WO1991000386A1

WO1991000386A1 - A method for bleaching kraft pulp with a mixture of oxygen and peroxide

Info

Publication number: WO1991000386A1
Application number: PCT/US1990/003600
Authority: WO
Inventors: David L. De Groot; Philip R. Campbell
Original assignee: Nekoosa Papers, Inc.
Priority date: 1989-06-30
Filing date: 1990-06-25
Publication date: 1991-01-10

Abstract

The invention concerns a process for the delignification and brightening of lignocellulosic pulp (1) by contacting chlorinated pulp with a base (25) and a hypochlorite (22) at a temperature of at least about 140°F prior to adding oxygen (31) and a peroxide (33).

Description

"_A M_ETHOD FOR BLEACHING KRAFT PULP WITH A MIXTURE OF OXYGEN AND PEROXIDE".

Field of the Invention

The present invention relates to a process for the delignification and brightening of lignocellulosic pulp.

More particularly, the present process relates to^' a method for improving brightness and maintaining fiber length and the strength properties of the pulp.

Background of the Invention

A number of multi-stage processes have been used to delignify and bleach lignocellulosic pulp. The most common commercial method has involved a process which has been designated "CEDED". This process involves a chlorination stage using chlorine, chlorine dioxide or a mixture of these materials, designated "C" . The chlorination step is normally followed by a caustic extraction stage designated "E". The extracted pulp is then contacted with chlorine dioxide in a stage designated "O" . Another caustic extraction stage is used to further remove dissolved lignins and spent chemicals with the resulting pulp passed to another chlorine dioxide stage. The bleached pulp is then washed. A number of variations of this CEDED sequence had been developed to reduce both operating and capital costs and improve pulp quality and comply with environmental constraints.

A modification to this bleaching process has been made involving the use of oxygen in the alkaline extraction stage. The addition of oxygen, as well as a small amount of peroxide, was disclosed in U.S. Patent No. 3,719,552 of Farley, et al. (1973). This process was also disclosed in U.S. Patent No. 4,568,420 of Nonni

(1986) . The latter patent further disclosed the addition of hypochlorite or peroxide prior to the oxygen-alkaline extraction stage. Environmental considerations have forced reductions in the use of chlorine in bleach plants. This has been achieved by substituting chlorine dioxide for at least a portion of the chlorine in the first chlorination stage. Shortened sequences have been developed commonly with three stages for hardwood and four stages for softwood. A typical, three-stage process involves a chlorination stage in which up to 60 percent of the chlorine has been replaced by chlorine dioxide followed by a caustic extraction in the presence of oxygen with a chlorine dioxide treatment being the third stage. Such a sequence is commonly designated as C/D-EO-D where there is a wash after each of the stages. A sequence for the treatment of softwood pulp might have an additional stage using hypochlorite or peroxide since more delignification is required than with hardwoods. These short sequence processes are sensitive to variations in operating conditions and must be carefully controlled. If each stage of the operation is not running at essentially optimum efficiency, substandard pulp can be produced. In addition to the need to reduce capital costs and operation chemical expenses and the environmental considerations, there is an increasing need for improved pulp quality. Pulp produced from bleach plants of today must be stronger, i.e., have longer fiber length, in order to be properly refined and modified to produce paper which will perform satisfactorily in high-speed printing presses, copiers, box machines and in other demanding end uses.

Accordingly, there is a need for a process which has the advantages of the short sequence, i.e., reduced capital and operating costs and decreased environmental impact while also providing improved quality pulp. It has been found that the instant process provides these advantages while also providing pulp with comparable or improved brightness and improved strength properties. Sum ary of the Invention

The present process comprises a method for bleaching kraft pulp which comprises contacting washed, chlorinated pulp with a base and a hypochlorite at a temperature of at least about 140°F for a time to provide an alkaline pulp stream. This alkaline pulp stream is mixed with oxygen and a peroxide to provide an alkaline mixture. This mixture is held at a temperature of at least 140°F for at least about 45 minutes to provide an extracted pulp.

In another embodiment, the instant invention involves a method for bleaching kraft pulp which comprises contacting the kraft pulp with sodium hydroxide in an amount sufficient to provide an alkaline solution with a pH of between about 10.5 and about 11.5 and with sodium hypochlorite in an amount equal to about 0.3 to about 0.6 weight percent. The contacting is accomplished at a temperature between about 140°F and about 180°F for between about 0.5 and about 2.0 minutes. About 0.3 to about 0.6 weight percent oxygen and about 0.1 to about 0.5 weight percent hydrogen peroxide are mixed with the resulting alkaline solution. This mixture is retained at a temperature between about 140°F and about 170°F for at least 45 minutes. The resulting alkaline extracted pulp can then be subjected to additional processing.

Brief Description of the Drawing

The Figure shows a block flow diagram of a preferred embodiment of the instant process.

Detailed Description of the Invention

It has been found that an unexpected improvement in the strength of pulp product can be obtained by initially contacting chlorinated pulp with a hypochlorite and a base. Oxygen and a peroxide are then added and the resulting mixture is retained for a time sufficient to allow the necessary reaction to occur. To obtain the improved results, it is necessary that the hypochlorite and base be in contact with the pulp for a time before the oxygen and peroxide are introduced. This process is described in more detail hereinbelow.

In the instant process, an aqueous northern or southern hardwood or softwood kraft pulp may be employed. The consistency of the pulp can be from about 8 to about 14 weight percent, based on the oven dry weight of pulp. The term "consistency" is used herein to refer to the weight percent of fiber in a slurry. It is preferred that the consistency be about 10 weight percent to about 12 weight percent. Unless otherwise stated, all weight percentages provided herein are based on oven dried pulp.

The pulp from the kraft process is normally subjected to a chlorination stage. If desired, an oxygen delignification stage can be used before the chlorination stage. In the chlorination stage, a mixture of chlorine and chlorine dioxide is commonly used, although the pulp can be subjected to chlorination using only one of these materials. Commonly, the temperature in the chlorination stage is between about 100°F and about 150°F, preferably about 135°F to about 145°F. If a mixture of chlorine and chlorine dioxide is used, the amount of chlorine used is preferably about 1.5 weight percent to about 3.5 weight percent and the amount of chlorine dioxide is equal to about 0.5 weight percent to about 1.5 weight percent. The pulp is normally subjected to the chlorination process for a time from about 10 to about 45 minutes, preferably about 20 to about 30 minutes.

The resulting chlorinated pulp is washed using a suitable washing apparatus such as a drum washer to remove spent chemicals and other materials, such as chlorinated hydrocarbons. Preferably, sufficient wash is provided to reduce the chlorine content of the pulp to less than about 0.1 weight percent.

A basic material, preferably sodium hydroxide or another suitable alkaline extractant, such as potassium hydroxide, calcium hydroxide, or sodium carbonate, or a mixture thereof, is added to the washed pulp stream. Additionally, alkaline medium from other steps in the pulp treatment process can be used such as the "white liquor" from the cooking stage of the process. The white liquor contains substantial amounts of sodium hydroxide and sodium sulfide. The terms "caustic" and "base" are used interchangeably herein to refer to alkaline or basic materials. A sufficient amount of the basic material is added to increase the pH to at least about 10.5, preferably in the range of about 11 to 12 and more preferably, about 11.0 to 11.5. Normally, this is an amount to between about 2.5 weight percent to about 4 weight percent base. The consistency of the pulp stream can be between about 8 weight percent and 15 weight percent, preferably about 10 weight percent to about 12 weight percent. Low-pressure steam is mixed with the pulp stream to increase the temperature of the stream to above about 140°F and preferably in the range of about 150°F to about 180°F.

In accordance with the instant invention, a hypochlorite is added to either the washed pulp stream before base is added, or after the base is added, or substantially simultaneously with the addition of the basic material. Hypochlorite is added in an amount of at least about 0.3 weight percent. Preferably the amount of hypochlorite added is in the range of about 0.4 weight percent to about 0.6 weight percent. These amounts are based upon sodium hypochlorite and can require adjustment if other hypochlorites are used. As indicated above, the hypochlorite can be added before the alkaline material and steam, after the base and steam or preferably simultaneously with the caustic and steam in a steam mixer.

An advantage of the instant process is that improved results in pulp strength can unexpectedly be obtained by contacting the pulp with the hypochlorite under alkaline conditions for a short period of time before adding oxygen and a peroxide. The required contact time is short enough that equipment such as a retention tower is not required. To obtain the desired results, the pulp should be contacted with the hypochlorite and base for a minimum contact period. Normally, this contact period is at least about 0.5 minutes. Preferably, the contact time is between about 1.0 and about 2.0 minutes. The preferred hypochlorites in the present process are the alkali and alkaline-earth salts, preferably of sodium, potassium or calcium hypochlorite and most preferably sodium hypochlorite. Mixtures of these hypochlorites can also be used. Any apparatus suitable for mixing these components can be used, for example a Peg-type mixer, a high-shear mixer, etc. The resulting heated alkaline stream containing pulp and the hypochlorite is maintained for the desired contact period at a temperature of at least about 140°F. Preferably the temperature is between about 140°F and about 160^βF.

Oxygen and a peroxide are then mixed with the heated alkaline stream. While either the oxygen or the peroxide can be added initially, followed by the addition of the second component, it is preferred that the oxygen and peroxide be mixed with the alkaline stream essentially simultaneously, preferably in a chemical mixer. Any apparatus suitable for mixing these components can be used, including a high shear mixer, a refiner or a medium consistence pump. Sufficient oxygen is added to provide the desired level of delignification and brightening. Preferably, at least about 0.2 weight percent and more preferably about 0.4 to about 0.6 weight percent oxygen is added. The preferred peroxide in the instant process is hydrogen peroxide, although sodium peroxide, other inorganic peroxides such as potassium peroxide or organic peroxides such as acetyl peroxide, benzoyl peroxide and the like or compounds which, in situ, produce such peroxides, can be used. Mixtures of such peroxides can also be used. If hydrogen peroxide is used, it is added in an amount of at least about 0.1 weight percent and preferably about 0.2 weight percent to about 0.5 weight percent. If other peroxides are used, they are added in amounts to be comparable in activity to the aforesaid amount of hydrogen peroxide.

The resulting alkaline stream containing oxygen, peroxide and some hypochlorite is retained for a time sufficient to substantially complete the reaction of the pulp with these chemicals which is normally a period of at least about 45 minutes, preferably a time in the range of about 60 to about 90 minutes, at a temperature of at least about 140°F, preferably in the range of about 150°F to about 170"F.

The alkaline pulp stream to which the oxygen, peroxide and sodium hypochlorite have been added can be retained in any conventional tower used for extraction or in a pressurized vessel such as a digester, or any other manner suitable for providing the desired residence time. It can also be conducted under a constantly declining pressure head as disclosed in U.S. Patent No. 3,832,276, incorporated herein by reference, and can also be accomplished by applying a constant partial pressure of oxygen to the pulp.

After completion of the retention period, the pulp is washed with water to remove dissolved lignins, waste chemicals and bleaching by-products, such as chlorinated -8- hydrocarbons. The washing conditions and washing media used can be those well-known to those skilled in the art. It is understood that recycled or recovered streams can also be used as appropriate in the washing 5 stages disclosed herein. For example, caustic filtrate recovered from the alkaline extraction stage can be used as a wash medium where desired. The washed pulp can then be subjected to additional bleaching stages, for example, using a peroxide, a hypochlorite or chlorine

10 dioxide. Preferably a chlorine dioxide bleaching stage is used. The conditions for these bleaching stages are well-known to those skilled in the art. Typical conditions for the chlorine dioxide stage involves temperatures of about 160°F to 180°F, time of about 2 to

15 4 hours, pH of about 2.5 to about 4.5 (adjusted typically with sulfuric acid) , chlorine dioxide addition in amounts of about 0.5 weight percent to about 1.5 weight percent, and a pulp consistency of about 10 weight percent to about 12 weight percent. The pH is

20 adjusted as necessary for subsequent treatments with the final bleached pulp being washed.

A preferred embodiment of the instant process is depicted in the Figure. Kraft pulp 1 is contacted with chlorine 3 and chlorine dioxide 5 in a chlorination zone

25 7. The resulting chlorinated pulp is transferred 9 to a wash zone 11 in which the pulp is contacted with water 13. Waste chemicals, dissolved lignins, chlorinated hydrocarbons, soluble inorganic chemicals and other soluble contaminates are removed as a waste stream 15

30 and the washed pulp is transferred 17 to mixing zone 19. Sodium hypochlorite 23 and caustic 25 are introduced into the mixing zone and combined with the pulp. The temperature of the pulp stream is increased by the addition of steam 21, preferably to a temperature of

35 about 160"F. Sufficient caustic is introduced to provide a solution pH of about 11.0. Sodium hypochlorite is added in an amount of about 0.5 weight percent. The resulting mixture is transferred 27 during a period of about 1.5 minutes to a second mixing zone 29. Oxygen 31 and hydrogen peroxide 33 are introduced into the mixing zone 29 and thoroughly mixed with the alkaline pulp stream. About 0.4 weight percent oxygen and about 0.25 weight percent hydrogen peroxide are added. The resulting mixture is passed 35 to a retention zone 37 where the mixture is retained for about 60 minutes.

The alkaline extracted product is transferred 39 to a wash zone 41 in which water 43 is used to wash dissolved lignin, waste chemicals and remove chlorinated hydrocarbons not removed in a previous stage from the pulp to provide a waste stream 45. The resulting washed pulp is transferred 47 to another mixing zone 49 where steam 51 is introduced to increase the temperature of the stream to about 165°F. The heated stream is passed 53 to another bleaching zone 55 in which chlorine dioxide 57 is contacted with the pulp. Chlorine dioxide is added in an amount equal to about 0.7 weight percent of the oven-dried pulp. The pulp is retained for about 120 minutes. The resulting bleached pulp 59 is passed to a washing zone 61 where the pulp is contacted with water 63 to remove waste chemicals, dissolved lignin, soluble organic and inorganic chemicals, and other soluble contaminants in a waste stream 65. The product pulp stream 67 is removed from the wash zone and transferred to additional processing stages as commonly used in the art.

The following examples are intended by way of illustration and not by way of limitation.

Southern hardwood kraft pulp was produced having a permanganate number of 11.0 as determined using Technical Association of the Pulp and Paper Industry (TAPPI) Useful Method 251. The washed kraft pulp having a consistency of about 3.0 to 3.5 percent based on oven dried pulp was chlorinated by adding a mixture of chlorine and chlorine dioxide simultaneously to a chemical mixer. The consistency (concentration) of the pulp suspended in the stream was determined using TAPPI Official Test Method T240 om-81. Amounts of chlorine and chlorine dioxide was added equal to 3.0 weight percent chlorine and 0.5 weight percent chlorine dioxine based on oven dried pulp. The resulting stream was passed through a retention tower with a hold time of about 25 to 30 minutes at a temperature of about 140°F and then washed using recycled filtrate from the extraction stage and mill water having a temperature of about 145"F to about 165^βF on a rotary drum washer. The brightness of the chlorinated stock was determined using an Elrepho brightness meter. The viscosity of the washed pulp as fed to the chlorination stage (inlet viscosity) was tested using TAPPI Official Standard T230 os-76. Chlorinated pulp produced under these conditions was used in the following runs.

Run No. .1

The washed chlorinated kraft pulp having a consistency of 10 to 12 percent based on oven dried pulp at a temperature of 130"F and a pH of 4.0 was mixed with steam and an aqueous solution containing about 15 weight percent sodium hydroxide. Sufficient steam was added using a steam mixer to provide a pulp slurry temperature of about 170°F. Sufficient sodium hydroxide solution was added to provide a concentration of about 3.0 weight percent sodium hydroxide based on oven dried pulp content and increase the pH to about 11.0. The resulting heated alkaline stream was then conveyed during a 1.0 to 2.0 minute period to a chemical mixer where oxygen was introduced to provide a dissolved oxygen concentration of 0.4 to 0.5 weight percent based -li¬ on oven dried pulp. This alkaline slurry containing oxygen was passed into a retention tower for an average hold time of about 50 to 70 minutes. The resulting oxygen extracted pulp product was washed with mill water having a temperature of about 145°F to about 165°F using a rotary drum filter. The pulp product was tested for brightness using an Elrepho brightness meter. The results are shown in Table 1. The washed oxygen extracted pulp having a consistency of 10 to 12 percent based on oven dried pulp at a temperature of 140°F and a pH of 10.0 was mixed with steam in a steam mixer to provide a slurry temperature of about 180°F. Sufficient chlorine dioxide was added using chemical mixed to provide a concentration of 0.65 to 0.75 weight percent based on oven dried pulp. Concentrated sulfuric acid was added simultaneously to the chemical mixer to provide a concentration of about 0.4 weight percent based on oven dried pulp and reduce the pH to a range of about 3.5 to 4.0. The resulting slurry was conveyed to a retention tower for an average hold time of 120 to 150 minutes. The resulting pulp product was washed using mill water at ambient temperature and tested for final viscosity using TAPPI Official Standard T230 os-76 and final brightness using TAPPI suggested method T525 sm-72 (diffuse illumination and 0 observation) . These results are given in Table 2.

Run No. 2

The same procedure was followed as in Run No. 1 except hydrogen peroxide in an amount of 0.2 to 0.3 weight percent based on oven dried pulp was added to the heated alkaline stream along with oxygen in the chemical mixer. The resulting oxygen extracted pulp product was washed and tested for brightness using an Elrepho meter. These results are given in TABLE 1. The final pulp product was washed and tested for viscosity and brightness. These results are given in Table 2.

Run No. 3 The same procedure was followed as in Run No. 1 except an aqueous solution containing about 2.5 weight percent sodium hypochlorite was added along with the sodium hydroxide and steam in the steam mixer. Sufficient sodium hypochlorite was added to provide 0.4 to 0.5 weight percent sodium hypochlorite based on oven dried pulp. The resulting oxygen extracted product was washed and tested for brightness using an Elrepho meter. These results are given in Table 1. The final pulp product was washed and tested for viscosity and brightness. These results are given in Table 2.

Run No. 4

In this process of the instant invention, the same procedure was followed as in Run No. 3 except an aqueous solution containing about 5 to 10 weight percent hydrogen peroxide was added in addition to the oxygen in the chemical mixer. Sufficient hydrogen peroxide solution was added to provide a concentration of hydrogen peroxide in the pulp stream of 0.2 to 0.3 weight percent based on oven dried pulp. The resulting oxygen extracted pulp product was washed and tested for brightness using an Elrepho meter. These results are given in Table 1. The final pulp product was washed and tested for viscosity and brightness. These results are given in Table 2. TABIE 1

OXYGEN EXTRACTED HJLP BRIGHINESS DEVEIOPMENT

(EIREHK) UNITS)

PROCESS C-HIOKENATED EXTRACTED NET RUN (*) HJLP HJLP INCREASE

EO 50.7 50.6

EPO 49.2 54.4 5.2

3 EHO 45.7 50.3 4.6

4 EHOP 51.2 58.9 7.7

These results show that the instant process as exemplified in Run 4 provided improved final brightness and decreased viscosity loss, i.e., improved pulp strength.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity and understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the invention, as limited only by the scope of the appended claims.

Claims

What is claimed is:

1. A method for bleaching kraft pulp, said method comprising:

(a) contacting chlorinated pulp with a base and a hypochlorite at a temperature of at least about 140°F to provide an alkaline pulp stream;

(b) mixing oxygen and a peroxide into said alkaline pulp stream; and

(c) retaining the mixture from step (b) at a temperature of at least about 140"F for at least about 45 minutes.

2. The method of Claim 1 wherein said base is sodium hydroxide, said peroxide is hydrogen peroxide, and said hypochlorite is sodium hypochlorite.

3. The method of Claim 1 wherein said base is present in an amount sufficient to provide a solution pH of at least about 10.5.

4. The method of Claim 1 wherein said base and said hypochlorite are combined with said pulp for a time of at least about 0.5 minutes before introducing the oxygen.

5. The method of Claim 1 wherein said oxygen is mixed in an amount sufficient to provide at least about 0.3 weight percent oxygen in said alkaline pulp stream.

6. The method of Claim 1 wherein at least about 0.2 weight percent peroxide is mixed into said alkaline pulp stream.

7. The method of Claim 1 wherein said base is sodium hydroxide in an amount sufficient to provide a solution pH of between about 11.0 and 11.5, said hypo¬ chlorite is sodium hypochlorite in an amount sufficient to provide a concentration of between about 0.4 and about 0.6 weight percent, said oxygen is present in an amount sufficient to provide a concentration of about 0.4 to about 0.6 weight percent and said peroxide is hydrogen peroxide in an amount sufficient to provide about 0.2 to about 0.5 weight percent.

8. The method of Claim 1 wherein said peroxide is selected from the group consisting of hydrogen peroxide, sodium peroxide, potassium peroxide, and mixtures thereof.

9. The method of Claim 1 wherein said base and said hypochlorite are combined with said pulp for a time of about 1.0 to about 2.0 minutes before introducing the oxygen.

10. The method of Claim 1 wherein said pulp is contacted with said hypochlorite before contacting with said base.

11. The method of Claim 1 wherein said chlorinated pulp is contacted with steam to provide said pulp at said temperature of at least 140°F.

12. The method of Claim 1 wherein said chlorinated pulp is contacted with a base selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium carbonate, sodium hydroxide and sodium sulfide, and mixtures thereof in an amount sufficient to provide a solution pH of about 10.5 to about 12, a hypochlorite selected from the group consisting of sodium hypochlorite, potassium hypochlorite and calcium hypochlorite or mixtures thereof, and steam in an amount sufficient to provide a solution temperature of at least about 140^βC and provide said alkaline pulp stream said contacting occurring for at least about 0.5 minutes before said oxygen is mixed with said alkaline pulp stream said oxygen being mixed in an amount between about 0.2 and about 0.6 weight percent and said peroxide being selected from the group consisting of hydrogen peroxide, sodium peroxide, potassium peroxide and mixtures thereof in an amount of between about 0.2 and about 0.5 weight percent.

13. A method for bleaching kraft pulp, said method comprising:

(a) contacting chlorinated kraft pulp with (i) sodium hydroxide in an amount sufficient to provide an alkaline solution with a pH of between about 10.5 and about 11.5 and (ii) sodium hypochlorite at a concentration of about 0.3 to about 0.6 weight percent said contacting accomplished at a temperature between about 140°F and about 180°F for between about 0.5 and about 2.0 minutes;

(b) mixing oxygen and hydrogen peroxide with the product of step (a) to provide an oxygen concentration of about 0.3 to about 0.6 weight percent and a peroxide concentration of about 0.1 to about 0.5 weight percent; and

(c) retaining said oxygen and peroxide containing mixture at a temperature of about 140°F to about 170°F for a time of about 45 to about 90 minutes.

14. The method of Claim 13 wherein said pH is between about 11.0 and 11.5 and said peroxide concentration is between about 0.2 and about 0.5 weight percent.