US3733247A - Bleching of cellulose fibers with an aqueous solution of a water-soluble hydroperoxypentacyanocobaltate salt - Google Patents

Abstract

CELLULOSE FIBERS ARE BLEACHED BY CONTACT WITH AN AQUEOUS SOLUTION OF A WATER-SOLUBLE HYDROPEROXYPENTACYANOCOBALTRATE SALT. THE SPENT COBALTATE SALT IS REGENERATED AND RECYCLED.

Description

United States Patent Office 3,733,247 Patented May 15,, 1973 3,733,247 BLEACHING F CELLULOSE FIBERS WITH AN AQUEOUS SOLUTION OF A WATER-SOLU- BLE HYDROPEROXYPENTACYANOCOBALTATE SALT John Allan Fetchin, Stamford, Conn., assignor to American Cyanamid Company, Stamford, Conn. No Drawing. Filed Nov. 18, 1971, Ser. No. 200,163 Int. Cl. D21c 9/16 US. Cl. 16278 10 Claims ABSTRACT OF THE DISCLOSURE Cellulose fibers are bleached by contact with an aqueous solution of a water-soluble hydroperoxypentacyanocobaltate salt. The spent cobaltate salt is regenerated and recycled.

The present invention relates to the bleaching of cellulose fibers by use of a bleaching agent. The invention includes the process wherein the bleaching agent is regenerated and if desired recycled.

Paper (including paperboard, cardboard and similar products) is commonly made by slurrying wood pulpin water, flowing the resulting fibrous suspension upon a screen to form a wet web, removing the web from the screen and drying the web. It is particularly desired that the final product should have a brilliant white color and to this end it is common practice to subject the pulp to a bleaching step. Moreover, paper is increasingly manufactured from recycled paper, the fibers of which may carry a dye and this presents a bleaching problem. 'In the manufacture of cotton textiles, it is frequently desired that the product have a maximum attainable whiteness.

The bleaching of fibers intended for use in the manufacture of paper and textiles is commonly performed by the use of strong oxidizing agents, for example, sodium hypochlorite, hypochlorous acid, tertiary butyl hydroperoxide, sodium peroxide, sodium peroxyacetate, and benzoyl peroxide. It is a disadvantage of these processes that it is not practical to recycle or otherwise reuse spent oxidizing solution, and accordingly the efiiuent liquor from the oxidizing step is commonly discarded. Strong oxidizing agents are detrimental to life, and the introduction of these agents into lakes and streams causes an ecological problem. Moreover, the discharge of such liquors involves a waste of the unconsumed peroxide which they generally contain.

The discoveries have now been made that the watersoluble hydroperoxypentacobaltate salts are highly effective bleaching agents for cellulose, capable of causing a substantial increase in the brightness even of cellulose fibers which have been bleached by conventional processes; that they perform this bleaching action rapidly and safely (if desired at room temperature); that the cobaltate in reduced state in the liquor coming from the process can be oxidized to peroxy form by a simple and industrially practical process, thereby permitting the liquor to be recycled for the treatment of subsequent batches of fiber, and that this oxidation can be performed in apparatus which does not require pressure.

Broadly, then the invention is the bleaching of either bleached or unbleached cellulose fibers by contacting the fibers with an aqueous solution of a water-soluble hydroperoxypentacyanocobaltate salt. In a limited sense the invention is the bleaching step followed by regeneration and, if desired, the recycling of the bleaching solution.

The invention is performed cyclically by contacting the fibers to be bleached with an aqueous alkaline solution of a hydroperoxypentacyanocobaltate salt thereby bleaching at least part of the fibers and simultaneously reducing at least a part of said cobaltate to hydroxypentacyanoco- 'baltate salt, separating the solution from the fibers, converting at least a part of the latter cobaltate back to a hydroperoXy-pentacyanocobaltate salt, and recycling the solution.

It has further been found that regeneration of the spent hydroperoxypentacyanocobaltate from the bleaching step can be performed by reducing the spent cobaltate to a pentacyanocobaltate salt, still further reducing said pentacyanocobaltate salt to a hydridopentacyanocobaltate salt, and oxidizing said hydridopentacyanocobaltate. The product is the desired hydroperoxypentacyanocobaltate salt, which can be recycled.

It has still further been found that conversion of said spent hydroperoxypentacyanocobaltate salt to a hydroperoxypentacyanocobaltate salt is performed by reacting said hydroxypentacyanocobaltate salt with hydrogen and aluminum, thereby forming a pentacyanocobaltate salt, and reacting said pentacyanocobaltate with molecular hydrogen thereby forming a hydridopentacyanocobaltate salt, and reacting said latter salt with molecular oxygen.

The aforesaid reactions can be performed with gas pressures between atmospheric pressure and 100 atmospheres and at temperatures from 0 C. to room temperature or higher.

The theoretical reactions involved in the regeneration of the reduced cobaltate to the active peroxy form are as follows (assuming potassium to be the cation), together with the names of the anions formed. In the equations the term PCC designates pentacyanocobaltate.

K3HOOC0(CN)5 color K3HOCO(CN)5 bleached Hydride P CO Hydroperoxy P C C The above reactions are also applicable to any watersoluble cobaltate salt, especially the alkali and alkaline earth metals salts, for example, the sodium, tetramethylammonium, calcium and magnesium salts. The sodium and potassium salts are preferred because they are inexpensive and provide good results. Any of the previously mentioned cations can be employed in place of the potassium cations shown in the equations above, with substantially the same results.

The bleaching step is carried out simply by contacting pulp with an aqueous solution of a hydroxypentacyanocobaltate salt. Even a small amount of the cobaltate elfects some bleaching, from which it appears that there is no amount of cobalate however small or duration of contact however short which will not effect at least some brightening. From the generic point of view therefore the invention does not depend upon amounts, times, temperatures, etc. However, in practice between about 0.001% and 20% of the cobaltate (based on the dry weight of the fibers) is needed to make the process economic, and best bleaching occurs when the amount of the cobaltate is in the range of 0.01% and 10% based on the fibers. With regard to the concentration of the cobaltate in the bleaching solution, the data indicate that bleaching proceeds better when the concentration of the cobaltate in the solution is low rather than high.

The bleaching step can be performed at room temperature. However, more rapid and extensive bleaching occurs when the step is performed at elevated temperature, e.g., in the range of 40 C.- C.

The bleaching process can be performed over the pH range of 3 to 13, which includes the papermaking range.

The alkali metal salts of hydroperoxypentacyanocobaltate are basic, and therefore the solutions used in the process of the present invention inherently have an alkaline pH unless otherwise desired. The solution acts as a bleach even when it has a content of free alkali. For this purpose any alkali metal or soluble alkaline earth metal hydroxide can be added, as well as soluble quaternary ammonium hydroxides.

Upon completion of the bleaching step the liquor is separated from the fibers in any convenient manner, for example, by centrifugation, filtration, compression, etc.

The resulting fibers may be processed directly into paper or textiles. However, better brightness is generally obtained when the fibers are washed after the bleaching step and before it is introduced into the papermaking process. Evidently, the step of washing removes decomposition products from the pulps as well as unreacted and spent cobalt complexes which to some extent remain lightly 2 afiixed to the cellulose. Best washing is achieved with water.

The conversion of the reduced peroxy cobaltate in the liquor from the bleaching step to peroxy state can be performed according to the process of the present inven- 2 tion in several steps by several methods. In the following description the term salt includes mixtures of any water-soluble salts, and references to potassium are for exemplary purposes only.

In the first step the spent bleaching component is reduced to a pentacyanocobaltate salt (see Equation 2 above). Suitable reducing agents are the combination of molecular hydrogen with a water-soluble hydridopentacyanocobaltate salts or molecular hydrogen with palladium on charcoal, aluminum powder, or sodium borohydride. Catalysts such as (e.g., Raney nickel) are used for hydrogenation reactions are also suitable. If desired, before or during this reaction, the solutions may be photolyzed to hydrolyze any K Co(CN) or similar salts to or K HOCo(CN) or similar salts, which are more reactive. It is also possible to reduce the spent cobaltate component electrolytically.

In the second step (cf. Equation 3 above) the solution is subjected to the action of hydrogen alone. Pressures from atmospheric to at least 100 atmospheres are suitable. This step is automatically performed by the action of molecular hydrogen. In this step the pentacyanocobaltate salt is converted to the corresponding hydridopena tacyanocobaltate salts.

In the third step (Equation 4 above) the hydridopentacyanocobaltate salt is reacted with molecular oxygen at pressures from atmospheric pressure to 100 atmospheres or more and at temperatures from 0 C. to room tempera- 0o ture. In this step the hydropentacyanocobaltate salt is oxidized to an appropriate hydroperoxypentacyanocobaltate.

The process is applicable in general to the conversion of a hydride ligand to a hydroperoxy ligand coordinated to cobalt.

The solutions resulting from the foregoing treatment methods can be recirculated directly to the bleaching step without intervening treatment.

The invention is further described by the examples 6 which follow. These examples illustrate the invention and are not to be construed as limitations thereon.

EXAMPLE 1 The following illustrates the effectiveness of the process of the present invention in brightening a pulp which has previously been bleached.

3.0 g. (dry basis) of a kraft hardwood pulp which had previously been bleached to a brightness of 48.4% (reflectance of pulp as the percent of the reflectance of BaCO 457 mg, is slurried in 97 cc. of water at 60 C. containing 3.3 g. of tripotassium hydroperoxypentacyanocobaltate [for preparation see J. Chem. Soc. 397 (1968); J. Catalysis 3, 123 (1964); and Aust. J. Chem. 16, 954 (1963)]. The slurry is gently stirred for a few moments to distribute the cobaltate solution uniformly throughout the pulp, and the slurry is allowed to stand for 60 minutes at 60 C. The fibers are then recovered by filtration, washed three times with water and once 10 with very dilute (0.1%) sulfuric acid, and dried. They have a brightness of 62.2% (an increase of 13.3 percentage points).

EXAMPLE 2 The following illustrates the bleaching action of potassium hydr0peroxypentacyanocobaltate at various dilutions on kraft hardwood pulp.

An aqueous solution is used containing 3% by weight of potassium hydroperoxypentacyanocobaltate. The pH of the solution is alkaline. Aliquots are taken from a hot laboratory stock of bleached hardwood fibers having a brightness of 48.4% and a consistency of 3% heated to 75 C. To these are added a sufficient amount of an aqueous solution of tripotassium hydroperoxypentacyanocobaltate to provide the amount of the cobaltate shown in the table below. The aliquots are agitated gently to distribute the cobaltate therein. The aliquots are maintained at 75 C. for 90 minutes, after which the fibers are recovered by filtration, washed and dried, and their brightness redetermined. The filtrate is analyzed to determine the percent of hydroperoxy s-ubstituents which were reduced.

Results are as follows:

Based on dry Weight of pulp.

b Percent of cobaltate consumed (i.e., reduecd).

c Reflectance against barium carbonate standard.

d Grains cobaltate required to increase brightness of 100 g. of pulp by 1.0 percentage point.

EXAMPLE 3 The following illustrates the treatment of a solution of tripotassium hydroperoxypentacyanocobaltate containing tripotassium hydropentacyanocobaltate (the spent liquor from the bleaching of cellulose pulp) to oxidize the hydroxypentacyanocobaltate content thereof to hydroperoxycobaltate. The solution contains 1.6% of tripotassium hydroxypentacyanocobaltate.

To the solution is added 0.054 g. of powdered aluminum as reductant and through the solution with stirring is bubbled hydrogen (from a porous disc sparger) for 105 minutes. Much of the hydroxypentacyanocobaltate is thereby converted to tripotassium hydridopentacyanocobaltate.

The unreacted aluminum is removed by filtration. The solution is then subjected to a pressure of 85 1b./in. of O in a rocking autoclave at room temperature, converting a part of the latter cobaltate to the desired hydroperoxide.

Analysis shows that this treatment converts 71% of the starting reduced cobaltate to the desired peroxy a cobaltate.

This solution is used for the brightening of pulp by the process of claim 1. Substantially the same brightening is achieved.

EXAMPLE 4 The following illustrates a method for the regeneration of the hydroperoxycobaltate by a method wherein an alkali metal hydridopentacyanocobaltate and molecular hydrogen are used together to reduce the hydropentacyanocobaltate.

A pulp is bleached with a solution of potassium hydroperoxypentacyanocobaltate as shown in Example 1. A part of the resulting aqueous medium is preserved, and the remainder is treated with a reducing agent to produce a solution of tripotassium pentacyanocobaltate which in turn is reduced to tripotassium hydridopentacyanocobaltate, also as shown in Example 1 and illustrated by Equations 1-3.

Equimolar quantities of the tripotassium hydridopentacyanocobaltate and the tripotassium pentacyanocobaltate solutions are mixed under a blanket of nitrogen and the mixture sparged with hydrogen at room temperature for an hour. The following reactions occur:

The resulting solution containing the K HCo(CN) produced by Equation 3 is reacted with molecular oxygen by the method of Example 3. Titration of the reulting solution shows that 34.4% of the original K3H0C0(CN)5 is converted to K HOCo (CN) by this method.

I claim:

1. Process for the bleaching of cellulose fibers which comprises contacting said fibers with an aqueous solution of a water-soluble hydroperoxypentacyanocobaltate salt thereby bleaching said fibers.

2. A process according to claim 1 wherein the cobaltate salt is trisodium hydroperoxypentacyanocobaltate.

3. A process according to claim 1 wherein the pH of the solution is within the range of 9.5 to 12.5.

4. A process according to claim 1 wherein the fibers are unbleached kraft hardwood fibers.

5. A process according to claim 1 wherein the fibers are kraft hardwood fibers which have previously been bleached.

6. A cyclic process for the bleaching of cellulose fibers which comprises contacting oil-color fibers with an aqueous alkaline solution of a hydroperoxypentacyanocobaltate salt thereby bleaching at least a part of said fibers and simultaneously converting at least a part of said cobaltate to a hydroxypentacyanocobaltate salt, separating said solution from said fibers, converting at least a part of said latter cobaltate back to a hydroperoxypentacyanocobaltate salt, and recycling said solution.

7. A process according to claim 6 wherein conversion of said hydroxypentacyanocobaltate salt to a hydroxyperoxypentacyanocobaltate salt is performed by further reducing said hydroxypentacyanocobaltate to a pentacyanocobaltate salt, still further reducing said pentacyanocobaltate salt to a hydridopentacyanobaltate salt, and oxidizing said hydridopentacyanocobaltate salt.

8. A process according to claim 6 wherein conversion of said hydroxypentacyanocobaltate salt to a hydroperoxypentacyanocobaltate salt is performed by reacting said hydroxypentacyanocobaltate salt with hydrogen and aluminum, thereby forming a pentacyanocobaltate salt, reacting said pentacyanocobaltate salt with hydrogen thereby forrning a hydridopentacyanocobaltate salt, and reacting said hydridopentacyanocobaltate salt with molecular oxygen.

9. A process according to claim 8 wherein the reactions of said hydrogen are performed between atmospheric pressure and atmospheres.

10. A process according to claim 8 wherein the reaction of said oxygen is performed between atmospheric pressure and 100 atmospheres.

References Cited UNITED STATES PATENTS 2,394,989 2/1946 Dreyfus 162-78 X 3,156,654 11/1964 Konecny et al. 252-186 X ROBERT L. LINDSAY, JR., Primary Examiner A. L. CORBIN, Assistant Examiner US. Cl. X.R.