US1787395A - Method for bleaching cellulose of any kind - Google Patents

Description

Patented Dec. 30, 1930 UNITED STATES PATENT OFFICE LADISLAV RYS, OF RUZOMBEROK, CZECHOSLOVAKIA METHOD FOR BLEACHING CELLULOSE OF ANY KIND No Drawing. Application filed December 20, 1928, Serial No. 327,458, and in Czechoslovakia December This invention relates to a' method for bleaching cellulose of any kind.

Aqueous solutions of alkali-metal hypochlorites or bleaching powder are the usual bleaching agents for bleaching cellulose. It

1 is known in the art, to subdivide the bleachis added at once to the cellulose to be bleached.

This saving of chlorine is due to the fact that a certain amount of oxidizable substances, liberated by the pre-bleaching process are removed by the intermediate washing process.

It is further known, to improve the bleaching process by using cellulose concentrations. of 15 to 24% instead of 5 8% as formerly, thereby increasing the rate of reaction during the bleaching process, avoiding heating of 25 the reaction mixture and reducing the amount of chlorine necessary. The disadvantage of a bleaching process with high cellulose concentration is, however, to,be seen in the fact, that the mixing of the viscous cellulose with the bleaching liquor is somewhat troublesome.

It is also known to bleach cellulose,'which if necessary has been subjected to a preliminary treatment with alkali metal or alkaline earth metal hydroxides or with alkali metal sulfitcs or bisulfites, by treatment with chlorine, repeated if necessary, before or after the bleaching process with alkaline peroxides with the addition of alkali metal silicates. It is also known to subject material, which has 40 been first bleached in the usual way, for'ex ample. with alkali hypochlorites, to a subsequent bleaching, repeated, if necessary, with chlorine. 7

However, these processes involve intensive treatment with considerable amounts of chlorine and for this reason injury to the material treated cannot be avoided even though the excess chlorine remaining in the material is removed by subsequenttreatment with sodium hyposulfite. for example. Furthermore, the cost of the comparatively large amount of chlorine used in these processes makes them expensive.

I have found that excellent results are obtained if, in the prebleaching with chlorine, the chlorine is used in the gaseous form or as chlorine water only in an amount up to about 90% of the total amount necessary for complete bleaching and it the bleaching is then completed by applying the remainder of the necessary chlorine in the form of hypochlorite. In this manner, the bleaching can be performed with greatest simplicity, with greatest sparing of the fibers of the material treated and with great saving of materials.

I have found further that the amount of chlorine used in the pre-bleaching can be still further substantially limited if the material to be bleached is washed with dilute alkalies such as sodium hydroxide, milk of lime and the like, beforethc chlorine treatment, whereby at the same time it is possible to avoid coloration of the fiber under the influence of the chlorine, which coloration would require the use of an increased amount of chlorine for its removal. In this washing operation the action of the dilute alkali solutions can be assisted by the addition of 02(- idizing agents. such as bleaching powder, for example, or alkali metal sulfites or wetting agents such as soap, saponine. organic solvents and the like, or by the addition of several of these agents.

As compared to bleaching processes heretofore known in the art, all varieties of cellulose may be bleached by the process of the present invention with a saving of 25-30); of active chlorine, and a product of better color and higher chemical purity is obtained. As the process may be carried out in the cold in a short time, it may be used as a continu ous process without it being necessary to work with high cellulose concentrations.

Example Partially concentrated pulp is digested with a solution of alkali-metal or earth-alkalimetal hydroxide at temperatures between 10 and 80 C. The pulp concentration may be from 1 to 25% ;-the amount of alkaline reacting substances present maybe calculated as sodium hydroxide, 0.2 to 2% by weight of the When the digestion is finished, the liquid is drawn off and the pulp is washed with water. The cellulose obtained is then treated, preferably in a closed vessel, with a measured amountof aqueous solution of chlorine, containing about 70 to 90% of the total chlorine necessary for bleaching. The concentratioi'i of the cellulose may be 120% in this treat ment. In about tol hour and at a temper: ature of between 5 to 20 C. the chlorine present is completely consumed.

l/Vhen this preliminary bleachi g step is finished, the treated cellulose mass is at once, or better after a thorough washing with water, digested-in the same manner with alkaline reacting washing liquids as described above and washed with water.

The pre-bleached cellulose mass thus obtained is partially concentrated and finished by bleaching it in the known manner with neutral or alkaline reacting hypochlorite solutions.

This after-bleaching step may be accelerated, by the addition of inorganic or organic salts of catalytic metals, such as copper,

. nickel, cobalt and the like, along with the chlorine water in the preliminary bleaching step. Because of the prevailing weakly acid reaction these salts remain in solution with- .lutionsof alkaline reacting compounds, subjecting the digested cellulose to a preliminary bleaching with an aqueous solution of gaseous chlorine in a measured amount less than the total necessary for compleie bleaching, washing the pre-bleached cellulose and subjecting the washed cellulose to an aften bleaching with hypochlorites. l 2. A method for bleaching cellulose of any klnd which consists in digesting the cellulose to be bleached with diluted aqueous solutions of alkaline reacting compounds containing an oxidizing agent, an alkali-metal sulfite, a moistening agent and an organic solvent, subjecting the digested cellulose to a preliminary bleaching with an aqueous solution of gaseous chlorine in a measured.

amount less than the t'otalwnecessary for complete bleaching, washing the pie-bleached .cellulose and subjecting the washed cellulose to an after-bleaching with hypochlorites.

3. A method for bleaching cellulose of any kind which consists in digesting the cellulose to be bleached with diluted aqueous solutions of alkaline reacting compounds, subjecting the digested cellulose to a preIiminary bleaching with an aqueous solution of gaseous chlorine-in a measured amount less than the total necessary for complete bleaching, digesting the pre-bleached cellulose with diluted aqueous solutions of alkaline reacting compounds and subjecting the digested cellulose to an after-bleaching with hypochlorites.

4. A method for bleaching cellulose of any kind which consists in digesting the cellulose to be bleached with diluted aqueous solutions of alkaline reacting compounds, subjecting the digested cellulose to a preliminary bleaching with an aqueous solution of gaseous chlorine in a measured amount less than the total necessary for complete bleaching, digesting the pre-bleached cellulose with diluted aqueous solutions oFalkaline reacting compounds containing an "oxidizing agent, an alkali-metal sulfite, a moistening agent and an organic solvent and subjecting the digested cellulose toan after-bleaching with hypochlorites.

5. A method for bleaching cellulose of any kind which consists in digesting the cellulose mass to be bleached with diluted aqueous solutions of alkaline reacting compounds containing an oxidizing agent, an alkalimetal sulfite, a moistening agent, and an organic solvent, washing the digested cellulose with water,subjecting the washed cellulose to a preliminary bleaching with an aqueous solution of gaseous chlorine containing about -9()% of the chlorine necessary for complete bleaching and small amounts of salts of metals catalyzing the decomposition of hypochlorites, washing the pre-bleached cellulose with water, digesting the washed cellulose with diluted aqueous solutions of alkaline reacting compounds containing an oxidizing agent, an alkali-metal sulfite, a

moistening agent and an organic solvent,

washing the digested cellulose with water and subjecting the washed cellulose to an after-bleaching with solutions of hypochlorites containing the remaining amount of active chlorine necessary for bleaching.

In testimony whereof I aflix 'my signature. LADISLAV R YS,