AU611115B2

AU611115B2 - Aeration apparatus

Info

Publication number: AU611115B2
Application number: AU82629/87A
Authority: AU
Inventors: Graeme John Jameson
Original assignee: Newcastle Innovation Ltd
Current assignee: Newcastle Innovation Ltd
Priority date: 1986-12-16
Filing date: 1987-12-16
Publication date: 1991-06-06
Anticipated expiration: 2007-12-16
Also published as: AU8262987A

Description

COMMONWEALTH OF AUSTiALIA PATENTS ACT 1 '52 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE 1115 Form Short Title: Int. CI: Application Number: Lodged: PH-09531 16 December 1986 Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: TO BE COMPLETED BY APPLICANT 0000 o 00a 00 00 0 o0 0 0 o 0 0 0 a 00 C 0 t~ Nacne of Appileant: GRAEME JOHN JAMhSON Address of Applicant: 34 Curzon Road, New Lambton, New South Wales Z305, Australia Actual Inventor: GRABMENE JOHN JAMESON Address for Service: GRIFFITH HASSEL FRAZER 71 YORK STREET SYDNEY NSW 2000

AUSTRALIA

Complete Specification for the Invention entitle: A T7 RATION APPARATUS The following stateraent is a full descilption of tI .is invention, including the best method of performing it known to me/us 6853A ~C C 4 C V 00 t a I0 BACKGROUND OF THE INVENTION This invention relates to aeration apparatus and more particularly to an improved apparatus for the production of small gas bubbles in a liquid in order to create a large interfacial area between the gas and the liquid, thereby increasing tuie effficiency of processes such as flotation and gas liquid mass transfer.

Although the apparatus may be of value in other fields such as aeration and gas absorption, the invention will be described in relation to the flotation process.

The art of flotation generally involves the aeration and agitation of a slurry or suspension in water of finely divided ore part",.cles in a cell or apparatus of suitable design. The mineral may be regarded as a mixture of valuable minerals or "values", and clay, rock or other unwanted "qangue" particles. The object of the process is to remove the values f rom the gangue, and this may be achieved by conditioning the slurry with chemical reagents which have the effect of rendering the values selectively 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 the liquid where it forms a froth which may be scraped off into a launder, thereby conveying the values out of the cell and separatin~g them from the gangue, which remains with the 0 0.

00 0 liquid in the cell. To assikst in the formation of a 0 00 stable f roth, it may be necessary to add chemical frothing 4 0 0:0, agents.

00 30 Flotation machines as customarily constructed consist of a tank i n the base of which is an aerating rotor and a 4 e q7 concentric stator. Air is introduced into the vicinity of the rotor which rotates on a suitably placed shaft, and is broken up into small bubbles by the action of blades or 35 fingers mounted on the rotor, which is frequently of a disc formation. The rotor provides the additional function of keeping the mineral particles in suspension.

Since the valuable mineral is removed from the cell 4700S/rs-2t I il_:iui- i ;Yiu ii at the surface of the gas bubbles, it is evident that the rate of removal of the particles will depend on the total gas-liquid interfacial area produced in the cell. Thus for a given rate of introduction of air, the smaller the gas bubbles produced by the rotor, the greater will be the interfacial area and. hence the more efficient will be t'ne removal process.

The need for more efficient flotation of mineral particles, especially of fine particles smaller than ten microns in diameter, 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 deposits are exploited, the ores are complex and require fine grinding to release the individual mineral particles. Further, in existing mineral concentrators there is a need to improve the recovery cf fine particles which hitherto had been allowed to go to tailings, in order to improve the economic performance of the mine.

It is therefore desirable to provide a flotation mechanism which can divide large quantities of gas into very small bubbles to achieve high metallurgical efficiency, while at the same time providing sufficient agitation to maintain the solids in suspension and yet produce a relatively calm surface between the froth layer and the slurry in the flotation cell.

oot The mechanism should also satisfy practical oo o requirements such as simplicity of construction and ao 0 Q 0 operation, long life, easy maintenance and repair, and 0 *s should be able to be made of wear-resistant and 30 corrosion-resistant materials.

SUMMARY OF THE INVENTION The present invention therefore provides aeration apparatus of the type comprising a rotor mounted at the lower end of a hollow drive shaft, adapted to be immersed 35 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 (4 4 00Sr 3 4700S/rs 3 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 issum~ on the underside of the rotor, the air is broken up into bubbles by the blades on the rotor, characterised by the configura-4o of the blades extending continuously outwardly on the underside of the disc from a point adjacent the shaft to the periphery the disc, and the height of the blades increasing uniforri,( r with distance from the shaft over at least a significant proportion of the radius of the disc.

Preferably the height of ear.i blade is determined at any point along the length of the blade in conjuction with 15 th desired speed of rotation o. the rotor to give a bubble size in the range of 100 to 500 Lm.

Preferably the height of the blade, at least toward the outer edge of the disc, is determined by the formula: 0000 0 0 0 00 0 00 0 0 0 0 0 0 0 0 00 00 0 00 00 00 0 o 0 0 0 0 0 00 0 0 0 0 0 0 d b 1-9.

2 h 1/

+C)

0 where db is the desired bubble diameter 00 0b 0 0 U is the velocity of the tip of the blade through the Do 0 25liquid, generally equal to 2 7CNr where N is the 00 00rotational frequency of the rotor in c.p.s. and r is 0 0 the greatest radius of the blade.

yis the surface tension of the liquid.

o: 300 is the viscosity of the liquid.

0 a 0C 30h is the height of the blade.

np is the density of the liquid.

C pis the drag coefficient on the blade (generally having a value of 1 to 2).

Preferably 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.

7 /rs -4- Preferably 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.

Preferably the number and thickness of the stator blades approximate the number and thickness of the rotor blades.

Preferably the stator blades extend radially outwardly beyond the periphery of the rotor, and extend upwardly beyond the outer ends of the rotor blades.

Preferably the aeration apparatus is incorporated in an improved flotation cell having a xotor-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 rotor-stator pump assembly positioned in the vessel beneath the slurry surface, a depending support means for supporting the rotor body 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 gaseous fluid below the slurry surface to the rotor body for dispersal in the slurry, means for introducing a slurry to the vessel, o means for removing a froth from the surface of the slurry, a 0 0 o and means for removing the slurry from the vessel. The S0 i r:otor body includes a top plate member and a plurality of °30 blade members extending transversely from the axis of the cc rotor.

DESCRIPTION OF THE DRAWINGS Notwithstanding any other forms that may fall within c its scope, one preferred form of the invention will now be 35 described by way of example only with reference to the accompanying drawings, in which:- Fig. 1 is a sectional elevation of one form of flotation separation apparatus incorporating aeration 4700S/rs 5 c lt ww apparatus according to the invention; Fig. 2 is a enlarged vertical cross-sectional view of the rotor of the aeration apparatus shown in Fig. 1; Fig. 3 is a plan view of the rotor and blades taken on a line 1-1 of Fig. 2; Fig. 4 is a plan view of the stator of the aeration apparatus shown in Fig. 1; and Fig. 5 is an enlarged side elevation of the rotoz-stator combination and part of the cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows a general view of a flotation cell generally designated 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 whe!:e it is contacted with air bubbles. The bubbles carrying the floatable patticles rise to the top of the slurry 1.4 to form a layer of froth 15 which then flows over a lip 16 into a suitably placed launder as the concentrate. The remaiinder of the slurry leaves 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 shaft 19 is supported by a suitable mounting system 00 containing also a means for introducing the air into the so 0 rotating shaft, and for driving the shaft at the desired 00 a 0 rotational speed, none of which is shown.

0 At the lower end of the hollow shaft 19 is attached a rotor 20 which rotates within a stator 21. The rotor exerts a pumping action on the contents of the cell and serves to break up the air flow into a multitude of small bubbles. The stator reduces the swirling motion of the liquid both before and after it passes through the rotor.

The rotor (Figs. 2, 3) comprises a top pl~ate or disc 22 from which depends a plurality of blades 23. The disc 22 is attached to the lower end of the hollow shaft 19 by a bolted flange or other svt0-ble means, and contains a 4700S/rs 6 r central co-axial opening 24 to allow air to pass from the shaft to the blades 23.

The blades 23 extend radially outwardly from the opening 24 to the periphery of the disc, although curved (backward- or forward-facing) blades may also be used with varying effects on t~he pumping capacity of the rotor. The straight blade has advantages for simplicity of construction. It is also possible for the blade to be discontinuous over its length, i.e. to incorporate a number of vertical cuts or slots in'the blade or other holes or apertures therethrough. Such 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 blade increases with transverse distance outward from the axis of the disc 22. Although it is preferable, and simpler, for the height of the blade to increase continuously over the length of the blade it will be realised that a similar benefit or effect could be achieved by increasing the height of the blade with distance from the shaft over at least a significant proportion of the radius of the disc. The height of the.~ blade at the periphery of the disc (25 of Fig. 2) shoild preferably be smaller than the disc radius. The thic;'tness 26 of the blades (Fig. 3) should preferably be no greater than the blade h~eight To provide increased efficiency in the operation of the flotation cell, it is desired to configure the blades on the rotor so that the '.,ubbles generated by the rotor are generally very small in size and preferably in the range of 100 to 500 pim. Tt has been found that this can be determ'Lned for a desired speed of rotation of the rotor by determiaing the blade height at any poilit along the length of the blade in accordance with the i1ollowing formul~a: o 0 00 0 0 0 0 a0 46 7 db= 11.89

U

1/3 where db is the desired bubble diameter U is the velocity of the blade through the liquid, generally equal'to 2 7rNr where N is the rotational frequency of the rotor in c.p.s. and r is the radius at any specific point on the blade.

y is the surface tension of the liquid.

p is the viscosity of the liquid.

h is the height of the blade.

p is the density of the liquid.

C is tha drag coefficient on the blade (generally

P

haiving a value of 1 to 2).

units are used throughout the formula, e.g. kg, m, s, N, etc.).

If the blade is attached to a rotating disc, it is clear that the velocity U will depend on the radial distance r, i.e. U 2jNr where N rotational frequency, and r is the radius. Thus in the equation if all else is constant, db ah 1 3 h 1 /3 db i- Ct U 2 FNr so for constant db the ratio h 1 /3/r should also be constant. In practice it is easiest to design as follows: choose the bubble size desired (preferably very small ones in the range 100 to 500 urm).

(ii) choose a tip speed Uti p from practical 30 experience based mainly on the wear properties of the materials of construction in the range 5 to 10 m/s.

(iii1 calculate the blade height h which will give the desired bubble size.

3 (iv) by determining the blade height in accordance with the formula, the configuration of the blade will be generally increasing in height from the centre of the rotor toward the 7, 8 0000 00 0 00 0 0 4@ 44 te C If periphery and should in theory have a concave lower edge. In practice it is sufficient to shape the blade so that its height increases linearly toward the outer edge of the impeller as this is simpler to manufacture and has almost thP same effect as a blade shaped in accordance with the theoretical formu.'.

It is also apparent from the formula given above that small bubbles are favoured by a high C 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 the blade.

Using the design criterion given above and taking water as an example of the fluid in the flotation cell, the following constants can be substituted to give an approximate formula for the bubble size.

Y 0.072 N/m (surface tension) P 1000 kg/m 3 (density) 1 10 3 Pa-s (viscosity) and C 1.25 from measurements, thu d b =h 3 0.072 x 10- x 2 3 (11.89) U (103 2 2.25 4.79 x 10 3 h1/ 3 metres.

U

where h is in metres and U is m/s.

0t C *0 n order to give a practical range of values with t this formula, it is possible to re-write the formula for 30 practical purposes in the following form.

Sdb a h u" where a is in the range 2 to 15 x 10 n is in the range 0.25 to m is it the range 0.7 to 1.3 In this way the size and configuration of the blades on the rotor can be designed to give the required small bubble effect in the flotation cell.

4700S/rs 9-

I

In order to optimize the efficiency of the aeration apparatus and to reduce swirl in the flotation 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 transvercely on lines drawn radially from an axis which is co-axial wiLth the centre of the rotor. It 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 diameter as the opening 24 in the rotor is provided. The stator is recessed so that the rotor assembly 29 may be placed within it, with the level kf the top of the rotor disc 22 being at or below the highest part 30 of the stator. suitable clearances are necessary 0000 between the rotor and the stator, and the stator and the o 1 00 0000 0base 18 of the cell. The statolc may be mounted on 0o suitably placed posts 31 t6 raise it off the cell bottom.

0 h20 The part 32 of the statcir blade generally beneath the 0 00 0 000 rotor may be shaped to meitch the slope of the rotor blades 0 0 00 0 etsetialy radisas clearnc 33 between the protore and 0 0 00 0 .00000 stator. The height 34 of the stator beneath the impeller should preferably be not less than the length of arc 36 between the stator b).ades in the plane of the rotor top 0 000000 0 0 0 000Q 0 0 00 0 0 0 :3 0 plate (Fig, at the same transverse distance from the rotor axis.

In operation, slurry is drawn by the pumping action of the rotating rotor 20 through the lower part 32 of the stator, and discharges through the upper part 35 of the stator. Air flows iito the eye of the rotor 24 and is sucked into vortices which develop at the edges of the blades 23. The production of small bubbles is enhanced by increasing the shear intensity of the vortices, and this intensity is improved by the presence of the vertical stator blades beneath the rotor, which serve to minimize the swirling motion about the rotor axis, of the slurry entering the rotor. After being discharged from the rotor, the mixture 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 formation of swirl vortices in the cell which would disturb the interface between the slurry 14 and the froth 15 and have a deleterious effect on cell performance and operation.

6 a 0 I 00 I Ii S, 7 S/r n o o s r s 1 1

Claims

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 depe'-ing 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 is broken up into bubbles by the blades on the rotor, characterised by the configuration of the blades extending c-ntinuously 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 increasing uniformly 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 exl:ends generally radially outwardly on the underside of the disc.

3. Aeration apparatus as claimed in either claim 1 or claim 2, wherein the height of each blade is determined at any point along the length of the blade in conj iction with the desired speed of rotation of the rotor to give a S bubble size in the range of 100 to 500 [Lm.

4. Aeration apparatus as claimed in any one of the U preceding claims, whereir, the height of the blade, at least toward the outer edge of the disc, is determined by the formula: db 3 2 1/3 U (l+C where db is the desired bubble diameter U is the velocity of the tip of the blade through the PAU liquid, S 00 s 12 NI -MM U A m ii 4 I gene:ally equal to 2 frNr where N is the rotational frequency off the rotor in c.p.s. and r is the greatest radius of the blade. Y is the surface tension of the liquid. p is the viscosity of the liquid. h is the height f the blade. p is the density of the liquid. C is the drag coefficient on the blade (generally P having a value of 1 to 2).

5. Aeration apparatus as claimed in any one of the preceding claims, wherein the aeration apparatus further comprises a stator mounted adjacent.the rotor and incorporating a plurality of substanhially 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 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 either claim or claim 6, wherein the number and thickness of the stator blades approximate the number and thickness of the rotor blades. O00 000 0 0 0 00 o 0 000 00 a 000 00 o o0 6 00 00 0 00 25 of 009 W4 f 0O0 30 IO t

8. Aeration apparatus as claimed in any one of claims 5 to 7, wherein the stator blades extend radially outwardly beyond the periphery of the rotor, and extend upwardly beyond the outer ends of the rotor blades.

9. Aeration apparatus substantially as described herein .ith reference to the accompanying drawings. DATED this SIXTEENTH day of DECEMBER 1987 GRABME JOhN JAMESON By his Patent Attorneys GRIFFITH [ASSBL FRAZER f C C 0 1 0 4700S/rs 13