FI91289C - Method and apparatus for bleaching the pulp - Google Patents

Description

i 91289

The present invention relates to a method for bleaching a pulp, in which the oxidized pulp is passed stepwise to an oxidation washing step, a mixture, an alkaline extraction solution, an extraction and an extraction step washing step, and a casting step obtained from the latter washing step.

Part of the extraction waste liquor is recycled in connection with the bleaching of the chemical pulp, usually to recover the heat and to replace the acidic chlorination waste liquor. The recycled extraction broth is added to the chlorinated pulp as it exits the scrubber from the chlorination stage scrubber through the scrubber jets and this recycled broth travels with the pulp back to the extraction stage (Histed, JA and Nelson, Jr., GG: The Bleaching of The Bleaching of 3rd Edition, TAPPI Press, Atlanta, 1979, p. 393; Histed, JA and Nicolle, FMA, Pulp Paper Mag. Can. 74 (5) T171 (1974); Wartiovaara, I., Paper and Wood 62 (5) 319 (1980), and Wartiovaara, I., Pulp Paper Can. 81 (7) T167 (1980)).

Alalia has also been previously known (Burkart, L.F., Paper Trade J. 156 (2) 33 (1972), and Azad, A.M. and Burkart, L.F.,

Pin 59 (4) 140 (1976)) uses sodium hydroxide-fortified recycled broth as the first extraction broth. This has been found to reduce the amount of sodium hydroxide used in the extraction step by about 16%.

The object of the present invention is to improve the process and the apparatus for reducing the amount of chemical required in the extraction step of the chemical pulp bleaching treatment.

2

The process according to the invention is characterized in that the pulp passes after the oxidation washing step of the additional washing step before mixing with the alkaline extraction solution and that the slurry obtained from the washing step of the extraction step is recycled to the mass immediately after the oxidation washing step.

According to the invention, the bleaching process, in which the pulp is treated alternately in an oxidation step, an oxidation washing step, an alkali solution mixing step (usually sodium hydroxide and steam) and an extraction step followed by an extraction washing step, is thus improved. The waste liquor from the extraction washing step is recycled to the pulp immediately after the oxidation washing step. The effluent obtained from the additional step is recycled for use in the oxidation washing step.

The invention also relates to a bleaching apparatus comprising an oxidizing tank, an oxidizing scrubber, means for mixing steam and an alkali extraction solution into the pulp, an extraction tower, an extraction scrubber and a line for recycling the effluent of the scrubber in the apparatus. The apparatus is characterized in that there is an additional scrubber between the oxidizing scrubber and the mixing means, that the apparatus comprises a line for circulating the broth of said additional scrubber to the oxidant scrubber, and that the scrubber of the extraction scrubber is

The invention will now be described in more detail with reference to the accompanying figures, in which: FIGURE 1 is a simplified flow diagram of a circulating system of a conventional bleaching plant in which the extraction step follows a chlorination step.

FIGURE 2 is a flow chart similar to Figure 1 showing an improved version of the present invention.

Detailed Description of Figures 1 and 2: The chlorination described herein comprises various forms of the chlorination step in which k1 chlorine dioxide partially or completely replaces chlorine. Chlorine and chlorine dioxide are examples of oxidizing cores that can be used before the new step. Each new step, preceded by an oxidizing step, can be improved by the present invention.

More specifically, as shown in Figure 1, a conventional chlorine bleaching system with a recycle stage includes a chlorination tower 10, tubes 12 leading to a scrubber scrubber 14, tubes 16 leading to a mixer 18 where the alkali solution and steam are mixed into the pulp, and further 20 and a transport tube 24 to the extraction stage scrubber 26. A method, 28, is required to transport the broth from the extraction stage scrubber 26 to the chlorination stage scrubber 14.

The present invention is shown in Fig. 2, in which reference numerals are the same as in Fig. 1. However, it should be noted that Fig. 2 further shows a scrubber 30 interposed between the chlorination scrubber and the mixer 18. In this case, the effluent 4 of the extraction scrubber 26 is recycled through the line 28 into the pulp through the point between the scrubbers 14 and 30. At this point, it is advantageous to use a turbulent slurry to ensure proper mixing i. The effluent from the additional scrubber 30 is recycled to the chlorination scrubber 14 via line 32.

The effluent used for the jets of the additional scrubber 30 is obtained via line 34 and may be leachate or water (not shown). The washing step is important because it removes the NaHCO 3 and CO 2 (together with NaCl) formed in the reaction between the chlorinated mass and the extraction broth. If NaHCO 3 and CO2 remain in the pulp, they consume, but do not clot, much of the chemical charge in the extraction step.

Example 1

Example 1 describes the sodium hydroxide preservatives which are possible by washing the leachate treated with the pulp.

The spruce sulphate pulp (Kappa number 27.5) was chlorinated, washed well and pressed into a 20% composition. The pulp was then diluted to 10% with the broth from the first extraction step. This required 5 ml of recycled broth for each gram of mass. This 5 ml / g could also have been added to replace the broth associated with the pulp or simply a thinner mass with a diluent. The extraction effluent, which had a temperature of 60 ° C, was allowed to act on the pulp for 5 minutes. The mass was washed with water and extracted with liquid alkali. The second sample was chlorinated, washed thoroughly and pressed into a 20% composition. The pulp was then diluted to 10% composition first with the broth from the second extraction step. The mass of the extraction waste at 60 ° C was allowed to act on the mass for 5 minutes, alkali was added and the extraction was completed. As a control sample, a pulp sample was used, which, after washing and chlorination, was extracted without pre-aging. All 3 samples were then bleached according to the partial DED system.

When the broth recycling and washing were used according to the present invention, a final brightness of 89% could be easily achieved, although the chemical consumption of stage E1 was reduced by one third, as shown in Table 1. In a conventional cycle where the circulating broth is not washed away from the pulp, no advantages could be found in terms of CE Kappa number or NaOH consumption.

E ^ -lieraen NaOH charge CE ISO Brightness TABLE 1 6 rotation in the extraction stage Kappa%% oven dried figure D2 ___mass ta________ ____

No rotation 3.0 5, A 81.0 89.5 2.0 6.1 76.7 1.0 9.0 56.5 84.6

Normal 3.0 5.5 81.4 89.6 rotation 2.0 5.8 76.6 87.9 (no washing) 1.0 8.4 60.7 85.5

Rotation and 3.0 5.1 82.2 89.9 This is 2.0 5.3 81.7 89.6 R a av awashing 1, 0 ____________ 5.8 ___ 7 9.6_88.7

Unbleached pulp: Black sulphate pulp, Kappa number 27.5 C-step: 5.5% of C1 2 oven-dried pulp, 45 min at 25 ° C, 3.5% composition E-step: 90 min 70 ° C: 10% composition

D1 or v: 1% C102 and 0.55% NaOH

oven-dried pulp; 3 h at 60 ° C, 10% composition E2 step: 1% NaOH from oven dried pulp; 1 h at 60 ° C; 10% composition D2 step: 0.5% NaOH from oven dried pulp; 3 h at 60 ° C; 10% composition 91289 7

The extraction broth used for recycling was quenched by extracting the same chlorinated pulp with 3% NaOH based on the oven-dried pulp.

Example 2

It can be seen from Table II Kay that the alkali savings shown in Example 1 are achieved when the rotation system reaches a steady state.

In run 1, a batch of spruce sulphate (Kappa number 27.5) was used, which was chlorinated, washed thoroughly and extracted (3.0% NaOH in the oven dried pulp) without prior leaching. In run 2, another sample from the same mass, which was chlorinated, was washed thoroughly and compressed into a 20% composition. Thereafter, the pulp was diluted in the extraction broth obtained from the extraction step of Run 1. The extraction broth was allowed to act on the pulp at 60 ° C for 5 minutes.

The pulp was washed with water and extracted with only 2% NaOH from the oven dry pulp. In run 3, a third sample from the same mass was used, which was chlorinated, washed thoroughly and compressed to a 20% composition. Thereafter, the pulp was diluted with extraction broth from Ajo 2. The pulp was washed again with water and extracted with only 2% NaOH from the oven-dry pulp. The method in which the broth from the previous run was utilized was repeated. It can be seen from Table II that the CE Kappa number remains unchanged even if the NaOH input is reduced from 3.0% to 2% of the oven-dried pulp in Times 2, 3 and 4.

8

Run 5 used a pulp sample that was chlorinated, washed thoroughly, and extracted with only 2% NaOH from the oven-dried pulp. The CE Kappa number was higher than in the experiment according to the invention, waste 1 cycle and washing.

The 2% NaOH charge according to the present invention, based on the oven dry mass, clearly corresponds to the 3% NaOH used in the usual way.

TABLE II

Process- Driving Round NaOH input CE

type no. used in the extraction step Kappa waste waste% of oven dried figure _________mass t a____

Normal 1 No rotation 3.0 4.3 linen

Cycle / 2 Run 1 2.0 4.3

Washing

Cycle / 3 Run 2 2.0 4.1

Washing

Cycle / 4 Run 3 2.0 4.0

Washing

Normal 5 No rotation 2.0 5.2 lines________________

Conditions: As in Table I, footnote.

91289 9

Example 3 This example shows that changes in time and temperature during manual production of chlorinated pulp with recycled extraction broth do not affect the final result of the process.

In the example, a spruce sulphate pulp (Kappa number 27.5) was used, which was chlorinated, washed thoroughly and pressed into a 20% composition. The pulp was then diluted to a 10% composition with the broth from the first extraction step. A combination of time and temperature was chosen for the reaction between the chlorinated mass and the recycled extraction broth. The pulp was washed with water and then extracted with 2.0% NaOH based on the oven-dried pulp. The experiment was repeated in terms of the time and temperature of the reaction between the different chlorinated pulp and the recycled extraction broth.

It can be seen from Table III that a serviceable reaction between the chlorinated pulp and the recycled extraction broth is achieved in 30 seconds at a temperature of 25 ° C and above. It is important here that the reaction can be carried out in a tube and that no special reaction is required.

TABLE III

1 o

Recycled recycled temperature Temperature Contact time CE

° C min Chapter 1 0.5 25 3.9 60 0.5 4.0 60 1.0 4.0 60 5.0 4.1 ___ 60 ___ 10.0 ______ 3_j_9_ Conditions for phases C and E: As in the footnote to Table I.

Example 4

The addition of a small amount of chlorine dioxide to the pulp in the chlorination step is generally used to minimize the disadvantages caused by chlorination.

It can be seen from Table IV that, as usual, the CE Kappa number of the treated pulp does not change very much, even if a 10% CLC® compensation is used. In addition, by treating the chlorinated pulp according to the present invention, the amount of NaOH in step E can be reduced to 2% or less, regardless of whether C102 is used or not.

TABLE IV

1 1 91289

Process C ^ substitution NaOH input CE

type C-stage uu11or hees sa Kappa-%% of oven-dried figure _____ ___________mass________

Ordinary 0 3.0 4.3

Spin / Wash 0 2.0 4.2

Spin / Wash 0 1.5 4.2

Rotation / Washing 0 1.0 4.1

Ordinary 10 3.0 4.1

Spin / Wash 10 2.0 4.0

Cycle / Wash 10 1.5 4.3

Cycle / Wash ____ 1 £ ______ 1, 0 _____ 4.4_

Unbleached pulp: Black six sulphate pulp,

Kappa number 27.5 C-step: 5.5% CI2 in the oven-dried mass; 45 min at 25 ° C; 3.5% composition

Cp step: 5.0% CI2 and 0.2% C102 from the oven dried pulp; 45 min at 25 ° C; 3.5% composition.

Step E: 90 min at 70 ° C; 10% composition.

The extraction broth used for recycling was prepared by extracting the same chlorinated mass with 3% NaOH based on the oven-dry mass.

1 2

It is to be understood that the embodiments described above are intended only to illustrate the invention and that changes and modifications may be made by those skilled in the art.

It is also clear that a substitute bleaching system can be used to carry out the steps of the invention.

The method of using the extraction medium as described above does not depend on the chemicals added to the extraction step. The addition of oxygen, peroxide or hypochlorite to the extraction step does not affect the principle of reducing the sodium hydroxide charge by post-recycle and pre-extraction washing.

Claims (8)

91289 A process for bleaching a pulp, wherein the oxidized pulp l & pikHy is a stepwise oxidation washing step (14), a mixture (18) to an alkaline extraction solution, an extraction step (22) and an extraction step washing step (26), and the process is recycled (28) to the bleaching process. to the step, characterized in that the pulp is added to the pulp after said additional oxidation washing step (14) before mixing (18) with the alkaline extraction solution (30) and that the slurry obtained from the extraction step washing step (26) is recycled to the pulp (28) after the oxidative washing step. A method according to claim 1, characterized in that said additional washing step (30) is performed with a liquid or an extraction roller gel. A method according to claim 2, characterized in that said alkaline extraction solution feeds a mixture of sodium hydroxide and steam. A method according to claim 1, characterized in that the broth of the excess washing step (30) is used in the oxidation washing step (14) and added to the pulp in the oxidation washer (14). A method according to claim 1, characterized in that turbulence is provided at the core of the extraction liquor to provide efficient mixing of the extraction liquor and the oxidized washed mass. The method of claim 1, wherein the oxidized pulp in the oxidizer washer (14) of the oxidation washing step, characterized in that: (a) the pulp is conveyed to an excess scrubber (30); (b) washing the pulp in said excess washer (30); (c) mixing the alkaline extraction solution into the pulp after said additional washing step (30); (d) a pulp sheet extraction step (22); (e) the extracted pulp is washed in an extraction stage scrubber (26); (f) circulating (28) the broth of the extraction stage scrubber (26) to the pulp at a point prior to said additional scrubber (30); and (g) conveying (32) the broth of the top scrubber (30) to the oxidation scrubber (14). Process according to one of Claims 1 to 6, characterized in that the oxidized pulp is chlorinated pulp. A bleaching apparatus comprising an oxidation tower (10), an oxidizing scrubber (14), means (18) for mixing hydride and an alkali extraction solution into a pulp, an extraction tower (22), an extraction scrubber (26) and a line (28) for circulating the extraction scrubber dispenser in the apparatus, that there is an extra scrubber (30) between the oxidizing scrubber (14) and the mixing members (18), that the apparatus feeds a line (32) for circulating said extra scrubber dispenser to the oxidizing scrubber and that the extraction scrubber (26) , which is in the silence of the oxidation scrubber and said excess scrubber. 91289