US1987212A - Bleaching of cellulose fiber - Google Patents

Description

Patented Jam-8, 1935 BIEACH'ING OF OELLULOSE FIBER George A. Richter, Berlin, N. 11., assignor to Brown Company, Berlin, N. 11., a corporation of Maine No Drawing. Application July 12, 1928, Serial No. 292,325

80laims.

The bleaching of cellulose pulp is usually ef-' fected by the use of oxidizing agents, lrvpochlorite bleach being generally employed for this pur- When cellulose pulps are bleached by hypoqhlorite bleach, it is possible to attain moderate whiteness, with some pulps, without materially degrading the cellulose fibers physically or chemically. The bleachability of different pulps, however, varies widely and while it is possible, for example, to bleach sulphite pulp to moderate whiteness with hypochlorite bleach without materially injuring the pulp, it is impossible to attain the same whiteness in the case of kraft pulp without using such a large amount of hypochlorite bleach as to injure the pulp seriously. There are various methods of facilitating the bleaching of pulp to make possible the attainment of high whiteness with comparatively low bleach usage. For instance, the bleaching of chemical wood pulp such as sulphite or kraft is facilitated by refining the pulp to high alpha cellulose content before bleaching is effected. The refining of such pulp may be eflected by first treating the unbleached pulp with chlorine water,

- which serves to react with ligneous and other non-alpha cellulose components present in the pulp, either removing them or making them susceptlble to solution in an alkaline liquor, and then the pulp in an'alkaline liquor to re- .mo ve chlorine reaction products, less-resistant celluloses, pentosans, resins, and other non-alpha cellulose components. The resulting refined pulp is more readily bleachable than the original pulp,

Q but even in such case it is difllcult to produce a product of high whiteness, possessing high strength, tear resistance, folding endurance, and

low copper number. In U. S. Patent No. 1,639,704,

ilssued August 23, 1927, to Milton 0. Schur and myself, there is disclosed a process of bleaching pulp to a finished product of high whiteness and having excellent physical and chemical characteristics. According to that process, the pulp is bubiected to hypochlorite bleach liquor, which is maintained distinctly alkaline throughout the bleaching operation. Such a bleaching process is especially elective whensulphite pulp in refined or unrefined condition is the raw-material. Certain pulps, however, such as kraft, are so re- I sistant to bleaching that it is impossible to attain high whiteness without injuring the pulp, even alter thepulp has been refined and when a distinctly alkaline bleaching operation is carried out. For instance, according to my patent application Serial No. 189,l,' filed Mar 6, 1027, it

is possible to produce a product of only moderate whiteness having the strength, tear resistance, and folding endurance of the original kraft pulp, by subjecting a refined kraft pulp to: a

series of distinctly alkaline bleaching treatments, 5

each treatment or which involves a lower concentration of alkaline bleach than would be necessary if it were attempted to bleach the pulp by one treatment. Specifically, the bleached, refined pulps' produced according to that process 10 have a color of about 98 to 100, and possesses substantially the same physical characteristics as the original kraft, whereas a well-bleached, unrefined sulphite pulp has a.color of about 103' to 104 and a bleached, refined sulphite pulp a 15 color of about 104 to 110, on the same scale.

I have discovered that it is possible to realize high whiteness in pulps quite readily, if bleaching is effected by first using hypochlorite liquor to produce moderate whiteness, then treating the bleached pulp with comparatively dilute solutions' of certain salts of the acids of manganese, such as the manganates or permanganates, and finally removing colored reaction products of the last treatment. Such a process involves certain desirable features, among which are the following:

' (1) Hypochlorite, which is the less expensive chemical, is used to do most of the bleaching.

(2) comparatively little manganate or permanganate'is necessary to produce a high degree of whiteness in the bleached pulp.

(3) Substantial degradation of the pulp is avoided.

While a bleaching process such as described may be applied to advantage in bleaching cellulose flbers of various origins to high whiteness, it is especially advantageous, in a pulp-refining process where kraft pulp is employed as a raw material, for in such case after .the kraft pulpfm has been chlorinated and digested in'alkaline liquor,- it may be subjected to one or more alkaline bleaching treatments while preserving subs'tantially the physical characteristics of the r original kraft, and then converted to high w iteness without materially injuring the pulp.

Let us assume that it is desired to produce a refined pulp of high whiteness having the stren tear, and folding endurance of kraft pulp, suitable for the manufacture of the highest quality bond, ledger, and writing papers. Such a product may be produced by applying the process of the present invention specifically as' follows. The kraft pulp employed as a raw mas terisl is first refined as described in m applica- ,deepen its color.- The pulp is washed and thendigested for, say, about one hour at a consistency of about 10%, in a 5% to 10% caustic soda solution at a temperature of from 10 to 10 C. In lieu of using caustic soda solution, other alkaline solutions may be employed, for instance, the socalled white liquor employed for the production of kraft pulp and which contains sodium hy-' droxide and sodium sulphide and generallyhas a sulphidity of from 20% to After digestion, the pulp is washed. The washed pulp is of high alpha cellulose content, but is usually not much whiter than the original krait pulp. When such pulp is subjected to bleaching, it is found that it bleaches much more readily than the original pulp, but when bleached to whiteness as ordinarlly, it becomes seriously degraded. In accordance with the process of the present invention, the pulp is first bleachedto moderate whiteness in hypochlorite liquor, and then treated with a manganate or permanganate liquor. The bleaching in hypochlorite liquor is preferably effected while such liquor is maintained distinctly alkaline throughout the bleaching operation. It is also; preferable to efiect the hypochlorite bleaching in two or more treatments, preferably washing the pulp with hot alkaline water, between the bleach-- ing treatments, to produce a bleached product having a color of about 98 to 100. The bleached pulp is washed and then mixed at a consistency of from 6% to 10% with, say, about 0.5% to 1% permanganate, based on pulp. Inasmuch as permanganate is a vigorous oxidizing agent, it is necessary to distribute it rapidly through the pulp, in order to avoid local oxidation and iniuryof the fibers. This may be accomplished by preparing a comparatively dilute solution of the permanganate and adding the solution to the pulp. The pulp is maintained in contact with the permanganate liquor, preferably with agitation, for about two to six hours at temperatures of about 70 to 80 F. During this time, the purple permanganate solution in contact with the pulp progressively lightens in color as the permanganate is consumed and the fiber assumes a yellow or brown color, as a result of the reduction of the permanganate by the organic reducing components of the pulp to manganese oxides which become fixed on the fiber. It is necesary to remove the manganese oxides from the fiber to obtain a white product, but, inasmuch as the manganese oxide is precipitated on the fiber in finely divided condition, some oxide remains on the fiber, even after washing. The removal of the oxides from the fiber is accomplished by first washing the fiber and then treating the fiber with a suitable reducing agent to form colorless manganous salts. For example, the fiber may be treated with a comparatively dilute sulphurous acid or bisulphite solution at room temperature. The product, when washed free of reaction products, has a bright white color of about 104 to 106 falling on the bluish side of the color scale, and possesses substantially the same strength, tear resistance, and, folding endurance as that of the original kraft pulp.

in carrying out the bleaching processpf the present invention, various modifications may be made. For instance, rather than using a neutral permanganate solution, one may make the solution slightly acid to increase the rate of bleaching by the liberation of free permanganic acid, without materially affecting the quality of the resulting product. So, too, one may make the solution slightly alkaline, but in such case care must be exercised to avoid injuring the pulp, as alkaline permanganate solutions are vigorous oxidants and may injuriously affect the physical and chemical characteristics of the pulp. For the preparation of certain cellulose derivatives, however, where a low-viscosity fiber is desired as the raw material, the use of alkaline permanganate superbleaching solutions may be advantageous, as such solutions result in fibers of lower viscosity-than when neutral or slightly acid solutions are employed.

If desired, the permanganate may be produced in situ in the pulp. For. instance, the pulp may be treated with finely pulverized manganese dioxide, and then treated with a suitable oxidizing agent such as sodium peroxide. The oxidation may be effected by adding successive doses of sodium peroxide, sufiicient time being allowed between doses to permit substantially complete reduction of the resulting permanganate by the organic reducing material in the pulp to manganese dioxide, so that less manganese dioxide is required than if all the permanganate required were produced at .once. The manganese dioxide may be removed from the fiber by a sodium sulphide solution rather than by a sulphurous acid or bisulphite solution, in which case it is preferable to use the solution at elevated temperature, as under such conditions the action is hastened. It is preferable, in such case, then, to treat the fiber with dilute sulphurous acid to enhance brightness of color and avoid the yellow side of the color scale. 1 I

Owing to the lack of a better generic term, it is to be understood that by the expression "permangana as used in the specification and appended claims, I mean to include the manganates, as well as the permanganates. Either or both the sodium -or potassium salts are satisfactory for the purposes of the bleaching process of the presentinvention.

There may be other changes of the process of the present invention which will suggest themselves to those skilled in the art without departing from the spirit or scope of invention as defined in the appended claims.

What I claim is:'

-1. A process which comprises pulp with bleach liquor containing hypochlorite in amount to whiten the pulp without material physical or chemical degradation of the fibers, and then treating the pulp in aqueous suspension at a consistency below about 10% with about 0.5% to 1% permanganate, based on; the weight of the pulp to enhance the-whiteness of the pulp.

-2. A process which comprises treating wood pulp with bleach liquor containing hypochlorite in amount to whiten the pulp without material physical or chemical degradation. of the fibers,

and modifications treating woos treating the pulp in aqueous suspension at a consistency below about 10% with about 0.5% to 1% permanganate, based on the weight of the pulp to enhance the whiteness of the P lp. washing the pulp free from residual permanganate, treating the washed pulp with a reducing agent to form colorless manganese salts from the manganese dioxide deposited on the fibers, and again washing the pulp free from reaction products. I

3. A process which comprises subjecting wood pulp to the action of hypochlorite bleach liquor,

and then treating the bleached pulp in aqueous suspension at a consistency below about 10% with neutral permanganate in the amount oi! about 0.5% to 1%, based on the weight of the pulp.

4. A process which comprises subjecting wood pulp to the action of hypochlorite bleach liquor, and then treating the bleached pulp in aqueous suspension at a consistencybelow about 10% with acid permanganate solution containing about 0.5% to 1% permanganate, based on the weight of the pulp.

5. A process which comprises bleaching cellulose pulp in hypochlorite bleach liquor, treating the bleached pulp in aqueous suspension at a consistency below about 10% with about 0.5% to 1% permanganate, based on the weight of the pulp, and then treating the pulp with a solution of sodium sulphide to remove manganese dioxide resulting from the reaction of the organic reducing materials in the pulp and the permanganate.

6. A process which comprises bleaching cellulose pulp in hypochlorite bleach liquor, treating the bleached pulp in aqueous suspension at aconsistency below about 10% with about 0.5% .to 1% permanganate, based on the weight of the pulp, then treating the pulp with a hot solution of sodium sulphide to remove manganese dioxide resulting from the reaction of the organic reducing materials in the pulp and the permanganate,

on the weight of the pulp.

8. Aprocess which comprises digesting chemical wood pulp in an alkaline liquor to refine such pulp to higher alpha cellulose content, bleaching the refined wood pulp with hypochlorite bleach liquor to whiteness without material physical or chemical degradation of the pulp fibers, said bleach liquor containing sufilcient free alkali to remain distinctly alkaline throughout the bleaching operation, and enhancing the whiteness of the pulp by suspending it at a consistency not exceeding about 10% in a permanganate solution containing not more than about 0.5% to 1% permanganate, based on the weight of the pulp.

GEORGE A. RICHTER.