US2377524A - Method of and means for separating solid particles in pulp suspensions and the like - Google Patents

Description

June 5, 1945. E. w. SAMSON ET AL 2,377,524

METHOD OF AND MEANS FOR SEPABATING SOLID PARTICLES IN PULP SUSPENSIONS AND THE LIKE Filed Nov. 21; 1939 2 Sheets-Sheet l FIGJ.

INVENTORS EH! SAMSON A. b- GROUP ATTORNEY June 1945. E. w. SAMSON ET'AL 2,377,524

METHOD OF AND MEANS FOR SEPARATING SOLID PARTICLES IN PULP SUSPENSIONS AND THE LIKE Filed Nov. 21, 1939 2 Sheets-Sheet 2 498 um'rs INVENTORS 6'. 14 SAMSON 45 AJACHOUP ATTORNEY Patented June 5, 1945 METHOD AND MEANS FOR SEPARATING SOLID PARTICLES IN PULP SUSPENSIONS AND THE LIKE Edward W. Samson and Alfred B.

Group, Erie,

Pa., assignors to Hammermill Paper Company, Erie, Pa., a corporation of Pennsylvania Application November 21, 1939, Serial No. 305,449

' 20 Claims. ('01. 92--28) This invention relates to a centrifugal method and means of separatingfrom or in a fluid suspension, solid particles whose reactions to centrifugal force are only slightly different from each other or from that of the suspending fluid.

These centrifugal force reactions depend upon the relative specific weights of the particles but they depend very largely also upon the shape and other factors which play a part in the ease or diificulty with which the particles are propelled by centrifugal force through the fluid.

In the purification of pulp for the manufacture of paper, an extremely diflicult problem is presented by certain impurities or forms of dirt of a character unsuited for removal by screening and whose specific gravity or reaction to centrifugal force are so close to that of the pulp fibers themselves as to seriously hamper centrifugal separation. Such impurities include so-called pitch, i. e., resinous and fatty materials, and also fine gritty materials, bark and, to some extent, shives. Shives, which are small bundles of fibers themselves that have not been fully separated and which, if left in the pulp, will produce a defect in the paper, have a reaction to centrifugal force which is so nearly the same as that of the individual fibers that they are hardly separable by centrifugal action from the regular fibers. This is particularly true of the longer shives, but these, fortunately, may be removed fairly well byscreening. The present invention is capable of eliminating a part of the shives. especially the shorterones, and is particularly adapted for the rapid and efficient separation and removal from the pulp suspension, at convenient mill consistencies, of a large percentage of the other impurities mentioned, either before or after the addition of paper-making ingredients, such as size, dyes, and fillers.

It should be pointed out that numerous factors enter into the ease or difficulty with which solids may be removed by centrifugal action from liquids. Specific gravity has, of course, some effect but particles which are relatively heavy may be less responsive to centrifugal force than lighter particles. The shape and nature of the surface of the particle have an important .bearing. Round, smooth particles respond more rapidly to centrifugal action than flat or elongated part cles, particularly when the latter have rough surfaces. For this reason pulp flbers,'which are considerably denser than many of the impurities in pulp. are less responsive to centrifugal action than the impurities. An accurate comparison between the responsiveness of different types of 65 particles to centrifugal action can only be made by reference to some standard equivalent particle, such as one of spherical form, of given size or range of sizes, and of given specific gravity. Thus, the impurities which the present invention is particularly designed to remove from pulp may be said to be equivalent to spherical particles'of 40 to mesh size, having a specific gravity of 1.01 to 1.08 and capable of settling in still water at the rate of between 5 and 25 centimeters per minute. In fact, by the invention, such equivalent particles having a settling rate down to 3 cm. per minute may be removed from the pulp to a certain extent but particles of slower settling rate are hardly separable. In this connection it may be stated that it is considerably more difficult to separate particles of a, given. type from a. pulp suspension than from plain water, this being due to the interference offered by the fibers of the pulp to the movement of the dirt particles. This interference, moreover, increases with the consistency of the pulp. For the removal of the finer dirt particles it is desirable, therefore, to dilute the pulp, before the centrifugal separating action, as far as practicable; it is hardly practicable, however, to deal with pulp of a consistency less than 0.2% and ordinarily it is not desirable to go much below 0.5% consistency for economic reasons. Convenientmill consistencies usually run between 0.3 and 1.0 per cent, although so far as the invention is concemed good results in the separation of impurities-from pulp may be obtained at consistencies of from 0.1%,

or even less, to about 1.5%, or slightly more.

For practical purposes it may be said that the settling rate of particles in still water gives in itself a fair indication as to their reaction to centrifugal force since this settling rate is affected by considerations quite similar to those which enterinto the centrifugal force reaction. Now, since some of the impurities which the present invention is capable of effectively removing from pulp at convenient mill consistencies will settle in still water at a rate as low as 5 centimeters per minute, or even lower, and since many of the fibers or small flocks of fibers and other desirable ingredients have almost the same settling rates, the difliculty of removing the impurities by centrifugal action will be appreciated. In fact, due to the overlapping between impurities and fibers in their responsiveness to centrifugal force, it is not possible to effect a clear and complete separation between them. All that can be expected is to produce a dirty fraction, containing a large percentage of the impurities and a smaller percentage of fibers, and a clean fraction, containing a large percentage of the fibers and a, small percentage of the impurities. Incidentally, the centrifugal action contemplated by the invention serves to break up the larger flocks normally existing in pulp, and which would have a much higher settling rate, but it probably does not break up, completely, the smaller flocks.

More particularly, the invention involves the subjection of a stream of pulp or other suspension to centrifugal action, induced by the stream itself, without revolving mechanical devices, under such conditions as to bring about the continuous separation of a substantially purified pulp or other suspension or fluid from a fraction containing most of the impurities. This separation, in the case of pulp purification, may be effected just after the coarse screening of the pulp in the mill or at some subsequent stage. It may, if desired, be eflected just in advance of the delivery of the pulp to the paper machine, in which case the impurities must be separated not only from the fibers but also from the other paper-making ingredients.

Devices have heretofore been employed or suggested for the separation of dirt or dust from air or other gases and for the separation of relatively heavy particles, such as sand, grit and the like, from liquid suspensions. Some of these prior devices or suggestions have involved centrifugal action induced by the tangential discharge of a stream of the gas or liquid into a circular chamber or vessel. But none of these prior devices are adapted for the removal of particles having so nearly the same specific gravity'as the suspending fluid nor for the separation between particles in suspension having so nearly the same reaction to centrifugal force as do the particles which the present invention is intended to separate from each other or from the suspending medium.

Briefly, the invention calls for the tangential introduction of the pulp, or similar material to be Purified, into a chamber having a small, substantially cylindrical portion at'its top and a long,

' conical, gradually tapered portion at the bottom.

An outlet for the constant withdrawal of a fraction, containing the bulk of the impurities, is provided at the bottom of the chamber and an outlet for the purified material, which preferably extends downwardly into the chamber, is provided at the center of the top. The construction and arrangement is such that as the material, for example a water suspension, is introduced into the chamber at a relatively high velocity, in the neighborhood of 25 feet per second, a vortex or vortical whirl is created and a centrifugal force is set up upon the particles, those having a slightly higher specific gravity, or which are otherwise thrown more readily and forcibly outward, being brought against or near the wall of the cone while the lighter particles, or those affected less by the centrifugal action, are carried or remain more nearly toward the inner surface of the downwardly moving and rotating stream. At the same time the vortex or vortical whirl causes an upwardly moving and whirling stream at the center of the chamber within the downwardly moving stream.

It should be stated at this point that in mentioning the upward and downward directions, we refer to the operation of the device with'its axis vertical and the point of the cone at the bottom. The device will, however, operate substantially with equal efliciency regardless of the disposition of its axis and regardless of whether the cone points upwardly or downwardly or strictly horizontally. Gravity has substantially no eflect pon the separating operation. Our reference to the direction of movement of the streams and the relative disposition of the parts of the device is simply for convenience in explaining the operation.

Now, it will be understood that the centrifugal force acting upon the particles varies directly with the square of the velocity and inversely with the radius of the path of the motion. The velocity tends to decrease slightly as the material advances toward the apex of the cone, due to the retarding effect of friction, but this may be more than offset by the tendency to increase the velocity as the pressure of the fluid drops and is converted into kinetic energy. It will be understood that a substantial pressure drop will take place between the inlet and outlet of the chamber. At the same time the radius of the whirling motion is decreasing, with the effect of increasin the centrifugal force. Therefore, in a device properly constructed for maximum efficiency, the centrifugal force upon the particles will not decrease but will, in fact, increase as the material progresses along the conical wall. Moreover, whatever increase takes place will be more or less uniform, i. e., there will be no abrupt change.

This centrifugal action tends to bring about a desired separation between the slightly heavier impurities or dirt particles and the regular fibers and other paper-making ingredients. The heavier particles will be thrown to the wall of the chamber on the downward whirling movement of the mass but some of the effectively lighter impurities which are more nearl like the fibers themselves in their response to centrifugal force will be carried by the inward component of the velocity of movement of the material into the upwardly moving stream at the center of the vortex. Since the centrifugal forces in this stream are even greater than in the outer stream, some of the impurities carried into the upwardly moving stream will be thrown out again into the downof an elongated cone having an exceedingly gentle taper, the inward component of the velocity is reduced to a minimum and the separation of a maximum portion of the impurities is made pos- S1 e.

The upwardly moving stream at the center of the vortex has a hollow core extending from the lower to the upper outlet. If the lower'outlet is in free communication with the atmospherathe interior of the hollow core is maintainedat substantially atmospheric pressure but if the lower end ofthe device is immersed in liquid, there is a tendency to produce a sub-atmospheric pressure within the core due to the removal of air bubbles in the stream leaving the top of the device. This has the further tendency of reducing the diameter of the hollow core by the pumping up of liquid from below the lower outlet, thereby al so tending to carry dirt back into the ascending stream.

It has been discovered that a most effective separation of the dirt from the pulpfibers may be brought about through the maintenance of a constant, positive outward the device.

flow at the apex of This positive outward flow causes a large percentage of the dirt particles to be washed outwardly through the opening at the apex and prevents this dirt from being swept into the ascending vortex. When the dirt to be removed reacts very nearly like the fibers to centrifugal force, it is desirable to maintain a sub: stantial outward flow at the apex so that a fairly high percentage of fibers will be removed along with the greater part of the dirt. The fibers may then be recovered by appropriate fractionation, as will be explained. It the apex of the device is arranged to discharge into the free atmosphere, so that the center or the vortex is filled with 'air at atmospheric pressure, the outflow at the apex need not be quite as reat as if the latter is immersed in the liquid and an opportunity is afiorded for the dirt to be carried back into the ascending stream.

To insure a suificiently rapid outflow of the material at the apex to prevent the dirt from being swept into the ascending vortex, it is important to provide a relatively small opening at the apex, i. e., one whose diameter isnot more than about /1, and preferably between /a and /20, of

the major diameter of the device. If the opening were much larger than this. too large a percentage of the total input with its proportionate share of the good fibers would need to be discharged through the apex to maintain the desired rate of flow or velocity and the device would be ineliicient for its intended purpose.

, A further tendency to interfere with the desired separation of the impurities results from the turbulence of the stream. In order to bring about the required centrifugal forces and to render the device practical for use in the separation of impurities from large quantities of pulp, it is necessary that the flow of the material within the ,device he at such a velocity that a certain amount ofturbulence is produced. This turbulence, of course, tends to stir up the particles suspended in the stream and thus partially oil'sets or interferes with the separating effect of the centrifugal action. The extent to which turbulence is created and interferesw'ith the desired separation depends upon the construction and condition of the vessel itself. To minimize the turbulence efiect it has been found important to reduce friction as far as possibleand to avoid any sudden applica tion of forces tending to abruptly changethe direction of movement of the suspension. It has been found that best reslllts are obtainable by eliminating all obstructions from the interior of theseparating chamber and by bringing about a continuous, smoothflow of the material within. and out of the chamber. i

In view of the foregoing and other considera-' tions. the eii'ectivev andefiicient elimination from pulp of impurities of the type indicated above requires, the observance of certain rather definite relationships between various parts of the de-,

vice. Thus, the size of the discharge opening at the apex ofthe conical portion of the chamber, in relation to other factors, is of considerable imefiiciency will be discussed in tain parts, the conditions or operation and the of eliminating a certain percentage of a standardized material constituting the impurityfl-i'rom a given quantity of a standard type of Pulp in suspension. All factors, such as the recover of fibers from the dirty fraction, have been taken into account in making the emciencydeterminations. The bearin 01 certain factors upon such reater detail hereinafter.

we have found that a very substantial purification of a pulp suspensiomand one highlyefilcient from the standpoint of cost, maybe brought about through the continuous passage oi' the same through a single separating chamber of proper form, such as hereinafter more fully described, the impurities being continuously withdrawn at the apex of the conical portion of the chamber and containing between and 95 per cent of the particles of impurities in a fraction 'constituting about 20 per cent by dry weight of the entire pulp introduced into the chamber. The remaining 80 per cent of the pulp containing between 5 and 40 per cent of the impurities may be continuously withdrawn at the opposite end of said conical portion. In general, the

lighter the dirt or the more diificult it is to 'remove, the smaller is the percentage of it which will be withdrawn at the apex and the greater is the percentage which will remain in. the fraction withdrawn at the top of the device, under given operating ,conditions. If further-purification of the pulp is desired, the partially purified materialmay be passed through another similar separator and thus subjected to] further purification. As many successive treatments of this character may be carried out as required to bring about the desired ultimate purity'of the pulp. when this fractionation method is employed in two or more stages, the separation in a single unit need not be carried to the maximum possible extent. A smaller percentage of the impurities maybe removed in each stage. 80 also,

the fraction containing the bulk of the impurities withdrawn from the apex of the conical portion of the separator may be subjected to further treatment to avoid the loss of the good fibers necessarily included in the dirty fraction. In this case, a somewhat greater portionof the original suspension may be taken as the dirty fraction in the first stage thus insuring greater purity of the clean fraction; and the subsequent repetition of the treatment upon the dirty. fraction will prevent the excessive loss of good fibers. By an appropriate assemblyand interconnection of a may be obtained with but a small loss of the desirable constituents of the original stock. Any number of successive stages of purification may be employed consistent with economical operation. For the handling of a large quantity of material, a

pluralityof units may be operated in parallel portance, as alreadyexplained. It must be such.

the result of numerous experiments we have dis- 1 covered that changes in the dimensions of cer,-

for eachstage of the treatment, a smaller numher being required for the successive stages.

Other features, objects and advantages of the invention will appear from the detailed description of an illustrative form of apparatus, particularly suited for use in carrying out the invention, which will now be given in conjunction with the accompanying drawings, in which arrangement and interconnection of a plurality of separating units designed for the handling of a large quantity of pulp and purifying it in successive stages.

As best shown in Figs. 1 and 2, the separating chamber is preferably of very simple construc-' tion, having a substantially cylindrical top portion 10 and-an elongated, conical, lower portion H. The inner wall oi?v the chamber, particularly the inner surface of the conical portion H,

is made as smooth as possible by "appropriate.

machining, grinding, electroforming or the like,

so as to reduce the frictional resistance to the flow of the pulp to a minimum. The chamber is formed, preferably, of a suitable metal capable of resisting the corrosive action of the pulp i or other material to be handled. If desired, the inner surface of the chamber may be simply coated or otherwise protected for this purpose.

An inlet I2 is arranged to permit the tangential introduction of the material to be purified or fractionated into the cylindrical portion of the separator; This inlet may be disposed in a plane perpendicular to the axis of the chamber or may be inclined slightly toassist in directing the streamof material toward the apex of the cone as it whirls around the inner surface of the chamber. Other special arrangements for insuring the smooth and uniform introduction of the material tangentially into the chamber may be employed, if desired. At the lower end or apex of the conical portion ll there is provided From these pressures'the various dimensions and the nature of the material, the velocity and other conditions of flow of the material at various points may be determined with the aid of empirical data.

While the specific dimensions of the separator may be varied considerably within limits, particularly when a change in one dimension is oflset by corresponding changes in other dimensions or in operating conditions, it has been found that very good results may be obtained from a construction in which the cylindrical'portion or the chamber is approximately 3'inches in diameter and about 1% inches in depth while the conical portion, considering the apex as if extended to a point, has an altitude'of about 33 inches. In this typical construction the inlet I2 may have a diameter of approximately half an inch while the outlet l3 may be slightly less than a quarter of an inchin diameter and the outlet I! may have a diameter of slightly more thanhalf an inch. The outlet tube IS in this typical unit extends to a point about 1.75 inches below the center line of the inlet I2, andabout three-quarters of an inch below the juncture of the'cylindrical and conical portions. The center line of the inlet 42, in turn, is about three-eighths of an inch below the end l6 of the chamber. already explained, these dimensions are simply those of a construction which has been demonstrated to be particularly satisfactory but the invention is not to be considered as'restricted to' the employment of these specific dimensions. A device with the particular dimensions specified operates best when handling pulp of a consistency of about 0.5 per cent. In dealing with pulp of a materially different consistency or character, certain changes are desirable. Inde'aling with beaten pulp the optimum size of the lower outlet I I is an opening i3 which may, if desired, be variable I or adjustable as to size,'although it should be kept constantly open to permit the continuous discharge of a portion of the material introduced into the chamber. A pipe l4 may be connected in any convenient way with the tip of the conical portion, to receivefthe. material discharged through the opening 13 and convey it to a closed chamber (not shown) or to any other desired point. The pipe l4 should be so formed and consomewhat smaller than forunbeaten pulp.

Now, in a typical operation of the particular separator specified, let us assume that the pulp, at a consistency of about'0.5 per cent, is introduced under a suitable pressure of, say, 38 pounds nected as not to present any sort of obstruclargerpipe I40 which is. spaced from the cone to allow-free passage of air therebetween. The pipe M0 may be supported in any convenient way, as by means of a bracket l4! engaging a flange I42 on the pipe. Beneath the lower end of the pipe there may be'provided an open trough or tank I43 to receive the fraction discharged through the opening 13.

At the top. of the chamber an outlet tube I5 is provided at the center or a closure member l6 which otherwise closes the top of the cylindrical portion In. The tube l5 preferably extends into the chamber to aP int inward of the axis of the inlet pipe i2. Suitable pressure gauges l1. l8 and I9 may conveniently be provided in the several tubes l2, I4 and I5, respectively, to en- I able the determination of the pressure of the material entering the chamber at the inlet and being withdrawn from the chamber at the two outlets.

' the apex of the cone.

per square inch at such a velocity as to cause a flow of about 18 gallons per minute. Due to the tangential introduction of the material, a whirling motion is created within the chamber, giving rise to a vortex. Thus'the material travelscircumferentially around the inner wall of the vessel and at the same time advances axially toward Centrifugal force tends to throw the suspended particles outwardly against the wall of the vessel and, as explained, this tendency is slightly greater with respect to the impurities. At the same time the inward taper of the conical portion ll ofthe vessel exerts a gradual inward force upon the material, which urges it toward the outlet I3. By thus providing a gradual and continuous inward taper from a point adjacent the inlet of the material toward the outlet l3, there is avoided any abrupt' change in the movement of the material and hence any asrasac centrifugal action, will be withdrawn continuously through the outlet II. In a typical operation of good cfiiciency the pressure of the material so discharged may be about 12 pounds per square inch.

It has been found that in the operation of the unit just described, utilizing as the pulp material to be purified a standard, bleached, unbeaten sulfite pulp, made from spruce and balsam wood, containing a standardizedtype of impurities, (l. e., 40 to 60 mesh spherical rosin wax particles having a specific gravity of about 1.02 and a median settling rate in still'wat'er. of 10 per minute, with 10 per cent settling slower than 7.4 cms. per minute), it 18.7 gallons per minute are introduced through the inlet l2, at 0.494 per cent consistency, 2.21 gallons containing 94.7 per cent of the impurities may be withdrawn at the outlet I! along with 23.1 per cent of the good fibers while the remaining portion of the original charge is withdrawn through the outlet ii.

Another unit which has been found even more efilcient for such light dirt, when operated under the same conditions as specified above with respect to pulp, consistency and pressures, has the following dimensions, referring to Figure 1: D=3.08 in.; 1:46 in.; k=0.375 in.; h=1.01 in.; s=1.75 in.; a=0.50. in.; :056 in.; and g=0.25 in. In a typical run on this unit at a pulp consistency of 0.486 per cent, making use of the same standardized impurities as in the last mentioned experiment, 97.4 per cent of the impurities were emitted at the outlet 13, along with 32.8 per cent of the good fibers, and 21 per cent of the volume input of 20 gallons per minute. When operating similarly with pulp at a consistency of 0.495 per cent containing dirt hav-' ing a median settling velocity of 12.7 cms. per minute, with per cent settling less than 8.6 cms. per minute, and a specific gravity of about 1.035, the fraction withdrawn at outlet l3 contained 99.8 per cent of the impurities and 33. per cent of the good fibers.

. 5 ing upon the character of the impurities, the

consistency of the pulp, the pressures to be main-- tained at the inlet and outlets, and certain relationships between the dimensions themselves. The efiiciency of the device could be kept above of the maximum with D varying between 2.0

and 4.2 inches, 1 varying between 17 and 46 inches, k varying between 0.5 the value of a and 6 inches, h varying between 0.5 the value of a and 7 inches, 3 varying between 0 and 4.5 inches, a varying between 0.175 and 0.65 inch,

5 varying between 0.37 and 0.75 inch. and g be tween 0.14 and 0.40 inch. It should ,be understood that the above specified permissible variations in any dimension are on the assumption that the other dimensions, operating conditions, type of dirt, etc., will remain the same, i. e., substantially as given for the first unit and operation described. Should two or more dimensions be varied simultaneously, the deviation from maximum efiiciency will be the cumulative effect of all the changes.' In some instances a change in one dimension may partially or completely oilset a change in another so that the efilciency varies little or even remains substantially the same but in other instances the several changes may combine to produce a relatively great deviation from maximum efliciency. By way of example, it may be stated that the various linear dimensions given for the first model described may be varied simultaneously in the same ratio from about 0.65 to 1.36 times the specified values without reducing the efilciency below two-thirds of the maximum. Taking into account this permissible range of variations in the several linear dimensions as a whole,- it will be possible to vary any single dimension through a somewhat greater total range than indicated above, without unduly reducing the efilciency of the device.

It has also been found that with a given div ameter, say D=3.0 inches, the highest efiiciencies In another operation of the same unit as used in the first of the three operations mentioned above, dealing with unbeaten, unbleached sulfite pulp, the inlet and outlet pressures being .the

same as stated above and the consistency of the pulp being 0.482 per cent, it was found that of a mixture of dirt, short shives and long shives 96.9 per cent of the visible dirt was removed along with 23.5% of the good fibers in the dirty fraction which constituted 13.4% by volume of the total input. 43 per cent of the shives were, broken up into pulp by the mechanical action of the separator and 44.7 per cent of the balance of the shives were removed in the dirty fraction.

Our experiments indicate that under appropriate conditions the various dimensions may be varied within reasonable limits and still provide an efiicient and eflective construction for the separation of the specified impurities from a pulp suspension, particularly if a change in one dimension is offset by a corresponding change in one or more of the other dimensions and operating conditions, the optimum values dependare obtained, particularly for a light type of dirt, if the length, l is relatively great, say 45 inches. In fact, the results indicate that an even greater length may be desirable for the removal of very light dirt. Thus the diameter B may be increased to as much as five inches and the length I may be correspondingly increased, maintaining the ratio of D to l as l to 15. The only limitation upon length appears to be that it should not be so great as to increase'the .iric- 'tional resistance to such an extent that the velocity of movement and hence the centrifugal force developed would be reduced too much at the lower end of the device. The increase in efiiciency, due to increase in the length of the cone, is more pronounced for light dirt than for the heavier dirt. Mbreover, for maximum em ciency in the removal of effectively light dirt it is desirable to reduce the consistency of the pulp as the lighter particles are dealt with. It is also de sirable toincrease the percentage of fibers entering the dirty fraction. Thus, from the standpoint of expense in attaining a desired degree of purification, it has been found best to permit a greater amount of the good fibers to be taken off with the dirty fraction in each stage of a multi-stage operation as the dirt to be removed becomes ef fectively lighter. While it is preferable, as indicated by the suggested range for the dimension 8, to locate the inner end of the outlet [5 at or below the axis of the inlet I2, this is not essential. The lower end of the outlet l5 may, if de: sired, be located fiush with the closure member l6 or at any intermediate point. So also, the maximum values permissible for the h and 7:

dimensions may well exceed the limits specified without seriously reducing the efficiency of the device. The pressure of the material at the inlet, as indicated by the gauge II, may be varied widely, say from about 6 to 80 pounds per square inch, although it is preferably kept between 15 and 65; the pressure at the outlet l or b may be varied from about 2 to 18 or even 22 pounds per square inch, while the pressure atthe outlet l3 or g is preferably atmospheric. The velocity of the material entering the chamber through the inlet l2 may be varied between about and 70 feet per second. The mean outward component of the velocity over the lower outlet may be varied from 6 to 55 feet per second, and that over the upper outlet between 8 and 45 feet per second without reducing the efilciency below two-thirds of the maximum, provided the velocity in question is produced by a proper choice of dimensions and pressures and consistency within the preferred ranges given elsewhere. The volume of flow through the outlet l3 should be. between 2 and 35% of the volume of all input in order to maintain an efiiciency of the character mentioned. The result of varying the diameter of the outlet I5 is to increase the purity of the clean fraction of the stock as the diameter decreases but this is at the expense of the percentage of the total stock which is removed as the clean fraction. As lighter impurities are dealt with, it has been found desirable to use slightly larger openings, 9, at the lower outlet. While, as indicated above, certain conditions and dimensions are most favorable from the standpoint of cost efficiency, it may be said, in general, that the extent of purification of the material passing out through the outlet I 5 may be increased, at a possible sacrifice of cost efliciency, by increasing the input pressure, or by decreasing the consistency of the pulp treated, or by increasing the percentage removed through the outlet I3, as the dirty fraction, through increasing the diameter of this outlet or decreasing the diameter of the outlet l5.

Aplurality of the separators may be employed in parallel where the capacity of a single separator is insufficient to handle the amount of pulp to be purified in a given time. Moreover, when a higher degree of purification is desired than is obtainable by a single passage of the material through the separator, a system of fractionation may be employed in which a certain extent of purification is obtained in each of the successive stages. Thus, referring to Fig. 4, a typical in- .stallation may comprise a multiplicity of the separators, some in parallel to increase the capacity of the installation as whole and some in series to improve the purity of the final product and reduce the loss of good fibers. Let us as: sume that the installation is intended to have a maximum capacity of 7580 gallons per minute at 0.528 per cent consistency. The pulp to be purified is delivered through a pipe 20 to a pump or series of pumps 2| adapted to discharge the pulp at a consistency of say, 0.5 per cent through a line 22 to a plurality of separators 23, each being of the-type specifically described above. About 569 of these separators in parallel will be required to handle the 7580 gallons of pulp per minute along-with the re-cycled stock, which will be presently mentioned. The purified pulp is withdrawn through a line 24 connected with the tops of the plurality of separators 23 and may deliver the pulp to any convenient point. A fairly large dirty fraction may be withdrawn through a line 25 connected with the apex of each of the separators 23 or connected with a vessel into which the apices of the separators discharge. In a typical operation about 2 0 gallons per minute of pulp containing the removed impurities may be withdrawn through the line 25. '-This material is delivered to a pump 26, which also receives fresh water to theextent of about 1163 gallons per minute through a line 21 and delivers the mixture through a line "to a further group of separators 29 in parallel. About 180 units in the group 29 will be required to handle the material delivered. by the pump 26. The purified fraction from the units 29 may be withdrawn through a line 30 and passed to the pump 2| for mixture with the original stock from the line 20. About 2920 gallons per minute may be so delivered through the line 30. The heavier fraction, containing the impurities to be removed, will be withdrawn through the, apex of each of the units 29 and passed through a line 3| to a pump 32. About 410 gallons per minute may be so withdrawn and. diluted at the pump with about 380 gallons per minute of fresh water delivered through a line 33. The mixture is then.

delivered through a line 34 to a group of separators 35, there being about 54 units in this group. The cleaner fraction withdrawn from the tops of the separators 35 is passed through a line 36 to a pump 26 for re-cycling through the units 29. The heavier, dirty fraction from the separators 35 is withdrawn through the line 31 and passed to a pump 38 where it is diluted with Water from a line 39. In the typical operation about 877 gallons per minute of the purified material will be drawn off through the line 36 while about 123 gallons per minute will be drawn off through the line 31 and diluted with about 88 gallons per minute from the line39. This mixture is then delivered through line 40 to another group of separators 4|, thirteen being sufiicient for this stage of the operation. The purified fraction in the amount of about 210 gallons per minute is delivered to the pump 32 while the dirty fraction in the amount of about 29.5 gallons per minute is withdrawn through the line 43, connecting with the apex of each of the separators. Thus it will be seen that through the use of 816 of the small separator units, arranged in a four-stage fractionation system, pulp in the amount of 7580 gallons per minute may be purified by discarding only between 29 and 30 gallons per minute with the bulk of the impurities contained therein.

If desired the clean fraction discharged from the first group of separators 23, through the line 24', may be subjected to a further separating treatment. This is in the event that a higher purity of the final product is sought. Thus, the partially purified material may be delivered to a pump 44, which will discharge itthrough a line 45 into another group of separators 46, about 498 in number, from which the purified fraction may be withdrawn through a line 41 and delivered to any convenient point. The dirty fraction withdrawn through a line 48 may, in this case, -be deliveredto the pump 2| to be mingled with the material delivered to the first group of separators 23. Should this additional since this percentage of the original pulp will need to be replaced by thedirty fraction delivered from the separators. 40 to the pump II. It gill be understood that'under this mode of operation suilicient water should be added to the fraction returned from the separators I. or otherwise added in advance the separators II to insure the proper consistency of the material delivered to these separatorafl The number of separator units 40 could, if desired, be reduced to 430 if a pulp thickener is used tolraise the consistency of the ingoing pulp to 0.5fper cent as for the other units.

While certain illustrative forms, of separating I devices and methods of operating the same have been described in considerable detail, it will be greatest diameter, said cylindrical portion hav-;

ing an end closure member, an outlet tube having a diameter between one-eighth and onequarter thatof said cylindrical portion extend ing through said closure member and axially into said chamber, a continuously unobstructed outlet at the apex of said conical portion of ap-, proximately one-quarter'the cross-sectional area of said outlet tube, and an inlet disposed tan gentially of said cylindrical portion.

2. In apparatus for separating solid particles from liquid a centrifugal separating chamber which comprises a cylindrical portion and a conical portion in axial alinement, said conical portion having its greatest diameter between about 2 and 5 inches the length of said conical portion being between about five and fifteen times its greatest diameter and the length of said cylindrical portion being less than its diameter, said cylindrical portion having anend closure mem= her, an outlet tube extending through said closure member into said chamber, said tube having a passage therethrough of a diameter between one-eighth and one-quarter the greatest diameter of said conical portion, a continuously unobstructed outlet at the apex of said conical portion of smaller area than said outlet tube, and an inlet disposed tangentially of saidcylindrlcal portion. a

' 3. In apparatus for separating solid particles from liquid suspension a chamber having a short cylindrical portion of between 2.0 and 4.2 inches in diameter and having a long conical portion exaxiaily from said cylindrical portion toward an apex, said conical portion having a length between 5 and 15 times the diameter of gentially of said cylindrical portion and having a diameter of between 0.175 and 0.65 inch, an

of between 2.0 and 4.2 inches and.

tion extending axially from said cylindrical portion toward an apex, said conical portion having a length between 10 and times its greatest diameter, an inlet arranged tangentially of said cylindrical 'portion and having a diameter of about half an inch, an outlet at the apex of said conical portion having a diameter of about onequarter of an inch, and an outlet at the center ofsaid cylindrical portion about half an inch.

5. A method of separating, from a pulp suspension, solids having a settling rate of as low as 5 centimetersper minute in still water which comprises spinning saidsuspension to produce a vortex of gradually decreasing diameter, continuously withdrawing a portion or the suspension a diameter of I from thepoint of minimum diameter, and causameter.

ing the balance or the suspension to return freely and without obstructionin the opposite direction through the interiorot the vortex from points removed from-the point of minimum di- 6. A method of separating, from a pulp suspension, solids having a settling rate of between 5 and 25 centimetersper minute in still water which comprises spinning said suspension .in an elongated conical path to produce avortex of gradually and constantly decreasing diameter; continuously withdrawing a portion of the suspension from the point of minimum diameter,

and causing the balance of the suspension to return freely and without obstruction in the opposite direction through the interior of the vortex from points removed from the point of minimum diameter.

7. A method of separating, from, a pulp suspension, substances having a settling rate of between 5 and 25 centimeters per minute in still water which comprises spinning said suspension and causing it to advance from the base to the apex of a conical path having an angle at its apex of not more than.l5 degrees, continuously withdrawing a portion of the suspension at the apex of said conical path, causing the balance of the suspension to return freely and without obstruction toward thebase of the conical path at the interior thereof, and withdrawing at least a part of said balance of the suspension from a point along the axis of the cone.

8. A method of separating, from a pulp suspension, substances having a settling rate of between 5 and 25 centimeters per minute in still water which comprises spinning said suspension in a conical path starting with a diameter of 'said cylindrical portion, an inlet arranged tanoutlet at the apex of said conical portion having a diameter of between 0.14 and 0.40 inch, and an outlet at the center of said cylindrical portion having a diameter of between 0.37 and 0.75 inches 2.0 and 4.2 inches in diameter, a long conical pornot more than about 5 inches at its base and advancing toward the apex at an angle to the axis of said path of not more than about .7 degrees,

continuously withdrawing a portion of the sus- V pension, at the apex of said conical path, causing the balance of the suspension to return freely and without obstruction toward the base of the cone at the interior of said conical path, and

withdrawing at least a part of said balance of the suspension from a point along the axis of the cone.

9,-A method of separating, from a pulp suspension, substances having a settling rate of less than 25 centimeters per minute in still water which comprises introducing said suspension tangentially into an elongated vessel tapering gradually from a point adJacent the point of introduction toward an apex at a turbulence creating velocity so as to produce a vortex, continuously withdrawing a portion of the suspension at said apex, causing the balance of the suspension to return freely and without obstruction in the opposite direction at the interior of the vortex from points inward ofsaid apex, and withdrawing at least a part or said balance at a'point inwardly of the plane of the axis along which the suspension is introduced.

10. A method of removing'from a pulp suspen sion impurities having nearly the same centrifugal force reaction as the pulp fibers, which comprises continuously introducing a stream of'the pulp suspension tangentially into a closed chamber, confining said stream to a 'path of constantlybut gradually decreasing radius, thereby exerting a; gradual inward force in opposition to the centrifugal force generated upon the impurities, and causing thepulp suspension to move without interruption along the wall of the chamber toward thepoint of minimum radius and then in the-opposite direction adjacent the-axis of the chamber, continually withdrawing a portion at' the. point of minimum radius in the path of said stream, and continually withdrawing anpulp suspension tangentially into a closed cham-' ber at a velocity between and 70 feet per second, confining said stream to a path 01', iconstantly but gradually decreasing radius, thereby exerting a gradual inward force in opposition to .the centrifugal force generated upon the impurities, and causing the pulp suspension to'advance without interruption along the wall of thechamber toward the point of minimum radius and then in the opposite direction adjacent the axis of the chamber, continually withdrawing a portion at the point of minimum radius in the path of said stream, and continually withdrawing another portion of the material at the center of the chamber in a region of relatively great radius.

12. A method of removing from a pulp suspension impurities of nearly the same centrifugal force reaction as the pulp fibers, which comprises continuously introducing a stream of the pulp suspension tangentially into a closed chamber, confining said stream to a path of constantly but gradually decreasing radius, thereby exerting a gradual inward force in opposition to the centrifugal force generated upon the'impurities. and causing the pulpsuspension to advance without interruption along the wall of'the chamber toward the point of minimum radius and then in the opposite direction adjacent the axis of the chamber, the axial distance from the plane of maximum radius to the plane of minimum radius of said path being at least 10 times said maximum radius, continually withdrawing a portion at'the point of minimum radius in the path of said stream, and continually withdrawing another portion of the material at a point alon the axis of the chamber in a plane of relatively great radius.

13. A method of removingfrom a pulp suspension impurities of nearly the same centrifugal force reaction as the pulp fibers which comprises subjecting a continuous stream of the suspension to an uninterrupted whirling motion along an elongated conical path and then freely in the opposite axial direction within the conical path but leaving an air space in the region of said axis, continually withdrawing into free air a sufaera e-i ficient fraction of the suspension from a point adjacent the apex of said path to cause a positive outward flow, returning the balance of the suspension constituting the major fraction of the original stream adjacent the axis of the vortex toward the larger end of the conical path, and continually withdrawing at least a part of. said major fraction froma point along said axis removed from said apex.

14; A method of separating, from a pulp suspension, substances having a settling rate of less than 25 centimeters per minute in still water which comprises spinning said liquid suspension and causing it to advance from the base to the apex of a long conical path having a major diameter of not more than five inches and an angle at its apex of not more than 15 degrees, continuously withdrawing a portion of the suspension at the apex of said conical path, causing the balance of the suspension to return freely and without obstruction toward :the base of the conical path at theinterior thereof, withdraw.- ing at least a part of said balance of the suspension from a point along the axis ofthe' conical path' removed from the apex, and subject; ing at least one of the withdrawn portions to the same separating treatment.

15. Apparatus for the separation of solids from liquid suspension which comprises a pluralityof separator units, each unit comprising a vessel having an elongated conical'portion having a maximum diameter not exceeding 5 inches and a length at least five times the maximum diameter, a tangentially disposed inlet adjacent the larger endof said conical portion, and outlets adjacent the smaller and larger ends of said conical portion for the removal of fractions of the liquid suspension, means for delivering streams of said liquid suspension under pressure to a portion of said units in parallel, and means connected with the outlets at the larger ends of said units for delivering the fractions of said suspension removed through said outlets in streams under pressure to another portion of said plurality of units.

16. Apparatus for separating solid particles from liquid suspension which comprises a chamber of circular transverse cross-section, a relatively large portion of said chamber being of gradually decreasingradius from one end thereof to the other, the interior wall of said portion of said chamber being smooth and free from ob structions and abrupt changes in direction and having a maximum diameter not exceeding five inches, a tangentially disposed inlet adjacent the larger end of said portion for the introduction of a suspension to be purified, an outlet for the removal of a fraction of the suspension adjacent the smaller end of said portion of the chamber, and a larger outlet adjacent the center of the larger end of said portion for the removal of another fraction of the suspension, the length of said portion of the chamber being at least four times but not more than about fifteen times its maximum diameter.

17. A centrifugal separator for removing solids from liquid suspension comprising a chamber which consists primarily of a conical portion whose length is more than four times but not more than about fifteen times its greatest diameter, an inlet disposed tangentially of said chamber adjacent the larger end of said conical portion, an outlet at the apex of said conical portion, a second outlet through the opposite end of said chamber axially alined with said first mentioned outlet, said second outlet being 01' greater area than said outlet at the apex and having a diameter less than one-quarter the greatest diameter of said conical portion.

18. A centrifugal separator for removing solids from liquid suspension comprising a chamber which consists primarily of a. conical portion 1 whose length is more than four times but not more than about fifteen times its greatest diameter, an inlet disposed tangentially of said chamber adjacent the larger end of said conical portion. an outlet at the apex of said conical portion, a second outlet through the opposite end of said chamber axially alined with said first mentioned outlet, said outlet at the apex having a diameter less than one-sixth the greatest diameter oi said conical portion.

19, A centrifugal separator for removing solids from liquid suspension comprising a chamber which consists primarily of a conical portion whose length is more than four times but not more than about fifteen times its greatest diameter, an inlet disposed tangentially of said chamber adjacent the larger end of said conical portion, an outlet at the apex of said conical portion, a second outlet through the opposite end of said chamber axially alined with said first mentioned outlet, said outlet at the apex having a diameter less than one-sixth the greatest diamter of said conical portion, and said second outlet having a diameter less than one-fourth of the greatest diameter of said conical portion.

20. A method of removing irom a liquid suspension of solids impurities having nearly the same centrifugal force reaction as said solids, which comprises continuously introducing a stream of the suspension tangentially into a closed chamber, confining said stream to a path of constantly but gradually decreasing radius, thereby exerting a gradual inward force in opposition to the centrifugal force generated upon the impurities, and causing the suspension to move without interruption along a confining wall toward the point of minimum radius and then in the opposite direction adjacent the axis of the path, continually withdrawing a portion at the point of minimum radius in the path of said stream, and continually withdrawing another portion of the material at the center of the chamber in a region of relatively great radius.

EDWARD W. SAMSON. AIFRED H. GROUP.

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