US2017985A

US2017985A - Mancfacttjbe of cellulose

Info

Publication number: US2017985A
Authority: US
Publication date: 1935-10-22
Anticipated expiration: 1952-10-22

Description

5 One of its objects is a process of obtaining an Patented Oct. 22, 1935,

UNITED STATES PATENT OFFICE .Aktiengesellsehaft, Frankfort onthe Main, Germany, a corporation of Germany No Drawing. Application Jnne'9, I933, Serial No.

675,116. In Germany June 15, 1932 U '14 Claims.

Our present invention relates to the manufacture of cellulose and, more particularly, contemplates an improvement in, the manufacture of cellulose byalkaline digestion.

improved cellulose by conducting the chlorinating treatment .which follows the alkaline diges- In the same proportion as the chlorination protion in a manner as will be" described hereafter. Another object is the. improved cellulose obtainable by our new process. Further objects will be seen from the detailed specification following I hereafter.

It is known that hard sulfite pulps o1 sulfate pulps, more particularly so-called kraft pulps,'are preferably subjected to a preliminary treatment with chlorine whereupon the chlorination products are removed by treatment with a weakly alkaline solution in order to promote their degree of digestion and thereby render them more accessible to the subsequent bleaching operation.

Careful studies have proven that the pulps, more particularly sulfate pulps, digested in an alkaline medium, suffer a reduction of their viscosity already when subjected to the aforesaid clilorina'ting treatment andytherefore, a loss of solidity. The reason for this'behavior of the pulp lies in'the fact that the reaction between the chlorine which is necessary in rather large quantities for the chlorination of such strongly incrusted pulps and the ligneous matter occurs but comparatively slowly, so. that, when conducting the chlorination under normal conditions, the

' ceeds to the inner layers the chlorination of the water-insoluble incrusts which have already been attacked by the chlorine, becomes more 'difflcult. The chlorine is more and more prevented from reacting with the inner incrusted layers, so that the bath gradually is enriched with chlorine. The consequence of this is a reduction of the viscosity and tensile strength of the cellulose. 'I'hisdrawbackmay be overcome by causing the ligth which becomes acid in the treatment will ntain an excessive amount of free chlorine for chlorine to react at such a 'slow' rate that it is continuously andcompletely absorbed by the incrusts of the cellulose pulp, so-that the incrusts are practically chlorinated in a bath which is free from chlorine. But because in this case the chlorine,'especially during the addition of the last third of the required amount,is consumed only very slowly, the duration of the chlorination would have to be prolonged too long from the commercial point of view.

According to this invention the time of chlorination may be considerably reduced without damaging the fibers by taking care "that in the same proportion as the chlorine .is brought into reaction, the chlorination products "formed are 1 dissolved by the addition of an exactly dosed amount of alkali such that the pH-value of the bath is maintained between .6 and 9, preferably between 7.5 and-8.5. c

It has already been proposed to react oncellulose pulp first with alkali and then with chlorine. gasor to react on cellulose pulps with alkali'and' chlorine simultaneously. In both cases", however.

the amount of alkali present was so large that hypochlorite was formed. Therefore, the condi- 25 tions of reaction of these known processes were completely different from those of a chlorination according to this invention and, as experiments have proven, by the known processes, the cellulose pulp suifered a strong reduction of 'viscosity and the resistance was greatly injured. In contradistinction thereto, according to this invention, the formation of hypochlorite is avoided by controlling the addition of the alkali, so to dis-' solve the added quantity is just suificient that the chlorinated 'lignous substances which are soluble in an alkaline solution. When proceeding in this way, the amount of chlorine which may be brought into reaction in a shorter period time,.is greater than that used for an ordinary chlorination in an acid medium. f

This fact can be explained from what has been set forth above- By maintaining'a' pH-valu'eof about 8 the chlorinated ligneous substances which are soluble in alkali are dissolved oil and the following layers are now readily accessible to the action of the chlorine. The digestion, therc-v 1 fore, takes place in a more umroma more in? tense, and a more rapid manner. In consequence, the bath will not becomeenriched with. chlorine, and the cellulose is not attached. By these means not only an improvement of the 'Sieber number is obtained while the viscosity remains unchanged, butthe treatment conductedunder such mild conditions yields a cellulose of an increased resistance also against chemical influences in'the finishing of the pulp. The new process also involves the advantage that it completely avoids foaming caused by the formation of carbonic acid which is abimdantly produced particularly in the treatment of kraft pulps rich in ligneous substances. The new process can be carried out in any receptacle which guarantees a thorough and rapid mixture of the pulp with the chemical agents, the density of the pulp depending on the capacity of the mixing tank. By avoiding the formation of larger quantities of free chlorine in the bath, it also becomes possible to work in open vessels, without inconvenience by odors. Moreover, by keeping the reaction of the bath in a more or less neutral state, any wear of the armatures, washing drums, and so on is avoided. I

The experiments set forth in the following table have been carried out in such a manner that the bath, after finishing the chlorination step, was alkaline so as to correspond with an 0.05 per cent. caustic soda solution. After this treatment the pulp was washed. The advantages presented by the process according to this invention may be seen from the following table:

as may be efiected, for instance, by lowering the" washing drum into the hollander, because the products already chlorinated absorb a further amount of chlorine besides the amount of chlorine necessary for the'r dissolution in alkali. This observation has b en verified by an experiment carried out while simultaneously washing with water under which conditions with the application of only 4 per cent. of chlorine, a cellulose was produced having a copper viscosity of 791 and a Sieber numberof 13.

Because with sulfate pulp only asmall part of the chlorination products are soluble in water, the continuous washing with water, as-has likewise been established by experiments, when carried out according to the method hitherto in practice, that is without a continuous addition of alkali, could not lead to an improvement.

A. further improvement of the process according to this invention maybe obtained by performing the chlorination in at least two steps with the intercalation of a wash with a solution of alkali whereby the chlorine brought into reaction in the first step may amount to 60 to per cent. of the quantity of chlorine required. Since in the first chlorination, according to our observations, the cellulose is not injured, this 'step may, if required, be carried out in the usual manner. It is only in the second step that the chlorination must be conducted according to the new process, 1. e. simultaneously neutralizing the bath. This is clearly demonstrated by the following two comparative experiments.

This mode of operation also involves a considerable reduction of the cost for alkali.

As a matter of fact the second step can be carried out with continuous washing with water.

The following example illustrates the invention:

1000 kilos of kraft pulp having a Sieber number of 60 are chlorinated with 3.7 per cent. of chlorine in the form of chlorine-water in a hollander provided with a washing drum. To avoid any excess of chlorine in the bath, the chlorination lasts about 2 hours. Then the pulp is thickened, washed, and mixed with such an amount of alkali that the bath contains about'0. 05 per cent. of NaOH. After the alkali has reacted half an hour, the content of the hollander is thoroughly washed. This treatment is followed by a second chlorination with 1.1 per cent. of chlorine while simultaneously adding alkali. The chlorine water is run in, for instance, at a substantial distance before the bucket or the propeller of the holiander near the bottom. The alkali required, which may be any soluble alkali, used alone or in mixture, however preferably caustic soda, is added in a highly dilute condition at a substantial distance .behind the mixing device where the chlorine is ractically consumed. During the whole chlorination process the bath must be maintained at a pH-value of 6.5 to 9.0. The quantity of NaOH necessary for this efi'ect amounts to about 1.3 per cent., calculated on the quantity of the pulp. This chlorination step likewise lasts about 2 hours, and its duration depends on the intensity of the mixing device or the speed of propulsion of the hollander. When the chlorination is complete the fiber pulp is mixed with so much alkali that its content of NaOH amounts to 0.05 per cent. and after half and hour's reaction the'pulp is thoroughly washed. The cellulose thus obtained has a viscosity of 740 which is practically its original viscosity but the Sieber number has been reduced to about 8.

The advantages involved in the method described reside among others in the high quality of the unbleached material, inasmuch as the resistibility of its fibers complies with the highest requirements. At thesame time the pulp has a lower Sieber number than any known, commercial, highly bleachable sulflte pulp or sulfate pulp. Due to its light yellow color, the flexibility of its fibers and their excellent resistibility, its freedom from resin and its low content of incrusts, it is well suited for the preparation of special papers oi any kind.

Due to its low content of ligneous substance such a pulp may be transformed. without strongly impairing its viscosity and the resistibility of the fibers by oxidizing agents such as the chloride of lime or sodium hypochlorite, when employed in the usual concentration, into a product which can' be bleached perfectly white. It is clear that the higher the viscosity of the starting material the more intense may be the bleaching operation for the production of a highly white material. This, also, is one of the special advantages obtained by the new cautious yet vigorous chlorination. The bleaching operation is preferably carried out as it 6 is described in U. S. Patent 1,801,876, 1. e. while maintaining the reaction slightly alkaline in the bleaching operation by the addition of a small quantity of a caustic alkali, a hydroxide of the alkaline earths including magnesium hydroxide. The bleaching is conducted preferably at a temperature above 40 0., for instance, between 40 C. and 80 C.

All the advantages presented bythe present invention come out, however, much more distinctly, if an unbleached sulfatep'ulp, after chlorinating the same in the manner described, to dissolve out its hemi-cellulose and increase its contents of alpha-cellulose, is first subjected to an. alkaline refinement and then to a bleaching operation. 80 Thus there can be obtained 'highly viscous improved celluloses which not only aresuitable for the preparation of special papers, but can also be employed for chemical processes, for instance the manufacture of artificial silk, such as viscose silk I or cellulose acetate silk. The process, furthermore, involves the advantage that such refined cellulose, the viscosity of which has been greatly spared, can be transformed into products of a lower viscosity which are exceptionally white, by.

l. A process of manufacturing cellulose which comprises treating a cellulose pulp obtained by alkaline digestion with chlorine while'maintain- 5 ing during the chlorination a pH-value between 6 and 9. by continuous addition of alkali.

2. A process of manufacturing cellulose which comprises treating a cellulose pulp obtained by alkaline digestion with chlorine while maintain- 50 ing during the chlorination a pH-value between 6 and 9 by continuous addition or alkali, removing part of the treating liquid, and adding a quantity of water equal to the quantity of treating liquid removed. I

55 3. A process of manufacturing cellulose-which comprises treating in a first step a cellulose pulp obtained by alkaline digestion with 60 to 80 per cent.' of the chlorine to be employed in the chlorlnating treatment, washing the pulp free from Q acid, treating the pulp in a second step with. the

further to 40 per cent. of chlorine while maintaining a pH-value between 6 and 9 by continuous addition of alkali.

4. A process of manufacturing cellulose which I '5 comprises treating in a first step'a cellulose pulp obtained by alkaline digestion with 60 to 80 per cent. of the chlorine to be employed in the chlorinating treatment, washing the pulp free from acid, treating the, pulp in a second step with the 79 further 20 to 40 per cent. of chlorine while maintaining a pH-value between 6 and 9 by continuous addition of alkali, adding to the pulp so much alkali thatits concentration amounts to at least 0.05 per cent., and washing the pulp with water.

1 5. A process of manufacturing cellulose which comprises treating a cellulose pulp obtained by alkaline digestion with chlorine while maintaining during the chlorination a pH-value between 6 and 9 by continuous addition of alkali, removing part of the treating liquid, and adding a quantity 5 of water equal to the quantity of treating liquid removed, adding to the pulp so much alkali that its concentration amounts to at least 0.05 per cent and washing the pulp with water.-

6. A process of manufacturing cellulose which 10 further 20 to 40 per cent. of chlorine while maintaining a pH-value between 6 and 9 by continuous addition of alkali, adding to the pulp so much alkali that its concentration amounts to at least 0.05 percent, washing the pulp with water, and 2,0; bleaching the pulp while maintaining the .pulp slightlyv alkaline.

'I. A process of manufacturing cellulose which comprises treating in a. first step a cellulose pulp obtained by alkaline digestion with 60 255 to 80 per cent. of the chlorine to be employed in the chlorinating treatment, washing the pulp free fromacid, treating the pulp in a second step with the further 20 to 40 per cent. of chlorine while maintaining a. pH-value between 6 and 9 by continuous addition of alkali, adding to the pulp so much alkali that its concentration amounts to at least 0.05 per cent., washing the pulp with water, and bleaching the pulp at a temperature of to 80 C. while maintaining $5 the pulp slightly alkaline.

8. A process of manufacturing cellulose which comprises treating in a first step a cellulose pulp obtained by alkaline digestion with 60 to 80 per cent. 01 the chlorine to be employed in the chlo- 40 rinating treatment, washing the pulp free from acid, treating the pulp in a second step with the further 20 to 40 per cent. of chlorine while maintaining a pH-value between 6 and 9 by continuous addition of alkali, adding to the pulp so much alkali that its concentration-amounts to at least 0.05 per cent., washing the pulp with water, treating the pulp with a solution of caustic alkali of 5 to 12 per cent. strength, and washing the pulp with water.

9. Aprocess of manufacturing cellulose which comprises treating in a first'step a cellulose pulp obtained by alkaline digestion with 60 to 80 per cent of the chlorine to be employed in the chlorinating treatment, washing the pulp free from $5 acid, treating the pulp in a second step with thefurther 20 to 40 per cent. of chlorine while maim taining a pH-value between 6 and 9 by continu ous addition of alkali, adding to the pulp so much alkali that its concentrationamounts to at least 60 0.05 per cent., washing the pulp with water, treating the pulp with a solution of caustic alkali of 5 to 12 per cent. strength, washing the pulp with water, and bleaching the pulp while maintaining the pulp slightly alkaline.

10. A process ofmanufacturing cellulose which comprises treating in a first step a cellulose pulp obtained by alkaline digestion with to per cent. of the chlorine to be employed in the chlorinating treatment, washing the pulp free from 7 acid, treating the pulp in a second step with the further 20 to 40 per cent. of chlorine while maintaining a pH-value between 6 and 9 by continuous addition of alkali, adding to the pulp so much alkali that its concentration amounts to at least (5 0.05 per cent, washing the pulp with water, treating the pulp with a solution of caustic alkali of 5 to 12 per cent. strength, washing the pulp with water, and bleaching the pulp at a temperature of 40 to 80 C. while maintaining the pulp slightly alkaline.

11. A process of manufacturing cellulose which comprises treating a cellulose pulp obtained-by alkaline digestion with chlorine while maintain-' ing during the chlorination a pH-value between 6 and 9 by continuous addition of alkali, removing part of the treating liquid, and adding a quantity of water equal to the quantity of treating liquid removed, adding to the pulp so much alkali that its concentration amounts to at least 0.05 per cent., washing the pulp with water, treating the pulp with a solution of caustic alkali of 5 to 12 per cent. strength, and washing the pulp with water. Y

12. A process of manufacturing cellulose which comprises treating a cellulose pulp obtained by alkaline digestion with chlorine while maintaining during the chlorination a pH-va-lue between 6 and 9 by continuous addition of alkali, removing part of the treating liquid, and adding a quantity of water equal to the quantity of treating liquid removed, adding to the pulp so much alkali that its concentration amounts to at least 0.05 per cent, washing the pulp with water, treating the pulp with a solution of caustic alkali of 5 to 12 per cent. strength, washing the pulp with water,

and bleaching the pulp while maintaining the pulp slightly alkaline.

- 13. A process of manufacturing cellulose which comprises treating a cellulose pulp obtained by alkaline digestion with chlorine while maintaining during the chlorination a pH-value between 6 and 9 by continuous addition alkali, removing part of the treating liquid, and adding a quantity of water equal to the quantity oiv treating liquid removed, adding to the pulp so much alkali that its concentration amounts to at least 0.05 per cent., washing the pulp with water, treating the pulp with a solution of caustic alkali of to 12 per cent. strength, washing the pulp with water, and bleaching the pulp at a temperature of 40 to 80 C. while maintaining the pulp slightly alkaline.

14. An unbleached cellulose pulp produced by alkaline digestion and to a great extent freed from impurities, said pulp having a viscosity of about 700 and a Sieber-number of about 10.

' ERICH OPFERMANN.

GUSTAV-ADOLF FELDTMANN.