US2203212A - Bleaching pulp - Google Patents

Description

Patented June 4, 1940 UNITED STATES PATENT OFFICE BLEACHING PULP No Drawing. Application February 14, 1939, Serial No. 256,359

5 Claims.

This invention or discovery relates to improvements' in bleaching pulp; and it comprises an improved method ofbleaching pulp, particularly kraft pulps, to a high degree of whiteness with 5 minimum loss of strength, which includes the steps of subjecting pulp to at least one chlorination step followed by washing, subjecting the Washed pulp usually in concentrated suspension, to the action of caustic soda in s all amount,

1 allowing time for extraction of coloring matter to take place, treating the alkaline pulp in dilute condition by an addition of a calcium hypochlorite liquor in amount sufficient to perform a substantial amount of bleaching under the strongly 15 alkaline conditions and giving the pulp a final bleach with a small amount of bleaching agent sufiicient to remove residual color and secure a high degree of whiteness but without substantially affecting the strength of the pulp; all as 20 more fully hereinafter set forth and as claimed.

In a general way, it may be said that the object in all processes of bleaching paper pulp is, of course, to secure the maximum whiteness, the minimumloss in strength and the most economi- 25 cal use of chemicals, that is, of chlorine and alkali; to secure the whitest and strongest product in the simplest manner. The total amount of chemicals required for complete bleaching of course varies with the darkness of the pulp.

30 Mostly the commercial processes operate in a plurality of successive stages and the way in which the alkali and the chlorine are distributed in the stages varies. larly important in obtaining the strongest and 35 whitest product to carefully control the propor-, tion of chlorine used in each stage; that is, the distribution among the stages of the total chlorine to be used. The sensitiveness of the cellulose fiber to attack, with loss of strength and degra- 40 dation, increases with the disappearance of coloring matter; the greatest amount of harm to the fiber is done in the final stages of bleaching. In all bleaching there is some loss of strength in the fiber, and the loss of strength may be rela- 45 tively great in securing very light color in pulp. Kraft pulp is dark colored and has exceptional strength; but it is difficult to bleach it completely without substantial loss of strength.

In most modern bleaching methods some of 50 the chlorine is used as such in water solution, that is, as an acid solution of hypochlorous acid; but the final step is usually a treatment with an alkaline bleach solution; that is, with an alkaline solution of hypochlorite.

65 In a prior Patent No. 2,127,765, I have dis- I have found it to be particuligible because of the high alkalinity and also closed and claimed an improved method of bleaching wood pulps and in particular kraft pulps under such conditions as to obtain a light-colored pulp with minimum loss in strength. In'some respects the present invention may be regarded as an improvement on this patent.

In the present invention '1 have rearranged and modified the steps in a multiple stage bleaching operation using particular proportions of the total chlorine in the several steps, achieving my object of producing a maximum whiteness with a minimum loss in strength. I find that with any particular total amount of chlorine, the ratio between the amounts used in successive steps 7 is important in securing full bleaching without loss of strength. Operating in the hereinafter described way, I can indeed use in the final steps relatively expensive bleaching agents; giving little attack on the fiber, such as chlorites and peroxides, without prohibitive expense. At this step, bleaching is so near completion that the amount of costly reagent required in a finishing step is not prohibitive. However I can secure about the same results with carefully used chlorine and hypochlorites without resorting to costly reagents even in minimal amount.

I find that I cansecure a good white pulp, without degradation of the pulp, by interpolating in processes such as that of my prior patent a step of bleaching with calcium hypochlorite liquor unan der strongly alkaline conditions; under conditions protecting the fiber strength of the pulp. The oxidizing effect upon the cellulose fiber is negbecause bleaching is stopped short of the attainment of a high white color.

In the present invention I find that with proper regulation most of the bleaching can be performed in this step with substantially no degradation of the pulp strength, but I stop short of a com- 40 plete bleaching. Then, in the last step, the pulp is bleached completely using only a small amount of bleach, because the bulk of the bleaching has already been accomplished. In the final step the amount of bleach used can be sufficiently small as to achieve the desired high brightness without any deterioration of the fiber. In fact, the amount of bleach necessary for the final whitening is reduced to such an unexpectedly small proportion that it is as stated feasible to use some commercial costly, highly efficient bleaching agents which are ruled out by expense considerations in ordinary processes. For example, in the last step instead of using hypochlorite liquor I can use sodium or potassium chlorite or peroxides. The net result is a pulp as white and as strong as is obtained in ordinary processes using large amounts of these costly agents, but with a total cost for reagents small enough to make the new process strictly competitive with ordinary kraft bleaching processes which do not give so white a pulp.

In sum, by the present invention there is produced a fiber of remarkably high strength and brightness, with a comparatively small number of bleaching stages; four stages in thebest embodiment. For all the simplicity of the new process the results compare favorably with those secured in complex six and seven stage bleaching processes.

In the following example there is set forth a specific embodiment of the invention directed to producing a bright, high strength paper-making fiber from a raw southern yellow pine kraft pulp.

Example 1.In such an operation, 1000 pounds of the pulp (air-dry basis) in water at a consistency of 4 per cent (25,000 pounds of suspensioniwere subjected to the action of elemental chlorine in amount 5 per cent by weight on the pulp, that is, 50 pounds of chlorine, in a circulating apparatus such as described in the Mo- Combs Patent 1,953,022. This amount of chlorine was less than that .required for a complete chlorination of the non-cellulose constituents of the pulp. The chlorination required about an hour. The pulp was then washed and brought again to the same consistency. About 2.5 per cent (25 pounds) of chlorine were introduced into the circulating pulp during the course of half an hour. This amount of chlorine was in excess of the amount required for chlorination of the impurities. A suspension of 3'7 pounds of hydrated lime (3.7 per cent by weight on the pulp) was added tothe pulp, without washing, to form a hypochlorite bleaching liquor in place, in conjunction with the residual chlorine. The pulp was agitated for 30 minutes to allow hypochlorite bleaching to take place. The pulp was then washed, introduced into a bleacher capable of circulating a high-consistency pulp, such as commercial Fletcher type bleachers, and brought to a higher consistency, about 15 per cent. Ten pounds (i. e., 1 per cent) of caustic soda (NaOH) were added and the temperature was brought to 130 F. The mixture was agitated and circulated for an hour under which conditions the caustic soda dissolved away substantial amounts of the remaining foreign matters. The pulp was then rediluted to a 5 per cent consistency which operation brought the temperature down to 100- 110 F. To the suspension was added a calcium hypochlorite bleach liquor in amount sufficient to give pounds (1 per cent) available chlorine and the suspension was circulated for approximately two hours under which conditions extensive but not complete bleaching took place. The pulp during this step was in a highly alkaline condition which protected it from degradation with loss of fiber strength. The pulp was then washed and treated with a small amount of hypochlorite bleach liquor corresponding to 0.2 per cent available chlorine (2 pounds). This final bleach was carried out in an ordinary manner and the bleached pulp washed. The final pulp was found to be very strong and remarkably white. The pulp produced showed advantages as regards strength and whiteness over that by the process of my acknowledged patent.

In lieu of using hypochlorite in the final step, a chlorite, such as sodium chlorite, can be subrination steps and of tile construction for the alkaline treatments. The temperatures employed are ordinary factory temperatures, say 70-75 F., unless otherwise stated.

The following tabulation shows comparative characteristics of the raw unbleached pulp, the pulp as bleached according to Example 1, and a pulp as produced from the same raw pulp accordmg to the acknowledged prior patent:

Unbleached Example 1 Prior patent Minutes to bring pulp to 80 percent ireeness 48 30 30 To 60 percent frccnessx. 82 54 55 Initial bursting strength (Mulle 80 52 53 At 80 percent freeness 121 115 107 At 60 percent freeness 127 126 117 Initial tear 161 166 154 Tear at 80 percent l'rcencss. 114 105 99 Tear at 60 percent freeness 109 100 95 The above described four-stage process gives paper-making pulp fiber characterized by an exceptionally high brightness and strength. In some cases it is desirable to simplify the process to some extent, in view of apparatus limitations; especially when such exceptionally bright pulps are not required. My method is readily adapted for such simplification.

Example 2.In a modification which produces a pulp stronger than ordinary pulps but not quite so bright as that in Example 1, the second chlorination step of Example 1 is omitted. That is, in a typical operation, a batch of 1000 pounds of the southern yellow pine kraft pulp mentioned above was subjected in 4 per cent suspension to 5 per cent elemental chlorine by weight on the pulp as described in the first step of Example 1. After the chlorination the pulp was washed. The pulp was made up to higher consistency, about per cent and caustic soda added directly. The amount of caustic soda required wasv about 1.5 per cent (15 pounds). The suspension was agitated at a temperature of 130 F. for an hour, then reduced to 5 per cent consistency. A calcium hypochlorite bleach liquor was added to the dilute pulp in amount sufiicient to give 4 per cent (40 pounds) available chlorine and the suspension was circulated for a couple of hours to allow bleaching to take place. As in Example 1, the highly alkaline condition of the pulp protected it from degradation. The pulp was then washed and treated with sodium chlorite in amount (3 pounds) yielding 0.5 per cent available chlorine. For this treatment the temperature was brought to 130 F. and the pH of the pulp was brought down to 4 by addition of an acid such as HCl. After bleaching was complete the pulp was washed. It was almost as bright as the pulp obtained in Example 1 and was very strong.

The amount of total chlorine required in this example is rather'higher than in Example 1, but this is ofiset in many cases by the simplification of apparatus made possible and in any event the amount of reagents compares favorably with that required in conventional processes which yield a pulp neither so strong nor so bright. Hy-

' pochlorite can be used in the final step in lieu of chlorite. in which case 5 pounds of hypochlorite are employed.

In the foregoing examples the pulp is described as being concentrated to high consistency prior to the caustic soda treatment. This is desirable from the standpoint of economy of reagents, but the same ultimate results can be secured by treating the pulp with the caustic soda without preliminary concentration, for a longer time. The temperature need not be raised above ordinary bleaching temperatures, 100-110 F. Such operation requires more time, and hence apparatus of greater capacity, but it avoids the necessity of providing a special piece of apparatus for handling high consistency pulps.

This modification makes possible a variation which has certain advantages. Thus in Example 1 the bleaching of the causticized pulp is performed with calcium hypochlorite liquor, added separately, but some of the advantages of the invention can be secured if the hypochlorite is made in situ. As stated, if the NaOH treatment is performed on a dilute pulp, say a 5 per cent pulp suspension, more NaOH is required. Two or 3 per cent NaOH may be required in lieu of the 1 per cent required when the suspension is concentrated to 15 per cent consistency. Not all this caustic is required for maintaining the pulpv alkaline during bleaching, so instead of adding extra calcium hypochlorite, the NaOH-treated pulp can simply be treated with elemental chlorine under conditions such as to form a suitable amount of hypochlorite with the excess NaOH.

'In lieu of caustic soda other alkalis or bases can be employed in the alkaline extraction treatment of the partially bleached pu l for example soda ash or hydrated lime. Such other alkalis yielda pulp nearly as white as with caustic soda and substantially as strong. However for convenience and simplicity of operation I regard caustic soda as the most useful alkali .at this stage.

What I claim is: y

1. A method of bleaching pulps to high brightness which comprises subjecting an aqueous pulp suspension to the action of free chlorine in substantial amount but less than that required for complete chlorination or the non-cellulose impurities, washing the pulp, treating the pulp again with free chlorine in excess of the amount required for direct chlorination of the remaining impurities, making the pulp alkaline so as to form hypochlorite from the residual chlorine and allowing time for action of the hypochlorite, washing the pulp, treating the washed pulp with an alkali in an amount equal to at least one per cent by weight of the dry pulp, subjecting the alkaline pulp to the action of an alkaline hypochlorite bleach liquor for a period of the order of one to two hours to effect a further bleaching, the amount of alkali present being at least one per cent by weight of the dry pulp and being suiiiciently high to minimize attack on the cellulose and preclude complete bleaching, and the amount of bleach liquor being such as to be substantially exhausted at the end of one to two hours, washing and then subjecting the washed pulp to a final bleaching to high brightness with a small amount of a bleaching agent such asto yield the equivalent of about 0.2 to 0.5 per cent available chlorine by weight 'on the dry pulp; whereby a pulp of high brightness and strength is obtained with a minimum degradation of the cellulose.

2. The method of claim 1 wherein the finalbleaching agent is a hypochlorite.

3. The method of claim 1 wherein the final bleaching agent is a peroxide.

4. The method of claim 1 wherein the final bleaching agent is a chlorite.

5. In bleaching washed extensively chlorinated pulps which have been subjected to the action of chlorine in excess of the amount required to react with the non-cellulose constituents of the pulp, the improvement which comprises subjecting such an extensively chlorinated pulp to the action of an alkaline hypochlorite bleach liquor for a period of the order of one to two hours to effect a further bleaching, the amount of alkali present being at least one per cent by weight of the dry pulp and being sufficiently high to minimize attack on the cellulose and precluding complete bleaching, and the amount of bleach liquor being such as to be substantially exhausted at the end of one to two hours, washing and then subjecting the washed pulp to a final bleaching to high brightness with a small amount of a bleaching agent such asto yield the equivalent of about 0.2 to 0.5 per cent available chlorine by weight on the dry pulp; whereby a pulp of high brightness and strength is obtained with a minimum degradation of the cellulose.

' CA SCIANI.