JPH0364588A - Washing of digested pulp - Google Patents

Abstract

PURPOSE:To decrease the kappa-value after washing, reduce the pulp bleaching load and waste-water treatment load in latter stage and improve the quality of pulp by placing a parallel flow washing tank before a washing machine and subjecting the pulp to parallel flow washing with a filtrate discharged from the washing machine. CONSTITUTION:Pulp and black liquor discharged from a digester 1 are passed through a slurry channel 4 and introduced into a parallel flow washing tank 2, in which the pulp, etc., are transferred upward together with a filtrate discharged from a vacuum washing machine 3 via the channel 4. In the course of ascending the pulp slurry together with the filtrate, the black liquor included in the cavity of the pulp fiber is permeated and diffused into the filtrate. The pulp slurry reaching the top of the parallel flow washing tank is separated into solid and liquid in a filtrate extraction zone 9. Separately, the pulp free from the black liquor in the cavity is transferred through the channel 4 to a vacuum-type multi-stage washing machine 3. The pulp washed by the machine is transferred through a channel 5 to a bleaching step.

Description

[Detailed description of the invention] (A) Industrial application field The present invention relates to a method for cleaning pulp after cooking.

For example, cooking chemicals and cooking chemicals are extracted from cooking black liquor produced in the pulp cooking process.
A more efficient and sophisticated method of cleaning by applying known methods in order to improve the recovery of combustible organic substances and further reduce the burden of subsequent pulp bleaching.

(B) Conventional technology The purpose of cleaning pulp slurry after cooking is to remove cooking black liquor, which contains lignin, resins, organic acids, and residual cooking chemicals solubilized by wood cooking, from the pulp. be. Particularly in the Kraft method, almost all of the cooking black liquor is recovered and reused. To wash the pulp after cooking, a washing process using various washing machines is adopted, and typical examples thereof will be explained below.

FIG. 2 shows a cleaning process in which several vacuum type cleaning machines 3, which are typical cleaning machines, are arranged in series. Pulp slurry accompanied by cooking black liquor discharged from the steaming kettle 1 passes through a flow path 4 and is introduced into the first stage of several successive vacuum type washers 3, where it is filtered from the subsequent vacuum type washer. Cleaning is done using The vacuum washer 3 is a device that once dilutes the pulp slurry to a pulp weight concentration of about 1 ounce, and then performs washing while concentrating and dewatering the pulp slurry to a pulp weight concentration of 14 to 16% on a rotating drum screen 2. Similar cleaning is performed continuously in the subsequent vacuum type cleaning machine. In this washing machine, the washing wave enters through the channel 6, and the washed pulp passes through the channel 5 and is led to the next process, which is a bleaching process for boat fishing. Further, the filtration after cleaning passes through the flow path 8 to the upper concave portion. As described above, countercurrent multi-stage washing is performed as a whole process (edited by Paper and Pulp Technology Association, Kraft Pulp and
Non-aqueous phase pulp P129-137). On the other hand, Figure 3 shows a cleaning process using a diffusion washer 10, in which a cleaning liquid is introduced while the pulp slurry rises almost uniformly between the thin partition walls installed in the washer, and the black liquor is diffused and cleaned. (edited by Paper and Pulp Technology Association, Craft Pulp and Non-Wood Pulp, pages 125-128). Furthermore, Figure 4 shows a process in which a cleaning liquid is introduced into the high heat zone 1↓, which is installed continuously in the digester 1, and is flowed countercurrently against the pulp slurry, thereby cleaning the pulp slurry at high temperature and under high pressure. Tester Industry Edition, Illustrated Paper Making Encyclopedia P102-103
).

(C) Problems to be Solved by the Invention The general cleaning process as described above is suitable for a short-time and continuous method of separating the cooking black liquor between pulp fibers. However, the lumen of the pulp fiber is filled with solubilized cooking black liquor, and in addition, the pulp fiber only has extremely small voids called membrane pores that communicate the fiber lumen with the outside world. Therefore, in the conventional method, it is difficult to completely remove the cooking black liquor that is solubilized and remains inside the fibers. Therefore, a considerable amount of cooking black liquor remains in the lumen of the pulp fibers even after washing, which causes an increase in the pulp bleaching load in the subsequent stage, insufficient recovery of cooking chemicals, a decrease in pulp quality, and an increase in the wastewater treatment load. cause problems. Water washing was invented to solve the above problems.

(D) Means for solving the problem, that is, the present invention, in washing the pulp after cooking, arranges a parallel current washing tank at the front stage of a multi-stage vacuum washing machine, and in this, washes the pulp after cooking as well as from the multi-stage vacuum washing machine. This is a method of cleaning while introducing a cleaning filter and moving in parallel.

The present invention will be explained in detail below based on the drawings.

FIG. 1 is a schematic diagram of a cleaning process showing an embodiment of the present invention. Pulp and black liquor discharged from the digester 1 pass through a slurry flow path 4 and enter a co-current washing tank 2.

In the parallel current cleaning tank 2, the water rises together with the filtered wave guided from the vacuum cleaner 3 via the flow path 4. While this pulp slurry rises in the washing tank for a predetermined period of time together with the filtration, the black liquor in the pulp fiber lumen percolates and diffuses toward the filtration side. After a predetermined residence time has elapsed in this manner, the pulp slurry that has reached the upper part of the co-current washing tank is separated into solid and liquid in a filter extraction zone 9 equipped with a strainer. The separated filter goes to the recovery process via the channel 8, while the pulp from which the black liquor in the lumen has been removed is then passed between the fibers.
Alternatively, in order to clean the black liquor on the surface of the fibers, the fibers are passed through the flow path 4 to an ordinary vacuum type multi-stage washer 3. After being washed several times in the washer 3, the pulp passes through the channel 5 to the next step, - a bleaching step for boat fishing. 6 is a flow path for cleaning liquid in the final stage of the vacuum type multi-stage cleaning machine.

In order to effectively utilize the present invention, the free lignin in the cooking black liquor that remains in the pulp fibers after cooking is softened in the co-current washing tank. It is necessary to prevent adsorption from occurring. Furthermore, the membrane pores on the pulp fibers that communicate the pulp fiber lumen with the outside world are extremely small, and the residual black liquor in the pulp fiber lumen cannot be sufficiently leached and diffused by contact with the cleaning liquid for a very short time. Therefore, in parallel current cleaning method 3 -1-
By obtaining a long residence time in the washing tank, preferably 2 hours or more, while providing the temperature conditions described above, the cooking black liquor that fills the inside cavity of the pulp fibers remains in a solubilized state and passes through the membrane pores on the fibers to the outside of the pulp fibers. Utilizes the effect of leaching and diffusion.

(E) Action Water washing utilizes the phenomenon of leaching and diffusion of black liquor remaining in the pulp fiber lumen, and in order to quickly apply this phenomenon, temperature conditions are required to solubilize the lignin in the cooking black liquor. . Furthermore, the cooking black liquor surrounding the pulp fibers after normal cooking (liquid ratio 2 to 4) has a concentration of 5 to 20% by weight, including chemicals and lignin content, and has a high flow resistance during washing and dewatering. Countercurrent cleaning is not an efficient method because sufficient cleaning fluid flow is not obtained. However, in the present invention, by flowing the cleaning liquid in parallel flow while providing the above-mentioned temperature conditions, the leaching and diffusion of residual black liquor is utilized.As a result, a high cleaning efficiency that cannot be obtained in the past can be obtained, and as a result, pulp bleaching that continues in the subsequent stage is achieved. This can reduce the load on water and wastewater treatment.

(F) Examples Examples and comparative examples of the method of the present invention will be described below, but the present invention is not limited thereto in any way.

Example 1 and Comparative Examples 1 to 3 Tasmanian eucalyptus hardwood chips were kraft-cooked under conditions of a liquid ratio of 2 to 4, an active alkali content of 16% based on the pulp, and a maximum cooking temperature of 170 degrees to produce cooked black liquor. A pulp slurry containing

After cooking, the pulp slurry was washed by the method of the present invention shown in FIG. 1 and the conventional method shown in FIGS. 2-4. The amount of cleaning fluid used at this time is all dilution factor (DF) 2.
．． It was carried out so that the result was 0. The results are shown in Table 1.

As shown in the table, in the cleaning according to the present invention, the amount of residual lignin in the pulp measured by the kappa number is smaller than that in the conventional method, and the degree of cleaning is improved.

In addition, although the residence time in the co-current washing atomization tank in the present invention is effective for a long time, the effect almost reaches its upper limit after 3 hours, and the deterioration of pulp quality becomes worse.
It turns out that about an hour is preferable. From the above results, by applying water washing to the pulp slurry after cooking, it is possible to reduce the burden of subsequent bleaching, improve pulp quality, and reduce the wastewater treatment load (f).

(Margins below) (Table 1) (G) Effects of the invention In cleaning the pulp after cooking, the cleaning liquid is introduced into the cleaning tank in parallel flow while maintaining the conditions of 1.40'C or more and 2 hours or more. The kappa number of can be reduced by about 20 to 30%. This effect is due to the pulp bleaching load in the latter stage and the negative effects of wastewater treatment.
I can be reduced by 30 to 40% and the pulp l'F+ quality can be significantly improved.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the cleaning process of the present invention. Figure 2.3.4 is a schematic diagram of a conventional cleaning process. 1: Digester, 2: Co-current washing tank, 3: Vacuum washer, 4 Pulp slurry flow path, 5: Washed pulp slurry flow path to the next process, 6: Washing liquid flow path, 7 Filter flow Path, 8: Flow path to the next step of filtration, 9: Filtering extraction zone, 10: Diffusion washer 11-・High heat washer Total 31 children-638 Rod 41D

Claims (1)

[Claims] 1. A method for washing cooked pulp, which is characterized in that, in washing the pulp after cooking, a co-current washing tank is provided upstream of a washing machine, and co-current washing is carried out using filtering waves from the washing machine.