WO2008061310A1

WO2008061310A1 - Regeneration of a lixiviant

Info

Publication number: WO2008061310A1
Application number: PCT/AU2007/001796
Authority: WO
Inventors: Marcus Worsley Richardson; James Andrew Davidson
Original assignee: Heathgate Resources Pty Ltd
Priority date: 2006-11-24
Filing date: 2007-11-22
Publication date: 2008-05-29

Abstract

A method and apparatus for the regeneration of a depleted lixiviant used to recover a metal from a metal ore such as in situ leach mining. The method includes the steps of passing a feed lixiviant stream (12) containing chloride ions and ferrous ions into an electrolytic cell assembly (19) comprising an anode (28a) and a cathode (28b), oxidising chloride ions to chlorine at the anode and allowing the chlorine to oxidise the ferrous ions to ferric ions to form a regenerated lixiviant. The electrolytic cell can be a bipolar cell or a flow cell (40, 60).

Description

REGENERATION OF A LIXIVIANT

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for the regeneration of lixiviant used, for instance, in the leaching of a metal such as uranium.

BACKGROUND OF THE INVENTION

Leaching of uranium ore involves circulating leach fluid through a bed of the ore.

The leach fluid, or lixiviant, is acidified for instance with sulphuric acid and an oxidising agent is added to cause oxidation of the ore. The action of the oxidising agent is to cause conversion to the lower oxidation states of uranium in the ore to the hexavalent uranyl ion, which together with the complexing action of the sulphuric acid dissolves the ore to produce a pregnant lixiviant. The uranium in this pregnant lixiviant is then extracted using an ion exchange or solvent extraction process, and in either case, the captured uranium is further chemically processed to produce a solid uranium oxide product ("yellowcake") which is the valuable product of the operation.

For instance in the mining underground deposits of uranium ore by the acid in- situ leach (ISL) method (also referred to as solution mining) involves circulating leach fluid through the ore using a pattern of wells in the well-field. The leach fluid, or lixiviant, is acidified for instance with sulphuric acid and an oxidising agent is added to cause oxidation of the ore. The action of the oxidising agent is to cause conversion to the lower oxidation states of uranium in the ore to the hexavalent uranyl ion, which together with the complexing action of the sulphuric acid dissolves the ore to produce a pregnant lixiviant. The uranium in this pregnant lixiviant is then extracted using an ion exchange or solvent extraction process. The oxidising agents usually used in ISL mining are oxygen, sodium chlorate or hydrogen peroxide. These oxidising agents regenerate the oxidising capacity of the depleted lixiviant. Sodium chlorate has the disadvantage that its use adds sodium chloride to the lixiviant which is a drawback when ion exchange resins are used to capture the uranium from the pregnant lixiviant (chloride ions compete with the uranium compound for active sites on the resin therefore a larger quantity of resin is required to extract the uranium). Hydrogen peroxide is relatively costly and requires special handling precautions in its concentrated form.

It is an object of the present invention to provide a method for regenerating a depleted leach lixiviant which does not require the addition of oxidizing agents such as oxygen, sodium chlorate or hydrogen peroxide.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method for regenerating a depleted lixiviant used to recover a metal from a metal ore, the method including the step of: passing a feed lixiviant stream containing chloride ions into an electrolytic cell assembly to form an anolyte and a catholyte; oxidising chloride ions in the anolyte to chlorine in the anode cell; combining the anolyte including chlorine with the catholyte from the cathode cell to form a regenerated lixiviant capable of oxidising ferrous ions to ferric ions.

According to another aspect of the present invention, there is provided a method for regenerating a depleted lixiviant used to recover a metal from a metal ore, the method including the step of: separating a feed lixiviant stream containing chloride ions into at least two streams; causing a first stream to pass through an anode cell of an electrolytic cell assembly to form a anolyte and a second stream to pass though a cathode cell of an electrolytic cell assembly to form a catholyte; oxidising the chloride ions in the first stream to chlorine in the anode cell; combining the anolyte including chlorine from the anode cell with the catholyte from the cathode cell to form a regenerated lixiviant capable of oxidising ferrous ions to ferric ions.

Preferably, the anode cell and cathode cell are separated from one another in the electrolytic cell assembly by a separating means which assists in preventing the chlorine formed at the anode from reaching the cathode.

Preferably, the regenerated lixiviant is used to recover a metal from a metal ore in an in-situ leach process. Alternatively the ore may be a milled ore.

The metal recovered from a metal ore is preferably uranium.

Preferably, a chemical species oxidised in the anode cell are ferrous ions which are oxidised to ferric ions.

Further preferably, the dissolved chlorine in the anolyte can oxidise ferrous ions to ferric ions in the combined catholyte and anolyte steams or with further untreated lixiviant which is not passed through the electrolytic cell assembly.

Preferably the separating means is a membrane or diaphragm. More preferably, the membrane or diaphragm defines a plurality of micropores which inhibit bulk fluid flow across the diaphragm in the electrolytic cell. The separating means can be formed from a porous polypropylene material. Alternatively, the separating means can be a porous poly tetrafluoroethylene PTFE membrane or a porous polyvinylchloride PVC membrane.

Preferably, the chemical species restricted from passing through the separating means include chlorine and ferric ions so that these do not reach the cathode.

There can be a plurality of anode and cathode cells. For example, there may be up to 150 cells arranged in stacks for receiving the separate streams of feed lixiviant.

Preferably, the electrodes used in the electrolytic cell are bipolar. Preferably, the electrodes consist of titanium, bare on the cathode side and coated with a mixed metal oxide on the anode side.

In a preferred from of the invention, ferric ions become ferrous ions as they oxidise the uranium ore to provide a soluble uranium ion which can be subsequently extracted as a valuable mining product. The present invention provides a means for re-oxidizing ferrous ions back to ferric ions, quickly, and without the use of expensive oxidizing agents.

Instead of adding oxidizing agents to the lixiviant, ferrous ions in the lixiviant are oxidized by dissolved chlorine in the lixiviant. The chlorine is formed electrochemically from chloride ions in the lixiviant solution which react at the anode of the electrolytic cell assembly to form free chlorine. The dissolved, free chlorine oxidizes the ferrous ions to ferric ions and both species are separated from the cathode by a diaphragm to restrict reduction of them back to the component chloride ions or ferrous ions. By separating the ferric ions from the cathode and therefore preventing the reduction of ferric ions, the concentration of ferric ions in the lixiviant is optimized thereby enhancing the recovery of uranium once the lixiviant is re-circulated into the well-field. According to another aspect of the invention there is provided a method for generating dissolved chlorine in a lixiviant used to recover a uranium metal from a uranium ore in an in-situ leach process, the method including the step of: separating a feed lixiviant stream containing chloride ions into at least two streams; causing a first stream to pass through an electrolytic cell assembly and a second stream to bypass the electrolytic cell assembly; allowing the chloride ions in the first stream to be oxidised to form chlorine in an anode cell of the electrolytic cell assembly; combining the anolyte including chlorine produced from the anode cell with the bypassed feed lixiviant stream to form a regenerated lixiviant used to recover a uranium metal from a uranium ore in the in-situ leach process.

According to another aspect of the invention there is provided a lixiviant regeneration apparatus comprising; an electrolytic cell assembly including a plurality of alternate anode cells and cathode cells; alternate cells being separated by a bipolar electrode and the other alternate cells being separated by an electrolytically conductive separating means which prevents bulk transport of chemical species formed in the anode cell reaching the cathode cell; a depleted lixiviant supply; separation means to separate the depleted lixiviant into at least a cathode side stream and an anode side stream and to supply the cathode side stream to respective cathode cells and the anode side stream to the anode cells; withdrawal means to withdraw catholyte from the cathode cells and to withdraw anolyte from the anode cells; and mixing means to mix the withdrawn anolyte and catholyte. Preferably the lixiviant supply includes a bypass and further including a further mixing means to mix the withdrawn anolyte and catholyte with the bypassed lixiviant.

There may be further including means to recirculate the cathode side stream and the anode side stream through respective cathode cells and anode cells.

In an alternative form the invention is said to comprise a lixiviant regeneration apparatus comprising an electrolytic cell comprising an upstream metal mesh cathode and a downstream anode, a depleted lixiviant supply and means to flow the depleted lixiviant through the electrolytic cell through the cathode and then through the anode.

Throughout this specification unless the context requires otherwise, the words 'comprise' and 'include' and variations such as 'comprising' and 'including' 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.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that such prior art forms part of the common general knowledge.

A specific embodiment of the invention will now be described in some further detail with reference to and as illustrated in the accompanying drawings. This embodiment is illustrative, and is not meant to be restrictive of the scope of the invention. Suggestions and descriptions of other embodiments may be included within the scope of the invention but they may not be illustrated in the accompanying figures or alternatively features of the invention may be shown in the figures but not described in the specification. While the invention is generally discussed in relation to the leaching of uranium ore, it is not so limited and other metal ores which can be extracted using the present method and apparatus are within the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

An illustrative embodiment of the present invention will be discussed with reference to the accompanying drawings and examples wherein:

FIGURE 1 is a schematic of the process according to a preferred embodiment of the present invention;

FIGURE 2 is a schematic of the process according to a preferred embodiment of the present invention for in situ leaching;

FIGURE 3 is a schematic of the process according to a preferred embodiment of the present invention for countercurrent leaching; FIGURE 4 shows detail on one form of electrolytic cell useful for the present invention; and

FIGURE 5 shows an alternative embodiment of an electrolytic cell useful for the present invention.

FIGURE 6 shows an alternative embodiment of an electrolytic cell useful for the present invention. ; and

FIGURE 7 shows schematic flow chart for use of the electrolytic cell of

Figure 6.

DESCRIPTION OF BACKGROUND ART AND PREFERRED EMBODIMENT It is recognized that the chemical species responsible for oxidizing the uranium ore in the leach fluid (or lixiviant) is ferric ions (Fe³⁺). Ferric ions may be naturally present in the acidified leach fluid and/ or may be deliberately added to it (in the form of ferric ions or ferrous ions or both). The reaction is as follows:

Leaching of uranium ore: 2Fe^S+ + 2e- = 2Fe²⁺ (1)

U^IVQ₂ = U^ViO₂ ²⁺ + 2e- (2)

(U^IVO2 can here can more generally represent the uranium contained in leachable ores such as coffinite)

(UO₂)²⁺ + 2(SO₄)²- = UO₂(SO₄)2²- (3)

The sulfato complex ion [UO₂(SO^s]⁴- is also formed by a reaction similar to reaction (3).

The uranyl sulphato anions, including [(UO₂) (SO₄)₂]²' and [(UO₂) (SO₄)3]⁴", are absorbed onto cationic ion exchange resin in a uranium recovery plant, and the uranium eluate from the ion exchange columns is treated (with hydrogen peroxide) to precipitate uranium peroxide (studtite and/ or metastudtite), which is the valuable product produced at the mine.

Once ferric ions have oxidized the uranium ore, they become ferrous ions. The present invention provides a means for re-oxidizing the ferrous ions back to ferric ions, quickly, and without the addition of oxidizing reagents to the lixiviant solution. Instead of adding oxidizing agents to the lixiviant, the ferrous ions are indirectly oxidized by dissolved chlorine (or free chlorine) in the lixiviant in accord with the following reaction:

do + _Fe²⁺ = Cl- + Fe³⁺ (4)

The chlorine is produced by an electrochlorination process i.e. a process where chlorine is generated electrolytically at an anode cell of an electrolytic cell assembly from chloride ions in solution. Preferably, the chloride ions are already present in the lixiviant (this may be the case in areas having highly saline water in the aquifer containing the ore). It is an option, that chloride ions can be added to the lixiviant. The flow rate of the solution through the electrolytic cell assembly can be controlled (at the same cell current) in order to control the concentration of chlorine in the anolyte. For example, a slower flow rate (at the same cell current) will allow more contact time with the anode and therefore create a higher concentration of free chlorine in the anolyte. The dissolved chlorine in solution will exist as free, elemental chlorine.

Figure 1 is a schematic diagram of the electrolytic process 10 in a flowing cell type arrangement and Figure 4 shows detail of the electrolytic cell.

A barren lixiviant stream 12, flows from uranium recovery plant 14 at a rate of approximately 300 litres per second (L/ s). The lixiviant is referred to as 'barren' because the uranium recovery plant 14 has extracted a substantial portion of the valuable constituents from the lixiviant (i.e. uranium).

Part of the barren lixiviant is separated into at least two side streams to undergo electrolytic treatment in an electrolytic cell assembly. Side streams 16 and 18 are shown separated from the circulating barren lixiviant stream 12. Side stream 16 is a feed lixiviant directed through a manifold towards cathode cells 21 (hereinafter the catholyte side stream) and side stream 18 is a feed lixiviant directed through a manifold towards anode cells 17 (hereinafter the anolyte side stream). The anode and cathode cells are more clearly shown in Figure 4. The anolyte is the liquid which flows through the anode cell and is chemically changed by the electrolytic process(es) at the anode. The catholyte is the liquid which flows through the cathode cell and is chemically changed by the electrolytic process(es) at the cathode. Dissolved chlorine is produced in the anolyte side stream 18 by electrolytic oxidation of chloride ions in the anode cell 17 of the electrolytic cell assembly 19. Hydrogen is evolved at the cathode by reduction of hydrogen ions in the acidic lixiviant solution and discharged from surge tank 13 at 33. To prevent the pH from increasing, sulphuric acid 15 is added to the catholyte surge tank 13 from which lixiviant is circulated in the electrolytic cell assembly 19. The hydrogen gas 33 can be recovered if desired.

Preferably, more lixiviant is separated into the anolyte side stream 18 than to the catholyte side stream 16. The efficiency of the process is improved if any ferric ions which exist in the lixiviant are kept from the cathode (since they will be reduced to ferrous ions). The flow rate of the anolyte side stream may be approximately 20 L/ s. the lixiviant flows into anolyte surge tank 23 and from there is passed into electrolyser or electrolytic cell 19. The flow rate of side stream 16 may be approximately 2 L/s, the lixiviant flows into catholyte surge tank 13 and from there is passed into electrolyser or electrolytic cell 19.

Electrolytic cell assembly 19 comprises a number of cells, in Figure 1 there are four cathode cells and four anode cells shown, however, there are preferably 150 cells in three stacks of 50. Figure 4 shows more detail of the electrolytic cell assembly 19. Each cell has a pair of bipolar titanium electrodes 28 with a mixed metal oxide coating on the anode side 28a (the right-hand side of the electrodes as shown schematically in Figure 1). The cathode side 28b of the electrode is the left- hand side of electrode 28 as shown schematically in Figure 1 and may be bare metal. Each bipolar electrode 28 is therefore shared by an adjacent electrolytic cell, except for the electrodes at the end of each stack. The end electrodes have current connections to the external circuit which includes DC power supply 29.

Preferably a substantial portion of the ferric ions in the lixiviant are used to oxidize the uranium ore in the well-field, however, before they are able to oxidize the ore it is possible that the cations will move to the cathode of the electrolytic cell assembly and be reduced back to ferrous ions. Furthermore, it is also possible that the free chlorine produced in solution will be reduced back to its composite ions at the cathode before it has been able to oxidize ferrous ions. In order to prevent his from occurring and to thereby increase the likelihood that uranium is efficiently oxidized, the ferric ions and the free chlorine are restricted from moving to the cathode by a separating means.

Figures 1 and 4 show a porous separating means or diaphragm 30 separating the anode cell from the cathode cell. The chemical species in the anolyte and catholyte are therefore separated which minimises the extent of undesirable reactions (described above) that reduce the efficiency of the system.

The separating means may be a membrane or a diaphragm, or the like, which permits fluid communication throughout the electrolytic cell assembly but which reduces or eliminates the transport of ferric ions and chlorine chemical species from the vicinity of the anode to the vicinity of the cathode. Preferably the separating means is a material which defines a plurality of micropores and which inhibit bulk flow within the electrolytic cell but does not substantially reduce the electrolytic conductivity of the electrolytic cell. The separating means should have the mechanical strength to withstand conditions in the cell and should be durable in the acidic and chlorinated conditions that exist. In the preferred embodiment, the separating means is a polypropylene felt material. Alternatively, the separating means can be a porous polytetrafluoroethylene (PTFE) membrane or a porous polyvinylchloride (PVC) membrane.

Internal manifolds and flow-directors (not shown) are part of the internal structure of electrolytic cell assembly 19. An advantage of using bipolar electrodes is that they do not have external current conductors to provide a current path between the anode side and the cathode side. This leads to simplification of the cell design. Recirculation pumps 31 and 32 increase the flow velocity of the side stream 16 and 18 electrolyte across the electrode surfaces, which increases the efficiency by reducing concentration gradients.

Anolyte side stream 16 having increased levels of dissolved chlorine (and associated hypochlorite ions in equilibrium) and catholyte side stream 18 are recombined with barren circulating lixiviant stream 12 at location 20. The chlorine in the lixiviant rapidly oxidises ferrous ions in the lixiviant stream. The treated or regenerated lixiviant stream 22, which is rich in ferric ions, circulates through a leach step 24 for the extraction of uranium from a uranium containing ore.

The anolyte which is combined with circulating lixiviant 12 at location 20 preferably contains dissolved chlorine at a concentration of around 2g/L. The oxidation of the ferrous ions re-reduces the free chlorine back to chloride ions. Accordingly, the concentration of chloride ions remains unchanged and the process can be referred to as autogenous.

After the lixiviant has passed through the leach step, the lixiviant including the dissolved uranium (referred to as the pregnant lixiviant) 26 flows from leach step 24, and into the uranium recovery plant 14. The uranium product is removed by ion exchange methods in recovery plant 14 (or solvent extraction plant if this is preferred or necessary). The barren lixiviant 12 is again released as stream 12 which is circulated as described above and the process continues.

Figure 2 is a schematic diagram of the electrolytic process 10 in a flowing cell type arrangement for use with in situ leaching. The embodiment shown in Figure 2 is similar to that of Figure 1 in many respects and the same reference numerals are used for corresponding items. A barren lixiviant stream 12 flows from uranium recovery plant 14 at a rate of approximately 300 litres per second (L/ s). The lixiviant is referred to as 'barren' because the uranium recovery plant 14 has extracted a substantial portion of the valuable constituents from the lixiviant (i.e. uranium).

Part of the barren lixiviant is separated into at least two side streams to undergo electrolytic treatment in an electrolyser or electrolytic cell assembly. Side streams 16 and 18 are shown separated from the circulating barren lixiviant stream 12. Side stream 16 is a feed lixiviant directed through a manifold towards cathode cells 21 (hereinafter the catholyte side stream) and side stream 18 is a feed lixiviant directed through a manifold towards anode cells 17 (hereinafter the anolyte side stream). The anolyte is the liquid which flows through the anode cell and is chemically changed by the electrolytic process(es) at the anode. The catholyte is the liquid which flows through the cathode cell and is chemically changed by the electrolytic process(es) at the cathode.

Dissolved chlorine is produced in the anolyte side stream 18 by electrolytic oxidation of chloride ions in the anode cell 17 of the electrolytic cell assembly or electrolyser 19. Hydrogen is evolved at the cathode by reduction of hydrogen ions in the acidic lixiviant solution and discharged from surge tank 13 at 33. To prevent the pH from increasing, sulphuric acid 15 is added to the catholyte surge tank 13 from which lixiviant is circulated in the electrolytic cell assembly 19. The hydrogen gas 33 can be recovered if desired.

Preferably, more lixiviant is separated into the anolyte side stream 18 than to the catholyte side stream 16. The efficiency of the process is improved if any ferric ions which exist in the lixiviant are kept from the cathode (since they will be reduced to ferrous ions). The flow rate of the anolyte side stream may be approximately 20 L/ s. the lixiviant flows into anolyte surge tank 23 and from there is passed into the electrolytic cell assembly 19. The flow rate of side stream 16 may be approximately 2 L/s, the lixiviant flows into catholyte surge tank 13 and from there is passed into the electrolytic cell assembly 19.

Anolyte side stream 16 having increased levels of dissolved chlorine (and associated hypochlorite ions in equilibrium) and catholyte side stream 18 are recombined with barren circulating lixiviant stream 12 at location 20. The chlorine in the lixiviant rapidly oxidises ferrous ions in the lixiviant stream. The treated or regenerated lixiviant stream 22, which is rich in ferric ions, circulates through the in-situ leach well-field pattern 24a for the extraction of uranium from the underground ore deposit.

After the lixiviant passes through the leach step, the lixiviant including the dissolved uranium (referred to as the pregnant lixiviant) 26 flows from leach step 24a and into the uranium recovery plant 14. The uranium product is removed by ion exchange methods in recovery plant 14 (or solvent extraction plant if this is preferred or necessary). The barren lixiviant 12 is again released as stream 12 which is circulated as described above and the process continues.

Figure 3 shows an alternative embodiment of metal recovery process utilising the regeneration process of the present invention. The embodiment shown in Figure 3 is similar to that of Figure 1 in many respects and the same reference numerals are used for corresponding items.

A barren lixiviant stream 12 flows from uranium recovery plant 14 at a rate of approximately 300 litres per second (L/ s). The lixiviant is referred to as 'barren' because the uranium recovery plant 14 has extracted a substantial portion of the valuable constituents from the lixiviant (i.e. uranium).

Preferably, more lixiviant is separated into the anolyte side stream 18 than to the catholyte side stream 16. The efficiency of the process is improved if any ferric ions which exist in the lixiviant are kept from the cathode (since they will be reduced to ferrous ions). The flow rate of the anolyte side stream may be approximately 20 L/s. the lixiviant flows into anolyte surge tank 23 and from there is passed into electrolyser or electrolytic cell 19. The flow rate of side stream 16 may be approximately 2 L/s, the lixiviant flows into catholyte surge tank 13 and from there is passed into electrolyser or electrolytic cell 19.

Anolyte side stream 16 having increased levels of dissolved chlorine (and associated hypochlorite ions in equilibrium) and catholyte side stream 18 are recombined with barren circulating lixiviant stream 12 at location 20. The chlorine in the lixiviant rapidly oxidises ferrous ions in the lixiviant stream. The treated or regenerated lixiviant stream 22, which is rich in ferric ions, circulates through the counter-current leaching process for the extraction of uranium from milled ore. The counter-current leaching process comprises a plurality of leach tanks 25 with ore 27 progressing between tanks in one direction and the lixiviant 22 passing in the other direction.

After the lixiviant passes through the leach process, the lixiviant including the dissolved uranium (referred to as the pregnant lixiviant) 26 flows from leach step 25 and into the uranium recovery plant 14. The uranium product is removed by ion exchange methods in recovery plant 14 (or solvent extraction plant if this is preferred or necessary). The barren lixiviant 12 is again released as stream 12 which is circulated as described above and the process continues.

Figure 5 shows an alternative embodiment of electrolyser useful for the present invention. The electrolytic cell assembly 40 comprises a flow tube 42 having a plurality of alternate anodes and cathodes 43 spaced across the flow path through the flow tube 42. First anode 46 and last cathode 48 are connected to a DC power supply 50. Each cathode and anode 43 is preferably combined as a titanium plate having an anode side 44 and a cathode side 45.

Figures 6 and 7 show an alternative embodiment of electrolytic cell, suitable for the present invention. In this embodiment a flow through electrolytic cell 50 comprises a tube 62 with an upstream cathode 64 and a downstream anode 66. Each of the anode and cathode are formed from an electrically conductive mesh such as a pressed metal mesh which have the advantage of high surface area and angles to induce turbulence for good mixing and for good electrical contact. A DC power supply 68 is connected between the anode and cathode. The flow rate of lixiviant through the cell is preferably greater than the drift velocity of ions from the anode to the cathode such that ferric ions produced adjacent to the anode from reaction with chlorine produced at the anode cannot drift under the electric field between the anode and cathode to the cathode and be reconverted back to ferrous ions again.

As shown in Figure 7 there may be a plurality of electrolytic cells 60 arranged in parallel with a flow of barren lixiviant 70 split between them. The electrolytic cells are arranged electrically in series with the anode of the first electrolytic cell assembly and the cathode of the last electrolytic cell assembly connected to the power supply and the intermediate cathodes connected to the anode of the next cell by electrical connection 73.

The invention will now be further explained with the use of several examples.

Oxidation of ferrous iron in acid in situ leaching barren lixiviant by dissolved chlorine.

The replacement of hydrogen peroxide as oxidant for the barren lixiviant at an in situ leaching mine was tested by adding a solution of chlorine water (water containing dissolved chlorine, prepared by bubbling chlorine gas through water- dissolved chlorine is taken to mean the amount of chlorine gas taken up in solution and available for oxidation, as shown by iodimetric titration) to 400 ml of unoxidised barren lixiviant taken from the uranium recovery plant. The chlorine water was added in increments to the stirred solution and the oxidation process monitored by the use of an oxidation/ reduction potential (ORP) electrode.

The barren lixiviant had a total iron concentration of 0.366 g/L as determined by X-ray fluorescence, and a residual uranium concentration of 13 mg/L. The oxidation/ reduction potential increased from 475 mV (Pt electrode and AgCl/ saturated KCL reference electrode) to 550 mV on addition of 9.5 ml of chlorine water containing 4.7 g/L dissolved chlorine. An ORP of 550 mV corresponds to the value normally maintained in the oxidised barren lixiviant by the addition of hydrogen peroxide. The reaction with the chlorine water added was very fast such that a time delay between the addition of the chlorine water and the rise in ORP measured with the electrodes could not be observed. By 'barren lixiviant' is meant the lixiviant used in the in-situ mining process from which uranium has been extracted and which is recirculated through the underground mineralised uranium deposit to react with and extract the uranium.

Electrolytic oxidation of ferrous iron in diaphragm cells

The electrolytic production of dissolved chlorine was demonstrated in cells where a porous ceramic diaphragm was used to separate the cell into an anode and a cathode compartment. The anodes comprised titanium sheet or mesh coated with a mixed metal electrocatalytic coating, and the cathodes comprised bare titanium metal sheet or mesh. Positive displacement peristaltic pumps were used to provide flows of electrolyte through the anode and the cathode compartments to form separate streams of treated anolyte and catholyte, respectively.

The electrolyte treated in this manner was unoxidised barren lixiviant obtained from an in situ leaching mine. Additional sulphuric acid was added to the catholyte stream to maintain the pH of the treated lixiviant and to prevent the formation of precipitates in the cathode compartment of the cells. To simulate the electrochemical process whereby the ferrous iron in the lixiviant is oxidised the anolyte and catholyte was combined after the two separate streams had passed through the electrolytic cell assembly.

Results of a test:

Untreated, unoxidised lixiviant:

PH 1.67

ORP 477 mV

Fe concentration 0.3 g/L (by XRF)

Cl concentration 5 g/L (by XRF) Volume treated in test run 25 L

Anolyte:

Flow rate 500 ml/min

ORP after electrolysis 1.080 V (free chlorine pr

^■ Catholyte: Sulphuric acid addition 2 g/L concentrated acid

Flow rate 50 ml/min

Electrolysis:

Cell voltage 4.9 V

Cell current 4.5 A Re-mixed anolyte + catholyte streams, after electrolysis: pH 1.65

ORP 575 mV

Flow-through cell tests with mesh electrodes and no diaphragm. Tests were made with coaxial, tubular anodes and cathodes. The central cathode was made from titanium mesh and had a diameter of 70 mm and a length of 250 mm. It was separated from the outer tubular anode by silicone rubber spacers. The anode was a tube of titanium metal mesh of diameter 85 mm with a mixed metal oxide coating. The bottom of the coaxial tubular electrode assembly was closed off by a plastic cup, causing the lixiviant introduced through the central axial plastic distributor tube to pass first through the spaces in the mesh cathode then through the mesh anode.

Untreated barren lixiviant (unoxidised by hydrogen peroxide) was introduced to the coaxial electrode arrangement through a 20 mm diameter PVC tube closed at the bottom end and having 66 holes of 5 mm diameter evenly spaced along the submerged length. The whole arrangement was placed in a vertical plastic tube closed at the bottom and with an arrangement that enabled overflow and collection of the treated electrolyte that had flowed through the electrodes from inside to outside.

Prior to the test, the lixiviant was acidified with 2 ml of concentrate sulphuric acid per litre. The flow rate of lixiviant through the electrolytic cell assembly was set to 1 L/ minute.

Untreated, unoxidised lixiviant: pH 1.6

ORP before electrolysis 472 mV

Fe concentration 0.32 g/L (by XRF). Cl concentration 5.6 g/L (by XRF)

Flow rate 1.0 L/min

Volume treated in test run 90 L Treated lixiviant:

ORP after electrolysis 1.15 V (free chlorine present). Electrolysis:

Cell voltage 3.7 V

Cell current 12 A

On addition of 90 ml of untreated lixiviant with 500 ml of electrolysed lixiviant and mixing, the ORP of the mixture dropped without any observable delay to 560 mV, which is the typical value for the barren lixiviant at the Beverley Plant after oxidation with hydrogen peroxide.

Dissolved chlorine as oxidant for milling uranium ore - comparison with other oxidants.

The following test results support the claim that dissolved chlorine produced by electrolysis of a lixiviant containing chloride, sulphuric acid and iron can be advantageously used in a milling process to extract uranium from ore containing uranium minerals.

So-called surface milling processes are known in which crushed and ground uranium ore is leached using a lixiviant containing sulphuric acid and dissolved iron, and to which an oxidant is added to maintain the dissolved iron in the ferric state as it oxidises the ore. Commonly used oxidants are hydrogen peroxide, sodium chlorate and manganese dioxide. It is also known that chlorine can fulfil the role of oxidant, in which case it could be added to the leaching process as gaseous chlorine obtained by the evaporation of liquid chlorine transported to the mine and held in pressurised vessels.

The benefit of using electrolytically produced dissolved chlorine in the lixiviant used for surface milling would be that it would avoid the transportation and handling of liquid chlorine in pressurised tanks and equipment, and potentially be more cost effective provided the cost of electricity for the electrolysis process was not excessive. Ore sample for leach tests:

Ore Coffinate, coated on sand grains. From subterranean ore deposit in the vicinity of the Beverley Uranium Mine. Lixiviant:

Sulphuric acid 0.01 M, approximately pH 2

Iron content 2 g/L added as FeSO₄.7H₂0 Oxidants:

Chlorine Solution of chlorine in water

Hydrogen peroxide 5ml 70% H₂O₂ diluted to 1 L.

Concentration assayed by iodimetric titration.

Sodium chlorate 0.25 % solution.

Manganese dioxide Solid powder.

Agitation leach tests:

100 g ore plus 350 ml lixiviant in 500 ml beaker, stirred at about 300 rpm. Tests repeated two to three times.

Temperature Tests done at 40 C, except for some additional chlorine leaching tests at 60 C

Oxidant addition Started after 30 minutes, to allow for decomposition of any sulphides or calcite present. Subsequently added during leach test to maintain an ORP of 528 mV (Pt electrode against Ag/ AgCl/ satd KCl reference electrode).

Sulphuric acid addition Added to maintain pH 2 during course of leach test. Sampling 0.5, 2, 6, 12, 18 hours after oxidant addition.

Summary of results: At 40 C the leaching with oxidant was over 80% complete after four hours.

* Includes cost estimate for electrochemical generation of dissolved chlorine in the lixiviant.

Although preferred embodiments of the apparatus of the present invention has been described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention. Modifications and variations such as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.

Co-pending Patent Application entitled "Modifying a Lixiviant" filed concurrently with this application provides details of processes for modifying lixiviants by removal of chemical species particularly chloride from solution for uranium extraction processes and the teachings therein is incorporated herein in their entirety.

Claims

1. A method for regenerating a depleted lixiviant used to recover a metal from a metal ore, the method including the step of: passing a feed lixiviant stream containing chloride ions and ferrous ions into an electrolytic cell assembly comprising an anode and a cathode; oxidising chloride ions to chlorine at the anode; allowing the chlorine to oxidise the ferrous ions to ferric ions to form a regenerated lixiviant,

2. A method according to Claim 1 wherein the regenerated lixiviant is used to recover a metal from a metal ore in an in-situ leach process.

3. A method according to Claim 2, wherein the metal recovered from the metal ore is uranium.

4. A method according to Claim 1 wherein the regenerated lixiviant is used to recover a metal from a metal ore in a counter-current leaching process.

5. A method according to Claim 4, wherein the metal recovered from the metal ore is uranium.

6. A method according to Claim 1 wherein the electrolytic cell comprises an anode cell and a cathode cell and the feed lixiviant stream is divided up into at least two streams and a first stream is passed through the anode cell to form an anolyte and a second stream is passed though the cathode cell to form a catholyte.

7. A method according to Claim 6, wherein title electrolytic cell comprises an anode cell and a cathode cell which are separated from one another in the electrolytic cell assembly by a separating means.

8. A method according to Claim 7 wherein the separating means is a membrane formed from a porous material selected from the group including polypropylene, polytetrafluoroethylene (PTFE) and polyvinylchloride (PVC).

9. A method according to Claim 6, wherein a plurality of anode and cathode cells are arranged in stacks for receiving the separate streams of feed lixiviant.

10. A method according to Claim 1, wherein electrodes used in the electrolytic cell are bipolar.

11. A method according to Claim 10 wherein the electrodes comprise titanium, bare on the cathode side and coated with a mixed metal oxide on the anode side.

12. A method as in Claim 1 wherein the electrolytic cell is a flow cell comprising an upstream cathode and a downstream anode.

13. A method as in Claim 12 wherein the upstream cathode and the downstream anode are expanded metal electrodes.

14. A method for regenerating a depleted lixiviant used to recover a metal from a metal ore in a leaching process, the method including the step of: separating a feed lixiviant stream containing chloride ions into at least two streams; causing a first stream to pass through an anode cell of an electrolytic cell assembly to form an anolyte and a second stream to pass though a cathode cell of an electrolytic cell assembly to form a catholyte; oxidising the chloride ions in the first stream to chlorine in the anode cell; combining the anolyte including chlorine from the anode cell with the catholyte from the cathode cell to form a regenerated lixiviant capable of oxidising ferrous ions to ferric ions.

15. A lixiviant regeneration apparatus comprising; an electrolytic cell assembly comprising an anode cell with an anode and a cathode cell with a cathode; a depleted lixiviant supply; means to withdraw at least a proportion of the depleted lixiviant supply and to split the withdraw depleted lixiviant supply into a first side stream and a second side stream means to flow the first side stream of depleted lixiviant through the anode cell over the anode; means to flow the second side stream of depleted lixiviant through the cathode cell over the cathode, and means to withdraw the first side stream and the second side stream from the anode cell and cathode cell respectively and to recombine the first side stream and the second side stream to form the regenerated lixiviant.

16. A lixiviant regeneration apparatus as in Claim 15 wherein the depleted lixiviant supply includes a bypass and further including a further mixing means to mix the withdrawn regenerated lixiviant with the bypassed lixiviant.

17. A lixiviant regeneration apparatus as in Claim 15 wherein the electrolytic cell assembly includes a plurality of alternate anode cells and cathode cells; alternate cells being separated by a bipolar electrode and the other alternate cells being separated by an electrolytically conductive separating means which prevents bulk transport of chemical species formed in the anode cell reaching the cathode cell.

18. A lixiviant regeneration apparatus as in Claim 17 further including means to recirculate the first side stream and the second side stream through respective anode cells and cathode cells.

19. A lixiviant regeneration apparatus comprising; an electrolytic cell comprising an upstream metal mesh cathode and a downstream anode; a depleted lixiviant supply; and means to flow the depleted lixiviant through the electrolytic cell through the cathode and then through the anode.