WO2004046045A1

WO2004046045A1 - Radioactive species removal process

Info

Publication number: WO2004046045A1
Application number: PCT/AU2003/001534
Authority: WO
Inventors: Brian Grigg
Original assignee: Aquenox Pty Ltd
Priority date: 2002-11-15
Filing date: 2003-11-17
Publication date: 2004-06-03
Also published as: AU2002952678A0

Abstract

A process to remove radioactive species, in particular uranium, from wastewater using electrocoagulation technology. The process comprises the following steps: (a) adjusting the pH of the wastewater to fall within the range 5.5-8.5; (b) adjusting the conductivity of the wastewater to fall within the range 300-20,000 µS/cm; (c) passing the wastewater through an electrolytic reaction chamber comprising a plurality of reaction plates; and (d) separating solids from the wastewater. The volts and amps applied to the electrolytic cell can be adjusted to obtain maximum uranium removal from the wastewater.

Description

RADIOACTIVE SPECIES REMOVAL PROCESS

FIELD OF INVENTION

This invention is concerned with methods of removing contaminants,

5 additives and/or hazardous materials from aqueous waste streams from industrial, mining or nuclear processes, or from contaminated bodies of water. In particular, the invention relates to a method of removing metal species from wastewater. More particularly, the invention relates to a process of the removal of uranium and other radioactive species from a o waste steam or effluent of an industrial, power-generating or mining process.

BACKGROUND OF THE INVENTION Uranium is a naturally occurring element that is commonly found dissolved in water at varying concentrations. Due to its radioactivity and therefore potentially hazardous effects, strict regulations have been s established worldwide stating the maximum permissible levels of uranium that can be contained in different types of waters. Much higher levels of uranium and other radioactive species can be found in the wastewater or effluent produced in mining and industrial operations or from the cooling water used in power generation plants. Therefore a number of processes o have been developed to remove uranium contaminants from water to comply with increasingly stringent regulations and to ensure the water can be safely discharged into public waters or used for human consumption.

As discussed in US Patent 6,419,832, uranium is a naturally occurring radioactive element that is commonly found dissolved in water at varying concentrations. The United States Environmental Protection Agency and other state and federal agencies have regulations that establish the maximum permissible levels of soluble uranium that can be contained in different types of waters. Hence, it is often necessary to remove some or most of the dissolved uranium from water to comply with the applicable regulations before the water can be safely discharged, recycled or used for human consumption.

A number of processes have heretofore been developed for removing dissolved uranium from water. Such processes include the use of anion exchange resin, activated alumina, granular activated carbon, lime softening, reverse osmosis and under certain conditions, conventional coagulation using aluminum sulfate or iron salts. Many of these prior art processes require that the pH of the water being treated be raised to a very high level (over a pH of 10) or reduced to a very low level (lower than a pH of 3). Other of the processes require some type of pretreatment of the water. As a result, very few of the processes have been converted to practice on a large scale either because of unsatisfactory uranium removal efficiency and/or high costs associated with the implementation of the process technology. US Patent 6,419,832 describes a process for removing dissolved uranium from water that comprises the steps of (a) mixing phosphoric acid with the water, (b) mixing a source of calcium such as calcium hydroxide with the phosphoric acid-water mixture produced in step (a) to thereby form calcium hydroxy apatite (synthetic bone ash) which in turn reacts with and complexes as well as occludes at least a portion of the uranium in the water to form a precipitate thereof and (c) separating the precipitate from the water. However the process of US Patent 6,419,832 while considered to be more efficient then conventional processes as described above was still considered to be disadvantageous because of the large costs involved with construction of a plant using this process and use of lime still required generation of excessive sludges during the process and removal of such sludges was not only inefficient but also expensive. US Patent No.4,872,959, in the name of Herbst and Renk, describes a process (CURE System) that removes contaminants (including metals) from aqueous solution using electrolysis. The process induces coagulation and precipitation of contaminants by a direct current electrolytic process followed by flocculant settling. The water is pumped through concentric tubes that act as electrodes around a centrally-located anode solid rod. However, a major disadvantage of the CURE System is that it only removes 32-52% of uranium, 63-99% plutonium and 69-99% americium as described on page 14 of the website www.epa.gov/ORD/SITE/reports/540r96502.pdf. It is therefore apparent that these removal efficiencies are not satisfactory for commercial operation.

It is therefore an object of the invention to provide a process for removal of radioactive species from water that may alleviate the disadvantages over the prior art. SUMMARY OF INVENTION In a first aspect, the invention provides a process to remove radioactive species from wastewater that comprises the steps of:

(a) adjusting the pH of the wastewater to fall within the range 5.5 - 8.5;

(b) adjusting the conductivity of the wastewater to fall within the range 300 - 20,000 micro Siemens per centimeter (μS/cm);

(c) passing water through an electrolytic reaction chamber which comprises a plurality of reaction plates or electrodes disposed within said reaction chamber and spaced apart from each other wherein at least two of said plates are electrically connected to a power supply; and

(d) separating solids from the wastewater.

In a preferred embodiment, wherein one or more of the variables governing performance of the electrolytic reaction chamber in step (c) which include voltage, amperage, number of electrically connected electrodes, flow rate and conductivity can be determined from the following relationships: (1) if the voltage, number of electrically-connected electrodes, and the flow rate are fixed, conductivity and amperage are variable; (2) if voltage, conductivity and flow rate are fixed, the number of electrically connected electrodes and amperage are variable; and (3) if the flow rate, conductivity and number of electrically connected electrodes are fixed the amperage and voltage are variables. The above relationships (1), (2) and (3) apply if the total number of electrodes, total wetted surface of the electrodes in the cell, size of the gap between the electrodes, pH of the wastewater and cell residence time are kept constant. Thus, for example, in relationship (1) when conductivity and amperage are the only variables, one of these variables may be determined on an experimental basis if the other is kept constant or vice versa. A similar conclusion applies to relationships (2) and (3).

Thus, it will be appreciated that relationship (3) will be the most commercially valuable relationship since this will apply in by far the large majority of cases. Thus appropriate voltages or amps relative to the chemical composition of a particular radioactive effluent or waste can be determined by empirical observation of the electrocoagulation cell (EC) especially when micro-flocs or particles of the complexes of the radioactive metals are formed in the discharged liquid from the EC cell. This empirical observation will also apply to relationships (1) and (2). Preferably, the radioactive species is uranium. In step (a) the wastewater may be obtained from an industrial process or any suitable contaminated water body such as ground water, a reservoir, spring, river, pool or other water body.

Preferably, if present, suspended solids are removed prior to step (c). Preferably, the variations in pH are controlled to within +/- 0.5 units of the desired value. Preferably, the conductivity of the wastewater falls within the range 1000 - 3000 μS/cm prior to step (c).

Preferably, the variations in conductivity of the wastewater are controlled to within +/- 10% of the desired value. 5 Preferably, the electrolytic reaction chamber is part of a conventional electrocoagulation cell as described in International Patent Application WO 01/53568 or US Patent 6,139,710. Preferably, direct current (DC) is applied to the reaction chamber. This has the advantage of using a smaller number of electrodes than is the case of alternating current. l o The electrocoagulation cell is preferably orientated vertically so that the outlet conduit is located at the top of the reaction chamber and the inlet conduit is located at the bottom of the reaction chamber. However, this does not preclude the use of an electrolytic cell arranged horizontally, such as described in, for example, WO 96/28389 or in US Patent No. 5,611 ,907. is Suitably, the water will make a single pass or multiple passes through the electrolytic reaction chamber but it is also possible for the water to be circulated throughout the cell in a serpentine fashion in either a vertical or horizontal orientation.

The electrolysis cell may comprise any number of electrodes or

20 reaction plates but at least three are used and two are electrically coupled to a power supply. The total number of electrodes used can be calculated depending on the total wetted surface area of electrodes in the cell, the number of electrodes electrically connected to a power source, the gap between the electrodes, and the flow rate, conductivity and cell residence time of the solution. Preferably, each side of the electrode plate is contacted by wastewater.

Preferably, the voltage and current applied to the cell can be varied. Preferably, a flow rate of 50-5000 LJmin and more preferably, 500-

2000 L/min is used. Also 2-25 reaction plates in the electrolysis cell may be used. Of these reaction plates, 2-24 may be connected to a power supply.

The wastewater enters the electrolysis cell through one inlet and is discharged through a single outlet. Preferably, the reaction plates extend from one end of the electrolysis cell to another which allows substantial contact between the water and reaction plates which thus increases the total wetted area of the reaction plates. Preferably, at least 50% and more preferably, 80-100% of the reaction plates on each side are contacted by the water passing through the reaction cell. In a preferred embodiment in step (c) the voltage and amperage applied to the electrocoagulation cell and more particularly the electrodes thereof are calculated depending on the flow rate and conductivity of the wastewater.

Preferably, the electrodes are manufactured from mild steel, iron, aluminium or iron alloys.

More preferably, the electrodes are manufactured from grade 250 mild steel.

Preferably, flocculant is added in step (d) to increase the rate of solid separation.

Preferably, greater than 95%, and more preferably greater than 99% of the radioactive species may be removed from the wastewater by the electrocoagulation process. More preferably, greater than 99.5% of the radioactive species has been removed from the wastewater by the electrocoagulation process.

Throughout this specification, "comprise", "comprises" and "comprising" are used inclusively rather than exclusively, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1: Flow diagram showing wastewater pH pretreatment, and separation and removal of solids pretreatment from uranium- contaminated wastewater. Fig. 2: Flow diagram showing wastewater conductivity pretreatment, electrocoagulent treatment and separation and removal of the solids from the wastewater. Fig. 3: Flow diagram showing an alternative method of separation and removal of solids from the wastewater. Fig. 4: Schematic showing an electrocoagulation electrode configuration and its connection to a power supply. There are 25 electrodes and nine electrode connections to the DC power supply. 3 denotes bipolar electrodes. DETAILED DESCRIPTION OF INVENTION The present invention provides a method of removing radioactive species from wastewater. The method provides improved removal efficiency of the radioactive contaminants after one pass through the system. For the purposes of this invention, by "water body" is meant any body of water, such as creeks, springs, rivers, reservoirs, ponds and groundwater. By ''wastewater¹' is meant any liquid or solution for treatment, for example, drinking water, a waste stream from industrial, mining or nuclear processes, or any contaminated body of water. By "radioactive species" is herein defined to include any elemental oxide, halide or salt which either comprises a radioactive isotope or is capable of forming a radioactive isotope. Suitable radioactive species may include but may not be limited to the following isotopes and compounds: All species that contain U₂38, including U₃O₈, KUF₅, UF₃, UF₄, UF₆, UO₂(OH)₂ UCl3, UCI₄, UCI₅, UO₂(IO₃)₂, UO2C2O4.3H2O, UO2CO3.2K2CO3, UO₂CO₃.2Na2CO₃, UO₂SO₄.3H₂O, 2(UO₂SO₄).7H₂O and any or all of U₂₃8 decay products of alpha and/or beta emissions including: U234, Pa234,Th₂34,

Th₂3o, Ra₂26, r>2i8, Rri222, At₂-i8, P0218, P0214, Bi₂i4, Pb2.4, P0210, Bi ιo, Pb209 and Pb₂io- All species that contain Th₂32 including Th(S0 )2, Thl₄, ThBr_4ι ThCI ,

Th(SO₄ )2-4H₂o and any or all of Th₂32 decay products of alpha and beta emissions including : Th₂2β, Ac 28, Ra228, Ra224, Rrtøo, P0216, P0212, Bi₂i2, Pb₂i2 All species that contain Ra₂26 including RaBr , RaCO3, RaCI₂, Ra(IO₃)2, Ra(NO₃)₂, RaSO₄, RaBr₂2H₂O, RaCI₂.2H₂O and any or all of Ra₂₂e decay products of alpha and beta emissions including : Rn₂22, t₂is, P0218, n2is,

PO214, BΪ214 Pb214, Pθ210,Bi210, Pb210, l210, Pb209, l206and Hg206. All species that contain Pa₂₃i including PaCU, PaF₄, Paθ₂, Pa2θ3 and any or all of Pa₂₃i decay products of alpha and beta emissions including :

Th₂27, AC227, Ra223, Rn2i9, P0215, P0211, Bi₂i2, Pb2iι ,Tl207, Fr₂23, At₂i9, At₂i25and Pb₂ιι.

All species that contain Np₂37 including NpBr3, NpCI_4ι NpC , NpF₆, NpF_4, NpF₃, Npl_3, NpO₂, Np₃Oβ and any or all of Np₂₃₇decay products of alpha

and beta emissions including : U233, Pa233, Th229, AC225, Ra225, Rn2i7, P0213, BΪ213, Pb209 I209, Fr22i and At₂i7.

All species that contain PU244 including PuBr_3ι PuC , PuF₆, PuF₄, PuF_3] Pul_3ι PuN, PuO₂, Pu(SO₄)₂, Pu(SO₄)₂.4H O and any or all of Pu₂₄4 decay products of alpha and beta emissions including: PU240, U236, U240, Np₂4o, Th₂₂s,

Th₂32, AC228, Ra224, Ra22β, Rn22o, P0216, P0212, Bi₂i2, Pb2i2and TI208.

All species that contain A1TI24₃ including AmBr₃, AmC , AmF₃, Am , A1T.O2, Am₂O₃ and any or all of Aιτ.243 decay products of alpha and beta

emissions including:PU₂39, U235, Np₂39, Pa23i, h₂27, h₂3i, AC227, Ra223, Fr₂23, Rn2i9, At2i5, At.219, P0215, P0211, B1211, BΪ215, Pb₂n and TI207.

All species that contain Cm₂₄7 including CmO₂, Cm₂θ_3ι CmF_3ι CmCI_3, CmBr₃ and any or all of Cm₂₄₇ decay products of alpha and beta emissions

including: PU₂₄3, Am₂₄3, Np₂₃9, PU239, U235, Pa23i, Th₂3i, Th₂ 7, AC227, Ra₂23, Fr₂23, Rn2i9, At₂i5, At₂ιg, P0215, Pθ2iι, B1211, B.215, Pb2iι and TI207.

All species that contain Cf₂₅i including CfC , CfOCI, Cf₂O3 and any or all of Cf₂₅i decay products of alpha and beta emissions including:

ClTl247, AlT.243, PU243, PU239, Np₂39, U₂35, Pa231, Th₂31, Th₂27, Ac₂27, Ra₂23, Fr₂23, Rn₂ιg, At₂i5, At₂ιg, P0215, Po₂n, Bi₂n, Bi₂i5, Pb2n and TI207.

All species that contain Es₂54 including and any or all of Es₂54 ecay products of alpha and beta emissions including: Cf₂5o, Bk₂so, Cm₂46, PU242, U₂38, U234, Pa₂34, Th₂₃4, Th₂3o, Ra226, Rn2i8, At₂ιs P0218, P0211, Bi₂ιι, BΪ215, Pb2iι and TI207. All species that contain Frr.257 including and any or all of Frτ.257 decay products of alpha and beta emissions including: ES₂53, Cf2₅₃, Bk249, Cf₂₄₉,

Cm₂45, Am₂ 5, PU241, Am₂₄i, U237, U233, Pa233, h₂29, Ac ₂5, Ra₂25, Fr₂₂ι, Rn₂i7,

All species that contain Pruo including Pr₂O₃ and any or all of Prι₄₀ decay products of alpha and beta emissions including Ce^o. To achieve maximum uranium species removal from the wastewater an initial sample of a particular wastewater stream is taken at a site at which the electrocoagulation system is to be installed. The conductivity of this sample is checked and the general chemical composition of the wastewater is assessed, for example, the sample pH, percentage of suspended solids, solid particle size and whether the sample comprises multiple contaminants.

It will be appreciated that the conductivity of the sample will be dependent on the chemical nature of the wastewater, for example, the conductivity of water comprising ionic or conductive contaminants such as metals, including heavy metals, and anions such as cyanide and fluoride, will be higher than water comprising polar or non-polar contaminants.

It will be appreciated that testing the sample chemical composition prior to installing an electrocoagulation system at a factory site which processes wastewater ensures the most appropriate, efficient and cost effective system is installed. Wastewater streams from the same site may have varying chemical characteristics and the electrocoagulation system must be designed to process all of them efficiently. If the configuration of the electrocoagulation cell is not set-up correctly taking into account the abovementioned characteristics for the specific contaminant composition of the wastewater, efficient removal of the contaminants will not occur. The results of the initial sample assessments will demonstrate whether it is advisable to install electrocoagulation pre-treatment steps, such as hydrocyclones or filters to remove suspended solids, and pH monitors to ensure the pH is maintained within a suitable range. The optimum pretreatment process steps can be chosen for the particular wastewater stream. Depending on the chemical nature of the wastewater it may be necessary to pre-treat the wastewater prior to its passing through the electrocoagulation process. Preferably, the pre-treatment process involves removal of excess suspended solids, and adjusting the pH and the conductivity of the wastewater. For example, if the conductivity of the wastewater is high (greater than 5,000 μS/cm), for example, when the wastewater comprises high concentrations of metal ions, such as lead, zinc, copper and manganese, in addition to uranium, the wastewater may be pre- treated by addition of a base or source of OH^' to remove the metal ions by removal of the resulting solid residue or precipitate, and reduce conductivity of the wastewater prior to uranium removal treatment. High concentrations of manganese in the wastewater, and to a lesser extent copper, sulphate and magnesium, prevents efficient uranium removal. An example of a suitable pre-treatment is shown in Example 6 hereinafter.

If the wastewater comprises a high percentage of suspended solids (greater than 1 g/L) the wastewater enters the pre-treatment process at tank 10 (Fig. 1) where the wastewater is maintained in an agitated condition to maintain the solids in suspension. An agitator, such as an impeller 11 prevents settling of the solids and maintains the solids in suspension. The impeller 11 has a shaft 11A and is driven by motor 11B. Each tank is also provided with one or more baffles 12 to facilitate maintaining the solids in suspension.

The wastewater is transferred from tank 10 into a hydrocyclone 14 through conduit 13 which has a pump 13D which is actuated by a motor 14A.

The hydrocyclone 14 uses centrifugal force to separate suspended solids from the liquid stream and thus provide a solids concentration of 200 mg/l or less.

Preferably, the cyclone is a non-mechanical scroll-like cylinder.

Preferably, the diameter of the cyclone is less than 0.5 m. A plurality of cyclones in parallel may be used to handle large flow streams that would exceed the capacity of a single cyclone.

The underflow or residue from the cyclones is directed to a sludge thickener tank 15. The liquid overflow, which can still comprise suspended solids is directed to tank 16.

The sludge thickener tank 15 uses an agitator and thickening agents to agglomerate the solid particles.

Preferably, the thickening agents or flocculants are long chain hydrocarbons (up to 100 carbon atoms in length). More preferably, the thickening agents are polyelectrolytes. A preferred polyelectrolyte is an anionic based polyelectrolyte (ie two long chain hydrocarbon having anionic end groups) such as AE1125 available from G E Betz (formerly known as Betz Dearborn).

Other flocculants which may be added to the sludge thickener tank 15 to promote particle growth and rapid sedimentation include KlarAid, Novus

CB Aqueous Dispersion Polymers and Novus CE Emulsion Polymers. The sludge is disposed though conduit 15A for disposal. The underflow from hydrocyclone 14 is passed into tank 15 through conduit 18.

The liquid overflow in hydrocyclone 14 is transferred to tank 16 through conduit 19. Agitators mix the wastewater in tanks 16 and 17 as shown and a pH sensor 20 is located in tank 17, below the water level, to measure the pH of the wastewater on a continuous basis using an automated controller or microprocessor 21. A signal proportional to the pH of the wastewater is transmitted via a conductor 22 to controller 21 that governs the operation of a control valve 13E that controls the flow of acid or alkali solution from tank 16A to tank 16.

If the pH of the wastewater is greater than 8.5, an acid solution prepared in tank 16A is added to mixing tank 16. Preferably, the acid solution is a mineral acid, such as sulphuric or hydrochloric acid.

If the pH of the wastewater is less than 5.5, alkali solution is added to mixing tank 16. Preferably, the alkali solution is sodium hydroxide or potassium hydroxide or an alkaline earth species such as calcium hydroxide or lime. This system maintains the pH within the desired range for electrocoagulation and the pH can be adjusted to fall within any desired range. For example, maintaining the pH between the limits of 5.5 and 8.5 pH may further enhance the electrocoagulation performance. The variations in pH can be controlled by the automated control system to be within +/- 0.5 pH units of the desired pH.

If the suspended solid particle size in the wastewater is greater than 10 μm, the wastewater may be passed via conduit 25 through a pair of filters 24 (FIG. 2) which clarifies the wastewater by removing micrometer and submicrometer (0.5-1.5 μm) particles. The filter can be a belt press or plate and frame type filter, and preferably at least two filters are necessary in case maintenance is required on a filter. Reference is also made to flow control valves 26 which control flow of influent into and out of filters 24 through conduits 25, 29 and 30. It will be appreciated the filters can be modified to remove particles of any size.

The solids from the filter underflow are collected and disposed through conduits 27, 28 and 28A. The liquid overflow from filters 24 can be transferred to tank 32 through conduit 31. Agitators mix the wastewater in tanks 32 and 33 and a conductivity detector 34 is located in tank 33, below the water level, to measure the conductivity of the wastewater on a continuous basis. The conductivity of the wastewater is controlled using an automated controller or microprocessor 35. A signal proportional to the conductivity of the wastewater solution is transmitted via conductor 36 to the automated controller or microprocessor 35 that governs the operation of a control valve 13E that controls the flow of an ionic solution from tank 22A into mixing tank 32 through conduit 37. Preferably, the ionic solution is an ionic salt of any kind such as a soluble alkali or alkaline earth salt inclusive of NaCI, KCI or CaNO₃. The wastewater may be transferred from tank 32 into tank 33 through conduit 38.

If the conductivity of the wastewater is greater than 20,000, more suitably 5,000, and more preferably 3,000 μS/cm, flow of the ionic solution into mixing tank 32 may be restricted by the shutdown of the pump 13D through conduit 37. If the conductivity of the wastewater is less than 300 and more preferably 1000 μS/cm, ionic solution flow to mixing tank 32 may be increased. This system maintains the conductivity within the desired range for electrocoagulation of a particular species and the conductivity can be adjusted to fall within any desired range. For example, a preferred conductivity range of the wastewater is 1500-2250 μS/cm. Variations in wastewater conductivity are controlled to within +/- 10% of the desired value. The outflow from mixing tank 32 then passes into the electrocoagulation treatment zone 39 to undergo electrocoagulation through conduit 40 to feed tank 41. A preferred embodiment of an electrocoagulation cell is described in WO 01/53568.

The electrocoagulant treatment zone 39, which incorporates tank 41 , electrocoagulation cell 42, coagulation tank 43 and conduits 44 and 45, treats the wastewater to induce uranium and/or other radioactive species to precipitate, coalesce, coagulate or otherwise separate from the wastewater. An electric current is passed through the wastewater in electrocoagulation cell 42 via a plurality of flat plate metal electrodes to induce excitation of uranium ions and simultaneously release electrode cations and water anions. An electrochemical reaction occurs whereby uranium or other metals present in the wastewater complex out of solution and form a solid coagulant. In most cases, inert forms of the metal (usually as oxides) are formed in the treatment of soluble metal solutions. For effective removal of the maximum amount of uranium from the wastewater, a critical current and voltage may be applied via the electrodes to the wastewater. The current and voltage values are dependent on the following critical parameters: (i) number of electrodes used; (ii) total wetted surface area of electrodes in the cell; (iii) number of electrode connections to a DC power source; (iv) size of the gap between the electrodes; (v) pH of the wastewater;

(vi) conductivity of the wastewater; and

(vii) flow rate and cell residence time of the wastewater through the cell.

The optimum parameters can be determined experimentally by a skilled person. If a fixed wastewater flow rate is used, a critical factor in determining the current and voltage values is the cell configuration, i.e. the number of electrodes in the cell, the gap between the electrodes and the number of electrodes that are connected to a DC power source. Electrodes are electrically connected to the DC power source via suitably rated cables and a bus bar arrangement (1), which is bolted directly onto each unipolar electrode (2) by bolts (2A) as required (Fig.4). Preferably, parameters such as the flow rate, total number of electrodes in the cell, the size of the gap between the electrodes and the number of electrodes that are connected to a DC power supply will be fixed. Preferably, the reaction plates extend across the width or length of electrolysis cell 42 and each side of the electrode is contacted by the water to allow maximum contact with the water flowing through the cell (total wetted surface area of cell). If a low flow rate is used (less than 10 IJmin), a smaller cell design with a lower number of electrodes and less total power (voltage and current) is required. If a high flow rate is required (greater than 500 L/min), a large cell design with a greater number of electrodes and more total power (voltage and current) is necessary.

Other important factors are the linear velocity of the solution through the cell and the cell residence time. For example, laminar flow is preferred based upon a cell residence time of 6.99 seconds and (a) a linear flow velocity through the cell, (b) orientated solution entry at the bottom of the cell, and (c) solution output vertically above the solution entry point and at the top of the cell.

If the flow rate is fixed, another determining factor is the conductivity of the solution. For example, the higher the conductivity of the solution, fewer electrodes may be required to be electrically connected to a DC power source to give the optimum current for a determined voltage. If the total number of electrodes and electrodes connected to a DC power source are fixed, the voltage value may decrease while the current value increases as the solution conductivity increases.

The method of determination of optimum voltage and current values is facilitated from knowledge of the chemical composition of the wastewater sample having regard to the criteria given above and use of a variac which is adjustable to increase or decrease voltage and current. Optimum voltage and current values can be determined experimentally by visually observing the reaction point in the electrocoagulation process, i.e. the formation of coagulant in the form of micro-flocs in the discharged wastewater.

Preferably, if an electrocoagulation cell comprises 25 electrodes with a gap of 3 mm between each electrode and 9 electrode connections to a DC power source, the voltage applied to the electrocoagulation cell falls within the range 35-110 volts (DC).

Preferably, if an electrocoagulation cell comprises 25 electrodes with a gap of 3 mm between each electrode and 9 electrode connections to a DC power source, the current applied to the electrocoagulation cell falls within the range 250-490 amps.

The power system controlling the electrocoagulation system may be automated to facilitate precise control and to provide flexibility in controlling the electrocoagulation treatment zone 39.

Preferably, the electrocoagulation treatment zone 39 and associated power system is designed to be compact and portable to facilitate transport to and use in industrial plants and mines.

The outflow from the electrocoagulation cell 42 passes to the coagulation tank 43 through an enclosed, sealed plastic conduit 45 that connects the discharge spout of the cell to the coagulation tank or mixing tank 43. The sealed conduit 45 ensures that water vapour or aerosols do not enter the atmosphere and are drawn into mixing tank 43 by a partial vacuum induced by the flowing stream of water within the conduit 45. The wastewater may be transferred to a settling tank 47 through conduit 46. Flocculating agents may be added to settling tank 47 to accelerate gravity separation of solid particles contained in the discharged wastewater and also to neutralise the anionic or cationic charge of the wastewater. Examples of suitable flocculants can be found in the GE Betz catalogue (www.gebetx.com) and include cationic emulsion polymers and anionic emulsion polymers, such as Novus CE emulsion polymers, polyelectrolytes as described above and AE 1125.

The wastewater may remain in settling tank 47 for one hour. The wastewater may then be passed to a sludge thickener tank 48 through conduit 48A in which particle growth and rapid sedimentation and gravity separation of solid particles occurs. The underflow from the sludge thickener tank 48 may be de-watered or dried prior to disposal by passage through conduit 49 to a suitable disposal location. As shown in Fig. 3, in an alternative to the embodiment shown in Fig.

2, liquid from the settling tank 47 may also be transferred through conduit 51 to a sand filter or other mechanical filtration system 50, for example, a plate and frame type filter press or belt filter that is used to clarify the liquid stream from the settling tank 47. This may be accomplished by pump 13D drawing the liquid through conduit 51.

The outflow from the sludge thickener tank 48 or filtration system 50 may be transferred through conduits 50A or 50B respectively to a holding tank 54 from which the wastewater may again be treated by passage to the electrocoagulation treatment zone 39 by passage through conduit 55 after passage through conduit 58. Any discharge may be passed through conduit 56. It will be appreciated that any second pass of the wastewater through the electrocoagulant treatment zone 39 may not be required, subject to the efficiency of uranium removal through the first pass treatment cycle. An alternative to discharging the treated wastewater would be to reuse, or recycle the treated water. This is particularly advantageous for industrial applications where water usage is high. Any recycled water may be passed through conduit 59. Purified water may also be discharged back into the contaminated water body source.

Modifications may be made to the uranium removal process. Any of the pre-treatment or post-treatment steps may be omitted subject to the nature or composition of the wastewater.

Preferably, radioactive contaminant plant 1 and associated power system is designed to be compact and portable to facilitate transport to and use in mines, industrial plants or at contaminated water bodies. Preferably, it can be mounted on ground engaging wheels or a skid. There also may be provided a generator for providing electrical power to the power supply.

While the invention has been described with the particular reference to uranium species removal it will be understood that, in a modified form, the invention may also be used for the removal of other radioactive species from wastewater. Additional or modified process steps may be required when treating wastewater comprising other radioactive species, to contend with differing chemical properties of the radioactive species. Different voltage and current values, different flow rate and cell residence time, different electrocoagulation cell design, different wastewater pH and conductivity, and different electrodes may be required for effective electrocoagulation. So that the invention may be more readily understood and put into practical effect, the skilled person is referred to the following non-limiting examples.

EXAMPLES The following examples apply to all radioactive species including uranyl oxide such as U3O8. Example 1

The raw wastewater contained 1366 μG/litre of uranium containing compounds and had a conductivity of 630 μS/cm and pH 5.6. The following electrocoagulation parameters were used for a fixed flow rate of 1 litre/minute:

The electrocoagulation cell comprises 8 electrodes with electrical connections to a DC power supply made to 4 electrodes.

The cell required 63 volts and 10 amps for 99% removal of uranium contaminants. Example 2

Electrocoagulation parameters for a fixed flow rate of 100 litres/minute and cell residence time of 6.99 seconds

The electrocoagulation cell comprises 25 electrodes with a gap of 3 mm between each electrode. The total wetted electrode surface area is 8.16 m². Electrical connections to a DC power supply are made to 9 electrodes. If the solution conductivity is 330 +/- 15% μS/cm, the cell requires 75 +/- 15 volts and 314 +/- 15% amps. Example 3

The electrocoagulation cell comprises 25 electrodes with a gap of 3 mm between each electrode. The total wetted electrode surface area is 8.16 m². Electrical connections to a DC power supply are made to 9 electrodes. If the conductivity is 630 μS/cm, the cell requires 55 +/- 15% volts and 404 +/- 15% amps. Example 4

The electrocoagulation cell comprises 25 electrodes with a gap of 3 mm between each electrode. The total wetted electrode surface area is 8.16 m². Electrical connections to a DC power supply are made to 9 electrodes. If the conductivity is 1230 μS/cm, the cell requires 39 +/- 15% volts and 490 +/- 15% amps. Example 5 Electrocoagulation parameters for a fixed flow rate of 100 litres/minute

The electrocoagulation cell comprises 24 electrodes with a gap of 3 mm between each electrode. The total wetted electrode surface area is 7.82 m². Electrical connections to a DC power supply are made to 2 electrodes.

If the conductivity is 20,000 μS/cm, the cell requires 102 +/- 15% volts and

306 +/- 15% amps. Example 6

The wastewater was obtained from an uranium mine pH = 4.5 conductivity = >20,000 μS/cm

The sample contained high concentrations of manganese (> 13 g/L), magnesium (> 3 g/L), copper (> 4 g/L), lead (> 3 g/L) and zinc (> 2.5 g/L), in addition to uranium (> 0.06 g/L).

The wastewater was pre-treated to remove the soluble metal ions

Pre-treatment

The pH was adjusted to pH 10.3 by the addition of NaoH plus lime, and allowed to settle The supernatent was siphoned after settling to remove the metal hydroxides

The pH was adjusted to pH 5.5-6.5

Electrocoagulation treatment pH = 6.15 conductivity = 2,000-3,000 μS/cm

Flow rate = 1 Umin 8 iron electrodes and electrical connections to a DC power supply are made to 2 electrodes

Volts used = 31 Amps used = 10.1

Less than 2 μg/L uranium was present in the treated wastewater (99.97% removal efficiency)

Example 7

The wastewater was obtained from an uranium mine pH = 6.9 conductivity = 1 ,230 μS/cm No pre-treatment was carried out

Flow rate = 1 LJmin

8 iron electrodes and electrical connections to a DC power supply are made to 4 electrodes

Volts used = 39 Amps used = 10.0 99.68% of the uranium was removed

Example 8

The wastewater was obtained from an uranium mine pH = 6.9 conductivity = 330 μS/cm

No pre-treatment was carried out Flow rate = 1 L/min

8 iron electrodes and electrical connections to a DC power supply are made to 2 electrodes

Volts used = 75 Amps used = 6.4

96.9% of the uranium was removed Example 9

The wastewater was obtained from an uranium mine pH = 5.6 conductivity = 630 μS/cm

No pre-treatment was carried out Flow rate = 1 Umin

Volts used = 63 Amps used = 10.2 5 99.92% of the uranium was removed

The advantages of this invention are as follows:

(i) the wastewater is treated on a continuous flow treatment basis, enabling the rapid treatment of large l o volumes of water;

(ii) electrocoagulation destabilizes and therefore removes suspended, emulsified or dissolved contaminants in an aqueous medium; (iii) the process removes > 99% uranium or other is radioactive species from the wastewater;

(iv) the process results in a low volume, aqueous stable sludge that is readily separated from a liquid stream for subsequent disposal. Typically the process generates a much lower amount of sludge compared to conventional 20 methods, such as lime treatment;

(v) a minimum amount of chemical are used in the process; (vi) the process can perform effectively the simultaneous treatment of multiple contaminants; (vii) the system is automated, compact and portable; and

(viii) the compact, mobile plant allows transportation to the industrial site or close to the site of contamination and minimizes costs.

Claims

1. A process to remove radioactive species from wastewater that comprises the steps of:

(a) adjusting the pH of the wastewater to fall within the range of 5.5 - 8.5;

(b) adjusting the conductivity of the wastewater to fall within the range of 300 - 20,000 micro Siemens per centimeter (μS/cm);

(d) separating solids from the wastewater.

2. The process of claim 1 wherein one or more of the variables governing performance of the electrolytic reaction chamber in step (c) which include voltage, amperage, number of electrically connected electrodes, flow rate and conductivity is determined from the following relationships:

(1 ) if the voltage, number of electrically-connected electrodes and the flow rate are fixed, conductivity and amperage are variables; (2) if voltage, conductivity and the flow rate are fixed, the number of electrically connected electrodes and amperage are variables; and (3) if the flow rate, conductivity and the number of electrically connected electrodes are fixed, the amperage and voltage are variables subject to other variables comprising total number of electrodes, total wetted surface of the electrodes in the cell, size of the gap between the electrodes, pH of the wastewater and cell residence time are kept constant.

3. The process of claim 2 wherein use is made of relationship (3).

4. The process of claim 1 wherein the radioactive species is uranium.

5. The process of claim 1 wherein suspended solids, if present in the wastewater, are removed prior to step (c).

6. The process of claim 1 wherein the conductivity of the wastewater falls within the range of 1000 - 3000 μS/cm prior to step (c).

7. The process of claim 1 wherein the electrolytic reaction chamber comprises at least three electrodes and wherein at least two electrodes are electrically coupled to a DC power supply.

8. The process of claim 1 wherein the flow rate of the wastewater through the electrolytic cell falls within the range 50-5000 IJmin.

9. The process of claim 10 wherein the flow rate falls within the range 500- 2000 Umin.

10. The process of claim 1 wherein the electrolytic cell comprises twenty five electrodes and nine electrodes are electrically connected to a DC power source.

11. The process of claim 10 wherein the voltage applied to the electrolytic cell falls within the range 35-110 volts.

12. The process of claim 11 or 12 wherein the current applied to the cell falls within the range 250-490 amps.

13. The process of claim 1 wherein the electrodes are manufactured from a metal selected from the group consisting of mild steel, iron, aluminium and iron alloys.

14. The process of claim 15 wherein the electrodes are manufactured from grade 250 mild steel.

15. The process of claim 1 wherein flocculant is added in step (d) to increase the rate of solid separation.

16. The process of claim 1 wherein greater than 99% of the radioactive species has been removed from the wastewater after step (d).

17. The process of claim 1 wherein the wastewater is pre-treated by addition of base or a source of OH^" to remove contaminant metals.

18. The process of claim 18 wherein contaminant metals include one or more of zinc, copper, manganese, lead and magnesium.