US3650887A

US3650887A - Wood pulp bleaching process utilizing peroxide-silicate bleaching solution

Info

Publication number: US3650887A
Application number: US826756*A
Authority: US
Inventors: Donald H Grangaard
Original assignee: Kimberly Clark Corp
Current assignee: Kimberly Clark Corp
Priority date: 1969-04-21
Filing date: 1969-04-21
Publication date: 1972-03-21
Anticipated expiration: 1989-03-21

Abstract

A process of bleaching defibered hardwood pulp with a peroxidesilicate composition to attain in a single bleaching step a brightness (General Electric) of 80+. The procedure requires a plurality of controlled conditions including bleaching time, temperature, pulp consistency, pH, sodium silicate and alkalinity concentrations, and particularly a peroxide solution concentration in actual contact with the pulp of between about 0.4 and 1.2 percent based on the weight of the aqueous bleach solution itself including any water in the pulp. The bleaching takes place at a consistency such that initially at least 3 percent by weight of peroxide based on the air dry pulp weight is present. Brightnesses well in excess of 80 are attained in a single bleaching step, and pre-treatment of pulp with barium chloride is effective in the attainment of such brightnesses at relatively low temperatures with reasonable peroxide consumptions.

Description

United States Patent [151 3,650,887

Grangaard 1*Mar. 21, 11972 [54] WOOD PULP BLEACHING PROCESS 3,382,149 5/1968 Hoh ..162/78 UTILIZING PEROXIDE-SILICATE 3,492,199 1/1970 Kindron et al. .I ..162/78 X BLEACHING SOLUTION 3,507,744 4/1970 Grangaard ..162/78 [72] Inventor: Donald H. Grangaard, Appleton, Wis. P i E i -B 1 5. Leon Assistant Examiner-Thomas G. Scavone [73] Asslgnee' a g Clark Corporation Neenah Attorney-Daniel J. Hanlon, Jr. and Raymond J, Miller Notice: The portion of the term of this patent sub- [57] ABSTRACT sequent to Apr. 21, 1987, has been disclaimed A process of bleaching defibered hardwood pulp with a peroxide-silicate composition to attain in a single bleaching step a 1 Filedl P 1969 brightness (General Electric) of The procedure requires [21 1 App] No 826 756 a plurality of controlled conditions including bleaching time,

' temperature, pulp consistency, pH, sodium silicate and all d [1,3, A li ti D t kalinity concentrations, and particularly a peroxide solution concentration in actual contact with the pulp of between about 0.4 and 1.2 percent based on the weight of the aqueous bleach solution itself including any water in the pulp. The

bleaching takes place at a consistency such that initially at 162/78 162/80 least 3 percent by weight of peroxide based on the air dry pulp [63] Continuation of Ser. No. 546,066, Apr. 28, 1966,

abandoned.

[52] U.S.Cl.....

[51] Int. Cl ..D2lc 9/16 welght IS present, Brightnesses well in excess of 80 are at- [58] Field of Search ..162/78, 80, 89, 8/1 11 mined in a single bleaching Step and pre n eatmem of p p [56] References Cited with barium chloride is effective in the attainment of such brightnesses at relatively low temperatures with reasonable UNITED STATES PATENTS Peroxide consumptions- 3,193,445 7/1965 Parker et al ..162/78 X 5 Claims, 2 Drawing Figures GE. BRIGHTNESS TIME (MIN) PAIENIEBI-IARZI I972 6.5 BRIGHTNESS- L06. RESIDUAL H202 CONQXIO TIME (M|N.)

FIG. I

Ist BLEACHING REACTION 2nd BLEACHING REACTION TIME (MIN) FIG. 2

WOOD PULP BLEACIIING PROCESS UTILIZING PEROXIDE-SILICATE BLEACHING SOLUTION This application is a continuation of my copending application Ser. No. 546,066 filed Apr. 28, 1966, and now abandoned.

My present invention relates to the bleaching of wood pulp. The invention is particularly directed to the attainment of high brightness levels in a single bleaching step utilizing a peroxidealkali and/or silicate system. More particularly, the invention is directed, but not restricted, to the economical bleaching of high yield (80 to 85 percent) hardwood neutral sulfite semichemical wood pulps to a brightness level in excess of 80+ as measured by the General Electric Reflectance Meter method.

Illustrative of the utility of my procedure is the bleaching to high brightness levels of semi-chemical pulps derived from hardwoods such as poplar, maple, beech and birch. Semichemical wood pulp is obtained by reducing the wood in log form to chips, cooking the chips in a digester with a chemical such as sodium sulfite to soften the chips, and thereafter mechanically working the softened chips to fibers. The yield of pulp by such a procedure varies with the extent of the.

digestion time and the specific woods. A variation of the semichemical procedure, sometimes termed chemi-mechanical pulp, involves essentially the same variables but always provides yields at the high end of the pulp yield range (85 percent or more) and is differentiated in the art from other neutral sulfite semi-chemical pulps in that there is said to be no major change in the lignin of the fibers. These semi-chemical pulps (including the chemi-mechanical) have been bleached to useful brightness ranges by multistage bleaching processes. Such bleaching processes either result in a significant loss in yield or, if this is not the case, the brightness levels obtained are lower than desired. The loss in yield may be as high as 30 percent of the unbleached pulp.

l have found that the bleaching of the semi-chemical pulps, and particularly including the chemi-mechanical (80 percent to 85 percent or more yield) to brightness ranges in excess of 80+ may be accomplished with only a slight loss (viz. 4 to 5 percent) in yield while utilizing basically a peroxide-silicate system. For my purposes it is essential that certain conditions be closely observed. Initially, it should be noted that bleaching to a range of 80+ is to be distinguished from bleaching to a range of up to about 78 or 79 and sometimes 80 (from as low as 50) as my researches indicate that significantly different bleaching conditions are required above and below the 80 limit. Specifically, it appears that when bleaching the semichemical pulps particularly to the range of 80+ to 90, the bleaching of the colored bodies for the color in the 80-90 range requires relatively high peroxide concentrations, closely controlled alkalinities and closely controlled temperatures if the peroxide consumption is to remain within what is believed to be economically feasible limits. The lower the cost of peroxide the greater can be the consumption of peroxide and still remain within what is considered to be economically feasible limits.

It is well known that during the bleaching of a pulp with bleach formulations of the alkali-silicate-peroxide type two opposing reactions are occurring. As a result of what I have termed the actual bleaching reaction, the brightness level of the pulp is increased. As a result of what I have termed the non-bleaching reaction, the brightness level of the pulp tends to be decreased. In many instances it is the degree to which this latter reaction occurs which governs the ultimate brightness level obtained. Particularly this is the case if the peroxide becomes entirely consumed during the course of the bleaching reaction. The reaction in question is that of the colored bodies, which are produced as a result of the action of the alkali contained in the bleach formulation itself upon the ul p :pparently unbeknown before, my researches indicate that during what I term the actual bleaching reaction itself there are, essentially, two bleaching reactions which occur (FIG. 1 As a result of a first of these bleaching reactions, brightness levels of the order of about 78 GE. brightness can readily be obtained. Illustrative of this reaction are the various examples of peroxide bleaching given by the prior art which appears in the bleaching literature. In order to consistently obtain brightness levels above G.E., however, the bleaching conditions must be such that the rate of the other of the bleaching reactions is substantially increased. In contrast to the first bleaching reaction, in order to increase its rate of reaction, this second reaction requires substantially higher peroxide concentrations. Further, due to the relatively slower rate of bleaching obtained even though higher peroxide concentrations are used, substantially longer bleaching times are required in order to obtain the full benefits possible from this reaction. This fact is readily seen when attempts are made to further bleach a pulp that has been prebleached to the 78-80 G.E. brightness range. Only upon the addition of the peroxide at a relatively high concentration can such a pulp be bleached to significantly higher brightness levels, as will be indicated hereinafter.

Specifically, I have found that in order to attain high bleach brightnesses consistently in a single step with only a slight loss in yield, and at reasonable peroxide consumptions, it is necessary that a plurality of factors be coordinated in the bleaching operation. The pulp should be treated with a peroxide solution in which:

a. The concentration of the peroxide solution initially in actual contact with the pulp is in the range of about 0.4 to 1.2 percent hydrogen peroxide. This concentration is measured in grams of actual hydrogen peroxide (H 0 per 100 cc. of solu tion (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 dry pulp weight.

b. The actual amount of peroxide in terms of the amount of pulp is of secondary importance. The amount must be such that, after the desired bleaching reaction has taken place, sufficient peroxide remains to compensate (or prevent) the darkening of the pulp caused by the residual alkalinity of the bleaching solution. Since this will vary considerably with the nature of the pulp and the degree of bleaching required, it cannot be rigidly defined. In general, it is of the order of 0.5 to 1 percent on the air dry weight of the initial pulp. In view, then, of the amount of peroxide consumed in the actual bleaching reaction (plus the amount lost by decomposition) together with the amount required to be present in the system at the end of the bleaching reaction, the total amount of peroxide required is in excess of about 3 percent peroxide on the air dry weight of the pulp. Air dry weight pulp is pulp having about 4 to 5 percent moisture.

c. The amount of sodium silicate on a weight basis should be between about 2 parts to 4.2 parts per part of peroxide expressed as hydrogen peroxide. Preferably, the silicate provides the greater portion of the alkalinity of the system as this tends to minimize peroxide consumptions.

d. Empirically, it has been found, provided an excess of peroxide is .present in the system, that if the ratio Gms. peroxide/100 cc. Gms. pulp/100 cc.

is greater than about 3.5, brightnesses greater than 80 invariably are obtained.

e. Suitably, the overall alkalinity of the bleach formulation expressed as parts by weight (grams) of sodium hydroxide per liter of bleach solution should be in the range of about 7 to 15 grams.

f. The time of bleaching is dependent upon several factors including peroxide concentration and bleaching temperature and is commonly between about 30 minutes and 120 minutes.

g. The pH of the system should be in the range of between about 9.0 to l 1.3 at the start of the bleaching procedure.

h. The bleaching temperature may be relatively low, as low as 50 C., or as high as C. At this latter temperature, however, the consumption of hydrogen peroxide increases quite disproportionately to the brightness increase. The preferred temperature appears to lie in the range of 60 to 70 C.

i. The consistency of the bleaching system should be in the range of about 7% to about 20 percent.

j. The bleach solution may also include magnesium sulfate or magnesium sulfate -7H O (Epsom salts) or the like to serve in known manner as a stabilizer.

k. For economy, based primarily upon current peroxide costs, the overall peroxide consumption should not be greater than about 3 grams per 100 grams of pulp.

The invention will be more fully understood with respect to the following description and accompanying drawings wherein:

FIG. 1 is a graph illustrating the relationship of GE. brightness with bleaching time at various solution concentrations of peroxide; and

FIG. 2 is a graph illustrating the interrelationship of bleaching time with residual peroxide concentration, that is, the peroxide concentration remaining at the end of the bleaching action, the graph particularly illustrating the different reaction rates involved as the residual concentration decreases.

EXAMPLE 1 Poplar pulp is prepared by the neutral sulfite semi-chemical pulping process by first barking the logs and then chipping the wood in conventional manner. The chips are screened to about inch in size, and fed to a digester for cooking. The digester is fed with a suitable neutral sulfite having sufficient sodium carbonate or caustic to serve as a buffer. The wood is then cooked to a yield of about 85 percent followed by mechanical defibration in equipment such as a disc refiner.

The resultant pulp contained about 85 percent by weight of the original wood. This pulp was screened. Handsheets made from this pulp had a GE. brightness of 66.88.

A hydrogen peroxide-silicate bleach formulation was made with the following constituents:

Parts By Weight (grams) Magnesium sulfate -7H,O 3

Sodium hydroxide (dry) 10 Sodium silicate (io -42 Be) 33.3 Hydrogen peroxide (307: H 20 Water to make up to a volume of l liter To 20 grams of the dry percent moisture) neutral sulfite semi-chemical high yield poplar pulp of an unbleached brightness of 66.88 there is added 200 cc. of the above identified bleach solution and 50 cc. H 0. The consistency was about 7.4 percent. This provides a concentration of peroxide solution in actual contact with the pulp ofabout 0.48 grams per 100 cc. or a quantity of about 6.0 grams of peroxide per 100 grams of dry pulp. This suspension is heated for 120 minutes at 90 C. to effect the bleaching.

The bleached pulp is then separated from the liquid by filtering. The pulp is washed with water, and the washings added to the filtrate. An analysis of the combined filtrate and washings indicates that 14.7 percent of the original peroxide was recoverable. The actual peroxide consumption was 5.125 g./l00 gms. pulp.

The bleached pulp was washed with water, treated with dilute SO H O, and rewashed. This pulp is then formed into handsheets. The GE. brightness was 84.22, a gain of about 18 points.

This example illustrates that relatively high brightness levels can be obtained at peroxide concentrations of the order of 0.5 g./100 cc. As to be expected, the peroxide consumption is also rather high. This drastically limits the utility of such a process for commercial bleaching purposes at existing commercial peroxide costs.

EXAMPLE 2 To 20 grams of dry (5 percent moisture) neutral sulfite semi-chemical high yield poplar pulp prepared similar to that of Example 1, with an initial brightness of 66.88, was added 200 cc. of the peroxide silicate-caustic solution shown in Example 1. No additional water was added. The consistency was 9.1, and the concentration of actual peroxide in the peroxide solution in contact with the pulp was 0.6 g./l00 cc., or a quantity of about 6.0 grams of peroxide per 100 grams dry pulp. After heating at 90 C. for 2 hours, filtering off the excess peroxide, and washing the pulp first with H O, then with SO I-I O solution, the pulp obtained had a GE. brightness of 87.24.

This example illustrates that on further increasing the concentration at which the peroxide is applied to the pulp still higher brightness levels may be obtained.

EXAMPLE 3 This example illustrates the effect of bleaching time upon the peroxide consumption and brightness levels. Further, the results illustrate that there are essentially two bleaching reactions involved; namely, a relatively fast and a relatively slow bleaching reaction. Additionally, the following data indicate that where brightness above was obtained, the peroxide residual was low, too low to overcome some color reversion due to the action of the alkali on the pulp. While the desired brightness of 80+ was obtained, a somewhat greater residual peroxide would have permitted higher brightnesses as in Example 2.

To 20 grams dry (5 percent moisture) unbleached poplar high yield NSSC pulp of an initial unbleached brightness of 62.46 prepared substantially according to Example 1 was added 200 cc. of the bleach formulation of Example 1.

The peroxide solution concentration was 0.6 g./l00 cc., the peroxide level 6 g./100 g. of pulp (dry basis), and the consistency was 9.1 percent. The pulp dispersion was then heated for various periods of time, at a temperature of C., and the peroxide consumption compared with the brightness level obtained. The results are shown in Table 1.

As illustrated by the data in Table 1, there is a brightness reversion at approximately 120 minutes, at which point the data shows there remains 0.68 g. of peroxide per grams of pulp, or about 10 percent peroxide, that is, 10 percent of the initially applied peroxide.

FIG. 2 also indicates that there are two reactions proceeding at different rates in a bleaching operation in which the residual peroxide decreases. The curve of FIG. 2 is predicated on bleaching a poplar pulp prepared in accordance with Example and bleached at a consistency of 3 percent at a temperature of about 88-90 C. for varying periods of time. The first reaction, though occurring at a more rapid rate and largely within the first 15 minutes, raises the brightness materially but not to the desired 80+ range. The second and slower reaction requires, even at 90 C., a driving force such as time and significant residual peroxide concentration.

EXAMPLE 4 The necessity of a high initial peroxide concentration in order to obtain brightness levels of the order of 78 to 80 in a relatively short period of time, as well as the necessity for the need of a high initial peroxide concentration if brightness values above the range are to be obtained upon bleaching for extended periods of time is indicated by FIG. 1. This FIG. 1 is based upon the data obtained on treating 20.0 g. samples of air dried unbleached NSSC poplar pulp, prepared according to Example 1, with water plus various amounts of peroxide solution, prepared essentially according to that of Example 1, such that the indicated peroxide concentrations were obtained at a consistency of 7.4 percent. The respective solutions were then heated to 90 C. and the required amount of peroxide solution to attain the desired concentration added. Heating was continued for the indicated periods of time, the mixture quickly cooled, and the residual bleach solution removed from the pulp by filtering. The pulp was then washed with water and brightness handsheets made. The curve in FIG. 1 designated A is based on a concentration in solution of only 0.12 g./l cc. (1.5 g./l00 g. pulp), and it will be noted that an 80+ brightness was not obtained even with an extensive time of heating at 90 C. Increasing the concentration to 0.24 g./100 cc. (3.] g./l00 g. pulp) raised the brightness somewhat for a given bleaching reaction time. Curve C" 0.496 g./100 cc. (6.2 g./l00 g. pulp) indicates that brightnesses above 80+ are attainable as the peroxide solution concentration is increased generally, however, at the expense of higher peroxide consumptions (see also Table 1). The minimum suitable solution concentration appears to be about 0.4 percent by weight of the solution if brightness levels above 80 are to be obtained; that is, l have found that brightnesses above 80 are attained only if the peroxide solution concentration is greater than about 0.4 g. peroxide/100 cc. solution.

EXAMPLE The effect of the necessity of a high initial peroxide concentration is also shown by the following experiments. In this instance a comparison has been made between bleaching in one step at high concentration of peroxide with bleaching in four steps employing the same quantity of peroxide but with onefourth of the quantity applied in each step.

A bleach solution was made up in accordance with Example 1. Two 200 g. samples of the unbleached NSSC poplar were suspended in 250-300 cc. H O for about one-half hour to stabilizc the pulp for the comparative bleaching tests. The pulp was well mixed, then filtered, and pulled as dry as possible on a filter. The resulting pulp mats were placed in separate beakers and treated as follows:

EXAMPLE 5A 200 cc. of the bleach solution was added and the pulp heated for 2 hours at 90 C. The mixture was cooled, filtered and washed with water. Brightness samples were then made.

EXAMPLE 5B 150 cc. of H 0 and 50 cc. bleach solution were added to the second g. pulp sample and the suspension was heated at 90 C. for minutes. The suspension was filtered and the pulp washed with I1 0. The pulp is now suspended in 150 cc. 11 0 and the suspension heated to 90 C. 50 cc. bleach solution was now added and the suspension heated for 30 minutes at 90 C. The pulp suspension was filtered and the pulp washed with H O. This bleaching operation was now repeated two more times. Brightness samples were then made.

The results obtained are shown in Table 2.

TABLE 2 EXAMPLE 6 A pulp prepared similar to that of Example 1 and with an unbleached brightness of 66.88 was first bleached to a brightness of 78.12 using a peroxide concentration of 0.15 g./l00 cc. (Temperature 88 C., time 2 hours.)

Attempts were then made to further bleach this pulp using a concentration of 0. l 23 g.H O 100 cc. and a concentration of 1.018 g.H O 100 cc. In each case a large excess of peroxide was used. The results are shown in Table 3. This example illustrates further the necessity of a high peroxide concentration (g./ 100 cc.) rather than merely that of a large quantity of peroxide (g./ l 00 g. pulp).

Table 3 Applied Solution Concentration G.H,O,/l00 pulp G.H,O,/l00 cc. Brightness EXAMPLE 7 This example illustrates the effect of alkalinity upon the peroxide consumption and brightness level.

To 87.0 grams of wet (about 20 g. dry) unbleached poplar neutral sulfite semi-chemical high yield pulp, prepared according to the conditions set forth in Example 1, was added 100 cc. of the indicated amounts of the various bleach formulations shown in Table 4.

TABLE 4 Gms. Grams, Grams, sodium cc. H1O: Total MgSOt NaOH silicate (30%) alkalinity Formula M 6. 0 20. 0 66. 5 40. 0 24. 24 N 0. 5 10. 0 50 40. 0 15. 96 O 0. 5 0. 0 50 40. 0 7. 2 P 0. 5 0. 0 100 30. 0 11.52

In each instance water was added to the tabled compositions to give a total volume of 1 liter. Total alkalinity was obtained by titrating the bleach solutions and is given as grams of sodi- All peroxide added consumed.

The pH recorded above and in other examples is measured on the pulp suspension, that is, the pulp with the bleach solution added. The foregoing data indicate that the initial! pH should be below 11.5 and the total alkalinity well below 24 in order to obtain brightnesses in excess of EXAMPLE 8 Using 87.0 gram samples of wet (approximately 20 g. dry) unbleached poplar neutral sulfite semi-chemical high yield pulp prepared according to the conditions as set forth in Example l, and treated similar to the bleaching conditions shown in Example 7 but at a temperature of 50 C., the results as shown in Table 6 were obtained.

TABLE 6 70 Exp. Total applied g. 11,0 1m x 1 i 1 Peroxide B i b W No. Bleaching Sequence per 100 g. of pulp Brightness F i gg r alkai l glty 5A Bleached in p 33-04 11. 7 11. 6 5. 529 77. 96 24. 24 SB Bleached in four steps 6.0 79.42 0- 0 9- 5 1- 79- 0 7- 2 l0. 1 9. 9 0. 8165 78. 73 11. 52

These results indicate that 50 C. at 2 hours is too low a temperature for the desired bleaching effect unless the pulp is pretreated as in Example 10. The example further illustrates that even at temperatures of 50 C. the alkalinity, when high, materially contributes to peroxide consumption without aiding brightness increase. Also, the stabilizing effect of silicate itself on the peroxide is indicated.

EXAMPLE 9 The bleaching conditions used in this example were similar to those used in Example 7, except that prior to the bleaching treatment the pulp was pre-extracted with a dilute acid (HCl) solution. The following data clearly indicate that the acid treatment is effective at a bleaching temperature of 90 C. in attaining improved brightnesses at lower peroxide consumptions provided the pH and alkalinity are maintained within a fairly limited range.

The bleaching conditions used in this example were similar to those in Examples 7 and 8, except that the pulp was pre-extracted with BaCl followed by washing with water. Preextraction with a heavy metal salt such as BaCl appears efiective to permit the attainment of high brightness at relatively lower temperatures of bleaching at quite reasonable peroxide consumptions.

a loss of about 3.86 percent. Therefore, less than about 0.5 percent of the wood is lost due to the actual bleaching reaction.

Further repeated tests show that, while there is some variation in the ultimate brightness levels obtained due to the inherent nature of the woods, the trends illustrated by the various examples nevertheless occur. in the instances where the brightness level may not be quite as high as desired, increasing the concentration at which the peroxide is applied to the pulp invariably tends to increase the brightness level.

Recovery of peroxide not consumed in the reaction may be practiced if desired. Such peroxide is re-usable. i prefer to recover the peroxide by subjecting the pulp, after bleaching. to a filtration and/or squeezing operation so that as much excess liquid as possible is obtained. Such hydrogen peroxidecontaining liquid may be concentrated and/or fortified by the addition of fresh peroxide.

it will be understood that this invention is susceptible to modification in order to adapt to different usages and conditions and, accordingly, it is desired to comprehend such modifications within the invention as may fall within the scope of the appended claims.

1. The process of bleaching defibered hardwood pulp which comprises subjecting the pulp at a consistency of between about 7 to about 20 percent to an aqueous stabilized peroxide bleaching solution wherein the solution in actual contact with the pulp, including any water in the pulp, consists essentially by weight of between about 0.4 and 1.2 grams of hydrogen peroxi e per 100 cc. of bleach solution at a pulp consistency such that initially at least 3 percent by weight of peroxide based on the air dry pulp weight is present and the solution has between about 2 to 4.2 parts by weight of sodium silicate per part of peroxide (expressed as hydrogen peroxide), the said solution having an initial pH of between about 9.0 and 1 L3 and an alkalinity expressed in parts by weight of sodium hydroxide per liter of bleach solution of between about 7 to TABLE 8 'Iempera- Peroxide Time, ture, Initial Final consump- Bright- Total hours 0. pH pH tion ness alkalinity Formula:

EXAMPLE l 1 i5, bleaching the wood pulp in a single bleaching step at said To 84.0 g. of unbleached high yield maple pulp (approximately 20 g. dry) prepared essentially according to the pulping conditions set forth in Example 1 and having an unbleached brightness of 57.42 was added 100 cc. of the silicate bleach formulation as shown in Example 7 (Formulation P). The consistency was 10.8 percent and the g.H O /lOO pulp was 4.81 percent. Upon bleaching at C. for 4 hours, the brightness obtained was 79.90. The peroxide consumption was 2.6855 g./l00 g. Upon pre-extraction of the pulp with dilute HCl acid, a brightness of 77.30 was obtained at a peroxide consumption of 1.0895 g./l00 g.; and upon pre-extraction with BaCl a brightness of 80.64 was obtained at a peroxide consumption of 2.043 grams. Again, with maple a pre-extraction with BaCl appears most effective if low bleaching temperature is employed.

Repeated experiments have indicated that the total loss in weight of the pulp under the most drastic bleaching conditions set forth hereinbefore is less than about 5 percent. in general, the average is about 4.3 percent. Further experiments have indicated that the loss in weight is due substantially solely to an alkali extraction and not to the bleaching reaction itself. Repeated tests show that where a loss in weight of about 4.3 percent on the average occurred upon bleaching, the same pulp weight, treated with the same solution, minus the peroxide and under the same conditions of time and temperature, exhibited consistency to attain a bleached brightness as measured by the General Electric Reflectance method in excess of by carrying out the bleaching step for at least about 30 minutes at a temperature of between about 50 C. and C., and not longer than about 120 minutes at a temperature of about 90 C., and removing the so-bleached pulp from the bleaching solution while the solution still contains about 10 percent of the initially applied peroxide.

2. A process according to claim 1 and wherein the pulp subjected to the bleaching solution is a hardwood pulp defibered by chemical cooking and mechanical refining of hardwood chips to a yield of at least about 80 percent of the wood of the chips.

3. A process according to claim 1 in which the bleaching temperature is between about 60 C. and 70 C.

4. A process according to claim 1 in which the unbleached pulp is pre-extracted with a dilute acid and then washed with water to eliminate acid, the said pulp being then subjected to the bleaching solution at a temperature of about 90 C. and at an alkalinity of between about 7.2 and 11.52.

5. A process according to claim 1 wherein the ratio of X is at least 3.5.

Claims

2. A process according to claim 1 and wherein the pulp subjected to the bleaching solution is a hardwood pulp defibered by chemical cooking and mechanical refining of hardwood chips to a yield of at least about 80 percent of the wood of the chips.
3. A process according to claim 1 in which the bleaching temperature is between about 60* C. and 70* C.
4. A process according to claim 1 in which the unbleached pulp is pre-extracted with a dilute acid and then washed with water to eliminate acid, the said pulp being then subjected to the bleaching solution at a temperature of about 90* C. and at an alkalinity of between about 7.2 and 11.52.
5. A process according to claim 1 wherein the ratio of