Methods of treating chemical cellulose pulp

Abstract

A single pulp bleaching stage comprises an acid treatment, A, a neutralization treatment, n, and a chlorine dioxide treatment, D0. The AnD0 bleaching stage may optionally include washing between treatments, either between each treatment or only some treatments, represented by A-n-D0 which indicates there is washing between the acid (A) and the neutralization (n) treatments and the neutralization (n) and the chlorine dioxide (D) treatments, or An-D0 which indicates there is washing only between the neutralization (n) and the chlorine dioxide (D) treatments. The AnD0 stage can be followed by washing and further bleaching stages. Preferably, the (AnD0) stage is followed by an extraction stage Eop with caustic in the presence of oxygen and a peroxide.

Description

FIELD OF THE INVENTION

The present invention relates to a method of treating chemical cellulose pulp from any chemical pulping process. In preferred embodiments, the present invention relates to treating chemical cellulose pulp in a stage with an acid treatment, a subsequent neutralization treatment and a chlorine dioxide treatment following the neutralization treatment.

BACKGROUND AND SUMMARY OF THE INVENTION

Pulp bleaching sequences using chlorine dioxide as the bleaching chemical are popular and environmentally feasible. Many practical and economic reasons exist for pulp bleaching sequences utilizing chlorine dioxide. For example, the price of chlorine dioxide is very competitive compared with that of other bleaching and delignifying chemicals. Also, the strength and brightness of the pulp obtained by chlorine dioxide bleaching are good, in fact at least approximately of the same order as those obtained by using peroxide at the same chemical consumption level (kg/admt).

Conventional chlorine dioxide bleaching is usually preformed at a temperature of 50-80° C., and with a chlorine dioxide dosage of about 10-30 kg per ton of pulp. Typical sequences used are, for example, D_oED₁ED₂ and OD_oED₁ED₂ and modifications thereof. The treatment time in a D_o stage is usually shorter as compared to the treatment time in the other D stages, being for example, between about 30-90 minutes. The pH of the pulp decreases to the range of 1-3 towards the end of the D_o stage. In the D, and D₂ stages the treatment time is 2-3 hours and the pH is slightly higher than in the D_o stage. The chlorine dioxide stage (all types) is usually performed at a temperature of about 70° C., while the treatment time in the D_o stage is 0.5-2 hours and in the D, and D₂ stages 2-3 hours. Higher temperatures than about 70° C. have typically been avoided as the low final pH in the range of 1-3 in the D stage combined with a high temperature and long treatment time deteriorates the strength properties of the pulp fibers.

Although conventional chlorine dioxide sequences have achieved widespread acceptance in the industry, some improvements are still desired. For example, improvements in the final brightness achieved by chlorine dioxide bleaching sequences are continually sought. It is towards fulfilling such improvements that the present is directed.

The present invention is especially embodied in a single bleaching stage which comprises an acid treatment, A, a neutralization treatment, n, and a chlorine dioxide treatment, D₀. The AnD₀ bleaching stage of the present invention may optionally include washing between treatments, either between each treatment or only some treatments, represented by A-n-D_o which indicates there is washing between the Acid (A) and the neutralization (n) treatments and the neutralization (n) and the chlorine dioxide (D) treatments, or An-D₀ which indicates there is washing only between the neutralization (n) and the chlorine dioxide (D) treatments. The AnD_o stage can be followed by washing and further bleaching stages. Preferably, the (AnD_o) stage is followed by an extraction stage E_op with caustic in the presence of oxygen and a peroxide.

When compared to a stage that is an acid treatment followed by a chlorine dioxide treatment without neutralization between the treatments (AD₀), using this AnD_o bleaching stage of the present invention gives the surprising results of higher brightness after the chlorine dioxide treatment and higher brightness and lower Kappa number after a subsequent E_op stage. This AnD_o bleaching stage also results in a higher final brightness at substantially identical bleaching conditions when bleaching does not include such an AD₀ or D₀ stage only.

Additionally, it has been found that reduced brightness reversion (that is, the measure of the loss of brightness of bleached pulp or paper over a period of time) occurs for pulp bleached in accordance with the present invention. In this regard, it is highly desirable for the bleached pulp to retain its white (“bleached”) color over prolonged time periods and thus reduction of brightness reversion of bleached pulp is highly desirable. Thus, brightness reversion of bleached pulp from the AnD₀ bleaching stage when compared to a conventional AD₀ stage is lower. It has also been found that the AnD₀ bleaching stage at a given brightness level requires less bleaching chemicals, especially chlorine dioxide. All of the improvements are a result of the introduction of a neutralization treatment, n, between the A and D₀ treatments.

Further, it has been found when compared to a stage that is an AnD₀, using an An-D₀ bleaching stage (acid treatment followed by neutralization followed by washing followed by chlorine dioxide) of the present invention gives even more surprising results of an even higher brightness after the chlorine dioxide treatment and higher brightness and lower Kappa number after a subsequent E_op stage. This An-D₀ bleaching stage also results in an even higher final brightness at substantially identical bleaching conditions when bleaching does not include such a stage.

As with the AnD₀, it has been found that the An-D₀ results in a reduced brightness reversion (that is, the measure of the loss of brightness of bleached pulp or paper over a period of time) for pulps bleached in accordance with the present invention. In this regard, it is highly desirable for the bleached pulp to retain its white (“bleached”) color over prolonged time periods and thus reduction of brightness reversion of bleached pulp is highly desirable. Thus, brightness reversion of bleached pulp from the An-D₀ bleaching stage when compared to a conventional AD₀ stage or even an AnD₀ is lower. It has also been found that the An-D₀ bleaching stage at a given brightness level requires less bleaching chemicals, especially chlorine dioxide. All of the improvements are a result of the introduction of a neutralization treatment, n, between the A and D₀ treatments with washing following the neutralization.

These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Reference will hereinafter be made to the accompanying drawings, wherein like reference numerals throughout the various FIGURES denote like structural elements, and wherein;

FIG. 1 is a schematic view of the AnD₀ process;

FIG. 2 is a schematic view of the An-D₀ process;

FIG. 3 is a bar graph showing pulp brightness for chlorine dioxide bleaching sequences with and without a neutralization, n, stage and with and without a caustic extraction stage in the presence of oxygen and a peroxide (E_op);

FIG. 4 is a bar graph showing the impact of a neutralization stage on E_op Kappa Number for chlorine dioxide bleaching stages;

FIG. 5 is a graph of final brightness (% ISO) versus ClO₂ charge in the D₁ stage; and

FIG. 6 is a graph of final brightness (% ISO) versus Total ClO₂ charge.

DETAILED DESCRIPTION OF THE INVENTION

A schematic view of a system 10 according to the present invention is depicted in accompanying FIG. 1. In this regard, the pulp slurry of the target consistency enters the treatment vessel 12 via supply line 14 so that the pulp can be subjected to an acid treatment, A. The acid treated pulp is discharged from the vessel 12 via line 16 and pumped by slurry pump 18 to a mixer 20. An appropriate amount of a neutralizing chemical (e.g., NaOH) is added to the pulp slurry at the mixer 20. The neutralized pulp slurry may then be introduced into treatment vessel 22 via supply line 24. The vessel 22 is sized to accommodate the desired residence time for the pulp slurry to be neutralized therein. The neutralized pulp slurry is then withdrawn from the vessel 22 via line 26 and directed to a mixer 28 wherein the bleaching charge of chlorine dioxide ClO₂ is added. The pulp slurry is thus introduced into the bleaching vessel 30 via line 32. The bleaching vessel 30 is sized so as to allow the pulp slurry to reside for a time sufficient to bleach the pulp whereby the bleached pulp may then be discharged therefrom via line 34.

The present invention is especially embodied in a single bleaching stage which comprises an acid treatment, A, a neutralization treatment, n, and a chlorine dioxide treatment, D₀. The (AnD₀) bleaching stage of the present invention may optionally include washing between treatments represented by (A-n-D₀), or between only the acid and neutralization treatments represented by (A-nD₀) or between only the neutralization and chlorine dioxide treatments represented by (An-D₀).

FIG. 2 shows the schematic of the system 10 where washing occurs at a wash stage 40 between neutralization n (which occurs in vessel 22) and the addition of chlorine dioxide at mixer 28. This wash stage may involve a press (not shown). The wash stage 40 (which can occur with or without a press) includes the addition of wash liquid (preferably fresh water at about 20° C. to about 100° C.) via line 41 to the pulp after the neutralization stage such that the material leaving the wash stage 40 via line 26-1 is at a consistency of between about 4 to about 35%, preferably at a consistency of between about 10 to about 14%. The wash liquid may be extracted from the wash stage 40 via line 42.

The preferred acid treatment will be conducted at a pH of between about 1 to about 5, and more preferably between about 2 to about 3.5. The acid used in such acid treatment may be any of those acids conventionally employed in the art, such as for example, sulfuric, hydrochloric acid and the like. Hydrofluoric acid may also be employed. The pulp retention time for the acid treatment will typically range between about 30 seconds to about 5 hours, and more preferably between about 2 to about 4 hours. The acid treatment will most preferably be conducted at an elevated temperature of between about 40 to about 100° C., and more preferably between about 80 to about 90° C. (e.g., about 85° C.). The consistency of the pulp slurry during acid treatment is preferably between about 0.1 to about 50%, more preferably about 4 to about 15%, and most preferably between about 10 to about 12%.

Particularly preferred acid treatments may be those described more fully in U.S. Pat. No. 6,776,876 B1, the entire content of which is expressly incorporated hereinto by reference. Specifically, the preferred acid treatment stage (A) will include treating chemical cellulose pulp produced by alkaline delignification having a kappa number under 24 and a solids consistency of between 0.1-50% by treating the pulp at a temperature over 85° C. and at a pH between about 2-5 for sufficient time to remove at least 50% of the hexenuronic acid and to reduce the kappa number by at least 2 units.

The acid treatment of this invention is followed by a neutralization treatment stage (n) and optionally may include an intermediate washing stage. As will be discussed below, especially preferred embodiments subject the acid treated pulp to the neutralization treatment without an intermediate wash (i.e., so as to achieve reduced brightness reversion). The purpose of the neutralization treatment is to neutralize the pulp slurry to a pH of between about 7 to about 14 (preferably about 9.5 pH) using an alkaline chemical, such as sodium hydroxide. The preferred neutralization treatment is accomplished by adding the neutralizing chemical directly to the pulp slurry directly after the acid treatment without washing at a temperature of between about 40 to about 100° C., more preferably between about 60 to about 90° C., most preferably 80 to about 90° C. The pulp retention time for the neutralization treatment will typically range between about 30 seconds to about 2 hours, and more preferably between about 5 to about 20 minutes. The consistency of the pulp slurry during the neutralization treatment is preferably between about 4 to about 15%, more preferably between about 10 to about 12%.

A chlorine dioxide treatment, D₀, directly follows the neutralization treatment. As discussed previously, intermediate washing of the pulp slurry may or may not occur between the n and D₀ treatments. The chlorine dioxide treatment is conducted at a pH of between about 1 to about 7, preferably between about 2.5 to about 3.5. The target pH is achieved by adding chlorine dioxide and/or an acid such as sulfuric, hydrochloric, or hydrofluoric acids. The temperature of the chlorine dioxide treatment will be between about 40 to about 10° C., more preferably between about 60 to about 90° C., most preferably 80 to about 90° C. Pulp retention times during the chlorine dioxide treatment will range between about 30 seconds to about 2 hours, preferably between about 15 to about 30 minutes. A chlorine dioxide charge of between about 0.05 to about 2.5% on oven dry pulp weight (odpt) or Kappa number 3 to 24 times the Kappa factor (Kappa factor of 0.05 to 1, preferably 0.15 to 0.25) will typically be employed at beginning of the D₀ treatment. Pulp slurry consistency during the D₀ treatment will be between about 4 to about 15%, preferably between about 10 to about 12%. The chlorine dioxide treatments that may be employed in the practice of this invention are described more fully in US Patent Application Publication 2002/0056533 A1 (the entire content of which is expressly incorporated hereinto by reference).

The AnD₀ or An-D₀ stage can be followed by washing and further bleaching stages. Preferably, the AnD_o or An-D₀ stage is followed by an alkaline extraction, preferably by an E_op stage with caustic in the presence of oxygen and a peroxide. Preferably the E_op stage is as disclosed more fully in U.S. Pat. No. 4,568,420 (the entire content of which is expressly incorporated hereinto by reference).

As noted previously, intermediate wash treatments may optionally be practiced between the neutralization treatment and each of the acid and chlorine dioxide bleaching treatments or between the acid and neutralization treatment or between the neutralization and chlorine dioxide treatment. Such a stage can be stated as A-n-D₀ or A-nD₀ or An-D₀. Suitable wash liquids, typically at a temperature of 20-100° C. for such intermediate wash treatments may be virtually any suitable water from the pulp mill, such as fresh water or filtrate from the last D (chlorine dioxide) stage or it can be condensate (either clean or dirty) from elsewhere in the mill, washer filtrate, waste water treatment plant effluent or combinations of any of these liquids, but preferably is fresh water.

Alternatively, an initial quantity of chlorine dioxide may be added to the acid treatment, A, resulting in a stage (A_DnD₀). According to this embodiment of the invention, the total charge of chlorine dioxide would be split between the A_D and D₀ treatments so that chlorine dioxide is added in two treatments. Specifically, chlorine dioxide would be added during or with the acid treatment as well as after the neutralization treatment.

Preferably, according to this embodiment, a treatment A_D is practiced to achieve an acid treatment with the addition of between 1 and 99% of the total chlorine dioxide to be used in the stage. For example, if 10% of the total chlorine dioxide to be added in this stage is added with the acid, A, treatment, then the remaining 90% of the chlorine dioxide is introduced in the subsequent chlorine dioxide, D₀, treatment. The operating conditions of this A_D treatment are preferably substantially the same as those of the acid treatment described previously, that is a pH of 1 to 4 preferably 2 to 3.5; a retention time for the treatment of 30 seconds to 5 hours, preferably 2 to 4 hours; a temperature of treatment 40 to 100° C., preferably 80 to 90° C.; and a pulp slurry consistency during treatment of 4 to 15%, preferably 10 to 12%. Preferably, the acid used is sulfuric, hydrochloric or hydrofluoric.

The A_D treatment is followed directly by a neutralization treatment without intermediate washing. That is, there is no washing of the pulp slurry between the A_D and n treatments. The D₀ treatment follows directly the neutralization treatment without intermediate washing. That is, there is no washing of the pulp slurry between the n and the D₀ treatments. The operating conditions for the n and the D₀ treatments are substantially the same as those described previously with the neutralization and D₀ treatments except of course that the balance of the chlorine dioxide charge is added in the D₀ treatment which depends on the amount of chlorine dioxide added in the A_D treatment.

Although no intermediate washings of the pulp are preferred, intermediate wash treatments may optionally be practiced between the neutralization treatment and each of the A_D and D₀ treatments. Such a sequence can be stated as (A_D-n-D₀). Suitable wash liquids for such intermediate wash treatments may be virtually any suitable water from the pulp mill, such as washer filtrate, waste water treatment plant effluent and the like.

Another embodiment in accordance with the present invention involves having successive chlorine dioxide treatments whereby bleaching is accomplished first at a relatively high pH value and then at a relatively low pH value. Such a sequence is identified as (AnD_HID_LO). In this regard the acid and neutralization treatments are preferably practiced substantially the same as described previously. Preferably, no intermediate washing of the pulp slurry occurs either before or during the chlorine dioxide treatments D_HI, D_LO. The total chlorine dioxide charge is split between the D_HI and D_LO treatments. Thus, between about 1 to about 99% of the total chlorine dioxide charge will be added in the D_HI treatment, whereas the balance (i.e., between 99% to 1%) of the total chlorine charge is then added in the D_LO treatment.

The D_HI chlorine dioxide treatment is conducted at a relatively high pH of between about 4 to about 7, preferably between about 5 to about 6. The target pH is achieved by adding sodium hydroxide or other alkaline chemicals with pH adjustment occurring after the addition of 1 to 99% of the chlorine dioxide charge. The D_HI treatment is preferably practiced at a temperature 40 to 100° C., preferably 60 to 90° C.; a retention time at high pH of 30 seconds to 2 hours, preferably 5 to 30 minutes; and a pulp slurry consistency of 4 to 15%, preferably 10-12%.

The D_LO chlorine dioxide treatment is practiced at a relatively low pH, without intermediate washing of the pulp slurry after the D_HI treatment, of between about 1 to about 4, preferably between about 2 to about 3. The pH adjustment is achieved by adding a suitable acid, such as sulfuric acid, hydrochloric acid and/or hydrofluoric acid. The addition of acid for pH adjustment can occur concurrently with or subsequent to the addition of the balance of the chlorine dioxide charge. Preferably, the D_LO chlorine dioxide treatment is practiced at a temperature 40 to 100° C., preferably 60 to 90° C.; a retention time at low pH of 30 seconds to 2 hours, preferably 5 to 30 minutes; and at a pulp slurry consistency of 4 to 15%, preferably 10-12%.

Although no intermediate washings are preferred, it is possible to practice intermediate washings between each of the treatments, specifically between the neutralization treatment and each of the acid and high pH chlorine dioxide treatments as well as between the high and low chlorine dioxide treatments. Such a stage can be stated as (A-n-D_HI-D_LO). Wash liquid could be any water from the mill, such as washer filtrate, waste water treatment plant effluent, and the like.

The present invention will be further understood from the following non-limiting Example.

EXAMPLE 1

Hardwood pulp was treated in accordance with Table 1 below to compare the results of the AnD₀ treatment in accordance with the present invention, with and without a subsequent E_op stage, with a conventional AD₀ stage. The results appear in Table 1 below and are graphically presented as FIGS. 3-5.

As can be seen, improvements in the final brightness are achieved by chlorine dioxide bleaching stages according to the present invention.

TABLE 1
Furnish	Mill O2 Pulp
Sample ID 2004-00	102403
Kappa Number	10.9
Viscosity, mPa · s	37.6
BS HexA, meq/kg	63.9
A Do Stage: 90° C. + 85° C.,
180 + 15 min., 10% cons.
Sample	ADo	AnDo
Kappa Factor	0.16	0.16
H2SO4, %	1.32	1.32
NaOH, %	—	1.1
ClO2, %	0.37	0.37
% Consumed ClO2	0.37	0.37
Final pH	2.3 + 2.2	2.3 + 9.9 + 4.1
Brightness, ISO	64.7	70.7
Kappa number	—	—
HexA, meq/kg	—	—
Eop Stage: 60 min., 85° C.,
40 psi., 10% cons.
H2O2, %	0.3	0.3
NaOH, %	0.8	0.8
Final pH	10.6	10.8
% Consumed H2O2	0.3	0.27
Kappa Number	3.0	2.5
Brightness, % ISO	78.0	83.8
D Stage: 70° C.,
180 min., 10% cons.
ClO2, %	0.2	0.4	0.6	0.2	0.4	0.6
NaOH, %	—	—	0.08	—	—	0.08
H2SO4, %	0.11	0.03	—	0.11	0.03	—
Final pH	4.2	3.8	3.9	4.2	4.1	3.4
ClO2 Consumed, %	0.2	0.4	0.6	0.2	0.4	0.6
ISO Brightness, %	85.80	87.6	89.0	88.4	89.6	90.6
Reverted ISO units	2.3	2.3	2.5	2.5	2.1	2.3
4 Hrs @ 105 C.

EXAMPLE 2

Pulp was treated in accordance with Table 2 below to compare the results of the AnD₀ and An-D₀ treatments in accordance with the present invention, with and without a subsequent E_op stage, with a conventional AD₀ stage. The results appear in Table 2 below and are graphically presented as FIG. 6. Hardwood pulp was used in the study summarized in Table 2.

As can be seen, improvements in the final brightness are achieved by chlorine dioxide bleaching stages according to the present invention. Specifically, as depicted in FIG. 6, a comparison of three bleaching sequences (AD₀), (AnD₀) and (An-D₀) reveals that the addition of the neutralization treatment between the acid and the chlorine dioxide treatments results in improved final brightness over the sequence with acid and chlorine dioxide treatments alone. It is also apparent the addition of washing between the neutralization and chlorine dioxide treatments results in an even higher final brightness than the case of neutralization between the acid and chlorine dioxide treatments without washing.

TABLE 2
Furnish	Hardwood Mill O2 Pulp
Sample ID 2004-00	102640
Kappa Number	10.4
Viscosity, mPa · s	40.3
HexA, meq/kg	63.9
AD0 Stage: 90° C. + 85° C.,
180 + 15 min., 10% cons.
AnD0 Stage: 90° C. + 90° C. + 85° C.,
120 + 5 + 15 min., 10% cons.
An-D0 Stage: 90° C. + 90° C. + 85° C.,
120 + 5 + 15 min., 10% cons.
Sample	AD0	AnD0	An-D0
Kappa Factor	0.18	0.18	0.18
H2SO4, %	1.42	1.96	1.25
NaOH, %	—	1.1	0.9
ClO2, %	0.42	0.4	0.42
% Consumed ClO2	0.42	0.4	0.42
Final pH, each Stage	2.5; 2.4	2.4; 10.8; 2.6	2.4; 9.4; 2.6
Brightness, ISO	65.9	66.0	72.8
Eop Stage: 60 min.,
85° C., 40 psi., 10% cons.
H2O2, %	0.5	0.5	0.5
NaOH, %	0.8	0.8	0.8
Final Ph	10.8	10.7	10.7
% Consumed H2O2	0.5	0.5	0.5
Kappa Number	2.6	2.5	2.4
Brightness, % ISO	81.5	81.3	84.5
D Stage: 70° C., 180 min.,
10% cons.
ClO2, %	0.2	0.4	0.6	0.2	0.4	0.6	0.2	0.4	0.6
NaOH, %	—	—	0.08	—	—	0.08	—	—	0.08
H2SO4, %	0.11	0.03	—	0.11	0.03	—	0.11	0.03	—
Final pH	4.5	4.0	3.9	4.2	3.9	3.8	4.2	4.0	3.9
% Consumed ClO2	0.2	0.4	0.6	0.2	0.4	0.6	0.2	0.4	0.6
Brightness, % ISO	87.3	89.0	89.9	87.7	89.6	90.4	88.7	89.9	91.1
Reverted ISO units, 4 Hrs @ 105° C.	2.7	2.5	2.8	2.3	2.5	2	1.7	1.5	1.9

Claims

1. A method of treating chemical cellulose pulp produced by alkaline delignification, comprising: (a) acid treating the pulp at a pH of between about 1 to about 5 and at a temperature of between about 40 to about 100° C.; (b) neutralizing the acid treated pulp according to step (a) with an alkaline chemical to achieve a pH of between about 7 to about 14; and thereafter (c) bleaching the neutralized pulp.

2. The method of claim 1, wherein step (c) is practiced by subjecting the neutralized pulp to at least one chlorine dioxide (D0) bleaching stage.

3. The method of claim 1, further comprising (d) washing the pulp after the acid treatment stage.

4. The method of claim 3, wherein the step of (d) washing the pulp is practiced between steps (a) and (b).

5. The method of claim 3, wherein the step of (d) washing the pulp is practiced between steps (b) and (c).

6. The method of claim 3, wherein the step of (d) washing the pulp is practiced between steps (a) and (b) and between steps (b) and (c).

7. The method of claim 1, wherein step (c) comprises bleaching the pulp with chlorine dioxide at a relatively high pH of between about 4 to about 7 (DHI), and thereafter bleaching the pulp with chlorine dioxide at a relatively low pH of between 1 to about 4 (DLO).

8. The method of claim 7, wherein DHI and DLO are practiced without intermediate washing.

9. The method of claim 1, wherein the chemical cellulose pulp has a kappa number of less than 24, and a solids consistency of between about 0.1 to about 50%, and wherein step (a) is practiced for a time sufficient to remove at least 50% of the hexenuronic acid and to reduce the kappa number of the pulp by at least 2 units.

10. A method of treating chemical cellulose pulp produced by alkaline delignification, consisting essentially of: (a) acid treating the chemical cellulose pulp at a pH of between about 1 to about 5 and at a temperature of between about 40 to about 100° C.; (b) neutralizing the pulp treated in step (a) with an alkaline chemical to achieve a pH of between about 7 to about 14; (c) washing the neutralized pulp; and thereafter (d) bleaching the washed and neutralized pulp.

11. The method of claim 10, further consisting essentially of (a1) washing the pulp between steps (a) and (b).