WO1988001661A1

WO1988001661A1 - Pulp bleaching process

Info

Publication number: WO1988001661A1
Application number: PCT/US1987/002130
Authority: WO
Inventors: John K. Rogers; Ronald K. Baysden
Original assignee: James River-Norwalk, Inc.
Priority date: 1986-08-28
Filing date: 1987-08-25
Publication date: 1988-03-10
Also published as: FI881930A0; FI881930A; EP0279845A4; EP0279845A1

Abstract

A multistage pump bleaching process comprising at least three bleaching stages including oxygen extraction (20) in an alkaline medium (EO) of a chlorine (C) or chlorine/chlorine-dioxide (CD) bleaching step (5) followed by a hot alkaline hypochlorite treatment (H*/E), (28) or followed by a chlorine dioxide (D) treatment with simultaneous treatment with chlorine dioxide (H*/ED) in the sequence EOH*/ED or EO(H*/ED). The novel bleaching sequences result in significant savings in time and operating costs as compared with conventional bleaching sequences capable of producing products of comparable brightness.

Description

PULP BLEACHING PROCESS

This invention relates to a multistage process for bleaching alkaline digested cooked pulps. such as wood pulps prepared by the kraft. alkaline sulphite and soda precesses. The process ot this invention comprises a sequence of delighification and bleaching stages in which the initial stage involves treatment with chlorine (C) or chlorine and chlorine dioxide (C_D) and in which the final stage comprises a chlorine diixide

(D) bleaching step. In the process of this invention, the initial chlorine (C) or chlorine/chlorine dioxide

(C_D) bleaching stage is followed sequentially by a combined alkali extraction/oxygen bleaching step (E_O) and a hot hypochlorite extraction step (H*/E), followed by chlorine dioxide (D) bleaching. In a preferred embodiment, chlorine dioxide bleaching is combined with the hot hypochlorite treatment (H*/ED).

Although the various treatment steps are known, per se, including the treatment with oxygen in an alkaline medium, the. combination of alkali extraction coupled with an oxygen bleaching step, followed by hot hypochlorite/alkali extraction is believed to be novel. The benefits of the particular combinations of steps forming the basis of this invention will be apparent in the light of the following more detailed description of the process.

A typical bleaching sequence in present practice, forming a part of the prior art with respect to the present invention, involves the sequence of steps C_DEHD wherein C_D represents treatment of an aqueous slurry of wood pulp with a mixture of chlorine and chlorine dioxide, E represents an alkali extraction step (usually with sodium hydroxide) , H represents a hypochlorite treatment step, and D represents treatment with chlorine dioxide.

The conversion of wood chips into wood pulp sui table for the manufacture of paper products generally consists of a series of chemical treatments necessary for the separation of relatively pure cellulose fibers from the various non-cellulosic components of the wood, most notably lignins. In the manufacture of high grade paper products, particularly highly absorbent tissue and the like, it i s necessary to separate the lignin ini ti ally combined with wood fibers f rom the cellulose fibers with minimal degradation of the cellulose fibers. In practice , raw wood, either softwood or hardwood, is first reduced to chips, loaded into a pressure vessel and digested or cooked in an aqueous chemical solution under conditions of elevated temperature and pressure dissolving at least a part of the lignin and liberating the wood fibers. The cellulose fibers are dark in color due to the presence of lignin which has not been removed completely from the fibers by the digestion process. The pulp resulting from digestion is referred to as unbleached pulp. unbleached pulp may be used directly in the manufacture of kraft paper useful in the production of paper bags, corrugated board, and the like.

Other grades of paper, particularly high grade facial tissue, toilet tissue, towels, and the like require bleaching of the pulp to produce a colorless or white product. A number of processes have been developed and utilized heretofore for bleaching of wood pulp. Among the more common bleaching process steps which are utilized commercially are chlorination, alkaline extraction , hypochlorite bleaching, and chlorine dioxide bl eaching operations. More recently, oxygen bleaching methods have Seen developed to replace or supplement chlorine bleaching operations. As the bleaching reagents are mostly oxidative in nature, the bleaching steps are carried out under mild reaction conditions to minimize oxidation and consequent degradation of the cellulose fibers .

The first step in a multistage bleaching process is usually a chlorination step in which pulp from a kraft digestion process, after water washing, is contacted with chlorine at a temperature in the range of about 20 to 60C for a period of time within the range of about 10 to 60 minutes at a pulp consistency ranging from 3 to 10 weight percent. The initial chlorination step, by chlorinating, oxidizing or otherwise solubilizing the lignins, aids in the subsequent removal of lignin from the pul p from the chlorination system and in a subsequent alkaline extraction step. The chlorination step may comprise treatment with a mixture of chlorine and chlorine dioxide and is usually followed by caustic extraction with a sodium hydroxide solution at a pulp consistency in the range of 8 to 14 weight percent for one hour or longer at a temperature in the range of 50 to 70C. A hypochlorite treatment step in which the pulp is treated with sodium hypochlorite at about the same pulp consistency and a temperature in the range of 35 to 40C commonly follows the caustic extraction step, and sometimes is combined with the caustic extraction step. Finally, chlorine dioxide bleaching at 75 to 85C is commonly used with kraft pulps to bring the brightness to the range of about 85 to 90% GE. Brightness of the pulp is a measure of the reflectance of light in the blue range (457 mμ ) compared with magnesium oxide as a standard. Brightness standard scales vary in different countries depending upon the standard light meter employed in determining brightness. In the united States, the General Electric meter is standard and brightness is reported as percent GE. For very high quality tissue, a pulp brightness of 85% GE or higher is desirable.

Pulp supplied directly from a kraft digestion process, after washing, typically has a GE brightness of 13 to 30% whereas highly bleached pulp has a GE brightness in the range of 85 to 92%.

As reported in the following examples, brightness was measured by a standard ISO meter; ISO brightness values are slightly lower than corresponding GE values. The Kappa number is a standard measure of lignin content. The Kappa number is determined by the amount of potassium permanganate consumed by a standard sample of pulp and represents a measurement of its retained lignin content. Higher Kappa numbers indicate the presence of greater amounts of lignin and vice versa. The extent of delignification which occurs in a bleaching step may be evaluated by comparison of Kappa numbers of samples taken before and after the bleaching step. Kappa numbers are commonly used as a measure of the lignin content of the cooked pulp prior to bleaching and of preoxidized and chlorine treated pulps. Brightness numbers are more generally used to determine the extent of removal of color bodies, including lignins and their degradation compounds, in the later stages of a bleaching sequence. Chlorine dioxide bleaching is a highly desirable process step in that it results in a high brightness coupled with good color stability without significantly deteriorating pulp strength. Chlorine dioxide is essentially non-degrading to cellulose and is capable of producing very bright pulp without degrading the cellulose or hemlcelluloses. Due to the highly corrosive and toxic nature of chlorine dioxide, it is generated at the pulp mill as a gas and then absorbed in water. The resulting solution is then brought in contact with the pulp. The reaction is conducted at a temperature in the range of 60 to 80C for two to six hours at a pH of about 4 and a pulp consistency of about 8 to 14 percent. In a typical CEDED sequence, a second caustic extraction followed by second chlorine dioxide stage is employed to bring kraft pulp brightness to the desired level. A chlorine dioxide bleaching step may be used as the first step in the bleaching sequence, but more commonly, it is used in the first step in combination with chlorination and is usually followed by caustic extraction or a hypochlorite treatment.

Oxygen bleaching usually has been employed as a prebleach stage before chlorination, oxygen bleaching being compatible with spent liquors from the kraft recovery system. In such operations, with pulp consistencies of 10 to 35 weight percent and 2 to 4 weight percent concentration sodium hydroxide (on pulp) at 85 to 125C for a period of 20 to 40 minutes, the lignin content of soft wood kraft pulp may be reduced from about 6 weight percent to about 3 weight percent. Oxygen bleaching is usually carried out in the presence of a magnesium salt, e.g. magnesium carbonate, to prevent oxidative degradation of the pulp. More recently, methods have been proposed for oxygen bleaching at pulp consistencies of the order of 2 to 3 weight percent in an aqueous alkaline suspension of 0.1 normal sodium hydroxide at a temperature in the range of 70 to 120C and a pH in the range of 9 to 14 at a pressure in the range of 40 to 135 psig for one hour without the need for the use of a magnesium salt to prevent depolymerization of the cellulose during the oxygen treatment step. The process is disclosed in U.S. 3,832,276, incorporated herein by reference, as a replacement for conventional chlorine delignification and extraction treatment, referred to in the paper industry as the C_DE stages of a bleaching sequence, the symbol C_D indicating chlorination with a mixture of chlorine and chlorine dioxide and the symbol E indicating sodium hydroxide extraction of the chlorinated pulp.

We have now found, unexpectedly, that improved bleaching of alkaline pulps, such as kraft pulps, may be effected by a sequence in which treatment of the pulp with a mixture of chlorine and chlorine dioxide (C_D) is followed by the sequence of sodium hydroxide extraction in the presence of oxygen (E_O) and hot hypochlorite/sodium hydroxide treatment (H*/E) and finally, in a preferred embodiment, completed in a chlorine dioxide (D) step in the sequence C_DE_O(H*/E)D. In another preferred embodiment, chlorine dioxide is employed in connection with hot hypochlorite/sodium hydroxide treatment (H*/ED) as the final step in the bleaching sequence C_DE_O(H*/ED). Although the individual processing steps are known in the art, insofar as we have been able to determine, the sequences E_O(H*/E) and E_O(H*/ED) are novel. These sequences are particularly advantageous when substituted for existing EH sequences in a typical C_DEHD sequence or C_DEHED sequence in conventional process bleaching operations as will be more fully described hereinafter.

In the process of our invention, the extraction step and hypochlorite bleaching steps common in existing bleaching sequences are modified to achieve chemical and energy savings as well as savings in capital expenditures through the use of lower dosages of reagents and shorter treating time sequences while maintaining pulp strength. By combining oxidative extraction with hot hypochlorite bleaching in accordance with the method of this invention, improved pulp brightness may be obtained as well. Preferred embodiments of the process of this application are illustrated in the accompanying drawings illustrating its application to conventional plant apparatus.

Fig. 1 is a flow diagram illustrating one embodiment of the present invention; and

Fig. 2 is a flow diagram illustrating another embodiment of this invention.

With reference to Fig. 1 of the drawings, unbleached kraft pulp at a consistency of about 3 weight percent wood fibers on an air dried basis is supplied from make-up tank 4 to chlorine-chlorine dioxide bleaching tower 5 by pump 6. Chlorine and chlorine dioxide are introduced into the bleaching tower 5 from line 7. Bleaching conditions in tower 5 are preferably 15 to 30 minutes residence time at a temperature in the range of

40 to 50C (about 100 to 125F) with about 3 to 7 weight percent chlorine and 0.2 to 0.25 weight percent chlorine dioxide, basis the air dried weight of the pulp. The pulp stream from tower 5 is passed through line 9 to washer 10. After washing, pulp from washer 10 is passed through line 11 to chemical/steam mixer 12 where it is heated and mixed with an aqueous solution of sodium hydroxide from line 13. As is well known in the art, other alkaline agents, especially sodium carbonate or ammonium hydroxide, may be employed in place of sodium hydroxide for extraction of liqnins from kraft pulp. In the preferred examples of this invention, sodium hydroxide is the preferred alkaline agent and in this stage is employed in an amount within the range of 2 to 5 weight percent basis the air dried (a.d.) weight of the pulp, at a pulp consistency of about 12 weight percent.

The alkaline pulp slurry from chemical/steam mixer 12 is passed through line 14 to a high shear mixing device 16. Oxygen in an amount equivalent to about 0.5 weight percent basis air dried pulp is supplied to the mixing device 16 from line 18 and thoroughly dispersed in the alkaline slurry. The resulting oxygenated alkaline pulp slurry is passed through transfer line reactor 19 to tower 20 where further delignification of the pulp takes place. Preferred conditions in the E_O stage are a minimum of 3 minutes at 20 psig or greater in transfer line reactor 19, followed by 60 to 120 minutes of retention time at 50 to 65C in the extraction tower 20. Pulp slurry is withdrawn from the bottom of the alkaline extraction tower 20 and passed by pump 21 through line 22 to washer 23.

Washed pulp from washer 23 is passed through line 24, mixed with aqueous sodium hydroxide and sodium hypochlorite from line 26, heated by means not illustrated and passed by pump 27 through reaction tube 28 where further amounts of lignin are removed from the cellulose by a hot caustic hypochlorite extraction. Preferred reaction conditions in the H*/E stage reaction tube 28 are in the ranges of 70 to 75C (160 to 17 OF) for a period of about 10 minutes with from about 0.7 to about 1.1 weight percent sodium hypochlorite and 0.4 to 0.6 weight percent sodium hydroxide basis air dried pulp.

From the hot hypochlorite extraction (H*/E) tube 28 the delignified pulp is passed through line 29 to washer 30. Washed delignified pulp from washer 30 is passed through line 31, mixed with chlorine dioxide from line 32, and passed to tower 40 where the final chlorine dioxide (D) bleach takes place. Preferred operating conditions in bleach tower 40 include a pulp consistency of about 12 weight percent, a holding time of 120 to 180 minutes at about 70C, and a dosage of chlorine dioxide in the range of 0.35 to 0.9 weight percent basis the air dried weight of the pulp, optionally including up to about 0.2 weight percent sodium hydroxide.

The bl eached pulp sl urry from the chlor ine dioxide (D ) bleach stage is withdrawn from the top of tower 40 and passed by pump 41 through l ine 42 to washer 43 .

Bleached pulp is discharged from washer 43 through l ine 44 to pump 45 and passed through l ine 46 to storage , not illustrated.

With ref erence to Fig. 2 , wherein l ike reference numerals indicate like elements of the system illustrated di agrammatically in Fig. 1 , the washer 30 of Fig. 1 is eliminated and the hot hypochlorite extraction H*/E of Fig. 1 combined with chlorine dioxide bleaching D in a H*/ED stage. As illustrated in Fig. 2, pulp from the extraction tower 20, after washing in washer 23, is passed through line 24, mixed with aqueous sodium hypochlorite and sodium hydroxide from line 26, and passed by pump 47 into the bottom of transfer line reactor 48 having a hold up time of about 5 to 10 minutes. From transfer line reactor 48, the entire pulp slurry is passed through line 49 to pump 50, ehlorine dioxide is added to the. hot hypochlorite mixture from line 51, and the mixture pumped directly into the bottom of tower 52 where the hot hypochlorite/chlorine dioxide bleaching (H*/ED) operation takes place. Preferred operating conditions in tower 52 include a temperature of about 70C and a holding time of 120 to 180 minutes with from 0.5 to 0.9 weight percent sodium hypochlorite, 0.2 to0.4 weight percent sodium hydroxide, and about 0.4 weight percent chlorine dioxide basis on the air dried weight of the pulp in the charge to the tower. Bleached pulp is discharged from the top of tower 52 through line 53, washed in washer 54, and discharged by pump 55 through line 56 to storage, not illustrated.

The advantages of the present invention are illustrated in the following examples.

EXAMPLES

Samples of softwood and hardwood pulps from the James River Naheola Mill, Pennington, Alabama, were treated in sequences comprising a chlorination treatment with mixtures of chlorine and chlorine dioxide C_D; sodium hydroxide extraction with the addition of oxygen E_O ; and hot sodium hypochlorite-sodium hydroxide extraction H*/E followed by or combined with chlorine dioxide D treatment, i. e. C_DE_Q (H*/E) D (Sequence B) and C_DE_O(H*/ED) (Sequence C). As a standard for comparison, a conventional mill sequence (Sequence A) of chlorine-chlorine dioxide C_D sodium hydroxide extraction E; hypochlorite H; and chlorine dioxide D treatment steps, i.e., C_DEHD, were applied to part of the samples.

To facilitate a comparison between the bleaching sequences, samples of the unbleached softwood and hardwood kraft pulps were bleached to 86.8% and 83.3% ISO brightness, respectively in a C_DEHD sequence as reported in Tables II and IV. The chemical charges, consumption and properties during and after bleaching are listed in Table II (softwood pulp) and Table IV (hardwood pulp).

EXAMPLES 1 TO 3 SOFTWOOD KRAFT PULP

Samples of softwood pulp, having Kappa number of 29.9, and a 0.5% CED viscosity of 26.9 mPa.s were bleached in a conventional sequence (Sequence A) in Example 1, and by sequences of this invention. Sequence B in Example 2 and Sequence C in Example 3. The treatment steps and process conditions for these examples are shown in Table I. In the examples. Sequence C differs from

Sequence B in omitting washing between stages H*/E and D, effectively combining these stages into a single H*/ED stage.

(1) Partial pressure of oxygen in the E_O stage was 1.36 atm supplied for 3 minutes of the extraction stage.

The unbleached pulp had a Kappa number of 29.9 and a viscosity of 26.9 mPa.s. Results of the tests are summarized in Table II.

(1) No wash between stages H*/E and D

It will be noted from the above examples that the sequences of this invention resulted in improved pulp brightness without significant degradation of the pulp as indicated by viscosity. The sequence E_O followed by a hot hypochlorite stage H*/E at 160F (71C) for 10 minutes (Examples 2 and 3) is as effective as the more conventional sequence of Example 1 in which the hypochlorite stage is carried out at 100F (38C) for 90 minutes. Example 3 demonstrates that the wash step may be omitted between the H*/E stage and the D stage without significant loss of brightness. EXAMPLES 4 TO 7 HARDWOOD KRAFT PULP

Samples of hardwood pulp having a Kappa number of 14.8 were bleached in a conventional mill sequence (Sequence A) and by methods of this invention (Sequences B and

C). In the examples. Sequence C differs from Sequence B in omitting the wash step between stages H*/E and D. Treatment steps and process conditions for these examples are shown in Table III with results of the tests reported in Table IV, Examples 4 to 7.

(1) No Wash between H*/E and D

It will be noted from these examples that Sequence B of Example 5 produces pulp of significantly improved brightness as compared with Sequence A of Example 4 with a savings in both sodium hypochlorite requirements and in time of treatment. Example 6 demonstrates that, for equivalent pulp brightness, less chlorine dioxide is required also even when the wash step between H*/E and D is omitted. Example 7 demonstrates that Sequence C is capable of producing a pulp brightness comparable to that of Sequence B (Example 5) with a small increase in the sodium hypochlorite requirement in the H*/E stage while still saving approximately half that required in Sequence A (Example 4).

Claims

1. In a multistage process for bleaching alkaline wood pulps wherein the pulp is sequentially treated with an aqueous chlorine or chlorine and chlorine dioxide solution, an aqueous alkaline solution, a hypochlorite solution, and an oxidizing bleach solution of chlorine dioxide, the improvement which comprises subjecting a chlorine or chlorine and chlorine dioxide pretreated pulp to extraction with an aqueous alkaline solution in the presence of oxygen, and subjecting the resulting pulp after washing to an alkaline hypochlorite treatment at a temperature in the range of 60 to 90C.

2. In a multistage process for bleaching a chlorine or chlorine/chlorine dioxide preoxidized wood pulp to produce a pulp having a very high degree of whiteness in a limited number of treatment steps without degrading the quality of the pulp, the combination of successive steps comprising a) oxidative extraction with oxygen and an alkali at a temperature in the range of 60 to 70C for a period of time within the range of 20 minutes to one hour at a pulp slurry consistency in the range of 8 to 14 weight percent; b) washing the pulp from step a); c) hot hypochlorite bleaching of the washed pulp from step b) in the presence of an alkali at a temperature in the range of 65 to 90C for a period of time in the range of 5 to 15 minutes with a pulp slurry consistency in the range of 8 to 14 weight percent; d) washing the pulp from step c); e) bleaching the washed pulp from step c) with chlorine dioxide at a temperature in the range of about 60 and 90C for a period of time in the range of about 1 to about 3 hours.

3. A process as defined in Claim 2 wherein the pulp slurry from hot hypochlorite bleaching stage c) is charged to the chlorine dioxide bleaching stage e) without an intermediate washing step d).

4. A process according to Claim 2 wherein unbleached kraft pulp is first treated with a mixture of chlorine and chlorine dioxide at a temperature in the range of 20 to 60C for a period of time in the range of 30 to 60 minutes at a pulp consistency in the range of 3 to 10 weight percent with an amount of chlorine in the range of 2 to 6 weight percent and withfrom 0.1 to 0.3 weight percent chlorine dioxide basis the air dried weight of the pulp.

5. A process according to Claim 2 wherein step a) is carried out with 2 to 3 weight percent sodium hydroxide and 0.5 to 1 weight percent oxygen based on the air dried weight of the pulp and the resulting pulp slurry is treated for a minimum of 3 minutes at 20 psig or greater.

6. A process according to Claim 2 wherein step c) is carried out with a concentration of sodium hypochlorite in the range of 0.5 to 1 weight percent and a sodium hydroxide concentration in the range of 0.2 to 0.5 weight percent based on the weight of the air dried pulp for a period of time within the range of 5 to 15 minutes and that step e) comprises introducing chlorine dioxide in an amount within the range of 0.15 to 0.5 weight percent into the pulp slurry from step c) and subjecting the pulp to the combined action of sodium hypochlorite , sodium hydroxide , and chl orine dioxide at a temperatur e in the range of 60 to 90C for a time period in the range of one to thr ee hour s .

7. A mul tistage process for del ignifying and bl eaching al kaline wood pulps comprising subj ecting unbl eached pulp to a chlorine C or a chlorine/chlor ine dioxide C_D bleaching step followed by ther bleaching and extraction sequence E_OH*/ED

8 . In a multistage process for del ignifying and bl eaching al kal ine wood pulps , the sequence C_DE_OH*/ED.

9. In a mul tistage process for del ignifying and bleaching al kal ine wood pulps , the sequence C_DE_O (H*/ED) .

10. In a multistage process for bleaching chlorine or chlorine/chlorine dioxide treated alkaline wood pulps, the sequence E_OH*/ED.

11. In a multistage process for bleaching chlorine or chlorine/chlorine dioxide treated alkaline wood pulps, the sequence E_O(H*ED).

12. In a multistage sequential process for bleaching alkaline digested cooked wood pulps, wherein the cooked pulp is subjected to an oxidizing treatment stage with chlorine or a mixture of chlorine and chlorine dioxide prior to washing and the washed pulp from the oxidizing treatment stage is treated to an oxidative extraction and further washed, the improvement which comprises subjecting the washed pulp from the oxidative extraction step to a hot alkaline extraction with alkaline hypochlorite at a concentration within the range of 0 .5 to 1 weight per cent and an al kal i hydroxide at a concentration of 0 .2 to 0 .5 weight percent basis air dried weight of the pulp for a period of time in the range of about 5 to 15 minutes at a temperature in the range of 60 to 90C after which chlorine dioxide in an amount within the range 0 .15 to

0 .5 weight percent basis the air dried weight ot the pulp is added to the pulp in the hot hypochlorite treatment stage and the combined hot hypochlorite and chlorine dioxide treatment continued for an additional period of time in the range of one to three hours.