CA1043514A - Oxygen bleaching in the presence of a catalyst - Google Patents
Oxygen bleaching in the presence of a catalystInfo
- Publication number
- CA1043514A CA1043514A CA246,970A CA246970A CA1043514A CA 1043514 A CA1043514 A CA 1043514A CA 246970 A CA246970 A CA 246970A CA 1043514 A CA1043514 A CA 1043514A
- Authority
- CA
- Canada
- Prior art keywords
- pulp
- transition metal
- oxygen
- accordance
- slurry
- 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
ABSTRACT OF THE DISCLOSURE
During the delignification and bleaching of lignocellulosic pulp fibers with oxygen in an alkaline medium, the rate of deligni-fication can be increased, while simultaneously protecting the pulp against excessive losses in viscosity, by contacting a slurry of lignocellulosic pulp fibers with a solution of water-soluble salt of a divalent transition metal selected from the group consisting of mangenous, nickelous, cobaltous, and vanadous ions having a concentration at least about 0.27% to about 1.10%, based on the weight of oven-dried (O.D.) pulp.
During the delignification and bleaching of lignocellulosic pulp fibers with oxygen in an alkaline medium, the rate of deligni-fication can be increased, while simultaneously protecting the pulp against excessive losses in viscosity, by contacting a slurry of lignocellulosic pulp fibers with a solution of water-soluble salt of a divalent transition metal selected from the group consisting of mangenous, nickelous, cobaltous, and vanadous ions having a concentration at least about 0.27% to about 1.10%, based on the weight of oven-dried (O.D.) pulp.
Description
BACKGROUND OF THE I~VENTION
The present invention relates to a ~rocess for the bleaching of lignocellulosic pulp with oxygen in the presence of alkali, and more particularly to an oxygen/alkali bleaching process wherein the rate of delignification is increased while at the same time protecting the cellulosic portion against excessive losses in viscosity.
Pulp delignification with alkaline solutions of molecular oxygen, or "oxygen bleaching", like any other bleach-ing process, represent a compromise situation, whereby one seeks to obtain the maximum extent of delignification, while minimizing the degree of oxidative degradation, i.e,, depoly-merization of the cellulosic component.
Industrial utilization of oxygen bleaching was retarded for a number of years because the above-referred to compromise was heavily weighted on the side of cellulose de-gradation. m us, though acceptable delignification could be achieved with oxygen/alkali bleaching, the pulp mechanlcal properties were uniformly unacceptable. The discovery by Robert et al., U.S. 3,384,533, that magnesium compounds could effect a certain degree of cellulose protection against oxida-tive degradation, provided the impetus for the establishment of the first commercial oxygen bleaching system at Enstra, South Africa. While other researchers have from time to time found that other alkaline earth metals, aside from magnesium, could achieve a certain degree of cellulose protection, none have attained commercialization. Indeed, only the potassium iodide protection disclosed by Minor and Sanyer in the 104;~514 Journal of Polymer Science, Part C, No. 36:73 (1971) affords the degree of protection which is comparable with that of magnesium, or the complexes of magnesium.
Use of magnesium as a protection compound for cel-lulose during oxygen bleaching presents a number of practical problems. Not the least of these problems is the expense of the compound. The action of magnesium is purely specific to cellulose, and appears to have no effect on the rate or extent of delignification that can be achieved during oxygen bleach-ing. Another problem associated with the use of magnesium is its tendency to impart scale on the process equipment thus resulting in problems of encrustation.
Consequently, the pulp bleaching industry has sought new and improved methods by which oxygen bleaching can be con-ducted, preferably without using magnesium compounds. An ad-ditive which also catalyzes delignification, aside from pro-ducing non-profit cellulose protection is, accordingly, highly desirable. A step in that direction was provided by Roymoulik et al., U.S. 3,832,276, which describes a low con-sistency oxygen bleaching process that does not require the use of cellulose protectors. The process reduces cellulose de-gradation by lowering the concentration of hydroxyl ions through the use of low pulp consistency and through the use of declin-ing oxygen pressures during reaction.
Disclosure of catalysts which will increase the rate of deliqnification have been rather limited to date. Minor and Landucci have reported, (see International Pulp Bleaching Conference, 1973, Vancouver, B.C., p. 83) that the rate of ` ~O~Si4 delignification of pulp b~ ox~gen/alkali could be increased by the use of manganese (the form was not specified). Data was presented showing that whereas delignification of a southern pine groundwood pulp was considerably accelerated by the ad-dition of 0.01% manganese (basis not specified), only a small effect was noted with a kraft pulP containing an initial lignin content of 7%. A subsequent publication by Landucci, Minor and Sanyer, (Fourth Canadian Wood Chemistry Symposium, 1973, Quebec, Canada, p. 71) concerning the use of manganese to accelerate delignification of southern pine, stated that "manganese has no deleterious effect on carbohydrates", pre-sumably meaning that pulp viscosities were similar to those obtained when manganese was not used.
Although the catalytic effect of the manganese on deiignification is not difficult to understand, the finding that viscosity was unaffected is quite surprising. A number of investigators have quite conclusively demonstrated that manganese, like a number of other transition metal ions, has a very damaging effect on the viscosity of cellulose oxygen delignification. For example, Ericsson et al. (Svensk Papper-stidning, Volume 74 (22), November 1971, p. 757) determined that, whereas 40 parts per million (as manganese, based on pulp weight,) of manganous sulfate had little in1uence on the vis-cosity of cotton linters, i.e., pure cellulose, during oxygen/
alkali treatment, repeating the experiment in the presence of 5% lignin (based on pulp weight) physically added to the mix-ture gave a disastrous decrease in the viscosity of the cotton linters. Other heavy metals, such as iron and copper were found to behave in a similar manner. Concern about the presence `` ~0~35i4 of manganese during oxygen bleaching is of critical industrial importance. Myburgh, for example, reported (TAPPI: Vol. 57 [5], p. 131, 1974) that the Enstra, South Africa, oxygen bleach plant required that unbleached pulp be prewashed with acid to dissolve out heavy metals, prior to entering the oxygen bleach reactor. Myburgh states that the unbleached pulp at Enstra contains 120-160 parts per million of manganese, which must be reduced blow 30 parts per million, or the metal will have "have a catalytic degrading effect on cellulose during oxygen bleach-ing."
To further confuse the seeming discrepancy betweenLanducci's claim on the one hand that manganese had no effect on pulp viscosity, and Ericsson's and Myburgh's claim on the other hand that manganese had disastrous consequences on pulp viscosity, the work of Gilbert, et al. (TAPPI: Vol. 56 [6], p. 95, 1973) should be cited. This work presented evidence that oxygen/alkali treatment of cotton linters in the presence of iron present in small quantities gave the degradation effect noted elsewhere. But, upon further additions of iron, to a level of 1,000 ppm. (on pulp basis) no further degradation was noted. However, upon extending the addition to a level of greater than 103 ppm., a prounouced increase in the pulp vis-cosity was observed. The authors further stated that a similar phenomenon was noted with manganese, though no data were given.
The observation was evidently not believed by the authors to be of practical im~ortance, due to the discoloration of the pulp by the heavy metal ions. They state "the orange color of the pulp samples is, of course, objectionable."
.~ , ~`
It is, accordingly, an object of the present invention to provide a process for the deiignification and bleaching of lignocellulosic pulp fibers with oxygen in an alkaline medium while protecting the pulp against excessive losses in viscosity.
It is another object of the present invention to provide a process for the delignification and bleaching of lignocellulosic pulps with oxygen in an alkaline medium in the presence of a compound which simultaneous catalyzes the rate of delignification and protects the pulp against excessive losses in viscosity.
It is a further object of the present invention to provide a continuous process for the delignification and bleaching of lignocellulosic pulps with oxygen in an alkaline medium in the presence of a compound which simultaneously catalyzes the rate of delignification and protects the pulp against excessive losses in viscosity and, also provides for the regeneration, recovery, recycling and reuse of the compound.
In one aspect of this invention, there is provided a process for the delignification and bleaching of ligno-cellulosic pulp fibers with oxygen in an alkaline medium while protecting the pulp against excessive losses in viscosity, which comprises the following steps in sequence:
(a) contacting a slurry of lignocellulosic pulp fibers, having a consistency of less than 50%, by weight of oven-dried pulp, with a solution of a water-soluble salt of a divalent transition metal selected from the group consisting of manganous, nickelous, cobaltous, and vanadous ions having a concentration of from 0.27% to about 1.10%, by weight of oven-dried pulp, for a period of time ~ufficient to allow A
104;~Si4 the divalent metal ion to penetrate the walls of the pulp fiber7 ~ b) delignifying the pulp by mixing the lignocellulosic pulp slurry containing the divalent transition metal ion with oxygen, in an alkaline medium resulting in a slurry pH
between about 10 and about 13, while oxidizing the divalent transition metal salt to a water-insoluble trivalent transition metal oxide and precipitating a viscosity protective coating of said trivalent transition metal oxide on the pulp fibers;
and then (c) washing the pulp with acid to reduce the trivalent transition metal oxide and thereby regenerating the divalent transition metal salt.
In another aspect of this invention, there is provided a continuous process for the delignification and bleaching of lignocellulosic pulp fibers with ~xygen in an alkaline medium while protecting the pulp against excessive losses in viscosity, which comprises the following steps in sequence:
(a) contacting a slurry of lignocellulosic pulp fibers, having a consistency of less than 50%, by weight of oven-dried pulp, with a solution of a water-soluble salt of a divalent transition metal selected from the group consisting of manganous, nickelous, cobaltous, and vanadous ions having a concentration of from about 0.27~ to about 1.10%, by weight of oven-dried pulp, for a period of time sufficient to allow the divalent metal ion to penetrate the walls of the pulp fibers;
(b) delignifying the pulp by mixing the lignocellulosic pulp slurry containing the divalent transition metal ions with oxygen, in an al~aline medium resulting in a slur~y pH
~ - 6~a) -`` i()4;~S14 between about 10 and about 13, while oxidizing the divalent transition metal salt to a water-insoluble trivalent transition metal oxide and precipitating a viscoslty protective coating of said trivalent transition metal oxide on the pulp fibers;
(c) washing the pulp to free it of anions;
(d) washing the pulp with acid to reduce the trivalent transition metal oxide and thereby regenerating the divalent transition metal salt;
(e) recovering the regenerated divalent transition metal salt and recycling it for reuse in (a).
Other objects, aspects and advantages of this invention will be apparent to those skilled in the art from the present specification when taken in conjunction with the appended drawings, in which:
FIG. 1 is a flow diagram illustrating the present invention.
-~FIG. 2 is a graph in which the effect of sodium hydroxide concentration in the present invention is shown by plotting the viscosity of the bleached pulp versus the l~app~ Nomber of the blea~hed pulp.
- 6(b) -A
~043514 GENERAL DESCRIP~ION OF THE INVENTION
It has been found that during the delignification and bleaching of lignocellulosic pulp fibers with oxygen in an alkaline medium that the rate of delignification can be in-creased, while simultaneously protecting the pulp against excessive losses in viscosity, by contacting a slurry of ligno-cellulosic pulp fibers with a solution of a water-soluble salt of a divalent transition metal selected from the group con-sisting of manganous, nickelous, cobaltous and vanadous ions having a concentration at least about 0.27% to about 1.10~, based on the weight of oven-dried ~O.D.) pulp. These di-valent metal salts function both as a catalyst of delignifica-tion and as a protector against excessive losses in viscosity.
This was highly unexpected, since as previously noted, other investigators have reported losses in viscosity, thus signi-fying excessive depolymerization of cellulose.
The present invention also provides a continuous pro-cess in which water-soluble divalent transition metal salts selected from the group consisting of manganous, nickelous, cobaltous and vanadous ions function as a catalyst to increase delignification and as a protector compound against excessive losses in viscosity during oxygen bleaching in an alkaline medium and, also, provides for the recovery, regeneration, re-cycling and reuse of the metal salts. Thus, there is provided a continuous process which enables the divalent transition metal salt to be reused effectively and economically and, further, removes objectionable color, which adversely effects brightness and interferes with subsequent bleaching stages.
In accordance with the process of the present inven-tion, a slurry of unbleached lignocellulosic pulp fibers havinga consistency of less than about 50%, based on the weight of oven-dried pulp, preferably between about 1% ana about 30%, 10435~4 and most preferably, between about 1% and about 10%, is brought into intimate contact with a water-soluble salt of a divalent transition metal for a period of time sufficient to allow the divalent metal to penetrate the walls of the pulp fiber.
I~hile it is preferred to use paper grade pulp or dis-solving grade pulp prepared by the kraft;process, pulps pre-; pared by other pulping processes, such as alkaline sulfite, neutral sulfite, soda, or semichemical, can be employed ad-vantageously. The lignocellulose utilized in the aforementioned pulping processes to provide the pulp for the process of the present invention can vary widely and can include hardwoods, softwoods, bagasse, etc.
The intimate contact between the Pulp and the metal is effectively achieved by using a soluble solution of the metal compound which can readily penetrate the walls of the pulp fibers. It is preferred to employ the sulfate, acetate and carbonate salts of the manganous, nickelous, cobaltous, and vanadous ions, since these anions are compatible with the pulp mill recovery system. It is especially preferred to employ manganous sulfate in view of the excellent results achieved with respect to increasing the rate and extent of delignifi-cation while, also, and most surprisingly, simultaneously functioning to retard or minimize excessive losses in viscosity, which reflects the extent of depolymerization or degradation ; caused by the bleaching process.
It is to be understood that while the specification will hereinafter only make reference to the terms "manganous sulfate" and "manganous" and "manganese" when referring to the divalent transition metal salts and the ions of the present invention, it is done solely in the interest of brevity and 1~435i4 clarity of exposition, and it is to be further understood that when such terms are used they are also intended to refer to and include the water-soluble salts of the nickelous, cGbal-tous, and vanadous ions, the ions themselves, and the other water-soluble salts of the manganous ion.
The manganous ion is functioning both as a catalyst of delignification and as a protector of cellulose viscosity.
It has been found that to function effectively in both ca-pacities, the concentration of manganous ion should be from about 0.27% to about 1.10%, based on the weight of oven-dried (o.~) pulp. At lesser concentrations than 0.27%, it has been found that the degradation of the pulp is actually catalyzed as measured by losses in viscosity, while at concentrations greater than 1.10% no added benefits in either viscosity pro-tection or increased delignification are realized. While con-centrations greater than 1.10% have no negative effect on vis-cosity or rate of delignification, an economic penalty is in-curred by using excessive amounts of manganous sulfate. It is preferred to employ a manganous ion concentration of about 0.55%, based on O.D. pulp, since maximum benefits are obtained at that concentration.
The aqueous pulp slurry, whose consistency has been diluted by the solution of manganous sulfate is then further diluted with a stream of liquor containing both sodium hydroxide and dissolved and intimately dispersed oxygen. The latter solution has been prepared by separately mixing oxygen and sodium hydroxide in a mixing device, which insures efficient distri-bution and dispersion of the oxygen and also assures satisfac-tory reaction with the manganous ion, along with steam, to give the desired reaction temperature.
la43s~
Upon contacting the puip slurry containing manganous ions, the oxygenated caustic stream must be mixed with the pulp slurry. This may take place in a separate mixing vessel, or it may occur in a small section of the main reactor.
Alkali is present in the oxygenated liquid in suf-ficient quantity to elevate the pH of the pulp solution to between about 10 and about 13, preferably above pH 11, and about 20 to 30 pounds of oxygen are admitted per ton of pulp.
The manganous ion, which has a valence of +2, is oxidized to a higher valence state after a protecti~e coating of manganous hydroxide has been deposited by precipitation onto the surface of the pulp fibers, The precipitation and oxidation of the manganous ions occurs only after contact of the pulp slurry with the oxygenated caustic solution. While various alkaline agents can be employed, such as sodium car-bonate, sodium hydroxide, ammonia, kraft white liquor, etc., it is preferred to employ sodium hydroxide as the alkali.
Then the oxygenated alkaline pulp slurry, is intro-duced into a reaction vessel. The vessel can be a large, high-pressure reactor equipped with a mixing or stirring device, or a downflow tower such as those used for caustic extraction in a pulp bleach plant, or an upflow tower such as those used - for chlorination in a pulp bleach plant. The advantages to be gained by the use of an upflow tower when oxygen bleaching ; pulp having consistencies less than about 10%, are disclosed in Roymoulik, U.S. 3,832,276. The process of the present invention, however, is not limited to operating at low pulp consistencies, since both high and low pulp consistencies can be utilized advantageously.
When a large, high pressure reactor is employed in ~043514 accordance with the process of the present invention, 20 to 30 pounds of dissolved and intimately dispersed oxygen per ton of pulp along with sodium hydroxide having a concentration of from about 1 gram per liter to about 20 grams per liter, pre-ferably from about 2 to about 4 grams per liter, are added to the pulp slurry in the reactor. The contents, which are at a pH of from about 11 to about 13, with p~ 12 being preferred, are heated at a temperature of from about 80C. to about 130C., preferably from about 95C. to about 105C., and at a pressure of from about 10 psig to about 300 psig, preferably at 150 psig, for a period of from about 1 minute to about Ç0 minutes, with from about 4 minutes to about 10 minutes being preferred.
It is believed, but it is not known with certainty, and is therefore only offered as a postulate, that during the residence time in the reactor there are three separate and distinct competing oxidation reactions taking place. These reactions are oxidation of the manganous ion, oxidation of the lignin, and oxidation of the cellulose. The manganous ion is oxidized to hydrous manganese (III) oxide which is believed to be the active species which affords viscosity protection to the cellulose and acts to catalyze the rate of delignification.
It is also believed that the oxidized metal combines with alkali and specifically directs its attack on the lignin, since experimental evidence indicates that the rate of delignifica-tion is dependent upon alkali concentration. It has unexpec-tedly been found that use of manganese allows the use of reduced concentrations of sodium hydroxide to achieve an extent of delignification that ordinarily coula only be achieved at much higher sodium hydroxide concentrations in the absence of man-ganese.
104;~5~4 ~ t the discharge of the reactor, oxygen bleached pulp falls into a washer~ e.~ acuum ~ilter~ which washes the pulp mat with fresh water, filters the residual alkali, and recirculates it back into the bleaching system. During washing, the manganese flock is retained in the mat of pulp fibers Since sorbed manganese ion on the pulp gives it a high degree of coloration, which both diminishes brightness and interferes with subsequent bleaching stages, it is effec-tively removed by washing the pulp mat on a filter with an acid in which manganese is soluble. This can be accomplished effectively at a pH of about 2 and at a temperature of from about room temperature up to about 50C. T~hile it is pre-ferred to employ sulfurous acid, other acids such as sulfuric, hydrochloric, or acetic can also be used.
The pulp mat which has now been freed of the cation can, if desired, be bleached in subsequent stages to a higher degree of brightness by using conventional bleaching sequences, such as CEHDED, CEDED, CEHD, or DE~.
The washing of t~e pulp mat with sulfurous acid on the washer reduces the manganese ion to manganous, and re- .
generates manganous sulfate, which is recovered from the fil-trate from the second washer and recycled for reuse with new unbleached pulp. Thus, there is provided a complete, contin-uous and economic process for the use, regeneration, recovery, recycle, and reuse of the manganous sulfate.
Referring to FIG. 1 of the drawings, describing one form of apparatus and embodiment of the process, an unbleached pulp slurry of the desired consistency and a solution of man-ganous sulfate are admitted into high density storage chest 1.
104~S14 The manganous sulfate solution is desirably regenerated man-ganous sulfate solution which is recycled from stream 2. The recycled manganous sulfate is introduced so as to economically reuse the same in a continuous manner.
After dilution in the high density storage chest, the manganous sulfate having penetrated the fiber wall of the unbleached lignocellulose pulp, the pulp slurry and the man-ganous sulfate liquor are then pumped via stream 3 into mixer 4.
The mixer is u~ed to pretreat the pulp slurry for a short period of time with dissolved and intimately dispersed oxygen and fresh sodium hydroxide uner pressure. Fresh sodium hydroxide (NaOH) is pumped into and introduced via stream 5 to steam and oxygen mixer 6 where the temperature of the mix-ture, i.e., sodium hydroxide, oxygen and dissolved organics, is raised to 220F. to 230F. and then pumped via stream 7 into mixer 4.
The pulp and liquor then pass from mixer 4 and are admitted via stream 8 into reactor 9, which, as previously stated, can be a high pressure reactor, an upflow tower, such as that used for chlorination in a bleach plant, or a downflow tower. Dissolved and intimately dispersed oxygen and alkali are admitted into the reactor and under elevated pressure and temperature the oxidation of the lignocellulosic pulp proceeds.
Residence time will vary depending upon the type of pulp, the amount of alkali employed, and the pressure and temperature employed.
The oxygenated pulp having entrained Na2SO4 and Mn2O3 is then carried by stream 10 to washer 11 where the pulp mat is washed with fresh water causing the Na2SO4, NaOH and 10435i~
organic sodium salts to pass into the filtrate, stream 12, where they are collected in washer seal tank 13. The major amount of the filtrate containing the above referred to compounds is pumped to the screen room deckers or the brown stock washers via stream 14, with a minor portion thereof being diverted to stream 14' and then to washer 11 and another minor portion being recycled into the oxygen delignification system via stream 5.
The washed pulp mat containing the remaining en-trained Mn2O3 is then transferred via stream 15 to a secondwasher 16 where the pulp mat is washed with an acid at a pH
of about 2. The acid wash reduces the sorbed manganese back to its original valence state of ~2, and it passes out of washer 16 as manganous sulfate filtrate via stream 17 and then passes into seal tan~ 18.
Optionally, stream l9 from seal tank 18, containing predominately manganous sulfate, can be partially diverted to stream 20 and the manganous sulfate can be used to dilute the pulp consistency on the discharge side of washer 11. The main portion of stream 19 feeds into stream 2 and the manganous sul-fate is thus recycled for reuse with new unbleached pulp in high density storage chest l.
One of the unexpected advantages of the present inven-tion is shown by FIG. 2 of the appended drawings. That figure represents a graph plotting the pulp viscosity versu~ Kappa Number for a series of runs showing the effect of sodium hydrox-ide at varying concentrations, namely, 2, 4, 6, and 8 grams per liter, in the absence of manganous sulfate and in the presence of 0.5~% manganous sulfate. FIG. 2 presents a graphical representation of the results tabulated in Table II hereinafter, based upon Examples 14-21, inclusive, hereinafter.
1()4;~
Viscosity represents a measurement of the average degree of polymerization of the cellulose in the pulp sample, i.e., the average chain length o the cellulose. Thus, de-creases in viscosity values represent the extent of depoly-merization or degradation caused by the bleaching process.
Excessive degradation is to be avoided since it provides un-desirable physical properties in any paper made from the pulp.
Kappa Number is determined by the potassium perman-ganate consumed by a sample of pulp and represents a measure-ment of its retained lignin content. The higher the KappaNumber, the less bleached and delignified is the pulp. By comparing Kappa Numbers of samples before and after bleaching treatment, one can obtain an evaluation of the extent of de-lignification which has taken place.
As can be seen from an examination of FIG. 2, as the concentration of sodium hydroxide is increased from 2 to 8 grams/liter there is, as expected, a decrease both in Kappa Number and viscosity. However, unexpectedly, with manganese present the decrease in viscosity is much less drastic, and in each and every instance where manganese is present the viscosity at a given Kappa Number is significantly greater. Of even greater importance, perhaps, is the fact that the amount of sodium hydroxide required to achieve a given Kappa Number is less when manganese is present. This reduces the amount of sodium hydroxide necessary for the process and, hence, the cost of the process is appreciably reduced.
DETAILED DES~RIPTION OF THE INVENTION
In order to disclose more clearly the nature of the present invention, the following examples illustrating the in-vention are given. It should be understood, however, that this is done solely by way of example and is intended neither todelineate the scope of the inVention nor limit the ambit of the appended claims.
15 grams, oven-dried basis, of an unbleached Southern hardwood pulp was diluted with water to yield 1.5 liters of a pulp slurry, equal to a pulp consistency of 1%.
The pulp slurry was then placed in the reaction chamber of a Parr reactor. Examples 1-7, inclusive, employed dissolving grade pulp and Examples 8-13, inclusive, emPloyed paper grade pulp. The dissolving grade pulp had a brightness of 43.1, a permanganate number of 8.5 and a viscosity of 37.8. The paper grade pulp had a brightness of 26.1, a Kappa number of 17.0 and a ViSCOsIty of 42.3.
To the pulp slurry there was then added manganous sulfate, in solution, in the amounts indicated below in Table 1.
Following this there was then added sufficient sodium hydroxide - to yield a 2 gram per liter solution. The addition of the manganous sulfate and the sodium hydroxide to the pulp slurry was accompanied by gentle stirring to keep the solution as homogeneous as possible.
The pressure stirring head was then positioned over the reaction chamber and the entire unit was sealed. The reactor was then placed in a heating mantle and slowly brought to a final temperature of 95C. A slow, steady mixing a~com-panied the temperature rise. When temperature was achieved, pressurized oxygen gas was introduced at 40 psig. During this time, the fastest mixing speed possible, approximately 2000 rpm, was utilized. After one minute of combined rapid mixing and oxygen pressure at 40 psig, the mixing was stopped and the reactor was removed from the heating mantle. Oxygen pressure 1l)4;~S14 was maintained for nine minutes before a slow gradual depres-surization of 1 psig per minute was undertaken for a total period of 40 minutes. After complete depressurization, the pulp was withdrawn, washed, filtered, and made into sheets.
The pulp sheets were diluted to a consistency of 5%
with sulfurous acid at pH 2 and were placed in plastic bags which were maintained in a constan~ temperature bath at 49C.
for one-half hour.
The pulp was then water-washed, filtered and dried and the brightness, Kappa Number and viscosity were measured and are tabulated in Table I below.
1043Sl~
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0 ~1 N r4 ~1 ~ ~I N O rl N ~ ~ N ~ N
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~0435~4 In the series of experiments shown in Table I both delignification catalysis and ~iscosity protection are evident with both ty~es of pulp. The viscosity protection was ob-served with a manganese application of 50 ppm and above.
The pulp, as expected, became increasingly dark and of a highly objectionable color. Contacting the pulp with a sulfurous acid solution at pH 2, removed the color caused by the manganese.
15 grams, oven-dried basis, of an unbleached Southern hardwood paper grade pulp having a Kappa Number of 17.0, and a viscosity of 42.3, was diluted with water to yield 1.5 liters of a pulp slurry, equal to a pulp consistency of 1%. The pulp slurry was then placed in the reaction chamber of a Parr re6ctor.
To the pulp slurry there was then added manganous sulfate, in solution, in the concentrations indicated below in Table II. Following this there was then added sodium hy-droxide in the concentrations indicated in Table II. The sodium hydroxide present is also calculated as a percentage of oven-dried pulp. The addition of the manganous sulfate and the sodium hydroxide to the pulp slurry was accompanied by gentle stirring to keep the solution as homogeneous as possible.
The pressure stirring head was then positioned over the reaction chamber and the entire unit was sealed. The reactor was then placed in a heating mantle and slowly brought to a final temperature of 95C. A slow, steady mixing ac-companied the temperature rise. When temperature was achieved, pressurized oxygen gas was introduced at 40 psig. During this time, the fastest mixing speed possible, approximately 2000 rpm, ~043S14 was utilized. After one minute of combined rapid mixing and oxygen pressure at 40 psig, the mixing was stopped and the reactor was removed from the heating mantle. Oxygen pressure was maintained for nine minutes before a slow gradual depres-surization of 1 psig per minute was undertaken for a total period of 40 minutes. After complete depressurization, the pulp was withdrawn, washed, filtered, and made into sheets.
The pulp sheets were diluted to a consistency of 5%
with sulfurous acid at pH2 and were placed in plastic bags which were maintained in a constant temperature bath at 49C.
for one-half hour.
The pulp was then water-washed, filtered and dried and the brightness, Kappa Number and viscosity were measured.
~43S14 ~ ~P ~1 0 U~ O U~
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.q ' ~P, Z c~ a~ ~ In ~ CD ~ ~ '~
td P ~ ~ o o --I co a~
P
~O
~q H J~ 1~ 'J e1' 1` o _1 ~ a~
~ ~ ~ ~r er m o P~ O O O O O O O o :C ~
o o o O O O O O O
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.
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o P
o o o o In u~ In In _ o o O o . ~ o ~1 x z i(~4~S~4 ~ t can be seen from Table ~I that the decrease in Kappa Nu~ber and ~iscosity is less pronouncedwith manganese present as the concentration of sodium hydroxide is increased.
In each case, it should be noted, the viscosity at a given Kappa Number is superior. Perhaps even more important, the amount of sodium hydroxide required to achieve a given Kappa Number is less when manganese is present. This reduces the expense of sodium hydroxide for the reaction.
It can be seen that the combination of 2.0 grams per liter of sodium hydroxide and 0.55% manganese gave delignifi-cation almost equal to that of 4.0 grams per liter, of sodium hydroxide without manganese, but the viscosity of the manganese protected run was vastly superior to the controls. The value of manganese also lies in the reduction of sodium hydroxide losses in washing the pulp. If only one washer were available to wash the oxygen bleached pulp, operation at 4.0 grams per liter would result in a loss of about 35 pounds of sodium hydroxide, whereas only about 17 pounds would be lost when manganese is used in combination with 2.0 grams per liter of sodium hydroxide.
1100 grams oven-dried basis of an unbleached EBK
("Easy Bleaching Kraft") hardwood aper grade pulp having a brightness of 19.8, a Kappa number of 13.5, and a viscosity of 18.7 was diluted with water to yield 36 liters of a pulp slurry, equal to a pul~ consistency of 3%. The pulp slurry was then placed in the reaction chamber of a Pfaudler reactor.
To the pulp slurry there was then added manganous sulfate in solution in the amounts indicated below in Table III.
Following this there was then added sodium hydroxide in the 1(~4;~514 amounts indicated below in Table ~II. The addition of the manganous sulfate and the sodium hydroxide to the pulp slurry was accompanied by gentle stirring to keep the solution as homogeneous as possible.
The reactor was then slowly brought up to a final temperature of 95C. A slow, steady mixing accompanied the temperature rise. When the temperature was achieved, pres-surized oxygen gas was introduced at 100 psig. During this time, the fastest mixing speed possible, approximately 300 rpm was utilized. After two minutes of combined mixing and oxygen pressure at 100 psig, the mixing was stopped and the oxygen pressure was released to 40 psig. Thereafter, over a period of 40 minutes slow gradual depressurization from 40 psig to 0 psig, at 1 psig per minute, was accomplished. After com-plete depressurization the pulp was withdrawn, washed with water, filtered and made into sheets.
The pulp sheets were diluted to a consistency of 5%
with sulfurous acid at pH 2 and were placed in plastic bags which were maintained in a constant temperature bath at 49C.
for one-half hour.
The pulp was then water-washed, filtered and dried and the brightness, Kappa Number and viscosity were measured.
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The results in Table III indicate that use of manganese allowed a reduction in the amount of sodium hydro-xide from 13.3% (based on O.D. pulp) to 6.7% (based on O.D.
pulp), ~hile simultaneously achieving improved pulp viscosity at virtually the same Kappa number.
The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and des-cribed or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.
The present invention relates to a ~rocess for the bleaching of lignocellulosic pulp with oxygen in the presence of alkali, and more particularly to an oxygen/alkali bleaching process wherein the rate of delignification is increased while at the same time protecting the cellulosic portion against excessive losses in viscosity.
Pulp delignification with alkaline solutions of molecular oxygen, or "oxygen bleaching", like any other bleach-ing process, represent a compromise situation, whereby one seeks to obtain the maximum extent of delignification, while minimizing the degree of oxidative degradation, i.e,, depoly-merization of the cellulosic component.
Industrial utilization of oxygen bleaching was retarded for a number of years because the above-referred to compromise was heavily weighted on the side of cellulose de-gradation. m us, though acceptable delignification could be achieved with oxygen/alkali bleaching, the pulp mechanlcal properties were uniformly unacceptable. The discovery by Robert et al., U.S. 3,384,533, that magnesium compounds could effect a certain degree of cellulose protection against oxida-tive degradation, provided the impetus for the establishment of the first commercial oxygen bleaching system at Enstra, South Africa. While other researchers have from time to time found that other alkaline earth metals, aside from magnesium, could achieve a certain degree of cellulose protection, none have attained commercialization. Indeed, only the potassium iodide protection disclosed by Minor and Sanyer in the 104;~514 Journal of Polymer Science, Part C, No. 36:73 (1971) affords the degree of protection which is comparable with that of magnesium, or the complexes of magnesium.
Use of magnesium as a protection compound for cel-lulose during oxygen bleaching presents a number of practical problems. Not the least of these problems is the expense of the compound. The action of magnesium is purely specific to cellulose, and appears to have no effect on the rate or extent of delignification that can be achieved during oxygen bleach-ing. Another problem associated with the use of magnesium is its tendency to impart scale on the process equipment thus resulting in problems of encrustation.
Consequently, the pulp bleaching industry has sought new and improved methods by which oxygen bleaching can be con-ducted, preferably without using magnesium compounds. An ad-ditive which also catalyzes delignification, aside from pro-ducing non-profit cellulose protection is, accordingly, highly desirable. A step in that direction was provided by Roymoulik et al., U.S. 3,832,276, which describes a low con-sistency oxygen bleaching process that does not require the use of cellulose protectors. The process reduces cellulose de-gradation by lowering the concentration of hydroxyl ions through the use of low pulp consistency and through the use of declin-ing oxygen pressures during reaction.
Disclosure of catalysts which will increase the rate of deliqnification have been rather limited to date. Minor and Landucci have reported, (see International Pulp Bleaching Conference, 1973, Vancouver, B.C., p. 83) that the rate of ` ~O~Si4 delignification of pulp b~ ox~gen/alkali could be increased by the use of manganese (the form was not specified). Data was presented showing that whereas delignification of a southern pine groundwood pulp was considerably accelerated by the ad-dition of 0.01% manganese (basis not specified), only a small effect was noted with a kraft pulP containing an initial lignin content of 7%. A subsequent publication by Landucci, Minor and Sanyer, (Fourth Canadian Wood Chemistry Symposium, 1973, Quebec, Canada, p. 71) concerning the use of manganese to accelerate delignification of southern pine, stated that "manganese has no deleterious effect on carbohydrates", pre-sumably meaning that pulp viscosities were similar to those obtained when manganese was not used.
Although the catalytic effect of the manganese on deiignification is not difficult to understand, the finding that viscosity was unaffected is quite surprising. A number of investigators have quite conclusively demonstrated that manganese, like a number of other transition metal ions, has a very damaging effect on the viscosity of cellulose oxygen delignification. For example, Ericsson et al. (Svensk Papper-stidning, Volume 74 (22), November 1971, p. 757) determined that, whereas 40 parts per million (as manganese, based on pulp weight,) of manganous sulfate had little in1uence on the vis-cosity of cotton linters, i.e., pure cellulose, during oxygen/
alkali treatment, repeating the experiment in the presence of 5% lignin (based on pulp weight) physically added to the mix-ture gave a disastrous decrease in the viscosity of the cotton linters. Other heavy metals, such as iron and copper were found to behave in a similar manner. Concern about the presence `` ~0~35i4 of manganese during oxygen bleaching is of critical industrial importance. Myburgh, for example, reported (TAPPI: Vol. 57 [5], p. 131, 1974) that the Enstra, South Africa, oxygen bleach plant required that unbleached pulp be prewashed with acid to dissolve out heavy metals, prior to entering the oxygen bleach reactor. Myburgh states that the unbleached pulp at Enstra contains 120-160 parts per million of manganese, which must be reduced blow 30 parts per million, or the metal will have "have a catalytic degrading effect on cellulose during oxygen bleach-ing."
To further confuse the seeming discrepancy betweenLanducci's claim on the one hand that manganese had no effect on pulp viscosity, and Ericsson's and Myburgh's claim on the other hand that manganese had disastrous consequences on pulp viscosity, the work of Gilbert, et al. (TAPPI: Vol. 56 [6], p. 95, 1973) should be cited. This work presented evidence that oxygen/alkali treatment of cotton linters in the presence of iron present in small quantities gave the degradation effect noted elsewhere. But, upon further additions of iron, to a level of 1,000 ppm. (on pulp basis) no further degradation was noted. However, upon extending the addition to a level of greater than 103 ppm., a prounouced increase in the pulp vis-cosity was observed. The authors further stated that a similar phenomenon was noted with manganese, though no data were given.
The observation was evidently not believed by the authors to be of practical im~ortance, due to the discoloration of the pulp by the heavy metal ions. They state "the orange color of the pulp samples is, of course, objectionable."
.~ , ~`
It is, accordingly, an object of the present invention to provide a process for the deiignification and bleaching of lignocellulosic pulp fibers with oxygen in an alkaline medium while protecting the pulp against excessive losses in viscosity.
It is another object of the present invention to provide a process for the delignification and bleaching of lignocellulosic pulps with oxygen in an alkaline medium in the presence of a compound which simultaneous catalyzes the rate of delignification and protects the pulp against excessive losses in viscosity.
It is a further object of the present invention to provide a continuous process for the delignification and bleaching of lignocellulosic pulps with oxygen in an alkaline medium in the presence of a compound which simultaneously catalyzes the rate of delignification and protects the pulp against excessive losses in viscosity and, also provides for the regeneration, recovery, recycling and reuse of the compound.
In one aspect of this invention, there is provided a process for the delignification and bleaching of ligno-cellulosic pulp fibers with oxygen in an alkaline medium while protecting the pulp against excessive losses in viscosity, which comprises the following steps in sequence:
(a) contacting a slurry of lignocellulosic pulp fibers, having a consistency of less than 50%, by weight of oven-dried pulp, with a solution of a water-soluble salt of a divalent transition metal selected from the group consisting of manganous, nickelous, cobaltous, and vanadous ions having a concentration of from 0.27% to about 1.10%, by weight of oven-dried pulp, for a period of time ~ufficient to allow A
104;~Si4 the divalent metal ion to penetrate the walls of the pulp fiber7 ~ b) delignifying the pulp by mixing the lignocellulosic pulp slurry containing the divalent transition metal ion with oxygen, in an alkaline medium resulting in a slurry pH
between about 10 and about 13, while oxidizing the divalent transition metal salt to a water-insoluble trivalent transition metal oxide and precipitating a viscosity protective coating of said trivalent transition metal oxide on the pulp fibers;
and then (c) washing the pulp with acid to reduce the trivalent transition metal oxide and thereby regenerating the divalent transition metal salt.
In another aspect of this invention, there is provided a continuous process for the delignification and bleaching of lignocellulosic pulp fibers with ~xygen in an alkaline medium while protecting the pulp against excessive losses in viscosity, which comprises the following steps in sequence:
(a) contacting a slurry of lignocellulosic pulp fibers, having a consistency of less than 50%, by weight of oven-dried pulp, with a solution of a water-soluble salt of a divalent transition metal selected from the group consisting of manganous, nickelous, cobaltous, and vanadous ions having a concentration of from about 0.27~ to about 1.10%, by weight of oven-dried pulp, for a period of time sufficient to allow the divalent metal ion to penetrate the walls of the pulp fibers;
(b) delignifying the pulp by mixing the lignocellulosic pulp slurry containing the divalent transition metal ions with oxygen, in an al~aline medium resulting in a slur~y pH
~ - 6~a) -`` i()4;~S14 between about 10 and about 13, while oxidizing the divalent transition metal salt to a water-insoluble trivalent transition metal oxide and precipitating a viscoslty protective coating of said trivalent transition metal oxide on the pulp fibers;
(c) washing the pulp to free it of anions;
(d) washing the pulp with acid to reduce the trivalent transition metal oxide and thereby regenerating the divalent transition metal salt;
(e) recovering the regenerated divalent transition metal salt and recycling it for reuse in (a).
Other objects, aspects and advantages of this invention will be apparent to those skilled in the art from the present specification when taken in conjunction with the appended drawings, in which:
FIG. 1 is a flow diagram illustrating the present invention.
-~FIG. 2 is a graph in which the effect of sodium hydroxide concentration in the present invention is shown by plotting the viscosity of the bleached pulp versus the l~app~ Nomber of the blea~hed pulp.
- 6(b) -A
~043514 GENERAL DESCRIP~ION OF THE INVENTION
It has been found that during the delignification and bleaching of lignocellulosic pulp fibers with oxygen in an alkaline medium that the rate of delignification can be in-creased, while simultaneously protecting the pulp against excessive losses in viscosity, by contacting a slurry of ligno-cellulosic pulp fibers with a solution of a water-soluble salt of a divalent transition metal selected from the group con-sisting of manganous, nickelous, cobaltous and vanadous ions having a concentration at least about 0.27% to about 1.10~, based on the weight of oven-dried ~O.D.) pulp. These di-valent metal salts function both as a catalyst of delignifica-tion and as a protector against excessive losses in viscosity.
This was highly unexpected, since as previously noted, other investigators have reported losses in viscosity, thus signi-fying excessive depolymerization of cellulose.
The present invention also provides a continuous pro-cess in which water-soluble divalent transition metal salts selected from the group consisting of manganous, nickelous, cobaltous and vanadous ions function as a catalyst to increase delignification and as a protector compound against excessive losses in viscosity during oxygen bleaching in an alkaline medium and, also, provides for the recovery, regeneration, re-cycling and reuse of the metal salts. Thus, there is provided a continuous process which enables the divalent transition metal salt to be reused effectively and economically and, further, removes objectionable color, which adversely effects brightness and interferes with subsequent bleaching stages.
In accordance with the process of the present inven-tion, a slurry of unbleached lignocellulosic pulp fibers havinga consistency of less than about 50%, based on the weight of oven-dried pulp, preferably between about 1% ana about 30%, 10435~4 and most preferably, between about 1% and about 10%, is brought into intimate contact with a water-soluble salt of a divalent transition metal for a period of time sufficient to allow the divalent metal to penetrate the walls of the pulp fiber.
I~hile it is preferred to use paper grade pulp or dis-solving grade pulp prepared by the kraft;process, pulps pre-; pared by other pulping processes, such as alkaline sulfite, neutral sulfite, soda, or semichemical, can be employed ad-vantageously. The lignocellulose utilized in the aforementioned pulping processes to provide the pulp for the process of the present invention can vary widely and can include hardwoods, softwoods, bagasse, etc.
The intimate contact between the Pulp and the metal is effectively achieved by using a soluble solution of the metal compound which can readily penetrate the walls of the pulp fibers. It is preferred to employ the sulfate, acetate and carbonate salts of the manganous, nickelous, cobaltous, and vanadous ions, since these anions are compatible with the pulp mill recovery system. It is especially preferred to employ manganous sulfate in view of the excellent results achieved with respect to increasing the rate and extent of delignifi-cation while, also, and most surprisingly, simultaneously functioning to retard or minimize excessive losses in viscosity, which reflects the extent of depolymerization or degradation ; caused by the bleaching process.
It is to be understood that while the specification will hereinafter only make reference to the terms "manganous sulfate" and "manganous" and "manganese" when referring to the divalent transition metal salts and the ions of the present invention, it is done solely in the interest of brevity and 1~435i4 clarity of exposition, and it is to be further understood that when such terms are used they are also intended to refer to and include the water-soluble salts of the nickelous, cGbal-tous, and vanadous ions, the ions themselves, and the other water-soluble salts of the manganous ion.
The manganous ion is functioning both as a catalyst of delignification and as a protector of cellulose viscosity.
It has been found that to function effectively in both ca-pacities, the concentration of manganous ion should be from about 0.27% to about 1.10%, based on the weight of oven-dried (o.~) pulp. At lesser concentrations than 0.27%, it has been found that the degradation of the pulp is actually catalyzed as measured by losses in viscosity, while at concentrations greater than 1.10% no added benefits in either viscosity pro-tection or increased delignification are realized. While con-centrations greater than 1.10% have no negative effect on vis-cosity or rate of delignification, an economic penalty is in-curred by using excessive amounts of manganous sulfate. It is preferred to employ a manganous ion concentration of about 0.55%, based on O.D. pulp, since maximum benefits are obtained at that concentration.
The aqueous pulp slurry, whose consistency has been diluted by the solution of manganous sulfate is then further diluted with a stream of liquor containing both sodium hydroxide and dissolved and intimately dispersed oxygen. The latter solution has been prepared by separately mixing oxygen and sodium hydroxide in a mixing device, which insures efficient distri-bution and dispersion of the oxygen and also assures satisfac-tory reaction with the manganous ion, along with steam, to give the desired reaction temperature.
la43s~
Upon contacting the puip slurry containing manganous ions, the oxygenated caustic stream must be mixed with the pulp slurry. This may take place in a separate mixing vessel, or it may occur in a small section of the main reactor.
Alkali is present in the oxygenated liquid in suf-ficient quantity to elevate the pH of the pulp solution to between about 10 and about 13, preferably above pH 11, and about 20 to 30 pounds of oxygen are admitted per ton of pulp.
The manganous ion, which has a valence of +2, is oxidized to a higher valence state after a protecti~e coating of manganous hydroxide has been deposited by precipitation onto the surface of the pulp fibers, The precipitation and oxidation of the manganous ions occurs only after contact of the pulp slurry with the oxygenated caustic solution. While various alkaline agents can be employed, such as sodium car-bonate, sodium hydroxide, ammonia, kraft white liquor, etc., it is preferred to employ sodium hydroxide as the alkali.
Then the oxygenated alkaline pulp slurry, is intro-duced into a reaction vessel. The vessel can be a large, high-pressure reactor equipped with a mixing or stirring device, or a downflow tower such as those used for caustic extraction in a pulp bleach plant, or an upflow tower such as those used - for chlorination in a pulp bleach plant. The advantages to be gained by the use of an upflow tower when oxygen bleaching ; pulp having consistencies less than about 10%, are disclosed in Roymoulik, U.S. 3,832,276. The process of the present invention, however, is not limited to operating at low pulp consistencies, since both high and low pulp consistencies can be utilized advantageously.
When a large, high pressure reactor is employed in ~043514 accordance with the process of the present invention, 20 to 30 pounds of dissolved and intimately dispersed oxygen per ton of pulp along with sodium hydroxide having a concentration of from about 1 gram per liter to about 20 grams per liter, pre-ferably from about 2 to about 4 grams per liter, are added to the pulp slurry in the reactor. The contents, which are at a pH of from about 11 to about 13, with p~ 12 being preferred, are heated at a temperature of from about 80C. to about 130C., preferably from about 95C. to about 105C., and at a pressure of from about 10 psig to about 300 psig, preferably at 150 psig, for a period of from about 1 minute to about Ç0 minutes, with from about 4 minutes to about 10 minutes being preferred.
It is believed, but it is not known with certainty, and is therefore only offered as a postulate, that during the residence time in the reactor there are three separate and distinct competing oxidation reactions taking place. These reactions are oxidation of the manganous ion, oxidation of the lignin, and oxidation of the cellulose. The manganous ion is oxidized to hydrous manganese (III) oxide which is believed to be the active species which affords viscosity protection to the cellulose and acts to catalyze the rate of delignification.
It is also believed that the oxidized metal combines with alkali and specifically directs its attack on the lignin, since experimental evidence indicates that the rate of delignifica-tion is dependent upon alkali concentration. It has unexpec-tedly been found that use of manganese allows the use of reduced concentrations of sodium hydroxide to achieve an extent of delignification that ordinarily coula only be achieved at much higher sodium hydroxide concentrations in the absence of man-ganese.
104;~5~4 ~ t the discharge of the reactor, oxygen bleached pulp falls into a washer~ e.~ acuum ~ilter~ which washes the pulp mat with fresh water, filters the residual alkali, and recirculates it back into the bleaching system. During washing, the manganese flock is retained in the mat of pulp fibers Since sorbed manganese ion on the pulp gives it a high degree of coloration, which both diminishes brightness and interferes with subsequent bleaching stages, it is effec-tively removed by washing the pulp mat on a filter with an acid in which manganese is soluble. This can be accomplished effectively at a pH of about 2 and at a temperature of from about room temperature up to about 50C. T~hile it is pre-ferred to employ sulfurous acid, other acids such as sulfuric, hydrochloric, or acetic can also be used.
The pulp mat which has now been freed of the cation can, if desired, be bleached in subsequent stages to a higher degree of brightness by using conventional bleaching sequences, such as CEHDED, CEDED, CEHD, or DE~.
The washing of t~e pulp mat with sulfurous acid on the washer reduces the manganese ion to manganous, and re- .
generates manganous sulfate, which is recovered from the fil-trate from the second washer and recycled for reuse with new unbleached pulp. Thus, there is provided a complete, contin-uous and economic process for the use, regeneration, recovery, recycle, and reuse of the manganous sulfate.
Referring to FIG. 1 of the drawings, describing one form of apparatus and embodiment of the process, an unbleached pulp slurry of the desired consistency and a solution of man-ganous sulfate are admitted into high density storage chest 1.
104~S14 The manganous sulfate solution is desirably regenerated man-ganous sulfate solution which is recycled from stream 2. The recycled manganous sulfate is introduced so as to economically reuse the same in a continuous manner.
After dilution in the high density storage chest, the manganous sulfate having penetrated the fiber wall of the unbleached lignocellulose pulp, the pulp slurry and the man-ganous sulfate liquor are then pumped via stream 3 into mixer 4.
The mixer is u~ed to pretreat the pulp slurry for a short period of time with dissolved and intimately dispersed oxygen and fresh sodium hydroxide uner pressure. Fresh sodium hydroxide (NaOH) is pumped into and introduced via stream 5 to steam and oxygen mixer 6 where the temperature of the mix-ture, i.e., sodium hydroxide, oxygen and dissolved organics, is raised to 220F. to 230F. and then pumped via stream 7 into mixer 4.
The pulp and liquor then pass from mixer 4 and are admitted via stream 8 into reactor 9, which, as previously stated, can be a high pressure reactor, an upflow tower, such as that used for chlorination in a bleach plant, or a downflow tower. Dissolved and intimately dispersed oxygen and alkali are admitted into the reactor and under elevated pressure and temperature the oxidation of the lignocellulosic pulp proceeds.
Residence time will vary depending upon the type of pulp, the amount of alkali employed, and the pressure and temperature employed.
The oxygenated pulp having entrained Na2SO4 and Mn2O3 is then carried by stream 10 to washer 11 where the pulp mat is washed with fresh water causing the Na2SO4, NaOH and 10435i~
organic sodium salts to pass into the filtrate, stream 12, where they are collected in washer seal tank 13. The major amount of the filtrate containing the above referred to compounds is pumped to the screen room deckers or the brown stock washers via stream 14, with a minor portion thereof being diverted to stream 14' and then to washer 11 and another minor portion being recycled into the oxygen delignification system via stream 5.
The washed pulp mat containing the remaining en-trained Mn2O3 is then transferred via stream 15 to a secondwasher 16 where the pulp mat is washed with an acid at a pH
of about 2. The acid wash reduces the sorbed manganese back to its original valence state of ~2, and it passes out of washer 16 as manganous sulfate filtrate via stream 17 and then passes into seal tan~ 18.
Optionally, stream l9 from seal tank 18, containing predominately manganous sulfate, can be partially diverted to stream 20 and the manganous sulfate can be used to dilute the pulp consistency on the discharge side of washer 11. The main portion of stream 19 feeds into stream 2 and the manganous sul-fate is thus recycled for reuse with new unbleached pulp in high density storage chest l.
One of the unexpected advantages of the present inven-tion is shown by FIG. 2 of the appended drawings. That figure represents a graph plotting the pulp viscosity versu~ Kappa Number for a series of runs showing the effect of sodium hydrox-ide at varying concentrations, namely, 2, 4, 6, and 8 grams per liter, in the absence of manganous sulfate and in the presence of 0.5~% manganous sulfate. FIG. 2 presents a graphical representation of the results tabulated in Table II hereinafter, based upon Examples 14-21, inclusive, hereinafter.
1()4;~
Viscosity represents a measurement of the average degree of polymerization of the cellulose in the pulp sample, i.e., the average chain length o the cellulose. Thus, de-creases in viscosity values represent the extent of depoly-merization or degradation caused by the bleaching process.
Excessive degradation is to be avoided since it provides un-desirable physical properties in any paper made from the pulp.
Kappa Number is determined by the potassium perman-ganate consumed by a sample of pulp and represents a measure-ment of its retained lignin content. The higher the KappaNumber, the less bleached and delignified is the pulp. By comparing Kappa Numbers of samples before and after bleaching treatment, one can obtain an evaluation of the extent of de-lignification which has taken place.
As can be seen from an examination of FIG. 2, as the concentration of sodium hydroxide is increased from 2 to 8 grams/liter there is, as expected, a decrease both in Kappa Number and viscosity. However, unexpectedly, with manganese present the decrease in viscosity is much less drastic, and in each and every instance where manganese is present the viscosity at a given Kappa Number is significantly greater. Of even greater importance, perhaps, is the fact that the amount of sodium hydroxide required to achieve a given Kappa Number is less when manganese is present. This reduces the amount of sodium hydroxide necessary for the process and, hence, the cost of the process is appreciably reduced.
DETAILED DES~RIPTION OF THE INVENTION
In order to disclose more clearly the nature of the present invention, the following examples illustrating the in-vention are given. It should be understood, however, that this is done solely by way of example and is intended neither todelineate the scope of the inVention nor limit the ambit of the appended claims.
15 grams, oven-dried basis, of an unbleached Southern hardwood pulp was diluted with water to yield 1.5 liters of a pulp slurry, equal to a pulp consistency of 1%.
The pulp slurry was then placed in the reaction chamber of a Parr reactor. Examples 1-7, inclusive, employed dissolving grade pulp and Examples 8-13, inclusive, emPloyed paper grade pulp. The dissolving grade pulp had a brightness of 43.1, a permanganate number of 8.5 and a viscosity of 37.8. The paper grade pulp had a brightness of 26.1, a Kappa number of 17.0 and a ViSCOsIty of 42.3.
To the pulp slurry there was then added manganous sulfate, in solution, in the amounts indicated below in Table 1.
Following this there was then added sufficient sodium hydroxide - to yield a 2 gram per liter solution. The addition of the manganous sulfate and the sodium hydroxide to the pulp slurry was accompanied by gentle stirring to keep the solution as homogeneous as possible.
The pressure stirring head was then positioned over the reaction chamber and the entire unit was sealed. The reactor was then placed in a heating mantle and slowly brought to a final temperature of 95C. A slow, steady mixing a~com-panied the temperature rise. When temperature was achieved, pressurized oxygen gas was introduced at 40 psig. During this time, the fastest mixing speed possible, approximately 2000 rpm, was utilized. After one minute of combined rapid mixing and oxygen pressure at 40 psig, the mixing was stopped and the reactor was removed from the heating mantle. Oxygen pressure 1l)4;~S14 was maintained for nine minutes before a slow gradual depres-surization of 1 psig per minute was undertaken for a total period of 40 minutes. After complete depressurization, the pulp was withdrawn, washed, filtered, and made into sheets.
The pulp sheets were diluted to a consistency of 5%
with sulfurous acid at pH 2 and were placed in plastic bags which were maintained in a constan~ temperature bath at 49C.
for one-half hour.
The pulp was then water-washed, filtered and dried and the brightness, Kappa Number and viscosity were measured and are tabulated in Table I below.
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~0435~4 In the series of experiments shown in Table I both delignification catalysis and ~iscosity protection are evident with both ty~es of pulp. The viscosity protection was ob-served with a manganese application of 50 ppm and above.
The pulp, as expected, became increasingly dark and of a highly objectionable color. Contacting the pulp with a sulfurous acid solution at pH 2, removed the color caused by the manganese.
15 grams, oven-dried basis, of an unbleached Southern hardwood paper grade pulp having a Kappa Number of 17.0, and a viscosity of 42.3, was diluted with water to yield 1.5 liters of a pulp slurry, equal to a pulp consistency of 1%. The pulp slurry was then placed in the reaction chamber of a Parr re6ctor.
To the pulp slurry there was then added manganous sulfate, in solution, in the concentrations indicated below in Table II. Following this there was then added sodium hy-droxide in the concentrations indicated in Table II. The sodium hydroxide present is also calculated as a percentage of oven-dried pulp. The addition of the manganous sulfate and the sodium hydroxide to the pulp slurry was accompanied by gentle stirring to keep the solution as homogeneous as possible.
The pressure stirring head was then positioned over the reaction chamber and the entire unit was sealed. The reactor was then placed in a heating mantle and slowly brought to a final temperature of 95C. A slow, steady mixing ac-companied the temperature rise. When temperature was achieved, pressurized oxygen gas was introduced at 40 psig. During this time, the fastest mixing speed possible, approximately 2000 rpm, ~043S14 was utilized. After one minute of combined rapid mixing and oxygen pressure at 40 psig, the mixing was stopped and the reactor was removed from the heating mantle. Oxygen pressure was maintained for nine minutes before a slow gradual depres-surization of 1 psig per minute was undertaken for a total period of 40 minutes. After complete depressurization, the pulp was withdrawn, washed, filtered, and made into sheets.
The pulp sheets were diluted to a consistency of 5%
with sulfurous acid at pH2 and were placed in plastic bags which were maintained in a constant temperature bath at 49C.
for one-half hour.
The pulp was then water-washed, filtered and dried and the brightness, Kappa Number and viscosity were measured.
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~q H J~ 1~ 'J e1' 1` o _1 ~ a~
~ ~ ~ ~r er m o P~ O O O O O O O o :C ~
o o o O O O O O O
:: I ` ' ' ` ` ` ` `
.
'., Z ~ ~ ~ ~D ~ ~ ~ ~D
o P
o o o o In u~ In In _ o o O o . ~ o ~1 x z i(~4~S~4 ~ t can be seen from Table ~I that the decrease in Kappa Nu~ber and ~iscosity is less pronouncedwith manganese present as the concentration of sodium hydroxide is increased.
In each case, it should be noted, the viscosity at a given Kappa Number is superior. Perhaps even more important, the amount of sodium hydroxide required to achieve a given Kappa Number is less when manganese is present. This reduces the expense of sodium hydroxide for the reaction.
It can be seen that the combination of 2.0 grams per liter of sodium hydroxide and 0.55% manganese gave delignifi-cation almost equal to that of 4.0 grams per liter, of sodium hydroxide without manganese, but the viscosity of the manganese protected run was vastly superior to the controls. The value of manganese also lies in the reduction of sodium hydroxide losses in washing the pulp. If only one washer were available to wash the oxygen bleached pulp, operation at 4.0 grams per liter would result in a loss of about 35 pounds of sodium hydroxide, whereas only about 17 pounds would be lost when manganese is used in combination with 2.0 grams per liter of sodium hydroxide.
1100 grams oven-dried basis of an unbleached EBK
("Easy Bleaching Kraft") hardwood aper grade pulp having a brightness of 19.8, a Kappa number of 13.5, and a viscosity of 18.7 was diluted with water to yield 36 liters of a pulp slurry, equal to a pul~ consistency of 3%. The pulp slurry was then placed in the reaction chamber of a Pfaudler reactor.
To the pulp slurry there was then added manganous sulfate in solution in the amounts indicated below in Table III.
Following this there was then added sodium hydroxide in the 1(~4;~514 amounts indicated below in Table ~II. The addition of the manganous sulfate and the sodium hydroxide to the pulp slurry was accompanied by gentle stirring to keep the solution as homogeneous as possible.
The reactor was then slowly brought up to a final temperature of 95C. A slow, steady mixing accompanied the temperature rise. When the temperature was achieved, pres-surized oxygen gas was introduced at 100 psig. During this time, the fastest mixing speed possible, approximately 300 rpm was utilized. After two minutes of combined mixing and oxygen pressure at 100 psig, the mixing was stopped and the oxygen pressure was released to 40 psig. Thereafter, over a period of 40 minutes slow gradual depressurization from 40 psig to 0 psig, at 1 psig per minute, was accomplished. After com-plete depressurization the pulp was withdrawn, washed with water, filtered and made into sheets.
The pulp sheets were diluted to a consistency of 5%
with sulfurous acid at pH 2 and were placed in plastic bags which were maintained in a constant temperature bath at 49C.
for one-half hour.
The pulp was then water-washed, filtered and dried and the brightness, Kappa Number and viscosity were measured.
10~
~dP O ~
~rl ~
u~ ~ ~ l`
8 '' ,, ~1 ~n ~ "
o ~C ~ I`
d~
:~: ~ ,~
o ~ ~ ~9 ~ o ,~ .~
~.
o _~ o o :~
:, H
, Z
; H
, . :-j H
~` ~ ~ R u~
m ~
: ~: ~ ~ . O O
E~ ~ a ~o ~:
~'~
.
~ Q
'X O O O
P.
X O
Z
lO'~S~I
The results in Table III indicate that use of manganese allowed a reduction in the amount of sodium hydro-xide from 13.3% (based on O.D. pulp) to 6.7% (based on O.D.
pulp), ~hile simultaneously achieving improved pulp viscosity at virtually the same Kappa number.
The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and des-cribed or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.
Claims (18)
1. A process for the delignification and bleaching of lignocellulosic pulp fibers with oxygen in an alka-line medium while protecting the pulp against exces-sive losses in viscosity, which comprises the fol-lowing steps in sequence:
(a) contacting a slurry of lignocellulosic pulp fibers, having a consistency of less than 50%, by weight of oven-dried pulp, with a solution of a water-soluble salt of a divalent transition metal selected from the group consisting of manganous, nickelous, cobaltous, and vanadous ions having a con-centration of from 0.27% to about 1.10%, by weight of oven-dried pulp, for a period of time sufficient to allow the divalent metal ion to penetrate the walls of the pulp fiber;
(b) delignifying the pulp by mixing the ligno-cellulosic pulp slurry containing the divalent transi-tion metal ion with oxygen in an alkaline medium resulting in a slurry pH between about 10 and about 13, while oxi-dizing the divalent transition metal salt to a water-insoluble trivalent transition metal oxide and precipi-tating a viscosity protective coating of said trival-ent transition metal oxide on the pulp fibers; and then (c) washing the pulp with acid to reduce the trivalent transition metal oxide and thereby regen-erating the divalent transition metal salt.
(a) contacting a slurry of lignocellulosic pulp fibers, having a consistency of less than 50%, by weight of oven-dried pulp, with a solution of a water-soluble salt of a divalent transition metal selected from the group consisting of manganous, nickelous, cobaltous, and vanadous ions having a con-centration of from 0.27% to about 1.10%, by weight of oven-dried pulp, for a period of time sufficient to allow the divalent metal ion to penetrate the walls of the pulp fiber;
(b) delignifying the pulp by mixing the ligno-cellulosic pulp slurry containing the divalent transi-tion metal ion with oxygen in an alkaline medium resulting in a slurry pH between about 10 and about 13, while oxi-dizing the divalent transition metal salt to a water-insoluble trivalent transition metal oxide and precipi-tating a viscosity protective coating of said trival-ent transition metal oxide on the pulp fibers; and then (c) washing the pulp with acid to reduce the trivalent transition metal oxide and thereby regen-erating the divalent transition metal salt.
2, A process in accordance with Claim 1 wherein in the step (b) the oxygen is present in an amount from about 20 to 30 pounds per ton of pulp.
3. A process in accordance with Claim 1 or 2 wherein the consistency of the pulp slurry is from about 1% to about 30%, by weight of oven-dried pulp.
4. A process in accordance with Claim 1 or 2 wherein the consistency of the pulp slurry is from about 1% to about 10%, by weight of oven-dried pulp.
5. A process in accordance with Claim 1 or 2 wherein the water-soluble divalent transition metal salt is manganous sulfate.
6. A process in accordance with Claim 1 or 2 wherein the water-soluble divalent transition metal salt is manganous sulfate and the concentration of the manganous sulfate in solution is about 0.55%, by weight of oven-dried pulp.
7. A continuous process for the delignification and bleaching of lignocellulosic pulp fibers with oxygen in an alkaline medium while protecting the pulp against excessive losses in viscosity, which comprises the following steps in sequence:
(a) contacting a slurry of lignocellulosic pulp fibers, having a consistency of less than 50%, by weight of oven-dried pulp, with a solution of a water-soluble salt of a divalent transition metal selected from the group consisting of manganous, nickelous, cobaltous, and vanadous ions having a concentration of from about 0.27% to about 1.10%, by weight of oven-dried pulp, for a period of time sufficient to allow the divalent metal ion to penetrate the walls of the pulp fibers;
(b) delignifying the pulp by mixing the lignocellulosic pulp slurry containing the divalent transition metal ions with oxygen, in an alkaline medium resulting in a slurry pH
between about 10 and about 13, while oxidizing the divalent transition metal salt to a water-insoluble trivalent transition metal oxide and precipitating a viscosity protective coating of said trivalent transition metal oxide on the pulp fibers;
(c) washing the pulp to free it of anions;
(d) washing the pulp with acid to reduce the trivalent transition metal oxide and thereby regenerating the divalent transition metal salt;
(e) recovering the regenerated divalent transition metal salt and recycling it for reuse in (a).
(a) contacting a slurry of lignocellulosic pulp fibers, having a consistency of less than 50%, by weight of oven-dried pulp, with a solution of a water-soluble salt of a divalent transition metal selected from the group consisting of manganous, nickelous, cobaltous, and vanadous ions having a concentration of from about 0.27% to about 1.10%, by weight of oven-dried pulp, for a period of time sufficient to allow the divalent metal ion to penetrate the walls of the pulp fibers;
(b) delignifying the pulp by mixing the lignocellulosic pulp slurry containing the divalent transition metal ions with oxygen, in an alkaline medium resulting in a slurry pH
between about 10 and about 13, while oxidizing the divalent transition metal salt to a water-insoluble trivalent transition metal oxide and precipitating a viscosity protective coating of said trivalent transition metal oxide on the pulp fibers;
(c) washing the pulp to free it of anions;
(d) washing the pulp with acid to reduce the trivalent transition metal oxide and thereby regenerating the divalent transition metal salt;
(e) recovering the regenerated divalent transition metal salt and recycling it for reuse in (a).
8. A process in accordance with Claim 7 wherein in the step (b) the oxygen is present in an amount from about 20 to 30 pounds per ton of pulp.
9. A process in accordance with Claim 7 or 8 wherein the consistency of the pulp slurry in (a) is from 1% to about 30%, by weight of oven-dried pulp.
10. A process in accordance with Claim 7 or 8 wherein the consistency of the pulp slurry in (a) is from about 1 to about 10%, by weight of oven-dried pulp.
11. A process in accordance with Claim 7 or 8 wherein the water-soluble divalent transition metal salt is manganous sulfate.
12. A process in accordance with Claim 11 wherein the water-soluble divalent transition metal salt is manganous sulfate and the concentration of the manganous sulfate is about 0.55%, by weight of oven-dried pulp.
13. A process in accordance with Claim 7 or 8 wherein the alkaline medium is sodium hydroxide having a concentration of from about 1 gram per liter to about 20 grams per liter.
14. A process in accordance with Claim 7 or 8 wherein the alkaline medium is sodium hydroxide having a concentration of from about 2 grams per liter to about 4 grams per liter.
15. A process in accordance with Claim 7 or 8 wherein the delignifying is conducted at a pH between about 11 and about 13.
16. A process in accordance with Claim 7 or 8 wherein sulfurous acid is used to acidify the pulp.
17. A continuous process for the delignification and bleaching of lignocellulosic pulp fibers with oxygen in an alkaline medium while protecting the pulp against excessive losses in viscosity, which comprises the following steps in sequence:
(a) contacting a slurry of lignocellulosic pulp fibers, having a consistency of from about 1% to about 10%, by weight of oven-dried pulp, with a solution of manganous sulfate having a concentration of from about 0.27% to about 1.10%, by weight of oven-dried pulp, for a period of time sufficient to allow the manganous ions to penetrate the walls of the pulp fiber;
(b) delignifying the pulp by mixing the lignocellulosic pulp slurry containing the manganous ions with oxygen, in the presence of sodium hydroxide resulting in a slurry pH
between about 11 and about 1%, while oxidizing the manganous sulfate to manganese oxide and precipitating a viscosity protective coating of said manganese oxide on the pulp fibers;
(c) washing the pulp with water to free it of water-soluble compounds while retaining the water-insoluble manganese oxide on the pulp fibers:
(d) washing the pulp with sulfurous acid to reduce the manganese oxide and thereby regenerating the manganous sulfate; and then (e) recovering the regenerated manganous sulfate and recycling it for reuse in (a).
(a) contacting a slurry of lignocellulosic pulp fibers, having a consistency of from about 1% to about 10%, by weight of oven-dried pulp, with a solution of manganous sulfate having a concentration of from about 0.27% to about 1.10%, by weight of oven-dried pulp, for a period of time sufficient to allow the manganous ions to penetrate the walls of the pulp fiber;
(b) delignifying the pulp by mixing the lignocellulosic pulp slurry containing the manganous ions with oxygen, in the presence of sodium hydroxide resulting in a slurry pH
between about 11 and about 1%, while oxidizing the manganous sulfate to manganese oxide and precipitating a viscosity protective coating of said manganese oxide on the pulp fibers;
(c) washing the pulp with water to free it of water-soluble compounds while retaining the water-insoluble manganese oxide on the pulp fibers:
(d) washing the pulp with sulfurous acid to reduce the manganese oxide and thereby regenerating the manganous sulfate; and then (e) recovering the regenerated manganous sulfate and recycling it for reuse in (a).
18. A process in accordance with Claim 17 wherein in the step (b) the oxygen is present in an amount from about 20 to 30 pounds per ton of pulp.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US59157275A | 1975-06-30 | 1975-06-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1043514A true CA1043514A (en) | 1978-12-05 |
Family
ID=24366990
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA246,970A Expired CA1043514A (en) | 1975-06-30 | 1976-03-02 | Oxygen bleaching in the presence of a catalyst |
Country Status (9)
| Country | Link |
|---|---|
| JP (1) | JPS525302A (en) |
| AU (1) | AU504749B2 (en) |
| BR (1) | BR7602753A (en) |
| CA (1) | CA1043514A (en) |
| FI (1) | FI761496A (en) |
| FR (1) | FR2316376A1 (en) |
| NO (1) | NO761777L (en) |
| SE (1) | SE7601935L (en) |
| ZA (1) | ZA762002B (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1998020199A1 (en) * | 1996-11-06 | 1998-05-14 | Macmillan Bloedel Limited | Vanadyl catalyzed oxygen treatment of lignocellulosic materials |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| SE380298B (en) * | 1974-03-14 | 1975-11-03 | Mo | PROCEDURE FOR DELIGNIFICATION BY ACID-ALKALI TREATMENT OF LIGNOCELLUOSA-CONTAINING MATERIAL |
| JPS5039161B2 (en) * | 1974-12-23 | 1975-12-15 |
-
1976
- 1976-02-19 SE SE7601935A patent/SE7601935L/en not_active Application Discontinuation
- 1976-03-02 CA CA246,970A patent/CA1043514A/en not_active Expired
- 1976-04-02 AU AU12617/76A patent/AU504749B2/en not_active Expired
- 1976-04-02 ZA ZA762002A patent/ZA762002B/en unknown
- 1976-05-04 BR BR7602753A patent/BR7602753A/en unknown
- 1976-05-20 FR FR7615276A patent/FR2316376A1/en active Pending
- 1976-05-25 NO NO761777A patent/NO761777L/no unknown
- 1976-05-27 FI FI761496A patent/FI761496A/fi not_active Application Discontinuation
- 1976-05-28 JP JP51062188A patent/JPS525302A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| FR2316376A1 (en) | 1977-01-28 |
| NO761777L (en) | 1977-01-03 |
| SE7601935L (en) | 1976-12-31 |
| AU504749B2 (en) | 1979-10-25 |
| ZA762002B (en) | 1977-04-27 |
| BR7602753A (en) | 1977-05-10 |
| FI761496A (en) | 1976-12-31 |
| AU1261776A (en) | 1977-10-06 |
| JPS525302A (en) | 1977-01-17 |
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