JPH0149836B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for bleaching kraft wood pulp using the C-E-H-P continuous process in a pulp mill. Wood pulp produced by the kraft process produces very strong paper products. For many applications,
It is desirable to purify and bleach the kraft pulp to a reflectance or brightness of 80 or higher. Unbleached kraft pulp is very dark in color, and it is common practice in pulp mills to refine or bleach the pulp in several steps to produce pulp with the desired brightness at minimal cost.
One step can be defined as a series of steps starting with mixing the pulp with an aqueous solution of chemicals to form a slurry, allowing the chemicals to react with the pulp, and then removing at least a portion of the liquid from the pulp and washing the pulp slurry. The entire process used is called the bleaching process. An improved bleach reaction method for the bleaching process is described by Schroeder et al., tappi 39 :9, September 1954, 376-
It is published in "New method for bleaching kraft pulp using sodium peroxide" on page 384. The Schrader process uses a C-E-H-P bleaching sequence consisting of chlorination, extraction, hypochlorite and peroxygen steps.
It uses an "acid post-treatment" step between the hypochlorite step and the peroxygen step. Although the Schrader et al. patent claims improved peroxygen efficiency, the method is not applicable to any pulp mill. It will be appreciated that this method requires additional chemicals to neutralize the alkalinity after the hypochlorite, i.e. the acid, as well as a mixing device and a device for metering the acid into the system. The present invention provides a method that overcomes the limitations of Schroeder et al.'s method by eliminating sufficient water from the C-E-H-P bleaching sequence and the pre-chlorination step that incorporates the pulp slurry during the peroxygen bleaching step. It consists of an additional step of mixing the liquid into the pulp slurry from the hypochlorite step to adjust the pulp slurry PH to greater than (but not including) 3 and less than 7. The operating conditions for each step of the conventional bleaching process are those commonly used in industry. In the chlorination step (C), elemental chlorine is added to the pulp and water suspension to form a slurry in the chlorinated liquid. The rate of addition depends on the pretreatment of the pulp. Generally 50% of the total chlorine requirement of the pulp during the process
supply up to 90%. Pulp usually has a density of 3 to 4
%, temperature 20 to 40°C, and pH usually 2 or less. During chlorination, a portion of the chlorine is usually converted to enough hypochlorite to adjust the pH to the desired range. After chlorination, the chlorinated pulp is separated from the spent chlorine bleach solution and washed to remove hydrochloric acid, soluble chlorinated organic products, and other products evolved from the pulp during chlorination. After the chlorination step, the pulp is contacted with an alkaline solution in an extraction step (E) to remove chlorinated lignin from the pulp. Generally, the alkaline solution has an alkalinity of 0.7 to 2.5%, and the pulp slurry is used at a concentration of 10 to 18% and a temperature of 60 to 80°C. The pH of this solution is usually about 11 to 11.5.
Sometimes the extraction step may be alkaline extraction as well as bleaching and extraction of chlorinated lignin materials by a combined E/O step of elemental oxygen. The E or E/O step ends with separation of the pulp and alkaline extract and washing of the pulp. One or more hypochlorite bleaching steps (H) can be used after the extraction step. The concentration of the liquid used in this process varies from 2% to 5% available chlorine. Pulp slurries generally have a consistency of 5 to 18%. The PH is kept at about 9 or above to prevent hypochlorite formation, which is known to degrade cellulose. Temperatures are usually 35-50°C for appropriate times. After separating the pulp slurry from at least a portion of the hypochlorite bleaching solution, the pulp is washed. In the present invention, it is important to contact the pulp slurry from the hypochlorite step with sufficient liquid from the chlorination step to adjust the PH to below 7 before the pulp slurry enters the peroxygen bleaching step. It is. Since the liquid from the chlorination process usually has a pH of 1 to 2, the resulting pulp slurry usually has a pH of 2 to 7. The pulp slurry should be adjusted to a pH of 6 or less (but greater than 3), more preferably about 5. The pulp slurry concentration should be between 1 and 10 so that the liquid from the chlorination process can be thoroughly and uniformly mixed into the pulp slurry.
It is preferable to adjust it to %. Although it can be operated at concentrations below 1%, it may be necessary to remove the excess amount of aqueous solution before subjecting the pulp slurry to the peroxygen bleaching process. The optimum concentration will vary from factory to factory, but can be easily determined by those skilled in the art without extensive experimentation. To obtain improved results in the subsequent peroxygen bleaching step, all that is required is to mix the chlorinated solution into the pulp slurry so as to adjust the pulp slurry pH to less than 7. It was found that a short contact time of about 10 minutes was effective. Furthermore, this method does not require the use of any energy during the contact time to obtain good results. After contacting the pulp with the liquor from the chlorination step, the pulp is thickened if necessary and contacted with a peroxygen bleach solution. Usually, the peroxygen process white liquor contains 0.2 to 1% hydrogen peroxide based on the oven-dried pulp, and the pulp concentration is 4 to 20%. PH is about 10 to
11.5 and the temperature is about 35 for 1/2 to 5 hours.
The temperature is maintained between 80℃ and 80℃. However, according to the present invention, the proportion of hydrogen peroxide used in the process can be reduced to about 0.1% (based on oven-dried pulp), and a substantial improvement in brightness can be obtained. After the peroxide bleaching step, the pulp is washed and made suitable for use in the next papermaking process. Reuse of wash water is not uncommon in pulp bleaching processes. However, the wash water source contains only water from subsequent bleaching or purification steps and not liquid from previous bleaching or purification steps. After all, the pulp flow was created by Rapson's U.S. patent.
As described in No. 3,698,995, the entire wash water stream is used in the pulp stream and the reaction stream as it proceeds through a continuous pulp bleaching process. It was completely unexpected that the use of liquor separated from the chlorination step could improve the subsequent peroxide bleaching step. Specific examples of carrying out the present invention will become clear from the following examples, but the present invention is not limited thereto. The pulp was obtained from the Kraft mill after the C-E/OH bleaching step and mixed to make it homogeneous. A sample equivalent to 25 g of oven-dried pulp was used for all tests. The pretreated sample was diluted to a concentration of 3% with water and/or sufficient liquid to adjust the PH within the desired range from the Kraft chlorination bleaching process. The pulp slurry was suspended by vigorous stirring for 10 minutes at the experimental PH and then concentrated to a concentration of 10%.
Peroxygen bleaching process chemicals were added at the weight percent listed in the table on an oven dry (OD) basis. The peroxygen bleaching step was kept at 71°C for 2.5 hours. A handsheet was made and Ellefo luminance was measured. Experimental conditions and results are shown in the table. Figure 1 shows the effect of pre-treatment of pulp slurry to pH5 (inventive tests 2, 4, 7, 8 and 9) on peroxygen bleaching results, with the pulp concentration being 3% without adjusting the chlorinated bleach solution pH. Compare with reduced trials 1, 3 and 6. Comparative test 5 is a normal C- test as a white test.
It consists of a continuous E-/O-H bleaching process. Exams 10 and 11 are
This shows that although adjustment to PH3.0 is still effective in some cases, it does not significantly improve the subsequent P step bleaching results and is at the borderline (outside the invention). C-E-H-P means chlorination-extraction-hypochlorite-peroxygen. C-E/O-H is chlorinated-
Extract/Oxygen - means hypochlorite. C-E/
O-H-P means chlorination-extraction/oxygen-hypochlorite-peroxide. 【table】

[Brief explanation of drawings]

Figure 1 is a curve diagram showing the effect of pH adjustment on the peroxygen bleaching results of the C-E/O-H kraft pulp of the present invention.
Contains ₂ % H ₂ O.

Claims (1)

[Claims] 1. Chlorination-extraction-hypochlorite-peroxygen (C-
In the bleaching method for kraft wood pulp consisting of the E-H-P) process, sufficient water from the chlorination process is used before the pulp slurry is subjected to the peroxygen bleaching process containing 0.1% by weight or more of hydrogen peroxide based on the oven-dried pulp. 1. A method for bleaching kraft wood pulp, which comprises mixing a liquid into a pulp slurry from a hypochlorite step to adjust the pH of the pulp slurry to more than 3 and less than 7. 2. The method according to claim 1, wherein the pulp slurry concentration is 1 to 10%. 3. The method according to claim 1 or 2, which maintains PH at 6 or less. 4 Claim 1 or 2 that maintains PH at 5
The method described in section. 5. The method according to any one of claims 1 to 4, wherein the liquid mixed into the pulp slurry from the hypochlorite step is a spent chlorinated liquid.