WO2015079210A1

WO2015079210A1 - Method and apparatus for treatment of aqueous dispersion

Info

Publication number: WO2015079210A1
Application number: PCT/GB2014/053468
Authority: WO
Inventors: Daniel Thomas Exley RITCHIE; Roger Nicholas HENSBY
Original assignee: Surewaters Consultancy Limited
Priority date: 2013-11-29
Filing date: 2014-11-25
Publication date: 2015-06-04
Also published as: GB2520733A; GB201321081D0

Abstract

Method and apparatus for separation of dispersed particles from an aqueous dispersion involves subjecting the aqueous dispersion to electrocoagulation (EC) treatment to promote formation of a flocculate comprising the particles, collecting the flocculate comprising the particles as a flocculated layer floating on a clarified aqueous solution and separating the flocculate from the remaining clarified aqueous solution. An aqueous solution of a gas is blended with the aqueous dispersion of particles, prior to separating the flocculate from the remaining clarified aqueous solution, with the aqueous solution arranged to be supersaturated with the gas on blending with the aqueous dispersion. The blending of the EC-treated aqueous dispersion with the supersaturated solution of gas speeds and facilitates separation of flocculate from clarified aqueous solution.

Description

Method and Apparatus for Treatment of Aqueous Dispersion

FIELD The present invention relates to methods and apparatus for electrocoagulation treatment of aqueous dispersions, in particular for electrocoagulation treatment of aqueous dispersions or slurries in order to facilitate removal of particles therefrom by flocculation. The invention is particularly concerned with improvements to electrocoagulation treatment when applied to purification of effluent streams of varying concentrations or to extraction of materials from aqueous dispersions by flotation.

BACKGROUND

The stabilisation and aggregation of colloidal dispersions or emulsions of particles in water or in aqueous solutions, has been explained in terms of DLVO theory (an acronym for the workers Derjaguin, Landau, Verwey and Overbeek who developed the theory) which combines the effects of van der Waals attraction with electrical double layer repulsion between dispersed, charged colloidal particles. Commonly charged colloidal particles (i.e. colloidal particles having the same sign of charge) are stabilised in colloidal dispersions by mutual electrostatic repulsion forces exceeding the attractive van der Waals attraction.

The charged particles may attract counterions, of opposite charge, to their charged surfaces, from their aqueous surroundings, resulting in the formation of an electrical double layer (EDL) at the particle surface. This EDL screens the electrical repulsion between particles, and so by formation of a suitable EDL, the electrostatic repulsion between the commonly charged colloidal particles may be sufficiently screened in order to allow van der Waals forces to drive coalescence of the particles into larger, bulk agglomerates or floes.

Typically, for water purification, or for winning of desired materials, such as heavy metals, from an aqueous dispersion or slurry, in order to remove colloidal particles from water by flocculation, modification of the EDL may be achieved by addition of electrolyte to the colloidal dispersion to be flocculated. However, for water purification, this has the disadvantage that high levels of dissolved electrolyte may remain in the water remaining after flocculated particles of material have been removed.

Electrocoagulation is based upon the use of electrochemical dissolution of an electrode by electrolytic oxidation with OH^" to form counterions of high charge, at the anodes, which can aid flocculation (typically cations such as Fe or Al for flocculation of fatty particles) without the need for addition of corresponding salt-derived anions into the aqueous dispersion to be treated (typically OH^" will be the counterions formed in the electrocoagulation process). In parallel with the formation of the cations formed at the anode, gas bubbles (hydrogen) are also formed at the cathode.

For a typical electrocoagulation system, opposed electrodes may be used to provide a voltage difference across one or more sacrificial electrodes positioned between the opposed electrodes, usually with the sacrificial electrodes not electrically connected to each other or to the opposed electrodes other than through the aqueous dispersion. This results in an electrical field being set up across the sacrificial electrodes, causing them to have cathodic and anodic surfaces and causing a current to flow between them and the opposed electrodes, typically with the material of the sacrificial electrodes oxidising and dissolving at the anodic surfaces and hydrogen bubbles being generated at the cathodic surfaces. For instance with sacrificial electrodes of aluminium, aluminium hydroxide is formed at the cathode and can lead to flocculation or co-precipitation of colloidal particles within the aqueous dispersion to be treated.

A problem with electrocoagulation systems is the need to provide large settling tanks arranged to permit flocculated materials to form and separate from the remaining aqueous solution following passage through an electrocoagulation chamber and donation of flocculant ions. Where the flocculated material is denser than the remaining solution, it will collect below the solution, but for many processes, the flocculated material may float on the surface of the solution to allow for convenient separation from the solution by skimming the flocculated material from the top of the settling tank whilst the solution is drained away below the surface. However, material that has not yet joined the floating surface layer may be lost with the draining solution.

When the floe is a waste that needs to be separated from an effluent stream to generate purified water for disposal, particles of waste remaining in the water may lead to contamination levels that may prevent the water being disposed of in the environment or local sewage systems or lead to damage if the water is allowed to enter the environment or local sewage system. Alternatively, re-use of the water for low-grade functions (such as toilet flushing, garden watering) may be prevented if the contamination levels are too high. When the floe is used for extraction of desired materials from an aqueous slurry, such as in the ore processing industry, then failure to capture all of the particles may lead to unnecessary waste and poor extraction efficiency. Hence, there is a need for improvement in the speed of separation of floes generated by electrocoagulation.

SUMMARY

It is one aim of the present invention, amongst others, to provide electrocoagulation methods and apparatus which allow for efficient flocculation of particulates from aqueous dispersions or slurries and efficient and rapid separation of flocculated material from remaining aqueous solution. It is also an aim of the invention to provide electrocoagulation methods and apparatus which address problems known from prior art electrocoagulation systems or which address other problems, such as those mentioned herinafter or otherwise present for electrocoagulation systems. For instance, one aim of the invention is to provide an electrocoagulation system suitable for treatment of both dilute and concentrated waste water streams from plant operations, such as food plant or polymer recycling operations. In particular, it is an aim of the invention to provide electrocoagulation apparatus and methods suitable for purification of water by flotation separation of fatty matter from a waste water stream. Another aim of the invention is to provide electrocoagulation apparatus and methods suitable for use in separating particulate matter from an aqueous slurry or dispersion as part of a process for winning and extracting desired materials, such as heavy metals. It is also an aim of the invention to provide an electrocoagulation method and apparatus which may be used continuously to treat effluent streams of varying concentrations and which reduce or eliminate risk of polluting waste entering the public sewage system or environment. Another aim of the invention is to provide an alternative to prior art method and apparatuses. According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

Throughout this specification, the term "comprising" or "comprises" means including the component(s) specified but not to the exclusion of the presence of other components. The term "consisting essentially of or "consists essentially of" means including the components specified but excluding other components except for components added for a purpose other than achieving the technical effect of the invention. The term "consisting of or "consists of means including the components specified but excluding other components.

Whenever appropriate, depending upon the context, the use of the term "comprises" or "comprising" may also be taken to include the meaning "consists essentially of" or "consisting essentially of, and also may also be taken to include the meaning "consists of or "consisting of. The optional features set out herein may be used either individually or in combination with each other where appropriate, and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention, as set out herein, are also applicable to any other aspects or exemplary embodiments of the invention where appropriate. In other words, the skilled person reading this specification should consider the optional features for each aspect or embodiment of the invention as interchangeable and combinable between different aspects or exemplary embodiments of the invention.

A first aspect of the invention provides a method for separation of dispersed particles from an aqueous dispersion of particles, the method comprising sequentially:

a) subjecting the aqueous dispersion to electrocoagulation treatment by flowing the aqueous dispersion through a region comprising sacrificial electrodes and located between opposed electrodes, and applying a voltage across the opposed electrodes to pass a current through the sacrificial electrodes whereby the sacrificial electrodes donate cations to the aqueous dispersion to promote formation of a flocculate comprising the particles; and

b) collecting the flocculate comprising the particles as a flocculated layer floating on a clarified aqueous solution and separating the flocculate from the remaining clarified aqueous solution; wherein the method further comprises blending an aqueous solution of a gas with the aqueous dispersion of particles, prior to separating the flocculate from the remaining clarified aqueous solution; and wherein the aqueous solution of gas is arranged to be supersaturated with the gas on blending with the aqueous dispersion. The term "electrocoagulation" as used herein is also meant to encompass electroprecipitation.

The sacrificial electrodes may be of any suitable material for electrochemical dissolution, depending upon the nature of the aqueous dispersion to be treated. Typically, the sacrificial electrodes may be of metal, and may comprise or consist essentially of aluminium or iron (e.g. steel). Aluminium-based electrodes (i.e. of an alloy comprising aluminium as a major component) may be particularly useful for the treatment of waste water in order to provide coagulation and coalescence of fatty materials dispersed therein whereby purification by bulk separation of fatty material and purified water may be facilitated. The first and second electrodes may suitably be of a material having a higher resistance to electrochemical dissolution then the sacrificial electrodes. For instance, if the sacrificial electrodes are of aluminium, the opposed electrodes may be of steel. If the sacrificial electrodes are of one grade of steel, the opposed electrodes may be of a different grade of steel, more resistant to electrolytic dissolution than the steel of the sacrificial electrodes. The term aqueous dispersion as used herein refers to any liquid suitable for application of electrocoagulation treatment, and includes flowable dispersions or slurries of particulate solids or liquids present in a continuous phase of solvent or solution typically including water as a component. Typically the solvent or solution may be aqueous solvent or solution, for instance comprising 50% or more by weight of water. The term particle merely means "small portion" and particles may be of liquid or solid, so for instance the oil droplets in an oil-in-water emulsion used as liquid are referred to herein as oil particles dispersed in a continuous aqueous phase. Typically the particles may have a diameter, for instance as measured by light scattering techniques, from 1 to 10,000nm.

The invention is of use for particles removed by flotation following flocculation, wherein the particles may have a density less than that of water or less than that of the aqueous solution in which they are originally dispersed. However, the presence of entrained gas from the electrocoagulation process may allow the invention to be applicable to particles of higher density, with the gas bubbles entrained with the particles to provide flotation. The invention is particularly useful when the particles are oils or fats such as natural fat from a renewable resource (such as an animal or vegetable fat or oil) or petrochemical oil and/or fat and/or wax. The term fat is used herein to describe such materials and typically, fats will have a density which is less than that of water.

Typically, for the method of the first aspect of the invention, the sacrificial electrodes are not electrically connected to each other or to the opposed electrodes other than through the aqueous dispersion. For instance, the sacrificial electrodes may be supported between the opposed electrodes by being held in an electrically insulating carrier.

The region comprising the opposed electrodes and sacrificial electrodes may be within a flow- through assembly comprising: sacrificial electrodes retained within a chamber, for instance held within an insulating frame; an inlet port and an outlet port arranged for flow of the aqueous dispersion through the chamber, into the inlet port, over the sacrificial electrodes, and out of the outlet port; a first electrode on an inner face of the chamber and a second electrode positioned opposite to the first electrode, such that the sacrificial electrodes are located between the opposed (first and second) electrodes.

In one suitable arrangement, an insulating frame for holding the sacrificial electrodes between the opposed electrodes in use may comprise a pair of opposed jambs or pillars of electrically insulating material having one or more sheets forming the sacrificial electrodes each having opposed edges retained in a respective slot in each opposed jamb. The sheets may typically be rectangular in shape, though this is not essential to the invention. The insulating frame may act as a replaceable cartridge to facilitate rapid replacement of the sacrificial electrodes when they are spent or damaged.

It will be understood that any suitable arrangement may be used for the opposed electrodes, for instance with an inner electrode located within a surrounding outer electrode to provide the opposed electrodes. For instance the inner electrode may be a rod with an outer electrode as a coaxial cylinder surrounding it and the sacrificial electrodes may be cylinders of various diameters coaxially positioned between the opposed electrodes. The method of the invention involves applying a voltage across the opposed electrodes whereby a current is passed between the electrodes through the sacrificial electrodes whereby the sacrificial electrodes donate cations to the aqueous dispersion. This current passes through the aqueous dispersion and will lead to the sacrificial electrodes having anodic and cathodic surfaces as a result of the applied electrical field.

In order to prevent excessive build-up of oxide/debris on the sacrificial electrodes, the method of the invention may involve periodically reversing the polarity of the voltage applied across the first and second electrodes with an interval T between the current having zero amplitude at each reversal. It will be understood that this switches the cathodic surfaces to become anodic surfaces and vice versa for the opposed electrodes and for the sacrificial electrodes. The interval T is suitably from 1 to 60 minutes, such as from 2 to 30 minutes. Shorter intervals than 1 minute may not allow sufficient time for removal of oxide/debris layers from the electrodes following reversal, whereas intervals longer than 1 hour can lead to excessive consolidation of oxide/debris layers whereby removal is more difficult.

The voltage may be applied, for instance, by means of an electrical power supply arranged across the opposed electrodes. Typically, a voltage of up to 600V, say 1 to 550V may be applied, with a direct current in the range from up to 60 Amperes (A), say 1 to 55 A, passing between the opposed electrodes. Usually, a voltage of 200 to 550V may be applied, with a direct current from 5 to 50 A, such as 10 to 25A, passing between the opposed electrodes.

The aqueous solution of gas may be formed by subjecting an aqueous solution, or water, to the gas at a pressure that is higher than the pressure of the aqueous dispersion upon blending with the solution of gas. By the term "supersaturated" it is meant that the gas is present at a concentration in excess of its solubility under the conditions after blending with the aqueous dispersion, with the consequence that gas bubbles may be nucleated as the gas comes out of solution following blending with the aqueous dispersion. Typically this may be achieved by the gas bubble generation apparatus comprising a chamber arranged for dissolving a gas into aqueous solution under pressure (i.e. with the gas at a pressure in excess of atmospheric pressure, such as at 2 Bar or more, for instance 5 Bar or more, to form the aqueous solution of gas solution in a supersaturated state). Any suitable gas which is soluble in water may be employed as the gas, for instance carbon dioxide, nitrogen or air. Air is a preferred gas as this is readily available commercially as compressed air, or may be generated in situ by means of an air compressor.

The aqueous solution of gas may be formed from a recirculated portion of the clarified aqueous solution resulting from the separation of the flocculated layer of particles from the aqueous dispersion, i.e. by using the recirculated portion as solvent for the gas.

It will be understood that at start-up of a method according to the invention, there may be no clarified aqueous solution available from the process of the method of the invention, in which circumstances, either water may be used to provide the aqueous solution of gas for start-up, or the formation of aqueous solution of gas may be delayed until clarified water is available from the process operating initially without blending of the aqueous solution of gas and the aqueous dispersion.

A second aspect of the invention provides an apparatus for separation of dispersed particles from an aqueous dispersion of particles, the apparatus comprising:

a) a flow-through assembly for electrocoagulation treatment of the aqueous dispersion, the assembly comprising:

a flow-through chamber comprising opposed electrodes and sacrificial electrodes positioned therebetween; and

a power supply arranged to apply a voltage across the electrodes and to cause a current to flow therebetween through said aqueous dispersion in use;

b) a gas solution generation apparatus arranged to generate an aqueous solution of a gas for blending with the aqueous dispersion; and

c) a floc-separation apparatus arranged for separation and removal from a remaining clarified aqueous solution of a flocculated layer formed by flocculation of the particles after electrocoagulation treatment of, and blending of the aqueous solution of gas with, the aqueous dispersion.

The gas solution generation apparatus may comprise a chamber arranged for dissolving the gas into an aqueous solution or water, with the gas at a pressure in excess of atmospheric pressure, to form the aqueous solution of gas in use.

The gas solution generation apparatus may be arranged to form the aqueous solution of gas from a portion of the clarified aqueous solution output from the floc-separation apparatus in use. The apparatus of the second aspect of the invention may further comprise a pump arranged for transferring said aqueous solution of gas for blending with said aqueous dispersion through a valve arranged to maintain the aqueous solution of gas at a pressure higher than the pressure of said aqueous dispersion on blending.

The valve may be positioned to blend the aqueous solution of gas with the aqueous dispersion after the aqueous dispersion has passed through the flow-through assembly for electrocoagulation but before, or at, entry into the floc-separation apparatus.

The floc-separation apparatus may comprises a settling tank arranged for floating the flocculated layer over the remaining clarified aqueous solution and arranged for separate removal of the aqueous clarified solution and the flocculated layer from the settling tank. For removal of dispersed particulate matter from water, the presence of gas bubbles from the cathodic portions of the electrocoagulation apparatus, subsequently entrained within the resulting flocculate of particulate matter, may assist in removal of the particulate matter by flotation and bulk separation. The particulate matter, particularly if fatty matter, may typically have lower density than water after flocculation and the presence of entrained gas bubbles from the EC treatment may further reduce the density of the flocculate formed, assisting in speeding separation by flotation of the flocculate to form a separate layer for subsequent removal to leave purified water.

Without wishing to be bound by any theory, it is believed that the further combination of a supersaturated aqueous solution of gas with the post-electrocoagulation aqueous dispersion may lead to a synergistic generation of gas bubbles having a size suitable to assist in the flotation and separation of flocculated fatty particles in the floc-separation apparatus. It has been found that this effect can be used to provide more rapid flow of the aqueous dispersion through a floc-separation apparatus whilst still achieving the same degree of clarification at the outlet of the floc-separation apparatus.

The floc-separation apparatus of the first aspect of the invention may comprise a settling tank arranged for floating the flocculated fat-containing layer over the remaining clarified aqueous solution and arranged for separate removal of the aqueous clarified solution and the fat- containing flocculated layer from the settling tank. For instance, the floc-separation apparatus may include a scraper blade arranged for to-and-fro horizontal motion in order to scrape the flocculated fat-containing layer into a drainage sump for subsequent removal to waste, with the clarified aqueous solution merely arranged to drain from a low point of the separation tank for pumping to the next part of the plant. The plant of the first aspect of the invention may further comprise a particulate filtration apparatus arranged downstream of the floc-separation apparatus and arranged to have filters to remove fine particulate solids from the clarified aqueous solution The particulate filters may suitably be configured to remove particulate material having a diameter of 2 μηι or more such as 5 μηι or more, say 10 μηι or more. For instance, the particulate filters may have an absolute pore diameter (the diameter of the smallest sphere capable of passing the filter), of from 2 to 500 μηι, such as from 5 to 200 μηι. Further filters may additionally be present downstream of the particulate filters, such as for ultrafiltration or reverse osmosis filtration.

It will be evident that the invention may be put into effect by means of pumps arranged to cause the flow of the waste stream, aqueous dispersion, clarified aqueous solution and filtered clarified aqueous solution. The pumps may be under the control of a controller such as a programmable computer apparatus programmed to put the method of the invention into effect on a suitable plant.

The controller may be arranged to control the power supply to vary the voltage and the current and may also be arranged to control the flow of aqueous dispersion through the electrocoagulation treatment step, for instance by controlling pumps and/or flow control valves arranged to direct the flow of the waste steam and the components derived from it.

For cleaning of the particulate filters, a controller may suitably be arranged to control flow through the filtration apparatus to effect cleaning by sediment removal using reverse flow through the particulate filters. The controller may be normally arranged to cause the clarified solution received from the floc-separation apparatus to flow, through a first valve, and then through a first filter, to a second location, in a first direction of flow, whilst monitoring a pressure of the clarified solution at a first pressure monitor located upstream of the filter, to provide filtered clarified aqueous solution at the second location, when the pressure is less than a first value. The controller may also be arranged to cause a cleaning solution to flow through the first filter, and then through the first valve, from a third location, in a second direction of flow, opposite to the first direction of flow, when the pressure attains or exceeds the first value and for a predetermined period of time thereafter, whilst halting the flow of the clarified solution through the first filter, and arranging the first valve to divert the cleaning solution to a fourth location when flowing in the second direction. When the predetermined time period has elapsed, the controller may be arranged to halt the flow of the cleaning solution and causing the clarified aqueous solution to flow again in the first direction of flow, from the floc-separation apparatus to the second location, through the first valve and the first filter, whilst monitoring the pressure of the clarified solution at the first pressure monitor. Preferably, the second location may be a holding tank adapted for retaining filtered clarified aqueous solution, and may be the same as the third location, such that the cleaning solution is filtered clarified aqueous solution which has already passed at least once through the filtration apparatus. The fourth location may an inlet to the floc-separation apparatus, so that the mixing of the dislodged sediment with the aqueous dispersion, prior to separation of flocculated particles and their removal, may lead to the dislodged sediment becoming entrapped within the separated flocculated particulate material for disposal.

The filtration apparatus may comprises one or more further filters in addition to the first filter, each further filter having a respective valve and a respective pressure monitor, with the controller arranged to control flow through the further filters in the same manner that it is arranged to control flow through the first filter, mutatis mutandis.

The controller may be arranged to halt the flow of the clarified solution in the first direction through one or more of the filters, and also be arranged to divert the flow of the clarified aqueous solution to pass though other filters for which flow is not halted.

In other words, each filter may be cleaned by halting filtration, and sending a reverse flow of cleaning solution, for a predetermined time, through the filter, to dislodge sediment deposited on the filters, when the pressure monitored at the inlet to the filter reaches a predetermined value. The filtered solution may be recirculated as cleaning solution and the solution containing dislodged sediment may be recirculated to be trapped in the flocculated component, allowing for a self-contained self-cleaning system. The plant of the first aspect of the invention may include a plurality of flow-through assemblies for electrocoagulation treatment arranged in parallel, with the controller arranged to direct the waste water flow through each flow-through assembly according to demand and/or a maintenance schedule. Hence, if the passage through an electrocoagulation (EC) flow-through assembly is a rate determining step at certain times, the controller may switch the plant to utilise two or more flow-through electrocoagulation assemblies. When one of the flow-through electrocoagulation assemblies has become inefficient over a usage period, for instance due to need for cleaning or for replacement of sacrificial electrodes, then the flow may be switched from the inefficient EC flow-through assembly to a different EC flow-through assembly so that the EC flow-through assembly may be cleaned without halting the continued operation of the waste stream treatment plant.

The controller may be arranged to control the power supply to vary the voltage and the current for electrocoagulation treatment and may also be arranged to control the flow of aqueous dispersion through the flow-through cell(s) of the electrocoagulation assembly, for instance by controlling pumps arranged to pump the aqueous dispersion. The controller may also be arranged to control the blending of the aqueous gas solution with the aqueous dispersion, for instance by controlling the pumps and valves involved in effecting such blending. Aspects of the invention may be implemented in any convenient form. For example computer programs may be provided to carry out the methods described herein. Such computer programs may be carried on appropriate computer readable media which term includes appropriate tangible storage devices (e.g. discs). Aspects of the invention can also be implemented by way of appropriately programmed computers, for instance as the control unit for use in aspect of the invention.

Embodiments of the invention are of use in plant operations, such as food plant operations, abattoir operation or polymer recycling operations, where waste water containing fatty materials may be generated, with the invention useful to remove fatty waste prior to permit disposal or reuse of the remaining waste water.

DETAILED DESCRIPTION

For a better understanding of the invention, and to show how exemplary embodiments of the same may be carried into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:

Figure 1 schematically depicts a schematic representation of an embodiment of a waste stream treatment apparatus according to the first aspect of the invention and employing the method of the first aspect of the invention;

Figure 2 schematically depicts a more detailed cross-sectional side view of the flow- through electrocoagulation (EC) assembly for use in the apparatus of the first embodiment as shown in Figure 1 .

Common reference numerals have been used throughout the Figures, and in the description, as set out below, reference is made to the same embodiment of the invention with the various features of the embodiment illustrated in the Figures. For the sake of clarity, connections between the controller 10 and the various pumps P, meter 7, power supply 27 and flow control valves 12, 22 have not been shown in the Figures. It should be understood that such connections will be present in the embodiment as set out in the Figures, even though not indicated directly, and the connections may be implemented as hard-wired connections, wireless connections or a mixture of these. Turning to Figure 1 , which shows a schematic depiction of an embodiment of a waste treatment apparatus according to the first aspect of the invention, a waste water stream enters the plant of the embodiment at an inlet I from an adjacent manufacturing site. The waste water stream is first passed through a pre-treatment apparatus which consists of a skimming tank 1 for removal of oil from the waste water stream by skimming a floating oil layer 2 from the waste water stream. This is indicated in Figure 1 by an oil collection gutter 3 which removes the surface layer of floating oil 2 and any other matter trapped in the floating layer of oil by collecting the oil from the surface of the skimming tank l and draining it to waste W. The remaining aqueous dispersion, of particles dispersed in an aqueous solution, is transferred from the skimming tank 1 by a pump P1 to a nozzle 37 through which it is sprayed onto a screening apparatus 6 having a cylindrical sieve in the form of a drum 4 arranged to rotate about an axle 5.

Coarse particulate matter is captured on the outer surface of the rotating sieve drum 4 and jets of water 32 are emitted from the axle 5 in order to dislodge the coarse particulate matter collected on the outer face of the sieve drum 4 so that the particles can be washed to waste W.

From the screening apparatus 6, the aqueous dispersion, now containing predominantly colloidal particles, is transferred by a further pump P2 through a pH and conductivity meter 7 into a balancing tank 1 1 . The balancing tank 1 1 is provided with a recirculation pump P3 and reagent dosing tanks 8, 9 are positioned to pump reagent (i.e. solutions of chemicals in this case) into the balancing tank 1 1 through pumps P8 and P9. The dosing of the reagents from dosing tanks 8, 9 via the pumps P8, P9 is controlled by the controller 10 in response to the values of pH and conductivity measured by the meter 7 and transmitted from the meter 7 to the controller 10. As explained below, the controller 10 may also dose the reagents in response to the measured fat content of the output clarified aqueous solution from the plant, for instance to increase conductivity so that more current can be supplied to the electrocoagulation treatment in order to increase flocculation without excessive power drain.

A baffle, 33 is positioned at the outlet to the balancing tank 1 1 in order that the circulation pump P3 mixes the reagents with the aqueous dispersion prior to exit from the balancing tank 1 1 . The baffle is positioned to present direct flow of reagent from the dosing tanks 8, 9 through the outlet of the balancing tank 1 1 .

The aqueous dispersion is transferred from the balancing tank 1 1 by pump P1 1 through a flow control valve 12 which determines through which of two flow-through electrocoagulation chambers 13, 14, the aqueous dispersion will flow. A power supply 27 is provided to supply a voltage across the opposed electrodes 28 of the flow-through electrocoagulation chambers 13, 14 and this is shown in more detail in Figure 2 for the flow-through electrocoagulation chamber 13. For this embodiment, the opposed electrodes 28 are of steel while the sacrificial electrodes 29 are of aluminium. With such an arrangement, the steel electrodes may endure through many replacement, or refurbished, sets of aluminium sacrificial electrodes.

A voltage is applied, by power supply 27, across the opposed electrodes 28 and the resulting electric field causes the sacrificial electrodes 29 to have cathodic and anodic surfaces, with the material of the sacrificial electrodes oxidising and dissolving at the anodic surfaces and hydrogen bubbles being generated at the cathodic surfaces. Typically, a voltage of 50 to 600V may be applied, with a direct current in the range from 5 to 20A passing between the opposed electrodes 29 and through the sacrificial electrodes. In order to prevent excessive build-up of oxide on the sacrificial electrodes, the direct current may be reversed at intervals in order to switch the cathodic surfaces to become anodic surfaces and vice versa. The controller 10 controls the aqueous dispersion to flow through either one of, or both of, the flow-through electrocoagulation chambers 13, 14 depending upon circumstances, such as total volume of waste stream entering at the inlet I, the condition of the electrodes (e.g. whether cleaning is required or whether the sacrificial electrodes 29 are nearly spent), or optionally the fat content of the outlet aqueous solution from the plant.

From the flow-through chambers for electrocoagulation treatment 13, 14, the aqueous dispersion, now also comprising hydrogen bubbles and dissolved cations from the sacrificial electrodes 29, passes to a floc-separation apparatus 15 in the form of a settling tank 15 through which the aqueous dispersion 17, following the electrocoagulation treatment, gently flows from an inlet 38 to an outlet 35 in order to allow time for the fat particles dispersed within the aqueous dispersion to flocculate and so to form a flocculated fat-containing layer 16 of lower density than the remaining clarified aqueous solution 17.

The settling tank is provided with a blade 18 arranged to move to-and-fro while positioned over the surface of the settling tank 15 in order to scrape the flocculated fat-containing layer 16 over a rim of the settling tank into a sump 30 for collection and disposal to waste W.

In addition to the outlet 35 arranged for the exit of the bulk of clarified aqueous solution 17 from the settling tank 15, a further outlet 34 is provided in the base of the settling tank 15, positioned to collect sediment, so that any sediment which is denser than the clarified aqueous solution will collect at this outlet 34 and can be removed at intervals using pump P12 to go to waste W. From the outlet 35, a pump P 13 transfers the clarified aqueous solution 17 to a filtration tank 36 and from the filtration tank 36 the clarified aqueous solution 17 passes through one or more of the particulate filters 20, 21 in accordance with flow control valve 19, which is controlled by the controller 10 to deliver the clarified aqueous solution 17 to the particulate filters 20, 21 depending upon the demand and/or maintenance schedule for unblocking of the particulate filters 20, 21 .

A further flow control valve 22 directs the bulk of the remaining resulting clarified aqueous solution, after particulate filtration, to the outlet of the plant S and from there to one or more of: i) re-use within the factory or manufacturing site from which it came, or ii) into the local sewage system, or iii) into the environment.

The flow control valve 22 also directs a portion of the clarified aqueous solution into a pressure tank 23 through the pump P22. A gas source 24 provides pressurised gas 25 over clarified aqueous solution 26 held in the pressure tank 23, and in this embodiment a gas pressure of 5 bar is used in order to dissolve the gas (in this case air) into the clarified aqueous solution 26 to form a supersaturated aqueous gas solution 26. Pump P26 is arranged to pump the resulting supersaturated aqueous gas solution through pressure control valve 31 under the control from the controller 10 in order to blend the supersaturated aqueous gas solution with the aqueous dispersion at the pressure control valve 31 before the aqueous dispersion enters the floc-separation tank 15.

As explained hereinbefore, the blending of the supersaturated aqueous gas solution with the aqueous dispersion 17, which already includes hydrogen bubbles generated during the electrocoagulation process, results in improved flotation and separation of the fat-containing layer 16 following flocculation, and it is thought that this may be due to improved nucleation of gas bubbles of a suitable size and their incorporation within the flocculated fat containing layer 16, improving its buoyancy and so improving rate of separation from the remaining clarified aqueous solution 17. The plant according to the invention may also include a meter to monitor the remaining fat content of the clarified aqueous solution, for instance a clarity meter, such as a nephelometer, arranged to measure the turbidity of the solution at the exit S, and the controller 10 may adjust the electrocoagulation conditions (current/voltage) and may adjust the reagent dosing at the balancing tank 1 1 in order to control the fat content to meet a specific requirement whilst minimising the electrical power input used in the electrocoagulation assembly.

The invention provides methods and apparatus for separation of dispersed particles from an aqueous dispersion involving subjecting the aqueous dispersion to electrocoagulation (EC) treatment to promote formation of a flocculate comprising the particles, collecting the flocculate comprising the particles as a flocculated layer floating on a clarified aqueous solution and separating the flocculate from the remaining clarified aqueous solution. An aqueous solution of a gas is blended with the aqueous dispersion of particles, prior to separating the flocculate from the remaining clarified aqueous solution, with the aqueous solution arranged to be supersaturated with the gas on blending with the aqueous dispersion. The blending of the EC- treated dispersion with the supersaturated solution of gas speeds and facilitates separation of flocculate from clarified aqueous solution. Although a few exemplary embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention, as defined in the appended claims. For instance, although in this embodiment the invention has been described with reference to a waste treatment plant, where the aqueous dispersion to be treated contains fatty particles, it will be understood that the invention would also be applicable to a plant intended for extraction of particulate compounds or elements from an aqueous ore dispersion by flotation.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1 . A method for separation of dispersed particles from an aqueous dispersion of particles, the method comprising sequentially: a) subjecting the aqueous dispersion to electrocoagulation treatment by flowing the aqueous dispersion through a region comprising sacrificial electrodes and located between opposed electrodes, and applying a voltage across the opposed electrodes to pass a current through the sacrificial electrodes whereby the sacrificial electrodes donate cations to the aqueous dispersion to promote formation of a flocculate comprising the particles; and b) collecting the flocculate comprising the particles as a flocculated layer floating on a clarified aqueous solution and separating the flocculate from the remaining clarified aqueous solution; wherein the method further comprises blending an aqueous solution of a gas with the aqueous dispersion of particles, prior to separating the flocculate from the remaining clarified aqueous solution; and wherein the aqueous solution is arranged to be supersaturated with the gas on blending with the aqueous dispersion.

2. A method according to claim 1 wherein the solution of gas is formed by subjecting an aqueous solution or water to the gas at a pressure that higher than the pressure of the aqueous dispersion on blending with the solution of gas.

3. A method according to claim 1 or claim 2 comprising the aqueous solution of gas being formed from a recirculated portion of the clarified aqueous solution resulting from the separation of the flocculated later of particles from the aqueous dispersion.

4. An apparatus for separation of dispersed particles from an aqueous dispersion of particles, the apparatus comprising: a) a flow-through assembly for electrocoagulation treatment of the aqueous dispersion, the assembly comprising:

a power supply arranged to apply a voltage across the electrodes and to cause a current to flow therebetween through said aqueous dispersion in use; b) a gas solution generation apparatus arranged to generate an aqueous solution of a gas for blending with said aqueous dispersion; and c) a floc-separation apparatus arranged for separation and removal from a remaining clarified aqueous solution of a flocculated layer formed by flocculation of said particles after electrocoagulation treatment of, and blending of said aqueous solution of gas with, said aqueous dispersion.

5. An apparatus according to claim 4 wherein the gas solution generation apparatus comprises a chamber arranged for dissolving the gas into an aqueous solution or water, with the gas at a pressure in excess of atmospheric pressure, to form said aqueous solution of gas in use.

6. An apparatus according to claim 4 or claim 5 wherein the apparatus is arranged to form the aqueous solution of gas from a portion of the clarified aqueous solution output from the floc-separation apparatus in use.

7. An apparatus according to any one of claims 4 to 6 further comprising a pump arranged for transferring said aqueous solution of gas for blending with said aqueous dispersion through a valve arranged to maintain the aqueous solution of gas at a pressure higher than the pressure of said aqueous dispersion on blending.

8. An apparatus according to claim 7 wherein the valve is positioned to blend said aqueous solution of gas with said aqueous dispersion after said aqueous dispersion has passed through the flow-through assembly for electrocoagulation but before or at entry into the floc- separation apparatus.

9. An apparatus according to any one of claims 4 to 8 wherein the floc-separation apparatus comprises a settling tank arranged for floating said flocculated layer over said remaining clarified aqueous solution and arranged for separate removal of said aqueous clarified solution and said flocculated layer from the settling tank.

10. An apparatus or method substantially as hereinbefore described with reference to and as shown in the accompanying figures.