CA1298001C - Aeration apparatus - Google Patents

Abstract

ABSTRACT
"AERATION APPARATUS"
Aeration apparatus for use in recovering values from slurries in a flotation cell wherein the impeller has a plurality of generally radial downwardly extending blades on its lower surface, each blade extending from the hollow drive shaft of the rotor to the periphery of the impeller and generally increasing in height radially outwardly along the length of the blade. In the preferred form of the invention a stator is provided having corresponding radial blades located beneath the impeller.

Description

BAC~GROUND OF THE INVENTION
This invent~on relates to aeration apparatus and more particularly to an impro~ed apparatus for the production of small gas bubbles in a liquid in order to create a large interfacial area betw~en the gas and the liquid, thereby increasing ~he efficiency of processes such as flotation and gas liquid mass transfer.
~ lthough the apparatus may be of value in other fields such as aeration and gas ab~orption, the invention will be described in relation to the flotation process.
The art of flotation qenerally involYes the aeration and agitation of a slurry or suspension in water of finely di~ided ore particles in a cell or apparatus of suitable design. The mineral may be regarded as a misture of valuable minerals or ~alues~, and clay, rock or other unwanted ~gangue~ particles. The object of the process is to remove the values from the gangue, and this may b~
achieved by co~ditioning the slurry with chemical reag~nts which ha~e the effect of rend~ring the values selectively ;. 20 hydrophobic or water repellent, while leaving the gangue particles hydrophilic or wettable. Thus when a hydrophobic particle comes into contact with a bubble, it will attach and rise with it to the surface of t~e liquid where it forms a froth which may be scraped off into a launder, thereby conveying the value~ out of the cell and separating th~m rom the ganque, wh;ch remains with the liquid in the cell. ~o assist in the formation of a stable froth, it may be necessary to add chemical frothing agents.
Flotation machines as customarily constructed consist o a tank in the base of which is an aerating rotor and a concentric stator. Air is introduced into the vicinity of the rotor wbich rotake~ on a suitably placed shaft, and is broken up into small bubbles by the action o blades or ingers mounted on the rctor, whi~h is frequ~ntly of a disc for~a~ion. The rotor provides the additional ~unction of ke~ping th~ mineral particles in suspen~on.
; Since the valuable mineral i~ removed from the cell 4700S/rs - 2 -~2~8~

at the sur~ace of ~e gas bubbles, it is ~vident that the rate of removal of the particl.es will depend on th~ ~otal gas-liquid interfacial area produced in the cel~. Thus for a given rate of introduction of airO th~ smaller the gas bubbles produced by the rotor, the ~reater will be the interfac al are~ a~ hence the more eficient will be the removal process.
The need for more eficient flotation of mineral particles, especially of fine particles smaller than ~en microns in diamet~r, has become more evident in recent times with the depletion of easily worked mineral deposits around the world. It is increasingly found that when new deposit~ are e~ploited, the ores are comples and require fins grinding to release the individual mineral particles. Further, in existing mineral conce~trators th~re is a need to improYe th~ recovery of fine particles which hi~herto had been allowed to go to tailings, in order to improYe the eco~omic performance o the mine.
It is thereore desira~l~ to pro~ide a flotation mecha~ism which can divide large quantities of gas into very small bubbles to achieve high metallurgical effi~i~ncy, while at the same time providing sufficient agitation ~o maintain the ~olids in suspension and yet pro~uce a relatively cal~ surace between the froth layer and the slurry in the flotation c~ll.
Th~ mechanism should also satisfy practical requiremen~s such as simplicity of construction and operation~ long life, easy maintenance and repair, and should be able to be made of wear-resistant and corrosion-resistant materials.
SUMNARY OF THE INVENTIO~
The present invention ther~or~ provides aeration apparatus o~ the type comprising a rotor mountad at th~
lower en~ o a hollow ~riYe ~ha~t, adapted to be immers~d in a liquld with the drive sha~ tending substantially : ~ertically upwardly from ~e rotor, the rotor comprising a di~c located in a plane at right angles to the asis of the shaft and havi~g a plurality of bla~es depending 4700S/rs - 3 -downwardly from the lower face of the disc, the interior of the hollow drive shat opening to the area beneath the disc such that when the rotor is rotated in a liguid by the drive shaft, and air is forced down the hollow drive shaft to issue on the underside of the rotor, the air being broken up int~ bubbles by the blades on the rotor, characterised by the configuration of the blades e~tending outwardly on the underside of the disc from a point adjacent the shaft to the periphery of the disc, and the height of the blades generally increasing with distance from the shaft over at least a significant proportion of the radius of the disc.
Preferably the height of each blade is determined at any point along the length of the blade in conjuction with lS the desired speed of rotation of the rotor to give a bubble size in the ranqe of 100 to 500 ~m.
Preferably the height of the blade, at least toward the outer edge of the disc, is determined by the formula:

db ~ 2 ~ YY . 2h ~ 1~3 U ~p2 (l~Cp)J

where db is the desired bubble diameter U is the velocity of the blade through the liquid, generally equal to 2 ~Nr where N is the rotational frequency of the rotor in c.p.s. and r is the greatest radius of the blade.
Y is the surface tension of the liquid.
~ is the viscosity of the liquid.
h is the height of the blade.
P is the density of the liquid.
Cp is the drag coefficient on the blade (generally havinq a value of 1 to 2).
Preferably the aeration apparatus further comprises a stator mounted adjacent the rotor and incorporatinq a plurality of substantially vertical blades estending radially outwardly from an area beneath thc opening from the hollow drive shaft of the rotor.

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Preferably the upper edges of the stator blades correspond with the profile of the lower edges of the rotor blades and are spaced a prede~ermi~ed distance therebelow.
SPreferably the number and thickness of the stator blades approsimate ~he number and thickness of the rotor ~ blades.
Preferably the stator blades e~tend radially outwardly beyond the periphery of the rotor, and e~tend upwardly b~yond the outer ends of the rotor blades.
Preferably the aeration apparatus is incorporated in an improved flotation cell having a rotor-stator pump assembly submerged in a slurry and in which a rotor body comprises plate and blade members for dispersing gas in the pumped slurry. A gas stream which is conveyed to the rotor is entrained into a trailing surface of each rotating blade where it is dispersed in the slurry.
Preferably the flotation cell comprises a vessel for supporting the slurry, a ro~r-stator pump asse~bly .
positioned in the vessel beneath the slurry surfa~e, a depending support means for supporti~g the rotor ~ody within a cavity formed by the stator~ means for supporting the stator, means for causing rotation of the rotor body in the vessel, means for conveying g~seous fluid below the slurry surface to the rotor body for dispersal in the slurry, means for introducing a slurry to the vessel, means for removing a froth from the surface of the slurry, and means for removing the slurry from the vessel. The rotor ~ody includes a top plate member and a plurality of blade members extending trans~ersely from the a~is of the rotor.
DESCRIPTION OF TH~ DæAWINGS
Notwithstanding any other forms that may fall within its ~cope, one preferred form of the invention will now be described by way o~ esa~ple only with rsference to the : accompanying drawings, in which:
:Fig. 1 ;s a sectional ~levatior~ of one form of flotation separation app~ratu~ incorporating aeration . 4700S~s - 5 -... . . . .. .. .... .... .. . . . . . . . . ... . .....

apparatus according to the invention;
Fig. 2 is a enlarged vertical cross-sectional view of th~ ro~or of the aeration apparatus ~hown in Fig. l;
Fig. 3 is a plan view of the rotor and blades taken on a line 1-1 of Fig. 2;
Fig. 4 is a pran view of the stator of the aera~ion apparatus shown in ~ig. l; and Fig. 5 is an enlarged side elevation of the rotor-stator combination and part of the cel l .
DESCRXPTION OF THE PR~FERR~D EMBODIME~T
Fig. 1 shows a general view of a 1Otation cell generally desig~ated as 10~ The suitably conditioned mineral slurry enters a feed box 11 and thence through an opening 12 into the body 13 of the cell itself where it is contacted with air bubbles. The bubbl~s carrying the floatable particles rise to the top of the slurry 14 to form a layer of froth 15 which then flows over a lip 16 into a suitably placed launder as the conc~trate. The remainder of the slurry lea~es the cell through an opening 17 as the tailings. The form of the cell 10 may be square, rectangular or cylindrical, and the base 18 may be flat, curved, hemispherical or U-shaped. The gas is introduced through the hollow shaft or spindle 19 which also acts as the driving shaft for the rotor 20. The 25 sha f t 19 is supported by a suitable mounting system containing also a means for introducing the air into the rotating shaft, and for driving the shat at the desired rotational speed, none of which is shown.
At the lower end of the hollow shaft 19 is attached a rotor 20 which rotates within a stator 21. The rotor e~erts a pumping action on the contents of he cell and serve~ to break up ths air flow into a multitude o small bubble~. The stator reduces th~ swirling ~otion of the liquid bo~h beore and aft~r it passes through th~ rotor.
The rotor (Fig~. ~, 3) comprises a top plate or disc 22 from which depen~s a plurality of blades 230 The disc 22 is attached to the lower end of the hollow shaft 19 by a bolted ~lange or other suitable means, and contains a 4700S/rs - 6 -~29~

cen~ral co~a~ial opening ~4 to allow air to pass from the shaft to the blades 23.
The blades 23 extend radially outwardly from the opening 24 ~o the periphery of ~he disc, although curved (backward- or forward-facing~ blsdes may al~o be used with varying e~ects on ~he pu~ping capacity of the rotor. The straight blade has advantages ~or simplicity of construction. It is also possible for the blade to be discontinuous over its lenyth, i.e. to incorporate a number of vert;cal cuts or slots in the blade or other holes or apertures therethrough. Su~h variations will not detract from the overall performance of the blade, but it is generally felt to be simpler to form the blade as a straight and continuous blade.
As shown in Fig. 2, the height of the blad~
preferably increases with transverse distance outward from the asis of the disc 22. Although it is preferable, and simpler, for ~he height of the blade to increase continuously over the length of the blade it will be realised that a similar benefit or efect could be achieved by increasing the height of the blade with distance rom the ~haft over at least a significant proportion of the radius of the disc. The heiqht of the blade at the periphery of ~he disc ~25 of Fig. 2~ should preferably be smaller than the disc radius. The thickness 26 of ~he blades (Fig. 3) s~ould preferably be no greater than the blade heiyht 25.
- To provide increased efficiency in the operation of the flotation cell, it is desired to configure the blades on the rotor so that the bubbles generated by the rotor are generally very small in size and preferably in the range of 100 to 500 ~m. It has been found that this can be determined for ~ desired ~peed of rotation of the rotor by deter~ining t~e blade height at any point along the 35 length of ~h~3 blade in accor~lance with the following formula:

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'12 ~

db ~ Y~. 2h ~ l/3 U ~ P ~l~Cp wher~ db is the desired bubble diam~ter U is the Yelocity of the blade through the liquid, generally equal to 2 ~Nr where N is the rotational frs~ueney of the rotor in c.p.s. and r is the radius at any specific point on the blade.
y is the surface tension o~ the liquid.
~ is the Yiscosity of the liquid.
h is the height of the blade.
p is the density of the liquid.
Cp is the drag coefficient on the blade (generally having a value of l to 2).
(S.I. units are used throughout the for~ula, e.g. kg, m, s, N, etc.).
If the blade is attached ~o a rotating disc, it is clear that the velocity U will depend on the radial distance r, i.e. U - 2~ Nr where N = rotational frequency, c.p.s.~ and r is the radius. Thus in the ~quation if all else is constant, d h V3 hl/3 U 2 ~Nr so for constant d~, the ratio hl/3~r should also be const3nt. In practice it is easiest to design as follows:
~i) choose the hubble size desired ~preferably very small ones in the range lO0 to S00 ~m).
(ii) choose a tip speed Utip from practical esperience - based mainly on the wear properties o~ the materials of eonstruction -in the:range 5 to lO m/s.
(iii) calculate the blade hei~ht h which will give the de~ir~d bubble size.
(iv~ by ~etermining the blade h~ight in accordance with the formula, the con~iguration of the blads will b~ generally i~crea~ing in height rom the centre of the r~tor toward the ~ ~ ~ 4700S/rs - 8 -;~:

periphery and should in theory have a concave lower edge. In practice it is suficient to ~hape th~ blade so that its hei~ht increases linearly to~ard the outer edge of the impeller S as this i~ simpler to manufacture and has almo~t t~ same efect as a blade shaped in accordanc~ with the theoretical for~ula.
It is also apparent from the formula given above that small bubbles are favoured by a high Cp factor, i.e.
high drag shapes. This is generally achieved by blades of small breadth to height ratio, i.e. where the thickness of the blade is considerably less than~the height of th~
blade.
Usiny the design criterion gi~an above and ta~ing water as an e~ample o the fluid in the flotation cell, the following con~tants can be substituted to give an approsima~e formula for the bubble size.
Y = 0.072 N/m (surface tension) P ~ lOOO kg~m3 (density) ~ = 10-3 Pa-s (~iscosity) and C - 1.25 rom measurements, thus db = hl~3 ~072 s lU 3 x 2~ tll.89 U \ (103~ 2.25 ~

- 4.79;x 10-3 hl~3 ~tres.
U
where h is in metres and U is m/s.
: ~ In order to gi~e a practical range of values with : : this formula, it is possible to re-write the formula for ~30 practical purposes in the olliowing form.
: ~ db ~ a h : wher~ a i~ in the range 2 to lS s lO 3 n ~s i~ th~ range 0 . 25 to l.0 ~35~ ~ ~ m:is ln th~ rangB 0.7 to 1.3 In this way tha ~;ze and configuration of the blades : on the rotor ca~be ~asigned to givei the requir2d small bubble effect in the ~lo~ation cell.

:
4700S~rs ~ 9 ~
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In order to optimize the efficiency of the aeration apparatus and to reduce swirl in the flo-tation cell to a minimum, it is also desirable -to operate the rotor in conjunction with a stator of novel configuration.
Referring to Figs. 4 and 5, the stator 21 consists of a plurality of vertical blades 27 which extend transversely on lines drawn radially from an axis which is co-axial with the centre of the rotor. ~t is not necessary for the blades to extend to the axis of the rotor-stator system and -there could be advantages in manufacturing if a cylindrical opening 28 of approximately the same diame-ter as the opening 24 in the rotor is provided. The stator is reC8SSed 80 tha-t the rotor assembly 29 may be placed within it, with the level of the top of the rotor disc 22 being at or below the highest part 30 of the stator. Suitable clearances are necessary between the rotor and the stator, and the stator and the base 18 of the cell. The stator may be mounted on suitably placed posts 31 to raise it ofE the cell bottom. The part 32 of the stator blade generally beneatll the rotor may be shaped to match the slope of the rotor blades at the same radius as shown in Fig. 5, to pxovide an essentially constant clearance 33 between the rotor and stator. The height 34 of the stator beneath the impeller should preferably be not less than the length of arc 36 between the stator blades in the plane of the rotor top -~; - 10 -,,~ ~., .,.

plaSe (Fig. 4), at the same transverse distance fro~ the rotor a~is.
In operation, slurry is drawn by the pumping action of the ro~ating rotor 20 through the lower part 32 of the stator, and discharges through the upper part 35 of the stator. Air flows i~to the eye of the rotor 24 and is sucked in~o vortices which develop at the edges of the blades 23. The production of small bubbles is enhanced by increasing the shear intensity of the Yortices, and this intensity is impro~ed by ~he presence of the vertical stator blades beneath the rotor, which serve to minimize the swirling motion about the rotor asis, of the slurry entering the rotor. After being discharged from the rotor, the~mi~ture of slurry and air bubbles passes into the upper part 35 of the stator where the swirling motion in the discharge flow pattern is essentially eliminated.
This is necessary to minimize the for~ation of swirl vortices in the cell which would disturb the interf ace between the slurry 14 and the froth 15 and have a deleterious effect on cell performance and operation.

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Claims (8)

1. Aeration apparatus of the type comprising a rotor mounted at the lower end of a hollow drive shaft, adapted to be immersed in a liquid with the drive shaft extending substantially vertically upwardly from the rotor, the rotor comprising a disc located in a plane at right angles to the axis of the shaft and having a plurality of blades depending downwardly from the lower face of the disc, the interior of the hollow drive shaft opening to the area beneath the disc such that when the rotor is rotated in a liquid by the drive shaft, and air is forced down the hollow drive shaft to issue on the underside of the rotor, the air being broken up into bubbles by the blades on the rotor, characterised by the configuration of the blades extending outwardly on the underside of the disc from a point adjacent the shaft to the periphery of the disc, and the height of the blades generally increasing with distance from the shaft over at least a significant proportion of the radius of the disc.

2. Aeration apparatus as claimed in claim 1, wherein each blade is continuous from the point adjacent the shaft to the periphery of the disc and extends generally radially outwardly on the underside of the disc.

3. Aeration apparatus as claimed in claim 1, wherein the height of each blade is determined at any point along the length of the blade in conjuction with the desired speed of rotation of the rotor to give a bubble size in the range of 100 to 500 µm.

4. Aeration apparatus as claimed in claim 1, wherein the height of the blade, at least toward the outer edge of the disc, is determined by the formula:

where db is the desired bubble diameter U is the velocity of the blade through the liquid, 4700S/rs - 12 -generally equal to 2 .pi.Nr where N is the rotational frequency of the rotor in c.p.s. and r is the greatest radius of the blade.
.gamma. is the surface tension of the liquid.
µ is the viscosity of the liquid.
? is the height of the blade.
? is the density of the liquid.
Cp is the drag coefficient on the blade (generally having a value of 1 to 2).

5. Aeration apparatus as claimed in claim 1, wherein the aeration apparatus further comprises a stator mounted adjacent the rotor and incorporating a plurality of substantially vertical blades extending radially outwardly from an area beneath the opening from the hollow drive shaft of the rotor.

6. Aeration apparatus as claimed in claim 5, wherein the upper edges of the stator blades correspond with the profile of the lower edges of the rotor blades and are spaced a predetermined distance therebelow.

7. Aeration apparatus as claimed in claim 5, wherein the number and thickness of the stator blades approximate the number and thickness of the rotor blades.

8. Aeration apparatus as claimed in claim 5, wherein the stator blades extend radially outwardly beyond the periphery of the rotor, and extend upwardly beyond the outer ends of the rotor blades.

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