US2120678A - Bleaching and purifying process - Google Patents

Description

Patented June 14, 1938 BLEACHING AND PURIFmG PROCESS John 12'. Parsons and Donald T. Jackson, Erie, Pa., assignors to Hammermill Paper Company, Erie, Pa", a corporation of Pennsylvania No Drawing.

8 Claims.

This invention relates to an improved bleaching and purifying process particularly adapted for the bleaching and purification of cellulose fibers derived from wood or the like by any of the known fiber-liberation processes. While it is capable of general application to pulps of all types, it is especially suited for the treatment of kraft or sulfate pulps. The resulting product is excellently suited for use in the paper industry and in the rayon, film, and similar industries.

may be bleached to a high white color without serious injury to the fibers through the proper use of a chlorite in lieu of part or all of the hypo- '25 chlorites ordinarily employed. In fact, through the use of-a three-stage process, one stage of which involves a chlorite treatment, the strength of the bleached and purified product will be equivalent or superior to that of the unbleached 30 pulp. Other improvements will also be noted. In attempting to bleach and purify pulp by the usual methods, new impurities in the form of degraded cellulose are produced. Such new impurities are not produced, however, .in the prac- 35 tice of the present improved process.

The three-stage process referredto involves a preliminary treatment of the pulp with free chlorine. While such a treatment alone-is quite unsatisfactory for producing a pulp of high white 40 character, especially kraft pulp, it will be found to pave the way excellently forthe chlorite treatment which follows. Between the chlorine and the chlorite treatments there is preferably, though not necessarily, added to the pulp a su'fil- 45 cient quantity of any suitable alkali to react with all of the available chlorine and the pulp is'then washed. In reacting with the chlorine the alkali will form a hypochlorite which in turn will quickly be consumed in reaction with the pulp. The

50 alkali also reacts directly with certain impurities Application March so, 1936, Serial No. 71,690

in the pulp, largely produced by the chlorination, and renders these impurities soluble. If desired the alkali treatment may be preceded as well as followed by a washing of the pulp and it may be extended over a considerable period, say an hour 5 or even more. Following the chlorite treatment, which will be described in greater detail hereinafter, the pulp is again washed and then a relatively small amount of calcium or sodium hypochlorite or the like is added to complete the 10 bleach. In many cases a satisfactory whiteness will be attained without the final hypochlorite treatment or by substituting for it a second chlorite treatment. Excellent results have been obtained by the use of a mixture of a chlorite with hypochlorite following the chlorine treatment or as the final treatment of the pulp following a treatment with a chlorite alone. Also in many cases, particularly when dealing with sulfite and soda pulps, very good results may be obtained without the preliminary chlorine treatment, in which case the process will preferably involve a chlorite treatment followed by a hypochlorite treatment.

Inasmuch as the chlorite treatment forms the most important feature of the improved process the preferred manner of carrying it out will now be described in some detail. We have found sodium chlorite to be especially suited for the purposes of this stage of the process although many otherchlorites might also be used, particularly the chlorites of the alkali and alkaline-earth metals and also ammonium chlorite. The quantity of the chlorite to be used depends upon the character of the other steps of the process. Or-

dinarilythe bleaching of a particular type of pulp to a particular state of whiteness will require a certain amount of available chlorine. The precise amount of available chlorine to be supplied through the chlorite will depend upon how much, 40

sufficient of the chlorite to provide about onefourth of the total available chlorine used. This may readily be varied, however, according to circumstances so that anywhere from 10 to 50% or more of the total available chlorine is provided through the chlorite.

In treating the stock with a chlorite we have employed pulp consistencies of 5 to 10 per cent. If desired the consistency might be made lower. e. g. 1 or 2%, but this will entail additional expense due to the greater bulk of the equipment required, the greater input of heat required, and various other considerations. In general it is preferable to employ higher consistencies and, if the equipment available for the handling of the stock is appropriate, much higher consistencies may be employed. In fact consistencies oi. as much as 20 to 30% or even more may then be utilized.

The chlorite treatment should be carried out in or with a solution that is on the acid side and preferably one that has a pH value of about 4.0. At this degree of acidity the reaction will take place at a relatively rapid rate with very satisfactory results. However, good results may be obtained at any pH between 3.0 and 6.0. At the higher pH values the reaction becomes ome what slower while at a pH below 3.0 there is a tendency for the chlorites to decompose rather rapidly. During the course of the chlorite treatment the solution has a tendency to become more strongly acid, 1. e. if it starts with a pH of 4.0 it will conclude with a pH around 3.5. For the purpose of establishing the desired acidity a suitable quantity of any appropriate acid, such as hydrochloric or sulfuric acid, may be added. Hydrochloric acid is preferred, due to the soluble salts formed and its strength, but a variety of other acids, particularly the mineral acids, might be used.

For best results the mass should be heated to a temperature of about 50 C. for the chlorite treatment. The reaction is considerably more rapid at a slightly elevated temperature of this character than at ordinary room temperatures. However, if the acidity of the mixture is sufficiently great a reasonable rapid reaction will take place at ordinary temperatures of around 20 C. n the other hand, so long as the acidity is not too great, i. e. not below a pH of about 4.0, the temperature may well be carried up to 60 C., or even higher. Above 60 C. at a pH of 4.0 the decomposition of the chlorite will become objectionably rapid but if the pH is higher'the temperature may be correspondingly higher without too rapid decomposition of the chlorite. Accordingly the temperature may well be varied between 20 and 60 C., or even broader limits, depending upon the other conditions prevailing.

Atypical operation of a three stage process conducted in accordance with the invention will now be described. A sulfite pulp requiring about 3.5 per cent available chlorine, based on the weight of oven dry pulp, to produce a satisfactory white color when bleached with calcium hypochlorite in a single stage process was subjected to the following treatments: For the first stage suflicient chlorine, in the form of chlorine water,

of the remaining available chlorine and furnish an alkaline solution. For this purpose 2.4% of calcium hydroxide, based on the weight of the dry pulp, was found suflicient. Corresponding amounts of other alkalis such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide or in fact any hydroxide of the alkali and alkaline-earth metals, or carbonates of the alkali .metals, might have been substituted for a part or all of the calcium hydroxide. Following the addition of alkali the pulp was washed and it was i then ready for the second stage of chlorite treatment. In preparation for this it was first reduced to a consistency of and was then heated to a temperature of about 50 C. Prior to the heating a sufilcient quantity of sodium chlorite was added to provide available chlorine to the extent of 375% of the weight of the dry pulp" and suflicient hydrochloric acid was added to bring about a pH value of 4.0. This chlorite treatment was continued for a. period of three and three quarters hours during which the mass was agitated continuously. At the conclusion of this treatment the pulp was thoroughly washed and then reduced again to a 5% consistency preparatory to the third stage treatment. In the third stage sufilcient calcium hypochlorite was added to provide an available chlorine content of 0.40 per cent of the weight of the dry fiber. The temperature during this stage in the typical operation was maintained at about 38 C. After a period of two hours, during which the pulp was continuously agitated, the bleached material was againthoroughly washed.

The product resulting from the foregoing three stage treatment exhibited properties different from and superior to the products of prior known bleaching processes. One startling characteristic of the new bleached product was that paper produced from it had a greater folding endurance, by the Schopper test, than paper of the same weight and character produced from the unbleached sulflte pulp. Thus the paper formed from the new product displayed a folding endurance of .1170 while paper formed from the same stock prior to bleaching displayed a folding endurance of only 910. This is quite remarkable inasmuch as paper formed from the same stock bleached by a two stage, chlorine and hypochlorite, process developed a folding endurance of only 680. This is the ordinary experience, i. e. prior bleaching processes have tended to reduce the folding endurance of the stock greatly while the product of our new process has an increased folding endurance, as compared with the original unbleached stock. In the determination of the values given above, standard methods and equipment were employed in connection with 20 lbs. basis weight sheets, (1'7" x 22"-500 sheets).

So also from the standpoint of strength, as determined. by the standard Mullen bursting I strength test applied to the same 20 lb. basis weight sheets, the new product is superior to the product of prior known bleaching processes.

Paper formed in the standard way from the new bleached product has a Mullen bursting strength of 52 as compared with a strength of 53 for the unbleached sulfite pulp and 50 for the same pulp bleached by a two stage chlorine-hypochlorite process.- The foregoing physical tests were con-e ducted at a temperature of 70 F.- and a relative humidity of 50%.

The. following table giving a comparison of the results obtained in certain tests upon the raw, unbleached sulfite pulp of the typical example, the

areas-rs bleached product of the typical example, and the same initial stock subjected to a two stage, chlorine-hypochlorite bleaching process, discloses the superiorityvof the new process and product in the foregoing and other respects:

' Two 315?; cl l i i ie blealfihed bleaching h P p 115 s chlorite 5: 23:3

Total percent av. added 3. l 2. 7 5 Total percent av. 01, consumed 2 85 2. 61 Bursting strength (Mullen) 53 52 50 Folding endurance (Schgp 910 1170 680 Viscosity in cuprammo 69 47 v 32 Percent alpha cellulose 85. 5 85. 8 85. 4 Copper number l. 7 l. 4 1. 3 Percent hot alkali soluble 20. 9 19. 6 2i. 9 Brightness (percent reflectance with ll filter in G. E. reflection meter) 59 90 87 In the foregoing the viscosity in cuprammonium solution and the alpha cellulose content have been determined according to the methods adopted by the Techanlcal Association of the Pulp and Paper Industry. The copper number has been determined by the Schwalbe-Hagglund method and the "per cent hot alkali soluble has been dewith 0.5 per cent of calcium hydroxide, based on the dry pulp weight. In the first stage suflicient chlorine was consumed to constitute 7.66% based upon the-weight of the dry fiber. In the next stage sufficient sodium chlorite was consumed to provide available chlorine to the extent of 5.96% of the dry pulp weight, while in the final stage sufilcient calcium hypochlorite toprovide 0.85%

available chlorine was consumed. Tests of the same character as outlined above; together with a standard (Thwing) tearing test applied to 20 lb. basis weight paper (17" x 22" 500 sheets) at- 50% relative humidity and 70 F. gave the following results:

Um Three stage bleached $33 pulp chlorite Bursting strength (Mullen) 61 Folding endurance (Schopper) 1630 1800 Tearing (Thwing) 93 104 Viscosity in cuprarnmon 112 30 Brightness (percent reflectance with #1 filter in G. E. reflection meter) 25 83 From the foregoing it will be seen that the bleached kraft resulting from the improved process not only displayed greater folding endurance but also better strength and better tear resistance than the initial, unbleached stock. This is directly contrary to the usual experience with products of other bleaching processes. Furthermore, this is in spite of; the fact that the kraft pulp. was bleached to an exceptionally high, white color. The high viscosity of the highly bleached product is also noteworthy. It is desirable for various purposesto maintain the viscosity value specified as high as possible, preferably over 25, but

, 3 prior bleaching processes in dealing with kraft or sulfate pulp have failed to achieve this end. It has'not been considered possible, heretofore, to combine high 'strength, high viscosity and a high white color in a kraft or sulfate pulp product.

While the preferred three stage processhas been described in considerable detail it will be understood that wide latitude is permissible in the practice of the invention. The preliminary chlorine treatment may be carried out in any known manner and may provide any desired portion of the total available chlorine used. Preferably 50% of more of the total available chlorine is supplied in this way but, if preferred, only 25% of the total need be so supplied. Furthermore,

good results can be obtained with sulfite pulp if the chlorine treatment is omitted altogether in which case the process will preferably consist of one or two chlorite treatments followed by a hypochlorite treatment. The permissible latitude in the chlorite treatment has already been referred to. While it is preferred to conclude the three stage process with a hypochlorite treatment of any suitable character, this is not essential and the hypochlorite may be replaced by a second chlorite treatment. When the hypo chlorite is used it should preferably supply only about 5 to 15% of the total available chlorine consumed. Should the preliminary chlorine treatment be omitted, however, a somewhat greater quantity of hypochlorite may-be em-' ployed, to supply, say up to 35% of the total required available chlorine. If desired the chlorite and hypochlorite treatments may be combined into a single stage or treatment with a chlorite alone may be followed by a treatment with a mixture of a chlorite and a hypochlorite. In any case the combined available chlorine should be substantially as specified and preferably the relative proportions of chlorite and hypochlorite should be as stated, although when a mixture is used the chlorite and hypochlorite may, if desired, be present in about equal proportions. When the mixture is used the temperature of the stock may suitably be maintained around 35 to 38 C'. and the pH at between 7 and 9 although these conditions may be varied to suit the particular circumstances. When the chlorite predominates in the mixture the conditions should approach more nearly those preferred for the chlorite treatment alone. It will be understood that appropriate washing and alkali treatmentswill be. employed whether the chlorine treatment is followed by separate chlorite and hypochlorite treatments or by treatment with a mixture. The duration of the various stages of the process will depend upon the amount of available chlorine supplied in each stage and uponthe tem- The losic material which comprises first subjecting the cellulosic material to the action of chlorine, then treating the same with a chlorite selected from the class consisting of an alkali metal and an alkaline earth metal, and finally subjecting the cellulosic material to the action of an alkaline hypochlorite.

losic material which comprises subjecting the same in a plurality of stages to the action of a plurality of agents having available chlorine, at least one of said agents being in the form of a chlorite selected from the class consisting of an alkali metal and an alkaline earth metal, and another of said agents being in the form of a hypochlorite, the final stage comprising treatment with an alkaline hypochlorite.

3. A method of bleaching and purifying cellulosic material which comprises successively subjecting the same to the action of chlorine, a chlorite selected from the class consisting of an alkali metal and an alkaline earth metal, and a hypochlorite, in the sequence specified, the chlorite treatment being conducted in the acid state at a temperature of about 50 C, and the hypochlorite treatment being conducted in the alkaline state at a temperature of about 38 C.

4. A method of bleaching and purifying cellulosic material which comprises successively subjecting the same to the action of chlorine, sodium chlorite, and calcium hypochlorite in the sequence specified, said hypochlorite being in alkaline solution.

5. A method of bleaching and purifying cellulosic material which comprises subjecting the same to the action of chlorine and then subjecting it to the action of a mixture of a chlorite and a hypochlorite.

6. A method of bleaching and purifying cellulosic material which comprises subjecting the same to the action of a mixture of a chlorite with a hypochlorite at a pH of at least 7.0.v

7. A method of bleaching and purifying cellulosic material which comprises subjecting the material to the action of chlorine, a chlorite and a hypochlorite in a plurality of stages, the hypochlorite being used at least in part in the final stage and in a quantity to provide available chlorine to the extent of not more than 1% of the dry weight .of the material being bleached, said final stage being .carried out in alkaline solution.

8. A method of bleaching and purifying cellu losic material which comprises subjecting the same in a plurality of stages to the action of a plurality of agents having available chlorine, at least one of said agents being in the form of a chlorite and providing from 10 to 50% of the available chlorine consumed, and another of said agents being a hypochlorite and providing from 5 to 35% of the available chlorine consumed, said hypochlorite being used in an alkaline state in the final bleaching stage.

JOHN L. PARSONS. DONALD TUJACKSON.