CA1178382A - Apparatus and method for separating particles from a fluid suspension - Google Patents

Abstract

Abstract An apparatus and method for separating particles from a fluid, particulate suspension by flotation in a centrifugal field. The apparatus includes a vertically oriented, cylindrical vessel having a tangential inlet and a tangential outlet. The particulate suspension is introduced into the vessel through the inlet and swirls around the inner surface of the vessel in a thin fluid layer. Air is sparged through a porous wall formed in the vessel and into the thin fluid layer of the particulate suspension. Small bubbles are generated at the surfact of the porous wall. The directed motion of the particles in the thin layer of particulate fluid suspension results in a high probability for collision and a rapid flotation. The air bubbles and particles form bubble/particle aggregates which migrate towards the axial center of the apparatus and into a froth phase in the core of the apparatus. The particle-containing froth travels upwardly, countercurrently to the thin fluid layer, and is removed coaxially through a vortex finder.

Description

l 1~8382 APPARATVS AND METHOD FOR
S~PARATING PARTICLES FROM
A FLUID S~SPENSION

~ack~round 1. The Field of the Invention The presen~ invention relates to a novel apparatus and method for separating particles from a fluid, particulate s~spension by flotation in a centrifugal field.

2. Related ~plications This application is related to my copending Canadian application Se~ial No. 383,739, filed August 12, 1981, entitled FLOTATION APPARATUS AND METHOD FOR ACHIEVING
FLOTATION IN A CENTR~FUGAL FIELD, and my Canadian applica-tion Serial No. 364,658, filed November 14, 1980, entitl~d AIR-SPARGED HYDROCYCLONE AND METHOD, which corresponds to U.S. Patent No~ 4,279,743, issued on July 21, 1981.

3. The Prior Art A. Flotation Processes Flotation is a process in which the apparent density of one particulate constituent of a suspension of ~inely dispersed particles is reduced by the adhesion of gas bubbles to that respective particulate constituent. The buoyancy of the bubble/particle aggregate is such that it rises to the surface and is thereby separated by gravity from the remainins particulate constituents. While the particulates l 178382 1 which attract air form bubble/particle aggregates which 2 "float" to the surfac~, the other particulates of the 3 suspension do not attract air and, therefore, remain

4 suspended in the liquid phase of the suspension.
The preferred method for removing the floated material 6 is to form a froth or foam to collect the bubble/particle 7 aggregates. The froth containing the collected 8 bubble/particle aggr~gates can then be removed from the top 9 of the suspension. This process is called froth flotation and is conducted as a continuous process in equipment called 11 flotation cells. It is important to realize that froth 12 flotation is encouraged by voluminous quantities of small 13 bubbles (such as in the range of one to two millimeters in 14 diameter).
In conventional processes, the success of flotation has 16 depended upon controlling conditions ln the suspension so 17 that the air is selectively retained by one particle 18 constituent and rejected by the other consti~uents of the 19 particulate suspension. To attain this objective, the feed must be treated by the addition of small amounts of known 21 chemicals which render one constituent hydrophobic, thus 22 causing that constituent to be repelled by the aqueous 23 environment and attracted to the air bubbles, thereby 24 enhancing the formation of bubble/particle aggregates as that constituent. Thus, a complete flotation process is 26 conducted in several steps: (1) the feed is ground, usually 27 to a size less than about 28 mesh; (2) a slur~y containing ~ 1783~2 1 about 5 ~o 40 percent solids in water is prepared; (3) the 2 necessary chemicals are added and sufficient agitation and 3 time pro~ided to distribute the chemicals on the surface of 4 the particles to be floated; (4) the ~reated slurry is aerated in a flotation cell by agitation in the presence of a 6 stream of àir or by blowing air in fine streams through the 7 slurry; and (5) the aerated particles in the froth are 8 withdrawn from the top of the cell as a froth product 9 (frequently referred to as ~he "concentrate") and the remaining solids and water are discharged from the bottom of 11 the cell (frequently referred to as the "tailing product").
12 Chemic~ls useful in creating the froth phase for the 13 flotation process are commonly referred to as "frothers."
14 The most common frothers are short chain alcohols such as meth~l isobutyl carbinol, pine oil, and cresylic acid. The 16 criteria for a good frother revolves around the criteria of 17 solubility, toughness, texture, froth breakage, and non-18 collecting techniques. In practical flotation tests, the 19 sizè, number, and stability of the bubbles during flotation may be optimized at given frother concentrations.
21 Much scientific endeavor has been expended toward 22 analyzing the various factors which relate to improving the 23 conditions during flotation for improved recovery of 24 particles. One particular phenomenon that has been known for some time is the poor flotation response of fine particles.
26 This becomes economically important when flotation 27 separation methods are used in the processing of minerals.

~ 178~82 1 Generally, prior art processes have a~hieved effective 2 flotation for both metallic and non-metallic minerals having 3 a particle size in the range of between 10 and 1000 microns.
4 In these processes, the minimum recoverable particle size has been anywhere from 10 to 100 microns, depending on the 6 particular mineral sought to be recovered. Frequently, the 7 mineral industries have thus been forced to discard the 8 smaller, unrecovered mineral particles since it is 9 uneconomical to concentrate or recover them.
The economic losses suffered by the mineral industries 11 due to this inability to recover very fine minerals by 12 conventional flotation tec~niques is staggering. For 13 example, in the Florida phosphate industry, approximately 14 1/3 of the phosphate is lost as slime. Roughly 1/5 of the 15 world's tungsten and about half of Bolivian tin is lost due 16 to the inefficiencies of present flotation techniques in 17 recoverin~ these minerals. It will thus be appreciated that 18 any process that could recover particles smaller than those 19 recovered in existing prior art processes would have a tremendous economic impact on these and other mineral 21 industries.
22 The inability of prior art flotation processes to 23 recover fine particles is also importan~ in the coal 24 industry. Flotation processes for separating ash and sulphur from coal have been used with greatly increased 26 frequency during recent years. However, in these flota~ion 27 separation processes, significant amounts of very fine coal --`

1 1783~2 1 particles go unrecovered. As a result, coal fines may be 2 lost in the reject stream. Not only is this a waste of a 3 valuable resource, but disposal of coal-containing reject 4 streams is frequently a serious environmental problem.
Another factor which further complicates the 6 effectiveness of conventional flotation is that conventional 7 flotation cells generally require a minimal retention time 8 of at least two minu~es for successful separation. This is 9 particularly disadvantageous because such relatively long retention times required for conventional flotation 11 processes limit plant capacity and result in large floor 12 space demands.
13 Surface chemical factors are also important with 14 respect to the potentiality for formation of bubble/particle aggregates in the flotation process. The qualitative 16 interrelationships between hydrophobicity, contact angle, 17 and flotation response are fairly well understood, but there 18 is little quantitative information available on the 19 relationship between hydrophobicity and induction time.
Induction time can be defined as the time taken for a 21 bubble to form a three-phase contact at a solid surface after 22 an initial bubble/particle collision. Alternatively, 23 induction time may be regarded as the time required after ~4 collision for the liquid film between a particle and air bubble to thin to its rupture thickness. In~uction ~imes 26 which are characteristics of good flotation conditions are 27 known to be of the order of about 10 milliseconds. Whereas 1 the contact angle between a bubble and a particle appears to 2 be an intrinsic characteristic of the surface chemical 3 forces, in an actual flotation system , induction time is 4 dependent on physical factors such as particle sizP, temperature (in certain circumstances), and inertial 6 efects, as well as being dependent on surface chemical 7 forces. Consequently, in considering bubble-particle 8 contact and adhesion, any calculations involving an 9 induction time factor must, to some extent, be speculative.
Nevertheless, such calculations may provide a useful guide 11 to the significance of the induction time factor on affecting 12 flotation rates and the general flotation response of any 13 particle.
14 Additional discussions relating ~o flotation and fine particles processing m~y be found in the following 16 publications:
17 M.C. Furstenau, editor, Flotation, (vols. 1 and 18 2), American Institute of Mining, Metallurgical, and 19 Petroleum Engineers, Inc. (1976); and P. Somasundaran, editor, Fine Particles 21 Processin~, Proceedings of the International Symposium 22 on Fine Particles Processing, Las Vegas, Nevada, 23 February 24-28, 1980 (vols. 1 and 23, ~merican 24 Institute of Mining, Metallur~ical, and Petroleum Engineers, Inc. (1980).
26 In addition to conventional froth flotation, variations 27 in flotation techniques sometimes include the addition of an ~ ~783~2 1 emulsion, such as by the addition of about three to five 2 percent or more oil to enhance the formation of oil 3 droplet/coal particle aggregates. When a slurry of ground 4 coal is flocculated with the oil, the flocs which float are S separated from the refuse material by skimmin~ fr~m the 6 surface. While this technique does no~ utilize air bubbles 7 for flotation, the adaptation of this system to froth 8 flotation has been used both for coal and a variety of ores 9 such as manganese dioxide and ilmeni~e (an oxide mineral of iron and titanium). In this latter process, a collector and 11 fuel oil are added to the ore slurry, often with an 12 emulsifier. The conditions of the process are ad~usted so 13 that when the slurry is aerated, the dispersed oil/particle 14 suspension inverts from that of oil-in~water in the slurry to one of water-in-oil in the froth. This process, therefore, 16 occupies a middle position between froth flotation and the 17 foregoing oil flotation process. The quantity of oil used is l& usually much lower than that used for the bulk oil or 19 spherical agglomeration processes; for e~ample only one to several pounds of oil per ton of ore processed is generally 21 used. Such modifications of conventional froth flotation 22 processes are referred to in the art as emulsion or oil 23 flotation.
24 Flotation techniques can be applied where conventional Z5 gravity separation techniques fai~. Indeed, flotation bas -26 supplanted the older gravity separation methods in solving a 27 number of separation problems. Originally, flotation was r ~ ~78382 1 used to separate sulphide ores of copper, lead and zinc from 2 associated gangue mineral particles. However, flotation is 3 also used for concentrating nonsulphide ores, for cleaning 4 coal, for separating salts from their mother liquors, and for recovering ele~ents such as sulphur and graphite.
6 B. Cyclonic Separators 7 The cyclonic separator (sometimes referred to as a 8 hydrocyclone~ is a piece of equipment which utilizes fluid 9 pressure energy to create rotational fluid motion. This rotational motion causes relative movement of the particles 11 suspended in the fluid, thereby permitting separation of 12 particles, one from another or from ~he fluid.
13 The rotational fluid motion is produced by tangential 14 injection of fluid under pressure into a vessel. At the point of entry for the fluid, the vessel is usually 16 cylindrical and can remain cylindrical over its entire 17 length, though it is more usual for a portion o the vessel to 18 be conically shaped.
19 In many instances, the hydrocyclone is used successfully for dewatering a suspension or for making a size 21 separation between the particulates in the suspension 22 (classifying hydrocyclone). However, equally important is 23 its use as a gravity separator. Hydrocyclones have been used 24 extensively as gravity separators in coal preparation plants, and design features have been established for such 26 applications which emphasize the difference in particle 27 gravity rather than the differences in particle size. Two ~ :~78382 1 ~eneral categories of hydrocyclones used for gravity 2 separation can be distinguished by their design features 3 particularly with respect to their feed and discharge ports 4 and, to a lesser extent,'by the presence or absence of a conical section.
6 The first type of hydrocyclone generally has three 7 inlet and outlet ports and consists of a cylindrical vessel 8 ranging in size (as found in industry) from 2 to 24 inches in 9 diameter with a conical or bowl-shaped bottom. Variations do exist in the shape, dimensions, bo~tom design, vortex 11 finder, and similar parameters. The choice of the various 12 parameters of the cyclone design depend upon the size of the 13 particles to be treated and the efficiency desired. Thus, 14 the major operating variables of the hydrocyclone are: (a) the vertical clearance between the lower orifice edge of the 16 vortex finder and the cyclone bottom, (b) the vortex finder 17 diameter, (c) the apex diameter, (d) the concentration of 18 feed solids, and (e) the inlet pressure.
19 `In operation, the particle-bearin~ water slurry is introduced tangentially and under pressure into the 21 cylindrical section of the cyclone where centrifugal ~orces 22 act upon the particles in proportion to their mass. As the 23 slurry moves downwardly into the conical section of the 24 cyclone, the centrifugal force acting on the particles increases with the decreasing radii of the cyclone. With 26 such a design, the heavy density particles o~ a given size 27 tend to move outwardly toward the descending water spiral ~ ~78~2 1 much more rapidly than their lighter density counterparts.
2 Consequently, as these lighter density particles approach 3 the apex of the cone, they are drawn into an upwardly 4 flowing, inner water spiral which envelopes a central air core. These lighter density particles then move towards the 6 vortex finder where they are removed as overflow product.
7 The heavier particles in the outer spiral along ~he cyclone 8 wall move towards the apex orifice of the hydrocyclone where 9 they are removed as an underflow product. Admlttedly, this is an oversimplified description of the separation affected 11 in a hydrocyclone which is, in fact, a very complex 12 interaction of many physical phenomena including centrifugal 13 acceleration, centripetal drag of the fluid, and mutual 14 impact of particles.
The second type of hydro~yclone used for gravity 16 separation has four inlet/outlet ports and consists of a 17 straight-wall cylindrical vessel of specified length and lB diameter and is usually operated at various inclined 1~ positions ranging between the horizontal and the vertical. A
suspen~ion of particles enters the vessel through a coaxial 21 feed pipe (generally at the ~pper end of the vessel) while a 22 second fluid (typically, water or a heavy media suspension) 23 enters the vessel tangentially, under pressure, through an 24 inlet adjacent the lower end of the vessel. The pumped second fluid thus creates a completely open vortex within the 26 vessel as it transverses the vessel toward a tangential 27 reject discharge adjacent the upper or inlet end. The 3 8 ~

1 cyclonic action created in the vessel transports the heavier 2 particles to the reject discharge while the lower density 3 particles are removed from the ~essel through a coaxial 4 outlet (vortex finder) at the lower end of the vessel.
Either of the foregoing devices can be used with or 6 without den~e media. Hydrocyclones used without dense media 7 for gravity separations are referred to as water-only 8 hydrocyclones and those that are used with dense media are 9 referred to as heavy media hydrocyclones. The dense media usually consists of an aqueous suspension of finely ground 11 magnetite or ferrosilicon to control the specific gravity of 12 the media between the specific gravities of the two 13 components of the feed ma~erial. The finely ground media 14 material is recovered from both the overflow and the under-flow streams by screening and recycling. As will be readily 16 appreciated, this requirement adds to the cost and 17 complexity of the separation and limits the process with 18 respect to the size of particles which can be separated.
19 ~dditional information regarding hydrocyclone separators and their operation may be found in the following 21 publications:
22 D. Bradley, The Hydrocyclone, Pergamon Press 23 (19653;
24 P. Sands, M. Sokaski, and M.R. Geer, "Performance of the Hydrocyclone as a Fine-Coal Cleaner", Bureau of 26 Mines Report of Investigations No. 7067, ~nited States 27 Department of the Interior (1968);

1 1~838~

1 A.W. Deur~rouck and J. Hudy, Jr., "Performance 2 Characteristics of Coal-Washing Equipment; Dense-3 Medium Cyclones", Bureau of Mines Report of 4 Investigations No. 7673, United States Depar~ment of the Interior, 1972;
6 A.W. Deurbrouck, "Performance CharacteristicS of 7 Coal-Washing Equipment Hydrocyclones", Bureau of Mines 8 RePort of Investi~ations, No. 7891, United States 9 Departmen~ of the Interior, (1974~; and E~Jo O'Brien and K.H. Sharpeta, "Water-Only 11 Cyclones; Their Functions and Performance", Coal Age at 12 110-14 (January 1976).
13 Surprisingly, it has been discovered that flotation can 14 be accomplished in a centrifugal field for improved efficiencies in the recov~ry of particles, particularly with 16 respect to those particles which are conventionally 17 considered too small to be recovered by gravity separators 18 and which do not respond well in conventional froth flotation 19 systems in a gravitational field. An apparatus and method implementing this discovery is disclosed and claimed herein.
21 Brief Summary and ObJects of the In~ention 22 The present invention relates to a novel apparatus and 23 method for separating particles from a fluid, particulate 24 suspension by flotation in a centrifugal field. The apparatus includes a vertically oriented, cylindrical vessel 26 having a tangential inlet at the upper end thereof for 27 introducing the particulate suspension under pressure into 3 ~ ~
1 th~ vessel in a generally tangential fashion, and a 2 tangential outlet at the lower end thereof for directing 3 fluid discharge from the particulate suspension out of the 4 vessel in a generally tangéntial fashion.
The novel configuration of the vessel with its 6 tangential inlet and outlet, directs the particulate 7 suspension around the vessel in a swirling motion such that 8 the particulate suspension forms a thin fluid layer around 9 the inner surface of the vessel wall. The unique design of the apparatus also directs the flow of ~he particulate 11 suspension so as to create a forced vortex in the vessel; the 12 forced vortex, in turn, forms a centrifugal field. A portion 13 of the vessel wall is formed as a porous wall, and the porous 14 wall is surrounded by an air plenum in communication with an air source.
16 The particulate suspension is first introduced into the 17 vessel through the tangential inlet and forms a thin fluid 18 layer against the inside surace of the vessel wall. Air l9 inside the air plenum is then injected through the porous wall and into the thin fluid layer of particulate suspension 21 within the Yessel. The air bubbles and particles within the 22 fluid suspension form bubble/particle aggregates which 23 float to the "top" of the centrifugal force field, i.e., the 24 axial center of the apparatus. As air is sparged through the porous wall in~o the thin fluid layer~ very smaIl air bubbles `26 are formed by the high shear velocity of the fluid suspension 27 against the porous wall. Aq the air bubbles form, they are ^ 13 -~ 17~38~ ~

1 constrained momentarily against the porous wall so as to 2 increase the collision rate between the air bubbles and the 3 particles in the fluid suspension. The remaining fluid then 4 exits the tangential outlét as discharge. The rate of the

5 fluid discharged through the outlet can be regulated so as to

6 control the water split within the vessel.

7 Because of the thin fluid layer in which flotation

8 occurs, flotation is achieved rapidly in this novel

9 apparatus and method, and the retention ~ime for the entire

10 separation process is on the order of seconds, depending on

11 the length of the vessel.

12 It is, therefore, an object of the present invention to

13 provide an apparatus and method for separating particles

14 from a fluid suspension by flotation in a centrifugal field

15 which achieves separation of fine particles which are

16 significantly smaller than particles separated by prior art

17 methods and apparatus.

18 Another object of the present invention is to provide an

19 apparatus and method for separating particles from a fluid

20 suspension by flotation in a cPntrifugal field in which

21 flotation occurs in a thin fluid layer of the particulate

22 suspension and which significantly increases the collision

23 rate between the particles and the air bubbles, thereby

24 substantially increasing the degree of separation achieved r

25 and allowing the separation process to be performed rapidly. J
2~ Still another object of the present invention is to 27 provide an improved flotation apparatus and method in which ~ 1~83~2 1 the fluid flow forms a forced vortex so as to enhance the 2 formation of a quiescen~ froth and optimize recovery of the 3 particles from the fluid suspension.
4 A further object of the present invention is to provide a 10tation apparatus and method which achie~es a fa~orable - 6 water split and which allows the water split to be 7 controlled.
8 Yet another object of the present invention is to 9 provide an apparatus for separating particles from a fluid suspension by flotation in a centrifugal field which is 11 relatively compact and does not require large amounts of 12 floor space.
13 These and other objects of the present invention will 14 become more fully apparent from the following description and appended claims 9 taken in conjunction with the 16 accompanying drawing.
17 Brief Description of the Drawing 18 Figure 1 is a graph comparing the minimum particle 19 diameter that will impact an air bubble as a function of the force field at a critical Stokes number of 0.2.
21 Figure 2 is a perspective view of a preferred embodiment 22 of the novel apparatus of the present i~vention.
23 Figure 3 is a longitudinal cross-sectional view of the 24 preferred embodiment of the apparatus of Figure 2 taken along line 3-3, which further illustrates the operation of

26 that apparatus in separating hydrophobic particles from a

27 fluid, particulate suspension containing both hydrophobic

28 and hydrophilic particles.

l 178382 Figure 4 is a par~ial 9 lon~itudinal cross-sectional view of the preferred embodiment of the apparatus of Figure 2, enlarged to better show the operation of that apparatus in separating hydrophobic particles from a fluid, particulate suspension containing only hydrophobic particles.
Figure 5 is a chart showing the tangential ~elocity distribution oi different types of vortices created by the rotational motion of the fluid flow in different hydrocyclone devices. The shaded areas correspond to the magnitude of the tangential velocity alon~ the diameter of each different apparatus. (Note that V = tangential velocity and r = radius.) De~ailed DescriPtion of the Preferred Embodiment The present in~ention is better understood in view of my related applications entitled "FLOT~TION APPAR~TUS AND

METHOD FOR ACHIEVING FLOTATION IN A CENTRIFUGAL FIELD", and "AIR-SPARGED HYDROCYCLONE AND METHOD", (U.S. Patent No.

4,279,743), which applications and patent are mentioned on page l.

The present invention can be best understood ~y reference to the drawing wherein like parts are designated by .
like n~merals throughout.
General Discussion As discussed in detail in my- copending application entitled "FLOTATION APPARATUS ~ND METHOD FOR ACHIEVING

1 17838~
1 FLOTATION XN A CENTRIFUGAL FIELD,"experimental studies with 2 certain embodim~n~s of the apparatus and m~hod disclosed in 3 that application have shown that the rate of flotation 4 generally increases as bubble size decreases. This phenomenon is also true of the novel apparatus and method of 6 the present invention. Moreover, further experimentation 7 with the apparatus and method of the present invention 8 indicates that by performing flotation in a centrifugal 9 field the rate of flotation can be increased and the size limit of fine particles which can be separated is extended.
11 Considering the probability of collision and attachment 12 of a particl~ to a bubble from the standpoint of inertial 13 impaction, a lower limit on particle size can be defined 14 below which impaction will not occur. Those particles smaller than the size limit have insufficient inertia to 16 deviate from the fluid streamlines. The Stokes number, which 17 is a measure of the ratio of inertial forces to viscous 18 forces, is a convenient criterion to determine the extent to 19 which particles will deviate from streamlines and undergo intertial impaction with a bubble. The minimum or critical 21 size of the particles which may be separated by flotation 22 depends to a large extent on the magnitude of the force field 23 experienced by the particles in the fluid suspension.
24 It has been recognized in the literature that from theoretical considerations (which have been experimentally -26 verified) that a Stokes number in the range of below about 27 0.2 is the critical number below which inertial impaction of ~78382 1 the particles with the bubbles will not occur. Assuming a 2 critical Stokes number of 0.2, Figure 1 shows the relation of 3 the critical particle size to the force field experie~ced.
4 Note that as the force field increases, the critical particle size needed for initial impastion drops significantly. For a 6 force field of 1 ~, the critical particle size for impaction 7 is on the order of from 10-100 microns, this is the range in 8 which the prior art flotation apparatus function. However, 9 as the force field increases, the critical particle size drops dramatically, reaching a size of 1 micron at about 11 100 Gs.
12 In one embodiment of the present invention, centrifugal 13 force fields of at least 50 Gs or greater can be achieved, 14 extending the fine particle flotation size limit towards 1 micron, and increasing the rate of flotation to about 300 16 times the rate experienced in the existing prior art 17 apparatus and processes.
18 The Apparatus 19 One preferred embodiment of the present invention is best illustrated in Figures 2 and 3. The apparatus for 21 separating particles from a fluid suspension by flotation in 22 a centrifugal field, generally designated 10, includes a 23 generally cylindrical housing or vessel which is preferably 24 vertically oriented. A generally tangential inlet 12 is formed at the upper end of cylindrical flotation vessel 11 26 for receiving the particulate suspension to be treated. A
27 generally tangential outlet 14 is formed at the lower end of 1 1783~2 1 vessel 11 for directing fluid discharge from the particulate 2 suspension out of vessel 11 in a generally tangential 3 fashion. A valve 36 is installed in outlet 14 to regulate the 4 flow rate of fluid discharge therethrough.
A portion of the wall of vessel 11 is formed as a porous 6 wall, generally designated 20, having an outer surface 19 and 7 an inner surface 21. An annular air plenum 18 is formed 8 around porous wall 20 so as to completely enclose porous wall 9 20 and form an air plenum chamber 17 therebetween. An air inlet 22 formed in air plenum 18 is in communication with an 11 air source (not shown) to accommodate introduction of air 12 into chamber 17.
13 A cylindrical vortex finder 16 is mounted to the upper 14 end of flotation vessel 11. Ports 15a and 15b are formed in the ends of the hollow vortex finder 16 to permit the passage 16 of froth therethrough.
17 The Method 18 The operation of flotation apparatus 10 and one 19 prefèrred embodiment of the novel method of the present invention are best understood by reference to Figure 3. A
21 particulate suspension (sometimes referred to as a "slurry 22 feed") containing finely divided hydrophilic particles 24 23 ~illustrated by light-colored boxes) and hydrophobic 24 particles 26 (illustrated by dark-colored boxes) is introduced into vessel 11 through tangential inlet 12 so as `26 to follow the course indicated by spiral pathway 28. (Prior 27 to the introduction of the particulate suspension into i ~L78382 1 vessel 11, if particles 26 are not naturally hydrophobic, 2 they should be made hydrophobic hy methods known in the art.) 3 The particulate suspension is injected into inlet 12 under 4 pressure and in a generally tangential fashion so as to assume the swirling path illustrated by spiral pathway 28.
6 Thus iniected, ~he particulate suspension forms a thin 7 fluid layer 40 (see Figure 4) against the inner surface 21 of 8 porous wall 20 (to be explained in more detail hereinafter).
9 Air is then introduced from air inlet 22 into chamber 17 of air plenum 18 and is sparged through porous wall 20 into the 11 thin fluid layer 40 of the particulate suspension.
12 Upon entry into thin fluid layer 40, the air forms small 13 bubbles which attach to and/or trap the hydrophobic 14 particles 26 and transport them ou~ through the centrifugal field to the axial center of flotation apparatus 10. The 16 hydrophilic particles 24 are not ~rapped by the air bubbles 17 and follow the swirl flow of the thin fluid layer 40 in the 18 centrifugal field along the inner surface 21 of porous wall 19 20. Hydrophilic particles 24 follow the thin fluid layer 40 downward and leave the vessel 11 tangentially with the fluid 21 discharge through tangential outlet 14. The hydrophobic 22 particle-containing bubhles congregate at the core of 23 vessel 11 to form a froth 34 (see Figure 4) which travels 24 upwardly through the vessel 11 and is discharged from the vessel th~ough vortex finder 16.
26 The apparatus and method of the present invention serve 27 to maximize the attachment of hydrophobic particles 26 to air ~ 1783~2 1 bubbles and thus increase the degree of separation of the 2 hydrophobic particles 26 from the particulate suspension.
3 This is due in part to the fac~ that flotation occurs in a 4 centrifugal field where the probability of collision and subsequent attachment of the air bubbles to hydrophobic 6 particles 26 is greatly enhanced. Thus, the novel apparatus 7 and method of the present invention take full advantage of 8 the affinity of the hydrophobic particles 26 for the air 9 bubbles in achieving maximal separation of the hydrophobic particles 26.
11 It will be appreciated that the same apparatus and 12 method may be used to separate finely divided hydrophobic 13 particles, or finely divided particles which are made 14 ~ydrophobic, from a particulate suspension containing no other particles. This is the application which is set forth 16 in Figure 4. In this process, which will be described in more 17 detail hereinafter, there are of course, no hydrophilic 18 particles particles 24 in the fluid discharge.
19 There are several significant advantages associated with the novel apparatus and method for separating particles 21 from a fluid suspension by flotation in a centrifugal field.
22 For example, the generally tangential orientation of inlet 23 12 and outlet 14 cause the injected particulate suspension to 24 form a forced vortex within vessel ll--the forced vortex creating a centrifugal field. Figure 5 iliustrates the 26 tangential velocity distribution in forced, free, and 27 combination forced-free vortex systems. As seen in Figure 5, ~ 1783~32 1 in a free vorte~ system, the tan~ential velocity is maximal 2 at an inter~ediate distance from the center of the apparatus.
3 Free vortices tend to occur in systems where the majority of 4 the flow leaves the apparatus axially.
In a forced vortex system, the whole fluid system 6 rotates at the same angular velocity. Hence,-a forced vortex 7 system results in a wheel-like motion with the ~angential 8 velocity of the fluid decaying to zero in the direction of 9 the axial center of the apparatus. Consequently, a quiescent froth 34 is more easily formed and stabilized in a forced 11 vortex system. Forced vortices tend to occur in systems 12 where the majority of the fluid flow leaves the apparatus 13 tangentially, such as in the preferred embotiment of the 14 novel apparatus discussed hereinabove.
A combination forced-free vortex system can be created 16 by combining the features characteristic of forced vortex 17 and free vortex systems, yielding a tangential velocity 18 distribution which is somewhat of a hybrid of the forced and 19 free vortex systems (see Figure 5). It should be emphasized that the novel apparatus and method of the present invention 21 serve to optimize the forced nature of the vortex created, 22 which in turn enhances the formation of a quiescent froth and 23 optimizes the quantity of bubblelparticle aggregates which 24 are recovered from the particulate suspension.
Another important advantage of the novel apparatus and -26 method of the present invention is the low water split which 27 is achieved. The water split may be defined as the ratio of 1 1783~2 1 the amount of water in the particle-containing froth 34 to 2 the amount of water in the slurry feed. It is highly 3 desirable to minimize the amount of water in the froth 34, 4 thereby minimizing the watér split.
One important factor to achieving a low water split is 6 the tangential orientation of both inlet 12 and outlet 14. A
7 tangential inlet and outlet assures that the particles will 8 be subjected to sufficient centrifugal forces to keep water 9 out of the froth phase. Other geometries such as conventional hydrocyclones with conical bottoms and axial 11 discharge result in significant transport of water through 12 the vortex finder into overflow product. Moreover, the 13 vertical orientation of vessel 11 is in part responsible for 14 the advan~ageously low water split achieved in the present invention. The vertical orientation of the vessel 11 16 maximizes the drainage o fluid from the froth 34 and the 17 overflow product which is moving upwardly in a vertical 18 direction, because the vertical orientation utilizes gravity 19 to its maximum extent and gravity is a major force acting on the water in the froth 34.
21 As the bubble/particle aggregates reach the core of 22 vessel 11, they congregate to form froth 34 which travels 23 upwardly towards the vortex finder 16, exiting vessel 11 24 therethrough. Due to the favorable water split obtained by the present invention, the particle-containing froth 34 -26 rontains a minimum amount of water. Since froth 34 travels 27 countercurrently to the thin fluid layer 40 and since the ~ ~7~2 1 vessel 11 is vertically oriented, water drainage from froth 2 34 is further enhanced, thus resulting in the low water 3 split.
4 Another structural feature of the presen~ invention which helps achieve the desired low water split is valve 36 6 which controls the flow of the fluid discharge through outlet 7 14. By opening up valve 36, the fluid discharge can be 8 removed at a rate sufficient to prevent the bottom portion of 9 vessel 11 from filling up with the fluid discharge. This, in turn, helps to maintain a quiescent froth 34 in the core of 11 the vessel 11. With valve 36 adjusted to a more reduced 12 outlet flow, the froth 34 can occupy more than 90% of the 13 volume of vessel 11 inside thin fluid layer 40. It will be 14 appreciated that valve 36 is shown by way of example only, and that any con~entional means for regulating the rate of 16 fluid flow through outlet 14 may be employed.
17 Referring now to Figure 4, the importance of the thin 18 fluid layer 40 and the separation of particles 32 from a 19 particulate suspension is illustrated. Fine particles 32, if not already hydrophobic, can be made hydrophobic by 21 treatment with certain chemicals, such treatment making 22 possible the separation of fine particles 32 by flotation.
23 (Fine particles 32 shown in Figure 4 thus correspond to 24 hydrophobic particles 26 shown in Figure 3.~ As seen in Figure 4, as the air is introduce~ from chamber 17 of air -26 plenum 18 through porous wall 20 into the thin fluid layer 27 40, small air bubbles are formed along the inner surace 21 ~ 17~82 1 of porous wall 20. The high shear velocity of the 2 particulate suspension in thin fluid layer 40 creates a 3 continual generation of very small air bubbles and provides 4 for intense contact of particles 32 with bubbles 30.
Mor~over, during their formation, air bubbles 30 are 6 momentarily constrained against the inner surface 21 of 7 porous wall 20. This temporary fixation of the air bubbles 8 30 to inner porous wall surface 21, together with the 9 directed motion of the particulate suspension toward the constrained air bubbles 30, considerably increases the 11 probability of collision between air bubbles 30 and 12 particles 32 in the thin fluid layer 40. In a conventional 13 flotation cell, air bubbles and particles are mixed together 14 at random, and the probability that a particle and bubble will meet with sufficient velocity to form a particle/bubble 16 aggregate is considerably less than the probability that 17 such an occurence will take place in the thin fluid layer 18 system of the present invention.
19 Additionally, since the thin fluid layer 40 of the present invention occupies less than 10% of the volume o~
21 vessel 11, flotation is achieved rapidly. This is because 22 the bubbles 30 need only arrive at the boundary between thin 23 fluid layer 40 and froth 34 before flotation is comple~e.
24 Indeed, flotation is achieved up to 300 times faster in the present invention than in most conventional flotation cells:
`26 It will be appreciated, as discussed above, that the 27 tangential outlet 14 and discharge re ulating valve 36 - 25 ~

l ~83~2 1 accommodate the maintenance of thin fluid layer 40 as well ~ the froth 34, by permitting discharge in such a ~anner and at 3 such a rate so as not to disturb the thin fluid layer 40 or 4 froth 34.
It will be understood that the generation of a large 6 number of very small air bubbles 30 (accomplished in part by 7 the high shear velocity of the particulate suspension) and 8 the constrained particle/bubble interaction (instead of 9 random particle/bubble interaction~ within thin fluid layer 40 are very important ~o the novel apparatus and method of 11 the present invention and ehe ex~raordinary flotation 12 results obtained thereby~
13 It should also be noted that the separation achieved by 14 the novel apparatus and method of the present invention has been shown experimentally to be due primarily to the improved 16 flotation techniques of the present inventionj not to be due 17 to factors which would cause separation of the particles by 18 size. That is to say, the present invention does not show 19 evidence of ~eparating by size the particles which are in suspension; on the contrary, the present invention separates 21 the particles according to flotation principles. This means 22 that a particulate product can be recovered from a fluid 23 suspension by the flotation techniques of the present 24 invention even though that product is comprised of particles over a broad range of particie sizes and even though there `26 may be other components in the suspension within the same 27 range of particle sizes.

1 1783~%
l As mentioned previously, the retention time of the 2 particulate suspension from ~he time it enters inlet 12 to 3 the time the fluid discharge exits outlet 14, is a matter of 4 seconds, thus providing for a much more rapid separation than is achieved in con~entional flotation cells. This, in turn, 6 allows flotation apparatus 10 ~o be constructed much smaller 7 than conventional flotation cells~ thereby eliminating the 8 need for large flo~r space to operate ~he apparatus. It will 9 be appreciated that the retention time is also influenced by the length of the porous wall 20 and the amount of air sparged 11 therethrough. Consequently, porous wall 20 may be 12 constructed with a length that will provide the most 13 desirable retention tlme for a given application.
14 It will be understood that the invention may be embodied in other specific forms without departing from its spirit or 16 essential characteristics; For e~ample, instead of the 17 verticle orien~a~ion of flotation apparatus 10, as shown in 18 Figures 2 and 3, the flotation apparatus 10 may be inclined l9 slightly as desired. The described embodiments are thus, to be considesed in all sespects only as illustrati~e and not 21 restrictive. The scope of the invention is, therefore, 22 indicated by the appended claims rather than by the foregoing 23 description. All changes-which come within the meaning and 24 range of equivalency cf the claims are to be embraced within their scope.
26 What is claimed and desired to be secured by 27 Canadian Letters Patent is:

Claims (15)

1. An apparatus for separating particles from a particulate suspension by flotation in a centrifugal field, comprising:
a generally vertically oriented vessel having a generally cylindrical configuration;
an inlet at the upper end of the vessel for introducing a particulate suspension under pressure into the vessel in a generally tangential fashion;
an outlet at the lower end of the vessel for directing fluid discharge from said particulate suspension out of the vessel in a generally tangential fashion, said inlet and outlet directing fluid flow so as to create a forced vortex in the vessel, said forced vortex forming a centrifugal field; and means for introducing air into said particulate suspension, the air forming small bubbles which separate the particles from the particulate suspension by flotation in the centrifugal field.

2. An apparatus as defined in claim 1 wherein the inlet is configurated so as to direct the particulate suspension around an inner surface of the vessel in such a manner that the particulate suspension forms a thin fluid layer within the vessel.

3. An apparatus as defined in claim 1 further comprising means for controlling flow rate of the fluid discharge through the outlet.

4. An apparatus as defined in claim 1 wherein at least a portion of a wall of the vessel comprises a porous wall, and wherein the air introducing means comprises an air plenum surrounding the porous wall portion of the vessel, the porous wall providing for the passage of air from the air plenum into the particulate suspension within the vessel.

5. An apparatus as defined in claim 1 wherein the air bubbles congregate within the vessel to form a froth and further comprising a vortex finder positioned at the upper end of the vessel so as to guide the froth coaxially out of the vessel.

6. An apparatus for separating particles by flotation in a thin fluid layer, comprising:
a generally vertically oriented vessel having a generally cylindrical configuration, at least a portion of a wall of the vessel comprising a porous wall;
an inlet at the upper end of the vessel for introducing a particulate suspension under pressure into the vessel in a generally tangential fashion, the inlet being configurated so as to direct the particulate suspension into the vessel in such a manner that the particulate suspension forms a thin fluid layer around the inner surface of the wall of the vessel;
an outlet at the lower end of the vessel for directing fluid discharge from the particulate suspension out of the vessel in a generally tangential fashion, the inlet and the outlet directing fluid flow so as to create a forced vortex in the vessel, the forced vortex forming a centrifugal field;
means for controlling flow rate of the fluid discharge through the outlet; and an air plenum surrounding the porous wall portion of the vessel, the porous wall providing for the passage of air from the air plenum into the thin fluid layer within the vessel.

7. A method for separating particles from a fluid suspension by flotation in a centrifugal field, comprising the steps of:
obtaining a generally vertically oriented vessel having a generally cylindrical configuration;
introducing a fluid suspension into the upper end of the vessel in a generally tangential fashion;
forming a centrifugal field in the vessel by creating a forced vortex in the vessel;
sparging air into the fluid suspension, the air forming small bubbles which separate the particles from the fluid suspension leaving a fluid discharge; and directing the fluid discharge out of the lower end of the vessel in a generally tangential fashion.

8. A method as defined in claim 7 further comprising the step of regulating flow rate of the fluid discharge from the vessel.

9. A method as defined in claim 7 further comprising the step of maintaining the fluid suspension within the vessel as a thin fluid layer against an inner surface of the vessel.

10. A method as defined in claim 7 wherein at least a portion of a wall of the vessel comprises a porous wall and wherein the sparging step comprises introducing air through the porous wall and into the fluid suspension within the vessel, the air forming bubbles within the fluid suspension.

11. A method as defined in claim 10 further comprising the step of generating the air bubbles at the porous wall so as to promote directed collision between the air bubbles and the particles in the fluid suspension.

12. A method as defined in claim 7 further comprising the steps of forming a particle-containing froth within the vessel and removing the froth from a coaxial outlet formed in the top of the vessel.

13. A method for separating particles from a fluid suspension by flotation in a centrifugal field, comprising the steps of:
obtaining a vessel having a generally circular cross-section;
introducing a fluid suspension into the vessel so as to form a thin layer against an inner surface of the vessel and so as to create a forced vortex in the vessel, the forced vortex forming a centrifugal field;
and sparging air into the thin layer of fluid suspension, the air forming bubbles which separate the particles from the fluid suspension by flotation.

14. A method for separating particles from a fluid suspension of particles by flotation in a thin layer of the fluid suspension, comprising the steps of:
obtaining a generally vertically oriented vessel having a generally cylindrical configuration, at least a portion of a wall of the vessel comprising a porous wall;
introducing a fluid suspension into the vessel in a generally tangential fashion such that the fluid suspension swirls around an inner surface of the wall of the vessel and forms a thin layer thereagainst;
forming a centrifugal field in the vessel by creating a forced vortex in the vessel;
sparging air through the porous wall and into the thin layer of fluid suspension within the vessel, the air forming small bubbles which separate the particles from the fluid suspension leaving a fluid discharge;
directing the fluid discharge out of the vessel in a generally tangential fashion;
regulating flow rate of the fluid discharge from the vessel;
forming a particle-containing froth within the vessel; and removing the froth coaxially from the vessel.

15. A method as defined in claim 14 further comprising the step of generating the air bubbles at the porous wall so as to promote directed collision between the air bubbles and the particles in the fluid suspension, thereby increasing the rate of collision of the air bubbles with the particles in the fluid suspension.