WO1992003218A1

WO1992003218A1 - Aeration of liquids

Info

Publication number: WO1992003218A1
Application number: PCT/AU1991/000397
Authority: WO
Inventors: Graeme John Jameson
Original assignee: The University Of Newcastle Research Associates Limited
Priority date: 1990-08-27
Filing date: 1991-08-27
Publication date: 1992-03-05
Also published as: KR930702065A; ZA916774B; MX9100829A; CN1060453A; WO1992003219A1; WO1992003220A1; EP0546033A4; JPH06500260A; ZA916775B; CA2090619A1; MX9100830A; EP0546033A1

Abstract

Method and apparatus for the aeration of liquids, typically for wastewater treatment or pollution control. Liquid is introduced into the top end of a vertical pipe (3) having its lower end immersed in a body of liquid (4) through a nozzle (2) forming a downwardly moving jet (8). The jet entrains air (for example through inlet (6)) forming a mixture of fine bubbles which issues from the lower end (5) of the conduit into the body of liquid. A diffuser plate (10) may be provided to disperse the bubbles in the body of liquid and draft tubes may be provided within the pipe to control the jet and the formation of bubbles.

Description

"AERATION OF LIQUIDS" TECHNICAL FIELD This invention relates to aeration of liquids especially to effect absorption of a gas into the liquid, stripping of a dissolved gas from the liquid, the separation by flotation of droplets or particles suspende in the liquids, or the formation of a cloud of bubbles to be transported into a reservoir of liquid to carry out an of the above functions. BACKGROUND ART

In each of the operations referred to above it is advantageous to first disperse the gas into fine bubbles in order to create a large interfacial area for transfer of matter between phases, or for collection of the particles by flotation. Having created a dense dispersion of fine bubbles in the liquid, it is further advantageous to inject the gas-liquid mixture into a reservoir in which further transport operations such as absorption, stripping or flotation, can take place. In the past this has been achieved by many different types of diffusers, nozzles, or jets or various forms of mechanical impellers and other aerators, but they all have significant limitations in the size of the bubbles produced, the flow rate of bubbles, and the dispersion of the bubbles into the body of liquid to be aerated.

It is the purpose of the method and apparatus for the aeration of liquids according to this invention to provide a device for the dispersion of a flow of gas into a mixture of fine bubbles in a liquid, and then to provide a means for efficient usage of the bubbles in a large reservoir of the liquid.

DISCLOSURE OF INVENTION Accordingly in one aspect the invention consists in a method of aerating liquids comprising the steps of passing a liquid in a downwardly moving jet into the upper part of a downwardly extending conduit, the lower end of which is submerged in a body of liquid, forming a foam of bubbles within the conduit and causing the foam to move downwardly and issue from the lower end of the conduit into the body of liquid.

Preferably gas is provided to the upper part of the conduit and entrained into the jet forming the foam of bubbles.

Preferably the upper part of the conduit is enclosed and the gas is provided at atmospheric pressure through a flow rate controlling constriction.

Alternatively the upper part of the conduit is enclosed and the gas is provided under pressure via a pump or blower.

In one form of the invention the liquid passing into the upper part of the conduit incorporates a gas in supersaturated solution within the liquid. The gas may be any convenient or desired gas but in many applications comprises air.

In a further aspect the invention consists in apparatus for aerating liquids comprising a substantially vertically extending conduit having an open lower end, liquid supply means arranged to supply liquid under pressure to a downwardly facing nozzle located and arranged within the upper part of the conduit so as to form a downwardly issuing jet of liquid within the conduit, and support means arranged to support the conduit with the lower end immersed in a body of liquid.

Preferably the conduit is provided with a diffuser extending outwardly from the exterior of the conduit and positioned to be immersed in the body of liquid, the diffuser being arranged to disperse bubbles issuing from the lower end of the conduit outwardly away from the conduit as the bubbles rise in the body of liquid.

Preferably the diffuser comprises a plate surrounding the conduit and extending outwardly and upwardly from a predetermined position on the exterior of the conduit.

Preferably the plate is provided with a plurality of holes therethrough sized to allow a predetermined flow rate of bubbles through each hole. Preferably a gap is provided between the inner periphery of the plate and the exterior of the conduit and a diverting ring is positioned on the exterior of the conduit below the gap, arranged to divert upwardly moving bubbles outwardly away from the conduit and away from the gap between the conduit and the plate.

In one form of the invention the interior of the conduit is provided with a draft tube having its axis parallel to the axis of the conduit and being configured to direct the flow of gas within the conduit and/or constrain the jet within the conduit.

Preferably the conduit is substantially circular in cross-section or is of other cross-section having minor and major lateral axes of the same order and having an effective diameter equal to the diameter of a circle of equivalent area, and wherein the diameter of the conduit is in the range 2 to 20 times the diameter of the nozzle.

More preferably the diameter of the conduit is within the range 3 to 12 times the diameter of the nozzle. The liquid to be aerated may be either the liquid which is passed into the upper end of the conduit through the nozzle in a downwardly moving jet or alternatively may be the liquid comprising the body of liquid into which the lower end of the conduit is immersed. In many applications the liquid issuing into the upper part of the conduit will be the same as the liquid in the body of liquid.

Examples of the use to which the invention may be put include the following: (a) Aeration of a pond of wastewater or effluent, in which it is desired to create a dispersion of fine bubbles in the wastewater, so as to provide oxygen for the growth of microorganisms used to remove the noxious components and hence reduce the biological oxygen demand prior to discharging into a sewer or river.

(b) Treatment of a water or effluent stream containing finely-dispersed oil droplets, so that the oil droplets attach themselves to the bubbles, which rise to the surface of the liquid in a pond or containing vessel, removing the oil droplets out of the main body of the liquid, which is therefore purified.

A fine dispersion of bubbles is created in the conduit or pipe which is substantially vertical, by the action of a jet of the liquid impinging into a column of dense foam in the vertical pipe. The bubbles are created at least in part by the shearing action between the high-speed jet of liquid and the relatively quiescent dense foam.

The velocity of the jet is sufficiently high to disperse the air entrained into the dense foam into very small bubbles, of maximum size 500 μm approximately. Such fine bubbles will provide a medium with a high interfacial area. Furthermore, in the absence of larger bubbles, the two-phase mixture thus created is found to flow as an essentially homogeneous stable mixed fluid, creating a favourable environment for absorption, flotation and kindred processes. The jet of liquid is supplied to the top of the vertical pipe, so that the two-phase mixture created in the pipe is forced to travel downward, and then to discharge into a large reservoir. When the mixture discharges out of the end of a parallel-walled vertical pipe, it has been found that the resulting flow is not conducive to the most effective use of the bubbles. The finely-dispersed dense foam tends to act as a single homogeneous phase whose density is very much less than that of the liquid into which it is discharging, and accordingly the bubbly foam tends to cling to the outer wall of the vertical pipe and rise quickly to the surface of the liquid in the reservoir.

Better use of the bubbles can be made if the homogenous foam flowing from the bottom of the vertical pipe, can be made to mix with the surrounding liquid, so that the bubbles become separated from each other and ris as if they were essentially in an infinite body of liquid. Being very small, the rise time of the individua bubbles is much greater than the rise time of the homogeneous mixture issuing from the bottom of the vertical pipe, and accordingly, the time available for molecules to diffuse in or out of the bubbles is greatly increased.

It is one of the purposes of this invention, to provide a means for the efficient dispersion of a gas into a stream of fine bubbles intimately mixed into a liquid stream to form a dense foam, through the use of the shearing action of a high-speed liquid jet confined in an essentially vertical pipe which discharges into a reservoir of liquid.

It is a further purpose of this invention, to provide simple and effective means for the individual bubbles in the essentially homogenous mixture issuing from the bottom of the vertical pipe, to be separated from each other so as to rise slowly to the surface, giving a greatly enhanced time of contact between the bubbles and the liquid in the reservoir, and hence giving maximum opportunity for transfer of material between the phases, or capture of fine particles by the bubbles. BRIEF DESCRIPTION OF DRAWINGS Notwithstanding any other forms that may fall within its scope, one preferred form of the invention and variations thereof will now be described with reference to the accompanying drawings, in which:

Figure la is a diagrammatic cross-sectional elevation through a basic form of aeration device according to the invention; Figure lb is a view similar to Figure la showing the behaviour of a bouyant plume of dense foam in the absence of a diffuser;

Figure lc is a sectional plan view of the diffuser incorporated in the apparatus of Figure la; Figure 2a is a diagrammatic cross-sectional elevation through the lower part of the conduit forming part of the apparatus of Figure la showing an alternative embodiment of the diffuser; Figure 2b is a plan view of the diffuser shown in Figure 2a;

Figure 3a and Figure 3b are diagrammatic cross-sectional elevations of alternative forms of diffuser;

Figure 4a is a diagrammatic cross-sectional elevation showing aeration apparatus with multiple nozzles and internal baffles in the conduit;

Figure 4b is a sectional plan view of the nozzle and baffle arrangement shown in Figure 4a;

Figure 5 is a diagrammatic cross-sectional elevation of a form of the invention incorporating a draft tube within the conduit;

Figure 6a and Figure 6b are views similar to Figure 5 showing alternative configurations of draft tube; and Figure 7 is a view similar to Figure 5 showing an alternative draft tube configuration.

MODES FOR CARRYING OUT THE INVENTION In the form of the invention shown in Figure la, liquid enters through an entry pipe (1) and a nozzle assembly (22) which terminates in an orifice (2) which faces essentially vertically downwards. The nozzle is mounted in the top of a conduit or pipe (3) which is essentially vertical. In operation, the liquid issues from the orifice (2) in the form of a high-speed jet which can move downwardly through the pipe (3).

The vertical pipe is mounted by way of support means (3a) so that its lower end is submerged in a reservoir of liquid (4). The liquid may or may not be the same as the liquid entering through the entry pipe (1) . Initially, before commencement of operation, the liquid levels in the reservoir and inside the vertical pipe (3) are the same. When the high-speed liquid jet is first established by the orifice (2), it travels downwards through the pipe (3) and plunges into the liquid, entraining gas which is inside the pipe and carrying it out of the lower extremity (5), to rise in the reservoir (4) in the form of fine bubbles. The vertical pipe (3) fills rapidly with a dense foam of bubbles dispersed in the feed liquid, and the pressure in the head space of the pipe drops below the ambient pressure outside the pipe. Accordingly, new gas, which is typically but not necessarily air, is drawn into the pipe through the air entry (6) .

The gas flow is regulated by a conveniently placed control valve (7) or other suitable means so that the rate at which air enters through the entry pipe (6) is less than the maximum amount which can be entrained by the plunging jet (8). In this way, the vertical pipe (3) remains filled with a dense foam which provides a favourable environment for interaction between the gas and liquid phases. The pipe (3) in which the bubbly mixture is produced should preferably be substantially vertical, i.e. within 15 of the vertical. It is however possible for the system to perform well in some cases when the axis lies further from the vertical than the limit stated, depending on the degree of coalescence of the bubbles which takes place within the pipe as the dense bubbly foam travels downward toward the pipe exit (5). If the bubbles coalesce, they will rise in the form of large slugs of gas in the uppermost parts of the sloping pipe, to the head space of the pipe (3), as a form of internal gas recycle. On reaching the head space in the pipe (3), they will displace the dense foam, and may cause the collapse of the bubbly mixture in the pipe. Accordingly, the pipe (3) will perform best if it is substantially vertical. Although the invention is described with reference to a circular pipe (3), it is not restricted to this form, and indeed the pipe may be replaced by a vertical duct of any cross-section. Best results will be found however with a regular polygon, or a section for which the ratio of the major to minor lateral axes is close to unity.

The apparatus has been described in terms of the formation of a dispersion of gas bubbles in a liquid, by entrainment of the gas into a dense foam mixture by the action of a high-speed liquid jet. In some applications, the gas is supplied entirely by the entrainment process. However, in the flotation of fine particles it is advantageous to supply gas not only in the form of bubbles created by the high-speed jet, but also by growth from a supersaturated feed solution. There is a substantial drop in pressure as the liquid passes through the injection nozzle. Thus if the feed liquid is saturated with the flotation gas prior to passing through the nozzle, it will become supersaturated downstream of the nozzle and the dissolved gas will come out of solution in the form of very fine bubbles. The presence of these bubbles aids the flotation process because the surface area per unit volume of such bubbles is very large, and also because the bubbles may preferentially nucleate on the surface of the fine hydrophobic particles which it is desired to float. Accordingly, the ability to float the fine particles with bubbles will be enhanced by the use of supersaturated feed liquid. When dissolved gas is used to enhance the flotation process, there is a synergistic effect between the bubbles produced by the shearing action of the plunging jet, which are normally around 500μm in diameter, and the bubbles produced by growth from the supersaturated solution, which are often in the range 20 to 50μm in diameter. Because the latter bubbles are so small, their rise velocity is also small, and hence the time required for them to rise to the surface of the liquid after issuing from the botto of the circular pipe (3) can be very large. However, whe there is a mixture of bubble sizes, coalescence can occur between the small and large bubbles to produce bubbles of even larger diameter, and so the rise time to the surface can be reduced, and hence the volume of the reservoir int which the pipe (3) is discharged can also be reduced. In some applications, the only supply of gas may b by way of dissolved gas in the liquid passing through the nozzle assembly (22). The top of the conduit or pipe (3) may be sealed and the inlet (6) omitted. The gas-liquid mixture which forms in the vertical pipe (3) has a voidage of up to 60 percent by volume approximately, and behaves as a.homogeneous fluid whose density is much less than the liquid in the reservoir (4). Accordingly, in the absence of any special precautions, it tends to rise as a buoyant plume, hugging the outer wall of the pipe (3) and rising rapidly to the surface (9), as shown in Figure lb. Consequently, the bubbles in the plume do not mix well with the liquid in the reservoir, and the possibility of maximum contact of the bubbles with this liquid is lost. To prevent the formation of the plume, it has been found advantageous to mount a shield or diffuser (10) outside the vertical pipe (3), which has the effect of breaking up the dense foam which issues from the pipe exit (5), and allowing the gas bubbles to rise individually in the liquid in the reservoir (4) rather than as a plume.

The bubble diffuser can conveniently be made from a plate in the form of a frustum of a cone, inverted so that the wide end is uppermost. The cone is perforated with a multiplicity of holes as shown in Figure lc. In operation, the buoyant plume issuing from the exit (5) of the vertical pipe (3),rises and spreads over the underside of the diffuser (10), to pass through the individual holes (11) in the diffuser (10). After passing through the individual holes (11), the bubbles of gas rise individually through the liquid in the reservoir (4) as shown in Figure la.

The diameter of the conical diffuser (10) shown in Figure lc should be in the range 1.5 to 10 times the outer diameter of the pipe (3), with good practical results being found when the cone diameter is 2 to 3 times the pipe diameter. The half-angle of the cone from which the frustum is formed is conveniently in the range 30 to 60°. The diameter of the holes (11) can be in the range 1 to 30 mm, and the holes should be distributed evenly over the conical surface to give an open area in the range 1 to 15% of the area of the surface. It is not necessary for the diffuser to be in the shape of a cone. Other shapes will perform as well, providing the elevation of the underside of the diffuser above the exit (5) of the vertical pipe (3), always increases as the radial distance from the axis of the pipe increases. If the underside of the diffuser surface is at any stage horizontal or tending to dip downwards as radial distance from the vertical pipe increases, there will be a tendency for the bubbles to collect in this area and coalesce, and thereby form less surface area per unit of gas volume, which will reduce the efficiency of any contacting operation between the gas and the liquid.

An alternative form of the diffuser is shown in Figures 2a and 2b, where the diffuser (12) takes the shape of part of a circular dish, with holes (13) perforating the dish. The radius of curvature of the dish can conveniently be in the range 2 to 20 times the diameter of the vertical pipe (3), and the diameter of the diffuser (12) shown in Figure 2b should be in the range 1.5 to 20 times the outer diameter of the pipe (3), with good practical results being found when the diffuser diameter is 2 to 3 times the pipe diameter. The diameter of the holes (13) can be in the range 1 to 30 mm, and the holes can conveniently be distributed evenly over the surface to give a total area of the perforations in the range 1 to 20% of the area of the diffuser surface.

A difficulty which can arise with the diffuser arrangements as shown in Figures la and 2a is that solid matter which may have been suspended in the liquid in the reservoir (4), may tend to settle on the upper face of th diffuser (10) or (12) and build up into a thick layer which may block the holes (11) or (13). This possibility can be avoided by mounting the diffuser (10) or (12) so that an annular gap (15) (Figures 3a and 3b) exists between the diffuser and the pipe wall (3). The conical diffuser is advantageous in this application because it will provide a surface of constant angle to the horizontal, down which any solids which may have deposite - li ¬ on the upper surface may slide toward the axis of the cone. The half-angle of the cone, of which the diffuser is a frustum, should be such that the solids will slide toward the axis and hence fall through the annular gap (15).

It is advantageous to use a diverting ring (14) or (16) in conjunction with the annular space (15). The purpose of the diverting ring is to prevent the dense foam rising from the open end (5) of the pipe (3), from entering the annular gap (15) and thereby evading the diffuser (10) or (12). The ring is mounted on the outer wall of the vertical pipe (3), and may conveniently be of triangular section (14) as shown in Figure 3a or of semi-circular section as in Figure 3b at (16). Alternative embodiments are now described which will improve the action of the liquid jet (8) shown in Figure 1. When the system is in operation, the jet plunges into the dense foam which fills the vertical pipe (3), and gas which enters through the inlet (6) is entrained into the dense foam by the shearing action at the edge of the jet. The velocity of the jet should be in the range 3 to 40 metres/sec. If the velocity is too low, the volume of air which can be entrained relative to the volume of liquid supplied will be too little, whereas if the velocity is too high, the energy demand will be excessive. Good practical operational velocities are in the range 12 to 20 metres/sec.

The jet diameter is fixed by practical considerations in that if it is too small, there is the possibility of it becoming blocked by adventitious material in the feed liquid. The minimum diameter should be such that matter suspended will pass through it. The diameter of the vertical pipe (3) should be in the range 2 to 20 times the jet diameter, with satisfactory operation being found in the range 3 to 12 times the jet diameter.

Bubbles produced by the plunging jet are generated by the shearing forces caused by the difference in velocity between the jet and the dense foam into which it plunges. An important determinant of the ultimate size of the bubbles is the power dissipated per unit volume of fluid contained in the generating device. To define this volume, use is made of the observation that the impinging jet tends to spread out as it travels downwards in the dense foam, giving up its forward momentum, and at a certain point, the expanding jet comes into contact with the wall of the vertical pipe (3). The jet has been observed to expand as a cone, whose included angle is in the range 10 to 20°. Thus for present purposes the volume in which the energy contained in the jet is essentially dissipated can be defined as the volume of fluid contained in the vertical pipe (3), between the entry point of the jet and the point at which the jet just begins to touch the wall of the vertical pipe (3).

The distance between the entry point of the jet and the point at which the jet just begins to touch the wall of the vertical pipe (3), is referred to as the impingement distance, and it is desirable that the length of the confining vertical pipe (3) should be greater than the impingement distance. When it is, the initial momentum of the jet is spread across the cross-section of the pipe, and a two-phase mixture which is essentially homogeneous has been created. It will be evident that since the jet expands slowly with distance from the orifice (2), the impingement distance and hence the height of the vertical pipe (3) could become excessive. One solution to this problem is to use a multiplicity of nozzles in a single vertical pipe, and divide the flow between them, as shown in Figure 4a, in which the liquid is fed through an entry pipe (1) to a chamber (17), before issuing through a multiplicity of nozzles or orifices. The number of jets is determined by practical considerations, because the greater the number of jets, the smaller will be the jet diameter for a given flow rate, and the minimum jet size should be such that it will not become blocked by solids suspended in th liquid. The jet velocity is determined by the pressure i the chamber (17), and is therefore the same for each jet.

A further improvement can be made by the installation of vertical baffles in a multi-jet system, so as to confine each jet within its own interior vertical duct, as shown in Figure 4a and 4b. Without such baffles, the individual jets are bounded mainly by the turbulent fluid in neighbouring jets, and in part by the wall of the confining pipe (3). The vertical baffles (20) provide a solid physical boundary which fixes the region of energy dissipation around each jet, and assists it to perform its task of dividing up the entrained gas into fine bubbles. The area of each section (21) of the cross-sectional area of the pipe confined by the vertical baffles (20), should be approximately the same. Note that in Figure 4a, the nozzles (2) are shown mounted on the lower extremity of short pipes (22), being similar in construction to the single orifice case depicted in Figure la. The purpose of the pipe piece (22) is to allow the jets to commence at a level within the pipe (3), which is below the air entry line (6). In operation, the containing pipe (3) may fill with dense foam up to the level of the entry point of the jet, and hence liquid may flow back up the air line (6), in which solids may be deposited. Accordingly, it is deisrable for the individual liquid injection nozzles (2) to be below the air inlet pipe (6).

The performance of the bubble dispersion system can be enhanced in various ways depending on the interfacial properties of the gas-liquid system. In order for a stable two-phase mixture to fill the vertical pipe (3), it is necessary that there should be little coalescence of the bubbles as they are forced to flow downwards toward the exit (5). Where coalescence occurs, very small bubbles aggregate with others and grow into bubbles so large that they may bridge the pipe (3), and cause collapse of the two-phase mixture within the pipe.

Coalescence is prevented or inhibited by the presence of salts, dissolved matter and especially surface-active agents dissolved in the liquid, as well as the presence of particles of solid or insoluble liquids such as oils and greases. Since the properties of each gas-liquid system will be different, it is unlikely that any single bubble dispersion device will be optimum for all cases, and it may be necessary to modify the design to cope with individual circumstances. A number of modifications are now described which may be usefully employed. Figure 5 shows an arrangement in which the jet is enclosed by a draft tube (30) in the form of an open-ended cylinder, which extends down the axis of the vertical pipe at least as far as the impingement point described above, where the jet has expanded to occupy the full cross-sectional area of the draft tube. The purpose of the draft tube (30) is to restrict the volume of gas-liquid mixture in the immediate vicinity of the jet, so as to intensify the rate at which energy is dissipated per unit volume of fluid, leading to smaller bubbles in the vertical pipe (3). The diameter of the draft tube can conveniently be in the range 2 to 10 times the jet diameter, with satisfactory operation being found in the range 3 to 8 times the jet diameter.

The upper end of the draft tube may conveniently be left open, or for ease of construction, it can be made in the form of a cylindrical pipe attached to the head of the pipe (3) as shown in Figure 5, with a communicating opening (31) being provided above the level of entry of the liquid jet, in order to enable gas to recirculate around the draft tube.

In a variation on this improvement, the draft tube may be pierced with holes (32) which may occupy up to 20% of the outer area of the tube as shown in Figure 5. The purpose of these holes is to permit circulation of gas which may return to the head space of the vertical pipe (3) due to coalescence of the bubbles, while at the same time providing sufficient integrity to the draft tube to have a substantial confining action on the fluid within i Another variation which has been found useful is depicted in Figure 6a, which shows a draft tube which is mounted within the vertical pipe (3) . The top of the draft tube is placed near the exit orifice (2) and the sides of the upper part of the tube (41) are sloping so that its area increases to a point with increasing distance down the vertical pipe (3) . At a convenient point (42), the area of the draft tube begins to contract with distance down the pipe, and the lower part (43) of the tube terminates at a convenient point so that the area of the tube exit is larger than the cross-sectional area of the liquid jet at the same level. The purpose of this form of draft tube, which first expands and then contracts in area, is to permit any large bubbles or slugs of gas which may have formed lower down in the vertical pipe (3), to rise and bypass the jet, and then be re-entrained through the entrance (45). Another form of this type of draft tube is shown in Figure 6b, in which the draft tube at its widest point essentially forms a seal against the inner wall of the vertical pipe (3). Thus any large bubbles which have risen up the walls of the pipe (3) are trapped in the annular space (44) between the draft tube and the pipe, and are re-entrained through the ring of openings (45) located immediately beneath the widest part of the draft tube (44).

The alternative configuration shown in Figure 7 is intended where minor coalescence takes place, and the bubbles are not so large as to cause collapse of the two-phase medium within the vertical pipe (3), but which may benefit from further exposure to the high-speed jet. In this alternative, the draft tube first contracts in area as distance increases down the pipe (3), to reach a minimum at the point (52), below which it increases in area. The cross-sectional area of the open tube at the point (52) should be greater than the cross-sectional area of the expanding liquid jet at this point. Annular spaces (53) and (54) may be left between the draft tube and the inner wall of the pipe (3), or the draft tube may be sealed to the walls of the pipe (3).

The dimensions of the vertical pipe (3) relative to the diameter of the jet, have an important bearing on the operation of the bubble dispersing system. For satisfactory operation, it has been found that the diameter of the vertical pipe (3) should be in the range 2 to 20 times the jet diameter, with satisfactory operation being found in the range 3 to 12 times the jet diameter. In many cases, the operating gas will be air at atmospheric pressure, and it is advantageous if the device can operate by drawing air from the atmosphere without th need for a blower or compressor. This can be achieved if the pressure in the vicinity of the jet orifice (2) is less than the atmospheric pressure. In other situations, however, air may be supplied into the upper part of the conduit or pipe (3) under pressure via a blower or compressor. This arrangement ma be applicable where it is desired to submerge the pipe (3) to a larger degree within the body of liquid (4) , so requiring a greater "head" within the top of the pipe (3) to cause the foam to move downwardly within the pipe and issue from the lower end (5) against the pressure head in the body of liquid (4).

Although the invention has been described in one preferred form using air as the gas for "aeration" it wil be appreciated that other gases may be used in certain situations where it is required to "aerate" a liquid with a gas other than air.

Although the invention has been described with reference to the aeration of wastewaters, it is also suitable for the flotation of mineral particles so as to remove the valuable minerals from unwanted waste matter, by contacting them with fine bubbles in a suspension of the mineral in water, so that the particles which it is desired to remove have been rendered non-wetting by the liquid while the particles which are to remain in the liquid are rendered wettable by the liquid. The valuable particles then adhere to the surface of the fine bubbles and rise with them to the surface of the liquid, from which they may be removed as a froth.

Claims

CLAIMS : -

1. A method of aerating liquids comprising the steps of passing a liquid in a downwardly moving jet into the upper part of a downwardly extending conduit the lowe end of which is submerged in a body of liquid, forming a foam of bubbles within the conduit and causing the foam t move downwardly and issue from the lower end of the conduit into the body of liquid.

2. A method as claimed in claim 1 wherein gas i provided to the upper part of the conduit and entrained into the jet forming the foam of bubbles.

3. A method as claimed in claim 2 wherein the upper part of the conduit is enclosed and the gas is provided at atmospheric pressure through a flow rate controlling constriction.

4. A method as claimed in claim 2 wherein the upper part of the conduit is enclosed and the gas is provided under pressure via a blower or compressor.

5. A method as claimed in any one of the preceding claims wherein the liquid passing into the uppe part of the conduit incorporates a gas in supersaturated solution within the liquid.

6. Apparatus for aerating liquids comprising a substantially vertically extending conduit having an open lower end, liquid supply means arranged to supply liquid under pressure to a downwardly facing nozzle located and arranged within the upper part of the conduit so as to form a downwardly issuing jet of liquid within the conduit, and support means arranged to support the condui with the lower end immersed in a body of liquid.

7. Apparatus as claimed in claim 6 wherein the conduit is provided with a diffuser extending outwardly from the exterior of the conduit and positioned to be immersed in the body of liquid, the diffuser being arranged to disperse bubbles issuing from the lower end o the conduit outwardly away from the conduit as the bubble rise in the body of liquid.

8. Apparatus as claimed in claim 7 wherein the diffuser comprises a plate surrounding the conduit and extending outwardly and upwardly from a predetermined position on the exterior of the conduit.

9. Apparatus as claimed in claim 8 wherein the plate is provided with a plurality of holes therethrough sized to allow a predetermined flow rate of bubbles through each hole.

10. Apparatus as claimed in either claim 8 or claim 9 wherein a gap is provided between the inner periphery of the plate and the exterior of the conduit.

11. Apparatus as claimed in claim 10 wherein a diverting ring is positioned on the exterior of the conduit below the gap, arranged to divert upwardly moving bubbles outwardly away from the conduit and away from the gap between the conduit and the plate.

12. Apparatus as claimed in any one of claims 6 to 11 wherein the interior of the conduit is provided with a draft tube having its axis parallel to the axis of the conduit and being configured to direct the flow of gas within the conduit and/or constrain the jet within the conduit.

13. Apparatus as claimed in claim 12 wherein the draft tube has one or more holes therein positioned to allow gas rising within the conduit, between the conduit and the draft tube to reenter the draft tube via the holes.

14. Apparatus as claimed in either claim 12 or claim 13 wherein the draft tube has a relatively narrow upper end positioned adjacent the nozzle, flaring downwardly and outwardly to a relatively wide mid-section, and then tapering downwardly and inwardly, terminating in an open lower end wider than the upper end but narrower than the mid-section.

15. Apparatus as claimed in either claim 12 or claim 13 wherein the draft tube has a relatively wide upper end positioned adjacent the nozzle, tapering downwardly and inwardly to a relatively narrow mid-section, and then flaring downwardly and outwardly, terminating in an open lower end.

16. Apparatus as claimed in any one of claims 7 to 15 wherein the conduit is substantially circular in cross-section, or is of other cross-section having minor and major lateral axes of the same order and having an effective diameter equal to the diameter of a circle of equivalent area, and wherein the diameter of the conduit is in the range 2 to 20 times the diameter of the nozzle.

17. Apparatus as claimed in claim 16 wherein the diameter of the conduit is in the range 3 to 12 times the diameter of the nozzle.