US2013042A - Process and apparatus for preparing cellulose fibers - Google Patents

Description

Sept. 3, 1935. J, P FOSTER 2,013,042

PROCESS AND APPARATUS FOR PREPARING CELLULOSE FIBERS A L WEHKSM Sra/aas:

WO'M

J. P. FOSTER Sem.. 3, 1935.

PROCESS AND APPARATUS FOR PREPARING CELLULOSE FIBERS Filed Feb. 5, 1934 2 Sheets-Sheei'l 2 /M 20mi l@ www@ 0' M Patented Sept. 3, 1935 UNITED "STATES -...invrialt'r OFFICE PICES AND'APPARATUS FOR PREPARING CELLULOSE FIBERS 19 Claims.

This invention relates to processes and apparatus for preparing useful cellulose fibers from bagasse andV vegetable grasses of like character, and it comprises processes wherein the alpha cellulose fibers in bagasse are liberated by treating the bagasse in sequence first with a weak a1- kali, then with a dilute nitric acid solution, and finally'with a stronger alkali, suitable Washing steps being interposed between and following these treatments; it further comprises cyclic processes wherein a plurality `of batches of bagasse are subjected to the aforesaid treatments, as will be more fully described; and it further comprises an advantageous type of beater, for use in treating the bagasse with the above chemicals, the beater being one which avoids drastic mechanical attack on the bagasse and its contained fibers, and hence permits of high yields of alpha cellulose.

The present invention, in its broadest aspect, has for its object the recovery of industrially useful cellulose fibers from bagasse. Such fibers can be used in the manufacture of paper, paper board, and similar felted fibrous materials. They may also be used in place of alpha cellulose fibers derived from other sources, such as wood pulp. My invention, therefore, relates to the liberation of such cellulose fibers from sugar-cane bagasse and grasses of like nature.

Bagasse is a waste material produced in large quantities in the recovery of sugar therefrom. There have been various proposals for the utilization of the bagasse. These are generally of two kinds. One involves the utilization of the bagasse as such for the manufacture of building boards and the like. The .second proposal is directed to methods by which the ultimate cellulose fiber in the bagasse can be isolated and recovered for use in paper-making and generally as a substitute for cellulose fibers from other sources.

There is perhaps no cheaper or more abundant source of cellulose than sugar-cane bagasse. In Hawaii, for instance, the cane crop is harvested at an age of about eighteen months; in Louisiana, its age is about eight months. Great quantities of bagassc are available in every crop and the time from planting to harvesting is many times less than most other commercial sources of cellulose with the exception of cotton.v Accordingly, any satisfactory method of recovering cellulose bers in useable form from the bagasse is of very real commercial significance.

Methods to this end have been proposed hitherto. In some instances the bagasse is subjected to severe and drastic digestion with alkalis. Most of these methods, however, do not fully take into account the very great differencesbetween bagasse and other sources of cellulose, such as wood. Bagasse is perhaps unique among the celluloseyielding plants and methods of removing the in- 5 crusting substances which bind the individual bers must take note of the character of these substances.'

Bagasse contains in addition to the lignins, considerable quantities of sucrose, pentosans and similar carbohydrates. Heavy treatment with alkaline fiber-liberating reagents, for example in ways analogous to the liberation of cellulose from wood, converts the carbohydrates to gelatinous compounds which are difficult to remove from the pulp. If the alkali treatment is drastic, some of the available cellulose fibers are destroyed, that is, degradation of the available ce1- lulose occurs. Despite modifications in the alkali treatment methods hitherto employed, loss of available cellulose has been relatively large and the expense of bleaching recovered fiber has been high.

I, have now found that the yield of alpha-cellulose from bagasse can be markedly increased, and the expense of bleaching such cellulose, greatly decreased provided the bagasse be treated with both alkaline and acid reagents and my process is characterized by a three stage chemical treatment involving the use of alkalis and nitric acid. I am aware that it is not new to liberate cellulose from plant materialswith nitric acid, and it is not new to follow such acid treatment by an alkaline treatment, and I make no claim to this method.

My invention departs from the aforesaid process by rst subjecting the bagasse to treatment with Weak alkali, advantageously about a 0.5 percent solution of caustic soda. Instead of using caustic soda, I can use a 1 percent solution 40 ofsodium carbonate, or, if desired, a mixture of both caustic soda (sodium hydroxide) and the carbonate. f

This preliminary treatment is a very important step in my process and effects a pronounced removal of color. The amount of actual fiberliberating reagents required in subsequent steps in markedly decreased. The final bleaching step f' is much easier and requires less bleaching agent. This will be of particular signicance to those skilled in the art since it is well understood that the ease and perfection of the bleaching process is the standard by which the value of the pulping process is measured.

On the attached sheets of drawings I have 55,

illustrated, in Figure l, my invention'in iiowsheet form;

In Figure 2 I have shown an advantageous beater especially useful in my process, the view being a vertical cross-section;

And in Figure 3 I have illustrated a side view of the beater.

Before proceeding to describe the flow-sheet process illustrated, I shall outline my process as applied to a single batch oi bagasse.

A quantity of bagasse is iirst shredded and screened to reduce its particle size and remove dirt. The batch is then placed in a container, such as that shown in Figures 2 and 3, and a quantity of an 0.5 solution of sodium hydroxide run in. 'I'he quantity should be suiiieient to fully saturate the bagasse and give a fluid pulp which can be stirred or agitated. The bagasse is treated with the alkali for from one to three hours at a temperature of around 160 F. and then the solution is drained oi. y

As stated above, much of the undesirable color bodies are removed in this step. These are substances soluble in the weak alkali under the conditions of the treatment. This alkali treatment is not sufficiently drastic to materially attack the cementitious substances binding the alpha cellulose iibers. Generally the drained-off alkali is run to waste and I then wash the alkalitreated bagasse with water. This is to remove adhering alkali solution.

Without removing the washed bagasse, I next steep it for two to four hours in a 2 percent solution of nitric acid, the temperature being maintained at about 160 F. The acid solution is then drained oir and the bagasse again thoroughly washed with water. Now the acid-treated and Washed bagasse is subjected to the action of a stronger alkali solution. I nd it best to use about a 5 percent solution of sodium hydroxide or sodium carbonate, or mixtures of each. The temperature during this step is reduced to about 120 F. and the time of treatment is usually from three to six hours. Following this, the alkali solution is drained off, the ber washed with water, and the aqueous Washings recovered for reuse as will be more fully described.

The actual times, concentrations, and temperatures in the foregoing three chemical treatments will, of course, vary somewhat, each should be correlated with the others so that optimum yields of fiber are obtained. Naturally the source of the bagasse should be taken into account. The treatment of Louisiana bagasse, which is harvested in eight months, will differ in minor details from the treatment of Hawaiian bagasse. But the essential conditions, that is, preliminary weak alkali treatment, acid treatment, stronger alkali treatment, will not be modified.

After the fiber has been liberated by the above three steps, the washed liber is bleached. Various bleaches can be used. Advantageously the bleach is a 1 to 3 percent solution of calcium or other hypochlorite. Five hours time is sucient. Chlorine itself can be used and it is best to conduct the bleaching operation at atmospheric temperature. Finally the fiber is washed free of bleaching agent. The fact that the fiber can be bleached at ordinary temperature, and by using such dilute concentrations of bleaching agents, is one of the marked advantages in my process. In fact, the consumption o f bleaching agent is but a iifth of that hitherto necessary when other methods of fiber liberation are employed. Aside from decreased consumption of aoiaoaa bleach, the lower temperature and concentration greatly lessens the degradation and loss oi available ber. Hence the yield is higher.

These advantages are, in part, attributable to my discovery that the cementitious substances in bagasse, as well as the impurities therein are more readily removed when the hydrogen ion concentration of the ber-liberating reagents is changed from acid to alkaline. Some of the noncellulosic materials are more readily soluble in acid solution; others are more readily soluble .in alkaline solution. In consequence the reversal in hydrogen ion concentration employed in my process is of great value in the removal of cementitious materials.

Moreover, the acid treatment step is so regulated by controlling temperature and concentration, that all residual sugars in the bagasse are hydrolyzed and removed before subjecting the bagasse to an alkaline treatment strong enough to convert the sugars to difiicultly removable gelatinous materials. The temperature of the alkaline fiber-liberating treatment (following the acid treatment) is also so low that little of the alpha cellulose hydrolyzes, or is converted to hydrated cellulose. Hence no loss of cellulose is met with. On the other hand, single treatment with acid alone, or alkali alone, would have to be far more drastic' than that described herein in order to get the same degree of purication. But such drastic treatment would destroy much of the available cellulose.

While I have more specifically referred to the use of caustic soda solutions of 0.5 percent and 5 percent strength, it is to be understood that I am not limited to these exact concentrations. In the preliminary treatment, the caustic soda can vary from about 0.5 to 1.5 percent and that used in the third step can vary from about 4 to 8 percent. Likewise the concentration of the nitric acid can vary from about 1.5 to 4 or 5 percent.

Likewise the temperatures recited are merely illustrative and should be varied. depending upon the character and source of the bagasse. Usually a temperature of around 160 F. is best for the rst alkali treatment and a somewhat lower temperature for the second alkali treatment. All temperatures given above can vary from 10 to 20 F. above or below those recited.

I have broadly described my invention in connection with the treatment of a single batch of bagasse. I now refer to Figure l on the attached drawings and will describe a commercial installation of my process as adapted to thc treatment of a. plurality of batches.

On this figure, A, B, C, D and E are ilve containers for bagasse. These containers are advantageously of the type to be subsequently described. The drawing also indicates a weak soda solution storage, the nitric acid storage, the stronger soda storage, water storage and bleaching solution. All of these have legends on the drawing for simpliiication in understanding it. Five pumps, labelled I, 2, 3, 4 and 5 serve to pump liquids from the various storage tanks to the bagasse containers or beaters.

Each beater is provided with an agitator or propeller, a screen as shown in dotted lines, and other elements as described later on. For simplication these details have been omitted in the flow-sheet.

In the operation of the apparatus set up illustrated (and it will, of course, be understood that the number of beaters can be increased or detill creased as desired) ,beater A is filled with fresh, shredded and screened bagasse. Pump i is started and weak sodium hydroxide or sodium carbonate or a mixture of sodium hydroxide and sodium carbonate is pumped through heater il to the beater A until the solution reaches the desired level in the latter, at whichtime the runoi in the bottom of beater A is opened into the weak soda storage and adjusted to maintain a constant level of liquid in the beater, the weak soda, thus circulating through beater A in a closed cycle. Heater i is best heated by steam coils, not shown, and serves to elevate the temperature of the solution to around 160 F. Alternatively, the beater can bev provided with a heating coil, as illustrated in Figures 2 and 3, but ordinarily a separate heater will do.

As the bagasse will oat in the circulating solution, it will be found desirable to start the propeller rotating so that the bagasse will be drawn down into the center of the beater. After a suitable time, as described above in connection with the treatment of a single batch of bagasse, the propeller and the circulation of solution is stopped and the weak solution is drained oft to storage or to the gutter. This drained on solution will be heavily charged with coloring matter,

pith fragments, dirt, etc;, so it is best to discard the solution.

Then the piping connections, illustrated by lines and cross-overs on the drawing are changed so that weak soda is circulated through beater B which has meanwhile received a charge of bagasse. The procedure with B is the same as with A. After weak soda has drained from A, pump t is started and warm water (heated by heater d) is circulated through the bagasse in A. Washing is continued until the water runs clear and all wash water is run to the gutter or discarded.

Piping `connections are then again changed and nitric acid from its storage tank is circulated through the washedbagasse in beater A by means of pump t, the acid being heated in heater t to about 160 F. After the acid treatment, the bagasse is again washed with water, the first water washings being sent to an acid recovery plant not shown. Finally, when the vwater washings indicate that `adhering acid has been removed, pump 3 is started, and 5% sodium hydroxide circulated through heater 3 (Where it is heated to about F.) and beater A.

After the strong alkali treatment, the fiber in A is again washed and, by opening valves (not shown) in the various pipe linesindicated diagrammatically, the rst water washings are caused to ow to the weak soda storage. in this way somel of the adhering'caustic soda is recovered for reuse.

rllhe liberated ber is then bleached with bleaching solution pumped from its storage tank by pump li and the ber finally washed again with water pumped by pump d.

Meanwhile, the bagasse in beater B has been going through a similar sequence of process steps and beaters C, D and E have also received charges ot bagasse. Thus, assuming all beaters are charged, when the charge in beater E is being bleached, that in D is receiving strong soda treatment, that in C ls being treated with acid, that in B is being treated with weak soda solution, and the liberated iiber in A, which has received its nal water wash after the bleaching operation,

is being discharged from A through a pulp outlet,

schematically shown at the bottom ot "ne screen indicatedbydotted lines.

Ihave purposely avoided confusing the Adrawlng with many reference characters. It will be understood that the various pipe lines are provided `with suitable valves to control the ow of liquids from the Various storage tanks and the drawing is simply-intended to illustrate one advantageous way of carrying out my process on a large scale using a plurality ci beaters.

l shall now describe an advantageous form of beater to be used in connection with my process.

lin conventional pulping processes, it is customary to subject the cellulosic material, after digestion, to the action of beating engines. The well known Jordan engine is an example. These devices break down the ber bundles thereby freeing the oers. Not only do the engines consume much power, but their severe mechanical attack on the bers tends to form hydrated cellulose which in turn is more readily hydrolyzed and broken down. This effect is even more pronounced in the case of relatively soft material such as bagasse.

Accordingly, l have devised a beating engine especially useful in connection with the treatment of bagasse as described above since, when using it, the hydration` and consequent degradation or the cellulose bers is reduced to a minimum. This of course means an increased yield of alpha cellulose.

in my device, I do not subject the bers to mechanical attrition by,` and contact with, mechanical runners or grinding wheels. l obtain a breaking down of the ber bundles by subjecting a liquid slurry or suspension of the ilbers to violent agitation whereby the beating of the material is accomplished by the impact of the bagasse particles on each other, and by the movement of the suspending liquid. This suspending liquid will invariably have a rate of speed different from that of the bagasse particles and will assist in tearing the particles apart and freeing the bers.

Referring to Figures 2 and 3 of the drawings, thebeater preferably employed in the process heretofore described comprises a metal tank it having a, substantially cylindrical main body portion li and a conicalbottom it. Disposed within the tank is a perforated container i3, which may advantageously be formed of screening. The container, which is of substantially the same shape as the tank, has a cylindrical body portion portion ifi and a conical bottom i5 respectively spaced from the corresponding parts of the tank.

The lower end or apex of the conical bottom i5 of the container is provided with a rigid metal cap it having a centrally disposed discharge opening il. IlThe cap i6 forms convenient means to which bracing elements it, which are rigid with the conical bottom l2 of the tank, may be attached for supporting the container within the tank in spaced relation to the latter.

Disposed within the annular space i9 between the body portions of the tank and receptacle is a heating coil 2t for receiving any suitable heating medium or fluid, such as steam. Communicating with the heating coil are suitable inlet and outlet pipes 2ly and 22, respectively, in which are disposed valves 23 for controlling the passage of the heating fiuid.

Extending downwardly into the beater is a rotatable shaft 2d having propeller blades 25,

which are preferably disposed adjacent the bottom of the container I3. The blades are preferably mounted in a manner, not shown, such that their pitch may be adjusted. Rotation may be imparted to the shaft 24 and consequently the propeller blades 25 by any convenient means, such as an electric motor 26, which may be supported above the beater by suitable framework designated by the reference character 21. The amount of power required to rotate the propeller will depend upon various factors, such as the density of the suspension, the speedand diameter of the propeller and the pitch and number of blades employed. It has been found that the power required to start the suspension in suitable motion is approximately one horse power per one hundred gallons, but, after the suspension is once in motion, considerably less power is required to maintain it in motion. Best results have been obtained in using the beater by .so adjusting the pitch of the propeller blades and their speed (revolutions per minute) that the propeller will draw the suspension down the center of the tank, forming a funnel-shaped vortex extending downwardly to between one-half and two-thirds of the depth of the suspension. This vortiginous motion will be found to assume a compound character; vthat is to say, the suspension will be not only in circulation horizontally but also in circulation downwardly at the center and upwardly adjacent the periphery and at an angle which is the resultant of the two forces simultaneously applied. It is to be understood that my invention is not limited to this particular type of circulation as, in some of the operations intended to be performed by the beater, it is desirable to have a horizontal circulation or a circulation tending to the horizontal. To support the lower end of' the rotatable shaft 24 and prevent it from vibrating, its lower end is preferably journaled in a bearing 28. This bearing has radiating arms 29, which are rigidly secured to the metal cap I6 of the container. Immediately above the discharge opening I1 of the cap, the shaft is preferably provided with a clearance blade or blades 30 to restrict the discharge opening and thereby prevent large articles from becoming lodged in the latter.

Communicating with the lower end of the tank II and the discharge opening I1 of the metal cap I6 are suitable valved passageways 3| and 32, respectively. The suspension liquid passing through the perforations of the container is adapted to be drained from the tank throimh the passageway 3|, while the material retained within the container is adapted to discharge through the passageway 32.

From the foregoing, it will be perceived that the beater thus described is particularly advantageous for use in the treatment of bagasse and other vegetable grasses of like character.v

Having thus described my invention, what I claim is:

1. The process of treating bagasse which comprises subjecting the bagasse to the action of a weak alkaline solution, then subjecting it to the action of a dilute nitric acid solution, and then subjecting it to the action of an alkaliney solution stronger than that first employed.

v2. The process of treating bagasse which comprises treating the bagasse with an alkaline solution equivalent in strength to approximately a 0.5 per cent solution of caustic soda at moderately elevated temperature, then treating the bagasse with a dilute nitric acid solution, and

then treating the bagasse with a solution of caustic soda stronger-than that used in the rst step.

Y 3. The process as in-claim 2 wherein the nitric acid solution has a concentration of about 2 percent.

4. The process of treating bagasse to liberate alpha cellulose therefrom which includes, as a preliminary step, treating shredded bagasse prior to fiber liberation with a weak alkaline solution at a moderately elevated temperature but below the boiling point, the alkaline solution being of the order of caustic soda having a concentration of about 0.5 to 1.5 percent, washing the thus treated bagasse, and then treating it with fiberliberating reagents.

k5. The process as in claim 4 wherein the caustic soda has a temperature of about 160 F.

6. 'I'he process of treating bagasse which comprises treating the bagasse with a weak caustic soda solution having a concentration of the order of 0.5 percent, washing the bagasse, treating it with a dilute nitric acid solution, washing the bagasse, treating it with a caustic soda solution having a concentration of the order of about 5 percent, washing the bagasse, and finally bleaching. Y

7. 'I'he process as in claim 0 wherein the nitric acid has a concentration of the order of two percent.

8. The process as in claim 6 wherein the acid and alkali treatments are conducted at moderately elevated temperatures.

9. The process as in claim 6 wherein the temperature of the first alkali treatment is about 160 F., the acid treatment about 160 F. and the second alkali treatment about 120 F.

10. The process of treating bagasse which comprises providing a plurality of batches of bagasse each batch being maintained ina separate treating receptacle, and subjecting each batch of bagasse to a sequence of steps wherein the ba'- gasse is first treated with an alkaline solution of the order of 0.5 percent caustic soda solution, then washed with water, then treated with dilute nitric acid solution having a concentration of the order of 2 percent, then washed with water,

' then treated with a caustic soda solution having a concentration of the order of 5 percent, lthen washed with water, and finally bleaching the liberated bers and washing free of bleaching agent, the water washings resulting from the washing of the bagasse after treatment thereof with the 5 percent caustic soda solution being used for the first alkali treatment of subsequent batches of bagasse.'

11. 'Ihe process of liberating alpha cellulose from bagasse which comprises first subjecting the bagasse to preliminary purification and removal of color bodies by treating the bagasse with a. very weak alkali solution and then subjecting the thus treated bagasse to a fiber-liberation treatment wherein the bagasse is treated with nitric acid and then with caustic soda.

12. The process of liberating alpha cellulose from bagasse which comprises first subjecting shredded and screened bagasse to the action of a weak alkaline solution at a moderately elevated temperature but below theboiling point prior to fiber liberation to remove color bodies without materially attacking the cementitious substances binding the alpha cellulose fibers, and then subjecting the thus treated bagasse to a fiber-liberation treatment and bleaching the alpha cellulose iiberthus obtained.

action of a dilute nitric acid solution, and a dilute caustic soda solution to liberate alpha cellulose,

vwashing the treated fibers after the acid and alkali treatment, and utilizing the Wash water from said dilute alkali treatment to preliminarily treat further quantities of bagasse.

14. The process of treating bagasse to liberate alpha cellulose therefrom which includes, as a preliminary step prior to ber liberation, treating the shredded bagasse with a weak alkali solution at a moderately elevated temperature but below the boiling point to effect removal of color bodies, pith, and dirt whereby subsequent bleaching of the liberated alpha cellulose is facilitated and the consumption of bleaching agent reduced.

15. A beater for the treatment of bagasse and the like involving a tank, a perforate container disposed Wi thin the tank in spaced relation thereto, heating elements in the space between said tank and the container, and a rotatable shaft extending downwardly into said container to near the bottom thereof, said shaft being provided with a propeller device adjacent its lower end and said tank and container having individually controlled discharge outlets.

16. A beater for the treatment of bagasse and the like involving a tank having a conical bottom, a perforate container also having a conical bottom disposed within the tank in spaced relation to the latter, the lower end of said container having a rigid cap, a rotatable shaft extending downwardly into said container and provided with propeller blades adjacent its lower end, means rigid with said cap aiording a bearing for the lower end of said shaft, and discharge outlets respectively located in the lower ends of said tank and container.

17. A beater as described in claim 16 having container provided with a propeller device disposed adjacent the bottom of the container.

19. A beater as described in claim 18, in which the lower end of the rotatable shaft isr provided with means spaced above the discharge opening of the cap for-restricting said opening.

JOHN P. FOSTER.

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