US2355091A - Apparatus for the treatment and removal of chemicals from cooked or digested fiber pulp - Google Patents

Description

Aug. 8, 1944. M. c. M DONALD 2,355,091

APPARATUS FOR THE TREATMENT AND REMOVAL OF CHEMICALS FROM COOKED OR DIGESTED FIBER PULP Filed March 16, 1939 3 Sheets-Sheet l INVENTOR ,Mimuel C. Mbflonald,

ATTORNEY Aug. 8, 1944.

M. c. M DONALD 2,355,091 APPARATUS FOR THE TREATMENT AND REMOVAL OF CHEMICALS FROM COOKED OR DIGESTED FIBER PULP Filed March 16, 1939 3 Sheets-Sheet 2 INVENTOR- Manuzl C Mfiamla} ATTORNEYS.

Aug. 8, 1944. M. c. MCDONALD 2,355,091

APPARATUS FOR THE TREATMENT AND REMOVAL OF CHEMICALS FROM COOKED OR DIGESTED FIBER PULP Filed March 16, 19:59

3 Sheets-Sheet 3 m k r a. w y

d M Qq SSSgEE-m-SEEEEES INVENTOR: Manuel 0 Patented Aug. 8, 1944 APPARATUS FOR THE TREATMENT AND BE- MOVAL OF CHEMICALS FROM COOKED OR DIGESTED FIBER PULP Manuel 0. McDonald, Monroe, La., assignor to The Brown Paper Mill Company, Inc., Monroe, La., a corporation of Delaware Application March 16, 1939, Serial No. 262,160

Claims.

My invention relates to the manufacture of pulp, and especially to the production of pulp by cooking or chemical digestion of wood or other fibrous material. The invention is more particularly concerned with the removal from the cooked or digested fiber pulp stock of various chemicals or substances that are present therein after the cooking or digestion process,--whether reagents employed for the purposes of digestion or cooking, or products liberated or produced in that operation. Such removal frees the stock of substances whose presence in the ultimate paper or fiber board might be undesirable, and permits the recovery of the chemicals or substances in question. My invention involves both a novel removal process and a novel system and apparatus for the purpose. The invention is especially useful in connection with the manufacture of fiber pulp for paper, fiber board, or the like.

It is an object of my invention to provide a simple and eflicient process for thoroughly removing or extracting from wood or fiber pulp stock the chemicals (such as hereinbefore indicated) that are present therein at the termination of the digesting or cooking process, so that such chemicals may be more economically reclaimed or recovered for reuse, or for any use of which they are susceptible. I also aim to obviate the loss or waste of appreciable quantities of chemicals or of pulp stock as an incident of the extraction process, so that large and rapid manufacturing operations may be carried on at minimum expense, and pollution of streams or other natural drains may be avoided. By treating the pulp stock throughout the extraction process at a higher consistency (i. e., with a smaller liquid content relative to fiber) than heretofore, in accordance with my invention, the bulk of material to be handled is greatly reduced, and it becomes practicable to continue extractive treatment after the chemical content has become relatively low. The treatment of the pulp stock at high consistency avoids extreme attenuation of the stock with liquid as an incident of the extraction, and the handling of large quantities of liquid in this process, My invention also permits of avoiding excessive dilution of chemicals prior to recovery, as well as the enormous consumption of fresh extract ng liquid Water) heretofore necessary. The economy in the use of liquid also makes it more practicable to use liquids other than water, if desired, since such 1iquid(s) may themselves be recovered in the chemical recovery plant, and need not be completely purified to be capable of reuse for extraction purposes. Moreover, the

process can be carried out with the pulp and its components substantially as hot as when received from the digester. My invention affords the further advantage that the extraction can be carried out as a continuous process, and under pressure, if desired.

In the cooking or digestion of wood and other fibrous materials into pulp for the manufacture of paper, fiber board, and the like, the wood or other fibrous material is usually cut or crushed into chips or pieces so small that the pieces of the material will be rapidly and thoroughly acted upon in the digesting or cooking process. This process, according to present methods, consists, briefly, in mixing the chips or pieces with a solution of caustic soda or other digesting chemical (if such a, chemical is used to assist or augment the cooking effect) in a cooking vessel or tank known as a digester, closin this vessel, and then cooking the mass therein by passing steam under pressure into the digester. The particular pressure and temperature of the steam and the period of cooking depend on the nature of the wood or other fibrous raw material being treated, the character of the pulp to be produced, and the particular kind or type of paper, fiber board, or the like that is to be made. When the chips or pieces of fibrous material have been cooked in the solution of soda or other digesting chemical to the extent desired, the contents of the digester are discharged, and are then treated to remove from the digested or cooked fiber stock (as completely as possible) all trace of the digestion liquor, comprising the caustic soda or other digesting chemicals, or such substances as may have been liberated or produced in the digesting or cooking process.

According to the most generally used extraction processes, this is accomplished batch-wise, by discharging each batch of pulp from the digester into a tank or vessel (open or closed) having a filter bottom, either at digester consistency or attenuated with added liquid; allowing the strong liquor to drain out of the stock by gravity, or forcing it out by pressure applied above the stock; and then successively washing the stock with weak liquor and with water, which are successively introduced into the filter tank above the stock and allowed to run through the latter by gravity, or are forced through under pressure. The weak liquor used for such washing of the stock is the water that has been used to wash a previous batch, which, of course, carries some of the chemicals of the pulp in solution. The stronger liquor drained from the filter tank is delivered to an evaporating plant or other system for the recovery or reclamation of the chemicals in it. The great amount of water required in these processes reduces the strength of the liquor or chemical-laden liquid from the later phases of the washing to such weakness that it would not pay to attempt to recover the chemical therefrom, so that a large part of such weaker liquor is customarily run to waste. Also, large quantities of the finely comminuted fiber stock are lost in the washing operations, as well as in the liquor that is run to waste.

In another extraction process, the pulp from one or more digesters is accumulated in a reservoir, and is thence run continuously over a series of rotary vacuum filters, on each of which the pulp spreads out in a rather thin mat, which is continuously stripped off. Thus a certain amount of liquor is extracted from the pulp at each filter, and the amount of chemicahs) in the pulp is reduced. In order that a satisfactory pulp mat may be formed on the face of each filter, great dilution or attenuation of the digester pulp is necessary before its introduction to the first filter, and likewise great dilution or reattenuation of the pulp stripped from the face of each filter and introduced to the next succeeding filter. Accordingly, large quantities of extracting liquid (water) are added to the pulp ahead of the filters, and to the pulp mat on each filter, where it serves to attenuate and wash the pulp. The proportion of the chemical in the pulp extracted by each filter is relatively small. The liquor or filtrate extracted from the pulp at the first filter is led off to the chemical recovery plant or other point of reclamation or reuse.

Besides requiring the use and handling of very large quantities of liquid, this process has the drawback that it must be carried out at comparatively low temperatures, to permit of maintaining sufficiently high vacuum at the filters; accordingly, much of the heat in the stock as it comes from the digester is lost. Moreover, air is apt to be entrained with the liquor extracted from the pulp at the filters, tending to create large quantities of foam that is very troublesome, both to handle and to break or dissipate.

In another and somewhat similar extraction process, the pulp from one or more digesters is accumulated in a reservoir, and is thence run continuously (with very great attenuation) through a rotary vacuum filter, where additional extraction liquid is supplied to the pulp mat. After stripping from the filter, the pulp is again attenuated with water, and then undergoes mechanical pressure in a rotary screw press to squeeze out some of the liquid, before going to the usual refinement treatment. The filtrate from the rotary filter goes to the recovery plant, while the liquid from the screw press may be returned to the rotary filter as part of the attenuating and washing liquid there used, if it is not run to waste.

In carrying out my invention as hereinafter described, I extract the chemicahs) from cooked or digested fiber stock by alternately and repeatedly compressing the stock mechanically, while suitably enclosed or confined and in the state of pulp, so as to express or squeeze out part of the chemical-laden liquid or liquor from the pulp, and introducing to the stock other liquid capable of taking up or displacing chemicals remaining in the stock after the previous expressing step,- hereinafter generally referred to as extractant or "liquid." Even when kept enclosed throughout the process, as hereinafter described, the stock-or stock and liquid extractant-can be compressed as often as desired by pressure of the enclosing walls. Preferably, the stock is just as received from the digester when my process begins,without having been attenuated as above described-so that the liquor expressed in the first stage of the process is of full digester strength or concentration. However, some dilution of the pulp before this first stage is permissible, and of course results in correspondingly weaker liquor from this stage. Broadly speaking, the concentration of the liquor in the pulp as received into the extraction system controls or influences the concentration of the liquor expressed from the stock at all stages throughout the process; but the extractant introduced during the process also controls or influences the concentration of the liquor expressed in the later stages. Any suitable liquid may be used as extractant, although an aqueous extractant is cheapest, and the following description generally refers to its use.

Dehydration by mechanical expression, as I have found, is not only applicable to the pulp as it comes from the digester at usual consistencies of some 10-13% fiber, more or less, but also permits of keeping the fiber stock at relatively high consistency throughout the process, and avoids any necessity for cooling the components. Indeed, the entire system of apparatus in which the process is carried out can be closed pressure-tight, from its pulp-receiving connection to the digester(s)or to a (closed) tank into which the digester(s) may discharge-clear through to the point where the chemical-free pulp is discharged for refining or other treatment,-exc1uding air from the material, and allowing the same to be kept under controlled conditions of pressure and temperature. Temperatures higher than are possible under mere atmospheric pressure increase the chemical-removing or solvent power of the extractant. or are otherwise advantageous,

A pulp consistency of some 10-13% fiber before dehydration contrasts with a consistency of something like 1 to 2% fiber to which the contents of the digester are attenuated in the treatment on rotary vacuum filters in certain processes described above,which gives some idea of the very great economy in the use and handling of liquid that is permitted by my process. However, it will be understood that if desired the pulp may be treated at lower than digester consistency in my process, if preferred for any reason: e, g., a consistency of some 6-8% fiber in the pulp.

In the dehydration operations, the concentrated pulp stock or magma may be brought each time to a consistency as high as some 30-40% fiber; though magna consistencies lower or higher than this are also permissible. In general, it pays to raise the consistency of the magma to about 35%, rather than to a lower degree than this, because of the greater quantity of liquor of full strength thus obtained from the stock; but to carry the concentration above 40% is generally uneconomical, because of the rapid increase in power required to effect the further compression and concentration, and because the recovery of liquor is proportionately less as the consistency is increased above this value. In general, the power required to increase the consistency of the stock above 50% is excessively high, and the stresses in the machinery employed are excessive, so that at present I do not recommend such extreme consistencies.

Hereinafter, I have generally referred to the TREATING APPARATUS.

combination of fiber and liquid dealt with in my process as stock simply, without regard to its consistency as determined by the quantity of liquid associated with the fiber, or to the presence or absence of chemicals; while for purposes of distinction, I have referred to stock of relatively low consistency (including digester consistency) as "pulp, and to stock of the higher consistency (temporarily) brought about by compression of the pulp in my process as magma.

Mechanical expression of liquor from the pulp is preferably effected by a continuously operating dehydrating press in which the stock is confined and is progressively fed forward through a constrictive permeable-walled passage of progressively reduced cross-section. A rotary screw type of press is preferred, and it should preferably have a pervious-walled barrel of uniform bore, grooved longitudinally to prevent the material from turning with the feed-screw or worm, and a worm channel of progressively diminishing cross-section--the worm-channel and press casing or barrel forming the progressively reduced and permeable-walled constrictive passage for the stock, as well as the means for feeding it. The introduction of liquid to the stock, for further recovery of chemical therefrom, takes place at a section of the press passage where the pulp has been compressed and thickened to magma in a caked condition, and at an area of the passage wall suitably removed from its wall area that is permeable,-the pressure on the stock due to the feed being relaxed at the point of introduction of the liquid. The liquid may be introduced between the stock cake and an enclosing wall, at one side of the cake. In the feed of the material through the passage beyond this point, the liquid so introduced is progressively squeezed, pushed or forced into or through the stock cake in the passage, by the compressive pressure of the enclosing walls on the liquid and cake, and a corresponding amount of liquor is expelled from this stock-consisting of original liquor of the pulp more or less diluted with the liquid squeezed into the stock as just described.

It is possible to pass liquid through the continuous, unbroken cake of stock in this forcible manner, without disrupting or channelling through it, because the previous thickening of the stock brings or felts its fibers together into an integrally matted, continuous magma cake, sealed with the enclosing walls at its edges. As hereinafter described, this continuous cake is preferably maintained unbroken and sealed at its edges from its formation throughout all the subsequent treatment.

The introduction of liquid and its expression into or through the stock as above described may be repeated as often as necessary in order to bring about the desired freedom of the stock from chemical. Of course the section of the press passage in which the pulp is thickened and caked and the section or sections where liquid is introduced and squeezed into or through the stock may be structurally separate rather than one continuous passage; but it. is better that the cake should not be broken in transition from one passage'or section to another, since that would result in by-passing the liquid through the open ings thus formed, without proper penetration of the liquid throughout the cake. Yet if found desirable for any reason, the cake may be broken up after several compressions and intervening introduction(s) of liquid, and the stock may then be repulped with added liquid before undergoing further compressions and intervening introduction(s) of liquid,-as explained more fully hereinafter.

In the case of a rotary screw type of press, the relaxation of feed pressure on the stock is preferably produced by deepening the feed screw channel at a point where the material has been brought to magma of suitably high consistency (say 30-40% fiber, more or less), and the extractant may be introduced through an internal hollow or duct in the screw, with radial outlets therefrom delivering at points distributed along the bottom of a channel convolution. Beyond the portion of the channel into which these outlets open, the channel depth may be diminished progressively toward the delivery end of the press, very much as in the antecedent portion of the feed screw.

Instead of introducing fresh or other extrinsic extracting liquid to the magma each time (01' even using it as the principal extractant), and passing the liquor from each expressing stage or operation directly to the chemical recovery plant, I prefer to introduce into one or more of the earlier stages the liquor expelled from the stock in later expressing stage(s), only using fresh or extrinsic liquid extractant in the last stage(s), and only passing the relatively strong liquor from the first expressing operatioms) or stage(s) directly to the chemical recovery plant. This, of course, means a further economy in the use of liquid (water) ,besides that due to working the stock at higher consistency, as above explained. Hence the amount of fresh or extrinsic extracting liquid required is only a fraction of that generally used, and none of the expressed liquid need be run to waste. Loss of fiber stock in the extraction process is obviated, as well as loss of chemicals.

Of course it will be understood that the systems here illustrated and particularly described might be either contracted or expanded, so as to have a smaller or a larger number of reexpressing or dehydrating stages and liquid-reintroducing stages, and that in an extended or amplified system, with a greater number of such stages, the points of introduction of fresh or extrinsic liquid might be varied or multiplied, as well as the points of liquor withdrawal, and the connections for passing expressed liquor from later stages in the sequence of expressing operations to earlier stages in such sequence. More specifically, expressed liquor may be withdrawn from any stage of the process, or extrinsic liquid may be introduced at any stage. It will also be understood that if desired some fresh liquid may be combined with the liquor to provide the extractant introduced to the fiber stock in one or more stages of the process.

The introduction of fresh or other extrinsic liquid into the system affords a means of controlling and regulating its temperature at all points ultimately reached by liquid extractant: e. g., by suitably varying the temperature of this fresh liquid, the temperature in the system can be kept equal to that of the entering pulp, or made either higher or lower, as desired. The amount of liquid so introduced not, only affords additional control over the temperature, but also affords a means of controlling the concentration of the liquor expressed from the stock in subsequent stages of the process.

Various other features and advantages of the invention will appear from the following description of species or forms of embodiment, and from the drawings. So far as novel over the prior art, all of the features and combinations shown or described are of my invention.

In the drawings, Fig. I is a schematic and diagrammatic general view of an extraction system for the purpose of my invention, including a diagrammatic vertical mid-section view of a rotary screw press of my invention; and Fig. II shows a cross section through the screw press, taken as indicated by the line and arrows II--II in Fig. I.

Fig. III is a schematic and diagrammatic general view of an extraction system including a series of rotary screw Dresses such as shown in Fig. I; and Fig. IV is an end view of the presses shown in Fig. III from the right of that figure, with parts in front of those shown broken away and removed-illustrating the interconnection of the presses for the passage of stock from one to another, with provision for repulping the stock between them.

Fig. V is a general schematic and diagrammatic view, similar to Fig. I, illustrating a more elaborate form of extraction system and rotary screw press, for producing more complete purification of fiber pulp.

Fig. I shows diagrammatically a plurality of pressure digesters l which may be operated batchwise in such staggered relation to one another that the reservoir or accumulating tank II into which they periodically discharge always contains a substantial amount of digested or cooked fiber pulp. From the tank ll, this pulp (preferably at digester consistency) is continuously run or fed into the extraction system, into the housing inlet l2 of the rotary screw press l5. Preferably, the tank II and its (valved) connections from the digester l0 and to the screw press inlet l2 may be closed pressure-tight, so as to exclude the atmospheric air from the stock and allow controlled pressure higher than atmospheric (and even more or less approximating that in the digesters) to be maintained in the extraction system, from. the inlet l2 clear to the seal where the stock. is discharged out of the extraction system, as explained hereinafter.

As shown in Fig. I, the rotary screw press l5 comprises a feed and pressing screw or worm l6 revolving in a double-walled barrel structure or casing ll, preferably of uniform internal bore from end to end. The stock to be treated, at the consistency of pulp, enters the press casing ll through the top inlet l2 at the left-hand end of the casing, and after traversing the worm I6 is discharged out of the extraction system at the right hand end of the casing, through an outlet hood l8, that delivers downward. The mechanical compression of the stock in the press l5 maintains it in a compact mass at the discharge end of the casing 11, thus sealing the extraction system pressure-tight at this point. As shown, the worm shaft extends out through a suitable pressure-tight packing 2| at the left hand end of the casing. The worm It may be driven by any suitable means (not shown) connected to the shaft 20, and suitable thrust bearings (not shown) may be provided to take the end thrust of the worm. A suitable speed of revolution for the worm is about -60 R. P. M. As here shown, the worm l6 also has a (hollow) shaft 22 extending from its right hand end through a packing 23 in the wall of the hood l8,--for a purpose hereinafter described. On this shaft 22 is mounted a four-vane paddle wheel 24, which revolves with the worm and chops and breaks up into small pieces the compressed and caked magma discharging from the right hand of the casing II. To facilitate the subsequent handling and transportation of the concentrated stock or magma discharging through the hood l8, it may be thinned with water or liquid (such as paper-mill white-water) supplied through a valved pipe 25 delivering downward through the top of the hood l8.

Aside from features hereinbefore or hereinafter described or indicated, the press l5 and its construction and accessories may correspond substantially to ordinary rotary screw press practice.

As shown in Fig. I, the inner casing wall 26 (defining the bore or tubular housing in which the worm l6 revolves) consists of a foraminous plate pierced with a multitude of fine holes 21, preferably tapered outward so as to be selfclearing. These holes may be about in diameter at their small inner ends. The perforate plate 26 may preferably be as thin as is consistent with adequate strength, and may be supported at suitable intervals by ribs 28 integral with the casing IT, as is usual in the construction of presses of this character. Preferably, the inner surface of the casing bore (formed by the wall 26) has a number of shallow longitudinal grooves 29 such as indicated in Figs. I and II, to resist rotation of the stock by the rotating worm I6, so that the latter may propel and press the stock more effectively.

As shown in Fig. I, the continuous external screw channel or groove 30 of the worm I6 is progressively reduced in cross-sectional area from the inlet !2 toward the right, substantially to a point corresponding to an annular partition 3| in the interspace between the inner wall 26 and the outer wall of the casing II. This partition 3| divides the interspace into two separate filtrate chambers 32 and 33. The worm channel 30 diminishes in both axial width and radial depth as far as the last worm channel convolution 34 at the left of the partition 3|. Beyond this point, the worm channel 30 remains of substantially the same width (measured axially of the worm) but in the next succeeding convolution 35 (to the right of the partition 3!), the depth of the channel 30 is increased rather suddenly, as indicated at 36 in Fig. II. This increase in depth at 36 may preferably be as abrupt as consists with considerations of mechanical strength of the worm I6. Beyond this point, the worm channel 30 is again progressively reduced in depth and constricted (toward the right), until at the end of its last convolution 31 at the extreme right, the crosssectional area of the channel is substantially the same as shown for its convolution 34.

It will be seen, therefore, that the press l5 and worm or feed-screw I6 may be regarded as comprising two structurally united but functionally distinct sections: a left-hand dehydrating section A ending with the worm-channel convolution 34 at its delivery end, and a right-hand supplemental section or extension B beginning with the worm-channel convolution 35 at its receiving end. In a way, indeed, the corresponding portions or sections of the worm channel 30 may be regarded as distinct serially interconnected constrictive passages. Nevertheless. the dehydrating and supplemental treating sections A, B together present a continuous screw channel, in such wise that the deeper receiving end portion 35 of the screw thread in the supplemental screw section or extension B forms a direct IHFA INH AllAhril Ubcontinuation of the screw thread in the main dehydrating screw section A from its shallow delivery end portion 34, and remains of substantially the same width as the latter throughout the section B, though decreasing in depth. As shown in Fig. II, the increase in depth from the delivery end portion 34 of the screw channel of the section A to the receiving end portion 35 of the screw channel of the extension B is made on an oilset at 36; or, in other words, the wall portion at 36 which connects the delivery and receiving thread portions at the bottoms thereof is offset relative to the normal continuation of said thread portions. However, notwithstanding the relatively abrupt increase in depth and crosssectional area at the transition 36 from section A to section B, the worm or screw-channel 30 may be sufiiciently even-sided throughout to sustain, transmit, and guide the stock cake formed in the section A (as hereinafter described) in its transition from section to section without breaking it up,-though temporarily relaxing the mechanical pressure on the stock where the channel enlarges at this transition.

As shown in Fig. I, the worm I6 has a central longitudinal internal chamber or duct 38 extending inward from its right-hand or "discharge" end, and coextensive with the section B. This is connected through a series of some dozen radial outlet holes 42 (more or less) into the bottom of the channel 30, at points suitably distributed along the channel bottom around the convolution 35,--or even on into the succeeding channel convolution(s) to the right. Th outlet hole or port 42 nearest the transition from one section to the other at 36 connects the internal duct or passage-way 38 of the screw extension B with this offset wall portion 36 and with the bottom portion X of the screw channel of section B at its deep receiving end 35, and thus serves (along with the adjacent ports 42) to introduce or deliver treating fiuid or liquid from said passageway 38 to the pressed material in its travel from the pressing screw A to the extension B. The chamber 38 is also connected through the interior of the hollow right-hand shaft or duct 22 to a stationary box 39 whereto is connected a valved liquid supply pipe 46. A pressure-tight joint may be made between the stationary box 39 and the revolving shaft 22 in any suitable way,-diagrammatically indicated in Fig. I by a washer 4| attached to the shaft, with its margin held fluidtight against the interior of the box 39 by the hydraulic pressure in the box.

As shown in Fig. I, the filtrate chamber 32 has multiple outlets which unite in a discharge pipe 43, and the filtrate chamber 33 has a discharge outlet pipe 44. These liquor discharge pipes 43 and 44 are connected through multiway valves 45, 46 to a liquor pipe line 41, which connects at one end into th pulp inlet l2 of the press l5, and leads off at its other end to any desired point of disposal, such as a storage tank or the usual chemical recovery plant of a paper mill. Another pipe line 48 leading to any desired point is shown connected to the valve 46. The valves 45, 46 allow the liquor from either or both the press chambers 32 and 33 to be passed either to the recovery plant or to the pipe line 48, and the valve 46 also permits of passing the liquor from the chamber 33 to the pulp inlet l2 of the press l5. Suitable liquor-handling pumps may be provided in the pipe line 41, as shown.

In the initial dehydrating press section A, the stock (entering at pulp consistency) is progressively compressed and fed along toward the right, and its chemical-laden liquid or liquor is progressively expressed or squeezed out, through the holes in the perforated housing plate 26, into the surrounding liquid chamber 32, until in the channel convolution 34 the stock is brought to a magma of a consistency that is determined by the particular design of the worm I6 and the consistency of the pulp entering at 12. In cases where a press designed for relatively thin pulp is used to treat a much thicker pulp, such thick I pulp may be thinned with liquid supplied to the press inlet l2 through the pipe 41, so that the stock shall not reach an excessive consistency in the press l5. The liquid thus introduced to the pulp at l2 controls or influences the temperature throughout the process, as well as the consistency of the stock and the concentration and temperature of the liquor expressed at various stages of the process. As the stock becomes sufllciently thickened in the press section A, it forms a cake of suflicient consistency and cohesion to engage effectively in the grooves 29 of the housing bore 26, Fig. II, which thus resist any tendency of the stock to turn with the worm IS in its further travel to the right. In the press section B, the mechanical pressure hitherto maintained on the stock by the worm I6 is relaxed, owing to the increase in depth and area of the channel 30 at 36, leaving a space X (indicated by dot and dash lines) in the bottom of the convolution 35 and perhaps a subsequent convolution, which is filled and kept filled by extraction liquid introduced thereto through the holes 42. Illustratively, assuming a pulp of 13% consistency at the inlet l2, and that in the convolution 34 it is brought to a consistency of about 35%, the space at X may correspond in volume to a stock consistency of about l7%,supposing (contrary to fact) that the liquid introduced into this space were fully and uniformly combined with the pulp in the corresponding length of the worm channel 30. This liquid can be introduced into the space at X under a very slight pressure head, just suflicient to induce the required rate of flow through the various pipes and passages. In the worm channel convolution 35 and the succeeding convo1ution(s), liquid is taken up or absorbed by the stock cake through its inner side: at any rate, whether the foregoing is exactly the correct explanation or not,-the liquid introduced through the openings 42 is very quickly sealed in; so that as the contents of the screw channel 36 are progressively compressed during further travel to the right in the progressively diminishing channel 30, the liquid does not flow back into the channel convolution 35 where it entered, nor create any back pressure there, but is forced or squeezed into and through the cake of stock. Thus further amounts of chemical-laden liquid or liquor are forced out and expressed through the holes of the perforated plate 26 into the chamber 33. This further recovery of liquor results, as I believe, from displacement of the original liquor of the pulp (entering the press at l2) by the liquid supplied at X as above described, with some dilution of such liquor in the cake by the liquid at X. The relaxation of pressure on the stock cake just beyond the point 36 and the opportunity for the cake to expand are very briefonly about 1%; see. when the press shown in Figs. I and II operates at some 50-60 R. P. M., as above mentionedbefore pressure on the stock (and liquid) is renewed in the succeeding constriction of the screw channel section B,

which favors displacement of the liquor in the pulp by the liquid introduced at X, rather than dilution.

The liquor dischargin from the chamber 32 through the pipe 43 is of the same concentration as in the pulp entering the screw I 6 from the inlet I2, and may be at substantially the same temperature: 1. e., this liquor is at the full digester concentration, if the pulp received at I2 is the undiluted digester pulp. The liquor discharging from the chamber 33 through the pipe 44 is generally of somewhat lower concentration, as explained above; while its temperature depends on the amount and temperature of the liquid supplied through the pipe 40 and introduced at X, as well as on the initial temperature of the entering pulp. Accordingly, the temperature of this liquor may be controlled and regulated by suitably varying the temperature of the liquid supplied at 40, and may be kept equal to that of the entering pulp, or made higher or lower, as desired from time to time. When the liquor discharged at 44 is introduced into the pulp intake I2 through the pipe 41, the temperature of the pulp and of the liquor discharged at 43 are also controlled or influenced, as already mentioned.

It will be seen that in the operation of the press as above described, the cake of stock in the Worm channel 30 is everywhere maintained, guided, transmitted, and sustained at its edges by the worm screw walls, and need never be broken, or disturbed in its edge sealin contact with the sides of the channel, from its formation somewhere in the press section A, clear up to the last worm convolution 3'I,-whence it discharges into the hood I8. This is, of course, important for the best efficiency of the liquor recovery in the press section B, since if the cake were broken in transition from the press section A to the press section B, or its contact with the worm channel sides were disturbed, the liquid supplied at X would pass out through the cracks or openings thus formed, instead of penetrating properly throughout the cake itself. The bore of the barrel or casing I'I being even throughout its length, without any abrupt change in size, and the feed screw I6 being conformable t the bore, this bore guides and sustains the cake in transition from section A into section B. Thus by the even casing bore and the even lips or edges of the screw channel 30, the cake is continuously and adequately guided and sustained (during the transition as well as in the rest of its travel) against all disruptive influences, including the outward pressure of the liquid introduced at 42, as well as the centrifugal force due to rotation of the feed screw I6, and the friction with all engaging surfaces.

Experience has shown that with proper operation of a press I such as shown in Fig. I, as high a recovery of chemical can be obtained as is achieved in the ordinary practice of many paper mills. Where a greater recovery of chemical is desired, or a greater purification of the stock,--or both,-a plurality of presses may be used in series, as illustrated in Figs. III and IV, where various parts and features are marked with the same reference characters as in Figs. I and II (with the addition of a letter where distinction appears necessary), as a means of dispensing with repetitive description. As in Fig. I, the pulp from the digesters or storage tank (not shown in Fig. III), enters the first press I5 through the top inlet I2, and discharges downward from outlet hood I8, which delivers through a closed connection, preferably pressure-tight, into the top inlet I2a of the second press I5a, which in turn delivers downward through its outlet hood IBa out of the extraction system. As it may not be convenient to arrange the pulp inlet I2a of the second press I5a directly under the discharge hood I8 of the first press I5, a lateral connection is shown between them, consisting of an upward-sloping conduit 50 equipped with a suitably driven rotary feed screw 5|, to lift the stock into the inlet I2a of the second press I5a without any back pressure on the first press I5. To facilitate the transfer or feed of the stock by the conveyor 50, it is desirable to thin it with liquid, which may be supplied through a valved pipe line 25 with a valved branch 55, the former delivering, like the connection 25 in Fig. I, into the top of the outlet hood I8, and the latter shown as delivering through the closed lower end of the conduit or conveyor casing 50. The conveyor screw 5| acts to remix the liquid supplied as just described with the fiber stock, thus repulping it. This repulping may bring the stock substantially to the consistency of the original pulp entering at I2, or to a somewhat lower or higher consistency, (e. g., 17%) if preferred. The action in the second press I5a is substantially like that in the press I5 in Fig. I, resulting in further purification of the stock, and further recovery of chemical therefrom. In cases where extremely high purification of the stock is desired, either or both the presses I5, I50, in Figs. III and IV may be like that shown in Fig. V and described hereinafter.

There is shown in Fig. III a unified system of extractant and liquor connections for the presses I5, I5a, intended to apply the purest extractant always to the purest stock, upon the countercurrent principle. For this purpose, (hot) water is supplied to the second press I5a at 40a, just as in Fig. I, and the discharge pipe 44a from the second section B of this press I5a is connected through the valve 46a to the pipe line 25 which supplies thinning or repulping fluid into the top of the hood I8 of the first press I5, this pipe line 25 being shown as provided with a liquor pump. The liquor discharge pipe 430. from the section A of the second press I50. is connected through the valve 45a to the pipe line 40 that supplies liquor extractant to the second section B of the first press I5, this pipe being shown as provided with a liquor pump. It will be seen that the extractant introduced to the stock at all stages of the process-except the very last-is the (hot) liquor expressed therefrom in the second-ensuing compression of the stock. As shown, the valved liquor discharge pipes 43, 44 for both the sections A and B of the first press I5 are connected to the pipe line 41 leading to any desired point of liquor disposal, such as a storage tank or the usual chemical recovery plant of a paper mill, and this pipe also extends and is connected into the pulp inlet I2 of this press.

With an extraction system such as illustrated in Figs. III and IV, including a couple of presses I5, I5a operating in series, the fiber can be brought to satisfactory purity for paper making, and may be transferred from the outlet I80. of the second press I5a directly to the refiner. To secure the most eiiicient division of work between two such presses I5, I So that are just alike, it may be desirable to operate the second press I5a at somewhat lower speed than the first press.

As will readily be seen from Fig. III, the recovery of extracting chemical with such a system is complete, since the only discharge of liquid from the system is through the pipe line 41, leading to storage or directly to the chemical recovery plant.

The use of separate presses in series as just described in connection with Figs. III and IV results, of course, in breaking the stock or magma cake formed in the first press I5, which necessitates repulping the stock to lower consistency and rethickening and recaking it in the second press la. While the liquid extractant used for repulping the stock serves a useful purpose in recovering chemical from the stock in the first stage Aof the second press l5a, this procedure is obviously more complicated and ineflicient than to effect the purification without breaking and having to reform the cake. The drawback can be avoided by performing the complete purification operation in a single longer press 15b, such as illustrated in Fig. V, having sections A and B like those shown in Figs. I and II, and also additional sections C and D which are duplicates of the section B. The action of these sections C and D is to express still further amounts of chemical-laden liquid from the stock with the aid of extractant introduced thereto just as in the section B. As shown in Fig. V, the screw sections A, B, C, D all together present a continuous screw channel which decreases in crosssectional area in the dehydrating section A, just as in Fig. I, and in the several supplemental sections or extensions B, C, D decreases in depth (and cross-sectional area) while remaining throughout of substantially the same width as at the delivery end of the section A, like the screw channel of the section B in I, but increases abruptly in depth (and cross-sectional area) at the transition from screw section to screw section, again as in Fig. I. The cake, therefore, remains unbroken from its formation in the section A all the way through the succession of supplemental treating sections B, C, and D to its discharge from the last convolution 31b of section D into the hood l8b. Of course such a press may have a greater or less number of sections than the four shown in Fig. V.

As shown in Fig. V, the supply of liquid to the supplemental press sections B, C, D is similar to that in Fig. I, through the interior of the hollow discharge-end of the worm or feed-screw l6b at its right, and into the bottom portion X of the screw channel in each supplemental section, at its deep receiving end, just as in the case of the supplemental section B in Fig. I. Accordingly, the worm I6b has internal longitudinal chambers or ducts 38b, 58, and 59 corresponding to the sections B, C, and D, and separated by transverse partitions or septa 60. Liquid is supplied to the chamber 38b of the section B through a central axial pipe-duct 62 corresponding essentially to the hollow shaft 22 in Fig. I. Liquid is supplied to the chamber 58 of the section C through a larger concentric pipe-duct 63. Liquid is supplied the cavity 59 in the section D through a still larger concentric hollow shaft or duct 64, substantially similar to the hollow shaft 22 in Fig. I. The ends of the shaft 64 and of the pipes 63 and 62 are connected to the pipes of the liquid extractant system through stationary boxes 39b, 39c, and 39d, that make suitable fluid-tight joints with the rotating parts to which they connect, and around the parts which extend through them.

Of course other arrangements might be made for conveying liquid to the cavities 38b, 58, 59, if preferred. The liquid extractant piping is arranged and connected substantially like that shown in Fig. III, and its corresponding parts are marked with the same reference characters, as a means of dispensing with repetitive description,-- a distinctive letter being added where such distinction appears necessary.

It will be seen that the process is substantially continuous, and may be made a substantially closed cycle of operation as regards liquid extractant. The use of air pressure or vacuum is entirely avoided, as well as foaming and all difficulties or losses incident thereto. Substantially all the chemicahs) employed, liberated, or produccd in the digesting or cooking process and present in the stock at the conclusion of that process are extracted and discharged from the system in relatively concentrated state; while the fiber stock is discharged in a relatively pure state, suitable for the manufacture of paper, fiber board, or the like, and at high consistency. The process can be carried out very expeditiously and economically, since there is no need of allowing the fiber stock or the extracted liquor to cool; the amount of liquid that must be handled is minimized; the consumption of fresh water is relatively very low; the loss of fiber is minimized; and only the small amount of chemical allowed to remain in the fiber stock as it leaves the extraction system is lost. The process is simple and efiicient, requires minimum handling of the material and minimum expenditure of power, as well as a minimum of manual labor, and is particularly well adapted to large scale manufacturing operations. Furthermore, the rotary screw presses here shown have a certain refining action on the fiber of the stock, thus reducing the amount of work to be done on the stock afterward in the refiner.

Having thus described my invention, I claim:

1. In a device of the class described, the combination of a tubular perforate housing, a pressing screw rotatable therein for propelling and pressing material fed into said housing, said screw having an extension on its delivery end provided with a continuation of the thread of said screw, the receiving portion of the thread on the extension being deeper than the delivery portion of the thread on the pressing screw, said delivery and receiving thread portions being connected by a wall portion at the bottoms thereof which is offset relative to the normal continuation of said thread portions, said extension being provided with an internal passageway and a port connecting said passageway with said offset wall portion for delivering fluid from said passageway to pressed material in its travel from said pressing screw to said extension.

2. In a rotary screw press of the character described, the combination with a permeable-walled barrel presenting an even bore, without abrupt change in size, of coacting rotary sectional feedscrew means conformable to said bore and presenting an even-lipped continuous screw channel which is progressively reduced in cross-sectional area in the several screw sections, but is increased in depth and cross-sectional area at the transition from section to section, said feed screw means cooperating with said barrel to progressively feed wet material being treated through said screw channel and to progressively express liquid or liquor from the material, thickening and caking the material in the first screw section, and also cooperating with said barrel to guide and sustain the cake against all disruptive influences in transition from section to section, thus maintaining the cake unbroken; and means for introducing treating liquid for the material into the screw channel where the cross-sectional area is increased as aforesaid, at the opposite side of the unbroken cake from the permeable barrel wall; so that in the feed of the material beyond where the liquid is thus introduced, this liquid is mechanically forced into and through the cake, expelling further amounts of liquor therefrom by displacement. 3. The combination of claim 2, wherein the rotary sectional feed screw'embodies a continuous external screw-channel having substantially even lips or edges throughout and is also of substantially uniform width beyond the first screw section, said screw being progressively reduced in cross-sectional area in the several sections and relatively abruptly increased in depth at its bottom, and said feed-screw also embodying means defining internal flow passage for treating liquid into the screw-channel where the latter is of maximum depth.

4. The combination of claim 2, wherein the bore of the permeable-walled barrel is provided with shallow longitudinal grooves, effective to resist rotation of the material progressed by the rotary sectional feed-screw means.

5. In the treatment of digested or cooked fiber stock, for the removal of chemicals resent therein, as it comes from the digester to caked magma; means for confining the stock pulp in a constrictive permeable passage substantially as it comes from the digester; means for compressing the stock pulp and expressing chemical-laden liquid or liquor therefrom by pressure; means for thickening and caking the stock into a magma; means for introducing liquid extractant for chemical into the stock pulp as the latter passes from the feed-in portion of the constrictive passage into the succeeding portion; means for maintaining the stock cake formed in said feed-in portion continuous throughout the rest of the treatment without breaking it up; and said means for introducing the liquid extractant, as aforesaid, to the unbroken cake being so arranged in relation to said cake and to the constrictive permeable passage that said liquid is mechanically forced outwardly into and through said cake in the succeeding portion of said passage, thus expelling liquid in the cake by displacement, rather than merely by dilution and squeezing out as when a formed cake is broken, repulped, and recompressed.

MANUEL C. McDONALD.

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