US2625320A - Centrifuge for rapid extraction of fine particles from suspensions - Google Patents

Description

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Jan. 13, 1953 June 20, 1950 S CENTRIFUGE FO FINE! PARTICI..

Filed Patented Jan. 13, -1953 CENTRIFUGE FOR RAPID EXTRACTION F FINE PARTICLES FROM SU SPENSIONS:

Sanford C. Lyons, Bennington, Vt., assignor to Bird Machine Co., South Walpole, Mass., a corporation of Massachusetts Application June 20, 1950, Serial No. 169,149

7 Claims.

l The present invention is concerned with means for extracting fine solid particles from liquid suspensions. More especially the invention has to do with a machine for economically and expeditiously operating on liquid suspensions of solid particles of varying degrees of fineness in such a manner as either to dewater the suspension by separating out substantially clear liquid or to classify the particles by causing the liquid to carry off particles below a selected degree of fineness. The machine embodying the present invention can be employed to operate satisfactorily on any one of a number of different solid materials. By way of illustration and not limitation, the machine is hereinafter described as operating on a slurry of kaolin or china clay.

I have discovered that solid particles of a variety of substances in a range of particle sizes and from which ultrafine particles have been excluded, are highly valuable and useful for coating the surfaces of manufactured products. One class of manufactured goods to which the invention is applicable is paper coated with clay for receiving the imprint of printing types in the manufacture of books, magazines, newspapers, and other goods. I have found that an aqueous adhesive composition of clay particles within the range of 2 or 3 microns equivalent spherical diameter at the maximum and .25 micron equivalent spherical diameter at the minimum when combined with an adhesive substance of the character commonly used for paper coating compositions can be applied in a consistency which is sufiiciently free flowing to spread evenly over the surface of paper webs in the course of making, and suiiiciently low in proportion of water content to be dried while passing through standard paper drying equipment without marring of the coating. The clay fraction is also whiter and brighter, with greater covering power than clay fractions containing all the ultrafines and particles larger than 2 to 3 microns. l

The invention is embodied in a machine in the nature of a continuously operating centrifuge having a helicoidal conveyor, the elements of which arel radial, that is, perpendicular to the axis of rotation, which machine includes, as a novel feature, a number of relatively closely spaced auxiliary helicoidal varies located between convolutions of the conveyor. The marginal portions of these auxiliary vanes are inclined with respect to radial planes'. A centrifuge' illustrating the principles of the invention is shown in th accompanying drawings and described in the following specification. The primary object 2 of this invention is to hasten the effective subsidence rate of fine particles in an aqueous or similar suspension, in which they are suspended. With the use of prior art centrifuges, the average free subsidence path of particles of the dimensions herein contemplated is so long that the quantity of solid material within the prescribed particle size range sedimented to the centrifuge wall is so small as to be uneconomical. These machines are large and costly, wherefore a substantially large tonnage of clay per hour must be recovered to pay for their use.

In the drawings,

Figure l shows a longitudinal section of a centrifuge illustrating the salient features of this'invention;

Figure 2 is a cross section on line 2-2 of Figure 1 and an elevation of the left hand head of the centrifuge;

Figure 3 is a cross section on the line 3 3 of Figure 1 and a right hand head elevation of the centrifuge;

Figure 4 is a fragmentary longitudinal section of the centrifuge shown in Figure 1, represented on a somewhat larger scale; and

Figure 5 is a portion of Figure 4 shown on a still larger scale.

Like reference characters designate the same parts wherever they occur in all the figures.

The centrifuge illustrated on the drawing is of the kind known in the industry as the solid bowl type such as is shown in my Patents Nos. 2,085,538 and 2,097,420, this type being characterized chiefly by an imperforate bowl and a concentric helical conveyor blade within the bowl which is relatively rotatable to push solids toward one end. The bowl of the centrifuge is designated,

by the numeral I0. It iscylindrical with end heads perpendicular toits axis and is mounted in use to rotate at high speed'with its axis horizontal. A tubular hub I2 projects axially from the front head I4 of the centrifuge and carries a gear I6 by which the centrifuge is rotated. A tubular shaft IB is fitted within the hub l2 and extends axially through the centrifuge bowl. This shaft carries a gear 20 by which it is rotated. The hubs may be journalled in suitable standards 22 and 24 by which the entire centrifuge is supported. The portion of the shaft I8 -within the bowl is enlarged to form a hollow hub 26 on which is mounted a helicoidal conveyor blade 30. The shape of the blade can be described as the surface generated by a` straight line segment which revolves around a 'xedaxis 'while 'it advances in the axial direction and always remains perpendicular to the axis. The elements of this surface are the successive positions occupied by the line segment as it generates the surface. Both the bowl l0 and the blade 30 are rotated at high speed, by which the material introduced into the bowl is subjected to powerful centrifugal force; but with a differential of 'speed such that the turns of the blade 30 act as a screw conveyor forcing toward the front end of the centrifuge the relatively non-fluent material which has been deposited by centrifugal force upon the wall of the bowl. head of the centrifuge for the discharge of solids, the circumferential boundaries or thresholds 34 of which are equidistant from the axis. l

A stationary feed pipe 36 enters the front end of the centrifuge through the shaft I8 and discharges into the chamber within the hollow hub 26, from which outlets 60 deliver the suspension supplied through the pipe 3B into the interior of the bowl, preferably near the front end. The solid matter which settles on the wall of the bowl l0 is forced out by the conveyor blade over the thresholds of the solids discharge ports 32 in the front head of the centrifuge, while the liquid overflow which may be clear or may contain particles too ne to settle under desired operating conditions overilows through ports l2 in the opposite head of the centrifuge, the latter ports being further from theaxis of rotation than are the thresholds 34 of the solids discharge ports 32.

The line M designates the free surface of the pool of suspension on the wall of the centrifuge bowl. Its location is determined by theradial distance of the eiiiuent discharge ports 42 from the cylinder dened by the inner surface of the wall of the centrifuge bowl. Since the stream of suspension is introduced into the centrifuge near the front end thereof and the overflow of liquid is through the rear head thereof, there is during the operation of the centrifuge a constant rearward flow of liquid within the bowl.

When a cla-y slurry is operated on, the coarser particles settle out fast enough by centrifugal force alone and are deposited on the wall of the bowl for a considerable portion of its length and are propelled by the conveyor along the wall to the front end of the centrifuge for discharge through the ports 32. The finer particles are carl ried by the liquid stream toward the rear end of the centrifuge. For example, in actual operation such particles of clay may be of the order of 1 micron or less, the free subsidence rate of particles in this lsize range being too slow for economical operation. According to the present invention, I provide means for hastening the sedimentation of 'such fine particles, such means consisting of auxiliary helicoidal vanes 5B a number of which are mounted in parallel array between convolutions of the conveyor blade 36. These auxiliary vanes differ radically from the conveyor blade in that whereas the elements of the blade are radial from hub to periphery, the

elements of the marginal portions of the vanes 58 f:

are sharply inclined with respect to radial planes and thus intercept radially subsiding particles. The vanos may be regarded as having been generated by a broken line revolving about a fixed axis as it advances in an axial direction, the inner segment of the generating line being always perpendicular to the axis, the outer segment making an oblique angle with the inner segment. The inner portion is useful merely as a support for the outer portion which dips into the pool of slurry There are ports 32 in the front and is the significant part of the vanes. The vanes are preferably spaced close together `so that particles suspended in liquid between these vanes have a relatively short radial distance to travel to reach a vane surface. As the finer particles collect on vane surfaces they form on such surfaces streams of substantially higher concentration of solids and hence of greater specic gravity than the residual suspension. For example, it has been found that Isuch a concentrated stream may have a specific gravity of 1.7 'as compared with a specic gravity of 1.1 of the residual suspension. It has 'been empirically demonstrated that the centrifugal force acts on such suspensions in proportion to their specific gravities. In other Words, the rate of subsidence of each particle in the example given is drastically increased by reason of the close proximity of myriade of other fine particles. The result of such concentrated streams of ne particles on the inclined marginal portions of the auxiliary vanes Eil is to promote the subsidence of such flnestoward the wall of the bowl where they are propelled toward and disfcharged from the front head of the centrifuge. Thus all or a very considerable portion of the fines which would ordinarily be carried out with the effluent liquid and lost because their rate of free subsidence as individual particles is too slow to enable them to reach the bowl wall Within a reasonable time when in an ordinary centrifuge, are concentrated on the inclined surfaces with a resulting increase of subsidence rate which-brings them to the bowl wall before they can be carried out with the eilluent stream.

The stream-s of more concentrated slurry which form on the vanes Ecw toward the-edges of 'the vanes, yand thence radially toward the wallI of the centrifuge. The stream of thinr slurry which is introduced into the centrifuge near the front end thereof and which flows to 'the rear end 'thereof as it becomes progressivelyv thinner by reason of the separation of solid particles therefrom necessarily flows ina helical course between successive convolutions of the blade 30. Thus at any point where this stream touches any of the vanes 5B, its direction of iiow at that point is at right angles to the direction of flow of the stream of concentrated slurry on the vane at the same point. Hence the current or flow of the thin slurry in no way impedes the ow of the slurry having the thicker consistency.

The auxiliary vanes 59 may be mounted between any selected convolutionsof theconv'eyor blade 38 or between all of them. As illustrated, they are located between some of the convolu tions toward the rear end of the centrifuge. The auxiliary vanes dip well into the pool of slurry within the bowl but preferably do not come near enough to the wall of the bowl to disturb the sedimented solids-which areleing propelled along the wall by the conveyor blade.

By the use of auxiliary vanes 50 as described, a very consider-able proportion of fine clay particles having a diameter of l micron or les-s canbe economicallyrecovered. Since the practical purpose of the invention is economical operation, some of the ultra fine particles may be allowed to escape in the effluent even though the machine could be operated to make the separation substantially complete.

The optimum angle of inclination of the marginal portions of the auxiliary vaneshas been found to vary with the different kinds of .slurries or materials used. For example, inthe case of clay Vslurries, vane surfaces inclined .about 45j to a radial plane are best for slurries having 25% solids, whereas a 60 inclination is best for clay slurries having 8% or so of solids.

I claim:

1. A centrifuge for continuous operation comprising a bowl rotatable about its axis, said bowl having a head at one end with discharge ports for lsolid particle concentrates and a head at the other end with discharge ports for eliluent liquid,

all said ports being spaced radially inward of the bowl wall to maintain a pool of suspension in said bowl during operation, a helicoidal conveyor blade rotatably mounted in said bowl with its peripheral edge nearly in contact with the inner surface of the bowl, a plurality of helicoidal auxiliary vanes mounted between convolutions of said blade to rotate therewith, the elements of said vanes having outer portions immersed in said pool and inclined to the radial direction, and means for rotating the bowl and the blade and vane assembly at different speeds.

2. A centrifuge for continuous operation comprising a cylindrical bowl rotatable about its axis, said bowl having a head at one end with discharge ports for solid particle concentrates and a head at the other end with discharge outlets for eflluent liquid, all said ports being spaced radially inward of the bowl wall to maintain -a pool of suspension in said bowl during operation, a helicoidal conveyor blade rotatably mounted in said bowl, the elements of said blade being radial, the helical periphery of said blade being nearly in contact with the inner surface of said bowl, a series of closely spaced helicoidal vanes mounted between convolutions of said blade to rotate therewith, the elements of said vanes having outer portions inclined to the radial direction immersed in said pool, yand means for rotating the centrifuge bowl and the blade and vane assembly at high speed in the same direction but with a speed differential to propel sedimented particles toward the discharge end for solid particle concentrates.

3. A conveyor for a rotatable centrifuge comprising a hub, a helicoidal blade mounted on said hub with the elements of its surface radial to the axis of the hub, and a plurality of helicoidal vanes between convolution-s of the conveyor, the radius of said vanes being somewhat less than that of said blade and the elements of said vanes having c outer portions inclined to the radial direction.

4. A rotatable centrifuge having a cylindrical bowl and two heads, one said head having discharge openings for solid particle concentrates,

the other said head having openings further from L.

the axis for the discharge of liquid, a helicoidal blade rotatably mounted in said bowl with its peripheral edge nearly in contact with the wall thereof, means for rotating said centrifuge and blade at different speeds whereby the blade acts as a conveyor to propel solids deposited against the wall toward the solids discharge end of the centrifuge, helicoidal auxiliary vanes between convolutions of said blade near the liquid disch-arge end of the centrifuge, the peripheral margins of said auxiliary vanes being inclined toward the solids discharge end of the centrifuge and sufficiently spaced from the wall o1' the bowl to avoid resuspension of the particles deposited on said wall, and means for continuously feeding a stream of suspension into the bowl.

5. A centrifuge comprising a bowl with end heads to maintain a pool of suspension in the bowl, one head having openings for the discharge of particles therethrough, the other head having openings for the discharge of liquid, a helicoidal blade rotatable in said bowl with a helical edge nearly touching the wall of the bowl, helicoidal auxiliary vanes mounted between convolutions of said blade, said vanes having spaced apart peripheral portions immersed in said pool and and overlapping both radially and longitudinally, whereby to intersect the radial sedimentation paths of the particles in the suspension, and means for rotating the blade and vanes relatively to the bowl in a direction to propel the solids deposited on the wall of the bowl toward the solids discharge openings.

6. A conveyor for a rotatable centrifuge comprising a hub, a helicoidal blade mounted on said hub, a plurality of helicoidal vanes between some of the convolutions of said blade, the diameter of said vanes being somewhat less than that of the adjacent convolutions of the blade, each vane having at least its outer peripheral portion inclined at an oblique angle to the radial direction.

7. A method of hastening the sedimentation of the nner particles in a liquid suspension being processed in a continuously operating centrifuge of the type comprising a rotatable bowl with end heads to maintain a pool of suspension in the bowl, one head having openings for the discharge of particles therethrough and the other head having openings for the discharge of liquid, and a helicoidal blade rotatable in and relative to the bowl with a helical edge nearly touching the wall of the bowl and with the elements of the blade being radial, the steps comprising: deecting the particles from their radial sedimentation paths of movement between the convolutions of the blade into sedimentation paths of movement inclined to the radial; and maintaining movement of the deflected particles in said inclined paths for an appreciable distance to collect the particles into streams of slurry of increased concentration within the suspension.

SANFORD C. LYONS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 775,320 Van Kirk Nov. 22, 1904 1,005,800 Berrigan Oct. 17, 1911 1,057,443 Miller Apr. l, 1913 2,435,623 Forsberg Feb. l0, 1948 FOREIGN PATENTS Number Country Date 456,353 Great Britain Nov. 9, 1936

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