WO1996027705A1

WO1996027705A1 - Peroxide bleaching

Info

Publication number: WO1996027705A1
Application number: PCT/CA1996/000074
Authority: WO
Inventors: Douglas John Mckenzie; Marco Solinas; Thomas Howard Murphy
Original assignee: Macmillan Bloedel Limited; Harmac Pacific Inc.
Priority date: 1995-03-03
Filing date: 1996-02-07
Publication date: 1996-09-12

Abstract

A method of bleaching of pulps using peroxide as the primary bleaching agent in an aqueous medium having between 10-50 % ethylene glycol at a consistency of between 8 and 35 % to thereby produce a peroxide bleached pulp.

Description

PEROXIDE BLEACHING

Field of the Invention

The present invention relates to bleaching of chemical paper making pulps, more particularly the present invention relates to an improved process for peroxide bleaching of such pulps. Background to the Present Invention

Hydrogen peroxide is a chemical commonly used for the bleaching of pulps. Generally the pulp is bleached at high, or medium consistency in an aqueous medium containing the required amount of peroxide thoroughly mixed with the pulp by retaining the pulp at the desired pH and temperature in a bleaching vessel for the required time to produce the bleached pulp. At the end of the bleaching operation residual peroxide is removed and the pulp soured. In some cases the separated residual peroxide is used in a further bleaching operation or otherwise depending on the amount of peroxide available and its purity.

There has been much activity recently devising techniques for reducing the effluent from pulp mills and particularly from the bleach plant of a chemical pulp mill These efforts have to a significant degree been related to the elimination of chlorine from the bleach plant effluent and have resulted in processes that are chlorine element free i.e eliminate the use of chlorine from the bleaching (EFC) operation or totally chlorine free

(TCF) wherein the effluent if any is totally free of chlorine compounds, i.e. even the conventionally used chlorine dioxide stages have been replaced.

The above referred to peroxide bleaching operations provide one of the known alternative techniques that reduce amount of chlorine that used in the bleaching operation.

Another bleaching chemical conventionally used for bleaching is ozone, however, it is known to significantly degrade the strength of the pulp, i.e. lowers the viscosity of the pulp, thus the strength characteristics of the pulp significantly more than for example chlorine dioxide. A recent development utilizes an alcohol such as methanol ethanol in the ozone stage (or other alcohol etc.) in the ozone stage to protect the pulp by reducing the decrease in viscosity of the pulp for a given ozone application so that the so bleached pulp has a higher viscosity at a given brightness and thereby overcome some of the major drawbacks of ozone bleaching see International Patent Publication no. WO 94/10377 published 11 May, 1994 inventor Solinas et al.

It has also recently been determined that the use of ethylene glycol in an oxygen stage improved the oxygen pulping or bleaching operation by protecting the pulp and thereby produces a pulp having a higher viscosity (strength) at a given brightness or kappa number. Such a process is described in the United States patent application serial

# 08/234,028 filed April 28 1994 by Solinas et al.

Development work continues to be performed to produce a bleached pulp using a process that is totally effluent free (TEF) and having a brightness and strength similar to that to which the industry has become accustomed and which has been produced using chlorine or chlorine compounds as the principal bleaching chemical. Brief Description of the Present Invention

It is an object of the present invention to provide a peroxide bleaching stage wherein the viscosity of the pulp at a given brightness is significantly improved.

It is also an object of the present invention to provide a new peroxide bleaching stage that is compatible with recent developments using alcohol as part of the aqueous medium in either an ozone or an oxygen or both stages.

Broadly the present invention relates to a method of bleaching a pulp comprising forming a mixture of said pulp in an aqueous medium to form a pulp slurry, said aqueous medium containing between 10-50% ethylene glycol by weight of said medium and said pulp slurry having a consistency of between 8-35%, applying at least 1% peroxide (measured as H₂O₂) to said pulp and bleaching said pulp at a temperature of 70-120°C for a time sufficient to complete the desired bleaching of said pulp. Preferably caustic will be applied to said bleaching in an amount so that the end pH of said pulp slurry after said bleaching is between 8 and 11.5.

Preferably said end pH of said pulp slurry will be in the range of 9 to 10.5.

Preferably said aqueous medium will contain between 25-35% ethylene glycol

Preferably said consistency will be in the range of 10-15%. Preferably the temperature during said bleaching will be maintained between

100-110°C. Preferably said bleaching will be carried out under a gaseous pressure of between 60 and 100 psig under gas pressure. Brief Description of the Drawings

Further features, objects and advantages will be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which

Figure 1 is a schematic flow diagram of a process in which the present invention may be applied.

Figure 2 is a plot of percent ethylene glycol in the aqueous medium versus viscosity and brightness showing the optimum percent ethylene glycol concentration in the aqueous medium for the pulp being processed. Description of the Preferred Embodiments

Figure 1 shows a specific flow diagram of a process into which the present invention has been incorporated and one which facilitates the closing of the system to minimize effluent discharged from the process.

As illustrated chemical pulp is introduced as indicated 10 into an oxygen stage

12 that preferably is operated in accordance with the invention described in the said US patent application #08/234,028 with the pulp contained in an aqueous medium containing ethylene glycol during the oxygen stage and thus is thoroughly mixed with glycol to form an aqueous glycol medium surrounding the pulp.

The pulp after the oxygen stage 12 is washed as indicated at 14 and then subject to an ozone bleaching stage as indicated at 16. Preferably the ozone stage 16 will also be a stage wherein the aqueous medium surrounding the pulp contains glycol as the alcohol and wherein the pulp is bleached to the desired degree in accordance with teachings as described in said World Publication WO 94/10377. After the ozone stage

16 the pulp is fed to a second washer 18 wherein the pulp is again washed.

Washed ozone bleached pulp is then treated in accordance with the present invention in a peroxide stage 20 wherein the medium surrounding the pulp is an aqueous medium containing glycol. The peroxide stage 20 further enhances the brightness of the pulp while minimizing the reduction in strength thereby to produce a pulp of the desired brightness. The peroxide bleached pulp is then fed to a third wash stage 22 and to produce the cleaned and bleached pulp which leaves the washers indicated at 24.

With the process as above described fresh water is introduced at the washer 22 and used to wash the bleached pulp. The effluent from this washer 22 is directed back to the washer 18 as indicated by line 28 and used to wash the ozone bleached pulp from the ozone stage 16 and the effluent from the washer 18 is directed via line 30 to the washer 14 and is used to wash the oxygen bleached or brightened pulp. Then the effluent from the washer 14 which will have a relatively high concentration of glycol is collected as a mixture of water plus glycol plus contaminants as indicated at 32 and is then fed to recovery system as indicated at 34.

The glycol will be separated from the water and the contaminants using any suitable process such as evaporation and used as one of the sources of glycol introduced and mixed with the pulp 10 in the oxygen glycol stage 12 as indicated at 11.

The above sequence facilitates the recovery of glycol but is not imperative to the operation of the present invention which relates primarily to the peroxide and glycol bleaching stage 20.

It has been found the addition of an alcohol particularly ethylene glycol during pressurized hydrogen peroxide bleaching stage improves the resultant pulp viscosity at a given brightness. To carry out the present invention one adjusts the percent of glycol in the aqueous medium surrounding the pulp to a concentration in the range of 10-50% ethylene glycol by weight in the aqueous medium. Preferably the amount of ethylene glycol will be between 25-35% by weight of the aqueous medium.

This ethylene glycol concentration adjustment if required is made as indicated by the addition of ethylene glycol at 36 in Figure 1. The peroxide may also be added at the same time or after the concentration of glycol in the surrounding aqueous medium has been adjusted as required. This addition of peroxide is indicated schematically by the arrow 38 in Figure 1.

The amount of peroxide added will generally be greater than 1% based on the dry weight of the pulp being bleached and normally will be applied in the amounts generally used in the application of peroxide to pulp in conventional peroxide bleaching operations i.e. will be in the range of about 3-5% peroxide (measured as hydrogen peroxide) based on the pulp. Peroxide used will normally be a hydrogen peroxide although other peroxides may be used if desired, however, the percentages used will have to be adjusted accordingly, i.e. the measurement of peroxide addition based on the equivalent hydrogen peroxide addition.

Temperature in the peroxide stage 20 will generally be in the range of 70-120°C preferably about 100-110°C and the pulp will be retained in this stage for sufficient time to obtain the desired bleaching. The time requirement is in part temperature dependent thus the higher temperature the lower the residence time required. The residence time is also to some degree concentration dependent. Generally the residence time during which peroxide bleaching occurs time will not be less than 45 minutes or three quarters of an hour and normally will not exceed about six hours the conventionally used time in the peroxide stage is about 2 hours and this has been found adequate.

It also been found that the end pH, i.e. the pH in the slurry leaving the peroxide bleaching stage should not be too high. It has been found that the pH should be in the range of about 8-1 1.5 preferably about 9-10.5.

The peroxide stage may also be operated and preferably will be operated at above atmospheric pressure by adjusting the gas over pressure within the bleaching tank up to about 100 psig, but it is preferred to operate a pressure of 75-90 psig. It also preferred to incorporate chelating agents, in fact, in some cases it has been found essential to apply a chelating agent as is conventional in peroxide bleaching of pulp i.e. generally prior to the application of the peroxide. The use of sodium silicate which is also conventional in many peroxide bleaching stages has been found to be useful in high consitency applications but not essential in medium consistency applications as will be described in further detail here in below.

Examples

Tests were carried out on a 30.2 ml kappa number pulp having a viscosity of 35.3 cp made from hemlock which was bleached to a kappa number of 6.7 ml and a viscosity of 24 cp in an alcohol (glycol) oxygen stage. In most cases this was followed by a chelating stage carried out a pH of 4.5-5 included a DTP A (diethylenetriamine pentaacetic acid (penta sodium salt)) addition of 0.5%. For the purposes of the present invention the alcohol (ethylene glycol) concentration in the aqueous medium is based on weight on weight of liquid for example a 50% concentration would be defined as illustrated by the following example; pulp weight 100 grams; consistency 10%; total weight of the pulp slurry 1000 gm then the amount of liquid equals 1000 -100 = 900 gm. and the amount of alcohol (900 X 0.5) = 450 gm. Example 1

Different concentrations of ethylene glycol were used in the aqueous medium in a peroxide stage operated at a temperature of 110°C for a period of 120 minutes at a consistency of 10%. The results are presented in Table 1.

TABLE 1 Effect of Ethylene Glycol Concentration on P_G Stage

Ethylene Glycol 0 10 20 33 33 40 50

Concentration, %

H₂O₂, % 4.0 4.0 4.0 4.0 4.0 4.0 4.0

Residual, % o.d. 0.77 0.75 0.56 0.76 0.94 0.18 0.21

NaOH, % 1.4 1.6 1.6 1.6 1.4 1.6 1.6

DTP A, % 0.3 0.3 0.3 0.3 0.3 0.3 0.3

MgSO₄, % 0.25 0.25 0.25 0.25 0.25 0.25 0.25

End pH 10.1 9.9 9.9 10.0 9.9 9.9 10.0

Brightness, % ISO 84.9 85.1 83.5 84.4 84.0 81.7 81.9

Viscosity, cp 16.0 18.1 18.8 19.4 21.1 20.2 19.9

These results are also plotted in Figure 2.

It can clearly be seen that the optimum concentration of ethylene glycol in the aqueous medium is between 30-40%, i.e. at 33% the brightness was 84% ISO and the viscosity was the highest found namely 21.1 cp. Example 2 The effect of increased temperature (assuming that the alcohol or ethylene glycol functioned to protect the pulp) was determined by experiments carried out at the optimum glycol concentration of 33% and the retention time and consistency in the peroxide stage were the same as used in Example 1. It became apparent that the viscosity tended to drop as the temperature increased although the brightness seemed to increase see Table 2. TABLE 2 Effect of Temperature on the PG Stage

Temperature, °C 110 120 130

H₂O₂, % 4.0 4.0 4.0

Residual, % o.d. 0.94 0.48 0.43

NaOH, % 1.4 1.6 1.6

DTP A, % 0.3 0.3 0.3

MgSO₄, % 0.25 0.25 0.25

End pH 9.9 9.8 9.4

Brightness, % ISO 84.0 84.9 85.8

Viscosity, cp 21.1 17.8 17.6

Example 3

The effectiveness of a chelating stage when operating a pressurized peroxide stage in an aqueous glycol medium was determined using the conditions describe in Example 1 but where a chelating agent namely DTPA was added in the amount of 0.3% percent weight on weight of pulp. The results as presented in Table 3 and clearly indicate that without the chelating agent the brightness is severely affected, at least for the particular pulp tested, and it is preferred to ensure the pulp is chelated to remove or neutralize the effect of metal ions.

TABLE 3 Effect of Chelation Stage on Following P Stage

Chelation Stage ? Yes No

H₂O₂, % 4.0 4.0

Residual, % o.d. 0.94 0

Ethylene Glycol, % 33 33

NaOH, % 1.4 1.6

DTPA, % 0.3 0.3

MgSO₄, % 0.25 0.25

End pH 9.9 9.7

Brightness, % ISO 84.0 67.6

Viscosity, cp 21.1 20.0 Example 4

An alcohol other than ethylene glycol was tried as the alcohol in the aqueous medium surrounding the pulp in the peroxide stage. 33% propanediol was used in the medium in place of the glycol while the other conditions used during the peroxide stage were essentially the same as those used in the previous example. The results obtained are given in Table 4.

TABLE 4 Effect of Alternate Polyhydric Alcohol

Alcohol Control Ethylene Glycol, Propanediol, 33% 33%

H₂O₂, % 4.0 4.0 4.0

Residual, % o.d. 0.77 0.94 0.35

NaOH, % 1.4 1.4 1.6

DTPA, % 0.3 0.3 0.3

MgSO₄, % 0.25 0.25 0.25

End pH 10.1 9.9 10.0

Brightness, % ISO 84.9 84.0 83.3

Viscosity, cp 16.0 21.2 17.4

It is apparent that the propanediol was significantly less effective in protecting the pulp as the viscosity of the pulp was 17.4 cp with the propanediol compared with 21 cp for ethylene glycol. The propanediol did show as slight increase in viscosity over the control, however a loss of 1.5% ISO was incurred which indicates delignification was being impaired. Example 5

High consistency pressurized peroxide bleaching was carried out with and without the presence of ethylene glycol using a bleaching sequence O_G QPG* where O_G = an oxygen stage using ethylene glycol in the medium,

Q = a chelating stage,

P_G* = high consistency pressurized alcohol peroxide stage, and compared with a similar process but wherein the ethylene glycol was not present in the peroxide stage The high consistency operations were carried out at consistency of about 25% and the medium consistency operations were conducted at 10%. The results are reported in Table 5.

TABLE 5

Effect of Ethylene Glycol and Sodium Silicate on HC Pressurized Peroxide Bleaching

Consistency and Medium Medium Cons. High High High Consistency

Additives Consistency + Ethylene Consistency Consistency Ethylene Glycol,

Glycol Control Ethylene Glycol Sodium Silicate

H₂O₂, % 4.0 4.0 4.0 4.0 4.0

Residual, % 0.77 0.77 0.70 0.02 0.71

NaOH, % 1.4 1.6 1.0 1.6 1.0

Ethylene Glycol, % - 33 0 30 30

Na₂SiO₃, % 0 0 3.0 0 3.0

DTPA, % 0.3 0.3 0.3 0.3 0.3

MgSO^ % 0.25 0.25 0.25 0.25 0.25

Time, min 120 120 60 60 45

Temperature, °C 110 110 110 110 110

Consistency, % 10.0 10.0 25.0 25.0 25.0

Pressure, psig 90 90 90 90 90

End pH 10.1 10.0 9.0 9.4 9.0

Brightness, % ISO 84.9 84.4 87.2 80.3 85.6

Viscosity, cp 16.0 21.1 10.9 15.6 16.8

Using the conventional systems the higher consistency operation produced a higher brightness but a major loss in viscosity (compare high consistency control with medium consistency control). Using ethylene glycol in the medium operating at high consistency there is a gain in brightness but when using sodium silicate in the bleaching liquor as compared with a similar high consistency operation without sodium silicate in the liquor. Most surprising are the results obtained operating at medium consistency with ethylene glycol in the medium or liquor, but without sodium silicate in the medium

- compare the 84.4% ISO brightness at a viscosity of 21.1 cp obtained at medium consistency with no silicate with the 85.6% ISO brightness at viscosity of 16.8 cp obtained at high consistency with sodium silicate present.

Experiment 6 The effect of end pH on brightness is illustrated by tests conducted applying different amounts of sodium hydroxide to the pulp entering the peroxide bleaching stage so that end pH, i.e. the pH of the pulp and medium (pulp slurry) at the end of the bleaching operation was changed. The results are reported in Table 6.

TABLE 6 Effect of End pH on Brightness and Viscosity for HC Peroxide Bleaching

It will be apparent that at the lower end pH of 9 when alcohol was present a bleached pulp having a brightness of 85.6% ISO was produced while a viscosity of 16.8 cp was retained. When this pH was increased by 1 point to a pH of 10 the brightness dropped one full point and the viscosity 4 points thereby clearly indicating that the lower pH at the end of the bleaching stage produced a better end result.

By the term pressurized hydrogen peroxide stage the pressure used in the above examples was 90 psig using oxygen as the over pressure gas to maintain the pressure.

Having described the invention, modifications will be evident to those skilled in the art without departing with the spirit of the invention as defined in appended claims.

Claims

CLAIMS We claim:

1. A method of bleaching a pulp comprising forming a mixture of said pulp in an aqueous medium to form a pulp slurry, said aqueous medium containing between 10- 50% ethylene glycol by weight of said medium and said pulp slurry having a consistency of between 8-35%, applying at least 1 % peroxide (measured as H₂O₂) to said pulp and bleaching said pulp at a temperature of 70-120°C for a time sufficient to complete the desired bleaching of said pulp.

2. A method as defined in claim 1 wherein caustic is applied to said pulp in said bleaching in an amount so that the end pH of said pulp slurry after completion of said bleaching is between 8 and 11.5. 3. A method as defined in claim 2 wherein said end pH is in the range of 9 to 10.5

4. A method as defined in claims 1, 2 or 3 wherein said aqueous medium contains between 25-35% ethylene glycol by weight.

5. A method as defined in claims 1, 2, 3 or 4 wherein said consistency is in the range of 10-15%. 6. A method as defined in claims 1, 2, 3, 4 or 5 wherein the temperature during said bleaching is maintained between 100-110°C.

7. A method as defined in claims 1, 2, 3, 4, 5 or 6 wherein said bleaching is carried out under a gaseous pressure of between 60 and 100 psig.

AMENDED CLAIMS

[received by the International Bureau on 20 May 1996 (20.05.96); original claims 1 and 4 amended; remaining claims unchanged (1 page)]

1. A method of bleaching a pulp comprising forming a mixture of said pulp in an aqueous medium to form a pulp slurry, said aqueous medium containing between 10- 50% glycol by weight of said medium and said pulp slurry having a consistency of between 8-35%, applying at least 1 % peroxide (measured as H₂O₂) to said pulp and bleaching said pulp at a temperature of 70-120°C for a time sufficient to complete the desired bleaching of said pulp.

2. A method as defined in claim 1 wherein caustic is applied to said pulp in said bleaching in an amount so that the end pH of said pulp slurry after completion of said bleaching is between 8 and 1 1.5.

3. A method as defined in claim 2 wherein said end pH is in the range of 9 to 10.5

4. A method as defined in claims 1, 2 or 3 wherein said aqueous medium contains between 25-35% glycol by weight.

5. A method as defined in claims 1, 2, 3 or 4 wherein said consistency is in the range of 10-15%.

6. A method as defined in claims 1, 2, 3, 4 or 5 wherein the temperature during said bleaching is maintained between 100-1 10°C.

STATEMENT UNDER ARTICLE 19

Prior to the examination of this application, please amend this application as follows: In the Claims

Claim 1. line 3. please delete "ethylene^".

Claim 4. line 2. please delete "ethylene^".

A new Claim page 1 1 incorporating the changes is enclosed for substitution.

Remarks

Claims 1 to 7 inclusive are being prosecuted.

By this amendment, the term "ethylene^" has been deleted in claim 1 and 4. It is apparent that although ethylene glycol was the principal material used, it is well recognized that other glycols behave in a similar manner, thus limiting the scope of the patent to ethylene glycol is unduly limiting.

This amendment does not add any new matter to the application as filed.

Examination of this application on its merits is respectfully requested. Respectfully submitted.