AU2002101010B4

AU2002101010B4 - Treatment process and apparatus

Info

Publication number: AU2002101010B4
Application number: AU2002101010A
Authority: AU
Inventors: Christopher John Conway; Graeme John Jameson; Timothy Wentworth Lewis
Original assignee: Environmental Group Ltd
Current assignee: ENVIRONMENTAL GROUP Ltd
Priority date: 2001-12-12
Filing date: 2002-12-12
Publication date: 2004-09-09
Anticipated expiration: 2010-12-12

Description

12/12 '02-TU 17:37 FAX 81 2 99255996 GRIFFITH HACK a 005

AUSTRALIA

Patents Act 1990 COMPLETS SPECIFICATION INNOVATION PATENT Applicant: THE ENVIRONMENTAL GROUP LIMITED Invention Title: TREATMENT PROCESS AND APPARATUS The following statement is a full description of this invention, including the beet method of performing it known to me/us: /1/12 '02'THU 17:37 FAX 61 2 99255996 GRIFFITH HACK [4008 -2- TREATMENT PROCESS AND APPARATUS Field of the Invention The present invention relates to a treatment process and apparatus therefor to increase the concentration of dissolved gas in an aqueous dispersion prior to a separation process. The process and apparatus can be applied in the pressurisation treatment of an aqueous dispersion of a substance to enhance the separation of a portion of the substance in a subsequent flotation step and will primarily be described with reference to this context.

The invention has broad use in the enhanced separation of substances such as suspended solids, oil and grease, biomass, algae and the like contained in industrial and municipal waste water, contaminated water and effluents prior to all manner of flotation separation techniques.

Background Art The known methods of induced gas flotation involve passing a plunging jet of inlet fluid containing particulate material into a downward moving co-current flow of bubbles and liquid confined in a column. A film of gas is induced by suction into the jet and is broken up into smaller bubbles in this turbulent and high shear fluid regime. The intimate mixing of bubbles, particulates and fluid in the column can give a highly efficient gas/solid contact mixing through the bubble lamellar film so that improved recovery of desired species occurs when the mixture is then allowed to separate by flotation in a cell (for example, US4938865).

In the flotation separation of finely dispersed materials such as suspended solids, oil and grease, biomass and algae contained in industrial and municipal waste water and effluents, a separate coagulation or flocculation step is often required prior to flotation. Such a step can increase the effective particulate size so that efficient 12/12 '02'TLHU 17:38 FAX 61 2 99255996 GRIFFITH HACK R007 3 gas/solid contact mixing through the bubble lamellar film can occur, since very fine particles have difficulty contacting the bubble surface in flotation processes.

However, the action of high shear rates can be destructive to these coagulated materials.

Summary of the Invention In a first aspect the present invention provides a process for the flotation of an aqueous dispersion of a substance, the process including the steps of: pressurising a portion of the aqueous dispersion to increase the concentration of gas in the dispersion; passing the pressurised dispersion through a flow constriction device so as to reduce the pressure of the aqueous dispersion; inducing a gas into the reduced pressure dispersion to provide a foam bed; and introducing an unprocessed aqueous dispersion at or into the foam bed.

When the term 'unprocessed aqueous dispersion' is used it refers to an aqueous dispersion of a substance that has not been pressurised to increase gas concentration in that aqueous dispersion.

In any subsequent flotation separation step the introduction of the unprocessed aqueous dispersion into the foam bed results in reduced shear stress on each particulate substance in that dispersion and, together with the increased concentration of gas in the dispersion, flotation of the substance is enhanced.

In a second aspect the present invention provides a process for the treatment of an aqueous dispersion of a substance to be subsequently separated by flotation, the process including the steps of: pressurising a portion of the aqueous dispersion to 12/12 '02'THU 17:38 FAX 61 2 99255996 GRIFFITH HACK 00oo 4 increase the concentration of gas in the dispersion; and passing the pressurised dispersion through a flow constriction device whereby the dispersion is released through said device at a pressure below atmospheric.

Subsequent flotation of the substance can then occur.

Preferably in the process of the first or second aspects the pressurising of the aqueous dispersion involves the addition of gas by means of compression, blowing or induction.

Preferably the process of the first or second aspects further involves absorption of at least some of the gas into the dispersion, the absorption being effected in a chamber into which the dispersion can flow and have a residence time.

Preferably the dispersion of the first or second aspects is pressurised to in excess of 200kPa gauge pressure. Most preferably the dispersion is pressurised to between about 300 to 1000kPa gauge pressure.

In a third aspect the present invention provides an apparatus for flotation of an aqueous dispersion of a substance, the apparatus including: means for pressurising the dispersion to increase the concentration of gas in the dispersion; a flow constriction device arranged for receiving the pressurised aqueous dispersion therethrough so as to reduce the pressure of the aqueous dispersion; means for introducing a gas into the reduced pressure aqueous dispersion to provide in use a foam bed for gas flotation; and means for introducing an unprocessed aqueous dispersion into the foam bed.

In use such apparatus reduces shear stress on the 12/12 '02'THU 17:38 FAX 61 2 99255996 GRIFFITH HIACK Mooa 5 substance and, together with the increased concentration of gas in the dispersion, can enhance flotation of the substance.

In a fourth aspect the present invention provides an apparatus for the treatment of an aqueous dispersion of a substance to be subsequently separated by flotation, the apparatus including: means for pressurising the dispersion to increase the concentration of gas in the dispersion; and a flow constriction device being arranged to receive the pressurised aqueous dispersion whereby the dispersion is released through said device at a pressure below atmospheric.

Subsequent flotation of the substance can then occur.

In a fifth aspect the present invention provides an apparatus for the treatment of an aqueous dispersion of a substance, the apparatus including: means for pressurising the dispersion to increase the concentration of gas in the dispersion; and a flow constriction device being arranged for receipt of a vertically oriented flow of the pressurised aqueous dispersion and wherein induced gas is introduced into the aqueous dispersion after passage through the constriction device which, together with the increased concentration of gas in the dispersion, enhances subsequent flotation separation of the substance.

Preferably the means for pressurising the dispersion of the third, fourth or fifth aspects is a pump which increases the pressure of the dispersion to in excess of 200kPa gauge pressure.

Preferably the apparatus of the third, fourth or fifth aspects further includes a chamber being arranged to permit absorption of at least some of the gas into the pressurised dispersion.

12/t2 'O2'THU 17:39 FAX 61 2 99255996 GRFIHAC GRIFFITH RACK -6 preferably the apparatus of the% third, fourth or fifth aspects further includes means for introducing gas into the pressurised dispersion. Most preferably the means for introducing the gas includes a blower, a compressor or a gas induction device such as a venturi.

Preferably the flow constriction of the third, fourth or fifth aspects includes an orifice plate or a venturi.

Preferably the apparatus of the third, fourth or fifth aspects further includes a confined chamber located on an opposing aide of the flow constriction device f or receiving the flow of treated dispersion and an induced flow of a flotation gas. Preferably a pipe is positioned to allow unprocessed dispersion into the confined chamber.

Preferably the confined chamber i's in fluid comunication with and partly located within an adjacent containment for flotation separation, the upper surface of the containment being open to atmosphere.

Brief Description of the Drawings Notwithstanding any other f onus which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure I shows a schematic view of one embodiment of a process for the treatment of an aqueous dispersion of a substance in accordance with the invention.

Figure 2 shows a schematic view of a further embodiment of a process for the treatment of an aqueous dispersion of a substance in accordance with the invention.

Modes for Carrying out the invention Referring to the drawings, apparatus 10 is shown for the treatment of an aqueous dispersion of a substance, to be subsequently separated by an induced gas flotation step.

Typically the substance is suspended solids, oil and grease, biomass or algae or a combination thereof, 12/12 '02THU 17:39 FAX 81 2 99255996 GRIFFITH HACK 011 7 dispersed in industrial or municipal waste water or effluents, although other very finely dispersed minerals and clays can also be fed to the process.

The process involves the pressurisation of the dispersion to increase the concentration of dissolved gas therein so as to enhance the flotation of the substance.

The typical means for pressurising the dispersion is a pump 12 which increases the pressure of the dispersion to in excess of 200kPa and preferably pressurised to between about 300 to 10OO0kPa gauge pressure. At least some of the introduced gas is absorbed into the dispersion in a chamber typically in the form of a pipe 14 or a pipe and tank 16 combination into which the dispersion can flow and have an adsorption residence time. Any shape of pipe or tank is able to be used to facilitate gas absorption. Typically the gas and fluid dispersion flows into a pipe or tank at an upper end so that gas is forced to flow through a resident volume of dispersion to facilitate gas absorption.

The gas can be added into the dispersion via the pump inlet pipe 18 by means of a compressor or a blower in fluid communication with the inlet pipe 18, or by way of a gas induction device such as a venturi located in the pump outlet pipe The pressurised and gasified dispersion passes from the pipe 14 and/or tank 16 combination through a flow constriction device in the form of an orifice plate (or nozzle) 22 in order to release the pressure to levels typically in the range OkPa to -30kPa gauge pressure (or 70-90 kPa absolute pressure). One or more orifice plates or nozzles may be fitted as part of a flow constriction device. At least part of the absorbed gas is caused to nucleate as gas bubbles in the dispersion or at the surface of the substance. The thus depressurised aqueous dispersion passes as a jet into a confined vessel in the form of a column or downcomer 24 in which gas for the flotation step is induced therein via a downcomer side wall 12/12 '02"THU 17:40 FAX 61 2 99255996 GRIFFITH HACK 1012 a inlet 26 and the dispersion meets a rising foam bed of gas bubbles 28. Alternatively a flow constriction device such as a venturi can be used wherein the flotation gas is introduced into the flow at the constriction.

The dispersion aerated by the induced gas forms a downward moving foam bed 28 comprising bubbles, liquid and the substance, confined in the column 24. As the gas bubbles are buoyant they will rise against the generally downward flow of fluid in the column giving a gentle but high efficiency contact zone for substance and bubbles.

The aerated dispersion then flows from the base 30 of the column 24 into a containment in the form of a flotation cell 32 where flotation separation of a portion of the substance can occur. The column 24 is in fluid communication with and partly located within the flotation cell 32, the upper surface 34 of the cell 32 where the flotation product is collected being open to atmosphere.

In further embodiments the cell may be enclosed for gas collection and re-use. The cell may be of any particular shape and configuration and may be agitated or aerated by various techniques, some of which are described later in this description.

In one embodiment shown in Figure 1 the flow of unprocessed aqueous dispersion is introduced to the process at or into the foam bed 28 by a pipe 36 from a feed liquid coagulation stage. In this way the coagulation or flocculation of the feed dispersion is not destroyed by pumping or the high shear of a plunging jet, and the coagulated form of the substance is preserved in the flotation cell as the flotation gas bubbles are encountered. Furthermore the feed to the pressurisation process is at least partly comprised of any substance which was not separated by flotation in the cell 32 and which is passed to the pump via the pump inlet pipe 18.

In further embodiments of the invention some flotation gas can be introduced to the depressurised flow of the

I

12/12 '02'THU 17:40 FAX 81 2 99255996 GRIFFITH HACK Io013 9 dispersion by other means, for example, by spargers, or induced by rotor/stator combination apparatus or other aeration distribution devices in the flotation cell (such as Cominco spargers, Agitair or Denver flotation cells etc).

In experimental measurements it has been found that the bubbles formed as a result of the nucleation of part of the absorbed gas at the surface of the substance typically average 20-50 micrometres in diameter. The bubbles resulting from the induced or other introduced gas flotation typically average 300 micrometres in diameter.

It is surmised that the nucleated bubbles perform several synergistic functions in the flotation operation which are an improvement over the known processes in the art, as will now be discussed.

Firstly, although the location of bubbles on the surfaces of the finely dispersed materials alone may not cause their flotation, the presence of the surface bubbles improves the likelihood of contact of these materials with the larger flotation gas bubbles in the froth bed 28. The small size of the nucleated bubbles is not normally achievable by the shearing of induced gas by a plunging jet, for example, and so the flotation recovery of small particulates can be improved.

In some biological treatment processes the coagulation and flocculation of feed dispersions can involve the use of high reagent dosages in order to coagulate very fine materials. In the present example, the flocculant polymer and other reagent dosages required are considerably reduced when the nucleated absorbed gas is present because the location of bubbles on the surfaces of the finely dispersed materials improves the likelihood of contact and coagulation with another like material in the froth bed 28.

Additionally, as already stated, the coagulation of very fine material in a flow of unprocessed aqueous dispersion is preserved in the froth phase of the flotation 12/-12 '0-THET 17:41 PAL 61 2 99255996 GRIFFITH RACE [IN14 10 cell in the arrangement shown in Figure 1 thus improving their flotation recovery since it is known that coarser particulates generally exhibit improved flotation rates and recoveries due to improved interaction with the lamellar film of a rising flotation gas bubble.

The materials of construction of the pressurisation apparatus 14, 16 can comprise any suitable materials which can be shaped, formed and fitted in the manner so described, such as metal to give a structurally sound apparatus that can withstand high pressures.

The embodiments of the invention shown can provide an improved efficiency of flotation recovery over the known techniques which typically are poor at recovering suspended solids, oil and grease, biomass or algae or a combination thereof, when finely dispersed in industrial or municipal waste water or effluents.

Whilst the invention has been described with reference to a number of preferred embodiments it should be appreciated that the invention can be embodied in many other forms.

It is to be understood that, if any prior art information is referred to herein, such reference does not constitute an admission that the information forms a part of the cormon general knowledge in the art, in Australia or any other country.

Claims

2. A process for the treatment of an aqueous dispersion of a substance to be subsequently separated by flotation, the process including the steps of: pressurising a portion of the aqueous dispersion to increase the concentration of gas in the dispersion; and -passing the pressurised dispersion through a flow constriction device whereby the dispersion is released through said device at a pressure below atmospheric.
3. An apparatus for flotation of an aqueous dispersion of a substance, the apparatus including: means for pressurising the dispersion to increase the concentration of gas in the dispersion; a flow constriction device arranged for receiving the pressurised aqueous dispersion therethrough so as to reduce the pressure of the aqueous dispersion; means for introducing a gas into the reduced 112 '02THU 17:41 FAX 81 2 99255996 GRIFFITH HACK o016 12 pressure aqueous dispersion to provide in use a foam bed for gas flotation; and means for introducing an unprocessed aqueous dispersion into the foam bed.
4. An apparatus for the treatment of an aqueous dispersion of a substance to be subsequently separated by flotation, the apparatus including: means for pressurising the dispersion to increase the concentration of gas in the dispersion; and a flow constriction device being arranged to receive the pressurised aqueous dispersion whereby the dispersion is released through said device at a pressure below atmospheric. An apparatus for the treatment of an aqueous dispersion of a substance, the apparatus including: -means for pressurising the dispersion to increase the concentration of gas in the dispersion; and a flow constriction device being arranged for receipt of a vertically oriented flow of the pressurised aqueous dispersion and wherein induced gas is introduced into the aqueous dispersion after passage through the constriction device which, together with the increased concentration of gas in the dispersion, enhances subsequent flotation separation of the substance. Dated this 12th day of December 2002 THE ENVIRONMENTAL GROUP LIMITED by its Patent Attorneys GRIFFITH HACK