CA1211871A

CA1211871A - Flotation machine

Info

Publication number: CA1211871A
Application number: CA000419390A
Authority: CA
Inventors: Richard B. Weber; Robert H. Forsyth
Original assignee: Zimpro Inc
Current assignee: Zimpro Passavant Inc
Priority date: 1982-01-14
Filing date: 1983-01-13
Publication date: 1986-09-23
Also published as: AU1006683A; GB8300639D0; SE8300137D0; ZA82237B; AU554801B2; SE8300137L; JPS58180250A; GB2114469A

Abstract

ABSTRACT
An apparatus and method for flotation of minerals and non-minerals are disclosed which incorporate the advantages of both dissolved air flotation and sparged air flotation. A flotation machine is provided consisting of a flotation chamber and a rotating gas diffuser. The flotation chamber has a unique design and features which create an efficient lower mixing chamber and an upper quiescent zone for maximizing float accumulation. The rotating diffuser produces ultra fine bubbles as small as 50 microns in diameter. The size of the bubble diameter can be controlled and produced in any size from less than 50 microns to greater than 3 mm to enable flotation of various materials and/or various sized fractions of materials. The ultra fine bubbles are produced by the diffuser in the same size range as bubbles produced by dissolved air flotation, but without the necessity of pressurizing gas in a first tank followed by depressurizing in a second tank. Since the diffuser is located within the flotation chamber and can vary bubble size, it also provides the advantage of sparged air flotation. This rotating diffuser configuration is a low drag thin disc which, when combined with a specially designed flotation chamber, embodies a flotation machine which produces higher product yields over a wider range of feed size fractions utilizing significantly less energy than the devices of the prior art.

Description

~21187~

Flotation is a process which has been practiced for many years in, for example, ore-dressing. In this process bubbles of the gas are introduced into a slurry or liquid mixture to which flotation agents are usually added to cause selected solid or liquid materials to become attached to the gas bubbles and rise to the surface as a froth. The froth, in which selected materials are concentrated, is skimmed from the surface of the slurry or liquid mixture, or is allowed to overflow. Materials which do not float are also removed from the cell as a stream, usually termed "tailings".
Flotation has been used to separate and concentrate many different types of materials, such as metallic ores and minerals, coal, grains and fl0ur, pigments, paper pulps, oils, and sewage sludge. Flotation is a phenomenon related to the surface characteristics of the materials to be separated and/or concentrated; chemicals are usually added to selectively enhance or depress floatability, produce froth, and/or deflocculate mineral surfaces.
Two basic types of flotation processes are used. In one process, generally called dispersed air flotation, air is sparged into the slurry or mixture or introduced below a rapidly revolving impeller. The other type of process, known as dissolved air flotation, has found considerable utility in concentrating se~age sludge solids and other organic materials. Air or other gas is dissolved into the slurry in a first chamber operating at superatmos-pheric pressure, typically 20 - 80 psig. In a second chamber, a flotation cell operating at atmospheric or subatmospheric pressure, the dissolved gases are released by a pressure reduction, forming a froth which carries the selected material upward. Both the air ~or other gas) and slurry or mixture must be pressurized.
In dispersed air flotation, intense agitation is required ~o produce 1211~1 the small bubbles which are required.
In dissolved air flotation, intense agitation is required in the pressurized chamber to dissolve the gas into the slurry or mixture. Furthermore, there are pumping energies expended in compressing both the gas and liquid.
In either case, it can be seen that the energy require-ments are high. Furthermore, control over bubble size is very limited, making the flotation processes inefficient.
A rotating gas diffuser useful for oxygenating waste-waters is described in Ihrig et al United States Patent No.3,992,491. Further improvements, shown in Hise United States Patent No. 4,228,112, have been found to beneficially effect flotation.
The invention relates to a flotation machine for selective flotation of materials from a body of liquid, said apparatus comprising: a flotation cell with a generally flat bottom and generally vertical side walls, having a lower mixing zone and an upper froth collection zone, said cell being adapted to hold said body of liquid; a rotating diffuser having a hollow shaft, immersed in said body of liquid for introducing a gas stream in the form of bubbles, of between 0.05 and 10 millimeters controllable nascent diameter, into said body of liquid; means for injecting a gas through the diffuser; means for introducing raw materials to be separated, frothers and other chemicals into said cell; and means for removing and recovering froth of floated materials and tailings of unfloated materials.

. ~

lZ111~7~
In drawings illustrating the invention:-Figures 1 and 2 are drawings of one embodiment of theapparatus of the invention;Figure 3 is a side view of a further embodiment of the apparatus - 2a -,, b,, ~2~ 71 of the invention;
Figures 4 and 5 are top and side views showing one embodiment of a baffle which is useful in the apparatus of the invention;
Figures 6 and 7 are top and side views of a further embodiment of a baffle which is useful in the apparatus of the invention;
Figure 8 is a side view of the rotating active diffuser and drive mechanism of this invention;
Figure 9 is a plan view of the disk like rotating active diffuser;
Figure 10 and 11 are top and side views of a preferred embodiment of the rotating active diffuser;
Figures 12 - 14 are side views of further preferred embodiments of the rotating active diffuser;
Figures 15 and 16 are top and side views of a froth collecting weir of this invention.
This invention embodies a flotation machine which contains a flotation chamber (cell), a rotating active diffuser, a means for injecting a gas through the diffuser, a m~ans for controlling bubble size and a means for providing the normal flotation cell functions of introducing raw materials, reagents and frothers; together with means for recovering the product and removing the tail-ings.
As shown in Figures 1 through 3, the flotation cell consists of a square, round or rectangular shaped tank 1 with a flat bottom 2 and generally vertical side walls. The side walls may have a unique shape. One such shape is shown on Figures l and 2, having a horizontal baffle 3 with a curved surface 4 on the bottom and an outwardly tapered surface 5 at the top. The horizontal baffle generally divides the flotation cell into lower mixing zone 6 and upper 12118~1 quiescent froth collection zone 7.
Vertical baffles 8 are optionally placed between the cell floor 2 and the horizontal baffle 3 to eliminate swirling of the cell contents. Long radius inserts 9 are optionally placed in the corners of the cell to form a smooth radius rather than a sharp corner. The bottom 2 of the cell may have a recess 10 to accommodate the rotating diffuser, or the rotating diffuser (not shown) may be located just above the floor 2.
Water or other fluid may be introduced beneath the disk like member of the rotating diffuser through line lOa, thereby preventing solids accumula-tion under the diffuser and thus reducing the energy required to turn the dif-fuser.
Another side wall shape which may be used is shown in Figure 3. In this particular configuration vertical side walls 11 enclose ~he mixing zone 12, whilst sloping walls 14 give a gradually increasing cross-section to the froth collection zone 13. Froth paddles 15 assist the froth removal.
Other side wall shapes may also be used.
In Figure 3, the gas diffusion means is shown as comprising a rotating disk like member 16 having porous diffusion members 16a through which gas is diffused into the body of liquid within the cell. Mixing blades 16b may be in-corporated into the upper and/or lower surfaces of the disk 16 to enhance mixing.
Gas is supplied to disk 16 from a source (not shown) through rotating seal 17 and hollow shaft 18. In the Figure 3 embodiment, disk 16 is rigidly con-nected to shaft 18, and the assembly is rotated by motor 19 acting through drive l9a.
Optionally, rotating diffuser 16 may be driven from below with shaft 18 passing through the cell floor. Disk 16 may be recessed into the cell floor~

lZ11871 For most practical flotation applications, baffles are required in the mixing zone to reduce the swirling motion of the cell contents, and to dis-sipate the energy in the upward gas-liquid flow created by the rotating diffuser.
The novel baffle designs shown in Figures 4 - 7 accomplish the necessary action by changing the flow direction one or more times with the final flow direction near the vertical but different than the direction of rotation of the rotating diffuser. For the sake of clarity, the diffuser, hollow shaft and other parts of the flotation machine are omitted in these drawings.
One baffle embodiment which accomplishes the desired energy dissipa-tion is shown in Figures 4 and 5. The "chevron" configuration of baffle 21, best seen in Figure 5, imposes a single change in flow direction. The proper liquid and gas flow patterns are achieved by varying baffle length, width and angle. In this baffle configuration the upper baffle element 21a and lower baffle element 21b, arranged respectively at angles 21c and 21d from the hori-zontal, are connected at 21e. The baffles may be placed on all side walls as shown in Figures 4 and 5, or alternately arranged on two of the side walls. The rotating diffuser, located near the bottom of the cell, generates radial and upward movement of gas and liquid. A properly designed baffle provides a com-pletely quiescent surface in the upper froth collection zone over wide ranges of particle sizes to be floated and mixing energies generated by the diffuser.
In another baffle configuration 31, shown in Figures 6 and 7, angled baffles are arranged in rows 31a, 31b and 31c. While up to five rows may be employed, two or three rows are presently preferred. The baffles in adjacent rows are attached to the side wall at differing angles to change the gas-liquid flow direction and dissipate the vertically directed forces. The baffles may be arranged on two or four sides of a rectangular flotation cell, or on any portion 1211~371 of the circumference of a round cell for example.
The baffle arrangement of Figures 6 and 7 imposes a dual change in flow direction. In this configuration the open space between baffles provides some bypassing of the fluid that would otherwise impinge on the baffle and, thus, permits an adjustment in the amount of energy dissipated.
As shown in all of Figures 4 - 7, the cell is preferably constructed to include a sl~ped side wall in the froth collection zone to accomodate a froth skimmer such as vane 15, and a weir 24.
Flotation cells require a continuous flow of water to the cell to maintain liquid level as the floated material exits over the outlet weir.
Another unique feature of the float chamber embodied herein is to in-troduce a small water flow underneath the rotating active diffuser (RAD) which reduces solids accumulation under the RAD and reduces horsepower by providing a clean water bearing. Optionally, any fluidizing agent, liquid or gas, includ-ing flotation reagents, may be used.
As we have seen in Figure 3, the RAD comprises a rotating seal, a drive mechanism, a hollow shaft, and a thin disk with mixing blades and diffu-sion media through which gas enters the float cell. Outside the diffusion media there is a flat or tapered edge which provides a surface for the ultra fine bubbles to continue a laminar flow path into the liquid without coalescence.
The fine bubbles are produced by the shearing action of the liquid at the sur-face of the diffusion media as the diffuser rotates. The speed of rotation is critical to the development of ultra fine bubbles, and bubble size may be varied by changing the rotation speed and/or by varying the air flow rate through the ~ porous media. ~le porosi~y of the media, together with the total medium surface L~ S ~ le ct~
a area, are also ~ k~e~o~ to provide the desired bubble sizes.

12~

Although the rotating disk like member may be rotated by using pres-surized gas as the motive force, the preferred embodiment has a rotatable hollow shaft attached to and turning the diffuser disk. Such an arrangement is shown in Figures 8 and 9.
In the diffusion apparatus 20 shown in Figures 8 and 9, the rotating active diffuser comprises: a hollow shaft, said shaft defining a main feed line for the gas to be introduced into said body of liquid, a disk like member 22 mounted on said shaft 26 perpendicular to the axis of rotation thereof, said member defining at least one gas plenum 70 in operative connection with said main feed line, at least one wall of said plenum being porous to permit the passage of gas through said wall to the outer surface of said disk like member so that the introduction of gas into said main feed line when the gas diffuser is rotating about its axis of rotation 27 while immersed in said body of liquid will cause gas to flow into said gas plenum 70 and out through said porous wall thereof to produce nascent gas bubbles on the surface of the disk like member 22, which nascent bubbles are sheared off to form fine gas bubbles by the vis-cous shear forces exerted by said liquid as the disk like member 22 rotates in said body of liquid, and means for minimizing the coalescence of said fine sheared bubbles, resulting from turbulence in the wake and jet surrounding the rotary disk like member that is caused by vortices shed from the edge 72 of the disk like member 22 and spiraling outward from said edge, said means including said disk like member, wherein the ratio of the overall diameter of said disk like member to the maximum thickness thereof in the gas diffusing area occupied by said gas plenums is at least about 10:1.
Preferably both the upper and lower walls of the gas plenum or plenums are porous.

lZ11871 The ratio of overall diameter of the disk to the maximum thickness of the disk in the gas diffusing area occupied by the plenums is at least about 10 : 1. Improved results are obtained when this ratio is at least about 48 : 1, more improvement is obtained with a ratio of at least about 64 : 1, and still greater improvement with a ratio of at least about 128 : 1. The preferred value of the ratio is at least about 256 : 1.
In the Figure 8 arrangement, the hollow shaft 26 is rotated by a motor 30 acting through drive 28. Gas is introduced through a rotary seal 49. Op-tionally, a sleeve 32 carrying a helical float submerging screw 40 may be used to recycle froth adjacent the shaft to the mixing zone.
The disk of Figure 9 may have top and bottom surfaces which are sub-stantially parallel to each other throughout their areas.
Optionally, the top and bottom surfaces may be substantially parallel to each other except at the outer edge portion 72 of the disk, where the disk tapers in cross-section to a smaller thickness at the perimeter, as illustrated in Figure 8.
In a further embodiment, mixing blades 42 and/or 44 are incorporated on the upper and/or lower surface of the disk 22 to increase the mixing energy exerted by the rotating diffuser and to decrease bubble size by increasing the shear velocity of the liquid across the porous walls of the gas plenum or plenums.
Another mixing blade according to the invention is shown in Figures 10 and 11. These Figures show straight radial mixing blades 25 which may be located on one or both sides of the disk 22. The blades may optionally be curved~
Figure 11 shows the disk 22 as having substantially parallel surfaces 8~1 throughout the surface areas.
Figures 12 through 14 show other embodiments with tapered outer edge portions 39; these embodiments are preferred.
The disk of Figure 14 is recessed into the bottom of the flotation cell, and its top surface is substantially level with the cell bottom surface.
In each of these embodiments, the diffusion media 23 may be located on *he top surface only, or may be on bottom surfaces, or, indeed, on both sur-faces.
The means for collecting and removing the floated froth from the flotation cell may vary. One form shown in Figures 3 - 7 comprises movable vanes 15 on one or more sides of the flotation cell which act to cause froth to overflow via weir 24.
A further embodiment is shown in Figure 15 where the weir is located at approximately the center of the froth collection zone, and may surround the shaft of the rotating active diffuser.
The rotating active diffuser 22 is further defined as:
a disk like member having a gas inlet and defining at least one gas plenum in operative connection with said inlet and at least one wall of said plenum being porous to permit the passage of gas through said wall to the outer surface of said disk like member so that the introduction of gas into said gas inlet when the gas diffuser is rotating about its axis of rotation while immersed in said body of liquid will cause gas to flow into said plenum and out through said porous wall thereof to produce nascent gas bubbles on the surface of said disk like member, which nascent bubbles are sheared off to form fine gas bubbles by the viscous shear forces exerted by said liquid as the disk member rotates in said body of liquid~ and means for minimizing the coalescence of said fine ~2118~

sheared bubbles, resulting from turbulence in the wake and jet surrounding the rotary disk like member that is caused by vortices shed from the edge of the disk like member and spiraling outward from s-aid edge, said means including said disk like member, wherein the ratio of the overall diameter of said disk like member to the maximum thickness thereof in the gas diffusing area occupied by said gas plenums is at least about 10 : 1.
The reason the relationship between disk diameter and disk thickness in the gas diffusi~g area has such importance appears to be the fact that for some reason the extent of coalescence that is caused by the shedding of large vortices at the edge of a rotating disk is many times greater than the coales-cence that would usually be expected from more typical turbulence present under other conditions. The rapidly rotating vortices cause entrainment of the bubbles carried in the wake following the vortices, and then the turbulent colli-sion of closely spaced bubbles in the vo~tex centers causes coalescence, but at a very much greater rate than would be expected.
With the turbulence usually present under other conditions, perhaps half a dozen to a dozen fine bubbles may be caused to coalesce at one time, but the volume of the resulting larger bubble is not so great as to defeat the aim of achieving a rapid rate of gas dissolution. However, the coalescence of fine bubbles that occurs with high gas flow rates in the vortices produced at the edge of a rotating disk can take place at such a level as to defeat completely the goal of maintaining a large number of very fine bubbles that will produce a high dissolution rate. Thus, tests have shown that typically thousands of sheared bubbles ~a number having an order of magnitude three times the order of magnitude of the number of bubbles that tend to coalesce in the usual situation) will, with high gas flow rates, coalesce into single bubbles within the vortices 1:~11871 in the wake and jet emanating from the disk. This, of course, has a very marked effect on the volume of the resulting coalesced bubbles.
It appears that the critical relationship between a very thin disk and the efficient formation and maintenance of fine gas bubbles expressed above is due to a number of factors:
1. As indicated by Figure 4 on page 259 of an article by Paul Cooper entitled "Turbulent Boundary Layer on a Rotating Disk Calculated with an Effec-tive Velocity", which appeared in the AIAA Journal, Vol. 9 (1971), pages 255 -261, the thickness of the boundary layers formed at the upper and lower surfaces of a rotating disk may be represented with a reasonable degree of accuracy by the formula:
~ = 0.26 DRe 2 where ~ = the boundary layer thickness, D = the overall diameter of the disk, and Re = the Reynolds number of the particular apparatus and liquid involved.

2. The Reynolds number can be represented by the following formula:
R = wD2/4v where w = the rotation speed of the disk, D = the overall diameter of the rotating disk, and v = the kinematic viscosity of the liquid in which the disk is rotating.

3. Substituting this expression for the Reynolds number in the formula given in (1.) above gives the following formula:

0.26D
D2 0.125 4v

4. The disk diameters and rotation speeds of current practical interest in the froth flotation field span the range from about a 13 inch diameter disk 1211~71 rotating at 600 r.p.m. to about a 10 foot diameter disk rotating at 50 r.p.m.
The Reynolds number factor (wD2/4v)0 125 in the denominator of the formula for determination of boundary layer thickness, given in the paTagraph numbered 3 above, varies from about 6.4 for a 20 inch disk to about 7.7 for the 10 foot disk, at the respective rotation speeds just indicated.

5. Because the Reynolds number raised to the 0.125 power remains essen-tially constant, despite changes in the overall diameter and speed of rotation of the rotating disk, the thickness of the boundary layer flow across a rotating disk immersed in a given liquid and rotated at about the indicated optimum speeds can be represented roughly as some constant times the disk diameter, i.e., ~ = K D

6. It is believed (a) that the amount of bubble coalescence occurring within the vortices at the edge of a rotating, disk gas diffuser is proportional to the rate at which bubbles are ingested into, or in other words are transport-ed to the center of, these vortices, (b) that this rate, in turn, is generally proportional to the pressure differential between the inside and outside of in-dividual vortices, and (c) that the ratio of disk thickness in the gas diffusing area to boundary layer thickness (d/8) controls the indicated pressure differen-tial. Thus, the smallest possible value for the ratio of disk thickness (in the gas diffusing area) to boundary layer thickness (d/8) should be sought.

7. From the relationship expressed in the paragraph numbered 5 above, it would seem to follow that the smallest possible value for the ratio of maximum disk thickness in the gas diffusing area to overall disk diameter ~d/D) should also be sought. Conversely, the largest possible ratio of disk diameter to maximum disk thickness in the gas diffusing area (D/d) would appear to be pre-ferred, and this is confirmed by the discovery already noted above that a ratio ~Z11~71 of disk diameter to maximum disk thickness in the gas diffusing area of approxi-mately 10 : 1 produces quite satisfactory bubble shearing results with a rotat-ing gas diffuser, and that a higher ratio, up to approximately 256 : 1, or even higher, produces the best results.
Example 1 A coarse phosphate ore which was sized 75 percent -35 to ~150 mesh, and 25 percent -20 to ~ 35 mesh, was separated in a small scale flotation appQratus of this invention. A rotat-ing active diffuser disk of 13 inch diameter was located in a baffled flotation cell with dimensions 18 inches wide, 18 inches deep and 13 inches high. Disk speed and air flow rates were varied to determine their effects on float grade and recovery of bone phosphate of lime (BPL). The results shown below indicate highest phosphate recovery at 300 r.p.m. and an air flow rate of 156 cubic feet per hr (cfh).
Concentrat~

RAD SPEED Air Flow Grade BPL Recovery BPL
(rpm) (cfh) (%) (o) 300 31 70.4 80.0 300 93 69.5 86.9 300 156 68.4 92.5 375 31 67.4 81.3 375 156 64.8 79.1 440 93 64.8 72.5 440 156 65.0 72.5 Example 2 Using the same phosphate ore and apparatus as in Example 1, the effect of mixing blades attached to the rotating active disk was studied. The results for this particular ore indicated deterioration in both the quality and percent recovery of BPL with reduced RAD speed and/or use of mixing blades.

_ 13 -~Z11871 Concentratc - --RAD Speed Mixing Blade Grade BPL Recovery BPL
~rpm) Size t%~ (%) 300 Large 63.8 79.4 375 Large 65.5 87.7 400 None 70.0 90.9 520 None 66.~ 93.2 375 Small 64.4 90.0 420 Small 66.1 90.9 Example 3 BPL was separated from an intermediate pebble phosphate ore ~-14 to +20 mesh) under optimized conditions, using the apparatus of Example 1 in com-parison with a flotation machine of Denver Equipment design. Fatty acid was added to promote flotation. The results indicated that significantly higher BPL recoveries were achieved with the flotation machine of the present invention.
The grade of BPL recovered was 70 - 72 percent for all tests.

Fatty Acid BPL Recovery Machine tlb/ton) (5) RAD 1.0 88.5 Denver 1.0 70.6 RAD 1.2 91.8 Denver 1.2 77.7 RAD 1.6 95.3 Denver 1.6 90.3 Example 4 Flotation of various mesh sizes of an Iowa coal was determined with the apparatus of Example 1. The results indicated that each size of coal was successfully floated.
Product - -Coal Size Air Flow Yield Ash Sulfur ; tMesh) (scfh) RPM (%) BTU/lb t%) t%) -65 +150 24 560 91 12,306 11.3 2.8 -150 +325 24 560 --- 12,275 12.8 3.3 -325 24 560 93 12,627 12.6 2.8

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An apparatus for selective flotation of materials from a body of liquid, said apparatus comprising: a flotation cell with a generally flat bottom and generally vertical side walls, having a lower mixing zone and an upper froth collection zone, said cell being adapted to hold said body of liquid; a rotating diffuser having a hollow shaft, immersed in said body of liquid for introducing a gas stream in the form of bubbles, of between 0.05 and 10 millimeters controllable nascent diameter, into said body of liquid; means for injecting a gas through the diffuser;
means for introducing raw materials to be separated, frothers and other chemicals into said cell; and means for removing and recover-ing froth of floated materials and tailings of unfloated materials.

2. An apparatus according to claim 1, including means for controlling the nascent bubble diameter of gas produced by said diffuser by adjustment of rotational speed of said diffuser, gas flow rate therethrough, or a combination thereof.

3. An apparatus according to claim 1, including means for introducing liquid/fluid into the cell to replace liquid/fluid removed from the cell in said froth and/or tailings.

4. An apparatus according to claim 3, wherein said liquid/-fluid is introduced in a stream beneath the rotating diffuser to prevent solids accumulation under said diffuser and reduce rotational energy.

DN 2408B (SDI 4.2-745) 15

5. An apparatus according to claim 1, wherein said flotation cell has in its mixing zone at least two baffles extend-ing inwardly from opposing side walls thereof for reducing the swirling motion of liquid and dissipating the energy in upward gas-liquid flow resulting from the action of the rotating diffuser.

6. An apparatus according to claim 5, wherein said baffles are vertical.

7. An apparatus according to claim 5, having two or more horizontal rows of baffles extending inwardly from opposing side walls, wherein baffles of adjacent rows are attached to the side wall at differing angles to change the gas-liquid flow direction and dissipate the energy in the upward gas-liquid flow.

8. An apparatus according to claim 5, wherein said baffles are comprised of connected upper and lower elements arranged at differing angles from the horizontal.

9. An apparatus according to claim 1, wherein said mixing zone and froth collection zone are separated at the periphery of said cell by a generally horizontal baffle.

10. An apparatus according to claim 9, wherein the lower surface of said horizontal baffle curves from a horizontal attitude at its inner edge to a generally vertical attitude congruent with the side walls of the flotation cell.

11. An apparatus according to claim 9, or 10, wherein the upper surface of said horizontal baffle is tapered outward to form a sloping wall between the inner edge of the horizontal baffle and the upper edges of the side walls.

12. An apparatus according to claim 1, wherein the means for removing and recovering the froth of floated materials includes a continuous overflow weir located adjacent the center of upper froth collection zone.

13. An apparatus according to claim 12, wherein said over-flow weir surrounds the rotating diffuser shaft.

14. An apparatus according to claim 1, wherein said rotating diffuser comprises: a hollow shaft, said shaft defining a main feed line for the gas to be introduced into said body of liquid, a disk like member mounted on said shaft perpendicularly to the axis of rotation thereof, said member defining at least one gas plenum in operative connection with said main feed line, at least one wall of said plenum being porous to permit the passage of gas through said wall to the outer surface of said disk like member so that the introduction of gas into said main feed line, when the gas diffuser is rotating about its axis of rotation while immersed in said body of liquid, will cause gas to flow into said gas plenum and out through said porous wall thereof to produce nascent gas bubbles on the surface of said disk like member, which nascent bubbles are sheared off to form fine gas bubbles by the viscous shear forces exerted by said liquid as the disk member rotates in said body of liquid, and means for minimizing the coalescence of said fine sheared bubbles, resulting from turbulence in the wake and jet surrounding the rotary disk like member that is caused by vortices shed from the edge of the disk like member and spiraling outward from said edge, said means including said disk-like member, wherein the ratio of the overall diameter of said disk like member to the maximum thickness thereof in the gas diffusing area occupied by said gas plenums is at least about 10:1.

15. An apparatus according to claim 1, wherein said rotating diffuser comprises: a disk like member having a gas inlet and defining at least one gas plenum in operative connection with said inlet and at least one wall of said plenum being porous to permit the passage of gas through said wall to the outer surface of said disk-like member so that the introduction of gas into said gas inlet, when the gas diffuser is rotating about its axis of rotation while immersed in said body of liquid, will cause gas to flow into said gas plenum and out through said porous wall thereof to produce nascent gas bubbles on the surface of said disk like member, which nascent bubbles are sheared off to form fine gas bubbles by the viscous shear forces exerted by said liquid as the disk member rotates in said body of liquid, and means for minimiz-ing the coalescence of said fine sheared bubbles, resulting from turbulence in the wake and jet surrounding the rotary disk like member that is caused by vortices shed from the edge of the disk like member and spiraling outward from said edge, said means including said disk like member, wherein the ratio of the overall diameter of said disk-like member to the maximum thickness thereof in the gas diffusing area occupied by said gas plenums is at least about 10:1.

16. An apparatus according to claim 14 or 15, wherein said disk like member is recessed into the floor of said cell.

17. An apparatus according to claim 14 or 15, wherein said disk like member is recessed into the floor of said cell, and wherein the upper surface of said disk like member is flat.

18. An apparatus according to claim 14, or 15, wherein the ratio of the overall diameter of said disk like member to the maximum thickness thereof in the gas diffusing area occupied by said gas plenums is at least about 48:1.

19. An apparatus according to claim 14, or 15, wherein the ratio of the overall diameter of said disk like member to the maximum thickness thereto in the gas diffusing area occupied by said gas plenums is at least about 64:1.

20. An apparatus according to claim 14, or 15, wherein the ratio of the overall diameter of said disk like member to the maximum thickness thereof in the gas diffusing area occupied by said gas plenums is at least about 128:1.

21. An apparatus according to claim 14, or 15, wherein the ratio of the overall diameter of said disk like member to the maximum thickness thereof in the gas diffusing area occupied by said gas plenums is at least about 256:1.

22. An apparatus according to claim 14, or 15, wherein the top wall, the bottom wall, or both said walls of, said gas plenums are porous and permit the passage of gas therethrough.

23. An apparatus according to claim 14, or 15, wherein the top surface and bottom surface of said disk like member are substantially parallel to each other throughout their area.

24. An apparatus according to claim 14, or 15, wherein the top surface and bottom surface of said disk like member are substantially parallel to each other except at the outer edge portion of said disk where the disk tapers in cross section to a smaller thickness at the perimeter of said member.

25. An apparatus according to claim 14, or 15, wherein mixing blades are incorporated on at least one surface of said disk to increase the mixing energy exerted by the rotating diffuser and to decrease bubble size by increasing the shear velocity of the liquid across the porous wall.

26. An apparatus according to claim 14, or 15, wherein said hollow shaft is rigidly connected to said disk whereby to rotate the latter.

27. An apparatus according to claim 14, or 15, wherein the top wall, the bottom wall, or both said walls of, said gas plenums are porous and permit the passage of gas therethrough, and wherein the area and/or porosity of walls which are porous are selected to provide bubble sizes of between 0.05 and 10 millimeters diameter.

28. An apparatus according to claim 1, or 14, or 15, wherein said rotating diffuser is at the lower end of said hollow shaft, and said shaft is supported above the liquid level in said cell.

29. An apparatus according to claim 1, or 14, or 15, wherein said rotating diffuser is at the upper end of said hollow shaft, and said shaft extends upwardly through the bottom wall of said cell.