US1899106A - Method of and apparatus for treating pulp - Google Patents

Description

Feb. 28, 1933. G. A. RICHTER ET AL 1,399,106

METHOD OF AND APPARATUS FOR TREATING PULP Filed Dec. 13 1924 5 Sheets-Sheet 1 P11 022? :1: ///a '(/0(/ 41/123, 620 a a; iZ/r/z/fir Feb. 28, 1933. G. A. RICHTER ET AL NE'II'WJD OF AND APPARATUS FOR TREATING PULP Filed Dec. 13, 1924 5 Sheets-Sheet 2 \\\\Y //1\ \\Y A Jim (7m 11 @412,

a 63:7 (1 12/271751. W M

Feb. 28, 1933. G. A. RICHTER ET AL 1,399,105

METHOD OF AND APPARATUS FOR TREATING PULP Filed Dec. 13, 1924 5 Sheets-Sheet 3 grams 727570: mil/00,2 me,

G'wzya (1/ 1212/1751 Feb. 28, 1933. G. A.. RICHTER ET AL METHOD OF AND APPARATUS FOR TREATING PULP Filed Dec. 13, 1924 5 Sheets-Sheet 4 'IIII'IIIII Feb. 28, 1933. G. A. RICHTER ET AL 1,899,106

METHOD OF AND APPARATUS FOR TREATING PULP Filed Dec. 15. 1924 5 Sheeis-Sheet 5 723731 142 adj J 660506 2 SQQPQ O I I l lllll Patented Feb. 28, 1933 UNITED STATES P TaN-ToFF-ma GEORGEA. RICH TEE AND ELWOOD EBIE, F BERLIN, NEW HAHIBHIBE, ASSIGNOhS TO-BBOWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION 01' mum METHOD OF AND APPARATUS FOR TREATING- PULP Application filed December 18, 1924. Serial No. 755,780.

[0 closed, and the cooking operation is carried on at an elevated temperature and high pressure for a sufficient length of time to insure the liberation of the cellulose fibers from the encrusting or cementitious content of the [5 wood. After the digesters are blown and the pulp is separated from the wasteor spent cooking liquor and is then washed, the solid material is subjected to a screening operationin which the stock, together with an exa cess of water, is caused to flow over screens,

and the pulp is withdrawn by pumps through the screens, the knots, chips and other coarse foreign particles in the stream bein carried by the water ast the screen an being collected in the orm of screenings so-called. The screened pulp ordinarily is now subjected to a bleaching operation in the case of sulphite or soda pulp, no bleaching ordinarily being practiced in connection with sulphate or kraft pulp because of its diflicult blcachability.

The screened pulp (whether bleached or unbleached), which constitutes the raw material in the rocem which we shall describe,

contains fibrillae, broken fibers, certain resinous bodies, and colloidal coloring matter; and moreover the fibers themselves vary in length, this variation in length being clearly observable under the microscope.

Broadly speaking, the object of the present invention is mechanically to separate from the pulp those bodies which are therein contained which are of a foreign nature such as the resinous, colloidal or coloring bodies to which we have just referred; and, as a further refinement, one of the objects of the present invention is also mechanically to separate from the perfect fibers the broken fibers or fibrillae which constitute a portion of the 5D pulp.

A still further ob'ect of the invention is mechanically to gra e, separate and recover separately the cellulose fibers of different lengths. In the practice of the process which we shall hereinafter describe, the extent to which the body of pulp is thus'separated depends upon circumstances and to some extent upon the character of the pulp which is bein treated.

rimarily, in carrying out our process, it is desirable ,to separate from the pulp those bodies which have a coloring efiect thereon such as the resins or resinous bodies, the colloidal coloring bodies and the like.- This is particularly advantageous in the treatment of soda or sulphite pulp where the. final roduct is to be white in color, and in sodomg it is possible at the same time to separate and remove more or lessof the fibrillae and broken fibres, since these are ordinarily of a darker color than the perfect fibers, probably due to their absorption of the reslns and colloidal coloring matter. It is quite evident that, if unbleached pulp be thus treated, the pulp after treatment may be bleached more easily and with a less consumption of bleaching agent than if the pulp had not thus been previously treated; and, even if, prior to its treatment, the pulp had been bleached, nevertheless the treatment results in separating and freeing from the pulp those foreign orcoloringbodies which resisted the action of the bleaching agent.

In practicing the process, however, it is possible by the use of the proper instrumentalities to separate'the pulp into two grades, one containing cellulose fibers of an average length considerably longer than the average length of fibers in the other grade, and in such case the last-mentioned grade may containvthe coloring matter if it be not desired initially to separate that from the P P- The strength of paper made from pulp depends, among other things upon the length of the cellulose fibers of w ich the paper is made. Consequently, by separating or grading the pulp into different masses of different average fiber lengths, it is possible by using that mass having the longest fibers to roduce a a r havin a strength (all other thin beil ig qual), e xceeding that which has en secured in paper made from the same pulp which has not been separated. Again, the longer fiber portion, being large- 1y freed of resistant coloring impurities is more suitable for conversion into cellulose derivatives, such as the nitrate, acetate, and xanthate, as it is desirable that such derivatives be free of side reaction products of such impurities, which impart cloudiness and color to solutions prepared from the cellulose derivatives.

The proces, which forms the subject of this application, may be considered in one aspect of the case as a process of classifying and separating the pulp into different grades. Carried to its ultimate conclusion, the different bodies resulting from the classification and separation might comprise, first only those coloring matters (including a certain amount of broken fibers and fibrillae originally in the pulp) second a pulp body in which the average length of fibers is relatively short; and third, a pul body the average length of fibers of which is long,

' and this can beaccomplished either before or after bleaching the the starting material.

We have found that it is possible to effect this classification and separation of the initial body of pulp mechanically by the use of screens provided the screening operation be carried on in such wise as to preclude the formation of a mat u on the active screen surface such as would interfere with the operation of the screen. B employing a screen having a mesh so smal that only colloidal coloring matter, resins, fibrillae, and short lengths of broken fibers will pass therethrou h, it will be seen that, by subjecting the imtial body of pulp to the screening operation in such way as to preclude the formation of a mat thereon, these bodies can be separated and removed from the pulp, thereby resulting in a pulp which is substantially free therefrom. In accomplishin this operation, the pulp must be suspended in a body of water and the suspension must be so agltated or otherwise acted upon as to permit the passage of the impurities through the screen without the formation of a mat of the perfect fibers thereon which would prevent the functioning of the screen. B causing the aqueous suspension to be broug t successively into contact with the screens of the proper and varying fineness of mesh while applying pressure or force on the pulp in addition to the pressure afforded by the liquid head of the water associated with the pulp which constitutes pulp, the entire body of pulp may not only be re ed from the foreign material but may also be separated and recovered in bodies of pul which vary from each other according to t e average length of the fibers.

In the screening operation which has ordinarily been carried on in connection with pulp as delivered from the digesters, the cellulose fibers themselves, together with the fine coloring matter, resins, broken fibers and fibrillse, are subtracted from the stream of water and are caused to pass through the screen. Our process differs therefrom in this important respect, in that according to our process the perfect fibers pass over the screens and the coloring matter is caused to pass through the screens, and subsequently, if desired, the fibers are clasified by permitting only a portion thereof not exceeding a given length to pass through the screen.

So far as we are aware, it has never heretofore been proposed mechanically to separate from the screened pulp by mechanical o eration, resins, colloidal coloring matter, fibrillae and broken fibers, or mechanically to separate the screened pulp into separate bodies of pulp difi'ering according to the fiber length.

It is possible, of course, to use various forms of instrumentality for carrying out our process according to the ultimate results desired to be achieved, and consequently we do not desire to be understood as limiting the present invention to the employment of any specific form of apparatus. Again the prac-' tice of the process may be varied, all without departing from the spirit and scope of the invention as defined in the claims.

In order that those who are skilled in the art may practice the process to the best advantage, we have illustrated upon the drawings various forms of instrumentalities which may be used and which we shall describe in detail, and the practice of the process may be varied to some extent in accordance with the particular apparatus which is employed.

Referring to the accompanying drawings,

Figure 1 is a longitudinal section of one form of apparatus which may be employed in the practice of our process.

Figure 2 is a section on line 22 of Figure 1. v

Figure 3 is a longitudinal section of another form of apparatus which may be used.

Figure 4 is a section on line 44 of Figure 3.

Figure 5 is a longitudinal section through a preferred form of apparatus utilizing mechanical means other than the forceof the wash .water for agitating and feeding the pulp over the screens.

Figure 6 is a section on line 66 of Figure 5.

Figure 7 is a side elevation partially broken away of another form of separator mechanism involving mechanical pulp agitating and feeding means.

Figure 8 is an end elevation of the same.

Figure 9 is a vertical section on line 9-9 of Figure 7 Figure is an enlarged sectional detail of one treating unit of the machine shown in Figures'7 to 9.

Figures 11, 12 and 13 are diagrammatic views illustrating certain groupings of the fibrous material as treated by successive units.

Referring to the construction particularly shown in Figures 1 and 2, at 1 is indicated a substantially rectangular tank having a screen 2 arranged substantially parallel to the base 3 of the tank and spaced thereabove. As shown, the screen 2 is supported directly on the upper face of a coarse wire mesh screen 4, fixed at its margins to a rectangular frame 5 fitting within the side and end walls 6 and 7 of the tank. The screen 2 is of fine mesh and may be formed of wire, or, if desired,

of a sheet metal plate perforated with openings of the desired small size. When washing out the shortest fibers, together with resins, colloidal coloring matter and fibrillae, the size of openings of this screen may be from 0.02 to 0.1 of an inch in diameter, or, if in addition to the coloring matter, resins, fibrillae and broken fibers, short fibers are to be removed, the screen 2 will be of a somewhat larger mesh. The screen 4 merely forms a support for the fine screen and has no function so far as the separation of the fibers is concerned.

Supported above the top of the tank 1 is a pipe 8 having a series of depending branches 9, carrying at their lower ends laterally extending jet or spray pipes 10, which extend across the tank a short distance from the upper face of the screen 2 and are provided with apertures for spraying water under considerable pressure down against the screens and preferably somewhat toward one end of the tank, herein shown at the right hand end. Provision is aso made by which the spray pipes may be reciprocated in order that the water jets may strike the pulp supported on the screen over an extended area. As shown, this is accomplished by means of a crank 11 rotated as by means of the belt 12 passing about the pulley 13 carried by the crank shaft, this crank being connected by means of a link 14 to a sliding rod 15 which supports the pipe 8. A flexible hose connection 16 may be employed to supply wash water to the jet pipes while they are being reciprocated by the crank. Adjacent to the right hand end of the tank, as shown in this figure, is a dam 17, preferably vertically adjustable in order to fix the depth of pulp on the screen 2. The pulp escapes over this dam to the outlet chamber 18, from which extends a pipe 19, through which the pulp which has been freed from impurities and fibers of short length is removed. A pipe 20 communicates with the interior of the tank beneath the screen 2 and serves to lead therefrom the wash water, together with the short pulp and impurities which have been forced throu h the screen by the force of the water jets; W'here the jets are inclined in the d1-, rection of progress of the pulp, they not only'serve to drive the short fibers through the screen (if the screen mesh is of sufiicient size) but to impel the entire mass of pulp from the entering to the exit end of the tank, so that the apparatus may be used continuously to treat the pulp. The force of the water jets also acts to break up any mat formation on the screen, which would naturally result from the passage of the wash water therethrough, and causes the fibers to-strike the screen again and again so that the short fibers (or only the coloring matter, resins, broken fiber and fibrillze, as the case may be) are separated from the long fibers and washed through the screen. Where the screen having openings of approximately the sizes above mentioned (from 0.02 to 0.1 of an inch in diameter) is employed, only the shorter fibrillae, broken fibers and impurities pass through the screen. In this case the sus ension of the starting pulp in an excess 0 water, is agitated by force of V the water jets, and the tendency of the perfect fibers to form a mat is overcome by the water jets, so that during the passageof the pulp over the active screen surface, practically all of the fibers are brought into forcible contact with the screen, and the impurities originally entrained therein are caused to pass with the Water through the screen.

Another form of apparatus employing high pressure wash water jets is shown in Figures 3 and 4. Referring to these figures, it will be seen that it comprises a casing substantially rectangular in cross section indicated at having therein a cylindrical screen 31 mounted to rotate on rollers 32 carried on a series of shafts 33 extending longitudinally within the casing. As shown, these rollers 32 engage ring track members 33 adjacent to either end of the screen. For the purpose of rotating this screen one of the shafts 33 as shown is extended beyond the tank end 34, where it is provided with a belt pulley 35 by which it may be driven by any suitable source of power. At is shown a pipe which extends into the top of the casing and then inwardly where it opens as at 41 within the screen adjacent to one end thereof, so that the starting pulp passed downwardlyftherethrough is deposited on the inner face of the screen. At 42 is shown a wash water supply pipe which extends from the same end of the screen a substantial distance into the interior thereof as shown at 43. The portion 43 is provided with a. plurality of openings so formed as to direct the wash water toward the opposite end of the screen from the supply pipe 42 and against the screen in a direction opposite to. its direction of rotation as driven from pulley 35. By this means, as the screen is rotated the water impinging on the pulp cuts in under and removes any matted layer from the inner face ofthe screen, agitates the pulp, and brings the fibers rapidly again and a in into contact with the inner face of t e screen and forces the broken fibers, fibrillie, resins and coloring matter therethrough and at the same time causes the remaining pulp to be moved away from the supply end of the screen to-- ward the opposite en from which it may be removed through the pipe 45. A bafile 38 extending across the up r portion of the exit end of the screen cylin er acts to prevent the pulp from splashing out at the upper portion of the cylinder, permitting it to escape only by flowing over the end of the screen. At 46 is shown a pipe opening beneath the screen cylinder through which the white water, together with the broken fibers and other impurities, is removed. While onl one water supply pipe 43 has been shown, it is often preferable to use more than one in this type of apparatus. The screen cylinder may be formed, if desired, of a fine mesh inside supported by a relatively heavy coarse screen on the outside.

Where the force of-the jets of wash water is employed to agitate and break up the pulp to prevent it from clogging the openings of the screen, and to cause the fibers to be presented forcibly thereto in order that the shorter ones may escape therethrough, it is found that a very large quantity. of water under heavy pressure is necessary to effect the washing operation as thoroughly as is needed. In order to economize in the use of water, mechanism such as shown in Figures 5 to 10 inclusive has been devised. In these mechanisms the force of the water jets is not relied upon to agitate the pulp, other mechanical means being substituted therefor.

Referring to Figures 5 and 6, which indicate a preferred form of apparatus, at 50 is shown a substantially rectangular tank having at 51 a platform extending between its side walls 52 and 53 lengthwise thereof and termlnating somewhat within its end walls 54 and 55. The space beneath this platform is divided into a number of washing units b vertical partitions 56, extending laterally o the tank and spaced longitudinally thereof, as herein shown, three such units being shown as at 57. A eater or less number might be employed if desired. The platform 51 is provided with a series of openings or slots 58 extending thereacross, each slot opening substantially centrally of each washing unit or compartment 57 Each of these compart ments has positioned therein a trou hshaped screen 60 depending from the side edges of the platform 51 at the correspond ing slot 58. The screen is supported at suitable-intervals by stirrup frame pieces 61 s aced apart to permit the read passage of t e white water and fibers t erebetween. Within each of these screens and arranged substantially concentric therewith is a roll 62, having a series of substantially axially arranged ribs 63 on its periphery, so that the surface of the roll is substantially fluted. There rolls are supported on shafts 62 which extend outwardly of the side wall'52 of the tank, where they are provided with suitable means. (no shown) by which they may be rotated.- I Between one end of the tank as 54 and the adjacent partition 56, a pul supply pipe is shown at 70. The pulp to e se arated is passed into the tank through t is pipe, passes over the adjacent end of the platorm 51 to the first fluted roll of the series and the first screen. Substantially as it reaches the roll, wash water may be supplied as herein shown, this being led to the roper position through a pipe 72, having ownwardly extending branches 73, terminating in horizontally arranged jet or spray pipes 74 having openings positioned to direct the wash water downwardly against the pulp. The roll 62 is driven in a direction to carry the pulp downwardly over the surface of the screen, the flutes being spaced sufliciently therefrom-approximately one inch being found satisfactor to cause the pulp and washing water to he forced against the screen with sufiicient force to break up any mat of fibers which might be formed thereon and to present the fibers with suflicient force a ainst the face of the screen to cause the s orter fibers and impurities to pass therethrough, the longer fibers being carried around and thrown up at the o posite side of the screen to the next succeeding roll of the series over the platform 51, this action being repeated at each successive washing unit. Each of the rolls is of sufliciently large diameter so as to occupy a substantial partof the cross sectional area defined by the screen in which it is placed so that the pulp is caused to pass over the screens in relatively thin layers thus to brin as large a proportion of the fibers as possible into forcible contact with the screen so that those fibers, fibrillae, broken fibers and specks which are of a size to pass through the screens shall do so instead of being carried along with the pulp of longer fiber length. By spacing the roll a comparatively short distance from the screen, for instance, about one inch, a substantial force or pressure in addition to that of the liquid head of the water associated with the pulp, is afforded over a large surface of the pulp, thereby greatly acceleratin the rate at which the short and broken bers and associated impurities pass through the screen. The depth of the pulp over each roll may be adjusted as by means of the deflecting vanes shown at 75, pivoted at 76 between the sides 52 and 53 of the tank, theilower edge of each vane being fixed at the pro er height b means of the adjustin bars 7 pivoted t ereto and provided wit any suitable means by which they may be fixed in adjusted position to the side walls of the tank. As shown, they are rovided with a series of spaced perforations 8, through which pins may be passed into mating openings in the adjacent tank walls. After passing rom the last screen the pulp passes over into the tank be ond the last partition 56 and between it an the end wall 55 and is drawn off through the pipe 80. The white water from the various washing compartments or units is taken off from beneath the screens by pipes as at 82, each pipe as shown leading to'the lower end of a box or compartment 83 having a vertically adjustable dam 84 partially dividing it into two compartments, the white water and short fibers being led into one, passing over the top of the dam, and led out of the other through the ipe 85. By adjusting the height of the 8am, the white water may be made to fill the corresponding compartment to the desired extent, it being found sometimes desirable with this form of apparatus to maintain the screens submerged in the white water.

In Figures 7 to 10 a somewhat different form of apparatus employing a mechanical means rather than the force of the wash water for a 'tating and conve ing the pulp is shown. fiferring to these gures, it will be seen that the apparatus comprises a series o superposed washing units, each one being indicated generally by 90. As shown best in Figure 10, each ofthese units comprises a compartment formed of a base 91, rear wall 92, an intermediate wall 93 having openings 94 therethrough at its lower end, and a forward wall 95. Between the forward and intermediate walls 95 and 93 is positioned an adjustable dam 96. From the upper end of the intermediate wall 93 extends at one end of the units except the top one, a forwardly and upwardly inclined directing plate 97. Supported at the upper end of intermediate wall 93 and on a longitudinal support at 98 carried by the back wall 92, is shown a troughshaped screen as at 100, and arranged therein substantiallyconcentrically thereto is a rotary roll 101. Instead of the axially extending flutes as in the form shown in Figures 5 and 6, the roll 101 is rovided with a helical rib 102 on its perip ery, this being formed, if desired, of a strap wound about the periphery of a lane cylindrical roll in helical fashion. This strap should approach the upper face of the screen to within about one inch. Units as thus constructed arranged in vertical series are supplied with pul to be treated through the pipe 102 shown in re 7 at one end of the u per roll 101. This roll is rotated in such a irection that the helical rib 102 thereon'acts to carry the pulp lengthwise of the screen 100 to the opposite end of this unit. Adjacent this end 18 a dam 103 of such a height that the pul and wash water are kept at the desired leve with relation to the screen and the roll, and as this material passes over this dam, it falls to the directing plate 97 at the adjacent end of the unit beneath and is guided to the screen of that unit where it is taken by the rotating roll therein and moved toward the opposite end thereof or toward the end at which the pulp was supplied to the top unit. Water is supplied to each of the units, as shown a pipe 105 extending vertically at the rear face of the series of units and provided with a downwardly extended jet 106, opening above each roll 101 at any suitable point along its length being employed for this purpose. After the stock passes the length of the unit adjacent to the top unit, it passes over a dam similar to the dam 103 and asses in like manner to the adjacent end 0 the roll beneath, the pulp thus proceeding in a zigzag path lengthwise of each of the units untll it is finally discharged from the final end of the last unit as through the pipe 107. The white water with the short fibers and impurities asses through the openings 94, thence over t e dam 96 and out through the pipe 108 which communicates with each unit. As shown, each of the rolls 101 is fixed to a shaft which extends beyond the unit at one end and carries a pulley 110, by which it may be rotated, the direcf tion of rotation and the direction of winding of the helical rib on the roll being such as to cause the pulp to pass first in one direction through one unit and then in the opposite direction through the next adjacent unit.

Any of the mechanisms above described may be built up in as many washing units as desired and ma be manipulated for the ur-. pose of removlng the short fibers, fibrillae, broken fibers, many of the impurities such as resins and specks, leaving the remainder of the pulp containing a substantially greater proportion of the longer fibers, or it may be employed, if desired, to separate the pulp into any desired number of grades, from a grade containing fibrillae, broken fibers, resin and specks, and of very little value, through grades having average fiber lengths of increasing amounts and sultable for making paper 0 better qualities. For example, Figures 11,12 and 13 show in diagrammatic form various arrangements of units for accomplishing various desired results. Thus, in Figure 11, five washing units are indicated, each having a screen having openings therethrough of a diameter of substantially 0.02 of an inch. The initial pulp, which we may assume has an avera e fiber length of 0.6 mm., is passed throug these washing units, the broken fibers and fibrillae washed out with the washing water averaging in length from 0.15 to 0.2 mm., 'while the fibers passing from the last unit show an average length of approximatel 0.95. This resultant pulp is also considers ly whiter than the pul before-it is passed through the washer an will be found to be somew at freer on the paper machine. The pul passing off wit the white water is foun to contam a lar e number of yellow fibrillaa and broken fi rs, to-

ther with resins and specks, will have a sfibwness considerably higher than that of the pulpl passing off from the last unit in its was ad condition and is of oor quality.

In Figure 12, five units a so are indicated, each having its screen openings substantially 0.1 mm. in diameter. Assuming the initial stock of an average fiber length of 0.6 mm., the stock remaining after the washin operation may have an avera e length of mm 0.8 to 1.0 mm., while the bers removed in the white water may be found to have an average length of from 0.03 to 0.5 mm. The stock coming off in the white water from the several units may be united and in this case is suitable for book paper, while the stock passing over the final screen is high grade stock sultable for bond paper. In both these cases, as shown in Figures 11 and 12, the pulp has been divided but once, one portion having an average fiber length relatively lon while the other has an average fiber lengt much shorter and contains practically no long fibers.

In Figure 13, five units have been indicated, the first two having screens the openings of which are 0.06 inch in diameter and the last three having screens with openings 0.1 inch in diameter. initial stock of an average fiber length 0 0.6 mm. as before, the fibers assing through the first two screens with t e white water may be mixed to ether to form a pulp having an avera e fi er length of about 0.3 mm., which may e suitable for certain uses, while the fiber passing through the next three screens with white water may be mixed to form stock having an average fiber length of sub stantially 0.5 mm., which is substantially the average length of commercial sulphite stock, while the fibers passing over the last screen have an average fiber length of substantially 1.0 mm., which is suitable for high ade bond aper. Where it is desired to o tain pulp aving a long fiber, the classification may be carried out before the"'entire bleaching operation has been effected, the longer .fibers being more readily bleached than the short fibers, since they are found in practice to contain less coloring matter. It is thus iplos'sible to complete the bleaching of Ion ber stock with a much smaller quantity 0 bleach than would be necessa 1f the unclassified pulp were to be bleac ed. Hence, there is not only a saving in the amount of bleach used, but a small amount of bleach causes the long fibered stock to be freer and y in better condition. The long fiber stock is also found to contain a larger amount of alpha cellulose than the rtions of the stock containing the shorter fi rs.

In the ractice of our process it will be apparent mm the fore oing that the initial stock may be treated either before or after a bleaching operation ,or both, or that pulp, such as su phate or Kraft pulp which is not bleached ordinarily may be treated, and divided and recovered in two or more difierent bodies, one containin fibers havin an average length greatly fimger than t ose contained in the other body or bodies of pulp. Conseqfilently a Kraft aper can be made from t e first mentioned body of long fiber pulp, which will greatly exceed the strength of aper made of the initial or starting pulp WhlCh has not thus been treated.

Our process, however, is possibly of greater value, when practiced in connection with sulphite pul when the latter is to be bleached an used in the manufacture of white paper, or in the manufacture of cellulose esters, such as viscose, cellulose nitrate and other cellulose roducts. If the unbleached sulphite pu is treated as herein described, with a screen of appropriate mesh, there is removed therefrom the resins or resinous bodies, the colloidal colorin bodies, and the broken fibers and fibrillae which are ordinarily of darker color than the perfect fibers, thereby producing a pulp having a greater average fiber length, a greater strength, a greater freeness and a whiter color, and roducing a paper of softer texture than t e same grade of unbleached pulp not so treated. But the ulp so produced may now be bleached, and in the bleaching operation, a much less drastic bleaching action is required and consequently less bleach and a shorter period of operation are required, and there is less danger of overbleaching in securing the desired degree of whiteness. --The bleached stock has practically no specks, has a higher proportion of alpha cellulose, and is of a much higher or finergrade, than pulp produced from stock not so treated by our process. If desired, after the stock has been treated and bleached, it may be again processed for the removal of an? impurities resulting from or left after the b caching operation.

It may be reiterated that while we have illustrated and described certain forms of apparatus useful in the practice of our process, many other instrumentalities may be employed in lieu thereof. So far as is now apparent to us, a necessary instrumentality for the practice of our process is a screen into contact with which the aqueous pulp suspension is brought, and having a mesh through which the perfect fibers exceeding a given length will not pass (except adventitiously) and provisions of some kind for preventing the matting of the fibrous pulp on the screen in such way as to prevent its functioning.

Having thus described certain methods and mechanism for practicing this invention it will be evident to those skilled in the art that various changes and modifications may be made therein without departing from its s irit or scope as defined by the appended c aims. y

We claim:

1. A mechanism of the class described comprising a tank, a series of trough-shaped screens of sufliciently coarse mesh to permit the passage of pulp fibrous therethrough in said tank, means of supplying pulp'to the screen at one-end of said series, means for removing the pulp from the screen at the opposite end of said series, said screens being arran ed to pass pulp from'one to the other end 0% the series, a roll substantially concentric'ally arranged in each of said screens and whose periphery is spaced a comparatively short distance from the screen, means on the periphery of said roll to cause a movement of the pulp, means for supplying wash water to certain of said screens, and means for rotating said rolls.

2. A mechanism of the class described comprising a tank, a series of trough-shaped screens in said tank, means for supplying pulp to the screen at one end of said series, means for removing the pulp from the screen at the opposite end of said series, said screens being arranged to pass pulp from one to the other end of the series, a roll'substantially concentrically arranged in each of said screens and whose periphery is spaced a comparatively short distance from the screen,

means on the periphery of said roll to cause a movement of the pulp, means for supplying wash water to certain of said screens, means for rotating said rolls, and means for retaining said screens submerged in the white water passingtherethrough to a predetermined extent.

3. A mechanism of the class described comprising a tank, a series of trough-shaped screens in said tank, means of supplying pulp to the screen at one end of said series, means for removing the pul from. the screen at the opposite end of sa1d series, said screens beingarranged to pass pulp from one to the other end of the series, the screen nearest the pulp-supplying end of the series having smaller openings therethrough than those near the pulp-removing end, a roll substantially concentrically arranged in each of said screens and whose periphery is spaced a comparatively short distance from the screen, means on the periphery of said roll to cause a movement of the pulp, means for supplywash water to certain of said screens, and ns for rotating said rolls.

mechanism of the class described comprising a tank, a series of trough-shaped screens in said tank, means of su plying pulp to the screen at one end of sai series, means, for removing the pulp from the screen at the opposite end of sa1d series, said screens being arran ed to pass pulp from one to the other end 0 the series, the screen nearest the pulp-supplying end of the series having smaller openings therethrough than those near the pulp-removing end, a roll substantially concentrically arran ed in each of said screens and whose perip e is spaced a comparatively short distance rom the screen, means on the peripher of said roll to cause a movement 0 the pu p, means for supply ing wash water to certain of said screens, means forrotating said rolls, and means for retaining said screens submerged in the white water passing therethrough to a predetermined extent.

5. A mechanism of the class described comprising a-tank having a longitudinally extending platform intermediate its height, vertical partitions dividing the space beneath said platform into .a series of compartments arranged lengthwise of said tank, said platform having a laterally extending slot communicating with each compartment, a trough-shaped screen depending into each compartment and bridging the corresponding slot, a fluted roll within and substantially concentric with each of said screens, means for supplying pulp to one end of said tank to flow over said screens in sequence, means at the opposite end of said tank for removing the pulp, means for supplying wash water in advance of each roll, vertically adjustable aprons beyond each roll to determine the depth of pulp thereover, and means for removing the water passing. through said screens into each compartment including means for maintaining a predetermined level of white water therein.

6. A mechanism of the class described comprising a tank having a longitudinally extending platform intermediate its height, vertical partitions dividing the space beneath said platform into a series of compartments arranged lengthwise of said tank, said platform having a laterally extending slot communicating with each compartment,a troughshaped screen depending into each compartment and bridging the corresponding slot, a fluted roll withinand substantially concentric with each of said screens,means for supplying pul to one end of said tank to flow over sai screens in sequence, means at the opposite end of said tank for removing the pulp, means for supplying wash water in advance of each roll, vertically adjustable aprons beyond 8 memos in pulp, subjecting the ulp to water jets of sufiicient velocity .to dis odge and carry of! minute adherent particles of impurities and short fibers.

8. In a process as described in claim 7, in which the pulp is subjected to jets of water of suflicient velocity to dislodge and carry off minute adherent particles of impurities and short fibers, by passing the pulp in the form 10 of a layer or sheet under the water jets, which latter are directed downwardly into the flowin? layer of pulp from above.

11 testimony whereof we have afiixed our signatures.

GEORGE A. RICHTER. ELWOOD EBIE.

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