US2119519A - Process of bleaching cellulose - Google Patents

Description

UNITED STATES .iRUixJi Utiisi) PATENT OFF-ICE PROCESS OF BLEACHING' CELLULOSE Alfred 0. Bragg, Jersey City, N. J., assignor to Kuehne Chemical Company, Elizabeth, N. J., a corporation of New Jersey No Drawing. Application May 22, 1935, Serial No. 22,785

16 Claims.

This invention relates to the bleaching of cellulose. While the invention is applicable to the bleaching of cellulose in any form, one preferred application of the invention is to the bleaching of cellulose fibers for use as paper pulp. The term cellulose fibers includes all of the various forms in which cellulose'ma'y be employed for paper pulp and, by way of illustration but not in any sense as limitation, includes rag fibers, old paper stock, soda pulp, sulphite pulp, and sulphate pulp. One object of the invention is to provide a marked reduction of the time required to bleach to any particular standard of whiteness, with consequent increase in capacity of the bleacher, and with a marked conservation of the original strength of the pulp.

Heretofore cellulose pulp has been bleached with chlorine, hypochlorites, potassium permanganate, etc., or mixtures of the same with other ingredients. Such substances give good bleaching, but in many cases result in undesired byproducts and the bleaching time required is long. In the practice of my invention the pulp is treated with an aqueous solution of an oxidizing bleaching agent inthe presence of a titanium compound, in which the titanium has a valence of less than four, while maintaining an alkaline pH. Any of the hypochlorites ordinarily employed as bleaching agents may be used as, for example, sodium hypochlorite, potassium hypochlorite, calcium hypochlorite, and magnesium hypochlorite. Other oxidizing bleaching agents may also be used, such as potassium perman ganate, potassium dichromate, sodium perborate,

etc. These are used in approximately the same amounts and concentrations at which they have been heretofore employed. The titanium compound may be titanium sulphate, T126003, ti tanium dichloride, TiC12, titanium hydroxide, Tl(OI-I)2, etc. The materials may be added in a dry state, in solution, or in suspension. The best results are obtained by adding a solution of the titanium compound to the pulp suspension, alkali, and hypochlorite. It will, of course, be understood that the bleaching operation is carried out with the pulp iii-aqueous suspension in the usual way. The titanium compound may be one which is soluble in water and/or in acids. I preferably employ the inorganic titanium compounds. In general, the amount of titanium compound present is relatively small as compared to the amount of hypochlorite used and is believed to exert an activating function in modifying the course of the hypochlorite oxidation of the bleaching reaction to obtain the objects hereinbefore set forth. The bleaching process must be carried out on the alkaline side, suitable addition of alkali being made from time to time, for the purpose of holding the pulp in the desired alkaline condition. Alkalinity may be effected by any of the Well known alkaline or alkaline earth hydroxides or other compounds, such as sodium hydroxide, or carbonate, potassium hydroxide or carbonate, calcium oxide or hydroxide, magnesium oxide, hydroxide or carbonate, etc.

It is of course obvious that mixtures of titanium compounds and mixtures of alkali and/or hypochlorites, can be used. Also if the pulp, water,

hypochlorite, or other ingredient of the furnish contains free alkali, or substance capable of forming free alkali, the amount of alkali added can be diminished or entirely dispensed with.

It is necessary, that enough excess alkali, over and above that normally found in the hypochlo- 5 be maintained on the alkaline side, since, on acidification, hypochlorous acid and chlorine are formed, 'chlorinating the pulp to a yellow or orange color. This is difiicult and expensive to remove.

The part of the process involving the bleaching acceleration by the titanium compound may be carried out at temperatures commonly used in hypochlorite bleaching. Although, in some cases it appears possible to further accelerate the rate of bleaching, and obtain a consequent reduction in the time required to reach any degree of whiteness, by carrying out the bleaching at a slightly higher temperature. If cold water is used for cooling, the pulp is first furnished'at as large a density as possible, to compensate for the dilution.

The amounts of pulp, water, titanium com pound, alkali, and hypochlorite may vary within wide limits, and this invention is not limited to any particular ratio of ingredients. Since the amount of titanium compound and excess alkali used will depend oii'individual circumstances and cost factors, it is impossible to state definite amounts. However, in general, the amount of titanium compound used would be at least, in the case of the 25% titanous sulphate solution, 8-10 lbs. per ton of pulp. The amount of alkali required depends on the type, amount, and acidity of the titanium compound used. Titration of the titanium compound against the alkali soluearcii tioom tion used, will give the necessary amount of alkali solution required.

On the addition of the titanium compound, an instantaneous, permanent whitening is noticed. In the presence of sufiicient alkali to prevent acidification, the degree of this whitening depends, among other factors, on the amount of the titanium compound added.

Laboratory tests have shown that equal-weight samples of sulphite pulp, which have been treated with equal amounts of calcium hypochlorite in the absence of the titanium compound, will require thirty minutes to develop whiteness. In the presence of the titanium compound, bleaching is instantaneous.

My bleached pulp has a greater strength, than pulp bleached by other processes. This is especially true of kraft pulp, which undergoes a decided loss in strength when bleached by methods now in commercial use.

Because of the reduced bleaching time, the bleaching cost is less. Power, labor, steam, maintenance costs, and other cost items, per ton of pulp bleached, are reduced.

A bleached alpha cellulose pulp of high quality can also be obtained.

In some cases it is desirable to increase the amount of the excess alkali, beyond the point re quired for prevention of acidity development. This is especially the case in bleaching sulphitepulp for alpha-cellulose pulp, the action of the excess alkali being to hydrolyze any colored compounds in the fibers to be bleached; combining with the colorless products of the oxidation; and destroying traces of resins, fats, sugars, acids, and other saponifiable and/or hydrolyzable matter present. Any traces of chlorinated substances are also removed.

In carrying out this invention, it is essential that the available chlorine does not become exhausted from the hypochlorite, since, in thepresence of the titanium compound, reversion and darkening of the color occur. Hence, it is necessary that there be a slight residual amount of available chlorine present at the completion of the bleaching, i. e. at the time the stock is dumped from the bleachers and thinned out with water. It is advisable to mix the titanium compound in with the pulp as quickly as possible.

I. have also found that the titanium compounds applicable to this process, should be kept as free from air and/or, oxygen as possible.

Better results are also obtained if the pulp and water are free from dissolved air or oxygen, and to eiTect this condition de-aeration by mechanical means, or vacuum, avoidance of splashing, etc. are advisable.

The titanium compound, and/or the hypochlorite in solution or suspension may be added continuously to the other materials. If desired, the pulp may first be treated with alkali to dissolve undesired impurities, washed with water, and then bleached.

By way of laboratory illustration, an aqueous suspension of unbleached sulphite pulp of five per cent density is brought to a pH of about ten with sodium hydroxide (about three to four pounds of dry sodium hydroxide per ton of dry pulp, depending on the acidity of the pulp), then preferably heated for one-half to one and onehalf hours at about 30 C., while maintaining a pH of about ten. Four per cent of the dry pulp of available chlorine, as calcium hypochlorite, is now added. Eight to ten pounds of a twentyfive per cent solution of titanium sulphate,

Ti2(SO4)3, per ton of dry pulp is added while still maintaining a pH of ten, and such addition is preferably continuous. The bleached pulp is then thinned with Water, and Washed.

In general, this process provides a means for improving the efliciency of alkaline hypochlorite bleaching, efiecting an instantaneous bleaching, resulting in savings in time and bleaching costs. It also provides a purer product than would. otherwise be obtainable at the same cost, from ordinary hypochlorite bleaching. The process is applicable to any bleaching processes in which alkaline hypochlorites are employed, such as the bleaching of cotton, jute, wool, silk, etc. The foregoing description is for purposes of illustration and not of limitation; and it is, therefore, my intention that the invention be limited only by the appended claims or its equivalent, wherein I have endeavored to claim broadly all inherent novelty.

I claim:

1. The process of bleaching cellulose comprising subjecting said cellulose to the action of a titanium compound in which the titanium has a valence of less than founin the pt'esenfioihn oxidizing bleaching agent and water while maintaining an alkaline pH.

2. The process of bleaching cellulose pulp comprising subjecting said pulp to the action of a titanium compound in which the titaniumlias a valence of less than four, in the presence of an oxidizing bleaching agent and water while maintaining an alkaline pH.

3. The process of bleaching cellulose comprising subjecting said cellulose to the action of a titanium compound in which the titaniuinhas a valence of less than four, in the presence of a hypochlorite and Water while maintaining an alkaline pH.

4. The process of bleaching cellulose pulp comprising subjecting said pulp to the action of a titanium compound in which the titanium has a valence of less than four, in the presence 'of calcium hypochlorite and Water while maintaining an alkaline pH.

5. The process of bleaching cellulose pulp comprising subjecting said pulp to the action of a titanium compound in which the titanium has a Valence of less than four, in the presence of sodium hypochlorite and water while maintaining an alkaline pH.

6. The process of bleaching cellulose pulp comprising subjecting said pulp to the action of a titanium compound in which the titaniunrhas a valence of less than four, in the presencebf magnesium hypochlorite and water while maintaining an alkaline pH.

7. The process of bleaching cellulose comprising subjecting said cellulose to the action of a titanium sulphate in which the titanium has a valence of less than four, in the pfience of an oxidizingbleaching agent and water whhemaintaining an alkaline pH.

8. The process of bleaching cellulose comprising mixing said cellulose, an alkali, an oxidizing bleaching agent and water, and thereafter adding a titanium compound in which the titanium has a valence of less than four.

9. The process of bleaching cellulose pulp domprising mixing said pulp, an alkali, an oxidizing bleaching agent and water, and thereafter adding a titanium compound in which theti-tanium has a valence of less than four.

10. The process of bleaching cellulose pulp WWW comprising mixing said pulp, sodium hypochlorite, an alkali and water, and thereafter adding a titariiumsulphate in which the titanium has a valence of less than four.

11. The process of bleaching cellulose pulp comprising mixing said pulp, calcium hypochlorite, an alkali and water, and thereafter adding a titanium sulphate in which the titanium has a valence of less than four. V

12. The process of bleaching cellulose pulp comprising mixing said pulp, magnesium hypochlorite, an alkali and water, and thereafter adding a titanium sulphate in which the titanium has a valence of less than four. M t.

13. The process of bleaching cellulose comprising subjecting said cellulose to the action of a titanium compound in which the titaniummhas a valence of less than four, in the presence of an oxidizing bleaching agent, and water while maintaining an alkaline pH, and terminating the (3L UHZ'JNU Lulu M1 Uuu iflva'dl New I 1 u N OF lEXlELES 61 HEEES,

bleaching while there is still residual available chlorine present.

14. The process of bleaching cellulose comprising adding an oxidizing bleaching agent to an aqueous suspension of cellulose of an alkaline pH, and thereafter adding a titanium compound in which the titanium has a i' alencelof less than four, while still maintaining an alkaline pH.

15. The process of bleaching cellulose comprising adding a hypochlorite to an aqueous suspension of cellulose of an alkaline pH, and thereafter adding a titanium compound in which the titanium has avalence of less than four, while still maintaining an alkaline pH.

16. The process of bleaching cellulose comprisin-g adding a hypochlorite to an aqueous suspension of cellulose of an alkaline pH, and thereafter adding a titanium sulphate in which the titanium has a valerxee of" less than four.

ALFRED O. BRAGG.