US1855332A - Method for purification and bleaching of cellulosic materials - Google Patents

Description

Patented Apr. 26,, 1932 UNITED STATES" PATENT orrlce CHARLES A. BLODGETT, OF SOUTH BREWER, MAINE, ASSIGNOR TO EASTERN MANU- FACTURING COMPANY, OF SOUTH BREWER, MAINE, A CORPORATION OF MASSAe CHU SETTS METHOD FDR PURIFICATION AND BLEAGHING F CELLULOSIG MATERIALS N0 Drawing.

This invention relates-to a method for the purification and bleaching of cellulosic pulps and the like, which is more or less generally applicable, but especially adaptable to those pulps which have been produced by an initial reduction with acid sulfite liquors.

In the art of reducing cellulosic raw materials to the condition of a pulp fromwhich paper or other products are to be made, the softening and disintegrating solutions usually employed may be qualitatively divided into two classes, namely those which are essentially acid, such as the bisulfites of the alkalis and alkaline earths which are commonly used in the sulfite processes, and those which are alkaline, such as caustic soda, sodium sulfide, etc., which are used in the soda and sulfate processes, respectively.

A fundamental diflerence between the 2 processes of pulp reduction as thus classified, lies in the differences in the properties of the acid and alkaline solutions used and in'the corresponding several effects of these classes of reagents upon the raw fibrous materials which are treated therewith. These are in general well recognized, but it is here to be especially pointed out and emphasized that, whereas an initial treatment of raw stocks with strongly caustic or alkaline solutions not only liberates and dissolves the various components of the wood which are associated with the fibres'but also tends to react with and degenerate the alpha-cellulose constituent of the fibres themselves, the acid or bisulfite solution, on the other hand and by way of contrast with the caustic treatments, are less reactive with respect to the alpha-cellulose component of the fibres, although potent reagents for liberating and removing most of the associated substances which are present in the raw wood.

Practical application of these observations has been made in the process described in oopending application of Theodore Clark. SerialNo. 1,078, filed January 7, 1925, for

the production'of a pure alpha-cellulose pulp. The procedure there disclosed typically includes a preliminary reduction of wood to pulp, treating the resulting pulp with a caustic soda solution having an alkali concentra Application med February 16, 1926. Serial No. 88,684.

tion of the order of 10 to 15% sodium hydroxide or its equivalent, then washing and bleaching as by the addition of a mineral acid which is subsequently removed by washing. The finished pulp is dried or used in the customary manner for the purpose desired.

It has also been found (co-pending application of Charles A. .Blodgett and Hugo H. Hanson Serial No. 87,806, filed February 12, 1926) that a procedure somewhat analogous to that above outlined may be effected wherein after a sulfite cook, preferably leaving the pulp somewhat raw, a second [or alkaline] stage of operations is conducted by digesting the pulp with a solution characterized by containing a reactive sulfur compound in alkaline solution, preferably a reducin solution, such as sodium sulfite, [thi-s1ilfate,] alkali sulfides, etc. or the like or a mixture of these. In the aforesaid co-pending application this is followed by washlng the pulp and bleaching in the customary manner. But

the bleaching of pulp prepared by the process thus outlined may be especially efiectively carried out by the procedure herein to be described.

Another pulp-making process producing pulp particularly adapted to the bleaching process herein disclosed, is that of Vance P. Edwardes, U. S. Patent No. 1,310,694.

It has now been found, as a part of the bleaching agent added, or alternatively by checking short of completion by anti-chlor. This oxidizing'bleach is then followed by a treatment with a reducing agent such as an alkali or alkaline-earth bi-sulfite bleach, and finally by washing of the pulp' with Water preferably until free from reagents. The process is especially appropriate to those pulps which have been previously cooked, in

any well known manner, with reducing acid liquors, such as the acid-sulfites, subsequently washed and then subjected to treatment with an alkaline reagent solution, for the further and more complete removal of initially associated lignin and other non-cellulose components, and of degenerated cellulosic components, of the pulp.

The process of the invention includes or presupposes apreliminary reduction of appropriate cellulose-bearing materials such as Woods, more especially exemplified by those from the coniferous species of trees, to the condition of a uniform fibrous pulp by partial digestion with a reducing acid solution, for example one or more of the alkaline earth bi-sulfites, and further digestion in an alkaline reagent, followed by subjecting to an initial reducing bleach, typically a solution essentially containing sulphurous acid or a bi-sulfite (which also neutralizes any residual amounts of the alkaline reagent or solution which the pulp may retain) treating with an oxidizing bleach such as bleaching powder, exhausting (or checking) the oxidizing bleaching reaction short of completion (preferably appreciably short of the complete or maximum bleaching effect obtainable by oxidizing action) whereafter a second and characteristically reducing bleach is effected by the addition of a reducing bleaching reagent, typically a solution containing sulfurous acid or a bi-sulfite, the excess of which is finally separated and removed from the residual purified and highly bleached alphacellulose pulp, as by washing.

It is to be understood that certain other operations may be interspersed with the several stages of procedure, as here outlined, for the procurement of specific or improved results, to effect economies of time, material, labor, etc., and also as incidental requirements due to any specific raw material employed and the variations of quality and composition which it manifests with respect to the several treatments involved.

Atypical and preferred application of the invention will be described as carried out upon a pulp which has been prepared by the sulfite process, e. g. from woods of the species 1 of conifers, such as spruce, etc.,and which is further treated after the manner of the disclosures above referred to, for the procurement of a highly bleached alpha-cellulose of high purity.

The wood is first reduced mechanically to the form of chips which are then mixed with a customary solution of alkaline earth bisulfite,-usually containing both calcium and magnesium bi-sulfites and free sulfurous acid, and digested under the conditions of heat and pressure, in the manner well known to those skilled in the art of producing sulfite pulp as by Mitscherlich or quick cook (Ritter-Kellner) processes. These conditions are maintained until sufficient action is had upon the woodchips to soften and reduce them to the condition of a uniform fibrous pulp. The various species of wood are adapted to different cooking methods, and hence a given batch should consist of woods adapted to the sulfite cook, and of a single species or of close- 1y related species only, and not of mixed dissimilar woods indiscriminately. Coniferous woods such as spruce, fir and hemlock for example are well known to be adapted to the sulfite cook, while woods of the broadleaf or deciduous group such as poplar are not well adapted to it on account of their inherent characteristics and the southern pines are not available without the preliminary extraction of pitch.

1n the instant case, where coniferous wood such as spruce is under treatment, the digestion may be continued an appreciably less period of time than that required for the usual complete sulfite digestion so that only a somewhat incomplete dissolution and removal of the lignin and other materials associated with the cellulose therein is effected, so that the resulting pulp is of the character known as raw.

At this stage the alpha-cellulose constituent is left not only substantially completely unchanged chemically but essentially without material physical alteration or deterioration of its fibrous structure. On the other hand, such treatment penetrates the interstices between the fibres, dissolving most of the lignin and other non-cellulose constituents normally present therein, and isolating the cellulose fibres but without causing the formation of appreciable quantities of degenerated celluloses or otherwise chemically affecting the principal or alpha-cellulose constituent of the wood.

The pulp so produced is next washed preferably to approximate neutrality and may then be subjected directly to the oxidizing bleach, although if intended to be so bleached (without the alkaline secondary cook herein referred to) it will ordinarily be more completely cooked to produce an easy bleaching grade.

For the manufacture of a higher grade of alpha-cellulose pulp, it is desirable at this point to subject the pulp first to an alkaline treatment in accordance with the procedures above alluded to, e. g. with a solution of sodium hydroxide or its equivalent, or with a relatively dilute solution (e. g. 3%. more or less) of a reactive sulfur compound characterized by being alkaline,either per se or through the addition of an alkali,such as a mixture of sodium sulfide and sodium bi-sullite. In the latter instance it is desirable to accompany the alkaline treatment with an increase of temperature and pressure. The batch so treated is then separated and washed substantially free from alkaline liquor.

The improved bleaching procedure in preferred form is next described as applied to such a pulp. 1

The residual traces of alkaline reagent which remain associated with the charge are neutralized or rendered slightly acidic by the addition of a reducing acid reagent such as a soluble bi-sulfite or sulfurous acid,conveniently freshly prepared cooking liquor of the sulfite process, effecting a preliminary bleaching of the pulp which has been somewhat darkened by the alkaline secondary cook. It is then ready for the oxidizing bleach. The oxidizing bleach may be effected in the usual manner, but with a bleach liquor containing available chlorine in total amounts of the order of only 2 to 2 5% of v the weight of dry fibre instead of the usual much larger amount. Oxidizing bleaching agents not only react upon the coloring components of the pulp and the portions which it is desirablel to oxidize, render soluble, and remove, but also if 1n sufficient amount and concentration, tend to act in course of time upon the cellulosic constituentsincluding' th'e alpha-cellulose, producing oxy-celluloses, etc., (and also imparting an additional coloration thereto). Moreover, overbleaching with such reagents also lowers the physical strength of the fibres to an appreciable extent and reduces the desirable properties of the pulp generally.

It is now recommended, for the purposes of the present invention and to avoid such degradation of the pulp, to exhaust or check the bleaching action considerably short of its ultimate possible effects upon the pulp, and indeed appreciably short of that point where the customary bleachingis obtained. This is done preferably by limiting the quantity of oxidizing agent to about the small amount stated above, but may alternatively be done by the addition when bleaching has gone far enough, of an anti-chlor, usually sodium bi-sulfite, in an amount sufiicient to react with the residual oxidizing agent, and hence to neutralize or overcome its further eflects upon the charge of pulp which is in process.

As a matter of regular control in commercial operations, and for the further insurance of preferred results in the ultimate product, the requisite amount of bleach may be so calculated as to effect the partial bleaching only of the batch under treatment, as for example to carry the batch to approximately two-thirds of the usual bleach, and this quantity only may be used. When this is done the bleaching or oxidizing capacity of the bleach expends itself at the desired point without the variations incident to checking the progress of an incomplete reaction, with anti-chlors and the like.

Her'etofore, the pul after being subjected to a complete c lorine or oxidizing bleachin v dition o an alkaline pulp, comprising neubleach for a suitable length of time to. pro duce the desired color, was. either drained and washed, or treated with an anti-chlor or reducing agent whereby to prevent the further action of the bleaching agent, and the soluble reagentsthen washed out, leaving the finished product. By the present invention it is found that a much higher quality of product is obtained by following such an incomplete oxidizing bleach as that disclosed above, with a reducing bleach as the final treatment.

To this end, therefore, a reducing bleach (containing SO as an essential component) is now added, such as a usual bi-sulfite cooking liquor, which is permitted to react for a short period (usually'a few minutes'only is adequate) followed by separation of the pulp and washing the pulp free from any substantial quantity of the reagents, with water. The bleaching action of this reagent und eir these conditions is marked and very rapi The resultant pulp is composed of long, relatively pure and strong alpha-cellulose fibres. They are essentially unaltered in their physical properties but are separate and essentially free from other components, associated materials and solids normally present in the woods from'which it was prepared, and with very little degenerated products of the cellulosic substance itself. It is moreover characterized by higher color, that is by a more brilliant white, than any other pulp which is of equal strength (as manifested for example by paper made therefrom) obtain able by oxidizing bleaching processes.

The process is unique in the preservation of a high yield of the original alpha-cellulose, and the simultaneous production of a highly purified pulp. The process further presents the great advantage of permitting the early exhaustion or early discontinuance of the oxidizing bleach and hence of eliminating its further and undesirable destructive action upon the pulp, which is not otherwise permissible where a commercially satisfactory white pulp is to be made.

I claim:

1. A method for the purification and bleaching of cellulosic materials, comprising treating the material with an acid-reducing agent, subjecting to an incomplete oxidizing bleach, and finally 2..A method for the bleachin of cellulosic materials, in the condition of an alkaline pulp, comprising neutralizing and bleaching the material with an acid reducing bleach, subjecting to an incomplete oxidizing bleach, and treating with a reducing bleaching agent.

3. A method for the purification and of cellulosic materials in the conpurification and tralizing the pulp with an acid reducing a ent, subjecting to an incomplete oxidizing b each, and treating with a reducing bleaching reagent, followed by freeing the pulp from associated reagent materials.

4. A method for the purification and bleaching of cellulosic materials, reduced to the condition of a pulp by a sulfite process, comprising subjecting the pulp to an alkaline treatment, neutralizing residual alkali with a reducing acid, subjecting to an incomplete oxidizing bleach, and treating with a reducing bleaching agent.-

5. A method for the purification and bleaching of cellulosic materials, reduced to the condition of a pulp by a sulfite process, comprising subjecting the pulp to an alkaline treatment,'neutralizing residual alkali with a reducing acid, sub ecting to an incomplete oxidizing bleach, and treating with a reducing bleaching reagent, said reducing acid and reducing bleach being the same reagent. I

6. A method for the purification and bleaching of cellulosic materials, reduced to the condition of a pulp by a sulfite process, comprising subjecting the pulp to an alkaline treatment, neutralizing residual alkali with a reducing acid, subjecting to an incomplete oxidizing bleach, and treating with a reducing bleaching reagent followed by freeing the pulp from associated reagent materials.

7. A method for the purification and bleaching of cellulosic materials, reduced to the condition of a pulp by a sulfite process, comprising subjecting the pulp to an alkaline treatment, neutralizing residual alkali with a reducing sulfur dioxide bleach, subjecting to an incomplete oxidizing bleach, and treating with a reducing bleaching reagent.

8. A method for the purification and bleaching of cellulosic materials, reduced to the condition of a pulp by a sulfite process, comprising subjecting the pulp to an alkaline treatment, neutralizing residual alkali with a reducing solution containing sulfurous acid, subjecting to an incomplete oxidizing bleach, and treating with a reducing bleaching reagent.

9. A method for the purification and bleaching of cellulosic materials, reduced to a condition of a'pulpby a sulfite process, comprising subjecting the pulp to an alkaline treatment, neutralizing residual alkali with a reducing sulfite bleach, subjecting to an incomplete oxidizing bleach, and treating with a reducing bleaching reagent.

10. A method for the purification and bleachin of cellulosic materials, reduced to the condition of a pulp by a sulfite process, comprising subjecting the pulp to an alkaline treatment,"neutralizing residual alkali with a. reducing acid sulfite, subjecting to an incom lete oxidizing bleach, and treating with a re ucing bleaching reagent.

11. A method for the purification and bleachin of cellulosic materials, reduced to a condition of a pulp by a sulfite process, comprising subjecting the pulp to an alkaline treatment, neutralizing residual alkali with a reducing solution of calcium and magnesium bi-sulfite, subjecting to an incomplete oxidizing bleach, and treating with a reducing bleaching reagent.

12. A method for the purification and bleaching of cellulosic materials, reduced to a condition of a pulp by a sulfite process, comprising subjecting the pulp to an alkaline treatment, neutralizing residual alkali with a reducing acid, subjecting to an oxidizing bleach, controlling the oxidizing bleach by adding the bleaching agent thereto in such amount as to be exhausted appreciably short of normal bleaching effect, and thereafter treating with a reducing bleaching reagent.

Signed by me at South Brewer, Maine, this 30th day of January, 1926.

CHARLES A. BLODGETT.