WO1995008021A1

WO1995008021A1 - Eop stage for chlorine free bleaching

Info

Publication number: WO1995008021A1
Application number: PCT/US1994/008077
Authority: WO
Inventors: Kaj Henricson; Joseph R. Phillips; J. Robert Prough; Aki Vilpponen; Jian Er Jiang
Original assignee: Kamyr, Inc.
Priority date: 1993-09-15
Filing date: 1994-07-19
Publication date: 1995-03-23
Also published as: AU7243594A

Abstract

A treatment sequence for paper pulp having a Kappa Number of 20 or less (e.g. 14 or less) may be AEopZ (the Z stage may be a (ZP) stage). The treatment sequence may be only the AEopZ sequence, or there can be an intervening ozone stage between the A and Eop stages, and/or subsequent oxidative treatment stages. During the A stage, or other metals removal stage, acid, chelating agent, and/or enzymes may be added. The Eop stage may be practised at superatmospheric pressure during at least part of the stage, at a temperature of 60-95 °C, with an oxygen charge of about 2-6 kg/adt and a H2O2 charge of about 10-20 kg/adt, and with a pH of 9-12, for 2-8 hours.

Description

EOP STAGE FOR CHLORINE FREE BLEACHING

BACKGROUND AND SUMMARY OF THE INVENTION

In the production of bleached kraft pulp there have been increasing regulatory and market demands to reduce or eliminate the presence of chlorinated organic compounds in pulp products and bleaching effluents. In order to properly react to such demands, it is necessary to eliminate the use of chlorine gas or any other chlorine containing compound (e. g. chlorine dioxide). However the removal of the chlorine based bleaching agents makes it extremely difficult to achieve desired brightness levels, especially if the pulp produced is to have acceptable strength properties. For example oxygen delignification must be utilized, and multiple stage oxygen delignification - especially with chelating treatment to control deleterious metal ions - has been shown to offer advantages in delignification and selectivity, especially when there is between-stage washing (see U.S. patent 4,946,556). However there are practical limits governing both the extent of delignification and the quality of pulp obtainable using oxygen delignification alone.

Of other available bleaching agents, perhaps the most promising is ozone. It has been demonstrated that ozone is a highly effective delignifying agent, however ozone has seen only marginal commercial acceptance to date in the pulp industry. The main obstacles to commercial utilization of ozone have been the chemical cost of ozone when large doses are needed, and the degradation of pulp strength that typically occurs relative to conventionally bleached pulps.

Another successful bleaching stage is an Eop stage. This stage can be especially effective following hot alkali extraction, or metals removal, and with a preceding or subsequent Z stage.

Thus, there has long been a need for bleached kraft pulp with commercially acceptable strength properties without the use of chlorine based bleaching agents. According to the present invention, and that of the parent applications, it is possible to finally achieve that goal.

According to the present invention, the basic approach that is taken is to have a pulp with minimum Kappa Number and maximum strength before it is subjected to non-chlorine, bleaching sequences. This is accomplished according to the present invention by utilizing kraft pulp produced from extended delignification processes. Continuous digesters sold by Kamyr, Inc. of Glens Falls, New York under the trademark "EMCC" practice a process known as extended modified continuous cooking (as described in copending application SN 07/583,043 filed September 17, 1990). Such a process can produce softwood pulps having a very low Kappa Number, easily about 18-20, and as low as 14 or less, while maintaining a high pulp viscosity — comparable to that for a conventional kraft (CK) pulp at a Kappa Number of 30. In the practice of the method according to the invention, a hot alkali extraction may be utilized to remove lignin from the pulp after treatment in an "EMCC" digester, metals are typically removed, and then the pulp is subjected to ozone bleaching stages.

According to one aspect of the present invention there is provided a method of chlorine-free bleaching of paper pulp having a Kappa Number of 20 or less, comprising the steps of substantially sequentially: (a) Subjecting the pulp with a Kappa Number of 20 or less to acid washing to remove metals. (b) Bleaching the pulp in an Eop stage. And, (c) bleaching the pulp in a Z stage. The Eop stage may immediately follow the acid wash, without any intervening bleaching stage. The method may consist essentially of steps (a) through (c), that is essentially no further bleaching stages being practiced, the pulp from step (c) being the final pulp produced During the practice of step (b) about 2-6 kilograms of oxygen per air dried ton ("adt") of pulp, preferably about 5 kg, about 10-20 kg hydrogen peroxide/adt, preferably about 15 kg, are provided as the chemical charge. Sufficient alkali is added so that a pH of between 9-12 is provided. Step (b) is practiced at a temperature of about 60-90°C for about 2-8 hours. Step (b) also may be practiced by adding magnesium as a protector, a chelating agent, or both.

The Z stage may be a (ZP) stage, and there may be other oxidative bleaching stages. Step (a) may be practiced so that the pulp initially has a Kappa Number of about 14 or less, and step (b) may be practiced at superatmospheric pressure (e. g. about 25-150 psig) for at least part of the stage.

According to another aspect of the present invention, a method of chlorine-free bleaching paper pulp having a Kappa Number of 20 or less may comprise the following substantially sequential steps: (a) Subjecting the pulp with Kappa Number 20 or less to a metal treatment to remove hydrogen peroxide-consuming metals in the pulp. And, (b) bleaching the pulp in an Eop stage.

Step (a) may be practiced by adding the material selected from the group consisting essentially of acid, a chelating agent, an enzyme, and combinations thereof. The conditions for the Eop stage, and materials added thereto, may be as described above. There may be the further step (c) of, just prior or subsequent to step (b), ozone bleaching the pulp. For example step (c) may be just prior to step (b) and may comprise a Z stage combined with a heavy metal removal step (a), or step (c) may be just subsequent to step (b) and comprise a (ZP) stage. The pulp is typically, although not necessarily, subjected to at least one additional oxidative bleaching stage.

It is the primary object of the present invention to provide for the production of kraft pulp having acceptable brightness (e.g. greater than 90 CPPA) and commercially acceptable strength, without utilizing chlorine containing bleaching compounds. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.

BRIEF DESCRIFΗON OF THE DRAWINGS

FIGURES LA and IB provide a schematic view showing apparatus in which an extended modified continuous cooking process is employed to produce pulp, followed by hot alkali extraction and non-chlorine bleaching sequences;

FIGURE 2 is a schematic view of the wash aspects of the treatment sequence of FIGURES lA & IB showing the liquid flow interconnections between the various stages; and

FIGURES 3-6 are schematic representations of various treatment sequences, including an Eop stage, that may be practiced according to the invention.

DTTATT ED DESCRIPTION OF THE DRAWINGS

An exemplary apparatus for kraft pulping according to the invention is illustrated in FIGURE lA Entirely standard components include a chips bin 10 for receiving a feed of chips or like cellulosic comminuted fibrous material. From the chips bin 10, the material goes to a horizontal steaming vessel 12, and a vertical conduit 13, kraft white liquor from the source 14 being added to the material in the conduit 13 to slurry the material as it is fed to the high pressure feeder 15. From the high pressure feeder 15 the material entrained in kraft white liquor passes to an optional impregnation vessel 16, and then in

5 line 17 to the top 18 of a continuous digester 19. At the top of the digester 19 there is a liquid/material separation device 20, which preferably comprises a plurality of bull screens with switching withdrawal from the screens, or the like. The digester 19 also includes a central portion 21 and a bottom 22. At a portion of the digester 19 between the top 18 and the central 0 portion 21 thereof one or more screens 23, and conduit 24, are provided as means for extracting black liquor from the digester 19. The black liquor is typically passed in conduit 24 to a series of flash tanks, as is conventional. At the central portion 21 of the digester 19, withdrawal screens 27 are also provided, being connected by a withdrawal conduit to a pump 28 and a heater

15 29 for re-introduction of the withdrawn liquid at point 30 of a re-introduction conduit, the point 30 typically being slightly above the screens 27. At the bottom 22 of the digester, kraft pulp is withdrawn in conduit 33 (a scraper or like conventional components can also be utilized), and wash liquor from source 34 is added in introduction conduit 35.

20 Adjacent the bottom of the digester 19 a wash screen (one or more rows) 37 is provided, liquid being withdrawn therethrough in a withdrawal conduit under the influence of pump 38, and then passing through heater 39 into a re-introduction conduit to be introduced at point 40 just above the screen 37. From the point 40 up to the screens 23, liquid flows countercurrently to

25 the material — as designated by arrows 42 — while above the screens 23 it flows cocurrently.

If desired, white liquor having higher sulfidity may be supplied to the vessel 16 than to the digester 19; or at other desired points in the system, the wood chips may first be treated with high sulfidity liquor, and then normal

30 white liquor, such as shown in copending application Serial Number 07/788,151 filed November 5, 1991. That is, white liquor added prior to the beginning of the cooking reaction may have enhanced sulfidity relative to the average sulfidity of the mill's white liquor supply (14), and also preferably has an elevated level of sodium polysulfide. What has been described so far, and including a main conduit 44 for adding kraft white liquor in conduit 45 to the first recirculation loop at digester central portion 21, is conventional in the Kamyr MCC™ system. As described in copending application Serial No. 07/583,043, in accordance with the present invention, a conduit 46 is provided for interconnecting the main conduit 44 to the withdrawal conduit in the second wash recirculation loop provided by elements 37 through 40.

In the preferred process, sufficient kraft white liquor, and additional heat, are added in conduit 46 so as to achieve significantly enhanced viscosity and strength properties of the pulp produced compared to the practice of the same method without the introduction of kraft white liquor in conduit 46. For example, the amount of kraft white liquor added in conduit 46 is at least about 5% of the total amount of kraft liquor utilized to effect kraft pulping, and typically is about 10-20% (e.g. about 15%). Kraft white liquor preferably is also added — as is known per se - in conduit 45 in addition to conduit 46, the amount added in conduit 45 being at least about 10%, and preferably about 10-20%.

The apparatus of FIGURE 1A also includes a recirculation loop (not shown) at a central portion — indicated generally by reference numeral 49 — of the impregnation vessel 16, including a screen, withdrawal conduit, pump, and re-introduction conduit/point. Kraft white liquor from source 14 also is preferably added as indicated by lines or points 52, 53, 54 to the recirculation loop in the impregnation vessel 16, and to the recirculation conduits from the digester to the impregnation vessel 16, and from the impregnation vessel 16 to the high pressure feeder 15. Normally the majority of the white liquor used in the conventional continuous kraft pulping process is added at the points or conduits 52 - 54. The extended delignification pulp in line 33 is then subjected to an optional further wash stage 58, and other treatments.

After optional wash stage 58, the pulp - having a Kappa Number of 20 or less (e.g. 14 or less) — passes to a hot alkali extraction vessel/stage 60. Hot alkali extraction (HAE) is shown per se in U.S. patent 5,203,963 (the disclosure of which is hereby incorporated by reference herein), and includes treatment of the pulp with a mild alkali solution at a temperature of about 150°C for a time sufficient to extract residual lignin from the pulp (typically about thirty minutes or less). The pulp may pass from HAE stage 60 to a pressure difϊuser 61 or the like (e. g. a DD™ washer sold by Ahlstrom Recovery of Roswell, Georgia) for washing. In the diffύser 61, white liquor may be added to enhance delignification, as shown in U. S. Patent 5,021,127. The pulp may then pass to oxygen stages 62, 63, which may be of the type shown per se in U.S. patent 4,946,556. At this point, the Kappa number of the pulp has been reduced at least between about 10 and 5 units compared to CK softwood pulp, and between about 5 and 2 units compared to CK hardwood pulp.

After the optional second (or subsequent) oxygen stage 63, the pulp passes to a pressure relief vessel 64, and then to diffusion washer 65 or the like on top of vessel 66. Acid and enzymes may be added to the pulp in the tank 66 to facilitate removal of heavy metals in a subsequent acid wash stage. Separation of undesirable particles and screening take place at 67, 68, typically at a consistency of about 3-4 % and under pressure. Alternatively, the screening system (like structures 67, 68) may be located in front of the oxygen vessels 62, 63 in the process sequence.

In the acid wash stage 69 (FIGURE IB), some of the wash filtrate is removed and passes to cooking liquor preparation 70. This removed filtrate contains heavy metals that would consume bleaching chemical, and is accomplished to prevent buildup of heavy metals in the pulp. The pulp then is ready to be bleached with non-chlorine (e.g. chlorine and chlorine dioxide free) bleaching chemicals, yet it will produce pulp comparable to CK pulp produced by a DEDED sequence.

Although chlorine free treatment is vastly preferred, according to the invention better quality pulp can also be produced by using the EMCC™ treatment followed by hot alkali extraction.

The first bleaching stage is preferably an ozone (Z) stage. 71, which is practiced with the pulp at medium consistency (i.e. about 5-18%). This Z stage is preferably practiced as disclosed in copending application Serial No. 07/498,205 filed March 23, 1990, the disclosure of which is hereby incorporated by reference herein, the ozone in an oxygen carrier gas being introduced under super atmospheric pressure, typically with about 2-14% of the introduced gas comprising ozone.

After ozone stage 71, the pulp passes to tanks 72 or the like, is washed as indicated at 73, and then subjected to an extraction stage (E) in vessels 74 and/or 75. iMtematively, there can be no washing between 71, 74, i.e. a (ZE) stage instead of ZE stages. A (ZE) stage is known per se from copending U. S. application Serial Number 07/721,780 filed June 28, 1991. The vessels 74, 75 typically provide an oxidative peroxide extraction stage (sometimes designated Eop). After the extraction stage wash sequence at 76, bleaching preferably continues in Z stage 77 (comparable to stage 71), which is connected to tank 78 which maintains the pulp in contact with added peroxide, and after the ozone stage 77 and washing in washer 79, the pulp passes to peroxide (P) stage 80.

After P stage 80 washing in washer 81, the pulp may pass to another, optional, Z stage 82 (the same as stages 71, 77, with medium consistency treatment), and from storage tanks 83 pulp may be withdrawn in the bleached pulp discharge line 84. The pulp in line 84 has properties comparable to conventional (CK) pulp subjected to a DEDED sequence. The bleaching chemical consumption is also comparable to a DEDED sequence. The following Table I provides an analysis of expected chemical consumption from the OOAZEZPZ sequence described above, while Table II indicates the expected Kappa number and yield loss associated with each stage of the sequence.

TABLE I Ozone bleaching - chemical consumption

Stage 0 A Z P Z 1

^E%

Oxygen kg/adt 15 5 2

Ozone kg/adt 4 2 2

Peroxide kg/adt 15 5 5

Ox. White liquor kg/adt 12

Alkali kg/adt 18 6 6

H₂S0₄ kg/adt 7 3 1

MgS0₄ kg/adt 3 1 1 1

Na₂EDTA kg/adt 1.5 0.5 0.5 0.5

TABLE II

Process conditions The oxygen (with small amounts of ozone) gas from the various stages 71, 77, 82 is captured in an oxygen recovery system 84'. After scrubbing to remove fiber, and after ozone destruction, some of the recycled oxygen may be passed to oxygen bleaching (e.g. stages 62, 63) via line 85', while the rest, in

5 line 86', passes to the atmosphere or other uses within the pulp mill, or is recycled to the ozone production system.

FIGURE 2 illustrates details of the washing sequences associated with and or between the various stages described with respect to FIGURES 1A and IB. The pulp in line 85 is from oxygen delignification, and is further washed

10 in diffusion washer 86, with liquid return via line 87 to the brown stock washer. The washed pulp from diffusion washer 86 passes to acid washer 88 (comparable to 69 in FIGURE IB), and is supplied with fresh water at 89. The filtrate in line 90, which comprises a variable, but significant, amount of the total filtrate from washer 88, is sent to cooking liquor preparation (e.g. 70 in

15 FIGURE IB), to prevent buildup of heavy metals.

From washer 88, the pulp ultimately flows to washer 91 (comparable to 73 in FIGURE IB) after the first ozone stage, with wash liquid from that washer being fed by line 92 to the washer 88 to provide part - with the fresh water added at 89 — of the wash liquid thereto. The pulp then ultimately flows

20 to the washer 93 after the extraction stage, with recycle via line 94 back to the washer 91, and as the wash liquid for diffusion washer 86. Fresh water also provides part of the wash water to washer 93 (comparable to 76 in FIGURE IB).

From washer 93 the pulp ultimately passes to washer 95 (comparable to

25 79 in FIGURE IB) after the next Z stage, with recycle via line 96, and then to washer 97 (comparable to 81 in FIGURE IB) after the P stage, with recycle via line 98. Fresh water is added at 99 to the washer 97 as the wash liquid. The pulp discharged in line 100 may pass to a final Z stage, and then becomes the final pulp (in line 84 from FIGURE IB). The washers 69, 73, 76, 79, 81, 88, 91, 93, 95, and 97 may be drums, diffusers, belts, presses, or the like, as long as they provide adequate washing efficiency. The washer 61 must be a pressurized washer.

A portion of the washing filtrate from lines 92, 94, 96, 98, 101, 102, and/or 103 may be purged from the system to the acid or caustic sewer in order to maintain acceptable dissolved solids and metals concentration levels in the system.

FIGURE 3 illustrates a particular sequence generally as described above, only in which the acid stage 110 to which low Kappa Number pulp (e.g. 20 or below, 18 or below, or preferably 14 or below) is supplied followed immediately by the Eop stage 111, with no intervening bleaching stage. The Eop stage 111 may be followed (after washing) by a Z or (ZP) stage 112, which, after washing, produces the final bleached pulp, or, optionally, as followed by other oxidative stages (typically peroxide or ozone stages). Acid (e. g. sulfuric acid) and a chelating agent (e. g. EDTA) may be added at 110, as seen for the A stage in TABLE I. The acid wash stage 110 is like the stages 69, 88 earlier described, and is one way to remove bleaching chemical- consuming metals.

FIGURE 4 schematically illustrates another treatment sequence in which a bleach chemical-consuming metal removal stage 114 (either a T or Q stage) is followed by an Eop stage 111, and then a Z or (ZP) stage 112, producing a final bleached pulp. In the stage 114, acid, such as sulfuric acid, a chelating agent, such as Na₂EDTA,- and/or enzymes are added to remove metals, such as iron, copper, etc., which will consume hydrogen peroxide bleaching chemical in the Eop stage 111.

FIGURE 5 illustrates a sequence similar to that in FIGURE 4 in which a metals removal stage 110, 114 is followed by an Eop stage 111 and an optional additional oxidative bleaching stage 115, with an ozone or (ZP) stage 112 between the stages 110, 114 and 111. FIGURE 6 illustrates a bleaching sequence similar to that of FIGURE 5 except that the metals removal and ozone stages 110, 114 and 112 are combined in the stage 116 (that is there is no washing between the metals removal and the ozone treatments). The conditions during the Eop stage may vary, and the stage 111 may be practiced at atmospheric pressure or at superatmospheric pressure (e.g. 2 bar or more, more particularly about 25-150 psig), e.g. being at least partially pressurized (that is being pressurized during part of the treatment). A typical oxygen charge during stage 111 is 2-6 kg oxygen/adt (e.g. 5 kg/adt - see Table I), while a typical peroxide charge is about 10-20 kg adt, for example about 15 kg adt (see Table I). Alkali is added as necessary (see Table I) so that the pH during stage 111 is preferably in the range of 9-12. The temperature during the practice of stage 111 is preferably between 60-95°C, and the treatment time is typically 2-8 hours. Magnesium may also be added during the stage 111, for example in the form of MgSO₄ (see Table I) as a protector, and a chelating agent such as Na₂EDTA (see Table I) may alternatively or also be added at stage 111.

It will thus be seen that according to the present invention a high brightness, high strength bleached kraft pulp can be produced without chlorine based bleaching compounds. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent processes and methods.

Claims

WHAT IS CLAIMED IS:

A method of chlorine-free bleaching paper pulp having a Kappa Number of 20 or less, comprising the steps of substantially sequentially: (a) subjecting the pulp with a Kappa Number of 20 or less to acid washing to remove metals; (b) bleaching the pulp in an Eop stage; and (c) bleaching the pulp in a Z stage.

2. A method as recited in claim 1 wherein the Eop stage immediately follows step (a), without any intervening bleaching stage.

3. A method as recited in claim 1 consisting essentially of steps (a)-(c), essentially no further bleaching stages being practiced, the pulp from step (c) being the final pulp produced.

4. A method as recited in claim 1 wherein during the practice of step (b) about 2-6 kg oxygen/adt, and about 10-20 kg hydrogen peroxide/adt are used, and step (b) is practiced at a temperature of about 60-95 degrees C and a pH of 9-12, for about 2-8 hours.

5. A method as recited in claim 3 wherein step (a) is practiced by adding with sulfuric acid and a chelating agent to the pulp.

6. A method as recited in claim 3 wherein step (b) is further practiced by adding magnesium as a protector, or a chelating agent, or both.

7. A method as recited in claim 3 wherein the Z stage is a (ZP) stage.

8. A method as recited in claim 1 wherein the pulp introduced in step (a) initially has a Kappa Number of about 14 or less.

9. A method as recited in claim 1 wherein step (b) is practiced at superatmospheric pressure for at least part of the stage.

10. A method of chlorine free bleaching paper pulp having a Kappa Number of 20 or less comprising the steps of substantially sequentially: (a) subjecting the pulp with Kappa Number 20 or less to a metal treatment to remove hydrogen peroxide-coiisuming metals in the pulp; and (b) bleaching the pulp in an Eop stage.

11. A method as recited in claim 10 wherein step (a) is practiced by adding a material selected from the group consisting essentially of acid, a chelating agent, an enzyme, and combinations thereof.

12. A method as recited in claim 10 wherein step (b) is practiced at superatmospheric pressure for at least part of the stage.

13. A method as recited in claim 10 comprising the further step (c) of, just prior or subsequent to step (b), ozone bleaching the pulp.

14. A method as recited in claim 13 wherein step (c) is just prior to step (b), and comprises a Z stage combined with the heavy metal removal step (a).

15. A method as recited in claim 13 wherein step (c) is just subsequent to step (b), and comprises a (ZP) stage.

16. A method as recited in claim 10 comprising the further step of subjecting the pulp to at least one additional oxidative bleaching stage.

17. A method as recited in claim 10 wherein the introduced in step (a) has a Kappa Number of about 14 or less.

18. A method as recited in claim 17 wherein step (b) is practiced with an oxygen charge of about 2-6 kg adt, and a peroxide charge of about 10-20 kg/adt, and wherein a magnesium protector, or a chelating agent, or both are added.

19. A method as recited in claim 10 wherein during the practice of step (b) about 2-6 kg oxygen/adt, and about 10-20 kg hydrogen peroxide/adt are used, and step (b) is practiced at a temperature of about 60-95 degrees C and a pH between 9 and 12, for about 2-8 hours.

20. A method as recited in claim 11 wherein step (a) is practiced by adding sulfuric acid during acid washing of the pulp.