US1970065A - Process of bleaching pulp and other lignified materials - Google Patents

Description

Aug. 14, 1934. "G. H. TOMLINSON 1,970,065

I PROCESS OF BLEACHING PULP AND OTHER LIG'NIFIED MATERIALS Filed Sept. 28, 1931 2 Sheets-Sheet 1 ATTORNEY M 2193*. G. H. TQMLINSO 1,970,665

PROCESS OF BLEACHING, PULP AND OTHER LIGNIFIED MATERIALS Filed Sept. 28, 1931 2 Sheets-Sheet 2 INVENTQR GEORGEHIOMUNSON ATTORNEY LO P .4 4

Patented Aug. 14, 1934 PATENT OFFICE PROCESS OF BLEACHING PULP AND OTHER LIGNIFIED MATERIALS George H. Tomlinson, Montreal, Quebec, Canada 7 Application September'28, 1931, Serial No. 565,485

10 Claims.

This invention relates to processes of bleaching wood pulps; and it comprises a method of removing lignin and color from wood pulp wherein the pulp is first treated with chlorine in a more or less hydrolyzed aqueous solution, then with hypochlorous acid in an aqueous solution iree'of hydrochloric acid, and finally with hypochlorite in an alkaline solution; treatment being ordinarily in transit with a fiowing stream of pulp in aqueous suspension and with an initial addition of all the chlorine needed, an intermediate addition of calcium carbonate to neutralize HCl, but not H001, and a later addition of caustic alkali to neutralize the H001 and make an alkaline solution; said method also being applicable to making clean strong cellulose directly from wood; all as more fully hereinafter set forth and as claimed.

The present invention provides a rapid and economical process for removing lignin and coloring matter from various wood pulps, including those classed as hard bleaching pulps, without impairing the strength or other desirable characteristics of the cellulose fibres.

Wood pulps are frequently classed as easy or hard bleaching according to the effects of the cooking or chemical process by which they are produced. If the process is carried to the point where practically all of the lignin and related compounds are removed from the cellulose, the result is an easy bleaching pulp. However, with the methods now available for the production of this class of pulp, it is necessary to subject the material to such drastic conditions of treatment as to impair the'strength and other characteristics of the cellulose fibres. In producing what -is'known as hard bleaching pulps the cooking or pulping process is usually conducted under such conditions that the chemical action upon the cellulose is minimized; is reduced to a point where degradation of quality is substantially avoided. This produces a strong pulp but one containing residual lignin and related compounds or coloring matter which are exceedingly diflicult to remove. Where this removal is attempted by the usual methods of bleaching attackinglignin, an oxidation of the cellulose occurs with consequent degradation or loss of strength in the final product. i

The problem of removing residual lignin and related compounds from hard bleaching pulps without direct oxidation has been the subject of considerable investigationby-workers in this art and numerous processes have been developed in which hard bleachingpulps are subjected to the action of solutions of chlorine under varying conditions of operation. These processes are primarily based upon the assumption that chlorine directly combines with lignin to produce chlorinated derivatives of undefined composition and properties. It is now generally recognized that these chlorinated derivatives or reaction products exhibit somewhat altered characteristics such as an increased solubility in alkalies and this recognition has lead to the development of processes involving various combinations of chlorine and alkaline treatments. So far as I am aware none of these processes has achieved any considerable commercial success due, apparently, to the fact that, in the absence of specific knowledge concerning the nature of the reactions between lignin and chlorine, it has been impossible'to avoid undesirable secondary reactions which have the efiect of causing the final product to exhibit undesirable characteristics which can be corrected only by resorting to expensive and complicated after treatments such as alkaline treatments to which reference has been previously made.

It is known that lignin has the characteristics of an aromatic phenolic substance as in its be- 30 havior toward caustic soda and the like, and in the light of recent investigation, I have been able to establish the fact that lignin will react with a dilute aqueous solution of chlorine and also with a dilute solution of hypochlorous acid to produce. in each instance, a chlorinated product which is insoluble in water but somewhat soluble in dilute alkalies. Another characteristic of these products is that, under the influence of alkaline bleaching or oxidizing agents, they undergo a transforma- 9 tion into the deleterious colored substances. The purpose of the bleaching is,.of course, to obtain a pulp consisting only of pure cellulose free from the lignin and from the coloring matters which are originally present in the wood or are pro- 9 duced by secondary reactions occurring during the bleaching process. The present invention ac complishes thisby a methodical use of chlorine, the pulp or other lignified material being treated under conditions yielding a final chlorinated lignin derivative which is readily soluble in alkaline solutions and in alkaline oxidizing'media without production of color.

It is now definitely established that when pulp containing lignin or related lignin compounds is treated with chlorine in a dilute acid solution it reacts very rapidly to produce a chlorinated derivative or reaction product which is somewhat soluble in the presence of alkaline bleaching agents but yields diflicultly removable color formations. Treatment with chlorine in an acid solution, whether the acidity results from hydrolysis of chlorine or is due to another-cause, is equivalent to the action of chlorine as such. I have discovered that a primary chlorinated derivative or reaction product so produced, after the neutralization of any hydrochloric acid present or formed in the reaction, is capable of undergoing a further energetic change or reaction with free hypochlorous acid. Free hypochlorous acid results when the solution still containing free chlorine is neutralized with calcium carbonate or like neutralizing agent incapable of reacting with free HOCl. This second reaction yields a hitherto unknown secondary chlorinated derivative or'reaction product which has entirely different properties as compared with the product of the initial reaction. Practically speaking, the most important properties characterizing this new secondary reaction product are (1') its greater solubility in alkaline oxidizing media and (2) the fact that it does not yield colored products when treated with such reagents. The formation of such a secondary product and its properties has not been previously recognized. The lignin of unbleached pulp reacts in two stages. The first stage of reaction occurs in acid solution; usually in the presence of hydrochloric acid resulting from hydrolysis of chlorine, while the second stage is brought about by subjecting the primary chlorinated product to the action of hypochlorous acid in the absence or substantial absence of hydrochloric acid. This two-stage reaction is utilized in the present invention wherein pulp is first acted on by chlorine in an acid solution, and then the chlorinated products formed at the expense of the lignin, are acted on by hypochlorous acid in a solution substantially free of otheracids; that is, in a neutral or substantially neutral medium. Both the successive reactions proceed at relatively high velocity and there is formed a new chlorinated reaction product which can be completely and easily bleached to a white color with very small quantities of alkaline bleaching agents and without danger of impairing the strength or other desirable characteristics of the cellulose. This result is presumably due to the fact that the chlorination of the pulp, in two stages as described herein, avoids undesirable secondary reactions, including the production of resistant color. The new reaction product has substantially greater solubility in alkaline bleaching agents than the products of chlorinating methods in the prior art.

Proceeding now to a more detailed description reference will be had to the accompanying drawings, wherein- Figure 1 is a curve diagram illustrating the.

progress of the reactions characterizing my improved process in comparison with other. processes, and

Figure 2 is a diagrammatic view of a simple form of apparatus which'may be used in carrying out the process in 'a continuous manner.

The graphs of Fig. 1 are plotted from data obtained in a number of tests on various ways of bleaching, including this invention. In order to make the results strictly comparable so far as possible, conditions were made identical as regards amount of chlorine, amount of water, proportion of dry pulp, agitation, time, temperature,

etc. All tests were made with the same unbleached p'ulp: a particular kraft pulp containing a minimum of 4 per cent lignin. In each case, treatment was with a solution containing 4 per cent chlorine based on the bone dry weight of the pulp.. An aqueous suspension was made containing 5 parts of dry pulp to 800 parts of water. with the stated amount of chlorine; 4 per cent of the bone dry weight of the pulp.

- In Fig. 1, the ordinates represent the amount of chlorine consumed; the proportion of the total 4 per cent which was used in the time indicated. For example, 3.6 per cent on this curve'would represent per cent consumption of the 4 per cent used. The abscissa: represent the time in minutes in these particular tests. The lowermost fullline curve A represents an ordinary bleaching operation in alkaline hypochlorite un-. der these particular conditions, the-aqueous sus pension being treated with 4 per cent of chlorine in the presence of a slight excess of alkali. Curve B shows the rate of chlorine consumption when the pulp was treated with 4% of chlorine in the presence of hydrochloric acid. Curve C shows the rate of chlorine consumption when the product obtained by the treatment represented by curve B was further subjected to the action of hypochlorous acid in the solution resulting from the addition of an excess of calcium carbonate to the mixture after the reaction accord- This was treated at room temperature ing. to curve B had proceeded for a period of four minutes, the curve of the reaction during such period being represented by the full line portion of the curve B. Curve D shows the rate of chlorine consumption following the addition of an excess of alkali to the mixture left after the reaction represented by curve C had proceeded for a period of four minutes, as indicated by the full line portion of said curve C. These determinations apparently support the conclusion that the rapid consumption of chlorine occurring during the first few minutes of the first and second stages, as represented by the full line portions of curves B and C is presumably due to the combination of lignin with chlorine. Following this, the reactions occurring in each of the stages represented by curves B and C are presumably secondary chlorination and progressive oxidation. As indicated on curve chart it is also found that the rate of chlorine consumption, when the original pulp was treated with 4% of chlorine, in a solution containing an excess of calcium carbonate, follows substantially the same curve as curve B.

In a modification of the present process I begin with a larger percentage of chlorine than that used in the described experiment (4%). A proportionately greater amount of chlorine is available for alkaline oxidation in stage 3 and produces conditions under which complete bleach-.

flcult to bleach, I commence the process with' less than the total "amount of chlorine which may be required toremove the lignin and then, after subjecting the pulp to the preliminary re- I actions provided for by stages 1, 2 and 3 I wash the pulp and complete the bleaching in a fourth stage, in which a fresh supply of alkaline bleach is added to the washed pulp. In carrying out this fourth or final stage, I prefer to employ from 2%. to 6% or more of 35% bleaching powder based upon the dry weight of the pulp. This final stage can be conveniently carried out with the aid of standard bleaching equipment and in accordance with the methods usually employed in the handling of exceptionally easy bleaching pulps. In carrying out this fourth stage a batch system is preferably employed, thus facilitating the preparation of a pulp with a standard degree of whiteness. In view of the extreme rapidity with which the various reactions, more particularly those involved in stages 1 and 2, proceed to completion it is obvious that the process of this invention is particularly adapted to be carried out in a continuous manner and with the aid of comparatively simple and inexpensive equipment. I

Considering the different stages of the process in the order of their occurrence, it will be understood that stage 1 involves the-addition ofchlorine to a pulp suspension in a neutral liquid so that the chlorine reacts in equilibrium with water in accordance with the formula that is, is more or less completely hydrolyzed. The chlorine may be added either as an aqueous solution or as a gas introduced directly into the pulp suspension. The method usually employed is to introduce chlorine gas into the flowing stream of pulp withviolent agitation which tends towards rapid promotion of the desired reactions. The amount of chlorine used should be substantially enough to remove the amount of lignin contained in the pulp. The desired reaction which takes place at this stage of the process is exceedingly rapid and requires only a contact period of from one todlve minutes which may be conveniently provided for by the employment of a reaction vessel through which the pulp is caused to pass, said vessel being of such size as to afford a contact period of the desired duration. In stage 2 of the process, a regulated supply of calcium carbonate is introduced into the flowing pulp stream as it leaves the reaction vessel employed in stage 1. This reagent may be in the form of a suspension of ground marble, precipitated chalk, or a colloidal form of calcium carbonate and should be added in such quantity as to provide a .slight excess over the amount necessary to neui that it avoids the necessity for closely controlling -pleteness of t e reaction. The-present invention is not, ho ever, limited to the use of calcium carbonate in stage 2 but contemplates the use of any other'agent-capable of bringing about substantially complete neutralization of the hydrochloric acid while leaving hypochlorous acid in the free state, It is desired tohave the liquid in a substantially neutral state during stage 2 as regards all other acids than HOCl. During the When the reactions occurring in stage 2 have proceeded to the desired point as represented by vthe full line portion on curve C, the reaction in stage 3 is initiated by adding an excess of lime or any other suitable caustic alkali to the flowing pulp suspension as it leaves the reaction vessel employed in stage 2. A reaction vessel is also used in stage 3 to provide for a contact period of from ten to thirty minutes or even longer. The purpose of this stage is to bring about a definite alkaline condition in the mixture immediately after the completion of stage 2 in order to convert the free hydrochlorous acid into hypochlorite and prevent undesirable secondary reactions.

In commencing stage 1, it is desirable to add chlorine in excess of the amount actually consumed in stages 1 and 2 so that suflicient chlorine is available in stage 3 for the production of the class of pulp desired. According to one modification of my invention, stage 3 may-be eliminated and the pulp washed at the end of. stage 2 and then subjected to the action of a. fresh alkaline bleaching reagent. This washing and bleaching of the pulp with fresh alkaline bleaching reagents may also be resorted to at any suitable point in the progress of stage 3. However, the method regarded as best is to proceed with stage 3 to a rather advanced point, until the reaction slackens materially, as indicated in the curve-diagram at the point corresponding to a chlorine consumption of 3.5 per cent and a time of 20 minutes. The pulp may be washed in a suitable way as, for example, by passing the pulp over a rotary vacuum filter. At this point the pulp may be sumciently bleached to answer the purpose for which it is to be used but, if a particularly bright color is desired, it is advisable to follow stage 3 with the fourth stage treatment previously mentioned herein as an alternate form of my process. This fourth stage lends itself to'the use of standard bleaching equipment and is preferably conducted, by a batch process, in the presence of an excess of alkali and under conditions usually characterizing the handling of very easy bleaching pulps.

When handling certain classes of pulp, it may be desirable to modify the process so that stage 1, as represented by curve B, is preceded by a preliminary treatment of the pulp with an alkaline hypochlorite so that the initial reaction proceeds to a certain point along the line-of curve .11 prior to initiation of the reaction stage represented by curve B. 'Under certain conditions and when dealing with certain classes of pulp, this initial treatment reduces the time required forbleaching the pulp 'to a particular degree of whiteness and does not appear to adversely affect the reactions occurring in the subsequent stages originally designated as stages 1 and 2.

Proceeding now to a description of the apparatus appearing in Fig. 2, 11 designates a pulp supply regulating box divided into. three compartments by weirs 12. The stock is delivered to the central compartment 13 through pipe 14,

,and flows over the weirs into the overflow compartment 15 or into the discharge compartment 16. From the compartment 15 the stock is returned to the source of supply through an over fiow pipe 1'1. The stock delivered to the discharge compartment 16 is conducted through pipe 18 to the inlet of a centrifugal pump 19 which it is subjected to violent agitation.

A regulated quantity of chlorine, preferably in the form of gas, is introduced into the pulp stream as the latter enters the zone of violent agitation afforded by the pump 19. In the present instance the chlorine is conveniently conducted to the inlet side of the pump through a pipe 20 under the control of a suitable regulating device indicated at '21. The mixture of pulp stock and chlorine leaving the pump 19 is conducted through a pipe 22 to a reaction vessel 23 which is designed to afiford the duration of contact required for thecompletion of stage 1.

In carrying out stage 2 of the process the pulp is first conducted from the reaction vessel 23 through'a pipe 24 to a mixing vessel 25 where calcium carbonate or other suitable alkali is added to the mixture to neutralize the hydrochloric acid. This addition of calcium carbonate or other agent may be conveniently accomplished through the medium of the pipe 26 and the controllable feeding device 27. The resulting mixture passes from the mixing vessel 25 through a pipe 28 to the inlet side of a pump 29 which afiords a second zone of violent agitation in which the calcium carbonate is mixed with .the reaction product of stage 1. From the pump 29 the mixture is conducted through a pipe 30 to a second reaction vessel 3l. designed so as to allow the mixture to remain: therein long enough to complete stage 2.

The product of stage 2 is then conducted through a pipe 32 into a second mixing vessel 33 wherein an excess of lime or other suitable alkali is added to the mixture as by means of the pipe 34 and the feeding device 35. In leaving the mixing vessel 33 the pulp mixture is conducted through a pipe 36 to a 'third reaction vessel'3'l which afiords the duration of contact required for the completion of stage 3 in the course of which the mixture is brought to a definite alkaline condition. The product of stage 3 is then conducted through a pipe 38 to a vacuum filter or washer 39 and from thence to any suitable form of conveyor 40. If the third stage is to be followed by the fourth stage of treatment previously mentioned herein, the pulp after being washed is delivered by the conveyor 40 to any standard type of bleaching apparatus in which the fourth stage of treatment is to be conducted.

In addition to its usefulness, in connection with the removal of lignin and color formations from previously prepared pulps, the process described herein is advantageous when applied to the treatment of raw lignified materials including hard woods, excelsior and the like. In this particular application the removal of lignin and color formations is accomplished without injury to the cellulose fibres so that the final pulp product is found to contain practically all of the hemicelluloses as well as the higher celluloses. The

present invention. therefore, provides a process of treating 'raw lignified materials which gives a greater yield as compared with the pulp processes now in use and which also makes possible the economical production of satisfactory pulp from hard woods and other lignified materials which have heretofore been considered more or less unsuitable for this purpose. In this particular application of the process, the raw lignified material, after being shredded or otherwise reduced to a desired state of fineness, is preferably boiled in water or otherwise hydrated to produce a certain swelling of the fibres. Following this,

the material is treated in accordance with the.

process previously described herein and it is found that this treatment removes the lignin and color formations in such a way that an exceptionally high yield of cellulose-is obtained. The

amount of chlorine employed in this treatment of raw lignified material is preferably enough to react with the total amount of lignin contained in said material.

Having thus described my invention, what I claim is:-

1. A continuous process of bleaching pulp in transit'which comprises conducting a moving stream of pulp in aqueous suspension and containing added chlorine through a plurality of successive reaction zones, these zones including (1) a zone where the chlorine is allowed to act until reaction slackens, (2) a zone in which HCl acidity of the moving stream is neutralized and the resulting HOCl is allowed to act until its activity slackens and (3) a zone where base is added in sufiicient amount to make the stream alkaline.

2. The processof bleaching wood pulp which comprises subjecting such pulp to the action of chlorine in dilute aqueous solution until the action slackens and then neutralizing the HCl acidity only and thereby subjecting the pulp to a treatment with hypochlorous acid in an aqueous solution substantially free of other acids.

3. The process of bleaching-wood pulp which comprises subjecting such pulp to the action of an excess of chlorine in dilute aqueous solution, then neutralizing hydrochloric acid in such solution, leaving hypochlorous acid in a free state, and subjecting the wood pulp to the action of such hypochlorous acid.

4. In the process of claim 3, neutralizing hydrochloric'acid with calcium carbonate.

5. The process of bleaching wood pulp which comprises subjecting such pulp to the action of chlorine in a dilute aqueous solution, continuing the treatment of the pulp with hypochlorous acid in a dilute solution substantially free of other acids and bleaching the resultant products in an alkaline medium.

6. The process of bleaching wood pulp which comprises subjecting such pulp to the action of chlorine in a dilute aqueous solution, continuing the treatment of the pulp with hypochlorous acid in a dilute solution substantially free of other acids and bleaching the resultant product in an alkaline medium, the amount of chlorine initially added being at least sufficient to react with the lignin of the wood pulp and to supply the hypochlorous acid of the second step and the alkaline bleach of the third step.

7. The process of bleaching pulp fiber which comprises subjecting such pulp to the action of chlorine in a dilute aqueous solution, continuing the treatment of the pulp with hypochlorous acid in a dilute solution substantially free of other acids, bleaching the resultant product in an alakline medium, washing the bleached product and subjecting the washed product to fur- $181 bleaching with fresh alkaline bleaching soluion.

8; In the removal of lignin from cellulose in fibrous material containing both, the process which comprises exposing such fibrous material to the action of an aqueous solution of chlorine until action slackens substantially, then neutralizing hydrochloric acid present without neutralizing hypochlorous acid, continuing the exposure. of the fibrous material till the new action instituted slackens materially, then neutralizing the hypochlorous acid with caustic alkali and forming an alkaline solution, and continuing the exposure of the fibrous material to the alkaline solution to remove chlorinated products.

9. A process as claimed in claim 1 in which the pulp is subjected to violent agitation in zone 1 coincident with or immediately following the contact of the pulp with chlorine and to further violent agitation in zone 2 coincident with or 5 immediately following the introduction of the acid neutralizing agent.

10. The process of bleaching pulp fiber which comprises subjecting such pulp to the action of and subjecting the pulp to further bleaching with fresh alkaline bleaching solution.

' GEORGE H. TOMLINSON.

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