TWI643998B - Process for reducing fluorescence in pulp - Google Patents

Abstract

The present invention relates to a method for reducing fluorescence in recycled pulp, wherein the method comprises: contacting a pulp slurry containing a fluorescer with chlorine dioxide; and contacting the pulp slurry with an amount sufficient to reduce the fluorescence in the pulp. The sodium hypochlorite component is contacted; wherein the pulp is contacted with the sodium hypochlorite component while or after being contacted with chlorine dioxide.

Description

Ways to reduce fluorescence in pulp

The field of the present invention relates to the manufacture of pulp and paper (e.g. recycled paper) from a pulp source containing a fluorescer. More specifically, it relates to reducing or eliminating fluorescence in pulp and paper.

Fluorescence is an optical phenomenon whereby light of an invisible wavelength (such as ultraviolet light) is absorbed and then re-emitted at a visible wavelength. Chemicals commonly called "fluorescent brighteners" or "optical brighteners" or "FWA" or "OBA" are added to the ingredients used in the manufacture of many fine papers (such as printing or carbon paper) as well as tissue and hand towel grade products In order to enhance the apparent brightness of the product. FWA is usually a compound based on stilbene and its chemical derivatives.

When FWA-containing paper products are regenerated, most of the fluorescent compounds remain with the fibers through the regeneration method. Some operating problems have been reported in deinked pulp mills and paper mills using recycled fibers due to color and brightness control. FWA can modify the apparent color of deinked pulp and damage or change the final quality of the final product. The presence of FWA can affect the response of UV-based online color sensors and can adversely affect the performance of the control loop.

In addition, it has been suspected that FWA can cause potential health hazards by leaching out of paper products made from recycled fibers. FWA is not allowed in some food-grade packaging. Therefore, FWA will need to be removed from recycled fibers intended for these food-grade applications.

Therefore, the industry still needs to reduce or remove FWA from recycled pulp in an effective manner.

It has been found that the fluorescence in recycled pulp can be reduced by a method involving a pulp containing a fluorescer and chlorine dioxide (ClO ₂ ) and sodium hypochlorite (or a sodium hypochlorite derivative) sufficient to reduce the amount of fluorescence in the pulp. A combination contact wherein the pulp is contacted with sodium hypochlorite at the same time or after the pulp is contacted with ClO ₂ .

In one embodiment, the ClO ₂ and sodium hypochlorite components are simultaneously added to the pulp or the sodium hypochlorite component is added shortly after the ClO _{2 is} added, wherein the sodium hypochlorite component contains sodium hypochlorite, a sodium hypochlorite derivative or a mixture thereof.

In one embodiment, ClO ₂ is added to the pulp simultaneously with sodium hypochlorite or sodium hypochlorite is added shortly after the ClO _{2 is} added. In the embodiment of the present invention, the sodium hypochlorite 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 less after adding ClO ₂ . Add in about 1 second. In one embodiment, ClO ₂ is added to the pulp simultaneously with sodium hypochlorite.

In the embodiment of the present invention, the amount of ClO ₂ added to the pulp is about 4 kg / t to about 15 kg / t (1000 kg) of dry pulp, or about 5 kg / t to about 10 kg / t or about 6 kg / t to about 8 kg / t. In the embodiment of the present invention, the amount of sodium hypochlorite added to the pulp is about 2 kg / t to about 12.5 kg / t dry pulp, or about 2.5 kg / t to about 8 kg / t or about 3 kg / t to about Within 6 kg / t. In one embodiment, the amount of ClO ₂ added is greater than the amount of sodium hypochlorite (by weight). In the embodiment of the present invention, the weight ratio of ClO ₂ to sodium hypochlorite is in the range of 1: 1 to 3: 1, or 1.1: 1 to 2: 1, or 1.2: 1 to 1.8: 1, or about 3: 2. In other embodiments of the present invention, lower chemical loads are also covered, and even if the fluorescence is not reduced to zero or near zero, a small reduction in fluorescence is also covered in the embodiments of the present invention.

Additional objectives, advantages, and novel features will become apparent to those skilled in the art after reviewing the following description.

In one aspect, the method of the present invention involves treating waste paper to form a pulp slurry containing pulp and a fluorescer (e.g., FWA), and applying chlorine dioxide to a process stream containing fluorescence before or simultaneously with sodium hypochlorite (pulp) ,paper). The pulp slurry may have a consistency in the range of about 4% to about 20%, or about 6% to about 18%, or about 8% to about 16% before the addition of chlorine dioxide. Can be operated at a consistency of less than 4% to, for example, match existing process streams in a pulp mill or paper mill and still benefit from the present invention, however, it is believed that a consistency of less than 1% will lose the benefit of adding sodium hypochlorite . In an embodiment, the delay between the two chemical additions does not grow so long that residual chlorine dioxide is no longer present. Preferably, there should still be a reaction time of at least 5 minutes after the addition of sodium hypochlorite.

In the examples of the present invention, the method is described as contacting the pulp with chlorine dioxide and sodium hypochlorite. "Contacting" means adding a chemical (contact pulp) to a pulp slurry or forming a chemical in situ by a reaction that forms a precursor chemical of a desired chemical. For example, according to the present invention, chlorine dioxide or sodium hypochlorite may be formed in situ in a pulp slurry and another chemical may be added to the slurry, or both chlorine dioxide and sodium hypochlorite may be formed in situ. In embodiments of the present invention, chlorine dioxide or sodium hypochlorite or both may be formed in situ in a separate process equipment or process stream and then added to the pulp slurry.

Although this article deals with contact with sodium hypochlorite (pulp) or the addition of hypochlorous acid Examples of the present invention for sodium (to pulp), but it should be understood that other embodiments of the present invention use sodium hypochlorite derivatives in place of (or in combination with) sodium hypochlorite. "Sodium hypochlorite derivative" means one or more chemical compounds produced according to the present invention due to the introduction of sodium hypochlorite together with chlorine dioxide into a pulp slurry, the (or) compound being dissociated by sodium hypochlorite during the fluorescence reduction process Or produced by the reaction of sodium hypochlorite with other compounds in the slurry, compared with chlorine dioxide alone, the presence of sodium hypochlorite derivatives improves fluorescence reduction. For example, it is believed that under certain conditions sodium hypochlorite will form hypochlorous acid in the solution of the pulp slurry. Therefore, hypochlorous acid is regarded as a derivative of sodium hypochlorite and the hypochlorous acid alone or in combination with sodium hypochlorite and the addition of chlorine dioxide to pulp are included in the scope of the embodiments of the present invention.

An example of the process flow is provided in Figure 1 below. Referring to FIG. 1, the chlorine dioxide system enters before conveying an equal consistency pump (MC) in a 12% consistency pulp. Sodium hypochlorite is added directly to the dynamic chemical mixer in less than 1 second. At high consistency, air pockets will usually form around the impeller. To avoid this, install a degassing pump. The neutralization chemicals used in one form of the chlorine compound are fed to a degassing pump, that is, applied to a gas rather than a pulp stream. The chlorine dioxide and sodium hypochlorite were thoroughly mixed into the pulp for about 20 minutes before the upflow column.

The time delay between the two chemicals can affect optical properties other than fluorescence, and strategies can be developed to optimize those optical properties related to the practice of the invention.

In one embodiment, the reaction of the two chemicals occurs in a single stage. Although a reaction vessel is specified in FIG. 1, in other embodiments, since The reaction time required to practice the present invention is relatively short, so a pipe is sufficient.

In the embodiment of the present invention, the pH of the chlorine dioxide / sodium hypochlorite / pulp flow can be adjusted with a pH adjusting chemical (acid, caustic or buffer). In the embodiment of the present invention, the pH is adjusted to a value in the range of about 2 to about 7. The specific target pH will usually depend on the system. For example, in a cleaning system with a fluorescence of about 5, the pH can be in the range of about 2 to about 5 or about 2 to about 4. If, for example, the system is buffered vigorously with calcium carbonate, the pH can be higher, for example in the range of about 4.5 to about 7. Moreover, the chlorine dioxide loading will decrease the pH and the sodium hypochlorite loading will increase the pH.

In the embodiment of the present invention, the temperature of the fluorescence reduction process (treated pulp slurry) is maintained in a range of about 40 ° C to about 85 ° C or about 65 ° C to about 80 ° C.

Although MC pulp and chemical mixers are used in some embodiments, it is expected that some fluorescence reduction will occur regardless of whether mixing is achieved.

It is believed that the combined application of chlorine dioxide and sodium hypochlorite requires less chemicals to remove fluorescence than the individual application of chlorine dioxide or sodium hypochlorite, and only a single stage is required.

When compared to sodium hypochlorite alone, the method of the present invention has the added benefit of being able to achieve zero or near zero fluorescence. It is believed that sodium hypochlorite alone is unlikely or at least extremely difficult to achieve zero fluorescence. Therefore, in one embodiment of the present invention, a sufficient amount of chlorine dioxide and sodium hypochlorite are added and the obtained pulp is maintained under conditions sufficient to achieve substantially zero or zero fluorescence. Essentially zero means that when measured with Technidyne Color Touch (CTH-ISO), the fluorescence is 0.25 Or lower.

When compared to chlorine dioxide alone, the method of the present invention reduces chemical loading while also preventing gas generation. Gas generation is caused by high levels of residual chlorine dioxide at the end of the process and is potentially dangerous if workers inhale it. According to the present invention, when chlorine dioxide is added in combination with sodium hypochlorite, when it is desired to reduce the maximum fluorescence, the residual ClO _{2 is} reduced, for example, the remaining is even less than one tenth of the initial amount.

In one embodiment, the fluorescence reduction process is implemented separately from any bleaching or brightening stage, wherein the goal of bleaching / brightening is to increase brightness and / or whiteness. In this embodiment, the fluorescent process does not increase the brightness or only increases the brightness relatively little. In the embodiment of the present invention, the brightness increase due to the practice of the fluorescence reduction process is less than about 5 ISO or less than about 0 ISO. In one embodiment, the fluorescence reduction process is performed before any bleaching or brightening stage.

In the embodiment of the present invention, the fluorescence reduction process uses an upflow tower or a pipe after adding both chlorine dioxide and sodium hypochlorite to the pulp slurry in less than about 30 minutes, or less than about 25 minutes, or less than about 20 minutes. Minute retention time to implement. In one embodiment, the residence time can be as small as 7 minutes, where excellent fluorescence reduction results have been observed.

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

The following examples include evaluating the effects of the time delay of adding sodium hypochlorite after adding ClO ₂ , the effects on the long reaction time of adding ClO ₂ alone, the effects of the amount of chemicals added, and the practice of the invention on the brightness and whiteness of paper influences.

Example 1 D-H two stages, thickening between D and H

The pulp is 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 hypochlorite was 1.785 kg / ton of dry pulp. The total combined reaction time for the two phases was 7 minutes. The previous work showed that the reaction was complete after 7 minutes. Chlorine dioxide time is 0, 2, 3.5 and 5 minutes. Zero indicates the basic situation of adding two chemicals at the same time. The consistency is 11% and the temperature at which the chlorine dioxide reacts with sodium hypochlorite is 71 ° C. The results are shown in FIG. 2.

Observation of Figure 2 reveals that fluorescence is increased due to a delayed, even as little as 2 minute, delay in the addition of sodium hypochlorite. Furthermore, once this loss occurs, the increase in delay has no additional effect on fluorescence.

Paper color can be defined by three coordinates L *, a *, and b * in a system called CIELAB, where L * is lightness, a * is red-green, and b * is yellow-blue. Measure these values and other optical properties with the delay of sodium hypochlorite addition. The effect on the L value is shown in FIG. 3 and the effect on hue is shown in FIG. 4.

Observation of Figures 3 and 4 reveals that brightness, L * value, and whiteness increase in a similar manner to fluorescence. These increases are for values with and without UVin.

The color tone is not the same as that of the fluorescent light, which not only increases during the first delay, but also continues to increase due to the increase in the delay of the addition of sodium hypochlorite.

The effect on the a and b values is shown in Figure 5. Observation of Figure 5 reveals the delay pair The a * value has little or no effect, but the b * value decreases, and the UVin decreases more than UVex.

Example 2 H-D without washing or thickening between D and H

Mixed office waste (MOW) is treated with a combination of chlorine dioxide and sodium hypochlorite in two separate methods. In the first method, chlorine dioxide is mixed with paper and sodium hypochlorite is added after 5 minutes, and then further mixed. The total residence time was 20 minutes and the temperature was 71 ° C.

In the second method, chlorine dioxide was added and allowed to react for 20 minutes alone. Sodium hypochlorite was then added to the paper without intermediate washing or thickening. The residence time at this stage is also 20 minutes, and the total residence time is 40 minutes. The results are shown in FIG. 6.

Observation of FIG. 6 reveals that, although having only half the residence time, both fluorescence and residual chemicals are lower than the first method. The D + H method has lower brightness and whiteness.

Example 3 FR method-zero fluorescence reduction

The recycled paper containing mixed office waste paper was treated with a combination of chlorine dioxide and sodium hypochlorite. The paper had an initial brightness of 89.95% ISO (including UV) and a fluorescence of 12.4, in which two chemicals were added simultaneously.

The load of chlorine dioxide varies from 0 kg / t to 15.5 kg / t, while sodium hypochlorite is added at 2.9 kg / t and 5 kg / t. The paper was bleached for 15 minutes in a constant temperature bath set at 68 ° C. The pH is not adjusted and allowed to fluctuate with the reaction. Measurement of brightness and fluorescence using D65 illuminator with Technidyne Color Touch instrument Light. The results are shown in FIG. 7.

Observation on Figure 7 reveals that when used with 5 kg / t sodium hypochlorite, the fluorescence reduction under a load of 6 kg / t chlorine dioxide is less than zero. When sodium hypochlorite is reduced to 2.9 kg / t, zero fluorescence cannot be achieved.

Measure the effect of the amount of ClO ₂ on pH and residual ClO ₂ . The results are shown in FIGS. 8 and 9.

Observations on Figures 8 and 9 reveal that for both sodium hypochlorite loads, the ClO ₂ residue is less than 0.01 gpl. Additional sodium hypochlorite was also used to increase the pH from 3.2 to 4.5 under 0.5 fluorescence.

Example 4 D → H single stage, no thickening or washing between D and H

The recycled paper is reacted with chlorine dioxide and sodium hypochlorite in a single stage. Addition of sodium hypochlorite differs from 0, 2, 5, 10, 15 and 18 minutes, where zero means simultaneous addition (basic case). No washing or thickening occurs between the two chemicals. For all operations, the total residence time is 20 minutes, the consistency is 11% and the temperature is 71.1 ° C. The effect of changes in the time delay of the addition of sodium hypochlorite on brightness and whiteness was evaluated. The results are shown in FIGS. 10 and 11.

Observation of Figure 10 reveals that all runs have a fluorescence between 1.6 and 2.1. Although the spread of the data seems obvious, most points have a lower fluorescence than the base case, indicating that the delay has no harmful effects or has only minor harmful effects.

Observations on Figures 10 and 11 further reveal that in the case where the initial delay of sodium hypochlorite addition is 2 minutes, both the brightness and whiteness of both UVin and UVex increase, and the hue of the first delay also increases. For increased sodium hypochlorite In terms of latency, all parameters remain relatively constant.

Figure 1 is a schematic diagram of a fluorescence reduction method using an upflow reaction vessel.

Figure 2 is a graph showing the effect of retardation of hypochlorite addition on brightness and fluorescence in Example 1.

Figure 3 is a graph showing the effect of the retardation of hypochlorite addition on the L value in Example 1.

Figure 4 is a graph showing the effect of retardation of hypochlorite addition on whiteness and hue in Example 1.

Figure 5 is a graph showing the effect of the retardation of hypochlorite addition on the a and b values in Example 1.

Figure 6 is a graph showing the effect of the chemical sequence of Example 2 on fluorescence.

Figure 7 is a graph showing the effect of chlorine dioxide loading on fluorescence in Example 3.

FIG. 8 is a graph showing the effect of chlorine dioxide loading on pH and residual chlorine dioxide in Example 3. FIG.

FIG. 9 is a graph showing the effect of chlorine dioxide loading on pH and residual chlorine dioxide in Example 3. FIG.

FIG. 10 is a graph showing the effect of the retardation of sodium hypochlorite added on brightness and fluorescence in Example 4. FIG.

FIG. 11 is a graph showing the effect of retardation of sodium hypochlorite addition on whiteness and hue in Example 4. FIG.

Claims (10)

A method for reducing fluorescence in recycled pulp, the method comprising: a) adding chlorine dioxide to a pulp slurry containing a fluorescer; and b) adding a sodium hypochlorite component sufficient to reduce the amount of fluorescence of the pulp slurry to In the pulp slurry, the sodium hypochlorite component is selected from sodium hypochlorite, at least one sodium hypochlorite derivative, or a combination thereof; characterized in that the sodium hypochlorite component is added at the same time or after the chlorine dioxide is added to the pulp slurry, wherein The sodium hypochlorite component is added to the pulp slurry less than 2 minutes after the chlorine dioxide is added. The method of claim 1, wherein the sodium hypochlorite component is added to the pulp slurry less than 30 seconds after the chlorine dioxide is added. The method of claim 2, wherein the sodium hypochlorite component is added simultaneously with the chlorine dioxide. The method according to any one of claims 1 to 3, wherein the amount of the chlorine dioxide added to the pulp slurry is within a range of 4 kg / t to 15 kg / t (1000 kg) of dry pulp. The method according to any one of claims 1 to 3, wherein the amount of the sodium hypochlorite component added to the pulp slurry is within a range of 2 kg / t to 12.5 kg / t (1000 kg) of dry pulp. The method according to any one of claims 1 to 3, wherein the weight ratio of the chlorine dioxide to the sodium hypochlorite component is in the range of 1: 1 to 3: 1. The method according to any one of claims 1 to 3, wherein the weight ratio of the chlorine dioxide to the sodium hypochlorite component is in the range of 1.2: 1 to 1.8: 1. The method of any one of claims 1 to 3, wherein the pulp slurry has a consistency in the range of 4% to 20% before the chlorine dioxide is added. The method of any one of claims 1 to 3, further comprising adding an amount of a pH adjusting chemical to adjust the pH of the pulp slurry to a value in the range of 2 to 7. The method according to any one of claims 1 to 3, wherein the sodium hypochlorite component is sodium hypochlorite.