US2478379A - Cellulose bleaching - Google Patents

Description

Patented Aug. 9, 1949 CELLULOSE BLEACHING Alfred Mead Dodson, Hopewell, Va., aasignor to Hercules Powder Company, Wilmington, DeL, a corporation of Delaware No Drawing.

Application July 30, 1948,

Serial No. 41,725

Claims.

This invention relates to processes for bleaching cellulose-and more particularly to a chlorination step improved by reducing cellulose degradation for a given bleach action or by effecting a greater bleach action for a given degree of cellulose degradation.

Cellulose fibers are customarily bleached in aqueous media with chlorine and hypochlorites.

Best results usually require at least one chlorination step of controlled severity. Chlorination conditions are acid to begin with or quickly become acid as chlorination takes place. Such chlorination causes a variable degradation of the cellulose depending upon the conditions of treatment. Hence, at elevated temperatures, high chlorine concentration, extended bleaching periods, etc., cellulose degradation is relatively large for the bleach action attained and may render the treatment impractical in some cases.

Thus, for example, excessive degradation of the cellulose may be encountered for the degree of bleaching accomplished where high chlorine concentrations with respect to the chlorine demand of the pulp are used, or where the concentration of the chlorine in the aqueous medium is high due to high pulp consistencies being employed. High water temperatures, which may be encountered as a seasonal variation in certain supplies, can have undesirable efiects on the cellulose properties when employed in a chlorination step. The range of operating conditions to which cellulose material can be subjected in chlorination, makes control of the properties of the ultimate product quite difllcult.

Now, in accordance with this invention, it has been discovered that the cellulose degradation inherent in the chlorination of cellulose materials suspended in acidic solution is decreased so that it is no longer excessive for the degree of bleaching action accomplished and the variations in degradation with changes in condition of chlorination are minimized. This is accomplished by a treatment in which a bleachable cellulosic material is suspended in water, containing a small amount of a nitrogen compound containing hydrogen on a nitrogen atom, and chlorinated while the system is acid in the pH range of 1.0 to 4.5 in the presence of free chlorine. Thus, the cellulose fibers may be given a chlorination much as in the past but where, considerable cellulose degradation formerly occurred, this is now greatly decreased by means of the small amount of nitrogen compound present. The nitrogen compound used is generally a compound with a basic nitrogen atom as exemplified by ammonia, substituted ammonia, and salts or chlorine compounds of ammonia or substituted ammonia.

The treatment in accordance with this invention is to be distinguished from other chlorine bleach operations at higher pH where treatment is dependent upon hypochlorite or other compounds such as chloramines as such treatments function differently. It is also to be distinguished from the use of ammonia or amines in large concentration to cause decomposition of hypochlorite in bleaching or to serve as basic substances in neutralizing acids to achieve a pH at ave 4.5. The function of the nitrogen compound is quite different in the present invention wherein relatively small amounts are required to accomplish the objectives of the invention.

The bleaching method in accordance with this invention will be illustrated by the following specific examples thereof.

Example I Four hundred and fifty grams of unbleached cotton linters having a viscosity of 6450 seconds by the Standard American Chemical Society method at 2.5% concentration were mixed with water to form a suspension having a consistency of 3%. The cotton linters had previously been digested in dilute caustic soda to remove fats, waxes and other caustic soluble impurities. To the slurry so formed there was added sulfuric acid in a quantity of 0.06% by weight of the slurry and an amount of ammonium hydroxide equivalent to 1.0 part per million of NH3 on the basis of the solution. The slurry was agitated for several minutes to insure uniform distribution of the reagents. The slurry at this point had a temperature of 86 F. Chlorine was then added in a quantity of 0.03% by weight of the solution. The chlorine was added in the form of calcium hypochlorite which reacted with the excess acid present to form free chlorine. The temperature remained at 86 F. to bleach the cellulose with the chlorine. The liquid was then drained from .the slurry and the bleached cotton linters were usual manner with alkaline hypochlorite to complete purification in the usual manner,

A third sample of the same unbleached cotton linters was bleached in the same manner as in the described example omitting the ammonia and reducing the chlorine present to 0.025% so as to obtain a bleached product having the same viscosity as that obtained with the use of ammonia and chlorine in a quantity of 0.03%. Although the limited viscosity drop was duplicated, the color of the product was darker and the amount of organic dirt remaining after chlorination was considerably greater than inthe case in which 1.0 part per million of ammonia had been used.

Example II Two hundred and fourteen grams of unbleached cotton linters having a moisture content of 6.6%, a viscosity of 4040 seconds by the Standard American Chemical society method at 2.5% concentration, and a bleach demand of about 0.18 gram' of available chlorine per 100 grams of bone-dry cotton were weighed into a stainless steel container and placed in a water bath maintained at a temperature of 35 C. The cotton linters had previously been digested in dilute caustic soda to remove fats, waxes and other caustic soluble impurities.

Seven hundred and eighty-six grams of a bleaching solution containing 1.80 grams of available chlorine (500% of the bleach demand of the cotton), 0.036 gram of ammonia (2% of the chlorine) and 2.6 grams of sulfuric acid in tap water was prepared in a separate container by rapidly mixing the water, 3% sulfuric acid solution, 0.3% ammonia solution, and 3% (available chlorine) sodium hypochlorite bleach in the above order. The reagents were brought to a temperature of 35 C. before mixing.

The bleaching solution was then immediately poured over the cotton in the stainless steel container and the cotton was thoroughly wet out by mixing to form a mixture containing 20% dry cotton. The temperature of the mixed cotton and bleach solution at this point was 35 C., and it was maintained at this temperature for 30 minutes. A portion of the liquid squeezed from the cotton at the end of the 30-minute period 1 had a pH of 2.50.

After bleaching as aforesaid the cotton was washed free of chlorine, centrifuged, fluffed and oven-dried to a moisture content of approximately The bleached cotton had a viscosity of 2960 seconds, a soda soluble of 1.89%, and a color expressed as an Excitation Purity of 6.3%.

Another portion of the same unbleached cotton linters was bleached at the same time and by the same procedure as outlined above except that no ammonia was used. The pH at the end of the 30-minute period was 2.62. This material, after bleaching, had a viscosity of 1030 seconds, a soda soluble of 3.15%, and a color expressed as an Excitation Purity of 6.0%.

Example III Three hundred and forty-three grams of unbleached cotton linters similar to that of Example II were placed in a bleaching vessel equipped with central draft tube and propeller agitator in bottom neck of the tube. An amount of cold water suiilcient to make a slurry of 2% bone-dry 4 in the following step. The slurry was allowed to mix for 5 minutes following the addition of the ammonia.

Gaseous chlorine was then injected into the 5 slurry in the region of the propeller to effect dispersion of the chlorine under considerable hydrostatic head. Chlorine was added as rapidly as dispersion and solution would permit. The quantity of chlorine added to the slurry was 57.6 grams, equivalent to 100 times the bleach demand of the dry cotton. The bleaching process was carried on for 30 minutes following the addition of the chlorine. The temperature at this point was C. and the pH was 2.0.

The bleach liquor was drained off and the cotton washed free of chlorine with cold water. The cotton was then centrifuged, fluifed and dried to approximately 5% moisture. The bleached cotton had a viscosity of 1545 seconds, a soda soluble of 1.95% and a color expressed as an Excitation Purity of 5.5%.

A second portion of the same unbleached linters was treated by the same process described above except that no ammonia was used. The bleached material had a viscosity of 880 seconds, a soda soluble of 2.47% and a color expressed as an Excitation Purity of 5.5%.

Example IV Eight hundred and twenty-nine grams of digested unbleached cotton linters having a viscosity of 7900 second-s, a bleach demand of about 0.25 gram of chlorine per hundred grams of bone-dry cotton, and a moisture content of 6.9% were placed in an autoclave .equipped for liquor circulation. An amount of tap water sufficient to make a final slurry (after addition of all reagents) containing 2% bone-dry cotton was then added. A 10% sulfuric acid solution was then added containing 13.85 grams of sulfuric acid and the liquid was circulated for two minutes. An amount of 0.3% ammonia solution was then added which contained 0.06 gram of NH: or 2% of the chlorine to be added in the following step. The solution was allowed to circulate through the mass for three minutes followingthe addition of the ammonia.

A 5.08% (available chlorine) sodium hypochlorite bleaching solution containing 3.0 grams of chlorine was then added. The amount of chlorine was equal to 1 times the measured bleach demand of the dry cotton. The autoclave was then sealed and the temperature raised to 110%C. and maintained at that temperature for 30 minutes, The pH of the slurry in the autoclave was 2.5.

At the end of the 30 minutes'of bleaching, the autoclave was cooled, the bleach solution was then drained off and the cotton thoroughly washed with cold tap water. The cotton was then centrifuged, fluffed and dried to approximately 5% moisture. The bleached cotton had a viscosity of.335 seconds, a soda soluble of 3.0% and a color expressed as an excitation purity of 5%.

A second portion of the same unbleached linters was treated by the same process described above except that no ammonia was used. The bleached niaterial had a viscosity of 124 seconds, a soda soluble of 4.2% and an Excitation Purity of 5.9%.

Example V Three hundred and forty-three grams of unbleached cotton linters similar to that of Example II were placed in a bleaching vessel equipped with a screened false bottom and a paddle-type arm agitator. An amount of tap water sufficient to make a final slurry (after addition of all reagents) containing 2% bone-dry cotton was then added. Ten grams of sulfuric acid were then added as 3% sulfuric acid solution and allowed to mix for 5 minutes. An amount of 0.3% ammonium sulfate solution was then added which contained 0.0225 gram of (NH) 2304, this being the molecular equivalent to 0.0058 gram of NH: or 0.1% of the chlorine to be added in the following step. The slurry was allowed to mix for 5 minutes following the addition of the ammonium sulfate solution.

A 3% (available chlorine) sodium hypochlorite bleaching solution containing 5.8 grams of chlorine was then added. This amount of chlorine was times the measured bleach demand of the dry cotton. The temperature at this point was 35 C. and the pH of the slurry was 2.5. The bleaching process was carried out for 30 minutes following the addition of the bleach.

The bleach solution'was then drained of! and the cotton washed free of chlorine with cold tap water. The cotton was then centrifuged, fluifed and dried to approximately 5% moisture. The bleached cotton had a viscosity of 2920 seconds, a soda soluble of 2.0% and an Excitation Purity of 5.9%.

Another portion of the same unbleached linters was treated by the same process described above except no ammonium sulfate was used. The bleached material had a viscosity of 535 seconds, a soda soluble of 2.7% and an Excitation Purity of 5.9%.

Example VI Three hundred and ninety-four grams of unbleached cotton linters similar to that of Example II were placed in a bleaching vessel equipped with a screened false bottom and a paddle-type arm agitator. An amount of tap water sufllcient to make a final slurry (after addition of all reagents) containing 2% bone-dry cotton was then added. Sulfuric acid in the amount of 9.37 grams was added as a 3% solution and the slurry allowed to mix for 5 minutes. An amount of 0.3% ammonia solution was then added which contained 0.33 gram of NH: or 10.0% of the chlorine to be added in the following step. The slurry was allowed to mix for 5 minutes following the addition of the ammonia.

A 3% (available chlorine) sodium hypochlorite bleaching solution containing 3.3 grams of chlorine was then added. This amount of chlorine was 5 times the bleach demand of the bone dry cotton. The temperature of the slurry at this point was 35 C. and the pH 2.5. The bleaching process was carried out for 30 minutes following the addition of the bleach.

The bleach solution was then drained off and the cotton washed free of chlorine with tap water. The cotton was then centrifuged, fluifed and dried to approximately 5% moisture. The bleached cotton had a viscosity of 3890 seconds, a soda soluble of 1.0% and an Excitation Purity of 6.4%.

Another portion of this same unbleached linters was treated by the same process described above except that 8.75 grams of sulfuric acid were added instead of 9.35 grams, and no ammonia was used. The pH of the bleaching solution in this case was also 2.5. The bleached material had a viscosity of 2025 seconds, a soda soluble of 1.6% and an Excitation Purity of 6.5%.

Example VII In this example, a sample of commercial unbleached sulflte wood pulp having a viscosity of 290 seconds, a soda soluble of 22.2% and a bleach demand of about 3.34 grams of chlorine per 100 grams of bone-dry pulp was bleached.

Two hundred and twenty-five grams of the above unbleached sulfite pulp having a moisture content of 11.0% were placed in a stainless steel container and an amount of tap water added such that the consistency of the final slurry after addition of all reagents was 10%. To this slurry was added an amount of 10% sulfuric acid solution containing 18.65 grams of sulfuric acid and the slurry was agitated for 5 minutes. An amount of 0.3% ammonia solution was then added which contained 0.267 grams of NH: or 2% of the chlorine to be added in the following step and the slurry was agitated for five minutes.

A 5.89% (available chlorine) sodium hypochlorite bleaching solution was then added which contained 3.34 grams of chlorine and the bleaching process started. After five minutes an additional 3.34 grams of chlorine were added, and again until a total of 13.36 grams of chlorine were added. This total amount of chlorine is equal to twice the bleach demand of the bone-dry sulfite pulp. After all the chlorine had been added the bleaching process was allowed to continue for 30 minutes. The temperature of bleaching in this example was maintained at 36 C. and the pH of the final bleach liquor was 1.9.

The pulp was then washed free of chlorine with tap water and centrifuged. The pulp was then given a caustic steep for 45 minutes at 5% consistency in a 0.25% caustic soda solution at 63 C. It was again washed and centrifuged and given an agitated steep for 15 minutes at 3% pulp consistency in a water slurry, the pH of which was adjusted to 2.5 with sulfuric acid, after which the pulp was washed, centrifuged and made into 10 inch x 12 inch hand sheets from a tap water slurry. The bleached and extracted pulp had a viscosity of seconds, a soda soluble of 21.2% and an Excitation Purity of 10.0%.

The second portion of the same unbleached pulp was given the same bleaching and extraction treatments as described above except no ammonia was used and the final pH of the bleachingsolution was 2.0 instead of 1.9 as found in the previous case. This bleached material had a viscosity of 14 seconds, a soda soluble of 25.8% and an Excl tation Purity of 9.9%.

Example VIII Sixty-four hundred and forty-seven grams of bleaching solution containing 12.8 grams of sulfuric acid, 0.267 gram of ammonia (2% of the chlorine) and 13.36 grams of available chlorine (200% of the bleach demand) were made by adding 10% sulfuric acid solution, 0.3% ammonia solution and 5.48% (available chlorine) sodium hypochlorite bleaching solution to tap water in the above sequence. The temperature of this solution was 35 C. and the pH was 4.5.

To this solution in a stainless steel vessel were added 219 grams of the same unbleached sulflte pulp as described in Example VII but at 8.6% moisture content instead of 11% as previously.

The bleaching process was continued for 30 minutes with agitation during which time the pH changed to a final value of 2.7. The pulp was then washed with tap water and centrifuged. The pulp was then given a caustic steep for 45 from tap water slurry. The bleached and ex-,

tracted pulp had a viscosity of 90 seconds, a soda soluble of 22.2% and an Excitation Purity of 8.2%.

Another portion of the same unbleached pulp was given the same bleaching and extraction treatments as described above except that no ammonia was used. The bleached material had a viscosity of 18 seconds, a soda soluble of 28.0% and an Excitation Purity of 8.6%.

Example IX Sixty-four hundred and forty-seven grams of bleaching solution containing 17.6 grams of sulfuric acid, 0.267 gram of ammonia (2.0% of the chlorine) and 13.36 grams of available chlorine (200% of the bleach demand) were made by adding 10% sulfuric acid solution, 0.3% ammonia solution and 5.24% (available chlorine) sodium hypochlorite bleach solution to tap water in the above sequence. 'The temperature of this solution was 36 C. and the pH 2.50.

To this solution in a stainless steel vessel were added 219 grams of the same unbleached pulp as described in Example VIII. The bleaching process was continued for hours with intermittent agitation. During the course of the bleaching the pH gradually lowered until it was 2.15 at the end of the 5 hours. The pulp was then washed with tap water and centrifuged. The pulp was then given a caustic extraction for 45 minutes at 5% pulp consistency in a 0.25% caustic soda solution at 63 C. It was again washed and centrifuged and given an agitated steep for 15 minutes at 3% pulp consistency in a water slurry, the pH of which was adjusted to 2.5 with sulfuric acid, after which the pulp was'washed, centrifuged and made into inch x 12 inch hand sheets from a tap water slurry. The bleached and extracted pulp had a viscosity c176 seconds (AC8 2.5 g. conc.), a soda soluble of 22.2% and an Excitation Purity of 8.6%.

Another portion of the same unbleached pulp was given the same bleaching and extraction treatments as described above except that 18.1 grams of sulfuric acid were used instead of 17.6 grams and that no ammonia was used. The bleached and extracted material had a viscosity 01' 81 seconds (ACS 5.0 g. conc.), a soda soluble of 36.2% and an Excitation Purity of 9.5%.

Example X Two hundred and nineteen grams of the same unbleached suliite pulp as used in Example IX were placed in a stainless steel vessel. An amount of tap water suflicient to make a final slurry (after the addition of all of the reagents) containing 3% bone-dry pulp was then added. Then 8.75 grams of sulfuric acid were added as a 10% An amount of 0.3% ammonia solution was added which contained 0.134 gram of NH; (2.0% of the chlorineto be added in the following step) and the slurry again agitated for 5 minutes.

A 6.47% (available chlorine) sodium hypochlorite bleaching solution containing 6.68 grams of chlorine was then added. The amount of chlorine added was just equal to the bleach demand. The slurry temperature at this time was 35 C.

.solution and the slurry agitated for 5 minutes.

8 and the pH 2.40. The bleaching process was continued for 10 minutes with constant agitation. At the end of the 10 minutes of bleaching the pH was 2.16.

The bleach solution was then drained OH and the pulp washed free of chlorine with tap water and formed into 10 inch x 12 inch hand sheets and dried to approximately 5% moisture. A portion of the sheeted pulp was torn into small pieces. reslurried in water, centrifuged and given a caustic extraction at 5% pulp consistency for 45 minutes in 0.25% caustic soda solution at 63C. It was again washed and centrifuged and given an agitated steep for 15 minutes at 3% pulp consistency in a water slurry, the pH of which was adjusted to 2.5 with sulfuric acid after which the pulp was washed, centrifuged and made into 10 inch x 12 inch hand sheets from a tap water slurry. The bleached and extracted pulp had a viscosity of 125 seconds, a soda soluble of 22.1% and an Excitation Purity of 8.6%.

Another portion of the same unbleached pulp was given the same bleaching and extraction treatments as described above except that no ammonia was used. This material after bleaching and extraction had a viscosity of 80 seconds, a soda soluble of 21.1% and an Excitation Purity of 9.0%.

Example XI An amount of 0.3% ammonia solution was added which contained 0.26 gram of NH: (2.0% of the chlorine to be added in the following step) and the slurry again agitated for 2 minutes.

A 5.30% (available chlorine) sodium hypochlorite bleaching solution containing 13.0 grams of chlorine was then added. The amount of chlorine added was of the bleach demand of the bone-dry pulp. The temperature of the slurry at this time was 35 C. and the pH was 1.0. The bleaching reaction was carried out for 30 minutes with intermittent agitation. The pH at the end of the reaction was still 1.0.

The pulp was then washed free of chlorine with tap water and centrifuged. The pulp was then given a caustic extraction at 5% pulp consistency in 0.26% caustic soda. solution for 45 minutes at 63 C. The pulp was then washed and centrifuged and then given an agitated steep for 15 minutes at 3% pulp consistency in a water slurry, the pH of which was adjusted to 2.5 with sulfuric acid. After this the pulp was washed, centrifuged and made into 10 inch x 12 inch hand sheets from a tap water slurry. The bleached and extracted pulp had a viscosity of 135 seconds and an Excitation Purity of 22.8%.

Another portion of the same unbleached pulp was given the same bleaching and extraction treatments as described above except that no ammonia was used. This material had a viscosity of seconds and an Excitation Purity of 23.1%.

Example XII One hundred and twenty grams of unbleached placed in a stainless steel container 'and an amount of tap water added such that the consistency of the final solution after addition of all reagents would be 2% cloth. The temperaturewae adjusted to 36 C. The mixture was stirred manually.

- A 3% sulfuric acid solution was added to a pH of 2.3. This was followed by the addition of 0.3% ammonia solution containing 0.048 gram of ammonia (2% of the chlorine) and an amount of 3% of sodium hypochlorite bleach solution containing 2.4 grams of available chlorine at intervals of 2 minutes. Bleaching was continued with occasional stirring for 30 minutes at 35 C. The final pH of the solution was 2.4.

The cloth was washed and soaked in water for 16 hours after which it was steeped in a 0.25% solution of caustic soda at 145 F.for 45 minutes at a consistency of 2%. It was then washed again and given a steep in cold water, the pH of which was adjusted to 2.5 with sulfuric acid for 20 minutes. Itwas then washed, centrifuged and oven-dried. The bleached cloth had a viscosity of 7100 seconds, a soda soluble of 1.6% and an Excitation Purity of 3.7%.

A second portion of the same unbleachedmuslin was bleached by an identical process as described above except that no ammonia was used. This material after bleaching had a viscosity of 2900 seconds, a soda soluble of 2.43% and an Excitation Purity of 3.5%.

Example XIII A sample of cotton linters, previously digested in 2% NaOH, was bleached in the same manner as in Example I except that instead of the ammonium hydroxide, 20 parts per million (on the solution basis) of aniline hydrochloride were used. The bleached cellulose product obtained had a viscosity of 630 seconds as compared with pulpwood, ramie, bagasse,- cotton textiles and the 50 like and at any stage of treatment of the recovered cellulosic material where a chlorination step might be used.

The present invention is particularly adapted to purification procedures in which the conservation of viscosity or of strength of the cellulose fibers or ultimate products derived therefrom, or in which the maximum improvement in the color of the pulp by bleaching methods, or both; are of importance. conditions are drastic enough to produce appreciable cellulose degradation, the use of relatively small amounts of a nitrogen-containing 'compound such as ammonia will inhibit or reduce this degradation while at the same time permitting the desired or even increased bleaching actions to take place. Furthermore, where the conditions of treatment are changed from time to time the variations in eifects on the properties of the pulp which normally issue may be substantiallyminimized by the use of a small amount of ammonia or other nitrogen-containing compound in accordance with the present invention.

The bleaching step of the present invention is applied to a chlorination under acid conditions 7 10 in the presence of free chlorine and is distinguished thereby from hypochlorite or other of the more alkaline treatments. Thus, the treatment will be carried out at a pH which may vary from about 1 to about 4.5 and preferably from about 1.5 to about 2.5.

The acidity of the treatment is brought about as the normal result of adding gaseous chlorine to water siurries or by the addition of acids such as sulfuric acid, hydrochloric acid and the like in a quantity sufficient to make the treating solution acid and to maintain it at an acid pH of about 4.5 or' lower in spite of the possible addition of substances of an alkaline nature. Where alkaline substances are present or are to be added so that acid must be added to the suspension, it is preferable to add the acid to the suspension of the cellulose and to mix it thoroughly therewith before any chlorine is added to insure a uniform and invariable condition of acidity.

The chlorine used in the acid bleaching step according to this invention may be added as such to the suspension of cellulose in water or it may be previously dissolved in the water before the addition of the cellulose. Instead of liquid or gaseous chlorine, a compound such as sodium hypochlorite, calcium hypochlorite, etc., which reacts with acid to form free chlorine, may be added to the acidified cellulose slurry (already containing the nitrogen addition compound) whereupon the excess acid present reacts with the hypochlorite to produce free chlorine equivalent to available chlorine in the added hypochlorite. This procedure provides an acid chlorination with elemental chlorine for the bleach.

The quantity of chlorine used with this will depend upon a number of factors, such as the bleach demand of the material treated, the slurry consistency, the temperature and time of treatment, the degree of purification desired and the degree of cellulose degradation to be tolerated, etc. The advantage of the treatment of this invention is the much wider latitude for the quan tity of chlorine calculated as parts per part of 5 cellulose pulp. The quantity of chlorine used Thus, whenever the bleaching may vary from one fourth to several times the bleach demand of the pulp, for example, five times the bleach requirement of the cellulosic material being treated. However, in some cases, it may be considerably more than this, for example, up to about times the bleach demand of the cellulosic material and still realize the improved results according to the invention. The concentration of the chlorine in the water phase is determined by the quantity of chlorine employed in terms of the pulp and the consistency of the pulp in the slurry. The concentration of chlorine in the slurry cannot of course exceed at any instant the solubility of chlorine in water for the conditions employed but it has been found in this invention that the higher concentrations of chlorine in the water phase can be tolerated and high pulp consistencies may be employed. Such drastic treatments are not possible in prior art processes since the cellulose degradation is so great as to render the resulting products unsatisfactory for most uses.

The nitrogen compound utilized according to this invention to inhibit or substantially reduce the cellulose degradation for the degree of bleaching accomplished, is in general a compound of nitrogen of the basic type. The compound will contain a nitrogen atom having at least one hydrogen atom thereon. Ordinarily, ammonia, conveniently in the form of ammonium hydroxide or 11' 1 ammonium salts, is preferably used because of its efficacy and cheapness. Primary or secondary amines have likewise been found suitable. Such amines are, for example, aniline, toluidine, methyl aniline, benzylamine, diphenylaminc, diamylamine, methylamine, ethylamine, diethanolamine, and the like. Acidamides, such as acetamide, formamide, butyric amide, and the various monosubstituted alkyl and aryl amides have been found suitable. Similarly, compounds of the urea type, such as urea, methyl urea, ethyl urea, diphenyl urea, diethyl urea, the urethanes, and other substituted ureas or thioureas; also hydroxylamines, substituted hydroxylamines, hydrazines, substituted hydrazines, guanidines, substituted guanidines, chloramine, and substituted chloramines, are likewise suitable. It will be understood that in referring to the ammonia, amine or other nitrogen compound having a hydrogen atom, such nitrogen compound may .be in the basic form or in the'form of a salt either before addition to the bleaching solution or after such addition.

It will also be appreciated that the nitrogen compound is chlorinated to form a chloramine upon being subjected. to the elemental chlorine of the acid chlorination. Again, the nitrogen compound will be referred to as anammonia or amine even though it probably functions during the actual bleach as a chlorine derivative. Instead of adding a salt, ammonia or amine, the nitrogen compound may be added in the form of the chlorine derivative. Thus, instead of ammonia, chloramine may be added. Similarly, instead of the more complex nitrogen compounds, chloramines derived therefrom (or salts of such chloramines) may be added to the bleach solution.

The amount of the nitrogen compound to be used is largely dependent on the results desired and the nature and conditions of the particular treatment. However, in all cases it has been found that exceedingly small quantities of the nitrogen compound are highly effective in minimizing viscosity drop in chlorination systems having free chlorine present. Thus, a concentration as small as 0.1 parts per milllion (as NHa) based on the solution has been found to be effective in materially cutting down viscosity drop. Ordinarily a concentration between about 0.2 and about 40 parts per million, based on the solution, will be used. Based on the chlorine available for the chlorination, a quantity between about 0.03% and about 6% will ordinarily be utilized. Large amounts, up to about 16%, based on the chlorine available forchlorination or about 0.1% based on the solution, may be used and still attain the advantages of the invention. However, quantities of the nitrogen compound in excess of about 16% of the chlorine progressively decrease the effectiveness of the bleach treatment. Hence, the use of nitrogen compounds in'excess of about 16% by weight of available chlorine does notgive the desired degree of purification and in addition, is uneconomical since a much smaller quantity is usually sufiicient for the effect desired.

The acid chlorination step of this invention may be carried out at any of the usual operating temperatures, such as, 32 to 95 F. In fact, it permits the use of higher temperatures, even under pressure above the boiling point of water, these higher temperatures usually being impractical in normal acidic bleaching because of the viscosity drop involved. At temperatures between about 32 and about 70 F., the advantages derived from the present invention are largely 12 those of better control and uniformity of bleaching as temperature is varied. The advantages of smaller viscosity drop for a given bleach action are found particularly at temperatures above 70 F., with the range between about 70 F. and about 95 F. being especially important. This range is a practical, highly desirable bleaching temperature range under certain conditions except for the fact that normally excessive and uncontrollable viscosity drops are encountered. In accordance with this invention, bleaching in this range is quite practical, and the benefits of higher temperature are derived while controlling viscosity drop within acceptable limits.

The length of the bleaching step may be any period between about five minutes and about six hours depending upon the temperature and the other conditions of treatment utilized. In most cases, the chlorination wil be continued until substantial consumption of the elemental chlorine or, where an excess of chlorine is used, until there is no substantial improvement discernible upon further chlorination.

The chlorination step of the present invention is ordinarily utilized in a sequence of bleaching operations. Thus, it may be the initial, final and/or intermediate step in a sequence employing hypochlorite bleach, chlorination without ammonia, alkaline extraction treatments at any temperature. Similarly, additional bleaching steps with bleaching agents, such as peroxides, permanganates, chlorites and the like may also be used.

It has been mentioned that the method according to this invention greatly minimizes the degradation of the cellulose in chlorination for the degree of bleach action attained and that this invention brings about good process control with variations in temperature and other conditions of treatment. Measures of cellulose degradation are several, the more pertinent ones are cellulose viscosity, soda soluble, copper number, alpha cellulose, carboxyl and aldehyde. content, stability to heat and/or light as measured by viscosity and color retention, etc. These measures are in many-cases interdependent and warrant specific consideration for specific applications. However, as a general rule reliance may be placed principally on viscosity and soda soluble to measure degradation.

The degree of bleach action may also be measured variously. Color is of course an important measure. For this, excitation purity as defined in the International Commission on Illumination (I. C. I.) may be used. In measuring bleach W action of an early stepin a bleaching process,

color may not be sumciently developed for direct measurement. It may then be necessary to determine the bleach action by submitting the treated samples to a finishing bleach to develop 60 the latent color of the samples. Bleach action may be measured also by determining the remaining bleach demand or permanganate number of the pulps by methods standardized by the Technical Association of the Pulp and Paper Industry. In some instances the major function of chlorination is conditioning the contaminants for subsequent removal. Under these conditions the color-forming substituted ammonia compounds may be utilized where the final color of 70 the cellulose product is unimportant or where the color is modified in subsequent treatment.

The term cellulosic material" as used in the specification and includes textiles and other forms of cell ose rs in addition to com- 75 mon forms, such as, wood pulp and cotton linters.

' tion Serial No. 459,957, filed September 28, 1942, and now forfeited.

What I claim and desire 'to protect by Letters Patent is:

1. A method of treating a cellulosic material with an acidicaqueous solution containing chlorine which comprises preparing an aqueous solution having a pilot from about 1.5to about 4.5

and containing chlorine and a product of dissolving a nitrogen-containing compound therein, said nitrogen-containing compound being selected from the group consisting of ammonia, monosubstituted ammonia, disubstituted ammonia, salts of ammonia, salts of monosubstituted ammonia, and salts of disubstituted ammonia, and being present in an amount from about 0.03% to about 16%, expressed as molecular equivalent ammonia, based on the weight of the chlorine, and subjecting the cellulosic material to treatment with said solution.

2; A method of inhibiting the viscosity reduction of cellulosic material when subjected to treatment with an aqueous solution with a pH of from about 1.5 to about 4.5 and containing a small proportion of chlorine, based on the weight of the solution, which comprises carrying out the treatment with said solution after dissolving therein from about 0.03% to about 16%, expressed as molecular equivalent ammonia, based on the weight of the chlorine, of a nitrogen-containing compound selected from the group consisting of ammonia, monosubstituted ammonia, disubstituted ammonia, salts of ammonia, salts of monosubstituted ammonia, and salts of disubstituted ammonia.

3. A method of inhibiting the viscosity reduction of cellulosic material when subjected to treatment with an aqueous solution with a pH of from about 1.5 to about 4.5 and containing a small proportion of chlorine, based on the weight of the solution, which comprises carrying out the treatment with said solution after dissolving therein from about 0.03% to about 6.0%, expressed as molecular equivalent ammonia, based on the weight of the chlorine, of a nitrogen-containing compound selected from the group consisting of ammonia, monosubstituted ammonia, disubstituted ammonia, salts of ammonia, salts of monosubstituted ammonia, andisalts of disubstituted ammonia.

4. A method of inhibiting the viscosity reduction of cellulosic material when subjected to treatment with an aqueous solution with a pH of from about 1.5 to about 4.5 and containing up to about 0.1% chlorine, based on the weight of the solution, which comprises carrying out the treatment with said solution after dissolving therein from about 0.03% to about 6.0%, expressed as about 95' 1'. with an aqueous solution with a pH of from about 1.5 to about 4.5 and containing up to about 0.1% chlorine, based on the weight oi. the solution, which comprises carrying out the treatment with said solution after dissolving therein from about 0.03% to about 6.0%, expressed as molecular equivalent ammonia, based on the weight of the chlorine. of a nitrogen-containing compound selected from the group con? sisting of ammonia, monosubstituted ammonia, disubstituted ammonia, salts of ammonia, salts of monosubstituted ammonia, and salts of disubstituted ammonia.

6. A method of inhibiting the viscosity reduction of cellulosic material when subjected to treatment with an acidic aqueous solution with a pH oi from about 1.5 to about 4.5 and containing chlorine which comprises carrying out the treatment in the presence of a minor amount in' the order of 0.2-20 parts per million, expressed as molecular equivalent ammonia, based on the weight of the solution, to inhibit substantially the viscosity reduction of the cellulosic material, of a nitrogencontaining compound dissolved in the solution and selected from the group consisting of am- .monia, moncsubstituted ammonia, disubstituted 1.5 to about 4.5 and containing chlorine which molecular equivalent ammonia, based on the weight of the chlorine, of a nitrogen-containing comprises carrying out the treatment in the presence of a minor amount, in the order of 02-20 parts per million, expressed as molecular equivalent ammonia and based on the weight of the solution to inhibit substantially the viscosity reduction of the cotton linters, of a nitrogen-containing compound dissolved in the solution and selected from the group consisting of ammonia, monosubstituted ammonia, disubstituted ammonia, salts of ammonia, salts of monosubstituted ammonia, and salts of disubstituted ammonia.

8. A method of inhibiting the viscosity reduction of wood pulp when subjected to treatment with an acidic aqueous solution with a pH of from about 1.5 to about 4.5 and containing chlorine which comprises carrying out the treatment in the presence of a minor amount in the order of 02-20 parts per million, expressed as molecular equivalent ammonia, based on the weight of the solution, to inhibit substantially the viscosity reduction of the wood pulp, of a nitrogen-containing compound dissolved in the solution and selected from the group consisting of ammonia, monosubstituted ammonia, disubstituted ammonia, salts of ammonia, salts of monosubstituted ammonia, and salts of disubstituted ammonia.

9. A method of inhibiting the viscosity reduction of cellulosic material when subjected to treatment with an aqueous solution with a pH of from about 1.5 to about 4.5 and containing chlorine which comprises carrying out the treatment in the presence of a minor amount of ammonia, in the order of 0.2-20 parts per million based on the weight of the solution to inhibit 1 substantially the viscosity reduction oi the cellulosic material.

10. A method of inhibiting the viscosity reduction of cotton linters when subjected to treatmentwith an aqueous solution with a pH of from about 1.5 to about 4.5 and containing chlorine which comprises carrying out the treatment in the presence of a minor amount of ammonia, in the order of 0.2-20 parts per million based on E the weight of the solution to inhibit substantially'the viscosity reduction of the cotton linters.

11. A method of inhibiting the viscosity reduction of cotton linters when subjected to treatment with an aqueous solution with a pH of from about 1.5 to about 4.5 and containing chlorine which comprises carrying out the treatment in the presence of a minor amount of chloramine, in the order of 02-20 parts per million, expressed as molecular equivalent ammonia and based on the weight of the solution to inhibit substantially the viscosity reduction of the cellulosic material.

12. A method of inhibiting the viscosity reduction of cotton linters when subjected to treatment with an aqueous solution with a pH of from about 1.5 to about 4.5 and containing chlorine which comprises carrying out the treatment in the presence of a minor amount of a salt of ammonia in the order of 02-20 parts per million, expressed as molecular equivalent ammonia, based on the weight of the solution, to inhibit substantially the viscosity reduction of the cotton linters.

13. A method of inhibiting the viscosity reduction of cellulosic material when subjected to treatment at a temperature of from about 70 1 F. to about 95 F. with an acidic aqueous solution with a pH of from about 1.5 to about 4.5 and I containing chlorine which comprises carrying ammonia, salts of monosubstituted ammonia, and

salts of disubstituted ammonia.

14. A method of inhibiting the viscosity reduction of cotton linters when subjected to treatment at {temperature of from about F. to about F. with an acidic aqueous solution with a pH of from about 1.5 to about 4.5 and containmg chlorine which comprises carrying out the treatment in the presence of a minor amount in the order of 02-20 parts per million, expressed as molecular equivalent ammonia, based on the weight of the solution, to inhibit substantially the viscosity reduction of the cotton linters, of a nitrogen-containing compound dissolved in the solution and selected from the group consisting of ammonia, monosubstituted ammonia, disubstituted ammonia, salts of ammonia, salts of monosubstituted ammonia, and salts of disubstituted ammonia.

15. A method of inhibiting the viscosity reduction of cotton linters when subjected to treatment at a temperature of from about 70 Fv'to about 95 F. with an aqueous solution with a pH of from about 1.5 to about 4.5 and containing chlorine which comprises carrying out the treatment in the presence of a minor amount of a salt of ammonia in the order of 02-20 parts per million, expressed as molecular equivalent ammonia, based on the weight of the solution, to inhibit substantially the viscosity reduction of the cotton linters.

ALFRED MEAD DODSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number. Name Date 1,737,104 Bergmann Nov. 26, 1929 1,765,581 Hall June 24, 1930 1,957,938 Campbell May 8, 1934 2,070,893 Glass Feb. 16, 1937 2,174,534 Shipp Oct. 3, 1939 2,299,200 Adams Oct. 20, 1942