US3438490A - Method and apparatus for wet sizing finely divided solid materials - Google Patents

Description

p 1969 J. LE ROY PETERSON 3,438,490

METHOD AND APPARATUS FOR WET SIZING FINELY DIVIDED SOLID MATERIALS Sheet Filed July 24, 1964 JNVENTOR.

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JOSEPH LEROY PETERSON BY ATTORNEYS April 1969 J. LE ROY PETERSON 3,438,490

METHOD AND APPARATUS FOR WET SIZING FINELY DIVIDED SOLID MATERIALS Filed Juiy 24, 1964 Z ors Sheet llllallvlllillrllrr INVENTOR. JOSEPH LEROY PETERSON ATTORNEYS April 15, 1969 LE ROY PETERSON 3,

METHOD AND APPARATUS FOR WET SIZING FINELY DIVIDED SOLID MATERIALS Filed July 24, 1964 'Sheet of s INQVENTOR. JOSEPH LEROY PETERSON AT TORNE Y5 United States Patent 6 3,438,490 METHOD AND APPARATUS FOR WET SIZING FINELY DIVIDED SOLID MATERIALS Joseph Le Roy Peterson, Salt Lake City, Utah, assignor to W. S. Tyler, Incorporated, Cleveland, Ohio, a corporation of Ohio Filed July 24, 1964, Ser. No. 384,896 int. Cl. B03l) 3/12 US. Cl. 209-17 20 Claims ABSTRACT OF THE DISCLOSURE Apparatus for separating finely divided solids has a classifying vessel and overlying screens resiliently mounted for reciprocatory vibration. Bafiles divide laminar overflow from the upper part of the classifying vessel into separate flows. Diflerent ones of the separate flows are directed to overflow, across the faces of the screens or through the screens.

This invention is concerned with the wet sizing of various, finely divided, solid materials, such as ores which have been crushed and ground for processing, natural sands containing both fine and coarse grains, and a variety of other industrial materials where relatively fine particles must be separated from relatively coarse particles. It has to do with both method and apparatus.

The invention involves the broad concepts disclosed in United States Patent No. 3,064,806 granted to Risto Tapani Hukki on Nov. 20, 1962, entitled Apparatus for Wet Sizing of Solid Materials, and constitutes an improvement on what is specifically shown therein In the Hukki apparatus, screens are provided in the upper part of a vessel adapted to contain a liquid pulp of solid particles of different sizes. There is a continuous feed of pulp to the vessel and agitation of the body of pulp in the lower part of the vessel, usually by means of an impeller, so relatively fine particles will rise and be Washed through the screens, counter to the force of gravity, by liquid which overflows and carries such particles with it, while relatively coarse particles will remain in the lower part of the vessel and be discharged therefrom separately from the screened fines.

One very important aspect of the present invention is sharp reclassification of the feed pulp before it reaches the screens, so the maximum size of particles reaching the screens is to a great extent predetermined. Thus, if screens having 48 wires to the inch are being utilized to pass minus 48 mesh material, preclassification in accordance with this invention will limit the maximum size of solid particles reaching the screens to, say, minus 35 mesh or even minus 42 mesh. This will bring the fines along for screening, without excessively loading the screens with oversize tive separation of differing solid materials in certain instances where substantially 100% of the particles of one of the materials are of such size as to pass through the screens, while the particles of the other material are oversize with respect to the screens but classify identically with the particles of the first material under the conditions prevailing in the machine.

Such preclassification is accomplished in the apparatus of the invention by preferably a combination of factors. Thus, as one of the factors, the pulp is merely agitated in the lower part of the containing vessel and is raised to screen level primarily by reason of the hydrostatic head. If an impeller is utilized for pulp agaitation, it is of neutral type; that is to say, it does not pump the pulp upwardly as does the impeller shown in the Hukki patent. Rather than producing turbulence, which would churn all particles into a heterogeneous mixture, the emphasis is preferably material, and will accomplish eflecon attaining laminar flow in the upper portion of the containing vessel in accordance with the teaching of Hukki British Patent No. 962,386 of July 1, 1964, and the corresponding Hukki US. Patent No. 3,351,195.

As a second factor, pulp is preferably introduced into the agitation zone by means of a pipe sloping downwardly through the upper portion of the containing vessel and terminating immediately above such zone in an elongate discharge opening that cuts across the pipe on the bias and extends substantially vertically to discharge a roughly Iglassified feed, with the fines above and the coarse material e ow.

A third aid to preclassification is the removal from the pulp of the air that is inevitably entrained in the feed and which, if not otherwise removed, would rise to the surface of the body of pulp in the form of bubbles capable of picking up and carrying, into the vicinity of the screens oversize particles which would otherwise not reach the screens. This is effectively accomplished in accordance with the invention by venting the feed as it enters the body of pulp in the containing vessel.

Venting is preferably accomplished by a bias-cut vent pipe similar to the pulp feed pipe and extending similarly and desirably in diametric opposition thereto.

Preclassification is preferably further facilitated by utilization of various other features of the aforementioned Hukki British Patent No. 962,386, and the corresponding Hukki US). Patent No. 3,351,195, especially the use of a conical formation for the bottom portion of the containing vessel and the use of multiple baflies above the agitation zone for inducing laminar flow. In the latter connection, it should be realized that the pulp feed pipe and the air vent pipe effectively serve, in and of themselves, as bafiles for braking spiral motion of the pulp as it rises in the containing vessel.

Although preclassification is very important for the attainment of best results, it to couple this with special precautions against blinding of the screens by near-mesh-size solid particles which tend to wedge in the screen openings. Ordinary vibration of the screens has been found to be ineffective under the circumstances.

In accordance with the invention, the screens are resiliently mounted for high frequency reciprocatory vibration along respective lines approximately normal to the surfaces thereof, and each screen is preferably provided with an overlying and contacting whipping screen of larger mesh, which vibrates at a different frequency and beats against the underlying, sizing screen. In addition, the screens are advantageously given a rocking movement of considerably lower frequency, as by offsetting the mounting of the vibrators with respect to the screens, and the downstrokes of such rocking movement are advantageously abruptly halted, i.e. interrupted, in a pounding action which tends to dislodge any particles wedged in the mesh.

It is also strongly advisable to guard against blinding of the screens by trash, such as wood chips, 'bits of roots and other vegetation, paper, blasting wire insulation, etc.

This is accomplished in accordance with the invention by overflowing part of the rising pulp from the containing vessel, rather than attempting to pass all of it through the screens. Advantageously, controlled quantities of the pulp rising within the containing vessel are directed away from the screens and toward overflow weirs, and other portions of the pulp in the vicinity of the screens are overflowed from the containing vessel laterally of such screens.

From the standpoint of apparatus, rising pulp is directed away from the screens by the provision of battles extending upwardly from the lower or classification portion of the containing vessel to at least overflow-weir height and placed intermediate of the respective spaces between screens and adjacent walls of the containing vessel, so as to divide such spaces into respective sets of separate flow passages, one of such passages leading to the particular screen concerned and the other leading to the overflow weir provided by the particular lateral wall concerned of the containing vessel. A damper type of valve within the latter passage provides the desired quantitative control of flow over the weir.

The baffles and the passages formed thereby perform an additional function, in that they accelerate the rate of settling of the oversize solid particles and thereby facilitate their collection in and discharge from the lower part of the containing vessel.

A feature of the invention, so far as apparatus is concerned, is the fact that the screens can easily be replaced by imperforate plates when it is desired to utilize the machine strictly as a classifier.

There is shown in the accompanying drawings apparatus presently regarded as the best mode of placing the inven tion into practice. From the detailed description of this specific embodiment, other objects and features of the invention will become apparent.

In the drawings:

FIG. 1 represents a fragmentary, cut-away, perspective view of the apparatus, showing various parts in vertical section and revealing internal structure;

FIG. 2, a top plan view drawn to a smaller scale;

FIG. 3, a front elevation, showing certain internal parts by dotted lines;

FIG. 4, a right side elevation taken from the standpoint of FIG. 3",

FIG. 5, a fragmentary vertical section taken on the line 5-5 of FIG. 2 and drawn to a considerably larger scale;

FIG. 6, a similar section taken on the line 6-6 of FIG. 2;

FIG. 7, a fragmentary view showing one of the screens, its mounting, and its vibrator in front elevation;

FIG. 8, a right side elevation of the structure shown in FIG. 7, except for the vibrator, which is removed to reveal parts in the background;

FIG. 9, an end elevation of the structure shown in FIG. 8, except for the vibration bar and related components; and

FIG. 10, a transverse vertical section taken on the line 1010 of FIG. 7.

Referring to the drawings:

The method of the invention is best carried out by means of the apparatus illustrated, which comprises a containing vessel 12 for pulp to be treated and a neutral, i.e. nonpumping, impeller 13 mounted on the lower end of a vertical impeller shaft 14. This shaft reaches deeply into the vessel 12 and places the impeller in the lowermost section 15 of the vessel, which is of conical formation having its apex directed downwardly and provided with a discharge opening 16 for coarse sands that descend in the vessel.

Above the conical section 15 of the vessel 12 is an intermediate section 17, preferably of cylindrical formation, in which are positioned closely spaced, radial vanes 18 that define an annular series of flow channels 19, through which pulp introduced into the containing vessel rises. The vanes 18 serve as brakes with respect to the circular flow induced by the impeller and the flow through channels 19 tends to be laminar, all substantially in accordance with the teachings of the aforementioned I-Iukki British Patent No. 962,386 and the corresponding I-Iukki U.S. Patent No. 3,351,195.

Above intermediate section 17, the vessel 12 flares outwardly in a top section 20 for the purpose of accommodating screening assemblies 21 and related structure, which includes a pair of mutually spaced launders 22 and an interconnecting transverse launder 23, FIG. 2, for receiving and discharging outflowing material that has passed through the screens from flow passages 24a and 25a, FIGS. 1 and 3; also baflies 24 and 25 for directing trash away from the screens and into by- pass channels 24b and 25b, respectively, FIGS. 1 and 2; also weirs 26, FIGS.

1, 2, 3 and 5, and 27, FIGS. 2 and 6, for overflowing, from the interior of the vessel and from by- pass channels 24b and 25b, respectively, pulp carrying such trash; also a pair of launders 28 and an interconnecting transverse launder 29 for receiving and discharging outflowing material which overflows weirs 26 and 27; and also weirs 30 and 31 (see especially FIGS. 5 and 6) for overflowing, into launder 29, some of the material which would normally be directed against the screens, so any trash which does flow towards the screens will tend to be diverted by lateral surface flow along the entry face of the screen and by overflow into launder 29.

As illustrated, the top of vessel 12 is open, but it could be closed if for any reason this is necessary or desirable.

Impeller shaft 14 is mounted in any suitable manner, for example, by means of bearings 32 secured to superstructure 33 of supporting framework 34, which mounts vessel 12 and its associated structure. It is driven at selected speeds in any suitable manner, as by means of an electric motor 35 mounted on a pivoted supporting structure 36, FIG. 2, having crank and screw mechanism 37 for swinging it toward or away from driven pulley 38 rigid on the impeller shaft, and, further, by means of a variable speed reducer in the form of a variable pitch drive sheave 351 having possible variation in pitch diameter of, say, 3 to 1.

For feeding pulp into vessel 12 and concomitantly removing entrained air from the feed in the desirable manner (from the standpoint of particle preclassification) previously indicated, pipes 49 and 41, respectively, are run downwardly through the open top of the vessel, on mutually opposite inclines and in diametric mutual opposition, to respective terminations immediately above impeller 13 in vertically elongate openings 40a and 41a, respectively, FIG. 1', formed by cutting the ends of such pipes on the bias. The upper end of feed pipe 40 is connected to a source of supply of pulp (not shown) in any suitable manner, and the upper end of vent pipe 41 is left open to the atmosphere, as shown in FIGS. 1 and 4, for the dissipation of air removed from the pulp entering vessel 12.

Attached to the downwardly directed apex 15a, FIG. 1, of conical section 15, positioned over and controlling underflow discharge of coarse sands or other coarse particles from opening 16 thereof, is a variable orifice valve 42, FIGS. 3 and 4, which may be any one of many readily obtainable on the open market.

Discharge nipples 43, FIGS. 3 and 4, lead from launder 23 for connection with suitable discharge piping (not shown) for the sized material which passes the screens. Similarly, discharge nipples 44 lead from launder 29 for connection with suitable discharge piping (not shown) for the trash-carrying overflow. The latter piping will usually recirculate the pulp to the feed supply after passage through some suitable device for separating the trash therefrom. In instances where the apparatus is used for classification without screening, as previously indicated, the pulp discharging from nipples 44 can be run into the flow from launder 23 following elimination of the trash.

Baffles 24- are preferably longer than baffles 25 and are bent transversely, to strategically place their lower ends above and with respect to the laminar flow channels 19. The lower ends of baffles 25 are similarly strategically placed to intercept the upward, substantially laminar flow of pulp.

The flow channels 24b, formed between the respective baflles 24 and walls of the top section 20 of vessel 12, are preferably provided with butterfly or other type of damper valves 46, FIG. 3, which afford control of the quantity of pulp flowing upwardly through such channels and aid in controlling the pulp level in the vessel and separation size of solids in the pulp flowing over the weirs 26. It is entirely possible in some instances to so control such separation size with this arrangement that the overflow will be compatible with the screened material discharging from launder 23, in which case the discharge from launder 29 can be run into the screened material discharge from launder 23, following removal of trash, to greatly increase the output capacity of sized material.

The weirs 26 and 27 are preferably adjustable in height in well known manner, for example, by the rovision of removable and replaceable weir bars or plates. Weirs 30 and 31, FIGS. 5 and 6, are topped by openings 47 and 48 formed through otherwise imperforate walls of the top section 20 of the vessel 12, laterally of the baffles 24 and 25, respectively, and communicating with launder 29.

The screening assemblies 21, see particularly FIGS. 710, are of unique structural and functional character for the purpose of dislodging particles of near-mesh size that tend to become wedged in the mesh openings of the screens. These assemblies each include sizing and "yyhipping screens, 49 and 50, respectively, FIG. and

mounting, vibrating, and pounding means therefor.

The sizing wire cloth of screen 49 and the overlying, coarse mesh, whipping wire cloth of screen 50 are applied to their respective rectangular frames 49a and 50a in tensioned condition, as by soldering them, tightly stretched, to such frames. The resulting screens are placed in a receptacle frame 51, with the wire cloths face-to-face, as shown in FIG. 10.

Receptacle frame 51 is secured to the inside margin of a resilient mounting frame 52 of rubber or other suitable material providing a floating support capable of vibrating back and forth at high frequency, e.g. 7500 strokes per minute. Securement is advantageously accomplished by bolts 53, which pass through frames 51, 52, and a retainer frame 54.

Resilient mounting frame 52 of each screening assem- -bly 21 is secured, along its outside margin, to the inside margin of a rigid receiving frame 55, FIGS. 1 and 10, forming part of the structural framework of the apparatus. Securement is advantageously accomplished by means of bolts 56, which pass through a retainer frame 57, and through frames 52 and'55.

The screens are preferably clamped tightly into place in receptacle frame 51 for rapid removal and replacement when it is necessary to change the wire cloths. Single cam type of tool-makers clamps 58, having throw levers 58a, are advantageously utilized for the purpose. They are fastened by means of bolts 581 to mounting brackets 59, rigidly carried by receptacle frame 51, and

have respective clamping members 58b. Certain of such clamps are equipped with offset lugs 58c, for bearing down against the whipping screen frame 50a, while another is positioned to bear down upon one end of a transverse vibration bar 60 extending across the width of the screens intermediate their lengths and resting directly upon whipping screen frame 50a.

The other end of vibration bar 60 is firmly held down upon the screens by means of a bolt 61 whose shank is welded to receptacle frame 51, see FIG. 10. Resilient mounting frame 52 is put in tension by adjustment of tuning bolts 62 acting in cooperation with anvil bolts 63, the former being secured in vibration bar 60 and the latter in retainer frame 57.

Mounted on vibration bar 60 is a conventional type of vibrator 64, which may be either pneumatically or electrically actuated. In order to obtain a desirable rocking and pounding action during the vibration of the screens, vibrator 64 is offset from the longitudinal axis of the screens and is raised above bar 60 on an inclined post 65 rooted on such axis, so the vibratory action is directed along an axis that is inclined to, rather than normal to, the face planes of the screens, as is clearly indicated in FIG. 10. The heads of bolts 62 and 63 are desirably case hardened, so as to withstand the pounding action there-between when the ''ibi'atory-rocking strokes of the screen are suddenly interrupted thereby.

For making it possible to quickly and easily remove and properly replace vibrator bar 60, the shank of clamping bolt 61 is passed through a marginal notch (not shown) in bar 60, and an alignment .post 66, FIG. 10, at the opposite side of receptacle frame 51, is accommodated by a receiving hole through bar 60. It is only necessary to loosen the nut of bolt 61 on its shank and to loosen the proper clamp 58 in order to pivot bar 60 about post 66 and lift it free of bolt 61 and post 66 for removing or replacing the screens.

In operation, pulp is continuously fed through pipe 40 into the lower part of containing vessel 12 immediately above impeller 13, where it is given a swirling motion by the impeller and rises in upward helical or spiral flow until such flow is broken by the vanes 18 and tends to become laminar.

Some preclassification of solid particles takes place during feed through pipe 40 and emergence through opening 40a. Additional classification takes place due to the changes in flow as the pulp rises. Elimination of entrained air through vent pipe 41 prevents interference with the classifying action.

Screening is rapid.a.ttd-. positive by reason of the bypassing from the' scr6e'as.- 'and the elimination from the containing vessel df. fiasn'which might otherwise blind the screens, and btfrea's'on of the peculiar vibrating and rocking movemen and the pounding action applied to the screens, together with the beating action of the whipping screens onthe sizing screens.

Control of pulp level by means of the weirs 26 and 27 and by means of the damper valves 46 enables reajsionably close size control of solid particles in the over- In those instances where it is desired to use the machine strictly a classifier, it is only necessary to replace the screening assemblies 21 by imperforate metal plates which are bolted to the respective receiving frames in the same manner as the resilient mounting frames 52.

Whereas there are here illustrated and described procedures and apparatus which are presently regarded as the best modes of carrying out the invention, it should be understood that various changes can be made without departing from the inventive concepts particularly pointed out and claimed herebelow.

I claim:

1. Apparatus for wet sizing solid particles from a pulp containing solid particles of different sizes, comprising:

a classification vessel having means in the lower part thereof for the discharge of an underfiow product, respective means in the upper part thereof for the separate discharge of overflow products, and means in the lower part thereof for the sharp classification of solid particles in a pulp so that relatively coarse particles flow downwardly toward the underflowproduct-discharge-means and relatively fine particles flow upwardly toward the overflow-products-discharge-means;

means for introducing into the lower part of said vessel a pulp containing the solid particles to be classified;

means in the upper part of said vessel defining-separate, substantially non-communicating flow passages for conducting respective upward flows of relatively-fineparticle-containing portions of the pulp to the re spective discharge means for the overflow products, at least one of asid passages being positioned and arranged to receive most of any floating trash that may be present in the pulp introduced in said vessel; and

screen means in the path of pulp fiow through another of said passages for ermitting flow to its corresponding discharge means, of pulp containing only particles that pass through said screen means.

2. The apparatus set forth in claim 1, wherein the means for introducing the pulp into the vessel comprises '2' a feed pipe extending downwardly through the interior of the vessel, on an incline, to a termination within the lower part of the vessel, said termination being a bias cut extending vertically and defining a vertically elongate, discharge opening, so as to aid particle classification.

3. The apparatus set forth in claim 2, wherein there is additionally included a vent pipe extending and terminating similarly to the feed pipe and defining a vertically elongate intake opening for air entrained in the feed material.

4. The apparatus set forth in claim 3, wherein the classification means include a neutral impeller mounted for rotation on a vertical axis in the lower part of the vessel, and the feed and vent pipes terminate immediately above said impeller.

5. The apparatus set forth in claim 4, wherein the classification means also include radial bat'fles above the terminations of the feed and vent pipes, for braking rotary movement of pulp within the vessel and thereby aiding particle classification.

6. The apparatus set forth in claim 1, including means resiliently mounting the screen for vibration at a high frequency; vibrating means for the screen; means for rocking the screen from side to side during vibration; and means for suddenly interrupting the vibratory-rocking strokes of the screen in one direction to free said screen of near-mesh particles which tend to wedge therein.

7. The apparatus set forth in claim 6, wherein the means for rocking the screen comprises an elevated, 0&- center mounting for the vibrating means, said mounting being carried by the screen, and the axis of said vibrating means being inclined to the face plane of said screen.

8. The apparatus set forth in claim 6, wherein a second screen is closely associated with the particle-sizing screen in face-to-face, overlying relationship-therewith.

and supported by the resilient mounting; said second screen being of coarser mesh than the particle-sizing screen and adapted to vibrate at a lower frequency, so as to whip the particle-sizing screen and aid in keeping it free from blinding.

9. The apparatus set forth in claim 1, wherein the respective means for the separate discharge of overflow products include weir means laterally of the screen means and at the entry face thereof for causing a portion of the pulp flowing toward said screen means to flow across the entry face of said screen means and out of the vessel.

10. The apparatus set forth in claim 9, additionally including valve means in the said at least one of said passages.

11. Apparatus for wet sizing solid particles, comprising:

a containing vessel for a liquid pulp of said solid particles, said containing vessel having a conical lower section with an open apex directed downwardly, an intermediate section, and an outwardly flared upper section; means for supporting said vessel in a vertical position; a neutral impeller rotatably mounted in said conical lower section of the vessel on a vertical axis;

means for rotating said impeller;

a feed pipe extending downwardly through the interior of the vessel, on an incline, to a termination within the lower part of the vessel, said termination being a biased cut extending vertically and defining a vertically elongate, discharge opening so as to aid in particle classification;

a vent pipe extending and terminating similarly to the feed pipe and defining a vertically elongate, intake opening for air entrained in the feed material;

a circumferential series of radial baflles mounted in said intermediate section of the vessel and surrounding the feed and the vent pipes;

sets of screening assemblies mounted in said upper section of the vessel and including particle-sizing screens mounted so that part of the pulp overflowing the vessel must pass through said screens;

valve means for controlling discharge flow through the apex opening of the conical lower section of the vessel;

overflow weirs in the said upper section of the vessel for portions of the pulp rising in the vessel during the feeding thereof;

and baffle means dividing the upper section of the vessel into substantially non-communicating flow passages and serving to partially divert flow of pulp from the screens and toward said weirs, to carry off trash.

12. The apparatus set forth in claim 11, wherein the screening assemblies each comprise, in addition to the particle-sizing screen, an overlying whipping screen of coarser mesh; means resiliently mounting the screens for vibration at a high frequency; vibrating means for the screens; means for rocking the screen during vibration; and means for suddenly halting the vibratory-rocking strokes of the screen in one direction to free said screen of near-mesh particles which tend to wedge therein.

13. A screening assembly for the wet sizing of solid particles comprising:

'a' particle-sizing screen;

means resiliently mounting said screen for vibration at a high frequency;

vibrating means for the screen;

means mountingthe screen for rocking during vibration;

and means for suddenly interrupting the vibratoryrocking strokes of the screen in one direction to free said screen of near-mesh particles which tend to wedge therein.

14. The screening assembly set forth in claim 13,

wherein the means mounting the screen for rocking comprises an elevated, elf-center mounting for the vibrating means, said mounting means being carried by the screen, and the axis of said vibrating means being inclined to the face plane of said screen.

15. The screening assembly set forth in claim 13, wherein a second screen of coarser mesh is mounted in face-to-face, overlying relationship with the particle-sizing screen, for vibration at a different rate than the latter screen and so as to whip said latter screen during vibration of the assembly.

16. A method for wet sizing solid particles from a pulp containing solid particles of different sizes, comprising introducing said pulp into a particle classification vessel;

subjecting the pulp to sharp classification in the lower part of said vessel to produce an underflow portion of the pulp made up predominantly of relatively coarse particles and an overflow portion ofthepulp made up predominantly of relatively fine particles; dividing said overflow portion into separate, substantially non-communicating flows in the upper part of said vessel, at least one of which carries most of any floating trash that may be present in said pulp; passing another of said flows to screening means; screening a portion of said another of said flows; overflowing from the vessel the said one flow with whatever floating trash it carries; and separately overflowing from the vessel the said portion, of the said another of said flows, that passes through said screen.

17. The method set forth in claim 16, including the additional step of Whipping the entry face of the screen with another screen of relatively coarse mesh during the screening operation.

18. The method set forth in claim 16, including additional steps of passing the portion of the said another of said flows that does not pass through said screen across the entry face of the screen; and

the

separately overflowing said portion from the vessel at a location in advance of said entry face of the screen to aid in keeping said entry face clean.

19. The method set forth in claim 18, including the additional steps of combining the discharge-d one flow and the portion of the said another flow that is separately overflowed in advance of said entry face of the screen;

removing any floating trash from the pulp of the combined flows; and

returning the trash-free pulp of the combined flows to the vessel along with the pulp introduced into said vessel,

20. The method set forth in claim 16, including the additional steps of removing any floating trash from the pulp of the discharged one flow; and

returning the trash-free pulp of said discharged one flow to the vessel along with the pulp introduced into said vessel.

References Cited UNITED STATES PATENTS Stanley 209-157 Stanley 209-17 Reynolds 209-403 X Glasco 209365 X Gisler 209-401 Katz 209159 Hukki 20917 Nickel 209-4125 Case 209383 X Humphreys 209-211 FOREIGN PATENTS FRANK W. LUTTER, Primary Examiner.

US. Cl. X.R.

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