DE1296503B - Process to avoid the formation of volatile, malodorous substances in the manufacture of sulphate pulp - Google Patents

Description

variable amount of black liquor from a pre- io compounds when cooking pulp after the contains going cooking process. However, avoid using the sulfate method. The surprising result a very annoying discomfort during cooking has been fully proven by practical experiments Odor. The malodorous substances exist in the first place.

Line made of methyl mercaptan (CH ₃ SH), methyl sulfide The wood that is pretreated with caustic soda can

(CH ₃ ) ₂ S and methyl disulfide (CH ₃ ) ₂ S ₂ . The first 15 occur in various forms, e.g. B. as used to these substances smells the most intense and is therefore very easy to cook schnitzel, mechanically divided schnitzel

or specially mechanically processed chips, so-called fiber-dispersed wood (also together with Caustic soda processed). If the ao wood has a high resin content, the resin is dissolved by the caustic soda, one part by weight of NaOH dissolving 7.6 parts by weight of rosin acid. Instead of wood, you can use straw, grass, etc. use.

The lignin part of the wood comes from the wood in an apparatus used to manufacture it. Furthermore, it is kept processed by so-called fiber-dispersed wood, like that it is plausible that this ether is used during cooking, this processing is expedient with the ge

difficult to remove in a satisfactory manner.

An explanation for the origin of methyl mercaptan results from the formula

RO- CH ₃ + NaSH -> RONa + CH ₃ SH

Presumably the other two substances are secondary to methyl mercaptan. RO-CH ₃ is an ether, and it is believed that this comes from the

(when the temperature is increased) decomposes and that it _{gives partly CH 3} SH partly CH ₃ OH according to the following formulas:

ROCH ₃ + NaOH - ► RONa + CH ₃ OH ROCH ₃ + NaSH -> RONa + CH ₃ SH

If the cooking is only carried out with NaOH (pure alkali cooking process), it is well known that no malodorous substances.

A method is already known according to which the wood chips are impregnated with black liquor during the production of sulphate pulp in order to suppress the development of mercaptans and other malodorous products which occur during cooking. It is in particular so proceeded that are treated the wood chips with the black liquor over a period of 45 minutes at a temperature of 12O ⁰ C and a pressure of 8.42 kg / cm ² and impregnated. Such a treatment reduces the development of the mercaptan, but this effect is dependent on the fact that a cooking liquor with said pretreatment is carried out using caustic soda, whereby the two processes take place simultaneously.

The treatment of the chips with the pretreatment liquid, e.g. B. caustic soda, can (except through mechanical processing) can be carried out in various ways, for example as follows:

a) The chips are immersed in the pretreatment liquid and then in the digester convicted;

b) the chips are mixed with the pretreatment liquid when they are filled in the digester splashed, after which the liquid drains off;

c) the chips are z. B. in a special apparatus or in the cooker, after filling it with the schnitzel, splashed or wetted, after which the Pre-treatment liquid is allowed to run off;

d) the pretreatment liquid is separated from the chips after the pretreatment and, if necessary after adding a new amount of

particularly low sulphide content is injected gradually during the cooking process. This sulfide 50
content is unfavorable for the following cooking process. Incidentally, you understandably wish
a treatment with normal cooking liquor without
a sulphide content is also added. In the end
when using this known method, as is now the case (there are no disadvantageous noticeable that the high treatment-malodorous compounds).

treatment liquid reapplied.

After the pretreatment, it is boiled in the usual way with white liquor (NaOH + Na ₂ S), which is mixed with black liquor.

The by-products in sulphate pulp manufacture can be more easily refined according to the invention

temperatures and working pressures make expensive systems necessary and careful treatment of the good, among other things, because of the long treatment time.

The object of the invention is to provide a method for avoiding the formation of volatile, malodorous and sulphurous substances in the production of sulphate pulp, which ensures a gentle pretreatment of the wood fiber material with the usual cooking liquor and can be carried out with the help of simple and cheap equipment. The invention is essentially characterized in that the amount by weight of sodium hydroxide solution (NaOH) required for the pretreatment is about ¹ I _{10 of} the amount of NaOH that is consumed when carrying out the sulphate boiling according to the usual method. However, the pretreatment according to the invention does not result in an increase in the consumption of alkali, since Vio of the same is used for the pretreatment and ⁹ J ₁₀ for the cooking itself.

The apparatus used for the pretreatment can be used as a special vessel for this process (if necessary with stirrer and heating device), or it can also be the pretreatment

carried out in the sulphate cooker as a special treatment immediately before the sulphate cooking itself will. The pretreatment liquid is expediently included in a larger or smaller amount a circulation pump circulated in the usual way, and one or more containers for Treatment liquid can be connected to the pump.

When the schnitzel is treated with the alkaline liquid, part of the alkali is consumed, see above that the alkali content decreases from the original concentration to a final concentration. In this case, alkali can be added to the solution during the pretreatment in a known manner. Farther the solution can be used again in a known manner after the addition of alkali, and likewise can you use several solutions one after the other and with different alkali contents.

After the pretreatment has taken place, at least some of the pretreatment liquid must be removed from the chips ao removed to allow cooking liquid for sulphate cooking (e.g. a mixture of white liquor and black liquor) can find space in the cooker. It is however, it is not necessary that all of the pretreatment liquid is drawn off, but rather that of the Liquid adhering to chips and absorbed in them can remain there. The tapped pretreatment liquid can be processed to produce resin and the like.

With regard to the alkali concentration, tests have been carried out with acceptable results with between 3 and 25 g NaOH per liter with pretreatment of 7.5 kg pulp (calculated as dry pulp) with about 50% water in these chips and 45 1 pretreatment solution, i.e. 6001 Pretreatment solution used per 100 kg of dry pulp; however, the most suitable alkali concentration range is between 5 and 15 g NaOH per liter. If it is less than 5 g, the reaction is slower and therefore time consuming; on the other hand, an amount of 15 g sometimes means unnecessarily much alkali. The best is 6 to 10 g NaOH per liter. Furthermore, alkalis other than NaOH can be used, e.g. B. KOH or Na ₂ CO ₃ .

The best temperature range for the pre-treatment is between 50 and 7O ⁰ C, but it can up and down, are changed to 40 to 9O ⁰ C for example. The lower the temperature, the slower the pretreatment reaction. At a temperature of more than 8O ⁰ C chemical side reactions may occur. Also from heat-economic reasons, the area between 50 and 7O ⁰ C is suitable.

The treatment time has been studied in detail by taking into account the pretreatment with the solution in the Circulation samples of the solution are taken after 5, 10, 15, 20, 30, 40, 50, 60, 70 and 80 minutes and these Samples have been analyzed for the amount of NaOH, creating curves over the chemical course of the Pretreatment can be obtained. It has been shown that the reaction time to 20 to 30 minutes and in limited to 10 to 15 minutes in certain cases can. A short pre-treatment time is of course desirable for economic reasons.

It is of particularly great value that the pretreatment is advantageous with various black liquor Concentrations can be run. However, this black liquor is said to be derived from a previous one Sulphate boiling originate in which the schnitzels have been pretreated according to the invention. A very The great advantage is that the pretreatment with black liquor is faster than with caustic soda alone. The time required for black liquor is around half the time for pretreatment with caustic soda, when the same temperature and concentration of NaOH is applied at the start of the pretreatment will. The pre-treatment time can thus be reduced to 5 to 8 minutes. The black liquor contains always a bit effective alkali, between 5 and 8 g NaOH per liter, which is good for this pretreatment is just appropriate. If you want to use an even higher alkali content in the black liquor, so you only need to add the desired additional amount of NaOH to the black liquor.

The short pre-treatment time when using black liquor makes it possible to carry out the pre-treatment in practice to be carried out in the sulphate cooker itself without increasing the circulation time of the cooker. Black liquor is fed into the digester at the same time as the schnitzel filling (either from below into the Cooker or z. B. by injection when filling schnitzel) introduced.

However, the schnitzels in the upper part of the cooker require a full pretreatment time, calculated from Time of completion of filling. This time can be shortened significantly by Add a new amount of black liquor to the top of the cooker and an equal amount Leaks black liquor from the bottom of the cooker. So much black liquor has to go to the cooker on it that there is space for the amount of white liquor to be added (this white liquor has quite high temperature). This removal of black liquor and supply of white liquor can take place at the same time, if the withdrawal and supply are carried out in different levels and the specific Weights of the liquids are measured in such a way that the white liquor does not mix with the black liquor which is drawn off mixes. Only then does the circulation process begin, with the supply of heat and cooking in the usual way Way to be carried out.

Example 1 with NaOH as pretreatment liquid

(at 5O ⁰ C)

7.5 kg dry pulp
+ 5.5 kg of water as moisture in the schnitzel

13.0 kg

45.01 pretreatment solution (NaOH solution) + 5.51 wood moisture

50.51
- 39.5 1 deducted after the end of the pretreatment

11.01 solution in the chips + 10.41 white liquor
+ 22.6 1 black liquor

44.01 Cooking liquor with sulphate boiling itself NaOH added during pretreatment .... 126 g

NaOH added with the black liquor 82 g

NaOH added with the white liquor (11.9 1, per liter 140 g NaOH) 1666 g

1874 g which gives 251 g NaOH per 1.0 kg dry pulp.

As a result of this pretreatment, after the subsequent sulphate boiling, you get a bad smell Substances completely free cellulose or such a cooking liquor.

Example 2

with black liquor as pretreatment liquid (at 50 ⁰ C)

7.5 kg dry pulp
+ 5.4 kg wood moisture

10

12.9 kg

45.01 pre-treatment solution in the form of

Black liquor
+ 5.41 wood moisture

50.41

- 37.41 deducted after the pretreatment has ended

13.01

+ 10.01 black liquor from previous cooking

+ 9.11 water

+ 11.9 1 white liquor is added

44.01 when boiling sulphate ^a5

NaOH added during pretreatment 126 g

NaOH added with the black liquor 82 g

NaOH supplied with the white liquor

(11.9 1, per liter 140 g) 1666 g

1874 g which results in 250 g NaOH per 1 kg dry pulp.

In this case, too, one obtains after the subsequent sulphate boiling (which is carried out in the usual way was) a cellulose or a cooking liquor that is free from malodorous substances.

To get the required impregnation of the last poured To reach schnitzel, namely those left near the top of the cooker, the cooker can have a lye splash at the top and a sieve to draw off lye at the top Bottom and be provided with a circulation system that enables pumping.

It can sometimes be expensive to use stoves for pretreatment if you're not already out too large a digester volume available for other reasons.

When pretreating with black liquor, its concentration can be varied within wide limits, e.g. B. from 2 to 15 ° Be (at 6O ⁰ C temperature) or even more. The operating conditions decide what to choose appropriately. Experiments with good results with 4 and 10 ⁰ Be have been carried out.

The black liquor penetrates the wood faster than the NaOH solution. In addition, the organic Separate substances from the black liquor NaOH by hydrolysis. Therefore, if part of the free NaOH the black liquor has been used up, new amounts of NaOH are released. This is done in the Inside the wood chip wall, so that the course of the chemical reaction in the entire wood mass becomes homogeneous. The methyl groups of the entire wood mass are therefore with the pretreatment Black liquor converted very quickly.

Claims (5)

Patent claims: 1. A method for avoiding the formation of volatile, malodorous sulfur-containing substances, such as methyl mercaptan, methyl sulfide and methyl disuffide, in the production of sulfate pulp by boiling wood fiber material with alkali and sulfide, characterized in that the wood material before this boiling in a 0.075 normal to 0.625 normal alkaline Liquid (calculated as NaOH, which corresponds to 3 to 25 g of NaOH per liter of solution) at a temperature of 40 to 90 ^° C., in particular from 50 to 70 ^° C., immersed or in some other way with a reaction time of about 10 to 30 minutes it is pretreated. 2. The method according to claim 1, characterized in that the pretreatment liquid before the subsequent cooking is separated. 3. The method according to claim 1 or 2, characterized in that an excess of liquor from Wood fiber material is separated and reapplied. 4. The method according to any one of claims 1 to 3, characterized in that part of the for Cooking required amount of lye, for example Via »for the pretreatment of the wood fiber material is applied. 5. The method according to any one of claims 1 to 4, characterized in that the pretreatment applied alkali solution is essentially free of sulfide compounds and, if applicable consists of a molten soda obtained from an earlier sulphate boiling, which is purified and is causticed.