sensitivity to acidification characteristic of sodium silicates, data obtained according to this invention gener-ally indicate that significant advantages can be obtained by increasing silica/sodium oxide ratio of the protector sys~em beyond abou, 2:1 and up to the limits of colloidal stability for water glass (e.g. 4:1 on a mole basis or 3.9:1 on a weight basis) (` 1062~358 There is a slight loss in brightness with the higher silica protec-tors, but this loss is insignifica~t when weighted against the ad-,vantages. ~sing surface-hydroxylated colloidal silica sols, data have been obtained for a silica/sodium oxide ratio ~by weight) of almost 100:1. These data indicate the,possibility of a slight yield increase obtainable at this high ratio, but even this slight increase is obtained with significant sacrifices, e.g. higher protector cost, lower pulp viscosity, and lower brightness as compared to some water glass protectors, e.~. those containing more than 75 weight % silica.on a dry basis.
.
'The Process Once the pulp has been obtained and the protector system has been formulated, the most effective way to protect the pulp is to add the protector system of this invention to the unbleached pul~
before it is introduced into the oxygen bleaching zone. The pro-tector system is preferably not introduced directly into the reactor, i.e. oxygen bleaching zone. Since the oxygen bleaching zone is typically a closed reactor under superatmospheric pressure, the more effective procedure is also the simpler one.
The semi-bleached pulp which is obtained from the,oxygen bleaching step has suf'ficient brightness and adequate color for use in a variety of papers ~hich do not require a high level of brightness. Alternativel~ ~and moxe typically) the semi-bleached pulp is brought to a fully bleached state ~ith one or more additional bleaching steps involving chlorine, chlorine dioxide, hypochlorite salts (e.g. sodium hynochlorite), peroxides (hydrogen peroxide, alkali metal and alkaline earth metal peroxides, etc.), and the like. ~ny of the known oxygen bleach sequences can be used, including O, ODED, O~, OCED, OCEDED, etc.
I~ is preferred not to use chlorine as such in any of these subse~uent bleaching stages, since this ~iould defeat some of the relatively non-polluting characteristics of these ~xygen bleach sequences.
The mechanism by which the advantages of the protector system of this invention are obtained is complex and not fully understood. Although this invention is not bound by any theory, it is presently believed that at least three factors are involved:
(a) retention of inorganic substances, (b) reduction in carbohydrate degradation, and (c) re~oval of colcred or color-forming impurities.
Observations made in connection with the practice of this invention suggest that the relatively high silica-to-sodium oxide xatio and the colloidal nature of the silicate (e.g. water glass) are related in some manner to the achievement of these improvements ~or example, both pulp yield and pulp viscosity increase with increasing silica/sodium oxide ratios, at least for water glass.
The pulp viscosity obtained with a "~udox" HS-40 (trademark) protector is less than for water glasses with silica/sodium oxide weight ratios above 3.25:1, however. Stated another way, excellent overall performance can be obtained when the silicate solids comprise about 75 80 wt. % silica ~e.~ 3.25:1 - 4:1 silica-to-sodium oxide). Studies have been conducted in connection with this invention to investigate the increase in pulp yield in an effort to analyze this increase into inorganic and organic components. The results of these studies point to the conclusion that the increased yield is due in part to ~he retention of inorganic substances and in part to the reduced ioss of carbo-hydrates. Both of these yield increase factors are believed to be significant. The greater retention of carbohydrates is believed ( 106Z85~3 to indica-te that cellulose, hemicelluloses, and the like are given a higll level of protection protection during the oxygen bleaching step. The increased inorganic component of the yield is believed to indicate that some part of the protector (llence the resulting protective effect) is retained and is not lost or carried away in the e~fluent from the oxygen bleaching stage, washing stages, extraction stages, or the like. ~vailable data indicate that, in the typical practice of this invention, about half of the protector material is taken up by the pulp prior to or during oxygen bleaching, and at least part of this uptake remains with the pulp throughout any additional processing steps, including subsequent bleaching stages. As pointed out previously, the protector system which is not taken up by the pulp and goes into the spent liquox from the oxygen reactor or subsequent effluents is recoverable fron and/or recycleable in the process.
Improvements in factors such as yield and viscosity have been noted in comparisons with systems containing prior art pro-tectors and systems with no protector at all. It is believed to be p~rticularly surprising that very significant improvements have been noted with respect to the performance of other types of alkali metal silicates, e.g. sodium metasilicate. Although this invention is not bound by any theory, it is believed that the hydro~yl groups of the colloidal micelles of silica bond with the hydroxyl groups of the cellulose in the pulp, thus forming some type o~ chemical bond between the protector and the cellu-lose chain. It is this portion of the protector (i.e. the portion chemically linked up with the cellulose molecule) which is believed to account mainly for the increased inorganic component of the yield. That is, the chemically bound portion of the protector system is apparently not removed by washing or subsequent blcaching and shoJs up as increased ash content in the fully bleached pulp.
106Z858 ( - For example, it has been found that the ash content of a semi-bleached aspen krat pulp o~tained from the oxygen bleach stage was 2.7~ by weight. That of the same oxygen pulp taken through subsequent chlorine dioxide and hypochlorite bleaching stages (i.e. the entire 0-D-~I sequence) was 1.0~. The same aspen pulp bleached with the conventional CE/IID sequence had an ash content of only 0.3%. These facts are believed to support a ~ hvpothesis that hydroxyl groups of the silica bond to or with h~droxyl groups of the cellulose in the pulp. Further~ore, the chemical b~nd thus formed is presently beleived to block oxidation sites on the cellulose. A further hypothesis is that high-silica colloidal silicates can s~abilize or complex with organic intermed-iates produced by the reaction of carbohydrates with oxygen, thereby stopping or slowing down the conversion of the intermediates to the degradation products.
Protector systems of this invention also improve brightness and color reversion properties of oxygen bleached pulps. The hypo-thesis for this phenomenon is that metal ions, resins, methyl group-containing fragments, and other colored impurities are absorbed by the colloidal silicates. Apparently, these colored impurities are carried away by chemically unbound protector when the pulp is washed or are transformed into colorless compounds.
In any event, the superiority o protector systems of this invention as compared to sodium metasilicate is believed to provide some insight into the mechanism of protective action provided by this invention. It is theorized that the lower silica content and higher water solubility of sodium metasilicate prevents formation of polymeric micelles of silica capable of bonding reactions or other chemical effects with carbohydrates which reduce loss o carbohydrates. This conclusion is evidenced by the yields of the metasilicate-treated pulps (Table I, Example C).
lO~Z85fl The process of this invention can be carried out ~Jithin the broad framewor}~ of the preceding discussion. There are, of course, preferred procedures and process conditions which maximize the effectiveness of a protector system of this invention. ~ typical detailed step-by-step procedure is as follows:
(1) Commercial water glass (e.g. 35 - 59 Be.) is mixed _ with a roughly equal amount of water ~e.g. 50 - 200 parts per hundred by volume, depending upon the concentration of dissolved or dispersed solids in the water glass, the desired pH, etc.).
A praticularly suitable medium for diluting the commercial water glass is recycled spent liquor effluent from the oxygen bleaching staga.
(2) The diluted water glass protector system is added together or in sequence with kraft cooking liquor, i.e. white liquor ~or some other suitable al~aline medium such as an aqueous solution of 1 - 10 wt. % NaOH) evenly onto an unbleached pulp which has been obtained from either a separate or an in-plant pulp mi~l.
(3) The diluted protector, the caustic medium, and the pulp are thoroughly mixed.
(4) The resulting mixture is then passed through a suitable feeder and fluffer device into the oxygen reactor (i.e.
the oxygen bleaching zone). The following are the ranges of desirable operating conditions for the oxygen bleaching zone:
Pulp Consistency (T 240 su-67): 8 - 30v by weight Water Glass Addition (as SiO2): 0.1 - 4% by weight White Liquor (as ~aOH): 1 - 5% by weight Temperature: 95 - 135C, Time: 10 - 80 minutes Total Pressure: 80 - 190 psig (about 5.5 - 13.5 ~g/cm2 gauge) Final pH: 9 - 11 ` ( 106Z858 (`
~ s is known in the ar-t, higher concentrations of caustic soda and ~roader ranges of pressure (e.y~ 70 - 225 psig) can be used. Oxygen cons~ption and oY~ygen concentrations are comparable to those of the prior art and typically range from about 2 to about 4%.
the oxygen bleaching zone). The following are the ranges of desirable operating conditions for the oxygen bleaching zone:
Pulp Consistency (T 240 su-67): 8 - 30v by weight Water Glass Addition (as SiO2): 0.1 - 4% by weight White Liquor (as ~aOH): 1 - 5% by weight Temperature: 95 - 135C, Time: 10 - 80 minutes Total Pressure: 80 - 190 psig (about 5.5 - 13.5 ~g/cm2 gauge) Final pH: 9 - 11 ` ( 106Z858 (`
~ s is known in the ar-t, higher concentrations of caustic soda and ~roader ranges of pressure (e.y~ 70 - 225 psig) can be used. Oxygen cons~ption and oY~ygen concentrations are comparable to those of the prior art and typically range from about 2 to about 4%.
(5) The semi-bleached pulp obtained from the oxygen reactor is typically washed prior to any subsequent bleaching stages such as chlorine, chlorine dioxide, hypochlorite, and peroxide bleaching stages. Other washing steps can be used as desired in this process. For example, the brownstock or pulp is typically washed prior to being blended with the caustic medium and the protector system.
Re-cycling of unconsumed protector remaining in the effluent from the oxygen reactor and in other effluents can be carried out in a number of ways. For example, the fresh commercial water glass can be stored in any suitable manner and pumped to a makeup tank.
The flow from the makeup tank can be combined with the flow of-the recycled alkaline, excess protector-containing liquor from the oxygen reactor and passed through a seal tank into a high consistency press and conveyor, which is receiving washed brown-stock. Another stream of makeup protector combined with aqueous caustic soda or with white liquor passes from the seal tank throuyh a steam mixer into the feed system for the oxygen reactor. Mean-while, protector material from the high consistency press and conveyor can be sent to a dilution chest, where it can be combined with additional recycled unconsumed protector of the oxygen xeactor ef1uent and sent to the brownstock washer. The semi-bleached pulp washer can also be a source of unconsumed, recycleable protector.
106285~
A white liquor used in step (2), prior to any diiution or combination with other materials, typically has a p}l above 13 and can comprise ahout 50 - 130 gram!liter of NaOH. Total solids content of the white liquor is, of course, much higher, but these e~tra solids do not adversely affect the process of the invention.
In fact, it is preferred to use white liquor in step (2) if a suitable souxce is available.
Characteristics of Bleached and Semi-Bleached Pulp Obtained Accordinq To The Process of This Invention An outstanding advantqge of the process of this invention is its effect upon oxygen bleached pulp yield. Since the amoun~
of delignification is comparable to prior art oxygen bleaching processes, this increased yield would be significant in itse~f, even if other properties of the pulp (e.g. brightness, color, oolor stability, viscosity, strength, opacity, etc.) and of the effluent (color, pollutant levels, etc.) were no better than prior art processes. For example, a yield increase of 1% or even 0.5% by weight has economic significance. In this invention, yield increases of 2% or more are obtained by comparison to protectox-free oxygen bleach processes and processes using prior art pro-tectors such as epsom salt, magnesium carbonate, or sodiu~ meta~
silicate. Perhaps the most surprising finding is that an O-D-~I
sequence using this invention can provide a yield increase compared to the conventional C-E-H-E-D sequence for pine }craft pulp. The yield increase was further confirmed by comparing an O-D-H sequence of this invention with a conventional C-E/H-D sequence for aspen ]craft pulp. These i~provements in yield can be obtained without sacrificing bri~htness, viscosity, etc.
As pointed out previously, a study ~as made to determine the a~ount of the yield increase which is ash-free. T'nat is, the amount of increased yield attributable to an increase in ash content was determined (using TAPPI ash determination test T211 m-58) and then subtracted from the yield increase to deter-mine the organic yield increase. Ash-free yield increases were still significant, e.g. 0.5~ by weight or more, even when compared to oxygen stage bleached kraft pulp protected by a prior art epsom salt protector. The organic yield increase was particularly significant (more than 2% or even more than 3%) when compared to the ash-free pulp yield of a kraft pulp bleached with oxygen - in the presence of sodium metasilicate as the protector. Similaxly, ash-free yield increases were obtained in a comparison betweer, an O-D-H sequence of this invention and a C-E/~-D sequence of the prior art. To provide an oxygen bleach sequence with an ash-free pulp y~ield equal to conventional chlorine'bleach sequences would be a significant achievement; this invention has been observed to exceed even that goal.
In one experiment, three batches of unbleached aspen kraft pulp were treated with equal amounts of white liquor solids (3.8~ by weight) resulting in equal amounts of sodium hydroxide on the pulp (2% by weight). Each batch was then treated with a different protector system, i.e. sodium metasilicate ~3~ applied, expressed SiO2), epsom salt (1.2~ by weight applied), and 3.25:1 (silica/sodium oxide) water glass (3~ applied, as SiO2). The ash-free pulp yield from the oxygen bleaching stage using sodium metasilicate protector was 92.2%. Adding on the ash content of the yield (2.2~ by weight), the total yield was 94.4%. The yield obtained ~lith the epsom salt protector ~las somewhat better: 95.0%
ash-free yield, 1.0% ash conten-t, and 96.0% total yielcl. The ~ 062~3513 ash-free pulp yicld for the protector of this invention was 95.8~ by weight, even thou~h the ash content was the largest of the three (2.7~). The total yield (98.5%) ~as significantly higher, even as compared to the epsom salt run.
Using the same protector system of this invention in a O-D-H
sequence, an ash-free yield of 95.2~ and a final ash content of 1.3~ were obtained, for a total yield of 96.5~. The ash-free yield from a conventional C-E/H-D sequence was only 94.8%. Adding in the small ash content from this conventional sequence, (0.3~) the total pulp yield came to only 95.1%. The C-E/H-D sequence was repeated with somewhat better xesults (~6% total pulp yield), but the O-D-H sequence of this invention still appeared to be superior.
The present invention not only increases pulp yield, but also improves pulp birghtness as compared to oxygen bleach processes using the epsom salt protector or no protector at all. (The TAPPI test for optical properties is T 218 os-69.) Sodium metasilicate protectors can increase pulp brightness to a slightly higher level than obtained with water glass protectors of this invention, but this improved brightness is obtained at the expense of a significant sacrifice in yield.
So far as can be determined from presently available data, pulp brightness is a factor which does not appear to increase with increasing silicajsodium oxide ratios in the water glass protectors of this invention. Nevertheless, brightness data throughout the practical silica/sodium oxide range are satis-factory. For pine kraft pulp, G.E. brightness measurements above 40% (e.g. about 50 - 60~) are readily obtained in practice for the oxygen stage pulp. The G.E. brightness for oxygen stage aspen kraft pulp of this invention is typically above 60%, e.g. about 65 - 75%. (The G.E. brightness for unbleached kraft pulp is typically less than 35u.) The color, as well as the brightness, of semi-bleached and fully bleached pulps of this invention is satisfactory and is resistant to reversion. It has been found that the water glass protector systems of this invention can stabilize the brightness of the oxygen bleached pulp. The color xeversion _ for an aspen kraft pulp fully bleached in accordance with this invention in an O-D-H sequence was less of a problem than for the same aspen pulp fully bleached hy the conventional`C-E/H-D
sequence. Improvements in resistance to color reversion were also noted in comparing an aspen kraft pulp fully bleached by the O-D-H sequence using magnesium sulfate as the protector. Further comparisons were made with C-E/H and C-E-H-E-D sequences, and the findings again supported the proposition that superior brightness stability can be obtained with this invention. The improved brightness stability simplifies the bleaching sequence J by eliminating the need to bleach to a very high initial brightness level.
This invention has also achieved improvements in pulp viscosity and pulp strength, at least with respect to prior art oxygen bleaching processes using prior art protectors such as epsom salt and sodium metasilicate. The conventional viscosity measurement (TAPPI T230 su-66) makes use of a suitable solvent to provide a viscous fluid medium whose viscosity can be measured and reported in centipoise (cps). This viscosity test is somewhat similar to t'ne intrinsic viscosity ~ests used to determine the relative molecular weights of polymers. The higher the viscosity is, the higher the average molecular weight is, and the lower are the levels of degradation ol cellulose. Increases of at least ~062858 about 10~ in viscosity for water glass treated pulps of this inven-tion has been noted, as cor.lpared to the viscosity values of the epsom salt and sodium metasilicate treated pulps. Viscosity increases over prior art protectors ranging from 10% to 40 or 50%
can be obtained according to this invention. It has been found that there is a positive correlation between silicaJsodium oxide ratio and viscosity, at least up to 3.75:1 (by weight). In experiments with "Ludox" HS-40 (duPont's trademark for sùrface-hydroxylated colloidal silica) and various silicates, a plot o pulp viscosity vs. SiO2/Na2O ratio for ratios from 1:1 up to almost 100:1 was obtained for southern pine kraft pulp. The plot indicates a positive correlation from 1:1 to 3.75:1.
~owever, the viscosity at 93:1 is about equal to that obtained with the 2.55:1 ratio.
The general strength properties are improved when mag-nesium salts are used as the protector for the cellulosic fibersr Further improvement is still desirable in tear strength, however.
The protectors of this invention can provide this further improve-ment. ~hen tear factor is plotted against Scott bond for an O-D-H bleached pine kraft pulp of this invention, and when this plot is superimposed upon the same plot for a C-E-H-E-D bleached pine kraft pulp, it is difficult to detect any significant differences between the two plots. This comparison also indicates that the increase in ash content resulting from the use of a pro-tector system of this invention does not adversely affect the strength properties of the pulp.
Xraft pulp bleached according to the teachinys of this inven~ion can have higher opacity than conventional bleached pulp. ~ligher opacity is desirahle in higher quality printing paper. ~s in the com~arison o~ strength properties, the plot `` 106Z8~8 for an O-D-~I pine bleached kraft pulp of this invention was super-imposed within the same coordina-tes upon a C-E-H-E-D plot from the same type of pine bleached kraft pulp. Each plot was for opacity in percent vs. Canadian Standard Freeness ~CSF) in ml.
For a freeness ranging from 325 to 700 ml, the opacity for the C-E~ E-D pulp ranged from about 57 to about 73%. The O-D-H
plot of this invention tended to show an improvement of about ~1 - 2% throughout the freeness range.
Many of the pulp properties discussed previously can be affected by the lignin content of the pulp. Lignin is the major non-carbohydrate constituent of wood and woody plants and can comprise as much as 1/3 of untreated wood. Lignin is generally thought to be a three-dimensional polymer containing phenyl propane units. Although the lignin content of wood chips is significantly reduced in the pulping process tparticularly in kraft and soda pulping), the pulp is further delignified by an al~aline oxygen bleach process. Delignification can be accurately measured by chemical analysis. A simplified analysis for lignin content involves oxidation with potassium permanganate to obtain the "R" or kappa number (TAPPI test T214 su-71) -- a reasonably relia~le indicator of lignin content. Chemical pulps bleached according to the teachings of this invention are delign~-fied to approximately the same extent as pulps bleached by prior art oxygen processes. For exam~le, bleached pine kraft pulp obtained fro~ th~ oxygen bleaching stage of the process of this invention can be more than 60% delignified, and is generally 50 - 75~ delignified. T~ith higher silica/sodium oxide ratios (e.g. above 3.0 on a weight/weight basis), the amount of delig-nification is rougllly comparable to the prior art. ~urthermore, the lignin content can actually bP lower as compared to oxygen - 2~ -(` 1062fl58 ( bleached pulp whexein the protector used is sodium metasilicate~
For oxygen-bleached aspen };raft pulp, delignification is in excess of 40%, frequently 45 - 60~. Again, these results are comparable to or even superior to the prior art oxygen bleached pulps.
The effluent from the process of this invention appears to meet very high anti-pollution standards. Oxygen demand levels are decreased, and the color of spent oxygen bleach liquor is improved.
E ~ lPLES
In the following non-limiting Examples, protectors of this invention were selected to illustrate the use of colloidaIly dispersed compounds of the formula:
Na~O(siO2)x wherein x is 1.6 (see Example 11) or, more typicallyr larger than 2.0 (on a mole or weight basis). This was done by select-ing agueous silicates wherein x (on a mole basis) is about 2.6, about 3.35, and about 3.9; or, as x is more commonly expressed, 2.~5, 3.25, and 3.75 on a weight ~asis. Commercial water glass products from the duPont Company and from Philadelphia Quartz Company were used. (In Example 15, data on the silica/sodium oxiae weight ratio of 93:1 were obtained through the use of "Ludox" HS-40, trademark of duPont Company for surface-hydroxylated siIica.) Comparisons were made with:
(1) aqueous solutions of sodium metasilicate, obtained by dissolving C.P. grade sodium metasilicate nonahydrate cryst~ls in water, and (2) C. P. grade epsom salt.
Except for G.~. brightness data, all parts or percentages 0 are by weight. Standard TAPPI tests were used, including viscosity - ( 1062858 ( (T230 su-66), K No. (T214 su-71), optical properties (T218 os-69), ash content (T211 m-58), and consistency (T240 su-67).
Examples 1 - 3 and Controls (Pine ~raft Pulp) Essentially the same pine kraft pulp was bleached in this set of Examples and Controls. For Controls A and B
and Examples 1 and 2, the properties of the unbleached pulp were as follows:
K. No. 18.8 Viscosity (cps) 25.1 Brightness, G.E. ~ 24.6 For Example 3 and Controls C and D, the characteristics of the unbleached pine kraft pulp were as follows:
K. No. 18.9 Viscosity tcps) 24.8 Brightness, G.E. % 24.9 The oxy~en stage bleaching conditions were the same for ~xamples 1 - 3 and the Controls, except for the protector.
The protectors selected for t~le Controls ~ere as follows:
Control ~: No protector Control ~: Epsom salt ~1.2% applied on the pulp, expressed as magnesium sulfate heptahydrate; 0.6~
soli~s applied on the pulp) Control C: 3.0% ~y wt. sodium metasilicate, expressed as SiO2; 6.2 wt.%
~ solids applied on the pulp.
Control D: 2.5% sodium metasilicate; 5.0%
solids applied on the pulp.
The protector s~stems for Examples 1, 2, and 3 were as f~llo~.~s:
Si2/Na2 Ratio Percent Applied on Pulp Example By Weight As Sio7 As Solids . 1 3.25 3.0 4.0 2 3.25 1.9 2.5 3 2.55 3.0 4.2 The oxygen bleach conditions were as follows:
Consistency of Pulp: 25 ~hite Liquor Solids added r as NaOH: 4~
Maximum Pressure:170 psig Maximum Temperature: 120C.
Time To ~Iaximum Temperature:6 minutes Time at ~Iaximum Temperature:30 minutes The resulting semi-bleached pulps were tested for delignification (and F. No.), viscosit~ (standard TAPPI test), i.nitial ~.E. brightness, ~.E. brightness after 18 hours at 105C., and pulp yield. The results are set fortn in Table I.
106Z85~ -~ U~ N N
td ~¦ a~ N ~ O L~
h N 1/1 N O O O ~r h p I tY) CO L~ 0~ 1~ t`
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.
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, nS~
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~8 ,) ) 1062~58 Examples ~ ~ 9 and Controls (Aspen Xraft Pulp) The unbleached aspen ~raft pulp used in this entire set of Examples and Controls had the following characteristics.
K ~o.: 10.3 Viscosity, centipoise: 35.7 G.E. Brightness, %: 34.9 In this set of Examples and Controls, all bleaches were carried out at 170 psig and 130C. for 30 minutes.
Approximately 6 minutes ~ere required to bring the reactor and the pulp to these temperature and pressure conditions. The alkaline medium for the oxygen bleaching was a white liquor.
The amount of solids applied to the pulp from the white liquor was 3.8%; expressed as applied sodium hydroxide this amounted to 2%.
The following is a description of the controls.
Control E: No protector.
Control F: Epsom salt protector (1.2% as magnesium s~lfate heptahydrate;
0.6~ solids applied to the pulp).
Con~rol G: 2.5~ sodium metasilicate, as SiO2; 5.0~ solids applied to the pulp.
Con,rol H: 3.0~ sodium metasilicate, as sio2; 6.2~ solids applied to the pulp .
Examples 4 - 9 are described in the following Table.
SiO2~i~a2O Percent Applied on Pulp Ratio 30E~ample By Weight As SiO~ ~s Solids 4 2.55 3.0 4.2 3.25 3.0 4.0
Re-cycling of unconsumed protector remaining in the effluent from the oxygen reactor and in other effluents can be carried out in a number of ways. For example, the fresh commercial water glass can be stored in any suitable manner and pumped to a makeup tank.
The flow from the makeup tank can be combined with the flow of-the recycled alkaline, excess protector-containing liquor from the oxygen reactor and passed through a seal tank into a high consistency press and conveyor, which is receiving washed brown-stock. Another stream of makeup protector combined with aqueous caustic soda or with white liquor passes from the seal tank throuyh a steam mixer into the feed system for the oxygen reactor. Mean-while, protector material from the high consistency press and conveyor can be sent to a dilution chest, where it can be combined with additional recycled unconsumed protector of the oxygen xeactor ef1uent and sent to the brownstock washer. The semi-bleached pulp washer can also be a source of unconsumed, recycleable protector.
106285~
A white liquor used in step (2), prior to any diiution or combination with other materials, typically has a p}l above 13 and can comprise ahout 50 - 130 gram!liter of NaOH. Total solids content of the white liquor is, of course, much higher, but these e~tra solids do not adversely affect the process of the invention.
In fact, it is preferred to use white liquor in step (2) if a suitable souxce is available.
Characteristics of Bleached and Semi-Bleached Pulp Obtained Accordinq To The Process of This Invention An outstanding advantqge of the process of this invention is its effect upon oxygen bleached pulp yield. Since the amoun~
of delignification is comparable to prior art oxygen bleaching processes, this increased yield would be significant in itse~f, even if other properties of the pulp (e.g. brightness, color, oolor stability, viscosity, strength, opacity, etc.) and of the effluent (color, pollutant levels, etc.) were no better than prior art processes. For example, a yield increase of 1% or even 0.5% by weight has economic significance. In this invention, yield increases of 2% or more are obtained by comparison to protectox-free oxygen bleach processes and processes using prior art pro-tectors such as epsom salt, magnesium carbonate, or sodiu~ meta~
silicate. Perhaps the most surprising finding is that an O-D-~I
sequence using this invention can provide a yield increase compared to the conventional C-E-H-E-D sequence for pine }craft pulp. The yield increase was further confirmed by comparing an O-D-H sequence of this invention with a conventional C-E/H-D sequence for aspen ]craft pulp. These i~provements in yield can be obtained without sacrificing bri~htness, viscosity, etc.
As pointed out previously, a study ~as made to determine the a~ount of the yield increase which is ash-free. T'nat is, the amount of increased yield attributable to an increase in ash content was determined (using TAPPI ash determination test T211 m-58) and then subtracted from the yield increase to deter-mine the organic yield increase. Ash-free yield increases were still significant, e.g. 0.5~ by weight or more, even when compared to oxygen stage bleached kraft pulp protected by a prior art epsom salt protector. The organic yield increase was particularly significant (more than 2% or even more than 3%) when compared to the ash-free pulp yield of a kraft pulp bleached with oxygen - in the presence of sodium metasilicate as the protector. Similaxly, ash-free yield increases were obtained in a comparison betweer, an O-D-H sequence of this invention and a C-E/~-D sequence of the prior art. To provide an oxygen bleach sequence with an ash-free pulp y~ield equal to conventional chlorine'bleach sequences would be a significant achievement; this invention has been observed to exceed even that goal.
In one experiment, three batches of unbleached aspen kraft pulp were treated with equal amounts of white liquor solids (3.8~ by weight) resulting in equal amounts of sodium hydroxide on the pulp (2% by weight). Each batch was then treated with a different protector system, i.e. sodium metasilicate ~3~ applied, expressed SiO2), epsom salt (1.2~ by weight applied), and 3.25:1 (silica/sodium oxide) water glass (3~ applied, as SiO2). The ash-free pulp yield from the oxygen bleaching stage using sodium metasilicate protector was 92.2%. Adding on the ash content of the yield (2.2~ by weight), the total yield was 94.4%. The yield obtained ~lith the epsom salt protector ~las somewhat better: 95.0%
ash-free yield, 1.0% ash conten-t, and 96.0% total yielcl. The ~ 062~3513 ash-free pulp yicld for the protector of this invention was 95.8~ by weight, even thou~h the ash content was the largest of the three (2.7~). The total yield (98.5%) ~as significantly higher, even as compared to the epsom salt run.
Using the same protector system of this invention in a O-D-H
sequence, an ash-free yield of 95.2~ and a final ash content of 1.3~ were obtained, for a total yield of 96.5~. The ash-free yield from a conventional C-E/H-D sequence was only 94.8%. Adding in the small ash content from this conventional sequence, (0.3~) the total pulp yield came to only 95.1%. The C-E/H-D sequence was repeated with somewhat better xesults (~6% total pulp yield), but the O-D-H sequence of this invention still appeared to be superior.
The present invention not only increases pulp yield, but also improves pulp birghtness as compared to oxygen bleach processes using the epsom salt protector or no protector at all. (The TAPPI test for optical properties is T 218 os-69.) Sodium metasilicate protectors can increase pulp brightness to a slightly higher level than obtained with water glass protectors of this invention, but this improved brightness is obtained at the expense of a significant sacrifice in yield.
So far as can be determined from presently available data, pulp brightness is a factor which does not appear to increase with increasing silicajsodium oxide ratios in the water glass protectors of this invention. Nevertheless, brightness data throughout the practical silica/sodium oxide range are satis-factory. For pine kraft pulp, G.E. brightness measurements above 40% (e.g. about 50 - 60~) are readily obtained in practice for the oxygen stage pulp. The G.E. brightness for oxygen stage aspen kraft pulp of this invention is typically above 60%, e.g. about 65 - 75%. (The G.E. brightness for unbleached kraft pulp is typically less than 35u.) The color, as well as the brightness, of semi-bleached and fully bleached pulps of this invention is satisfactory and is resistant to reversion. It has been found that the water glass protector systems of this invention can stabilize the brightness of the oxygen bleached pulp. The color xeversion _ for an aspen kraft pulp fully bleached in accordance with this invention in an O-D-H sequence was less of a problem than for the same aspen pulp fully bleached hy the conventional`C-E/H-D
sequence. Improvements in resistance to color reversion were also noted in comparing an aspen kraft pulp fully bleached by the O-D-H sequence using magnesium sulfate as the protector. Further comparisons were made with C-E/H and C-E-H-E-D sequences, and the findings again supported the proposition that superior brightness stability can be obtained with this invention. The improved brightness stability simplifies the bleaching sequence J by eliminating the need to bleach to a very high initial brightness level.
This invention has also achieved improvements in pulp viscosity and pulp strength, at least with respect to prior art oxygen bleaching processes using prior art protectors such as epsom salt and sodium metasilicate. The conventional viscosity measurement (TAPPI T230 su-66) makes use of a suitable solvent to provide a viscous fluid medium whose viscosity can be measured and reported in centipoise (cps). This viscosity test is somewhat similar to t'ne intrinsic viscosity ~ests used to determine the relative molecular weights of polymers. The higher the viscosity is, the higher the average molecular weight is, and the lower are the levels of degradation ol cellulose. Increases of at least ~062858 about 10~ in viscosity for water glass treated pulps of this inven-tion has been noted, as cor.lpared to the viscosity values of the epsom salt and sodium metasilicate treated pulps. Viscosity increases over prior art protectors ranging from 10% to 40 or 50%
can be obtained according to this invention. It has been found that there is a positive correlation between silicaJsodium oxide ratio and viscosity, at least up to 3.75:1 (by weight). In experiments with "Ludox" HS-40 (duPont's trademark for sùrface-hydroxylated colloidal silica) and various silicates, a plot o pulp viscosity vs. SiO2/Na2O ratio for ratios from 1:1 up to almost 100:1 was obtained for southern pine kraft pulp. The plot indicates a positive correlation from 1:1 to 3.75:1.
~owever, the viscosity at 93:1 is about equal to that obtained with the 2.55:1 ratio.
The general strength properties are improved when mag-nesium salts are used as the protector for the cellulosic fibersr Further improvement is still desirable in tear strength, however.
The protectors of this invention can provide this further improve-ment. ~hen tear factor is plotted against Scott bond for an O-D-H bleached pine kraft pulp of this invention, and when this plot is superimposed upon the same plot for a C-E-H-E-D bleached pine kraft pulp, it is difficult to detect any significant differences between the two plots. This comparison also indicates that the increase in ash content resulting from the use of a pro-tector system of this invention does not adversely affect the strength properties of the pulp.
Xraft pulp bleached according to the teachinys of this inven~ion can have higher opacity than conventional bleached pulp. ~ligher opacity is desirahle in higher quality printing paper. ~s in the com~arison o~ strength properties, the plot `` 106Z8~8 for an O-D-~I pine bleached kraft pulp of this invention was super-imposed within the same coordina-tes upon a C-E-H-E-D plot from the same type of pine bleached kraft pulp. Each plot was for opacity in percent vs. Canadian Standard Freeness ~CSF) in ml.
For a freeness ranging from 325 to 700 ml, the opacity for the C-E~ E-D pulp ranged from about 57 to about 73%. The O-D-H
plot of this invention tended to show an improvement of about ~1 - 2% throughout the freeness range.
Many of the pulp properties discussed previously can be affected by the lignin content of the pulp. Lignin is the major non-carbohydrate constituent of wood and woody plants and can comprise as much as 1/3 of untreated wood. Lignin is generally thought to be a three-dimensional polymer containing phenyl propane units. Although the lignin content of wood chips is significantly reduced in the pulping process tparticularly in kraft and soda pulping), the pulp is further delignified by an al~aline oxygen bleach process. Delignification can be accurately measured by chemical analysis. A simplified analysis for lignin content involves oxidation with potassium permanganate to obtain the "R" or kappa number (TAPPI test T214 su-71) -- a reasonably relia~le indicator of lignin content. Chemical pulps bleached according to the teachings of this invention are delign~-fied to approximately the same extent as pulps bleached by prior art oxygen processes. For exam~le, bleached pine kraft pulp obtained fro~ th~ oxygen bleaching stage of the process of this invention can be more than 60% delignified, and is generally 50 - 75~ delignified. T~ith higher silica/sodium oxide ratios (e.g. above 3.0 on a weight/weight basis), the amount of delig-nification is rougllly comparable to the prior art. ~urthermore, the lignin content can actually bP lower as compared to oxygen - 2~ -(` 1062fl58 ( bleached pulp whexein the protector used is sodium metasilicate~
For oxygen-bleached aspen };raft pulp, delignification is in excess of 40%, frequently 45 - 60~. Again, these results are comparable to or even superior to the prior art oxygen bleached pulps.
The effluent from the process of this invention appears to meet very high anti-pollution standards. Oxygen demand levels are decreased, and the color of spent oxygen bleach liquor is improved.
E ~ lPLES
In the following non-limiting Examples, protectors of this invention were selected to illustrate the use of colloidaIly dispersed compounds of the formula:
Na~O(siO2)x wherein x is 1.6 (see Example 11) or, more typicallyr larger than 2.0 (on a mole or weight basis). This was done by select-ing agueous silicates wherein x (on a mole basis) is about 2.6, about 3.35, and about 3.9; or, as x is more commonly expressed, 2.~5, 3.25, and 3.75 on a weight ~asis. Commercial water glass products from the duPont Company and from Philadelphia Quartz Company were used. (In Example 15, data on the silica/sodium oxiae weight ratio of 93:1 were obtained through the use of "Ludox" HS-40, trademark of duPont Company for surface-hydroxylated siIica.) Comparisons were made with:
(1) aqueous solutions of sodium metasilicate, obtained by dissolving C.P. grade sodium metasilicate nonahydrate cryst~ls in water, and (2) C. P. grade epsom salt.
Except for G.~. brightness data, all parts or percentages 0 are by weight. Standard TAPPI tests were used, including viscosity - ( 1062858 ( (T230 su-66), K No. (T214 su-71), optical properties (T218 os-69), ash content (T211 m-58), and consistency (T240 su-67).
Examples 1 - 3 and Controls (Pine ~raft Pulp) Essentially the same pine kraft pulp was bleached in this set of Examples and Controls. For Controls A and B
and Examples 1 and 2, the properties of the unbleached pulp were as follows:
K. No. 18.8 Viscosity (cps) 25.1 Brightness, G.E. ~ 24.6 For Example 3 and Controls C and D, the characteristics of the unbleached pine kraft pulp were as follows:
K. No. 18.9 Viscosity tcps) 24.8 Brightness, G.E. % 24.9 The oxy~en stage bleaching conditions were the same for ~xamples 1 - 3 and the Controls, except for the protector.
The protectors selected for t~le Controls ~ere as follows:
Control ~: No protector Control ~: Epsom salt ~1.2% applied on the pulp, expressed as magnesium sulfate heptahydrate; 0.6~
soli~s applied on the pulp) Control C: 3.0% ~y wt. sodium metasilicate, expressed as SiO2; 6.2 wt.%
~ solids applied on the pulp.
Control D: 2.5% sodium metasilicate; 5.0%
solids applied on the pulp.
The protector s~stems for Examples 1, 2, and 3 were as f~llo~.~s:
Si2/Na2 Ratio Percent Applied on Pulp Example By Weight As Sio7 As Solids . 1 3.25 3.0 4.0 2 3.25 1.9 2.5 3 2.55 3.0 4.2 The oxygen bleach conditions were as follows:
Consistency of Pulp: 25 ~hite Liquor Solids added r as NaOH: 4~
Maximum Pressure:170 psig Maximum Temperature: 120C.
Time To ~Iaximum Temperature:6 minutes Time at ~Iaximum Temperature:30 minutes The resulting semi-bleached pulps were tested for delignification (and F. No.), viscosit~ (standard TAPPI test), i.nitial ~.E. brightness, ~.E. brightness after 18 hours at 105C., and pulp yield. The results are set fortn in Table I.
106Z85~ -~ U~ N N
td ~¦ a~ N ~ O L~
h N 1/1 N O O O ~r h p I tY) CO L~ 0~ 1~ t`
I .-..... -~ ~ U~_lOOOU~
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m ~ ~ ~ N N--i U~
.
d N ~`S
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, nS~
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~8 ,) ) 1062~58 Examples ~ ~ 9 and Controls (Aspen Xraft Pulp) The unbleached aspen ~raft pulp used in this entire set of Examples and Controls had the following characteristics.
K ~o.: 10.3 Viscosity, centipoise: 35.7 G.E. Brightness, %: 34.9 In this set of Examples and Controls, all bleaches were carried out at 170 psig and 130C. for 30 minutes.
Approximately 6 minutes ~ere required to bring the reactor and the pulp to these temperature and pressure conditions. The alkaline medium for the oxygen bleaching was a white liquor.
The amount of solids applied to the pulp from the white liquor was 3.8%; expressed as applied sodium hydroxide this amounted to 2%.
The following is a description of the controls.
Control E: No protector.
Control F: Epsom salt protector (1.2% as magnesium s~lfate heptahydrate;
0.6~ solids applied to the pulp).
Con~rol G: 2.5~ sodium metasilicate, as SiO2; 5.0~ solids applied to the pulp.
Con,rol H: 3.0~ sodium metasilicate, as sio2; 6.2~ solids applied to the pulp .
Examples 4 - 9 are described in the following Table.
SiO2~i~a2O Percent Applied on Pulp Ratio 30E~ample By Weight As SiO~ ~s Solids 4 2.55 3.0 4.2 3.25 3.0 4.0
6 3.75 1.5 1.9
7 3.75 0.5 0.6 ~ 2.55 1.5* 2.1 'J 2.55 2.1* 2.9 *The oxygen hleach spent liquor from Example 4 ~as used as recyclinc liquor to replace fresh water for ma};ing up to 25~- consistency. Thus, the Eresn water glass requirement was reduced considerahly.
The results of these ~leaching processes are cJiven in ~able II.
o 1~ (~, ~ o~
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O ~ O
~v 5~ ~ * ~7 ~ v ~1 1,7 ,~ ~ O ~ ~ ~ q ~ * ~ ~ ~
O~ "7,~ ~q~~ ,~ ,~ v v v 3~
- Examples 10 Complete Bleaching Sequence (O-D-H) .
~ In this comparison, the semi-bleached pine pulp of Example 3 was carried through a chlorine dioxide stage and a hypochlorite stage to produce a fully bleached pulp. ~The same O-D-H sequence was used for Control B.
Conditions in the chlorine dioxide stage were as follows.
Chlorine Dioxide Applied: 1.25~
Temperature: 150F.
Time: 180 minutes Consistency: 6%
The conditions for the hypochlorite stage were as follows~
- Sodium Hypochlorite Applied: 0.4~
Sodi~m Hydroxide Applied: 0.05%
Time: 70 minutes Tempera~ure: 110F.
Consistency: 6%
The results of the complete bleach sequences are given in Table III.
Table III
Results of O-D-~ Bleaching of Pine Kraft Pulp Bleached Pulp Control BExample 3 Yield, ~ 92.0 94.3 Viscosi~y, cps 7.8 - 11.
Bxightness, G.E. %
(initial) -- 84.0 Brightness,-G.E. %
(faded, 18 hr/105C.) 7~.0 78.
E~amples 11 - 15_ Oxygen Bleached Southern Pine Kraft Pulp In these Exam les, all bleaches were carried out under the condi Lions used for ~xamples 1 - 3, except that white li~uor solids (as NaOII) were 3 wt. % instead of 4 wt. %. A no-protector Control (similar to Control A) and a sodium metasilicate Control (si~lilar to Control C) were used. Examples 11 - 14 illustra-te ( 106Z85~ ( the silica/sodium oxide weight ratio range of 1.6 to 3.75:1.
In Example 15, the protector was "Ludox" ~S-40 (see introductory paragraph prior to Example 1), hence this ratio was 93:1.
The unbleached Southern Pine ~raft pulp used in the Controls and Examples 11 - 15 had a K No. of 22.9, a viscosity of 27.7 cps, and a brightness of 21.7% G.E. In the sodium metasilicate control and Examples 11 - 15, the amount of protector applied on the pulp texpressed as SiO2) was 3~ by weight.
The results for the Controls and this series of Examples are set forth in Table IV.
.
U~I ~ CO o ~ ~ o C~
. ...... ..
.. . . . . . . . .. .
U~
C~ G~ ~ O ~ o ~ . . ...... ..
~ o ~ o ~ ~ ~r~ ~ , . ~
U~
r~ u~ ~ o ~ ~ ~ ~r . . ...... ..
O ~ ~ ~ ~D U~ ~ ~ ~1 .
~ ~I N
O ~ N ~ I O
~ ~ t~
P ~ ~ I~
H q~ ~1 ~ ~~ ~ ~ ~ o ~
~3 O .~ o o ~ n o ~ ~ ,i o ~:1 Ut E~ ~ ~: . ' U . ' , 1 a~ . _I
O . ~ .
C) U O
~J I ~ .~ N ~ C;~ Lt~ ~D O O C) >t ~
O h I ~ ~ ~ o ~ oo o h O Z .' U
J~ ~ ' d. h _l ~ . p~
~S ~ ~ lC a~ I
Ul _ O ~: d. Ul O
~ a) ~ ~
_~ ~ ~ rd U
'- O ~,o Q) J:
I O ~ h H
O r~ U ~ S~d~ U O
h ~ ~ ~ O u~ S
H
~ 0~ ~ U t~
o a~ ~ ~ ~-rl ~
c~ O a) u ~ ~1 ~ h ~ ~ ~ .,~
s~ ~ o a ~ ~ cJ ~ ~ri Q) O X cn ~ t~ ~ ~ g h 0 aJ o ~ o tr~ O ~ ~ 4 U
~1 J~ ~~;-r~ O ~ p~d I
C) ~1 U~
~ O u~ C) ~ o ~ - ~ ~ ~ ~ m X r~ ~1 X
O ~C
Example 16 Potassium Water Glass An experiment essentially similar to Example 12 was repeated with "Kasil" grade water glass, which is reported to be an aqueous potassium silicate with a silica/potassium oxide ratio of 2.5:1.
The pulp was Southern Pine Kraft pulp. The resulting data were co~parable to Example 12, the numexical differences (e.g. the ~ slightly higher yield) being considered insignificant.
- 3~s -
The results of these ~leaching processes are cJiven in ~able II.
o 1~ (~, ~ o~
.... .. .
L') ~D ~ G~
I~ ~I L') ~
.... .. . . tq ~1 o0 0~ 1 h ~ ^
ao ~ ~ o Ir~ r` ~J ,~
~n 1~1 11'7 0 000 L~l 1~ ~ X ~ U~
~ . ~ .
.
u~ n tn t~ ~
K
o a~ d ~ o~
Q~ O S:-l Q a) ~r In o ~ o~ o ,4 Lq ~ 1 . . . .. . . . C) L') ~1 ~
n -O q tl5 ~
~ ~~ ~o H ~ N N ~t O 00 ~rt~l O ,~
H .,1 ~ .... .. , , ~; ~
2 o ~ ~ N ~1 1 D ~a No O 15~ N ~1 ~ ID p~W ~ ~n ~q ~ er U~ O~ O G~ ~:r ~ o G~ rl U
tn u ~ ~
~ ~ ~ ~ ~ O O N
X ~ i o ~ ~ U
O Q ord ~q ~ co N O ~ ~v U
~ ~ P ," ~
Pl 1:: 0 ~i U d~ 3 æ N ~
h v ` ~q O ~J Gl Q) 6) ~1 ~ ~ X
O~1 ~ I,q ~ ,L~ > ~ O G) o ~ir~ O-~ ~ ~O ~l~
O. ~ I,q ~ ~v--~ ~v U).rl rd 'v W ~ ~ ~ h V
O ~ O
~v 5~ ~ * ~7 ~ v ~1 1,7 ,~ ~ O ~ ~ ~ q ~ * ~ ~ ~
O~ "7,~ ~q~~ ,~ ,~ v v v 3~
- Examples 10 Complete Bleaching Sequence (O-D-H) .
~ In this comparison, the semi-bleached pine pulp of Example 3 was carried through a chlorine dioxide stage and a hypochlorite stage to produce a fully bleached pulp. ~The same O-D-H sequence was used for Control B.
Conditions in the chlorine dioxide stage were as follows.
Chlorine Dioxide Applied: 1.25~
Temperature: 150F.
Time: 180 minutes Consistency: 6%
The conditions for the hypochlorite stage were as follows~
- Sodium Hypochlorite Applied: 0.4~
Sodi~m Hydroxide Applied: 0.05%
Time: 70 minutes Tempera~ure: 110F.
Consistency: 6%
The results of the complete bleach sequences are given in Table III.
Table III
Results of O-D-~ Bleaching of Pine Kraft Pulp Bleached Pulp Control BExample 3 Yield, ~ 92.0 94.3 Viscosi~y, cps 7.8 - 11.
Bxightness, G.E. %
(initial) -- 84.0 Brightness,-G.E. %
(faded, 18 hr/105C.) 7~.0 78.
E~amples 11 - 15_ Oxygen Bleached Southern Pine Kraft Pulp In these Exam les, all bleaches were carried out under the condi Lions used for ~xamples 1 - 3, except that white li~uor solids (as NaOII) were 3 wt. % instead of 4 wt. %. A no-protector Control (similar to Control A) and a sodium metasilicate Control (si~lilar to Control C) were used. Examples 11 - 14 illustra-te ( 106Z85~ ( the silica/sodium oxide weight ratio range of 1.6 to 3.75:1.
In Example 15, the protector was "Ludox" ~S-40 (see introductory paragraph prior to Example 1), hence this ratio was 93:1.
The unbleached Southern Pine ~raft pulp used in the Controls and Examples 11 - 15 had a K No. of 22.9, a viscosity of 27.7 cps, and a brightness of 21.7% G.E. In the sodium metasilicate control and Examples 11 - 15, the amount of protector applied on the pulp texpressed as SiO2) was 3~ by weight.
The results for the Controls and this series of Examples are set forth in Table IV.
.
U~I ~ CO o ~ ~ o C~
. ...... ..
.. . . . . . . . .. .
U~
C~ G~ ~ O ~ o ~ . . ...... ..
~ o ~ o ~ ~ ~r~ ~ , . ~
U~
r~ u~ ~ o ~ ~ ~ ~r . . ...... ..
O ~ ~ ~ ~D U~ ~ ~ ~1 .
~ ~I N
O ~ N ~ I O
~ ~ t~
P ~ ~ I~
H q~ ~1 ~ ~~ ~ ~ ~ o ~
~3 O .~ o o ~ n o ~ ~ ,i o ~:1 Ut E~ ~ ~: . ' U . ' , 1 a~ . _I
O . ~ .
C) U O
~J I ~ .~ N ~ C;~ Lt~ ~D O O C) >t ~
O h I ~ ~ ~ o ~ oo o h O Z .' U
J~ ~ ' d. h _l ~ . p~
~S ~ ~ lC a~ I
Ul _ O ~: d. Ul O
~ a) ~ ~
_~ ~ ~ rd U
'- O ~,o Q) J:
I O ~ h H
O r~ U ~ S~d~ U O
h ~ ~ ~ O u~ S
H
~ 0~ ~ U t~
o a~ ~ ~ ~-rl ~
c~ O a) u ~ ~1 ~ h ~ ~ ~ .,~
s~ ~ o a ~ ~ cJ ~ ~ri Q) O X cn ~ t~ ~ ~ g h 0 aJ o ~ o tr~ O ~ ~ 4 U
~1 J~ ~~;-r~ O ~ p~d I
C) ~1 U~
~ O u~ C) ~ o ~ - ~ ~ ~ ~ m X r~ ~1 X
O ~C
Example 16 Potassium Water Glass An experiment essentially similar to Example 12 was repeated with "Kasil" grade water glass, which is reported to be an aqueous potassium silicate with a silica/potassium oxide ratio of 2.5:1.
The pulp was Southern Pine Kraft pulp. The resulting data were co~parable to Example 12, the numexical differences (e.g. the ~ slightly higher yield) being considered insignificant.
- 3~s -
Claims (15)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method for bleaching chemical or semi-chemical cellulosic pulp comprising subjecting said pulp to the bleaching action of oxygen gas in an oxygen gas bleaching zone under super-atmospheric pressure and elevated temperatures in the presence of an alkaline medium and a yield-increasing amount of a protector, said protector consisting essentially of an alkali metal colloidal silicate having a molar ratio of silica to alkali metal oxide of at least 1.6:1.
2. A method according to claim 1 wherein said colloidal silicate is a water glass having a silica:sodium oxide molar ratio within the range of about 2:1 to 4:1.
3. A method according to claim 1 comprising the additional step of obtaining said chemical or semi-chemical cellulosic pulp prior to said subjecting step and wherein said chemical pulp is kraft pulp.
4. A method according to claim 2 comprising the additional steps of:
(a) recovering a semi-bleached pulp from said bleaching zone, (b) further bleaching said semi-bleached pulp in at least one additional bleaching step with a bleaching agent selected from the group consisting of chlorine, chlorine dioxide, hypochlorite, and peroxide.
(a) recovering a semi-bleached pulp from said bleaching zone, (b) further bleaching said semi-bleached pulp in at least one additional bleaching step with a bleaching agent selected from the group consisting of chlorine, chlorine dioxide, hypochlorite, and peroxide.
5. A method according to claim 4 wherein the complete bleaching sequence for producing fully bleached pulp is oxygen-chlorine dioxide-hypochlorite.
6. A method according to claim 2 wherein the amount of addition of water glass to said pulp, expressed as SiO2 on a weight basis, is 0.1 - 4% on pulp.
7. A method according to claim 6 wherein the pulp consistency ranges from about 8 to about 30% by weight, and the time, temperature, and pressure in said oxygen bleaching zone range from about 10 to 80 minutes, about 95 to 135°C, and 70 - 225 psig, respectively; and wherein the final pH at the completion of said subjecting step ranges from 9 to 11.
8. A method according to claim 6 wherein the increase in semi-bleached pulp yield based on unbleached pulp, at the completion of said subjecting step, is at least about 1.0%
when compared to the yield obtained without the presence of any protector.
when compared to the yield obtained without the presence of any protector.
9. A method according to claim 8 wherein said semi-bleached pulp is fully bleached by additional bleaching steps subsequent to said subjecting step, and wherein there is an increase in fully bleached pulp yield of at least 1.0% based on unbleached pulp, when compared to the yield obtained without the presence of any protector.
10. A method according to claim 8 wherein the TAPPI
T230 su-66 viscosity measurement of the semi-bleached pulp at the completion of said subjecting step is increased as compared to the viscosity measurement for semi-bleached pulp obtained under the same conditions but in the absence of any protector, and wherein said viscosity increase is at least about 10%.
T230 su-66 viscosity measurement of the semi-bleached pulp at the completion of said subjecting step is increased as compared to the viscosity measurement for semi-bleached pulp obtained under the same conditions but in the absence of any protector, and wherein said viscosity increase is at least about 10%.
11. A method according to claim 6 wherein said pulp is a kraft pulp and wherein the spent liquor effluent from said oxygen bleaching zone is combined with fresh water glass and recycled to said oxygen bleaching zone.
12. A method according to claim 2 wherein said chemical or semi-chemical pulp is mixed with water glass and the resulting mixture is then introduced into said oxygen bleaching zone for bleaching according to said subjecting step.
13. In a method for bleaching a chemical or semi-chemical cellulosic pulp by subjecting the pulp to the action of oxygen gas in the presence of an alkaline medium at elevated temperature and super-atmospheric pressure, the improvement which comprises:
protecting the cellulose of said pulp from degradation and increasing the yield of bleached pulp by adding to said chemical or semi-chemical pulp a cellulosic-protecting amount of an alkaline material selected from the group of alkali metal colloidal silicates consisting of water glass and a surface-hydroxylated silica sol.
protecting the cellulose of said pulp from degradation and increasing the yield of bleached pulp by adding to said chemical or semi-chemical pulp a cellulosic-protecting amount of an alkaline material selected from the group of alkali metal colloidal silicates consisting of water glass and a surface-hydroxylated silica sol.
14. A method according to claim 13 comprising the following steps:
(a) mixing unbleached chemical or semi-chemical pulp with white liquor or caustic and a said alkaline material, said material comprising about 5 - 60% by weight of solids having a silica/sodium oxide ratio ranging from about 2 to about 100, said solids being colloidally and stably dispersed in water, (b) introducing the resulting mixture into an oxygen bleaching zone wherein said pulp is bleached to a G.E. brightness of at least 40% by the action of oxygen gas;
(c) washing the resulting semi-bleached pulp;
(d) bleaching said semi-bleached pulp further with at least one additional bleaching step utilizing a bleaching agent selected from the group consisting of chlorine dioxide and sodium hypochlorite.
(a) mixing unbleached chemical or semi-chemical pulp with white liquor or caustic and a said alkaline material, said material comprising about 5 - 60% by weight of solids having a silica/sodium oxide ratio ranging from about 2 to about 100, said solids being colloidally and stably dispersed in water, (b) introducing the resulting mixture into an oxygen bleaching zone wherein said pulp is bleached to a G.E. brightness of at least 40% by the action of oxygen gas;
(c) washing the resulting semi-bleached pulp;
(d) bleaching said semi-bleached pulp further with at least one additional bleaching step utilizing a bleaching agent selected from the group consisting of chlorine dioxide and sodium hypochlorite.
15. A method according to claim 14 comprising the further step of re-cycling at least a portion of the spent liquor effluent from said step (b) to said step (a), whereby at least part of the water glass medium used in said step (a) comprises said spent liquor effluent.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US55027475A | 1975-02-18 | 1975-02-18 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1062858A true CA1062858A (en) | 1979-09-25 |
Family
ID=24196466
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA234,066A Expired CA1062858A (en) | 1975-02-18 | 1975-08-25 | Oxygen bleaching of pulp in presence of high-silica protector |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS5220562B2 (en) |
| CA (1) | CA1062858A (en) |
| DE (1) | DE2552931A1 (en) |
| FR (1) | FR2301632A1 (en) |
| GB (1) | GB1519072A (en) |
| SE (1) | SE7600243L (en) |
| ZA (1) | ZA76274B (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102007036376A1 (en) * | 2007-07-31 | 2009-02-05 | Voith Patent Gmbh | Bleached pulp |
| SE543640C2 (en) * | 2019-09-04 | 2021-05-11 | Stora Enso Oyj | Fluff pulp from oxygen delignified pulp |
-
1975
- 1975-08-25 CA CA234,066A patent/CA1062858A/en not_active Expired
- 1975-11-26 DE DE19752552931 patent/DE2552931A1/en not_active Withdrawn
-
1976
- 1976-01-09 GB GB784/76A patent/GB1519072A/en not_active Expired
- 1976-01-12 SE SE7600243A patent/SE7600243L/en unknown
- 1976-01-16 JP JP51003295A patent/JPS5220562B2/ja not_active Expired
- 1976-01-16 FR FR7601074A patent/FR2301632A1/en active Granted
- 1976-01-19 ZA ZA00760274A patent/ZA76274B/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| FR2301632A1 (en) | 1976-09-17 |
| ZA76274B (en) | 1976-12-29 |
| SE7600243L (en) | 1976-08-19 |
| JPS5220562B2 (en) | 1977-06-04 |
| DE2552931A1 (en) | 1976-09-02 |
| GB1519072A (en) | 1978-07-26 |
| JPS5196505A (en) | 1976-08-24 |
| FR2301632B1 (en) | 1980-06-20 |
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