US1957938A - Bleaching process - Google Patents

Description

y 1934- J. CAMPBELL ET AL v 1,957,938

BLEACHING PROCESS Filed Dec. 9, 1931 INVENTOR- WIW Patented May 8, 1934 BLEAOHING PROCESS John Campbell and Lancelot 0. Bolleaton, Glens Falls, N. Y., assignors to International Paper Company, New York, N. Y., a corporation of New York Application December 9,, 1931, Serial No. 579,896

This invention relates to an improved method of bleaching fibrous cellulose materials such as paper pulp, wood pulp, straw, bagasse, esparto grass, cane fiber, and similar materials. More particularly it has reference to a process of the type disclosed in our pending application Serial No. 487,758 filed October 10, 1930, in which the material to be bleached is subjected to the action of a gaseous bleaching agent, such as free chlorine gas, under suitable conditions at one or more stages of the process.

It is contemplated that the material shall first be placed in a condition in which it is made available for thorough impregnation with the gaseous agent. This involvesreducing the ma- -terialtoacertainstateofdrynesswhileatthe same time maintaining the fibers or the like in a relatively loose condition so that air may be present within the individual cells of the material and on their outer surfaces in the spaces or interstices between the'cells. The water in the material which may suitably make up somewhere between 15% and 85% of the entire composition of the pulp, and preferably between and 75%, should simply be in the form of a thin film lining the inner and outer surfaces of the hollow tubes or envelopes constituting the cells of the material leaving room both within and without for air and, at certain stages in the process, for the gaseous bleaching agent.

Fibrous cellulose may conveniently be brought into this state in a variety of different ways. For example pulp taken from the roll of an ordinary wet machine and allowed to expand will be found to carry an appropriate amount of moisture distributed in the manner indicated. So also the material may be brought to the desired state by passing a comparatively low density solution, containing, say, 4 parts of the solid substance and the balance water in 100 parts of solution, into a centrifugal machine of suitable construction and whirling the material until it is reduced to the proper state of dryness. In the expulsion of a large part of the water by centrifugal .action in this way it will be found that the remaining water will simply,line the inner and outer surfaces of the cells and air will enter the material to fill up the voids. Still another method of producing pulp in the desired state is to squeeze a relatively wet pulp in a suitable screw press by forcing it through a perforated plate by the action of a screw. In this way a desired portion of the water may be squeezed from the material and when it is allowed to expand after compression it will take in a corresponding amount of air to fill the spaces within and between the cells. The water remaining inthe material when produced in any of the foregoing ways willbe inthe form ofathinfilm of finely divided particles.

The bringing of the material into the definite state pointed out above is an important step in the process as a whole since the reaction with the gaseous agent can not otherwise be effectively and efiiciently carried out. It will be found that those skilled in the art will be capable of so controlling the operation of a wet machine, a centrifugal machine, or a screw press or any other machine adapted for the same purpose as to produce the desired result without undue attention.

Material which has been prepared in any of these ways will be well suited to a bleaching operation, involving the introduction of a gaseous agent into a treating zone maintained under a partial vacuum, as will be explained more fully hereinafter. A more rapid and more effective bleach may be brought about, however, if the material in the general condition mentioned contains, prior to the bleaching operation, certain chemical substances which will react with the gaseous bleaching agent to increase the activity of the agent. For example where free, gaseous chlorine is employed as the bleaching agent it will be found that the prior addition of certain alkaline substances and various salts of such substances will greatly increase the activity of the chlorine. Among the substances particularly suited for this purpose may be mentioned sodium, potassium, calcium or magnesium oxides,

or suspension of the fibrous material before it is fed to the centrifugal machine, screw press, or the like, for the removal of the excess water. In the fibrous material which is ready for the bleaching action the chemical substances may be in solution or suspension in the particles of water which line the inner and outer surfaces of the cells or a part or all of these substances may be chemically combined with the fibrous material itself. It is probable that these substances will combine with the free chlorine, later introduced, to form suitable hypochlorites which contribute their bleaching effect.

Having brought the pulp or similar material into the particular state specified, either with or without the addition of chemical substances of the nature mentioned, the bleaching reaction with a gaseous agent may be carried out in the way set forth in our pending application hereinbefore referred to. For example the relatively dry material may be introduced into a suitable vessel which may then be closed and made gastight. A vacuum pump may be connected with the interior of the vessel to remove a large percentage of the air including that previously held within the cells and in the interstices between the cells. A vacuum of any suitable degree, preferably around 25 inches of mercury or even greater may advantageously be produced within the vessel. Simultaneously with the removal of air from the vessel at one point free chlorine gas may be introduced at another point or if desired the introduction of the gas may be made subsequent to the removal of the desired portion of the air. At first the chlorine will react with the chemical substances added to the pulp, and with certain constituents of the pulp which impart an objectionable color to it. As a result no appreciable increase in pressure takes place. When this reactionhas been carried to a certain point, however, pressure is gradually built up within the vessel until substantially atmospheric pressure is attained. The bleaching reaction is ,al-

- most instantaneous since the gas is brought very quickly into quite intimate contact with all portions of the material. It is not necessary to leave the chlorine in contact with the material for any appreciable time. As soon as the desired pressure has been built up within the vessel, which may be just atmospheric or slightly above or below, depending upon the difllculty with which the reaction takes place and upori the harmful reactivity of the material to the gas, the excess chlorine may be removed and again replaced by air.

A simple way in which this may be efi'ected is to blow air under pressure, of, say, 15 or 20 lbs. through the material. This will, in a large measure, drive out the free gas and replace it with air. In order to avoid a wastage of the free chlorine and also to avoid a discharge of this gas into the atmosphere with resulting hardship upon the operatives we have found it desirable to pass the mixture of air and gas, leaving the vessel containing treated pulp, into a second vessel containing pulp to be treated. The untreated or partially treated pulp serves as a filtering agent for the unused chlorine since this will immediately react with the pulp or the chemical substances with which it may be impregnated. The air may simply be permitted to blow through this second vessel into the atmosphere and it will be found to be in a quite pure state upon discharge. In lieu of forcing the excess gas by means of air under pressure, as explained, it may be drawn from the first vessel into the second by opening up a communication between the two and connecting the second chamber, containing the fresh material to be bleached, with the vacuum pump.

After a batch of material has been bleached to a desired degree in the manner indicated it is ness of the original batch by the appropriate use of a centrifugal machine, wet machine, screwpress, or the like. The partially bleached and washed material, preferably impregnated with a new quantity of alkaline substances, may then be given a second bleaching action by the use of vacuum and the introduction of chlorine or other gaseous bleaching agent in the same manner as before. After this second bleaching action the material may again be washed and, if desired, treated in the same way as before to carry on a third bleaching reaction under substantially the same conditions. It has been found that a much more thorough bleach may be effected without deteriorating the fibrous structure of the material by carrying out the process in several stages. A two stage treatment, each stage being preferably of substantially the same intensity, will be found to work very good results with most forms of pulp. In dealing with some forms of material, however, such as kraft pulp which it is desired to bleach entirely white, it may be found that a three stage treatment, all stages being preferably substantially the same will produce the most favorable results. A greater number of similar treatments may be employed should it be found necessary to effect a more gradual bleach upon each stage or should some difliculty be encountered in attempting to fully bleach the material in fewer stages of treatment.

Following the final bleaching reaction the material is subjected to a further washing to remove impurities and to a treatment with caustic soda or similar strong alkali for the purpose of removing all traces of chlorinated products. Certain of the converted products resulting from the chlorination of the coloring matters will not dissolve in water and can not be effectively removed without special treatment. A solution of caustic soda will be found particularly effective for this purpose. Finally the caustic soda and converted substances should be removed from the pulp by a thorough washing. The pulp is then ready for further disposition of any nature as for the production of paper or other similar products.

For the purpose of more clearly illustrating the preferred sequence of operations a diagrammatic view has been prepared in which the various pieces of equipment that may be advantageously employed in the process are shown schematically. Referring now to this drawing pulp in the form of slush may be received from the mill through apipe 10and dumpedintoamixbox ilofany suitable construction. If alkaline substances of the nature already specified are to be added to the pulp prior to chlorination they may be introduced through suitable connections from a tank 12 containing a strong solution of the alkali. A caustic soda solution has been found particuuarly emcacious for the purpose. It has been found that an appropriate mixture in the mix box may consist of from 2 to 5 parts of caustic soda in 1000 to 3500 parts of water containing 100 parts of the dry pulp. If other alkaline substances are substituted for the caustic soda they should preferably be in such proportions as to produce substantially the same degree of alkalinity. It should be understood, however, that larger or smaller quantities of the alkali than specified may be added and that, if desired, the alkali may be omitted altogether.

The relatively low density mixture present in the mix box 11 may be discharged into the hopper 13 of any suitable form of water extracting equipment. This hoppermay, for example, be

associated with the chamber of a screw press 14 which is adapted to squeeze the solid material through a series of perforations 15 at the forward end of the press. The liquor which is squeezed from the pulp may be withdrawn through a pipe 16 communicating with the bottom of the screw press chamber and may dispose of this liquor in any suitable way. It is preferably re-used at some point in the process where its chemical content may be used to advantage or it may be simply concentrated by evaporation of a large part of the water and then. passed to the tank 12. The small lumps or nodules forced through the open ings 15 of the press may be discharged into any suitable form of open container 1'7. Here the pulp fibers will be permitted to expand with the result that air will be introduced into the cells and into the interstices between the cells.

From the bottom of the hopper or storage vessel 17 the pulp may be led by suitable chutes 18 and 19 into any one of a plurality of chlorination chambers. Two chambers or vessels 20 and 21, are shown for this purpose. Only one chamber is required for the treatment of a particular batch but the provision of two or more is considered advantageous inasmuch as .it permits a more or less continuous operation. Assuming that a quantity of the material is discharged through the chute 18, by a suitable trapdoor control, into the chamber 20, the latter is thenmade gas-tight by the application of a cover 22, which may propriately clamped down for this we chambers 20 and 21 are preferably mounted for rotation about horizontal axes so that at a later stage in the operation, the material maybe dumped through the opening at the top after removal of the cover 22. For this purpose the containers are preferably provided with trunnions at abouttheir midpoints adapted to rotate within bearings provided in supports 23.

Having filled the vessel 20 and made it gastight by the application of the cover 22, the air is exhausted from the interior of the vessel by means of a suitable vacuum pump 24. This pumpmay be connected with the interior of the vessel by a line 25 terminating in a pipe extending through one of the supporting trunnions. As shown, the construction may conveniently be such that the same line 25 is connected with a pipe extending between the interiors of the two chambers 20 and 21. Suitable valves 26 may then be provided to place the appropriate one of the chambers under the control of the vacuum pump. After the air has been exhausted to the desired degree, so that a vacuum of, say, 25 inches of mercury, more or less, is created within the vessel 20, free chlorine gas may be admitted as, for example, through the opposite trunnion of the vessel, or the chlorine may be admitted simultaneously with the withdrawal of the air. For this purpose a tank 27 containing chlorine in liquid form under pressure may be connected by lines 28 to pipes 29 passing through these opposite trunnions. By proper adjustment of the valves 281 in the lines 28 chlorine may be admitted to the vessel until the desired pressure. preferably atmospheric, is estab lished within the vessel. Almost immediately thereafter the free chlorine which has not reacted 'with the pulp or-chemicals added to the pulp may be forced out of the vessel by air introduced under pressure. For this purpose a blower 30 may be provided, this.being connected by a line 31 with the same pipe 29 which islconnected with the chlorine tank. The air under a suitable pressure,

of say 20 pounds, sweeps throught-he material tight, the material is subjected to a vacuum by 150 within the vessel 20 and dilutes and removes a large portion of the remaining free chlorine. The air with its chlorine content is preferably passed from the vessel 20 into the vessel 21, this being permitted by opening of both of the valves 26 and closing of a valve 32 in the line 25. The mixture of air and gas in the vessel 21 will be subjected to a filtering action by contact with the fresh charge of pulp previously introduced into the vessel. The air may be permitted to discharge into the atmosphere through a valve controlled outlet 33 or, if desired, the cover 22 for this vessel may simply be left partially or completely open during this stage of the operat;on.' Subsequently the material in the vessel 21 will be subjected to the same treatment previously. given the material in vessel 20 and the remaining unspent chlorine in vessel 21 will be blown through a new batch of material in vessel 20. By the alternate use of a plurality of vessels in this way the operation may be made substantially continuous. I

The material in the vessel 20, which has now been partially bleached, is dumped into a hopper or storage chamber 34. This may be effected in the construction shown by simply removing the cover 22 and rotating the vessel upon its supporting trunnions, the pipe connections 25 and 29 being suitably attached and packed to permit this rotation. Any suitable means, such as a conveyor screw 35, may be employed to feed the material, which is still in a substantially dry state, into an inclined chute or passageway 36 which discharges at its lower end into a storage tank 37. A suitable spray device 38 may be provided to discharge a relatively large quantity of water into the pulp as it descends the chute 36 or as it reaches the tank 37. Sufficient water may be added in this way to restore the pulp to its original low density condition in which it carries from 3 to 10% fibrous material and 90 to 97% water. If desired, the spray- 38 may be supplied with a suitable aqueous solution of caustic soda or other alkaline substances employed to react with the chlorine. The pulpmay be subjected, if desired, to a preliminary washing operation before the dilution to the state mentioned and before the addition of alkaline substances which constitutes preparation of the material for a subsequent bleaching operation. The relatively low densitypulp mixture in the tank 37 may be forced by a pump 39 through a line 40 into a second mix box 41. Here a quantity of caustic soda or other alkaline agent may be added from a tank 42 to bring the alkalinity of the pulp to substantially the same state as is maintained in the mix box 11. If sufficient alkali has been added through the spray 38 none will be required from the tank 42. While it is deemed preferable to obtain the same degree of alkalinity in the box 41 itwill be understood that this may suitably be made either weaker or stronger or the alkali may be omitted altogether at'this point. I n

The pulp mixture from the mix box 41 may be subjected to substantially the same treatment as the original pulp taken from the mix box 11. It may be first passed 'to a water extracting machine, such as a screw press 43, which discharges the pulp in a relatively dry state, containing between 20 and 85% of water, into a hopper or storage vessel 44. From this hopper the material may be alternately introduced into one or another of a pair of chlorination chambers 45 and 46. Assuming that it is passed to the chamber 45 and the cover is applied to this chamber to render it gasthe action of a second vacuum pump 47, or, if desired, by a direct connection with the vacuum pump 24. Upon expulsion of the air to a desired degree from the'material and the interior of the vessel, chlorine may be introduced from a tank 48 through one of the lines 49. When sufiicient chlorine has been introduced to restore the pressure to substantially atmospheric, or any other desired pressure, the excess chlorine may be removed by air under pressure derived from the blower 30 or from a separate blower 50, as shown. Valves 51 and 52 will be suitably adjusted to direct the air-from the blower through one of the supporting trunnions of the vessel 45, assuming this is the one in which the chlorine treatment has just been completed. This air, together with the chlorine swept from the cells and interstices of the fibrous material, may then'be passed into the other vessel 46 by proper adjustment of valves 53 and 54. Here a new charge of material will serve to remove the excess chlorine. so that the air discharged into the atmosphere through a valve-controlled outlet 55 will be quite pure.

After the second bleaching reaction has been completed, the vessel 45 may be rotated upon its trunnions and the material may be dumped through the opening at the top of the vessel, upon removal of the cover, and discharged into a hopper or storage tank 56. From this tank it may be fed by a screw 57 to an inclined chute 58 which communicates at its lower end with a tank 59. A spray 60 adapted to direct a supply of wash water into the pulp may be located either adjacent the chute 58 or the tank 59. In the latter the pulp is maintained in a relatively low density solution and may be adapted to overflow to an inclined chute 61 which discharges the material into a mixing tank 62. Acaustie soda solution of suitable strength maybe introduced with the water through the spray 60 or it may be separately introduced through a spray 63 located adjacentthe chute 61. Inthe mixing tank the material will be thoroughly mixed by being forced tofollow a tortuous path around the ends of the bafiies shown. The chlorinated products which are not directly soluble in water will be converted by the caustic soda into water-soluble compounds and may subsequently be removed from the pulp by an appropriate washing treatment. The caustic soda treatment may be preceded, replaced, or supplemented by a wash with a solution. containing sulfur dioxide. For example suli'ur dioxide may be contained in the water introduced through either the spray 60 or 63. The material may be discharged through a pipe 64 and passed to any suitable point for further purification and manufacture into paper or other final products.

While a two-stage bleaching treatment has been illustrated in the drawing, it will 'be understood that three or more stages may be employed, if desired. On the other hand, if the material is relatively white at the outset, or is not to be bleached to a pure white color, it may be found that a single stage of treatment will be sufllcient. Ordinarily,'however. it will be found that a multistage treatment in which each stage is adapted to contribute its proportionate share to the final bleaching will produce the most satisfactory results. The production of a pure white pulp from a relatively dark, fibrous mass, resulting for example from the Kraftprocess, may be produced without destruction of the fibers by appropriate control of a multi-stage process. Furthermore, an economy in the use of the bleaching agent and the reacting alkaline chemicals may beeffected through the use of a multi-stage treatment.

While certain definite operating conditions and methods, as well as certain specific forms of apparatus, have been particularly pointed out it will be understood that various modifications may be made without departing from the general principles and scope of the invention. The terms and expressions employed herein are to be considered as terms of description and not of limitation.

What we claim is:

1. A method of bleaching fibrous cellulose material which ,compises successively subjecting it to a reaction with free chlorine gas in aplurality of stages, removing air from within and between the cells of the material prior to each stage of the reaction and maintaining the material in a substantially dry state during each stage of the chlorine reaction.

2. A method'of bleaching fibrous cellulose material which comprises .successively subjecting it to a reaction with free chlorine gas in a plurality of stages, removing air from withinand between the cells of the material prior to each stage of the reaction, maintaining the material in a substantially dry state during each stage of the chlorine reaction, and reducing the material to a fluid, aqueous suspension between the successive stages.

3. A method of bleaching fibrous cellulose material which comprises reducing a liquid suspension of the material to a substantially dry state but with the fibers relatively free to permit the retenti" of air, withdrawing air from the-materialfi's'ubjecting the material to reaction with a gaseous bleaching agent, adding water to the bleached material, again reducing the material to a substantially dry state and ithdrawing air therefrom, and again subjecting he material to reaction with" a gaseous bleaching agent.

4. A method of bleaching fibrous cellulose material which comprises adding caustic soda to an aqueous suspension of the material, reducing the material to a substantially dry state with air between the-fibers, withdrawing air from the material, subi'ecting the material to reaction with a gaseous bleaching agent, and washing the material with an aqueous solution of caustic soda.

- 5. A method of bleaching fibrous cellulose material which comprises adding caustic soda to an aqueous suspension of the material, reducing the material to a substantially dry state with air between the fibers, withdrawing air from the material, subjecting the material to reaction with a gaseous bleaching agent, creating an aqueous suspension of the bleached material and repeating the same sequence of steps.

6. A method of bleaching fibrous cellulose material which comprises adding an alkaline agent to an aqueous suspension of the material,-reducing the material to asubstantially dry state with air between the fibers, removing air from the material with an aqueous solution of an alkaline agent.

terial, subjecting the material to reaction with a gaseous bleaching agent, and washing the material which comprises successively subjecting it to a reaction with free chlorine gas in a plurality of stages, the material being maintained in a substantially dry state during each stage of the chlorine reaction and being subjected to a vacuum to remove air from within and between the fibers prior to reaction with chlorine in each stage.

9. A method of bleaching fibrous cellulose material which comprises removing most of the water from an aqueous suspension of the material to reduce it to a substantially dry state, removing air from within and between the fibers of said material, introducing a free gaseous bleaching agent, blowing air through the bleached material to remove the excess gaseous agent from between the fibers, and washing the material to remove impurities. I

10. A method of bleaching fibrous cellulose material which comprises adding an alkaline agent to an aqueous suspension of the material, removing most of the water from said aqueous suspension of the material to reduce it to a substantially dry state, removing air from between the fibers of said material, introducing a free gaseous bleaching agent, blowing air through the bleached-material to remove the excess gaseous agent from between the fibers, and washing the material to remove impurities.

11. A method of bleaching fibrous cellulose material which comprises adding caustic soda to an aqueous suspension of the material, removing water from the material and allowing it to expand to absorb air, removing the air by subjecting the material to a vacuum, introducing gaseous chlorine into and between the cells of the material to bleach the same, and subsequently treating the material with a caustic soda solution.

12. A method of bleaching fibrous cellulose material which comprises adding caustic soda to an aqueous suspension of the material, removing water from the material and allowing it to expand to absorb air, removing the air by. subjecting the material to a vacuum, introducing gaseous chlorine into and between the cells of the material to bleach the same, again forming an aqueous suspension of the material, and repeating the same series of steps.

13. A method of bleaching fibrous cellulose material which comprises adding caustic soda to an aqueous suspension 'of the material, removing water from the material and allowing it to expand to absorb air, removing the air by subjecting the material to a vacuum, introducing gaseous chlorine into and between the cellsot the material to bleach the same, again forming an aqueous suspension of the material, and repeating the same series of steps, and subsequently treating the material with a caustic soda solution.

JOHN CAMPBELL. LANCELOT 0. ROILESTON.

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