US3326459A - Particle classifier - Google Patents

Description

J1me 1967 J. P. LEROUX PARTICLE CLASSIFIER 3 Sheets-Sheet 1 Filed Oct.

VQ cos or I 2u/l2b 27 June 20, 1967 J, P, LERQUX 3,326,459

PARTICLE CLASSIFIER Filed Oct. 7, 1964 5 Sheets-Sheet June 20, 1967 J. P. LEROUX 3,326,459

PARTICLE CLASSIFIER Filed Oct. 7, 1964 3 Sheets-Sheet 3 United States Patent 3,326,459 PARTICLE CLASSIFIER Jean Paul Leroux, Hull, Quebec, Canada, assignor to Canadian Patents and Development Limited, Ottawa,

Ontario, Canada, a corporation of Canada Filed Oct. 7, 1964, Ser. No. 402,262 5 Claims. (Cl. 233-28) The present invention relates to a method and apparatus for classifying or separating particles according to their relative size, and in particular to such a method and apparatus employing centrifugal forces and hydrodynamic 'forces.

The present invention provides a solution to the problem of separating particles which are suspended in a liquid in two or more fractions according to their size. In many instances it is necessary to separate particles accurately and quickly in size ranges from 0.5 micron up to microns, where 1 micron is 10- cm. Prior to the present invention the separation of such particles into fractions by size in the size range up to 10 microns has been carried out by decantation and gravitational elutriation which requires between 10 and 500 hours to completely separate one gram of particles. Accordingly the great length of time required has been unsatisfactory for separation of such fractions. In addition it has required considerable space, equipment and many gallons of liquid, the temperature of which must be very critically controlled during the process. In accordance with the prior art the fractional decantation time may be considerably reduced by centrifugation, but since the centrifugal force acting on the particles increases with the square of the radius of gyration, the settling velocity is not uniform and considerable time must be given to the calibration of the instrument for each type and definite size of particles.

Accordingly none of the prior art devices can simply and reliably separate particle fractions in the range from one-half to 10 microns.

The present invention provides a simple and reliable particle classification centrifuge which may be used accurately to separate fractions of a particle suspension in a liquid and which operates on principles not heretofor used in such a particle classifier. In accordance with the invention, liquid containing particles to be classified is rotated at a predetermined angular speed, the suspension to be classified flowing outwardly from the axis of rotation and being suddenly diverted for a short distance in a direction parallel to the axis of rotation. The suspension is then caused to flow inwardly toward the axis of rotation whereby those particles whose diameter is larger than a predetermined size will be accelerated radially away from said axis and those particles less than said diameter will be transported toward the axis by the liquid. The apparatus of the invention gives a sharp definition between those particles which are accelerated away from the axis and deposited on the wall of the centrifuge and those which are transported by the liquid toward the axis of rotation. In accordance with a further aspect of the invention more than one stage of separation may be provided in a particle classifier and separated particle fractions may be obtained from the deposit on the walls of the classifier at each successive stage. In accordance with a further feature of the invention means are provided in the centrifuge for collecting the liquid after particle classification and for recirculating the liquid back to the inlet of the centrifuge.

In the operation of a centrifuge in accordance with the present invention, the centrifuge is accelerated to its proper rotational speed, the flow of liquid through it is adjusted to a predetermined value and the suspension is then introduced into the centrifuge and classified therein. It is essential that the suspension not be introduced into the centrifuge until the proper operating conditions are obtained otherwise the classification will not be accurately carried out, since the process of the present invention is dependent upon the rotation rate and the velocity of flow of the liquid through the centrifuge, as is detailed more fully below.

It has been experimentally verified that the drag on a spherical body in a fluid is equal to Q=31rvV d 1) where V terminal constant velocity of particle, u=viscosity of the fluid, and

d=diameter of the particle,

Also, the centrifugal force acting on a spherical body moving circularly with a fluid around an axis is expressed as where R=radius of gyration of the particle, w=angular speed in radians of the system, =specific gravity of the particle, =specific gravity of the fluid, d=diameter of the particle.

Equating the two forces Q and C in (1) and (2) one obtains Equations 3 and 4 indicate that a particle of diameter d suspended in a streamline fluid moving radially at constant flow towards the center of rotation between two parallel disks would be in equilibrium, neglecting gravity, at a radius R as long as the angular speed of the system remains constant. By extension it can be stated that a particle having a diameter a greater than d would start moving away from the axis of rotation being accelerated under the resultant force of both the increasing centrifugal force C and the decreasing drag force Q. On the other hand, a particle having a diameter d less than d, would be accelerated towards the axis under the resultant of both the increasing drag force Q and decreasing centrifugal force C.

The theoretical situation just described shows that a very sharp separation should be obtained with some device which would actually approach the conditions im posed by Equations 3 and 4. The present invention provides such a device.

In drawings which illustrate the principles and embodiments of the invention,

FIGURE 1 is a schematic diagram illustrating the principle of separation of the invention in relation to Equati-ons l and 2,

FIGURE 2 is a schematic diagram illustrating the principle of operation of separation apparatus according to the present invention,

FIGURE 3 is a cross-section of a preferred form of apparatus in accordance with the invention,

FIGURE 4 is a cross-section of a multi-stage separator, and

FIGURE 5 is a schematic diagram showing the use of apparatus according to the present invention.

FIGURE 1 illustrates schematically the separation principle of apparatus of the present invention. A particle ideally introduced at point P at radius R from the axis of rotation 10, and positioned between plates A and B is under two influences. The first of these is the flow toward the axis 10, the drag force of which is represented by the vector Q. The other influence is the centrifugal force due to rotation of the apparatus, represented by the vector C. The drag force as discussed above may be represented by the equation Q=31rvV d, and 51mllarly the centrifugal force is equal to If these two forces are equal and opposite then a particle of diameter d will remain motionless at the position P. Particles with a diameter less than d will be accelerated toward the axis of rotation and particles with a diameter greater than d will be accelerated away from the axis 10. Accordingly this principle of operation may be embodied in apparatus pursuant to the present invention and may be used to separate particles in difierent size ranges in a faster and more efficient manner than could previously be achieved.

FIGURE 2 schematically illustrates the actual separation of particles in apparatus according to the invention. As in FIGURE 1, the apparatus is rotating about an axis of rotation 10. The apparatus consists of a lower plate 11, an upper plate 12 and a baflie 13 which is positioned between the upper and lower plates. The apparatus also includes an area 14 in which the fluid is stagnated and caused to rotate with the members 11 and 12 so that there is no relative motion between the area 14 and the members 11 and 12. Thus a fluid channel 15, 16 and 17 is formed with the fluid flowing radially outwardly in the area 15, vertically in the area 16 and inwardly towards the axis of rotation in the are-a 17. It will be noted that the portion of the channel at 16 is narrower than at 15 or 17 and accordingly the flow will achieve its highest velocity in the area marked by the reference numeral 16. correspondingly the lowest flow velocity will occur in the area marked by the reference numeral 17. If a particle suspension is flowing through the channel 15, 16 and 17 the drag force will be highest in the area 16 and lowest in the area 17. Preferably the height in the area 17 is one-third of the height in the area 15. A particle suspended in the fluid and at point P would have a velocity indicated by the vector V cos a which will have an axial component V It will be appreciated that the particle is not moving directly toward the axis of rotation, however the error introduced is very small and accordingly very little difference in particle size d can be found when calculated according to Equation 3 at radius R2 and R1 respectively. Accordingly a particle with a diameter less than a arriving at point P will be directed to the left following the trajectory I -P since the drag force Q will be greater than the centrifugal force C. Similarly if a particle of diameter d larger than d arrives at point P the centrifugal force will be greater than the drag force Q and the particle will follow a path P P directed toward the right and away from the axis of rotation. It has been found in practice that Equation 4 may be used directly for calculating the required operating conditions of the apparatus provided that the velocity vector VQ cosec a of the stream between the points R1 and R2 is 1.412 times greater than the velocity vector V Under this condition the flow at P is larger than V by a factor of approximately 1.412 and Equation 4 may be used in its A original form at a predetermined value of R with a good approximation when all other factors of the apparatus are known.

The apparatus of the invention may be constructed in such a manner that Equation 4 can be used to predict the operation of the apparatus, and the flow F, the radius of gyration R at which separation takes place, the number of revolutions per minute N and the diameter d of the particles (considered as spheres) can all be considered as independent variables.

The conditions which must be controlled to give most accurate particle separation are, the dimensions of the separating chamber, the suspending fluid, the dilution of the suspension and the stability of flow.

As explained in the above in relation to FIGURE 2, in order to obtain sharp separation at point P and avoid turbulence in the separation Zone, bafile 13 should be as thin as possible as well as the spaces between plates 11 and 12 and channel 15 respectively.

Liquids being practically incompressible and considerably more viscous than gasses, they constitute when mixed with particles a more stable system for rotation at high speed and for avoiding turbulence in the separating chamber which would impair the ideal conditions imposed by Equation 4 for proper separation of particles.

As proved by many authors a dilution of 1% or preferably 0.5% weight/ volume is of paramount importance in elutriation techniques under gravity or inertial forces. If particles should arrive recombined in the critical zone of separation because of agglomeration they would simply settle at once as particles of larger diameters. A dispersion device accordingly should be used with the present invention. It must be noted here that this condition is diflicult to control with air-borne material where agglomeration may be complicated by electric charges built up by friction along the channel walls and by residual moisture especially with particles smaller than 10 microns.

Laminar flow in the separating zone is required because of the almost instantaneous nature of the separation process. The small dimensions of the separating chamber ensure a non-turbulent motion of the liquid and the self feeding and discharging device used with the apparatus of the invention also ensure constant flow. Any drop of pressure in the feeding system can result in a change of velocity of the suspension and impair the separation. In the apparatus of the present invention the pressure difference between any two points selected on a radial plane of the rotating liquid mass will remain constant as long as the angular speed is kept constant; this condition is diflicult to realize in the case of a compressible gas suspension moving at high velocity in multi-directional paths.

FIGURE 3 is a cross section, which is partly schematic, illustrating the structure of apparatus in accordance with the present invention. It will be appreciated that the apparatus is circular in plan view being symmetrical about the axis of rotation 10. As shown in FIGURE 3 the apparatus consists of a base member 11, a central member 12 and an upper member 18. The apparatus is provided with a fluid input tube 19 and an outlet tube 20. The base member 11 is provided with a bore 21 concentric with the axis of rotation 10, in which a shaft may be positioned to rotate the apparatus. A further bore 22 is provided in which a locking screw may be fitted to lock the apparatus to a shaft inserted in the bore 21. The base member 11, central member 12 and the upper member 18 are held together in any suitable fashion such as by bolts 23 passing through holes in the upper and central members 18 and 12 respectively and threaded into holes in the base member 11. As illustrated in FIG- URE 3 O- ring seals 24 and 25 are preferably provided to seal the joints between the central member 12 and the upper and lower members 11 and 18.

The central member 12 is provided with baflle 13 which corresponds to the battle 13 in FIGURE 2 around which the fluid suspension is constrained to flow. The passage 26 is provided in the central member 12 through which the fluid passes to the upper member 18 where the particles of a diameter less than the critical diameter and not deposited in the area 14 are coated on the wall 27. Baflles 28 and 29 are only two of a series of deceleration bafiles which are uniformly spaced about the axis of rotation and which serve to prevent the particles deposited in the area 14 and on the wall 27 from being swept away when the apparatus is decelerated after a fluid suspension has been passed therethrough. In actual practice the passageway 26 will not be a continuous cylindrical passageway but will be made up of discontinuous slots or a series of holes through which the fluid flowing around the baffle member 13 may reach the upper member 18.

The central portion of the member 12 is provided with an upstanding boss 30 through which the stationary water inlet tube 19 protrudes. The interior wall of the boss 30 has an outward and downward slope so that any fluid from the inlet 19 which arrives on the interior wall of the 'boss will be caused by centrifugal force to flow through the passage and will not migrate back along the outside of the tube 19. As shown a nut 31 is provided on the outer portion of the boss 30 to clamp the baflle 32 to the central member 12.

The outlet is provided with a scoop shaped probe 33 which faces toward the direction of rotation of the apparatus and collects the fluid which builds up within the upper member 18 during rotation.

By virtue of the centrifugal action of the apparatus all particles smaller than the critical diameter which are carried through the passage 26 to the upper member 18 are deposited on the inner wall 27 of the member 18 so that only liquid substantially free from particles is picked up by the probe 33.

The operation of the apparatus illustrated in FIGURE 3 is substantially as discussed in relation to FIGURE 2. The apparatus is rotated about the axis of rotation 10, fluid is introduced through the inlet 19 thus establishing the predetermined flow rate around the baffle 13 and is recirculated from the probe 33 back to the inlet 19. The particle suspension to be classified is then introduced into the inlet 19 while maintaining a constant flow rate throughout the system. Particles larger than the critical diameter will be deposited in the area 14 and particles less than the critical diameter will be coated on the wall 27. The separation of the suspension will take place very quickly. 0nce the suspension has all been introduced into the inlet 19 the flow of fluid through the apparatus may be stopped and the rotation of the apparatus subsequently stopped. The baflles 28 and 29 will prevent the particles deposited in the area 14 and on the wall 27 from being washed off the wall by the fluid in the decelerating apparatus. Preferably prior to stopping rotation of the apparatus the rotation rate is increased more firmly to coat the particles on the walls of the apparatus and ensure that all particles are at rest before the apparatus is stopped.

FIGURE 4 is a partial cross section of an alternative form of apparatus constructed in accordance with the invention which may be used for performing multiple separations. As before the apparatus is rotated about an axis of rotation 10 and has a lower member 11 and an upper member 18. A plurality of central members 12, 12a and 1212 are provided each being associated with a baflle 13, 13a and 13b respectively. Collecting areas 14, 14a and 14b are also provided in which particles greater than the critical diameter for the flow conditions at the baflies 13, 13a and 1312 respectively are deposited. By now the operation of the apparatus will be self evident. The largest particles will be deposited in the area 14, particles of a smaller size will be deposited in the area 140, particles of a still smaller size will be deposited in the area 14b, and residual particles will be coated on the wall 27 of the upper member 18. A nut 31 as before is used to hold the central portion of the apparatus together.

FIGURE 5 is a schematic representation of the use of the apparatus of the present invention. The separator 40 is shown connected by tubing to a flow meter 41, a three way valve 42 and the dispersion chamber 43. In accordance with the practice outlined above the separator is caused to rotate at its predetermined rotation rate in accordance with Equation 4. The flow through the separator 40 is adjusted until the proper flow reading is obtained on the flow meter 41 with the fluid flowing between the separator 40 and the flow meter 41 by an appropriate setting of the three way valve 42. When the appropriate flow is obtained the valve 42 is turned so that the suspension of particles in the dispersion chamber 43 is caused to flow through the separator and the particles are classified. The flow of fluid through the separator 40 may then be stopped by an appropriate setting of the valve 42 and the separator is then accelerated to coat the particles on the walls of the chambers before stopping. The separator may then be stopped and the particles greater than the critical diameter will be found to be separated from the particles less than the critical diameter.

I claim:

1. A centrifugal particle classifier for separating solid particles of different diameters and the same specific gravity into at least two discrete size fractions, said particles being suspended in a liquid, said classifier comprising a rotating portion having a lower plate, an upper plate, and a baffie positioned therebetween, said upper plate, said lower plate, and said batfle being circular, and connected together for rotation about a common axis passing through the centre thereof, a wall connecting the rim of the upper plate to the rim of the lower plate, said baflie being spaced from said upper plate, said lower plate and said wall adjacent the outer end of said baflie to form a flow channel about the end of said baffle and means to cause said liquid with said par-ticles suspended therein to flow laminarly between said lower plate and said baflle in a direction radially away from said axis of rotation, around the end of said baflle and radially towards said axis of rotation between said upper plate and said baffle, particles larger than a predetermined diameter being taken out of suspension and deposited on said wall, and particles smaller than said predetermined diameter remaining suspended in said liquid, said predetermined diameter being dependent on the flow, the specific gravity of the particles, the specific gravity of the liquid, the distance between said baflie and said upper plate, the radius of said batfle, the viscosity of the liquid, and the rate of rotation of said rotating portion.

2. Apparatus according to claim 1 wherein said predetermined diameter is d, and wherein the conditions of operation are defined by 3 Mi gee ulan where F is the flow of said liquid p is the specific gravity of the particles p is the specific gravity of said liquid H is the distance between the baflle and the upper plate R is the radius of said baflie 11 is the viscosity of the liquid, and

N is the rate of rotation of said rotation portion 3. Apparatus according to claim 1 including deceleration baflies to prevent relative rotary motion between said liquid and said Wall.

4. Apparatus as claimed in claim 1 wherein said means to cause flow of said liquid includes inlet means communicating with the flow channel between said lower plate and said batfie for introducing the liquid suspension, and outlet means communicating with the flow channel between the upper plate and the baflle for removing liquid from said apparatus.

5. Apparatus as claimed in claim 4 wherein said inlet means and said outlet means are connected in a closed liquid circuit having two branches, a first branch including a dispersion chamber in which the suspension to be classified may be contained and a second branch including a fiow meter for measuring the flow therethrough, said branches being connected by a three way valve permitting 15 flow through either said first or said second branch, or for stopping flow in said circuit.

References Cited UNITED STATES PATENTS 7/1897 Linders 23328 7/1902 Ten Winkel 233-28 X 8/1903 Keiper 233-28 5/1914 Resines 23344 3/1915 Curtis et al 223-32 X 11/1923 Eccleston 23327 X 3/1925 Coleman 23318 X 7/1944 Banning 23346 FOREIGN PATENTS 10/1924 France.

5/1891 Great Britain.

M. CARY NELSON, Primary Examiner.

HENRY T. KLINKSIEK, Examiner.

Claims (1)

1. A CENTRIFUGAL PARTICLE CLASSIFIER FOR SEPARATING SOLID PARTICLES OF DIFFERENT DIAMETERS AND THE SAME SPECIFIC GRAVITY INTO AT LEAST TWO DISCRETE SIZE FRACTIONS, SAID PARTICLES BEING SUSPENDED IN A LIQUID, SAID CLASSIFIER COMPRISING A ROTATING PORTION HAVING A LOWER PLATE, AN UPPER PLATE, AND A BAFFLE POSITIONED THEREBETWEEN, SAID UPPER PLATE, SAID LOWER PLATE, AND SAID BAFFLE BEING CIRCULAR, AND CONNECTED TOGETHER FOR ROTATION ABOUT A COMMON AXIS PASSING THROUGH THE CENTRE THEREOF, A WALL CONNECTING THE RIM OF THE UPPER PLATE TO THE RIM OF THE LOWER PLATE, SAID BAFFLE BEING SPACED FROM SAID UPPER PLATE, SAID LOWER PLATE AND SAID WALL ADJACENT THE OUTER END OF SAID BAFFLE TO FORM A FLOW CHANNEL ABOUT THE END OF SAID BAFFLE AND MEANS TO CAUSE SAID LIQUID WITH SAID PARTICLES SUSPENDED THEREIN TO FLOW LAMINARLY BETWEEN SAID LOWER PLATE AND SAID BAFFLE IN A DIRECTION RADIALLY AWAY FROM SAID AXIS OF ROTATION, AROUND THE END OF SAID BAFFLE AND RADIALLY TOWARDS SAID AXIS OF ROTATION BETWEEN SAID UPPER PLATE AND SAID BAFFLE, PARTICLES LARGER THAN A PREDETERMINED DIAMETER BEING TAKEN OUT OF OF SUSPENSION AND DEPOSITED ON SAID WALL, AND PARTICLES SMALLER THAN SAID PREDETERMINED DIAMETER REMAINING SUSPENDED IN SAID LIQUID, SAID PREDETERMINED DIAMETER BEING DEPENDENT ON THE FLOW, THE SPECIFIC GRAVITY OF THE PARTICLES, THE SPECIFIC GRAVITY OF THE LIQUID, THE DISTANCE BETWEEN SAID BAFFLE AND SAID UPPER PLATE, THE RADIUS OF SAID BAFFLE, THE VISCOSITY OF THE LIQUID, AND THE RATE OF ROTATION OF SAID ROTATING PROTION.

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