Title:
MULTI-STEP PULP BLEACHING
Kind Code:
A1


Abstract:
A method for bleaching cellulosic pulp is disclosed, with one embodiment revealing a three-step bleaching method for cellulosic pulp comprising an activating bleach step, an alkaline peroxide step, and a reductive bleach step. The cellulosic pulp treated according to the present invention may possess a greater GE brightness gain than if treated according to conventional methods, while not suffering a significant loss of lignin. The cellulosic pulp may comprise recycled fibers, including those with high groundwood content. The bleaching method may also decrease the pH of the pulp.



Inventors:
Lee, Jeffrey A. (Neenah, WI, US)
Keen, Stephanie A. (Myrtlewood, AL, US)
Thomas, Charles H. (Green Bay, WI, US)
Sumnicht, Daniel W. (Green Bay, WI, US)
Application Number:
11/548584
Publication Date:
04/17/2008
Filing Date:
10/11/2006
Assignee:
Fort James Corporation (Atlanta, GA, US)
Primary Class:
Other Classes:
162/65, 162/70, 162/71, 162/76, 162/78, 162/82, 162/158
International Classes:
D21B1/32; D21C3/22
View Patent Images:
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Primary Examiner:
CALANDRA, ANTHONY J
Attorney, Agent or Firm:
Georgia-Pacific LLC (Atlanta, GA, US)
Claims:
What is claimed is:

1. A method for bleaching recycled fibers, comprising (1) providing recycled fibers with a Kappa number of less than about 40; and (2) contacting the recycled fibers with at least an activating bleach step, an alkaline hydroxide step, and a reductive bleach step, wherein the activating bleach step comprises contacting the recycled fibers with at least one activating bleaching agent chosen from peracetic acid, peroxymonosulfuric acid, and an acylamide.

2. The method according to claim 1, wherein the recycled fibers contain groundwood.

3. The method according to claim 1, wherein the method results in an increase of at least 10 GE brightness points from the initial recycled fibers.

4. The method according to claim 1, wherein the activating bleach step further comprises contacting the recycled fibers with oxygen.

5. The method according to claim 4, wherein contacting the recycled fibers with oxygen comprises contacting the recycled fibers with oxygen peroxide.

6. The method according to claim 3, wherein the acylamide comprises N,N,N′,N′-tetraacetylethylenediamine.

7. The method according to claim 6, wherein the acylamide further comprises a peroxy compound.

8. The method according to claim 7, wherein the peroxy compound is hydrogen peroxide.

9. The method according to claim 1, wherein the activating bleaching agent is present in an amount of about 0.1 to 0.2 percent by weight of fibers.

10. The method according to claim 1, wherein the alkaline hydroxide step comprises contacting the recycled fibers with at least one alkaline peroxide agent chosen from a mixture of an alkaline hydroxide and a peroxy compound.

11. The method according to claim 10, wherein the alkaline hydroxide is sodium hydroxide.

12. The method according to claim 10, wherein the peroxy compound is chosen from at least one of hydrogen peroxide and oxygen peroxide.

13. The method according to claim 10, wherein the alkaline hydroxide step further comprises contacting the recycled fibers with at least one of peracetic acid, peroxymonosulfuric acid, and an acylamide.

14. The method according to claim 13, wherein the acylamide comprises N,N,N′,N′-tetraacetylethylenediamine.

15. The method according to claim 1, wherein the reductive bleach step comprises contacting the recycled fibers with a reductive bleaching agent chosen from at least one of formamidine sulfinic acid, hydroxy methyl sulfinic acid, sodium borohydride, and sodium hydrosulfite.

16. The method according to claim 1, wherein the activating bleach step occurs before the alkaline hydroxide step.

17. The method according to claim 16, wherein the recycled fibers are carried from the activating bleach step to the alkaline hydroxide step without removal of the activating bleaching agent.

18. The method according to claim 1, wherein the activating bleach step occurs after the alkaline hydroxide step.

19. The method according to claim 1, wherein the recycled fibers are washed before the reductive bleaching step.

20. The method according to claim 19, wherein the activating bleach step is followed by the alkaline hydroxide step, a wash of the recycled fibers, and then the reductive bleach step.

21. The method according to claim 20, wherein the activating bleach step comprises contacting the recycled fibers with peracetic acid.

22. The method according to claim 20, wherein the alkaline hydroxide step comprises contacting the recycled fibers with a mixture of sodium hydroxide and hydrogen peroxide.

23. The method according to claim 20, wherein the reductive bleach step comprises contacting the recycled fibers with sodium hydrosulfite.

24. The method according to claim 1, wherein the activating bleach step and the alkaline hydroxide step occur simultaneously.

25. The method according to claim 1, wherein the time, temperature, and concentration of the activating bleach step and the alkaline hydroxide step are controlled such that the change in Kappa number of the recycled fibers is no more than about 10 points but the increase in GE brightness of the recycled fibers is at least about eight points.

26. The method according to claim 1, wherein the recycled fibers experience a decrease in Kappa number of no more than about 10.

27. The method of claim 1, wherein the recycled fibers contact the activating bleaching agent at a temperature of at least 170° F.

28. The method of claim 28, wherein the method further comprises a filtrate step.

29. The method of claim 26, wherein the filtrate step comprises contacting the recycled fibers with a mixture of filtrate from the alkaline hydroxide step and an acylamide.

30. The method of claim 29, wherein the acylamide comprises N,N,N′,N′-tetraacetylethylenediamine.

31. A method for bleaching recycled fibers, comprising (1) providing recycled fibers with a Kappa number of less than about 40, and (2) contacting the recycled fibers with peracetic acid, an alkaline oxygen peroxide, and a reductive bleaching agent in one or more steps.

32. The method of claim 31, wherein the peracetic acid is produced by the reaction of N,N,N′,N′-tetraacetylethylenediamine, hydrogen peroxide, and a caustic, optionally including a chelant.

33. The method of claim 32, wherein the caustic is sodium hydroxide.

34. The method of claim 32, wherein the chelant is dimyristoleoyl phosphonomethyl trimethyl ammonium.

35. The method of claim 31, wherein the alkaline oxygen peroxide comprises at least one of hydrogen peroxide and sodium peroxide.

36. The method of claim 31, wherein the contacting occurs at a temperature of 170° F. or less.

37. The method of claim 33, wherein said contacting occurs at a temperature of 140° F. or less.

38. The method of claim 31, wherein the reductive bleaching agent comprises at least one of sodium hydrosulfite and hydroxymethane sulfinic acid.

39. The method of claim 31, wherein the recycled fibers are washed before contacting the reductive bleaching agent.

40. The method of claim 31, wherein the alkaline oxygen peroxide contacts the recycled fibers after the peracetic acid.

41. The method of claim 40, wherein the reductive bleach agent contacts the recycled fibers after the alkaline oxygen peroxide.

42. A method for bleaching cellulosic pulp, comprising: providing cellulosic pulp with a Kappa number of less than about 40; contacting the cellulosic pulp with a first step chosen from one of an activating bleach step and an alkaline hydroxide step; contacting the cellulosic pulp with a second step, different from the first step, chosen from one of an activating bleach step and an alkaline hydroxide; collecting the filtrate from the second step; and, contacting the cellulosic pulp with a reductive bleach step after the second step, wherein the first step including contacting the cellulosic pulp with the filtrate from the second step, and wherein the activating bleach step comprises contacting the cellulosic pulp with at least one activating bleaching agent chosen from peracetic acid, peroxymonosulfuric acid, and an acylamide.

43. The method of claim 42, wherein the cellulosic pulp is recycled pulp.

44. The method of claim 42, wherein the at least one activating bleaching agent is peracetic acid.

45. The method of claim 44, wherein the peracetic acid is produced by the combination of N,N,N′,N′-tetraacetylethylenediamine, hydrogen peroxide, a caustic, and optionally a chelant.

46. The method of claim 45, wherein the caustic is hydrogen peroxide.

47. The method of claim 45, wherein the chelant is dimyristoleoyl phosphonomethyl trimethyl ammonium.

48. The method of claim 45, wherein the filtrate from the second step comprises N,N,N′,N′-tetraacetylethylenediamine.

49. The method of claim 42, wherein the alkaline hydroxide step comprises contacting the cellulosic pulp with at least one alkaline peroxide agent.

50. The method of claim 49, wherein the at least one alkaline peroxide agent is sodium hydroxide.

51. The method of claim 49, wherein the alkaline hydroxide step further comprises contacting the cellulosic pulp with at least one peroxy compound.

52. The method of claim 51, wherein the at least one peroxy compound is chosen from hydrogen peroxide and oxygen peroxide.

53. The method of claim 42, wherein the reductive bleach step comprises contacting the cellulosic pulp with at least one reductive bleaching agent.

54. The method of claim 53, wherein the at least one reductive bleaching agent is chosen from sodium hydrosulfite and hydroxymethane sulfinic acid.

55. A method for reducing the pH of a cellulosic pulp, comprising: providing cellulosic pulp with a pH of greater than about 10; contacting the cellulosic pulp with a first step chosen from one of an activating bleach step and an alkaline hydroxide step; contacting the cellulosic pulp with a second step, different from the first step, chosen from one of an activating bleach step and an alkaline hydroxide; and, contacting the cellulosic pulp with a reductive bleach step after the second step, wherein the pH of the cellulosic pulp after the reductive bleach step is less than about 8, and wherein the activating bleach step comprises contacting the cellulosic pulp with at least one activating bleaching agent chosen from peracetic acid, peroxymonosulfuric acid, and an acylamide.

56. A process for providing pulp to a paper-making machine, comprising: subjecting a cellulosic pulp to an alkaline peroxide step, comprising contacting cellulosic pulp at a pH greater than about 8 with at least one alkaline peroxide agent; thereafter subjecting the cellulosic pulp to an activating bleach step comprising contacting the cellulosic pulp with peracetic acid, wherein the activating bleach step reduces the pH of the cellulosic pulp; thereafter subjecting the cellulosic pulp to a reductive bleach step comprising contacting the cellulosic pulp with sodium hydrosulfite, wherein the reductive bleach step reduces the pH of the cellulosic pulp to about 6.5 to about 8; and, thereafter passing the cellulosic pulp to a paper-making machine.

57. The process of claim 56, wherein the pH of the cellulosic pulp monotonically declines from at least about 10.5 to from about 6.5 to about 8.

58. A process for providing cellulosic pulp to a paper-making machine, comprising: (i) subjecting a cellulosic pulp to an alkaline peroxide step at a pH greater than about 8; (ii) thereafter subjecting the cellulosic pulp to an activating bleach step, wherein the activating bleach step reduces the pH of the cellulosic pulp; (iii) thereafter subjecting the cellulosic pulp to a reductive bleach step, wherein the reductive bleach step reduces the pH of the cellulosic pulp to about 6.5 to about 8; and, (iv) thereafter passing the cellulosic pulp to a paper-making machine.

59. The process of claim 58, wherein the pH of the cellulosic pulp before the alkaline peroxide step is at least about 9.5.

60. The process of claim 58, wherein the activating bleach step reduces the pH of the cellulosic pulp to about 9.5.

61. The process of claim 58, wherein the activating bleach step reduces the pH of the cellulosic pulp to about 9 to about 10.5.

62. The process of claim 58, wherein the pH of the cellulosic pulp before alkaline peroxide step is at least about 10.5 and the pH of the cellulosic pulp monotonically declines to from about 6.5 to about 8 before passing to the paper-making machine.

63. The process of claim 58, wherein the alkaline peroxide step comprises contacting the cellulosic pulp with at least one alkaline peroxide agent.

64. The process of claim 63, wherein the activating bleaching step comprises contacting the cellulosic pulp with peracetic acid.

65. The process of claim 64, wherein the reductive bleaching step comprises contacting the cellulosic pulp with sodium hydrosulfite.

66. The process of claim 58, wherein the pH of the cellulosic pulp before the alkaline peroxide step is about 10.5, the pH of the cellulosic pulp before the activating bleach step is about 9.5, and the pH of the cellulosic pulp before the reductive bleach step is about 8.5.

67. A process for bleaching cellulosic pulp, comprising subjecting a cellulosic pulp with a GE brightness of less than about 67 and a pH of greater than about 9 to an activating bleach step and at least one step chosen from an alkaline peroxide step and a reductive bleach step, whereafter the cellulosic pulp has a GE brightness of at least about 74 and a pH of less than about 7.5.

68. The process of claim 67, wherein the steps are an activating bleach step and an alkaline peroxide step.

69. The process of claim 68, wherein the activating bleach step comprises contacting the cellulosic pulp with peracetic acid and the alkaline peroxide step comprises contacting the cellulosic pulp with at least one alkaline peroxide agent.

70. The process of claim 67, wherein the pH of the cellulosic pulp monotonically declines while the cellulosic pulp is subjected to the activating bleach step and the at least one step.

71. The process of claim 67, wherein the cellulosic pulp comprises recycled fibers.

Description:

BACKGROUND OF THE INVENTION

Methods for bleaching cellulosic fibers, including secondary or recycled fibers, are disclosed. In one embodiment, a method for bleaching recycled pulp fibers increases the brightness of the fibers without significant delignification. In a further embodiment, a method for bleaching cellulosic fibers involves at least two steps. In another embodiment, a method for bleaching cellulosic fibers reduces the pH of the pulp fibers by at least about 2 while also increasing the GE brightness by at least about 9.

Considerable resources have been invested in determining effective ways to recycle the increasingly vast amount of waste paper generated each day. Presently, almost 100 percent of the highest quality grades of paper, like printer's clippings and white office waste, is recycled. Those grades are considered premium secondary or recycled fiber sources as the original, high quality of the chemically produced virgin paper fibers makes it relatively inexpensive to recycle those fiber sources into a high brightness pulp. The secondary or recycled fibers from these sources also command premium prices.

Recycling of other waste papers, like those from curbside residential recycling, is more costly and difficult. Because those papers tend to include substantial groundwood content, the current technology used to recycle premium fibers is ill-suited for these lower grades of waste paper. More specifically, the lower grade pulp fibers used in the original production of those waste paper products suffer from a decreased brightness over the premium virgin or premium recycled fibers. Thus, the recycled fibers from those waste paper products cannot easily be used to create premium or near-premium quality consumer products like bath tissue, facial tissue, paper towels, and napkins, since consumers tend to prefer higher brightness fibers in these products. Such products are considered premium or near-premium due to, for example, their high brightness and/or low ink concentration. Even though current technologies make it possible to recycle some of those waste papers, the high chemical cost makes their use unattractive. In fact, in certain instances, the lower purchase cost of those waste papers is offset by the higher cost of chemical treatments needed to produce acceptable brightness levels.

The consumer tissue product industry chooses between paying premium prices for premium recycled fibers with sufficient brightness, if a sufficient quantity even exists, and paying lower prices for lower quality fibers possessing a brightness beneath consumer demand. As described above, in some instances high processing costs can offset any gains in the lower cost of lower quality fibers. Previous processes to increase the brightness of the lower quality fibers to premium brightness levels are generally extremely, if not prohibitively, expensive.

The use of peracetic acid has been suggested as a component of elemental chlorine-free bleaching and/or delignification sequences for chemical pulps. U.S. Pat. No. 3,720,577 to Roymoulik discusses a two-stage process that may involve a chlorine dioxide bleaching step followed by washing and a peracetic acid bleaching step. A three-stage process is also discussed using chlorine dioxide followed by peracetic acid followed by an additional chlorine dioxide bleaching step. Pulp washing is practiced after each bleaching step. In U.S. Pat. No. 3,695,995, Roymoulik further discusses a two- or three-stage elemental chlorine free process for bleaching chemical pulps that utilizes an oxygen stage as the first bleaching step, which must have a “protector” compound added to prevent degradation of the cellulose fibers. The protector is a polysulfide having the formula Na2Sx, where x is an integer from 1 to 4, and Na2S2O4.

A three-stage oxidative bleaching stage for bleaching chemical lignocellulosic pulps is discussed in U.S. Pat. No. 4,372,812 to Phillips et al. That process uses an oxygen bleaching stage followed by a peroxide bleaching stage followed by at least one ozone bleaching stage. The peroxide bleaching stage may use alkaline hydrogen peroxide, acid hydrogen peroxide, or a peracid bleaching agent. The patent discusses that each bleaching stage should be followed by a washing stage to remove residual chemicals and bleaching byproducts, and also discusses the wash filtrates may be utilized in a countercurrent flow where the filtrate from the following stage is utilized as the wash water in the preceding stage.

U.S. Pat. No. 4,400,237 to Kruger et al. discusses a process for bleaching cellulose using a two-step process, wherein organic peracids are applied to the pulp at an acid pH followed by a hydrogen peroxide step at alkaline pH.

U.S. Pat. No. 4,548,674 to Hageman discusses the problem of removing polymeric tape contaminants from wastepaper during a recycling process. Peracetic acid is applied to contaminated wastepaper at an acid pH (between 2.5 to 6.5) and aids in the breakdown of the adhesive polymers. No brightness gain is achieved through the peracetic acid treatment at the disclosed operating conditions.

U.S. Pat. No. 5,387,317 to Parthasarathy et al. discusses a method to delignify chemical pulp “brown stock” using a high temperature, high pressure process combining peracetic acid, ozone, and oxygen under acetic conditions.

U.S. Pat. No. 5,589,032 to Chang et al. discusses a process to increase the generation efficiency of peracids from hydrogen peroxide. That method adds peroxymonosulfuric acid (Caro's acid) to a reaction vessel containing concentrated hydrogen peroxide and acetic acid. The addition of Caro's acid supposedly increases the generation of peracids. U.S. Pat. No. 5,693,185 also to Chang et al. discusses the use of a mixed peracid solution to brighten lignocellulosic and cellulosic pulps, for example, delignified wood pulps and cotton or cotton by-products.

U.S. Pat. No. 5,645,686 to Troughton et al. discusses a three- to six-step bleaching process for chemical pulps, in which at least one of the steps involves contacting the pulp with an enzyme. Troughton et al. does not discuss the use of a reducing agent.

U.S. Pat. No. 5,733,412 to Markham et al. discusses a method to decolorize unbleached Kraft “brown” fibers that may be a contaminant in mixed wastepapers. Markham's method requires fine screening mixed wastepaper, followed by a two-step bleaching process. That two-step process must alternate oxidative and reductive bleaching, where if the first step is oxidative then the second step must be reductive, and alternately if the first step is reductive then the second step must be oxidative. The first bleach step is carried out in a dispersion machine and at least one step must be carried out at high temperature (above 100° C.) and high pressure (exceeding one atmosphere). While Markham et al. discloses several oxidative and reductive agents, in its bleaching steps only a first FAS (thioureas dioxide) step followed by a second hydrogen peroxide step is preferred and practiced in the examples.

Several patents discuss using solid TAED as a component of a detergent composition, which are intended to be used to clean textiles and clothing. The issues addressed in these patents is achieving long term stability of the composition and activation of the bleach for final use. For example, U.S. Pat. No. 6,225,276 to Gassenmeier et al. coats the TAED compound with a polymeric acid, which then dissolves when the detergent composition is added to water to allow release of bleaching compounds. In addition, U.S. Pat. No. 4,283,302 to Foret et al., U.S. Pat. No. 4,338,210 to Clements et al., U.S. Pat. No. 4,938,889 to Wilsberg et al., U.S. Pat. No. 6,080,710 to Withenshaw et al., and U.S. Pat. No. 5,478,357 to Madison et al. all discuss the need to produce dry, shelf-stable detergent compositions that generate active bleaching compounds when wetted.

U.S. Pat. No. 6,569,286 to Withenshaw et al. discusses the formation of a solution of peracetic acid by reacting solid (N,N,N′,N′-tetraacetylethylenediamine, TAED) granules in water in the presence of hydrogen peroxide, a chelant, and caustic soda. The solution may be used to bleach a pulp in a single stage under alkaline conditions. Withenshaw et al. also discusses that the use of its pre-reacted TAED solution is superior to use of TAED either directly added to the pulp mixture or pre-reacted with any other type of peroxide solution. The single stage process is also supposedly superior to a process using an alkaline peroxide bleaching stage.

The present invention provides, in part, a method for bleaching cellulosic fibers including lower quality recycled or secondary pulp fibers, and in one embodiment those containing groundwood, to achieve premium brightness levels. A premium brightness level may be at least about 75 GE, although the skilled artisan will recognize that acceptable premium brightness levels depend on the particular application. For example, a premium brightness level for a commercial product (such as tissue and towel products for use in office buildings and restaurants) may be in the range of about 75 to about 82 GE. As another example, a premium brightness level for a consumer product (such as tissue and towel products for use in a home) may be in the range of about 78 to about 90 GE. However, brightness levels lower than at least about 75 GE may be considered premium depending on the particular application.

Bleaching processes, like the present invention, should endeavor to present substantially minimal to no damage to the physical structure of the pulp fibers, such as by shortening their length, increasing the fines content, decreasing the lignin content, in an effort to maintain the quality of the paper produced from the pulp fibers. In one embodiment of the invention, the present invention provides in part a method for bleaching fibers, recycled fibers, or secondary pulp fibers of any quality to achieve premium brightness levels, thus affecting the brightness of the pulp fibers, but without significantly changing other properties of the fibers, such as fiber length, fines content, and lignin content.

In a further embodiment of the invention, the fibers or recycled fibers may contain color bodies from lignin content or from dyes, pigments, inks, and other colorizers, the types and amounts of which may impact the brightness gain achievable by the present invention. In such an embodiment, the use of one or more reductive stages may assist in decolorizing the compounds and fibers.

The amount of lignin present in pulp or fibers directly impacts the weight of the pulp or fibers; because virgin and recycled pulp or fibers are generally sold on a per ton basis, an increase brightness levels without significant delignification may have a dual commercial or economic impact. In addition, removed lignin may appear in a wastewater stream and possess a biological oxygen demand (BOD) that may increase the treatment cost of that effluent. Moreover, delignification generally reduces the product yield, which increases the operating cost for a particular product.

In another embodiment of the invention, pulp fibers are subjected to an increase in brightness without the significant removal of lignin from the fibers (also called “delignification”). In one such embodiment, the fibers experience less than about a 20% decrease in lignin content when subjected to a process according to the present invention. In another such embodiment, the fibers experience less than about a 10% decrease in lignin content. In a further such embodiment, the fibers experience less than about a 5% decrease in lignin content. In yet another such embodiment, the fibers experience no substantial decrease in lignin content.

In yet another embodiment, the method for bleaching of the present invention provides at least a three point increase in GE brightness over currently used methods to increase the brightness of recycled pulp fibers, such as a peroxide/hydrosulfite process that, when applied to starting pulp fibers with an initial brightness of 53 GE and a Kappa number of 43, would yield a brightness of 64 GE.

In yet a further embodiment of the present invention, the brightness gain of the recycled fibers is directly related to the type of pulp used in the disclosed method.

In another embodiment of the present invention, the total chemical cost necessary to achieve a particular brightness increase for a given recycled pulp is less than the total chemical cost necessary to achieve about the same or a similarly acceptable brightness increase for that recycled pulp using prior art or known bleaching methods. The present invention contemplates that lower total chemical cost may be more commercially important than overall brightness increase. As such, the present invention allows the skilled artisan to achieve a target brightness increase or brightness value using a chosen or economically appropriate quantity and kind of chemicals.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic drawing of a system configured for bleaching cellulosic pulp, according to the conventional practice.

FIG. 2 is a schematic drawing of one embodiment of a system configured for bleaching cellulosic pulp, according to the present invention.

FIG. 3 is a schematic drawing of another embodiment of a system configured for bleaching cellulosic pulp, according to the present invention.

FIG. 4 is a schematic drawing of another embodiment of a system configured for bleaching cellulosic pulp, according to the present invention.

DESCRIPTION OF THE INVENTION

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the claimed invention. Combinations and variants of the individual embodiments discussed are both fully envisioned and intended. As used herein, the expression “at least one” refers to one or more and thus includes individual components as well as mixtures/combinations. Other than in the examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, physical or chemical properties or measured values, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained herein. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches. Any section labels used in the specification are intended for general reference only and should not restrict the interpretation, scope, or construction of any parameter, stage, component, or embodiment herein.

Recycled pulp fibers for use with the present invention may include non-virgin cellulosic fibers from any process and from any source. As used herein, recycled pulp fibers, recycled fibers, non-virgin fibers, secondary pulp fibers, and secondary fibers are used interchangeably. In one embodiment, recycled fibers include fibers from waste paper, including but not limited to printer's clippings and white office waste (also known as premium and/or near premium fibers), as well as fibers from lower grades of waste paper, including but not limited to curbside recycling, newsprint, “junk mail,” magazines, corrugated materials, packaging, and other mixed papers. In another embodiment, recycled fibers include fibers of waste paper that contains greater than about 5% groundwood content. Recycled newsprint fibers may include groundwood contents in the range of about 10% to about 20% or greater. In a further embodiment, the recycled fibers include groundwood pulp with a Kappa number of less than about 10. Recycled fibers may also possess a Kappa number in the range of about 10 to about 20. In yet another embodiment, the fibers include those from both premium sources and lower grades of waste paper.

In general, cellulosic pulp is produced by either chemical means, mechanical means, or a combination thereof (“chemi-mechanical”). Pulp produced by chemical or chemi-mechanical means generally possesses high brightness levels on the order of about 80 GE or greater and is produced using brightening agents (such as hydrogen peroxide) that do not substantially delignify the fibers. As used herein, groundwood pulp refers to papermaking pulp fibers produced primarily or substantially using mechanical means. Groundwood pulp may contain an amount of lignin higher than in other forms of pulp or fibers. Groundwood pulps include, for example, bleached chemi-thermal-mechanical pulp (“BCTMP”) and alkaline peroxide mechanical pulp (“APMP”). Historically, groundwood pulp is not bleached and possesses a brightness of about 60 GE, making it a good choice for newsprint use. Currently, however, bleached groundwood pulp is available with a brightness of about 80 GE or greater, which allows bleached groundwood pulp to be substituted for pulp produced by chemical or chemi-mechanical means. That bleached groundwood pulp generally has a lower price than chemical or chemi-mechanical pulp, and the groundwood pulp is not easily distinguished from chemical or chemi-mechanical pulp in the waste paper supply.

Unfortunately, bleached mechanical pulps tend to “yellow” under alkaline conditions or hypochlorite-based recycled fiber bleaching systems. Yet because no delignifying chemical treatment is carried out, those groundwood pulps tend to retain most of the lignin contained in the source wood or plant raw material. Lignin is a naturally occurring polymeric material with cellulose and is generally considered to be responsible for the strength of wood. The removal of lignin from papermaking pulp fibers generally increases the GE brightness of the fibers. Yet, as discussed above, bleaching processes should endeavor to present substantially minimal to no damage to the physical structure of the pulp fibers, such as by decreasing the lignin content, in an effort to maintain the quality of the paper produced from the pulp fibers. Therefore, certain embodiments of the present invention seek to increase the brightness of pulp without significant loss of lignin. In one embodiment of the present invention, the disclosed process may increase the brightness of the pulp without the removal of significant amounts of lignin. In another embodiment where the recycled pulp comprises mechanical or thermomechanical groundwood pulp fibers, the disclosed process may increase the brightness of the pulp without the removal of significant amounts of lignin.

Brightness, as used herein, refers to GE brightness. GE brightness measures the amount of light reflected from the surface of a pulp and is highly dependent not only on the type of pulp but also on the degree to which it is bleached. GE brightness is measured by comparing the amount of essentially parallel light beams reflected by a pulp surface illuminated at an angle of 45°, to the amount of same light beams reflected by the surface of magnesium oxide, which is the standard of 100%. The process for measuring GE brightness as used herein is disclosed in TAPPI T-452, “Brightness of Pulp, Paper, and Paperboard (Directional Reflectance at 457 nm).” In one embodiment, the change in the GE brightness of pulp subjected to the present invention is at least about 5. In another embodiment, the change in the GE brightness of pulp subjected to the present invention is at least about 8. In a further embodiment, the change in the GE brightness of pulp subjected to the present invention is at least about 15.

Kappa number (K), as used herein, refers to the amount of lignin in a pulp. The lower the Kappa number, the lower the amount of lignin in the pulp. Multiplying the Kappa number by 0.15% yields the percentage of lignin in the pulp. Specifically, according to the Scandinavian standard method, the Kappa number is determined by the following equation:

K=654×ligninmasstotalsolidmass

With that equation in mind, a higher amount of lignin in a pulp yields a higher Kappa number. In one embodiment of the present invention, the fibers for use in the process may possess a Kappa number of less than about 70. In another embodiment, the recycled fibers may possess a Kappa number of less than about 10. In a further embodiment, the recycled fibers may possess a Kappa number in the range of about 10 to about 20. In yet another embodiment, recycled fibers with a Kappa number of greater than about 20 are diluted with fibers possessing a lower Kappa number before use in the present invention.

As mentioned above, bleaching processes such as the present invention should endeavor to present substantially minimal to no damage to the physical structure of the pulp fibers, such as by decreasing the lignin content, which is revealed by a decrease in Kappa number. In one embodiment, the change in Kappa number of a pulp subjected to the present invention is less than about 20. In another embodiment, the change in Kappa number of a pulp subjected to the present invention is less than about 10. In a further embodiment, the change in the Kappa number of a pulp subjected to the present invention is less than about 5. In yet another embodiment, the change in Kappa number of a pulp subjected to the present invention is less than about 3. In still a further embodiment, there is no substantial change in the Kappa number of a pulp subjected to the present invention. Changes in Kappa number may also be expressed in terms of the initial Kappa number. In one embodiment, the change in Kappa number of a pulp subjected to the present invention is less than about 20%. In another embodiment, the change in Kappa number of a pulp subjected to the present invention is less than about 10%. In another embodiment, the change in the Kappa number of a pulp subjected to the present invention is less than about 5%.

The method in accordance with the present invention provides for the bleaching and/or decolorizing of recycled fibers. Bleaching, as used herein, means an increase in the GE brightness. Decolorizing, as used herein, means a reduction in color, either by hue or chroma or both, and may result in an overall color that is more white. While the invention is described for use with secondary or recycled fibers, it may also be used to increase the brightness of virgin fibers. However, the use of the present method on virgin fibers may result in greater delignification than when recycled fibers are used. According to one embodiment, this invention accomplishes bleaching of the pulp fibers without significant removal of lignin, which may be important if the fibers were subjected to delignification during an original pulping process. In another embodiment of the present invention, the disclosed bleaching process may affect the chromophores of the recycled fibers, in part leading to increased brightness, without significantly affecting the lignin content. In a further embodiment, the present invention is used with a mixture of virgin fibers and recycled fibers.

According to one embodiment, the present invention is a method for bleaching recycled fibers comprising an activating bleach step, an alkaline peroxide step before or after the activating bleach step and with or without the addition of oxygen, followed by a reductive bleach step. Each step in the method of this embodiment may affect the overall brightness of the treated pulp differently, each leading to an overall increased brightness between the beginning recycled fibers and end product. The terms step, stage, and treatment are interchangeable throughout this specification and represent contacting a fiber pulp with an active component agent or agents.

Activating Bleach Step

The activating bleach step comprises contacting the recycled fibers with at least one agent known to produce active oxygen, called hereinafter an activating bleaching agent. As an example, the CRC Manual (1989 edition) defines active oxygen as “a measure of the oxidizing power of a substance expressed in terms of oxygen with a gram equivalent weight of 8.0.” Activating bleaching agents may include any compounds capable of generating active oxygen. In particular, activating bleaching agents may include those oxygen-containing compounds that are capable of reacting with (e.g., accepting electrons from) colored bodies or substances in fibers or pulp and reducing at least one of the hue, chroma, and brightness of those colored bodies or substances. Such activating bleaching agents include, but are not limited to, molecular oxygen (O2), hydrogen peroxide, alkaline hydrogen peroxide in the form of perhydroxly anion (OOH—), peroxymonosulfuric acid (PMSA, also known as permonosulfuric acid and Caro's acid), peracetic acid (PAA, also known as peroxyacetic acid, ethaneperoxic acid, per acid, and periacetic acid), and a mixture of TAED and a peroxy compound such as hydrogen peroxide. Other activating bleaching agent may and will be known to those skilled in the art. In one embodiment, the activating bleaching agent is capable of producing from about one to about nine pounds of active oxygen per ton of pulp. In another embodiment, the activating bleaching agent produces less active oxygen than prior art processes, yet the inventive process yields substantially similar or greater brightness increases.

In one embodiment, the activating bleaching agent is a mixture of at least one acylamide and at least one peroxy compound. In one such embodiment, the at least one acylamide is TAED. TAED refers to N,N,N′,N′-tetraacetylethylenediamine, or C10H16O4N2, an acylamide that may be available commercially under the Clairant trade name PERACTIVE®-P or under the Warwick International trade name PEROXYBOOST®. In another such embodiment, the at least one peroxy compound is chosen from hydrogen peroxide and alkaline oxygen peroxide. The direct addition or mixture of at least one acylamide and at least one peroxy compound may enhance the brightening reactions of the activating bleach step. In one embodiment, a mixture of TAED and at least one peroxy compound produces at least one of hydrogen peroxide, peracetic acid, peroxymonosulfuric acid, and molecular oxygen. In another embodiment, a mixture of TAED and at least one peroxy compound produces two or more of hydrogen peroxide, peracetic acid, peroxymonosulfuric acid, and molecular oxygen. In a further embodiment, a mixture of TAED and at least one peroxy compound produces hydrogen peroxide, peracetic acid, peroxymonosulfuric acid, and molecular oxygen. In any one of the above embodiments, the at least one peroxy compound may be alkaline hydrogen peroxide in the form of the perhydroxly anion (OOH—). In one embodiment, TAED and a peroxy compound are mixed before addition to the pulp. In another embodiment, either TAED or a peroxy compound are added to the pulp followed by addition of the other. In a further embodiment, TAED and a peroxy compound are added simultaneously to the pulp.

In one embodiment of this invention, when a mixture of TAED and hydrogen peroxide is used as the at least one activating bleaching agent, the activating bleach step may be carried out at a lower temperature than if only hydrogen peroxide is used. Lower temperatures for the activating bleach step may have the benefit of lower energy costs, which increases the economic and commercial impact of the inventive process. In one embodiment, the activating bleach step is conducted at or about room temperature, e.g., 70° F. In another embodiment, the activating bleach step is conducted at a temperature above room temperature to increase the effectiveness (i.e., enhance the bleaching and/or decolorizing effect) of the activating bleaching agent.

In another embodiment, the at least one activating bleaching agent is TAED directly added to hydrogen peroxide. In this embodiment the recycled fibers may be conducted or added, without washing, to an alkaline peroxide step. For example, in one embodiment, the recycled fibers may be conducted, without washing, to an alkaline peroxide step, in which the at least one alkaline peroxide agent may be a mixture of sodium hydroxide and hydrogen peroxide.

In another embodiment, the at least one activating bleaching agent is peracetic acid (PAA). In a further embodiment, the at least one activating bleaching agent is PAA produced from a mixture comprising TAED, hydrogen peroxide, and a caustic (for example, sodium hydroxide), also optionally including a chelant. That production of PAA may be conducted according to the Warwick Process of Warwick International, Flintshire, United Kingdom. Such a process may include that described in U.S. Pat. No. 6,569,286, which is incorporated by reference herein in its entirety. A PAA production process, and in particular the Warwick Process, may also produce diacetylethylenediamine (DAED) as a byproduct. The addition of the TAED, hydrogen peroxide, caustic, and optional chelant, along with the resulting PAA (and any DAED that may be present), may serve to increase further the brightness of the recycled fibers versus the three (or four) compounds alone. In one embodiment, TAED directly added to a peroxy compound may yield higher brightness gains as the least one activating bleaching agent according to the present invention than would peracetic acid alone produced through the Warwick Process. In another embodiment, TAED, hydrogen peroxide, chelant, and water are mixed and then a caustic (for example, sodium hydroxide) is added, resulting in the production of PAA that then contacts the recycled fibers. Of course, any unreacted starting compounds for PAA production may also contact the recycled pulp, in addition to any DAED that is produced, which may also serve to increase the brightness of the pulp.

In one embodiment, finely divided granules of TAED are suspended or dispersed in water in an amount of about 1% by weight prior to mixing with the other components. In another embodiment, the hydrogen peroxide is diluted or dissolved in a bleach solution to 1% before it is mixed with the other components. In a further embodiment, a chelant is included and diluted to less than 5 PPM in bleach solution. One non-limiting example of a chelant is PQ 100, a form of dimyristoleoyl phosphonomethyl trimethyl ammonium available from PQ Chemical. In embodiments where a caustic may be used, the caustic may be added in an amount sufficient, for example 0.5% by weight, to adjust the pH of the solution from 8.5 to 9.0 for the reaction to produce greater amounts of PAA. The mixture of TAED, hydrogen peroxide, chelant, water, and any caustic may be adjusted for the addition of the mixed solution to the fibers to a temperature of 120° F. and a pH of from 7.5 to 8.0, resulting in the addition of a 2% PAA in water solution to the fibers.

Molecular oxygen (O2) may be added to the activating bleach step. Molecular oxygen is a relatively low cost source of active oxygen. In one embodiment, when peracetic acid is present as an activating bleaching agent, it may not be desirable to add molecular oxygen to the activating bleach step, as it may result in the initial yellowing of the recycled pulp fibers. In another embodiment, molecular oxygen may be added to the activating bleach step so that a lower amount of the at least one activating bleaching agent may be used to achieve about the same equal active oxygen application and/or brightness gain. In a further embodiment, molecular oxygen may be added to the activating bleach step to supplement the amount of the at least one activating bleaching agent, achieving a greater active oxygen application and/or brightness gain. In yet another embodiment, the at least one activating bleaching agent is peracetic acid and molecular oxygen is added in the activating bleach step. In still another embodiment, the at least one activating bleaching agent is hydrogen peroxide and molecular oxygen is added in the activating bleach step. In still a further embodiment, the at least one activating bleaching agent is molecular oxygen.

The amount or concentration of the at least one activating bleaching agent may be controlled either by its addition rate, by controlling its level of production, or both. High amounts or concentrations of the at least one activating bleaching agent, or the components of the at least one activating bleaching agent, may result in the loss of significant amounts of lignin, damage or depolymerize the cellulose, and increase the chemical cost without necessarily providing further gains in brightness levels. In one embodiment, the amount or concentration of the at least one activating bleaching agent is controlled so that it does not result in loss of a significant amount of lignin. In one such embodiment, the fibers experience less than about a 20% decrease in lignin. In another such embodiment, the fibers experience less than about a 10% decrease in lignin. In a further such embodiment, the fibers experience less than about a 5% decrease in lignin. In another embodiment, the amount or concentration of the at least one activating bleaching agent is controlled so that it does not lead to yellowing of the recycled fibers.

In one embodiment, the amount of the at least one activating bleaching agent is from 0.05 to 0.4 percent by weight of recycled fibers. In another embodiment, the amount of the at least one activating bleaching agent is less than 0.25 percent by weight of recycled fibers. In a further embodiment, the amount of the at least one activating bleaching agent is any amount capable of providing a desired increase in brightness. In yet another embodiment, the amount of the at least one activating bleaching agent is set to achieve the desired level of brightness increase with a minimum amount of residual agent passing to the next step. In all embodiments, excess amounts of the at least one activating bleaching agent should not negatively impact the effectiveness of the disclosed process. If loss of lignin is not a factor in a given embodiment, then increased amounts of the at least one activating bleaching agent may be used for a larger brightness increase.

The recycled fibers may or may not be washed between the activating bleach step and the alkaline peroxide step, regardless of which step is performed first. In one embodiment, the recycled fibers are washed between the activating bleach step and the alkaline peroxide step. In another embodiment, the recycled fibers are not washed between the activating bleach step and the alkaline peroxide step. In prior art processes where peracetic acid is used as a bleaching agent, the pulp is washed to remove lignin. However, since the bleaching process of the present invention may be practiced on recycled pulp fiber, it is often undesirable to delignify the recycled fibers, making a wash unnecessary. In some embodiments it may be desirable that at least some quantity of the at least one activating bleaching agent be present in the recycled fibers at the time of contact with an alkaline peroxide step. In additional embodiments, it may be desirable that at least some quantity of the least one activating bleaching agent be present in the recycled pulp at the end of the activating bleach step. In one such embodiment, at least about 10% of the at least one activating bleaching agent is present the recycled fibers at the end of the activating bleach step. In another such embodiment, at least about 10% of the at least one activating bleaching agent is present the recycled fibers at the end of the activating bleach step. In a further such embodiment, from about 5% to about 10% of the at least one activating bleaching agent is present the recycled fibers at the end of the activating bleach step.

Alkaline Peroxide Step

The alkaline peroxide step comprises contacting the recycled fibers with at least one alkaline peroxide agent. In one embodiment, the at least one alkaline peroxide agent is a mixture of hydrogen peroxide and at least one alkaline hydroxide agent. In another embodiment, the at least one alkaline peroxide agent is chosen from hydrogen peroxide and sodium peroxide. In yet another embodiment, the at least one alkaline peroxide agent is chosen to generate perhydroxyl anions (OOH—) from hydrogen peroxide at a pH from about 10 to about 11. The at least one alkaline hydroxide agent may be one of or a mixture of known caustic compounds, including, but not limited to, sodium hydroxide, calcium hydroxide, and magnesium hydroxide. In one embodiment, the at least one alkaline hydroxide agent is sodium hydroxide. In another embodiment, the at least one alkaline hydroxide agent is magnesium hydroxide. In a further embodiment, the at least one alkaline hydroxide agent is a mixture of sodium hydroxide and magnesium hydroxide. In still a further embodiment, the at least one alkaline hydroxide agent is a mixture of hydrogen peroxide and sodium hydroxide.

The alkaline hydroxide agent may be added in any amount to effect the desired increase in brightness of the fibers. In one embodiment, the peroxide is added in the amount of 0.5% on fiber dose. In another embodiment, the peroxide is added in the amount of less than about 2% on fiber dose. In a further embodiment, the peroxide is added in the amount of less than about 1% on fiber dose.

The time, temperature, and concentration of the activating bleach step and the alkaline peroxide step can be controlled to effect a desired change in the Kappa number and GE brightness of the recycled fibers. The residence time of both the activating bleach step and the alkaline peroxide step is a function of the temperature and chemical concentration of those steps. In general, increases in the temperature and/or the chemical concentration increase the reaction rate of a given step and decrease the reaction time.

In one embodiment, the reaction time is chosen to achieve about 90% consumption of the at least one activating bleaching agent. In another embodiment, the reaction time is chosen to achieve less than about 95% consumption of the at least one activating bleaching agent, as greater than about 95% consumption may result in brightness reversion due to alkaline darkening of the lignin. In an embodiment of the present invention in which the activating bleach step follows the alkaline peroxide step, the reaction time is chosen to achieve about 50% consumption of the at least one alkaline peroxide agent, such that the unreacted portion of the at least one alkaline peroxide agent may react with the at least one activating bleaching agent (or at least one of the components of the at least one activating bleaching agent) to generate additional active oxygen.

In general, use of an increased temperature for the alkaline peroxide step may result in an increased brightness gain. Such an increased temperature may require a pressurized reaction vessel and would generally require increased energy consumption. Such an increased temperature may also result in heat management issues within the pulp system. In one embodiment, the temperature of the alkaline peroxide step is less than about 250° F. In another embodiment, the temperature of the fibers is increased between the activating bleach step and the alkaline peroxide step. For example, in one embodiment the temperature of the contacting at the activating bleach step is 140° F., while the temperature of the contacting at the alkaline peroxide step is 170° F. However, increased temperatures are not necessary to achieve increased brightness levels with the present invention. In a further embodiment, the activating bleach step, whether before or after the alkaline peroxide step, allows the alkaline peroxide step to be operated at a reduced temperature for a given or similar brightness gain, as opposed to a bleaching operation without an activating bleach step. In general, lower temperatures reduce energy costs, minimize undesirable heat build-up in pulp systems, require lower capital demands (such as, through not requiring pressurized equipment), and increase the safety of pulp system operating personnel.

The bleaching process for recycled fibers disclosed herein is relatively tolerant of the different mixing abilities of various pulping and/or papermaking systems. In general, the mixing of the pulping and/or papermaking system used for the present invention should be sufficient to allow contact of the chemicals of a given step with the pulp fibers at the lowest intensity possible to avoid any mechanical damage to the fibers. In one embodiment, where the pulping and/or papermaking system can provide adequate mixing in all three steps (if used) of the process disclosed herein, either the activating bleach step or the alkaline peroxide step may be first, followed by the other, and then followed by the reductive bleach step. In another such embodiment of adequate mixing in all three steps, the activating bleach step may be followed by the alkaline peroxide step, then a wash of the fibers, and then the reductive bleach step. In a further embodiment, where a pulping and/or papermaking system provides better mixing in the first step than in subsequent steps, the alkaline peroxide step may be followed by the activating bleach step, then a wash of the fibers, and then the reductive bleach step.

The pulp subjected to treatment according to the present invention may be washed between one or more steps. Such a wash may be used to remove residual chemicals or agents that would interfere with a subsequent step or steps. Such a wash may also be used to remove any undesirable byproducts of a step or steps, such as organics, free lignin, and lignin byproducts. Removal of those undesirable byproducts may help prevent any interference of those byproducts with chemicals used in the paper-making process. Removal of those undesirable byproducts may also help prevent the formation of biological slimes or other deposits in the paper-making machinery. In one embodiment of the invention, the pulp is washed before the reductive bleach step. In such an embodiment, the wash may assist in the removal of residual oxidative chemicals that would interfere with the reductive bleach step (for instance, by consuming the at least one reductive bleach agent). In another embodiment of the invention, the pulp is not washed before the reductive bleach step. In a further embodiment, the pulp is not washed between an activating bleach step and an alkaline peroxide step.

Wash filtrate from the alkaline peroxide step may be recovered and used in the activating bleach step and/or the alkaline peroxide step. Such a filtrate step may reduce overall operating cost because unreacted chemicals increase the overall chemical concentration and thus may help increase the brightness of pulp, instead of running out with any wastewater or effluent. In addition, such a filtrate step may allow for the generation of active oxygen in an activating bleach step. Further, such a filtrate step may allow a higher addition rate of an alkaline peroxide agent to the alkaline peroxide step, without increased chemical cost.

In one embodiment, residual amounts of the at least one alkaline peroxide agent, or residual amounts of any compounds in combination with the at least one alkaline peroxide agent, may react with the at least one activating bleaching agent to generate active oxygen compounds. In another embodiment, the filtrate from the alkaline peroxide step may be reacted with any TAED used in the activating bleach step, and that mixture contacted with the pulp in the activating bleach step. In such an embodiment, residual amounts of the at least one alkaline peroxide agent may generate additional active oxygen when contacting and reacting with the TAED. Moreover, in such an embodiment where alkaline peroxide agent is present in the activating bleach step, lower reaction temperatures may be used in the alkaline peroxide step to achieve similar brightness gains. In addition, in such an embodiment, the pulp may be washed before the reductive bleach step, such that any residual chemicals from the alkaline peroxide step and the activating bleach step may be reused in either step.

Reductive Bleach Step

The reductive bleach step comprises contacting the recycled pulp with at least one reductive bleaching agent. In one embodiment, the reductive bleach step removes or decreases the color hue, intensity, or brightness of any dye that may be present in the pulp. The at least one reductive bleaching agent may be any one of or a mixture of agents known to those of ordinary skill in the art, including one or more of zinc hydrosulfite, sodium hydrosulfite (SHS, Na2S2O4, also known as hydrolin, sodium dithionite, sodium sulfoxylate, and sodium hydrosulphite), formamidine sulfinic acid (FAS, also known as thioureas dioxide), hydroxyl methyl sulfinic acid (HAS), sodium hydrosulfite, or borohydrides including, but not limited to, sodium borohydride (NaBH4, aka sodium tetrahydridoborate). In one embodiment, the reductive bleaching agent is sodium hydrosulfite.

The reductive bleach agent may be added in any amount necessary to effect the desired increase in brightness of the fibers. In one embodiment, the reductive bleach agent is added in an amount of less than about 2% on pulp dose. In another embodiment, the reductive bleach agent is added in an amount of 0.1% to 0.8% on pulp dose.

The reaction conditions of the reductive bleach step may be modified to effect greater increases in brightness. In a further embodiment, brightness gains in the pulp from the activating bleach step and the alkaline peroxide step allow at least one of (1) the use of less at least one reductive bleach agent, (2) less reaction time of the pulp with the at least one reductive bleach agent, and (3) a lower temperature for the reductive bleach step.

In one embodiment, the temperature of the reductive bleach step is about the same or greater than the temperature of the alkaline peroxide step and/or the activating bleach step. Such high temperatures may result in faster and more complete bleaching of the pulp in the reductive bleach step, but may also require higher amounts of energy and higher capital costs. In another embodiment, the temperature of the reductive bleach step is less than the temperature of the alkaline peroxide step and/or the activating bleach step.

Many paper-making machines run at an overall pH generally in the range of about 7 to about 8. A pH of less than about 7 may result in greater amounts stress on the paper-making equipment, for instance, in increased corrosion. A pH of less than about 7 may also affect the pulp in that the lower pH may result in a lower brightness level, particularly for wastepaper-derived pulps. Such an acidic pH may result in a lower brightness level due to the yellowing of residual lignin or yellowing of non-fiber elements, such as clays or carbonates. Conversely, a pH of greater than about 8 may affect the pulp in that the higher pH may result in a lower brightness level, particularly for pulps with a high lignin content. Such an alkaline pH may result in a lower brightness level due to alkali darkening of the pulp.

The conditions of one or more of the activating bleach step, the alkaline peroxide step, and the reductive bleach step may cause the pH of the pulp to drop. In one embodiment, one or more of the activating bleach step, the alkaline peroxide step, and the reductive bleach step individually may cause a drop in pH of about 1. In another embodiment, one or more of the activating bleach step, the alkaline peroxide step, and the reductive bleach step individually may cause a drop in pH of about 0.5. In a further embodiment, one or more of the activating bleach step, the alkaline peroxide step, and the reductive bleach step may cause the pH to monotonically decline. “Monotonically decline,” as used herein, means that the cellulosic pulp experiences only reductions in pH due to one or more of the activating bleach step, the alkaline peroxide step, and the reductive bleach step. In yet another embodiment, the pH of the cellulosic pulp may monotonically decline as it is subjected to each of an activating bleach step, an alkaline peroxide step, and a reductive bleach step, regardless of the order of those steps.

In those embodiments in which one or more steps of the present invention individually may cause a reduction in pH, those pH reductions may be used to adjust the pH of the pulp. Further, those pH reductions may be used to adjust the pH of the pulp before entering the paper-making machinery without the need for additional chemicals to raise or lower the pH to a desired level. In one embodiment, pulp enters an exemplary process of the present invention at an alkaline pH and, through the use of one or more steps of the present invention, the pH of that pulp may be reduced while achieving the desired increase in brightness. In another embodiment, pulp enters the process of the present invention at a pH of about 10.5; after an alkaline peroxide step, the pH of the pulp may fall to about 9.5; after an activating bleach step, the pH may fall to about 8.5; after a reductive bleach step, the pH may fall to about 7.5, which is within the general overall range for pH for a paper-making machine. In a further embodiment, pulp enters the process of the present invention at a pH greater than about 8; after an alkaline peroxide step, the pH of the pulp may fall to about 9 to about 10.5; after an activating bleach step, the pH may fall to about 8.5; after a reductive bleach step, the pH may fall to about 6.5 to about 8. In yet another embodiment, pulp enters the process of the present invention at a pH greater of about 10.5; after an alkaline peroxide step, the pH of the pulp may fall to about 9 to about 10.5; after an activating bleach step, the pH may fall to about 7.5 to about 9; after a reductive bleach step, the pH may fall to about 6.5 to about 8.

The pH of the resulting pulp after treatment according to the present invention may desirably be raised through use of one or more alkaline or alkaline-producing compounds before entering the paper-making machine. The skilled artisan readily knows such alkaline or alkaline-producing compounds that may be used to effect a desired increase in pH. In one embodiment, the pH of the resulting pulp after treatment according to the present invention is less than about 7; in such an embodiment, the pH of the pulp may desirably be raised through use of one or more alkaline or alkaline-producing compounds before entering the paper-making machine. In one embodiment, the pH of the resulting pulp after treatment according to the present invention is less than about 8; in such an embodiment, the pH of the pulp may desirably be raised through use of one or more alkaline or alkaline-producing compounds before entering the paper-making machine.

The pH of the resulting pulp after treatment according to the present invention may desirably be lowered through use of one or more acidic or acidic-producing compounds before entering the paper-making machine. The skilled artisan readily knows such acidic or acidic-producing compounds that may be used to effect a desired decrease in pH. In one embodiment, the pH of the resulting pulp after treatment according to the present invention is greater than about 8; in such an embodiment, the pH of the pulp may desirably be raised through use of one or more acidic or acidic-producing compounds before entering the paper-making machine. In one embodiment, the pH of the resulting pulp after treatment according to the present invention is greater than about 7; in such an embodiment, the pH of the pulp may desirably be raised through use of one or more acidic or acidic-producing compounds before entering the paper-making machine.

In one embodiment, the pH of the pulp before a reductive bleach step is less than about 9. In another embodiment, the pH of the pulp before a reductive bleach step is less than about 8.5. In a further embodiment, the pH of the pulp after a reductive bleach step is about 7.5. In yet another embodiment, the pH of the pulp after a reductive bleach step is from about 7 to about 8.

FIG. 1 shows a recycled fiber bleaching system in accordance with the prior art. Recycled fibers in a steam mixer 1 contact an alkaline peroxide agent 2 (such as hydrogen peroxide and sodium hydroxide) and pass to a pump 3. Optional oxygen 5 may be added at mixer 4 and the fiber mixture is passed to a holding tank 6 for contacting in an alkaline peroxide step. The fiber is then put through a press 7 to remove excess alkaline peroxide agent, the filtrate of which is passed to a dissolved air floatation device (“DAF”) 8. The fibers from press 7 pass to a steam mixer 9 and through a pump 10 to a mixer 11, where the fibers are reacted with hydrosulfite 12. The fibers are then passed to holding tank 13 for contacting in a reductive bleach step. The fibers are then passed to the remaining parts of a papermaking system 14, or to storage tanks or any other holding facility readily known by one of ordinary skill in the art.

FIG. 2 shows one embodiment of a recycled fiber bleaching system in accordance with the present invention. Recycled fibers in a steam mixer 1 pass to a pump 5 after the addition of peracetic acid 4 (PAA), the PAA created from a mixture of 2 of TAED, peroxide, caustic, and chelant in a PAA makedown tank 3. Optional oxygen 7 may be added at mixer 6 and the fiber/PAA mixture is passed to a holding tank 8 for contacting in an activating bleach step. The fiber mixture is then reacted with peroxide 9 (which may comprise at least some of filtrate 13) and passed through a mixer 10 to a holding tank 11 for contacting in an alkaline peroxide step. The fiber is then put through a press 12 to remove excess activating bleaching agent and alkaline peroxide agent, the filtrate 13 of which is passed to a dissolved air floatation device (“DAF”) 14. The fibers from press 12 then pass to a steam mixer 15 and through a pump 16 to a mixer 16, where the fibers are reacted with hydrosulfite 18. They are then passed to holding tank 19 for contacting in a reductive bleach step. The fibers are then passed to the remaining parts of a papermaking system 20, or to storage tanks or any other holding facility readily known by one of ordinary skill in the art.

FIG. 3 shows another embodiment of a recycled fiber bleaching system in accordance with the present invention. Recycled fibers in a steam mixer 1 pass to a pump 3 after the addition of peroxide 2 (which may comprise at least some of filtrate 12). The fiber/peroxide mixture is passed to a holding tank 4 for contacting in an alkaline peroxide step. The fiber mixture from that step then passes to a mixer 8 after the addition of peracetic acid 7 (PAA), the PAA created from a mixture of 5 of TAED, peroxide, caustic, and chelant in a PAA makedown tank 6. Optional oxygen 9 may be added at mixer 8 and the fiber/PAA mixture is passed to a holding tank 10 for contacting in an activating bleach step. The fiber is then put through a press 11 to remove excess activating bleaching agent and alkaline peroxide agent, the filtrate 12 of which is passed to a dissolved air floatation device (“DAF”) 13. The fibers from press 11 then pass to a steam mixer 14 and through a pump 15 to a mixer 16, where the fibers are reacted with hydrosulfite 17. They are then passed to holding tank 18 for contacting in a reductive bleach step. The fibers are then passed to the remaining parts of a papermaking system 19, or to storage tanks or any other holding facility readily known by one of ordinary skill in the art.

FIG. 4 shows another embodiment of a recycled fiber bleaching system in accordance with the present invention. Pulp papermaking fibers, which may include at least partly recycled fibers, in a thickener 1 pass to a steam mixer 3 into which steam 2 is presented. After sufficient contact with steam 2, the fibers pass to a pump/chemical mixer 5 where they are mixed with at least one alkaline peroxide agent 4 and are passed to an upflow tower 6 for contacting in an alkaline peroxide step. The fibers are then passed to a flow discharger/mixer 8 where they are mixed with at least one activating bleaching agent 7. For instance, the activating bleaching agent 7 may comprise peracetic acid created from a mixture comprising TAED, peroxide, caustic, and chelant in a PAA makedown tank not shown in FIG. 4. The fibers are then passed to a downflow tower 9 for contacting in an activating bleaching step. The fibers may optionally be washed between the flow discharger/mixer 8 and the downflow tower 9 by any apparatus or method known to the skilled artisan, for example a drum washer, which is not represented in FIG. 4. Oxygen, which is not represented in FIG. 4, may also be optionally added to the flower discharger/mixer 8 or pump/mixer 5. After the activating bleaching step in downflow tower 9, the fibers are passed to a drum washer 10 for washing and then to a steam mixer 12, into which steam 11 is presented. After sufficient contact with steam 11, the fibers are passed to a pump/mixer 12 where they are mixed with at least one reductive bleaching agent 13. The fibers are then passed to a downflow tower 15 for contacting in a reductive bleaching step. After the reductive bleaching step, the fibers are passed to a drum washed 16 for washing and then to the remaining parts of a papermaking system 17, or to storage tanks or any other holding facility readily known by one of ordinary skill in the art. The letters A through G represent points during in the bleaching system, among others not shown, at which operational parameters and characteristics of the fibers may be measured, for instance, the pH of the pulp, the temperature of the pulp, the presence of any agents or chemicals (including residual agents from steps), and/or the brightness of the pulp.

The invention will now be described through the following examples, which do not restrict the invention but merely further illustrate the best mode contemplated by the applicants for the practice of the invention.

EXAMPLE 1

The starting fibers in Example 1 were recycled cellulosic fibers collected prior to any bleaching step. The initial brightness of the recycled cellulosic fibers was 66.5 GE; the Kappa number of the recycled cellulosic fibers was 26.3; the delta a* of the recycled cellulosic fibers was 19.65. A description of the a* value and processes for determining that value are found in U.S. Published Patent Application Nos. 2004/0000383 and 2004/0079497, which are assigned to the assignee of this application and which are incorporated by reference herein in their entireties.

Those starting fibers were then bleached according to a conventional Eop/Y process. In particular, the fibers were bleached in a Quantum mixer under an alkaline peroxide step, with hydrogen peroxide as the alkaline peroxide agent, in conditions listed in Table 1 as “EOP Conditions.” The two 15-minute charges of oxygen at 60 PSIG involved pressurizing the quantum mixer to 60 PSIG for 15 minutes, relieving the mixer to reduce the pressure to atmospheric pressure, and then re-pressurizing the mixer to 60 PSIG for another 15 minutes. Those oxygen conditions approximate an oxygen charge of about 2 pounds per ton of pulp, or about a 1% charge. The brightness of the fibers was measured to be 74.7 GE, an 8.2 gain in GE brightness over the initial brightness of the starting fibers. To continue the conventional process, the fibers were then displacement washed on a Buchner funnel and passed through a reductive bleach step, using a mixture of sodium hydrosulfite (SHS) and hydroxymethane sulfinic acid (HAS) as the reductive bleaching agent, in conditions listed in Table 1 as “Reductive Conditions.” The brightness of the fibers was then measured to be 75.5 GE, a 9.0 GE brightness gain over the initial brightness of the starting fibers.

TABLE 1
Example 1 Bleaching Conditions
MethodConditions
EOPTemperature170° F.
ConditionsConsistency12%
OxygenTwo 15 minute
charges at 60PSIG
Hydrogen Peroxide32 lb/ton
Sodium Hydroxide12.8 lb/ton
DTPA2.5 lb/ton
Retention120 minutes
Measured Brightness74.7 GE
ReductiveTemperature170° F.
ConditionsConsistency8%
SHS/HAS10 lb/ton
Retention60 minutes
Measured Brightness75.5 GE

EXAMPLE 2

In Example 2, 0.3 grams of TAED powder (Clariant trade name PERACTIVE®-P) was added to a sample of the same starting recycled fibers used in Example 1. The fibers were then bleached using alkaline hydrogen peroxide under the EOP conditions listed in Example 1. The fibers were sampled after the first 15 minute oxygen charge and the fibers were measured to have a 74.7 GE brightness. After the second 15 minute oxygen charge, the fibers were measured to have a 76.4 GE brightness. The fibers were also tested and found to contain substantially no residual oxidant, indicating that bleaching was complete. The Eop/direct addition of TAED process of this Example 2 resulted in a larger brightness gain than the conventional Eop/Y bleaching process of Example 1.

EXAMPLE 3

In Example 3, an additional 0.5% hydrogen peroxide charge, at 10 lb/ton, was added to resulting fibers of Example 2 and retained under atmospheric pressure for two hours. The fibers surprisingly achieved a 78.5 GE brightness, or an additional 2.1 GE points over the resulting fibers from Example 2. The addition of hydrogen peroxide after complete bleaching, such as in Example 2, generally does not result in any significant brightness increase. However, this Eop/TAED+P process of Example 3 resulted in an unexpected additional brightness gain over the Eop/TAED process of Example 2, due to the continued bleaching effects of TAED in the presence of the additional hydrogen peroxide charge.

The fibers from this example contained 0.2 g/L, or approximately 0.2%, residual peroxide. That residual peroxide amounted to about 6% of the additional peroxide added, which is a good amount of consumption for a bleaching process.

EXAMPLE 4

In Example 4, a sample of the starting fibers from Example 1 were bleached using a conventional, single-stage oxidative bleaching step in which the oxidative bleaching agent was peracetic acid (at a concentration of about 15 grams/liter). After the treatment, the fibers were sampled and measured to have a GE brightness of 67.3, a brightness gain of 0.8 GE from the initial fibers. This example shows the general ineffectiveness of single-stage Paa bleaching.

EXAMPLE 5

In Example 5, the fibers bleached with peracetic acid from Example 4 were further bleached with peroxide, as is conventional in the art. After the peroxide treatment, the fibers were sampled and measured to have a GE brightness of 72.4 due to the overall Paa/P bleaching process. Brightness levels according to Paa/P conditions, such as Example 5, generally vary depending on the pulp and reaction conditions; therefore, Paa/P bleaching may result in slightly higher brightness increases than indicated in this Example.

EXAMPLE 6

Example 6 was designed to help show the effect of residual peracetic acid and peroxide in combination with TAED, versus direct addition, to yield additional active oxygen and further GE brightness gain. The fibers from Example 5 were then dewatered on a buchner funnel, using a #415 filter pad, to about 30% solids; approximately 750 mL of filtrate was extracted from those fibers. A new sample of the starting fibers from Example 1 were diluted to about 20% solids with tap water and placed in a Quantum Mixer. Those fibers were then preheated to 170° F. The filtrate from the Example 5 fibers and 0.3 grams of TAED powder (Clariant trade name PERACTIVE®-P) were the added to the mixer, which was then sealed and the fibers agitated to mix to a 12% consistency. Two oxygen charges at 60 PSIG, in a manner similar to Example 1 but after 15 minutes and 45 minutes of reaction time, were added to the fibers. After an additional 45 minutes, a sample of the fibers yielded a GE brightness of 70.8, an increase of 4.3 GE from the initial fibers.

EXAMPLE 7

In Example 7, a charge of 1% OP hydrogen peroxide and 0.3% OP (on pulp) sodium hydroxide was added to the fibers of Example 6 and retained for an additional two hours. The fibers were then brightness tested and displacement washed, yielding a GE brightness of 74.4, an increase of 7.9 from the initial fibers.

EXAMPLE 8

In Example 8, the fibers of Example 7 along with the fibers of Example 4 were reductively bleached at 170° F., 8% consistency, with 1% HAS/Y applied. The fibers had a GE brightness of 77.6, a gain of 11.1 GE from the initial fibers.

EXAMPLE 9

Example 9 was designed as a lower temperature version of Example 2. In Example 9, a sample of the starting fibers from Example 1 were placed in a Quantum Mixer and preheated to 140° F. 0.3 grams of TAED powder (Clariant trade name PERACTIVE®-P) (0.12% OP active dose) was added along with a 2% OP hydrogen peroxide and 1% OP sodium peroxide. The mixer was sealed and run for 8 seconds to mix; no oxygen charge was used. After 10 minutes of retention the GE brightness was measured to be 72.8; after 20 minutes the GE brightness was 75.1; after 40 minutes the GE brightness was 75.7. Surprisingly, the lower temperature Eop/TAED bleaching of this Example 9 resulted in a GE brightness level almost as high as the higher temperature Eop/TAED bleaching of Example 2, which is significant as it unexpectedly indicates that lower amounts of energy and heat may still result in similar brightness.

The peroxide residual was measured as 0.2 g/L, or about 10%, indicating that bleaching was essentially complete after about 40 minutes. The pH after 40 minutes was measured to be 10.15, indicating sufficient residual alkalinity to add additional hydrogen peroxide.

EXAMPLE 10

In Example 10, an additional 1% OP hydrogen peroxide charge was added to the fibers from Example 9 approximately 60 minutes after the initial alkaline peroxide addition according to Example 1. The fibers were held for 120 minutes. After 60 minutes the brightness was measured to be 77.9 GE; after 120 minutes the brightness was 79.1 GE. Surprisingly, the lower temperature Eop/TAED+P bleaching of this Example 10 resulted in a GE brightness level almost higher than the higher temperature Eop/TAED+P bleaching of Example 3, which is significant as it unexpectedly indicates that lower amounts of energy and heat may still result in greater brightness levels.

The residual peroxide at 120 minutes was measured at 0.72 g/L, indicating only about 30% of the additional peroxide charge had been consumed.

Table 2 summarizes the brightness levels from Examples 1-10.

TABLE 2
Summary of Examples 1 to 10
BrightnessBrightness
ExampleConditionsGEGain
Unbleached Pulp66.5
1Conventional Eop/Y Bleach74.78.2
2Eop/Direct Addition of TAED76.49.9
30.5% peroxide added to Example 278.512
4Peracetic Acid Bleached67.30.8
5Peracetic Acid/Peroxide Bleached72.45.9
6TAED + Residual Filtrate70.84.3
7Additional Peroxide to Example 674.47.9
8Reductive Bleaching77.611.1
9Low T, Eop/direct addition of TAED75.79.2
10Additional Peroxide Added to 979.112.6

EXAMPLE 11

In Example 11, starting pulp fibers were harvested from a paper-making system and measured to have a brightness of 53.5 GE and a kappa number of 43. These fibers were then hypochlorite bleached according to current practice of those skilled in the art (5% consistency, 125 OF, 2% OP hypochlorite, 60 minute retention). The hypochlorite used was sodium hypochlorite commercially available as Clorox® bleach. Table 3 displays the results of this Example, indicating a GE brightness gain of −0.9. This Example 11 shows that hypochlorite bleaching on high Kappa pulp is not effective to increase brightness, due to yellowing of the lignin present.

EXAMPLE 12

In Example 12, a sample of starting fibers from Example 11 were hydrosulfite bleached according to current practice of those skilled in the art (8% consistency, 170° F., 1% OP hydrosulfite, 60 minute retention). The hydrosulfite used was 100% sodium hydrosulfite powder commercially available from Fisher Scientific. Table 3 displays the results of this Example, indicating a GE brightness gain of 3.6.

EXAMPLE 13

In Example 13, a sample of starting fibers from Example 11 were bleached in a two step peroxide/hydrosulfite sequence according to conventional practice of those skilled in the art. The peroxide step used 12% consistency, 165° F., 2% OP peroxide, and 60 minute retention; the hydrosulfite step used 8% consistency, 175° F., 1% OP hydrosulfite, and 60 minute retention. The peroxide used was 30% hydrogen peroxide in water, the hydrogen peroxide commercially available from Fisher Scientific. Table 3 displays the results of this Example, indicating a GE brightness gain of 10.7.

EXAMPLE 14

In Example 14 a sample of starting fibers from Example 11 were bleached with a two step hydrosulfite/peracetic acid sequence as is conventional in the art. The hydrosulfite step used 8% consistency, 175° F., 1% OP hydrosulfite, and 60 minute retention; the peracetic acid step used 12% consistency, 160° F., 0.2% OP peracetic acid, and 120 minute retention. The hydrosulfite and peracetic acid used were the same as previous examples. Table 3 displays the results of this Example, a GE brightness gain of 5.9, and reveals this Example 14 had a lower GE brightness gain than that of Example 13.

EXAMPLE 15

In Example 15 a sample of starting fibers from Example 11 were bleached with a two step peracetic acid/hydrosulfite sequence as is conventional in the art. The peracetic acid step used 12% consistency, 160° F., 0.2% OP peracetic acid, and 120 minute retention; the hydrosulfite step used 8% retention, 175° F., 1% OP hydrosulfite, and 60 minute retention. The hydrosulfite and peracetic acid used were the same as previous examples. Table 3 displays that results of this Example 15 indicated a GE brightness gain of 8.5. Example 15, a Paa/Y process, displayed a higher GE brightness gain than Example 14, a Y/Paa process, indicating the higher brightness from first completing a peracetic acid step.

Table 5 reveals that of Examples 11-15 representing current or conventional processes, Example 13 had the best GE brightness gain.

TABLE 3
Summary of Examples 11–15
BrightnessBrightness
ExampleSequenceGEGain
Starting Pulp Fibers53.5
11Hypochlorite (H)52.6−0.9
12Hydrosulfite (Y)57.13.6
13Peroxide/Hydrosulfite (P/Y)64.210.7
14Hydrosulfite/Peracetic Acid59.45.9
15Peracetic Acid/Hydrosulfite628.5

EXAMPLE 16

In Example 16 a sample of starting fibers from Example 11 were bleached with a three step peracetic acid/peroxide/hydrosulfite sequence according to the present invention. The activating bleach step used peracetic acid as the at least one activating bleaching agent. That step used 12% consistency, 160° F., 2% OP peracetic acid, and 120 minute retention. The activating bleach step was followed (without a wash) by the alkaline peroxide step, which used hydrogen peroxide as the at least one alkaline peroxide agent and used 12% consistency, 165° F., 2% OP hydrogen peroxide, and 180 minute retention. The alkaline peroxide step was followed (without a wash) by a reductive bleach step, which used sodium hydrosulfite as the at least one reductive bleaching agent and used 8% consistency, 175° F., 1% OP sodium hydrosulfite, and 60 minute retention. The peracetic acid, peroxide, and hydrosulfite used were the same as previous examples. Table 4 displays the results of this Example 16, a GE brightness gain of 16.9. This Example 16 resulted in a higher GE brightness gain than Examples 14-15.

Table 4 further compares the results of Example 13, which showed the highest overall GE brightness gain of Examples 11-13, and Example 16, which showed the highest overall GE brightness gain of Examples 14-16. Table 5 illustrates that the three step bleaching process of Example 16 effected a greater GE brightness gain than the best prior art method, shown by Example 13.

TABLE 4
Example 13 and 16 Brightness Results
BrightnessBrightness
ExampleSequenceGEGain
Starting Pulp Fibers53.5
13Peroxide/Hydrosulfite64.210.7
(Conventional Practice)
16Peracetic Acid/Peroxide/Hydrosulfite70.416.9
(According to the Present Invention)
Difference in GE Brightness Gain from 16 to 136.2

EXAMPLE 17

In Example 17, starting fibers were harvested from a different papermaking system than in Example 11 and were measured to have a GE brightness of 66 and a Kappa number of 16. A sample of these starting pulp fibers were bleached using a two step oxygen peroxide/hydrosulfite sequence according to a conventionally practiced mill system, as shown in FIG. 1. The oxygen peroxide step used 12% consistency, 170° F., 2% OP oxygen peroxide, and 180 minute retention time; the hydrosulfite step used 12% consistency, 170° F., 1% OP hydrosulfite, and 60 minute retention time. The peroxide and hydrosulfite used were the same as previous examples. Table 5 displays the results of this Example 17, indicating a GE brightness gain of 10.6.

EXAMPLE 18

In Example 18 a sample of starting fibers from Example 17 were bleached according to the present invention using peracetic acid, followed by a wash, then bleached with a sequence of oxygen peroxide and hydrosulfite. The activating bleach step used peracetic acid as the at least one activating bleaching agent. That step used 12% consistency, 170° F., 0.2% OP peracetic acid, and 60 minute retention. The activating bleach step was followed by a wash and then by the alkaline peroxide step, which used hydrogen peroxide as the at least one alkaline peroxide agent and used 12% consistency, 175° F., 2% OP hydrogen peroxide, and 180 minute retention. The alkaline peroxide step was followed (without a wash) by a reductive bleach step, which used sodium hydrosulfite as the at least one reductive bleaching agent and used 12% consistency, 170° F., 1% OP sodium hydrosulfite, and 60 minute retention. The peracetic acid, peroxide, and hydrosulfite used were the same as previous examples. Table 5 displays the results of this Example, indicating a GE brightness gain of 13.4. In comparison with Example 17, Example 18 shows the increased brightness displayed according to the present invention.

EXAMPLE 19

In Example 19 a sample of starting fibers from Example 17 were bleached according to the present invention using peracetic acid, not followed by a wash, then bleached with a sequence of oxygen peroxide, a wash, and hydrosulfite. The activating bleach step used peracetic acid as the at least one activating bleaching agent. That step used 12% consistency, 170° F., 0.2% OP peracetic acid, and 60 minute retention. The activating bleach step was followed (without a wash) by the alkaline peroxide step, which used hydrogen peroxide as the at least one alkaline peroxide agent and used 12% consistency, 175° F., 1% OP hydrogen peroxide, and 180 minute retention. The alkaline peroxide step was followed by a wash and then by a reductive bleach step, which used sodium hydrosulfite as the at least one reductive bleaching agent and used 12% consistency, 170° F., 2% OP sodium hydrosulfite, and 60 minute retention. The peracetic acid, peroxide, and hydrosulfite used were the same as previous examples. Table 5 displays the results of this Example, a GE brightness gain of 15.3, and reveals that Example 19 showed the best GE brightness gain of Examples 17-21, particularly in comparison with Example 18. Without wishing to be bound by theory, it is believed that the peracetic acid may act as an activation agent for the subsequent alkaline peroxide bleaching.

EXAMPLE 20

In Example 20 a sample of starting fibers from Example 17 was bleached according to the present invention using an oxygen peroxide/0.2% OP peracetic acid/hydrosulfite sequence. The alkaline peroxide step used oxygen peroxide as the at least one alkaline peroxide agent and used 12% consistency, 170° F., 2% OP hydrogen peroxide, and 180 minute retention. The alkaline peroxide step was followed (without a wash) by the activating bleach step, which used peracetic acid as the at least one activating bleaching agent and used 12% consistency, 170° F., 0.2% OP peracetic acid, and 60 minute retention. The activating bleach step was followed (without a wash) by a reductive bleach step, which used sodium hydrosulfite as the at least one reductive bleaching agent and used 12% consistency, 170° F., 1% OP sodium hydrosulfite, and 60 minute retention. The peroxide, peracetic acid, and hydrosulfite used were the same as previous examples. Table 5 displays the results of this Example 20, indicating a GE brightness gain of 11.5.

EXAMPLE 21

In Example 21 a sample of starting fibers from Example 17 was bleached according to the present invention using an oxygen peroxide/0.4% OP peracetic acid/hydrosulfite sequence. The alkaline peroxide step used oxygen peroxide as the at least one alkaline peroxide agent and used 12% consistency, 170° F., 2% OP hydrogen peroxide, and 180 minute retention. The alkaline peroxide step was followed (without a wash) by the activating bleach step, which used peracetic acid as the at least one activating bleaching agent and used 12% consistency, 170° F., 0.4% OP peracetic acid, and 60 minute retention. The activating bleach step was followed (without a wash) by a reductive bleach step, which used sodium hydrosulfite as the at least one reductive bleaching agent and used 12% consistency, 170° F., 1% OP sodium hydrosulfite, and 60 minute retention. The peroxide, peracetic acid, and hydrosulfite used were the same as previous examples. Table 5 displays the results of this Example 21, indicating a GE brightness gain of 12.4. A double dose of PAA in Example 21 lead to only a slight increase in GE brightness over Example 20.

Table 5 shows that any of the Examples 18-21, with a three-step bleaching process according to the present invention, showed improved brightness over the conventional two-step bleaching practice of Example 17.

TABLE 5
Examples 17–21 Brightness Results
BrightnessBrightness
ExampleSequenceGEGain
Starting Pulp Fibers66
17Oxygen Peroxide/Hydrosulfite76.610.6
(Conventional Practice)
18Peracetic Acid/Wash/79.413.4
Oxygen Peroxide/Hydrosulfite
19Peracetic Acid/Oxygen Peroxide/81.315.3
Wash/Hydrosulfite
20Oxygen Peroxide/0.2% Peracetic77.511.5
Acid/Hydrosulfite
21Oxygen Peroxide/0.4% Peracetic78.412.4
Acid/Hydrosulfite

EXAMPLE 22

In Example 22, starting fibers were harvested from a different papermaking system than Examples 11 and 17 and measured to have a GE brightness of 58.6 and a Kappa number of 12. A sample of those starting fibers was bleached with a two step peroxide/hydrosulfite sequence as is conventional in the art. The peroxide acid step used 12% consistency, 160° F., 2.5% OP hydrogen peroxide, and 120 minute retention; the hydrosulfite step used 8% retention, 165° F., 1.25% OP hydrosulfite, and 60 minute retention. The peroxide and hydrosulfite used were the same as previous examples. Table 6 displays the results of this Example 22, indicating a GE brightness gain of 12.3.

EXAMPLE 23

In Example 23, a sample of starting fibers from Example 22 was bleached with a two step peroxide/hydrosulfite sequence as is conventional in the art. The peroxide acid step used 12% consistency, 160° F., 2.5% OP hydrogen peroxide, and 120 minute retention; the hydrosulfite step used 8% retention, 165° F., 1.875% OP hydrosulfite, and 60 minute retention. The peroxide and hydrosulfite used were the same as previous examples. Table 6 displays the results of this Example 23, indicating a GE brightness gain of 13.5.

EXAMPLE 24

In Example 24, a sample of starting fibers from Example 22 was bleached with a two step peroxide/hydrosulfite sequence as is conventional in the art. The peroxide acid step used 12% consistency, 160° F., 2.5% OP hydrogen peroxide, and 120 minute retention; the hydrosulfite step used 8% retention, 165° F., 1% OP hydrosulfite, and 60 minute retention. The peracetic acid and hydrosulfite used were the same as previous examples. Table 6 displays the results of this Example 24, indicating a GE brightness gain of 12.1.

EXAMPLE 25

In Example 25, a sample of starting fibers from Example 22 was bleached according to the present invention according to the present invention using a peracetic acid/2.5% OP hydrogen peroxide/1.25% OP hydrosulfite sequence. The activating bleach step used peracetic acid as the at least one activating bleaching agent and used 12% consistency, 160° F., 0.2% OP peracetic acid, and 60 minute retention. The activating bleach step was followed (without a wash) by the alkaline peroxide step, which used hydrogen peroxide as the at least one alkaline peroxide agent and used 12% consistency, 160° F., 2.5% OP hydrogen peroxide, and 120 minute retention. The alkaline peroxide step was followed (without a wash) by a reductive bleach step, which used sodium hydrosulfite as the at least one reductive bleaching agent and used 8% consistency, 165° F., 1.25% OP sodium hydrosulfite, and 60 minute retention. The peracetic acid, peroxide, and hydrosulfite used were the same as previous examples. Table 6 displays the results of this Example, indicating a GE brightness gain of 14.4.

EXAMPLE 26

In Example 26, a sample of starting fibers from Example 22 was bleached according to the present invention according to the present invention using a peracetic acid/2.5% OP hydrogen peroxide/1.875% OP hydrosulfite sequence. The activating bleach step used peracetic acid as the at least one activating bleaching agent and used 12% consistency, 160° F., 0.2% OP peracetic acid, and 60 minute retention. The activating bleach step was followed (without a wash) by the alkaline peroxide step, which used hydrogen peroxide as the at least one alkaline peroxide agent and used 12% consistency, 160° F., 2.5% OP hydrogen peroxide, and 120 minute retention. The alkaline peroxide step was followed (without a wash) by a reductive bleach step, which used sodium hydrosulfite as the at least one reductive bleaching agent and used 8% consistency, 165° F., 1.875% OP sodium hydrosulfite, and 60 minute retention. The peracetic acid, peroxide, and hydrosulfite used were the same as previous examples. Table 6 displays the results of this Example 26, indicating a GE brightness gain of 13.4.

EXAMPLE 27

In Example 27, a sample of starting fibers from Example 22 was bleached according to the present invention using a peracetic acid/2.5% OP hydrogen peroxide/2.5% OP hydrosulfite sequence. The activating bleach step used peracetic acid as the at least one activating bleaching agent and used 12% consistency, 160° F., 0.2% OP peracetic acid, and 60 minute retention. The activating bleach step was followed (without a wash) by the alkaline peroxide step, which used hydrogen peroxide as the at least one alkaline peroxide agent and used 12% consistency, 160° F., 2.5% OP hydrogen peroxide, and 120 minute retention. The alkaline peroxide step was followed (without a wash) by a reductive bleach step, which used sodium hydrosulfite as the at least one reductive bleaching agent and used 8% consistency, 165° F., 1.5% OP sodium hydrosulfite, and 60 minute retention. The peracetic acid, peroxide, and hydrosulfite used were the same as previous examples. Table 6 displays the results of this Example 27, indicating a GE brightness gain of 13.7.

Table 6 further reveals that Example 25 possessed the highest GE brightness gain of Examples 22-27. While Examples 23 and 26 shared nearly equal brightness gains, Examples 25 and 27, with the addition of an activating bleach step in accordance with the present invention, showed clearly higher brightness gain over Examples 22 and 24 at the same respective caustic ratios.

TABLE 6
Example 22–27 Brightness Results
CausticBrightnessBrightness
ExampleSequenceRatioGEGain
Starting Pulp Fibers58.6
22Peroxide/Hydrosulfite0.570.912.3
23(Conventional)0.7572.113.5
241.070.712.1
25Peracetic0.573.014.4
26Acid/Peroxide/Hydrosulfite0.7572.013.4
27(Present Invention)1.072.313.7

EXAMPLE 28

In Examples 25 to 27, it was noted that high peroxide residuals were measured where the peracetic acid was applied prior to the peroxide. For Example 28, a sample of starting fibers from Example 22 was bleached with a 0.5% peroxide/hydrosulfite sequence according as is conventional in the art. The peroxide acid step used 12% consistency, 160° F., 0.5% OP hydrogen peroxide, and 120 minute retention; the hydrosulfite step used 8% retention, 175° F., 1.5% OP hydrosulfite, and 60 minute retention. Table 7 displays the results of this Example 28, indicating a GE brightness gain of 4.7.

EXAMPLE 29

In Example 28, a sample of starting fibers from Example 22 was bleached with a 1.0% peroxide/hydrosulfite sequence according as is conventional in the art. The peroxide acid step used 12% consistency, 160° F., 1.0% OP hydrogen peroxide, and 120 minute retention; the hydrosulfite step used 8% retention, 175° F., 1.0% OP hydrosulfite, and 60 minute retention. The peroxide and hydrosulfite used were the same as previous examples. Table 7 displays the results of this Example 29, indicating a GE brightness gain of 6.5.

EXAMPLE 30

In Example 30, a sample of starting fibers from Example 22 was bleached according to the present invention according to the present invention using a 0.1% OP peracetic acid/0.5% OP hydrogen peroxide/0.5% OP hydrosulfite sequence. The activating bleach step used peracetic acid as the at least one activating bleaching agent and used 12% consistency, 160° F., 0.1% OP peracetic acid, and 60 minute retention. The activating bleach step was followed (without a wash) by the alkaline peroxide step, which used hydrogen peroxide as the at least one alkaline peroxide agent and used 12% consistency, 160° F., 0.5% OP hydrogen peroxide, and 120 minute retention. The alkaline peroxide step was followed (without a wash) by a reductive bleach step, which used sodium hydrosulfite as the at least one reductive bleaching agent and used 8% consistency, 175° F., 1.5% OP sodium hydrosulfite, and 60 minute retention. Table 7 displays the results of this Example, indicating a GE brightness gain of 10.4.

EXAMPLE 31

In Example 30, a sample of starting fibers from Example 22 was bleached according to the present invention according to the present invention using a 0.1% OP peracetic acid/1.0% OP hydrogen peroxide/0.5% OP hydrosulfite sequence. The activating bleach step used peracetic acid as the at least one activating bleaching agent and used 12% consistency, 160° F., 0.1% OP peracetic acid, and 60 minute retention. The activating bleach step was followed (without a wash) by the alkaline peroxide step, which used hydrogen peroxide as the at least one alkaline peroxide agent and used 12% consistency, 160° F., 1.0% OP hydrogen peroxide, and 120 minute retention. The alkaline peroxide step was followed (without a wash) by a reductive bleach step, which used sodium hydrosulfite as the at least one reductive bleaching agent and used 8% consistency, 175° F., 1.5% OP sodium hydrosulfite, and 60 minute retention. Table 7 displays the results of this Example, indicating a GE brightness gain of 11.9.

In Examples 28 and 29, it was noted that very high peroxide residuals, on the order of about 50% and greater, were measured than in the peracetic acid bleached samples of Examples 30 and 31. In comparison with Examples 25 and 27, Examples 30 and 31 reveal the synergy of the activating bleach step and the alkaline peroxide step according to the present invention, even at lower peroxide doses.

TABLE 7
Example 28–31 Brightness Results
ExampleSequencePeroxideBrightness GEGain
Starting Pulp Fibers58.6
28Peroxide/Hydrosulfite0.5%63.34.7
29(Comparative)1.0%65.16.5
30Peracetic Acid/0.5%69.010.4
31Peroxide/Hydrosulfite1.0%70.511.9

EXAMPLE 32

In Example 32, starting recycled fibers were harvested from the same papermaking system as those in Example 11 and measured to have a brightness of 66.5 GE and a Kappa number of 18.7. A sample of these starting fibers were bleached according to the present invention by a sequence of peracetic acid+oxygen/peroxide. The activating bleach step used peracetic acid as the at least one activating bleaching agent and used 12% consistency, 170° F., 0.15% OP peracetic acid, and 60 minute retention. The peracetic acid charge was followed (without a wash) by the alkaline peroxide step, which used hydrogen peroxide as the at least one alkaline peroxide agent in combination with sodium hydroxide and used 12% consistency, 170° F., 1.0% OP hydrogen peroxide, and 60 minute retention. No reductive bleach step was used. The peracetic acid and peroxide used were the same as previous examples. Table 8 lists in greater detail the reaction conditions for Example 32 and also displays the results for this Example, indicating a GE brightness gain of 9.2.

EXAMPLE 33

In Example 33, a sample of starting fibers from Example 32 was bleached according to the present invention by the identical sequence and conditions of Example 32, except that oxygen charges were used as follows. After adding the peracetic acid charge, the fibers were sealed in a quantum mixer and pressurized with 60 PSIG oxygen gas for 15 minutes. The pressure was relieved and a second 60 PSIG oxygen charge was added. After 45 total minutes, the oxygen pressure was relieved and followed (without a wash) by the alkaline peroxide step. Table 8 lists in greater detail the reaction conditions for Example 33 and also displays the results for this Example, indicating a GE brightness gain of 9.3. Example 33 revealed that there may be no increase in overall brightness when oxygen is added to the activating bleach stage, and that oxygen may actually decrease the overall brightness gain that may be expected with an activating bleach step comprising peracetic acid.

EXAMPLE 34

In Example 34, a sample of starting fibers from Example 32 was bleached according to the present invention by a sequence of peracetic acid+oxygen/peroxide. The activating bleach step used peracetic acid as the at least one activating bleaching agent and used 12% consistency, 170° F., 0.15% OP peracetic acid, and 60 minute retention. The peracetic acid charge was followed (without a wash) by the alkaline peroxide step, which used hydrogen peroxide as the at least one alkaline peroxide agent in combination with at sodium hydroxide and used 12% consistency, 170° F., 1.0% OP hydrogen peroxide, and 60 minute retention. No reductive bleach step was used. The peracetic acid and peroxide used were the same as previous examples. Table 8 lists in greater detail the reaction conditions for Example 33 and also displays the results for this Example, indicating a GE brightness gain of 9.3.

Table 9 also reveals that the gains in GE brightness after the peracetic acid step for Examples 32 and 34, with oxygen charges, were slightly higher than in Example 33 without oxygen. In addition, the increase in sodium hydroxide in Example 34 with oxygen charges resulted in the same brightness gain as in Example 33 with less sodium hydroxide and no oxygen charges.

TABLE 9
Examples 32–34 Conditions and Results
Activating Bleach Step
Paa DoseOxygenTempStartingFinalInitialPaa
Exp% OPPSI° F.% CpHpHBRBRBR GainResidual Paa
320.150170127.777.1466.567.81.30.00
330.1560170127.987.4166.567.40.90.03
340.150170127.537.4766.568.31.80.02
Alkaline Peroxide Step
PeroxideNaOHTempStartingFinalInitialBRResidual% OP
Exp% OP% OP° F.% CpHpHBRGainH2O2Consumption
321.00.3170129.528.9575.79.20.550.46
331.00.3170129.319.0074.68.10.570.43
341.00.4170129.849.5675.89.30.510.49

EXAMPLE 35

In Example 35, a sample of starting fibers from Example 32 was bleached using a conventional Eop/Y sequence. The reaction conditions and results of this Example 35 are reported in Table 10. Table 10 also reports the results for this Example 35, a GE brightness gain of 8.2.

Table 11 shows the brightness gain for the peroxide and oxygen peroxide steps along with the total peroxide consumption (% peroxide applied minus residual) for Examples 32-35. From this table, it is apparent that the GE gain was about three times higher per unit reacted for the peracetic acid bleached pulps in Examples 32-34, compared to the oxygen peroxide bleached control in Example 35.

TABLE 10
Example 35 Conditions and Results
Temperature170° F.
Consistency12%
Oxygen60 PSI
Peroxide1.6% on pulp
Caustic0.64% on pulp
DTPA0.3% on pulp
Silicate0.02% on pulp
Retention180 minutes
Initial Brightness66.5 GE
Final Brightness74.7 GE
Brightness Gain8.2 GE
Residual Peroxide0.139 g/L

TABLE 11
Peroxide Brightness Gain and Consumption
GE Initial at Peroxide −Peroxide % OPGain/
ExperimentGE Final from PaaConsumedConsumed
327.90.45517.4
337.20.43216.7
347.50.48715.4
358.21.4615.6

EXAMPLE 36

In Example 36, a commercial-scale papermaking system as presented in FIG. 4 was used. The papermaking fibers included recycled fibers. The activating bleaching agent 7 was peracetic acid created from a mixture comprising TAED, peroxide, caustic, water, and chelant in a PAA makedown tank not shown in FIG. 4. The optional oxygen was not added to, and the optional wash did not follow, mixing at the flow discharger/mixer 8. Table 12 presents the agents used and the operational parameters and pulp characteristics measured. The results show that the papermaking fibers experienced, on average, a brightness gain of about 10 points. In addition, the results show a progressive and monotonic decrease in pH at each measurement point, resulting in an average drop in pH of about 3.5.

TABLE 12
AlkalineActivatingReductive
TestPeroxideBleachingBleachingPoint
InformationPoint AAgent 4Point BPoint CAgent 7Point EAgent 13FPoint G
TestBright-H2O2NaOHTempBright-PAAResidualHydrosulfiteTempBright-
NumberDayness% OP% OP° F.pHpHness% OPpHBrightnessH2O2% OP° F.pHness
1168.11.60.3170.510.410.074.20.068.275.80.10.51537.482.2
2170.61.60.3174.010.310.075.90.068.476.40.20.51537.482.3
3262.21.50.2170.59.88.868.00.057.469.60.30.51527.274.5
4675.10.80.6168.510.29.577.50.048.779.60.30.51597.782.7
5675.30.80.6172.09.89.678.40.048.879.70.30.51577.782.7
6870.80.80.6168.510.89.974.20.058.876.20.30.51587.682.1
7870.70.80.6174.010.19.874.00.058.875.50.30.51587.780.6
8973.81.60.8174.510.19.778.10.069.179.30.50.51617.783.1
9974.21.60.8174.510.09.778.50.069.279.50.60.51617.883.4
10 1072.51.61.0178.010.410.178.50.079.679.50.60.51638.084.2
11 1072.71.61.0175.010.010.178.00.079.577.90.50.51628.281.5
12 1368.51.40.8180.010.19.472.90.058.773.70.40.51607.777.8
Average71.21.30.6173.310.29.775.70.18.876.90.40.51587.781.4

Other embodiments of this invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.