WO1995027557A1

WO1995027557A1 - Method of generating gas bubbles in a liquid and apparatus for the implementation of the method

Info

Publication number: WO1995027557A1
Application number: PCT/FI1995/000209
Authority: WO
Inventors: Heikki Kauppinen
Original assignee: Ekokehitys Oy
Priority date: 1994-04-12
Filing date: 1995-04-12
Publication date: 1995-10-19
Also published as: FI941674A7; FI941674L; FI941674A0; AU2259595A

Abstract

The present invention relates to a method and an apparatus for generating gas bubbles in a liquid, in which method gas and liquid are taken into a dispersion space (2) situated inside a dispersion nozzle device (1), and at least a portion of the dispersion operation is carried out in the dispersion space and in a discharge channel (3) exiting thereof using liquid-gas-liquid dispersion. In the method and apparatus according to the invention, flexible and accurate control of gas bubble size is achieved through altering the length (A) of the dispersion space (2) with the help of an adjuster means (4; 5; 10), or alternatively, through altering simultaneously the length (A) of the dispersion space (2) and the size of the gas infeed opening (12) to the dispersion space with the help of the adjuster means (4; 5; 10).

Description

Method of generating gas bubbles in a liquid and apparatus for the implementation of the method

The present invention relates to a method of generating gas bubbles in a liquid, in which method gas is taken into a dispersion space situated inside a dispersion nozzle device, and at least a portion of the dispersion operation is carried out in the dispersion space and in a discharge channel exiting thereof using liquid-gas-liquid dispersion. Furthermore, the invention concerns an apparatus for the implementation of the method.

Various applications in the industry and environmental technology require the dispersion of different liquids and gases. Such applications include, e.g., aeration or oxygenation of water or aqueous solutions/suspensions, and the froth flotation and/or flotation of minerals, solids, sludges and similar components from liquids containing these particles.

The small bubbles of air required in the flotation purification of liquids are generally produced using a so-called dissolved-air method in which a solution saturated with air or gas in a pressure vessel with the help of elevated pressure is subsequently taken along a pipe to the space containing the liquid to be purified. The small bubbles (with a size of < 100 μm) thus gener¬ ated float up the solids, sediment and similar particles. The flotated material is accumulated immediately below the liquid surface forming a relatively consistent super¬ natant froth which is removed at timed intervals by means of a scraper or similar means over the edge of the flota¬ tion cell. Dissolved nutrients or other undesirable ele¬ ments contained in the liquid can be removed by first coagulating them with suitable chemicals and then remov¬ ing by flotation the formed sediment, to which also solids contained in the liquid generally readily attach. The microbubbles required for flotation are still today conventionally generated by the dissolved-air method in a pressure vessel, wherefrom the air/gas-saturated liquid mixture is taken via piping to the purification tank. In addition to their high purchase and operating costs, the pressure vessels require annual inspections and mainte¬ nance.

In the concentration of minerals, air bubbles are often generated by means of a mechanical impeller, whereby air is injected via the blades of the revolving impeller into the slurried ore. Also other methods described in the prior art based on injecting compressed air into water, particularly using the microbubble generation methods, are used in the mining industry. In mineral flotation an amphiphatic collector reagent is used with one end of the reagent molecule attaching to the desired valuable miner¬ al and the other, aerophilic end attaching to an air bubble. Then, the air bubble floats the adhering mineral particles up to the surface of the solution and further along with the overflow of the formed froth, over the edge of the cell. The bubble size required in mineral froth flotation is larger than in flotation, typically being ≥IOOO μm. A disadvantage herein is a high specific energy consumption and the need of repetitive maintenance and servicing of the mechanical rotors. Moreover, the adjustment of the air bubble size in impeller aeration is problematic. Prior-art methods of controlling the air bubble size include change of air feed rate and/or bubble size control by means of various chemicals. Both control methods of bubble size are complicated and frequently cause operating deviations and disturbances in the process that rather deteriorate than improve the desired outcome.

As grinding in mineral concentration often ends up in overgrinding (resulting in partially excessive comminu- tion of the valuable mineral) or undergrinding, it would be advantageous to have a method of controlling the aver¬ age size of the dispersion bubbles during the process to a value which follows the grinding result with a rela- tively high accuracy. Today, it is impossible to achieve a sufficiently flexible and controlled bubble size adjustment, whereby optimal results of the process cannot usually be attained.

Apparatuses are used in the art in which the liquid and the gas are mixed in a separate dispersion vessel prior to taking them to the treatment tank or similar point of use. Conventional dispersion methods are suited for generating either microbubbles for flotation, or more typically, only larger bubbles for froth flotation, but not for making both bubble sizes controlledly by means of the same apparatus.

In general, it can be said that prior-art arrangements are based on controlling the bubble size through air flow rate adjustment. When smaller bubbles are desired, the air flow rate must be reduced, whereby also the total volume of air entrapped in the bubbles may possibly fall below the critical volume required for the process. Also a contrary problem may occur if the process demands a high volume of air. Then, increasing the air flow rate may bring the bubble size excessively large for the process. In other words, the bubble size in conventional arrangements is very strongly dependent on the air flow rate.

It is an object of the present invention to provide a method and apparatus capable of offering essential improvement over conventional techniques. It is a parti- cular object of the invention to provide a method suited for adjusting the size of gas bubbles in a manner which is flexible, controlled, quick and independent of the air flow rate as required by the intended application. It is still another object of the invention to provide an apparatus having a simple and reliable construction which is cost-efficient in manufacture and operation.

The goal of the invention is achieved by means of a method and apparatus characterized by what is stated in the appended claims.

In the method according to the invention, the size of generated gas bubbles is controlled through altering the length of the dispersion space with the help of an adjus¬ ter means, or alternatively, through altering simul¬ taneously the length of the dispersion space and the size of the gas infeed opening to the dispersion space with the help of the adjuster means. The dispersion space is formed into a separate space, and bubble generation and size of bubbles generated therein can be adjusted flexibly according to the invention. The size of the gas infeed opening is either kept constant while the length of the dispersion space is altered, or alternatively, the size of the gas infeed opening is adapted variable in a predetermined fashion. When the method according to the invention is applied, the bubble size can be adjusted in a manner that causes less variation in the air flow rate than conventional arrangements. The method according to the invention is superior to conventional dispersion arrangements in that the present method makes it possible to generate both small bubbles (microbubbles) for flota- tion and also larger bubbles for froth flotation in a controlled manner. In the generation of both small and larger bubbles, the bubble size can be controlled accurately as required and also the fine-tuning of the bubble size is possible. Particularly in mineral froth flotation such a bubble size fine-tuning facility is an excellent benefit as it may be employed to reduce production losses caused by errors in ore grinding. The bubble size can be adjusted mechanically for a larger or smaller size in order to improve the flotation efficiency.

The aeration apparatus according to the invention is simple and easy to operate and maintain. In the method according to the invention, the kinetic energy of the liquid is effectively utilized by implementing the liquid-gas-liquid dispersion inside the apparatus and the gas-liquid dispersion outside the apparatus in the liquid to be dispersed. As a result a gas-liquid mixture with abundant bubbles is achieved at a low cost of bubble generation. The method according to the invention is suited for the dispersion of all kinds of liquids and gases as well as their numerous applications.

When the dispersion space is implemented as a separate space, changes can be controlled and the size of gas bubbles adjusted. In a preferred embodiment of the invention, the fine-tuning of the size of gas bubbles generated in the dispersion space and the discharge channel can be complemented with bubble size control based on varying the flow and/or pressure ratios of the infeed liquid and/or gas, which are conventionally known coarse adjustment methods of bubble size.

The apparatus suited to implement the method^■according to the invention, which apparatus is installable in an operating space such as an aeration tank, a dispersed-air flotation cell or a dissolved-air froth flotation cell, has an extremely simple construction and comprises no wearing mechanisms. The manufacturing and maintenance costs of the apparatus are low. Moreover, the replacement of the apparatus or a part thereof with a new one takes only a few minutes, whereby such a replacement operation cannot generally cause any major disturbance to the process supported by the apparatus. The disturbance states related to the maintenance operations of the apparatus according to the invention are short-term and rare. By contrast, the process must generally be run down for service or maintenance operations when a conventional apparatus is used, whereby in addition to maintenance costs, economical losses will occur from process run-downs, restarts and production losses.

The specific energy consumption of the apparatus accord- ing to the invention is extremely modest. For example, in comparison to conventional impeller cell flotation, the energy consumption of the apparatus according to the invention is less than 10 %. Also the operating costs of the dispersion apparatus according to the invention remain appreciably smaller than those of the prior-art.

Microbubbles employed in the conventional flotation tech¬ niques of water purification can be generated by means of the method according to the invention and utilizing the apparatus implementing the method without the use of a pressure vessel for gas saturation. Additionally, the same apparatus can be used for generating larger gas bubbles suited for separation of sludge and solids through froth flotation, whereby the sludge, solids or similar particulates are carried by the bubbles up to the surface of the sludge-containing liquid and further away from the liquid being purified along with the overflow of the formed froth, over the edge of the cell. As the froth flotation operation is appreciably faster (needing shorter time for purification) than conventional flota¬ tion, the purification equipment requires an essentially smaller footprint in the water treatment premises. Hence, froth flotation applications using the embodiment accord¬ ing to the present invention require smaller investment costs in building and equipment with regard to those of conventional flotation applications. Moreover, the oper- ating costs remain herein smaller than in conventional flotation.

In the following the invention is described in more detail with reference to the attached drawings, in which!

Figure 1 is a longitudinally sectioned side view of an embodiment of the dispersion nozzle device according to the invention;

Figure 2 is a longitudinally sectioned side view of another embodiment of the dispersion nozzle device according to the invention;

Figure 3 is a longitudinally sectioned side view of a third embodiment of the dispersion nozzle device according to the invention;

Figure 4 is a schematic diagram illustrating an aeration application of the method suited for use in the oxidiza¬ tion of, e.g., water taken from natural supplies;

Figure 5 is a schematic diagram illustrating another aeration application of the method suited for aeration, and conventional flotation or froth flotation in, e.g., tanks;

Figure 6 is a froth flotation/conventional flotation application of the apparatus according to the invention in a partially sectioned side view;

Figure 7 is a top view of the apparatus shown in Fig. 6; and

Figure 8 is a partially diagrammatic side view of another further application of a froth flotation/conventional flotation apparatus according to the invention. Referring to Fig. 1, the dispersion nozzle device 1 com¬ prises a body part 6 incorporating a discharge channel 3, an end part 4 incorporating a liquid infeed connection 8 and a liquid channel 9, and an intermediate part 10 adapted between the body part and the end part. Between the intermediate part, the end part and the discharge channel is arranged a dispersion space 2. The end part 4 is axially adjustably connected to the intermediate part 10 by means of threads, for example, thus providing stepless adjustment of the length A of the dispersion space 2 by moving the end part with respect to the inter¬ mediate part. Similarly, the intermediate part 10 is adjustably connected to the body part 6 thus permitting the adjustment of the length of the dispersion space 2 also by moving the intermediate part relative to the body part. A gas infeed connection 7 is adapted to the body part. The wall of the intermediate part is provided at the gas infeed connection with an opening 12 through which the gas can enter a gas space 11 between the intermediate part and the end part, whereby the opening forms a round, slit-like gas entrance orifice into the dispersion space. The opposing walls of the intermediate part and the end part are arranged tapering in this space. The walls are extended tapering almost parallel, thus giving an effective control means for the gas flow rate through the adjustment of the axial distance between the intermediate part and the body part. The gas connec¬ tion line 7 is provided with a regulator for the adjust¬ ment of the gas flow rate and pressure. Additionally, the liquid connection line 8 may be provided with a regulator for the adjustment of the liquid pressure and flow rate.

The discharge channel 3 is detachably connected to the body part 6 thus facilitating easy replacement. Similar- ly, the end part is adapted detachable from the inter¬ mediate part and the intermediate part detachable from the body part. Hence, a certain application may readily be provided with a size- and shape-optimized liquid channel, discharge channel and other parts. An extra benefit herein is the easy replacement of worn parts.

In the use of the dispersion nozzle device, the disper¬ sion liquid is taken via the liquid connection 8 to the liquid channel 9 of the end part of the dispersion nozzle device and therefrom further to the dispersion space 2, into which the gas is taken via the gas infeed connection 7 and the gas space, that is, the gas infeed opening 12. The gas infeed opening 1? is adapted to surround the liquid channel in an annular fashion. In the dispersion space the pressurized liquid jet meets the gas entering from about it, whereby liquid-gas-liquid dispersion occurs. The dispersed liquid-gas mixture discharges further via the discharge channel, undergoing further dispersion in the channel, into the liquid to be gas- aerated or into a liquid containing particulates to be subjected to purification, flotation or other similar treatment thereby causing the dispersion of the liquid with the injected gas-liquid dispersion. In the embodi¬ ment shown in Fig. 1, the length and volume of the dis¬ persion space are adapted adjustable through an axial movement of the intermediate and end part combination relative to the body part, or alternatively, of the intermediate part relative to the end part, while the size of the gas space, that is, the gas infeed opening can be simultaneously adjusted if required by an axial movement of the end part relative to the intermediate part. In this embodiment the longitudinal adjustment of the dispersion space can be implemented either with the gas infeed openings 12 staying constant during the adjustment, or alternatively, making the openings variable in a predetermined fashion during the adjustment of the dispersion space. Now referring to Fig. 2, the embodiment of the dispersion nozzle device shown therein comprises a body part 6, an end part 4 detachably adapted to one end thereof, and a tip part 5 also detachably adapted to the other end of the body part. To the body part is attached a gas infeed connection 7. The end part is provided with a liquid infeed connection 8, and axially extending through the end part is made a liquid channel 9. In the inside on the device, between the body part, the end part and the tip part is arranged a separate dispersion space 2. The gas connection 7 is located to the side of the body part so as to exit into a gas space 11 situated between the end part and the body part. The body part is provided on the outer wall of the gas space with an appropriately upward slightly tapering, conical inner-diameter-reducing section. The outer diameter of the constant-diameter tip section of the end part that extends to the gas space is larger than the smallest inner diameter of the conical reducer section. In this application the length of the dispersion space 2 and the bubble size can be adjusted by means of an axial movement of the end part and/or the tip part relative to the body part. Additionally, the gap between the gas space and the dispersion space can be adjusted by moving the tip part 5 or the end part 4 or both simultaneously relative to the body part.

Referring to Fig. 3, a. simple embodiment of the disper¬ sion nozzle device is shown longitudinally sectioned. In this embodiment the body part 1 also forms the tip part. Then, both the volume of the gas space 11 and the length and volume of the dispersion space 2 are changed by the same adjustment when the end part is moved relative to the body part, whereby this type of bubble size adjustment facility is sufficient for some applications.

Referring to Fig. 4, an aeration application according to the invention is shown. In this application the liquid is fed to the dispersion nozzle device 1 by means of a pipe 13 forming a circle, eclipse or other desired shape. A required number of dispersion nozzle devices 1 are con¬ nected to the pipe. In this application the gas is fed to the dispersion nozzle device via a hose 14. The disper¬ sion nozzle devices are mounted so that dispersion occurs outward and downward slanting from the perimeter of the shape-folded joint manifold, whereby the gas-liquid mixture is discharged to a maximally large area and simultaneously the retention time of the dispersion mixture in the liquid to be dispersed is long and the gas treatment can be extended maximally close to the bottom yet leaving a sufficient upward distance so that the bottom sludge layer will not be evoked up with the turbu- lence. The adjustment of the dispersion nozzle devices is carried out locally according to the intended use.

Referring to Fig. 5, another aeration application accord¬ ing to the invention is shown in which the pipe 15 feed- ing the liquid and the hose 16 feeding the gas are con¬ figured to a shape complying with the treatment tank 17 or similar point of application, and a required number of dispersion nozzle devices 1 according to the invention are mounted on said liquid infeed pipe. Also in this application, optimal alignment of the discharge openings of the dispersion nozzle devices is capable of increasing the retention of the dispersion mixture in the liquid and thus aiding the gas treatment of the liquid. Moreover, individual alignment of the discharge openings of the device sets or single devices permits aiming the disper¬ sion mixture to critical points in the liquid space as required by the treatment process. The adjustment of the dispersion nozzle devices is carried out locally accord¬ ing to the intended use.

The above-described application is particularly suited to points of use where the dispersion is carried out in tanks or cells as are conventionally employed in the froth flotation of minerals, sludge and other solids or similar particulates, in flotation or in water treatment through aeration or oxidization. Obviously, also this embodiment of the invention may be varied widely not only as required by the treatment space parameters but also according to the specifications of the intended applica¬ tion.

In the application shown in Figs. 6 and 7, the apparatus is placed on the bottom of a froth flotation or conven¬ tional flotation cell 18 having a conical bottom section. The dispersion nozzle devices 1 are placed at a distance from each other along the perimeter of the conical section with one of the devices located on the bottom of the conical section. In the froth flotation/conventional flotation application shown in Fig. 8, the dispersion nozzle devices 1 are located in rows on the bottom of a tank 19 and its flanked walls.

In the foregoing the invention is described with refer¬ ence to a few applications only. To a person versed in the art, it is obvious that the invention can be varied widely within the inventive scope and spirit of the appended claims.

Claims

Claims :

1. A method of generating gas bubbles in a liquid, in which method gas and liquid are taken into a dispersion space (2) situated inside a dispersion nozzle device (1), and at least a portion of the dispersion operation is carried out in the dispersion space and in a discharge channel (3) exiting thereof using liquid-gas-liquid dispersion, c h a r a c t e r i z e d in that the size of generated gas bubbles is controlled through altering the length (A) of the dispersion space (2) with the help of an adjuster means (4; 5; 10), or alternatively, through altering simultaneously the length (A) of the dispersion space (2) and the size of the gas infeed opening (12) to the dispersion space with the help of the adjuster means (4; 5; 10).

2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that the size of bubbles generated in the dispersion space (2) and the discharge channel (3) as well as the air feed rate are controlled by varying the flow and/or pressure ratios of the infeed liquid and/or gas.

3. A method as defined in claim 1 or 2, c h a r a c ¬ t e r i z e d in that a number of the dispersion nozzle devices (1) are used.

4. An apparatus suited for implementing the method according to claim 1, said apparatus incorporating a dispersion nozzle device (1) comprising a body part (6) with a gas infeed connection (7) and a liquid infeed connection (8), a dispersion space (2) inside the device with said gas infeed connection (7) and said liquid infeed connection (8) communicating with said space, and a discharge channel (3) exiting from said dispersion space to a treatment tank or similar space, c h a r - a c t e r i z e d in that said dispersion nozzle device includes adjuster means (4; 5; 10) for controlling the length of the dispersion space and/or the cross section of the gas infeed opening.

5. An apparatus as defined in claim 4, c h a r a c ¬ t e r i z e d in that said dispersion nozzle device com¬ prises an end part (4) extending into said dispersion space and having said liquid infeed connection (8) in it, and in that said end part (4) is axially movably attached to said body part (6) for controlling the volume of said dispersion space.

6. An apparatus as defined in claim 5, c h a r a c - t e r i z e d in that said dispersion nozzle device comprises an intermediate part (10) placed between said end part (4) and said body part (6) to which intermediate part said end part (4) is movably attached and that said dispersion space is situated between said body part (6), said intermediate part (10) and said end part (4).

7. An apparatus as defined in any of foregoing claims 4-6, c h a r a c t e r i z e d in that said dispersion nozzle device incorporates a tip part (5) forming one wall of said dispersion space and having a discharge channel (3) and that said tip part is movably attached to said body part for controlling the volume of said dispersion space.

8. An apparatus as defined in any of foregoing claims

4-7, c h a r a c t e r i z e d in that said gas infeed connection (7) is adapted to communicate with a gas space (11) remaining between said body part or intermediate part and said end part, whereby said gas space forms a gas infeed opening and has a cross section tapering toward said dispersion space (2).

9. An apparatus as defined in any of foregoing claims 4-8, c h a r a c t e r i z e d in that said apparatus incorporates a plurality of said dispersion nozzle devices (1) connected to each other and located at a distance from each other in a treatment tank or similar point of use.