US3069309A - Pulp bleaching - Google Patents

Description

Dec. 181,. 1962i F. LE ROY FENNl-:LL 3,069,309

PULP BLEAQHING Filed Een. 1, 1951, s' sheets-sheet s 54 a LLI 52 PERoxmE 11mm amen-2% 110011 111 c111Ps 5.o 511101115 10.9% 112o29.9%s1o21 4 l l l l l PULP 111011 1155111511 1 1 1 H2504 USED I 1 TpoiAL 12.1 1.2.1 12.0 11.0 11.1 11,0 11.4 11b 11.0 10.0 101.0 10.5 9.9 9.6 "00H20 2.4 2.2 2.0 1.a 1.6 1.4 1.2 1.0 0.0 0.0 0.4 ozmvoglTOR FRANCIS L. FENNELL ATTORNEY 3,069,309 Patented Dec. 18, 1962 tice 3,069,309 PULP BLEACHING Francis Le Roy Fennell, Wilmington, Del., assignor t E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Feb. 1, 1961, Ser. No. 86,553 1 Claim. (Cl. 162-26) This invention relates to pulp bleaching and more particularly it relates to the bleaching of cold soda pulp chips during the refining of the chips.

Mechanical wood pulp has for years been produced from the softwoods and semi-hardwoods, such as aspen, by grinding. The softwoods have become largely depleted but substantial supplies of hardwoods are available for wood pulp production. Hardwoods cannot, however, be used to produce mechanical pulp by the conventional process of grinding on stones. Many attempts have recently been made to first produce hardwood chips and then refine these chips in a so-called refiner which will disintegrate the 1t-inch square chips into a pulp mass composed of small fibers suitable for paper manufacture. In most cases the hardwoods must be chemically treated to soften the chips so they may be adequately refined in one to three refining stages. Probably the most satisfactory chemical treatment comprises a treatment with a strong alkaline agent such as NaOH. Such treatment of hardwood chips, however, produces a darker pulp than that obtained `from mechanically ground softwoods.

The softened hardwood chips (softened by soaking in NaOH solution) containing residual NaOH are customarily fed at a controlled rate to a rener, or a series of refiners, for disintegration into pulp. The pulp from the renner varies in brightness from 40 to 50 G.E. units as measured by a General Electric brightness meter. The brightness will to some extent depend upon the wood species used, the concentration of NaOH used in the softening treatment and the chipstorage time prior to refining. Due to the brightness limitations of this process, the resulting pulp requires bleaching for subsequent use in most grades of paper and paperboard manufacture.

Such pulp can, of course, be bleached in conventional bleaching stages involving large bleaching towers, pulp washers, pulp mixing equipment, pulp pumps, piping, etc. Such pulp bleaching processes involve a large capital investment in equipment and require many hours of bleaching to attain the desired bleached brightness.

Attempts have been made to bleach with H2O2 the hardwood chips in the refining operation, i.e., during the process of disintegrating the chips into a pulp mass. Such attempts have heretofore been oftentimes unsatisfactory because the total alkalinity of the chips entering the refiner cannot be readily controlled. The alkalinity of the chip varies as a result of the following variables:

(l) The hardwood chips require different concentrations of NaOH to develop suitable pulp strength.

(2) The caustic demand will vary with the species of wood used and with changes in wood chip mixtures.

(3) The temperature, time and chemical concentration varies as a result of variations in hardwood chips and thereby affects the residual caustic control of the wood chips entering the refiner.

Variation in total alkalinity of the chips not only produces nonuniform bleaching results during the refining operation but when unduly alkaline, there will be a great waste of H2O2 by decomposition without bleaching of the pulp.

It is an object of this invention to bleach in a satisfactory uniform manner hardwood chips with H2O2 during the rening of the chips.

It is another object of this invention to bleach hardwood chips with H2O2 during the refining thereof with a uniform bleaching effect and without substantial waste Of H202.

It is another object of this invention to provide a new and improved bleaching process for the lbleaching of cold soda pulp within a time period of 10 to 30 minutes.

Other objects of this invention will appear hereinafter.

The objects of this invention may be accomplished, in general, by finely disintegrating cold soda hardwood pulp chips in the presence of a sufiicient quantity of a sodium silicate-containing hydrogen peroxide bleaching solution and an adjusted quantity of sulfuric acid to produce a pulp mass having a consistency of 3 to 20% and a total alkalinity as it passes from the disintegrating step of 0.5% to 1.6% NaOH.

Hardwood pulp chips are composed of poplar, oak, elm, gum, beech and other hardwoods. The chips are produced by gouging out wood flakes usually about 1/2 inch square and having a thickness varying from 1/32 to 1/s inch. Before these chips or flakes can be further comminuted or disintegrated to form pulp they must be chemically treated to soften and somewhat loosen the bonding material of the wood, holding the fibers together. The chips are most satisfactorily treated with caustic soda and the pulp made therefrom is referred to as cold soda pulp. Regardless of how carefully these chips are treated they will contain varying proportions of NaOH. If the amount of NaOH added during the treatment is insufficient, then the chips will not be readily disintegrated and the power input for refining operation will be unduly high, and pulp quality will suffer. It is, therefore, customary to add an excess of caustic soda to the chips before they are disintegrated in a pulp refiner. The excess amount of NaOH added is objectionably wasteful of hydrogen peroxide of a hydrogen peroxide bleach solution without obtaining the desired degree of bleaching brightness.

It has been found, in accordance with this invention, that the objectionable effect of excess caustic may be avoided and the pulp bleached without waste of H2O2 by adding a controlled amount of H2804 to the H2O2 bleaching solution with which the pulp chips are treated during the refining operation. Moreover, the refined pulp can be thereby bleached with a holding period of between 10 and 30` minutes.

The hydrogen peroxide bleaching solution for use in accordance with this invention is one that contains a sodium silicate buffering agent with or without a stabilizing agent, for example, Epsom salts. The buffering agent used in the bleaching solution of this invention is sodium silicate, however, other known buffering agents such as sodium pyrophosphate, sodium tripolyphosphate, sodium tetraphosphate, sodium hexapolyphosphate, or borax may be used in addition to sodium silicate. Epsom salts may be added, if desired, to inhibit the decomposition of H2O2 by the catalytic effects of metals such as copper, manganese or iron that maybe present. The use of Epsom salts is desirable if the bleach solution is stored for a period of hours before using. On the other hand, if the bleach solution is used as it is mixed, the use of Eps-om salts is unnecessary.

The bleaching solution of this invention may have a greatly varied composition. In the first place, the bleaching solution may be added in such amount that the pulp mass passing from the refiner has a consistency between 3% and 20%. Secondly, the pulp should be treated with such quantity of bleaching solution that between 0.15% and 2% of H2O2, based on the bone dry weight of the pulp, is added. Similarly, the amount of sodium silicate added should be between 3% and 7.5%, the Epsom salts,

Moreover, the bleach solution may be produced in a more concentrated form and water added simultaneously with the addition of the bleaching solution, or alternatively, the bleaching solution may be diluted with the water before passing the same into the reliner. The bleaching solution may contain the sulfuric acid before mixing with the pulp in the rener or the hydrogen peroxide. Alternatively, hydrogen peroxide, Epsom salts, sodium silicate and a portion of the water may be passed into the reiiner and the sulfuric acid and a lfurther portion of water simultaneously passed into the refiner in a separate stream whereby the several constituents of the bleaching solution are mixed in the presence of the pulp mass in the reiiner.

If other buffering agents than sodium silicate are used in addition to sodium silicate, they should be present in an amount so that the total is chemically equivalent to the above-said amount of silicate. Small quantities of other compatible bleaching ingredients may, of course, be present, as desired.

The sulfuric acid constituent should be added to the bleach solution in such amount that the total caustic content of the pulp mass immediately as it emerges from the rener is between 0.5 and 1.6 weight percent regardless of the consistency of the pulp mass which, as stated, may vary between 3% and 20%. This corresponds approximately to a pulp mass pH of between about 10.4 to 11.5. The total caustic content measure is more accurate than pH value since variations in pH value may be present without corresponding variation in total caustic content.

The pulp mass flowing from the rener, therefore, should contain between 0.15% and 2% of H202, between 3% and 7.5% sodium silicate (based on a Na20 content of 8.9%), if used between 0.05% and .5% Epsom salts, and a total caustic content of 0.5 to 1.6%, all based on the bone dry weight of the pulp.

The details of the invention will be more clearly apparent by reference to the following description when taken in connection with the accompanying illustrations, in which:

FGURE 1 is a side elevational, diagrammatic View of an embodiment of apparatus suitable for use in accordance with this invention;

FGURE 2 is a graphic curve showing the peak brightness to be obtained with bleach having a given residual caustic content in the rener pulp mass and a pulp consistency of 5%; Y

FIGURE 3 is a similar graphic curve showing the peak brightness of a similar bleach having a pulp consistency of 12%.

Referring to FIGURE 1, reference numeral 3 designates a pulp reiiner comprising two oppositely disposed and oppositely rotating comminuting pulp defibering discs having a multitude of opposing teeth. The oppositely disposed discs are rotated in the retiner housing by means of shafts 6 and 6a `driven by motors 5 and 5a, mounted on base 4.

In another excellent form of pulp rener one debering disc is stationary and an opposing disc is rotated in the Vrefiner housing, thus requiring only a single motor.

The pulp chips 10 are metered and passed into the mouth of funnel 7v by means of an endless belt 9, as shown. The bleaching solution and H2804, with sufficient water to provide a pulp consistency of between 3 and 20, is also metered, eg., through valves and 17, and passed into `funnel 7 simultaneously with Vthe passage of the pulp chips. The refined material is passed from the refiner through outlet 30 from which it is passed to a holding receptacle (not shown) at the desired consistency.

The pulp mass is preferably kept in the holding receptacle for a period of 10 to 30 minutes, whereby it is further bleached by the residual H202 contained therein. After completion of the bleaching operation the pulp is usually neutralized with S02 to a pH of between 5 and 7, screened, cleaned and forwarded to subsequent paper- B making operations. If desired, the pulp may be washed prior to neutralization.

The hydrogen peroxide, sodium silicate and Epsom salts of the bleach solution may Vbe mixed with a desired amount of water in a mixing tank 18 and then iiowed through pipe 19 to H202 bleaching solution storage tank 16. From storage tank 16 the H202 bleaching solution is passed through pipeline 12, and H2802 is passed from storage tank 1lithrough pipe line 13, the two passing toether through common pipe line 11 into `funnel 7. The H2804V solution should have such an adjusted strength that the pulp mass passing from the refner will have a total alkalinity of between 0.5 and 1.6% NaOH based on the bone dry weight of the pulp. The concentration of H2204 passed into the reiner will, of course, depend upon the 'caustic content of the chips and the percentage of silicate in the bleach solution. The H2804 may, if desired, `be mixed with the bleaching solution in tank 16 or, alternatively, it may pass directly into funnel 7 and the bleaching solution and, if necessary, additional water passed independently into the funnel and the two mixed in the renner. The H202 and silicate may be mixed in mixing tank 18 and passed directly into funnel 7, in `which case no stabilization agent such as Epsom salts will be required.

FIGURES 2 and 3 clearly show the peak brightness to `be obtained when the total NaOH of the pulp as it emerges from` the reiiner varies between 0.5% and 1.6%. Both illustrations show a peak brightness of over 58 when the bleaching solution contains between these percentages of total NaOl-I content regardless of the fact that the pulp consistency of the pulp mass may vary from 5% to 12%. The peak Ibrightness will be obtained between these percentages of total NaOH content regardless of the hydrogen peroxide or sodium silicate content within the limits above set forth although the total brightness rray ybe somewhat enchanged by higher concentrations of H202. In order to obtain an economically useful result by a peroxide lbleaching operation, the bleached brightness for hardwood pulp for most paper or paperboard operations should be at least 58 G.E. units.

The following examples are given to illustrate certain preferred embodiments of the process of this invention.

In fourteen tabulated examples set forth below, hardwood pulp chips composed of equal parts of birch, beech and maple having an average unbleached brightness of 42 G.E. units as -measured on a General Electric brightness meter are passed into a rener. These chips had an average of 2% NaOH total alkalinity.

At the same time a peroxide bleaching mixture consisting of the following ingredients is passed into the reiiner.

The bleaching soiution was fed at a rate of 240 gals. per ton of moisture-free pulp. This solution feed will deliver to the reiiner 0.05% Epsom salt, 5% sodium silicate and 1.25% H202 ibasis) all based on the weight of moisture-free pulp.

Sulfurc acid, V66" B., was added in such amount that the residual total alkalinity, measured as percent NaOH varied in the examples between 0% and 2.6%.

The fourteen examples were carried out at a pulp consistency of 5% and repeated with a puip consistency of 12%.

The results are given in Tables'l and II below. These results are'graphically represented in FGURES 2 and 3.

In all of the examples the reiiner was operated to maintain in the pulp mass liowing from the renner a temperature of approximately C. which temperature is generated by the disintegrating action of rthe rener. In

some cases, with low density pulp, it may be necessary to maintain the elevated temperature by the addition of steam. The pulp mass owing from the rener was in each case passed into a relatively small-capacity holding receptacle in which the pulp was held for 15 minutes to complete the bleaching operation.

The bleached pulp is then passed from the holding receptacle, diluted to a consistency of about 3%, and neutralized to a pH of 6.0 (Becknan pH meter) with SO2.

The pulp is then diluted with distilled Water to a consistency of about 0.3%, the pH adjusted to between 4 and 4.5 with dilute H2804, made into a handsheet and airdried, all in accordance with TAPPI standard procedure. The brightness of the sheets was then measured on a G.E. brightness or reflectance meter. i

C@ alkali the H2804 will be varied inv accordance with the varying amount of alkali to provide a total residual a1- kalinity in the pulp of between 0.5% and 1.6% NaOH.

This is a continuation-in-part of my copending application Serial No. 861,369, and now abandoned, led December 22, 1959.

Throughout the specification and claims, any reference to parts, proportions and percentages refers to parts, proportions and percentages by weight unless otherwise specified.

Since it is obvious that many changes and modications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claim.

Table 1,-Refz`ner Bleaching With Peroxide-5% Consistency Pulp (Results Shown in FIGURE 2) [Series 1-14 examples at varying alkalinity Chemicals/M .F. Pulp From Renner Pulp pH Pulp Brightness Total Al- Residual Ex. kalinitv Bleach Pulp Peroxide 2 No. Na OH in Epsom Sodium H2O2, T.A 1 Time, Temp., After 15 Oven- Chips to Salts, Silicate, Per- H2504, Percent Min. F. From After Min., Air Dry Renner, Pcr- Percent cent Percent Renner Min. Percent Dry 1 hr Percent cent 105 C 2.0 2.0 180 12. 1 44. 9 43. 2 2. 0 0. 05 5. 0 1. 25 2. 6 l5 180 1l. 8 0. 0 55. 3 53. 8 2. 0 0. 05 5. 0 1. 25 5 2. 4 15 180 11. 8 11. 6 0. 0 56. 0 54. 5 2. 0 0. 05 5. 0 1. 25 0. 50 2. 2 15 180 11. 7 11. 2 0. 0 57. 6 55. 4 2.0 0. 05 5. 0 1. 25 0. 75 2.0 15 180 11.5 11.4 3. 0 56. 8 55. 1 2. 0 0.05 5.0 1. 25 1.00 1. 8 15 180 11.4 11. 1 3. 0 56. 9 55.7 2. 0 0. 05 5. 0 1. 25 1. 25 1. 6 15 180 11. 4 11. 2 10. 0 58. 5 57. 3 2.0 0. 05 5. 0 1. 25 1. 50 1.4 15 180 11.2 11.0 19.0 59.5 58.0 2. 0 0.05 5. 0 1. 25 1. 75 1. 2 15 180 11.2 10. 7 26. 0 60. l 59. 1 2.0 0.05 5.0 1. 2.00 1.0 15 180 11.0 10.5 47. 0 60.4 58.8 2. 0 0. 05 5. 0 1. 25 2. 25 0. 8 15 180 10. 9 10. 1 47. 0 59. 7 58. 7 2.0 0.05 5.0 1. 25 2. 50 0.6 15 180 10.7 10. 1 51.0 59.8 58.2 2. 0 0. O5 5. 0 1. 25 2. 75 0. 4 15 180 10. 5 10. 0 85. 0 55. 8 55.4 2. 0 0.05 5.0 1. 25 3. (l0 0.2 15 180 10. 1 9.8 85.0 54. 3 53. 6

1 'F.A. includes NaOH from chips plus NaOH from silicate minu 2 Percent residual peroxide as percent of total H2O2 applied to refinar.

s NaOH neutralized by H2804.

Table 11.-Rener Bleaching With Peroxide-12% Consistency Pulp (Results Shown in FIGURE 3) [Series 2-14 examples al; varying alkalinity] Chemicals/MF. Pulp From Redner Pulp pH Pulp Brightness Total Al- Residual EX, kalmitv Bleach Pulp Peroxide 2 No. NaOH in Epsom Sodium H2O2, 'I.A., 1 Time, Temp., After 1o Oven- Chips to Salts, Silicate, Per- HaSOl, Percent Min. F. From After Min., Air Dry Renner, Per- Percent cent Percent Redner 15 Min. Percent Dry 1 Percent cent 105 C 2.0 2.0 180 44. 1 43. 6 2. 0 0. 05 5. 0 1. 25 2. 6 15 180 12. 1 0. 0 55. 1 53. 8 2.0 0.05 5.0 1. 25 0.25 2. 4 15 180 12. 1 0.0 55.3 54. 9 2.0 0.05 5. 0 1. 25 0.50 2. 2 15 180 12.0 0. 0 56. 0 55. 3 2. 0 0.05 5.0 1.25 0. 75 2.0 15 180 11.8 0. 0 56. 6 56. 2 2.0 0. 05 5. 0 l. 25 1.00 1. 8 15 180 11.7 0. 1 58.0 57. 3 2.0 0.05 5. 0 1. 25 1. 25 1. 6 15 180 11.6 10.0 59. 8 58. 5 2.0 0.05 5. 0 l. 25 1.50 l. 4 15 180 11.4 16.0 60.1 59.0 2. 0 0. 05 5. 0 1. 25 1. 75 1. 2 15 1S() 11. 3 22.0 59. 1 58. 8 2.0 0. 05 5. 0 1. 25 2. 00 1. 0 15 180 11.0 21.0 61.0 G0. 3 2.0 0.05 5.0 1. 25 2. 25 0.8 15 180 10.8 24.0 61.8 61. 1 2.0 0.05 5.0 1. 25 2. 50 0. 6 l5 180 10.6 32.0 59.6 59. 2 2.0 0.05 5.0 l. 25 2. 75 0.4 15 180 10.3 63. 0 59.9 59. 9 2. 0 0.05 5.0 1. 25 3.00 0.2 15 180 9. 9 89.0 56. 7 56. 3 2. 0 0. 05 5.0 1. 25 3. 25 0. 0 15 180 9. 6 100. 0 53. 2 51. 9

From the tables and FIGURES 2 and 3 it will be seen I claim:

that the H2SO4 of the bleach must be added in such quantity, based upon the NaOH content of the silicate and NaOH content of the chips to produce in the pulp mass passing from the rener a total alkalinity of between 0.5% and 1.6% NaOH in order that the bleached brightness will be over about 58.

The process of chemically and mechanically rening hardwood chips previously softened with NaOH which comprises finely disintegrating said softened hardwood chips containing in excess of 1.6% NaOH based on the dry weight of the chips and simultaneously passing in contact therewith 0.3% to 5% H2504 and a sufficient Where the hardwood chips have a varying content of quantity of aqueous hydrogen peroxide containing be- 53,0693? G9 t7 a o tween "0.15% and 2% H2O2, between A3% and 7.5%

taining said pulp mass in the aforesaid condition for besodium-si1icate (based on 8.9% Nao and 29.9% sioz), tween wand 30 minutes ali based on the dry weight .'of said chips t0 produce a References 'Cited in the 'le of this patent .pulp mass having a Consistency between 3% and 20% 5 UNITED STATES PATENTS and a total alkalinity, as it emerges from the disintegrats 2,514,503 McEwen July 11, 1950 1ngvstep, of between 0.5% and `1.67% NaOH, and mam 2,838,459 Sprout June 10, 1958

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