CS213345B2 - Method of preparation of chemical pulp - Google Patents

Description

The present invention relates to a process for the preparation of very high cellulose-cellulose chemical pulp using a kraft process.

Chemical pulp with high cellulose content is needed for the production of soluble pulp. The technically acceptable soluble pulp contains at least 90% by weight of cellulose.

Currently, soluble pulp is prepared by a sulfite batch. The high cellulose content of the soluble pulp is achieved by bleaching the pulp with hot alkali at a suitable alkali concentration and temperature.

The preparation of soluble pulp by the sulfite process is very expensive due to the low yield of the method and the content of extracted material in the pulp. In addition, sulphite plants are very old and would require large investments to be effective. In addition, environmental solutions are very expensive in the sulphite process.

In preparing soluble pulp by known methods, the other pulp components dissolve in the mercerization solution, which reduces the yield of soluble pulp and prevents the formation of high quality dissolving pulp.

There is a growing demand for synthetic fibers worldwide and hence the need for soluble pulp.

According to Finnish Patent No. 52 230, soluble pulp is produced from unbleached raw pulp by removing hemicelluloses with an alkaline solution containing 4 to 13 why. sodium hydroxide at 0 to 120 ° C. This process requires the use of a relatively large amount of alkali (600% based on pulp) and a relatively long batch time (about 230 min). Alkaline extraction cannot take place simultaneously with bleaching because the process is not able to consume all the alkali used.

The present invention provides a process for preparing chemical pulp containing more than 90 why. by weight of kraft cellulose by a process of cooking wood raw material, such as sawdust, in the presence of a significant excess of alkali, characterized in that the total alkali content is 20 to 40% by weight, preferably 30% by weight, calculated as Na ₂ and the sulphide content is 20 to 50% by weight, preferably 30 to 40% by weight, based on the amount of Na &_lt; 20, the batch time is 5 to 20 minutes, preferably 10 minutes and the temperature is 180 to 190 ° C. the hemicelluloses are dissolved, whereupon the pulp obtained is bleached, preferably using a bleaching solution.

Qj) a stream containing 5 to 10% by weight, preferably 7% by weight of alkali, and chlorine dioxide, chlorine, sodium hydroxide and hydrogen peroxide at a temperature of 20 to 30 ° C.

The process of the invention is based on the preparation of soluble pulp by the kraft process. The kraft batch is carried out continuously in the presence of a considerable excess of alkali.

The batch time is very short and the temperature is quite high. Under these conditions, most of the hemicellulose and other wood substances dissolve during the batch, which is very disadvantageous with respect to pulp for papermaking, but highly desirable with respect to dissolving materials. This dissolution of the hemicellulose increases the N-cellulose content directly on the batch to an extremely high value (up to 89%). In a normal kreit process, the cellulose content is about 80%. if the cellulose content increases during the batch, the reactivity of the pulp improves, which contributes to the preparation of high-quality soluble cellulose from the pulp.

After the batch, the pulp is exposed to cold alkali while bleaching using about 7 why. 20 to 30 ° C; at a fairly low alkali content, and at a particularly low temperature. Altering the alkali content affects the α-cellulose content obtained. The cellulose content of the pulp thus prepared is at least 9 (1%, usually about 92% w / w).

The yield of soluble pulp prepared according to the invention is about 37%, calculated on the amount of absolutely dry wood, and pulp. prepared in this way does not contain any extracted materials, the presence of which in the soluble pulp is detrimental;

The normal cellulose content of the bleached pulp prepared by the kraft process is about 84% by weight.

The best raw material is sawdust arising from • sawing wood that has been used for a variety of secondary purposes. Other wood raw materials may also be considered.

According to the invention, sawdust or shavings are produced under Bauer batch chemical pulp having a very high cellulose content after batch, and an ultra-high cellulose content is obtained by bleaching in a cold alkali environment with an alkali content of only 7% . The product obtained can be used as such as a raw material in a dissolution process. This process is not environmentally harmful compared to current methods of preparing soluble pulp.

Example

Preparation of chemical pulp, containing more than 92% by weight of cellulose, from sawdust.

Sawdust from the Kaukas plant is used in the tests.

Batch:

total alkali Na ₂ O 27% sulphide Na ₂ O 30%

Brewing curve min 182 ° C kappa number about 30

The cooking liquid is preheated before being pumped into the digester.

Performance ratio:

wood: liquid = 1: 5

The batch is carried out in a 50 L cooker using direct steam.

After the batch, the pulp in the digester is washed, disintegrated in a separator and passed through a flat screen.

The amount of wood in each batch was 5 kg of absolutely dry wood.

The results of post-batch pulp analysis are shown in Table I.

Bleaching:

Bleaching is performed using a packaging combination

D — C — A — H — P — SO ₂ where

D is the pretreatment of chlorine dioxide carried out in a small mixer using up to half the amount of chlorine in the overall chlorination stage. Chlorine content is 2.2%, consistency 2.5% and temperature 20 (>.

C denotes the action of chlorine at 20 ° C, a chlorine content of 2.2%, a consistency of 2 why. and a reaction time of 35 min,

A denotes alkali treatment with 7.0% NaOH, 30 ° C, 10% consistency, 180 min reaction time, pH 11.0 to 11.3,

H denotes hypochlorite treatment; active chlorine content 1.8%, temperature 40 ° C, consistency 10 ° / o, reaction time 180 min, pH 10.0 to 10.5; hypochlorite is used for + i H _n ,

P denotes hydrogen peroxide treatment; hydrogen peroxide content 1.5%, temperature 80 degrees Celsius, 10 O / o consistency, reaction time 180 min, pH 10.0 to 10.5 and

SO ₂ means acidification with sulfur dioxide-containing water, pH below 4.5, temperature 20 degrees Celsius, consistency 3%, reaction time 15 min.

The amount of pulp at each bleaching was 1 kg of absolutely dry pulp.

After each step, washing with ten times the amount of water was performed.

The results of the bleached pulp analysis are shown in Table II.

Results of pulp analysis after batch

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Comparative example

Preparation of chemical pulp from sawdust using a batch batch.

The saws from the Kaukas sawmill are used in the tests.

Batch:

active alkali Na ₂ O 16% for wood sulphide Na ₂ O 30%

Cooking curve:

° C / 30min, 80 ° C / 120min, 170 ° C / 70min. At 170 ° C a kappa number of about 30.

Performance ratio:

wood: liquid = 1: 4.5.

The amount of wood per batch was 2 kg of absolutely dry wood.

After the batch, the pulp is defibrated and passed through a flat sieve with a 0.25 mesh plate.

The results of post-batch pulp analysis are shown in Table III.

Bleaching and semi-bleaching bleaching combination:

Cl ₂ —N — Cl ₂ —A — H 1 —SO ₂

(b) full whitening of the bleaching combination:

and Cl ₂ —N — Cl ₂ —A — H [- · H — —SO ₂ bj Cl ₂ —-N — Cl ₂ —A — Hj — P — SO ₂

The batches described above (5j) are combined into a single sample and bleaching is performed, each batch used corresponding to 1 kg of absolutely dry pulp.

Bleaching conditions:

Cl ₂ : temperature 20 ° C, consistency 2%, reaction time 30 min, pH at the end of the step about 2.0.

N: NaOH feed 23 kg / h, temperature 45 ° C, consistency 2%, reaction time 20 min, pH

10.5 to 11.3.

II Cl ₂ : 20 ° C, 6% consistency, reaction time 45 min, pH at end of stage 2.2.

A: NaOH feed 15 kg / h, 50 ° C, 10% consistency, reaction time 120 min, pH at end of stage about 11.3.

Hp semi-white temperature 27 ° C at kappa number 20, 30 ° C at kappa number 33, at full bleaching at both hardnesses of 45 ° C, consistency 8%, reaction time 120 min, pH 10.0 to 10.5.

H _n : 45 ° C, 8% consistency, reaction time 120 min, pH 10.0 to 10.5.

EDTA wash: EDTA feed 2 kg / h, temperature 45 ° C, consistency 3%, reaction time 15 min, pH 9.5 to 10.0.

P: 60 ° C, 10% consistency, reaction time 120 min, pH 9.0 to 10.0. Sodium silicate additives 50 kg / h (41 ° Béj and MgSO ₄ .7 H ₂ O 1.0 kg / h.

SO ₂ : acidification with sulfur dioxide-containing water (pH below 4.5 µ, temperature about 20 ° C, consistency 3%, reaction time 15 min.

The results of the bleached pulp analysis are shown in Table IV.

Table III

Results of sawdust pulp analysis

Batch No Number kappa Viscosity cm ³ / kg CH ₂ Cl-extract why. Yield% sifter buniěina K 1356 33.7 - 0.03 45.0 K 1357 35.1 - - 0.03 45.1 K 1358 33.8 - - 0.06 44.2 YK 1360 30.4 - - 0.08 44.0 K 1361 31.8 - 0.06 44.4

Results of bleached pulp analysis

Bleach No Kappa Number ........ PC Yellowing Viscosity CH ₂ Cl ₂ -extract, to ° / o Yield% calculated unbleached SCAN clarity, Clarity ° / o KCL cm ³ / g why. yield on wood pulp why. _bleaching__

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Claims (2)

Process for preparing chemical pulp containing more than 90% by weight of cellulose by a kraft process by boiling a wood raw material such as sawdust in the presence of a substantial excess of alkali, characterized in that the total alkali content is 20 to 40 why. by weight, preferably 30% by weight, calculated as Na ₂ O per amount of wood raw material, and the sulfide content is 20 to 50% by weight, preferably 30 to 40% by weight, calculated on Na ₂ O, batch time 5 to 20 min, preferably 10 min and the temperature is 180 to 190 ° C, with a larger portion of the hemicellulose being dissolved, whereupon the pulp obtained is bleached, preferably using a bleaching solution containing 5 to 10% by weight, preferably 7% by weight of alkali, and chlorine dioxide, chlorine, sodium hydroxide and hydrogen peroxide at 20-30 ° C. 2. The process of claim 1, wherein chlorine is used in an amount equal to up to half the total amount of chlorine in the chlorination stage and the pulp is treated with chlorine in two stages.