KR20140107640A - Process for reducing fluorescence in pulp - Google Patents

Abstract

The present invention relates to a method for producing a pulp slurry comprising contacting a pulp slurry containing a fluorescent agent with chlorine dioxide; A method for reducing fluorescence in a regenerated pulp, comprising contacting the pulp slurry with sodium hypochlorite in an amount sufficient to reduce fluorescence of the pulp, wherein the pulp is contacted with chlorine dioxide Contacted with the sodium hypochlorite component after contact.

Description

PROCESS FOR REDUCING FLUORESCENCE IN PULP < RTI ID = 0.0 >

The field of the invention relates to the production of pulp and paper from pulp sources containing fluorescent agents such as recycled paper. More particularly, the field of the invention relates to reducing or eliminating fluorescence in pulp and paper.

Fluorescence is an optical phenomenon in which light is absorbed at non-visible wavelengths (eg ultraviolet) and then re-emitted at visible wavelengths. A chemical referred to as a "fluorescent whitening agent" or "optical brightening agent" or "FWA" or "OBA" is often used in a number of fine papers, Or to the furnish in the production of tissue and towel-grade products as well as in copying, thereby enhancing the apparent brightness of the product. FWAs are typically stilbene-based compounds and their chemical derivatives.

When a paper product containing FWA is recycled, most of the fluorescent compound will be with the fiber through the recycling process. This has been reported to cause some operational problems due to color and brightness control in de-inked pulp mills and in paper mills using recycled fibers. FWA not only can change the apparent color of the deinked pulp, but it can also inhibit or change the final quality of the finished product. The presence of the FWA can affect the response of a UV-based on-line color sensor and can adversely affect the performance of the control loop.

It is also suspected that FWA may leak out of paper products made from recycled fibers and present potential health hazards. FWA is not allowed in some food-grade packaging. Thus, the FWA will need to be removed from the regenerated fiber intended for such food-grade applications.

Thus, there is a need to degrade or remove FWA from recycled pulp in an efficient manner.

Fluorescence in regenerated pulp is obtained by contacting the pulp containing the fluorescent agent with a combination of chlorine dioxide (ClO ₂ ) and sodium hypochlorite (or a derivative of sodium hypochlorite) in an amount sufficient to lower the fluorescence of the pulp, Or by contacting the pulp with sodium hypochlorite at the same time as contacting the pulp with ClO ₂ or after contacting the pulp with ClO ₂ .

In one embodiment, the ClO ₂ and sodium hypochlorite components are added to the pulp simultaneously, or a short time after the sodium hypochlorite component is added to the ClO ₂ , and the sodium hypochlorite component is added to the sodium hypochlorite, A derivative of sodium hypochlorite, or a mixture thereof.

In one embodiment, ClO ₂ and sodium hypochlorite are added to the pulp simultaneously, or a short time after the sodium hypochlorite component is added to the ClO ₂ . In embodiments of the present invention, sodium hypochlorite is ClO ₂ is added to the paper is less than about 2 minutes, or less than about 1 minute, or less than about 30 seconds, or less than about 10 seconds, or less than about 5 seconds, or from about 1 Lt; / RTI > In one embodiment, ClO ₂ and sodium hypochlorite are added simultaneously to the pulp.

In embodiments of the present invention, the amount of ClO ₂ added to the pulp is from about 4 kg / t to about 15 kg / t (1000 kg), or from about 5 kg / t to about 10 kg / t, or from about 6 kg / / t. In embodiments of the present invention, the amount of sodium hypochlorite added to the pulp is from about 2 kg / t to about 12.5 kg / t, or from about 2.5 kg / t to about 8 kg / t, or from about 3 kg / 6 kg / t. In one embodiment, ClO ₂ is added in an amount greater than the amount of sodium hypochlorite (by weight). In embodiments of the present invention, the weight ratio of ClO ₂ to sodium hypochlorite may range from 1: 1 to 3: 1, or from 1.1: 1 to 2: 1, or from 1.2: 1 to 1.8: to be. In another embodiment of the invention, a lower charge of the chemical is also contemplated, and even a lesser reduction in fluorescence, even when the fluorescence is not zero (0) or nearly zero, is also considered to be within embodiments of the present invention do.

Additional objects, advantages and novel features will become apparent to those skilled in the art upon examination of the following description.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a fluorescence reduction process using an upflow type reaction vessel. FIG.
Fig. 2 is a graph showing the influence of the delay of hypochlorite addition on the brightness and fluorescence of Example 1. Fig.
3 is a graph showing the effect of delayed addition of hypochlorite to the L-value of Example 1. Fig.
4 is a graph showing the effect of delayed addition of hypochlorite on the whiteness and hue of Example 1. Fig.
FIG. 5 is a graph showing the effect of the hypochlorite addition delay on the values of a and b in Example 1. FIG.
Figure 6 is a graph showing the effect of chemical order on fluorescence of Example 2;
7 is a graph showing the effect of chlorine dioxide filling on fluorescence of Example 3;
Figure 8 is a graph showing the pH of Example 3 and the effect of chlorine dioxide charge on remaining chlorine dioxide.
9 is a graph showing the pH of Example 3 and the effect of chlorine dioxide charge on the residual chlorine dioxide.
10 is a graph showing the effect of retardation of addition of sodium hypochlorite on brightness and fluorescence of Example 4. FIG.
11 is a graph showing the effect of retardation of sodium hypochlorite addition on the whiteness and hue of Example 4. Fig.

In one embodiment, the process according to the present invention comprises processing the waste paper to form a pulp slurry comprising pulp and a fluorescent agent, for example, FWA; And applying chlorine dioxide to the process stream (pulp, paper) containing the fluorescence before or simultaneously with the sodium hypochlorite. The pulp slurry may have a consistency ranging from about 4% to about 20%, or from about 6% to about 18%, or from about 8% to about 16%, prior to the addition of chlorine dioxide. It is possible to operate at a consistency lower than 4%, for example, to obtain advantages from the present invention while matching with existing process streams in pulp or paper mills, but the advantage of adding sodium hypochlorite at less than about 1% Is lost. In one embodiment, the delay between the addition of the two chemicals is not too long so that the residual chlorine dioxide is no longer present. Preferably, the reaction time remaining after addition of sodium hypochlorite should be at least 5 minutes.

In embodiments of the present invention, the process is described as contacting the pulp with chlorine dioxide and sodium hypochlorite. By "contacting" is meant that the chemical (in contact with the pulp) is added to the pulp slurry or formed in situ by reaction of the precursor chemicals that form the desired chemical. For example, chlorine dioxide or sodium hypochlorite may be formed in a pulp slurry in situ, other chemicals may be added to the slurry in accordance with the present invention, or both chlorine dioxide and sodium hypochlorite may be added to the slurry, As shown in FIG. In embodiments of the present invention, chlorine dioxide or sodium hypochlorite, or both, may be added to the pulp slurry after being formed in situ in a separate process facility or process stream.

While embodiments of the present invention are described for contacting pulp with sodium hypochlorite or adding sodium hypochlorite to pulp, other embodiments of the present invention include, but are not limited to, It is to be understood that the use of sodium hypochlorite derivatives is contemplated. The term "sodium hypochlorite derivative" refers to a derivative of sodium hypochlorite in a slurry, either as a result of the introduction of sodium hypochlorite into the pulp slurry according to the invention with chlorine dioxide, by dissociation of the sodium hypochlorite, Quot; refers to one or more compounds that are produced by reaction with other compounds and whose presence improves the fluorescence degradation relative to the sole chlorine dioxide. For example, under certain conditions, sodium hypochlorite is thought to form hypochlorous acid in solution in pulp slurry. Thus, hypochlorous acid is considered to be a derivative of sodium hypochlorite, and it is contemplated that addition of hypochlorous acid alone or in combination with sodium hypochlorite to pulp and chlorine dioxide is within the scope of embodiments of the present invention.

1, an example of a process flow is provided. Referring to Figure 1, chlorine dioxide enters the medium consistency pump (MC), which transports pulp at 12% consistency. Sodium hypochlorite is added directly to the dynamic chemical mixer in less than one second. In high consistency, air pockets are typically formed around the impeller. In order to prevent this, a degassing pump is installed. Some types of neutralizing chemicals for chlorine compounds are supplied to the degassing pump. That is, it is applied to a gas other than a pulp stream. Both chlorine dioxide and sodium hypochlorite are thoroughly mixed into the pulp prior to a top-upflow tower of about 20 minutes.

The time delay between the two chemicals can affect optical properties, not fluorescence, and a method for optimizing such optical properties in connection with the practice of the present invention can be developed.

In one embodiment, the reaction of the two compounds occurs in a single stage. Although the reaction vessel is shown in Fig. 1, a pipe may suffice because a relatively short reaction time is required in the practice of the present invention in other embodiments.

In an embodiment of the present invention, the pH of the chlorine dioxide / sodium hypochlorite / pulp stream can be adjusted with a pH controlling chemical (acid, caustic or buffer material). In embodiments of the present invention, the pH is adjusted to a value in the range of from about 2 to about 7. The specific target pH will generally depend on the system. For example, in a cleaning system having a fluorescence of about 5, the pH may range from about 2 to about 5, or from about 2 to about 4. If the system is highly buffered, for example, with calcium carbonate, the pH may be higher, for example, in the range of about 4.5 to about 7. Also, filling the chlorine dioxide will reduce the pH, and sodium hypochlorite filling will increase the pH.

In embodiments of the present invention, the temperature of the fluorescence degradation process (treated pulp slurry) is maintained in the range of about 40 캜 to about 85 캜, or about 65 캜 to about 80 캜.

While MC pulp and chemical mixers are used in some embodiments, some fluorescence degradation is believed to occur regardless of the mix achieved.

The simultaneous application of chlorine dioxide and sodium hypochlorite requires less chemistry to remove fluorescence than a separate application of chlorine dioxide or sodium hypochlorite, and only a single stage is considered necessary.

Compared to single sodium hypochlorite, the process according to the present invention has the added advantage of being able to achieve zero or almost zero fluorescence. Zero fluorescence is not possible with, or at least very difficult to achieve with, sodium hypochlorite alone. Accordingly, in one embodiment of the present invention, chlorine dioxide and sodium hypochlorite are added in sufficient amounts and the resulting pulp is maintained under conditions sufficient to achieve zero or substantially zero fluorescence. Substantially zero means fluorescence is 0.25 or less when measured with Technidyne Color Touch (CTH-ISO).

Compared to a single chlorine dioxide, the process according to the present invention also prevents gas generation from occurring while reducing chemical loading. Gas generation is caused by a high level of residual chlorine dioxide at the end of the process and presents the operator with a potential hazard when breathing. When chlorine dioxide and sodium hypochlorite are added in combination according to the present invention, the residual ClO ₂ is lowered, for example, only about one-tenth of the original amount is left when maximum fluorescence reduction is desired.

In one embodiment, the fluorescence degradation process is carried out separately from any bleaching or bleaching stage wherein the purpose of bleaching / bleaching is to increase brightness and / or whiteness. In such embodiments, the fluorescence process does not cause an increase in brightness or causes only a relatively small increase in brightness. In embodiments of the present invention, as a result of performing the fluorescence degradation process, the brightness increase is less than about 5 ISO, or less than about 0 ISO. In one embodiment, the fluorescence degradation process is performed before any bleaching or thickening stage.

In an embodiment of the present invention, the fluorescence degradation process is performed in such a way that both the chlorine dioxide and the sodium hypochlorite are added to the pulp slurry, Or pipes. In one embodiment, the residence time may be as low as 7 minutes, with good fluorescence degradation results observed.

The foregoing general discussion of the present invention will be further illustrated by the following specific, non-limiting examples.

The following Examples in the case of the addition of sodium hypochlorite was added in the ClO ₂ Effect of the time delay, the effect of the separate addition of a long reaction time for the ClO ₂ alone, the amount of influence of the chemicals added, and the paper And evaluating the effect of the practice of the present invention on brightness and whiteness.

Example  One

D-H 2-stage, between D and H Thickening

The pulp was first reacted with chlorine dioxide, then thickened to about 25% consistency, diluted to 11% consistency, and further treated with sodium hypochlorite. The amount of ClO ₂ added was 2.5 kg / ton (1000 kg) of dry pulp and the amount of sodium chlorite was 1.785 kg / ton of dry pulp. The total reaction time for both stages was 7 minutes. The work indicated that the reaction was complete after 7 minutes. The chlorine dioxide time was 0, 2, 3.5, and 5 minutes. Zero indicates the basic case where two chemicals are added simultaneously. The consistency for both chlorine dioxide and sodium hypochlorite reactions was 11% and the temperature was 71 ° C. The results are shown in FIG.

A review of FIG. 2 shows that fluorescence increased with delayed addition of sodium hypochlorite, even with a delay as short as two minutes. Also, when this loss occurs, the increased delay has no further effect on fluorescence.

The paper color can be specified by three coordinate systems L *, a * and b * in a so-called CIELAB system where L * is lightness, a * is red-green and b * is yellow-blue. These values, as well as other optical properties, were measured as a function of delay time in sodium chlorite addition. The effect on the L value is shown in Fig. 3, and the effect on the hue is shown in Fig.

3 and 4 show that the brightness, L * value, and whiteness increased in a manner similar to fluorescence. This increase was both in the values for UV-containing (UVin) and UV-free (UVex).

Unlike fluorescence, the hue was not only increased with the first delay in sodium hypochlorite addition, but increased with increasing delay.

The effect on the values of a and b is shown in Fig. An examination of FIG. 5 shows that the delay did not affect or had little effect on the a * value, the b * value decreased, and UVin decreased more than UVex.

Example  2

Washing or washing between D and H Increase  No H-D

Mixed office waste (MOW) was treated in two separate processes in combination with chlorine dioxide and sodium hypochlorite. In the first process, chlorine dioxide was mixed with the paper, and after 5 minutes sodium hypochlorite was added, followed by further mixing. The total residence time was 20 minutes and the temperature was 71 ° C.

In the second step, chlorine dioxide was added and allowed to react alone for 20 minutes. Thereafter, sodium hypochlorite was added to the paper without intermediate washing or thickening. The residence time for this stage was also 20 minutes and the total residence time was 40 minutes. The results are shown in Fig.

The discussion of FIG. 6 shows that both the fluorescence and residual chemistry were degraded in the first process, even though the residence time was only half. Both brightness and whiteness were lower in the D + H process.

Example  3

FR  Process - zero fluorescence degradation

Recycled paper containing a mixed office paper with an initial brightness of 89.95% ISO containing 12.4 of fluorescence and UV was treated with a combination of chlorine dioxide and sodium hypochlorite, at which time both chemicals were added simultaneously.

Chlorine dioxide charge was varied from 0 kg / t to 15.5 kg / t while sodium hypochlorite was added at 2.9 kg / t and 5 kg / t. The paper was bleached for 15 minutes in a thermostat set at 68 占 폚. The pH was not adjusted and the reaction was allowed to float. Brightness and fluorescence were measured using a Technidyne Color Touch meter and a D65 emitter. The results are shown in FIG.

An examination of FIG. 7 shows that fluorescence falls below zero when 6 kg / t of carbon dioxide chlorine charge is accompanied by 5 kg / t of sodium hypochlorite. When the sodium hypochlorite was lowered to 2.9 kg / t, zero fluorescence was not achieved.

the pH and the influence of the amount of ClO ₂ to the residual ClO ₂ was measured. The results are shown in Figures 8 and 9.

Analysis of Figures 8 and 9 show that it was a remnant of ClO ₂ 0.01gpl less than about two charge of sodium hypochlorite. Additional sodium hypochlorite also acted to increase the pH from 3.2 to 4.5 at a fluorescence of 0.5.

Example  4

D → H single stage, between D and H Thickening  Or no flushing

Recycled paper was reacted with chlorine dioxide and sodium hypochlorite in a single stage. The addition of sodium hypochlorite was varied to 0, 2, 5, 10, 15 and 18 minutes, where zero represents simultaneous addition (basal case). No washing or thickening was performed between the two chemicals. For all operations, the total residence time was 20 minutes; The consistency was 11% and the temperature was 71.1 ° C. The effects on brightness and whiteness were evaluated as a function of time delay in the addition of sodium hypochlorite. The results are shown in Figures 10 and 11.

A review of Figure 10 shows that all work has fluorescence between 1.6 and 2.1. Although the data range appears to be significant, most values have lower fluorescence than the baseline, indicating that the delay does not have a deleterious effect or only a small deleterious effect.

10 and 11 further indicate that both the UVin and UVex were increased in the case of the initial retardation of the addition of the sodium hypochlorite with a brightness and whiteness of 2 minutes and that the hue also increased with respect to the first delay. All parameters remained relatively constant for increased sodium hypochlorite delay.

Claims (11)

a) contacting a pulp slurry containing the fluorescent agent with chlorine dioxide;
b) contacting said pulp slurry with a sodium hypochlorite component in an amount sufficient to reduce fluorescence of said pulp,
Wherein the sodium hypochlorite component is selected from sodium hypochlorite, one or more sodium hypochlorite derivatives or combinations thereof,
Wherein the pulp is contacted with the sodium hypochlorite component either simultaneously with contact with chlorine dioxide or after contact with chlorine dioxide. The method of claim 1 wherein the pulp is contacted with the sodium hypochlorite component after less than about 2 minutes of contact with the chlorine dioxide. 3. The method of claim 2, wherein the pulp is contacted with the sodium hypochlorite component after less than about 30 seconds of contact with the chlorine dioxide. 4. The method of claim 3, wherein the pulp is simultaneously contacted with the sodium hypochlorite component and chlorine dioxide. 5. A process according to any one of claims 1 to 4, wherein the amount of chlorine dioxide component in contact with the pulp ranges from about 4 to about 15 kg / t (1000 kg) of dry pulp. 6. The method of any one of claims 1 to 5 wherein the amount of said sodium hypochlorite component in contact with the pulp ranges from about 2 to about 12.5 kg / t (1000 kg) of dry pulp. 7. A process according to any one of the preceding claims, wherein the weight ratio of chlorine dioxide to the sodium hypochlorite component ranges from 1: 1 to 3: 1. 8. The process according to any one of claims 1 to 7, wherein the weight ratio of chlorine dioxide to the sodium hypochlorite component ranges from 1.2: 1 to 1.8: 1. 9. The method according to any one of claims 1 to 8, wherein the pulp slurry has a consistency in the range of about 4% to about 20% before being contacted with chlorine dioxide. 10. The method of any one of claims 1 to 9, further comprising adding a predetermined amount of a pH control chemical to adjust the pH of the pulp slurry to a value in the range of 2 to 7. 11. The method according to any one of claims 1 to 10, wherein the sodium hypochlorite component is sodium hypochlorite.

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