US 3507744 A
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April 21, 1970 WOOD PULP BLEACHING PROCESS UTILIZING PEROXIDE PYROPHOSPHATE HIGH CONCENTRATION COMPOSITIONS D. H. GRANGAARD 3,507,744
Filed. April 1, 1966 BRIGHTNESS VS TIME OF REACTION BRIGHTNESS (G.E.)
30 so 90 I20 I50 REACTION TIME IMIN.)
BRIGHTNESS VS PYROPHOSPHATE CONCENTRATION BRIGHTNESS (G.E.)
POPLAR MAPLE o 2 4 s 8 IO MOLE RATIO H202 PYRO 5 .25 I6 .I2 01 MOLE RATIO PYROH O FIGIZ United States Patent 3,507,744 WOOD PULP BLEACI-IING PROCESS UTILIZING PEROXIDE PYROPI-IOSPHATE HIGH CONCEN- TRATION COMPOSITIONS Donald H. Grangaard, Appleton, Wis., assignor to Kimberly-Clark Corporation, Neenah, Wis., a corporation of Delaware Filed Apr. 1, 1966, Ser. No. 539,535 Int. Cl. D21c 9/16 U.S. Cl. 162-78 5 Claims ABSTRACT OF THE DISCLOSURE A wood pulp bleaching process particularly for the attainment of high brightness wood pulps of 80 G.E. or greater in a single bleaching step without a substantial loss in yield. The system involves providing a high concentration of a soluble peroxide and a pyrophosphate in actual contact with the pulp slurry at temperatures of at least 80 C. at relatively high pulp concentrations. The peroxide may be between 0.6% and 1.49% and the pyrophosphate at least 6% by weight of the bleaching solution in actual contact with the pulp. Pre-treatment of the pulp is effective to minimize peroxide consumption.
The present invention relates to the bleaching of wood pulp. More particularly, though not solely, the invention is concerned with the bleaching of the so-called high yield pulps prepared from woods from the hardwood species. By such high yield pulps I mean those termed semi-chemical (including those termed chemi-mechanical) produced by a combination of chemical and mechanical processes and in which the yield of pulp is 80 to 85% or more of the original wood, that is, about twice the yield of conventional chemical pulping processes.
The bleaching of mechanical pulps and semi-chemical pulps to brightnesses in excess of- 80 (General Electric Reflectance Meter Scale) has posed a problem in the art. Bleaching of such pulps in several steps to brightness levels of the order of 7578 is well known. The bleaching of such pulps in one bleaching step to a brightness level in excess of this range, without a substantial loss in yield, is currently one of the prime objectives of pulp research laboratories. It is one of the objects of this invention to bleach such pulps to high brightness levels. Further, it is one of the objects of this invention to accomplish this bleaching with no signficant loss in yield and to attain the bleaching action in a single bleaching step.
I have found that semi-chemical, as well as many other types of pulps, may be readily bleached in a simple single bleaching step to brightness levels in the range of 80 to 90 GE. if certain conditions previously believed to be impracticable are practiced. Such conditions are several and relate to: the amount and nature of the stabilizer which is utilized with the peroxide; the actual concentration of the peroxide solution in actual contact with the pulp, which must be substantially higher than practiced and believed practicable in the prior art, and which preferably must be substantially maintained throughout the bleaching process; the time and temperature at which the bleaching reaction is carried out; and the consistency of the bleaching system. Additionally, recovery of the unused peroxide may be practiced.
The effect of the bleaching treatment, when practiced as specifically noted hereinafter, is such that the pulp is bleached without a substantial loss in yield. Further, quite unexpectedly. I have found that brightness levels in excess of 80 GE. can be obtained wherein the peroxide consumption in the bleaching action is only of the order of l to 3% of the pulp weight (dry) and results are reproducible. The unused peroxide of the bleaching solu- 3,507,744 Patented Apr. 21, 1970 tion may be recovered and, after fortification, may be utilized as the bleaching agent for the bleaching of a previously unbleached pulp or may be used for other bleaching purposes.
Essentially, I have found that, by employing relatively high solution concentrations of peroxide (viz. H 0 accompanied by relatively high pyrophosphate stabilizer concentrations and wherein the pyrophosphate is the sole source of the alkalinity, additional bleaching reactions occur which do not apparently occur when bleaching at the peroxide concentrations as taught in the prior art. Further, although these high peroxide concentrations are used, the actual consumption of peroxide in the bleaching action need not be greater than 1 to 3% of the dry pulp weight and frequently is less than 1%.
On bleaching pulps there appears to be, in essence, at least two main bleaching reactions involved. The first reaction appears to involve the brightness range of from about 60 to or somewhat less; the second, the range of 80+ to and above. Inspection of the prior art reveals that it is concerned with the range of 60 to about 80, although the desirability of higher brightness is readily recognized. In fact, on practicing the teachings of the prior art, I have found that the brightness levels obtained invariable are in the range of 60 to about 78 with only occasional brightnesses of 79 and slightly above attained.
Apparently, on bleaching in the range of 60 to 78 brightness, high brightness changes per small changes in the applied peroxide concentration occur. Thus, at the low concentration of peroxide and pyrophosphate as taught in the prior art, differences due to wood species and initial brightness level of the unbleached pulp, etc., tend to effect the ultimate brightness level attainable.
In contrast, this invention involves bleaching high yield and other pulps in the range of 80+ to 90 brightness in a single step. To accomplish this, peroxide solution concentrations and pyrophosphate concentrations far in ex cess of that taught by the prior art are required. Further, in this brightness range the brightness change is relatively small per change in peroxide concentration. The nature of the reaction thus is substantially different. Stated somewhat differently, when bleaching at high peroxide and pyrophosphate concentrations, the bleaching potential unexpectedly was found to be such that one can apparently bleach certain colored bodies which are not affected when bleaching at the lower concentrations as taught in the prior art. Identification of the specific colored bodies involved is presently not possible, but the results clearly indicate that the colored bodies which must be bleached in order to obtain brightness levels in excess of about 80+ are not affected when bleaching at low peroxide and/or pyrophosphate concentrations.
I have also found that the results of my process may by further favorably influenced by the utilization of certain additional features. Thus, I have found that it is very beneficial to dry the pulp to a very low moisture content, preferably about 4 to 8%, and to subject the pulp to bleaching at this low moisture content. The lower the moisture content, the higher is the solution concentration of the peroxide and pyrophosphate in contact with the actual pulp which may be obtained. This, in turn, results in higher brightness levels. Drying of pulp for baling and shipment prior to bleaching is a common practice, and my method suits this condition well as the dry pulp may be bleached at its destination.
Additionally, I have found that pretreatmentof the pulp by extraction sequentially with an acid and an alkaline earth chloride or other salt of higher pH value than the acid, with washing of the pulp following each extraction, are effective to substantially reduce the peroxide consumption without in turn producing a pulp more difiicult to bleach. Treatment of the pulp with a mineral acid alone, followed by washing out of the acid is effective to reduce the peroxide consumption but the pulp appears to be more difiicult to bleach. By this I mean that the effect of the acid treatment alone is such that the same brightness level is not obtained with the same peroxide concentration for the same bleaching time and temperature conditions. One or another or all must be increased following an acid treatment and wash to reach the same brightness level but this is then at the expense of increased peroxide consumption. The effect of the succeeding alkaline earth chloride or other treatment, however, is to restore to the pulp its bleachability, while retaining the property of reduced peroxide consumption for the obtaining of a given brightness level at a given peroxide concentration, bleaching time and temperature.
My process is applicable to a wide variety of pulps from various hardwoods produced by a variety of processes to attain large brightness increases. Thus, even un bleached kraft pulp may be increased 20 points or more H in brightness; such is not a practical application of my one step procedure, however, because the brightness is still very low and not satisfactory commercially. Unbleached balsam sulfite pulp may, however, be bleached to a GE. brightness value of 85 in a single stage bleaching procedure at relatively low peroxide consumptions. Groundwood, produced solely by mechanical attrition, likewise may be bleached in a single stage to brightnesses of 85 to 90 by my procedure. But, surprisingly, the high yield pulps produced by semi-chemical and chemi-mechanical processes from the hardwoods such as poplar, maple, beech and birch, previously considered by the art to be unbleachable to brightness levels in excess of about 78 GE, may be bleached readily to brightnesses in excess of 80 and usually to 83 to 85 (GB) or better.
In more specific aspect, the pulp, pretreated or otherwise, is subjected to a bleach formulation such that:
(1) the concentration of the peroxide solution in actual contact with the pulp is in the range of about 0.5% to 1.2% hydrogen peroxide. This concentration is measured in weight units, for example grams, of actual hydrogen peroxide (viz H per 100 cc. of solution (including any water in the pulp itself), and is to be distinguished from the conventional recitation of peroxide concentration wherein the amount of peroxide by weight is commonly stated on the basis of the dry pulp weight;
(2) the pyrophosphate E 0) concentration (also measured on the solution basis) is between about 6% to about 8% or higher. Preferably, the pyrophosphate is present to the extent that the solution is close to saturation as to the pyrophosphate but not such that precipitation of pyrophosphate will occur under bleaching conditions; both this pyrophosphate concentration and the peroxide concentration must be maintained during the reaction;
(3) the pulp consistency, measured in parts by weight of pulp on the basis of the total weight of the pulp slurry, is between about 10 to (4) no other stabilizer other than the pyrophosphate is necessary or desirable since the addition of alkali tends to decrease the brightness attainable, all other conditions being equal; further, either the sodium or potassium salt may be used. When using the more soluble potassium salt, substantially higher peroxide and pyrophosphate concentrations may be used;
(5) the pH is maintained in the range of about 8.0 to about 9.2; (the pyrophosphate concentration employed is sufficiently high to insure this usually);
(6) the bleaching temperature should be at least 80 and not more than 100 C.; and
(7) the bleaching time should be at least minutes with most wood pulps.
The invention will be more fully understood by reference to the following detailed description and accompanying drawings wherein:
FIG. 1 is a graph illustrating brightness change in wood pulp treatment in accordance with the invention over a range of bleaching reaction time, particularly indicating the presence of two difference reaction rates; and
FIG. 2 is a graph illustrating the brightness change which occurs as the peroxide to pyrophosphate ratio changes.
FIG. 1 is predicated upon bleaching a poplar high yield N.S.S.C. pulp at a temperature of 90 C. using a peroxide concentration of 0.6 g./l00 cc., and wherein the peroxide to pyrophosphate molar ratio was 2:1. The consistency was 9.1%.
As indicated by FIG. 1, the reaction time should usually be at least 30 minutes in accordance with the invention. While the time and temperature will vary somewhat with the specific nature of the wood pulp, I have found that for consistently reproducible results a bleaching time of at least 30 minutes at a temperature of at least C. is commonly necessary. Whether a pretreatment is required for a specific wood pulp to attain a desired brightness in the 80+ range within the bleach time is readily determined by simple test.
FIG. 2 is predicated upon bleaching a high yield N.S.S.C. poplar pulp at a temperature of C. for a time of 2 hours employing a peroxide concentration of 0.674 g./ cc.
As will be noted, the brightness decreases below the 80 Value as the pyrophosphate concentration relative to the peroxide decreases. I have found that the pyrophosphate to peroxide mole ratio should usually be between about 1 to 1.5 to 3.0 and preferably is l to 2. These figures may vary somewhat with specific wood pulps and bleach conditions but are generally applicable, and optimum factors are readily determinable to accommodate the process to specific species by simple testing as indicated by the following examples.
In the following examples the yields of Wood pulp, unless otherwise specified, were not significantly reduced by the actual bleaching process and commonly were less than 1%; always at least 97% of the pulp introduced to the peroxide bleaching solution was removed from the solution at the end of the bleach period.
EXAMPLE 1 Poplar pulp is prepared by the neutral sulfite semir chemical pulping process by first barking the logs and then chipping the wood in conventional manner. The chips are screened to about A" x in size, and fed to a digester for cooking. The digester is fed with a suitable neutral sulfite having sutficient sodium carbonate or caustic to serve as a buffer. The wood is then cooked to a yield of about 85% followed by mechanical defibration in equipment such as a disc refiner.
The resultant pulp contains about 85% by weight of the original wood. This plup is screened to remove any shives of the like. Handsheets made from such pulp had a General Electric brightness of 65.42.
A hydrogen peroxide bleach formulation was made in the laboratory as follows:
Parts by weight (grams) To 94.5 grams of wet N.S.S.C. high yield poplar pulp of unbleached brightness 65.42 (20 grams pulp on a dry basis) there is added 100 cc. of the bleach solution. This provides a concentration of peroxide solution in actual contact with the pulp of 0.73 g./ 100 cc., or a quantity of 6.392 grams of peroxide per 100 grams of dry pulp. The sodium pyrophosphate (l0 H O) concentration of the aqueous system in contact with the pulp is about 4.6 g./100 cc., and it is present to the extent of 40 grams sodium pyrophosphate (1O H O) per 100 grams of dry pulp. The pH of the system is 9.1. The suspension is heated for minutes at 90 C. to effect 5 the bleaching. During bleaching the pH falls to about 8.0.
The bleached pulp is then separated from the liquid by filtering. The pulp is washed with water (100 cc.) and the washings added to the filtrate. An analysis of the combined filtrate and washings shows that 0.8396 gram of the original 1.2784 grams of peroxide were recovered, or 4.198 grams per 100 grams of pulp (dry). The actual consumption of perioxide was thus 2.194 g./100 gms. pulp.
The bleached pulp is then washed, soaked in dilute acetic acid solution, and rewashed. This pulp is then formed into handsheets. The General Electric brightness was 82.58, a gain of over 17 points.
The combination of filtrate and washing which exhibits a hydrogen peroxide concentration of about 0.21 gram per 100 cc. (pH 8.0) is then concentrated in vacuo and fortified by the addition of suflicient hydrogen peroxide and pyrophosphate to provide a solution of the original concentrations for use in the bleaching of another quantity of pulp.
EXAMPLE 2 Example 1 is repeated except that the pulp is dried to a moisture content of about 5% before bleaching. The pulp weight dry is 20' grams.
Due to the dry condition of the pulp, the concentration of the peroxide solution in actual contact with the pulp is 1.278 grams/1 cc., although the amount of peroxide (g./100 gms. pulp) added to the system is the same as in Example 1, namely 6.392 g./100 grams of pulp on a dry basis. The pyrophosphate concentration of the aqueous solution in contact with the pulp is 8 g./100 cc. or about 40 grams per 100 grams of dry pulp.
The bleached pulp in this instance exhibits a somewhat higher G.E. brightness of 84.30; and about 2.274 grams H 0 per 100 grams of dry pulp was consumed during the bleaching action. The amount of peroxide recovered was 4.118 grams.
The solution (filtrate plus washings) are concentrated in vacuo, fortified by hydrogen peroxide and pyrophosficult to bleach. Thus, in order to obtain a given brightness level, an increase in the bleaching time and/or peroxide concentration and/or temperature is necessary.
Soluble barium salts such as barium chloride, when employed on the pulp without the acid treatment, are effective to a lesser degree than the acid alone but do not make the pulp more difiicult to bleach. Also, the barium salts, whether used alone or following acid pretreatment, are more effective than corresponding calcium salts.
The pretreatment in each instance was carried out by simply immersing the pulp in the pretreating or extracting reagent for a period not exceeding 30 minutes; the pulp was then removed from the reagent and washed substantially free of the pretreating reagent with water. After removing the excess water by vacuum filtration, 100 cc. of the pyrophosphate-peroxide bleach formulation as shown in Example 1 was added, and the mixture heated at 90 C. for 2 hours. The results obtained in terms of amount of applied peroxide, actual peroxide consumption, and brightness level is shown in Table 1. Where two treating agents are mentioned in the table, the treatment was with each agent in succession with a Washing step with water interposed.
The concentration of the pyrophosphate in Example 1 is much higher than commonly employed in peroxide TABLE 2 Poplar Maple G/Pyrophos- G./H2O 4 phate (10 1120)] [100 gms. Consumption. Consumption, Test 100 gms. dry pulp dry pulp g./100 pulp Brightness g./100 Brightness phate addition, and utilized for bleaching in the same manner a new batch of pulp. Due to the initial dry condition of the pulp, the volume of the recovered bleach solution (plus washings) is substantially less than when using a wet pulp as in Example 1. In fact, in certain instances, wherein the pulp is carefully washed, the volume of recovered bleach solution is slightly less than the volume of the bleach solution which was initially applied to the pulp. Such a recovered bleach solution, after fortification with H 0 and pyrophosphate, can be used directly for the bleaching of a new batch of pulp.
EXAMPLE 3 Example 1 is repeated except that the pulp is pretreated prior to bleaching. The pretreatments preferably include: a sequence of treatments with a dilute acid and an alkaline earth salt. The effect of these treatments in sequence is to materially decrease the amount of peroxide consumed on bleaching under a given set of conditions of time, temperature and peroxide concentration.
The acids and alkaline earths are also effective to some extent if employed individually as now noted. The dilute mineral acids, while effective in themselves to reduce the peroxide consumption, tend to produce a pulp more difbleaching systems as based on the dry pulp weight. The effect of varying the pyrophosphate concentration, while maintaining the procedure of Example 1 in other respects, when bleaching a sample of poplar and a sample of maple high yield N.S.S.C. pulp, is shown in Table '2.
EXAMPLE 5 EXAMPLE 6 Example 5 was repeated using a dry rather than a wet pulp. A brightness of 84.50 was obtained with a consumption of 3.052 g. H 0 per 100 grams of dry pulp; the residual H 0 recovered was 3.340 g./100 grams of dry pulp.
'5 EXAMPLE 7 Example 5 was essentially repeated except that the pulp was pie-extracted with dilute HCl acid. A brightness of 80.78 was obtained at a peroxide consumption of 0.706 g./l grams of dry pulp.
EXAMPLE 8 Example was essentially repeated except that the pulp was pre-extracted with dilute BaCl solution. A brightness of 83.24 was obtained at a peroxide consumption of 1.0375 g./l00 gms. of dry pulp.
EXAMPLE 9 Example 5 was essentially repeated except that the pulp was pro-extracted first with dilute HCl acid, washed, then re-extracted with dilute BaCl solution. A brightness of 82.64 was obtained at a peroxide consumption of 0.746 g./l00 gms. of dry pulp.
EXAMPLE 10 Example 1 was repeated employing birch as the wood source. The pulp yield from the wood treatment process was 89.8%. The initial unbleached brightness of the pulp obtained was 58.38. At a peroxide concentration of 0.72 g./100 cc. a brightness of 81.78 G.E. was obtained; the peroxide consumption was 5.0195 g./ 100 pulp, however Under similar bleaching conditions, but wherein the pulp was pre-extracted with BaCl only and then washed with water, a brightness of 84.18 was obtained at a peroxide consumption of 2.594 g./100 pulp.
EXAMPLE 1 1 Example 1 was repeated employing beech as the source of the pulp; the pulp yield was 88.3%. The initial brightness of the pulp so obtained was 52.32. A brightness of 75.36 GE. was obtained at a peroxide solution concentration of 0.835 g./100 cc. The peroxide consumption was 4.1865 g./100 gms. pulp. Under similar bleaching conditions, but wherein the pulp was pre-extracted with BaCl and washed, a brightness of 78.04 at a peroxide consumption of 2.259 g./ 100 pulp was obtained. Through the use of peroxide concentrations of about 1.0 g./100 cc. a brightness of 81.04 was'obtained. This illustrates the necessity of the need of substantially higher peroxide concentrations when bleaching certain hard to bleach pulps.
EXAMPLE 12 Example 1 was repeated with the temperature of bleaching reduced to 50 C. and the bleaching time extended to 4 hours. The brightness was 76.2 and the consumption of peroxide 0.920 g./100 gms. pulp. The recovered peroxide was 5.455 g./100 gms. of bone dry pulp. This is indicative that temperatures as low as 50 C. are not particularly useful with the present system in bleaching high yield pulp.
EXAMPLE 13 Example 1 was repeated except that an equal molar amount of potassium pyrophosphate was substituted for the sodium pyrophosphate. A brightness of 81.84 was obtained with a consumption of 3.5655 g. of H 0 per 100 gms. of dry pulp.
EXAMPLE 14 Example 1 was repeated except that the concentration of potassium pyrophosphate was 8.26 g./100 cc. and the hydrogen peroxide concentration was 1.495 g./ 100 cc. A brightness of 85.64 was obtained.
EXAMPLE 15 A series of tests were conducted with equal amounts of various phosphates to determine suitable substitutes for the pyrophosphate. A summary of some of such tests is included in the following Table 3. It will be noted that none other than the pyrophosphate was found useful.
TABLE 3 Amount 11202 Amount H2O: applied consumed Phosphate /100 pulp I100 pulp Brightness Sodium pyrophosphate 6. 1. 79 82. 8t Monosodiuin phosphate. (1. 35 O. 6875 (34. U4 Disodium phosphate 6. 35 0. 9880 67. .24 Trisodiurn phosphate (i. 35 3. 9305 70. 88 50% disodium and 50% trisodiuln G. 35 3. 397 79. 26
Only the pyrophosphate appears to respond well at low peroxide consumptions for a given brightness.
EXAMPLE 16 A sample of unbleached poplar groundwood (brightness 67.78) was bleached essentially according to the conditions used in Example 1. A brightness of 85.62 was obtained at a peroxide consumption of 4.44 g./ 100 pulp. Through pretreatment of the pulp with dilute HCl acid, prior to the bleaching sequence, a brightness of 85.62 was obtained at a peroxide consumption of 3.03 g./ 100 pulp. Upon using BaCl for the pretreatment, a brightness of 86.08 was obtained at a consumption of 3.839 g./100 pulp.
In the bleaching of an N.S.S.C. pulp at temperatures of the order of 50 C. versus 90 C. a substantial decrease in the brightness level occurs; poplar groundwood, in distinct contrast however, bleaches to relatively high brightness levels at temperatures as low as 50 C. It also behaves differently insofar as pie-extraction is concerned. On no pre-extraction a brightness of 84.16 was obtained at a consumption of 3.378; on HCl extraction, a brightness of 84.42 at a consumption of 1.755, indicating that acid pretreatment does not result in a pulp which is more difficult to bleach.
EXAMPLE 17 A sample of unbleached balsam sulfite pulp (brightness 66.30) was bleached according to the conditions used in Example 1. A brightness of 85.30 was obtained at a peroxide consumption of only 1.369 g./ gms. pulp. Through pretreatment of the pulp with dilute HCl acid, prior to bleaching, upon bleaching a brightness of 84.70 was obtained at a peroxide consumption of only 0.905 g./100 pulp. Upon using BaCl for the pretreatment, a brightness of 86.34 was obtained at a consumption of 0.9685 g./l00 pulp.
Other pulps, as for example, maple, yellow birch, white plus yellow birch magnefite pulps, in the yield range of 50 to 52%, have also been successfully bleached to high brightness levels by this procedure.
Further, in instances where it is desirable to bleach to brightness levels only of the order 78 to 80, brightness levels of this order of magnitude may readily be obtained in relatively short periods of time (viz. 30-45 minutes) and at peroxide consumptions of the order of 0.7 to 1.0 g/100 pulp. Under present currently used commercial bleaching conditions, brightness levels of this order require essentially a two stage process. In the first stage, the pulp is bleached first with an alkaline peroxide solution. This is followed by a second stage, wherein the pulp is further bleached by a reagent as sodium hydrosulfite. My procedure overcomes the necessity for two stage bleaching in such instances.
What is claimed is:
1. A bleaching process which consists in treating wood substantially unbleached pulp with an aqueous stabilized bleaching solution wherein the bleaching solution in actual contact with the pulp contains by weight: between about 0.67% and 1.49% of hydrogen peroxide, at least 6% by weight of a pyrophosphate selected from the group consisting of sodium and potassium pyrophosphates and mixtures thereof, the pyrophosphate present to the extent that precipitation of the pyrophosphate will not occur, the balance of said solution being water, said pyrophosphate constituting the only alkali contributing component of the bleaching solution and the mole ratio of pyrophosphate to peroxide being in the range of 1 to between about 1.5 to 3.0, said solution having a pH in the range of about 8.0 to 9.2; bleaching the wood pulp as a slurry with the said solution at a temperature in the range of at least 80 C. but not more than 100 C. at a pulp consistency in the bleach solution of between about 520% by weight based on the total weight of bleach solution and pulp including any water in the pulp; after at least 30 minutes of bleaching under the stated conditions, removing the pulp from the bleach solution while the solution still contains a substantial quantity of hydrogen peroxide and while the wood pulp is present to the extent of at least about 97% of the weight of pulp introduced to the bleaching solution; and thereafter washing the pulp to remove substantially all residual peroxide and pyrophosphate from the pulp.
2. A bleaching process according to claim 1 wherein the pulp is dried to a moisture content of not more than about 5% and then subjected to treatment with the bleaching solution.
3. A bleaching process according to claim 1 wherein the wood pulp is pretreated prior to subjection to the bleaching solution, said pretreatment including applying a mineral acid to the pulp, washing the pulp and then applying an inorganic salt selected from the group consisting of water soluble salts of barium and calcium and washing said pulp to prepare it for presentation to the bleaching solution.
4. A bleaching process according to claim 1 wherein the wood pulp is derived from hardwoods selected from the group consisting of poplar, birch, beech and maple.
5. A bleaching process according to claim 1 wherein the wood pulp prior tosubjection to the bleaching solution is treated with a solution of barium chloride and washed prior to treatment with the bleaching solution.
References Cited UNITED STATES PATENTS 2,510,595 6/1950 McEwen et a1. 16278 XR 2,822,236 2/1958 Sheldon et a1 16278 XR 3,382,149 5/1968 Hoh 162-78 2,687,943 8/1954 Pete 16278 XR S. LEON BASHORE, Primary Examiner R. H. TUSHIN, Assistant Examiner US. Cl. X.R. 8111; 162-80